Formation kinetics of iron oxide in mould flux during continuous casting

The present work aims to understand the mechanism and rate of iron oxide formation in molten mould flux initially having no iron oxide to discuss how to keep the mild cooling performance of the flux. The concentration change of iron oxides in mould fluxes was measured at 1 891 K as a function of time in variations of three physico-chemical conditions: (i) oxygen partial pressures in atmospheres (air and argon), (ii) SiO₂ activities in mould fluxes and (iii) oxygen concentrations in molten irons, the last being provided by Al-killed and non-killed operations on electrolytic iron samples. The following findings have been obtained: (i) iron oxide concentrations increase with holding time and reach ca 3.5 mass% within 1.8 ks, independently of atmospheres, (ii) higher SiO₂ activity leads to higher viscosity of mould flux and lower iron oxide concentrations and (iii) iron oxide concentrations decrease to ca 0.5 mass% due to lower oxygen concentrations in molten Al-killed iron. Thus, the following mechanism has been proposed: oxygen dissolved in molten iron is primarily oxygen source and reacts with iron to form iron oxides at the metal/flux interface, which oxides diffuse into molten flux phase. A kinetic discussion has given the total reaction rate constant as k = 7.5 . 10-6 cm·s-1 and suggested that the rate be dominated by iron oxide transfer through the boundary layer. To suppress iron oxide formation, additions of reducing agents would be more efficient than controls of oxygen partial pressures.