Patent ID: 11898219
Assignee: NIPPON STEEL CORPORATION
Field: Materials, metallurgy (Chemistry)
Classification: CPC C  B | IPC B  C

Claim 2:
3. A steel sheet including a steel micro-structure containing, in volume fraction,
tempered martensite: 85% or more, retained austenite: 5% or more to less than 15%, and ferrite, pearlite, bainite, and as-quenched martensite being less than 10% in total,
a chemical composition of the steel sheet comprising in mass %:
C: 0.18% or more to 0.38% or less,
Si: 0.80% or more to 2.50% or less,
Mn: 0.6% or more to 5.0% or less,
P: 0.0200% or less,
S: 0.0200% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0% or more to 1.000% or less,
Cr: 0% or more to 2.0% or less,
Mo: 0% or more to 0.50% or less,
Ti: 0% or more to 0.10% or less,
Nb: 0% or more to 0.10% or less,
B: 0% or more to 0.0100% or less,
V: 0% or more to 0.50% or less,
Cu: 0% or more to 0.50% or less,
W: 0% or more to 0.100% or less,
Ta: 0% or more to 0.100% or less,
Ni: 0% or more to 1.00% or less,
Co: 0% or more to 0.50% or less,
Sn: 0% or more to 0.050% or less,
Sb: 0% or more to 0.050% or less,
As: 0% or more to 0.050% or less,
Mg: 0% or more to 0.050% or less,
Ca: 0% or more to 0.050% or less,
Y: 0% or more to 0.050% or less,
Zr: 0% or more to 0.050% or less,
La: 0% or more to 0.050% or less,
Ce: 0% or more to 0.050% or less, and
the balance: Fe and unavoidable impurities,

when a content of Mn and a content of C in the retained austenite are denoted by MnA and CA, respectively, and when a content of Mn and a content of C in a matrix are denoted by MnM and CM, respectively, following Formulas (1) to (3) are satisfied, and
when a region measuring 20000 μm2 and centered about a t/4 point (t denotes a thickness of the steel sheet) from a surface of the steel sheet is observed, the number of carbides having an equivalent circle radius of 0.1 μm or more is 100 or less, and the steel sheet has a tensile strength of 1100 MPa or more,

MnA/MnM≥1.2  (1)

CA/CM≤5.0  (2)

CA≤1.0  (3).