During the continuous-casting process, retarded cooling of the strand surface below oscillation marks and surface depressions results in the formation of coarse austenite grains. These coarse grains have proven to dramatically reduce the ductility of steel within the second ductility trough, and thus increase the risk of surface crack formation. In addition to the thermal history the composition of the steel, in particular the content of carbon and precipitation-forming elements, plays a decisive role in the development of the austenite grain size. The present work addresses the development and validation of an experimental and numerical model for predicting the austenite grain size in the continuous-casting process. In a first step, the previous austenite grain size on the surface of slabs was determined by metallographic examinations for several slabs with various carbon content. Next, a solidification experiment was adjusted in order to simulate the cooling conditions in the mold of a slab caster, but also to suppress the precipitation of nitrides and carbo-nitrides by subsequent accelerated cooling. Thus, it was possible to study the influence of steel composition on austenite grain growth at temperatures close to the solidus temperature, unaffected by precipitates. The results of both the plant and laboratory experiments point to a maximum austenite grain size with a carbon content of approximately 0.17 mass pct.The parameters of a grain size prediction model were fitted to the results of the experiment. The resultant model was coupled with a precipitation model and then applied to the slab casting process. The measured and calculated grain size values at the surface and immediately below the surface of the slabs agree very closely. The model was finally applied to calculate the grain growth in the center of a virtual oscillation mark under simplified assumptions. Although only the surface temperature in the mold diverges significantly from the original solution, the difference of the initial cooling conditions results in an increase of the final grain size by up to 40 pct.

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We present a study on temperature dependent spectroscopic data for Yb:KGW, Yb:KYW and Yb:YLF between 80 K and 280 K and Yb:YAP between 100 K and 300 K. Absorption and emission cross sections are determined. The latter ones are obtained by using a combination of the McCumber relation and the Füchtbauer-Ladenburg equation. Fluorescence lifetimes are measured within a setup optimized for the suppression of re-absorption and compared to the radiative lifetimes calculated from the previously determined cross sections to cross check the validity of the measurements. The cross sections are evaluated with regard to the materials' potential for supporting the generation of ultra-short laser pulses, low quantum defect lasing and requirements for suitable diode laser pump sources.

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For continuous casting of steel the flow inside the liquid core is important for the quality of the end product. Optimal velocities at the solidification front are desired to enhance a transition from columnar to equiaxed solidification. Other benefits are the reduction of inclusions and segregations. In-mould electromagnetic stirring (M-EMS) is a way to achieve these velocities. It is a widely used tool to modify the flow in round bloom strands and is used for most products at Voestalpine Stahl Donawitz. Because measurements at the plant are complicated and physical models are difficult or expensive to build, numerical simulations are nearly the only applicable way to gain insight in the flow in the liquid core of the strand. In this work the full coupling between flow field and magnetic field is considered. While the flow is calculated using a finite volume CFD-solver, the magnetic field is simulated with a finite element solver. The temperature distribution in the mould has a large impact on the shielding effect and hence modifies the magnetic flux density and Lorentz-force density inside the mould. The results will be compared with other (simpler) methods that are commonly used in the literature. Numerische Simulation des Kokillenrührens beim Rundstranggießen. Beim Stranggießen von Stahl hat die Strömung im flüssigen Kern einen großen Einfluss auf die Qualität des Endproduktes. Erwünscht sind Geschwindigkeiten an der Erstarrungsfront, die den Übergang von gerichteter zu globularer Erstarrung fördern. Weitere positive Auswirkungen sind die Reduktion von Einschlüssen und Seigerungen. Ein möglicher Weg um diese tangentialen Bewegungen zu realisieren ist das elektromagnetische Rühren in der Kokille. Es ist ein weit verbreitetes Werkzeug um die Strömung beim Rundstranggießen zu beeinflussen und wird für die Mehrzahl der Produkte bei Voestalpine Stahl Donawitz verwendet. Da Messungen im Stahlwerk kompliziert sind und der Aufbau physikalischer Modelle sehr schwer und/oder teuer ist, sind numerische Simulationen die nahezu einzige Möglichkeit, einen tieferen Einblick in die Stahlströmung zu erhalten. In dieser Arbeit wird die volle Kopplung zwischen Strömungsfeld und Magnetfeld berücksichtigt. Die Stahlströmung wird mit Hilfe eines auf Finiten Volumen basierenden Strömungssimulationsprogramms berechnet, wogegen das Magnetfeld mit der Methode der Finiten Elemente simuliert wird. Die Temperaturverteilung in der Kokille hat einen großen Einfluss auf die magnetische Abschirmwirkung der Kokille und ändert dadurch die magnetische Flussdichte und des Weiteren die Lorentz-Kräfte im Strang. Die Ergebnisse der gekoppelten Simulationen werden mit anderen (simpleren) Methoden, die oft in der Literatur Anwendung finden, verglichen.

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