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Étude des mécanismes de germination et de croissance d'oxydes sélectifs sur un acier ferritique

Abstract : Continuous galvanizing process involving immersion in a molten zinc bath is commonly used to form zinc coatings on steel sheets. Before hot-dip galvanizing, the steel sheets are annealed at a temperature of 800 °C in a N2 atmosphere containing 5 vol.% of H2, with only traces of water. This heat treatment is used to recrystallize the steel substrate and to reduce the iron oxides in order to improve the wettability by liquid zinc. At the same time, the less-noble alloying elements of the steel preferentially oxidize and di_use towards the surface. The aim of this study is a better understanding of selective oxidation of ferritic steels. Nucleation is observed at a temperature of 700 °C. The measurements of the geometrical characteristics of the precipitates show that their diameter increases and the number of precipitates per unit surface area decreases during annealing. The oxide particles are found to be composed of MnO, SiO2, Mn2SiO4, MnAl2O4 and B2O3. External oxidation is also dependent on the crystallographic orientation of the substrate. Nucleation on {111} faces takes place earlier than nucleation on {100} faces. Surface energies can explain this observation. There is a difference in geometrical characteristics of the external particles on grains of main planes of body-centered cubic iron substrate. Growth on {111} and {211} orientation planes is ahead (surface density is lower and equivalent diameter is higher) of growth on {110} grains. The smallest number of particles is found on {100} grains. Diffusional information are calculated from obtained depth concentration profiles by means of simple models. Calculated global diffusion coefficients take into account diffusion in grain boundaries and in dislocations. Cold rolling steels contain lots of these easy paths of diffusion.
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Amélie Ollivier. Étude des mécanismes de germination et de croissance d'oxydes sélectifs sur un acier ferritique. Autre. Ecole Centrale Paris, 2009. Français. ⟨NNT : 2009ECAP0009⟩. ⟨tel-00453245⟩



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