Improvement of Mechanical Strength of a Hot-Worked Twinning-Induced Plasticity Steel through an Optimum Secondary Annealing Treatment

Abstract

The microstructure of the twinning-induced plasticity (TWIP) steel microalloyed by Ti (TWIP-Ti) exhibited a wide grain size distribution after applying a thermomechanical treatment. The presence of fine and coarsest grains affected mechanical twinning and properties. This research was directed toward elucidating the effect of an optimum secondary annealing treatment to produce a homogenous microstructure through the release of remnant energy stored during the hot deformation processing of a TWIP-Ti steel. A comprehensive analysis of the effect of annealing parameters on the microstructure and its enhanced mechanical properties in terms of grain size distribution, precipitation strengthening and annealing twin fraction was performed. The microstructural characterization included light optical microscopy and scanning electron microscopy (SEM). SEM line scans were performed to characterize precipitate particles found in the TWIP-Ti steel. Microhardness and tensile tests were performed on the base material (as-solution condition) and post-annealing samples. The Taguchi method was applied to optimize the annealing parameters. Furthermore, image processing was employed to quantify the grain size distribution and annealing twin fraction. The analysis results revealed that upon completion of the second stage of annealing treatment with the optimal parameters, the grain size heterogeneity decreased due to the effect of coarsest grain subdivision. The low SFE of the TWIP-Ti steel, homogenous grain size microstructure and precipitation strengthening produced yield strength and ultimate tensile strength increases of 27% and 8.5%, respectively. The homogenous grain size distribution had a more substantial effect on improving the mechanical properties than the annealing twin fraction.