ArcelorMittal S.A.
Method for producing a high strength coated steel sheet having improved strength and ductility and obtained sheet

Abstract:

A method for producing a high strength coated steel sheet having a yield strength YS of at least 800 MPa, a tensile strength TS of >1180 MPa, a total elongation >14% and a hole expansion ratio HER >30%. The steel contains in weight %: 0.13%.ltoreq.C.ltoreq.0.22%, 1.2%.ltoreq.Si.ltoreq.1.8%, 1.8%.ltoreq.Mn.ltoreq.2.2%, 0.10%.ltoreq.Mo.ltoreq.0.20%, Nb.ltoreq.0.05%, Al.ltoreq.0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at a temperature TA higher than Ac.sub.3 but less than 1000.degree. C. for more than 30 s, then quenched by cooling temperature QT between 325.degree. C. and 375.degree. C., at a cooling speed sufficient to obtain a structure consisting of austenite and at least 60% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85 and 97% of the sum 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 cooled to the room temperature. Coated sheet obtained.