1. A steel sheet which is a cold-rolled steel sheet, the steel sheet comprising, as a
chemical composition, by mass%,
C: 0.01% to 0.4%,
Si: 0.001% to 2.5%,
Mn: 0.001% to 4.0%,
'41: 0.001% to 2.0%,
P: limited to 0.15% or less,
S: limited to 0.03% or less,
N: limited to 0.01% or less,
0: limited to 0.01% or less, and
a balance consisting of Fe and unavoidable impurities,
wherein: an average pole density of an orientation group of {100)<011> to
15 (22314 lo>, which is a pole density represented by an arithmetic average of pole
densities of each crystal orientation (1 00)<01 I>, (1 16)<1 lo>, (1 1.414 lo>,
{112)<1 lo>, and (223)<1 lo>, is 1 .O to 5.0 and a pole density of a crystal orientation
{332}<113> is 1.0 to 4.0 in a thickness central portion which is a thickness range of 518
to 318 based on a surface of the steel sheet;
20 a Lankford-value rC in a direction perpendicular to a rolling direction is 0.70 to
1.50 and a Lankford-value r30 in a direction making an angle of 30" with the rolling
direction is 0.70 to 1.50; and
the steel sheet includes, as a metallographic structure, plural grains, and includes,
by area%, a ferrite and a bainite of 30% to 99% in total and a martensite of 1% to 70%.
25
2. The cold-rolled steel sheet according to claim 1, further comprising, as the
chemical composition, by mass %, at least one selected from the group consisting of
Ti: 0.001% to 0.2%,
Nb: 0.001% to 0.2%,
5 B: 0.0001% to 0.005%,
Mg: 0.0001% to 0.01%,
Rare Earth Metal: 0.000 1 % to 0.1 %,
Ca: 0.0001% to 0.01%,
Mo: 0.001% to 1.0%,
10 Cr: 0.001% to 2.0%,
V: 0.001% to 1.0%,
Ni: 0.001% to 2.0%,
Cu: 0.001% to 2.0%,
Zr: 0.0001% to 0.2%,
15 W: 0.001% to 1.0%,
As: 0.0001% to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001 % to 0.2%,
Pb: 0.0001% to 0.2%,
20 Y 0.001% to 0.2%, and
Hf 0.001% to 0.2%.
3. The cold-rolled steel sheet according to claim 1 or 2,
wherein a volume average diameter of the grains is 5 ym to 30 ym.
25
wherein the average pole density of the orientation group of { 100) <0 11 > to
(223)<1 lO> is 1 .O to 4.0, and the pole density of the crystal orientation {332}<113> is
1.0 to 3.0.
5. The cold-rolled steel sheet according to claim 1 or 2,
wherein a Lankford-value rL in the rolling direction is 0.70 to 1.50, and a
Lankford-value r60 in a direction making an angle of 60" with the rolling direction is
0.70 to 1.50.
6. The cold-rolled steel sheet according to claim 1 or 2,
wherein, when an area fraction of the martensite is defined as fM in unit of
area%, an average size of the martensite is defined as dia in unit of pm, an average
distance between the martensite is defined as dis in unit of pm, and a tensile strength of
15 the steel sheet is defined as TS in unit of MPa, a following Expression 1 and a following
Expression 2 are satisfied,
dia 5 13 pm . . . (Expressioa I),
TS / flM x dis / dia L 500 . . . (Expression 2).
20 7. The cold-rolled steel sheet according to claim 1 or 2,
wherein, when an area fraction of the Inartensite is defnled as fM in unit of
area%, a major axis of the martensite is defined as La, and a minor axis of the martensite
is defined as Lb, an area fraction of the martensite satisfying a following Expression 3 is
50% to 100% as compared with the area fraction fM of the martensite,
La / Lb 5 5.0 . . . (Expression 3).
8. The cold-rolled steel sheet according to claim 1 or 2,
wherein the steel sheet includes, as the metallographic structure, by area%, the
5 bainite of 5% to 80%.
9. The cold-rolled steel sheet according to claim 1 or 2,
wherein the steel sheet includes a tempered martensite in the martensite.
10 10. The cold-rolled steel sheet according to claim 1 or 2,
wherein an area fraction of coarse grain having grain size of more than 35 pm is
0% to 10% among the grains in the metallographic structure of the steel sheet.
11. The cold-rolled steel sheet according to claim 1 or 2,
15 wherein, when a hardness of the ferrite or the bainite which is a primary phase is
measured at 100 points or more, a value dividing a standard deviation of the hardness by
an average of the hardness is 0.2 or less.
12. The cold-rolled steel sheet according to claim 1 or 2,
20 wherein a galvanized layer or a galvannealed layer is aranged on the surface of
the steel sheet.
13. A method for producing a cold-rolled steel sheet, comprising:
first-hot-rolling a steel in a temperature range of 1000°C to 1200°C under
25 conditions such that at least one pass whose reduction is 40% or more is included so as to
d * &.,.- :.i a -
.: [ p ~ l,\ ; I - .
control an average grain size of an austenite in the steel to 200 pm or less, wherein the %
steel includes, as a chemical composition, by mass%,
C: 0.01% to 0.4%,
Si: 0.001% to 2.5%,
Mn: 0.001% to 4.0%,
Al: 0.001% to 2.0%,
P: limited to 0.15% or less,
S: limited to 0.03% or less,
N: limited to 0.01% or less,
0: limited to 0.01% or less, and
a balance consisting of Fe and unavoidable impurities;
second-hot-rolling the steel under conditions such that, when a temperature
calculated by a following Expression 4 is defined as T1 in unit of "C and a ferritic
transformation temperature calculated by a following Expression 5 is defined as AT3 in
15 unit of "C, a large reduction pass whose reduction is 30% or more in a temperature range
of Tl + 30°C to T1 + 200°C is included, a cumulative reduction in the temperature range
of Tl + 30°C to T1 + 200°C is 50% or more, a cumulative reduction in a temperature
range ofAr3 to lower than T1 + 30°C is limited to 30% or less, and a rolling finish
temperature is A r 3 or higher;
20 first-cooling the steel under conditions such that, when a waiting time from a
finish of a final pass in the large reduction pass to a cooling start is defined as t in unit of
second, the waiting time t satisfies a following Expression 6, an average cooling rate is
50 "Clsecond or faster, a cooling temperature change which is a difference between a
steel temperature at the cooling start and a steel temperature at a cooling finish is 40°C to
. . I >
,f"+>'" " J . ,
-. %? * + p -1 5 * I . . t-•
~J-,z" . 5' T r d t , ) q- $ .*au 140°C, and the steel temperature at the cooling finish is TI + 100°C or lower; 1 'L ;
20\3
second-cooling the steel to a temperature range of a room temperature to 600°C
after finishing the second-hot-rolling;
coiling the steel in the temperature range of the room temperature to 600°C;
pickling the steel;
cold-rolling the steel under a reduction of 30% to 70%;
heating-and-holding the steel in a temperature range of 750°C to 900°C for 1
second to 1000 seconds;
third-cooling the steel to a temperature range of 580°C to 720°C under an
10 average cooling rate of 1 "Clsecond to 12 "Clsecond;
fourth-cooling the steel to a temperature range of 200°C to 600°C under an
average cooling rate of 4 "C/second to 300 "Clsecond; and
holding the steel as an overageing treatment under conditions such that, when an
overageing temperature is defined as T2 in unit of OC and an overageing holding time
15 dependent on the overageing temperature T2 is defined as t2 in unit of second, the
overageing temperature T2 is within a temperature range of 200°C to 600°C and the
overageing holding time t2 satisfies a following Expression 8,
TI = 850 + 10 x ([C] + m]) x [Mn]. .. (Expression 4),
here, [C], [N], and [Mn] represent mass percentages of C, N, and Mn
20 respectively,
Ar3 =879.4-516.1 x [C] - 65.7 x [Mn] +38.0 x [Si] +274.7 x [PI ...
(Expression 5),
here, in Expression 5, [C], [Mn], [Si] and [PI represent mass percentages of C,
Mn, Si, and P respectively,
3 ,
-7
/7e 1 2 4
!. "a ,
t I 2.5 x tl . . . (Expression 6), ?.?\"
here, t 1 is represented by a following Expression 7,
tl = 0.001 x ((Tf- TI) x P1 / 100)~- 0 .109 x ((Tf-TI) x PI /100)+3.1 ...
(Expression 7),
5 here, Tf represents a celsius temperature of the steel at the finish of the final pass,
and P1 represents a percentage of a reduction at the final pass,
log(t2) 5 0. 0002 x (T2 - 425)2 + 1.18.. . (Expression 8).
14. The method for producing the cold-rolled steel sheet according to claim 13,
10 wherein the steel further includes, as the chemical composition, by mass%, at
least one selected fi-om the group consisting of
Ti: 0.001% to 0.2%,
Nb: 0.001% to 0.2%,
B: 0.0001% to 0.005%,
15 Mg: 0.0001% to 0.01%,
Rare Earth Metal: 0.000 1 % to 0.1 %,
Ca: 0.0001% to 0.01%,
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
20 V: 0.001% to 1.0%,
Ni: 0.001% to 2.0%,
Cu: 0.001% to 2.0%,
Zr: 0.0001% to 0.2%,
W: 0.001% to 1.0%,
25 As: 0.0001% to 0.5%,
Co: 0.000-1% to LO%,
Sn: 0.0001% to 0.2%,
Pb: 0.0001% to 0.2%,
Y: 0.001% to 0.2%, and
Hf 0.001% to 0.2%,
wherein a temperature calculated by a following Expression 9 is substituted for
the temperature calculated by the Expression 4 as T1,
T 1 = 8 5 0 + 1 0 x ( [ C ] + ~ ] ) x [ M n ] + 3 5 0 x ~ ] + 2 5 0 x [ T i ] + 4 0 x [ B ] + 1 0 x
[Cr] + 100 x [Mo] + 100 x [V] . . . (Expression 9),
here, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent mass
percentages of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively.
15. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
15 wherein the waiting time t fbrther satisfies a following Expression 10,
0 I t < tl . . . (Expression 10).
16. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
20 wherein the waiting time t further satisfies a following Expression 11,
tl l t l tl x 2.5 ... (Expression 11).
17. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
25 wherein, in the first-hot-rolling, at least two times of rollings whose reduction is
Lr .
' .,<
40% or more are conducted, and the average grain size of the austenite is controlled to
100 pm or less.
18. The method for producing the cold-rolled steel sheet according to claim 13 or
5 14,
wherein the second-cooling starts within 3 seconds after finishing the
second-hot-rolling.
19. The method for producing the cold-rolled steel sheet according to claim 13 or
10 14,
wherein, in the second-hot-rolling, a temperature rise of the steel between passes
is 18°C or lower.
20. The method for producing the cold-rolled steel sheet according to claim 13 or
15 14,
wherein the first-cooling is conducted at an interval between rolling stands.
21. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
20 wherein a final pass of rollings in the temperature range of TI + 30°C to TI f
200°C is the large reduction pass.
22. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
wherein, in the second-cooling, the steel is cooled under an average cooling rate
23. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
5 wherein a galvanizing is conducted after the overageing treatment.
24. The method for producing the cold-rolled steel sheet according to claim 13 or
14,
wherein: a galvanizing is conducted after the overageing treatment; and
10 a heat treatment is conducted in a temperature range of 450°C to 600°C after the
galvanizing.
Dated this 2211 112013
(NEHA SWASTAVA)
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS