Sıcak haddeleme yük hesabı metodlarının karşılaştırılması ve uygun metodun pratik bir uygulaması

Abstract

Yassı çelik mamullerin sıcak haddelenmesinde, had¬ deleme yükünün hesaplanması için, çeşitli araştırmacılar tarafından geliştirilmiş ve literatürde önerilmiş birçok metod vardır, önerilen bu metodlarla hesaplanan haddeleme yükleri ile, ölçülen haddeleme yükleri arasında az yada çok daima bir sapma bulunmaktadır. Bunun nedeni; haddeleme yükünü etkileyen mukavemet, sürtünme katsayı¬ sı, ezme miktarı ve haddeleme hızı gibi faktörlerin karmaşık oluşu ve aldıkları değerin tam olarak belirlenmesindeki zorluğun yanısıra, haddeleme yükü hesaplanmasın¬ da önemli bir yere sahip olan ezme bölgesi geometrisinin, haddelemede kullanılan hadde tezgahının özelliklerine göre değişmesidir. Yapılan bu çalışmada, yassı çelik mamullerden, şerit haddelenmesinde, haddeleme yüküne etki eden faktörler incelenmiş ve üç ayrı metodla (Ride, Ekelund, Gele- ji) hesaplanan haddeleme yüklerinin, ölçülen fiili yüklerle karşılaştırılması yapılmıştır. Çalışmada Fe-33, Fe-37 ve Fe-44 kalite çeliklerin dörtlü tersinir hadde tezgahında haddelenmesinde alınan veriler kullanılmıştır. Haddeleme sırasında alınan veriler kullanılarak her metod için yazılan bilgisayar programları yardımı ile, haddeleme yükü, deformasyon direnci, akma gerilme¬ si, deformasyon hızı, sürtünme katsayısı, sürtünme kuvveti, ezme bölgesi geometrisi ve bunlara etki eden faktörler incelenerek bu değerler hesaplatılmış ve haddeleme yükünün hesaplanmasında kullanılmıştır. Hesaplanan haddeleme yükleri ile ölçülen haddeleme yükleri, çizilen grafiklerde karşılaştırılarak incelenmiştir.Yazılan bilgisayar programları yardımı ile, teorik yüklerle fiili yükler arasındaki korelasyon ilişkisi, en küçük kareler metodu ile hesaplatılarak her metodun ölçülen yükle olan bağıntısı elde edilmiştir. Sıcaklık, deformasyon hızı,geometrik faktör ve karbon miktarının deformasyon direnci üzerindeki etkileri grafikler halinde incelenmiştir. Ayrıca,sıcak haddelem de etkili olan faktörlerin oda sıcaklığındaki mekanik özelliklere etkisi,çekme deneyleri ile incelenmiştir. Metodlara göre hesaplanan yükler ile fiili yüklerin karşılaştırılması sonunda belirlenen en uygun metod referans alınarak, bir sıcak haddeleme paso dizayn programı yazılmış ve ERDEMÎR tesislerinde pratik uygulamaya sunulmuştur.

in the present work, three different load calcula- tion methods for the höt rolling of flat products are investigated. On the other hand, a computer program has been developed by the method that gave the best results compared the measured values for using in ERDE- MÎR höt rolling work station. The factors used in these methods that affect rol¬ ling load are studied. These factors are the deformati- on zone geometry, external friction in the deformation zone and the factors affecting strength of the material. Standard terms used in theories of flat rolling are; average workpiece thickness, draft, relative reduction, roll bite angle and roll contact length. Many factors affect rolling load, öne of them is aspect ratio of the rolling deformation zone which is described in öne of the following three terms; a) Aritmetic average aspect ratio (Za). b) Parabolic average aspect ratio (Zp). c) Geometric mean aspect ratio (Zg). A number of solutions have been proposed for type and magnitude of coefficient of friction and distributi- on of frictional force in the roll bite of höt mili. Although distribution of frictional force is different at the roll-strip interfaces according to different the¬ ories, there are two types of friction. These are slip- ping and sticking friction. Friction between the rolls and the vrorkpiece is necessary to transmit deformation energy from the rolls to the strip. Excessive friction tends to restrain the deformation and results in undesirably high rolling for- ce and spindle torgues. On the other hand, too little friction results İn either roll slippage ör the failure of workpiece to enter the roll bite. The value of friction coefficient depends on tempe- rature, scala conditions of workpiece surface, type of roll, surface condition and state of lubricant. The va- lues of the effective coefficient of friction in the rol! bite of a höt mili changes from 0.2 to 0.5 in the roll bite. Ih order to compute the rolling force in a particu- lar stand of a höt strip mili, it is necessary to know the flow stress at the temperature and strain rate asso- ciated with the deformation at that stand. A number of expressions have been developed empirically relating the flow stress to the temperature and the average strain rate för steel. Ekelund derived an eguation for yield stress of the rolled material corresponding to a given temperature and chemical composition. Geleji proposed an eguation to calculate yield stress of the rolled material based on rolling temperature. The most important factor affecting flow stress ör yield stress of rolled material is temperature. The tem¬ perature distribution within a coil at any instant du- ring the rolling process is determined by a number of factors which may be classified into two groups; those that impart heat to the work piece and those that cool it. The piece may acguire heat by; 1) Its deformation 2) Frictional effects in the roll bites 3) Oxidation ör scaling of the vrorkpiece surfaces 4) Physical and metallurgical changes according in in the piece 5) Heating such as may be introduced into heat-shi- elds located on crop shear tables. Heat may be lost by the piece by; sr 1) Direct condition of the work rolls and table rolls 2) Radiation 3) Air cooling 4) Heat conduction within the piece. The rate of deformation of an element of a workpie- ce as it passes through a roll bite decreases as the elements moves from the entry to the exit end of the bite. For computational purposes, however, it is desi rable, for reasons of mathematical simplicity, to use an effective average value of the strain rate. Four solutions are proposed for mean strain rate. These are as follows; 1) Ford and Alexander's solution 2 ) Sims ' solution 3) Orovan and Pascoe's solution 4) Wusatowski's solution. Although a number of equation were derived to cal culate strain rate, they were expressed as a function of the roll speed and the reduction divided by the length of the contact area. Many mathematical methods have been proposed to calculate rolling load for hot rolling of steel. Three of these methods are studied and used in this study for rolling load calculation of steel flat products. These methods are as follows; 1) Ride's method 2) Ekelund's method 3) Geleji's method. It is important to know that, Ekelund's method is valid for the following conditions: Minimum rolling spe ed is 7 m/s and maximum manganese content is 1%. Another restriction about yield stress used in Ge leji's method is that yield stress value is valid for ?sr carbon steels of a tensile strength up to 60 kg/mm2 and in a temperature range of 800 to 1300 °C. Ride has utilized a statistical analysis of the rolling mill data in order to derive an empirical formu la for the calculation the roll seperating force. In this formula, each of the parameters affecting the roll seperating force was included as a parabolic func tion. These parameters are roll roll peripheral speed, strip temperature, per cent reduction and roll contact length. The resulting eguation is obtained from a cor relation by the method of least squares. Data used in this work were collected from the 1676 mm reversible hot strip mill in Ereğli Iron and Steel Works consists of a 4-high powered by two 2500 hp drive motors. Tests with different rolling conditions on plain carbon steels (Fe-33, Fe-37, Fe-44) were performed and data is evaluated by a computer programm in order to find adequate rolling load calculation method or methods for hot flat rolling of steel strip which gives satis factory results when compared with measured loads. The following rolling parameters were measured; 1) Incoming temperature and thickness of bar for each pass 2) Rolling load 3) Outgoing thickness for each pass 4) Strip width after last pass. Data collected from hot strip mill were stored in a computer (Appendix-1). Rollingload for each of the three methods used in this work was calculated by computer programs and compa red with measured load. Strain rate and deformation resistance were calcu lated according to the equations given in each of these methods. At the same time, the deformation resistance are also calculated by using measured load and compared with calculated deformation resistance values of the methods. 3E Cooperative curves drawn for measured and calcula ted deformation resistance snowed that there is always some deviation between calculated and measured rolling loads for each of these methods because of the difficul ties to determine accurately the factors affecting roll- ling load such as flow stress, draft, coefficient of friction and geometry of deformation zone in the hot rolling of steel flat products. The results of this study showed that Geleji's met hod gave the best result for calculation of rolling load when its calculated values compared to measured load va lues. Deformation resistance that measured and calculated by the methods decreased with increasing rolling tempe rature- In low temperatures, Geleji's method gave the best results compared to measured values, but in high temperatures, Ride's and Ekelund's methods gave the best results. Flow stress which depends on temperature and chemi cal composition in Ekelund's method gave higher values than flow stress which depends on temperature in Celeji' s method. Deformation resistance that measured and calculated by methods increased with increasing deformation rate at constant temperature. Geleji's method gave the best re sults compared to measured values. Deformation resis tance calculated by Ride's and Ekelund's methods are mo re affected from deformation rate than Geleji's. The deformation resistance of Fe-33 steel increased with increasing geometric factor and deformation speed. Âs a result of this study, it is also obtained that the rolling parameters which are rolling temperature, rolling speed and draft in the studied range did not clearly affect the room temperature yield strength of the material.