Steel sheet and plated steel sheet

A steel sheet with a tensile strength of 780 MPa or more, wherein

• a chemical composition comprises, by mass %,
• C:
• 0.08 to 0.40%,
• Si:
• 0.4 to 2.0%,
• Mn:
• 0.1 to 5.0%,
• sol. Al:
• 0 to 2.0%,
• P:
• 0.0300% or less,
• S:
• 0.0300% or less,
• N:
• 0.0100% or less,
• B:
• 0 to 0.0100%,
• Ti:
• 0 to 0.1500%,
• Nb:
• 0 to 0.150%,
• V:
• 0 to 0.150%,
• Cr:
• 0 to 2.00%,
• Ni:
• 0 to 2.00%,
• Cu:
• 0 to 2.0000%,
• Mo:
• 0 to 1.00%,
• W:
• 0 to 1.000%,
• Ca:
• 0 to 0.1000%,
• Mg:
• 0 to 0.100%,
• Zr:
• 0 to 0.100%,
• Hf:
• 0 to 0.100%,
• REM:
• 0 to 0.1000% and
• a balance of Fe and impurities,
• in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 3 μm or more,
• in a depth direction from the steel sheet surface, a thickness of a layer with an area ratio of cementite of 10% or less is 5 μm or more, and
• a surface roughness of the steel sheet is an arithmetic average roughness Ra of 3.0 μm or less.