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

Abstract:

A method for manufacturing a high-strength sheet having improved formability and ductility accord which the chemical composition of the steel contains, in percent by weight: 0.25%<C.ltoreq.0.4% 2.3%.ltoreq.Mn.ltoreq.3.5% 2.3%.ltoreq.Si.ltoreq.3% Al.ltoreq.0.040% the remainder being Fe and unavoidable impurities, the method comprising the steps of annealing a rolled sheet made of said steel by soaking it at an annealing temperature AT higher than the Ac.sub.3 transformation point of the steel, quenching the sheet by cooling it down to a quenching temperature QT between Ms-65.degree. C. and Ms-115.degree. C., being in order to obtain a final structure containing at least 65% of martensite and at least 5% of residual austenite, the sum of the ferrite and bainite contents being less than 10%, Ms being the Ms transformation point of the steel according to the Andrews formula, heating the sheet up to an overaging temperature PT between 360.degree. C. and 500.degree. C. and maintaining it at said temperature for a time Pt between 10 s and 600 s, and cooling the sheet down to the ambient temperature.