Étude de l’influence de l’état métallurgique de l’austénite sur la microstructure de transformation de phase dans les aciers bas carbone

Abstract : The thesis aims to understand the influence of the metallurgical state of austenite at high temperature on the phase transformation mechanisms in two model grades of low carbon steel: 0.06%C-2.4%Mn-0.3%Si (CMn-1) and 0.2%C-2.4%Mn-0.3%Si (CMn-2). A series of heat treatments enabled a thorough crystallographic analysis of the transformation products according to the [gamma] grain size. According to the numerous microstructures obtained, the advantages and limits of the crystallographic reconstruction of austenite from EBSD data are defined according the type of inherited microstructures. Finally, hot compression tests at 800°C (Ar3+100°C) are performed on the grade CMn-1 to follow the evolution of the austenitic microstructures and microtextures based on strain conditions. The characteristics obtained by crystallographic reconstruction are confirmed by an analysis of the 30%Fe-70%Ni hot-rolled model alloy. We identified high misorientations in the vicinity of grain boundaries and two characteristics deformation substructures: grains homogeneously deformed by planar slip mechanism and grains with deformation microbands. These structures depend on the components of texture developed in the austenite. It was found that the bainitic laths are promoted as the deformation increases to the detriment of the martensite formed by quenching, resulting in a decrease of the hardness. Moreover a granular structure also grows along prior [gamma] grains boundaries and spreads across the grain during isothermal bainite transformation at 525°C. Lamellar microstructures are aligned along the most active slip plans {111} [gamma] in homogeneously deformed grains, while the nucleation of small bainitic areas with different orientations is promoted in the microbands
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Élodie Boucard. Étude de l’influence de l’état métallurgique de l’austénite sur la microstructure de transformation de phase dans les aciers bas carbone. Autre. Université de Lorraine, 2014. Français. ⟨NNT : 2014LORR0245⟩. ⟨tel-01751262⟩

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