Metallurgical aspects in cold rolled high strength steel sheets
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Box annealing of c o m m e r c i a l cold rolled steel sheets is conventionally c a r r i e d out at the t e m p e r a t u r e just below A 1 t r a n s f o r m a t i o n t e m p e r a t u r e for the purpose of making the a s - c o l d - r o l l e d sheet r e c r y s t a l l i z e d and softened. This p r o c e s s is favorable for producing sheet with m o d e r a t e strength and high ductility. It is well known that high strength cold rolled sheet with high ductility is not obtained by only changing the condition of box annealing without changing the chemical composition of the sheet. Therefore, the contribution of chemical compositions, in addition to annealing condition, was examined. Continuous annealing provides m o r e rapid cooling f r o m annealing t e m p e r a t u r e s than box annealing, and some continuous annealing equipment m a y p e r m i t heatt r e a t m e n t s of normalizing and quench-tempering. On this basis, continuous annealing is considered to be s u p e r i o r to box annealing in producing cold rolled high strength steels. Although the continuous annealer will be used, depending on the demand of the high strength sheets, the various factors to develop the high strength steel sheets which a r e applicable to box annealing were investigated. Cold working r a i s e s strength at the expense of ductility. For instance, as rolled c o m m e r c i a l cold rolled steel sheet has a yield strength of about 600 N/ram 2 but an elongation of about 3 pct, which is not p r a c t i cal for forming. Usually t e m p e r rolling is p e r f o r m e d not to r a i s e yield strength but to lower it, and to e l i m inate yield elongation. Large reduction in t e m p e r r o l l ing helps to i n c r e a s e yield strength but generally the loss of ductility is too large to produce high strength steel sheet for forming. Aging r a i s e s yield strength. However, f r o m p r e v i -
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Fig. l--Tensile test specimen (ram).
VOLUME 6A, AUGUST 1975-1613
ous s t u d i e s 1-z the i n c r e m e n t due to aging is not l a r g e enough to p r o d u c e cold r o i l e d high s t r e n g t h s t e e l s h e e t . This p a p e r on the d e v e l o p m e n t of cold r o i l e d high s t r e n g t h s t e e l s h e e t s d e a l s with the p r o d u c t i o n of s h e e t s by the c o m b i n a t i o n of a l l o y i n g and box a n n e a l ing.
2. E F F E C T O F ALLOYING E L E M E N T S ON STRENGTHENING The e f f e c t s of c h e m i c a l c o m p o s i t i o n on s t r e n g t h were investigated by using laboratory melts. They w e r e f o r g e d and r o l l e d hot to s l a b s of about 6 m m in t h i c k n e s s . A f t e r h e a t i n g at 1250~ they w e r e hot r o l l e d to about 2.7 m m and c o l d r o l l e d to final gage at 0.8 m m . A n n e a l i n g w a s p e r f o r m e d in a r g o n a t m o s p h e r e b y h e a t i n g at the r a t e of 50~ keeping for 2 h, and c o o l i n g at the r a t e of about 50~ Tensile t e s t s p e c i m e n i s shown in F i g . 1. Yield s t r e n g t h r e f e
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