The effect of carbon in slag on steel reoxidation by CaO-SiO2-Al2O3-MgO-MnOFetO slags

An equilibrium study was carried out at 1873 K to ascertain the effect of carbon in CaO-SiO2-Al2O3-MgO-MnO-Fet(t)O slag systems on their FetO and MnO activity coefficients, representing the slags' thermodynamic potential for steel reoxidation. Both γ(FetO) and γ(MnO) showed not only a sharp increment but also a simultaneous slow decrement by increasing carbon content in slag, suggesting opposite roles of the carbon according to its stable forms. XPS (X-ray photoelectron spectroscopy) was introduced to clarify the stable forms of carbon in slag. XPS results prove that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. It was concluded that carbonate ions increase γ(FetO) and γ(MnO), but that carbide decreases them. This paper suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (FetO + MnO) in slag to some allowable amount.