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We report an investigation on the corrosion behavior of ZrO 2 -graphite insert in submerged entry nozzles (SEN) during its high temperature interaction with the mould flux focused specifically on the role of glaze coating. A detailed metallographic examination of the representative samples collected at different stages of its usage was carried out to examine the transient progress of corrosion. The interaction between the glaze material and the base refractory at the typical preheating temperature of SEN initiated the degradation of the partially stabilized ZrO 2 grains due to the formation of reaction products. Clear evidence is presented to show that the reaction product and the original layer of glaze coating survived after 90 min of casting. Early stages of corrosion involved the progressive congruent dissolution of reaction products and the residual glaze phase into the mould flux. Due to the absence of the glaze layer in the later stages of operation (480 min), corrosion was caused by the fragmentation of the aggregate phase and structural damage. This work highlights the importance of compositional control and coating thickness of the glaze material on the ZrO 2 -graphite refractory insert.

IntroductionRefractory degradation is a complex phenomenon involving chemical wear (corrosion) and physical/mechanical wear (erosion/abrasion). Refractory corrosion results in the recession of "hot face" (working surface) due to chemical reactions and molten metal penetration. 1) Alumina-carbon refractories are commonly used in steelmaking applications due to their excellent high-temperature strength and thermal shock resistance. [2][3][4][5] The corrosion resistance of Al 2 O 3 -C refractories is influenced by its reactions with liquid iron, which leads to the dissolution of refractory constituents into the melt. 6) Carbon in the form of graphite is used in refractories since it enhances corrosion resistance and thermal shock resistance. 7) However, the degradation of carbon-based refractories occurs through carbon depletion, resulting in decreased refractory resistance to chemical attack, increased carbon pickup by steel and extensive metal penetration within the refractory. 8) Material inhomogeneity and porosity can promote corrosion and allow corrosive agents to penetrate and cause erosive damage, resulting in severe degradation. 9) Iron oxide formed can react with alumina to form hercynite (FeO-Al 2 O 3 ), which builds up in the refractory over time, eventually causing uncontrolled expansion of saturated refractory and hot face spalling. 10) The corrosion of alumina-carbon refractories is primarily influenced by their chemical reactions with molten iron/steel, which proceed through a direct contact with the metal. The composition of the refractory, its physical texture, the nature of the bonding phases, and the characteristics of melt and reaction products are known to influence reaction kinetics. 11) Refractory-iron reactions lead to molten metal penetration; the penetration depth is believed to be influenced by the chemical composition of the refractory, metal and physical properties like porosity of the refractory. The penetration depth may range from a few microns to several millimetres depending upon these factors, and the quantity of molten metal. 1) Carbon is picked up by Fe resulting in carbon dissolution from the refractory and the alumina left behind tends to resist Fe penetration. The regions which experience carbon depletion are subsequently filled with molten iron, which contributes to the corrosion. In the liquid iron/alumina-graphite system, carbon transfer from graphite to liquid iron is a well known phenomena, while alumina is considered to be unreactive to iron at steelmaking temperatures (1 550°C). 12) The influence of wettability on carbon dissolution into molten iron suggests that the rate of dissolution is significantly affected by the contact area available for the carbon-transfer. 13) The depth of penetration of molten iron into the refractory and the strength of bonding between the metal and the refractory 804 © 2010 ISIJ ISIJ International, Vol. 50 (2010), No. 6, pp. 804-812 The interfacial behaviour of alumina/carbon refractories with liquid iron was investigated at 1 550°C, w...