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

• Main Authors: AOUAFI, Anis, LUCAS, Emmanuel, COBO, Sebastian, BEAUVAIS, Martin, ALLELY, christian
• Format: Patent
• Language: English; French
• Published: 06.12.2018
• Subjects: ALLOYS; CHEMICAL SURFACE TREATMENT; CHEMISTRY; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATIONOR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY IONIMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; COATING MATERIAL WITH METALLIC MATERIAL; COATING METALLIC MATERIAL; DIFFUSION TREATMENT OF METALLIC MATERIAL; FERROUS OR NON-FERROUS ALLOYS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUSMETALS OR ALLOYS; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION INGENERAL; MAKING METAL MALLEABLE BY DECARBURISATION, TEMPERING OR OTHERTREATMENTS; METALLURGY; METALLURGY OF IRON; MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THESURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; TREATMENT OF ALLOYS OR NON-FERROUS METALS

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 Δ, such that: Nisurf > Ninom, Ninom being the nominal nickel content of the steel, and such that, Nimax being the maximum nickel content in Δ: (Nimax + Ninom) x (Δ) ≥ 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, 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 Mn Cr Sicarbone, manganèse, chrome et silicium satisfont à : Formule 1, la 5.3 13 15composition 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 Δ, 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 Δ : (Formule (II), et telle que : Formule (III) et la densité surfacique de toutes les particules Di et la densité surfacique des particules de taille supérieure à 2 micromètres D(>2µm) satisfont, au moins sur une profondeur de 100 micromètres au voisinage de la surface de ladite tôle, à : Di + 6,75 D(>2µm) < 270 Di et D(>2µm) étant exprimées en nombre de particules par millimètres carrés, et lesdites particules désignant l'ensemble des oxydes, sulfures, nitrures, purs ou mixtes tels que les oxysulfures et carbonitrures, présents dans la matrice de l'acier.