Study On The Design Method And Application Of Autobody Structure Based On High Strength Steel

Facts showed that high-strength steel (HSS) has become quite competitive automotive lightweight materials, which has greater advantages on anti-collision performance, processing technology and material cost than other materials. As the mechanical properties of HSS are quite different from ordinary steel, which leading to poor forming performance of autobody panels; on the other hand, large rebound of HSS is a major problem when applied on autobody that hasn’t been solved yet, and dimensional accuracy is difficult to control, which seriously affects the assembly precision of automotive products. In addition, how should a reasonable distribution of various levels of HSS on autobody, which parts can be replaced with HSS, and how to match HSS between different components, that need to be replaced selectively in the premise of ensuring the overall performance of autobody unaffected, in order to take the advantage of the proper material selected for the right part based on meeting the original performance and reducing the autobody weight.(1) With HSS tailor-welded cylindrical pieces as an example, the influence of material matching law on formability including material flow, weld-line movement and strain path changes was divided into three cases: 1) different thickness with the same material, 2) the same thickness with different materials and 3) different thickness with different materials, which provides the basis for material selection on autobody tailor-welded blank (TWB). While with the 1st case considered, the influence of blank holder force (BHF) distribution on thick/thin sides on its forming limit, weld-line movement and strain path at the cracking point was studied, which provides the basis for BHF control on TWB forming.(2) A step-BHF based on rupture criterion rather than wrinkle criterion was presented to increase effective plastic strain region and reduce the amount of springback with no rupture appeared; with TWB-front rail inner panel as an example, orthogonal experimental method was used to determine the influence of various BHF parameters on the amount of springback, the optimization results showed that the control effect is very significant for HSS TWB-sidewall and flange springback angle.(3) As the CAE simulation of crashworthiness has reached a mature stage relatively, the simulation accuracy to continue to improve subjected to many constraints. On the basis of precision comparison of one-step forming method and incremental method, material property changes such as thinning and plastic strain hardening were included into crash simulation analysis by incremental forming method and grid mapping technology. The influence of forming effects on structure deformation, energy absorption rate and acceleration, etc of a single cap-shaped pieces and the whole vehicle was studied, the simulation results considering forming effects were more consistent with experimental results, the changes of material properties during forming process should be considered in accurate crash simulation.(4) The ideal acceleration-displacement curve under deformation control of crash simulation was proposed. With a certain own-brand SUV front rail as an example, a simplified model of front rail on crashworthiness problem was established; the weld-line location of TWB front rail was optimized to obtain a reasonable crushing order and absorb more impact energy; two design cases and mathematical optimization models of lightest and optimal energy absorption were proposed, experimental design, approximation model and adaptive response surface method were combined to obtain the optimal TWB lightweight solution while satisfying the crashworthiness requirements.(5) With the accuracy of the CAE model verified through modal experiments of body in white and real 100% frontal crash test, a simplified model of the frontal energy-absorbing structures was established with efficiency and accuracy both taken into account, with the material deformation path and strain rate effect considered, the Pareto optimal solution set was obtained through multi-objective genetic algorithm base on lightweight and crashworthiness optimization. As the optimized thickness and material distribution more reasonable, the 100% frontal crash safety was greatly improved, thus verified the feasibility of multi-objective optimization of the lightweight and crashworthiness issue based on multi-objective genetic algorithm.This study involved the influence of material matching on forming, springback and structural crashworthiness; the purpose is to provide material selection method for structural lightweight, and promote the application of HSS on own-brand autobody.