Electromagnetic Mould Stirring with Higher Supply Frequency
The use of mould stirring in the continuous casting of steel yields a better surface quality of the cast strands. 1-3) On the one side, a more uniform thickness of the initial solidified shell is realized by homogenizing the temperature of the molten steel. 1,2) On the other side, due to the "washing effect" non-metallic inclusions like larger alumina particles and Ar bubbles are removed from the surface layer of the cast strand,      where their presence can generate cracks and
... enerate cracks and slivers in the subsequent processes.    As the magnetic field of the stirrer must penetrate through the inner copper wall of the mould, just low supply frequencies can be employed. Therefore, large stirrers and frequency converters of high power have to be used ( Table 1 and Fig. 1) . This paper presents a mould stirring configuration allowing the usage of higher frequencies, e.g. 50 Hz and consequently, of smaller inductors with direct network supply. 6) The mould stirring can be performed with an outer inductor if all mould walls are made of copper (Fig. 2) . The electric currents, i e , induced in the exterior mould wall by the inductor's rotational magnetic field are passed through the interior wall, where the inner currents, i i , appear. These currents produce a second rotational magnetic field inside the mould, which stirs the molten core of the strand. 6) If the thickness of the mould walls is smaller than the penetration depth of the electromagnetic field, d, an intermediate magnetic core shown in Fig. 2 (a) must be used as a return path for both outer and inner rotational magnetic fields. Figure 2 (b) shows a mould with vertical walls thicker than d, where an intermediate magnetic core is no longer neces-Note Fig. 1. Mould stirrer of the SMS Elotherm: (a) the three-phase inductor; (b) the stirrer and the frequency converter. Fig. 2. Mould stirring by an outer inductor inside (a) a thin-wall and (b) a thick-wall mould (schematic): 1, copper mould; 2, laminated core of the inductor; 3, cast strand; 4, intermediate magnetic core. Table 1. Characteristics of industrial mould stirrers.