The 1975 Howe Memorial Lecture The Iron and Steel Society of AIME

Continuous Casting Update

CHARLES R. TAYLOR

A critical survey of available information on primary and secondary cooling as affecting solidification constants, and their validity in mathematical modeling.A review of the mathematics of reciprocation. Comments on the functionand mode of mold lubrication. The important effects of the above parameters and of liquid metal handlingprocedures on the internal and external quality of strand cast billets and slabs~ THE first continuous casting of a metal would appear to have been done in 1840 by an American, George S e l l e r s , "~ a l m o s t 50 y e a r s b e f o r e B e s s e m e r conducted his trials on steel. The metal was lead and the product was pipe. The commercial adoption of the continuous casting of steel is now over twenty years old, so it took about

dary cooling below the mold, mold lubrication, and mold reciprocation. We will then discuss the quality problems encountered in making slabs and billets as t h e y r e l a t e to t h e s e a n d o t h e r p a r a m e t e r s .

135 years to make the transition! At any rate, it appeared that it would be useful to examine the state of the art as to where we stand and to propose areas where information is needed. We will, of course, only be able to skim the surface of such a massive topic. Adequatetreatment would require a book, not a one hour lecture. So if we overlook a facet of the process which some of you considervital, please bear with the omission. We will discuss primary cooling in the mold, secon-

The factors controlling the formation of the first few millimeters of steel in continuouscasting have been the subject of numerous investigations. A thorough understanding of what goes on in the mold is definitely important in establishing the quality of the product and the reliability of the process. An overall description of what goes on would appear to be deceptively simple. Steel is introduced into the mold, a shell forms and is withdrawn at the same mass rate as it is introduced and the casting process proceeds

The Howe Memorial Lecture was established in 1923 by the Iron and Steel Division, now the Iron and Steel Society, o f AIME. The Lecturer is selected for his outstanding contributions to the science and practice o f iron and steel metallurgy or metallography.

D.Sc. in physical chemistry from the University of Cincinnati in 1941. His many professional activities in the AIME include active participation in the Physical Chemistry of Steelmaking Committee (now Process Technology Committee) from 1942 to date, (Chairman 1950); Chairman of the Iron and Steel Division of The Metallurgical Society of AIME in 1964; Board of Directors ISD1962-66; Board of Directors-The Metallurgical Society- 1966-73; Board of Directors-A1ME 1968-73; President of The Metallurgical Society- 1971-72; McCune'Award-1946-Iron and Steel Division, AIME; ASM Fellow-19749 He is holder of several patents and author of numerous papers; co-author of a Chapter in "Electric Furnace Steelmaking."

CHARLES R. TAYLOR, now retired, delivered this lecture as Manager of Melting, Refining, and Refractories Research, Research and Technology, Armco Steel Corporation, Middletown, Ohio, having served the firm in various technical and managerial capacities since 1933. He received his B.S. degree in chemistry from Ohio State University in 1932, followed by an M.S. in 1933. As Armco Steel Corporation Fellow, he earned a METALLURGICAL TRANSACTIONS B

P R I M A R Y COOLING

VOLUME 6B,SEPTEMBER 1975-359

to its c o n c l u s i o n - - a n e m p t y ladle, and a u s a b l e p r o d uct. L e t ' s take a look at what goes on in the m o l d under s t e a d y s t a t e conditions such as e x i s t w e l l into a c a s t (Fig. 1). Since, s o m e s u p e r h e a t above the liquidus is n e c e s s a r y to get the s t e e l through the tundish n o z z l e , this e n e r g y m u s t be r e m o v e d b e f o r e s o l i d i f i c a t i o n can p r o c e e d . T h e r e is t h e r e f o r e a funite length of the m o l d w h e r e no s o l i d i f i c a t i o n o c c u r s . The length of this z o n e is r e l a t e d to the d e g r e e of s u p e r h e a t and the c a s t i n g speed. As soon as the s u p e r h e a t is d i s s i p a t e d s o l i d i f i c a t i o n s t a r t s . T h e s h e l l t r i e s to c o n t r a c t , but t h i s c o n t r a c tion is opposed by liquid m e t a l p r e s s u r e . The m a n n e r in which the c o n t r a c t i o n o c c u r s depends in the s h a p e of the c a s t i n g . Slice Fig. 1 s o m e w h a t below the m i n i s cus. N e a r the c o r n e r s , heat is e x t r a c t e d m o r e r a p i d l y , the skin b e c o m e s t h i c k e r and can o p p o s e m e t a l p r e s s u r e m o r e e a s i l y so an " a i r " gap f o r m s h e r e f i r s t (Fig. 2). At s o m e point below the i n i t i a l s o l i d i f i c a t i o n point the c a s t i n g would no l o n g e r look l i k e a s q u a r e METAL STREAM

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(for b i l l e t s ) but would take an odd shape. As soon as an a i r gap f o r m s h e a t flow is inhibited in the c o r n e r s , and the s i d e s s o l i d i f y m o r e r a p i d l y r e s u l t i n g in g r e a t e r s t r e n g t h , and e v e n t u a l l y the a i r gap e x i s t s a r o u n d the c o m p l e t e p e r i p h e r y of the c a s t i n g . In l a r g e s l a b s (Fig. 3) the p i c t u r e is m o r e c o m p l e x b e c a u s e of the b r o a d a r e a e x p o s e d to liquid p r e s s u r e , and the shape b e c o m e s m o r e u n c e r t a i n . Since end plate t a p e r is u n i v e r s a l l y used on s l a b m a c h i n e s , the p i c t u r e is e v e n m o r e c o m p l e x . On v e r y wide s l a b s it is doubtful w h e t h e r an a i r gap e v e r f o r m s in the c e n t e r p a r t of the b r o a d face. In an e f f o r t to m e a s u r e what was happening in the mold, (Fig. 4) Rudoi 2 p l a c e d a n e t w o r k of " c o n t a c t " s e n s o r s in a 175 x 420 m m (6.9 x 16.5 in.) mold, (Fig. 4), 58 on each b r o a d f a c e and 30 on each n a r r o w one. This was a t y p i c a l l y long, 1500 m m R u s s i a n mold. Each s e n s o r was c o n n e c t e d to a l i g h t bulb which was lit when c o n t a c t was m a d e and photos of the l i g h t banks w e r e m a d e . Rudoi concluded that no t r u e a i r gap r e a l l y e x i s t e d , but that the s t e e l s h e l l c o n t i n u a l l y m a d e and b r o k e c o n t a c t with the m o l d wall and t h e r e f o r e h e a t was e x t r a c t e d in p u l s e s r a t h e r than c o n t i n uously. As can be seen, t h e r e is no r e a l i n d i c a t i o n of a t r u e gap. One m i g h t s a y that at 600 m m and below, the c o n tact was i n f r e q u e n t enough to call it an a i r gap. Gordienko 3 e t a l (Fig. 5) m a d e c o n t a c t m e a s u r e m e n t s v s t i m e on a c u r v e d m o l d m a c h i n e 130 x 145 m m at two l e v e l s . When pct c o n t a c t t i m e s a r e c o m p a r e d s i m u l t a -

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Fig. 5--Relative contact time for two mold sizes on all four sides of the mold. METALLURGICAL TRANSACTIONS B

n e o u s l y for opposite faces, no evidence of an i n v e r s e r e l a t i o n s h i p a p p e a r s . That is, if one face has a low p e r c e n t a g e of contact, the opposite face does not n e c e s s a r i l y have a high p e r c e n t a g e contact. F a c e s appear to act somewhat independently, which r e i n f o r c e s the "wet s o c k " concept of s t r a n d b e h a v i o r in the mold. This technique is open to s o m e q u e s t i o n b e c a u s e of v i b r a t i o n which m i g h t c a u s e contact loss independently of a i r gap f o r m a t i o n . Folk and W u n n e n b e r g (Mannesman) studied the a v e r a g e heat t r a n s f e r along a mold by dividing the mold into eight h o r i z o n t a l w a t e r p a s s a g e s , m e a s u r i n g water quantity and AT for each (Fig. 6). As might be expected the heat flux v a r i e d with height, and the p a t t e r n v a r i e d with c a s t i n g speed. We b e l i e v e that the i n c r e a s e in heat flux toward the bottom of the mold was due to the p e n e t r a t i o n of water f r o m the s e c o n d a r y cooling s p r a y s into the space between the c a s t ing and the mold w h e r e it e n d o t h e r m i c a l l y d i s s o c i a t e s into hydrogen and FeO, p r o d u c i n g a hydrogen r i c h gas having e x c e l l e n t t h e r m a l conductivity. This p a t t e r n

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a g r e e s r e m a r k a b l y well with m o r e r e c e n t work at Nippon s (Fig. 7) with t h e r m o c o u p l e s in the mold wall. I n t e g r a t i n g the Folk and W u n n e n b e r g c u r v e s give the o v e r a l l heat flux shown in Fig. 8, which is in a g r e e m e n t with other w o r k e r s who m e a s u r e d the total flux. When " f l u x e s " were s u b s t i t u t e d for oil as a l u b r i cant total heat t r a n s f e r was affected only to a m i n o r d e g r e e as can be seen in Fig. 9, but the p a t t e r n (Fig. 10) of heat t r a n s f e r was affected to a m a j o r d e g r e e , p a r t i c u l a r l y when a "high m e l t i n g " powder was used. The shape at the lower end of the mold c u r v e s m a y be r a t i o n a l i z e d to support the w a t e r p e n e t r a t i o n c o n cept, s i n c e low m e l t i n g fluxes would " w e t " the s u r f a c e of the c a s t i n g b e t t e r than high m e l t i n g fluxes and tend to p r e v e n t the p r o d u c t i o n of a high c o n d u c t i v i t y h y d r o gen a t m o s p h e r e . In an effort to find out what is going on in the mold, c o n s i d e r a b l e ingenuity has b e e n exhibited by m a n y i n v e s t i g a t o r s . Additions of s u l f u r , r a d i o a c t i v e P, S, As, Ag, and so forth, have b e e n m a d e to the mold and s u l fur p r i n t s and auto r a d i o g r a p h s made to study s o l i d i fication r a t e s . Tungsten c a p s u l e s loaded with Cobalt 60 have been dropped in and m o l t e n lead has b e e n added in an effort to d e l i n e a t e the l a s t m e t a l to s o l i d fly. All of these t e c h n i q u e s have c o n t r i b u t e d to our

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Fig. 9 - - E f f e c t of m o l d l u b r i c a n t s on s h e l l t h i c k n e s s v s d i s t a n c e f r o m t h e m o l d top. VOLUME 6B,SEPTEMBER 1975-361

u n d e r s t a n d i n g of the heat and m a s s flow in a c a s t i n g machine. T a r m a n and F o r s t e n e r 6 e x a m i n e d the u s e of s u l f u r and isotope additions to the mold. They concluded, b e c a u s e of the unknown effect of m i x i n g t i m e , and the p r e s e n c e of a m u s h y zone, that these methods w e r e u n r e l i a b l e . T h e i r p r e f e r r e d method was to r u n the m a chine s o m e w h a t f a s t e r than design, so that when the cut off t o r c h i n t e r s e c t e d the liquid core, the m e t a l would r u n out. Using this r a t h e r h e r o i c method they found s o m e r a t h e r s u r p r i s i n g effects d u r i n g the e a r l y p h a s e s of s o l i d i f i c a t i o n (Fig. 11). F o r the f i r s t 1 - 1 / 4 m i n u t e s s o l i d i f i c a t i o n was s o m e what e r r a t i c as shown in the figure. This f i g u r e is plotted on the a s s u m p t i o n that the s q u a r e r o o t law is obeyed; and r e s u l t s in a r t i f i c i a l s o l i d i f i c a t i o n cons t a n t s if it is not. It is evident that, d u r i n g e a r l y s o l i d i f i c a t i o n this " c o n s t a n t " can v a r y widely depending on the contact b e t w e e n the s o l i d i f y i n g s h e l l and the mold, b e c a u s e as the contact t i m e m e a s u r e m e n t r e f e r r e d to e a r l i e r shows, the a i r gap is highly e r r a t i c . When the a i r gap is c o n t r o l l e d , by the s u p e r - p o s i t i o n of a c o n t r o l l e d b a r r i e r l a y e r , s o l i d i f i c a t i o n obeys the s q u a r e r o o t law to t i m e z e r o . In the e a r l y 1950's when we w e r e i n v e s t i g a t i n g the Hazelett method of c o n t i n u ous c a s t i n g we m e a s u r e d s o l i d i f i c a t i o n c o n s t a n t s in v e r y s m a l l s p e c i m e n s b y i m m e r s i n g a r e f r a c t o r y cov-

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Casting Speed: m/min

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ered w a t e r cooled probe u s i n g c a r e f u l t i m i n g d e v i c e s , into heats of v a r i o u s a n a l y s i s (Fig. 12). T i m i n g was automated and a c c u r a t e and a c c o u n t was taken of the t i m e of i m m e r s i o n and e x t r a c t i o n , as well as dwell t i m e .

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Solidification of Mild Steel on Water Cooled Diaphragm

Fig. 1 0 - - E f f e c t of m o l d l u b r i c a n t s in t h e d i s t r i b u t i o n of h e a t flux f r o m top to b o t t o m of t h e mold.

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Fig. 1 4 - - S o l i d i f i c a t i o n c o n s t a n t f o r t h e f i r s t f N e s e c o n d s of t h e s o l i d i f i c a t i o n of m i l d s t e e l on a w a t e r e o o l e d u n c o a t e d diaphragm. METALLURGICAL TRANSACTIONS B

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Solidification of Stainless Steel in Water Cooled Steel Diaphram Copper B l o c k

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Solidification of Mild Steel on Copper Block Coated with AI203 and Shellac Wash u)

.1--

--

N o n - u n i f o r m contact was obvious (Fig. 13) f r o m the c a s t i n g s produced. This can be s e e n in the photograph of two s p e c i m e n s , one u s i n g b a r e s t e e l . On the other, with a thin A1203-shellac c o a t i n g w h e r e b y a c o n t r o l l e d a i r gap was produced, s o l i d i f i c a t i o n was quite u n i f o r m , but s o l i d i f i c a t i o n constants w e r e a lot s m a l l e r . By m e a s u r i n g the t h i c k e s t s p o t s on the u n c o a t e d s p e c i m e n s , we found that the s q u a r e r o o t law was obeyed, r e s u l t i n g in a c o n s t a n t of 0.11 for b a r e s t e e l , s t r o n g l y w a t e r cooled. ( T i m e s e c o n d s , t h i c k n e s s inches). The data is shown in F i g . 14 and s c a t t e r of the points is evident. Stainless s t e e l on an u n c o a t e d d i a p h r a g m gave good data (Fig. 15) with a c o n s t a n t of 0.090. When the c o a t ing was used, (Fig. 16), the c o n s t a n t b e c a m e 0.086. Fig. 17 shows the e f f e c t of s u p e r h e a t on K for e x p e r i m e n t s using the wash. U n f o r t u n a t e l y we did not m e a s u r e the height to which the liquid s t e e l c o v e r e d the b l o c k ; so did not get any m e a s u r e of d e l a y t i m e f r o m contact to s o l i d i f i c a t i o n s t a r t i n g . We have, h o w e v e r , o b s e r v e d this e f f e c t when r i m m i n g s t e e l was being c o n t i n u o u s l y c a s t . T h e r e was c o n s i d e r a b l e t u r b u l e n c e in the mold, not a n o r m a l r i m m i n g action, so on o c c a s i o n the i n i t i a l s o l i d i f i c a tion point could be o b s e r v e d as a h o r i z o n t a l l i n e about 2 to 3 cm below the a v e r a g e top of the m e t a l pool. The net of all this d i s c u s s i o n is that: while under s t a t i c conditions s o l i d i f i c a t i o n constants a r e useful, in continuous casting, w h e r e the s a m e spot in a c a s t ing is being s u b j e c t e d to v a r i a b l e gap e n v i r o n m e n t s , s o l i d i f i c a t i o n constants a r e a r a t h e r poor guide as to what is going on in the m o l d . We a r e m o s t c o n c e r n e d with the t h i n n e s t s o l i d i f i e d spot, b e c a u s e this is the l i k e l y point for a b r e a k o u t . U s e of s o l i d i f i c a t i o n constants in the m o l d to d e s c r i b e e a r l y s o l i d i f i c a t i o n is analagous to the man who d r o w n e d while wading a c r o s s a r i v e r with an a v e r a g e depth of two feet. T a r m a n e t a l 6 did show that a f t e r things had quieted down in the l o w e r p a r t of the mold, and into the s e c o n d a r y cooling s e c t i o n , the s q u a r e r o o t law was obeyed, (Fig. 18) and w e r e able to show an e f f e c t of s u p e r h e a t on the s o l i d i f i c a t i o n constant. By g r a p h i c a l methods Square

0/

,

I 1

I 2

3

29

Seconds

0

28

Fig. 16--Solidification constant for the first five seconds of the solidification of mild steel on a copper block plus AlzO3shellac wash.

27 - - ~ 26

"O

'

0 ~c)~C.~..u, CL~O u

2s .11

I

I

Sections

3o

"~

~'-~

-v...~ ~.

24

I

.IG f20 ,.., = .09 .08

§

§ +80 Superheat

+100 +120"C

i

r

"~ + 2

~ .07 i

4

c .06 -o

E

--'

.os -

.~ . 0 4 -= "o 2 6 0 0

27O0

2800

2900

3OO0

Temperature "F.

Fig. 17--Effect of metal temperature on the solidification constant of stainless steel with an AlzO3-shellac wash. METALLURGICALTRANSACTIONS B

-2

~--15

20 25 Castin9 Speed

3 0 m/rain

Fig. 18--Effect of super heat and casting speed on the solidification constant. VOLUME 6B, SEPTEMBER 1975-363

they w e r e also able to show an independent effect of c a s t i n g s p e e d - - a l b e i t a r a t h e r s m a l l one. S u m m i n g up, it a p p e a r s that the s q u a r e root law is obeyed to z e r o t i m e , when the a i r gap e n v i r o n m e n t is c o n t r o l l e d . Since this is a p p a r e n t l y not the case in the mold, s o l i d i f i c a t i o n c o n s t a n t s can only be a guide to s o l i d i f i c a t i o n d u r i n g this t i m e period, and cannot be r e l i e d on to p r e d i c t skin t h i c k n e s s at all positions a r o u n d the p e r i p h e r y of a c a s t i n g d u r i n g its t r a v e r s e of the mold. This is c e r t a i n l y an a r e a where m o r e i n f o r m a t i o n is needed. In o r d e r to i n c r e a s e our u n d e r s t a n d i n g of what is going on in the mold we have an e x p e r i m e n t underway. A mold is b e i n g i n s t r u m e n t e d with s e v e r a l h u n d r e d t h e r m o c o u p l e s . The output of t h e s e t h e r m o couples will be s c a n n e d e l e c t r o n i c a l l y about once a second. By e l e c t r o n i c l e g e r d e m a i n t h e s e r e a d i n g s will be i n t e r p o l a t e d , and the output d i s p l a y e d on a cathode r a y tube as i s o t h e r m s , o r p e r h a p s color coded m a p s . Data will s i m u l t a n e o u s l y be r e c o r d e d on tape. Hopefully, we will be able to watch how the a i r gap r e sponds to change in speed, water usage, v a r i o u s fluxes, and so forth, while the m a c h i n e is r u n n i n g . I n s t a n t r e p l a y s and slow motion will be a v a i l a b l e as called for.

Since they were unable to c o n t r o l c a s t i n g r a t e and m e t a l s u p e r h e a t at the planned l e v e l s , due to the e x i g e n c i e s of m e l t i n g and the b r e a k o u t h a z a r d s at high speeds with low s e c o n d a r y water, they used g r a p h i c a l c o r r e l a t i o n methods to s e p a r a t e t h e s e v a r i a b l e s f r o m each o t h e r . Since they used fixed o r i f i c e n o z z l e s , i n c r e a s i n g 1/kg could only be a t t a i n e d b y i n c r e a s i n g p r e s s u r e s . It is t h e r e f o r e l i k e l y that up to 0.2 1/kg the d r o p l e t s did not p e n e t r a t e the s t e a m b o u n d a r y l a y e r and impinge on the s u r f a c e of the casting. Muller and J e s c h a r s (Fig. 20) s t u d i e d heat t r a n s f e r e x p e r i m e n t a l l y by m e a s u r i n g the power r e q u i r e d to m a i n t a i n s u r f a c e t e m p e r a t u r e of a s t e e l plate when s p r a y s w e r e used. Their data shows the effect of water q u a n t i t y (1/m z s) on the heat e x t r a t i o n coeffic i e n t for six water v e l o c i t i e s . If we s l i c e t h e i r d a t a at 2 I / m e s, (Fig. 21) we see that the heat e x t r a c t i o n coefficient is l i n e a r l y dependent on the w a t e r v e l o c ity, and it t u r n s out, independent of the t e m p e r a t u r e of the s u r f a c e (radiation cooling not included). T h e i r data can be e x p r e s s e d as a m a t h e m a t i c a l equation:

1600

/ 0 32 ~,/ x'=.l

Water Discharge Velocity m/sec: 1401 ,4

SECONDARY COOLING

J

~E 120(

Once the c a s t i n g has left the mold s o l i d i f i c a t i o n m u s t p r o c e e d in such a m a n n e r that p h y s i c a l and m e t a l l u r g i c a l i n t e g r i t y of the c a s t i n g is m a i n t a i n e d . Since we a r e free of the e r r a t i c effect of a i r gap e n v i r o n m e n t , much b e t t e r c o n t r o l of heat e x t r a c t i o n can be attained. Heat is e x t r a c t e d , and s o l i d i f i c a t i o n r a t e controlled, by the s p r a y s , m o s t l y f r o m the b o i l i n g of the water. S i m u l t a n e o u s l y of c o u r s e , heat is l o s t to the s u r r o u n d ings b y r a d i a t i o n . H o l z g r u b i e r and T a r m a n n , 7 on a 140 m m s q u a r e (Fig. 19) u s i n g the q u i c k d r a i n t e c h nique v i a the cutoff torch, have showed the effect of cooling w a t e r r a t e on the s o l i d i f i c a t i o n constant.

r~_

i

,,g

,000

.,,

._u L)

800

IJ

600

~

Distance

Between Nozzle and

Heating Face, > 100 and < 200 rnm 400

Width of Heated Face, >20 and <_-60mm

200

Difference Between Heated Face and Boiling Temperatures, 900 K 1

i 3

2

i 4

i 5

i 6

i 7 Water Impact Density, L/m2s

9

10

F i g . 2 0 - - E f f e c t of w a t e r q u a n t i t y a n d v e l o c i t y on h e a t e x t r a c t i o n in the s e c o n d a r y s p r a y s e c t i o n .

800

/'•

27

25

i 8

700

/

-700

/I

~d 2""

600

J

23

~22

60(]

0 0.1 0.2 0.30 O~ 0.6 07 0.8 0.9 1.0 1.1 1.21,3 Quontily ot C~lin s Water Wle c (tltrelkg)

lfllllllll ~.21N1' ~("211 II

IIr l~ ~[[ i I I [ li I III1~ ,I 1,3(.3)! I !

2,1tNII'! ~ o l l l P k l II II ~., I~ II

~cl'iM.dl

I

~211 I f [ I I1 b) ~ 1 [ I [ I Ic)ll

-a

%3!LL.L,L.U ,!3,!. ,LL ,!. ,!.' Meto~ $upe,heal. C

Castro9 Speed v(m/mm )

F i g . 1 9 - - E f f e c t of w a t e r q u a n t i t y , s u p e r h e a t a n d s t r a n d s p e e d on the s o l i d i f i c a t i o n c o n s t a n t ( m u l t i p l e r e g r e s s i o n ) . 3 6 4 - V O L U M E 6B, SEPTEMBER 1975

/

5O0

/

-500 ,4 o.

~,X

-400

/ 400

:lg

j~,.

/ 300

-300

,X

200 10

12

14

16

18 20

2 2 24 M sec

26 28 30

32

34

Fig. 21--Slice through Fig. 20 at 2 I/m2s showing the linear effect of water velocity on heat extraction. METALLURGICAL TRANSACTIONS B

W a t t s / m 2 K = (0.688 m / s + 107) 1/m2s + 10 m / s m = meters

Sudace Terno C 12 10 It

Mo,0:C

s = seconds 1 = liters k = ~ Kelvin After the shell has reached a thickness of s o m e w h e r e between 50 and 70 ram, spray cooling loses its effectiveness because of the poor thermal conductivity of steel, and the use of water sprays subsequent

to this p o s i t i o n is a waste of water and m a c h i n e design costs. In the cooling of wide s l a b s , the u n i f o r m i t y of s p r a y c o v e r a g e b e c o m e s i m p o r t a n t , p a r t i c u l a r l y j u s t under the mold w h e r e the c a s t i n g is thin, and heat t r a n s f e r l a r g e . R u s s i a n work 9 has indicated that t h e r e is r e l a tively little i n t e r f e r e n c e between d r o p l e t s in o v e r l a p ping s p r a y s , and that the i m p a c t p r e s s u r e and t h e r e fore heat e x t r a c t i o n is n e a r l y p r o p o r t i o n a l to the s u m of the t w o nozzles in the overlap area. The net result is that at 10 to 15 pct overlap in sprays can compensate for the cosine effect and the inherent non-uniformity of individual spray patterns and heat extraction patterns across a wide slab can be quite uniform. The upper limit of water cooling high in the machine, not considering any effect on crack formation, is set by the p r e s s u r e effect of the s p r a y s in opposing the bulging due to liquid m e t a l p r e s s u r e on the shell. P r e s s u r e , u s i n g cold m o d e l s , have been m e a s u r e d , ~~ but these m e a s u r e m e n t s do not take into account the a c c e l e r a t i o n p r e s s u r e s e x e r t e d on the c a s t i n g s u r f a c e by s t e a m b e i n g f o r m e d and l e a v i n g the i m p a c t zone. Since it t u r n s out that i n t e n s e water cooling cannot be t o l e r a t e d b e c a u s e of c r a c k i n g and " s e g r e g a t i o n z o n e s , " such m e a s u r e m e n t in a hot model would be of doubtful u t i l i t y anyhow. N u m e r o u s m a t h e m a t i c a l m o d e l s have been w r i t t e n for the continuous c a s t i n g o p e r a t i o n with v a r y i n g d e gree of s o p h i s t i c a t i o n . ~2'2s As we have pointed out, the m e a s u r e m e n t of s o l i d i fication c o n s t a n t s , except by the d u m p i n g technique, a r e open to question. They should not be used to a s s e s s the v a l i d i t y of a m a t h e m a t i c a l model. A much m o r e p r a c t i c a l method is to use s u r f a c e t e m p e r a t u r e s . These t e m p e r a t u r e s m u s t be m e a s u r e d by a p r o p e r l y d e s i g n e d two color p y r o m e t e r to have any validity. When the heat and m a s s flow model is p r o p e r l y w r i t t e n , u s i n g the m o r e s o p h i s t i c a t e d finite e l e m e n t m a t h e m a t i c a l method, good a g r e e m e n t can be obtained between c a l c u l a t e d and m e a s u r e d s u r f a c e temperatures. A l b e r n e y ~2 (Fig. 22) got f a i r l y good a g r e e m e n t b e tween c a l c u l a t e d and m e a s u r e d t e m p e r a t u r e s at v a r i ous l e v e l s in the m a c h i n e , c o n s i d e r i n g the hostility of the e n v i r o n m e n t , u s i n g G a u t i e r ' s 2s m a t h e m a t i c a l model. A l b e r n e y ' s m a c h i n e was designed for e x p e r i m e n t a l s e p a r a t i o n of cooling s p r a y s , and the t o r t u o u s t e m p e r a t u r e t r a c e would not be t o l e r a b l e on a c o m m e r c i a l m a c h i n e , w h e r e s p r a y b a n k s a r e not s e p a r a t e d f a r t h e r than one r o l l spacing. F u r t h e r , due to the s i m p l i f i e d m a t h e m a t i c a l a p proach p o s s i b l e on a c y l i n d r i c a l casting, Gautier did not need the m o r e advanced c o m p u t e r techniques, but his data does indicate that s u r f a c e t e m p e r a t u r e s can be c a l c u l a t e d with s u f f i c i e n t a c c u r a c y , (Fig. 23) w h e r e a point by point c o m p a r i s o n of e x p e r i m e n t a l and m e a s u r e d t e m p e r a t u r e s is plotted. METALLURGICALTRANSACTIONSB

13

(O0.s) 14 15

Surlace Temp, C M01d

Zone i ~

,

Zone' Zonell

zo~

Zone Ill

~

-T7

Withdrawal Rolls,

l-

.

i i/

Zone iV . . . . . .

Withdrawal q Rolls C

(O0,s)

:

L ~ . ,

Zone II I

Zone IV

I

k

t

i

Cut off

Calculated

Experimental

Fig. 22--A comparison of the calculated vs observed surface temperature pattern-full machine length. 1400

1300

/

/

P ~1200

S

/ m__

-- n -

~

r

a

m

-

-

e ~,.I f~

//

0 1100 5

n

/(3 1000 loO0

!

1100

1~00

I

1300

1400

Meas.TerrCL'C Fig. 23--Direct comparison of calculated vs measured surface temperature from Fig. 22.

ili I ~

8

\~3o

,~ !

ll ,P'G llllI,I J IIIlz,,

' 0

20

40

r

["

~

:

!

~

:

I

~

!

I t

-

60 80 100 FREQUENCY N / M I N

:- ~-

120

140

160

Fig. 24--Graphical representation of the reciprocation equation.

We conclude that the m a t h e m a t i c a l m o d e l i n g of s e c o n d a r y cooling is adequate, and f u r t h e r work is not needed. RECIPROCATION We would cause some complishes,

like to discuss reciprocation briefly, beconfusion seems to exist as to what it ac-

and why it is n e c e s s a r y . VOLUME 6B, SEPTEMBER 1975-355

R e c i p r o c a t i o n m a d e continuous c a s t i n g a p r o d u c t i o n p r o c e s s . B e c a u s e of f r i c t i o n a l f o r c e s , s m a l l b r e a k outs o c c u r - - r e c i p r o c a t i o n allows t h e m t i m e to heal. The t i m e i n t e r v a l , when the r e l a t i v e m o t i o n between the mold and the c a s t i n g is z e r o or c o m p r e s s i v e is called " h e a l t i m e " . Many m o d e s of r e c i p r o c a t i o n have been t r i e d . With cam d r i v e s the m o l d p r o c e e d s downward s o m e w h a t f a s t e r than the c a s t i n g , t h e r e b y putting the skin under c o m p r e s s i o n . In o r d e r to r e coup this e x t r a downward m o t i o n r e t u r n t i m e had to be s h o r t e r than down t i m e . A c c e l e r a t i o n f o r c e s w e r e high, and b e a r i n g m a i n t e n a n c e was f r e q u e n t l y a p r o b l e m . H o w e v e r , it b e c a m e a p p a r e n t that s i n u s o i d a l m o t i o n would be adequate if the s t r o k e and f r e q u e n c y w e r e p r o p e r l y s e l e c t e d . The equation r e l a t e s t h e s e variables.

Heal t i m e s will v a r y along the h o r i z o n t a l l i n e s . F o r e x a m p l e , if we wish to o p e r a t e at 60 ipm, and m a i n tain a heal t i m e of at l e a s t 0.4 s, we can o p e r a t e at 40 c y c l e s p e r minute, and use a 3/4 in. s t r o k e . Data on the e f f e c t of heal t i m e on the i n c i d e n c e of t r a n s v e r s e c r a c k s o r b r e a k o u t s is s p a r s e , s i n c e m o s t o p e r a t o r s p r e f e r to play it s a f e . R u s s i a n w o r k ~5 i n d i cates that a h e a l t i m e of 0.3 s m a y be r e q u i r e d a s a m i n i m u m , but s m a l l e r b i l l e t m a c h i n e s s e e m to be able to o p e r a t e at as low as 0.2 s without undue p r o b lems. R e c i p r o c a t i o n m a r k depth is i n c r e a s e d as f r e q u e n c y is r e d u c e d , so it m a y be d e s i r a b l e to o p e r a t e at s h o r t e r s t r o k e lengths and h i g h e r f r e q u e n c i e s to m i n i m i z e the effect. MOLD L U B R I C A T I O N

V = ~ F S cos (3FT) 1

T= ~ V /~ S T

= = = =

V a r c cos 7rFS

Strand speed i n . / m i n F r e q u e n c y of o s c i l l a t i o n Stroke length (in.) Heal t i m e , s.

G r a p h i c a l r e p r e s e n t a t i o n of the e f f e c t of c y c l e s / rain on h e a l t i m e for a 3/4 in. s t r o k e and v a r i o u s c a s t i n g s p e e d s a r e shown in Fig. 24. You will note that for each s p e e d (at c o n s t a n t s t r o k e ) t h e r e is a f r e q u e n c y that g i v e s m a x i m u m h e a l t i m e . F o r e x a m p l e with a s t r o k e of 0.75 in. and 100 ipm speed, the m a x i m u m h e a l t i m e that can be attained is 0.25 s at a f r e q u e n c y of about 65 c y c l e s per m i n u t e . Heal t i m e is s o m e t i m e s r e p o r t e d in p e r c e n t of c y cle t i m e . We p r e f e r to use r e a l - t i m e n u m b e r s in d i s c u s s i n g h e a l t i m e , s i n c e we a r e a t t e m p t i n g to c o n t r o l a s o l i d i f i c a t i o n p r o c e s s which t a k e s p l a c e in r e a l t i m e . T h e r e a r e m a n y ways to p r e s e n t the e q u a t i o n so that it will be useful to o p e r a t o r s . F o r e x a m p l e , l e t ' s a s s u m e that for any s p e e d , we w i s h to o p e r a t e at a s t r o k e length and f r e q u e n c y which will g i v e m a x i m u m heal t i m e . Such a c h a r t is shown in Fig. 25. Each speed has a b r o a d c o m b i n a t i o n of f r e q u e n c i e s and s t r o k e s which will r e s u l t in m a x i m u m h e a l t i m e for a p a r t i c u l a r p a i r of s e t t i n g s .

As we have indicated, d u r i n g the o p e r a t i o n of a m a chine t h e r e is a l m o s t continuous r e l a t i v e m o t i o n b e tween the c a s t i n g and the mold. T h e r e f o r e a d v a n t a g e is taken of the s u b s t a n t i a l r e d u c t i o n in the c o e f f i c i e n t of f r i c t i o n , m o v i n g v s s t a t i c . In the c a s e of oil l u b r i c a t i o n , used m o s t l y on s m a l l e r s i z e s , oil can p r o c e e d into the gap by w e t t i n g the m o l d w a l l - - o b v i o u s l y it cannot wet the hot s t e e l . Since it is not c o m p l e t e l y d e s t r o y e d by the hot s t e e l it a l s o cont r i b u t e s to the a t m o s p h e r e in the " a i r g a p " , a l l o w i n g m i c r o b r e a k o u t s to heal without oxidation. When m o l d o v e r f i l l s o c c u r , and the c a s t i n g is a l l o w e d to s o l i d i f y in situ, the s u r f a c e , when f i n a l l y r e m o v e d , is b r i g h t and v e r y a c t i v e , s i n c e it r u s t s e a s i l y . The c h a r a c t e r i s t i c s of g l a s s y " m o l d p o w d e r " l u b r i cation is m u c h m o r e c o m p l e x . H e r e the w e t t i n g b e h a v ior is r e v e r s e d , and the flux wets the s t e e l and does not wet the mold. Fig. 26 shows the c o n f i g u r a t i o n that

~'~

_ _ z. _ _

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, '

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GAS(I)

~

31

--

"r,3 cosy= ~2 cose +'r23r Fig. 26--Schematic representation of a flux droplet floating on liquid steel.

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Fig. 25--Graphical representation of the effect of stroke, frequency and strand speed on maximum heal time. 366-VOLUME 6B,SEPTEMBER 1975

ing fluxes

0005 6 I/K Z 8 9 Fig. 27--Effect of temperature on log viscosity for two glasses and three casting fluxes. METALLURGICAL TRANSACTIONS B

would be a s s u m e d by a flux d r o p l e t floating on liquid steel. T h r e e p h a s e s a r e p r e s e n t : gas, flux, liquid steel. If Y~3 cos r is g r e a t e r than the sum of Ylz cos 0 + Y~ cos 4, complete s p r e a d i n g will occur. No m e a s u r e m e n t s of these i n t e r r a c i a l t e n s i o n s have been made, m o s t l y b e c a u s e of the e x t r e m e difficulty of making them. It would appear that while i n t e r f a c i a l t e n s i o n is a factor, it m a y act m o r e or l e s s like a gate at the j u n c t u r e between the liquid flux-liquid m e t a l i n t e r f a c e and the mold. If some wetting o c c u r s flux will r e a d i l y p e n e t r a t e into the slot. If wetting does not o c c u r , the flux will not p e n e t r a t e . Flux usage is in the o r d e r of 1.2#/NT cast. When solidified flux was collected j u s t below the f i r s t water s p r a y s , the m e a s u r e d t h i c k n e s s agreed well with consumption f i g u r e s , i n d i c a t i n g complete coverage. V i s c o s i t y is i m p o r t a n t in c o n t r o l l i n g the t h i c k n e s s of the flux l a y e r . Dr. Y. K. Chuang, of our l a b o r a t o r i e s , calculated the t h i c k n e s s , c o n s i d e r i n g s t r a n d speed, buoyancy effects, l a t e r a l p r e s s u r e through the shell, and v i s c o s i t y .

3nV

t = L 4 g (p

n V g p

]i/z

Fg---hF l u x ) ]

= Viscosi.~y = Strand speed = Gravitational constant = Density

This equation p r e d i c t s a flux cover t h i c k n e s s about 20 pct t h i c k e r than m e a s u r e d , but the t e m p e r a t u r e of these fluxes at the m o l d - m e t a l - f l u x i n t e r f a c e is not known. To give us s o m e feel for t h e s e v i s c o s i t i e s , Fig. 27 is a plot of v i s c o s i t y vs t e m p e r a t u r e for g l a s s e s , the upper one a low sodium g l a s s , the lower one a high sodium glass, and some c a s t i n g fluxes. The i m p o r t a n t thing to note is that t h e r e is no m e l t i n g point per s e - we should only r e f e r to v i s c o s i t y at any given t e m p e r ature. A necessary constituent of these fluxes is graphite or carbon. Fig. 28 shows the effect of pct graphite on the time for the melting of 200 g of flux on top of liquid steel in a laboratory induction furnace. The effect of

graphite is a p p a r e n t l y one of c o n t r o l l i n g m e l t i n g r a t e . As the flux in contact with the s t e e l m e l t s , graphite floats out, providing a graphite r i c h l a y e r between the m o l t e n flux and the unmelted powder. The i n t e r p o s i tion of this graphite r i c h l a y e r acts to block r a d i a n t and p o s s i b l y conductive heat t r a n s f e r into the u n melted powder. If graphite is not used, the flux m e l t s too r a p i d l y and t h e r m a l i n s u l a t i o n is lost. The g r a p h ite m u s t be of such a p a r t i c l e size that it floats out r e a d i l y . If it is too fine, the flotation is too slow. We feel that the flux should have s e v e r a l c h a r a c t e r istics: 1) Flowability as a powder. 2) Should not r e a c t with the steel. High i r o n oxide content should be avoided when a l u m i n u m b e a r i n g s t e e l s a r e cast. 3) Should m e l t homogeneously, 2a so that lower m e l t ing components do not " l e a c h o u t " d u r i n g the c a s t i n g p r o c e s s - - d e s t r o y i n g " g l a s s i n e s s " and r a i s i n g the v i s cosity. 4) Should r e m a i n " g l a s s y " over the full t e m p e r a t u r e r a n g e f r o m liquid s t e e l t e m p e r a t u r e to exit skin t e m p e r a t u r e , and should have such a v i s c o s i t y that the f o r c e s r e q u i r e d to s h e a r the film, as l a t e r a l cont r a c t i o n of the c a s t i n g takes place, a r e s u b s t a n t i a l l y l e s s than the force r e q u i r e d to pull the s u r f a c e of the casting into a longitudinal c r a c k . This m u s t be t r u e from the m i n i s c u s to the m o l d exit t e m p e r a t u r e . This is a n a r e a where much m o r e work is needed. We need a c c u r a t e t e m p e r a t u r e - - v i s c o s i t y c u r v e s for fluxes c o v e r i n g the t e m p e r a t u r e r a n g e f r o m s t e e l liquidus t e m p e r a t u r e s to s u r f a c e t e m p e r a t u r e s at the mold exit. We need to know the effect of the s c a v e n g ing of deoxidation and r e o x i d a t i o n products on the v i s cosity. For example, on high m a n g a n e s e s t e e l s we need to know what happens to the m a n g a n e s e content of the flux, and how this affects v i s c o s i t y . F r a n k l y , t h e r e is too much w i t c h c r a f t in this a r e a . Having d i s c u s s e d some of the e n g i n e e r i n g f a c t o r s involved in continuous casting, we will now d i s c u s s how some of them and other f a c t o r s affect the quality of the c a s t i n g s produced. We will s t a r t with b i l l e t quality.

16 15 14 13

7:

i

-I

9 ?

A

z

tlJ

i

-1

10

9 I

i(3 _z

E u0

8

,g o Suitable ! 9 Allowable 9 Improper [

7

[

5 4 4

I

O

I

[

I

L

2

3

4

5

6

~ GRAPHITE

Fig. 28--Effect on pct graphite on the melting rate of a continuous casting flux. METALLURGICALTRANSACTIONS B

t

40 60 80100120140160180 Billet Size (mm) Fig. 29--Relation between corner radius and billet size as affecting longitudinal cracks in billets. VOLUME6B, SEPTEMBER 1975-367

B I L L E T QUALITY E x t e r n a l Longitudinal C o r n e r C r a c k s Akita e t a l . z5 (Fig. 29) indicate that the effect of c o r n e r r a d i u s on longitudinal c r a c k s depends on b i l l e t s i z e - - t h e l a r g e r the size, the m o r e generous the r a d i u s can be. F o r a given mold design, (Fig. 30) t h e r e is a liquid m e t a l s u p e r h e a t that should not be exceeded. This effect is also well known for ingots. Segregation Streaks When the solidifying s t e e l is s t r e s s e d by m e c h a n i c a l or t h e r m a l m e a n s , i n t e r n a l c r a c k s can f o r m . These a r e i m m e d i a t e l y filled by sulfur r i c h m e t a l that is d r a i n e d out of the i n t e r d e n d r i t i c s p a c e s i n the s o l i d i l y i n g m a s s . They a r e e a s i l y detected by s u l f u r p r i n t s . Since they have s t r o n g effects on such p r o p e r t i e s as cold heading, twist t e s t s and so forth, they have been e x t e n s i v e l y studied. R u s s i a n work in 195826 c l e a r l y pointed out the effect of s u r f a c e t e m p e r a t u r e r e h e a t i n g on this defect. This effect was quantified by Grill, B r i m a c o m b e , and W e i n b e r g . z7 If the s p r a y p a t t e r n in the s e c o n d a r y cooling is not u n i f o r m , s u r f a c e r e h e a t i n g will o c c u r . As the s u r face expands, i n t e r n a l t e n s i l e force is g e n e r a t e d . Since s t e e l s have zero ductility at s o l i d i f i c a t i o n t e m p e r a bares, i n t e r n a l c r a c k s r e s u l t . The a m o u n t of r e h e a t i n g the c a s t i n g can t o l e r a t e depends on the grade. High Mn/S r a t i o s d i m i n i s h the effect, s i n c e the m e l t ing point of the i n c l u s i o n s i n c r e a s e s up to a Mn/S of about 40, at which point only MnS e x i s t s . M e c h a n i c a l d e f o r m a t i o n , u s u a l l y due to r o l l p r e s s u r e , c a n also produce the defect, a s can liquid or m u s h y c o r e bending.

,

_~ 60 U

Carbon is an i m p o r t a n t f a c t o r , zs (Fig. 31) p a r t i c u l a r l y in the p e r i t e c t i c r e g i o n when hot d u c t i l i t y is much r e d u c e d . When a s q u a r e b i l l e t is produced, i r r e g u l a r mold wear will r e s u l t in the s q u a r e s e c t i o n b e c o m i n g r h o m boidal (Fig. 32) at the mold exit. This shape is the r e sult of uneven heat e x t r a c t i o n . The d i f f e r e n c e in the diagonals can be used to m e a s u r e this u n e v e n n e s s . As this d i f f e r e n c e i n c r e a s e s , the " c r a c k s " b e c o m e longer and t h e i r s t a r t i n g l o c a t i o n is c l o s e r to the s u r f a c e . "-9 Speed d e c r e a s e s the depth (Fig. 33) at which the c r a c k s a r e located, and higher m e t a l t e m p e r a t u r e s lengthen c r a c k s and make t h e m s h a l l o w e r . Axial P o r o s i t y Since a c o n t i n u o u s l y cast b i l l e t is r e a l l y a long thin ingot, feeding cannot be a c c o m p l i s h e d b e c a u s e of b r i d g e f o r m a t i o n . Axial p o r o s i t y r e s u l t s . High speed, higher t u n d i s h t e m p e r a t u r e s , all i n c r e a s e axial p o r o s i t y . Knight and O s b o r n e 3~ (Fig. 34) have shown that i n c r e a s i n g the s e c o n d a r y water amount, at c o n s t a n t size and speed i n c r e a s e s the d i a m e t e r of the hole in the c e n t e r of the ingot. Cleanliness In b i l l e t c a s t i n g it has b e e n well d o c u m e n t e d that the m a j o r c a u s e of l a r g e i n c l u s i o n s is s t r e a m o x i d a tion, m o s t l y o c c u r r i n g b e t w e e n the tundish and the mold. Since s o l i d i f i c a t i o n t i m e s a r e s h o r t c o m p a r e d to ingots, and s i n c e the s u r f a c e / m a s s r a t i o of s t r e a m exposure to a i r is much higher than on ingots, this is a serious problem.

,[i

35 ,. E 30

| ! ~401 '

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9 j . ~

p/

2 L, ngth

/I

O~o~_cc -J

i/

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~ o"~ =~,q,,

J mI

r

% 1%14701480% 150~510%1530

~r~

Casting Tern peratu re{~

Fig. 30--Effect of casting temperature on longitudinal surface cracks of billets.

5

10 Shell

15 20 Distortion,mm

25

30

Fig. 32--Effect of a rhomboidity on length and position of segregation streaks.

20

-

1

t5 u

.~ uO ~

10

8

os

'% 0%

~>Plastic

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o 0-10

0"20

0"30

0.40

Carbon Content, ~'o

Fig. 31--Effect of carbon on hot ductility at a temperature near the solidus. 368-VOLUME6B, SEPTEMBER 1975

"- 4 ~ . . ~" 0 0.4 0.8 Withdraw Speed, m/min.

.

.

. 1.2

l 1.6

Fig. 33--Effect of strand speed on the depth of segregation streaks for two billet sizes. METALLURGICALTRANSACTIONS B

Various methods are used to avoid this oxidation. Shielding by argon enclosures such as the use of a "Pollard Shroud''~I (Fig. 35) or submerged entry through a suitable refractory tube (large billets only) all seem to accomplish the desired result. Residence time in the tundish must be long enough to allow ladle to tundish oxidation products to float o u t , a l t h o u g h t h i s is u s u a l l y o n l y a m i n o r e f f e c t .

5 84

/

D-diameter Of Central Hole InThe Billet In Inches

4

Segregation Bands Occasionally, on transverse sections either deep etched or sulfur printed, ~'circumferential" light and dark bands occur. This can persist on sections rolled from such billets for surprisingly high percentages of hot reduction. The cause is usually uneven secondary spraying. If this problem is rectified, these bands or zones disappear.

1 D2 xlO0

R e f r a c t o r y q u a l i t y of t u n d i s h w e l l s a n d t u n d i s h t u b e s m u s t b e s u f f i c i e n t to a v o i d s u b s t a n t i a l m a n g a n e s e s i l i c a t e f o r m a t i o n f r o m t h e r e a c t i o n of t h e m a n g a n e s e in t h e s t e e l w i t h s i l i c a in t h e r e f r a c t o r i e s . It w o u l d a p p e a r t h a t a s l o n g a s t h e A 1 2 0 3 / S i O 2 r a t i o 48 e x c e e d s t h e m u l l u t e c o m p o s i t i o n , ( F i g . 36) t h i s c r i t e r i o n i s s a t i s f i e d . S i l i c a t e b o n d i n g of t h e r e f r a c t o r y g r a i n m u s t a l s o be avoided. Dense zircon is also satisfactory and can be used more than once when nonsequence casting is being employed.

3.

2 1

SLAB QUALITY 120

~ 160 180 200 220 SecondaryWater Cooling - Gal./rain.

240

Fig. 34--Effect of s e c o n d a r y w a t e r usage on the square root of the d i a m e t e r of the c e n t r a l porosity in a s t r a n d cast billet. Tundish

_

Cma

Line ~ H

External Cracks C r a c k i n g p r o b l e m s in s l a b s a r e m o s t c o m m o n l y l o n g i t u d i n a l , s o t h i s d i s c u s s i o n w i l l b e l i m i t e d to t h i s c l a s s i f i c a t i o n . O p e r a t i n g p a r a m e t e r s c o n t r i b u t i n g to this effect would indicate that the proximate cause is

non-uniform solidification in the mold. Russian32'33 and Japanese works4 indicated that stream entry position had a strong effect, (Table I) the interpretation being that wall thinning results from improper metal entry position. T u n d i s h t e m p e r a t u r e i s i m p o r t a n t as'~6 ( F i g . 37), w i t h higher temperatures being sharply worse when a critical tundish temperature is reached. D u c t i l i t y of t h e m e t a l a t a t e m p e r a t u r e s o m e w h a t b e low t h e s o l i d u s i s i m p o r t a n t . F i g . 31 s h o w s t h e e f f e c t

Table I. 200 X 640 mm Slabs ---

Liquid Metal

Courtesy

Total Cracks

oB J & L

Fig. 35--Diagramatic r e p r e s e n t a t i o n of the " P o l l a r d Shroud".

Single Nozzle in Center Single Nozzle in End 2 Nozzles400 mm Apart 2 NozzlesClose to Ends 2 Nozzles Inclined Toward Ends

30

567 mm/M 187 mm/M 263 mm/M 59 mm/M 149 mm/M

Longitudinal Cracks mm/m / /

2O

J

I

J

10 I

OJ: ~ 1550

Fig. 36--Effect of A1203 content of r e f r a c t o r i e s on metal e r o sion in manganese b e a r i n g steel. METALLURGICAL TRANSACTIONS B

1560

1570 Temp.T.D.

1580

I 1590

Fig. 37--Effect of tundish t e m p e r a t u r e on longitudinal c r a c k ing of slabs. VOLUME 6B, SEPTEMBER1975-369

of c a r b o n on elongation in this t e m p e r a t u r e r e g i o n . E v i d e n t l y the p e r i t e c t i c r e a c t i o n is a f f e c t i n g e l o n g a tion. We should not e x p e c t a s h a r p e f f e c t of carbon on cracking because interdendritic micro segregation would p r o d u c e a m i x e d s t r u c t u r e as t h e s e carbon c o n tents a r e a p p r o a c h e d . This e f f e c t is shown in Fig. 38. Langford ~? has shown that the p e r c e n t sulfur (Fig. 39) has a s t r o n g e f f e c t on elongation at high t e m p e r a t u r e s . L a n g f o r d ' s data indicated a b e n e f i c i a l e f f e c t of M n / S r a t i o when the pct S was above the s o l u b i l i t y l i m i t at the 1093~ t e s t i n g t e m p e r a t u r e . His data in-

d i c a t e s that at about 0.010 pct S, Mn had no effect. The d u c t i l i t y being e x c e l l e n t e v e n at low Mn l e v e l s . This is p r o b a b l y the s o l u b i l i t y l i m i t at this t e m p e r a ture. Fig. 40 shows the e f f e c t of sulfur on the c r a c k i n g p r o p e n s i t y of one s t e e l . Slab shape (Fig. 41) a f f e c t s c r a c k i n g p r o p e n s i t y , a~ also. The left graph shows the e f f e c t of width at c o n s t a n t t h i c k n e s s ; the r i g h t graph the e f f e c t of t h i c k n e s s at constant width. We have found an i n t e r a c t i o n (Fig. 42) b e t w e e n p c t Slab Thickness lOOmm

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0.15 0.16 0.17 0.18 Carbon Content,%

0.19

t

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110

150160170180190 200 Slab Thickness, mm

Fig. 41--Effect of slab dimensions in "extent of cracking" of slabs.

% REDUCTION OF AREA AS C A S T ~ 2 0 0 0 " F

.70 65

\\

'~ 120

w

Q20 O21 Q22

Fig. 38--Effect of carbon content in the peritectic range on the extent of cracking of slabs.

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N a r ~ow ( 4 0 " - 4 4 " )

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Fig. 39--Effect of pet S and pet Mn as the reduction of area of low carbon steeIs at 1093~

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~ 2 4 .025

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Fig. 42--Effect of sulfur and width on pct cracked first slabs at 10 in. thickness.

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22

24

26 28 30 32 Sulfur Content, ~/o

34

36

38

40

Fig. 40--Effect of sulfur on "extent of cracking" of slabs. 3 7 0 - V O L U M E 6B, SEPTEMBER 1975

I

I

I

0.6

I

0.8

I

I 1.o

Casting Rate, m / m i n

Fig. 43--Effect of flux type and strand speed on temperature variation in the mold wall. METALLURGICAL TRANSACTIONS B

S and width on the c r a c k i n g of low carbon deep d r a w ing s t e e l s . This i n t e r a c t i o n can be a l t e r e d by flux type and mold w a t e r usage. The m e l t i n g b e h a v i o r of flux can also e x e r t an e f f e c t on c r a c k i n g . S p r i n g o r u m za r e p o r t e d on the e f f e c t of i n h o m o g e n e o u s m e l t i n g of fluxes on c r a c k s . As the l o w e r m e l t i n g c o n s t i t u e n t m e l t s , it is " e x t r a c t e d " down the s l o t between the c a s t i n g and the mold so that the v i s c o s i t y of the flux i n c r e a s e s to the point w h e r e n o n - u n i f o r m s o l i d i f i c a t i o n and wall thinning can o c c u r , pulling into c r a c k s as the m e t a l c o n t r a c t s l a t e r a l l y d u r i n g s o l i d i f i c a t i o n . It has also been r e p o r t e d that a sulfur content of the flux > 0.4 pct will produce c r a c k s - - t h e explanation is o b v i o u s l y the p r o d u c t i o n of a s u l f u r r i c h s u r f a c e on the casting. R e c e n t J a p a n e s e w o r k ~ (Fig. 43) using a network of

o

sol-

/o

t h e r m o c o u p l e s in the mold wall has i n d i c a t e d the e f f e c t of speed and flux type on the t e m p e r a t u r e f l u c t u a tions o b s e r v e d during casting. Note powder " C " . T h e s e t e m p e r a t u r e fluctuations (Fig. 44) w e r e found to affect conditioning l o s s e s , b e c a u s e they a f f e c t e d cracking. Longitudinal c r a c k s a r e f r e q u e n t l y l o c a t e d in l o n g i tudinal " d e p r e s s i o n s " , indicating a too e a r l y s e p a r a tion of the solid face f r o m the mold. End plate t a p e r m u s t be adjusted to the s p e e d r a n g e u s e d to p r e v e n t e a r l y air gap f o r m a t i o n due to l a t e r a l c o n t r a c t i o n with consequent breakout hazard. Inclusions B e c a u s e of the s h o r t s o l i d i f i c a t i o n t i m e s on c o n t i n u o u s l y c a s t slabs as c o m p a r e d to ingots, the p u r i f i c a tion that o c c u r s in ingots due to the floating out of inc l u s i o n s is r e l a t i v e l y i n s i g n i f i c a n t . It soon b e c a m e e v i d e n t that liquid m e t a l handling t e c h n i q u e s would h a v e to be modified on continuous c a s t i n g m a c h i n e s . As in b i l l e t s , the two m a i n s o u r c e s of i n c l u s i o n s w e r e identified as a i r oxidation p r o d u c t s , p r o d u c e d e i t h e r between the l a d l e and the tundish or the tundish and the mold, and the r e a c t i o n of liquid s t e e l with the r e f r a c t o r i e s in the s a m e a r e a s . R u s s i a n work ~ (Fig. 45) showed the e f f e c t of m e t a l 28

.

~

24

I

Frequency Of Excessive Temp. Variation/Meter Fig. 44--Effect of temperature variation on surface conditioning due to cracks.

. . o . - ~

-

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6D

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. " ~

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8

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650mm

0

~'~

0 10 ' Top (8)

20 '

50 .

.

40.

. 30

20

Percent Removed

1b 0 Bottom

Fig. 47--Curved mold machine-pattern of manganese silicate inclusion through the thickness of a slab.

0

1520-1535 1540-1560 SteelTemp. InTundish,~ Fig. 45--Effect of tundish temperature and metal depth in the tundish on rejections due to manganese silicates.

::i(:

i

~10) :t:

~0 r e.o

Fig. 46--Effect of tundish temperature and casting speed on rejections due to manganese silicate inclusions9

4.0 u

9 v:

u .;:,-s-'.-

,~.: .~.

ffl

.~_.

15401540-1550 ",:i550

MetalTemp. InTundish'C

METALLURGICALTRANSACTIONSB

Fig. 48--Photomicrograph of an A1203 cluster--as cast9 Magnification 33 times. VOLUME 6B, SEPTEMBER 1975-371

t e m p e r a t u r e , and depth in the tundish, and c a s t i n g speed (Fig. 46) on r e j e c t s due to i n c l u s i o n s on Si-Mn deoxidized s t e e l s . C o n s i d e r a t i o n s on r e f r a c t o r y quality are, of course, the s a m e as d i s c u s s e d u n d e r b i l l e t quality. The p a t t e r n of i n c l u s i o n s on c u r v e d m a c h i n e s shows a c o n s i d e r a b l e g r a v i t a t i o n a l effect. (Fig. 47) The conc e n t r a t i o n of i n c l u s i o n s in the " u p p e r s i d e " is quite d i f f e r e n t f r o m that on the bottom, ~ due to flotation of i n c l u s i o n s up a g a i n s t the solidifying m e t a l on the c u r v e d casting. This p a r t i c u l a r study was on s t a i n l e s s steel, and the e l e c t r o - p o l i s h i n d i c a t i o n s w e r e MnO. SlOe s t r i n g ers. The case of a l u m i n u m deoxidized s t e e l s , such as Special Killed for deep d r a w i n g applications, is corn-

plicated by the phenomenon of a l u m i n a c l u s t e r f o r m a tion and tundish tube clogging. Alumina, e i t h e r from air oxidation or d e o x i d a l i o n , tends to form c l u s t e r s (as cast) (Fig. 48). Fig. 49 shows the a p p e a r a n c e of a c l u s t e r on a s c a n n i n g e l e c t r o n m i c r o s c o p e after r e m o v i n g m e t a l with a b r o m i n e methyl alcohol etch. Since c h e m i c a l a n a l y s e s of the c l u s t e r s i n d i c a t e a l m o s t p u r e a l u m i n a , the q u e s t i o n i m m e d i a t e l y a r i s e s , why do they stick together at 1550~ when the s o f t e n ing point of pure AlzO~ is 1750~ J a p a n e s e work ~~ indicated that each AlzOz r o d a n d / or d e n d r i t i c c r y s t a l is coated with a thin s i l i c a f i l m . P y r o m e t r i c cones made f r o m e x t r a c t e d AlzO~ tend to r e i n f o r c e this by an a n a m o l o u s low t e m p e r a t u r e s i n t e r i n g . Since the c l u s t e r s a r e u s u a l l y located by s u l fur p r i n t i n g , this e l e m e n t m a y also be involved. At any r a t e , the c l u s t e r s a r e " s t i c k y " . A c c u m u l a t i o n in tundish n o z z l e s b e c a u s e of th~s

Fig. 51--Cluster count--straight down entry.

Fig. 49--SEM photo of part of a cluster--bromine-methyl alcohol etch.

o2a

I-

024 I ~ 0.20!

o,e[.

o2e

;:if:!:'

016~. ' ~008

" ..

~

.__~ 0 " 1 2 t

,.;:;]::-:' : : ' : =

.

~ 008

'/

r

-1

"J" ~

.~'~

8O

/ Steel between Ladle and Mold

Fig. 50--Cluster count distribution on a east slab showing effeet of bifurcated down vs bifurcated up entry. 372-VOLUME 6B, SEPTEMBER 1975

Fig. 52--Cluster count--Bifurcated down plus tundish dam. METALLURGICALTRANSACTIONS B

~1 9 I

~

c~ "-x

I,

Fig. 53--Cluster count--effect of casting speed9

.9 ...,

2=

i /. . $

Fig. 54--Sulfur print showing cluster concentration due to flotation on a curved mold machine.

s t i c k i n e s s o c c u r s independently of r e f r a c t o r y type. If the a c c u m u l a t i o n b e c o m e s s u f f i c i e n t l y m a s s i v e it m a y c l o s e off the n o z z l e c o m p l e t e l y . A h a r m f u l e f f e c t can o c c u r if m a s s i v e chunks of this m a t e r i a l b r e a k off for s o m e r e a s o n and e n t e r the liquid pool. This b e h a v i o r has been o b s e r v e d on our pilot c a s t e r , a single s t r a n d m a c h i n e . Since we used s t o p p e r c o n t r o l for c o n t r o l of liquid m e t a l , if a " n o z z l e f l u s h " o c c u r r e d , the o p e r a t o r i m m e d i a t e l y had to t h r o t t l e back to avoid o v e r filling the mold. C l u s t e r c o n c e n t r a t i o n s in the slab a r e a f f e c t e d by speed and m o d e of m e t a l entry. L i s t h u b e r e t a l 4~ have d e m o n s t r a t e d this beautifully. They d e m o n s t r a t e d , on a " s t r a i g h t with b e n d i n g " m a c h i n e , the e f f e c t of tundish d a m s , m e t a l e n t r y m o d e and c a s t i n g s p e e d . Fig. 50 shows the e f f e c t of b i f u r c a t e d " d o w n " v s b i f u r c a t e d " u p " e n t r y on the c l u s t e r p a t t e r n . While the twin peaks inboard f r o m the s u r f a c e s of the slab a r e i n t e r e s t i n g f r o m the point of view of g r a v i t a t i o n a l effects, the i m p o r t a n t peaks as far as s u r f a c e q u a l i t y is c o n c e r n e d , t u r n e d out to be those c l o s e to the s u r f a c e of the b r o a d f a c e s . Note that b i f u r c a t e d " u p " is s u p e r i o r in this a r e a . Fig. 51 shows the e f f e c t of s t r a i g h t down e n t r y on c l u s t e r d i s t r i b u t i o n . Note that b i f u r c a t e d down is much b e t t e r than s t r a i g h t down e n t r y . Fig. 52 shows the e f f e c t of r e s i d e n c e t i m e in the tundish on c l u s t e r p a t t e r n for a b i f u r c a t e d down e n t r y a tundish dam being u s e d to f o r c e the m e t a l to take a l o n g e r path b e f o r e the tundish n o z z l e is e n c o u n t e r e d . Fig. 53 shows the e f f e c t of m a c h i n e speed for a s t a n d a r d tundish and b i f u r c a t e d down mold entry. Note that h i g h e r speed s u b s t a n t i a l l y d e c r e a s e s b r o a d f a c e c l u s t e r count peaks at the s u r f a c e . On a c u r v e d mold, the s o m e w h a t a s y m e t r i c p a t t e r n d e m o n s t r a t e d by L i s t h u b e r is much e x a g g e r a t e d , as the sulfur p r i n t in F i g . 54 shows. C l u s t e r s a r e a l m o s t c o m p l e t e l y confined, on a 245 m m thick slab m a c h i n e , to an a r e a f r o m 20 to 50 m m below the " u p p e r " s u r face. H o w e v e r , the s i z e of t h e s e c l u s t e r s and t h e i r depth below the slab s u r f a c e m a k e it doubtful that t h e s e p a r t i c u l a r c l u s t e r s account for g r o s s s u r f a c e d e f e c t s . Our e x a m i n a t i o n of hot r o i l e d coil d e f e c t s (Fig. 55) by s u r f a c e polishing, (Fig. 56), i n d i c a t e s that AlzO3 c l u s t e r s a g r e a t d e a l m o r e m a s s i v e m u s t be involved.

,~ y:0:=:~ .....

-~b,::"

./.

"

/

Fig. 55--"Open seam" defect on a hot roiled coil. Magnification 0.65 times. METALLURGICALTRANSACTIONS B

Fig. 56--A1203 cluster from an "open seam" defect polished parallel to the surface. Magnification 32.5 times. VOLUME 6B,SEPTEMBER 1975-373

We feel that the g r o s s defects we c l a s s i f y as " o p e n s e a m s " r e s u l t p r i m a r i l y from v a r y i n g d e g r e e s of the " n o z z l e f l u s h " phenomenon. The use of an " e m u l s i o n " of a r g o n and steel obt a i n e d by i n j e c t i n g a r g o n into the t u n d i s h s t r e a m , when o b s e r v e d on a water model, i n d i c a t e s that the b e n e f i cial effect of this technique r e s u l t s f r o m the p r o d u c tion of a higher d e g r e e of t u r b u l e n c e in the tube. Again f r o m the model it does not appear to make any d i f f e r ence where the a r g o n is introduced. Anywhere from the tube wall to the c e n t e r of the s t o p p e r rod has the s a m e effect. We have not, to date, m a d e m a c h i n e t r i a l s on this technique, but unofficial r e p o r t s we have

Slab

200 X 2.100 m

3~0

9

g2 oJ

~ v

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/~o

u.~

me~,o 10

0

,~%

u

ooc8

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0 /

,m

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3c)

40

r e c e i v e d from other u s e r s i n d i c a t e that the technique produces a m a j o r i m p r o v e m e n t in s u r f a c e quality. This r e i n f o r c e s the m e c h a n i s m proposed, that this t u r b u l e n c e avoids nozzle f l u s h i n g b y p r e v e n t i n g the a c c u m u l a t i o n of c l u s t e r s on the r e f r a c t o r y s u r f a c e s . SLAB QUALITY Center Unsoundness This defect does not appear to be harmful in sheet and strip. Evidently the 98 to 99 pct hot work in going from slab to hot rolled coil smears the central unsoundness out sufficiently so that it does not affect physical properties. If plate steels are to be made in some thicknesses where the hot work is <12 times, center unsoundness can be of some concern. Asano e t al 4z rated this defect on as cast slabs on the basis of sulfur prints. They show the effect of TD temperature AT, (Tundish Temp-Liquidus Temp) and secondary cooling water on this rating (Fig. 57). They showed the effect of a bent roll (Fig. 58) and

50

Degree of Superheating of Liquid Metal AT.Oc 2.6 -% 4

n

"~3-

n

%.

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OLarge Casting Bulging

2.2

-~c~

E ~

Q Slight Bulging X Shape Bulging, 8ram

Slab 250ram ~'hick

C

0.6 0,8 Cooling WaterRatio, liter/kg

10

.g

12

Fig. 57--Center unsoundness rating as affected by superheat and secondary coolingwater.

s~

~

I t.4

uO

lO

s,,~ 2so x a.laa..

ip,~--x w

+~

~

~x o

x.

o

=

0.6

80

m

100

125 140 Distance from surface, mm

160

180

Fig. 60--Sulfur segregation in the center of a slab. 5

10

15

20

25

Slab Length, m

F i g . 5 8 - - E f f e c t of a b e n t s u p p o r t r o l l on c e n t e r u n s o u n d n e s s rating.

Slob 2S0 x 2,|OOmm AT 10 X 20"C

.~

/

/

(

3

/

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88C)

OO

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)--O.--~ /

~9

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-1

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7

9

Fig. 59--Effectof bulging on center unsoundness rating. 3 7 4 - V O L U M E 6B, SEPTEMBER I975

Fig. 6 1 - - T e s t i n g method for short t r a n s v e r s e plate.

d u c t i l i t y of

METALLURGICAL TRANSACTIONS B

a n d to A r m c o S t e e l C o r p o r a t i o n publish this study.

i!

53

D OJ

._c

"%D b

41

~b 40

30 20 Sheet Thicknessl'mm)

10

Fig. 6 2 - - E f f e c t of hot r e d u c t i o n r a t i o on s h o r t t r a n s v e r s e ductility.

b u l g i n g ( F i g . 59) d u e to m a c h i n e m i s a l i g n m e n t on the defect. A c t u a l s u l f u r a n a l y s e s ( F i g . 60) s h o w a n e n r i c h m e n t a s h i g h a s 2 . 4 t i m e s t h e l a d l e s u l f u r in t h e d e f e c t z o n e (Ref. 43). The major physical property affected is short transv e r s e d u c t i l i t y . R u s s i a n w o r k e r s 44 d e v i s e d a t e s t ( F i g . 61) a n d b y i t s u s e f o u n d t h a t p l a t e s r o l l e d f r o m 1 7 5 to 180 • 1 0 4 0 m m s l a b s g a v e m a x i m u m d u c t i l i t y a t 30 m m , or about a 6X reduction (Fig. 62). This effect was attributed to a combined effect of center unsoundness with very high sulfur (0.01 pct S ladle, with as high as 0.05 pct S at the center) and the plastic deformation of silicate inclusions when the hot reductions were higher. Summing up, a massive amount of work of variable quality has been done in studying the continuous casting of steel. More information is needed about primary solidification in the mold. Factors affecting uniformity of solidification need to be studied. Hopefully our approach will shed some light on this matter. Secondary cooling, using surface temperatures to verify the model, can be handled adequately by modern mathematical techniques. F a c t o r s a f f e c t i n g c l e a n l i n e s s w o u l d a p p e a r to b e well enough known. Cracking of very wide slabs needs more work to determine the factors affecting its occurrence, particularly in the area of casting fluxes. So t h e r e i s a l o t o f l i g h t b u t s t i l l - - e n o u g h d a r k n e s s to p r o v i d e j o b s e c u r i t y f o r s e v e r a l y e a r s .

ACKNOWLEDGMENTS I w o u l d l i k e to a c k n o w l e d g e h e l p r e c e i v e d f r o m E . D. S c h e r r e r , Y. K. C h u a n g , a n d J o h n d ' E n t r e m o n t ,

METALLURGICAL TRANSACTIONS B

to

REFERENCES

"r 49

~ 45

for permission

1. L. S. Rudoi, et al.." lzv. Vyssh. Ucheb. Zaved., ChertL Met., 1971, vol. 12, p.41, BI51 #10151. 2. L. S. Rudoi, et al.: lzv. Vyssh. Ucheb. Zaved., Cher~ Met., 1962, vot. 2, p. 52, Hb #5567. 3. M. S. Gordienko, etal.: Sb. Nauck Tr,, 1965, vol. 11, p. 109, HB #7220. 4. Volk& Wunnenberg: KleipzigFachber, 1972, vol. 80, p. 491, HB #9010. 5. T. Miyake: Tetsu-To-Hagane, 1974, vol. 60, HB #9318. 6. Tarmann and Forstener: Radex Rundch., 1966, vol. 1, p. 51, HB #6867. 7. W. Holzbruber and B. Tarmann: Steel Times, 1967, Aug. 25, vol. 195, p. 217. 8. H. Muller and R. Jeschar: Arch. Eisenhuettenw., 1973, vol. 44, p. 589, HB #9126. 9. H. Graichen: Die Technique, 1967, vol. 22, p. 500, HR #7357. 10. U. S. Patent #1908, George Sellers. 11. Junk: NeueHuette, 1972, vol. 17, p. 13, HB #8740. 12. E. A. Mizikar: Tran~ TMS-AIME, 1967, vol. 239, p. 1747. 13. W. R. Irving: J. Iron Steellnst., 1967, vol. 205, p. 271. 14. K. Cliff and R. J. Dain: J. Iron Steel Inst., 1967, vol. 205, p. 278. 15. V. T. Lebedev and D. P. Evteev: Stal., 1973, vol. 4, p. 315. 16. J. Savage and W. H. Prichard: J. Iron Steellnst., 1954, vol. 178, p. 269. 17. M. Bundeis and H. Muller: B e ~ Huettenman~ Montash., 1965, vol. 114, p. l15, HB#8191. 18. H. G. Baumann: StahlEisen, 1969, vol. 89, p. 1467, BISI #8392. 19. A. Monyama and I. Muchi: Tetsu-To-Hagane, 1969, vol. 55, p. 682, BISI #8034. 20. L. S. Rudoi and A. P. Kongaklin: Izv. Vyssh. Ucheb. Zaved, Chem Met., 1971, vol. 12, p. 4, BISI #10151. 21. M. A. Soliman, J. Szekely, and Walt Ray: unpublished research. 22. R. Albemy: Circ. InforrrL-TechnoL, 1973, vol. 3, p. 763, BISI #11633. 23. J. J. Gautier, et aL: J. Iron Steel Inst., 1970, vol. 208, p. 1053. 24. Springorum: Steel Times, 1969, vol. 197, p. 727. 25. Y. Aketa, et al.: Tetsu-To-Hagane, 1959, vol. 45, p. 1341, HB #5223. 26. V. S. Rutes, et al.: Liteinoe Proizvod., 1958, vol. 9, p. 11, HB #4554. 27. Grill, Brimacombe, and Weinberg: presen ted at the 104th AIME Meeting, February 17, 1975. 28. L. 1. Morozenskii, et aL: Stal., 1965, vol. 4, p. 272. 29. N. G. Gladyshev: lzv. Vyssh. Ucheb. Zaved., Cheni Met., 1965, vol. 5, p. 40, HB #6507. 30. T. Oita: Iron Steel Eng., Sept. 1965, vol. 42, p. 169, discussion by D. J. Knight and F. T. Osborne of Atlas Steel Co. 31. M. L. Samways, etal.: J. Metals, 1974, vol. 26, no. 10, p. 28. 32. A. I. Chizhikov: lzv. Vyssh. Ucheb. Zaved., Chem. Met., 1968, vol. 1 I, p. 53, HB #7420. 33. E. A. lodko: lzv. Vyssh. Ucheb. Zaved, Chem. Met., 1960, vol. 12, p. 31, HB #5082. 34. E. A. lodko: lzv. Vyssh. Ucheb. Zaved, CherrL Met., 1961, vol. 11, p. 60 HB #5472. 35. A. A. Skvortsov: Izv. Vyssh. Ucheb. Zaved., Chem. Met., 1961, vol. 17, p. 78, HB #5347. 36. T. V. Sladkoshteev, et aL: Continuous Casting o f Steel, p. 126, Metallurgizdat Press, Moscow, 1970, HB #8594. 37. D. B. Evteev: Berg. HuettenmanrL Montash., 1966, vol. 11, p. 238 HB #6973. 38. W. T. Langford: Met. Trans., 1972, vol. 3, p. 1331. 39. J. Vrsek: Hum. Lis~, 1967, vol. 22, p. 88, HB #7387. 40. W. F. Pontius and C. R. Taylor: Proc. Elect. Furnace Conf., 1967, vot. 25, p. 40. 41. K. Kawakami, etaL: Tetsu-To-Hagane, 1972, vol. 58, p. S-402, HB #8907. 42. F. Listhuber, et aL: Iron Steel Eng., 1974, rot. 51, #492. 43. K. Asano: Tetsu-To-Hagane, 1973, vol. 59, p. 582, HB #9099. 44. K. Asano, etal.: Tetsu-To-Hagane, 1974, vol. 60, HB. 45. V. 1. Askol'dov, et aL: StaL, 1971, vol. 11, p. 996, BISI #10020. 46. R. Shultz: Bull Amer Ceram. Soc., 1973, vol. 52, p. 833, #I 1.

VOLUME 6B, SEPTEMBER 1975-375