Abstract

The understanding of the reactions between filter material and molten steel is essential to improve the purity of casted products by filtration. Thus, this contribution illuminates the kinetics of interactions between carbon-bonded filters and molten steel. In addition, a thermodynamic model accompanies the experiments to describe the impact of carbon on the reactions. For the experiments, carbon-bonded alumina filters were coated either with pure alumina or with a mixture of alumina and 4wt% carbon (Al2O3+4C). Two methods were applied to evaluate the time dependence of the interactions. First, the coated filters were immersed in molten steel for 10s up to 120s. These short-time tests are performed using a special steel casting simulator providing close-to-reality conditions. Secondly, long-time trials up to 60min using spark plasma sintering (SPS) equipment complement the investigations. In the SPS test, the flow of molten steel is minimized, which enables an almost undisturbed examination of the chemical reactions at the interface. The phase composition and the distribution of phases at the metal/ceramic interface were characterized by means of optical microscopy, SEM with EDX, WDX and EBSD. Carbon seems to accelerate the layer buildup during steel contact, as the in situ formed layers were already observed after immersing filters coated with Al2O3+4C for 10s into the molten steel. In comparison, the in-situ formed layers were only found after 30s immersion of alumina coated samples. Furthermore, the long-term SPS experiments revealed enhanced chemical interactions of Al2O3+4C coated filters as compared to the alumina coated ones. These findings indicate that an accelerated dissolution and precipitation of alumina from steel is triggered by the presence of carbon, in excellent agreement with predictions of the thermodynamic model. Interestingly, very fine inclusions with the size of a few micrometers were found on the surface of alumina coated filters after 60s and even more of these inclusions after 120s immersion. They are considered to be deposited directly from the molten steel, which would be the desired filtration effect. In summary, the carbon content of alumina based filter material significantly influences the steel melt filtration. The thermodynamic model clarified the impact of carbon on these interactions and will be beneficial to tailor future filter material compositions.