Steel sheet and plated steel sheet

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

• C:
• 0.05 to 0.30%,
• Si:
• 0.01 to 2.50%,
• Mn:
• 0.80 to 3.00%,
• Al:
• 0.010 to 2.000%,
• P:
• 0.1000% or less,
• S:
• 0.1000% or less,
• N:
• 0.0300% or less,
• O:
• 0.010% or less,
• B:
• 0 to 0.0100%,
• Ti:
• 0 to 0.100%,
• Nb:
• 0 to 0.100%,
• V:
• 0 to 0.100%,
• Cr:
• 0 to 1.00%,
• Ni:
• 0 to 0.10%,
• Cu:
• 0 to 0.10%,
• Mo:
• 0 to 0.50%,
• W:
• 0 to 0.50%,
• Ca:
• 0 to 0.100%,
• Mg:
• 0 to 0.100%,
• Zr:
• 0 to 0.100%,
• Hf:
• 0 to 0.100%, and
• REM:
• 0 to 0.100% and
• having a balance comprising Fe and impurities and
• having a thickness of 0.4 to 2.0 mm,
• in which steel sheet,
• a tensile strength is 550 to 1500 MPa,
• an internal oxidation layer and decarburized layer are contained,
• wherein the decarburized layer is a region from a surface of the steel sheet to a region inside of the steel sheet in which a C concentration becomes lower compared with a bulk C concentration of the steel sheet,
• wherein when a thickness of the decarburized layer per side of the steel sheet:
• A (μm), the bulk C concentration of the steel sheet:
• Cb (%), and a thickness of the steel sheet:
• t (μm),
• 0.01≤A/t≤0.15 per side of the steel sheet, and
• wherein a C concentration at the A/2 position is Cb/2 or more per side of the steel sheet.