Effect of ZrO2 on the Melting, Viscosity, and Crystallization Behaviors of Fluorine-Free Mold Flux for Casting Medium Carbon Steels

The effect of ZrO2 on the melting, viscosity, and crystallization behaviors of fluorine (F)-free mold flux designed for the casting of medium carbon steels is studied by using Single/Double Hot Thermocouple Technology (SHTT/DHTT) and high temperature rotary viscometer in this paper. The results showed that the melting temperature, viscosity at 1300 degrees C, and break temperature increases with the addition of ZrO2 content in the F-free mold flux system. Besides, both the Continuous Cooling Transformation (CCT) and Time Temperature Transformation (TTT) diagrams showed that the ZrO2 can promote the crystallization ability of F-free mold flux; and the analysis from the Scanning Electron Microscope (SEM) and X-ray diffraction (XRD) verified that the enhancement of ZrO2 on the mold flux crystallization is due to the fact that it could act as heterogeneous nucleation agent to trigger the heterogeneous crystallization of F-free mold flux system, due to its limited solubility in the molten slags.