The present invention relates to continuous casting of molten metal. More particularly, the invention relates to a process and apparatus for top-feed casting of metals and for improving the surface quality of aluminum and other hard-to-cast alloys thereof.
Strand casting, also known as continuous casting, involves a process in which molten metal is introduced into a mold. During its residence time in the mold, the metal solidifies in contact with the wall of the mold and can be drawn downward via a movable bottom portion of the mold. A reserve of molten metal is positioned above the mold in what is referred to as a feeder head or a “hot-top”. A parting agent, or release agent, is applied to the surface of the continuous casting being formed, such that direct contact with the surface of the mold is avoided, thereby facilitating easy removal of the casting. Parting agents may include mixtures of oils and gases. It is particularly desirable if the oil-gas mixture is first formed close to the mold.
Hot-top molds of the kind described above are well known in the art. U.S. Pat. No. 5,320,159 to Schneider et al. discloses a hot-top mold where a parting agent, by way of a parting agent distributor, reaches the surface of the cast strand. Two different parting agents such as oil and gas, may be fed separately or as a mixture.
Certain hard-to-cast alloys, such as aluminum alloys containing lead, zinc, tin and copper, pose casting problems that can result in poor surface quality. Recently, such alloys have been gaining importance in the production of special alloys and machining alloy stock, which are to be used at a high cutting speed. Additionally, when the parting agent supply is insufficient or not uniformly regulated surface and edge structural defects can develop in the casting. These include, in particular, surface irregularities or in homogeneities in the structure near the surface.
One problem which contributes to the surface irregularities spoke about above and determines whether the parting agent reaches the entire surface of the metal strand is the precise control of the gas pressure. Pressure fluctuations can result in surface flaws, and pressure that is too high may pose a risk that gas might escape through the molten metal.
U.S. Pat. No. 4,732,209 to Pechiney attempts to solve this problem by using a graphite ring on the inside of the mold. The porosity of the graphite ring is established so that a gas under pressure is forced from the outside through the open pores of the graphite material to the inside of the mold and thereby acts as a parting agent between the surface of the forming metal strand and the mold surface. However, while this solution addresses the problem of controlling gas pressure, this solution cannot be applied to hard-to-cast alloys.
The present invention is directed to a hot-top mold that permits different types of alloys, in particular hard-to-cast alloys, to be made with an improved surface quality.
The present invention comprises a hot-top ring, which lies on top of a parting agent distributor and presses it against the surface of a mold. An overhang on the inner surface of the hot-top ring extends beyond the parting agent distributor in the downward direction of the movement of the strand and forms an annular gap with the running surface of the mold. A function ring is position on the inner surface of the mold and forms a function surface with the parting agent distributor through which gaseous and liquid parting agents are delivered.
The hot top can be disassembled for use with different alloys. A gaseous parting agent and a liquid (e.g. oil) are introduced into the parting agent distributor. The parting agent distributor and function ring may produce a foam layer to be used as a parting agent which permits improved surface quality, particularly for hard-to-cast alloys.
a is a top view of a parting agent distributor plate illustrated in
b is a bottom view of a parting agent distributor plate illustrated in
As shown in
Rings 1a, 1b of hot-top mold 1 are easy to assemble and disassemble, facilitating the quick exchange of different parting agent distributors 2 and function rings 5 according to the desired type of alloy. Loosening clamping ring 6 and outer ring 4 permits replacement of the parting agent distributor 2 and function ring 5.
Referring to
Function ring 5 includes surfaces facing the parting agent distributor 2 and the mold 3. Function ring 5 may comprise copper or copper alloys. It may also comprise ceramic, composite materials or graphite. The function ring surfaces (function surfaces) provide walls with a defined roughness for the radial channels on parting agent distributor 2. It is especially desirable if a function surface forms one wall face for the liquid-bearing channel. With a precisely adjusted surface roughness and temperature of the function surfaces, a defined change in the viscosity of the fluid parting agent can be achieved for producing a stable foam layer.
According to a preferred embodiment, a tight porosity of function ring 5 and a specific density is chosen within narrow limits, such as a closed porosity of 0-20% and a density of 1.5-10 g/cc. Additional improvements in the stability of the parting agent foam can be achieved by cooling function ring 5 in order to keep the viscosity properties on the function surfaces constant. The construction and manner of operation of the function surfaces and the cooling is further explained below.
Referring to
The formation of certain function surfaces, such as the gas and liquid carrying passages described above as well as annular channel 11 and the area directly inside the function ring 5, is critical to the formation of a stable parting agent foam. The annular channel 11 is in direct contact with the passages of the parting agent distributor 2 and cooperates in the formation of the foam.
As an innovative and supplementing feature of the function surfaces, activator ring 7, which can be made of various materials, lies between outer ring 4 and the top 2a of the parting agent distributor 2. It thus covers the top of the gas-carrying passages made in parting agent distributor 2. Its roughness values differ from those of the hot-top ring 1b and can be adjusted to the particular requirements of the parting agent, and thereby control the thermal gradient of parting agent distributor 2.
Function ring 5 is preferably cooled by arranging cooling passages 15 (
In this primary cooling zone, a temperature is established for the optimum action of the parting agent. An adjoining annular gap 14 forms a secondary cooling zone providing rapid removal of heat in the annular gap 14 and the foam layer 16. This secondary cooling zone runs in the direction of the descent of the strand and into a slot nozzle 53. Here, the pressure of the coolant is lowered and is directed into contact with the aluminum strand permitting heat dissipation thereof.
Mold 3 is supplemented by an outer ring 4 and a function ring 5. By means of a clamping ring 6, the system parts are assembled together with the inclusion of an activator ring 7. The mold is completed by a bottom part 8 and a pressure plate 9. Pressure plate 9 stands in connection on its top end with mold 3 and on its bottom end with the bottom part 8. The pressure plate 9 supports mold 3 above a liquid reservoir 42 (used as a coolant) and is used to avoid deflections in the mold 3. The use of pressure plate, as used in this fashion, is a common measure taken to avoid deflections of the supported member, e.g., to avoid deflection of the mold 3.
Initially, the viscosity of the parting agent foam 12 is controlled by the surface roughness in the gas and liquid passageways and is an essential factor in the formation of the foam. Furthermore, the pressure and flow rate of the delivered gaseous and liquid parting agents can be controlled, thereby regulating the composition of the parting agent foam within wide limits. This control over the gaseous and liquid parting agents provides for a controlled heat removal and is particularly advantageous for hard-to-cast alloys.
Referring to
In operation, gas and liquid are ejected from the respective top and bottom radial channels of parting agent distributor 2 through the annular channel 11 to the meniscus. Various types of gases which can be used include, but are not limited to: air, nitrogen, argon, CO2 and Freon. The force of the gas traveling through annular channel 11 causes parts of the liquid separating agent or lubricant, e.g., oil, to entrain with the gas. A parting agent foam or emulsion of gas and lubricant/separating agent forms and travels in laminar flow below the overhang 30 in the draw-off direction (the direction of the flowing cast metal) and parallel to the radial inside surface of function ring 5.
As the cast metal flows downwardly, the molten starts to solidify. Along the inner edge of the function ring 5, the cast metal is at least partially solidified. At the beginning of a casting run the solid metal first presented along the function ring 5 is called the starter block. The parting agent foam travels along the gap present between the starter block and running surface 38 of mold 3. Below the running surface 38 a water stream passes from the cooling passage 15 through the annular gap 14.
Cooling passage 15 extends proximate the agent distributor 2 and the function ring 7 and provide two cooling functions. First, coolant passing through passage 15 acts to cool distributor 2 and function ring 7, forming a first cooling zone. The liquid conducted through cooling passage 15 is directed into a slotted nozzle (slot nozzle 53) formed by annular gap 14 and is sprayed on the descended strand forming a second cooling zone. Unlike the first cooling zone, this liquid stream provides rapid heat removal from the cast strand through the foam layer 16. Additionally, leading the water stream into slot nozzle 53 lowers the pressure of the coolant.
Annular gap 14 is directed towards the starter block and a water stream is forced through annular gap 14 towards the block. The direction of flow of the water stream causes the formation of a vacuum, drawing the parting agent foam downwardly along running surface 38 and the edge of the starter block. The vacuum causes the formation of a foam veil, which completely shields the function ring 5 and wall 38 from the liquid metal, presently above the starter block.
The mixing of the gas with the lubricant and /or separating agent occurs variably between 2 and 10 mm before the outlet opening of annular channel 11. The ability to predetermine the point of mixing allows the mixing process to be optimized. The actual point of mixing is dependent on the configuration of the parting agent distributor. Also, different oils having different viscosities exhibit different mixing behavior with various gases.
Number | Date | Country | Kind |
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10115999.4-24 | Mar 2001 | DE | national |
This patent application claims the benefit of priority, under 35 U.S.C. § 120, as a continuation-in-part of U.S. patent application Ser. No. 10/115,808, filed Mar. 28, 2002, entitled “Mold with a function ring” which is herein incorporated by reference in its entirety.
Number | Date | Country | |
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Parent | 10115808 | Mar 2002 | US |
Child | 10951950 | Sep 2004 | US |