Relationship between microstructure, texture and ridging in ferritic stainless steels

• Main Author: Lefebvre, Guillaume
• Language: English
• Published: University of British Columbia 2014
• Online Access: http://hdl.handle.net/2429/46161

The influence of cold rolling and final annealing on the development of ridging during tensile deformation of an industrial AISI 445 ferritic stainless steel has been investigated. The relationship between microstructure, (micro)-texture and ridging was evaluated by comparing full-field crystal plasticity calculations (VPFFT) to experimental measurements of surface roughness and microstructure. Results showed that the major parameter responsible for ridging is the through-thickness fraction of orientations with high out-of-plane shear strain rate. This was found to dictate the amplitude of the surface displacement and the spacing between corrugations observed on the sheet surface. To examine the origins of these regions, the process of annealing from the cold rolled state to the final product was next characterized by means of electron back-scattered diffraction (EBSD), with a focus on the formation of regions with similar shearing behaviour as defined by the crystal plasticity calculations. The combined effect of preferential nucleation and growth advantage of {111}<112> orientations from deformed {111}<112> grains is able to explain the bulk texture change from the deformed to the recrystallized state. On a microscopic scale, these orientations (within +/- 15° of the ideal orientation) have low out-of-plane shearing intensity of both negative and positive sign. Their nucleation and growth during annealing leads to the replacement of large grains with high out-of-plane shearing intensity with finer grains having lower out-of-plane shearing tendency. As a consequence, both the intensity and the spatial distribution of orientations with various out-of-plane shearing tendency are modified, leading to a reduction of ridging. As the final processing stage of the steel sheet (cold rolling and final annealing) reduces the non-random distribution of grains having a particular out-of-plane shearing tendency (“clustering”) and because ridging is still present in the final product, it was concluded that ridging originates in the upstream process. A generalized description of the origins of ridging and the heredity of microstructure and texture from the slab to the final product is proposed based on simplified crystal plasticity calculations and microstructure observations of the casting and the transfer bar.