Steel sheet, member, and method for producing steel sheet, and method for producing member

A steel sheet comprising:

• a chemical composition containing, by mass %,
• C:
• 0.12% or more and 0.40% or less,
• Si:
• 0.8% or less,
• Mn:
• 1.7% or less,
• P:
• 0.05% or less,
• S:
• 0.010% or less,
• sol.Al:
• 1.00% or less,
• N:
• 0.010% or less,
• Ti:
• 0.002% or more and 0.080% or less, and
• B:
• 0.0002% or more and 0.0050% or less,
• and optionally further containing, by mass %, at least one selected from the group consisting of
• Cu:
• 0.01% or more and 1.00% or less,
• Ni:
• 0.01% or more and 1.00% or less,
• Mo:
• 0.005% or more and 0.350% or less,
• Cr:
• 0.005% or more and 0.350% or less,
• Zr:
• 0.005% or more and 0.350% or less,
• Ca:
• 0.0002% or more and 0.0050% or less,
• Nb:
• 0.002% or more and 0.060% or less,
• V:
• 0.005% or more and 0.500% or less,
• W:
• 0.005% or more and 0.200% or less,
• Sb:
• 0.001% or more and 0.100% or less,
• Sn:
• 0.001% or more and 0.100% or less,
• Mg:
• 0.0002% or more and 0.0100% or less, and
• REM:
• 0.0002% or more and 0.0100% or less,
• with the balance being Fe and inevitable impurities;
• a metallic structure in which an area ratio of martensite to an entire microstructure is 95% or more, and a ratio LS/LB satisfies a following formula (1), where LS denotes a length of a sub-block boundary and LB denotes a length of a block boundary:
• 0
• .11
• /
• [
• C
• ⁢
• %
• ]
• 0.8
• ≤
• L
• S
• /
• L
• B
• ≤
• 0
• .16
• /
• [
• C
• ⁢
• %
• ]
• 0.8
• ,
• (
• 1
• )
• where [C %] represents a C content in the chemical composition of the steel sheet in mass %;
• and
• a tensile strength of 1310 MPa or more.