[0001]The present invention relates to a cold rolled steel sheet and a method for producing the
same. More particularly, it relates to a cold rolled steel sheet excellent in shape fixability and
10 workability and a method for producing the same.
BACKGROUND
[0002]
Improvement of the fuel efficiency of automobiles and securing safety in collision have
15 been simultaneously sought. Steel sheet for automotive use has been made lighter in weight and
higher in strength. Therefore, as steel sheet for automotive use, high strength steel sheet having higher tensile strength has been used in large amounts.
[0003]
Further, steel sheet for automotive use is mostly shaped by press-forming, and therefore the
20 high strength steel sheet used for steel sheet for automotive use is required to have excellent
workability. To secure excellent workability, uniform elongation enabling shaping without cracking at the time of shaping is sought. Furthermore, in high strength steel sheet, shape fixability is sought for forming it into the shape of the targeted part with a high dimensional precision.
25 [0004]
In particular, in recent years, there has been rising demand for lightening the weight of automobiles. In high strength steel, for example, high strength steel having a tensile strength of 1180 MPa or more, steel sheet having the above such excellent shape fixability and workability is required.
30 [0005]
However, along with the higher strength of steel sheet, it has become difficult to sufficiently secure ductility which is in a tradeoff relationship with strength. Further, if steel sheet becomes higher in strength, the problem arises that the amount of springback at the time of press-forming becomes greater and it becomes difficult to form the steel sheet into a shape of the
35 targeted part with a high dimensional precision. “Springback” means the phenomenon where
when steel sheet is press-formed by a die, the bent part receiving the bending moment in the

constrained state deforms when detached from the die so that the moment becomes smaller and
deviation occurs in the shape of the press-formed steel sheet from the die shape. Such springback
becomes more remarkably the higher the strength of steel sheet and becomes a problem in press-
forming. To suppress springback, it is effective to decrease the yield ratio (yield point/tensile
5 strength), and therefore in high strength steel sheet used for portions where high dimensional
precision is sought, a low yield ratio is strongly demanded.
[0006]
To deal with such a problem, PTL 1 describes a method for producing a composite structure type high strength cold rolled steel sheet. Specifically, PTL 1 discloses a method for producing a
10 composite structure type high strength cold rolled steel sheet comprising hot rolling and cold
rolling a steel containing C: 0.003 to 0.03%, Si: 0.1 to 1%, Mn: 0.3 to 1.5%, Ti: 0.02 to 0.2%, Al: 0.01 to 0.07% and having (Si%+2-Mn%)=1 to 3% and an (effective Ti)/(C+N) atomic concentration ratio of 0.4 to 0.8, then continuously annealing it by heating it to a temperature of the Ac1 transformation point or more and 900°C or less for 30 seconds to 10 minutes and
15 cooling it by a cooling rate of 30°C/s or more. PTL 1 teaches that by this production method,
steel sheet having composite structures comprised of ferrite and second phases of martensite and/or bainite is obtained and that this steel sheet is steel sheet having a r-value of 1.4 or more and a yield ratio of 50% or less and excellent in balance of tensile strength-elongation.
[0007]
20 On the other hand, PTL 2 discloses a cold rolled steel sheet having a chemical composition
containing C: 0.01% or more and 0.15% or less, Si: 0.01% or more and 1.5% or less, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less, S: 0.01% or less, Al: more than 0.10% and 1.5% or less, and N: 0.010% or less and having an a-value prescribed by a=Mn+Six0.5+Alx0.4 of 1.9 or more, and steel structures having a ferrite volume ratio of 40% or more and a martensite volume
25 ratio of 3% or more at a position of 1/4 depth of the sheet thickness from the surface of the steel
sheet. Further, PTL 2 teaches that the cold rolled steel sheet has composite structures of ferrite and martensite having excellent mechanical properties by making the average crystal grain size dF (nm) of the ferrite at the position of 1/4 depth of the sheet thickness finer to 4.5 um or less. [0008]
30 However, in the art described in PTL 1, the content of C is small and it is difficult to obtain
high strength steel having a high tensile strength such as 1180 MPa or more. On the other hand, by just the refining of the average crystal grain size of ferrite such as described in PTL 2, it is difficult to maintain a high tensile strength while reducing the yield ratio.
[0009]
35 In general, as a technique for realizing higher tensile strength, strengthening of the
microstructure using martensite or tempered martensite has been known. However, if using
2

martensite or tempered martensite, while the strength is high, the uniform elongation is extremely low and good workability is difficult to secure. Further, with single phases of martensite, it is difficult to reduce the yield ratio and difficult to secure shape fixability as well.
[0010]
5 As high strength steel sheet for solving this problem, a composite structure steel sheet
comprised of soft phases (ferrite) and hard phases (martensite and tempered martensite) may be considered. In composite structure steel sheet, ductility is secured by the soft phases and strength is secured by the hard phases. Further, due to the difference in strengths between the soft and hard phases, the yield phenomenon occurs earlier at the soft phase side, and therefore it is
10 possible to greatly reduce the yield point. However, in such composite structure steel sheet, to
secure a higher tensile strength of the steel sheet, it is necessary to sufficiently raise the volume ratio of the hard phases. If making the volume ratio of the hard phases increase, in the prior art, a drop in the uniform elongation and increase in the yield ratio are unavoidable, and therefore in the high strength class such as 1180 MPa or more, there was the problem that realization of
15 excellent uniform elongation and a low yield ratio was extremely difficult.
[0011]
PTL 3 proposes a steel sheet comprising a steel sheet microstructure mainly comprised of
ferrite and including martensite, having a volume ratio of ferrite of 60% or more, having a block
size of martensite of 1 [im or less, and having a concentration of C in the martensite of 0.3% to
20 0.9%, whereby the strength of the martensite structures is raised without increasing the volume
ratio of the hard structures of martensite and thereby the volume of ferrite contributing to securing ductility is secured while a maximum tensile strength of 900 MPa or more (900 to 1582 MPa) and a yield ratio (YR) of 0.75 or less are secured.
[0012]
25 However, in the art shown in PTL 3, the particle sizes of the ferrite and martensite are
controlled, but the morphology is not controlled at all. There was still room for improvement in regard to the improvement of the tensile strength and the reduction of the yield ratio.
[0013]
In relation to a composite structure steel sheet, PTL 4 describes a method for producing a
30 cold rolled steel sheet comprising a metallic structure with main phases comprised of low
temperature transformed phases, and second phases including retained austenite and polygonal
ferrite, the method comprising (A) a hot rolling step including hot rolling a slab having a
chemical composition comprising, by mass%, C: more than 0.020% and less than 0.30%, Si:
more than 0.10% and 3.00% or less, Mn: more than 1.00% and 3.50% or less, P: 0.10% or less,
35 S: 0.010% or less, sol. Al: 2.00% or less and N: 0.010% or less, and a balance of Fe and
impurities and ending the rolling in a temperature region of the Ar3 point or more to obtain hot
3

rolled steel sheet, cooling the hot rolled steel sheet down to a temperature region of 780°C or
less within 0.4 second after the end of the rolling, and coiling it in a temperature region of less
than 400°C; (B) a hot rolled sheet annealing step including hot rolled sheet annealing by heating
the hot rolled steel sheet obtained in the above step (A) to a temperature region of 300°C or
5 more to obtain a hot rolled annealed steel sheet; (C) a cold rolling step including cold rolling the
hot rolled annealed steel sheet to obtain a cold rolled steel sheet; and (D) an annealing step
including soaking the cold rolled steel sheet at a temperature region of (Ac3 point-40°C) or
more, then cooling it to a temperature region of 500°C or less and 300°C or more and holding it
at the temperature region for 30 seconds or more. Further, PTL 4 describes that according to the
10 above method, it is possible to obtain a high strength cold rolled steel sheet having sufficient
ductility, work hardenability, and stretch flangeability enabling it to be applied to press-forming and other working operations.
[0014]
However, PTL 4 does not necessarily sufficiently study maintaining a high strength while
15 decreasing the yield ratio as a perspective. Therefore, in the invention described in PTL 4, there
still has been room for improvement of the shape fixability, etc.
[CITATIONS LIST]
[PATENT LITERATURE]
20 [0015]
[PTL 1] Japanese Unexamined Patent Publication No. 58-39736 [PTL 2] Japanese Unexamined Patent Publication No. 2014-65975 [PTL 3] Japanese Unexamined Patent Publication No. 2011-111671 [PTL 4] Japanese Unexamined Patent Publication No. 2013-14824 25
SUMMARY [TECHNICAL PROBLEM]
[0016]
Therefore, the technical problem to be solved by the present invention is to provide a high
30 strength cold rolled steel sheet excellent in workability and shape fixability having excellent
uniform elongation and improved in yield ratio YR by a novel constitution and a method for producing the same.
[SOLUTION TO PROBLEM]
35 [0017]
The inventors engaged in intensive research to solve the above problem and produce a high
4

strength cold rolled steel sheet excellent in workability and shape fixability. Below, details of the present art will be explained.
[0018]
The inventors engaged in intensive studies and as a result discovered that by making the
5 metallic structure of steel sheet a microstructure including soft phases and hard phases and by
making the phases uniformly and finely disperse and controlling the morphology to one having
interfacial shapes where hard phases and soft phases are intricately intertwined, it is possible to
improve the ductility by the soft phases and secure the strength by the hard phases in a
complementary manner to the maximum extents. They discovered that by controlling the
10 chemical composition and area ratios of the phases in appropriate ranges and further controlling
the recrystallization rates of the soft phases in addition to controlling the particle sizes and
morphology of the interfacial shapes of the two phases, a steel sheet having both a 1180 MPa or
more tensile strength and excellent uniform elongation and having a yield ratio (YR) of 60% or
less can be realized.
15 [0019]
The inventors discovered that by integrally controlling the (a) hot rolling step-(b) tempering
step-(c) cold rolling step-(d) annealing step, it is possible, in a way not able to be realized in the
prior art, to obtain a microstructure in which the soft phases and hard phases are made to
uniformly and finely disperse and in which the interfacial shapes of the two phases are
20 controlled to intricately intertwined forms. Specifically, the inventors discovered a method for
producing a cold rolled steel sheet excellent in workability and shape fixability comprising (a) a
hot rolling step controlling the microstructure to low temperature transformed phases imparting
certain accumulated strain (for example, martensite phases), (b) a tempering step causing iron
carbides to uniformly and finely precipitate, (c) a cold rolling step imparting driving force for
25 recrystallization of ferrite, and (d) an annealing step for causing ferrite to sufficiently
recrystallize during heating and for pinning the recrystallized ferrite grain boundaries by iron
carbides and promoting growth of austenite along the grain boundaries to thereby make the soft
phases and hard phases uniformly and finely disperse and control the morphology to one where
the interfacial shapes of the two phases are intricately intertwined.
30 [0020]
The gist of the present invention is as follows:
[1] A cold rolled steel sheet having a chemical composition consisting of, by mass%,
C: 0.15% or more and 0.40% or less,
Si: 0.50% or more and 4.00% or less,
35 Mn: 1.00% or more and 4.00% or less,
sol. Al: 0.001% or more and 2.000% or less,
5

P: 0.020% or less,
S: 0.020% or less,
N: 0.010% or less,
Ti: 0% or more and 0.200% or less,
5 Nb: 0% or more and 0.200% or less,
B: 0% or more and 0.010% or less,
V: 0% or more and 1.00% or less,
Cr: 0% or more and 1.00% or less,
Mo: 0% or more and 1.00% or less,
10 Cu: 0% or more and 1.00% or less,
Co: 0% or more and 1.00% or less,
W: 0% or more and 1.00% or less,
Ni: 0% or more and 1.00% or less,
Ca: 0% or more and 0.010% or less,
15 Mg: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less, and
balance: iron and impurities, and
a metallic structure consisting of ferrite phases, hard second phases consisting of martensite
20 phases and retained austenite phases, and remaining phases consisting of cementite phases and
bainite phases, wherein
an area ratio of the ferrite phases is 35% or more and 65% or less,
an area ratio of the hard second phases is 35% or more and 65% or less,
an area ratio of the remaining phases is 0% or more and 5% or less,
25 60% or more of the ferrite phases are recrystallized ferrite phases,
an average crystal grain size defined by 15° grain boundaries is 5.0 um or less,
a maximum connecting rate of the hard second phases is 10% or more, and
a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.
[2] The cold rolled steel sheet according to [1], wherein the chemical composition comprises,
30 by mass%, one or more selected from the group consisting of
Ti: 0.001% or more and 0.200% or less,
Nb: 0.001% or more and 0.200% or less,
B: 0.0005% or more and 0.010% or less,
V: 0.005% or more and 1.00% or less,
35 Cr: 0.005% or more and 1.00% or less,
Mo: 0.005% or more and 1.00% or less,
6

Cu: 0.005% or more and 1.00% or less,
Co: 0.005% or more and 1.00% or less,
W: 0.005% or more and 1.00% or less,
Ni: 0.005% or more and 1.00% or less,
5 Ca: 0.0003% or more and 0.010% or less,
Mg: 0.0003% or more and 0.010% or less,
REM: 0.0003% or more and 0.010% or less, and
Zr: 0.0003% or more and 0.010% or less.
[3] The cold rolled steel sheet according to [1] or [2], having a hot dip galvanized layer on the
10 surface thereof.
[4] The cold rolled steel sheet according to [1] or [2], having a hot dip galvannealed layer on
the surface thereof.
[5] A method for producing the cold rolled steel sheet according to [1] or [2], comprising:
a hot rolling step of rough rolling a slab having the chemical composition according to [1]
15 or [2], then finish rolling it wherein a rolling reduction of a final stage of the finish rolling is
15% or more and 50% or less and an end temperature of the finish rolling is Ar3°C or more and 950°C or less, cooling it down to a coiling temperature of less than 400°C by an average cooling rate of 50°C/s or more, and coiling it at the coiling temperature,
a tempering step of tempering the hot rolled steel sheet in a temperature region of 450°C or
20 more and less than 600°C under conditions of a tempering parameter £ defined by following
Formula 1 of 14000 to 18000,
a cold rolling step of pickling the tempered steel sheet, then cold rolling it by a rolling reduction of 30% or more, and
an annealing step of heating the cold rolled steel sheet in a temperature region of 500°C to
25 Ac1°C by an average heating rate of 5.0°C/s or less up to a maximum heating temperature of
(Ac1+10)°C or more and (Ac3-10)°C or less, holding it at the maximum heating temperature for
60 seconds or more, then cooling it in a temperature region of (Ac1-50)°C to a cooling stop
temperature by an average cooling rate of 20°C/s or more down to the cooling stop temperature
of Ms°C or less:
30 Formula 1: ^=(T+273)[log10 (t/3600)+20]
T [°C]: tempering temperature, t [s]: tempering time
Ac1 [°C]=751-16x[%C]+35x[%Si]-28x[%Mn]
Ac3 [°C]=881-353x[%C]+65x[%Si]-24x[%Mn]
Ar3 [°C]=910-203x[%C]+44.7x[%Si]-24x[%Mn]-50x[%Ni]
35 Ms [°C]=521-353x[%C]-22x[%Si]-24x[%Mn]
where, %C, %Si, %Mn, and %Ni are contents [mass%] of C, Si, Mn, and Ni.
7

[6] The method for producing the cold rolled steel sheet according to [5], further comprising
cooling down to the cooling stop temperature of Ms°C or less, then holding at a temperature of
200°C or more and 450°C or less for 60 seconds or more and 600 seconds or less.
[7] The method for producing the cold rolled steel sheet according to [5] or [6], further
5 comprising hot dip galvanization at a temperature of 430°C or more after the annealing step to
produce the cold rolled steel sheet according to [3].
[8] The method for producing the cold rolled steel sheet according to [5] or [6], further
comprising hot dip galvanization at a temperature of 430°C or more after the annealing step,
then alloying treatment at 400°C or more and 600°C or less to produce the cold rolled steel sheet
10 according to [4].
[ADVANTAGEOUS EFFECTS OF INVENTION]
[0021]
According to the present invention, it is possible to obtain a high strength cold rolled steel
15 sheet excellent in workability and shape fixability having a tensile strength TS of 1180 MPa or
more and excellent uniform elongation and having a yield ratio YR of 60% or less.
BRIEF DESCRIPTION OF DRAWINGS
[0022]
20 FIG. 1 is a view schematically showing a maximum connected region in a steel sheet
microstructure.
FIG. 2 is a schematic view of a binarized image explaining a two-dimensional isoperimetric constant.
25 DESCRIPTION OF EMBODIMENTS
[0023]

Below, a cold rolled steel sheet according to one embodiment of the present invention will
be explained in detail. However, the present invention is not limited to only the configuration
30 disclosed in the present embodiment and can be changed in various ways within a range not
deviating from the gist of the present invention. Further, the ranges of limitation of the numerical
values described below include the lower limit values and upper limit values in those ranges.
Numerical values described with “more than” or “less than” are not included in the ranges of the
numerical values. The “%” relating to the contents of the elements mean “mass%”.
35 [0024]
[Chemical Composition]
8

First, the chemical composition and reasons of limitation of the cold rolled steel sheet
according to the present invention will be explained. The cold rolled steel sheet of the present
embodiment contains, as its chemical composition, basic elements and, as required, optional
elements and has a balance of iron and impurities.
5 [0025]
In the chemical composition of the cold rolled steel sheet according to the present embodiment, C, Si, Mn, Al, P, S, and N are the basic elements.
[0026]
(C: 0.15% or More and 0.40% or Less)
10 C (carbon) is an element important in securing the strength of the steel sheet. To
sufficiently obtain such an effect, the content of C is 0.15% or more, preferably 0.17% or more or 0.20% or more, more preferably 0.23% or more, still more preferably 0.25% or more. On the other hand, if excessively containing C, sometimes the weldability becomes poor. Therefore, the content of C is 0.40% or less, preferably 0.35% or less, more preferably 0.30% or less.
15 [0027]
(Si: 0.50% or More and 4.00% or Less)
Si (silicon) is an element important in holding the cementite up to a high temperature. If the content of Si is low, cementite dissolves during the heating and sometimes refining the crystal grains becomes difficult. Therefore, the content of Si is 0.50% or more. Preferably, it is 0.80% or
20 more or 0.90% or more, more preferably 1.00% or more. On the other hand, if excessively
containing Si, sometimes deterioration of the surface properties is caused, and therefore the content of Si is 4.00% or less. The content of Si is preferably 3.50% or less or 3.20% or less, more preferably 3.00% or less.
[0028]
25 (Mn: 1.00% or More and 4.00% or Less)
Mn (manganese) is an element effective for raising the hardenability of steel sheet. To sufficiently obtain such an effect, the content of Mn is 1.00% or more. The content of Mn is preferably1.20% or more or 1.50% or more, more preferably 2.00% or more. On the other hand, if excessively adding Mn, due to Mn segregation, sometimes the microstructure becomes
30 heterogeneous and the stretch flangeability falls. Therefore, the Mn content is 4.00% or less,
preferably 3.50% or less or 3.00% or less, more preferably 2.80% or less or 2.60% or less.
[0029]
(Sol. Al: 0.001% or More and 2.000% or Less)
Al (aluminum) is an element having the action of deoxidizing steel and making the steel
35 sheet sounder. To reliably obtain such an effect, the content of sol. Al is 0.001% or more.
However, if deoxidation is sufficiently necessary, the content of sol. Al is more preferably
9

0.010% or more, still more preferably 0.020% or more or 0.025% or more. On the other hand, if
the content of sol. Al is too high, sometimes the fall in weldability becomes remarkable, the
oxide-based inclusions increase, and the deterioration of the surface properties becomes
remarkable. Therefore, the content of sol. Al is 2.000% or less, preferably 1.500% or less, more
5 preferably 1.000% or less, most preferably 0.800% or less or 0.600% or less. “sol. Al” means
soluble Al which is not Al2 O3 or another oxide and can dissolve in acid.
[0030]
(P: 0.020% or Less)
P (phosphorus) is an impurity generally contained in steel. If the content of P is excessive,
10 the deterioration in the weldability becomes remarkable. Therefore, the content of P is 0.020%
or less. The content of P is preferably 0.015% or less or 0.010% or less. The lower limit of the content of P is not particularly limited and may even be 0%, but from the viewpoint of the producing costs, the content of P may be more than 0%, 0.0001% or more, or 0.001% or more.
[0031]
15 (S: 0.020% or Less)
S (sulfur) is an impurity generally contained in steel. From the viewpoint of weldability, the less, the better. If the content of S is excessive, the drop in weldability becomes remarkable and the amount of precipitation of MnS increases resulting in a drop in the bendability and other workability. Therefore, the content of S is 0.020% or less. The content of S is preferably 0.010%
20 or less, more preferably 0.005% or less. The content of S may even be 0%, but from the
viewpoint of the desulfurization costs, the content of S may be more than 0%, 0.0001% or more, or 0.001% or more.
[0032]
(N: 0.010% or Less)
25 N (nitrogen) is an impurity generally contained in steel. From the viewpoint of weldability,
the less, the better. If the content of N is excessive, the drop in weldability becomes remarkable. Therefore, the content of N is 0.010% or less. The content of N is preferably 0.005% or less, more preferably 0.003% or less. The content of N may even be 0%, but from the viewpoint of the producing costs, the content of N may be more than 0%, 0.0001% or more, or 0.001% or
30 more.
[0033]
The cold rolled steel sheet according to the present embodiment may contain the following
optional elements in addition to the basic elements explained above. For example, instead of part
of the balance of Fe explained above, as optional elements, one or more of Ti, Nb, B, V, Cr, Mo,
35 Cu, Co, W, Ni, Ca, Mg, REM, and Zr may be contained. These optional elements may be
contained in accordance with the objective. Accordingly, there is no need to specify lower limit
10

values of these optional elements. The lower limit values may also be 0%. Further, even if the selective elements are contained as impurities, the effects of the present embodiment are not impaired.
[0034]
5 (Ti: 0% or More and 0.200% or Less)
Ti (titanium) is an element precipitating as TiC during the cooling of steel sheet and
contributing to improvement of the strength. Therefore, Ti may be contained. On the other hand,
if excessively adding Ti, this becomes a cause of worse low temperature embrittlement of the
steel sheet. Therefore, the content of Ti is 0.200% or less. The content of Ti is preferably 0.180%
10 or less, more preferably 0.150% or less. To reliably obtain this effect, the content of Ti may be
0.001% or more. The content of Ti is preferably 0.020% or more, more preferably 0.050% or more.
[0035]
(Nb: 0% or More and 0.200% or Less)
15 Nb (niobium), like Ti, is an element which precipitates as NbC and improves the strength.
Therefore, Nb may also be included. On the other hand, if excessively containing Nb, texture develops and the anisotropy of the material sometimes becomes greater. Therefore, the content of Nb is 0.200% or less. The content of Nb is preferably 0.150% or less, more preferably 0.100% or less. To reliably obtain this effect, the content of Nb may be 0.001% or more. The content of
20 Nb is preferably 0.005% or more, more preferably 0.010% or more.
[0036]
The cold rolled steel sheet according to the present embodiment preferably includes, as its
chemical composition, by mass%, at least one of the elements among Ti: 0.001% or more and
0.200% or less and Nb: 0.001% or more and 0.200% or less.
25 [0037]
(B: 0% or More and 0.010% or Less)
B (boron) segregates at the grain boundaries to improve the intergranular strength and
thereby can improve the toughness of the material. Therefore, B may also be included. On the
other hand, even if the content of B is too high, the above effect becomes saturated and
30 economically disadvantageous, so the upper limit of the content of B is 0.010%. The content of
B is preferably 0.005% or less, more preferably 0.003% or less. To reliably obtain this effect, the content of B may be 0.0005% or more or 0.001% or more.
[0038]
(V: 0% or more and 1.00% or less)
35 (Cr: 0% or more and 1.00% or less)
(Mo: 0% or more and 1.00% or less)
11

(Cu: 0% or more and 1.00% or less)
(Co: 0% or more and 1.00% or less)
(W: 0% or more and 1.00% or less)
(Ni: 0% or more and 1.00% or less)
5 V (vanadium), Cr (chromium), Mo (molybdenum), Cu (copper), Co (cobalt), W (tungsten),
and Ni (nickel) are all elements for stably securing strength. Therefore, these elements may also
be contained. However, even if one of these elements is contained in excess, the effect due to the
above action becomes easily saturated and sometimes economically disadvantageous. Therefore,
the contents of these elements are respectively 1.00% or less. The contents of these elements are
10 respectively preferably 0.80% or less, more preferably 0.50% or less. To more reliably obtain the
effect according to the above action, for any of the elements, the content need only be 0.005% or more, preferably 0.01% or more, more preferably 0.05% or more.
[0039]
In the cold rolled steel sheet according to the present embodiment, as the chemical
15 composition, by mass%, at least one type of element among V: 0.005% or more and 1.00% or
less, Cr: 0.005% or more and 1.00% or less, Mo: 0.005% or more and 1.00% or less, Cu: 0.005% or more and 1.00% or less, Co: 0.005% or more and 1.00% or less, W: 0.005% or more and 1.00% or less, and Ni: 0.005% or more and 1.00% or less is preferable.
[0040]
20 (Ca: 0% or more and 0.010% or less)
(Mg: 0% or more and 0.010% or less) (REM: 0% or more and 0.010% or less) (Zr: 0% or more and 0.010% or less)
Ca (calcium), Mg (magnesium), REM (rare earth elements), and Zr (zirconium) are all
25 elements which have the action of contributing to control of inclusions, in particular fine
dispersion of inclusions, and of raising the toughness. Therefore, these elements may also be
contained. However, in each element, if excessively contained, sometimes the surface conditions
remarkably deteriorate. Therefore, the contents of these elements are respectively 0.010% or
less. The contents of these elements are respectively preferably 0.008% or less or 0.005% or
30 less, more preferably 0.003% or less. To obtain the effect due to this action more reliably, for
each element, 0.0003% or more is sufficient. REM are the overall name for rare earth elements, i.e., Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The content of REM means the total content of these elements.
[0041]
35 In the cold rolled steel sheet according to the present embodiment, as the chemical
composition, by mass%, at least one type of element among Ca: 0.0003% or more and 0.010%
12

or less, Mg: 0.0003% or more and 0.010% or less, REM: 0.0003% or more and 0.010% or less, and Zr: 0.0003% or more and 0.010% or less is preferably contained.
[0042]
In the cold rolled steel sheet according to the present embodiment, the balance other than
5 the above constituents is comprised of Fe and impurities. The impurities are constituents, etc.,
entering due to various factors in the producing process such as the ore, scrap and other raw materials when industrially producing a cold rolled steel sheet.
[0043]
The above chemical composition of the steel may be measured by general analytical
10 methods of steel. For example, the chemical composition of the steel may be measured using
inductively coupled plasma-atomic emission spectrometry (ICP-AES). C and S may be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-nondispersive infrared absorption method.
[0044]
15 The metallic structure of the cold rolled steel sheet according to the present embodiment
will be explained.
[0045]
[Metallic Structure]
The cold rolled steel sheet according to the present embodiment has a metallic structure
20 consisting of ferrite phases, hard second phases consisting of martensite phases and retained
austenite phases, and remaining phases consisting of cementite phases and bainite phases, wherein an area ratio of the ferrite phases is 35% or more and 65% or less, an area ratio of the hard second phases is 35% or more and 65% or less, an area ratio of the remaining phases is 0% or more and 5% or less, 60% or more of the ferrite phases are recrystallized ferrite phases, an
25 average crystal grain sizes defined by the 15° grain boundaries is 5.0 um or less, a maximum
connecting rate of hard second phases is 10% or more, and a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.
[0046]
(Area Ratio of Ferrite Phases: 35% or More and 65% or Less)
30 The cold rolled steel sheet according to the present embodiment has an area ratio of 35% or
more and 65% or less of ferrite phases. By having such structures, the soft phases contributing to improvement of the ductility can be sufficiently secured and an excellent uniform elongation and 60% or less yield ratio (YR) can be achieved. If the area ratio of the ferrite phases is less than 35%, the hard second phases become the main structures and excellent uniform elongation and
35 YR of 60% or less cannot be achieved. The area ratio of the ferrite phases may for example be
38% or more, 40% or more, or 45% or more. On the other hand, if the area ratio of ferrite phases
13

is more than 65%, the area ratio of the hard second phases becomes insufficient, so a 1180 MPa or more tensile strength cannot be achieved. The area ratio of the ferrite phases may be, for example, 60% or less, 58% or less, or 55% or less.
[0047]
5 (Area Ratio of Hard Secondary Phases: 35% or More and 65% or Less)
The cold rolled steel sheet according to the present embodiment has an area ratio of 35% or more and 65% or less of hard second phases. The hard second phases are comprised of fresh martensite phases, tempered martensite phases, and retained austenite phases. When simply describing “martensite phases”, it includes both “fresh martensite” and “tempered martensite”.
10 By having such structures, it is possible to sufficiently secure the hard phases contributing to
improvement of the strength and achieve a 1180 MPa or more tensile strength (TS). If the area ratio of hard second phases is less than 35%, the martensite phases and retained austenite phases supporting the strength become insufficient and 1180 MPa or more tensile strength cannot be achieved. The area ratio of the hard second phases may for example be 38% or more, 40% or
15 more, or 45% or more. On the other hand, if the area ratio of hard second phases is more than
65%, the area ratio of the soft phases of the ferrite phases is insufficient, so excellent uniform elongation and a YR of 60% or less cannot be achieved. The area ratio of the hard second phases is for example 63% or less, 60% or less, or 55% or less.
[0048]
20 (Area Ratio of Remaining Phases: 0% or More and 5% or Less)
The cold rolled steel sheet according to the present embodiment has an area ratio of 0% or more and 5% or less of remaining phases. The remaining phases are comprised of cementite phases and bainite phases. If the cementite or bainite unavoidably included in the remaining part is more than 5%, the balance of the strength and uniform elongation falls, so it is not possible to
25 realize excellent uniform elongation and a low yield ratio while maintaining the strength. For
this reason, the area ratio of the remaining phases is 0% or more and 5% or less. Preferably, the area ratio of the remaining phases is 4% or less, 3% or less, 2% or less, or 1% or less.
[0049]
(Recrystallized Ferrite Phases: 60% or More of Ferrite Phases)
30 In the present invention, the ferrite phases are classified into recrystallized ferrite phases not
including dislocations in the grains due to recrystallization and nonrecrystallized ferrite phases including high densities of dislocations introduced in the grains by working in the cold rolling step. In dual-phase structure steel including ferrite phases and hard second phases, the yield point is strongly affected by the strength of the soft ferrite phases, so it is preferable to realize a
35 low yield ratio and then control most of the ferrite phases to softer recrystallized ferrite phases.
Therefore, in the present invention, 60% or more of the ferrite phases is recrystallized ferrite
14

phases, preferably 70% or more, more preferably 80% or more, is recrystallized ferrite phases. If
the recrystallized ferrite phases in the ferrite phases is less than 60%, the yield point of the ferrite
phases rises and a yield ratio of 60% or less can no longer be achieved. Further, excellent
uniform elongation is liable to be unable to be realized. The upper limit of the ratio of the
5 recrystallized ferrite phases among the ferrite phases is not particularly prescribed and may be
100%, 95%, or 90%.
[0050]
(Method of Measurement of Area Ratios of Phases)
The area ratios of the phases of the metallic structure are evaluated as follows by the SEM-
10 EB SD method (electron backscatter diffraction method) and examination of the SEM secondary
electron image.
[0051]
First, a sample having a cross-section of thickness parallel to the rolling direction of the steel sheet as the observed surface was taken, the observed surface was machine polished to a
15 mirror finish, then the surface was electrolytically polished. Next, in one or more observed fields
in a range of 1/8 thickness to 3/8 thickness centered about 1/4 thickness from the surface of the base metal steel sheet at the observed surface, a total area of 2.0x 10-9 m2 or more is analyzed for crystal structure and orientation by the SEM-EBSD method. For analysis of the data obtained by the EBSD method, “OIM Analysis 6.0” made by TSL is used. Further, the distance between
20 evaluation points (steps) is 0.03 to 0.20 um. Regions judged as FCC iron from the results of
observation are deemed retained austenite. Further, boundaries with differences in crystal orientations of 15 degrees or more are deemed grain boundaries to obtain a crystal grain boundary map.
[0052]
25 Next, the same sample as that examined by EBSD is corroded by Nital and examined by a
secondary electron image for the same fields as the examination by EBSD. To examine the same fields as the time of measurement by EBSD, Vickers indentations and other marks may be provided in advance. The obtained secondary electron image is used to measure the area ratios of the ferrite, retained austenite, bainite, tempered martensite, fresh martensite, and cementite. The
30 regions having lower structures in the grains and having cementite precipitating in several
variants are judged to be tempered martensite. The regions with small luminance and with no lower structures observed are judged to be ferrite. The regions with large luminance and with no lower structures revealed by etching are judged to be fresh martensite and retained austenite. The regions not corresponding to any of the above regions are judged to be bainite. The area ratios
35 are calculated by the point counting method to obtain the area ratios of the phases.
[0053]
15

(Method of Measurement of Ratio of Recrystallized Ferrite Phases)
In all of the ferrite regions found above, using a field emission type scanning electron
microscope (FE-SEM) and OIM crystal orientation analysis apparatus, the regions of
recrystallized ferrite were observed at the same regions as the regions observed by the SEM
5 above. Measurement surface 100 um square regions were examined at 0.2 um intervals to obtain
groups of crystal orientation data. The obtained groups of crystal orientation data were analyzed
by analysis software (TSL OIM Analysis). Regions with kernel average misorientations (KAM
values) between first neighboring side measurement points in the ferrite crystal grains of 1.0° or
less are defined as recrystallized regions. The area ratio of those regions with respect to all
10 regions is calculated and the ratio of recrystallized ferrite phases in the ferrite phases are
determined.
[0054]
(Average Crystal Grain Size Defined by 15° Grain Boundaries: 5.0 um or Less)
By refining the crystal grain sizes, the strength of the metallic structure can be improved. In
15 dual phase structure steel containing ferrite phases and hard second phases, the effect in making
the deformation uniform is large. By the deformation being made uniform, uniform elongation can be secured while strength can be secured. If the average crystal grain size defined by the 15° grain boundaries is more than 5.0 um, deformation easily unevenly occurs and realizing both strength and uniform elongation becomes difficult. For this reason, the average crystal grain size
20 defined by the 15° grain boundaries is 5.0 um or less. Preferably, it is 3.0 um or less, more
preferably 2.5 um or less. In the present invention, the grain boundaries of the ferrite phases and hard second phases can both be judged for individual grains by the 15° grain boundaries, so the areas of the grains discriminated by the 15° grain boundaries are calculated as the circle equivalent diameters and used as the grain sizes.
25 [0055]
(Method of Measurement of Average Crystal Grain Size)
The average crystal grain size was measured by the SEM/EBSD method. A sample was taken at 1/4 thickness from the surface of the steel sheet with a cross-section of thickness parallel to the rolling direction of the steel sheet as the observed surface. The surface of the steel sheet
30 was polished to a mirror finish and polished by colloidal silica. A field emission type scanning
electron microscope (FE-SEM) and OIM crystal orientation analysis apparatus were used to obtain groups of crystal orientation data for measurement surface 200 um square regions at 0.2 Um intervals. The obtained groups of crystal orientation data were analyzed by analysis software (TSL OIM Analysis), the interfaces having differences of orientations of 15° or more were
35 defined as crystal grain boundaries, the crystal grain sizes were calculated as circle equivalent
diameters from the areas surrounded by the crystal grain boundaries, and the average crystal
16

grain size was calculated as the median diameter (D50) from the histogram of these crystal grain sizes.
[0056]
In the present invention, to simultaneously achieve a 1180 MPa or more tensile strength,
5 excellent uniform elongation, and 60% or less yield ratio, in addition to the above-mentioned
chemical composition, area ratios of the phases, ratio of the recrystallized ferrite phases in the
ferrite phases, and average crystal grain size, control of the morphology of the steel sheet is the
most important point. That is, as explained above, by controlling the dual phase structure
containing soft recrystallized ferrite phases and hard second phases (martensite phases or
10 retained austenite phases) in constant amounts or more to become a morphology where
improvement of the ductility by the ferrite phase and securing of the strength by the hard second phases are realized in a complementary manner, it is possible to realize the above-mentioned target properties.
[0057]
15 The inventors discovered that for realizing both improvement of the ductility by the ferrite
phases and securing of the strength by the hard second phases in a complementary manner to the maximum extents, it is effective that these two phases have mutually intricately intertwined structures.
[0058]
20 The microstructure having intricately intertwined structures is characterized by the hard
second phases being connected and by the interfacial area being greater than with true circular shaped particles having the same areas. When having intricately intertwined structures, while the reasons why the above-mentioned effects are obtained are not necessarily clear, it is surmised that the localization of deformation is suppressed to distribute the deformation between soft and
25 hard phases and a uniform yield phenomenon occurs in the microstructure as a whole. In the
present invention, “the maximum connecting rate of the hard second phases” is used as an indicator showing the hard second phases are connected together and the “two-dimensional isoperimetric constant of the hard second phases” is used as an indicator of a large interfacial area of the soft phases and hard phases.
30 [0059]
(Maximum Connecting Rate of Hard Secondary Phases of 10% or More)
To obtain the above-mentioned effect, the maximum connecting rate of the hard second phases has to be 10% or more. If the maximum connecting rate of the hard second phases is 10% or more, the soft phases and hard phases have sufficiently intricately intertwined structures, so a
35 yield phenomenon evenly occurs in the metallic structure as a whole and a TS of 1180 MPa or
more and a YR of 60% or less can be simultaneously achieved. The maximum connecting rate of
17

the hard second phases is preferably 15% or more, more preferably 20 or more, still more preferably 25% or more, most preferably 30% or more. The upper limit is not particularly defined, but may be 100% or less, 90% or less, 80% or less, or 70% or less.
[0060]
5 (Two-Dimensional Isoperimetric Constant of Hard Secondary Phases: 0.20 or Less)
Further, to obtain the above-mentioned effect, the two-dimensional isoperimetric constant of the hard second phases has to be 0.20 or less. If the two-dimensional isoperimetric constant of the hard second phases is 0.20 or less, the metallic structure forms a sufficiently uniform network, so it is possible to secure strength by the hard second phases, draw upon the ductility of
10 the ferrite phases at the time of deformation, and simultaneously realize a TS of 1180 MPa or
more and a YR of 60% or less. The two-dimensional isoperimetric constant of the hard second phases is preferably 0.15 or less, more preferably 0.12 or less, still more preferably 0.10 or less. The lower limit is not particularly defined, but may be 0.01 or more, 0.02 or more, or 0.03 or more.
15 [0061]
The maximum connecting rate of the hard second phases and the two-dimensional isoperimetric constant will be explained in more detail below. FIG. 1 schematically shows a maximum connected region 1 in a steel sheet microstructure. The maximum connected region 1 is a microstructure where hard second phases are continuously connected with each other in a
20 mesh form. In FIG. 1, the finely hatched part is the maximum connected region 1, the white
parts are the ferrite structure regions 2, and the roughly hatched part is a hard second phase region 3 not the maximum connected region 1 (nonmaximum connected region 3). To facilitate their differentiation, the maximum connected region 1 and nonmaximum connected region 3 are shown with opposite slants of hatching. In the maximum connected region 1, the plurality of
25 ferrite regions (white parts) are present in mutually separate states. Further, the nonmaximum
connected region 3 is separated from the maximum connected region 1. The nonmaximum connected region 3 is surrounded by ferrite regions (white parts).
[0062]
The maximum connecting rate of the hard second phases is determined by the following
30 method. A secondary electron image measured by FE-SEM by 1000X (measured surface 200
Um square region) at a region down to the position of t/2 depth from the position of the depth
3/8t from the surface (t: thickness of steel sheet) is binarized by the above method and one pixel
showing a hard second phase region is selected in the binarized image. Further, if a pixel
adjoining the thus selected pixel (pixel showing hard second phase region) in any direction of
35 the four directions of up, down, left, and right shows a hard second phase region, these two
pixels are judged to be the same connected region. In the same way, it is successively judged if
18

the pixels adjoining it in the up, down, left, and right directions are connected regions to
determine the range of a single connected region. If the adjoining pixel is not a pixel showing a
hard second phase region (i.e., if the adjoining pixel is a pixel showing a ferrite region), the part
becomes a part of the edge of the connected region. The region having the greatest number of
5 pixels in the connected region of hard second phases is identified as the “maximum connected
region”.
[0063]
The area ratio of the maximum connected region of hard second phases with respect to all
of the hard second phase regions, i.e., the maximum connecting rate of the hard second phases
10 Rs, is obtained by finding the area Sm of the maximum connected region and calculating it from
the ratio Rs=Sm/Ss of the sum hard second phase regions with the area Ss. [0064]
The maximum connecting rate Rs (%) is calculated by the following formulas:
Rs={area Sm of maximum connected region of hard second phases/area Ss of sum hard
15 second phase regions}x 100
Area Ss of sum hard second phase regions=area Sm of maximum connected region + total area Sm’ of nonmaximum connected region
[0065]
The two-dimensional isoperimetric constant K is calculated by the following formulas. The
20 perimeter Lm of the maximum connected region can be measured in the structural image
measured by the above FE-SEM. However, when calculating the perimeter, if any of the four
sides of the image data frame corresponds to part of the perimeter of the maximum connected
region, the length of the corresponding frame is also treated as part of the perimeter of the
maximum connected region.
25 ^.(Lm/2^)2.K=Sm
K=4^Sm/Lm2 Lm: perimeter of maximum connected region of hard second phases
[0066]
FIG. 2 is a schematic view of a binarized image for explaining a two-dimensional
30 isoperimetric constant. FIG. 2(a) is a schematic view showing the case where the maximum
connected region of hard second phases is substantially circular. On the other hand, FIG. 2(b) is
a schematic view showing the case where the maximum connected region has the same area
(Sm) as FIG. 2(a) and has an interfacial shape of hard phases and soft phases intricately
intertwined. For example, for the structure of FIG. 2(a), if measuring the perimeter Lm of the
35 maximum connected region and calculating the two-dimensional isoperimetric constant K based
on the above formula, K=0.92. On the other hand, FIG. 2(b) is the same in area Sm of the
19

maximum connected region as FIG. 2(a), but the perimeter Lm of the maximum connected
region is long, and therefore if similarly calculating the two-dimensional isoperimetric constant
K, K=0.03. From the explanation relating to FIG. 1 and a comparison of FIGS. 2(a) and (b), etc.,
it will be understood that, as prescribed in the present embodiment, by making the maximum
5 connecting rate of the hard second phases 10% or more while making the two-dimensional
isoperimetric constant of the hard second phases 0.20 or less, it is possible to form a relatively
large maximum connected region having an interfacial shape of hard phases and soft phases
intricately intertwined in the metallic structure. For this reason, according to the present
embodiment, it becomes possible to realize the improvement in ductility by the soft phases and
10 the securing of strength by the hard phases in a complementary manner.
[0067]
The cold rolled steel sheet according to the present invention may also have a hot dip
galvanized layer or hot dip galvannealed layer on the surface for the purpose of improvement of
the corrosion resistance.
15 [0068]
Next, the mechanical properties of the cold rolled steel sheet according to the present embodiment will be explained.
[0069]
[Mechanical Properties]
20 (Tensile Strength TS: 1180 MPa or More)
The cold rolled steel sheet according to the present embodiment preferably has sufficient
strength for contributing to lighter weight of automobiles. For this reason, the tensile strength
(TS) is 1180 MPa or more. The tensile strength is preferably 1270 MPa or more, more preferably
1370 MPa or more. The tensile strength is preferably high, but with the configuration of the
25 present embodiment, more than 1780 MPa is difficult, so the substantive upper limit becomes
1780 MPa. The tensile test may be performed based on JIS Z2241 (2011). The sample for the
tensile test use may be obtained from 1/4 of the width direction of the cold rolled steel sheet so
that the direction vertical to the rolling direction (C direction) becomes the longitudinal direction
(JIS No. 5 test piece).
30 [0070]
(Excellent Uniform Elongation uEL)
The value of excellent uniform elongation differs depending on the class of strength of the
steel sheet. In the cold rolled steel sheet according to the present invention, the tensile strength is
1180 MPa or more, but the uniform elongation sought differs according to the class of strength.
35 Explaining this specifically in cold rolled steel sheet with a tensile strength of 1180 to 1370
MPa, tensile strength and excellent uniform elongation become necessary. On the other hand, if
20

the tensile strength is more than 1370 MPa, a higher tensile strength is demanded even if the
uniform elongation characteristic is sacrificed somewhat. Therefore, in the present invention, the
steel sheet having “excellent uniform elongation” is steel sheet satisfying the following
conditions with respect to the tensile strength. The uniform elongation is obtained, in the same
5 way as the case of tensile strength, by performing a tensile test using a JIS No. 5 test piece taken
from a position of 1/4 of the width direction of the cold rolled steel sheet so that the direction vertical to the rolling direction (C direction) becomes the longitudinal direction based on the provisions of JIS Z 2241 (2011).
•In the case where tensile strength TS: 1180 to 1370 MPa
10 uniform elongation uEL> 10.0%
•In the case where tensile strength TS: more than 1370 MPa
uniform elongation uEL>7.0% [0071]
(Yield Ratio YR<60%)
15 The cold rolled steel sheet according to the present embodiment has sufficient strength for
contributing to lighter weight of automobiles while must be provided with excellent shape
fixability and workability. The yield ratio YR is 60% or less. Preferably, the YR is 58% or less,
more preferably the YR is 55% or less. The yield ratio YR is the ratio of the yield point YS to
the tensile strength TS and is expressed by YR (%)=(YS/TS)x100. The yield point as well, in the
20 same way as the case of the tensile strength, is found by using a JIS No. 5 test piece taken from a
position of 1/4 in the width direction of cold rolled steel sheet so that the direction vertical to the rolling direction (C direction) becomes the longitudinal direction and performing a tensile test based on the provisions of JIS Z 2241 (2011).
[0072]
25 Next, the method for producing the cold rolled steel sheet according to the present
embodiment will be explained.
[0073]

In the present invention, by the four steps of (a) a hot rolling step storing rolling strain
30 while controlling the microstructure to uniform low temperature transformed phases (upper
bainite phases, martensite phases, or mixed phases comprised of the same), (b) a tempering step
making the iron carbides uniformly and finely precipitate, (c) a cold rolling step imparting a
driving force for recrystallization of ferrite, and (d) an annealing step for making ferrite
sufficiently recrystallize during heating, pinning the recrystallized ferrite grain boundaries by
35 iron carbides, and promoting growth of austenite along the grain boundaries to thereby make the
soft phases and hard phases uniformly and finely disperse and control the interfacial shape of the
21

two phases to an intricately intertwined morphology, it is possible to make the ferrite phases of
the soft phases and the hard second phases comprised of martensite phases and retained austenite
phases present in desired area ratios, disperse the phases uniformly and finely, and control the
interfacial shapes to an intricately intertwined morphology. Explained more specifically, by
5 arranging iron carbides on the recrystallized ferrite grain boundaries and pinning the
recrystallized ferrite grain boundaries, it is believed that not only are the crystal grains refined, but also the directions of growth of the austenite follow along the grain boundaries, and therefore intricately shaped austenite can be formed between the spaces of ferrite along the grain boundaries. Therefore, it is possible to control the metallic structure of the finally obtained steel
10 sheet to a morphology where soft phases and hard phases are intricately intertwined and as a
result, for example, it becomes possible to obtain the characteristic metallic structure according to the present invention where the two-dimensional isoperimetric constant of the hard second phases is 0.20 or less. Below, the steps in the method for producing the cold rolled steel sheet according to the present invention will be explained in detail.
15 [0074]
The producing step preceding the hot rolling step is not particularly limited. That is, it is sufficient to perform smelting by a blast furnace or electric furnace, etc., then perform various secondary refining, then perform casting by the usual continuous casting, casting by the ingot method, thin slab casting, or another method. In the case of continuous casting, the cast slab is
20 cooled once to a low temperature, then again heated and then hot rolled. It is also possible to not
cool the cast slab down to a low temperature, but hot roll it as is after casting. For the raw material, scrap may also be used. The chemical composition of the slab is adjusted to a chemical composition such as explained above.
[0075]
25 The cast slab is heat treated. In this heating step, for example, the slab may be heated to
1200°C or more and 1300°C or less temperature, then held there for 30 minutes or more. If the heating temperature is less than 1200°C, the Ti- and Nb-based precipitates are not sufficiently melted, so sufficient precipitation strengthening is liable to be unable to be obtained at the time of the later step of hot rolling. Further, sometimes these remain in the state as coarse carbides
30 whereby the shapeability is degraded. Therefore, the heating temperature of the slab is
preferably 1200°C or more. 1220°C or more is more preferable. On the other hand, if the heating temperature is more than 1300°C, the amount of formation of scale increases and the yield is liable to fall, so the heating temperature is preferably 1300°C or less, more preferably 1280°C or less. To make the Ti- and Nb-based precipitates sufficiently melt, the sheet is preferably held at
35 this temperature range for 30 minutes or more. For example, it may also be held there for 45
minutes or more, 60 minutes or more, 90 minutes or more, or 120 minutes or more. Further, to
22

suppress excessive scale loss, the holding time is preferably 10 hours or less, more preferably 5 hours or less.
[0076]
[Hot Rolling Step]
5 (Rough Rolling)
In the hot rolling step in the present invention, the sheet is rough rolled, then rolled by
multi-stage finish rolling. First, the heated slab is rough rolled. In this rough rolling, the slab
need only be rendered the desired dimensions and shape. The conditions are not particularly
limited. The thickness of the rough rolled steel sheet has an effect on amount of temperature
10 drop from the front end to tail end of the rolled sheet occurring from the time of start of rolling
to the time of end of rolling in the finish rolling step, and therefore is preferably determined considering this.
[0077]
(Finish Rolling)
15 In the finish rolling, by controlling the rolling reduction of the final stage in the multistage
finish rolling to 15% or more and 50% or less and controlling the rolling end temperature of the final stage to Ar3°C or more and 950°C or less, it becomes important to raise the stored strain of the prior austenite grains at the time of hot rolling and increase the density of the nucleation sites of the iron carbides.
20 [0078]
(Rolling Reduction of Final Stage of Finish Rolling: 15% or More and 50% or Less) If the rolling reduction of the final stage of the finish rolling is less than 15%, the amount of stored strain of the prior austenite grains is not sufficient, the precipitation sites of the iron carbides decrease, refinement of the crystal grains cannot be achieved in the annealing step after
25 the cold rolling step, and the desired tensile strength and uniform elongation cannot be
simultaneously obtained. Therefore, the rolling reduction of the final stage of the finish rolling is 15% or more. The rolling reduction of the final stage of the finish rolling is preferably 16% or more, more preferably 18% or more, still more preferably 20% or more. On the other hand, if the rolling reduction of the final stage of the finish rolling is more than 50%, the steel sheet
30 remarkably deteriorates in shape and rolling becomes difficult, so the rolling reduction of the
final stage of the finish rolling is 50% or less. The rolling reduction of the final stage of the finish rolling is preferably 45% or less, more preferably 40% or less.
[0079]
(End Temperature of Finish Rolling: Ar3°C or More and 950°C or Less)
35 If the end temperature of the finish rolling becomes less than Ar3°C, ferrite and pearlite are
formed, uniform low temperature transformed phase structures cannot be realized, the
23

recrystallized ferrite grain boundaries cannot be pinned by the iron carbides, and the shape of the
boundaries of the soft phases and hard phases is liable to be unable to be controlled to an
intricately intertwined morphology. For this reason, the end temperature of the finish rolling is
Ar3°C or more. On the other hand, if the end temperature of the finish rolling is more than
5 950°C, the stored strain of the prior austenite grains is reduced by the recovered recrystallization
whereby the precipitation sites of iron carbides are reduced and the interfacial shapes of the soft phases and hard phases are liable to be unable to be controlled to an intricately intertwined morphology. For this reason, the finish rolling end temperature is 950°C or less. The finish rolling end temperature is preferably (Ar3+10)°C or more, more preferably (Ar3+20)°C. The
10 finish rolling end temperature is preferably 940°C or less, more preferably 930°C or less.
[0080]
(Average Cooling Rate: 50°C/s or More)
The hot rolled steel sheet after the finish rolling is cooled down to the coiling temperature. If the average cooling rate after the finish rolling is less than 50°C/s, ferrite and pearlite
15 precipitate during cooling, uniform low temperature transformed phase structures cannot be
obtained, and a fine intricately intertwined morphology cannot be obtained, so the average cooling rate is 50°C/s or more. The average cooling rate is preferably 70°C/s or more, more preferably 100°C/s or more. The upper limit of the average cooling rate is not particularly prescribed, but from the viewpoint of stable production, it is preferably 200°C/s or less.
20 [0081]
(Coiling Temperature: Less Than 400°C)
If coiling at 400°C or more temperature, ferrite and pearlite or bainitic ferrite precipitates, whereby it is not possible to control the microstructure of the hot rolled steel sheet to uniform low temperature transformed phases and a fine and intricately intertwined morphology cannot be
25 obtained. For this reason, the coiling is performed at less than 400°C temperature. The coiling
temperature is preferably 380°C or less, more preferably 350°C or less, still more preferably 100°C or less. [0082]
[Tempering Step]
30 In the present invention, pinning of recrystallized ferrite grain boundaries by iron carbides
and formation of austenite from the pinning particles of iron carbides are utilized to thereby
realize a fine and intricately intertwined morphology. Therefore, control of the iron carbides in
the tempering step of hot rolled steel sheet is an extremely important control process even in the
present application.
35 [0083]
By tempering the hot rolled steel sheet after coiling, an amount of iron carbides required for
24

pinning the recrystallized ferrite grain boundaries is made to precipitate. Here, the pinning force
of recrystallized ferrite grain boundaries by iron oxides is proportional to the amount of
precipitation of the pinning particles of iron oxide and inversely proportional to the particle size
of iron carbides, so to effectively cause the pinning force to be generated, it is preferable to
5 cause fine iron carbides to precipitate in large amounts. On the other hand, the larger the particle
size of the iron carbides, the higher the frequency of nucleation of austenite starting from the iron carbides on the grain boundaries, so from the viewpoint of obtaining both the pinning force and austenite nucleation, it is necessary to control the particle size of the iron carbides to a suitable range.
10 [0084]
In the present invention, the inventors discovered that by performing tempering in suitable ranges of temperature and heat treatment time, it becomes possible to suitably control the amount of precipitation and particle size of the iron carbides, secure the pinning force of the recrystallized ferrite grain boundaries, and utilize the iron carbides on the grain boundaries as
15 austenite nucleation sites. Specifically, the tempered heat treatment is performed at a tempering
temperature of 450°C or more and less than 600°C in temperature range so that the tempering parameter £ becomes 14000 to 18000. By performing such heat treatment, it is possible to sufficiently obtain the pinning effect by iron carbides to obtain a fine and intricately intertwined morphology. As a result, for example, it becomes possible to obtain a metallic structure with a
20 two-dimensional isoperimetric constant of the hard second phases of 0.20 or less.
[0085]
(Tempering Temperature: 450°C or More and Less Than 600°C)
The tempering temperature is 450°C or more and less than 600°C. If the tempering
temperature is less than 450°C, the particle size of the iron carbides becomes excessively fine,
25 the effect as a nucleation site of austenite cannot be sufficiently obtained, and a fine and
intricately intertwined morphology cannot be obtained. For this reason, the tempering
temperature is 450°C or more. The tempering temperature is preferably 500°C or more. On the
other hand, if 600°C or more, Ostwald ripening of the iron carbides causes the pinning force of
the iron carbides to remarkably fall and a fine and intricately intertwined morphology cannot be
30 obtained. For this reason, the tempering heat treatment temperature is less than 600°C. The
tempering temperature is preferably 550°C or less. [0086]
(Tempering Parameter £: 14000 or More and 18000 or Less)
If the tempering parameter £ is less than 14000, the amount of precipitation of iron carbides
35 becomes insufficient, the pinning force of recrystallized ferrite grain boundaries by iron carbides
becomes insufficient, and a 5.0 um or less average particle size cannot be realized. On the other
25

hand, if the tempering parameter £ is more than 18000, excessive growth of the iron carbides
causes the pinning force of the recrystallized ferrite grain boundaries to become insufficient and
5.0 um or less average grain size to be unable to be realized. For this reason, the tempering
parameter £ is 14000 or more and 18000 or less. Preferably, the tempering parameter is 14500 or
5 more, 15000 or more, or 15500 or more. Further, preferably, the tempering parameter is 17500
or less, 17000 or less, or 16500 or less. The tempering parameter £ can be found by following Formula 1.
Formula 1: ^=(T+273)[log10(t/3600)+20]
T [°C]: tempering temperature, t [s]: tempering time
10 [0087]
[Cold Rolling Step]
(Rolling Reduction: 30% or More)
The steel sheet tempered in the above way is pickled, then cold rolled. If the rolling
reduction of the cold rolling step is less than 30%, the driving force of recrystallization of the
15 ferrite is not sufficient and nonrecrystallized ferrite remains, so the rolling reduction of the cold
rolling step is 30% or more. The rolling reduction is preferably 35% or more, more preferably
40% or more, more preferably 45% or more. On the other hand, no upper limit of the cold
rolling reduction is particularly provided, but if a more than 70% rolling reduction, sometimes
the rolling load becomes too high and rolling does not become possible or there is a danger of
20 the steel sheet fracturing during rolling, so the rolling reduction is preferably 70% or less.
[0088]
In the present invention, due to the iron oxides made to precipitate due to tempering the steel sheet after the hot rolling step, the recrystallized ferrite grain boundaries are pinned and softening of the base phase ferrite and refinement of the crystal grains are achieved. Due to the
25 austenite transformation using the iron carbides on these grain boundaries as nucleation sites, the
morphology becomes intricately intertwined. The reason why the morphology becomes an intricately intertwined shape by using the iron carbides on the grain boundaries as austenite nucleation sites is not necessarily clear, but it may be that the main reason is that anisotropy occurs in the direction of growth of the austenite due to the difference in the grain boundary
30 diffusion coefficient due to the tilt angle of the ferrite grain boundaries contacting the iron
carbides. That is, by utilizing the austenite transformation from the iron carbides on the grain boundaries, it is possible to realize not a microstructure like in the past where the martensite or other hard second phases completely cover the surroundings of the ferrite grains, but a morphology where the ferrite and hard second phases are intricately intertwined.
35 [0089]
[Annealing Step]
26

(Average Heating Rate From 500°C to Ac1C: 5.0°C/s or Less)
The steel sheet cold rolled in the above way is annealed by heating it to the maximum
heating temperature, holding it there, then cooling it. In the heating process from 500°C to
Ac1°C, the ferrite phases are recrystallized after cold rolling and the recrystallized ferrite grain
5 boundaries are pinned by the iron carbides. If the average heating rate from 500°C to Ac1C is
more than 5.0°C/s, the recrystallization of ferrite does not sufficiently occur and further
sufficient iron carbides cannot be placed on the recrystallized ferrite grain boundaries, and
austenite transformation is started, so it is not possible to obtain a morphology where soft phases
and hard second phases are sufficiently intricately intertwined. For this reason, the average
10 heating rate from 500°C to Ac1C is 5.0°C/s or less. The average heating rate is preferably
4.0°C/s or less, more preferably 3.0°C/s or less. [0090]
(Maximum Heating Temperature: (Ac1+10)°C or More (Ac3-10)°C or Less)
If the maximum heating temperature of the annealing step is less than the (Ac1+10)°C, it is
15 not possible to secure 35% or more hard second phases, so the maximum heating temperature is
the (Ac1+10)°C or more. On the other hand, if more than (Ac3-10)°C, austenite transformation
excessively proceeds and the structural fraction of hard second phases becomes more than 65%,
so the maximum heating temperature is (Ac3-10)°C or less. The maximum heating temperature
is preferably (Ac1+20)°C or more, more preferably (Ac1+30)°C or more. Further, the maximum
20 heating temperature is preferably (Ac3-20)°C or less, more preferably (Ac3-30)°C or less.
[0091]
(Holding Time at Maximum Heating Temperature: 60 Seconds or More)
If the holding time at the maximum heating temperature is less than 60 seconds, the melting
time of the iron carbides becomes insufficient, iron carbides remain unmelted as impurities, i.e.,
25 the area ratio of the remaining phases becomes higher, so the holding time is 60 seconds or
more. On the other hand, if the holding time is more than 1200 seconds, production is interfered
with and an increase in costs is led to, so the heating holding time is preferably 1200 seconds or
less. Preferably, the holding time at the maximum heating temperature is 120 seconds or more,
180 seconds or more, 240 seconds or more, or 300 seconds or more.
30 [0092]
(Average Cooling Rate From (Ac1-50)°C to Ms°C or Less Cooling End Temperature: 20°C/s or More)
If the average cooling rate from (Ac1-50)°C to an Ms°C or less cooling end temperature is
less than 20°C/s, this becomes a factor due to which pearlite and bainitic ferrite are formed
35 during cooling, the area ratio of the remaining phases increases, and the desired yield ratio can
no longer be obtained, so the average cooling rate is 20°C/s or more. The average cooling rate is
27

preferably 30°C/s or more, 40°C/s or more, or 50°C/s or more. Further, the upper limit of the average cooling rate is not particularly limited, but for example may be 100°C/s or less. [0093]
(Cooling End Temperature: Ms°C or Less)
5 If the cooling end temperature is more than Ms°C, after the cooling, pearlite and bainitic
ferrite are formed, the area ratio of the remaining phases increases, and the balance of tensile
strength and the uniform elongation falls. For this reason, the cooling end temperature is the Ms
point or less. Preferably, the cooling end temperature is (Ms-10)°C or less, (Ms-20)°C or less, or
(Ms-30)°C or less. The lower limit of the cooling end temperature is not particularly limited, but
10 may also be room temperature or so (for example, 20°C).
[0094]
The above-mentioned transformation points: Ac1 (°C), Ac3 (°C), Ar3 (°C), and Ms (°C) are calculated by the following formulas:
Ac1 [°C]=751-16x[%C]+35x[%Si]-28x[%Mn]
15 Ac3 [°C]=881-353x[%C]+65x[%Si]-24x[%Mn]
Ar3 [°C]=910-203x[%C]+44.7x[%Si]-24x[%Mn]-50x[%Ni]
Ms [°C]=521-353x[%C]-22x[%Si]-24x[%Mn]
Here, %C, %Si, %Mn, and %Ni are the contents of C, Si, Mn, and Ni [mass%].
[0095]
20 The cold rolled steel sheet according to the present invention can be obtained by the above
four steps, i.e., the hot rolling step, tempering step, cold rolling step, and annealing step. In addition to these steps, the following additional steps, i.e., a reheating step, hot dip galvanization step, hot dip galvanization step, and alloying step, may be performed.
[0096]
25 [Reheating Step]
(Reheating Temperature of 200°C or More and 450°C or Less) In the annealing step, after cooling to the Ms°C or less temperature, for the purpose of improving the uniform elongation, the steel sheet may be reheated to 200°C or more and 450°C or less temperature. If the reheating temperature is less than 200°C, sometimes the effect of
30 raising the uniform elongation cannot be effectively exhibited. If the reheating temperature is
more than 450°C, cementite precipitates, i.e., the area ratio of the remaining phases increases and sometimes a yield ratio YR of 60% or less can no longer be achieved, so the reheating temperature is preferably made 200°C or more and 450°C or less. The reheating temperature is preferably 250°C or more, more preferably 300°C or more. Further, the reheating temperature is
35 preferably 400°C or less, more preferably 350°C or less.
[0097]
28

(Holding Time at Reheating Temperature: 60 Seconds or More and 600 Seconds or Less)
If the holding time at the reheating temperature is less than 60 seconds, the effect of raising
the uniform elongation cannot be sufficiently obtained, so the holding time is preferably 60
seconds or more. On the other hand, if the holding temperature at the reheating temperature
5 becomes more than 600 seconds, the yield point is improved and a yield ratio YR of 60% or less
is liable to be unable to be obtained. For this reason, the holding time is preferably 600 seconds or less. More preferably, the holding time at the reheating temperature is 550 seconds or less, 500 seconds or less, 450 seconds or less, or 400 seconds or less.
[0098]
10 [Hot Dip Galvanization Step]
In the hot dip galvanization step, the cold rolled annealed sheet after the annealing step is hot dip galvanized by heating it from the Ms point or less cooling temperature to a predetermined temperature suitable for hot dip galvanization, then dipping the cold rolled annealed sheet in a hot dip galvanization bath to form a hot dip galvanized layer on the surface.
15 The hot dip galvanization conditions do not particularly have to be limited. The cold rolled
annealed sheet is dipped in the hot dip galvanization bath and formed on its surface with a predetermined thickness of hot dip galvanized layer. Any of the usual hot dip galvanization conditions may be applied. For example, the hot dip galvanization may be performed at 430°C or more. If the temperature of the steel sheet when entering the hot dip galvanization bath falls
20 below 430°C, there is a possibility of the zinc deposited on the steel sheet aggregating, so if the
austempering temperature falls below 430°C, the steel sheet is preferably heated to a predetermined temperature before entering the hot dip galvanization bath. Further, after the hot dip galvanization, wiping may also be performed to adjust the amount of coating deposition in accordance with need. The temperature of the hot dip galvanization may, for example, be 500°C
25 or less.
[0099]
[Alloying Step]
The hot dip galvanized steel sheet formed with the hot dip galvanized layer may also be
alloyed in accordance with need. In this case, if the alloying temperature is less than 400°C, the
30 alloying rate becomes slow and the productivity is impaired. Not only that, unevenness occurs in
the alloying, so the alloying temperature is 400°C or more. On the other hand, if the alloying temperature is more than 600°C, the alloying excessively proceeds and sometimes the coating adhesion of the steel sheet deteriorates. Therefore, the alloying temperature is 600°C or less.
35 EXAMPLES
[0100]
29

(Preparation of Samples of Cold Rolled Steel Sheet)
Slabs having the chemical compositions shown in Table 1 were processed by a hot rolling
step, tempering step, cold rolling step, and annealing step under the conditions shown in Table 2
to obtain thickness 1.5 mm cold rolled steel sheets. Sample Nos. 19 to 21 and 34 were processed
5 by a reheating step after the annealing step. Sample No. 22 was hot dip galvanized at 450°C.
This was indicated as “GI” in Table 2. Sample No. 42 was hot dip galvanized at 450°C, then was alloyed at 460°C. In Table 2, this is shown as “GA”. Further, in Table 2, “RT” means “room temperature”.
[0101]
10 The transformation points Ac1 (°C), Ac3 (°C), Ar3 (°C), and Ms (°C) of Table 1 and Table
2 were calculated from the following formulas:
Ac1 [°C]=751-16x[%C]+35x[%Si]-28x[%Mn]
Ac3 [°C]=881-353x[%C]+65x[%Si]-24x[%Mn]
Ar3 [°C]=910-203x[%C]+44.7x[%Si]-24x[%Mn]-50x[%Ni]
15 Ms [°C]=521-353x[%C]-22x[%Si]-24x[%Mn]
[0102]
The tempering parameter of Table 2 was calculated from following Formula 1.
Formula 1: ^=(T+273)[log10(t/3600)+20]
T [°C]: the tempering temperature, t [s]: tempering time
20 [0103]
(Determination of Metallic Structures)
The area ratios of the phases of the metallic structures of Table 3 were evaluated by the
SEM-EBSD method and observation of SEM secondary electron images. Specifically, first, in
each, a sample was taken with the cross-section of thickness parallel to the rolling direction of
25 the steel sheet as the observed surface. The observed surface was machine polished to a mirror
finish, then was electrolytically polished. Next, at five observed fields in the range of 1/8
thickness to 3/8 thickness centered at 1/4 thickness from the surface of the base metal steel sheet
at the observed surface, a total area of 1.0x10-8 m2 was analyzed for crystal structure and
orientation by the SEM-EBSD method. For analysis of the data obtained by the EBSD method,
30 “OIM Analysis 6.0” made by TSL was used. Further, the distance between evaluation points
(steps) was 0.10 um. From the results of observation, the regions judged to be FCC iron were
deemed retained austenite and further the boundaries with differences of crystal orientations of
15 degrees or more were deemed grain boundaries to obtain a crystal grain boundary map. Next,
samples the same as those examined for EBSD were corroded by Nital and secondary electron
35 images were observed at the same fields as the observation by EBSD. The obtained secondary
electron image is used to measure the area ratios of ferrite, retained austenite, bainite, tempered
30

martensite, fresh martensite, and cementite. The regions having lower structures in the grains
and having cementite precipitating in several variants are judged to be tempered martensite, the
regions with small luminance and with no lower structures observed are judged to be ferrite, and
the regions with large luminance and with no lower structures revealed by etching are judged to
5 be fresh martensite and retained austenite. The regions not corresponding to any of the above
regions are judged to be bainite. The area ratios are calculated by the point counting method to obtain the area ratios of the phases.
[0104]
(Measurement of Ratio of Recrystallized Ferrite Phases)
10 In all of the ferrite regions found above, the regions of recrystallized ferrite were observed
at the same regions as the regions observed by the SEM above using an FE-SEM and OIM crystal orientation analysis apparatus. Measurement surface 100 [im square regions were examined at 0.2 [im intervals to obtain groups of crystal orientation data. The obtained groups of crystal orientation data were analyzed by analysis software (TSL OIM Analysis). Regions with
15 KAM values between first neighboring side measurement points in the ferrite crystal grains of
1.0° or less are defined as recrystallized regions. The area ratios of those regions with respect to all of the regions is calculated and the ratio of recrystallized ferrite phases in the ferrite phases are determined. The ratio of the obtained recrystallized ferrite is shown in Table 3.
[0105]
20 (Measurement of Average Crystal Grain Size)
The average crystal grain size was measured by the SEM/EBSD method. In each case, a sample was taken at 1/4 thickness from the surface of the steel sheet with a cross-section of thickness parallel to the rolling direction of the steel sheet as the observed surface. The surface of the steel sheet was polished to a mirror finish and polished by colloidal silica. A field
25 emission type scanning electron microscope (FE-SEM) and OIM crystal orientation analysis
apparatus were used to obtain groups of crystal orientation data for measurement surface 200 um square regions at 0.2 [im intervals. The obtained groups of crystal orientation data were analyzed by analysis software (TSL OIM Analysis), the interfaces having differences of orientations of 15° or more were defined as crystal grain boundaries, the crystal grain sizes were calculated as
30 circle equivalent diameters from the areas surrounded by the crystal grain boundaries, and the
average crystal grain size was calculated as the median diameter (D50) from the histogram of these crystal grain sizes.
[0106]
(Measurement of Maximum Connecting Rate of Hard Secondary Phases)
35 (Measurement of Two-Dimensional Isoperimetric Constant of Hard Secondary Phases)
The maximum connecting rate of the hard second phases was determined by the following
31

method. A structural image measured by FE-SEM by 1000X at a region down to the position of
t/2 depth from the position of the depth 3/8t from the surface (t: thickness of steel sheet) was
binarized and one pixel showing the hard second phase region was selected in the binarized
image. Further, if a pixel adjoining the thus selected pixel in any direction of the four directions
5 of up, down, left, and right showed a hard second phase, these two pixels were judged to be the
same connected region. In the same way, it is successively judged if the pixels adjoining it in the
up, down, left, and right directions are in a connected region to determine the range of a single
connected region. The region having the greatest number of pixels in the connected region of the
hard second phases determined in this way was specified as the “maximum connected region”.
10 The area ratio of the maximum connected region of the hard second phases with all of the hard
second phase regions, i.e., the maximum connecting rate Rs of the hard second phases, was calculated from the ratio Rs=Sm/Ss of Area Sm of the maximum connected region found and the area Ss of the sum hard second phase regions.
[0107]
15 The maximum connecting rate Rs (%) was calculated by the following formulas:
Rs={area Sm of maximum connected region of hard second phases/area Ss of sum hard second phase regions}x 100
Area Ss of sum hard second phase regions=area Sm of maximum connected region + total
area Sm’ of nonmaximum connected region
20 [0108]
The two-dimensional isoperimetric constant K was calculated by the following formula.
The perimeter Lm of the maximum connected region was measured in a structural image
measured by the above FE-SEM.
^•(Lm/2^)2.K=Sm
25 K=4^Sm/Lm2
Lm: perimeter of maximum connected region of hard second phases
[0109]
(Measurement of Mechanical Properties)
The tensile strength, yield point, and uniform elongation were measured as follows: A JIS
30 No. 5 test piece taken from a position of 1/4 of the width direction of the cold rolled steel sheet
in a direction perpendicular to the rolling direction (C direction) as the longitudinal direction was
used for conducting a tensile test based on the provisions of JIS Z 2241 (2011) to find the yield
point (0.2% yield strength) YS, tensile strength TS, and uniform elongation uEL. Further, the
yield ratio YR was found using YR=(YS/TS)x100. When the tensile strength TS was 1180 MPa
35 or more, the uniform elongation uEL was 10.0% or more (TS: 1180 to 1370 MPa) or 7.0% or
more (TS: more than 1370 MPa), and the yield ratio YR was 60% or less, the steel sheet was
32

evaluated as high strength cold rolled steel sheet excellent in workability and shape fixability.
[0110]
The underlined numerical values in Tables 1 to 3 show values outside the scope of the
present invention.
5 [0111]
In Tables 2 to 3, Sample Nos. 1 to 3, No. 5, No. 9, No. 19, No. 22, No. 23, and Nos. 28 to 44 are steel sheets of the present inventions satisfying all of the conditions of the present invention.
[0112]
10 In the invention examples, the chemical composition is satisfied and the structural fractions
and particle size and morphologies are suitable, so cold rolled steel sheets with tensile strengths of 1180 MPa or more, with excellent uniform elongations, and with yield ratios YR of 60% or less are obtained.
[0113]
15 Sample No. 26 has a chemical composition of the steel outside the scope prescribed in the
present invention so an excellent 1180 MPa or more tensile strength cannot be obtained. Further, No. 27 does not satisfy the chemical composition of the steel prescribed in the present invention, so excellent uniform elongation and a low yield ratio are not obtained.
[0114]
20 Sample No. 4, No. 6 to 8, No. 10 to 18, No. 20, No. 21, No. 24, and No. 25 have producing
conditions outside the scope prescribed in the present invention, so a 1180 MPa or more tensile strength and excellent uniform elongation and low yield ratio cannot be simultaneously obtained.
[0115]
33

[Table 1] Table 1
Steel type Chemical composition (units: mass%, balance Fe and impurities) Transformation points (°C)

C Si Mn sol. Al P S N Others Ac1 Ac3 Ar3 Ms
A 0.18 1.40 2.10 0.030 0.010 0.001 0.003 738 858 886 376
B 0.22 2.01 2.80 0.029 0.010 0.001 0.002 739 867 888 332
C 0.27 2.20 2.90 0.031 0.008 0.001 0.003 742 859 884 308
D 0.30 3.10 3.30 0.025 0.010 0.002 0.002 B:0.001 762 897 908 268
E 0.26 1.20 1.20 0.025 0.010 0.001 0.002 755 838 882 374
F 0.11 1.00 2.40 0.029 0.010 0.001 0.003 717 850 875 403
G 0.21 0.40 2.10 0.020 0.012 0.003 0.003 703 782 835 388
H 0.24 1.80 1.80 0.029 0.011 0.002 0.003 B:0.001 760 870 899 353
I 0.18 1.20 2.10 0.025 0.010 0.001 0.002 Nb:0.002 731 845 877 381
J 0.17 0.90 2.50 0.029 0.010 0.002 0.003 Ti:0.010 710 819 856 381
K 0.20 1.60 2.40 0.030 0.011 0.001 0.003 V:0.01 737 857 883 358
L 0.24 1.40 2.00 0.025 0.010 0.001 0.003 Cr:0.40 740 839 876 357
M 0.19 2.10 1.80 0.020 0.012 0.001 0.003 Mo:0.05 771 907 922 365
N 0.23 1.90 3.10 0.020 0.010 0.001 0.002 Cu:0.01 727 849 874 324
O 0.24 2.11 2.01 0.030 0.010 0.001 0.003 Co:0.10 765 885 907 342
P 0.19 1.00 2.20 0.029 0.010 0.001 0.003 W:0.01 721 826 863 379
Q 0.28 3.10 1.80 0.020 0.012 0.003 0.003 Ni:0.80 805 940 909 311
R 0.18 2.80 2.50 0.030 0.011 0.001 0.003 Ca:0.006 776 939 939 336
S 0.28 1.20 2.70 0.021 0.013 0.001 0.002 Mg：0.002 713 795 842 331
T 0.35 2.80 3.30 0.030 0.011 0.002 0.003 REM:0.005 751 860 885 257
U 0.28 2.10 1.20 0.130 0.014 0.001 0.002 Zr:0.002 786 890 918 347
V 0.30 1.20 2.90 0.580 0.010 0.002 0.003 707 784 833 319
W 0.28 0.70 2.81 0.710 0.010 0.001 0.003 692 760 817 339

[0116]
5 [Table 2-1]

34

Table 2-1

Sample no. Chemical comp. Transformation points Hot rolling step Tempering step Cold rolling step Annealing step Reheating step Hot dip galvan-ization step Remarks

Ac1
(°C) Ac3
(°C) Ar3
(°C) Ms (°C) Final stage rolling reduction (％) Finish
rolling
end
temp.
(°C) Average
cooling
rate
(°C/s) Coiling temp.
(°C) Tempering temp.
(°C) Time (h) Tempering parameter
% Rolling
reduction
(％) Heating rate
(°C/s) Max. heating temp.
(°C) Heating
holding
time
(s) Cooling rate
(°C/s) Cooling stop temp.
(°C) Reheat temp.
(°C) Reheat time (s)

1 A 738 858 886 376 20 920 100 RT 500 1.0 15460 63 3.8 770 600 50 RT - - - Inv. ex.
2 B 739 867 888 332 21 920 100 150 550 1.0 16460 55 3.9 800 600 50 RT - - - Inv. ex.
3 C 742 859 884 308 17 900 80 RT 500 1.0 15460 48 3.9 760 1000 50 RT - - - Inv. ex.
4 A 738 858 886 376 10 920 100 80 530 1.0 16060 52 3.7 770 500 45 300 - - - Comp. ex.
5 D 762 897 908 268 20 930 100 300 500 3.0 15829 40 4.2 780 300 30 250 - - - Inv. ex.
6 A 738 858 886 376 19 870 100 150 500 1.0 15460 50 4.2 770 300 50 300 - - - Comp. ex.
7 J 710 819 856 381 22 1000 100 100 500 1.5 15596 53 3.8 750 500 50 100 - - - Comp. ex.
8 A 738 858 886 376 21 910 25 200 490 1.2 15320 55 3.8 810 600 50 100 - - - Comp. ex.
9 A 738 858 886 376 20 910 100 385 500 1.0 15460 50 2.5 790 600 50 50 - - - Inv. ex.
10 B 739 867 888 332 18 920 100 150 400 12.0 14186 45 4.0 800 600 50 100 - - - Comp. ex.
11 L 740 839 876 357 21 915 100 100 630 0.5 17788 50 4.1 790 500 50 100 - - - Comp. ex.
12 B 739 867 888 332 20 920 120 200 580 30.0 18320 60 4.5 790 500 50 100 - - - Comp. ex.
13 C 742 859 884 308 20 920 100 120 500 1.0 15460 20 4.5 795 100 50 100 - - - Comp. ex.
14 C 742 859 884 308 20 930 90 RT 500 1.0 15460 50 10.0 780 70 50 150 - - - Comp. ex.
15 A 738 858 886 376 20 920 100 RT 500 1.0 15460 63 3.8 770 30 50 100 - - - Comp. ex.
16 C 742 859 884 308 20 920 100 100 520 1.2 15923 45 3.6 795 300 15 100 - - - Comp. ex.
17 A 738 858 886 376 20 920 100 RT 500 1.0 15460 53 3.8 790 300 30 400 - - - Comp. ex.
18 A 738 858 886 376 20 920 70 600 500 2.0 15693 50 4.0 790 600 50 100 - - - Comp. ex.
19 B 739 867 888 332 25 920 100 RT 500 3.0 15829 50 3.9 800 400 50 RT 300 300 - Inv. ex.
20 C 742 859 884 308 22 900 120 250 510 2.0 15896 55 3.6 780 150 40 100 500 200 - Comp. ex.
21 C 742 859 884 308 20 930 100 100 500 1.0 15460 45 4.0 775 300 40 150 400 700 - Comp. ex.
22 A 738 858 886 376 17 930 100 200 500 1.5 15596 50 4.5 800 600 40 350 - - GI Inv. ex.
[0117]
[Table 2-2] 5
35

Table 2-2

Sample no. Chemical comp. Transformation points Hot rolling step Tempering step Cold
rolling
step Annealing step Reheating step Hot dip galvan-ization step Remarks

Ac1
(°C) Ac3
(°C) Ar3
(°C) Ms (°C) Final stage rolling reduction (％) Finish
rolling
end
temp.
(°C) Average
cooling
rate
(°C/s) Coiling temp.
(°C) Tempering temp.
(°C) Time (h) Tempering parameter
% Rolling
reduction
(％) Heating rate
(°C/s) Max. heating temp.
(°C) Heating
holding
time
(s) Cooling rate
(°C/s) Cooling stop temp.
(°C) Reheat temp.
(°C) Reheat time (s)

23 E 755 838 882 374 17 900 100 200 500 1.0 15460 50 3.6 810 100 40 200 - - - Inv. ex.
24 A 738 858 886 376 21 920 120 200 500 3.0 15829 45 4.1 745 120 40 300 - - - Comp. ex.
25 E 755 838 882 374 19 930 100 RT 500 1.0 15460 50 3.9 830 150 40 250 - - - Comp. ex.
26 F 717 850 875 403 21 900 100 RT 520 1.0 15860 55 3.5 750 600 50 100 - - - Comp. ex.
27 G 703 782 835 388 22 910 100 100 500 1.0 15460 50 2.5 740 300 50 300 - - - Comp. ex.
28 H 760 870 899 353 25 920 60 300 550 0.5 16212 45 4.1 790 600 30 RT - - - Inv. ex.
29 I 731 845 877 381 20 910 100 100 500 1.0 15460 50 3.9 780 300 50 300 - - - Inv. ex.
30 J 710 819 856 381 17 920 110 RT 500 1.5 15596 50 4.2 770 600 50 RT - - - Inv. ex.
31 K 737 857 883 358 21 920 100 120 500 1.0 15460 50 4.2 760 600 50 RT - - - Inv. ex.
32 L 740 839 876 357 23 900 100 100 480 1.0 15060 50 4.1 780 400 50 100 - - - Inv. ex.
33 M 771 907 922 365 19 930 100 120 510 1.0 15660 55 3.8 820 600 50 RT - - - Inv. ex.
34 N 727 849 874 324 40 890 120 RT 520 1.0 15860 50 4.2 790 300 50 250 350 60 - Inv. ex.
35 O 765 885 907 342 23 920 100 120 500 1.0 15460 45 4.5 800 180 50 RT - - - Inv. ex.
36 P 721 826 863 379 33 900 100 120 500 1.0 15460 50 3.6 750 300 45 RT - - - Inv. ex.
37 Q 805 940 909 311 18 920 100 RT 520 1.0 15860 45 4.5 890 80 50 250 - - - Inv. ex.
38 R 776 939 939 336 22 940 120 100 500 1.2 15521 50 4.2 850 100 50 RT - - - Inv. ex.
39 S 713 795 842 331 21 930 100 120 500 1.0 15460 50 4.1 760 300 50 100 - - - Inv. ex.
40 T 751 860 885 257 30 920 100 RT 500 1.0 15460 60 4.3 800 300 55 RT - - - Inv. ex.
41 U 786 890 918 347 19 940 100 RT 500 1.0 15460 50 3.6 810 300 50 RT - - - Inv. ex.
42 A 738 858 886 376 17 920 100 RT 500 1.0 15460 55 4.5 810 400 40 320 - - GA Inv. ex.
43 V 707 784 833 319 17 890 90 RT 525 1.0 15960 50 4.0 755 550 50 RT - - - Inv. ex.
44 W 692 760 817 339 18 900 90 RT 500 1.0 15460 55 4.0 720 500 50 RT - - - Inv. ex.
[0118]
[Table 3-1] 5
36

Table 3-1
Metallic structure Mechanical properties
Sample no. Chem. comp. Ferrite phase
area ratio
(％) Hard second
phase area ratio
(％) Remaining phase area
ratio
(％) Average
crystal grain
size
(|am) Recrystallized
ferrite fraction
(％) Hard second
phase max.
connecting rate
(％) Hard second
phase 2D
isoperimetric
constant Yield point
YS
(MPa) Tensile
strength TS
(MPa) Uniform
elongation
uEL
(％) Yield ratio YR (％) Remarks
1 A 53 45 2 1.9 79 32 0.06 594 1238 11.0 48 Inv. ex.
2 B 42 55 3 1.7 83 55 0.13 686 1491 8.7 46 Inv. ex.
3 C 39 60 1 1.3 76 89 0.04 759 1686 7.1 45 Inv. ex.
4 A 60 38 2 8.0 84 3 0.46 595 976 14.0 61 Comp. ex.
5 D 39 58 3 1.6 83 92 0.05 747 1778 7.0 42 Inv. ex.
6 A 58 40 2 6.3 78 19 0.35 833 1141 11.0 73 Comp. ex.
7 J 53 45 2 3.9 89 8 0.31 727 1101 10.1 66 Comp. ex.
8 A 35 63 2 12.0 85 5 0.15 690 1095 9.8 63 Comp. ex.
9 A 49 50 1 4.2 79 24 0.04 540 1201 12.0 45 Inv. ex.
10 B 43 55 2 6.9 91 6 0.28 811 1308 6.0 62 Comp. ex.
11 L 41 57 2 12.1 73 8 0.25 736 1098 14.0 67 Comp. ex.
12 B 46 53 1 5.8 75 19 0.23 715 1135 12.0 63 Comp. ex.
13 C 41 55 4 6.3 52 18 0.18 887 1431 6.2 62 Comp. ex.
14 C 44 54 2 4.7 58 18 0.63 1055 1551 5.9 68 Comp. ex.
15 A 53 40 7 2.1 85 24 0.05 607 1103 15.0 55 Comp. ex.
16 C 45 40 15 3.6 80 14 0.08 755 1218 8.6 62 Comp. ex.
17 A 50 18 32 4.1 95 9 0.41 961 1095 6.1 88 Comp. ex.
18 A 52 45 3 14.0 87 8 0.29 619 983 14.0 63 Comp. ex.
19 B 38 60 2 1.9 90 79 0.14 806 1390 9.0 58 Inv. ex.
20 C 42 51 7 2.6 87 18 0.09 1020 1416 10.1 72 Comp. ex.
21 C 48 44 8 2.2 78 28 0.19 909 1377 13.1 66 Comp. ex.
22 A 36 60 4 2.3 95 35 0.09 730 1258 12.0 58 Inv. ex.
[0119]
[Table 3-2]
37

Table 3-2
Metallic structure Mechanical properties
Sample no. Chem. comp. Ferrite phase
area ratio
(％) Hard second
phase area ratio
(％) Remaining phase area
ratio
(％) Average
crystal grain
size
(|am) Recrystallized
ferrite fraction
(％) Hard second
phase max.
connecting rate
(％) Hard second
phase 2D
isoperimetric
constant Yield point
YS
(MPa) Tensile
strength TS
(MPa) Uniform
elongation
uEL
(％) Yield ratio YR (％) Remarks
23 E 42 55 3 2.4 85 19 0.05 770 1539 7.2 50 Inv. ex.
24 A 65 31 4 4.6 75 21 0.24 681 1098 14.0 62 Comp. ex.
25 E 13 83 4 3.2 81 98 0.23 1229 1731 5.2 71 Comp. ex.
26 F 60 38 2 4.5 92 11 0.18 556 1011 16.0 55 Comp. ex.
27 G 35 63 2 6.3 88 85 0.23 953 1401 6.2 68 Comp. ex.
28 H 46 52 2 3.6 80 25 0.19 686 1399 8.2 49 Inv. ex.
29 I 47 50 3 1.9 90 19 0.08 681 1381 7.2 49 Inv. ex.
30 J 36 62 2 1.7 87 31 0.17 800 1403 7.3 57 Inv. ex.
31 K 55 44 1 2.5 72 13 0.18 700 1399 8.3 50 Inv. ex.
32 L 46 52 2 3.1 77 21 0.09 723 1315 10.6 55 Inv. ex.
33 M 36 63 1 2.9 65 44 0.07 585 1299 11.6 45 Inv. ex.
34 N 39 58 3 2.8 81 89 0.03 876 1511 9.8 58 Inv. ex.
35 O 49 49 2 3.1 88 35 0.16 697 1394 7.8 50 Inv. ex.
36 P 56 41 3 2.1 71 14 0.09 699 1295 12.5 54 Inv. ex.
37 Q 41 57 2 2.4 80 54 0.07 721 1413 8.4 51 Inv. ex.
38 R 38 59 3 3.2 80 73 0.13 826 1501 7.2 55 Inv. ex.
39 S 50 49 1 3.1 78 56 0.19 689 1406 9.1 49 Inv. ex.
40 T 40 58 2 2.1 80 98 0.01 943 1779 8.2 53 Inv. ex.
41 U 36 61 3 1.9 95 98 0.03 836 1671 7.4 50 Inv. ex.
42 A 37 59 4 2.4 92 41 0.11 710 1196 10.4 59 Inv. ex.
43 V 48 50 2 1.8 79 79 0.07 771 1547 8.5 50 Inv. ex.
44 W 40 59 1 2.1 75 77 0.06 781 1699 8.7 46 Inv. ex.
38

INDUSTRIAL APPLICABILITY
[0120]
According to the above aspects of the present invention, it is possible to obtain cold rolled
steel sheet having a 1180 MPa or more tensile strength (maximum tensile strength), excellent in
5 workability, and excellent in shape fixability. Therefore, the industrial applicability is high.
REFERENCE SIGNS LIST
[0121]
1 maximum connected region
10 2 ferrite structure region
3 nonmaximum connected region
39

WE CLAIMS

A cold rolled steel sheet having a chemical composition consisting of, by mass%,
C: 0.15% or more and 0.40% or less,
5 Si: 0.50% or more and 4.00% or less,
Mn: 1.00% or more and 4.00% or less,
sol. Al: 0.001% or more and 2.000% or less,
P: 0.020% or less,
S: 0.020% or less,
10 N: 0.010% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more and 0.010% or less,
V: 0% or more and 1.00% or less,
15 Cr: 0% or more and 1.00% or less,
Mo: 0% or more and 1.00% or less,
Cu: 0% or more and 1.00% or less,
Co: 0% or more and 1.00% or less,
W: 0% or more and 1.00% or less,
20 Ni: 0% or more and 1.00% or less,
Ca: 0% or more and 0.010% or less,
Mg: 0% or more and 0.010% or less,
REM: 0% or more and 0.010% or less,
Zr: 0% or more and 0.010% or less, and
25 balance: iron and impurities, and
a metallic structure consisting of ferrite phases, hard second phases consisting of martensite phases and retained austenite phases, and remaining phases consisting of cementite phases and bainite phases, wherein
an area ratio of the ferrite phases is 35% or more and 65% or less,
30 an area ratio of the hard second phases is 35% or more and 65% or less,
an area ratio of the remaining phases is 0% or more and 5% or less,
60% or more of the ferrite phases are recrystallized ferrite phases,
an average crystal grain size defined by 15° grain boundaries is 5.0 um or less,
a maximum connecting rate of the hard second phases is 10% or more, and
35 a two-dimensional isoperimetric constant of the hard second phases is 0.20 or less.
40

[Claim 2]
The cold rolled steel sheet according to claim 1, wherein the chemical composition comprises, by mass%, one or more selected from the group consisting of
Ti: 0.001% or more and 0.200% or less,
5 Nb: 0.001% or more and 0.200% or less,
B: 0.0005% or more and 0.010% or less,
V: 0.005% or more and 1.00% or less,
Cr: 0.005% or more and 1.00% or less,
Mo: 0.005% or more and 1.00% or less,
10 Cu: 0.005% or more and 1.00% or less,
Co: 0.005% or more and 1.00% or less,
W: 0.005% or more and 1.00% or less,
Ni: 0.005% or more and 1.00% or less,
Ca: 0.0003% or more and 0.010% or less,
15 Mg: 0.0003% or more and 0.010% or less,
REM: 0.0003% or more and 0.010% or less, and
Zr: 0.0003% or more and 0.010% or less.
[Claim 3]
20 The cold rolled steel sheet according to claim 1 or 2, having a hot dip galvanized layer on
the surface thereof.
[Claim 4]
The cold rolled steel sheet according to claim 1 or 2, having a hot dip galvannealed layer on
25 the surface thereof.
[Claim 5]
A method for producing the cold rolled steel sheet according to claim 1 or 2, comprising:
a hot rolling step of rough rolling a slab having the chemical composition according to
30 claim 1 or 2, then finish rolling it wherein a rolling reduction of a final stage of the finish rolling
is 15% or more and 50% or less and an end temperature of the finish rolling is Ar3°C or more and 950°C or less, cooling it down to a coiling temperature of less than 400°C by an average cooling rate of 50°C/s or more, and coiling it at the coiling temperature,
a tempering step of tempering the hot rolled steel sheet in a temperature region of 450°C or
35 more and less than 600°C under conditions of a tempering parameter £ defined by following
Formula 1 of 14000 to 18000,
41

a cold rolling step of pickling the tempered steel sheet, then cold rolling it by a rolling reduction of 30% or more, and
an annealing step of heating the cold rolled steel sheet in a temperature region of 500°C to
Ac1°C by an average heating rate of 5.0°C/s or less up to a maximum heating temperature of
5 (Ac1+10)°C or more and (Ac3-10)°C or less, holding it at the maximum heating temperature for
60 seconds or more, then cooling it in a temperature region of (Ac1-50)°C to a cooling stop temperature by an average cooling rate of 20°C/s or more down to the cooling stop temperature of Ms°C or less:
Formula 1: ^=(T+273)[log10(t/3600)+20]
10 T [°C]: tempering temperature, t [s]: tempering time
Ac1 [°C]=751-16x[%C]+35x[%Si]-28x[%Mn]
Ac3 [°C]=881-353x[%C]+65x[%Si]-24x[%Mn]
Ar3 [°C]=910-203x[%C]+44.7x[%Si]-24x[%Mn]-50x[%Ni]
Ms [°C]=521-353x[%C]-22x[%Si]-24x[%Mn]
15 where %C, %Si, %Mn, and %Ni are contents [mass%] of C, Si, Mn, and Ni.
[Claim 6]
The method for producing the cold rolled steel sheet according to claim 5, further
comprising cooling down to the cooling stop temperature of Ms°C or less, then holding at a
20 temperature of 200°C or more and 450°C or less for 60 seconds or more and 600 seconds or less.
[Claim 7]
The method for producing the cold rolled steel sheet according to claim 5 or 6, further
comprising hot dip galvanization at a temperature of 430°C or more after the annealing step to
25 produce the cold rolled steel sheet according to claim 3.
[Claim 8]
The method for producing the cold rolled steel sheet according to claim 5 or 6, further
comprising hot dip galvanization at a temperature of 430°C or more after the annealing step,
30 then alloying treatment at 400°C or more and 600°C or less to produce the cold rolled steel sheet
according to claim 4.