Cold rolled, annealed and tempered steel sheet and method of manufacturing the same

Abstract

A cold rolled, annealed and partitioned steel sheet, made of a steel having a composition including, by weight percent: C: 0.05-0.18%, Mn: 6.0-11.0%, Mo: 0.05-0.5%, B: 0.0005-0.005%, S≤0.010%, P≤0.020%, N≤0.008%, and including optionally one or more of the following elements, in weight percentage: Al<3%, Si≤1.20%, Ti≤0.050%, Nb≤0.050%, Cr≤0.5%, V≤0.2%, the remainder of the composition being iron and unavoidable impurities resulting from the smelting, said steel sheet having a microstructure including, in surface fraction, from 0% to 30% of ferrite, such ferrite, when present, having a grain size below 1.0 μm, from 8% to 40% of retained austenite, the fraction of austenite islands with a size above 0.5 μm being below or equal to 5% from 30 to 92% of partitioned martensite less than 3% of fresh martensite, a carbon [C]and manganese [Mn]content in austenite, expressed in weight percent, such that the ratio ([C]×[Mn])/(C %×Mn %) is below 18.0, C % and Mn % being the nominal values in carbon and manganese in weight %.