Fluorine‐Free Mould Powders for Slab Casting: Crystallization Control in the CaO–SiO2–TiO2–Na2O–Al2O3 System

The main problem related to the development of fluorine‐free mould powders for slab casting is effectively controlling the heat transfer between the steel shell and mould. In commercial mould powders crystallization of cuspidine (Ca₄Si₂O₇F₂) from mould slag has a great effect on heat‐transfer control. In industrial process the crystallization rate for a fluorine‐free mould slag should be similar to the crystallization rate of cuspidine. To evaluate the crystallization rate for slags time‐temperature‐transformation (TTT) diagrams can be constructed using the Single Hot Thermocouple Technique by in situ observation. In the present work, fundamental information related to crystallization control in the CaO–SiO₂–TiO₂–Na₂O–Al₂O₃ system was obtained. It was observed that the addition of Na₂O in CaO–SiO₂–TiO₂ slags dramatically shortens the crystals' incubation times in TTT diagrams to the range of seconds. It is possible to control the crystallization kinetics in CaO–SiO₂–TiO₂ slags by changing the Na₂O content. Some observations for the crystals' morphology are reported.