High tensile strength steel sheet and its production

PURPOSE: To provide a high tensile strength steel sheet excellent in workability, corrosion resistance, and surface treatability. CONSTITUTION: The steel sheet has a composition which contains 0.05-0.3% C, ≤2% Si, 0.05-4.0% Mn, ≤0.1% P, ≤0.1% S, 0.1-2% Cu in the range not lower than Si(%)/5, 0.1-2% Al, ≤0.01% N, and Ni by the amount not lower than Cu(%)/3 (Ni is not needed when Cu is =0.5%) and satisfies [Si(%)+Al(%)≥0.5] and [Mn(%)+Ni(%)≥0.5]. Further, this steel sheet has a structure containing retained austenite by ≥5% by volume ratio. This steel sheet can b< produced by subjecting a slab of a steel with the above composition to hot rolling, to coiling at 300-720°C, to descaling treatment, and to cold rolling at 30-80% draft and then subjecting the resulting steel sheet to heating up to a temp. in the region between the Ac 1 transformation point and the Ac 3 transformation point in the subsequent continuous annealing or continuous hot-dip galvanizing stage and to holding, in the course of cooling, at a temp. in the range between 550 and 350°C for ≥30sec or to slow cooling through the temp. region at ≤100°C/min cooling rate. COPYRIGHT: (C)1993,JPO&Japio