GB2140720A

GB2140720A - Feeding molten metal through a nozzle to a casting mould

Info

Publication number: GB2140720A
Application number: GB08413639A
Authority: GB
Inventors: Kurt Buxmann; Martin Bolliger
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1983-06-01
Filing date: 1984-05-29
Publication date: 1984-12-05
Also published as: GB2140720B; IT8421170A0; IT8421170A1; GB8413639D0; FR2546782A1; SE8402845D0; LU85381A1; NL8401742A; IT1174119B; SE8402845L; DE3320322C2; DE3320322A1; JPS606251A; CH662073A5; BE899772A

Abstract

Molten metal (7) is supplied through a passage (6) in a nozzle (1) into a casting zone between casting mould elements (2, 3), e.g. belts, rolls, and an air cushion is maintained in a space (J) between the nozzle and mould elements so as to prevent molten metal from flowing backwards into that space. The gas influences the radius of curvature of the metal leaving the nozzle; the gas preferably issues from channels (14) in the nozzle body. <IMAGE>

Description

SPECIFICATION Process for feeding molten metal One of the most difficult problems encountered in the continuous casting of metals is the nozzle by means of which the molten metal is fed into the gap between the rolls, moulds, endless belts or the like of the casting machine.

The nozzle, which may need to be of small dimensions particularly if endless belts are used to cast thin strips of for example 20 mm or less, can if desired include a separate nozzle mouthpiece, but in this specification the term nozzle is used to mean a nozzle with or without a separate nozzle mouthpiece.

The nozzle is exposed to great risk because of the very high temperature of the metal flowing through it. There are only a few materials which can withstand erosion or dissolution in the metal. One of the few materials which meets these requirements is graphite.

Graphite, however, suffers the disadvantage of high thermal conductivity with the result that the heat is conducted away so quickly from the melt that the metal has a tendency to solidify in the nozzle. Another refractory material is a mixture of 30% diatomaceous earth (almost pure silica in the form of microscopic cells), 30% long asbestos fibres, 20% sodium silicate (dry mixture) and 20% chalk (to form calcium silicate). A nozzle formed of this material is used widely for casting aluminium; for casting steel on the other hand nozzles made of ZrO2 or ZrSiO4 are normally employed.

The nozzle must withstand not only the thermal stresses which arise due to the temperature of the metal being cast, but, equally, withstand the resultant chemical attack and the mechanical stresses due to fluctuating movements of the mould or rolls and bending of the nozzle due to the relatively large weight of the molten metal passing through it, this bending leading to friction between the nozzle and the rolls, belts or the like and thus to destruction of the nozzle.

Swiss patent CH-PS 508 433 discloses a nozzle which features, on the outside close to the outer edge of its mouthpiece, inserts which run round the whole periphery and are made of a self-lubricating material. These inserts project just beyond the surface of the mouth piece so that they prevent any direct contact between the mouthpiece and the mould halves, and prevent the molten metal penetrating the small gap (play) between the mouthpiece and the mould halves. In practice, however, it has been found to be a disadvantage that rubbed-off traces of the graphite inserts behave as "activated" strips which produce faster solidification and a corresponding non-uniform structure, often resulting in surface cracks.

A further problem which arises is flow-back of metal at the nozzle. The metal emerging from the nozzle forms, in the region between the outlet of the nozzle and the place of first contact with the moving wall of the rolls, moulds, belts or the like, a radius of curvature the size of which depends essentially on the surface tension of the metal, the metallostatic pressure of the metal leaving the nozzle and the rate of movement of the walls of the related parts of the casting machine. As a result of premature solidification of the metal a barrier can form causing the metal to flow back behind the outlet of the nozzle. This is very troublesome when it occurs as it disturbs the continuity of casting and hinders the cooperative behaviour of the nozzle and the casting machine.

The object of the present invention is therefore to hinder any flow-back of metal at the nozzle and at the same time if necessary prevent any friction between the nozzle and the casting machine.

In accordance with a first aspect of the present invention, a process for feeding molten metal, through at least one passage in a nozzle into a gap between rolls, moulds, belts or the like of a casting machine, comprises forming an air or gas cushion in a space between the nozzle and the rolls, moulds, belts or the like, the cushion influencing a radius of curvature produced by the molten metal between an outlet of the nozzle and a point of contact of the molten metal with the rolls, moulds, belts or the like.

The liquid metal, which emerges from the outlet in the nozzle with a specific pressure Pmet, determined by the Bernoulli law by the static and dynamic pressure, would produce a radius of curvature R = a/pmOtw where a is the surface tension of the metal. As a result of the air or gas cushion, a counter-pressure p is produced, whic causes the melt to adopt a radius of curvature R = a/ (pmet-p). This enables a larger space to be produced between the nozzle and the rolls, moulds, belts or the like thereby discouraging metal flowing backwards behind the nozzle. This way there is also less less risk of the nozzle rubbing on the rolls, moulds, belts or the like.Furthermore, the air or gas cushion offers the advantage that it initiates the cooling of the metal before it comes into contact with the rolls, moulds, belts or the like and so accelerates this cooling.

Preferably, the cushion acts under the hydrodynamic paradox to hold the nozzle in a pre-determined position with respect to the rolls, moulds, belts or the like. The hydrodynamic paradox is an effect that is fully discussed in our Swiss patent application 7410/82 (GB 2 132 354 A) and concerns jetting a compressed gaseous fluid in such a manner that a constant distance is maintained between a surface from which the jet issues and a surface against which the jet is directed. Also preferably, the pressure of the cushion is controllable, the cushion is formed of an inert gas, and the inert gas is argon.

The air or gas forming the cushion may issue from one or more channels extending through the nozzle, with the one or more channels for the air or gas being separated from the at least one passage through the nozzle for the molten metal.

In accordance with a second aspect of the present invention, a casting machine nozzle assembly for feeding molten metal, through at least one passage in a nozzle into a gap between rolls, moulds, belts or the like of a casting machine, comprises means for form ing an air or gas cushion in a space between the nozzle and the rolls, moulds, belts or the like, the cushion influencing a radius of curvature produced by the molten metal between an outlet of the nozzle and a point of contact of the molten metal with the rolls, moulds, belts or the like.

Further advantages, features and details of the invention are revealed in the following description of an exemplified embodiment and the accomanying drawing which shows schematically a cross-section through a nozzle 1 in its operating position between two moving mould belts 2 and 3 of an endless track type mould.

The nozzle 1 features an upper nozzle wall 4 and a lower nozzle wall 5, between which is situated a passage 6 for a melt 7 of molten metal. This melt 7 leaves the nozzle 1 via outlet 8 and is introduced into the gap between the two mould belts 2 and 3 such that it comes into contact with their cooled mould walls 9 and 10. The cooling has the effect of causing the melt 7 to solidify from its outside to form metal layers 11 briefly after contact with the mould walls 9 and 10 is made.

The melt 7 will make contact with the mould walls 9 and 10 at points 1 2 which can be further from or closer to the outlet 8 depending on the speed of the belts 2 and 3 with respect to the speed of the emerging melt 7 and the surface tension of the melt 7.

In general the speed of movement of the belts 2 and 3 remains constant, whereas the exit speed of the melt 7 depends on the metallostatic pressure Pmet A radius of curvature r of the arc between the outlet 8 and a point of contact 1 2 is then determined essentially by the ratio of the surface tension a to the metallostatic pressure Pmet Provided in the walls 4 and 5 of the nozzle are channels 14 which connect up via outlets 1 5 to a space J between the mould walls 9 and 10 and the nozzle 1. Air or gas introduced into the space J from a controllable source of pressure (not shown) forms a cushion which influences the radius of curvature r and does so approximately in accordance with the ratio of surface tension off to metallostatic pressure Pmet less the pressure p of the cushion. This reduces the chance of flow-back of the melt 7 into the space J and also maintains the nozzle 1 out of contact with the mould belts 2 and 3.

The above-discussed exemplified embodiment is particularly relevent to the disclosure of our Swiss patent application 3010/83, and falls within the claims thereof, and if a nozzle is employed in the present invention of the kind disclosed in said Swiss patent application 3010/83, the channels for the air or gas are preferably provided in the metallic strengthening parts of the nozzle.

Claims

1. A process for feeding molten metal, through at least one passage in a nozzle into a gap between rolls, moulds, belts or the like of a casting machine, comprising forming an air or gas cushion in a space between the nozzle and the rolls, moulds, belts or the like, the cushion influencing a radius of curvature produced by the molten metal between an outlet of the nozzle and a point of contact of the molten metal with the rolls, moulds, belts or the like.

2. A process according to claim 1, in which the cushion acts under the hydrodynamic paradox to hold the nozzle in a predetermined position with respect to the rolls, moulds, belts or the like.

3. A process according to claim 1 or claim 2, in which the pressure of the cushion is controllable.

4. A process according to any preceding claim, in which the cushion is formed of an inert gas.

5. A process according to claim 4, in which the inert gas is argon.

6. A process according to any preceding claim, in which the air or gas forming the cushion issues from one or more channels extending through the nozzle, with the one or more channels for the air or gas being separate from the at least one passage through the nozzle for the molten metal.

7. A process substantially as hereinbefore described with reference to the accompanying drawings.

8. A casting machine nozzle assembly for feeding molten metal, through at least one passage in a nozzle into a gap between rolls, moulds, belts or the like of a casting machine, comprising means for forming an air or gas cushion in a space between the nozzle and the rolls, moulds, belts or the like, the cushion influencing a radius of curvature produced by the molten metal between an outlet of the nozzle and a point of contact of the molten metal with the rolls, moulds, belts or the like.

9. A casting machine nozzle assembly according to claim 8, in which said means is controllable to control the pressure of the cushion.

10. A casting machine nozzle assembly according to claim 8 or claim 9, in which the air or gas for forming the cushion issues from one or more channels extending through the nozzle, with the one or more channels for the air or gas being separate from the at least one passage through the nozzle for the molten metal.

11. A casting machine nozzle assembly substantially as hereinbefore described with reference to the accompanying drawing.