Abstract: A mold slag based on CaO-SiO₂-Al₂O₃ system was designed for casting high-aluminum peritectic steels. Single hot thermocouple technique (SHTT) was applied to study the impact of w(CaO)/w(Al₂O₃) on crystallization behaviors. Double hot thermocouple technique (DHTT) was used to simulate the formation of slag film and define the impact of w(CaO)/w(Al₂O₃) on solidification fraction. The results show that the slags can be divided into two groups according to crystallization characters: the group with w(CaO)/w(Al₂O₃) higher than 1.30 (Group Ⅱ) has a higher tendency of crystallization than that with w(CaO)/w(Al₂O₃) lower 1.30 (Group Ⅰ). Cuspidine (Group Ⅰ) and dicalcium silicate (Group Ⅱ) were precipitated respectively in these two groups during isothermal process. As the w(CaO)/w(Al₂O₃) increased， the crystallization tendency increased sharply which causes the solidification fraction in slag film increased. The slags were also compared with industrial slag used for casting common peritectic steels， showing that the slags with w(CaO)/w(Al₂O₃)≤1.30 have similar properties with industrial slag. The designed CaO-SiO₂-Al₂O₃ slag can be used in casting high-aluminum peritectic steels.

Key words: CaO-SiO₂-Al₂O₃ mold slag, w(CaO)/w(Al₂O₃), crystallization behaviors, slag film simulation, solidification fraction