METHOD AND SYSTEM FOR THE MAINTENANCE OF SURFACES

Abstract: Method for the maintenance of rolls/shells for rolling materials, e.g. light metals/aluminium, and casting of such materials in rolling mills, characterized in that the small ridges which are created on the surface of the roll during operation are removed continuously or periodically with a suitable machining tool being adapted to leave the surface layer of the roll essentially intact. The control of the machining tool may either be an active system which localizes the positions of the ridges, or a passive system which in a chosen way follows the surface.

Description: METHOD AND SYSTEM FOR THE MAINTENANCE OF SURFACES This invention relates to the rolling of materials, e.g. metallic materials, and especially light metals, and/or casting such materials with so-called roll casting or band casting. More precisely the invention is related to a method for avoiding the problems leading to surface roughness on the produced material increasing over time as a result of the creation of defects on the roll surface. It is known that the rolling process and especially roll casting generates deformations and cracks in the roll surface, which in turn leads to deterioration of the quality of the surface of the produced product. Such roughness in the surface may seam to be insignificant, but leads to many unwanted effects, such as corrosion, rubbing off/smearing (slip) of material during handling, as well as pollution of rolling oils and pretreatment baths. Therefore, in roll casting, the rolls are after a certain production time taken out of production and treated so that a new surface without surface cracks is obtained. Rolls and especially roll shells, being used e.g. in roll casting of aluminium, may at times be subject to extreme strain due to variations in temperature. Local crack-generation in the roll surface has its natural explanation in a thermal cycle as the roll surface use to roll casting is an internally water cooled pipe being subject to a liquid aluminium melt. The thermal expansion exceeds the yield point of the material and results in rivetting and bulging of the surface metal of the roll. Later cooling leads to contraction with tensile stress leading to thermal fatigue and crack generation. In addition an outer oxide layer is, among other things, generated at the surface of the roll. This has proven to have a favourable effect as it reduces the tendency of the rolled material to stick to the oxide layer compared to the untreated surface of a new roll. The prevailing opinion of the abovementioned problem is that the cracks in the roll are the origin of the problems with increasing roughness on the product. To be able to produce an even surface quality it is therefore usual to performe a periodical reparation of the rolls by removing up to several mm, so that a new crack-free, or essentially crack-free surface is obtained. Some times it is chosen not to remove the deepest cracks to spare the thickness of the material. This method helps temporarily, but since it in the case of roll casting means that one is working with decreasing thickness of the material in the roll, the temperature gradient through the material is increasingly more dramatic, and the production time between each time the problem arises decreases. One is working in a vicious circle, which is caused by not realizing the real centre of the problem. Another way to try to reduce the problem, which is used today, is to add increasing amounts of slip? agent? to the rolls. By increasing the added amount of slip agent the problem may be reduces temporarily. The amounts of slip agent used today represents, however, a considerable environment and pollution problem, as well as unwanted effects of the material, at the same time as it does not solve the problem on the long term because it does not remove the cause of the problem. In relation to cold and hot rolling it is known to use certain techniques for running treatment an maintenance of the roll surface. Prom German patent publication 41 05 079 it is thus known a device for grinding a roll surface in order to be sure that the roll surface always may be considered to be new. According to this publication it is an object to work with a roll surface without an oxide layer, something that, however gives additional sticking between the roll and the material, and a reduced quality surface quality on the material. Another aspect of the method is that it demands a relatively voluminous equipment, which may be difficult to fit in in relation to roll casting. A third aspect of such rough equipment is that it grinds off a considerable quantity of material, which produces a pollution problem in the production line. The German patent application 44 09 300 teaches another variant of such grinding of rolls. It do, as far as it goes, have the same disadvantages concerning size and grinding amount. It is of course both possible and preferable not to let such equipment be used continuously, so as to reduce the pollution problem, but any way such equipment because if its size will not be very suitable in roll casting. A completely different essential disadvantage with the apparatus according to this application is - as in the abovementioned case - that the grinding method is so coarse that the oxide layer is removed completely, which is strongly unwanted because of the sticking effect that thereby arises. In EP 154.319 an apparatus is described for grinding of parts of rolls. The background for this is that rolls being used for rolling materials with different width will be worn differently. When a wide sheet of material is to be rolled there is a risk, according to this patent publication, that the rolling is uneven. There arises a need for grinding of parts of the rolls to obtain an equal quality over the entire width. The grinding is performed using grindstones, with an accuracy in the μm range. The solution is aimed at another problem than -the present invention comprising deeper grinding, which among other things results in a bulky and complicated system which would not be suitable in this regard. In addition such a grinding will primarily be aimed at the areas that are used the least, so that the difference in the surface quality in the form of protruding areas, will increase with the treatment described in this patent publication. US 4.575.972 discloses a grinding machine for rolls using an abrasive belt. This is not suitable regarding the protruding areas this invention is aimed at, since the paper band will follow the surface and thus grind between the protruding areas. This in turn affects the oxide layer, with following problems with sticking. No of the abovementioned methods are, however, known to have been used in any rolling mill, something that probably is connected to the abovementioned disadvantages. The real crux of this problem regarding roughness in the material in roll casting and warm/cold rolling, the recognition of which is the basis of this of this invention, is not the previously mentioned crack generation in the rolls, but on the contrary the generation of small protrusions (swells) or ridges in the roll gods, along the edges of the cracks. The ridges are a natural cause of the same natural cycling that causes the crack generation. In addition to the mechanism mentioned above material will be deposited on the roll from slip agents and the passing material, which like the ridges will lead to protrusions on the roll surface. To this date there has not been suggested any really good method for securing a constant quality in the rolled material over time, based upon the recognition that the protrusions are the cause of the problem with surface roughness. It is therefore the recognition of this invention the these protrusions are the main cause of the accumulation of materials arising on the produced product, which surface therefore gets a torn, and visually featured, rough quality. As will be described below, this invention do not only provide a method which, with simple means secure a better surface quality over time, but it also represents a breakthrough concerning the understanding, recognition of the problem. The method according to the invention allows for production of rolled material with a nearly perfect surface for longer periods of time without much maintenance, at the same time as the rolls/roll shells is improved considerably. Especially in roll casting, which is performed with water filled roll shells with a relatively limited shell thickness, the improvement is substantial. The invention is more specifically related to a method and a system for maintenance of rolls/-shells for rolling of material, such as light metal/aluminium, and casting of such material in so-called roll casting, characterized as stated in the accompanying independent claims. The preferred embodiments of the invention are described in the dependant claims. By only - or mainly - removing the abovementioned protrusions from the roll surface it is shown that the casted/produced product obtains a considerable improvement. It is therefore not necessary to grind the roll surface in depth to maintain a good surface quality what roughness on the product concerns, and neither is it necessary to treat the whole roll surface to secure good quality. The cracks in the rolls have the same effect as expansion joints for the local thermal expansion in the material when contact is made between the roll shell and the material melt, and causes in itself no significant harm on the produced product. Thus there is no need to remove these cracks. As mentioned above the naturally occurring oxide layer on the rolls gives an advantage, as it reduces the sticking of the material on the roll. In stead of the oxide layer, which normally occurs after used over some time, a layer with similar effect may be provided on the roll before use. Such a layer may be made in different way and of different materials, such as ceramic layers, or bimetallic layers in which case the roll is treated with a different metal type which diffuses into the roll and changes its characteristics. The latter also occurs naturally as elements of the passing material diffuses into the roll. This will occur in addition to the generation of an oxide layer. Thus it is an essential feature of the method according to this invention that it provides such a gentle treatment of the rolls that the surface layer of the roll is maintained at all times. This is in contrast with the known devices for the treatment of roll for warm- or cold rolling described above. Below the invention is described in detail with reference to figure 1. The figure shows a section of rolls and melt in roll casting. The figure shows a section of a roll mill with a roll shell 1, a nozzle 4, melt in to the roll by 2, and casted band 3. The figure shows only the upper roll, but the lower roll is positioned under the broken symmetry line. While the rotation speed of the roll is constant the surface of the material which is casted and which is directly in contact with the roll, will gain speed towards the most narrow cross section between the upper and the lower rolls. As is illustrated (encircled and enlarged) on the figure the mentioned protrusions on the surface of the roll at first make a small feature in the material to be moulded. During the reduction of the materials cross section, when the material gain speed, the protrusion will retain the material because of the relative difference in speed between the roll and the material, and thus cause a depression in the material being considerably longer in the longitudinal direction than size of the protrusion alone should imply. Where the roll swings outwards and loses contact with the material scraped out material will be left as a small protrusion behind the described depression. This is illustrated in the enlarged portion (encircled) of the figure. For rolls with a relatively low rotational speed, as is the case e.g. in roll casting, it is most appropriate to use equipment which continuously or automatically removes the unwanted material during production, from the parts of the rolls that at a given time is not in contact with the material. The material is removed from the rolls is eliminated using known techniques so that they do not contaminate the melt or the finished, casted or rolled material. It is not necessary to treat the whole roll surface area, as the regions giving problems in the form of roughness are located at a relatively small part of the surface. In principle it is natural to limit the removal of material to the problematic regions. This has the advantage that less material must be removed, but on the other hand it demands a more sophisticated system being able to locate the protrusions. Choosing to remove material from the roll only limited in depth, not in area, one may manage with a passive system or a following system passively following the surface of the rolls. Even the so-called passive system must, however, be sophisticated enough to compensate for "slow" changes in the surface, e.g. because of deviations in the circumference of the roll away from a perfect cylinder. The system is in this case designed such that the treatment tool essentially "floats" upon all slow changes in the contour of the roll surface, but cuts straight through sharper peaks in the surface. The figures 2 and 3 shows simplified sketches of tools being use to remove the protrusions on the roll surface. In both cases the tools are light and do not occupy much space or require big changes in existing roll systems. Both will follow the surface of a roll if it deviates from a perfect cylinder, and may be made in small units covering only a part of the surface, but may be moved along this in a few revolutions in order to cover all of it. In figure 2 a wagon 7 is shown comprising grinding tool, here e.g. a whetstone. The grinding tool is held with a pressure against or a distance from the surface providing a grinding effect on the protrusions. It may be noted that the protrusions need not be removed entirely on each passing of the tool, so that the force with which the tool is held against surface do not have to be big. The grinding tool is kept in contact with the surface with wheels 5 positions in relation to a wagon 5 or similar which keeps the right position in relation to the surface. The wagon may be provided with equipment for rotation of the grinding tool to give a grinding effect, and may be provided with means for adjusting the height of the grinding tool to adapt to wear on the grinding tool, and to perform the grinding at chosen times or intervals. The rotation and height of the grinding tool may be controlled by mechanical settings or actively by known, e.g. electrical, control systems. The grinding tool in figure 2 may cover the whole length of the roll or only a part of it. In the latter case it will preferably be capable of movement in the axial direction of the roll, or may be one of a plurality of tools positioned along the roll, which will give an opportunity to grind at a chosen area on the roll. Figure 3 shows another tool comprising a whetstone 4 positioned in a holder 9. The whetstone may be spring loaded to keep a constant force against the surface and may, similar to the solution in figure 2, comprise devices for lifting and lowering the whetstone in relation to the surface. The holder 9 may also comprise channels for supplying grinding liquids and removal of this together with grinding dust, so that the dust do not pollute the rolled material. To be capable of movement in relation to the roll the holder 9 is connected to a guiding screw 6 with a groove 8 so that the grinding tool may be moved smoothly in the axial direction to, after a while, be able to grind the whole roll, or so that it may be moved to a chosen area that needs grinding, and be activated there. This invention is, however, not limited to a certain tool for removing of material, as an important feature of the invention is in the recognition that it is the protrusions and not the cracks, that are the key to the problem. Thus any way to remove such protrusions, it being manual or automatic, while the equipment is in use or temporarily stopped, when the roll surface is essentially not touched, is within the frame of this invention. Removal of the protrusions may, as mentioned, be done in many different way, using cutting, scraping or grinding tools. An edge positioned close to the surface of the roll will cut off the tops without touching the surface layer, or protrusions may be scraped off by scraping against the surface with a chosen pressure. Laser may also be used to cut the protrusions. What constructively differs this method from known methods is that the removal of material is done in a limited degree. While other known methods use coarse equipment the treating according to the invention is limited at least in depth, and possibly also in relation to the area of the roll surface. This gives the present invention a great advantage in relation to size and complexity. By limited in depth it is understood that one at any time removes at little that the surface layer is essentially intact. Any method removing the protrusions and at the same time leaves the surface layer essentially intact, is within the limits of this invention. It is not important if all of the roll surface is given a mild treatment, as longs as the surface layer is spared in most of the surface area. There are different technical possibilities regarding both active and passive control systems for the treatment tool, and this embodiments is not limited to certain way to do this. Such control systems may typically be based on such principles as capacitive systems, optical systems, mechanical systems, systems based upon an air cushion principle or combinations of such systems, depending on the accuracy and sophistication with which one wants to control the treatment. Such systems may easily be applied with the present technology in the micro mechanical area. Electrical, optical and/or acoustic measuring systems may be used in a known way to classify the surface, and thus detect the need for treatment over the whole, or parts of, the surface. In certain situations and for some kinds of equipment, the situation will be such that continuous removal with the production going is out of the question. This may for example be situations in which the need for space or the consideration of other marginal limitations make it impossible or impractical to work continuously. For certain rolls the rotation will be to big to allow for treatment of the roll while going, e.g. in warm or cold rolling. One may then perform the treatment in periods between the rolling of two pieces, e.g. blocks or coils, as the rolls then will hold a reduced speed. The invention is primarily aimed at casting or rolling of materials with a relatively high melting point, such as metals or typical light metals. The concept of the invention holds, however, as previously mentioned, for any type of materials casted/rolled between rotating rolls.

Claims: C l a i m s 1. Method for treatment of the surface of rolls for rolling materials, e.g. light metal/aluminium, and/or for casting of such materials, the roll surface comprising an outer layer, e.g. of an oxide layer, and protruding areas in the shape of pegs or ridges, c h a r a c t e r i z e d in that a tool adapted for removal of the protruding areas is positioned by the surface of the roll, and that the tool continuously or in periods is positioned with a distance from, or a pressure against, the surface of the roll which, by movement of the roll in relation to the tool, gives at least a partial removal of the protruding areas by each passing by the tool, essentially without affecting the outer layer. 2. Method according to claims 1, c h a r a c t e r i z e d in that the chosen distance to and/or pressure against the roll surface is chosen passively, e.g. by using a chosen spring constant and/or by mechanically maintaining a chosen distance between the tool and the surface to maintain the chosen distance between the tool and the surface. 3. Method according to claim 1 or 2, c h a r a c t e r i z e d in that the position and possibly pressure against the surface is controlled actively by using per se known distance and/or pressure sensors with corresponding actuators for controlling the tool. 4. Method according to any one of the claims 1-3, c h a r a c t e r i z e d in measuring of the characteristics of the roll surface for the detecting and locating of the protruding areas, and positioning of the tool in the located areas. 5. Method according to claim 4, c h a r a c t e r i z e d in that the measuring of the roll surface is performed using optical or acoustic measuring methods, e.g. for measuring the distance to the surface. 6. Method according to claim 4, c h a r a c t e r i z e d in that the measuring of the roll surface is performed using electromagnetic equipment. 7. System for maintenance of the surface of rolls for rolling materials, e.g. light metals/aluminium, and/or for casting of such materials by roll casting, c h a r a c t e r i z e d in a cutting, scraping or grinding tool provided by the roll surface, the roll surface comprising an outer layer, e.g. an oxide layer, and protruding areas in the form of pegs or ridges, and a device for continuous, or in chosen periods, positioning of the tool with a distance to, or possibly a pressure against, the roll surface which, by movement of the surface in relation to the tool, provides an at least partial removal of the protruding area at each passing of the protruding areas by the tool, essentially without affecting the surface layer. 8. System according to claim 7, c h a r a c t e r i z e d in that it comprises mechanical and/or pneumatic devices for maintaining of the tool at a chosen distance from, or possibly pressure against, the roll surface. 9. System according to claim 7 or 8, c h a r a c t e r i z e d in that it comprises per se known devices for active measuring of the position of the tool and possibly the pressure against the surface of the roll, and control means for controlling the position of the tool and possibly the pressure against the surface. 10. System according to any one of the claims 7-9, c h a r a c t e r i z e d in that it comprises per se known measuring devices, e.g. optical or capacitive, for the detection of protruding areas, and control means for positioning of the tool by the given positions for removing the protruding areas.

Inventors: BERG GISELA E, BERG, GISELA, E.
Applicants: BERG GISELA E, BERG, GISELA, E.
Application year: 1997
Main classification: B21B28
IPC codes: B21B28, B24B5