Study On The Second Phase Precipitation Behavior And Microstructure Evolution Of HRB500E Seismic Reinforcement

HRB500E seismic reinforcement is a kind of low-carbon pearlite and ferrite reinforcement,which can delay the collapse of buildings and resist certain earthquake load when earthquake occurs.HRB500 E seismic reinforcement has high strength and good plasticity.However,it is difficult to meet the strength and plasticity requirements of HRB500 E seismic reinforcement only by adding a certain amount of vanadium element.Therefore,the addition of niobium element can improve HRB500 E seismic reinforcement and save a certain production cost.The microstructure,morphology and second phase precipitates of HRB500 E seismic steel were observed and analyzed by optical microscope,SEM,TEM and EDS.The results show that the size of the second phase precipitate containing niobium in HRB500 E seismic steel is about 50 nm,and the morphology is approximately circular,mainly distributed near the grain boundary.The thermodynamic precipitation temperature of different second phases in HRB500 E seismic steel was calculated by thermodynamic calculation,dynamic calculation and two-dimensional lattice mismatch.The results show that the second phase containing niobium and vanadium precipitates completely in the process of austenite transformation ferrite.The PTT curve of Nb element carbonitride precipitated in austenite of HRB500 E seismic steel is in the form of C curve.The nose point temperature of grain boundary nucleation is about 1000-1030 ?,and that of homogeneous nucleation and dislocation nucleation is about 850-870 ?.The mismatches of Nb C with ?-Fe,?-Fe and ?-Fe heteromorphisms were 8.71%,11.59% and 13.2%,respectively;the mismatches of VC with ?-Fe,?-Fe and ?-Fe heteromorphisms were 4.40%,1.70% and 18.80%,respectively.Nb C and ?-Fe,VC and ?-Fe are easier to nucleate.The change of grain size of HRB500 E seismic steel was further studied by adding different content of element Nb.The grain size of ferrite in the test steel with vanadium added only was between 3.6?m and 8.3 ?m;the grain size of ferrite in the test steel with 0.013% niobium was between 2.6?m and 5.3?m;the grain size of ferrite in the test steel with 0.017% niobium was between 2.4?m and 4.8?m;the grain size of ferrite in the test steel with 0.023% niobium content was between 2.4?m and 4.8?m The ferrite grain size of the tested steel is between 2.6?m and 4.3?m.The addition of Nb element makes the grain size of HRB500 E seismic steel significantly refined and the tensile strength increased from 500 MPa to 560 MPa.