| With excellent mechanical properties and formability,the third-generation of automobile medium manganese steel can well meet the increasing demands of automobile lightweight and collision safety applications,and is regarded as a very promising material in modern automobile industry.At present,the research on medium manganese steel focuses on the microstructure regulation and microstructure evolution.In this paper,medium manganese steel developed by the CISRI is taken as the test material to explore the springback problem in the bending process.Through different direction of uniaxial tensile tests,different conditions of V-bending tests,cyclic loading-unloading test and V-bending test finite element simulation considering constant/variable elastic modulus,this paper mainly analyses the mechanical properties,bending springback variation and unloading elastic modulus variation of medium manganese steel.Also,the importance of considering the variation of unloading elastic modulus for the accuracy of V-bending springback simulation of medium manganese steel is investigated,and the following conclusions are obtained.(1)Medium manganese steel exhibits excellent combination of strength and plasticity in different directions,with high tensile strength and high elongation after fracture.At the same time,the anisotropic tendency of medium manganese steel is small,thus medium manganese steel can be regarded as isotropic material.DP780 high strength steel also has good tensile strength and elongation after fracture.TRIP effect plays an important role in medium manganese steel with high strength and high plasticity as well as yield plateau.(2)The influence of rolling direction on the springback angle of medium manganese steel and DP780 high strength steel is negligible.With the increase of bending angle,due to the corresponding deformation degree and the decrease of positive springback accumulation,the springback angle decreases gradually,and the positive springback tends to negative springback.In contrast,the springback of medium manganese steel as a whole is more inclined to negative springback.The springback angle of the two materials increases with the increase of the punch radius.The springback obtained by the test are all positive springback.At the same time,the positive springback angle of medium manganese steel is larger and larger than DP780 high strength steel.With the increase of punching speed,the springback angle of the two materials decreases gradually.(3)The cyclic loading-unloading test with an interval of 0.03 strain is carried out on medium manganese steel,and the cyclic loading-unloading curve of medium manganese steel is obtained.The variation of inelastic recovery and unloading elastic modulus with strain are explored,and the relationship between unloading elastic modulus and strain is fitted with the relevant model.The obtained model parameters of the relationship between the unloading elastic modulus of medium manganese steel and the true plastic strain lay a good foundation for the subsequent numerical simulation of V-bending test of medium manganese steel considering the variable elastic modulus.(4)UMAT subroutine with constant elastic modulus and variable elastic modulus are established for medium manganese steel,and the accuracy of UMAT subroutine is verified by uniaxial tensile test and loading-unloading test simulation.The finite element analysis software of ABAQUS is used to simulate the springback results of V-bending test under different test conditions,and the results are compared with the test values.Through the UMAT subroutine with constant elastic modulus and variable elastic modulus,the simulated springback result of V-bending test under different test conditions has the same change trend as the test result,and the simulated value obtained by using the UMAT with variable elastic modulus is closer to the test value,which proves the importance of considering the change of unloading elastic modulus in the simulation. |
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