Effect of Zr on the microstructure and properties of high-strength Ti-Mo microalloyed steel after quenching and tempering

Abstract

Investigation on the effects of Zr and quenching and tempering processes on the microstructure and properties of high-strength Ti-Mo microalloyed steel using SEM, TEM, mechanical property testing, and First-principles calculations. The results show that within the tempering temperature range of 550–650 °C, the microstructures of Ti-Mo microalloyed steel and Ti-Zr-Mo microalloyed steel consist of ultra-low carbon martensite and carbides. Specifically, the strength of Ti-Mo microalloyed steel initially increases and then decreases with the increase in tempering temperature, while the strength of Ti-Zr-Mo microalloyed steel increases with the increase in tempering temperature. During tempering, the addition of Zr element affects the diffusion of C element, delaying the martensitic tempering transformation process, and slowing down the nucleation, growth, and coarsening processes of alloy carbides. First-principles calculation results indicate that ZrC preferentially forms during tempering, resulting in a certain sequential gradient of precipitated microalloy carbides, thereby enhancing the tempering stability of microalloyed steel. Compared to Ti-Mo steel, Ti-Zr-Mo steel exhibits narrower lath martensite width and finer dispersed precipitation phases after tempering at 600 °C and 650 °C.