Rolling bearing

Abstract

Provided is a rolling bearing that can be used in a low-temperature, lean-lubrication environment and can achieve exceptional durability by further improving wear resistance. A rolling bearing (1) comprises a bearing ring composed of an outer ring (10) and an inner ring (20), and a plurality of rolling elements (30) rollably held between the outer ring (10) and the inner ring (20), the rolling bearing (1) being used in a low-temperature, lean-lubrication environment. At least one of the outer ring (10), the inner ring (20), and the rolling elements (30) is formed using a steel material containing 0.1-0.7 mass% of C, 1.1-1.8 mass% of Cr, 0.7-1.3 mass% of Si, 0.3-1.1 mass% of Mn, and 0.6-1.3 mass% of Mo, and is subjected to a carbonitriding process and a quenching / tempering process in the stated order. The surface carbon concentration is 0.6-1.5 mass%, and the surface nitrogen concentration is 0.05-0.8 mass%.

Description

Rolling bearing

[0001] The present invention relates to a rolling bearing, and more particularly, to a rolling bearing used in a lean and lubricated environment in which a liquid having a viscosity of VG2 or less at 40°C flows between the outer ring, the inner ring, and the rolling elements at a low temperature of -30°C or lower (hereinafter, the low temperature means a temperature of -30°C or lower).

[0002] In recent years, hydrogen, which does not generate any greenhouse gas, has attracted attention in order to achieve a decarbonized society. In addition, hydrogen can be produced from various resources and the procurement sources can be dispersed both at home and abroad, so it is an energy with low supply and procurement risks. Generally, by cooling hydrogen to -253°C and liquefying it, a large amount of hydrogen can be stored and transported. Specifically, liquefied hydrogen becomes about 1 / 800 of the volume of hydrogen gas, so by liquefying it, a large amount of hydrogen can be stored and transported. However, in bearings used at a temperature of -253°C for liquefying hydrogen, for example, various risks peculiar to low temperatures occur.

[0003] For example, conventionally, various devices have been proposed for pumping low-temperature fluids such as liquefied natural gas (LNG) and liquefied petroleum gas (LPG). Patent Document 1 discloses a submerged pump in which a throttle structure is provided on the downstream side of the bearing, thereby increasing the pressure around the bearing, preventing the vaporization of the handling fluid around the bearing, and preventing poor lubrication of the bearing. Patent Document 2 proposes a rolling bearing having rolling elements made of ceramics with a smaller Young's modulus than when made of silicon nitride. According to Patent Document 2, it is described that wear resistance and corrosion resistance can be improved while suppressing a decrease in life.

[0004] Japanese Patent No. 4300088 Japanese Unexamined Patent Application Publication No. 2010-1984

[0005] As mentioned above, rolling bearings in liquefied gas pump systems and the like are used in low-temperature environments because they come into contact with liquefied gas. Furthermore, if lubricating oil is mixed with the liquefied gas, the liquefied gas becomes contaminated, making it impossible to lubricate the rolling elements of the rolling bearings with ordinary lubricating oil. Consequently, rolling bearings incorporated into liquefied gas pump systems operate under extremely harsh conditions, making them prone to wear between the rolling elements and the outer and inner ring raceways.

[0006] The structures and bearings described in Patent Documents 1 and 2 above aim to prevent lubrication failure and improve wear resistance and corrosion resistance. However, in recent years, there has been a demand for further improvements in wear resistance, and there is room for improvement, including a review of the materials used.

[0007] The present invention has been made in view of the above problems, and aims to provide a rolling bearing that can be used in low-temperature and dilute-lubricated environments and that can achieve even greater wear resistance and superior durability.

[0008] The above objective of the present invention is achieved by the following configuration [1] relating to a rolling bearing.

[0009] [1] A rolling bearing comprising a raceway consisting of an outer ring and an inner ring, and a plurality of rolling elements held so as to be rotatable between the outer ring and the inner ring, used in a dilute lubrication environment where a liquid with a viscosity of VG2 or less at 40°C flows between the outer ring, the inner ring and the rolling elements at a low temperature of -30°C or lower, wherein at least one of the outer ring, the inner ring and the rolling elements is formed from a steel material containing C: 0.1% by mass or more and 0.7% by mass or less, Cr: 1.1% by mass or more and 1.8% by mass or less, Si: 0.7% by mass or more and 1.3% by mass or less, Mn: 0.3% by mass or more and 1.1% by mass or less, Mo: 0.6% by mass or more and 1.3% by mass or less, and is subjected to carburizing and nitriding treatment and quenching and tempering treatment in that order, and has a surface carbon concentration of 0.6% by mass or more and 1.5% by mass or less, A rolling bearing characterized by having a surface nitrogen concentration of 0.05% by mass or more and 0.8% by mass or less.

[0010] Furthermore, preferred embodiments of the present invention relating to rolling bearings are described in the following [2] to [5].

[0011] [2] The rolling bearing according to [1], characterized in that the Vickers hardness at room temperature of the outer ring, the inner ring, and the rolling elements, which are made of the steel material, is HV700 or higher and HV800 or lower.

[0012] [3] The rolling bearing according to [1] or [2], characterized in that each of the raceways has a raceway surface on which the rolling elements roll, at least one of the outer ring and the inner ring is formed using the steel material, and the volume fraction of retained austenite in the region at a depth of 50 μm from the surface of the raceway surface of the raceway formed using the steel material is 2% or less.

[0013] [4] A rolling bearing according to any one of [1] to [3], characterized in that the Vickers hardness at -196°C of the outer ring, the inner ring, and the rolling elements, which are made of the steel material, is HV800 or higher and HV900 or lower.

[0014] [5] A rolling bearing according to any one of [1] to [4], characterized in that it is used in a pumping device for liquefied gas.

[0015] According to the present invention, it is possible to provide a rolling bearing that can be used in low-temperature and dilute-lubrication environments, such as a rolling bearing incorporated into a liquefied gas pump system, and that can achieve even greater wear resistance and superior durability.

[0016] Figure 1 is a schematic cross-sectional view showing a rolling bearing according to an embodiment of the present invention. Figure 2 is a schematic diagram showing an example of a liquefied gas pump device using the rolling bearing according to this embodiment. Figure 3 is a schematic diagram showing a method for measuring the wear depth of a test material. Figure 4 is a graph showing the comparison results of the wear depth of Comparative Example No. 1 and Invention Example No. 1. Figure 5 is a microstructure observation photograph taken after corroding the surface of a groove formed in the test material of Comparative Example No. 1. Figure 6 is a graph showing an example of the surface shape of the test material of Comparative Example No. 2 after a thrust test.

[0017] As a result of diligent research by the inventors of this invention, they found that when bearings are used in low-temperature and dilute environments, carbides on the track surface detach, and this detachment of carbides causes deterioration of the raceway surface. In other words, when carbides that contribute to improved wear resistance detach, the mechanical strength of the bearing raceway surface or the surface of the rolling elements decreases, and wear resistance is significantly reduced.

[0018] As a result of further investigation by the inventors of the present invention, they found that the wear resistance of bearing members can be improved by appropriately controlling the size and content of carbides contained in the raceway surfaces of the outer ring and inner ring. Specifically, it is believed that reducing the size of carbides in a specific region of the bearing member and reducing the carbide content can reduce the collapse of the raceway surface due to carbide shedding.

[0019] Furthermore, the inventors of this invention hypothesized that eutectic carbides, in particular, have a significant influence on the acceleration of wear among carbides, and conducted further investigations. As a result, they found that eutectic carbides, which are expected to contribute to improved wear resistance under normal temperature conditions, may conversely work to increase the amount of wear under low temperature conditions. This invention is based on the above findings.

[0020] [Rolling Bearing] The rolling bearing according to the present invention will be described in detail below with reference to the drawings. Note that the present invention is not limited to the embodiments described below, and can be modified as appropriate without departing from the spirit of the invention. Furthermore, the rolling bearing according to the present invention is a rolling bearing used in a lean lubrication environment at low temperatures of -30°C or below, in which a liquid with a viscosity of VG2 or less at 40°C flows between the outer ring, the inner ring, and the rolling elements. VG2 represents the viscosity grade number specified by ISO, and a viscosity of VG2 or less means that the kinematic viscosity is 2.42 (mm²). ２ This indicates that the value is less than or equal to / s. In this specification, the operating environment of the rolling bearing according to the present invention may be referred to as a low-temperature, dilute lubrication environment.

[0021] Figure 1 is a schematic cross-sectional view showing a rolling bearing according to an embodiment of the present invention. As shown in Figure 1, the rolling bearing 1 has a raceway consisting of an outer ring 10 and an inner ring 20, and a plurality of rolling elements 30 that are held to roll freely between the outer ring 10 and the inner ring 20. More specifically, an outer ring raceway surface 10a is provided on the inner diameter surface of the outer ring 10, and an inner ring raceway surface 20a is provided on the outer diameter surface of the inner ring 20, and the outer ring 10 rotates relative to the inner ring 20 as the rolling elements 30 roll between the outer ring raceway surface 10a and the inner ring raceway surface 20a.

[0022] In this embodiment, at least one of the outer ring 10, inner ring 20, and rolling element 30 is formed using a steel material having a specific composition. When the outer ring 10 is formed using the steel material described below, it is sufficient that at least the main portion of the outer ring 10, including the outer ring raceway surface 10a, is made of the following steel material. Similarly, when the inner ring 20 is formed using the steel material described below, it is sufficient that at least the main portion of the inner ring 20, including the inner ring raceway surface 20a, is made of the following steel material.

[0023] In this embodiment, a steel material with appropriate alloy components added is used, and the bearing material is subjected to a carburizing and nitriding treatment to reduce eutectic carbides. As a result, the wear resistance of the rolling bearing in low-temperature and dilute lubrication environments can be maintained, ensuring durability. Furthermore, since it is preferable for the components of a rolling bearing used in low-temperature environments to have a stable structure and strong resistance to temperature changes, it is preferable for the retained austenite in the components of the rolling bearing to be low. The components contained in the steel material and their content, as well as the heat treatment conditions for the rolling bearing material, will be described in detail below.

[0024] <C: 0.1% by mass or more and 0.7% by mass or less> If the carbon (C) content in the steel material is less than 0.1% by mass, the required level of cleanliness for a rolling bearing cannot be obtained. Therefore, the carbon (C) content in the steel material should be 0.1% by mass or more, preferably 0.2% by mass or more, and more preferably 0.3% by mass or more, relative to the total mass of the steel material. On the other hand, if the carbon (C) content in the steel material exceeds 0.7% by mass, the amount of retained austenite increases, reducing the dimensional stability of the bearing, or eutectic carbides are formed, resulting in a shorter lifespan. Therefore, the carbon (C) content in the steel material should be 0.7% by mass or less, preferably 0.6% by mass or less, and more preferably 0.5% by mass or less, relative to the total mass of the steel material.

[0025] <Cr: 1.1% by mass or more and 1.8% by mass or less> Cr (chromium) is an element that improves fatigue life in environments where metal contact occurs. If the Cr content in the steel material is less than 1.1% by mass, the structural stability will decrease, and surface fatigue may occur. Therefore, the Cr content in the steel material should be 1.1% by mass or more, preferably 1.2% by mass or more, and more preferably 1.3% by mass or more, relative to the total mass of the steel material. On the other hand, if the Cr content in the steel material is too high, cold workability, machinability, and carburizing properties will decrease, and manufacturing costs will increase significantly. In addition, coarse eutectic carbides may be formed, which may significantly reduce the fatigue life and strength of rolling bearings. Therefore, the Cr content in the steel material should be 1.8% by mass or less, preferably 1.7% by mass or less, and more preferably 1.6% by mass or less, relative to the total mass of the steel material.

[0026] <Si: 0.7% by mass or more and 1.3% by mass or less> Silicon (Si) is an essential element for forming Si-containing carbides or carbonitrides, or Si-X (where X is at least one of Mn, Mo, and Cr), which are effective in suppressing seizing and wear due to friction. If the Si content in the steel material is less than 0.7% by mass, the deoxidation effect during steelmaking cannot be sufficiently obtained, making it difficult to improve seizing resistance and wear resistance. Therefore, the Si content in the steel material should be 0.7% by mass or more, preferably 0.8% by mass or more, and more preferably 0.9% by mass or more, relative to the total mass of the steel material. On the other hand, if the Si content in the steel material is too high, the effect of improving the seizing resistance and wear resistance of carbides or carbonitrides decreases. Therefore, the Si content in the steel material should be 1.3% by mass or less, preferably 1.2% by mass or less, and more preferably 1.1% by mass or less, relative to the total mass of the steel material.

[0027] <Mn: 0.3% by mass or more and 1.1% by mass or less> Manganese (Mn), like Si, is an element necessary for deoxidation during steelmaking. In addition, by being added to the raw material as a deoxidizing agent during steelmaking, Mn improves hardenability and contributes to improving strength and rolling fatigue life after heat treatment. If the Mn content in the steel material is less than 0.3% by mass, the deoxidation effect during steelmaking cannot be sufficiently obtained, making it difficult to improve seizure resistance, wear resistance, strength after heat treatment, and rolling fatigue life. Therefore, the Mn content in the steel material should be 0.3% by mass or more, preferably 0.4% by mass or more, and more preferably 0.5% by mass or more, based on the total mass of the steel material. On the other hand, if the Mn content in the steel material is too high, retained austenite, which is harmful to dimensional stability, is generated, and workability also deteriorates. Therefore, the Mn content in the steel material should be 1.1% by mass or less, preferably 1.0% by mass or less, and more preferably 0.9% by mass or less, based on the total mass of the steel material.

[0028] <Mo: 0.6% by mass or more and 1.3% by mass or less> Mo (molybdenum) is an element that dissolves in the martensitic matrix and enhances hardenability, tempering softening resistance, and corrosion resistance. Mo also forms fine carbides, preventing grain coarsening during heat treatment, stabilizing the structure, and, like Cr, improving fatigue life in environments with metal-to-metal contact. Therefore, the Mo content in steel materials should be 0.6% by mass or more, preferably 0.7% by mass or more, and more preferably 0.8% by mass or more, relative to the total mass of the steel material. On the other hand, if the Mo content in steel materials is excessive, cold workability and machinability decrease, and manufacturing costs increase significantly. In addition, coarse eutectic carbides may be formed, which can significantly reduce the fatigue life and strength of rolling bearings. Therefore, the Mo content in the steel material should be 1.3% by mass or less, preferably 1.2% by mass or less, and more preferably 1.1% by mass or less, relative to the total mass of the steel material.

[0029] In this embodiment, the components whose content is controlled among the components contained in the steel material are as described above, and the remainder of the components of the steel material consists of Fe (iron), components whose content is not controlled, and unavoidable impurities. Examples of components whose content is not controlled include Ti, Cu, Ni, S, P, O, etc. Ti appears as a nonmetallic inclusion in the form of TiN. Because this TiN is hard and has low plastic deformability, it becomes a source of stress concentration and reduces the lifespan. Therefore, it is preferable to reduce the Ti content in the steel material as much as possible, and specifically, it is preferable that the Ti content in the steel material be 40 ppm or less relative to the total mass of the steel material. Cu and Ni are elements mixed into the scrap that is the raw material for steel, and if their content relative to the total mass of the steel material exceeds 0.4 mass%, the amount of retained austenite after quenching becomes too large, and dimensional stability decreases. Therefore, it is preferable that the Cu and Ni content be 0.4% by mass or less, relative to the total mass of the steel material.

[0030] S is a component that causes the formation of sulfide-based nonmetallic inclusions such as MnS. Because MnS has low hardness and high plastic deformability, it acts as an initiation point for cracks during pre-processing such as rolling and forging. Therefore, in order to prevent crack formation during pre-processing such as forging and enable stronger processing, it is preferable to reduce the S content in the steel material as much as possible. Specifically, it is preferable that the S content in the steel material be 0.020% by mass or less relative to the total mass of the steel material. P is an element that has the effect of reducing the impact resistance of manufactured members. Therefore, it is preferable to reduce the P content in the steel material, and it is preferable that it be 0.040% by mass or less relative to the total mass of the steel material. O is an element that forms nonmetallic inclusions in the steel material and is extremely harmful to rolling fatigue life, so it is preferable that the O content in the steel material be 10 ppm or less relative to the total mass of the steel material.

[0031] <Carburizing and Nitriding Treatment> In this embodiment, in order to adjust the surface carbon concentration and surface nitrogen concentration of the bearing member and to ensure the necessary characteristics as a rolling bearing, a carbonizing and nitriding treatment is performed on the bearing material obtained by processing a steel material having the above composition. The conditions for the carbonizing and nitriding treatment are not particularly limited, as they can be appropriately designed depending on the content of each component contained in the steel material, the size of the bearing material, the type of furnace used for the carbonizing and nitriding treatment, etc. For example, conditions in which the bearing material is heated to a temperature of 850°C or higher and then rapidly cooled can be used. Specifically, the carbonizing and nitriding treatment is performed by selecting conditions such that the surface carbon concentration and surface nitrogen concentration are within the range described later.

[0032] <Quenching and Tempering Treatment> In this embodiment, in order to improve the wear resistance of the bearing in a low-temperature and dilute-lubricated environment, the bearing material that has undergone the above-described carburizing and nitriding treatment is subjected to quenching and tempering treatment. The conditions for quenching and tempering treatment are not particularly limited, as they can be appropriately designed depending on the content of each component contained in the steel material, the size of the bearing material, the type of furnace used for quenching and tempering treatment, etc. For example, as the quenching treatment, conditions can be used in which the bearing material is heated to a temperature of 850°C to 1200°C and then rapidly cooled. Furthermore, if the heat treatment temperature for tempering is set to a temperature of 250°C or higher, residual austenite, which is harmful to dimensional stability, can be decomposed. Therefore, as the tempering treatment, it is preferable to use conditions such as heating the quenched bearing material at a temperature of 250°C or higher and then air-cooling it.

[0033] (Surface carbon concentration: 0.6% by mass or more and 1.5% by mass or less) If the surface carbon concentration of a bearing member processed using steel material having the above composition is less than 0.6% by mass, it indicates that the carbonitriding treatment has not been sufficiently applied, and the rolling fatigue strength required for a bearing cannot be obtained. Therefore, the surface carbon concentration of the bearing member that has undergone carbonitriding treatment among the outer ring, inner ring, and rolling elements should be 0.6% by mass or more, preferably 0.7% by mass or more, and more preferably 0.8% by mass or more. On the other hand, if the surface carbon concentration of the bearing member exceeds 1.5% by mass, the carbonitriding treatment is excessive, and eutectic carbides are easily formed. Such eutectic carbides can become defects, which can reduce the rolling fatigue life. Therefore, the surface carbon concentration of the bearing member that has undergone carbonitriding treatment should be 1.5% by mass or less, preferably 1.4% by mass or less, and more preferably 1.3% by mass or less.

[0034] (Surface nitrogen concentration: 0.05% by mass or more and 0.8% by mass or less) Nitrogen has the effect of improving wear resistance and seizure resistance. When the surface nitrogen concentration of a bearing member processed using a steel material having the above composition is 0.05% by mass or more, it indicates that the carbonitriding treatment has been sufficiently applied, and seizure resistance can be significantly improved. Therefore, the surface nitrogen concentration of a bearing member that has undergone carbonitriding treatment should be 0.05% by mass or more, preferably 0.075% by mass or more, and more preferably 0.1% by mass or more. On the other hand, if the surface nitrogen concentration of the bearing member exceeds 0.8% by mass, the carbonitriding treatment is excessive, making it difficult to grind, and the productivity of the polishing process, which is the finishing process of the bearing member, decreases. Therefore, the surface nitrogen concentration of a bearing member that has undergone carbonitriding treatment should be 0.8% by mass or less, preferably 0.7% by mass or less, and more preferably 0.6% by mass or less.

[0035] Furthermore, the surface carbon concentration and surface nitrogen concentration can be measured by analyzing the material using an electron probe microanalyzer (EPMA) with an accelerating voltage of, for example, 15 kV.

[0036] In this embodiment, the composition of the steel material, as well as the surface carbon concentration and surface nitrogen concentration, are controlled as described above in order to satisfy the characteristics of being resistant to temperature changes in low-temperature environments and having wear resistance in dilute lubrication environments. Furthermore, in this embodiment, it is preferable that the Vickers hardness at a predetermined temperature and the volume fraction of retained austenite at a predetermined position are controlled. The preferred ranges for Vickers hardness and the volume fraction of retained austenite are described below.

[0037] <Vickers hardness at room temperature: HV700 or higher, HV800 or lower> For example, when a steel material is held in a temperature environment of -196°C, the hardness increases and the brittleness decreases compared to when it is held at room temperature. In order to obtain the desired hardness at a temperature of -196°C, it is preferable that the Vickers hardness at room temperature of the components formed using the above-mentioned steel material, among the outer ring 10, inner ring 20, and rolling elements 30, be HV700 or higher. Furthermore, in order to prevent the hardness from increasing too much and the brittleness from decreasing even at a temperature of -196°C, it is preferable that the Vickers hardness at room temperature of the components formed using the above-mentioned steel material be HV800 or lower. In this embodiment, room temperature refers to 25°C.

[0038] <Vickers hardness at -196°C: HV800 or higher, HV900 or lower> As described above, in a temperature environment of -196°C, the hardness of steel materials increases and their brittleness decreases. To obtain the desired hardness at a temperature of -196°C, it is preferable that the Vickers hardness at -196°C of the outer ring 10, inner ring 20, and rolling elements 30, which are made using the above-mentioned steel material, be HV800 or higher. Furthermore, in order to prevent the hardness from increasing too much and the brittleness from decreasing even at a temperature of -196°C, it is preferable that the Vickers hardness at -196°C of the outer ring 10, inner ring 20, and rolling elements 30, which are made using the above-mentioned steel material, be HV900 or lower.

[0039] Furthermore, the Vickers hardness at room temperature (25°C) and -196°C can be measured in accordance with the "Vickers hardness test - test method" described in JIS Z 2244:2009.

[0040] <Volume fraction of retained austenite: 2% or less> When the rolling bearing according to this embodiment is applied to a device that uses liquefied gas as a lubricant, the operating environment of the rolling bearing becomes extremely harsh. That is, it is presumed that rapid temperature increases and decreases occur locally, resulting in significant temperature changes. Therefore, if an unstable structure such as retained austenite, which undergoes structural changes due to temperature changes, is present in the steel material, dimensional changes will occur, hindering the smooth rotation of the rolling elements. For this reason, it is preferable to reduce the retained austenite in at least one of the raceways of the outer ring 10 and the inner ring 20 formed using the above steel material. Accordingly, the volume fraction of retained austenite in the region at a depth of 50 μm from the surface of the raceway of the raceway formed using the above steel material is preferably 2% or less, and more preferably 1.5% or less.

[0041] Furthermore, the volume fraction of retained austenite at a depth of 50 μm from the surface of the raceway can be measured by electrochemically polishing the bottom of the groove of the bearing inner ring to a depth of 50 μm and analyzing the surface using an X-ray diffractometer.

[0042] [Method for Manufacturing a Rolling Bearing] The method for manufacturing the rolling bearing 1 according to this embodiment is briefly described below. First, a steel material having the above-described predetermined composition is used as the material for at least one of the components of the outer ring 10, inner ring 20, and rolling element 30, and each component is processed into a desired shape to produce a bearing material. Next, the bearing material is subjected to carburizing and nitriding treatment and quenching and tempering treatment in this order. Examples of treatment conditions in the carburizing and nitriding treatment and heat treatment conditions in the quenching and tempering treatment are as described above. It is preferable to design the specific treatment conditions appropriately so that the surface carbon concentration and surface nitrogen concentration, the Vickers hardness of the component surface at a predetermined temperature, and the volume fraction of retained austenite at a predetermined position are adjusted to the above-described range. After that, the rolling bearing 1 can be manufactured by assembling the outer ring 10, inner ring 20, and rolling element 30.

[0043] [Liquefied Gas Pump Device] The rolling bearings 1 according to the present embodiment are excellent in wear resistance in a low-temperature and lean lubrication environment, and thus are suitable for a liquefied gas pump device. FIG. 2 is a schematic diagram showing an example of a liquefied gas pump device in which the rolling bearings according to the present embodiment are used. As shown in FIG. 2, a liquefied gas pump device 100 includes a main body 110 having a suction port 111 and a discharge port 112, a housing 101, a main shaft 102 inserted inside the housing 101, and the above-described rolling bearings 1, 1 interposed between the housing 101 and the main shaft 102 and arranged at an axial interval.

[0044] The outer rings of the rolling bearings 1, 1 are mounted inside the housing 101, the inner rings are fitted to the main shaft 102, and the main shaft 102 is rotatably supported with respect to the housing 101 by the rolling bearings 1, 1. And near one end of the main shaft 102, an impeller 103 for pumping liquefied gas is provided, and near the other end, a motor 104 having a stator 105 and a rotor 106 for rotationally driving the main shaft 102 is attached.

[0045] In the liquefied gas pump device 100 configured as described above, the liquefied gas sucked from the suction port 111 is passed through the main body 110 by the motor 104 and discharged from the discharge port 112. At this time, the low-temperature liquefied gas always flows through the rolling bearings 1, 1. Further, when using the liquefied gas pump device 100 to transfer, for example, liquefied hydrogen, liquefied hydrogen flows between the outer ring, the inner ring, and the rolling elements instead of lubricating oil. Therefore, the rolling bearings 1, 1 operate in a low-temperature and lean lubrication environment.

[0046] The rolling bearings 1, 1 according to the present embodiment are excellent in wear resistance and are unlikely to cause rotation problems even when used in a low-temperature and lean lubrication environment. Therefore, when used in the liquefied gas pump device 100, rotation problems of the main shaft 102 are unlikely to occur, and durability can be improved.

[0047] The present invention will be described more specifically below with reference to inventive examples and comparative examples. However, the present invention is not limited to these examples, and it is also possible to make modifications within the scope that conforms to the gist thereof and implement them. All of them are included in the technical scope of the present invention.

[0048] [Example 1] <Production of test materials> Using a steel material with the content of each component within the range defined in the present invention, it was processed into various shapes, subjected to carbonitriding treatment, and then quenched and tempered to produce a test material of Inventive Example No. 1. Also, using a steel material of SUS440C specified in JIS G 4303:2012, it was processed into a predetermined shape and quenched and tempered without subjecting it to carbonitriding treatment to produce a test material of Comparative Example No. 1.

[0049] As the carbonitriding treatment of Inventive Example No. 1, the bearing material was heated at a temperature of 890°C and then rapidly cooled. As the quenching treatment, the bearing material was heated at a temperature of 840°C and then rapidly cooled. As the tempering treatment, after heating the quenched bearing material at a temperature of 300°C, it was air-cooled. The compositions of the steel materials of Inventive Example No. 1 and Comparative Example No. 1 are shown in Table 1 below. The remainder of the components shown in Table 1 below is Fe and unavoidable impurities. Also, in Table 1 below, "-" indicates that the element is not added or is below the detection limit.

[0050]

[0051] <Measurement of test materials> (Vickers hardness) For the test materials produced as described above, in accordance with the "Vickers hardness test - test method" described in JIS Z 2244:2009, the Vickers hardness at room temperature (25°C) and -196°C was measured.

[0052] (Volume ratio of retained austenite) For the test materials produced as described above, the surface was electrochemically polished to 50 μm, and the volume ratio of retained austenite was measured by analyzing that surface with an X-ray diffractometer.

[0053] (Surface carbon concentration and surface nitrogen concentration) For the test material prepared as described above, the surface carbon concentration and surface nitrogen concentration were measured using EPMA with an acceleration voltage of 15 kV.

[0054] <Evaluation of Wear Resistance> (Thrust Test) Nine rolling elements, held in a holder, were placed on a test material processed into a disc shape for the thrust test. An upper plate was placed on top of this, so that the rolling elements were sandwiched between the upper plate and the test material. In this state, the test material and the upper plate were pressed together in a direction that brought them closer to each other so that contact stress was applied between the test material and the rolling elements. After that, the test material was rotated at a predetermined rotation speed while under pressure, and the surface of the test material was observed using a non-contact three-dimensional shape measuring machine to measure the wear depth. The test conditions are shown below.

[0055] • Load: 100 kgf • Rolling elements: 9 x 3 / 8-inch SUJ2 balls • Surface pressure: 2.3 GPa • Rotational speed: 2000 rpm • Lubricant: Liquid nitrogen • Temperature: -196°C

[0056] Figure 3 is a schematic diagram showing a method for measuring the wear depth of a test material. As shown in Figure 3, by rolling an unshown rolling element on the surface of the test material 40, the raceway portion that receives the most pressure from the rolling element wears down, and a groove 41 is formed. In this embodiment, the difference between the surface position S1 of the test material 40 and the bottom position S2 of the groove 41 is defined as the "wear depth," and Invention Example No. 1 and Comparative Example No. 1 were compared.

[0057] Figure 4 shows a graph comparing the wear depth of Comparative Example No. 1 and Invention Example No. 1, and Figure 5 shows the surface observation results of Comparative Example No. 1 after the thrust test. Figure 5 is a microstructure observation photograph taken after corroding the surface of the groove formed in the test material of Comparative Example No. 1. The measurement results are shown in Table 2.

[0058]

[0059] As shown in Table 1 above, Comparative Example No. 1's steel material composition falls outside the range specified in the present invention, and no carbonitriding treatment was performed. As shown in Table 2 above, the surface nitrogen concentration is below the lower limit specified in the present invention, and the material contains eutectic carbides. Therefore, as shown in Figure 4, the wear depth was approximately five times that of Invention Example No. 1. Furthermore, as shown in Figure 5, when the surface of the groove formed in the test material of Comparative Example No. 1 was corroded, the eutectic carbides fell out, holes were formed, and the wear resistance decreased.

[0060] In contrast, Invention Example No. 1 uses steel material whose composition is within the range specified in this invention, and is subjected to carburizing and nitriding treatment, with the material composition adjusted so that eutectic carbides are absent. Therefore, Invention Example No. 1 exhibits a wear depth of approximately 1 / 5 that of Comparative Example No. 1, achieving extremely superior wear resistance compared to Comparative Example No. 1. Furthermore, because the composition of Invention Example No. 1 is controlled, it was possible to obtain the required Vickers hardness. It should be noted that if the above-mentioned carburizing and nitriding treatment and quenching / tempering treatment are performed and the surface carbon concentration and surface nitrogen concentration are controlled, it is considered that the wear resistance will not be affected regardless of the value of the content of each component within the range specified in this invention.

[0061] [Example 2] <Preparation of Test Material> Using SUJ2 steel material specified in JIS G 4805:2019, a test material for Comparative Example No. 2 was prepared by processing it into a predetermined shape and then quenching and tempering it without applying carburizing and nitriding treatment. The components contained in the steel material of Comparative Example No. 2 and their content are shown in Table 1 above.

[0062] <Measurement of Test Material> Vickers hardness, volume fraction of retained austenite, and surface carbon and nitrogen concentrations were measured in the same manner as in Example 1.

[0063] <Evaluation of wear resistance> (Thrust test) A thrust test was performed on the test material of Comparative Example No. 2, which was processed into a disc shape, in the same manner as described above. After that, the test material was rotated at a predetermined rotation speed while under pressure, and the surface shape of the test material was observed using a non-contact three-dimensional shape measuring machine. The test conditions were the same as in Example 1.

[0064] In Comparative Example No. 2, the composition of the steel material used was outside the range specified in the present invention, and since no carburizing or nitriding treatment was performed, the surface nitrogen concentration was below the lower limit specified in the present invention. Figure 6 is a graph showing an example of the surface shape of the test material of Comparative Example No. 2 after the thrust test. The volume fraction of retained austenite in Comparative Example No. 2 was 12%. The surface of the test material of Comparative Example No. 2 showed rapid temperature increases and decreases locally, suggesting significant temperature changes. Therefore, unstable structures such as retained austenite, which are prone to structural changes due to temperature changes, were present in the steel material, and as shown in Figure 6, irregularities were formed on the surface. As a result, it is thought that bearing members created by the same method as the test material of Comparative Example No. 2 are prone to dimensional changes, hindering the smooth rotation of the rolling elements.

[0065] This application is based on the Japanese Patent Application No. 2024-179204 filed on October 11, 2024, the contents of which are incorporated by reference within this application.

[0066] 1 Bearing 10 Outer ring 10a Outer ring raceway surface 20 Inner ring 20a Inner ring raceway surface 30 Rolling element 100 Liquefied gas pump device 101 Housing 102 Main shaft 103 Impeller 104 Motor 105 Stator 106 Rotor 110 Body 111 Inlet 112 Outlet

Claims

1. A rolling bearing comprising a raceway consisting of an outer ring and an inner ring, and a plurality of rolling elements held so as to be rotatable between the outer ring and the inner ring, used in a dilute lubrication environment where a liquid with a viscosity of VG2 or less at 40°C flows between the outer ring, the inner ring and the rolling elements at a low temperature of -30°C or lower, wherein at least one of the outer ring, the inner ring and the rolling elements is formed from a steel material containing C: 0.1% to 0.7% by mass, Cr: 1.1% to 1.8% by mass, Si: 0.7% to 1.3% by mass, Mn: 0.3% to 1.1% by mass, Mo: 0.6% to 1.3% by mass, and subjected to carburizing and nitriding treatment and quenching / tempering treatment in that order, with a surface carbon concentration of 0.6% to 1.5% by mass, A rolling bearing characterized by having a surface nitrogen concentration of 0.05% by mass or more and 0.8% by mass or less.

2. The rolling bearing according to claim 1, characterized in that the Vickers hardness of the steel material of the outer ring, inner ring, and rolling elements at room temperature is HV700 or higher and HV800 or lower.

3. The rolling bearing according to claim 1, characterized in that each of the raceways has a raceway surface on which the rolling elements roll, at least one of the outer ring and the inner ring is formed using the steel material, and the volume fraction of retained austenite in the region at a depth of 50 μm from the surface of the raceway surface of the raceway formed using the steel material is 2% or less.

4. The rolling bearing according to claim 1, characterized in that the Vickers hardness at -196°C of the outer ring, the inner ring, and the rolling elements, which are formed using the steel material, is HV800 or higher and HV900 or lower.

5. A rolling bearing according to any one of claims 1 to 4, characterized in that it is used in a pumping device for liquefied gas.

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