Abstract

During corrosion experiments on refractories for steel ingot casting, at the interface between refractory and steel, large ceramic aggregates were found. It seemed that the aggregates attached onto a thin layer. The purpose of this study thus is to investigate this supposedly reactive interfacial layer and its effect on the attached aggregates or inclusions for the subsequent application in steel melt filtration. Therefore, this study relates the oxygen content of the melt, the surface properties of the refractories and interface reactions as well as the amount and distribution of inclusions in the steel to each other.

The used steel was an ingot casting steel (17CrNiMo76) with a high Al content and high casting temperature of 1580 °C. The refractory crucibles were Al2O3, Al2O3-C, Al2O3 doped with ZrO2 and TiO2 (AZT), AZT-C and AZT-C with carbon nanotubes and alumina nanosheets. The corrosion experiments were conducted in a metal casting simulator under argon atmosphere. It was heated to the casting temperature, which was hold for 20, 30 or 60 min. About every 10 min the temperature and oxygen content were measured. For the investigation of the refractories mainly scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX) were used. The inclusions in the steel were determined by a special Aspex-SEM.

The oxygen contents in the steel melt were generally lowest when melted in carbon containing refractories. The course of the oxygen content at the casting temperature correlated with the temperature, except for the crucibles AZT and AZT-C with nanoscaled additives. Supposedly, additional chemical reactions took place. Furthermore, the correlation between the oxygen content and the total corrosion showed strong linearity. The mean oxygen content during the holding time correlated with the total area of inclusions. This means that the inclusions respectively observed aggregates were already formed at this temperature. As the inclusion situation correlates with the mean oxygen content, the conclusions drawn from the mean oxygen content apply also on the inclusions: Carbon bonded refractories lead to considerably lower inclusions and an increasing total corrosion leads to increasing inclusion areas. In the carbon containing samples with increasing reactivity of the system by additional dopants and nanoscaled additives, the attachment of aggregates (based on alumina) increased as well as the formation of the thin layer (also mainly alumina). As also the Al contents in the steel were strongly reduced after the melting experiments compared to before, it seems that the inclusions precipitated.

Consequently, an increasing reactivity of the system by adding nanoscaled additives and dopants to the alumina increased the efficiency of collecting aggregates. Thus, AZT-carbon with multi-walled carbon nanotubes and alumina nanosheets has a high potential for steel melt filtration.