Application of exchangeable carbon-bonded alumina foam filters in an industrial tundish for the continuous casting of steel

Abstract Various melt refining techniques were developed in the past and led to ongoing improvement of casting quality. One of these processes is the so-called metal melt filtration, which became state of the art especially in aluminum foundry operations by using ceramic foam filters. Attempts to adapt the technique for the continuous casting of steel showed numerous difficulties due to the severe thermomechanical conditions and the high quantity of processed melt. Premature blockage or failure of ceramic foam filters implemented in the tundish promoted the application of flow control devices and multi-hole filters as alternatives. In the present study, a new approach for ceramic foam filters in the continuous casting of steel was tested. Thereby, the main disadvantage of limited filter service lifetime was circumvented by the application of an exchangeable filter system without disturbing the underlying continuous casting process. Cylindrical carbon-bonded alumina foam filters were prepared and mounted on alumina-carbon shafts using different adhesives and slag protection measures. The filters were immersed by a crane into the steel melt in an industrial tundish with 80 t capacity. During each test, one batch with approximately 380 t of steel was cast at over 1550 °C before removing the filter. The application of high-alumina mortar as adhesive and a compact filter geometry without macro channels proved to be essential for the successful operation. Samples of the corresponding filter were analyzed by digital light microscopy, X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy and electron backscatter diffraction. The complementary investigations revealed the formation of pronounced clogging layers based on aluminum-rich calcium aluminates, which can be clearly distinguished from the synthetic slag material containing silicon and other trace elements. At the bottom of the filter pronounced clusters of coral-like and plate-like inclusions were observed, which approached the filter surface by buoyancy.