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1、ISSN 0967 0912, Steel in Translation, 2011, Vol. 41, No. 1, pp. 4147. Allerton Press, Inc., 2011. Original Russian Text G.N. Elanskii, I.F. Goncharevich, 2011, publishedtal,” 2011, No. 1, pp. 1421.Improving Mold Operation in Continuous Casting MachinesG. N. Elanskiia and I. F. GoncharevichbaMoscow S

2、tate Evening Metallurgicaltitute, Moscow, RussiabRussian Engineering Academy, Moscow, RussianAbstractMold operation with spring suspension and a programmable hydraulic drive is considered. Com puter methods of investigating the moldbillet interaction are developed. A new mold with longitudinal trans

3、verse vibration and dynamic stabilization has been developed for continuous casting machines.DOI: 10.3103/S0967091211010049The mold in a continuous casting machine is a complexmultifunctionalsystem.Thissystemincludes the mold itself, which is primarily a thermal unit, con trolling the heat transfer

4、from the steel melt that will form the continuous cast billet; a suspension system, which ensures specified billet trajectory and, if it is elastic (usually a spring system), also somewhat com pensates the dynamic loads; a drive ensuring specified vibration of the mold; and a supply system for the s

5、lag forming mixture. On melting in the gaps between the casing of the continuous cast billet and the mold walls, the slag forming mixture performs a series of functions, such as control of the thermal processes; lubrication with reduced drag on the billet as it moves through the mold; and removal of

6、 nonmetallic inclu sions from the melt. Note the important role of the lubricant in transforming frictional forces that do not depend on the speed (dry friction) into viscous fric tion. Viscous friction between the mold and the billet is essential for effective asymmetric vibration. Effec tive opera

7、tion of the mold system depends on smooth interaction of all the subsystems, which must perform their assigned functions.When continuous casting was first introduced, the mold was motionless. However, it was quickly estab lished that this process is problematic. Then recipro cating motion (rocking)

8、of the mold along the cast bil let was induced, with significant improvement in the process. The benefits of reciprocating motion are largely due to the replacement of static friction between the casing of the continuous cast billet and the mold walls by dynamic friction, which is less pro nounced a

9、nd more stable.When using large rocking speeds (higher than the billets extrusion rate), the interaction between the casing of the continuous cast billet and the mold walls is fundamentally changed. ome stages of motion, the mold walls outpace the billet, which was not previ ously the case. The excl

10、usively tensile force on the cas ing of the continuous cast billet in the stationary mold is replaced by compressive force ome stages of mold motion. It is also found that the damage to thebillet casing that sometimes appears when the mold outpaces the billet (when its speed exceeds the billets extr

11、usion rate) will be partially or completely elimi nated. This is a fundamental benefit of a rocking mold over a stationary mold.Detailed study of defect amelioration permits the development of special rocking conditions to maxi mize this effect. To this end, very complex suspension mechanisms and dr

12、ives are required. In practice, how ever, many of these systems are unwieldy and difficult to maintain; in other words, their operational effi ciency is poor. Special rocking conditions with unsmooth motion and dramatic changes in mold speed create considerable dynamic loads in the drive mechanisms,

13、 on account of the associated accelera tion. Therefore, in industry, molds with balllever sus pension operate predominantly mooth slow har monic processes characterized by low frequency and large amplitude, which are more stable and less dynamic.When steel plants began to increase the billets extrus

14、ion rate so as to improve productivity, it was nec essary to increase the molds rocking speed. This required increasing the rocking frequency (to which the rocking speed is proportional), because the rock ing amplitude could not be increased without impair ing the billet surface. As a result, the ac

15、celeration sharply increased (in proportion to the square of the rocking frequency) and hence the dynamic load increased. In many hinges with fixed technological gaps, impact loads arose, breaking the suspension 1. In those conditions, hinged lever suspension systems provedimpracticableand were repl

16、aced bydeformable elastic suspensions (specifically, spring suspensions), which had long been used in vibrational engineering. Thanks to the lack of free play, such suspensions per mit the mold to precisely track the specified billet con figuration (linear or curvilinear). Elastic spring sus pension

17、s proved extremely effective in practice, both in technological terms and in terms of simplifying the design of the continuous casting machine and reduc ing its cost.4142ELANSKII, GONCHAREVICHTo improve the billet produced by traditional con tinuous casting machines, we need to investigate the facto

18、rs responsible for unsatisfactory product quality. Industrial experience indicates that, in outdated con tinuous casting machines, the main factor reducing billet quality is imperfection of the molds hinged lever suspension. However, analysis shows that such suspensions cannot be replaced by spring

19、suspensions without changing many auxiliary systems. In particu lar, old and new continuous casting machines differ fundamentally in design. Therefore, replacing any sin gle component of an old system by new mechanisms unavoidably entails the tallation of appropriate auxiliary equipment, at greatcos

20、t.Accordingly, a low cost option is not to replace the entire suspension but to use elastic hinges, which have been satisfactorily employed in cyclic systems 1.As well as the equipment, continuous casting tech nology has been radically changed. In new molds with elastic suspension that are switched

21、to high frequency operation with low amplitude, asymmetric vibration proves more effective in technological terms. To ensure reliable maintenance of complex nonharmonic mold oscillation, programmable electrohydraulic drives are employed.Thus, there is a qualitative shift from traditional molds with

22、rigid kinematic elements and undeform able eccentric drives to machines with deformable linksandnonrigidhydraulicdrivesandfromharmonic vibration to spectrally more complex nonharmonic vibration. Whereas the rocking conditions are rigidly specified in molds with eccentric drives (provided there are n

23、o gaps in the hinges), the presence of elastic links in the new molds means that their motion is determined not only by the drive but also, to some extent, by the dynamic properties of the whole system consisting of the billet, the mold, and the continuous casting machines drive mechanisms.In develo

24、ping new casting processes, designers must take full account of these design changes and the new scope for the operation of continuous casting machines. Special research is required to make sense of the wide range of parameters for the new molds, to clarifythediversecriteriafortheassessment of casti

25、ng efficiency, and to reconcile the sometimes contradic tory technological and dynamic requirements. Thus, new approaches are required in view of the radical changes in design principles for the new molds and the powerful and continuous dynamic relation between the process and the operating conditio

26、ns of the equip ment.The further development of continuous casting technology requires the consideration of the whole complex machineload system. Note that the con tinuing increase in casting speed entails appropriate increase in amplitude of the rocking speed in any con ditions (including harmonic

27、conditions, which still predominate). On account of technological considerations, this entails increasing the carrier frequency of the vibrations, with considerable increase in dynamic loads in all the components of the system and in the mold drive.If we use special asymmetric nonharmonic vibra tion

28、s (containing higher harmonics), which are techno logically more effective, the mold acceleration and the corresponding inertial forces increase considerably. This increase in inertial forces is even greater than in harmonic conditions, since it is proportional to the square of the oscillation frequ

29、encies in the nonhar monic motion (including the higher harmonics). Accordingly, methods of reducing the dynamic load on the continuous casting machines must be developed.The dynamic loads may be reduced if the inertial forces of the rocking masses are compensated by the elastic forces of the spring

30、 suspension. The inertial forces are completely balanced when the eigenfre quency of the mold (determined by the rigidity/mass ratio of the spring suspension) matches the drive fre quency (in resonant conditions).Reduction in the dynamic load of drives in contin uous casting machines by ensuring res

31、onant condi tions with asymmetric rocking is complicated that the system only has only operating frequency, whereas asymmetric rocking of the mold is a polyfrequency process. Another difficulty is that the eigenfrequencies of existing molds are constant, specified in the design process (by the mold

32、mass and the rigidity of the spring suspension), and cannot be adjusted during mold operation, whereas the oscillation frequency is deter mined by the selected technological conditions and varies widely in the course of operation Therefore, the eigenfrequency of the mold must be established by optim

33、al design with inconsistent quality criteria 2, 3. Partial dynamic balancing of the mechanisms of con tinuous casting machines that operate in asymmetric polyharmonic conditions has been developed. Meth ods that permit maximum possible reduction in dynamic load by selecting optimal parameters of the

34、 spring suspension have also been formulated. It has been shown that continuous variation in oscillation frequency of the drive within each cycle imposes fun damental constraints that prevent the complete bal ancing of dynamic loads within the vibrating parts of the continuous casting machine.Thus,w

35、itching new generation molds of continuous casting machines to effective nonharmonicoperation, it is important to develop an optimal design method for continuous casting such that the dynamiccomplications may be reconciled. At present, progress is being made in that areain particular, thanks to intr

36、oduction of special biharmonic mold vibrations. We will now focus attention on the optimal combination of effective operation and dynamic bal ancing of the continuous castingmachine.STEEL IN TRANSLATIONVol. 41No. 12011IMPROVING MOLD OPERATION IN CONTINUOUS CASTING MACHINES43The measures considered n

37、ext facilitate the use of highlyefficientnonharmonicvibrationandsimultaneous reduction in dynamic loads within the molds drive.opposite direction. For mold motion in the same direction at a speed exceeding the extrusion rate, the mold walls outpace the billet, and the frictional force between them b

38、ecomes a motive force, with corre sponding decrease in mean drag forces over the cycle. According to theavailabledata, thecompressivestress in the casing is associated with 2030% decrease in the defects arising at the billet surface in the case of opposite motion of the mold and billet, when tensile

39、 forces act in the billet casing. With increase in the ratio between the times of mold operation in the same direction as the billet (positive motion) and the oppo site direction (negative motion), mold rocking becomes more effective, in technological terms. Anal ysisshowsthehighefficiencyofasymmetr

40、icrockingin new generation molds, especially when viscous drag predominates. Thus, with sufficiently asymmetric vibration, the speed may be significantly higher in the positive part of the cycle than in the negative part. The mean drag over the cycle also changes on account of mold rocking. The avai

41、lable data indicate relatively effi cient mold operation in asymmetric conditions, in terms of reduced mean drag (predominantly viscous drag, with a modest dry friction component) on the bil let as it travels through the mold. Because of the greater efficiency with viscous drag, it is expedient to o

42、rganize reliable lubrication in asymmetric vibration. Note that asymmetric conditions tend to increase the lubricant supply. In correctly selected conditions, the new gener ation molds more effectively reduce the mean drag on the billet in comparison with traditional molds.To assess the effectivenes

43、s of the rocking condi tionsin particular, to determine the stress in the bil let casing and the lubricant supplyphenomenologi cal inertial elastoviscoplastic models and correspond ing systems of nonlinear differential equations have been developed. The methods used in developing the models were out

44、lined in 412. These models may be used to select optimal rocking conditionsin terms of minimal internal stress of the billet casingwithout impairment of the systems dynamic properties. On that basis, there is a real possibility of selecting non harmonic rocking conditions while reducing the dynamic

45、loads on the drives of the continuous casting machine.Note that it is impossible to eliminate dynamic loads in the drives of the continuous casting machine with asymmetric polyharmonic operation, because there is only a single operating frequency, whereas the asymmetric rocking of the mold is polyha

46、rmonic; within a single cycle, the drive frequency varies con tinuously, while the eigenfrequency of the existing mold systems is constant.At present, a possible approach to efficient rocking of the mold and reduction in the dynamic loads on the driveistodevelopspecialbiharmonicmoldvibrations. As sh

47、own by computer experiments, this approach is relatively effective, both in technological terms and inINTERACTION OF THE CONTINUOUS CAST BILLET WITH THE MOLDS WALLSThe interaction of the continuous cast billet with the molds walls is affected not only by the conditions of mold vibration but also by

48、the supply of slag form ing mixture and its properties. According to current concepts, the slag forming mixture dissolves in the melt within the mold and mixes with the solid particles to form a coating with lubricant properties. Close to the meniscus, it acts as a viscous lubricant, and mea suremen

49、ts show that viscous friction forces predomi nate in this region. These forces are proportional to the relative velocity of the frictional pair (the billet and the mold wall).On moving away from the meniscus, viscoplastic friction (viscousdry friction) is observed; this force depends less on the rel

50、ative speed of the billet and mold and beg to depend on the pressure of the billet casingat the wall. A proportion of viscoproportion of dry friction fassert that dry friction largely acts w the mold; its magnitude depends onhe nd the Specialistshe billet leaves ce pressingthe continuous cast billet

51、 agaWemay also assume that this effect is due to the maximum ferrostatic pressure on the billet casing at its exit from the mold. Thus, in model research, these experimental laws should be reproduced.Note that these processes are also accompanied by increase in thickness of the billet casing. Accord

52、ingly, the stress in the casing declines on moving toward the molds exit, despite the increase in frictional forces. The casing usually breaks down in the meniscus region, especially on account of the increase tress due to the unfavorable balance of the forces acting and the strength of thecasing.St

53、udy of the formation of drag on the billet in the mold is important not only to develop preventive mea sures, but also so as to reduce the extrusion forces of the blank and reduce the load in the tractional mech anism. This requires appropriate selection of the com position of the slag forming mixtu

54、re, its delivery con ditions, and the rocking parameters of the mold. In addition, it is important to formulate rocking condi tions corresponding to sufficient lubricant supply, specified billet motion, and reduced frictional forces.The interaction of the billet with the mold walls depends primarily

55、 on their relative speed, which deter mines the viscousdry frictional forces. When their relative speed is reversed, the direction of action of the frictional forces changes. The efficiency of mold oper ation is characterized by the ratio of the times of mold operation in the same direction as the b

56、illet and theSTEEL IN TRANSLATIONVol. 41No. 1201144ELANSKII, GONCHAREVICH40200(a)5Ps PsDGP 2040600012(b)345550024625Phase angle of drive0P PFig. 2. Uncompensated dynamic load G from the rocking load that acts on the continuous casting machines drive when using hinged lever suspension and load D comp

57、en sated by the restoring forces of the spring suspension with biharmonic mold vibration.255075012(c)345mold. In other words, we analyze the feasibility and expediency of combining the initial stage of reduction with casting. We briefly review the necessary precondi tions for such an approach.The op

58、erational efficiency of the mold is primarily determined by its rocking conditions along the billet axis. The reduction cell is pressed aga t the mold by transverseforces. Sincerockingoccurs alongthebillet axis, the frictional forces between the mold walls and the billet produce tensioncompression s

59、tress in the billet casing. This will considerably affect the billet quality and the overall stability of the process.In longitudinal rocking, the frictional force at the moldbillet casing may only be regulated when the forces between them depend on their relative speed (that is, viscous friction forces). In t