Tracing the Origin of Non-ferrous Oxides in Lamination Defects on Hot-Rolled Coils: Mold Slag Entrainment vs Submerged E
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TRODUCTION
THE quality of flat steel products is often compromised by the presence of surface defects such as slivers. While many slivers are related to downstream strip rolling, a significant fraction is known to arise from operational faults or instabilities during casting of the steel. This is especially true for slivers containing an array of high-temperature oxide phases (other than iron oxides) or slags. The analysis of microstructure and chemical composition of such slivers, to identify the specific circumstances of process disturbance or entrapment, has a long tradition.[1] Microstructure investigations of various forms of slivers and related surface defects continue to be published regularly (e.g. References 2 through 5). The sources of non-metallic materials entrained in the steel shell at a continuous caster are usually subdivided[6] into ‘‘endogenous’’ and ‘‘exogenous.’’ Endogenous are non-metallic inclusions
SABRI SENGO and ENNO ZINNGREBE are with Tata Steel, R&D, P.O. Box 10000, 1970 CA IJmuiden, The Netherlands. Contact e-mail: [email protected] PATRICIA ROMANO TRIGUERO and FOKKO MENSONIDES are with Tata Steel, Strip Products Mainland Europe, P.O. Box 10000, 1970 CA IJmuiden, The Netherlands. Manuscript submitted May 26, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
(NMI) formed within the liquid steel during secondary metallurgy treatments, and exogenous are fragments of contact materials to the liquid steel that in some way get entrained. Among the latter, those commonly expected are refractory contact materials to the steel (such as SEN or other cast ware), or more often slags that are thought to become entrained into flowing steel liquid. The most relevant of these is mold slag formed from mold powder cover, for which a variety of different entrainment mechanisms at the slag/steel interface have been recognized (e.g., References 7 and 8) including direct capture by the growing shell tip (e.g., Reference 9). Given this variety of material origins and entrainment mechanisms of non-metallic material, it is of high importance from an operational viewpoint to be able to establish the likely active material and process source for a given occurrence of slivers from direct observations of these slivers, so that the operational causes can be addressed. Many attempts to do so rely on the chemical composition of the oxides (ferrous or ceramic) that are encountered, and/or their specific phase compositions.[10–12] This can become problematic if, as often observed, the encountered materials in the slivers are not a perfect match with the expected compositions of slags or refractories used in the casting system. It is further complicated by the fact that the contact materials themselves can change their composition and phase makeup due to interactions with steel during the cast.
Thus, the identification of specific material precursors causing defect formation in the rolling state is not a trivial task. In this study, we intend to show how detailed characterization and analysis of enclosed materials can p
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