Steel sheet
Provided are a steel sheet having a predetermined chemical composition and having the following steel structure, and a method of manufacturing the steel sheet. (1) In area ratio %, ferrite: 0 to 5%, martensite: 90 to 100%, a ratio of tempered martensite to total martensite: 80 to 100%, and retained austenite: 0.5 to 6.0% are contained. (2) The number density of inclusions satisfying the maximum diameter≥3 μm is 40 inclusions/mm 2 or less. (3) When the number density of the inclusions satisfying the maximum diameter≥3 μm in each section is calculated, the number density in the section where the number density of inclusions is in the top 10% is 80 inclusions/mm 2 or less. (4) Formula (A) is satisfied. Vγ′/V γ≥0.1 (A) Vγ: Initial retained austenite, Vγ′: Retained austenite after deep cooling at −196° C. (5) The tensile strength is 1470 MPa or more.
1 . A steel sheet comprising a chemical composition that contains, in mass %:
C: 0.18% to 0.35%,
Si: 0.01% to 1.00%,
Mn: 0.10% to 2.40%,
P: 0.050% or less,
S: 0.0050% or less,
Al: 0.001% to 1.00%,
Ti: 0.001% to 0.050%,
B: 0.0005% to 0.0050%,
N: 0.0100% or less,
O: 0.0050% or less,
Cr: 0% to 1.00%,
Mo: 0% to 0.50%,
Cu: 0% to 0.50%,
Ni: 0% to 0.50%,
Co: 0% to 0.50%,
W: 0% to 0.50%,
Sn: 0% to 0.50%,
Sb: 0% to 0.50%,
Nb: 0% to 0.050%,
V: 0% to 0.50%,
Ca: 0% to 0.0100%,
Mg: 0% to 0.0100%,
Ce: 0% to 0.0100%,
Zr: 0% to 0.0100%,
La: 0% to 0.0100%,
Hf: 0% to 0.0100%,
Bi: 0% to 0.0100%,
REM: 0% to 0.0100%, and
a balance consisting of Fe and impurities, and
that satisfies the Formula: 0.001≤Ti−(47.88/14.01)×N≤0.040, wherein each element symbol represents mass % of the corresponding element, and
a steel structure in a cross section in a rolling direction of a steel sheet and in a range of ⅛ thickness to ⅜ thickness centered on ¼ thickness from a surface of the steel sheet,
that contains, in area ratio %,
ferrite: 0 to 5%,
martensite: 90.0 to 99.5%,
a ratio of tempered martensite to total martensite: 80.0 to 100.0%, and
retained austenite: 0.5 to 6.0%,
and
in a measurement region defined by a range of ⅛ thickness to ⅞ thickness from the surface of the steel sheet×25 mm in the cross section in the rolling direction of the steel sheet, a number density of inclusions satisfying a maximum diameter≥3 μm is 40 inclusions/mm 2 or less,
when the measurement region is divided into 100 sections obtained by dividing the measurement region into 2 sections in a thickness direction and 50 sections in a width direction, and the number density of the inclusions satisfying the maximum diameter≥3 μm in each section is calculated, the number density in a section where the number density of inclusions is in a top 10% is from 2 inclusions/mm 2 to 80 inclusions/mm 2 ,
the following Formula (A) is satisfied, and
a tensile strength is 1470 MPa or more:
Vγ′/Vγ≥ 0.1 (A),
wherein Vγ: area ratio of initial retained austenite, Vγ′: area ratio of retained austenite after deep cooling at −196° C.
2 . The steel sheet according to claim 1 , wherein the number density of inclusions satisfying the maximum diameter≥3 μm is 30 inclusions/mm 2 or less in the measurement region.
3 . The steel sheet according to claim 2 , wherein the number density in the section where the number density of inclusions is in the top 10% is 60 inclusions/mm 2 or less.
4 . The steel sheet according to claim 2 , satisfying the following Formula (A2):
Vγ′/Vγ≥ 0.2 (A2),
wherein Vγ: area ratio of initial retained austenite, Vγ′: area ratio of retained austenite after deep cooling at −196° C.
5 . The steel sheet according to claim 1 , wherein the number density in the section where the number density of inclusions is in the top 10% is 60 inclusions/mm 2 or less.
6 . The steel sheet according to claim 5 , satisfying the following Formula (A2):
Vγ′/Vγ≥ 0.2 (A2),
wherein Vγ: area ratio of initial retained austenite, Vγ′: area ratio of retained austenite after deep cooling at −196° C.
7 . The steel sheet according to claim 1 , satisfying the following Formula (A2):
Vγ′/Vγ≥ 0.2 (A2),
wherein Vγ: area ratio of initial retained austenite, Vγ′: area ratio of retained austenite after deep cooling at −196° C.