Bearing steel

Abstract

To suppress the damage of a bearing caused by the formation of a white structure derived from hydrogen under the rolling fatigue environment of the bearing.SOLUTION: A steel for a bearing of a high-frequency quenching type containing, by mass%, C:0.52% or more and 0.80% or less, Si: 0.15% or more and 0.65% or less, Mn: 0.30% or more and 0.8% or less, and Cr: 0.70% or more and 3.5% or less, with the balance being Fe and inevitable impurities, wherein an average value of carbon concentrations when a carbon concentration is obtained at an 8 μm pitch in a predetermined analysis range including a band-like region of a positive segregation band and a negative segregation band in the bearing steel is defined as an average concentration, when a zone having a carbon concentration higher than an average concentration is defined as a positive segregation zone, a zone having a carbon concentration lower than the average concentration is defined as a negative segregation zone, and a negative segregation zone included in top 20% having a wide zone width among these negative segregation zones is defined as a wide negative segregation zone, an average value of the zone widths of these wide negative segregation zones is 100 μm or less, and an average value of the carbon concentrations of the respective wide negative segregation zones is 0.500 mass% or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an induction-hardened bearing steel that has excellent rolling contact fatigue properties in an environment where hydrogen penetrates. [Background technology]

[0002] Bearings are required to have an excellent rolling fatigue life, and are manufactured by applying an appropriate hardening heat treatment to the bearing steel. Conventionally, methods such as through quenching and tempering or carburizing, quenching and tempering have been used, but in recent years, the need for carbon neutrality has become a priority in manufacturing processes, and induction hardening has attracted attention as a bearing heat treatment method that produces less CO2 emissions.

[0003] A key feature of induction hardening is the short heating time. This allows for lower CO2 emissions compared to furnace heating. Meanwhile, the short heating time characteristic of induction hardening makes the microstructure after hardening more non-uniform than heat treatments that require relatively long times, such as through hardening or carburizing. With short heating times, the effects of component segregation in the steel before induction hardening lead to differences in hardenability, which manifest as differences in the microstructure after hardening. The inventors have discovered that this characteristic may have a negative impact on the rolling contact fatigue life of bearings in environments where hydrogen penetrates.

[0004] It has been pointed out that hydrogen, which is involved in rolling contact fatigue, may be generated through the decomposition of bearing lubricating oil due to tribochemical reactions on the rolling surfaces. When this hydrogen penetrates and accumulates in the steel and the bearing is subjected to rolling contact fatigue, the hydrogen accelerates fatigue, promoting the initiation of cracks and the growth of larger cracks due to accelerated plastic deformation around the cracks. It is well known that in an environment where hydrogen is present, repeated microscopic plastic deformation around cracks due to rolling contact fatigue causes a change in the microstructure known as white structure change, which may lead to early spalling (see, for example, Patent Document 1).

[0005] Patent Document 2 discloses a method for preventing cracks during induction hardening by controlling microsegregation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6481652 [Patent Document 2] Patent No. 6102183 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to suppress bearing damage caused by the formation of white structure derived from hydrogen in a rolling contact fatigue environment of the bearing. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention provides a bearing steel comprising: (1) a bearing steel containing, by mass%, C: 0.52% to 0.80%, Si: 0.15% to 0.65%, Mn: 0.30% to 0.85%, Cr: 0.70% to 3.5%, the balance being Fe and unavoidable impurities; wherein the average value of carbon concentrations obtained at 8 μm intervals in a predetermined analysis range that includes band-like regions of positive segregation bands and negative segregation bands in the bearing steel is defined as the average concentration; bands with carbon concentrations higher than the average concentration are defined as the positive segregation bands; bands with carbon concentrations lower than the average concentration are defined as the negative segregation bands; and negative segregation bands that are included in the top 20% of these negative segregation bands in terms of width are defined as wide negative segregation bands; the average band width of these wide negative segregation bands is 100 μm or less, and the average carbon concentration of each wide negative segregation band is 0.500 mass% or more.

[0009] (2) The bearing steel according to (1) above, further containing, by mass%, one or two of Ni: 0.03% or more and 0.50% or less, and Mo: 0.03% or more and 0.50% or less.

[0010] (3) The bearing steel according to (1) or (2) above, further containing, by mass%, V: 0.05% or more and 0.20% or less.

[0011] (4) The maximum value of the orthogonal shear stress detected at the position corresponding to the raceway position of the bearing made from the bearing steel is defined as SS max When the above definition is made, the predetermined analysis range is the maximum value SS of the perpendicular shear stress. max The bearing steel according to (1) or (2) above, wherein the ratio of the total surface area to the total surface area is within a range where the ratio is 50% or more of the total surface area. [Effects of the Invention]

[0012] According to the present invention, it is possible to suppress bearing damage caused by the formation of white structure derived from hydrogen in a rolling contact fatigue environment of the bearing. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram (perspective view) of bearing steel (thrust type). [Figure 2] 2 is a cross-sectional view of the bearing steel sheet of FIG. 1 cut in the axial direction along the dashed line. [Figure 3] 1 is a graph showing a schematic diagram of the carbon concentration obtained at each acquisition position (negative segregation zones are hatched). [Figure 4] In FIG. 3, the wide negative segregation zone is hatched in this graph. [Figure 5] 1 shows the test results of a thrust type rolling fatigue test. DETAILED DESCRIPTION OF THE INVENTION

[0014] The bearing steel of the present invention is a bearing steel whose rolling surfaces have been hardened by induction hardening, and refers to a raw material (steel material) or semi-finished product that serves as the base material for bearings. With the intention that this is a steel for induction hardening to be used in bearing applications in mind, the chemical composition of the steel material is limited as follows. The reasons for limiting each component are described below. In the following description, all "%" indications regarding the content of each element mean "% by mass."

[0015] (Essential element) C: 0.52% or more and 0.80% or less Carbon (C) is effective in improving the hardness of induction-hardened parts during induction hardening. Carbon is also an essential element necessary for ensuring the strength of areas not affected by induction hardening (e.g., the core of a steel part heated from the outer periphery by induction, where high-frequency induced current does not flow). Furthermore, carbon also affects the forgeability of steel when it is processed into intermediate or final products, as well as the machinability of steel when it is shaped into a part. A carbon content of less than 0.52% results in insufficient core hardness and reduced strength, so the addition of 0.52% or more is necessary. A carbon content exceeding 0.80% undesirably reduces the toughness of the martensitic structure in the induction-hardened part and also impairs workability, such as machinability and forgeability. Furthermore, as described below, the carbon content significantly affects the behavior of elemental segregation defined in the present invention, and therefore the carbon content must be limited within a specified range. The carbon content is preferably 0.60% or more and 0.80% or less, more preferably 0.64% or more and 0.76% or less.

[0016] Si: 0.15% or more and 0.65% or less Silicon is an essential element for deoxidation, and it also increases the strength of steel materials in high-temperature environments, inhibits structural changes, and improves rolling fatigue life. To fully achieve these effects, it is necessary to add 0.15% or more of silicon. If the silicon content exceeds 0.65%, the hardness of the steel material increases, hindering workability such as machinability and forgeability.

[0017] Mn: 0.30% or more and 0.85% or less Manganese is an essential element necessary for ensuring induction hardenability, and to fully achieve this effect, it is necessary to add 0.30% or more of manganese. If the manganese content exceeds 0.85%, the hardness of the steel increases, hindering workability such as machinability and forgeability. Furthermore, when manganese combines with sulfur to form manganese sulphide (MnS), this can become the starting point for white microstructure changes due to hydrogen-induced fatigue, which is undesirable. Limiting the manganese content to 0.85% or less is intended to prevent the formation of manganese sulphide, which in turn promotes manganese deficiency around the MnS, making it more likely to form microsegregations. A more desirable manganese content is 0.60% or less.

[0018] Cr: 0.70% or more and 3.5% or less Cr is an essential element necessary for ensuring induction hardenability, and also an element that enhances the effect of suppressing white structure changes caused by hydrogen. To fully obtain this effect, Cr must be added in an amount of 0.70% or more, preferably 1.5% or more, and more preferably 2.0% or more. If the Cr content exceeds 3.5%, coarse carbides are formed when the steel is solidified, and white structure changes caused by hydrogen are likely to occur around these coarse carbides. Therefore, the Cr content must be 3.5% or less. As will be described later, the Cr content has a significant effect on the behavior of the element segregation specified in the present invention, and from this viewpoint as well, it is essential that the Cr content be in the amount specified in the present invention.

[0019] Bearing steels consisting of the above essential elements must have a hypereutectoid composition. In this specification, the term "hypereutectoid composition" refers to the intersection of the A3 line and the Acm line when a phase diagram for a given alloy content of C, Mn, Cr, etc. is created using commercially available thermodynamic calculation software utilizing the CALPHD method, and the hypereutectoid composition is defined as +0.1% from the eutectoid point. The reason for specifying the "+0.1%" range is to take into account the component fluctuations in the negative segregation zone, which will be described later. Hypereutectoid steel is generally a steel with a C content of more than approximately 0.8% in the Fe-C binary phase diagram. However, since Cr is an element that exerts a γ-closed type action in the temperature range where austenitization (γ) occurs, it has the effect of shifting the Acm line to the low C side. Therefore, by adding a large amount of Cr, it is possible to achieve a hypereutectoid composition even on the low C side. In other words, even if the C content is approximately the lower limit of 0.52%, by increasing the Cr content within the range of the present invention, it is possible to produce bearing steel with a hypereutectoid composition.

[0020] In contrast to hypereutectoid steel, there is hypoeutectoid steel, which has a carbon content below the eutectoid point. In the case of hypoeutectoid steel, ferrite bands are likely to form in areas where negative segregation of alloy elements occurs during hot-working and cooling of parts. In particular, the ferrite band area has a low carbon content, and short-term heating by induction hardening does not allow sufficient carbon diffusion and homogenization into the ferrite band area, resulting in insufficient martensite fatigue strength and reduced bearing life. In other words, hypoeutectoid steels are prone to poor fatigue properties in the negative segregation zone, potentially resulting in reduced bearing life. Therefore, the bearing steel of the present invention has a hypereutectoid composition.

[0021] By controlling the C and Cr contents within the ranges of the present invention to obtain a hypereutectoid composition and by carrying out the heat treatment described below, it is possible to produce segregation (described below) that is useful for bearing steel.

[0022] (1st arbitrary element) The following elements may be contained as the first optional element. Ni: 0.03% or more and 0.50% or less Ni has the effect of suppressing microstructural changes during the rolling contact fatigue process and improving the rolling contact fatigue life. The addition of Ni is also effective in improving toughness. Therefore, Ni may be added as an optional element. To fully obtain these effects, it is necessary to add 0.03% or more of Ni. If the Ni content exceeds 0.50%, a large amount of retained austenite is generated during quenching of the steel, making it impossible to obtain the desired hardness and increasing the cost of the steel.

[0023] Mo: 0.03% or more and 0.50% or less Adding Mo not only further suppresses the formation of white structure due to rolling contact fatigue caused by hydrogen, but also improves induction hardenability. Therefore, when manufacturing larger parts, it is a good idea to add Mo to the steel as an additive element. To fully obtain these effects, it is preferable to add Mo at 0.03% or more. The effect of Mo saturates at 0.50%, and costs also increase, so the upper limit is set at 0.50%. Here, one of Ni and Mo may be added as the first optional element, or both of them may be added as the first optional elements.

[0024] (2nd arbitrary element) The following elements may be contained as second optional elements. V: 0.05% or more and 0.20% or less V, which forms precipitates such as carbides and carbonitrides after induction hardening, keeps the crystal grains fine during austenitization, ensuring the toughness required for bearing parts. Furthermore, V precipitates trap and fix hydrogen, preventing it from diffusing into areas susceptible to rolling contact fatigue and accelerating fatigue. To achieve a sufficient effect, it is preferable to add 0.05% or more of V. The effect of V saturates at 0.20%, but if the V content exceeds 0.20%, coarse carbides and carbonitrides precipitate, which actually promotes structural changes and adversely affects fatigue life. Therefore, it is recommended that the V content be 0.20% or less. V may be added together with the first optional element, or V alone may be added without the first optional element.

[0025] The balance is Fe and unavoidable impurities. (Inevitable impurities) P is a harmful element that embrittles steel materials and reduces fatigue strength, so it is desirable to reduce it as much as possible. If the P content exceeds 0.020%, the steel materials are embrittled and fatigue strength is significantly reduced. Therefore, the P content is 0.020% or less, and more preferably 0.015% or less. In particular, when using iron scrap as a raw material, it is difficult to industrially reduce the P content to 0%, so P is inevitably contained.

[0026] S is also a harmful element that impairs the hot workability and cold workability of steel materials and reduces fatigue strength, so it is desirable to reduce it as much as possible. If the S content exceeds 0.02%, the impact of reduced workability and fatigue strength will become significant. Therefore, S should be 0.02% or less. More preferably, it should be 0.015% or less. Even more preferably, it should be 0.008% or less. However, it is difficult to industrially reduce the S content to 0%, and in many cases, it is unavoidable to include S.

[0027] In the bearing steel of the present invention, the segregation of carbon is specified. Here, the arithmetic mean value of the carbon concentration obtained by measuring the carbon concentration at 8 μm intervals in a predetermined analysis range of the bearing steel is defined as the average concentration Ave. The analysis range will be described in detail with reference to Figures 1 and 2. A wavelength dispersive X-ray analyzer using an electron beam (electron probe microanalyzer, also known as EPMA) can be used as the analysis device.

[0028] FIG. 1 is a schematic diagram (perspective view) of bearing steel processed into a test bearing shape (thrust type). FIG. 2 is a cross-sectional view of the bearing steel in FIG. 1 cut in the axial direction along the dashed line. Referring to these figures, bearing steel 10 has an inner diameter surface 11 and an outer diameter surface 12, and the thickness direction corresponds to the rolling direction. The hatched area in FIG. 2, as an example, is a "2 mm horizontal x 400 μm vertical area" in the cross section directly below the position corresponding to the raceway position in the thrust type test bearing, which is the specified analysis range. The "horizontal" corresponds to the radial direction of bearing steel 10, and the "vertical" corresponds to the axial direction (rolling direction) of bearing steel 10. Within this analysis range, an electron beam is scanned radially at 8 μm intervals, and the carbon concentration is obtained based on the relationship (calibration curve) between the carbon concentration and the detected amount of characteristic X-rays, which is previously determined based on the detected amount of characteristic X-rays. In the direction perpendicular to the radial direction (thickness direction), the scanning is repeated by shifting the beam by 8 μm and then scanning again in the radial direction (hereinafter referred to as the "repeated scanning method") to obtain the carbon concentration within an analysis area of ​​2 mm wide x 400 μm long. The above electron beam scanning method is one example, and any scanning method that covers the entire analysis range can be selected.

[0029] The concept of the specified analysis range will be explained. In the direction of the depth of the lower part of the raceway where the rolling element is in rolling contact with the bearing, the perpendicular shear stress (hereinafter, this value is referred to as the maximum value SS max This maximum value SS max is considered to be the stress that governs the rate of crack growth that leads to delamination due to rolling fatigue. max The size and depth of the contact stress depend on the diameter of the rolling element, the curvature of the raceway, and the maximum Hertzian contact stress acting on the contact area between the rolling element and the raceway, and can be calculated based on the Hertzian elastic contact theory. max The distribution of the orthogonal shear stress around the depth where the value of the orthogonal shear stress reaches the maximum value SS is calculated in the same manner. maxThe analysis range can be selected from within a depth region that is 50% or more of the original depth. Therefore, the position of the analysis range exemplified in the specification of the present invention (a 2 mm wide × 400 μm long region shown schematically in FIG. 2) is merely an example, and the numerical values ​​of the analysis range may be changed depending on the size of the bearing steel, etc. Analysis processing within the analysis range can also be performed at the stage of the steel before it is processed into the bearing shape. In that case, analysis may be performed at a position in the original steel that corresponds to the position that will become the raceway when processed into the bearing shape (in other words, the intended rolling position).

[0030] In a 2 mm analysis range, carbon concentration data can be obtained at 8 μm intervals to obtain 250 points of carbon concentration data (an example of a scanning method that covers the entire analysis range), and the arithmetic mean value of these can be used as the average concentration Ave. In addition, with the repeated scanning method, carbon concentration data can be obtained from an area 2 mm wide x 400 μm long (250 points in the radial direction x 50 points in the thickness direction = 12,500 points), and the arithmetic mean value of these can be used as the average concentration Ave.

[0031] The graph in Figure 3 shows a schematic representation of the carbon concentration obtained at each acquisition position, with the carbon concentration acquisition position indicated by the distance from the acquisition start position. The horizontal dotted line indicates an example of the average concentration Ave.

[0032] A zone with a carbon concentration lower than the average concentration Ave is defined as a negative segregation zone. In Figure 3, negative segregation zones are shown in gray. On the other hand, a zone with a carbon concentration higher than the average concentration Ave is defined as a positive segregation zone. Negative and positive segregation zones alternate within the analysis range. With the repeated scanning method described above, multiple carbon concentration data (50 points in the above example) are obtained from the same radial acquisition position. Therefore, the arithmetic mean value of these data is compared with the average concentration Ave, and if the arithmetic mean value is lower than the average concentration Ave, it is evaluated as a negative segregation zone, and if the arithmetic mean value is higher than the average concentration Ave, it is evaluated as a positive segregation zone. In other words, the carbon concentration data plotted in Figure 3 may be a single carbon concentration data (a scanning method that covers the entire analysis range) or may be the arithmetic average of multiple carbon concentration data obtained at the same radial position (a repetitive scanning method). Here, when each negative segregation zone is ranked based on its width (larger negative segregation zones are ranked higher), negative segregation zones in the top 20% are defined as particularly wide negative segregation zones. Note that the width refers to the radial length (length on the horizontal axis) of each negative segregation zone.

[0033] Specifically, X is calculated based on the following formula (1), and when Y is an integer smaller than and closest to X, the negative segregation zone from the 1st position to the Yth position can be defined as the wide negative segregation zone. X = 20 × number of negative segregation zones ÷ 100 (1) In Figure 3, there are 16 negative segregation bands, so X = 3.2 and the nearest integer Y is 3. In this case, the negative segregation bands ranked 1st to 3rd in width can be considered as wide negative segregation bands. Figure 4 is a graph corresponding to Figure 3, and wide negative segregation bands within the negative segregation bands are shown in gray.

[0034] The segregation characteristics specified for the bearing steel of the present invention are defined as follows: "the average width of the wide negative segregation bands is 100 μm or less (first condition), and the average carbon concentration of each wide negative segregation band is 0.500 mass% or more (second condition)." In the example shown in Figure 4, there are three wide negative segregation zones, and the first condition for the segregation definition is that the arithmetic mean value of the zone widths of these wide negative segregation zones is 100 μm or less (preferably 80 μm or less, and more preferably 70 μm or less). Furthermore, since each wide negative segregation zone is composed of multiple carbon concentration data, the arithmetic mean value of the carbon concentration of each wide negative segregation zone can be calculated. The second condition for the segregation definition is that all of these arithmetic mean values ​​are 0.500 mass% or more. It should be noted that the "first" in the first condition and the "second" in the second condition do not indicate which condition is better, and the first condition and the second condition are equally important.

[0035] The reason why the first and second conditions are set as the prescribed conditions for segregation will be explained below. The inventors discovered that hydrogen that penetrates steel accumulates in large amounts in negative segregation regions. In a rolling fatigue environment involving hydrogen, this accumulated hydrogen promotes hydrogen embrittlement, making fatigue cracks more likely to initiate in negative segregation regions than in positive segregation regions. This phenomenon becomes more pronounced as the carbon concentration in the negative segregation regions decreases. Whether microcracks that initiate in embrittled regions grow and lead to macroscopic fracture of the bearing, i.e., spalling, can be considered to depend on the size of the region susceptible to embrittlement, i.e., the width of the negative segregation band. Therefore, because fracture is more likely to occur when the negative segregation band is wide, we focused on the average carbon concentration of the wide negative segregation band.

[0036] Unlike deep hardening or carburizing, induction hardening is characterized by the fact that it is performed after heating for a significantly shorter period of time, making it difficult to eliminate the effects of elemental segregation after induction hardening. Therefore, the chemical composition is specified and the microstructure before induction hardening is controlled to prevent the formation of wide negative segregation zones, which have a greater adverse effect on life, even in the negative segregation areas where fatigue cracks are likely to occur preferentially. Therefore, the feature of the present invention is that it does not focus on the minimum value of C in the segregated area, but rather focuses on regulating the band width of the negative carbon segregation band, thereby attempting to prevent macroscopic damage to the bearing.

[0037] If the average width of the wide negative segregation band is 100 μm or less, the presence of a positive segregation band adjacent to the wide negative segregation band (a region where hydrogen is less likely to accumulate and is resistant to hydrogen-related fatigue) makes it easier for cracks to stop, preventing the cracks from growing larger and thereby extending the bearing's lifespan. If the average width of the wide negative segregation band exceeds 100 μm, the fatigue cracks that occur will be long and will propagate within the adjacent positive segregation band, causing macroscopic fracture and shortening the bearing's lifespan. Therefore, the first condition for specifying segregation is that "the average width of the wide negative segregation band is 100 μm or less." Furthermore, as mentioned above, the occurrence of fatigue cracks promoted by hydrogen embrittlement becomes more pronounced as the carbon concentration in the negative segregation zone decreases. To prevent this, the second condition in the segregation specification is that "the average carbon concentration in each wide negative segregation zone must be 0.500 mass% or more."

[0038] Next, a method for producing the bearing steel of the present invention will be described. The steel having the above-described chemical composition of the present invention is melted using a vacuum induction melting apparatus or the like, and then hot forged to obtain a round bar having a predetermined cross-sectional area, in this case a diameter of 65 mm. A hot-forged steel with a diameter of 65 mm is normalized and then annealed. The annealing process spheroidizes the carbide particles in the steel. The bearing steel according to the present invention has spheroidized carbide particles and satisfies the first condition of segregation (the average width of the wide negative segregation zones is 100 μm or less) and the second condition (the average carbon concentration of each wide negative segregation zone is 0.500 mass% or more). Annealing conditions for obtaining a steel with this configuration require holding within a temperature range on the phase diagram where austenite and carbide coexist in the hypereutectoid composition region and slow cooling from that temperature range. For example, holding at 800°C for 1 hour followed by cooling at 15°C / h facilitates the spheroidization of the carbides. The end temperature of slow cooling is preferably such that no austenite remains at the end of the slow cooling; for example, slow cooling to approximately 500°C is sufficient. The annealing conditions are not limited to those exemplified above, and conditions that allow the treatment to be completed in the minimum time required depending on the steel type can be adopted. Here, when a steel having the above-mentioned composition is annealed, a large amount of spheroidized carbides are formed in the positive segregation zone, and a smaller amount is formed in the negative segregation zone. Before the annealing, a soaking treatment may be performed as needed. The soaking treatment is a process in which the steel is held at 1100°C to 1300°C for 3 to 5 hours. By performing the soaking treatment under these conditions, carbon diffusion between the positive and negative segregation zones is promoted, thereby mitigating segregation. This reduces the difference in carbon concentration between the positive and negative segregation zones while maintaining the arithmetic mean carbon concentration of the wide negative segregation zone at 0.500 mass% or more. This reduces the difference in carbon concentration between the positive and negative segregation zones, thereby improving the surface hardness of the steel as a whole while maintaining the tendency toward longer life.

[0039] If high-temperature annealing, which simply involves slow cooling from the austenite region, is performed instead of the above-mentioned annealing treatment, it is not possible to produce bearing steel that satisfies the first condition for segregation (the average width of the wide negative segregation bands must be 100 μm or less) and the second condition (the average carbon concentration of each wide negative segregation band must be 0.500 mass% or more). After the above-mentioned annealing treatment, the round bar is roughly machined to obtain a ring-shaped steel material.

[0040] The obtained ring-shaped steel material is subjected to induction hardening. The hardening position may include a position that includes the raceway position (in other words, the position where the rolling elements roll on the raceway surface 11). Therefore, induction hardening may be performed on a part of the ring-shaped steel material that includes the raceway position, or on the entire ring-shaped steel material.

[0041] When induction hardening is performed, the short heat treatment time suppresses the dissolution of carbides in the positive segregation zone with a high carbon concentration (in other words, the large amount of alloying elements slows the diffusion of carbon and suppresses dissolution), while in the negative segregation zone with a low carbon concentration, carbides dissolve relatively quickly, resulting in the amount of carbon dissolved in the positive and negative segregation zones being equalized. This homogenizes the microstructure of the steel after induction hardening, enabling a stable improvement in hydrogen environment life.

[0042] It is common knowledge that bearing steel requires a sufficiently high surface hardness, specifically a hardness of at least 58 HRC on the Rockwell hardness scale, and is manufactured to meet the required surface hardness for that purpose. Furthermore, because bearings must withstand heavy loads during operation, they require a sufficiently high hardness from the surface to a certain depth. This depth can be referred to as the effective hardened depth, and the required depth varies depending on the size and design of the part. The induction hardening temperature is adjusted to meet these quality requirements, but this temperature also varies depending on the composition of the steel. In other words, when manufacturing bearing steel from steel satisfying the above-mentioned component ranges, the induction hardening temperature is adjusted according to the composition to meet the aforementioned quality requirements (effective hardened depth and surface hardness). The quality requirements (effective hardened depth and surface hardness) vary depending on the application of the bearing steel, and are not limited in this specification. If induction hardening is performed at a temperature that does not satisfy the above-mentioned quality conditions, it becomes difficult to produce bearing steel that satisfies the first and second segregation conditions.

[0043] After induction hardening, the steel is tempered and then subjected to surface finish polishing. These tempering and surface finish polishing may be carried out by any commonly used method, and therefore detailed description thereof will be omitted.

[0044] In the above embodiment, thrust bearing steel has been described, but the present invention is not limited to this and can be applied to other bearing steels (for example, radial bearing steel). The above heat treatment should be performed so as to satisfy the first and second conditions for segregation within the analysis range defined above.

[0045] (Example) Next, the present invention will be specifically described with reference to examples. A 100 kg steel ingot was melted using a vacuum induction melting apparatus or the like. The composition of the steel ingot is shown in Table 1 (however, Table 1 does not list Fe and unavoidable impurities). As mentioned above, the elements that affect the component segregation are C and Cr, so the contents of the other essential elements (Si, Mn) and optional elements (Ni, Mo) were fixed (except for Example 4, where Mo was set to 0 mass%) to allow for appropriate comparison. Assuming that they are present as unavoidable impurities, the P content was set to 0.01% and the S content was set to 0.005%. [Table 1]

[0046] The molten steel was hot forged to obtain a φ65 mm round bar, which was then subjected to a normalizing treatment. The normalizing treatment was performed by heating at 900°C for 1 hour and then air-cooling. After the normalizing treatment, the round bar was subjected to an annealing treatment to spheroidize the carbides (Examples 1 to 5). After the normalizing treatment, the round bar was subjected to an annealing treatment to spheroidize the carbides (Examples 6 to 7). (Hereinafter, unless there is a need to distinguish between Examples 1 to 7, these will also be referred to as "Example materials"). Finally, the round bar was subjected to a high-temperature annealing treatment (Comparative Examples 1 and 2). (Hereinafter, unless there is a need to distinguish between Examples 1 to 7, these will also be referred to as "Comparative materials"). For the soaking treatment in Examples 6 and 7, the round bar was held at 1200°C for 4 hours. Five of each Example material and each Comparative material were prepared. The compositions of the starting materials for Examples 1 to 7 and Comparative Examples 1 and 2 are shown in Table 1. The maximum temperature of the annealing treatment to spheroidize the carbides was 800°C, which corresponds to the two-phase region of ferrite and austenite, and the holding time at that temperature was 1 hour, followed by slow cooling at a rate of 15°C / h to 500°C. The maximum temperature of the high-temperature annealing treatment was 870°C, which corresponds to the austenite single-phase region, and the holding time at that temperature was 1 hour, followed by slow cooling at a rate of 30°C / h to 500°C.

[0047] Ring-shaped test specimens were prepared by rough machining of the working material and the comparative material (hereinafter, referred to as test specimens when there is no need to distinguish between them). The dimensions of the ring-shaped test specimens were outer diameter: 52 mm, inner diameter: 20 mm, and thickness: 6.8 mm. Induction hardening was performed so as to include a position (φ38.5 mm) corresponding to the raceway diameter of the ring-shaped test specimens. The specimens were then subjected to a thrust-type rolling fatigue test to simulate the fatigue state of bearings. The quality requirements for proper testing were set to "surface hardness of 700 Hv or more, effective hardening depth of 1 mm or more," and the induction hardening temperature was set to satisfy these quality requirements.

[0048] After induction hardening, the specimens were tempered (held at 170°C for 2 hours, then air-cooled), and the surfaces were polished to a mirror finish (average surface roughness Ra = 0.004 μm) to obtain thrust-type rolling fatigue test specimens.

[0049] The Vickers hardness of the surface at the induction hardened position was measured. The load during measurement was 300 gf. Table 2 shows the results of measuring the Vickers hardness of the surface of the test specimens corresponding to each example and comparative example. [Table 2]

[0050] In order to measure the spalling life in a hydrogen penetration environment, hydrogen was introduced into the test specimen using the cathodic charging method, and then a thrust type rolling fatigue test was conducted under specified conditions (maximum contact pressure: 4.5 GPa, rolling elements: three 3 / 8 inch steel balls, lubrication: ISO VG68 oil bath lubrication), and the rolling fatigue life (number of cycles, in this case L 50 The lifespan was evaluated. 50 The lifespan is the number of revolutions (or time) at which 50% of the specimens, for example, when fatigue testing 10 specimens simultaneously, continue to rotate without delamination, and can be considered the average lifespan. To introduce hydrogen into the specimens, they were electrolytically charged (cathodic charging) in an aqueous solution containing an electrolyte, with the specimens acting as cathodes. The conditions were: aqueous solution: 3% sodium chloride, 0.3% ammonium thiocyanate; specific solution volume: 500 mL per specimen; temperature: 50°C; average current density: 0.2 mA / cm; duration: 8 hours. The lifespan of the specimens was determined by detecting the onset of delamination using a vibration meter attached to the thrust-type rolling contact fatigue testing machine. The number of revolutions at which the testing machine automatically stopped due to a mechanism linked to the vibration corresponding to delamination was defined as the delamination lifespan.

[0051] FIG. 5 shows the test results of the thrust type rolling fatigue test, with the horizontal axis representing the number of cycles and the vertical axis representing the cumulative failure probability. The data plotted with ● represents the results of Example 2, the data plotted with ■ represents the results of Example 7, and the data plotted with ▲ represents the results of Comparative Example 2. 50 The lifespan is summarized in Table 3. However, the L 50 The life is the same as that of SUJ2, a general-purpose bearing steel. 50 The values ​​are relative to the lifespan, which is set at 1.00. [Table 3] The experimental material has a better peeling life than the comparative material.

[0052] The first and second segregation conditions were evaluated using the methods described in the embodiments. An electron probe microanalyzer (EPMA) was used, an EPMA-1600 manufactured by Shimadzu Corporation. Tables 4 and 5 show the test results. Table 4 shows the width of each wide negative segregation zone and the average value of the width. Table 5 shows the average carbon concentration of each wide negative segregation zone. [Table 4] [Table 5]

[0053] In all of Examples 1 to 7, the average width of the wide negative segregation zone was 100 μm or less, and the average carbon concentration in all of the wide negative segregation zones was 0.500 mass % or more. In contrast, in Comparative Example 1, the average carbon concentration was 0.500 mass% or more in all of the wide negative segregation zones, but the average width of the wide negative segregation zones was 102.7 μm. In Comparative Example 2, the average width of the wide negative segregation zones was 131.2 μm, and wide negative segregation zones with an average carbon concentration of less than 0.500 mass% were confirmed. [Explanation of symbols]

[0054] 10 Bearing steel 11 Inner diameter surface 12 Outer diameter surface

Claims

1. A bearing steel containing, by mass%, C: 0.52% or more and 0.80% or less, Si: 0.15% or more and 0.65% or less, Mn: 0.30% or more and 0.85% or less, Cr: 0.70% or more and 3.5% or less, the balance being Fe and unavoidable impurities, The average value of the carbon concentrations obtained at 8 μm intervals in a predetermined analysis range including the positive segregation zone and the negative segregation zone in the bearing steel is defined as the average concentration, A zone having a carbon concentration higher than the average concentration is defined as the positive segregation zone, A zone having a carbon concentration lower than the average concentration is defined as the negative segregation zone, When the negative segregation zones included in the top 20% of these negative segregation zones with the widest zone width are defined as wide negative segregation zones, The average width of these wide negative segregation zones is 100 μm or less, and the average carbon concentration of each wide negative segregation zone is 0.500 mass% or more. Steel for bearings.

2. Further, in mass%, it contains one or two of Ni: 0.03% or more and 0.50% or less, and Mo: 0.03% or more and 0.50% or less, The bearing steel according to claim 1.

3. Further, V: 0.05% or more and 0.20% or less by mass%, 3. Bearing steel according to claim 1 or 2.

4. The maximum value of the orthogonal shear stress detected at a position corresponding to the raceway position of a bearing made from the bearing steel is defined as SS. ｍａｘ When we define The predetermined analysis range is the maximum value of the perpendicular shear stress SS ｍａｘ Included in the area where it is 50% or more of 3. The bearing steel according to claim 1 or 2.

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