R E F R A C T O R I E S F O R C O N T I N U O U S S T E E L

A . K . K a r k l i t , Y u . V . M a t e r i k i n , a n d N . M . F r o l o v s k i i

C A S T I N G

The USSR has developed and, during the pas t 10 yea r s , extensively introduced into the industry the method of continuous s teel cast ing for obtaining cast blanks which have a more homogeneous s t ruc tu re mad chemical composi t ion than ingots cast in molds. The continuous s teel casting plant produces round, square , rec tangula r , and hollow tube blanks, etc. Carbon kil led and r i m m e d s tee ls can be cast , including those with highly ductile p r o p e r t i e s for the product ion of cold- and ho t - ro l led sheet, low- and high-alloyed, e l ec - t r i ca l engineer ing s tee l s and al loys.

The re l iabi l i ty of the opera t ion of continuous s teel cast ing plant l a rge ly depends on the s e rv i ce of the r e f r a c t o r i e s that are employed. The speci f ic f ea tu res of continuous cast ing involve s t r i c t e r r e q u i r e m e n t s being p laced on the r e f r a c t o r i e s in all sect ions of the p roce s s , f rom the s teel cast ing ladle to feeding the me ta l into the mold.

Lining Stee l -Cast ing Ladles. The t e m p e r a t u r e of the s teel tapped during continuous s teel cast ing is 30-60~ higher than during ingot cast ing. The duration of the cast ing is 1-2 h, which is 20-30% g r e a t e r than with top cast ing into ingot molds f r o m ladles of the same capacity. The re fo re the r e s i s t ance of the linings in ladles for continuous cast ing is reduced. In o rde r to improve the life of the linings it is n e c e s s a r y to use ladle r e f r a c t o r i e s of improved quality (KShP-39 and KShP-37). Highly aluminous products made f r o m natura l raw ma te r i a l s , for example, Arka lyksk raw mate r ia l s , a re ve ry effective, espec ia l ly in the s lag belt ; the life of the lining can be inc reased to 14-16 heats . It is also planned to make linings f r o m acid r a m m i n g bodies [1]. The working l ayer of the acid lining is conver ted into a viscous g l a s sy mass , which is densif ied and consolidated by the p r e s s u r e of the iron. In the p re sence of a continuous layer , the s lag cannot pene t ra t e the c racks , and hinders the diffusion of the s l ag - fo rming oxides at the grain boundar ies . Volume diffusion, and accordingly solution of the r e f r ac to ry , occurs slowly, so the life of the lining can be brought to 15-25 or m o r e heats , depending on the thickness and the working conditions, iV[echanization of the filling and tamping of the r a m m i n g bodies however r equ i res the thickening of the lining to 150-250 ram, and also a sufficiently accura te shape and r igidity in the housing of the s tee l cast ing ladle.

Lining the In te rmedia te Ladle. The main cause of fa i lure is damage done during the r emova l of the adhering meta l res idues , which aggrava te the e ros ion in the joints of the s t ruc ture , reaching th icknesses of 6-8 m m because of the complex shape of the ladles. Exper ience showed that it would be effect ive to use different l inings. In the continuous s tee l cast ing plant in the open hea r th shops of the Donetsk MetaUurgJtcal and other f ac to r ies for cast ing one heat, the max imum res i s t ance (up to 28 heats) was achieved by using a r ig id b r i ck s t ruc tu re c lamped at the top with a meta l band welded to the upper par t of the ladle housing. Curved r e f r a c t o r y shapes a re used in the rounded (small radius of curvature) sect ions to e l iminate cutting of s tandard products and to reduce the thickness of the joints. In some cases it is des i rab le to use s e m i - acid ladle products , and also k y a n i t e - s i l l imani te , which expand during heating, thus closing the joints.

At the Novo-Lipe t sk Metal lurgical Fac to ry for cast ing seve ra l heats successfu l use has been made of h igh-a lumina r a m m i n g bodies (19-40 heats) . In in te rmedia te ladles there is ve ry li t t le s lag and no s t i ck- ing of s lag on the lining containing high A1203 concentrat ions which is usual for s teel cast ing ladies . The chemical r e s i s t a n c e of the h igh-a lumina bodies is higher than that of acid bodies, and so they are p r e f e r r e d for in te rmedia te ladles, but it is a lso poss ib le to opera te with acid bodies.

All-Union Insti tute of R e f r a c t o r i e s . I .P. Bardin Sc ien t i f i c -Resea rch Insti tute of F e r r o u s Metal lurgy. T rans l a t ed f r o m Ogneupory, No.9, pp.4-10, September , 1971.

�9 1972 Consultants Bureau, a division of Plenum Publishing Corporation, 227 West 17th Street, New York, N. Y. 10011. All rights reserved. This article cannot be reproduced [or any purpose whatsoever without permission of the publisher. A copy of this article is available from the publisher for $15.00.

559

TABLE 1. Proper t ies 9f Stopper Tubes

R e f r a e t o ~ Content, OZ~ ~ ~nder lo.d 212 I ~ l ~ Pha~e oompo-

Chamotte, Semiluks 37"38 ~2 1720 [ 111101__[112 Refractories Plant . . . . . 1390 1445 1600 0,15 22,7 2,02 - -

High- alumina bonded (witheleetrocorundum) 52--6011,2--1770 1445 1520 1660 0 21,0 2,20--

I I 12 I 1,7 ' i ! l ]

TABLE 2. Characteristics of Plugs for Intermediate Ladies

Plugs*

High-alumina with a Polozhe kaolin bond . . . . . . . . . . . .

Electrocorundum and Lamensk c l a y . . . . . . . . . . . . . . . . .

High-alumina from the Vnukov and Semiluks Refractories Plants . . . . . . . . . . . . . . . .

Content, %

A1203 tF%03

51,90 1,15

71,70 1,45

72--73 0,70

1790 1500 21,6

1880 1560 19,4

~1800 1570 19--21

5"-- I < "N t)

2,18 3--4

2,62 3--4

2,45 10--15

* All plugs were thermally shock resistant (in accordance with the requirements of GOST 550O- 64).

Stopper Tubes. In the steel casting ladles of the continuous steel casting plant the tubes operate in heavier conditions than when the casting is done in steel ingots. The f i rec lay tubes stand up sat isfactor i ly if the individual routine control is str ict , and when each tube is of a high quality. Using e lectromagnet ic waves of superhigh f r equency it is possible to evaluate the uniformity of the s t ructure ac ross the section of the tube, in combination with the specified average porosi ty [2]. The bond which in f i reclay products contains a large quantity of glass phase and mullite of less perfect crystal l izat ion is less c o r r o s i o n - r e s i s - tant. The res is tance of the bond is inereased if it is enriched with alumina by using high-aIumina kaolinite - h y d r a r g i l l i t e ores or adding commerc ia l aluminas to the clay with e lec t r ic - fused corundum in the form of a combined-grinding mixture. Table 1 shows the charac ter i s t ics of such products. The wear (reduction in diameter) during the hot time of the tubes with the high-alumina bond is 50~c less than in the chamotte products.

The serv ice of the stopper tubes in intermediate ladies is improved with air cooling of the rod (core). During the casting of a large number of heats in success ion it is possible to use the above improved quality tubes, and also high-alumina tubes.

Nozzles and Plugs for Steel Ladies. Magnesite f ired and unfired nozzles are sueeessful ly used in these ladles. The installation of the nozzles is done outside the ladle which means that nozzles have to be made with a reversible cone, and the ladles fitted with the appropriate devices. The plugs operate with a large number of coverings (up to 60 per cast); repeated changes in the flow conditions of the metal enhance the erosive wear of the plug and the seating of the nozzle. Chamotte plugs serve satisfactorily for casting killed, alloyed, and electricalengineeringsteels. For the corrosive steels (manganese, low-carbon rimmed med) it is possible to use plugs with a higher content of alumina (plugs for special purposes or high-alumi-

na specified by GOST 5500-64).

560

3O

22

14

I

I

lJO0 140G 00 ~

Fig.1. Relationship between the viscosity of synthetic specimens of contact layer and the composi- tion of the slags and temperature:

No.

of curve

Plugs for Intermediate Ladles* With a highly res is tant batching device, covering of the jet with the plug in the intermediate ladle is s ca rce ly ever required, but the long casting period makes it necessa ry in certain cases to use special high-alumina plugs. These plugs were developed by the East Institute of Refrac tor ies (Table 2) and have been successful ly tested for casting up to 10 heats of killed steel at the Novo-Lipetsk Metallurgical Factory, and are now being produced by r e f r ac to ry concerns.

Nozzles and Batching Devices for Intermediate Ladles. The batching equipment is a complex and cri t ical section of the contin- uous steel casting plant. This device should meet the following r e - quirements : the delivery of steel into the mold should be in the form of a compact and stable jet without spattering; there should be r e s i s - tance against corros ion and tightening, that is, a constant cross section in the channel; it shouldbe possible to r e ta rd the jet, espe- cially with an automatic cycle, and there should be reliable covering or shut off; another requirement is s implici ty and at the same time reliable operation over a per iod of severa l hours during the casting of "heat on heat."

The resul ts of the service of batching devices are determined by the physicochemical p rocesses at the contact zone between the

Mix composition for determina- steel (slag) and the re f rac to ry . The essential p rocess factors are : tion of viscosity, %

chemical react ion between the oxide components of the steel with AltOs SiO~ FeO-;-Fe~O3 ~'v~nO the components of the re f rac to ry ; the formation of a contact layer

20 50 20 10 between the jets of steel and the r e f rac to ry in conditions of contin- 20 40 20 20 uous diffusion exchange with the medium boundaries; the development 20 35 20 25 20 35 30 15 of dynamic equilibrium depending on the viscosity of the contact ~0 30 40 20 layer and the ratio of the adhesion forces between the contact layer, 10 20 40 30 30 10 40 20 the re f rac to ry , and the steel, which is in turn connected with the

surface tension forces .

Investigations of the react ion between aluminosil icate re f rac to r ies and steel and slag in the sys tems CaO- SiO 2 - AI203 and AI203- SiO 2- MnO- FeO- Fe203, have shown that the solubility of the refractory diminishes linearly with an increase in the content of alumina, the activity of the slags, and with a reduc- tion in their basicity. The solution of the refractory is accelerated with an increase in the content of iron and manganese oxides in the steel, and a reduction in the content of alumina and silica.

The compositions of the contact layer vary in wide limits (Table 3). The viscosity of the melts syn- thesized from oxides and close in composition to a number of specimens from the contact layer of alumino- silicate refractories are shown in Fig.1. The viscosity depends on the content of silica in the melt, and with an increase in this there is a reduction in the ratio O:Si, which is accompanied by an enlargement of the silicon-oxygen anions and an increase in the viscosity [10]. The effect of the alumina concentration on the viscosity is less important. The predominance of the lower oxides of manganese and iron causes a reduction in the viscosity, which is also explained by the change inthe size of the anions [10].

The adhesion properties of the contact layer depend on the surface tension and the wetting angle on the boundaries with the refractory and steel:

tV = ~st (1 +cos 0),

where est is the surface tension of the contact layer , and 0 is the wetting angle of the r e f r ac to ry or steel by the contact layer .

Wear of the batching devices will occur if the adhesion of the contact layer to the steel is g rea te r than to the re f rac to ry . In the oposite ease constr ic ted flow may occur. F rom the viscosi ty and surface

* Nonstopper casting is not considered in this article.

561

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{91

proper t ies of the melt corresponding in composition to the contact layer it is possible to evaluate the ex- pected res i s tance of a par t icu lar r e f r ac to ry mater ia l in batching devices.

During investigations of the magnesite re f rac to r ies the effect of the quantity of impurit ies in the per ic lase was noted. In magnesite containing 91-92% magnesium oxide the ratio of the impurit ies is un- favorable, and after f ir ing forms about 10% fusible si l icates (monticellite etc.). With an increase in the calcium oxide content (CaO:SiO2 > 2) we observe cracking of the re f rac tor ies due to the polymorphic inver- sions of the dicalcium si l icate . In per ic lase re f rac tor ies made f rom sea water magnesia (95-97% MgO) the p resence of about 2.5% of uniformly distributed monticellite has no great effect on the res is tance. Exper i - ments have shown that inser ts fitted to the nozzles p r epa red f rom sea water magnesia, have a g rea te r res is tance, and can be used for casting low-carbon r immed steels for up to 2-3 heats, one after the other.

F r o m investigations of r e f rac to r ies in the sys tems ZrO 2 - SiC 2 - M n O - F e O - F e 2 0 ~ the increase in the viscosi ty and adhesion of the contact layer to the r e f rac to ry with an increase in the content of ZrO 2 and SiC 2 has been well established. This suggests that z irconium-containing re f rac to r ies are promis ing for use in continuous steel casting plant. Until now only zircon products have been widely used.

Depending on the proper t ies and behavior of the contact layer, batching devices can be classified 1into four groups. In the batching devices of the f i rs t group, a highly fluid contact layer is formed during the p roces s of casting, which is continuously wetted by jets of steel. Inthe batchers of the second group a viscous contact layer is formed, whose adhesion to the steel is g rea te r than to the re f rac tory . The wear of the batchers of both types during casting is substantial. In the batchers of the third group a viscous con- tact layer develops whose adhesion to the r e f r ac to ry is g rea te r than to the steel, and the wear is only moderate . The use of these batchers is most effective. Inthe batchersof the fourth group the difference in the values of the adhesion between the contact layer and the r e f r ac to ry and the steel is very great, which causes construct ion of the channel. Classifying r e f r ac to ry mater ia ls of batchers in a par t icular group depends of course on the type of steel.

On the basis of brief sc ient i f ic-exper imental data, a ser ies of r e f rac to r ies was developed for batch- ing devices, ensuring the casting of steels of different grades. For killed steel type St.3, r immed carbon (2 kp-3kp) manganese, and also steels with an increased content of aluminum the zircon nozzles are r e - commended. The t r a n s f o r m e r steel and also small quantities of killed steel (up to 30 ton per single batcher) can be successful ly cast through f i rec lay nozzles. The casting of steels with a very high degree of co r - ros iveness (low-carbon r immed) requi res the use of composite batching devices made f rom high-alumina nozzles (82% A1203) and corundum or per ic lase inser t -ba tchers .

The design of the devices is very important. Composite devices made f rom different mater ia ls , and in which the more res is tant mater ia l is used for the inserts, have proved themselves .

One of the most complicated problems is in the casting of steel deoxidized by a large quantity of aluminum (more than 1 kg per ton of steel), since the balance of the diffusion of the oxides into the contact layer causes an increase in its viscosity, and an unfavorable ratio of the adhesion forces . The best sob~- tion at the present t ime is to use nozzles with a channel having an increased diameter, whose dimensions are determined by the tightening-up kinetics. The necessa ry velocity of flow is regulated by retarding it with a plug, and the compactness of the jet is p re se rved by using an additional nozzle lowered into the mold and being an extension of the f i rs t (the so-cal led "head-free" casting). Work is also continuing with the testing of certain types of r e f rac to r i e s with reduced adhesion to the contact layer.

Nozzles for Incorporat ing the Jet under the Steel Level in the Mold. Covering the jet improves the quality of the metal. In the casting of e lect r ical engineering steels through an elongated nozzle the content of oxygen in the metal diminished f rom 0.0027 to 0.0022%, nitrogen f rom 0.0113 to 0.0093%, and nonmetal- lic inclusions f rom 0.00745 to 0.00042%. The compactness of the jet is not disturbed during retardation, which makes it possible to reduce the danger of constr ic ted flow in the nozzle. Complete coverage of the jet during the route f rom the steel casting ladle to the mold is prospect ive for the near future. For the in- termedia te l a d l e - mold section, r e f rac to r i e s have been developed on the basis of c l a y - g r a p h i t e mixtures, and also f rom fused quartz. With lower tapping of the jet the service conditions of the nozzles are made somewhat eas ier . With side tapping best resul ts were obtained with nozzles made f rom fused quartz. The choice of the type of nozzle and the content of alumina in graphite nozzles depends on the type of steel being cast, and the casting conditions.

563

C O N C L U S I O N S

As a resu l t of work ca r r i ed out by inst i tutes and e n t e r p r i s e s in the r e f r a c t o r i e s and meta l lu rg ica l industry of the USSR, var ious types of r e f r a c t o r i e s have been developed for continuous s teel cast ing plant that is widely used in product ion conditions. The speeding up of the work of continuous cast ing plant, the extension of the range of s tee ls being cast , and the development of continuous cast ing plant of var ious designs all r equ i re decisions to be made in the field of r e f r a c t o r y ma te r i a l s . It is n e c e s s a r y to continue r e s e a r c h into the mechan i sm respons ib le for the fo rmat ion of the contact l ayer between the s teel and the r e f r ac to ry , the way in which it reac ts , and the movement in var ious boundary conditions; it will also be n e c e s s a r y to develop fur ther scient i f ic exper imenta l theor ies for the contact layer , and through the accu- mulat ion of the n e c e s s a r y data to approach the p rob l ems of construct ing a ma themat ica l model for the p r o c e s s .

The main p rob lem is to improve r e f r a c t o r i e s used in batehing setups in o rde r to c rea te re l iable chan- nels for t r anspor t ing the s tee l to the mold at a control lable velocity. For the important e lements of the complex, that is, the unit respons ib le for the flow of meta l f r o m the ladle and its control, and also for in- t rodueing the moIten meta l under the meniscus into the mold it is n e c e s s a r y to develop r e f r a c t o r i e s with the opt imum phys icoehemica l p r o p e r t i e s , ensur ing continuous operat ion of the plant for sufficiently long per iods , m e a s u r e d as a function of the type of s tee l in t e r m s of 4-12 h or more .

LITERATURE CITED

1. V.G. Flyagin et al., Ogneupory, No.3, 27 (1971). 2. R.S. Bernshte in et al., Ogneupory, No.12, 13 (1970). 3. J .R. Rait, T rans . Bri t . Ceram. Soc., 42, No.4, 57 (1943). 4. E.I. Isaev, in: Trudy Dnepropetrov. Metal. In-ta, No.45, 71 (1961). 5. M.S. Kamenichnyi, Ogneupory, No.2, 73 (1948). 6. T .Wahlberg , Je rnkon to re t s Annaler, 137., No. l , 1 (1953). 7. V.A. Bron et al., Trudy VostIO, No. 2 83 (1960). 8. Trudy VIO, No.35, 52 (1963). 9. N.V. Pi tak and V.M. Strele ts , Ogneupory, No. 1, 15 (1962).

10. Sh.M. Mikiashvil i et al., Izv. Akad Nauk SSSR, Otd. Tekh. Nauk, 1, 115 (1957).

564