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: 0.12-0.50 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. 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, as well as 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 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] 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, in the manufacturing process of general-purpose atmospheric melted steel, it is not easy to excessively control the inclusion diameter to 50 μm or less, and pursuing excessive reduction in the diameter of non-metallic inclusions as a measure to extend the 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. 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 the steel, it was 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. Therefore, it was found that rolling fatigue life can be improved without excessively refining the nonmetallic inclusion diameter to 50 μm or less, as in the bearing steel described in Patent Document 1, and the present invention was completed.

[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: 0.12-0.50 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, 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 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, 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]

[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 steel for bearings according to this embodiment can be used for all or part of the components (bearing components) constituting the bearing, or for components subjected to rolling fatigue (rolling fatigue similar to the operating mechanism of the bearing).

[0016] The steel for bearings according to this embodiment has the chemical components described below and, as the balance, Fe and unavoidable impurities. The chemical components are determined by molten steel analysis (JIS G0320) or chemical analysis of steel materials and components. The chemical components in the steel for bearings can be adjusted in the molten steel refining process in the manufacturing process of the steel for bearings. Hereinafter, the content ratios of the respective chemical components will be described.

[0017] (Content ratio of C) The content ratio of C is 0.12 to 0.50% by mass. C is an element that affects the hardenability involved in the hardness of the core of the component when generating the component from the steel for bearings, the forging properties in hot and cold forging, and the machinability. By setting the content ratio of C to 0.12 to 0.50% by mass, it is possible to ensure the strength of the core of the component made of the steel for bearings and suppress the inhibition of the workability such as machinability and forging properties.

[0018] When the content ratio of C 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, when the content ratio of C is higher than 0.50% by mass, the workability such as machinability and forging properties is inhibited due to the increase in the hardness of the steel material. Here, the content ratio of C is preferably 0.18% by mass or more.

[0019] (Content ratio of Si) The Si content is 0.15 to 0.65 mass%. Si is an element necessary for deoxidation and also increases the strength of steel, contributing to the suppression of structural changes in steel due to rolling fatigue and the improvement of 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 Mn content is 0.30 to 1.80 mass%. Mn is an element necessary for ensuring hardenability, and by increasing the amount of retained austenite when steel is carburized or carbonitrided, it is possible to suppress the development of white structure changes in the matrix that appear due to fatigue progression caused by hydrogen in rolling fatigue. In order to obtain these effects, the Mn content must be 0.30 mass% or more. Here, the Mn content is preferably 0.50 mass% or more. More preferably, the Mn content is 0.75% or more.

[0021] On the other hand, if the Mn content exceeds 1.80 mass%, the hardness of the steel increases, which impairs machinability and forgeability. Furthermore, the combination of Mn with S to form MnS can cause stress concentration during rolling fatigue and may become the starting point for hydrogen-induced white structure changes. Here, it is preferable that the Mn content be 1.20 mass% or less.

[0022] (P content) The P content shall be 0.030 mass% or less. P is an unavoidable impurity element, and if the P content is higher than 0.030 mass%, it will cause embrittlement of the steel and reduce its fatigue strength. Here, the P content is preferably 0.020 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.

[0023] (S content) The sulfur content should be 0.030 mass% or less. S is an unavoidable impurity element, and if the sulfur content is higher than 0.030 mass%, it will hinder cold workability and reduce 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 sulfur and other unavoidable impurities other than the P mentioned above.

[0024] (Cr content) 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 must be 0.70 mass% or more. Preferably, the Cr content is 1.00% or more, and more preferably 1.60% 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 can be 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 fatigue progression of the matrix in environments where hydrogen penetrates. To obtain these effects, it is preferable that the Mo content be 0.06 mass% or more, and more preferably 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 that the Mo content be 0.70 mass% or less. Here, it is more preferable that the Mo content be 0.50 mass% or less, and more preferably 0.37% 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 can be 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, and more preferably 0.40 mass% or more. 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. Considering this point, it is preferable to set the Ni content to 2.00 mass% or less. Here, the Ni content is more preferably 1.80 mass% or less, and more preferably 1.50 mass% or less.

[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 changes in the white structure caused by hydrogen. Furthermore, Nb 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 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 changes in the white structure 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, which has an adverse effect. Therefore, it is best to keep the Nb content below 0.20% by mass. Thus, 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] (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.

[0032] 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 50is a region where, in a component made of the shaft steel of the present embodiment or a component similar to the operating mechanism of a bearing part or bearing made of the shaft steel of the present embodiment, where rolling fatigue is applied, more than 50% of the maximum shear stress generated with the application of rolling fatigue acts. Also, the target region A 50 is a region 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).

[0033]

Equation

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] (Hardness) The surface hardness of bearing steel can be 58 HRC or higher. This hardness is measured 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, it 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.

[0044] (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]

[0045] (Preparation of test specimens) For the examples (Examples 1-8 described later) and comparative examples (Comparative Examples 1 and 2 described later), steel was prepared with the chemical compositions shown in Table 1 below, with Fe and unavoidable impurities as the remainder. The steel type of the comparative example (Steel Material 2) is SUJ2 as specified in JIS G4805.

[0046] [Table 1]

[0047] 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.

[0048] Next, a micro-drill with a tip diameter of 0.20 mm was used to create drill holes with a diameter of 0.20 mm and a depth of 1 mm on the buffed surface of the test specimen. Artificial spherical Al2O3 particles were placed into these drill holes to simulate nonmetallic 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, Comparative Example 2), and 200 μm (Example 8).

[0049] 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.

[0050] After HIP processing, Examples 1-8 and Comparative Example 1, which used steel material 1, were subjected to normalizing in the same manner as described above, while Comparative Example 2, which used steel material 2, was subjected to normalizing and spheroidizing annealing in the same manner as described above. Next, for Examples 1-8 and Comparative Example 2, the test pieces (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 found to be 100% (no gap between Al2O3 particles and the matrix). Furthermore, for Comparative Example 1, in order to reproduce the gap around nonmetallic inclusions that can inevitably occur in the manufacturing process of normal 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 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. Observations showed that the ratio R of the gap between the Al2O3 particles and the matrix phase was 0% (a gap existed between the Al2O3 particles and the matrix phase). Subsequently, the hardness of the steel was adjusted to 62 HRC by carburizing (carburizing at 930°C for 3 hours followed by oil cooling) and then tempering (heating at 180°C for 1.5 hours followed by air cooling). The hardness of the steel was adjusted to 62 HRC by quenching (heating temperature 835°C, followed by oil cooling) and then tempering (heating at 180°C for 1.5 hours followed by air cooling).

[0051] 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.

[0052] 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.

[0053] [Table 2]

[0054] (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.

[0055] 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.

[0056] 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.

[0057] [Table 3]

[0058] 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.

[0059] 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").

[0060] On the other hand, as mentioned above, target area A 50 Regarding the above-mentioned ratio R for the gap between the Al2O3 particles, which have been adjusted to be internal, and the surrounding matrix, if the ratio R is 85% or more, it is marked as "○", and if the ratio R is less than 85%, it is marked as "×".

[0061] 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.

[0062] [Table 4]

[0063] According to Table 4 above, in Examples 1-8 and Comparative Example 2, matrix phase delamination was dominant, while in Comparative Example 1, inclusion-initiated delamination was observed. 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%. Furthermore, in Example 7 and Comparative Example 2, although the diameter of the Al2O3 particles (maximum inclusion diameter √area_max) was the same (150 μm), the delamination life differed due to the difference in the chemical composition of steel materials 1 and 2. The delamination life of Comparative Example 2 was approximately equivalent to the basic rated life of the bearing (a 90% reliability life that yields a bearing life of 1 million rotations), demonstrating that the bearing steel having the chemical composition of the present invention is effective in extending the life of matrix phase delamination and inclusion-initiated delamination in a hydrogen-ingress environment.

[0064] Regarding the values ​​(lifetime / average lifetime (reference)) shown in Table 4 above, Comparative Example 1 showed a significant decrease compared to Examples 1-8. 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.

[0065] Based on these results, in order to further investigate the effect of 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 and Comparative Example 1), 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 test specimens were subjected to carburizing treatment (carburizing treatment at 930°C for 3 hours followed by oil cooling), and then tempered (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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] [Table 5]

[0070] 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. 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 higher, crack propagation originating from nonmetallic inclusions remains localized, 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, if the ratio R is 85% or higher, it is possible to suppress short-life delamination due to inclusion-initiated delamination.

[0071] 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.

[0072] 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, 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 composition of Ni: 0.30 to 2.00% by mass.

3. The bearing steel according to claim 1 or 2, characterized in that the bearing steel contains one or more 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 formed from bearing steel according to any one of claims 1 to 4, which is subjected to rolling fatigue.

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