| 摘要 |
The present invention provides a method for the fabrication of a steel sheet with a completely martensitic structure which has an average lath size of less than 1 micrometer and an average elongation factor of the laths is between 2 and 5. The elongation factor of a lath is defined as a maximum dimension divided by and a minimum dimension 1max. The steel sheet has a yield stress greater than 1300 MPa and a mechanical strength greater than (3220(C)+958) megapascals. A composition of a semi-finished steel product includes, expressed in percent by weight, is, 0.15%≦̸C≦̸0.40%, 1.5%≦̸Mn≦̸3%, 0.005%≦̸Si≦̸2%, 0.005%≦̸Al≦̸0.1%, 1.8%≦̸Cr≦̸4%, 0%≦̸Mo≦̸2%, whereby: 2.7%≦̸0.5 (Mn)+(Cr)+3(Mo)≦̸5.7%, S≦̸0.05%, P≦̸0.1%, optionally: 0%≦̸Nb≦̸0.050%, 0.01%≦̸Ti≦̸0.1%, 0.0005%≦̸B≦̸0.005%, 0.0005%≦̸Ca≦̸0.005%. The semi-finished product is reheated to a temperature T1 in the range between 1050° C. and 1250° C., then subjected to a roughing rolling at a temperature T2 in the range between 1000 and 880° C., with a cumulative rate of reduction &egr;a greater than 30%, to obtain a sheet with a completely recrystallized austenitic structure with an average grain size less than 40 micrometers and preferably less than 5 micrometers. The sheet is then partially cooled to prevent a transformation of the austenite at a rate VR1 greater than 2° C./s to a temperature T3 between 600° C. and 400° C. in the metastable austenitic range, and subjected to a finishing hot rolling at the temperature T3 of the partially cooled sheet, with a cumulative rate of reduction &egr;b greater than 30% to obtain a sheet that is then cooled at a rate VR2 which is greater than the critical martensitic quenching rate. |
