GB2263427A - Refractory stopper rod with a lined channel - Google Patents

Abstract

A stopper rod (3) has a refractory body in which at least one channel (20; 40) is formed which is bounded by a refractory membrane (6; 10) which is substantially gas-impermeable and that membrane has a contiguous barrier (7; 11) between it and the surrounding refractory material to act as a buffer zone to counteract the effects of differential expansion and contraction arising from mechanical or thermal stresses. The refractory membrane may be formed of dense alumina or silicon nitride-bonded silicon carbide materials. The barrier layer may be formed of a fibrous ceramic material co-pressed into the refractory body to form an inseparable boundary layer between dissimilar refractory materials of the lining and the bulk of the stopper. Alternatively it may be formed of co-pressed mullite or corundum. <IMAGE>

Description

STOPPERS FOR USE IN MOLTEN METAL HANDLING This invention relates to a stopper used as part of a valve mechanism in the control of flow of molten material from a vessel through a submerged outlet. More particularly it relates to refractory monoblock stoppers, i.e. one piece ceramic stopper rods as currently used to control the flow of molten metal exiting from a nozzle mounted in the bottom of a melt-containing vessel, e.g. ladle or tundish. This is typically applied in the casting of steel through an opening in the base of the tundish via nozzles and shrouds into a water-cooled mould.

Typically, such stoppers consist of an elongate cylindrical refractory ceramic body of isostatically-pressed graphite/alumina having at the lower end a rounded or tapered profile suitable for engagement in the throat of a corresponding exit nozzle, and at the upper end some form of connecting means to fasten the stopper onto an external lifting mechanism by which the flow is controlled.

Operation of the stopper is simple in principle. A mechanical lifting system is used to vertically lift the stopper rod from a seating position on the nozzle to ease or restrict the volume of the molten metal flowing through the nozzle. However, in practice, such a stopper rod has to operate under harsh environmental conditions such as being submerged in the molten metal for long periods of time and must be able to withstand the high thermal shocks encountered in the pouring processes.

In the already known processes for the continuous casting of steel, in addition to the use of a one-piece refractory stopper rod for controlling the flow of molten metal there are associated steps for controlling the quality of the melt. These include separate monitoring of the temperature of the melt by insertion of protected probes e.g. within ceramic sheaths, and the addition of alloying additives e.g. via injectors penetrating the side wall of the vessel. Furthermore, it is now common practice to provide a stopper rod with means for injecting an inert gas via an axial bore through the stopper rod into the nozzle to prevent fouling of the nozzle by deposition of alumina or other contaminants e.g. non-metallic oxides and to exclude in so far as possible the ingress of air into the casting pathway.This type of stopper, a gas-ducted stopper", has a gas supply line fastened to the upper end of the throughbore which then acts as a gas duct to convey inert gas to the stopper nose which may be porous or have a narrow gas vent.

The ducts and vents are normally formed within the stopper body by widely used methods involving use of arbors and sacrificial void formers the latter burning off during firing of the green refractory body.

Our earlier patent publications such as GB-A-2 095 612 GB-A 2 210 305, EP-A-0 074 988, EP-A-0 179 837 and US-A4 706 944 describe refractory monoblock stoppers and the contents thereof are hereby incorporated by reference.

The industry is very competitive and continual efforts are being made to improve the already high grade of product to meet ever tightening quality specifications. Desired objectives are the improved control of melt temperature, alloying additives and removal of inclusions (trapped impurities).

Thus it is an object of this invention to provide means for addressing these areas of product control and thereby obviate or mitigate the disadvantages of existing equipment and processes without resorting to use of expensive high purity refractory materials or metals such as molybdenum, nor complex assemblies.

According to this invention there is provided a stopper rod having a refractory body in which at least one channel is formed which is bounded by a refractory membrane which is substantially gas-impermeable and that membrane has a contiguous barrier between it and the surrounding refractory material to act as a buffer zone to counter the effects of differential expansion and contraction arising from mechanical or thermal stresses. -.

In a preferred stopper the channel is a blind bore within which a temperature sensor e.g. thermocouple may be housed. Preferably the channel is bounded by a gasimpermeable oxide refractory sheath such as a co-pressed mullite or corundum membrane situated within the refractory body but protected from direct contact with the bulk refractory material by an intervening barrier layer of ceramic fibre for example which is capable of absorbing forces developed during thermal expansion and contraction of the stopper body which due to the differing nature of the respective materials of the sheath and refractory body could lead to separation arising from differential expansion and contraction capabilities.

Preferably the membrane and barrier do not extend fully over the blind end of the channel so that the thermal path through the stopper body to the sensor is not interrupted.

Preferably also the region of refractory surrounding the said blind end housing the tip of the sensor contains a thermal conductivity enhancing material e.g. higher levels of graphite.

As well as thermal sensors utilising electrical conductivity such as thermocouples to provide signals for monitoring of melt conditions, the invention may additionally include or substitute optical sensors for this purpose.

In a further embodiment of the invention a channel is provided within the refractory body which has an outlet in the side of the stopper body at a position which in use would be below the level of slag in the vessel, said channel having a wear resistant lining of for example dense alumina or silicon nitride-bonded silicon carbide materials and also having a surrounding contiguous barrier layer between it and the surrounding refractory material to act as a buffer zone to counter the effects of differential expansion and contraction arising from mechanical or thermal stresses.

This channel is conveniently used for feeding alloying additives directly into the melt below the slag level thereby deiivering same in a more efficient manner without the difficulties and risks associated with penetration of a vessel wall. The additives may be fed as continuous wire, chopped wire, fibrils, or in otherwise particulate or powdered form.

The barrier layer used in this invention may be a fibrous ceramic material of a type such as used in drysealing joints but in this case the barrier layer is copressed into the refractory body to form an inseparable boundary layer between dissimilar refractory materials of the membrane or liner and the bulk of the stopper body.

The method of manufacture of the stopper of the invention is known in general terms in that methods for copressing differing refractory materials into a single composite refractory monoblock are known. Thus those skilled in the art are aware of packing methods to include gas-impermeable membranes. Our own earlier patent publications EP-A-0 143 822 and US-A-4 668 554 relate to refractory pouring nozzles incorporating gas-impermeable membranes. However the invention described therein does not address the problems of parameter sensing or addition of alloying additives to baths of molten material.

The invention will now be further described by way of example with reference to the accompanying drawings in which: Fig 1 shows an axial section through a stopper in accordance with the invention; and Fig. 2 shows a cross-section on the line A-A of Fig. 1.

In a first embodiment of the invention a tundish 1 has a nozzle 2 located in the base thereof and a stopper rod 3 is mounted in a typical support system overhead. The mounting system for the stopper rod comprises a threaded support rod 16 extending from the upper portion of the stopper rod to a support cross bar (not shown). The stopper is mounted on the threaded support bar in a known manner.

The stopper rod comprises a refractory body having an axial bore 31 having a restricted section 32 in the lower region of the stopper leading to a vent 33 in the tip of the stopper. An inert gas from an external supply fliot shown) is fed through coupler 15 and small bore channel 17 into the bore 31. Additional small bore channels 13 including restrictor means 14 provide further inert gas vents 34 in the stopper side wall.

This embodiment of the stopper also provides additional features for process control by virtue of further small bore channels 20 and 40, one of which channels 20 is formed as a blind bore in which a thermocouple (Pt/PtRh) is housed. The channel 20 is bounded by a refractory membrane or sheath 6 which is substantially gas-impermeable to exclude harmful gaseous or vapour contaminants evolving under operational conditions from hydrocarbon and silicon additives normally present in the refractory materials making up such a stopper body. A barrier layer 7 of ceramic fibre material provides an intervening buffer zone to separate the membrane from direct contact with the surrounding refractory material to counter the effects of differential expansion and contraction arising from mechanical or thermal stresses in normal operational use.

In this embodiment the membrane or sheath is made from a refractory oxide such as a co-pressed mullite or corundum membrane. The membrane and barrier layer do not extend fully over the blind end of the channel 20 so that a favourable thermal path through the stopper body from the exterior to the thermocouple contained therein is available.

Also the region 8 of refractory surrounding the said blind end housing the tip 5 of the thermocouple contains a thermal conductivity enhancing material e.g. higher levels of graphite. The head 4 of the thermocouple is connected to signal analysis equipment and microprocessor control systems (not shown) to enable substantially real time control of the process conditions and melt quality.

A channel 40 is provided within the refractory body and extends from an upper portion through the body to an outlet port 12 in the side of the stopper body at a position which in use would be below the level of slag in the vessel. This channel is provided with a wear resistant lining 10 of dense alumina or silicon nitride-bonded silicon carbide materials and a surrounding contiguous barrier layer 11 of fibrous ceramic material between it and the surrounding refractory material to act as a buffer zone to counter the effects of differential expansion and contraction arising from mechanical or thermal stresses. An overhead wire feed mechanism 9 feeds wire into this channel to provide alloying additives directly into the melt below the slag level.Of course the additives may be fed other than as continuous wire, e.g as chopped wire, fibrils, or in otherwise particulate or powdered form.

Advantages of this invention include: (i) the ability to monitor conditions of the melt close to the actual pouring point rather than at a surface or remote part of the melt standing in the tundish or similar vessel; (ii) the ability to introduce additives to the melt at a point below the slag line to enable more effective utilisation of these usually expensive additives without prejudicing the safety of operatives and vessel integrity; (iii) the ability to automate the process and provide more rapid adjustment of the melt quality without resorting to removing of samples for analysis by utilising continuous feedback of electrical signals from the optical and thermal analysis equipment to microprocessor control systems to provide reactive continuous monitoring. This represents a substantial benefit in that real time process monitoring has hitherto been unattainable commercially by certain doubts as to the value of measurements carried out "live" in the melt by surface probes or that additives poured in from above or injected through the side of a vessel were actually being efficiently used in the melt and not simply floating back up to become lost in the slag. Verification has involved removing samples which necessarily involve a delay resulting in a "snap shot view of the part of the melt sampled only.

Claims (9)

1. A stopper rod having a refractory body in which at last one channel is formed which is bounded by a refractory membrane which is substantially gas-impermeable and that membrane has a contiguous barrier between it and the surrounding refractory material to act as a buffer zone to counteract the effects of differential expansion and contraction arising from mechanical or thermal stresses.

2. A stopper rod according to claim 1, wherein the channel is a blind bore within which a temperature sensor is housed.

3. A stopper rod according to claim 1 or 2, wherein the channel is bounded by a gas-impermeable oxide refractory sheath situated within the refractory body but protected from direct contact with the bulk refractory material by an intervening barrier layer of ceramic fibre.

4. A stopper rod according to claim 3, wherein the oxide sheath is a co-pressed mullite or corundum membrane.

5. A stopper rod according to claim 4 wherein the membrane and barrier do not extend fully over the blind end of the channel so that the thermal path through the stopper body to the sensor is not interrupted.

6. A stopper rod according to any one of claims 2,3 or 4, wherein the region of refractory surrounding the blind end housing the tip of the sensor contains a thermal conductivity enhancing material.

7. A stopper rod having a refractory body in which at least one channel is formed, the channel being provided with an outlet in the side of the stopper body at a position which in use would be below the level of slag in a vessel, the channel having a wear resistant lining of dense alumina or silicon nitride-bonded silicon carbide materials and also having a surrounding contiguous barrier layer between it and the surrounding refractory material to act as a buffer zone to counter the effects of differential expansion and contraction from mechanical or thermal stresses.

8. A stopper rod according to claim 7 wherein the barrier layer is of fibrous ceramic material co-pressed into the refractory body to form an inseparable boundary layer between dissimilar refractory materials of the lining and the bulk of the stopper.

9. A stopper rod substantially as hereinbefore described and as shown in Figures 1 and 2 of the accompanying drawings.