EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Typical commercial calcium silicate based mould powders (CaO-SiO₂-Al₂O₃-CaF₂) are used in casting high aluminium (Al) steels. The dissolved Al in liquid steel reacts with the mould slag during casting. In high Al alloyed steels, SiO₂ of the slag is partially reduced and high amount of alumina (Al₂O₃) up to 30 % is formed. The resulting change in mould slag composition causes modified physico-chemical properties. Especially viscosity and solidification behavior are affected. Also, lubrication and heat transfer conditions in the mould deteriorate due to the changing SiO₂/Al₂O₃ ratio. This are the reasons why various kinds of casting problems such as increased number of breakout alarms, longitudinal cracks and depressions. Therefore, a calcium aluminate based mould powder (CaO-Al₂O₃-CaF₂-Li₂O) has been developed by thyssenkrupp Steel Europe AG and Imerys Metalcasting GmbH. A comparison with a commercial calcium silicate based mould powder was performed. The new calcium aluminate based mould powder reaches the operating point much faster than the calcium silicate based mould powder. Operational tests show that slag composition, viscosity, crystallization temperature and heat transfer rate reach a steady state within about 20 min. Mineralogical phases and fabric of formed slag films were characterized and compared on polished cross sections from both mould slags. Measurements of crystallization temperatures and viscosities of mould slags were performed in laboratory trials. Further optimization of physico-chemical properties was achieved by tuning the chemical and mineralogical composition of the new calcium aluminate mould powder. This optimization procedure was supported by thermodynamic calculations of viscosity and melting behavior for different compositions. The operating tests carried out with the new calcium aluminate mould powder showed an improved surface quality of the slabs with decreased number of longitudinal cracks. It is planned to establish the new mould flux in series production.