• PDF / 1,619,536 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 24 Downloads / 265 Views

DOWNLOAD

REPORT

---

D U A L - P H A S E s t e e l s , so called b e c a u s e they consist e s s e n t i a l l y of a m i x t u r e of f e r r i t e and m a r t e n s i t e , have a r o u s e d c o n s i d e r a b l e i n t e r e s t in the last few y e a r s b e c a u s e they offer the p o s s i b i l i t y of i n c r e a s e d f o r m a b i l i t y at a given tensile s t r e n g t h . 1'2 F o r example, a d u a l - p h a s e s t e e l with a t e n s i l e s t r e n g t h of 655 M P a (95 ksi) has a total elongation of about 28 pct, while a conventional high s t r e n g t h low alloy (HSLA) s t e e l of s i m i l a r t e n s i l e strength has a total elongation of about 20 pct. 3 It has been proposed that the p r o p e r t i e s of the f e r r i t e a r e r e s p o n s i b l e for the enhanced ductility of d u a l - p h a s e steels; 4 the f e r r i t e should p o s s e s s a fine grain s i z e for maximum strength, and be f r e e of fine p r e c i p i t a t e s and i n t e r s t i t i a l alloying e l e m e n t s , so as to m a x i m i z e the ductility. The main r o l e of the m a r t e n s i t e is to control the o v e r a l l s t r e n g t h level of the composite d u a l - p h a s e steel; the strength of the d u a l - p h a s e s t e e l s is given by a law of m i x t u r e s b e tween a s t r o n g m a r t e n s i t e and a weak f e r r i t e . 5 The main opportunity to optimize the p r o p e r t i e s of a d u a l - p h a s e s t e e l , that is to m a x i m i z e the ductility at a given t e n s i l e strength, has been shown to be by i n c r e a s i n g the s t r e n g t h of the f e r r i t e while m a i n t a i n ing a high ductility.4,s The p r i n c i p l e ways to i n c r e a s e f e r r i t e s t r e n g t h a r e by grain r e f i n e m e n t and solid solution strengthening; the p r e s e n t d u a l - p h a s e s t e e l s a r e a l r e a d y f i n e - g r a i n e d (grain diam