In this paper, the local distribution of austenite grain size (AGS) was experimentally determined by conducting single round-oval and square-diamond pass hot bar rolling experiments of AISI4135 steel. The rolling experiments were carried out using the laboratory mill. The local distribution of AGS was also determined numerically. In order to predict AGS distribution, the AGS evolution model was combined with three dimensional non-isothermal finite element analyses by adopting a modified additivity rule. AGS evolution model was experimentally determined from hot torsion test according to Hodgson's model. The predicted results were in a reasonably good agreement with experimental results.
In bar or rod rolling process, improvement of mechanical properties of the hot rolled products requires numerical prediction of austenite grain size (AGS) for better controlling the microstructural evolution. In this study, a fully three-dimensional finite element (FE) program, which can simulate three-dimensional deformation and heat transfer was integrated with an AGS evolution model available in the literature. It was applied to a four-pass round-oval-round rolling sequence to characterize the AGS distributions depending on the change of roll gap and rolling speed. The predicted AGS distribution obtained from the FE based approach was compared with that obtained from the approximate analytical approach based on elementary theory of plasticity, developed for practical purpose. It was found out that reducing roll gap and increasing the rolling speed leads to fine and uniform grain distribution and recrystallization behavior divided into meta-dynamic and static recrystallization region, respectively. In addition, AGS predicted from the approximate analytical approach was in agreement with that from the FE based approach, but showed discrepancies at higher rolling speed conditions investigated in the present work.KEY WORDS: austenite grain size; bar or rod rolling; finite element method; rolling speed; approximate analytical approach.parameters for each pass in an average sense were used as input values for the AGS evolution model proposed by Hodgson and Gibbs. 6) In this study various strategies to achieve grain refinement were discussed but only average value of the AGS was predicted.Compared to plate rolling, it is well known that deformation mechanics for shape rolling is rather complex in the roll gap because of the material flow in all three directions. To predict the microstructural change in shape rolling process by utilizing the AGS evolution model, the deformation of material during rolling must be accurately determined in couple with temperature and then the AGS model should be integrated with it. In this aspect, the FE method, which can provide the detailed information on material flow during rolling, has been widely employed. Several research groups applied the FE analysis to model the deformation in shape rolling and to predict the AGS distribution in rolled sections. Glowacki et al. 10) predicted the AGS distribution using two-dimensional FE simulations based on the generalized plane strain assumption in consideration of computational efficiency. The predicted AGS distribution for a square-oval rolling pass was compared with the experimental results.To make better prediction of the AGS distribution, more accurate information of the material flow and temperature distribution during rolling is required. With this in mind, in this study, a fully three-dimensional FE analysis, which can calculate the three-dimensional heat transfer, was conducted and the results were integrated with an AGS evolution model proposed by Hodgson and Gibbs. 6) The proposed approach was then applied to four-pass round-oval...
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