Liquid Metal Model for Continuous Casting (LIMMCAST)
Modeling and Electromagnetic Control of the Continuous Casting Process
MotivationFluid flow in the mould cavity of the continuous casting process can be controlled by the application of magnetic fields. For instance, AC magnetic fields are employed as electromagnetic stirrers (EMS) for a homogenization of the melt and a promotion of the double-roll flow pattern in the mould, which is supposed to diminish many slab defects such as the entrapment of bubbles and non-metallic inclusions and to improve therefore the quality of the solidified steel strand. Another approach to ensure high quality slabs under high casting speeds is the utilisation of DC magnetic fields as electromagnetic brake (EMBR). The DC field should directly reduce the velocity in the mould region and suppress recirculating flows which arise from the high intensity jet flow poured into the mould from the submerged entry nozzle (SEN). But the electromagnetic braking effect in highly turbulent, complex flow configurations is not fully understood until now.
A multitude of numerical studies have been carried out considering different magnetic field configurations for the continuous steel casting, but the reliability of the numerical results is insufficiently confirmed by accompanying experimental activities. Experimental studies on industrial scale with hot metallic melts may require formidable effort and expense. The main drawback is the extremely limited availability of measuring techniques which are able to provide reliable quantitative data from the flow being relevant for numerical code validations. Cost-saving model experiments using low melting point metallic melts permit detailed investigations of the flow structure and related problems with a high grade of flexibility.
As the quality of steel and the casting speed in continuous casting is mainly determined by the flow structure in the submerged entry nozzle and the mould, the subproject B10 investigates the liquid steel flow in these two important components of continuous casting by the means of experimental modelling and numerical simulations. The main purpose is the design of the respective magnetic field which provides a well-directed control on the flow. The experimental investigations are carried out at the new LIMMCAST-facility. The metal used as model liquid is a tin-bismuth-alloy which allows an operation of the facility at a temperature range of 150 to 300 °C. The investigations are focused on flow measurements in the mould or the characterization of the two-phase-flow in the submerged entry nozzle. In this subproject the effect of diverse magnetic fields on the jet flow in the mould is of special interest. Both versions of DC magnetic fields, the so-called electromagnetic brakes (EMBR) and AC magnetic fields as electromagnetic stirrers will be considered. The injection of argon at the stopper rod is a usual application in the steel casting industry for preventing the nozzle from clogging.
The so-called mini-LIMMCAST facility is used as small-scale experiment working with an eutectic GaInSn alloy at room temperature, which makes the experiments rather convenient because heating becomes unnecessary. During constructing and commissioning of LIMMCAST first investigation of the mould flow were performed at mini-LIMMCAST.
In parallel to the experimental activities, numerical calculations were performed by means of the software package CFX with an implemented RANS-SST turbulence model. The non-isotropic nature of the MHD turbulence was taken into account by specific modifications of the turbulence model. The numerical results were validated by flow measurements at the mini-LIMMCAST facility.
The following key issues were addressed by our numerical and experimental modelling:
- Flow investigations in mould, tundish and submerged entry nozzle (SEN)
- Influence of electromagnetic fields: electromagnetic brakes as well as electromagnetic stirrers
- Two-phase-flows in mould and SEN
Application of low melting alloysThe employment of low melting alloys for process modelling has several advantages, compared to the use of water models or industrial trials in real steel flows:
- Suitable measuring techniques and sensors are available for that temperature range
- Modelling of magnetic field effects are possible
- Realistic modelling of two-phase flows and flows at significant temperature gradients
- Solidification experiments are possible
|Fluid||Melting Temperature||Density||Viscosity||Electrical Conductivity||Heat Conductivity|
|[Ts] = °C||[ρ] = kg/dm3||[ν] = 10-6 m2/s||[σ] = 106 A/Vm||[λ]=W/Km|
|Steel||1480 - 1510||7,9||0,85||0,77||35,0|
|Sn60Bi40||138 - 170||8,3||0,19||1,05||~25,0|
|Water||0||1,0||1,00||0,05 * 10-6||0,60|
Some exemplary resultsThe diagrams below show the flow at the nozzle exit. The two-dimensional flow field displays the colour-coded horizontal velocity in the half section of the mould. Blue colours indicate a flow towards the outer wall on the right side of each diagram. The flow direction towards the SEN is marked by a red colour.
The horizontal, green lines represent the position of the magnet pole-shoes. Here, a steady magnetic field of 310 mT was applied to the mould flow.
Our measurements deliver an authentic reproduction of the location and extension of the emergent jet and disclose the temporal behaviour of the flow inside the jet as well as in the recirculating zones. An important result of our study is the feature that a static magnetic field may give rise to non-steady, non-isotropic large-scale flow perturbations. This problem requires further investigation, because the concept of an EMBR in the continuous casting process relies on a certain damping effect by the applied magnetic field. Moreover, the electrical conductivity of the side walls has a striking impact on the mould flow if it is exposed to a magnetic field. In particular, the jet undergoes significant fluctuations in case of a non-conducting mould, whereas an efficient damping of velocity fluctuations becomes obvious in a conducting channel at sufficiently high Hartmann numbers.
AcknowledgementThe research is supported by the German Helmholtz Association in frame of the Helmholtz-Alliance “LIMTECH”.
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Our experimental work provides a valuable data base for code validations which has also been used by external groups, see for instance: