EP1716945A1

EP1716945A1 - Immersed pour tube, installation comprising it, process of manufacture and use thereof.

Info

Publication number: EP1716945A1
Application number: EP05447092A
Authority: EP
Inventors: Eric Hanse
Original assignee: Vesuvius Crucible Co
Current assignee: Vesuvius Crucible Co
Priority date: 2005-04-26
Filing date: 2005-04-26
Publication date: 2006-11-02

Abstract

This invention relates to an immersed pour tube (1) for molten metal comprising a body (2) of refractory material, the body (2) having a flow passage (3) for the molten metal, to an installation comprising such a pour tube, to a process for the manufacture of such a pour tube and to the use of such a pour tube in a process for preventing an incident due to the erosion of the pour tube during pouring of a molten metal. According to the invention, the immersed pour tube is characterized by a chamber (4) comprised in the body (2) and connected to a duct (5), the chamber (4) being located in a region of the body which, in use, is under or at the level of the molten metal pool or the slag layer floating on the molten metal.

This immersed pour tube can be used to pour molten metal until as close as possible to the break point, without however, reaching the break point of the immersed pour tube and would allow to monitor the erosion of the tube and to take appropriate measures as soon as a break condition is detected.

Description

This invention relates to an immersed pour tube for molten metal comprising a body of refractory material, the body having a flow passage for the molten metal, to an installation comprising such a pour tube, to a process for the manufacture of such a pour tube and to the use of such a pour tube in a process for preventing an incident due to the erosion of the pour tube during pouring of a molten metal.
Pour tubes conduct molten metal from one metallurgical vessel into a mold or another vessel. Immersed pour tubes have at least one end of the tube, typically the downstream end, immersed in a pool of molten metal. Examples of such tubes include sub-entry nozzles (SENs), sub-entry shrouds (SESs) and ladle shrouds (LSs), which find particular utility in the continuous casting of molten steel.
In the continuous casting of steel, a stream of molten steel is typically transferred via an immersed pour tube from a first metallurgical vessel into a second metallurgical vessel or mold. The downstream end of the pour tube is immersed in a pool of molten steel, and has at least one sub-surface outlet(s) below the surface level of the molten steel. Such outlet(s) permit(s) the steel to pass from the first vessel to the second vessel or mold without contacting air or slag. This reduces oxidation and limits contamination by slag.
Pour tubes are typically preheated before use, but although preheated, the tubes are relatively cold compared to the molten steel. The molten steel passing through or around the tube subjects the tube to thermal shock, which can cause the tube to fracture. Consequently, pour tubes typically comprise thermal shock-resistant refractories.
During casting, an immersed pour tube extends through a layer of slag floating on the molten steel. Slag may comprise glasses, fluxes, mold powders or various impurities. Slag is corrosive, and the pour tube may erode more quickly where it comes in contact with the slag, that is, at the slag-line, than the remainder of the pour tube. The tube may fracture where such erosion occurs. A fractured tube permits slag to mix with the molten steel and also exposes the steel to oxidation. Additionally, a pour tube immersed in a mold often has sub-surface outlets designed to affect flow patterns and crystallization of the molten steel. Loss of the downstream end having the sub-surface outlets may thereby compromise steel quality and, in some cases, may permit breakout in the frozen steel strand issuing from the mold.
Attempts to prevent erosion of an immersed pour tube involve the use of collars fitted around the pour tube at the slag-line. Such collars, or slag-line sleeves, protect the tube from contact with corrosive slag. The sleeve may move relative to the outside surface of the tube, and permit the sleeve to rise and fall with changes in the molten steel level. A slag-line sleeve may be connected to a mechanism capable of raising or lowering the sleeve in response to melt level. The sleeve may even form a type of crucible surrounding the pour tube. The crucible has at least one opening communicating with a sub-surface outlet in the pour tube.
Sleeves may also be fixedly attached to the outside of the pour tube. In practice, sleeves have been mortared, threaded, or copressed onto the pour tube. A mortared construction involves cementing an erosion resistant sleeve onto the exterior of a pour tube. Alternatively, a threaded, erosion-resistant sleeve may be screwed onto the outer surface of the tube. Copressing involves pressing together two refractory mixes or one refractory mix and a pre-fired component, and then firing into a single piece.
Slag-line sleeves often comprise erosion-resistant refractories, such as zirconia, zirconia-graphite, silicon nitride, boron nitride, and zirconium diboride. Additional sleeve compositions include magnesia, magnesia-graphite, magnesia-alumina spinels and dense alumina. Unfortunately, such erosion-resistant refractories often have poor thermal shock-resistance. This property is especially detrimental with pour tubes having fixedly attached sleeves. Attempts to improve thermal shock-resistance by modifying the composition of the sleeve, for example, by adding graphite, frequently compromises erosion-resistance.
For economical reasons, the operators try to exploit as long as possible the pour tube (SEN, SES or LS), provided or not with a slag-line sleeve, and to bring it as close as possible to the break point where the erosion of the tube is so severe that the pour tube breaks. While desirable from the point of view of the economy, such an extended use of the pour tube raises some problems as to the security of the operations.
It is therefore an objective of the present invention to provide an immersed pour tube of the above described type which can be used safely as long as possible. A further objective of the invention is to provide an immersed pour tube which can be used to pour molten metal until as close as possible to the break point, without however, reaching the break point of the immersed pour tube. Another objective of the invention is to provide an immersed pour tube which would allow to monitor the erosion thereof and to take appropriate measures as soon as a break condition is detected (replacement of the pour tube in case of a SES or a LS or stopping of the casting operations in case of a SEN).
Attempts to monitor the erosion of an immersed pour tube have already been disclosed in the prior art. For example JP-A-58-163561 suggests using X-rays to assess the internal condition and erosion of a pour tube. Such a process is highly impractical and cannot be used in the severe environment of a metallurgical plant.
Other attempts to solve this problem involve the use of sensors embedded into the refractory material constituting the pour tube. For example, the following documents disclose sensors embedded into the refractory material: WO-A1-98/56524 , JP-A1-60-089701 : electrically conductive metal wires, JP-A1-08-027506 : optical fibers and JP-A1-08-094264 and JP-A1-62-080216 thermocouples.
Desirably, the invention should provide an immersed pour tube that would be easy and simple to manufacture and would not involve complicated measuring devices.
These objectives and others are reached with an immersed pour tube for molten metal comprising a body of refractory material, the body having a flow passage for the molten metal, as characterized in claim 1. According to the invention, the body of the immersed pour tube comprises thus a chamber connected to a duct, the chamber being located in a region of the immersed pour tube body which in use is under or at the level of the molten metal pool where a severe erosion takes place, or of the slag layer floating on the molten metal.
Thereby, in use (once the regime temperature has been reached) any change in the physical properties of the gas comprised in the chamber indicates that the gas comprised in the chamber can flow out of the said chamber and that a critical level of erosion of the body, at the level of the chamber, has been reached.
The US patent No. 4,668,554 describes already a pour-tube according to the preamble of claim 1. However, the chamber is designed to inject gas into the molten metal and its walls are made from porous material and the chamber is located far from the regions where the erosion should be monitored so that the pour-tube described in this document could not be used as described herein.
According to the invention, the chamber in the body can have any shape. For example, it can be a plenum chamber or a simple slit provided in the wall of the body. It can be cylindrical and substantially surround the flow passage or just located in a particular portion of the pour tube body. The opposite walls of the chamber may also be bridged locally.
It must be understood that the present invention permits to detect either the erosion of the external surface of the pour tube inward or the erosion of the bore outward. Both aspects are covered under the present application.
As indicated above, the chamber can be located in a region of the immersed pour tube body which in use is under or at the molten metal pool level where a severe erosion takes place, for example, at the level of the sub-surface outlets of the pour tube. Thereby, in case the pour tube is a SEN or SES, it is possible to detect when the sub-surface outlets designed to affect flow patterns and crystallization of the molten steel have become so eroded that they can no longer fulfil their function and may thereby compromise steel quality and, in some cases, may permit breakout in the frozen steel strand issuing from the mold. Alternately, the chamber can be located in a region of the immersed pour tube which in use is located in a region of the body which, in use, is under or at the level of the slag layer floating on the molten metal. Thereby, it is possible to monitor the corrosive effect of the slag layer on the pour tube. A particularly advantageous location for the chamber, when the body further comprises an erosion resistant sleeve which, in use, extends at the level of the molten metal pool or at the level of the slag layer floating on the molten metal pool is inside the body behind the erosion-resistant sleeve. Thereby, it is possible to determine when the slag line sleeve has been eroded. In a variant, the chamber is located inside the erosion-resistant sleeve so that when the break condition is detected, there remain some security.
According to the invention, the immersed pour tube can be a SEN, a SES or a LS for any known metal pouring application, including thin strip casting.
The duct generally comprises a conduit extending through the wall of the body. This conduit can be made from metal or a refractory material for example. Preferably it will extend through the body until a region which, in use, is above the molten metal pool level.
According to a particular embodiment, the chamber and/or the duct are gas-tight. This embodiment permits a simpler monitoring of the system. It is generally admitted that a refractory material is porous when its specific permeability is greater than or equal to 10^-15 m². Suitable porous refractory materials used to inject gas into a melt have a specific permeability within that range. In the sense of the present invention, a material must be regarded as gas-tight when its specific permeability is lower than 5.10^-16 m².
The invention also relates to an installation comprising such an immersed pour tube and a pressure measuring device - connected to the gas duct (optionally through an external gas feed line) - so that the pressure in the chamber can be monitored. Any sudden drop in the gas pressure in the chamber indicates that a critical level of erosion of the body, at the level of the chamber, has been reached.
In such an embodiment, it is not mandatory to connect the duct to a gas source if the chamber and the duct are gas-tight. Indeed, in this case, the chamber, the duct and the pressure measuring device (for example a pressure gauge) can form a sealed unit. The pressure rise can be monitored as the pour tube is heated up until the air contained in this sealing unit has expanded. The pressure then remains stable until the sealed unit (in particular the gas-tight chamber) is breached.
The invention also relates to an installation comprising such an immersed pour tube wherein the gas duct is connected to a gas source through an external gas feed line. The connection between the immersed pour tube and the external gas feed line can be realized for example as described in WO-A1-01/83138 or in WO-A1-2004/069451 or by any other means. The nature of the gas is not critical as long as the gas does not interact significantly with the components of the pour tube. Suitable gas are air, carbon dioxide, nitrogen. Inert gas such as argon are preferred.
In a variant, the installation might comprise a flow measuring device on the external gas feed line. Thereby, when a critical level of erosion of the body, at the level of the chamber, has been reached and the gas comprised in the chamber can flow out of the said chamber, there is a flow of fresh gas from the gas source through the external gas feed line which can be measured at the level of the flow measuring device.
Advantageously, the installation further comprise means for emitting a signal (sound, light, and the like) in case of significant and sudden variation of the measured pressure or flow.
According to another of its aspects, the invention relates to the use of a pour-tube as described in claim 1 in a process for monitoring the erosion of such pour-tube.
According to its last aspect, the invention relates to a process for preventing an incident due to the erosion of a pour tube during pouring of a molten metal which comprises

a) using an installation as above described;
b) monitoring the pressure or flow measured respectively by the pressure or flow measuring device;
c) taking the appropriate steps when significant variation of the measured pressure or flow is detected.

The appropriate steps can be the replacement of the pour tube in case of a SES or stopping of the casting operations (with a sliding gate valve or a stopper) in case of a SEN.
The invention will now be better described with reference to the enclosed drawings which are only provided for the purpose of illustrating the invention and not to limit its scope.

Fig. 1 shows a cross-sectional view of an immersed pour tube according to the invention;
Fig. 2 and 3 show cross-sections of the pour tube of Fig. 1 respectively at the level of lines I-I and II-II;
Fig. 4 shows an enlargement of the circle III of Fig. 1.

An immersed pour tube 1 for molten metal as been represented on these figures. A SES has been depicted on these figures but the same would also apply for a SEN. The immersed pour tube 1 comprises a body 2 of refractory material, the body 2 has a flow passage 3 for the molten metal. The body 2 also comprises a chamber 4 connected to a duct 5. The chamber 4 is formed as a cylindrical slit circumscribing the flow passage 3 located in a region of the body which, in use, is under or at the molten metal pool level. As a matter of fact, the chamber 4 extends in a region of the body which, in use, is at the level of the slag layer floating on the molten metal.
The pour tube of the shown embodiment comprises an erosion resistant sleeve 6 which, in use, extends at the level of the molten metal pool or at the level of the slag layer floating on the molten metal pool. The chamber 4 is located inside the body 2, just behind the erosion-resistant sleeve 6 so that it is possible to determine when the slag line sleeve 6 has been eroded.
The other elements of the installation (external gas feed line connected at the level of the outlet 7, pressure or flow measuring device, gas source, signal emitter, etc.) are not visible on the figures.

Claims

An immersed pour tube (1) for molten metal comprising a body (2) of refractory material, the body (2) having a flow passage (3) for the molten metal, characterized in that the body (2) comprises a chamber (4) connected to a duct (5), the chamber (4) being located in a region of the body which, in use, is under or at the level of the molten metal pool or of the slag layer floating on the molten metal.
A pour tube according to claim 1, characterized in that the body (2) further comprises an erosion resistant sleeve (6) which, in use, extends at the level of the molten metal pool or at the level of the slag layer floating on the molten metal pool and in that the chamber (4) is located inside the body (2) behind or inside the erosion-resistant sleeve (6).
A pour tube according to claim 1 or 2, characterized in that the gas duct (5) extends through the body (2) until a region which, in use is above the molten metal pool level.
A pour tube according to any one of claims 1 to 3, characterized in that the chamber (4) and/or the duct (5) are gas-tight.
Installation comprising a pour tube (1) according to any one of claims 1 to 4, wherein the gas duct (5) is connected to a gas source through an external gas feed line.
Installation according to claim 5, characterized in that the external gas feed line comprises a flow measuring device and/or a pressure measuring device.
Installation comprising a pour tube (1) according to claim 4, wherein the gas duct is connected to a pressure measuring device.
Installation according to any one of claims 5 to 7 further comprising a signal emitter for emitting a signal in case of significant variation of the measured pressure or flow.
Use of a pour-tube according to any one of claims 1 to 4 for monitoring the erosion of said pour-tube.
Process for preventing an incident due to the erosion of a pour tube (1) during pouring of a molten metal which comprises
a) using an installation according to any one of claims 5 to 8,

b) monitoring the pressure or flow measured respectively by the pressure or flow measuring device;

c) taking the appropriate steps when significant variation of the measured pressure or flow is detected.