GB2157600A

GB2157600A - Producing continuous-casting moulds

Info

Publication number: GB2157600A
Application number: GB08509938A
Authority: GB
Inventors: Horst Gravemann
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1984-04-21
Filing date: 1985-04-18
Publication date: 1985-10-30
Also published as: IN163575B; FI851566L; SE8501910D0; SE8501910L; KR920002009B1; ATA115385A; GB8509938D0; CA1240479A; CH667412A5; IT1184754B; DE3415050A1; AT387405B; FR2563130A1; JPS60234958A; FR2563130B1; IT8520414A0; FI851566A0; ZA852966B; BR8501892A; KR850007817A

Abstract

In the manufacture of moulds from copper materials for steel continuous-casting installations, a wear-protection layer is produced as a result of the action of a laser. Material for forming the wear-protection layer initially is disposed on the copper mould cavity surface and then subjected to a laser beam. The resulting layer is fused to the mould body and, hence, is resistant to faking off. The material may comprise nickel or chromium, and may include additional metals, non-metals or metallic carbides etc. Solid lubricants may also be added.

Description

SPECIFICATION Producing continuous-casting moulds This invention relates to the production of moulds from materials comprising copper, suitable for steel continuous- casting systems, with a wear-resistant layer on the shaping surface(s) defining the mould cavity.

As is known, continuous-casting moulds which are made from materials comprising copper because of the high thermal conductivity of these materials are used for the continuous casting of metals of high melting point, e.g. iron and steel.

Having regard to the intended use, a distinction must be made between one-part and multi-part moulds; one-part moulds are produced from seamlessly pressed or cast tubes or from welded sheets or strips, whereas multi-part moulds consist of a plurality of components clamped together in a frame to form the mould cavity.

A common feature of all these mould designs is that, because of the high heat throughout in the region of the bath level, the cast billet or the slab solidifies rapidly in the outer zone facing the mould wall, thereby forming a thin shell, which lifts off from the mould wall and is pressed on again by the melt flowing along after it. The resulting uneven cooling conditions prevailing around the periphery and along the length of the extrusion cause mechanical stresses in the shell of the extrusion, which can lead to hot cracks and perforations.

However, continuous-casting mould inserts made of materials comprising copper also undergo considerable wear in the mould cavity as a result of friction with the solidified shell of the extrusion, and because of slag particles flowing in between the extrusion and the mould cavity. The resulting dimensional change in the inside measurements of the mould significantly shortens the possible service life of the moulds. Another disadvantage is that certain grades of steel absorb copper, thus resulting in grain-boundary diffusion and consequently the feared red-shortness of the steel.

To avoid these difficulties, it has already been proposed to apply wear-resistant coverings to the melt-contacing inner surfaces of the mould, or to provide, in the region of the bath level, inserts consisting of a material having lower thermal conductivity than the material of the mould body (German Laid- open Specification (Offenlegungsschrift) 1,957,332). The coverings are intended to increase the abrasion resistance of the mould and consequently its service life, and the inserts offer the possibility of regulating the heat throughput as a function of the mould height.

For example, a chronium or nickel layer has already been applied electrolytically to a mould surface coming in contact with the melt. However, despite its high hardness, chromium has the disadvantage that only relatively thin layers, for example up to 250 microns, can be applied, since otherwise this layer flakes off because of the different coefficients of expansion of the mould material and the coating material, and because of low adhesion. Another disadvantage of the thin electrolytically applied layers of usually 60 to 150 microns' thickness is that, because of the low strength of the supporting base layer of copper material, they can easily be perforated when subjected to mechanical stress by hard objects.

Nickel or nickel alloys alloyed, for example, with cobalt or iron behave in a more favourable way than chromium in this respect, and even layers of up to a few milimetres can be applied to plane or gently shaped plates. However, the hardness of such layers is low in comparison with chromium, and in addition the layer thickness obtainable in the case of rectangular or square moulds, for example mould tubes for producing billets, is restricted to approximately 150 to 200 microns because of the unfavourable dispersibility of the electrolytes, since otherwise the inner contour and dimension accuracy would change unduly.

In addition to the process for electrolytic or chemical coating, for example nickel with dispersions of phosphorous, boron or the inclusion of hard materials, for example silicon carbide, it has also been proposed to employ thermal spraying processes, such as wire flame-spraying, powder flame-spraying, plasma-spraying or detonation plasma-spraying. Molybdenum, aluminium bronzes, manganese bronzes and hard metals of the nickel-chromium-boron-silicon-iron-carbon type have been used as coating materials in these cases.

However, the adhesion of these layers applied by means of spraying proceses has proved inadequate, and moreover such spray layers are more or less porous, have microcracks, and are heterogeneous and anisotropic. There is no question of achieving a possible improvement in the adhesion between the moult material and the applied layer as a result of preheating of the mould insert, since this would cause a softening of the cold- formed mould, and, where open work is concerned, that is to say work without a protective-gas shield, there is also inadmissible oxidation of the shaping mould walls.

This being the state of the art, it is an object of the present invention to enable layers of wear-resistant materials of high quality to be applied permanently to the shaping surfaces of mould inserts.

According to the invention, there is provided a process for producing moulds from materials comprising copper, suitable for steel continuous-casting installations, the moulds having a wear resistant layer on the shaping surface(s) defining the mould cavity, wherein the wear-resistant layer is produced by means of a laser beam. The high energy-density of the beam results in zonal melting of the layer material, but also of the mould surface on the interspace region. This leads to a metallic bond forming between the copper material of the mould and the layer materials. A mechanically firm and permanent clamping of the adjoining metal layers is consequently achieved, and the wearing layer is prevented from flaking off even over a relatively long period of operation.The present zonal heating, being a narrowly restricted and relatively brief heating, avoids softening of the cold-formed mould, and a further particular advantage is that the layers applied or the layers hardened and tempered after application are practically free of cracks and pores.

In putting the invention into practice, one may initially apply, with bonding, a metal layer or a metal-containing layer to the shaping surface(s) of the mould, and subsequently unite this layer metallurgically with the copper material of the mould by fusion under the influence of the laser beam.

The said layer, which can, for example, be an electrolytically applied nickel or chromium layer, can be treated by means of a laser beam over the entire surface of the mould cavity, but this laser beam treatment can also be restricted to predetermined regions, viz. the regions of the mould which are subjected to particularly high mechanical stress. For example, it can often be sufficient simply to unite the wear-resistant metal or metal- containing layer metallurgically with the mould material only in the region of maximum mechanical stress caused by the continuous -casting extrusion, for example in a lower part of the mould.

This consequently prevents the applied layer of e.g. nickel or chromium from flaking off even in this region, and at the same time affords a hardening and tempering of the layer, improving it in terms of freedom from cracks and pores.

In producing wear-resistant layers by means of a laser beam in accordance with the present invention, one may also initially apply a metal or metalcontaining layer, with bonding, to the shaping surface(s) of the mould and subsequently add to the material of this layer, under the influence of the laser beam, one or more alloying additives which increase wear resistance. For example, an electrolytically applied nickel layer as mentioned above can be used for alloying, and suitable alloy powders, e.g. tungsten carbide, chromium, silicon, iron, or cobalt can be added, alone or in mixture,by means of a laser.

By means of a process according to the invention, wear- resistant layers up to a thickness of several millimetres can be applied permanently and with excellent quality without difficulty.

It is also possible, in accordance with the present invention, to produce the wear-resistant layer by applying the requisite wear-resistant material to the shaping surface(s) of the mould by means of the laser beam by a build-up welding technique.

Almost all the materials known in spray deposition processes are suitable for this purpose, e.g. up to 90% by weight of tungsten carbide on a cobalt base, tungsten carbide with a nickel base, tungsten carbide with a nickel-cobalt-chromium base, selfflowing materials on a nickel base with chromium, boron, silicon, iron and carbon and with additives of tungsten carbide up to 75% by weight, as well as stellites.

An appropriate procedure here is to apply the said material, in the form of metal powder or metal-containing powder, to the shaping mould surface(s) in a locality immediately ahead of the point of impact of the laser beam and to fuse it on in that locality; the feed devices usually employed in (e.g.) plasma processes may be utilised here. As a result of the simultaneous fusion of the mould wall in the region near the boundary, a perfect metallic diffusion bond with the copper material of the mould wall can be achieved.Such build-up welds are practically pore-free, and also it is no longer possible to separate the coating and the base material during bending tests.

A further possibility faliling within the ambit of the invention is to apply the wear-resistant material to the shaping surface(s) of the mould, for example by means of an ordinary spray deposition process, and afterwards to remelt it zone- wise by means of a laser beam, the metallic bond between the layer initially applied and the base material of the mould being made only during this remelting operation.

A still further possibility falling within the ambit of the invention is one in which incorporation or embeding of one or more materials, increasing wear resistance, into the superficial region of the shaping mould surface(s), is effected under the influence of the laser beam. For this purpose, the wear- resistance increasing materials, e.g. carbides or nitrides, can be incorporated or embedded in the above-mentioned metal or metal-containing layer, under the influence of the laser beam; thus they can be incorporated mechanically into e.g. an electrolytically applied layer of nickel or chromium while fusion is being affected in this layer be means of a laser.However, wear resistance increasing materials can also be incorporated or embedded in this way directly into the superficial regions of the shaping mould surface(s), so that the wear-resistant layer is formed by the solid particles incorporated or embedded in the copper material of the mould.

Certain "self-lubricating" continuous-casting moulds are known (German Patent Specification 2,930,572), and these employ a composite superficial layer consisting of a solid lubricant, e.g. graphite, and one or more of chromium, nickel and cobalt. The function of this layer is to provide a low coefficient of friction at high temperatures, without additional lubricants, so that the draw-off resistance of the continuous-casting extrusion is reduced.The present invention offers a further advantageous possibility here. In particular, by means of the laser, one or more solid lubricants, e.g. boron mitride, or chromium nitride, as also chromium carbide, tungsten carbide, molybdenum disulphide or tungsten disulphide, can be additionally incorporated in the wear-resistant layer at the same time as this layer is being produced.

As already indicated, the above-mentioned metal or metal- containiong layer which may be applied to the shaping surface(s) of the mould with bonding may be applied electrolytically, for example as a pure nickel or chromium layer.This layer can subsequently be united metallurgically with the mould wall by means of a laser. Also, however, it can serve for alloying with additional constituents, or be used as a matrix for incorporating wearing bodies.

Alternative techniques for the application of the said metal or metal-containing layer include plasma-spraying, flame- spraying and flame shockspraying (detonation spraying); known devices can be used for this purpose. Spray-deposited layers, for example comprising tungsten carbide, chromium carbide or titanium carbide, or again aluminium oxide or chromium oxide, with or without alloying additives, e.g. cobalt, nickel, chromium or molybdenum, can be applied by these processes.

Another possibility is to apply the said metal or metal- containing layer by explosion plating. Treatment by means of a laser beam for the relevant purposes can then be carried out subsequently.

Depending on the requirements applicable in any particular case, a wide variety of materials or combinations of materials can be used for the purposes of the invention. Thus, a nickel- chromiumcobalt alloy can be used advantageously as a base for the above-mentioned metal or metal-containing layer, but "multi- component" cobalt-chromium alloys (stellites) have also proved valuable for the purposes of the invention. The said metal or metalcontaining layer can consist of a metal-base matrix in which are incorporated bodies of wear-resistance increasing materials, e.g. carbides or nitrides; again, it can contain metal oxides, e.g. chromium oxide, titanium oxide or aluminium oxide, or a mixture of two or more thereof.

The wear-resistant layers formed in accordance with the invention on mould surfaces will be disposed in typical cases in at least the base or lower region of the mould, this being the region exposed to the highest wearing stressed during operation.

In connection with the above-mentioned proposal (German Laid- open Specification (Offenlegungsschrift) 1,957,332) to provide inserts in the region of the bath level, in order thereby to obtain a lower thermal conductivity of the mould or a higher wall temperature, the invention affords additional advantages. Thus, in accordance with the present invention, it is possible to fill, for example, a rectangular or wedge-shaped groove in the region of the bath level of a plate mould with an appropriate metal powder and to fuse it into the groove by the use of a laser beam. Gaps do not in this case form between the groove wall and the introduced insert in as much as the insert is in practice integrated metallurgically into the mould wall in the groove region as a result of a metallic bond between the mould and insert, near their interface.

The invention will be explained in greater detail with reference to the accompanying diagrammatic drawing, in which each of Figures 1 to 6 is a crosssection of a mould wall.The Figures show various mould inserts, comprising not only complete layers but also tubular seamless inserts and welded inserts, as indicated more specifically below.

Thus, in the case of Figure 1, the shaping surface of a mould wall 1 has been provided, by means of a 5-W laser, with a wear- resistant coating 2 extending over the entire height.For this purpose, the mould wall 1, e.g. a plate of CuAg, had a powdery material composed of a mixture of 75 % by weight of WC, 21 % by weight of Ni, 3.5 % by weight of Cr, 0.5 % by weight of B, 0.8 % by weight of Si and 0.8 % by weight of Fe, applied to it by means of an appropriate form of metering device ahead of the point of impact of the laser beam on the mould wall so that the powdery material was fused on, the metering device and laser beam being moved in synchronism. At a working speed of approximately 0.3 to 0.6 cm2/sec, it was thus possible to apply a wear-resistant layer in a thickness of 1.5mm.Perfect metallic diffusion bonding with the base material of the mould, with a hardness of the alloying layer of HV 1070, was achieved, and the layer was practically pore-free. No operation in the nature of "material build-up by flame or plasma spraying" was needed here.

The coating provided in accordance with the invention can also be used particularly advantageously in combination with a layer produced electrolytically. Thus, for example, a nickel layer 0.7mm thick can be deposited on the entire inner surface of the mould to prevent copper diffusion, and subsequently this layer (or merely a region of it subjected to the highest wearing stress) can be alloyed by means of a suitably formulated alloying powder e.g. comprising tungsten carbide, chromium, silicon, iron or cobalt, under the influence of the laser.

In the case of Figures 2 to 4, a recess has been provided in the base region of a "plate mould" 3.

A chromium layer 5 (Figure 3) of approximately 80 microns thickness has been applied electrolytically to the entire mould plate including the recess 4. To increase wear resistance in the base region, a powder mixture, for example consisting of 60% by weight of tungsten carbide, 29% by weight of nickel, 2% by weight of boron, 2 % by weight of silicon, 2 % by weight of iron and 0.5 % by weight of carbon, has subsequently been introduced additionally into the recess 4 and fused on by means of a laser, so that a hard-metal layer 6 (Figure 4) has been obtained in this region. An advantage of this procedure is that the chromium of layer 5 already present in the recess 4 can contribute to the alloying of the wear-resistant layer 6. It has been found possible to achieve by this procedure degrees of hardness of HV 1020.

Useful hardness values can also be obtained if the powder mixture incorporates a combination of nickel, boron, silicon, iron and carbon without tungsten carbide, or employs cobalt instead of nickel, or even used a nickel/cobalt combination.

For the purposes of the present invention, it is also possible to replace the chromium coating 5 by, for example, a nickel coating, formulating the alloy powder accordingly. Moreover, TiC or Cr can be used successully instead of tungsten carbide.

In the case of Figure 5, a mould wall 7 consisting of a material comprising copper has an insert 8 of rectangular cross- section in the region of the bath level. When a powder mixture, for example an Ni Al combination with 3 to 5 % by weight of Al, was introduced into an appropriately prepared groove in the mould wall and this powder was subsequently fused in by means of a laser-beam arrangement, it was possible to achieve a durable anchoring efgect.The invention thus also provides a solution to the problem of producing thicker securely bonded coverings or inserts, without the danger of gaps forming between an insert such as that shown at 8 and an adjoining mould wall. This applies particularly to the mould regions in which lower thermal conductivity or a higher wall temperature of the mould is desired, as in the casting of sensitive steels, often.

Finally, Figure 6 shows a wedge-shaped insert 10 composed of a material of lower thermal conductivity disposed in the region of the bath level of a mould wall 9. The insert too is securely embedded in the mould wall, a diffusion bond having been formed with the adjoining surfaces of the mould wall as a result of fusion by means of a laser.

Claims

1. Process for producing moulds from materials comprising copper, suitable for steel continuouscasting installations, the moulds having a wear-resistant layer on the shaping surface(s) defining the mould cavity, wherein the wear-resistant layer is produced by means of a laser beam.

2. Process according to claim 1, wherein the wear-resistant layer is produced by initially applying a metal layer or a metal-containing layer, with bonding, to the shaping surface(s) of the mould, and subsequently uniting this layer metallurgically with the copper material of the mould by fusion under the influence of the laser beams.

3. Process according to claim 2, wherein the said metal or metal-containing layer is united metallurgically with the copper material of the mould, under the influence of the laser beam, only in a predetermined region, this being a region exposed to particularly high mechanical stress in continuous-casting extrusion.

4. Process according to claim 1 or 2, wherein the wear-resistant layer is produced by initially applying a metal or metal- containing layer, with bonding, to the shaping surface(s) of the mold, and subsequently adding one or more alloying additives which increase wear resistance to the material of this layer under the influence of the laser beam.

5. Process according to claim 1, wherein the wear-resistant layer is produced by applying the requisite wear-resistant material to the shaping surface(s) of the mould by means of the laser beam by a build-up welding technique.

6. Process according to claim 5, wherein the said material, in the form of metal powder or metal-containing powder, is applied to the shaping mold surface(s) in a locality immediately ahead of the point of impact of the laser beam and is fused on in that locality.

7. Process according to any of claims 1 to 6, wherein incorporation or embedding of one or more materials increasing wear resistance, into the superficial regions of the shaping mould surface(s), is effected under the influence of the laser beam.

8. Process according to any of claims 2 to 4 wherein incorporation or embedding of one or more material increasing wear resistance, in the said metal or metal-containing layer, is effected under the influence of the laser beam.

9. Process according to any of claims 1 to 8, wherein one or more solid lubricants are additionally incorporated in the wear-resistant layer produced by means of the laser beams.

10. Process according to any of claims 4 to 9, wherein the addition andjor incorporation or embedding of alloying additives or other materials increasing the wear resistance andlor build-up welding are carried out, by means of the laser beam, only in a predetermined region, this being a region of particularly high mechanical stress in continuous-casting extrusion.

11. Process according to any of claims 2 to 4 and 7 to 10, wherein the metal or metal-containing layer is applied electrolytically.

12. Process according to any of claims 2 to 4 and 7 to 10, wherein the metal or metal-containing layer is applied by plasma-spraying, flame-spraying or flame shock-spraying.

13. Process according to any of claims 2 to 4 and 7 to 10, wherein the metal or metal-containing layer is applied by explosion plating.

14. Process according to any of claims 1 to 13, wherein nickel or chromium is a principal constituent of the wear-resistant layer.

15. Process according to any of claims 1 to 13, wherein an alloy of nickel, chromium and cobalt is a principal constituent of the wear-resistant layer.

16. Process according to any of claims 1 to 13, wherein a multi-component alloy of cobalt and chromium (ie., a stellite) is a principal constituent of the wear-resistant layer.

17. Process according to any of claims 1 to 16, wherein the wear-resistant layer comprises a metallic matrix in which are incorporated or embedded bodies of a carbide or nitride or other wearresistant material.

18. Process according to any of claims 1 to 17, wherein the wear-resistant layer contains one or more metal oxides.

19. Process according to claim 1, substantially as described with reference to any Figure (s) of the accompanying drawing.

20. Continuous-casting mould produced by a process according to any of claims 1 to 19 and having a wear-resistant layer on the shaping inner surface(s) of the mould wall, wherein the wear-resistant layer has been at least partially metallurgically united with the mould wall after the fusion of at least a superficial region of the mould wall.

21. Mould according to claim 20, wherein at least a lower region of the mould has a wear-resistant layer united metallurgically with the mould wall.

22. Mould according to claim 21, wherein the wear-resistant layer provided in the said lower region of the mould has a greater thickness than that provided in its remaining region.

23. Mould according to claim 20, wherein a wear-resistant layer, which additinally is of lower thermal conductivity, is provided in the region of the bath level.

24. Mould according to claim 23, wherein the wear-resistant layer of lower thermal conductivity is provided in the form of inserts disposed in the region of the bath level.

25. Mould according to claim 24, wherein the inserts have a cross-section which is wedgeshaped, or in the form of a part of a wedge, and have their boundary surfaces converging in the intended direction of flow of the extrusion.

26. Mould according to claim 20, 21, 22, 23, 24 or 25, wherein the region of the mould wall which has solidified after the said fusion contains alloying constituents differing, in nature and/or proportion, from those contained in the regions in its vicinity.

27. Continuous-casting mould substantially as described with Figure 1, Figures 2-4, Figure 5 or Figure 6 of the accompanying drawing.

28. A continuous casting process wherein the metal being cast is cast in a mould as claimed in any of claims 20 to 27.