Thermal measurement and degradation quantification of teeming ladle refractories and the effects on the process

Abstract

The key objective of the thesis was to quantify the heat loss caused to the liquid steel due to the cooling effect of the teeming ladle refractories. It was previously hypothesised that the in-situ degradation of insulation layer would increase this cooling effect. To determine the cooling effect of the degraded insulation material it was first thermally characterised with in-situ thermocouple measurements. Post-mortem samples were recovered from the teeming ladles used for the thermocouple measurements during their regular production cycles in a BOS plant. The post-mortem samples were then tested for their thermophysical properties. From this it was possible to determine the density increased from 260kg/m3 to 759.6 kg/m3, the thermal conductivity increased from 0.039W/m.K to 0.15W/m.K and the specific heat capacity decreased by 40% compared to its original state. These findings were then used to calculate the increased heat loss rate of the refractory material in the teeming ladle, which then in turn causes increased heat loss to the steel transported by the ladle. A thermal model was used to determine the heat flux stored in a fully saturated ladle and then different time periods of cooling with and without a lid. The effect of teeming ladle lids reduced the heat losses by up to 11°C per cycle compared to a ladle without a lid. Whereas the heat loss due to the insulative layer degradation was calculated to be <1°C for the initial heats before the ladle reached production temperatures and, therefore, had minimal effect. However, the degradation did show an increase in teeming ladle shell temperatures, which needs to be taken into account for service temperature monitoring. The thermal profiles of the modelled scenarios showed that if an accurate hot face measurement could be achieved it would be possible to accurately predict the cooling effect of each teeming ladle in production. This study was able to accurately measure the refractories and slag taken from a teeming and utilise the geometry of the ladle to reduce the error from thermal imaging. Previously predictions were used that could cause errors up to ±175°C when taking thermal images of the teeming ladle hot face. Through the method adopted in this study it was possible to take accurate measurements of the hot face within ±5°C. This can now be utilised by a thermal model to make accurate real time predictions of the heat loss caused by teeming ladle refractories. Thereby reducing the reheating required and improving the quality of steel produced.