ECCC-68振痕形成的同一性机理

a)
b)
Figure 1: Oscillation marks appearance on, a) finished slab surface after Däcker and Sohlgren [1] and b) cross section schematics based on findings by Le Papillon et al. [2], Hill et al. [3] and Schwerdtfeger et al. [4]. Although these marks do not represent an impediment for casting themselves, they are related to problems such as segregation, sub-surface hooks, nucleation of cracks, etc. and thus, they are regarded as defects. All these defects are highly dependent on the heat transfer, metal-flow dynamics and mould oscillation conditions inside the CC mould (Figure 2).
Introduction
Oscillation marks (OM) are typical surface defects of continuously cast products (such as slabs, billets and blooms); consisting of regularly spaced depressions along their transverse section (Figure 1).
Mould performance and initial solidification
Session 8
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A UNIFIED MECHANISM FOR OSCILLATION MARK FORMATION
Pavel E. Ramirez-Lopez1, Kenneth C. Mills2, Peter D. Lee2 and Begoña Santillana3
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Düsseldorf, 27 June – 1 July 2011
Mould performance and initial solidification
Session 8
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This generates solidification along a curved front, and consequently, heat removal is much more complex than that envisaged in the typical horizontal heat-transfer approach. Casting without defects entails a delicate balance between heat extraction, shell growth, stresses in the shell and the lubrication provided by the slag. Such a balance is well known for a specific set of casting conditions, which conform to the process operational window. However, departures from this window to increase productivity (by increasing casting speed, slab size, etc) or the introduction of grades which are difficult to cast (such as peritectic and alloyed steels with very particular segregation behaviour) result in the loss of this balance and lead to defects. Figure 2: Initial shell formation in CC. The hot metal enters the mould from the submerged entry nozzle (SEN) and forms a jet which later divides into upper and lower rolls. Then, solidification occurs as heat is extracted from the liquid metal through the copper mould walls to form a thin steel skin (i.e. shell). A protective layer of ceramic powder or granules (i.e. powder-bed) is added on top of the metal bath to provide thermal insulation and avoid re-oxidation. As the carbon and moisture in the powder burns-off, the powder sinters (to. form a sintered layer) while the slag in contact with the steel melts to form a liquidpool. This pool works as a reservoir that supplies liquid slag to the gap between mould and the recentlyformed shell. The liquid slag infiltrates into the gap and partially freezes forming a thin solidified film in the gap which sticks to the mould, whereas the slag closer to the shell remains in the liquid state and provides lubrication for the strand. Temperatures tend to be lower towards the top of the mould which causes the solid slag-film to enlarge and form a rim (which moves up and down during mould oscillation). Moreover, this slag-rim interacts with the slag-metal interface (e.g. meniscus) and causes pressure changes during oscillation through the liquid slag-film, which have significant influence on the formation of defects such as OM’s. Later, the shell suffers shrinkage, which combined with a lack of lubrication and excessive mould friction could lead to cracking; and in the worst scenario, to a breakout. The typical approach adopted for the analysis of the heat transfer in the mould is to split it into vertical and horizontal components. The vertical heat transfer takes place through the liquid metal and slag-bed while the horizontal heat transfer occurs through the shell, slag-film and mould. OM’s are commonly attributed to variations in horizontal heat transfer ignoring the influence of the slag-bed. Nevertheless, a common feature in all the published mechanisms that explain OM formation is the growth of the initial shell along a round meniscus profile (as depicted in Figure 2)[3-6].
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Keywords: Oscillation marks, slag infiltration, continuous casting, solidification, mould oscillation
Abstract
Oscillation marks (OM’s) are regular, transverse indentations formed on the surface of continuously cast products. Although they do not pose much of a problem themselves, they are widely considered as defects since they can be associated with segregation and transverse cracking. In this study we apply a novel multiphase model to study the interaction of the molten steel with the liquid/solid slag-bed and slag-film. Application of this model to heat-transfer, fluid-flow and solidification coupled with mould oscillation allows predictions of the transient heat flux, powder consumption, pressure, slag film thickness and OM profile during the cycle. Using this analysis we explicitly capture the first stages of OM formation. OM’s are formed when the mould starts to descend and cold liquid-slag infiltrates the shell-mould gap. As the mould descends closer to the shell tip, this cold, downward flow has two effects it produces (i) an “indentation” in the liquid adjacent to the shell tip and (ii) rapid solidification of the “indentation” to form an OM. On the return cycle when the mould is ascending, the flow of slag in the gap is in an outward direction; such slag flow over the meniscus is warm and results in little shell solidification during this period. This mechanism is consistent with the industrial practices followed to reduce oscillation mark depth. Moreover the model provides quantitative results regarding the influence of slag infiltration on shell solidification. Controlling the precise moment when infiltration occurs during the oscillation cycle can enhance mould powder consumption and reduce OM’s incidence.
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Casting and Flow Simulation Group, Swerea MEFOS Aronstorpsvägen 1, SE-974 37 Luleå, SWEDEN pavel.ramirez.lopez@mefos.se
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Hale Waihona Puke Department of Materials, Imperial College London, Prince Consort Road, SW7 2AZ, London, UK ls@ & p.d.lee@ Research Development & Technology, Tata Steel 1970 CA, IJmuiden, NETHERLANDS begona.santillana@