The role of natural convection in ladles as affecting tundish temperature control in continuous casting
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AND
J. H. CHEN
An a p p r o x i m a t e a n a l y s i s i s p r e s e n t e d to d e s c r i b e the flow f i e l d in m o l t e n s t e e l held in a l a d l e , a s c a u s e d by n a t u r a l c o n v e c t i o n . It is shown that a t h i ck (say 8 to 14 in.) s l a g l a y e r is r e q u i r e d to s u p p r e s s e x t e n s i v e v e r t i c a l m i x i n g within the l a d l e . It is a l s o shown that n a t u r a l c o n v e c t i o n c a u s e d by the c o n t a c t of the m e t a l with the cold l ad l e w a l l s m a y p r o d u c e m a s s flow r a t e s of the o r d e r of 2 to 3 t o n s p e r m i n f o r t y p i c a l 100 to 150 ton l a d l e s . If l a d l e s a r e tapped at c o m p a r a b l e r a t e s , then this p e h n o m e n o n can e f f e c t i v e l y p r e v e n t m i x i n g in the bulk. The r e s u l t a n t s t r a t i f i c a t i o n m a y be d e s i r a b l e in tundish t e m p e r a t u r e c o n t r o l .
I N the operation of continuous casting units it is desirable to maintain a uniform temperature in the tundish which feeds molten metal to the mold. Usually the tundish is fed from a ladle, thus in principle we may calculate the tundish temperature, provided the teeming temperature is known and that an estimate may be made of the heat losses while the metal is held in the ladle. The role of a slag cover in limiting the heat loss from the top metal surface in the ladle has been r e cently examined I and it was found that a slag layer of about 2 in. thick would be sufficient to prevent significant heat loss from the bulk of the metal. In a recent experimental investigation Hlinka and Miller,2 using both real hot metal systems and properly scaled models, measured continuously the tundish temperature, fed from a ladle. It was found that a thin slag cover (say 1 to 3 in.) led to characteristic maxima in the tundish temperature at intermediate values of the teeming time, whereas a thick slag cover (say 6 in. or more) gave a relatively uniform tundish temperature throughout the teeming process. This behavior is illustrated in Fig. l.showing the results of two typical runs with 7~- ton heats. Hlinka and Miller explained their findings by suggesting that: i) The presence of a thick slag cover prevented mixing (due to natural convection) within the bulk of the metal, but natural convection did take place causing downward flow in the vicinity of the cold vertical walls of the ladle. The metal discharged from the ladle was drawn primarily from this downward flowing stream, so that the teeming itself did not cause appreciable mixing in the bulk. ii) The downward stream was cooled through its contact with the ladle walls, but this effect became diminished for increasing times, as the ladle walls became progressively hotter; at the same time heat was lost from the bulk of the metal so that these two complementary effects led to a relatively uniform temperature for the stream teemed from the ladle. This mechanism was supported by visual observations, in the water model tests; lateral mixing i
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