Research On Time-dependent Springback Behavior Of High Elastic And High Strength Bar Roller Straightening

Titanium alloys and other high-elastic and high-strength metal materials are widely used in high-tech fields such as petroleum,chemical industry,aviation,aerospace and so on because of their excellent mechanical properties.As a process of bar finishing,the straightening of two cross-rolls straightening is the key technology to ensure the straightness of bar after straightening.There are two problems in bar straightening in practical production.The first is that the shape of bar is unstable after forming due to the large residual stress and uneven distribution after straightening,the subsequent processing process is difficult,and even the production cost caused by de-stress annealing is large.Second,the smaller the straightness that can be measured after straightening and springback,the greater the final Time-dependent springback is,and the quality of the finished product is not necessarily good.In this paper,the high strength metal 0cr18ni9 alloy is taken as the research object.Aiming at the phenomenon that the residual stress is too large and the distribution is not uniform after straightening,the Time-dependent springback of the bar after straightening is studied by means of theoretical analysis,finite element simulation and experimental method.Based on the elastic power strengthening material model,the relationship between the bending moment and the elastic zone ratio and the original bending curvature of the bar is deduced.Combined with the straightening and deformation mechanism of two cross-rolls straightening,the paper put forward to adopt equal curvature reverse bending roll shape instead of empirical roll shape designed by principle of pressure straightening in production process to solve the problem of large residual stress and uneven distribution of bar after straightening.Two-layer viscoplasticity model can well characterize the time-dependent viscoelastic behavior of metal deformation.In this paper,the parameters of 0cr18ni9 metal Two-layer viscoplasticity model are obtained by uniaxial tensile and stress relaxation experiments of0cr18ni9 alloy bar.The material parameters are provided for the finite element simulation of Time-dependent springback of 0cr18ni9 metal bar.By selecting the ratio of elastic zone 0.75,0.6,0.45,0.3,0.15 to these five values,themagnitude of residual stress after straightening and the residual stress of time-dependent springback after straightening and the straightness of the corresponding time of bar after seven days of straightening are selected respectively in the case of 0.15,0.3,0.45,0.6,0.75 for these five kinds of values,respectively.The linearity is simulated by finite element method,The simulation results show that the smaller the elastic zone ratio is,the better the straightness is measured immediately,but the larger the residual stress is,the greater the Time-dependent springback is,and the final straightness of the bar is not always up to the standard.The size and distribution of residual stress and the straightness of bar after seven days time-dependent springback were compared between the empirical roll shape and the equal curvature reverse bending roll shape.The results of experiment and simulation show that the residual stress of bar is obviously less than that of experiential roll after the correction of equal curvature reverse bending roll,and the distribution of residual stress of bar after correction of equal curvature reverse bending roll is relatively uniform.After seven days of delayed rebound,the straightness of bar after equal curvature reverse bending was less than that of the bar after experience roll straightening.Therefore,it is possible to solve the problem that the residual stress is too large and the distribution of the bar is not uniform in the actual production by means of equal curvature reverse bending roll shape,because of the excessive residual stress after straightening.