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

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: 0.93-1.10 mass%, Si: 0.15-0.75 mass%, Mn: 0.25-1.70 mass%, Cr: 0.90-2.05 mass%, P: 0.025 mass% or less, S: 0.030 mass% or less with the balance being Fe and unavoidable impurities. In an area of interest to be affected by 50% or more of maximum shear stress resulting from a load of rolling fatigue, a ratio R of Nc to Nt is 85% or more, where Nt means the total number of non-metal inclusions that are present in the area of interest and are have a size of more than 5 μm, and Nc means the total number of non-metal inclusions that have a size of more than 5 μm in a state of blocked interface with the base material.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a bearing steel that suppresses delamination originating from nonmetallic inclusions in environments where hydrogen is present, thereby exhibiting excellent rolling fatigue life, and to bearings, bearing components, and components formed from this bearing steel. [Background technology]

[0002] In recent years, hydrogen-induced delamination has become a problem in bearings used in wind power generation and other applications. It is known that in environments where hydrogen penetrates the steel material during bearing use, delamination can occur not only from common nonmetallic inclusions, but also from the formation of a characteristic fatigue structure in the matrix phase due to hydrogen involvement (matrix delamination).

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

[0004] Here, the steel cross-section is 100 mm. 2 Among the non-metallic inclusions, the largest inclusion diameter was measured at 30 locations, and the predicted maximum diameter of 30,000 mm was determined using extreme value statistics. 2 The predicted maximum inclusion diameter (√area max) is 50 μm or less. Furthermore, when the steel is carburized, quenched, and tempered, or carburized, nitrided, quenched, and tempered, the total amount of dissolved Si, Mn, Cr, Ni, and Mo in the matrix components at a position 100 to 300 μm from the outermost surface of the steel is 3.0% or more. The residual γ content is 25 to 50 vol%, and the remainder is mainly martensite. [Prior art documents] [Patent Documents]

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

[0006] While there are various steel grades for bearings, such as SUJ2, SUJ3, and SUJ5 as specified in JIS G4805, the chemical composition described in Patent Document 1 differs from that of SUJ2, SUJ3, and SUJ5. Therefore, to improve the rolling fatigue life of bearing steels using steel grades such as SUJ2, SUJ3, and SUJ5 (i.e., general-purpose steel grades) in environments where hydrogen is present, a different approach from that described in Patent Document 1 is required. In general rolling fatigue environments, it is known that reducing the maximum inclusion diameter √area_max is effective in suppressing delamination associated with rolling fatigue. However, it is not easy to excessively finely control the inclusion diameter to 50 μm or less in the manufacturing process of general-purpose atmospheric melted steel materials, 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 diligently conducted research and, through investigations focusing on the influence of the state of the interface between nonmetallic inclusions and the matrix phase on rolling fatigue life in an environment where hydrogen penetrates steel, have found that by closing the gaps at the interface when gaps exist, premature delamination originating from nonmetallic inclusions can be suppressed, thereby improving rolling fatigue life, and have completed 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, bearing components, and 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: 0.93-1.10 mass%, Si: 0.15-0.75 mass%, Mn: 0.25-1.70 mass%, Cr: 0.90-2.05 mass%, P: 0.025 mass% or less, S: 0.030 mass% or less, with the remainder being Fe and unavoidable impurities. Furthermore, in the region (target region) where 50% or more of the maximum shear stress generated due to rolling fatigue load acts, the ratio (R) of the total number of nonmetallic inclusions (Nc) with a size greater than 5 μm and where the interface between the base material and the nonmetallic inclusions is closed, to the total number (Nt) of all nonmetallic inclusions (Nc) with a size greater than 5 μm present in the target region, is 85% or more.

[0010] The bearing steel can also contain an additional chemical component of Mo: 0.06 to 0.60 mass%.

[0011] 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.

[0012] Using the bearing steel of the present invention, bearings, bearing components that constitute a part of a bearing, or components subjected to rolling fatigue (rolling fatigue similar to that of the bearing's operating mechanism) can be formed. [Effects of the Invention]

[0013] According to the present invention, delamination originating from nonmetallic inclusions can be suppressed in environments where hydrogen is present. [Modes for carrying out the invention]

[0014] The bearing steel of this embodiment can be used in all or some of the components (bearing parts) that constitute a bearing, or in parts that are subjected to rolling fatigue similar to that of a bearing's operating mechanism.

[0015] The steel for shafts according to this embodiment has the chemical components described below and, as the balance, Fe and unavoidable impurities. Examples of the steel types of the steel for shafts according to this embodiment include SUJ2, SUJ3, and SUJ5 defined in JIS G4805. The chemical components are determined by molten steel analysis (JIS G0320) or chemical analysis of steel materials and parts. The chemical components in the steel for shafts can be adjusted in the molten steel refining process in the manufacturing process of the steel for shafts. Hereinafter, the content rates of the respective chemical components will be described.

[0016] (Content rate of C) The content rate of C is 0.93 to 1.10 mass%. C is an element that affects the hardness of the steel for shafts, the hardenability in the core part, the forging properties in hot and cold working, and the machinability. By setting the content rate of C to 0.93 to 1.10 mass%, the hardness of the steel for shafts can be ensured, and it is possible to suppress the inhibition of workability such as machinability and forging properties. When the content rate of C is less than 0.93 mass%, sufficient hardness cannot be obtained on the surface or the core part, and the strength decreases. On the other hand, when the content rate of C is higher than 1.10 mass%, the workability such as machinability and forging properties is inhibited due to an increase in the hardness of the steel material.

[0017] (Content rate of Si) The content rate of Si is 0.15 to 0.75 mass%. Si is an element necessary for deoxidation, and also an element that increases the strength of the steel material and contributes to the suppression of the structural change of the steel material accompanying rolling fatigue and the improvement of the rolling fatigue life. In order to obtain these effects, it is necessary to set the content rate of Si to 0.15 mass% or more. On the other hand, when the content rate of Si is higher than 0.75 mass%, the workability such as machinability and forging properties is inhibited due to an increase in the hardness of the steel material.

[0018] (Content rate of Mn) The Mn content is 0.25 to 1.70 mass%. Mn is an element necessary to ensure hardenability, and a content of 0.25 mass% or more is required. On the other hand, if the Mn content is higher than 1.70 mass%, the hardness of the steel increases, which hinders machinability and forgeability, and Mn combines with S to form MnS, which becomes the starting point for white structure changes caused by hydrogen. Here, the Mn content is preferably 1.2 mass% or less, and more preferably 0.50 mass% or less.

[0019] (P content) The P content is 0.025% by mass or less. P is an unavoidable impurity element, and if the P content is higher than 0.025% by mass, it causes embrittlement of the steel and reduces its fatigue strength. Here, the P content is preferably 0.020% by mass or less. Note that the bearing steel of this embodiment may contain unavoidable impurities other than P and S, which will be described later.

[0020] (S content) The sulfur content is 0.030 mass% or less. S is an unavoidable impurity element, and if the sulfur content is higher than 0.030 mass%, it inhibits cold workability and reduces fatigue strength. Here, the sulfur content is preferably 0.025 mass% or less, and more preferably 0.010 mass% or less. Note that the bearing steel of this embodiment may contain unavoidable impurities other than sulfur and the P mentioned above.

[0021] (Cr content) The Cr content is 0.90 to 2.05 mass%. Cr is an element necessary for ensuring hardenability. Furthermore, Cr is effective in forming fine, homogeneous retained austenite and can enhance the effect of suppressing white structure changes caused by hydrogen. To obtain these effects, the Cr content needs to be 0.90 mass% or higher. On the other hand, if the Cr content is higher than 2.05 mass%, coarse carbides may form, which may promote crack propagation due to rolling fatigue. Therefore, the Cr content should be 2.05 mass% or less, and more preferably 1.75% or less.

[0022] (Mo content) In addition to the chemical components described above, the bearing steel of this embodiment may contain Mo. Mo is an element that enhances hardenability and may be added as needed. The Mo content is 0.06 to 0.60 mass%. Here, the Mo content is preferably 0.10 mass% or more. Excessive addition of Mo increases the cost of the steel and reduces its machinability, so taking this into consideration, the Mo content is set to 0.60 mass% or less. Here, the Mo content is preferably 0.35 mass% or less, and more preferably 0.26 mass% or less.

[0023] (The state of blockage of the gaps around nonmetallic inclusions) Gaps may or may not exist between nonmetallic inclusions in steel and the base material (interface). When the gaps are closed, the nonmetallic inclusions and the base material are in close contact. In the bearing steel of this embodiment, the ratio R described below is 85% or more. If the ratio R is 85% or more, it is possible to suppress the occurrence of delamination originating from nonmetallic inclusions (hereinafter referred to as "inclusion-initiated delamination") in an environment where hydrogen is present. This is because, when the ratio R is 85% or more, even if cracks occur due to the concentration of stress around nonmetallic inclusions during the rolling fatigue process, it is easier to suppress them from propagating significantly forward and backward in the rolling direction or from connecting with other cracks and becoming larger, thus limiting the crack area to a locally restricted range.

[0024] The proportion R is given by the region A (hereinafter referred to as the "target region"). 50 In this region, it is the ratio of the total number of nonmetallic inclusions (Nc) to the total number of nonmetallic inclusions (Nt). Target region A 50 This is the region in which 50% or more of the maximum shear stress generated due to rolling fatigue is applied in the bearing steel of this embodiment, or in bearing components or bearing operating mechanisms similar to those made from the bearing steel of this embodiment. Also, target region A 50is an area that affects the formation of the fatigue structure of steel and where fatigue preferentially progresses in an environment where hydrogen penetrates. That is, the ratio R is represented by the following formula (1).

[0025]

Number

[0026] In the above formula (1), the total number Nt is the total number (number) of all non-metallic inclusions exceeding 5 μm in size existing within the target area A 50 . The size of the non-metallic inclusions is determined by √(major diameter a × minor diameter b). Non-metallic inclusions with a size of 5 μm or less are not considered because their involvement in promoting damage due to rolling fatigue is relatively small even when they serve as the starting point of the white structure change in the parent phase that occurs during the rolling fatigue process where hydrogen penetrates. The total number Nc is the total number (number) of non-metallic inclusions exceeding 5 μm in size with the interface with the base material in a closed state. According to the above formula (1), the higher the ratio R, the higher the proportion of the number of non-metallic inclusions in a closed state within the target area A 50 means.

[0027] The method for obtaining the above-mentioned ratio R will be described below.

[0028] First, taking a test piece made of the steel of this embodiment for a thrust-type rolling fatigue test (details will be described later), which is used as one of the means for evaluating the rolling fatigue life characteristics, as an example, the specification of the target area A 50 will be described. The target area A 50 is specified by the depth from the raceway surface in the cross-section of the steel material based on the position of the raceway of the rolling element. In the thrust-type rolling fatigue test described later, at a predetermined depth position from the raceway surface corresponding to the diameter of the rolling element used in the test and the maximum Hertz contact stress acting on the portion where the rolling element contacts the raceway surface of the flat thrust test piece, the shear stress is maximum. Therefore, based on this depth position, the range up to where the shear stress at each depth decreases to 50% of the maximum shear stress is the target area A 50 .

[0029] Next, target area A 50 After mirror-polishing the cross section perpendicular to the orbital surface including the area A, 50 The process involves identifying non-metallic inclusions larger than 5 μm in size within the material, and determining whether or not a gap exists between each identified non-metallic inclusion and the base material (interface). The identification of non-metallic inclusions and the determination of gaps are performed using an optical microscope or scanning electron microscope on a mirror-finished target area A. 50 This is done by observing the area A of the object being observed. 50 The range is at least 25mm in total. 2 Preferably 50mm 2 Let's assume that.

[0030] Furthermore, gaps between non-metallic inclusions and the matrix phase are mainly formed when steel is processed (rolling, forging, etc.), and gaps are likely to be observed around non-metallic inclusions in the direction in which the steel is stretched during processing such as rolling or forging. Therefore, when preparing a thrust-type rolling fatigue test specimen so that the raceway surface is perpendicular to the direction of stretching due to processing such as rolling or forging of the steel, it is sufficient to simply use a cross section perpendicular to the raceway surface as the observation cross section. If it is not perpendicular (for example, parallel), it is sufficient to use a cross section perpendicular to the direction of stretching due to the processing as the observation cross section. The observation cross section for bearing components and components subjected to rolling fatigue similar to that of bearing operating mechanisms is determined in the same manner. Target region A is the depth region in which 50% of the maximum shear stress acts in those components. 50 This can be determined by stress calculations.

[0031] For nonmetallic inclusions that exist as multiple groups, the distance d between two adjacent nonmetallic inclusions is compared with the size of the smaller nonmetallic inclusion (√(major axis a × minor axis b)). If the size of the smaller nonmetallic inclusion (√(major axis a × minor axis b)) 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 of the smaller nonmetallic inclusion (√(major axis a × minor axis b)) is less than the distance d between the nonmetallic inclusions, the two nonmetallic inclusions are considered to be separate entities. The major axis a is the longest side connecting the ends of the nonmetallic inclusions, and the minor axis b is the maximum width of the nonmetallic inclusion enclosed by a line parallel to the side of major axis a. These are taken into consideration when calculating the total number of nonmetallic inclusions in the evaluation of the gap ratio between the nonmetallic inclusions and the matrix.

[0032] If no gap (at least partially) is observed between the non-metallic inclusion and the base material, it is determined to be a blocked state. Conversely, if a gap (at least partially) is observed between the non-metallic inclusion and the base material, it is determined not to be a blocked state. Target area A 50 Within the sample, the total number Nt of nonmetallic inclusions (larger than 5 μm) is counted, and the total number Nc of nonmetallic inclusions (larger than 5 μm) that are judged to be in an obstructed state is also counted. This allows the proportion R to be determined.

[0033] By determining the ratio R for a manufactured bearing steel, or a bearing component or bearing operating mechanism made from that bearing steel that is subjected to rolling fatigue similar to that of a bearing, as described above, it is possible to confirm whether the ratio R is 85% or higher. Furthermore, with respect to bearing steel, or a bearing component or bearing operating mechanism made from that bearing steel that is subjected to rolling fatigue similar to that of a bearing, it is possible to design the manufacturing conditions and manufacture the bearing steel, or a bearing component or bearing operating mechanism made from that bearing steel that is subjected to rolling fatigue similar to that of a bearing, under those manufacturing conditions, thereby achieving a ratio R of 85% or higher.

[0034] (Hardness) The 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 bearing steel is less than 58 HRC, the rolling fatigue life will be significantly reduced. More preferably, the hardness of 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.

[0035] (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 5 ppm. [Examples]

[0036] (Preparation of test specimens) As examples (Examples 1-8) and comparative examples (Comparative Examples 1 and 2), steel was prepared with the chemical compositions shown in Table 1 below, with Fe and unavoidable impurities as the remainder. This steel is steel grade SUJ2 as specified in JIS G4805, and is an example of bearing steel in this embodiment.

[0037] [Table 1]

[0038] 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. This steel was then subjected to normalizing by holding it at 865°C for 1 hour and then air-cooling, and spheroidizing annealing by holding it at the maximum heating temperature (800°C) and then slowly cooling. Next, the steel was 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 was buffed to produce a thrust-type rolling fatigue test specimen.

[0039] Next, a 0.2 mm diameter, 1 mm deep drill hole was created in 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 to simulate non-metallic inclusions. The Al2O3 particles used were those with diameters of 52 μm (Example 1), 60 μm (Example 2), 70 μm (Example 3), 80 μm (Example 4), 100 μm (Example 5), 120 μm (Example 6), 150 μm (Example 7, Comparative Example 1), and 200 μm (Example 8, Comparative Example 2).

[0040] Next, the Al2O3 particles and the matrix were bonded together using HIP (Hot Isostatic Pressing). Specifically, the Al2O3 particles were first secured to prevent them from falling out, then the test specimen was placed in a low-carbon steel case, a mandrel was inserted into the inner diameter hole of the test specimen, and the case was sealed. After vacuuming the inside of the case, it was held at 147 MPa and 1170°C for 5 hours, and then slowly cooled to bond the Al2O3 particles and the matrix, creating a gap-free state.

[0041] After HIP processing, normalizing and spheroidizing annealing were performed as described above, and then, for Examples 1 to 8, the test specimens (outer diameter 56 mm, inner diameter 20 mm, thickness 4.8 mm) were reprocessed. When the ratio R of the gap between the nonmetallic inclusions and the matrix was evaluated at this time, it was 100% (no gap between Al2O3 particles and matrix). For Comparative Examples 1 and 2, in order to reproduce the gaps around nonmetallic inclusions that can inevitably occur in the manufacturing process of ordinary steel materials and parts, the intermediate material was first processed to have a shape (outer diameter φ54 mm, thickness 6.2 mm) that would allow for tensile processing to be applied so that the adhesion between the Al2O3 particles and the matrix would be incomplete, and then tensile processing was applied to form a gap between the embedded Al2O3 particles and the matrix. When the ratio R of the gap between the Al2O3 particles and the matrix was observed at this time, it was 0% (a gap existed between Al2O3 particles and matrix). Subsequently, the hardness of the steel was adjusted to 60 HRC by performing quenching and tempering on the test specimens (oil cooling at 835°C for 0.5 hours, followed by air cooling at 180°C for 1.5 hours).

[0042] Next, after removing the oxide scale from the heat treatment by surface grinding, the position of Al2O3 particles in the test specimen was identified using 50 MHz ultrasonic testing (UT). Based on this positional information, grinding and buffing were performed, and in the thrust-type rolling fatigue test described later, the depth region where more than 50% of the maximum shear stress acts (the target region A mentioned above) was identified. 50 The Al2O3 particles were adjusted to be positioned within (corresponding to) the given space.

[0043] Table 2 below summarizes the diameter of Al2O3 particles and the hardness of the test specimens for Examples 1-8 and Comparative Examples 1 and 2. Here, the diameter of the artificially embedded Al2O3 particles is larger than the nonmetallic inclusions that are inevitably generated and contained during the process of melting the steel, which is the base material of the test specimens, so it can be considered as the maximum inclusion diameter √area_max. The maximum inclusion diameter √area_max is the largest size of nonmetallic inclusions contained in the steel material.

[0044] [Table 2]

[0045] (Thrust-type rolling fatigue test) For the thrust-type rolling fatigue test, test specimens (Examples 1-8 and Comparative Examples 1,2) 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 Al2O3 particles so that the rolling elements passed directly above the embedding position of the Al2O3 particles.

[0046] 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, as described above, the embedding depth of the Al2O3 particles inside the test specimen was pre-adjusted so that the Al2O3 particles were located at a position (depth) where more than 50% of the maximum shear stress was acting.

[0047] Prior to the thrust-type rolling fatigue test, hydrogen was added to the test specimens by the cathode charging method under the conditions shown in Table 3 below, in order to measure the delamination life in an environment where hydrogen was present. This hydrogen charging was performed on the test specimens of Examples 1 to 8 and Comparative Examples 1 and 2.

[0048] [Table 3]

[0049] 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.

[0050] On the other hand, the causes of delamination were investigated, and the types of delamination were classified. The appearance after delamination was observed, and the types of delamination were classified into two types 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). If delamination occurred from the area where Al2O3 particles (simulation of non-metallic inclusions) were embedded, it was judged to be "inclusion-initiated delamination." If it was not "inclusion-initiated delamination," it was judged to be delamination originating from fatigue tissue formed in the matrix (hereinafter referred to as "matrix delamination").

[0051] On the other hand, as mentioned above, target area A 50Regarding the above-mentioned ratio R for the gap between the Al2O3 particles, which have been adjusted to be internal, and the surrounding matrix, a value of "○" was used if the ratio R was 85% or more, and a value of "×" was used if the ratio R was less than 85%.

[0052] Table 4 below shows the diameter of the Al2O3 particles (considered as the maximum inclusion diameter √area_max), whether the percentage R is 85% or more, the delamination factor, and the delamination lifetime (lifetime / average lifetime (reference)) relative to the average lifetime as a reference value (hereinafter referred to as "average lifetime (reference)"). Here, the average lifetime (reference) is the average value of the delamination lifetime (matrix delamination) for Examples 1 to 8, and for Examples 1 to 8, the lifetime / average lifetime (reference) was set to "1.0" as shown in Table 4 below.

[0053] [Table 4]

[0054] According to Table 4 above, in Examples 1 to 8, matrix delamination was dominant, while in Comparative Examples 1 and 2 were predominantly inclusion-initiated delamination. In Example 7 and Comparative Example 1, although the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) was the same (150 μm), the delamination factors differed depending on whether the ratio R was 100% or 0%. Similarly, in Example 8 and Comparative Example 2, although the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) was the same (200 μm), the delamination factors differed depending on whether the ratio R was 100% or 0%.

[0055] Regarding the values ​​(lifetime / average lifetime (reference)) shown in Table 4 above, Comparative Examples 1 and 2 showed a significant decrease compared to Examples 1 to 8. From this, it became clear that regardless of the size of the Al2O3 particles (maximum inclusion diameter √area_max), when the percentage R is 0%, the delamination life decreases in an environment where hydrogen penetrates the steel.

[0056] Based on these results, in order to further investigate the effect of the ratio R on rolling fatigue life, a small amount of artificial spherical Al2O3 particle powder of various sizes from 5 to 150 μm was mixed with steel powder (particle size less than 500 μm) having the composition of the bearing steel of this embodiment (same as Examples 1 to 8, Comparative Examples 1 and 2), and the mixture was completely sintered by HIP processing to make the ratio R related to the gap 100%. Then, thrust-type rolling fatigue test specimens were prepared using the same test specimen preparation method as described above (excluding the melting and forging processes). Furthermore, tensile processing was applied to the prepared thrust-type rolling fatigue test specimens in the same manner as described above, and the ratio R related to the gap between the Al2O3 particles contained in the test specimen and the matrix was varied by adjusting the amount of tensile processing. Subsequently, the hardness of the steel material was adjusted to 60 HRC by quenching and tempering the test specimens in the same manner as described above (oil cooling at 835°C for 0.5 hours followed by air cooling at 180°C for 1.5 hours).

[0057] In this embodiment, the bearing steel is manufactured with the purpose of verifying the effect of the ratio R on the gap between the Al2O3 particles and the matrix phase, by treating the numerous artificially embedded Al2O3 particles as nonmetallic inclusions. Because the matrix steel is made from powder as the starting material, oxygen adheres to it during sintering, resulting in an oxygen content exceeding 8 ppm after complete sintering by HIP processing. However, the oxygen produced by this process forms oxides smaller than 1 μm, which do not affect rolling fatigue in environments where hydrogen is present. Conversely, the oxygen content when the bearing steel of this embodiment is manufactured via a melting process is assumed to be 8 ppm or less. The rolling fatigue life of these thrust-type rolling fatigue test specimens in an environment where hydrogen was present was evaluated using the same method as described above.

[0058] Furthermore, with respect to thrust-type rolling fatigue test specimens, the aforementioned target region A in its cross-section. 50 The proportion R of the Al2O3 particles was determined and is shown in Table 5 below.

[0059] In Table 5 below, the delamination life is shown as the ratio of delamination life (life / average life (reference)) to the average delamination life due to matrix delamination in thrust-type rolling fatigue tests for examples where the proportion R was 85% or more, with 1.0 as the baseline.

[0060] [Table 5]

[0061] As shown in Table 5 above, with respect to (lifetime / average lifetime (reference)), Comparative Examples 3-7 showed a decrease compared to Examples 9 and 10. This result indicates that when the ratio R is less than 85%, the delamination life decreases in an environment where hydrogen penetrates the steel material. The difference in delamination life due to the ratio R is related to the range of crack propagation originating from nonmetallic inclusions. When the ratio R is 85% or more, crack propagation originating from nonmetallic inclusions remains within a localized range, and therefore does not lead to inclusion-initiated delamination, but rather to delamination of the matrix phase. On the other hand, when the ratio R is less than 85%, cracks originating from nonmetallic inclusions tend to propagate in both forward and backward directions of rolling, resulting in inclusion-initiated delamination. Therefore, in the bearing steel of the present invention, if the ratio R is 85% or more, it is possible to suppress premature delamination due to inclusion-initiated delamination.

[0062] In the embodiment described above, while HIP processing was shown as a means of making the gap between the nonmetallic inclusion and the matrix phase tightly bonded (making the ratio R 100%), the means are not limited to this, and the target region A is necessary for suppressing inclusion-induced delamination. 50 In order to make the proportion R within the material 85% or more, other industrial means may be used to apply compression processing to the gap between the nonmetallic inclusions and the matrix phase so that the proportion R satisfies this requirement.

[0063] The present invention makes it possible to provide a bearing steel that offers an excellent balance between properties and cost, particularly in its ability to suppress premature delamination initiated by non-metallic inclusions during rolling fatigue in a hydrogen-ingressed environment.

Claims

1. A bearing steel having a surface hardness of 58 HRC or more, C: 0.93 to 1.10% by mass, Si: 0.15 to 0.75% by mass, Mn: 0.25 to 1.70% by mass, Cr: 0.90 to 2.05% by mass, P: 0.025% by mass or less, S: 0.030% by mass or less, The remainder consists of Fe and unavoidable impurities. A bearing steel characterized in that, in a target region where 50% or more of the maximum shear stress generated due to rolling fatigue load acts, the ratio of the total number of nonmetallic inclusions larger than 5 μm in size that exist within the target region and have a closed interface with the base material is 85% or more of the total number of nonmetallic inclusions larger than 5 μm in size that exist within the target region.

2. The bearing steel according to claim 1, characterized in that it contains a chemical component of Mo: 0.06 to 0.60 mass%.

3. The bearing steel according to claim 1 or 2, characterized in that the oxygen content in the bearing steel is 8 ppm or less.

4. A bearing formed of bearing steel according to any one of claims 1 to 3.

5. A bearing component made of bearing steel according to any one of claims 1 to 3.

6. A component subjected to rolling fatigue, formed from bearing steel according to any one of claims 1 to 3.

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