ArcelorMittal S.A.
Method for Producing a High Strength Coated Steel Sheet having Improved Strength, Ductility and Formability

Abstract:

A method is for producing a high strength coated steel sheet having a yield stress YS>800 MPa, a tensile strength TS>1180 MPa, and improved formability and ductility. The steel contains: 15%.ltoreq.C.ltoreq.0.25%, 1.2%.ltoreq.Si.ltoreq.1.8%, 2%.ltoreq.Mn.ltoreq.2.4%, 0.1%.ltoreq.Cr.ltoreq.0.25%, Al.ltoreq.0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature higher than Ac3 and lower than 1000.degree. C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 250.degree. C. and 350.degree. C., to obtain a structure consisting of at least 60% of martensite and a sufficient austenite content such that the final structure contains 3% to 15% of residual austenite and 85% to 97% of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 430.degree. C. and 480.degree. C. and maintained at this temperature for a partitioning time Pt between 10 s and 90 s, then hot dip coated and cooled to the room temperature.