| The high-speed railway has been developing rapidly in our country,but the bearing for high-speed railway still depends on imports now.GCr15 bearing steel is often used to produce bearing rolling elements for high-speed railway.Because the research on GCr15 bearing steel starts quite late in our country.The quality of domestic GCr15 bearing steel is quite poorer compared with that in some developed countries,such as Sweden,Japan and Germany,which is one of the important reasons restricting the localization of bearing for high-speed railway.With the development of smelting process,the purity of domestic GCr15 bearing steel has been improved significantly.However,there is still a gap between our country and other developed countries in terms of hot working and heat treatment process of GCr15 bearing steel.This work is supported by the National High Technology Research and Development Program(863 program):key technology development of bearing steel for major equipment.In this work,the hot working and heat treatment process of GCr15 bearing steel for high-speed railway was studied.The purpose of this work is to reveal the microstructure evolution behavior of GCr15 bearing steel during the hot working and heat treatment process,and improve its microstructure and properties through the optimization of hot working and heat treatment process,and at last accumulates basic data for achieving the localization of GCr15 bearing steel for high-speed railway.The main works are as follow:(1)The equilibrium phase diagram of GCr15 bearing steel was calculated,and the continuous cooling transformation curve was measured,and the isothermal precipitation behavior of proeutectoid carbide under the condition of hot deformation was analyzed,and the phase transformation behavior GCr15 bearing steel was clarified.The results indicated that under the equilibrium state,the precipitation temperature of proeutectoid carbide and transformation temperature of pearlite were within the range of 750?916? and 740?750?,respectively.Under the condition of continuous cooling,the precipitation temperature of proeutectoid carbide and transformation temperature of pearlite were within the range of 568?869? and 492?671?,respectively.In addition,when the continuous cooling rate exceeded 3?/s,the microstructure was martensite.In the cooling process after hot deformation,the proeutectoid carbide precipitated little at the temperature above 810?,and it precipitated obviously within the range of 700-770?.(2)The hot rolling process of GCr15 bearing steel was simulated with thermo-mechanical simulator,and the microstructure evolution behavior was analyzed.Based on the simulated results,the experiment of thermo-mechanical control process was carried out.The results showed that when the final hot deformation temperature decreased from 870? to 770?,both the pearlite colony size and grain size decreased,and the pearlite interlamellar spacing increased slightly,and the degree of carbide network decreased first and then increased;when the cooling rate increased from 2?/s to 25?/s,the pearlite colony size,grain size and pearlite interlamellar decreased slightly,and the precipitation of proeutectoid carbide was inhibited obviously as the cooling rate exceeded 8?/s;when the final cooling temperature decreased from 700? to 600?,the grain size changed slightly,and both the pearlite colony size and degree of carbide network decreased slightly,and martensite appears in the microstructure when the final cooling temperature was below 500?.In order to obtain finer hot rolled microstructure and decrease the degree of carbide network,the final rolling temperature can be controlled around 830?,and the water-cooling rate can be set to 10?/s,and the final water-cooling temperature can be set to 680?.The DICTRA software was used to analyze the precipitation behavior of proeutectoid carbide in the continuous cooling process,and it was indicated that the precipitation rate of proeutectoid carbide increased first and then decreased upon the cooling process after hot rolling,and a carbon-depleted region will be formed in the austenite contacted with proeutectoid carbide,which corresponds to the thin ferrite film at room temperature.(3)The effects of isothermal spheroidization annealing parameters and initial hot rolled microstructure parameters on the final spheroidized microstructure of GCr15 bearing steel were studied.The relationship between the undissolved carbide formed in the austenitizing process and the final spheroidized carbide was analyzed.The microstructure evolution behavior during isothermal spheroidization annealing process was analyzed.A mathematical model about the relationship between mean diameter of spheroidized carbide and number of carbide per unit area was proposed.The results showed that when the austenitizing temperature increased from 760? to 880?,or the isothermal temperature increased from 680? to 720?,the mean diameter of spheroidized carbide increased,and with the prolonging of austenitizing time or isothermal time,the mean diameter of spheroidized carbide also increased.There is a positive correlation between the undissolved carbide and the final spheroidized carbide in terms of number of carbide per unit area and carbide size.The refining of pearlite interlamellar spacing of initial hot rolled microstructure is beneficial to obtaining finer and dispersed spheroidized carbide.During the austenitizing process of isothermal spheroidization annealing,accompanied with the carbide dissolution,the carbide lamellae will fragment and granulate gradually,and with the prolonging of austenitizing time,the size of undissolved carbides increased;During the cooling process after austenitization and initial stage of isothermal process,austenite will transform into ferrite and granular carbide through divorced eutectoid transformation;At the final stage of isothermal process,the carbides with larger size will coarsen at the expense of carbides with smaller size,and the number of carbide per unit area decreases continuously.(4)The online spheroidization annealing process of GCr1 5 bearing steel was proposed,which includes online intercritical and subcritical spheroidization annealing.For the online intercritical spheroidization annealing,GCr15 bearing steel can be cooled to 650? by water after hot rolling,and then heated to 800? for isothermal spheroidization annealing;For the online subcritical spheroidization annealing,it can be cooled to 450?500? after hot rolling,which will form degenerate pearlite,and then heated to around 700? for,subcritical annealing.Compared with the intercritical spheroidization annealing,the online subcritical spheroidization annealing can obtain finer spheroidized carbide and smaller ferrite grain size.(5)The effects of austenitizing parameters,tempering parameters and initial spheroidized microstructure on the quenched microstructure and property of GCr15 bearing steel were studied,and carbide dissolution behavior and variation of size of undissolved carbide during the austenitizing process were analyzed.The results indicated that underhigher austenitizing temperature or longer austenitizing time,the volume fraction of undissolved carbide was smaller,and the prior austenite grain size was larger,which were detrimental to the final impact toughness.In the early stage of austenitizing process,the dissolution rate of carbide is quite large,and then it decreases gradually.The dissolution process of carbide in austenite is controlled by the diffusion of Cr element.During the austenitizing process,the mean diameter of undissolved carbide decreases first and then increases,and when the volume fraction of undissolved carbide is around 4.9%,the mean diameter of undissolved carbide has the smallest value.When the tempering temperature was within the range of 160? to 250?,with the increase of tempering temperature or prolonging of tempering time,the impact toughness of experimental steel improves slightly.In order to exert the effect of precipitation strengthening,the tempering temperature can be controlled within the range of 160? to 180?.In addition,the refinement of initial spheroidized carbide accelerates the dissolution of carbide during austenitizing process,and results in the refinement of undissolved carbide.There is a positive correlation between the ferrite grain size of initial spheroidized microstructure and the prior austenite grain size of quenched microstructure.(6)Spheroidized and quenched microstructures,with different microstructure parameters,were obtained through adjusting the hot rolling and spheroidization process.The fatigue property of experimental steels after low temperature tempering was measured.The results showed that compared with the process of air-cooling after rolling,after spheroidization and quenching,both the final prior austenite grain size and undissolved carbide size of the experimental steel in the process of water-cooling after rolling are smaller.For the experimental steels in the processes of air-cooling after rolling+isothermal spheroidization,and water-cooling after rolling+isothermal spheroidization,and water-cooling after rolling+online subcritical spheroidization annealing,when number of cycles to failure was 107,the corresponding fatigue limit was around 789,908 and 986 MPa,respectively.Compared with air-cooling after rolling+isothermal spheroidization,much finer quenched microstructure can be obtained in the processes of water-cooling after rolling+isothermal spheroidization and water-cooling after rolling+online subcritical spheroidization,which is beneficial to improve the fatigue property of experimental steel after low temperature tempering.(7)Using the GCrl5 bearing steel in the process of water-cooling after rolling+isothermal spheroidization,bearing sample was fabricated and then bench test was carried out by our cooperation partner.The duration time of bench test was 624 h and the maximum velocity was 300 km/h.The results of thermal performance and durability test indicated that the current of main motor was quite stable,and bearing sample run smoothly,and no abnormal temperature rise was observed during the bentch test.The bearing rolling elements and bearing race did not seize up,and the phenomena of color change,deformation and crack were not observed after test,which satisfy the requirements of bearing for high-speed railway. |
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