| High-carbon chromium bearing steel has been widely used for bearings of precision machine tools,rail transit,mining machinery and other fields due to its good comprehensive property,simple production process and low cost.Given the severe working conditions and long service life requirement of bearings,the performance optimization of high carbon chromium bearing steel has been a focus of continuous research in the materials filed,especially the improvement of its metallurgical quality.In the past few decades,technical progress in steel metallurgy has significantly improved the metallurgical quality of high-carbon chromium bearing steel,and steel cleanliness and non-metallic inclusions have been effectively controlled,but the contradiction between the demand of prolonging bearing fatigue life and the bottleneck of improving metallurgical quality of bearing steel has also become increasingly prominent.Theoretically,rare earth elements have the functions of purifying liquid steel,improving inclusions and microalloying.However,the previous high carbon chromium bearing steel treated by rare earth always encounters the problems of performance fluctuation and nozzle clogging.In consideration of the possible influence of inclusions in rare earth materials on metallurgical quality,studying the role of high purity rare earth metals in high clean bearing steel has an important guiding significance for the analysis of the mechanism of rare earth metals in steel and the development of long-life rare earth bearing steel.Based on the possible failure modes of bearings in service and quality requirements of bearing steel,the influence mechanism of high purity rare earth metals on inclusions,microstructure,impact toughness and fatigue property of bearing steel was systematically investigated in the thesis.The main research contents and conclusions are summarized as follows:The inclusions in experimental bearing steels with different rare earth contents were analyzed,on the basis of which the modification behaviors of rare earth to inclusions and the evolution sequence of inclusion type were discussed.The results indicate that rare earth elements can modify Al2O3 and MnS inclusions in high-carbon chromium bearing steel into rare earth inclusions.For bearing steels with low S/O ratio,the reaction sequence of rare earth elements and inclusion forming elements appears to be O,S,As,P and C successively.The evolution sequence of rare earth inclusion type is mainly RE2O3?RE2O2S?RES?RE-O-S-As?RE-S-As?RE-S-As-P?RE-O-S-As-P-C?RE-O-As-P-C?RE-O-P-C and RE-O-C.By contrast,For bearing steel with high S/O ratio,trace amount of rare earth tends to preferentially modify MnS inclusions into RE3S4 inclusions in bearing steel.RE3S4 inclusions can not only form independently or at the nuclei of Al2O3 inclusions during the steelmaking process,but also precipitate together with MnS inclusions in the form of RE3S4·yMnS(y<1)complicated inclusions during the solidification process,also taking Al2O3 inclusions as the nuclei.Under the condition of complete modification of Al2O3 and MnS inclusions,the evolution sequence of rare earth inclusion type is expected to be RE2O3,RE2O2S,RES,RE-S-As,RE-As(-P)/RE-O-As(-P),RE-P-C/RE-O-P(-C)and RE-O-C.The relatively high As and P contents as well as low oxygen content in high S/O bearing steels increase the possibility of the formation of simple rare earth inclusions without oxygen,thereby leading to the different evolution sequence of inclusion type with that in low S/O bearing steels.The inclusions,microstructure,grain size and impact property of bearing steels with different rare earth contents were systematically studied to clarify the influence of rare earth on impact toughness.The results reveal that excessive rare earth addition to bearing steel will generate a large amount of rare earth inclusions containing C and reduce carbide size and the volume fraction of carbides under the conventional heat treatment process.Apart from that,however,rare earth addition has no obvious effect on the phase fraction,carbide size and grain size of bearing steel.An appropriate amount of rare earth addition may modify elongated MnS inclusion and its complex inclusion into regular rare earth inclusions distributed evenly in bearing steel,thus significantly improving the impact property and isotropy of bearing steel without rare earth addition.The increase of rare earth content within a certain range can weaken grain boundary segregation of harmful elements and strengthen grain boundary by enhancing the forming ability of rare earth inclusions containing As and P.on the other hand,the increasing trend of inclusion size and the volume fraction of inclusions also promotes the variation of impact crack growth path.Both of them are able to improve the transverse and longitudinal impact absorbed energy.However,excessive rare earth addition results in the formation of a great amount of large rare earth inclusions,which may cause grain boundary cracks and promote crack growth.As a result,the impact property of bearing steel is seriously deteriorated.Very high cycle fatigue properties of industrial bearing steel ingots were investigated by ultrasonic fatigue test,and the mechanism of rare earth elements in very high cycle fatigue failure of bearing steel was analyzed.The results show that rare earth addition can reduce inclusion size and the volume fraction of inclusions,which increases the fatigue limit of rare earth bearing steel at 109 cycles by about 9.4%and prolongs its fatigue life by more than 10 times.Complex rare earth inclusions derived from the modification of rare earth to CaO-Al2O3-MgO-SiO2-CaS inclusions in bearing steel have weak internal binding force and interface binding force with the matrix,so the crack initiation life of rare earth bearing steel at inclusions is short.However,small rare earth inclusions may give rise to large fine grain area,making the crack propagation life of rare earth bearing steel much longer than bearing steel without rare earth addition.Transverse and longitudinal fatigue properties of continuous casting bearing steels were explored in the very high cycle fatigue regime,which helps reveal crack initiation and propagation behavior caused by inclusions with different shape and the influence mechanism of rare earth elements.The results show that very high cycle fatigue failure caused by inclusion particle indicates almost synchronous crack propagation in all directions from inclusion particle,and the length-width ratio of instant crack keeps close to 1.In contrast,for very high cycle fatigue failure induced by long strip type inclusion,the initial crack initiates in the wider region of the inclusion strip,and the width of instant crack plays an important role in crack propagation.The length-width ratio of instant crack continuously decreases with crack propagation until it approaches 1 or crack reaches the edge of fatigue specimen.In the very high cycle fatigue regime,effective inclusion area and effective inclusion size can be determined from the perspective of crack initiation and propagation.For very high cycle fatigue fracture surface with FGA,the effective inclusion area corresponds exactly to the inclusion area included in FGA.Rare earth addition may modify common inclusions into complex rare earth inclusions that deform more easily during hot rolling to reduce the effective inclusion size at fatigue source.Therefore,rare earth addition can improve the fatigue property of continuous casting bearing steel,especially the longitudinal fatigue property. |
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