| Bainite steels were considered as promising candidates for the next generation of premium wheel materials because of its good strength-toughness matching and superior rolling contact fatigue resistance.Exploring the wear mechanism of bainite steel and improving its wear resistance has become one of the hotspots in the research of wheel materials.Here,the relationship between microstructure and wear properties of self-developed bainite wheel steel has been discussed,and wear resistance improvement of wheel steel by tailoring microstructure has been studied.The main contents of present work include composition and phase transformation characteristics,relationship between microstructure and properties,wear resistance improvement,industrial trial production,and heavy load line service.Results about the role of elements and phase transformation characteristics of employed steel shows that the Fe-C-Mn-Si-Ni-Mo-Cu-V carbide-free bainite(CFB)wheel steel demonstrates high bainite hardenability.Bainite or bainite/martensite multiphase can be obtained in a wide continuous cooling rate range 0.25-30℃/s.The bainite obtained during continuous cooling process includes granular bainite(GB),lath-shaped bainite(Lath-B)and leaf-shaped bainite(Leaf-B).After isothermal holding at different temperatures,the microstructure is mainly composed of GB,Lath-B,Leaf-B and martensite/austenite(M/A)island.The isothermal transformation can be accelerated through prior continuous cooling transformed Leaf-B.In addition,the existence of Leaf-B can improve the strength and toughness of isothermal transformed bainite wheel steel simultaneously.Using machine learning method,predict models were built for retained austenite(RA)content and yield strength of CFB steels based on the correlation analysis of composition,process,microstructure and properties.The model predicts that it is possible to obtain the most RA volume fraction in bainite wheel steel when isothermal holding at 370-410℃.The error between the predicted yield strength and the experimental yield strength is within 4%for some Mn-series CFB steel,which means that the model has high accuracy.Result of sliding wear test shows that bainite wheel steel has better wear resistance than CL65 pearlite wheel steel.The main failure form of both wheel steels is abrasive wear.Weight loss of tested steels is not only related to the initial hardness,but also closely related to the hardness of the surface after work hardening.The higher the work hardening ability of steel,the higher the potential to improve its wear resistance.RA in bainite steel improves the wear resistance by enhancing its work hardening ability.High RA content steel(MC-BIT steel)performs greater wear resistance than that of the steel with low RA content(MC-QT steel),although MC-BIT steel has a lower hardness than MC-QT steel.Moreover,vanadium addition significantly improves the wear resistance of bainite steel.The weight loss of LCV steel(with vanadium)is lower than that of LC steel(without vanadium)with equivalent strength level.LCV steel shows higher surface hardness after wear test.Similar to the quenching process of existing pearlite wheel,bainite wheel’s quenching process during industrial production includes two stages,intermittent water spraying at first and then air cooling.This water spraying process can avoid the appearance of high temperature ferrite in wheel rim.The following slow air cooling can not only promote bainite ferrite formation,but also improve the stability of RA.Corresponding microstructure of steels at wheel rim is bainite with different morphologies,martensite and RA.The tempering process of wheel steel was designed based on related microstructure evolution.RA stability in steel was further enhanced by 300℃tempering treatment.As-treated bainite wheel steel shows excellent strength-toughness matching.The yield strength mainly comes from~′phase(containing martensite and bainite ferrite).The increase of dislocation density and the decrease of effective grain size are beneficial for yield strength improvement of~′phase.Bainite packets with various direction,RA and plastic deformation all play an important role in improving the toughness.In rolling wear test,the wear resistance of industrial bainite wheel steel is better than that of CL65 pearlite wheel steel and while slightly weaker than that of CL70pearlite wheel steel.Compared with CL70 pearlite wheel steel,there is big space for bainite wheel steel’s work hardening ability improvement further.In addition,actual service result of bainite wheel shows that some cracks can be observed at the contact zone between wheel tread and brake shoe.These are thermal fatigue cracks caused by the friction during braking.Therefore,attention should be also paid to thermal stability of microstructure while further improving the wear resistance of wheel steel. |
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