| 摘要 |
The invention relates to a method for the production of a steel sheet having a fully martensitic structure with an average lathe size of less than 1 micrometre, the average elongation factor of the lathes being between 2 and 5, wherein the elongation factor of a lathe of maximum dimension lmax and minimum dimension Imin is defined asImax / Imin, with a yield point greater than 1300 MPa, and mechanical strength greater than (3220(C)+958) megapascals, (C) denoting the carbon weight content of the steel. The method comprises the following steps consisting in: supplying a semi-finished steel product having a composition containing, expressed as 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%, wherein 2.7% = 0.5 (Mn)+(Cr)+3(Mo) = 5.7%, S = 0.05%, P = 0.1 % and, optionally, 0% = Nb = 0.050%, 0.01 % = Ti = 0.1 %, 0.0005% = B = 0.005%, 0.0005% = Ca = 0.005%, the remainder of the composition being formed by iron and the inevitable impurities resulting from production; heating the semi-finished product to a temperature T1 between 1050°C and 1250°C and, subsequently, subjecting the heated semi-finished product to rough rolling at a temperature T2 between 1000 and 880°C, with a cumulative reduction rate ea greater than 30%, such as to obtain a sheet having an austenitic structure that is totally recrystallised, with an average grain size of less than 40 micrometres and preferably less than 5 micrometres; and partially cooling the sheet, such as to prevent the 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, subsequently, subjecting the not completely cooled sheet to final hot rolling at temperature T3, with a cumulative reduction rate eb greater than 30%, such as to obtain a sheet that is cooled at a rate VR2 above the critical cooling rate. |
