US 3212142 A
Description (OCR text may contain errors)
Oct. 19, 1965 G. E. MORITZ 3,212,142
CONTINUOUS CASTING SYSTEM Filed Feb. 15, 1962 FIGI J31 32 ALUMINUM M V .6
9 6"DIA 2 I l I 00 2 3 4 SINCHES EFFECTIVE MOLD LENGTH (FOR cc MOLD) H63 SOLID HEAD (FOR HOT GRAPHITE MOLD) LIQUID METAL HEAD (FOR DC MOLD) INVENTOR.
GUNTH ER E. MQRITZ A TTORNE Y! United States Patent 3,212,142 CONTINUOUS CASTING SYSTEM Gunther E. Moritz, Henrico County, Va., assignor to Reynolds Metals Company, Richmond, Va., a corporation of Delaware Filed Feb. 15, 1962, Ser. No. 173,508 Claims. (Cl. 22-57.2)
This invention relates generally to a system tor the continuous of metal and, more particularly, to an ingot casting mold employing a tapered hot graphite liner.
-In the past, continuous casting of metal ingots has been perto-rmed by apparatus of the direct chill (DC) type as disclosed by Ennor in U .8. Patent 2,301,027 issued November 3, 1942, and the controlled cooling (CC) type described in Moritz in U.S. Patent 2,983,972 issued May 16, 1961. Still another continuous casting apparatus is the liquid top mold disclosed 'by Smart, J r. in U.S. Patent 2,740,177, wherein a graphite tube serves as the die torming portion of the mold and that tube cooled to remove heat laterally from the metal contained Within the mold.
When using the Enno-r apparatus, a close relationship must be maintained between the diameter of the ingot and the depth of the pool of molten metal. "In the Smart apparatus, the graphite mold is very long; and the ingot withdrawing means must be mechanically closely controlled because of the lfrictional problems involved and the criticality of axial alignment between the ingot and the mold. In addition, the Smart device forms Ia solidified shell around the ingot while it is within the mold and solidification is propagated Largely in a radial direction by cooling the outer surface of the resulting embryo ingot.
In accordance with the present invention, however, there is provided an improved, relatively inexpensive continuous casting apparatus which permits the metal ingot to be directly cooled to remove heat therefrom substantially in the longitudinal direction, while at the same time eliminating frictional problems and criticali-ty of axial alignment between the ingot and the mold. The mold incorporates a relatively short, tapered graphite liner or insert which acts to limit the radial removal of heat, there- 'by substantially avoiding the tormation of a shell of solidified metal at the ingot periphery (which is characteristic of the DC process). The length of the graphite liner may be varied to increase greatly the range of permissible ingot withdrawing speeds as compared with the prior art devices. While this mold is particularly suitable for the continuous casting of aluminum and aluminum alloys, it may also be used for the casting of other metals.
Fora better understanding oi the invention and its various objects, advantages, and details, reterence is now made to the present preferred embodiment of the invention which is shown, for purposes of illustration, inthe accompanying drawings, wherein:
FIGURE 1 is a cross-sectional view of a preferred casting apparatus;
FIGURE 2 is a cross-sectional fragmentary view showing a modification of the preferred apparatus illustrated in 'FIGU RE =1; and
FIGURE *3 is a graph showing the relationship between ingot dropping speed and characteristics of this mold and certain prior art molds.
The mold shown in FIGURE 1 is designed to produce cylindrical ingots and, theretore, the mold is generally annular and the tapered passageway therein is generally conical. However, it is to be understood that other suitably shaped molds may be used within the scope of this invention to produce ingots of different cross-sections.
Mold 10 comprises an entrance end 16 to which molten metal is supplied to form a molten metal pool 18 within ice the mold. A lowering device (not shown) withdraws the solid ingot 20 from the exit end 22 of mold .10.
Mold 10 comprises an outer aluminum shell 24 which has a lower extension (26 whose inner surface 28 may be either slightly divergent toward exit end 22 of mold 10 with respect to the longitudinal axis 31 of the mold, as shown in FIGURE 1, or else substantially parallel to axis 31. The extension 26 is provided to establish a clear boundary or demarcation above which the cooling fluid is excluded from contact with the ingot.
Mounted within the annular aluminum shell 24 is an annular liner 30 of insulating material such as Marin-ite, a composition of asbestos and an inorganic binder, Insulating liner 30 has an annular recess therein which contains an annular graphite liner 34 whose inner surface 36 is tap cured at an angle of about ten degrees. Surfiaces 32 and 36, together with the inner surface 28 of shell extension 26, define the passageway of the mold. The length of liner 34 is substantially less than the maximum interior dimension of the mold passageway normal to 1 e mold axis. It is important to note that heat insulator 30 contains a portion 38, and graphite liner 34 is entirely insulated from aluminum shell 24.
A perforated ring 42 is connected to a water supply (not shown) to direct a water spray against ingot 20. The ring 42 with respect to the extension 26 determines the point at which the cooling water impinges upon ingot 20. As will be explained below, the dimension labeled as the solid head in FIGURE 1 is the distance between the lower end of extension 26 and the highest point 50 ot ireeze line 44 where it contacts the tapered inner surface 36 of the hot graphite liner 34. Although the demarcation between molten and solidified metal is referred to herein as a freeze line, it will be understood that the ingot portion underlying boundary 4 4 ordinarily includes a zone of metal which is in la mushy or semisolid state.
The operation of mold 10 is different from that or the prior art molds previously mentioned. Since the annular graphite liner 34 is heat insulated by means of the insulating liner 30, the temperature of the graphite liner 34 ap proaches the temperature of molten metal 18 contained in mold 10 during casting, and substantially no latent heat is removed through the graphite. An embryo ingot is first torrned by plugging the exit end of the mold, and then ingot 20 is solidified by direct chilling with the cooling Water so that latent heat is removed substantially longitudinally through ingot '20. The combination and cooperation of the hot graphite liner -34 and the direct chilling of ingot v20 result in a crater shape shown schematically in FIGURE 1.
When ingot 20 is dropped or withdrawn from mold 10, the highest point 50 of freeze line 44 terminates within the mold at the inner surface 36 of graphite liner 34. Below the freeze line 44 the ingot is virtually solid, and a gap 46 exists between the exterior surface 47 of ingot 20 and the tapered graphite liner. Ingot 20 never touches the inner surface 28 of the aluminum shell 24, the only function of shell extension 26 being to determine the point at which the cooling water impinges upon the exterior surface 47 of ingot 20.
If the withdrawal speed of ingot 20 is reduced excessively, freeze line 44- tends to move toward the meniscus surface, thereby causing cold shutting and other trouble; however, the minimal speed is still very much lower than that attainable with DC molds at which the same trouble occurs. For example, when casting 6-inch diameter ingots the lowering speed may be reduced to less than one inch per minute. When the dropping speed becomes too high, freeze line 44 will move downwardly beyond graphite liner 34 and contact the cold aluminum shell extension 26 to change the operation of the mold and destroy the ad vantages provided by the hot graphite liner. However, an important feature of this invention is that the range between these two extremes of dropping speeds can be made virtually as great as desired by suitably varying the longitudinal length of graphite liner 34.
The distance between the bottom 48 of mold and the highest solid point 50 in ingot during casting is called the solid or cold head and depends on the dropping speed of the ingot or, conversely, the range of operative dropping speeds can be varied by altering the mold dimensions. Such a relationship is caused by the fact that the aluminum shell extension 26 determines the point at which the cooling water impinges upon ingot 20 and, therefore, affects the rate at which the ingot is cooled.
FIGURE 2 shows that it is necessary only that the graphite liner 34a be tapered. The inner surfaces of shell 24a, Marinite insert a and insert portion 38a may be cylindrical. The important requirement is that portion 38a and extension 26a do not protrude inwardly beyond the surface 36a of the graphite liner 34a. It is also important to note that the level of molten metal within the mold is not critical and may be high enough to engage the upper portion of the Marinite insert.
The taper angle of the graphite liner is designed to be great enough to compensate for any misalignment between the emerging ingot and the mold surfaces 36, 40 and 28, and to assure that gap 46 exists. For this purpose, the taper angle ordinarily will be at least 3-5 degrees, but is preferably about ten degrees or less. Any greater angle affords not particular advantage, and it is not necessary that the angularity of the hot graphite surface be sufiicient to cause formation of a meniscus in the metal adjacent the freeze line (as in the CC mold).
FIGURE 3 is a graph which compares operational characteristics of various continuous casting molds: (a) circular area 52 indicates the range of dropping speeds, depending upon the liquid metal head, for DC molds; (b) large hatched area 54 shows the range of combinations of effective mold lengths and dropping speeds that can be realized with controlled cooling or CC molds; and (c) the points along curve 56 define operative dropping speeds and corresponding solid head for the hot graphite mold 10.
The mold shown in FIGURE 1 has been used, for example, to produce six-inch diameter ingots of an aluminum alloy containing 5% magnesium. The dropping speed was two and one-half inches per minute. The solid head was about one and one-half inches and the cooling water was supplied at a rate of nineteen gallons per minute. The surface of the ingots was smooth and free of bleeding or cold shuts. The graphite liner was not preheated, and after the casting of about a five inch length of the ingot, the graphite liner reached substantially the temperature of the molten metal. However, the first five inches cast in the cooler mold did not have as smooth an exterior surface as the rest of the ingot.
From the foregoing description and accompanying drawing, it can be seen that there has been provided an improved continuous casting apparatus particularly useful for producing aluminum alloy ingots. Due to the tapered surface 36 of the mold, friction between the ingot and the mold is reduced, thereby eliminating the main cause for hot tearing. Most significantly, radial withdrawal of heat is minimized and the resulting metallurgical structure is consequently improved. Loss of heat may be virtually eliminated, furthermore, by providing means for heating the liner. This may take the form of electrical heaters incorporated directly in the mold. In general, however, the use of insulation alone has been found to be sufiicient for this purpose.
A further advantage of the tapered mold is that the axial alignment between the lowering device and mold is not critical in that the solid ingot 20 has very limited engagement with the mold. Furthermore, since the requirements for dimensional accuracy of the inside of the hot graphite mold 10 shown in FIGURE 1 are not as high as in the liquid top apparatus disclosed in the Smart patent, molds for sheet ingots may be easily built. Additionally, since the complete inner surface 36 of the mold is maintained at a high temperature during casting, a salt or salt mixture with melting point below the solidus point of the metal being cast can be used as a lubricant.
While a present preferred embodiment of the invention has been illustrated and described, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.
1. Apparatus for the continuous casting of molten metal, comprising:
an annular mold having a passageway therethrough with an entrance end for receiving molten metal and an exit end from which solidified metal is withdrawn;
a graphite member having an annular interior surface constituting the major portion of said passageway, said surface being divergent toward the exit end of the mold at an angle sufiicient to maintain a gap between the solidified metal and adjacent portions of the mold, and the length of said surface longitudinally of the mold being no greater than the maximum interior dimension of said passageway normal to the longitudinal axis of the mold;
means for maintaining the graphite surface substantially at the temperature of the molten metal to avoid solidification of the metal through heat transfer into said graphite member; and
means for causing solidification of the metal to produce a freeze line therein terminating against said graphite surface, including means for applying cooling fluid directly against the solidified metal as it emerges from the mold, whereby solidification of the molten metal is accomplished by heat removal in the longitudinal direction.
2. Apparatus according to claim 1, wherein said graphite surface is tapered at an angle of about 10 from the longitudinal axis of the mold.
3. Apparatus for the continuous casting of metal ingots, comprising:
a mold having a passageway extending longitudinally therethrough, with an entrance end of the mold adapted to receive molten metal and an exit end from which solidified metal is withdrawn, said mold including a shell portion extending around the periphery of said passageway at the exit end of the mold;
means for applying cooling fluid directly against the solidified metal as it emerges from the mold, said means being so located relative to the mold exit that the position of contact between the metal and cooling fluid is established by the coaction of said fluid applying means and said shell portion of the mold;
a graphite liner disposed within said mold and having an annular interior surface divergent toward the mold exit at an angle of about 3 to 10 degrees, said surface defining at least a portion of the mold passageway, and that portion of the mold passageway enclosed within said surface having a length substantially less than its maximum dimension normal to the longitudinal mold axis, whereby said mold is adapted to accommodate substantial misalignment between the resulting ingot and the mold passageway, and a gap is maintained around the solidified metal for the entire range of ingot withdrawal rates producing a freeze line terminating in the outer periphery of the metal against said surface.
4. In the continuous casting of metal ingots, including the steps of feeding molten metal into the entrance end of mold having a longitudinal passageway therethrough and withdrawing substantially solidified metal at the exit end thereof, said mold having an annular interior surface divergent toward said exit end at an angle of about and disposed to intercept the freeze line between molten and solidified portions of the metal being cast, the cross-section of the resulting ingot being variable in size depending on the position at which said freeze line contacts the divergent surface, the method which comprises: applying cooling fluid directly against the solidified metal emerging from the mold, and limiting heat transfer radially from the metal into the surrounding 'portions of the mold by maintaining said devergent surface substantially at the temperature of the molten metal, thereby causing solidification of the molten metal by heat removal in the longitudinal direction; and coordinating said application of cooling fluid and the rate at which metal is withdrawn from the mold, to cause said freeze line to terminate against said divergent surface at a substantially fixed position in the mold corresponding to the desired ingot size. 5. In the continuous casting of metal ingots, including the steps of feeding molten metal into the entrance end of a mold having a longitudinal passageway therethrough and withdrawing substantially solidified metal at the exit end thereof, said mold having an annular interior surface disposed to intercept the freeze line between molten and solidified portions of the metal being cast, said surface constituting a major portion of the mold passageway and being divergent toward the exit end of said mold at an angle of about 10 sufiicient to maintain a substantial peripheral gap between the solidified metal and adjacent portions of the mold, whereby the cross-section of the resulting ingot is variable in size depending on the position of said freeze line relative to said divergent surface, the method which comprises:
applying cooling fluid directly against the metal ingot emerging from the mold, to produce a solid head of metal adjacent the exit end of said mold and prevent molten metal from escaping peripherally of the solidified portions thereof, while continuously maintaining said gap around the solidified metal; controlling the dropping speed at which the metal is withdrawn from the mold, and maintaining the solid head and dropping speed in predetermined relationship, to assure that the freeze line between molten and solidified portions of the metal being cast terminates in the outer periphery of the metal against the aforesaid divergent surface at a position defining the desired ingot size.
References Cited by the Examiner UNITED STATES PATENTS 2,126,808 8/38 Phillips 2257.2 2,131,070 9/38 Poland 2257.2 2,135,465 11/38 Eldred 2257.2 2,136,394 11/38 Poland 2257.2 2,242,350 5/41 Eldred 2257.2 2,245,224 6/41 Poland 2257.2 2,983,972 5/61 Moritz 2257.2
MARCUS U. LYONS, Primary Examiner.
WINSTON A. DOUGLAS, MICHAEL V. BRINDISI, Examiners.