Modeling the behavior of inclusions in plastic deformation of steels

Abstract: This doctoral thesis presents a modeling method fordemonstrating the behavior of inclusions and their surroundingmatrix during plastic deformation of steels.Inclusions are inescapable components of all steels. Moreknowledge about their behavior in processes such as rolling andforging is necessary for carrying out the forming processes ina more proper way so that the properties of the final productare improved. This work is focussed on deformation ofinclusions together with void formation at the inclusion-matrixinterface. The topic of the work is analyzed by differentFE-codes.The relative plasticity index is considered as an importantmeasure for describing the deformability of inclusions. Theindex could be analyzed quantitatively, enabling a deeperunderstanding of the deformation mechanisms. The workingtemperature is found to be an important process parameter. Thisis very clear when the deformation of silicate inclusions in alow-carbon steel is studied during hot rolling. Here a narrowtransition temperature region exists, meaning that theinclusion behaves as non-plastic at lower temperatures and asplastic at higher. The results are in agreement withexperiments published by other authors.Regarding void formation, the simulations have been carriedout by utilizing an interfacial debonding criterion. Thedifference in yield stress between the matrix and the inclusionis one common reason for void initiation and propagation.During large compressive deformation the evolution of voidsgoes through a sequence of shapes, from convex with two cuspsto concave with three cusps together with self-welding lines.It is concluded that the formation of voids is alwaysassociated with a large relative sliding between the inclusionand the matrix.In order to study the local behavior of the material closeto inclusions during hot rolling a mesomechanical approach isused. Uncoupled macro- and micro- models have been developed.By means of the macro-model, the stress-strain historythroughout each sub-volume of the steel is evaluated. Thestress components or velocity fields are recorded with respectto time as history data. No consideration is taken to theexistence of inclusions. The micro-model, which includes bothinclusion and steel matrix, utilizes the stress components orthe velocity fields from the macro-model as boundaryconditions.Keywords: Inclusion; Steel; Plastic deformation; Void;Rolling; Forging; Finite Element; Mesomechanical approach.