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1. INTRODUCTION P RODUCTION of s l a b s by the t h r e e - p h a s e ESR p r o c e s s r e s u l t s in a complex d i s t r i b u t i o n of t e m p e r a t u r e and e l e c t r i c a l c u r r e n t b e t w e e n the e l e c t r o d e s , slag, mold, and s o l i d i f y i n g m e t a l . While it has b e e n d e m o n s t r a f e d 1 that the p r o c e s s will p r o d u c e high q u a l i t y s l a b s for s u b s e q u e n t r o l l i n g to plate, a knowledge of the t e m p e r a t u r e p a t t e r n s and c u r r e n t paths is r e q u i r e d to m a x i m i z e the a d v a n t a g e s of the p r o c e s s r o u t e . The p r e s e n t work was d e s i g n e d to produce i n f o r m a t i o n r e lating to the heat flow t h r o u g h the mold wall and b a s e plate, which would p r o v i d e data for t h r e e - p h a s e s l a b p r o d u c t i o n and f o r m the b a s i s of a t h e r m a l model of the p r o c e s s . A c c u r a t e v a l u e s of the heat flux through the mold wall a r e of d i r e c t r e l e v a n c e to: i) The d e s i g n of the ESR mold and its c a m p a i g n life, an i m p o r t a n t factor in the r e m e l t i n g c o s t s . ii) The s o l i d i f i c a t i o n p a t t e r n of the s l a b . In c o l l a r mold o p e r a t i o n the t h i c k n e s s of solid s t e e l s h e l l s u r r o u n d i n g the liquid m e t a l pool, below the bottom line of the mold is a function of the r e m e l t i n g c / c a s t i n g speed. iii) The e l e c t r i c a l e f f i c i e n c y of the s l a g . Although a l e s s i m p o r t a n t cost factor than i) above, the s p e c i f i c power c o n s u m p t i o n of the s l a g p l a y s a s i g n i f i c a n t p a r t in the e c o n o m i c s of the ESR p r o c e s s . F r o m a knowledge of the heat flux v a l u e s and t h e i r v a r i a t i o n it was p l a n n e d to build up a d d i t i o n a l , m o r e i n d i r e c t , i n f o r m a t i o n r e l a t i n g to: i) U n i f o r m i t y of t e m p e r a t u r e d i s t r i b u t i o n within the s l a g bath and the s o l i d i f y i n g s l a b . ii) The p o s s i b i l i t y of u s i n g heat flux or mold wall t e m p e r a t u r e m e a s u r e m e n t s a s a w o r k i n g guide to s l a g (and p o s s i b l y metal) pool depths, the p r o g r e s s of hottopping c y c l e s , and the safe o p e r a t i o n of the mold. 2. E X P E R I M E N T A L Sixteen s l a b ingots, 660 • 152 m m in c r o s s - s e c t i o n and weighing a p p r o x i m a t e l y 0.6 t o n n e s , w e r e p r o d u c e d in the 700 kVA t h r e e - p h a s e ESR unit at B i r l e c L i m i t e d , A l d r i d g e . The s e c o n d a r y windings of the t r a n s f o r m e r were connected in delta, and c o n s e q u e n t l y t h e r e was no r e t u r n c i r c u i t via the m o l d / b a s e p l a t e . A r a n g e of c o m m e r c i a l l y used s l a g s in the CaF2-CaO-AlzO3 s y s D. N. POCKLINGTONis Technical Manager-ESR, British Steel Corporation, Scottish Laboratory, Motherwell ML1 1QB, Scotland and B. P