GB2034217A

GB2034217A - Moulds with Roughened Surface for Casting Metals

Info

Publication number: GB2034217A
Application number: GB7937893A
Authority: GB
Original assignee: Alusuisse Holdings AG; Schweizerische Aluminium AG
Current assignee: Alcan Holdings Switzerland AG
Priority date: 1978-11-03
Filing date: 1979-11-01
Publication date: 1980-06-04
Also published as: DE2856472C2; IT1125524B; IS1102B6; IT7926655A0; IS2517A7; FR2440236A1; FR2440236B1; ZA795516B; IN151740B; SE441420B; ES485112A1; CA1143922A; CH633206A5; NL7907798A; AU5212179A; DE2856472A1; BE879721A; GB2034217B; JPS5564950A; NO793517L

Abstract

A continuous casting mould with roughened surface is such that, on first contact with the melt, heat transfer is controlled by virtue of the melt coming into contact only with the peaks (f) of the projections on the surface and an air gap is formed between the melt and the valleys in the surface. Trapped gases can escape along the parallel and continuous valleys to the edge of the mould the mould can be used in conventional continuous casting and in caterpillar type mould belts. <IMAGE>

Description

SPECIFICATION Moulds with Roughened Surface for Casting Metals The invention relates to moulds with roughened surface for casting metals, in particular for casting aluminium and its alloys, where the heat transfer on first contact with the melt is regulated in such a way that the melt comes into contact only with the roughness peaks on the mould surface and an air gap forms between the melt and the valleys.

In continuous casting with moving moulds the melt solidifies by coming directly into contact with the mould. Quality requirements make it necessary to control the transfer of heat accurately when the melt first makes contact with the mould. When the heat is extracted too quickly, as is the case with smoothly ground moulds, there are often cold shuts in the cast product, which then leads to scrap. The transfer of a large amount of heat through the mould at the start also means high thermal stresses in the mould which can lead to cracks forming in the mould surface.

In the present state of the art there are two methods which are used to regulate the heat transfer between the melt and the mould. These are: 1. The surface of the mould is coated with a thermally insulating, protective layer.

2. The surface of the mould is roughened mechanically.

The use of insulating, protective layers often involves spraying a coat of lining material on the mould before casting commences. Ceramic layers which can be deposited by plasma spraying is another possibility. Experience has shown however that there are also disadvantages associated with the use of linings.

The lining must be deposited after each casting. It is then especially important that the surface of the mould is coated uniformly, which depends of course on the skill of the operator.

Non-uniform coating leads to areas in the cast strand or strip, where the rate of initial solidification differs. In most materials this leads to casting flaws which mostly-appear in the form of surface porosity and surface cracks. Another problem is that there is always the danger of pickup of particles from the coating material. For many products (e.g. foils) this leads to unacceptable contamination of the surface.

Experience has also shown that many aluminium alloys can be cast in continuously moving moulds only if the initial solidification is sufficiently fast that the cell size at the surface of the cast strip is 10-20 ,um. The normal coatings however produce milder solidification conditions which then lead to surface flaws surface porosity in particular.

Permanent ceramic layers have the disadvantage in view of the high coating costs that they exhibit only limited service lives. It is also difficult using this method of coating to achieve an-initial solidification rate which is sufficiently fast for casting alloys.

In the case of a mechanically roughened mould the heat transfer is regulated by creating a suitably rough surface. When the melt comes into contact with a mould surface which, for example, has been roughened by shot peening with steel balls, then, if the metallostatic head is not too high, it comes into contact only with the peaks on the roughened surface, while an air cushion forms between the melt and the valleys on the roughened surface.

By appropriate dimensioning of the relative contact surface

where F= the contact surface of a peak on the surface F0=the total mould surface area n= the number of peaks on the surface and by controlling the depth of roughness and the average spacing of neighbouring peaks, the heat transfer through the mould can be regulated.

In the present state of the art there are two methods for mechanically roughening continuously moving moulds: a) Grooves are created in the surface by means of chip forming processes (mIlling, planing). This method however exhibits various disadvantages.

Because the demand for uniformity of heat transfer through the mould surface is very high, the demand for uniformity in the grooves is also very high. Modern machine tools can satisfy these requirements only for grooves spaced at about 1 mm or more apart. When the grooving is to be finer it is difficult to maintain uniform depth and uniform contact surface area. Furthermore, the machining costs increase markedly with increasing fineness of the grooves. Also, the surface to be machined in a continuous casting unit with moving moulds is very large indeed in a unit with moving, caterpillar track type moulds, where the casting width is 2 m and the length 3 m, the mould surface area is about 30 m2.

Coarse grooving, i.e. a groove spacing of > 0.5 mm, leads to cracks, especially when casting wide strip, as too deep penetration of the metal in the valleys of the grooves results in rubbing between the solidified melt and the mould, to such an extent that the shrinkage on solidification is hindered.

brby striking the mould with hardparticles steel balls in particular the surface is indented.

This method leads to a uniform reduction in heat transfer which, at a suitable metallostatic pressure, permits the casting also of highly alloyed alloys (e.g. AIMg 4.5) with moving moulds, in particular if the mould is made of copper. Practical experience has however revealed another disadvantage of this process which is described in the following: During long production runs, it is unavoidable that impurities gather in the recesses formed by peening or otherwise impacting, and these decompose to produce gases when heated. These impurities include organic substances, hydroxides and various salts which contain water of crystallisation. If, on casting, the metal comes into contact with such contaminated area, then gas is produced.At high casting speeds in particular, this gas is trapped between the melt and the mould; the reason is that, because of the special feature of the roughening (craters adjacent to each other but separated by ridges), the flow of the gas parallel to the mould surface is greatly hindered as soon as the melt touches the surface.

Bubbles of gas trapped between the mould and the solidifying metal, however, lead to flaws in the cast strip, which generally result in the strip being scrapped. It has also been found that the removal of these impurities by the various cleaning methods taking into account the safety measures required in production does not provide a suitable remedy.

It is therefore an object of the invention to develop a mould with roughened surface for use in the casting of metal, whereby the said surface provides the requisite uniform, and exact reduction in heat transfer between the melt and the mould, at the same time avoiding flaws in the surface of the cast product which are caused by gas trapped between the melt and the mould.

This object is achieved by way of the invention in that the valleys in the roughened surface are interconnected in such a way that gases produced in the valleys, when the melt comes in contact with the mould, can escape without hindrance parallel to the mould surface, with the result that the melt is not raised from the mould surface as a result of excessively high gas pressure in that region.

Preferably the roughened surface comprises a regular pattern of pyramidal or blunted cone shaped projections.

When the molten metal flows onto this mould surface, then it comes into contact only with the crest surfaces of the projections, that is to say surfaces lying parallel to the surface of the mould.

Consequently, the heat transfer during the initial stages of solidification can be chosen via the equation d2 The distance dbetween neighbouring projections is defined here as the distance between the centres of the surfaces in question, each of area f.

It is advantageous if a satisfies the condition: 0.05 < a < 0.5, preferably 0.1 < a < 0.25 where the distance dequals 0.05 to 1 mm, preferably 0.2 to 0.5 mm.

It has also been found advantageous to choose the height h between the surfaces fand the plane represented by the lowest points in the valleys such that this height h lies within the limits: 0.1 d < h < d, preferably 0.15 d < h < 0.4 d Extensive production trials with various aluminium alloys on a casting unit with moving, caterpillar track type moulds have shown that using moulds with surfaces roughened in this manner avoids the entrapment of gases and therefore allows top quality of the cast strip to be produced.

The improvement in the quality of the cast strip by using the moulds in accordance with the invention can be explained as follows. The gas which forms when the melt first comes into contact with the mould surface is able to flow freeiy in the connecting channels between the projections and is therefore able to escape.

There are special methods which are suitable for producing the necessary roughness pattern; these start from a smooth mould surface and do not involve any mechanical deformation of the mould surface. Preferred, is the etching of the requisite patterns into the mould surface.

It has been found particularly advantageous to produce an exactly defined roughness pattern by etching via photochemical etching processes, such as are used in the manufacture of printing rolls for the textile industry or for printed circuits in the electronic industry.

Trials with various aluminium alloys on a casting unit with moving, caterpillar-track type moulds have shown that photochemical etching methods for producing a defined roughness pattern are to be preferred over mechanical methods, in particular when the moulds are made of copper. Mechanically roughened copper surfaces always feature a certain amount of surface deformation. Experience shows that these are more susceptible to corrosion, and to hydrogen and oxygen embrittlement. Also, mechanically roughened surfaces exhibit creep characteristics which can have an adverse effect on the geometry of the moving mould. All these negative effects are not observed with the surface which has been photochemically roughened and is absolutely free of deformation.

Furthermore, trials have shown that photochemically roughened mould surfaces for reasons similar to those in casting with moving moulds also lead to a considerable improvement in surface quality of the casting product when casting into chill moulds, and in continuous D.C.

casting with moulds where there is sliding contact between the metal being cast and the mould wall.

This improvement means lower finishing costs.

The principle of the invention will now be explained in greater detail with the help of the accompanying schematic drawings viz., Figure 1 A cross section through a part of a mould of the prior art, the surface of which has been roughened by shot peening with steel balls; and Figure 2 A perspective view of a portion of the surface of a mould in accordance with the invention.

In Figure 1 the melt 2 is in contact with a mould surface 1 which has been roughened by shot peening with steel balls. The melt therefore comes into contact only with the areas around the tips 3 projecting upwards, and there is a cushion of air 5 between the melt 2 and each of the depressions 4 in the surface.

By appropriately dimensioning the relative contact surface area, i.e. here the ratio of the sum of the surfaces F, to F4, to the total surface area F,, and by selecting the depth of roughness t and the average distance a between neighbouring peaks, the heat transfer between the melt and the mould can be regulated.

The mould in Figure 2 with a surface in accordance with the invention features pyramidal shaped projections 6. These projections are characterised by a height h and a surface flying parallel to the surface of the mould. Neighbouring projections are spaced a distance dapart.

From Figure 2 it is clear that such a uniform pattern of roughness allows exact and reproducible control of the heat transfer between the melt and the mould, whilst at the same time the interconnecting system of channels between the individual projections ensures unhindered escape of the gases formed.

Claims

1. Mould with roughened surface for casting metals, in particular for casting aluminium and its alloys, where the heat transfer on first contact with the melt is regulated in such a way that the melt comes into contact only with the roughness peaks on the mould surface and an air gap forms between the melt and the valleys, the said mould being such that the valleys in the roughened surface are interconnected in such a way that gases produced in the valleys, when the melt comes into contact with the mould, can escape without hindrance parallel to the mould surface, with the result that the melt is not raised from the mould surface as a result of excessively high gas pressure in that region.

2. Mould according to claim 1, in which the roughened surface comprises a regular pattern of pyramidal or blunted cone shaped projections.

3. Mould according to claim 2, in which neighbouring projections are spaced apart a distance dof 0.05 to 1 mm, preferably 0.2 to 0.5 mm, are of height h such that 0.1 d < h < d, preferably 0.15 d < h < 0.4d, and the pyramidal or blunted cone shaped crest surfaces each have an area fwhich satisfies the condition 0.05 < f/d2 < 0.5, preferably 0.1 < f/d2 < 0.25.

4. Process for manufacturing a mould in accordance with any of claims 1 to 3, in which the roughness is a pattern produced without mechanical deformation, starting from a smooth mould surface.

5. Process according to claim 4, in which the pattern of roughness is produced by etching.

6. The use of a mould in accordance with any of claims 1 to 3, for continuous casting of ingot or strip with moving moulds.

7. The use of a mould in accordance with any of claims 1 to 3, for continuous casting of ingot or strip with moving caterpillar track type mould belts.

8. The use of a mould according to any of claims 1 to 3, for castings produced in stationary moulds.

9. The use of a mould according to any of claims 1 to 3, for continuous casting with moulds where there is sliding contact between the metal being cast and the moulds.