Research On Controlled Cooling Technology For Semi-Hot Forming Based On Microstructure Evolution

Semi-hot precision forming is the process whose forming temperature range is between warm forming and hot forming. The semi-hot forming has some advantages: lower the forming resistance, reduce the equipments and processes and make the products acquire good internal overall properties due to combining controlling precision forming and controlled cooling technology. In this thesis, the flow stress behavior, the austenite recrystallization behaviors, the flow stress model, the continuous cooling transformation and the phase transformation dynamics model during semi-hot forming for XC45 steel were investigated by means of thermo-simulated tests, tensile tests, metallographic analysis, theoretical inference, numerical simulation and so on. The effect of semi-hot forming processing parameters on the recrystallization behaviors, grain size and continuous cooling transformation were investigated. The constitutive model and a phase transformation dynamics model were established. The backward extrusion simulation and test were carried out. The experimental results were compared with FE simulation and calculated results. It was showed that the predicted volume fraction of ferrite were consistent with experimental results, which verified the stabilization and reliability of phase transformation dynamics model. The results on this study can be applied to provide scientific evidence to the selection of the forming and controlled cooling parameters of semi-hot forming.The results in this dissertation can be summarized as follows:(1) The flow stress behavior and the dynamic recrystallization(DRX) behaviors of XC45 steel during semi-hot deformation were investigated by means of single pass thermal-simulated compression tests. It was illustrated that the peak stress and strain increased with the decrease of deformation temperature and increase of strain rate, According to the creep equation, the activation energy and the flow stress model had been established.(2) According to the hot compression tests and controlled cooling tests by thermal-simulated test machine, the static and dynamic CCT diagrams were drew by dilatometric curves. The results show that volume fraction of ferrite rises with the decrease of cooling rate and deformation temperature and the increase of strain and strain rate based on the quantitative analysis of volume fraction of ferrite through metallographic and MATLAB programming. According to the JMAK model and Scheil’s additivity rule and the experimental result, a phase transformation dynamics model was established. The mean errors of the calculated results and experimental results are approximately 8.78%, which validates the accuracy of the phase transformation dynamics model.(3) According to the tensile tests and notched tensile tests, the yield strength and tensile strength fall observably with the increase of the volume fraction of ferrite after phase transformation, but the elongation and cold forge ability growth with the increase of the volume fraction of ferrite. The relationship between the yield strength, cold forge ability and the volume fraction of ferrite was established.(4) The backward extrusion experiments and simulation of XC45 steel were carried out to analyze the effect of deformation parameters on microstructure. The volume fractions of ferrite after phase transformation in different positions were obtained through by the phase transformation dynamics model. The mean errors of the calculated results and experimental results are approximately 15.76%, which verified the accuracy of the model built by this paper and provided scientific guidance for defining the process parameters of semi-hot deformation and controlled cooling technology.