Effect of mass fraction of Y₂O₃ and w(CaO)/w(Al₂O₃) on physicochemical properties of electro-slag remelting slag systems

To systematically investigate the influence of mass fraction of Y₂O₃ and w(CaO)/w(Al₂O₃) on the physicochemical properties of the CaF₂-CaO-Al₂O₃-MgO-Y₂O₃ slag systems, this study addresses the severe burning loss of the Y element during the electro-slag remelting process of Y-containing rare earth steels. It proposes improving slag system performance by adding Y₂O₃ and adjusting the w(CaO)/w(Al₂O₃), thereby increasing the yield of Y. Experiments were conducted to prepare five slag systems with different mass fractions of Y₂O₃ and four slag systems with different w(CaO)/w(Al₂O₃) values. Various testing and analytical methods were comprehensively utilized, including an X-ray diffractometer, an X-ray fluorescence spectrometer, hemispherical melting temperature testing, rotating cylinder viscosity measurement, a Fourier transform infrared spectrometer, and a scanning electron microscope, to conduct a systematic study on the phase composition and content, melting characteristics, viscosity changes, structural features, and precipitated phase characteristics of the slag system. The aim is to provide theoretical support for optimizing slag composition, suppressing the burning loss of the Y element, and improving its yield. The results indicate that the addition of Y₂O₃ promotes the formation of the CaYAlO₄ phase. As the mass fraction of Y₂O₃ increased from 0 to 20%, the melting temperature of the slag first decreased and then increased, while the viscosity first increased and then decreased, with the optimal Y₂O₃ mass fraction of 15%. With the increase of the w(CaO)/w(Al₂O₃) from 0.8 to 1.4, the characteristic peaks of the CaYAlO₄ phase in the slag systems gradually intensified, the melting temperature and viscosity of the slag systems decreased, and the proportion of needle-like CaYAlO₄ in the slag systems increased. The decrease in viscosity might be primarily attributed to the depolymerization ofAlO_nF_4-n^- tetrahedral complexes and the transformation ofAlO₄^5- tetrahedra intoAlO₆^9- octahedra induced by the elevated w(CaO)/w(Al₂O₃). The low viscosity led to the reduction of ionic clusters migration resistance, which in turn reduced the energy potential barrier for nucleation and crystal growth, accounting for the increase in the percentage of CaYAlO₄. The needle-like morphology of CaYAlO₄ may be affected by the growth mechanism controlled by screw dislocations. Based on the above discussion, the optimal slag composition is determined as follows, Y₂O₃mass fraction of 15%, w(CaO)/w(Al₂O₃) is 1.4. The melting temperature of this slag ratio is 1 346 ℃, and its viscosity at 1 600 ℃ is 0.28 Pa·s.