The mechanical properties of zero- carbon ferrite-plus-martensite structures
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1. INTRODUCTION
IN the
l a s t few y e a r s a new type of steel, c o n s i s t i n g of about 15 to 20 pct m a r t e n s i t e in a f e r r i t e m a t r i x , has b e c o m e of technological i n t e r e s t b e c a u s e at a c o n s t a n t t e n s i l e s t r e n g t h it exhibits g r e a t e r f o r m a b i l i t y than a s t a n d a r d h i g h - s t r e n g t h l o w - a l l o y (HSLA) steel. 1-5 F o r example, when VAN-80 (a J o n e s & Laughlin Steel Co. HSLA steel) is obtained in the m a r t e n s i t e plus f e r r i t e condition, the yield s t r e s s d e c r e a s e s f r o m 550 to 380 M P a (80 to 55 ksi) while t h e r e a r e i n c r e a s e s in the total elongation f r o m about 18 to 26 pct and in the s t r a i n h a r d ening exponent, n, f r o m about 0.12 to 0.22; the t e n s i l e s t r e n g t h r e m a i n s a p p r o x i m a t e l y constant, independent of s t r u c t u r e , at 700 M P a (100 ksi). 2~ T h e s e s t e e l s with a m a r t e n s i t e plus f e r r i t e s t r u c t u r e a r e often called d u a l phase s t e e l s . '-5 Mileiko 6 and G a r m o n g and T h o m p s o n 7 have developed a theory of the m e c h a n i c a l p r o p e r t i e s of f i b e r c o m p o s ites of two d u c t i l e p h a s e s tested in t e n s i o n p a r a l l e l to the fiber axis; good a g r e e m e n t b e t w e e n e x p e r i m e n t and theory was found for Ni-W, Cu-W and A g - s t a i n l e s s s t e e l c o m p o s i t e s . 6'7 It has b e e n shown 4'5 that the changes in t e n s i l e s t r e n g t h and d u c t i l i t y of dual p h a s e s t e e l s as a function of the p e r c e n t m a r t e n s i t e a r e in a g r e e m e n t with this theory even though the m a r t e n s i t e is not in a f i b r o u s f o r m . The v a l u e s of the t e n s i l e s t r e n g t h and ductility of the component f e r r i t e and m a r t e n s i t e used to c o m p a r e e x p e r i m e n t with theory w e r e obtained by extrapolation; a m a j o r p r o b l e m in m e a s u r i n g the p r o p e r t i e s of the p h a s e s i n d i v i d u a l l y is that h i g h - c a r b o n m a r t e n s i t e s (>0.3 pct C), as found in d u a l - p h a s e VAN-80, a r e v e r y notch s e n s i t i v e and have v e r y l i m i t e d t e n s i l e ductility. In addition t h e r e may be m o v e m e n t of alloying e l e m e n t s into and out of the f e r r i t e and thus its c o m p o s i t i o n is not e a s i l y d e t e r m i n e d . To m o r e d i r e c t l y check the a p p l i c a b i l i t y of the c o m p o site theory to d u a l - p h a s e alloys a z e r o c a r b o n F e - 3 N i 3Mo alloy was chosen for i n v e s t i g a t i o n . This alloy can be obtained in e i t h e r the fuU m a r t e n s i t i c or fully f e r R. G. DAVIES is Senior Staff Scientist, Ford Motor Company, Dearborn, MI48121. Manuscript submitted September 8, 1977. METALLURGICALTRANSACTIONSA
r i t i c s t a t e s 8 or in the d u a l - p h a s e condition. This m a k e s it p o s s i b l e to m e a s u r e the p r o p e r t i e
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