Interaction between deformation, recrystallization and precipitation in niobium steels
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R. DORVEL,
AND
R. A. PETKOVIC
T h e p r e s e n t study was u n d e r t a k e n in o r d e r to e s t a b l i s h the e x t e n t to which n i o b i u m ( c o l u m b i u m ) inhibits r e c o v e r y and r e c r y s t a l l i z a t i o n of m i c r o a l l o y e d a u s t e n i t e while p r e s e n t as solute on the one hand, and as c a r b o n i t r i d e p r e c i p i t a t e s on the o t h e r . T h r e e s t e e l s w e r e used; the f i r s t was an HSLA s t e e l c o n t a i n i n g 0.054 pct Nb, 0.05 pct C and 0.92 pct Mn. T h e s e c o n d was p r e p a r e d by t r e a t i n g the f i r s t in wet h y d r o g e n at l l 0 0 ~ so as to r e d u c e the C and N content to about 10 ppm by wt. T h e t h i r d m a t e r i a l was a plain c a r b o n s t e e l containing 0.055 pct C and 0.41 pct Mn. T h e i s o t h e r m a l r e c o v e r y and r e c r y s t a l l i z a t i o n of t h e s e m a t e r i a l s , a f t e r an i n t e r v a l of hot w o r k i n g , w as s t u d i e d by m e a n s of i n t e r r u p t e d c o m p r e s s i o n t e s t s . S a m p l e s w e r e p r e s t r a i n e d at 10 -2 and 10-~s -1 at 900, 950 and 1000~ to n a t u r a l s t r a i n s of 0.10 and 0.25, and held i s o t h e r m a l l y p r i o r to r e l o a d i n g . T h e r e s u l t s obtained in t h i s way, i n d i c a t e that 0.56 pet of s u b s t i t u t i o n a l s o l u t e can g i v e r i s e to an o r d e r of m a g n i t u d e d e c r e a s e in the r a t e of r e c r y s t a l l i z a t i o n . When p r e c i p i t a t i o n of Nb(CN) t a k e s p l a c e e i t h e r d u r i n g d e f o r m a t i o n or in s t r a i n e d a u s t e n i t e the m e a n p r e c i p i t a t e s i z e is ~ 20 n m . Th e p r e s e n c e of s u c h p a r t i c l e s inhibits b o t h s t a t i c r e c o v e r y and r e c r y s t a l l i z a t i o n ; the m a g n i t u d e of the e f f e c t b e i n g d e p e n d e n t on the v o l u m e f r a c t i o n of p r e c i p i t a t e . When the v o l u m e f r a c t i o n r e a c h e s about 0.02 pct, s t a t i c r e c r y s t a l l i z a t i o n is c o m p l e t e l y s u p p r e s s e d f o r the p r e s e n t p r e s t r a i n i n g c o n d i t i o n s . 1. INTRODUCTION
IT
is now w e l l e s t a b l i s h e d that the a d d i t i o n of n i o b i u m to plain c a r b o n s t e e l s is e f f e c t i v e in r e t a r d i n g both d y n a m i c and s t a t i c r e c r y s t a l l i z a t i o n . l - 1 5 A d e b a t e c o n t i n u e s , h o w e v e r , on the o r i g i n of the o b s e r v e d r e t a r d a t i o n . Many a u t h o r s 1-11 have d e m o n s t r a t e d the i n hibition of r e c r y s t a l l i z a t i o n by fine p r e c i p i t a t e s (< 25 n m in diam), while o t h e r s have a r g u e d that solute i n t e r a c t i o n s a l s o play a s i g n i f i c a n t r o l e in the c o n t r o l of 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 . '~-1S Since s o l ute i n t e r a c t i o n s , if p r e s e n t , would be e f f e c t i v e at h i g h e r t e m p e r a t u r e
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