Application of phase diagrams to predict phases formed during deoxidation of steel
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E , A R L Y s t u d i e s of the deoxidatton of s t e e l w e r e b a s e d on the a s s u m p t i o n that the r e a c t i o n s b e t w e e n the d e o x i dant and oxygen r a p i d l y p r o c e e d t o w a r d e q u i l i b r i u m with the m o s t s t a b l e oxide p h a s e . In r e c e n t y e a r s t h e s e c o n c e p t s have b e e n c h a l l e n g e d on the g r o u n d s that k i ne ti c and p h y s i c a l l i m i t a t i o n s p r e v e n t such an e q u i l i b r i u m f r o m being a p p r o a c h e d r a p i d l y . The p u r p o s e of this c o n t r i b u t i o n is to d e m o n s t r a t e that by p r o j e c t i n g the path of r e a c t i o n s with the aid of p h a s e d i a g r a m s , the m e t a s t a b l e o r n o n e q u i l i b r i u m conditions can be a n t i c i p a t e d in p r a c t i c e . Some e x p e r i m e n t a l and p r a c t i c a l o b s e r v a t i o n s can be r a t i o n a l i z e d u s i n g this method of a n a l y s i s . We s h a l l f i r s t d i s c u s s i s o t h e r m a l d e o x i d a tlon of s t e e l by a l u m i n u m alone, with m a n g a n e s e , and with m a n g a n e s e plus s i l i c o n , and then c o n s i d e r c h a n g e s o c c u r r i n g d u r i n g c o o l i n g and s o l i d i f i c a t i o n of s t e e l s d e o x i d i z e d by s i l i c o n o r a l u m i n u m . I S O T H E R M A L DEOXIDATION Strabe et al. ~ m a d e s o m e i n t e r e s t i n g o b s e r v a t i o n s tn s a m p l e s taken f r o m induction m e l t s (Table I). T h e s e e x p e r i m e n t s m a y be c o n s i d e r e d as i s o t h e r m a l d e o x i d a tion with a l u m i n u m . F o r low m a n g a n e s e s t e e l s (< 0.08 pc t Mn), the i n c l u s i o n s had a c o r e containing s u b s t a n t i a l a m o u n t s of F e - M n o x i d e s , which p e r s i s t e d f o r m o r e than 30 s e c . The I n c l u s i o n s o b s e r v e d in the s a m p l e s taken i m m e d i a t e l y a f t e r adding a l u m i n u m w e r e liquid tn n a t u r e . F o r h i g h e r m a n g a n e s e (> 0.35 pct) the p r o p o r t i o n of M n - o x i d e in the c o r e was s u b s t a n t i a l l y h i g h e r . Strabe el al. a l s o m a d e the o b s e r v a t i o n that in the two h i g h e r m a n g a n e s e s t e e l s the c o r e had the c o m p o s i t i o n of g a l a x i t e i n c l u s i o n s while the s u r f a c e was a l m o s t p u r e a l u m i n a . T h e s e o b s e r v a t i o n s p r o v i d e an e x c e l l e n t e x a m p l e of a d e o x i d a t i o n p h e n o m e n o n that we can e x pl ai n by the use of a p h a s e d i a g r a m . We will s t a r t with the F e - A 1 - O p h a s e d i a g r a m , f o r which a s c h e m a t i c i s o t h e r m a l s e c t i o n at 1600~ is shown In Fig. 1. 2 Due to the e x t r e m e l y low c o n c e n t r a -
ttons of a l u m i n u m (10 -8 to 10 -s pet) which a r e tn e q u i l i b r i u m with s u b s t a n t i a l l y l a r g e r c o n c e n t r a t i o n s of oxygen (10 -s to 10 -x pct), the d i a g r a m is
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