Some new aspects on inclusion separation in tundishes

Abstract

The removal of non-metallic inclusions due to buoyancy forces in tundishes in continuous casting systems is considered. The maximal theoretical removal rate is determined by the flow rate through the tundish, the magnitude of the tundish fluid surface area and the particle terminal rising velocity, depending on the particle size. Two reasons, why the particle separation is worse than the maximal possible one are an unsuitable fluid flow pattern and the turbulent particle diffusion. An analytical discussion of simple flow patterns (parallel flow with different velocity profiles, dead regions, swirling flows) and diffusion shows how they influence the particle removal. Using these simple considerations, it is demonstrated that the often used RTD (residence time distribution)-curves are inappropriate to estimate the particle separation in tundishes; only the direct measurement and CFD-simulation of particle removal should be used. The common CFD-methods are affected with numerical errors such as numerical diffusion for particle concentration simulations with finite volume methods and interpolation errors for particle trajectory calculations. These errors influence significantly the calculated particle removal curves; they are nonsystematic and difficult to quantify. Neue Aspekte zur Partikelabscheidung in Verteilern. Das Abscheidungsverhalten von nichtmetallischen Partikeln in StranggieBverteilern infolge von Auftrieb wird untersucht. Zunachst wird die maximal moqllchs Abscheidung bestimmt, die sehr einfach aus Durchsatz, Verteileroberflache und Partikeldriftgeschwindigkeit (abhanqlq von der PartikelgroBe) bestimmt werden kann. Dann werden zwei Ursachen untersucht, die tur eine nicht optimale Abscheidung verantwortlich sind: ungOnstige Stromungsform und turbulente Partikeldiffusion. Es wird mit analytischen Methoden aufgezeigt, wie sich verschiedene einfache Grundstromungsformen (Parallelstromunq mit verschiedenen Stromungsprofilen, Totraurne, Drallstromungen) und Diffusion auf das Abscheideverhalten auswirken. Anhand dieser einfachen Beispiele kann gezeigt werden, daB die oft verwendeten Verweilzeitkurven im allgemeinen ungeeignet sind, urn das Abscheideverhalten eines Verteilers zu beurteilen. Statt dessen bleibt nur die direkte Messung oder Simulation von Abscheidevorgangen. Bei der Oblichen CFD-Simulation ist allerdings Vorsicht geboten: Die numerischen Fehler der verschiedenen Verfahren (numerische Diffusion bei finiten Volumen, Interpolationsfehler bei Partikeltrajektorien) vertalschen das Ergebnis deutlich und stellen einen unsystematischen und schwer quantifizierbaren Fehler in den Abscheidekurven dar.

Simulation of Nonmetallic Inclusions in a Continuous Casting Strand

“…The whole liquid volume inside the strand shell and inside the nozzle is modelled (figure 1); it starts at the mould level and ends with zero thickness at the point where the solidification fronts meet each other. The shape of the liquid domain inside the solidified strand is calculated via the square root law for the growth of the shell thickness a part is still transported to the tundish [1,2] and the mould surface, where they are absorbed by the covering slag if this is properly conditioned. This separation effect increases if the casting velocity is lowered.…”

Mathematical Analysis of the Touching Inclusions Parameters at the Tundish Free Surface to Predict More Realistic Inclusion Removal Rates

“…On the other hand, the inclusions removal rates, that are going to be discussed in the following sections, will be validated using the published results in the theoretical range and those obtained by physical modeling …”

“…In these previous studies, the efforts were focused on the understanding of inclusions behavior and its flotation, accomplishing important advances in the qualitative description of the removal. Nevertheless, it is clear that a fraction of these inclusions that touches the free surface are not removed and continues its trajectories, mainly the small ones . Wikstrom et al stresses that not all the inclusions reaching the steel–slag interface are removed from the melt, because they can remain at the interface or oscillate, increasing the risk that these inclusions follow the steel flow and get back to the bath.…”

Study of Inclusion Bands in Continuously Cast Steel Billets for Rolling Thermomechanically Treated Rebars

“…This oxide formation mostly increases in the temperature interval where solidification happens (two-phase regions) as the solubility of oxygen drops discontinuously during solidification. Using computational fluid dynamics models, the authors [1][2][3] have shown that most of the inclusions resulting from the metallurgical treatment are separated already in the ladle, but some remaining inclusions are still transported to the tundish and the mold surface where they are absorbed by the covering slags, if they are properly conditioned. This separation effect increases with lower casting velocity.…”