Deoxidation mechanisms in liquid steel.

Quantitative chemical and scanning electron microscopical techniques have been employed to investigate the deoxidation kinetics and changes in oxidation product morphology in low carbon steel melts. The techniques have been used to study the deoxidation processes associated with aluminium, titanium, silicon, zirconium and a calcium-aluminium alloy.After the addition of the deoxidant, the total oxygen concentrations of all melts rapidly decreased corresponding with a decrease in the size and number of inclusions observed. This continued to a plateau level of total oxygen concentration and mean inclusion diameter.Samples removed from the melts prior to deoxidation were found to contain globular MnO-FeO inclusions. It was discovered that the morphological sequence for single element deoxidants involved a progressive evolution from liquid globular to solid spherical inclusions followed by polyhedral, dendritic and coralline morphologies. Finally, sintered agglomerates were formed when inclusion clusters collapsed. The extent to which the oxidation products went down the sequence depended on: the dissolution characteristics of the deoxidant; the thermodynamic affinity of the deoxidant for oxygen in the melt; the inclusion/melt interfacial energy characteristics; the refractoriness of the oxidation products and intermediate compounds; and the degree of turbulence experienced by the melt. Explanations have been postulated which elucidate the behaviour of the different deoxidants, as not all displayed the whole morphological sequence. Silicon deoxidation produced spherical silicates, whereas the zirconia inclusions were either spherical or dendritic and the titanium oxidation products had spherical or polyhedral morphologies. Aluminium exhibited all morphologies in the sequence. Deoxidation with the calcium-aluminium alloy was found to have preceded by a two stage process. The initial stage was dominated by the formation of aluminium rich solid oxides followed by the progressive reduction by calcium, resulting in an adhesive liquid calcium-aluminate surface coating.The role of refractory crucible as a collecting surface for the capture and removal of deoxidation products from the melt was investigated, which confirmed that the inclusions were generally incorporated into the low melting point matrix phases. Turbulence also increased the probability that emergence would take place at these capture sites.