Effect Of Nb On Austenite Grain Size And Recrystallization Law Of U71Mn Heavy Rail Steel

U71Mn steel is widely used in railway transportation because of its excellent performance.However,the rapid development of railway transportation industry has increased the performance requirements of rail.Microalloying is an economical and effective method to improve the performance of rail.As a residual element in iron ore,Nb exists in heavy rail steel,and its existence has a certain influence on austenite grain size,second phase precipitation and recrystallization behavior.This thesis added different contents of Nb element to U71Mn steel by microalloying technology,and analyzed the effects of Nb element on heating behavior and recrystallization process of U71Mn steel.It can provide a theoretical basis for the composition and production process parameter optimization of Nb-containing U71Mn steel in actual production.This thesis was based on the composition ratio of U71Mn steel,added different proportions of Nb in the steel（Nb content is 0%,0.015%,0.02%,respectively,mass fraction）.Obtained different Nb contents experimental steels by smelting（No.1#,2#,3#）.First,calculated the complete solution temperature of Nb C in the steel by JMat Pro software.On this basis,formulated heat treatment processes to perform different heat treatments on the experimental steel,and compared the influence of Nb on the microstructure and austenite grain size of experimental steel under different heat treatment conditions.Combined with the production process,this thesis analyzed the influence of different holding time on the solid solution law of Nb C at 1120°C.Secondly,combined with the actual production,used a laboratory two-high reversing rolling mill to carry out a single-pass rolling experiment,and explored the effect of Nb element on recrystallization behavior.Furthermore,used the Gleeble-3500 thermal simulator to carry out high-temperature hot compression experiment on the experimental steel,and used the the stress-strain curve to study the effect of Nb on the thermal deformation behavior at high temperature.Finally,combined with the computational deduction,this thesis studied the precipitation kinetics of the second phase Nb C under uniform nucleation,grain boundary nucleation and dislocation nucleation.And analyzed the law of Nb C precipitation changing with temperature and time,as well as the law of relative nucleation rate changing with temperature.By comparing the microstructure of the three experimental steels after quenching at the same temperature,it can be seen that the microstructure of the quenched samples are acicular martensite+residual austenite.At the same temperature,the average grain size decreases with the increase of Nb content in steel.For example,at 1250℃,compared with the grain size of 1#steel,the grain size of 2#steel decreases by 20.7%,and the 3#steel decreases by 31.5%.At the same holding time,the average grain size decreases with the increase of Nb content.For example,at 1120℃for 1h,compared with the grain size of 2#steel,the grain size of 3#steel decreases by 15.7%.At the same time,the increase of Nb content will also reduce the Nb solution ratio.For example,the solution ratio of 2#and 3#steel is 52.3%and 42.6%respectively after holding for one hour.Then observe the second phase particles microscopic observation and diffraction spot by TEM,it can be seen that the second phase is Nb C.The single-pass hot compression experimental study shows that when the deformation temperature is 870℃,the deformation is 70%,and the strain rate is 0.1s^-1,the work hardening of the 1#steel and the 3#steel is dominant,and the complete recrystallization has not been completed.In the same strain the stress of Nb-containing steel is larger.The double-pass hot compression experiment shows that under the same holding time,the softening rate decreases with the increase of Nb content.For example,under 120s,the softening rate of 1#steel is 86.1%,and the softening rate of 3#steel is 45.7%.By comparing the grain size of the experimental steel at the same final rolling temperature,it can be seen that the average grain size decreases with the increase of the Nb content in the steel.For example,at 870℃,compared with the grain size of 1#steel,the grain size of 2#steel decreases by 22.8%,and the 3#steel decreases by 32.8%.The study of the second phase precipitation kinetics shows that in the precipitation curve,it can be seen that at the temperature corresponding to the fastest precipitation time is about 760℃for uniform nucleation,180℃higher for grain boundary nucleation and40℃lower for dislocation nucleation.Increasing the deformation storage energy will increase the temperature.