The activity of SiO 2 in the CaO-Al 2 O 3 -SiO 2 (-10%MgO) systems was measured at the region of %SiO 2 Ͻ10%. The melts were equilibrated with a reference metal of Cu in a controlled oxygen partial pressure at 1 873 K. Iso-SiO 2 activity contours were drawn from the measured values. For the CaO-Al 2 O 3 -SiO 2 system, the measurement result showed positive deviation from the previous data. It was also found that the SiO 2 activity decreased as CaO content increased. In the CaO-Al 2 O 3 -SiO 2 -10%MgO system, the composition dependence of the SiO 2 activity seemed to be similar as the ternary system. On the other hand, the activity values were much lower than that of the ternary system. It could be attributed to the strong basic behaviour of MgO. Based on the result of the SiO 2 activity measurement, the equilibrium between the inclusions and liquid steel during ladle treatment was discussed.KEY WORDS: activity of SiO 2 ; CaO-Al 2 O 3 -SiO 2 system; CaO-Al 2 O 3 -SiO 2 -MgO system; deoxidation equilibrium; ladle process.

During refining of stainless-steel melt using the AOD (Argon Oxygen Decarburization) process, slag composition and melt stirring can significantly affect the decarburization behavior and reduction of Cr2O3 in slag. An 18 wt%Cr-8 wt%Ni stainless-steel melt containing 0.2 wt%C was decarburized at 1650 ℃ by different types of slag containing Cr2O3. Melt stirring by Ar or Ar+O2 gas bubbling through a submerged nozzle enhanced the decarburization rate significantly. Pure solid Cr2O3 and liquid slag containing Cr2O3 were far more effective than a solid CaO-Cr2O3 mixture for decarburization. The formation of a stable CaO․ Cr2O3 phase in the slag was confirmed by XRD analysis after the decarburization experiment with CaO-Cr2O3 slag. Therefore, in order to improve the decarburization of stainless-steel melt in low-carbon-content regions, it is essential to increase the liquid fraction of slag.

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