Formation of MgO·Al₂O₃ Inclusions in High Strength Alloyed Structural Steel Refined by CaO–SiO₂–Al₂O₃–MgO Slag

抄録

Present paper focused on the formation of MgO·Al₂O₃ inclusions in high strength 42CrMo alloyed structural steel refined by CaO–SiO₂–Al₂O₃–MgO slag. Soluble aluminum contents in molten steel were less than 0.0100% when steel/slag equilibrium was established.
It is found by pre-equilibrium experiments that 90 min are sufficient for the attainment of equilibrium of slag–metal at 1873 K. Spinel inclusions dispersed widely in the steel, with sizes between 2 μm and 4 μm. Magnesium content in molten steel influences the morphology of spinel inclusions greatly. With the rise of magnesium content in steel, globular spinel inclusions were modified into angular ones. However, trace amount of Ca in inclusions is effective to change the spinels from angular shape to globular shape. Solubility of MgO in slag was also discussed. It is shown that MgO in slags are saturated and activities of MgO can be considered as unity. Observed log(X_MgO/XAl₂O₃) of inclusions increases with the growth of observed log[a_[Mg]/(a²_[Al]·a²_[O])] of molten steel as well as the rise of observed log(a_MgO/aAl₂O₃) in slag, both exhibiting good linear relation.
MgO–MgO·Al₂O₃–Al₂O₃ stability phase diagram was obtained. It is found that aluminum and magnesium contents in molten steel mainly position in the MgO·Al₂O₃ formation zone, which is essentially pre-requisite for the formation MgO·Al₂O₃ in steel. Silica in slag is helpful to stabilize MgO·Al₂O₃ phase, because it would react with calcium and aluminum in molten steel. In the scope of soluble aluminum contents, magnesium content influences the transformation among MgO–MgO·Al₂O₃–Al₂O₃ ternary phases greatly, which is the reason for the formation of rectangular MgO inclusions.