| Ferrite/martensite dual phase steel has been widely used in automobile industry because of its advantages of high strength,high formability and high impact absorption energy.Because the heat treatment process of the dual phase steel involves complex physical and metallurgical phenomena affecting the volume fraction,carbon content and distribution position of each phase in the dual phase steel and all of these play important roles in the final mechanical properties of the dual phase steel.Therefore,in order to study the above influencing factors from the microscopic point of view,it is aimed to study the microstructure evolution and the deformation mechanism during tensile process of DP590.The main contents are listed as below:(1)Firstly,martensite/ferrite heterogeneous dual phase steel was prepared by designing a feasible heat treatment process.The microstructure evolution and mechanical behavior were characterized by experiments and numerical simulation,respectively.Firstly,the experimental results show that when the annealing temperature of the two-phase zone is 780 ℃,the equilibrium microstructure obtained after quenching is a dual phase with the volume ratio of martensite to ferrite of 3:7,and the average thickness of lath martensite is 0.88 μm.Furthermore,the yield strength and tensile strength of the DP590 is 596 MPa and 810 MPa,respectively,showing a good match of strength and plastisity.With the optimum annealing temperature of 780 ℃,we further obtain the best holding time of 10 min to realize the final equilibrium state of ferrite and martensite.At the same time,the cellular automata tool was exerted to simulate the microstructure evolution of DP590 during heat treatment.It is found that the morphology and distribution of austenite at 780 ℃ are qualitatively consistent with those of martensite observed in the experimental results.It reveals that the martensite obtained by quenching was almostly inherited from the precursor austenite.Consistent with the experimental results,the volume fraction of austenite also trends to be steady within 1 min.(2)The deformation mechanism of DP 590 was investigated by DIC technique systematically.Macroscopically,the strain distribution of ferrite and martensite dual phases is heterogeneous at the initial stage.Microscopically,the softer ferrite bears a larger strain,the angle between which and loading direction is usually 45°.The distribution of ferrite and martensite will make difference the direction and shape of the deformation band.The larger the volume fraction of martensite is,the greater the probability of deformation band passing through the martensite is.With the increase of the annealing temperature,the strain heterogeneity distributing in ferrite and martensite decreases.At the same time,the morphology of the fracture section of the as-prepared DP590 was analyzed.It is found that the deformation band caused by strain localization in the ferrite grain expends along the interface,which provides the first candidate region for local strain.It also proves that ferrite bears more strain,which is consistent with the micro DIC results.(3)In order to further explore the deformation mechanism of DP590 during tension,the effects of volume fraction,microstructure uniformity and boundary conditions on the strain distribution were studied by finite element simulation.The simulation results show that:(a)The larger the volume fraction of martensite is,the larger the core strain zone becomes,resulting in the greater possibility of martensitic phase failure.(b)The more uniform the microstructure is,the more uniform the strain distribution is.Therefore,the strain concentration induced deformation should be avoided.(c)With the plane strain state of the free side boundary,the shear strain is dominant;while if the side boundary is strictly constrained,the failure mode will become a splitting mode perpendicular to the loading direction.This provides important information for us to understand the micromechanical behavior in materials. |
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