ArcelorMittal S.A.
Cold rolled and heat treated steel sheet, method of production thereof and use of such steel to produce vehicle parts

Abstract:

A cold rolled and heat treated steel sheet having a composition including the following elements, expressed in % by weight: 0.1%.ltoreq.carbon.ltoreq.0.6% 4%.ltoreq.manganese.ltoreq.20% 5%.ltoreq.aluminum.ltoreq.15% 0.ltoreq.silicon.ltoreq.2% aluminium+silicon+nickel.gtoreq.6.5% and can possibly contain one or more of the following optional elements: 0.01%.ltoreq.niobium.ltoreq.0.3%, 0.01%.ltoreq.titanium.ltoreq.0.2% 0.01%.ltoreq.vanadium.ltoreq.0.6% 0.01%.ltoreq.copper.ltoreq.2.0% 0.01%.ltoreq.nickel.ltoreq.2.0% cerium.ltoreq.0.1% boron.ltoreq.0.01% magnesium.ltoreq.0.05% zirconium.ltoreq.0.05% molybdenum.ltoreq.2.0% tantalum.ltoreq.2.0% tungsten.ltoreq.2.0% the remainder being composed of iron and unavoidable impurities caused by elaboration, wherein the microstructure of said steel sheet includes in area fraction, 10 to 50% of austenite, the austenite phase optionally including intragranular kappa carbides, the remainder being regular ferrite and ordered ferrite of D03 structure (Fe,Mn,X).sub.3Al, optionally including up to 2% of intragranular kappa carbides (Fe,Mn).sub.3AlC.sub.x said steel sheet presenting a ultimate tensile strength higher than or equal to 900 MPa. It also deals with a manufacturing method and with use of such grade for making vehicle parts.