[Technical Field]
[0001] The present invention relates to a steel sheet.
[Background Art]
[0003] In recent years, efforts have been being made to reduce carbon dioxide emission in
a number of fields from the viewpoint of the global environment protection. Automobile
manufacturers are also actively developing techniques for weight reduction in vehicle
bodies for the purpose of fuel consumption reduction. A decrease in the weight of
steel to be used, such as a decrease in the sheet thickness of a steel sheet, makes
it possible to easily decrease the weight of vehicle bodies. However, in the case
of automobiles, improvement in the impact resistance is also emphasized in order to
ensure passenger safety, and thus weight reduction in vehicle bodies by a decrease
in the weight of steel to be used or the like, which is easy, cannot be adopted, and
weight reduction in vehicle bodies is not easy. Accordingly, studies are underway
to thin members using high strength steel sheets in order to satisfy both weight reduction
in vehicle bodies and collision resistance. Incidentally, steel sheets to be applied
to vehicle components are formed into component shapes, and, normally, the formability
deteriorates as the strengths of the steel sheets increase. Therefore, there is a
strong desire for steel sheets to be applied to vehicle components to have both a
high strength and excellent formability. Specifically, for steel sheets that are used
for inner sheet members, structural members, suspension members, and the like of automobiles,
stretch flanging (hole expansion) or bending is often used, and thus the steel sheets
need to have a high strength and to be excellent in terms of elongation, stretch flangeability
and bending workability.
[0004] For example, as described in Patent Document 1, as a steel sheet from which excellent
elongation can be obtained, a dual-phase steel sheet (hereinafter, DP steel) composed
of a composite structure of soft ferrite and hard martensite is known. However, the
DP steel sheet is excellent in terms of elongation, but cracks occur in some cases
due to the formation of voids in the interface between ferrite and martensite, which
have significantly different hardness, and thus there is a case where the DP steel
sheet is poor in terms of stretch flangeability or bending workability.
[0005] In addition, Patent Document 2 proposes a high strength hot-rolled steel sheet that
is obtained by setting the cooling rate in a temperature range from the solidification
of a slab to 1300°C to 10 to 300 °C/min and, after finish rolling, coiling the slab
at 500°C or higher and 700°C or lower and has a steel structure composed of a ferrite
single phase and a tensile strength of 1180 MPa or more. Patent Document 2 discloses
that the high strength hot-rolled steel sheet is excellent in terms of the bending
workability. However, the high strength hot-rolled steel sheet described in Patent
Document 2 is manufactured by reheating a slab without cooling the slab to lower than
900°C where ferrite begins to be formed and hot-rolling the slab. Therefore, there
is a problem in that segregation formed during solidification is not sufficiently
reduced and there is a case where the bending workability is not stable. In addition,
in Patent Document 2, the stretch flangeability is not taken into account.
[0006] Patent Document 3 proposes a method for manufacturing a steel sheet having a ferrite
area fraction of 80% or more and a tensile strength of 980 MPa or more by completing
hot rolling within five hours after continuous casting to form a solid solution of
Ti exceeding the solubility in γ and precipitating fine TiC together with ferritic
transformation during coiling at 550°C or higher and 700°C or lower and a high strength
hot-rolled steel sheet that is obtained by the manufacturing method. However, even
in Patent Document 3, since continuous casting through the completion of hot finish
rolling is performed in an austenite region to suppress the precipitation of coarse
TiC, there has been a case where the bending workability deteriorates due to Mn segregation.
In addition, in Patent Document 3 as well, similar to Patent Document 2, the stretch
flangeability is not taken into account.
[Citation List]
[Patent Documents]
[Summary of the invention]
[Problems to be Solved by the Invention]
[0008] The present invention has been made in consideration of the above-described problems,
and an object of the present invention is to provide a steel sheet having a high strength
and being excellent in terms of elongation, stretch flangeability and bending workability.
Here, the steel sheet of the present invention also includes steel sheets having a
cover such as a plating layer on the surface.
[Means for Solving the Problem]
[0009] The present inventors studied steel sheets that are favorable in all of the strength,
the elongation, the stretch flangeability and the bending workability. As a result,
it was found that a steel sheet having a high strength and being excellent in terms
of elongation, stretch flangeability and bending workability can be manufactured by
optimizing the chemical composition and manufacturing conditions to control the microstructure
of the steel sheet and Mn segregation and controlling the precipitation form of a
Ti-based carbide.
[0010] The present invention has been made based on the above-described finding, and the
gist of the present invention is as described below.
- [1] A steel sheet according to an aspect of the present invention contains, as a chemical
composition, by mass%, C: 0.050% to 0.250%, Si: 0.005% to 2.000%, Mn: 0.10% to 3.00%,
P: 0.100% or less, S: 0.0100% or less, sol. Al: 0.001% to 1.00%, Ti: 0.150% to 0.400%,
N: 0.0010% to 0.0100%, Nb: 0% to 0.100%, V: 0% to 1.000%, Mo: 0% to 1.000%, Cu: 0%
to 1.00%, Ni: 0% to 1.00%, Cr: 0% to 2.00%, W: 0% to 1.000%, B: 0% to 0.0020%, Ca:
0% to 0.0100%, Mg: 0% to 0.0100%, REM: 0% to 0.0100% and Bi: 0% to 0.0200% with a
remainder of Fe and impurities, in which Ex. C obtained by the following formula (1)
is 0.020% or less, a microstructure at a 1/4 depth position of a sheet thickness from
a surface contains 60% or more of ferrite, 0% to 5% of MA and a total of 0% to 5%
of pearlite and cementite with a remainder of bainite in terms of area fractions,
in the microstructure, the average crystal grain diameter is 10.0 µm or less, the
average aspect ratio of crystal grains is 0.30 or more, the standard deviation of
a Mn concentration is 0.60 mass% or less, a Ti-based carbide having a Baker-Nutting
orientation relationship in the ferrite is precipitated in a semi-coherent state,
and a tensile strength is 980 MPa or more.

Here, "%Ti*" in the formula (1) is obtained from the following formula (2).

%C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the
amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%.
- [2] The steel sheet according to [1] may contain, as the chemical composition, by
mass%, one or more selected from the group consisting of Nb: 0.001% to 0.100%, V:
0.005% to 1.000%, Mo: 0.001% to 1.000%, Cu: 0.02% to 1.00%, Ni: 0.02% to 1.00%, Cr:
0.02% to 2.00%, W: 0.02% to 1.000%, B: 0.0001% to 0.0020%, Ca: 0.0002% to 0.0100%,
Mg: 0.0002% to 0.0100%, REM: 0.0002% to 0.0100%, and Bi: 0.0001% to 0.0200%.
- [3] The steel sheet according to [1] or [2], in which a plating layer may be formed
on a surface.
- [4] The steel sheet according to [3], in which the plating layer may be a hot-dip
galvanized layer.
- [5] The steel sheet according to [4], in which the hot-dip galvanized layer may be
a hot-dip galvannealed layer.
[Effects of the Invention]
[0011] According to the above-described aspect of the present invention, it is possible
to provide a steel sheet having a high strength and being excellent in terms of elongation,
stretch flangeability and bending workability. The steel sheet of the present invention
is preferable as a material that is used in uses for automobiles, home appliances,
mechanical structures, construction and the like, and, in particular, when the steel
sheet is used as a material for components such as inner sheet members, structural
members, suspension members, and the like of automobiles, not only is a contribution
made to weight reduction in vehicle bodies and improvement in impact resistance but
the steel sheet is also easily worked into component shapes.
[Embodiments of the Invention]
[0012] Hereinafter, a steel sheet according to an embodiment of the present invention (the
steel sheet according to the present embodiment) will be described below in detail.
However, the present invention is not limited only to the configuration disclosed
in the present embodiment and can be modified in a variety of manners within the scope
of the gist of the present invention.
[0013] First, the chemical composition of the steel sheet according to the present embodiment
will be described.
[0014] Numerical value limiting ranges expressed below using "to" include the values at
both ends as the lower limit and the upper limit in the ranges. However, numerical
values expressed with 'less than' or 'more than' are not included in numerical value
ranges. In the following description, "%" regarding the chemical composition of the
steel sheet indicates "mass%" in all cases.
<Chemical composition of steel sheet>
(C: 0.050% to 0.250%)
[0015] C is an element that bonds to Ti or the like to form a carbide, thereby increasing
the tensile strength of steel. When the C content is less than 0.050%, it becomes
difficult to obtain a tensile strength of 980 MPa or more. Therefore, the C content
is set to 0.050% or more. The C content is preferably set to 0.070% or more.
[0016] On the other hand, when the C content is more than 0.250%, there is a concern about
a deterioration of the weldability. Therefore, the C content is set to 0.250% or less.
The C content is preferably 0.220% or less, more preferably 0.200% or less and still
more preferably 0.180% or less.
(Si: 0.005% to 2.000%)
[0017] Si is an element having an action of increasing the tensile strength of steel by
solid solution strengthening and the enhancement of hardenability. In addition, Si
is an element that also has an action of suppressing the precipitation of cementite.
When the Si content is less than 0.005%, it becomes unlikely for the above-described
action to be exhibited. Therefore, the Si content is set to 0.005% or more. The Si
content is preferably 0.010% or more.
[0018] On the other hand, when the Si content is more than 2.000%, the surface properties
of the steel sheet significantly deteriorate due to surface oxidation in a hot rolling
step. Therefore, the Si content is set to 2.000% or less. The Si content is preferably
1.500% or less and more preferably 1.300% or less.
(Mn: 0.10% to 3.00%)
[0019] Mn is an element having an action of increasing the tensile strength of steel by
solid solution strengthening and the enhancement of hardenability. When the Mn content
is less than 0.10%, ferritic transformation is excessively promoted, and a Ti-based
carbide is coarsely precipitated together with the ferritic transformation at high
temperatures. In this case, it becomes difficult to obtain a tensile strength of the
steel sheet of 980 MPa or more. Therefore, the Mn content is set to 0.10% or more.
The Mn content is preferably 0.30% or more and more preferably 0.50% or more.
[0020] On the other hand, when the Mn content is more than 3.00%, ferritic transformation
and bainitic transformation are delayed, and a desired ferrite area fraction cannot
be obtained. In this case, the elongation deteriorates, and the formation of MA degrades
the stretch flangeability or the bending workability. Therefore, the Mn content is
set to 3.00% or less. The Mn content is preferably 2.50% or less, more preferably
2.00% or less and still more preferably 1.50% or less.
(sol. Al: 0.001% to 1.00%)
[0021] Al is an element having an action of cleaning steel by deoxidation in a steelmaking
stage. When the sol. Al content is less than 0.001%, it becomes difficult to exhibit
the above-described action. Therefore, the sol. Al content is set to 0.001% or more.
The sol. Al content is preferably 0.01% or more, more preferably 0.02% or more and
still more preferably 0.03% or more.
[0022] On the other hand, even when the sol. Al content is set to more than 1.00%, the effect
of the above-described action is saturated, and the refining cost increases. Therefore,
the sol. Al content is set to 1.00% or less. The sol. Al content is preferably 0.80%
or less and more preferably 0.60% or less. sol. Al refers to acid-soluble Al.
(Ti: 0.150% to 0.400%)
[0023] Ti is an element that bonds to C to form a Ti-based carbide and contributes to increase
in the tensile strength of the steel sheet. In addition, Ti is an element having an
action of refining the microstructure by forming a Ti nitride to suppress the coarsening
of austenite during the reheating and hot rolling of a slab. When the Ti content is
less than 0.150%, it becomes difficult to obtain a tensile strength of 980 MPa or
more due to the lack of the precipitation hardening amount. Therefore, the Ti content
is set to 0.150% or more. The Ti content is preferably 0.170% or more, more preferably
0.190% or more and still more preferably 0.210% or more.
[0024] On the other hand, when the Ti content becomes excessive, a coarse Ti-based carbide
remains in austenite in an undissolved state, which degrades the elongation or the
bending workability, and the amount of a Ti-based carbide having a Baker-Nutting orientation
relationship contributing to the strength, which decreases the strength. Therefore,
the Ti content is set to 0.400% or less. The Ti content is preferably 0.380% or less
and more preferably 0.350% or less.
(N: 0.0010% to 0.0100%)
[0025] N is an element having an action of refining the microstructure by forming a Ti nitride
to suppress the coarsening of austenite during the reheating and hot rolling of a
slab. When the N content is less than 0.0010%, it becomes difficult to exhibit the
above-described action. Therefore, the N content is set to 0.0010% or more. The N
content is preferably 0.0015% or more and more preferably 0.0020% or more.
[0026] On the other hand, when the N content is more than 0.0100%, a coarse Ti nitride is
formed, and the stretch flangeability of the steel sheet deteriorates. Therefore,
the N content is set to 0.0100% or less. The N content is preferably 0.0060% or less
and more preferably 0.0050% or less.
(P: 0.100% or less)
[0027] P is an element that is contained in steel as an impurity and has an action of degrading
the stretch flangeability or bending workability of the steel sheet. Therefore, the
P content is set to 0.100% or less. The P content is preferably 0.060% or less, more
preferably 0.040% or less and still more preferably 0.020% or less. P is mixed from
a raw material as an impurity, and the lower limit thereof is not particularly limited,
but the P content is preferably as small as possible from the viewpoint of ensuring
the bending workability. However, when the P content is excessively decreased, the
manufacturing cost increases. From the viewpoint of the manufacturing cost, the P
content is preferably 0.001% or more and more preferably 0.005% or more.
(S: 0.0100% or less)
[0028] S is an element that is contained in steel as an impurity and has an action of degrading
the stretch flangeability or bending workability of the steel sheet. Therefore, the
S content is set to 0.0100% or less. The S content is preferably 0.0080% or less,
more preferably 0.0060% or less and still more preferably 0.0030% or less. S is mixed
from the raw material as an impurity, and the lower limit thereof is not particularly
limited, but the S content is preferably as small as possible from the viewpoint of
ensuring the bending workability. However, when the S content is excessively decreased,
the manufacturing cost increases. From the viewpoint of the manufacturing cost, the
S content is preferably 0.0001% or more, more preferably 0.0005% or more and still
more preferably 0.0010% or more.
[0029] The remainder of the chemical composition of the steel sheet according to the present
embodiment includes Fe and impurities. In the present embodiment, the impurity means
a substance that is mixed from ore as a raw material, a scrap, the manufacturing environment
or the like and is allowed to an extent that the steel sheet according to the present
embodiment is not adversely affected.
[0030] The steel sheet according to the present embodiment may contain the following optional
elements instead of some of Fe. Since the steel sheet according to the present embodiment
is capable of solving the problems even when the optional elements are not contained,
the lower limit of the amount of the optional elements is 0%.
(Nb: 0% to 0.100%)
[0031] Nb is an optional element. Nb is an element having effects on the suppression of
the coarsening of the crystal grain diameters of the steel sheet and an increase in
the tensile strength of the steel sheet by the refinement of the ferrite grain diameters
or precipitation hardening attributed to the precipitation of Nb as NbC. In order
to obtain these effects, the Nb content is preferably set to 0.001% or more. The Nb
content is more preferably 0.005% or more and still more preferably 0.010% or more.
[0032] On the other hand, when the Nb content exceeds 0.100%, the above-described effects
are saturated, and there is a concern about an increase in the rolling force during
finish rolling. Therefore, in a case where Nb is contained, the Nb content is set
to 0.100% or less. The Nb content is preferably 0.060% or less and more preferably
0.030% or less.
(V: 0% to 1.000%)
[0033] V is an optional element. V is an element having effects on an increase in the tensile
strength of the steel sheet by the formation of a solid solution in steel and increase
in the tensile strength of the steel sheet by precipitation hardening attributed to
the precipitation of V as a carbide, a nitride, a carbonitride or the like in steel.
In order to obtain these effects, the V content is preferably set to 0.005% or more.
The V content is more preferably 0.010% or more and still more preferably 0.050% or
more.
[0034] On the other hand, when the V content exceeds 1.000%, a carbide is likely to become
coarse and there is a case where the bending workability deteriorates. Therefore,
in a case where V is contained, the V content is set to 1.000% or less. The V content
is preferably 0.800% or less and more preferably 0.600% or less.
(Mo: 0% to 1.000%)
[0035] Mo is an optional element. Mo is an element having effects on the high-strengthening
of the steel sheet by the enhancement of the hardenability of steel and the formation
of a carbide or a carbonitride. In order to obtain these effects, the Mo content is
preferably set to 0.001% or more. The Mo content is more preferably 0.005% or more,
still more preferably 0.010% or more and far still more preferably 0.050% or more.
[0036] On the other hand, when the Mo content exceeds 1.000%, there is a case where the
cracking sensitivity of a steel material such as a slab is enhanced. Therefore, in
a case where Mo is contained, the Mo content is set to 1.000% or less. The Mo content
is more preferably 0.800% or less and still more preferably 0.600% or less.
(Cu: 0% to 1.00%)
[0037] Cu is an optional element. Cu is an element having an effect on improvement in the
toughness of steel and an effect on an increase in the tensile strength. In order
to obtain these effects, the Cu content is preferably set to 0.02% or more.
[0038] On the other hand, when Cu is excessively contained, there is a case where the weldability
of the steel sheet deteriorates. Therefore, in a case where Cu is contained, the Cu
content is set to 1.00% or less. The Cu content is preferably 0.50% or less and more
preferably 0.30% or less.
(Ni: 0% to 1.00%)
[0039] Ni is an optional element. Ni is an element having an effect on improvement in the
toughness of steel and an effect on an increase in the tensile strength. In order
to obtain these effects, the Ni content is preferably set to 0.02% or more.
[0040] On the other hand, when Ni is excessively contained, the alloying cost increases,
and there is a case where the toughness of the steel sheet in a welded heat-affected
zone deteriorates. Therefore, in a case where Ni is contained, the Ni content is set
to 1.00% or less. The Ni content is preferably 0.50% or less and more preferably 0.30%
or less.
(Cr: 0% to 2.00%)
[0041] Cr is an optional element. Cr is an element having an effect on an increase in the
tensile strength by the enhancement of the hardenability of steel. In order to obtain
this effect, the Cr content is preferably set to 0.02% or more. The Cr content is
more preferably 0.05% or more and still more preferably 0.10% or more.
[0042] On the other hand, when the Cr content become excessive, the chemical convertibility
deteriorates. Therefore, in a case where Cr is contained, the Cr content is set to
2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or
less and still more preferably 0.50% or less.
(W: 0% to 1.000%)
[0043] W is an optional element. W is an element having an effect on an increase in the
tensile strength by the formation of a carbide or a carbonitride. In order to obtain
this effect, the W content is preferably set to 0.020% or more.
[0044] On the other hand, even when more than a certain amount of W is contained, the effect
of the above-described action is saturated, and thus the alloying cost increases.
Therefore, in a case where W is contained, the W content is set to 1.000% or less.
The W content is preferably 0.800% or less.
(B: 0% to 0.0020%)
[0045] B is an optional element. B is an element having an effect on an increase in the
tensile strength of the steel sheet by grain boundary strengthening or solid solution
strengthening. In order to obtain this effect, the B content is preferably set to
0.0001% or more. The B content is more preferably 0.0002% or more.
[0046] On the other hand, even when more than 0.0020% of B is contained, not only is the
above-described effect saturated, but the alloying cost also increases. Therefore,
in a case where B is contained, the B content is set to 0.0020% or less. The B content
is more preferably 0.0015% or less.
(Ca: 0% to 0.0100%)
[0047] Ca is an optional element. Ca is an element having an effect on the refinement of
the microstructure of the steel sheet by the dispersion of a number of fine oxides
in molten steel. In addition, Ca is an element having an effect on improvement in
the stretch flangeability of the steel sheet by fixing S in molten steel as spherical
CaS to suppress the formation of an elongated inclusion such as MnS. In order to obtain
these effects, the Ca content is preferably set to 0.0002% or more. The Ca content
is more preferably 0.0005% or more and still more preferably 0.0010% or more.
[0048] On the other hand, when the Ca content exceeds 0.0100%, the amount of CaO in steel
increases, and there is a case where the toughness of the steel sheet deteriorates.
Therefore, in a case where Ca is contained, the Ca content is set to 0.0100% or less.
The Ca content is preferably 0.0050% or less and more preferably 0.0030% or less.
(Mg: 0% to 0.0100%)
[0049] Mg is an optional element. Similar to Ca, Mg is an element having effects on the
suppression of the formation of coarse MnS by the formation of an oxide or a sulfide
in molten steel and the refinement of the microstructure of the steel sheet by the
dispersion of a number of fine oxides. In order to obtain these effects, the Mg content
is preferably set to 0.0002% or more. The Mg content is more preferably 0.0005% or
more and still more preferably 0.0010% or more.
[0050] On the other hand, when the Mg content exceeds 0.0100%, an oxide in steel increases,
and there is a case where the toughness of the steel sheet deteriorates. Therefore,
in a case where Mg is contained, the Mg content is set to 0.0100% or less. The Mg
content is preferably 0.0050% or less and more preferably 0.0030% or less.
(REM: 0% to 0.0100%)
[0051] REM is an optional element. Similar to Ca, REM is also an element having effects
on the suppression of the formation of coarse MnS by the formation of an oxide or
a sulfide in molten steel and the refinement of the microstructure of the steel sheet
by the dispersion of a number of fine oxides. In the case of obtaining these effects,
the REM content is preferably set to 0.0002% or more. The REM content is more preferably
0.0005% or more and still more preferably 0.0010% or more.
[0052] On the other hand, when the REM content exceeds 0.0100%, an oxide in steel increases,
and there is a case where the toughness of the steel sheet deteriorates. Therefore,
in a case where REM is contained, the REM content is set to 0.0100% or less. The REM
content is preferably 0.0050% or less and more preferably 0.0030% or less.
[0053] Here, REM (rare earth metal) refers to a total of 17 elements including Sc, Y, and
lanthanoids. In the present embodiment, the REM content refers to the total amount
of these elements.
(Bi: 0% to 0.0200%)
[0054] Bi is an optional element. Bi is an element having an effect on improvement in the
formability of the steel sheet by the refinement of the solidification structure.
In order to obtain this effect, the Bi content is preferably set to 0.0001% or more.
The Bi content is more preferably 0.0005% or more.
[0055] On the other hand, when the Bi content exceeds 0.0200%, the above-described effect
is saturated, and the alloying cost increases. Therefore, in a case where Bi is contained,
the Bi content is set to 0.0200% or less. The Bi content is preferably 0.0100% or
less and more preferably 0.0070% or less.
(Ex. C: 0.020% or less)
[0056] C is precipitated as a Ti-based carbide and contributes to the high-strengthening
of the steel sheet. However, when the amount of C contained is larger than the amount
of C to be precipitated as a Ti-based carbide, excess C forms pearlite, cementite,
MA or the like and consequently degrades the stretch flangeability or the bending
workability.
[0057] Ex. C that is obtained by the following formula (1) corresponds to the amount of
C contained more than the amount of C to be precipitated as a Ti-based carbide. In
the steel sheet according to the present embodiment, this Ex. C is set to 0.020% or
less. Ex. C is preferably 0.018 or less and more preferably 0.015% or less. The lower
limit is not particularly limited.

[0058] Here, "%Ti*" in the formula (1) is obtained from the following formula (2).

%C, %V, %Nb, %Mo, %W, %Ti, %N and %S in the formula (1) and the formula (2) are the
amounts of C, V, Nb, Mo, W, Ti, N and S in the steel sheet by mass%, respectively.
[0059] Next, the microstructure of the steel sheet will be described. In the steel sheet
according to the present embodiment, the microstructure at a 1/4 depth position of
the sheet thickness from the surface contains 60% or more of ferrite, 0% to 5% of
MA and a total of 0% to 5% of pearlite and cementite with a remainder including bainite.
In addition, in the microstructure, the average crystal grain diameter is 10.0 µm
or less, the average aspect ratio of crystal grains is 0.30 or more, and the standard
deviation of the Mn concentration is 0.60 mass% or less. In addition, a Ti-based carbide
having a Baker-Nutting orientation relationship in the ferrite is precipitated in
a semi-coherent state.
[0060] Here, the reason for regulating the microstructure at the 1/4 depth position of the
sheet thickness in the sheet thickness direction from the surface of the steel sheet
(a t/4 position from the surface in a case where the sheet thickness is represented
by t) is that the microstructure at this position is a typical microstructure of the
steel sheet.
(Area fraction of ferrite: 60% or more)
(Area fraction of MA: 0% to 5%)
(Total area fraction of pearlite and cementite: 0% to 5%)
(Remainder: Bainite)
[0061] Ferrite is required to obtain favorable elongation. When the area fraction is less
than 60%, the elongation deteriorates. Therefore, the area fraction of ferrite is
set to 60% or more. The area fraction of ferrite is preferably 70% or more, more preferably
80% or more and may be 100% (that is, a ferrite single phase).
[0062] There is a case where the microstructure contains, in addition to ferrite, a small
amount of MA, which is allowed as long as the area fraction is 5% or less. The area
fraction is preferably 4% or less, more preferably 3% or less and most preferably
2% or less. In addition, there is a case where pearlite and cementite are precipitated,
which is allowed as long as the total area fraction is 5% or less. The total area
fraction is preferably 4% or less, more preferably 3% or less and most preferably
2% or less. When the area fraction of MA is more than 5%, the bending workability
and the hole expandability deteriorate. Alternatively, when the area fraction of pearlite
and cementite is more than 5%, the hole expandability deteriorates.
[0063] In the microstructure, the remainder other than the above-described structures includes
bainite. The hardness difference is small between bainite and ferrite that has been
precipitation-hardened by a Ti-based carbide. Therefore, bainite has a small effect
on the degradation of the hole expandability compared with MA (Martensite-Austenite
constituents), pearlite and cementite. Therefore, bainite is contained as the remainder
in microstructure.
(Average crystal grain diameter: 10.0 µm or less)
[0064] When the average crystal grain diameter is large, the bending workability deteriorates.
Therefore, in the microstructure, the average crystal grain diameter is set to 10.0
µm or less. The average crystal grain diameter is preferably 8.0 µm or less. Since
the average crystal grain diameter is preferably as small as possible, the lower limit
is not particularly limited. However, it is technically difficult to refine crystal
grains by ordinary hot rolling such that the average crystal grain diameter becomes
less than 1.0 µm. Therefore, the average crystal grain diameter may be set to 1.0
µm or more.
[0065] "The average crystal grain diameter" in the present embodiment refers to the average
value of crystal grain diameters for which a region that is surrounded by grain boundaries
having a crystal orientation difference of 15° or more and has a circle equivalent
diameter of 0.3 µm or more in a material having a bcc crystal structure, that is,
ferrite, bainite, martensite, and pearlite is defined as a crystal grain, and the
crystal grain diameters of residual austenite are not included in the average crystal
grain diameter.
(Average aspect ratio of crystal grain: 0.30 or more)
[0066] In the present embodiment, the average aspect ratio of bcc crystal grains is 0.30
or more. The aspect ratio is a value obtained by dividing the length of the minor
axis of a crystal grain by the length of the major axis and has a value of 0 to 1.00.
As the average aspect ratio of crystal grains becomes smaller, the crystal grains
become flatter, and, as the average aspect ratio becomes closer to 1.00, it is indicated
that a crystal grain becomes more equiaxial. When the average aspect ratio of the
crystal grains is less than 0.30, there are a number of flat crystal grains, the anisotropy
of the material becomes large, and the stretch flangeability and the bending workability
deteriorate. Therefore, the average aspect ratio of the crystal grains excluding residual
austenite is set to 0.30 or more. As the crystal grains become more equiaxial, the
anisotropy becomes smaller, and the workability becomes superior, and thus the average
aspect ratio of the crystal grains excluding residual austenite is preferably as close
to 1.00 as possible.
[0067] In the present embodiment, the average crystal grain diameter, the average aspect
ratio of the crystal grains and the area fractions of the microstructure are obtained
by the scanning electron microscopic (SEM) observation and the electron back scattering
diffraction (EBSD) analysis of the microstructure at the 1/4 depth position of the
steel thickness from the surface of the steel sheet of a cross section of the steel
sheet parallel to a rolling direction and the sheet thickness direction using an EBSD
analyzer composed of a thermal field emission scanning electron microscope and an
EBSD detector. In a region that is 200 µm long in the rolling direction and 100 µm
long in the sheet thickness direction and has the 1/4 depth position of the sheet
thickness from the surface of the steel sheet at the center, crystal orientation information
is acquired at 0.2 µm intervals while differentiating fcc and bcc. Crystal grain boundaries
having a crystal orientation difference of 15° or more are specified using the software
attached to the EBSD analyzer ("OIM Analysis (registered trademark)" manufactured
byAMETEK, Inc.). Regarding the average crystal grain diameter of bcc, the average
crystal grain diameter is obtained by defining a region that is surrounded by crystal
grain boundaries having a crystal orientation difference of 15° or more and has a
circle equivalent diameter of 0.3 µm or more as a crystal grain.
[0068] A crystal grain boundary having a crystal orientation difference of 15° or more is
mainly a ferrite grain boundary or a block boundary of martensite and bainite. In
a method for measuring ferrite grain diameters according to JIS G 0552: 2013, there
is a case where a grain diameter is calculated even for a ferrite grain having a crystal
orientation difference of less than 15°, and furthermore, a block of martensite or
bainite is not calculated. Therefore, as the average crystal grain diameter in the
present embodiment, a value obtained by EBSD analysis as described above is adopted.
In the EBSD analysis, since the length of the major axis and the length of the minor
axis of each crystal grain are also obtained at the same time, the average aspect
ratio of bcc crystal grains is also obtained by adopting the present method.
[0069] The area fraction of ferrite is measured by the following method. Here, a region
that is surrounded by grain boundaries having a crystal orientation difference of
5° or more and has a circle equivalent diameter of 0.3 µm or more is defined as a
crystal grain. Among such crystal grains, for crystal grains for which a value that
is obtained by an analysis with Grain Average Misorientation analysis equipped in
OIM Analysis (GAM value) is 0.6° or less, the area fraction is calculated. The area
fraction of ferrite is obtained by such a method. The reason for defining a boundary
having a crystal orientation difference of 5° or more as a grain boundary at the time
of obtaining the area fraction of ferrite is that there is a case where it is not
possible to differentiate different microstructures formed as close variants from
the same prior austenite grain.
[0070] The area fraction of pearlite and cementite is obtained by observing microstructures
revealed by Nital etching with a SEM. The area fraction of MA is obtained by observing
a microstructure revealed by LePera etching with an optical microscope. The area fraction
may be obtained by an image analysis or may be obtained by a point counting method.
For example, for pearlite and cementite, the area fractions may be obtained by the
point counting method with lattice spacings of 5 µm after three or more visual fields
(100 µm × 100 µm/visual field) in a region at the 1/4 depth position of the sheet
thickness from the surface of the steel sheet are observed at a magnification of 1000
times. In addition, the area fraction of MA may be obtained by the point counting
method with lattice spacings of 5 µm after two or more visual fields (200 µm × 200
µm/visual field) in a region at the 1/4 depth position of the sheet thickness from
the surface of the steel sheet are observed at a magnification of 500 times.
(Standard deviation of Mn concentration: 0.60 mass% or less)
[0071] The standard deviation of the Mn concentration at the 1/4 depth position of the sheet
thickness from the surface of the steel sheet according to the present embodiment
is 0.60 mass% or less. In such a case, a local unevenness in the tensile strength
attributed to Mn segregation is reduced, and it is possible to stably obtain favorable
bending workability. The value of the standard deviation of the Mn concentration is
desirably as small as possible, but the substantial lower limit is 0.10 mass% due
to restrictions in the manufacturing process.
[0072] The standard deviation of the Mn concentration can be obtained by collecting a sample
such that a cross section parallel to the rolling direction and the sheet thickness
direction of the steel sheet becomes an observed section, mirror-polishing the observed
section and then measuring the 1/4 depth position of the sheet thickness from the
surface of the steel sheet with an electron probe microanalyzer (EPMA). As the measurement
conditions, the acceleration voltage is set to 15 kV, the magnification is set to
5000 times, and a distribution image in a range that is 20 µm long in the rolling
direction of the sample and 20 µm long in the sheet thickness direction of the sample
is measured. More specifically, the measurement intervals are set to 0.1 µm, and the
Mn concentrations at 40000 or more sites are measured. Next, the standard deviation
is calculated based on the Mn concentrations obtained from all of the measurement
points, thereby obtaining the standard deviation of the Mn concentration.
(Ti-based carbide)
[0073] In the steel sheet according to the present embodiment, a carbide containing Ti (Ti-based
carbide) is precipitated in ferrite. Ti is an element having a high driving force
for the precipitation of a carbide in ferrite, and the control of the content and
a heat treatment make it easy to control the precipitation state of a carbide. In
addition, the Ti-based carbide also has a high precipitation hardening capability.
Here, the Ti-based carbide refers to a carbide having a NaCl-type crystal structure
containing Ti. In a case where such a carbide contains Ti, even when a small amount
of other carbide-forming alloying elements are contained, the above-described driving
force is not significantly weakened, and thus the effect can be obtained. Within the
range of the chemical composition that is regulated by the present embodiment, the
Ti-based carbide may contain other carbide-forming alloying elements, for example,
Mo, Nb, V, Cr and W. Furthermore, even when the Ti-based carbide is a carbonitride
in which some of carbon atoms have been substituted with nitrogen atoms, the precipitation
state does not change, and thus the effect can be obtained.
(Precipitation of Ti-based carbides in ferrite in semi-coherent state)
[0074] In a case where the proportion of Ti-based carbides for which the interface with
ferrite is a semi-coherent interface to the Ti-based carbides precipitated in ferrite,
which have the Baker-Nutting orientation relationship, is 50% or more, the stretch
flangeability of the steel sheet becomes stably favorable. The state where "the Ti-based
carbides are precipitated in a semi-coherent state" mentioned in the present embodiment
refers to such case. In a case where the Ti-based carbides are not precipitated in
a semi-coherent state, the hole expandability deteriorates.
[0075] Whether or not the Ti-based carbides having the Baker-Nutting orientation relationship
are in a semi-coherent state is determined as described below. That is, an annular
dark-field scanning transmission electron microscopic image, for which the detection
angle of an annular detector is set to 60 mrad or more and 200 mrad or less in the
scanning transmission electron microscopy (magnification: 910,000 times to 5,100,000
times), is captured by injecting electron beams into a thin film sample for a transmission
electron microscope produced from the 1/4 depth position of the sheet thickness from
the surface along a [001] orientation of ferrite. When a particle forming a plate-like
form having a (100) plane of ferrite in the matrix as a habit plane and a particle
forming a plate-like form having a (010) plane of ferrite as a habit plane are regarded
as the Ti-based carbides having the Baker-Nutting orientation relationship, a case
where the numbers of the crystal planes of a {010} plane of ferrite and a {01-1 }
plane of the Ti-based carbides that sandwich the habit plane of the (100) plane of
the particle forming a plate-like form having a (100) plane of ferrite in the matrix
as a habit plane and the habit plane of the (100) plane of the particle forming a
plate-like form having a (010) plane of ferrite as a habit plane coincide with each
other is determined as a coherent state, and a case where the numbers of the crystal
planes do not coincide with each other is determined as the semi-coherent state. In
a case where 20 or more Ti-based carbides are observed and 50% or more is in the semi-coherent
state, the Ti-based carbides having the Baker-Nutting orientation relationship in
steel from which the observed thin film sample for a transmission electron microscope
has been collected are determined to be in the semi-coherent state.
[0076] Regarding the sizes of the Ti-based carbides, ordinarily, as the carbides become
larger, the number density tends to become smaller. In the present invention, from
the viewpoint of ensuring the number density of the Ti-based carbides that are precipitated
in ferrite to have the Baker-Nutting orientation relationship, the thickness of the
Ti-based carbide needs to be 1 nm or more and 5 nm or less.
[0077] The thickness of the Ti-based carbide is measured by the following method.
[0078] A thin film sample for a transmission electron microscope is produced from the 1/4
depth position in the sheet thickness direction from the surface of the steel sheet
and observed with a scanning transmission electron microscope (hereinafter, also referred
to as "STEM"). In a Ti-based carbide forming sheet surfaces on the (100) plane and
the (010) plane of ferrite observed in a STEM image captured by injecting electron
beams along the [001] orientation of ferrite, the length of a small side between the
sizes of the Ti-based carbide measured along the [100] and [010] orientations of ferrite
is regarded as the thickness. In addition, at the time of evaluating the thickness
of the Ti-based carbide, a scale is corrected such that the interatomic distance,
which is as long as 10 unit lattices, becomes 2.866 nm in each of the [100] orientation
and the [010] orientation of ferrite in a site where no precipitates are shown in
the image.
<Mechanical properties>
(Tensile Strength: 980 MPa or more)
[0079] The steel sheet according to the present embodiment has a high strength and is excellent
in terms of the elongation, the stretch flangeability, and the bending workability
by the control of the microstructure, the precipitation form of the Ti-based carbide
and Mn segregation. However, when the tensile strength of the steel sheet is small,
an effect on weight reduction in vehicle bodies, rigidness improvement or the like
is small. Therefore, the tensile strength (TS) of the steel sheet according to the
present embodiment is set to 980 MPa or more. The tensile strength is preferably 1080
MPa or more. Although the upper limit is not particularly regulated; however, as the
tensile strength increases, press forming becomes more difficult. Therefore, the tensile
strength may be set to 1800 MPa or less.
[0080] From the viewpoint of the formability, the steel sheet according to the present embodiment
aims at TS × λ, which serves as an index of the balance between the strength and the
stretch flangeability, of 50000 MPa·% or more and aims at TS × El, which serves as
an index of the balance between the strength and the elongation, of 14000 MPa·% or
more. TS × El is more preferably 15000 MPa·% or more. TS × λ is more preferably 55000
MPa·% or more, still more preferably 60000 MPa % or more and far still more preferably
65000 MPa % or more.
[0081] The tensile strength and elongation of the steel sheet are evaluated by the tensile
strength and the total elongation at fracture (El) using a No. 5 test piece regulated
in JIS Z 2241: 2011. The stretch flangeability of the steel sheet is evaluated with
the limiting hole expansion ratio (λ) regulated in JIS Z 2256: 2010.
<Manufacturing method>
[0082] The reason for limiting the conditions for manufacturing the steel sheet according
to the present embodiment will be described.
[0083] The present inventors are confirming that the steel sheet according to the present
embodiment can be obtained by a manufacturing method including a heating step, a hot
rolling step, a cooling step and a coiling step as described below.
[Heating step]
[0084] First, a slab or steel piece having the above-mentioned chemical composition is heated.
The slab or steel piece may be a slab or steel piece obtained by continuous casting
or casting and blooming or may be also a slab or steel piece obtained by additionally
performing hot working or cold working on the above-described slab or steel piece.
(Retention time in temperature range of 700°C to 850°C during heating: 900 seconds
or longer)
[0085] When the slab or steel piece that is to be subjected to hot rolling is heated, the
slab or steel piece is caused to retain in a temperature range of 700°C to 850°C for
900 seconds or longer. In austenitic transformation occurring in the temperature range
of 700°C to 850°C, Mn is distributed to ferrite and austenite. Therefore, when the
transformation time is extended by extending the retention time, it is possible to
diffuse Mn in the ferrite region. This eliminates Mn microsegregation that is unevenly
distributed in the slab and significantly reduces the standard deviation of the Mn
concentration.
(Heating temperature: 1280°C or higher and SRT (°C) or higher)
[0086] The heating temperature of the slab or steel piece that is to be subjected to hot
rolling is set to 1280°C or higher and a temperature SRT (°C) represented by the following
formula (3) or higher. When the heating temperature is lower than 1280°C, there is
a case where the reduction in the standard deviation of the Mn concentration due to
the diffusion of Mn during heating becomes insufficient. In addition, the heating
temperature is lower than the SRT (°C), the solutionizing of a Ti carbonitride becomes
insufficient, and, in any cases, the tensile strength or bending workability of the
steel sheet deteriorates. Therefore, the temperature of the slab or steel piece that
is to be subjected to hot rolling is set to 1280°C or higher and the SRT (°C) or higher.
Here, the fact that "the temperature of the slab or steel piece is 1280°C or higher
and the SRT (°C) or higher" means that the temperature of the slab or steel piece
is higher than the higher temperature of 1280°C and the SRT (°C) or the higher temperature
of 1280°C and the SRT (°C) is the same as the temperature of the slab or steel piece.
[0087] On the other hand, when the heating temperature is higher than 1400°C, there is a
case where a thick scale is formed to decrease the yield or significantly damage heating
furnaces. Therefore, the heating temperature is preferably 1400°C or lower.

[0088] Here, [element symbol] in the formula (3) indicates the amount of each element by
mass%.
[Hot rolling step]
[0089] In the hot rolling step, multi-pass hot rolling is performed on the slab or steel
piece after the heating step using a plurality of rolling stands to produce a hot-rolled
steel sheet. The hot rolling step is divided into rough rolling and finish rolling
that is performed after the rough rolling.
[0090] The multi-pass hot rolling can be performed using a reverse mill or a tandem mill;
however, at least several stages from the end are preferably performed using a tandem
mill from the viewpoint of the industrial productivity.
(Time from beginning of rough rolling to completion of finish rolling: 600 seconds
or shorter)
[0091] Since rolling promotes the precipitation of the Ti-based carbide and makes the precipitation
begin, when the time taken until the completion of the finish rolling is too long,
a large amount of a coarse Ti-based carbide is precipitated in austenite. In this
case, a fine Ti-based carbide that contributes to high-strengthening and is precipitated
in ferrite after the finish rolling reduces, the tensile strength of the steel sheet
significantly decreases, and the bending workability deteriorates. Therefore, the
time from the beginning of the rough rolling to the completion of the finish rolling
is set to 600 seconds or shorter. The time is preferably 500 seconds or shorter, more
preferably 400 seconds or shorter and most preferably 320 seconds or shorter.
[0092] Normally, in hot rolling steps, the rolling reduction and the rolling temperature
are controlled depending on the specification of a roller, the sheet thickness and
sheet width of a coil to be manufactured and a desired material, but the time from
the beginning of rough rolling to the completion of finish rolling is not comprehensively
controlled. The present inventors newly found that the time from the beginning of
the rough rolling to the completion of the finish rolling affects the precipitation
state of the Ti-based carbide.
(Total rolling reduction within temperature range of 850°C to 1100°C: 90% or larger)
[0093] When hot rolling is performed in a manner that the total rolling reduction within
a temperature range of 850°C to 1100°C becomes 90% or larger, mainly recrystallized
austenite is refined, and the accumulation of the strain energy in the non-recrystallized
austenite is promoted. As a result, the recrystallization of austenite is promoted,
the diffusion of Mn atoms is promoted, and the standard deviation of the Mn concentration
becomes small. Therefore, in the hot rolling, the total rolling reduction (cumulative
rolling reduction) within the temperature range of 850°C to 1100°C is set to 90% or
larger.
[0094] The total rolling reduction within the temperature range of 850°C to 1100°C can be
represented by (t0 - t1)/t0 × 100 (%) where the inlet sheet thickness before the first
pass in rolling within this temperature range is indicated by t0 and the outlet sheet
thickness after the final pass in the rolling within this temperature range is indicated
by t1.
(Finish rolling completion temperature FT (°C): TR (°C) or higher and 1080°C or lower)
[0095] When the FT (°C) is lower than TR (°C) represented by the following formula (4),
significantly flat austenite is formed before cooling after the finish rolling, the
microstructure is elongated in the rolling direction in the final product steel sheet,
the average aspect ratio of crystal grains excluding residual austenite and having
a bcc structure becomes smaller, and the plastic anisotropy becomes large. In this
case, the elongation, stretch flangeability and/or bending workability of the steel
sheet deteriorates. Therefore, the FT (°C) is set to the TR (°C) or higher.
[0096] On the other hand, when the FT (°C) exceeds 1080°C, the structure becomes coarse,
and the bending workability of the steel sheet deteriorates. Therefore, the FT (°C)
is set to 1080°C or lower. The FT (°C) is preferably 1060°C or lower.
[0097] The temperature during the finish rolling refers to the surface temperature of steel
and can be measured with a radiation-type thermometer or the like.

[0098] Here, [element symbol] in the formula (4) indicates the amount of each element by
mass%, and zero is assigned in a case where the corresponding element is not contained.
[Cooling step]
[0099] The method for manufacturing the steel sheet according to the present embodiment
has, as the next step of the hot rolling step, a cooling step of cooling the hot-rolled
steel sheet with water to a temperature range of 650°C to 800°C at an average cooling
rate of 45 °C/second or faster. In addition, in the method for manufacturing the steel
sheet according to the present embodiment, the cooling step is begun within 3.0 seconds
after the end of the hot rolling step (after the completion of the finish rolling).
(Time from completion of finish rolling to beginning of water cooling: 3.0 seconds
or shorter)
[0100] When the time from the completion of the finish rolling to the beginning of water
cooling is longer than 3.0 seconds, the tensile strength or the bending workability
deteriorates due to the growth of the refined austenite crystal grains or the coarse
precipitation of a carbonitride of Ti or the like. Therefore, in the method for manufacturing
the steel sheet according to the present embodiment, the water cooling is begun within
3.0 seconds after the completion of the finish rolling. The time is preferably 2.0
seconds or shorter and more preferably 1.5 seconds or shorter.
(Average cooling rate from beginning of water cooling after completion of finish rolling
to water cooling stop temperature of 650°C to 800°C: 45 °C/second or faster)
[0101] When the average cooling rate to a water cooling stop temperature of 650°C to 800°C
is slower than 45 °C/second, a coarse Ti-based carbide is precipitated in non-transformed
austenite or in transformed ferrite grains, and it becomes difficult to obtain a desired
strength. Therefore, the average cooling rate is set to 45 °C/second or faster. The
average cooling rate is preferably 50 °C/second or faster and more preferably 55 °C/second
or faster. The upper limit is not particularly limited, but is preferably 300 °C/second
or slower from the viewpoint of the facility cost. The average cooling rate is a value
obtained by dividing the amount of temperature dropped from the beginning of the water
cooling after the completion of the hot rolling to the stopping of the water cooling
by the required time.
(Retention time within temperature range of 650°C to 800°C: 5 to 50 seconds)
[0102] After cooled to 650°C to 800°C at an average cooling rate of 45 °C/second or faster,
the steel sheet is caused to retain in the corresponding temperature range. When the
retention time at 650°C to 800°C is short, since it becomes difficult to obtain a
desired ferrite area fraction, the retention time needs to be five seconds or longer.
The retention time is preferably seven seconds or longer. On the other hand, when
the retention time is long, pearlite is formed, and the hole expandability deteriorates.
Therefore, the retention time within this temperature range is set to 50 seconds or
shorter. The retention time is preferably 40 seconds or shorter.
[0103] In addition, while the steel sheet is caused to retain at 650°C to 800°C, ferritic
transformation progresses, and a Ti-based carbide having a semi-coherent interface
is precipitated in ferrite. As the results, a steel sheet being excellent in terms
of the tensile strength and the hole expandability can be obtained. When the Ti-based
carbide is precipitated at a temperature higher than 800°C, the Ti-based carbide is
coarsely precipitated, a desired number density cannot be obtained, and it becomes
difficult to obtain a desired tensile strength. On the other hand, when the Ti-based
carbide is precipitated at a temperature lower than 650°C, a Ti-based carbide having
a coherent interface is precipitated, and the hole expandability deteriorates.
(Average cooling rate within temperature range of 550°C to 650°C: 45 °C/second or
faster)
[0104] After the retention, the steel sheet is cooled to a temperature of 550°C or lower
(coiling temperature) in a manner that the average cooling rate within a temperature
range of 550°C to 650°C becomes 45 °C/second or faster. When the average cooling rate
is slower than 45 °C/second, a Ti-based carbide having a coherent interface is precipitated
during the cooling, and the hole expandability deteriorates. The upper limit of the
average cooling rate is not particularly limited, but is preferably 300 °C/second
or slower from the viewpoint of the facility cost.
[Coiling step]
(Coiling temperature: 350°C or higher and lower than 550°C)
[0105] After the cooling step, the steel sheet is coiled at 350°C or higher and lower than
550°C. When the coiling temperature is lower than 350°C, non-transformed austenite
transforms into martensite, and the hole expandability or the bending workability
deteriorates. On the other hand, when the coiling temperature becomes 550°C or higher,
a Ti-based carbide having a coherent interface is formed after the coiling, and the
hole expandability deteriorates. The coiling temperature is preferably 400°C or higher
and lower than 500°C.
[0106] In the present embodiment, a plated steel sheet having a plating layer may be produced
by performing plating on the surface of the steel sheet after the coiling step. Even
in a case where plating is performed, there is no problem in performing the plating
as long as the conditions for the method for manufacturing the steel sheet according
to the present embodiment are satisfied. The plating may be any of electroplating
and hot-dip plating, and the plating type is also not particularly limited, but is
ordinarily zinc-based plating including zinc plating and zinc alloy plating. As examples
of the plated steel sheet, an electrolytic zinc-plated steel sheet, an electrolytic
zinc-nickel alloy-coated steel sheet, a hot-dip galvanized steel sheet, a galvannealed
steel sheet, a hot-dip zinc-aluminum alloy-coated steel sheet and the like are exemplary
examples. The plating adhesion amount may be an ordinary amount. Before the plating,
Ni or the like may be applied to the surface as pre-plating.
[0107] At the time of manufacturing the steel sheet according to the present embodiment,
well-known temper rolling may be performed as appropriate for the purpose of shape
correction.
[0108] The sheet thickness of the steel sheet according to the present embodiment is not
particularly limited, but is preferably 8.0 mm or less since, in a case where the
sheet thickness is too thick, microstructures formed in the surface layer and the
inside of the steel sheet significantly differ. The sheet thickness is more preferably
6.0 mm or less. On the other hand, when the sheet thickness is too thin, since threading
during hot rolling becomes difficult, ordinarily, the sheet thickness is preferably
1.0 mm or more. The sheet thickness is more preferably 1.2 mm or more.
[Examples]
[0109] Next, the effect of one aspect of the present invention will be more specifically
described using examples, but conditions in the examples are simply examples of the
conditions adopted to confirm the feasibility and effect of the present invention,
and the present invention is not limited to these examples of the conditions. The
present invention is capable of adopting a variety of conditions within the scope
of the gist of the present invention as long as the object of the present invention
is achieved.
[0110] Steel materials having a chemical composition (unit mass%, the remainder was Fe and
impurities) shown in Table 1A and Table 1B and having a sheet thickness of 250 mm
were hot-rolled under conditions shown in Table 2A and Table 2B, thereby producing
hot-rolled steel sheets having a sheet thickness of 2.5 to 3.5 mm. On some of the
obtained hot-rolled steel sheets, a hot-dip galvanizing treatment with an annealing
temperature of 700°C and, furthermore, an alloying treatment were performed to produce
hot-dip galvanized steel sheets (GI) or galvannealed steel sheets (GA).
[Table 1A]
Steel |
(Mass%: remainder is Fe and impurities) |
C |
Si |
Mn |
P |
S |
sol. Al |
Ti |
N |
Nb |
V |
Mo |
A |
0.082 |
0.620 |
1.32 |
0.015 |
0.0020 |
0.06 |
0.310 |
0.0024 |
|
|
|
B |
0.062 |
0.052 |
1.35 |
0.010 |
0.0012 |
0.05 |
0.225 |
0.0033 |
|
|
|
C |
0.120 |
1.561 |
1.32 |
0.013 |
0.0019 |
0.08 |
0.185 |
0.0045 |
|
|
|
D |
0.084 |
0.053 |
1.29 |
0.011 |
0.0009 |
0.09 |
0.316 |
0.0039 |
|
|
|
E |
0.081 |
0.064 |
1.32 |
0.009 |
0.0026 |
0.05 |
0.297 |
0.0032 |
0.025 |
|
|
F |
0.088 |
0.111 |
1.42 |
0.010 |
0.0017 |
0.07 |
0.248 |
0.0043 |
|
0.106 |
|
G |
0.087 |
0.042 |
1.40 |
0.009 |
0.0012 |
0.05 |
0.278 |
0.0032 |
|
|
0.110 |
H |
0.084 |
0.063 |
1.31 |
0.014 |
0.0002 |
0.06 |
0.311 |
0.0034 |
|
|
|
I |
0.082 |
0.025 |
1.30 |
0.010 |
0.0027 |
0.09 |
0.313 |
0.0037 |
|
|
|
J |
0.072 |
0.079 |
1.25 |
0.016 |
0.0020 |
0.03 |
0.302 |
0.0044 |
|
|
|
K |
0.076 |
0.368 |
1.35 |
0.010 |
0.0027 |
0.04 |
0.279 |
0.0037 |
|
|
|
L |
0.075 |
0.032 |
1.36 |
0.016 |
0.0018 |
0.06 |
0.279 |
0.0029 |
|
|
|
M |
0.075 |
0.025 |
1.40 |
0.010 |
0.0024 |
0.04 |
0.285 |
0.0030 |
|
|
|
N |
0.073 |
0.039 |
1.32 |
0.010 |
0.0019 |
0.08 |
0.291 |
0.0038 |
|
|
|
O |
0.075 |
0.025 |
2.56 |
0.012 |
0.0015 |
0.05 |
0.262 |
0.0031 |
0.021 |
|
|
P |
0.071 |
0.102 |
' 0.68 |
0.008 |
0.0021 |
0.03 |
0.283 |
0.0034 |
|
|
|
Q |
0.041 |
0.209 |
1.38 |
0.014 |
0.0017 |
0.07 |
0.256 |
0.0030 |
|
|
|
R |
0.052 |
0.498 |
1.26 |
0.016 |
0.0006 |
0.08 |
0.138 |
0.0041 |
|
|
|
S |
0.125 |
0.046 |
1.35 |
0.011 |
0.0026 |
0.07 |
0.450 |
0.0031 |
|
|
|
I |
0.208 |
0.517 |
1.30 |
0.015 |
0.0027 |
0.05 |
0.252 |
0.0039 |
|
|
|
U |
0.083 |
0.498 |
3.25 |
0.014 |
0.0030 |
0.06 |
0.301 |
0.0034 |
|
|
|
V |
0.091 |
0.045 |
1.32 |
0.011 |
0.0015 |
0.09 |
0.292 |
0.0025 |
0.028 |
|
|
W |
0.125 |
0.510 |
0.52 |
0.012 |
0.0021 |
0.05 |
0.225 |
0.0025 |
0.020 |
0.235 |
0.104 |
X |
0.082 |
0.025 |
1.30 |
0.010 |
0.0027 |
0.09 |
0.320 |
0.0037 |
|
|
|
Y |
0.078 |
0.201 |
1.58 |
0.015 |
0.0032 |
0.36 |
0.251 |
0.0021 |
|
|
|
Z |
0.088 |
0.950 |
1.72 |
0.008 |
0.0036 |
0.07 |
0.275 |
0.0036 |
|
|
|
[Table 1B]
Steel |
(Mass%: remainder is Fe and impurity) |
ex. C (T) |
SRT (°C) |
TR (°C) |
Cu |
Ni |
Cr |
W |
B |
Ca |
Mg |
REM |
Bi |
A |
|
|
|
|
|
|
|
|
|
0.007 |
1300 |
924 |
B |
|
|
|
|
|
|
|
|
|
0.009 |
1245 |
892 |
|
|
|
|
|
C |
|
|
|
|
0.078 |
1287 |
876 |
D |
|
|
|
|
|
|
|
|
|
0.009 |
1303 |
927 |
E |
|
|
|
|
|
|
|
|
|
0.007 |
1295 |
934 |
F |
|
|
|
|
|
|
|
|
|
0.005 |
1286 |
900 |
G |
|
|
|
|
|
|
|
|
|
0.007 |
1295 |
912 |
H |
0.06 |
0.04 |
|
|
|
|
|
|
|
0.009 |
1302 |
925 |
I |
|
|
0.20 |
|
|
|
|
|
|
0.008 |
1300 |
926 |
J |
|
|
|
|
|
0.0022 |
|
|
|
0.001 |
1285 |
921 |
K |
|
|
|
|
0.0015 |
|
|
|
|
0.010 |
1283 |
912 |
L |
|
|
|
|
|
|
0.0023 |
|
|
0.008 |
1282 |
912 |
M |
|
|
|
|
|
|
|
0.0021 |
|
0.007 |
1284 |
915 |
N |
|
|
|
|
|
|
|
|
0.0018 |
0.004 |
1283 |
917 |
O |
|
|
|
|
|
|
|
|
|
0.010 |
1276 ' |
918 |
P |
|
|
|
|
|
|
|
|
|
0.004 |
1278 |
914 |
Q |
|
|
|
|
|
|
|
|
|
-0.020 |
1220 |
904 |
R |
|
|
|
|
|
|
|
|
|
0.021 |
1186 ' |
858 |
S |
|
|
|
|
|
|
|
|
|
0.016 |
1371 |
978 |
T |
|
|
|
|
|
|
|
|
|
0.149 |
1365 |
902 |
U |
|
|
|
|
|
|
|
|
|
0.012 |
1298 |
921 |
V |
|
|
|
|
|
|
|
|
|
0.017 |
1303 |
934 |
W |
|
|
|
|
|
|
|
|
|
0.001 |
1309 |
903 |
X |
|
|
|
0.050 |
|
|
|
|
|
0.003 |
1302 |
928 |
Y |
0.10 |
0.06 |
|
|
|
0.0020 |
|
|
0.0015 |
0.018 |
1276 |
902 |
Z |
|
|
|
0.101 |
|
|
|
0.0025 |
|
0.017 |
1295 |
911 |
[Table 2A]
Test No. |
Steel |
Retention time at 700°C to 850°C during heating (seconds) |
Heating temperature (°C) |
Time from beginning of rough rolling to completion of finish rolling (seconds) |
Total rolling reduction in temperature range of 850°C to 1100° |
Finish rolling completion temperature FT (°C) |
Time from finish rolling to beginning of water cooling (seconds) |
1 |
A |
1137 |
1383 |
431 |
91 |
951 |
0.8 |
2 |
A |
1242 |
1375 |
330 |
95 |
957 |
2.4 |
3 |
A |
1132 |
1373 |
359 |
93 |
952 |
1.6 |
4 |
A |
1227 |
1380 |
320 |
95 |
954 |
1.0 |
5 |
A |
1145 |
1375 |
557 |
94 |
965 |
23 |
6 |
A |
834 |
1385 |
431 |
95 |
943 |
1.1 |
7 |
A |
1485 |
1228 |
446 |
95 |
927 |
0.9 |
8 |
A |
1518 |
1380 |
682 |
95 |
949 |
1.6 |
9 |
A |
1562 |
1373 |
469 |
83 |
932 |
1.1 |
10 |
A |
1469 |
1380 |
314 |
94 |
847 |
0.8 |
11 |
A |
1084 |
1380 |
328 |
93 |
958 |
4.1 |
12 |
A |
1202 |
1376 |
321 |
93 |
942 |
0.9 |
13 |
A |
1010 |
1365 |
305 |
93 |
940 |
1.2 |
14 |
A |
989 |
1383 |
337 |
94 |
941 |
1.2 |
15 |
A |
993 |
1365 |
342 |
94 |
951 |
1.3 |
16 |
B |
1120 |
1348 |
418 |
95 |
902 |
1.3 |
17 |
C |
1463 |
1350 |
322 |
94 |
905 |
1.1 |
18 |
D |
1272 |
1380 |
353 |
95 |
977 |
13 |
19 |
E |
1532 |
1350 |
308 |
94 |
982 |
1.4 |
20 |
F |
1070 |
1350 |
418 |
92 |
918 |
0.9 |
21 |
G |
1285 |
1350 |
402 |
93 |
922 |
1.3 |
22 |
H |
1142 |
1350 |
390 |
93 |
953 |
1.4 |
23 |
I |
1052 |
1350 |
261 |
94 |
939 |
1.2 |
24 |
J |
1317 |
1350 |
355 |
96 |
953 |
1.2 |
25 |
K |
1194 |
1350 |
317 |
94 |
931 |
1.3 |
26 |
L |
1500 |
1350 |
263 |
95 |
936 |
1.4 |
27 |
M |
1336 |
1350 |
344 |
93 |
933 |
1.3 |
28 |
N |
1563 |
1350 |
378 |
95 |
944 |
1.6 |
29 |
O |
1133 |
1350 |
281 |
93 |
937 |
1.5 |
30 |
P |
1484 |
1350 |
408 |
95 |
946 |
1.2 |
31 |
Q |
1310 |
1350 |
362 |
94 |
920 |
1.4 |
32 |
R |
1148 |
1350 |
391 |
93 |
887 |
1.1 |
33 |
S |
1102 |
1380 |
375 |
93 |
989 |
1.6 |
34 |
T |
978 |
1380 |
276 |
93 |
923 |
1.3 |
35 |
U |
1210 |
1350 |
301 |
93 |
956 |
1.8 |
36 |
V |
1023 |
1350 |
305 |
93 |
964 |
1.5 |
37 |
w |
1065 |
1380 |
318 |
93 |
952 |
1.8 |
37 |
X |
1253 |
1380 |
400 |
93 |
942 |
1.2 |
38 |
Y |
1546 |
1350 |
293 |
94 |
963 |
0.7 |
39 |
Z |
1032 |
1350 |
284 |
92 |
951 |
0.5 |
[Table 2B]
Test No. |
Cooling rate from beginning of water cooling after comletion of finish rolling to
water cooling stop temperature of 650°C to 850°C (°C/s) |
Retention time at 650°C to 800°C (seconds) |
Average cooling rate from 550°C to 650°C (°C/s) |
Coiling temperature (°C) |
SRT (°C) |
TR (°C) |
1 |
87 |
7 |
88 |
402 |
1300 |
924 |
2 |
80 |
18 |
89 |
450 |
1300 |
924 |
3 |
54 |
12 |
68 |
443 |
1300 |
924 |
4 |
75 |
11 |
69 |
456 |
1300 |
924 |
5 |
71 |
12 |
62 |
489 |
1300 |
924 |
6 |
77 |
12 |
57 |
415 |
1300 |
924 |
7 |
60 |
12 |
62 |
451 |
1300 |
924 |
8 |
56 |
12 |
56 |
431 |
1300 |
924 |
9 |
60 |
16 |
66 |
426 |
1300 |
924 |
10 |
58 |
16 |
64 |
476 |
1300 |
924 |
11 |
62 |
12 |
51 |
431 |
1300 |
924 |
12 |
12 |
15 |
80 |
425 |
1300 |
924 |
13 |
68 |
3 |
71 |
425 |
1300 |
924 |
14 |
59 |
14 |
0.1 |
625 |
1300 |
924 |
15 |
65 |
14 |
1 |
456 |
1300 |
924 |
16 |
50 |
12 |
69 |
482 |
1245 |
892 |
17 |
72 |
16 |
94 |
465 |
1287 |
876 |
18 |
57 |
11 |
66 |
395 |
1303 |
927 |
19 |
77 |
15 |
76 |
415 |
1295 |
934 |
20 |
80 |
13 |
80 |
426 |
1286 |
900 |
21 |
87 |
14 |
87 |
457 |
1295 |
912 |
22 |
88 |
12 |
77 |
432 |
1302 |
925 |
23 |
71 |
11 |
71 |
419 |
1300 |
926 |
24 |
76 |
18 |
66 |
427 |
1285 |
921 |
25 |
79 |
14 |
85 |
461 |
1283 |
912 |
26 |
81 |
12 |
68 |
428 |
1282 |
912 |
27 |
68 |
16 |
69 |
409 |
1284 |
915 |
28 |
83 |
15 |
102 |
428 |
1283 |
917 |
29 |
52 |
35 |
59 |
437 |
1276 |
918 |
30 |
59 |
11 |
79 |
451 |
1278 |
914 |
31 |
82 |
11 |
97 |
468 |
1220 |
904 |
32 |
60 |
13 |
81 |
456 |
1186 |
858 |
33 |
72 |
13 |
62 |
451 |
1371 |
978 |
34 |
74 |
40 |
66 |
465 |
1365 |
902 |
35 |
55 |
45 |
59 |
468 |
1298 |
921 |
36 |
68 |
14 |
84 |
410 |
1303 |
934 |
37 |
80 |
13 |
89 |
459 |
1309 |
903 |
37 |
85 |
15 |
92 |
432 |
1302 |
928 |
38 |
65 |
14 |
54 |
410 |
1276 |
902 |
39 |
70 |
16 |
58 |
489 |
1295 |
911 |
[0111] Regarding the obtained steel sheets (the hot-rolled steel sheets and the plated steel
sheets), the microstructures at the 1/4 depth positions of the sheet thicknesses from
the surfaces of the steel sheets were observed, and the area fractions of individual
structures, the average crystal grain diameters and average aspect ratios of the crystal
grains having a bcc structure and the standard deviations of the Mn concentrations
were obtained.
[0112] The area fractions of the microstructure at the 1/4 depth position of the sheet thickness
from the surface of the steel sheet, the average crystal grain diameter and average
aspect ratio of the crystal grains having a bcc structure were obtained by the scanning
electron microscopic (SEM) observation and electron back scattering diffraction (EBSD)
analysis of the microstructure at the 1/4 depth position of the sheet thickness from
the surface of the steel sheet of a cross section of the steel sheet parallel to a
rolling direction and the sheet thickness direction using an EBSD analyzer composed
of a thermal field emission scanning electron microscope and an EBSD detector.
[0113] At that time, in a region that was 200 µm long in the rolling direction and 100 µm
long in the sheet thickness direction and had the 1/4 depth position of the sheet
thickness from the surface of the steel sheet at the center, crystal orientation information
was acquired at 0.2 µm intervals while differentiating fcc and bcc. Crystal grain
boundaries having a crystal orientation difference of 15° or more were specified using
the software attached to the EBSD analyzer ("OIM Analysis (registered trademark)"
manufactured by AMETEK, Inc.). Regarding the average crystal grain diameter of bcc,
the average crystal grain diameter was obtained by defining a region that was surrounded
by crystal grain boundaries having a crystal orientation difference of 15° or more,
was identified as bcc and had a circle equivalent diameter of 0.3 µm or more as a
crystal grain.
[0114] The area fraction of ferrite was measured by the following method.
[0115] A region that was surrounded by crystal grain boundaries having a crystal orientation
difference of 5° or more, was identified as bcc and had a circle equivalent diameter
of 0.3 µm or more was defined as a crystal grain. Among such crystal grains, for crystal
grains for which a value that was obtained by an analysis with Grain Average Misorientation
analysis equipped in OIM Analysis (GAM value) was 0.6° or less, the area fraction
was calculated.
[0116] The area fraction of pearlite and cementite was obtained by the point counting method
with lattice spacings of 5 µm after the microstructure revealed by Nital etching in
a region at the 1/4 depth position of the sheet thickness from the surface of the
steel sheet was observed at three visual fields using a SEM at a magnification of
1000 times. In addition, the area fraction of MA was obtained by the point counting
method with lattice spacings of 5 µm after the structure revealed by LePera etching
in the region at the 1/4 depth position of the sheet thickness from the surface of
the steel sheet was observed at two visual fields using an optical microscope at a
magnification of 500 times.
[0117] While not shown in the table, the remainders of the microstructures were bainite.
[0118] The standard deviation of the Mn concentration was obtained by mirror-polishing a
cross section of the steel sheet parallel to the rolling direction and the sheet thickness
direction and then measuring the 1/4 depth position of the sheet thickness from the
surface of the steel sheet with an electron probe microanalyzer (EPMA). As the measurement
conditions, the acceleration voltage was set to 15 kV, the magnification was set to
5000 times, and a distribution image in a range that was 20 µm long in the sample
rolling direction and 20 µm long in the sample sheet thickness direction was measured.
More specifically, the measurement intervals were set to 0.1 µm, and the Mn concentrations
at 40000 or more sites were measured. Next, the standard deviation was calculated
based on the Mn concentrations obtained from all of the measurement points, thereby
obtaining the standard deviation of the Mn concentration.
[0119] In order to evaluate the mechanical properties of the obtained steel sheets, the
tensile strengths TS (MPa) and the total elongations at fracture El (%) were measured
according to JIS Z 2241: 2011. In addition, the limiting hole expansion ratios (λ)
were measured according to JIS Z 2256: 2010.
[0120] The bending workability was evaluated by a 90° V bend test in which the bend radius
was set to twice the sheet thickness.
[0121] Table 3A and Table 3B show the microstructures and the test results of the mechanical
properties.
[0122] The tensile strength was evaluated as a high strength in a case where the tensile
strength was 980 MPa or more.
[0123] The elongation was evaluated as excellent in a case where the product of the tensile
strength and the total elongation at fracture (TS × El) was 14000 MPa·% or more. In
addition, in a case where TS × λ was 50000 MPa·% or more, the stretch flangeability
was evaluated as excellent. The bending workability was evaluated as excellent bending
workability (OK) when cracking did not occur in all test pieces during the bend test
performed three times and evaluated as insufficient bending workability (NG) when
cracking occurred in one or more test pieces.
[Table 3A]
Test No. |
Steel |
Microstructure |
Ferrite area fraction (%) |
MA area fraction (%) |
Pearlite and cementite area fraction (%) |
Average crystal grain diameter (µm) |
Average aspect ratio |
Standard deviation of Mn concentration (mass%) |
Coherent/semi-coherent |
1 |
A |
62 |
2 |
0 |
6.2 |
0.61 |
0.52 |
Semi-coherent |
2 |
A |
95 |
0 |
0 |
5.8 |
0.60 |
0.42 |
Semi-coherent |
3 |
A |
77 |
0 |
0 |
6.5 |
0.59 |
0.45 |
Semi-coherent |
4 |
A |
74 |
0 |
0 |
6.1 |
0.61 |
0.43 |
Semi-coherent |
5 |
A |
71 |
0 |
0 |
6.7 |
0.58 |
0.43 |
Semi-coherent |
6 |
A |
77 |
1 |
0 |
5.2 |
0.56 |
0.71 |
Semi-coherent |
7 |
A |
83 |
0 |
0 |
6.2 |
0.41 |
0.65 |
Semi-coherent |
8 |
A |
74 |
0 |
0 |
6.2 |
0.58 |
0.27 |
Semi-coherent |
9 |
A |
76 |
0 |
0 |
6.0 |
0.51 |
0.64 |
Semi-coherent |
10 |
A |
90 |
0 |
0 |
7.3 |
0.12 |
0.31 |
Semi-coherent |
11 |
A |
78 |
0 |
0 |
10.5 |
0.66 |
0.45 |
Semi-coherent |
12 |
A |
89 |
0 |
0 |
10.3 |
0.56 |
0.45 |
Semi-coherent |
13 |
A |
5 |
5 |
0 |
8.5 |
0.54 |
0.46 |
Semi-coherent |
14 |
A |
99 |
0 |
1 |
8.1 |
0.58 |
0.40 |
Coherent |
15 |
A |
84 |
0 |
0 |
8.6 |
0.58 |
0.42 |
Coherent |
16 |
B |
84 |
0 |
0 |
6.1 |
0.56 |
0.34 |
Semi-coherent |
17 |
C |
69 |
1 |
6 |
5.4 |
0.56 |
0.28 |
Semi-coherent |
18 |
D |
78 |
0 |
0 |
6.1 |
0.68 |
0.29 |
Semi-coherent |
19 |
E |
89 |
0 |
0 |
5.6 |
0.65 |
0.22 |
Semi-coherent |
20 |
F |
80 |
0 |
0 |
5.3 |
0.62 |
0.45 |
Semi-coherent |
21 |
G |
81 |
0 |
0 |
6.3 |
0.58 |
0.42 |
Semi-coherent |
22 |
H |
79 |
0 |
0 |
5.8 |
0.60 |
0.41 |
Semi-coherent |
23 |
I |
78 |
0 |
0 |
6.8 |
0.56 |
0.38 |
Semi-coherent |
24 |
J |
100 |
0 |
0 |
6.3 |
0.57 |
0.33 |
Semi-coherent |
25 |
K |
85 |
0 |
0 |
6.0 |
0.59 |
0.42 |
Semi-coherent |
26 |
L |
81 |
0 |
0 |
7.0 |
0.62 |
0.26 |
Semi-coherent |
27 |
M |
91 |
0 |
0 |
5.8 |
0.62 |
0.30 |
Semi-coherent |
28 |
N |
90 |
0 |
0 |
5.6 |
0.59 |
0.31 |
Semi-coherent |
29 |
O |
78 |
0 |
0 |
6.1 |
0.61 |
0.39 |
Semi-coherent |
30 |
P |
98 |
0 |
0 |
5.6 |
0.56 |
0.29 |
Semi-coherent |
31 |
Q |
79 |
0 |
0 |
4.5 |
0.63 |
0.48 |
Semi-coherent |
32 |
R |
90 |
0 |
0 |
4.2 |
0.62 |
0.42 |
Semi-coherent |
33 |
S |
79 |
0 |
0 |
7.4 |
0.60 |
0.35 |
Semi-coherent |
34 |
T |
66 |
6 |
9 |
5.4 |
0.63 |
0.46 |
Semi-coherent |
35 |
U |
32 |
4 |
0 |
5.9 |
0.60 |
0.55 |
Semi-coherent |
36 |
V |
85 |
4 |
0 |
5.8 |
0.54 |
0.47 |
Semi-coherent |
37 |
W |
80 |
1 |
0 |
6.4 |
0.56 |
0.50 |
Semi-coherent |
37 |
X |
90 |
0 |
0 |
5.4 |
0.52 |
0.35 |
Semi-coherent |
38 |
Y |
78 |
0 |
1 |
6.2 |
0.63 |
0.31 |
Semi-coherent |
39 |
Z |
91 |
0 |
1 |
6.8 |
0.55 |
0.44 |
Semi-coherent |
[Table 3B]
Test No. |
Characteristics |
Plating |
Note |
TS (MPa) |
El (%) |
λ (%) |
TS × El (MPa·%) |
TS × λ (MPa-%) |
Bending workability |
1 |
1210 |
11.9 |
46 |
14399 |
55660 |
OK |
- |
Invention Example |
2 |
1120 |
15.7 |
78 |
17584 |
87360 |
OK |
- |
Invention Example |
3 |
1194 |
13.2 |
55 |
15761 |
65670 |
OK |
GI |
Invention Example |
4 |
1182 |
13.5 |
53 |
15957 |
62646 |
OK |
GA |
Invention Example |
5 |
1052 |
14.2 |
55 |
14938 |
57860 |
OK |
- |
Invention Example |
6 |
1192 |
13.1 |
55 |
15615 |
65560 |
NG |
- |
Comparative Example |
7 |
845 |
18.0 |
71 |
15210 |
59995 |
NG |
- |
Comparative Example |
8 |
954 |
16.1 |
69 |
15359 |
65826 |
NG |
- |
Comparative Example |
9 |
1180 |
13.2 |
67 |
15576 |
79060 |
NG |
- |
Comparative Example |
10 |
1085 |
15.1 |
35 |
16384 |
37975 |
NG |
- |
Comparative Example |
11 |
985 |
15.5 |
60 |
15268 |
59100 |
NG |
- |
Comparative Example |
12 |
932 |
17.5 |
75 |
16310 |
69900 |
OK |
- |
Comparative Example |
13 |
958 |
10.5 |
65 |
10059 |
62270 |
OK |
- |
Comparative Example |
14 |
1185 |
14.2 |
28 |
16827 |
33180 |
OK |
- |
Comparative Example |
15 |
1199 |
14.4 |
36 |
17266 |
43164 |
OK |
- |
Comparative Example |
16 |
1008 |
16.3 |
73 |
16430 |
73584 |
OK |
- |
Invention Example |
17 |
1025 |
14.2 |
35 |
14555 |
35875 |
OK |
- |
Comparative Example |
18 |
1308 |
12.8 |
51 |
16742 |
66708 |
OK |
- |
Invention Example |
19 |
1170 |
14.9 |
68 |
17464 |
79531 |
OK |
- |
Invention Example |
20 |
1138 |
14.5 |
60 |
16503 |
68286 |
OK |
- |
Invention Example |
21 |
1158 |
14.5 |
61 |
16791 |
70638 |
OK |
- |
Invention Example |
22 |
1214 |
13.9 |
56 |
16879 |
68000 |
OK |
- |
Invention Example |
23 |
1195 |
14.2 |
57 |
16963 |
68090 |
OK |
- |
Invention Example |
24 |
1204 |
15.1 |
82 |
18178 |
98717 |
OK |
- |
Invention Example |
25 |
1211 |
14.2 |
61 |
17200 |
73886 |
OK |
- |
Invention Example |
26 |
1096 |
15.3 |
63 |
16767 |
69040 |
OK |
- |
Invention Example |
27 |
1166 |
15.2 |
70 |
17717 |
81592 |
OK |
- |
Invention Example |
28 |
1082 |
15.9 |
74 |
17192 |
80088 |
OK |
- |
Invention Example |
29 |
1298 |
12.2 |
51 |
15839 |
66214 |
OK |
- |
Invention Example |
30 |
1033 |
17.1 |
88 |
17671 |
90938 |
OK |
- |
Invention Example |
31 |
865 |
18.2 |
75 |
15743 |
64875 |
OK |
- |
Comparative Example |
32 |
825 |
19.2 |
92 |
15840 |
75900 |
OK |
- |
Comparative Example |
33 |
942 |
15.0 |
54 |
14130 |
50868 |
NG |
- |
Comparative Example |
34 |
1002 |
15.2 |
23 |
15230 |
23046 |
NG |
- |
Comparative Example |
35 |
1160 |
11.5 |
45 |
13340 |
52200 |
OK |
- |
Comparative Example |
36 |
1189 |
14.5 |
44 |
17241 |
52316 |
OK |
- |
Invention Example |
37 |
1265 |
13.1 |
55 |
16572 |
69575 |
OK |
- |
Invention Example |
37 |
1212 |
14.9 |
58 |
18059 |
70296 |
OK |
- |
Invention Example |
38 |
1181 |
13.1 |
54 |
15471 |
63774 |
OK |
- |
Invention Example |
39 |
1189 |
13.9 |
52 |
16527 |
61828 |
OK |
- |
Invention Example |
[0124] As shown in Table 3A and Table 3B, in the invention examples where the requirements
of the present invention were satisfied, all of TS, TS × El and the bending workability
were excellent. On the other hand, in the comparative example where at least one of
the requirements of the present invention was not satisfied, at least one of TS, TS
× El and the bending workability was poor.
[Industrial Applicability]
[0125] According to the present invention, it is possible to provide a steel sheet having
a high strength and being excellent in terms of elongation, stretch flangeability
and bending workability. The steel sheet of the present invention is preferable as
a material that is used in uses for automobiles, home appliances, mechanical structures,
construction and the like, and, in particular, when the steel sheet is used as a material
for components such as inner sheet members, structural members, suspension members,
and the like of automobiles, not only is a contribution made to weight reduction in
vehicle bodies and improvement in impact resistance but the steel sheet is also easily
worked into component shapes. Therefore, the steel sheet of the present invention
makes an extreme industrial contribution.