Inclusion removal by bubble flotation in a continuous casting mold
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NONMETALLIC inclusions in molten steel can lead to serious defects in the final product, and the continuous casting process is the last chance to remove them. Gas injection is commonly applied to many secondary metallurgical processes such as ladle treatment, RH degassing, and submerged entry nozzles (SEN). Although it is well known that gas injection helps to remove inclusions, the mechanisms and removal rates have not been quantified. This work presents fundamental models to quantify the removal of inclusions by bubbles in molten steel and applies them to the continuous casting mold for typical conditions. The problem of modeling the multiple size and time scales involved in inclusion removal by bubbles is handled by separating the phenomena into models at two different scales. A small-scale model is used to quantify the attachment probability of individual inclusions to individual bubbles. The results are then used in a large-scale coupled model of turbulent fluid flow in the entire metallurgical vessel, including the transport of bubbles and inclusions. After briefly reviewing previous work on four relevant topics, the models and corresponding results are presented in three sections: fundamental inclusion-bubble interactions and attachment probabilities, bubble trajectories, and inclusion removal. II.
INCLUSION ATTACHMENT TO BUBBLES IN MOLTEN STEEL
A. Defects Gas injection processes in steel refining focus on achieving two conditions: fine bubbles and good mixing.[1–6] LIFENG ZHANG, Professor, is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology, Høgskoleringen 8, Alfred Getz vei 2, N-7491 Trondheim, Norway. Contact e-mail: [email protected] JUN AOKI, formerly a Visiting Scholar, with the University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801, U.S.A., is now Researcher with Nippon Steel, Japan. BRIAN G. THOMAS, Professor, is with the University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801, U.S.A. Manuscript submitted August 4, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
During steel secondary refining, finer bubbles provide a larger gas/liquid interfacial area and higher attachment probability of inclusions to bubbles.[3,6] Good mixing enhances the efficiency of the transfer of the alloy elements. Bubbles injected into the SEN and continuous casting mold affect steel quality in several ways: (1) Helping to reduce nozzle clogging (2) Influencing the flow pattern in the mold (3) Generating top-surface-level fluctuations and even slag emulsification if the gas flow rate is too large (4) Capturing inclusions moving within the molten steel, agglomerating them, and removing some of them into the top slag[3,7,8,9] (5) Entrapping bubbles and inclusion clusters into the solidified shell, eventually leading to line defects such as surface slivers, blisters, pencil pipes, or internal defects in the rolled product.[7,8,10,11,12] Aided by surface tension forces from nonwetting contact, most solid inclusions
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