ISSN 0967-0912, Steel in Translation, 2016, Vol. 46, No. 5, pp. 343–348. © Allerton Press, Inc., 2016. Original Russian Text © A.A. Korostelev, N.S. S’emshchikov, A.A. Chernyshev, A.E. Semin, K.N. Bel’maz, A.N. Bozheskov, V.V. Kazakov, A.V. Kosonogov, 2016, published in Stal’, 2016, No. 5, pp. 16–22.
Advantages of Stoppers with MgO-Based Heads in Continuous Casting Machines A. A. Korosteleva, *, N. S. S’emshchikova, A. A. Chernysheva, A. E. Seminb, K. N. Bel’mazc, A. N. Bozheskovd, V. V. Kazakovd, and A. V. Kosonogove a
OOO VPO Stal’, Odintsovo, Russia Institute of Steel and Alloys, Moscow, Russia c Corwintec, Dalian, China d OAO Volzhskii Trubnyi Zavod (VTZ), Volzhskii, Russia eAO OMK-Stal’, Vyksa, Russia *e-mail:
[email protected] bMoscow
Received May 5, 2016
Abstract—Stoppers are under test in continuous casting machines at various Russian enterprises so as to determine the influence of the material from which the stopper head is made on the stability of casting and the fluctuations of the metal level in the crystallizer. Keywords: stopper, cast steel, continuous casting, metal level in crystallizer, nonmetallic inclusions, clogging DOI: 10.3103/S0967091216050077
In the continuous casting of steel, it is important to ensure stable dosing of metal from the tundish to the crystallizer and prolonged maintenance of a constant metal level in the crystallizer. Dosing by means of a stopper and a tundish nozzle is most common in continuous casting; the resulting casting process is very stable. The stopper is an important component of the tundish, ensuring dosed supply of steel to the crystallizer and shutoff of the tundish nozzle when necessary [1]. Precise calculation of the stopper head is very important for stable dosing over a long period, especially with automated maintenance of the metal level in the crystallizer. The main types of load on the stopper are as follows: (1) Thermal loads: thermal shock at the beginning of casting, when the tundish is filled with liquid steel; and temperature gradients between the parts of the stopper that are and are not immersed in the steel. (2) Flexural loads associated with the repulsive force when the stopper is immersed in the liquid steel; with the influence of the metal flux in the tundish; and with closing of the tundish nozzle by the stopper (when the stoppers are displaced from the steady vertical axis). (3) Vibrational impact associated with stopper vibration in casting. This may be due to the conditions of metal flow through the tundish nozzle.
(4) Change in gas pressure within the stopper cavity on account of nonuniform (or interrupted) supply of argon injected into the steel. (5) Erosive wear of the stopper in the slag line, which reduces the cross-sectional area and the total strength of the cylindrical section. In practice, the stopper is usually attached to the rod by means of a metal or ceramic nut pressed into the stopper body. In stopper installation, the steel rod is screwed into that nut and then held in place by an external nut. That permits rapid and reliable connection of the stopper and the steel rod (Fig. 1). An important feature of stoppers is the ability to inject argon into the metal being cast, so as to stabilize the metal flow from the tundish to the crystallizer and reduce clogging of the steel-casting channel by aluminum oxides. By adjusting the amount of argon injected, the penetration depth of the steel jet into the liquid bath within the mold may be regulated. Argon is supplied through a hole in the stopper head (diameter 5–7 mm), through a gas-permeable plug close to the surface of the part, or by both means. The use of both methods together is most common and most safe. The gas-permeable plug prevents the penetration of liquid steel into the interior of the stopper in the event of interruptions in the argon supply. In addition, prolonged maintenance of the specified gas flow rate is possible, and the injection of small inert-gas bubbles is ensured.
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Stopper mechanism
Stopper
Metal flux
Tundish nozzle Fig. 1. Dosing of steel melt from the tundish into the crystallizer by the stopper and the tundish nozzle.
In casting, with maintenance of the metal level in the crystallizer by the stopper mechanism, strict requirements are imposed on the stopper kinematics and on the automatic system: in particular, high casting rates are required for billet of small cross section in bar casting or single-strand casting. In steel casting, the head of the stopper is subject to mechanical wear and chemical erosion. Therefore, it is made from strong material based on magnesium or aluminum oxide and graphite, with a carbon binder. These materials are entirely compatible with the main stopper body and may be pressed together. The stopper with a head based on Al2O3 is stronger at room temperature than that based on MgO: ~9 and ~7 N/mm2, respectively. However, at operational temperatures, the difference in strength of the corundum–graphite compositions disappears, and the stopper life is determined by the chemical composition of the cast steel. The stopper life is shortest when casting steel reduced by means of materials containing calcium [2]. Therefore, for long cast steel, it is preferable to use stoppers with a head based on MgO. The production of such stoppers to withstand the applied loads even at high casting speeds calls for high-technology equipment and high-quality magnesia. In addition, the graphite must be effectively protected from oxidation during operation of the part and during its production (heat treatment). Particular benefits of magnesia–carbon components include their mechanical strength at elevated temperatures and their resistance to chemical erosion, which are associated with the highly refractory character of periclase and the presence of an effective carbon
matrix reinforced by antioxidant additives. The main deficiency of magnesia–carbon components is their sensitivity to the internal mechanical stress due primarily to thermal impact. However, this may easily be eliminated if the recommendations regarding the heating of components before the onset of casting are observed. In steel production today, ladle treatment by materials that contain calcium is essential, on account of the influence of calcium on the physicochemical state of the metal, the macro- and microstructure of the billet, and the quality and properties of the steel produced. The treatment of the steel melt with materials that contain calcium, as well as various other additives, will affect the life of the refractory in contact with the molten steel. The Al2O3 in the stopper head reacts with the calcium in the steel and the nonmetallic inclusions to form various compounds, including calcium aluminates and calcium alumosilicates with different melting points. Intense chemical reactions not only increase expenditures on replacement materials but also reduce the safety of the continuous casting machine and the ability of the stopper and tundish nozzle to ensure precise adjustment of the steel flux from the tundish at any moment and also to ensure uninterrupted flow of the liquid steel. This confirms the benefits of materials based on MgO in the stopper head. Treatment of the steel with calcium imposes special requirements on the selection of refractories in the stoppers, tundish nozzles, submerged entry nozzles, and other components. Accordingly, in the present work, we assess the effectiveness of stoppers with a magnesia–carbon head at Russian steel plants. MgObased refractories are characterized by high resistance to the thermochemical action of steel treated with calcium. At PAO Taganrogskii Metallurgicheskii Zavod, Corwintec stoppers with MgO-based heads have been in successful use for a long time [3]. In the sheet-rolling plant at AO OMK-Stal’, research on the selection of refractory for the stopper head was undertaken between 2012 and 2015, so as to ensure stable, troublefree casting. At that plant, a single strand is used for casting from the tundish to the crystallizer. The mass of a one heat is ~160 t. The casting sequence may cover 15 heats (10–15 h, depending on the billet cross section). In this case, ~2400 t of steel passes through a single casting element in the stopper–nozzle system. The steel throughput in this case is 4 t/min. In those circumstances, the stopper experiences extreme physicochemical stress associated with the metal fluxes in the tundish, vibration of the stopper itself, and the influence of inclusions in the cast steel. With such a flux through the casting channel, clogging of the steel line at AO OMK-Stal’ is unlikely. STEEL IN TRANSLATION
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(b) (a)
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Fig. 2. Stopper heads based on Al2O3 before casting (a) and after casting five heats of 09Г2С steel (b), five heats of 13ХФА steel (c), and seven heats of 22ГЮ steel (d).
Fig. 3. Stopper heads based on MgO before casting (a) and after casting nine heats of 22ГЮ steel (b), seven heats of steel 20 (c), and six heats of 22ГЮ steel (d).
At OOO VPO Stal’, stoppers with different head materials are tested: alumina–carbon and periclase– carbon. It is evident from Fig. 2 that the head profile shows considerable wear after 5–7 heats when the material based on Al2O3 is used. The wear rate in the contact section is 1.0–1.6 mm/heat, which doesn’t ensure the sequence required by the customer and may lead to interruption of the sequence. As we see in Fig. 3, the profile of Corwintec stopper heads based on MgO shows no signs of wear after 6–9 melts. In Fig. 4, we show stopper heads made from materials based on Al2O3 (A) and MgO (B) in casting 22ГЮ steel. In the course of casting, the position of the stopper with an MgO-based head is stable, since the cross section through which the steel passes remains constant and hence there are no marked fluctuations of the metal level in the crystallizer (Fig. 5); accordingly, emergency shutdown of the casting strand is not a serious concern here. The use of stoppers with MgO-based heads requires careful preheating. Corwintec stopper heads based on MgO outperform stoppers with Al2O3-based heads at AO OMK-Stal’ and may be recommended for industrial use. In October and November 2015, one-piece stoppers with MgO-based heads were used on the continuous bar-casting machine at OAO Volzhskii Trubnyi Zavod (VTZ) so as to improve the casting properties of the steel (Fig. 6). Previously, research was undertaken at OAO VTZ with a view to reducing the clogging of
the steel-casting channel between the tundish and the crystallizer. Such clogging was attributed to the adhesion of Al2O3-based nonmetallic inclusions at the channel wall and at the corundum head section of the stopper [4]. This research established that the clogging may be reduced by supplying argon to the steel-casting channel of the tundish nozzle and improving the casting properties of the steel. In the second stage of the tests, stoppers with MgO-based heads were used to ensure more stable casting and moderate the fluctuations of the metal level in the crystallizer. Those fluctuations were attributed to the adhesion of nonmetallic inclusions to the Al2O3-based stopper head. Adjust-
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Fig. 4. Stopper heads after casting 22ГЮ steel. Heads A and B are made from Al2O3- and MgO-based materials, respectively.
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(а)
80 60 65.3
69.9
40 20 0 1 511 733 1 511 734 1 511 735 1 511 736 1 511 737 1 511 738 1 511 739 1 511 740 (b)
80 60
A
55.1
40
59.0
20 0 1 511 757
1 511 758
1 511 759
1 511 760
1 511 761
1 511 762
1 511 763
Fig. 5. Position of stopper with MgO-based head after casting of 8 heats of 22ГЮ steel with a cross-sectional width of 1580 mm (a) and after casting of 7 heats of K-52 steel with a cross-sectional width of 1750 mm.
Values of Δ and n for stoppers with heads based on Al2O3 (left columns) and MgO (right columns) Melt No.
Fig. 6. Head of Corwintec stopper used at OAO VTZ.
Δ
n
260084 260085 260086 260087 260088 260089 260090 260091 260095 260096 260097 260098 260099 260100 260101 260102 260115 260116 260117 260118
1.4 1.8 1.7 1.8 0.6 0.4 0.6 0.7 1.2 0.9 0.9 1.2 0.6 1.1 0..9 1 0.9 0.7 0.8 1.2
3 4 4 3 2 2 2 3 2 3 2 4 3 2 2 4 3 3 4 3
Àverage value:
1.02
2.9
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Melt No. 256200 256201 256202 256203 256204 256205 256210 256211 256212 256213 256124 160137 260138 260139 260140 260141 260142 260143 260144 Average value:
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1.1 1.4 0.8 1.1 0.7 0.7 1.3 0.6 1.1 0.8 0.4 1.2 0.8 1.2 0.8 0.8 1.3 1.2 0.8 0.95
2 1 2 1 2 1 2 2 1 2 2 3 2 3 2 1 3 3 2 1.9
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96 88 80 72 64 56 48 40 32 24 16 8 0 7:53:51 8:02:11 8:10:31 8:18:51 8:27:11 8:35:31 8:43:51 8:52:11 9:00:31 9:08:51
06.01.16 96 88 80 72 64 56 48 40 32 24 16 8 0 17:48:32 17:57:52 18:06:12 18:14:32 18:22:52 18:31:12 18:39:32 18:47:52 18:56:12 19:04:36
27.12.15
(b) 48 44 40 36 32 28 24 20 16 12 8 4 0 19:30:36 19:37:49 19:45:02 19:52:16 19:59:29 20:06:42 20:13:56 20:21:09 20:28:22 20:35:36
07.01.16 48 44 40 36 32 28 24 20 16 12 8 4 0 10:25:35 10:32:48 10:40:01 10:47:15 10:54:28 11:01:41 11:08:55 11:16:08 11:23:21 11:30:35
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Fig. 7. Stopper position in the casting of 06ГФБА and 13ХФА steel by means of stoppers with heads based on MgO (a) and current corundum stoppers (b).
ment of the stopper position in casting ensures constant metal level in the crystallizer. The results in Fig. 7 indicate stable casting when using MgO-based heads. The table presents results for 19 heats of 06ГФБА and 13ХФА steel cast by means of stoppers with heads based on MgO and 20 melts of the same steels cast by means of current corundum stoppers with Al2O3based heads. In the table, Δ is the difference between the maximum and minimum stopper positions in the course of casting; and n is the number of sudden adjustments in the stopper position (jolts). STEEL IN TRANSLATION
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n 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0
0.4 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.7 1.8 Δ Fig. 8. Mean number of jolts n for stoppers with heads based on Al2O3 (j) and MgO (j) at different values of Δ (the difference between the minimum and maximum stopper positions).
It is evident from the table that, when using stoppers with MgO-based heads, the magnitude of the change in stopper position is 7% less than when using stoppers with Al2O3-based heads. Hence, the fluctuations in the metal level in the crystallizer will also be of smaller amplitude. That indicates more stable casting of the steel. The number of considerable jolts of the stopper is also 34% less, on average. The data in the table are presented as a histogram in Fig. 8. CONCLUSIONS If stoppers with MgO-based heads are adopted in place of the standard stoppers with Al2O3 heads currently in use, we observe more stable casting of steel and less pronounced fluctuations of the metal level in the mold. Operation of the continuous-casting machine is trouble-free. This benefit is particularly pronounced in the casting of steel treated with calcium. ACKNOWLEDGMENTS Financial support was provided by the Russian Ministry of Education and Science (contract 14.578.21.0023, June 5, 2014; unique identifier RFMEF157814X0023). REFERENCES 1. Smirnov, A.N., Kuberskii, S.V., Podkorytov, A.L., et al., Nepreryvnaya razlivka sortovoi zagotovki: Monografiya (Continuous Casting of Profiled Billets: Monograph), Donetsk: Tsifrovaya Tipografiya, 2012. http://steeltimes.ru/books/casting/sortccm/sortccm.php. Accessed December 17, 2015. 2. Dyudkin, D.A. and Kisilenko, V.V., The interaction of calcium-containing materials with refractory materials at the out-of-furnace treatment and casting of steel.
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3. Korostelev, A.A., S”emshchikov, N.S., Chernyshev, A.A., et al., Application of new developments of Corwintec company for steel casting at the Taganrog Metallurgical Plant, Nov. Ogneupory, 2014, no. 9, pp. 10–13.
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4. Bozheskov, A.N., Kazakov, V.V., Korostelev, A.A., S”emshchikov, N.S., Implementation of batcher glasses with argon purging to improve the steel casting, Stal’, 2015, no. 7, pp. 13–16.
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5. Aksel’rod, L.M., Razrabotka i vnedrenie kompleksa meropriyatii po snizheniyu intensivnosti formirovaniya otlozhenii v stalerazlivichnom trakte pri nepreryvnoi razlivke metalla na MNLZ (Development and Implementation of Measures for Reduction of Sedimentation Intensity in Steel-Casting Groove at the Continuous Metal Casting Machine), Moscow, 2007. http:// www.dissercat.comcontent/razrabotka-i-vnedrenie-
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kompleksa-meropriyatii-po-snizheniyu-intensivnostiformirovaniya-otlo. Povolotskii, D.Ya., Kudrin, V.A., and Vishkarev, A.F., Vnepechnaya obrabotka stali (Out-of-Furnace Treatment of Steel), Moscow: Mosk. Inst. Stali Splavov, 1995, pp. 196–199. Allenstein, J., Taschenbuch Feuerfeste Werkstoffe: Aufbau—Eigenschaften—Prüfung, Routschka, G. and Wuthnow, H., Eds., Essen: Vulkan, 2007. Guzman, I.Ya., Khimicheskaya tekhnologiya keramiki (Chemical Technology of Ceramics), Moscow: Stroimaterialy, 2003, pp. 416–422. Kashcheev, I.D., Strelkov, K.K., and Mamykin, P.S., Khimicheskaya tekhnologiya ogneuporov (Chemical Technologies of Refractories), Moscow: Intermet Inzhiniring, 2007.
Translated by Bernard Gilbert
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