Thermodynamics on the formation of spinel (MgO center dot Al2O3) inclusion in liquid iron containing chromium

Abstract

The stability diagram of MgO, spinel solid solution (MgO.(AlXCr1-X)(2)O-3), and sesquioxide solid solution ((AlgammaCr(1-gamma))(2)O-3) as a function of Mg, Al, and O contents at a constant chromium content (18 mass pct) in liquid iron is drawn at 1873 K. The interaction parameters between Mg and other solutes (Al, Cr, Ni, Ti, Si, and C) are determined by the experimental method, which assures equilibrium between Mg vapor and liquid iron, were applied to calculate the diagram. Titanium deoxidation is not recommended for the prevention of spinel formation, because Ti accelerates Mg dissolution from Ti refractory or slag due to its high affinity for Mg (e(Mg)(Ti) = -0.64). The standard Gibbs free energies of formation for the three inclusions (periclase, spinel, and sesquioxide solid solutions) and the tielines between two solid solutions were calculated with the aid of the regular solution model and the thermochemical F*A*C*T database computing system, respectively. The phase stability regions and oxygen content in steel for the current Fe-Mg-Al-Cr (18 mass pct)-O system are compared with those of the previous non-Cr system. Detailed information on the spinel composition according to Mg and Al contents is also available from the present stability diagram.