Patent ID: 7540928

Claim:
A process for manufacturing bake hardening steel sheet comprising: the smelting of a steel, the composition of which comprises, expressed in % by weight: 0.03≦C≦0.06 0.50≦Mn≦1.10 0.08≦Si≦0.20 0.015≦Al≦0.070 N≦0.007 Ni≦0.040 Cu≦0.040 P≦0.035 S≦0.015 Mo≦0.010 Ti≦0.005 it being understood that the steel also contains boron in an amount such that: 0.64 ≤ B N ≤ 1.60 the balance of the composition consisting of iron and impurities resulting from the smelting; the casting of a slab of this steel, this slab then being hot rolled in order to obtain a sheet, the end-of-rolling temperature being above that of the Ar3 point; the coiling of said sheet at a temperature of between 500 and 700° C.; then the cold rolling of said sheet with a reduction ratio of 50 to 80%; a continuous annealing heat treatment which is carried out for a time of less than 15 minutes; and a skin pass which is carried out with a reduction ratio of between 1.2 and 2.5%, wherein the continuous annealing heat treatment comprises a reheat of the steel until it reaches a temperature of between 750 and 850° C., isothermal soak followed by a first cooling operation comprising a stow first part carried out at a rate of less than 10° C./s, followed by a rapid second cooling operation carried out at a rate of between 20 and 50° C./s.