ISSN 0967-0912, Steel in Translation, 2008, Vol. 38, No. 7, pp. 556–559. © Allerton Press, Inc., 2008. Original Russian Text © R.S. Takhautdinov, Yu.A. Bodyaev, S.V. Gorostkin, D.V. Yurechko, V.M. Korneev, 2008, published in “Stal’,” 2008, No. 7, pp. 24–27.
Development of Continuous Casting at OAO MMK R. S. Takhautdinov, Yu. A. Bodyaev, S. V. Gorostkin, D. V. Yurechko, and V. M. Korneev OAO Magnitogorskii Metallurgicheskii Kombinat (MMK) Abstract—The introduction of continuous casting at OAO MMK was a historic event, marking the end of uneconomical chill casting. Thanks to the introduction of continuous casting in the oxygen-converter shop, improvement in casting-machine design, and expansion of the range of steel that can be cast, unique casting characteristics have been attained: high productivity (more than 10 million t/yr), long casting runs (up to 208 melts), infrequent accidents, low rejection rates of the steel, and high quality of the continuous-cast slabs. The introduction of continuous casting in the converter shop provided a stimulus to the reconstruction of the openhearth shop as an electrosmelting shop and the installation there of two bar-casting machines and one slab-casting machine. DOI: 10.3103/S0967091208070140
Converter 1 and continuous-casting machine 1 went into operation in November 1990, followed by continuous-casting machines 2–4 in December 1990, September 1991, and April 1994, respectively. Operation revealed certain design deficiencies of their components, which posed some difficulties. Thus, on account of the weak gear systems for moving the narrow walls and the insufficient compressive force of the broad walls of the adjustable mold, the narrow walls became uncontrollable. The bandaged rollers had to be replaced by one-piece rollers, since the bandages unrolled and the ingots were left in the machine. In addition, during the initial period, failure of the intermediate bearings provoked numerous shutdowns of the casting machine, on account of jamming of the 556
12
250
10
200
8
150
6 4
b
100
a 50 2 0 0 1990 1992 1995 1998 2001 2004 2007 Year
Fig. 1. Output of continuous-cast steel in the oxygen-converter shop between 1990 and 2007 and the mean (a) and maximum (b) length of the casting run.
Number of melts in run
slabs and pronounced blemishes on their surfaces. This was eliminated by systematic acidic flushing of the intermediate bearings before installation and redesign of the cooling and lubrication system for the intermediate bearings. During the startup phase, those who made important contributions included V. D. Kiselev, A. V. Boyarintsev, O. V. Ishmaev, and V. N. Kotii. From the beginning, attention focused on increasing the productivity of the casting machines and reducing downtime and accidents. From year to year, steel output and the capacity of the casting machines increased (Fig. 1). In 1994, all four machines were in operation, and casting in four strands was perfected. Between 1995 and 1998, the casting of a wide range of steels was introduced, including low-carbon steel (08û), carbon and high-carbon steel (50ïÉîÄ, 65É, and 70), and electrical-engineering steel (dynamo steel, transformer steel, relay steel). In addition, the introduction of continuous casting was accompanied by extension of the
Steel output, 106 t
Until the 1990s, at Magnitogorsk Metallurgical Works (now OAO MMK), steel was smelted in openhearth shops and chill-cast. In the mid-1980s, it was decided to construct an oxygen-converter shop. The first step was to introduce two converters and four continuous-casting machines for slab production at the rate of 5 million t/yr. Two- and four-strand curvilinear continuous-casting machines were designed by specialists of Uralmash production facility, with a basic radius of 8 m. To ensure high-quality slabs, a rigid roller system, and independent casting rates when producing narrow slabs, all the rollers are characterized by middle bearings (discontinuous bearings). The continuous-casting machines are equipped with four gas-cutter units. The intermediate ladles, which hold more than 40 t when the metal level is 1100–1200 mm, are equipped with rotating three-plate gates. The roller sections of the secondary-cooling zone have separate drives for each of the four strands, while each drive has three gear systems. All the components of the continuous-casting machines (the mold and the transport modules) were adjusted at the TsRMO center before installation.
DEVELOPMENT OF CONTINUOUS CASTING
casting runs to 150 melts, without impairment of the billet macrostructure and surface, taking account of the requirements on the state of the casting machine. In 2007, the mean length of a run was 40.8 melts, with a maximum of 183 melts. Another goal was to increase the maximum slab width to 2520 mm. Steel casting through molds of asymmetric cross section was also introduced. By 2001, thanks to the introduction of automated systems and other measures, the total output of continuous-cast steel was more than 7.9 million t, and the productivity of the continuous-casting machines was 269.3 t/h. Given that more than 50% of the converter shop’s output is cast slab of width 1250–1350 mm, continuous-casting machine 3 was modernized at the end of 2001 so as to increase total output, and continuouscasting machine 3 was reconstructed in February 2003. As a result, the total steel production in the continuouscasting department reached 10 million t/yr. Working together, OAO MMK and Uralmash specialists developed the concepts for a high-performance continuous-casting machine equipped with state-ofthe-art automatic systems. Various organizational measures permitted the dismantling of the old machine and the installation of more than 3000 t of new equipment within 19 days, a record time for Russian metallurgy. Notable mechanical aspects of the continuous-casting machine include the following: simultaneous casting in four strands through two double molds with a common oscillation mechanism for the pairs of strands; a roller-conveyer zone and intake transport line with complete separation of the strands in terms of casting speed and cutting of the billet into measured lengths; new molds with optimal copper-plate dimensions and independent cooling of each narrow wall and the pairs of broad walls; automatic delivery of water for cooling of the mold and sprayer cooling of the ingot at the mold and in sections 1 and 2; simultaneous transportation of two ingots (width 1350 mm) over the existing TOL roller conveyers, without reconstruction, by introducing a section for positional introduction of the slabs in the castingmachine line; standardization with the existing continuous-casting machines in terms of the systems for adjustment of the roller-conveyer equipment, the types of rollers, and the repair technology for the copper walls of the molds. For the new continuous-casting machine, the basic equipment in the casting section from the previous machine is employed: in particular, the casting stand, the intermediate-ladle trolleys, the intermediate ladles, and the heating systems. The basic components and mechanisms of the gas-cutting machines remain the STEEL IN TRANSLATION
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Characteristics of curvilinear continuous casting machines with radial mold Characteristic
Before reconstruction
After reconstruction
Number of strands 2–4 4 Cross section of cast 250 × 750–2350 250 × 1250–1350 slabs, mm Technological 35.8 28.0 length, m Length of circular 8.79 8.43 section, m Length of straighten7.03 6.72 ing section, m Casting rate, m/min 0.2–1.5 0.2–1.0 Mold height, m 1.20 0.95 Oscillation frequency Up to 120 Up to 160 of mold, min–1 Amplitude of oscilla± 4–6 ±3 tions, mm Design output, 106 t 1.25 3.00
same. Only the mechanisms for moving the machines and supports and the sensor assemblies are modified. Automatic Gega equipment is used for slab cutting. The mold is significantly redesigned: its length is reduced by 250 mm. The thickness of the copper plates is 50 mm for the narrow walls and 70 mm for the broad walls. The narrow walls were first pressed into the mold by means of spiral screws and finally by composite springs. In the broad walls, the cooling channels are bored; in the narrow walls, they are of slot structure. Separate regulation of the water flow in these channels is possible. At the lower end of the mold, there are rollers of diameter 140 mm: one row over the broad face, and two rows over the narrow faces. The rollers are cooled by secondary water sprays. In the lower part of the mold, there are lines for the first secondary-cooling zone of the ingot. The table summarizes the basic characteristics of continuous-casting machines 2 and 3 before and after reconstruction. The machine is equipped with automatic systems for secondary cooling (precise water delivery to the zones), for temperature monitoring of the water entering and leaving the mold walls and also the water leaving the intermediate and center bearings, and for diagnostics of the electrical current. Automated systems permit monitoring of the continuous-casting process and the operation of the basic components, which is extremely important for long casting runs. The first melt was sent to continuous-casting machine 3 on December 15, 2001. In January and February 2002, the modernized machine cast 1148 melts (400000 t of steel).
TAKHAUTDINOV et al. 12
Number of templates with surface defects, % 76.4 80 73.6
10
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1995
1998
2001 Year
2004
2008*
0
48.6
0 1991 1992 1995
13.2
21.3
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15.2
17.0
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9.3
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2
20
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40
30.2
6
60
28.2
8
67.9
21.9
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0
Number of tears
Rejection rate, %
558
2004
2007 Year
Fig. 2. Accidents (tears per 1000 melts) and rejection rate at continuous-casting machines in the oxygen-converter shop; the figure for 2008 refers only to the first four months.
Fig. 3. Surface quality of etched transverse templates between 1991 and the first four months of 2008.
The introduction of casting technology at continuous-casting machine 3 was accompanied by incidents associated with software failures and with shutdown of the roller system on account of electrical problems. The effects of these factors were eliminated and, by April 2002, output was 401.5 t/h, and the design capacity was exceeded by 2.4%. Thus, modernization of the continuous-casting machines at OAO MMK considerably increased their output, reduced the accident rate and the rejection rate of the cast steel, and improved working conditions. Data on accidents at the continuous-casting machines (tearing of the ingot skin) and rejection rates between 1991 and the first four months of 2008 for the oxygenconverter shop at OAO MMK are presented in Fig. 2. Problems with the fulfillment of small orders prompted the introduction of the casting of slab with different cross sections on a single machine and the separation of slabs of different steel within the same casting run by means of dividing slabs, as well as the replacement of the casting and intermediate ladles. The separation of a wide range of steel grades can now be ensured. On introducing steel casting, attention was paid to refining the smelting conditions, the casting rate and temperature, and slab cooling. Experience shows that a wide range of steel may be cast on such machines, with high productivity and metal quality, provided certain operational requirements are satisfied and monitoring continues. For the first time in Russia, a macrotemplate laboratory was designed for monitoring of the slab macrostructure and introduced in the department for the preparation of continuous-cast billet. This significantly reduced the time for evaluating metal quality. At shop startup, quality monitoring of the steel macrostructure in each series of melts was introduced; ~10% of all melts are evaluated. The results permit evaluation of the
state of the continuous-casting machine and the formulation of corrective measures. Common slab defects include internal cracks perpendicular to the slab surface, which are mainly due to disruption of the conveyer belt or wear of the rollers. Corrective measures prevent the development of defects larger than a certain size (corresponding to a score of 2). The monitoring data also establish the location where the defect is formed. If this is within the first and second sections, measures are taken to reduce the casting speed, as to eliminate tearing of the ingot casing. Axial liquation, axial cracks, and peeling arise in the curvilinear section. To reduce smaller defects (corresponding to a score less than 2), a preventive-maintenance schedule is established, along with periodic stretching of the roller belt. The largest defects at the slab surface are reticular and transverse cracks, mainly on slab surfaces of small radius. Experience shows that such defects, generally of depth no more than 3 mm, account for 95% of the damage. It appears that these cracks arise on account of unsatisfactory water–air spray cooling, water fluxes in the nonoperational zones, and other unforeseen water leakage. Corrective measures permit (3–4)-fold reduction in the incidence of such cracks (Fig. 3). Currently, defects whose depth is 5 mm or more account for no more than 1% of the total. During the introduction of continuous casting, ribbed transverse cracks along the oscillation folds in the mold were relatively common. These were due to unsatisfactory operation of the oscillation mechanism as a result of rupture of the motor cords, damage to the attachment holes, and the use of nonstandard material. Elimination of these problems greatly reduced the incidence of such cracks. The other surface defects of slabs—such as longitudinal and transverse cracks, slag inclusions, and STEEL IN TRANSLATION
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crimps—are not very common. As a role, they may be quickly detected, and their causes eliminated. The introduction of continuous casting in the converter shop has permitted the production of steel slab of satisfactory quality. Continuous casting has also been introduced outside the converter shop at OAO MMK. Until 2004, the openhearth shop relied on chill casting. Under a contract signed with VAI in 2003, two continuous bar-casting machines were built in record time—less than a year. At the same time, the aging unit for adjusting the steel’s composition was replaced by a state-of-the-art ladle– furnace unit. In 2004, after the completion of construction, debugging, and cold and hot testing, continuous-casting machines 1 and 2 went into operation. These machines (radius 9 m) are equipped with Diamold multicone molds, Dynaflex hydraulic rocking mechanisms for the mold, electromagnetic systems for mixing the metal in the molds, a water–air cooling system for the blank, and an intermediate push-rod unit for separating billets of flake-sensitive steel at ~900°C. The machines quickly reached their design output of continuous-cast billet: 2 million t/yr. The new casting machines were intended for the production of billet cross sections 100 × 100, 124 × 124, 120 × 150, and 152 × 170 mm. With the introduction of three new Danieli rolling machines, the production of 150 × 150 mm billet also became possible. To expand the scope for continuous casting, the capacity of the existing two-bath smelting units was reduced to 175 t. With further reconstruction of the open-hearth shop, these units were replaced by up-todate arc furnaces of similar capacity, the first of which went into operation in April 2006. These electric furnaces increased smelting capacity to 4 million t/yr, while the capacity of the bar-casting machines was 2 million t/yr. To further improve slab quality and the flexibility of production, a continuous slab-casting machine went into operation in the electrosmelting shop in August
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2006. In view of the four slab machines in the converter shop, this is referred to as continuous-casting machine 5. Casting machine 5 is a two-strand curvilinear unit with a vertical section, multipoint flexure, and straightening of the continuous ingot. It can produce ingots whose thickness and width are 250 and 1250– 2350 mm, respectively. The radius of the circular section is 8 m; the maximum casting speed (for 250 × 1250 mm slab) is 1.2 m/min. The overall length of the unit is 31.7 m; the length of its components is as follows: 0.9 m for the vertical mold with slot cooling channels; 2.8 m for the vertical section; 1.8 m for the flexure section; 8.3 m for the circular section; 6.6 m for the straightening section; and 12.1 m for the horizontal section. The vertical mold in casting machine 5 is equipped with slot cooling channels and a hydraulic rocker mechanism. Besides sinusoidal rocking, other motion is possible. The path of the mold is 0–8 mm, and the frequency is 20–25 min–1. To ensure the required slab quality, the new machine is equipped with up-to-date components such as a system for automatic maintenance of the metal level, water–air slab cooling in the secondary-cooling zone, automatic marking of the hot slabs, Gega gas cutters, and a Gidris system for measurement of the hydrogen in the intermediate ladle. The first melt (ëÚ3ÒÔ steel) was cast at the beginning of August 2006. Within six months, more than 500000 t of regular steel (ëÚ3ÒÔ, 29, St 37-2) and lowalloy and pipe steel (09É2ë, 17Éë, St 52-3) had been cast. The maximum run length is 105 melts. In 2007, OAO MMK signed a contract with SMS for the construction of another continuous-casting machine, intended to produce slab of thickness 190, 250, and 300 mm and width 1400–2700 mm. The machine will be equipped with the latest automatic systems and mechanisms producing slabs of pipe steel (strength class X80–X120) for the 5000 thick-sheet mill.