PROCESS FOR MANUFACTURING STEEL SHEETS FOR PRESS HARDENING, AND PARTS OBTAINED BY MEANS OF THIS PROCESS

The invention relates to a rolled steel sheet, for press hardening, the chemical composition of which comprises, the contents being expressed by weight: 0.24% = C = 0.38%, 0.40% = Mn = 3%, 0.10% = Si = 0.70%, 0.015% = Al = 0.070%, 0% = Cr = 2%, 0.25% = Ni = 2%, 0.015% = Ti = 0.10%, 0 % = Nb = 0.060%, 0.0005% = B = 0.0040%, 0.003% = N = 0.010%, 0.0001% = S = 0.005%, 0.0001% = P = 0.025%, it being understood that the titanium and nitrogen contents satisfy: Ti/N > 3.42, and that the carbon, manganese, chromium and silicon contents satisfy: formula (I), the chemical composition optionally comprising one or more of the following elements: 0.05% = Mo = 0.65%, 0,001% = W = 0.30%, 0.0005 % = Ca = 0.005%, the remainder consisting of iron and of unavoidable impurities originating from the production, the sheet containing a nickel content Nisurf at any point of the steel in the region of the surface of said sheet over a depth ?, such that: Nisurf > Ninom, Ninom denoting the nominal nickel content of the steel, and such that, Nimax denoting the maximum nickel content within ?: formula (II) and such that: formula (III) the depth ? being expressed in micrometres, the Nimax and Ninom contents being expressed as weight percentages. L'invention concerne une tôle d'acier laminée, pour durcissement sous presse, dont la composition chimique comprend, les teneurs étant exprimées en poids : 0,24% = C = 0,38%, 0,40% = Mn = 3%, 0,10% = Si = 0,70%, 0,015% = Al = 0,070%, 0% = Cr = 2%, 0,25% = Ni = 2%, 0,015% = Ti = 0,10%, 0 % = Nb = 0,060%, 0,0005% = B = 0,0040%, 0,003% = N = 0,010%, 0,0001 % = S = 0,005%, 0,0001 % = P = 0,025%, étant entendu que les teneurs en titane et en azote satisfont à :Ti/N >3,42, et que les teneurs en carbone, manganèse, chrome et silicium satisfont à : formule (I), la composition chimique comprenant optionnellement un ou plusieurs des éléments suivants: 0,05% = Mo = 0,65%, 0,001 % = W = 0,30%%, 0,0005 % = Ca = 0,005%, le reste étant constitué de fer et d'impuretés inévitables provenant de l'élaboration, la tôle contenant une teneur en nickel Nisurf en tout point de l'acier au voisinage de la surface de ladite tôle sur une profondeur ?, telle que :Nisurf > Ninom, Ninom désignant la teneur nominale en nickel de l'acier, et telle que, Nimax désignant la teneur maximale en nickel au sein de ? : formule (II) et telle que : formule (III) profondeur ? étant exprimée en micromètres, les teneurs Nimax et Ninom étant exprimées en pourcentages en poids.