HORIZONTAL CONTINUOUS CASTING MACHINE NOZZLES* L. M. Aksel'rod, N. I. Lisitsyna, V. P. Simonov, and V. S. Lisin

UDC 666.76:621.746.328.3.047

The determining role of the refractory part located in the mold of a horizontal continuous casting machine in formation of a high quality continuously cast billet has been discussed in detail in the domestic [i] and foreign [2, 3] literature. Preliminary stoppage of teeming Of steel in a horizontal continuous casting machine and low quality of the billets cast are caused by wear (failure) of the refractory nozzle. The reasons for wear may be insufficient heat resistance and mechanical strength, low resistance to molten steel, excess wettability of the refractory material by the molten steel, and high thermal conductivity. Necessary conditions determining the life of the nozzle in a horizontal continuous casting machine are the absence of a gap at the joint of the refractory nozzle with the mold and the presence in the composition of the refractory material of components not wetted by the molten metal (C, BN, SiaN~, etc.) [i, 4, 5]. As established experimentally, under the action of ferrostatic pressure of the molten metal in the metal reservoir (level of the metal 0.4-0.6 m) the metal flows into the gap between the refractory and the water-cooled surface of the mold with a gap thickness of 0.08-0.05 mm, forming a single whole with the solidifying shell of the ingot (Fig. la). In this case either the shell separates from the refractory with a gradual increase in the gap and accordingly poorer quality of the case billet and failure of the refractory (Fig. ib) or the shell does not separate from the refractory in each step of withdrawal and rupture of it occurs at the junction of the fronts or at the junction of the steps (Fig. Ic). The theory of nonwetting by molten metal of a refractory containing carbon and metal nitrides and carbides was presented in a preceding article [5]. During experimental use of refractory nozzles of different compositions it was established that the zone of greatest wear in the absence of flow of metal into the joint of the refractory with the mold is the face portion of the nozzle at a distance of 4-6 mm from the mold (Fig. 2). This zone corresponds to the region of the ingot shell and to the region of the two-phase composition (liquid-solid) of the phase diagram, which was also confirmed by calculation [i]. The high surface activity of oxygen and sulfur at the refractory-metal interface of the phases (in the two-phase zone) leads to formation of intermediate products of interaction with the elements of the refractories being oxidized all the way to formation of solid or gaseous oxides (sulfides). This phenomenon is also confirmed by'analysis of the microstructure of disks from the zone of the junctions of the steps, where gas bubbles and sulfide inclusions are recorded. This process also leads to local wear of the refractory (Fig. 3). Several designs of the refractory nozzle-mold joint and also different refractory materials containing carbon and oxygen-free compounds have been tested on a horizontal continuous casting machine. Three plans of the nozzle-mold joint were tested: a) attached refractory nozzle; b) cylindrical fit of the nozzle in the mold; c) placement of the nozzle in a tapered fit (Fig.

4). For teeming of commercial types of steel and further working of the ingots obtained on a small shape mill the plan with a cylindrical fit of the nozzle is being used successfully (Fig. 3b). The nozzle is made of chamotte-graphite refractory (21-23 wt.% carbon) and placed in the mold in mortar. First by machining the mounting fit is given a configuration corresponding to the mounting fit of the mold. *V. S. Babiev, V. u Bagrova, O. A. u A. u and V. N. Terekhov participated in the work.

Olenchenko, u

P. Nevzorov, u

M. Shukin,

All-Union Refractory Institute. Tula Ferrous Metallurgy Scientific and Production Union. Karaganda Metallurgical Combine. Translated from Ogneupory, No. 4, pp. 45-48, April, 1990.

0034-3102/90/0304-0243512.50

9 1990 Plenum Publishing Corporation

243

7

/

7

4

J

24J

e

2

4

%5

J

2

c

Fig. i. Plan of formation of defects in flow of metal into the joint of the refractory nozzle with the mode: i) refractory nozzle; 2) shell of the ingot; 3) mold; 4) gap between the ingot and the mold; Me) molten metal; I) direction of withdrawal of the ingot.

Fig. 2. Refractory nozzle after teeming of steel. Arrow shows the zone of wear.

I

2

J

~

z

5

Fig. 3. Plan of wear of the horizontal continuous casting machine refractory nozzle: i) mold; 2) refractory nozzle; 3) ingot shell; 4) two-phase zone; 5) zone of wear; Me) molten metal; S) solidus isotherm; L) liquidus isotherm. The assembly prepared using this method makes it possible to teem individual heats with a weight of metal of 15-20 tons. However, progressive wear of the face of the nozzle occurs during teeming. The quality of a round of square steel billet teemed on a horizontal continuous casting machine with use of a chamotte-graphite nozzle meets the requirements imposed on a commercial grade billet [6]. The use of refractory nozzles of nitride ceramic placed in a cylindrical fit showed low wear of them but poor stability of the teeming process as the result of flow of metal into the joint of the refractory with the mold. For the purpose of fulfilling the requirements of an increase in quality of the billets teemed two variations of the joint of the refractory nozzle with the mold made dry without a gap were tested.

244

TABLE i. Nozzle

material

Characteristics and Results of Use of Refractory Nozzles [Machin- ~-~-~Bend Method of | ability ]of the ~ s t r e n g t h , production ~mounting N/mm2 Ifit

Chamottegraphite

Forming, reductlon firing Boron nitride-Hot presssilicon di- mng

Good

~

Wear

heatdjtime, h l~e~med, ~ '

steel

22--24

4--6

I

1,5

15--20

10--25

0,7--1,5

1--3

85

1--3

1,5---4,0

<~48

<~.5

0,05--0,2

16--22

47--63

1--3

1,5--4,0

<~48

~.5

0,05--0,2

Superior

oxide

Boron nitmide-Fo.r~ing,;resilicon niactlonslntride tering

72

IZ

a

/

b

2

c

Fig. 4. Plans (a-c) of the refractory nozzle-mold joint. In testing the attached refractory nozzle (Fig. 4a) it was not possible to avoid local flows of metal into the joint of the previously ground surfaces of the refractory and the mold. Obviously the face of the mold experienced warping as the result of thermal stresses duringteeming. The use of a nitride-containing nozzle placed in the mold using a tapered fit (Fig. 4c) provided positive results in combination with continuous clamping of the nozzle to the mold during teeming. Warping of the mounting fit of the mold is recorded even after the first teaming. Therefore replacement of the nozzle after teeming without remachining of the mounting fit of the mold is undesirable. Basically two forms, hot-pressed of BN-SiO 2 system material and reaction sintered of BN-SiBN ~ system material, were tested as refractory nozzles installed dry without a gap. Metallographic investigations of the quality of continuously cast billets cast with use of the different forms of refractory nozzle-mold assemblies made it possible to draw the following conclusions: the use of a chamotte-graphite nozzle is always accompanied by quite intense erosion of the refractory and the corresponding exogenic inclusions, products of erosion of the refractory, were recorded at the junctions of the spacings of the ingot; a comparison of the results of use of nitride-containing nozzles indicates that in all cases with the use of a BN-Si3N4-base refractory a smaller number of exogenic inclusions were observed at the junctions of the spacings and less defectiveness of the joints of the spacings and the fronts of the ingots than with the use of BN-SiO2-base refractory; the quality of the junctions is better with installation of nitride-containing refractory using a tapered fit. Table 1 shows the characteristics and results of use of the tested refractory nozzles. The chamotte-graphite nozzles were used in the form of a one-piece nozzle, which after appropriate machining of the mounting fit is placed in mortar in the mounting fit of the mold. The nitride-containing nozzles were used in the form of 20-25 mm thick rings. Machining provided the mounting fit of the nozzle with the proper taper and it was clamped with a special device to the mounting fit of the mold and remained clamped during teeming. The metal was supplied from the metal receiver to the mold through a chamotte-graphite connecting nozzle. 245

During teeming of about 20 tons of constructional steel in the horizontal continuous casting machine with use of a chamotte-graphite nozzle its face portion located in the mold wore by 10-25 mm along the wall of the mold and further teeming became impossible. The service life of the nitride-containing refractory nozzles was in practice determined by the batch production of the heats. This is related to warping of the mounting fit of the mold after teeming and the appearance of a gap indefinite in configuration and location at the joint of the refractory with the mold. The wear of the nitride-containing nozzle after teeming of the first heat does not exceed 0.07 mm per ton of steel while in teeming of subsequent heats the local wear progresses and in time may reach 0.2 mm per ton of steel~ Failure of the nozzle occurs in removal of it from the mold after completion of teeming of a heat or series of heats. In the case of long continuous teeming of steel (heat after heat series) the specific wear of the nitride-containing nozzles will be less than that shown in Table i. Therefore as the result of production tests of the assembly for delivery of metal to the mold of a horizontal continuous casting machine the optimum configuration of the refractory nozzle-mold joint was determined and recommendations given on selection of the nozzle refractory materials. LITERATURE CITED I.

L.M.

Aksel'rod, V. L. Novikov, E. Ya. Litovskii, and V. P. Simonov, Ogneupory, No.

6, 36-40 (1988). 2. 3. 4. 5. 6.

246

Seiichi Yamaji, Tetsu-to-Hagane. J. Iron Steel Inst., 71, No. 4, 275 (1985). Noraki Teruhibo, Kawahara Minoru, Mori Takasube, and Yao Yastaka, Iron and Steel Eng., No. i0, 50-56 (1985). A . J . Zainer, Iron and Steel Eng., No. 12, 37-44 (1985). L . M . Aksel'rod, I. Ya. Dol'nikov, V. L. Novikov, and O. A. Val'dman, Ogneupory, No. i0, 55-58 (1984). A . I . Bogdanov, S. M. Kozachenko, N. V. Efremov, et al., Stal', No. 6, 22-23 (1988).