热轧带钢轧辊的破坏的一些研究外文翻译.doc

damage in hot rolling work rollsabstractsamples from a couple of work rolls employed in a hot rolling steel strip mill were examined by scanning electron microscopy in order to study their surface ,as well as the wear mechanisms responsible for their damage. it was found that a series of cracks develop within the rolls at the beginning of their operational cycle, these cracks follow the primary carbide network and are attributed to be caused either by thermal fatigue or contact stresses or by a combination of both. work rolls heat up as a result of contact with the steel strip and an oxide layer forms on them. it is observed that oxidation proceeds through the cracks, isolating healthy material with brittle layers, a characteristic which might explain the high wear rate encountered in some stands of the mill.1999 elsevier science s.a. all rights reserved. keywords: hot rolling; steel; wear; oxidation; work rolls1. introduction some of the most important segments in the operating costs of a modern hot rolling mill are related to the work rolls, since they are expensive and a large stock must bekept in hand to assure the continuous operation of the mill. the work rolls have to be changed frequently, accounting for downtime, and they must be ground to their required profile (crown) before being replaced into the mill. in many cases rolls are changed because their surface has deteriorated, rather than because of changes in shape or crown due to thermal expansion or wear. previous research has pointed out that thermal and mechanical fatigue, as well as abrasion due to hard iron oxides, are the most important factors affecting the rate of surface deterioration in work rolls of second generation type mills. new generation mills rolling strips to thinner gauges at higher speeds seems to benefit from the oxide formed at their surface,but there is also evidence that this oxidation might be deleterious in some cases.the aim of this work is to present the results from a series of microstructural observations, carried out in pieces obtained from spalled work rolls, which help to explain the conditions and mechanisms responsible for their surface deterioration.2. roll sampling it would be desirable to obtain samples from work rolls which have been subjected to different rolling conditions, but this would imply cutting and spoiling healthy and usable work pieces, therefore, it was decided to study the microstructure of work rolls which were broken or spalled during their campaign. care was taken to assure that theportion of the roll sampled for this particular study was not related to the spalling. samples from two different work rolls were obtained by these means, they were cut and prepared for observation by scanning electron microscopy using the secondary and back scattered detectors, x-ray analysis were made in selected regions.one sample came from a work roll that spalled early in its life, whereas the second one failed towards the end of it. both work rolls were mounted in different stands, table 1, of a six stand continuous strip mill which is fed by 50 mm thick continuously cast slab.table 2 shows the chemical composition, hardness and roughness of both rolls. hardness is referred to the shore c scale, whereas roughness is defined as to the arithmetical average of the deviation from a mean centerline, these data were measured in the work roll before being mounted in the mill.3. observations the samples were examined with detectors for secondary and back scattered electrons. although the former is the most common technique used in the study of surface details, the latter was employed in this study due to its ability to detect the concentration of different atomic species(lighter elements appear darker in photographs). fig.1 shows a region close to the surface in work roll a, its original surface appears towards the right side (the specimen was mounted in bakelite and, as was mentioned before, it appears black), the plane of view was cut at a 60angle with respect to the surface normal. different characteristics can be appreciated, among them a series of light and dark eutectic carbides, punctual x-ray analysis on them indicated that the light ones are rich in mo, whereas the others contain cr. a series of cracks starting from the surface seems to follow the carbides. some of this cracks penetrate to around 110m. fig.2 shows a higher magnification of a crack that surrounds a light carbide(the one towards mid-height on the right hand side in fig.1)and later on deviates along a darker one to follow a trajectory parallel to the roll surface(this branch is around 40m deep).a series of dark regions can be appreciated close to the roll surface, punctual x-ray analysis indicated the presence of oxygen in them. figs.3 and 4 were taken from work roll b (the surface is cut at a 458 angle from the one normal to the surface),as in previous figures, the roll surface is located at the rightside, but an irregular and porous dark grey layer is encountered, x-ray analysis indicated a high concentration of oxygen. a dark gray loop which develops from a crack originated at the roll surface can be seen in fig.3,x-ray analysis conducted in this loop indicated a similar concentration of oxygen in comparison to that in the dark superficial region. two light particles are seen close to the surface, one of them just in front of the loop, x-ray analysis indicated compositions similar to the light carbides found in work roll a, see figs.1 and 2. fig.4 is a view at higher magnification of the light particle found towards the top right hand side in fig.3,it can be seen that this particle is surrounded by the dark layer, more important, this dark compound follows the cracks present in the work roll leaving healthy material surrounded by it. some of the dark gray layer shown in figs.3 and 4 was removed with a file and the debris were analyzed by x-ray diffraction yielding to be made of magnetite and hematite,fig.5.4. discussion examination of table 1 indicates that work roll a spalled very early in its life, allowing for the study of an unoxidized surface. however, it is worth noticing the presence of cracks even after such a short campaign. figs.6 show the stress gradients within work roll a, as calculated from the solution proposed by boresi and sidebottom with data from the predictions from a hot rolling model .it can be seen in fig.6 that some of the component of stress (r andz) reach levels of 400mpa at the surface,a value which is around that expected for fracture at room temperature .this mechanical component of fatigue should be responsible, in part, for the origin of the cracks, however, the thermal component of fatigue should also be considered, since the surface will be subjected to thermal cycling similar to that shown in fig.8.this figure indicates that the roll surface can easily reach temperatures above 500,but due to the shallow penetration(as the contact time between roll and strip is short)it will be subjected to very sharp changes in temperature, and thermal stresses. some authors consider that the oxide layer found on the surface of the work roll is due to the adhesion of that on the strip being rolled, but, as can be seen from figs.3 and 4,the only mechanisms which will be able to explain the presence of oxide in cracks, and around carbide particles, are those based on diffusional growth .in such a case, the oxide thickening will be accelerated by the increase in temperature and by the cracks, see figs.3 and4,developed in the layer as result of thermal cycling .the presence of loops of oxide, like the one shown in fig.3,helps to explain the high wear rate, known as peeling, to which some mills are subjected to.in this case, the work roll will not wear by abrasion, but healthy material will come out in big pieces as result of the fracture of the oxide layers surrounding them.5. conclusions metallographic examination of regions close to the surface in samples from work rolls indicate the existence of cracks, due to either thermal or mechanical fatigue, at a very early stage.these cracks start at the surface and grow into the roll following the primary eutectic carbides network. oxidation of the work roll surface will be due to diffusional processes, this phenomenon will be able to proceed through cracks within the work roll. the oxide will be able, in some cases, to isolate healthy regions of