Reasearch On Microstructure Evolution And Controlling Of Network Carbide Of GCr15 Bearing Steel

Based on the conditions provided by Shaoguan Iron and Steel Co.,Ltd.,the thermal simulation of GCr15 bearing steel produced by Gleeble-3800 thermal simulator was studied.The complete CCT curve of the steel was measured by expansion method and metallographic method.This paper complements the work of our research group which has not measured the precipitation curve of the proeutectoid cementite,and the evolution process and regularity during its continuous cooling process was systematically studied.On this basis,the two-stage cooling process was adopted,and the effect of parameter(deformation,finishing rolling temperature and finishing cooling temperature)on the microstructure transformation was studied,which could provide a reference for inhibiting the precipitation of network carbide.The main work and results of this paper are as follows:(1)Through the Gleeble-3800 thermal simulator,the pearlite transformation and martensite transformation temperature and microstructure evolution of GCr15 bearing steel at different cooling rates were studied by expansion method.The results show that the pearlite phase transformation mainly occurs in the temperature range of570~710℃,and the phase transformation temperature decreases gradually with the increase of cooling rate.The critical cooling rate for complete pearlite phase transformation is 5℃/s.At low cooling rate,the structure at room temperature is mainly network carbide and pearlite.With the increase of cooling rate,the carbide network breaks,the pearlite microstructure is refined,and the microhardness increases,and martensite begins to form in the low temperature region.The phase transformation temperature is about 190℃.When the cooling rate reaches 8℃/s or above,the structure at room temperature is mainly martensite.(2)Based on the measured pearlite transformation start temperature,the Gleeble-3800 thermal simulator was used to measure the precipitation temperature of the proeutectoid cementite by the metallographic method,and the precipitation curve of proeutectoid cementite was supplemented.The dynamic CCT curve of GCr15 bearing steel was improved,and the precipitation mechanism of secondary carbide was studied and analyzed.The research shows that the CCT curve of GCr15 steel is mainly composed of five phase zones: single phase austenite zone,austenite+carbidezone,austenite+carbide+pearlite zone,carbide+pearlite zone and martensite zone.The precipitation temperature range of the proeutectoid cementite is about 700~900℃,and the critical cooling rate for inhibiting the precipitation of proeutectoid cementite is 8℃/s.when the cooling rate is greater than 3℃/s,the precipitation temperature of the proeutectoid cementite is lower than the pearlite phase transformation temperature,which indicates that the effect of cooling rate on the precipitation of the pre-eutectic cementite is greater than that on the pearlite transformation.The increase of cooling rate after rolling reduces the diffusion coefficient of C,Cr and other elements,and inhibits the diffusion to the grain boundary,reduces the precipitation of secondary carbide,and thus inhibits the formation of network carbide.(3)According to the complete CCT curve,the two-stage cooling process was adopted to suppress the precipitation of network carbides,and the effects of deformation,finishing rolling temperature and finishing cooling temperature on the precipitation of network carbide were studied.The experimental results show that the pearlite grain can be refined obviously by increasing the deformation,but it has little effect on improving the morphology of network carbide.The finish rolling temperature has obvious inhibitory effect on the precipitation of secondary carbide,and the effect of low temperature finishing rolling is remarkable,but the lower the temperature is,the greater the deformation resistance is,and the faults such as broken rolls are easy to occur,so the finishing temperature should not be too low.The precipitation degree of secondary carbide also reduces significantly with the decrease of the finish cooling temperature.When the finish cooling temperature decreases to600℃ or below,the precipitation amount of the secondary carbide is further inhibited,and dispersed as particles in the pearlite structure.Considering the above results,the precipitation of network carbide can be effectively inhibited by the two-stage cooling process of GCr15 bearing steel cooled to 600℃ with the cooling rate of 8℃/s after finishing rolling deformation at 820℃ and then slow cooling to room temperature at1℃/s.The pearlite structure required before spheroidizing annealing is obtained.