Evolution of ultra-fine austenite grains in Nb and Nb-V-Ti microalloyed low carbon steel under cyclic heating-quenching treatment

Effects of initial microstructure (warm-rolled ferrite/pearlite, warm-rolled tempered martensite and ferrite/bainite) and cycle times of repetitive heating-quenching treatment on the evolution of ultra-fine austenite grains in Nb and Nb-V-Ti microalloyed steel were investigated. The results indicate that much more significant refinement of austenite grains can be expected in Nb-V-Ti microalloyed steel by cyclic heating-quenching treatment, and the initial microstructure of warm-rolled ferrite/pearlite is favourable for the refining of austenite grains, and from which austenite grains in size of 1~2μm can be obtained for the steel with the repetitive treatment for 3~4 times. Otherwise, less times of repetitive treatment is required to reach the minimum value of austenite grain size for the Nb-steel compared with the Nb-V-Ti microalloyed steel. Effects of the microalloying on ultra-refinement of austenite grains greatly depend on the amount of dissolved vanadium and niobium, according to TEM observation and EDS analysis of second-phase particles in Nb-V-Ti steel.