Abstract
The mechanism of the accumulation of large non-metallic inclusions in the bottom portion of 23 ton Al-killed ingots has been investigated by changing the superheat in the vicinity of the meniscus of the melt in the ingots during the early period of solidification. The change has been made by applying either slow-burning conventional exothermic or newly developed quick-igniting highly exothermic hot-topping powder under a favorable condition that downward convectional metal flow along the solidifying front is not influenced by the change of the superheating.
With the conventional exothermic powder, marked development of the mushy-zone in the beginning of solidification is observed. The development is found to be due more than 70% to the showering of iron globulites from the vicinity of the meniscus, the balance to that from the solidifying fronts on the side walls. The showering of the globulites in cluster-like agglomerates is quantitatively shown to entrap floating large inclusions, pulling them down onto the mushy-zone.
Application of the quick-igniting powder is shown to retard the initiation of the showering from the meniscus, allowing the inclusions to float up, hence resulting in virtually no accumulation of inclusions in the mushy-zone.
