Microstructual Evolution of a Nb-Microalloyed Advanced High Strength Steel Treated by Quenching-Partitioning-Tempering Process

The recently developed "quenching and partitioning'' heat treatment and "quenching-partitioning-tempering'' heat treatment are novel processing technologies, which are designed for achieving advanced high strength steels (AHSS) with combination of high strength and adequate ductility. Containing adequate amount of austenite phase is an important characteristic of the above steel, and the partitioning treatment is a key step in Q&P or Q-P-T process during which the austenite phase is enriched with carbon and achieves thermal stability. However, the microstructural evolution of the steel during the partitioning process is rather complicated. In present study, evolution of complex microstructure in a low carbon steel containing Nb during the Q-P-T process has been studied in detail. The microstructural evolution of the steel was investigated in terms of X-ray diffraction, scanning electron microscope and transmission electron microscope. The experimental results show that the Nb-microalloyed steel demonstrates a complex multiphase microstructure which consists of lath martensite with high dislocation density, retained austenite, alloy carbide, transition carbide, and a few twin martensite after the Q-P-T process. The experimental results can be helpful for the design of Q-P-T heat treatment and for the control of mechanical properties of Q-P-T steel.