| 摘要 |
A method for producing a high strength steel sheet having a yield strength YS of at least 850 MPa, a tensile strength TS of at least 1180 MPa, a total elongation of at least 14% and a hole expansion ratio HER of at least 30%. The chemical composition of the steel contains: 0.15% ≤ C ≤ 0.25%, 1.2% ≤ Si ≤ 1.8%, 2% ≤ Mn ≤ 2.4%, 0.1 % ≤ Cr ≤ 0.25%, Nb ≤ 0.05 %, Ti ≤ 0.05 %, Al < 0.50%, the remainder being Fe and unavoidable impurities. The sheet is annealed at an annealing temperature TA higher than 850 °C but less than 1000°C for more than 30 s, by cooling it to a quenching temperature QT between 275°C and 325°C, at a cooling speed suffioint 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, heated to a partitioning temperature PT between 420 °C and 470 °C and maintained at this temperature for time between 50 s and 150 s and cooled to the room temperature. Obtained steel sheet. |
