Bearing steel with excellent rolling fatigue life in a hydrogen environment, bearings and bearing components formed from this bearing steel.

Abstract

To provide a steel for bearings having a long rolling fatigue life in an environment where hydrogen gets therein.SOLUTION: A steel for bearings contains C: 0.12-0.50 mass%, Si: 0.15-0.65 mass%, Mn: 0.30-1.80 mass%, Cr: 0.70-3.50 mass%, P: 0.030 mass% or less, and S: 0.030 mass% or less with the balance being Fe and unavoidable impurities. A maximum inclusion diameter, √area _max, is more than 50 μm and 120 μm or less. Here, the steel for bearings may further contain Mo: 0.06-0.70 mass% and / or Ni: 0.30-2.00 mass%, as chemical compositions.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a bearing steel that suppresses exfoliation starting from non-metallic inclusions and has excellent rolling fatigue life in an environment where hydrogen penetrates, and a bearing and bearing parts formed of this bearing steel.

Background Art

[0002] In recent years, in bearings used in wind power generation and the like, exfoliation involving hydrogen has become a problem. In an environment where hydrogen penetrates into steel during the use of bearings, not only exfoliation starting from general non-metallic inclusions occurs, but also exfoliation (matrix exfoliation) caused by the formation of characteristic fatigue structures in the matrix due to the involvement of hydrogen is known to occur.

[0003] Patent Document 1 discloses a high-purity bearing steel having excellent rolling fatigue life in a hydrogen environment. This bearing steel contains one or two chemical components selected from C (0.13 to 0.35% by mass), Si (0.20 to 0.65% by mass), Mn (0.50 to 1.20% by mass), P (0.030% by mass or less), S (0.030% by mass or less), Cr (2.30 to 3.50% by mass), Ni (0.10 to 0.50% by mass), and Mo (0.03 to 0.50% by mass), and the balance is steel composed of Fe and unavoidable impurities.

[0004] Here, among the non-metallic inclusions in a 100 mm cross-section of the steel material, the measurement of the maximum inclusion diameter is performed at 30 locations, and the predicted value √area max of the maximum inclusion diameter in 30000 mm predicted by the extreme value statistical method is 50 μm or less. Also, when the steel is carburized, quenched, and tempered or carburized and nitrided, quenched, and tempered, the total of Si, Mn, Cr, Ni, and Mo dissolved in the matrix component at a position 100 to 300 μm from the outermost surface of the steel material is 3.0% or more. And the residual γ amount is 25 to 50 vol%, and the balance is mainly a martensite-based structure.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Japanese Patent Publication No. 2018-053291 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Under typical rolling fatigue conditions, it is known that reducing the maximum inclusion diameter (√area_max) is effective in suppressing delamination associated with rolling fatigue. However, precisely controlling the inclusion diameter to 50 μm or less in the manufacturing process of general-purpose atmospheric melted steel is not easy, and pursuing excessive reduction in the diameter of non-metallic inclusions as a measure to extend delamination life was not practical.

[0007] The inventors of this invention have further diligently researched bearing steel containing chemical components similar to those described in Patent Document 1. Focusing on the size of nonmetallic inclusions present in the steel, they conducted a detailed study on the relationship between rolling fatigue life and the size of nonmetallic inclusions in an environment where hydrogen is present. They found that even in regions where the maximum inclusion diameter √area_max exceeds 50 μm, the nonmetallic inclusions do not act as a delamination initiation point, and the aforementioned matrix phase delamination preferentially occurs, resulting in delamination. Therefore, even if the nonmetallic inclusions are made finer than necessary, the matrix phase delamination life will be the rate-limiting factor in the life of the component. Thus, without reducing the maximum inclusion diameter √area_max to 50 μm or less, they found a maximum inclusion diameter √area_max that can suppress delamination initiated by nonmetallic inclusions in an environment where hydrogen is present, thereby ensuring a practically necessary rolling fatigue life. This led to the completion of the present invention.

[0008] The object of the present invention is to provide a bearing steel that suppresses delamination of nonmetallic inclusions in environments where hydrogen is present, thereby providing a bearing with excellent rolling fatigue life, and bearings and bearing components formed from this bearing steel. [Means for solving the problem]

[0009] The bearing steel of the present invention has a chemical composition of C: 0.12-0.50 mass%, Si: 0.15-0.65 mass%, Mn: 0.30-1.80 mass%, Cr: 0.70-3.50 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, with the remainder being Fe and unavoidable impurities. Furthermore, the maximum inclusion diameter √area_max is greater than 50 μm and 120 μm or less.

[0010] The bearing steel can further contain chemical components of Mo: 0.06 to 0.70 mass%. Furthermore, the bearing steel can further contain chemical components of Ni: 0.30 to 2.00 mass%.

[0011] The bearing steel may further contain one or more chemical components selected from V: 0.01-0.20 mass%, Nb: 0.01-0.20 mass%, and Ti: 0.01-0.20 mass%.

[0012] The hardness of the bearing steel can be 58 HRC or higher. Furthermore, the oxygen content in the bearing steel can be 8 ppm or less.

[0013] Using the bearing steel of the present invention, it is possible to form bearings, bearing components that constitute part of a bearing, or components having an operating mechanism similar to a bearing. [Effects of the Invention]

[0014] According to the present invention, in an environment where hydrogen is present, delamination originating from nonmetallic inclusions can be suppressed for a practically required lifespan. [Modes for carrying out the invention]

[0015] The bearing steel of this embodiment can be used in the entire bearing, or in some of the components that make up the bearing (bearing components), or in components that have an operating mechanism similar to a bearing.

[0016] The bearing steel of this embodiment has the chemical composition described below, with the remainder being Fe and unavoidable impurities. The chemical composition can be determined by molten steel analysis (JIS G0320) or chemical analysis of steel materials and parts. The chemical composition in the bearing steel can be adjusted during the molten steel refining process in the manufacturing process of the bearing steel. The content of each chemical component will be described below.

[0017] (C content) The carbon content is 0.12 to 0.50 mass%. Carbon is an element that affects hardenability, forgeability in hot and cold conditions, and machinability when manufacturing parts from bearing steel. By setting the carbon content to 0.12 to 0.50 mass%, it is possible to ensure the strength of the core of the bearing steel part while suppressing the impairment of machinability and forgeability.

[0018] If the carbon content is less than 0.12% by mass, sufficient hardness cannot be obtained in the core, and the strength of the core decreases. On the other hand, if the carbon content is higher than 0.50% by mass, the hardness of the steel increases, which impairs machinability and forgeability. Here, it is preferable that the carbon content be 0.18% by mass or higher.

[0019] (Si content) The Si content is 0.15 to 0.65 mass%. Si is an element necessary for deoxidation and also contributes to increasing the strength of steel, suppressing structural changes in steel due to rolling fatigue, and improving rolling fatigue life. To obtain these effects, the Si content must be 0.15 mass% or higher. On the other hand, if the Si content is higher than 0.65 mass%, the hardness of the steel increases, which can hinder workability such as machinability and forgeability, and can also inhibit carburizing, resulting in insufficient material strength even after carburizing or carbonitriding.

[0020] (Mn content) The content rate of Mn is 0.30-1.80% by mass. Mn is an element necessary for ensuring hardenability, and by increasing the amount of retained austenite when carburizing or carbonitriding the steel material, it can inhibit the development of the white structure change in the matrix structure that appears due to the progression of hydrogen-induced fatigue in rolling fatigue. To obtain these effects, it is necessary to set the content rate of Mn to 0.30% by mass or more. Here, the content rate of Mn is preferably 0.50% by mass or more. More preferably, the content rate of Mn is 0.75% by mass or more.

[0021] On the other hand, when the content rate of Mn is higher than 1.80% by mass, the hardness of the steel material increases, which inhibits the workability such as machinability and forging property, and Mn combines with S to generate MnS, which may cause stress concentration in rolling fatigue and become the starting point of the white structure change caused by hydrogen. Here, the content rate of Mn is preferably 1.20% by mass or less.

[0022] (Content rate of P) The content rate of P is set to 0.030% by mass or less. P is an element of inevitable impurity. When the content rate of P is higher than 0.030% by mass, it causes embrittlement of the steel material and reduces the fatigue strength. Here, the content rate of P is preferably 0.020% by mass or less. Note that the shaft steel of the present embodiment may contain inevitable impurities other than P and S described later.

[0023] (Content rate of S) The content rate of S is 0.030% by mass or less. S is an element of inevitable impurity. When the content rate of S is higher than 0.030% by mass, it inhibits the cold workability and reduces the fatigue strength. Here, the content rate of S is preferably 0.025% by mass or less, and more preferably 0.010% by mass or less. Note that the shaft steel of the present embodiment may contain inevitable impurities other than S and P described above.

[0024] (Content rate of Cr) The Cr content is 0.70 to 3.50 mass%. Cr is an element necessary for ensuring hardenability, and when steel is carburized or carbonitrided, it can suppress the white structure change in the matrix caused by hydrogen by increasing the amount of retained austenite. Cr is effective in forming fine and homogeneous retained austenite and can enhance the effect of suppressing the development of white structure changes caused by hydrogen. To obtain these effects, the Cr content needs to be 0.70 mass% or more. More preferably, the Cr content should be 1.00 mass% or more, and even more preferably 1.60 mass% or more.

[0025] On the other hand, if the Cr content is higher than 3.50% by mass, oxides are formed on the outermost surface of the steel during carburizing or carbonitriding, inhibiting carburizing and reducing the strength of the bearing steel. In addition, Cr forms coarse carbides during carburizing, and hydrogen-induced white structure changes are more likely to occur around these coarse carbides.

[0026] (Mo content) In addition to the chemical components described above, the bearing steel of this embodiment may contain Mo. In this case, the Mo content is 0.06 to 0.70 mass%. Mo enhances the hardenability of the steel, increases the amount of retained austenite when the steel is carburized or carbonitrided, and is effective in homogenizing the structure and uniformly distributing the retained austenite, as well as slowing down the progression of fatigue of the matrix in environments where hydrogen is present. To obtain these effects, the Mo content must be 0.06 mass% or more. More preferably, it should be 0.10 mass% or more. On the other hand, excessive addition of Mo increases the cost of the steel, and the effect of suppressing changes in the white structure saturates at a Mo content of 0.70 mass%, so it is preferable to keep the Mo content below 0.70 mass%. Here, the Mo content is preferably 0.50 mass% or less, and more preferably 0.37 mass% or less.

[0027] (Ni content) In addition to the chemical components described above, the bearing steel of this embodiment may contain Ni. In this case, the Ni content is 0.30 to 2.00 mass%. Ni can improve the hardenability of the steel and increase the amount of retained austenite when the steel is carburized or carbonitrided. To obtain these effects, the Ni content must be 0.30 mass% or more. Here, a Ni content of 0.40 mass% or more is preferable. On the other hand, excessive addition of Ni increases the cost of the steel. Also, excessive addition of Ni makes it easier for relatively large, lumpy retained austenite to form during carburizing or carbonitriding, and the effect of suppressing the development of white structure changes caused by hydrogen due to retained austenite is lost. Taking this into consideration, the Ni content is set to 2.00 mass% or less. Here, a Ni content of 1.80 mass% or less, and more preferably 1.50 mass% or less, is preferable.

[0028] (V content) In addition to the chemical components described above, the bearing steel of this embodiment may contain V. V is an element that refines the crystal grains and reduces the hydrogen concentration at the grain boundaries, thereby suppressing white structure changes caused by hydrogen. Furthermore, V functions as a hydrogen trap by forming submicron-order carbides and carbonitrides during carburizing or carbonitride, effectively suppressing white structure changes. To obtain a sufficient effect, the addition of V is 0.01% by mass or more. On the other hand, the effect of V in refining crystal grains and suppressing white structure changes due to the precipitation of carbides and carbonitrides saturates with the addition of V up to 0.20% by mass, and excessive addition of V leads to the precipitation of coarse carbides and carbonitrides, thus having an adverse effect. Therefore, it is preferable to keep the V content below 0.20% by mass. Thus, the V content is set to 0.01 to 0.20% by mass.

[0029] (Nb content) In addition to the chemical components described above, the bearing steel of this embodiment may contain Nb. Nb is an element that refines the crystal grains and reduces the hydrogen concentration at the grain boundaries, thereby suppressing white structure changes caused by hydrogen. Furthermore, Nb functions as a hydrogen trap by forming submicron-order carbides and carbonitrides during carburizing or carbonitride, which is effective in suppressing white structure changes. To obtain these sufficient effects, the addition of 0.01% by mass or more of Nb is necessary. On the other hand, the effects of Nb addition on crystal grain refinement and suppression of white structure changes due to the precipitation of carbides and carbonitrides saturate with Nb addition up to 0.20% by mass. Adding too much Nb will cause the precipitation of coarse carbides and carbonitrides, thus having an adverse effect. Therefore, it is preferable to keep the Nb content below 0.20% by mass. Accordingly, the Nb content is set to 0.01 to 0.20% by mass.

[0030] (Ti content) In addition to the chemical components described above, the bearing steel of this embodiment may contain Ti. Ti is an element that refines the crystal grains and reduces the hydrogen concentration at the grain boundaries, thereby suppressing changes in the white structure caused by hydrogen. Furthermore, Ti functions as a hydrogen trap by forming submicron-order carbides and carbonitrides during carburizing or carbonitriding, which is effective in suppressing changes in the white structure. To obtain these sufficient effects, it is necessary to add 0.01 mass% or more of Ti. On the other hand, the effect of grain refinement and suppression of changes in the white structure due to the precipitation of carbides and carbonitrides by the addition of Ti saturates at the addition of 0.20 mass% of Ti, and adding too much Ti will cause the precipitation of coarse carbides and carbonitrides, thus having an adverse effect. Therefore, it is best to keep the Ti content at 0.20 mass% or less. Thus, the Ti content is set to 0.01 to 0.20 mass%.

[0031] (Maximum inclusion diameter√area_max) The maximum inclusion diameter √area_max is greater than 50 μm and less than or equal to 120 μm. The maximum inclusion diameter √area_max is the largest size of nonmetallic inclusions contained in the steel material and can be determined by the method described later.

[0032] When the maximum inclusion diameter √area_max is greater than 120 μm, delamination originating from nonmetallic inclusions (hereinafter referred to as "inclusion-initiated delamination") is more likely to occur in environments where hydrogen is present. On the other hand, when the maximum inclusion diameter √area_max is 120 μm or less, although delamination originating from fatigue tissue formed in the matrix (hereinafter referred to as "matrix delamination") may occur, the occurrence of inclusion-initiated delamination in environments where hydrogen is present can be suppressed. This ensures the lifespan of practically necessary components. Here, it is even more preferable that the maximum inclusion diameter √area_max is 100 μm or less. Furthermore, by making the maximum inclusion diameter √area_max greater than 50 μm, it becomes unnecessary to finely control the inclusion diameter.

[0033] Next, we will explain how to find the maximum inclusion diameter √area_max.

[0034] A sample is taken from the middle circumference of the test specimen, and after the sample is polished to a mirror finish, it is observed using an optical microscope. Here, a 10 mm × 10 mm area is defined as one field of view (hereinafter referred to as the "reference microscopic area S0", where S0 = 100 mm²). 2 The largest non-metallic inclusion present within the reference microscopy area S0 is searched for. The size of the largest non-metallic inclusion found within the reference microscopy area S0 is measured, and the inclusion diameter is calculated using √(major axis a × minor axis b), thereby determining the maximum inclusion diameter √area_max_s within one reference microscopy area S0.

[0035] Here, we will explain how to determine the major axis a and minor axis b. For nonmetallic inclusions, the major axis a is defined as the longest side connecting the ends of a single nonmetallic inclusion or a group of nonmetallic inclusions, and the minor axis b is defined as the maximum width of the nonmetallic inclusion enclosed by a line parallel to the side of major axis a. Here, for nonmetallic inclusions that exist as a group, the distance d between two adjacent nonmetallic inclusions is compared with the size (√area) of the smaller nonmetallic inclusion. If the size (√area) of the smaller nonmetallic inclusion is greater than the distance d between the nonmetallic inclusions, the two nonmetallic inclusions are considered to be a single entity. On the other hand, if the size (√area) of the smaller nonmetallic inclusion is smaller than the distance d between the nonmetallic inclusions, the two nonmetallic inclusions are considered to be separate nonmetallic inclusions. These are taken into consideration when determining the major axis a and minor axis b of nonmetallic inclusions.

[0036] The above procedure is performed for 30 different fields of view, and the nonmetallic inclusions with the smallest maximum inclusion diameter √area_max_s are plotted on the extreme value statistical graph in descending order. The regression line of the maximum inclusion distribution is then obtained using the least squares method, and this regression line is used to predict the area S (S = 30,000 mm²). 2 The maximum inclusion diameter present within the ) is calculated (Reference: "Influence of Metal Fatigue Microdefects and Inclusions," by Takayoshi Murakami, Yokendo, 1993.3). The maximum inclusion diameter obtained in this way becomes the maximum inclusion diameter √area_max in this embodiment.

[0037] (Hardness) The surface hardness of bearing steel can be 58 HRC or higher. This hardness is determined by the Rockwell hardness test (JIS Z2245). If the hardness of the bearing steel is less than 58 HRC, the rolling fatigue life will be significantly reduced. More preferably, the hardness of the bearing steel should be 60 HRC or higher. This hardness may also be obtained by converting the hardness determined by a test method other than the Rockwell hardness test to the Rockwell hardness test hardness.

[0038] (oxygen content) The oxygen content as an impurity in bearing steel can be kept below 8 ppm. The oxygen content can be measured in accordance with the provisions of JIS G1239. By keeping the oxygen content below 8 ppm, the frequency of large nonmetallic inclusions in the steel can be reduced. More preferably, the oxygen content should be below 6 ppm. [Examples]

[0039] (Preparation of test specimens) For the examples (Examples 1-6 described later) and comparative examples (Comparative Examples 1-5 described later), steel was prepared with the chemical compositions shown in Table 1 below, with Fe and unavoidable impurities as the remainder. The steel grade (steel material 2) for Comparative Examples 3-5 is SUJ2 as specified in JIS G4805.

[0040] [Table 1]

[0041] The aforementioned steel (100 kg) was melted in a vacuum melting furnace, and the steel was forged to a diameter of 65 mm at 1150°C. For these steel materials, steel material 1 was normalized by holding it at 900°C for 1 hour and then air-cooling. Steel material 2 was normalized by holding it at 865°C for 1 hour and then air-cooling, and then spheroidizing annealing was performed by holding it at the maximum heating temperature (800°C) and then slowly cooling. Next, the steel materials were roughly machined to an outer diameter of 60 mm, an inner diameter of 20 mm, and a thickness of 8 mm, and one side of the steel material was buffed to produce thrust-type rolling fatigue test specimens.

[0042] Next, a 0.2 mm diameter, 1 mm deep drill hole was machined into the buffed surface of the test specimen using a micro-drill with a tip diameter of 0.20 mm. Artificial spherical Al2O3 particles were placed into this drill hole as a simulation of nonmetallic inclusions. The Al2O3 particles used were those with diameters of 52 μm (Example 1, Comparative Example 3), 60 μm (Example 2, Comparative Example 4), 70 μm (Example 3, Comparative Example 5), 80 μm (Example 4), 100 μm (Example 5), 120 μm (Example 6), 130 μm (Comparative Example 1), and 150 μm (Comparative Example 2).

[0043] Next, the Al2O3 particles and the matrix phase were bonded together using HIP (Hot Isostatic Pressing). Specifically, the Al2O3 particles were first secured to prevent them from falling out, and the test specimen was placed in a low-carbon steel case. A mandrel was then inserted into the inner diameter hole of the test specimen, and the case was sealed. After vacuum degassing the inside of the case, the specimen was held at 147 MPa and 1170°C for 5 hours, followed by slow cooling, which resulted in the Al2O3 particles and the matrix phase being bonded together. This intentionally created a state where there were no gaps at the interface between the non-metallic inclusions (Al2O3 particles) and the matrix phase.

[0044] After HIP processing, the test specimens made from steel material 1 (Examples 1-6, Comparative Examples 1 and 2) were subjected to the same normalizing process as described above, and the test specimens made from steel material 2 (Comparative Examples 3-5) were subjected to the same normalizing and spheroidizing annealing process as described above. Then, they were processed into intermediate materials having a shape for tensile working (outer diameter φ54 mm, thickness 6.2 mm), and tensile working was applied to form a gap between the embedded Al2O3 particles and the matrix. Subsequently, the test specimens made from steel material 1 (Examples 1-6, Comparative Examples 1 and 2) were subjected to carburizing (carburizing at 930°C for 3 hours followed by oil cooling), and then tempering (heating and holding at 180°C for 1.5 hours followed by air cooling) to adjust the surface hardness of the steel to 62 HRC. For the test specimens (Comparative Examples 3-5) made from steel material 2, the hardness was adjusted to 62 HRC by performing a quenching treatment (heating temperature 835°C, followed by oil cooling) and then a tempering treatment (heating at 180°C for 1.5 hours, followed by air cooling).

[0045] Next, after removing the oxide scale from the heat treatment by surface grinding, the positions of the Al2O3 particles in the test specimen were identified using 50 MHz ultrasonic testing (UT). Based on this positional information, grinding and buffing were performed to adjust the specimen so that the Al2O3 particles would be positioned within the high shear stress depth range in the thrust-type rolling fatigue test described later.

[0046] Table 2 below summarizes the diameter of artificially embedded Al2O3 particles and the hardness of the test specimens for Examples 1-6 and Comparative Examples 1-5. Here, since the diameter of the Al2O3 particles is larger than that of nonmetallic inclusions that inevitably form and remain in the steel during the manufacturing process and are included in thrust-type rolling fatigue test specimens, the diameter of the artificially embedded Al2O3 particles can be considered as the maximum inclusion diameter √area_max mentioned above.

[0047] [Table 2]

[0048] (Thrust-type rolling fatigue test) Test specimens (Examples 1-6 and Comparative Examples 1-5) were used as the lower plate, and a race (model number 51305) of a single thrust bearing made of SUJ2 was used as the upper plate. Three rolling elements (3 / 8-inch diameter SUJ2 steel balls) were placed between the upper and lower plates at equal intervals of 120°. In the test specimen (lower plate), the embedding position of the rolling elements was aligned with the track position of the rolling elements so that the rolling elements passed directly above the embedding position of the Al2O3 particles.

[0049] A load was applied to the contact area between the rolling element and the test specimen so that a maximum Hertz contact stress of 4.5 GPa was applied. The load cycle speed was set to 1800 cycles / min, and lubrication was performed by immersion in an ISO VG68 oil bath. Under these conditions, a thrust-type rolling fatigue test was conducted at room temperature. Here, the embedding depth of the Al2O3 particles inside the test specimen was pre-adjusted so that the Al2O3 particles were located at approximately 0.09 mm depth, where the shear stress is maximum under the conditions of this embodiment.

[0050] On the other hand, in order to measure the delamination life in an environment where hydrogen is present, hydrogen was added to the test specimens (Examples 1-6 and Comparative Examples 1-5) by the cathode charging method prior to the thrust-type rolling fatigue test under the conditions shown in Table 3 below.

[0051] [Table 3]

[0052] In the thrust-type rolling fatigue test described above, the number of stress cycles until delamination was measured, and this number of cycles was determined as the delamination life.

[0053] On the other hand, the causes of delamination were investigated, and the types of delamination were classified into two categories. The appearance after delamination was observed, and the types of delamination were classified into two categories based on the relationship of the delamination location. In addition, the starting point of delamination was identified by buffing the cross-section of the delaminated area, etching it with Nital, and observing it with an optical microscope or scanning electron microscope (SEM). When delamination occurred from the area in which Al2O3 particles (simulation of non-metallic inclusions) were embedded, it was judged to be "inclusion-initiated delamination." Furthermore, when delamination originated from fatigue tissue formed in the matrix phase, rather than from the embedded Al2O3 particles, it was judged to be "matrix phase delamination."

[0054] Table 4 below shows the diameter of the artificially embedded Al2O3 particles (considered as the maximum inclusion diameter √area_max), the delamination factors of each thrust test specimen, and the delamination lifetime (lifetime / average lifetime (reference)) relative to the average lifetime (hereinafter referred to as "average lifetime (reference)") for Examples 1-6 and Comparative Examples 1-5. Here, the average lifetime (reference) was the average value of the delamination lifetime (matrix delamination) for Comparative Examples 3-5.

[0055] [Table 4]

[0056] According to Table 4 above, when the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) was 120 μm or less (Examples 1-6), matrix delamination predominantly occurred. When the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) was 130 μm or more (Comparative Examples 1 and 2), inclusion-initiated delamination predominantly occurred.

[0057] Whether or not premature delamination occurs is determined by the diameter of the Al2O3 particles (maximum inclusion diameter √area_max), as shown in Table 4 above. The factors involved are the extent of crack propagation originating from nonmetallic inclusions generated due to rolling fatigue. When the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) is greater than 120 μm (Comparative Examples 1 and 2), crack propagation originating from nonmetallic inclusions extends over a wide area, leading to delamination originating from nonmetallic inclusions at an earlier time than matrix delamination. On the other hand, when the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) is 120 μm or less (Examples 1 to 6), crack propagation originating from nonmetallic inclusions is localized. As a result, matrix delamination originating from the white structure of the matrix occurs before delamination originating from nonmetallic inclusions, and the delamination lifetime is shown to be longer than that of delamination originating from nonmetallic inclusions.

[0058] Furthermore, regarding the values ​​(lifetime / average lifetime (reference)) shown in Table 4 above, Examples 1 to 6 were 5.5 to 6.5 times the average lifetime (reference) (Comparative Examples 3 to 5), indicating that the lifetime of matrix delamination differed due to the difference between steel materials 1 and 2. The average lifetime (reference) of Comparative Examples 3 to 5 was roughly equivalent to the basic rated life of the bearing (a 90% reliability life that yields a bearing life of 1 million rotations). Considering this point, it was demonstrated that the bearing steel having the chemical composition of the present invention is effective in extending the lifetime of matrix delamination and inclusion-initiated delamination in a hydrogen-ingress environment.

[0059] Therefore, as a countermeasure against early delamination in a hydrogen-ingressed environment, it has become clear that in addition to improving the rolling fatigue life in a hydrogen-ingressed environment by the chemical composition of the bearing steel of the present invention, controlling the diameter of non-metallic inclusions is important. Regarding the control of the diameter of non-metallic inclusions, since delamination of the matrix phase becomes dominant as the diameter of non-metallic inclusions decreases, it is not necessary to excessively reduce the maximum diameter of non-metallic inclusions √area max to 50 μm or less. However, as the diameter of non-metallic inclusions increases, short-life delamination originating from the non-metallic inclusions occurs, so in order to suppress this, it is necessary to keep the maximum inclusion diameter √area_max at least 120 μm or less.

[0060] The present invention makes it possible to suppress the increase in manufacturing costs required to excessively reduce the diameter of non-metallic inclusions in bearing steel, and to obtain bearing steel with excellent rolling fatigue life in environments where hydrogen is present.

Claims

1. A bearing steel having a surface hardness of 58 HRC or more, It has the following chemical composition: C: 0.12-0.50 mass%, Si: 0.15-0.65 mass%, Mn: 0.30-1.80 mass%, Cr: 0.70-3.50 mass%, P: 0.030 mass% or less, S: 0.030 mass% or less, Mo: 0.06-0.70 mass%. The remainder consists of Fe and unavoidable impurities. A bearing steel characterized in that the maximum inclusion diameter √area_max is greater than 50 μm and 120 μm or less.

2. The bearing steel according to claim 1, characterized in that it contains a chemical composition of Ni: 0.30 to 2.00% by mass.

3. The bearing steel according to claim 1 or 2, characterized in that it contains one or more chemical components selected from V: 0.01 to 0.20 mass%, Nb: 0.01 to 0.20 mass%, and Ti: 0.01 to 0.20 mass%.

4. The bearing steel according to any one of claims 1 to 3, characterized in that the oxygen content in the bearing steel is 8 ppm or less.

5. A bearing formed of bearing steel according to any one of claims 1 to 4.

6. A bearing component made of bearing steel according to any one of claims 1 to 4.

7. A component subjected to rolling fatigue, formed from bearing steel according to any one of claims 1 to 4.

Images