An analysis of the open-coil decarburization of steel
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ALTHOUGH
the e q u i l i b r i u m t h e r m o d y n a m i c s hz of a t m o s p h e r e c o m p o s i t i o n on the c a r b u r i z a t i o n o r d e c a r b u r i z a t i o n of s t e e l have b e e n a d e q u a t e l y i n v e s t i g a t e d , the c o m p l e x i t y of p a r a m e t e r s affecting d e c a r b u r t z a t i o n d u r i n g o p e n - c o i l a n n e a l i n g on a c o m m e r c i a l s c a l e h a s p r e c l u d e d o p t i m i z a t i o n of p r o c e s s efficiency. The i n a b i l i t y to a c c u r a t e l y m o n i t o r and c o n t r o l f u r n a c e a t m o s p h e r e c o m p o s i t i o n , the l a c k of i n f o r m a t i o n a v a i l a b l e on the effect of g a s flow r a t e and the c o m p l e x e f f e c t of t e m p e r a t u r e on both the k i n e t i c s of d e c a r b u r i z a t i o n and m e t a l l u r g i c a l p r o p e r t i e s have a l l c o n t r i b uted to the r e l a t i v e i n e f f i c i e n c y of the c o m m e r c i a l open-coil decarburization process. Decarburtzation of s t e e l s t r i p is w i d e l y e m p l o y e d in the s t e e l i n d u s t r y to p r o d u c e b a s e c o m p o s i t i o n s s u i t a b l e for e n a m e l i n g and e l e c t r i c a l l a m i n a t i o n a p p l i c a t i o n s . The c o m m e r c i a l p r o d u c t i o n of l a r g e t o n n a g e s of d e c a r b u r i z e d s t e e l s t r i p is n o r m a l l y a c h i e v e d by u t i l i z ing the o p e n - c o i l a n n e a l i n g (OCA) p r a c t i c e d e v e l o p e d b y Lee Wilson E n g i n e e r i n g Company. A unique f e a t u r e of t h i s p r o c e s s is the p r o v i s i o n f o r r e a d y a c c e s s of f u r n a c e g a s e s to the s t e e l s u r f a c e through s e p a r a t i o n of the individual c o i l l a p s b y m e a n s of w i r e . In a d d i tion to f a c i l i t a t i n g m o r e i n t i m a t e c o n t a c t between the s t e e l s u r f a c e and r e a c t i n g g a s e s , s e p a r a t i o n of the coil l a p s p e r m i t s m o r e r a p i d h e a t i n g and cooling. A s s u m i n g r e a d y a c c e s s and a v a i l a b i l i t y to r e a c t a n t g a s e s for oxidation of c a r b o n at the s t e e l s u r f a c e and, f u r t h e r m o r e , the a b s e n c e of r a t e - c o n t r o l l i n g s u r f a c e r e a c t i o n s , the d e c a r b u r i z a t i o n p r o c e s s m a y be d e s c r i b e d as diffusion c o n t r o l l e d . Depending on t e m p e r a t u r e (with r e s p e c t to At), the p r o c e s s is f u r t h e r c o m p l i c a t e d b y e i t h e r an a u s t e n i t e - f e r r i t e o r a F e 3 C - f e r r i t e p h a s e change. The g e n e r a l c o n c e p t s of diffusion a c c o m p a n i e d b y a p h a s e change h a v e b e e n d e s c r i b e d b y D a r k e n . a Smith 4 a n a l y z e d the r a t e of motion of the b o u n d a r y between ~ i r o n and t~ + y i r o n o r ~ + FeaC d u r i n g d e c a r b u r t z a t i o n at v a r i o u s t e m p e r a t u r e s b e t w e e n 773 K and 1138 K,
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