High-strength cold-rolled steel sheet and method for producing the same

1. A high-strength cold-rolled steel sheet consisting of, by mass %:

C: 0.075% to 0.300%;

Si: 0.30% to 2.50%;

Mn: 1.30% to 3.50%;

P: 0.001% to 0.050%;

S: 0.0001% to 0.0100%;

Al: 0.001% to 1.500%;

N: 0.0001% to 0.0100%;

Ti: 0.150% or less;

Nb: 0.150% or less;

V: 0.150% or less;

Cr: 2.00% or less;

Ni: 2.00% or less;

Cu: 2.00% or less;

Mo: 1.00% or less;

W: 1.00% or less; and

one or more of Ca, Ce, Mg, Zr, Hf and REM, wherein a total amount of the one or more of Ca, Ce, Mg, Zr, Hf and REM is 0.5000% or less; and

a balance consisting of Fe and unavoidable impurities,

wherein:

a surface microstructure, which is a microstructure in a surface of the steel sheet, contains 3% to 10% of residual austenite, 90% or less of ferrite, 15% or less of fresh martensite, and 5% or less of pearlite by volume fraction,

an inner microstructure, which is a microstructure at a depth of t/4, wherein t represents a sheet thickness of the steel sheet, contains 3% to 30% of residual austenite, 15% or less of fresh martensite, and 5% or less of pearlite by volume fraction,

the inner microstructure is different from the surface microstructure,

a ratio Hvs/Hvb between a surface hardness Hvs of the steel sheet surface and a hardness Hvb at a depth of t/4 satisfies: 0.75<Hvs/Hvb≤0.90,

a maximum tensile strength TS is 700 MPa or more,

a relationship between the maximum tensile strength TS and a total elongation EL satisfies TS×EL≥18000(MPa·%), and

a limit bending radius evaluated by a 90-degree bending test using a 90-degree die and a punch having a radius R of a tip end of 0.5 mm to 6 mm is 1 mm or less and a necking does not occur during the 90-degree bending test, or the limit bending radius is 0.5 mm or less.

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

wherein the volume fraction of the ferrite in the surface microstructure is 10 to 87%, and

wherein the surface microstructure further contains 10% to 50% of tempered martensite by volume fraction.

3. The high-strength cold-rolled steel sheet according to claim 1 ,

wherein the inner microstructure further contains 10% to 87% of ferrite, and 10% to 50% of tempered martensite by volume fraction.

4. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3 ,

wherein a coating film including an oxide containing phosphorus is formed on at least one surface.

5. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3 ,

wherein an electrogalvanized layer is formed on at least one surface.

6. The high-strength cold-rolled steel sheet according to claim 5 ,

wherein a coating film including an oxide containing phosphorus is formed on the electrogalvanized layer.

7. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3 ,

wherein a galvanized layer is formed on at least one surface.

8. The high-strength cold-rolled steel sheet according to claim 7 ,

wherein a coating film including an oxide containing phosphorus is formed on the galvanized layer.

9. The high-strength cold-rolled steel sheet according to any one of claims 1 to 3 ,

wherein a galvannealed layer is formed on at least one surface.

10. The high-strength cold-rolled steel sheet according to claim 9 ,

wherein a coating film including an oxide containing phosphorus is formed on the galvannealed layer.

11. A method for producing the cold-rolled steel sheet according to claim 1 , the method comprising:

heating a slab to 1050° C. or higher, the slab consisting of, by mass %, C: 0.075% to 0.300%; Si: 0.30% to 2.50%; Mn: 1.30% to 3.50%; P: 0.001% to 0.050%; S: 0.0001% to 0.0100%; Al: 0.001% to 1.500%; N: 0.0001% to 0.0100%; Ti: 0.150% or less; Nb: 0.150% or less; V: 0.150% or less; Cr: 2.00% or less; Ni: 2.00% or less; Cu: 2.00% or less; Mo: 1.00% or less; W: 1.00% or less; one or more of Ca, Ce, Mg, Zr, Hf and REM, wherein a total amount of the one or more of Ca, Ce, Mg, Zr, Hf and REM is 0.5000% or less; and a balance consisting of Fe and unavoidable impurities, performing hot rolling at a finish rolling temperature set to an Ar3 transformation point or higher, and then coiling in a temperature range of 750° C. or lower, to obtain a hot-rolled steel sheet;

cold-rolling the hot-rolled steel sheet under a reduction of 30% to 80% to obtain a cold-rolled steel sheet; and

annealing the cold-rolled steel sheet in a temperature range of (Ac1 transformation point +40)° C. to (Ac3transformation point +50)° C. in an atmosphere in which log (water partial pressure/hydrogen partial pressure) is −3.0 to 0.0 for 20 seconds to 600 seconds, cooling the steel sheet to a temperature range of 100-300° C. so that a cooling rate is 0.5-500° C./sec in a temperature range of 700-500° C., and then, heating the steel sheet to 350° C. to 500° C. and holding the steel sheet at 350° C. to 500° C. for 10 seconds to 1000 seconds to obtain a high-strength cold-rolled steel sheet.

12. The method for producing a cold-rolled steel sheet according to claim 11 , further comprising:

forming a coating film including an oxide containing phosphorus on at least one surface of the high-strength cold-rolled steel sheet.

13. The method for producing a cold-rolled steel sheet according to claim 11 , further comprising:

forming an electrogalvanized layer on at least one surface of the high-strength cold-rolled steel sheet.

14. The method for producing a cold-rolled steel sheet according to claim 13 , further comprising:

forming a coating film including an oxide containing phosphorus on the electrogalvanized layer.

15. The method for producing a cold-rolled steel sheet according to claim 11 , further comprising:

forming a galvanized layer on at least ogre surface of the high-strength cold-rolled steel sheet,

wherein in the forming of the galvanized layer, the high-strength cold-rolled steel sheet is immersed in a galvanizing bath, wherein the immersion temperature of the cold-rolled steel sheet is in a temperature range of (galvanizing bath temperature −40)° C. to (galvanizing bath temperature +50)° C., and cooled.

16. The method for producing a cold-rolled steel sheet according to claim 15 , further comprising:

forming a coating film including an oxide containing phosphorus on the galvanized layer.

17. The method for producing a cold-rolled steel sheet according to claim 11 , further comprising:

forming a galvannealed layer on at least one surface of the high-strength cold-rolled steel sheet,

wherein in the forming of the galvannealed layer, the high-strength cold-rolled steel sheet is immersed in a galvanizing bath, wherein the immersion temperature of the cold-rolled steel sheet is in a temperature range of (galvanizing bath temperature −40)° C. to (galvanizing bath temperature +50)° C., subjected to alloying at a temperature of 460° C. or higher, and then, cooled.

18. The method for producing a cold-rolled steel sheet according to claim 17 , further comprising:

forming a coating film including an oxide containing phosphorus on the galvannealed layer.

19. The high-strength cold-rolled steel sheet according to claim 1 ,

wherein a thickness of the high-strength cold-rolled steel sheet is 0.33 mm or more.

20. The high-strength cold-rolled steel sheet according to claim 1 ,

wherein a thickness of the high-strength cold-rolled steel sheet is 1.2 mm or more.

21. The high-strength cold-rolled steel sheet according to claim 1 ,

wherein an amount of residual austenite is 8% to 30% in the inner microstructure by volume fraction.

22. The method for producing a cold-rolled steel sheet according to claim 11 ,

wherein the steel sheet is cooled to 100° C. to 250° C. between annealing the cold-rolled steel sheet and holding the steel sheet.