MELTER

HOLDER

FURNACE

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HOLDER

FLYING SAW

\ILTER UNIT FILTER & DEGAS

\BELT CONVEYOR

CASTING UNIT

Fig. 1-Schematic horizontal continuous casting system. '

FILTER & DEGAS

CASTING UNIT

Fig. 2-Melting and melt treatment system for continuous casting.

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Continuous casting of aluminum by R. E. Spear and K. J. Brondyke Horizontal, continuous casting of aluminum ingot is discussed with special attention directed to metal supply, mold package, mold lubrication, ingot cooling, and alignment of equipment. In most instances, requirements are more rigorous than with vertical, discontinuous direct chill casting. Horizontal casting necessitates pressure-tight sealing during metal transfer to the mold and compensations for convection and gravity within the mold. Continuous casting requires a constant metal supply and mold equipment capable of producing acceptable ingot for extended periods without ingot quality deterioration.

R. E. SPEAR is Chief of the Ingot Casting Division and K. J. BRONDYKE is Assistant Director of Alcoa Research Laboratories, New Kensington, Pa. Paper presented at 1970 TMS Annual Meeting.

36-JOURNAL OF METALS, APRIL 1971

INTRODUCTION Since development of the direct chill (DC) process by W. T. Ennor of Alcoa in the 1930's,1 virtually all aluminum ingots have been cast by some variation of the DC process. In the past several years, considerable interest has developed in a horizontal version of DC casting. As one might expect, substantial changes must be made from normal DC casting to permit commercial horizontal casting. This paper discusses the unique features of horizontal casting and of continuous operation. A major advantage of horizontal casting is that long ingots can be cast without the necessity of increasing plant head room, digging deep casting pits or installing large capacity cranes for ingot removal from the pits. Justification for continuous rather than semicontinuous operation usually is dependent upon economic considerations. Recovery is normally higher because beginning and end scrap is a much smaller portion of the total ingot length. Automation is more readily achievable with the advantages of better control and reduction in manpower. Continuous casting also permits higher productivity per strand than discontinuous operation, primarily because a shorter portion of the overall casting cycle is devoted to starting and stopping. GENERAL DESCRIPTION A horizontal casting system for continuous Operation is illustrated schematically in Fig. 1. Molten metal is transferred from a holding furnace, preferably through a metal treating unit, to the casting basin and then into the water-cooled mold where a solid ingot shell is formed. The ingot is withdrawn on

INSULATION BOARD

Fig. 4-Feed slot in insulotion boord for horizontal casting .

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Fig. 3-Mold packoge for continuous costing.

METAL SUPPLY Metal supply for a continuous casting process is more critical than for a batch process. In the aluminum industry, furnace charging, melting, alloying, and to some extent, fiuxing in the furnace are basically batch operations. Organizing these operations so that continuous metal fiow is available at the casting unit requires careful planning and control. The rate a t which molten metal is supplied to the casting unit must satisfy the cast rate, otherwise the casting operation must be slowed or m et al must be poured which has not been properly prepared. In either event, acceptable production off the casting u nit will suffer. A metal supply system designed to accomm odate continuous casting is illustrated in Fig. 2. The complex consists of a melting furnace which feeds two holding furnaces. The holding furnaces are used for alloying and, if necessary, fiuxing of the charge. The holders operate so that while one is being tapped into the casting unit, the other is being supplied from the melter. Metal from the holder passes through an inline processing unit which filters and degasses the melt." Improved fiexibility can be achieved by cross linking the processing and casting units so tha t high quality m etal can be supplied to either or both cast-

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ME TAL ENTRY

a moving belt, which controls the casting speed, and conveyed to an automatic fiying saw. Sawed sections are carried to a fabricating unit or to stacking and packaging equipment. Ultrasonic inspection of the ingot near the casting unit is desirable to detect ingot defects which would result in scrap. Placement of the ultrasonic insp ection unit n ear the mold reduces the amount of ingot scrap before corrective measures can b e taken to eliminate the cause of scrap.

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ing units continuously. Because the processing units are located immediately upstream of the castin g unit, the metal is brought to its highest quality just before casting and subsequent deter ioration of quality is unlikely. By reducing or eliminating furnace fiuxing in-line treatment can decrease furnace cycle time, and improve the ability of a furnace complex to supply adequ ate amounts of malten metal to a continuous casting complex.

MOLO PACKAGE Molds for hor izontal, continuous casting are of necessity more complex than for DC casting. A typical mold p ackage is illustrated in Fig. 3. Metal is conveyed from the fiuxing unit into the pouring basin and then through a section of insulating boar d into the mold cavity. The p r imary function of the insulating board is to act as a seal against m etal leakage b etween the mold and the pouring basin. The insulating board also is designed as a means to con trol and direct metal fiow into the mold to compensate for the effect of convection. Fig . 4 shows a typical feed slot for casting 6-in. diameter ingot. The opening for this ingot size is a semicircular arc approximately lj4 -in. wide, and lfz in. fr om the mold wall-castin g su rface at the bottom. The d esign and location of this slot are important for the p r oduction of high quality ingot." If m etal entering the casting cavity rises too quickly, the ingot will have excessive lapping along the bottarn surface and will r un h ot on top. If t h e slot directs hot metal too near the mold APRIL 1971, JOURNAL OF METALS-37

4

bottom, the ingot shell will be weakened causing tearing and bleedouts. Between the insulating board and the mold proper (Fig. 3) there is a thin metal ring acting as a cover plate for the mold lubrication system! Under most casting conditions, the initial ingot shell will start to solidify on the bore surface of this cover plate. Consequently, a high heat flux, usually greater than 100 Btu/in.•-min, is conducted through the cover plate into the mold. Therefore, the plate must have high thermal conductivity and be firmly attached to the mold so that thermal resistance across the mold-eover plate interface is low. Otherwise, the cover plate will overheat, and shell tearing will result. Selection of mold material is dependent upon casting conditions. Formost applications, molds are made of aluminum 6061 alloy. However, in a few instances, copper has been used to accommodate an especially high heat load such as that associated with extremely fast casting rates. Mold lengths are typical of those used in DC casting and are generally in the range of 1lfz to 6 in. Unlike DC casting where grease or oil is manually applied to the mold wall between casts, in continuous casting the mold bore is continuously lubricated with oil.

MOLO LUBRICANT In horizontal continuous casting, control of mold lubricant is an important feature . In many instances, termination of casts can be ascribed to improper control of the lubrication. As shown in Fig. 3, oil is introduced through the mold body to an annular channel and then through small grooves to the mold bore. The grooves must be designed to provide uniform distribution to the mold bore. It is important that the cover plate fit tightly to the mold so that excess oil cannot flow to one area of the mold bore at the expense of flow elsewhere. The specific qualities which make an oil acceptable as a mold lubricant are not yet weil established. The lubricant should not form excessive amounts of vapor nor leave a varnish residue on the mold surface. Even a low rate of varnish formation can be detrimental to thermal transfer in a continuous casting system since the mold must operate for extended periods. A good mold lubricant also should have a small change in viscosity with temperature variations. Flowing through the mold, the oil may change in temperature from 40-300 °F. Table I shows the change in viscosity with temperature for two standard DC lubricants (lard oil and castor oil), and two new lubricants. Lard oil has an acceptable viscosity index but the 40 °F pour point could result in its "freezing" in the mold. The low viscosity index of castor oil makes attainment of uniform oil flow virtually impossible. Both of the new lubricants listed in 38-JOURNAL OF METALS, APRIL 1971

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Table I are superior to lard oil or castor oil in promoting uniform flow of lubricants to the mold bore. The influence of mold lubricant on cooling water contamination is an important consideration under any circumstances, but of major consequence with recirculated water. For these systems, oils should be used which strip easily from water to prevent buildup of oil concentration in the cooling water. Oil concentrations above 50 ppm can cause problems at the cooling tower as weil as in the mold . Water quality has always been a major consideration in DC casting. However, for continuous operation, water quality is even more important since molds must operate for many hours without cleaning the water system. A small amount of solid or lubricant buildup on the water side of a mold cooling system can inftuence thermal efficiency of the mold and cause ingot defects.

INGOT COOLING Heat removal requirements from most DC molds are high. However, horizontal systems place even higher heat loads on the mold because of the increased contact between ingot shell and mold wall

Table 1-Physical properlies of mold lubricants Lubrleant Lard Oll Castor 0!1 Castor Oil Derivative Synthetic Diester

Viseoslty-SSU IOOOF 2100F

Vlseoslty Index

Po ur Point OF

210 1508 565

50 101 69

129 80 105

40 to 45 -10to -25 -15

447

71

146

Below -15

NORMAL INGOT CONTOUR

Fig. 6-Dual belt withdrawal. BELT

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from pressure of the molten metal. Fig. 5 shows the heat flux at various locations in a mold and several in. downstream of the mold exit for a 16-in. thick horizontal ingot cast in a 2-in. long mold. The heat values were calculated from ingot shell measurements using mathematical analyses." An initial thermal peak occurs as the molten metal first contacts the cover plate and forms a solid shell. Since the exact heat flux magnitude at the moment of contact is not definable, these portions of the curves in Fig. 5 are dotted. After the initial shell forms. the heat flux drops appreciably along ingot top and bottom. However, the higher pressure along the bottom causes better mold contact and maintains a higher heat flux . This difference between top and bottom varies with ingot size and alloy. The values shown in Fig. 5 are typical only of large horizontal ingots. At the mold exit, water impinges upon the ingot. reducing the surface temperature to approximately 200°F. If the ingot shell is thin, as at the top, this will cause a considerable increase in heat flux. If the ingot shell is thicker ( as was the case for the ingot bottom illustrated in Fig. 5) thermal resistance within the shell will Iimit heat flux so that heat flux at the mold exit is less. A high heat flux at the mold exit requires extreme uniformity in water flow to prevent ingot shell cracking. Establishing this uniformity is complicated in horizontal casting because the angle at which water can impinge upon the ingot must be controlled to prevent water from backing up into the mold. Generally, the angle of impingement should not exceed 7 to 10 degrees.

ALIGNMENT In horizontal casting, the mold must be bolted to some rigid support and cannot float as in DC casting. Alignment between molds and the withdrawal unit is critical. Care must be exercised in mold setup to assure that the ingot pass-line on the withdrawal unit corresponds accurately with the mold bottom. The mold is placed below withdrawal unit pass-line by an amount equal to the total shrinkage of the ingot at the time that it comes to rest on the withdrawal belts minus the compression of the belt due to ingot weight. Molds for casting small ingots usually can be set with the bottom directly on the withdrawal unit pass-line because ingot shrinkage minus belt compression is small. However, these factors must be considered with !arger ingots.

Large reetangular ingots cast with the rolling faces in a horizontal plane create a special pass-Iine problern because ingot shrinkage will vary across the rolling faces. To obtain ingot with parallel rolling faces at room temperature, molds are contoured convex to take this shrinkage into account. The starting block and first section of ingot cast will have a barrel shape similar to the mold contour, and when withdrawn onto a flat belt will be deflected above passline creating a bend that is self perpetuating. The ingot produced is serpentine and is unsatisfactory for subsequent rolling operations. This problern is overcome by using a twin belt withdrawal system (as illustrated in Fig. 6) where only the outer edges of the ingot come in contact with the withdrawal unit.• Thus, the starting block and initial bulge in the center of the ingot lie between the belts and can pass through the unit without affecting ingot pass-line.

SAWING A comment should be made about the flying saw used so that the process can be continuous. The saw can be synchronized with the casting unit either hydraulically, electrically, or mechanically. All these techniques are successful if properly tuned. However, it is important that clamping and sawing equipment be precisely synchronized and in line with the casting unit. A remarkably small amount of disturbance at the saw can result in an ingot surface defect generated at the mold.

SUMMARY Horizontal, continuous casting requires more sophisticated equipment than vertical, discontinuous DC casting. The mold package, lubrication, and ingot cooling must be specially designed to allow for horizontal operation and to assure continuous casting over long time periods. Even supporting operations such as metal supply and ingot sawing require special consideration to attain continuous operation. On the plus side, horizontal continuous casting can produce a constant, high quality product in large volume with maximum automation and minimum cost.

REFERENCES Ennor, W. T.; U.S. Patent 2,301 ,027, November 3, 1942. 2 Brondyke, K . J., "nd Hess, P. D .; "Filtering and Fluxing Processes for Aluminum Alloys," JouRNAL oF METALS, February, 1965, pp. 146-149. •Brondyke, K. J., and Craig, R. T . ; U.S . Patent 3,286,309, November, 1966. • Gardner, G . R.; U .S. Patent 3,381,741, May, 1968. • Tikhonov, A . N ., and Shvidovskii, E. G.; "On the Theory of Continuous Casting," Zhurnal Teknecheskoi Fiziki, Vol. 17, no. 2, 1947, pp. 161-176. • Brondyke, K . J., and Cralg, R. T . ; U.S. Patent 3,349,837, October, 1967. 1

APRIL 1971, JOURNAL OF METALS-39