ISSN 1068-798X, Russian Engineering Research, 2009, Vol. 29, No. 12, pp. 1237–1240. © Allerton Press, Inc., 2009. Original Russian Text © A.V. Kuklev, I.F. Goncharevich, Yu.M. Aizin, 2009, published in Vestnik Mashinostroeniya, 2009, No. 12, pp. 42–46.
Efficient Lubricant Supply to Molds in Continuous-Casting Machines A. V. Kuklev, I. F. Goncharevich, and Yu. M. Aizin ZAO Korad Abstract—Software to provide information on continuous steel casting is based on a rheological model of the system consisting of the mold, the lubricant, and the casting. Using this software, the most effective mold configurations and parameter ranges may be selected; their influence on factors such as the formation of the mold surface and the resistance to steel passage through the mold may be investigated; and the supply of slag-forming mixture may be optimized. DOI: 10.3103/S1068798X09120090
The transmission of vibrations to the mold of a continuous-casting machine proved highly effective and was introduced on an industrial scale. Today, however, methods of improving mold vibration are required. The fact is that, in the continuous casting of steel, the frictional forces between the walls of the vibrating mold and the billet produce tension–compression stress in the shell of the ingot, which considerably affects the quality of the cast metal and the stability of the casting process as a whole. The stress and strain arising in the ingot shell also depend on its temperature, the type of steel being cast, and other factors, but the frictional force is the primary factor. With considerable frictional force, the ingot shell formed in the mold is deformed and may even rupture, since its strength is relatively low close to the melting point. The most widespread and serious disruption of the casting process is tearing of the metal. In most case, tearing occurs on leaving the mold, predominantly as a result of previous rupture of the ingot shell in the upper part of the mold. In practically any conditions of mold vibration, alternating compressive and tensile stress appears in the casting under the action of the frictional force at its wall. In the case of stable casting and stable ingot quality, tensile stress is undesirable. Therefore, various preventive measures have been developed, including special conditions of mold vibration to minimize this stress, especially in the weakest section near the meniscus, which is predisposed to rupture. It is also found that hazardous stress in the ingot shell arises in the case of adhesion or welding to the mold wall at individual points. It has been suggested that such phenomena may be reduced by polyharmonic vibrations with high-frequency harmonics. In mold design, particular attention focuses on the near-meniscus section, where adhesion of the casting shell to be mold is observed. In many cases, this leads to jamming of the ingot in the mold. Experimental data
indicate that the mold–casting interaction is extremely complex. Tight contact over the whole length is not encountered; likewise, a guaranteed gap is not maintained between the ingot and the mold. The contact zone is not constant. When the ingot jams in the mold, extreme extension of the shell leads to rupture in the near-meniscus region. Liquid metal penetrates into the gap that forms and solidifies there. If the patch formed within the gap in the shell cannot withstand the load required to break the adhesion, then the shell again ruptures. In ingot extension, the rupture moves toward the mold exit and is responsible for tearing of the metal, which disrupts the normal casting process. Several methods have now been developed to adjust the frictional force between the ingot and the mold wall—for example, special coatings for the mold walls or special vibration conditions. A very common approach is to use technological lubricant in the mold. The goal is to ensure stable casting by the adequate supply of lubricant to the space between the ingot and mold surfaces. The rate and uniformity of lubricant supply to the mold are the key factors here. The data suggest that well-organized lubricant supply reduces the drag by a factor of 1.5–2.5. Effective lubrication of the mold in a continuouscasting machine includes two aspects: (1) standardized uniform supply of casting powder and its distribution over the whole mold cross section; on melting, the casting powder forms the lubricant; (2) mold vibration so as to ensure the required supply of casting powder (and hence lubricant) to the gap between the mold wall and the casting. The influence of the configuration of the melt meniscus on the lubricant supply to the mold has been studied in sufficient detail in the foreign literature. In the present work, we will not dwell on these problems. Rather, we consider the interaction of the lubricant with
1237
1238
KUKLEV et al.
the mold walls and the casting shell over the whole contact section—from the mold input to the mold output. To this end, we have developed a computer model of the system consisting of the mold, the lubricant layer, and the casting. This system is represented by an inertial (two-mass) model of the lubricant in the continuous-casting machine, with three contact zones: between the mold wall and the first lubricant layer; between the first lubricant layer and the second lubricant layer; and between the second lubricant layer and the casting. The multilayer model permits analysis of the deformation processes in the lubricant layer and their influence on the formation of the casting shell and its defects. Comprehensive research permits the development of means of supplying casting powder and conditions of mold vibration such that the powder is supplied in the molten state—as lubricant—to the space between the casting and the mold wall. In current (mainly non-Russian) practice, granulated slag-forming mixture is employed. It is generally supplied to the mold by pneumatic means. However, this approach is not well suited to the supply of steelcasting powder at Russian plants, because there is a considerable risk that the mixture will become segregated during transfer, on account of the different aerodynamic properties of its components. Outside Russia, more complex and expensive equipment based mainly on screw conveyers are mainly used to supply casting powders to the mold. At ZAO Korad, a simple vibrational unit has been developed for the supply of slag-forming powders to the molds of continuous-casting machines. Bench tests show that a vibrational sensor of simple design ensures uniform distribution of the powder over the specified surface. An even simpler and cheaper mechanism is under development, in the form of a mass-produced screw conveyer with vibrating units distributing the steel-casting powder over the mold surface. The supply of mixture to the mold may be studied on the basis of inertial phenomenological models (the rheology of nonsteady processes), since the lubricant and melt that pass through a mold operating at high-frequency are subjected to inertial loads comparable with the other forces present. Therefore, ignoring these loads may lead to considerable errors. Note also that, at high frequency (vibrational-pulsed conditions), effects that are not seen in slow equipment appear in the system: inertial forces; change in the action of the gravitational forces; decrease in dry and viscous friction; decrease in the limit of plastic deformation; positive or negative change in the density of the disperse media; fast eddies and slow circulation of the solid, liquid, and hybrid (solid–liquid–gas) media, with accompanying fluidization and vibrational boiling; and acceleration of energy and mass transfer. The meniscus may be formed somewhat differently at a vibrating mold wall than at a stationary wall, although this possibility requires more detailed study. In high-frequency
perturbation, continuous casting may be accompanied by complete breakdown of the physicochemical structures in the disperse systems, with increase in the efficiency of many technological processes; and the casting structure may improve under intense vibration of the crystallizing metal. The proposed methods take into account that the interaction of the melt with the mold walls is accompanied by variation in the frictional force over the whole length, in the casting parameters, and in the stress– strain state. Since these changes are not known in advance, the model of the melt includes a logical system to track its configuration at all stages and to continuously adjust its structure and parameters. The selfconfiguration system of the model permits physically more reliable reproduction of technological processes of any complexity at all stages. Accuracy of the phenomenological models is ensured by calibration with full-scale test results. The surface of the cast metals depends significantly on the conditions in which the casting shell is formed in the mold; and on the rate and uniformity of casting-mixture supply to the mold. The need to improve the quality of the surface and the subsurface layer of the billet (to reduce the risk associated with mold vibration, to reduce the height of folds, to eliminate microscopic creases and cracks, etc.) entails multicriterial optimization of vibration in terms of lubricant supply to the mold. The methods developed permit analytical investigation of the influence of mold vibration on the formation of the ingot surface and the resistance to ingot motion through the mold; optimization of the supply of slagforming mixture; and selection of the most effective mold configurations and vibrational parameters. The corresponding software provides complete information on the casting process, in tabular and graphical forms: —kinematic and dynamic characteristics—deformation and displacement and the corresponding rates and accelerations; —stresses and forces—elastic, viscous, and elastoviscous stress; plastic stress with and without hardening; and normal and tangential forces at the mold wall; —energy characteristics—the energy consumption in elastic, viscous, and plastic deformation of the ingot and its mechanical displacement; circulation of potential and kinetic energy in the solidifying ingot during periodic deformation; and energy consumption in overcoming all the drag forces. Various special characteristics may also be tracked. We may consider the formation of asymmetric operating conditions and lubricant supply to new-generation molds with elastic spring suspension and a hydraulic drive, for the example of an asymmetric vibration pattern adopted in industry and some new possibilities developed by ZAO Korad. The results are analyzed and compared with harmonic mold oscillation at the same frequencies and amplitudes. Special mold vibration may permit very efficient operation of the continuous-
RUSSIAN ENGINEERING RESEARCH
Vol. 29
No. 12
2009
EFFICIENT LUBRICANT SUPPLY TO MOLDS
casting machine, in some respects. The proposed methods are universal and may be used for the analysis of existing and new patterns of mold vibration. The models take account of the influence of the dynamic properties of the system consisting of the billet, the mold, and the tractional mechanism on the operating conditions. The selection of the mold eigenfrequencies (the relations between the rigidities of the spring suspension and the vibrating mass) for operation with considerable asymmetry is considered. The next step is further discussion of the computer determination of the lubricant volume supplied to the cavity between the mold wall and the casting, with specified mold configuration and vibration parameters and specified casting speed. The lubricant is simulated by an inertial two-mass elastoviscoplastic model. The first lubricant layer is in viscoplastic contact with the mold wall; the second layer is in viscoplastic contact with the casting. The two lubricant layers are in elastoviscoplastic contact. The action of gravitational forces on the lubricant layers is taken into account. Thus, the model is able to reproduce the elastoviscoplastic and inertial forces within the lubricant layers and at the contacts with the mold wall and casting and the circulation and deformation processes within the two-layer lubricant. The resistance to vibration of the mold under load is determined, as well as the resistance to casting motion. The expanded model also takes account of the variation in lubricant supply under the influence of the ring of hardening lubricant and the melt meniscus above the mold; and the temperature-dependent change in viscoplastic characteristics of the lubricant over the height of the mold. The model of the lubricant layer between the mold wall and the casting is intended to permit rapid and inexpensive preliminary testing of newly designed mold configurations and vibration conditions, in terms of the efficiency of lubricant supply in the quantities required by the process. The software is universal and suitable for the investigation of any conditions of casting extension and mold vibration, when using slagforming mixtures of any composition. The software may be successfully used by plant personnel to modernize production processes on the basis of minimal consultations with its developer. We will present some examples in which we determine the required lubricant supply rate to the mold; the characteristics of the lubricant are assumed to be same for all the operating conditions considered. Harmonic vibration of the mold is considered, for a number of known asymmetric vibration conditions and some new special vibration conditions, with identical amplitudes and carrier frequencies but different configurations of mold motion. The lubricant parameters are arbitrary, but are the same in all cases. It is established with sufficient reliability that, in fact, the efficient vibration conditions are not universal but may only be estabRUSSIAN ENGINEERING RESEARCH
Vol. 29
No. 12
1239
vlu 0.1
(a)
0
vlu
vlu me
–0.1 –0.2 –0.3 vlu 0
(b) vlu
–0.1 vlu me
–0.2 –0.3 10 v1lu 0.2
15
20
25
30
(c)
0
v1lu
–0.2 –0.4 –0.6 15 σ 1
20
σ
25 (d) Δ = –1
30
0 –1 R 30 20 10 0 –10 –20 –30 10
(e)
Δ = –1
R
15
20 ϕ, rad
25
30
Figure.
lished if all the casting parameters are taken into account (in particular, the rheological properties of the lubricant). Additional research shows that the proposed method may be useful in the preliminary selection of the slagforming mixture for a specific process (selection of the slag-forming mixture for specific mold vibration and also modification of the mixture properties to ensure the required rate of supply). The software provides relatively detailed information on the processes in the lubricant layer between the 2009
1240
KUKLEV et al.
mold walls and the casting and analysis of the stresses that arise there, in terms of factors such as the bulk flow rate (vlu) of lubricant to the mold in special nonharmonic vibration (Fig. a) and harmonic vibration (Fig. b); the lubricant flow rate vlu at contact with the mold (Fig. c); the elastoviscoplastic deformation; the internal elastoviscoplastic stress σ in the lubricant layer (Fig. d); and the total viscoplastic force R applied to the mold (Fig. e). In Figure, ϕ denotes the angle of drive rotation; Δ is the asymmetry coefficient. The information obtained from the calculations may be used in solving related problems such as control of thermal conditions in the mold; improvement of the casting surface; prevention of crack formation in the casting; and decrease in the resistance to casting motion in the mold. Note that the analytical method and software considered in the present work permits analysis of the consequences of new designs prior to manufacturing the equipment or switching to the new operating conditions, with minimal expenditure of time and effort. Note also that, as a rule, the problems considered are multicriterial. In other words, a beneficial change in
one parameter will not always improve the others. Therefore, we draw the reader’s attention to the papers on the optimal design of metallurgical equipment that have been published in the journal Stal in recent years. REFERENCES 1. Kuklev, A.Z., Aizin, Yu., Goncharevich, I.F., and Danilov, V.L., Computer Methods of Estimation of Efficient Process Conditions of the CCM’s Casting Mold, Sixth European Conference on Continuous Casting (ECC), Rechione, Italy, 2008. 2. Gusev, B. and Goncharevich, I., Vibration and Wave Technologies, Aims of Future of Engineering: Proceedings, Hong Kong, 2005. 3. Gusev, B.V. and Goncharevich, I.F., Development and Identification of Bulk Inertial Elastoviscoplastic Phenomenological Models for the Investigation of Vibrational Processing of Disperse Media in the Production of Composites and Special Alloys, Fifth Moscow International Conference on the Theory and Practice of Technologies of Manufacturing Composite Materials and New Metal Alloy Products (TMCMM), Moscow, 2008.
RUSSIAN ENGINEERING RESEARCH
Vol. 29
No. 12
2009
