Method for production of martensitic steel having a very high yield point and sheet or part thus obtained

Abstract

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 Tin the range between 1050° C. and 1250° C., then the reheated semi-finished product is subjected to a roughing rolling at a temperature Tin the range between 1050 and 1150° C., with a cumulative rate of reduction ϵgreater 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 Vgreater than 2° C./s to a temperature Tin the range between 970° C. and Ar3+30° C., is then subjected to a finishing hot rolling at the temperature Tof the cooled sheet, with a cumulative rate off reduction ϵgreater than 50% to obtain a sheet, then the sheet is cooled at a rate Vwhich is greater than the critical martensitic quenching rate. Steel sheets are also provided.