WO1998008637A2

WO1998008637A2 - Roll caster parting agent

Info

Publication number: WO1998008637A2
Application number: PCT/GB1997/002308
Authority: WO
Inventors: Jean-Marie Bouzendorffer; Bruno Di Filippo; Philip Milroy Thomas
Original assignee: Davy International Ltd; Eurofoil SA
Current assignee: Kvaerner Engineering and Construction UK Ltd; Eurofoil Luxembourg SA
Priority date: 1996-08-27
Filing date: 1997-08-27
Publication date: 1998-03-05
Anticipated expiration: 1999-02-27
Also published as: AU4026097A; GB9617863D0; WO1998008637A3

Abstract

A parting agent for use in a roll caster such as an aluminium roll caster, which comprises a solution of a water-soluble oil, such as a gelled water-glycol mixture.

Description

ROLL CASTER PARTING AGENT

This invention relates to roll casting, in particular to the casting of metal strip.

Twin roll casting is an established technology for the production of metal strip. In the case of aluminium alloys, it is usual for the thickness of the cast strip to be in the range 6 - 10mm. In this casting process molten metal is fed into the bite of a pair of internally cooled, counter-rotating rolls. The molten metal solidifies in contact with the casting rolls and then undergoes a hot reduction before exiting the caster as solid strip.

Recently considerable efforts have been made to reduce the thickness of the cast strip and this has resulted in a fundamental change in the design of casting machines. It is well known that during roll casting the casting rolls deform under the action of the separating forces developed between the cast strip and the casting rolls. As the cast thickness is reduced the separating forces generated by the cast strip increase significantly. These increased separating forces can be accommodated by, either increasing the diameter of the casting rolls and roll necks or preferably by supporting the casting rolls with back- up rolls. With this preferred solution the casting rolls are sized to suit the requirements of the process and the back-up rolls are sized to accommodate the separating forces and torques generated during casting.

A major problem of the aforesaid process is the tendency of the cast strip to adhere to either or both of the casting rolls. It is known that the application of a parting agent, such as a suspension of colloidal graphite in water, to the casting rolls on the exit side of the machine is beneficial in reducing this sticking. The parting agent is usually applied to the casting rolls as an atomised spray.

Another feature of roll casting of aluminium strip is the formation of a complex aluminium/oxide coating on the surface of the casting rolls and it is known that the presence of a coherent coating is beneficial in minimising the occurrence of sticking. It is believed that the aluminium/oxide coating provides a substrate that is receptive to the application of the parting agent and that the parting agent is substantially removed with the cast strip. It is known that once the roll coating has developed, the amount of parting agent that has to be applied to control sticking can be reduced.

In twin roll casting, the surface coating is not deformed other than when it passes through the molten metal when any damage to the coating may be repaired. In four high casting however the surface coating and parting layer are subjected to a rolling action in the casting roll/backup roll bite and this can have the effect of damaging the combined surface coating and parting agent layer. The effect of "rolling" this combined layer is to cause the layer to fragment, and this can generate debris. Due to the damage caused to the coating/parting agent layer during this rolling operation a non-uniform roll surface is created. In some areas the roll coating will be compacted whereas in other areas the coating will be removed and a fresh steel surface exposed to the molten metal. This non-uniform coating will result in non-uniform sticking behaviour with sticking being encouraged in regions where the roll coating is removed. For convenience, this form of sticking will be described as micro or point sticking to differentiate it from the macro or gross sticking that can be encountered in conventional twin roll casting. This cycle is repeated every revolution and leads to a gradual degradation of the roll surface and strip surface quality.

From the preceding discussion it can be seen that the combined surface coating/parting agent layer are "damaged" by the rolling action of the back-up rolls. This problem is exacerbated by the use of colloidal graphite suspended in water as the parting agent. The parting agent suspension is made up by adding a known volume of graphite concentrate to water such that there is typically 1 - 2% of graphite in suspension. Although the particle size of the graphite is typically in the range 1 - 2 microns, there is a tendency for the graphite particles to agglomerate and come out of suspension. On passing between the casting roll/back-up roll bite, graphite particles will be fragmented and it is likely that a proportion of this will become detached from the casting rolls. It is an object of the present invention to provide a method of at least partially overcoming the above-mentioned problems when casting with a four high caster.

According to a first aspect of the invention a parting agent is employed which does not depend on solid particles to provide the required properties. Such a material could be water soluble oil with lubricating and load bearing properties. An example of a suitable oil is a gelled water glycol mixture containing adhesion promoters. The concentration of oil in the water is in the range 0 - 30% and can be varied depending on the precise casting conditions. Similarly the composition of the oil can be varied depending on the precise requirements of material being cast. For example, if high loads are anticipated, the composition of the oil can be adjusted to increase the load bearing characteristics. In particular applications, particles such as graphite may also be included as a component of the parting agent.

The parting layer can be applied on either the exit or entry sides of the machine. If it is applied on the exit side, a parting agent which is free from particles must be employed. If it is applied on the entry side it is preferred to employ a parting agent which is free from particles but if desired, a conventional parting agent including particles may be used or a mixture of parting agents having particles and not having particles respectively may be used. If the parting agent is applied on the entry side it is necessary to incorporate full width roll wipers to ensure that the parting agent is completely dry before entering the molten metal contact area.

According to a second aspect of the invention, to avoid damage of the aluminium/oxide coating on each casting roll, the surface of the roll is wiped to remove surface debris prior to that part of the roll entering the bite between the roll and its back-up roll.

The wiping can be achieved in a number of ways. For example the casting rolls can be wiped or brushed with abrasive materials or the debris can be removed by wiping with a totally non-abrasive medium such as cotton cloth held in a rigid support.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings which are diagrammatic side elevations of four-high roll casters in accordance with the invention.

The roll caster comprises a pair of steel casting rolls 1,2 rotatably supported in a rigid housing not shown. The casting rolls are arranged one above the other and each roll is backed-up by a backing roll 3,4 respectively. The rolls, particularly the casting rolls, are cooled usually by the flow of water internally thereof.

In use, the cooled rolls are rotated and molten metal, usually aluminium or aluminium alloy, is introduced into the bite between the casting rolls on the input side of the machine by means of a nozzle not shown. The cast strip exits the machine as indicated by the arrow.

In the embodiment of the invention shown in figure 1 a parting layer of a material free from particles in water is sprayed onto each casting roll 1,2 by a nozzle 5 positioned on the outgoing side of the machine. The parting layer is conveniently a water soluble oil with lubricating and load bearing properties.

Alternatively the parting layer can be sprayed onto the rolls on the entry side of the machine but in that case it is necessary to incorporate full width roll wipers to ensure that the parting agent is completely dry before entering the molten metal contact area. Also shown in figure 1 are wipers 6, one associated with each casting roll, which remove debris from a part of the surface of the casting roll prior to that part of the surface of the roll entering into engagement with its back-up roll.

In the arrangement shown in figure 2, the spray nozzles 5 are positioned on the entry side of the machine and wipers 7 ensure that the parting agent sprayed from the nozzles 5 is completely dry before it comes into contact with molten metal.

Additionally, wipers, indicated by reference numeral 8 are provided on the exit side of the machine to remove debris from the casting rolls before they come into contact with their associated back-up roll. The wipers 8 can take a number of different forms. For example the casting rolls can be wiped or brushed with abrasive materials or the debris can be removed by wiping with a totally non-abrasive medium such as cotton cloth held in a rigid support. The support can either be fixed or oscillated. If aggressive cleaning is required it is beneficial to incorporate an extraction system to remove any debris. If brushing is the preferred method, the brush can be made strongly abrasive by using a wire fill or if less aggressive brushing is required a synthetic fill impregnated with abrasive particles can be used. If very light brushing is required the brush can be made up of discs of a soft material such as cotton cloth. The brush can be the full width of the casting rolls but the preferred approach is to use a shorter brush that covers the roll face by traversing across the rolls. In this approach the parting agent can be applied through a plurality of nozzles attached to the traversing mechanism. The brushes can be loaded onto the roll using either position control or load control modes. In the event of a full width brush being used it is beneficial to oscillate the brush to prevent localised marking of the rolls. An alternative approach to remove loose debris is to use a full width doctor blade. The doctor blade can be used in conjunction with any of the aforementioned roll dressing devices and methods of applying the parting layer.

In the event of the rolls being cooled externally, the roll dressing equipment can conveniently be an integral part of the external cooling device.

Claims

1. A parting agent for use in roll casting of metal, comprising a water soluble oil.

2. A parting agent according to claim 1 comprising a gelled water glycol mixture containing adhesion promoters.

3. A parting agent according to claim 1 or claim 2 in which the concentration of oil is in the range 0% - 30%.

4. A parting agent according to any of claims 1 to 3 , further comprising a component including particles of a suitable lubricating solid.

5. A parting agent according to claim 4 in which the particles comprise graphite.

6. A parting agent according to claim 4 in which the particles comprise colloidal graphite, the parting agent comprising an emulsion of oil and water.

7. A method of reducing adhesion of a cast metal strip to either or both of the casting rolls in a roll caster, comprising applying a parting agent according to any of claims 1 to 3 to either of the entry or exit sides of the machine.

8. A method according to claim 7 in which the parting agent is applied on the exit side.

9. A method according to claim 7 in which the parting agent is applied on the entry side and includes a component containing particles, the component being in accordance with any of claims 4 to 6.

10. A method according to claim 7 or claim 9 in which full width roll wipers are used to ensure that the parting agent is completely dry before entering the molten metal area.

11. A method according to claim 7 in which roll wipers are provided to remove debris from a part of the surface of the casting roll, before that part enters into engagement with its corresponding back-up roll.

12. A method according to any of claims 7 to 11 in which the wipers comprise abrasive or non-abrasive media held in a rigid support.

13. A method according to any of claims 7 to 12 in which the wipers comprise brushes.

14. A method according to claim 13 in which the brushes include a wire fill or a synthetic fill depending on the required degrees of abrasiveness.

15. A method according to claim 13 in which the brushes comprise discs of soft material.

16. A method according to any of claims 7 to 11 in which the wipers are narrower than the rolls and are traversed across the rolls, in use.

17. A method according to claim 16 in which the parting agent is applied through a plurality of nozzles attached to the mechanism for traversing the rolls.

18. A method according to any of claims 7 to 11 in which the wipers are loaded onto the roll using either position control or load control modes.

19. A method according to claim 13 in which a full width brush is used and is oscillated to prevent localised marking of the rolls.

20. A method according to any of claims 7 to 19 in which a full width doctor blade is used for removing loose debris.

21 A roll-casting machine including a pair of casting rolls, each having at least one back-up roll; means for spraying a parting agent onto each of the casting rolls; and wiper means for each of the casting rolls and arranged to ensure that the parting agent is completely dry before it comes into contact with molten metal.

22. A roll-casting machine according to claim 21 and further comprising wiper means arranged to engage a region of each casting roll so as to prevent loose debris from being brought into engagement with its corresponding back-up roll.