High strength cold-rolled steel sheet and manufacturing method therefor

1. A cold-rolled steel sheet, comprising:

C: 0.05-0.19 mass %;

Si: 3.0 mass % or less, exclusive of 0 mass %;

Mn: 0.1-5.0 mass %;

P: 0.1 mass % or less, exclusive of 0 mass %;

S: 0.02 mass % or less, exclusive of 0 mass %;

Al: 0.01-1.0 mass %; and

N: 0.01 mass% or less, exclusive of 0 mass % respectively;

iron and inevitable impurities,

wherein

a microstructure comprises ferrite that is a soft first phase by 20-50% in terms of area ratio, and tempered martensite, tempered bainite, or both, that is a hard second phase;

a difference between area ratio Vαs of ferrite of a steel sheet surface layer section from a steel sheet surface to a depth of 100 μm and area ratio Vαc of ferrite of a center section of t/4-3t/4 ΔVα=Vαs−Vαc is less than 10%, where t is a sheet thickness; and

a ratio of hardness Hvs of the steel sheet surface layer section and hardness Hvc of the center section RHv=Hvs/Hvc is 0.75-1.0.

2. The steel sheet according to claim 1 , further comprising at least one group selected from groups (a)-(c): (a) Cr: 0.01-1.0 mass %,

(b) at least one element selected from the group consisting of Mo: 0.01-1.0 mass %, Cu: 0.05-1.0 mass %, and Ni: 0.05-1.0 mass %, and

(c) at least one element selected from the group consisting of Ca: 0.0001-0.01 mass %, Mg: 0.0001-0.01 mass %, Li: 0.0001-0.01 mass %, and REM: 0.0001-0.01 mass %.

3. A method of manufacturing the cold-rolled steel sheet according to claim 1 , comprising hot rolling, thereafter cold rolling, thereafter annealing, and tempering with respective conditions (A1)-(A4):

(A1) hot rolling condition:

finish rolling temperature: Ar 3 point or above

coiling temperature: above 600° C. and 750° C. or below;

(A2) cold rolling condition:

cold rolling ratio: more than 50% and 80% or less;

(A3) annealing condition:

holding at an annealing temperature of Ac 1 or above and below (Ac 1 +Ac 3 )/2for annealing holding time of 3,600 s or less, thereafter slow cooling with a first cooling rate of 1° C/s or more and less than 50° C/s from the annealing temperature to a first cooling completion temperature of 730° C. or below and 500° C. or above, and thereafter rapid cooling with a second cooling rate of 50° C/s or more to a second cooling completion temperature of Ms point or below; and

(A4) tempering condition:

tempering temperature: 300-500° C.

tempering holding time: 60-1,200 s within the temperature range of 300° C.-tempering temperature.

4. A cold-rolled steel sheet comprising:

C: 0.05-0.30 mass %;

Si: 3.0 mass % or less, exclusive of 0 mass %;

Mn: 0.1-5.0 mass %;

P: 0.1 mass % or less, exclusive of 0 mass %;

S: 0.02 mass % or less, exclusive of 0 mass %;

Al: 0.01-1.0 mass %; and

N: 0.0052-0.01 mass %;

iron and inevitable impurities,

wherein

a microstructure comprises ferrite that is a soft first phase by 20-50% in terms of area ratio, and tempered martensite, tempered bainite, or both, that is a hard second phase;

a difference between area ratio Vαs of ferrite of a steel sheet surface layer section from a steel sheet surface to a depth of 100μm and area ratio Vαc of ferrite of a center section of t/4-3t/4 ΔVα=Vαs−Vαc is less than 10%, where t is a sheet thickness; and

a ratio of hardness Hvs of the steel sheet surface layer section and hardness Hvc of the center section RHv=Hvs/Hvc is 0.75-1.0.

5. The cold-rolled steel sheet according to claim 1 , wherein ΔVα is less than 5%.

6. The cold-rolled steel sheet according to claim 1 , wherein RHv is 0.80-1.0.

7. The cold-rolled steel sheet according to claim 1 , wherein the microstructure comprises the soft first phase by 35-50% in terms of area ratio.

8. A cold-rolled steel sheet having a tensile strength of from 1,009 to 1,131 MPa, comprising:

C: 0.05-0.30 mass %;

Si: 3.0 mass % or less, exclusive of 0 mass %;

Mn: 0.1-5.0 mass %;

P: 0.1 mass % or less, exclusive of 0 mass %,

S: 0.02 mass % or less, exclusive of 0 mass %;

Al: 0.01-1.0 mass %; and

N: 0.01 mass % or less, exclusive of 0 mass% respectively;

iron and inevitable impurities,

wherein

a microstructure comprises ferrite that is a soft first phase by 20-50% in terms of area ratio, and tempered martensite, tempered bainite, or both, that is a hard second phase;

a difference between area ratio Vαs of ferrite of a steel sheet surface layer section from a steel sheet surface to a depth of 100μm and area ratio Vαc of ferrite of a center section of t/4-3t/4ΔVα=Vαs−Vαc is less than 10%, where t is a sheet thickness; and

a ratio of hardness Hvs of the steel sheet surface layer section and hardness Hvc of the center section RHv=Hvs/Hvc is 0.75-1.0.

9. The steel sheet according to claim 4 , further comprising at least one group selected from groups (a)-(c): (a) Cr: 0.01-1.0 mass %,

(b) at least one element selected from the group consisting of Mo: 0.01-1.0 mass %, Cu: 0.05-1.0 mass %, and Ni: 0.05-1.0 mass %, and

(c) at least one element selected from the group consisting of Ca: 0.0001-0.01 mass %, Mg: 0.0001-0.01 mass %, Li: 0.0001-0.01 mass %, and REM: 0.0001-0.01 mass %.

10. A method of manufacturing the cold-rolled steel sheet according to claim 4 , comprising hot rolling, thereafter cold rolling, thereafter annealing, and tempering with respective conditions (A1)-(A4):

(A1) hot rolling condition:

finish rolling temperature: Ar 3 point or above

coiling temperature: above 600° C. and 750° C. or below;

(A2) cold rolling condition:

cold rolling ratio: more than 50% and 80% or less;

(A3) annealing condition:

holding at an annealing temperature of Ac 1 or above and below (Ac 1 +Ac 3 )/2 for annealing holding time of 3,600 s or less, thereafter slow cooling with a first cooling rate of 1° C/s or more and less than 50° C/s from the annealing temperature to a first cooling completion temperature of 730° C. or below and 500° C. or above, and thereafter rapid cooling with a second cooling rate of 50° C/s or more to a second cooling completion temperature of Ms point or below; and

(A4) tempering condition:

tempering temperature: 300-500° C.

tempering holding time: 60-1,200 s within the temperature range of 300° C. -tempering temperature.

11. The cold-rolled steel sheet according to claim 4 , wherein ΔVα is less than 5%.

12. The cold-rolled steel sheet according to claim 4 , wherein RHv is 0.80-1.0.

13. The cold-rolled steel sheet according to claim 4 , wherein the microstructure comprises the soft first phase by 35-50% in terms of area ratio.

14. The steel sheet according to claim 8 , further comprising at least one group selected from groups (a)-(c): (a) Cr: 0.01-1.0 mass %,

(b) at least one element selected from the group consisting of Mo: 0.01-1.0 mass %, Cu: 0.05-1.0 mass %, and Ni: 0.05-1.0 mass %, and

(c) at least one element selected from the group consisting of Ca: 0.0001-0.01 mass %, Mg: 0.0001-0.01 mass %, Li: 0.0001-0.01 mass %, and REM: 0.0001-0.01 mass %.

15. A method of manufacturing the cold-rolled steel sheet according to claim 8 , comprising hot rolling, thereafter cold rolling, thereafter annealing, and tempering with respective conditions (A1)-(A4):

(A1) hot rolling condition:

finish rolling temperature: Ar 3 point or above

coiling temperature: above 600° C. and 750° C. or below;

(A2) cold rolling condition:

cold rolling ratio: more than 50% and 80% or less;

(A3) annealing condition:

holding at an annealing temperature of Ac 1 or above and below (Ac 1 +Ac 3 )/2 for annealing holding time of 3,600 s or less, thereafter slow cooling with a first cooling rate of 1° C/s or more and less than 50° C/s from the annealing temperature to a first cooling completion temperature of 730° C. or below and 500° C. or above, and thereafter rapid cooling with a second cooling rate of 50° C/s or more to a second cooling completion temperature of Ms point or below; and

(A4) tempering condition:

tempering temperature: 300-500° C.

tempering holding time: 60-1,200 s within the temperature range of 300° C.-tempering temperature.

16. The cold-rolled steel sheet according to claim 8 , wherein ΔVαis less than 5%.

17. The cold-rolled steel sheet according to claim 8 , wherein RHv is 0.80-1.0.

18. The cold-rolled steel sheet according to claim 8 , wherein the microstructure comprises the soft first phase by 35-50% in terms of area ratio.