Zusammenfassung
Der konventionelle Stranggießprozess ist der dominierende Prozess für das Vergießen herkömmlicher Kohlenstoffstähle. Die Erstarrung im Stranggießprozess, vor allem beim Brammenstranggießen, ist durch eine moderate Wärmeabfuhr und entsprechend geringe Erstarrungsgeschwindigkeiten und Kühlraten gekennzeichnet. Die Kopplung des Gieß- und Walzprozesses oder das direkte Gießen eines Produktes verlangt die Erhöhung der Gießgeschwindigkeit und deshalb auch die Erhöhung der Wärmeabfuhr. Da die Phasenumwandlungen, die beim konventionellen Prozess während des Abkühlens und Wiedererwärmens auftreten, beim gekoppelten Gießwalzen fehlen, kommt der Erstarrungsstruktur auch eine größere Bedeutung für den nachfolgenden Walzprozess zu. Am Christian-Doppler-Labor für Metallurgische Grundlagen von Stranggießen wurde in Zusammenarbeit mit Siemens-VAI Metals Technologies ein Versuchsstand zur Nachbildung der beschleunigten Erstarrung entwickelt und umgesetzt. Das Experiment beruht auf einem Tauchversuch in einem Vakuuminduktionsofen unter kontrollierter Gasatmosphäre. Die Apparatur wurde vor kurzem mit einem Pyrometer ausgestattet, um die Temperatur der erstarrten Proben während der Abkühlung bestimmen zu können. Auch das Nachstellen bestimmter Temperaturzyklen ist nunmehr möglich. Damit wurde es möglich, den thermischen Zyklus von Prozessen mit beschleunigter Erstarrung nachzuempfinden und die Auswirkung auf die Mikrostruktur und das Gefüge der Proben bei Raumtemperatur zu beurteilen. Abschließend wird exemplarisch ein Ergebnis für einen Kohlenstoffstahl vorgestellt.
Summary
Today, continuous casting is the common technology for casting commercial steel grades. Conventional casting processes, like slab casting, are characterised by a moderate heat withdrawal and a rather low solidification velocity and cooling rate. Linking the casting and the rolling process demands a higher casting velocity, an increased heat withdrawal and thus, a higher local cooling rate. The absence of phase transformations during cooling and reheating before the rolling process makes the solidification microstructure more important for the behaviour of the steel during rolling and for the final product properties. Over several years the Christian Doppler Laboratory for "Metallurgical Fundamentals of Continuous Casting Processes" has developed an experimental setup for the simulation of solidification at higher cooling rates. The experiment is based on the principle of a dipping test under inert gas atmosphere inside a vacuum induction furnace. Recently, this apparatus has been equipped with a pyrometer in order to measure the temperature of the solidified sample during the subsequent cooling phase and also with a furnace in order to simulate different cooling and heat treatment strategies. Thus, it is possible to reproduce solidification and subsequent cooling of the cast material in casting-rolling processes and to characterise the microstructure and the mechanical properties of the solidified samples. The present work will give an overview on heat transfer in conventional casting processes, present a laboratory scale simulation of solidification a higher cooling rate, touch some aspects like the numerical simulation of the experiment and conclude with some results and an outlook on further planned work.
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Linzer, B., Hohenbichler, G., Bragin, S. et al. Experimental Simulation of the Solidification of Steel at Higher Cooling Rates. Berg Huettenmaenn Monatsh 154, 498 (2009). https://doi.org/10.1007/s00501-009-0511-9
DOI: https://doi.org/10.1007/s00501-009-0511-9