Structural Transformation of Molten CaO–SiO₂–Al₂O₃–Fe_xO Slags during Secondary Refining of Steels

Abstract

According to the variation of compositions during steel secondary refining process, the ice-quenched samples of CaO–SiO₂–Fe_xO–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 Q²_Al, Q³_Al, Q⁴_Al coexisted in the molten slag and the lower polymerized units Q²_Al transform into the higher polymerized Q³_Al and Q⁴_Al along with the ratio of Al/(Al+Si) increasing from 0.41 to 0.85. For the Si⁴⁺ cations, Q⁰_Si, Q¹_Si and Q²_Si 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.