Effect of alloying elements on the microstructure and properties of a hot-rolled low-carbon low-alloy bainitic steel
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A t w o - l e v e l full f a c t o r i a l s t a t i s t i c a l e x p e r i m e n t c o n s i s t i n g of eight a l l o y s was c o n d u c t e d to d e t e r m i n e the effect of 2 pct cobalt, 1 pct n i c k e l and 1 p c t c h r o m i u m on the h o t - r o l l e d m i c r o s t r u c t u r e and p r o p e r t i e s of a b a i n i t i c s t e e l containing 0.2 pct C, 2 p c t Mn, 1 p c t Si, 0.75 pct Mo and 0.003 p c t B. The r e s u l t s i n d i c a t e that c h r o m i u m induced the f o r m a t i o n of the a c i c u l a r b a i n i t i c s t r u c t u r e w h i l e c o b a l t f a v o r e d m a s s i v e f e r r i t e f o r m a t i o n and r e s u l t e d in i s l a n d s of m a r t e n s i t e a n d / o r a u s t e n i t e . N i c k e l , when a d d e d singly, did not a p p e a r to influence the m i c r o s t r u c t u r e but in c o m b i n a t i o n with c h r o m i u m , enhanced the f o r m a t i o n of the l o w e r b a i n i t i c s t r u c t u r e . T h e m e c h a n i c a l p r o p e r t i e s w e r e s t a t i s t i c a l l y a n a l y z e d and s t a t i s t i c a l equations w e r e o b t a i n e d to p r e d i c t o p t i m i z e d c o m p o s i t i o n s . T h e s e e q u a t i o n s i n d i c a t e that c h r o m i u m i n c r e a s e d the t o u g h n e s s of t h e s e s t e e l s m o r e than n i c k e l . H o w e v e r , it was shown that with s i m i l a r b a i n i t i c s t r u c t u r e s , n i c k e l e n h a n c e d the t o u g h n e s s m o r e than c h r o m i u m . T h e r e s u l t s i l l u s t r a t e the s h o r t - c o m i n g of a p u r e s t a t i s t i c a l a p p r o a c h to the d e s i g n of a l l o y s .
THE d e m a n d
f o r h i g h - s t r e n g t h tough s t e e l s i s i n c r e a s i n g f o r c o n s t r u c t i o n a l , s t r u c t u r a l , p i p e l i n e and a u t o m o t i v e a p p l i c a t i o n s . M i n i m u m y i e l d s t r e n g t h s of 80 ksi* can be o b t a i n e d with l o w - c a r b o n m a n g a n e s e *A conversion table of units to SI is included in the Appendix.
s t e e l s with f e r r i t e - p e a r l i t e s t r u c t u r e s by m i c r o a l l o y ing with c o l u m b i u m , v a n a d i u m o r t i t a n i u m and b y cont r o l l e d hot r o l l i n g and cooling. Above 80 k s i m i n i m u m y i e l d s t r e n g t h , the s t e e l s have b a i n i t i c a n d / o r tempered martensitic structures. In this study, an a l l o y w a s d e s i r e d which would exhibit a 125 ksi m i n i m u m y i e l d s t r e n g t h with good t o u g h n e s s and w e l d i n g p r o p e r t i e s . G e r b e r i c h , e t a t 1 have shown that a m i x e d b a i n i t i c - m a r t e n s i t i c s t r u c t u r e in D6aC (0.26 p c t C, 0.70 p c t Mn, 1.14 p c t C r , 0.99 p c t Mo, 0.54 p c t Ni, 0.22 p c t Si and 0.02 p c t V) e x h i b i t e d b e t t e r t o u g h n e s s than a quenched and t e m p e r e d and a u s f o r m e d s t r u c t u r e . It is l i k e w i s e known that m i c r o s t r u c t u r e s c o n t a i n i n g p r o
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