High-strength cold-rolled steel sheet and method of manufacturing the same

1. A high-strength cold-rolled steel sheet having a composition and a microstructure, the composition comprising:

0.15% to 0.27% C, 0.8% to 2.4% Si, 2.3% to 3.5% Mn, 0.08% or less P, 0.005% or less S, 0.01% to 0.08% Al, and 0.010% or less N on a mass basis, the remainder being Fe and inevitable impurities, and

the microstructure comprising:

ferrite having an average grain size of 5 μm or less and a volume fraction of 3% to 20%, retained austenite having a volume fraction of 5% to 20%, and martensite having a volume fraction of 5% to 20%, the remainder being bainite and/or tempered martensite; and

the total number of retained austenite with a grain size of 2 μm or less, martensite with a grain size of 2 μm or less and a mixed phase thereof with a grain size of 2 μm or less being 150 or more per 2,000 μm 2 of a thickness cross section parallel to the rolling direction of the steel sheet.

2. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.10% or less V, 0.10% or less Nb, and 0.10% or less Ti on a mass basis.

3. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains 0.0050% or less B on a mass basis.

4. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.50% or less Cr, 0.50% or less Mo, 0.50% or less Cu, and 0.50% or less Ni on a mass basis.

5. The high-strength cold-rolled steel sheet according to claim 1 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

6. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 1 comprising:

preparing a steel slab having the composition according to claim 1 ;

hot-rolling the steel slab to produce hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:

holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,

holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:

heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet for 30 seconds or more,

cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,

heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature.

7. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 2 comprising:

preparing a steel slab having the composition according to claim 2 ;

hot-rolling the steel slab to produce hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:

holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,

holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:

heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet for 30 seconds or more,

cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,

heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature.

8. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 3 comprising:

preparing a steel slab having the composition according to claim 3 ;

hot-rolling the steel slab to produce hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:

holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,

holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:

heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet for 30 seconds or more,

cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,

heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature.

9. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 4 comprising:

preparing a steel slab having the composition according to claim 4 ;

hot-rolling the steel slab to produce hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:

holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,

holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:

heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet for 30 seconds or more,

cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,

heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature.

10. A method of manufacturing the high-strength cold-rolled steel sheet according to claim 5 comprising:

preparing a steel slab having the composition according to claim 5 ;

hot-rolling the steel slab to produce hot-rolled steel sheet;

pickling the hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet;

subjecting the cold-rolled steel sheet to a first annealing, the first annealing comprising:

holding the cold-rolled steel sheet at a first soaking temperature of 800° C. or higher for 30 seconds or more,

cooling the cold-rolled steel sheet from the first soaking temperature to 320° C. to 500° C. at a first average cooling rate of 3° C./s or more,

holding the cold-rolled steel sheet in a first holding temperature range of 320° C. to 500° C. for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature;

subjecting the cold-rolled steel sheet to a second annealing, the second annealing comprising:

heating the cold-rolled steel sheet to a second soaking temperature of 750° C. or higher at an average heating rate of 3° C./s to 30° C./s,

holding the cold-rolled steel sheet for 30 seconds or more,

cooling the cold-rolled steel sheet from the second soaking temperature to 120° C. to 320° C. at a second average cooling rate of 3° C./s or more,

heating the cold-rolled steel sheet to a second holding temperature range of 320° C. to 500° C., is held for 30 seconds or more, and

cooling the cold-rolled steel sheet to room temperature.

11. The high-strength cold-rolled steel sheet according to claim 2 , wherein the composition further contains 0.0050% or less B on a mass basis.

12. The high-strength cold-rolled steel sheet according to claim 2 , wherein the composition further contains at least one selected from the group consisting of 0.50% or less Cr, 0.50% or less Mo, 0.50% or less Cu, and 0.50% or less Ni on a mass basis.

13. The high-strength cold-rolled steel sheet according to claim 3 , wherein the composition further contains at least one selected from the group consisting of 0.50% or less Cr, 0.50% or less Mo, 0.50% or less Cu, and 0.50% or less Ni on a mass basis.

14. The high-strength cold-rolled steel sheet according to claim 11 , wherein the composition further contains at least one selected from the group consisting of 0.50% or less Cr, 0.50% or less Mo, 0.50% or less Cu, and 0.50% or less Ni on a mass basis.

15. The high-strength cold-rolled steel sheet according to claim 2 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

16. The high-strength cold-rolled steel sheet according to claim 3 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

17. The high-strength cold-rolled steel sheet according to claim 4 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

18. The high-strength cold-rolled steel sheet according to claim 11 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

19. The high-strength cold-rolled steel sheet according to claim 12 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

20. The high-strength cold-rolled steel sheet according to claim 13 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.

21. The high-strength cold-rolled steel sheet according to claim 14 , wherein the composition further contains at least one selected from the group consisting of 0.0050% or less Ca and 0.0050% or less of a REM on a mass basis.