Abstract: A method for producing a high strength steel sheet having a yield strength YS>850 MPa, a tensile strength TS>1180 MPa, a total elongation >13% and a hole expansion ratio HER>30%, by heat treating a steel sheet wherein the chemical composition of the steel contains: 0.13%?C?0.22%, 1.2%?Si?1.8%, 1.8%?Mn?2.2%, 0.10%?Mo?0.20%, Nb?0.05%, Ti<0.05%, Al?0.5%, the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA>865° C. and <1000° C. for a time of more than 30 s, then quenched by cooling it to a quenching temperature QT between 275° C. and 375° C., at a cooling speed >30° C./s in order to have, just after quenching, a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite without ferrite, then heated to a partitioning temperature PT between 370° C. and 470° C.

Abstract: A method for producing a high strength coated steel sheet having an improved ductility and an improved formability, the chemical composition of the steel containing: 0.13%?C?0.22%, 1.9%?Si?2.3%, 2.4%?Mn?3%, Al?0.5%, Ti<0.05%, Nb<0.05%, the remainder being Fe and unavoidable impurities. The sheet is annealed at temperature TA higher than Ac3 but less than 1000° C. for a time of more than 30 s, quenched by cooling to a quenching temperature QT between 200° C. and 280° C. in order to obtain a structure consisting of austenite and at least 50% of martensite, the austenite content being such that the final structure can contain between 3% and 15% of residual austenite and between 85% and 97% of the sum of martensite and bainite, without ferrite, heated up to a partitioning temperature PT between 430° C. and 490° C. and maintained at this temperature for a time Pt between 10 s and 100 s, hot dip coated and cooled to the room temperature.