Soğuk haddeleme işleminde termik olaylar

Abstract

ÖZET Çeliklerin soğuk haddelenmesi, başlıca, rulo haddeleme metodunun kullanıldığı tersinir haddelerde yapılır. 4-lü tek standlı- tersinir haddeler, 3 ila 6 mm arasındaki kalın bandları kullanarak, 0,5 mm kalınlığında ve bazen daha ince kalınlıklara kadar ezme yapılabilirler. Çok merdaneli standlerden oluşan haddeler, çok ince bandlar ve 0,18 mm' den daha ince olan teneke bant haddelenmesinde kullanılırlar. Stand sayılarının 5-6 ya kadar arttırılması veya 2-3 standlı kontini haddelerin ilavesiyle, çok standlı haddelerde çok ince folyolar elde edilir (0,08 mm). Düşük şekil alma kabileyetli alaşımlı çeliklerin haddelenmesinde, 12'li ve 20'li çok merdaneli haddeler de geniş şekilde kullanılırlar. Kontinü haddelerde, rulo, üstü açılan rulo çözücüye yerleştirilir ve rulonun kuyruğu, özel magnetik kaldırma makinalarıyla veya sıyırıcı tip kuyruk çekiciler vasıta - sıyla merdanelerin içine doğru beslenir, hadde düşük bir hızla çalışır. Bandın ucu bir kere rulo makinasına geçirildiğinde, normal çalışma hızına erişilinceye kadar haddenin hızı kademeli olarak arttırılır. Doğru akım motorlarıyla tahrik edilen kontinü soğuk haddeleme, geniş bir hız ara lığında ayar edilebilirdir. Soğuk haddelemede,, çeşitli mineral yağlar kullanılmış tır. Bazen hayvansal ve bitkisel yağlarda uygulamada yer alır. Sulu solüsyon şekillerinde eriyebilen yağlarda uygu lamada kullanılmıştır, örneğin, palmiye yağı, teneke bant haddelenmesinde uygulanmıştır. Sulu solüsyon şekillerinde eriyebilen yağlar, önemsiz mahiyette sürtünmeyi azaltır, ama sıkıştırılmış hava vasıtasıyla bile bant yüzeyinden kolaylıkla uzaklaştırılabilir. Teneke bant gibi oldukça ince bantlar haddelendiğinde sıcak sulu emilsiyon formlarındaki eriyemiyen yağlar kullanılır. Bu gibi yağların haddeleme sonrası uzaklaştırılmaları zordur, bu yüzden bu gibi durumlarda, kimyasal veya elektriksel yağ giderme usulü kullanılır. Tek tek ayrılmış veya kontinü tavlama hatlarında da yağ giderme yapılır.

SUMMARY THERMAL ASPECTS OF THE COLD ROLLING PROCESS Certain basic components are common to most types of mills these including: a b c d e f g h i J' k 1 m n o P q r s t the work rolls and their bearings the backup rolls and their bearings the mill housing the mill foundations roll balancing systems roll positioning systems mill roll changing devices mill protection devices roll cooling and lubrication systems spindles and couplings pinions gearing motor couplings drive motors elektrical power supply systems idler and bridle rolls the uncoller and coiler coil handling equipment mill instrumentation, and the operating controls for themill viOf all the mill components, none is more critical fith respect to the rolling operation than the mill rolls. They must be capable of withstanding the high compressive and shearing stresses needed to deform the strip without thenselves undergoing plastik deformation and, though they elastically distort under these stresses, the distortion must be controlled to such an extent that the rolled strip has satisfactory shape, and that the rolling forces aire maintained within reasonable ranges. Such stresses and distortions are directly related to the physical properties and the dimensions of the rolls and, accordingly, roll geometries and the metallurgical properties of the rolls are both of considerable importance with respect to mill design and operation. Mill rolls are manufactured to very diverse specifications. In diameter, they may range from a fraction of an inch (as in the case of cluster mills rolling very thin foils) to several feet (as in the case of large backup rolls used in wide tandem mills). The length of the barred (or face) may range from a few inches for narrow laboratory-typ mills to close fo 100 inches for the widest commercial cold strip mills. For small rolls in cluster mills, tungsten carbide is frequently utilized as the roll material, but the rolls of two, three and four-high mills are usually made of cast iron or steel. Most rolls are of single-piece construction, but `built-up` or sleeved rolls are finding increasing use. At the same time, a wide variety of surface finishes are given to the rolls, ranging from a rough, shot-blasted finish to mirror-like finishes with roughnesses as low as one or two micro- inches RMS. Similarly, a wide variety of crowns are used in practice to compensate for roll bending. Roll necks aire usually about one-third to three- quarters of the diameter of the main roll body and are machined to be fitted into bearings accommodated in the roll chocks which position the rolls in the mill housing. Two principal types of bearing are used* the sieve type or `Morgoil` bearing and the `antifriction` or roller bearing. The former generally featurer an additional thoust bearing of roller type and utilizes a constant flow of lubricant, whereas the latter may, in many cases, be packed with grease prior to use or may be mist- lubricated. Adequate mill instrumentation is desirable for a. number of reasons. First, for the protection of the mill equipment itself, as, for examples, in the prevention of excessive motor loads or rolling forces on the roll necks and the mill housing; second, to ensure that the strip is undamaged in processing by virtually eliminating strip breaks and cobbles due to improper tensions and rolling viiforces; thid, to provide the desired quality in the rolled product with respect to such properties as gage and shape, fourth, to facilitate the general operation of mill, particularly during its aceleration and decelera tion so that the amount of off -gage strip at the head and tail end of a coil is held to a minimum, fifth, to assist in the proper maintenance of the rolling facility by indicating when changes must be made with respect to various components of the mill such as rolls and lubricants; sixth, to provide, with the assistance of automatic and computer control systems utilized on the mill, reliable gage control and mill set-upe; last, to acquire experimental data for use in studies relating to rolling the eories, strip behavior, mill desing and the like. Cold rolling of steels is done mainly in reversing mills using the coil rolling method. Single stand four high reversing mills can operate on a thick strip plate (3-6 mm) and produce sheets 0,5 mm thick and sometimes even thinner ones. Multiroll mills are used for rolling ultra-thin.'j sheets and tin plate (less than 0,18 mm in thickness) with multistands mills thinner sheets (0,08 mm) are obtained by increasing the number of stands (upto 5-6) or by additional rolling in two-or-three stand continuous mills. Multiroll mills (twelve and twenty-high) are also widely used in rolling low- f o ratability alloy steels. In continuous mills, the coil mounted on the decoiler is opened up and its tail is fed into the rolls by means of special magnetic lifting devices or stripper-type tail pullers, the mill being run at a low speed. Once the co iler has been threaded with the head end of the strip, the mill is gradually speeded up until its normal (working speed is reached. Continuous cold rolling mills with powerwul main d.c. motors are adjustable within a wide range of speeds. In fair-stand mills, the maximum rolling speed reaches 25 m/sn and their output amounts to 1300 thousand tons per year. Five stand mills operate at a maximum speed of 37m/sn and have and out put of 500-100 thousand tons per year. Six-stand mills with a roll body length of up to 1420 mm which are designed to roll tin plate less than 0,16 mm in thickness from a strip plate about 2.0 mm thick operate art a speed of up to 35 m/sn, their output reaching 700 thousand tons per year. Tin plate less than 0,076 mm in thickness is produced by secondary rolling in two or three-stand mills. The rolling speed in these mills is up to 28 m/sn. and the output amonuts to 300 thousand tons per year. viiiAs is known, when rolling with tension, the roll pressure is reduced. Therefore, in cold rolling, it is very importand to properly selected the strip tension and to keep it constant. The strip tension is kep constant automatically by controlling the speed of rolls and coilers, this being accomplished by an automatic control system which uses tensometrie devices (tens ime ter rolls) as sensors. The present thickness of rolled strip is also automatically maintained by means of the roll-gap control system incorporating nucleonic thickness gauges. Of great importance for continous rolling is the coiffucient of friction between the strip and rolls which is determined by quality of the lubricantused, the condi tion of the roll surfaces and rolling speed. In cold rolling, special attention is paid to the quality of the lubricant used which should reduce friction, ensure good cooling of the rolls and be easy to remove from the sheet prior to heat treatment. Various mineral oils are used in cold rolling. Sometimes animal and vegetable oils also find application. For instance, palm oil is employed when rolling thin plate soluble oils in the form of aqueous solutions reduce friction insignif.... insignificantly, but are easily removed from the surface of the strip even by compressed air non soluable oils in the form of hot aqueous emulsions are used when rolling extremely thin sheets, e.g. tin plate. These lubricants are somewhat difficult to remove after rolling and there fore, in such cases, use is made of chamical or electrolytic degreasing, the degreasers being either included in continuous annealing lines or installed squaretly. Lately there has been a trend to inerease the diameter of the rolls of continuous cold rolling mills. The diameter of the work rolls has been increased from 450 to 585 mm. and that of the backup rolls, from 1200 to 1500 mm. As the work roll diameter is increased, the removal of heat from the rolls improves, rolling speed increases, roll wear reduces and the surface finish of the rolled sheet improves. The greater diameter of the backup rolls makes the stand more stiff. In the commercial utilization of sheet and strip product J. it is desirable that the gage of the material should be as uniform as possible and the material devoid of all types of shape defects. For example, in the fabrication of rocket engine casings and other aerospace hardware, the use of sheet of nonuniform gage could lead to serious discrepancies between the actual and computed, values for component weights and their centers of mass. Moreover, shape defects could result in undesirable stresses and the subsequent distortion of such components. ixFurthermore, their fabrication, by such methods as auto matic welding, could be rendered appreciably more difficult if the sheets to be welded together were to be of different gage. In some cases, sheet product slit and, as narrow strip, is further reduced by cold rolling. Where a coil with excessive crown is slit, the edge partions would possess a very slightly wedge-shaped cross section, and the strip, if rolled to uniform thickness, would exhibit a camber or sweep. On the other hand, if the roll gap were adjusted to avoid the occurrence of camber, the rolled workpiece might be difficult to satisfactorily coil on a mandril. Flatness of cold rolled products is especially important where they find application in such items as curtain walls, appliances, furniture, trailer bodies and mobile homes. Departures from flatness in large exposed areas become immediately apparent and objectionable from an aesthetic viewpoint. Moreover, in such applications, the sheet product must also exhibit certain desirable surface features. Shape defects may be enhanced by subsequent shearing or slitting of a workpiece. A rolled product that may appear to be of acceptable shape but possessing a slight fullness just within an edge may develop an unacceptable wavy edge when sidetrimmed. Strip, that may appear satisfactory in long lengths uncoiled on a flat surface, may sometimes be sheared into sheets which have an tendency to curl. This effect is decidedly disadvantageous where sheet or strip is to be sheared and stacked for further processing, as in can manufacture. Under these circumstan ces, shape defects, such as coil set, twist and crossbow, make it difficult for the sheared pieces to be successfully fed to punch presses and other machines in a sequential manner without jamming the machines. x