Effect of Initial Microstructure on Austenite Grain Size of Niobium Microalloyed Steels

To study the effect of different initial microstructures on the austenite grains of Nb containing trace element alloy steels, three experimental steels with the same chemical composition and different initial microstructures were obtained through heat treatment. After furnace cooling, air cooling and water quenching, the microstructure of the hot rolled experimental steel is ferrite and a large amount of pearlite, ferrite and pearlite, martensite and bainite, respectively. The research results indicate that under the heating rate of 0.5°C/s, the austenite grain size in the furnace cooled microstructure is the lowest (115 ± 8 μm). The austenite grains in the water quenched microstructure have significantly grown (146 ± 12 μm). Based on the expansion curve and the Johnson–Mehl–Avrami–Kolmogorov equation, determine the kinetic parameters k and n of austenite transformation under non isothermal conditions. The kinetic parameter k increases with the increase of heating rate, while the value of n remains almost unchanged. Based on the Kissinger formula, the activation energy of austenite transformation was calculated. The activation energy of austenite transformation in furnace cooled microstructure is the highest (520.04 kJ/mol), while the activation energy of austenite transformation in water quenched microstructure is the lowest (454.28 kJ/mol). The initial microstructure plays an important role in austenite phase transformation.