Method for producing high strength steel plate having improved ductility and formability and obtained steel plate

摘要

A method for producing a steel plate, comprising the following continuous steps: preparing a cold rolled steel plate, wherein the chemical composition of the steel is 0.15% ≦ C ≦ 0.23% by weight%, 2.0% ≦ Mn ≦ 2.8%, 1.0% ≦ Si ≦ 2.1%, 0.02% ≦ Al ≦ 1.0%, 0 ≦ Nb ≦ 0.035%, 0 ≦ Mo ≦ 0 .3%, 0 ≦ Cr ≦ 0.4%, provided that 1.7% ≦ Si + Al ≦ 2.1%, the remainder being Fe and inevitable impurities, at least 65% Annealing the steel sheet at an annealing temperature TA so as to obtain a structure comprising austenite and up to 35% of dual phase ferrite, from a temperature of at least 600 ° C. at a cooling rate of at least 20 ° C./s, from Ms-170 ° C. Quenching temperature QT included between Ms-80 ℃ Quenching the steel plate; heating the steel plate to a distribution temperature PT between 350 ° C. and 450 ° C .; maintaining the steel plate at this temperature for a distribution time Pt comprised between 80 seconds and 440 seconds; The steel sheet comprises 40% to 70% tempered martensite, 7% to 15% retained austenite, 15% to 35%, depending on the proportion of the surface. A method having a final microstructure consisting of ferrite, up to 5% fresh martensite and up to 15% bainite.