Hot stamped steel and method for producing the same

1. A hot stamped steel comprising, by mass %:

C: more than 0.150% to 0.300%;

Si: 0.010% to 1.000%;

Mn: 1.50% to 2.70%;

P: 0.001% to 0.060%;

S: 0.001% to 0.010%;

N: 0.0005% to 0.0100%; and

Al: 0.010% to 0.050%; and

optionally one or more of

B: 0.0005% to 0.0020%;

Mo: 0.01% to 0.50%;

Cr: 0.01% to 0.50%;

V: 0.001% to 0.100%;

Ti: 0.001% to 0.100%;

Nb: 0.001% to 0.050%;

Ni: 0.01% to 1.00%;

Cu: 0.01% to 1.00%;

Ca: 0.0005% to 0.0050%; and

REM: 0.0005% to 0.0050%; and

a balance including Fe and unavoidable impurities,

wherein, when [C] represents an amount of C by mass %, [Si] represents an amount of Si by mass %, and [Mn] represents an amount of Mn by mass %, a following expression a is satisfied,

a metallographic structure includes 80% or more of a martensite in an area fraction, and optionally, further includes one or more of 10% or less of a pearlite in an area fraction, 5% or less of a retained austenite in a volume ratio, 20% or less of a ferrite in an area fraction, and less than 20% of a bainite in an area fraction,

TS×λ which is a product of TS that is a tensile strength and λ that is a hole expansion ratio is 50000 MPa·% or more, and

a hardness of the martensite measured with a nanoindenter satisfies a following expression b and a following expression c,

(5×[Si]+[Mn])/[C]>10   (a)

1.005≦ H 2 /H 1<1.10   (b)

σHM<20   (c)

here, the H 1 represents an average hardness of the martensite in a surface portion, the H 2 represents the average hardness of the martensite in a center part of a sheet thickness that is an area having a width of ±100 μm in a thickness direction from a center of the sheet thickness, and the σHM represents a variance of the hardness of the martensite existing in the central part of the sheet thickness.

2. The hot stamped steel according to claim 1 , wherein an area fraction of a MnS existing in the metallographic structure and having an equivalent circle diameter of 0.1 μm to 10 μm is 0.01% or less, and a following expression d is satisfied,

n 2 /n 1<1.5  (d)

here, the n 1 represents an average number density per 10000 μm 2 of the MnS in a ¼ part of the sheet thickness, and the n 2 represents an average number density per 10000 μm 2 of the MnS in the central part of the sheet thickness.

3. The hot stamped steel according to claim 1 or 2 , wherein a hot dip galvanized layer is formed on a surface thereof.

4. The hot stamped steel according to claim 3 , wherein the hot dip galvanized layer includes a galvannealed layer.

5. The hot stamped steel according to claim 1 or 2 , wherein an electrogalvanized layer is formed on a surface thereof.

6. The hot stamped steel according to claim 1 or 2 , wherein an aluminized layer is formed on a surface thereof.

7. A method for producing a hot stamped steel comprising:

casting a molten steel having a chemical composition according to claim 1 and obtain a steel;

heating the steel;

hot-rolling the steel with a hot-rolling facility having a plurality of stands;

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 a following expression e;

annealing in which the steel is heated under 700° C. to 850° C. and cooled after the cold-rolling;

temper-rolling the steel after the annealing; and

hot stamping in which the steel is heated to a temperature range of 750° C. or more at a temperature-increase rate of 5° C./second or more, formed within the temperature range, and cooled to 20° C. to 300° C. at a cooling rate of 10° C./second or more after the temper-rolling,

1.5× r 1 /r+ 1.2× r 2 /r+r 3 /r> 1  (e)

wherein ri (i=1, 2 or 3) represents an individual target cold-rolling reduction in unit % at an i th stand (i=1, 2 or 3) based on an uppermost stand among the plurality of the stands in the cold-rolling, and r represents a total cold-rolling reduction in unit % in the cold-rolling, and

wherein an area fraction of a pearlite of the steel before the cold-rolling is 15% or more and the area fraction of the pearlite of the steel after the temper-rolling is 10% or less.

8. The method for producing a hot stamped steel according to claim 7 ,

wherein, when CT in unit ° C. represents a coiling temperature in the coiling;

[C] represents an amount of C by mass %, [Mn] represents an amount of Mn by mass %,

[Cr] represents an amount of Cr by mass %, and [Mo] represents an amount of Mo by mass % in the steel;

a following expression f is satisfied;

560−474×[C]−90×[Mn]−20×[Cr]−20×[Mo]<CT<830−270×[C]−90×[Mn]−70×[Cr]−80×[Mo]  (f).

9. The method for producing a hot stamped steel according to claim 7 or 8 ,

wherein, when T in unit ° C. represents a heating temperature in the heating, t in unit minutes represents an in-furnace time; and

[Mn] represents an amount of Mn by mass %, and [S] represents an amount of S by mass % in the steel,

a following expression g is satisfied,

T ×ln( t )/(1.7×[Mn]+[S])>1500  (g).

10. The method for producing a hot stamped steel according to claim 7 or 8 , further comprising:

galvanizing the steel between the annealing and the temper-rolling.

11. The method for producing a hot stamped steel according to claim 10 , further comprising:

alloying the steel between the hot dip galvanizing and the temper-rolling.

12. The method for producing a hot stamped steel according to claim 7 or 8 , further comprising:

electrogalvanizing the steel between the temper-rolling and the hot stamping.

13. The method for producing a hot stamped steel according to claim 7 or 8 , further comprising:

aluminizing the steel between the annealing and the temper-rolling.