Method for reconditioning rollers

[0001] The present invention relates to a method for refurbishing rolls of a hot rolling stand for aluminum materials for producing rolling mill products, in which the rolls are refurbished after or during usage by processing the roll surface.

[0002] It is known from the prior art that rolls for the production of rolling mill products have a roll surface adapted to the particular intended use. For example, work rolls for hot or cold rolling of metal strips have a roll surface having a defined roughness in order to ensure secure gripping of the rolling stock by the rolls and the production of a defined surface of the rolling stock. Other types of rolls, such as backup rolls, strip tension rolls, or the like, for example, also have a defined roll surface which ensures the perfect functioning of these rolls.

[0003] Due to high pressures arising during rolling of rolling mill products, such as strips, plates, etc., uniform deposits are formed on the roll surface which, after a certain period of usage of the rolls, lead to the rolling procedure no longer being able to operate properly and/or the rolling result no longer being satisfactory.

[0004] Especially tenacious deposits arise during hot rolling of aluminum strips in particular. The work rolls used during the hot rolling of aluminum strips have a grinding structure on their surface which ensures the production of desired surface properties of the rolling stock and contributes to a secure gripping of the rolling stock by the rolls, through which slipping in the roll gap is avoided. During usage of the rolls, aluminum oxide particles or aluminum particles or—depending on the alloy rolled—magnesium oxide particles or magnesium particles are deposited in the recesses of the grinding structure. These particles are compressed in the recesses by the high rolling pressure force in such a way that they fill up the recesses of the grinding structure compactly and very resistively. In turn, this leads to the grinding structure on the roll surface no longer being effective after a certain usage period of the roll during rolling. The deposits formed on the roll surface are also referred to as roll coating.

[0005] The filling up of the grinding structure of the roll surface leads to different effects. Firstly, secure gripping of the rolling stock by the rolls is no longer ensured and the rolls begin to slip. The aforementioned particles also start to accumulate more increasingly at a few points of the roll surface of the work rolls and lead to undesired imprints on the rolled strip surface. Sometimes, the accumulated and compacted particles also detach in larger units and remain on the surface of the strips as a so-called pickup.

[0006] The effects cited occur in varying degrees during hot rolling of different rolling mill products made of different aluminum alloys.

[0007] Since rolls are products which are costly to produce, according to the prior art, they are not completely replaced after the formation of roll scale or the like, but uninstalled and refurbished by processing of the roll surface. After the refurbishment, the rolls again have the desired properties on their roll surface and may be used again. Refurbishment of the rolls by processing the roll surface using mechanical abrasive methods, such as grinding, is known from the prior art. For this method, the deposited contaminants impurities and the material of the roll itself are removed in a defined way from the roll surface in order to produce an unused roll surface. Sometimes, the roll surfaces are also subjected to renewed coating or hardening after the mechanical abrasive treatment.

[0008] Since the roll diameter is continuously reduced upon repeated refurbishment due to the removal of roll material during the mechanical abrasive processing, the rolls are finally worn down after a specific number of refurbishments, i.e., not reusable even after refurbishment. This is caused by the change of the geometric properties, but also by reaching material layers of the roll material which are no longer suitable for further refurbishment due to insufficient strength or the like.

[0009] These methods known from the prior art for refurbishment rolls have different problems. On one hand, the rolls must be uninstalled for refurbishment and replaced by new and/or refurbished rolls. Shutdown times of the rolling mills, which are connected with significant costs, result therefrom. Furthermore, refurbishment of the rolls on location is typically not possible, so that the rolls to be refurbished must be transported over further distances. Finally, the processing of the rolls using mechanical abrasive methods leads to relatively rapid wear of the costly rolls. It is particularly to be noted in this case that, for example, work rolls in hot rolling stands must be refurbished multiple times per week because of the effects cited.

[0010] Furthermore, providing cleaning brushes, by means of which the surface of the rolls may be brushed in the installed state, preferably continuously, to extend the usage times of rolls in a hot rolling stand for aluminum materials is known. In this way, the formation of roll coating on the roll surface is at least reduced. The brushes cited are steel or plastic brushes whose use over a long period leads to a change of the roll surface—similar to polishing—through which the ability of the rolls to grip the rolling stock securely is reduced.

[0011] Originating from the related art described above and the problems connected therewith, the present invention has the object of specifying a method for refurbishing rolls for the production of rolled products which ensures simple, rapid, and cost-effective refurbishment of the rolls.

[0012] The object derived and described above is solved according to the present invention in that the roll surface is processed by means of a liquid jet and the pressure of the liquid for generating the liquid jet is set to approximately 50 to 250 MPa, preferably 70 to 150 MPa.

[0013] Surprisingly, it has been shown that the highly compressed particles and deposits accumulated on the roll surface may be removed nearly without residue by means of a liquid jet. The processing of the roll surface by means of a liquid jet is advantageous particularly because the original roll surface is nearly unchanged in this way, so that, for example, no reduction of the roll diameter occurs during the refurbishment. In this way, because the actual roll surface is nearly unchanged during the processing by means of a liquid jet, the roll remains dimensionally correct even after refurbishment. The demands on the processing by means of a liquid jet are therefore much lower than for mechanical abrasive methods, in which the dimensional correctness must be ensured. Finally, processing of the roll surface by means of a liquid jet may be performed significantly more rapidly than processing of the roll surface with mechanical abrasive methods, such as grinding. The method according to the present invention may also, however, be combined with the mechanical abrasive methods known from the related art.

[0014] Because the liquid pressure is set to approximately 50 to 250 MPa (106 Pa), preferably 70 to 150 MPa, during the production of the liquid jet, reliable removal of the residues and contaminations on the roll is ensured. To produce pressures of this type, mechanically driven piston pumps (plunger pumps) are used in the overwhelming number of cases, in part with the additional use of hydraulic pressure intensifiers.

[0015] A first advantageous embodiment of the method according to the present invention is distinguished in that the roll surface is processed exclusively by means of a liquid jet for refurbishment. It has been shown that the processing of the roll surface by means of a liquid jet for refurbishing rolls is sufficiently thorough that further processing of the roll surface, particularly by brushes, is unnecessary. In particular, coatings of the roll surface, which have been applied to increase the strength of the roll surface, for example, are typically retained during the processing by means of a liquid jet.

[0016] The properties of the method according to the present invention already described permit optimum further development of the method in regard to the shutdown times and the operating sequence if the roll surface is processed by means of a liquid jet with the roll being built-in. By refurbishing a roll being built-in the already mentioned high rate of the processing of the roll surface by means of a liquid jet, the shutdown times of a rolling mill caused by the refurbishment may be reduced drastically.

[0017] In case the processing is performed continuously or with interruptions during rolling, shutdown times for refurbishment of the rolls are avoided completely.

[0018] As an alternative to the embodiment just described, the teaching according to the present invention is advantageously further developed if the processing is performed during rolling breaks. In the rolling process, rolling breaks occur regularly, which result because of the system, e.g. from the transport of a new ingot to the rolling stand or something similar. These rolling breaks may be used without anything further in order to refurbish the rolls according to the present invention.

[0019] Both measures allow refurbishment of the rolls without using steel or plastic brushes which brush the rolls during rolling, as is known from the prior art. In this way, the useful life of the rolls is improved further, since the roll surface is not “polished” by the steel brushes.

[0020] If water is used as the liquid, the method according to the present invention is developed further in regard to environmental compatibility, operational safety, and the costs connected with the method. Maintaining explosion safety without problems is ensured by the use of water. In addition, the water arising during refurbishment may be treated without anything further in conventional waste disposal measures and/or may remain in a water cycle, for example, the emulsion water cycle of a hot roll. However, depending on the application, the use of alternative liquids for generating the liquid jet used for processing is also possible in principle. The quality of the water used is to be adapted to the particular application. For specific applications, the addition of particles in an adapted dosage is also conceivable, in order to elevate the cleaning force and/or to process the roll surface in a shape-changing way.

[0021] There are several possibilities for embodying and developing the method according to the present invention for refurbishment of rolls for the production of rolling mill products. In this regard, reference is made to the claims subordinate to claim 1 and to the description of an embodiment of the invention in connection with the drawing, for example.

[0022] In the drawing:

[0023]
FIG. 1 schematically shows a highly enlarged section through the roll surface of a work roll and

[0024]
FIG. 2 shows a perspective view of a roll during refurbishment by means of a method according to the present invention.

[0025] The highly enlarged section schematically illustrated in FIG. 1 shows the profile of a work roll of a hot rolling stand for aluminum alloys.

[0026] In the section shown in FIG. 1, the actual roll material 1 is shown hatched. In the present case, this is a steel roll. For other applications, rolls made of cast iron are also used, for example. It may be seen clearly that the roll surface is contoured in the scale shown and has multiple elevations 2 and recesses 3. These elevations 2 and recesses 3 provided in the roll material 1, referred to in general as roughness, ensure, among other things, reliable gripping of the rolling stock by the roll and a desired surface shape of the rolling stock during the use of the work roll illustrated, as already noted. This is ensured with force transmission by friction arising between the elevations 2 and the recesses 3 and the aluminum strip rolled.

[0027] It is also shown in FIG. 1 that chromium plating 4 has been produced on the roll surface. This chromium plating 4 over the entire surface is used to increase the hardness of the roll surface and therefore the durability of the elevations 2.

[0028] The roll surface shown in FIG. 1 belongs to a roll which has already been used to a significant extent. In the course of this use, aluminum oxide particles and aluminum particles, which have been compressed by the high rolling pressure into deposits 5, have accumulated in the recesses 3 during the hot rolling of aluminum strips. As may be seen clearly, the deposits 5 shown in FIG. 1 already cover a part of the recesses 3, through which the engagement between the roll surface and the rolling stock is reduced. In this way, effects such as slipping of the rolls, the detachment of deposits 5, which remain on the rolling stock surface, and increasing accumulation of deposits 5 in points, which leave imprints on the rolling stock surface, increasingly occur.

[0029] The roll surface shown in FIG. 1 must be refurbished to produce continued flawless rolled stock. According to the method according to the present invention, the refurbishment is performed by processing the roll surface by means of a liquid jet, as shown in FIG. 2. For a work roll of a hot rolling stand for aluminum, water is preferably used as the liquid, since a water/oil emulsion is used during the hot rolling itself, and water therefore does not represent a foreign material in the process.

[0030] A roll 6 illustrated in FIG. 2 shows, after a certain period of usage, a roll scale 7 made of the deposits 5 shown in FIG. 1, which must be removed during refurbishment.

[0031] According to the method according to the present invention, the refurbishment is performed by processing the roll surface 8 by means of a liquid jet 9, which exits a jet lance 10. In the surface region 11 already processed by means of the liquid jet 9, the roll scale has been removed.

[0032] The refurbishment according to the present invention shown in FIG. 2 may be performed in both the built-in and in the uninstalled state of the roll 6. In addition, the jet lance 10 may be controlled by hand or automatically. Positioning a device inside the rolling stand which guides the jet lance 10 automatically in a defined way over the roll surface 8 during refurbishing of the roll 6 in order to remove the roll scale—as soon as this is necessary—stands to reason. This is also possible during rolling procedure.

[0033] For the refurbishment according to the present invention of a work roll of a hot rolling stand for aluminum alloys, the following facts are to be observed. The roughness of a roll refurbished by grinding with the aid of methods known from the prior art has been reduced in the course of usage by approximately 55% before the undesired effects described, such as slipping or the like, occurred. Processing of the roll surface according to the present invention by means of a water jet has led to an increase of the roughness to approximately 95% of the original value. Therefore, a flawless further use of the refurbished roll is ensured.

[0034] For the processing, a water jet having a pressure of approximately 200 MPa without the addition of particles is used during the refurbishment as described.

[0035] For the embodiment described, i.e., a work roll for hot rolling of aluminum, it has been shown that the roll is to be refurbished conventionally, i.e., by mechanical abrasive methods such as grinding and subsequent chroming, after multiple refurbishments by means of the method according to the present invention in order to refurbish the roll surface fundamentally. This conventional refurbishment is necessary in the embodiment described after multiple refurbishments according to the present invention.

Number	Date	Country	Kind
101 52 718.7	Oct 2001	DE
01125177.4	Oct 2001	EP

Method for reconditioning rollers

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