Hot stamped steel and method for producing hot stamped steel

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

C: 0.030% to 0.150%;

Si: 0.010% to 1.00%;

Mn: 1.50% to 2.70%;

P: 0.001% to 0.060%;

S: 0.001% to 0.010%;

N: 0.0005% to 0.0100%;

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%;

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 after a hot stamping includes 40% to 90% of a ferrite and 10% to 60% of a martensite in an area fraction,

a total of an area fraction of the ferrite and an area fraction of the martensite is 60% or more,

the metallographic structure 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, and less than 40% of a bainite as a remainder in an area fraction,

a hardness of the martensite measured with a nanoindenter satisfies a following expression (B) and a following expression (C),

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

(5×[Si]+[Mn])/[C]>11  (A),

H 2 /H 1<1.10  (B),

σHM<20  (C), and

the H 1 is an average hardness of the martensite in a surface part of a sheet thickness after the hot stamping, the H 2 is an average hardness of the martensite in a central part of the sheet thickness which is an area having a width of 200 μm in a thickness direction at a center of the sheet thickness after the hot stamping, and the σHM is a variance of the average hardness of the martensite in the central part of the sheet thickness after the hot stamping.

2. The hot stamped steel according to claim 1 , wherein

an area fraction of MnS existing in the hot stamped steel and having an equivalent circle diameter of 0.1 μm to 10 μm is 0.01% or less,

a following expression (D) is satisfied,

n 2/ n 1<1.5  (D), and

the n1 is an average number density per 10000 μm2 of the MnS having the equivalent circle diameter of 0.1 μm to 10 μm in a ¼ part of the sheet thickness after the hot stamping, and the n2 is an average number density per 10000 μm2 of the MnS having the equivalent circle diameter of 0.1 μm to 10 μm in the central part of the sheet thickness after the hot stamping.

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

4. The hot stamped steel according to claim 3 , wherein a galvannealing is formed on a surface of the hot dip galvanizing.

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

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

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

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

heating the steel;

hot-rolling the steel with a hot-rolling mill including a plurality of stands;

coiling the steel after the hot-rolling;

pickling the steel after the coiling;

cold-rolling the steel with a cold-rolling mill including a plurality of stands after the pickling under a condition satisfying a following expression (E);

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

temper-rolling the steel after cooled following annealed;

hot stamping in which the steel is heated to a temperature range of 700° C. to 1000° C. after the temper-rolling, hot-stamped within the temperature range, and thereafter cooled to a room temperature or more and 300° C. or less,

1.5 ×r 1 /r +1.2× r 2 /r+r 3 /r >1.0  (E), and

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

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

when CT represents a coiling temperature in the coiling in unit ° C., [C] represents the amount of C by mass %, [Mn] represents the amount of Mn by mass %, [Cr] represents the amount of Cr by mass %, and [Mo] represents the amount of Mo by mass %, 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 the hot stamped steel according to claim 8 , wherein

when T represents a heating temperature in the heating in unit ° C., t represents an in-furnace time in the heating in unit minute, [Mn] represents the amount of Mn by mass % in the steel sheet, and [S] represents an amount of S by mass % in the steel sheet, a following expression (G) is satisfied

T× ln( t )/(1.7×[Mn]+[S])>1500  (G).

10. The method for producing the hot stamped steel according to any one of claims 7 to 9 , further comprising:

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

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

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

12. The method for producing the hot stamped steel according to any one of claims 7 to 9 , further comprising:

electrogalvanizing the steel after the temper-rolling.

13. The method for producing the hot stamped steel according to any one of claims 7 to 9 , further comprising:

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