Abstract: A method for producing a steel sheet having a microstructure including 71% to 91% martensite and bainite, 9% to 13% retained austenite, and at most 20% ferrite is provided. The method includes providing a cold-rolled steel sheet including, in weight percent: 0.13%?C?0.22%, 1.2%?Si?2.3%, 0.02%?Al?1.0%, with 1.25%?Si+Al?2.35%, 2.4%?Mn?3%, Ti?0.05%, Nb?0.05% and a remainder of Fe and unavoidable impurities, annealing the steel sheet to obtain 80% to 100% austenite and 0% to 20% ferrite, quenching the steel sheet at a cooling rate between 20° C./s and 50° C./s to a quenching temperature between 240° C. and 310° C., heating the steel sheet to a partitioning temperature between 400° C. and 465° C. and maintaining the steel sheet at the partitioning temperature for 50 to 250 seconds, then immediately cooling the sheet to room temperature. Steel sheets are also provided.

Abstract: A method for producing a steel sheet is provided. The steel sheet has a microstructure including, in area fraction, 20% to 50% intercritical ferrite, 10% to 20% retained austenite, 25% to 45% tempered martensite, 10% to 20% fresh martensite, and bainite. The sum of tempered martensite and bainite is between 30% and 60%. The method includes providing a cold-rolled steel sheet including, in weight percent, 0.18%?C?0.25%, 0.9%?Si?1.8%, 0.02%?Al?1.0%, with 1.00%?Si+Al?2.35%, 1.5%?Mn 2.5%, 0.010%?Nb?0.035%, 0.10%?Cr?0.40%, and a remainder including Fe and unavoidable impurities. The method further includes annealing the steel sheet to obtain 50% to 80% austenite and 20% to 50% of ferrite, quenching the sheet at a cooling rate between 20° C./s and 50° C./s to a quenching temperature between Ms-50° C. and Ms-5° C., heating the sheet to a partitioning temperature between 375° C. and 450° C.

Abstract: A method for producing a steel sheet is provided. The method includes providing a cold-rolled steel sheet including in weight %: 0.15%?C?0.23%, 1.4%?Mn?2.6%, 0.6%?Si?1.5%, 0.02%?Al?1.0%, with 1.0%?Si+Al?2.0%, 0?Nb?0.035%, 0?Mo?0.3%, 0?Cr?0.3%, and a remainder of Fe and unavoidable impurities, annealing the steel sheet at an annealing temperature between Ac1 and Ac3 to obtaining at least 40% austenite and at least 40% intercritical ferrite, quenching the sheet from at least 600° C. at a cooling rate of at least 20° C./s to a quenching temperature between 180° C. and 260° C., heating the sheet to a partitioning temperature between 375° C. and 470° C. and maintaining the sheet at this partitioning temperature for a partitioning time Pt between 25 s and 440 s, then cooling the sheet to room temperature. A steel sheet is also provided.

Abstract: A method for producing a steel sheet having a microstructure including 71% to 91% martensite and bainite, 9% to 13% retained austenite, and at most 20% ferrite is provided. The method includes providing a cold-rolled steel sheet including, in weight percent: 0.13%?C?0.22%, 1.2%?Si?2.3%, 0.02%?Al?1.0%, with 1.25%?Si+Al?2.35%, 2.4%?Mn?3%, Ti?0.05%, Nb?0.05% and a remainder of Fe and unavoidable impurities, annealing the steel sheet to obtain 80% to 100% austenite and 0% to 20% ferrite, quenching the steel sheet at a cooling rate between 20° C./s and 50° C./s to a quenching temperature between 240° C. and 310° C., heating the steel sheet to a partitioning temperature between 400° C. and 465° C. and maintaining the steel sheet at the partitioning temperature for 50 to 250 seconds, then immediately cooling the sheet to room temperature. Steel sheets are also provided.

Abstract: A method for producing a steel sheet, the method containing the following successive steps: providing a cold-rolled steel sheet, the chemical composition of the steel containing in weight %: 0.15%?C?0.23%, 2.0%?Mn?2.8%, 1.0%?Si?2.1%, 0.02%?Al?1.0%, with 1.7%?Si+Al?2.1%, 0?Nb?0.035%, 0?Mo?0.3%, 0?Cr?0.4%, the remainder being Fe and unavoidable impurities, annealing the steel sheet at an annealing temperature TA so as to obtain a structure comprising at least 65% of austenite and up to 35% of intercritical ferrite, quenching the sheet at a cooling rate of at least 20° C./s from a temperature of at least 600° C. down to a quenching temperature QT comprised between Ms-170° C. and Ms-80° C., heating the sheet up to a partitioning temperature PT between 350° C. and 450° C.