The carbon solubility of the CaO-Al2O3-FeO-SiO2-MgO slag, the main system formed in the
Ruhrstahl-Heraeus (RH) vacuum degasser, was measured at 1873 K to understand the effect of
slag compositions on the carbon contamination in the ultra-low carbon (ULC) steels. The stable form
of carbon was demonstrated to be carbonate by plotting the log [(wt%C)/aC] ratio against the log
p(O2) which ranged -11 < log p(O2,atm) < -9. The carbon solubility of the CaO-Al2O3-FeO-SiO2-MgO
slag can be estimated using thermodynamic model. The distribution ratio of carbon increased with
increasing concentration of FeO due to the FeO reduction reaction and increased basicity of slag.
The carbonate capacity of the CaO-Al2O3-FeO-SiO2-MgO slag can be expressed as a linear function
of the activity of lime and the optical basicity.