Thin steel plate and plated steel sheet, and hot rolled steel sheet manufacturing method, cold rolled full hard steel sheet manufacturing method, heat treatment plate manufacturing method, thin steel sheet manufacturing method and plated steel sheet manufacturing method

摘要

Provided is a thin steel sheet including a ferrite phase having a certain amount or more, a low yield ratio, a tensile strength of 780 MPa or more, and good bending fatigue characteristics. The thin steel sheet of the present invention has a specific component composition, an area ratio of ferrite phase of 20% or more and 80% or less, an area ratio of martensite phase of 20% or more and 80% or less, and an average ferrite particle diameter of the surface layer portion of the steel sheet is 5.0 µm or less, steel sheet When the surface layer portion has a steel structure having an inclusion density of 200 pieces/mm 2 or less, and the hardness at the position of 1/2 t (t is the thickness of the steel sheet) in the thickness direction from the steel sheet surface is 100%, the steel sheet surface hardness is 95% or more. .