that some form of shape correction will be necessary within the process route. Shape defects introduced at the cold Unacceptable shape defects in light gauge strip mill can be classified into two main types, the first being a products may be introduced as a result of differential surface area differential defect such as coilset or crossbow. reduction being applied across the width of the Second, edge to edge (crosswidth) length differentials are material at rolling mills. Consumer specifications generally associated with edge wave, centre buckle, camber, and quality procedures demand that the resulting and twist. defects are removed by subsequent processingThe means by which these defects are removed are operations. Such an operation is tension levelling, predominantly pure stretch levellers, stretch-bend levellers which attempts to remove the resulting defects (i.e. tension levellers), and multiroll levellers (roller levellers). such as edge waves by ensuring that all Pure stretch levelling processes require considerably high longitudinal material 'fibres' possess the same bridle drum tensions to flatten the incoming material. In length. Shape removal is achieved by subjecting this case, the tensions employed induce stresses in the material the material to a series of alternate bends under higher than the yield stress. Tension levellers demand somesuperimposed tension. Consequently, imbalanced what smaller line tensions of ~10-40% of the initial yield residual stress fields may be present in the as stress. Multiroll levellers need use only enough applied levelled strip, resulting in bowed material in a tension to pull the strip through the machine. In all cases, further downstream slitting operation. Much work selective elongation of the shortest material fibres ensures has been published on the mechanics of the that strain equalisation of all fibres takes place across the process, and many advances have been made, width, thus removing the initial shape defects. The magniespecially in the design of levelling systems to tude of tension required in each of these processes, to improve shape removal and reduce internal stress remove the incoming defect, is dependent upon the number levels. However, residual stresses and specifically and diameter of bending rolls (workstations) employed. In the imbalance of these stresses post-levelling is an pure stretch levelling no specific bending rolls are used. issue that has failed to be addressed in its entirety.High tensions, therefore, must be generated as a result of To investigate the generation and effects of residual considerable velocity differentials at large diameter bridle stresses owing to tension levelling, a finite element drums. Consequently, this can lead to plastic instabilities (FE) model has been developed with validation in the process and prevalent 'Lu ¨dering', or differential yielding, carried out on an experimental laboratory leveller.in the case of annealed strip. At progressively higher strain The analysis is performed in real time using the levels (1...
Tension levelling is a process used in the steel industry to remove any shape defects (i.e. out of flatness) present in coiled material. It is generally the final process before the cold rolled product is dispatched to the customer, and is therefore a process which plays an important role in delivering the desired material properties and the product standards required by the customer. In this paper a designed factorial analysis has been employed to study the effect of tension levelling process parameters on the shape characteristics of the levelled product. These characteristics include residual flatness (longitudinal curvature, referred to as longbow), centreline elongation and residual (or internal) stress imbalances; criteria that determine whether the customer accepts the material or not. It has been identified that in a basic five roll leveller the final (adjustable) roll wrap angle has the most significant influence on these characteristics in over 70% of cases. It has also been recognised that final flatness is dictated by many second order process interactions.
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