CPC C21D 9/46 (2013.01) [B22D 11/182 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0273 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C23C 2/06 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] | 16 Claims |
1. A steel sheet having a chemical composition comprising, by mass %:
C: 0.13% or more and 0.40% or less;
Si: 1.5% or less;
Mn: 1.7% or less;
P: 0.010% or less;
S: 0.0020% or less;
sol. Al: 0.20% or less;
N: less than 0.0055%,%;
O: 0.0025% or less;
Nb: 0.002% or more and 0.035% or less;
Ti: 0.002% or more and 0.10% or less;
B: 0.0002% or more and 0.0035% or less; and
the balance being Fe and incidental impurities,
wherein the steel sheet has a microstructure including martensite and bainite, a total area fraction of the martensite and the bainite being in a range of 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite, and
a total of (i) a density of inclusion particles having a long-axis length in a range of 20 μm or more and 80 μm or less and a minimum interparticle distance of more than 10 μm and (ii) a density of inclusion particle clusters each having a long-axis cluster length in a range of 20 μm or more and 80 μm or less and each including two or more inclusion particles having a long-axis length of 0.3 μm or more and a minimum interparticle distance of 10 μm or less is 5 pieces/mm2 or less,
a local P concentration in a region extending from a position 1/4 of a thickness of the steel sheet in a thickness direction from a surface of the steel sheet to a position 3/4 of the thickness of the steel sheet in the thickness direction from the surface of the steel sheet is 0.060% or less by mass, and a degree of Mn segregation in the region is 1.50 or less,
the steel sheet has a tensile strength of 1,320 MPa or more, and
formulae (1) and (2) are satisfied:
[% Ti]+[% Nb]>0.007 (1)
[% Ti]×[% Nb]2≤7.5×10−6 (2)
where, in each of formulae (1) and (2), [% Nb] and [% Ti] are a Nb content (%) and a Ti content (%), respectively, of the steel sheet.
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