Effects of the electromagnetic stirring on the removal of inclusions of oxide from liquid steel
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I.
INTRODUCTION
IN the field of steelmaking,
one of the metallurgist's primary aims is to obtain inclusion-free steels, by cheaper means than vacuum techniques. In fact, the presence of macroscopic inclusions of oxide containing mostly alumina or silica is the cause of the major portion of internal and surface defects encountered in final products. Although these inclusions are partly a result of oxidation which takes place while handling the liquid alloys, they essentially come from an insufficient removal of deoxidation products. Several methods are employed to increase the final cleanliness of steels. One of the most commonly used is the addition of complex deoxidizers ~ that modify the morphology and the physical properties of the inclusions in order to facilitate their elimination or to reduce their harmful effects in the ultimate use of the end products. As for the other methods, it seems that handling difficulties for inert gas bubbling through the melt 2 and technological problems for mechanical stirring 3 limit the use of these techniques. In the early seventies studies were undertaken on the effects of the electromagnetic stirring of molten metals by using a pulsating stationary field or a traveling field. 4'5 It has already been shown that medium frequency induction melting can be used in order to separate metallurgical phases, one of these being liquid. 6 On the other hand, several authors mention the effect of electromagnetic stirring on the deoxidation mechanism. 7'8'9 In the present work, the authors attempt to take advantage of the force field existing in a medium frequency coreless induction furnace in order to remove inclusions from a steel bath. The modeling of electromagnetic stirring has been studied elsewhere, 7'8'1~ and thus section 2 includes only a review of the effects of the electromagnetic field, and a dimensionless number is defined to characterize the frequency of the field in relation to the diameter of the ladle. The kinetic results and the morphology of the inclusions are PIERRE CREMER is Engineer at lmphy S.A., Aci6ries d'Imphy, 58160, Imphy, France. JEAN DRIOLE is with Laboratoire de Thermodynamique et Physico-Chimie M6tallurgiques, E.N.S.E.E.G., Domaine Universitaire, B.P. 44, 38401, Saint Martin d'H~res, France. Manuscript submitted April l, 1981. METALLURGICALTRANSACTIONS B
9
discussed in section 3. The localization of the inclusions in solidified ingots is examined in section 4.
II.
EFFECTS OF THE E L E C T R O M A G N E T I C FIELD AND T H E DEFINITION OF Ro,
There are two kinds of equations governing electromagnetic quantities: ~2 Maxwell's equations, which are independent of the material and result only from the definition of the electromagnetic quantities and from the principle of space charge conservation, and the constitutive equations for the magnetizing field (H), the electric displacement (D), and the current density (J). These equations, combined with the correct boundary conditions, lead to the determination of the electromagnetic field and the eddy currents
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