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There exists an increasing pressure on the metal-making and metal-using industry to remove solid and liquid inclusions such as deoxidation products (oxides), sulfides, nitrides carbides, etc. and thereby improve metal cleanliness. It is well known that size, type, and distribution of non-metallic inclusions in metal decrease dramatically the mechanical properties and especially the fracture toughness, the tensile strength, the ductility as well as the fatigue of the cast products resulting to excessive casting repairs or rejected castings. [1] In case of the oxide inclusions in steel melts, Wasai et al. [2] assigned the dendritic, maple-like and polygonal inclusions to the group of the primary inclusions generated directly after adding aluminum in the metal melt. In contrast, the network-like, coral-like, and spherical inclusions, which are composed of alumina, hercynite, and wü stite, are classified as secondary inclusions. The secondary inclusions are formed due to the lower solubility of oxygen in the melt as a function of the temperature above the liquidus temperature of the melt. In case of the secondary inclusions, the a-, g-, and d-alumina modification are more frequently detected in steel melts. Below the liquidus temperature tertiary and quartenary inclusions are generated that present the highest impact on fractures toughness of steel casts according to Ovtchinnikov. [3]

Understanding interactions between filter and molten steel is essential to improve the purity of casted products by filtration. Characteristic, in situ formed layers on the surface of carbon-bonded alumina filters result from these interactions. To comprehend their formation, this study illustrates the time dependency of the layer buildup. Therefore, reactions at the filter/steel interface under quasi static conditions are examined using spark plasma sintering (SPS) equipment. Immersion tests in a steel casting simulator, which provides close-to-reality conditions, complement these investigations. Microstructure and phase analyses reveal that interfacial reactions between filter and steel lead to a thin in situ formed layer on the filter surface. During a "reactive" stage, large polycrystalline alumina structures are formed. Thereby, material is transported both from the carbon-bonded material underneath (i.e., gaseous reaction products) and from the molten steel (i.e., precipitating particles and endogenous inclusions) to the filter/steel interface. The formation of these alumina particles comes to an end as soon as the carbon supply, which triggers the dissolution and precipitation processes at the interface, is cut-off. From that point on, endogenous inclusions are deposited on them ("active" stage). The filters were most efficient during the reactive stage, that is, as long as the interfacial reactions take place.

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In the present article, the performance and the efficiency of ceramic filters for continuous steel filtration in an induction crucible furnace, which is part of the steel casting simulator facility located at Technische Universität Bergakademie Freiberg, is investigated numerically. In order to achieve this objective, a macro-scale simulation for the melt flow in the crucible is coupled with a pore-scale simulation for the flow inside the ceramic filter that is adequately resolved by its detailed filter geometry, obtained from computed tomography scan images. The considerable influence of the filter on the flow field is indicated from the present results. Moreover, the efficiency of the employed filter is also determined and compared for two pore densities.

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