ISSN 09670912, Steel in Translation, 2012, Vol. 42, No. 11, pp. 784–786. © Allerton Press, Inc., 2012. Original Russian Text © V.N. Lebedev, A.A. Podosyan, A.E. Pozin, I.E. Petrov, K.N. Vdovin, 2012, published in “Stal’,” 2012, No. 11, pp. 62–66.

Molds with Narrow Walls of Variable Taper in ContinuousCasting Machines V. N. Lebedeva, A. A. Podosyana, A. E. Pozina, I. E. Petrova, and K. N. Vdovinb a

ZAO Mekhanoremontnyi Kompleks, Magnitogorsk, Russia Nosov Magnitorsk State Technical University, Magnitogorsk, Russia

b

Abstract—New designs are proposed for the narrow mold walls in continuouscasting machines at OAO Magnitogorskii Metallurgicheskii Kombinat. The taper of the new walls varies over the height so as allow for shrinkage of the cast billet. DOI: 10.3103/S0967091212110046

Effective maintenance and repair of metallurgical equipment is one of the key factors in ensuring the competitiveness of steel plants. Accordingly, well trained repair staffs must be on hand, capable of resolving problems of any complexity. At the same time, it is essential to introduce new technologies throughout Russian industry. That largely depends on the creative thinking of plant management. OAO Magnitogorskii Metallurgicheskii Kombinat (MMK) has always been at the forefront of engineer ing progress. Currently, the challenge facing the plant is to maximize the output of products with high added value. To this end, stateoftheart equipment has recently been introduced: the 5000 rolling mill, the MNLZ6 continuouscasting machine, the APP2 ladletreatment system, and the 2000 mill. OAO MMK uses an advanced model of service integration, so as to respond flexibly to emerging and potential risks. The repair divisions are united as ZAO Mekhanoremontnyi Kompleks (MRK), which includes uptodate machining, forging, casting, and repair shops, a metalworking shop, an engineering department, and auxiliary services. Dynamic solution of any problem is possible as a result of the rational organization of ZAO MRK and the breadth of techno logical capabilities. This new approach to the organi zation of metallurgical repair combines manufactur ing and repair principles. ZAO MRK serves all the shops in OAO MMK, including complex systems such as continuouscasting machines. One of the basic characteristics of continuous casting machines is the life of the equipment, espe cially the molds and the first sections, for which the maintenance protocols are minimal. Depending on whether it is of one or twostrand type, the mold con sists of four or eight broad and narrow copper walls attached to a steel base. The walls form a rectangular cavity, in which the metal solidifies on casting, with the formation of a crust around the developing slab. As it operates, the mold performs specified reciprocating

motion, while the slab, which is initially fixed at the starter bar, is gradually extruded from the mold, at fixed speed. Slagforming mixture is employed to facilitate extrusion and separation of the crust from the copper walls. In casting, that mixture is added to the mold in portions. Such operating conditions result in fast erosion of the copper walls—especially the narrow walls. Conse quently, the continuouscasting machine must be shut down frequently for mold replacement. Clearly, the losses associated with the interruption of casting and the need to keep a large stock of copper walls on hand pose a serious problem. Hence, analysis of the erosion in order to extend mold life must be a high priority. Currently MSr copper (with 0.8–1.2% silver) is used to produce the narrow mold walls at OAO MMK. The mold life between repairs is 135 melts for con tinuouscasting machines 2 and 3 and 100 melts for machines 1 and 4. Eight repairs are possible at the nar row walls, and fourteen repairs at the broad walls. At OAO MMK, it is found that the narrow walls are mainly worn in the lower part of the copper, as a result of abrasion of the copper over a length of 500– 700 mm, with further wear at the working ends and in the regions of radii 8000 and 7740 mm (in sections of width up to 70 mm). The mean wear at the narrow walls is 2.1 mm. For engineering calculations, an empirical formula may be used to calculate the wear H (mm) of the nar row mold walls, when the length of the copper section is 950 mm and the extrusion rate is 0.7 m/min H = 0.0003K 1.6782, where H is the value of wear and K is the number of melts. The wear is due to the difference between the mold parameters and the actual shape of the slab formed in extrusion (Fig. 1). In practice, the shrinkage of the slag (decrease in its size over the mold height) is compen sated by special inserts in the narrow copper walls.

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Distance from meniscus, mm

MOLDS WITH NARROW WALLS OF VARIABLE TAPER

785

α1

0 200 400 600 800 1000 1200 1400

Plane I

1

2

0

1

2 3 4 5 Shrinkage, mm

Plane II

6 Plane III

Fig. 1. Profiles of continuouscast ingot (1) and narrow mold wall with 1% taper (2).

The cross section through which the slab moves may be characterized as a trapezium, with its smaller base in the first section. The angle between the sides of the trapezium (the narrow copper walls) and the mold’s vertical axis is called the taper. The taper takes account of approximately 1% ingot shrinkage and is specially calculated for each slab cross section. Insuf ficient taper results in a gap between the ingot crust and the cooled wall, according to the data in [1]; excessive taper increases the frictional force, which may lead to ingot rupture. This approach takes no account of the actual slab shrinkage, which is nonlinear, as is clear from the empirical formula already given. As we see in Fig. 1, the slab begins to expand beyond the cavity formed by the narrow walls in the lower part of the mold. That results in wear of the walls and all the associated prob lems. Therefore, we need to develop an optimal mold profile corresponding to slab shrinkage. Operational experience shows that the wear rate of the walls decreases over time, as the working surface approaches its optimal form. According to data from Kolokol’tsev and Vdovin, the shrinkage in the solid state and the shrinkage of the continuouscast ingot depend on many parameters, such as the properties of the steel, the type of continuouscasting machine, and the casting conditions. The use of narrow copper walls with a complex par abolic profile matching slab shrinkage has been pro posed by Siemens VAI, the Central Research Institute of Ferrous Metallurgy, and OOO Korad. However, the manufacture of such walls requires expensive numeri cally controlled equipment, and the adjustment and monitoring of the walls entails the acquisition of appropriate measuring equipment. In addition, the machining time for the walls is much increased, and consequently the mold’s repair time is increased. A new design for the narrow mold walls in the con tinuouscasting machines at OAO MMK’s oxygen converter shop has been developed by ZAO MRK spe cialists, in collaboration with Nosov Magnitorsk State Technical University. The taper of these walls varies STEEL IN TRANSLATION

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α2 Fig. 2. Threeplane mold wall for continuouscasting machines 1 and 4.

(Fig. 2). No special equipment is required for the manufacture of such walls; standard milling machines may be employed. Likewise, there is no need to acquire additional measuring instruments; the walls are adjusted by standard instruments on the basis of taper measurement. First of all, four sets of walls with the new design were produced for continuouscasting machines 1 and 4, and four experimental molds were constructed. A special formula was used to calculate the corresponding cavity parameters. The life of the experimental molds was 129, 111, 117, and 113 melts, in comparison with a requirement of 100 molds. After operation, the wear of the narrow walls was measured. The mean wear was 0.5–0.9 mm (as against 1.8–2.5 mm for standard walls); again, that is better than the requirement (1.5 mm). The molds were removed from operation not because of wear but because of production requirements. The life of the new walls may exceed that of stan dard walls by a factor of 1.5–2, because the pressure distribution over the copper wall is more uniform on account of two additional planes set at the rational angles α1 and α2 (Fig. 2). In experimental operation, other benefits of the new walls were noted: in particu lar, a slab crust that is more isotropic and generally of higher quality, evidently on account of the improved heat transfer through the redesigned walls (Fig. 3). The positive experience with the new walls permitted their recommendation for mass production. Analysis of temperature data for the copper in differ ent conditions over the mold height indicates that the drop in water temperature (and hence ingot tempera ture) at the experimental walls is greater than normal in the upper section (at a height of 150–300 mm above the

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LEBEDEV et al. (a)

α1

(b)

260–8.75

B

1

2

2 A

3

√16

3 √16

1

4

4 α2

Wall temperature, deg

Fig. 3. Configuration of ingot and standard (a) and exper imental (b) narrow mold wall: (1) ingot; (2) watercooled copper plate; (3) narrow steel wall of mold; (4) ingot crust.

110

C 259–8.75

Fig. 5. Modified threeplane mold wall (with facets).

100 3

90 80

1

4

2

70 100 200 300 400 500 Height above upper section of copper walls, mm

Fig. 4. Temperature data for experimental left (1) and right (2) walls and standard left (3) and right (4) walls.

upper section). The difference in copper temperature at heights of 150 and 300 mm is 12.8°C for the standard walls and 17.3°C for the experimental walls (a differ ence of 35%). That indicates more intense heat transfer between the ingot and mold wall and consequently faster crust formation (and a thicker crust) in the upper zone of the mold (Fig. 4). OAO MMK has taken out a patent on the new wall design [2]. The next step in optimizing the narrow walls is modification of the threeplane molds on the basis of operational experience: specifically, the addition of lateral wedge sections so as to create more zones with reduced contact pressure of the billet edges on the nar row walls. Since the edges of the blank are harder than the faces, such zones will tend to reduce slab–wall friction, with further reduction in wear and increase in mold life. Special wedge facets create such zones at the lower plane of the walls over surfaces of radius 8000 and 7740 mm (Fig. 5). That wall design has also been patented [3]. The use of copper walls of variable taper increases mold life by a factor of ~1.5, without the need to apply hard coatings. The quality of the cast slab is also enhanced. The introduction of the new walls with wedge facets has extended mold life by 30% in OAO MMK’s oxygenconverter shop. There is the potential

for 100% increase in mold life. Further research (undertaken in collaboration with Nosov Magnitorsk State Technical University) is aimed at closer harmo nization of the narrow walls with slab shrinkage, on the basis of third and fourthorder curves. ZAO MRK is capable of generating such profiles by using the latest numerically controlled FRF200 machine tools. A new design of the narrow copper walls is currently under development (in collaboration with Nosov Mag nitorsk State Technical University). This profile includes special copper inserts in the shape of hexahe dral prisms; the base is an irregular hexagon. The work ing surface of these inserts, which conforms to the dis continuous contour of the three lateral faces, produces transition regions of the highly optimized multifaceted slab. In the back of the inserts, cooling channels of spe cial form are cut. As a result, the continuouscast ingot changes shape from rectangular to multifaceted, which reduces the likelihood of crack formation at the corners of the slab and reduces wall wear [4]. The economic benefit of the new mold walls in OAO MMK’s oxygenconverter shop is determined by the reduced downtime of the continuouscasting machines for mold replacement and the increased mold life. At present, the copper consumption in the molds has been reduced by at least 15%. REFERENCES 1. Podosyan, A.A., Pozin, A.E., Zav’yakov, V.I., et al., Development and Operation of Molds in Continuous Casting Machines with Narrow Walls of Variable Taper, Stal’, 2008, no. 7, pp. 40–41. 2. Vdovin, K.N., Zakharov, I.M., Sarychev, A.V., et al., Russian Patent 79815, 2009. 3. Berdnikov, S.N., Galkin, V.V., Vdovin, K.N., et al., Russian Patent 97660, 2010. 4. Galkin, V.V., Berdnikov, S.N., Vdovin, K.N., et al., Russian Patent 113684, 2012. STEEL IN TRANSLATION

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