The high tension where local ductility is superior the black sheet and the manufacturing method

摘要

PURPOSE: To stably provide a high tensile strength steel sheet excellent in workability including local ductility, capable of strength level regulation without causing great changes in C content, and having superior external appearance. ;CONSTITUTION: The steel sheet has a composition containing 0.05-0.3% C, ≤2.0% Si, 0.5-4.0% Mn, ≤0.1% P, ≤0.1% S, 0-5.0% Ni, 0.1-2.0% Al, and ≤0.01% N and satisfying [Si(%)+Al(%)=0.5] and [Mn(%)+1/3Ni(%)≥1.0] and also has a structure containing retained austenite by ≥5% by volume. A slab of steel with the above composition is hot-rolled, coiled at 300-720°C, descaled, and cold-rolled at 30-80% draft. The resulting steel sheet is subjected, in the course of subsequent continuous annealing stage, to heating up to a temp. in the region between the Ac₁ transformation point and the Ac₃ transformation point and then subjected, in the course of cooling, to holding at 550-350°C for ≥30sec or to slow cooling at ≤400°C/min cooling rate.;COPYRIGHT: (C)1993,JPO&Japio