Effects of B₂O₃ on Crystallization, Structure, and Heat Transfer of CaO-Al₂O₃-Based Mold Fluxes

The reaction between traditional CaO-SiO₂-based mold fluxes and high-Al steel inevitably changes flux composition, and, consequently, flux properties. This problem can be mitigated by using CaO-Al₂O₃-based mold fluxes. To maintain appropriate melting properties, CaO-Al₂O₃-based mold fluxes contain B₂O₃, which is an effective fluxing agent that decreases the liquidus temperature. In this article, the effects of B₂O₃ on crystallization behavior, structure, and heat transfer of CaO-Al₂O₃-based mold fluxes were studied using single/double hot thermocouple technique, Raman spectroscopy, and infrared emitter technique. The increase of B₂O₃ content from 7.6 to 13.1 mass pct suppressed the crystallization tendency of mold fluxes in continuous cooling experiments and isothermal experiments conducted over 1273 K (1000 °C). The isothermal crystallization below 1273 K (1000 °C) was also inhibited when B₂O₃ content increased from 7.6 to 9.6 mass pct; but a further increase of B₂O₃ content to 13.1 mass pct did not show a visible effect on the crystallization tendency. The increase of B₂O₃ content from 9.6 to 13.1 mass pct improved the heat fluxes under an incident thermal radiation of 1.6 MW/m²; however, the increase of B₂O₃ content from 7.6 to 9.6 mass pct slightly decreased the heat transfer rate. Crystallization of fluxes and heat transfer were discussed in relation to flux structure.