Effect of Cooling Rate on Phase and Crystal Morphology Transitions of CaO–SiO(2)-Based Systems and CaO–Al(2)O(3)-Based Systems

The phase and crystal morphology transitions of two typical types of mold fluxes were investigated fundamentally using differential scanning calorimetry (DSC) and confocal scanning laser microscopy (CSLM) techniques. For the traditional CaO–SiO(2)–CaF(2)-based mold flux, different cooling rates can change the phases and the crystal morphologies. Faceted cuspidine and CaSiO(3) are co-precipitated when the cooling rate is less than 50 °C·min(−1). The phases transform from Ca(4)Si(2)O(7)F(2) and CaSiO(3) to Ca(4)Si(2)O(7)F(2) at the cooling rate of 50 °C·min(−1). Cuspidine shows four different morphologies: faceted shape, fine stripe, fine stripe dendrite, and flocculent dendrite. The crystalline phases of CaAl(2)O(4) and Ca(3)B(2)O(6) are co-precipitated in the CaO–Al(2)O(3)-based mold flux. Neither the phases nor the crystal morphologies change in the low cooling rate range (5 °C·min(−1) to 50 °C·min(−1)). With decreasing temperature, the morphology of CaAl(2)O(4) firstly becomes dendritic, and then the dendritic quality gradually changes to a large-mesh blocky shape at the cooling rates of 100 °C·min(−1), 200 °C·min(−1), and 500 °C·min(−1). Different cooling rates do not show an obvious impact on the morphology transition of CaAl(2)O(4). The strong crystallization ability and large rate of crystallization affect the control of the heat transfer of the CaO–Al(2)O(3)-based mold flux during casting. The big morphology difference between primary crystals of the CaO–SiO(2)–CaF(2-)based mold flux and the CaO–Al(2)O(3)-based mold flux is probably one of the biggest factors limiting lubrication between the CaO–Al(2)O(3)-based mold flux and high-Al steel during casting.