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

Abstract:

A method is for producing a high strength coated steel sheet having an improved ductility and an improved formability, and a chemical composition containing: 0.13%.ltoreq.C.ltoreq.0.22%, 1.9%.ltoreq.Si.ltoreq.2.3%, 2.4%.ltoreq.Mn.ltoreq.3%, Al.ltoreq.0.5%, Ti.ltoreq.0.05%, Nb.ltoreq.0.05%, the remainder being Fe and unavoidable impurities. The sheet is annealed at temperature TA higher than Ac3 but less than 1000.degree. C. for a time of more than 30 s, quenched by cooling to a quenching temperature QT between 200.degree. C. and 280.degree. C. in order to obtain a structure consisting of austenite and at least 50% 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, heated up to a partitioning temperature PT between 430.degree. C. and 490.degree. C. and maintained at this temperature for a time Pt between 10 s and 100 s, hot dip coated and cooled to the room temperature.