Recrystallization of austenite
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J. J. BAUWIN,
AND
J. C. RALEY
A l a b o r a t o r y s i m u l a t i o n of h o t - r o l l i n g s c h e d u l e s , which e m p l o y s a s e q u e n c e of highspeed c o m p r e s s i o n s p e r f o r m e d over a r a n g e of t e m p e r a t u r e s , was used to d e t e r m i n e the effects of c o m p o s i t i o n and p r o c e s s i n g v a r i a b l e s on the s t a t e of the a u s t e n i t e at s e v e r a l points in the h o t - r o l l i n g p r o c e d u r e . The type and q u a n t i t y of c a r b i d e a n d / o r n i t r i d e f o r m i n g e l e m e n t s (Nb(Cb), V, A1) in the s t e e l s were v a r i e d to p r o d u c e p r e c i p i t a t e s at v a r i o u s points in the h o t - w o r k i n g r a n g e of t e m p e r a t u r e s . T i m e - t e m p e r a t u r e d e f o r m a t i o n s c h e d u l e s were d e s i g n e d a r o u n d s c h e d u l e s c u r r e n t l y used in the h o t - r e d u c t i o n of plate s t e e l s ; v a r i a t i o n s were i n t r o d u c e d to d e t e r m i n e the effects of p r o c e s s i n g v a r i a b l e s on the d e v e l o p m e n t of the a u s t e n i t e s t r u c t u r e s . At v a r i o u s points in the d e f o r m a t i o n s c h e d u l e , s p e c i m e n s w e r e quenched out and c h a n g e s in the a u s t e n i t e g r a i n s t r u c t u r e w e r e e x a m i n e d . C h a n g e s i n the v o l u m e and a s p e c t r a t i o of the p r i o r - a u s t e n i t e g r a i n s were used as m e a s u r e s of the d e g r e e of a u s t e n i t e r e c r y s t a l l i z a t i o n . At t e m p e r a t u r e s above the p r e c i p i t a t i o n - s t a r t t e m p e r a t u r e , light r e d u c t i o n s (< 10 pct) c a u s e d g r a i n c o a r s e n i n g . C o a r s e n i n g could be e l i m i n a t e d by i n c r e a s i n g the r e d u c t i o n p e r p a s s , d e c r e a s i n g the r o l l i n g t e m p e r a t u r e s , or i n c r e a s i n g the c o n c e n t r a t i o n of c a r b i d e or n i t r i d e f o r m i n g e l e m e n t s to r a i s e the p r e c i p i t a t i o n - s t a r t t e m p e r a t u r e . It is c l e a r f r o m t h e s e r e s u l t s that a t t e m p t s to study s t r u c t u r a l changes that o c c u r d u r i n g plate r o l l i n g by u s i n g a few h i g h - r e d u c t i o n p a s s e s m a y p r o d u c e e r r o n e o u s r e s u l t s . INTRODUCTION C O N T R O L L E D r o l l i n g has proved effective in r e fining the f e r r i t e g r a i n size of HSLA s t e e l s . T h i s r e s u l t s p r i n c i p a l l y f r o m c o n t r o l of the s i z e and shape of the a u s t e n i t e g r a i n s t r u c t u r e at the point of t r a n s f o r m a t i o n . P r e c i p i t a t i o n of a l l o y c a r b i d e s a n d / o r n i t r i d e s i n h i b i t s the growth of r e c r y s t a l l i z e d a u s t e n i t e g r a i n s at high (roughing) t e m p e r a t u r e s , and at the lower f i n i s h i n g t e m p e r a t u r e , p r e c i p i t a t e s a p p a r e n t l y inhibit r e c r y s
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