Patent ID: 8114227

Claim:
A process for manufacturing a part made of steel having a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each region of said part, which process comprises: cutting a blank from a strip of steel, the composition of which comprises, in % by weight: 0.05≦C≦0.50% 0.50≦Mn≦3.0% 0.001≦Si≦3.0% 0.005≦Al≦3.0% Mo≦1.0% Cr≦1.50% Ni≦2.0% Cu≦2.0% P≦0.10% S≦0.05% Ti≦0.20% V≦1.0%, the balance of the composition being iron and impurities resulting from smelting; the microstructure of the steel, after the part has been cooled, is a TRIP multiphase microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite; optionally, said blank undergoes prior cold deformation; said blank is heated so as to reach a soak temperature T s above Ac1 but below Ac3 and held at this soak temperature T s for a soak time t s adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% by area; said heated blank is transferred into a forming tool so as to hot-form said part; and the part is cooled within the tool at a cooling rate V such that the microstructure of the steel, after the part has been cooled, is a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each region of said part.