Contribution to Deoxidation of Austenitic Steels in a Vacuum Induction Furnace with Carbon

Deoxidation of steel with carbon under reduced pressure is often used for increasing the steel purity. Suitable units for this purpose in foundries are vacuum induction furnaces. Possibilities of increasing the steel purity by deoxidation with carbon in the vacuum induction furnace were studied for the steel for the petrochemistry of specific composition 25Cr/35Ni. The charge composed of the return material only was melted in the air. During melting the charge oxidized and the oxidizing slag formed. Chemical composition of steel, morphology, chemical composition of inclusions in the steel and chemical composition of slag after vacuuming were studied on the basis of samples taken before and after vacuuming. Temperature and oxygen activity were measured before and after vacuuming. Globular inclusions with dominant content of silicon and manganese were observed in steel before and after vacuuming. Contents of total oxygen in steel didn’t change significantly during vacuuming. On the basis of composition of inclusions and measured oxygen activity the activity of Cr₂O₃ in inclusions was calculated. A slag sample was taken after vacuuming and equilibrium oxygen activity in steel with regard to the Cr₂O₃ content in the slag was estimated from the slag composition. Equilibrium oxygen activity in relation to the Cr₂O₃ content in the slag was higher than equilibrium activity measured in the steel. For this reason it is not possible, under the studied conditions, to decrease oxygen content in steel during vacuuming.