The effects of experimental rolling on microstructure and mechanical properties of a Ni-Cu-Cb steel
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The effects of hot r o l l i n g and w a r m working on m e c h a n i c a l p r o p e r t i e s and m i c r o s t r u c t u r e of N i - C u - C b s t e e l were i n v e s t i g a t e d . In the hot r o l l e d condition o p t i m u m s t r e n g t h and t o u g h n e s s r e s u l t e d f r o m r o l l i n g in the upper p o r t i o n of the a u s t e n i t e plus f e r r i t e phase field. This t r e a t m e n t produced a f i n e - g r a i n e d s t r u c t u r e c o n s i s t i n g of equiaxed and a c i c u l a r f e r r i t e . U n u s u a l l y high s t r e n g t h - t o u g h n e s s c o m b i n a t i o n s were r e a l i z e d by w a r m working the hot r o l l e d s t e e l s at 1200~ I N r e c e n t y e a r s t h e r e has b e e n growing i n t e r e s t in the use of 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 to obtain i m p r o v e m e n t s in m e c h a n i c a l p r o p e r t i e s . A u s f o r m i n g is now only one of a v a r i e t y of 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 c u r r e n t l y b e i n g e m p l o y e d for i m p r o v i n g the m e c h a n i c a l p r o p e r t i e s of s t e e l . Of the v a r i o u s f a c t o r s which a r e influenced by 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 , g r a i n size is the m o s t i m p o r t a n t since a r e f i n e m e n t in g r a i n size can i m p r o v e both s t r e n g t h and i m p a c t r e s i s tance. T h e r e have b e e n n u m e r o u s e x p e r i m e n t a l v e r i f i c a t i o n s of the H a l l - P e t c h r e l a t i o n s h i p which p r e d i c t s that y i e l d s t r e n g t h i n c r e a s e s in d i r e c t p r o p o r t i o n to d - 1 ' 2 where d i s the g r a i n d i a m e t e r . Since c u r r e n t hot r o l l i n g p r a c t i c e for N i - C u - C b s t e e l and other s t e e l s that contain g r a i n r e f i n i n g e l e m e n t s y i e l d s f e r r i t e g r a i n s i z e s on the o r d e r of ASTM 7 to 9, the p r o p e r t i e s obtained to date c l e a r l y r e p r e s e n t only a f r a c t i o n of what might be a c h i e v e d if u l t r a f i n e g r a i n s i z e s of ASTM 14 or s m a l l e r could be developed. While t e c h n i q u e s a r e c u r r e n t l y a v a i l a b l e for developing a u s t e n i t e g r a i n s i z e s in the range ASTM 13 to 15 in a wide v a r i e t y of s t e e l c o m p o s i t i o n s , a these t e c h n i q u e s a r e not r e a d i l y a m e n a b l e to c o m m e r c i a l r o l l i n g p r a c t i c e . The p r e s e n t paper is c o n c e r n e d with a l t e r n a t i v e methods of obtaining fine g r a i n s i z e s , in p a r t i c u l a r r o l l i n g in the a u s t e n i t e plus f e r r i t e r e g i o n . T h i s work was p r o m p t e d by r e c e n t s t u d i e s of two phase s t a i n l e s s s t e e l s in which it was d e m o n s t r a t e d that hot r o l l i n g in the a u s t e n i t e plus f e r r i t e r e g i o n y i e l d e d a " m i c r o d u p l e x " s t r u c t u r e with e x c e p t i o n a l m e c h a n i c
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