Specific types of non-metallic inclusions are known to act as heterogeneous nuclei for the formation of acicular ferrite, which provides excellent toughness. By increasing the amount of acicular ferrite in the microstructure, the properties of HSLA steels can be optimized significantly.Although the formation of acicular ferrite caused by heat treatments (thermomechanical treatments or welding) is quite well described in literature, there is less information to find about the formation of acicular ferrite immediately out of the liquid melt. Within the present study experiments on laboratory scale are carried out simulating the influence of cooling conditions and Ti-content on size, chemical composition and morphology of non-metallic inclusions and consequently on the amount of acicular ferrite. All experiments were carried out with a dipping test simulator enabling very well controllable cooling conditions. Optical microscopy in combination with special etching methods as well as SEM/EDS-analysis was used for microstructure and inclusion characterization.
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. Experimentelle Simulation der Erstarrung von Stahl bei hohenKühlraten. 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 Wärmeab-fuhr. 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 Temperaturzykle...
Abstract. Cooling strategies in continuous casting of steel can vary from rapid cooling to slow cooling, mainly controlled by adjusting the amount of water sprayed onto the surface of the product. Inadequate adjustment however can lead to local surface undercooling or reheating, leading to surface and inner defects. This paper focuses on cooling efficiency of Air-Mist nozzles on casted steel and the experimental and numerical prediction of surface temperature distributions over the product width. The first part explains the determination of heat transfer coefficients (HTC) on laboratory scale, using a so called nozzle measuring stand (NMS). Based on measured water distributions and determined HTC's for air-mist nozzles using the NMS, surface temperatures are calculated by a transient 2D-model on a simple steel plate, explained in the second part of this paper. Simulations are carried out varying water impact density and spray water distribution, consequently influencing the local HTC distribution over the plate width. Furthermore, these results will be interpreted with regard to their consequence for surface and internal quality of the cast product. The results reveal the difficulty of correct adjustment of the amount of sprayed water, concurrent influencing water distribution and thus changing HTC distribution and surface temperature.
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