Formation of longitudinal, midface cracks in continuously-cast slabs
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T HE s u r f a c e q u a l i t y of a s t r a n d - c a s t s l a b can s u f f e r c o n s i d e r a b l y f r o m the p r e s e n c e of l o n g i t u d i n a l c r a c k s that often a p p e a r in the m i d d l e two t h i r d s of the b r o a d face, a s shown in F i g . 1. U n l e s s the c r a c k s a r e r e moved b y s c a r f i n g p r i o r to the s l a b b e i n g r o l l e d , r e s u l t a n t d e f e c t s w i l l show up in the f i n a l p r o d u c t ; and in c a s e s of e x t r e m e c r a c k i n g , the s l a b m u s t be s c r a p p e d . It is not s u r p r i s i n g , t h e r e f o r e , t h a t longitudinal, m i d f a c e c r a c k s have b e e n the s u b j e c t of n u m e r o u s s t u d i e s . A r e v i e w 1 of t h i s work, which h a s been cond u c t e d m a i n l y in plant, has shown that a l a r g e n u m b e r of o p e r a t i n g f a c t o r s can influence the f r e q u e n c y and s e v e r i t y of the c r a c k s : a) m o l d c o n d i t i o n s - i n adequate taper, improper mold powder, irregular mold o s c i l l a t i o n , i m p r o p e r w a t e r cooling, m o l d w e a r ; b) s u b m o l d c o n d i t i o n s - o v e r cooling o r u n d e r cooling in the u p p e r s p r a y z o n e s , type of s u p p o r t b e l o w the mold (grids, plates or roils), poor alignment between m o l d and s u b m o l d s u p p o r t a s s e m b l i e s ; c) s t e e l c o m p o s i t i o n - 0 . 0 6 < p c t C < 0.18, high p c t S, low M n / S , d) i n c r e a s i n g w i d t h / t h i c k n e s s of s l a b s ; e) high p o u r ing t e m p e r a t u r e ; f) m e t a l - s t r e a m i m p i n g e m e n t on the b r o a d face; and g) v a r y i n g o r i n c r e a s i n g c a s t i n g s p e e d . F r o m t h i s it i s c l e a r that the c r a c k s a r e g e n e r a t e d in the u p p e r p a r t of the c a s t i n g m a c h i n e . H o w e v e r , b e c a u s e l i t t l e b a s i c w o r k has b e e n u n d e r t a k e n , the p r e c i s e r o l e of e a c h of the f a c t o r s in the c r a c k i n g p r o c e s s r e m a i n s p o o r l y u n d e r s t o o d . In p a r t i c u l a r , the i n f l u e n c e of o p e r a t i n g v a r i a b l e s on the g e n e r a t i o n of t e n s i l e s t r e s s e s a t the s l a b s u r f a c e , and on the m e c h a n i c a l b e h a v i o r of the s o l i d i f i e d s h e l l at s t r a n d c a s t i n g t e m p e r a t u r e s have not b e e n e l u c i d a t e d c l e a r l y and r e l a t e d to a c t u a l c a s t i n g c o n d i t i o n s . In the p r e s e n t work, a d e t a i l e d i n v e s t i g a t i o n of t h i s problem has been undertaken, Samples containing longitudinal, m i d f a c e c r a c k s f r o m s l a b s c a s t at two J. K. BRIMACOMBE is Associate Professor, F. WEINBERG is Professor, and E. B. HAWBOLT is Associate Professor, Department of Metallurgical Engineering, University of British Columbia, Vancouver, B.C. V6T lW5 Canada. Manuscript submitted October 2, 1978. METALLURGICAL TRANSACTIONS B
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