Very high strength martensitic steel or part and method of fabrication

A method for the fabrication of steel sheet with a completely martensitic structure with an average lath size of less than 1 micrometer, an average elongation factor of the laths being between 2 and 5, an elongation factor of a lath having a maximum dimension lmax and a minimum dimension lmin being defined by

• l
• ⁢
• ⁢
• max
• l
• ⁢
• ⁢
• min
• ,
• the steel sheeting having a yield stress greater than 1300 MPa, a mechanical strength greater than (3220)(C)+958 megapascals, (C) designating a carbon content of the steel in percent by weight, the method comprising the steps of:
• providing a semi-finished steel product, a composition of the semi-finished steel product including, whereby the contents are expressed by weight,
• 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%,
• 2.7%≤0.5 (Mn)+(Cr)+3(Mo)≤5.7%,
• S≤0.05%, and
• P≤0.1%,
• a remainder of the composition including iron and the inevitable impurities resulting from processing;
• heating the semi-finished product to a temperature T1 between 1050° C. and 1250° C.;
• subjecting the heated semi-finished product to a roughing rolling at a temperature T2 between 1000 and 880° C., with a cumulative rate of reduction εa greater than 30% to obtain a sheet with a completely recrystallized austenitic grain structure with an average grain size less than 40 micrometers, the cumulative rate of reduction εa being defined by:
• Ln
• ⁢
• e
• ia
• e
• f
• a
• ,
• .
• where eia designates a thickness of the semi-finished product before hot roughing rolling and efa designates a thickness of the sheet after the roughing rolling;
• partially cooling the sheet to a temperature T3 between 600° C. and 400° C. in the metastable austenitic range at a rate VR1 which is greater than 2° C./s;
• subjecting the partially cooled sheet to a hot finish rolling at the temperature T3, with a cumulative rate of reduction εb greater than 30% to obtain a sheet, whereby the cumulative rate of reduction εb is defined by:
• Ln
• ⁢
• e
• ib
• e
• f
• b
• ,
• .
• where eib designates a thickness of the semi-finished product before hot finish rolling and efa a thickness of the sheet after the hot finish rolling;
• and
• cooling the sheet at a rate VR2 which is greater than the critical martensitic quenching rate.