Abstract
Air aspiration is an important cause of nozzle clogging and inclusions in final products of continuous casting of steel due to the presence of metal oxides (such as alumina) which occur through the reoxidation of molten steel. This problem is most likely to occur when the flow control system (slide-gate or stopper rod) causes the pressure inside the nozzle to drop below atmospheric pressure, drawing gas into the system through possible cracks or gaps in the refractory walls. In this work, a 1-D pressure-energy model of the complete metal delivery system from the tundish to the mold is developed to predict the pressure distribution and throughput under dynamic operating conditions and varying clogging conditions. The energy balance approach includes pressure losses in the slide-gate, wall friction, and nozzle geometry variations, including the effects of multiphase flow due to argon gas injection. The model also predicts air aspiration, oxide inclusion formation, and the time for clogging shutdown. The predicted pressure distribution is verified with a three-dimensional numerical simulation of multiphase turbulent flow, and is validated with plant measurements. Parametric studies with different submerged entry nozzle (SEN) designs revealed that a smaller SEN diameter may lessen negative pressure by redistributing the pressure loss from the slide-gate to the entire nozzle through increased friction losses. Under negative pressure, a submillimeter-thin gap was shown to cause considerable air aspiration. Clogging shutdown times were evaluated for several scenarios under static and dynamic operating conditions.
Highlights
Air aspiration during continuous casting is very detrimental, because it generates nonmetallic inclusions through reoxidation [1,2,3]
Air aspiration is caused by negative pressure developing near the narrow opening regulated by the flow control system [2,4]
The system throughput (Q = Vport A port ) depends on how much of the potential energy associated with the height difference between the liquid levels in the tundish and the mold level is converted into kinetic energy exiting the nozzle port outlets, relative to the pressure losses experienced during the journey
Summary
Air aspiration during continuous casting is very detrimental, because it generates nonmetallic inclusions through reoxidation [1,2,3]. Air aspiration is caused by negative pressure (i.e., lower than atmospheric pressure) developing near the narrow opening regulated by the flow control system (slide-gate or stopper rod system) [2,4]. The generated oxide particles either attach to the nozzle walls to cause clogging or are carried into the molten steel pool in the mold, where they may become entrapped in the solidifying steel shell as nonmetallic inclusions in the final product. Oxide particles formed due to air aspiration can lead to nozzle clogging, which is detrimental in many ways. Argon injection may lead to increased pressure inside the nozzle by causing the slide-gate to open further for a given molten steel throughput [3]. A better understanding of nozzle pressure distribution is needed to find and evaluate solutions to this important problem
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