| With the increasing demand for automobile safety and environmental protection,high strength steel has gradually entered the field of vision,the application of high strength steel in automobile is more and more extensive,which promotes the development of vehicle lightweight.As an important technology of high strength steel sheet forming,hot stamping has attracted more and more attention.The hot stamping of high strength steel plate is to ensure the austenitic steel plate in the cooling mould and rapidly cooled to get martensite structure,the strength of the obtained parts is up to 1500 MPa.The high strength steel sheet produced by hot stamping is applied to automobile parts,which can not only increase the strength of the automobile parts,but also reduce the weight of the car and achieve a good lightweight effect.For some automobile parts,different parts are required to have different strength.Such as the B-pillar of a car,in the event of a car collision,the upper and lower areas of the B-pillar should require better shaping and lower strength,the middle area should be with high strength,to resist the impact of shell invasion during car collisions,in order to ensure the safety of car drivers.Therefore,high strength steel hot stamping forming local hardening technology came into being.High strength steel hot stamping forming local hardening technology in the hot stamping process to different parts of the sheet material using different cooling rates,in order to achieve different parts of the molded parts have different strength.In this paper,cold rolled high strength steel B1500 HS is used as raw material,air is used as the cooling medium and H13 hot die steel is used as the mold material.The feasibility of local hardening of high strength steel hot stamping is studied by numerical simulation and experiments.The specific contents and results of this study are as follows:(1)High strength steel B1500 HS was isothermally stretched under different forming temperatures and different strain rates,and the Norton-Hoff model was used to multi-linearly fit the experimental results and a high temperature rheological model of high strength steel B1500 HS was established.The comparison fitting results and experimental results showed that the calculated values were in good agreement with the experimental values,indicating that the Norton-Hoff model could be more accurate.It can accurately describe the high temperature rheological behavior of high strength steel B1500 HS and take it as an important input parameter for high strength steel hot stamping model.(2)A finite element model of high strength steel hot stamping was established.The two stages of sheet metal forming and pressing quenching during high strength steel hot stamping process were simulated by finite element simulation software ABAQUS and FLUENT.The influence of different air velocity,holding time and holding pressure on the temperature field of sheet material under the conditions of multi process cycle was analyzed.A method for local hardening of high strength steel hot stamping is proposed,and the local hardening of high strength steel was numerically simulated,and the air velocity,holding time and holding pressure were obtained to meet the local hardening of high strength steel.(3)The local hardening mould for high strength steel hot stamping was drawn by CATIA,and the mould was processed,assembled and debugged.(4)According to the numerical simulation,the local hardening experiment of hot stamping was carried out.The hardness test,mechanical properties and microstructure of the forming parts under different conditions were analyzed.The feasibility of the local hardening of high strength steel hot stamping was verified by the comparison between the experiment and the simulation. |
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