| Advanced high strength steel has the characteristics of high strength,good plasticity and toughness,good formability and high initial work hardening rate,and is the first choice for realizing lightweight steel.Through the design of reasonable rolling and heat treatment processes,nano-scale Cu-rich precipitates are introduced into the fine-grained matrix to achieve the combination of high strength and good ductility,which has theoretical and practical significance for the realization of lightweight steel materials.In this thesis,a Cu-containing experimental steel was designed based on the reinforcement and plasticization concept of advanced high strength steel,and it was subjected to solid solution,rolling and isothermal aging treatment.The effects of different processes on microstructure and mechanical properties were investigated by changing the rolling deformation and aging time.The effects of rolling and aging processes on the microstructure evolution and Cu-rich phase precipitation behavior of the experimental steel substrate were analyzed by means of electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and atom probe tomography(APT).The contribution of fine grain structure and Cu-rich relative yield strength obtained by different preparation processes was discussed.At the same time,a quasi-3D representative volume element model was constructed based on EBSD data,and the distribution of stress and strain of matrix microstructure obtained by different preparation processes during tensile deformation was further analyzed by crystal plasticity simulation.The results of matrix microstructure and precipitated phase analysis show that with the increase of rolling deformation,the ferrite grains in the experimental steel elongated along the rolling direction,the volume fraction of martensite decreased,the small angle grain boundary increased,and the kernel average misorientation difference accumulation of the matrix increased accordingly.At the same time,the high density dislocation of the matrix provides the conditions for the precipitation of Cu-rich phase.During the aging process,the alloy elements Ni and Al segregated at the interface of precipitates,forming a shell distribution structure and improving the thermal stability of Cu-rich precipitates.The mechanical properties test results show that with the increase of aging time,the hardness of the experimental steel increases first and then decreases,and reaches the peak hardness at 60 min.The increase of rolling deformation promotes the growth of precipitation particles,the grain refinement of matrix microstructure,the increase of small angle grain boundaries,and the increase of dislocation density,which provides more contributions to the yield strength of experimental steel.The results of EBSD characterization and crystal plasticity simulation show that for the Cu-containing experimental steel with dual-phase structure after rolling deformation,during the tensile deformation,ferrite bears a large strain,and martensite bears a large stress.The phase boundary has a stronger hindering effect than the grain boundary.When the plastic deformation continues,the crack will first appear near the phase boundary and the ferrite grain boundary adjacent to martensite.At the same time,the increase of rolling deformation intensifies the uneven distribution of stress and strain,which accelerate the initiation and propagation of cracks. |
