Formation of MgO·Al<sub>2</sub>O<sub>3</sub> Inclusions in High Strength Alloyed Structural Steel Refined by CaO–SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub>–MgO Slag

Abstract

Present paper focused on the formation of MgO·Al 2 O 3 inclusions in high strength 42CrMo alloyed structural steel refined by CaO-SiO 2 -Al 2 O 3 -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 1 873 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 /X AlO3 ) of inclusions increases with the growth of observed log[a [Mg] /(a 2 [Al] ·a 2 [O] )] of molten steel as well as the rise of observed log(a MgO /a Al2O3 ) in slag, both exhibiting good linear relation. MgO-MgO·Al 2 O 3 -Al 2 O 3 stability phase diagram was obtained. It is found that aluminum and magnesium contents in molten steel mainly position in the MgO·Al 2 O 3 formation zone, which is essentially pre-requisite for the formation MgO·Al 2 O 3 in steel. Silica in slag is helpful to stabilize MgO·Al 2 O 3 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·Al 2 O 3 -Al 2 O 3 ternary phases greatly, which is the reason for the formation of rectangular MgO inclusions.