HIGH STRENGTH STEEL SHEET EXERTING EXCELLENT DEEP DRAWABILITY AT ROOM TEMPERATURE AND WARM TEMPERATURES AND METHOD FOR WARM WORKING SAME

摘要

This high strength steel sheet has a component composition containing in mass% 0.02 to 0.3% C 1 to 3% Si 1.8 to 3% Mn 0.1% or less P 0.01% or less S 0.001 to 0.1% Al and 0.002 to 0.03% N the remainder being iron and impurities. The high strength steel sheet has a structure containing in terms of area ratio relative to the entire structure each of the following phases: 50 to 85% beinitic ferrite; 3% or more retained austenite (?); 10 to 45% martensite and the aforementioned retained austenite (?); and 5 to 40% ferrite. The ratio between the Mn concentration (Mn) in the retained austenite (?) and the average Mn concentration (Mn) in the entire structure is 1.2 or more (Mn/Mn) based on the Mn concentration distribution obtained by means of EPMA line analysis. As a consequence the high strength steel sheet exhibits strength of 980 MPa or more and exerts excellent deep drawability.