Study On The Microstructure Evolution And Controlled Rolling And Controlled Cooling Process Of Ti Microalloyed Earthquake-resistant Steel Bar

Titanium is an important micro-alloying element,which is of great significance to the strengthening of earthquake-resistant steel bars.In this paper,based on the new national standard(GB/T1499.2-2018)rebar composition,6 kinds of Ti microalloyed seismic rebar test steels with different compositions are designed.Combined with the controlled rolling and controlled cooling process,the microstructure evolution,the contribution of strengthening mechanism,and the change of properties were studied.The equilibrium phase in the test steel and the change of the elements in the phase of temperature were calculated by using JMatpro software.The results show that the ferrite phase exists stably between 560 ℃ and 820 ℃,and the M(C,N)precipitates increase with the decrease of temperature.The TTT curve of the titanium microalloyed steel bar shows a "C" shape as a whole.The optimum incubation temperature of ferrite is 650 ℃,and the optimum incubation temperature of pearlite is585 ℃.The coiling temperature is controlled at about 575-705 ℃,and the microstructure of the test steel is mainly pearlite + ferrite.The effect of titanium content on the microstructure and mechanical properties of seismic steel bars was studied.The results show that the microstructure of the test steel is band-like along the rolling direction,mainly composed of pearlite and ferrite.Elliptical precipitates were formed in the test steel,which were titanium(nitrogen,carbon)second-phase particles.With the increase of Ti content,the ferrite content increased significantly,the microstructure was refined and the Vickers hardness increased,and the pearlite lamellar spacing was refined from 170.6 nm to 107.5 nm.When the crimping temperature is 600-700 ℃ and cooled to room temperature,the main parts of the microstructure are ferrite and pearlite,showing good ductile fracture;The diameter of Ti C particles decreases,the amount of precipitation increases,and the corresponding hardness and yield strength also increase.The Gleeble-3800 thermal simulator was used to simulate the continuous cooling transformation curve.The results show that the critical temperature of steel is Ac3=823.6 ℃,Ac1=718.5 ℃,when the cooling rate after rolling is less than 0.5 ℃/s,bulk ferrite and mixed organization of pearlite.As the cooling rate increases,the ferrite transformation temperature decreases and the hardness increases.