Steel sheet, member, and method for producing them

A steel sheet having a chemical composition comprising, by mass %:

• Si:
• 0.20% to 2.00%;
• Mn:
• 2.70% to 4.00%;
• C:
• 0.120% to 0.400%;
• P:
• 0.001% to 0.100%;
• S:
• 0.0200% or less;
• Al:
• 0.010% to 2.000%;
• N:
• 0.0100% or less;
• optionally at least one selected from the group consisting of:
• Sb:
• 0.200% or less, and
• Sn:
• 0.200% or less
• optionally at least one selected from the group consisting of:
• Ti:
• 0.200% or less,
• Nb:
• 0.200% or less,
• V:
• 0.100% or less,
• B:
• 0.0100% or less,
• Cu:
• 1.000% or less,
• Cr:
• 1.000% or less,
• Ni:
• 1.000% or less,
• Mo:
• 0.500% or less,
• Ta:
• 0.100% or less,
• W:
• 0.500% or less,
• Mg:
• 0.0200% or less,
• Zn:
• 0.020% or less,
• Co:
• 0.020% or less,
• Zr:
• 0.020% or less,
• Ca:
• 0.0200% or less,
• Ce:
• 0.0200% or less,
• Se:
• 0.0200% or less,
• Te:
• 0.0200% or less,
• Ge:
• 0.0200% or less,
• As:
• 0.0200% or less,
• Sr:
• 0.0200% or less,
• Cs:
• 0.0200% or less,
• Hf:
• 0.0200% or less,
• Pb:
• 0.0200% or less,
• Bi:
• 0.0200% or less, and
• REM (except Ce):
• 0.0200% or less;
• optionally an equivalent carbon content Ceq of 0.697% or more;
• the remainder being Fe and incidental impurities,
• wherein:
• the steel sheet has a microstructure including a ferrite area fraction in a range of 5% to 30%, a bainitic ferrite area fraction in a range of 5% to 35%, a tempered martensite area fraction in a range of 20% to 50%, and a retained austenite volume fraction in a range of 5% to 35%,
• in a region within 15.0 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more,
• the lowest Si concentration LSi and the lowest Mn concentration LMn in the region within 15.0 μm in the thickness direction from the surface of the steel sheet and a Si concentration TSi and a Mn concentration TMn at a quarter thickness position of the steel sheet satisfy the following formula (1):
• LSi+LMn≤(TSi+TMn)/4 (1)
• the steel sheet has a tensile strength of 1180 MPa or more, and
• optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.