A Study On Composition Design,Microstructure And Mechanical Properities Of 460 MPa Grade Construction Structural Steels

With the rapid development of construction industry,the construction structural steels are widely used.As a kind of low carbon bainitic steel,the construction structural steels have a good combination of strength and toughness,and been successfully found applications in large buildings and long span steel structure.For high-performance construction structural steel,however,there is a need in simultaneous maintainment for high strength and anti-seismic,fire-resistance and weathering.Also,there are some problems in quality control and production cost.Considering the above problems,therefore,the chemical composition was designed and optimized for the 460 MPa grade construction structural steel in this thesis.And the rolling and heat treatment processes were studied while the microstructure and mechanical properties of this steel were acquired.Finally,the 460MPa high strength construction structural steel was obtained with the required properties.The main research contents and resultant results in this thesis are summarized as follows.(1)The phases and mechanical properties of construction structural steel were calculated by JMatPro software.The influence of alloy element content on the precipitated phase characteristics and mechanical properties of construction structural steel was analyzed.The composition system and main chemical composition content of the 460 MPa grade construction structural steel were determined.The chemical composition(in wt.%)was 0.06 C,1.05 Mn,0.15 Si,0.02 Nb,0.012 Ti,0.20 Cr,0.30 Cu,0.30 Mo,0.40 Ni,0.002 B respectively.(2)The static and dynamic continuous cooling transformation behaviors of the 460 MPa grade construction structural steel were studied by phase transformation tester.Combined with hardness measurement and metallographic observation,the static and dynamic CCT curves were plotted.The results show that,the microstructure evolution in this steel was F(Ferrite)?GB(Granular Bainite)?LB(Lath Bainite)with the increase of cooling rate,and a full bainitic microstructure was obtained within the cooling rate range of 30?50 ?/s.The hardness of the steel increased with the increasing of the cooling rate.Due to pre-straining of austenite,the critical cooling rate of austenite was increased,the initial temperature of phase transformation was increased,the temperature range of phase transformation was expanded,and the bainite structure can be easily obtained in the experimental steel.(3)The rolling test of experimental steel was carried out using the two-stage controlled rolling and cooling process in the laboratory.The effect of final cooling temperatures on microstructure and mechanical properties of the steel was investigated.The results show that the strength of the experimental steel reduced,the yield ratio decreased,and the toughness increased first and then decreased with the increasing of final cooling temperature.When the final cooling temperature was 480?,the steel had a good combination of strength and toughness,and the microstructure of the steel was the lath bainite and granular bainite.(4)Combined with the effect of the final cooling temperature on the microstructure and mechanical properties of the experimental steel,the optimized rolling process of 460 MPa grade construction structural steel was obtained.The process was as follows:The forgings was austenitized at 1200? for 2 h.The start rolling temperature during the first stage was set to 1100? and rough rolling process was four-pass(reduction ratio of 39.3%).When temperature cooled to 930?,a second stage,rolling was performed,the final rolling temperature was 880?,and rough rolling process was eight-pass(reduction ratio of 70.5%).After the temperature reached to 780?,steel plate was cooled to 480? at the cooling rate of 20?/s.Finally,the steel plate was slowly cooled to room temperature in asbestos.(5)The influence of tempering temperature on the microstructure and mechanical properties of construction structural steel was studied.The results show that the yield strength increased,the tensile strength remained unchanged,and the impact toughness decreased first and then increased with the increasing of tempering temperature.The M-A islands in steel gradually decomposed,lath bainite merged and coarsened,and the content of granular bainite increased.The polygonal ferrite was formed at higher tempering temperature.The heat treatment process of construction structural steel after rolling was heated at 300? for 1 h.Thus,the yield strength of steel was 540 MPa,the tensile strength was 660 MPa,the yield strength ratio was 0.810,the elongation after fracture was 20.0%,and the impact energy were 172 J and 156 J at-20? and-40?,respectively.