The equilibration between CaO-SiO 2 -MgO-Al 2 O 3 -CaF 2 (-TiO 2 ) slag and Fe-11 mass pct Cr ferritic stainless steel melts was investigated at 1873 K in order to clarify the effect of Al and Ti addition as well as that of slag composition on the formation of complex oxide inclusions. The activity of oxygen calculated from the classical Wagner formalism changes from about a O = 0.0002 to 0.001 and the values of a O from [Al]/(Al 2 O 3 ) and that from [Si]/(SiO 2 ) equilibria are in relatively good agreement with each other with some scatters. The phase stability diagram of the inclusions and the equilibrium iso- [O] lines in the Fe-11 mass pct Cr-0.5 mass pct Si-0.3 mass pct Mn-0.0005 mass pct Mg steel melts was constructed by using FACTSAGE 5.5 program as a function of Al and Ti contents. The computed iso-[O] lines were slightly larger than the values estimated from the slag-metal equilibria. The composition of the inclusions could be plotted on the computed MgO-Al 2 O 3 -TiO x phase diagram. The inclusions in the steel melts equilibrated with the basic slags are located in the ''spinel + liquid'' region, while those in equilibrium with the less basic slags are mostly in the ''liquid'' single phase. This is in good accordance to the observed morphology of the inclusions. However, in cases of high concentration of Ti and Al, the inclusions were found to be spinel + liquid, even though the less basic slags are equilibrated. When plotted on logarithmic scales, the mole ratio X MgO  X Al 2 O 3 X Ti 2 O 3 À Á of the inclusions (spinel potential) was expressed as a linear function of a Mg  a 2 Al  a O a 2 Ti Ä Å of the steel melts with a slope of unity theoretically expected. Also, the spinel potential is very low and nearly constant when the activity of Al 2 O 3 is less than that of TiO 2 in the slag saturated by MgO, whereas it linearly increases by increasing the log a Al 2 O 3 =a TiO 2 ð Þat X Al 2 O 3 =X TiO 2 ð Þ >1.

A thermodynamic equilibrium between aluminum and oxygen and inclusion morphology in the Fe-16Cr stainless steel were investigated to understand the fundamentals of the aluminum deoxidation technology for ferritic stainless steels. Further, the effect of calcium addition on the changes in chemistry and morphology of inclusions was discussed. The measured results for the aluminum-oxygen equilibria exhibit relatively good agreement with the calculated values, indicating that an introduction of the firstand second-order interaction parameters, recently reported, is reasonable to numerically express the aluminum deoxidation equilibrium in a ferritic stainless steel. In the composition of dissolved aluminum content greater than about 60 ppm, pure alumina particles were observed, while the alumino-manganese silicates containing Cr 2 O 3 appeared at less than 20 mass ppm of dissolved aluminum. The formation of calcium aluminate inclusions after Ca treatment can be discussed based on the thermodynamic equilibria among calcium, aluminum, and oxygen in the steel melt. In the composition of steel melt with relatively high content of calcium and low aluminum, the log (X CaO /X Al 2 O 3 ) of inclusions linearly increases by increasing the with the slope close to unity. However, the slope of the line is significantly lower than the expected value in the composition of steel melt with relatively low calcium and high aluminum contents. log [a Ca /a Al 2 # a O 2 ]

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The phase diagram of the CaO-SiO2-CrOx-MgO-MnO system at moderately reducing oxygen partial pressure was calculated using a commercial thermochemical program and compared with the phase analysis for converter slags taken from the stainless steelmaking process. It was found that a (Mn,Mg)Cr2O4 solid solution and Ca2SiO4 phase are in equilibrium with the oxidation slags after the oxygen blowing period, which is consistent with thermodynamic calculations. Furthermore, it could be proposed that the thermodynamic properties of a MgCr2O4-MnCr2O4 binary spinel make it close to an ideal solution.