Study On Recrystallization And Phase Transformation Behavior Of Niobium Microalloyed HRB400E Steel Bar

Compared with the traditional process,the niobium microalloyed HRB400 E hot rolled ribbed steel bar produced by high-speed bar production process has the advantages of high precision and low cost,and is widely used in high-rise buildings and Bridges.In the present work,the recrystallization and phase transformation behaviors of niobium microalloyed HRB400 E steel bars were studied.The influence of high-speed bar finishing rolling and controlled rolling process on the microstructure and properties of steel bars was simulated by thermal simulation tester,and the controlled rolling process was optimized,which provides a reference for the development of niobium microalloyed HRB400 E steel bars based on high-speed bar technology.The main research results are as follow:(1)The stress-strain curves under different deformation conditions were obtained via single pass and double pass compression experiments.The influence of different deformation parameters on recrystallization of Nb microalloyed steel was analyzed.The Zener-Hollomon parametric equation,critical strain model of dynamic recrystallization,dynamic recrystallization dynamic model and static recrystallization dynamic model were established.The results show that Nb can increase the non-recrystallization temperature of austenite and inhibit the dynamic recrystallization.The nonrecrystallization temperature of Nb microalloyed steel is 945 ℃.The dynamic recrystallization activation energy of the experimental steel was calculated to be 368.40 k J/mol.(2)The dynamic continuous cooling transition behavior of Nb microalloyed steel was investigated,and the cooling rate and temperature Windows corresponding to different types of phase transitions were determined.The effects of different cooling modes on the microstructure and precipitation behavior of Nb microalloyed steel were studied.When 0.015%Nb is added,the cooling rate range of the pre-eutectoid ferrite and pearlite phase transitions decreases,and the bainite phase transitions move to the right of CCT curve.At the same time,adding Nb can refine the microstructure,and the hardness of steel containing Nb is greater than that of steel without Nb at each cooling rate.When the final cooling temperature is 600 ℃,the microstructure of niobium steel is ferritic and pearlite,the grain size of ferritic is 4.67μm and Nb C precipitates are finely dispersed.(3)The influence of different deformation parameters on the microstructure and properties of Nb microalloyed steel bar was studied by thermal simulation experiments based on the high-speed bar finishing control rolling process.The results show that when the deformation temperature is 850 ℃ and the second pass deformation is 35%,the yield strength of the experimental steel is 562.14 MPa,the tensile strength is 645.78 MPa,the elongation is 32%,the average grain size is 8.7 μm,the microstructure is relatively uniform and the comprehensive mechanical properties are the best.On this basis,the field TMCP process is optimized.After the optimization,the microstructure of HRB400 E steel bar is ferrite and pearlite,the ferrite grain size is 11,the precipitation is Nb C,and the precipitation size is mainly distributed in 20～60 nm.Refinement strengthening is the main strengthening method of the steel bar.