Effect of B2O3 on the Crystallization Behavior of CaF2-Based Slag for Electroslag Remelting 9CrMoCoB Steel

The non-isothermal crystallization characteristics of the electroslag remelting (ESR)-type slag with varied B2O3 contents were investigated by non-isothermal differential scanning calorimetry (DSC), field emission scanning electron microscopy (SEM-EDS), and X-ray diffraction (XRD). The crystallization mechanism of the B2O3-bearing slag was also identified based on kinetics analysis. The results showed that the primary crystalline phase was CaF2, there was no change in the type of the primary crystal as B2O3 content increased, and the morphology of the CaF2 crystal was mainly dendritic. The sequence of crystal precipitation during the cooling process was CaF2 to Ca12Al14O32F2 and MgO/MgAl2O4, followed by Ca3B2O6. The activation energy of CaF2 crystallization increased firstly, then decreased and reached stability, while the activation energy of Ca3B2O6 crystallization increased continuously with the increasing B2O3 content. The crystallization behavior of CaF2 was three-dimensional growth with a constant nucleation rate. The proper B2O3 content added into the CaF2-based ESR slag should be around 1.0% to limit the precipitation of the CaF2 crystal to attain good surface ingot quality and stable ESR operation.