Microstructures and Formation of Tundish Clogging Deposits in Ti-Alloyed Al-Killed Steel
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ONE of the longstanding problems in the production of steels is the occurrence of clogging of the continuous casting installations during casting of liquid steel. In aluminum killed steels, the clogging most often consists of alumina (Al2O3) accumulations found at various locations in the continuous casters, most notably in the ladle shroud, the tundish well, and/or the submerged entry nozzles or shrouds.[1–5] The fundamental mechanisms, to which the formation of inclusion accumulations are generally attributed, fall into two main groups. Since the steel is strongly non-wetting to most oxide surfaces, it is often assumed that pre-existing particles floating in the
ENNO ZINNGREBE, JAMES SMALL, SIEGER VAN DER LAAN, and ALBERT WESTENDORP are with Tata Steel, R&D, P.O. Box 10000, 1970 CA IJmuiden, The Netherlands. Contact e-mail: [email protected] Manuscript submitted July 31, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
steel liquid stick to refractory. This type of mechanism is ‘‘depositional’’, meaning the particles are not formed at the clog location (e.g., Refs. 2, 6 through 8). A second mechanism involves reoxidizing steel sidewall reactions, in which oxygen is locally added to the steel (from gases such as CO or ambient air, or from reducible oxides in the refractory). This oxygen then precipitates new alumina particles by recombining with the dissolved aluminum in the steel, creating a clog in situ. This mechanism is independent of the amount of previously existing particles in the steel melt (such as alumina from deoxidation) (e.g., Refs. 3, 4, 9 through 12 ). Both mechanisms can also be combined in the generation of a particular clog. However, there is a lack of studies that demonstrate the actual operation of either mechanism from observations of the real casting process. It is well known that Ti-alloyed Al-killed steels are especially sensitive to clogging (e.g., Ref. 13). It is not generally permissible to assume that the mechanisms leading to clog formation are the same in the different parts of the caster system. Specifically, it has to be expected that mechanisms of clog formation at ladle
Table I. Element ppm
Typical Composition of the Ti-Alloyed Al-Killed Steel Casts
C
N
S
Al
Si
Ti
Mn
15 to 20
20
70
200 to 400
30 to 50
400 to 500
1100
well/gate (including well filler sand); at tundish bottom and well; and in the SEN respectively are qualitatively different phenomena. Most published research on clogging deals with clogging in the SEN. From an operational standpoint however, a clogged SEN can be changed without significant interruption of the casting series. Clogging in the tundish terminates a casting series, and therefore has a larger operational effect, but is far less studied in the literature because of the difficulty of obtaining post-mortem samples. The tundish well is also where Ti-alloyed Al-killed steels tend to build-up a growing clog over many heats, limiting casting series lengths.[13] Therefore a research program has been carried out at Tata Steel Europe, IJm
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