Effects of Cooling Rate on the Crystallization Behaviors of CaO-Al2O3-B2O3 -CaF2-Based Mold Flux
During the casting of steel, the crystallization behaviors of mold flux have an important effect on the quality of cast. A new type of CaO-Al2O3-B2O3-CaF2-based mold flux for high aluminum steel casting has been designed. The effects of cooling rate on the crystallization behaviors were investigated using confocal scanning laser microscopy (CSLM) and a high temperature tube furnace. Both Ca3B2O6 and Ca12Al14F2O32 crystals precipitated at a cooling rate of 1 ℃/min, 5 ℃/min, and 20 ℃/min. The primary crystal morphology of Ca12Al14O33 gradually changed from an irregular massive block shape to a moderate faceted shape as cooling rates increased from 1 ℃/min to 20 ℃/min. The morphology of the Ca3B2O6 crystals also transformed from a large dendritic shape to a small faceted shape, and the size gradually decreased with the increasing cooling rate. The glassy phase also increased with the increasing cooling rate. Ca12Al14O33 showed four different morphologies at the cooling rates ranging from 1 ℃/min and 800 ℃/min: faceted shape, blocky-shape with a fine stripe, large sheet with flocculent, and dendrite. The crystallization temperature decreased from 1177.3 ℃ to 894.1 ℃ as the cooling rate increased from 30 ℃/min to 500 ℃/min. The crystallization mechanism of Ca12Al14O33 was deduced as the surface crystallization at the present slag.