On Physico‐Chemical and Technical Limits in Clean Steel Production

Great progresses in steel cleanliness have been attained during the last decades. In the measures of oxygen the lowest levels are approaching 5 ppm in O_tot i.e. close to the thermodynamic limit of Al deoxidation. As the thermodynamics of the reaction system is firmly anchored to the steel chemistry and thus to the properties of steel and the final product, it is useful to examine the thermodynamic constraints for selected steels in the refining and casting processes. In most steels Al‐O equilibrium determines the limit of deoxidation. Calculations by applying thermodynamic software showed that in selected “common” steels the equilibrium oxygen content varied from below 5 ppm up to 30 ppm or higher depending on the aluminium content, interaction effect of other alloying elements and temperature. For lower oxygen content there are several possibilities. The first one is to use stronger deoxidizing additions like Ca, Zr, Ce etc. However, they can be problematic as substitutes as they produce inclusions which influence steel properties. The second way is to intensify the deoxidizing power of certain elements e.g. Si by bringing the steel in intimate contact with a proper slag with low SiO₂ activity in ladle treatment with violent stirring. In the calculation example it was possible to decrease oxygen content from 30 ppm to the level below 10 ppm in C/Mn/Si steel at very low Al level. The third potential but unused process is vacuum deoxidation especially for medium and high carbon steels. Equilibrium oxygen contents below 1 ppm are thermodynamically easily attainable. But in practice the vacuum treatment should be designed to intensify the carbon‐oxygen reaction on the top surface of the stirred steel liquid. Also the eventual reactions with refractory materials should be suppressed.