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ABSTRACTThe computer aided development of the levelling technology for flat hot rolled products is the main aimof this work. Accuracy of the numerical simulations is important when they are used to develop a realmanufacturing technology. The nonlinear combined hardening model was selected to capture theinfluence of the Bauschinger effect that plays an important role during low cyclic deformation that ischaracteristic for levelling. 50293
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Themodel identification procedure on the basis of cyclic tension/compressiontests combined with inverse analysis is described first. Application of the developed model to designefficient leveller setups is presented in the second part of the paper. 1. IntroductionThe research presented in this paper is part of the larger RFCSOptifin project that is aimed at an improved understanding ofinternal stress/strain fields, evolution of microstructure, materialproperties, flatness and squareness during hot rectification, andcooling of plates and sections. This work is focused on develop-ment of an accurate and realistic model for simulation of the hotand cold levelling processes. Computer aided technology design isan efficient tool to achieve required mechanical and dimensionalproperties, thereby reducing or eliminating the need for furthercold rectification. Numerical modelling of material behaviourduring thermomechanical processing is widely used in experi-mental research and in industrial applications [1–3]. However,accuracy of simulations depends, to a large extent, on thecorrectness of the description of material properties, as well asmechanical and thermal boundary conditions. All the majormechanisms that are responsible for material deformation haveto be taken into account in the material model that is used duringsimulations. A discussion of mechanisms that occur during largecyclic deformation is given in [4]. Hot and cold levelling operationsare examples of processes with small cyclic deformation, and inthis case the Bauschinger effect has to be considered. That is why amaterialmodel based on a nonlinear combined hardening law thattakes into account the mentioned phenomenon is used duringindustrial simulations. Another important issue is accurateidentification of parameters of the selected material model. Twoapproaches based on half cycle test data and stabilised cycle testdata are usually used during the identification procedure inindustrial conditions. In these cases, the identification procedure isfast, but the question of accuracy remains open.
That is the reasonwhy in this work a solution based on inverse analysis is proposed,and the obtained results are compared with the mentionedapproaches. Finally, the proposed hardening model is used duringdevelopment of the levelling operations that will reduce the needfor additional cold levelling.2. Identification of hardening model for cyclic deformationThe Bauschinger effect is observed in most metals and isrelated to the strain path change during lowcyclic deformation. Inthe simplest case, when material is subjected to the tension andthen to the compression mode, the plastic deformation in asubsequent stage occurs earlier than in the previous stage [5].Under cyclic deformation, material can harden or softendepending on the deformation conditions. Modelling of thematerial behaviour under these cyclic loading conditions requiresapplication of the proper hardening law, which describes themovement of the yield surface corresponding to the kinematichardening and the change in the size of the yield surfacecorresponding to the isotropic hardening [6]. As mentioned,accuracy of numerical simulations is very important when theyare used to develop a real manufacturing technology. Creation ofan appropriatematerialmodel plays in this case a crucial role andit is composed of a series of laboratory tests that are the basis foridentification of the model parameters.2.1. Cyclic deformation laboratory testsThe cyclic tension/compression tests were performed to studythe effect of the applied hardening rule on the accuracy of theprediction ofmaterial behaviour subjected to cyclic loading duringcold and hot levelling. Axisymmetrical low carbon steel specimenswith gauge diameters of 6 mm and 4 mm were used. Ten cycles ofthe tension/compression with amplitudes of 0.25 and 0.125 mm were carried out on the Zwick Z250 and Gleeble 3800testing machines at room and elevated temperatures with the strainrate of 0.1 s 1. Examples of specimens used during the cyclic T/C testsare shown in Fig. 1. Numerical models were developed using thecommercial Abaqus Standard code. *
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