Hot-stamped steel, cold-rolled steel sheet and method for producing hot-stamped steel
A hot-stamped steel according to the present invention has a predetermined chemical composition, satisfies (5×[Si]+[Mn])/[C]>10 when [C] is the amount of C by mass %, [Si] is the amount of Si by mass %, and [Mn] is the amount of Mn by mass %, includes 40% to 95% ferrite and 5% to 60% martensite in area fraction, and optionally further includes 10% or less pearlite in area fraction, 5% or less retained austenite in volume fraction, and less than 40% bainite in area fraction. The total of the area fraction of ferrite and the area fraction of martensite is 60% or more, the hardness of martensite measured with a nanoindenter satisfies H2/H1<1.10 and σHM<20, and TS×λ which is product of tensile strength TS and hole expansion ratio λ is 50000 MPa·% or more.
1. A hot-stamped steel comprising, by mass %:
C: 0.030% to 0.150%;
Si: 0.010% to 1.000%;
Mn: 0.50% or more and less than 1.50%;
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 at least one 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 of Fe and impurities, wherein
when [C] is an amount of C by mass %, [Si] is an amount of Si by mass %, and [Mn] is an amount of Mn by mass %, a following expression (A) is satisfied,
the structure of the hot-stamped steel sheet consists of a ferrite, a martensite, and optionally one or more of a pearlite, a retained austenite, and a bainite,
an area fraction of the ferrite is 40% to 95% and an area fraction of the martensite is 5% to 60%,
a total of the area fraction of the ferrite and the area fraction of the martensite is 60% or more,
an area fraction of the pearlite is 10% or less, a volume fraction of the retained austenite is 5% or less, and an area fraction of the bainite is less than 40%,
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]>10 (A),
H2/H1<1.10 (B),
σHM<20 (C), and
the H 1 is an average hardness of the martensite in a surface portion of a sheet thickness of the hot-stamped steel, the surface portion is an area having a width of 200 μm in a thickness direction from an outermost layer, the H 2 is an average hardness of the martensite in a central portion of the sheet thickness of the hot-stamped steel, the central portion is an area having a width of 200 μm in the thickness direction at a center of the sheet thickness, and the σHM is a variance of the average hardness of the martensite in the central portion of the sheet thickness of the hot-stamped steel.
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 n 1 is an average number density per 10000 μm 2 of the MnS having an equivalent circle diameter of 0.1 μm to 10 μm in a ¼ portion of the sheet thickness of the hot-stamped steel, and the n 2 is an average number density per 10000 μm 2 of the MnS having an equivalent circle diameter of 0.1 μm to 10 μm in the central portion of the sheet thickness of the hot-stamped steel.
3. The hot-stamped steel according to claim 1 , 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 is alloyed.
5. The hot-stamped steel according to claim 1 , wherein an electrogalvanized layer is formed on a surface thereof.
6. The hot-stamped steel according to claim 1 , wherein an aluminized layer is formed on a surface thereof.