Steel sheet, and production method therefor

A steel sheet having a composition comprising, in mass %,

• C:
• 0.05% or more and 0.20% or less,
• Si:
• 0.60% or more and 1.65% or less,
• Mn:
• 1.8% or more and 3.5% or less,
• P:
• 0.05% or less,
• S:
• 0.005% or less,
• Al:
• 0.08% or less, and
• N:
• 0.0060% or less,
• the balance being Fe and inevitable impurities,
• the steel sheet having a metallographic structure containing, in terms of an area ratio, ferrite of 25% or more and 65% or less, martensite having iron-based carbides precipitated in the grains thereof of 35% or more and 75% or less, and the balance structure other than the ferrite and the martensite of 20% or less (including 0%) in total,
• average grain diameters of the ferrite and the martensite being respectively 5 μm or lower, and
• a total of concentrations of Si and Mn at interface between the ferrite and the martensite being, in terms of an atomic concentration, 5% or more, and
• the steel sheet having a tensile strength of 900 MPa or higher.