Effect Of Alloying And Microstructure On Corrosion Resistance Of Ultra-low Carbon Bainite Rail Steel

Corrosion of rails seriously affects the safety and comfort of railway transport.In the past,the strength and corrosion resistance can be improved via reducing the interlamellar spacing of pearlite,but the existing pearlite rails are difficult to meet the demands of highspeed railway.Ultra-low carbon bainitic steel(ULCB steel)has been attracted widespread attention due to their excellent low temperature toughness,weldability and corrosion resistance.Based on the research of Mn-series ULCB steels,the relationships between "composition,process,structure and property" of ULCB were studied.The microalloying and heat treatment processed were performed to offset the loss of strength caused by ultralow carbon content,and further to improve its corrosion resistance.Based on heat treatment process,the microstructure of ULCB steel was controlled.Combined with finite element simulation,the oil cooling process was finally obtained to attain the best combinations of strength and toughness: tensile strength: 910 MPa,total elongation: 20.1%,and room temperature impact: 190 J.In order to meet strength requirements of rails for passenger dedicated lines,namely tensile strength meets 980 MPa,the chemical composition of ULCB steel was designed via micro-alloying of adding0.1 wt.% V and 0.2 wt.% V.The tensile strength of V micro-alloyed ULCB steels exceeds1000 MPa after treating by oil cooling process,companied with excellent ductility and toughness.Corrosion behaviors of 9 kinds of test steels,including ULCB steels treated by four different heat treatments,ULCB steels with different V contents,U20 Mn bainitic rail steel,and U75 V pearlitic rail steel,were carried out by periodic immersion corrosion test,electrochemical test,and Kelvin probe force microscopy(KPFM)analysis.Results show that corrosion resistance of ULCB steel after annealing heat treatment is the best,with the corrosion rate of 0.023 mm/y.With the increase of cooling rate,the corrosion resistance of ULCB steels is reduced.The corrosion rate of water cooled ULCB steel is0.095 mm/y,which is the lowest among the four ULCB steels with different cooling rate.The grain size or lath width is refined from 9.1 μm to 1.5 μm with the increase of cooling rate,resulting in increased density of micro-batteries and enhanced corrosion tendency.It is found that the corrosion rate of oil cooled ULCB steel is reduced from 0.077 mm/y to 0.059 and 0.033 mm/y via addition of 0.1wt.% and 0.2 wt.% V,respectively.The lattice parameters of bcc phase in the ULCB steel with different V content are 2.86769 ?,2.86479 ? and 2.86410 ?,respectively,which is resulted from precipitation of V(C,N)in the micro-alloyed ULCB steels.The precipitation consumes C,resulting in decreased average content of C in the lath matrix,which is good for the corrosion resistance.The average C content in U20 Mn steel is 0.2 wt.%,while that in U75 V steel is 0.71 wt.% ～0.8 wt.%.The corrosion rates of U20 Mn and U75 V rail steels are 0.27 mm/y and 0.33mm/y,respectively,which is much higher than ULCB steels.It reveals that with the increase of C content,the density of the micro-battery is increased,and the corrosion tendency is enhanced.Based on the above findings,the influence of microstructure on the capability and density of micro-batteries was discussed.Consequently,the micro-mechanism of improving the corrosion resistance of bainitic rail steel was uncovered.