The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Study on the Layers in the Film Originating from the Casting Powder Between Steel Shell and Mould and Associated Phenomena in Continuous Casting of Stainless Steel
Paavo Hooli
Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Materials Science and Engineering for public examination and debate in Auditorium V1 at Helsinki University of Technology (Espoo, Finland) on the 16th of November, 2007, at 12 o'clock noon.
Dissertation in PDF format (ISBN 978-951-22-9031-4) [6150 KB]
Errata (in PDF format)
Dissertation is also available in print (ISBN 978-951-22-9030-7)
Abstract
Studies were conducted at Outokumpu Tornio Works in order to examine the properties of films between the mould and steel shell during the continuous casting of stainless steel. Film sampling was carried out in the tail-out phase at the end of the cast. The films were subjected to chemical analyses and respective compounds and phases were distinguished. Local heat fluxes were measured with thermocouples and the resulting data was subjected to analyses.
No previous study utilising equivalent sampling and analysis has been reported. Several massive samples were collected, a number of which were several millimetres in thickness and covered virtually the entire width of the mould, descending 40cm below the meniscus. Analysis showed that the flux films could explain some of the very low, local heat fluxes that were observed.
The most typical feature in the sampled films was the layer dominated by a cuspidine phase. The crystalline structure of these films had either formed during solidification, or via the devitrification process. Following a high cooling rate and short residence time prior to sampling, glassy structures were found. Most of these findings accord well with the literature. Such crystalline structures are to be expected in these conditions, via either direct formation or devitrification. Evidence from the literature confirms that the layers in the films were formed during the longer period of the casting process and not during tail-out.
The most unexpected feature was the formation of a separate NaF layer, and the formation of elemental sodium (Na), and probably sometimes potassium (K), against the mould wall. The study found that the films can have residence times of as much as several hours on the mould wall, and complex structures containing several sub layers, voids and pores can develop. With the aid of temperature recordings, evidence of fracturing was found in the sampled films. Variation was seen in the appearance of surfaces on the mould side. There were variations between samples from different heats, and local variations in centimetre scale.
This method can be used when evaluating the functioning of different casting powders.
Keywords: mould powders, continuous casting, flux film, gap, slag phases, cuspidine
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© 2007 Helsinki University of Technology
Last update 2011-05-26