TECHNICAL FIELD
[0001]
The present invention relates to a steel sheet and a method for producing the steel
sheet.
BACKGROUND ART
[0002]
In general, when the strength of a steel sheet is increased, elongation is reduced
and the formability of the steel sheet may decrease. Therefore, in order to use a highstrength
steel sheet as a component for a body of an automobile, it is necessary to increase
both strength and formability which are mutually conflicting properties, that is, a more
excellent strength-ductility balance is required. Further, a high-strength steel sheet to be
used for vehicle body components is required to have excellent bendability and impact
characteristics. Therefore, as the mechanical properties of such a steel sheet, the steel
sheet is required to have high strength and excellent formability, and also have excellent
bendability and impact characteristics.
[0003]
In order to improve elongation that affects the formability, thus far a so-called
"medium Mn steel" has been proposed in which Mn is positively added so as to contain
about 5% by mass of Mn in the steel sheet and form retained austenite in the steel, and
utilize the transformation induced plasticity thereof (for example, Non-Patent Document
1).
[0004]
Further, Patent Document 1 proposes steel in which Mn is contained in an
amount ranging from 2.6% or more to 4.2% or less. Since the aforementioned steel also
contains more Mn than general high-strength steel, retained austenite is easily formed and
2
the elongation is high, and the steel is even more excellent in bendability.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0005]
Patent Document 1: WO 2017/183348
NON PATENT DOCUMENT
[0006]
Non-Patent Document 1: Furukawa Takashi and Matsumura Osamu, Heat
Treatment, Japan, the Japan Society for Heat Treatment, 1997, vol. no. 37, no. 4, p. 204
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007]
However, because the content of Mn in the steel sheet disclosed in Non-Patent
Document 1 is high, the production cost increases. Therefore, a steel sheet having less
alloy elements and which, while having a smaller content of Mn, has both strength and
elongation in a compatible manner, and also has impact characteristics is needed.
[0008]
An objective of the present invention is to solve the problem described above
and provide a steel sheet which has high strength and is excellent in strength-ductility
balance, bendability and impact characteristics.
SOLUTION TO PROBLEM
[0009]
The present invention has been made to solve the problem described above, and
the gist of the present invention is a steel sheet and a method for producing the steel sheet
described in the following.
[0010]
(1) A steel sheet, having a chemical composition consisting of, in mass%:
3
C: more than 0.18% and less than 0.30%,
Si: 0.01% or more and less than 2.00%,
Mn: more than 2.50% and 4.00% or less,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
Cr: 0% or more and less than 2.00%,
Mo: 0 to 2.00%,
W: 0 to 2.00%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Zr: 0 to 0.010%,
REM: 0 to 0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and
the balance: Fe and impurities,
wherein:
in a cross section parallel to a rolling direction and a sheet thickness direction of
the steel sheet, a steel micro-structure at a position at a depth of 1/4 of a sheet thickness
from a surface is, in area%:
tempered martensite: 25 to 90%,
4
ferrite: 5% or less,
retained austenite: 10 to 50%, and
bainite: 5% or less;
at a position at a depth of 1/4 of the sheet thickness from a surface of a cross
section parallel to the rolling direction and the sheet thickness direction of the steel sheet,
a proportion of a total area of retained austenite grains having an area of 1 m2 or more
and having a grain shape circularity of 0.1 or more is less than 50% with respect to an
entire area of the retained austenite; and
an Mn concentration in the steel micro-structure at a position at a depth of 1/4 of
the sheet thickness from the surface satisfies formula (i) below:
CMn/CMn  1.2 ...(i)
where, meaning of each symbol in formula (i) above is as follows:
CMn: average Mn concentration (mass%) in retained austenite
CMn: average Mn concentration (mass%) in ferrite and tempered martensite.
[0011]
(2) The steel sheet according to (1) above, wherein:
the chemical composition contains, in mass%,
Si: 0.10% or more and less than 2.00%; and
an Si concentration in the steel micro-structure at a position at a depth of 1/4 of
the sheet thickness from the surface of a cross section parallel to the rolling direction and
sheet thickness direction of the steel sheet satisfies formula (ii) below:
CSi/CSi  1.1 ...(ii)
where, meaning of each symbol in formula (ii) above is as follows:
CSi: average Si concentration (mass%) in tempered martensite and ferrite
CSi: average Si concentration (mass%) in retained austenite.
[0012]
(3) The steel sheet according to (1) or (2) above, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
Cr: 0.01% or more and less than 2.00%,
5
Mo: 0.01 to 2.00%,
W: 0.01 to 2.00%,
Cu: 0.01 to 2.00%, and
Ni: 0.01 to 2.00%.
[0013]
(4) The steel sheet according to any one of (1) to (3) above, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%, and
V: 0.005 to 0.300%.
[0014]
(5) The steel sheet according to any one of (1) to (4) above, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
B: 0.0001 to 0.010%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%, and
REM: 0.0001 to 0.010%.
[0015]
(6) The steel sheet according to any one of (1) to (5) above, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
Sb: 0.0005 to 0.050%,
Sn: 0.0005 to 0.050%, and
Bi: 0.0005 to 0.050%.
[0016]
(7) The steel sheet according to any one of (1) to (6) above, having:
a hot dip galvanized layer on a surface of the steel sheet.
6
[0017]
(8) The steel sheet according to any one of (1) to (6) above, having:
a hot dip galvannealed layer on a surface of the steel sheet.
[0018]
(9) The steel sheet according to any one of (1) to (8) above, wherein:
a Charpy impact value at 20C is 20 J/cm2 or more.
[0019]
(10)A method for producing a steel sheet, that is a method in which a hot rolling
process, a cold rolling process, a primary annealing process and a secondary annealing
process are performed in that order on steel having a chemical composition according to
(1) or any one of (3) to (6) above, wherein:
the hot rolling process includes a process of performing finish hot-rolling using
a rolling mill having a plurality of stands, the number of which is four or more;
in the process of performing finish hot-rolling:
a sheet thickness reduction between before and after a last four stands among the
plurality of stands satisfies formula (iii) below,
a strain rate at a final stand of the last four stands and a rolling exit side
temperature at the final stand satisfy formula (iv) below, and
cooling to 750C at an average cooling rate of 100C/s or more is performed
within 1.0 s after rolling at the final stand, and cooling in a temperature range from 750C
to 300C is performed at an average cooling rate of 10C/s or more;
in the primary annealing process, after holding for 10 to 1000 s in a temperature
region of more than 750C and an Ac3 point or more, cooling is performed to a
temperature region of 300C or less under a condition that an average cooling rate to
300C is 10 to 2000C/s, and thereafter cooling is performed to a temperature region of
less than 100C; and
in the secondary annealing process, heating is performed to a temperature region
of 650C or more and less than the Ac3 point at an average heating rate of 1 to 40C/s,
and is held in the temperature region for 300 s or more:
1.2  ln(t0/t)  2.8 ...(iii)
7
11.0  log(vexp(33000/(273 + T)))  15.0 ...(iv)
where, meaning of each symbol in the above formulas is as follows:
t0: sheet thickness (mm) immediately before entering last four stands
t: sheet thickness (mm) immediately after exiting from last four stands
v: strain rate (/s) at final stand
T: rolling exit side temperature (C) at final stand.
[0020]
(11) The method for producing a steel sheet according to (10) above, wherein:
in the primary annealing process, after holding for 10 s or more in a temperature
region of more than 750C and the Ac3 point or more, cooling is performed at an average
cooling rate of 10 to 2000C/s to a temperature region of 300C or less, and after holding
for 10 to 1000 s in a temperature region of 100 to 300C, cooling is performed to a
temperature region of less than 100C.
[0021]
(12) The method for producing a steel sheet according to (10) or (11) above,
wherein:
after the secondary annealing process, cooling is performed, and a hot dip
galvanizing treatment is performed.
[0022]
(13) The method for producing a steel sheet according to (12) above, wherein:
after the hot dip galvanizing treatment is performed, a hot dip galvanized layer
is subjected to an alloying treatment in a temperature region of 450 to 620C.
ADVANTAGEOUS EFFECT OF INVENTION
[0023]
According to the present invention, a steel sheet can be obtained that has high
strength and is excellent in strength-ductility balance, bendability and impact
characteristics.
DESCRIPTION OF EMBODIMENT
8
[0024]
Hereunder, the respective requirements of the present invention are described in
detail.
[0025]
(A) Chemical Composition
The reasons for limiting each element are as follows. Note that, the symbol
"%" with respect to content in the following description means "mass%".
[0026]
C: more than 0.18% and less than 0.30%
C is a necessary element for raising the strength of the steel and stabilizing
retained austenite. On the other hand, if C is excessively contained, it becomes difficult
to maintain the weldability of the steel sheet. Therefore, the content of C is set to more
than 0.18% and less than 0.30%. The content of C is preferably 0.20% or more.
Further, the content of C is preferably 0.28% or less, and more preferably is 0.25% or less.
[0027]
Si: 0.01% or more and less than 2.00%
Si is an effective element for strengthening tempered martensite, making the
micro-structure uniform, and improving the strength-ductility balance. Further, Si also
has an action that suppresses the precipitation of cementite and promotes formation of
retained austenite. On the other hand, if Si is excessively contained, it becomes difficult
to maintain the plating properties and chemical treatment properties of the steel sheet.
Therefore, the content of Si is set to 0.01% or more and less than 2.00%. In order to
satisfy the aforementioned formula (ii) and further improve the impact characteristics of
the steel sheet, the content of Si is preferably 0.10% or more, more preferably is 0.20%
or more, further preferably is 0.40% or more, and further preferably is 0.70% or more.
Further, the content of Si is preferably 1.80% or less, and more preferably is 1.60% or
less.
[0028]
Mn: more than 2.50% and 4.00% or less
Mn is an element that stabilizes austenite. Further, in the present invention, Mn
9
is distributed throughout the austenite and stabilizes the austenite more. By making the
content of Mn 4.00% or less, non-uniformity of the content of Mn due to solidification
segregation can be lessened. Furthermore, even if Mn concentrates in retained austenite,
because non-uniformity of the strength in the microstructure is also lessened without the
steel sheet markedly hardening, the bendability can be improved. In addition, by
reducing the content of Mn, it is possible to keep down the production cost of the steel
sheet. Therefore, the content of Mn is set to more than 2.50% and 4.00% or less. The
content of Mn is preferably more than 3.00%, and more preferably is more than 3.20%.
Further, the content of Mn is preferably less than 3.85%, and more preferably is less than
3.70%.
[0029]
Sol. Al: 0.001% or more and less than 3.00%
Al is a deoxidizer, and also has an action that expands the temperature range of
a two-phase region during annealing and raises the material quality consistency.
Although the greater the content of Al, the greater the effect thereof, if Al is excessively
contained, it will be difficult to maintain the surface properties, coating properties, and
weldability. Therefore, the content of sol. Al is set to 0.001% or more and less than
3.00%. The content of sol. Al is preferably 0.005% or more, more preferably is 0.010%
or more, and further preferably is 0.020% or more. Furthermore, the content of sol. Al
is preferably 2.50% or less, and more preferably 1.80% or less. Note that, in the present
description the term "sol. Al" means "acid-soluble Al".
[0030]
P: 0.100% or less
P is an impurity, and lowers the weldability of the steel material. Therefore,
the content of P is set to 0.100% or less. The content of P is preferably 0.050% or less,
more preferably is 0.030% or less, and further preferably is 0.020% or less. Whilst it is
not particularly necessary to define a lower limit of the content of P, from the viewpoint
of suppressing an increase in the refining cost, the lower limit of the content of P may be
set to 0.001% or more, although the lower the content of P is, the more preferable it is.
[0031]
10
S: 0.010% or less
S is an impurity, and as the result of hot rolling, elongated MnS is formed and
reduces the bendability and hole expandability. Therefore, the content of S is set to
0.010% or less. The content of S is preferably 0.007% or less, and more preferably is
0.003% or less. Whilst it is not particularly necessary to define a lower limit of the
content of S, from the viewpoint of suppressing an increase in the refining cost, the lower
limit of the content of S may be set to 0.001% or more, although the lower the content of
S is, the more preferable it is.
[0032]
N: less than 0.050%
N is an impurity, and if the steel sheet contains N in an amount of 0.050% or
more, the low-temperature toughness decreases. Therefore, the content of N is set to
less than 0.050%. The content of N is preferably 0.010% or less, and more preferably
is 0.006% or less. Whilst it is not particularly necessary to define a lower limit of the
content of N, from the viewpoint of suppressing an increase in the refining cost, the lower
limit of the content of N may be set to 0.003% or more, although the lower the content of
N is, the more preferable it is.
[0033]
O: less than 0.020%
O is an impurity, and if the steel sheet contains O in an amount of 0.020% or
more, the elongation decreases. Therefore, the content of O is set to less than 0.020%.
The content of O is preferably 0.010% or less, more preferably is 0.005% or less, and
further preferably is 0.003% or less. Whilst it is not particularly necessary to define a
lower limit of the content of O, from the viewpoint of suppressing an increase in the
refining cost, the lower limit is preferably set to 0.001% or more.
[0034]
In addition to the elements mentioned above, one or more kinds of element
selected from Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and Bi may also
be contained in the steel sheet of the present invention.
[0035]
11
Cr: 0% or more and less than 2.00%
Mo: 0 to 2.00%
W: 0 to 2.00%
Cu: 0 to 2.00%
Ni: 0 to 2.00%
Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet,
and therefore one or more kinds selected from these elements may be contained. On the
other hand, if these elements are excessively contained, surface defects are liable to occur
during hot rolling, and furthermore, the strength of the hot-rolled steel sheet may become
too high and the cold rolling properties may deteriorate. Therefore, the content of Cr is
set to less than 2.00%, the content of Mo to 2.00% or less, the content of W to 2.00% or
less, the content of Cu to 2.00% or less, and the content of Ni to 2.00% or less.
[0036]
The content of Cr is preferably 1.50% or less, 1.00% or less, 0.60% or less, or
less than 0.30%. The content of Mo is preferably 1.50% or less, 1.00% or less, 0.50%
or less, or 0.10% or less. The content of W is preferably 1.50% or less, 1.00% or less,
0.50% or less, or 0.10% or less. The content of Cu is preferably 1.50% or less, 1.00%
or less, 0.60% or less, or 0.20% or less. The content of Ni is preferably 1.50% or less,
1.00% or less, or 0.50% or less. In order to more reliably obtain the effect produced by
the aforementioned action of these elements, it is preferable to contain at least any one of
the aforementioned elements in an amount of 0.01% or more.
[0037]
Ti: 0 to 0.300%
Nb: 0 to 0.300%
V: 0 to 0.300%
Ti, Nb, and V form fine carbides, nitrides, or carbo-nitrides, and are thus
effective for improving the strength of the steel sheet. Therefore, one or more kinds of
element selected from these elements may be contained in the steel sheet. On the other
hand, if these elements are excessively contained, in some cases the strength of the hotrolled
steel sheet increases too much and the cold rolling properties decrease. Therefore,
12
the content of Ti is set to 0.300% or less, the content of Nb to 0.300% or less, and the
content of V to 0.300% or less. The content of each of Ti, Nb, and V is preferably
0.200% or less, 0.100% or less, 0.060% or less, or 0.030% or less. In order to more
reliably obtain the effect produced by the aforementioned action of these elements, it is
preferable to contain at least any one of the aforementioned elements in an amount of
0.005% or more.
[0038]
B: 0 to 0.010%
Ca: 0 to 0.010%
Mg: 0 to 0.010%
Zr: 0 to 0.010%
REM: 0 to 0.010%
B, Ca, Mg, Zr, and REM (rare earth metal) improve the local ductility and hole
expandability of the steel sheet. Therefore, one or more kinds selected from these
elements may be contained. On the other hand, if excessively contained, these elements
may cause the workability of the steel sheet to decrease. Therefore, the content of B is
set to 0.010% or less, the content of Ca to 0.010% or less, the content of Mg to 0.010%
or less, the content of Zr to 0.010% or less, and the content of REM to 0.010% or less.
[0039]
The content of each of B, Ca, Mg, Zr, and REM is preferably 0.008% or less,
0.006% or less, or 0.003% or less. Further, while it suffices that the total content of one
or more kinds of element selected from B, Ca, Mg, Zr, and REM is 0.050% or less, the
total content is preferably 0.030% or less. In order to more reliably obtain the effect
produced by the aforementioned actions of these elements, it is preferable to contain at
least any one of the aforementioned elements in an amount of 0.0001% or more, and more
preferably in an amount of 0.001% or more.
[0040]
Here, REM refers to a total of 17 kinds of element including Sc, Y, and
lanthanoids, and the term "content of REM" refers to the total content of these elements.
In the case of lanthanoids, the lanthanoids are added industrially in the form of misch
13
metal.
[0041]
Sb: 0 to 0.050%
Sn: 0 to 0.050%
Bi: 0 to 0.050%
Sb, Sn, and Bi keep easily oxidizable elements such as Mn, Si, and/or Al in the
steel sheet from diffusing at the steel sheet surface and forming oxides, and thereby
improve the surface properties and plating properties of the steel sheet. Therefore, one
or more kinds selected from these elements may be contained. However, even if these
elements are contained in excess, the effect obtained by the aforementioned action will
be saturated. Therefore, the content of Sb is set to 0.050% or less, the content of Sn to
0.050% or less, and the content of Bi to 0.050% or less. The content of each of Sb, Sn,
and Bi is preferably 0.030% or less, 0.010% or less, 0.006% or less, or 0.003% or less.
In order to more reliably obtain the effect produced by the aforementioned action of these
elements, it is preferable to contain at least any one of the aforementioned elements in an
amount of 0.0005% or more, and more preferably in an amount of 0.001% or more.
[0042]
The balance in the chemical composition of the steel sheet of the present
invention is Fe and impurities. Here, the term "impurities" refers to components which,
during industrial production of the steel material, are mixed in from a raw material such
as ore or scrap or due to various causes during the production processes, and which are
allowed within a range that does not adversely affect the present invention.
[0043]
(B) Micro-structure of steel sheet
The micro-structure of the steel sheet according to the present invention is
described hereunder. In the following description, the symbol "%" with respect to area
fraction means "area%".
[0044]
In a cross section (also referred to as an "L cross section") parallel to a rolling
direction and a sheet thickness direction of the steel sheet according to the present
14
invention, the steel micro-structure at a position at a depth of 1/4 of the sheet thickness
from the surface includes 25 to 90% of tempered martensite, 5% or less of ferrite, 10 to
50% of retained austenite, and 5% or less of bainite. The fraction of each microstructure
changes depending on the conditions of the annealing, and have an effect on the
strength, elongation, and impact characteristics of the steel sheet. The reasons for
limiting each micro-structure are described in detail hereunder.
[0045]
Tempered martensite: 25 to 90%
Tempered martensite is a micro-structure that increases the strength of the steel
sheet and improves the strength-ductility balance and impact characteristics. In order to
preferably maintain the strength, elongation, and impact characteristics of the steel sheet
within the range of the target strength level, the area fraction of tempered martensite is
made 25 to 90%. The area fraction of tempered martensite is preferably 28% or more,
and more preferably is 50% or more.
[0046]
Further, from the viewpoint of hydrogen brittleness, the area fraction of
tempered martensite is preferably 80% or less, more preferably 75% or less, and further
preferably 70% or less. In addition, by controlling the content of tempered martensite
so that the area fraction thereof is 35 to 75%, it is possible to obtain a steel sheet having
high strength in which a higher level of both elongation and strength are achieved in a
compatible manner.
[0047]
Ferrite: 5% or less
In the steel sheet according to the present invention, it is important that the area
fraction of ferrite is low. If the area fraction of ferrite is high, the strength will decrease,
and if non-recrystallized ferrite remains, the strength-ductility balance will decrease.
Therefore, the area fraction of ferrite is made 5% or less. The area fraction of ferrite is
preferably 3% or less, and more preferably is 0%.
[0048]
Retained austenite: 10 to 50%
15
Retained austenite is a micro-structure that increases the strength-ductility
balance of the steel sheet by transformation induced plasticity. Further, since retained
austenite can be transformed to martensite by working that is accompanied by tensile
deformation, retained austenite also contributes to improving the strength of the steel
sheet. In addition, retained austenite also improves the impact characteristics of the steel
sheet. The higher the area fraction of retained austenite is, the more preferably it is.
However, in a steel sheet having the chemical composition described above, the upper
limit of the area fraction of retained austenite is 50%. Therefore, the area fraction of
retained austenite is made 10 to 50%. The area fraction of retained austenite is
preferably 18% or more, and more preferably is 20% or more.
[0049]
Bainite: 5% or less
In the steel sheet according to the present invention, it is important that the area
fraction of bainite is low. MA (Martensite-Austenite constituent) that is a hard microstructure
is present in bainite, and hence the strength-ductility balance decreases.
Therefore, the area fraction of bainite is made 5% or less. The area fraction of bainite
is preferably 0%. Tempered bainite can also be included in bainite, and no distinction
between bainite and tempered bainite is made in the description of the present application.
[0050]
In the steel sheet according to the present invention, fresh martensite (that is,
untempered martensite) is desirable as the other micro-structure than tempered martensite,
ferrite, retained austenite, and bainite. Further, with regard to pearlite, whilst pearlite
may be included, the possibility that pearlite will be included is low, and preferably the
area fraction of pearlite is less than 1%, and more preferably is 0%.
[0051]
Fresh martensite is a hard micro-structure, and is effective for securing the
strength of the steel sheet. In a case where importance is attached to strength, the area
fraction of fresh martensite is preferably more than 0%, more preferably is 1% or more,
and further preferably is 3% or more. However, the lower the area fraction of fresh
martensite is, the higher the bendability of the steel sheet will be. Therefore, from the
16
viewpoint of bendability, the area fraction of fresh martensite is preferably 55% or less,
more preferably is 45% or less, and further preferably is 20% or less. In a case where
particular importance is attached to bendability, the area fraction of fresh martensite is
preferably 10% or less.
[0052]
In the steel micro-structure at a position at a depth of 1/4 of the sheet thickness
from the surface of an L cross section of the steel sheet according to the present invention,
the total area of retained austenite grains having an area of 1 m2 or more and having a
grain shape circularity of 0.1 or more is less than 50% with respect to the entire area of
retained austenite.
[0053]
When the area fraction that the micro-structure of retained austenite in which the
area of the grains is 1 m2 or more and the grain shape circularity of the grains is 0.1 or
more occupies in the entire micro-structure of retained austenite is less than 50%, a steel
sheet that is excellent in strength-ductility balance, impact characteristics, and bendability
can be obtained. If retained austenite in which the area of the grains is large and the
grain shape circularity is large occupies 50% or more of the entire micro-structure of
retained austenite, the strength-ductility balance, impact characteristics, and bendability
of the steel sheet will decrease.
[0054]
In retained austenite in which the area of the grains is less than 1 m2, that is,
the grain size is small, since it is easy for Mn to uniformly concentrate in a short time
during annealing in the ferrite-austenite two-phase region and hence the stability is high,
transformation to the high strain side is delayed. Therefore, a steel sheet excellent in
strength-ductility balance and impact characteristics can be obtained.
[0055]
With respect to the retained austenite, even in the case of retained austenite in
which the area of the grains is 1 m2 or more, that is, the grain size is large, if the grain
shape circularity is less than 0.1, since most of the grains are present between martensite
or between tempered martensite laths, transformation to the high strain region side is
17
delayed due to spatial constraints from the surroundings. Therefore, a steel sheet that is
excellent in strength-ductility balance and impact characteristics can be obtained.
[0056]
Note that, grain shape circularity is expressed by the following formula (v).
Further, the grain shape circularity and area of grains can be measured by performing
electron back scatter diffraction patterns (EBSP) analysis with a standard function (Map
and Grain Shape Circularity) of OIM Analysis version 7 manufactured by TSL Company
Ltd.
Grain shape circularity = 4A/P2 ...(v)
Where, the meaning of each symbol in formula (v) above is as follows:
A: area of grain
P: circumferential length of grain
[0057]
In the steel material of the present invention, the Mn concentration in the steel
micro-structure at a position at a depth of 1/4 of the sheet thickness from the surface of
the L cross section satisfies the following formula (i).
CMn/CMn  1.2 ...(i)
Where, the meaning of each symbol in formula (i) above is as follows:
CMn: average Mn concentration (mass%) in retained austenite
CMn: average Mn concentration (mass%) in ferrite and tempered martensite
[0058]
By causing Mn to concentrate in the retained austenite, the retained austenite is
stabilized, and the strength-ductility balance and impact characteristics of the steel sheet
can be improved by the transformation induced plasticity. Therefore, the higher the
value of CMn/CMn is, the more preferable it is, and the value of CMn/CMn is 1.2 or more,
and preferably is 1.4 or more. Note that, whilst it is not necessary to set an upper limit
for the value of CMn/CMn, since the heat treatment time will be long, from the viewpoint
of productivity the upper limit is preferably 8.0 or less, and more preferably is 6.0 or less,
4.0 or less or 2.0 or less.
[0059]
18
In the steel material of the present invention, preferably the Si concentration in
the steel micro-structure at a position at a depth of 1/4 of the sheet thickness from the
surface of the L cross section satisfies formula (ii) below.
CSi/CSi  1.1 ...(ii)
Where, the meaning of each symbol in formula (ii) above is as follows.
CSi: average Si concentration (mass%) in tempered martensite and ferrite
CSi: average Si concentration (mass%) in retained austenite
[0060]
By causing Si to concentrate in the tempered martensite and ferrite, the tempered
martensite and ferrite are strengthened, and the strength and impact characteristics of the
steel sheet can be improved. To obtain the effect of strengthening the tempered
martensite and ferrite and improving the impact characteristics, the value of CSi/CSi is
made 1.1 or more, and preferably 1.2 or more.
[0061]
In order to make the value of CSi/CSi 1.1 or more, it is necessary for the content
of Si to be 0.1% or more. If the content of Si is less than 0.1 mass%, the value of
CSi/CSi will be less than 1.1. Further, in order to make the value of CSi/CSi 1.2 or
more, the content of Si is set to 0.7% or more. Note that, whilst it is not necessary to set
an upper limit for the value of CSi/CSi, since the heat treatment time will be long, from
the viewpoint of productivity the upper limit is preferably 1.8 or less.
[0062]
Methods for identifying the steel micro-structures and calculating the area
fractions are described hereunder.
[0063]

The area fraction of retained austenite is measured by the X-ray diffraction
method. First, from a center portion of the principal surface of the steel sheet, a
specimen is cut out which has a width (length in rolling direction) of 25 mm, a length
(length in direction orthogonal to rolling direction) of 25 mm, and a thickness that is the
thickness in the sheet thickness direction of the as-annealed sample. The specimen is
19
then subjected to chemical polishing to reduce the sheet thickness by 1/4 to obtain a
specimen having a chemically polished surface. X-ray diffraction analysis is performed
three times on the surface of the specimen using a Co tube.
[0064]
With regard to the fcc phase, the integrated intensities of the respective peaks of
(111), (200), and (220) are determined, and with regard to the bcc phase, the integrated
intensities of the respective peaks of (110), (200), and (211) are determined. By
analyzing the integrated intensities, and the averaging the results obtained by performing
X-ray diffraction analysis three times, the volume ratio of retained austenite is determined,
and the determined value is regarded as the area fraction of retained austenite.
[0065]

The area fractions of tempered martensite, ferrite, bainite, and fresh martensite
are calculated based on micro-structure observation using a scanning electron microscope
(SEM). After mirror-polishing the L cross section of the steel sheet, the microstructure
is revealed using 3% nital (3% nitric acid-ethanol solution). The microstructure in an
area of 100 m in length (length in sheet thickness direction)  300 m in width (length
in rolling direction) at a position at a depth of 1/4 of the sheet thickness from the surface
of the steel sheet is then observed using an SEM at a magnification of 5000, and the area
fraction of each micro-structure can be measured.
[0066]
The area fraction of tempered martensite is calculated by determining that,
among white micro-structure recognized in the observation using the SEM, a microstructure
whose substructure is confirmed within grains is tempered martensite. Ferrite
is distinguished as a grey base micro-structure, and the area fraction thereof is calculated.
In the observation by SEM, bainite is an aggregate of lath-shaped grains, and is
distinguished as a micro-structure in which carbides extend in the same direction in laths,
and the area fraction thereof is calculated.
[0067]
20
In the observation by SEM, fresh martensite is recognized as white microstructure,
similarly to retained austenite. Therefore, although it is difficult to
differentiate between retained austenite and fresh martensite in the observation by SEM,
the area fraction of fresh martensite is calculated by deducting the area fraction of retained
austenite measured by the X-ray diffraction method from the total area fraction of retained
austenite and fresh martensite obtained by observation by SEM.
[0068]

CMn, CMn, CSi, and CSi can be measured by EBSP, an SEM, and an electron
probe microanalyzer (EPMA). That is, using EBSP and an SEM, a region of 50 m 
50 m is observed at a magnification of 500, EBSP data is measured at measurement
intervals of 0.1 m, and with respect to five regions, retained austenite, ferrite, and
tempered martensite are identified. Next, with respect to the identified retained
austenite and tempered martensite, point analysis by EPMA measurement is performed at
five points in five regions, respectively, the measured values are averaged to calculate
CMn, CMn, CSi, and CSi, and CMn/CMn and CSi/CSi are determined.
[0069]
(C) Mechanical properties
Next, the mechanical properties of the steel sheet according to the present
invention are described.
[0070]
The tensile strength (TS) of the steel sheet according to the present invention is
preferably 780 MPa or more, more preferably is 980 MPa or more, and further preferably
is 1180 MPa or more. This is because, when using the steel sheet as a starting material
for automobiles, increasing the strength allows the sheet thickness to be reduced, which
contributes to weight reduction. Further, since the steel sheet according to the present
invention will be subjected to press forming, it is desirable that elongation after fracture
(tEL) is also excellent. The TS  tEL of the steel sheet according to the present invention
is preferably 22,000 MPa% or more, and more preferably is 25,000 MPa% or more.
[0071]
21
Further, the steel sheet according to the present invention has excellent impact
characteristics, and preferably the value of impact energy in a Charpy impact test at 20C
is 20 J/cm2 or more.
[0072]
(D) Production method
Next, a steel sheet according to one embodiment of the present invention can be
obtained, for example, by a production method including the processes described
hereunder.
[0073]

To produce the steel sheet according to the present invention, steel having the
aforementioned chemical composition is melted by a conventional method and cast to
prepare a steel material (hereunder, also referred to as a "slab"). As long as the steel
sheet according to the present invention has the aforementioned chemical composition,
the molten steel may be molten steel melted by a normal blast furnace method, or may be
molten steel for which the raw material includes a large amount of scrap, as in the case of
steel produced by an electric furnace method. The slab may be produced by a normal
continuous casting process or may be produced by thin slab casting.
[0074]

Hot rolling can be performed using a normal continuous hot rolling line. The
hot rolling process includes a rough rolling process and a finish rolling process.
[0075]
Slab heating temperature: 1100 to 1300C
The slab to be subjected to the hot rolling process is heated before the hot rolling.
By making the temperature of the slab to be subjected to hot rolling 1100C or more,
deformation resistance during hot rolling can be reduced more. On the other hand, by
making the temperature of the slab to be subjected to hot rolling 1300C or less, a decrease
in the yield due to an increase in scale loss can be suppressed. Therefore, the
temperature of the slab to be subjected to hot rolling is preferably made 1100 to 1300C.
22
Note that, in the description of the present application, the term "temperature" means the
surface temperature of a slab, a hot-rolled steel sheet, or a cold-rolled steel sheet.
[0076]
Whilst the holding time in the aforementioned slab heating temperature range is
not particularly limited, in order to improve the stability of the material quality the holding
time is preferably 30 minutes or more, and more preferably is 1 hour or more. Further,
to suppress excessive scale loss, the holding time is preferably set to 10 hours or less, and
more preferably 5 hours or less. In the case of performing hot direct rolling or hot charge
rolling, the slab may be subjected to hot rolling as it is without being subjected to a prior
heat treatment.

We claim:
1. A steel sheet, having a chemical composition consisting of, in mass%,
C: more than 0.18% and less than 0.30%,
Si: 0.01% or more and less than 2.00%,
Mn: more than 2.50% and 4.00% or less,
sol. Al: 0.001% or more and less than 3.00%,
P: 0.100% or less,
S: 0.010% or less,
N: less than 0.050%,
O: less than 0.020%,
Cr: 0% or more and less than 2.00%,
Mo: 0 to 2.00%,
W: 0 to 2.00%,
Cu: 0 to 2.00%,
Ni: 0 to 2.00%,
Ti: 0 to 0.300%,
Nb: 0 to 0.300%,
V: 0 to 0.300%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Zr: 0 to 0.010%,
REM: 0 to 0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%, and
the balance: Fe and impurities,
wherein:
in a cross section parallel to a rolling direction and a sheet thickness direction of
42
the steel sheet, a steel micro-structure at a position at a depth of 1/4 of a sheet thickness
from a surface is, in area%:
tempered martensite: 25 to 90%,
ferrite: 5% or less,
retained austenite: 10 to 50%, and
bainite: 5% or less;
at a position at a depth of 1/4 of the sheet thickness from a surface of a cross
section parallel to the rolling direction and the sheet thickness direction of the steel sheet,
a proportion of a total area of retained austenite grains having an area of 1 m2 or more
and having a grain shape circularity of 0.1 or more is less than 50% with respect to an
entire area of the retained austenite; and
an Mn concentration in the steel micro-structure at a position at a depth of 1/4 of
the sheet thickness from the surface satisfies formula (i) below:
CMn/CMn  1.2 ...(i)
where, meaning of each symbol in formula (i) above is as follows:
CMn: average Mn concentration (mass%) in retained austenite
CMn: average Mn concentration (mass%) in ferrite and tempered martensite.
2. The steel sheet according to claim 1, wherein:
the chemical composition contains, in mass%,
Si: 0.10% or more and less than 2.00%; and
an Si concentration in the steel micro-structure at a position at a depth of 1/4 of
the sheet thickness from the surface of a cross section parallel to the rolling direction and
the sheet thickness direction of the steel sheet satisfies formula (ii) below:
CSi/CSi  1.1 ...(ii)
where, meaning of each symbol in formula (ii) above is as follows:
CSi: average Si concentration (mass%) in tempered martensite and ferrite
CSi: average Si concentration (mass%) in retained austenite.
3. The steel sheet according to claim 1 or claim 2, wherein:
43
the chemical composition contains one or more kinds of element selected from,
in mass%,
Cr: 0.01% or more and less than 2.00%,
Mo: 0.01 to 2.00%,
W: 0.01 to 2.00%,
Cu: 0.01 to 2.00%, and
Ni: 0.01 to 2.00%.
4. The steel sheet according to any one of claim 1 to claim 3, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
Ti: 0.005 to 0.300%,
Nb: 0.005 to 0.300%, and
V: 0.005 to 0.300%.
5. The steel sheet according to any one of claim 1 to claim 4, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
B: 0.0001 to 0.010%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%, and
REM: 0.0001 to 0.010%.
6. The steel sheet according to any one of claim 1 to claim 5, wherein:
the chemical composition contains one or more kinds of element selected from,
in mass%,
Sb: 0.0005 to 0.050%,
Sn: 0.0005 to 0.050%, and
Bi: 0.0005 to 0.050%.
44
7. The steel sheet according to any one of claim 1 to claim 6, having:
a hot dip galvanized layer on a surface of the steel sheet.
8. The steel sheet according to any one of claim 1 to claim 6, having:
a hot dip galvannealed layer on a surface of the steel sheet.
9. The steel sheet according to any one of claim 1 to claim 8, wherein:
a Charpy impact value at 20C is 20 J/cm2 or more.
10. A method for producing a steel sheet, that is a method in which a hot rolling
process, a cold rolling process, a primary annealing process and a secondary annealing
process are performed in that order on steel having a chemical composition according to
claim 1 or any one of claim 3 to claim 6, wherein:
the hot rolling process includes a process of performing finish hot-rolling using
a rolling mill having a plurality of stands, the number of which is four or more;
in the process of performing finish hot-rolling:
a sheet thickness reduction between before and after a last four stands among the
plurality of stands satisfies formula (iii) below,
a strain rate at a final stand of the last four stands and a rolling exit side
temperature at the final stand satisfy formula (iv) below, and
cooling to 750C at an average cooling rate of 100C/s or more is performed
within 1.0 s after rolling at the final stand, and cooling in a temperature range from 750C
to 300C is performed at an average cooling rate of 10C/s or more;
in the primary annealing process, after holding for 10 to 1000 s in a temperature
region of more than 750C and an Ac3 point or more, cooling is performed to a
temperature region of 300C or less under a condition that an average cooling rate to
300C is 10 to 2000C/s, and thereafter cooling is performed to a temperature region of
less than 100C; and
in the secondary annealing process, heating is performed to a temperature region
45
of 650C or more and less than the Ac3 point at an average heating rate of 1 to 40C/s,
and is held in the temperature region for 300 s or more:
1.2  ln(t0/t)  2.8 ...(iii)
11.0  log(vexp(33000/(273 + T)))  15.0 ...(iv)
where, meaning of each symbol in the above formulas is as follows:
t0: sheet thickness (mm) immediately before entering last four stands
t: sheet thickness (mm) immediately after exiting from last four stands
v: strain rate (/s) at final stand
T: rolling exit side temperature (C) at final stand.
11. The method for producing a steel sheet according to claim 10, wherein:
in the primary annealing process, after holding for 10 s or more in a temperature
region of more than 750C and the Ac3 point or more, cooling is performed at an average
cooling rate of 10 to 2000C/s to a temperature region of 300C or less, and after holding
for 10 to 1000 s in a temperature region of 100 to 300C, cooling is performed to a
temperature region of less than 100C.
12. The method for producing a steel sheet according to claim 10 or claim 11,
wherein:
after the secondary annealing process, cooling is performed, and a hot dip
galvanizing treatment is performed.
13. The method for producing a steel sheet according to claim 12, wherein:
after the hot dip galvanizing treatment is performed, a hot dip galvanized layer
is subjected to an alloying treatment in a temperature region of 450 to 620°