Analysis And Optimization In Lightweight Of Autobody Structure Based On Hight Strength Hot Forming Steel

In order to satisfy the demand of vehicle lightweight design, the automobiledesign and manufacturing industry are keeping developing new materials andtechnologies. High-strength steel is receiving more and more attention, because of itsadvantage in strength, stiffness, impact resistance, recoverability and cost. In thispaper, light high-strength steel is taken as an object to carry out properties study andforming study on thermal problem; and an independent brand sedan body is taken as acarrier for lightweight structural analysis and optimization based on the MSOTmethod established.This paper studies the microstructure of high-strength hot forming steel by itssurface morphology and component spectrum, the core microstructure and componentspectrum. And we compare the microstructure and components before and afterheating. Based on the iron-carbon alloy phase diagram, this paper studies theheat-treatment process theory and thermal simulation on hypo-eutectoid hot formingsteel, and acquires the CCT curve of high-strength boronization steel. We test andanalyze the mechanical performance of hot forming boronization steel in differenttemperatures and shape-changing rates by means of cold-state tensile experiment andthermal simulation, and fit the stress-strain curve in normal temperature and afterquenching and the stress-strain curve in high temperature, with the help ofexponential constitutive equation and thermal state model respectively. We also studythe forming limit of hot forming boronization steel in600℃,700℃and800℃, andacquire forming limit diagram.According to the forming difficulty of high-strength structural parts, this paperstudies the numerical simulation and process parameters optimization among hotforming process. A hot stamping process is presented in this paper. And this paper alsoexplains the relationship among heat-treatment, phase transition and forming processin hot forming numerical simulation, and analyzes hot stamping finite deformationtheory, the relationship of thermodynamic parameters and solution algorithm. Besides,this paper establishes a hot stamping mechanical model and finite element analysismodel of a typical automotive body parts, and analyzes the parts expansion reverse,hot forming simulation solution and formability result.Using high-strength hot forming steel will make material composite more complex and strength grade range wider. It needs corresponding methods to solvethese two problems. In order to propose the performance objectives of lightweightautomotive body structure, this paper builds a strength and modal CAE model, andanalyzes and evaluates its bending stiffness, torsional stiffness and modalcharacteristics. This paper also builds an automotive body structure strength CAEmodel to study its structural strength under different limiting conditions. A whole carimpact CAE model is built to analyze and evaluate the40%ODB offset impact andside impact performance. After that, the mechanics characteristic distribution ofautomotive body structure high-strength steel is obtained, which can help to selecttarget parts for lightweight optimization.Steel plate used in vehicle body component will be thinner if using high-strengthhot forming steel. The whole body will be composed by different strength anddifferent thickness steel plate, so it’s very important to study how the thickness affectsstructural performance. Based on the basic theory of automotive body multi-objectiveoptimization and some related technology system about optimization algorithm, webuild approximate models and lightweight optimization mathematic models ondynamic and static performance, safety performance in frontal offset impact and sideimpact, and acquire the Pareto solution set of structural quality and torsional stiffness,structural quality and energy-absorbing, and structural quality and acceleration,respectively. Two projects are selected to carry on finite element model simulation.The result is compared with the simulation result before optimization, which showsthe optimization method works effectively.The comparisons between the test and simulation of BIW modal model and twocollision models show both the BIW model and whole car collision CAE modelsatisfy simulation precision demand. From different aspects, such as fatigue strength,safety performance in frontal impact and side impact, this paper compares thesimulation result of these two projects before and after optimization, which verifiesthe comprehensive effect of lightweight automotive body multi-objectiveoptimization.