| At present,China’s automotive industry has entered the fast lane of development.Energy consumption and environmental problems have received great attention from all walks of life.People become increasingly concerned about the energy efficiency,environmental protection and safety of cars.In this context,the automotive industry has conducted a lot of research on the lightweight design of body materials.High-strength steels ensure the stability and safety of vehicles while reducing body weight,which are a key focus of research into vehicle lightweighting today.However,as the strength level of high-strength steel increases,it also faces the problem of poor forming performance at room temperature and high springback,which seriously restricts its wide application in the automotive industry.It is worth mentioning that the warm forming process is effective in improving the forming properties and reducing the springback of highstrength steels.This paper takes DP980 high-strength steel sheet as the research object.The deformation modes of DP980 steel sheet with different center-hole diameters were analyzed.The effect of temperature on the forming process and deformation mode of the sheet with center-hole was investigated.And the split-ring springback of formed parts under different deformation modes was also investigated.The main research and conclusions of this paper are as follows.(1)The stress-strain data of DP980 steel sheets at different temperatures(25～500°C)and material orientations(0°,45° and 90° from the sheet rolling direction)were obtained by uniaxial tensile experiments,and a BP neural network constitutive model was developed to describe the warm deformation behaviour of DP980 steel sheets.The results show that DP980 steel sheet not only has negative temperature sensitivity,but also shows obvious anisotropy with the increase of temperature.The constructed BP neural network model has good prediction ability,and the correlation coefficients of the predicted and experimental values of both training and test data are above 0.999,and the average relative errors are also within 1%,indicating that the model can accurately predict the flow stresses of DP980 steel sheets at different temperatures and material orientations.(2)A finite element model for drawing-flanging of DP980 steel sheet with centerhole was established,and the deformation mode of the sheet under different center-hole diameters was determined by a combination of theoretical analysis,numerical simulation and experiment.The results show that the deformation mode is pure deepdrawing forming when the hole diameter is 0～10mm,the deformation mode is compound forming with deep drawing as the main deformation when the hole diameter is 15～20mm,the deformation mode is compound forming with hole flanging as the main deformation when the hole diameter is 25～35mm,and the deformation mode is pure hole flanging forming when the hole diameter is 40 mm.(3)The effect of initial sheet temperature on sheet forming quality and properties was analyzed under different deformation modes,and the split-ring springback of deformation modes dominated by deep drawing and hole flanging were analyzed.The results show that under the same deformation mode,the forming load,equivalent stress,thinning rate and split-ring springback of DP980 steel sheet are reduced with the increase of the initial temperature of the sheet,which indicates that warm forming can improve the forming performance of DP980 steel sheet.In addition,when the temperature-dependent Mises yield criterion and Hill48 yield criterion were used to predict the deformation modes dominated by deep drawing and hole flanging split-ring springback,it was found that the springback predicted by the Mises yield criterion coupled with the isotropic hardening model was closer to the experimental value. |
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