Method for the production of very high strength martensitic steel and sheet or part thus obtained

Abstract

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 1divided by and a minimum dimension 1. 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 Tin the range between 1050° C. and 1250° C., then subjected to a roughing rolling at a temperature Tin the range between 1000 and 880° C., with a cumulative rate of reduction εgreater 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 Vgreater than 2° C./s to a temperature Tbetween 600° C. and 400° C. in the metastable austenitic range, and subjected to a finishing hot rolling at the temperature Tof the partially cooled sheet, with a cumulative rate of reduction εgreater than 30% to obtain a sheet that is then cooled at a rate Vwhich is greater than the critical martensitic quenching rate.