Systems, apparatus, and methods for direct chill casting exhaust

Abstract

Provided herein are systems, apparatus, and methods for evacuating a direct chill casting mold by venting excess casting gas and retaining oxides from above the casting during the direct chill casting process. The method for evacuating casting gas from a direct chill casting mold includes supplying molten metal to the direct chill casting mold through a transition plate, supplying casting gas through a casting surface of the direct chill casting mold, and evacuating the casting gas from a gas pocket in the transition plate, wherein the evacuating of the casting gas from the gas pocket in the transition plate is performed in response to the pressure of the casting gas in the gas pocket reaching a predetermined pressure.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 073,523, filed September 2, 2020, the contents of which are incorporated herein by reference in their entirety.

[0002] The present disclosure relates to systems, apparatus, and methods for evacuating direct chill casting molds, and more particularly to evacuating excess casting gases and retaining oxides from above the casting during the direct chill casting process. [Background technology]

[0003] Metal products are formed in a variety of ways, but many methods initially require an ingot, billet, or other casting part that can serve as a raw material from which a final metal product can be produced, for example, by rolling, extrusion, or machining. One method of producing an ingot or billet is through a continuous casting process known as direct chill casting, in which a vertically oriented mold cavity is placed on a platform that translates vertically downward into a casting pit. To begin the casting process, at least initially, a starter block may be placed on the platform and form the bottom of the mold cavity. Molten metal is poured into the mold cavity, where it is cooled, typically using a cooling fluid. The platform with the starter block thereon is lowered into the casting pit at a predetermined rate, allowing the metal to exit the mold cavity and solidify as it descends with the starter block. As more molten metal enters the mold cavity and solid metal leaves the mold cavity, the platform continues to be lowered. This continuous casting process allows metal ingots and billets to be formed that follow the profile of the mold cavity and have lengths limited only by the depth of the casting pit and the hydraulically actuated platform that moves within it. Summary of the Invention

[0004] The present disclosure relates to systems, apparatus, and methods for venting direct chill gas cushion casting hot-top billet molds, and more particularly to venting excess casting gas and retaining oxides from above the casting during the direct chill casting process. Embodiments provided herein include a transition plate for a direct chill casting mold, the transition plate including a top surface and a bottom surface, the bottom surface having a casting gas pocket defined on an outer periphery of the bottom surface, and one or more vent holes defined within the casting gas pocket. In one exemplary embodiment, the transition plate includes a lip extending around the outer periphery of the transition plate and separated from the bottom surface by a gas pocket surface. In one exemplary embodiment, the one or more vent holes are defined in the gas pocket surface.

[0005] According to one exemplary embodiment of the transition plate, the lip is elevated relative to the bottom surface when the transition plate is positioned on the mold. A casting gas pocket is formed on the outer periphery of the transition plate by the lip and the gas pocket surface, and the exhaust holes are disposed closer to the bottom surface than the lip. According to one exemplary embodiment, in response to gas bubbles forming in the casting gas pocket, the multiple exhaust holes are configured to allow the casting gas to be exhausted before it reaches the bottom surface of the transition plate. In one exemplary embodiment, the gas pocket surface includes a chamfered surface relative to the bottom surface, and one or more exhaust holes are defined in the chamfered surface. In one exemplary embodiment, the multiple exhaust holes include a web of material that is gas permeable but impermeable to molten metal. In one exemplary embodiment, the multiple exhaust holes are vented to atmospheric pressure. In one exemplary embodiment, the multiple exhaust holes are associated with a valve that allows the multiple exhaust holes to be vented to atmospheric pressure in response to the pressure in the casting gas pocket meeting a predetermined value. According to one exemplary embodiment, the transition plate includes a lip, and the casting gas pocket is defined between the lip and the bottom surface.

[0006]

[0006] Embodiments provided herein include a method for venting casting gas from a direct chill casting mold, the method including: supplying molten metal to the direct chill casting mold through a transition plate; supplying casting gas through a casting surface of the direct chill casting mold; and venting the casting gas from a gas pocket in the transition plate, wherein venting the casting gas from the gas pocket in the transition plate is performed in response to the pressure of the casting gas in the gas pocket reaching a predetermined pressure. In one example embodiment, the predetermined pressure is determined based on a metal head static pressure of the molten metal being supplied to the direct chill casting mold. In one example embodiment, the method further includes supplying pressure to a plurality of exhaust holes in the transition plate to prevent flow of the molten metal through the exhaust holes and reducing or removing pressure on the plurality of exhaust holes to allow venting of the casting gas.

[0007] Embodiments provided herein include a system for venting a direct chill casting mold, the system including: a thimble through which molten metal is delivered to the direct chill casting mold; and a transition plate attached to the direct chill casting mold and having the thimble received therein, the transition plate including a gas channel and a plurality of vents disposed therein, wherein casting gas is vented through the gas channels of the transition plate in response to molten metal filling the direct chill casting mold. The transition plate in one exemplary embodiment includes a top surface and a bottom surface, and a casting gas pocket is defined on the periphery of the bottom surface.

[0008] In one exemplary embodiment, the transition plate includes a lip extending along the periphery of the transition plate and separated from the bottom surface by a gas pocket surface. In one exemplary embodiment, one or more exhaust holes are defined in the gas pocket surface. In one exemplary embodiment, the lip of the transition plate is elevated relative to the bottom surface when the transition plate is positioned on the mold. A casting gas pocket is formed by the lip and the gas pocket surface at the periphery of the transition plate, and the exhaust holes are disposed closer to the bottom surface than the lip. In one exemplary embodiment, in response to gas bubbles forming in the casting gas pocket, the plurality of exhaust holes are configured to allow the casting gas to be exhausted before it reaches the bottom surface of the transition plate. In one exemplary embodiment, the gas pocket surface includes a chamfered surface relative to the bottom surface, and one or more exhaust holes are defined in the chamfered surface. [Brief explanation of the drawings]

[0009] Having thus described the invention in general terms, reference is now made to the accompanying drawings, which are not necessarily drawn to scale.

[0010] [Figure 1] 1 illustrates an exemplary embodiment of a direct chill casting mold according to the prior art. [Figure 2] 1 illustrates an example of an early stage of direct chill or continuous casting according to an exemplary embodiment of the present disclosure. [Figure 3] 1 illustrates an exemplary embodiment of direct chill casting following an initial stage in accordance with an exemplary embodiment of the present disclosure. [Figure 4] 1 illustrates an exemplary embodiment of steady-state direct chill casting according to an exemplary embodiment of the present disclosure. [Figure 5] 1 illustrates air gap casting of a billet according to an exemplary embodiment of the present disclosure. [Figure 6] 1 illustrates a cast gas pocket configuration within a transition plate according to an exemplary embodiment of the present disclosure. [Figure 7] 1 illustrates a vent defined within a casting gas pocket according to an exemplary embodiment of the present disclosure. [Figure 8] 1 is a flowchart of a method for evacuating casting gases from a direct chill casting mold according to an exemplary embodiment of the present disclosure. [Figure 9] 1 illustrates a transition plate including an oxide dam according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0011]

[0013] Exemplary embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all, embodiments of the present disclosure are shown. Indeed, the embodiments described herein may take many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numbers refer to like elements throughout.

[0012] Embodiments of the present disclosure relate generally to systems, apparatus, and methods for evacuating direct chill casting molds, and more particularly to evacuating excess casting gases and retaining oxides from above the casting during the direct chill casting process.

[0013] Vertical direct chill casting, or continuous casting, is a process used to produce ingots or billets of various cross-sectional shapes and sizes for use in a variety of manufacturing applications. The direct chill casting process begins with a horizontal mold table or mold frame containing one or more vertically oriented molds disposed therein. Each mold defines a mold cavity, which is initially closed at the bottom with a starter block to seal the bottom of the mold cavity. Molten metal is introduced into each mold cavity through a metal distribution system to fill the mold cavity. Once the molten metal is near the bottom of the mold and solidifies adjacent to the starter block, the starter block moves vertically downward along a linear path into the casting pit. The movement of the starter block is driven by a hydraulic lowering platform to which the starter block is attached. The vertical downward movement of the starter block draws solidified metal from the mold cavity while additional molten metal is introduced into the mold cavity. Once initiated, the process operates in a relatively steady state due to the continuous casting process forming a metal ingot having a profile defined by the mold cavity and a height defined by the depth to which the platform and starter block are moved.

[0014] During the casting process, the mold itself is cooled to accelerate solidification of the metal before it exits the mold cavity as the starter block advances downward, and as the metal is cast, cooling fluid is introduced to the surface of the metal near the exit of the mold cavity to draw heat from the cast metal ingot and solidify the molten metal within the now solidified shell of the ingot. As the starter block advances downward, cooling fluid is sprayed directly onto the ingot to cool the surface and draw heat from within the center of the ingot.

[0015] FIG. 1 shows a cross-sectional schematic diagram of a direct chill casting mold 100 during a continuous casting process. The mold shown may be for, for example, a round billet or a substantially rectangular ingot. The cooling water spray patterns described herein are primarily intended for casting round billets. However, embodiments may be possible for use with substantially rectangular ingots, particularly if the corners of such ingots have some curvature. As shown, a continuous casting mold 105 defines a mold cavity in which a cast portion 110 is formed. The casting process begins with a starter block 115 sealing or substantially filling the bottom of the mold cavity against the mold walls of the continuous casting mold 105. The platform 120 moves downward along arrow 145 into the casting pit, and the cast portion 110 exits the mold cavity as the cast portion begins to solidify at its edge within the mold walls of the continuous casting mold 105. Metal flows from an injection trough 125, which in some embodiments includes a reservoir fed from a heated reservoir or furnace, into the mold cavity, for example, through a thimble 130. As shown, the thimble 130 is partially submerged in the molten metal pool 135 to avoid oxidation of the metal, which would occur if the metal were fed from above the molten metal pool 135. The solidified metal 140 constitutes the formed casting, such as an ingot. Flow through the thimble 130 is controlled within the injection trough 125, such as by a tapered plug fitting in an orifice connecting the cavity of the injection trough 125 with the flow path through the thimble 130. Conventionally, the injection trough 125, thimble 130, and mold cavity / mold wall of a continuous casting mold 105 are held in a fixed relationship from the start of a casting run to the end of the casting run. Metal flow through the thimble 130 continues as the platform 120 continues to descend into the casting pit along arrow 145. When the casting run ends, either because the platform is at the bottom of its travel, the metal supply is low, or the casting has reached its finished size, the flow of metal through the thimble 130 stops and the thimble assembled on the trough is removed from the molten metal pool 135, allowing the molten metal pool to solidify and complete the casting.

[0016] FIG. 2 illustrates an exemplary embodiment of a hot-top casting method for a direct chill casting process according to the present disclosure, including a continuous casting mold 105, a trough 125, and a thimble 130 for delivering molten metal from the trough to the mold cavity. The illustrated embodiment of FIG. 2 includes a starting position in which the thimble 130 or the tip of the thimble is positioned adjacent to a starter block 115 supported by a platform 120. The starter block 115 is positioned on the platform 120 and aligned to cooperate with the mold 105 to seal the mold cavity and prevent molten metal 107 from leaking between the continuous casting mold 105 and the starter block 115. The thimble 130 or the thimble is received within a transition plate 200 that is securely attached to the top of the mold 105, such as by a threaded engagement. The transition plate 200 of the exemplary embodiment is secured to the mold 105 by a metal ring that threads into a round opening on the billet mold 105 to hold the transition plate securely to the mold. The mold 105 in one exemplary embodiment is constructed from a metal such as aluminum, while the thimble 130 and transition plate 200 are generally formed from a heat-resilient, refractory material.

[0017] FIG. 2 illustrates the start of casting with the starter block 115 aligned with the continuous casting mold 105. As casting begins, as shown in FIG. 3, the platform 120 descends with the starter block 115 as molten metal flows from the trough 125 through the thimble 130, solidifying above the starter block 115 and at the bottom of the mold cavity forming the cast portion 140. Thus, as the starter block 115 descends away from the continuous casting mold 105, a cast portion, shown as 140 in FIG. 4, is formed. FIG. 4 illustrates the run-state phase, or steady-state portion, of the casting process, in which the platform 120 descends at a substantially constant rate and the cast portion 140 grows accordingly. FIG. 2 also illustrates spray jets 150, described in more detail below, which provide coolant or cooling fluid to the surface of the casting.

[0018] 2-4 with a transition plate 200 is effective, but has drawbacks. In particular, excess casting gases and oxides are trapped between the surface of the molten metal 107 and the transition plate 200.

[0019] According to an exemplary embodiment described herein, a billet mold casting technique for hot-top direct chill casting of aluminum, as shown in FIG. 5 , employs a graphite casting surface 210 where the initial solidification of the cast billet occurs. The permeable graphite material allows both casting gas and casting lubricant to flow over the casting surface, creating an air-slip casting condition involving a void 220 between the solidifying molten metal 107 in the mold cavity and the graphite casting surface 210. The casting lubricant reduces friction on the casting surface 210 to prevent sticking and tearing of the newly solidified shell of the cast part 140. The casting gas flow further helps reduce this friction while simultaneously providing a thin film of gas between the casting surface and the billet shell that reduces heat transfer from the molten aluminum to the casting surface. When properly balanced, the introduction of gas and oil produces an as-cast billet with a very smooth surface and a very narrow shell thickness compared to conventionally cast billets. Water or coolant flowing from coolant chamber 155 into spray jets 150 impinges on the shell of casting 140 and flows down the sides of the casting, as shown at 145, further cooling the casting.

[0020] The amount of casting lubricant used during casting is directly related to the surface area of ​​the billet. Balancing the amount of casting gas introduced through the casting surface is difficult. Due to the inherent shrinkage that occurs during the solidification process, the billet's shell shrinks slightly away from the casting surface 210, allowing gas to escape from the lower portion of the mold cavity. However, the density of casting gas is substantially lower than that of the molten metal, and excess casting gas that cannot escape from the lower portion of the mold tends to rise within the mold cavity and up through the molten metal above the mold in the casting system's injection trough 125 or "hot top" design. Additionally, in one example embodiment, air trap recesses or pockets are created in the transition plate 200 or graphite casting ring that forms the casting surface 210 to trap gas in pockets 230 at the corners of the mold cavity where the flowing liquid metal changes trajectory from a horizontal to a vertical trajectory and flows downward along the casting surface.

[0021] FIG. 6 shows a cross-sectional view of a portion of the mold 105, including a transition plate 200 secured to the mold by a threaded collar 205. Also shown are the graphite casting surface 210 and the corner pockets 230 that capture the rising casting gas. When properly balanced, a continuous flow of casting gas fills the pockets 230, and when the pressure increases to a point where it matches the metallostatic pressure of the metal in the trough 125, the gas flows downward through the void 220 without bubbling up through the thimble 130. Bubbling up through the molten metal must be reduced or prevented to prevent oxide films from being drawn into the metal above the mold and then down into the solidified billet. These oxide films are considered "inclusions" that can cause defects in subsequent downstream processing parts.

[0022] The gas pocket 230 in the direct chill casting systems described herein is the area where the transition plate 200 meets the casting surface 210. This is the area where molten aluminum flows out the metal feed opening in the thimble 130 toward the mold wall, then changes direction and flows downward to begin forming a solidified shell. In a hot-top casting configuration such as that shown in FIGS. 2-5, the metallostatic pressure of the liquid metal head above the mold forces the metal to completely fill this area and form a pocket of gas 230; the built-up gas pressure combined with the alloy and strength of the oxide creates a critical radius, commonly referred to as the "meniscus" radius. To aid in the formation of the meniscus radius and contain trapped gas, according to exemplary embodiments described herein, a recess is created in the transition plate at the casting surface interface.

[0023] The gas pocket 230 in the exemplary embodiment is designed so that its width is kept close to the natural meniscus radius where it will form. The depth of the pocket 230 in one exemplary embodiment is kept minimal to reduce the overall volume of the pocket. The edges of the pocket 230 in one exemplary embodiment are smoothed to reduce the tendency to tear the oxide layer as it moves along the hot metal surface and transitions to the pocket and meniscus radius. During casting using the hot-top method of direct chill casting described herein, a dynamic bulging or pulsating motion forms in the pocket 230 region. With the continued inflow of casting gas, the air bubble in the pocket increases in size and pressure until it can be forced down the gap 220 between the mold wall and the casting and escape out the bottom of the mold cavity. This increase in bubble volume forces metal back through the thimble or thimbles 130, thereby lowering the metal level as the gas pressure is released and the gas escapes. Rocking or rocking harmonics can occur in the mold located directly opposite the metal delivery runner in the trough 125. This periodic bulging of the meniscus should be reduced or kept to a minimum to prevent the formation of surge laps, which are accompanied by microstructural anomalies in the solidified billet shell commonly manifested as meniscus marks. These meniscus marks directly affect the overall width of the shell zone, and a thicker shell zone, if too pronounced, is undesirable for downstream processing.

[0024] A second reason for reducing or minimizing metal blistering is that as the size of the gas bubbles in the pocket 230 increases, the bubbles will extend over the edges of the pocket 230 and onto the hot metal surface adjacent the transition plate 200. When excess casting gas is released along the void 220 and the bubbles contract, the effect is to spread the oxide layer over the edges of the pocket. When this occurs, the oxide layer is often torn apart, which can result in metal buildup at the pocket edges along with random, non-uniform oxide release on the billet surface.

[0025] In a worst-case scenario for an exemplary embodiment of hot-top casting, the casting gas flow rate is too high for the natural gas release to travel downward and out through the bottom of the mold cavity, causing the excess gas to emerge over the edge of the thimble 130 opening and bubble upward through the melt above the mold. This sudden gas escape causes the gas pocket to violently collapse, completely filling that area with liquid metal. This event has several undesirable consequences that can lead to a deterioration in billet surface quality, including the resulting release of large, heavy oxides that create an uneven billet surface appearance. Breaking the protective oxide layer and allowing liquid metal to contact these surfaces increases the likelihood of these heavy oxides below the surface collapsing into the solidified shell and of adhesion to the transition plate pocket 230 area or the graphite casting surface 210. The collapse of the meniscus and exposure to liquid metal increases the likelihood of metal seeping into small gaps at the transition plate-to-graphite casting ring interface or any type of excess gas evacuation scheme. Metal buildup can result in scrap billets and potential bleed-out. The temperature of the casting surface temporarily increases during gas release from pocket collapse, which can lead to increased combustion of the casting lubricant and potentially produce varnish, another potential aluminum buildup site resulting in surface defects.

[0026] In addition to the above problems, casting gas bubbling up through the thimble 130 draws oxide films into the melt as oxygen in the casting gas bubbles is stripped off and reacts with the molten aluminum to form these oxide films. Billet quality is reduced by these oxides and surface problems resulting from casting gas migration. It is desirable to eliminate casting gas bubbling up through the melt throughout the casting process to prevent inclusion formation. In accordance with exemplary embodiments described herein, embodiments reduce or eliminate casting gas bubbling up through the thimble 130 and through the molten metal to prevent oxide film entrainment. Eliminating bubbling is a balancing act between allowing a sufficient flow rate of casting gas applied to the mold to maintain the casting conditions of the cavity 220 and limiting the rate at which escaped gas travels downward along the cavity interface and out through the lower portion of the mold, rather than up through the molten metal delivery system. The correct amount of casting gas is directly related to the thermal conditions of the casting surface. Colder casting conditions generally require higher casting gas flow rates than hotter casting conditions because at colder conditions, solidification of the billet occurs to a greater extent on the casting surface and more of the casting gas escapes out the bottom of the mold.

[0027] Higher temperature casting conditions move the solidification front further down the casting surface, allowing the casting gas to be more effective in maintaining the void 220. These conditions also reduce the ability of gas to escape from the bottom of the mold, thereby bubbling up through the thimble 130. This situation creates a challenge as many casting runs go through a metal temperature range that varies significantly from the beginning of casting to the end of casting, making it more difficult to optimize the casting gas flow rate to maintain the void 220 with minimal melt sloshing and no bubbling through the thimble 130. However, even when the melt temperature is stable, the casting gas flow rate window remains relatively narrow to maintain the best billet surface quality without losing the void 220, creating surge laps, or bubbling. Losing the void 220 produces a billet of inferior quality compared to billets with surge laps and can result in the entire billet being scrapped. Furthermore, losing the air gap over a period of time can cause the casting surface to overheat, burning off the casting oil and blocking the pores in the graphite casting surface 210, thereby impeding gas flow and requiring removal of the mold and replacement of the graphite casting ring.

[0028] The embodiments described herein include the ability to widen the casting gas flow rate window, which increases the robustness of the casting without creating the foaming problems described above. Venting excess casting gas as described herein allows for operation at higher casting gas flow rates, ensuring that the void 220 is maintained at lower casting conditions, while preventing foaming at higher temperatures.

[0029] According to one exemplary embodiment described herein and illustrated in FIG. 7 , a cross section of a portion of a transition plate 200 is shown and illustrated herein. The transition plate of the illustrated embodiment includes a top surface 238 and a bottom surface 248. The transition plate 200 further includes a rim 242 extending around the circumference of the transition plate, which in the illustrated embodiment includes a lip 244. When the transition plate 200 is in place within the casting mold 105, the lip 244 seals the top of the casting cavity against the mold. The lip 244 of the exemplary embodiment is shown elevated relative to the bottom surface 248 of the transition plate 200. The elevated position of the lip 244 relative to the bottom surface 248 of the transition plate 200 creates a casting gas pocket 230. The lip 244 is joined to the bottom surface 248 of the transition plate by a gas pocket surface. While the gas pocket surface (240) in the illustrated embodiment of FIG. 7 is a bevel or chamfer, embodiments include a fillet or a radiused surface.

[0030] As shown, the transition plate 200 includes exhaust holes 250 of a plurality of exhaust holes around the transition plate in the region of the pocket 230. The holes, in one exemplary embodiment 0.5 millimeters in diameter, are positioned along the slope of the gas pocket surface 240 of the transition plate in the recess of the gas pocket 230. The exhaust holes 250 exhaust to exhaust channels 260, which allow casting gas to escape from the casting mold 105. As the gas pocket bubble grows due to high gas flow rates, the edge of the bubble moves the meniscus 245 down the sloped surface of the pocket in the direction of arrow 255, receding the pocket edge and preparing it for bubbling through the melt. However, when the leading edge of the expanding bubble in the pocket 230 reaches the exhaust holes 250 on the slope of the gas pocket surface 240, the gas pocket self-exhausts excess gas. This type of system includes an orifice through which gas escapes that is small enough that the surface tension of the molten metal prevents metal from penetrating the orifice.

[0031] In another exemplary embodiment, vent 250 and / or vent channel 260 are filled with a porous material that allows gas to penetrate but not molten metal. Such materials include fibrous webs of material similar to filter cartridges. Vent 250 in one exemplary embodiment is filled with a porous material that provides a specific degree of resistance to gas flow, such that vents are optionally positioned at various locations within pocket 230 so that when gas pressure within the pocket reaches sufficient pressure, gas will escape through the vent without requiring a specific location for the bubble to recede before venting.

[0032] While passive venting is employed as described in the above embodiments, active venting of the gap in one exemplary embodiment provides a user-configurable alternative. One exemplary embodiment of such active venting includes a floating needle valve and seat arrangement designed to open at a specific gas pressure within the pocket 230 of the transition plate 200. The pressure in one exemplary embodiment is selected to be a predetermined pressure that approximately matches the metal head static pressure at the metal level above the mold. As the size and resulting pressure of the gas bubble in the pocket increase, the needle lifts from its seat, allowing excess casting gas to escape, thereby preventing the gas from bubbling up through the thimble 130. In one exemplary embodiment, such a pressure relief valve 265 is received within the channel 260 of the transition plate 200, as shown in FIG. 7 . The pressure relief valve 265 in one example is calibrated to a predetermined pressure determined to be lower than the pressure at which casting gas will bubble up through the molten metal, but higher than the pressure at which casting gas will escape in an undesirable manner. Additionally, for active evacuation of the gap, various pressure relief systems can be used to allow or prevent the flow of gas from the gas pocket 230 during casting. While the evacuation of casting gases from the gas pocket 230 can be to the atmospheric or ambient pressure of the casting environment, the evacuation of gas from the gas pocket in one example embodiment is also regulated by a pressure control to either decrease the pressure to keep the gas vent clear or increase the amount of gas evacuated by increasing the pressure as needed.

[0033] While the venting of the gas pockets in the previous embodiments is accomplished through the gas pocket vent holes as described above, embodiments optionally employ gas paths in the transition plate to guide the gas as it escapes from the gas pocket along a defined gas path. One embodiment includes engraved paths in the transition plate 200 and other refractory components, such as the thimble 130, to direct the gas along a path between the refractory pot shell and the liquid metal to prevent the formation of true bubbles that could rise through the thimble 130 and cause oxide entrainment. Another exemplary embodiment for creating a path for gas to escape is to create a chimney that allows the gas to bubble up toward the metal flow, out of the metal flow, and into the mold. In this embodiment, oxide films may be generated, but they are not entrained in the cast billet. The concept of venting excess casting gas allows a much wider window for casting gas flow rates for ease of multi-strand operation (multiple billets simultaneously), allows for reduced meniscus pulsation, and eliminates bubbling up through the melt.

[0034] FIG. 8 is a flowchart of a method for venting casting gases from a direct chill casting mold. As shown, molten metal is supplied to the direct chill casting mold through a transition plate, as shown at 310. In one exemplary embodiment, the molten metal is provided through a trough (e.g., trough 125) and a thimble (e.g., thimble 130). Casting gas is supplied through the casting surface of the mold, as shown at 320. The casting gas is supplied, for example, through the casting surface 220 of a graphite casting ring, as shown in FIGS. 2-6. Venting of the casting gas is performed from gas pockets in the transition plate, as shown at 330. The transition plate includes gas pockets that receive the casting gas, and as pressure builds, the casting gas is vented through the mechanism described above.

[0035] The flowchart blocks support combinations of means for performing the specified functions and combinations of operations for performing the specified functions. It will also be understood that one or more of the flowchart blocks, and combinations of flowchart blocks, may be implemented by various aspects of exhausting casting gases from a direct chill casting mold, as described above.

[0036] In some embodiments, certain of the above operations are modified or further extended. Additionally, in some embodiments, additional optional operations are included. The modifications, additions, or extensions to the above operations of one exemplary embodiment may be performed in any order and in any combination that facilitates venting of foundry gases as described herein.

[0037] In another embodiment, a valve system is used to pressurize the vent during the metal-filling stage of casting. Metal flowing into the mold creates turbulence that can force the liquid metal into small vent holes or porous media, effectively blocking the ability to vent excess casting gas. Applying a positive gas flow through the exhaust system helps alleviate this metal infiltration problem. The valve system switches from positive flow into the mold cavity to free-flow exhaust of gas pockets once the mold is filled with metal and the starter block begins to be lowered into the casting pit. This valve system can be a separately controlled and operated process, or it can be incorporated into an existing casting gas supply connected to the mold itself and use various casting gas pressures that alternate between applying positive flow to vent excess gas. This not only helps prevent metal infiltration during mold filling, but also helps prevent the vent from becoming blocked when the casting operator is applying a release coating to the hot metal surface of the transition plate 200 between casts.

[0038] Transition Plate Oxide Dam Additional transition plate embodiments include a transition plate "oxide dam." In the case of hot-top billet casting, the term "oxide dam" refers to an undercut recess in the transition plate extending from the thimble 130 or thimble area toward the mold bore. The use of an oxide dam creates a condition in which the majority of the oxide on the top of the billet is trapped and unable to break down and roll onto the as-cast billet surface. The hot metal surface is significantly reduced, and therefore the oxide layer is much thinner and remains mobile, easily flowing outward, rolling over the meniscus, and onto the as-cast billet surface. This results in a very uniform billet surface appearance, preventing random heavy oxide breakaways or "patches" from breaking free during casting and disrupting the billet's appearance. The narrow hot metal surface also helps eliminate the need to "hit" the mold hard with high gas flow rates to break free heavy oxides that form from the cascade metal during mold filling.

[0039] 9 shows two transition plates 200; the transition plate on the right is conventional and includes a pocket 230 on the periphery where the transition plate engages the mold cavity. The transition plate 200 on the left includes a pocket 230 on the periphery, but also includes an undercut 270 not present on the surface 280 of a conventional transition plate. The undercut provides an area for the thimble 130 to rest below the bottom of the undercut, providing an oxide dam as oxides on top of the molten metal are held within the undercut, while clean molten metal flows under the undercut, passes through the pocket 230, and migrates down the side of the casting.

[0040] To ensure oxide retention when the metal head gently bobs up and down under the light tension on the meniscus due to air gap casting conditions, Applicant has found an optimum undercut depth of approximately 12 millimeters in the transition plate of one exemplary embodiment. The hot metal surface is generally kept between approximately 12 and 20 millimeters. This distance is a compromise between helping to prevent gas bubbles forming at the meniscus from breaching the edge of the oxide dam and bubbling up through the opening in the thimble, and limiting the time the oxide thickness and strength must "grow" before rolling over the meniscus.

[0041] Many modifications and other embodiments of the inventions described herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. It is to be understood, therefore, that the invention is not limited to the specific embodiments disclosed, and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A transition plate (200) for engaging a direct chill casting mold (105), comprising: an upper surface (238); a bottom surface (248); a sloped gas pocket surface (240); a rim including a lip (244) extending around the periphery of the transition plate (200); the lip is joined to the bottom surface (248) by the inclined gas pocket surface (240); the lip (244) is configured to seal an upper portion of a mold cavity of the direct chill casting mold; the transition plate (200) meets the casting surface (210) of the direct chill casting mold (105), and a casting gas pocket (230) is formed at the outer periphery of the transition plate (200) by the lip (244) and the inclined gas pocket surface (240); The transition plate (200) further includes one or more exhaust holes (250) positioned along the slope of the inclined gas pocket surface (240) around the periphery of the transition plate, the exhaust holes (250) being configured to exhaust the casting gas pocket (230).

2. 2. The transition plate of claim 1, wherein the lip (244) is elevated relative to the bottom surface (248) when the transition plate is positioned on the direct chill casting mold (105), the elevated lip (244) defining the casting gas pocket (230), and the exhaust holes (250) are positioned closer to the bottom surface (248) than the lip (244).

3. 2. The transition plate of claim 1, wherein in response to bubbles forming in the casting gas pocket, the one or more exhaust holes are configured to allow the casting gas to be exhausted before the casting gas reaches the bottom surface of the transition plate.

4. The transition plate of claim 1 , wherein the one or more vent holes (250) comprise a web of gas permeable but molten metal impermeable material.

5. 2. The transition plate of claim 1, wherein the one or more exhaust holes are associated with a valve that enables the one or more exhaust holes to vent to atmospheric pressure in response to pressure within the casting gas pocket meeting a predetermined value.

6. 1. A method for evacuating casting gases from a direct chill casting mold, comprising: supplying molten metal (107) to the direct chill casting mold (105) through the transition plate (200) of claim 1; supplying a casting gas through a casting surface (210) of the direct chill casting mold; and venting the casting gas from a gas pocket (230) of the transition plate through one or more exhaust holes (250) positioned along a slope of a gas pocket surface (240) of the gas pocket (230) disposed around the periphery of the transition plate (200) where the transition plate (200) meets the casting surface (210) of the direct chill casting mold, and venting the casting gas from the gas pocket of the transition plate is performed in response to the pressure of the casting gas in the gas pocket reaching a predetermined pressure.

7. 7. The method of claim 6, wherein the predetermined pressure is determined based on a metal head static pressure of the molten metal (107) delivered to the direct chill casting mold (105).

8. applying pressure to the one or more exhaust holes (250) of the transition plate (200) to prevent the flow of molten metal through the exhaust holes; 7. The method of claim 6, further comprising reducing or removing pressure to the one or more vents to allow venting of casting gases.

9. 1. A system for evacuating a direct chill casting mold, comprising: a direct chill casting mold (105); a thimble (130) through which molten metal (107) is supplied to the direct chill casting mold; a transition plate (200) as recited in claim 1 attached to the direct chill casting mold and having the thimble received therein; In response to molten metal filling the direct chill casting mold, casting gases are exhausted through the one or more exhaust ports (250).

10. 10. The system of claim 9, wherein the lip (244) is elevated relative to the bottom surface (248) when the transition plate is positioned on the mold (105), and the exhaust holes (250) are disposed along a slope between the casting gas pockets closer to the bottom surface than the lip (244).

11. 11. The system of claim 10, wherein in response to casting gas bubbles forming in the casting gas pocket, the one or more exhaust holes are configured to allow the casting gas bubbles to be exhausted before the casting gas bubbles reach the bottom surface of the transition plate.

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