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MODELLING ELECTROMAGNETIC BRAKING IN CONTINUOUS-CASTING

BY:  Dr M. Malin, E. Lopez, CHAM Consultancy, July 99.

FOR: CSM (Centro Sviluppo Materiali)

  1. Introduction

  2. This project consisted in studying the solidification of a thin-slab steel caster. The aim of such a simulation is to determine the optimum fluid-dynamics conditions for the solidifying steel, in order to prevent process irregularities and achieve good quality of the product inner surface.
For this particular project, the interest was focused on testing the effect of a magnetic brake on the caster. The customer requirements was for CHAM to produce information on the solidification rate and velocity,  information that would enable them to assess the benefit of such an implementation, and to define the optimum value of the magnetic field to apply.
  1. Case 1: Steel solidification in a continuous thin-slab caster, without electromagnetic braking.

  2. Geometry:

    The caster modelled appears as a vertical thin slab (see Geometry 1) of 8 metres long (X direction for the simulation), 80 centimetres width (Y), and 7 centimetres thick (Z). The mould (top part) of the caster is funnel-shaped, justifying the use of Body Fitted Coordinates. Its shape is shown on the figure Geometry 2.

    Only one fourth of the caster was modelled for symmetry reasons. There are two symmetry plane on the border of the simulation domain shown in Figure  Geometry 2 and Geometry 3.

    The steel is injected through a submerged entry nozzle (see Geometry 3)., located in the middle, at the top of the whole caster. In the simulation domain, only one fourth of the nozzle is represented. The steel is injected downward but is driven to the side because of the nozzle design (Y direction), as shown on the figure.

    Modelling options:

    The model is a 3-dimensional, steady using Body Fitted Coordinates because of the funnel shape of the mould.

    The turbulence was modelled using the standard k-e model.

    The equations of mass, momentum, energy (temperature) were solved, as well as a the equation for caculations of solid fraction, based on the DARCY-type solidification model.

    Boundary Conditions:

    The liquid steel inlet was specified by its injection velocity and inflow. As the steel is solidified at the bottom of the casting, and its velocity known, the outlet was defined specifying the outflow and velocity. Two of the plane on the boarders of the domain had symmetry boundary conditions. On the remaining boarder faces of the domain (external faces), temperature boundary conditions were applied (heat sinks defined by the customers, varying with the location).

    Results:

    Behaviour at symmetry plane

    Figure 1: Velocity vectors at symmetry plane

    Figure 2: Solid fraction contour at symmetry plane

    Figure 3: Temperature contour at symmetry plane

    Behaviour on the external face

    Figure 4: Velocity vectors on the external surface

    Figure 5: Solid fraction contour on the external surface

    Figure 6: Temperature contour on the external surface

    Figure 1 shows the flow pattern through the nozzle.  The steel downward and is driven to the right due to the nozzle design. The liquid steel recirculates after exiting the nozzle. Lower down, the steel solidifies and the velocity becomes uniform across the caster.

    Figure 4, shows the velocity on the external face of the caster. In contact with the air, the steel is solidified and the velocity is almost uniform.

    Figures 2 and 5 shows the solid fraction comtours. At symmetry plane, the solidification is taking place, from liquid steel (blue) on the top of the caster to solid steel (red) at the bottom. The solidification is faster on the left side (opposite to the nozzle), as the left surface is an external face.

    Figures 3 and 6 shows the temperature contours. As expected the farest from the external surfaces, the highest the temperature we have.

  3. Case 2: Steel solidification in a continuous thin-slab caster, with electromagnetic braking.

  4. The settings (geometry, models, boundary conditions) for Case 2 are identical to Case 1, apart from the addition of the electromagnetic brake, not implemented in Case 1.

    The implementation of the electromagnetic brake consisted in applying the electromagnetic force on the mould. This was carried out by way of ground coding.

    This force is induced by the electromagnetic field applied to the caster. The electromagnetic field was provided by the customer, and the force deduced using Ohm's law and assumptions specified by the customer.
     
     

                                             (Ohm's Law)

    Results:
     
     

    Behaviour at symmetry plane

    Figure 7: Velocity vectors at symmetry plane

    Figure 8: Solid fraction contour at symmetry plane

    Figure 9: Temperature contour at symmetry plane

    Behaviour on the external face

    Figure 10: Velocity vectors on the external surface

    Figure 11: Solid fraction contour on the external surface

    Figure 12: Temperature contour on the external surface
     
     

    The general comments for Case1 are also valid for Case 2, however, the effect of the brake is noticeable in the display of solid fraction, and also evident on the flow pattern:

    Velocity vectors at symmetry plane (Case1)

    Velocity vectors at symmetry plane (Case2)

    The peak velocity is reduced, and the fluid velocity is effectively reduced in the mould area (where the brake is applied).
     
     

  5. Some additional graphics: