ArcelorMittal S.A.
Method For Producing an Ultra High Strength Galvannealed Steel Sheet and Obtained Galvannealed Steel Sheet

Abstract:

A method for producing a coated steel sheet having a tensile strength TS of at least 1450 MPa and a total elongation TE of at least 17% includes the successive steps of providing a cold rolled steel sheet made of a steel having a chemical composition comprising, in weight %: 0.34%.ltoreq.C.ltoreq.0.45%, 1.50%.ltoreq.Mn.ltoreq.2.30%, 1.50.ltoreq.Si.ltoreq.2.40%, 0%<Cr.ltoreq.0.7%, 0%.ltoreq.Mo.ltoreq.0.3%, 0.10%.ltoreq.Al.ltoreq.0.7%, and optionally 0%.ltoreq.Nb.ltoreq.0.05%, the remainder being Fe and unavoidable impurities, annealing the cold-rolled steel sheet at an annealing temperature AT higher than the Ac3 transformation point of the steel, quenching the annealed steel sheet by cooling it down to a quenching temperature QT lower than the Ms transformation point of the steel and comprised between 150.degree. C. and 250.degree. C., and reheating the quenched steel sheet to a partitioning temperature PT between 350.degree. C. and 450.degree. C. and maintaining the steel sheet at the partitioning temperature PT for a partitioning time Pt of at least 80s, and coating the steel sheet by galvannealing, with an alloying temperature GAT comprised between 470.degree. C. and 520.degree. C.