Hot-rolled steel sheet and associated production method

A hot-rolled steel sheet with an elastic limit greater than 690 MPa and less than or equal to 840 MPa, with a strength between 780 MPa and 950 MPa, an elongation at failure greater than 10% and a hole expansion ratio (Ac) greater than or equal to 50%, the chemical composition of which contains, whereby the contents are expressed in percent by weight:

• 0.040%≦C≦0.065%,
• 1.4%≦Mn≦1.9%,
• 0.1%≦Si≦0.55%,
• 0.095%≦Ti≦0.145%,
• 0.025%≦Nb≦0.045%,
• 0.005%≦Al≦0.1%,
• 0.002%≦N≦0.007%,
• S≦0.004%, and
• P<0.020,
• the remainder consisting of iron and unavoidable impurities resulting from processing, the microstructure of which contains cementite in an area percentage of less than 1.5%, granular bainite, ferrite, and carbonitrides of titanium and niobium,
• whereby the density of titanium nitrides of average size greater than 6 μm is less than or equal to 3/mm2, and
• a ratio between the grain size DL measured parallel to the rolling direction and a grain size DN measured perpendicular to the rolling direction is less than or equal to 1.4.

A method for the fabrication of a hot rolled steel sheet with an elastic limit greater than 690 MPa and less than or equal to 840 MPa, a strength between 780 MPa and 950 MPa and an elongation at failure greater than 10%, comprising:

• obtaining a liquid steel in a liquid metal form, the composition of which contains, expressed in percent by weight:
• 0.040%≦C≦0.065%,
• 1.4%≦Mn≦1.9%,
• 0.1%≦Si≦0.55%,
• 0.095%≦Ti≦0.145%,
• 0.025%≦Nb≦0.045%,
• 0.005%≦Al≦0.1%,
• 0.002%≦N≦0.007%,
• S≦0.004%, and
• P<0.020%,
• the remainder consisting of iron and unavoidable impurities,
• subjecting the liquid steel to a vacuum treatment or a SiCa treatment, provided that if the liquid steel is subjected to the SiCa treatment the composition contains 0.0005%≦Ca≦0.005%, expressed in percent by weight, wherein the quantities of titanium [Ti] and nitrogen [N] dissolved in the liquid steel satisfy the relation %[Ti]%[N]≦6·10−4%2,
• casting the liquid steel to obtain a cast semi-finished product and optionally reheating said semi-finished product to a temperature between 1160° C. and 1300° C., then
• hot rolling said cast semi-finished product with a temperature at the end of rolling between 880° C. and 930° C., a reduction rate of the penultimate pass being less than 0.25, the rate of the final pass being less than 0.15, the sum of the two reduction rates being less than 0.37, the starting temperature at the beginning of the penultimate pass being less than 960° C., to obtain a hot-rolled product, then
• cooling said hot-rolled product at a rate of between 50 and 150° C./s to obtain a hot-rolled steel sheet, and
• coiling said hot-rolled sheet at a temperature between 470 and 625° C.