Effect of basicity and B2O3 on the viscosity and structure of fluorine-free mold flux

The effect of basicity (weight ratio of CaO/SiO2) and B2O3 on the viscosity and structure of Fluorine-free mold flux for the casting of medium carbon steels was conducted in this article, through the rotating cylinder method combined with the Fourier transform infrared (FTIR) spectroscopy. The results showed that, with the increase of basicity, the viscosity of mold flux was attenuated dramatically, and then kept constant or slight increased in the low temperature region. The reason could be explained as the degree of polymerization (DOP) of the network structure was first reduced significantly with the addition of basicity, and then the further depolymerization is less pronounced with the further increase of basicity. Beside the formation of high melting point substance leads to the slight increase of viscosity. Moreover, it suggested that the viscosity of mold flux is decreased with the addition of B2O3 content, due to the fact that B2O3 is a low melting point oxide which could substantially lower the break temperature of mold flux. The result of FTIR indicated B2O3 acts as network former, and tends to form [BO3]-trihedral and [BO4]-tetrahedral structural units, which would connect with each other to form some simple network structure such as diborate or pentaborate. With the addition of B2O3, the free oxygen ions (O2 −) would depolymerize the diborate structural unit, and the depolymerized diborate would link again with free [BO3]-trihedral to form complex pentaborate groups. Moreover, the effect of above addition on the apparent activation energy for viscous flow and break temperature of mold flux also were discussed. The results obtained in this paper provide the detailed study of the structure evolution of Fluorine-free mold flux when B2O3 is added.