Grain refinement of Nb microalloyed steel during continuous casting under different thermal profiles

Coarse austenitic grains contribute to the heightened crack sensitivity in microalloyed steel. To better control austenite grain growth, this study focuses on Nb microalloyed steel and aims to optimise the grain growth process of continuous casting slabs through Gleeble-controlled thermal history experiments. The research investigates the influences of initial temperature, cooling rate during the primary phase change, and reheating temperature during the secondary phase change on grain refinement. Scanning electron microscopy and metallographic microscopy are employed for analysis. The results indicate that the optimal initial temperature for phase transformation is 1000 °C. As the cooling rate increases, the grain size initially decreases and then increases. The most effective cooling rate for phase transformation is identified as 5°C/s, which leads to the highest γ/α transition rate and carbon nitrides are found to be uniformly distributed within the grains. When the temperature of the secondary phase transformation reaches 1000 °C, the average austenite grain size is refined to 334.4 µm, and no significant mixed grain formations are observed.