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J. B. VANDER

SANDE,

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MORRIS

COHEN

T h e r e s p o n s e of a u s t e n i t e s to t h e r m o m e c h a n i c a l t r e a t m e n t s is s t u d i e d in a s e r i e s of n i o b i u m ( c o l u m b i u m ) HSLA s t e e l s . I n t e r a c t i o n s b e t w e e n c o m p o s i t i o n , p l a s t i c d e f o r m a tion, s t r a i n - i n d u c e d p r e c i p i t a t i o n , and 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 a r e d e s c r i b e d and r e l a t e d to p r e v i o u s w o r k in the f i e l d . N i o b i u m in s o l u t i o n p r i o r to d e f o r m a t i o n l e a d s to s i g n i f i c a n t r e t a r d a t i o n of s u b s e q u e n t 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 if Nb(C,N) p r e c i p i t a t i o n t a k e s p l a c e p r i o r to o r d u r i n g the e a r l y s t a g e s of r e c r y s t a l l i z a t i o n . Such s t r a i n i n d u c e d p r e c i p i t a t i o n p r o c e e d s in two s t a g e s : i n i t i a l l y at a u s t e n i t i c g r a i n b o u n d a r i e s and d e f o r m a t i o n b a n d s , and l a t e r on s u b s t r u c t u r a l f e a t u r e s in the u n r e c r y s t a U i z e d a u s t e n i t e . T h e l a t t e r p r e c i p i t a t i o n is a c c e l e r a t e d only if it o c c u r s in the u n r e c r y s t a l l i z e d a u s t e n i t e ; if r e c r y s t a l l i z a t i o n p r e c e d e s Nb(C,N) p r e c i p i t a t i o n , then the p r e c i p i t a t i o n r e a c t i o n i s much s l o w e r . T h u s , the Nb(C,N) p r e c i p i t a t i o n and 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 r e a c t i o n s a r e coupled p h e n o m e n a . T h e c o n d i t i o n s n e c e s s a r y f o r s u c h an i n t e r a c t i o n a r e a n a l y z e d , and it is p r o p o s e d that the l e v e l of s u p e r s a t u r a t i o n of Nb(C,N) in the a u s t e n i t e at the d e f o r m a t i o n t e m p e r a t u r e is a c r i t i c a l f a c t o r in d e t e r m i n i n g w h e t h e r o r not an e f f e c t i v e i n t e r a c t i o n w i l l o p e r a t e at that t e m p e r a t u r e . H I G H - S T R E N G T H , l o w - a l l o y (HSLA) s t e e l s c o n s t i tute a c l a s s i c a l m e t a l l u r g i c a l d e v e l o p m e n t in which a l l o y i n g a d d i t i o n s and t h e r m o m e c h a n i c a l p r o c e s s i n g have b e e n c o m b i n e d to a t t a i n d e s i r a b l e m e c h a n i c a l p r o p e r t i e s t h r o u g h m i c r o s t r u c t u r a l c o n t r o l . Such p r o c e s s and c o m p o s i t i o n a l v a r i a t i o n s can d e v e l o p y i e l d s t r e n g t h s t y p i c a l l y in the r a n g e of 350 tO 700 M P a (50 to 100 k s i ) , thus doubling the c o r r e s p o n d i n g y i e l d s t r e n g t h of m i l d s t e e l s . T h i s i n c r e a s e d s t r e n g t h , coupled with good t o u g h n e s s and w e l d a b i l i t y , h a s r e s u l t e d in a p p l i c a t i o n s in t r a n s p o r t a t i o n , p i p e l i n e , c o n s t r u c t i o n , and p r e s s u r e