EP0544931B1 - Method for continuous casting of metal ingots - Google Patents

Abstract

A method of continuous cord casting of molten metal (esp. Cu or its alloys) is claimed. In this process, a chiller cord casting mould (I) is used which is equipped with an inner lining plate (II) with high heat resistance such as graphite or BN and having thicknesses 0.2-15mm (esp. 1-5mm). The II is held flush with the side of I by vacuum applied via numerous boreholes through the I side. A thin layer of paste like material with high heat conducting capacity is inserted between the I and II. This material consists of fine Cu or graphite particles in molten Ga and/or In.

Description

The invention relates to a method for the continuous casting of metallic strands, in particular wide rectangular slabs made of copper or a copper alloy, with a cooling mold lined with highly heat-resistant material for pouring the metal melt.

In the continuous casting of a whole series of metals or metal alloys, molds consisting of cooled copper plates are usually used, which form the mold cavity. In order to avoid welding the mold plates to the melt, a highly heat-resistant material, for example graphite, can be arranged as a separating or lubricating agent, especially when continuously casting copper and copper alloys between the melt and mold plates (cf. EP-A-52947, SU-A- 806 237).

Thus, it is already known to coat the mold walls on the surface facing the melt with a graphite-containing slurry before casting, a relatively loose graphite layer having a thickness in the range from about 0.5 to 1 mm being formed. However, due to its low abrasion resistance, this coating is removed mechanically in the course of the casting process. After a relatively short time, the casting process must be stopped and the mold surface must be given a new coating.

Attempts have also already been made to screw thick graphite plates onto the cooled mold base plate on the surface of the mold facing the melt. For this, however, graphite plates with a thickness of at least 20 mm are required in order to be able to accommodate the screwing elements that are within the Graphite plate are anchored. Disadvantageously, such graphite plates are very expensive and have a relatively high thermal resistance, which considerably reduces the cooling of the mold. In addition, the graphite plates can bulge under the influence of thermal stresses, which further deteriorates the heat transfer between the graphite plates and the cooling mold. This effect is particularly noticeable in the case when strands with larger widths are to be cast.

The present invention has for its object to provide a method of the type mentioned in such a way that the lining material for the cooling mold has a low thermal resistance and still has sufficient abrasion resistance. Furthermore, the assembly and maintenance effort for the lining should be kept as low as possible.

This object is achieved in that a thin-walled lining plate is arranged on the surface of the cooling mold facing the molten metal, which is held on the surface of the cooling mold by means of negative pressure. Advantageous further developments result from the subclaims.

The method according to the invention surprisingly achieves a significantly longer lifespan for the lining. Due to the good heat dissipation of the cooling mold, the cast strands also have a very smooth strand shell, so that the surface finishing that has been customary up to now can be omitted.

Within the mold cavity, the mold has no fastening elements that could additionally hinder the formation of a smooth strand shell. Rather, the problem of fastening the lining plates was solved in that they are sucked onto the mold base plates by negative pressure. For this there are numerous holes within the mold base plates, which are connected to a suction or vacuum pump via a pipe system. In this way, a negative pressure can be generated in these holes relative to the ambient atmosphere if the holes on the melt-side surface of the mold base plates are covered and the suction pump is in operation. If a thin lining plate, for example an approximately 3 mm thick graphite plate, is placed on the surface of a mold base plate facing the melt, it is sucked in uniformly by the negative pressure in the bores. With this method it is possible to attach thin-walled lining panels to mold base panels without the need for disruptive connecting elements within these panels.

For example, since a lining plate made of graphite is usually slightly porous, it is necessary that the suction pump is also in operation during the casting process, in order to constantly ensure the negative pressure within the suction hole and thus the adhesion of the graphite plates to the mold base plates.

Provided that the negative pressure is lower than the ambient pressure, the negative pressure in the suction bores and channels can be within a wide range of values. A vacuum of significantly less than 10% of the ambient pressure, for example 0.1% of the ambient pressure, is expediently and technically feasible without too much effort. A further reduction in the negative pressure is possible without further ado, but does not bring any significant technological advantages. The preferred range for the negative pressure to be maintained in the intake system is within the limits of 0.1 to 10% of the ambient pressure (10,000 to 100 Pa). The compressive force with which the lining plate is pressed onto the base plate essentially depends on the area of the bore cross sections.

The invention is explained in more detail below using an exemplary embodiment.

A rectangular mold suitable for the continuous casting of a copper alloy consisted of two mold base plates for the long sides, which had a length of 400 mm and a width of 300 mm. In each of these two base plates, suction holes with a diameter of 4 mm were made at a distance of about 20 mm, which were arranged horizontally and vertically next to each other in rows.

If one assumes that the total cross-sectional area of the suction bores is approximately 4200 mm² and assumes a pressure difference of 50,000 Pa between ambient pressure and internal pressure in the bores, then the total pressure or contact pressure that was exerted on the lining plate is calculated from this information , to about 210 N.

This pressure force can be increased with the same pressure difference if the number of bores and / or their diameter is increased, since the cross-sectional area effective for the negative pressure is increased in this way. The same effect can also be achieved in that the melt-side surface of the mold base plate is covered with fine grooves or suction channels into which the suction holes open. It is particularly advantageous if the intake ducts have a depth of more than 0.05 mm and the width is in the range from 0.05 mm to 3 × d, d being the thickness of the lining plate.

The cooling capacity of a mold with a thin-walled lining plate depends to a large extent on the conditions of heat transfer from the lining plate to the cooled mold base plate. Good heat transfer requires intensive contact between the lining plate and the mold base plate. This is guaranteed by a high contact pressure.

A prerequisite for the suction of usually slightly porous lining plates made of graphite-containing material is that the suction pump has a sufficiently high output. Under unfavorable circumstances, however, these lining plates can corrode, in particular if air is sucked in from the surroundings and reacts with the lining material (e.g. graphite, boron nitride) in the pores of the lining plate.

In this case it is advantageous to use lining plates in which the porosity has been reduced or eliminated, for example, before installation by metal infiltration. The lining plate can also be provided on the suction side with a coating that reduces the gas permeability. Lining plates of this type then also have the advantage that the negative pressure in the suction channels can be better maintained, as a result of which both the suction behavior and the contact intensity are improved.

During mechanical processing of the contact surfaces, it is often impossible to avoid microscopic unevenness remaining even with the greatest technical effort. In order to improve the heat transfer even further, additional measures may be required in advantageous embodiments of the method according to the invention.

To increase the heat transfer, the contact surfaces of the mold base plate are coated with a mixture of liquid gallium and fine copper dust. A thin-walled lining plate made of graphite-containing material is then arranged on the mold base plate. The contact mixture penetrates into the microscopic cavities and thus improves the contact. This mixture solidifies as a result of alloying through heat treatment or during the first casting process. In a further process variant, the contact surfaces of the mold base plates made of copper or a hardenable copper alloy are first roughened and then before application of the lining plates coated with liquid gallium. The gallium fills the small cavities and later solidifies with the material of the mold base plate through the formation of alloys.

However, it is also particularly advantageous to use pasty contact media with higher thermal conductivity, which essentially retain their pasty properties even during the casting process. These pastes consist of small solid particles, preferably those that have a high thermal conductivity. These solid particles, for example copper powder particles, are in a liquid carrier medium, which also preferably has a higher thermal conductivity than the material of the lining plate. Such a pasty contact medium is applied to the contact surfaces of the mold base plate before the mold base and lining plate are pressed firmly together.

A paste has proven to be particularly favorable which consists of a mixture of graphite particles or graphite-coated copper powder particles and a carrier material consisting of indium and / or gallium.

The thickness of the lining plate can range from about 0.2 to 15 mm. A plate thickness of 1 to 5 mm is particularly preferred. The lining plate can consist both of graphite-containing material and of a composite material which contains, for example, graphite or boron nitride as a lubricant.

To increase the security against pressure losses in the intake ducts, it may be appropriate to provide an additional seal in the edge area of the intake plate, which guarantees a gas-tight seal. Either a thin film of a metal solder or such a pasty mixture can be used as the sealing material, which has also proven to be particularly favorable for improving the heat transfer.

Alternatively, a sealing tape can also be used, which has a slight flexibility and thus adjusts to possible unevenness when the lining plate is pressed on.

Frictional forces exerted on the lining plate by the strand shell of the moving cast strand. Despite sufficient adhesion of the lining plate to the mold base plate, frictional forces can cause the lining plate to shift somewhat in the pulling direction during operation. This would lead to the fact that the residual gas pressure in the intake duct system can rise due to the penetrating ambient air and the adhesion of the lining plate is deteriorated. If necessary, the casting process would then have to be interrupted in order to reposition the lining plate. In order to avoid this disadvantage, it is expedient to provide brackets on the mold base plate that reliably prevent the lining plate from slipping. In the simplest case, the mold base plate has a small projection at the lower end on which the lining plate is seated. In the case of molds which are moved oscillatingly in the withdrawal direction during the casting process, a holder can also be provided in the upper part of the mold. However, the brackets must be installed in such a way that the thermal expansion of the lining plate can in no way be impeded.

In general, the heat transfer resistance between two parallel interfaces can be changed by changing the contact pressure with which the two interfaces are pressed against each other. Accordingly, the local heat flow density within a cooling mold equipped with lining plates can be locally reduced, for example, by locally reducing the contact pressure or the suction pressure. Under certain conditions, this can be desirable for the use of a mold with lining plates.

As is known, the areas on the narrow sides of a mold and especially the corner areas are cooled more than the middle of the long sides, since the ratio of slab surface to slab volume is particularly large here. This finds expression in the fact that the solidification proceeds correspondingly faster in these more cooled areas.

An uneven or unfavorable solidification behavior can lead to stresses in the solidified material with certain alloys and to cracks or deformations if these stresses become too great.

In such cases, it may be important that the cooling conditions are adapted locally to the desired conditions and that certain partial areas of the lining plate are sucked onto the surface of the mold base plate with a greater contact force than the other areas. Locally different contact pressures on the mold base plate can be achieved, for example, by separate suction systems with different negative pressures or by changing the area density of the suction bores or suction channels in some areas.

Claims (13)

1. Process for continuous casting of metal strips, in particular broad rectangular slabs of copper or copper alloys, with a chill lined with a material resistant to high temperatures into which the metal melt is poured, characterised in that on the surface of the chill facing the metal melt is disposed a thin-walled lining plate which is held on the surface of the chill with the aid of vacuum.
2. Process for continuous casting of metal strips according to claim 1, characterised in that the lining plate exhibits a thickness lying in the range from 0.2 to 15 mm, preferably in the range from 1 to 5 mm.
3. Process for continuous casting of metal strips according to one of claims 1 or 2, characterised in that the lining plate essentially consists of graphite or boron nitride.
4. Process for continuous casting of metal strips according to one of claims 1 or 2, characterised in that the lining plate consists of a composite material which contains graphite or boron nitride as lubricant.
5. Process for continuous casting of metal strips according to one of claims 1 to 4, characterised in that a vacuum lying in the range from approximately 10,000 to 100 Pa is maintained to hold the lining plate on the chill.
6. Process for continuous casting of metal strips according to one of claims 1 to 5, characterised in that the lining plate is pressed on to the surface of the chill with the aid of suction holes and/or suction grooves disposed in the chill.
7. Process for continuous casting of metal strips according to one of claims 1 to 6, characterised in that on the suction side the lining plate is provided with a coating which reduces the gas permeability.
8. Process for continuous casting of metal strips according to one of claims 1 to 7, characterised in that a thin layer of a heat-conducting contact medium is disposed between the chill and the lining plate, the contact medium being solid at operating temperature.
9. Process for continuous casting of metal strips according to claim 8, characterised in that the contact medium consists of a copper alloy containing gallium.
10. Process for continuous casting of metal strips according to one of claims 1 to 7, characterised in that a thin layer of a heat-conducting contact medium is disposed between the chill and the lining plate, the contact medium being pasty at operating temperature.
11. Process for continuous casting of metal strips according to claim 10, characterised in that the contact medium consists of a mixture containing graphite particles and having gallium and/or indium as carrier material.
12. Process for continuous casting of metal strips according to claim 10, characterised in that the contact medium consists of a mixture which is composed of graphite-coated metal powder particles, preferably copper powder particles, and gallium and/or indium as carrier material.
13. Process for continuous casting of metal strips according to one of claims 1 to 7, characterised in that a least one part area of the lining plate is sucked onto the surface of the chill with a greater force than the remaining areas.