The fluid flow in a continuous casting tundish affects the separation of non‐metallic particles and the cleanliness of the steel. Today, laser‐optical investigations of water models are state of the art and enable detailed information about the effect of baffles, i. e. dams, weirs and turbo‐stoppers, on the flow. In this work 3D‐LDA and 2D‐DPIV‐investigations for different turbo‐stoppers in a water model on a scale of 1:1.7 of a 16 t single strand tundish are presented. Three circular turbo‐stoppers are investigated. Detailed measurements of the mean velocity and turbulence intensity in the tundish with and without turbo‐stopper are shown. With a suitable turbo‐stopper geometry the recirculation area in the tundish centre and short‐circuit flows along the side walls can be avoided and thus more favourable residence time distributions can be obtained. It is shown that the turbo‐stopper produces higher turbulence in the inlet region of the tundish, which is spatially more limited, however, in relation to the flow without turbo‐stopper. Thereby a more homogeneous flow is created at the discharge of the tundish with better conditions for the particle separation. The experimental data yield a good understanding of the flow phenomena in a tundish with turbo‐stopper and are used as validating criterion for numerical simulations (Fluent 5.5) on the basis of the Reynolds equations. The turbulence modelling is based on a two‐equation model (realizable k‐ε model).
Reactions at the refractory/melt interface during ingot casting of Ni-and Ni-Fe-alloys were studied. The casts were performed using different alumino-silicate bricks as refractory materials. Samples taken from the casting channel before and after casting were investigated using light and scanning electron microscopy with XPS. Thermodynamic calculations were performed with FactSage and the results were compared with the results from industrial tests. After the melt has infiltrated the surface layer of the bricks, refractory corrosion starts with an attack of Mn and Mg of the melt on Si0 2 and Fe203 of the refractory bonding matrix. Despite the presence of elements with higher oxygen affinity in the melt, low-melting alumino-silicate phases are predominantly built by the reaction with Mn and Mg. In a second step this liquid phase either traps non-metallic inclusions from the melt or, at higher contents of Zr, Ti, Mg, Y etc. in the melt, causes massive reoxidation and inclusion formation. The refractory materials investigated show an increasing trend for reoxidation with an increasing amount of Si0 2 in glassy phases of the refractory bonding matrix. By the use of a refractory material with higher mullite content in the bonding matrix or by use of alumina bricks a strong reoxidation of the melt and intense inclusion formation can be avoided. These observations are also valid for other alloys with higher contents of elements with high affinity to oxygen.
Abstract. In technical solidification processes like Electro Slag (ESR) or Vacuum Arc Remelting (VAR), freckles present a serious type of defects which limit the maximum ingot size for many grades of steels and superalloys. Therefore, modelling of freckle formation is an important task for optimizing industrial remelting processes. Practically all present freckle models are based on a critical Rayleigh number. They are inspired by the "classical" assumption that freckles are caused by an inversion of the liquid density in the semisolid region. Plumes of the lighter segregated liquid evolve through perturbation of the metastable layering of the melt in the mushy zone, a mechanism which motivates its description via Rayleigh criteria. But these models are not suitable for materials like Alloy 718 which do not show a liquid density inversion, but nevertheless are prone to freckle formation in technical remelting processes. In this paper, a criterion is developed which -instead of using a Rayleigh number -is based on the evaluation of the non-isothermal component of an instantaneous down-hill flow of heavy segregated melt in a melt pool with axial symmetry. With knowledge of the exact pool geometry and the shape and properties of the mushy zone, the occurrence of freckles can be predicted. The model is applied to a technical ESR casting for which temperature fields and microstructural parameters have been obtained using CFD and 3D phase-field modelling, respectively. Furthermore, the implications are discussed which the new model offers for the understanding of freckles in technical remelting processes.
In the KCB-S process-a modified AODprocess-process
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