Continuous Cooling Transformation And Microstructure-properties Of 690 MPa Grade Rebar Steel Microalloyed With V And Nb

At present,the reinforced concrete used in super high-rise buildings and long-span bridges our country has the actual problem that the bar performance is extremely mismatched with high-performance concrete,which has seriously affected the quality and service life of these buildings.The development of high-strength bar steels supporting high-performance concrete is very urgent and has huge economic and social benefits.Most of the high-strength bars（prestressed bars）currently used in China are obtained through cold working,which has serious shortcomings in strength,plasticity,and bond strength with concrete.In this thesis,the continuous cooling transformation behavior and heat treatment process optimization of V and Nb microalloyed high-strength steel bars are systematically studied.In order to prepare high-strength steel bars with excellent comprehensive mechanical properties and to provide theoretical support and technical reserves for accelerating the localization of bar steels for high-rise buildings.Four kinds of high-strength steels for high-strength bars with medium carbon without V and Nb,containing V,containing Nb,and adding V and Nb,respectively denoted as 6-0 steel,6-V steel,6-Nb steel and 6-V/Nb steel.The phase transition points of the four bar steels were measured.The A_c1temperature of 6-0 steel,6-V steel,6-Nb steel and 6-V/Nb steel is 705℃,725℃,720℃and 725℃,respectively.The A_c3temperature is 785℃,825℃,785℃and 780℃,respectively,and the M_stemperature is 300℃,275℃,280℃and 270℃,respectively.The CCT curves of the four bar steels were measured.The results show that whether the elements V and Nb are added independently or in combination,the high-temperature transformation part of the CCT curve is significantly moved to the right,and the critical cooling rate for martensite transformation is reduced,so that it is easier to obtain martensite or bainite in a larger cooling rate range.Moreover,the effect of adding V alone is stronger than adding Nb alone,and the combined effect of adding V and Nb is between them.For the simulated hot rolling of the bar steels,except 6-V/Nb steel,the mechanical properties of the three steels cannot meet the performance requirements of 690MPa grade bar steel.For 6-V/Nb steel of the simulated hot rolling,the tensile strength and yield strength are 1184 MPa and 783 MPa,respectively,and the total elongation is 14%.After reheating at 1200℃for 0.5 h,cooling in air and then tempering at 590℃for0.5 h,except for 6-0 steel,the mechanical properties of the other three steels all exceed the performance requirements of 690 MPa grade bar steel,and the strengthening effect of 6-V steel and 6-V/Nb steel is better than the 6-Nb steel.The tempering temperature between590 to 650℃has no significant effect on the mechanical properties of 6-V steel and 6-Nb steel.For the samples tempered 620℃for 0.5 h,fine and dispersed VC and Nb C precipitates were observed in 6-V steel and 6-Nb steel,respectively,and they have the most direct contribution to the mechanical properties of the tested 690 MPa grade bar steels.