Strain-Gradient Crystal Plasticity for Multiscale Modelling of Through-Thickness Strength Gradients in TMCP Rolled X100 Steel

This study presents a multi-scale model for thermo-mechanical control process (TMCP) rolling of X100 steel sheets at 900 ˚C to quantify through-thickness gradients in microstructure and mechanical properties. A macroscale plane-strain rolling model was developed to characterise the evolution of through-thickness compressive and shear strains and strain rates based on plane-strain Gleeble stress-strain testing. A micro-scale strain-gradient crystal plasticity fast Fourier transform model was developed to quantify the relative contributions of compression and shear to through-thickness evolutions in microstructure (slip, grain orientation, dislocation density) and the associated tensile response. Rolling-induced compressive strain was shown to primarily contribute to the accumulation of crystallographic slip and dislocation density, the formation of low-angle grain boundaries, crystal-morphology evolution, and associated tensile strength enhancement, whereas rolling-induced shear strain was shown to primarily contribute to through-thickness heterogeneity of the corresponding material properties. The simulated increase in dislocation density after rolling had a more significant effect on yield strength but a less pronounced effect on ultimate tensile strength than variations in crystal texture. Both rolling-induced compressive and shear strains increased the intensity of the orientation and reduced that of the orientation in the rolling direction, primarily near the surface, thereby causing texture hardening. The work is important for understanding thickness-dependent behaviour (e.g., tensile and fatigue) of TMCP steels. • Developed a novel strain-gradient crystal plasticity framework to capture the through-thickness microstructural strengthening gradients in TMCP-rolled X100 steel at 900 ˚C. • quantified the contributions of strain components to variations in crystal texture, dislocation density, and grain morphologies. • quantified the strengthening contributions of crystal texture, dislocation density, and grain morphologies to tensile properties through thickness.