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

Abstract:

A method for producing a coated steel sheet having a tensile strength TS of at least 1100 MPa, a total elongation TE according to ISO standard 6892-1 of at least 12%, the product TSxTE of the tensile strength by the total elongation being at least 14200 MPa %, and a hole expansion ratio HER according to ISO standard 16630:2009 of at least 25%, the method including the following successive steps: providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %: 0.15%.ltoreq.C.ltoreq.0.23%, 2.0%.ltoreq.Mn.ltoreq.2.7%, with C+Mn/10.gtoreq.0.420%, 0.ltoreq.Cr.ltoreq.0.40%, with Mn+Cr.gtoreq.2.25%, 0.2%.ltoreq.Si.ltoreq.1.6%, 0.02%.ltoreq.Al.ltoreq.1.2%, with 1.0%.ltoreq.Si+Al.ltoreq.2.2%, 0.ltoreq.Nb.ltoreq.0.035%, 0.ltoreq.Mo.ltoreq.0.1%, the remainder being Fe and unavoidable impurities, annealing the steel sheet at an annealing temperature T.sub.A so as to obtain a structure comprising at least 65% of austenite and at most 35% of intercritical ferrite, quenching the sheet from a temperature of at least 600.degree. C. at a cooling rate comprised between 20.degree. C./s and 50.degree. C./s down to a quenching temperature QT between 200.degree. C. and 270.degree. C., heating the sheet up to a partitioning temperature PT comprised between 400.degree. C. and 480.degree. C. and maintaining the sheet at this partitioning temperature PT for a partitioning time Pt comprised between 50 s and 250 s, hot-dip coating the sheet at a temperature less than 515.degree. C., cooling the coated sheet down to the room temperature, the steel sheet having a microstructure consisting of, in surface fraction: between 3% and 15% of retained austenite, at least 30% of tempered martensite, at most 5% of fresh martensite, at most 35% of bainite, the sum of the surface fractions of tempered martensite, fresh martensite and bainite being comprised between 55% and 92%, and between 5% and 35% of ferrite.