Effects of the cooling rate on the crystallization behaviors of the CaO–Al2O3–B2O3–CaF2-based mold flux

Abstract

In the casting of steel, the crystallization behavior of the mold flux has an important effect on the cast quality. A new type of CaO–Al₂O₃–B₂O₃–CaF₂-based mold flux for high aluminum steel casting has been designed. The effects of the cooling rate on the crystallization behavior were investigated using confocal scanning laser microscopy (CSLM) and a high temperature tube furnace. The Ca₃B₂O₆ and Ca₁₂Al₁₄O₃₃ crystals precipitated at cooling rates of 1, 5 and 20 °C min⁻¹. The primary crystal morphology of Ca₁₂Al₁₄O₃₃ gradually changed from an irregular massive block shape to a moderate faceted shape, as the cooling rate increased from 1 to 20 °C min⁻¹. The morphology of the Ca₃B₂O₆ crystals also transformed from a large dendritic shape into a small faceted shape, and their size gradually decreased with increasing cooling rate. The glassy phase also increased with increasing cooling rate. Ca₁₂Al₁₄O₃₃ showed four different morphologies at cooling rates ranging from 1 to 800 °C min⁻¹: faceted shape, block-shape with fine stripes, large sheet with cloudy dendrites, and dendrites. The crystallization temperature decreased from 1177.3 to 894.1 °C as the cooling rate increased from 30 to 500 °C min⁻¹. The crystallization mechanism of Ca₁₂Al₁₄O₃₃ was deduced to be surface crystallization for the present slag.