Steel sheet, plated steel sheet, method of production of hot-rolled steel sheet, method of production of cold-rolled full hard steel sheet, method of production of steel sheet, and method of production of plated steel sheet

摘要

A steel sheet, a plated steel sheet, and methods for producing a hot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steel sheet. The steel sheet has a specified composition and a microstructure including 0 to 60% of polygonal ferrite and 40 to 100% of a total of martensite, bainite, and residual austenite in terms of an area ratio within 20 μm from the steel sheet surface. The content of Mn in martensite present within 20 μm of the steel sheet surface ([Mn]SM) and the content of Mn in a bulk ([Mn]B) satisfy [Mn]SM/[Mn]B≤1.5. At a location 300 μm from the steel sheet surface, an area ratio of the martensite is in a range of 40 to 80%, and the polygonal ferrite and the bainite have an average crystal grain size of less than 15 μm.