This paper reviewed the development history of the first generation bearing steel GCr15, the second generation bearing steels M50 and M50NiL, and the third generation bearing steels Cronidur30 and CSS-42L. The fourth generation bearing alloy characterized by light weight is put forward. Based on the analysis of metallurgical quality and fatigue properties of traditional bearing steel, the direction of metallurgical quality control of bearing steel with fine quality and homogenization of large particle inclusions and carbide was proposed, and the contact fatigue control mechanism of bearing steel and two different anti-fatigue mechanisms of carbide control were revealed. According to the latest development of quality control technology and quantitative characterization technology for traditional bearing steel GCr15, the development direction of quality control for high-end bearing steel is proposed. Through the research on the overall heat treatment technology and surface carburizing technology of superfine matrix and carbide of bearing steel GCr15 and CSS-42L steel, the double heat treatment and surface superhardening heat treatment are innovatively developed, which can increase the contact fatigue life of bearing steel GCr15 at room temperature to 5 times and more than 10 times, respectively. Finally, it is pointed out that the application of quantitative inspection and testing technology is an important guarantee for high-performance bearing steel with good metallurgical quality and high performance.

The cleanliness of bearing steels produced in China has been greatly improved due to the significant progress in the steelmaking technologies in the past decade, leading to their total oxygen (T.O.) contents lowered to no more than 6×10-6. Under such a high cleanliness, it is then expected that the influence of non-metallic inclusions on fatigue property should be different from the previous knowledge, because both the size and quantity of inclusions are reduced greatly. Therefore, both inclusions and fatigue properties for three ultra-clean GCr15 (100Cr6) bearing steels containing T.O. around 6×10-6, which were manufactured via different industrial production processes, were studied for this purpose. First, inclusions were characterized by ASPEX SEM and then statically analyzed by the statistics of extreme values (SEV) and the generalized Pareto distribution (GPD). Next, their rolling contact fatigue lives (RCF) L10 and L50 were measured by flat washer tests. Only the largest inclusion in each sample is required for predicting the characteristic sizes of maximum inclusion (CSMI) for the three steels using the SEV method. The calculated CSMIs, however, are not consistent with the variation of either L10 or L50, indicating they are not relevant. In contrast, the types of inclusions above threshold (u) size can be classified and their number density of inclusions quantified when the GPD method is employed. In particularly, the CSMIs of different types of inclusions can be determined. In this case, it has been found that the CSMI of TiN inclusion, which is the most dangerous for initiating cracking, is in a good agreement with the low probability rolling fatigue life (L10), suggesting that they are very correlated. This, however, cannot explain the variation of high-probability fatigue life (L50). Instead, the density of total inclusions also played an important role on the L50 of ultra-clean bearing steels in addition to the CSMI of TiN inclusions. This is reasonable because cracking shall be initiated at not only the most dangerous TiN inclusion during the early failure but also some other highly dense inclusions particularly during the late failure. Therefore, it is then concluded that the L10 is much more related to the CSMI of most dangerous TiN inclusion; whilst the L50 is strongly affected by the number density of total inclusions.

以3种不同工艺工业生产的总O含量均≤6×10^-6的超洁净GCr15轴承钢为研究对象，通过推力片实验测试这3种钢的滚动接触疲劳寿命并获得额定寿命(L₁₀)和中值寿命(L₅₀)，通过ASPEX扫描电镜获得各工艺下的夹杂物样本数据并进行统计分析，使用极值法(SEV)和广义Pareto分布法(GPD)估算出样品中最大夹杂物特征尺(CSMI)，然后将其与实测L₁₀和L₅₀进行对比和分析。结果表明，SEV法仅检测每个样品的最大夹杂物，无法通过其获得的CSMI来合理解释3种钢L₁₀和L₅₀的变化，2者之间相关性较差；而GPD法分析夹杂物时，需要对阈值尺寸以上的所有夹杂物进行表征和统计分析，可以获得夹杂物的数量密度以及不同类型夹杂物的CSMI，GPD法所预测出的最危险类型TiN夹杂物的CSMI可以合理解释L₁₀的变化，2者之间有较好相关性，但无法据此解释L₅₀的变化；但将总的夹杂物数量密度与TiN夹杂物最大特征尺寸相结合，能合理解释3种钢的L₅₀差异，这是因为当更多样品失效时，裂纹萌生位置将不再仅仅局限于最危险类型夹杂物。因此，最危险类型夹杂物的CSMI与超纯净轴承钢中的早期疲劳失效的L₁₀相关性最强，而夹杂物的数量密度对高概率的中值疲劳寿命L₅₀也有重要影响。

Rare earth (RE) addition to steels to produce RE steels has been widely applied when aiming to improve steel properties. However, RE steels have exhibited extremely variable mechanical performances, which has become a bottleneck in the past few decades for their production, utilization and related study. Here in this work, we discovered that the property variation of RE steels stems from the presence of oxygen-based inclusions. We proposed a dual low-oxygen technology, and keeping low levels of oxygen content in steel melts and particularly in the raw RE materials, which have long been ignored, to achieve impressively stable and favourable RE effects. The fatigue life is greatly improved by only parts-per-million-level RE addition, with a 40-fold improvement for the tension-compression fatigue life and a 40% enhancement of the rolling contact fatigue life. We find that RE appears to act by lowering the carbon diffusion rate and by retarding ferrite nucleation at the austenite grain boundaries. Our study reveals that only under very low-oxygen conditions can RE perform a vital role in purifying, modifying and micro-alloying steels, to improve the performance of RE steels.© 2022. The Author(s), under exclusive licence to Springer Nature Limited.

High-carbon chromium bearing steels with different rare earth (RE) contents were prepared to investigate the effects of RE on inclusions and impact toughness by different techniques. The results showed that RE addition could modify irregular Al2O3 and MnS into regular RE inclusions. With the increase of RE content, the reaction sequence of RE and potential inclusion forming elements should be O, S, As, P and C successively. RE inclusions containing C might precipitate in molten steel and solid state, but the precipitation temperature was significantly higher than that of carbides in high-carbon chromium bearing steel. For experimental bearing steels, the volume fraction of inclusions increased steadily with the increase of RE content, but smaller and more dispersed inclusions could be obtained by 0.018% RE content compared with bearing steel without RE, whereas the continuous increase of RE content led to an increasing trend for inclusion size and a gradual deterioration for inclusion distribution. RE addition could improve the transverse impact toughness and isotropy of bearing steel, and for modified high-carbon chromium bearing steel by RE alloying, the increase of RE content continuously increased both transverse and longitudinal impact toughness until excessive RE addition.

为了提高G102Cr18Mo高碳不锈轴承钢的洁净度、细化碳化物组织,采用真空感应熔炼、两次真空自耗重熔、大锻压比锻造的工艺路线,研究了真空处理及大锻压比锻造对化学成分、气体含量、夹杂物分布、二次枝晶间距及碳化物颗粒度的影响。研究结果表明,真空感应熔炼过程(VIM)中,随着铝含量的增加,碳的脱氧能力大幅降低,即使铝质量分数为0.003%也对碳的脱氧能力有明显的阻碍作用;真空自耗重熔过程(VAR)由于高的真空度、高的重熔温度等热力学条件以及反应动力学条件的改善,氧含量显著降低,第一次自耗重熔后氧质量分数从0.001 49%降低至0.000 57%,降低了61.7%,第二次自耗重熔后氧质量分数降低至0.000 50%。真空感应熔炼、真空自耗重熔过程,夹杂物的成分变化不大,主要以Al-Si夹杂为主,其次为Al₂O₃夹杂,再次为MnS夹杂、Mg-Al-Ca、Mg/Ca-Al夹杂。双真空冶炼后,钢中夹杂物主要为0～5 μm的细小夹杂物,未发现大于20 μm的夹杂,含有少量10～20 μm的夹杂,钢的洁净度大幅度提高。在真空自耗锭横断面上,从边部向芯部二次枝晶的形貌变化不大,二次枝晶间距逐渐增大,但是变化趋势缓慢,二次枝晶间距为85～95 μm,这主要得益于低的自耗重熔速度。对真空自耗锭进行大变形处理,最终锻造成40 mm的圆棒,碳化物颗粒的最大尺寸不大于20 μm,平均尺寸为15 μm,且没有碳化物聚集的现象。低的自耗重熔速度和大锻压比锻造是碳化物细化的关键。

Bearing is one of the most technologically important engineering components in machines. With the development of several advanced steel-refining technologies to suppress the detrimental effect of nonmetallic inclusions on the mechanical properties of materials, the impact of carbides on the service life of bearings has gradually highlighted. The carbides have become a key factor in determining the performance of a bearing, particularly for primary carbides formed during the solidification of high carbon-chromium bearing steel. Therefore, exploring the formation mechanism of primary carbides and their control strategies is vital to improve the manufacturing process of bearing steel as well as the service life and reliability of bearings. To clarify the formation mechanism of primary carbides and the effects of the processing technique, as well as the addition of rare earth elements, a modified type of GCr15 high carbon-chromium bearing steel with and without rare earth elements was remelted and solidified at different cooling rates. After solidification, the quantity, area, average size, and chemical composition of the primary carbide in the as-cast bearing steel were characterized and analyzed via OM, EPMA, SEM, and XRD. The results show that the type of carbide in GCr15 series bearing steel is M3C cementite with high Cr content (more than 15%, mass fraction). The nucleation rate of M3C cementite increased with the increase in the cooling rate; thus, the number of carbides increased considerably. However, at very high cooling rates, the primary austenite was refined and the diffusion time of C and Cr elements required to form carbides declined; therefore, the size of carbides was reduced significantly, resulting in more uniform dispersion of the carbides. Moreover, the addition of rare earth elements could refine the primary austenite, and subsequently, refine the carbide to some extent. Considering the properties of the primary carbides at different cooling rates, the kinetic formation mechanism for the primary carbide in high carbon-chromium bearing steel during solidification is proposed.

为明确液析碳化物的形成与工艺的关系和产生机制,以及受添加稀土元素的影响,对添加稀土元素和未添加稀土元素的GCr15系连铸高碳铬轴承钢进行了不同冷却速率下的重熔凝固实验。试样凝固完毕后,采用OM、EPMA、SEM和XRD等表征和分析了铸态轴承钢中的液析碳化物的数量、面积、平均尺寸和化学成分等与冷却速率的关系,以及添加稀土元素的影响效果。结果表明,GCr15系轴承钢液析碳化物的类型为M₃C型渗碳体,Cr含量较高,可达15% (质量分数)以上,并且随冷速增加,其数量明显增加。但是,当冷却速率较快时,初生奥氏体细化,同时因形成碳化物所需的C、Cr元素扩散时间减少,碳化物尺寸显著减小,且分布更加弥散均匀。通过对比分析添加稀土元素对凝固组织的影响,发现稀土元素有细化奥氏体进而细化液析碳化物的作用。根据不同冷速下液析碳化物的特点,提出了高碳铬轴承钢凝固过程一次碳化物形成的动力学机制。

M50 bearing steel is widely used in the manufacture of aeroengine spindle bearings. The voids generated by the thermal processing of bearing steel can easily initiate fatigue cracks and lead to fatigue failure of the bearings. Thus, it is essential to understand the steel production conditions, void distribution in the steel, and effect of the subsequent treatment on the healing process of voids to improve the thermal processing and mechanical properties of the steel. In this work, the thermal deformation of the M50 bearing steel was conducted using a thermal simulation machine. The effects of the strain rate (0.001-1 s-1), deformation temperature (1000-1150oC) and strain (10%-50%) on the formation of voids and void healing during the subsequent thermal treatment were systematically studied using OM, SEM, EBSD, and in situ scanning methods. The results show that the formation of voids between the carbide and matrix is attributed to the different hardness values between the matrix and primary M2C and MC carbides. In addition, the carbide fractures can promote the formation of internal voids. The quantitative analysis of the voids indicated that most voids are generated under the following conditions: a high strain rate of 1 s-1, low deformation temperature of 1000oC, and medium deformation of 30%. Applying a heat treatment after deformation can significantly promote the void healing process, and the Cr element is enriched in the healing zone due to its rapid diffusion in γ-Fe.

M50轴承钢广泛应用于航空发动机主轴轴承的制造，轴承钢热加工产生的孔洞极易作为疲劳裂纹的萌生源导致轴承的疲劳破坏。本工作通过热模拟实验和OM、SEM、EBSD、原位扫描等方法系统研究了不同应变速率(0.001~1 s^-1)、变形温度(1000~1150℃)和应变(10%~50%)对M50轴承钢内部孔洞产生行为的影响，以及后续保温处理过程中孔洞的愈合机制。结果表明，轴承钢内部一次碳化物M₂C和MC与基体硬度的差异导致非协调变形，从而在碳化物与基体交界处产生孔洞，此外碳化物的破碎也会促使孔洞在其内部形成。通过对不同条件下孔洞的产生情况进行定量分析发现，在高应变速率(1 s^-1)、低变形温度(1000℃)、中等变形量(30%)的条件下，M50轴承钢内部产生的孔洞最多。变形后的保温处理可显著促进孔洞的愈合，其中保温处理后孔洞愈合区域富集Cr元素。

轻量化且高强韧化已成为汽车用钢的主要发展方向。在不同类型的轻质钢中,Fe-Mn-Al-C系奥氏体基低密度钢因具有更高的比强度和良好的塑韧性备受研究者们青睐。从成分体系与力学性能、多种强韧化机制、不同变形机制及工艺调控等方面总结了Fe-Mn-Al-C系奥氏体基低密度钢的研究进展,重点阐述了奥氏体基低密度钢中特有的B2相以及κ-碳化物、MC碳化物等析出相的析出行为对强韧性机制的影响。最后,结合目前国内外学者对Fe-Mn-Al-C系奥氏体基低密度钢的最新研究成果,对奥氏体基低密度钢进行了展望。