Ductility and the corrosion-resistant high tension cold rolling which is superior the black sheet and manufacturing method

摘要

PURPOSE: To stably provide a high tensile strength steel sheet excellent in workability including ductility, capable of regulating strength level without causing marked change in C content, and having superior corrosion resistance and external appearance. ;CONSTITUTION: The steel sheet has a composition containing 0.05-0.3% C, ≤2.0% Si, 0.05-4.0% Mn, ≤0.1% P, ≤0.1% S, 0.5-5.0% Cr, 0.1-2.0% Al, and =0.01% N and satisfying [Si(%)+Al(%)≥0.5] and [7.0≥Mn(%)+Cr(%)≥1.0] and also has a structure containing retained austenite by ≥5% by volume. This steel sheet can be produced by subjecting a slab of steel with the above composition to hot rolling, to coiling at 300-720°C, and to descaling treatment, applying cold rolling to the resulting steel sheet at 30-80% draft, and subjecting the resulting steel sheet to heating up to a temp. in the region between the Ac₁ transformation point and the Ac₃ transformation point in the subsequent continuous annealing or continuous hot-dip galvanizing stage and then to holding at a temp. in the region between 550 and 350°C for ≥30sec in the course of the cooling or to annealing through the temp. region at ≤400°C/min cooling rate.;COPYRIGHT: (C)1993,JPO&Japio