ArcelorMittal S.A.
METHOD FOR MANUFACTURING A HIGH-STRENGTH STEEL SHEET AND SHEET OBTAINED BY THE METHOD

Abstract:

A method is for manufacturing a high-strength steel sheet having a tensile strength of more than 1100 MPa and a yield strength of more than 700 MPa, a uniform elongation UE of at least 8.0% and a total elongation of at least 10%, made of a steel containing in percent by weight: 0.1%.ltoreq.C.ltoreq.0.25%, 4.5%.ltoreq.Mn.ltoreq.10%, 1%.ltoreq.Si.ltoreq.3%, 0.03%.ltoreq.Al.ltoreq.2.5%, the remainder being Fe and impurities resulting from the smelting, the composition being such that CMnIndex=C.times.(1+Mn/3.5).ltoreq.0.6. The method includes annealing a rolled sheet made of said steel by soaking it at an annealing temperature TA higher than the Ac.sub.1 transformation point of the steel but less than 1000.degree. C., cooling the annealed sheet to a quenching temperature QT between 190.degree. C. and 80.degree. C. at a cooling speed sufficient to obtain a structure just after cooling containing martensite and retained austenite, maintaining the steel sheet at an overaging temperature PT between 350.degree. C. and 500.degree. C. for an overaging time Pt of more than 5 s cooling the sheet down to the ambient temperature.