Thermodynamics of the Formation of MgO-Al₂O₃-TiO _x Inclusions in Ti-Stabilized 11Cr Ferritic Stainless Steel

Abstract

The equilibration between CaO-SiO₂-MgO-Al₂O₃-CaF₂ (-TiO₂) 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]/(Al₂O₃) and that from [Si]/(SiO₂) 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-Al₂O₃-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 ${\left( {{X_{{{\text{MgO}}}} \times X_{{{\text{Al}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} } \mathord{\left/ {\vphantom {{X_{{{\text{MgO}}}} \times X_{{{\text{Al}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} } {X_{{{\text{Ti}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} }}} \right. \kern-\nulldelimiterspace} {X_{{{\text{Ti}}_{{\text{2}}} {\text{O}}_{{\text{3}}} }} }} \right)}$ of the inclusions (spinel potential) was expressed as a linear function of ${\left\lfloor {{a_{{{\text{Mg}}}} \times a^{2}_{{{\text{Al}}}} \times a_{{\text{O}}} } \mathord{\left/ {\vphantom {{a_{{{\text{Mg}}}} \times a^{2}_{{{\text{Al}}}} \times a_{{\text{O}}} } {a^{2}_{{{\text{Ti}}}} }}} \right. \kern-\nulldelimiterspace} {a^{2}_{{{\text{Ti}}}} }} \right\rfloor }$ 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 Al₂O₃ is less than that of TiO₂ in the slag saturated by MgO, whereas it linearly increases by increasing the $\log \;{\left( {{a_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } \mathord{\left/ {\vphantom {{a_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } {a_{{{\text{TiO}}_{{\text{2}}} }} }}} \right. \kern-\nulldelimiterspace} {a_{{{\text{TiO}}_{{\text{2}}} }} }} \right)}$ at ${\left( {{X_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } \mathord{\left/ {\vphantom {{X_{{{\text{Al}}_{{\text{2}}} {\text{O}}{}_{{\text{3}}}}} } {X_{{{\text{TiO}}_{{\text{2}}} }} }}} \right. \kern-\nulldelimiterspace} {X_{{{\text{TiO}}_{{\text{2}}} }} }} \right)} > 1$.