Internal stresses or residual stresses remain in almost every part after manufacturing and/or further processing. Even if the entire stress state inside a system is in an equilibrium, single stresses due to their direction and strength may have positive or negative influences to the properties of a body. Especially in big parts, the residual stress state is relatively unknown, because it can only be determined by destructive methods as sectioning or slitting. The possibility of the use of non destructive measuring methods is only given for surface near regions or thin parts and not useful for the specification of the entire residual stress state inside a large compound work roll. This paper outlines an approach for the determination of residual stresses in centrifugal casted work rolls with an indefinite chill double poured or high speed steel shell. In several steps, different measurement techniques are tested and the results are to be presented. Beside the residual stress state, which is caused by manufacturing or heat treatment, these work rolls with different shell and core material differ in their thermophysical and elastic properties. These parameters in combination with the residual stress state and the thermal and load stresses, which arise during the hot rolling process, are causing a complex stress field that is presented by a combined model for work and backup rolls in operation.
The present work aims at the modelling and simulation of the hot rolling process for wire rod and bars. After the fundamentals of plasticity, which are essential for the understanding of the process characteristics have been described, typical section deviations that can be expected in wire rod and bar mills are calculated with help of a numerical simulation model. The model allows the calculation of section shapes under the influence of elastic rolling stand deformations and interstand tensions. From this computational assessment of section faults, the necessity of inline measurement and process control for wire rod and bar mills is shown. This work is part of the PIREF project which incorporates the development of sensors, control systems and process models in order to control the dimensional accuracy of hot rolled wire rod and bars. The metal forming process model, as described here is used internally as a model for the static and kinematic interactions in the rolling process inside of the control model.
In the joint project PIREF, the metal forming group of the University of Duisburg-Essen has collaborated with the University of Applied Sciences Ruhr-West Mülheim (Ruhr), the University of Siegen, EMG Automation GmbH and SMS group GmbH to develop sensors, for an online measurement of material velocity and cross section as well as control models for the rolling process of wire rod and bars. The University of Duisburg-Essen provided a metal forming process model for the rolling process to assess the influencing parameters on the rolled section precision. A technique was found to segregate height- from width- influencing parameters from a measured cross-sectional area and actual roll gap. With this measuring technology and with help of the process model, rules for control of the rolling process to achieve close tolerances were obtained. The modelling was accompanied by rolling trials on a laboratory rolling mill at the University of Duisburg-Essen, where a typical Round-OvalRound pass sequence was used for validation of the rolling model concerning lateral spread, inlet and outlet velocity as well as rolling force and torque calculation. The present paper shows how the material flow and the distribution of the velocity in the roll gap can be described. In subsequent rolling of bar and rod in a continuous rolling mill the dimensions can be influenced by application of longitudinal stresses and screwdown. The application of stress can be achieved by an inter-stand velocity mismatch. With the developed models the necessary velocity mismatch can be calculated.
Roll-drawing of full sections is an alternate process for wire drawing through a closed die, as it conserves less hard tool materials. It therefore enhances the resource efficiency of thin wire production. As the tools are rotating, the relative velocity is lower and friction is reduced. Apart from this, there is only little knowledge about the process mechanics inside the deformation region. According to the relative motion mentioned above, a neutral point should exist within the deformation region, but this hypothesis is unconfirmed until today. In the present work, a combination of an improved empirical model for the lateral spread in roll-drawing and a mechanical model for roll drawing based on the slab method is proposed. The mechanical model deviates from the well-known rolling theory by the fact that a zero deformation torque should exist on the rolls, it predicts the neutral angle, the forward-slip, the external drawing force and the roll separating force. The hypothesis of existence of a neutral point in the deformation region is supported by experimental measurements of the nonzero forward slip in the roll-drawing process. Measurement is done on copper wire, where 200 passes with initial round section sizes ranging from 2 mm to 1.3 mm with an increment of 0.1 mm are evaluated.
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