Hot Forming Properties And Hot Stamping Process Of Docol 1500 Bor Steel For Automobile

At present,the contradiction between the increase of car ownership and the reduction of vehicle emissions is becoming increasingly prominent.In order to reduce vehicle exhaust emissions and meet the requirements of green environmental protection,the most effective means is to reduce the weight of the car.Therefore,the car body steel structure,such as A-pillar,B-pillar etc.with patch plates are usually used to reduce the weight of the car body under the premise of meeting the performance requirements.Automobile body steel structure is usually made of ultra-high strength steel by stamping process,especially hot stamping process.The hot stamping process has the characteristics of good formability and high strength,but the die structure is complex and there are many factors affecting the stamping process.The main purpose of this project is to explore the optimization scheme of spot welding process of patch plate by systematically studying the microstructure transformation and hot deformation law of hot formed Docol 1500 Bor steel,and further optimize the hot stamping process of B pillar based on the optimization design of hot stamping cooling water channel.In this paper,CCT and TTT curves of Docol 1500 bor steel were measured by thermal expansion method.The phase transformation mechanism of Docol 1500 bor steel during continuous cooling and high temperature holding was determined by microstructure observation.Based on the analysis of the rheological curves at different temperatures and deformation rates,the hot deformation constitutive model of the test steel was established.The orientation analysis of the deformed microstructure under typical deformation conditions was carried out by using EBSD.The optimal hot stamping deformation process "window" was obtained,and the micro mechanism affecting the high temperature deformation behavior was clarified.Based on the Simufact-Welding software,the temperature / stress / microstructure coupling finite element simulation calculation of the patch plate resistance spot welding process was carried out.Based on the validity of the model,the influence of welding current,welding time and electrode pressure on temperature field,nugget size and penetration rate in the spot welding process of automobile plate with different thickness were systematically studied,and the process parameters were obtained.The influence of the parameters on the spot welding technology and usability of different specifications of automobile plate is studied.The fluid-structure coupling simulation model of the cooling pipe of stamping die was established by using FLUENT software.The influences of water channel diameter,water channel distance,distance between water channel and die surface,holding time,initial temperature of die,initial temperature of sheet metal and holding time on cold quenching ability of hot stamping die and stable process of temperature field of forming parts were discussed.The optimized process parameters and the geometric parameters of cooling pipe were obtained,which lay a foundation for further research on hot stamping process of B-pillar.The finite element model of temperature / stress / microstructure coupling calculation in hot stamping process of ultra-high strength steel sheet was established by using the professional forming software Simufact-Forming.The relationships among process parameters,temperature field,stress field and microstructure field were systematically studied.The influence of initial sheet temperature,die temperature and stamping speed on the above output characteristics was revealed and optimized Process parameters.On the basis of the above research,the industrial trial production of Docol 1500 bor ultra-high strength steel B-pillar was carried out by using optimized process parameters.The process of resistance spot welding and hot stamping,as well as the microstructure and properties of the workpiece,were tracked and studied.The results show that the stamping thinning rate,microstructure and mechanical properties of the trial products all meet the requirements.