| 摘要 |
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 and maintained at this temperature for a time Pt between 50 s and 150 s, then cooled to the room temperature |
