Thermodynamic fundamentals on the systems Fe-Al-O and Fe-Al-Ca-S-O being important to metallurgy of Ca-treated Al-killed steels are analysed and discussed. The formation of liquid Ca aluminates for the improvement of castability is pursued in view of the production of Ca-treated Al-killed steels. The "Liquid window" is theoretically analysed and experimentally investigated. The top slag is formed after tapping and its composition is fairly constant during ladle treatment for both with and without RH treatment, respectively. However, the top slag of the degassed heats shows a lower oxygen level compared to the non-degassed heats. It is found that there exists an "inclusion path" during the ladle treatment. The inclusion path for the heats with RH treatment considerably differs from that of the heats without RH treatment and it greatly influences the efficiency of Ca treatment.