EP0924011A1

EP0924011A1 - Casting mould for the continuous casting of metal, and method for continuous casting using this casting mould

Info

Publication number: EP0924011A1
Application number: EP98204304A
Authority: EP
Inventors: Arie Hamoen; Jeffrey Flemming; Marcelinus Cornelis Bernardus Van Vliet; Gerardus Theodorus Marie Stam
Original assignee: Hoogovens Staal BV
Current assignee: Tata Steel Ijmuiden BV
Priority date: 1997-12-18
Filing date: 1998-12-18
Publication date: 1999-06-23
Also published as: NL1007830C2; NO985984D0; NO985984L

Abstract

Casting mould for the continuous casting of metal, comprising a cylindrical wall which is made from copper or a copper alloy and is provided, on the outside, with water cooling means and, on the inside, with a refractory lining, characterized in that the refractory lining forms a sealed protective coating which has a thickness of less than 1 mm, a specific thermal resistance, as defined herein, which is between 4 and 7 times higher than that of the copper wall, and of which the free surface is difficult for liquid steel to wet.

Description

The invention relates to a casting mould for the continuous casting of metal, comprising a cylindrical wall which is made from copper or a copper alloy and is provided, on the outside, with water cooling means and, on the inside, with a refractory lining. Depending on the shape of the cylindrical wall, the casting mould can be used to cast, for example, slabs or billets. The novel casting mould can be used to cast various metals, such as steel or aluminium. Where the casting of steel is referred to below, this is merely intended to illustrate the invention.
Since, owing to the water cooling means, the casting mould is able to intensively dissipate heat from the liquid steel flowing through it, a solidified layer forms on the inside of the said casting mould, which layer becomes increasingly thick in the casting direction. On leaving the casting mould, this solidified layer is sufficiently thick and strong to support the liquid core, and the cast product can then be cooled and solidified further.
Various problems may arise in the continuous casting of steel. For example, it is necessary to prevent the solidifying steel from adhering to the casting mould, to prevent the heat transfer from the steel to the casting mould being so high in certain locations that the temperature of the casting mould rises so far that it begins to melt, and to prevent corrosion in the casting mould which is caused by reactions with the steel. In the case of billet-casting machines, such problems mean that the casting mould has an average service life of only, for example, in the order of magnitude of 100 hours, after which time it has to be replaced. Various measures have been proposed in order to prevent adhesion, such as causing the casting mould to oscillate in the casting direction or adding casting powder at the top of the casting mould, which is able to form a separating and lubricating liquid layer between the solidifying steel and the copper wall of the casting mould.
Billet-casting machines are often fed with oil-like material which forms a film of gas between the steel and the copper wall. This film of gas ensures a thermal barrier, with the result that the hot surface of the casting mould generally does not rise to temperatures higher than 300°C. However, even this method, however generally it is employed, has drawbacks. In particular, traces of water in the oil may lead to local explosions, or constituents of the oil may react with steel and/or copper. In the former case, the surface of the steel product will be contaminated.
European Patent No. EP-0,448,773 has proposed a structure of the known type mentioned above in which the casting mould is lined on the inside with tiles made from refractory material. This material is selected to be such that it has lubricating properties with respect to the steel moving along it.
A drawback of this structure consists in the fact that the refractory lining may become damaged in certain locations. Also, those parts of the lining which become detached in the event of such damage may cause contamination to the steel surface.
A further drawback of this structure is that the tiles have a considerable influence on the dissipation of heat, so that in fact it is necessary to develop a completely different way of operating the casting machine.
Finally, the attachment of the tiles has itself been known to lead to problems.
The object of the novel invention is to find a solution to the existing problems without having to adapt the operating conditions. In this context, the reference point used is the operating conditions used for casting billets, in which oil is injected between the casting mould and the steel which moves through it.
According to the invention, it has been found that a solution to this problem is found if the refractory lining forms a sealed protective coating which has a thickness of less than 1 mm, a specific thermal resistance, as defined herein, which is between 4 and 7 times higher than that of the copper wall, and of which the free surface is difficult for liquid steel to wet.
The specific thermal resistance of a layer is in this case understood to mean the quotient d/λ. In this quotient, d represents the layer thickness in mm and λ represents the coefficient of thermal conduction in W/m².sec of the material which forms the layer. A measure which is often used for the wettability of a surface by a liquid is the contact angle between a drop of the liquid and the surface in question. In the case of the proposed refractory lining, the contact angle between liquid steel and the free surface of the refractory lining is to be greater than 110°.
The European Patent Application EP-0 44 394 A2 describes a casting mould to which a ceramic protective coating is applied. However, this is a casting mould for the discontinuous casting of metal, for example die-casting. In this case, there is no thermal equilibrium established between molten metal, protective coating and casting mould, as is the case for continuous casting and as specified in this application.
This patent application also describes a porous protective coating which may offer advantages for die-casting but is certainly not acceptable for continuous casting.
Furthermore, it can be pointed out that in die-casting, unlike in continuous casting, there is no movement between the solidifying metal and the casting mould, which inevitably makes special measures necessary.
French Patent No. FR 7, 812, 760 describes a casting mould made from graphite which is only suitable for the continuous casting of non-ferrous metals. Damage to the graphite is prevented by applying a ceramic protective coating, the layer thickness of which, incidentally, is not given. In contrast to this, the protective coating according to the invention is applied to a copper casting mould, with the aim of preventing solidifying steel from adhering to the casting mould and of making the use of lubricant superfluous. In this context, the thermal resistance, and therefore also the layer thickness, of the protective coating and the wettability by liquid steel are of considerable importance.
Japanese Patent JP 56086655 describes a casting mould of the type which is known and is outlined in the preamble. However, this document does not indicate any specific thermal resistance of the protective coating or the wettability by liquid steel.
Various methods are known in the prior art for applying thin ceramic layers to metal surfaces. For the function which the refractory lining of the casting mould is to fulfil in this case, it has been found that the best results can be obtained if the refractory lining is applied to the copper wall by means of plasma spraying, in which case, first of all, a metallic adhesive coating which is known per se and has a layer thickness of less than 0.15 mm is applied, by plasma spraying.
Since the refractory lining is subjected to considerable thermal load, thermal shock load and mechanical load, it is important that the adhesive coating be able to form a very secure attachment both to the copper surface and to the refractory material. It is therefore recommended to use an adhesive coating which can be applied with a high heat transfer or which, during its formation during the plasma spraying, provides a very exothermic reaction. Good results have been obtained, by way of example, if molybdenum is used in the adhesive coating, owing to its high melting point. Thus, when molten molybdenum is applied, a considerable amount of heat is supplied, which when the molybdenum solidifies is able to lead to secure attachment.
ZrO₂ is a suitable material for the refractory lining which satisfactorily meets the demands imposed above and has a low reactivity with chemicals which are released during the casting of steel, such as sulphur. A drawback of this ZrO₂ consists in the fact that it passes through various phases when heated and cooled, which may give rise to internal stresses in the material. It is therefore preferred to use a material for the refractory lining which consists of a ZrO₂ which is at least partially stabilized with Y₂O₃.
Good results are obtained with a casting mould according to the invention in which the refractory lining has a layer thickness of less than 0.5 mm, in which the layer thickness of the adhesive coating is less than 0.1 mm, and in which the material of the refractory lining consists of ZrO₂, which is stabilized with more than 17% Y₂O₃.
The result of this stabilization is that this material will not pass through any phase transition when it is heated from room temperature to almost the liquid phase and then is cooled back down to room temperature.
The invention is explained below with reference to a number of figures, in which:
Fig. 1 shows a conventional casting mould for casting steel billets, in detail.
Fig. 2 shows the same detail, but in the design according to the invention.
Fig. 3 shows a graph illustrating the temperature curve of steel to cooling water when the known device is used.
Fig. 4 shows the same temperature curve when the invention is used.
Fig. 5 diagrammatically illustrates the layer structure of the refractory lining.
In Fig. 1, reference numeral 1 indicates, in longitudinal section, part of the wall of a casting mould for casting steel billets. In cross section (not shown), the casting mould is square, the length of the sides of the square being approx. 130 mm. The casting mould is made from copper. On its inside, the casting mould is provided with a layer of chromium which is applied by electrodeposition. Channel 2 diagrammatically depicts a cooling system through which cooling water is supplied via inlet opening 3 and discharged via outlet opening 4. Liquid steel is fed into the mould in the direction of arrow 5 and establishes itself at a level 6 by maintaining equilibrium between the feed and removal of the steel. In a conventional process form, oil is supplied between casting mould and the liquid steel. This is diagrammatically indicated in Fig. 1 by arrow 7. As a result, a gaseous layer 8 is formed against the casting mould, through which gaseous layer the heat from the steel is dissipated to the wall of the cooled casting mould. As a result, a solidified layer of steel 9 which grows as it progresses in the casting direction is formed. On leaving the casting mould, the solidified layer 9 is sufficiently thick and strong to be able to support the content of steel which is still molten. The billet, which solidifies further, is removed at a constant rate, so that the level 6 of the liquid steel can establish itself in a constant position.
In the casting mould according to the invention, as illustrated in Fig. 2 (also in detail), no oil is supplied, and therefore no film of gas 8 is formed. However, a layer of refractory lining 10 is applied. In the case illustrated, the thickness of this layer is 0.37 mm.
Fig. 3 diagrammatically depicts the temperature curve from the liquid steel to the cooling water. The reference numerals 11, 1, 8 and 12 respectively denote the cooling water; the copper wall; the film of gas and the steel. It can be seen from this that there is a considerable jump in temperature from the inside of the casting mould to the steel, certainly compared to the temperature curve through the copper wall.
Fig. 4 shows a corresponding temperature curve, but in this case the film of gas 8 is replaced by the refractory layer 10. By suitably selecting the material and the thickness of the refractory lining, it is sought to achieve an identical temperature curve to that shown in Fig. 3.
This allows many of the operational settings used for casting the billets to remain unchanged with respect to the earlier operating situation.
Fig. 5 diagrammatically depicts, on an enlarged scale, a cross section through the inner wall of the casting mould. On the copper wall 1, there is firstly a layer of adhesive coating 14, followed by the layer of refractory material 13. The layer of adhesive coating 14 has a thickness of approx. 0.08 mm. Both layers are applied by means of the plasma-spraying technique. Plasma-spraying is a known technique which is extensively described in numerous literature sources. Therefore, this technique is not described in further detail here. The adhesive coating consists primarily of molybdenum. The layer 13 consists of ZrO₂ which is stabilized with 20% Y₂O₃.
Measurements have shown that the layer formed in this way is very difficult for steel to wet, with the result that no problems involving adhesion between solidifying steel and the lining layer were found. Also, in operation, it was found that the surface of the billets formed was free of defects and inclusions.
A considerable improvement on the usual casting mould service live of 100 hours was also observed.

Claims

Casting mould for the continuous casting of metal, comprising a cylindrical wall which is made from copper or a copper alloy and is provided, on the outside, with water cooling means and, on the inside, with a refractory lining, characterized in that the refractory lining forms a sealed protective coating which has a thickness of less than 1 mm, a specific thermal resistance, as defined herein, which is between 4 and 7 times higher than that of the copper wall, and of which the free surface is difficult for liquid steel to wet.
Casting mould according to Claim 1, characterized in that the contact angle between liquid steel and the free surface of the refractory lining, which is used as a measure for the wettability of this free surface by means of liquid steel, is greater than 110°.
Casting mould according to Claim 1 or 2, characterized in that the refractory lining is applied to the copper wall by means of plasma spraying, in which case, first of all, a metallic adhesive coating which is known per se and has a layer thickness of less than 0.15 mm is applied, also by plasma spraying.
Casting mould according to one of Claims 1-3, characterized in that the material for the refractory lining consists of a ZrO₂ which is at least partially stabilized with Y₂O₃.
Casting mould according to Claim 4, characterized in that the refractory lining has a layer thickness of less than 0.5 mm, in that the layer thickness of the adhesive coating is less than 0.1 mm, and in that the material of the refractory lining consists of ZrO₂ which is stabilized with more than 17% Y₂O₃.
Method for the continuous casting of metal using a casting mould, characterized in that a casting mould according to one of Claims 1 to 5 is used.