Study On Dynamic Recrystallization Behavior And Finite Element Simulation Of Zirconium Alloy Sheet During Hot Deformation

Zirconium alloys are widely used in the nuclear industry due to their excellent nuclear properties,corrosion resistance and mechanical properties,such as positioning grids and cladding tubes in fuel elements.In this paper,Gleeble-3800 thermal simulation testing machine and four-high reversing hot rolling mill were used to conduct hot compression experiments and multi-pass hot rolling experiments on Zirlo zirconium alloy,respectively.At the same time,the microstructure was characterized by Zeiss-Axiocam MRC5 metallographic microscope and EBSD technology.The microstructure evolution law,deformation mechanism and dynamic recrystallization（DRX）mechanism of quenched Zirlo zirconium alloy under the experimental conditions of deformation temperature of 500～700°C,strain rate of 0.01 s^-1 and maximum deformation of 70%were studied.In addition,the finite element simulation of the multi-pass hot rolling process of the quenched Zirlo zirconium alloy at a rolling speed of 30-60 m·min^-1 and an initial temperature of 630°C was also studied.The research results can be used to guide the microstructure and texture control of Zirlo zirconium alloy and the optimization of hot rolling process parameters.Firstly,the DRX behavior of Zirlo zirconium alloy during hot deformation was analyzed.On the basis of considering the thermal effect of deformation,the true stress-true strain curve is corrected for temperature,and the peak stress model is established on the basis of Arrhenius equation.At the same time,the model was verified,and it was found that the predicted value was basically within the 10%error bar and the correlation coefficient reached 0.99432.The microstructure and texture of Zirlo zirconium alloy as quenched and compressed were characterized by EBSD technique.The main deformation mechanisms of Zirlo zirconium alloy were determined by the in-grain misorientation axes method（IGMA）and Schmidt factor（SF）with critical shear stress（CRSS）method.In addition,the main DRX mechanism of Zirlo zirconium alloy was identified by selecting characteristic grains.The results show that the texture evolves significantly with the increase of the deformation temperature.The DRX mechanism is dominated by continuous dynamic recrystallization（CDRX）at 500 and 650°C,while it is dominated by discontinuous dynamic recrystallization（DDRX）at 700°C.Secondly,the DRX model of Zirlo zirconium alloy during hot deformation is established.Using the corrected true stress-true strain curve,a critical condition model of DRX was constructed based on the Sellars model,including the peak strain model and the critical strain model.The DRX kinetic model was constructed based on the Yada model.At the same time,the predicted and experimental values of DRX volume fraction were compared.In addition,a DRX grain size model was established.The results showed that the predicted curve of DRX volume fraction was a typical“S”-shaped feature.The predicted value of DRX volume fraction was basically consistent with the experimental value,and the maximum error did not exceed 7%.Finally,the macroscopic deformation and microstructure evolution of Zirlo zirconium alloy during multi-pass hot rolling are simulated.The microstructure and texture of Zirlo zirconium alloy after hot rolling were characterized by EBSD technique.A multi-pass hot rolling finite element model is established,and the microstructure evolution model is imported into Deform software as a material model,which realizes the macroscopic simulation of the multi-pass hot rolling process and the prediction of the average statistical characteristics of the microstructure.The ten-pass hot rolling process of Zirlo zirconium alloy sheet from 104 mm to 19 mm thickness was simulated by finite element at the rolling speed of 30～60 m·min^-1 and the initial temperature of630°C,and the results of hot rolling experiment were carried out comparison.The results show that the simulation results are basically consistent with the experimental results.With the multi-pass hot rolling,the average grain size decreased from 8.5μm to 1.17μm,while the volume fraction of DRX increased from 0 to 45.3%.As the rolling speed increases,the surface temperature and core temperature of the rolled sheet increase,while the rolling force of the rolls and the final thickness of the rolled sheet decrease.