| Connecting-rod fracture splitting is the newest engine connecting-rod processinginternationally and it has irreplaceable superiority. The high duty materials are requiredfor decreasing the weight and improving the fatigue strength of engine connecting-rod.Appropriate brittleness and low ductile of connecting-rod materials can reduce thedistortion. Meanwhile, good machinability is required for increasing the life of machinetool, improving the surface quality of connecting-rod, and reducing the cost. In domestic,the research and development of connecting-rod fracture splitting technology just start andthe fracture splitting material also is mainly dependent on import. Therefore, thedevelopment of automobile engine fracture splitting materials with good performance hasbeen the focus of research and exploitation of automobile steels.In this paper, with the steel for engine fracture splitting connecting-rod as the studyobject, the effects of microalloying elements such as V, Ti and N on microstructure andmechanical properties of medium carbon microalloyed steel were studied. The V-Ti-Nmicroalloyed steel used for fracture splitting connecting-rod was developed. The effects ofdeoxidation and the composition of refining slag on its inclusion composition,morphology and distribution were studied by the refining process. The CCT diagram ofaustenite continuous cooling transformation was gained by the thermal expansion methods.Based on the diagram, the effects of the cooling rate after rolling and the forgingtemperature on microstructure and properties of the V-Ti-N microalloyed steel wereinvestigated. Finally, the fatigue performance of V-Ti-N microalloyed steel, C70S6BYand42CrMo for automobile engine connecting rod steel were comparative studied and theforming mechanism of fatigue cracks was explored. The following results were obtained:A new type of V-Ti-N microalloyed steel, whose chemical composition (wt%) wasC0.35～0.40，Si0.50～0.70，Mn0.90～1.20，S0.040～0.070，V0.20～0.30，Ti0.010～0.020， N0.0100～0.0150used for fracture splitting conrods was developed. Thestrengthening mechanism of microalloying elements mainly lie on that the V, Ti and Nelements precipitate a tiny V (C, N) phase in V-Ti-N microalloyed steel in the coolingprocess of rolling. At the same time, because of a suitable amount of Ti content, dispersedTiN particles could contribute to prevent austenitic graining excessively duringhot-working and obtain fine microstructure. The research of the refining technology of V-Ti-N medium carbon microalloyed steelshowed that, it was helpful for casting performance and reducing AlN inclusions in steelby improving the Si-Mn deoxidization and reducing Al deoxidization in the refiningprocess. By changing the refine slag basicity from high to low, the oxygen content of steelreached0.0016%, while the sulfur recovery reached75%. After calcium treatment,inclusion shape MnS inclusions were changed into ball shape inclusions.The results of the FORMASTOR-F test of V-Ti-N medium carbon microalloyed steelshowed that, with the cooling rate increasing, the time required for phase transformationwas shorten. The starting point and the end point of phase transformation tended to declineand the grains were refined gradually. At a cooling rate of0.1~5℃/s, austenitic firstlytransformed to ferrite and pearlite. The final microstructure was the mixture of ferrite andpearlite. At a cooling rate of5~30℃/s, proeutectoid ferrite was obtained on the boundaryof prior austenite grains without pearlite and bainite transformation. Instead, martensitetransformation took place and its final microstructure was composed of martensite and afew ferrites. When the cooling rate was up to50℃/s, martensite transformation happeneddirectly and the final microstructure was martensite. At the same time, as the cooling rateincreased, the number of tiny precipitations in steel increased. However, when the coolingrates increased to over10℃/s, precipitations were suppressed, leading to less precipitatedphase.The cooling rate after rolling and the forging temperature have important influenceson the composites, microstructure and performance properties of the V-Ti-N mediumcarbon microalloyed steel used as fracture splitting con-rods. The contents of pearlite andnumbers of precipitated phases increase while ferrite grain size and interlayer spacing ofpearlite decrease when the cooling rate and the forging temperature get higher. Thus thetensile strength and the yield strength improve. After higher cooling rate was appliedunder a forging temperature of1200℃, the yield strength and tensile strength went up to840MPa and1050MPa respectively. The precipitant phase in the steel gave a contributionof174MPa to the yield strength. The impact test shows typical brittle fracturecharacteristics with sulfide, carbon nitrides and oxides as the origins of the fracture cracks.Fatigue fracture and its mechanism have been studied by fatigue test of V-Ti-Nmedium carbon microalloyed steel, C70S6BY and42CrMo. The results show that thefatigue strength of V-Ti-N medium carbon microalloyed steel was about400MPa, whichis better than C70S6BY and42CrMo. Fatigue cracks in the V-Ti-N medium carbon microalloyed steel initiated in the boundaries of ferrite and pearlite, then expanded alongthe boundaries and expanded into the internal pearlite. The surface of the fatigue fractureshowed typical ductile fracture characters, which was dimple and necking. Fatigue cracksin C70S6BY initiated and expanded from the ferrites in the boundaries of a thin anduncontinuous network of proeutectoid ferrite, the surface of the cracks showed distinctbrittle fracture character. The microstructure of42CrMo steel was composed ofair-cooling bainite and ferrite, when applied under a condition of stress, the longbainite-ferrite stick in the boundaries of original coarsen austenite became the initiationsof fatigue micro cracks. |
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