HIGH-STRENGTH COLD-ROLLED STEEL SHEET, HIGH-STRENGTH PLATED STEEL SHEET, AND METHODS FOR THEIR MANUFACTURE

摘要

A high-strength cold-rolled steel sheet providing a product with a good surface condition after press forming, having excellent bake hardenability and anti room temperature aging property, and having a dual phase structure with a tensile strength of at least 340 MPa is provided. A high-strength cold-rolled steel sheet has a structure comprising a main phase which is a ferrite and a secondary phase which is a low temperature transformation product including a martensite and has a hardness distribution of the ferrite phase in an arbitrary cross section having a length of 10 mm in the widthwise direction of the sheet which satisfies the relationship prescribed by (Hv(max)−Hv(ave))<0.5×(Hv(ave). Hv(max) is the maximum Vickers hardness of ferrite grains in a region at a distance of from (⅛)t to (¼)t in the thickness direction from the surface when the thickness of the high-strength cold-rolled steel sheet is t, and Hv(ave) is the average Vickers hardness of ferrite grains in this region.