Flow Control with Local Electromagnetic Braking in Continuous Casting of Steel Slabs
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CONTINUOUS casting is used to solidify over 90 pct of the 1.1 billion tonnes of steel produced in the world each year.[1,2] Most of the defects affecting steel quality in this process are associated with fluid flow in the mold, so small improvements in flow pattern can have a big impact. Excessive surface velocity can entrain mold-slag inclusions and cause surface level variations and fluctuations with time that produce surface defects.[3] Insufficient surface flow allows meniscus freezing and related surface defects. Deep penetration of the jet entering the mold cavity encourages the capture of subsurface inclusions. Thus, the mold flow pattern must be carefully optimized to find windows of stable casting conditions which avoid these problems. Fluid flow in the mold is controlled by many design and operation conditions. These include the submerged entry nozzle (SEN) and port design shape, the SEN submergence depth (distance from top of nozzle ports to mold top surface), mold size, casting speed, position of the flow-control mechanism (slide gate or stopper rod), the rate of inert gas injection, and the application of electromagnetic forces. Each of these factors must be adjusted with regard to the other factors in order to produce a good flow pattern.[3] Although electromagnetics have great potential by improving the ability to control fluid flow in the mold cavity, their application is only effective as part of a complete flow-system design. KEVIN CUKIERSKI, MasterÕs Candidate, and BRIAN G. THOMAS, Wilkins Professor of Mechanical Engineering, are with the Department of Mechanical and Industrial Engineering, University of Illinois at Urbana–Champaign, Urbana, IL. Contact e-mail: [email protected] Manuscript submitted September 13, 2007. Article published online December 8, 2007. 94—VOLUME 39B, FEBRUARY 2008
Furthermore, flow in the steel caster is difficult to measure and computational models offer one of the few ways to understand the effect of electromagnetic forces on flow. This article applies validated computational models to improve understanding of how to apply electromagnetics to improve fluid flow in the mold cavity. Electromagnetics can produce stirring, accelerating, or braking of flow and can be divided into two categories: electromagnetic stirrers (EMS) and electromagnetic brakes (EMBr). Electromagnetic stirrers employ alternating current to generate a continuously-varying magnetic field to control flow in the mold cavity. Slab-mold EMS employs two stirrers on each wide side of the mold near the meniscus. These magnetic systems sequence the forces to circulate the flow around the mold perimeter, which homogenizes meniscus temperature, thus improving the quality of the finished slab.[5,6] Okazawa et al. used an large eddy simulation (LES) model and an experimental mercury model to study the effect of EMS magnet placement on flow circulation. The predicted velocities matched the measurements in the mercury model obtained with a Vives-type sensor.[4] Multimode EMS (MM-EMS) uses two stirrers on each wide side located near the SEN por
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