Ultrafine Grained Austenite in a Low Carbon Vanadium Microalloyed Steel

Ultrafine austenite grains with average size of 2 μm were successfully obtained by combining thermo-mechanical control process followed by reheating in a vanadium microalloyed steel. The mixed microstructure transformed from pancaked austenite formed during controlled rolling has a higher density of high angle boundaries, compared to that transformed from equiaxial austenite. It contributes to increasing nucleation density of austenite grain during the reheating process. A certain volume fraction of undissolved nano-sized (Ti, V)C particles, which are formed during the controlled rolling process and/or the reheating process, effectively inhibit austenite grain growth and consequently refine austenite grain size significantly. The critical grain size of austenite calculated by Gladman model agrees well with the experimental result.