Cold-rolled steel sheet and method for manufacturing same, and hot-stamp formed body

1. A cold-rolled steel sheet having a chemical composition comprising, in terms of mass %:

C: 0.1% to 0.3%;

Si: 0.01% to 2.0%;

Mn: 1.5% to 2.5%;

P: 0.001% to 0.6 %;

S: 0.001% to 0.01%;

N: 0.0005% to 0.01%;

Al: 0.01% to 0.05%;

B: 0% to 0.002%;

Mo: 0% to 0.5%;

Cr: 0% to 0.5%;

V: 0% to 0.1%;

Ti: 0% to 0.1%;

Nb: 0% to 0.05%;

Ni: 0% to 1.0%;

Cu: 0% to 1.0%;

Ca: 0% to 0.005%;

REM: 0% to 0.005%; and

rest including Fe and impurities, wherein

a structure before and after a hot-stamping includes

a ferrite: 30 area % to 90 area %,

a martensite: 0 area % or more and less than 20 area %,

a pearlite: 0 area % to 10 area %,

a retained austenite: 5 volume % to 20 volume %, and

a rest structure: a bainite,

a hardness of the retained austenite measured with a nano indenter before and after the hot-stamping satisfies Expression 1 and Expression 2,

a tensile strength and a ductility satisfy a relation of TS×El≥20000 MPa·%,

H 2/ H 1<1.1:  Expression 1,

σHM<20:  Expression 2,

and

the “H 1 ” represents the hardness of the retained austenite existing within a thickness surface portion before and after the hot-stamping, the thickness surface portion being an area within 200 μm in the thickness direction of a surface of the cold-rolled steel sheet, the “H 2 ” represents the hardness of the retained austenite existing within a thickness center portion before and after the hot-stamping, the thickness center portion being an area within ±100 μm along the thickness direction of a center plane of the cold-rolled steel sheet in the thickness direction, the “σHM” represents a variance of the hardness of the retained austenite within the thickness center portion before or after the hot-stamping, “TS” represents the tensile strength of the cold-rolled steel sheet in terms of MPa, and “El” represents the ductility of the cold-rolled steel sheet in terms of %.

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

the chemical composition includes one or more elements selected from a group consisting of, in terms of mass %:

B: 0.0005% to 0.002%;

Mo: 0.01% to 0.5%;

Cr: 0.01% to 0.5%;

V: 0.001% to 0.1%;

Ti: 0.001% to 0.1%;

Nb: 0.001% to 0.05%;

Ni: 0.01% to 1.0%;

Cu: 0.01% to 1.0%;

Ca: 0.0005% to 0.005%; and

REM: 0.0005% to 0.005%.

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

a hot-dip galvanized plating is formed on the surface of the cold-rolled steel sheet.

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

a galvannealed plating is formed on the surface of the cold-rolled steel sheet.

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

an electrogalvanized plating is formed on the surface of the cold-rolled steel sheet.

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

an aluminum plating is formed on the surface of the cold-rolled steel sheet.

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

casting a molten steel having the chemical composition into a steel;

hot-rolling after the casting, in which heating is performed on the steel in a furnace, and then rough-rolling and finish-rolling are performed on the steel under a condition satisfying Expression 3;

coiling the steel after the hot-rolling;

pickling the steel after the coiling;

cold-rolling the steel after the pickling with a cold-rolling mill having a plurality of stands under a condition satisfying Expression 4;

annealing after the cold-rolling, in which annealing is performed on the steel at 700° C. to 850° C., and then the steel is cooled;

temper-rolling the steel after the annealing, wherein

2<(1/5)×( h 1/ h 2)×(1/10)×( t 1+ t 2) 0.3 <6:  Expression 3,

1.5×( r 1/ r )+1.2×( r 2/ r )+( r 3/ r )>1:  Expression 4,

and

“h 1 ” represents a thickness of the steel before the rough-rolling in terms of mm, “h 2 ” represents the thickness of the steel after the rough-rolling in terms of mm, “t 1 ” represents a duration from a time at which the steel exits the furnace to a time at which the rough-rolling of the steel starts in terms of seconds, “t 2 ” represents a duration from a time at which the rough-rolling is finished to a time at which the finish-rolling starts in terms of seconds, and “ri” represents an individual target cold-rolling reduction of an i th stand (i=1, 2, 3 . . . ) from a first stand along the plurality of the stands in terms of %, and “r” represents a total target cold-rolling reduction in the cold-rolling in terms of %.

8. The method according to claim 7 for manufacturing the cold-rolled steel sheet according to claim 3 , further comprising

hot-dip galvanizing between the annealing and the temper-rolling, in which the hot-dip galvanized plating is formed on the steel.

9. The method according to claim 8 for manufacturing the cold-rolled steel sheet according to claim 4 , further comprising

galvannealing between the hot-dip galvanizing and the temper-rolling, in which the steel is galvannealed.

10. The method according to claim 7 for manufacturing the cold-rolled steel sheet according to claim 5 , further comprising

electrogalvanizing after the temper-rolling, in which the electrogalvanized plating is formed on the steel.

11. The method according to claim 7 for manufacturing the cold-rolled steel sheet according to claim 6 , further comprising

aluminizing between the annealing and the temper-rolling, in which the aluminum plating is formed on the steel.

12. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 1 .

13. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 2 .

14. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 3 .

15. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 4 .

16. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 5 .

17. A hot-stamp formed body obtained by using the cold-rolled steel sheet according to claim 6 .