Influence of non-uniform grain size distribution on mechanical properties of metal sheet

Abstract

however, certain grains may grow larger because of factors such as chemical composition, manufacturing processes, and heat treatment conditions, thereby resulting in a bimodal distribution of grain sizes. Previous studies on the bimodal distribution of grain sizes primarily focused on nanocrystalline, ultrafine-grained, or multi-phase materials, wherein the presence of coarse grains enhanced strain hardening ability, uniform elongation, and drawing formability. In contrast, our observations in practical industrial materials, which contain a mixture of fine and coarse grains, revealed a decrease in the strain hardening exponent and uniform elongation. In addition, unlike in previous research, coarse grains were found to be incapable of accommodating deformations. Therefore, the microstructural non-uniformity of grain size leads to localised deformation and a decline in stretch formability. In this study, we developed a chemical composition model for ultra-low carbon steel containing a small amount of niobium precipitates using Thermo-Calc software and determined appropriate heat treatment temperatures. Materials with a bimodal distribution of grain sizes were artificially obtained by dissolving precipitates after hot and cold rolling. The mechanical properties of these manufactured materials were evaluated using uniaxial tensile tests, limit dome height, and limit drawing ratio. We analysed microstructural changes before and after plastic deformation using scanning electron microscopy equipped with electron backscatter diffraction to gain insights into the microstructure, texture, and deformation distribution of these materials. We conducted tensile tests at various strain rates (0.002, 0.02, 0.2, and 2.0$^{-1}$) and derived the material constants for the crystal plasticity finite element method (CPFEM) analysis from these results. Subsequently, we created a virtual microstructure with varying sizes, fractions, and orientations of coarse grains and performed CPFEM analysis to evaluate the mechanical properties such as strain hardening exponent and uniform elongation. Our study confirms microstructures with grain sizes ranging from a few to tens of micrometres experience localised deformation influenced by the crystallographic characteristics of non-uniform microstructures and the presence of coarse grains.

The accumulation of dislocations in a cold-rolled sheet acts as the primary driving force for recovery and recrystallization during heat treatment. During continued heat treatment, grains tend to grow uniformly and continuously