ArcelorMittal S.A.
High strength and high formability steel sheet and manufacturing method

Abstract:

A cold-rolled and heat-treated steel sheet, having a composition including, by weight percent: 0.10%.ltoreq.C.ltoreq.0.40%, 3.5%.ltoreq.Mn.ltoreq.8.0%, 0.5%.ltoreq.Si.ltoreq.2.5%, 0.003%.ltoreq.Al.ltoreq.3.0%, with Si+Al.gtoreq.0.8%, 0.001%.ltoreq.Mo.ltoreq.0.5%, S.ltoreq.0.010%, P.ltoreq.0.020%, N.ltoreq.0.008%, and optionally one or more elements selected from a group comprising Cr, Ti, Nb, V and B, such that: 0.01%.ltoreq.Cr.ltoreq.2.0%, 0.010%.ltoreq.Ti.ltoreq.0.080%, 0.010%.ltoreq.Nb.ltoreq.0.080%, 0.010%.ltoreq.V.ltoreq.0.30%, 0.0005%.ltoreq.B.ltoreq.0.003%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting. The microstructure includes, in surface fraction, between 10% and 50% of retained austenite, at most 8% of fresh martensite, and tempered martensite. The retained austenite includes Mn-enriched austenite, having a Mn content higher than 1.3*Mn %, Mn % designating the Mn content of the steel sheet, a surface fraction of the Mn-enriched austenite with respect to the whole microstructure is between 2% and 12%, and Mn-poor austenite, having an Mn content between 0.5*Mn % and 1.3*Mn %.