Effects of clogging, argon injection, and continuous casting conditions on flow and air aspiration in submerged entry no
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I. INTRODUCTION
NOZZLE clogging is one of the most disruptive phenomena in the operation of the tundish-mold system in continuous casting of steel and has received much study.[1,2,3] Nozzle clogs are suspected to adversely affect product quality in several ways. First, the clog may change the flow pattern in the mold, which is usually carefully designed based on the assumption of no clogging. Mold level variations and unstable flow in the mold are more severe with clogging.[4] Second, the internal quality of the final product is seriously compromised whenever chunks of a nozzle clog break off and enter the flow stream. Clogs trapped in the solidifying steel form inclusion defects that drastically lower strength and toughness.[1] Even if it is not entrapped in the solidified steel, a large clog can be detrimental if it suddenly floats into the slag layer. It may cause sudden level surges, which are well known to cause surface quality problems. The alumina added from a clog can also disrupt the local slag composition and increase slag viscosity, which can make slag infiltration at the meniscus more difficult, and, thereby, lead to surface defects, such as longitudinal cracks. Finally, as the buildup progresses, the slide-gate opening must be increased to maintain the desired flow rate. Once the slide gate reaches its maximum position, production must stop and the nozzle must be replaced. Thus, it is important to find and understand ways to both detect and prevent clogging. Argon injection into the nozzle is widely employed to HUA BAI, Senior Research Engineer, is with the Dow Chemical Company, Freeport, TX 77541. BRIAN G. THOMAS, Professor, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted September 12, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
reduce nozzle clogging, even though its working mechanisms are still not fully understood.[1] In addition, the injected argon bubbles affect the flow pattern in the nozzle, and subsequently in the mold. Some bubbles may attach with small inclusions and become entrapped in the solidifying shell, resulting in “pencil pipe” and blister defects on the surface of the final product.[5,6,7] Other possible disadvantages of argon injection observed in operation include increased quality defects and nozzle slag-line erosion due to the increased meniscus fluctuation,[8,9] exposure of the steel surface and subsequent reoxidation,[10] entrapment of the mold power,[11] and emulsification of the flux layer, leading to flux-gas foams, which are easily entrained as inclusional defects.[12] Large gas injection flow rates might create a boiling action in the mold,[13] which can greatly intensify those adverse effects. Thus, it is important to optimize argon injection to the minimum amount needed to achieve its benefits. Air aspiration through cracks, joints, or porous refractory into the nozzle leads to reoxidation, which is an important source of inclusions and a cause of clogging.[13,14]
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