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Austenite grain growth does not only play an important role in determining the mechanical properties of steel, but certain surface defects encountered in the continuous casting industry have also been attributed to the formation of large austenite grains. Earlier research has seen innovative experimentation, the development of metallographic techniques to determine austenite grain size and the building of mathematical models to simulate the conditions pertaining to austenite grain growth during the continuous casting of steel. Oscillation marks and depressions in the meniscus region of the continuously casting mold lead to retarded cooling of the strand surface, which in turn results in the formation of coarse austenite grains, but little is known about the mechanism and rate of formation of these large austenite grains. Relevant earlier research will be briefly reviewed to put into context our recent in situ observations of the delta-ferrite to austenite phase transition. We have confirmed earlier evidence that very large delta-ferrite grains are formed very quickly in the single-phase region and that these large delta-ferrite grains are transformed to large austenite grains at low cooling rates. At the higher cooling rates relevant to the early stages of the solidification of steel in a continuously cast mold, deltaferrite transforms to austenite by an apparently massive type of transformation mechanism. Large austenite grains then form very quickly from this massive type of microstructure and on further cooling, austenite transforms to thin ferrite allotriomorphs on austenite grain boundaries, followed by Widmanstätten plate growth, with almost no regard to the cooling rate. This observation is important because it is now well established that the presence of a thin ferrite film on austenite grain boundaries is the main cause of reduction in hot ductility. Moreover, this reduction in ductility is exacerbated by the presence of large austenite grains.

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Die Betrachtung metallurgischer VorZusammenfassung: gänge bei Temperaturen von bis zu 1700 °C in einer infrarotbeheizten Hochtemperaturkammer durch ein Laser-Scanning-Konfokal-Mikroskop wird zunehmend zum Standardwerkzeug zeitgemäßer Stahlforschung. Im Rahmen eines Projekts am Leobener COMET K2-Zentrum wurde nunmehr durch das Materials Center Leoben ein Mikroskop angeschafft und -als erst drittes Gerät in Europa -am Lehrstuhl für Metallurgie der Montanuniversität installiert. Der vorliegende Artikel stellt die Methode und ihr Potenzial für die Stahlforschung vor und zeigt ausgewählte Beispiele für Untersuchungsmöglichkeiten aus der Literatur sowie erste eigene Ergebnisse. Im Besonderen wird auf die Beobachtung nichtmetallischer Einschlüsse in Stählen und Schlacken, auf die Vorgänge während der Erstarrung sowie auf Festkör-perphasenumwandlungen eingegangen. Weitere Anwendungsmöglichkeiten werden kurz angesprochen. First Results of the High-Temperature Laser Scanning Confocal Microscope at the Montanuniversitaet LeobenThe in-situ observation of metallurgical processes Abstract: at temperatures up to 1700 °C inside a mirror furnace with a laser scanning confocal microscope becomes more and more a standard tool for efficient steel research. Within the framework of a COMET K2-project the Materials Center Leoben acquired a microscope system. The system is installed at the Chair of Metallurgy and one of only three systems in Europe today. The article describes the principles and potential of method for steel research and presents selected examples for possible investigations from literature and some first own results. The focus lays on the behavior of nonmetallic inclusions in liquid steels and slags, processes related to solidification and phase transformations in the solid state. Other applications will be briefly mentioned.

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