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

Abstract:

A method for producing a steel sheet having a microstructure including 71% to 91% martensite and bainite, 9% to 13% retained austenite, and at most 20% ferrite is provided. The method includes providing a cold-rolled steel sheet including, in weight percent: 0.13%.ltoreq.C.ltoreq.0.22%, 1.2%.ltoreq.Si.ltoreq.2.3%, 0.02%.ltoreq.Al.ltoreq.1.0%, with 1.25%.ltoreq.Si+Al.ltoreq.2.35%, 2.4%.ltoreq.Mn.ltoreq.3%, Ti.ltoreq.0.05%, Nb.ltoreq.0.05% and a remainder of Fe and unavoidable impurities, annealing the steel sheet to obtain 80% to 100% austenite and 0% to 20% ferrite, quenching the steel sheet at a cooling rate between 20.degree. C./s and 50.degree. C./s to a quenching temperature between 240.degree. C. and 310.degree. C., heating the steel sheet to a partitioning temperature between 400.degree. C. and 465.degree. C. and maintaining the steel sheet at the partitioning temperature for 50 to 250 seconds, then immediately cooling the sheet to room temperature. Steel sheets are also provided.