Structural Transformation of Molten CaO-SiO2-Al2O3-FexO Slags during Secondary Refining of Steels

According to the variation of compositions during steel secondary refining process, the ice-quenched samples of CaO-SiO2-FexO-Al2O3 system were prepared, the structure were detected via the method of Raman spectroscopy, and the evolution of structural units were further analyzed. The results showed that, for the Fe3+ cation, two types of units of tetrahedral fourfold coordination ([FeO4]) and octahedral sixfold coordination ([FeO6]) coexisted in the molten slag, and the ratio of [FeO6]/[FeO4] increases with the decreasing ratio of CaO/(SiO2+Al2O3). For the Al3+ cations, four types of aluminum units of Q(Al)(2), Q(Al)(3), Q(Al)(4) coexisted in the molten slag and the lower polymerized units Q(Al)(2) transform into the higher polymerized Q(Al)(3) and Q(Al)(4) along with the ratio of Al/(Al+Si) increasing from 0.41 to 0.85. For the Si4+ cations, Q(Si)(0), Q(Si)(1) and Q(Si)(2) are the main types of [SiO4]-tetrahedral units, the Ca2+ cations of oxygen coordination in [SiO4]-tetrahedral units are gradually replaced by Al3+, which just act as network modifier, with the increase of Al/(Si+Al) ratio. Accordingly, the degree of polymerization of molten slag presented in NBO/T increases with the process of secondary steelmaking.