Slag movement in ESR of steel
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Considerable amount of work has been done on the theoretical prediction of the flow field in the slag and pool of the ESR system. I'2'3 Also, experiments have been performed on cold models. 4 From the qualitative visual observation of the slag surface in the ESR system it is known that the stirring velocities are in the order of a few centimeters per second. But, apparently, no systematic measurements of the flow pattern in the real ESR have been carried out. In the present work measurements performed at the surface of the slag are reported. The fluid flow in the ESR furnace is caused mainly by two effects. 2 First, there is natural convection. The hot slag below the electrode tends to move upward, whereas the cooler slag close to the mold tends to sink downward. A recirculation motion will develop which is directed toward the mold at the free surface of the slag, Second, forced convection is caused by the electromagnetic force F = [j x B]. Since the current density j is larger in the electrode than in the ingot, there results a curl of the force vector causing a circulatory movement which is directed toward the electrode at the free surface of the slag. Hence, this motion is in opposite direction to the natural convection. Under practical conditions there may also be a tangential flow component in addition to the radial and vertical components. The cause is not well known. It may be due to nonsymmetry because the electrode is not centered exactly. Another electromagnetic nonsymmetry may be caused by the exterior cables. Experimental Technique. The experiments were carried out with a laboratory unit using a movable mold of 240 mm internal diameter and electrodes of 100 m m diameter. The small fill ratio was applied in order to obtain the large free slag surface suitable for the velocity measurements. The steel grades used were C 35, St 37, and St 52. The slag had Table I.
Number of Experimental tteat E 185 E 215 E 217 E 218 E 223 E 224 E 225 *at
Type of Current DC or AC AC AC AC DC DC AC
the composition 30 pct CaO/30 pct A1203/40 pct CaFz or 33 pct CAO/33 pct A1203/33 pet CaF2. The flow velocity at the surface of the slag was determined by placing small bubble alumina spheres at the surface and by photographing their motion. Four to five pictures were taken per second in most experiments. The camera was connected electrically to a fast recorder with which the time spacings between the shots were registered. The starting location of the spheres at the slag surface was varied over the whole of the surface. Both AC of 10 Hz and DC (with electrode negative) were used. The data on current, voltage, power, and slag temperature are listed in Table I. The radial and tangential velocity components Vr and v~ were evaluated from the films. Vr is taken positive when the motion is to the mold; v, is taken positive when the motion is anticlockwise. For the representation of the data the slag surface is divided into quadrants (Figure 1). The first quadrant (0 to 90 deg) is the left one at the front. It was observed that with A
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