US3642058A

US3642058A - Mold apparatus for continuous casting

Info

Publication number: US3642058A
Application number: US11407A
Authority: US
Inventors: Fred J Webbere; Robert G Williams
Original assignee: Motors Liquidation Co
Current assignee: Motors Liquidation Co
Priority date: 1970-02-16
Filing date: 1970-02-16
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15
Also published as: DE2104616B2; CA922083A; FR2081000B2; DE2104616A1; JPS5027448B1; GB1310312A; FR2081000A2

Abstract

Apparatus for continuous casting including an open-ended mold with a first portion adjacent the inlet end having a low heat transfer capacity wherein no significant solidification of metal takes place, a second portion adjacent the first portion adapted to effect initial skin layer solidification and a third portion adjacent the second portion having a plurality of longitudinal grooves in its inner surface to vent gas released upon solidification from the second portion out the open end of the mold and wherein progressive solidification is accomplished to form solid bar stock.

Description

United States Patent Webbere et al. 5] Feb. 15, 1972 [54] MOLD APPARATUS FOR CONTINUOUS 3,210,812 10/1965 Berwick, Jr ..164/282 CASTING 3,286,310 11/1966 Dore et a1 ..l64/281 3,304,585 2/1967 Marchlik 164/281 [72] Inventors: Fred J. Webbere, Orchard Lake; Robert 3,438,426 4/1969 Parfit 164/282 Williams, Birmingham, both of Wm 3,459,255 8/1969 Kolle ..164/273 x [73] Assignee: glenzral Motors Corporation, Detroit, Primary Examiner k Spencer Annfar Att0rneySidney Carter and Peter P. Kozak [22] Filed: Feb. 16, 1970 [21] Appl. No.: 11,407 [57] ABSTRACT Apparatus for continuous casting including an open-ended Related US. Application Data mold with a first portion adjacent the inlet end having a low heat transfer capacity wherein no significant solidification of [63] Continuation-impart of Ser. No. 827,747, May 26, metal takes place a second portion adjacent the first portion 1969' adapted to effect initial skin layer solidification and a third portion adjacent the second portion having a plurality of Ion- [52] U.S. Cl ..l64/283, 164/ 82 gitudinal grooves in its inner Surface to vent gas released upon [51] Int. Cl ..B22d 11/00 lidifi ti f the Second portion out thc Open end f the Field of Search 138, 28], 283 mold and wherein progressive solidification is accomplished to form solid bar stock. [56] References Cited 5 Claims, 8 Drawing Figures UNITED STATES PATENTS Behrenudt 1 64/ 282 MOLD APPARATUS FOR CONTINUOUS CASTING This application is a continuation-in-part of our application, Ser. No. 827,747 filed May 26, 1969 entitled Mold Apparatus for Continuous Casting.

This invention relates to the continuous casting of bars or rods of various shapes and, more particularly, to a mold apparatus for continuous casting of bars formed of relatively high melting point metals which are unsaturated in carbon, such as steel, in which venting of the mold is provided for elimination of gases released from the metal upon solidification.

The apparatus shown and described in our application Ser. No. 827,747 included, in general, a horizontally disposed open-ended mold associated with a molten metal holding vessel or furnace including a low heat transfer first zone immediately adjacent the holding vessel which is effective to contain and convey the molten metal therethrough without appreciable solidification and which is substantially chemically inert to the molten metal, a relatively high heat transfer second zone adjacent the first zone which effects the initial solidification in the form of a thin skin of solidified metal progressively and coextensively, first at the interface of the first and second zones and progressively to the end of the second zone, and a third zone adjacent the second zone wherein the molten metal is further solidified to form a selfsustaining rod which may be progressively withdrawn from the mold by mechanical means. The apparatus also included means to mechanically pull the solidified rod from the opposite end of the mold continuously but intermittently in predetermined increments and with predetermined time intervals between the increments of length or segments of movement of the rod wherein each increment corresponds in length to the aforesaid second or initial solidification zone so that with each incremental pull of the rod the thin solidified skin layer or segment formed in the second zone is advanced into the third zone thereby exposing the second zone to the advance of molten metal from the first zone and the progressive soliuification of a new skin layer or segment is formed in the second zone. The time interval or dwell between the pulling intervals is sufficient to permit the forward end of the newly formed skin layer in the second zone to weld to the rod in the third zone, so that when the rod is again pulled, it will carry with it the newly formed segment into the third zone in a continuous rod solidification process.

One of the difficulties encountered in a continuous casting system in which the mold is attached directly to the casting vessel is the problem of venting small amounts of gas that may be released by the metal during solidification. Accumulation of this gas around the outside of the skin of solidified metal causes interference with normal heat transfer to the mold wall and, consequently, incomplete or an improper mode of solidification occurs. In the instant mold apparatus in which solidification takes place coextensive with the second zone and progressively in a downstream direction from the interface of the first and second zone and in which the solidified rod is advanced in an intermittent manner, continuous operation is dependent upon welding of the forward end of the newly formed skin layer in the second zone to the rod in the third zone at the juncture of the second and third zones. If the normal heat transfer characteristics of the second zone are interrupted by the accumulation of a layer of gas between the thin skin and the high heat transfer mold wall, a weld of sufficient strength will not form between the skin layer and the rod during the dwell between pulling intervals. On advance of the rod, then, the skin layer remains in the second zone and molten metal issues against the mold wall at the juncture of the second and third zones resulting in an improper solidification process. This phenomenon is known as a breakout" and may only be cured by stopping the casting process for an extended time to allow for healing of the rupture to take place. Therefore, in order to maintain a continuous and controlled solidification sequence during casting, it is highly desirable to eliminate these gases from within the'mold.

Accordingly, it is the principal object of this invention to provide for venting of gases released from the molten metal on solidification in such a manner that normal heat transfer within the mold is not interrupted.

It is a further object of this invention to provide venting means within the mold which vents accumulated gases in a downstream direction away from the primary solidification region without being in contact with molten metal during normal operation of the apparatus whereby surface imperfections or interruption of the casting process due to solidification in the vents is avoided.

These and other objects are accomplished by the provision of a horizontally disposed open-ended mold associated with a molten metal holding vessel or furnace of the type herein previously described in which the third zone wherein the molten metal is further solidified to form a self-sustaining rod is provided with a plurality of longitudinal grooves cut into the inside surface of the zone beginning at the juncture of

zone

2 and 3, downstream from the primary solidification zone, and extending to the open end of the mold whereby a longitudinal void is provided which provides for venting of gas released upon metal solidification in zone 2 even though the remainder of the cast surface is in intimate contact and in heat transfer relationship with the mold wall. Such an arrangement is particularly applicable to the mold herein described wherein the natural venting resulting from thermal contraction of the bar has been offset by tapering of the inside surface of the third zone of the mold.

Other objects and advantages of the invention will be apparent from the following description, reference being had to the drawings in which:

FIG. 1 is a cross-sectional view of a horizontal continuous casting apparatus;

FIG. 2 is an enlarged view of a portion of the mold shown in FIG. 1;

FIGS. 3-5 are fragmentary cross-sectional views of the mold at various stages of the casting process;

FIG. 6 is a view of the mold shown in FIG. 2 taken along line 6-6;

FIG. 7 is a diagram showing the internal dimensions of one embodiment of the mold; and

FIG. 8 is an enlarged view of a portion of the mold shown in FIG. 1 showing another embodiment of the mold.

Referring now to FIG. 1 of the drawings, the molding apparatus of this invention consists generally of a molten metal reservoir 10 shown as a fragment thereof and a horizontally disposed open-ended mold 12 supported adjacent an opening 14 near the base of the reservoir. The reservoir is of conventional construction including an outer metal shell (not shown) having a lining 16 of suitable refractory material for containing molten metal, such as steel. The opening or channel 14 in the reservoir is formed in a frustoconical refractory body 18 cemented to the lining 16. The refractory reservoir may include heating means (not shown), such as an induction heating coil or resistance heating element for maintaining the metal at a desired casting temperature.

Referring to FIG. 2, the mold 12 consists of three distinctly different portions with different heat transfer characteristics. The first portion immediately adjacent the reservoir consists of a noule portion 20 preferably formed of boron nitride having relatively low heat transfer characteristics such that it will contain the molten metal therein without any appreciable solidification. The second portion 22 is positioned immediately adjacent the nozzle 20 and is formed of material having relatively high heat transfer characteristics as, for example, beryllium-copper alloy. The third portion 24 is disposed immediately adjacent the second portion 22 and is preferably provided with a graphite liner 26. The third portion preferably has somewhat lower heat transfer characteristics than the second portion and is provided with a plurality of grooves 28 as will be hereinafter fully explained.

The second mold portion 22 as well as the third mold portion 24 are. both provided with coolant passages 30. The

second mold portion is preferably formed of a berylliumcopper alloy because of its high heat transfer characteristics. As will be explained hereinafter, the molten metal contacts the surface of the second mold portion 22 only in a transient manner.

The continuous casting of rods'in the apparatus after the casting process is started and is in continuous operation is characterized basically by the molten metal passing from the reservoir 10 through three successive zones in the mold 12. The molten metal is conveyed from the reservoir 10 into the first zone without significant solidification due to the sufficiently low heat transfer capacity of the zone and without exposure to air. As the metal flows into the second zone, a thin skin of solidified metal is progressively formed along the length of mold portion 22 due to the high heat transfer capacity thereof. The skin layer is then advanced as a segment or increment into the third zone or mold portion 24 wherein the molten metal is further solidified to form a self-sustaining rod 31 which is mechanically pulled out of the mold by suitable means such as rollers 32. As the aforesaid skin layer is advanced from the second zone to the third zone, a second skin layer is formed in the second zone which subsequently welds itself to the rod being solidified in the third zone. The second skin layer is advanced into the third zone as the rod is pulled incrementally whereby a continuous rod is formed in a continuous but incremental process.

The following detailed explanation will make the nature of the apparatus and the casting process involved more clear. The reservoir 10 is provided with a suitable quantity of molten metal such as steel so that its level extends substantially above the mold 12. The molten metal advances due to gravity into the mold through a boron nitride nozzle 20 which constitutes the aforementioned first zone. Since boron nitride is material of relatively low heat conductivity and is not provided with any cooling means, the molten metal does not significantly solidify therein.

As soon as the molten metal enters the second zone or the mod portion 22, an initial circumferential annulus solidifies against the mold surface portion 22 at the interface 25 of the nozzle 20 and the mold portion 22, as shown in FIG. 3. This occurs because the mold portion 22 is formed of material of relatively high heat conductivity and is cooled by means of a suitable coolant, such as water, circulating in the coolant passages 30 to provide a high heat transfer capacity whereby a film or skin of metal solidifies on the mold 22 the instant contact is made. As the molten metal advances into the second zone, a solidified skin layer 34, as shown in FIG. 3, forms progressively on the surface of the mold portion 22 in a downstream direction.

The skin layer 34 is then advanced as a segment into the third zone or the mold portion 24 of the mold where further solidification takes place to form the self-sustaining rod 31.

When the metal has advanced into the third zone, the solidified layer is of substantial thickness and is relatively cool so that no significant graphite diffusion occurs in consequence of the graphite liner 26 provided in the third zone of the mold. The use of graphite is advantageous because it is relatively soft and self-lubricating and it permits the solidified bar to be readily drawn through even though minor imperfections have occurred in the surface of the rod during solidification thereof. There is no need for fluid lubricants such as those commonly used in vertical continuous casting.

As the skin layer 34 begins to advance into the third zone, it must first break away or release from the nozzle 20, as shown in FIG. 4, to form a slight space 36 between the skin 34 and nozzle 20. This space is immediately filled with fresh molten metal flowing from the nozzle to initiate formation of the new skin layer 38 (FIG. at the interface 25 of the nozzle 20 and the mold portion 22 and to closely follow the advancing skin layer 34 and to progressively form the new skin layer 38. After the layer 34 has reached its full increment of movement, it is permitted to remain stationary for a time sufficient to permit the new layer 38. to weld to layer 34 as shown at 40 of FIG. 5.

It is essential to the successful operation of the process that the skin layer 34 part cleanly from the nozzle 20 and that it remain stationary in its advanced position for a time sufficient to permit the new skin layer 38 to weld thereto. If either of these steps is not performed properly, a break will occur in the successively formed skin layer causing molten metal to break out and issue against the mold portion 22 at 40 thereby preventing proper rod solidification.

As the fresh molten metal flowing from the nozzle into zone two contacts the mold wall of zone two and solidifies, gases absorbed in the molten metal are rejected from the solidifying metal. In addition, the freshly formed skin layer shrinks slightly from the mold wall due to the volume change involved in the liquid-to-solid transformation. As a result, the released gases collect against the mold wall in the form of a thin gaseous layer and accumulate downstream toward zone three as solidification progresses. Simultaneously, gas is being rejected from the center of the rod 31 in zone three as solidification progresses toward the center of the rod which passes along the liquid-solid interface 42 of the rod upstream and toward the mold wall finally accumulating against the mold wall in the region 40 at the juncture of zones 2 and 3 (FIG. 5). This combined release of absorbed gases on skin solidification in zone 2 and interior solidification in zone 3 results in a thin layer of gas along the surface of the mold portion or second zone 22 with a particularly large amount of gas buildup in the region 40 at the juncture of

zones

2 and 3.

The presence of this gas layer presents a thermal barrier at the surface of the high heat transfer second zone resulting in a decrease of heat transfer through the zone and consequently improper skin formation in zone 2. Furthermore, the buildup of gas at the juncture of

zones

2 and 3 presents a thermal barrier which inhibits the last metal to solidify during the dwell period between rod advances from forming a weld of sufficient strength to advance the freshly formed skin forward at the end of the dwell period. As a result, on advance of the rod in zone 3 a rupture occurs at the welding point, the freshly formed skin remains in place in zone 2, and molten metal issues against the mold wall at the point of rupture instead of at the juncture of the first and second zones as in normal operation. This breakout of metal forces the operation to be discontinued to allow for the rupture to heal.

This problem of gas accumulation on the mold wall is overcome by providing a plurality of longitudinal grooves 28 in the inner surface of zone 3 (FIG. 2) whereby accumulated gases just the heat transfer characteristics of zone 3.

The grooves being situated entirely within zone 3 and beginning at the juncture of

zones

2 and 3 provide not only effective venting of gases from the region of highest gas buildup as previously described and from along the surface of zone 2, but also eliminate the problem associated with molten metal solidifying within the grooves and the resulting retardation of forward advance of the rod. That is, since the grooves are entirely within zone 3 and the formation of the initial skin layer occurs entirely within zone 2, the grooved portion of the mold is only in contact with a solidified skin and, consequently, the grooves will not be closed by molten metal freezing therein. Occasionally, however, during the process a breakout may occur at the juncture of

zones

2 and 3 in which case a small amount of metal may penetrate and freeze in the grooves. However, with the grooves being parallel to the axis of extraction this will not result in high extraction forces since the ridges formed on the rod will follow the grooves along the direction of rod advance and out the open end of the mold.

The size of the grooves may vary according to operating conditions. Satisfactory casting has been experienced with grooves up to 0.030 inch wide and 0.0l5 inch deep with a pressure head of molten steel up to 24 inches.

ln practicing the process of this invention in casting round rods, for example, it is preferable that the inside diameter of the zone 3 portion of the mold vary in accordance with the progressive solidification of the rod so that the mold surfaces are continuously in close contact with the solidifying and shrinking rod to enhance roundness of the rod and to effect optimum heat transfer between the mold and the solidifying rod. FIG. 7 is a diagrammatic representation showing a preferred design. The zone 1 which includes the nozzle is shown to be of constant diameter although this is not necessary since the metal exists in this zone only in molten form. It has been found advantageous that the mold portion 22 have a gradually increasing diameter over its length in the direction of the outlet end of the mold in order to reduce the stress in the skin layer during the first critical advance into zone 3. As shown in FIG. 7, a 5-minute taper with a longitudinal axis in the mold wall produces desired results in the case where the length of the segment 34 is about 1 inch. The mold in the third portion 24 is provided with the grooved graphite liner 26 and is gradually decreased in diameter in the amount of 4% minutes to the longitudinal axis over the first three inch portion. For casting a 1 7/16 inch diameter bar, this mold portion continues to decrease in diameter in increments of 2 to 4 inches by about two-thousandths of an inch as shown in FIG. 7. The purpose of this taper is to compensate for the fact that the rod is undergoing substantial solidification as may be seen from inspection of FIG. 3. Then, for a final 26-inch interval the diameter of the mold remains constant. The zone 3 portion of the mold tapers in accordance with the increased solidification and decreasing temperature to compensate for shrinkage. The effect is to maintain roundness and good heat transmission from the bar to the mold. In the last 26-inch portion of zone 3 referred to above, the diameter is preferably maintained constant to slow down heat transfer from the bar to the mold. This is desirable to minimize the reheating of the bar surface and, hence, cracking thereof after the bar has emerged from the mold and to reduce frictional resistance to the moveme t of the bar. As may be seen, the use ofgrooves in the zone 3 portion is particularly applicable to the use of a tapered mold since the taper maintains the mold and the rod in continuous contact thus removing the gap between the mold and the rod caused by shrinkage of the rod which would otherwise by itself allow for some venting of gas.

It has been found that beneficial effects can be achieved by replacing from 1 to 6 inches of the initial portion of the graphite liner 26 with a corresponding metallic liner 44, shown in FIG. 8, having grooves 28 aligned with grooves 28 in the graphite liner 26. The metallic liner 44 may be heat shrunk into position within the mold and is preferably formed of molybdenum or a molybdenum alloy containing predominately molybdenum to enhance heat transfer and to provide more resistance to erosion wear than afforded by the graphite alone. The grooved metallic liner results in a more positive healing of occasional breaks that may occur in the skin adjacent to the juncture of zone 2 and zone 3 and contributes to improved life of the remaining graphite liner section. A molybdenum alloy containing 0.5 percent titanium and 0.08 percent zirconium, available commercially under the name TZM, a product of Climax Molybdenum Company, has been found to be a particularly suitable material for forming the grooved sleeve metallic liner.

In accordance with this invention, round bar stock may be cast in sizes of from about 1 to 3 inches in diameter with a roundness variation of 2% percent or less expressed in terms of the difference in major and minor diameters of the rod.

Although the apparatus has been disclosed in terms of a horizontal casting process, it will be apparent to those skilled in the art that the mold apparatus may readily be adapted for vertical casting.

Although this invention has been described in terms of specific examples, it is to be understood that other forms of the invention may be readily adapted within the scope of the invention.

What is claimed is:

1. Apparatus for the continuous casting of a metal rod comprising, in combination, a stationary open-ended mold having an inlet end and an outlet end and a molten metal reservoir associated with said inlet end in sealed fluid flow relationship,

said mold including a first portion, including said inlet end having a relatively low heat transfer capacity disposed adjacent said reservoir, a second portion adjacent said first portion having a relatively high heat transfer capacity, and a third portion adjacent said second portion,

said first portion having internal dimensions which are less than the internal dimensions of said second portion and being so arranged that the juncture of said first portion and said second portion within the mold cavity is defined by a radially extending wall of said first portion and an axial wall of said second portion,

the heat transfer capacities of said first portion and said second portion being related so that molten metal flowing through said mold is maintained in a substantially completely molten state within said first mold portion, the high heat transfer capacity of said second portion is operative to form at least a skin layer of solidified metal on the surface thereof beginning immediately at said juncture and the heat transfer capacity of said third portion is operative to further solidify the molten metal to form a self-sustaining rod, the portion of said first zone immediately adjacent said second zone being formed of a material which is substantially inert to said molten metal and is nonadherent to said skin layer,

said third portion having a plurality of spaced grooves in the inner surface thereof, said grooves being parallel to the longitudinal axis of said mold beginning at the juncture of said second portion and said third portion and terminating at said outlet end for venting gases accumulated between said skin layer and said surface of said second portion in a downstream direction away from said second portion and out said outlet end, and

means for advancing said rod formed in said stationary mold intermittently in predetermined segments and at predetermined time intervals.

2. The apparatus of claim 1 wherein the internal dimensions of said third portion progressively decrease in a downstream direction for at least a portion thereof substantially in proportion to the progressive decrease in the dimensions of the rod due to shrinkage of said rod solidified in said mold in such a manner that the mold is in snug contact with said rod to promote rod shape and the transmission of heat from said solidified rod to said mold.

3. The apparatus of claim 1 wherein said grooves are about 0.030 inch wide and about 0.0 l 5 inch deep.

4. Apparatus for the continuous casting of a cylindrical metal rod comprising, in combination, a stationary openended mold having an inlet end and an outlet end and a mo]- ten metal reservoir associated with said inlet end in sealed fiuid flow relationship,

said first portion including a refractory nozzle adjacent said second portion substantially inert to said metal in molten form having an internal diameter which is less than the internal diameter of said second portion and being so arranged that the juncture of said first portion and said second portion within the mold cavity is defined by a radially extending wall of said first portion and an axial wall of said second portion,

said third portion including in at least the initial portion thereof a metallic liner adjacent said second portion and a corresponding graphite liner adjacent said metallic liner and extending to said outlet end,

the heat transfer capacities of said first portion and said second portion being related so that molten metal flowing through said mold is maintained in a substantially the internal diameter of said third portion progressively decreasing in a downstream direction for at least a portion thereof substantially in proportion to the progressive decrease in the diameter of the roddue to shrinkage of said rod solidified in said mold in such a manner that the mold is in snug contact with said rod to promote roundness thereof and the transmission of heat from said solidified rod to said mold,

said third portion having a plurality of spaced grooves in the inner surface thereof, said grooves being parallel to the longitudinal axis of said mold beginning at the juncture of said second portion and said third portion and terminating at said outlet end, for venting gases accumulated between said skin layer and said surface of said second portion in a downstream direction away from said second portion and out said outlet end, and

5. The apparatus of claim 4 wherein said metallic liner is formed of a metal selected from the group consisting of molybdenum and molybdenum alloys containing predominately molybdenum.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIUN Petent No. 3,642,058 Q Y Dete'd February 15-, 1972 Inyentor(s) Fred J. Webbere, et. a1.

It is certified that error appears in the above-identified patent and that said Letters Patent'are hereby corrected as shown below:

Cancel the illustrative drawing on the cover sheet and substitute the followinq:

Signed. and sealed this 26th day of December 197.2.

.(sEAL) Attest:

EDWARD-M.FLETCHER,JR. i l ROBERT GOTTSCHALK' Attesting Officer v v Commissioner of'Pat ents- F ORM PO-105O (10-69) USCOMM-DC scam-p69 9 U 5. GOVERNMENT PRNYING OFFICE 1 969 O356'334, I

Claims

1. Apparatus for the continuous casting of a metal rod comprising, in combination, a stationary open-ended mold having an inlet end and an outlet end and a molten metal reservoir associated with said inlet end in sealed fluid flow relationship, said mold including a first portion, including said inlet end having a relatively low heat transfer capacity disposed adjacent said reservoir, a second portion adjacent said first portion having a relatively high heat transfer capacity, and a third portion adjacent said second portion, said first portion having internal dimensions which are less than the internal dimensions of said second portion and being so arranged that the juncture of said first portion and said second portion within the mold cavity is defined by a radially extending wall of said first portion and an axial wall of said second portion, the heat transfer capacities of said first portion and said second portion being related so that molten metal flowing through said mold is maintained in a substantially completely molten state within said first mold portion, the high heat transfer capacity of said second portion is operative to form at least a skin layer of solidified metal on the surface thereof beginning immediately at said juncture and the heat transfer capacity of said third portion is operative to further solidify the molten metal to form a self-sustaining rod, the portion of said first zone immediately adjacent said second zone being formed of a material which is substantially inert to said molten metal and is nonadherent to said skin layer, said third portion having a plurality of spaced grooves in the inner surface thereof, said grooves being parallel to the longitudinal axis of said mold beginning at the juncture of said second portion and said third portion and terminating at said outlet end for venting gases accumulated between said skin layer and said surface of said second portion in a downstream direction away from said second portion and out said outlet end, and means for advancing said rod formed in said stationary mold intermittently in predetermined segments and at predetermined time intervals.

3. The apparatus of claim 1 wherein said grooves are about 0.030 inch wide and about 0.015 inch deep.

4. Apparatus for the continuous casting of a cylindrical metal rod comprising, in combination, a stationary open-ended mold having an inlet end and an outlet end and a molten metal reservoir associated with said inlet end in sealed fluid flow relationship, said mold including a first portion, including said inlet end having a relatively low heat transfer capacity disposed adjacent said reservoir, a second portion adjacent said first portion having a relatively high heat transfer capacity, and a third portion adjacent said second portion, said first portion including a refractory nozzle adjacent said second portion substantially inert to said metal in molten form having an internal diameter which is less than the internal diameter of said second portion and being so arranged that the juncture of said first portion and said second portion within the mold cavity is defined by a radially extending wall of said first portion and an axial wall of said second portion, said third portion including in at least the initial portion thereof a metallic liner adjacent said second portion and a corresponding graphite liner adjacent said metallic liner and extending to said outlet end, the heat transfer capacities of said first portion and said second portion being related so that molten metal flowing through said mold is maintained in a substantially completely molten state within said first mold portion, the high heat transfer capacity of said second portion is operative to form at least a skin layer of solidified metal on the surface thereof beginning immediately at said juncture and the heat transfer capacity of said third portion is operative to further solidify the molten metal to form a self-sustaining ingot, the internal diameter of said third portion progressively decreasing in a downstream direction for at least a portion thereof substantially in proportion to the progressive decrease in the diameter of the rod due to shrinkage of said rod solidified in said mold in such a manner that the mold is in snug contact with said rod to promote roundness thereof and the transmission of heat from said solidified rod to said mold, said third portion having a plurality of spaced grooves in the inner surface thereof, said grooves being parallel to the longitudinal axis of said mold beginning at the juncture of saId second portion and said third portion and terminating at said outlet end, for venting gases accumulated between said skin layer and said surface of said second portion in a downstream direction away from said second portion and out said outlet end, and means for advancing said rod formed in said stationary mold intermittently in predetermined segments and at predetermined time intervals.