| With fast developent of output, lightweighting'and high performance of automobile,20CrMnTi grade, as an important vehicle-used gear steel, has to improve properties to fulfill the development. 20CrMnTi grade has fine grains, good carburizing-quenching property (direct quenching after carburization), reliable technological feature and low cost and so on, occupying large part in domestic gear steel market since.introduced from Soviet union in 50's last centrury. At present, it is still accounting for about 70% of carburizing gear steels.In practical application, under impact and alternating stress, gears occur fatigue failure mainly in the way of fracture of gear tooth and contact fatigue of tooth surface, so superior strength and toughness are wanted. Moreover, the wear resistance, heat treatment distortion, precision and low cost are wanted too for gear manufacturers. Main index for evaluating gear steels includes hardenability (band), purity and grain size, and also hot workability and cutting performance, et al.For 20CrMnTi grade, carbon content in 0.17%-0.23% is adopted to obtain the necessary centre hardness and toughness, elements Cr and Mn are used to improve hardenablility, Ti forms steady TiN or Ti(C,N) particles at high temperature and then the particles prevent grain growth. However, due to adopting Ti alloying, liquid precipitation of coarse TiN occurs frequently and decreases the contact fatigue life greatly due to fatigue crack initiation at boundary between TiN and matrix in gear application. In this paper, the composition of 20CrMnTi was optimized(by adjusting the content of Ti and/or N), as a result, the contact fatigue life was increased obviously by eliminating or inhibiting TiN liquid precipitation, meanwhile, the volume fraction of Ti(C,N) is much enough to prevent grain growth. Furthermore, the effect of the optimization on grain size, Ti nitrides, carburizing and quenching technological properties, and related fundamental theories were studied too.Solubility of TiN in liquid iron is the theoretical support for controlling liquid precipitation of TiN at solidifaction temperature of liquid iron and dendrite spacing. According to the composition of 20CrMnTi, the melting temperature is about 1512℃(1785K), the real solidifaction temperature is about 1500℃(1773K) and for dendrite solidification it may be lowed to about 1400℃(1673K). In terms of the solubility product for TiN in liquid iron, the solubility product equals 0.0003509 at 1500℃(1773K). Therefore, for electric products(content of N = 0.008%), Ti amount should not exceed 0.043%, and for transfer products(content of N = 0.004%), Ti amount should be lower than 0.086%. At 1400℃(1673K), solubility product is 0.00009685, for electric furnace products(content of N = 0.008%), Ti amount should be below 0.012%, and for transfer furnace products(content of N = 0.004%), Ti amount should be below 0.024%. Based on the analyses above, four-heat tested materials with different Ti×N products (for specific Ti×N product value:No.1:0.0001066, No.2: 0.0001352, No.3:0.0004320, No.4:0.0008280) had been prepared. In order to compare the titanium nitride inclusions, one-heat commercial 20CrMnTi steel (Ti×N=0.0004128)was choosen from LaiWu Iron and Steel Group. In order to studied the contact fatigue life, one-heat transfer furnace 20CrMnTi and one-heat electric furnace product were obtained (product of titanium and nitrogen are 0.0003500和0.0004640 respectively) from LaiWu Iron and Steel Group too.Experiments on titanium nitrides indicate that the mean size, distribution and size homogeneity had been improved by optimization of composition. With decreasing product of titanium and nitrogen, the mean size of TiN decreases, indicating obvious relationship. Dynamic precipitation theory shows that only the product of Ti and N below 0.00016 can the effective precipitation temperature for titanium carbonitrides lower than or equal the molten temperature, i.e., precipitation within grains. Moreover, for steels with different titanium and nitrogen contents, the ripening rate m was calculated, which reveals the Ostwald rippening effect on precipitate.size can be neglected when it is carburized at 930℃.The fine grains of 20CrMnTi were obtained by Ti(C,N) pinning grain boundaries,' so related tests were carried out. The experimental results indicate that after optimization, No.1 tested steel (Ti×N = 0.0001066) occurs growth abnormaly, the grain size of other tested materials(No.2, No.3 and No.4) reaches ASTM 7. Meanwhile, in terms of Zener-Gladman equation, the combination of size and volume fraction of the titanium carbonitrides had been theoretically calculated.Hardenability and carburization(or carbonitriding) are important heat-treatment. technological properties. Results of experiment on hardenability showed that the optimization had no harm to 20CrMnTi steels, and carburization and carbonitriding tests showed that both carburizing and carbonitriding had steady diffusion-layer depth, fine microstructures, so the optimization of composition of 20CrMnTi steel in this work could fulfill the practical production.According to the contact fatigue test, compared with 20CrMnTi grade produced by converter furnace, the product from electric furnace has little higher fatigue life(increased by-66%), the optimized No.2 test steel(Ti×N = 0.0004320) increased by 5 times and No.3 (Ti×N = 0.0001352) about 23 times. Therefore, the optimization here is much helpul for improving the contact fatigue life of 20CrMnTi grade.Finally, based on the experimental results and related theoretical analyses, the reasonable composition range and technological control for enhancing the contact fatigue life of commercial 20CrMnTi steels had been given:1) the product of Ti and N should be controlled in range of 0.00012-0.00032, at the same time, the lowest content of Ti should be controlled too.2) for electric furnace product,0.035% 0.07% of Ti content would be good and for converter product it should be in 0.025%-0.04%.3) in commercial production process, aluminium-deoxidizing is respected, and the acid-soluable Al content should be controlled in the range of 0.02%-0.05%. 4) before continuous casting, liquid iron could have higher temperature, and in subsequent continuous casting reasonably fast cooling rate was wanted.
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