STOPPER ROD FOR REGULATING THE FLOW OF A LIQUID, HAVING A FREE SPACE FED WITH GAS

Abstract: The stopper rod for regulating the flow of a liquid has a body (2) traversed by an axial passage (6) and having at one end a porous nose (4) fed with gas. This nose (4) has a free space (8, 108) fed with gas , e.g., a slot or a mesh network (108) comprised of a network of channels. Preferably, bridges of material (10) connect the two faces of the free space.

Description: The invention concerns a stopper rod for regulating the flow of a liquid, having a porous part fed with gas according to the preamble of claim 1. Stopper rods are frequently used in industry for opening and closing an orifice of a receptacle containing a liquid. By more or less removal of the stopper rod from the orifice of the receptacle, the flow rate of the liquid can be regulated. In some cases, these regulating stopper rods make it possible to blow a gas into the liquid contained in the receptacle. Thus, the use of the stopper rod for controlling the flow of a molten metal emerging from a tundish into a water-cooled continuous-casting mold is well known. This stopper rod is used to introduce an inert gas, generally argon, into the molten metal. The purpose of the argon is to eliminate the inclusions contained in the molten metal. Another purpose is to reduce the deposits of alumina that occur in the casting elements. Finally, the injection of argon makes it possible to avoid the development of a vacuum inside of the casting element, this vacuum being capable of leading to an aspiration of air that in turn causes an oxidation of the molten metal. According to an initial familiar actualization, the argon is injected by means of an axial channel that passes through the stopper rod and passes out at the end of the latter. The injection of a gas by means of a porous stopper sealed in the refractory material at the end of the stopper rod is also well known. In the case of the first solution the hole has a substantial diameter, of the order of 2-3 mm. Consequently, a rise in the liquid metal can occur through this orifice in the case where the pressure of the gas would be interrupted for any reason. Furthermore, the injection is localized at one point and induces large bubbles that are less effective for eliminating the impurities contained in the metal. The second solution certainly makes it possible to produce small bubbles distributed on the surface of the porous stopper. Nevertheless, there is the risk of unsealing of this stopper that would lead to a rise of the molten metal in the axial channel of the stopper rod. A stopper rod having a porous nose isostatically co-pressed at the same time as the body is also known (U.S. Patent, 4,791,978 upon which the preamble of claim 1 is based.). The porous nose has a composition similar to that of the body, but its permeability is much higher. The copressing makes it possible to avoid the risk of loosing the porous nose. Nevertheless, in this actualization the internal surface of the end of the axial channel of the stopper rod is small. In addition, the thickness of the porous material to be passed through is substantial, e.g. 40 mm, for a stopper rod of current dimensions. These characteristics lead to a limitation of the argon flow rate obtained at elevated temperatures. Thus, the maximum flow rate of argon obtained at 1500° C is ca. 6-8 Nl/mn for a counterpressure of 2.8 bar. This flow rate is insufficient in some cases and also the relatively substantial counterpressure that is necessary is dangerous for the axial channel, the connection of the stopper rod and the gas feed piping. The principal risk is the bursting of the nose during the casting, the catastrophic consequence of which would be loss of control of the molten metal flow. The second problem is a high risk of leakage in the connections, leading to the inefficacy of the porous nose. Given that a high resistance to corrosion by steel is necessary, all the attempts made to increase the permeability of the porous material constituting type nose failed because an increase in the number of pores and/or an increase in their size result in an unacceptable reduction in the resistance of the porous material. The present invention is precisely to remedy these shortcomings of the prior art. The object of the invention is to create a stopper rod for regulating the flow of a liquid that preserves the advantages of stopper rods of the prior art, while permitting an increase in the flow rate of gas; this flow rate should be obtained for a lower counterpressure. This result is obtained according to the features as said out in claim 1. This free space has the effect of bringing the gas closer to the outside surface of the stopper rod. Consequently, the flow rate of gas is increased for a same counterpressure value because the thickness of material to be traversed is decreased. The free space is preferably a slit that can be continuous or discontinuous. Preferably, bridges of material connect the two faces of the slits. These bridges avoid the loss of the outer part of the porous nose in the case where erosion due to the steel would reach the slit. According to another preferential characteristic, the free space has a surface greater than the inside surface of the porous nose. Due to this characteristic, the contact surface between the inert gas and the porous material is increased. The passage cross section offered this gas is thus increased. Consequently, the flow rate of the gas is increased for the same value of the counterpressure. According to one actualization variant, the space left free in the porous stopper is comprised of a network of channels or a mesh network provided in the porous material. It is obvious that the configuration of the free space is not limited to these examples, but can be chosen freely as a function of the needs of the user and the application. Finally, in some applications the thickness of the porous material separating the outer surface of the stopper of the free space is chosen so as to define at least a preferential zone of blowing. In other words, the distance that separates the free space, e.g., the slit, from the outer surface of the porous nose in not constant. It can be less in a given zone in order to obtain a greater flow rate of gas in this zone, the passage of the gas being facilitated by the decrease in the thickness of the wall to be traversed. Preferably, the free space is contained entirely in the porous stopper in order to avoid the development of fragile zones at the level of the junction surface between the porous nose and the body of the piece. The invention will now be described in greater detail with reference to the attached drawings, which present only one mode of execution, given for the sake of illustration. Figure 1 is a cross sectional view of a stopper rod having a porous nose according to the invention. Figure 2 is a cross sectional view of a stopper rod according to the invention, having a free space provided in the porous nose. Figure 3 is a detailed view that shows another actualization example of the free space. Figure 4 shows a slot shaped so as to create a zone of preferential blowing. Figure 5 is a view of a molding form that facilitates development of a free space. Figure 1 shows a cross sectional view of a stopper rod designed to regulate the casting of a liquid, e.g., a molten metal according to a known prior art (U.S. Patent 4,791,978). This stopper rod is comprised of a body 2 of general cylindrical form and a nose 4 located at a lower end of the body 2. The body is traversed by a longitudinal passage channel 6. The entrance end of the channel 6 is threaded to fit a connection for feeding the stopper rod with an inert gas, e.g., argon. The channel 6 is prolonged into the porous nose 4, to which it facilitates bringing the inert gas. The nose 4 for example has a hemispherical or ogival form. It is found that in this embodiment the exchange surface between the end of the passage channel 6 and the porous nose is reduced. Furthermore, the thickness of the porous material 4 that is to be traversed by the inert gas is relatively substantial, of the order of 40 mm in a current exemplary embodiment. Consequently, for a stopper rod of this type the argon flow rate obtained at a temperature of 1500 °C does not exceed 8 Nl/mn for a counterpressure of 2.8.10 5 N/m2 (2.8 bar). Although such a flow rate is adequate in some cases, it is dangerous to work with such a high counterpressure in the stopper rod and in the gas feed system, as explained previously. In effect, there are risks of leakage at the level of the connection and the pipes, as well as the risk of bursting of the stopper rod. Figure 2 shows a cross sectional view of a stopper rod according to the invention, having a free space in the porous material. It is distinguished from the stopper rod of the prior art, shown in Fig. 1, through the fact that it has a free space designated by 8, effected in the porous material comprising the nose 4. In the example shown, the free space 8 is comprised of a slot of essentially hemispherical or ogival shape and which is essentially parallel to the outer surface 9 of the porous nos 4. It is noted that bridges 10 connect the two edges of the slot. These bridges avoid the loss of the outer part of the porous nose in the case erosion due to the steel would reach the slot 8. It should be noted that the junction 11 between the porous material and the nonporous material is not traversed by the slot 8 in order to avoid weakening this critical zone in which stresses are present by reason of the slightly different nature of the materials in contact. In the exemplary embodiment shown in Fig. 2 the apex of the slot is tangent to the lower part of the casting channel 6. Thus, the free space 8 is fed with gas directly through the casting channel 6, as well as through a number of radial passages 12 that permit feeding the other end of the slot. However, it is not necessary that the slot be tangential to the end of the passage channel 6. In one actualization variant this slot could be included entirely in the material constituting the porous nose. The slot 8 that was just described makes it possible to obtain two different effects : 1) It reduces the thickness of the porous material to be traversed by the gas. 2) Due to the fact that the surface of the slot is greater than the surface of the interface between the passage channel 6 and the lower part of the porous nose 4, the exchange surface is increased. These two factors contribute to increasing the flow rate of argon for the same counterpressure value. Figure 3 shows an actualization variant of the invention in which the free space instead of being comprised of an essentially continuous slot is a lattice network, designated by 108, obtained by means of a network or netting of wax wire, which are eliminated during baking. In this actualization example the surface of the free space is smaller than in the preceding example. Consequently, the exchange surface with the central passage 6 is also smaller. However, the advantage that consists in bringing the gas close to the outer surface of the porous nose is preserved so that the flow rate of the gas can at least be increased for an identical value of the counterpressure. Finally, Figure 4 shows another actualization variant of the invention, in which the shape of the free space was shaped to define a preferential blowing zone 15. In other words, the thickness of the porous material to be traversed by the gas is not essentially constant. On the other hand, this thickness is sharply diminished in a zone where one wishes to obtain a preferential blowing. In order to obtain this result, the shape of the slot is modified in the desired zone, as shown in Fig. 4. It is obvious that the shape of the free space is not limited to the two actualization examples just described, but the technician can adapt the shape of this space as a function of his particular needs. The actualization of the stopper rod shown in Figs. 2-4 is as follows. A molding form 16 is placed on a mandrel (see Fig. 5); its shape corresponds to the shape of the free space 8 that one wishes to obtain, e.g., a continuous slot (Fig. 2) or a mesh network (Fig. 3) or any other form desired. This molding form is made of expendable material, i.e., a material that will be eliminated in a subsequent stage of the process. In addition, centering rods 14 also of an expendable material, assure the positioning of the molding form on the pressing mandrel. Then, in a classic manner, the pressing mandrel with the molding form on it is placed in a pressing envelope that is filled first with nonporous materials and then porous materials. After isostatic pressing, the stopper rod is heated moderately or dried and then baked in a furnace. During the heating and/or baking operation, the molding form of wax is eliminated, leaving vacant the empty space desired as well as the radial channels 12 for the passage of the gas. EXAMPLE : A stopper rod according to the invention, having a slot of the type shown in Fig. 2, has been produced. With this stopper rod it was possible to measure an increase in the argon flow rate up to 20 Nl/mn at a temperature of 1500°C for a counterpressure less than 2.10 5 N/m2 (2 bar). As we recall from the introductory portion, for a stopper rod of the prior art of identical dimensions, at the same temperature of 1500°C, it was not possible to obtain an argon flow rate more than 8 Nl/mn for a counterpressure of 2.8.10 5 N/m2 (2.8 bar). This comparison consequently shows that the invention makes it possible to multiply the argon flow rate by 2 or more. Although the invention was described principally with reference to a stopper rod, it is equally applicable to any casting of a fluid, comprised of a porous part and a nonporous part. It can be advantageously applied to a nozzle having a porous sleeve on the inside for the injection of a gas.

Claims: Stopper rod for regulating the flow of a liquid, said stopper rod comprising an elongated body (2) made of a refractory material, and a substantially rounded nose (4) at one end of said elongated body (2), said body and said substantially rounded nose being isostically co-pressed in one single operation, and a longitudinal passage channel (6) traversing said body (2) and ending in said porous nose (4), said longitudinal passage channel being able to be connected to a gas feed duct for injecting said gas into the liquid, characterized in that the porous nose (4) has a free space (8-108) which communicates with said longitudinal passage channel but which does not communicate with the outer surface of the nose, said free space having a shape which substantially matches that of the substantially rounded nose the shape of the free space being designed so as to reduce the thickness of the material to be traversed by the gas, the shape of the free space being also designed such that its surface is several times larger than the internal surface of the channel passage (6) ending in the porous nose. Stopper rod according to claim 1, characterized in that the free space is a slot. Stopper rod according to claim 1 or 2, characterized in that bridges of material (10) connect the two faces of the faces of the free space. Stopper rod according to claim 1 or 3, charaterized in that the free space is a network of channels (108) provided in the porous material. Stopper rod according to any one of claims 1-4, characterized in that the thickness of the porous material separating the outer surface of the porous nose (4) from the free space in chosen so as to define at least one preferential blowing zone (15). Stopper rod according to any one of claims 1-5, characterized in that the free space (8, 108) is contained entirely in the porous nose (4).

Inventors: BUBOIS, PASCAL, DUMAZEAU, CLAUDE
Applicants: VESUVIUS FRANCE S.A.
Application year: 1990
Main classification: B22D41
IPC codes: B22D11, B22D41