An analytical description of the jet finishing process for hot-dip metallic coatings on strip
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JET
finishing is a method of obtaining a coating of controlled weight and uniformity on hot-dip coated s t r i p by p a s s i n g the s t r i p through an elongated jet of gaseous fluid as it l e a v e s the molten pool. The p r o c e s s is shown s c h e m a t i c a l l y in F i g . 1. J e t finishing p e r m i t s the use of higher line speeds, l i g h t e r coating weights, and thinner gage s t r i p than a r e p o s s i b l e with r o l l s . * P r e v i o u s d e s c r i p t i o n s of the jet finishing p r o -
JET FINISHING PROCESS
*Coating rolls limit line speeds to less than 250 to 300 fpm (76 to 92 m/min, zinc coating weights to greater than about 0.60 oz/ft 2 (183 g/m2), (0.5 mil or 12.7 #/side), and strip thicknesses to a minimum of about 28 gage (15.3 mil or 0.389 ram).
JET DAM \
c e s s have been e m p i r i c a l in n a t u r e . 1 A p h y s i c a l d e s c r i p t i o n is p r e f e r r e d for design and s c a l i n g studies, p a r t i c u l a r l y when e x t r a p o l a t i n g to c o n s i d e r a b l y higher line s p e e d s and lighter coating weights than a r e c u r r e n t l y being used. The specific p u r p o s e s of this p a p e r are: 1) To b r i e f l y d e s c r i b e an a n a l y t i c a l t r e a t m e n t which r e l a t e s the solidified coating t h i c k n e s s to: 9 The coating m a t e r i a l (viscosity), e t h e jet finishing fluid (ratio of s p e c i f i c heats), 9 the s t r i p speed, e t h e nozzle-to-strip separation, 9 the nozzle plenum p r e s s u r e , 9 the nozzle slot opening, 9 the nozzle efficiency, 9 an e m p i r i c a l s c a l i n g p a r a m e t e r a s s o c i a t e d with the stagnation p r e s s u r e p r o f i l e that develops when a j e t flow i m p a c t s on a flat s u r f a c e . 2) To r e p o r t on c o m p a r i s o n between the p r e d i c t i o n s of the a n a l y t i c a l d e s c r i p t i o n and the r e s u l t s of l a b o r a t o r y and m i l l t e s t s . 3) To d i s c u s s two engineering c o n s i d e r a t i o n s as cons e q u e n c e s of the a n a l y t i c a l d e s c r i p t i o n : 9 the use of a i r v s steam, e t h e p r o s p e c t s f o r s u b s t a n t i a l l y i n c r e a s e d line s p e e d s and r e d u c e d coating weights. ANALYTICAL MODEL G e n e r a l Approach The e s s e n t i a l f e a t u r e of the model is that liquid coating m a t e r i a l is e n t r a i n e d onto the moving s t r i p by v i s c o u s s h e a r f o r c e s acting against g r a v i t y and the JOHN A. THORNTON is VicePresident,Researchand Development, Telic Corporation, Santa Monica, CA 90404. HART F. GRAFF is Principal Research Engineer, Armco Steel Corporation, Middletown, OH 45042. Manuscript submitted December 4, 1975. METALLURGICAL TRANSACTIONS B
STRIP MOTION
/
STRIP
1
--METERED COATING THICKNESS
FREE JET VELOCITY PROFILE
F
G
ATERIA
MOLTEN BATH
Fig. 1--Schematic
representation
of j e t f i n i s h i n g p r o c e s s .
p r e s s u r e g r a d i e n t s e s t a b l i s h e d by the j e t i n t e r a c t i o n . T
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