Structural effects and band segregate formation during the electromagnetic stirring of strand-cast steel
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I.
INTRODUCTION
THEincreasing use of electromagnetic stirring (EMS) over recent years has brought with it increased interest in the problem known as white bands. The white band is a zone of negative segregation (appearing white on sulfur prints) often found in electromagnetically stirred products, and corresponding to the position of the solidification front during stirring) The negative segregation is readily apparent, but less visible is an attendant solute enriched zone corresponding to the end of stirring.~2s The visual appearance of the segregation has not only given rise to the name, but is probably also the white band's most undesirable feature. The extent of negative segregation at the white band is less than the positive segregation along the center line, but its continued presence after hot working can result in a deterrent to customer acceptance, mostly on cosmetic grounds. The most widely quoted mechanism for white band formation is the "solute washing" mechanism. 4~sThis proposes that the turbulent flows caused by EMS penetrate the dendrite mesh and sweep out enriched interdendritic liquid (Figure 1). However, in order to maintain this action it is necessary to assume that the removed solute is very rapidly dispersed throughout the remaining liquid. This being so, it is difficult then to explain the observed solute enrichment at the end of stirring. A variation on this principle has been proposed by Kollberg, 6 who argues that it is the acceleration and deceleration of the liquid which is important. This idea has been supported by observations made on slabs, where, by extending the recirculation pattern of transverse stirring (Figure 2), white bands were reduced in intensity. This result is similar to the observations of Hurtuk and Tzavares,-' where stirring for a long time resulted in no significant white band formation. M.R. BRIDGE is a Research Officer at the Department of Materials Science and Engineering, University of Surrey, Great Britain. G.D. ROGERS is Plant Metallurgist with the Goodyear Tire and Rubber Co. Wire Plant, Asheboro, NC 27204. During the period of this work the authors were Research Engineers at the United States Steel Company Research Laboratories, Monroeville, PA 15146. Manuscript submitted August 24, 1983. METALLURGICALTRANSACTIONS B
Kor I has suggested an alternative explanation, in which the white band is the result of changes in the growth rate at the start and end of stirring. It was proposed that the onset of EMS causes the growth rate to decrease with a subsequent increase at the end of stirring. The mechanism was justified on the basis of solute partition at a planar interface, with KE~r changing as a result of alterations in the diffusion profile ahead of the interface. While only an approximate solution, the agreement between theory and experiment (using the same rotary stirrer as the current investigation) was quite
DIFFUSION---J,/ V D''I ~,""~,1 \',
/
FLOW
SOLIDUS Fig. 1--Schematic illustration of the solute washing mechanism for white band formation.
Fig, 2-
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