High-strength steel sheet and method for manufacturing same

1. A high-strength steel sheet having a tensile strength of 1,180 MPa or more, the high-strength steel sheet having a chemical composition comprising, by mass %:

C: 0.09% or more and 0.37% or less;

Si: more than 0.70% and 2.00% or less;

Mn: 2.60% or more and 3.60% or less;

P: 0.001% or more and 0.100% or less;

S: 0.0200% or less;

Al: 0.010% or more and 1.000% or less;

N: 0.0100% or less; and

the balance being Fe and incidental impurities,

wherein the steel sheet has a steel structure in which an area fraction of martensite having a C concentration in a range of more than 0.7×[% C] and less than 1.5×[% C] is 55% or more,

an area fraction of tempered martensite having a C concentration of 0.7×[% C] or less is in a range of 5% or more and 40% or less,

a ratio of a C concentration in retained austenite to a volume fraction of retained austenite is in a range of 0.05 or more and 0.40 or less, and

the martensite and the tempered martensite each have an average grain size of 5.3 μm or less, where [% C] represents the content, by mass %, of compositional element C in the steel sheet.

2. The high-strength steel sheet according to claim 1 , wherein in the steel structure, a thickness of a surface softened layer is in a range of 10 μm or more and 100 μm or less.

3. The high-strength steel sheet according to claim 1 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of:

Ti: 0.001% or more and 0.100% or less,

Nb: 0.001% or more and 0.100% or less,

V: 0.001% or more and 0.100% or less,

B: 0.0001% or more and 0.0100% or less,

Mo: 0.010% or more and 0.500% or less,

Cr: 0.01% or more and 1.00% or less,

Cu: 0.01% or more and 1.00% or less,

Ni: 0.01% or more and 0.50% or less,

Sb: 0.001% or more and 0.200% or less,

Sn: 0.001% or more and 0.200% or less,

Ta: 0.001% or more and 0.100% or less,

Ca: 0.0001% or more and 0.0200% or less,

Mg: 0.0001% or more and 0.0200% or less,

Zn: 0.001% or more and 0.020% or less,

Co: 0.001% or more and 0.020% or less,

Zr: 0.001% or more and 0.020% or less, and

REM: 0.0001% or more and 0.0200% or less.

4. The high-strength steel sheet according to claim 1 , further comprising a coating layer disposed on a surface of the steel sheet.

5. A method for manufacturing the high-strength steel sheet according to claim 1 , the method comprising annealing a cold-rolled steel sheet obtained by performing hot rolling, pickling, and cold rolling,

wherein the annealing includes:

heating under conditions that an average heating rate in a temperature range of 250° C. or higher and 700° C. or lower is 10° C./s or more, and a heating temperature is in a range of 850° C. or higher and 950° C. or lower, and

subsequently, cooling under conditions that a holding time in a temperature range of 50° C. or higher and 400° C. or lower is in a range of 70 s or more and 700 s or less, and an average cooling rate in a temperature range of 50° C. or higher and 250° C. or lower is 10.0° C./s or less.

6. The method for manufacturing the high-strength steel sheet according to claim 5 , wherein, in the heating temperature range, an oxygen concentration is in a range of 2 ppm or more and 30 ppm or less, and

a dew point is −35° C. or higher.

7. The method for manufacturing the high-strength steel sheet according to claim 5 , further comprising, after the annealing, performing coating treatment.

8. The high-strength steel sheet according to claim 2 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of:

Ti: 0.001% or more and 0.100% or less,

Nb: 0.001% or more and 0.100% or less,

V: 0.001% or more and 0.100% or less,

B: 0.0001% or more and 0.0100% or less,

Mo: 0.010% or more and 0.500% or less,

Cr: 0.01% or more and 1.00% or less,

Cu: 0.01% or more and 1.00% or less,

Ni: 0.01% or more and 0.50% or less,

Sb: 0.001% or more and 0.200% or less,

Sn: 0.001% or more and 0.200% or less,

Ta: 0.001% or more and 0.100% or less,

Ca: 0.0001% or more and 0.0200% or less,

Mg: 0.0001% or more and 0.0200% or less,

Zn: 0.001% or more and 0.020% or less,

Co: 0.001% or more and 0.020% or less,

Zr: 0.001% or more and 0.020% or less, and

REM: 0.0001% or more and 0.0200% or less.

9. The high-strength steel sheet according to claim 2 , further comprising a coating layer disposed on a surface of the steel sheet.

10. The high-strength steel sheet according to claim 3 , further comprising a coating layer disposed on a surface of the steel sheet.

11. The high-strength steel sheet according to claim 8 , further comprising a coating layer disposed on a surface of the steel sheet.

12. A method for manufacturing the high-strength steel sheet according to claim 2 , the method comprising annealing a cold-rolled steel sheet obtained by performing hot rolling, pickling, and cold rolling,

wherein the annealing includes:

heating under conditions that an average heating rate in a temperature range of 250° C. or higher and 700° C. or lower is 10° C./s or more, and a heating temperature is in a range of 850° C. or higher and 950° C. or lower, and

subsequently, cooling under conditions that a holding time in a temperature range of 50° C. or higher and 400° C. or lower is in a range of 70 s or more and 700 s or less, and an average cooling rate in a temperature range of 50° C. or higher and 250° C. or lower is 10.0° C./s or less.

13. A method for manufacturing the high-strength steel sheet according to claim 3 , the method comprising annealing a cold-rolled steel sheet obtained by performing hot rolling, pickling, and cold rolling,

wherein the annealing includes:

heating under conditions that an average heating rate in a temperature range of 250° C. or higher and 700° C. or lower is 10° C./s or more, and a heating temperature is in a range of 850° C. or higher and 950° C. or lower, and

subsequently, cooling under conditions that a holding time in a temperature range of 50° C. or higher and 400° C. or lower is in a range of 70 s or more and 700 s or less, and an average cooling rate in a temperature range of 50° C. or higher and 250° C. or lower is 10.0° C./s or less.

14. A method for manufacturing the high-strength steel sheet according to claim 8 , the method comprising annealing a cold-rolled steel sheet obtained by performing hot rolling, pickling, and cold rolling,

wherein the annealing includes:

heating under conditions that an average heating rate in a temperature range of 250° C. or higher and 700° C. or lower is 10° C./s or more, and a heating temperature is in a range of 850° C. or higher and 950° C. or lower, and

subsequently, cooling under conditions that a holding time in a temperature range of 50° C. or higher and 400° C. or lower is in a range of 70 s or more and 700 s or less, and an average cooling rate in a temperature range of 50° C. or higher and 250° C. or lower is 10.0° C./s or less.

15. The method for manufacturing the high-strength steel sheet according to claim 12 , wherein, in the heating temperature range, an oxygen concentration is in a range of 2 ppm or more and 30 ppm or less, and

a dew point is −35° C. or higher.

16. The method for manufacturing the high-strength steel sheet according to claim 13 , wherein, in the heating temperature range, an oxygen concentration is in a range of 2 ppm or more and 30 ppm or less, and

a dew point is −35° C. or higher.

17. The method for manufacturing the high-strength steel sheet according to claim 14 , wherein, in the heating temperature range, an oxygen concentration is in a range of 2 ppm or more and 30 ppm or less, and

a dew point is −35° C. or higher.

18. The method for manufacturing the high-strength steel sheet according to claim 6 , further comprising, after the annealing, performing coating treatment.

19. The method for manufacturing the high-strength steel sheet according to claim 12 , further comprising, after the annealing, performing coating treatment.

20. The method for manufacturing the high-strength steel sheet according to claim 13 , further comprising, after the annealing, performing coating treatment.

21. The method for manufacturing the high-strength steel sheet according to claim 14 , further comprising, after the annealing, performing coating treatment.

22. The method for manufacturing the high-strength steel sheet according to claim 15 , further comprising, after the annealing, performing coating treatment.

23. The method for manufacturing the high-strength steel sheet according to claim 16 , further comprising, after the annealing, performing coating treatment.

24. The method for manufacturing the high-strength steel sheet according to claim 17 , further comprising, after the annealing, performing coating treatment.