Effect of chromium and molybdenum segregation on the ridging behavior of type 434 stainless steel
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ridging by aiding the retention of a band-like distribution of grains having a cube-on-face orientation in finished sheet of this steel is discussed. F E R R I T I C s t a i n l e s s s t e e l s , such as AISI T y p e s 4 3 0 (17 p c t C r ) a n d 434 (17 p c t C r , 1 p c t M o ) , a r e w i d e l y u s e d for decorative household and automotive i t e m s t h a t a r e f a b r i c a t e d by s e v e r e s t r e t c h i n g o r d e e p - d r a w ing operations. However, when s h e e t or s t r i p of t h e s e s t e e l s is s u b j e c t e d to t h e l a r g e s t r a i n s e n c o u n t e r e d d u r i n g s e v e r e forming, an u n d e s i r a b l e s u r f a c e defect c a l l e d " r i d g i n g " or " r o p i n g " often develops. This defect o c c u r s p a r a l l e l to t h e r o l l i n g d i r e c t i o n a n d a p p e a r s a s n a r r o w r a i s e d a r e a s , s i m i l a r to c o r r u g a t i o n s , on the s u r f a c e of a f o r m e d p a r t . B e c a u s e expensive g r i n d i n g a n d p o l i s h i n g o p e r a t i o n s a r e r e q u i r e d to e l i m i n a t e t h e r i d g e s , a l a r g e a m o u n t of r e s e a r c h h a s b e e n c o n d u c t e d by t h e s t e e l i n d u s t r y t o d e v e l o p m i n i m u m ridgingferritic stainless steels. R e c e n t l y , C h a o p r o p o s e d a m e c h a n i s m o f r i d g i n g .1'4 T h e m o s t s a l i e n t f e a t u r e o f his p r o p o s a l is t h a t s e v e r e r i d g i n g o c c u r s whenever a well-developed cube-on-face ( C F ) t e x t u r e ({001} ( 0 1 1 ) ) i s r e t a i n e d a n d d i s t r i b u t e d a s b a n d s within t h e cube-on-corner (CC) m a t r i x t e x ture ({111} (011) and {111} (112)) of finished s h e e t . C h a o c o n f i r m e d the p r e s e n c e a n d d i s t r i b u t i o n of t h e s e t e x t u r e s by a n e t c h - p i t t i n g t e c h n i q u e . W h e n t h e C F t e x t u r e was a b s e n t o r was r a n d o m l y d i s t r i b u t e d in t h e m a t r i x t e x t u r e , r i d g i n g was m u c h less s e v e r e . P o u i l l a r d and Osdoit2 have suggested that t h i s banded
distribution results from the replacement of large, elongated grains in the hot-rolled structure, after annealing, by aggregates of smaller grains with similar orientations. Although Chao has shown that ridging can occur in body-centered-cubic (bcc) materials that are entirely ferritic during hot rolling as well as in those, such as Type 430 steel, that consist of two phases (austenite and ferrite), the two-phased structure may still contribute to ridging. For example, because chromium and molybdenum are known to retard the recrystallization of cold-rolled iron during annealing,3 a band-like distribution of these elements could lead to retention of a similar distribution of the CF texture in finished Type 434 steel sheet. Evidence of the bandlike segregation of chromium and molybdenum in Type 434 stainless steel and the effect of this segregation pattern on ridging are discussed in the present paper. J
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