Process for manufacturing steel sheet

A process for manufacturing a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an Re/Rm ratio of less than 0.75 and an elongation at break of greater than 10%, the process comprising:

• supplying a steel composition comprising, the contents being expressed by weight:
• 0.10%≤C≤0.25%;
• 1%≤Mn≤3%;
• Al≥0.015%;
• Si≤1.985%;
• Mo≤0.30%;
• 0.3%≤Cr≤1.5%;
• S≤0.015%;
• P≤0.1%;
• Co≤1.5%;
• B≤0.005%;
• wherein 1%≤Si+Al≤2% and Cr+(3×Mo)≥0.3%, and
• a balance of the composition includes iron and inevitable impurities resulting from smelting;
• casting a semi-finished product from the steel composition;
• heating said semi-finished product to a temperature above 1150° C.;
• hot-rolling said semi-finished product in a temperature range in which the microstructure of the steel is entirely austenitic;
• then
• cooling the sheet thus obtained from a temperature TDR lying above Ar3 down to an intermediate temperature TI at a cooling rate VR1 of 70° C./s or higher, said temperature TI not exceeding 650° C.;
• then
• cooling said sheet from said temperature TI down to a temperature TFR, said temperature TFR being between B′s and Ms+50° C., B′s denoting a temperature defined relative to the bainite transformation start temperature Bs, and Ms denoting the martensite transformation start temperature, in such a way that the cooling rate between said temperature TDR and said temperature TFR is between 20 and 90° C./s;
• then
• cooling said sheet from the temperature TFR at a secondary cooling rate V′R between 0.08° C./min and 600° C./min down to the ambient temperature;
• said temperature B′s being equal to Bs when said rate V′R is between 0.08 and 2° C./min;
• and
• said temperature B′s being equal to Bs+60° C. when said rate V′R is greater than 2° C./min but does not exceed 600° C./min.

A process for manufacturing a hot-rolled steel sheet, the process comprising supplying a steel composition comprising, the contents being expressed by weight:

• 0.10%≤C≤0.25%;
• 1%≤Mn≤3%;
• Al≥0.015%;
• Si≤1.985%;
• Mo≤0.30%;
• 0.3%≤Cr≤1.5%;
• S≤0.015%;
• P≤0.1%;
• Co≤1.5%;
• B≤0.005%;
• wherein 1%≤Si+Al≤2% and Cr+(3×Mo)≥0.3%, and
• a balance of the composition includes iron and inevitable impurities resulting from smelting;
• casting a semi-finished product from the steel;
• heating said semi-finished product to a temperature above 1150° C.;
• hot-rolling said semi-finished product in a temperature range in which the structure of the steel is entirely austenitic;
• then
• the primary cooling start temperature TDR lying above Ar3, the primary cooling finish temperature TFR, the primary cooling rate VR between TDR and TFR and the secondary cooling rate V′R are adjusted in such a way that the microstructure of said steel consists of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.