Phase Transition Kinetics And Microstructure Transformation Of CL60 Steel With Trace Mo Element During Cooling

CL60 steel is a commonly used material for rail vehicles.Today,microalloying is an important way to improve the wheel performance.As a trace element in steel,Mo element can improve the overall performance of the wheel steel,but it can make the cooling transition curve shift to the right,and the tendency of the medium and low temperature to transform the structure increases at the wheel rim.Therefore,this paper used the expansion method,metallographic method and differential thermal analysis method to determine the continuous cooling curve and isothermal transformation curve of the forged CL60 steel with three Mo contents of 0.01,0.03 and 0.05%.The phase transition kinetics of continuous cooling,the isothermal transformation and the process of tissue transformation are studied.We obtained the effect of different content of Mo element on the phase transformation kinetics and microstructure transformation of CL60 steel,which could provide a theoretical basis for the formulation of water-cooling heat treatment process for trace amount of Mo element CL60 steel and the control of wheel rim microstructure.The continuous cooling curves of three kinds of Mo content CL60 steel in the range of 0.5-50°C/s were determined by expansion method and differential thermal analysis.The analysis of room temperature transformation showed that the critical cooling rate of ferrite and pearlite high temperature transformation microstructure or bainite and martensite non-high temperature transformation microstructure decreased gradually with the increase of Mo content.The specific parameters were as follows :(1)0.01% Mo CL60 steel,bainite structure is formed at a cooling rate of 5 °C/s,martensite structure is formed at a cooling rate of 7 °C/s;(2)0.03% Mo CL60 steel,Bainite structure is formed at a cooling rate of 5 °C/s,and martensite structure is formed at a cooling rate of 10 °C/s.(3)0.05% Mo CL60 steel,bainite structure started at a cooling rate of 2 °C/s.When the cooling rate is 5 °C/s,martensite structure starts to appear.Therefore,with the increase of the cooling rate,the content of high temperature tissue decreases,the content of non-high temperature tissue and the macroscopic hardness of the room temperature tissue gradually increase.With the increase of Mo content,the tissue transitions move to high temperature,and the high temperature transition and the medium temperature transition temperature range have a combined trend.The isothermal transformation curves of three kinds of Mo content CL60 steel in the range of 480-680 °C were determined by metallographic method and differential thermal analysis.It is found that the supercooled austenite gradually transforms into ferrite and pearlite with the prolongation of the holding time until the complete ferrite and pearlite transformation structure are obtained during the isothermal holding period under the experimental conditions in this paper.The transition volume with time changes is typical "S" shape.As the isothermal temperature decreases,the content of ferrite transformed structure gradually decreases,and the pearlite mass and lamellar spacing become fine.On the single "C" curve of isothermal transformation,the nose tip temperature of the isothermal transformation curve of CL60 steel with 0.01,0.03,and 0.05% Mo was 500,555,and 590 °C,respectively.As the Mo content increases,the nose tip incubation period first decreases and then increases.By comparison and analysis of three trace elements of Mo element CL60,It was found that the addition of Mo inhibited the transformation of ferrite and pearlite structure;refined the spacing of pearlite and the size of pearlite cluster,effectively refining the grain.Besides,it inhibited the precipitation of cementite,causing the cementite sheet in the pearlite to exhibit a discontinuous distribution,and the hardness of the pearlite structure is lowered.