| Summary: | 碩士 === 國立中正大學 === 機械工程學系暨研究所 === 99 === Hot rolling is a multi-pass plastic deformation prosess. During hot rolling process, the mechanical properties and microstructures of materials are strongly influenced by the hot-rolling parameters such as temperature, strain and strain rate. Recrystallization and grain growth of the hot rolled material thus highly depend on the process parameters . The effect at each pass on the microstructure evolution would be accummulated . The purpose of this study was to predict the grain size and mechanical properties of the slab after hot rolling. The C-Mn steel was used in this study. The effect of the initial grain size and strain condition at each pass on the microstructure evolution due to accumulated effect of the rolled steel slab was investigated. Based on Yanagimoto’s incremental analysis and Gleeble upset compression test results, we derived a mathematical model of recrystallization and microstructure evolution for C-Mn steel to predict the flow curves.
The hot rolling can be divided into a roll-bite and a inter-pass period between rolling passes. From the result of a single hot compression test, the equations for recrystallization have been proposed to predicted the flow curve for a single pass hor rolling of C-Mn steel. The predicted results are in good agreement with the experimental ones .
In order to study the accumulated effect on the microstructure evolution at each pass of hot rolling, the static recrystallization and metadynamic recrystallization behavious in C-Mn steel were investigated by two-step compression test. The effects of rolling temperature, strain, and interpass duration on the microstructure evolution during interpass period in hot rolling of C-Mn steel were expressed in terms of softening fraction. In this study, the mean flow stress was used to deternine softening fraction due to metadynamic and static recrystallization. The softening fraction increases with increasing deformation temperature, interpass duration and residual strain of the previous hot compression. However, the effect of deformation degree of first pass hot compression on the softening behavious is not marked. Then we combined the obtained recrystallization equations in this work with the metadynamic and static recrystallization equation proposed by Yada to establish a mathematical model to predict C-Mn recrystallization and the flow curve of two-step hot compression under multi-pass hot rolling. The predicted two-step rolling flow curves did not agree well with experimental flow curves. The mathematical model of multi-pass hot rolling still needs to be modified to predict the accumulated effect on the microstructure at each pass of hot rolling. The mathematical model for multi-pass hot rolling obtained in this work could predict the evolution of grain size and the results agree well with those grain size obtain from hot compression test on C-Mn steel.
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