| The advanced high strength steels have attracted great attentions with the development of lightweight steels in the automobile industry.Therein,TWIP(twinning induced plasticity)steels have been one of the most important materials due to their prominent comprehensive performance.It is known that TWIP steels are usually deformed under the strain rate of 10-1?101 s-1,which are far more less than the strain rate up to 102?103 s-1 as the collision happens.Therefore,in order to assess collision safety preferably,it is essential to explore microstructures and mechanical behavior of TWIP steels at varied strain rates and investigate the crushing energy absorbing of its structure components.In this work,the mechanical behaviors,microstructure morphologies and phase compositions of 600 MPa high yield strength TWIP steels and DP 1000 dual phase steels(as comparison)were analyzed by SEM,TEM,XRD,universal tester and high speed tensile testing machine.In addition,the crushing energy absorption characteristic of spot welding cap type thin-walled member was carried on the electro-hydraulic servo universal testing machines and drop-testing machine.The main work and results were drawn as follows.(1)At the strain rate ranges of 10-4?103 s-1,the yield strength of high strength TWIP steels monotonously increases with strain rates,whereas the tensile strength increases first and then decreases.Moreover,the tensile strength presents positive strain rate sensitivity(PSRS)varying from 10-4 to 103 s-1 and reaches the maximum value of 1168 MPa at the strain rate of 102 s-1,then reveals a decreasing trend with increasing strain rates showing negative strain rate sensitivity(NSRS).In comparison,both the yield and tensile strength of DP 1000 dual phase steels go steadily up with the increase of strain rates.In addition,the high strength TWIP steel shows lager strain rate sensitivity index(m)and lower activation volume v*at strain rate range of 10-2?102 s-1.As for TWIP steels,the work hardening rate in the homogeneous plastic deformation stage can be divided into two processes:i)there are barely fluctuations with the increase of strains at strain rate intervals from 10-4 to 10-2 s-1;ii)varying from 10-1 to 103 s-1.the work hardening rate increases with strains.Meanwhile,based to the Considere instability criterion,it can be easily found that the evolution of instability point with strain rates is consistent with that of the uniform plastic deformation properties for both the experimental steels.The localized deformation under dynamic loading(10-1?103 s-1)results in the adiabatic temperature rise,which partially offsets workhardening effect of strain rates,and increases the stacking fault energy(SFE)in the TWIP steel by 5?9 mJ/m2.When the strain rate exceeds 102 s-1,the fracture energy absorption intensity(?Efracture)of high strength TWIP steel and DP 1000 dual phase steel reaches above 50 GPa%and 20 GPa%,respectively.It is obvious that the TWIP steel parts possess better collision absorption characteristics than DP 1000.(2)Both the experimental steels present typical ductile fracture feature and contain a large amount of dimples.At low strain rates,the size and distribution of dimples in the tensile fracture surface are non-uniform,and the size deviation of dimples declines with the increase of strain rates.Plenty of large and deep dimples evolve in high strength TWIP steels at 102 s-1 and there are a mass of uneven and elongated dimples in DP 1000 dual phase steels at 103 s-1.Note that high strength TWIP steels represent varying degrees of TWIP effect at different strain rates without the occurrence of s-martensite transformation.Under the condition of quasi-static there still exist fewer deformation twins and relatively thicker twin lamellas,and the twin lamella thickness becomes uniform with increasing strain rates.When the strain rate reaches 102 s-1,it can be seen that a part of secondary deformation twins which intersect with primary deformation ones gradually evolve.With further increase of strain rates(5×102 s-1 and 103 s-1),the quantity reduction of deformation twins is attributed to the suppression of twinning effect caused by the increase of SFE resulted from the adiabatic temperature rise.Moreover,it is also found that the evolution of twin growth complies with DoseResp model at the stain rate of 100 s-1 and increases linearly at 102 s-1.However,in terms of DP 1000 dual phase steels the dislocation densities inside ferrites in the vicinity of M/F interface increase and the martensitic laths are elongated with increasing strain rates.The martensitic laths are broken to some extent at the strain rate of 103 s-1.(3)The index of strain rate sensitivity in Johnson-Cook model is corrected through the study of mechanical behavior and deformation mechanism under different strain rates in high strength TWIP steels.Then the modified Johnson-Cook equation is established which could well characterize the dynamic deformation behavior of the TWIP steel.The simulation results through ABAQUS finite element simulation are consistent with the experimental results.(4)Under the condition of quasi-static,DP 1000 steel components develop cracks in initial period of deformation;on the contrary,high strength TWIP steel components have a large compressible displacement,displaying stronger energy absorption capability.In comparison with DP 1000 steel components,high strength TWIP steel components can absorb equivalent collision kinetic energy at shorter crushing displacements,showing stronger energy absorption capability.During the quasi-static and dynamic compression,both the experimental steel parts produce ordered deformation in the axial direction and the energy is absorbed through the formation of complicated folds.Additionally,with increasing crushing speed,the first peak load of TWIP steel components shows a rising trend,and the average load at the early stage of deformation increases until a flat stage occurs at the late stage of deformation.At high crushing speed,there is no crack coming into being in high strength TWIP steel components compared with the equivalent compressional displacement under the quasi-static condition,demonstrating that high strength TWIP steel components can absorb more collision kinetic energy. |
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