| 摘要 |
The present invention provides a method for the fabrication of a martensitic steel sheet with a yield stress greater than 1300 MPa. The method includes the steps of obtaining a semi-finished steel product, the composition of which includes, whereby the contents are expressed in percent by weight: 0.15%≰C≰0.40%, 1.5%≰Mn≰3%, 0.005%≰Si≰2%, 0.005%≰Al≰0.1%, S≰0.05%, P≰0.1%, 0.025%≰Nb≰0.1%, and optionally: 0.01%≰Ti≰0.1%, 0%≰Cr≰4%, 0%≰Mo≰2%, 0.0005%≰B≰0.005%, 0.0005%≰Ca≰0.005%. The remainder of the composition is iron and the inevitable impurities resulting from processing. The semi-finished product is reheated to a temperature T1 in the range between 1050° C. and 1250° C., then the reheated semi-finished product is subjected to a roughing rolling at a temperature T2 in the range between 1050 and 1150° C., with a cumulative rate of reductionεa greater than 100%, to obtain a sheet with a not totally recrystallized austenitic structure with an average grain size less than 40 micrometers and preferably less than 5 micrometers. The sheet is then cooled to prevent a transformation of the austenite at a rate VR1 greater than 2° C./s to a temperature T3 in the range between 970° C. and Ar3+30° C., is then subjected to a finishing hot rolling at the temperature T3 of the cooled sheet, with a cumulative rate off reductionεb greater than 50% to obtain a sheet, then the sheet is cooled at a rate VR2 which is greater than the critical martensitic quenching rate. Steel sheets are also provided. |
