Metallurgist, Vol. 45, Nos. 9–10, 2001
FEATURES OF THE CONTINUOUS CASTING OF CORROSION-RESISTANT STEEL
V. M. Parshin, A. V. Larin, I. I. Sheinfel’d, B. A. Korotkov, and A. M. Chigrinov
UDC 621.74.047
The world demand for corrosion-resistant steel has grown nearly 50% in the last decade – from 8 to 13–13.5 million tons/yr – and it is expected to increase further [1]. If certain assumptions are made, it can be stated that the production and consumption of corrosion-resistant steel serves as an index of the industrial development of a country. In connection with this, manufacturing companies are actively engaged in the production and sale of corrosion-resistant steel and in investing in equipment modernization and improving technologies for the manufacture of this product. One element of the production process is continuous casting. Foreign and domestic experience in the production of corrosion-resistant steel shows that the continuous casting of titanium-bearing steel is the most labor-intensive and complex casting operation, based on the requirements that must be met to ensure a quality cast product. In this article, we examine technological aspects of the production of corrosion-resistant steel. Specific features of corrosion-resistant steel Kh18N10T make it difficult to obtain continuous-cast ingots of this steel that will have a good surface and a minimal number of macrostructural and microstructural defects. In particular, the steel has substantial concentrations of easily oxidized elements (Al, Ti, Cr, Si). Their reaction with carbon, nitrogen, and oxygen dissolved in the metal or present in the atmosphere results in the formation of oxides, nitrides, and carbonitrides that have a high melting point and adversely affect the fluidity of the steel. Ingots of steel Kh18N9-19T can have a defect known as edge porosity or “titanium” porosity, which is related to rotation of the solid skin formed on the surface of the metal in the mold. The defect is usually located about the periphery of the ingot and propagates into it to a depth ranging from 1–2 to 20–25 mm. An analysis of the existing literature data on the continuous casting of corrosion-resistant steel shows that the reports are of a general nature and do not include information on the industrial practice of continuously casting corrosion-resistant steel with titanium. The first studies made in this country of the continuous casting of steel 08-12Kh18N10T were done on experimental units at TsNIIchermet [2]. In 1952, the Volgograd Metallurgical Plant Krasnyi Oktyabr’ developed and introduced a experimental-commercial technology for the continuous casting of corrosion-resistant steel 08-12Kh18N10T [3]. In 1963, the Gorky Metallurgical Plant began introducing a commercial technology for continuously casting steel 08-12Kh18N10T into semifinished slabs 180 × (400–500) mm [4]. The plant resolved basic issues regarding the casting technology: protecting the metal from secondary oxidation on the ladle – tundish and tundish – mold paths; developing regimes for reciprocating motion of the mold; developing slag-forming mixtures (the plant chose an exothermic mixture of the following composition, mass %: SiCa – 26; CaF2 – 38; NaNO3 – 10; Fe2O3 – 20; Na2B4O7 – 6); delivering the metal to the mold. The advances that were made allowed manufacturers to significantly reduce the amount of metal lost during conditioning and begin industrial production of the steel. The positive experience gained in the production of steel 08-12Kh18N8-10T was later used in the delivery of a Russian-made vertical continuous caster to Italy (the Terni Works of the company ILVA-SPA). The caster was designed for Central Scientific Research Institute of Ferrous Metallurgy (TsNIIchermet) and the Limited Liability Company Kanpro. Translated from Metallurg, No. 10, pp. 43–46, October, 2001. 0026-0894/01/0910-0391$25.00 ©2001 Plenum Publishing Corporation
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1
2 3 7
4 5 6
1 4 6
7 5
a
b
Fig. 1. Unit for protecting metal from secondary oxidation on the pouring-ladle section (a) and the tundish–mold section (b): 1) pouring ladle and tundish; 2) slide gate; 3) collector; 4) upper part of unit; 5) lower part of unit; 6) protective tube and submersible nozzle; 7) mold.
Tundish
Submersible (steel-casting) nozzle Bath of liquid metal
End seal
Skin of solidifying strip
Pouring ladle Slide gate
Slide gate
Weight
Level
Force Stationary roll Movable roll
Tension Withdrawal stand
Thickness Water-cooled copper shell
Continuous-cast strip
a
b
Fig. 2. Diagram of the casting of strip on a two-roll continuous caster (a) and the main elements of the automation system (b).
casting corrosion-resistant steel into 140 × 600 – 200 × 1250 mm slabs from 50–180-ton ladles. The steel was made in electric furnaces operated with an AOD. An integrated technology for making steel Kh18N10T was put to use in the electric steelmaking shop of the company Severstal’ on vertical continuous casters with molds measuring 200 × (1070–1200) mm. The technology provided for hot rolling of the cast slabs on the 2000 mill at Severstal’ and subsequent cold rolling by the company Mechel. The following problems were resolved in the course of this work: • the lining of the tundishes was made using high-alumina materials containing at least 62% Al2O3; • the metal was provided with protection from secondary oxidation (Fig. 1); • the metal was supplied to the mold through a submersible corundum-graphite nozzle having discharge holes inclined upward at an angle of 10–15°; • the temperature and speed regimes of the casting operation were stabilized; 392
• compositions of slag-forming, exothermic, and heat-insulating mixtures were developed (the consumption of these mixtures ranges up to 1.2 kg/ton steel); • the regimes for secondary cooling of the slabs were corrected (with a unit water consumption of 0.4–0.5 liters/kg steel), which made it possible to obtain a slab surface temperature on the order of 880–920°C at the end of the cooling zone (roughly 8.5 m from the bottom end of the mold). The pouring operation and the process of cutting the slabs to measured lengths were also stabilized. In addition to these elements of the technology – which ensure the production of quality cast slabs of titanium-bearing corrosion-resistant steel – work was done to limit and stabilize the concentrations of aluminum, titanium, and gases (oxygen, nitrogen, and hydrogen) in the cast steel. The electric steelmaking shop at Severstal’ was the first shop to try casting steel 12Kh18N10T into a mold equipped with a device for electromechanical mixing (EMM). The device has been used to cast more than 6000 tons of steel. Comparison of the quality of the surface of experimental and control ingots confirmd that EMM has a positive impact. There is less folding of the surface and no distortion. The number of slag inclusions on the surface is reduced by a factor of 2.0–4.0 and does not exceed 0.5 inclusions per 1 m2. The size of the subsurface blowholes is cut in half and the total number of blowholes is reduced by three- to fourfold. The incidence of bleeders is 2–8 times lower. Taking into account both domestic and foreign experiences, the Chelyabinsk Metallurgical Combine built a special shop to make corrosion-resistant steel, chromium steel, and chromium-nickel steel (with and without titanium) and cast them on a curvilinear continuous caster into slabs with a cross section of (150–170) × (1350–1550) mm. Total production volume was to be about 400,000 tons a year. The technology used to make low-carbon corrosion-resistant steels 03Kh18N10T, 08Kh18N10, and 08Kh18N10T includes the use of a DSP-100 I7 electric steelmaking furnace to make a semifinished product with 1.5–2.0% C and contents of chromium, nickel, and other elements that are close to the chemical composition of the finished steel. The semifinished product is finished in a 100-ton argon-oxygen refining (AOR) unit and cast on a curvilinear single-strand continuous caster with a mold having a cross section of (150–170) × (350–1550) mm. Improvements in the methods used to make and cast corrosion-resistant steel have made it possible to also improve the quality of hot- and cold-rolled sheet. For example, while the yield of hot- and cold-rolled sheets with the highest rating for surface quality (under GOST 5582–75) was 37.3 and 50.1% based on the results for the first series of heats, the corresponding figures for the second and third series improved to 53.2 and 62.5%, respectively [5]. In addition to the traditional method for the continuous casting of corrosion-resistant steel, metallurgists abroad make use of the latest casting techniques: thin slab, thin sheet (two-roll casting), and horizontal casting. While the first two methods are best for casting steel which does not contain titanium, horizontal casting is nearly ideal for any type of corrosion-resistant steel. That is because the surface of the steel is formed in a special receiving vessel, and it is impossible for inclusions to be drawn into the body of the ingot. Unfortunately, there are still no thin-slab casters in Russia. However, positive results have been obtained in making roll-type units for casting sheet. TsNIIchermet and the company Module-Engineering have developed a two-roll continuous caster for the production of 2–6-mm-thick sheet made of silicon-alloyed carbon electrical steel, as well as austenitic and ferritic corrosion-resistant steel [6]. The unit is equipped with a special submersible nozzle which ensures a uniform distribution of the liquid steel along the rolls. The unit also has a device to feed gas into the caster in order to create a protective surface above the liquid bath, a mechanism that regulates the position of the roll-molds, a contact piece for the graphite-ceramic elements that restrict the lateral movement of the cast slab, and equipment to apply a functional coating to the rolls that reconditions them during the pauses between the castings without requiring disassembly of the unit. The 1200-mm-diam. roll-molds, with a width of 1000 mm, are made of steel and contain cavities (to reduce weight). The rolls also have bronze-alloy sleeves with a coating. The speed of the rolls is 0.2–1.05 m/sec. The chocks of one of the rolls is equipped with a hydraulic stop adjusted for a certain force. The other roll has a clamping mechanism with sensors that measure the force on the strip side of the rolls. That measurement is an indicator of the degree of solidification of the steel. The force is automatically kept constant so as to guarantee stable conditions for the casting operation. 393
TABLE 1. Comparative Characteristics of Steels and Alloys Cast on the Two-Roll Casting Machine at VNIImetmash Alloy
Electrical steel
Grade, composition of alloy
Advantages over traditional technologies and materials
Application
4–6.5% Si res. Step motors
Corrosion-resistant steel
Hard-to-deform resistive alloys
65Kh13
30% power reduction; 75% shorter manufacturing cycle
Electric-razor blades and grids
No crumbling of strip at carbide accumulation sites; 80% shorter manufacturing cycle
N70MKhYuE Electronic devices
Hard-to-deform magnetostrictive alloys
Improvement in service characteristics and the quality of the strip surface in micron thicknesses
Ultrasonic vibrators and generators
3–5-fold improvement in processing ductility and property uniformity. Replacement of nickel and cobalt alloys
1 2
3
6
4
5
6
Fig. 3. Diagram of technology for making continuous-cast semifinished products on a horizontal continuous caster with bidirectional withdrawal of the ingot: 1) tundish; 2) mold; 3) metal conduit; 4) ingot; 5) channnels; 6) withdrawal stand.
A mechanism to remove the sheet is installed under the rolls. After leaving the rolls, the sheet slides along a chute that directs it into a withdrawing stand. An automatic system (Fig. 2) regulates the tension on the sheet and synchronizes the speeds of the roll-molds and the rolls of the withdrawing stand. Thanks to its relatively small dimensions, the unit is easily incorporated into existing production facilities. Due to the compactness and linear layout of the equipment, the unit investment is 75% lower than for conventional continuous casters. There is also a 50% reduction in modernization costs, and there are added benefits from the elimination of hot rolling and other operations related to machining, conditioning, and heating of the slabs, etc. The organization VNIImetmash (All-Union Scientific Research, Planning, and Design Institute of Metallurgical Machinery) has developed the first and thus far only two-roll machine (with 600-mm diam. rolls; maximum heat size 160 kg) for casting thin strip [7]. This advance was preceded by a large amount of theoretical and experimental work done on a laboratory machine with 240–500-mm-diam. rolls having a working part 150 mm wide. For example, the developers performed trial castings of different grades of steels and alloys, such as electrical steel with a silicon content of 4.6–6.5%, corrosionresistant steel 65Kh13, resistive hard-to-deform alloy N70MKhYuE, and a magnetostrictive alloy of the iron-aluminum system (Table 1). The quality of the strip made on the experimental unit met the requirements established for the same product obtained by the conventional technology. Thus, Russian designers are meeting world standards. However, commercial application of their innovations is being held up by a lack of government support and manufacturers’ own shortage of financial resources. 394
As regards the casting of corrosion-resistant steel on horizontal casters, it should be pointed out that machines of this type are presently used mainly to cast squares and rounds with sides or diameters ranging from 70 to 360 mm. The units made by the company Technica Cuss cast semifinished products with sides (diameters) 35 mm or greater. As of January 1, 1999, there were just 20 horizontal continuous casters in the world that were capable of casting corrosion-resistant steel – in Austria, England, Germany, Spain, China, France, Japan, and the U.S. [8]. These machines are mostly two-strand casters, and the steel is poured from ladles ranging in capacity from 5 to 50 tons. The Edestahl plant of the firm Mannesmann–Demag (in Germany) has built a six-strand horizontal continuous caster for casting steel from 80-ton ladles. Significant strides have also been made in Russia in regard to the continuous casting of corrosion-resistant steel on horizontal casters. VNIImetmash was the first in world practice to conduct tests of a technology for the continuous casting of chromium-nickel corrosion-resistant steel containing 0.38–0.52% Ti in a 45 × 500 mm mold. The technology improves the surface quality of the cast semifinished products and eliminates the need for their conditioning. The yield of useable continuous-cast semifinished products is 5% greater than for the technology traditionally used to cast steels 12Kh18N10T, 10Kh17T, 0Kh23Yu5, and other grades. The tests involved casting slabs with a cross section of (45–50) × (500–1200) mm on an experimental horizontal caster [9]. TsNIIchermet and VNIImetmash subsequently collaborated on the development of a technology for casting corrosion-resistant steel 12Kh18N10T on a horizontal caster in 140 × 140 mm molds. Here, the cast ingots were withdrawn from the caster in two directions. The caster was designed by VNIImetmash (Fig. 3) and has the following specifications: Cross section of semifinished product, mm . . . . . . . . . 120 × (120–175) × 175 Productivity, tons/h . . . . . . . . . . . . . . . . . . . . . . . . . . . 15–30 Dimensions of unit, mm: length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25,000 width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6000 height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2200 Casting speed, m/min . . . . . . . . . . . . . . . . . . . . . . . . . 0.7–1.2 Weight of main equipment (without shears), tons . . . . 60 In the continuous casting of steel 12Kh18N10T on a horizontal caster with bidirectional withdrawal of the ingot, the main process load is applied to the tundish. Studies have shown that normal operating conditions for the components of the caster can be maintained by properly choosing the profile and dimensions of the tundish and the downtube, reliably servicing the refractories, and keeping the thermal loads on the metal small. A flawless (with respect to its surface and internal structure) semifinished product having a temperature high enough (720–750°C) to allow rolling with hot charging can be obtained by having the surface layers of the melt enter the tundish first, slowly cooling the semifinished product after the mold, and stabilizing the casting operation. The expediency and promise of the scheme “cast semifinished product – sheet bar – hot-rolled sheet” for producing corrosion-resistant steel was demonstrated in a test rolling performed at the Serp i Molot Metallurgical Plant [10]. The most important contribution being made to improving the production of corrosion-resistant steel is the use of continuous casting at the Mechel, Krasnyi Oktyabr’, and Serp i Molot metallurgical plants and the Volzhskii Pipe Plant. Studies conducted at Serp i Molot which involved changes in the titanium, oxygen, and nitrogen contents of steel 12Kh18N10T and examined the state of oxidation of the steel during refining and casting once again confirmed the need to carefully isolate the liquid metal from the surrounding atmosphere [11]. For the production of corrosion-resistant steels 10Kh18N9 and 30Kh13, the Volzhskii Pipe Plant has high hopes for optimally combining the kinetic features of steel production in an ultra-high-power arc furnace and steel treatment in a 150-ton VOD unit. The plant has devised a set of measures to re-equip the electric steelmaking shop so that it can make corrosion-resistant steel and cast it on a radial continuous caster [12]. It should be mentioned that the facilities used to treat steel outside the furnace play an important role in ensuring its high quality. Specifically, this pertains to treatments administered in a ladle-furnace unit in combination with VD, AOD, and VOD units. 395
Thus, in contrast to foreign practice, the continuous casting of corrosion-resistant steel in Russia is oriented mainly toward the production of steel that contains titanium. That makes it difficult to use continuous casting. A continuous-casting technology for such steel has been developed in this country, the technology including treatment outside the furnace, continuous casting, and preparation of the cast metal for rolling. Several plants – Serp i Molot, Mechel, Severstal’, and and Volzhskii Pipe Plant – have modern equipment for making products of this type of steel and satisfying the domestic and foreign demand for corrosion-resistant steel.
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A. Gardner, “State of the international market for stainless steel,” Novosti Chernoi Metallurgii za Rubezhom, No. 2, 3–7 (1996). M. S. Boichenko, V. S. Rutes, and V. B. Ful’makht, The Continuous Casting of Steel [in Russian], Metallurgizdat, Moscow (1961). B. Z. Kononov and A. G. Il’in, Stal’, No. 11 (1956). E. I. Astrov et al., The Continuous Casting of Steel [in Russian], Metallurgiya, Moscow (1974). A. N. Volkodaev, A. V. Tokarev, O. K. Gokovoi, et al., “Mastering a technology for making corrosion-resistant steel with treatment in an argon-oxygen refining unit,” Stal’, No. 8, 22–23 (1995). S. I. Vitik and V. M. Parshin, “Beginning of the development of a prototype unit for the high-speed casting of steel strip,” in: Trans. V Congress of Steelmakers, Chermetinfomatsiya (1999), pp. 395–397. V. I. Reshetov, “Development of the production of thin semifinished products on two-roll casters,” in: Trans. V Congress of Steelmakers. Chermetinformatsiya (1999), pp. 397–399. Continuous Casting Machines for Steel. World Survey. Situation January 1, 2000. Concast Standart/Documentation Center, Zurich. V. M. Sinitskii, A. I. Maiorov, L. P. Zakov, et al., Stal’, No. 12, 16–17 (1991). A. M. Chigrinov, V. M. Parshin, I. I. Sheinfel’d, et al., “Continuous casting fo steel 12Kh18N10T on a horizontal caster with bidirectional ingot withdrawal and no conditioning of the cast metal,” ibid., No. 1, 37–38 (1993). M. A. Filimonov, n. N. Perevalov, et al., “Effect of secondary oxidation on the quality of corrosion-resistant titanium-bearing steel,” ibid., No. 7, 33–35 (1994). V. Yu. Kuznetsov, V. V. Frolochkin, et al., “Mastering the production of austenitic corrosion-resistant steel and tubes made from it at the Volzhskii Pipe Plant,” in: Transactions of the Third International Congress of Metallurgists (Moscow, April 10–15, 1995), pp. 368–370.
