ArcelorMittal S.A.
HIGH STRENGTH AND HIGH FORMABILITY STEEL SHEET AND MANUFACTURING METHOD

Abstract:

A cold-rolled and heat-treated steel sheet, has a composition comprising, by weight percent: n0.10%.ltoreq.C.ltoreq.0.25%, 3.5%.ltoreq.Mn--.ltoreq.6.0%, 0.5%.ltoreq.Si.ltoreq.2.0%, 0.3%.ltoreq.Al.ltoreq.1.2%, with Si+Al.gtoreq.0.8%, 0.10%.ltoreq.Mo.ltoreq.0.50%, S.ltoreq.0.010%, P.ltoreq.0.020%, N.ltoreq.0.008%. The cold-rolled steel sheet has a microstructure consisting of, in surface fraction: between 10% and 45% of ferrite, having an average grain size of at most 1.3 .mu.m, the product of the surface fraction of ferrite by the average grain size of the ferrite being of at most 35 .mu.m %, between 8% and 30% of retained austenite, the retained austenite having an Mn content higher than 1.1*Mn %, Mn % designating the Mn content of the steel, at most 8% of fresh martensite, at most 2.5% of cementite and partitioned martensite.