Microstructural evolution and mechanical properties of a low-carbon V-N-Ti steel processed with varied isothermal temperatures

The influence of isothermal temperature on the microstructure and mechanical properties of a low-carbon V-N-Ti steel was investigated. The process, containing heating at 1200 degrees C, cooling at 50 degrees C/s to different isothermal temperatures (600 degrees C, 550 degrees C, 500 degrees C, and 450 degrees C), followed by air cooling, was performed on a Gleeble 3800 simulator. The correlation of microstructure and mechanical properties of the isothermal samples was studied. A mixed microstructure of granular bainitic ferrite (GBF), polygonal or acicular ferrite (PF or AF), and martensite/austenite (M/A) constituent formed in each sample. AF transformation occurred preferentially during the 50 degrees C/s cooling stage due to the heterogeneous nucleation, followed by the GBF transformation. The fraction of AF and the degree of supercooling increased with the decreasing isothermal temperature, leading to the decrease in the mean equivalent diameter (MED). The microstructural refinement can enhance the yield strength and the MED with the misorientation tolerance angles ranging from 2 degrees to 6 degrees was effective in governing the yield strength. Moreover, the area fraction and size of the M/A constituent decreased because of the insufficient diffusion of carbon during the isothermal stage. Accordingly, the yield ratio increased and the impact toughness was improved with the decreasing isothermal temperature.