Thermodynamics of rare earths in steelmaking
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MICROPROBEand X - r a y
d i f f r a c t i o n a n a l y s i s of a l u m i n u m deoxidized s t e e l s which have b e e n t r e a t e d with r a r e e a r t h s i l i c i d e o r m i s c h m e t a l in m a g n e s i t e c r u c i b l e s i n d i c a t e that i n c l u s i o n s of the type (RE)xSy and (RE)202S a r e f o r m e d . 1 No p u r e R E - o x i d e s have b e e n found. The p o r t i o n of the C e - O - S p h a s e d i a g r a m g i v e n i n F i g . 1, i s a s c h e m a t i c r e p r e s e n t a t i o n b a s e d on the q u a l i t a t i v e work of E a s t m a n e t al, 2 and P . B e s a n c o n e t al, 3 and gives the type of p h a s e r e l a t i o n s h i p s one m i g h t expect to see i n i n c l u s i o n s found i n these rare earth treated steels. It is the s o l u b l e r a r e e a r t h , s u l f u r and oxygen cont e n t s in e q u i l i b r i u m with t h e s e p h a s e s , however, that a r e of m o r e i n t e r e s t to the s t e e l m a k e r and the followi n g t r e a t m e n t , p r i m a r i l y b a s e d on the t h e r m o d y n a m i c d a t a of G s c h n e i d n e r et al, 4,s p r o v i d e s the b a s i s for p r e d i c t i n g the t h e r m o d y n a m i c b e h a v i o r of r a r e e a r t h s in steelmaking. The r e s u l t s of t h i s t r e a t m e n t and t h e i r c o r r e l a t i o n with p r a c t i c e have b e e n p u b l i s h e d e l s e w h e r e 6 and the p u r p o s e of t h i s p a p e r i s p r i m a r i l y to p r o v i d e a d e tailed thermodynamic background. The t h e r m o d y n a m i c data u s e d i n t h i s t r e a t m e n t a r e s u m m a r i z e d i n T a b l e I. The s t a n d a r d f r e e e n e r g y c h a n g e s for the c o r r e s p o n d i n g s t e e l m a k i n g r e a c t i o n s a r e given i n T a b l e II, w h e r e [ ] r e p r e s e n t s a c o m p o n e n t i n s o l u t i o n i n s t e e l and the s u b s c r i p t 1 wt pct r e p r e s e n t s the s t a n d a r d s t a t e . The d e r i v a t i o n of t h e s e f r e e e n e r g y changes is given i n the Appendix. The e s t i m a t e d e r r o r s i n the t h e r m o d y n a m i c data u s e d in this p a p e r a r e also given i n T a b l e I and a r e of s u f f i c i e n t m a g n i t u d e to j u s t i f y only a p p r o x i m a t e c a l c u l a t i o n s . T h e s e c a l c u l a t i o n s m a y s t i l l be used, however, to p r e d i c t the b e h a v i o r of r a r e e a r t h s in steelmaking. Although one m i g h t expect only m i n o r d i f f e r e n c e s i n the t h e r m o d y n a m i c b e h a v i o r of i n d i v i d u a l r a r e e a r t h s in s t e e l , s o m e of t h e s e d i f f e r e n c e s a r e n e v e r t h e l e s s s i g n i f i c a n t . C e r i u m has b e e n c h o s e n as the r e p r e s e n t a t i v e r a r e e a r t h b e c a u s e of the a v a i l a b i l i t y of t h e r m o d y n a m i c data, but c o m p a r i s o n s with l a n t h a n u m , the o t h e r m a j o r c o n s t i t u e n t of r a r e e a r t h s i l i c i d e and m i
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