Effect of casting speed rising on the cleanliness of IF steel slab during the initial casting stage

During the initial casting stage of the IF steel, the cleanliness of the first slab deteriorates. Steel plants treat the initial casting slabs as waste or demoting products. Moreover, the cleanliness deterioration of initial casting slabs leads to unstable quality and low product yield. In the present study, to investigate the cleanliness of continuous casting slabs and the optimization measures during the initial casting stage of the IF steel, the cleanliness of the initial casting slabs is evaluated by field sampling and experimental analysis. Moreover, the effect of the casting speed rising method on the cleanliness of the IF steel slab during the initial casting stage is investigated. Additionally, numerical simulations are conducted to reveal the level of fluctuations of the molten steel in the mold during the initial casting stage. The results show that the variations in TO, N, and the content of microinclusions in the slab decrease obviously along the casting direction. The contents of TO and N in the slab reach the normal slab level of 6 m (approximately 13×10⁻⁶) and 7 m (approximately 19×10⁻⁶) away from the dummy bar head, respectively. Nevertheless, the effects of the casting speed rising method on the contents of TO, N, and microinclusions in the initial casting slabs are inapparent during the open casting process. For the three conditions of open casting (different casting speed rising methods), the contents of the macroinclusions in the slab reach the normal slab level of around 2 m (approximately 2 mg/10 kg) away from the dummy bar head, respectively. Subsequently, the fluctuation of the macroinclusion content can be determined. Furthermore, under the condition of “quick-slow” casting speed rising, the fluctuation can be quickly alleviated, and the macroinclusion content normalizes. Numerical simulations reveal that the variations in the fluctuation level of the molten steel at different positions (near the nozzle, a quarter of the width, and near the narrow face) in the mold during the initial casting stage of the three casting speed rising modes are similar. First, the fluctuation level is extremely intense. Then, the fluctuation level gradually weakens for some time. Finally, the fluctuation level of the molten steel at different positions in the mold stabilizes. Additionally, the numerical simulation results reveal that the slab cleanliness can quickly normalize (430 s after initial casting and about 5 m away from the dummy bar head) under the “quick-slow” casting speed rising mode during the initial casting stage.