Research On Process And Structure Of Car Body Ultra High Strength Steel Hot-stamped Part And Hot-stamping Die Based On Reliability Optimization Method

The rapid development of the automobile industry has made lots of issues increasingly serious including environmental pollution,energy crisis and traffic safety.The realization of lightweight design for automobiles is proved to be one of the effective ways to solve the above problems.In order to satisfy the collision standards during the lightweight design for automobiles,new materials and processes are increasingly used in car body design.The use of advanced high-strength steel in car body can reduce the part weight efficiently without damaging the crashing behavior.As a typical lightweight material,high-strength steel has been widely used in key parts of the body,such as structural parts reinforcements.However,as the tensile strength of high-strength steel increases,the plasticity is greatly reduced.There would be many defects such as cracking,wrinkling and springback during the cold stamping process,which limits popularization of high-strength steel in vehicle body.The hot-stamping technology effectively combines the stamping process with the heat treatment process,and fully utilizes the plastic strengthening characteristics of the material under high temperature to solve the problem of high strength steel's poor formability at room temperature.The material after quenching procedure of the hot-stamping process has exhibited high strength and plasticity,which greatly improves the structural strength of the vehicle body and effectively reduces the part mass.It is noted that the hot-stamping technology has a good light weight prospect.However,there are still some blind spots in the current hot-stamped part design and performance analysis,cooling system of hot-stamping and die service performance research area,which affect the development and popularity of hot-stamped parts.Therefore,the researches on the structure,process parameters,equipment development and service performance of the hot-stamped parts have important theoretical significance and practical application value.To explore the key properties of hot-stamped part for guiding the development of parts efficiently,the "structure-process-performance" coupled analysis method is first introduced for the structure and process design of hot-formed parts considering hot stamping effect.Subsequently,the cooling system of hot-stamping die is designed and optimized to improve the design efficiency which takes the die structural strength,cooling capacity and cooling uniformity into consideration.After then,the service performance evaluation method of hot-stamping die considering the wear amount,structural strength and cooling performance was proposed,and the influence of key process parameters on the service performance was analyzed.The specific research contents of this paper are listed as follows:(1)Research on the structure-process-performance coupling simulation analysis method of hot-stamped parts.Firstly,the mapping method of hot-stamping effects to crashing model is proposed.Based on the Voce model,the hot-stamping material property after quenching is precisely defined.With the above procedures,the coupling simulation method with hot-stamping effects is put forward.On this basis,a multi-objective reliability-based design and optimization method is developed based on meta-model technology,Monte Carlo method and genetic algorithm.Subsequently,the typical hot-stamped parts were chosen for research including sill and front door beam.The design and optimization is conducted including the part structure,process and performance.The optimization results show that the deterministic optimization design improves the crashing behavior of typical hot-stamped parts while the part's formability is relatively low,resulting in the lower reliability.On the contrary,the part obtained by the reliability optimization design sacrifices some of the crashing performance,but exhibits better reliability and robustness,which can be more effective on the practical project development.(2)Research on design method of hot-stamped tailored rolled blank(TRB)parts considering hot-stamping effects.To further explore the application prospect of hot-stamping technology in lightweight design of car body,the hot-stamped TRB part is studied to improve the design efficiency and reduce the risk of failure.The "process-structure-performance" design and optimization method is conducted to explore the influence of the key process parameters and thickness distribution on the crashworthiness and formability of the hot-stamped part.Based on the multi-objective deterministic and reliability design optimization method,the eff ects of various important parameters of the hot-stamped parts on the performance are studied.Subsequently,the hot-stamped TRB B-pillar is selected for design and optimization.The crashworthiness indexes of the parts are obtained through the three-point bending test and the partial impact test.The optimization results show that the reliability design solution has better formability without reducing the crashworthiness.Meanwhile,the reliability design exhibits better robustness and applicability.(3)Research on high-efficiency design method for hot-stamping die.To improve the design efficiency of hot-stamping die and reduce the risk of design failure,a design method is proposed fully considers the structural strength,cooling capacity and cooling uniformity.Firstly,the key design parameters of the cooling channel of the hot-stamping die are extracted,and the design space of the parameters is preliminary confirmed by the energy balance principle.Subsequently,the evaluation index of the key performance of the hot stamping die was proposed to achieve an accurate evaluation of the hot stamping die's performance.On this basis,a method integrated particle swarm optimization algorithm and Monte Carlo simulation method is put forward to achieve the optimal design solution.The research results show that the reliability design scheme greatly improves the cooling performance and cooling uniformity without reducing the die structural strength.The parts produced by the hot-stamping die have better comprehensive mechanical properties which show good guidance during project development.(4)Research on service performance of hot-stamping die considering process parameters.To explore the service performance of hot-stamping die and study the influence of various key process parameters on the overall performance,a novel method is proposed using the multiobjective reliability-based optimization method.This method solves the contradiction between hot-stamping die wear,cooling capacity and structural strength.Firstly,a numerical simulation method is carried out based on the Archard method,die service life prediction method and hot stamping process finite element model for the whole procedure.On this basis,the multi-objective deterministic and reliability optimization design method is used to obtain the optimal design scheme.This method is verified by the practical project development.The research results show that the proposed scheme effectively reduces the wear of the hot-stamping die,improves the structural strength and cooling capacity,which showed good engineering applicability.