US2871534A

US2871534A - Method of continuous casting

Info

Publication number: US2871534A
Application number: US579622A
Authority: US
Inventors: Hans C P Wieland
Original assignee: Wieland Werke AG
Current assignee: Wieland Werke AG
Priority date: 1956-04-20
Filing date: 1956-04-20
Publication date: 1959-02-03
Anticipated expiration: 1976-02-03

Description

Feb. 3, 1959 H. c. P. wlELAND METHOD OF CONTINUOUS CASTING 2 Sheets-Sheet 1 Filed April 20, 1956 N w m m m w R C s N A H o MH 2 m l O MN .l N ON .w.. vm NN ATTORNEY Feb. 3, 1959 Filed Apr-11 2o, 195e H. C. P. WIELAND METHOD OF CONTINUOUS CASTING 2 Sheets-Sheet 2 60 INVENTOA HANS c? W/ELAND United States Patent O METHOD F CONTINUOUS CASTING Hans C. P. Wieland, Ulm, Germany, assigner to Wieland- Werke A. G., Ulm (Danube), Germany, a corporation of Germany Application April 20, 1956, Serial No. 579,622

1 Claim. (Cl. 22--200.1)

This invention relates to a continuous casting mold, its manufacture and use in the continuous casting of metal. More particularly, it relates to a billet mold, its manufacture and use for the continuous casting of copper, especially oxygen-bearing copper such as tough pitch copper, and particularly large size billets of such metal.

This application is a continuation-in-part of my application, Serial No. 235,987, filed July l0, 1951, entitled Continuous Casting of Metals, now Patent No. 2,772,459, granted December 4, 1956, and of my application, Serial No. 533,590, tiled September l2, 1955, entitled Continuous Casting Mold, Its Manufacture and Use.

In continuous casting procedures, molten metal is introduced into one end and the casting is withdrawn from the other end of-an openended chill mold. During the formation of the casting the molten metal freezes into a shell which increases in thickness and rigidity until the shell shrinks away from contact with the mold due to the contraction of the solidified metal in the shell. In the mold, greatest heat transfer takes place in that portion of the mold wall which is in contact with the molten metal and said shell thereof to the point where the latter shrinks away from direct contact with the mold.

In prior attempts to use molds having a graphite liner disposed in a fluid-cooled jacket, it has been found diicult, if not impossible, especially in molds for casting shapes having a large cross-sectional dimension, to maintain contact between the jacket and the liner during the casting operation, particularly in the area corresponding to the area of greatest heat transfer between the liner and the metal being cast in the mold. It is believed that such lack of contact is due to differences in distortion, caused at least in part by difference in expansion, of the liner as compared with the jacket at the temperatures existing in the mold under the casting conditions.

The principal advantage of the invention resides in the fact that it provides a method and means for enhancing contact between the liner and the jacket, particularly in the said area of greatest heat transfer. It has been found that practice of the invention results in improved surface characteristics in the casting and at the same time per mits the use of higher casting rates than would oherwise be possible. These and other advantages and objects will be more apparent from4 the following detailed description of the invention.

Broadly, the invention contemplates the continuous casting of molten metal in an open-ended mold, having a huid-cooled jacket with a graphite liner disposed therein which defines a mold cavity, while causing the jacket to press against the liner to maintain the latter preloaded in compression by said jacket during the ycasting operation, at least in the area of greatest heat transfer between the liner and the jacket thereby enhancing contact between these two elements during the casting operation.

The invention also contemplates an open-ended mold comprising a mold cavity-defining graphite liner having a convexedly curved outer surface and a uid-cooled metal jacket having a'correspondingly concavely curved inner surface, said liner being disposed in said jacket with said convex liner surface presented to said concave jacket surface and with the jacket preloading the liner in compression at least to the extent required to maintain the liner under compression at the temperatures encountered in the mold under the casting conditions, in the area in which greatest heat transfer takes place between the liner and the jacket.

The liner may be fabricated of any grade or quality of graphite including materials containing graphite such as graphite coated carbons which are not wetted by the molten metal being cast. Although the jacket may be made of any metal or alloy, it preferably is fabricated of steel, copper or copper based metal with copper being the most preferred material for making the jacket. Preferably also, the liner is a thin unitary graphite tube and the jacket comprises a tubular copper sleeve, the outer surface of which is adapted to be cooled by direct contact with water. In the most preferred mold the entire surface of the liner facing the sleeve is maintained under compression by the sleeve. For convenience, this fit between jacket and liner will sometimes be referred to as a compression fit. This may take the form of a force fit or a shrink lit, as discussed below.

In making the mold by using a force t, a graphite core is positioned in a jacket which is adapted to be preloaded in tension, with the jacket compressing the core to preload the latter in compression at least to the extent set forth above. For best results in making the mold, an oversize and over thick graphite core is prepared having outer dimensions which are larger than the inner dimensions of the jacket in which the core is to fit. The core is then forced axially into pla-ce in the jacket thereby placing the jacket under tension and the core under compression. Thereafter, material from the interior of the core is removed to obtain a graphite liner of the size v and shape for the desired casting. A core of sufficient oversize is selected so as to establish therein the requisite compression as it is forced into the jacket. Preferably,

the core and jacket are at ordinary room temperaturev during said forcing step.

In making a mold by using a shrink fit, the jacket may be heated, or the core may be cooled, or both of these latter expedients may be employed. The core may then be inserted inside the jacket without forcing it'. No subsequent machining of the inside of the coreis required in the case of a shrink fit, but the liner may be made of the final wall thickness before assembly.

It will be understood that the shrink fit may also be used in conjunction with the force t. Also, instead of the core being a unitary hollow tubular shape, the core may be solid, or of segmented hollow tubular shape.

Preferably, the mold according to the invention is separate from the holding furnace, so that the molten metal in the mold may have a free surface; the metal to be cast may be fed to the mold by a suitable conduit, or the metal may be introduced as a freely falling-stream. The mold according to the invention may also be attached to the bottom of a metal holding furnace with the metal flowing directly into the mold, so that a colum-n of metal extends from the mold up to the top of the liquid metal in the holding furnace.

The invention is further illustrated in the examples and in the accompanying drawings which form a part of this specification. lt should be understood however that the examples and drawings are given for purposes of illustration and the invention in its broader aspects is not limited thereto.

In the' drawings:

Fig. l is a diagrammatic elevation with parts in section, illustrating a continuous casting process according to the invention;

' assises Fig. .2. is a view along `line 2 2 ,or Fig. 1 taken in the direction of the arrows;

Fig. 3 is an enlarged vertical section of a mold for practicing the invention;

Fig. 4 is a view along the line 4 4 of Fig. 3 taken in .the direction of the arrows;

Fig. 5 is a diagrammatic view in vertical section illustrating the first step in the preferred method of mounting an oversize graphite core in a mold jacket;

Fig. 6 is similar to Fig. J shoving the core partly in Yduit 11 which may be a metallic tube lined with graphite 12. The casting issues from the bottom of the mold and passes into and through tank 13. The system may be provided below the tank 13 with power-driven rolls and a saw (not shown), for lowering the casting at a controlled rate and cutting the withdrawn casting into desired lengths.

The mold 19 comprises, in general, a water-cooled jacket 20 having a graphite liner 2l. The mold may be supported by a reciprocable frame indicated in general by numeral 22. The reciprocating mechanism may comprise a set of four bell cranked levers 23 pivoted to a stationary support by pivots 24. Tie rods 25 connect the pairs of bell cranks .23. Links 26 connect the bell cranks to the frame 22. A motor 27 drives a crank 28 connected by connecting rods 29 to the bell cranks 23.

The motor 27 may be of variable speed or some other device may be provided, to control the number of vertical strokes per minute of time imparted to the mold. By adjusting the length of the lever arms or the eccentricity of the crank 28, the length of the stroke may be varied.

Water tank 13 may be suitably independently supported in a position below but adjacent the bottom of the mold. A rubber water seal 33 is provided which works against the casting. The casting is cooled suticiently by the time it leaves the bottom of the tank so that no overheating of the rubber seal is encountered.

`Cooling water may be supplied to the water jacket through supply pipe 34; the water entering the jacket through tangential inlet 35. The water passes through the mold in a circular path and issues from the bottom thereof through restricted annular orice 36 which, because of its relatively small cross-section in relation to the volume of water delivered to the mold through pipe 34, causes the water passage 37 in the jacket to be lled with water. issuing from the water jacket against the outside surface of the casting issuing from the bottom of the mold. The water then dropsv into water tank 13 from which it is withdrawn through pipe 38 at a rate to maintain a desired level of water in tank 13. The advantage of this arrangement is that there is substantially no contact of air with the casting until the latter emerges from tank 13 at which point the casting is suihciently cooled to prevent, or at least drastically reduce oxidation of the surface of the casting.

Referring now to'Figs. 3 and 4, the mold will now be described in detail. The mold lil corresponds to mold 10, jacket 20 to jacket 20, liner 21' to liner 21, and inlet to inlet 35. Jacket 20 is formed of an inner cylindrical member 41 telescoped within outer cylindrical member 42.

Plates

43 and 44 suitablyk attached, as by welding, to the top and bottom respectively of member The annular orifice 36 directs the water i 42, .Serve .2,15 @Ildwlosing flanges to form a water jacket between

members

41 and 42. The upper end portion of member 41 may be provided with a shoulder 45 having a ange portion 46, which form a seal and a support with plate 43. The lower outer edge of member 41 may be beveled and spaced from the beveled inner edge of plate 44 by means of set screws 47. The position of member 4?. maybe adjusted to adjust the size of the annular orifice 36; this latter being adjusted to a crosssectional area which is less than that of the inlet 35 thereby maintaining the cooling jacket full of cooling fluid.

The liner 21 is inserted under pressure into member 4l., in a manner to be described more 'fully hereinafter. The bottom of the inserted liner may abut shoulder 48 with which member 41 may be provided adjacent its lower inner edge. Retainer ring 49 may be mounted on the top of the mold by screws 50 which may be threaded into plate 43. Ring 49 may project over the top edge of liner 21', as shown, to assist in maintaining the iiner as well as member 41 in position in the mold. [t will be noted that there is substantially no interference with the longitudinal and transverse expansion and contraction, including thermal expansion and contraction, of huid-cooled inner sleeve 41 by outer sleeve 42 and

plates

43 and 44 and set screws 47.

The preferred mode of mounting the liner in member 41 under the requisite pressure is illustrated in Figs. 5 through 8. In lthese gures also, the various parts are shown diagrammatically for simplicity. Referring to Fig. 5, ythemember 41 is placed on the bed 55 of .a press, and oversize graphite core 5 6 is placed on top of said member with the axis of the core aligned with that of member 41. vEither or both of theouter lower edge of core 56 or the upper inner edge Aof member 41 mayv be appropriately beveled to assist in starting the movement of the core into the member 41. The head 57 of the press is then brought to bearagainst the top of the core. Thereafter suicient force is applied to the press to force the core into the member 41 until' the core is in the position shown in Fig. 7; the core being compressed and its .dimensions reducedas it enters the member 4l, as illustrated in Fig. 6.

The size of dimension A of core 56 (see Fig. 5) is such that the graphite is maintained preloaded in compression by the member 41 at the temperatures to which the graphite and said member .are exposed in the mold under the casting conditions. Preferably the entire length L of the core possesses the oversized dimension A. If desired, however, the core may be oversized only in that part B of its length which represents that portion of the resulting liner in which greatest heat transfer takes place in the mold `from the metal 'being cast to the liner and from thelatter to the member 41.

In accordance lwith the invention, the inner dimension `C of the-core (see` Fig. 5) issuch as to provide a core lthickness T which is greater than that of the desired liner thickness after the core is disposed in member 41. The thickness T is selected to insure suicient core strength to permit insertion of the core into said member without breakage. After insertion, the core is machined away to the desired dimension D to provide the liner thickness E shown in Fig. 8. As shown in this latter tigure, this may be accomplished by placing the core mounted in the member 41 in the chuck`60 of va suitable lathe after which the core is turned down with cutting tool 6l to liner thickness E.

'In starting the casting process, a rod having dimensions to iit the-mold `cavity defined bylinerll may be inserted into mold 10 through the bottom of tank 13 in Fig. l. 'Thereafter the feeding of cooling water into the jacket through conduit 34 and` molten metal through conduit A11 is begun; the rod being withdrawn as the metal freezes. The head of the rod Amay berprovided with projecting means, such Yas a bolt, around'which the initial casting to the casting rate.

.entre from contact with liner 21 at a point Z, due to the contraction ofthe frozen metal in the shell. Accordingly, the area of greatest heat transfer in the mold is confined to the liner area W between the top of the metal level in the mold and the point Z as in this area the liner and the metal being cast, are in direct contact with each other.

Below point Z, heat is transferred from the casting to the liner at a reduced rate due to the loss of contact. To `insure against the danger of spillage of molten metal due to rupture of shell X, the bottom of the mold is located well away from point Z; the location of which point and the size of area W being governed by the rate at which heat is withdrawn from the casting in relation In general a mold having a length greater than about inches insures against this danger.

In any particular mold the distance to which the pool l of unfrozen metal Y extends into the casting is controlled by the casting rate i. e. the slower the rate of withdrawal of the casting the shallower the pool and vice versa. Any casting rate may be used including a rate which establishes a shallow pool of metal whose bottom is close to the top of the mold as well as one with which the pool extends well beyond the lower edge of the mold. vIn general, however, and particularly in casting tough pitch copper, it is preferred to adjust the casting rate so that the bottom of the pool of unfrozen metal is adjacent th bottom of the mold. i

The heat which is extracted by the mold from the metal being cast flows into the cooling water in jacket 20 passing through liner 21 and jacket wall 41 both of which possess high thermal conductivity and permit a high rate of heat ow when in contact with each other. However, if contact between these two members is reduced or destroyed flow of heat is seriously impaired inasmuch as even an extremely thin gap or gas layer possesses a remarkable insulating capacity. Any such impairment of heat ow between the liner and the jacket drastically affects the temperature gradient across and between these members, and causes a large increase in `the temperature of the liner surface facing the metal being cast.

In the absence of the present invention, it is believed that contact between the liner Iand wall 41, especially in the area of greatest heat transfer in the mold, is not maintained but instead is reduced or destroyed during the casting of metal in the mold. It is believed that such reduction or loss of contact is caused by a difference in expansion between these two members which, due to the difference in their coeicients of expansion, is sufficient to cause the member 41 to expand away from the liner even though the liner temperature is higher than that of member 41. As a consequence, flow of heat in the mold is impaired and the temperature of the liner becomes excessive thereby resulting in the production of inferior surface characteristics in the casting and at the same time a reduction in the casting rates that can be employed in any particular mold.

On the other hand, with the present invention, contact between the liner and the jacket is maintained during I the casting procedure due to the fact that the expansion ofthe liner is as great as that of jacket sleeve member 41. The difference in thermal expansion in these two members is made up by elastic expansion of the liner as the preload lieved that this accounts for the twin benefits of increased casting rates together with improved surface characteristics that are obtained when the invention is practiced.

The invention may be employed to cast any metal or alloy. It is most useful for casting metals such as steel, silver, nickel, aluminum, magnesium and particularly copper. It vis especially useful for casting oxygen-bearing copper such as tough pitch copper in any desired size; and for casting large size billets (i. e. those greater than about three inches in diameter) of coppers free of oxygen such as oxygen-free or phosphorous deoxidized copper. Heretofore it has not been possible to cast tough pitch copper successfully in the large quantities and with the quality required by industry.

The term oxygen-bearing copper, as used herein, is intended to include tough pitch copper, as well as copper containing a lesser amount of oxygen; it is intended to include any copper in which oxygen is in available form for attacking the graphite liner if the operating temperature of the liner is exceeded.

On the other hand, the term copper free of oxygen, asuse'd herein, is intended to cover those coppers known as l phosphorous deoxidized copper containing both high phosphorous and low residual phosphorous, any other deoxidized copper such as copper deoxidized by lithium, boron, calcium, etc., and also those coppers referred to as oxygen-free, in other words, any copper in which there is no oxygen available for attacking the graphite liner at operating temperatures.

In practicing the invention, the mold may be reciprocated at a suitable rate and stroke, and metal introduced to maintain the level thereof below the top of the mold. Reciprocation is required for casting coppers free of oxygen and it is desirable for casting oxygen-bearing coppers. If desired, a stationary mold may be used for vcasting oxygen-bearing coppers.

,fed by a freely falling stream of metal.

For'casting coppers free of oxygen, it is preferred to maintain a protective layer of discrete particles of carbonaceous material, such as flake graphite, lamp black, pulverized anthracite, etc., floating on the surface of the molten metal in the crater Y. On the other hand. for casting oxygen-bearing coppers, a cover of reactive carbonaceous material must not be used.

The casting rate may vary with the material being cast, so as to maintain the temperatureof the graphite liner at the proper value to obtain good casting characteristics. In the case of coppers free of oxygen, the maximum temperature of the graphite liner should be kept less than 1400 F. In the case of oxygen-bearing coppers, this temperature should be kept at less than l112 F. (600 C.).

The casting rnay be withdrawn at a uniform rate, or intermittently, from the mold. A casting of indefinite length may be cast and cut to desired lengths as it is being withdrawn; this is the preferred procedure with relatively small shapes. With relatively large shapes, however, it is preferred to stop the casting operation when a desired A cylindrical graphite liner was inserted into a cylindrical copper jacket as illustrated in Figs. S through 8,

.,C) was 6 inches. jacket member 41 was v6.999 inches; its outside diam- The Outside .diameter yO f the'graphte core (dimension A,

Fig. V) was 7.0158 inc hegits Vinside,diameter (dimension The inside diameter of the copper eter was 7.754 inches. Contrary to expectations, it was `found that the graphite core was vreadily axially forced `into member 41 without breaking. After being forced Example II The cylindrical graphite liner 21' and cylindrical copper jacket sleeve 41 were assembled by the shrink t above described. Before assembly, the outside diameter of the `graphite liner was 8.696 inches; its inside diameter was 8.122 inches; the inside diameter of the copper jacket sleeve was 8.675 inches; the outside diameter of the jacket `sleeve was 9.002 inches.

To assemble, the copper sleeve 41 was heated to a temperature of 40G-450 F. while the liner was maintained at room temperature. The liner was then easily inserted linto the expanded jacket and the jacket was then allowed to cool. The jacket sleeve contracted to tightly grip the inserted liner so that good surface contact was maintained between these members during the casting process described below.

An indication of the pressure generated between the contacting surfaces of copper sleeve and graphite liner is given by the fact that, after assembly, When cold, the outer diameter of the copper sleeve was 9.004 inches, showing an expansion of .002 inch, and the inside diameter of the liner was 8.103 inches, showing a compression of 0.019 inch. Liner thickness was 0.287 inch.

No machining was done to the inner liner surface of the assembled jacket which was of a size to cast a billet of approximately 8 inches in diameter.

. If desired, for assembling copper sleeve and liner, the copper sleeve may be heated to a temperature higher than that given above, and the outside diameter of the liner before assembly may have a still greater oversize so that the compression lit will generate still greater pressure between the contacting surfaces.

Example Ill Figs. 3 and 4, which are drawn approximately to scale. l

The length of the mold was about l0 inches. This mold was used in the continuous casting system shown in Fig. 1.

The mold was reciprocated at a frequency of about 60 cycles per minute with a stroke of about 4 mm. Cooling water at ordinary temperatures was circulated through the passage 37 ofthe mold.

Molten phosphorous deoxidized copper was introduced into the mold from the forehearth of a heating furnace through conduit 11 (Fig. l) which had an outlet diameter of about 0.2 inch. Pouring and speed of withdrawal of the cast billet was so arranged that the level of the molten metal in the mold was maintained about one inch below the upper edge of the graphite liner. The upper edge of the crater shell X formed by the congealing metal was maintained at about surface of the molten metal. The crater appeared to extend about to they lower edge of the mold and the liner 2 1 remained tight in sleeve 41 during the entire casting procedure.

The feed conduit 11 was kept submerged under the surface of the molten metal in the crater and a cover of flake graphite was maintained on the surface of the molten metal.

The cooling and casting rates vwere so adjusted as to maintain the hottest point Vof the liner at a temperature below about 760 C. (l400 E). A phosphorus fdeoxidized billet was continuously castat the rate of two tons per hour. The surface of the casting was smooth and free of annular folds and overlapping. The smoothness of the surface was such that substantially no water leaked through the rubber seal 33 in the bottom of .water tank 13. There was no discernible reduction in the thickness of the liner even after a considerable period of use in the mold.

Example IV The following is an example of casting toughpitch copper. The jacket and liner were assembled as set forth in Example Il. The assembly was incorporated in the mold following the teachings of Figs. 3 and 4. The mold length was about l0 inches. The mold was used in the continuous system following the teachings of Fig. l.

The mold was reciprocated at a frequency of about sixty cycles per minute with a stroke of about 4 mm. Cooling water at ordinary temperature ywas .circulated through the mold.

Molten tough pitch copper was introduced into the moldfrom the fo-rehearth of a heating furnace through conduit l1 (Fig. l) which Vhad an outlet diameter of elevenasixteenths inch. Pouring and speed of withdrawal of the cast billet was so arranged that the ,level of the molten metal in the mold was maintainedabout one .inch below the upper edge of the graphite liner. The UPPer edge of the shell formed by the congealed metal was maintained at about the surface of the molten metal. The crater appeared to extend about to the lower edge of the mold and the liner 21 remained tight in the sleeve during the entire casting procedure.

The feed conduit 11 was kept submerged under the surface of the molten metal in the crater but no -oating cover of any kind was used on the free surface of the molten metal in the crater.

The cooling and casting rates were so adjusted as lto maintain the hottest point of the liner at a temperature below about 600 C. (1li2 F1). The surface of the cast billet was smooth and free of annular folds and overlapping. The smoothness of the surface was such that substantially no water leaked through the rubber seal 33 in the bottom of water tank 13. There was no discernible reduction in the thickness of the liner even after a considerable period of use in the mold.

The temperature of the graphite liner may be measured in any desirable or convenient manner. For example, for `measuring the temperature of a liner having a thickness of 0.287v inch as in Example Il, a hole of onesixteenth inch in diameter was drilled vertically in the top edge of the graphite liner,lengthwise of the liner and midway across the liner thickness; a thermocouple -was inserted in such hole, and moved up and down, to measure the temperature along the length of -the liner. In this way the hottest point on the liner was measured conveniently. Maximum temperature usually occurs in the area indicated by W in Fig. l.. It will be noted that with a liner of the above thickness, the distance be tween the hole and the casting surface of the liner is approximately 0.102 inch.

The exact thickness ofthe liner is not especiallycritical Within the following functional limits. t is desired to have the liner thin enough to conform to any irregular changes in the shape of the copper jacket sleeve caused by expansion under casting conditions and yet thick enough to generate sufficient mechanical pressure against jacket from the compression fit described above. The exact thickness of the copper sleeve 4l is not ycritical so long as Ait is heavy enough to withstand the pressure generated by the compression t of the liner.

Experience has shown that the liner thickness may run from as low as 0.08 or 0.12 inch up to 0.287 inch and higher, with molds having inside diameter of the range of from about three to ten inches. By providing more material the thicker liners simplify drilling the hole lengthwise of the liner downwardly from its upper edge for the reception of the thermocouple as described above.

The compression tit between liner and jacket may be obtained, either by the axial force t or the shrink t, above described. Regardless of method of assembly, the resulting mold has the property of excellent heat transfer from liner to jacket. The compression fit provides a resulting structure in which the liner is under compression circumferentially, and the jacket is under tension circumferentially. The natural elasticity of the rigid, circumferentially-unitary walls of jacket and liner provides this preloading without requiring auxiliary loading means such as springs. j

The oversize liner exerting its expanding force against the jacket acts to exert uniformly distributed radial pressure against the jacket, insuring the maintenance vof tightness between liner and jacket under temperatures encountered under casting conditions. Maintenance of an excellent solid-to-solid heat path from liner to jacket is maintained. This makes possible a very high rate of heat removal comparable to the heat removing ability of an all-metal mold while retaining the self-lubricating action of graphite.

It should be noted also that the present invention relates to substantially a metal mold, and not to a graphite mold,

in that the relatively strong metal jacket provides mechanical strength and reinforcement for the relatively thin, fragile liner.

While certain novel features of the invention have been disclosed herein, and are pointed out in the annexed claim, it will be understood that various omissions, substitutions and changes may be made by those skilled in the art without departing from the spirit of the inven tion.

l0 What is claimed is: I The method of continuously casting phosphorized coppers in a mold, said mold having a solid graphite wall defining the mold cavity and an open top and an open bottom, said method comprising feeding the molten metal through a conduit into the top of said mold, withdrawing the congealed product from the bottom of the mold, intensively cooling the solid graphite wall and cooling said congealed withdrawn product, said feeding, withdrawing and cooling being at such rate that a top surface of molten metal is maintained in said mold above the lower end of said feed conduit, and the upper edge of the con gealing crater shell is maintained in close proximity to said top surface, introducing, in particle form, solid discrete carbonaceous material to maintain an appreciable protective layer of such discrete carbonaceous material on said top surface of molten metal and that portion of the mold wall contacted by said carbonaceous material, and axially reciprocating the mold.

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France Oct. 7, 1953