The high intensity which is superior in the local extension production manner null

摘要

PURPOSE:To produce a high strength steel sheet having high elongation and excellent in local elongation by subjecting a steel having a specific composition consisting of C, Mn, S, and Fe to hot rolling and to cold rolling at specific cold draft and applying specific heat treatment to the resulting steel sheet. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.02-0.1% C, 0.2-1.5% Mn, =0.01% S, and the balance iron with inevitable impurities and further containing, it necessary, at least one kind among 0.01-1.0% Si, 0.01-0.1% P, and 0.01-1.0% Cr or at least one kind among 0.001-0.005% B, 0.001-0.1% Nb, and 0.001-0.1% Ti is hot-rolled and further cold-rolled at =30% cold draft. The resulting steel sheet is heated at 800-1000 deg.C for 10sec-10min and cooled rapidly from =650 deg.C at =100 deg.C/sec cooling rate. After cooled to room temp. by the above procedure, the steel sheet is reheated to 200-500 deg.C for 5sec-10min, or, the steel sheet is cooled to 200-400 deg.C and held for 5sec-10min. By this method, the sheet metal composed essentially of tempered martensite containing retained austenite by 1.0-6.0%, excellent in local elongation, and having strength as high as =60kgf/mm2 can be obtained.