Model Investigations on the Stability of the Steel-Slag Interface in Continuous-Casting Process
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tion of either slag droplets in steel or steel droplet in slag is an area of interest in different metallurgical processes. The term emulsification is used for a mixture of two or more immiscible fluids. During the emulsification process, a huge number of droplets is formed, increasing the interfacial area and, therefore, the reaction rate between steel and slag. In the basic oxygen furnace, the emulsification of molten steel in the top slag or the reverse emulsification of the top slag in the molten steel takes place under certain blowing conditions and properties of liquid steel and slag. These phenomena significantly influence the rate of the reaction between molten steel and slag.[1,2] The effect of sulfur on emulsification behavior in steel was studied by El Gammal et al.[3] These authors pointed out a maximum of emulsification of steel in slag at the initial stage of desulfurization, i.e., at high sulfur content in RENE´ HAGEMANN, Ph.D. Student, and HANS P. HELLER, Dr.-Ing., Senior Engineer (Scientific Assistant), are with the Institute of Iron and Steel Technology, Freiberg University of Mining and Technology, 09599 Freiberg, Germany. Contact e-mail: rene.hagemann @iest.tu-freiberg.de RU¨DIGER SCHWARZE, Apl. Professor Dr.-Ing. habil., is with the Institute of Mechanics and Fluid Dynamics, Freiberg University of Mining and Technology. PIOTR R. SCHELLER, Professor Dr.-Ing. habil., is with the Faculty of Materials Science and Technology, Freiberg University of Mining and Technology. Manuscript submitted December 19, 2011. Article published online November 8, 2012. 80—VOLUME 44B, FEBRUARY 2013
steel calculating the flotation coefficient. A theoretical model for the emulsification of slag into steel in gasstirred ladles was developed by Wei and Oeters[4] as well as Mietz et al.[5] Using a balance of forces acting on fluids near the interface, the emulsification of slag in steel depending on critical flow velocity, size, and number of droplets formed per time was investigated. Figure 1 illustrates the situation in the continuouscasting mold. Shear stresses are generated between the steel flow and the liquid slag layer. At the steel-slag interface, two flows leading to slag entrainment: a macroscopic shear flow and a microscopic interfacial flow (Marangoni flow), which is generated by a local gradient of the interfacial tension caused, e.g., by chemical reactions.[6] In the continuous-casting process, the interface between steel and slag should be stable to avoid the entrainment of slag droplets forming nonmetallic inclusions, which deteriorate the product quality.[7] In the current article, the term entrainment is used to describe the formation of slag droplets at the steel-slag interface in the continuous-casting mold. Thomas[8] mentioned three types of mold slag entrainment: vortexing, high velocity flow that shears slag from the surface, and turbulence at the meniscus. To get an insight into slag entrainment, numerical simulation was found to be a useful tool to study the formation of defects in the casing mold related to flow phenomena.
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