Abstract

The reason why variant selection phenomena occur in ausforming treatments is still not known. For that reason, in this work, the effect of compressive deformation on the macro and micro-texture of a bainitic microstructure was analyzed in a medium-carbon high-silicon steel subjected to ausforming treatments, where deformation was applied at 520 °C, 400 °C and 300 °C. The as-received material presented a very weak leftlangle {3, 3, 1} rightrangle fiber texture along the rod axis, due to prior thermomechanical processing. For the samples isothermally heat-treated, it was detected that the bainitic ferrite inherited a leftlangle {1, 0, 0} rightrangle fiber texture from the leftlangle {1, 1, 0} rightrangle fiber texture present in the prior austenite. The intensity of this transformation texture was more pronounced as the deformation temperature decreased. Also, variant selection was examined at different scales by combining Electron-Backscattered Diffraction and X-ray Diffraction. The quantification of the fraction of crystallographic variants under certain conventions for every condition revealed variant selection in samples subjected to ausforming treatments, where these phenomena were stronger as the deformation temperature was lower. Finally, some of the theories proposed so far to explain these variant selection phenomena were tested, showing that variants were not selected based on their Bain group and that their selection can be better described in terms of their belonging to packets, if these are defined according to a global reference frame. This suggests that the phenomena might have to do with the effect of deformation mechanisms on the prior austenite.

Highlights

  • IntroductionDEFORMING austenite before the start of the bainitic reaction, by the so-called ausforming treatments, has been shown to lead to many benefits, including the acceleration of the bainitic transformation and the refinement of the microstructure.[1,2,3,4] this processing route is associated with some microstructural changes that could be relevant to the properties of the material, such as the austenite mechanical stabilization,[5,6,7] the formation of phases during the deformation step[8] or the selection of crystallographic variants.[2,9,10,11,12,13,14]

  • The as-received a global texture was measured by X-ray diffraction (XRD) and its most important texture components were identified

  • As Tdef is lowered, the values of area percentage of each variant (APVi) corresponding to the variants in P2 and P4 decreased way below 4.2 pct, except for the values corresponding to V9 and V21, which remain close to 4.2 pct. These results show that the selection of variants can be better described in terms of their belonging to packets, if variants are defined according to a global reference frame, rather than by the activation of certain Bain groups

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Summary

Introduction

DEFORMING austenite before the start of the bainitic reaction, by the so-called ausforming treatments, has been shown to lead to many benefits, including the acceleration of the bainitic transformation and the refinement of the microstructure.[1,2,3,4] this processing route is associated with some microstructural changes that could be relevant to the properties of the material, such as the austenite mechanical stabilization,[5,6,7] the formation of phases during the deformation step[8] or the selection of crystallographic variants.[2,9,10,11,12,13,14]. Because bainitic transformations are displacive, the bainitic ferrite keeps a given orientation relationship (OR) with the prior austenite, characterized by the parallelism of planes and directions of the parent and the daughter phase. Variant selection phenomena occur when not all the expected ferrite crystallographic orientations appear

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