Rej?actories and Industrial Ceramics
Vol. 38, Nos. 1 1 - 1 2 , 1997
R E F R A C T O R I E S AT THE USER UDC 666.762.2:621.746.328.3
HIGH-DURABILITY SUBMERGED FUSED SILICA NOZZLES FOR CONTINUOUS STEEL CASTING MACHINES: DESIGN, FABRICATION AND SERVICE E. V. R o z h k o v , 1 Y u . E . P i v i n s k i i , 1 V. I. K h a b a r o v a , 1 E. A. V i s l o g u z o v a , 2
and V. B. C h e r k a s o v 2
Translated from Ogneupory i Tekhnicheskaya Keramika, No. 12, pp. 22 - 25. December, 1997. Original article submitted July 21, 1997. A procedure is developed for the manufacture of submerged fused-silica nozzles with a ceramoconcrete structure by centrifugal molding. Made in open-bottom and blind-bottom designs, the nozzles can withstand the casting of 6 - 8 heats of steel with 0.8 - 1.0% manganese. The proposed nozzles offer a level of durability comparable with that of imported Vesuvius Co. nozzles at a quarter of their cost.
the mammoth effort put into research in the past 20 to 30 years, improvement in the quality of submerged nozzles remains a task yet to be performed. Moreover, as pointed out in the last analytical review [2, p. 140], this task "should be tackled through improvements not only in nozzle design, but also in the material used, and an ideal refractory remains a goal for the future." Unfortunately, the corundum-graphite submerged nozzles fabricated in Russia are inferior to those manufactured abroad. In this connection, it appears advisable to look for ways and means of extending the service life of fused-silica nozzles since they are free from the limitations inherent in their corundum-graphite counterparts. It is hoped that improvements in manufacturing practices and in the structure and properties of nozzles fabricated from high-fused silica slurties [ 4 - 6] by centrifugal molding [6 - 8] would significantly extend their service life, including the casting of steels relatively high in manganese ( 0 . 8 - 1.0%). At the Pervoural'sk Dinas Plant, R&D work on fused-silica refractories has been under way since 1987. Concurrently with improvements in manufacturing practices, their output has been increased. Whereas it was 270 metric tons in 1989 and 850 metric tons in 1992, the figure rose to about 3000 metric tons by 1995, which is enough to support about 80% of the total steel handled by continuous casters. Improvements in the quality and durability of fused-silica nozzles rely on the extension of ceramoconcrete technology
Although there has been a decline in steel output in Russia in recent years, the proportion of steel cast on continuouscasting machines (CCM) has been increasing steadily. Between 1994 and 1996, for example, it rose from 31.7 to 39.5%. Since continuously cast steel accounts for 70% of the amount produced worldwide, it is obviously necessary to make wider use of the method in Russia as well. Considering that for some time past it has been usual to cast anywhere from 8 to 10 heats in succession, the main requirement for the refractories used in continuous casters is to withstand this number of heats without replacement. In the circumstances, it is especially important to choose the right approaches to the manufacture and use of steel-casting refractories, especially nozzles. In view of the increasingly stringent requirements that submerged nozzles have to meet, the general tendency outside Russia has been to use corundum-graphite products of various compositions and designs [ 1 - 3 ] . This has been accompanied by a decrease in the use of fused-silica nozzles since they are less durable in the casting of manganese steels (with more than 0.5% Mn) [3, p. 44]. However, the corund u m - graphite nozzles manufactured and used in and outside Russia suffer from a number of important service limitations, such as low thermal endurance and low slag resistance. Also, their bore and orifices tend to close in service [1 - 3]. Despite 1 Pervoural'sk Dinas Plant JSC, Pervoural'sk, Russia. 2 Nizhny Tagil Iron and Steel Integrated Works JSC, Nizhny Tagil, Russia.
467 1083-4877/97/3812-0467518.009 1998PlenumPublishingCorporation
468
[6] to centrifugal molding. In the case of open-bottom nozzles, with the grain-size analysis and molding conditions properly optimized, it has been possible to apply a production process which imparts to the material a structure uniform over the cross-section, with coarse grains ( 0 . 0 6 3 - 1 mm across) accounting for 55 - 60% o f the total. This mediumgrained cerarnoconcrete has a porosity of 12 - 13% and a fine capillary structure which adds to durability in service. In the case o f blind-bottom nozzles, the bottom part is formed from a ceramoconcrete mix o f a certain composition. As a result, the bottom part has a shrinkage-free structure retained even in a long casting service. This is attributed to the fact that in the structure o f the fused-silica ceramoconcrete thus produced [6] the grains ( 1 - 10 m m across) o f fused-silica aggregate account for 60 - 70%. The cylindrical part of a nozzle suffers an amount of shrinkage in service, which acts to compact the bottom part and to eliminate the clearance forming between the bottom and wall o f the nozzle. Optimum results have been obtained with a ceramoconcrete mix whose composition (on a dry basis) is 3 5 - 4 0 % fused-silica slurry with a density o f 1 . 8 9 - 1.93 g / c m 3 and a multiple grain-size fused-silica aggregate with particles measuring 1 - 5 , 1 - 7 , and 1 - 10 m m across. The maximum aggregate grain size is decided by the desired diameter o f the nozzle bottom and the workability o f the mix. With a bottom diameter o f 50 - 70 ram, an aggregate composed of particles 1 - 5 m m across is good; with a bottom diameter of 7 0 90 m m and greater the aggregate o f choice is one consisting of particles I - 7 and 1 - 10 m m across, respectively. As to the manner in which the bottom part is fabricated, the nozzles are classed into those with a conical splitter and those with a straight bottom. After centrifugal molding and initial drying (for 4 - 6 h under natural conditions), nozzles with a conical splitter are placed with the flared end up, and porous rubber inserts are pushed in through the sides until they meet at the center line o f the nozzle. The nozzles are then filled to the brim with a ready-made ceramoconcrete mix with a moisture content o f 5 or 6% and left overnight to dry partly under natural conditions. The ceramoconcrete bottom acquires its structure because the mix loses some of its water as the partly dried walls suck it up and the open part o f the bottom is dried further. The mix hardens as its water content drops to as little as 1 - 1.5%. A straight-bottomed nozzle is put on a metal rod and a rubber gasket corresponding to the diameter o f the nozzle or its bottom is placed at the end o f the rod. An appropriate volume of the nozzle is then filled with a ceramoconcrete mix as in the former case. In the course o f centrifugal molding, suitable tools produce on the internal walls near the bottom a set of annular grooves which make up between them a labyrinth seal in the molded ceramoconcrete bottom. Green nozzles are finally dried at 1 0 0 - 1 2 0 ~ for 2 4 h. The dried nozzles are fired and held at the final temperature ( 1 0 5 0 - 1150~ for 2 h. The temperature is raised at a rate of 8 0 - 100~ per hour. The maximum firing tempera-
E.V. R o z h k o v et al.
ture is chosen so that the linear shrinkage of the nozzle wall will not be greater than 0 . 5 - 0.8%. Otherwise, cracks might develop in the wall due to an uncompensated shrinkage because the ceramoconcrete would not shrink in these circumstances. For comparison, fused-silica submerged nozzles were tested at the Nizlmy Tagil Iron and Steel Integrated Works in parallel with conventional corundum-graphite nozzles manufactured by Vesuvius Co. The latter have an outer fibrous shell to enhance thermal endurance and to reduce thermal conductivity, and an outer zirconia belt for better slag resistance. Before installation in the mold, the nozzles must be heated to 1000 - 1100~ (for at least 1 h). Open-bottom fused-silica nozzles were tested on a fourstrand continuous caster. The steels cast were grade K768 ( 0 . 8 - 0.9% Mn) and grade 20TR ( 0 . 3 5 - 0.65% Mn). Seven or eight heats were cast in succession with one tundish. The weight o f a heat was 155 metric tons. The casting speed was 0 . 4 5 - 0 . 5 5 m/rain for grade K768 and 0 . 5 0 - 0 . 5 5 m/min for grade 20TR. The temperature of the steel in the tundish was 1 4 9 0 - 1510~ for grade K768 and 1 5 4 0 - 1555~ for grade 20TR. With these grades of steel, the fused-silica nozzles withstood 7 - 8 heats, which corresponds to 2 7 0 310 metric tons of steel per nozzle. The service life o f the nozzles was thus 8 - 10 h. The typical wear of the fused-silica nozzles was the scouring of the internal surface accompanied by an insignificant wear at the slag belt. The Vesuvius Co. nozzles showed wear at the slag belt and bore closure. The blind (ceramoconcrete)-bottomed fused-silica nozzles were tested on a two-strand continuous slab caster o f the curved-strand type (with a 1515 • 240 mm mold) in casting killed steels 2sp and 3sp. The chemical analysis o f the slagforming mix for the mold was 32. I% CaO, 29.3% SiO 2, 6.5% A1203, 4.0 MgO, 6.0% R20, 8.0% F z, and 9.6% C. The slagforming mix was expended at a rate of 0.6 kg per metric ton of steel The fused-silica nozzles were immersed into the mold to a depth of 352 mm and the Vesuvius nozzles, to a depth o f 315 ram. More detailed results of the tests are given in Table 1. As can be seen, the fused-silica submerged nozzles withstood trouble-free from 3 to 6.5 successive heats (530 metric tons) o f steel with up to 0.65% Mn. The reduced durability shown by some nozzles (as few as 3 - 5 heats) was not related to their wear (the casting operation was discontinued for "metal-making" reasons). Interestingly, the nozzles removed from service had a fairly thick wall, which indicates that they still had a reserve of service life. The main form o f wear was the scouting o f the bore and orifices. The wear of the slag belt accounted for 2 0 - 3 0 % o f the total, which ranged from 0.030 to 0.035 mm per metric ton o f steel, depending on the grade o f steel involved. In the case o f the cor u n d u m - graphite Vesuvius Co. nozzles, which had a durability comparable with that o f their fused-silica counterparts, the main causes of failure were the closure o f the bore and orifices by nonmetallic inclusions and the wear o f the slag belt, which averaged 0.030 mm per metric ton o f steel.
High-Durability Submerged Fused Silica Nozzles for Continuous Steel Casting Machines
469
T A B L E 1. Service Conditions and Durability o f Submerged Nozzles Nozzle No. "1
Casting speed,
Casting duration,
m/rain
h - min
Steel tempera- Active metal content, % ture in tundish, ~
Mn
Quantity o f heats
A1
Steel cast per nozzle, metric tons
Total wear, m m per metric ton remaining.2 o f steel
Wall thickness, m m
original
Cause o f removal from service
Killed steel 2sp 11.18.2
0.55
2-44
15391573
0.4970.532
0.0200.055
3.2
237
33
30-21 25.5
0.032
Minor window scouring
V
0.55
3-04
15391573
0.4970.532
0.0200.055
3.5
310
27
1 9 - 17 18
0.029
Bore closure
11.19.2
0.55
5-30
15251597
0.4310.539
0.0260.075
5.4
415
30
4 - 15 9.5
0.049
Ladle change
V
0.55
5 - 10
15251597
0.431 0.539
0.0260.075
5.1
400
27
26 - 10 18
0.022
11.20.2
0.7
4 - 50
1534 1570
0.5450.597
0.021 0.067
4.7
365
32
10 - 0 5
0.073
Orifice scouring
11.19.2
0.63
2 - 10
15341582
0.5970.653
0.021 0.047
2.3
178.3
31
27 - 27 27
0.022
Ladle change
39.19.3
0.6
5 -20
15441556
0.4220.502
0.023 0.058
6.0
465
30
13 - 20 16.5
0.029
End of series
V
0.6
2 - 40
15441556
0.422 0.502
3.0
232
27
20 - 24 22
0.022
Bore closure
11.19.2
0.6
3 - I0
15491575
0.4280.536
0.0190.074
3.6
270
32
25 - 28 26.5
0.020
Ladle change
V
0.6
3 - 15
15491575
0.4280.536
0.0190.074
3.7
265
27
22 - 10 16
0.042
11.18.2
0.55
5 - 30
1537 1565
0.4440.560
0.02 0.059
6.5
530
33
6 - 20 13
0.039
Bottom dropout
11.18.2
0.55
3 - 10
1553 1565
0.4440.560
0.0290.059
3.5
267
33
31 - 10 20.5
0.047
Burn-through under prongs
11.19.2
0.55
2-40
15371557
0.5050.560
0.0200.059
3.0
232
30
20-28 24
0.024
Window scouring
11.20.2
0.53
4-00
1551 1580
0.3950.454
0.0100.050
4.5
348
32
20 - 24 22
0.029
Ladle change
11.20.2
0.53
4 - 10
1551 1580
0.395 0.454
0.0100.050
363
31
19 - 20 19.5
0.032
80.31.3
0.68
3-20
15351556
0.4730.573
0.0170.047
4.2
317.5
30
25-25 25
0.016
Bore closure
V
0.68
2-40
1535 1556
0.4730.573
0.0170.047
3.5
272
27
22 - 24 23
0.015
Inflow o f metal between nozzle and nonwhirl nozzle
11.19.2
0.65
2 - 30
15341581
0.5000.547
0.039 0.051
3.0
232.5
32
26 - 26 26
0.026
Ladle change
V
0.65
4-40
15341581
0.4870.572
0.0290.051
5.8
458
27
2- 5 3.5
0.051
Nozzle bottom failure
....
Killed steel 2sp
....
....
*l Vesuvius Co. nozzle. *2 Numerator) range; denominator) average.
In one case (nozzle 80.31.3), the bore of the fused-silica nozzle was clogged after 317.5 metric tons of steel had been cast. The specific wear was thus about half as great. 3 Another series o f fused-silica nozzles, used to cast killed steel 3sp (at a casting speed o f 0.45 - 0.60 m/min and at a steel temperature o f 1 5 4 7 - 1 5 9 0 ~ in the tundish), lasted 8 heats, which works out to 600 metric tons of steel per nozzle. Some steelmakers hold the view that fused-silica nozzles might pollute the steel with silica. However, even simple cal3
This result needs verification.
culations based on the durability of nozzles stated above and their real wear (2 - 4 kg per 500 metric tons o f steel) demonstrate that this pollution is a factor of tens o f thousands smaller than the actual Si content of the metal. For example, the silicon content of the steels made at the Orenburg Electrical Steel Works ranges between 0.20 and 0.35%. Thus, the fused-silica nozzles in question offer a level of durability comparable with that of corundum-graphite Vesuvius Co. nozzles at a quarter of their cost. Coupled with multiple tundish blocks from bauxite ceramoconcrete [9], these high-durability nozzles are another milestone in the
470
Dinur JSC effort to develop a complete range of high-durabil-
E . V . R o z h k o v et al.
REFERENCES
ity refractories for continuous steel casting. CONCLUSION A procedure has been developed for the manufacture o f high-durability fused-silica submerged nozzles with a ceramoconcrete structure. In casting steels with 0 . 8 - 1.0% Mn on a four-strand continuous steel casting machine, open-bottom nozzles withstood 7 - 8 successive heats (270 - 3 l0 mettic tons o f steel). When used on a two-strand continuous caster, blind-bottom nozzles withstood 6 - 8 heats ( 5 0 0 600 metric tons o f steel). In casting an aluminum-deoxidized steel, the fused-silica nozzles showed a significantly higher durability than did the c o r u n d u m - ~ a p h i t e nozzles. The fused-silica nozzles offer a level of durability comparable with that of imported c o r u n d u m graphite Vesuvius Co. nozzles at a quarter of their cost.
t. I. G. Ochagova, Nov. Chem. Metal. Rubezh., No. 4, 1 5 0 - 159 (1995). 2. S. Kataoka, Taikabutsu, 48(5), 212-217 (1996). 3. Proceedings of a Conference on Refractories for Continuous Steel Casting Machines [Russian translation], Metallurgiya, Moscow (1986). 4. Yu. E. Pivinskii, F. S, Kaplan, S. G. Semikova, et al., Ogneupory, No. 1, 3 9 - 4 3 (1985). 5. Yu. E. Pivinskii, Y. L Litovskaya, O. N. Samarina, et al., Ogneupory, No. 9, 40 - 4 4 (1989). 6. Yu. E. Pivinskii, Ceramic Cements and Ceramoconcretes [in Russian], Metalturgiya. Moscow (1990). 7. Yu. E. Pivinskii, T. I. Litovskaya, 1. B. Volchek, et al., Ogneupory, No. 11, 2 - 6 (19'-)1 I. 8. Yu. E. Pivinskii. T. 1 kito~skaya, F. S. Kaplan, et al., Ogneupo~y, No. 3 , 6 - 9 (1992). 9. Yu. E. Pivinskii, D. A. Dobrodon, E. M. Grishpun, et al., Ogneup. Tekh. Keram., No. 9.33 - 36 (1997).
