Previous study has shown that conducting walls, such as the solid steel shell surrounding the liquid cavity in continuous casting, have a stabilizing effect on the flow. However, the magnetic field can change the flow stability in non-obvious ways. The flow of a conducting fluid such as steel through a magnetic field generates a force opposing the motion, and thus should be self-stabilizing. The effect on transient flow has received less attention in the studies so far. When the EMF coil currents are adjusted to produce equal peak field strengths, this double-ruler configuration is commercially known as “Flow-Control-Mold” or “FC-Mold” ElectroMagnetic Braking or “EMBr.” The regions of the strongest magnetic fields tend to deflect the flowing steel, altering the time-averaged flow, which has been the subject of many previous modeling studies. Statically-applied electromagnetic-field (EMF) configurations include local, single-ruler (a rectangular field across the entire mold width), and double-ruler (two ruler-shaped fields, with one positioned across the mold near the meniscus and the other one aligned through or below the nozzle ports). In steel slab casting, both static and moving magnetic fields have been implemented. In addition, the electromagnetic forces change naturally in response to changes in the instantaneous, local turbulent flow. One of the few process parameters that potentially could be adjusted to respond to changes in the flow is the application of electromagnetic fields. Sudden increases in velocity, level fluctuations, vortex formation, and other intermittent flow events can lead to the entrainment of mold slag, the formation of surface defects, and other quality problems. The highly turbulent nature of flow in the mold causes transient behavior even during statistically steady-state operation. When the control parameters create conditions which fall on the borderline between single and double rolls, then complex unstable flow conditions are likely.
EMBR PITTSBURGH FREE
If the jet first impinges on the narrow face, and splits, flowing up toward the free surface, then a “double-roll” flow pattern is generated.
In conventional slab casting, if the jet impinges first on the free surface, a “single-roll” flow pattern is generated. The flow pattern depends on the nozzle geometry, casting speed, mold width, mold thickness, argon gas injection, and submergence depth. Maintaining stable flow conditions is well known to produce steel of the best quality.
The quality of steel products is greatly affected by the fluid flow near the top surface of the mold during the continuous casting process. Nail board measurements taken at this commercial caster, in the absence of the field, matched reasonably well with the calculated results, both quantitatively and qualitatively. Applying the magnetic field suppresses the unbalanced behavior, producing a more complex mold flow pattern, but with much lower surface velocities (~0.1 m/s), and a flat surface level with small level fluctuations (<☑ mm). In the absence of a magnetic field, a double-roll pattern is observed, with transient unbalanced behavior, high surface velocities (~0.5 m/s), surface vortex formation, and very large surface-level fluctuations (~☑2 mm).
EMBR PITTSBURGH CODE
Large eddy simulations with the in-house code CU-FLOW resolve the important transient behavior, using grids of over five million cells with a fast parallel solver. In the current study, transient flow in a typical commercial caster is simulated in the absence and in the presence of a double-ruler magnetic field, with rulers of equal strengths. The double-ruler electromagnetic field configuration, or “FC-Mold EMBr,” is popular in commercial slab casting as it provides independent control of the applied static field near the jet and free surface regions of the mold. Transient flow during nominally steady conditions is responsible for many intermittent defects during the continuous casting of steel.
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