A high-rotating bending fatigue strength bearing steel

Abstract

This invention discloses a high-rotational-bending-fatigue-strength bearing steel, whose chemical composition and mass percentage are as follows: C: 0.80%–0.85%, Cr: 4.00%–4.25%, Mo: 4.2%–4.5%, V: 0.90%–1.10%, Al: 0.1%–1.5%, Ni: ≤0.20%, Mn: ≤0.35%, Si: ≤0.35%, S: ≤0.005%, P: ≤0.008%, Co: ≤0.25%, W: ≤0.25%, Cu: ≤0.10%, Ti: ≤0.0025%, Ca: ≤0.001%, O: ≤0.0006%, N: 0.005%–0.008%, with the remainder being Fe and unavoidable impurity elements. The bearing steel provided by this invention exhibits a double-refined microstructure, excellent tensile properties, and high rotational-bending fatigue strength.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, and specifically relates to a bearing steel with high rotational bending fatigue strength. Background Technology

[0002] GCr4Mo4V steel is a fully hardened bearing steel based on secondary hardening. With its good hardenability, excellent hot hardness, and outstanding hardness retention in the temperature range of 120℃~320℃, it is widely used in the rings and rolling elements of the main shaft bearings of aero-engines below 316℃, and has become the most widely used high-temperature bearing steel in my country's aero-engines.

[0003] Fatigue failure is the typical service failure mode of bearing steel, so its fatigue characteristics directly determine its service life. Rotational bending fatigue testing is one of the most common methods for characterizing the fatigue properties of metallic materials. Compared with rolling contact fatigue testing, rotational bending fatigue specimens ultimately fail in the form of fracture. Therefore, the number of cycles of the specimen under a specific alternating load can be determined, and then the fatigue limit (i.e., fatigue strength) can be obtained through statistical calculations, serving as a reference indicator for improving the service performance of bearing steel.

[0004] Numerous studies have shown that the size and morphology distribution of non-metallic inclusions, the morphology and size of large carbides, and the grain size and uniformity in GCr4Mo4V steel all have a significant impact on fatigue performance. Among these, non-metallic inclusions are mainly oxide inclusions. Under current special metallurgical technology, the oxygen content in molten steel can be controlled to below 6 ppm, and in advanced countries it can even reach 3 ppm to 5 ppm. Therefore, the key to determining the fatigue performance of GCr4Mo4V steel lies in the size and distribution of large carbides (mainly primary carbides) and grain size. In particular, large carbides have the most significant impact on fatigue life: since GCr4Mo4V has an alloy element content as high as 9% and a carbon content of about 0.8%, it is inevitably affected by element segregation during solidification and crystallization, which easily leads to the formation of blocky, network, and large carbides. Alloy elements and carbon elements are locally enriched in the dendritic structure, causing inhomogeneity of the ingot's chemical composition. During subsequent processing, this is gradually inherited to the steel, forming white etched areas or even butterfly structures, further leading to a shortened service life and unstable performance of the steel.

[0005] Domestically produced GCr4Mo4V generally suffers from poor uniformity in the size and distribution of carbides and grains, significantly impacting the quality of final bearing components. This directly manifests as large fluctuations in the service life of main shaft bearings for Chinese aero-engines, with an average service life only half that of comparable Russian products and one-tenth that of comparable American products. Achieving refined and uniform microstructure control of GCr4Mo4V is of great significance for solving the "bottleneck" problem in my country's aero-engine industry and realizing the self-sufficiency of aero-engine technology as soon as possible.

[0006] Domestic and international efforts to improve the size distribution of large GCr4Mo4V carbides mainly focus on controlling segregation and plastic processing. The former includes continuous directional solidification ("Current Status of Metallurgical Quality and Fatigue Performance of Rolling Bearing Steel and Development Direction of High-End Bearing Steel", *Acta Metallurgica Sinica*, 2020, Vol. 56, No. 4, pp. 513-522) and additive manufacturing ("Mechanical Properties of High-Speed ​​Steel AISI M50 Produced by Laser Powder Bed Fusion", *Steel Research International*, 2020, Vol. 91, No. 5, 1900562), while the latter includes multi-directional forging ("Microstructure evolution and mechanical anisotropy of M50 steel ballbearing rings during multi-stage hot forging", *Chinese Journal*). "The Influence of Multi-directional Forging on the Primary Carbide Fracture Mechanism of M50 Steel", Vol. 34, No. 11, 2021, pp. 254-266; "The Influence of Multi-directional Forging on the Primary Carbide Fracture Mechanism of M50 Steel", China Metallurgy, Vol. 30, No. 9, pp. 98-103, 135. Although the above methods have played a positive role in improving the carbide size distribution, continuous directional solidification and additive manufacturing have disadvantages such as complex processes and high costs, while multi-directional forging has problems such as numerous forging processes, difficulty in control, and difficulty in controlling grain size. Therefore, it is urgent to adopt appropriate technical solutions to simultaneously improve the size distribution of carbides and grain structure in bearing steel, thereby achieving a significant improvement in fatigue performance. Summary of the Invention

[0007] The technical problem to be solved by this invention is to provide a bearing steel with high rotational bending fatigue strength, and the technical solution adopted is as follows: A high-rotational-bending-fatigue-strength bearing steel has the following chemical element mass fractions: C: 0.80%–0.85%, Cr: 4.00%–4.25%, Mo: 4.2%–4.5%, V: 0.90%–1.10%, Al: 0.1%–1.5%, Ni: ≤0.20%, Mn: ≤0.35%, Si: ≤0.35%, S: ≤0.005%, P: ≤0.008%, Co: ≤0.25%, W: ≤0.25%, Cu: ≤0.10%, Ti: ≤0.0025%, Ca: ≤0.001%, O: ≤0.0006%, N: 0.005%–0.008%, with the remainder being Fe and unavoidable impurity elements.

[0008] The high rotational bending fatigue strength bearing steel of this invention is prepared into ingots using a three-stage special metallurgical process of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". After the ingots are deformed by three upsetting and drawing processes in a forging press, they are transferred to an air hammer for elongation processing to obtain forgings. After spheroidizing annealing treatment, the forgings are obtained as annealed bearing steel. The annealed steel is then quenched and tempered to obtain bearing steel in heat-treated condition.

[0009] The bearing steel annealed material provided by this invention has a maximum carbide particle size of 1.36μm to 1.66μm and an average carbide particle size of 1.31μm to 1.52μm; the heat-treated bearing steel has an average grain size of 8.18μm to 9.67μm, a retained austenite volume fraction of 1.3% to 2.2%, a room temperature tensile strength of 2890MPa to 2958MPa, a room temperature yield strength of 2397MPa to 2612MPa, and a room temperature rotational bending fatigue strength of 1084MPa to 1123MPa.

[0010] Traditional bearing steels, to avoid the formation of oxide and nitride inclusions, limit the content of oxygen and nitrogen elements to the greatest extent possible in composition control, while avoiding the introduction of oxide-forming elements such as calcium and aluminum, as well as nitride-forming elements such as titanium. This invention reverses this traditional approach to bearing steel composition design. While employing a triple-stage special smelting process to ensure ultra-low oxygen, titanium, and calcium levels, it appropriately controls the nitrogen element within the range of 50ppm to 80ppm, and alloys it by introducing 0.1% to 1.5% aluminum, resulting in a completely new bearing steel composition design.

[0011] The design concept of this invention is as follows: (1) Adding aluminum to GCr4Mo4V bearing steel can not only play the traditional deoxidation function during the solidification process of bearing steel, but also effectively suppress the amount of large carbide precipitation in bearing steel, so that most of the carbides precipitate in the form of fine and dispersed secondary carbides, play the precipitation strengthening role of the second phase, and improve the mechanical properties of the material. (2) On the basis of ultra-low titanium regulation, the nitrogen content is appropriately increased. The aluminum and nitrogen elements combine to form the second phase of aluminum nitride at the grain boundary. By suppressing the migration of grain boundaries, the grain size is not too large and the mixed grain phenomenon is avoided due to long-term heating or repeated heating and heat preservation during forging, quenching and other processes. This further improves the mechanical properties of the material. (3) The addition of aluminum can weaken the austenite stability of GCr4Mo4V bearing steel. The content of residual austenite is less during quenching and tempering, the amount of martensite transformation is increased, and the mechanical properties of the material are improved compared with traditional GCr4Mo4V. Attached Figure Description

[0012] Figure 1 Metallographic photograph of carbide distribution in annealed bearing steel material according to Embodiment 1 of the present invention; Figure 2 This is a metallographic photograph of the grain structure of the bearing steel in the heat-treated state according to Embodiment 1 of the present invention; Figure 3 Metallographic photographs showing the carbide distribution in annealed bearing steel for comparison; Figure 4 Metallographic photographs of the grain structure of bearing steel in a comparative heat-treated state. Implementation

[0013] The present invention will be further described in detail below with reference to specific embodiments.

[0014] The original billets of GCr4Mo4V bearing steel used in each embodiment and comparative embodiment were φ300mm×700mm GCr4Mo4V vacuum consumable ingots smelted by the triple process (vacuum induction + protective atmosphere electroslag remelting + vacuum consumable remelting) under the same process conditions. The chemical composition of the steel used in each embodiment and comparative embodiment is shown in Table 1. The average and maximum carbide particle size after spheroidizing annealing, the mechanical properties after heat treatment, the average grain size and the volume fraction of retained austenite are shown in Table 2.

[0015] The spheroidizing annealing process and quenching and tempering heat treatment process for bearing steel are as follows: (1) Spheroidizing annealing treatment: heat to 840℃ and hold for 4 hours, cool to 730℃ at 30℃ / hour and hold for 4 hours, and air cool to 550℃ at 30℃ / hour; (2) Quenching and tempering treatment: After holding at 1120℃ for 30 minutes, oil quenching is performed, followed by air cooling tempering after holding at 530℃ for 2 hours three times. Example 1

[0016] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0017] Metallographic photographs of carbide distribution in annealed bearing steel, such as Figure 1 As shown, the microstructure consists of spherical pearlite with finely dispersed carbides, the largest carbide particle having a diameter of approximately 1.36 μm; the metallographic photograph of the grain structure of the heat-treated bearing steel is shown below. Figure 2 As shown, the microstructure consists of martensite, bainite, a small amount of retained austenite, and retained carbides, with an average grain size of 8.18 μm and uniform distribution. The proportion of retained austenite is 1.3%. The average and maximum carbide grain size after spheroidizing annealing, the mechanical properties after heat treatment, the average grain size, and the volume fraction of retained austenite are shown in Table 2. Example 2

[0018] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0019] Metallographic photographs of carbide distribution in annealed bearing steel and grain structure in heat-treated bearing steel are similar to those in Example 1 and will not be repeated. The average and maximum carbide particle size after spheroidizing annealing, mechanical properties after heat treatment, average grain size, and retained austenite volume fraction are shown in Table 2. Example 3

[0020] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0021] Metallographic photographs of carbide distribution in annealed bearing steel and grain structure in heat-treated bearing steel are similar to those in Example 1 and will not be repeated. The average and maximum carbide particle size after spheroidizing annealing, mechanical properties after heat treatment, average grain size, and retained austenite volume fraction are shown in Table 2. Example 4

[0022] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0023] Metallographic photographs of carbide distribution in annealed bearing steel and grain structure in heat-treated bearing steel are similar to those in Example 1 and will not be repeated. The average and maximum carbide particle size after spheroidizing annealing, mechanical properties after heat treatment, average grain size, and retained austenite volume fraction are shown in Table 2. Example 5

[0024] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0025] Metallographic photographs of carbide distribution in annealed bearing steel and grain structure in heat-treated bearing steel are similar to those in Example 1 and will not be repeated. The average and maximum carbide particle size after spheroidizing annealing, mechanical properties after heat treatment, average grain size, and retained austenite volume fraction are shown in Table 2.

[0026] Comparative Example

[0027] A φ300mm×700mm bearing steel ingot was prepared using a triple special metallurgical process consisting of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting". The chemical composition is shown in Table 1. After the ingot was deformed by three upsetting and drawing processes in a forging press, it was transferred to an air hammer for elongation processing to obtain forging material. The forging material was subjected to spheroidizing annealing treatment to obtain annealed bearing steel material. The annealed material was then subjected to quenching and tempering treatment to obtain bearing steel in heat-treated condition.

[0028] Metallographic photographs of carbide distribution in annealed bearing steel, such as Figure 3As shown, the microstructure consists of spherical pearlite, with coarse and unevenly distributed carbides, the largest carbide particle having a diameter of approximately 25.26 μm; the metallographic photograph of the grain structure of the heat-treated bearing steel is shown below. Figure 4 As shown, the microstructure consists of martensite, bainite, retained austenite, and retained carbides, with an average grain size of 19.66 μm and uneven distribution. The proportion of retained austenite is 8%. The average and maximum carbide grain size after spheroidizing annealing, the mechanical properties after heat treatment, the average grain size, and the volume fraction of retained austenite are shown in Table 2.

[0029] Table 1. Chemical composition and mass fraction (wt%) of bearing steel in each embodiment and comparative example.

[0030] Table 2 Microstructure and Properties of Bearing Steel in Each Example and Comparative Example

[0031] The above embodiments are only used to illustrate and not limit the technical solutions of the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention without departing from the spirit and scope of the present invention. Any modifications or partial substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A high rotational bending fatigue strength bearing steel, characterized in that, The mass fraction of the chemical elements in the bearing steel is as follows: C: 0.80%–0.85%, Cr: 4.00%–4.25%, Mo: 4.2%–4.5%, V: 0.90%–1.10%, Al: 0.1%–0.8%, Ni: ≤0.20%, Mn: ≤0.35%, Si: ≤0.35%, S: ≤0.005%, P: ≤0.008%, Co: ≤0.25%, W: ≤0.25%, Cu: ≤0.10%, Ti: ≤0.0025%, Ca: ≤0.001%, O: ≤0.0006%, N: 0.005%–0.008%, with the remainder being Fe and unavoidable impurity elements. The maximum carbide particle size of the annealed bearing steel is 1.36 μm to 1.66 μm, and the average carbide particle size is 1.31 μm to 1.52 μm. The average grain size of the heat-treated bearing steel is 8.18 μm to 9.67 μm, the volume fraction of retained austenite is 1.3% to 2.2%, the room temperature tensile strength is 2890 MPa to 2958 MPa, the room temperature yield strength is 2397 MPa to 2612 MPa, and the room temperature rotational bending fatigue strength is 1106 MPa to 1123 MPa.

2. The high rotational bending fatigue strength bearing steel as described in claim 1, characterized in that, The bearing steel production method is as follows: Ingots are prepared using a three-stage special metallurgical process of "vacuum induction melting + protective atmosphere electroslag remelting + vacuum consumable remelting"; after the ingots are deformed by three upsetting and drawing processes in a forging press, they are transferred to an air hammer for elongation processing to obtain forgings; the forgings are subjected to spheroidizing annealing treatment to obtain annealed bearing steel; the annealed steel is subjected to quenching and tempering treatment to obtain the final product.

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