The deformation behavior of a vanadium-strengthened dual phase steel
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with the t h i c k n e s s of the s h e e t s and the m e c h a n i c a l t e s t s p e r f o r m e d . T e n s i l e s p e c i m e n s 50 m m (2 in.) gage length by 12.5 m m (1/2 in.) wide were m a c h i n e d p r i o r to heat t r e a t m e n t s and w e r e t e s t e d in an I n s t r o n m a c h i n e at r o o m t e m p e r a t u r e at a c r o s s h e a d rate of a p p r o x i m a t e l y 2 • 10 -2 m m / s (0.05 i n . / m i n ) . Impact t e s t s a m p l e s were s t a n d a r d Charpy s i z e except for t h e i r t h i c k n e s s (~6 mm); the length of the s p e c i m e n s was made 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 and the notch was made p e r p e n d i c u l a r to the s u r f a c e of the sheet. S t a n d a r d hour g l a s s shaped s p e c i m e n s 8 with a 2.5 m m gage length were used for the fatigue study. All heat t r e a t m e n t s were c a r r i e d out in a high t e m p e r a t u r e n e u t r a l s a l t pot. F o r the i n t e r c r i t i e a l IDP t r e a t m e n t s a m p l e s were held for 10 m i n at the r e q u i r e d t e m p e r a t u r e while for the fully a u s t e n i t i z e d ADP t r e a t m e n t 5 m i n was sufficient. The a n n e a l i n g t e m p e r a t u r e s chosen a r e shown in Fig. 1, together with the e q u i l i b r i u m c o m p o s i t i o n s for the a and ~, p h a s e s e x i s t i n g at these t e m p e r a t u r e s ; this e q u i l i b r i u m d i a g r a m was d e r i v e d f r o m the data p r e s e n t e d by Bain and Paxton 9 and is applicable for alloys c o n t a i n i n g 1 to 2 wt pet Mn. S p e c i m e n s were e i t h e r a i r cooled (still or forced air) or b r i n e quenched as r e q u i r e d . U n l e s s noted o t h e r w i s e the IDP t r e a t m e n t was 10 rain at 790~ and the ADP t r e a t m e n t was 5 m i n at 870~ both followed by a i r cooling. Most s p e c i m e n s used for the m e t a l l o g r a p h i c e x a m i n a t i o n were t e m p e r e d at 400~ for 30 rain to produce a d a r k e n e d m a r t e n s i t e when etched in a n i t a l - p i c r a l solution.
RESULTS A. M e t a l l o g r a p h y
PROCEDURE
The c o m p o s i t i o n s of the v a r i o u s heats of the V s t e e l s h e e t s used in this study a r e p r e s e n t e d in T a b l e I along R. G. DAVIESis Staff Scientist with the MetallurgyDepartment, Engineeringand Research Staff, Ford Motor Company, Dearborn, MI 48121. Manuscript submitted April 25, 1977. METALLURGICALTRANSACTIONSA
The s t r u c t u r e of the s t a n d a r d V s t e e l is, as shown in Fig. 2, a m i x t u r e of f e r r i t e and u n r e s o l v e d p e a r lite. Rashid ~,1~ noted that in addition to the p e a r l i t e , t h e r e were many c e m e n t i t e p a r t i c l e s s i t u a t e d m a i n l y at g r a i n b o u n d a r i e s , and a fine d i s t r i b u t i o n of v a n a d i u m c a r b o n i t r i d e s within the f e r r i t e g r a i n s . Both the IDP and ADP t r e a t m e n t s produce the dual phase, f e r r i t e
ISSN0360-2133/78/0110-0041$00.75/0 9 1978
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