| Steel structure bridge has many advantages,such as light weight,erection convenience,environmentally-friendly,etc.With the expansion of the bridge construction area,the poor service conditions put forward higher requirements for the load,seismic and corrosion resistance of bridge steel.The development of the bridge steel is towards high performance,high strength,excellent low-temperature toughness and good corrosion resistance.The application of H-beam in the bridge construction can reduce the self-weight of the bridge.Meanwhile,it will significantly improve the load capacity,and reduce the cost of painting and maintenance of the steel bridge.Therefore,the development of new high-strength and weatherresistant H-beam will bring considerable economic benefits to society.In this paper,high strength and toughness hot rolled Q500qNHE weathering H-beam with ReL?500MPa?Rm?630MPa?A?18%?Akv(-40?)?120 J has been developed.Based on the design idea of low C,low P,and S,Nb-V microalloy elements,Cr,Ni,Cu were added to ensure the strength,toughness and weathering performance of the beam.Through the rolling experiment of H-beam flange in the laboratory,the strengthening and toughening mechanism,as well as the corrosion resistance of the experimental steel was studied under the simulated industrial atmosphere.The main work and conclusions are as follows:(1)Three experimental steels with different Nb-V alloy contents were designed considering the effects of different contents of C,Mn and corrosion-resistant elements on the mechanical properties and weathering resistance of H-beam.The transformation point and austenite continuous cooling transformation curve of the experimental steel(No.3 steel)were measured.The results showed that under the condition of no deformation,when the cooling rate was greater than 1?/s,the experimental steel can obtain the full bainite structure;under the condition of deformation,when the cooling rate was 1 ?/s,duplex microstructure with ferrite and bainite was obtained.With increasing the cooling rate,the volume fraction of bainite increased,while the volume fraction of ferrite decreased.When the cooling rate increased to 5?/s,the microstructure was full bainite;Deformation increased the ferrite transformation temperature,and inhibited the transformation of bainite.(2)The deformation behavior of experimental steel(No.3 steel)at high temperatures was studied.The influence of deformation temperatures and deformation rates on the deformation resistance of experimental steel was explored.The constitutive equation of experimental steel was established,and the deformation resistance model was regressed to provide theoretical guidance for the formulation of rolling process parameters and industrial production.The result showed that the recrystallization activation energy of the experimental steel was 399 kJ/mol.When the deformation temperature was less than 1100?,the dynamic recrystallization was difficult to occur.Because of the addition of Nb microalloy elements,the end temperature of non-recrystallization was increased and the rolling process window of H-beam in nonrecrystallization zone on universal rolling mill was enlarged.(3)The rolling process of simulated H-beam flange was applied to the three kinds of experimental steels with different alloy composition.The experimental results showed that it was feasible to increase Si and Mn content based on C-reduction,low P and S,and to adopt Nb and V microalloying.The composition of No.3 experimental steel was optimized.The optimum rolling process was that the reduction rate was greater than or equal to 88%,the final rolling temperature was 850?870?,the microstructure was ferrite and bainite,and the excellent mechanical property was ReL=579 MPa?Rm=877 MPa?A=21%?Akv(-40?)=185 J.All mechanical properties met the standard requirements.(4)The contribution of different strengthening mechanisms to the yield strength of the experimental steel was calculated by comparing the microstructure of the experimental steel with different compositions and volume fraction of retained austenite.The strengthening and toughening mechanism of the optimal composition of experimental steel(No.3 steel)were revealed.The results showed that the strengthening mechanism of experimental steel was mainly solution strengthening,precipitation strengthening,fine grain strengthening and dislocation strengthening.Under the optimal rolling process,the average grain size of ferrite was 4.31 ?m,and the fine grain strengthening was 230 MPa,accounting for 39.73%of the yield strength,toughness and strength of the experimental steel were significantly improved.At the same time,the crack tip stress was reduced owing to the 3.64%retained austenite and the dispersed M/A island structure with the size less than 1.6?m,which can consume a part of the propagation work,and effectively improve the low-temperature toughness of the steel.(5)The accelerated corrosion test was applied to investigate the corrosion resistance behavior and mechanism of the experimental steel(No.3 steel)under the simulated industrial atmosphere.The results showed that the corrosion rate of No.3 experimental steel was 54.4%relative to Q345B after 72 hours,which met the requirements of the standard.The atmospheric corrosion resistance process was composed of two stages:the stage of rapid decline of corrosion rate and the stage of stable corrosion rate.The corrosion rate decreased with the extension of the corrosion period,and finally reached stability.After 240 hours of corrosion,the weathering elements Cr,Cu and Ni were enriched in different degrees in the rust layer.Cu and Cr were mainly enriched in the inner rust layer.The peak content of Cu element enrichment reached 8.5%,and the peak content of Cr element enrichment reached 9.0%.The enrichment of corrosion-resistant elements Cu,Cr,Ni in the rust layer promoted the formation of oc-FeOOH in the dense rust layer,which effectively hindered the corrosion of corrosive ions to the matrix and improved the atmospheric corrosion resistance of the experimental steel. |
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