Multiphase Flow-Related Defects in Continuous Casting of Steel Slabs

Abstract

Multiphase flow phenomena greatly affect the quality of continuous cast steel. Air aspiration through a nozzle, due to negative pressure distribution, can cause reoxidation and non-metallic inclusions, which may build up on the refractory walls as nozzle clogging. Asymmetric jet flow from a clogged nozzle causes excessive surface velocities and vortexing at the top surface in the mold, resulting in entrainment of the mold slag into the steel pool. In addition, instability at the interface between the molten steel and surface slag, caused by jet wobbling, results in sudden level drops and slag entrapment into the solidifying shell at the meniscus region. This surface defect formation becomes more severe with meniscus freezing and the accompanying formation of subsurface hooks. Furthermore, particles such as argon gas bubbles, alumina inclusions, and entrained slag droplets can be transported deep into the strand and captured into the steel shell, especially on the inside radius wall during curved strand casting. This causes internal defects. To quantify the above defect formation mechanisms relevant to multiphase flow phenomena, high-resolution multiphase flow models validated with plant measurements and/or laboratory-scale model experiments are required. Finally, these multiphase flow-related defects can be lessened with appropriate choice of nozzle geometry and all of the casting conditions which control the flow. The specific effects of nozzle port angle, nozzle submergence depth, casting speed, gas injection, and electromagnetic forces are discussed.