In steel ingot casting, moulds are served via a refractory pipe system embedded in the ground, which is required to offer chemical inertia against the steel melt and excellent resistance against thermal shock and erosion. However, state of the art refractory clay, in particular free silica, interacts with the steel melt leading to the formation of non-metallic inclusions. While this is acceptable for low grade steel products, higher grade steel products require cutting-edge steel cleanliness. This study presents an alternative composite refractory material based on renewable resources, i.e. rice husk ash. Due to the high thermal insulation of the substrate material, the application of such material significantly improves heat- and energy balance during ingot casting. However, the chemical inertia against the steel melt has to be achieved by a functionalised ceramic coating via flame spraying.
This study investigates the feasibility for applying flame-spray coating on three different grades of rice husk ash substrate materials. The layer bonding of the applied coating is tested using pull-off tests. Moreover, the thermal expansion behaviour of the substrate materials and the ceramic coating was investigated by means of dilatometry measurements. In addition, high-temperature phase evolution during heating of the flame-sprayed ceramic layer was investigated using X-ray diffraction.
Based on these results, coefficient of thermal expansion of the ceramic layer was calculated, which showed good agreement to the dilatometry measurements performed on as-sprayed specimens.
All three grades of substrate materials were successfully coated by a ceramic layer via flame spraying. First pilot trials for the composite refractory material in contact with liquid steel were carried out by means of immersion trials in a steel casting simulator.