GB1581044A

GB1581044A - Preheating parts of molten metal handling vessels

Info

Publication number: GB1581044A
Application number: GB45971/76A
Authority: GB
Original assignee: Foseco International Ltd
Current assignee: Foseco International Ltd
Priority date: 1976-11-04
Filing date: 1976-11-04
Publication date: 1980-12-10
Also published as: BE860510A; ZA776605B

Description

(54-) PREHEATING PARTS OF MOLTEN METAL HANDLING VESSELS (71) We, FOSECO INTERNATIONAL LIM ITED, a British Company, of Long Acre, Nechells, Birmingham, B7 5JR, England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to preheating parts of molten metal handling vessels.

In the handling of molten metal, e.g.

molten steel, it is common practice to charge the metal into a ladle, tundish or like vessel provided with an outlet aperture which is closed by a so-called stopper rod. The aperture is normally in the base of the vessel and is surrounded by a socalled nozzle. The aperture is closed by reception of one end of the stopper rod in the nozzle, and the shank of the stopper rod extends upwards through the molten metal contained in the vessel and is accessible from above the molten metal surface.

To avoid chilling of the molten metal when it is charged into the vessel and to ensure that the stopper rod will function properly, it is necessary to preheat the nozzle and the stopper rod (particularly the end which fits into the nozzle) by means of burners.

The use of burners for heating stopper rods according to conventional practice has disadvantages. Such burners are usually placed inside the vessel together with supply pipelines for compressed air and fuel such as gas, and they must be removed before the vessel is filled with molten metal, causing a delay between preheating and molten metal inflow and accordingly a loss of temperature in the preheated parts. It can be awkward for the workers to manipulate the equipment and to remove it from the vessel before molten metal is poured. In the case of a tundish lined with an inner lining of refractory heat insulating material according to our British Patent Specification No.

1,364,665 less heat is required to heat up the vessel prior to filling with the molten metal as the only parts which require heating are the nozzles and the stopper rods.

In such a case the heat from the burners can scorch the lining located in the parts which do not need heating. Furthermore burners are costly to maintain and they also tend to be inefficient since much of the available heat energy which they produce is wasted. It would therefore be advantageous if the stopper rod could be preheated more efficiently and especially when the tundish is lined with an inner lining of refractory heat insulating material.

According to the present invention there is provided a method of preheating cooperating parts of a nozzle and stopper rod assembly in a molten metal handling vessel having a heat sensitive or scorchable inner surface which comprises locating within the vessel, about the nozzle and about the lower end of the stopper rod adjacent the nozzle, a heat barrier forming a substantially closed space around the lower end of the stopper rod, and supplying heat into the space within the barrier. The heat is preferably supplied through the nozzle.

The heat barrier is preferably dimensioned and arranged to extend up from the floor of the vessel surrounding the nozzle to surround the lower end of the stopper rod and contact the walls of the rod a short way up from its lower end. Preferably the heat is supplied from outside the vessel via the nozzle, which has the advantage of confining the heat to those parts of a tundish lined with a refractory heat insulating lining which require heating while at the same time avoiding damage to the lining itself.

As noted above, the invention is of particular value in connection with heating tundishes. When the vessel is a tundish having a casting tube adapted to extend below the level of molten metal to be contained in the vessel or in the mould below, the heat is preferably supplied to the space within the heat barrier via that tube.

The heat barrier is preferably of frustoconical shape and is preferably made from a mixture of a fibrous material, especially a refractory fibrous material such as asbestos, slag wool or rock wool, or aluminosilicate fibre, particulate refractory material and an organic binder. The barrier may be made by suitably arranging a suitably shaped piece of flexible cloth of such composition. Alternatively, the heat barrier may be generally cylindrical in form.

By the use of the heat barrier, the temperature attained both at the outer surface of the stopper rod and on the nozzle surfaces which cooperate therewith can be made substantially the same, and moreover the temperature can be raised to the desired high level quickly.

It has been found in practice and as shown by the evaluations reported below, that when the stopper rod and the nozzle have been heated to a sufficiently high temperature, because of the presence of the heat barrier, their temperature tends to fall only slowly over a period of about ten minutes thereafter, so allowing for any delay in use of the vessel between removal of the supply of heat and the pouring of molten metal into the vessel. In some cases, it may be desirable to boost the temperature of the nozzle and the base of the stopper rod by appliying a torch thereto just before the molten metal is poured into the vessel.

Most preferably the heat barrier is formed of a heat insulating and/or exothermic material as described in one of British Patent Specifications 1,117,977, 1,218,568 and 1,283,692. It is also preferable to use a barrier of duplex construction in which there is a heat-insulating layer and an exothermic layer.

Although in practice the heat barrier is preferably frustoconical, other shapes such as a cylinder with a lid may also be used, especially if they are easier to manufacture.

The heat barrier may be made of a plurality of parts which prior to use may be assembled to the required shape. In use there is preferably a small aperture, e.g. a slight annular clearance between the stopper rod and the top of the heat barrier, to allow for the escape of exhaust gases.

At the end of the preheating, the heat barrier may be left in position there to be destroyed by the molten metal being poured in, or it may be recovered for reuse, for example by being raised with the stopper rod when the rod is moved away from the nozzle.

The invention is of especial value in connection with the preheating of selected parts of an otherwise so-called "cold" tundish of the type described in British Patent Specifications 1,364,665 and 1,469,513.

Within such a tundish is an expendable layer formed of heat insulating slabs, and if these are heated accidentally by the burners prior to pouring molten metal into the tundish they can be damaged, for example by scorching. The invention is further of value when the tundish is used in slab casting in which case the nozzle will have a depending casting tube or shroud, formed for example of silica or graphitised alumina. The lower end of such a tube may have a plurality of outlets, each leading to the same continuous casting mould, to distribute the molten metal inflow into the mould.

Instead of supplying heat from within the tundish, according to the invention the burners are directed into the outlets of such a tube or shroud to warm up the tube and the inside surfaces of the nozzle and the lower end of the stopper rod. The presence of the heat barrier in such a case serves to speed up the attainment of the desired temperature; this is particularly important when parts made wholly or partly of silica are used since the faster the silica is transformed to cristobalite the greater the avoidance of damage.

The burner used to supply the heat up the casting tube or into the nozzle may be of any suitable type. It has been found in practice that oxyacetylene burners, oxypropane burners, or air/natural gas burners may be used. The optimum shape of the burner will depend on the particular circumstances; where the casting tube has a multiple outlet it is convenient to direct one burner up each outlet.

The invention is illustrated by way of example in the accompanying diagrammatic drawings, in which: Figure 1 is a part sectional view through the floor of a tundish showing a nozzle and stopper rod assembly, Figure 2 is a view as Figure 1 showing the location of temperature probes when certain tests were being done, and Figures 3 to 8 are graphs showing the time/temperature curves obtained in certain tests.

Referring to Figure 1, a tundish consists of an outer metal casing 11, lined with two layers of refractory brick 12, and an expendable inner lining formed of slabs of refractory heat insulating material 13. A refractory ring 14 is seated on the metal casing 11 over an aperture formed in the metal casing and a refractory nozzle 15 is located within the refractory ring 14, the nozzle 15 having a depending casting tube 19, which has, as shown in Figure 2, a bifurcated outlet. The ring 14 and nozzle 15 are fixed in position by refractory cement 16. A stopper rod 17 is mounted in the tundish by means, not shown, with the lower end of the rod located slightly above the nozzle. The rod is surrounded at its lower end by a heat barrier 18 of asbestos cloth the bottom of which rests on the inner lining 13 and the top of which lies adjacent the rod 17.

The barrier 18 may conveniently be fixed to the rod by means not shown and in this way the barrier can be held in compression between the rod and the tundish floor.

When the tundish is prepared to receive molten metal the nozzle 15 and the base of stopper rod 17 are preheated by means of gas burners located outside the tundish and directed through the apertures of the casting tube 19 up towards the nozzle 15 and the end of the stopper rod 17. The barrier 18 ensures that heat is retained in the area of the cooperating nozzle/stopper rod parts, and as a result the lower end of the stopper rod 17 and the inner surface of nozzle 15 heat up rapidly. The radiation, convection and conduction heat losses which occur if a burner located within the tundish is used are greatly reduced, and scorching of the expendable lining of refractory heat insulating material 13 is prevented.

When the nozzle 15 and stopper rod 17 have been sufficiently preheated the burners are switched off and are removed. The barrier 18 is left in position within the tundish and helps reduce heat losses from the stopper rod 17 and the nozzle 15 during the time lag between removal of the burners and when molten metal is charged into the tundish. During filling of the tundish the barrier 18 is gradually destroyed by the heat from the molten metal and residues from the material of the barrier 18 float up to the surface of the molten metal. The molten metal leaves the tundish via the casting tube 19.

A series of evaluations was carried out using the tundish as shown in Figure 1.

Six temperature probes, made of platinum/ rhodium (87:13), were used to take readings. Probes 5 and 6 were located on the inner surface of the tundish nozzle, probes 1 and 3 on the outer surface of the stopper rod and probes 2 and 4 within the stopper rod, all as shown in Figure 2. The assembly shown in Figure 1 was preheated for a period of 20 minutes and then left for a further 20 minutes without heating.

The temperature was continuously recorded on a chart recorder. Similar preheating and subsequent cooling runs were carried out using other arrangements, as set out below. Figures 3 to 5 and 7 are obtained using methods not according to the invention. Figures 6 and 8 are obtained using the method of the invention.

The graph of Figure 3 shows the temperature attained at the various probe positions in the absence of any heat barrier, but with heating by acetylene burners through the nozzle. It will be noted that the temperature differs from one position to another greatly during heating and that the heat loss is rapid; within 10 minutes following removal of the burners, all positions are at the same relatively low temperature. For these reasons heating up through the nozzle in the absence of any heat barrier is unsatisfactory.

The graph of Figure 4 shows the case where instead of the heat barrier shown in Figure 1, a sleeve open at each end is located about the nozzle. The presence of the sleeve substantially improves the temperature attained by the stopper rod, but not by the nozzle walls. The sleeve was formed by dewatering and drying portions of an aqueous slurry comprising the following solids components (relative parts by weight).

aluminium silicate fibre 55 resin binder 7 alumina 15 colloida silica sol 15 aluminium powder 8 The use of a heat insulating sleeve in this way is not especially advantageous; the heating is not uniform, as can be seen from Figure 4.

The graph of Figure 5 shows the results obtained when using a sleeve as in the test for which Figure 4 shows the results, but containing an exothermic fuel which generates heat to compensate for heat losses arising by virtue of the shape of the sleeve.

The sleeve was formed by dewatering and drying portions of an aqueous slurry containing the following solids components (relative parts by weight) ball mill dust 83 resin 4 paper 5 calcium silicate fibre 3 fluorspar 3 surfactants 2 It will be observed that while the temperature of the stopper rod was increased substantially compared with the use of an insulating sleeve, that of the nozzle area was not. This shows that improving the heat characteristics of the sleeve does not achieve uniform heating.

The graph of Figure 6 shows the results obtained when using the method of the present invention. The barrier 18 was formed of the first composition specified above. As can be seen from Figure 6, all the probes attained substantially the same temperature at substantially the same rate.

Moreover while the temperature fell following the removal of the burners, the rate of drop in temperature was considerably reduced compared with the corresponding portions of Figures 3 to 5 in which no pro tection was used or where a sleeve open at each end was used.

The graph of Figure 7 shows the results obtained in the absence of a heat barrier when using a gas burner of horseshoe shape. It will be noted that here the temperature of the nozzle is raised whereas that of the stopper rod is not raised to the same extent and is not evenly raised.

Figure 8 is the graph obtained correspondingly when a heat barrier 18 of the first composition specified above is used.

It will be noted that there is a tendency for a more uniform heating of both the nozzle and the stopper rod to occur.

WHAT WE CLAIM IS: - 1. A method of preheating cooperating parts of a nozzle and stopper rod assembly in a molten metal handling vessel having a heat-sensitive or scorchable inner surface which comprises locating within the vessel, about the nozzle and about the lower end of the stopper rod adjacent the nozzle, a heat barrier forming a substantially closed space around the lower end of the stopper rod and supplying heat into the space enclosed within the barrier.

2. A method according to Claim 1 wherein heat is supplied through the nozzle to the space within the heat barrier.

3. A method according to Claim 1 or 2 wherein the vessel is a tundish having a casting tube adapted to extend below the level of molten metal to be contained in the vessel or in the mould below, and wherein the heat is supplied via that tube to the space within the heat barrier.

4. A method according to any one of the preceding claims wherein the molten metal handling vessel is a tundish which has an outer metal casing, a relatively permanent refractory lining and an inner expendable lining of refractory heat insulating material.

5. A method according to any one of the preceding claims wherein the heat barrier extends from the floor of the molten metal handling vessel surrounding the nozzle to the walls of the stopper rod a short way above its lower end.

6. A method according to any one of the preceding claims wherein the heat barrier is of generally frusto-conical internal shape.

7. A method according to any of Claims 1 to 6 wherein the heat barrier is made from a mixture of refractory fibrous material, particulate refractory material and an organic binder.

8. A method according to any of Claims 1 to 7 wherein the heat barrier is made of a refractory fibrous cloth.

9. A method according to any of Claims 1 to 8 wherein the heat barrier is made partly of exothermic material.

10. A method according to any of Claims 1 to 9 wherein a small annular clearance for escape of exhaust gases is provided at the upper end of the barrier.

11. A method according to any one of the preceding claims wherein the heat is supplied to the interior of the barrier by means of an oxyacetylene flame.

12. A method of preheating the nozzle and stopper rod assembly of a molten metal handling vessel substantially as hereinbefore described with reference to the accompanying drawings.

13. -A molten metal handling vessel comprising a nozzle set in the base of the vessel and a refractory stopper rod adapted for cooperation with the nozzle and, surrounding the nozzle and the lowermost portion of the stopper rod, a heat barrier forming a substantially closed space around the upper end of the nozzle and the lower end of the stopper rod.

14. An assembly according to Claim 13 wherein the heat barrier is of generally frusto-conical internal shape.

15. An assembly according to Claim 13 or 14 wherein the heat barrier is formed of a composition comprising refractory fibre, particulate refractory material and binder.

16. An assembly according to Claim 13 or 14 wherein the heat barrier is formed of a refractory fibrous cloth.

17. A tundish substantially as hereinbefore described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tection was used or where a sleeve open at each end was used. The graph of Figure 7 shows the results obtained in the absence of a heat barrier when using a gas burner of horseshoe shape. It will be noted that here the temperature of the nozzle is raised whereas that of the stopper rod is not raised to the same extent and is not evenly raised. Figure 8 is the graph obtained correspondingly when a heat barrier 18 of the first composition specified above is used. It will be noted that there is a tendency for a more uniform heating of both the nozzle and the stopper rod to occur. WHAT WE CLAIM IS: -

1. A method of preheating cooperating parts of a nozzle and stopper rod assembly in a molten metal handling vessel having a heat-sensitive or scorchable inner surface which comprises locating within the vessel, about the nozzle and about the lower end of the stopper rod adjacent the nozzle, a heat barrier forming a substantially closed space around the lower end of the stopper rod and supplying heat into the space enclosed within the barrier.