[0001] The present invention relates to a high-strength cold-rolled steel sheet having a
tensile strength of 980 MPa or more and being excellent in formability and crashworthiness
and to a method for producing the same. In further detail, the present invention relates
to the high-strength cold-rolled steel sheet described above, a high-strength electrogalvanized
steel sheet having an electrogalvanized layer formed on a surface of the high-strength
cold-rolled steel sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip
galvanized layer formed on a surface of the high-strength cold-rolled steel sheet,
and a high-strength hot-dip galvannealed steel sheet having a hot-dip galvannealed
layer formed on a surface of the high-strength cold-rolled steel sheet, and to a method
for producing the same.
[0002] In order to achieve fuel cost reduction of automobiles, transport aircrafts and the
like, it is desired to reduce the weight of the automobiles, transport aircrafts and
the like. In order to achieve weight reduction, it is effective, for example, to reduce
the sheet thickness by using a high-strength steel sheet. However, when the steel
sheet is made to have a higher strength, the steel sheet comes to have poorer ductility
and stretch-flangeability, thereby degrading the formability into a product shape.
[0003] Also, in steel parts for automobiles, a steel sheet whose surface has been subjected
to galvanization such as electrogalvanization (which may hereafter be denoted as EG),
hot-dip galvanizing (which may hereafter be denoted as GI), or hot-dip galvannealing
(which may hereafter be denoted as GA), which may hereafter be comprehensively referred
to as galvanized steel sheet, is often used from the viewpoint of corrosion resistance.
In these galvanized steel sheets as well, increase in strength and formability is
demanded in the same manner as in the above high-strength steel sheet.
[0004] For example, Patent Literature 1 discloses a hot-dip galvannealed steel sheet having
a metal structure in which martensite and retained austenite are mixedly present in
ferrite and having a tensile strength TS of 490 to 880 MPa by reinforcement of the
complex structure thereof, thus having a good press formability.
[0005] Also, Patent Literature 2 discloses a high-strength cold-rolled steel sheet having
a TS (Tensile Strength) of 590 MPa or more and being excellent in formability, specifically,
with TS × EL (EL: Elongation, elongation) being 23000 MPa% or more, and being excellent
in corrosion resistance after application even in a severe environment such as a hot
salt water test, a salt water spraying test, or a combined cyclic corrosion test.
The metal structure of this steel sheet is a structure including ferrite, retained
austenite, bainite, and/or martensite. It is described that the retained austenite
has a function of enhancing ductility of the steel sheet, the function being known
as a TRIP effect.
[0006] In the meantime, it is demanded that the steel parts for automobiles are excellent
in crashworthiness which is an ability to efficiently absorb an impact generated when
the automobiles come into collision. There is known, for example, Patent Literature
3 as a technique for improving the crashworthiness. Patent Literature 3 discloses
a high-strength galvanized steel sheet having a maximum tensile strength of 900 MPa
or more and being excellent in collision absorption energy in which a dynamic/static
ratio as large as that of a steel sheet of 590 MPa class and a maximum tensile strength
of 900 MPa or more are compatible with each other, as well as a method for producing
the same. This production method is characterized in that, after performing galvanization,
cooling is performed, and rolling is performed with use of a roll having a roughness
(Ra) of 3.0 or less.
US 2014/182748 A1 discloses a method for manufacturing a high strength galvanized steel sheet with
excellent formability.
JP 2014 095155 A discloses a cold rolled steel and its manufacturing method.
[0007]
Patent Literature 1: Japanese Patent No. 3527092
Patent Literature 2: Japanese Patent No. 5076434
Patent Literature 3: Japanese Patent No. 5487916
[0008] According to the techniques disclosed in Patent Literatures 1 and 2, the formability
of a steel sheet can be improved. However, no consideration is made on the crashworthiness.
In contrast, according to the technique disclosed in Patent Literature 3, the crashworthiness
of the steel sheet can be improved. However, no consideration is made on the formability
as evaluated by ductility and stretch-flangeability.
[0009] The present invention has been made in view of the aforementioned circumstances,
and an object thereof is to provide a high-strength cold-rolled steel sheet having
a tensile strength of 980 MPa or more, having good formability as evaluated by ductility
and stretch-flangeability, and having excellent crashworthiness. Another object of
the present invention is to provide a high-strength electrogalvanized steel sheet
having an electrogalvanized layer on a surface of the high-strength cold-rolled steel
sheet, a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized
layer on a surface of the high-strength cold-rolled steel sheet, and a high-strength
hot-dip galvannealed steel sheet having a hot-dip galvannealed layer on a surface
of the high-strength cold-rolled steel sheet. Still another object of the present
invention is to provide a method for producing a high-strength cold-rolled steel sheet,
a high-strength hot-dip galvanized steel sheet, and a high-strength hot-dip galvannealed
steel sheet having the above properties in combination.
[0010] A high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or more
according to the present invention that has solved the aforementioned problems is
a steel sheet containing, in mass%, C: 0.10% or more to 0.5% or less, Si: 1.0% or
more to 3% or less, Mn: 1.5% or more to 7% or less, P: more than 0% to 0.1% or less,
S: more than 0% to 0.05% or less, Al: 0.005% or more to 1% or less, N: more than 0%
to 0.01% or less, and O: more than 0% to 0.01% or less, and optionally, as other elements,
one or more of any of (a) to (e) below in mass%:
- (a) at least one selected from the group consisting of Cr: more than 0% to 1% or less
and Mo: more than 0% to 1% or less,
- (b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or
less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less,
- (c) at least one selected from the group consisting of Cu: more than 0% to 1% or less
and Ni: more than 0% to 1% or less,
- (d) B: more than 0% to 0.005% or less,
- (e) at least one selected from the group consisting of Ca: more than 0% to 0.01% or
less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less, with
a balance being iron and inevitable impurities. Further, the gist lies in that a metal
structure at a position of 1/4 of a sheet thickness satisfies (1) to (4) below. The
term "MA" is an abbreviation for Martensite-Austenite Constituent.
- (1) When the metal structure is observed with a scanning electron microscope, an area
ratio of ferrite relative to a whole of the metal structure is more than 10% to 65%
or less, with a balance being a hard phase including quenched martensite and retained
austenite and including at least one selected from the group consisting of bainitic
ferrite, bainite, and tempered martensite, wherein the metal structure may, in addition
to ferrite and the hard phase, contain at least one selected from the group consisting
of pearlite and cementite.
- (2) When the metal structure is measured by X-ray diffractometry, a volume ratio Vγ of retained austenite relative to the whole of the metal structure is 5% or more
to 30% or less.
- (3) When the metal structure is observed with an optical microscope, an area ratio
VMA of an MA structure, in which quenched martensite and retained austenite are combined,
relative to the whole of the metal structure is 3% or more to 25% or less, and an
average circle-equivalent diameter of the MA structure is 2.0 µm or less.
- (4) A ratio VMA/Vγ of the area ratio VMA of the MA structure to the volume ratio Vγ of the retained austenite satisfies a formula (i) below:
and wherein
the value of tensile strength TS × elongation EL is 17000 MPa·% or more, the value
of TS × expansion ratio λ is 20000 MPa·% or more, and the value of TS × VDA bending
angle is 90000 MPa·° or more.
[0011] A high-strength electrogalvanized steel sheet having an electrogalvanized layer on
a surface of the high-strength cold-rolled steel sheet, a high-strength hot-dip galvanized
steel sheet having a hot-dip galvanized layer on a surface of the high-strength cold-rolled
steel sheet, and a high-strength hot-dip galvannealed steel sheet having a hot-dip
galvannealed layer on a surface of the high-strength cold-rolled steel sheet are also
comprised within the scope of the present invention.
[0012] The high-strength cold-rolled steel sheet having a tensile strength of 980 MPa or
more and being excellent in formability and crashworthiness according to the present
invention can be produced by subjecting a steel satisfying a component composition
described above to hot rolling with a rolling rate at a final stand of finish rolling
being 5 to 20% and with a finish rolling end temperature being the Ar
3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and heating and holding in a temperature region of the cooling stop temperature T+50°C
or higher and 550°C or lower for 200 seconds or more, and thereafter cooling to room
temperature, wherein the Ms point is calculated on the basis of the following formula
(iv), wherein brackets [ ] indicate the content of each element in mass% and Vf indicates
the area ratio of ferrite relative to the whole of the metal structure,
[0013] A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa
or more and being excellent in formability and crashworthiness according to the present
invention can be produced by subjecting a steel satisfying a component composition
described above to hot rolling with a rolling rate at a final stand of finish rolling
being 5 to 20% and with a finish rolling end temperature being the Ar
3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and a reheating of heating and holding in a temperature region of the cooling stop
temperature T+50°C or higher and 550°C or lower for 200 seconds or more, and after
performing hot-dip galvanizing within a holding time, cooling to room temperature,
wherein
in the reheating, there are three patterns of (I) to (III) as a combination of the
hot-dip galvanizing and only the heating without performing the hot-dip galvanizing,
- (I) only the heating is carried out, and thereafter, the hot-dip galvanizing is carried
out;
- (II) the hot-dip galvanizing is carried out, and thereafter, only the heating is carried
out;
- (III) only the heating is carried out, and thereafter, the hot-dip galvanizing is
carried out, and further, only the heating is carried out, in this order,
wherein the Ms point is calculated on the basis of the following formula (iv), wherein
brackets [ ] indicate the content of each element in mass% and Vf indicates the area
ratio of ferrite relative to the whole of the metal structure,
[0014] A high-strength hot-dip galvannealed steel sheet having a tensile strength of 980
MPa or more and being excellent in formability and crashworthiness according to the
present invention can be produced by subjecting a steel satisfying a component composition
described above to hot rolling with a rolling rate at a final stand of finish rolling
being 5 to 20% and with a finish rolling end temperature being the Ar
3 point or higher to 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and a reheating of heating and holding in a temperature region of higher than the
cooling stop temperature T+50°C or higher and 550°C or lower for 200 seconds or more,
and after performing hot-dip galvanizing within a holding time, further performing
an alloying treatment and thereafter cooling to room temperature, wherein
in the reheating, there are three patterns of (I) to (III) as a combination of the
hot-dip galvanizing and only the heating without performing the hot-dip galvanizing,
- (I) only the heating is carried out, and thereafter, the hot-dip galvanizing is carried
out;
- (II) the hot-dip galvanizing is carried out, and thereafter, only the heating is carried
out;
- (III) only the heating is carried out, and thereafter, the hot-dip galvanizing is
carried out, and further, only the heating is carried out, in this order,
wherein the Ms point is calculated on the basis of the following formula (iv), wherein
brackets [ ] indicate the content of each element in mass% and Vf indicates the area
ratio of ferrite relative to the whole of the metal structure,
[0015] According to the present invention, the component composition and the metal structure
are suitably controlled, so that a high-strength cold-rolled steel sheet, a high-strength
electrogalvanized steel sheet, a high-strength hot-dip galvanized steel sheet, and
a high-strength hot-dip galvannealed steel sheet having a tensile strength of 980
MPa or more and being excellent both in formability as evaluated by ductility and
stretch-flangeability and in crashworthiness can be provided. The high-strength cold-rolled
steel sheet, the high-strength electrogalvanized steel sheet, the high-strength hot-dip
galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet according
to the present invention is particularly excellent in ductility among the formability
properties. The present invention can also provide a method for producing the high-strength
cold-rolled steel sheet, the high-strength electrogalvanized steel sheet, the high-strength
hot-dip galvanized steel sheet, and the high-strength hot-dip galvannealed steel sheet
described above. The high-strength cold-rolled steel sheet, the high-strength electrogalvanized
steel sheet, the high-strength hot-dip galvanized steel sheet, and the high-strength
hot-dip galvannealed steel sheet according to the present invention are extremely
useful in the fields of industry such as automobiles.
[0016] FIG. 1 is a schematic descriptive view showing one example of a heat treatment pattern
performed in the Examples.
[0017] The present inventors have repeatedly made eager studies in order to improve all
of ductility, stretch-flangeability, and crashworthiness in a high-strength cold-rolled
steel sheet having a tensile strength of 980 MPa or more. As a result, the present
inventors have found out that, in order to improve the ductility while ensuring the
tensile strength by setting a ferrite fraction in the metal structure to be within
a predetermined range and setting the balance structure to be a hard phase, it is
effective to generate a predetermined amount of ferrite and to appropriately control
a ratio V
MA/V
γ of an area ratio V
MA of an MA structure, in which quenched martensite and retained austenite are combined,
to a volume ratio V
γ of retained austenite relative to the whole of the metal structure and that, in order
to improve the stretch-flangeability, it is effective to make the MA structure finer
and, in order to improve the crashworthiness, it is effective to make the MA structure
finer and to appropriately control the above ratio V
MA/V
γ, thereby completing the present invention.
[0018] First, the metal structure characterizing the present invention will be described.
[0019] The high-strength cold-rolled steel sheet according to the present invention is characterized
in that the metal structure at a position of 1/4 of the sheet thickness satisfies
(1) to (4) below.
- (1) When the metal structure is observed with a scanning electron microscope, the
area ratio of ferrite relative to the whole of the metal structure is more than 10%
and 65% or less, with the balance being a hard phase including quenched martensite
and retained austenite and including at least one selected from the group consisting
of bainitic ferrite, bainite, and tempered martensite, wherein the metal structure
may, in addition to ferrite and the hard phase, contain at least one selected from
the group consisting of pearlite and cementite.
- (2) When the metal structure is measured by X-ray diffractometry, the volume ratio
Vγ of retained austenite relative to the whole of the metal structure is 5% or more
and 30% or less.
- (3) When the metal structure is observed with an optical microscope, the area ratio
VMA of an MA structure, in which quenched martensite and retained austenite are combined,
relative to the whole of the metal structure is 3% or more and 25% or less, and an
average circle-equivalent diameter of the MA structure is 2.0 µm or less.
- (4) The volume ratio Vγ of the retained austenite and the area ratio VMA of the MA structure satisfy a formula (i) below:
[0020] The observation of the above metal structure is carried out all at the position of
1/4 of the sheet thickness, as representing the steel sheet.
[0021] Methods of measuring the fractions in the metal structure as defined in the above
(1) to (3) may differ from each other, so that a sum of the fractions may exceed 100%.
In other words, in the above (1), the metal structure is observed with a scanning
electron microscope, so that the measured area ratio is a ratio obtained when the
whole of the metal structure is assumed to be 100%. The area ratio measured with use
of a scanning electron microscope includes that of quenched martensite and retained
austenite as an area ratio of the hard phase. On the other hand, in the above (2),
the retained austenite fraction in the metal structure is calculated by X-ray diffractometry,
while in the above (3), the area ratio of the MA structure in which quenched martensite
and retained austenite are combined is observed with an optical microscope. For this
reason, the fraction of retained austenite and quenched martensite is measured in
a duplicated manner by a plurality of methods. Also, hereafter, the retained austenite
may be denoted as retained γ. Accordingly, a sum of the fractions in the metal structure
as defined in the above (1) to (3) may exceed 100%. Also, the structure in which quenched
martensite and retained γ are combined may be denoted as MA structure.
- (1) In the present invention, the area ratio of ferrite relative to the whole of the
metal structure is set to be more than 10% to 65% or less when the metal structure
is observed with a scanning electron microscope. Ferrite is a structure that particularly
enhances ductility among the formability properties of the steel sheet. In order that
such an effect may be exhibited, the area ratio of ferrite is set to be more than
10% in the present invention. The area ratio of ferrite is preferably 15% or more,
more preferably 20% or more. However, when ferrite is excessive in amount, the strength
of the steel sheet decreases, so that a tensile strength of 980 MPa or more cannot
be ensured. Accordingly, in the present invention, the area ratio of ferrite is set
to be 65% or less. The area ratio of ferrite is preferably 60% or less, more preferably
50% or less.
The balance of the above metal structure is a hard phase including quenched martensite
and retained γ as an essential structure and including at least one selected from
the group consisting of bainitic ferrite, bainite, and tempered martensite. These
hard phases constitute a structure that is harder than ferrite, so that, by generating
a predetermined amount of ferrite and making the balance structure be a hard phase,
the strength of the steel sheet can be enhanced to be 980 MPa or more. The reason
why quenched martensite and retained γ are contained as an essential structure is,
as described later, for the purpose of generating a predetermined amount of an MA
structure in which quenched martensite and retained γ are combined.
In addition to ferrite and the hard phase, the above metal structure may contain at
least one selected from the group consisting of pearlite and cementite. A sum area
ratio of pearlite and cementite is not particularly limited as long as the effect
of the present invention is not deteriorated; however, the sum area ratio is preferably,
for example, 20% or less. The sum area ratio is more preferably 15% or less, still
more preferably 10% or less.
The area ratio of the above metal structure may be calculated by performing observation
with a scanning electron microscope after the position of 1/4 of the sheet thickness
is corroded with nital, and the observation magnification may be set to be, for example,
1000 times.
- (2) In the present invention, when the metal structure is measured by X-ray diffractometry,
the volume ratio Vγ of retained γ relative to the whole of the metal structure is set to be 5% or more
to 30% or less. The retained γ produces an effect of suppressing concentration of
strain by receiving the strain so as to be deformed and transformed into martensite
when the steel sheet is processed, thereby promoting hardening of the deformed portion
during the processing. For this reason, the strength - elongation balance of the steel
sheet is enhanced, and the ductility can be improved. In order that such an effect
may be exhibited, it is necessary that the volume ratio of retained γ is set to be
5% or more. The volume ratio of retained γ is preferably 6% or more, more preferably
7% or more. However, when the volume ratio of retained γ increases excessively, the
stretch-flangeability becomes deteriorated. Accordingly, the volume ratio of retained
γ is set to be 30% or less in the present invention. The volume ratio of retained
γ is preferably 25% or less, more preferably 20% or less.
The above volume ratio of retained γ may be determined by measuring the position of
1/4 of the sheet thickness by X-ray diffractometry. The retained γ exists between
the laths of bainitic ferrite or by being included in the MA structure. The above
effect by the retained γ is exhibited irrespective of the existence form, so that,
in the present invention, the volume ratio was determined by calculating a sum of
the amounts of all the retained γ measured by X-ray diffractometry irrespective of
the existence form.
- (3) In the present invention, when the metal structure is observed with an optical
microscope, the area ratio VMA of the MA structure relative to the whole of the metal structure is set to be 3%
or more to 25% or less. The above MA structure is a structure that enhances the strength
- elongation balance of the steel sheet and can improve the ductility. In order that
such an effect may be exhibited, it is necessary that the area ratio of the MA structure
is set to be 3% or more. The area ratio of the MA structure is preferably 4% or more,
more preferably 5% or more. However, when the area ratio of the MA structure increases
excessively, the crashworthiness becomes deteriorated. Accordingly, the area ratio
of the MA structure is set to be 25% or less in the present invention. The area ratio
of the MA structure is preferably 23% or less, more preferably 20% or less.
Also, in the present invention, the average circle-equivalent diameter of the MA structure
is set to be 2.0 µm or less. By making the MA structure be finer, the stretch-flangeability
and the crashworthiness can be enhanced. In order that such an effect may be exhibited,
it is necessary that the average circle-equivalent diameter of the MA structure is
set to be 2.0 µm or less. The average circle-equivalent diameter of the MA structure
is preferably 1.8 µm or less, more preferably 1.5 µm or less. According as the MA
structure becomes finer, the stretch-flangeability and the crashworthiness will be
better, so that a lower limit of the average circle-equivalent diameter of the MA
structure is not particularly limited; however, from an industrial point of view,
the lower limit is about 0.1 µm.
The above MA structure is a structure in which quenched martensite and retained γ
are combined. The quenched martensite means a structure in a state in which untransformed
austenite is transformed into martensite during the process in which the steel sheet
is cooled from the heating temperature down to room temperature. By observation with
an optical microscope, the quenched martensite can be distinguished from the tempered
martensite that has been tempered by a heating treatment. In other words, when the
metal structure is observed with an optical microscope after being subjected to LePera
corrosion, the quenched martensite is observed to be white whereas the tempered martensite
is observed to be gray.
The quenched martensite and the retained γ are hardly distinguished from each other
by observation with an optical microscope, so that the structure in which quenched
martensite and retained γ are combined is measured as the MA structure in the present
invention.
The area ratio of the above MA structure is a value as measured at the position of
1/4 of the sheet thickness of the steel sheet.
The average circle-equivalent diameter of the MA structure is a value determined by
calculating a circle-equivalent diameter based on the area of each MA structure for
all the MA structures that are recognized in the field - of observation and calculating
an average of the obtained circle-equivalent diameters.
- (4) In the present invention, it is important that the ratio VMA/Vγ of the area ratio VMA of the MA structure to the volume ratio Vγ of the retained γ satisfies the following formula (i):
[0022] The ductility and the crashworthiness are rendered compatible with each other when
the value of the above ratio V
MA/V
γ is controlled to satisfy the above formula (i). In other words, as described above,
the retained γ is positively generated in the present invention in order to enhance
the strength - elongation balance that constitutes an index of ductility. As a result
of this, the MA structure is inevitably formed in the steel sheet. Further, upon further
studies on the strength - elongation balance, it has been found out that, when a predetermined
amount of retained γ is generated, it is good to control the area ratio V
MA of the MA structure so that the value of the above ratio V
MA/V
γ may become 0.50 or more. The value of the above ratio V
MA/V
γ is preferably 0.55 or more, more preferably 0.60 or more. However, when the value
of the above ratio V
MA/V
γ becomes excessively large, the MA structure is excessively generated. The quenched
martensite that exists in the MA structure is a very hard structure, so that, when
the MA structure is excessively generated, cracks are liable to be generated at the
interface to other structures at the time of collision, and accordingly, the crashworthiness
is rather deteriorated. Therefore, in the present invention, the value of the above
ratio V
MA/V
γ is set to be 1.50 or less in order to reduce the area ratio of quenched martensite
in the MA structure to ensure the crashworthiness. The value of the above ratio V
MA/V
γ is preferably 1.40 or less, more preferably 1.30 or less.
[0023] As shown above, the metal structure of the high-strength cold-rolled steel sheet
that characterizes the present invention has been described.
[0024] Next, the component composition of the high-strength cold-rolled steel sheet according
to the present invention will be described. Hereafter, "%" with regard to the component
composition of a steel sheet means "mass%".
[C: 0.10% or more to 0.5% or less]
[0025] C is an element that is necessary for ensuring the tensile strength of 980 MPa or
more and for enhancing the stability of retained γ to ensure a predetermined amount
of the retained γ. In the present invention, the C amount is set to be 0.10% or more.
The C amount is preferably 0.12% or more, more preferably 0.15% or more. However,
when the C amount is excessively large, the strength after hot rolling increases,
so that cracks may be generated during the cold rolling, or the weldability of a final
product may decrease. Accordingly, the C amount is set to be 0.5% or less. The C amount
is preferably 0.40% or less, more prefefably 0.30% or les, and still more preferably
0.25% or less.
[Si: 1.0% or more to 3% or less]
[0026] Si is an element that acts as a solute-strengthening element and contributes to a
higher strength of the steel. Also, Si suppresses generation of carbide and effectively
acts for generation of ferrite and retained γ, so that Si is an element that is necessary
for ensuring an excellent strength - elongation balance. In the present invention,
the Si amount is set to be 1.0% or more. The Si amount is preferably 1.2% or more,
more preferably 1.35% or more, and still more preferably 1.5% or more. However, when
the Si amount is excessively large, a considerable scale is formed during the hot
rolling to generate scale marks on the surface of the steel sheet, thereby degrading
the surface property. Also, the pickling property is degraded as well. Accordingly,
the Si amount is set to be 3% or less. The Si amount is preferably 2.8% or less, more
preferably 2.6% or less.
[Mn: 1.5% or more to 7% or less]
[0027] Mn is an element that contributes to a higher strength of the steel sheet by enhancing
the hardenability. Further, Mn is an element that is necessary for stabilizing γ to
generate retained γ. In the present invention, the Mn amount is set to be 1.5% or
more. The Mn amount is preferably 1.6% or more, more preferably 1.7% or more, still
more preferably 1.8% or more, and furthermore preferably 2.0% or more. However, when
the Mn amount is excessively large, the strength after hot rolling increases, so that
cracks may be generated during the cold rolling, or the weldability of the final product
may decrease. Also, when Mn is added in an excessively large amount, Mn is segregated
to deteriorate the ductility and the stretch-flangeability. Accordingly, the Mn amount
is set to be 7% or less. The Mn amount is preferably 5.0% or less, more preferably
4.0% or less, and still more preferably 3.0% or less.
[P: more than 0% to 0.1% or less]
[0028] P is an impurity element that is inevitably contained and, when contained in an excessively
large amount, deteriorates the weldability of the final product. Accordingly, the
P amount is set to be 0.1% or less in the present invention. The P amount is preferably
0.08% or less, more preferably 0.05% or less. The smaller the P amount is, the better
it is. However, it is industrially difficult to set the P amount to be 0%. A lower
limit of the P amount is 0.0005% from the industrial point of view.
[S: more than 0% to 0.05% or less]
[0029] As with P, S is an impurity element that is inevitably contained and, when contained
in an excessively large amount, deteriorates the weldability of the final product.
Also, S forms sulfide-based inclusions in the steel sheet, thereby causing deterioration
of the ductility and the stretch-flangeability of the steel sheet. Accordingly, the
S amount is set to be 0.05% or less in the present invention. The S amount is preferably
0.01% or less, more preferably 0.005% or less. The smaller the S amount is, the better
it is. However, it is industrially difficult to set the S amount to be 0%. A lower
limit of the S amount is 0.0001% from the industrial point of view.
[Al: 0.005% or more to 1% or less]
[0030] Al is an element that acts as a deoxidizer. In order that such an action may be exhibited,
the Al amount is set to be 0.005% or more in the present invention. The Al amount
is more preferably 0.01% or more. However, when the Al amount is excessively large,
the weldability of the final product is considerably deteriorated. Accordingly, the
Al amount is set to be 1% or less in the present invention. The Al amount is preferably
0.8% or less, more preferably 0.6% or less.
[N: more than 0% to 0.01% or less]
[0031] N is an impurity element that is inevitably contained and, when N is contained in
an excessively large amount, nitride is deposited in a large amount to deteriorate
the ductility, stretch-flangeability, and crashworthiness. Accordingly, the N amount
is set to be 0.01% or less in the present invention. The N amount is preferably 0.008%
or less, more preferably 0.005% or less. Since nitride in a small amount contributes
to a higher strength of the steel sheet, the N amount may be 0.001% or more.
[O: more than 0% to 0.01% or less]
[0032] O is an impurity element that is inevitably contained and, when contained in an excessively
large amount, deteriorates the ductility and the crashworthiness. Accordingly, the
O amount is set to be 0.01% or less in the present invention. The O amount is preferably
0.005% or less, more preferably 0.003% or less. The smaller the O amount is, the better
it is. However, it is industrially difficult to set the O amount to be 0%. A lower
limit of the O amount is 0.0001% from the industrial point of view.
[0033] The cold-rolled steel sheet according to the present invention satisfies the aforementioned
component composition, and the balance is made of iron and inevitable impurities.
The inevitable impurities may include the above-mentioned elements such as P, S, N,
and O, which may be brought into the steel depending on the circumstances of raw materials,
facility materials, production equipment, and the like, and may also include tramp
elements such as Pb, Bi, Sb, and Sn.
[0034] The cold-rolled steel sheet of the present invention may further contain, as other
elements,
- (a) at least one selected from the group consisting of Cr: more than 0% to 1% or less
and Mo: more than 0% to 1% or less,
- (b) at least one selected from the group consisting of Ti: more than 0% to 0.15% or
less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less,
- (c) at least one selected from the group consisting of Cu: more than 0% to 1% or less
and Ni: more than 0% to 1% or less,
- (d) B: more than 0% to 0.005% or less,
- (e) at least one selected from the group consisting of Ca: more than 0% to 0.01% or
less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less, and
the like.
[0035] These elements of (a) to (e) may be contained either alone or in an arbitrary combination.
The reason why such ranges have been set is as follows.
[(a) at least one selected from the group consisting of Cr: more than 0% to 1% or
less and Mo: more than 0% to 1% or less]
[0036] Cr and Mo are each an element that acts to improve the strength of the steel sheet
by enhancing hardenability. In order that such an action may be effectively exhibited,
the amount of each of Cr and Mo is preferably set to be 0.1% or more, more preferably
0.3% or more. However, when these elements are contained in an excessively large amount,
the ductility and the stretch-flangeability decrease. Also excessive addition leads
to higher costs. Accordingly, when Cr or Mo is contained alone, the amount is preferably
1% or less, more preferably 0.8% or less, still more preferably 0.5% or less. Cr and
Mo may be used either alone or in combination. When Cr and Mo are used in combination,
it is preferable that each amount is within the above range of the content when used
alone, and a sum of the contents of Cr and Mo is 1.5% or less.
[(b) at least one selected from the group consisting of Ti: more than 0% to 0.15%
or less, Nb: more than 0% to 0.15% or less, and V: more than 0% to 0.15% or less]
[0037] Ti, Nb, and V are each an element that acts to improve the strength of the steel
sheet by forming carbide and nitride in the steel sheet and to make prior γ grains
finer. In order that such an action may be effectively exhibited, the amount of each
of Ti, Nb, and V is preferably set to be 0.005% or more, more preferably 0.010% or
more. However, when these elements are contained in an excessively large amount, carbide
is deposited at the grain boundary, so that the stretch-flangeability and the crashworthiness
of the steel sheet are deteriorated. Accordingly, in the present invention, the amount
of each of Ti, Nb, and V is preferably set to be 0.15% or less, more preferably 0.12%
or less, and still more preferably 0.10% or less. These elements may be used either
alone or in combination of two or more that are arbitrarily selected.
[(c) at least one selected from the group consisting of Cu: more than 0% to 1% or
less and Ni: more than 0% to 1% or less]
[0038] Cu and Ni are each an element that acts effectively for generation and stabilization
of retained γ. Also, Cu and Ni act to improve the corrosion resistance of the steel
sheet. In order that such an action may be effectively exhibited, the amount of each
of Cu and Ni is preferably set to be 0.05% or more, more preferably 0.10% or more.
However, when Cu is contained in an excessively large amount, the hot formability
is deteriorated. Accordingly, when Cu is added alone, the amount of Cu is preferably
set to be 1% or less, more preferably 0.8% or less, and still more preferably 0.5%
or less. On the other hand, when Ni is contained in an excessively large amount, a
higher cost is invited, so that the amount of Ni is preferably set to be 1% or less,
more preferably 0.8% or less, and still more preferably 0.5% or less. Cu and Ni may
be used either alone or in combination. When Cu and Ni are used in combination, the
above action is more likely to be exhibited, and also, by incorporation of Ni, the
deterioration of hot formability caused by addition of Cu is more likely to be suppressed.
When Cu and Ni are used in combination, a sum of the amounts of Cu and Ni is preferably
set to be 1.5% or less, more preferably 1.0% or less.
[(d) B: more than 0% to 0.005% or less]
[0039] B is an element that improves hardenability and is an element that acts to allow
austenite to exist stably down to room temperature. In order that such an action may
be effectively exhibited, the amount of B is preferably set to be 0.0005% or more,
more preferably 0.0010% or more, and still more preferably 0.0015% or more. However,
when B is contained in an excessively large amount, boride may be generated to deteriorate
the ductility. Accordingly, the amount of B is preferably set to be 0.005% or less.
The amount of B is more preferably 0.004% or less, still more preferably 0.0035% or
less.
[(e) at least one selected from the group consisting of Ca: more than 0% to 0.01%
or less, Mg: more than 0% to 0.01% or less, and REM: more than 0% to 0.01% or less]
[0040] Ca, Mg, and REM are elements that act to finely disperse the inclusions in the steel
sheet. In order that such an action may be effectively exhibited, the amount of each
of Ca, Mg, and REM is preferably set to be 0.0005% or more, more preferably 0.0010%
or more. However, when these elements are added in an excessively large amount, the
castability, hot formability, and the like may be deteriorated. Accordingly, the amount
of each of Ca, Mg, and REM is preferably set to be 0.01% or less, more preferably
0.008% or less, and still more preferably 0.007% or less. These elements may be used
either alone or in combination of two or more that are arbitrarily selected. In the
present invention, REM is an abbreviation for Rare earth metal (rare earth element),
and is meant to include lanthanoid elements which are fifteen elements from La to
Lu, and Sc and Y.
[0041] As shown above, the high-strength cold-rolled steel sheet according to the present
invention has been described.
[0042] An electrogalvanized layer, a hot-dip galvanized layer, or a hot-dip galvannealed
layer may be formed on a surface of the high-strength cold-rolled steel sheet. In
other words, the scope of the present invention includes a high-strength electrogalvanized
steel sheet (which may hereafter be referred to as EG steel sheet) having an electrogalvanized
layer formed on a surface of the high-strength cold-rolled steel sheet, a high-strength
hot-dip galvanized steel sheet (which may hereafter be referred to as GI steel sheet)
having a hot-dip galvanized layer formed on a surface of the high-strength cold-rolled
steel sheet, and a high-strength hot-dip galvannealed steel sheet (which may hereafter
be referred to as GA steel sheet) having a hot-dip galvannealed layer formed on a
surface of the high-strength cold-rolled steel sheet.
[0043] Next, a method for producing the high-strength cold-rolled steel sheet according
to the present invention will be described.
[0044] The high-strength cold-rolled steel can be produced by subjecting a steel satisfying
a component composition described above to hot rolling with a rolling rate at a final
stand of finish rolling being 5 to 20% and with a finish rolling end temperature being
the Ar
3 point or higher and 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and heating and holding in a temperature region of the cooling stop temperature T+50°C
or higher and 550°C or lower for 200 seconds or more, and thereafter cooling to room
temperature.
[0045] Hereafter, the steps will be sequentially described.
[0046] [Rolling rate at a final stand of finish rolling: 5 to 20%]
[0047] First, a steel satisfying the aforementioned component composition is heated in accordance
with a conventional method. A heating temperature is not particularly limited; however,
the heating temperature is preferably set to be, for example, 1000 to 1300°C. When
the heating temperature is lower than 1000°C, solid solution of carbide is insufficiently
formed, and a sufficient strength is hardly obtained. On the other hand, when the
heating temperature is higher than 1300°C, the structure of the hot-rolled steel sheet
becomes coarse, and also the MA structure of the cold-rolled steel sheet is liable
to become coarse. As a result, the crashworthiness tends to decrease.
[0048] After the heating, hot rolling is carried out. In the present invention, it is important
that the rolling rate at a final stand of finish rolling is set to be 5 to 20%. When
the rolling rate is less than 5%, the austenite grain size after hot rolling becomes
coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled
steel sheet after annealing becomes large. As a result, the stretch-flangeability
decreases. Accordingly, in the present invention, it is necessary that the rolling
rate is set to be 5% or more. The rolling rate is preferably 6% or more, more preferably
7% or more, and still more preferably 8% or more. However, when the rolling rate exceeds
25%, the average circle-equivalent diameter of the MA structure also becomes large,
leading to deterioration of the stretch-flangeability and crashworthiness. The mechanism
therefor is not clear; however, this seems to be because the structure after hot rolling
is made non-homogeneous. In the present invention, it is necessary that the rolling
rate is set to be 20% or less.
[Finish rolling end temperature: Ar3 point or higher and 900°C or lower]
[0049] When the finish rolling end temperature is lower than the temperature of the Ar
3 point, the steel sheet structure after hot rolling becomes non-homogeneous, and the
stretch-flangeability decreases. On the other hand, when the finish rolling end temperature
exceeds 900°C, recrystallization of austenite occurs to make the crystal grains become
coarse, and the average circle-equivalent diameter of the MA structure in the cold-rolled
steel sheet becomes large. As a result, the stretch-flangeability decreases. Accordingly,
in the present invention, it is necessary that the finish rolling end temperature
is set to be 900°C or lower. The finish rolling end temperature is preferably 890°C
or lower, more preferably 880°C or lower.
[0050] The temperature of the Ar
3 point was calculated on the basis of the following formula (ii). In the formula,
brackets [ ] indicate the content of each element (mass%), and calculation may be
made by assuming that the content of an element that is not contained in the steel
sheet is 0 mass%.
[Coiling temperature: 600°C or lower]
[0051] When the coiling temperature exceeds 600°C, the crystal grains become coarse, and
the average circle-equivalent diameter of the MA structure in the cold-rolled steel
sheet becomes large. As a result, the stretch-flangeability decreases. Accordingly,
in the present invention, the coiling temperature is set to be 600°C or lower. The
coiling temperature is preferably 580°C or lower, more preferably 570°C or lower,
and still more preferably 550°C or lower.
[Cold rolling]
[0052] After the hot rolling, the steel sheet may be coiled, cooled to room temperature,
pickled by a conventional method in accordance with the needs, and subsequently cold-rolled
by a conventional method. The cold rolling rate in the cold rolling may be set to
be, for example, 30 to 80%.
[Annealing]
[0053] After the cold rolling, annealing is carried out by heating, at an average heating
rate of 10°C/sec or more, to a temperature region of 800°C or higher and lower than
the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more. When
the average heating rate of heating to the above temperature region after the cold
rolling is lower than 10°C/sec, the austenite grains grow and become coarse during
the heating, so that the average circle-equivalent diameter of the MA structure in
the cold-rolled steel sheet becomes large, and the stretch-flangeability decreases.
Accordingly, in the present invention, the average heating rate is set to be 10°C/sec
or more. The average heating rate is preferably 12°C/sec or more, more preferably
15°C/sec or more. An upper limit of the above average heating rate is not particularly
limited; however, the average heating rate is typically about 100°C/sec at the maximum.
[0054] By setting the soaking temperature to be 800°C or higher and lower than the Ac
3 point, a desired ferrite amount can be ensured. When the soaking temperature is lower
than 800°C, reverse transformation to austenite becomes insufficient, and the processed
structure remains in the cold-rolled steel sheet, so that the formability decreases.
Accordingly, in the present invention, the soaking temperature is set to be 800°C
or higher. The soaking temperature is preferably 805°C or higher, more preferably
810°C or higher. However, when the soaking temperature is higher than the temperature
of the Ac
3 point, a desired ferrite amount cannot be ensured, and the ductility is deteriorated.
Accordingly, in the present invention, the soaking temperature is set to be lower
than the temperature of the Ac
3 point. The soaking temperature is preferably (Ac
3 point - 10°C) or lower, more preferably (Ac
3 point - 20°C) or lower.
[0055] When the soaking time is less than 50 seconds, the processed structure remains in
the cold-rolled steel sheet, and the ductility is deteriorated. Accordingly, in the
present invention, the soaking time is set to be 50 seconds or more. The soaking time
is preferably 60 seconds or more. An upper limit of the soaking time is not particularly
limited; however, when the soaking time is too long, concentration of Mn into the
austenite phase proceeds, and the Ms point may decrease, leading to increase or coarsening
of the MA structure. Accordingly, the soaking time is preferably set to be 3600 seconds
or less, more preferably 3000 seconds or less.
[0056] Regarding the soaking holding in the above temperature region, the steel sheet need
not be thermostatically held at the same temperature, so that the steel sheet may
be heated and cooled in a fluctuating manner within the above temperature region.
[Cooling]
[0058] After the above soaking holding, the steel sheet is cooled to an arbitrary cooling
stop temperature T°C that lies in a temperature range of 70°C or higher and the Ms
point or lower. By cooling down to this temperature range, untransformed austenite
can be transformed to martensite and hard bainite phase, and the MA structure also
can be made finer. During this period, martensite exists as quenched martensite immediately
after the transformation; however, the martensite is tempered while being reheated
and held in a later step and remains as tempered martensite. This tempered martensite
does not give adverse effects on any of the ductility, stretch-flangeability, and
crashworthiness of the steel sheet. However, when the above cooling stop temperature
T exceeds the Ms point, martensite is not generated, and the MA structure generated
in the reheating holding step at a high temperature becomes coarse, so that the local
deformation capability decreases, and the stretch-flangeability cannot be improved.
Accordingly, in the present invention, the cooling stop temperature T is set to be
equal to or lower than the temperature of the Ms point. The cooling stop temperature
T is preferably (Ms point - 20°C) or lower, more preferably (Ms point - 50°C) or lower.
On the other hand, when the cooling stop temperature T is lower than 50°C, retained
γ and the MA structure are generated only in a slight amount, so that the ductility
cannot be improved. Accordingly, in the present invention, a lower limit of the cooling
stop temperature T is set to be 70°C or higher.
[0059] The temperature of the aforementioned Ms point is calculated on the basis of the
following formula (iv). In the formula, brackets [ ] indicate the content of each
element (mass%), and calculation may be made by assuming that the content of an element
that is not contained in the steel sheet is 0 mass%. Also, Vf in the formula indicates
the area ratio of ferrite relative to the whole of the metal structure.
[0060] After performing the above soaking and holding, it is important that an average cooling
rate down to the cooling stop temperature T that lies in the above temperature range
is set to be 10°C/sec or more. Excessive generation of ferrite can be suppressed by
appropriately controlling the cooling rate down to the cooling stop temperature T
after soaking and holding. In other words, when the average cooling rate is lower
than 10°C/sec, ferrite is excessively generated during the cooling, and the tensile
strength decreases. Accordingly, in the present invention, the average cooling rate
is set to be 10°C/sec or more. The average cooling rate is preferably 15°C/sec or
more, more preferably 20°C/sec or more. An upper limit of the above average cooling
rate is not particularly limited, and the steel sheet may be cooled by cooling with
water or cooling with oil.
[Reheating step]
[0061] After the steel sheet is cooled down to an arbitrary cooling stop temperature T°C
in the temperature range of 70°C or higher and the Ms point or lower, it is important
that the steel sheet is reheated to a temperature region of the cooling stop temperature
T+50°C or higher and 550°C or lower, and the steel sheet is held in this temperature
region for 200 seconds or more. By reheating to the temperature region of the cooling
stop temperature T+50°C or higher and 550°C or lower, the hard phase such as martensite
can be tempered, and untransformed austenite can be transformed to bainitic ferrite
or bainite. When the reheating is not carried out, the balance between the amounts
of generation of retained γ and the MA structure becomes degraded, and the ratio V
MA/V
γ of the area ratio V
MA of the MA structure to the volume ratio V
γ of the retained γ cannot be controlled to be within an appropriate range. As a result,
the crashworthiness cannot be improved. Further, the hard phase cannot be tempered,
and dislocation at a high density is generated. Accordingly, in the present invention,
the steel sheet is reheated to a temperature exceeding the cooling stop temperature
T after the steel sheet is cooled to the cooling stop temperature T. The reheating
temperature is (T + 50°C) or higher. However, when the reheating temperature exceeds
550°C, retained γ and the MA structure are generated only in a slight amount, so that
the tensile strength decreases, and the value of TS × λ becomes smaller, making it
impossible to improve the stretch-flangeability. Accordingly, in the present invention,
the reheating temperature is set to be 550°C or lower. The reheating temperature is
preferably 520°C or lower, more preferably 500°C or lower, and still more preferably
450°C or lower.
[0062] Here, in the present invention, "reheating" means, as it is stated, heating, that
is, raising the temperature from the above cooling stop temperature T. Accordingly,
the reheating temperature is a temperature higher than the above cooling stop temperature
T. Therefore, even if the reheating temperature is, for example, within a temperature
region of 50°C or higher and 550°C or lower, this does not fall under the category
of the reheating of the present invention if the cooling stop temperature T and the
reheating temperature are the same as each other or if the reheating temperature is
lower than the cooling stop temperature T.
[0063] After the steel sheet is reheated to the temperature region of the cooling stop temperature
T+50°C or higher and 550°C or lower, the steel sheet is held in the temperature region
for 200 seconds or more. When the reheating holding time is less than 50 seconds,
the MA structure is excessively generated, and the ductility cannot be improved. Further,
the MA structure becomes coarse, and the average circle-equivalent diameter cannot
be appropriately controlled, so that the stretch-flangeability cannot be improved
either. Also, the ratio V
MA/V
γ of the area ratio V
MA of the MA structure to the volume ratio V
γ of the retained γ cannot be appropriately controlled, so that the crashworthiness
cannot be improved either. Furthermore, the hard phase cannot be sufficiently tempered,
and also the transformation of untransformed austenite to bainitic ferrite or bainite
does not proceed sufficiently. Accordingly, in the present invention, the reheating
holding time is set to be 200 seconds or more. An upper limit of the reheating holding
time is not particularly limited. However, when the holding time is long, the productivity
decreases, and the tensile strength tends to decrease. From such viewpoints, the reheating
holding time is preferably set to be 1500 seconds or less, more preferably 1000 seconds
or less.
[0064] After the steel sheet is reheated and held, the steel sheet is cooled to room temperature.
An average cooling rate during the cooling is not particularly limited; however, the
average cooling rate is preferably, for example, 0.1°C/sec or more, more preferably
0.4°C/sec or more. Further, the average cooling rate is preferably, for example, 200°C/sec
or less, more preferably 150°C/sec or less.
[Plating treatment]
[0065] After the reheating holding, the high-strength cold-rolled steel sheet according
to the present invention obtained by cooling to room temperature may be subjected
to electrogalvanization, hot-dip galvanizing, or hot-dip galvannealing in accordance
with a conventional method.
[0066] The electrogalvanization may be carried out, for example, by subjecting the above
high-strength cold-rolled steel sheet to energization while immersing the steel sheet
into a zinc solution of 50 to 60°C (particularly 55°C) so as to perform an electrogalvanization
treatment. The plating adhesion amount is not particularly limited and may be, for
example, about 10 to 100 g/m
2 per one surface.
[0067] The hot-dip galvanizing may be carried out, for example, by immersing the above high-strength
cold-rolled steel sheet into a hot-dip galvanizing bath of 300°C or higher and 550°C
or lower, so as to perform a hot-dip galvanizing treatment. The plating time may be
suitably adjusted so that a desired plating adhesion amount can be ensured. The plating
time is preferably set to be, for example, 1 to 10 seconds.
[0068] The hot-dip galvannealing may be carried out by performing an alloying treatment
after the above hot-dip galvanizing. The alloying treatment temperature is not particularly
limited; however, when the alloying treatment temperature is too low, the alloying
does not proceed sufficiently, so that the alloying treatment temperature is preferably
450°C or higher, more preferably 460°C or higher, and still more preferably 480°C
or higher. However, when the alloying treatment temperature is too high, the alloying
proceeds too much, and the Fe concentration in the plating layer becomes high, thereby
deteriorating the plating adhesion property. From such a viewpoint, the alloying treatment
temperature is preferably 550°C or lower, more preferably 540°C or lower, and still
more preferably 530°C or lower. The alloying treatment time is not particularly limited
and may be adjusted so that the hot-dip galvanized layer may be alloyed. The alloying
treatment time is preferably, for example, 10 to 60 seconds.
[0069] A high-strength hot-dip galvanized steel sheet having a tensile strength of 980 MPa
or more and being excellent in formability and crashworthiness according to the present
invention can also be produced by subjecting a steel satisfying a component composition
described above to hot rolling with a rolling rate at a final stand of finish rolling
being 5 to 20% and with a finish rolling end temperature being the Ar
3 point or higher and 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and a reheating of heating and holding in a temperature region of the cooling stop
temperature T+50°C or higher and 550°C or lower for 200 seconds or more, and after
performing hot-dip galvanizing within a holding time, cooling to room temperature.
In other words, the steps until heating to the temperature region of the cooling stop
temperature T+50°C or higher and 550°C or lower are the same as those of the above-described
method for producing a high-strength cold-rolled steel sheet according to the present
invention, so that the hot-dip galvanizing and the holding for 200 seconds or more
that is carried out in the above temperature region of the cooling stop temperature
T+50°C or higher and 550°C or lower may be simultaneously carried out in the same
step.
[0070] The hot-dip galvanizing may be carried out within the holding time in the reheating
temperature region, that is, in the temperature region of the cooling stop temperature
T+50°C or higher and 550°C or lower, and a conventional method can be adopted as a
specific plating method. For example, the steel sheet heated to the temperature region
of the cooling stop temperature T+50°C or higher and 550°C or lower may be immersed
into a plating bath adjusted to have a temperature within the range of the cooling
stop temperature T+50°C or higher and 550°C or lower, so as to perform a hot-dip galvanizing
treatment. The plating time may be suitably adjusted so that a desired plating amount
can be ensured within the time of the reheating holding. The plating time is preferably
set to be, for example, 1 to 10 seconds.
[0071] There are the following three patterns of (I) to (III) as a combination of the hot-dip
galvanizing treatment; and only the heating and without performing the plating treatment,
in the reheating.
- (I) Only the heating is carried out, and thereafter, the hot-dip galvanizing treatment
is carried out.
- (II) The hot-dip galvanizing treatment is carried out, and thereafter, only the heating
is carried out.
- (III) Only the heating is carried out, and thereafter, the hot-dip galvanizing treatment
is carried out, and further, only the heating is carried out, in this order.
[0072] The reheating temperature at which only the heating is carried out and the temperature
of the plating bath used for performing the hot-dip galvanizing may be different from
each other. In the present invention, heating or cooling may be carried out from one
temperature to the other temperature. Furnace heating, induction heating, or the like
may be adopted as a method for the heating.
[0073] A high-strength hot-dip galvannealed steel sheet having a tensile strength of 980
MPa or more and being excellent in formability and crashworthiness according to the
present invention can also be produced by subjecting a steel satisfying a component
composition described above to hot rolling with a rolling rate at a final stand of
finish rolling being 5 to 20% and with a finish rolling end temperature being the
Ar
3 point or higher and 900°C or lower, coiling with a coiling temperature being 600°C
or lower, and cooling to room temperature; cold rolling; heating, at an average heating
rate of 10°C/second or more, to a temperature region of 800°C or higher and lower
than the Ac
3 point, and soaking by holding in the temperature region for 50 seconds or more; cooling
at an average cooling rate of 10°C/second or more, to an arbitrary cooling stop temperature
T°C that lies in a temperature range of 70°C or higher and the Ms point or lower;
and a reheating of heating and holding in a temperature region of the cooling stop
temperature T+50°C or higher and 550°C or lower for 200 seconds or more, and after-performing
hot-dip galvanizing within a holding time, further performing an alloying treatment
and thereafter cooling to room temperature. In other words, the steps until heating
to the temperature region of the cooling stop temperature T+50°C or higher and 550°C
or lower are the same as those of the above-described method for producing a high-strength
cold-rolled steel sheet according to the present invention, so that the hot-dip galvanizing
and the holding for 200 seconds or more that is carried out in the above temperature
region of the cooling stop temperature T+50°C or higher and 550°C or lower may be
simultaneously carried out in the same step, and thereafter the hot-dip galvanized
layer may be alloyed, followed by cooling down to room temperature.
[0074] The alloying treatment temperature is not particularly limited; however, when the
alloying treatment temperature is too low, the alloying does not proceed sufficiently,
so that the alloying treatment temperature is preferably 450°C or higher, more preferably
460°C or higher, and still more preferably 480°C or higher. However, when the alloying
treatment temperature is too high, the alloying proceeds too much, and the Fe concentration
in the plating layer becomes high, thereby deteriorating the plating adhesion property.
From such a viewpoint, the alloying treatment temperature is preferably 550°C or lower,
more preferably 540°C or lower, and still more preferably 530°C or lower.
[0075] The alloying treatment time is not particularly limited and may be adjusted so that
the hot-dip galvanized layer may be alloyed. The alloying treatment time is preferably,
for example, 10 to 60 seconds. The alloying treatment is carried out after performing
the hot-dip galvanizing treatment for a predetermined period of time within the temperature
region of the cooling stop temperature T+50°C or higher and 550°C or lower, so that
the time needed for the alloying treatment is not included in the holding time within
the temperature region of the cooling stop temperature T+50°C or higher and 550°C
or lower.
[0076] After performing the hot-dip galvanizing within the holding time in the temperature
region of the cooling stop temperature T+50°C or higher and 550°C or lower and performing
the alloying treatment in accordance with the needs, the steel sheet may be cooled
down to room temperature. The average cooling rate during the cooling is not particularly
limited; however, the average cooling rate is preferably, for example, 0.1°C/sec or
more, more preferably 0.4°C/sec or more. Further, the average cooling rate is preferably,
for example, 200°C/sec or less, more preferably 150°C/sec or less.
[0077] The high-strength cold-rolled steel sheet according to the present invention has
a tensile strength of 980 MPa or more. The tensile strength is preferably 1000 MPa
or more, more preferably 1010 MPa or more. Further, the above high-strength cold-rolled
steel sheet is excellent in formability as evaluated by ductility and stretch-flangeability,
and also is excellent in crashworthiness.
[0078] The ductility can be evaluated by strength-elongation balance. In the present invention,
those in which a product of the tensile strength TS (MPa) and the elongation EL (%)
is 17000 MPa·% or more are rated as acceptable. The value of TS × EL is preferably
17100 MPa·% or more, more preferably 17200 MPa·% or more.
[0079] The stretch-flangeability can be evaluated by strength - hole expansion ratio balance.
In the present invention, those in which a product of the tensile strength TS (MPa)
and the hole expansion ratio λ (%) is 20000 MPa·% or more are rated as acceptable.
The value of TS × λ is preferably 21000 MPa-% or more, more preferably 22000 MPa·%
or more.
[0080] The crashworthiness can be evaluated by strength-VDA bending angle balance. In the
present invention, those in which a product of the tensile strength TS (MPa) and the
VDA bending angle (°) is 90000 MPa·° or more are rated as acceptable. The value of
TS × VDA bending angle is preferably 90500 MPa·° or more, more preferably 91000 MPa·°
or more.
[0081] The thickness of the high-strength cold-rolled steel sheet according to the present
invention is not particularly limited; however, the steel sheet is preferably a thin
steel sheet having a thickness of, for example, 6 mm or less.
Examples
[0083] Hereafter, the present invention will be described more specifically by way of Examples;
however, the invention is not limited by the following Examples. No. 39 is for reference.
[0084] A steel containing the components given in the following Table 1 with the balance
being iron and inevitable impurities was prepared by ingot-making and subjected to
hot rolling, cold rolling, and continuous annealing to produce a cold-rolled steel
sheet. In the following Table 1, "-" means that the corresponding element is not contained.
The following Table 1 also show the temperature of the Ar
3 point calculated on the basis of the above formula (ii) and the temperature of the
Ac
3 point calculated on the basis of the above formula (iii). Further, FIG. 1 shows one
example of a heat treatment pattern that was carried out in the continuous annealing.
In FIG. 1, the reference sign 1 denotes a heating step, 2 a soaking step, 3 a cooling
step, 4 a reheating holding step, and 5 a cooling stop temperature.
[Hot rolling]
[0085] A slab obtained by ingot-making was heated to 1250°C, and hot rolling was carried
out to a sheet thickness of 2.3 mm with the rolling reduction in the final stand of
finish rolling being set to be a rolling reduction shown in the following Table 2-1
or 2-2 and with the finish rolling end temperature being set to be a temperature shown
in the following Table 2-1 or 2-2. After the hot rolling, the steel sheet was cooled
down to a coiling temperature shown in the following Table 2-1 or 2-2 at an average
cooling rate of 30°C/sec, followed by coiling. After the coiling, the steel sheet
was cooled in air to room temperature, so as to produce a hot-rolled steel sheet.
[Cold rolling]
[0086] After the obtained hot-rolled steel sheet was pickled to remove surface scale, cold
rolling was carried out to produce a cold-rolled steel sheet having a thickness of
1.2 mm.
[Continuous annealing]
[0087] The obtained cold-rolled steel sheet was subjected to continuous annealing based
on the heat treatment pattern shown in FIG. 1. That is, the obtained cold-rolled steel
sheet was heated as a heating step at an average heating rate shown in the following
Table 2-1 or 2-2 up to the soaking temperature shown in the following Table 2-1 or
2-2, and was held at the soaking temperature as a soaking step. The following Table
2-1 or 2-2 shows the soaking time. Further, the following Table 2-1 or 2-2 shows a
value calculated by subtracting the soaking temperature from the temperature of the
Ac
3 point.
[0088] After the soaking, the steel sheet was cooled as a cooling step at an average cooling
rate shown in the following Table 2-1 or 2-2 down to the cooling stop temperature
T°C shown in the following Table 2-1 or 2-2.
[0089] After the cooling, the steel sheet was heated to the reheating temperature shown
in the following Table 2-1 or 2-2 and was held at the reheating temperature as a reheating
holding step, followed by cooling down to room temperature to produce a test sample
material. The following Table 2-1 or 2-2 shows the reheating holding time. Also, the
following Table 2-1 or 2-2 shows a value calculated by subtracting the cooling stop
temperature T from the reheating temperature.
[0090] Nos. 8 and 11 shown in the following Table 2-1 are samples in which the reheating
holding step was not carried out after the cooling was stopped at the cooling stop
temperature T shown in the following Table 2-1. That is, in No. 8, the steel sheet
was cooled with the cooling stop temperature T set to be 480°C, and thereafter cooled
to 350°C, which was lower than that temperature, and held at 350°C for 300 seconds.
For the sake of convenience, the following Table 2-1 gives 350°C in the section of
the reheating temperature and gives 300 seconds in the section of the reheating holding
time. In No. 11, the steel sheet was cooled with the cooling stop temperature T set
to be 330°C, and thereafter cooled to 300°C, which was lower than that temperature,
and held at 300°G for 300 seconds. For the sake of convenience, the following Table
2-1 gives 300°C in the section of the reheating temperature and gives 300 seconds
in the section of the reheating holding time.
[Electrogalvanization]
[0091] No. 2 shown in the following Table 2-1 is a sample in which the above test sample
material was immersed into a galvanizing bath of 55°C to perform an electrogalvanization
treatment and thereafter washed with water and dried to produce an electrogalvanized
steel sheet. The electrogalvanization treatment was carried out with an electric current
density set to be 40 A/dm
2. The galvanizing adhesion amount was 40 g/m
2 per one surface. In the electrogalvanization treatment, washing treatments such as
degreasing with alkaline aqueous solution immersion, washing with water, and pickling
or the like were carried out as appropriate, so as to produce a test sample material
having an electrogalvanized layer on the surface of the cold-rolled steel sheet. In
the following Table 2-1, the section of classification for No. 2 gives "EG".
[Hot-dip galvanizing]
[0092] No. 36 shown in the following Table 2-2 is a sample in which the above test sample
material was immersed into a hot-dip galvanizing bath of 460°C to perform a hot-dip
galvanizing treatment, thereby to produce a hot-dip galvanized steel sheet. The hot-dip
galvanizing adhesion amount was 30 g/m
2 per one surface. In the following Table 2-2, the section of classification for No.
36 gives "GI".
[Hot-dip galvannealing]
[0093] No. 18 shown in the following Table 2-1 is a sample in which the above test sample
material was immersed into a hot-dip galvanizing bath of 460°C to perform a hot-dip
galvanizing treatment, followed by heating to 500°C to perform an alloying treatment,
thereby to produce a hot-dip galvannealed steel sheet. The hot-dip galvannealing adhesion
amount was 30 g/m
2 per one surface. In the following Table 2-1, the section of classification for No.
18 gives "GA".
[0094] Test sample materials in which none of the electrogalvanization treatment, hot-dip
galvanizing treatment, and hot-dip galvannealing treatment was carried out are denoted
as "cold-rolled" in the section of classification in the following Tables 2-1 and
2-2.
[0095] With respect to the obtained test sample materials, a metal structure was observed
by the following procedure.
[Observation of metal structure]
(Area ratio of ferrite and hard phase)
[0096] After the cross-section of the obtained test sample material was polished, the test
sample material was subjected to nital corrosion, followed by performing observation
at the position of 1/4 of the sheet thickness in three fields of view at a magnification
of 1000 times with a scanning electron microscope, so as to capture a photomicrograph
image. The observation field of view was such that one field of view had a size of
100 µm × 100 µm. With the lattice interval set to be 5 µm, the area ratio of ferrite
was measured by the point counting method with the number of lattice points being
20 × 20, and an average value Vf of the three fields of view was calculated. The calculation
results are shown in the following Tables 3-1 and 3-2. The area ratio of ferrite was
calculated by excluding the area ratio of the hard phase that existed in the ferrite
phase.
[0097] Further, the Ms point was calculated in accordance with the above formula (iv) based
on the component composition shown in the following Table 1 and the average area ratio
Vf of ferrite shown in the following Tables 3-1 and 3-2. The results are shown in
the following Tables 2-1 and 2-2. The following Tables 2-1 and 2-2 also show a value
obtained by subtracting the temperature of the Ms point from the cooling stop temperature
T.
[0098] In a similar manner, a sum area ratio of pearlite and cementite was measured by the
point counting method, and an average value of the three fields of view was calculated.
The calculation results are shown in the following Tables 3-1 and 3-2. The sum area
ratio of pearlite and cementite is denoted as "other structures" in the following
Tables 3-1 and 3-2.
[0099] In the present Examples, the structure other than ferrite, pearlite, and cementite
calculated by the point counting method was assumed to be a hard phase. In other words,
a value obtained by subtracting the area ratio of ferrite and the sum area ratio of
pearlite and cementite from 100% was calculated as an area ratio of the hard phase.
The results are shown in the following Tables 3-1 and 3-2.
[0100] As a result of observation of a specific structure constituting the hard phase, it
was found out that the hard phase included quenched martensite and retained γ and
included at least one selected from the group consisting of bainitic ferrite, bainite,
and tempered martensite.
(Volume ratio Vγ of retained γ)
[0101] The obtained test sample material was polished down to the position of 1/4 of the
sheet thickness with use of a sandpaper of #1000 to #1500, and further the surface
was subjected to electrolytic polishing down to the depth of 10 to 20 µm, followed
by measuring the volume ratio V
γ of retained γ with use of an X-ray diffractometer. Specifically, "RINT 1500" manufactured
by Rigaku Corporation was used as the X-ray diffractometer and, with use of a Co target,
a power of 40 kV - 200 mA was output to measure the range of 40° to 130° in terms
of 20. The volume ratio V
γ of retained γ was quantitated on the basis of the obtained bcc (α) diffraction peaks
(110), (200), and (211) and fcc (γ) diffraction peaks (111), (200), (220), and (311).
The results are shown in the following Tables 3-1 and 3-2.
(Area ratio VMA and average circle-equivalent diameter of MA structure)
[0102] After the cross-section of the obtained test sample material was polished, the test
sample material was subjected to LePera corrosion, followed by performing observation
at the position of 1/4 of the sheet thickness in three fields of view at a magnification
of 1000 times with an optical microscope, so as to capture a photomicrograph image.
The observation field of view was such that one field of view had a size of 100 µm
× 100 µm. The portion whitened by LePera corrosion was regarded as the MA structure.
With the lattice interval set to be 5 µm, the area ratio of the MA structure was measured
by the point counting method with the number of lattice points being 20 × 20, and
an average value of the three fields of view was calculated. The calculation results
are shown in the following Tables 3-1 and 3-2.
[0103] Upon subjecting the photomicrograph image captured with the optical microscope to
image analysis, the average circle-equivalent diameter d of each MA structure was
calculated, and an average value was determined. The results are shown in the following
Tables 3-1 and 3-2.
(Ratio of area ratio VMA of MA structure to volume ratio Vγ of retained γ)
[0104] The ratio V
MA/V
γ of the area ratio V
MA of the MA structure to the volume ratio V
γ of the retained γ was calculated on the basis of the volume ratio V
γ of the retained γ and the area ratio V
MA of the MA structure calculated by the above-described procedure. The calculation
results are shown in the following Tables 3-1 and 3-2.
[0105] Next, with respect to the obtained test sample material, the mechanical properties,
ductility, stretch-flangeability, and crashworthiness were evaluated by the following
procedure.
[Evaluation of mechanical properties and ductility]
[0106] A No. 5 test piece defined in JIS Z2201 was cut out so that the direction perpendicular
to the rolling direction of the obtained test sample material would be a longitudinal
direction. With use of this test piece, a tensile test was carried out so as to measure
the tensile strength TS and the elongation EL. The measurement results are shown in
the following Tables 3-1 and 3-2.
[0107] In the present Examples, the samples in which the tensile strength was 980 MPa or
more were evaluated as having a high strength and being acceptable, whereas the samples
in which the tensile strength was less than 980 MPa were evaluated as having an insufficient
strength and being a reject.
[0108] Also, the value of tensile strength TS × elongation EL was calculated on the basis
of the measured values of tensile strength TS and elongation EL. The calculation results
are shown in the following Tables 3-1 and 3-2. The value of TS × EL indicates a strength
- elongation balance and serves as an index for evaluating the ductility.
[0109] In the present Examples, the samples in which the value of TS × EL was 17000 MPa·%
or more were evaluated as having an excellent ductility and being acceptable, whereas
the samples in which the value of TS × EL was less than 17000 MPa·% were evaluated
as having a poor ductility and being a reject.
[Evaluation of stretch-flangeability]
[0110] In order to evaluate the stretch-flangeability of the test sample material, a hole
expansion test was carried out according to the Japan Iron and Steel Federation Standard
JFS T 1001, so as to measure the hole expansion ratio λ. The measurement results are
shown in the following Tables 3-1 and 3-2.
[0111] Also, the value of tensile strength TS × hole expansion ratio λ was calculated on
the basis of the measured values of tensile strength TS and hole expansion ratio λ.
The calculation results are shown in the following Tables 3-1 and 3-2. The value of
TS × λ indicates a strength - hole expansion ratio balance and serves as an index
for evaluating the stretch-flangeability.
[0112] In the present Examples, the samples in which the value of TS × λ was 20000 MPa·%
or more were evaluated as having an excellent stretch-flangeability and being acceptable,
whereas the samples in which the value of TS × λ was less than 20000 MPa·% were evaluated
as having a poor stretch-flangeability and being a reject.
[Evaluation of crashworthiness]
[0113] It is disclosed in the following literature that the crashworthiness is correlated
to a bending angle.
[0115] Accordingly, a bending test was carried out under the following conditions on the
basis of the VDA standard (VDA238-100) defined by the German Association of the Automotive
Industry. The displacement at the maximum load measured by the bending test was converted
into an angle according to the VDA standard, so as to determine the bending angle.
The conversion results are shown in the following Tables 3-1 and 3-2.
(Measurement conditions)
[0116] Test method: support with rolls, pressing-in of punch
Roll diameter: φ30 mm
Punch shape: tip end R = 0.4 mm
Distance between rolls: 2.9 mm
Punch pressing-in speed: 20 mm/min
Test piece dimension: 60 mm × 60 mm
Bending direction: direction perpendicular to the rolling direction
Testing machine: SIMAZU AUTOGRAPH 20 kN
[0117] Also, the value of tensile strength TS × VDA bending angle° was calculated on the
basis of the values of the tensile strength TS measured in the tensile test and the
VDA bending angle. The calculation results are shown in the following Tables 3-1 and
3-2.
[0118] In the present Examples, the samples in which the value of TS × VDA was 90000 MPa·°
or more were evaluated as having an excellent crashworthiness and being acceptable,
whereas the samples in which the value of TS × VDA was less than 90000 MPa·° were
evaluated as having a poor crashworthiness and being a reject.
[0119] On the basis of the above results, samples satisfying all of the requirements: the
value of TS being 980 MPa or more, the value of TS × EL being 17000 MPa·% or more,
the value of TS × λ being 20000 MPa·% or more, and the value of TS × VDA being 90000
MPa·° or more were regarded as the present invention examples and listed as being
acceptable in the totaltotal evaluation section of the following Tables 3-1 and 3-2.
On the other hand, samples in which one or more of the value of TS, the value of TS
× EL, the value of TS × λ, and the value of TS × VDA failed to satisfy the above acceptance
standard were regarded as the comparative examples and listed as being a reject in
the total evaluation section of the following Tables 3-1 and 3-2.
[Table 2-1]
No. |
Steel type |
Hot rolling step |
Annealing step |
Classification |
Heating |
Soaking |
Cooling |
Reheating holding |
Finish rolling end temperature (°C) |
Final stand rolling rate (%) |
Coiling temperature (°C) |
Average heating rate (° C/sec) |
Soaking temperature (°C) |
Ac3 soaking temperature (°C) |
Soaking time (sec) |
Average rate (°C/sec) |
Cooling stop Temperature T (°C) |
Ms point (°C) |
Cooling stop temperature T -Ms point (°C) |
Reheating temperature (°C) |
Reheating temperature - cooling stop temperature T (°C) |
Reheating holding time (sec) |
1 |
A |
880 |
15 |
550 |
15 |
820 |
61 |
300 |
15 |
250 |
349 |
-99 |
450 |
200 |
600 |
Cold-rolled |
2 |
A |
880 |
15 |
550 |
15 |
840 |
41 |
300 |
15 |
200 |
383 |
-183 |
380 |
180 |
600 |
EG |
3 |
A |
1000 |
15 |
550 |
15 |
820 |
61 |
300 |
15 |
300 |
323 |
-23 |
420 |
120 |
600 |
Cold-rolled |
4 |
A |
880 |
40 |
550 |
15 |
820 |
61 |
300 |
15 |
300 |
352 |
-52 |
450 |
150 |
600 |
Cold-rolled |
5 |
A |
880 |
3 |
550 |
15 |
820 |
61 |
300 |
15 |
250 |
344 |
-94 |
420 |
170 |
400 |
Cold -rolled |
6 |
B |
880 |
20 |
500 |
15 |
820 |
41 |
300 |
15 |
190 |
310 |
-120 |
420 |
230 |
600 |
Cold-rolled |
7 |
B |
880 |
20 |
550 |
15 |
900 |
-39 |
300 |
20 |
90 |
386 |
-296 |
380 |
290 |
600 |
Cold-rolled |
8 |
B |
880 |
20 |
500 |
15 |
850 |
11 |
300 |
15 |
480 |
372 |
108 |
350 |
-130 |
300 |
Cold-rolled |
9 |
C |
880 |
15 |
500 |
15 |
840 |
5 |
300 |
15 |
300 |
364 |
-64 |
420 |
120 |
600 |
Cold-rolled |
10 |
C |
880 |
15 |
500 |
15 |
840 |
5 |
300 |
15 |
25 |
369 |
-344 |
480 |
455 |
600 |
Cold-rolled |
11 |
C |
880 |
30 |
500 |
15 |
820 |
25 |
300 |
15 |
330 |
350 |
-20 |
300 |
-30 |
300 |
Cold-rolled |
12 |
D |
880 |
15 |
500 |
15 |
850 |
56 |
300 |
20 |
150 |
387 |
-237 |
380 |
230 |
600 |
Cold-rolled |
13 |
D |
880 |
7 |
500 |
15 |
850 |
56 |
300 |
20 |
200 |
384 |
-184 |
440 |
240 |
600 |
Cold-rolled |
14 |
D |
880 |
15 |
500 |
15 |
880 |
26 |
300 |
20 |
250 |
402 |
-152 |
450 |
200 |
5 |
Cold-rolled |
15 |
E |
880 |
10 |
550 |
20 |
850 |
64 |
300 |
20 |
250 |
387 |
-137 |
400 |
150 |
600 |
Cold-rolled |
16 |
E |
880 |
10 |
550 |
3 |
850 |
64 |
300 |
20 |
250 |
382 |
-132 |
450 |
200 |
600 |
Cold-rolled |
17 |
F |
880 |
15 |
550 |
15 |
830 |
17 |
300 |
15 |
170 |
353 |
-183 |
360 |
190 |
600 |
Cold-rolled |
18 |
F |
880 |
15 |
550 |
15 |
810 |
37 |
300 |
15 |
170 |
337 |
-167 |
440 |
270 |
300 |
GA |
19 |
F |
880 |
15 |
550 |
15 |
810 |
37 |
300 |
15 |
400 |
345 |
55 |
450 |
50 |
600 |
Cold-rolled |
20 |
G |
880 |
10 |
550 |
15 |
820 |
68 |
300 |
15 |
300 |
392 |
-92 |
450 |
150 |
600 |
Cold-rolled |
[Table 2-2]
No. |
Steel type |
Hot rolling step |
Annealing step |
Classification |
Heating |
Soaking |
Cooling |
Reheating holding |
Finish rolling end temperature (°C) |
Final stand rolling rate (%) |
Coiling temperature (°C) |
Average heating rate (°C/sec) |
Soaking temperature (°C) |
Ac3 - soaking temperature (°C) |
Soaking time (sec) |
Average cooling rate (°C/sec) |
Cooling stop temperature T (°C) |
Ms point (°C) |
Cooling stop temperature T -Ms point (°C) |
Reheating temperature (°C) |
Reheating temperature - cooling stop temperature T (°C) |
Reheating holding time (sec) |
21 |
H |
880 |
15 |
550 |
15 |
820 |
54 |
300 |
15 |
170 |
283 |
-113 |
450 |
280 |
600 |
Cold-rolled |
22 |
I |
880 |
20 |
550 |
15 |
850 |
20 |
300 |
20 |
100 |
257 |
-157 |
500 |
400 |
600 |
Cold-rolled |
23 |
J |
880 |
15 |
600 |
15 |
810 |
22 |
300 |
10 |
300 |
362 |
-62 |
450 |
150 |
600 |
Cold-rolled |
24 |
K |
880 |
15 |
550 |
15 |
820 |
45 |
300 |
10 |
200 |
350 |
-150 |
420 |
220 |
600 |
Cold-rolled |
25 |
K |
880 |
15 |
550 |
15 |
820 |
45 |
300 |
10 |
300 |
346 |
-46 |
600 |
300 |
1000 |
Cold-rolled |
26 |
L |
880 |
15 |
550 |
15 |
830 |
63 |
300 |
20 |
250 |
297 |
-47 |
420 |
170 |
600 |
Cold-rolled |
27 |
L |
880 |
15 |
550 |
15 |
830 |
63 |
300 |
1 |
300 |
106 |
194 |
420 |
120 |
600 |
Cold-rolled |
28 |
M |
880 |
15 |
550 |
15 |
840 |
29 |
300 |
20 |
250 |
333 |
-83 |
440 |
190 |
600 |
Cold -rolled |
29 |
M |
880 |
15 |
700 |
15 |
840 |
29 |
300 |
20 |
250 |
321 |
-71 |
460 |
210 |
600 |
Cold-rolled |
30 |
N |
880 |
15 |
600 |
15 |
810 |
34 |
300 |
10 |
200 |
345 |
-145 |
400 |
200 |
600 |
Cold -rolled |
31 |
O |
880 |
15 |
600 |
15 |
810 |
42 |
300 |
10 |
190 |
325 |
-135 |
480 |
290 |
600 |
Cold-rolled |
32 |
P |
880 |
15 |
600 |
15 |
805 |
30 |
300 |
10 |
150 |
278 |
-128 |
500 |
350 |
600 |
Cold-rolled |
33 |
Q |
880 |
15 |
500 |
15 |
840 |
68 |
300 |
20 |
200 |
443 |
-243 |
420 |
220 |
600 |
Cold-rolled |
34 |
R |
880 |
15 |
500 |
15 |
810 |
12 |
300 |
20 |
200 |
403 |
-203 |
420 |
220 |
600 |
Cold-rolled |
35 |
S |
880 |
15 |
500 |
15 |
830 |
72 |
300 |
10 |
200 |
247 |
-47 |
480 |
280 |
600 |
Cold-rolled |
36 |
E |
880 |
10 |
550 |
20 |
880 |
34 |
400 |
20 |
200 |
401 |
-201 |
440 |
240 |
200 |
GI |
37 |
F |
880 |
15 |
550 |
7 |
830 |
17 |
300 |
15 |
320 |
344 |
-24 |
460 |
140 |
600 |
Cold-rolled |
38 |
E |
880 |
10 |
550 |
20 |
820 |
94 |
300 |
7 |
300 |
204 |
96 |
400 |
100 |
300 |
Cold-rolled |
39 |
C |
880 |
15 |
500 |
15 |
830 |
15 |
300 |
15 |
210 |
356 |
-146 |
370 |
160 |
100 |
Cold-rolled |
40 |
B |
880 |
15 |
500 |
10 |
840 |
21 |
300 |
30 |
220 |
335 |
-115 |
380 |
160 |
600 |
Cold-rolled |
41 |
A |
880 |
15 |
550 |
20 |
830 |
51 |
100 |
20 |
220 |
380 |
-160 |
420 |
200 |
300 |
Cold-rolled |
[Table 3-1]
No. |
Structure fraction |
Average circle-equivalent diameter of MA structure (µm) |
VMA/Vγ |
Material properties |
Ferrite Vf (area%) |
Hard phase (area%) |
Other structures (area%) |
Retained γ Vγ (vol%) |
MA structure VMA (area%) |
TS (MPa) |
EL (%) |
λ (%) |
VDA (°) |
TS × EL (MPa·%) |
TS × λ (MPa·%) |
TS × VDA (MPa·°) |
Total evaluation |
1 |
48 |
50 |
2 |
12 |
15 |
1.5 |
1.25 |
1022 |
22 |
24 |
92 |
22484 |
24528 |
94024 |
Acceptable |
2 |
32 |
65 |
3 |
8 |
8 |
1.2 |
1.00 |
1216 |
17 |
31 |
76 |
20672 |
37696 |
92416 |
Acceptable |
3 |
56 |
41 |
3 |
14 |
18 |
2.5 |
1.29 |
1051 |
20 |
17 |
86 |
21020 |
17867 |
90386 |
Reject |
4 |
47 |
48 |
5 |
16 |
22 |
2.8 |
1.38 |
1073 |
22 |
15 |
79 |
23606 |
16095 |
84767 |
Reject |
5 |
50 |
47 |
3 |
12 |
15 |
2.2 |
1.25 |
1031 |
20 |
18 |
89 |
20620 |
18558 |
91759 |
Reject |
6 |
42 |
57 |
1 |
17 |
20 |
1.1 |
1.18 |
1055 |
26 |
32 |
98 |
27430 |
33760 |
103390 |
Acceptable |
7 |
1 |
97 |
2 |
5 |
6 |
0.7 |
1.20 |
1281 |
10 |
42 |
82 |
12810 |
53802 |
105042 |
Reject |
8 |
12 |
84 |
4 |
13 |
24 |
1.9 |
1.85 |
1057 |
19 |
21 |
75 |
20083 |
22197 |
79275 |
Reject |
9 |
24 |
73 |
3 |
13 |
18 |
1.2 |
1.38 |
1036 |
19 |
22 |
94 |
19684 |
22792 |
97384 |
Acceptable |
10 |
21 |
76 |
3 |
3 |
1 |
0.7 |
0.33 |
992 |
15 |
42 |
91 |
14880 |
41664 |
90272 |
Reject |
11 |
32 |
65 |
3 |
7 |
12 |
2.3 |
1.71 |
1354 |
13 |
18 |
61 |
17602 |
24372 |
82594 |
Reject |
12 |
28 |
68 |
4 |
10 |
13 |
1.1 |
1.30 |
1246 |
17 |
26 |
75 |
21182 |
32396 |
93450 |
Acceptable |
13 |
30 |
68 |
2 |
12 |
15 |
1.6 |
1.25 |
1053 |
20 |
21 |
88 |
21060 |
22113 |
92664 |
Acceptable |
14 |
16 |
83 |
1 |
14 |
35 |
3.4 |
2.50 |
1521 |
7 |
8 |
55 |
10647 |
12168 |
83655 |
Reject |
1.5 |
29 |
66 |
5 |
13 |
16 |
1.4 |
1.23 |
1041 |
21 |
26 |
92 |
21861 |
27066 |
95772 |
Acceptable |
16 |
32 |
66 |
2 |
12 |
17 |
2.4 |
1.42 |
992 |
21 |
17 |
91 |
20832 |
16864 |
90272 |
Reject |
17 |
26 |
71 |
3 |
9 |
7 |
1.0 |
0.78 |
1222 |
17 |
31 |
78 |
20774 |
37882 |
95316 |
Acceptable |
18 |
34 |
64 |
2 |
13 |
14 |
1.2 |
1.08 |
1062 |
22 |
28 |
86 |
23364 |
29736 |
91332 |
Acceptable |
19 |
30 |
66 |
4 |
13 |
16 |
2.3 |
1.23 |
1034 |
18 |
15 |
88 |
18612 |
15510 |
90992 |
Reject |
20 |
31 |
67 |
2 |
10 |
11 |
1.0 |
1.10 |
1029 |
17 |
31 |
91 |
17493 |
31899 |
93639 |
Acceptable |
[Table 3-2]
No. |
Structure fraction |
Average circle-equivalent diameter of MA structure (µm) |
VMA/Vγ |
Material properties |
Ferrite (area%) |
Hard phase (area%) |
Other structures (area%) |
Retained γ Vγ (vol%) |
MA structure VMA (area%) |
TS (MPa) |
EL (%) |
λ (%) |
VDA (°) |
TS × EL (MPa·%) |
TS × λ (MPa·%) |
TS × VDA (MPa·°) |
Total evaluation |
21 |
33 |
65 |
2 |
18 |
21 |
1.2 |
1.17 |
1238 |
18 |
22 |
76 |
22284 |
27236 |
94088 |
Acceptable |
22 |
13 |
86 |
1 |
22 |
24 |
1.2 |
1.09 |
1421 |
14 |
21 |
65 |
19894 |
29841 |
92365 |
Acceptable |
23 |
21 |
77 |
2 |
11 |
15 |
1.6 |
1.36 |
1053 |
17 |
23 |
87 |
17901 |
24219 |
91611 |
Acceptable |
24 |
24 |
74 |
2 |
12 |
15 |
0.9 |
1.25 |
1017 |
20 |
24 |
100 |
20340 |
24408 |
101700 |
Acceptable |
25 |
26 |
60 |
14 |
2 |
2 |
0.9 |
1.00 |
819 |
21 |
16 |
82 |
17199 |
13104 |
67158 |
Reject |
26 |
48 |
49 |
3 |
16 |
19 |
1.0 |
1.19 |
1004 |
25 |
28 |
95 |
25100 |
28112 |
95380 |
Acceptable |
27 |
73 |
22 |
5 |
11 |
16 |
1.5 |
1.45 |
921 |
29 |
16 |
101 |
26709 |
14736 |
93021 |
Reject |
28 |
42 |
56 |
2 |
13 |
16 |
1.6 |
1.23 |
997 |
25 |
26 |
92 |
24925 |
25922 |
91724 |
Acceptable |
29 |
46 |
50 |
4 |
12 |
17 |
2.3 |
1.42 |
1011 |
21 |
17 |
91 |
21231 |
17187 |
92001 |
Reject |
30 |
23 |
75 |
2 |
9 |
10 |
0.9 |
1.11 |
1194 |
16 |
32 |
77 |
19104 |
38208 |
91938 |
Acceptable |
31 |
18 |
82 |
0 |
13 |
17 |
1.5 |
1.31 |
1432 |
14 |
23 |
65 |
20048 |
32936 |
93080 |
Acceptable |
32 |
12 |
88 |
0 |
12 |
17 |
1.7 |
1.42 |
1501 |
13 |
25 |
61 |
19513 |
37525 |
91561 |
Acceptable |
33 |
32 |
66 |
2 |
3 |
5 |
1.0 |
1.67 |
1023 |
15 |
34 |
97 |
15345 |
34782 |
99231 |
Reject |
34 |
8 |
89 |
3 |
5 |
6 |
1.1 |
1.20 |
1038 |
14 |
56 |
102 |
14532 |
58128 |
105876 |
Reject |
35 |
72 |
20 |
8 |
11 |
14 |
1.7 |
1.27 |
938 |
25 |
15 |
97 |
23450 |
14070 |
90986 |
Reject |
36 |
19 |
77 |
4 |
11 |
14 |
1.6 |
1.27 |
1005 |
20 |
31 |
94 |
20100 |
31155 |
94470 |
Acceptable |
37 |
31 |
66 |
3 |
10 |
11 |
2.4 |
1.10 |
1027 |
17 |
17 |
93 |
17459 |
17459 |
95511 |
Reject |
38 |
73 |
22 |
5 |
13 |
16 |
1.8 |
1.23 |
932 |
22 |
19 |
102 |
20504 |
17708 |
95064 |
Reject |
39 |
29 |
69 |
2 |
10 |
9 |
1.0 |
0.90 |
1248 |
15 |
34 |
76 |
18720 |
42432 |
94848 |
Acceptable |
40 |
33 |
64 |
3 |
13 |
16 |
1.5 |
1.23 |
1214 |
16 |
27 |
76 |
19424 |
32778 |
92264 |
Acceptable |
41 |
34 |
63 |
3 |
10 |
12 |
1.3 |
1.20 |
1064 |
18 |
27 |
87 |
19152 |
28728 |
92568 |
Acceptable |
[0120] From Tables 1, 2-1, 2-2, 3-1, and 3-2, the following considerations can be made.
[0121] In Tables 3-1 and 3-2, all of the samples rated as "acceptable" in the total evaluation
section are steel sheets satisfying the requirements defined in the present invention,
and all of the value of TS × EL, the value of TS × λ, and the value of TS × VDA determined
in accordance with the tensile strength TS satisfy the acceptance standard values.
It will be understood that these steel sheets have good formability as evaluated by
ductility and stretch-flangeability, and are excellent in ductility in particular,
and also in crashworthiness.
[0122] In contrast, the samples rated as "reject" in the total evaluation section are steel
sheets that do not satisfy one or more of the requirements defined in the present
invention, and at least one of ductility, stretch-flangeability, and crashworthiness
could not be improved. The details are as follows.
[0123] No. 3 is a sample in which the MA structure was coarsened because the finish rolling
end temperature was too high. As a result, the value of TS × λ was small, so that
the stretch-flangeability could not be improved.
[0124] No. 4 is a sample in which the MA structure was coarsened because the rolling reduction
at the final stand during the finish rolling was too high and exceeded the range defined
in the present invention. As a result, the value of TS × λ was small, so that the
stretch-flangeability could not be improved. Also, the value of TS × VDA was small,
so that the crashworthiness could not be improved.
[0125] No. 5 is a sample in which the MA structure was coarsened because the rolling reduction
at the final stand during the finish rolling was too low and was below the range defined
in the present invention. As a result, the value of TS × λ was small, so that the
stretch-flangeability could not be improved.
[0126] No. 7 is a sample in which a ferrite amount within the range defined in the present
invention could not be ensured because the soaking was carried out at a high temperature
exceeding the temperature region of 800°C or higher and lower than the Ac
3 point. As a result, the value of TS × EL was small, so that the ductility could not
be improved.
[0127] No. 8 is a sample in which the value of V
MA/V
γ was too large because the cooling stop temperature T after the soaking was too high
and exceeded the temperature region of 50°C or higher and the Ms point or lower and
because the reheating holding was not carried out after the cooling. As a result,
the value of TS × VDA was small, so that the crashworthiness could not be improved.
[0128] No. 10 is a sample in which a predetermined amount of retained γ and the MA structure
could not be ensured because the cooling stop temperature T after the soaking was
below 50°C, so that the value of V
MA/V
γ was small and below the defined range. As a result, the value of TS × EL was small,
so that the ductility could not be improved.
[0129] No. 11 is a sample in which the MA structure was coarsened and the value of V
MA/V
γ was too large because the rolling reduction at the final stand during the finish
rolling was too high and exceeded the range defined in the present invention and because
the reheating holding was not carried out after the cooling. As a result, the value
of TS × VDA was small, so that the crashworthiness could not be improved.
[0130] No. 14 is a sample in which the MA structure was coarsened because the reheating
holding time was too short. As a result, the value of TS × λ was small, so that the
stretch-flangeability could not be improved. Further, the MA structure was generated
excessively. As a result, the value of TS × EL was small, so that the ductility could
not be improved. Also, the value of V
MA/V
γ was too large. As a result, the value of TS × VDA was small, so that the crashworthiness
was deteriorated.
[0131] Nos. 16 and 37 are samples in which the MA structure was coarsened because the average
heating rate after the coiling was too small. As a result, the value of TS × λ was
small, so that the stretch-flangeability could not be improved.
[0132] No. 19 is a sample in which the MA structure was coarsened because the cooling stop
temperature T after the soaking was too high and exceeded the temperature region of
50°C or higher and the Ms point or lower. As a result, the value of TS × λ was small,
so that the stretch-flangeability could not be improved.
[0133] No. 25 is a sample in which decomposition of austenite occurred and a predetermined
amount of retained γ and the MA structure could not be ensured because the reheating
temperature carried out after the cooling was too high. As a result, TS was small.
Also, the value of TS × λ was small, so that the stretch-flangeability could not be
improved. Further, the value of TS × VDA was small, so that the crashworthiness could
not be improved.
[0134] Nos. 27 and 38 are samples in which ferrite was excessively generated because the
average cooling rate after the soaking was too small. As a result, TS was small. Also,
the value of TS × λ was small, so that the stretch-flangeability could not be improved.
[0135] No. 29 is a sample in which the MA structure was coarsened because the coiling temperature
was too high. As a result, the value of TS × λ was small, so that the stretch-flangeability
could not be improved.
[0136] No. 33 is a sample in which the C amount was too small, so that a retained γ amount
within the range defined in the present invention could not be ensured, and the value
of V
MA/V
γ was so large as to exceed the range defined in the present invention. As a result,
the value of TS × EL was small, so that the ductility was deteriorated.
[0137] No. 34 is a sample in which the Si amount was too small, so that a ferrite amount
within the range defined in the present invention could not be ensured. As a result,
the value of TS × EL was small, so that the ductility was deteriorated.
[0138] No. 35 is a sample in which the Mn amount was too small, so that the hardenability
was insufficient, and ferrite was excessively generated. As a result, TS was low.
Further, the value of TS × λ was small, so that the stretch-flangeability was deteriorated.
Reference Signs
[0139]
- 1
- Heating step
- 2
- Soaking step
- 3
- Cooling step
- 4
- Reheating holding step
- 5
- Cooling stop temperature
1. Hochfeste kaltgewalzte Stahlplatte mit einer Zugfestigkeit von 980 MPa oder mehr und
welche eine ausgezeichnete Formbarkeit und Kollisionssicherheit aufweist, wobei die
hochfeste kaltgewalzte Stahlplatte enthält, in Massen-%:
C: 0,10% oder mehr bis 0,5% oder weniger,
Si: 1,0% oder mehr bis 3% oder weniger,
Mn: 1,5% oder mehr bis 7% oder weniger,
P: mehr als 0% bis 0,1 % oder weniger,
S: mehr als 0% bis 0,05% oder weniger,
Al: 0,005% oder mehr bis 1 % oder weniger,
N: mehr als 0% bis 0,01% oder weniger, und
O: mehr als 0% bis 0,01 % oder weniger, und
gegebenenfalls, als andere Elemente, eines oder mehrere von einem beliebigen der nachstehenden
(a) bis (e), in Massen-%:
(a) mindestens eines, ausgewählt aus der Gruppe, bestehend aus Cr: mehr als 0% bis
1% oder weniger und Mo: mehr als 0% bis 1% oder weniger,
(b) mindestens eines, ausgewählt aus der Gruppe, bestehend aus Ti: mehr als 0% bis
0,15% oder weniger, Nb: mehr als 0% bis 0,15% oder weniger, und V: mehr als 0% bis
0,15% oder weniger,
(c) mindestens eines, ausgewählt aus der Gruppe, bestehend aus Cu: mehr als 0% bis
1% oder weniger und Ni: mehr als 0% bis 1% oder weniger,
(d) B: mehr als 0% bis 0,005% oder weniger, und
(e) mindestens eines, ausgewählt aus der Gruppe, bestehend aus Ca: mehr als 0% bis
0,01% oder weniger, Mg: mehr als 0% bis 0,01% oder weniger und REM: mehr als 0% bis
0,01% oder weniger,
wobei ein Rest Eisen und unvermeidbare Verunreinigungen ist, wobei eine Metallstruktur
an einer Position von 1/4 einer Plattendicke die nachstehenden (1) bis (4) erfüllt:
(1)wenn die Metallstruktur mit einem Rasterelektronenmikroskop betrachtet wird, ein
Flächeninhaltsverhältnis von Ferrit relativ zu einer Gesamtheit der Metallstruktur
mehr als 10% bis 65% oder weniger beträgt, wobei ein Rest eine Hartphase ist, einschließend
abgeschreckten Martensit und Restaustenit und einschließend mindestens eines, ausgewählt
aus der Gruppe, bestehend aus bainitischem Ferrit, Bainit und getempertem Martensit,
wobei die Metallstruktur, zusätzlich zu Ferrit und der Hartphase, mindestens eines,
ausgewählt aus der Gruppe, bestehend aus Perlit und Zementit, enthalten kann,
(2) wenn die Metallstruktur mittels Röntgendiffraktometrie vermessen wird, ein Volumenverhältnis
Vγ von Restaustenit relativ zu der Gesamtheit der Metallstruktur 5% oder mehr bis 30%
oder weniger beträgt,
(3) wenn die Metallstruktur mit einem optischen Mikroskop betrachtet wird, ein Flächeninhaltsverhältnis
VMA einer MA-Struktur, in welcher abgeschreckter Martensit und Restaustenit kombiniert
sind, relativ zu der Gesamtheit der Metallstruktur 3% oder mehr bis 25% oder weniger
beträgt und ein mittlerer Kreisäquivalenzdurchmesser der MA-Struktur 2,0 µm oder weniger
beträgt und
(4) ein Verhältnis VMA/Vγ des Flächeninhaltsverhältnisses VMA der MA-Struktur zu dem Volumenverhältnis Vγ des Restaustenits einer nachstehenden Formel (i) gehorcht:
und wobei
der Wert der Zugfestigkeit TS × Streckung EL 17000 MPa·% oder mehr beträgt, der Wert
von TS × Expansionsverhältnis λ 20000 MPa·% oder mehr beträgt und der Wert von TS
× VDA Biegewinkel 90000 MPa·° oder mehr beträgt, wobei die Probe für TS gemäß JIS
Z2201 entnommen wurde und der VDA-Wert gemäß der Norm VDA238-100 bestimmt wurde.
2. Hochfeste elektrolytisch verzinkte Stahlplatte mit einer elektrolytisch verzinkten
Schicht auf einer Oberfläche der hochfesten kaltgewalzten Stahlplatte nach Anspruch
1.
3. Hochfeste feuerverzinkte Stahlplatte mit einer feuerverzinkten Schicht auf einer Oberfläche
der hochfesten kaltgewalzten Stahlplatte nach Anspruch 1.
4. Hochfeste feuerverzinkt-geglühte Stahlplatte mit einer feuerverzinkt-geglühten Schicht
auf einer Oberfläche der hochfesten kaltgewalzten Stahlplatte nach Anspruch 1.
5. Verfahren zum Herstellen einer hochfesten kaltgewalzten Stahlplatte mit einer Zugfestigkeit
von 980 MPa oder mehr und mit ausgezeichneter Formbarkeit und Kollisionssicherheit,
das Verfahren umfassend:
das Verwenden eines Stahls, welcher eine Bestandteilszusammensetzung, wie in Anspruch
1 ausgeführt, erfüllt,
das Warmwalzen des Stahls mit einer Walzrate bei einem Endzustand des Fertigwalzens
von 5 bis 20% und mit einer Endtemperatur des Fertigwalzens, welche ein Ar3-Punkt oder höher bis 900°C oder niedriger ist, das Aufrollen des Stahls mit einer
Aufrolltemperatur von 600°C oder niedriger und das Abkühlen des Stahls auf Raumtemperatur,
das Kaltwalzen des Stahls,
das Erwärmen des Stahls, mit einer mittleren Erwärmungsrate von 10°C/Sekunde oder
mehr, auf einen Temperaturbereich von 800°C oder höher und niedriger als ein Ac3-Punkt und das Durchwärmen des Stahls, wobei der Stahl für 50 Sekunden oder mehr in
dem Temperaturbereich gehalten wird,
das Abkühlen des Stahls bei einer mittleren Abkühlungsrate von 10°C/Sekunde oder mehr,
auf eine beliebige Endtemperatur des Kühlens T°C, welche in einem Temperaturbereich
von 70°C oder höher und einem Ms-Punkt oder niedriger liegt, und
das Erwärmen und Halten des Stahls in einem Temperaturbereich der Endtemperatur des
Kühlens T+50°C oder höher und 550°C oder niedriger für 200 Sekunden oder mehr und
danach das Abkühlen des Stahls auf Raumtemperatur,
wobei der Ms-Punkt auf Grundlage der folgenden Formel (iv) berechnet wird, wobei Klammern
[ ] den Gehalt eines jeden Elementes in Massen-% angeben und Vf das Flächeninhaltsverhältnis
von Ferrit relativ zu der Gesamtheit der Metallstruktur angibt,
6. Verfahren zum Herstellen einer hochfesten feuerverzinkten Stahlplatte mit einer Zugfestigkeit
von 980 MPa oder mehr und mit ausgezeichneter Formbarkeit und Kollisionssicherheit,
das Verfahren umfassend:
das Verwenden eines Stahls, welcher eine Bestandteilszusammensetzung, wie in Anspruch
1 ausgeführt, erfüllt,
das Warmwalzen des Stahls mit einer Walzrate bei einem Endzustand des Fertigwalzens
von 5 bis 20% und mit einer Endtemperatur des Fertigwalzens, welche ein Ar3-Punkt oder höher bis 900°C oder niedriger ist, das Aufrollen des Stahls mit einer
Aufrolltemperatur von 600°C oder niedriger und das Abkühlen des Stahls auf Raumtemperatur,
das Kaltwalzen des Stahls,
das Erwärmen des Stahls, mit einer mittleren Erwärmungsrate von 10°C/Sekunde oder
mehr, auf einen Temperaturbereich von 800°C oder höher und niedriger als ein Ac3-Punkt und das Durchwärmen des Stahls, wobei der Stahl für 50 Sekunden oder mehr in
dem Temperaturbereich gehalten wird,
das Abkühlen des Stahls bei einer mittleren Abkühlungsrate von 10°C/Sekunde oder mehr,
auf eine beliebige Endtemperatur des Kühlens T°C, welche in einem Temperaturbereich
von 70°C oder höher und einem Ms-Punkt oder niedriger liegt, und
ein Wiedererwärmen des Erwärmens und Haltens des Stahls in einem Temperaturbereich
der Endtemperatur des Kühlens T+50°C oder höher und 550°C oder niedriger für 200 Sekunden
oder mehr und nach dem Durchführen von Feuerverzinken innerhalb einer Haltedauer,
das Abkühlen des Stahls auf Raumtemperatur, wobei
es in dem Wiedererwärmen drei Muster von (I) bis (III) als eine Kombination des Feuerverzinkens
und nur des Erwärmens ohne das Durchführen des Feuerverzinkens gibt,
(I) nur das Erwärmen wird durchgeführt und danach wird das Feuerverzinken durchgeführt;
(II) das Feuerverzinken wird durchgeführt und danach wird nur das Erwärmen durchgeführt;
(III) nur das Erwärmen wird durchgeführt und danach wird das Feuerverzinken durchgeführt
und weiterhin wird nur das Erwärmen durchgeführt, in dieser Reihenfolge,
wobei der Ms-Punkt auf Grundlage der folgenden Formel (iv) berechnet wird,
wobei Klammern [ ] den Gehalt eines jeden Elementes in Massen-% angeben und Vf das
Flächeninhaltsverhältnis von Ferrit relativ zu der Gesamtheit der Metallstruktur angibt,
7. Verfahren zum Herstellen einer hochfesten feuerverzinkt-geglühten Stahlplatte mit
einer Zugfestigkeit von 980 MPa oder mehr und mit ausgezeichneter Formbarkeit und
Kollisionssicherheit, das Verfahren umfassend:
das Verwenden eines Stahls, welcher eine Bestandteilszusammensetzung, wie in Anspruch
1 ausgeführt, erfüllt,
das Warmwalzen des Stahls mit einer Walzrate bei einem Endzustand des Fertigwalzens
von 5 bis 20% und mit einer Endtemperatur des Fertigwalzens, welche ein Ar3-Punkt oder höher bis 900°C oder niedriger ist, das Aufrollen des Stahls mit einer
Aufrolltemperatur von 600°C oder niedriger und das Abkühlen des Stahls auf Raumtemperatur,
das Kaltwalzen des Stahls,
das Erwärmen des Stahls, mit einer mittleren Erwärmungsrate von 10°C/Sekunde oder
mehr, auf einen Temperaturbereich von 800°C oder höher und niedriger als ein Ac3-Punkt und das Durchwärmen des Stahls, wobei der Stahl für 50 Sekunden oder mehr in
dem Temperaturbereich gehalten wird,
das Abkühlen des Stahls bei einer mittleren Abkühlungsrate von 10°C/Sekunde oder mehr,
auf eine beliebige Endtemperatur des Kühlens T°C, welche in einem Temperaturbereich
von 70°C oder höher und einem Ms-Punkt oder niedriger liegt, und
ein Wiedererwärmen des Erwärmens und Haltens des Stahls in einem Temperaturbereich
der Endtemperatur des Kühlens T+50°C oder höher und 550°C oder niedriger für 200 Sekunden
oder mehr und nach dem Durchführen von Feuerverzinken innerhalb einer Haltedauer,
weiter das Durchführen einer Legierungsbehandlung und danach das Abkühlen des Stahls
auf Raumtemperatur, wobei
es in dem Wiedererwärmen drei Muster von (I) bis (III) als eine Kombination des Feuerverzinkens
und nur des Erwärmens ohne das Durchführen des Feuerverzinkens gibt,
(I) nur das Erwärmen wird durchgeführt und danach wird das Feuerverzinken durchgeführt;
(II) das Feuerverzinken wird durchgeführt und danach wird nur das Erwärmen durchgeführt;
(III) nur das Erwärmen wird durchgeführt und danach wird das Feuerverzinken durchgeführt
und weiterhin wird nur das Erwärmen durchgeführt, in dieser Reihenfolge,
wobei der Ms-Punkt auf Grundlage der folgenden Formel (iv) berechnet wird,
wobei Klammern [ ] den Gehalt eines jeden Elementes in Massen-% angeben und Vf das
Flächeninhaltsverhältnis von Ferrit relativ zu der Gesamtheit der Metallstruktur angibt,
1. Tôle d'acier à haute résistance mécanique laminée à froid offrant une résistance à
la traction de 980 MPa ou plus et qui présente une excellente aptitude à la mise en
oeuvre et résistance aux chocs, la tôle d'acier haute résistance mécanique laminée
à froid contenant, en % en masse:
C: de 0,10% ou plus à 0,5% ou moins,
Si: de 1,0% ou plus à 3% ou moins,
Mn: de 1,5% ou plus à 7% ou moins,
P: de plus de 0% à 0,1% ou moins,
S: de plus de 0% à 0,05% ou moins,
Al: de 0,005% ou plus à 1% ou moins,
N: de plus de 0% à 0,01% ou moins, et
O: de plus de 0% à 0,01% ou moins, et
éventuellement, comme autres éléments, un ou plus de l'un quelconque de (a) à (e)
ci-dessous, en % en masse:
(a) au moins un sélectionné parmi le groupe consistant en Cr: de plus de 0% à 1% ou
moins, et du Mo: de plus de 0% à 1% ou moins,
(b) au moins un sélectionné parmi le groupe consistant en Ti: de plus de 0% à 0,15%
ou moins, du Nb: de plus de 0% à 0,15% ou moins, et du V: de plus de 0% à 0,15% ou
moins,
(c) au moins un sélectionné parmi le groupe consistant en Cu: de plus de 0% à 1% ou
moins, et du Ni: de plus de 0% à 1% ou moins,
(d) du B: de plus de 0% à 0,005% ou moins, et
(e) au moins un sélectionné parmi le groupe consistant en Ca: de plus de 0% à 0,01%
ou moins, du Mg: de plus de 0% à 0,01% ou moins, et du métal terre rare: de plus de
0% à 0,01% ou moins,
le reste étant du fer et d'inévitables impuretés, dans laquelle
une structure métallique à une position de 1/4 d'une épaisseur de tôle satisfait à
(1) à (4) ci-dessous:
(1) quand la structure métallique est observée à l'aide d'un microscope à balayage
électronique, un rapport de surface de ferrite par rapport à la totalité de la structure
métallique va de plus de 10% à 65% ou moins, le reste étant une phase dure comprenant
de la martensite trempée et de l'austénite de trempe et comprenant au moins l'un sélectionné
parmi le groupe consistant en ferrite bainitique, bainite et en martensite revenue,
dans laquelle la structure métallique peut contenir, en plus de ferrite et de la phase
dure, au moins l'un sélectionné parmi le groupe consistant en perlite et cémentite,
(2) quand la structure métallique est mesurée par diffractométrie aux rayons X, un
rapport volumique Vγ d'austénite de trempe par rapport à la totalité de la structure métallique va de
5% ou plus à 30% ou moins,
(3) quand la structure métallique est observée avec un microscope optique, un rapport
de surface VMA d'une structure MA, dans laquelle de la martensite trempée et de l'austénite de trempe
sont combinées, par rapport à la totalité de la structure métallique va de 3% ou plus
à 25% ou moins, et un diamètre moyen de cercle équivalent de la structure MA va de
2,0 µm ou moins, et
(4) un rapport VMA/Vγ du rapport de surface VMA de la structure MA au rapport volumique Vγ de l'austénite de trempe satisfait à une formule (i) ci-dessous:
et dans laquelle
la valeur de la résistance à la traction TS x élongation EL est de 17 000 MPa•% ou
plus, la valeur de TS x rapport d'expansion X est de 20000 MPa•% ou plus, et la valeur
de TS x angle de pliage VDA est de 90000 MPa• °ou plus, dans laquelle l'échantillon
pour le TS a été pris selon JIS Z2201 et la valeur VDA a été déterminée selon la norme
VDA238-100.
2. Tôle d'acier haute résistance mécanique électrogalvanisée avec une couche électrogalvanisée
à la surface de la tôle d'acier haute résistance mécanique laminée à froid selon la
revendication 1.
3. Tôle d'acier haute résistance mécanique galvanisée par immersion à chaud avec une
couche galvanisée par immersion à chaud à la surface de la tôle d'acier haute résistance
mécanique laminée à froid selon la revendication 1.
4. Tôle d'acier haute résistance mécanique recuite après galvanisation par immersion
à chaud avec une couche recuite après galvanisation par immersion à chaud à la surface
de la tôle d'acier haute résistance mécanique laminée à froid selon la revendication
1.
5. Procédé de production d'une tôle d'acier haute résistance mécanique laminée à froid
offrant une résistance à la traction de 980 MPa ou plus et qui présente une excellente
aptitude à la mise en oeuvre et résistance aux chocs, le procédé comprenant:
l'utilisation d'un acier satisfaisant à une composition de composants telle que décrite
à la revendication 1,
le laminage à chaud de l'acier avec une vitesse de laminage à un état final du laminage
de finition qui est de 5 à 20% et avec une température finale du laminage de finition
qui est un point Ar3 ou plus à 900°C ou moins, l'enroulement de l'acier à une température d'enroulement
qui est de 600°C ou moins, et le refroidissement de l'acier à la température ambiante,
le laminage à froid de l'acier,
le chauffage de l'acier, à une vitesse de chauffage moyenne de 10°C/s ou plus, dans
une zone de température de 800°C ou plus, et inférieure à un point Ac3, et chauffage à cœur de l'acier tout en maintenant l'acier dans la zone de température
pendant 50 secondes ou plus,
le refroidissement de l'acier à une vitesse de refroidissement moyenne de 10°C/seconde
ou plus, à une température d'arrêt du refroidissement arbitraire T°C qui se situe
dans une plage de températures de 70°C ou plus et un point Ms ou
moins, et
le chauffage et le maintien de l'acier dans une zone de température de la température
d'arrêt du refroidissement T+50°C ou plus et 550°C ou moins pendant 200 secondes ou
plus, et après cela refroidissement de l'acier à la température ambiante,
dans laquelle le point Ms est calculé sur base de la formule suivante (iv), dans laquelle
les crochets [ ] indiquent la teneur en chaque élément en % en masse et Vf indique
le rapport de surface de ferrite par rapport à la totalité de la structure métallique,
6. Procédé de production d'une tôle d'acier haute résistance mécanique galvanisée par
immersion à chaud présentant une résistance à la traction de 980 MPa ou plus et qui
offre une excellente aptitude à la mise en oeuvre et résistance aux chocs, le procédé
comprenant:
l'utilisation d'un acier satisfaisant à une composition de composants telle que décrite
à la revendication 1,
laminage à chaud de l'acier à une vitesse de laminage à l'état final du laminage de
finition qui est de 5 à 20% et à une température finale du laminage de finition qui
est un point Ar3 ou plus à 900°C ou moins, l'enroulement de l'acier à une température d'enroulement
qui est de 600°C ou moins, et le refroidissement de l'acier à la température ambiante,
le laminage à froid de l'acier,
le chauffage de l'acier, à une vitesse de chauffage moyenne de 10°C/s ou plus, dans
une zone de température de 800°C ou plus, et inférieure à un point Ac3, et chauffage
à cœur de l'acier tout en maintenant l'acier dans la zone de température pendant 50
secondes ou plus,
le refroidissement de l'acier à une vitesse de refroidissement moyenne de 10°C/seconde
ou plus, à une température d'arrêt du refroidissement arbitraire T°C qui se situe
dans une plage de températures de 70°C ou plus et un point Ms ou moins, et
le réchauffage du chauffage et le maintien de l'acier dans une zone de température
de la température d'arrêt du refroidissement T+50°C ou plus et 550°C ou moins pendant
200 secondes ou plus, et après, réalisation d'une galvanisation par immersion à chaud
pendant une durée de maintien, le refroidissement de l'acier à la température ambiante,
dans lequel
dans le réchauffage, il y a trois modèles de (I) à (III) comme combinaison de galvanisation
par immersion à chaud et seul le chauffage sans réalisation de la galvanisation par
immersion à chaud,
(I) seul le chauffage est réalisé, et après cela, on réalise la galvanisation par
immersion à chaud;
(II) la galvanisation par immersion à chaud est réalisée, et après cela, on réalise
uniquement le chauffage;
(III) seul le chauffage est réalisé et après cela, la galvanisation par immersion
à chaud est réalisée, et plus loin, on réalise uniquement le chauffage, dans cet ordre;
dans laquelle le point Ms est calculé sur base de la formule suivante (iv), dans laquelle
les crochets [ ] indiquent la teneur en chaque élément en % en masse et Vf indique
le rapport de surface de ferrite par rapport à la totalité de la structure métallique,
7. Procédé de production d'une tôle d'acier haute résistance mécanique recuite après
galvanisation par immersion à chaud offrant une résistance à la traction de 980 MPa
ou plus et qui présente une excellente aptitude à la mise en oeuvre et résistance
aux chocs, le procédé comprenant:
l'utilisation d'un acier satisfaisant à une composition de composants telle que décrite
à la revendication 1,
le laminage à chaud de l'acier à une vitesse de laminage à l'état final du laminage
de finition qui est de 5 à 20% et à une température finale du laminage de finition
qui est un point Ar3 ou plus à 900°C ou moins, l'enroulement de l'acier à une température d'enroulement
qui est de 600°C ou moins, et le refroidissement de l'acier à la température ambiante,
le laminage à froid de l'acier,
le chauffage de l'acier, à une vitesse de chauffage moyenne de 10°C/s ou plus, dans
une zone de température de 800°C ou plus, et inférieure à un point Ac3, et chauffage à cœur de l'acier tout en maintenant l'acier dans la zone de température
pendant 50 secondes ou plus,
le refroidissement de l'acier à une vitesse de refroidissement moyenne de 10°C/seconde
ou plus, à une température d'arrêt du refroidissement arbitraire T°C qui se situe
dans une plage de températures de 70°C ou plus et un point Ms ou moins, et
le réchauffage du chauffage et le maintien de l'acier dans une zone de température
de la température d'arrêt du refroidissement T+50°C ou plus et 550°C ou moins pendant
200 secondes ou plus, et après réalisation d'une galvanisation par immersion à chaud
pendant une durée de maintien, réalisation plus loin d'un traitement d'alliage et
après cela refroidissement de l'acier à la température ambiante, dans lequel
dans le réchauffage, il y a trois modèles de (I) à (III) comme combinaison de galvanisation
par immersion à chaud et seul le chauffage sans réalisation de la galvanisation par
immersion à chaud,
(I) seul le chauffage est réalisé, et après cela, on réalise la galvanisation par
immersion à chaud;
(II) la galvanisation par immersion à chaud est réalisée, et après cela, on réalise
uniquement le chauffage;
(III) seul le chauffage est réalisé et après cela, la galvanisation par immersion
à chaud est réalisée, et plus loin, on réalise uniquement le chauffage, dans cet ordre;
dans laquelle le point Ms est calculé sur base de la formule suivante (iv), dans laquelle
les crochets [ ] indiquent la teneur en chaque élément en % en masse et Vf indique
le rapport de surface de ferrite par rapport à la totalité de la structure métallique,