Kontakt

Dr. Klaus Timmel

k.timmelAthzdr.de
Tel.: +49 351 260 2159

Dr. Sven Eckert

Lei­ter Mag­neto­hydro­dyna­mik
s.eckertAthzdr.de
Tel.: +49 351 260 2132

Liquid Metal Model for Continuous Casting (LIMMCAST)

The LIMMCAST-project with it's three experimental facilities is aiming on the modeling of the continuous casting of liquid steel. It is espacially focusing on the electromagnetic process control and the actual liquid metal-gas two-phase flow.

Panorama view from the lower LIMMCAST-platform

Motivation

Fluid 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 is supposed to 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 and can result in non-intuitive flow changes.

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.

Another issue is the injection of gas into the liquid steel, which can be done in actual casting for several reasons. One of it can be the mixing and homogenization of the metal melt using inertial gases. Another one might be the enhancing or triggering of chemical reactions within the melt. But once again, the actual kind of two-phase flow within the liquid metal is not really known. Espacially the inital stage of gas injection and bubble formatoin is affected by many aspects - like the surface tension of the liquid, the wetting condition at the solid wall material - which are very different from standard water model experiments. By this, numerical simulations are missing essential information for setting up corresponding simulations.

Experimental modeling

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 mold, the work in the LIMMCAST project 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 a fundamental understanding of the magnetic fields action on the liquid metal flow and finally the design of the respective magnetic field, which provides a well-directed control on the flow.

The experimental investigations can be carried out at the big 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 mold. The effect of diverse magnetic fields on the jet flow in the mold is of special interest. Further, the injection of argon at the stopper rod is a usual application in the steel casting industry for preventing the nozzle from clogging, which can be applied in this setup, too. Both aspects together, i.e. the combined action of magnetic fields and gas injection on the melt flow, is a strength of this experimental setup.

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. Especially single-phase liquid metal experiments can be done at this setup in an extensively and convenient manner.

The third setup is the X-LIMMCAST. This experimental facility is specialized for the visualization of Argon-melt two-phase flows in the submerged entry nozzle and in the mold by the means of X-ray imaging.

Photograph of the LIMMCAST facility Photograph of the mini-LIMMCAST facility Photograph of the X-LIMMCAST facility

Summing up, the following key issues were addressed by our experimental modelling:

  • Flow investigations in mold, tundish and submerged entry nozzle (SEN)
  • Influence of electromagnetic fields: electromagnetic brakes as well as electromagnetic stirrers
  • Two-phase-flows in mold, SEN and ladles

Application of low melting alloys

The 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
Comparison of material properties
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
GaInSn (eutectic) 10,5 6,3 0,33 3,29 24
Water 0 1,0 1,00 0,05 * 10-6 0,60

Further development of measurement techniques for liquid metals

The use of liquid metals as model fluid request peculiar methods for measurements. The opaque nature of liquid metals prohibits the use of the usal optical measurement methods, like the PIV or LDA. The most widely used velocity measurement technique at our model experiments about continuous casting is the Ultrasound Doppler Velocimetry (UDV). Further, the Contactless Inductive Flow Tomography (CIFT) is an developement from the HZDR and has been adapted for the specifics of the continuous casting models.

Some exemplary results

The 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.
Reference: Flow without magnetic field Flow with a magnetic field of B = 310 mT and insulating walls. Flow with a magnetic field of B = 310 mT and conducting walls.

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.

Further visualizations of experimental results done at the LIMMCAST setups can be found here:

Some selected Publications

Stirring in mold and SEN

Effect of static magnetic fields

Gas-liquid metal two-phase flows

related PhD-Theses

Material properties of the used alloys in the experiments