PRODUCTION OF STEEL MATERIAL REDUCED IN YIELD RATIO

摘要

PURPOSE:To improve strength and bendability and to provide low yield ratio without using particular alloying elements by subjecting a steel with a specific composition to hot rolling and to reheating and hardening treatment and further carrying out reheating, light draft, water cooling, and tempering. CONSTITUTION:A steel having a composition consisting of, by weight, 0.03-0.30% C, 0.05-0.60% Si, 0.50-2.5% Mn, 0.0005-0.1% Al, and the balance Fe with inevitable impurities is hot-rolled and subjected to reheating and hardening treatment. his steel is reheated up to a temp. between (Ac1+20 deg.C) and (Ac1+80 deg.C), by which the area ratio of ferrite is improved. Then, light draft at 1-30% draft is applied to the above steel. Successively, the above steel is water-cooled and subjected to tempering treatment at 200-600 deg.C. By this method, a steel material having a strength as high as about 50kgf/mm2, improved in bendability, having yield plateau, and reduced in yield ratio can inexpensively be produced while obviating the necessity of reheating after reduction.