In the master's thesis titled "Optimisation of VOD slag composition to obtain good castability of MKM CrMoW22 ferritic stainless steel in continuous casting process" we addressed the issue of castability of the MKM CrMoW22 steel on the continuous casting machine. This is a demanding ferritic stainless steel alloyed with chromium, molybdenum, and tungsten, and is used as an interface in solid oxide fuel cells. Poor castability is a result of clogging in the continuous casting nozzle between the tundish and the mould. The reason for this is the presence of non-metallic inclusions in the steel. These settle on the inner surface of the nozzle, reducing the cross-section and thereby impede the flow of the melt. We focused on the steel production process on the VOD device and the slag analysis during production, in combination with the chemical composition of the steel and the non-metallic inclusions present in the melt. We found a solution in the proper optimization of the slag composition formed during the secondary metallurgy, which was achieved with the addition of Calumet 70, as well as increasing the elements that reduce the presence of oxygen in the melt.
We compared the production process of four individual heats of MKM CrMoW22 steel. Slag samples were taken at different points of production for each heat. Liquidus and solidus temperatures were determined for heats 320753 and 323276 by DTA analysis. We compared the chemical composition from the end of the VOD process with samples taken from the solid slabs. The chemical analysis among the heats differed mainly in aluminum content, which varied from the first to the fourth heat, along with an increase in the yield of lanthanum and the success rate of the casting. Correspondingly, the content of aluminum oxide (Al2O3) in the slag increased, and was saturated with it by the end of the production of heat 323276. Through automated analyses of inclusions on a scanning electron microscope, we classified and counted the most alumina inclusions on the sample taken prior to casting, most of which contained lanthanum as well. The optimal conditions for achieving good castability were found with a combination of slag saturated with Al2O3 and an aluminium content of 0,253 % w/w in the melt.
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