WO1998056524A1

WO1998056524A1 - Casting nozzle

Info

Publication number: WO1998056524A1
Application number: PCT/EP1998/003718
Authority: WO
Inventors: Dirk Mooij
Original assignee: Hoogovens Staal B.V.
Priority date: 1997-06-13
Filing date: 1998-06-12
Publication date: 1998-12-17
Also published as: ZA985141B; AU8539798A; NL1006304C2

Abstract

Casting nozzle (2, 4) suitable for conveying molten metal from a first metallurgical vessel to a second metallurgical vessel such as from a ladle (1) to a tundish (3) or from a tundish (3) to a mould (5) in the case of casting molten steel, whereby the nozzle comprises detection means (19, 20, 21, 30, 31, 32, 33, 40, 41, 50, 60) which are suitable to be applied during use of the nozzle in detecting damage to the nozzle.

Description

CASTING NOZZLE

The invention relates to a nozzle suitable for conveying molten metal from a first metallurgical vessel to a second metallurgical vessel such as from a ladle to a tundish or from a tundish to a mould in casting molten steel.

The casting of steel can relate to casting thick or thin slabs, blooms or billets.

Apart from conveying the molten metal, one of the functions of the nozzle is to shroud the molten metal from the outside air in order to prevent oxidisation by oxygen from the outside air. In the case where the first metallurgical vessel is placed above the second metallurgical vessel, the nozzle therefore generally extends down beneath the surface of the bath of molten metal in the second metallurgical vessel. On the molten metal in the second metallurgical vessel a layer of a molten shrouding material such as slag, tundish powder and/or a casting flux is often maintained, among other things in order to shroud the surface of the molten metal from the oxygen from the outside air. In that case the nozzle extends through the layer of shrouding material into the molten metal. The nozzle is generally manufactured from a ceramic material comprising among others dolomite containing graphite or magnesite. During use, the nozzle wears away, especially from the outside inwards because of on the one hand erosion, but on the other hand to a far greater extent because of chemical attack. In order to retard the chemical attack it is known to provide the nozzle with a wear resistant layer such as zirconium oxide at least on the part that is in contact with molten material, metal or shrouding material.

During casting of the molten metal the wall of the nozzle can become worn as a result of erosion and/or chemical attack of the wall of the nozzle to such an extent that a hole develops in the wall of the nozzle, a phenomenon that is known as breakthrough. Molten metal then flows in an unpredicted manner through the developed hole occurring into the second metallurgical vessel and disturbs the desired or normal flow. This can result in inclusions such as solid particles of the shrouding material or oxygen or nitrogen going into the molten metal. This can lead to a quantity of cast metal having to be downgraded which means economic damage.

In an extreme case a part of the nozzle can even break away and go into the bath of molten metal and ultimately into the slab, bloom or billet obtained from it. In such a case an even greater quantity of metal has to be downgraded.

The object of the invention is to create a nozzle with which the problems described above caused by a breakthrough or the nozzle breaking away are prevented or at least to a considerable extent reduced. This object is achieved with a nozzle which is characterised in accordance with the invention in that the nozzle comprises detection means which are suitable to be applied during use of the nozzle in detecting damage to the nozzle, in particular damage in the region of the molten metal bath or in the region of a layer of material present on the molten metal such as a layer of slag or casting flux.

The detection means make it possible to signal that the nozzle is damaged by erosion or chemical attack so that measures may be taken in good time to prevent breakthrough or breaking away. The detection means are preferably active in the wall of the nozzle and more preferably in that part of the nozzle that during casting is in or comes into contact with molten steel or shrouding material.

One measure that is possible to take after damage is detected is to increase the distance between the two metallurgical vessels so that the damaged part of the nozzle comes out of the molten material and no longer becomes affected. It is known from patent abstracts of Japan, JP 08 027506 A to provide a tapping spout with an optical fibre sensor of an optical fibre temperature distribution measuring instrument. This optical fibre sensor is laid on the inner side of a heat insulating layer, in order to detect wear of the refractory material of the tapping spout in its early stages.

From patent abstracts of Japan, JP 60 089701 A it is known to embed conductors in the castable refractory material at the bottom of a tapping spout. These conductors detect variations in electric current flowing between the conductors and the earth while impressing an AC voltage between the conductors and the earth. The variations in electric current are used to monitor the erosion of the tapping spout.

Patent abstracts of Japan, JP 08 094264 A describes a method to detect severe damage to a refractory layer formed on an inner peripheral surface of an electric furnace. Therefore a temperature measurement sensor, which uses a thermocouple so as to measure the temperature, is installed between the shell of the electric furnace and the refractory layer or inside the refractory layer. The wear state of the refractory layer is estimated by measuring the changes of the temperature.

From patent abstracts of Japan, JP 03 002637 A, a sensor is known which detects the location of corrosion in refractory material at the bottom of a hot metal trough. The sensor detects a condition of an erosion of a refractory from a variation in insulation resistance of an insulating substance inside the sensor, which variation arises from a high temperature of the refractory due to erosion of the refractory.

A method to detect erosion of refractories of a vacuum refining furnace is known from patent abstracts of Japan, JP 62 080216 A. A thermocouple is placed inside a metallic protective pipe which is inserted into the refractories of the side wall parts and the furnace bottom part of the vacuum furnace. The thermocouple and the protective pipe conduct to each other through the molten steel and the resistance between both decreases sharply when the refractories are eroded- and the insulating material of the pipe is melted.

A preferred embodiment of the nozzle in accordance with the invention is characterised in that the detection means are suitable for producing about a change in an electrically measurable quantity in the event of damage to the nozzle.

This embodiment yields the advantage of a great choice in embodiments of the detection means. In addition electrical quantities are generally simply and remotely measurable and the measurement results are usually suitable to prepare in a simple manner for presentation to operating personnel or for intervening in the casting process .

A simple and sturdy embodiment of the nozzle in accordance with the invention is characterised in that the detection means comprise a resistance element.

Resistance elements, especially in the embodiment of wire, can in a simple manner be incorporated in the wall of the nozzle. The resistance elements can be manufactured from resistance wire or from good conductive metal such as copper or iron.

An embodiment of the nozzle which is particularly suitable for being made with resistance wire but which is not limited thereto is characterised in that the resistance element comprises a number of windings wound into a coil and embedded in the nozzle.

This embodiment is particularly suitable for use with a nozzle which is provided with a wear resistant layer. The windings are then wound on the nozzle before the wear resistant layer is applied over it.

While the wires are embedded in the wall of the nozzle, the windings wound into a coil have a certain resistance. At a certain moment, when the wall of the nozzle is attacked, one or more windings become exposed and come into contact with molten material that has a relatively low resistance, and therefore to a greater or lesser extent short-circuit the exposed windings. The change in resistance thus caused is measurable and detectable using simple means.

Another embodiment which is particularly suitable for being made in good conductive material, but not limited to it, is characterised in that the resistance element comprises a number of elements in the form of wires extending essentially in the longitudinal direction of the nozzle and embedded in the nozzle.

With this embodiment, while the wires remain embedded in the wall of the nozzle, the resistance between the resistance elements and the bath of molten material is relatively high. At a certain moment, when the wall of the nozzle is attacked, one or more wires become exposed and come into contact with molten material so that the electrical resistance between the resistance elements and the bath strongly decreases. This change in resistance is measurable and detectable in a simple and of itself known manner.

The resistance elements can be individual wires or can comprise one or more loops running parallel essentially in the longitudinal direction of the nozzle.

To the skilled person it will be clear that, also in the embodiment with windings which form a coil, the axis of which runs parallel to the longitudinal direction of the nozzle, use can also be made of the change in resistance between a resistance element and the bath when a nozzle becomes damaged. It will also be clear to the skilled person that, with loops running essentially in -the longitudinal direction of the nozzle, use can be made of the change in resistance occurring when, if a nozzle becomes damaged, the molten material from the bath short- circuits one or more loops to a greater or lesser extent.

Another embodiment of the nozzle in accordance with the invention is characterised in that the detection means comprise a temperature-sensitive element.

The temperature-sensitive element is preferably embedded in the wall of the nozzle. As the wall thickness reduces as a result of attack of the wall of the nozzle, the temperature increases at the location of the temperature-sensitive element. This embodiment is therefore also suitable for being used for determining the residual wall thickness, and indeed also for detecting in the manner described above that the wall has been attacked to such an extent that the molten material has reached the temperature-sensitive element.

A further preferred embodiment of the nozzle in accordance with the invention is characterised in that the temperature-sensitive element comprises a thermocouple.

Thermocouples are available for many ranges of temperature and for a wide variety of ambient conditions. The measuring techniques needed for measuring the thermoelectric voltages are very advanced and very reliable.

A higher output voltage of the detection means is obtained with a further embodiment of the nozzle in accordance with the invention which is characterised in that the temperature-sensitive element comprises a thermopile.

The invention is also embodied in a method for casting molten metal, whereby the molten metal is conveyed through a nozzle from a first metallurgical vessel to a second metallurgical vessel, whereby for detecting damage to the nozzle, use is made of a nozzle in accordance with one of the preceding Claims. At the same time the invention is embodied in a nozzle evidently suitable for use with this method.

Applying the method in accordance with the invention produces the advantage that an imminent breakthrough or break in the nozzle can be detected in good time so that suitable measures may be taken to prevent an actual breakthrough or break. This helps reduce the number of operational disruptions and the quantity of material to be downgraded.

The invention will now be illustrated by reference to the drawing in which the figures show several non- limitative embodiments of the nozzle in accordance with^" the invention.

In the drawing

Fig. 1 shows an apparatus for casting steel, whereby a nozzle in accordance with the invention is being used;

Fig. 2 shows a first embodiment of the nozzle in two variants in which the invention is embodied;

Fig. 3 shows a second embodiment of the nozzle in accordance with the invention;

Fig. 4 shows a third embodiment of the nozzle in accordance with the invention;

Fig. 5 shows a fourth embodiment of the nozzle in accordance with the invention;

Fig. 6 shows a fifth embodiment of the nozzle in accordance with the invention.

In Fig. 1, 1 is a ladle, a metallurgical vessel, out of which molten metal is being conveyed by means of a nozzle (2) to another metallurgical vessel, a tundish (3). Molten metal from tundish (3) flows through a nozzle (4) , also designated in this place as submerged nozzle, to yet another metallurgical vessel, the mould (5) . Mould (5) can be suitable for casting thick or thin slabs, blooms or billets. Depending on the product being cast, one or more moulds can be filled from one single tundish.

In the mould there is a bath of molten metal (7) within a shell (6) of already solidified metal. On the molten metal lies a layer (8) of molten material, in this case casting flux and possibly slag brought along by the molten metal. There are means of dosing (not drawn) in or on the ladle and/or tundish for controlling the level of the bath of molten metal in the mould.

In a few embodiments in accordance with the invention, nozzle (4) is illustrated in further detail in Figures 2- 6, and can be used either between ladle and tundish, or between tundish and mould.

The left part of each of the Figures 2-6 shows a nozzle with a homogeneous construction of wall (12) and the right part shows a wall a part of which close to base part (13) is provided with a wear resistant layer (15) .

Fig. 2 illustrates a nozzle comprising a coupling part (11) , a wall (12) and a base part (13) .

Base part (13) is provided with a closed bottom (16) and four outflow openings (17) , three of which are drawn.

Wall (12) is at the portion adjacent the base part (13) provided with a resistance element (19) comprising windings (19.1), (19,2), (19.3), (19.4), (19.5) and (19.6) wound into a coil and which lie embedded in wall (12). In the embodiment in which the wall is provided with a wear resistant layer, the windings are preferably placed on the boundary surface of the wear resistant layer and the part of the wall of the nozzle running within it.

The coil is provided with connecting wires (20) and

(21) extending outside the nozzle which are suitable for measuring the resistance of the coil.

When, as a result of wear or chemical attack, a part

(22) of wall (12) of the nozzle has disappeared, molten material, such as molten metal reaches windings (19.2) and

(19.3) and short-circuits them.

This short-circuiting is measurable as a change in electrical resistance at connecting wires (20) and (21) . To the skilled person it will be clear that the absence of part (22) is also detectable by measuring the electrical resistance between the bath of molten metal and the windings.

Fig. 3 shows another embodiment of the nozzle in accordance with the invention. In this embodiment the nozzle is provided with resistance elements (30) , (31) , (32) and (33) in the form of wires' running in the wall in the longitudinal direction of the nozzle, a first end of which resistance elements ends in bottom part (13), and a second part exits the nozzle close to or in the coupling part. The two ends of the resistance elements are suitable for connecting to an electrical resistance measuring apparatus. In this embodiment part (22) falling away can be detected as a change in resistance between two or more resistance elements in the form of wires, or as a change in electrical resistance between one or more resistance elements in the form of wires and the bath of molten metal .

Fig. 4 shows another embodiment of the nozzle in accordance with the invention which, like the embodiment in accordance with Fig. 2, needs only two connections for a measuring apparatus.

In Fig. 4 the resistance element is an element in the form of wires that is embedded in the wall of the nozzle in the form of a number of loops running in the longitudinal direction of the nozzle and connected together. The element in the form of wires starts and ends with connecting wires (40) and (41) onto which a measuring instrument can be linked. When part (22) disappears, molten metal or molten material will short-circuit one or more loops so that a measurable change in electrical resistance will occur at connecting wires (42) and (43) . Also if there is a contact between the elements in the form of wires and molten metal or material from the bath, the electrical resistance between bath and element in the form of wire will undergo a measurable change.

In the embodiment in accordance with Fig. 5, one or more thermocouples (50) are embedded as temperature- sensitive elements in the wall of the nozzle, preferably on the boundary surface of wear resistant layer (15) -and the part of the wall of the nozzle below it. Fig. 5 shows only one thermocouple. When the wall of the nozzle at zone (22) becomes thinner this results in the temperature in the area of the hot junction of the thermocouple increasing. The measured increase in temperature is an indication of the increasing wear of the nozzle. The temperature-sensitive element, for example in the form of a thermocouple or thermopile, can also be used in a measurement of the electrical resistance between the temperature-sensitive element and the bath in the manner described above.

Fig. 6 shows an embodiment in which the temperature- sensitive element is a temperature-dependent resistance (60) . The functioning and use of this embodiment correspond with those in the embodiment described above in accordance with Fig. 5.

Preferably the temperature-sensitive element is placed on the boundary surface of the wear resistant layer and the part of the wall of the nozzle below it.

To the skilled persons it will be clear that the elements can extend into base part (13) as shown in Figures 2 and 3, or can run just into wall (12) as shown in Figures 4 , 5 and 6.

Claims

1. Nozzle suitable for conveying molten metal from a first metallurgical vessel to a -second metallurgical vessel such as from a ladle to a tundish or from a tundish to a mould in the case of casting molten steel, characterised in that the nozzle comprises detection means which are suitable to be applied during use of the nozzle in detecting damage to the nozzle.

2. Nozzle in accordance with Claim 1, characterised in that the detection means are suitable for producing a change in an electrically measurable quantity in the event of damage to the nozzle.

3. Nozzle in- accordance with Claims 1 or 2 , characterised in that the detection means comprise a resistance element.

4. Nozzle in accordance with Claim 3, characterised in that the resistance element comprises a number of windings wound into a coil and embedded in the nozzle.

5. Nozzle in accordance with Claims 3 or 4 , characterised in that the resistance element comprises a number of elements in the form of wires extending essentially in the longitudinal direction of the nozzle and embedded in the nozzle.

6. Nozzle in accordance with one of the preceding Claims, characterised in that the detection means comprise a temperature-sensitive element.

7. Nozzle in accordance with Claim 6, characterised in that the temperature-sensitive element comprises a thermocouple .

8. Nozzle in accordance with one of the Claims 6 or 7 , characterised in that the temperature-sensitive element comprises a thermopile.

9. Method for casting molten metal, whereby the molten metal is conveyed through a nozzle from a first metallurgical vessel to a second metallurgical vessel, whereby for detecting damage to the nozzle, use is made of a nozzle in accordance with one of the preceding Claims.

10. Nozzle evidently suitable for use with a method in accordance with Claim 9.