Oscillation marks (OMs) are regular, transverse indentations formed on the surface of continuously cast (CC) steel products. OMs are widely considered defects because these are associated with segregation and transverse cracking. A variety of mechanisms for their formation has been proposed (e.g., overflow, folding, and meniscus freezing), whereas different mark types have also been described (e.g., folded, hooks, and depressions). The current work uses numerical modeling to formulate a unified theory for the onset of OMs. The initial formation mechanism is demonstrated to be caused by fluctuations in the metal and slag flow near the meniscus, which in turn causes thermal fluctuations and successive thickening and thinning of the shell, matching the thermal fluctuations observed experimentally in a mold simulator. This multiphysics modeling of the transient shell growth and explicit prediction of OMs morphology was possible for the first time through a model for heat transfer, fluid flow, and solidification coupled with mold oscillation, including the slag phase. Strategies for reducing OMs in the industrial practice fit with the proposed mechanism. Furthermore, the model provides quantitative results regarding the influence of slag infiltration on shell solidification and OM morphology. Control of the precise moment when infiltration occurs during the cycle could lead to enhanced mold powder consumption and decreased OM depth, thereby reducing the probability for transverse cracking and related casting problems.
A mathematical model of the continuous casting process has been developed which couples metal, slag and gas flow with heat flux and solidification. An extensive sensitivity study has been carried out with this model, studying the influence of changing casting conditions upon a number of quantifiable model predictions (i.e. responses). The casting conditions studied were: casting speed, mould flux properties (viscosity, break temperature), mould oscillation frequency and stroke, and superheat. The model was then applied to determine the influence of each of these parameters on the variations in: powder consumption (lubrication), heat flux (solidification) and oscillation mark formation (defects). It is shown that all three responses vary in a consistent manner through the cycle. Equations are derived for the powder consumption and heat flux, showing good agreement with prior experimental data.
The continuous casting (CC) mould may appear very peaceful when viewed from above, but the powder bed hides relentless fluctuations in the following phenomena: metal flow, thermal gradients, chemical reactions and multiple phase transformations. When observed separately, some of these phenomena seem to have a 'simple behaviour', which may appear easy to control through the main casting parameters (e.g. casting speed, pouring temperature and powder type) and associated control systems (e.g. mould level control, automatic powder feeding and mould oscillation). However, when combined, these phenomena exhibit periodic fluctuations in behaviour, which is both difficult to predict and control. For instance, the combination of casting speed, submerged entry nozzle design and slab size can cause the metal flow pattern to shift from double roll to single roll and back, which can cause unstable fluctuations in metal level, standing waves, etc. In this respect, the CC process closely resembles a meteorological system where both variations and local fluctuations in temperature, humidity, pressure, etc., can result in effects that are difficult to predict in the long term. This is equivalent to the famous Lorenz premise: 'Does the flap of a butterfly's wings in Brazil set off a tornado in Texas?' In this paper, we give some examples of the 'butterfly effect' in CC discussed below by using a mathematical model able to predict the slab solidification inside the mould in which various factors affecting the process stability are analysed and the probable sources of fluctuation are identified.
Hot cracking is one of the major defects in continuous casting of steels, frequently limiting the productivity. To understand the factors leading to this defect, microstructure formation is simulated for a low-carbon and two high-strength low-alloyed steels. 2D simulation of the initial stage of solidification is performed in a moving slice of the slab using proprietary multiphasefield software and taking into account all elements which are expected to have a relevant effect on the mechanical properties and structure formation during solidification. To account for the correct thermodynamic and kinetic properties of the multicomponent alloy grades, the simulation software is online coupled to commercial thermodynamic and mobility databases. A moving-frame boundary condition allows traveling through the entire solidification history starting from the slab surface, and tracking the morphology changes during growth of the shell. From the simulation results, significant microstructure differences between the steel grades are quantitatively evaluated and correlated with their hot cracking behavior according to the Rappaz-Drezet-Gremaud (RDG) hot cracking criterion. The possible role of the microalloying elements in hot cracking, in particular of traces of Ti, is analyzed. With the assumption that TiN precipitates trigger coalescence of the primary dendrites, quantitative evaluation of the critical strain rates leads to a full agreement with the observed hot cracking behavior.
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