TECHNICAL FIELD
[0001] The present invention relates to a hot press-formed product required to have high
strength, such as used for structural members of automobile parts, a process for producing
the same, and a thin steel sheet for hot press forming. In particular, the present
invention relates to a hot press-formed product that can be provided with a prescribed
shape and at the same time heat treated to have prescribed strength when a preheated
steel sheet (blank) is formed into the prescribed shape, a process for producing such
a hot press-formed product, and a thin steel sheet for hot press forming.
BACKGROUND ART
[0002] As one of the measures for fuel economy improvement of automobiles beginning from
global environmental problems, automobile body lightening has proceeded, and steel
sheets to be used for automobiles need to be strengthened as highly as possible. However,
highly strengthening of steel sheets for automobile lightening lowers elongation EL
or r value (Lankford value), resulting in the deterioration of press formability or
shape fixability.
[0003] To solve such a problem, a hot press-forming method has been adopted for production
of parts, in which method a steel sheet is heated to a prescribed temperature (e.g.,
a temperature for change in austenite phase) to lower its strength (i.e., make it
easily formable) and then formed with a press tool at a temperature (e.g., room temperature)
lower than that of the thin steel sheet, whereby the steel sheet is provided with
a shape and at the same time heat treated by rapid cooling (quenching), which makes
use of a temperature difference between both, to secure its strength after forming.
[0004] According to such a hot pressing method, a steel sheet is formed in a state of low
strength, and therefore, the steel sheet has decreased springback (favorable shape
fixability). In addition, the use of a material having excellent hardenability, to
which alloy elements such as Mn and B have been added, thereby obtaining a strength
of 1500 MPa class in terms of tensile strength by rapid cooling. Such a hot press-forming
method has been called with various names, in addition to a hot press method, such
as a hot forming method, a hot stamping method, a hot stamp method, and a die quench
method.
[0005] Fig. 1 is a schematic explanatory view showing the structure of a press tool for
carrying out hot press forming as described above (hereinafter represented sometimes
by "hot stamp"). In this figure, reference numerals 1, 2, 3, and 4 represent a punch,
a die, a blank holder, and a steel sheet (blank), respectively, and abbreviations
BHF, rp, rd, and CL represent a blank holding force, a punch shoulder radius, a die
shoulder radius, and a clearance between the punch and the die, respectively. In these
parts, punch 1 and die 2 have passage 1a and passage 2a, respectively, formed in the
inside thereof, through which passages a cooling medium (e.g., water) can be allowed
to pass, and the press tool is made to have a structure so that these members can
be cooled by allowing the cooling medium to pass through these passages.
[0006] When a steel sheet is subjected to hot stamp (e.g., hot deep drawing) with such a
press tool, the forming is started in a state where steel sheet (blank) 4 is softened
by heating to a temperature within two-phase region, which is from Ac
1 transformation point to Ac
3 transformation point, or a temperature within single-phase region, which is not lower
than Ac
3 transformation point. More specifically, steel sheet 4 is pushed into a cavity of
die 2 (between the parts indicated by reference numerals 2 and 2 in Fig. 1) by punch
1 with steel sheet 4 in high-temperature state being sandwiched between die 2 and
blank holder 3, thereby forming steel sheet 4 into a shape corresponding to the outer
shape of punch 1 while reducing the outer diameter of steel sheet 4. In addition,
heat is removed from steel sheet 4 to the press tool (punch 1 and die 2) by cooling
punch 1 and die 2 in parallel with the forming, and the hardening of the material
is carried out by further retaining and cooling steel sheet 4 at the lower dead point
in the forming (the point of time when the punch head is positioned at the deepest
level: the state shown in Fig. 1). Formed products with high dimension accuracy and
strength of 1500 MPa class can be obtained by carrying out such a forming method.
Furthermore, such a forming method results in that the volume of a pressing machine
can be made smaller because a forming load can be reduced as compared with the case
where parts of the same strength class are formed by cold pressing.
[0007] As steel sheets for hot stamp, which have widely been used at present, there are
known steel sheets based on 22MnB5 steel. These steel sheets have tensile strengths
of 1500 MPa and elongations of about 6% to 8%, and have been applied to impact-resistant
members (members neither deformed nor fractured as much as possible at the time of
impact). In addition, some developments have also proceeded for C content increase
and further highly strengthening (in 1500 to 1800 MPa class) based on 22MnB5 steel.
[0008] However, there is almost no application of steel grades other than 22MnB5 steel.
One can find a present situation where little consideration is made on steel grades
or methods for controlling the strength and elongation of parts (e.g., strength lowering
to 980MPa class and elongation enhancement to 20%) to extend their application range
to other than impact-resistant members.
[0009] In middle or higher class automobiles, taking into consideration compatibility (function
of, when a small class automobile comes to collide, making safe of the other side)
at the time of side or back impact, both functions as an impact-resistant portion
and an energy-absorbing portion may sometimes be provided in parts such as B pillars
or rear side members. To produce such members, there has mainly been used so far,
for example, a method in which ultra-high tensile strength steel sheets having high
strength of 980 MPa class and high tensile strength steel sheets having elongation
of 440 MPa class are laser welded (to prepare a tailor welded blank, abbreviated as
TWB) and then cold press formed. However, in recent years, the development of a technique
has proceeded, in which parts are each provided with different strengths by hot stamp.
[0010] For example, Non-patent Document 1 has proposed a method of laser welding 22MnB5
steel for hot stamp and a material that does not have high strength even if quenched
with a press tool (to prepare a tailor welded blank, abbreviated as TWB), followed
by hot stamp, in which method different strengths are provided so that tensile strength
at a high strength side (i.e., impact-resistant portion side) becomes 1500 MPa (and
elongation becomes 6% to 8%) and tensile strength at a low strength side (i.e., energy-absorbing
portion side) becomes 440 MPa (and elongation becomes 12%). In addition, as the technique
of providing parts each with different strengths, some techniques have also been proposed,
such as disclosed in Non-patent Documents 2 to 4.
[0011] The techniques disclosed in Non-patent Documents 1 and 2 provide a tensile strength
of not higher than 600 MPa and an elongation of about 12% to 18% at an energy-absorbing
portion side, in which techniques, however, laser welding (to prepare a tailor welded
blank, abbreviated as TWB) is needed previously, thereby increasing the number of
steps and resulting in high cost. In addition, it results in the heating of energy-absorbing
portions, which need not to be hardened originally. Therefore, these techniques are
not preferred from the viewpoint of energy consumption.
[0012] The technique disclosed in Non-patent Document 3 is based on 22MnB5 steel, in which
boron addition, however, adversely affects the robustness of strength after quenching
against heating to a temperature within two-phase region, making difficult the control
of strength at an energy-absorbing portion side, and further making it possible to
obtain only an elongation as low as 15%.
[0013] The technique disclosed in Non-patent Document 4 is based on 22MnB5 steel, and therefore,
this technique is not economic in that control is made in such a manner that 22MnB5,
which originally has excellent hardenability, is not hardened (control of press tool
cooling).
PRIOR ART DOCUMENTS
NON-PATENT DOCUMENTS
[0015] Non-patent Document 2: Usibor1500P(22MnB5) /1500MPa-8%-Ductibor500/550-700MPa-17%
[searched on April 27, 2013] Internet <http://www.arcelormittal.com/tailoredblanks/pre/seifware.p1>
[0016] Non-patent Document 3: 22MnB5/above AC3/1500MPa-8%-below AC3/Hv190-Ferrite/Cementite
Rudiger Erhardt and Johannes Boke, "Industrial application of hot forming process
simulation", Proc, of 1st Int. Conf. on Hot Sheet Metal Forming of High-Performance
steel, ed. By Steinhoff, K., Oldenburg, M, Steinhoff, and Prakash, B., pp83-88, 2008.
[0017] Non-patent Document 4:
Begona Casas, David Latre, Noemi Rodriguez, and Isaac Valls, "Tailor made tool materials
for the present and upcoming tooling solutions in hot sheet metal forming", Proc,
of 1st Int. Conf. on Hot Sheet Metal Forming of High-Performance steel, ed. By Steinhoff,
K., Oldenburg, M, Steinhoff, and Prakash, B., pp23-35, 2008.
JP 2007 016 296 discloses a steel plate for hot press forming into automobile parts.
SUMMARY OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0018] The present invention has been made in view of the above-described circumstances,
and its object is to provide a hot press-formed product in which balance between strength
and elongation can be controlled in a proper range and high ductility can be achieved,
a process useful for producing such a hot press-formed product, and a thin steel sheet
for hot press forming.
MEANS FOR SOLVING THE PROBLEMS
[0019] The hot press-formed product of the present invention, which can achieve the above
object, is a hot press-formed product, characterized by comprising a thin steel sheet
formed by a hot press-forming method, and having a metallic structure that contains
ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by area not including
0% by area, martensite at 30% by area or lower not including 0% by area, and retained
austenite at 3% to 20% by area.
[0020] In the hot press-formed product of the present invention, the chemical element composition
thereof is not particularly limited, typical examples of which may include the following
chemical element composition: C at 0.1% to 0.3% where "%" means "% by mass", and the
same applies to the below with respect to the chemical element composition; Si at
0.5% to 3%; Mn at 0.5% to 2%; P at 0.05% or lower not including 0% S at 0.05% or lower
not including 0% Al at 0.01% to 0.1%; and N at 0.001% to 0.01%, and the remainder
consisting of iron and unavoidable impurities.
[0021] In the hot press-formed product of the present invention, it is also useful to allow
additional elements to be contained, when needed; for example, (a) B at 0.01% or lower
not including 0% and Ti at 0.1% or lower not including 0% ; (b) one or more selected
from the group consisting of Cu, Ni, Cr, and Mo at 1% or lower not including 0% in
total; and (c) V and/or Nb at 0.1% or lower not including 0% in total. Depending on
the kind of element to be contained, the hot press-formed product may have further
improved characteristics.
[0022] When the hot press-formed product of the present invention is produced, the following
steps may be used, i.e., heating a hot-rolled steel sheet having a metallic structure
that contains ferrite at 50% by area or higher, or a cold-rolled steel sheet at a
reduction of 30% or higher, to a temperature not lower than Ac
1 transformation point and not higher than AC
1 transformation point x0.3+Ac
3 transformation point x 0.7 and then starting the forming of the hot-rolled steel
sheet or the cold-rolled steel sheet with a press tool to produce the hot press-formed
product, during which forming an average cooling rate of 20°C/sec or higher is kept
in the press tool, and which forming is finished at a temperature not higher than
bainite transformation starting temperature Bs - 100° . The forming finishing temperature
may preferably be controlled in a temperature range of not higher than bainite transformation
starting temperature Bs - 100°C and not lower than martensite transformation starting
temperature Ms point, in which temperature range the steel sheet may preferably be
retained for 10 seconds or longer, followed by the forming.
[0023] Alternatively, the following method may be adopted as the other method. When a thin
steel sheet is press formed with a press tool, the thin steel sheet may be heated
to a temperature not lower than Ac
3 transformation point and not higher than 1000°C, and then cooled to a temperature
not higher than 700°C and not lower than 500°C at an average cooling rate of 10°C/sec
or lower, and then the forming of the thin steel sheet may be started, during which
forming an average cooling rate of 20°C/sec or higher may be kept in the press tool,
and which forming may be finished at a temperature not higher than bainite transformation
starting temperature Bs - 100°C. Also in this method, the forming finishing temperature
may preferably be controlled in a temperature range of not higher than (bainite transformation
starting temperature Bs - 100° and not lower than martensite transformation starting
temperature Ms point, in which temperature range the steel sheet may preferably be
retained for 10 seconds or longer, followed by the forming.
[0024] The present invention further includes a thin steel sheet for hot press forming,
which is intended for producing a hot press-formed product as described above, and
this thin steel sheet is characterized by being a hot-rolled steel sheet having a
metallic structure that contains ferrite at 50% by area or higher, or a cold-rolled
steel sheet at reduction of 30% or higher.
EFFECTS OF THE INVENTION
[0025] The present invention makes it possible that: retained austenite can be allowed to
exist at a proper fraction to adjust the metallic structure of a hot press-formed
product by properly controlling the conditions of a hot press-forming method; a hot
press-formed product having more enhanced ductility (retained ductility) inherent
to the formed product as compared with the case where conventional 22MnB5 steel is
used; and strength and elongation can be controlled by a combination of heat treatment
conditions and pre-forming steel sheet structure (initial structure). In addition,
the control of heating temperature within two-phase region makes it possible to provide
different strengths and elongations freely.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] Fig. 1 is a schematic explanatory view showing the structure of a press tool for
carrying out hot press forming.
MODE FOR CARRYING OUT THE INVENTION
[0027] The present inventors have studied from various angles to realize a hot press-formed
product having prescribed strength and further exhibiting excellent ductility (elongation)
after forming when a steel sheet is heated to a prescribed temperature and then hot
press formed to produce the formed product.
[0028] As a result, the present inventors have found that a hot press-formed product having
excellent balance between strength and ductility can be achieved when the type of
a steel sheet, heating temperature, and forming conditions are properly controlled
so that its structure is controlled to contain retained austenite at 3% to 20% by
area in the hot press forming of a steel sheet with a press tool, thereby completing
the present invention.
[0029] The reasons for setting the ranges of the respective structures (basic structures)
in the hot press-formed product of the present invention are as follows:
[Ferrite at 30% to 80% by area]
[0030] High ductility of a hot press-formed product can be achieved by making its structure
composed mainly of fine and high-ductility ferrite.
[0031] From this viewpoint, the area fraction of ferrite should be controlled to 30% by
area or higher. However, when this area fraction is higher than 80% by area, prescribed
strength becomes not secured. The fraction of ferrite may preferably be not lower
than 40% by area as the preferred lower limit (more preferably not lower than 45%
by area) and not higher than 70% by area as the preferred upper limit (more preferably
not higher than 65% by area).
[Bainitic ferrite at lower than 30% by area (not including 0%)]
[0032] Bainitic ferrite is effective for strength improvement, but it causes a slight lowering
of ductility. Therefore, the fraction of bainitic ferrite should be controlled to
lower than 30% by area as the upper limit. The fraction of bainitic ferrite may preferably
be not lower than 5% by area as the preferred lower limit (more preferably not lower
than 10% by area) and not higher than 25% by area as the preferred upper limit (more
preferably not higher than 20% by area).
[Martensite at 30% by area or lower (not including 0%)]
[0033] Martensite is effective for strength improvement, but it causes a considerable lowering
of ductility. Therefore, the fraction of martensite should be controlled to not higher
than 30% by area as the upper limit. The fraction of martensite may preferably be
not lower than 5% by area as the preferred lower limit (more preferably not lower
than 10% by area) and not higher than 25% by area as the preferred upper limit (more
preferably not higher than 20% by area).
[Retained austenite at 3% to 20% by area]
[0034] Retained austenite is transformed into martensite during plastic deformation, thereby
having the effect of increasing work hardening rate (transformation-inducing plasticity)
to improve the ductility of a formed product. To make such an effect exhibited, the
fraction of retained austenite should be controlled to 3% by area or higher. When
the fraction of retained austenite is higher, ductility becomes more excellent. In
a composition to be used for automobile steel sheets, retained austenite that can
be secured is limited, of which upper limit becomes about 20% by area. The fraction
of retained austenite may preferably be not lower than 5% by area as the preferred
lower limit (more preferably not lower than 7% by area) and not higher than 15% by
area as the preferred upper limit (more preferably not higher than 10% by area).
[0035] When the hot press-formed product of the present invention is produced, a hot-rolled
steel sheet having a metallic structure that contains ferrite at 50% by area or higher,
or a cold-rolled steel sheet at a reduction of 30% or higher, may be used, and when
the hot-rolled steel sheet or the cold-rolled steel sheet is press formed with a pres
tool, the hot-rolled steel sheet or the cold-rolled steel sheet may be heated to a
temperature not lower than Ac
1 transformation point and not higher than (Ac
1 transformation point x 0.3 + Ac
3 transformation point x 0.7), and then the forming of the hot-rolled steel sheet or
the cold-rolled steel sheet may be started, during which forming an average cooling
rate of 20°C/sec or higher may be kept in the press tool, and which forming may be
finished at a temperature not higher than (bainite transformation starting temperature
Bs - 100°C). The reasons for defining the respective requirements in this process
are as follows:
[Using a hot-rolled steel sheet having a metallic structure that contains ferrite
at 50% by area or higher, or a cold-rolled steel sheet at a reduction of 30% or higher]
[0036] To obtain ferrite structure, which has high contributions to ductility, during heating
to a temperature within two-phase region, the type of a steel sheet (steel sheet for
forming) should properly be selected. When a hot-rolled steel sheet is used as the
steel sheet for forming, it is important to achieve that the fraction of ferrite is
high and ferrite is retained during heating to a temperature within two-phase region.
From this viewpoint, the hot-rolled steel sheet to be used may preferably have a metallic
structure that contains ferrite at 50% by area or higher. The fraction of ferrite
may preferably be not lower than 60% by area as the preferred lower limit (more preferably
not lower than 70% by area). When the fraction of ferrite in the hot-rolled steel
sheet becomes too high, the fraction of ferrite in the formed product becomes too
high. Therefore, the fraction of ferrite in the hot-rolled steel sheet may preferably
be not higher than 95% by area, more preferably not higher than 90% by area.
[0037] On the other hand, a cold-rolled steel sheet is used, it becomes an important requirement
that recrystallization occurs during heating to form dislocation-free ferrite, and
therefore, rolling (cold rolling) should be carried out at a prescribed reduction
or higher so that recrystallization occurs. In the case of a cold-rolled steel sheet,
it may have any structure. From this viewpoint, when a cold-rolled steel sheet is
used, it is preferable to use a cold-rolled steel sheet at a reduction of 30% or higher.
The reduction may preferably be 40% or higher, more preferably 50% or higher. The
"reduction" as used herein is a value determined by formula (1) below.
[Heating a steel sheet to a temperature not lower than Ac1 transformation point and not higher than (Ac1 transformation point x 0.3 + Ac3 transformation point x 0.7), and then starting the forming]
[0038] To cause the partial transformation, while retaining, of ferrite, which is contained
in the steel sheet, into austenite, the heating temperature should be controlled in
a prescribed range. The proper control of the heating temperature makes it possible
to cause transformation into retained austenite or martensite in the subsequent cooling
step to provide the final hot press-formed product with a desired structure. When
the heating temperature of the steel sheet is lower than Ac
1 transformation point, a sufficient fraction of austenite cannot be obtained during
heating, and therefore, a prescribed fraction of retained austenite cannot be secured
in the final structure (the structure of a formed product). When the heating temperature
of the thin steel sheet is higher than (Ac
1 transformation point x 0.3 + Ac
3 transformation point x 0.7), the fraction of transformed austenite is increased too
highly during heating, and therefore, a prescribed fraction of ferrite cannot be secured
in the final structure (the structure of a formed product).
[During forming, an average cooling rate of 20°C/sec or higher is kept in the press
tool, and the forming is finished at a temperature not lower than (bainite transformation
starting temperature Bs - 100°C)]
[0039] To change the austenite, which was formed in the above heating step, into a prescribed
fraction of retained austenite, while preventing the formation of cementite, the average
cooling rate during forming and the forming finishing temperature should properly
be controlled. From this viewpoint, the average cooling rate during forming should
be controlled to 20°C/sec or higher, and the forming finishing temperature should
be controlled to a temperature not higher than (bainite transformation starting temperature
Bs point - 100°C, sometimes abbreviated as "Bs - 100°C"). The average cooling rate
during forming may preferably be 30°C/sec or higher (more preferably 40°C/sec or higher).
With respect to the forming finishing temperature, the forming may be finished, while
cooling to room temperature at an average cooling temperature as described above.
Alternatively, the cooling is stopped after the cooling to a temperature not higher
than Bs - 100°C, and then the forming may be finished. The control of the average
cooling rate during forming can be achieved by a means of, for example, (a) controlling
the temperature of a press tool (using a cooling medium shown in Fig. 1 above) or
(b) controlling the thermal conductivity of a press tool (the same applies to the
cooling in the method described below).
[0040] As another method for producing the press-formed product of the present invention,
when a steel sheet is press formed with a press tool, the thin steel sheet may be
heated to a temperature not lower than Acs transformation point and not higher than
1000°C, and then the thin steel sheet is cooled to a temperature not higher than 700°C
and not lower than 500°C at an average cooling temperature of 10°C/sec or lower, and
then the forming of the thin steel sheet may be started, during which forming an average
cooling rate of 20°C/sec or higher may be kept in the press tool, and which forming
may be finished at a temperature not higher than (bainite transformation starting
temperature Bs - 100°C). The reasons for defining the respective requirements in this
process are as follows (the same as described above applies to the cooling finishing
temperature):
[Heating a thin steel sheet to a temperature not lower than Ac3 transformation point and not higher than 1000°C]
[0041] To properly adjust the structure of a hot press-formed product, the heating temperature
should be controlled in a prescribed range. The proper control of the heating temperature
makes it possible to cause transformation into a structure composed mainly of ferrite
while securing a prescribed fraction of retained austenite in the subsequent cooling
step to provide the final hot press-formed product with a desired structure. When
the heating temperature of the thin steel sheet is lower than Ac
3 transformation point, a sufficient fraction of austenite cannot be obtained during
heating, and therefore, a prescribed fraction of retained austenite cannot be secured
in the final structure (the structure of a formed product). When the heating temperature
of the thin steel sheet is higher than 1000°C, the grain size of austenite becomes
increased during heating, and therefore, ferrite cannot be formed in the subsequent
cooling.
[Cooling to a temperature not higher than 700°C and not lower than 500°C at an average
cooling rate of 10°C/sec or lower, and then starting the forming]
[0042] This cooling step is an important step for forming ferrite during cooling. When the
average cooling rate in this cooling step becomes higher than 10°C/sec, a prescribed
fraction of ferrite cannot be secured. The average cooling rate may preferably be
7°C/sec or lower, more preferably 5°C/sec or lower. The cooling stopping temperature
in this cooling step (this temperature may sometimes be referred to as the "cooling
rate changing temperature") should be controlled to not higher than 700°C and not
lower than 500°C. When the cooling stopping temperature becomes higher than 700°C,
a sufficient fraction of ferrite cannot be secured. When the cooling stopping temperature
becomes lower than 500°C, the fraction of ferrite becomes too high, and therefore,
prescribed strength cannot be secured. The cooling stopping temperature may preferably
be not higher than 680°C as the preferred upper limit (more preferably not higher
than 660°C) and not lower than 520°C as the preferred lower limit (more preferably
not lower than 550°C).
[0043] In any of these methods, the forming finishing temperature should be controlled to
not higher than (Bs - 100°C), but may preferably be controlled in a temperature range
of not lower than martensite transformation starting temperature Ms (a temperature
in this range may sometimes be referred to as the "cooling temperature changing temperature),
in which temperature range retention may preferably be carried out for 10 seconds
or longer. The bainite transformation can proceed from super-cooled austenite to form
a structure composed mainly of ferrite by retention in the above temperature range
for 10 seconds or longer. The retention time may preferably be 50 seconds or longer
(more preferably 100 seconds or longer). When the retention time becomes too long,
austenite starts to decompose, so that the fraction of retained austenite cannot become
secured. Therefore, the retention time may preferably be 1000 seconds or shorter (more
preferably 800 seconds or shorter).
[0044] Retention as described above may be any of isothermal retention, monotonic cooling,
and re-heating step, so long as it is in the above temperature range. With regard
to a relationship between such retention and forming, retention as described above
may be added at the stage when forming is finished. Alternatively, a retention step
may be added within the above temperature range during the finish of forming. After
forming is finished in such a manner, the steel sheet may be left as it is for cooling
or cooled at a proper cooling rate to room temperature (25°C).
[0045] The process for producing the hot press-formed product of the present invention can
be applied, not only to the case where a hot press-formed product having a simple
shape as shown in Fig. 1 above is produced (i.e., direct method), but also to the
case where a formed product having a relatively complicated shape is produced, even
if any of the methods described above is adopted. However, in the case of a complicated
product shape, it may be difficult to provide a product with the final shape by a
single press forming step. In such a case, there can be used a method of cold press
forming in a step prior to hot press forming (this method has been referred to as
"indirect method"). This method includes previously forming a difficult-to-form portion
into an approximate shape by cold processing and then hot press forming the other
portions. When such a method is used to produce, for example, a formed product having
three projections (profile peaks) by forming, two projections are formed by cold press
forming and the third projection is then formed by hot press forming.
[0046] The present invention is intended for a hot press-formed product made of a high-strength
steel sheet, the steel grade of which is acceptable, if it has an ordinary chemical
element composition as a high-strength steel sheet, in which, however, C, Si, Mn,
P, S, Al, and N contents may preferably be controlled in their respective proper ranges.
From this viewpoint, the preferred ranges of these chemical elements and the grounds
for limiting their ranges are as follows:
[C at 0.1% to 0.3%]
[0047] C is an important element for securing retained austenite. The concentration of austenite
during heating at a temperature within two-phase region or at a temperature within
single-phase region, which is not lower than Ac
3 transformation point, allows the formation of retained austenite after quenching.
It further contributes to an increase of martensite fraction. When C content is lower
than 0.1%, a prescribed fraction of retained austenite cannot be secured, making it
impossible to obtain excellent ductility. When C content becomes higher than 0.3%,
it results in that strength becomes too high. C content may more preferably be not
lower than 0.15% as the more preferred lower limit (still more preferably not lower
than 0.20%) and not higher than 0.27% as the more preferred upper limit (still more
preferably not higher than 0.25%).
[Si at 0.5% to 3%]
[0048] Si suppresses austenite after heating at a temperature within two-phase region or
at a temperature within single-phase region, which is not lower than Ac
3 transformation point, from being formed into cementite, and exhibits the action of
increasing the fraction of retained austenite. It further exhibits the action of enhancing
strength by solid solution enhancement without deteriorating ductility too much. When
Si content is lower than 0.5%, retained austenite cannot be secured at a prescribed
fraction, making it impossible to obtain excellent ductility. When Si content becomes
higher than 3%, the degree of solid solution enhancement becomes too high, resulting
in the drastic deterioration of ductility. Si content may more preferably be not lower
than 1.15% as the more preferred lower limit (still more preferably not lower than
1.20%) and not higher than 2.7% as the more preferred upper limit (still more preferably
not higher than 2.5%).
[Mn at 0.5% to 2%]
[0049] Mn is an element to stabilize austenite, and it contributes to an increase of retained
austenite. To make such an effect exhibited, Mn may preferably be contained at 0.5%
or higher. However, when Mn content becomes excessive, the formation of ferrite is
prevented, thereby making it impossible to secure a prescribed fraction of ferrite,
and therefore, Mn content may preferably be 2% or lower. In addition, a considerable
improvement of austenite strength increases a hot rolling load, thereby making it
difficult to produce steel sheets, and therefore, even from the viewpoint of productivity,
it is not preferable that Mn is contained at higher than 2%. Mn content may more preferably
be not lower than 0.7% as the more preferred lower limit (still more preferably not
lower than 0.9%) and not higher than 1.8% as the more preferred higher limit (still
more preferably not higher than 1.6%).
[P at 0.05% or lower (not including 0%)]
[0050] P is an element unavoidably contained in steel and deteriorates ductility. Therefore,
P content may preferably be reduced as low as possible. However, extreme reduction
causes an increase of steel production cost, and reduction to 0% is difficult in the
actual production. Therefore, P content may more preferably be controlled to 0.05%
or lower (not including 0%). P content may more preferably be not higher than 0.045%
as the more preferred upper limit (still more preferably not higher than 0.040%).
[S at 0.05% or lower (not including 0%)]
[0051] S is also an element unavoidably contained in steel and deteriorates ductility, similarly
to P. Therefore, S content may preferably be reduced as low as possible. However,
extreme reduction causes an increase of steel production cost, and reduction to 0%
is difficult in the actual production. Therefore, S content may preferably be controlled
to 0.05% or lower (not including 0%). S content may more preferably be not higher
than 0.045% as the more preferred upper limit (still more preferably not higher than
0.040%).
[Al at 0.01% to 0.1%]
[0052] Al is useful as a deoxidizing element and further useful for fixation of dissolved
N in steel as AlN to improve ductility. To make such an effect effectively exhibited,
Al content may preferably be controlled to 0.01% or higher. However, when Al content
becomes higher than 0.1%, it results in the excessive formation of Al
2O
3 to deteriorate ductility. A1 content may more preferably be not lower than 0.013%
as the more preferred lower limit (still more preferably not lower than 0.015%) and
not higher than 0.08% as the more preferred upper limit (still more preferably not
higher than 0.06%).
[N at 0.001% to 0.01%]
[0053] N is an element unavoidably incorporated in steel, and a reduction of N content may
be preferred, which has, however, a limitation in actual process. Therefore, the lower
limit of N content was set to 0.001%. When N content becomes excessive, ductility
is deteriorated by strain aging, or the addition of B causes deposition of N as BN,
thereby lowering the effect of improving hardenability by solid solution of B. Therefore,
the upper limit of N content was set to 0.01%. N content may more preferably be not
higher than 0.008% as the more preferred upper limit (still more preferably not higher
than 0.006%).
[0054] The basic chemical components in the press-formed product of the present invention
are as described above, and the remainder consists essentially of iron. The wording
"consists essentially of iron" means that the press-formed product of the present
invention can contain, in addition to iron, minor components (e.g., besides Mg, Ca,
Sr, and Ba, REM such as La, and carbide-forming elements such as Zr, Hf, Ta, W, and
Mo) in such a level that these minor components do not inhibit the characteristics
of the steel sheet of the present invention, and can further contain unavoidable impurities
(e.g., O, H) other than P, S, and N.
[0055] It is also useful to allow the press-formed product of the present invention to contain
additional elements, when needed; for example, (a) B at 0.01% or lower (not including
0%) and Ti at 0.1% or lower (not including 0%); (b) one or more selected from the
group consisting of Cu, Ni, Cr, and Mo at 1% or lower (not including 0%) in total;
and (c) V and/or Nb at 0.1% or lower (not including 0%) in total. The press-formed
product may have further improved characteristics depending on the kinds of elements
contained. When these elements are contained, their preferred ranges and grounds for
limitation of their ranges are as follows:
[B at 0.01% or lower (not including 0%) and Ti at 0.1% or lower (not including 0%)]
[0056] B is an element to prevent the formation of cementite during cooling after heating,
thereby contributing to the securement of retained austenite. To make such an effect
exhibited, B may preferably be contained at 0.0001% or higher, but even if B is contained
beyond 0.01%, the effect is saturated. B content may more preferably be not lower
than 0.0002% as the more preferred lower limit (still more preferably not lower than
0.0005%) and not higher than 0.008% as the more preferred upper limit (still more
preferably not higher than 0.005%).
[0057] On the other hand, Ti fixes N and maintains B in solid solution state, thereby exhibiting
the effect of improving hardenability. To make such an effect exhibited, Ti may preferably
be contained at least 4 times higher than N content. However, when Ti content becomes
excessive beyond 0.1%, it results in excessive formation of TiC, thereby causing an
increase of strength by precipitation enhancement but a deterioration of ductility.
Ti content may more preferably be not lower than 0.05% as the more preferred lower
limit (still more preferably not lower than 0.06%) and not higher than 0.09% as the
more preferred higher limit (still more preferably not higher than 0.08%).
[One or more selected from the group consisting of Cu, Ni, Cr, and Mo at 1% or lower
(not including 0%) in total]
[0058] Cu, Ni, Cr, and Mo prevent the formation of cementite during cooling after heating,
and effectively act the securement of retained austenite. To make such an effect exhibited,
these elements may preferably be contained at 0.01% or higher in total. Taking only
characteristics into consideration, their content may be preferable when it is higher,
but may preferably be controlled to 1% or lower in total because of a cost increase
by alloy element addition. In addition, these elements have the action of considerably
enhancing the strength of austenite, thereby increasing a hot rolling load so that
the production of steel sheets becomes difficult. Therefore, even from the viewpoint
of productivity, their content may preferably be controlled to 1% or lower. These
elements' content may more preferably be not lower than 0.05% as the more preferred
lower limit (still more preferably not lower than 0.06%) in total and not higher than
0.9% as the more preferred upper limit (still more preferably not higher than 0.8%)
in total.
[V and/or Nb at 0.1% or lower (not including 0%) in total]
[0059] V and Nb have the effect of forming fine carbide and make structure fine by pinning
effect. To make such an effect exhibited, these elements may preferably be contained
at 0.001% or higher in total. However, when these elements' content becomes excessive,
it results in the formation of coarse carbide, which becomes the origin of fracture,
thereby deteriorating ductility in contrast. Therefore, these elements' content may
preferably be controlled to 0.1% or lower in total. These elements' content may more
preferably be not lower than 0.005% as the more preferred lower limit (still more
preferably not lower than 0.008%) in total and not higher than 0.08% as the more preferred
upper limit (still more preferably not higher than 0.06%) in total.
[0060] The thin steel sheet for hot press forming of the present invention may be either
a non-plated steel sheet or a plated steel sheet. When it is a plated steel sheet,
the type of plating may be either ordinary galvanization or aluminium coating. The
method of plating may be either hot-dip plating or electroplating. After the plating,
alloying heat treatment may be carried out, or additional plating may be carried out
as multilayer plating.
[0061] According to the present invention, the characteristics of formed products, such
as strength and elongation, can be controlled by properly adjusting press forming
conditions (heating temperature and cooling rate), and in addition, hot press-formed
products having high ductility (retained ductility) can be obtained, so that they
can be applied even to parts (e.g., energy-absorbing members), to which conventional
hot press-formed products have hardly been applied; therefore, the present invention
is extremely useful for extending the application range of hot press-formed products.
The formed products, which can be obtained in the present invention, have further
enhanced residual ductility as compared with formed products, of which structure was
adjusted by ordinary annealing after cold press forming.
[0062] The following will describe the advantageous effects of the present invention more
specifically by way of Examples, but the present invention is not limited to the Examples
described below. The present invention can be put into practice after appropriate
modifications or variations within a range capable of meeting the gist described above
and below, all of which are included in the technical scope of the present invention.
EXAMPLES
[0064] Steel materials having respective chemical element compositions shown in Table 1
below were formed into slabs for experimental use by a vacuum fusion method, after
which the slabs were hot rolled, followed by cooling, and then wound. These rolled
sheets were further cold rolled into thin steel sheets, followed by quench treatment
so that they had respectively prescribed initial structures. In Table 1, Ac
1 transformation point, Ac
3 transformation point, Ms point, and (Bs - 100°C) were determined respectively on
the basis of formulas (2) to (5) described below (see, e.g., the
Japanese translation of "The Physical Metallurgy of Steels" originally written by
William C. Leslie, published by Maruzen, 1985). Table 1 further shows the calculated values of (Ac
1 transformation point x 0.3 + Ac
3 transformation point x 0.7) (these calculated values may hereinafter be referred
to as "A values").
where [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu], and [Ni] indicate
C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu, and Ni contents (% by mass), respectively. When
some element indicated in a certain term of formulas (2) to (5) above is not contained,
calculation is carried out under the assumption that the term does not exist in the
formula.
[0065] The steel sheets thus obtained were heated under the respective conditions shown
in Table 2 below, and then subjected to forming and cooling treatment using a high
speed heat treatment testing system for steel sheets (CAS series, available from ULVAC-RIKO,
Inc.), which can control an average cooling rate. The steel sheets to be subjected
to cooling treatment had a size of 190 mm x 70 mm (and a sheet thickness of 1.4 mm).
Test Nos. 1 to 14, 17 to 19, and 21 to 25 were the cases where hot-rolled steel sheets
were used as steel sheets for forming. Test Nos. 15, 16, and 20 were the cases where
cold-rolled steel sheets were used steel sheets for forming. The term "cooling 1"
shown in Table 2 indicates cooling from a heating temperature to a temperature of
700°C to 500°C. The term "cooling 2" shown in Table 2 indicates cooling from then
to a temperature range of [(Bs - 100°C) to Ms point] (In Test Nos. 19 to 23, forming
was started at this stage). When needed, the steel sheet was subjected to hot-dip
galvanization to obtain a hot-dip galvanized steel sheet (Test No. 25).
[0066] For the respective steel sheets after the above treatments (heating, forming, and
cooling), measurement of tensile strength (TS) and elongation (total elongation EL),
and observation of metallic structure (fraction of each structure), were carried out
by the methods described below.
[Tensile strength (TS) and elongation (total elongation EL)]
[0067] JIS No. 5 specimens were used for tensile tests to measure tensile strength (TS)
and elongation (EL). At that time, strain rate in the tensile tests was set to 10
mm/sec. In the present invention, the specimens were evaluated as "passing" when fulfilling
any of the conditions that: (a) tensile strength (TS) is from 780 to 979 MPa and elongation
(EL) is 25% or higher; and (b) tensile strength (TS) is from 980 to 1179 MPa and elongation
(EL) is 15% or higher.
[Observation of metallic structure (fraction of each structure)]
[0068]
- (1) For ferrite and bainitic ferrite structures in the steel sheets, the steel sheets
were each subjected to nital etching, and then observed by SEM (with a magnification
of 1000x or 2000x), in which ferrite and bainitic ferrite were distinguished to determine
their respective fractions (area fractions).
- (2) For the fraction (area fraction) of retained austenite in the steel sheets, the
steel sheets were each measured by an X-ray diffraction method, after grinding to
one-quarter thicknesses of the steel sheets and subsequent chemical polishing (see,
e.g., ISJJ Int. Vol. 33 (1933), No. 7, p. 776).
- (3) For the area fraction of martensite (as-quenched martensite), the steel sheets
were each subjected to repera etching, and assuming white contrast as a mixed structure
of martensite (as-quenched martensite) and retained austenite by SEM observation,
the area fraction of the mixed structure was measured. The fraction of as-quenched
martensite was calculated by subtracting the fraction of retained austenite, which
had been determined by an X-ray diffraction method, from the area fraction of the
mixed structure.
[0069] These results are shown in Table 3 below, together with the types of pre-forming
steel sheets (fraction of ferrite, and reduction of cold-rolled steel sheet).
[Table 2]
Test No. |
Steel grade |
Production conditions |
Plated or non-plated |
Steel sheet for forming |
Heating temperature (°C) |
Average cooling rate in cooling 1 (°C/sec) |
Cooling rate changing temperature from cooling 1 to cooling 2 (°C) |
Average cooling rate in cooling 2 (°C/sec) |
Retention time at [Bs - 100°C to Ms point] (sec) |
Forming finishing temperature (°C) |
Fraction of ferrite (% by area) |
Reduction (%) |
1 |
A |
60 |
- |
800 |
40 |
- |
- |
3.2 |
300 |
Non-plated |
2 |
B |
60 |
- |
800 |
40 |
- |
- |
3.0 |
300 |
Non-plated |
3 |
C |
60 |
- |
800 |
40 |
- |
- |
3.1 |
300 |
Non-plated |
4 |
D |
60 |
- |
800 |
40 |
- |
- |
3.3 |
300 |
Non-plated |
5 |
E |
60 |
- |
800 |
40 |
- |
- |
2.9 |
300 |
Non-plated |
6 |
F |
60 |
- |
800 |
40 |
- |
- |
3.0 |
300 |
Non-plated |
7 |
G |
60 |
- |
800 |
40 |
- |
- |
3.3 |
300 |
Non-plated |
8 |
H |
60 |
- |
800 |
40 |
- |
- |
3.0 |
300 |
Non-plated |
9 |
I |
60 |
- |
800 |
40 |
- |
- |
2.9 |
300 |
Non-plated |
10 |
J |
60 |
- |
800 |
40 |
- |
- |
3.0 |
300 |
Non-plated |
11 |
K |
60 |
- |
800 |
40 |
- |
- |
2.7 |
300 |
Non-plated |
12 |
L |
60 |
- |
800 |
40 |
- |
- |
3.2 |
300 |
Non-plated |
13 |
M |
50 |
- |
800 |
40 |
- |
- |
3.3 |
300 |
Non-plated |
14 |
N |
60 |
- |
800 |
40 |
- |
- |
2.9 |
300 |
Non-plated |
15 |
C |
30 |
50 |
800 |
40 |
- |
- |
3.1 |
300 |
Non-plated |
16 |
C |
30 |
20 |
800 |
40 |
- |
- |
3.1 |
300 |
Non-plated |
17 |
C |
60 |
- |
720 |
40 |
- |
- |
3.1 |
300 |
Non-plated |
18 |
C |
60 |
- |
900 |
40 |
- |
- |
3.1 |
300 |
Non-plated |
19 |
C |
60 |
- |
900 |
5 |
600 |
40 |
3.1 |
300 |
Non-plated |
20 |
C |
30 |
20 |
900 |
5 |
600 |
40 |
3.1 |
300 |
Non-plated |
21 |
C |
60 |
- |
800 |
40 |
450 |
3 |
13.3 |
300 |
Non-plated |
22 |
C |
60 |
- |
900 |
15 |
600 |
40 |
3.1 |
300 |
Non-plated |
23 |
C |
60 |
- |
900 |
5 |
450 |
40 |
3.1 |
300 |
Non-plated |
24 |
C |
60 |
- |
800 |
40 |
- |
- |
3.1 |
600 |
Non-plated |
25 |
C |
60 |
- |
800 |
40 |
- |
- |
3.1 |
300 |
Plated |
[Table 3]
Test No. |
Steel grade |
Structure of formed product (% by area) |
Tensile strength TS (MPa) |
Elongation EL (%) |
Ferrite |
Bainitic ferrite |
Martensite |
Retained austenite |
Others* |
1 |
A |
41 |
25 |
26 |
8 |
|
994 |
17 |
2 |
B |
43 |
29 |
22 |
6 |
|
1020 |
16 |
3 |
C |
48 |
23 |
21 |
8 |
|
994 |
17 |
4 |
D |
49 |
23 |
21 |
7 |
|
1002 |
17 |
5 |
E |
49 |
24 |
20 |
7 |
|
1023 |
17 |
6 |
F |
48 |
24 |
21 |
7 |
|
1031 |
17 |
7 |
G |
44 |
25 |
23 |
8 |
|
1028 |
17 |
8 |
H |
46 |
25 |
22 |
7 |
|
1011 |
17 |
9 |
I |
45 |
24 |
23 |
8 |
|
1019 |
17 |
10 |
J |
48 |
24 |
22 |
6 |
|
1018 |
17 |
11 |
K |
49 |
26 |
24 |
1 |
|
1022 |
12 |
12 |
L |
51 |
25 |
22 |
2 |
|
1302 |
14 |
13 |
M |
36 |
28 |
28 |
8 |
|
1095 |
16 |
14 |
N |
45 |
27 |
28 |
0 |
|
989 |
13 |
15 |
C |
48 |
24 |
20 |
8 |
|
1023 |
17 |
16 |
C |
25 |
45 |
25 |
5 |
|
1082 |
13 |
17 |
C |
81 |
- |
15 |
- |
θ:4 |
745 |
14 |
18 |
C |
- |
- |
95 |
5 |
|
1523 |
10 |
19 |
C |
65 |
8 |
20 |
7 |
|
984 |
17 |
20 |
C |
62 |
9 |
22 |
7 |
|
999 |
17 |
21 |
C |
43 |
26 |
22 |
9 |
|
1032 |
18 |
22 |
C |
12 |
61 |
20 |
7 |
|
1233 |
12 |
23 |
C |
83 |
17 |
- |
- |
|
921 |
14 |
24 |
C |
56 |
20 |
- |
- |
P:24 |
893 |
14 |
25 |
C |
47 |
23 |
23 |
8 |
|
994 |
17 |
* θ and P indicate cementite and pearlite, respectively. |
[0070] From these results, discussions can be made as follows: Test Nos. 1 to 10, 13, 15,
19 to 21, and 25 are Examples fulfilling the requirements defined in the present invention,
thereby indicating that parts having satisfactory balance between strength and ductility
were obtained.
[0071] In contrast, Test Nos. 11 to 12, 14, 16 to 18, and 22 to 24 are Comparative Examples
not fulfilling any of the requirements defined in the present invention, thereby deteriorating
any of the characteristics. More specifically, Test No. 11 was the case where steel
having insufficient C content (steel grade K shown in Table 1) was used, so that retained
austenite was not secured, thereby obtaining only low elongation (EL). Test No. 12
was the case where steel having insufficient Si content (steel grade L shown in Table
1), so that retained austenite was not secured, thereby obtaining only low elongation
(EL).
[0072] Test No. 14 was intended for conventional 2MnB5 equivalent steel (steel grade N shown
in Table 1), so that retained austenite was not secured, thereby obtaining only low
elongation (EL), although high strength was obtained. Test No. 16 was the case where
cold-rolled steel sheet having low reduction was used, so that the formed product
had a structure containing ferrite at 25% by area, thereby lowering elongation (EL).
Test No. 17 was the case where the heating temperature was lower than Ac
1 transformation point, so that the formed product had a structure containing ferrite
at 81% by area (the remainder was martensite and cementite) and retained austenite
was not secured, thereby lowering elongation (EL) and tensile strength. Test No. 18
was the case where the heating temperature was higher than A value, so that ferrite
and bainitic ferrite were not secured by excessive formation of martensite, thereby
lowering elongation (EL).
[0073] Test No. 22 was the case where the average cooling rate in cooling 1 was high, so
that ferrite was not secured by the formation of bainitic ferrite, thereby lowering
elongation (EL). Test No. 23 was the case where the average cooling rate in cooling
1 was low and the cooling rate changing temperature was low, so that the formed product
had a structure containing ferrite at 83% by area (the remainder was bainitic ferrite)
and retained austenite was not secured, thereby lowering elongation (EL). Test No.
24 was the case where the forming finishing temperature was high, so that pearlite
was formed in the structure of the formed product and retained austenite was not secured,
thereby lowing elongation (EL).
INDUSTRIAL APPLICABILITY
[0074] The present invention makes it possible to provide a hot press-formed product, including
a steel sheet formed by a hot press-forming method, and having a metallic structure
that contains ferrite at 30% to 80% by area, bainitic ferrite at lower than 30% by
area (not including 0%), martensite at 30% by area or lower (not including 0%), and
retained austenite at 3% to 20% by area, whereby balance between strength and elongation
can be controlled in a proper range and high ductility can be achieved.
DESCRIPTION OF REFERENCE NUMERALS
[0075]
- 1
- Punch
- 2
- Die
- 3
- Blank holder
- 4
- Steel sheet (Blank)