Study on Slag–Steel Reaction Between CaO–Al₂O₃–Ce₂O₃–5 PctMgO-7 PctSiO₂ Slag System and Fe–Al Steel System

In the steelmaking process of RE-containing steel, there exist several problems such as the instability of existing forms of RE elements and the nozzle clogging phenomenon caused by RE-containing inclusions. The indirect alloying method of RE elements is expected to solve these problems and deserves attention. In the current work, the slag–steel reaction between CaO–Al₂O₃–Ce₂O₃–5 pctMgO-7 pctSiO₂ slag system and Fe–Al steel system was studied. We first made a thermodynamic analysis based on the IMCT “activity” theory (Ion and Molecule Coexistence Theory) for the slag system and the Wagner’s interaction parameter formalism for steel systems. On this basis, high temperature experiments were performed to study the variation of T.Ce in steel with reaction time, the evolution of non-metallic inclusion type in steel, and the change of precipitate type in slag. The results indicate that the final content of T.Ce in steel increased with the increase of w(CaO)/w(Al₂O₃) and w(Ce₂O₃) of the slag, as well as with the increase of w[Al] of the steel. The restrictive step in the initial stage of the slag–steel reaction was the interfacial reaction, where the reaction rate was only affected by a(Ce₂O₃) of the slag and a_[Al] of the steel; for the medium stage, the accumulation rate of Ce content in steel decreased due to the consumption of reactants and the accumulation of products. Under the current experimental condition, a maximum of 80 ppm of Ce can be introduced into the steel during the slag–steel reaction. With the slag–steel reaction proceeding, the content of Ce and Mg in steel increased, while that of Al and O decreased, resulting in the partial or complete change of Al₂O₃ inclusions into MgAl₂O₄, CeAlO₃, CeAl₁₁O₁₈, and their combinations. MgAl₂O₄ is a metastable inclusion in some experimental groups, which disappeared in the final stage. Different types of precipitates were found in the final slags, including 2CaO·Al₂O₃·Ce₂O₃, 2CaO·3Al₂O₃·Ce₂O₃, CeAlO₃, MgAl₂O₄, and MgO. According to the type of inclusions in steel and precipitates in slag, the phase diagram showing the co-existence relation of CaO–Al₂O₃–Ce₂O₃–MgO slag system was determined. The present research results are helpful for further studying on the indirect alloying method of RE elements through RE-containing slag systems.