GB2068800A

GB2068800A - Method and apparatus for shielding a stream of metal

Info

Publication number: GB2068800A
Application number: GB7943317A
Authority: GB
Original assignee: BOC Ltd; MARSHALL REFRACTORIES Ltd
Current assignee: BOC Ltd; MARSHALL REFRACTORIES Ltd
Priority date: 1979-12-17
Filing date: 1979-12-17
Publication date: 1981-08-19
Also published as: GB2068800B

Abstract

Molten metal is poured from a ladle or, preferably, tundish into the mould of a continuous casting machine. Shielding gas is distributed from a distributor (10) surrounding the stream of metal as it falls from the tundish into the mould. The distributor (10) has a heat-resistant gas-permeable generally annular member (12) of ceramic material received in a casing which covers part only of the surface of the member (12). Contact between the ceramic member (12) and the inner surface of the casing is such that there is substantially no free gas passage therebetween. The casing has diametrically opposed gas inlets from which shielding gas passes through the ceramic member (12) issuing from the exposed faces thereof. <IMAGE>

Description

SPECIFICATION Method and apparatus for shielding a stream of metal This invention relates to a method and apparatus for shielding a stream of molten metal. In particular it is concerned with a method and apparatus for protecting with shielding gas a stream of molten metal issuing from a tundish or ladle. The purpose of the shielding gas is to prevent oxidation of the stream of metal. Such protection may be required for streams of molten steel passing from a tundish to a mould or a billet, bloom or slab caster. The need for protection from oxidation may also arise when pouring metal from a ladle to a tundish or from a ladle to an ingot mould. It is an aim of the method and apparatus according to the invention is suitable for use in such processes.

A large number of shielding or shrouding devices are described in published patent specifications. However, no one single kind of shrouding device is without drawbacks.

According to the present invention there is provided a method of shielding a stream of metal by means of a shielding gas, in which the shielding gas is distributed from a distributor comprising a gas-permeable heat-resistant member of ceramic material and a casing which covers part only of the surface of the member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

The invention also provides casting apparatus including a ladle or tundish, a receptacle for receiving a stream of molten metal issuing from a tundish or ladle, and a distributor for distributing a shielding gas so as to shield the stream of molten metal, the distributor comprising a gas-permeable heat-resistant member of ceramic material and a casing which covers part only of the surfaces of the member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

The invention additionally provides a gas distributor for use in the above-defined method and apparatus, which distributor comprises a gas-permeable heat-resistant member of ceramic material and a casing which covers part only of the surfaces of the member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

The gas distributor is an improvement over that described in German Offenlegungsschrift 2 550 831. This is because the gas distributor according to the invention has no free gas passage or chamber between the casing and the ceramic member. This makes it possible to form a ceramic member in situ in the casing and thus simplifies manufacture of the distributor.

Furthermore, the absence of a requirement for a gas chamber between the casing and the ceramic member makes it possible to reduce the diameter of the distributor in comparison with those described and shown in German Offenlegungsschrift 2 550 831.

Preferably the ceramic member of the or each distributor has (in its shielding position) an exposed surface facing upwards, an exposed surface facing downwards and an exposed surface facing the stream of molten metal. This makes it possible to create flows of shielding gas transversely to a vertically descending stream of molten metal, and vertically upwards and vertically downwards. In order to shield around the complete circumference of the stream of molten metal, more than one gas distributor may be used. However, it is possible to obtain adequate shielding around the complete circumference of the metal stream by employing a single distributor which has a generally annular ceramic member with a complementary casing, the stream being concentric with the annular ceramic member.Such an arrangement makes it possible to create a radial flow of shielding gas towards the stream of molten metal. Such a flow of gas helps to prevent the stream, after it has passed through the annular ceramic member, from diverging or becoming 'ragged' Preferably, in use, the or each distributor is positioned intermediate the tundish or ladle and the receptacle, the descending stream of metal being visible from above and below the distributor. The ability to see as much as possible of the stream is valued by steelmakers and, accordingly, such a method and apparatus according to this invention is advantageous over that described in the aforesaid German Offenlegungsschrift. Similarly, the method and apparatus according to the invention are advantageous over the use of long tubular shrouds which have previously been proposed for shielding streams of molten metal.

Typically, in a continuous casting plant the or each distributor is positioned about 5 cm below the tundish. By using a distributor with a generally annular ceramic member it is possible, as a result of the shielding gas leaving the upper exposed face of the ceramic member, to reduce or prevent air being drawn into contact with the stream of metal between the tundish and the distributor. The exposed face of the ceramic member facing the stream of metal directs gas at the stream so that a shroud of gas surrounds the stream can be formed. Some of this shielding gas is drawn downwards by the molten metal as it falls towards the mould or other receptacle in which it is received. Thus, a shroud or curtain of gas extending between the receptacle and the distributor is formed. Moreover, gas leaving the lower exposed face of the ceramic member contributes to the formation of this curtain.

The or each distributor is preferably readily removable from its shielding position. Typically, the or each distributor may be attached to the tundish, ladle or receptacle by a bayonet coupling. For example, the tundish may be provided with suitable projections which make a bayonet coupling with complementary projections from the casing of a distributor.

In continuous casting, it is preferred that the or each distributor be pivotally attached to the tundish. This enables the distributor to be readily moved into and out of a shielding position.

Preferably, in such an arrangement, the ceramic member and casing are not in the form of complete annuli but instead are formed with ends spaced circumferentially apart from one another. The space is desirably sufficiently large for the distributor to be able to be moved into and out of a shielding position while the stream of molten metal is flowing. Since a descending stream of molten steel issuing from a tundish may at first be ragged, it is preferred to establish a regular flow of molten metal from the tundish and then move the distributor into a shielding position. Use of a generally annular ceramic member with circumferentially spaced apart ends makes it possible for this to be done.

If a distributor is to be used to shield molten metal being poured from a ladle, it may be adapted for use on a sliding gate ladle nozzle. All such nozzles generally employ a bayonet or similar fitting extension tip holder. It is thus possible to provide the distributor with a coupling which fits the extension tip holder. In such an embodiment, the distributor need not have a ceramic member with an exposed surface facing upwards.

The member of ceramic material is typically formed of particles of refractory material of differing size. The refractory material may, for example, be alumina or magnesite. Typically the particles are chemically bonded together by means of a suitable bonding agent. By appropriately selecting the distribution of particles sizes within the chosen range, it is possible to form a porous ceramic member through which gas is able to flow at a chosen rate. Preferably, a distributor according to the invention is capable of distributing gas in an overall flow rate of at least 1 cubic foot per minute (approximately 0.035 cubic metres per minute). Typically, the distributor is formed by ramming a mixture of the ceramic particles and a bonding agent into the casing and then causing the bonding agent to set to form a rigid ceramic structure.Typically this can be done by heating the mixture of ceramic material and bonding agent. A distributor having a ceramic gas distribution member is advancageous over comparable ail-metal distributors in that the gas will leave the exposed faces of the ceramic material evenly and through a large number of pores so that the distribution of gas is relatively uneffected by molten steel splashing on to a part of the ceramic surface.

The method and apparatus according to the present invention is now described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic side view of continuous casting apparatus incorporating a distributor according to the invention; Figure 2 is a plan view of the distributor shown in Fig. 1, and Figure 3 is a side view, partly in section, of the distributor as shown in Fig. 1.

Referring to Fig. 1, a tundish 2 has in its base 4 a outlet 6 for a stream of molten steel. The stream of molten steel passes downwards under gravity and is received in the mould 8 of the continuous casting machine. A gas distributor 10 having a heat-resistant gas-permeable generally annular member 1 2 of ceramic material is positioned between the tundish and the casting machine. Shielding gas is distributed from the distributor 10 and helps to prevent oxidation of the molten steel between the tundish and the continuous casting machine.

The distributor 10 has an arm 1 4 extending horizontally. The arm 14 is pivotally connected to a lug 1 6 depending from the base 4 of the tundish 2. Distributor 10 can thus be readily moved into and out of a shielding position.

The distributor 1 0 is shown in more detail in Figs. 2 and 3. It has an inner generally annular member 1 2 of heat-resistant gas-permeable ceramic material with circumferentially spaced apart ends 18 and 19. This space 20 between the ends 18 and 19 of the ceramic member 12 (and a complementary space between complementary ends of an outer casing 22 of the distributor) enables the distributor 10 to be moved into and out of the shielding position even when there is an established stream of molten metal.Referring again to Fig. 1, the arrangement of the arm 14 and lug 1 6 is such that the centre of the member 1 2 (when the distributor is in its shielding position with the stream of molten metal concentric w;th the ceramic member 12) lies on an arc described by the mid-point of the space 20 as the arm 14 is rotated about its pivot. The ends 1 8 and 1 9 of the ceramic members 1 2 subtend a sufficient arc for the distributor 10 to be swung into and out of a shielding position without interrupting or contacting the stream of steel.

The ceramic member i 2 has an inner (cylindncal) surface 24 and an outer (cylindrical) surface 26 and an upper (planar) race 28 and a lower (plantar} race 30. The casing 22 is typically formed of one piece and is, for example, of mild steel The casing contacts and covers the whole of the outer surface 26. The casing has an upper annular member 32 which contacts and covers an outer region of the face 28 of the ceramic member 1 2. The casing 22 has a lower lip which contacts an outermost annular portion of the lower face 30 of the ceramic member 1 2. Contact between the ceramic member 1 2 and the inner surface of the casing 22 is such that substantially no free gas passage therebetween is permitted.A very small proportion of the gas passing through the casing (through diametrically opposite apertures 36 and 38) may seep between the inner surface of the casing 22 and the surfaces of the ceramic member 1 2 it contacts. If desired, therefore, a sealing material may be applied so as to form a circular seal between the periphery of the extension 32 and the upper face 28 of the ceramic member 1 2.

This seal is indicated by the reference numeral 29. It may, for example, be formed by applying a suitable grouting cement.

The casing 22 has inlet pipes 40 and 42 to enable gas to be fed into the distributor through the inlet apertures 36 and 38. The pipes 40 and 42 are welded to the casing.

The ceramic member may be of any suitable refractory ceramic material. Suitable materials include magnesite and tabular alumina. Typically, there is a range of particle sizes in the material from which the ceramic member 1 2 is formed. Typically the maximum particle size may be in the range 1 to 2 mm with the smallest particles being 0.5 mm or less in size. The ceramic member 1 2 is porous. The average pore size will depend on the distribution of sizes of the ceramic particles.

In order to make the distributor, a mixture of refractory ceramic particles and bonding agent is rammed hard into the casing. The casing is then placed in an oven which is raised in temperature to 200"C so as to cause the chemical bonding agent to cure and thereby to form the ceramic member 1 2. After, say, an hour, the casing may be removed from the oven and allowed to cool to ambient temperature. The gas pipes 40 and 42 may then be welded onto the casing. In addition, the arm 14 may be welded to the casing.

With reference to Fig. 1, in order to position the distributor as shown, the lug 1 6 is welded or otherwise secured to the base 4 of the tundish 2. The pivotal connection between the arm 1 4 and the lug 1 6 is then made. The gas pipes 40 and 42 are connected to a source of shielding gas by means of flexible hoses. (Both the hoses and the source of shielding gas are not shown in Fig. 1.) The distributor is typically positioned so that its top is 5 cm below the base 4 of the tundish 2. Typically, the gap between the top of the mould 8 and the bottom of the distributor 10 may be 10 to 15 cm.

Typically, a stream of molten steel is established before the distributor 10 is moved into its shielding position (as shown in Fig. 1). If necessary, an oxy-fuel torch is used to melt any steel that has solidified in the outlet 6 of the tundish 2 to enable the stream to be established. Once the stream has been satisfactorily established, the distributor can be swung into its shielding position with the stream of metal concentric with the ceramic member 1 2.

A supply of shielding as to the pipes 40 and 42 is then started. The shielding gas is typically argon or nitrogen. Argon has the advantage that it will shield the molten steel against pick-up of nitrogen. If nitrogen is used for the shielding gas, then obviously the nitrogen content of the steel will be increased as a result. Some steels are intended for uses in which the metallurgical consequences of an increased nitrogen content are not important. Nitrogen may be used as a shielding gas for such steels. Other steels are intended for uses which are sensitive to the nitrogen content.It may be possible to use nitrogen as a shielding gas for even these steels provided the steel which comes from the tundish is produced with a sufficiently low nitrogen concentration for the amount of nitrogen that is picked up during transfer of the molten steel from the tundish to the mould not to cause the chemical analysis of the steel to fall outside that normally specified for it.

The shielding gas entering the distributor 10 through the pipes 40 and 42 encounters those parts of the surface 26 of the ceramic member 12 that face the outlets of the pipes 40 and 42.

Substantially all the gas passes through the surface 26 and travels through the pores of the ceramic material until it leaves the distributor through either the face 24 or the exposed parts of the faces 28 and 30. Since the ceramic member 1 2 is porous, the shielding gas will leave it from a multitude of outlet points. Thus, should a part of the ceramic member 1 2 be splashed with molten steel, there will still be plenty of outlets for the shielding gas should some of the pores become blocked by the steel. Moreover, since the ceramic member 1 2 is heat-resistant, its being splashed with molten steel will not cause it to melt or otherwise disintegrate.

The gas leaving the ceramic member 1 2 by its surface 24 will travel radially inwards and tend to impinge upon the stream of molten steel. The pressure of the gas on the stream of molten steel helps to prevent the stream from subsequently diverging. In addition, the shielding gas will in the region of that part of the stream which is surrounded by the ceramic member 1 2 form a curtain of gas which shields the stream of molten steel against oxidation.

That part of the shielding gas leaving the ceramic member by the exposed part of its upper face 28 prevents air being drawn into contact with the stream of molten metal between the tundish and the distributor, or at least reduces the quantity of such air that is so drawn into contact with the stream of molten metal. That part of the shielding gas which leaves the ceramic member 1 2 by its lower face 30 helps to provide shielding for the molten stream of metal between the distributor 10 and the mould 8. Additional shielding is provided by shielding gas flowing downwards with the molten metal from the space surrounded by the inner face 26 of the ceramic member 12.

If desired, the distributor 10 may be swung out of its shielding position at any time.

Typically, the tundish may have more than one outlet and therefore more than one stream (or strand) of molten steel may be established. Each stream may be shielded by a gas issuing from its own distributor.

Typically, the surface 24 may have a diameter of 1 50 mm and the surface 26 a diameter of 250 mm. The axial length or height of the distributor 1 2 may typically be in the order of 100 to 200 mm.

The ceramic ring 1 2 may typically have one or other of the following set of properties (and dimensions): Typical properties Bulk density g/cmm3 2.2-2.4 2.45-2.6 Permeability to air (Cm2 sec-' Cm H20) 1-2 2-3 approx. maximum particle size (mm) 1.7 1.2 approx. maximum pore size (mm) 0.5 0.5 Typical gas flow (for each inlet) Applied pressure (psig) 36 37 Actual flow (lit/min) 500 450 Chemical analysis MgO 92 - Awl203 -- 97.3 SiO2 0.8 - CaO 2.1 P205 2.7 2.1

Claims

1. A method of shielding a stream of metal by means of a shielding gas, in which the shielding gas is distributed from a distributor comprising a gas-permeable heat-resistant member of ceramic material and a casing which covers part only of the surface of the member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

2. A method as claimed in claim 1, in which the ceramic member of the or each distributor has (in its shielding position) an exposed surface facing upwards, and an exposed surface facing downwards and an exposed surface facing the stream of molten metal.

3. A method as claimed in claim 2, in which there is employed a single distributor which has a generally annular ceramic member with a complementary casing, the stream being concentric with the annular ceramic member.

4. A method as claimed in claim 3, in which the ceramic member and the casing are formed with ends spaced circumferentially apart from one another.

5. A method as claimed in any one of the preceding claims, in which the stream of molten metal issues from a tundish or ladle and is received in a receptacle.

6. A method as claimed in claim 5, in which the or each distributor is positioned intermediate the tundish or ladle and the receptacle, the descending stream of metal being visible from above and below the distributor.

7. A method as claimed in claim 5 or claim 6, in which the or each distributor is pivotally attached to the tundish.

8. A method as claimed in claim 5 or claim 6, in which the or each distributor is attached to the tundish or ladle by a bayonet coupling.

9. A method of shielding a stream of molten metal substantially as herein described with reference to the accompanying drawings.

1 0. Casting apparatus including a ladle or tundish, a receptacle for receiving a stream of molten metal issuing from a tundish or ladle, and a distributor for distributing and shielding so as to shield the stream of molten metal, the distributor comprising a gas-permeable heatresistant member of ceramic material and a casing which covers part only of the surface of the ceramic member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

11. Casting apparatus as claimed in claim 10, in which the ceramic member of the or each distributor has (in its shielding position) an exposed surface facing upwards, an exposed surface facing downwards and an exposed surface facing the stream of metal.

1 2. Casting apparatus as claimed in claim 10 or claim 11, in which there is a single distributor which has a generally annular ceramic member with a complementary casing.

1 3. Casting apparatus as claimed in claim 12, in which the ceramic member and casing are formed with ends spaced circumferentially apart from one another.

14. Casting apparatus as claimed in any one of claims 10 to 13, in which the or each distributor is positioned intermediate the tundish or ladle and the receptable, the descending stream of metal being visible from above and below the distributor.

1 5. Casting apparatus as claimed in any one of claims 10 to 14, in which the or each distributor is attached to the tundish or ladle by a bayonet coupling.

16. Casting apparatus as claimed in any one of claims 10 to 14, in which the or each distributor is pivotally attached to the tundish.

1 7. Casting apparatus substantially as described herein with reference to, and as shown in, the accompanying drawings.

1 8. For use in the method as claimed in any one if claims 1 to 10, a gas distributor which comprises a gas-permeable heat-resistant member of ceramic material and a casing which covers part only of the surfaces of the member and which has at least one gas inlet communicating with the said member, there being no substantial free gas passage between the casing and the said member.

1 9. A gas distributor substantially as described herein with reference to, and as shown in, the accompanying drawing.