Steel sheet and method for producing same
Steel sheet low in cost and improved in fatigue characteristics without causing a drop in the cold formability, characterized in that it comprises an inner layer and a hard layer on one or both surfaces of the inner layer, a thickness of the hard layer is 20 μm or more and 40% or less of the thickness of the steel sheet, an average micro-Vickers hardness of the hard layer is 240 HV or more and less than 400 HV, an amount of C of the hard layer is 0.4 mass % or less, an amount of N is 0.02 mass % or less, a variation of hardness measured by a nanoindenter at a depth of 10 from the surface of the hard layer is a standard deviation of 2.0 or less, an average micro-Vickers hardness of the inner layer is 80 HV or more and less than 400 HV, a volume rate of carbides contained in the inner layer is less than 2.00%, and the average micro-Vickers hardness of the hard layer is 1.05 times or more the average micro-Vickers hardness of the inner layer.
1. A multi-layer steel sheet comprising
an inner layer and
a hard layer on one or both surfaces of the inner layer, the steel sheet characterized in that
a thickness of the hard layer is 20 μm or more and 40% or less of a thickness of the steel sheet,
an average micro-Vickers hardness of the hard layer is 240 HV or more and less than 400 HV,
an amount of C of the hard layer is 0.4 mass % or less,
an amount of N of the hard layer is 0.02 mass % or less,
a standard deviation of a variation of hardness measured by a nanoindenter at a depth of 10 μm from a surface of the hard layer is 2.0 or less,
an average micro-Vickers hardness of the inner layer is 80 HV or more and less than 400 HV,
a volume rate of carbides contained in the inner layer is less than 2.00%, and
the average micro-Vickers hardness of the hard layer is 1.05 times or more the average micro-Vickers hardness of the inner layer, wherein
the hard layer contains, by mass %,
C: 0.03 to 0.35%,
Si: 0.01 to 3.00%,
Mn: 0.70 to 10.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 0.500% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Cr: 0 to 2.000%,
Mo: 0 to 1.000%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.500%,
W: 0 to 0.100%,
Ta: 0 to 0.100%,
Ni: 0 to 0.500%,
Sn: 0 to 0.050%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Mg: 0 to 0.0500%,
Ca: 0 to 0.050%,
Y: 0 to 0.050%,
Zr: 0 to 0.050%,
La: 0 to 0.050%,
Ce: 0 to 0.050%, and
a balance of Fe and impurities and the inner layer contains, by mass %,
C: 0.001 to 0.300%,
Si: 0.01 to 3.00%,
Mn: 0.10 to 3.00%,
P: 0.0200% or less,
S: 0.0200% or less,
Al: 0.500% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Ti: 0 to 0.500%,
B: 0 to 0.0100%,
Cr: 0 to 2.000%,
Mo: 0 to 1.000%,
Nb: 0 to 0.500%,
V: 0 to 0.500%,
Cu: 0 to 0.500%,
W: 0 to 0.100%,
Ta: 0 to 0.100%,
Ni: 0 to 0.500%,
Sn: 0 to 0.050%,
Sb: 0 to 0.050%,
As: 0 to 0.050%,
Mg: 0 to 0.0500%,
Ca: 0 to 0.050%,
Y: 0 to 0.050%,
Zr: 0 to 0.050%,
La: 0 to 0.050%,
Ce: 0 to 0.050%, and
a balance of Fe and impurities,
wherein the thickness of the steel sheet is 0.2 to 2.4 mm; and the fatigue limit ratio of the steel sheet is 0.50 or more.
2. The multi-layer steel sheet according to claim 1 , wherein the tensile strength of the steel sheet is 535 MPa or more.
3. The multi-layer steel sheet according to claim 1 , wherein
the tensile strength of the steel sheet is 739 MPa or more,
the thickness of the hard layer is 21 μm or more and 25% or less of a thickness of the steel sheet, and
the fatigue limit ratio of the steel sheet is 0.58 or more.