NO OTHER ALTERNATIVE BUT THE COSTLY ONE

Casting-rolling stations revolutionize steel rolling processes.



NO OTHER ALTERNATIVE BUT THE COSTLY ONE

Valentin DANCHENCO, the National Metallurgical Academy of Ukraine

Casting-rolling stations revolutionize steel rolling processes.

The modern realities urgently call for restructuring of Ukrainian metallurgical mills to broadly apply continuous casting and set up casting-rolling stations in order to make sheets, plates, and bars. This will yield a 15% decrease in consumption of steel per ton of rolled metal and will slash power consumption by 30 to 50%. The latest achievements in smelting and continuous casting, together with effective combination of casting and rolling procedures, have led to creation of a new concept of manufacturing long metal products [1-5]. This concept provoked restructuring of metallurgical industries in the developed countries of the world.

Unfortunately, Ukraine looks rather pathetically against the background of effective transformations in the world metallurgy caused by broad application of continuous casting. In the beginning of the nineties Ukraine ranked the fifth in the world in terms of steelmaking with only 8% of its steel being continuous cast. Today introduction of new continuous casters (together with a simultaneous recession in steel output due to the economic downswing) has boosted this ratio to some 20%. Meanwhile, Western European countries continuous cast 87% of their total steel and Japan continuous casts 94.4% of its total.

Along with this, in Ukraine power consumption per ton of steel is 30% higher than in Japan. It takes 1.3 tons of steel to make a ton of rolled metal in Ukraine, 1.24 tons in Russia, and 1.044 tons in Japan.

This has brought about the situation when a significant portion of Ukrainian metal is noncompetitive on the world market, while low wages and small allocations for pollution control are the only factors enabling Ukraine to export metal to price-cautious clients at low prices.

The developed countries have launched broad restructuring of iron and steel industry, commenced decentralization of large metallurgical mills (e.g. British Steel), phasing out of obsolete facilities, and phasing in of mini-mills (producing up to 2 million metric tons of steel per year) that integrate advanced smelting, casting, and rolling technologies.

The developed economies with efficient financial and fiscal systems don’t keep their noncompetitive mills, divisions, and workshops idle for years, but modernize productive facilities or close them down completely. Along with this, owing to favorable laws that promote investments in efficient industrial sectors, new hi-tech enterprises come into being.

Obsolete metallurgical mills are upgraded on the basis of traditional technologies (blast-furnace – converter treatment – continuous casting – rolling or smelting in electric arc furnace – continuous casting – rolling) or applying advanced breakthrough technologies (ore-to-steel process; electric arc smelting – direct continuous casting and rolling).

Specialists have estimated [5] that even the developed countries will gradually abandon application of traditional technologies till the middle of the 21st century. This process is natural for lifetime of any technology doesn’t stop all at once, while renewal of old metallurgical technologies requires huge capital inputs. It is estimated that by 2013 direct-reduced iron will account for 1% of total steel output, while continuous casters will produce only 18% of the total thin slabs and 3% of the total strips, less than 10 mm thick.

Meanwhile, introduction of advanced technologies is getting less risky today. Thus, all the predicted achievements can actually take place much earlier.

Owing to technical backwardness of the Ukrainian as well as of the Russian [3] metallurgy, consumption of materials and power in these countries is 20 to 25% greater than in the leading global manufacturers. It is clear that development of this crucial industry needs thorough governmental support, as well as profound expert analysis of its evolutionary ways.

In this connection special attention should be paid to development of continuous casting, its integration with rolling facilities, to design of modular equipment, and casting-rolling stations. Casting-rolling technologies are also of paramount interest for production of flat-rolled products, taking into consideration the pattern of rolling in Ukraine (with 62.8% being contributed by various bars and 37.2% by sheets & plates) that didn’t correspond to the internationally-accepted practice even back in 1991. Flat-rolled metal makes up to 2/3 of the total output in the developed countries.

Casting-rolling stations combine casting and rolling within one production line. Creation of these stations could have been impossible without the latest achievements in curved-mold (the first type) and horizontal continuous casting (the second type), when ingots slide between the oscillating walls of the mold.

There have already been designed pilot industrial casting-rolling stations taking advantage of horizontal continuous casters where ingots move together with cooling surfaces of the mold (whether it is a conveyor belt or roll). Anyway, these mills haven’t won popularity yet.

Successful development of modern continuous casters with high level of automation and control, together with achievements in smelting of pure steel, in prevention of contamination during continuous casting processes, and of metal deformations during solidification), has enabled heading billets to the direct rolling mills without prior inspection, thereby saving the casting heat. Changes in the temperature of metal (as on figure 1) illustrate the advantages of the new technologies. Various countries were doing research in the sphere of continuous casting back in the 1930-1940s, though rapid development and industrial implication of this process dates back to the early 1960s. Becoming the most popular casting method in the world and being amplified with the rise of advanced technologies, continuous casting process is likely to undergo further proliferation in Eastern Europe. In 1995 the number of functioning continuous casters totaled 1,686 units throughout the world.

Figure 1. Changes in metal’s temperature on different stages of steel strip manufacturing (depending on different slab feeding to the rolling mill): I – feeding of cold alloys; II – feeding of hot alloys; III – direct rolling (1 – continuous casting, 2 – cutting, 3 – quality control, 4 – heating, 5 – scale removal, 6 – rolling, 7 – cooling, 8 – coiling)

 

Figure 2. Flat strips’ thickness depending on continuous casting rate (1 – slabs, 2 – thin slabs, 3 – strips, 4 – metal strips with amorphous structure)

 

Continuous casting can produce slabs, blooms, billets, as well as thin strips and blanks with cross sections close to the finished products. Thinning of cast strips speeds up the casting rate (see figure 2).

Casting production (from slabs to thin strips) required creation of various continuous casters (with the first and the second-type molds) with relatively high productivity.

In 1989 US-based NUCOR mill became the first company in the world that actually integrated continuous casting with hot rolling [6].

Casting of thin slabs, lowering of the overall hot strain, and production of HR sheets within one production line marked the dawn of a new stage of metalmaking evolution, that is the casting-rolling epoch.

Over the past decade a number of foreign companies launched their own casting-rolling projects. Metallurgical mills have most widely employed the thin slab technology consisting of such technological operations as continuous casting of steel, cutting of thin slabs, heating in a furnace, scale removal, hot rolling, cooling, cutting, and coiling.

Transition to modern S-type (curved-mold) continuous casters has resulted in significant complication of the casting technology and equipment architecture. Designers of casting-rolling stations had to tackle the problems of safety of continuous casting process, high surface and inner quality of strips, consistency of equipment’s operations under conditions of slow casting and rapid rolling, etc. Fundamental difficulties have been surmounted mainly owing to breakthrough steelmaking technologies to obtain pure steel, creation of new machinery and equipment, thorough automation of production lines, and computer-based casting control.

Mathematical simulation software has also contributed to prevention of liquid metal’s bleeding during solidification, prevention of crack formation, compacting of strips’ cores, and to manufacturing of sheets and plates with high precision and flatness.

It is possible to control the temperature profile of a thin slab’s cross section with the model of dynamic solidification during reduction and additional cooling in continuous caster’s driven rolls after leaving the mold (see figure 3). Owing to efficient management and stable casting conditions, slabs can be directly rolled without inspection or preparation.

Casting of low-carbon steels (0.08-0.16%) poses particular difficulties. These grades of steel are highly exposed to appearance of cracks.

Utilization of appropriate tundishes and ladle nozzles, backed with superposition of electromagnetic fields, stabilize casting conditions and prevent fluctuations during automatic control over metal’s cooling.

According to the data with MDM firm, Germany, high homogeneity of mechanical properties, texture improvement, and lowering of segregation indicate higher quality of strips manufactured in this way compared to quality of strips produced via traditional casting. This result is also backed with tundish heating, application of heat-exchanging nozzles, intensive secondary cooling of metal after it has left the mold, and usage of soft (moderate and selective) reduction of continuous-cast ingots in driven rolls. Precision control over the level of liquid phase and position of sinkhole has proved to be the key factor for enhancement of product quality.

Reduction of ingots with liquid core in driven roll guides reaches 70% [7], thus contributing to quick convergence of solidification fronts, to reduction in the distance between dendrite arms, and to increase in castings’ homogeneity.

Backward movement of reduced liquid metal has an effect similar to electromagnetic agitation, when columnar crystals are transformed into equilibrium ones with a homogeneous area in the center of the strip [7].

Narrowing the distance among dendrite arms, the casting of thin sections favorably influences internal strip quality (see figure 4), but also can deteriorate the quality of strip’s surface [8]. Nevertheless, the extensive experience of applying casting-rolling stations proves that it is always possible to obtain high-quality products regardless of the grade and purity of steel and irrespective of the casting approach.

It goes without question that only high-quality commodities can stay on the world market for metal products. Today, the experience of NUCOR mill equipped with two double-strand hot-rolling mills for sheets and plates, 1.5-12.7 mm thick, (the CSP method producing 4 million tons per year) insists that quality of such sheets is higher than quality traditionally-made flats.

Various casting-rolling stations are capable of manufacturing a wide range of steels, namely carbon, structural, corrosion-resistant, electrical, bearing, and tool steels.

In 1968 Algoma Steel Co. located in Canada became the first to produced cast billets with dimensions very much alike to finished products (mainly for manufacturing of heavy sections, such as beams, rails, etc). More than 15 casting-rolling stations of this kind are already functioning in the USA, Japan, China, and Germany (all of them were made by Germany’s SMS company). Rough sections are finish-rolled on universal mills.

In contrast to production of sheet steel, hot rolling will obviously keep on being the keystone for manufacturing of bars and seamless tubes.

It should be noted that Germany’s MDM company has managed to produce high-quality bars using 3-4-strand horizontal continuous casters with fixed molds and a special oscillating stand [9].

The data with MDM company show that metals with uniform structure and with casting-free core were obtained during manufacturing of rods and skelp.

Further progress of the continuous casting process will make it possible to produce hollow sections, which will render completely useless the piercing stage and lower hot strain during hot finishing of tubes and pipes. Casting of hollow sections with diameters and wall thickness close to parameters of cold-finished tubes will considerably improve the cyclic recurrence of their production.

Since cross sections of billets became smaller, rolling mills have started producing much less overall hot strain. This made reduction mills (slabbings and bloomings) and rough stands go out of business.

Casting-rolling stations that manufacture sheet steel consist of (see figure 5): 1 to 7 stands within continuous finishing mill (CSP), or an upgraded Steckel mill [10] with reels located in the furnace area (TSP, ISP, MDH), and Platzer planetary mill (MDS).

Synchronized work of the continuous caster – furnace – rolling mill system rests on the principle of a heating furnace operating in the casting mode. Long 200-m tunnel furnace, or induction heaters, or the so-called coil-boxes (compact furnaces equipped with reel devices) are applied to do high-speed coiling of HR flat metal. Coil-boxes are able to process strips, less than 35 mm thick. Quarto stands that are applied in casting-rolling stations have become more complicated to meet the growing demands for more steel grades involved and more types of sheet dimensions (1-25 mm thick and 600-2,000 mm wide). To meet the quality requirements, casting-rolling stations have been specially equipped with the necessary facilities to regulate sheets’ dimensions and attain the needed flatness.

Financial performance is the major driving force for development of casting-rolling stations for thin sections and for minimum hot strain during rolling. Continuous casting of billets with cross sections close to dimensions of the finished products can be the key to enhancement of metal’s competitiveness in the 21st century. Further development of casting-rolling stations will lead to establishment of new enterprises and will fundamentally change the pattern of metalmaking.

Figure 3. Temperatures of flat strips during casting in a curved-mold continuous caster

Figure 4. Distance among dendrite arms (s) depending on strip thickness (h):

(I – extra-thin slabs (quick solidification), II – strips, III – thin slabs, IV – slabs, V – semi-continuous-cast slabs)

 

Therefore, Ukraine should pay special attention to direct casting of flat strips and, what is more, to roll casting of steel strips. Direct casting of strips is based on application of mold with a moving surface (the second-type mold on figure 6).

Despite the fact that the technology and the project of direct rolling of cast liquid metal on the cooled rotating rollers were suggested by G. Bessemer as far back as in 1860, certain results in this sphere were achieved only a hundred years later [11]. Development of this process continues even at this point of time and flat-strip casters have already been created. In 2005 Russia plans to apply direct rolling to produce 1 million metric tons of steel. A number of countries have already applied this technology to cast steel plates, 2 to 8 mm in thick and 1,000 mm wide [12].

Krupp Stahl and VDM Nickel Technologies have introduced the process of double-roll continuous casting of high-alloy steels and nickel alloys [13]. Steel is cast between the two water-cooled rolls, 600 mm (the upper one) and 950 mm in diameter. Stand tilt and control over the level of liquid metal in the feeding unit make it possible to equalize the length of metal on both rolls. After convergence of solidified layers and reduction, the rolls produce a strip, 1.5-4.5 mm thick and 700-1,050 mm wide, at the speed of 5-60 m/sec and temperature of 1,100oC.

KELEIN-HUTTEN technology is one of the fresh techniques that employ direct continuous casting [12]. Moderate investments, low power consumption, small length of the mill, short metal-treating time, moderate number of operating personnel, absence of intermediate storage, and comparative simplicity of equipment are the major advantages of this technology.

Being equipped with internally-cooled rolls, 400-1,000 mm in diameter, that rotate synchronously, the double-roll units cast steel at the speed of 35 m/min and cool it at the speed of 100-1,000oC/sec, which results in uniform thin structure of solidified metal.

Special features of metal’s solidification during cooling and reduction in double-roll units (with rolls being located vertically (obliquely) and horizontally), as well as strip-drawing in a single-roll machine, require creation of control systems to manufacture broad and narrow strips (see figure 7).

Uneven build-up of the shell can lead to appearance of internal cracks during direct casting and rolling [12]. If a double-roll machine reduces strips in the end point of solidification zone, it can cause negative axial segregation and central ‘white strip’ defects.

Constant temperature control over the whole strip’s surface and appropriate computer monitoring of casting, cooling, and reduction can avert such flaws as non-flatness, waviness, heterogeneousness, unequal strip thickness, and other surface defects.

Figure 5. Development of casting-rolling technologies to manufacture HR flat strips: a – slab technology; b, c, d – thin-slab technologies (CSP, ISP, TSP respectively); e – strip manufacturing technology; f – direct thin-slab casting (1 – continuous caster, 2 – torches, 3 – quality control, 4 – furnace, 5 – coil box, 6 – rough mill, 7 – continuous mill, 8 – Steckel mill, 9 – planetary mill, 10 – cooling device, 11 – coiler)

 

Bearing in mind further cold rolling of thin strips, one should remember that steel would always rival light metals when it comes to production of homogeneous thin products with high strength and satisfactory cost.

To prevent scaling when casting carbon steel strips, NSC company has begun to use cooling chambers filled with inert gas.

Manufacturing of low-alloy steel sheets at BHP company features additional treatment of strips after direct casting and solidification. The cast strips are cooled down to 700oC, then reheated to 900-1,100oC, and cooled to 650oC at the speed of 200oC/sec. Subsequently, strips are coiled or additionally reheated and rolled at the temperature of 1,100oC in quarto stand with 30 to 50% reduction (to compact metal and obtain precise strip thickness). As a result, ferrite grains in such structures have the dimensions of 5-10 micrometers.

Other approaches to treatment of cast strips may include cold rolling as well.

The accumulated experience of direct casting of thin strips inspires many machine-building companies. The Russians have developed a new technology of direct casting in a double-roll machine with rolls, 800 mm (upper) and 1,200 mm (lower) in diameter [14]. Internal evaporative cooling of rolls lowers consumption of cooling water and accelerates the speed of casting up to 1 m/sec. This export-oriented casting-rolling station, only 20 m long from tundish to reel, is expected to produce up to 0.5 million metric tons of 5-61,000-mm strips using carbon and high-alloy steels (see figure 8). It should also be noted that it weighs (not including the lifting-turnover device for the casting ladle) only 140 metric tons.

Novokramatorsk Machine Building Plant designs casting-rolling stations equipped with direct casting and rolling in Ukraine.

It is pretty interesting to consider researches conducted by foreign scientists, in particular the comparative data [15] dealing with the construction cost of an aluminum smelter to manufacture aluminum sheets.

According to these calculations, a traditional smelter would cost more than USD 1 billion, while a mini-mill with the same capacity equipped with devices for high-speed direct casting and rolling of thin 1-mm strips would cost only USD 45 million.

The coming decade will be marked with greater introduction of steelmaking technologies related to direct reduction of iron, introduction of new types of electric arc furnaces, secondary steelmaking, continuous casting, production of billets with cross sections close to the finished products, and subsequent finish-rolling. This will result in creation of compact smelting-casting-rolling stations, which will revolutionize the concept of modern metalmaking mini-mills.

One can’t help but share the opinion [3] that today Ukraine should import technologies of thin slab production and fully automatic computer-controlled casting-rolling stations. At the same time, in the 21st century domestic machine-building companies will have to compete in manufacturing of a new generation of smelting-casting-rolling stations that produce strips with dimensions close to finished products.

Figure 6. Location of main components of a casting-rolling station that manufactures HR flat thin strips: a – casting of thin strips with subsequent rolling; b – direct casting (with possible reduction) with no subsequent rolling (continuous caster: 1 – curved-mold, 2 – conveyer-type, 3 – direct; 4 – rolling mill, 5 – coiler; hn and hr – thickness of cast and hot-rolled strips, P – productivity (output) per year)

 

To meet this purpose, Ukraine will have to adjust the priorities of scientific research in the sphere of metallurgy, develop automation and computer monitoring devices for metallurgical processes, establish a machine-building corporation (for instance, on the basis of Novokramatorsk Machine Building Plant) to deal with manufacturing of new advanced machinery featuring European quality and servicing.

Radical restructuring of Ukrainian metallurgical mills requires tremendous investments. This task is expensive, but there are no alternatives. Thus, attention should be given to step-by-step development and financing. Along with this, the upgraded mills will need widely skilled specialists in metallurgy.

Bibliography

Mazur V.L. // Metallurgy and mining. – 1998. – No.3. – p.1-3

Balvd V. // MPT – Metallurgical Plant and Technology. – 1991. – p. 68-85

Shevtsov A.Z. // Steel. – 1998. – No.5. – p. 2-4

Nill P., Entuan A. // MPT – Metallurgical Plant and Technology. – 1992. – p. 50-64

Molotilov B.V., Brozov A.A., Motorin V.N. // Steel. – 1997. – No.9. – p. 1-5

Iverson F.K., Busse K. // Stahl u. Eisen, 111. – 1991. – No.1. – p. 37

Kobagashi H., Masaoka T., Susuki M. // 6-th IISC, ISIJ. – 1990. – No.3. – p. 285

Wolf М.М. // Workpapers of metallurgical conference “Ferrous metallurgy in Russia and the CIS in the 21st century”. – Moscow, 1994. – book 3. – p. 142-147

Stagler L., Schnakenburg N.F. Horizontal continuous casting – abilities of new production lines. –МDН, 1998

Rosentahl P. // Steel Times International. – 1994. – No.1

Stepanov A.A., Zilberg Y.V., Neustuyev A.A. // Making sheet steel from liquid metal – Moscow: Metallurgy, 1978. 158 p

Fink F., Bandgiesen // Metallurgia, 37 – 1998. No.2. – p. 87-91

Hointrich R., Dunke M., Phunk Y. // Ferrous metal – 1991. – No.2. – p. 3-6

Timokhin О.А. // Steel. – 1998. – No.5. – p. 67-77

Latest technical developments in continuous casting. Aluminium, 73. Jahryang. – 1997. – No.10. – p. 685-690

Figure 7. Ways to direct continuous casting of flat thin strips: a – with side feed of liquid metal; b – with feed of liquid metal from the top; c – with strip drawing in a single-roll machine. (1 – liquid metal, 2 – strip, 3 – cooled rolls, 4 – feeder)

 

Figure 8. Casting-rolling technologies with a double-roll continuous unit to cast thin strips: a – by Krupp Stahl and VDM Nickel Technologies; b –by Tissen Stahl on the casting – compacting – rolling principle; c – by OZ firm and VNIIMetMash (1 – ladle, 2 – mold, 3 – compacting rolls, 4 – rolling mill, 5 – water-cooled rolls, 6 – looper, 7 – draw rolls, 8 – furnace, 9 – scale remover, 10 – rolling mill, 11 – torches, 12 – cooling device, 13 – coiler)

 

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