EP0028569A1 - Process for agitating a molten metal by injection of gases - Google Patents

Abstract

In the homogenization of molten metal with a gas, the gas is delivered to agitate the molten metal in a vessel, through an injector tube having a fine bore effective to prevent penetration of molten metal into and along the bore; when injection of gas is discontinued, molten metal is allowed to solidify on the tip of the tube to seal it; when gas is to be delivered to a second load of molten metal in the vessel, it is delivered, at least initially, at a pressure effective to unseal the tube at the tip to permit entry of the gas into the second load to agitate the molten metal and thereby homogenize it; the injector tube can be used for successive batches of molten metal without maintenance.

Description

The present invention relates to the treatment of molten metals by methods which involve the intervention of a gas. It relates more particularly to a method and a device for stirring a molten metal, which can be used in particular for the homogenization or degassing of steel or other metals.

With the development of continuous steel casting and the demand for better quality steels, the use of inert gas to ensure the agitation of molten steel is increasing. The inert gas is used to homogenize the molten steel, in its chemical composition and in temperature, after its extraction from the refining furnace and before its casting in ingots or in continuous casting devices. Mixing with a gas in the ladle makes it possible to obtain a steel whose characteristics are more uniform in all respects.

Several methods of brewing molten steel are known. In a first method, the molten steel is stirred, by means of a steel ingot which is lowered using a traveling crane, into the molten steel contained in the ladle. The movement of the bridge causes a displacement of the ingot and the agitation of the metal. This process is impractical, it takes time and it is not effective enough.

Other methods are known which involve agitation by injection of gas and which are safer. They differ from each other in the way the gas is introduced. In one of these methods, a stopper is used which consists of a rod which is hollow, so that it can convey gas. The stopper rod has at its end small orifices arranged radially, allowing the outlet of the gas. It is protected by a refractory sleeve. In operation, the stopper system, instead of being attached to the pocket, is attached to a fixed raised horizontal beam; the rod is placed upside down and the gas is introduced into it by a gas inlet at the top and it exits at the bottom, through the head orifices. A pocket containing molten metal is brought, by a crane, into position under the sys stopper rod and it is lifted so that the stopper rod is immersed in the molten metal. The gas exiting the head of the stopper spears through the molten metal, thereby producing the desired agitation.

This process is used industrially. However, such a stopper device only lasts from 5 to 10 operations, after which it must be replaced. On the other hand, it cannot be certain that all the gas flow passes into the molten metal, because the refractory sleeves constitute a poor quality seal against the steel stopper, and, as a result, the gas imprints the path of less resistance between the stopper rod and the refractory sleeves, rather than passing through the molten metal. Finally, sufficient agitation can only be ensured by examining the surface of the molten metal to adjust the gas flow rate until the desired degree of agitation is observed.

In another known method, a porous refractory is used to introduce the gases into the molten metal. This process is implemented industrially. It uses a refractory brick or a porous plug having the property of being permeable to gas under pressure but substantially impermeable to molten metal. The porous plug constitutes part of the internal lining of the bag, in a place where it is submerged when the bag is filled with molten metal. The gas is introduced into the molten metal through the porous plug and the desired degree of agitation is obtained by action on the gas flow rate. Under unfavorable local conditions, such porous plugs can have a life expectancy of 10 to 25 cycles, and in the case of the composition of molten iron, the life can be from 50 to 200 heating cycles.

In another known method, implemented in some installations, a metal tube is used, made of steel, which is embedded in the refractory lining of a ladle, for example at the bottom or near the bottom. This tube passes through the steel outer casing and the internal lining of refractory material and ends at the surface interior of the latter. The introduction of gas begins before the metal is poured into the pocket and after obtaining the desired agitation, the introduction of gas is stopped. The metal then flows back into the tube and solidifies. Generally, the tube should be replaced after each cycle. Sometimes the tube can be cleaned after use, by broaching using a steel bar or by drilling and removing the solidified metal, and it can be reused until it becomes too short.

The porous refractory and metal tube processes are also applied for the introduction of a relatively non-reactive gas at the bottom of a bath of molten metal in the steelworks field, in certain oxygen converters of the LD type. In electric ovens, reverberatory ovens and the like, these gas distribution stirring devices can also be used to stir the molten metal.

The subject of the present invention is a method and a device for stirring by introducing pressurized gas into a molten metal, which does not require any maintenance intervention between successive heaters. The method and the device according to the invention can be used for stirring molten metal, in order to homogenize the latter, but they can also be used to partially or completely expel a particular gas dissolved in the molten metal. Furthermore, if the gas can be an inert gas, playing only a stirring role, it can also be a reactive gas in the presence of the molten metal, for example a reducing gas or an oxidizing gas, or any other gas to be introduced into the molten metal or to be mixed with it.

According to the invention, the dimensions of the metal tube through which the gas is injected are chosen so that the molten metal solidifies at the end of the tube and completely closes the mouth, the inlet of molten metal in the inner conduit of the metal tube thus being prevented. The closed end is easy to unclog by gas pressure when the metal tube is put into service.

The metal tube according to the invention therefore makes it possible to avoid the drawbacks of the metal tubes used in the prior process in that it has a long service life and can be used in successive heating cycles without requiring maintenance. This distinguishes it from the metal tubes according to the prior art, of larger diameter, which must be replaced after each use. In addition, there is no need for a special device to prevent the entry of molten metal into the gas supply circuit, since the metal tube itself prevents this entry.

The invention thus relates to a device for stirring a molten metal by gas injection, characterized in that it comprises a tank for containing the molten metal, comprising an outer casing provided with a refractory lining and at minus a metal injection tube which passes through this envelope and ends at the right of the interior surface of the refractory lining, means for admitting a gas under pressure into the tank through the injection tube, from a source of supply of gas under pressure external to the tank and means for interrupting the injection by closing a connection pipe between the injection tube and said source, and means for creating at least temporarily an inlet pressure for the gas sufficient to expel a solidified metal plug possibly formed at the end of the tube during an interruption of the injection.

The invention also relates to a method of stirring molten metal by means of the preceding device, which essentially consists in using a tank for containing the molten metal, comprising an outer casing provided with a refractory lining and at least one tube. of metal injection which crosses this envelope and ends at the right of the interior surface of the refractory lining, introduce a first charge of molten metal in the tank, send gas through the injector tube in the molten metal, under a pressure capable of ensuring the mixing of the molten metal, closing said conduit to interrupt the sending of this gas and allowing the molten metal to solidify on the end of the tube, by sealing it, remove the homogenized molten metal from the tank, introduce a second charge of molten metal into the tank, send gas through the injection tube, at least temporarily under a pressure capable of causing the tube to be uncorked at its end, in order allow the entry of this gas into the second charge, and continue the admission of gas under a pressure ensuring mixing of the second charge of molten metal.

The injection tube advantageously has a small diameter to prevent the penetration of the molten metal into the tube, the maximum value of which is determined by the condition that the molten metal must not enter the tube. The maximum diameter can be determined experimentally, for particular molten metals. In the case of ferrous metals such as iron and steel, the maximum admissible diameter of the interior passage of the tube is of the order of 2.5 mm. The minimum admissible diameter is determined by the condition that the injection tube is capable of distributing an inert gas at a suitable rate and under pressure to ensure mixing of the molten metal. In fact, the internal diameter may generally be between 0.25 and 2.5 mm, over a length of at least 2 mm, and in particular over the entire thickness of the lining, which it passes through, of the order of 100 mm to 1 meter. The thickness of the wall of the injector tube is determined to ensure sufficient mechanical strength of the tube for normal handling.

The injection tube can be made of any metal which does not deform or soften under the operating conditions. For example, the injection tube can be made of stainless steel, low carbon steel or copper. The conditions of treatment can be modified according to the nature of the metal used to manufacture the tubes.

In the application for example to a ladle, the injection tube passes through the refractory lining, so that the end or gas outlet mouth of the tube stops at the right of the interior surface of the lining. The injection tube must not extend beyond the refractory lining, taking into account the high temperatures to which it would be subjected. Molten metal, for example molten steel, is poured into the pocket and the injection tube then becomes closed by the metal which solidifies on the free end. When the temperature of the injection tube increases, the tube becomes very weak mechanically.

When gas mixing is required, gas pressure is applied to the tube to unclog it. The unblocking can take place by expulsion of the only solidified metal on the end, or by bursting of the hot part, therefore weak, of the tube, near the end where it approaches the temperature of the molten metal. Once the gas pressure has cleared the tube from the solidified metal, the gas can flow through the tube. When the desired agitation of the molten metal has been obtained, the introduction of gas is stopped and the molten metal again closes the end of the tube, by solidification on the mouth. In the following cycle, the hot molten metal which is poured into the pocket heats the injection tube and the solidified metal plug, so that this plug is expelled or the tube bursts as described above, which allows the introduction normal brewing gas in the pocket.

It is in practice, of little importance whether or not the end of the tube is broken and expelled with the solidified metal. This may depend on the nature of the metal constituting the tube. In the case of a stainless steel injection tube used for the treatment of steel molten, the most likely is that the solidified metal sealing the mouth is only driven by gas pressure, since stainless steel resists relatively well at high temperatures. In the case of a copper injection tube used for the treatment of molten steel, the most likely is that the tube itself bursts at its end, copper being a metal of lower mechanical resistance, in particular to high temperatures.

Copper and low carbon steel tubes have the advantage that a lower gas pressure is required to unclog them compared to stainless steel tubes. In general, while the supply pressure suitable for stirring is normally between 1 and 12 bars, it is desirable to have an installation making it possible to apply a pressure of 10 to 100 bars, at the start of the injection, to expel the solidified metal, and when the gas supply is stopped by closing the supply duct, the pressure in this duct is balanced with that of the bath and the plug hardly forms for 2 to 3 mm at the end of the tube.

It is understood that with continuous use, there may be some erosion of the injection tube at the end. However, it can be seen that this erosion is slight and that the tube remains intact, even after prolonged service.

On the other hand, it will be noted that the same device according to the invention can also be used with profit in converter installations for refining cast iron, although in this case it is not necessary to interrupt the gas injections. between two charges and that we can therefore avoid the formation of the solidified metal plug.

The invention will be better understood in the light of the description of its non-limiting embodiments, illustrated by the accompanying drawings.

Figure 1 is a vertical section through a ladle containing a metal bath and equipped with an injection tube according to the present invention, this tank can be an LD type converter, an electric oven or a street lamp.

Figure 2 is a horizontal section along line 2-2 of Figure 1, a bag equipped with several injection tubes.

Figures 3a and 3b are respectively a side view and an end view of an injection tube usable in the invention.

Figure 4 is a partial vertical section through a pocket, showing a group of injection tubes.

Figure 5 is a partial section, on a larger scale, illustrating an injection zone in a tank bottom for the reception of molten metal.

Figure 6 is a longitudinal section through another embodiment of the injection tube according to the invention, the tube being housed in a thin metal sleeve.

Figure 7 is a cross-section along line 7-7 of the tube of Figure 6.

In the drawings, the ladle is generally designated by the reference B. The ladle B has a steel casing 15 and a refractory lining 17 and it contains a bath C of molten metal. The bottom of the bag B is equipped with a refractory piece 19, through which an injection tube D enters for the introduction of inert gas into the bath C. The bag B is provided with a cover 21, coated on the inside of a refractory lining 23.

The lid 21 protects the surface of the stirred metal C melt, against moisture and oxygen ^- 'ambient atmosphere. To increase the effectiveness of this protection, it may be desirable to introduce an additional inert gas through the cover 21. An inlet pipe, not shown, can be provided for this purpose. The cover 21 also reduces heat loss by radiation. For example, in an installation treating 60 tonnes of steel heated under argon, the temperature drop rate is reduced from around 7 ° C / min without cover 21, to around 3.8 ° C / min with cover 21.

FIG. 2 represents a bag B equipped with four injection tubes D, D ₁ , D ₂ , D ₃ , and a pouring nozzle 31.

The distribution of the gas flow between several sources, as shown in FIG. 2, makes it possible in total to use a much greater gas flow, without excessive projection of the metal.

The nature of the injection tube D and its use in the present invention are described below in detail. The diameter of the interior passage of the injection tube D should not be greater than about 2.5 mm when the bath C is a molten ferrous metal. A diameter greater than this value can allow drips of molten metal penetrating further into the tube and causing internal blockages which cannot be removed by the pressure of gas alone. Such blockages would require cleaning the tube D mechanically or replacing it between successive uses. The admissible internal diameter, up to a maximum of 2.5 mm, is determined by the gas flow rate and the pressure for expulsion. In practice, the minimum internal diameter is of the order of 0.25 and preferably 0.8 mm.

The wall thickness of tube D must be between 0.25 mm and 4 mm. The minimum wall thickness is determined by the mechanical strength required for the tube. The maximum wall thickness is determined by the combination of the temperature of the molten metal with which the tube comes into contact, the gas pressure and the type of refractory that surrounds the tube. The heat transmission, under the conditions prevailing in the area surrounding the injection tube D, the refractory and the contact with the molten metal, is extremely complex. Consequently, the optimal dimensions must be specified experimentally.

The injection tube D can be a single tube, as shown in FIG. 3. If a greater flow rate is necessary to obtain the desired agitation in the molten metal, several tubes can be used.

Another way to get a larger gas flow is to bundle the injection tubes. In each group, two or more tubes are used connected to a common inlet in the pocket. Figure 4 illustrates this device. It can be seen in this figure that a common inlet tube 1 leads to a bundle of tubes D, D ₁ , through a steel casing 15 and a refractory lining 17. This makes it possible to multiply the gas flow, in any conditions, while remaining for the individual tubes within suitable dimensional tolerances.

To obtain an even greater flow rate, multiple bundles of injection tubes can be provided, arranged for efficient operation. The number of tubes D can be chosen as a function of several factors, and makes it possible to overcome certain difficulties inherent in the use of a single source of gas for expulsion.

In most steel mills, the ladle has a dimension to receive a full load of steel from the refining furnace. The distribution of the gas in a bundle of injection tubes makes it possible to ensure the mixing of such a complete charge, while the use of a single source could lead to projections which, to be avoided, would require reducing the quantity of metal in the pocket so as to increase the upper free space for projections. Of course, the projection conditions in an LD converter, an electric oven and a reverberatory oven are less strict than in a ladle.

FIG. 5 is a view, on a larger scale, of an injection tube D through the steel casing 15 and the refractory lining 17. The tube D is surrounded by refractory elements 33 and 35 which may be prefabricated bricks from compressed or suspended refractory material. The refractory elements 33 and 35 constitute a part of the refractory lining 17. The end E of the tube D does not extend beyond the free interior surface of the refractory element 35.

To simplify the removal and replacement of the injection tubes, their ends can be modified so as to receive a material of lower resistance than that of the adjacent refractory, for example graphite, which can burst or break during the withdrawal of the worn tube.

FIGS. 6 and 7 show such a tube D5 provided with a thin sleeve 8.

Although the invention has been described with reference to inert or relatively inert gases, it is understood that it is not limited to the use of such gases. It can be used with reducing gases, for example natural gas, propane, etc. Liquid hydrocarbons can also be used. Active oxidizing gases, for example oxygen, can be sent through the injection tube not in commercially pure form, but diluted with an inert gas, for example argon, helium, nitrogen, etc. Oxygen can be present up to a content of 75% by volume. In a particular embodiment, the gas contains by volume 70% of molecular oxrygene.

The molten metal treated by the process according to the invention can be subjected to a lower or higher pressure than atmospheric pressure, depending on the desired results.

The gas pressure necessary to open the closed end of the injection tube can advantageously be between approximately 10 bars and approximately 50 bars, although higher pressures can be used. Once the tube is opened, which is almost immediate, the gas pressure can be reduced to the desired value, which essentially depends on the agitation useful for homogenizing the molten metal.

The invention is illustrated below with reference to an exemplary implementation.

A mild steel injection tube is used, having an outside diameter of 3.2 mm, a wall thickness of 0.8 mm, and an inside pass diameter of 1.55 mm. The tube is embedded in the refractory lining at the base of a pocket B, as shown in Figure 1. 112 kg of molten iron are poured into pocket B and the metal solidifies on the exposed end of the tube, sealing the latter. Argon is sent under a pressure of 15.7 bars, the plug on the tube is completely expelled and the metal is stirred with argon. The gas supply is stopped by closing the supply duct and the end of the tube is again closed by the molten metal. The latter is then emptied. In the following cycle, 112 kg of molten metal are poured into pocket B. Argon pressure is applied and the normal gas flow is established. The cycle can be renewed continuously. The effective service life of the tube depends on the repair campaign of the pocket refractory.

It is understood that modifications of detail can be made in the form of implementation of the method and of the device according to the invention, without departing from the scope thereof.

Claims (9)

1. Device for stirring a molten metal by gas injection, characterized in that it comprises a tank for containing the molten metal, comprising an outer casing (15) provided with a refractory lining (17), and at least one metal injection tube (D) which passes through this envelope and ends at the right of the interior surface of the refractory lining, means for admitting a gas under pressure into the tank through the injection tube, from a source of gas supply under pressure external to the tank and means for interrupting the injection by closing a connection pipe between the injection tube and said source, and means for creating at least temporarily a pressure d gas admission sufficient to expel a solidified metal plug possibly formed at the end of the tube during an interruption of the injection. 2. Device for stirring a molten metal by gas injection according to claim 1, characterized in that said injection tube has a small internal diameter, of the order of 0.25 to 2.5 mm, on a length of at least 2 mm, to prevent penetration of the molten metal into the tube. 3. Device for stirring a molten metal by gas injection according to claim 2, characterized in that said tube has said small diameter over the entire passage of the lining, with a thickness of the order of 100 mm to 1 metre. 4. Device for stirring a molten metal by gas injection according to claim 1, 2 or 3, characterized in that the internal material (38) of the injection tube is fragile and in that an external sleeve (8) ensures its mechanical strength. 5. A method of stirring a molten metal by gas injection, involving the use of a device according to claim 1, 3 or 3, characterized in that it consists in using a tank to contain the molten metal , comprising an outer casing (15), provided with a refractory lining (17), and at least one metal injection tube (D) which passes through this casing and ends at the right of the inner surface of the refractory lining, introduce a first charge of molten metal (C) in the tank, send gas through the injection tube into the molten metal, under a pressure capable of ensuring the mixing of the molten metal, close said conduit to interrupt the send this gas and allow the molten metal to solidify on the end of the tube, by closing it, remove the homogenized molten metal from the tank, introduce a second charge of molten metal in the tank, send gas through the injection tube, at least temporarily under a pressure capable of causing the unclogging of the tube at its end, in order to allow the entry of this gas into the second charge, and to continue the admission of gas under a pressure ensuring the mixing of the second charge of molten metal. 6. Method according to claim 5, characterized in that the gas is an inert gas with respect to the molten metal. 7. Method according to claim 5 or 6, characterized in that the molten metal contains a dissolved gas which is at least partially removed from the molten metal with the gas introduced under pressure through the injection tube. 8. Method according to any one of claims 5 to 7, characterized in that the ferrous metal is refined with oxygen introduced to the upper part of the molten metal bath. 9. Method according to any one of claims 5 to 8, characterized in that the gas contains 70% of molecular oxygen, by volume.

Images