Ductility and plasticity of ferritic-pearlitic steel after severe plastic deformation

Abstract

A ferritic-pearlitic steel was subjected to severe plastic deformation (SPD) through Equal Channel Angular Pressing (ECAP) at room temperature, obtaining strengths greater than 1 GPa. Steel constituents were identified by light microscopy displaying similar grain sizes around 14 μm for the pearlite, and 13 μm and 18 μm for the ferrite before and after heat treatment, respectively. Texture changed from a rolling type to a simple shear texture with higher intensity after 4 ECAP passes. After different severe plastic deformation magnitudes, an ultra-fine grain structure was obtained with grain sizes between 0.9 μm-0.36 μm. The substantial grain size reduction was related to heterogeneous Geometrically Necessary Dislocations (GNDs) distribution in the as-received material with pearlite showing higher GNDs densities than ferrite. The remarkable strength increase after ECAP processing was found to be dependent on both small grain sizes and high dislocation densities. On the other hand, the low ductility of the ultrafine-grained (UFG) material was associated with a high annihilation rate of mobile dislocations at deformations greater than 0.39. Additionally, excellent plasticity properties were associated with a high density of immobile dislocations as well as high GND densities inside the deformed grains.