THIN STEEL PLATE AND PLATED STEEL PLATE, HOT ROLLED STEEL PLATE MANUFACTURING METHOD, COLD ROLLED FULL HARD STEEL PLATE MANUFACTURING METHOD, HEAT-TREATED PLATE MANUFACTURING METHOD, THIN STEEL PLATE MANUFACTURING METHOD AND PLATED STEEL PLATE MANUFACTURING METHOD

摘要

Provided are a steel sheet and so forth including a certain amount or more of a ferrite phase and having a low yield ratio, a tensile strength of 780 MPa or more, and good bending fatigue properties.;A steel sheet includes a specific component composition and a steel microstructure having an area percentage of a ferrite phase of 20% or more and 80% or less and an area percentage of a martensite phase of 20% or more and 80% or less, the area percentage being determined by microstructure observation, in which a surface layer portion of the steel sheet has an average ferrite grain size of 5.0 µm or less and an inclusion density of 200 particles/mm^-2 or less, and in which the steel sheet has a surface hardness of 95% or more when the steel sheet has a hardness of 100% at a position 1/2t (where t represents the thickness of the steel sheet) away from a surface of the steel sheet in the thickness direction.