Study on the hot ductility behavior and control technology during continuous casting of a low alloy high strength steel slab

Carbonitrides and austenitic grain size significantly influence the hot ductility and surface cracking of continuously cast steel. This study analyzes the precipitation and redissolution behaviors of carbonitrides, austenite grain growth behavior, and hot ductility curves at different grain sizes in AH36 steel slabs under various temperatures. It proposes a method to calculate the third brittle temperature zone, considering the actual coarse austenite grains in the surface layer of the slabs. The results indicate that carbonitrides precipitate as a composite phase within the temperature range of 1573–1173 K, in the sequence of TiN (at 1473 K) → Nb(C,N) (at 1373 K) → AlN (at 1173 K), with particle sizes increasing with extended holding times. Compared to 1373 K, when the temperature is raised to 1623 K, the austenite grain size significantly increases, and the hot ductility drops below 40%, resulting in a typical third brittle zone. By integrating the continuous cooling grain growth model with the reduction of area estimation model, a hot ductility prediction model for conditions involving coarse austenite grain size has been developed. This model was employed to calculate the potential upper limit temperature of the brittle zone for AH36 steel under continuous casting conditions, and this temperature was used to optimize the continuous casting cooling regime.