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HE e q u i l i b r i a involving the liquid phase i n the F e - M n - S s y s t e m a r e of f u n d a m e n t a l i m p o r t a n c e for the u n d e r s t a n d i n g of d e s u l f u r i z a t i o n of m o l t e n s t e e l and the f o r m a t i o n of s u l f i d e s d u r i n g s o l i d i f i c a t i o n . The phase d i a g r a m was thus studied in c o n s i d e r a b l e d e t a i l a l r e a d y d u r i n g the 1930's (Ref. 1-6) and the m a i n f e a t u r e s w e r e d e s c r i b e d . In m o r e r e c e n t t i m e s c o n s i d e r a b l e a t t e n t i o n has been paid to the s o l u b i l i t y of s u l f u r at the i r o n - r i c h c o r n e r 7-1~ and, r e c e n t l y , an a t t e m p t was m a d e to r e e x a m i n e the phase d i a g r a m and some of the phase e q u i l i b r i a w e r e studied q u a n t i t a t i v e l y u s i n g m i c r o p r o b e m e a s u r e m e n t s ) 1' 12 In spite of a l l the i n v e s t i g a t i o n s of the F e - M n - S s y s t e m , the q u a n t i t a t i v e i n f o r m a t i o n is s t i l l v e r y m e a g e r and in a r e c e n t study of the c o n d i t i o n s for the f o r m a t i o n of s u l f i d e s d u r i n g s o l i d i f i c a t i o n of s t e e l , it was n e c e s s a r y to b a s e the d i s c u s s i o n on a r a t h e r hypothetical p h a s e diagram.IS T h a t e x p e r i e n c e i n i t i a t e d an a t t e m p t to d e r i v e the p r o p e r t i e s of the t e r n a r y F e - M n - S s y s t e m by e x t r a p o l a t i o n f r o m the b i n a r y s y s t e m s . T h i s a t t e m p t will be d e s c r i b e d in this r e port. Such an e x t r a p o l a t i o n is s t r a i g h t - f o r w a r d for a s u b s t i t u t i o n a l s o l u t i o n p h a s e if it can be d e s c r i b e d by the r e g u l a r s o l u t i o n m o d e l on the t h r e e b i n a r y s i d e s . H o w e v e r , as soon a s the b e h a v i o r of the b i n a r y s y s t e m s is m o r e c o m p l i c a t e d , t h e r e e x i s t s e v e r a l conflicting m e t h o d s of e x t r a p o l a t i o n ) 4 As a c o n s e q u e n c e , a p a r t i c u l a r m e t h o d has to be c h o s e n . In this r e p o r t , a new method will be p r e s e n t e d which m a y be r e g a r d e d as a n a t u r a l f i r s t a p p r o x i m a t i o n f o r the p a r t i c u l a r kind of s y s t e m which is h e r e b e i n g d i s cussed. In o r d e r to obtain an adequate d e s c r i p t i o n of the F e - M n - S s y s t e m , it is m o s t i m p o r t a n t to have a good r e p r e s e n t a t i o n of the p r o p e r t i e s of the liquid p h a s e , b e c a u s e it extends o v e r the c o m p l e t e r a n g e of c o m position. I n a r e c e n t a n a l y s i s of the F e - S s y s t e m Is it was found that an i n t e r s t i t i a l m o d e l could be s u c c e s s fully applied up to about 49 at. pct of s u l f u r i n spite of the fact that the c h e m i c a l n a t u r e of the s o l u t i o n of s u l f u r in liquid i r o n is c e r t a i n l y quite d i f f e