ISSN 0967-0912, Steel in Translation, 2008, Vol. 38, No. 1, pp. 35–38. © Allerton Press, Inc., 2008. Original Russian Text © A.V. Kushnarev, M.A. Tret’yakov, 2008, published in “Stal’,” 2008, No. 1, pp. 18–21.
Continuous Casting of Steel at OAO NTMK A. V. Kushnarev and M. A. Tret’yakov OAO Nizhnetagil’skii Metallurgicheskii Kombinat (NTMK) DOI: 10.3103/S0967091208010129
At OAO Nizhnetagil’skii Metallurgicheskii Kombinat (NTMK), which produces a complex range of rolled steel, continuous casting increases the quality of the steel and saves resources over the whole metallurgical cycle. In the oxygen-converter shop, with four 160-t converters, all the steel is continuous-cast. The history of this shop at NTMK allows us to trace how the design of continuous-casting machines has developed at integrated enterprises with large-capacity shops. The need to switch from ingot casting of steel to continuous casting was evident even during the design and construction of the first Russian converter shop at NTMK in the period 1960–1963. The design of a shop for duplex smelting of steel from vanadium hot metal was based on the construction of two parallel converter departments. In the first department, gas injection of the hot metal was planned, with the production of vanadium slag and a carbon intermediate product, which was converted to steel in the second department. A continuous-casting department was planned between the two parallel converter departments, permitting the smelting of steel in either of the converters, with delivery of the steel in ladles to the continuous-casting machine. However, this design was only implemented in 1963 to the extent of creating a single shop with 130-t converters and ingot casting of steel. At the beginning of the 1990s, high-output machines for continuous casting of steel to large bar ingots were still not sufficiently reliable. Then, in 1995, the basic continuous-casting machines of combined type went into operation in the NTMK converter shop: four-strand bar-casting machine 1, producing rectangular and round billet for rail, wheel, and pipe manufacture; two-strand machine 2 for casting slabs, with conversion so as to cast four bar billets; two-strand machine 3 for casting complex billet (of dog-bone type), with conversion to cast rectangular bar billet; and single-strand slab-casting machine 4, with conversion to double-slab casting. An important stage in the development of continuous casting in the Russian steel industry began with the construction of a single-strand curvilinear continuous slab-casting machine, with stepping rollers, in the NTMK open-hearth shop in 1968. Design innovations and technological features were debugged on this machine, which was able to cast from 150-t ladles. A
license for this machine was purchased in Japan, where a corresponding unit operated for a number of years. On the basis of operational experience with this continuous-casting machine, Uralmash designed and introduced the first large Russian curvilinear slab machine in OAO NLMK oxygen-converter shop 2, for casting 370-t melts. In the open-hearth shop, the NTMK machine cast 300 000 t/yr of slab, which was sent to the thick-sheet mill. After 25 years, continuous-casting machine 1 was decommissioned when the open-hearth shop was closed. Thus, the benefits of continuous casting were well understood at NTMK. Therefore, on attaining economic independence in 1992, with the introduction of a market economy, complete conversion to converter production and continuous casting of steel was adopted as a priority in the program for reconstruction and development of NTMK between 1992 and 2000. The selection of continuous-casting machines was based on the need to produce billet for wheel, rail, thick-sheet, and universal-beam rolling machines. Continuous casting was introduced in the converter shop between 1995 and 2000. With the shutdown of open-hearth shop 2 (producing 1.7 million t/yr of steel), the first priority was to switch the production of wheel and sheet steel to the converter shop; this determined the selection of the first continuous-casting machines. In the converter shop, at that time, four 160-t converters were in operation. One produced a carbon intermediate product and vanadium slag from vanadium hot metal. Two produced an intermediate product without the use of metal scrap and, when vanadium hot metal was scarce, produced steel in a single process from regular hot metal. With a long lining life, steel could be cast in three converters. Given the lack of experience in the design and construction of machines for continuous-casting the large round billet required for railroad wheels, bandages, and annular components, Russian specialists turned to foreign expertise. As a result, VAI (Austria) was commissioned to supply the equipment for continuous-casting machine 1, with the assistance of Uralmashzavod. According to the 1992 contract, the design output was 500000 t/yr; the calculated output was 696000 t/yr. This was the most productive continuous-casting machine of its type at that time. To ensure satisfactory 35
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quality of the cast wheel steel, a ladle–furnace unit and a circulatory vacuum unit were installed. Even during the construction of the continuous-casting machine, VAI specialists were asked to redesign the machine so as to produce rectangular billet for rails, in view of the growing demand. In 1995, the first stage went into operation: the ladle–furnace unit and continuous-casting machine 1 for producing 300 × 360 mm blooms and round billet (diameter 430 mm). Somewhat later, the circulatory vacuum unit went into operation. All the units were designed by VAI and produced by VAI and Uralmashzavod. Subsequently, again with the participation of VAI and Uralmash, three more continuouscasting machines were built, as well as two ladle–furnace units and a second (two-position) RH vacuum unit, on the basis of SMS Mevac designs and equipment. Continuous-casting machine 1 is a four-strand curvilinear machine with a base radius of 12 mm. The distance between the strands is 1750 mm. The radial mold is of length 700 mm. The metallurgical length of the machine is 32.6 m, which permits casting of round billet at 0.45 m/min. (A rate of 0.50 m/min is theoretically possible, with billet solidification for ~63 min.) For rectangular (bloom) billet, the casting rate is up to 0.6 m/min; the theoretical casting rate may be 1.05 m/min with a solidification time of 31 min. However, on account of the short supporting section (1100 mm), billet stretching is observed above 0.6 m/min. An electromechanical rocking mechanism is employed. In the course of operation, the mold’s rocking frame was redesigned, and the secondary-cooling and other modules were modified. Continuous-casting machine 2 is a two-strand slab machine of UZTM type, with a design productivity of 1100000 t/yr. It went into operation in 1996. The machine casts 240 × 1500 mm slabs; when inserts are introduced in the molds, it may also produce four rectangular (240 × 310–575 mm) bar billets. The vertical mold is of length 1000 mm. Beyond the mold, the secondary-cooling zone includes a vertical section of length ~3.0 m and a radial section (radius 8 m). The metallurgical length is 29.5 m. To ensure product quality, the casting rate must be no more than 1.0 m/min for slabs and 1.1 m/min for blooms. Technologically, rates of 1.2–1.4 m/min are possible. The rocker mechanism for the mold is electromechanical, of lever type. The distance between the strands is 5 m. The roller-conveyer sections are reinforced for stable operation of the machine and the production of high-quality billet. Also, provision is made for centralized lubrication and external cooling of the conveyer rollers; the attachment of the gears to the sections is modified, along with the secondary-cooling system; the temperature difference is taken into account in the design of the roller-conveyer sections, and gas cutting is employed. In 2000, a third machine went into operation. This machine, based on VAI designs and equipment, is of
two-strand radial type (base radius 12 m) and is intended for the casting of complex billet of several types for rolling column profiles and large broad-strip beams (height 600–1000 mm) on a universal-beam machine. When the molds in the machine are replaced, 200 × 500–550 mm rectangular blooms may be produced. The length of the supporting-roller zone in casting the beam blank is 13.1 m; the metallurgical length is 28 m. The casting rate is 1.0–1.2 m/min. The distance between the strands is 3.5 m. The rocker mechanism is analogous to that in the first VAI machine. The maximum casting rate for beam blank is 1.13 m/min, according to the design. In fact, at rates above 0.90 m/min, stretching of the blank is seen. In the course of operation, the design of the machine was somewhat modified: in particular, temperature shrinkage was taken into account in adjusting the rollers in the conveyer segments, secondary cooling was introduced, and the conveyer rollers were redesigned. Continuous-casting machine 4, designed by VAI, went into operation in 2004 and was one of the most upto-date machines used in metallurgy. Its output is 1500000 t/yr. Even in the first months of operation, its design output was exceeded: the machine cast 129000 t of billet. It is intended, in particular, for the production of slabs from carbon structural, low-alloy, and special alloy steel, for use in rolling sheet of strength classes K65 (X80) and K80 (X100) on thick-sheet mills. Machine 4 is a single-strand slab machine, with the possibility of double-slab casting. In single-strand operation, the slab produced is of thickness 200– 300 mm, width 1500–2700 mm, and length 6000– 10000 mm (later, of length 3000–4200 mm). In doubleslab casting, the slabs are of width 1150–1280 mm. The fourth continuous-casting machine is inclined at 90° to the other three. This makes the best use of the limited space in the converter shop. The fourth machine incorporates the most promising design innovations available at the time of its construction. The straight mold of the continuous-casting machine (of height 900 mm) is mounted on a hydraulic rocker unit (flexible plate springs). The rocking parameters (distance, frequency) may be adjusted in the course of casting, which ensures high surface quality. The mold is followed by a set of rollers supporting the broad and narrow sides of the slab and then the sections of the secondary-cooling zone. Hydraulic devices at the mold permit change in billet width in the course of casting. The mold is equipped with the MoldExpert tear-prevention system. The Dynaflex hydraulic rocker mechanism and the optimal secondary-cooling conditions (selected by means of the Dynacs model) ensure the required surface quality of the slabs. For the best removal of nonmetallic inclusions, the vertical section of the continuous-casting machine, together with the mold, is of height ~3 m. This is followed by a bending system, which prepares the slab for the radial section (radius 10 m) by means of a special curve. Beyond the radial zone, there are straightening devices, which proSTEEL IN TRANSLATION
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CONTINUOUS CASTING OF STEEL AT OAO NTMK
duce horizontal slab. From the curvilinear zone to the end of the solidification zone, the individual sections are equipped with hydraulic cylinders for hot adjustment of the roller configuration in accordance with the temperature shrinkage of the billet and for slight reduction of the continuous-cast slab. To facilitate billet motion, the conveyer rollers consist of several segments on a single shaft, with two intermediate bearings. The machine is equipped with a system for mild reduction of the billet. The casting rate is up to 1.3 m/min. The metallurgical length of the machine is 27.2 m. A rotary unit for burr removal is followed by a labeling unit, a machine for turning the slabs, and gas–oxygen cutters. The 35-t intermediate ladle is fitted with partitions. A system for fast nozzle rotation allows this operation to be performed at working speeds in the intermediate ladle, thereby preventing delays in the process and reducing the subsequent trimming required. The automatic control system for all the continuouscasting machines in the converter shop was developed and supplied by VAI (now Siemens–VAI). The design output of the machines has been reached and exceeded. The actual output is 828700 t/yr for machine 1, 1533700 t/yr for machine 2, 711700 t for machine 3, and 1391200 t/yr for machine 4. Runs of 80–100 melts are required, whereas 140 melts are possible in practice. The use of four different machines, casting ten types of billet (from round and rectangular to complex) is due to the wide range of rolled products at NTMK and the need for billet best matching the specifics of the rolling mills,. This complicates the organization of castingmachine operation, since each machine specializes in particular grades of cast steel and it is not possible to switch from one machine to another in the course of a melt. The significant design differences between the continuous-casting machines complicate the standardization of spare parts and the organization of maintenance and repairs. The differences in the operating cycles of the machines create certain difficulties in planning the operation of the converters, the ladle-treatment systems, and the machines themselves. Thus, for continuous-casting machine 1, the casting cycle is 80–90 min; the cast metal (mainly rail and wheel steel) requires treatment in the ladle–furnace unit and the vacuum unit. The casting time on machine 2 is 30–35 min for slabs and 50–55 min for blooms. About half of the metal used for blooms requires vacuum treatment. The casting cycles for machines 3 and 4 last 90–100 and 30–40 min, respectively. The same problems had to be solved in introducing each continuous-casting machine and bringing it to the design power: selection of staff, elimination of weak spots and design deficiencies in the machines, creation of an up-to-date casting technology, and organization of specialized repair shops. STEEL IN TRANSLATION
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The production of a wide range of continuous-cast billet must be preceded by the elimination of design defects in the machines, the development of a smelting technology in the converter department, development of means of preparing the steel for ladle treatment and subsequent casting, and increase in staff skills and vigilance. Production technologies for high-quality wheel-, rail-, tube-, and sheet-steel billet have been created. Synchronization of the equipment in the converter shop is based on a dynamic-planning system, which permits real-time monitoring and control of the production process. This system is introduced in the information network of the large-scale metallurgical control system developed at NTMK. Later continuous-casting machines are redesigned on the basis of operational experience and design innovations so as to permit expansion of the product range and greater productivity. New intermediate ladles, elongated molds, spring–hydraulic rocker mechanisms, and extended supporting sections are introduced. The use of magnetic mixers in machines casting square (round) billet is considered. The quality of continuous-cast billet depends on the structure of the continuous-casting machine, the current state of the mold, the roller conveyers, and the secondary-cooling zone, the slag-forming mixtures employed, the possibility of regulating individual parameters in the course of casting, and other factors. The billet quality also depends on the technology used in smelting the steel and ladle treatment. Experience in steel production shows that all the relevant parameters must be consistently maintained within narrow tolerances. In the NTMK converter shop, steel production includes familiar stages and specific measures associated with the processing of vanadium hot metal, as illustrated in the figure. 1. Production of a carbon intermediate product and commercial vanadium slag from vanadium hot metal. Removal of vanadium and silicon from carbon product, which contains 3.0–3.5% C, <0.03% V, traces of Ti, Si, and Mn, <0.02% S, and <0.10% P. The intermediate product does not contain other impurities that impair steel quality. 2. Removal of sulfur in a refining unit, by injecting lime and magnesium powder. Reduction of sulfur content to 0.002–0.005%, depending on the steel specifications. 3. Treatment of the intermediate product in the converter to the required carbon content for the specified steel. 4. Discharge of metal from the converter and reduction of the steel in the ladle, with simultaneous argon injection through a porous bottom tuyere (nozzle). 5. Transfer of the ladle with the metal to one of the three ladle–furnace units, where the oxidized slag is neutralized with additions of lime and fluorspar, the
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Vanadium hot metal
Converter no. 1
Converter no. 2
Converter no. 3
LFU no. 1
LFU no. 2
CCM no. 1
Converter no. 4
LFU no. 3
RH no. 1
RH no. 2
CCM no. 2
CCM no. 3
CCM no. 4
Production system for converter steel: LFU, ladle–furnace unit; CCM, continuous-casting machine.
chemical composition is corrected, and the composition and temperature of the steel are further corrected. Treatment is accompanied by bottom argon injection. 6. Degassing of the steel at one of the two RH vacuum-treatment units is necessary for steel that is sus-
ceptible to flaking (rail steel, wheel steel, etc.) and special steel. 7. Continuous casting of the steel, with monitoring of the parameters by the automatic control system. Billets or billet segments with deviant parameters are rejected. In steel production in the NTMK converter shop, no metal scrap is employed; this limits the introduction of undesirable elements in the steel. Until recently, NTMK was the only Russian source for primary steel based on hot metal that is produced from impurity-free Kachkanarsk ore. Continuous-cast billet from the first three machines is sent to the rolling mills. Costs are significantly reduced here. The steel consumption is 1064.4 kg/t in producing R-65 rail, 1288–1380 kg/t (depending on the type) in wheel manufacture (full-profile treatment), and 1060–1100 kg/t in the production of broad-base beam; this is 200–250 kg less than in the previous technologies and corresponds to best world practices. Between 2007 and 2009, the NTMK converter department will be reconstructed, with a second supply line for friable materials (in operation since 2007) and replacement of the converters and gas-exhaust channel. Provision will be made for bottom injection of the converters and an automated system for segregation of the oxidized slag on discharging steel into the ladle. Rebuilt converter 1 went into operation in 2007. The creation of a modern system for continuous casting and ladle treatment of steel in the NTMK converter shop is an example of large-scale transformation in Russian metallurgy. In combination with the unique production technology and the use of up-to-date information technologies in management and planning, specialist experience may be fully utilized in the reconstruction and development of metallurgical systems.
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