高温渗碳齿轮钢的研究进展

常用齿轮钢渗碳温度为930℃,提高渗碳温度至1000～1050℃能显著缩短渗碳时间,但易引起晶粒长人,因此发展了通过Nb、Ti、B微合金化,细化钢原奥氏体晶粒的高温渗碳齿轮钢。文中介绍了国内外高温渗碳齿轮钢的钢种成分、工艺特点、高温渗碳层组织控制和钢的疲劳性能的研究进展。     

高温渗碳齿轮钢的晶粒粗化行为

  为了开发适合980 ℃高温渗碳的齿轮钢，利用伪渗碳方法，研究了铌质量分数为0、0.036%、0.060%和0.100%的18Cr2Ni2Mo渗碳齿轮钢在930和980 ℃的晶粒粗化行为。结果表明，由于析出NbC钉扎晶界，铌微合金化可以显著细化试验钢在930和980 ℃奥氏体化后的晶粒尺寸，且随着铌质量分数增加，铌微合金化明显抑制试验钢在980 ℃长时间奥氏体化晶粒粗化倾向。添加0.100%Nb的18Cr2Ni2Mo齿轮钢在980 ℃奥氏体化20 h后，平均晶粒尺寸仍然在26 μm左右，适合于980 ℃高温长时间渗碳。     

齿轮钢高温渗碳后的疲劳性能

利用旋转弯曲疲劳试验方法,研究了Nb和Ti微合金化20CrMn齿轮钢(20CrMnNb钢:w(Nb)0.077%;20CrMnTiNb钢:w(Nb)0.048%+w(Ti)0.038%)经1 000℃高温渗碳后的疲劳性能。结果表明,Nb-Ti复合微合金化的20CrMnTiNb钢中析出相尺寸小、数量多,其渗碳层原奥氏体晶粒平均尺寸明显低于Nb微合金化的20CrMnNb钢,因而20CrMnTiNb钢的疲劳极限高于20CrMnNb钢。疲劳断口观察发现,20CrMnNb和20CrMnTiNb钢主要以近表面基体方式起裂,渗碳层中疲劳裂纹沿晶界扩展,因而晶粒尺寸较细的20CrMnTiNb钢的疲劳性能较高。     

微合金化渗碳齿轮钢的研究进展

通过添加Al、Nb、Tj、N和B等微合金化元素可细化渗碳齿轮钢的晶粒,提高切削性能和钢的疲劳性能;同时通过添加Nb、Ti等元素,可缩短渗碳时间,精简工序,降低成本。介绍了微合金化冷锻齿轮钢,高温渗碳齿轮钢,高强韧性齿轮钢的钢种、化学成分、生产工艺特点和研究进展。     

含铌齿轮钢的晶粒长大动力学

 利用金相试验方法和理论模型研究了几种Nb微合金化齿轮钢的奥氏体晶粒长大动力学。结果表明,试验钢中Nb含量分别为w(Nb)=0%、004%、006%和008%,奥氏体化温度在900~1 200 ℃范围,奥氏体化保温时间为15~600 min条件下,由于NbC颗粒的钉扎晶界作用,齿轮钢中添加微量Nb,可有效阻止奥氏体晶粒粗化,而且随Nb含量的增加,晶粒细化效果越明显。     

Nb微合金化高碳钢奥氏体晶粒长大原位观察

摘要：以往研究表明Nb析出相钉扎和固溶Nb溶质拖曳作用共同阻碍奥氏体晶粒长大。采用高温共聚焦显微镜研究了Nb对一种高碳含Nb钢奥氏体晶粒长大的影响,对含Nb钢加热过程组织演变进行原位观察。结果表明,Nb在没有钉扎作用下（即高温条件下）仍能起到阻碍奥氏体晶粒长大的作用,该阻碍效果主要是固溶Nb的溶质拖曳作用引起的。采用2种模型对奥氏体晶粒长大行为进行拟合,给出了不同加热温度下Nb微合金化高碳钢的Beck长大方程,同时考虑到加热温度和保温时间的共同影响,根据原位观察结果得到实验钢的奥氏体晶粒长大动力学模型,该模型能够较准确地预测Nb微合金化高碳钢奥氏体晶粒长大行为。     

AlN和Zr微合金化对齿轮钢热塑性及渗碳处理晶粒度的影响

齿轮钢渗碳过程常采用AlN控制奥氏体晶粒长大，然而，AlN大量析出将恶化其热塑性和裂纹敏感性，且1 050℃高温渗碳时AlN回溶，钉扎作用减弱。通过设计不同AlN浓度积([%Al][%N])以及Zr微合金化的钢种，研究AlN和Zr微合金化对于20Cr系齿轮钢热塑性和渗碳处理奥氏体晶粒长大行为的影响，结果表明：对于20Cr系齿轮钢，提高AlN浓度积会使其热塑性恶化，塑性低谷(750～900℃)向高温区移动。而渗碳过程钢中AlN浓度积较低或Ostwald熟化现象，均会减弱AlN的钉扎晶界效果。因此，将钢中的[%Al][%N]控制在2.9×10^-4～5.1×10^-4之间偏下限控制较为合适。此外，齿轮钢中的Zr在凝固过程中会优先析出粗大的Zr(C,N)，而造成AlN析出量减少，削弱钉扎晶界的效果。故需要对N元素含量进行控制，避免粗大高温析出相的出现对AlN造成影响。     

钛铌微合金化齿轮钢的奥氏体晶粒长大研究

 采用热模拟渗碳方法研究了Ti、Ti-Nb微合金化的20CrMnTi和20CrMnTiNb渗碳齿轮钢在930~1200℃的奥氏体晶粒长大规律。结果表明,添加0. 038%(质量分数,下同)的钛和0. 048%的铌的20CrMnTiNb钢中含有铌和钛的析出相,其粒子间距为0. 361μm;而含0. 054%的钛的20CrMnTi钢中仅含有较大尺寸的TiN析出相,粒子间距为0. 471μm,前者奥氏体晶粒粗化倾向明显低于后者。20CrMnTiNb钢经1000℃奥氏体化10h后奥氏体晶粒长大不明显,且无混晶现象,适合高温渗碳工艺。     

中碳微合金化钢高温延塑性的研究

借助Gleeble1500热模拟试验机测试了含Nb和含Nb、Ti两种中碳微合金化钢的高温力学行为,分析了析出物、相变、动态再结晶对微合金化钢高温延塑性的影响。结果表明:试验钢种无第Ⅱ脆性区出现;含Nb钢第Ⅲ脆性区的温度范围为950~700℃,含Nb、Ti钢第Ⅲ脆性区的温度范围为900~725℃;微合金化元素Ti的加入可以细化奥氏体晶粒使含Nb微合金化钢高温塑性槽变窄、变浅;析出物沿晶界多而细小的析出和γ→α相变是第Ⅲ脆性区微合金化钢高温延塑性变差的主要原因。实际生产中通过优化二冷区水量,采用弱冷,可以有效降低微合金化钢表面微裂纹的发生率。     

铝氮比对轿车用渗碳钢晶粒混晶的影响

在转炉长流程生产工艺条件下,研究了铝含量、氮含量和铝氮比对轿车用20Mn Cr5渗碳钢晶粒度混晶的影响。结果表明,在规定的热处理工艺条件下,20Mn Cr5渗碳齿轮钢产生混晶的主要原因是钢中氮含量偏低。当20Mn Cr5钢中铝含量≥0.025%且铝氮比≥3时,可以达到轿车渗碳钢对奥氏体晶粒度的要求。     

Microstructure and Mechanical Properties of Gear Steels After High Temperature Carburization

High temperature carburization is becoming more and more attractive because it can remarkably reduce processing time and increase productivity. However, the commonly used gear steels which are microalloyed by Al are not suitable for high temperature carburization due to abnormal grain coarsening. The gear steel 20CrMnTiNb, which is microalloyed with 0. 048% Nb and 0. 038% Ti, has been compared with the gear steel 20CrMn in terms of microstructure in the case of hardened layer and in the core after carburizing at 1000 °C for 4 h and mechanical properties after carburizing and pseudo-carburizing. The results indicate that the fine austenite grains exist in the carburized case of 20CrMnTiNb steel, while there is abnormal coarsening and duplex grain structure in the case and core of steel 20CrMn. The average prior austenite grain sizes are 19.5 and 34.2 μm for the steels 20CrMnTiNb and 20CrMn, respectively. In addition, the mechanical properties of 20CrMnTiNb steel are superior to those of 20CrMn steel. In particular, the HV hardness of the former is higher than that of the latter by about 40–70 in the range of less than 0.7 mm in depth. Therefore, the steel 20CrMnTiNb is suitable for high temperature carburization.     

Microalloyed Steels for Heat Treating Applications at Higher Process Temperatures

The potential is considered for use of microalloyed bar steels,in conjunction with thermomechanical processing,to enhance the properties of steels heat treated at higher process temperatures than have been used historically.Two examples are highlighted:microalloyed spring steels with enhanced resistance to tempering and Nb-modified gear steels for high temperature vacuum carburizing,e.g.on the order of 1050℃ versus 930℃ for a typical gas carburizing operation.In the spring steel example,the Nb+V steel results in significantly finer prior austenite grain sizes than the other steels considered,enhanced fatigue performance,and improved toughness.In the Nb-modified carburizing steel,Nb additions up to 0.1 wt pct to a Ti-modified 8620 steel,in conjunction with thermomechanical processing to control initial precipitate distributions prior to carburizing,are shown to lead to materials with improved resistance to abnormal austenitic grain growth at the higher process temperatures.Alloy content and heating rate to the carburizing temperature were shown to be important variables and suppression of abnormal grain growth was correlated with the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature leading to improved fatigue performance in steels with fine and uniform grain structures.Opportunities for extending the results of this study to alloy design and controlled rolling in bar mills are assessed.     

Nb(C,N)溶解对高铌奥氏体晶粒长大行为的影响

为了定量研究铌对高铌钢加热过程奥氏体晶粒长大的影响，采用化学溶解过滤分离及电感耦合等离子光谱测定不同加热温度两种试验钢固溶铌质量分数，并对比研究了奥氏体晶粒长大行为。结果表明，在低温条件下，低铌钢固溶铌质量分数高于高铌钢；随加热温度升高，高铌钢固溶铌质量分数快速增加，但即使在1 300 ℃时，铌也不能完成固溶，少量铌存在于(Ti,Nb)(N,C)析出相中；奥氏体晶粒快速长大的温度与固溶铌质量分数快速增加的温度有关。随铌质量分数由0.082%增加到0.120%，奥氏体晶粒快速长大的临界温度由1 050升高到1 150 ℃。高铌钢在1 150～1 250 ℃加热温度范围内，奥氏体晶粒尺寸小于100 μm。     

Austenite in the process of thermomechanical treatment of microalloyed spring steels

On a classical CrV-bearing spring steel 50CrV4 (1.8159) the effects of thermomechanical treatment (TMT) and additional microalloying with titanium and niobium on the processes in austenite were investigated. The aim of this study was to achieve an austenite state, that promises -after subsequent quench and tempering- mechanical properties of spring steels superior to those after conventional treatment. For laboratory tests the hot deformation simulator Wumsi was employed. It was found that after reheating to usual austenitization temperatures the austenite microstructure of steel 50CrV4 is widely uninfluenced by the initial as-delivered microstructure. Despite the finer austenite grain size occurring after austenitization, additional microalloying with Ti and Nb raises the recrystallization temperature considerably. The finding that the incubation time of austenite recrystallization can be prolonged up to several minutes by microalloying is decisive for the practical applicability of TMT. In this way the substructured state of polygonized austenite beneficial for martensite formation can be preserved over the time needed for additional production steps between hot rolling and hardening of leaf springs. Moreover, the deformation of a not recrystallizing austenite may favourably influence the distribution of undesirable tramp elements in spring steels.     

Development of microalloyed steels for high temperature carburizing

The potential for use of microalloy additions to suppress abnormal austenite grain growth and produce steels with enhanced bending fatigue resistance after high temperature vacuum carburizing was investigated in a series of Ti-modified SAE 8620 steels with w(niobium) additions up to 0.1%.Results are considered from a series of papers at the Advanced Steel Processing and Products Research Center on the effects of Nb content,heating rate, rolling history,and processing temperature on the evolution of austenite grain structures in carburizing steels. Emphasis is placed on understanding the effects of alloying and processing on each stage in the annealing process including the as received laboratory rolled conditions,during the onset of carburizing after annealing at different heating rates,and after annealing for various times at carburizing temperatures up to 1 100℃.Heating rate to the carburizing temperature was shown to be an influential variable and suppression of abnormal grain growth was dependent on the development of a critical distribution of fine NbC precipitates,stable at the austenitizing temperature.The importance to industrial carburizing practice of heating rate effects on precipitates and austenite grain size evolution are discussed and correlated to selected data on fatigue performance.     

高温渗碳齿轮钢铝氮含量对奥氏体尺寸的影响

 轨道交通用高端齿轮钢往往要求长时间高温渗碳处理以提高其表面硬度与耐磨性,利用合适的铝、氮含量实现AlN粒子对奥氏体晶界的有效钉扎对保证齿轮的晶粒度、力学性能与尺寸精度至关重要。在通常的渗碳温度下,AlN已经发生了部分固溶,为了保证高温渗碳后奥氏体晶粒细小,齿轮钢中的酸溶铝质量分数一般需要保持在0.02%~0.055%以保证析出足量细小的AlN第二相粒子来钉扎晶界,且氮质量分数要求为0.01%~0.016%。这一元素含量范围较广,因此有必要研究钢在高温渗碳时所需要的恰当铝氮积与铝氮比,也就是钢中w(Al)与w(N)的乘积和比值的取值范围,还需要研究AlN粒子对于奥氏体的钉扎作用。针对不同含铝含氮轨道交通用齿轮钢进行了伪渗碳试验与AlN第二相粒子Ostwald熟化和Gladman钉扎模型计算研究,揭示了奥氏体晶粒不均匀性因子Z与加热温度T的定量关系式。研究了含铝含氮齿轮钢高温保温过程奥氏体晶粒半径R_A的变化规律,以及不同铝氮积和铝氮比对奥氏体晶粒生长的影响。结果表明,加热温度T在1 173~1 273 K范围内,此类微合金高强钢的奥氏体晶粒长大不均匀性因子服从线性规律Z=3.742 97-0.001 76T;保温时间t一定时,lnR_A与1/T大致呈二次多项式关系;加热温度T一定时,lnR_A与lnt呈线性关系,奥氏体生长时间指数为0.33;当钢中铝氮积大于4.77×10-4、对应奥氏体晶粒粗化温度T_C>1 263 K时,在T=T_C-10 K温度下保温6 h后,铝氮比为1.5~3.8的钢均可保证其奥氏体晶粒度达到7级以上;当铝氮积或T_C为定值时,同样在T=T_C-10 K加热温度下保温6 h后,钢的奥氏体晶粒大小与其铝氮比呈线性正相关;铝氮比在1.5~3.8范围内,其奥氏体晶粒度相差约在1级以内。     

重载齿轮钢的高温性能

 通过Gleeble 1500D热模拟试验机高温拉伸试验,对比研究了17Cr2Ni2MoVNb和17Cr2Ni2Mo钢的高温性能。结果表明：因微合金元素V（0.1%,质量分数,下同）、Nb（0.036%）产生细晶强化及固溶强化,17Cr2Ni2MoVNb 钢的抗拉强度比17Cr2Ni2Mo钢稍高。在低N（0.0057%）含量的17Cr2Ni2MoVNb钢中,V和Nb对热塑性的危害很小。而高N（0.0130%）含量的17Cr2Ni2Mo钢在600～900 ℃及1050～1200 ℃温度区间塑性低于17Cr2Ni2MoVNb钢。N含量及相变温度不同导致第二期AlN析出量不同及铁素体先后析出,是造成两试验钢塑性差别的主要原因。