Sulfide shape control in high strength low alloy steels
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AMONGs t r u c t u r a l
s t e e l s , the f a m i l y of hot r o l l e d high s t r e n g t h low alloy (HSLA) s t e e l s , with g u a r a n t e e d m i n i m u m y i e l d s t r e n g t h s f r o m 45,000 to 80,000 p s i , s h o w s a m o s t r e m a r k a b l e g r o w t h . ~ In d e v e l o p i n g t h e s e s t e e l s , s t r e n g t h i s only one of the p r o p e r t i e s c o n s i d e r e d . To s a t i s f y d i v e r s i f i e d c u s t o m e r r e q u i r e m e n t s and b e c o m p e t i t i v e , a ' % a l a n c e d p a c k a g e of p r o p e r t i e s " m u s t be o f f e r e d , T a b l e I. Two of t h e s e p r o p e r t i e s - - t o u g h n e s s and bend f o r m a b i l i t y - - a r e a d v e r s e l y a f f e c t e d by i n c l u s i o n s e l o n g a t e d d u r i n g hot r o i l i n g . Fig. 1 i l l u s t r a t e s t h i s effect on t o u g h n e s s ; i m p a c t c u r v e s a r e p l o t t e d f o r both l o n g i t u d i n a l and t r a n s v e r s e C h a r p y V - n o t c h , 1/2 s i z e s p e c i m e n s . In l o n g i t u d i n a l s p e c i m e n s , the i m p a c t e n e r g y at 100 p c t s h e a r , c o m m o n l y c a l l e d the s h e l f e n e r g y , i s in e x c e s s of 40 f t - l b ; in t r a n s v e r s e s p e c i m e n s , it i s 15 R - l b . T h i s d i f f e r e n c e i s c a u s e d p r i m a r i l y by the p r e s e n c e of e l o n g a t e d i n c l u s i o n s w h i c h a r e o r i e n t e d p a r a l l e l to t h e p l a n e of f r a c t u r e in the t r a n s v e r s e s p e c i m e n s . F i g . 2 shows the e f f e c t of e l o n g a t e d i n c l u s i o n s on b e n d f o r m a b i l i t y . In t r a n s v e r s e s p e c i m e n s (bend a x i s p a r a l l e l to the r o l l i n g d i r e c t i o n ) , s e v e r e c r a c k s a p p e a r in a s a m p l e bent o v e r a r a d i u s e q u a l to two t i m e s the p l a t e t h i c k n e s s , b u t in a l o n g i t u d i n a l s p e c i m e n b e n t o v e r the s a m e r a d i u s , no c r a c k i n g o c c u r s . T h i s e f f e c t of d i r e c t i o n a l i t y c a u s e d by flat, e l o n g a ted i n c l u s i o n s w a s h i g h l i g h t e d d u r i n g the d e v e l o p m e n t of an 80,000 p s i y i e l d s t r e n g t h low a l l o y s t e e l . The s t e e l w a s an a l u m i n u m k i l l e d , M n - V - N t y p e , in w h i c h the p r e d o m i n a n t i n c l u s i o n s w e r e i d e n t i f i e d a s m a n g a n e s e s u l f i d e s . To a s s u r e an a d e q u a t e bend f o r m i n g p e r f o r m a n c e , the s u l f u r c o n t e n t w a s l o w e r e d to a p p r o x i m a t e l y 0.010 p c t by c a r e f u l l y c o n t r o l l e d s t e e l m a k i n g practices. For some advanced part designs however, it w a s d e s i r a b l e to d e v e l o p b e n d f o r m i n g p e r f o r m a n c e c o m p a r a b l e to t h a t of m i l d s t e e l . LEON LUYCKX, JOHN R. BELL, and MICHAEL KORCHYNSKY are Senior Research Metallurgists and Assistant Director of Research, respectively, Graham
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