Influence of martensite composition and content on the properties of dual phase steels
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a p p r o x i m a t e l y 1.2 m m (0.050 in.);, the a n a l y z e d c o m p o s i t i o n s of t h e s e a l l o y s is shown in T a b l e I. A f e r r i t e g r a i n s i z e of about 10 ~ m was obtained by a n n e a l ing f o r 5 min j u s t above the A3 t e m p e r a t u r e f or the al l o y and a i r cooling; the dual p h ase s t r u c t u r e was o b t a i n e d by holding f o r 10 rain at t e m p e r a t u r e s within l i m i t s shown by the shaded a r e a of the ~ + y p h a s e field, F i g . 1, 6 and then q u e n c h i n g into a b r i n e s ol ut i on. It is p o s s i b l e to c o n s i d e r t h e s e a l l o y s as p s e u d o b i n a r i e s b e t w e e n F e - M n and C b e c a u s e Mn would not have t i m e to p a r t i t i o n to any s i g n i f i c a n t e x t e n t d u r i n g the i n t e r c r i t i c a l anneal; it can be c a l c u l a t e d that a f t e r 10 min at 740~ Mn would diffuse l e s s than 0.03 p m in a u s t e n i t e . 7 A l l heat t r e a t m e n t s w e r e c a r r i e d out in a n e u t r a l salt bath. 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 w e r e 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 r a t e of a p p r o x i m a t e l y 2 • 10 -2 m m / s (0.05 in. p e r min). 3. R E S U L T S AND DISCUSSION 3.1. S t r e n g t h a) V a r i a t i o n With M a r t e n s i t e Content. Th e flow s t r e s s as a function of quench t e m p e r a t u r e and c a r b o n content of the al l o y is shown in F i g . 2. It can be s e e n that at a given quench t e m p e r a t u r e , the flow s t r e s s is
Table I, Composition (Wt Pct) of Alloys Studied, Balance Fe
No.
C
Mn
1 2 3 4 5 6 7 8 9 10
0,06 0.10 0.15 0.16 0.20 0.20 0.22 0.27 0.28 0.29
1.37 1.36 1.37 1.30 1.30 1.38 1.20 1.40 1.38 1,50
Other* -
No.
C
Mn
l1 12 t3 14 15 16 17 18 19 20
0.31 0.36 036 0.42 0.47 0.52 0.14 0.14 0.14 0.15
1.34 1.45 1.30 1.25 1.30 1.25 1.47 0.72 0.72 1.49
Other* 0.25Mo, 0.06 Nb 0.26Mo, 0.05 Nb 0.28Mo 0.06Nb
*Allalloys contain approximately0.25 pct Si and 0.07 pct aluminumfor deoxidation.
ISSN 0360-2133/78/0510-0671500.75/0 9 1978 AMERICANSOCIETY FOR METALSAND THE METALLURGICALSOCIETY OF AIME
VOLUME 9A, MAY 1978-671
Fig. 1--The derived equilibrium diagram for Fe-1 wt pct M n - C a l l o y s ; the s h a d e d a r e a i n d i c a t e s the r e g i o n in w h i c h alloys were heat treated.
l i n e a r l y dependent upon the c a r b o n content, and that at a given c a r b o n level the higher the quench t e m p e r a t u r e the l a r g e r is the flow s t r e s s . However the m a r t e n s i t e ~ontent of any alloy is a function of the c a r b o n content and the quenching t e m p e r a t u r e . A t y p i c a l s e r i e s of m i c r o g r a p h s for alloys quenched f r o m 740, is shown in Fig. 3; the alloys were t e m p e r e d at 400~ for 1/2 h p r i o r to p o l i s h i n g so as to e n h a n c e the c o n t r a s t b
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