Evolution Of Seismic Reinforcement 500MPa High Strength Tensile Deformation In The Organization Of The Multi-phase Process

High strength seismic reinforcement could improve the safety of structures. In this paper, the effect of multiphase structure on the mechanical properties and evolution of microstructure were analyzed by according to four different Nb-V microalloyed high strength steel500MPa at room temperature tensile test by using he universal tensile testing machine mechanical, metallographic microscope, microhardness tester,scanning electron microscope and transmission electron microscopy, hoping to provide reference for the development and application of seismic reinforcement with good seismic performance.Microstructure analysis show that, the microstructure in core of four kinds of tested rebars is characterized by ferrite, pearlite and bainite, bainite volume fraction of50%,15%,8%and3%, respectively. The effects of multiphase structure on strain hardening and fracture behavior indicate that the strain hardening exponent and strain hardening rate decreases with the increasing strain. The yield plateau length decreases, ratio of tensile strength and yield strength increases, uniform elongation and tatal elongation increases and then decreases, the tensile fracture surface dimple size and depth increases and then decreases,with increasing volume fraction of bainite. The yield platform disappeared while the volume fraction of bainite is50%. Fracture test of reinforcement is ductile fracture, inclusion at the crack is mainly MnS and oxide. The microstructure of No.1tested rebar with50%bainite and No.3tested rebar with8%bainite at different deformation was analyzed. The microstructure is elongated along the loading direction, which is expressed by the zonal distribution. The ferrite deformation is the most obvious. Pearlite is changed from lump to strip, the lamellar spacing decreases with the the deformation. The bainite is dispersed in the ferrite matrix microstructure, the lath width is from6to20μm. In the early stages of deformation, the deformation of bainite is not obvious. When the deformation is15%, the bainite lath elongates along the tensile axis and the lath width decreases slightly. The fracture behavior for pearlite at different deformation stages of No.3rebar was analyzed. The deformation and fracture of pearlite is shear fracture, propagation of crack perpendicular with perpendicular lamellar or as the inclination angle with lamellar. The analysis results of dislocation structure at different deformation shows, tested bars in deformation mechanism during tensile deformation is mainly dislocation slip mechanism, The dislocation density increases with the development of dislocation. The density of dislocation is the highest when deformation amount of15%, and dislocation interaction between dislocations and precipitates. The dislocation line is formed not only in the ferrite grains, but also in the grain boundaries. Precipitated phase analysis results show that the different deformation, the morphology and size of precipitation phase were different, with a large number of carbon nitride precipitation. The microhardness of different deformation stages was measured by the microhardness testing. The results show, microstructure hardness values of each phase in the deformation process increase slightly.The mechanical properties of No.3tested steel rebar with8%bainite in different tensile rates was researched by tensile testing machine (strain rate wit1×10-5s-1,5×10-5s-1,1×10-4s-1,5×10-4s-1,1×110-3s-1). The results show that, The yield strength and tensile strength of tested steel rebar increase, total elongation and elongation in the maximum stress decreased, dimple size and depth decreases, fiber shear lip area enlarges, with increasing of strain rate.