Understanding the thermal behavior of mold powders during continuous casting of steel is essential to deliver a good quality final product. Although a considerable number of studies have been conducted to evaluate such properties, some disagreements still persist.
In this study, the Cold Finger technique was used to better estimate the thermal properties of industrial mold powders used in the steel industry. However, because the results of the traditional Cold Finger experimental route are greatly affected by convection in the melted slag, opposite to heat transfer in the mold, a new experimental approach was designed.
While only the measurement of heat flux is not enough to characterize the thermal properties of mold powders, it was observed that the thickness of the slag film recovered after each test depends on the slag composition and the temperature of the liquid slag. Hence, after several trials, an improved Cold Finger experimental route was achieved. In this new route, three different temperatures of the melted slag were used for each sample, a new probe was designed to improve the consistency of the slag films, and an appropriate experimental time was chosen to ensure closer approach to steady state. The details regarding this new approach are discussed in the Chapter 5 of this document.
Several mold powders used in the industry were tested using the new experimental route proposed for the Cold Finger technique. The results shown in Chapter 6 indicate that besides the chemical composition of the powders, the temperature of the melted slag is also responsible for changes in the interfacial resistance of the solidified slag film, hence temperature experienced by the flux during casting can influence its thermal properties.
Typical mold powders used in continuous casting have a significant amount of fluorine ranging from about 5 wt.% to 10 wt.%. However, given environmental and safety concerns that arise from the evaporation of fluorine, several efforts have been made to substitute regular industrial mold powders with fluorine-free versions. In chapter 7, two different samples of fluoride-free mold powders were tested with the new Cold Finger technique approach. The results indicate that, although these samples can substitute low basicity mold powders, the lack of crystallization observed might be a problem to substitute mold powders used in the casting of crack sensitive steel where, in order to prevent surface defects, heat withdrawn in the caster needs to be reduced.
In chapter 8 of the document, for two of the regular samples, a comparison between SEM images of slag rims recovered during the continuous casting of steel and slag films recovered after the Cold Finger tests showed similar microstructure and phase formation. In the same way, for the same samples, the thermal properties estimated using data from the casting process and the results from the Cold Finger test are quite similar, indicating that the new technique presented is a suitable tool to estimate the thermal properties of mold fluxes.
In summary, the new approach proposed in the present work enables better characterization of the thermal properties of silicate melts and can be a useful tool to deeper understand the thermal behavior of mold fluxes.
|Advisor:||Pistorius, Petrus C.|
|School:||Carnegie Mellon University|
|School Location:||United States -- Pennsylvania|
|Source:||DAI-B 78/08(E), Dissertation Abstracts International|
|Keywords:||Casting, Cold finger, Heat flux, Mold flux|
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