Mold with a function ring

Abstract

The invention relates to a hot-top mold for a strand casting apparatus, consisting of a hot-top, which lies on the upper side of a parting agent distributor and presses it with its underside against the surface of a mold, an overhang being formed on the radially inner surface of the hot-top, which protrudes beyond the parting agent distributor in the direction of take off of the strand, the hot-top being centered and held by an outer ring, which is releasably fastened to the mold. The mold surrounds a function ring toward the bearing surface of the mold which, together with the parting agent distributor, forms function surfaces with adjustable roughness on the surfaces. In the parting agent distributor, radial channels are formed at the upper side and the underside, the channel cross sections at the upper side and the underside being in a ratio of 1:3 to 1:5 to one another.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to an apparatus for strand casting of molten metal, and more particularly, to a hot-top mold for improving the surface quality of hard-to-cast alloys.

[0002] 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. The reserve is also 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.

[0003] Hot-top molds of the kind described above are well known in the art. European Pat. No. EP 0566 865 to VAW 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.

[0004] 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.

[0005] Another problem is the precise control of the gas pressure, which determines whether the parting agent reaches the entire surface of the metal strand. 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.

[0006] 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.

SUMMARY OF THE INVENTION

[0007] 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.

[0008] 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.

[0009] 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.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 is an exploded view of a hot-top mold, in accordance with a preferred embodiment of the invention; and

[0011] FIG. 2 is a representation of a hot-top mold used in strand casting, in accordance with the preferred embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENT

[0012] As shown in FIG. 1, a hot-top mold 1 comprises an upper ring 1a and a lower ring 1b. Rings 1a, 1b are secured within the radially inside surface of an outer ring 4 which is releasably fastened to a mold 3. Mold 3 surrounds at its inner surface a function ring 5, whose upper end forms a function surface with a parting agent distributor 2. A clamping ring 6 secures hot-top rings 1a, 1b, parting agent distributor 2, mold 3, and outer ring 4. An activator ring 7 lies between outer ring 4 and the top section 2a of parting agent distributor 2. A bottom part 8 and pressure plate 9 compete the lower portion of mold 3.

[0013] Rings 1a, 1b of hot-top mold 1 are easy to assemble and disassemble, facilitating the quick exchange of different parting agent distributors and function rings 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.

[0014] Parting agent distributor 2 comprises a plate provided on both sides with radial channels. The cross sections of the channels may be in a ratio of 1:3 to 1:5 between top and bottom sides. This ratio is important for controlling the composition of the parting agent mixture. By matching the surface and channel cross sections, foam production is easily possible. Foam is especially effective as a highly viscous parting agent and permits an improved surface quality, particularly for hard-to-cast alloys.

[0015] Function ring 5 includes surfaces facing the parting agent distributor 2 and the mold surface. Function ring 5 may comprise copper or copper alloys. It may also comprise ceramic, composite materials or graphite. The function ring surfaces form walls with a defined roughness for the channels formed by the parting agent distributor 2. It is especially desirable if the 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.

[0016] It is advantageous if a defined, 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 the 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.

[0017] The top surface 2a of parting agent distributor 2 is used preferably to form gas-carrying passages, and bottom surface 2b of parting agent distributor 2 is used for a liquid parting agent. The passages incorporated into the upperside 2a are connected to a pressurized gaseous medium, while the passages on the underside 2b are connected to a pressurized liquid reservoir. The passages must be made with very great precision, which can be achieved by laser machining, a chemical etching method, or other techniques. The cross-sectional shape of the radial passages is important for the air content in the parting agent foam, and the surface area ratio of the passages is important for the formation of a parting agent foam distributed uniformly over the circumference. It is preferred that the lower radial passages be configured in the manner of a diffuser, with an approximately square cross section being given to the radially outward lying passage carrying parting agent distributor 2, and also a rectangular cross section with a surface ratio of at least 1:2 formed on the radially inward lying exit side. Under these circumstances the parting agent is formed with especially fine cells and hence it is strong, thereby reducing the liquid component of the parting agent.

[0018] Function ring 5 is preferably cooled by arranging cooling passages 15 (See FIG. 2), in mold 3, which extend into the area underneath parting agent distributor 2 and function ring 5. In this primary cooling zone, a temperature is established for the optimum action of the parting agent. An adjoining secondary cooling zone permits a rapid removal of heat in the annular gap 14 through a foam layer 16, since the cooling passages here run in the direction of the descent of the strand and lead into a slot nozzle. Here, the pressure of the coolant is lowered, so that the coolant is in contact with the aluminum strand, thereby permitting heat dissipation. Lower ring 1b has a thermal conductivity of approximately 1.5 to 2.0 times greater than that of upper ring 1a.

[0019] The formation of certain function surfaces is critical to the formation of a stable parting agent foam. The function surfaces are situated above and below the parting agent distributor as well as on the inside of the function ring 5. The latter is in direct contact with the passages of the parting agent distributor 3 and cooperates in the formation of the foam.

[0020] 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 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. At the same time, the thermal gradient of the parting agent distributor 2 underneath the hot-top 1b can thereby be controlled.

[0021] Mold 3 is supplemented by an outside 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 3 is completed by a bottom part 8 and a pressure plate 9.

[0022] FIG. 2 illustrates the basic construction of the hot-top mold. An annular gap 11 exists underneath the overhang of hot-top ring 1b, the side walls of annular gap 11 being formed by the hot-top overhang and by the inside radius of the parting agent distributor 2 and by the function ring 5. Upon the introduction of gaseous and liquid parting agents within the volumetric ratio of the invention, a stable parting agent foam 12 is formed, which develops as a continuous layer of foam between the mold 3 and the metal strand 10.

[0023] Initially, the viscosity of the parting agent is controlled by the surface roughness in the gas region as well as in the liquid region. The viscosity of the parting agent is the essential factor in the formation of the foam. Furthermore, the pressure and rate of flow of the delivered gaseous and liquid medium can be controlled, so that the composition of the parting agent foam can be regulated within wide limits. In turn, a controlled heat removal can be performed, or else an insulating effect can be produced with the parting agent. This is particularly advantageous for hard-to-cast alloys.

Claims

We claim:

1. A hot-top mold for a strand casting apparatus comprising a mold (3), a parting agent distributor (2), a hot-top ring (1a, 1b) lying on the top (2a) of the parting agent distributor (2) and pressing its bottom (2b) against the top surface (3a) of the mold (3), an overhang being formed at the radially interior surface of the hot-top ring (1a, 1b) protruding beyond the parting agent distributor (2) in the withdrawal direction of the strand and forming an annular gap (11) with the mold surface, an outer ring (4) releasably fastened to the mold (3), for centering and holding the hot-top ring, a function ring (5) surrounded by the mold positioned toward the inner surface of the mold, the function ring and parting agent distributor (2) forming function surfaces with adjustable roughnesses on the surfaces, and radial passages formed on the top and bottom surfaces (2a, 2b) of the parting agent distributor, the passage cross sections between the upper and bottom surfaces being in a ratio of 1:3 to 1:5.

2. The hot-top mold of claim 1, wherein the function ring (5) includes copper or copper alloys.

3. The hot-top mold of claim 1, wherein the function ring (5) includes ceramic or composite materials.

4. The hot-top mold of claim 1, wherein the function ring (5) includes graphite material.

5. The hot-top mold of claim 1, wherein a clamping ring (6) is disposed above the hot-top (1a, 1b), by which the hot-top (1a, 1b), the parting agent distributor (3), the function ring (5) and the mold (3) are clamped.

6. The hot-top mold of claim 5, wherein the function ring (5) has a closed porosity of 0-20% and a density of 1.5-10 g/cc.

7. The hot-top mold of claim 6, wherein the radial channels, incorporated into the upper side 2a of the parting agent distributor (2), are connected to a pressurized gaseous medium and that radial passages are formed at the underside (2b) of the parting agent distributor (2) and are connected to a pressurized liquid reservoir.

8. The hot-top mold of claim 7, wherein the lower radial passages of the parting agent distributor are configured in the manner of a diffuser, an approximately square cross section being given to the radially outward lying passage carrying the parting agent distributor (2) and a rectangular cross section with a surface ratio of at least 1:2 being formed on the radially inward-lying exit side.

9. The hot-top mold of claim 8, wherein cooling passages (15) are disposed in the mold (3), which reach into the area underneath the parting agent distributor (2) and the function ring (5), heat removal being performed in the annular gap (14) through a foam layer (16) produced by the gas and liquid medium and issuing in the descending direction of the strand.

10. The hot-top mold of claim 9, wherein the bottom part of the thermal conductivity of the hot-top (1b) is 1.5 to 2.0 times greater than that of the upper part of the hot-top (1a).

11. The hot-top mold of claim 10, wherein the upper part (2a) of the parting agent distributor (2) is covered by an activator ring (7).