| Hot stamping is a kind of high temperature forming technology based on water-cooled die quenching,which can help car achieve lightweight and crashworthiness simultaneously.The hot stamping parts with good shape accuracy and uniform mechanical property are conductive to the welding and the strength enhancing of car body.Whereas,the basic hot stamping process and the straight cooling channels die cannot fully meet the requirements of complex shape parts for forming quality and uniform performance.Therefore,in order to improve the performance of hot stamping parts,it is of great significance to improve the basic hot stamping process,optimize the design of conformal cooling channels in hot stamping die by considering the coupling heat transfer between temperature field and flow field,and manufacture the hot stamping die with conformal cooling channels suitable for industrial application.In this dissertation,the systematic experiment and simulation studies on the hot stamping process,the interfacial heat transfer between blank and die,the coupling heat transfer,the optimization design of conformal cooling channels in hot stamping die,and the die manufacture methods are carried out.The research work mainly covers the following aspects:(1)The influences of blank thickness,initial forming temperature,dwell pressure and dwell time on the hardness and springback of U-shaped parts are studied during systematic hot stamping experiments.The cooling characteristics of the main features on U-shaped parts are also analyzed.Based on the basic hot stamping process,a rapid cooling hot stamping process is proposed.The process can improve the formability,hardness and its distribution uniformity,as well as reduce the springback of hot stamping parts.Moreover,under the premise of ensuring the parts have the similar performance with those formed by basic hot stamping process under the same experiment condition,the improved process also can reduce the dwell pressure and dwell time.Based on the CCT diagram of 22MnB5 steel and the microstructure of hot stamping parts,the microscopic mechanism of rapid cooling hot stamping process is revealed by analyzing the cooling rate of blank before stamping and the secondary cooling rate of blank after the beginning of martensitic transformation.Finally the rapid cooling process is applied to a hot stamping experiment of a typical B-pillar,which proves the performance improvement of hot stamping parts.(2)Based on the hot stamping experiment with differential heat transfer and the combination of finite element simulation and optimization,a method of solving the interfacial heat transfer coefficient(IHTC)in hot stamping is established.The influences of boundary condition type,initial design and optimization algorithm on the interfacial heat transfer coefficients between the blank and upper or lower die are studied.The results show that:both the IHTCs of upper and lower dies increase at first and then decrease with the decrease of blank temperature.At the beginning of heat transfer,the IHTC of upper die is larger than that of lower die,but when the blank temperature dropped to a certain value,the IHTC of lower die is larger than that of upper die.The IHTCs solved by the 1st and 2nd boundary are basically the same.The variation law of IHTC solved by the 3rd boundary is similar to those solved by the 1st and 2nd boundary,but the IHTC is small,which is due to the consideration of thermal properties of blank during the simulation.The IHTCs solved by the gradient optimization algorithms have strong sensitivity to the initial design,and the IHTCs solved by use of the three optimization algorithms,NLPQL,MOST and MISQP,are more stable if optimal searching from an initial design that complies with actual physical law.After considering the solution efficiency,the NLPQL is proved as the most suitable algorithm for the solution of IHTC in hot stamping.The equivalent IHTCs solved by the 3rd boundary and the NLPQL algorithm are substituted into a U-shaped hot stamping simulation,and the error of blank hardness between simulation and experiment is less than 6.5%.(3)Based on the discrete and aggregate principle,the design method of conformal cooling channels in hot stamping die is established.Based on the simulation analysis of fluid heat transfer,and combined the full factorial design and the multi-indicator comprehensive evaluation method,the cooling performance of hot stamping dies with different cooling channels is studied.The results show that the cooling performance of die mainly depends on the pattern of cooling channels and the mass flow rate in inlet.When at the low Reynolds number condition,the cooling performance of die with serpentine conformal cooling channels is the best,otherwise,that of die with longitudinal conformal cooling channels will be the best.In order to obtain more accurate temperature field of hot stamping,a coupling heat transfer simulation method of hot stamping is proposed.In this method,by introducing the time-averaged heat flux into the hot stamping model and the fluid heat transfer model and applying the grid data mapping method,the effects of IHTC(blank and die)and CHTC(die and cooling water)can be considered simultaneously during the hot stamping simulation.Compared with the temperature field of pure hot stamping simulation,by use of the coupling simulation method,the maximum temperatures of die,punch and blank are increased by 80%,82%and 75%,respectively.Furthermore,the temperature fields of die/punch/blank are no longer symmetrical,instead,the temperature near the inlet is low and near the outlet is high.In addition,the error of temperature between experiment and simulation is less than 10%.(4)The integrated optimization design method of the conformal cooling channels in hot stamping die is established based on the coupling heat transfer simulation of hot stamping,the optimal Latin hypercube design,the response surface method and the multi-objective optimization.By this method,the design of longitudinal conformal cooling channels in a typical B-pillar hot stamping die is optimized and the optimal Pareto frontier is obtained.Taking one optimal design point on the Pareto frontier as an example,the average temperature and temperature standard deviation of die surface respectively decrease by 11.7%and 28.2%after optimization.In order to manufacture the hot stamping die with conformal cooling channels,the two methods,ceramic core casting solution and 3D printing casting solution,are developed,and the detailed flow of casting mold making,pouring process and subsequent processing is made.The results show that both methods can successfully manufacture the industrial hot stamping die with conformal cooling channels.For the former method,the ceramic core has strong impact resistance to the molten steel and the casting yield is high.For the latter method,the conformal cooling channels of any shape,and the high precision and easy assembly casting box can be rapidly printed by precoated sand.The latter method is more suitable for the manufacture of die with complex cooling system.Finally,the die insert with the optimal longitudinal conformal cooling channels is applied to a hot stamping experiment of a typical B-pillar.By comparing the hardness and microstructure on the cross-section of the parts,the performance improvement of hot stamping parts is proved. |
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