US3435881A - Anisotropic continuous casting mold - Google Patents

Description

April 1, 1969 WQRSMITH UAL 3,435,881

lANISOTROPIC CONTINUOUS CASTING MOLD Filed Jan. '3; 1967 United States Patent O U.S. Cl. 164-283 4 Claims ABSTRACT F THE DISCLOSURE This disclosure relates t0 a continuous casting mold made of anisotropic material and having an inner liner made of vitreous graphite.

This invention relates generally to molds adapted for continuously casting primarily metals, or metal alloys, and particularly relates to a mold ofanisotropic material having an inner lining impervious to liquid metals.

The advantages obtainable by continuously casting metals are well known in the art. Thus, the number of steps required in treating the metal is greatly reduced. There is also a savings in labor, a reduction in the quantity of material which is being treated which results in a savings of storage space as well as in a reduction of the time required to process the metal and an increase in the yield of metal.

On the other hand, there are still problems attendant to the use of the continuous casting of metals. The primary problem is the removal of heat from the cooling metal during steady state of operation, This means that the heat must be transferred from the liquid metal contained in the mold into a suitable heat sink through the mold so that the metal solidies relatively rapidly to permit further handling. Obviously the more rapidly the heat can be removed from the solidifying metal, the more rapidly the lmetal can be cast. One of the limitations of the present techniques is the relatively slow casting rate which necessitates the provision of several molds disposed in parallel and arranged to be fed from the same ladle or tundish. i

These and related problems have been solved by the anisotropic mold liner disclosed in the copending application to James W. Warren, Ir., entitled Anisotropic Mold Liner for Continuous Casting of Metals, Serial No. 541,159, iiled on April 8, 1966, and assigned to the assignee of the present application. According to the Warren application the mold is made of or lined with a refractory material having anisotropic heat-conducting properties such, for example, as pyrolytically deposited boron nitride (BN) or mica. However, it has been found that pyrolytic graphite is particularly adapted for this purpose, In practising the invention disclosed and claimed in the Warren application, it has been found that the casting mold must be constructed of individual plates of pyrolytic graphite stacked on topof each other.

Pyrolytic graphite is deposited from a vapor containing carbon at elevated temperatures in random layers which are disposed like disarranged stacks of cards. This is the reason Why pyrolytic graphite has highly anisotropic characteristics. Its mechanical, thermal and electrical properties depend upon the direction. It has become conventional practise to designate as a-b axes, which in turn define a plane, those'in which the graphite is deposited. The c-axis is at right angles to the a-b plane. Pyrolytic graphite conducts heat very well in the a-b plane but is highly insulating in the c-axis or direction. Thus, the heat conductivity of pyrolytic graphite is about 250 times as great in the a-b plane as in the c-direction.

Pyrolytic graphite is deposited from a vapor which may be a chemical compound. This may, for example, be effected by dissociating methane (Cl-I4) under the inuence of heat. This is preferably done in a vacuum furnace at a pressure which may vary within a wide range but may, for example, be between about 1 and 10 mm. 0f mercury. The temperature of the furnace may also vary within a wide range but preferably is around 2200 F. The manner of depositiong pyrolytic graphite is, of course, Well known in the ait.

Plates of pyrolytic graphite which are made in accordance with the proces diclosed hereinabove, have their a-b planes oriented in the mold so as to remove heat rapidly from the molten metal through the walls of the mold. On the other hand, the c-axis is oriented so that `it serves as a thermal barrier between the container of liquid metal such as a tundish or ladle and a suitable heat lsink which may, for example, be a water-cooled copper lock.

It is extremely diicult and expensive to produce pyrof lytic graphite in thicknesses greater than 3A. Plate of Ipyrolytic graphite 1/2 thick on the other hand are standard production items, When the material is deposited aS described above it always Orients itself so that the ab plate is perpendicular to the direction of the thickness.

A continuous casting mold has a length which may, for example, be of the order of a foot. Hence, the length of the mold is many times the thickness of a pyrolytic graphite plate which can conveniently be manufactured. Accordingly, it is necessary to stack several graphite plates in order to form a continuous casting mold. It is very diflicult to have a perfect match between adjacent plates, and accordingly molten metalmay penetrate between individual graphite plates. This metal freezes between the plates and may cause tearing of the pyrolytic graphite plates and of the metal surface as the metal tends to move through the mold.

In addition, pyrolytic graphite has a high coetlicient of thermal expansion along the c-axis. This coeicient is of the order of 12.5 x 10*6 in./in./ F. Assuming for convenience that a mold for continuous casting were 12" long and attains a temperature of 1500" F., during operation it would grow in length from l2 to 12M-(12.5 X 10"*5) (12) (1500) or 12.225. Unless this rather substantial expansion is provided for, either the mold will rupture or a gap would open in the mold and again allow the liquid metal to penetrate into the mold. During experiments which have been conducted with the anisotropic mold disclosed and claimed in the Warren application, several of these difficulties have been observed.

It is accordingly an object of the present invention to provide an improved mold for continuously casting metals including metal alloys.

A further object of the present invention is the provision of a thin liner of vitreous carbon between the cast metal and the anisotropic mold to prevent molten metal from penetrating into openings or gaps of the mold and between plates of the anisotropic mold material. Another object of the present invention is to provide a continuous casting mold having anisotropic Iheatconducting properties and having a liner to allow for the large thermal expansion of the anisotropic mold.

'In accordance with the present invention there is provided a mold for continuously casting metals. It should be noted that the term metal or metals is meant to include metal alloys, such as brass, steel or the like. This mold has a portion which is disposed substantially between the area where the liquid metal is poured and the area where at least the outer surface of the metal is solidified. This portion of the mold consists of a refractory material having anisotropic heat-conducting properties. Preferably such a material consists of pyrolytic graphite although other material may be used instead. The refractory material is oriented so that it conducts heat relatively rapidly away from the metal across the Wall of the mold while it conducts heat relatively slowly along the path of movement of the cooling metal in the mold.

Finally an inner liner of vitreous carbon is provided which covers the refractory material to provide an impervious barrier to the liquid metal.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawing, wherein the single figure is a schematic cross-sectional view through a mold embodying the present invention.

Referring now to the single figure of the drawing, there is illustrated a mold generally shown at and disposed below a suitable crucible, ladle or tundish 11. The ladle 11 contains molten metal to be cast and is used to till the mold 10 at a predetermined rate. There is further provided a refractory backing material 12 which backs the mold proper and is followed by a heat sink 14. The heat sink may, for example, consist of a block of copper provided with siutable ducts l15 for passing cooling water through the copper block in a conventional manner.

The mold is provided Iwith a liner generally indicated at 2,0 and disposed at least between the area where the liquid metal is poured, that is, below the ladle 11 and the heat sinlk 14. 'This portion of the mold 20 consists of a refractory material having anisotropic heat-conducting properties. Preferably, such a material has a ratio between the heat conductivity in one plane and another plane at right angles thereto of 50 to 1 or greater. For example, pyrolytically :deposited boron nitride (BN) is a suitable material which is refractory and has anisotropic heatconducting properties as just defined. It is also feasible to utilize mica for this purpose. However, We prefer to utilizevfor this purpose pyrolytic graphite.

As pointed out before, the graphite consists of indi- Vidual plates or discs shown at 24 and which are stacked one on top of the other.

The pyrolytic graphite discs 24 are oriented in such a manner that they will conduct heat relatively rapidly away from the liquid metal within the space 21 and across the wall and into the heat sink 14. At the same time the pyrolytic graphite discs 24 will conduct heat relatively slowly along the path of movement of the cooling metal in the mold. In other words, this prevents the heat of the molten metal from the ladle 11 and above the mold from being conducted away directly into the heat sink 14 rather than permitting metal to cool slowly so that any given cross-section has a relatively uniform temperature.

To this end the a-b plane of the anisotropic material, such as pyrolytic graphite, extends in the horizontal direction. Hence, the c-axis is disposed along the direction shown by the arrow 22. This will accomplish precisely what is required, namely, to prevent a rapid conduction of heat in the vertical dtrection as shown by arrow 2-2, while at the same time conducting heat in the horizontal direction into the heat sink 14.

It should be noted that the heat conductivity of pyrolytic graphite in the a-b plane is equivalent to or higher than that of copper, `depending on the temperature. On the other hand, pyrolytic graphite in the c-direction is practically an insulator of heat.

lIn accordance with the present invention there is provided a liner 26 between the pyrolytic graphite discs 24 and the inner space 21 of the mold. In other words, this liner 26 forms a barrier for the liquid metal so that it cannot penetrate into the spaces between individual pyrolytic graphite discs 24 in the manner previously explained.

This liner 26 in accordance with the present invention consists of vitreous carbon. Glassy or vitreous carbon has been described, for example, in the British publication Nature in the issue of January 20, 1962, page 261, by S. Yamada and H. Sato. Vitreous graphite is a form of carbon which has the characteristics of glass. lt is fragile but has excellent chemical properties and is impermeable to gas and, of course, impermeable to liquids such as liquid metals. Its resistance to thermal shock is very good in view of its good thermal conductivity.

Vitreous carbon is obtained by carbonization and further thermal treatment of organic materials having strong transverse molecular bonds which produces a coke having a large crystalline disorder and submicroscopic pores. This explains its low density of approximately 1.5 compared to the theoretical density of graphite of approximately 2.26 and the density of pyrolytic graphite of about 2.2. Vitreous carbon is not graphitizable in the usual Sense of the term. Even upon attaining a temperature of 2500 C. or more there is little or no modification of the crystalline structure. The material may be heat treated at a temperature as high as 2500 C. Thus, it will easily stand the temperature of liquid copper or steel which is usually poured at a temperature of around 2430u F. It should be noted that vitreous carbon is not wetted by liquid metals. Hence, any molten metals cannot adhere to the liner 26.

As shown in the drawing, the vitreous carbon liner 26 preferably extends beyond the anisotropic mold 20 to accommodate the expansion of the mold 20 with increases in temperature. The thickness of the wall of the liner 26 may be of the order of 0.1". Thus, the wall thickness need not be very thick because the liner 26 has the primary purpose to prevent liquid metal from penetrating between the spaces between individual pyrolytic graphite plates 24 or between cracks in the mold. Also, the liner 26 prevents liquid metal from entering the pores of the graphite used for more conventional continuous casting molds.

The thermal conductivity of vitreous carbon of which the liner 26 consists is not as high as that of pyrolytic graphite in the a-b plane. On the other hand, it is higher than that of pyrolytic graphite along the c-axis. By keeping the liner 26 of vitreous carbon relatively thin the thermal block for the transfer of heat between the molten metal and the pyrolytic graphite may be minimized. Also the expansion of the plates 24 of pyrolytic graphite due to the high temperature of the liquid metal may readily be taken care of. Thus, the pyrolytic graphite plates 24 may slide up or down the vitreous carbon liner or sleeve 26.

There has thus been disclosed a mold for the continuous casting of metals. The mold consists of anisotropic material which conducts heat relatively rapidly away from the metal across the wall of the mold while conducting heat relatively slowly along the path of movement of the cooling metal in the mold. This refractory material is covered by a sleeve of vitreous carbon. This permits the pyrolytic graphite to expand and contract with changes in temperature without the possibility of liquid metal entering the gaps formed thereby. Also it prevents the liquid metal from entering the gaps between adjacent plates or discs of pyrolytic graphite. On the other hand since the sleeve of vitrous carbon may be made relatively thin, it minimizes the impedance to the transfer of heat between the liquid metal and the mold.

The invention and its attendant advantages will be understood from the foregoing description and it will be apparent that various changes may be made in the form construction and arrangement of the parts of the invention without departing from the spirit and scope thereof or sacrificing its material advantages the arrangementhereinbefore described merely by way of example and we do not Wish to be restricted to the specic form shown or uses mentioned except as defined in the accompanying claims wherein various portions have been separated for clarity of reading and not for emphasis.

We claim:

1. A mold for continuously casting metals, said mold having a portion disposed substantially between the area where the liquid metal is poured and the area where at least the outer surface of the metal is solidified.

(a) said portion consisting of a refractory material having anisotropic heat-conducting properties,

(b) said refractory material. being oriented so that it conducts heat relatively rapidly away from the metal across the wall of the mold and conducts heat rela tively slowly along the pathof movement of the cooling metal in said mold, and

(c) an inner liner of vitreous carbon covering said refractory material to provide an impervious barrier to the liquid metal.

2. A mold as defined in claim 1 wherein said refractory material consists of pyrolytic graphite.

3. A mold as delined in claim 2 wherein said pyrolytic graphite portion has its a-b axes oriented at right angles to said path of movement and has its c-aXis oriented substantially parallel to said path of movement.

4. A mold as defined in claim 2 wherein said pyrolytic graphite portion consists of a stack of individual plates.

References Cited UNITED STATES PATENTS 2,466,612 4/ 1949 Phillips et al 164-283 X 3,059,295 10/ 1962 Voss Kuehler 164-283 3,076,241 2/1963 Simonson et al. 164--89 X 3,210,812 10/1965 Berwick 164-282 3,304,585 2/ 1967 Marchlik 249-134 X 3,381,741 5/1968 Gardner 164-82 X OTHER REFERENCES Pyrolytic Graphite Engineering Handbook, 1963, General Electric Company, pp. 1-5 and 24.

J. SPENCER OVERHOLSER, Primary Examiner. R. SPENCER AN NEAR, Assistant Examiner.

U.S. Cl. X.R. 249-134 gg@ UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIGN Patent No. 3,165,881 Dated April l, 1969 Inventor-@WILLIAM H. SMITH, DANIEL M. wH-TTLEY, and EDGAR P.EATUN It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 13, "proces" should be proceSS-.

column une Q2, "Plate" should be P1ates--.

column 2, une 26, "plate" should be "plane".

Column 5, line '7, after "solidified", change the period to a comme SIGNED ma SEALED APR 2S 2h-P ,r'v-'. -fw're {SEAL} Attest:

Eawara M. Fletcher, 1'1" WILLIAM E. summum, m. nesting Qffiw Commissioner of Patents

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