Optimization of Submerged Entry Nozzle Structure: Synergistic Control of Longitudinal Crack on the Surface of φ400 mm Ultrasupercritical Steel Round Bloom Continuous Casting

To prevent surface longitudinal crack formation during the continuous casting of ultrasupercritical P91 steel, a coupled flow–heat–stress multiphysics model was developed and validated for a φ400 mm round bloom mold. Four submerged entry nozzle (SEN) structures—single‐port, four‐port, five‐port, and swirling flow—were compared in terms of flow behavior, free surface temperature, and level fluctuation. An evaluation index, the shell uniformity evaluation index (SUEI), was proposed to enable a more intuitive prediction of crack susceptibility. The findings suggest that the swirling flow SEN exhibits the optimal overall performance. In comparison to the other three SEN types, it demonstrates notable advantages in terms of balancing free surface temperature, improving level fluctuation, ensuring uniform shell growth, and reducing thermal stress difference. The functionality of this nozzle is such that it can be deployed to address surface longitudinal crack issue in ultrasupercritical steel. Furthermore, experimental studies, including mold flux melting experiments and industrial trials, provide substantial support for the validation of simulation models. This study provides theoretical and practical guidance for optimizing SEN design to improve the surface quality of ultrasupercritical steels.