Crystal plasticity investigation of ridging mechanisms in ferritic stainless steels

Abstract

The surface roughening and ridging behavior of ferritic stainless steel sheets with different roughness properties were studied in this work. First, two-dimensional experimental and numerical investigations were conducted on three material samples. The roughnesses of the deformed materials under uniaxial tension were measured and predicted using a rate-dependent crystal plasticity finite element method (CPFEM). The initial textures of the three materials, obtained by EBSD in the rolling and normal planes, were used as input. Severe ridging undulations were well predicted using the texture information from the RD plane in which the entire through-thickness grain orientations are considered. An out-of-plane shear strain in the material was shown to promote ridging. The abrupt change in the shear strain along the transverse direction created severe ridges or valleys. The shear strain distribution in the RD plane appeared to be an indication of the ridging mechanism. Additionally, the ridging properties under various deformation modes, such as plane strain and biaxial strain conditions, were simulated. Second, three-dimensional experimental investigations of the ridging in other ferritic stainless steel sheet samples, using serial sectioning, were conducted to enhance the accuracy of the CPFEM simulations. The measured roughness of uniaxially elongated specimens (up to 15%) in the rolling direction (RD) was compared with the prediction using a 3-D rate-dependent crystal plasticity FE model. The initial textures of the two ferritic stainless steel samples on five equi-spaced sequential RD planes were obtained using EBSD measurements. The initial textures were utilized as input parameters for the crystal plasticity model. Simulations were conducted either considering each individual layer separately or simultaneously. In the latter case, the five texture layers were combined into a three-dimensional structure, which was mapped onto the FE mesh. The ridging profiles predicted by the CPFEM using both individual single layer textures and the multilayer texture were compared to the experimental results. The ridging profile predicted using the five-layer EBSD mapping of a material exhibiting weak was in good agreement with the experimental result. Conversely, the prediction using only a single texture layer was able to estimate the ridging in a material that exhibited severe ridging due to the elongated grain clusters with analogous orientations in the RD. Finally, the effect of the γ-fibers on ridging in ferritic stainless steels was studied. A methodology based on a shear deformation mechanism was used to account for the ridging phenomenon. A viscoplastic crystal plasticity finite element modeling (CPFEM) was used to simulate ridging under uniaxial tension up to 15% strain. The CPFEM using the textures measured on the RD plane reasonably predicted the ridging magnitude compared with the experiments. Moreover, a shear strain map of the crystal orientations under RD tension was produced in full-scale Euler space using a simplified roping model (SRM). This map was used successfully to classify the crystal orientations in terms of the sign of the shear strain in 2D Euler space. The evaluation of the γ-fiber intensity and resultant shear strain under uniaxial tension was conducted, and it was deduced that specific orientations ({111}<341>) in the γ-fibers primarily induced strong shear deformations. Furthermore, stochastic analysis of the {111}<341> texture components with respect to the sign of the shear strain provided good agreement with the calculated ridging profiles.