Research On Strength And Toughness Properties Of Axle Steel With Nb-B Compound Microalloying Of High-speed Train

Nonuniform grain size and insufficient hardenability,which reduce strength,toughness and fatigue property of steel,are the main problems of 25CrMo large diameter axle of high-speed train.In order to solve the above problems,Nb and B elements are added in the experimental steels.The addition of Nb can refine grains and improve strength and toughness.Meanwhile,the refined grains resulting from Nb addition reduce hardenability.The addition of a little B in Nb containing steel can improve the hardenability and reduce the amount of big size ferrite in axle.Normally,the hot ductility of Nb containing steel is bad and the addition of B can improve the hot ductility of Nb containing steel.Firstly,the effect of compound addition of Nb-B on hot ductility of the experimental steel is studied.Secondly,the dissolution and precipitation behaviors of boron containing particles and their effects on hardenability and toughness under different heat treatment processes are studied,since the effect of boron in steel has relationship with heat treatment parameters.Based on the above studies,the effects of Mo and V contents on microstructure and precipitates of the experimental steels are also studied.All the above studies provide theoretical and technical supports for optimizing the chemical composition,hot working technology and heat treatment process of axle steel.When the strain rate is 0.5 s-1,the results of the hot tensile test show that the hot ductility is not deteriorated with addition of 0.04%Nb.With increasing boron content from 20 ppm to 36 ppm into Nb containing steel,the hot ductility is improved significantly due to the increment of boron atoms segeragating at grain boundaries.With addition of boron into Nb containing steel,the hot ductility of?+? two phase region is improved due to the inhabitation of ferrite transformation.The compound addition of Nb and B improves hot ductility of experimental steel significantly with relatively high strain rate,0.5 s-1.The study results of the effects of heat treatment processes on hardenability and toughness of Nb-B containing steels show that the hardenability of the experimental steel reduces with increasing boron content,austenitizing temperature or holding time.The higher austenitizing temperature or longer holding time promotes the dissolution of M2B into austenite.Excessive boron atoms segregating at grain boundaries promote the precipitation of new M2B particles which deteriorates the hardenability of boron containing steel.Meanwhile,the excessive boron atoms segregating at grain boundaries also promote precipitation of M2B during tempering.The double quenching process with lower austenitizing temperature improves the hardenability and toughness.The carbides coarsen with increasing tempering temperature or holding time.The carbides precipitating at grain boundaries and lath boundaries are larger than those precipitating in matrix.The main carbides in the experimental steel are M23C6,M7C3 and MC.M23C6 carbides tend to precipitate at grain boundaries and sub-grain boundaries.M7C3 and MC carbides tend to precipitate in matrix.The coarsening behavior of martensite structure during tempering reduces the amount of large size M23C6 carbides precipitating at boundaries,which reduces the negative effect of large size carbides on toughness.The excessive boron atoms segregating at grain boundaries also promote precipitation of M2B during tempering and reduce toughness.Hence,the tempering temperature and holding time should be optimized to reduce the precipitation of M2B.The effects of Mo and V contents on microstructure,precipitates and mechanical properties have also been investigated.The results show that the tensile strength increases and impact toughness decreases with increasing V content from 0%to 0.12%.According to thermodynamic calculation results,the amount of M23C6 carbides decreases and MC carbides increases with increasing V content.The fine MC carbides improve precipitation strengthening.Moreover,the solid dissolved V atoms also increase with increasing V content in the experimental steel.The compound effects of precipitation strengthening and solid solution strengthening deteriorate toughness.The martensite effective grain size and lath width are refined with increasing Mo content from 0%to 0.50%in 25CrMoNbB experimental steel.The CCT curves calculated by J Mat Pro show that the ferrite and pearlite transformation is delayed with increasing Mo content from 0%to 0.50%.Meanwhile,the Ms temperature is reduced.The martensite structure is refine and the dislocation density is increased with increasing Mo content.According to thermodynamic calculation results,the amount of M7C3 carbides reduces with increasing Mo content from 0%to 0.50%.The amount of MC carbides increases with increasing Mo content from 0%to 0.25%.The amount of M23C6 carbides increases with increasing Mo content from 0.25%to 0.50%.The fine MC carbides have obvious precipitation strengthening and deteriorate toughness significantly.Hence,the tensile strength increases and impact toughness decreases with increasing Mo content from 0%to 0.25%.Although the amount of M23C6 carbides increases with increasing Mo content from 0.25%to 0.50%,there is no obvious coarsening of M23C6 carbides due to the increment of carbides nucleation sites resulting from martensite structure refinement and the dislocation density increment.Besides,the improvement of thermal stability of M23C6 carbides enriched with Mo also contributes to carbides refinement.Moreover,the refined martensite structure is beneficial to improve toughness.Therefore,the strength is increased and the toughness is not deteriorated with increasing Mo content from 0.25%to 0.50%.The experimental steel 25CrMoNbB containing 0.50%Mo has excellent fatigue property.