Die Existenz der nichtmetallischen Einschlüsse in der Metallschmelze führt zur Verschlechterung der physikalischen Eigenschaften des gegossenen Stahls. Die im Rahmen des SFB 920 entstandene Arbeit beschäftigt sich hauptsächlich mit der Untersuchung der reaktiven Reinigung und aktiven Filtration der Metallschmelze im Induktionsofen mithilfe der numerischen Strömungsmechanik. Dazu werden im Rahmen der Arbeit neue numerische Simulationsmodelle entwickelt. Die Ergebnisse zeigen, dass die reaktive Reinigung einen ausschlaggebenden positiven Einfluss auf die Entfernung der Einschlüsse aus der Metallschmelze hat, wobei der Reinheitsgrad der Metallschmelze innerhalb einer kurzen Eintauchzeit des kohlenstoffgebundenen Filters ansteigt. Der in der reinen aktiven Filtration erzielte Reinheitsgrad ist deutlich niedriger als im Falle der reaktiven Reinigung. Des Weiteren befasst sich die Arbeit mit dem Effekt der Turbulenzmodellierung auf die Beschreibung der Schmelzeströmung und der Verteilung der Einschlüsse. Weiterhin stellt die Arbeit ein experimentelles Wassermodell zur Nachbildung der rezirkulierenden Schmelzeströmung im Induktionsofen vor.:Abstract List of figures List of Tables Nomenclature 1 Introduction 2 State of the Art 3 Numerical Model 4 Results 5 Summary and Conclusion 6 Outlook Bibliography / During the continuous casting, non-metallic inclusions such as deoxidation or reoxidation products may arise in steel melt due to different reactions. These inclusions have to be removed from the melt with the aim of improving the cleanliness of steel products. There are many approaches in continuous casting to enhance inclusion removal from the melt. However, this Phd thesis focuses particularly on inclusion removal by reactive cleaning and active filtration during the melt casting in an induction crucible furnace using ceramic foam filter developed in CRC 920. For this purpose, a numerical model is developed in OpenFOAM in order to simulate the melt flow and reactive cleaning as well as active filtration. Besides the numerical model, an experimental water model are created to validate the capability of the numerical model to predict the recirculating turbulent flow in the induction crucible furnace. The results show that reactive cleaning and the formation of carbon monoxide bubbles, which arise on the inclusions, have a positive impact on the melt cleanliness. Reactive cleaning can improve inclusion removal and enhance the melt cleanliness sufficiently. Based on the results, one can conclude that the rate of inclusion removal in case of reactive cleaning is considerably higher than that in case of active filtration. Furthermore, the results indicate the considerable influence of filter position and filter number on the flow field as well as on the cleaning efficiency in the induction crucible furnace. Moreover, the results shows that turbulence modeling has a considerable impact on the prediction of the melt flow and inclusion dispersion in the induction crucible furnace.:Abstract List of figures List of Tables Nomenclature 1 Introduction 2 State of the Art 3 Numerical Model 4 Results 5 Summary and Conclusion 6 Outlook Bibliography

In order to improve the purity of steel castings, the use of special reactive coatings on carbon-bonded ceramic foam filters was explored. Carbon nanotubes were dispersed in water by means of ultrasonic treatment, using xanthan gum to stabilize the nanotubes in suspension and control the rheological behavior. The coatings were applied by cold spraying and binding was achieved during heat treatment in reducing atmosphere, thanks to an artificial pitch added to the slurry. The coated filters were successfully immersed in molten steel for different times. The thickness of the first alumina layer generated at the interface was independent of the immersion time: concentration gradients through its thickness suggested that the formation of this structure is limited by diffusion. Investigation of the steel after solidification by means of ASPEX showed that the presence of the coating influenced the size as well as the chemical composition of the remaining inclusions. Nano-coated filters had the best filtration efficiency (up to 95% for alumina inclusions after 10 s), but longer tests resulted in worse performance. In addition, coatings based on calcium aluminates in combination with carbon showed an efficiency greater than 97% for steel samples taken directly from the melt.

In order to improve the purity of steel castings, the use of special reactive coatings on carbon-bonded ceramic foam filters was explored. Carbon nanotubes were dispersed in water by means of ultrasonic treatment, using xanthan gum to stabilize the nanotubes in suspension and control the rheological behavior. The coatings were applied by cold spraying and binding was achieved during heat treatment in reducing atmosphere, thanks to an artificial pitch added to the slurry. The coated filters were successfully immersed in molten steel for different times. The thickness of the first alumina layer generated at the interface was independent of the immersion time: concentration gradients through its thickness suggested that the formation of this structure is limited by diffusion. Investigation of the steel after solidification by means of ASPEX showed that the presence of the coating influenced the size as well as the chemical composition of the remaining inclusions. Nano-coated filters had the best filtration efficiency (up to 95% for alumina inclusions after 10 s), but longer tests resulted in worse performance. In addition, coatings based on calcium aluminates in combination with carbon showed an efficiency greater than 97% for steel samples taken directly from the melt.

In order to considerably improve the quality of steel products, efforts are being made to increase the cleanness of steel. It is known that the size, type and distribution of non-metallic inclusions (NMIs) in metals significantly influence their mechanical properties. Within the frame of the Collaborativ Research Centre 920 'Multifunc-tional filters for metal melt filtration – a contribution to zero defect materials.”, carbon-bonded alumina filters for steel melt filtration has been investigated. In the present thesis, the interactions between coated carbon-bonded alumina filters and a steel melt were investigated in more detail, with the porous coatings being based on alumina. The time-depended behaviour was evaluated by immersing coated filters for different dwell times. After the immersion tests, the microstructure of the filter surface and the NMIs remaining in the solidified steel were examined to comprehend the interactions. The here presented results imply that carbon of the carbon-bonded alumina sub-strate took part in the filter–steel interactions. The presence of dissolved carbon at the filter–steel melt interface is essential to promote alumina dissolution and pre-cipitation processes. Thereby, the melt was locally supersaturated with aluminium, which reacted with dissolved oxygen to form secondary corundum. During these interactions, a liquid oxide film was formed directly at the ceramic surface and provided nuclei for heterogeneous nucleation of secondary corundum. After immersion during cooling, a characteristic layer built-up formed at the filters surface. All alumina-based coated filters contributed to the filtration of especially alumina-based NMIs, and outperformed the uncoated carbon-bonded alumina filter. During the first experimental trials, it became obvious that the thermal shock resistance of the alumina coating has to be enhanced. For this purpose, a material combination was investigated which so far has not been used as a coating material to the author’s knowledge: alumina-zirconia-titania in the ration 95 : 2.5 : 2.5. This material is known for its excellent thermal shock resistance. Thereby, the influence of zirconia or titania doping of the coating were considered. The addition of titania enhanced wetting of this filter by the steel melt. As a result, alumina inclusions of the steel melt were modified: they were more in number, but distinctly smaller compared to trials without filter or the immersion of the other filter types. Especially, the decreased average area of the alumina inclusions is interesting because the particle size of NMIs strongly influences the fatigue life of a steel product. The deformability of a steel product, however, is determined by the amount of NMIs. Thus, the modification of alumina inclusions by adding titania to the filter coating might present a way to tailor these inclusions depending on the product’s application.