The Effect Of Holes On The Springback In Cold Roll Forming Of UHSS Car Threshold

With the development of automotive lightweight,ultra-high strength steel(UHSS)pre-notched sections are getting growing popularity in the automotive industry.A large number of studies on springback has been done by domestic and foreign scholars,and it is mainly discussed that the effects of cold roll forming process,process parameters,material and material model on the springback of metal strip without holes.A large number of auto parts need to be prefabricated before cold roll forming for the assembly requirements of parts,that is,machining pre-punching holes.However,the springback problem of metal strip with pre-punching holes in cold roll forming needs to be further researched.The springback of UHSS metal strip is larger in cold roll forming process,and the existence of holes also has a certain effect on the springback.Therefore,in order to improve the forming accuracy of automobile threshold parts,the effect of holes on the springback of UHSS in cold roll forming process is discussed by simulation and experiment.The main research contents of this thesis include:(1)The real stress and strain data of martensitic 1300(MS1300)UHSS are obtained through uniaxial tensile and cyclic loading tensile experiments.Then,the data fitting is carried out by using Matlab software,and it is obtained that the change curve and mathematical model between elastic modulus and plastic strain.(2)The cold roll forming scheme of the car threshold with holes is designed with the same forming conditions as the experiment,and the finite element model of this scheme is established.Then,the accuracy of the finite element model is verified through the continuous cold roll forming experiment of the car threshold with holes.The result shows that the error of the final forming angles between the finite element model and the experiment is less than 5%,so the accuracy of the finite element model meets the requirements.(3)The mathematical model of variable elastic modulus is applied to the finite element model,and then the springback results using constant elastic modulus and variable elastic modulus are compared.The result shows that the variable elastic modulus method can improve the prediction accuracy of springback by about 15% in the hole area of UHSS threshold parts.(4)Several forming schemes are designed to research the effect of different features on the springback in the hole region.In each scheme,one of the parameters is changed,including the shape of holes,hole position,hole diameter,distance between holes,number of stands,plate thickness and distance between stands.The results show that the existence of holes reduces the springback of trip and the effect is different at different positions of the car threshold.The influence of different shapes of holes on springback is also different.The springback in the hole region decreases with the increase of the number of stands,the strip thickness and the hole diameter,and with the decrease of the distance between stands and the distance between holes.