Cold-rolled steel sheet and production method thereof

1. A cold-rolled steel sheet comprising a steel sheet chemical composition including, by mass %,

C: 0.02% 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,

O: limited to 0.01% or less,

optionally one or more selected from

Ti: 0.2% or less,

Nb: 0.2% or less,

B: 0.005% or less,

Mg: 0.01% or less,

REM: 0.1% or less,

Ca: 0.01% or less,

Mo: 1.0% or less,

Cr: 2.0% or less,

V: 1.0% or less,

W: 1.0% or less,

Ni: 2.0% or less,

Cu: 2.0% or less,

Co: 1.0% or less,

Sn: 0.2% or less,

Zr: 0.2% or less, and

As: 0.5% or less, and

a balance including Fe and unavoidable impurities,

wherein a sum of the Si content and the Al content is 1.0% to 4.5% in the chemical composition of the steel sheet,

an average pole density of an orientation group from {100}<011> to {223}<110>, which is a pole density expressed by an arithmetic average of pole densities of respective crystal orientations {100}<011>, {116}<110>, {114}<110>, {112}<110>, and {223}<110>, is 1.0 to 6.5, and a pole density of a crystal orientation {332}<113> is 1.0 to 5.0 in a sheet thickness central portion within a range of ⅝ to ⅜ of a sheet thickness,

a microstructure of the steel sheet includes a plurality of grains,

the microstructure of the steel sheet includes, by an area ratio, 5% to 80% of ferrite, 5% to 80% of bainite, and 2% to 30% of retained austenite, in the microstructure, martensite is limited to 20% or less, pearlite is limited to 10% or less, and tempered martensite is limited to 60% or less, and

rC that is a Lankford value in a direction orthogonal to a rolling direction is 0.70 to 1.10, and r30 that is a Lankford value in a direction forming an angle of 30° with the rolling direction is 0.70 to 1.10.

2. The cold-rolled steel sheet according to claim 1 ,

wherein the steel sheet chemical composition further includes, by mass %, one or more selected from the group consisting of

Ti: 0.001% to 0.2%,

Nb: 0.005% to 0.2%,

B: 0.0001% to 0.005%,

Mg: 0.0001% to 0.01%,

REM: 0.0001% to 0.1%,

Ca: 0.0001% to 0.01%,

Mo: 0.001% to 1.0%,

Cr: 0.001% to 2.0%

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Ni: 0.001% to 2.0%,

Cu: 0.001% to 2.0%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Zr: 0.0001% to 0.2%, and

As: 0.0001% to 0.5%.

3. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein a volume average diameter of the grains is 2 μm to 15 μm.

4. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein the average pole density of the orientation group from {100}<011> to {223}<110> is 1.0 to 5.0, and the pole density of the crystal orientation {332}<113> is 1.0 to 4.0.

5. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein among the plurality of grains, an area ratio of grains which exceed 35 μm is limited to 10% or less.

6. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein among the plurality of grains, a proportion of grains, in which a value obtained by dividing a length of a grain in the rolling direction by a length of a grain in a sheet thickness direction is 3.0 or less, is 50% to 100%.

7. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein a Vickers hardness of the bainite is 180 HV or more, and an average concentration of C in the retained austenite is 0.9% or more.

8. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein the rL that is the Lankford value in the rolling direction is 0.70 to 1.10, and r60 that is a Lankford value in a direction forming an angle of 60° with the rolling direction is 0.70 to 1.10.

9. The cold-rolled steel sheet according to claim 1 or 2 ,

wherein a hot-dip galvanized layer or a galvannealed layer is provided on a surface of the steel sheet.

10. A production method of a cold-rolled steel sheet, the production method comprising:

a first hot-rolling process of performing a hot-rolling with respect to a steel having a steel chemical composition, so as to set an average austenite grain size of the steel to 200 μm or less, wherein the first hot-rolling process includes at least one pass of rolling reduction with a rolling reduction ratio of 40% or more in a temperature range of 1,000° C. to 1,200° C., and

the steel chemical composition including, by mass %,

C: 0.02% 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,

O: limited to 0.01% or less,

optionally one or more selected from

Ti: 0.2% or less,

Nb: 0.2% or less,

B: 0.005% or less,

Mg: 0.01% or less,

REM: 0.1% or less,

Ca: 0.01% or less,

Mo: 1.0% or less,

Cr: 2.0% or less,

V: 1.0% or less,

W: 1.0% or less,

Ni: 2.0% or less,

Cu: 2.0% or less,

Co: 1.0% or less,

Sn: 0.2% or less,

Zr: 0.2% or less, and

As: 0.5% or less, and

a balance including Fe and unavoidable impurities, and

wherein a sum of the Si content and the Al content is 1.0% to 4.5%;

a second hot-rolling process of performing a hot-rolling with respect to the steel, wherein the hot-rolling process includes a large-rolling-reduction pass with a rolling reduction ratio of 30% or more in a temperature range of T1+30° C. to T1+200° C. when a temperature calculated by a following Expression 1 is set to T1° C., an accumulative rolling reduction ratio in the temperature range of T1+30° C. to T1+200° C. is 50% or more, an accumulative rolling reduction ratio in a temperature range that is higher than or equal to Ar 3 ° C. calculated by a following Expression 4 and lower than T1+30° C. is limited to 30% or less, and a rolling termination temperature is Ar 3 ° C. calculated by the following Expression 4 or higher;

a first cooling process of performing a cooling with respect to the steel, such that a standby time t second, which is set as a time from a completion of a final pass among the large-rolling-reduction passes to a cooling initiation, satisfies a following Expression 2;

a coiling process of performing a coiling with respect to the steel in a temperature range of 650° C. or lower;

a pickling process of performing a pickling with respect to the steel;

a cold-rolling process of performing a cold-rolling with respect to the steel at a rolling reduction ratio of 30% to 90%;

a two-step heating process of performing a two-step heating with respect to the steel, in which an average heating rate HR1 in a temperature range from room temperature to 650° C. is 0.3° C./s or more, and an average heating rate HR2 in a temperature range from higher than 650° C. to Ac 1 ° C., when Ac 1 is calculated by a following Expression 5, is 0.5×HR1 or less, in units of ° C./s;

a retaining process of performing a retaining with respect to the steel within a temperature range of Ac 1 ° C. to 900° C. for 1 second to 300 seconds;

a cooling process of performing a cooling with respect to the steel until a temperature range of 580° C. to 780° C. at an average cooling rate of 1° C./s to 20° C./s;

a cooling process of performing a cooling with respect to the steel to a temperature T OA , which is within a temperature range of 350° C. to 500° C., at an average cooling rate of 5° C./s to 200° C./s; and either

(a) a retaining process of performing a retaining with respect to the steel so as to obtain a steel sheet, wherein the steel is retained within the temperature range of 350° C. to 500° C. for a time of t OA seconds or more, which is calculated by a following Expression 6, to 1000 seconds or less, or

(b) a cooling and retaining process of performing a cooling and retaining with respect to the steel so as to obtain a steel sheet, wherein the steel is further cooled to a temperature of 350° C. or lower, then the steel is reheated to the temperature range of 350° C. to 500° C., and the steel is retained within the temperature range of 350° C. to 500° C. for the time of t OA seconds or more, which is calculated by the following Expression 6, to 1000 seconds or less,

T 1=850+10×([C]+[N])×[Mn]+350×[Nb]+250×[Ti]+40×[B]+10×[Cr]+100×[Mo]+100×[V]  (Expression 1)

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,

t≦ 2.5× t 1  (Expression 2)

here, t1 is expressed by a following Expression 3,

t 1=0.001×(( Tf−T 1)× P 1/100) 2 −0.109×(( Tf−T 1)× P 1/100)+3.1  (Expression 3)

here, Tf represents a Celsius temperature of the steel at the time of the completion of the final pass, and P1 represents a percentage of the rolling reduction ratio during the final pass,

Ar 3 =879.4−516.1×[C]−65.7×[Mn]+38.0×[Si]+274.7×[P]  (Expression 4)

Ac 1 =723−10.7×[Mn]−16.9×[Ni]+29.1×[Si]+16.9×[Cr]+290×[As]+6.38×[W]  (Expression 5)

t

OA

=

10

0.0002

⁢

(

T

OA

-

425

)

2

+

1.18

(

Expression

⁢

⁢

6

)

.

11. The production method of a cold-rolled steel sheet according to claim 10 , wherein the steel chemical composition further includes, by mass %, one or more selected from

Ti: 0.001% to 0.2%,

Nb: 0.005% to 0.2%,

B: 0.0001% to 0.005%,

Mg: 0.0001% to 0.01%,

REM: 0.0001% to 0.1%,

Ca: 0.0001% to 0.01%,

Mo: 0.001% to 1.0%

Cr: 0.001% to 2.0%,

V: 0.001% to 1.0%,

W: 0.001% to 1.0%,

Ni: 0.001% to 2.0%,

Cu: 0.001% to 2.0%,

Co: 0.0001% to 1.0%,

Sn: 0.0001% to 0.2%,

Zr: 0.0001% to 0.2%, and

As: 0.0001% to 0.5%.

12. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein the standby time t second satisfies a following Expression 8 using t1

0≦ t<t 1  (Expression 8).

13. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein the standby time t second satisfies the satisfies a following Expression 9 using t1

t 1≦ t≦ 2.5× t 1  (Expression 9).

14. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein a cooling temperature variation that is a difference between a steel temperature at the time of the cooling initiation and a steel temperature at the time of the cooling termination is 40° C. to 140° C., in the first cooling, and the steel temperature at the time of the cooling termination is T1+100° C. or lower.

15. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein the first hot-rolling includes a pass having a rolling reduction ratio of 40% or more at least two or more times, so as to control an average austenite grain size of the steel to be 100 μm or less.

16. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein the second cooling is initiated within 10 seconds after the steel is passed through a final rolling stand and after terminating the first cooling.

17. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein a temperature rising of the steel between respective passes in the temperature range of T1+30° C. to T1+200° C. is set to 18° C. or lower in the second hot-rolling.

18. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein the first cooling is performed at between rolling stands.

19. The production method of a cold-rolled steel sheet according to claim 10 or 11 ,

wherein a hot-dip galvanized layer or a galvannealed layer is formed on a surface of the steel sheet.