| abstract |
Process for manufacturing a piece of steel, comprising the following successive stages: - casting a steel to obtain a semi-product, said steel having a composition comprising, by weight: 0.10% <= C <= 0.35% 0 .8% <= Si <= 2.0% 1.8% <= Mn <= 2.5% P <= 0.1% 0% <= S <= 0.4% 0% <= Al <= 1.0% N <= 0.015% 0% <= Mo <= 0.4% 0.02% <= Nb <= 0.08% 0.02% <= Ti <= 0.05% 0.001% <= B <= 0.005% 0.5% <= Cr <= 1.8% 0% <= V <= 0.5% 0% <= Ni <= 0.5% with the balance being Fe and unavoidable impurities resulting from the casting, - hot-rolling the semi-finished product at an initial hot-rolling temperature higher than 1000 °C and cooling the product thus obtained by air-cooling to room temperature to obtain a hot-rolled steel part, the cooling rate being in the core of the hot rolled product during air cooling from a final hot rolling temperature to ambient temperature temperature greater than or equal to 0.2 °C/s, said hot-rolled steel part having, after air-cooling at room temperature, a microstructure consisting, in surface fraction, of 70% to 90% bainite, 5% to 25% M/A compounds and a maximum of 25% martensite, with bainite and M/A compounds containing retained austenite such that the total content of retained austenite in the steel is between 5 % and 25%, and the carbon content of the retained austenite being between 0.8% and 1.5% by weight. |