Observational Study of Clogging Specimens from the Tundish Well Showing Origin and Growth of a Clog in an Al-Killed Ti-A
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IN the production of Al-killed steel grades, non-metallic inclusions (overwhelmingly alumina Al2O3) cause the impediment and ultimately blockage of flow in critical places of the casting installation, which is ubiquitously known as clogging phenomenon. While research on causes and countermeasures to clogging is ongoing since over 40 years,[1–5] still very little is known about the actual formation and aggregation processes of clogging deposits from observations in the industrial practice.[6–8] Most current research on the formation of clogging deposits assumes the operation of certain physical processes and explores their development over
BERNADETA KARNASIEWICZ is with the Niebylec, Poland and also with Tata Steel, R&D, P.O. Box 10,000, 1970 CA IJmuiden, The Netherlands ENNO ZINNGREBE is with Tata Steel, R&D. Contact e-mail: [email protected] Manuscript submitted December 13, 2018. Article published online June 10, 2019. 1704—VOLUME 50B, AUGUST 2019
time using various types of modeling.[9–13] Overall, the suggested mechanisms for the clogging origin fall into two conflicting hypotheses: 1. attachment of particles existing in the steel bath to the refractory or to pre-existing clogging deposit at the location of the clogging growth (depositional models[2,10,12–15]); 2. formation of new in situ particles at the location of the clogging deposit, due to chemical reactions such as reoxidation (sidewall reaction models[4,5,16–19]); both eventually causing the termination of liquid steel casting. Clogging by alumina particles is known to occur in two main structural types: – in the form of alumina powder along refractories and in powder-filled voids (gas bubbles[2,4,20,21]), most often in the Submerged Entry Nozzles; – as alumina grain aggregations within the steel matrix, which is often encountered in ladle or tundish nozzles
METALLURGICAL AND MATERIALS TRANSACTIONS B
and other surfaces of the casting installations, especially in the casting of Ti-alloyed steels.[1,2,7,14,16,22] While the contrast between these two forms of clogging deposits is well known in the industrial practice, the reasons for their respective occurrence and distribution in conventional casting systems are not well understood. For loose or only slightly sintered powder of alumina grains as often observed in SENs, reference is often made to the non-wetting character of the steel-to-alumina contact leading to phase separation of inclusions from liquid metal especially as argon is added as anti-clogging countermeasure through porous nozzles. Yet, in the same casts, fully metal-penetrated, aggregated alumina networks are found covering the nozzles at which argon is injected. Given this continuing lack of clarity on the microphysics underlying this difference, there have been renewed efforts to study the microstructures and formation of clogging deposits from metallographic specimens either of experimental samples[7] or of post-mortem samples directly from the industrial process,[16,23] making use of the improved capabilities of modern c
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