GB2094193A - Mould for direct-chill casting of metals - Google Patents

Abstract

A chromium plated layer (30) is provided on an inner surface of a mold (12) which is contacted with molten metal to be cast. The mold includes an overlying hot top (10) affording a feed- head smaller in cross-section than the mold cavity. Also included is a lubricating system in which a common rotary drive shaft operates several plunger- type pumps which pump lubricant to respective molds. <IMAGE>

Description

SPECIFICATION Apparatus for direct chill casting of metals This invention relates to an apparatus for direct chill casting of metals. More particularly this invention relates to a mold, used for continuous or semi-continuous casing of aluminium or its alloy ingot in a so-called hot-top casting method, and highly improved in lubrication efficiency, operational life thereof, and also substantial quality of the ingot surfaces, as well as a system capable of effectively and uniformly supplying a lubricating oil to the mold.

Metallic ingots such as those of aluminium or aluminium alloys have conventionally been casted by some well known continuous or semi-continuous casting methods. In recent days a so-called hot-top casting method, wherein a melt reservoir for receiving a molten metal which is made of a heat insulative refractory material is disposed on top of a mold so as to hold the melt or molten metal of high hydrostatic pressure above a solidified layer of metal, is rapidly prevailing above all. In one representative apparatus belonging to this category of methods, a direct chill casting mold open-ended cylindrical shape, with a melt reservoir made of a refractory material and formed into an overhang type for being disposed on the mold, wherein a lower end portion of the inner periphery of the melt reservoir is inwardly extended beyond the inner wall surface of the mold, is usually employed.While the molten metal to be cast is received for being held on one hand in the reservoir, the molten metal is imparted on the other hand a continuous primary cooling, while being held at a column state within the mold in close vicinity to a lubrication surface formed by lubricating oil supplied to the inner wall surface of the mold through a passage, by a cooling agent (water is generally used) communicated to be flowed in the mold, and the solidifying molten metal is further imparted a secondary cooling before it is continuously taken out of a lower opening of the mold as an ingot of column shape by a sprayed cooling agent so as to be completely solidified as far as the innermost portion of the ingot. A target ingot can be obtained in such a process.

This sort of direct chill casting method is highly advantageous in being released of the strict surface level adjustment of the molten metal in a casting mold, unlike in conventional casting methods, and other respects. However, the method is far from a completed art, it still is inherently defective in some aspects, such as occurrence of scars, ripples, cold shuts, etc., on the casting surface of the ingot obtained due to the strong primary cooling effect from the inner wall surface of the mold, increased area of ingredient segregation in the neighbourhood of the casting surface, and so on.

For the purpose of avoiding such problems some remedy propositions were made. For example, U. S.

Patent No. 4,157,728 disclosed a method of infusing air, nitrogen, or an innert gas from between the melt reservoir of a refractory material and the direct chill casting mold so as to diminish contact of the molten metal with the direct chill casting mold, and the British Patent No. 1,389,784 discloses in its specification a method of inserting a graphite sleeve at a portion of the direct chill casting mold where the molten metal contact the same so as to mitigate the adverse effects of the primary cooling to the ingot. Either of them, however, can not be said an ideal art from a practical view point, because they could not achieve sufficient improvement of the casting surface and the ingot surface organization, and besides they were too much complicated in the casting process and the structure of the casting mold.More specifically speaking, the gas infusion in the former method is problematic in its difficulty of controlling of the amount of infused gas, and in the latter method appearance of scars or ripples perculiarly caused by the graphite is an inevitable weak point.

This invention was made from such a background. The principal object of this invention is therefore to provide a direct chill casting apparatus highly enhanced in its lubrication capability.

A direct chill casting apparatus in accordance with this invention, which is aimed to accomplish the above-mentioned object, includes a melt reservoir for holding a molten metal, which is made of refractory material, and a direct chill casting mold disposed thereunder, wherein a lower end portion of the inner periphery of the melt reservoir is inwardly extended beyond the inner wall surface of the mold to be an overhang portion, for the purpose of gradually solidifying the molten metal held in the melt reservoir under cooling treatment while the same is continuously descended through the mold without changing its posture of a column status, and is characterized in that the inner wall surface of the mold is applied chromium plating thereover where the molten metal is contacted.This device of applying the chromium plating over the inner wall surface of the mold extremely enhances the lubrication capability thereof and diminishes the amount of the lubricating oil consumed there. Other effects of this device are: sticking or welding of molten metal to the inner wall surface of the mold can be effectively restricted; grinding process of the mold inner surface can be thereby reduced; remarkable improvement of the quality of the ingot surface portion, etc.

For describing the invention further in detail preferred embodiments.will be explained with reference to the appended drawings, in which: Figure 1 is a vertical sectional view of an embodiment of an apparatus for direct chill casting of metals according to this invention; Figure 2 is a block diagram of a multistrand casting system in which the apparatus of this invention is incorporated; and Figure 3 is a cross sectional view of a lubricating oil supplying apparatus used in the multistrand casting system shown in Figure 2.

Under an open-ended cylindrical melt reservoir 10 for receiving or holding molten metal 11 therein made of a refractory material an open-ended cylindrical direct chill casting mold 12 made of pure copper is disposed as illustrated in Figure 1, wherein the lower end portion of the melt reservoir 10 is inwardly extended or protruded beyond the inner wall surface of the mold 12 so as to form an overhang portion 14.

Within the mold 12 a cooling water chamber 16 is formed whereinto water as a cooling agent is supplied through a water supply conduit 18 for cooling the mold 12, and the cooling water is thereafter sprayed to a half-solidified metal ingot 22 of column state from a slit 20 circumferentially formed around a lower opening of the mold 12. Within the mold 12 a lubrication system, is further disposed, which consists of a lubrication oil (hereinafter called lubricant) reservoir 24 placed rather upper portion thereof, a lubricant groove 26 for leading the lubricant from the reservoir 24 to the upper portion of the inner wall surface of the mold 12, and a lubricant supply conduit 28. The lubricant continuously supplied from the system in the course of a casting operation in this way forms a lubrication surface at the inner wall of the mold 12.

Over the entire inner surface of the mold 12 where it is contacted with the molten metal 11 a layer of hard chromium plating 30 is formed. This hard chromium plating layer 30 is supplied on its surface with predetermined lubricant supplied from the lubricant reservoir 24through the lubricant groove 26.

In an apparatus for direct chill casting of metals according to this invention a column of the molten metal is formed, upon supplying of the molten metal 11, on a base plate 32, being held by the melt reservoir 10 and the inner wall of the mold 12. The column of the molten metal 11 is imparted the primary cooling by the cooling water flowed in the cooling water chamber 26 via the chromium plating layer 30 formed on the inner wall surface of the mold 12, and further imparted the secondary cooling by the cooling water sprayed through the slit 20 in the lower portion of the mold 12. An ingot 22 completely solidified as far as the innermost portion thereof can be obtained continuously in a column status in response to the descending of the base plate 32.The inner wall surface of the mold 12 which is applied the hard chromium plating layer 30 to be a flat smooth surface functions as an effective lubricant surface by being given the lubricant uniformly spread all over the surface. It not only enhances the lubrication efficiency but also greatly prolongs the life of the mold 12 itself. Formation ofthistype chromium plating layer 30 overthe inner wall surface of the mold 12 has resulted in remarkable reduction of the lubricant amount consumed.According to experiments by the invertors the reduction of the lubricant, for example colza oil or castor oil, by approximately 50% does not affect the uniform spreading thereof or the restraining function against the occurrence of defective casting surface such as quality deterioration of the ingot surface and appearance of scars, ripples, etc. there.

As another advantage of this invention decrease of sticking between the mold 12 and the molten metal 11 or ingot 22 can be pointed out, which decrease effectively prevents attaching of slag caused by the sticking on to the very surface of the mold 12 and thereby keeps the surface flat and smooth for a long time. It consequently eliminates grinding process of the surface which has been conventionally required fairly after, bringing about reduction of the labour force therefor and elongation of the life of the mold 12 a great deal.

The above-mentioned effects of this invention was eloquently proved by the foliowing experiment carried out by the inventors.

This Comparison Invention Case Diameter of Ingot (mm) 65 65 Casting Speed (mm/min.) 450 450 CoolingWaterAmount(i:/min.) 35 35 Chromium Plating (Thickness; ,u) 10 None Lubricant (castor oil cc/min.) 1.5-2 2-4 This experiment was aimed to observe the comparison of lubricant amount consumed according to existing and non-existing of the chromium plating layer on the inner wall surface of the mold.In the case without the chromium plating it was necessary to supply the lubricant, i.e. castor oil, at a rate of 2-4 cc/min., for preventing the occurrence of sticking or scars, ripples, etc. on the ingot, while the case with the chromium plating at a thickness of 10 F according to this invention caused substantially no sticking even when the amount of the lubricant was reduced by one half in comparison to the traditional ways, viz., to 1.5-2 cc/min., much less appearance of the defective casting surface. Besides, the mold with the chromium plating could maintain its flat and smooth surface even after a semi-continuous casting operation of as many as 300 times without requiring any grinding thereof. On the contrary, similar semi-continuous casting operation in the range of 30-50 times caused already sticking and other defective casting surfaces, requiring grinding operation on the surface.

Incidentally, it is difficult to definitely determine the thickness of the chromium plating layer 30 formed on the inner wall surface of the mold 12, as it largely depends on casting conditions, kinds of the molten metals, etc. It is preferable in general to be in the range of 5-30 cm. If it is too thin the plating layer 30 itself becomes meaningless, being useless for the purpose of this invention, and if it is too thick, it adversely affects the primary cooling function against expectation, being furthermore not preferable from either the technological and economical standpoint.

The chromium plating layer of this kind may be, generally speaking, formed by a known electroplating or a chemical plating, and it is generally applied over the entire inner wall surface of the mold 12. It may be however applied only on the upper portion of the surface where it is contacted with the molten metal or a shell of an ingot, leaving the lower portion of the inner wall surface unplated because of no contact being expected there.

An apparatus according to this invention is preferred to be utilized for producing thin rods of aluminium or its alloy of thickness in the order of 40-100 mm, but it is of course effectively employable for producing rod like ingots of larger diameter.

This invention can be more effectively applicable to a multistrand casting system, wherein casting operation is performed simultaneously in a plurality of molds set in one casting system, when it is realized in combination with an undermentioned lubricant supply system. The lubricant supply system applied to the multistrand casting system having a plurality of casting molds, to each of which predetermined amount of molten metal is supplied for being respectively cooled and solidified there so as to obtain simultaneously a plural of ingots, is characterized in that a lubricant supply system including a plurality of pump elements of constant discharge amount disposed parallelly, a drive shaft for driving the pump elements, and a motor variable in r.p.m., supplies lubricant in response to the driving of the common shaft at a predetermined amount from each of the pump elements to each of the casting molds through a respective lubricant supply conduit leading to each of the molds. By adopting such a lubricant supply system effectively in supplying lubricant to each of the molds, the effect of the chromium plating layer according to this invention in each of the molds can be realized to the maximum extent.

In short, the multistrand casting system includes a plurality of, and after several decades of, casting molds provided with the chromium plating layer for simultaneously producing ingots in all of them, one example being illustrated in Figure 2. Eight of molds 41 like the mold 12 in the previous embodiment are arranged therein in two rows, four for each row. Each of the molds is provided with a proper lubricant supply conduit 42, through which the lubricant, for example castor oil or colza oil, is to be led as far as the lubricant supply conduit 28 in the mold.

Another aspect of this invention resides in providing a novel lubricant supplying system to each of the lubricant supply conduit 42, which comprises a plurality of pump elements 43,43,--- of constant discharge amount arranged parallelly, a drive shaft 44 for driving the pump elements 43,43, - - - respectively, and a motor 45 variable in r.p.m. and connected to the drive shaft 44. In the lubricant supply system numbered 46, driving of the drive shaft 44 actuated by the motor 45 causes each of the pump elements 43 to be operated for respectively supplying the lubricant in a predetermined amount to each of the lubricant supply conduits 42.

More specifically describing the system, the lubricant supply system 46 illustrated in Figure 3 comprises the pump elements 43, each being a pump unit of plunger type arranged in axial direction of the drive shaft 44, and reciprocation of a plunger 47, actuated by the rotation driving of the drive shaft 44, which is immersed in an oil tank 48, transmitted by way of a cam 49 and a lever 50, make a predetermined amount of the lubricant stored in the oil tank 48 to be sucked via a pipe 51 to a pump portion 52. The lubricant sucked into the pump portion 52 of the lubricant supply system 46 is discharged into the lubricant supply conduit 42 in each of the pump elements 43. In the oil tank 48 a suitable heater 53 is disposed for heating the lubricant up to a predetermined temperature for reducing the viscosity thereof.The lubricant is maintained at a predetermined viscosity so that it may not adversely affect pumping operation of the pump elements 43.

Rotational range of the lever 50 which is rotated about a shaft 54 under actuation due to abutment of the cam 49 is regulated by a regulation member 55, which in turn regulates the amount of reciprocative movement of the plunger 47. In addition to the control of the lubricant discharge amount from the pump element 43, the lubricant supply amount to the lubricant supply conduit 42, which is generally regulated, is further respectively adjusted or controlled to be exact in accordance with the condition in each of the conduits 42.

In a casting operation employing a lubricant supply system 46 of this type, all of the pump elements 43 are simultaneoulsy driven by the rotational driving of the single drive shaft 44 rotated by the single motor 45, and all of the pump elements 43 never fail to discharge constant amount of lubricant to each of the lubricant supply conduits 42 because of little influence being imaginable from the difference in flow resistance in each of the lubricant supply conduits 42, which warrants that identical amount of lubricant which has been discharged from each pump element 43 is supplied to each of the molds 41, exactly the lubricant reservoir 24 of the mold 12 in Figure 1, through each respective lubricant supply conduit 42. It consequently ensures that each of the molds 41 can be operated casting under the same desired amount of the lubricant. There can therefore be little possibility of varying in the quality of the ingot casting surface, allowing production of ingots stable in the quality thereof.

The discharge amount of the lubricant from each of the pump elements can be varied easily and instantly.

This varying can be done similarly for all of them only by varying the number of rotation of the motor 45, which in turn makes it extremely easy to charge the supply amount of the lubricant to each of the molds 41.

This is a great merit for increasing the supply amount temporarily at the initial stage of casting operation and thereafter gradually decreasing the same, and particularly so because of the regulating operation being simple and easy.

This invented lubricant supply system 46 is very meritorious again in being compact in its structure, because all of the pump elements 43 can be driven by a single motor 45 and a single drive shaft 44. In addition, all of the molds share only one lubricant supply system 46, not having a proper system for each, which provides various advantages, such as elimination of installation problem of the system on a narrow casting mold table, elimination of operational problems regarding the drive adminstration of the casting apparatus, cost reduction by a iarge margine due to the installation of a single lubricant supply system, etc.

This invention is by no means limited to those embodiments disclosed above, but various modifications and variations can be made for those skilled in the art. For example, although the mold 12 in the above embodiments is of pure copper, it may be replaced by one made of a heat-conductive material selected from the group consisting of aluminium, an aluminium alloy, a copper alloy, etc. In case of casting a slab a mold of regular prismal shape is employed instead of a cylindrical one.As to the secondary cooling which is executed after the primary cooling by the inner wall surface of the mold, it is also permissible, instead of the conventional method by spraying the cooling water flowed in the mold from the opening formed in the lower part of the mold, to cool the molten metal by means of spraying a cooling agent from a cooling agent spraying pipe(s) disposed around the lower opening of the mold.

This invention is, in this way, featured in forming a chromium plating layer over the inner wall surface of the mold in an apparatus for direct chill casting of metals so that the lubricant may be continuously and uniformly spread or diffused over the inner surface of the mold, with the consequent result of forming an effective lubrication surface over the inner surface of the mold. The above-mentioned feature of this invention is of great industrial significance through such merits that lubrication effect can be attained to the maximum extent, consumption of the lubricant can be economized a great deal, and effective prevention of sticking of the molten metal to the inner surface of the mold accompanied by a side effect of eliminating the grinding process of the inner surface of the mold because of its being maintained always flat and smooth.

Claims (7)

1. An apparatus for direct chill casting of metals including a direct chill casting mold with a melt reservoir made of a refractory material disposed thereon for holding a molten metal to be cast, wherein the lower end portion of said melt reservoir is extended inwardly beyond the inner wall surface of said mold to form an overhang portion, for continuously cooling and solidifying said molten metal in said mold while it is held in a column status, said apparatus characterized in that an inner wall surface of said mold, where the same is contacted with said molten metal, is applied chromium plating to form a layer thereof over said wall surface.

2. An apparatus in accordance with claim 1, wherein said chromium plating layer is of thickness 5-30 U.

3. An apparatus in accordance with claim 1, wherein said molten metal is aluminium or its alloy.

4. An apparatus in accordance with claim 1, which further comprises a lubrication system for supplying a lubricant to said inner wall surface of said mold to form a lubricating surface.

5. An apparatus in accordance with claim 1, wherein said mold comprises a cooling chamber to flow a cooling agent and an opening formed in the lower part thereof to spray the cooling agent flowed in the cooling chamber to a metal ingot which is continuously taken out of said mold.

6. A casting system of multistrand casting type for continuously or semi-continuously casting a plurality of ingots by means of supplying predetermined molten metal in each of a plurality of such molds as claimed in claim 1 before solidifying the same by cooling, said casting system provided with a lubricant supply system comprising a plurality of pump elements of constant discharge amount arranged parallelly, one drive shaft for driving said pump elements, and one motor connected to said drive shaft and variable in the number of rotation, whereby a predetermined amount of the lubricant discharged from each of said pump elements driven by said common drive shaft is respectively led by way of each lubricant supply conduit to each of said plurality of molds.

7. An apparatus for direct chill casting of metals, substantially as hereinbefore described with reference to the accompanying drawings.