Formation and Modification of MgO·Al₂O₃ Spinel in Oil Casing Steel

The formation and modification mechanism of MgO·Al₂O₃ spinel in oil casing steel during secondary refining process was studied by industrial experiments and thermodynamic calculations.The results show that when dissolved Al are 0.02% and 0.05% respectively,the required amount of dissolved Mg for the formation of MgO·Al₂O₃ spinel are 1.5×10^-6and 2.8×10^-6accordingly.For molten steel with 0.02% dissolved Al,When the dissolved Mg ranges from 4×10^-6to 8×10^-6,MgO·Al₂O₃ inclusions would transform into CaO-Al₂O₃-MgO system inclusions under dissolved Ca content of 0.21×10^-6and 0.42×10^-6 respectively.MgO·Al₂O₃ spinel inclusions could be easier modified than Al₂O₃ inclusions.Because of the influence of slag-metal reaction and calcium treatment,the evolution of inclusion could be roughly described as follows:Al₂O₃→MgO-Al₂O₃ system inclusions→CaO-Al₂O₃-MgO or CaO-Al₂O₃ system liquid complex inclusions.