Investigation of Fluid Flow and Steel Cleanliness in the Continuous Casting Strand
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he continuous casting mold is important due to its effect on many phenomena related to steel quality, such as the transport of argon bubbles and inclusions, transient waves and fluctuations of the top surface, the transport of superheat, meniscus freezing, shell thinning from the jet impinging upon the solidifying shell, thermal stress, and crack formation. The entrainment LIFENG ZHANG, Professor, is with the Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), 7491 Trondheim, Norway. Contact e-mail: [email protected]. SUBO YANG, Chief Researcher, is with the Technical Research Center, Panzhihua Iron and Steel Company, Panzhihua 617023, Sichuan Province, People’s Republic of China. KAIKE CAI, Professor, and JIYING LI and XIAOGUANG WAN, Graduate students, are with the School of Metallurgy, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China. BRIAN G. THOMAS, Professor, is with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801, USA. Manuscript submitted June 14, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS B
of mold slag due to excessive surface velocities and level fluctuations is one of the most important causes of defects found in steel products.[1–9] The main flow-related phenomena that cause slag entrainment and surface quality problems are shown in Figure 1.[10] If the jet from the SEN outport strongly impinges on the narrow face and splits to flows upward along the narrow face, it will lift the level of the molten steel, changing its profile and also generating large level fluctuations near the meniscus. This also pushes slag away from the narrow face, leading to surface quality problems.[10–17] Direct jet impingement of the jet onto the steel-slag interface associated with a single-roll flow pattern, such as induced by excessive gas bubble injection,[18] is even more detrimental. Excessive velocity of the molten steel across the top surface may shear off fingers of slag into the steel.[19–27] Turbulence and level fluctuations at the top surface can induce slag entrainment at the meniscus and surface defects. Flow problems, such as uneven flow discharge from opposite ports of the SEN,[10] may cause asymmetric and VOLUME 38B, FEBRUARY 2007—63
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INVESTIGATION METHODS
A. Similarity Criterion of Water Model Experiments
Fig. 1—Mechanisms of slag entrainment in the continuous casting mold.[10]
unsteady flow in the mold[28] or periodic oscillations of the level. High speed surface flows or asymmetrical flow in the mold may also induce vortices near the SEN.[10,29] These are other important causes of slag entrainment. Emulsification of the slag/steel interface, such as caused by the rupture of bubbles floating to this interface,[16,30] is also dangerous. The slag foam is easily entrained into the steel flow. Alternatively, slag may be sucked down along the SEN wall due to flow recirculation and the low pressure region just above the SEN port exits.[10] Mold slag
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