Hot Forming Partial Hardening Experiment And Temperature Simulation For High Strength Steel

All the time automobile industry has been faced with a series of problems like safety,energy saving and environmental protection.Using high-strength steel has become a major turning point in the automotive industry.At present,the automotive industry development focuses lightweight.The lightweight not only means thickness thinning for car certain parts,but also needs to ensure that the strength can meet the requirements of suction collision,and maximize the protect the safety of passengers as much as possible.The technology of high strength steel hot stamping has opened up a new path for the automotive industry,and it is more and more large that the proportion of high strength steel stamping parts are in the automobile.More and more researchers were also involved in this field,mainly related to the cooling channel inside the mold to change the cooling rate of sheet in the forming process.Through the contact with the surface of die,the heat transfer is carried out via the forming and holding pressure,and the quenching is realized.Therefore,the dissertation can control temperatures of the mold according to the austenite continuous cooling transformation curve,and indirectly control the cooling speed of the sheet.Finally,the local hardening of the sheet metal is achieved.In this paper,cold rolled B1500 HS steel sheet was used as the raw material,and the hot stamping die steel was made of H13.The hot stamping partial hardening experiments and finite element software were used to verify the analysis.The main contents were as follows:(1)Based on the optimization,the partial hardening model of the hot stamping forming was established.The paper complete the selection of mold assembly and test equipment according to the experimental condition,and curves were drawn by recording the mold temperature during the experiments.Connected with the continuous cooling transformation curves found the results: the reduction of martensite transformation’s degree and cooling rate of sheet was reduced via heating die,and obtained the better bainite structure.The heating rate of die could be controlled by air cooling,and obtained the martensite.The mutual influence of mold temperature in cooling and heating zone was reduced by the method of prefabricated gap,and improved the controllability of local strength of sheet.(2)The mechanical properties and microstructures of the formed parts were measured and analyzed quantitatively under different conditions of hot stamping,the following conclusions were obtained: The cooling and transition zone of the forming sheet were unaffected by the heating temperature of the heating zone and attained most martensite structures.Fracture morphology can be judged as brittle fracture,the tensile strength was 1400 Mpa,and the hardness was up to 460 HV.With the increase of heating temperature,the content of martensite decreased,the content of bainite and ferrite increased,gradually.The fracture mode of the material was ductile fracture,and the fracture surface was cup cone.The tensile strength was reduced from 1185.98 Mpa to 620.44 Mpa,and the hardness decreased from 397 HV to 210 HV.There was little difference between U type bottom and sidewall materials.For partial hardening hot stamping sheet,the mechanical properties and microstructures showned gradient changes,mainly distributed in cooling zone,transition zone and heating zone.(3)For the heating zone,a two-dimensional U model was established based on ABAQUS,the article analyzed the effects such as different holding time and heating temperature on the sheet forming,and obtained the temperature field of die and sheet.For the cooling zone,a three-dimensional U model was established by the means of Fluent,and analyzed the effect of air flow rate on hot forming sheet,got the temperature field of die and sheet.Based on the continuous cooling transformation curves,the following conclusions were obtained: the optimum holding time was 10 s,heating die could reduce the degree of martensite transformation.When the air velocity in the mold was higher than a certain flow rate,the sheet material could be completely martensite.The optimum air flow velocity was 6m/s...