HIGH-TENSILE COLD-ROLLED STEEL SHEET, HIGH-TENSILE PLATED STEEL SHEET AND PROCESS FOR PRODUCING THEM

摘要

A high-tensile cold-rolled steel sheet with composite structure that excels in the surface properties of product after press forming and exhibits excellent baking hardenability and resistance to natural aging, and that has a tensile strength of = 340 MPa. There is provided a high-tensile cold-rolled steel sheet having a structure comprising a ferrite phase as a main phase and a low-temperature transformation phase containing a martensite phase as a second phase, wherein at any arbitrary cross section of 10 mm length along the direction of sheet width, the hardness distribution of the ferrite phase satisfies the relationship defined by the expression Hv_(max)-Hv_(ave) < 0.5×HV_(ave). Hv_(max) is the maximum Vickers hardness of ferrite grains exhibited when the distance from surface in the direction of depth is in the range of (1/8)t to (1/4)t wherein t is the thickness of high-tensile cold-rolled steel sheet, and Hv_(ave) is the average Vickers hardness of ferrite grains in the above range.