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 <111> orientation and reduced that of the <110> 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.