ArcelorMittal S.A.
METHOD FOR PRODUCING A HIGH STRENGTH STEEL SHEET HAVING HIGH DUCTILITY, FORMABILITY AND WELDABILITY, AND OBTAINED STEEL SHEET

Abstract:

A method for producing a high-strength steel sheet having high ductility, formability and weldability includes providing a cold-rolled sheet, with a composition containing: 0.15% .ltoreq.C.ltoreq.0.23%, 1.4% .ltoreq.Mn.ltoreq.2.6%, 0.6% .ltoreq.Si.ltoreq.1.3%, with C+Si/10.ltoreq.0.30%, 0.4% .ltoreq.Al.ltoreq.1.0%, with Al.gtoreq.6(C+Mn/10)-2.5%, 0.010% .ltoreq.Nb.ltoreq.0.035%, 0.1% .ltoreq.Mo.ltoreq.0.5%, annealing the sheet at 860.degree. C.-900.degree. C. to obtain a structure consisting of at least 90% austenite and at least 2% intercritical ferrite, quenching to a temperature between Ms-10.degree. C. and Ms-60.degree. C. at a rate Vc higher than 30.degree. C./s, heating to a temperature PT between 410.degree. C. and 470.degree. C. for 60 s to 130 s, hot-dip coating the sheet, and cooling to room temperature. The microstructure includes 45% to 68% of martensite, consisting of 85% to 95% partitioned martensite having a C content of at most 0.45%, and fresh martensite; 10% to 15% retained austenite; 2% to 10% intercritical ferrite; 20% to 30% lower bainite.