Steel sheet, member, and methods for producing them

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.