US3580328A - Mold for improved control of heat transfer in casting plate or strip products - Google Patents

Abstract

An open-ended mold is disclosed for continuously casting metal plate or strip, the mold having a graphite casting cavity liner formed of two parallel plates separated at two of their opposing edges with spacer members so as to define an open ended cavity of substantially rectangular cross section, and two cooled metal jackets each tightly clamped against a liner plate. The two principal directions of heat transfer from the mold cavity defined by the liner are through the respective oppositely disposed graphite plates into the adjacent cooled jackets. Improved control over the rate of heat transfer from the mold cavity is obtained by providing a slight convex curvature or crown of suitable height on either the internal surfaces of each cooled jacket or the adjacent external surfaces of the liner plates so that when a crowned surface is tightly clamped against an adjacent flat surface positive control is maintained over the area of contact between the surfaces during casting despite the inherent tendency of the graphite liners to bow to the mold cavity when in contact with hot metal due to thermal expansion.

Description

United States Patent [72] Inventors Robert E. Eppich Southiield; Fred J. Webbere, Orchard Lake, Mich.

[21] Appl. No. 823,706

[22] Filed May 12, 1969 [45] Patented May 25, 1971 [73] Assignee General Motors Corporation Detroit, Mich.

[54] MOLD FOR IMPROVED CONTROL OF HEAT TRANSFER IN CASTING PLATE OR STRIP PRODUCTS 5 Claims, 6 Drawing Figs.

[52] US. Cl 164/283,

[51] Int. Cl B22d 11/00 [50] Field of Search 164/82,

[56] References Cited UNITED STATES PATENTS 3,410,331 11/1968 Miller et a1. 164/51 3,412,784 11/1968 Wieland 249/134X 3,502,135 3/1970 Wertli l64/82X 3,511,305 5/1970 Wertli 3,519,062 7/1970 Wertli Primary Examinerl. Spencer Overholser Assistant ExaminerR. Spencer Annear AnorneysWilliam S. Pettigrew and George A. Grove ABSTRACT: An open-ended mold is disclosed for continuously casting metal plate or strip, the mold having a graphite casting cavity liner formed of two parallel plates separated at two of their opposing edges with spacer members so as to define an open ended cavity of substantially rectangular cross section, and two cooled metal jackets each tightly clamped against a liner plate. The two principal directions of heat transfer from the mold cavity defined by the liner are through the respective oppositely disposed graphite plates into the adjacent cooled jackets. Improved control over the rate of heat transfer from the mold cavity is obtained by providing a slight convex curvature or crown of suitable height on either the internal surfaces of each cooled jacket or the adjacent external surfaces of the liner plates so that when a crowned surface is tightly clamped against an adjacent flat surface positive control is maintained over the area of contact between the surfaces during casting despite the inherent tendency of the graphite liners to bow to the mold cavity when in contact with hot metal due to thermal expansion.

MOLD FOR IMPROVED CONTROL OF HEAT TRANSFER IN CASTING PLATE R STRIP PRODUCTS The subject invention is related to the continuous casting of flat plate or strip products through an open-ended mold. More particularly the subject invention is related to a mold structure formed of a liner inset and an adjacent cooled metal jacket wherein improved means for controlling the surface contact between the jacket and liner and therefore the rate of heat transfer therebetween is obtained.

It is now common metallurgical practice to continuously cast strip or plate products ofiron, steel, copper, aluminum or the like through a suitable open ended mold in either a vertical or horizontal mode. Typically the mold contains a liner portion of graphite or like material and a cooled jacket member in thermal contact with the liner. Graphite is frequently employed liner material because it has a very high melting point, has a relatively high coefficient of thermal conductivity and provides lubricity for the withdrawal of the cast product from the mold. The metal jacket is frequently formed of copper base alloys because of their high thermal conductivity and is generally cooled by the passage of water or other suitable fluid therethrough. Having had a certain degree of success in the continuous casting of metal products there is now interest in providing molds which permit a substantial increase in the rate of casting and/or an improvement in the metallurgical structure of the cast product. One example of a situation in which both desiderata are sought is in the casting of certain aluminum-babbitt alloys wherein high casting rates are desired in combination with the high degree of control over the character of the dispersion of the babbitt phase in the aluminum matrix. US. Pat. No. 3,410,331 described such alu minum-babbitt alloys and a method of casting the same in a horizontal mode to obtain a desired gradient in the concentration of the lead babbitt phase throughout the aluminum matrix.

It is an object of the present invention to provide an openended mold for the continuous casting of plate or strip product, the mold having structural means for improved control over the rate of heat transfer from the mold cavity so that the rate of casting of the strip can be substantially increased and/or improved metallurgical quality of the cast product can be obtained.

It is a more specific object of the present invention'to provide a mold structure for casting metal product of substantially rectangular cross section, the mold having a graphite liner portion and a cooled jacket member wherein the character of the engaging surfaces of the graphite liner and the cooled jacket are arranged and constructed to provide an increased and more affirmatively controlled rate of heat transfer from the mold cavity through the graphite liner to the cooled jacket notwithstanding the inherent tendency of the graphite liner to bow inwardly away from the jacket due to thermal expansion.

In accordance with a preferred embodiment of our invention these andother objects are accomplished by forming a liner of graphite or other suitable material comprising two parallel plates of suitable dimensions separated at two opposite edges by spacer members to define an open-ended mold cavity rectangular in cross section. Two metal jackets adapted for the passage of cooling water or other suitable cooling fluid are provided and adapted to be clamped against the two liner plates. Either the outer surface of a liner plate or the adjacent surface of the cooled jacket member is prepared with a convex curvature (or crown) of predetermined curvature and height in a manner that will subsequently be described in detail. Preferably, the maximum height of the convex surface is adjacent the center of the mold cavity. When the crowned surface is of suitable height and the jacket members and liner are tightly bolted or clamped together, improved control over the rate of heat transfer is realized during casting in the two opposing directions extending from the mold cavity through the two liner plates into the cooled jacket members. Surface contact is maintained between the convex surface and the adjacent planar surface, and therefore control over heat transfer is attained at the surface boundaries of the cooled jacket and the graphite liner, despite the inherent tendency of the liner plates to bow inwardly due to the relatively high temperature of the metal inthe mold cavity. A beneficial increase is obtained in the removal of heat from the mold cavity, particularly at the center portions of the two large surfaces of the flat product.

These and other objects and advantages of the subject invention will be more apparent from a detailed description thereof which follows. In the description reference will be made to the attached drawings, not necessarily drawn to scale, in which:

FIG. I is an elevation view, partly in section, of a suitable holding furnace and attached horizontally disposed open ended mold arranged and constructed in accordance with the practice of our invention;

FIG. 2 is a sectional view of a mold of our invention taken along line 2-2 of FIG. 1 showing a convex surface provided on the coolingjackets ofthe mold;

FIG. 3 is a sectional view of the mold as in FIG. 2 except a different embodiment is shown in which the convex surface is provided on the liner plates;

FIG. 4 is a plan view of rectangular cast strip produced by molds in which only flat mold surfaces are employed;

FIG. 5 is a plan view of cast strip produced by a mold of our invention; and

FIG. 6 is an elevation view, somewhat schematic in nature, depicting apparatus suitable for providing a crown surface on a mold member in accordance with our invention.

In FIG. I is shown an induction melting or holding furnace 10 in which a reservoir 12 of molten metal is maintained at a suitable casting temperature. The reservoir 12 is arranged and constructed in the embodiment shown so that molten metal may be withdrawn from an outlet 14 at the bottom thereof through a horizontally disposed open-ended mold 16. Depending upon the capacity of the reservoir 12, a cast strip 22 of substantial length, up to several hundred feet or more, may be produced from a single melt. The mold is detachable joined to the induction furnace 10 by suitable bolt means 18, the details of which are not critical to the practice of the subject invention. Mold 16 comprises a liner 20 and cooling jackets 28. The liner is formed of graphite or other suitable material and defines an open ended mold cavity (shown in FIGS. 1, 2 and 3 to be filled with molten or solidified metal) and which is substantially rectangular in a cross section transverse to the direction of flow of the cast metal and withdrawal of the cast strip product 22. The peripheral surface of a transverse cross section of the liner 20 is also generally rectangular in configuration. That is at least the top and bottom surfaces of the liner are planar unless they are provided with a slight convex curvature or crownas will be described. The character of the outer side surfaces of the liner is not critical to the practice of this invention. Since it is generally necessary to periodically replace the liner or insert portion of a continuous casting mold, for simplicity of construction and ease of assembly the liner is preferably formed of two substantially rectangular flat plates 24 separated with suitable spacer members 26 at the longitudinal edges thereof (FIGS. 2 and 3). Two cooling jacket members 28 of copper, brass or the like are formed and bolted 36 one each to the liner plates 24. The cooling jackets 28 are provided with internal passages 30 defined by webs 52 for the flow of cooling water or other suitable cooling fluid. Preferably, the cooling water or other fluid traces a curvilinear path through each of the jackets coursing back and forth several times across the width of the jacket as it progresses from one end of the mold 16 to the other.

In FIGS. 2 and 3 it is seen that each jacket member 28 has an inner surface 32 engaging the adjacent outer surface 34 of the graphite liner. In accordance with our invention the engaging surfaces are characterized in that either the liner surface 34 or the jacket surface 32 is shaped into a smooth convex curvature or crown and the other surface is substantially flat. As shown in FIG. 2 the crown has been formed on the interior surface 32 of each of the jacket members 28. In general it is preferred that the crown be formed on the inner surface 32 of the jacket member 28 because the liner member 20 must be replaced periodically and a new crown surface would have to be prepared in each instance. FIG. 3 illustrates the mold 16' of our invention in which the crown surfaces have been alternatively provided on the outer surface of the liner plates 34'. In

the assembled position, ready for casting operation, the liner plates 24 are assembled with the spacers 26 therebetwcen at the longitudinal edges and mold jackets 28 are placed above and below the graphite liner 20. The complete assembly is either bolted 36 together as shown or clamped together by C- clamps or other suitable device.

By way of example the mold of our invention may be employed to cast strip 4 to 8 inches wide and three-eighth inch to 1 inch thick. The height of the spacer members 26 generally determines the thickness of the cast strip 22 and the width of the spacer member provides the support for the edges of the plate. In producing cast strip of about 5 to 7 inches in width, we have found that crowns of 0.005 to 0.02 inch maximum height, the center of the crown being located at a point above center of the cast strip, are generally suitable.

In the startup of a casting operation the outlet opening of the mold is plugged until the molten charge can be prepared and heated to a suitable casting temperature. The molten metal is permitted to flow into the inlet opening 54 of the horizontally disposed mold 16 and heat is extracted therefrom through the liner 20 into the cooled metal jacket 28. The metal solidifies and is then withdrawn through the outlet 56 of the mold. In general, the cast strip is not withdrawn in a continuous motion from the mold but is initially advanced a predetermined increment over a brief period of time and then stopped for a predetermined period of time. In some instances it may be desirable to push the cast strip back into the mold a fraction of the distance it has been withdrawn prior to further advancement thereof. These features of the continuous casting process are generally recognized by those skilled in the art and may be employed with our improved mold. The utility of our mold is not dependent on a particular casting cycle or cast product removal cycle.

During the operation of the mold the highest temperatures are obtained within the mold cavity. Heat is transferred from the molten metal in the mold cavity through the graphite liner, through the inner portion of the cooled jacket and to the circulating cooling fluid. Of course, there is a sustantial temperature differential across each of the mold components between the mold cavity and the cooling fluid. Because only its cavity defining surfaces are in contact with the hot metal there is a definite tendency for the liner plates to bow inwardly into the cavity. There is also generally a lesser tendency (because it is at a lower temperature) for the adjacent surface of the jacket to bow as well. When the liner bows inwardly it tends to move out of physical contact with the adjacent cooling jacket adversely affecting heat transfer therebetween. In prior art continuous casting molds reliance has been placed on the tendency of both the jacket and the liner to bow inwardly to a similar extent and on additional clamping forces to maintain some physical contact for heat transfer between the adjacent surfaces. However, we have found that markedly improved casting results are obtained by providing a convex surface, as above indicated, on either the outer surface of the liner or the inner adjacent surface of the jacket and then bolting or clamping the two members securely together.

Preferably, the maximum height of the convex surface is adjacent the approximate center of the mold cavity and, thus, of the strip to be cast. in the continuous casting of flat strip or plate it is desirable and necessary to remove a substantial portion of the heat through the major surfaces of the mold liner and particularly through the center portion of these surfaces. Thus, with the crown surface on either the liner or the jacket and the two members bolted together, substantial contact can be maintained at the center portion of the major surface area of the mold despite the tendency of the liner plates to bow away from the jacket in mold operation.

connection with casting aluminum-lead bearing alloys by the method described in U.S. Pat. No. 3,410,331. In casting twophase alloys of this type wherein it is desired to obtain a specific reproducible gradient of one phase within another there is a particular need for obtaining control over the rate of heat transfer from the mold cavity. We found that when conventional molds are employed, wherein the engaging surfaces of both the mold liner and the mold jacket are flat, extended parabolic solidification fronts 38 are visually observable on the surface of the cast strip 22 such as is depicted in FIG. 4. These solidification fronts are formed at each increment of advance of the cast strip. Extended parabolic solidification fronts indicate that the center ofthe cast strip solidifies significantly later than the edges of the strip and that the rate of heat removal across the width of the mold cavity is far from being uniform. When an attempt is made to increase the casting rate above a particular relatively low value, tearing is noted at the solidification fronts in the center of the strip as indicated at 40 in FIG. 4. On the other hand, when the mold of our invention is employed having a crowned surface of 0.015 inch height on the cooling jackets, substantially square solidification fronts 42 as illustrated in FIG. 5, were obtained. The squared-off solidification front indicates that the rate of heat removal is generally uniform across the whole width of the casting area and that the molten metal was solidifying to form the cast strip at the same longitudinal position in the mold uniformly across the width of the casting. Moreover, we found that significantly higher casting rates could be employed without tearing the strip. Of even more critical significance in the case of aluminum-lead alloys, however, than the improved casting rate is the fact that the subject mold provided uniformity in the concentration of dispersed lead particles at a given level in the casting throughout its cross section.

A few specific examples will further illustrate the utility and advantages of the subject mold. It was desired to produce cast strip of aluminum-lead alloy by the process identified in the above-identified patent. The strip was to be 5% inch wide by three-eighth inch thick. Therefore, the span of the graphite liner plate members 24 between the spacers 26 was 5% inch and the height of the spacers was three-eighth of an inch. The graphite liner plates were 0.350 inch thick. The coolingjacket was formed of hard copper. The mold from molten metal inlet to cast product outlet was 9 inches long. Cooling water from a common source was admitted to both the top and bottom cooling jackets at a rate to each of 300 gallons per hour. The design of the cooling jacket was such that the streams of cooling water coursed back and forth nine times across the width of a jacket in proceeding from the inlet end adjacent the furnace to the outlet end adjacent the mold outlet. A 0.015 inch crown 34' was provided on each of the two graphite liner plate members 24 so that the mold 16' was of the type depicted in FIG. 3. The height of the crown is considered to be the difference in height between the edge of the plate and the middle thereof as indicated in FIG. 6, the maximum height of the crown being in the center of the plate adjacent the center of the mold cavity.

A charge of molten aluminum based, lead containing alloy was prepared and maintained at about 1,700F. in holding furnace 10. A cast strip was produced on a continuous basis by advancing the solidified casting at a rate of approximately feet per hour in accordance with the following cycle. The strip was initially advanced 1 inch in a half second and halted for 2% seconds, the cycle being repeated on a continuous basis. The cast strip was produced having square solidification fronts 42 visible in the surface thereof as shown in FIG. 5. The temperature of the cast strip emerging from the mold was 320 F. and a temperature rise in the cooling water of 30 F. was noted in both the top and bottom cooling jackets.

The experiment was repeated with the same cast alloy under the same conditions except that flat graphite plates were employed against flat jacket surfaces and the following results were noted. When an attempt was made to cast at the above rate, 100 feet per hour, parabolic solidification fronts 38 of the type depicted in FIG. 4 were obtained. Moreover, center tearing 40 as depicted in FIG. 4 was obtained rendering the plate unusable. The exit temperature of the plate was 400F. and an increase in only F. was noted in the cooling water temperature which indicates that less total heat was removed from the metal. Only when the rate of casting was reduced to about 60 feet per hour was tear-free strip produced. Even then, however, the solidification fronts were still highly parabolic. While the substantial sacrifice in casting rate employing the prior art mold is a distinct disadvantage, of more critical significance with respect to casting aluminumlead alloy was the finding that the values of the lead content at a particular depth within the casting varied from 3-6 percent over the width of the casting. That this variation would be objectionable in a bearing alloy wherein the function of the lead is to provide a substantial portion of the lubricating properties can well be appreciated particularly in view of the teachings of the above-identified patent.

Thus, we have found that a convex or crowned surface on either the liner member or the adjacent jacket member produces substantial improvement in the continuous casting of flat plate or strip in terms of casting rate and in terms of metallurgical quality. ln FIG. 6 is depicted a device which is suitable for preparing the crown surfaces required for the mold components in our invention. A commercial grinding machine 44 is available on which the grinding wheels 46 vertical position is continuously determined by a mechanical follower 48 which traces a path over a template 50. Thus a curved surface may be produced on either the mold liner plate or the mold jacket by traversing the mechanical follower 48 over a simply supported single-point loaded beam template 50 which has been deflected to produce a crown of the desired height over the width of the plate. The beam can also be adjusted so that the point of maximum deflection of the beam will coincide with the longitudinal axis of the plate or jacket that is being ground. It will be recognized that the desired height of the crown on a given mold member will depend on a number of variables including the temperature of the molten metal, the thickness and width of the composite graphite plate, the thickness of the jacket in contact with the plate and the elastic and thermal properties of the two contacting members. As indicated above, in casting aluminum-lead alloys 4 to 8 inches in width, employing graphite plates of about 0.350 inch to 0.500 inch in thickness, a crown of 0.005 inch to 0.020 inch is satisfactory. However, the height of the convex curvature in a particular application of our mold irrespective of the metal being cast or the mode of casting, whether vertical or horizontal, may be determined by experiment. The crown permits a marked increase in thermal conductivity from the center of the mold cavity along the full length of the mold which is extremely important respect to rectangular strip for high casting rates and product quality.

While our invention has been described in terms of a few specific embodiments it will be appreciated that other forms might readily be adapted by those skilled in the art. Therefore the scope of our invention is considered limited only by the following claims.

We claim:

1. A mold for the continuous casting of metal comprising a cooled metal jacket disposed in surface to surface contact heat transfer relationship with a graphite mold liner,

said liner defining a cavity having a first open end into which molten metal is fed and a second open end from which cast product is withdrawn, said cavity being rectangular in cross section in a plane transverse to the direction of flow of said metal through said cavity,

the said contacting surfaces of said jacket and said liner being characterized in that one of said surfaces is substantially flat and the adjacent contacting surface is convex with respect to said flat surface. the curvature of said convex surface being predetermined such that controlled surface to surface contact is maintained between said jacket and said liner for purposes of efficient heat transfer during the operation of said mold.

2. A mold for continuous casting of elongated metal strip or plate of rectangular cross section comprising a graphite mold insert member having a substantially rectangular outer peripheral surface in a plane transverse to the direction of flow of said cast metal and defining a mold cavity of rectangular cross section, said cavity having an inlet for the introduction of molten metal and an outlet for the withdrawal of cast product therefrom, and a cooled metallic jacket engaging at least the two sides of greatest area of said rectangular peripheral outer surface,

the two engaging surfaces of said jacket and a said side of said insert being characterized in that one of said surfaces is substantially flat and the engaging surface is convex with respect to said flat surface, the curvature of said convex surface being predetermined such that controlled surface to surface contact between said insert and said jacket is maintained for efficient heat transfer during a casting operation.

3. A mold for continuously casting flat metal plate or strip comprising two rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends,

and a cooled jacket member tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for the efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets,

the engaging surfaces of a said plate and a said jacket being characterized in that one said engaging-surface is substantially flat and the other is so convex with respect to said flat surface that in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for purposes of efficient heat transfer despite the tendency of said plate member to bow into said mold cavity and out of surface contact with said jacket due to thermal expansion.

4. A mold for continuously casting flat metal plate or strip of up to about 8 inches in width and of substantially greater but indefinite length comprising two substantially flat rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends, the width of said cavity being up to about 8 inches, the height of said cavity being up to about 1 inch,

and a cooled metal jacket tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets,

the engaging surfaces of a said plate and a said jacket being characterized in that said surface of said plate is substantially flat and said surface of said jacket is prepared with a slight convex curvature with respect to said flat surface, the maximum height of said convex curvature being about 0.005 inch to 0.020 inch, the location of said maximum height being adjacent the center of said mold cavity whereby in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for efficient heat transfer.

5. A mold for continuously casting flat metal plate or strip of up to about 8 inches in width and of substantially greater but indefinite length comprising two substantially flat rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends, the width of said cavity being up to about 8 inches, the height of said cavity being up to about 1 inch,

and a cooled metal jacket tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for the efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets,

imum height being adjacent the center of said mold cavity whereby in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for efficient heat transfer.

Claims (5)

1. A mold for the continuous casting of metal comprising a cooled metal jacket disposed in surface to surface contact heat transfer relationship with a graphite mold liner, said liner defining a cavity having a first open end into which molten metal is fed and a second open end from which cast product is withdrawn, said cavity being rectangular in cross section in a plane transverse to the direction of flow of said metal through said cavity, the said contacting surfaces of said jacket and said liner being characterized in that one of said surfaces is substantially flat and the adjacent contacting surface is convex with respect to said flat surface, the curvature of said convex surface being predetermined such that controlled surface to surface contact is maintained between said jacket and said liner for purposes of efficient heat transfer during the operation of said mold.

2. A mold for continuous casting of elongated metal strip or plate of rectangular cross section comprising a graphite mold insert member having a substantially rectangular outer peripheral surface in a plane transverse to the direction of flow of said cast metal and defining a mold cavity of rectangular cross section, Said cavity having an inlet for the introduction of molten metal and an outlet for the withdrawal of cast product therefrom, and a cooled metallic jacket engaging at least the two sides of greatest area of said rectangular peripheral outer surface, the two engaging surfaces of said jacket and a said side of said insert being characterized in that one of said surfaces is substantially flat and the engaging surface is convex with respect to said flat surface, the curvature of said convex surface being predetermined such that controlled surface to surface contact between said insert and said jacket is maintained for efficient heat transfer during a casting operation.

3. A mold for continuously casting flat metal plate or strip comprising two rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends, and a cooled jacket member tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for the efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets, the engaging surfaces of a said plate and a said jacket being characterized in that one said engaging surface is substantially flat and the other is so convex with respect to said flat surface that in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for purposes of efficient heat transfer despite the tendency of said plate member to bow into said mold cavity and out of surface contact with said jacket due to thermal expansion.

4. A mold for continuously casting flat metal plate or strip of up to about 8 inches in width and of substantially greater but indefinite length comprising two substantially flat rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends, the width of said cavity being up to about 8 inches, the height of said cavity being up to about 1 inch, and a cooled metal jacket tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets, the engaging surfaces of a said plate and a said jacket being characterized in that said surface of said plate is substantially flat and said surface of said jacket is prepared with a slight convex curvature with respect to said flat surface, the maximum height of said convex curvature being about 0.005 inch to 0.020 inch, the location of said maximum height being adjacent the center of said mold cavity whereby in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for efficient heat transfer.

5. A mold for continuously casting flat metal plate or strip of up to about 8 inches in width and of substantially greater but indefinite length comprising two substantially flat rectangular plates of graphite separated by spacer members at two opposite edges of said plates so as to define a mold cavity rectangular in cross section and open at two ends, the width of said cavity being up to about 8 inches, the height of said cavity being up to about 1 inch, and a cooled metal jacket tightly clamped in heat transfer relationship to the outer surface of each of said plates to thereby provide a path for the efficient removal of heat from said mold cavity through each of said plates into each of said cooled jackets, the engaging surfaces of a said plate and a said jacket being characterized in that the said surface of said jacket is substantially flat and the said surface of said plate is prepared with a slight convex curvature with respEct to said flat surface, the maximum height of said curvature being about 0.005 inch to 0.020 inch, the location of said maximum height being adjacent the center of said mold cavity whereby in the operation of said mold in a casting process surface contact to a predetermined degree between the said engaging surfaces is maintained for efficient heat transfer.

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