摘要
随着汽车产量、轻量化及高性能化的发展,作为重要的车用渗碳齿轮钢,20CrMnTi钢亟需提高性能才能符合车辆的快速发展要求。20CrMnTi钢具有晶粒细、渗碳淬火性能良好、工艺性能成熟可靠且成本低廉等优点,所以自上世纪50年代从前苏联引进至今,在我国齿轮用钢量中始终占据着很大的份额,目前.生产量大致占渗碳齿轮钢的70%。
齿轮在使用过程中,担负着传递动力的任务,在冲击、交变应力等作用下以齿根断裂和齿面接触疲劳为主要失效形式,因此齿轮钢应有良好的强韧性、耐磨性以承受冲击、弯曲和接触应力；此外,还要求变形小、精度高,噪声低,对齿轮生产企业而言还要在保证性能的前提下成本尽量低等特点。衡量齿轮钢质量的主要为末端淬透性带宽、钢材纯净度和晶粒度及热加工性和切削性能等。
20CrMnTi钢中适当的碳含量(D.17%-0.23%)有利于齿轮渗碳后具有良好的心部强度和韧度,铬和锰可明显提高钢的淬透性,而钛形成的高温下非常稳定的TiN或Ti(C,N)颗粒可明显阻止渗碳时晶粒的粗化。另一方面,由于体系采用Ti合金化,常导致液析TiN产生,使得齿轮在使用过程中,TiN和基体的交界处易萌生疲劳裂纹,引起齿轮过早失效。据此,本文通过对20CrMnTi钢进行成分优化(调整钢中的Ti和/或N含量),在保证有足够体积分数的Ti(C,N)颗粒明显阻止晶粒粗化的前提下消除或抑制液析TiN,从而明显地提高了20CrMnTi的接触疲劳性能,同时,研究了成分优化对晶粒度、氮化钛、以及渗碳、淬火等工艺性能的影响,并进行了相关的基础理论研究。
液态铁中TiN的溶度积公式是控制铁液凝固及枝晶凝固温度下不发生TiN液析的理论依据。按照钢的成分,计算得到钢的熔点约在1512℃(1785K),实际凝固温度约在1-500℃(1773K),枝晶间最后凝固区域的凝固温度可能低至1400℃(1673K)。结合TiN在钢液中的溶度积公式可知,当凝固开始钢水温度为1500℃时,对电炉生产(典型N含量约为0.008%),溶度积约为0.0003509,则Ti含量应低于0.043%,而对转炉生产(典型N含量约为0.004%),Ti含量应低于0.086%：当钢液最后凝固温度为1400℃时,对电炉钢(N含量约为0.008%),其钛含量超过0.012%时就有可能析出液析TiN,而对转炉钢(N含量约为0.004%)萁钛含量超过0.024%时将可能有液析TiN析出。为此,冶炼了钛氮乘积不同的四炉试验钢(钛氮乘积分别为1号：0.0001066、2号：0.0001352、3号：0.0004320、4号：0.0008280),为研究试验钢中对疲劳寿命影响严重的氮化钛夹杂尺寸和分布特点,随机选取了一炉莱钢工业生产20CrMnTi钢(钛氮乘积为0.0004128)进行比较；为研究试验钢的接触疲劳寿命,随机选取了莱钢转炉和电炉生产的两炉钢(溶度积分别为0.0003500和0.0004640)以作比较。
对氮化钛尺寸、分布及均匀性试验测定结果表明成分优化后碳氮化钛的尺寸均匀性和平均尺寸得到改善,分布更加均匀；随钢中钛氮乘积的减小,氮化钛的平均尺寸不断减小,二者具有明显的相关性。碳氮化钛动力学析出理论分析结果表明只有钛氮乘积小于0.00016时,碳氮化钛的有效析出温度才可能等于或低于钢的凝固温度,即才可能发生固态下的晶内析出。此外,对不同钛、氮含量的Ostwald熟化速率m值进行了计算,结果表明在930℃进行常规渗碳时,Ostwald熟化作用对颗粒尺寸的影响可以忽略不计,最终得到的碳氮化钛颗粒的尺寸主要取决于沉淀析出相变完成时的初始尺寸。
20CrMnTi钢的细晶粒是依靠Ti的第二相钉轧作用获得的。晶粒度试验表明成分优化后钛氮乘积最小的1号试验钢发生了混晶,其余三炉试验钢晶粒度级别均达到7级以上且无混晶现象；根据Zener-Gladman公式,计算了所需晶粒度级别下第二相的尺寸与体积分数匹配。
淬透性与渗碳或碳氮共渗是20CrMnTi钢生产过程中重要的性能指标。淬透性试验表明成分优化未明显改变钢的淬透性,均符合GB/T 5216-2004要求；热处理工艺试验表明试验钢无论是渗碳还是碳氮共渗工艺,其渗层深度稳定,组织均匀细小,完全满足实际工业生产需要。
接触疲劳对比试验表明,与莱钢转炉生产20CrMnTi钢相比,莱钢电炉钢疲劳寿命稍高,成分优化的2号试验钢(溶度积Ti×N=0.0004320)疲劳寿命提高了近5倍,而3号试验钢(溶度积Ti×N=0.0001352)则提高了23倍,表明本实验条件下成分优化对疲劳寿命具备明显的有利作用。
最后,通过对理论与试验分析,给出了20CrMnTi齿轮钢工业生产的优化成分范围及工艺控制建议：钢中钛氮乘积控制在0.00012-0.00032,同时对最低钛含量有一定的控制要求;若采用电炉冶炼,钢中Ti含量应控制在0.025%-0.04%的范围：,若采用转炉冶炼,则Ti含量应控制在0.035%-0.07%的范围；生产中采用铝脱氧,酸溶铝量控制在0.02%-0.05%；浇注前适当提高钢水温度；连铸过程中适当快冷。
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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