Research On The Effects Of Controlled Rolling And Controlled Cooling On The Microstructure Regulation And Mechanical Properties Of High-strength Earthquake-resistant Steel Bars

Rebar is a very important material in major engineering structures,and its fine-grained structure determines the strength and seismic resistance of rebar.The combination of microalloying elements and controlled rolling and controlled cooling has an important influence on the microstructure transformation of high-strength anti-seismic rebars.The Gleeble-3800 thermal simulator was used to measure the phase transformation law and thermal expansion curve under the conditions of different cooling rates,and obtained the continuous cooling transformation(CCT)curve of high-strength anti-seismic rebars.The results showed that when the cooling rate were 0.3 ℃/s～3 ℃/s,the experimental steel with 0.017% Nb and 0.023% Nb occurred ferrite transformation and pearlite transformation.The microhardness of two experimental steels increased slowly with the increase of cooling rate.When the cooling rate was 0.5 ℃/s,the main precipitates of 0.017% Nb and 0.023% Nb experimental steels were(Nb,V,Ti)C,which were circular in shape,and the average particle size ranges were 10～20 nm and 10 nm,respectively.The rolling process conditions also have an important influence on the microstructure and mechanical properties of high-strength anti-seismic rebars.Four experimental steels with different compositions were designed,and the Gleeble-3800 thermal simulator was used to simulate thermal deformation and final cooling temperature control.The results showed that the microstructure of 1#～4#experimental steels were mainly composed of ferrite and pearlite.As the final cooling temperature decreased,the tensile strength and yield strength of 1#,2#,3#,and 4#experimental steels increased.The tensile strength and yield strength of 4#experimental steel reached the maximum at the final cooling temperature of 650℃under the conditions of process I and process II,which were 712.94 MPa,562.7 MPa and 655.62 MPa,522.76 MPa,respectively.The fractures of 1#,2#,3#,and 4#experimental steels belonged to the ductile fractures,and the depth of equiaxed dimples first increased and then decreased.The main precipitated phases in the 3#experimental steel were(V,Nb,Ti)C and(Nb,V,Ti)C,which were round and square,and the average particle size ranges were 2-5 nm and 20-30 nm,respectively.The main precipitation phases in the 4# experimental steel were(Nb,Ti,V)C,(Ti,Nb,V)C and(V,Nb,Ti)C,which were round and long,and the average particle size ranges were 10-20 nm,30-50 nm and 2-5 nm,respectively.