METHOD FOR PRODUCING HIGH-STRENGTH STEEL PARTS WITH IMPROVED DUCTILITY, AND PARTS OBTAINED BY SAID METHOD

The invention relates to a rolled steel sheet for press hardening, the chemical composition of which comprises, in weight percentages: 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%, the titanium and nitrogen contents satisfying: Ti/N > 3.42, and the carbon, manganese, chromium and silicon contents satisfying: 2.6C + + + = 1.1%, 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 unavoidable impurities resulting from the preparation, the sheet having a nickel Nisurf content at any point of the steel in the vicinity of the surface of said sheet over a depth ?, such that: Nisurf > Ninom, Ninom being the nominal nickel content of the steel, and such that, Nimax being the maximum nickel content in ?: (Nimax + Ninom) x (?) = 0.6, and such that: (Nimax - Ninom) = 0.01 2 ?, and the surface density of all of the particles Di and the surface density of the particles larger than 2 micrometres D(>2µm) satisfy, at least over a depth of 100 micrometres in the vicinity of the surface of said sheet: Di + 6,75 D(>2µm) < 270, Di and D(>2µm) being expressed as a number of particles per square millimetre, and said particles being all of the oxides, sulfides, and nitrides, either pure or mixed such as oxysulfides and carbonitrides, present in the steel matrix. L'invention concerne une tôle d'acier laminée, durcissement sous presse, dont la composition chimique comprend, les teneurs étant exprimées en poids : 0,24%