Steel sheet and plated steel sheet

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

• C:
• 0.05 to 0.40%,
• Si:
• 0.2 to 3.0%,
• Mn:
• 0.1 to 5.0%,
• sol. Al:
• 0 to less than 0.4000%,
• P:
• 0.0300% or less,
• S:
• 0.0300% or less,
• N:
• 0.0100% or less,
• B:
• 0 to 0.010%,
• Ti:
• 0 to 0.150%,
• Nb:
• 0 to 0.150%,
• V:
• 0 to 0.150%,
• Cr:
• 0 to 2.00%,
• Ni:
• 0 to 2.00%,
• Cu:
• 0 to 2.00%,
• Mo:
• 0 to 1.00%,
• W:
• 0 to 1.00%,
• Ca:
• 0 to 0.100%,
• Mg:
• 0 to 0.100%,
• Zr:
• 0 to 0.100%,
• Hf:
• 0 to 0.100%,
• REM:
• 0 to 0.100% and
• a balance of Fe and impurities, wherein
• a surface layer of the steel sheet contains grain boundary oxides,
• a Ratio A of a length of the grain boundary oxides projected on a surface of the steel sheet to a length of the surface of the steel sheet when examining a cross-section of the surface layer of the steel sheet is 50% or more and 100% or less,
• a number density of granular oxides is less than 4.0/μm2,
• the steel sheet comprises an Si—Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet, and
• Si and Mn contents of the Si—Mn depleted layer at ½ position of the thickness and not containing oxides are respectively less than 10% of the Si and Mn contents at a sheet thickness center part of the steel sheet.