Effect of initial cooling temperature on multiphase microstructure, strength-toughness and yield ratio of V–Ti–N microalloyed weathering steel

For a novel V-Ti-N microalloyed weathering steels (WSs) with high heat input weldability, this study systematically investigated the influence of initial cooling temperatures (800°C, 780°C, 760°C, 740°C) in Thermo-Mechanical Controlled Process (TMCP) on the microstructural evolution and mechanical properties via a Gleeble-3800™. The experimental results revealed that all samples exhibited a multiphase microstructure comprising acicular ferrite (AF), bainitic ferrite (BF), polygonal ferrite (PF), and martensite/austenite (M/A) constituents. However, lowering the initial cooling temperature prolongs the high-temperature holding time between the final rolling temperature and the initial cooling temperature, thereby promoting the nucleation and growth of coarse PF. Meanwhile, it reduces the number of deformation shear bands and dislocation substructures formed during rolling, thus inhibiting the formation of AF. Crystallographic analysis reveals that the microstructure evolution-induced coarsening of grains defined by low-angle grain boundaries and the attenuated dislocation strengthening contribution constitute the primary mechanisms for strength degradation under decreasing initial cooling temperature. The reduction in high-angle grain boundary fraction, declined dislocation density and decreased proportion of low-Σ coincidence site lattice boundaries collectively impair crack propagation resistance by weakening crack deflection, crack-tip blunting and intergranular crack inhibition; meanwhile, the coarsening of M/A constituents aggravates local microstrain concentration and elevates crack initiation susceptibility, synergistically reducing the impact energy. However, the increased areal fraction of M/A constituents enhances work hardening capacity via strain partitioning, thereby lowering the yield-to-tensile ratio.