HOT-ROLLED STEEL SHEET SUPERIOR IN FORMABILITY AFTER WELDING, WITH HIGH STRENGTH HAVING TENSILE STRENGTH OF 780 MPa OR HIGHER, OF MAKING HEAT AFFECTED ZONE HARDLY BE SOFTENED, COLD-ROLLED STEEL SHEET WITH HIGH STRENGTH, AND SURFACE-TREATED STEEL SHEET WITH HIGH STRENGTH

摘要

PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet with high strength superior in press formability after welding, which can control reduction in strength of a welded heat affected zone, and to provide a cold-rolled steel sheet with high strength, and a surface-treated steel sheet with high strength.;SOLUTION: These steel sheets comprise, by wt.%, 0.05-0.20% C, 0.005-1.3% Si, 1.0-3.2% Mn, 0.001-0.05% P, 0.0001-0.01% S, 0.0005-0.01% N, 0.001-0.1% Al, 0.05-0.5% Mo, one or two elements of 0.005-0.05% Nb and 0.001-0.05% Ti, and the balance Fe with unavoidable impurities, wherein a microstructure consists of martensite of 5-40% by area rate, the rest consists of one or more of ferrite, retained austenite, bainite, and tempered martensite, and amounts of C, Si, Mn, and Mo, and amounts of C, Si, Mn, and Mo, and amounts of C, Si, Mn, Mo, Nb, and Ti, respectively satisfy particular expressions (A) and (B).;COPYRIGHT: (C)2003,JPO