Study On Numerical Simulation, Microstructure And Properties Of Tailored Hot-stamped U-shaped Parts Based On Temperature Field Control

In the past two or three decades,the demand for automobiles has been increasing,and the requirements for weight reduction and safety have been increasing.Ultra high strength steel hot stamping parts came into being and have been widely used in automobile industry.Ultra high strength steel hot stamping parts have all martensitic structure with tensile strength up to 1500 MPa.Although the strength of parts manufactured by traditional hot stamping process is very high,the toughness and crashworthiness are poor.Some parts of passenger cars need different performance in different parts,such as B-pillar.In the event of an accidental collision,the intrusion control area in the middle of the B-pillar has a high strength so as not to deform,while the energy absorption area at both ends absorbs the energy generated by the collision,so the cooperation of the two can protect the safety of the passengers.Therefore,the variant of traditional hot stamping parts,which have tailored performance,appears in people's field of vision and meets the requirements of some automotive parts.In this paper,B1500 HS steel is taken as the research object.After the ideal process parameters of tailored hot stamping are solved by the finite element software simulation,the continuous tailored hot stamping experiment is carried out by using the U-shaped die with high-speed air flow device,and the microstructure and mechanical properties of tailored U-shaped parts are analyzed.The main research contents and conclusions are as follows:(1)The quenching zone and tailored zone of the U-shaped part are simulated by FLUENT and ABAQUS respectively.Based on the simulation results of temperature field in FLUENT and ABAQUS,the ideal air velocity and holding time of continuous tailored hot stamping experiment are determined.The conclusion is as follows: When the air velocity is 1,3,4.5,5.5 and 7 m/s,the temperature of the die in the quenching zone shows a more and more slowly rising trend in the continuous hot stamping process,and it is stable at the 13 th,11th,9th,7th and 6th hot stamping respectively.With the increase of air velocity,the temperature of the blank after pressure holding quenching decreases continuously,but the decreasing range is smaller and smaller.At the same time,the temperature field of the plate is more and more uniform.However,when the air velocity is more than 5.5 m/s,the temperature and homogenization of the blank at the end of pressure maintaining quenching do not change significantly when the air velocity continues to increase,and the equipment cost for larger air velocity is higher.So it's not that the higher the air velocity,the better the result.Through numerical simulation analysis,it can be determined that the ideal air velocity is 5.5 m/s.If the holding time is too short,the austenite structure in the quenching zone can not be completely transformed into martensite,and the temperature difference of each part of the part will be large,resulting in poor performance and quality of the formed part.Besides,the short holding time is not conducive to the lower bainite transformation with better comprehensive properties.However,a longer holding time will affect the production efficiency.Therefore,the holding time needs to meet the above requirements.Through theoretical analysis and numerical simulation,it can be determined that the pressure ideal holding time is 10 s.(2)Based on the analysis of the holding time of 10 s,the die temperature is set to 100,200,300,400 and 500? respectively,and the hot stamping process in the tailored area is simulated.The temperature distribution of sheet metal is obtained by simulation results.The conclusion is as follows: Under the ideal holding time,the temperature distribution of sheet metal is more uniform and higher than the set die temperature.By adjusting the set die temperature,the ideal heating temperature for obtaining the bainite-based structure transformation process in the customized zone is 400? and 500?.(3)Based on the ideal process parameters obtained from the simulation results,the die temperature in the tailored zone is set to 100,200,300,400,and 500?,respectively,and continuous tailored hot stamping experiments are performed.The microstructure,micro-hardness,tensile strength and elongation at break of quenched and tailored U-shaped parts are analyzed.The results are as follows: For the quenching area,the quenching phase is mainly martensite,the micro-hardness can reach 485 HV,and the tensile strength can reach 1498 MPa.For the tailored area,with the increase of die temperature from 100? to 500?,the micro-hardness and tensile strength decrease,at the same time,the area fraction of bainite increases and that of martensite decreases.