The strain aging characteristics of reinforcing steel with a range of vanadium contents
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The s t r a i n aging c h a r a c t e r i s t i c s of a r e i n f o r c i n g s t e e l made with vanadium ranging between z e r o and 0.1 pct and with two aluminum l e v e l s have been investigated for the a s hot r o l l e d condition. It has been shown that vanadium contents of 0.04 to 0.06 pct (a V/N r a t i o of 7 to 9) will r e s u l t in the combination of a l m o s t a l l the active nitrogen as vanadium nitride and s u p p r e s s e s n a t u r a l s t r a i n aging. Vanadium in e x c e s s of this level r e suits in the p r e c i p i t a t i o n of vanadium c a r b i d e and the consequential p r e c i p i t a t i o n h a r dening gives an i n c r e a s e in the yield strength, t e n s i l e strength, and impact t r a n s i t i o n t e m p e r a t u r e without i m p a r t i n g f u r t h e r b e n e f i c i a l effect with r e g a r d to s t r a i n aging. The mechanical p r o p e r t i e s have been shown to be g e n e r a l l y unaffected by the two different aluminum l e v e l s , and no g r a i n r e f i n e m e n t r e s u l t e d f r o m e i t h e r the vanadium or aluminum additions. T h i s absence of g r a i n r e f i n e m e n t and the p r e c i p i t a t i o n hardening r e s u l t s in an i n c r e a s e in the impact t r a n s i t i o n t e m p e r a t u r e with i n c r e a s i n g vanadium content, although this i n c r e a s e is i n i t i a l l y slow whilst the active nitrogen content is being reduced.
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d u c t i l i t y r e q u i r e m e n t s for r e i n f o r c i n g s t e e l s used in earthquake r e s i s t a n t concrete s t r u c t u r e s a r e significantly m o r e demanding than those used for n o r mal r e i n f o r c e d s t r u c t u r e s . This is consequential to the philosophy of design codes for s e i s m i c loading which r e q u i r e s t r u c t u r e s to r e s i s t only r e l a t i v e l y m o d e r a t e earthquakes e l a s t i c a l l y ; whereas, in m a j o r e a r t h q u a k e s r e l i a n c e is placed on the a v a i l a b i l i t y of sufficient ductility a f t e r yielding to enable a s t r u c t u r e to survive without c a t a s t r o p h i c failure.~ This d u c t i l i t y is e n s u r e d by the design of " p l a s t i c h i n g e s " into the s t r u c t u r e at p r e f e r r e d locations, i.e. r e g i o n s of l o c a l i z e d p l a s t i c deformation where the s t r u c t u r e yields i n i t i a l l y when loaded beyond the e l a s t i c limit, and r e q u i r e s r e i n f o r c i n g s t e e l s with large L u d e r s s t r a i n s (generally g r e a t e r than 16 t i m e s the e l a s t i c s t r a i n at the yield s t r e s s ) . Although this ductility r e q u i r e m e n t is g e n e r a l l y adequately provided for by c a r b o n - m a n g a n e s e r e i n f o r c i n g s t e e l s in the hot r o l l e d state, r e i n f o r c i n g b a r is also p r e s e n t in the p l a s t i c a l l y s t r a i n e d state in many regions of these s t r u c t u r e s , i.e. at design bends, r e t u r n s , s t i r r u p s , and so forth.
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