Fundamental investigations on high temperature behaviour of continuous steel casting metal delivery refractories in the presence of liquid slags

摘要

Employed either in metal holding ladles/ tundishes, stopper rods, or metal delivery shrouds/ nozzles; the service life of refractory bodies plays an important role in the commercial feasibility of continuous casting of steel. A suitable selection of oxide–graphite refractory material for a specific application is usually based on the fundamental understanding of its high temperature corrosion behaviour. Slag attack is particularly important. The structural strength of refractory may be critically reduced by the solvent action of liquid slags. Typically, in the conventional method of the continuous casting of liquid steel whether refining slags in the metal holding ladle, or casting powder in mould – they are almost always composed of high basic oxide constituent (CaO) concentration. Therefore, corrosion studies having CaO as one of the constituent oxides, has been the subject of many research work. However, with the advent of modern near–net–shape continuous casting of steel not only thin walled refractory bodies are being employed but role and composition of meniscus slags has also changed making the control of corrosion a big challenge. The lack of available information on the corrosive action of the menisci slags of the modern continuous casting of steel, which are usually composed of high MnO and SiO2 contents (having no or very little concentration of CaO), on the refractory bodies needs to be thoroughly studied. In general, uncertainties also exist regarding the fraction of graphite content in the refractory mix because an increase of the graphite content of refractory does not always minimize corrosion process.An experimental study, using static corrosion test, was conducted at the usual steel casting temperature (1550 C) to develop a fundamental understanding by comparing the transient progresses of corrosion reactions of several commonly available oxide–graphite refractory in their interaction with menisci slags that are generated during the near–net–shape casting of liquid steel.Factors such as the presence of solid fractions and the low wetting characteristic of molten slag phase seems to promote the entrapment of gas bubbles at theviinterface, which was found to favourably impact the corrosion resistance property of refractory by minimizing its contact with the corrosive slag phase. MgO–graphite refractories showed higher resistance to the corrosive actions of both the menisci slags due to the formation of two inhibitive layers: a thin layer of dense MgO and Olivine phase. It has also been found that the corrosion performance of some oxide–graphite refractory completely changes with the change of meniscus slag composition. Exhibiting different corrosion behaviours against the two menisci slags, refractories composed of (Y–PSZ) ZrO2 as oxide aggregate phase while performed poorly in interaction with ternary meniscus slag, a satisfactory corrosion performance of the refractory was noted when it interacted with binary meniscus slag composition. Examining the effect of graphite content on the corrosion performances of refractories by ternary meniscus slag (MnO–SIO2–Al2O3), we found that while an inverse relationship existed for Al2O3–graphite refractory, the corrosion performance of MgO–graphite refractory improved when these were composed of higher graphite contents.