The Role of Mullite-based Refractory Chemical Interactions on the Formation of Exogenous Non-metallic Inclusions

Abstract

have the most deleterious effect on steel properties, becoming the major cause of failure of mechanical components in service. These inclusions are usually entrapped in steel during teeming and solidification and tend to concentrate in the regions of the steel section that solidify most rapidly or in zones from which their escape by flotation is in some way hampered; in the case of forging ingots this region corresponds to the ingot bottom. The majority of exogenous inclusions originates from mould fluxes entrapment, reoxidation processes, which occur when molten steel comes into contact with external sources of oxygen, such as casting atmosphere, and refractories wear. Considering the large amount of inclusions sources, it is sometimes very difficult for a steelmaker to assess what is the precise origin of macroinclusions, in order to improve the process quality. This investigation is also complicated by the fact that the chemical characterization of macroinclusions is not easy, because the inclusions have large size and multiphase composition, with a chemical analysis which can change from point to point. For these reasons a careful analysis of this matter is always needed. In the present work a precise characterization of macroinclusions which came from refractories, occurred in forging steel ingots, was carried out by means of SEM-EDS and non conventional X-Ray Microdiffraction technique. In order to explain the origin and the chemical evolution of these inclusions the analysis of ex-service refractory materials was carried out and a thermodynamical model taking in account the steel-refractory interactions was implemented.