Study On Rare Earth Microalloying And Structure Property Of Medium Carbon Medium Alloy Steel ZG70Cr2MnNiSi

ZG70Cr2MnNiSi steel is a new kind of medium carbon medium alloy steel, it canmeet the demand of large ball mill scaleboard. In the paper, the temperature point ofphase-transition of this wear-resistant steel is measured，CCT(Continuous CoolingTransformation) curve of the steel was drawn.Based on this, two kinds of alloy Sr-Si-Fe and RE-Ca-Ba were chosen,they were used to modify the ZG70Cr2MnNiSi steel.Then the three kinds of cast steel experienced annealing and quenching based on theproposed process. Then four kinds of tempering temperature were chosen fortempering. XRD、SEM、TEM and others methods were used to indicate structureand property of samples. Hardness and tensile property was tested, abrasion resistanceproperty of test steel was tested. Influence of tempering process on the microstructureand properties of ZG70Cr2MnNiSi Steel was analysed. Effect of rare earth on themicrostructure and property of medium carbon medium alloy ZG70Cr2MnNiSi steelwas analysed. This paper can give a basis for in-depth study and application ofmedium carbon medium alloy steel.The test results are as follow. Because alloying element dissolving into theaustenite are more, in the CCT curve, pearlite and bainite’s transformation range of thetest steel is compared on the right. When the cooling rate was less than0.15℃/s,transmutation products were ferrite, pearlite and bainite; when the cooling rate reachand surpass0.5℃/s, transmutation product was martensite. Alloy steel went through860℃quenching, phase transformation products were lath martensite and retainedaustenite and carbide. The test steel went through860℃×2h air quenching and590℃×2h tempering, the metallurgical structure was tempered sorbite, its hardnesswas44.83HRC, tensile strength was1285.35MPa，the lower yield strength was1188.99MPa. It had good Comprehensive mechanical properties, and it can adapt toworking conditions which demands strength and toughness. SEM observation showed, there were carbide particles on the ferrite matrix. With the increase of the temperingtemperature, the size of carbide got smaller, the dispersion degree of alloy carbide gotbigger.Two kinds of modification by adding Sr-Si-Fe and RE-Ca-Ba played a goodeffect, Sr-Si-Fe’s effect was better.The test steel went through Sr-Si-Femodification， its tensile strength and lower yield strength both increased about50MPa, the percentage reduction of area reduction of area and elongation both increasedto varying degrees, the relative amount of wear decrease10%compared to pre-modification. XRD analysis showed, adding rare earth didn’t change the phasecomposition of tempering sample, the carbide included cementite, M2C,M3C,M7C3.Fracture morphology analysis showed that, modification improved toughness of thetest steel, the fracture mechanism changed from quasi-cleavage fracture to dimple andtear ridges mixed fracture. After composite modification, the metallographic structureof ZG70Cr2MnNiSi steel was well distributed, its grain was fine, and its strengthand toughness both improved to varying degrees. The reason was that, adding rareearth can promote martensite to form high density dislocation, promote carbide’smuch diffusion separate out in the lath martensite. And there was retained austeniteamong lath martensite.