Research On Hot Stamping Process For Thick-walled Ultra High Strength U-shape Parts By Numerical Simulation

Ultra-high strength steel parts have been used to reduce vehicle weight，increase strengthand impact resistance of vehicle body structure. Hot forming is the most effective technologyto manufacture ultra-high strength steel parts. In the hot stamping process，the quenchableboron alloyed steel sheet is heated to a temperature above Ac3and kept the temperature for aperiod of time，after sufficient austenization the steel sheet is transfered to die with coolingsystem to be quenched and formed，then ultra-high strength parts with lath martensitemicrostructure will be obtained. Nowadays，hot stamping steel of2mm or thinner is widelyused in vehicle body structure，while the thick parts of vehicle underpan have not been formedwith hot stamping process，if hot stamping blank of4mm thickness instead of steel parts of6mm or thicker can be used in heavy-duty truck underpan，the load bearing capacity ofunderpan system will be significantly increased which also leads to the weight reducing of theunderpan appropriately. In this paper，the4mm22MnB5is used as experimental material，thehot formability of U-shaped part formed by thick steel sheet is analyzed systematically bynumerical simulation and experiment.Pam-Stamp is used for numerical simulation in the stamping and quenching processunder different process parameters. With initial forming temperature850℃and initial mouldtemperature20℃，the numerical simulation about stamping speed indicate that diffusivephase transition occurs before quenching process when stamping speed is less than75mm/s，in the stamping speed of100mm/s，the lowest temperature of blank after stamping process is580℃and no solid phase transition occurs before quenching process，the highest stressvalue is225MPa and the maximum strain value is0.15at the end of the stamping process.The numerical simulation results of die fillet indicate that small die radius led to theinadequate martensite transition in the side wall of U-shape part with initial punch radius 3mm，the hot formability is good when the die radius is11mm， the peak stress value isabout256MPa and the maximum strain value is0.12after stamping process， full martensitecan be obtained after quenching process，the largest thinning ratio is9.5%. The numericalsimulation results of mold clearance show that diffusive phase transition occurs duringquenching process in the mold clearance of4.1mm，the best mold clearance of thick-walledU-shape part is4.0mm～4.05mm. The numerical simulation results of dwell time forquenching show that the best dwell time is9.5s.The hot stamping die is designed for U-shaped part with4mm thickness，and optimizingof cooling system is analyzed by FLUENT. K-type thermocouple temperature tests are carriedout to determine the optimum parameters for the cooling system：cooling channel diameter is8mm，cooling channel spacing is6mm，distance between cooling channel and mold surface is6mm and the speed of cooling water is higher than650mm/s. The thick-walled U-shape partsare manufactured under the best process parameters acquired by numerical simulation，highdimensional accuracy parts are obtained，the maximum thinning rate is6.5%，almost nospringback occurs，microstructure analysis indicate that full lath martensite transition occurs，and the martensite lath width of wing side and bottom is less than side wall，after quenchingprocess，the hardness of wing side，bottom and side wall is486.1HV，481.2HV，461.2HV，respectively，and the tensile strength is1544.72MPa，1536.48MPa and1511.42MPa.