Research On Strengthening And Toughening Mechanisms And Corrosion Resistance Properties Of High Performance Bridge Steels

High-performance bridge steel is designed with low-carbon microalloyed composition,high strength,high toughness,excellent weldability and good corrosion resistance,which has promoted the continuous upgrading of materials used for bridges in China and effectively promoted the development of large-span,large-width and heavy-load steel structural bridges in China.At the same time,it also requires the continuous improvement of the quality grade and thickness of high-performance bridge steels,and puts forward higher requirements of strength,toughness,weldability,corrosion resistance,and manufacturing process etc of high-performance bridge steels.This thesis has studied the strengthening,toughening,corrosion resistance mechanisms,obtained manufacturing processes,and carried out the research on mechanical properties,weldability,and corrosion resistance etc.Taking Q500q E steel as the research object,impact test and microstructure analysis were carried out by using optical microscope,scanning electron microscope,transmission electron microscope（TEM）,EBSD energy spectrum analyzer and tensile impact testing machine.The effect of different deformation pass distribution in austenite recrystallization zone on low temperature toughness of Q500q E steel due to rolling 56 mm thick steel plate with different width was studied.The results show that when the pass compression rate of austenite recrystallization zone of steel plate with a width of 3500 mm decreases,the effective grain size is larger,the proportion of large angle grain boundary decreases,the grain area of<001>orientation increases,and the low-temperature impact toughness is lower than that of steel plate with a width of2650 mm.However,the transverse and longitudinal strength difference of the steel plate is small,and the anisotropy is relatively reduced.When the deformation temperature and cooling rate are the same,the distribution of different deformation passes in the austenite recrystallization zone has little effect on the shape and quantity of M/A islands.Reducing the passes and increasing the compression ratio of the passes promote the uniformity,fine and dispersion of precipitated phases,which is beneficial to improving the low-temperature impact toughness of steel.Reducing the yield ratio of low carbon bainitic steel is a difficult point in TMCP process control at present.How to deal with the holding time between rolling and cooling,that is,relaxing time,is an important way to adjust the yield ratio.The effect of different relaxation time after rolling on the microstructure,second phase precipitation and mechanical properties of 500 MPa high strength bridge steel was studied by using optical microscope,transmission electron microscope（TEM）,EDS energy spectrum analyzer and tensile impact tester.The results show that the relaxing after rolling is beneficial to the refinement of bainite grains,and can obtain granular bainite and lath bainite dual-phase structure.With the extension of the relaxing time after rolling,lath bainite decreases,tensile strength decreases,the appearance,increase and growth process of granular bainite,and the yield strength firstly increases and then decreases.At the same time,the relaxing after rolling is also conducive to the precipitation of the second phase.With the extension of the relaxing time,the precipitation of the second phase is more sufficient,and the precipitates continue to gather and grow.When the relaxing time after rolling is 60～80 s,and then cooled to400℃at a speed of 20℃/s,good comprehensive mechanical properties can be obtained,and the properties meet the standard requirement of Q500q E.Thermo-mechanical control process（TMCP）is the key process for the manufacture of high-performance structural steel.Especially,the cooling rate is used to control different microstructure and obtain different quality grades of steel,which is the basis for the realization of flexible rolling of medium and heavy plates.The effect of cooling rate on microstructure,second phase precipitation and mechanical properties of high strength bridge steel was studied by means of static undercooled austenite continuous cooling transformation（CCT）curve test,thermal simulation test and industrial rolling test,using optical microscope,scanning electron microscope（SEM）,transmission electron microscope（TEM）,energy spectrum analyzer（EDS）and tensile and impact test.The results show that with the increase of cooling rate in the range of 1～50℃/s,the microstructure after cooling changes from massive ferrite+pearlite to granular bainite and lath bainite,but M/A islands are dispersed on the ferrite matrix or boundary.At the cooling rates above 5℃/s,with the increase of cooling rate,the bainitic ferrite grain is refined,the number of M/A islands is reduced,the size is refined,and the distribution is more dispersed.With the increase of cooling rate,the proportion of large angle grain boundaries in the microstructure decreases firstly and then increases.In the ferrite+pearlite structure after slow cooling at 1℃/s,a large number of massive ferrite has large angle orientation difference.In the cooling range of 10～50℃/s to form bainite structure,the increase of cooling rate promotes the formation of large angle grain boundaries（orientation difference angle>15°）.The increase of cooling rate hinders the precipitation of carbides and nitrides of Nb and Ti,and the morphology of intragranular precipitates changes from granular or irregular block to fine needle and spherical shape.When the cooling rate increases in the range of 1～30℃/s,the yield strength and tensile strength increase,the elongation decreases,and the impact absorpted energy firstly increases and then decreases.At the cooling rate of 1～30℃/s,the flexible process of Q420q E,Q500q E and Q550q E industrial production can be realized by adjusting the cooling rate.In order to study the corrosion resistance of high performance bridge steels Q500q E,Q370q D and normalized Q370q E,the phase potential difference was measured,and the corrosion behavior of three test steels under marine atmospheric environment was simulated by electrochemical and salt spray test methods.The results show that the potential difference between bainitic ferrite phase and M/A island is lower than that between cementite phase and ferrite phase.Higher pearlite content and banded structure increase the number of micro-corrosion cells formed by cementite and ferrite.The corrosion products of the test steel are mainly composed ofα-Fe₂O₃,γ-Fe OOH,Fe₃O₄andα-Fe OOH.More Cr,Ni,Cu and alloy elements are added to the Q500q E steel to promoteγ-Fe OOH transition toα-Fe OOH.The microstructure of Q370q D steel is more uniform than that of Q370q E steel,and the rust layer structure is more uniform and compact after long-term corrosion.The order of corrosion current density in 0.5%Na Cl solution is Q500q E<Q370q D<Q370q E,and the value of charge transfer resistance Rct is Q500q E>Q370q D>Q370q E.Description of corrosion resistance is in the sequence of Q500q E>Q370q D>Q370q E.The manufacturing process of high performance bridge steel is formed by smelting technology of ultra clean steel with low P,S and low inclusion,high-precision controlled rolling and controlled cooling technology（TMCP）,and high uniformity heat treatment technology.Q500q E bridge steel has been subjected to manual welding oblique Y-type crack resistance test,maximum hardness test,manual welding,gas shielded welding butt joint performance test,submerged arc welding different wire and flux matching and different heat inputs welding butt joint performance test.The results show that the welding joint has excellent low-temperature impact performance.