Inclusion evolution in molten and solidifying steel

Cleanliness, with respect to impurities and nonmetallic inclusions in the melt, is an important issue in steel production. The commercial interest in controlling steel cleanliness has been growing rapidly, because clean steel exhibits a highly attractive combination of corrosion resistance, good formability, pleasing appearance, and a wide range of strength levels. In order to satisfy the requirements for the degree of cleanliness in steel, controlling the size distribution, chemistry and shape of inclusions are of great importance in the steelmaking process. A knowledge of the formation of nonmetallic inclusions and their chemical and morphological evolution during the steelmaking and casting process is necessary in order to minimize the inclusion size and also try to promote potentially beneficial properties of inclusions, such as grain-refining.; In this research, the evolution of inclusions in molten and solidifying steels was investigated through in-situ observations using a high temperature Confocal Scanning Laser Microscope (CSLM). The study focused on solid Al₂O₃ and liquid Al ₂O₃-CaO inclusions on low carbon steel melt surfaces.; Firstly, the agglomeration and clustering of inclusions on steel surfaces were quantified and compared to predictions according to capillary depression driven attraction forces. A strong agglomeration was observed between the solid Al₂O₃3 particle pairs. However, the liquid Al ₂O₃-CaO inclusions were not prone to agglomeration due to their lens-like morphology, which causes the absence of capillary force.; Secondly, the pushing vs. engulfment and entrapment of both liquid Al ₂O₃-CaO and solid Al₂O₃ inclusions by advancing planar and cellular δ-ferrite solidification fronts was studied and compared to model predictions based on the force balances acting on the inclusions at the solid/melt interface. The critical velocity, above which the inclusions get engulfed, was observed to be slower at the cellular front than at the planar interface for liquid Al₂O₃-CaO inclusions. This indicates that these inclusions tend to more easily get engulfed at inter-cellular boundaries. However, there was no appreciable difference observed about the critical velocity for the large solid Al₂O ₃ inclusion clusters at the cellular boundary with that at the planar interface.; The pushed liquid Al₂O₃-CaO inclusions were subject to a chemical and morphological change during solidification. (Abstract shortened by UMI.)...