Rolling bearings

Abstract

The present invention provides a rolling bearing that can be used in low-temperature and low-lubrication environments, further improving wear resistance and achieving excellent durability. [Solution] 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, and is used in a low-temperature and dilute-lubricated environment. At least one of the outer ring 10, inner ring 20 and rolling elements 30 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, and Mo: 0.6% to 1.3% by mass, and is subjected to carburizing and nitriding treatment and quenching and tempering treatment in that order, with a surface carbon concentration of 0.6% to 1.5% by mass and a surface nitrogen concentration of 0.05% to 0.8% by mass.

Description

Technical Field

[0001] The present invention relates to a rolling bearing. More specifically, it relates to a rolling bearing used in a dilute and lubricated environment where 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 refers to a temperature of -30°C or lower).

Background Art

[0002] In recent years, hydrogen, which does not generate any greenhouse gas, has attracted attention in order to realize 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, it becomes possible to store and transport a large amount of hydrogen. However, for example, in bearings used at a temperature of -253°C for liquefying hydrogen, various risks specific 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. In addition, Patent Document 2 proposes a rolling bearing having rolling elements made of ceramics having 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.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[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 are designed 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. [Means for solving the problem]

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

[0009] [1] A raceway comprising an outer ring and an inner ring, and a plurality of rolling elements held to roll freely between the outer ring and the inner ring, A rolling bearing used in a dilute 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, At least one of the outer ring, the inner ring, and the rolling element is 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, Formed using a steel material containing Mo: 0.6% by mass or more and 1.3% by mass or less. Carburizing and nitriding treatments, followed by quenching and tempering treatments, are performed in this order. The surface carbon concentration is 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 formed using the steel material, is HV700 or higher and HV800 or lower.

[0012] [3] Each of the raceway wheels 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. The rolling bearing according to [1] or [2], characterized in that the volume fraction of retained austenite in a region 50 μm deep from the surface of the raceway of the raceway ring 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] The rolling bearing according to any one of [1] to [4], characterized by being used in a pump device for liquefied gas.

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a rolling bearing that can be used in a low-temperature and thin lubrication environment, such as a rolling bearing incorporated in a pump device for liquefied gas, further improving wear resistance and obtaining excellent durability.

Brief Description of the Drawings

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

Embodiments for Carrying Out the Invention

[0017] As a result of intensive studies by the inventors of the present application, it has been found that when a bearing is used in a low-temperature and thin environment, carbides on the running track fall off, and due to this fall-off of carbides, collapses on the raceway surface are formed. That is, when the fall-off of carbides that contributes to improving wear resistance occurs, the hardness of the surface of the raceway or rolling elements of the bearing decreases, and the 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 suspected that eutectic carbides, in particular among carbides, have a significant influence on accelerating wear, and conducted further investigations. As a result, they found that eutectic carbides, which are expected to contribute to improved wear resistance in a normal temperature environment, may conversely work to increase the amount of wear in a low-temperature environment. This invention is based on the above findings.

[0020] [Rolling bearings] 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 defined by ISO, and a viscosity of VG2 or less means that the kinematic viscosity is 2.42 (mm²). 2 This indicates that the operating environment of the rolling bearing according to the present invention 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, low-lean lubrication environment.

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

[0022] In the present embodiment, at least one of the outer ring 10, the inner ring 20, and the rolling elements 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 part including the outer raceway surface 10a of the outer ring 10 is composed 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 part including the inner raceway surface 20a of the inner ring 20 is composed of the following steel material.

[0023] In the present embodiment, a steel material added with appropriate alloy components is used, and a carbonitriding treatment is performed on the bearing material to reduce eutectic carbides. As a result, the wear resistance of the rolling bearing in a low-temperature and lean lubrication environment can be maintained, and the durability can be ensured. In addition, as a member of a rolling bearing used in a low-temperature environment, it is preferable that the structure is stable and has characteristics resistant to temperature changes. Therefore, it is preferable that the retained austenite of the member of the rolling bearing is less. Hereinafter, the components contained in the steel material and their contents, and the heat treatment conditions of the bearing material will be described in detail.

[0024] <C: 0.1 mass% or more and 0.7 mass% or less> If the C (carbon) content in the steel material is less than 0.1 mass%, the cleanliness required for the rolling bearing cannot be obtained. Therefore, the C content in the steel material is 0.1 mass% or more with respect to the total mass of the steel material, preferably 0.2 mass% or more, and more preferably 0.3 mass% or more. On the one hand, when the C content in the steel material exceeds 0.7% by mass, the retained austenite increases, reducing the dimensional stability of the bearing, or eutectic carbides are formed, resulting in a short service life. Therefore, the C content in the steel material should be 0.7% by mass or less based on the total mass of the steel material, preferably 0.6% by mass or less, and more preferably 0.5% by mass or less.

[0025] <Cr: 1.1% by mass or more and 1.8% by mass or less> Cr (chromium) is an element that has the effect of improving the fatigue life in an environment where metal contact occurs. When the Cr content in the steel material is less than 1.1% by mass, the tissue stability decreases, and there is a risk of surface fatigue. Therefore, the Cr content in the steel material should be 1.1% by mass or more based on the total mass of the steel material, preferably 1.2% by mass or more, and more preferably 1.3% by mass or more. On the other hand, if the Cr content in the steel material is too high, the cold workability, machinability, and carburizing treatment property decrease, and the manufacturing cost increases significantly. In addition, coarse eutectic carbides may be generated, and the fatigue life and strength of the rolling bearing may be significantly reduced. Therefore, the Cr content in the steel material should be 1.8% by mass or less based on the total mass of the steel material, preferably 1.7% by mass or less, and more preferably 1.6% by mass or less.

[0026] <Si: 0.7% by mass or more and 1.3% by mass or less> [[ID=I4]] Si (silicon) is an essential element for forming carbides or carbonitrides containing Si, or carbides or carbonitrides containing Si-X (X is at least one of Mn, Mo, and Cr), which is effective in suppressing seizure and wear due to friction. When the Si content in the steel material is less than 0.7% by mass, the effect of deoxidation during steelmaking cannot be fully obtained, and it becomes difficult to improve seizure resistance and wear resistance. Therefore, the Si content in the steel material should be 0.7% by mass or more based on the total mass of the steel material, preferably 0.8% by mass or more, and more preferably 0.9% by mass or more. On the one hand, if the Si content in the steel material is too high, the effect of improving the seizure resistance and wear resistance of carbides or carbonitrides will decrease. 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, based on the total mass of the steel material.

[0027] <Mn: 0.3% by mass or more and 1.1% by mass or less> Mn (manganese), like Si, is an element necessary for deoxidation during steelmaking. Also, when Mn is added to the raw materials as a deoxidizer during steelmaking, it enhances hardenability and contributes to improving the strength and rolling fatigue life after heat treatment. If the Mn content in the steel material is less than 0.3% by mass, the effect of deoxidation during steelmaking cannot be fully obtained, and it becomes 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 harmful to dimensional stability is generated, and the 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 has the effect of dissolving in the martensite matrix to enhance hardenability, tempering softening resistance, and corrosion resistance, etc. Also, Mo forms fine carbides to prevent grain coarsening during heat treatment, stabilizes the structure, and like Cr, is a component that has the effect of improving fatigue life in an environment where metal contact occurs. Therefore, the Mo content in the steel material should be 0.6% by mass or more, preferably 0.7% by mass or more, and more preferably 0.8% by mass or more, based on the total mass of the steel material. On the other hand, if the Mo content in the steel material 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% each, 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 mass% or less each 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 steel materials as much as possible. Specifically, it is preferable that the S content in steel materials be 0.020 mass% or less relative to the total mass of steel materials. P is an element that has the effect of reducing the impact resistance of manufactured components. Therefore, it is preferable to reduce the P content in steel materials, and it is preferable that it be 0.040 mass% or less relative to the total mass of steel materials. O is an element that forms nonmetallic inclusions in steel materials and is extremely harmful to rolling fatigue life, so it is preferable that the O content in steel materials be 10 ppm or less relative to the total mass of steel materials.

[0031] <Carburitting and nitriding treatment> In this embodiment, in order to adjust the surface carbon concentration and surface nitrogen concentration of the bearing member and ensure the necessary characteristics as a rolling bearing, a bearing material obtained by processing a steel material having the above composition is subjected to a carbonitriding treatment. The conditions for the carbonitriding 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 carbonitriding treatment, etc. For example, conditions in which the bearing material is heated to a temperature of 850°C to 950°C and then rapidly cooled can be used. Specifically, the carbonitriding treatment is performed by selecting conditions such that the surface carbon concentration and surface nitrogen concentration fall within the ranges described later.

[0032] <Heat treatment and tempering> 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 treatment 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 mass% or more and 1.5 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 carburizing and nitriding 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 carburizing and nitriding 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 mass%, the carbonitriding treatment is excessive, making it easier for eutectic carbides to form. Such eutectic carbides can become defects, potentially reducing the rolling fatigue life. Therefore, the surface carbon concentration of the carbonitriding-treated bearing member should be 1.5 mass% or less, preferably 1.4 mass% or less, and more preferably 1.3 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. A surface nitrogen concentration of 0.05% by mass or more in a bearing member processed using steel material having the above composition indicates that sufficient carburizing and nitriding treatment has been performed, and seizure resistance can be significantly improved. Therefore, the surface nitrogen concentration of a bearing member that has undergone carburizing and nitriding 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 reducing the productivity of the polishing process, which is the finishing process for the bearing member. Therefore, the surface nitrogen concentration of the carbonitriding-treated bearing member 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 them using an electron probe microanalyzer (EPMA) with an accelerating voltage of, for example, 15kV.

[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, its hardness increases and its brittleness decreases compared to when it is held at room temperature. To obtain the desired hardness at -196°C, it is preferable that the Vickers hardness at room temperature of the components formed using the steel material among the outer ring 10, inner ring 20, and rolling elements 30 is HV700 or higher. Furthermore, to prevent the hardness from increasing too much and the brittleness from decreasing even at -196°C, it is preferable that the Vickers hardness at room temperature of the components formed using the steel material is HV800 or lower. In this embodiment, room temperature refers to 25°C.

[0038] <Vickers hardness at -196℃: HV800 or higher, HV900 or lower> As described above, at a temperature of -196°C, the hardness of the steel material increases and its brittleness decreases. To obtain the desired hardness at -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, is HV800 or higher. Furthermore, to prevent the hardness from increasing too much and the brittleness from decreasing even at -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, is HV900 or lower.

[0039] Furthermore, 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 for the rolling bearing becomes extremely harsh. That is, it is presumed that rapid temperature increases and decreases occur locally, resulting in significant temperature fluctuations. Therefore, if unstable structures such as retained austenite, which undergo structural changes due to temperature changes, are 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, the outer ring 10 and the inner ring 20, formed using the above-mentioned 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-mentioned 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] [Manufacturing method for rolling bearings] A brief description of the manufacturing method for the rolling bearing 1 according to this embodiment is given 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 the bearing material. Next, the bearing material is subjected to carburizing and nitriding treatment and quenching and tempering treatment in that 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] [Pumping equipment for liquefied gas] The rolling bearing 1 according to this embodiment is suitable for liquefied gas pump devices because it has excellent wear resistance in low temperature and dilute lubrication environments. Figure 2 is a schematic diagram showing an example of a liquefied gas pump device using the rolling bearing according to this embodiment. As shown in Figure 2, the liquefied gas pump device 100 comprises a main body 110 having an inlet 111 and a discharge port 112, a housing 101, a main shaft 102 inserted inside the housing 101, and the rolling bearings 1, 1 interposed between the housing 101 and the main shaft 102 and arranged with an axial gap between them.

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

[0045] In the liquefied gas pump device 100 configured as described above, liquefied gas drawn in 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, low-temperature liquefied gas is always passed through the rolling bearings 1, 1. Furthermore, when using the liquefied gas pump device 100 to transfer, for example, liquefied hydrogen, liquefied hydrogen is passed between the outer ring and inner ring and the rolling elements instead of lubricating oil. For this reason, the rolling bearings 1, 1 operate in a low-temperature and lean-lubrication environment.

[0046] The rolling bearings 1, 1 according to this embodiment have excellent wear resistance and are less prone to rotational malfunctions even when used in low-temperature and low-lubrication environments. Therefore, when used in a liquefied gas pump device 100, rotational malfunctions of the main shaft 102 are less likely to occur, and durability can be improved. [Examples]

[0047] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples, and can be implemented with modifications to the extent that is consistent with its spirit, and all such modifications are included within the technical scope of the present invention.

[0048] [Example 1] <Preparation of test specimens> Test specimen No. 1 of the Invention was prepared by processing steel material in which the content of each component was within the range specified in the Invention into various shapes, subjecting it to carburizing and nitriding treatment, and then quenching and tempering it. In addition, test specimen No. 1 of the Comparative Example was prepared by processing SUS440C steel material specified in JIS G 4303:2012 into a predetermined shape and quenching and tempering it without subjecting it to carburizing and nitriding treatment.

[0049] For the carburizing and nitriding treatment of Invention Example No. 1, the bearing material was heated to a temperature of 890°C and then rapidly cooled. For the quenching treatment, the bearing material was heated to a temperature of 840°C and then rapidly cooled. For the tempering treatment, the quenched bearing material was heated to a temperature of 300°C and then air cooled. The composition of the steel materials for Invention Example No. 1 and Comparative Example No. 1 is shown in Table 1 below. The remainder of the components shown in Table 1 below consists of Fe and unavoidable impurities. In Table 1 below, "-" indicates that the element is not added or is below the detection limit.

[0050] [Table 1]

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

[0052] (Volume fraction of retained austenite) The test material prepared as described above was electrochemically polished to a thickness of 50 μm, and the volume fraction of retained austenite was measured by analyzing the surface using an X-ray diffractometer.

[0053] (Surface carbon concentration, 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 15kV.

[0054] <Evaluation of wear resistance> (Thrust test) Nine rolling elements, held in a holder, were placed on a disc-shaped test specimen for thrust testing. An upper plate was then placed on top, sandwiching the rolling elements between the upper plate and the test specimen. In this state, the test specimen and the upper plate were pressed together to apply contact stress between them. After that, the test specimen was rotated at a predetermined speed while under pressure, and the surface of the test specimen was observed using a non-contact three-dimensional shape measuring machine to measure the wear depth. The test conditions are as follows.

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

[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 receiving the most pressure from the rolling element wears down, forming a groove 41. In this embodiment, the difference between the surface position S1 of the test material 41 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 results of each measurement are shown in Table 2.

[0058] [Table 2]

[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 since no carbonitriding treatment was performed, 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. In contrast, Invention Example No. 1's steel material composition is within the range specified in the present invention, and carbonitriding treatment has been performed, adjusting the material composition so that eutectic carbides are absent. Therefore, the wear depth was approximately 1 / 5 that of Comparative Example No. 1, and extremely superior wear resistance was obtained compared to Comparative Example No. 1. It should be noted that if the above carbonitriding 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 each component within the range specified in the present invention.

[0060] [Example 2] <Preparation of test specimens> Comparative Example No. 2 test material was prepared by processing SUJ2 steel material specified in JIS G 4805:2019 into a predetermined shape, and then quenching and tempering it without carburizing or nitriding treatment. The components and their contents contained in the steel material of Comparative Example No. 2 are also shown in Table 1 above.

[0061] <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.

[0062] <Evaluation of wear resistance> (Thrust test) A thrust test was performed on the disc-shaped test material of Comparative Example No. 2 in the same manner as described above. After that, the test material was rotated at a predetermined 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.

[0063] 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 irregularities were formed on the surface as shown in Figure 6. 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. [Explanation of Symbols]

[0064] 1 bearing 10 Outer ring 10a Outer ring raceway surface 20 Inner circle 20a Inner ring raceway surface 30 Rolling element 100 Liquefied Gas Pumping System 101 Housing 102 Main axis 103 Impeller 104 Motor 105 Stator 106 Rotor 110 Main Unit 111 Inlet 112 Discharge port

Claims

1. It has a raceway consisting of an outer ring and an inner ring, and a plurality of rolling elements that are held to roll freely between the outer ring and the inner ring, A rolling bearing used in a dilute lubrication environment at low temperatures of -30°C or below, 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 least one of the outer ring, the inner ring, and the rolling element is 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, Formed using a steel material containing Mo: 0.6% by mass or more and 1.3% by mass or less. Carburizing and nitriding treatments, followed by quenching and tempering treatments, are performed in this order. The surface carbon concentration is 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.

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. Each of the aforementioned raceway wheels has a raceway surface on which the rolling element rolls, At least one of the outer ring and the inner ring is formed using the steel material. The rolling bearing according to claim 1, characterized in that the volume fraction of retained austenite in a region at a depth of 50 μm from the surface of the raceway of the raceway ring 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 made of 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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