Study On Controlling Of Mechanical And Corrostion-resistance Proeperties Of Q500EHPS Steel For Bridge

The morden steel bridges in China need to be built with large span and integral welding connections and satisfy the requirements for supporting high-speed and heavy load.Correspondingly,the current bridge steels should be upgraded to a high-performance level,which is constituted by high strength,excelent toughness,low yield ratio,good weldability and corrosion resistance.Among them,the high-performance Q500 EPHS steel with yield strength of 500 MPa class has been chosen as the key construction material for building Hutong Yantze River Bridge and Wuhu Second Yantze River Bridge,which both are the large-span,heavy-loaded and high-spead railway bridges.Developing such a Q500 EPHS steel and simultaneously balancing all the described performances came crossed a great technical chanllenge.This dissertation systematically dealt with the effects of chemical compositions of alloying elements and rolling parameters on the microstructure and mechanical properties of Q500 EPHS steel,discussed their relationships,as well as optimized the chemical compositions and rolling parameters.The effects of Cr-Ni-Cu alloying elements,microstructure and non-metallic inclusions on the uniform and pitting corrosion behaviors were also investigated.The systematic solutions to controlling the comprehensive mechanical and corrosion-resistance properties were determined accordingly.Based the above investigations,the industrially trial production of Q500 EPHS steel plates and their property evaluations were finally carried out.Firstly,the effects of C,Cr,Mo,Nb,etc on the tensile and impact properties of Q500 EHPS steel were examed via smelting and rolling in laboratory,and the optimized chemical compositions satisfying the overall mechanical properties were determined as 0.055C-0.25Si-1.55Mn-0.20Cu-0.15Cr-0.25Ni-0.20Mo-0.040Nb-0.030V-0.015 Ti.The effects of thermal-mechanical controlled processing(TMCP)papameters on microstructure and comprehensive mechanical properties were investigated by simulating the TMCP of the optimizaed steel in Gleeble-3500 system,micro-sample tension,standard impact and microstructure characterization.The fininish rolling temperature(FRT),start cooling temperature(SCT),controlled cooling rate(CCR)and re-reddening temperature(RRT),as the key TMCP parameters were involved.The results reveal that an excellent mechanical property combination of high yield strength(Rp0.2?500 MPa),high impact energy at-40 oC(Ej?200 J)and low yield ratio(YR?0.84)can be obtained while the TMCP parameters are controlled at FRT =830 °C,SCT=780 °C,CCR=15-25 °C/s and RRT=500-550 °C.It is also found that the yield strength is strongly influenced by the grain boundaries with low misorientation angle,dislocations,precipitated particles and M/A islands.The mean equivalent diameter of quasi-polygonal ferrite and granular bainite ferrite grains grains with boundaries at low misorientation tolerance angle 2°-15°,MED-1/22°???15°,has a linear relationship with the yield strength,which is accordant with Hall-Petch equation.The contribution of M/A islands to tensile strength is larger than that to yield strength,resulting in an increase of YR.The low temperature toughness can be improved with the refinement of quasi-polygonal ferrite,granular bainite ferrite and M/A islands,as well as an increase of the ratio of high angle grain boundaries(MTA>15°).The effects of Cr-N i-Cu alloying elements,microstructure and non-metallic inclusions on the uniform and pitting corrosion behaviors,as well as the mechanisms in controlling corrosion behaviors were also investigated using electrochemical polarization curve measurement,weight loss calculation from cyclic dry/wet and immersion testing,SEM/EDS/XRD characterization of rust layer,and in-situ LSCM observation.The results indicate that the Cr-Ni-Cu alloying elements can enhance the thermal stability of the steel by enriching in the rust layer,increase the valume rato of stab le ?-FeOOH phase to metastable ?-FeOOH phase and make the inner rust layer more dense and stable,which leads to an improved corrosion resistance.It is also indicated that the corrosion resistance is dependent on the microstructure morphololy of experimental steel with an identical chemical composition.In detail,the corrosion resistance of mixed microstructure of QPF+GBF+M/A is superior to that of the dual-phase microstructures of F+P and F+M,but inferior to that of the homogeneous microstructure of bainite.The initial pitting corrosion caused by the spherical Ca-Al-O-S complex inclusion is an essential galvanic corrosion,and the pitting corrosion of experiemenal steel in 0.01 M NaHSO3 is much slighter than that in 3% NaCl solution.It is further revealed that the sulfide contained in the Ca-Al-O-S complex inclusion plays an important role in initiating the pitting corrosion due to the mechanism of enhancing the conduction of electricity.The industrially trial production of Q500 EPHS steel plates was finally carried out based the above investigations.The results demonstrate that the overall mechanical properties of Q500 EHPS have fully satisfied the requirements regulated by the standard of GB/T 714 with the Rp0.2?500 MPa,the YR?0.85 and the Ej ?200 J at-40 oC,indicating an excelant combination of strength and tougness.This newly developed steel is easy to be welded with the Ceq(%)?0.45 and the Pcm(%)?0.20 and has fully satisfied the welding process specification.It also has a better corrosion resistance than a plain steel for bridge with an elevated corrosion resistant index of IJ ?1.0.