JP2014152927A - Bearing part and rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive bearing part having improved yield strength and service life, and a rolling bearing including the bearing part.SOLUTION: Bearing parts (an outer wheel 11, an inner wheel 12 and a ball 13) comprise a bearing steel, and surface layer parts 11B, 12B, 13B containing an outer wheel rolling surface 11A and an inner wheel rolling surface 12A or a rolling contact surface 13A are nitrided. Nitrogen concentrations in the surface layer parts 11B, 12B, 13B are 0.4 mass% or more. A residual austenite amount in the surface layer parts 11B, 12B, 13B is 8 vol.% or less.

Description

  The present invention relates to a bearing component and a rolling bearing, and more particularly to a bearing component having a nitrided surface layer including a rolling surface or a rolling surface, and a rolling bearing including the bearing component.

  One of the important functions of the rolling bearing is to reduce the torque during the rotational movement of the machine. In order to achieve this, it is important to reduce energy loss during rotation. Causes of energy loss during the rotation of the rolling bearing include rolling viscous resistance, differential sliding, elastic hysteresis loss, or lubricating oil stirring resistance. Compared to ball bearings and bearings in rolling bearings, ball bearings have less energy loss due to the difference in rolling viscous resistance. Therefore, it can be said that the ball bearing is a more preferable structure from the viewpoint of reducing the torque during the rotational movement of the machine. On the other hand, another important function of the bearing is the life, and a method of performing an efficient carbonitriding process to improve the life has been proposed (for example, see Patent Document 1).

  Roller bearings have a larger contact area between bearing parts (bearing rings and balls) than ball bearings, and therefore contact pressure applied to the bearing parts is smaller. Therefore, a roller bearing is effective from the viewpoint of supporting a large load. On the other hand, if a large load is supported in the ball bearing, the contact surface pressure becomes excessive and plastic deformation occurs in the race and the ball. As a result, the rotational motion of the machine is increased in torque, and abnormal noise is generated and rotational accuracy is lowered. As a result, it becomes difficult to satisfy the function as a bearing. Thus, the excessive load (contact surface pressure) that leads to malfunction of the bearing is referred to as “static load capacity” of the rolling bearing. Therefore, it can be said that in reducing the torque of the rolling bearing, it is preferable to employ a ball bearing using a material that is difficult to plastically deform (high yield strength).

  A typical material that hardly undergoes plastic deformation includes a ceramic material such as silicon nitride. However, since this ceramic material is difficult to plastically deform but has difficult workability, there is a problem that when it is used as a bearing material, the processing cost increases. Therefore, in place of the ceramic material, it is required to develop a material having a low total cost including a processing cost and hardly causing plastic deformation. The “difficulty to cause plastic deformation” is not related to the hardness expressed by HV hardness or HRC hardness but to a region where the amount of plastic deformation close to the elastic limit, yield strength, etc. is minute.

JP 2007-154293 A

  As described above, conventionally, in ball bearings having a structure advantageous in reducing torque, it has been difficult to prevent plastic deformation without using a material with high processing cost such as silicon nitride. Moreover, it is necessary to improve the life of the bearing as well as to reduce the torque.

  Therefore, an object of the present invention is to provide a bearing component that is inexpensive and has improved yield strength and life, and a rolling bearing including the bearing component.

  A bearing component according to an aspect of the present invention is a bearing component made of bearing steel and having a rolling surface or a surface layer portion including the rolling surface nitrided. In the bearing component, the nitrogen concentration in the surface layer portion is 0.4% by mass or more. Moreover, in the said bearing component, the area ratio of the precipitate in the inside which is not nitrided is 11% or more.

  The present inventor has conducted detailed studies on measures for improving the yield strength and the life of bearing components used for rolling bearings without using materials with high processing costs. As a result, the following knowledge was obtained and the present invention was conceived.

  The bearing parts are manufactured by carbonitriding and quenching, and then further tempering. According to the study of the present inventor, by increasing the heating temperature during the tempering process as compared with the prior art, the area ratio of precipitates in the inside that is not nitrided by the carbonitriding process (hereinafter referred to as an unnitrided region) is 11% or more To increase the yield strength of the material. That is, in the bearing component in which the area ratio of precipitates in the non-nitrided region is increased, plastic deformation of the material can be suppressed without using a material such as silicon nitride that has a high processing cost.

  Moreover, in a bearing component, the lifetime can be improved by increasing the nitrogen concentration in the surface layer portion by carbonitriding. However, in conventional bearing parts, the yield strength of the material decreases as the nitrogen concentration in the surface layer increases, so it has been difficult to achieve both improved yield strength and improved life. On the other hand, according to the study by the present inventor, when the tempering temperature is made higher than before, the reciprocal relationship between the yield strength and the life is eliminated. That is, by raising the tempering temperature and setting the area ratio of the precipitates in the non-nitrided region to 11% or more, not only the yield strength is improved, but the nitrogen concentration in the surface layer portion is increased to 0.4% by mass or more. Even when it is increased, it is possible to suppress a decrease in yield strength. Therefore, according to the bearing component according to the one aspect of the present invention, the nitrogen concentration in the surface layer portion is 0.4% by mass or more and the area ratio of precipitates in the non-nitrided region is specified to be 11% or more. Thus, it is possible to provide a bearing component that is inexpensive and has improved yield strength and life.

  In the bearing component, the area ratio of precipitates may be 12% or more. When the tempering temperature is further increased, the area ratio of precipitates in the non-nitrided region increases to 12% or more. Thereby, the yield strength of the material can be further improved.

  In the bearing component, the retained austenite amount in the surface layer portion may be 8% by volume or less.

  As described above, when the tempering temperature is increased as compared with the conventional case, not only the area ratio of precipitates in the non-nitrided region increases, but also the amount of retained austenite in the surface layer portion decreases to 8% by volume or less. In this case, a bearing component in which the yield strength of the material is similarly improved can be obtained.

  In the bearing component, the amount of retained austenite may be 5% by volume or less. As described above, when the tempering temperature is further increased, not only the area ratio of precipitates in the non-nitrided region is further increased, but also the amount of retained austenite in the surface layer portion is reduced to 5% by volume or less. In this case, the yield strength of the material can be further improved similarly.

  The bearing part may be made of JIS standard SUJ2. JIS standard SUJ2, which is a typical bearing steel, is suitable as a constituent material for bearing parts according to one aspect of the present invention.

  A bearing component according to another aspect of the present invention is a bearing component made of bearing steel and having a rolling surface or a surface layer portion including the rolling surface nitrided. In the bearing component, the nitrogen concentration in the surface layer portion is 0.4% by mass or more. Moreover, in the said bearing component, the amount of retained austenites in the said surface layer part is 8 volume% or less.

  As described above, when the tempering temperature is made higher than before, the amount of retained austenite in the surface layer portion is reduced to 8% by volume or less, and as a result, the yield strength of the material is improved. In addition, when the tempering temperature is higher than the conventional case, the reciprocal relationship between the yield strength and the life of the material is eliminated, and even when the nitrogen concentration in the surface layer is increased to 0.4 mass% or more, the yield strength is improved. The decrease can be suppressed. Therefore, according to the bearing component according to another aspect of the present invention, it is possible to provide a bearing component that is inexpensive and has improved yield strength and life, similar to the bearing component according to one aspect of the present invention. it can.

  In the bearing component, the amount of retained austenite in the surface layer portion may be 5% by volume or less.

  When the tempering temperature is further increased as described above, the amount of retained austenite in the surface layer portion is reduced to 5% by volume or less. Thereby, the yield strength of the material can be further improved.

  The bearing part may be made of JIS standard SUJ2. JIS standard SUJ2, which is a representative bearing steel, is also suitable as a constituent material for bearing parts according to the other aspects of the present invention.

  In the above bearing component, the grain size number of the prior austenite crystal grains in the non-nitrided interior may be 9 or more and 11 or less.

  The size of the prior austenite crystal grains depends on the heating temperature during carbonitriding. Therefore, if the size of the prior austenite crystal grains in the bearing component is within the above range, it can be confirmed that the carbonitriding process is performed at an appropriate temperature.

In the bearing component, the surface layer portion may be nitrided by carbonitriding. Then, the carbonitriding treatment, the activity of a _c of carbon in the bearing _part, the undecomposed ammonia concentration in the heat treatment furnace in which the bearing parts are arranged in the carbonitriding process when the C _N, gamma = may be implemented to the value of γ is defined by a c _/ C _N is in the range of 2 to 5.

In bearing steel (steel having a carbon concentration of 0.8% by mass or more), when the value of γ is 5, the penetration rate of nitrogen into the bearing part is maximum, and when the value of γ is 5 or less, The penetration speed is constant. That is, by setting the value of γ to 5 or less, it is possible to maximize the penetration rate of nitrogen into the bearing component made of bearing steel. Here, a _c is a value calculated by the following equation (1), P _CO is the partial pressure of carbon monoxide (CO), P _CO2 is the partial pressure of carbon dioxide (CO ₂ ), and K is < It is an equilibrium constant in C> + CO ₂ ⇔2CO.

  On the other hand, when the value of γ is less than 2, the supply rate of ammonia to the heat treatment furnace increases, and accordingly, the partial pressure of carbon monoxide in the heat treatment furnace decreases. Therefore, in order to maintain the carbon potential, it is necessary to increase the amount of enriched gas introduced into the heat treatment furnace, resulting in sooting (soot in the heat treatment furnace and sticking to bearing parts). There is a risk that quality defects such as surface carburization may occur in the bearing parts. For these reasons, the value of γ is preferably 2 or more and 5 or less.

  The rolling bearing according to the present invention has an outer ring having a rolling surface on the inner peripheral surface, an inner ring having a rolling surface on the outer peripheral surface and disposed inside the outer ring, and a contact with the rolling surface on the rolling surface. And a plurality of rolling elements arranged side by side on an annular track. In the rolling bearing, at least one of the outer ring, the inner ring, and the rolling element is a bearing component according to the present invention.

  The rolling bearing according to the present invention comprises a bearing component that is inexpensive and has improved yield strength and life. Therefore, plastic deformation due to excessive contact surface pressure being applied to the bearing component is suppressed, and as a result, a reduction in torque during the rotational movement of the machine can be achieved. Therefore, according to the rolling bearing according to the present invention, it is possible to provide a rolling bearing which is inexpensive and can reduce the torque of the rotational movement of the machine and has an improved life.

  As is apparent from the above description, according to the bearing component according to the present invention, it is possible to provide a bearing component that is inexpensive and has improved yield strength and life. In addition, according to the rolling bearing according to the present invention, it is possible to provide a rolling bearing that is inexpensive, can reduce the torque of the rotational movement of the machine, and has an improved life.

It is a schematic sectional drawing which shows the structure of a rolling bearing. It is a schematic sectional drawing which expands and shows the structure of a rolling bearing. It is a flowchart which shows roughly the manufacturing method of a rolling bearing, and the manufacturing method of bearing components. It is a figure for demonstrating the detail of the hardening hardening process included in the manufacturing method of a bearing component. It is a figure which shows an example of the heating pattern in the heating pattern control process included in the carbonitriding process of FIG. It is a figure which shows the relationship between nitrogen concentration, tempering temperature, and residual indentation depth. It is a figure which shows the Weibull plot of an indentation origin type | mold peeling life. It is a figure which shows the rapid acceleration / deceleration pattern in a hydrogen resistance life test. It is a figure which shows the Weibull plot of a hydrogen-resistant lifetime. It is a figure which shows the relationship between tempering temperature and a retained austenite amount. It is a figure which shows the relationship between tempering temperature and the area ratio of a precipitate. It is a photograph of the precipitate when the tempering temperature is 180 ° C. It is a photograph of the precipitate when the tempering temperature is 210 ° C. It is a photograph of the precipitate when the tempering temperature is 240 ° C. It is a photograph of the precipitate when the tempering temperature is 260 ° C. It is a photograph of prior austenite crystal grains in an unnitrided region.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and description thereof will not be repeated.

  First, the structure of a deep groove ball bearing which is a rolling bearing according to an embodiment of the present invention will be described. Referring to FIG. 1, a deep groove ball bearing 1 according to the present embodiment is disposed between an annular outer ring 11, an annular inner ring 12, and an outer ring 11 and an inner ring 12. And a plurality of balls 13 which are held rolling elements. The outer ring 11 has an outer ring rolling surface 11A on the inner peripheral surface. The inner ring 12 has an inner ring rolling surface 12A on the outer peripheral surface, and is arranged inside the outer ring 11 so that the inner ring rolling surface 12A faces the outer ring rolling surface 11A. The plurality of balls 13 are in contact with the outer ring rolling surface 11A and the inner ring rolling surface 12A on the rolling surface 13A. The plurality of balls 13 are arranged on an annular track of the outer ring 11 and the inner ring 12 by the cage 14 and are arranged at a predetermined pitch in the circumferential direction. Thereby, the plurality of balls 13 are held so as to roll freely on the track. With such a configuration, the outer ring 11 and the inner ring 12 of the deep groove ball bearing 1 are rotatable relative to each other. In addition, the outer ring | wheel 11, the inner ring | wheel 12, and the ball | bowl 13 are bearing components which concern on this Embodiment mentioned later.

  Next, the bearing components (outer ring 11, inner ring 12 and ball 13) according to the present embodiment will be described in detail. The bearing part is made of bearing steel such as JIS standard SUJ2. Referring to FIG. 2, in the region including the surfaces of the bearing parts (the outer ring rolling surface 11A, the inner ring rolling surface 12A, and the rolling surface 13A), nitrogen having a higher nitrogen concentration than the inside 11C, 12C, and 13C. Enriched layers 11D, 12D, and 13D are formed, respectively. Surface layers 11B, 12B, and 13B including outer ring rolling surface 11A, inner ring rolling surface 12A, and rolling surface 13A, which are the surfaces of nitrogen-enriched layers 11D, 12D, and 13D, are nitrided by carbonitriding, The concentration is 0.4% by mass or more. Thereby, the lifetime of the said bearing components (the outer ring | wheel 11, the inner ring | wheel 12, and the ball | bowl 13) is improving. The surface layer portions 11B, 12B, and 13B are regions within a distance of 20 μm from the outer ring rolling surface 11A, the inner ring rolling surface 12A, and the rolling surface 13A.

  The amount of retained austenite in the surface layer portions 11B, 12B, and 13B of the bearing component is 8% by volume or less, and preferably 5% by volume or less. In the bearing component, the area ratio of precipitates in the non-nitrided interiors 11C, 12C, and 13C is 11% or more, and preferably 12% or more. Thereby, the yield strength of the material of the bearing component is improved as will be described later. The “precipitate” is an iron carbide or a carbonitride in which part of carbon of the carbide is replaced with nitrogen, and includes an Fe—C-based compound and an Fe—C—N-based compound. Moreover, this carbonitride may contain the alloy component contained in steel, such as chromium.

  In the bearing component, the size of the old austenite crystal grains in the interiors 11C, 12C, and 13C is 9 or more and 11 or less in the JIS standard. The size of the prior austenite crystal grains depends on the heating temperature during carbonitriding. Therefore, if the size of the prior austenite crystal grains in the bearing component is within the above range, it can be confirmed that the carbonitriding process is performed at an appropriate temperature (850 ° C.).

  Next, a method for manufacturing a rolling bearing and a method for manufacturing a bearing component according to the present embodiment will be described. Referring to FIG. 3, in the method for manufacturing a rolling bearing according to the present embodiment, steps (S10) to (S50) are sequentially performed, whereby deep groove ball bearing 1 according to the present embodiment is manufactured. The Moreover, in the manufacturing method of the rolling bearing which concerns on this Embodiment, process (S10)-(S40) is implemented as a manufacturing method of the bearing components which concerns on this Embodiment.

  With reference to FIG. 3, a steel material preparation process is first implemented as process (S10). In this step (S10), first, a steel material made of over-steeled steel such as JIS standard SUJ2 is prepared. And the said steel material is shape | molded by the schematic shape of a bearing component. For example, a steel bar, steel wire or the like is used as a raw material, and the steel bar, steel wire or the like is subjected to processing such as cutting, forging, turning, etc., so that the outer ring 11, inner ring 12 and ball 13 which are bearing parts are roughly outlined. A steel material formed into a shape is prepared (see FIG. 1).

Next, a quench hardening process is implemented as process (S20). In this step (S20), the carbon steel prepared in the above step (S10) is subjected to carbonitriding, and then the temperature of the A ₁ transformation point (hereinafter simply referred to as A ₁ point) or higher is set to the M _s point (Martens). Site transformation start point) Cooled to the following temperature. Carbonitriding treatment, the activity of a _c of carbon in the bearing _part, the undecomposed ammonia concentration in the heat treatment furnace in which the bearing part is arranged in the carbonitriding process in case of the _{C N, γ = a c /} C N It is carried out so that the value of γ defined in (1) is in the range of 2 or more and 5 or less, preferably 5. Thereby, the surface layer portion of the bearing component is nitrided. Details of this step (S20) will be described later.

Next, a tempering step is performed as a step (S30). In this step (S30), the steel material hardened and hardened in the above step (S20) is subjected to a heat treatment at a temperature of A ₁ point or less. More specifically, after carbonitriding, the steel material is held for a predetermined time (for example, 2 hours) at a temperature of 240 ° C. or higher, preferably 240 ° C. or higher and 260 ° C. or lower, which is a temperature of A ₁ or lower. The steel material is tempered. Then, the steel material is cooled by air at room temperature (air cooling). Thereby, the toughness etc. of steel materials can be improved.

  Next, finishing is performed as a step (S40). In this step (S40), grinding and finishing are performed on the outer ring rolling surface 11A, the inner ring rolling surface 12A, and the rolling surface 13A of the tempered steel material. Thereby, the outer ring | wheel 11, the inner ring | wheel 12, and the ball | bowl 13 which are the bearing components which concern on the said this Embodiment are manufactured (refer FIG. 1).

  Next, an assembly process is performed as a process (S50). In this step (S50), the outer ring 11, the inner ring 12 and the ball 13 manufactured in the steps (S10) to (S40) are combined with the separately prepared cage 14 and the like, and the deep groove in the above-described embodiment. The ball bearing 1 is assembled (see FIG. 1).

  Next, details of the quench hardening step (S20) will be described. FIG. 4 is a diagram for explaining the details of the quench hardening process included in the method of manufacturing a bearing component according to the present embodiment. Moreover, FIG. 5 is a figure which shows an example of the heating pattern in the heating pattern control process included in the carbonitriding process of FIG. In FIG. 5, the horizontal direction indicates time, and the time elapses toward the right. In FIG. 5, the vertical direction indicates the temperature, and the higher the temperature, the higher the temperature. With reference to FIG. 4 and FIG. 5, the detail of the hardening hardening process contained in the manufacturing method of the bearing component which concerns on this Embodiment is demonstrated.

Referring to FIG. 4, in the quench hardening process of the bearing component manufacturing method according to the present embodiment, first, a carbonitriding process is performed in which a steel material that is a workpiece is carbonitrided. Thereafter, the cooling step of the steel is cooled to a temperature below M _S point from the temperature of the above _point A is performed.

  The carbonitriding process includes an atmosphere control process in which the atmosphere in the heat treatment furnace is controlled, and a heating pattern control process in which the temperature history applied to the workpiece in the heat treatment furnace is controlled. The atmosphere control step and the heating pattern control step can be performed independently and in parallel. The atmosphere control step includes an undecomposed ammonia concentration control step for controlling the undecomposed ammonia concentration in the heat treatment furnace, and a partial pressure control for controlling a partial pressure of at least one of carbon monoxide and carbon dioxide in the heat treatment furnace. Process.

In the partial pressure control step, by controlling the partial pressure of at least one of carbon monoxide and carbon dioxide in the heat treatment furnace, the carbon activity (a _C ) value in the steel material is controlled, and the γ value is As well as being adjusted, the carbon potential (C _P ) value is adjusted. Furthermore, in the atmosphere control step, the undecomposed ammonia concentration control step and the partial pressure control step are performed so that the value of γ is in the range of 2 to 5, preferably 5.

Specifically, in the undecomposed ammonia concentration control step, first, an undecomposed ammonia concentration measurement step for measuring the undecomposed ammonia concentration (C _N ) in the heat treatment furnace is performed. The measurement of the undecomposed ammonia concentration can be performed using, for example, a gas chromatograph. Then, an undecomposed ammonia concentration determination step for determining whether or not an ammonia supply amount adjustment step for increasing or decreasing the supply amount of ammonia gas to the heat treatment furnace based on the undecomposed ammonia concentration measured in the undecomposed ammonia concentration measurement step is performed. Is implemented. The determination is made by comparing the target undecomposed ammonia concentration determined in advance so that the value of γ (a _C / C _N ) is in the range of 2 to 5, and the measured undecomposed ammonia concentration. Is done.

  When the undecomposed ammonia concentration is not equal to the target undecomposed ammonia concentration, an ammonia supply amount adjusting step for increasing or decreasing the undecomposed ammonia concentration in the heat treatment furnace is performed, and then the undecomposed ammonia concentration measuring step is performed. Will be implemented again. The ammonia supply amount adjusting step includes, for example, a mass flow controller attached to the pipe for the amount of ammonia flowing into the heat treatment furnace per unit time from an ammonia gas cylinder connected to the heat treatment furnace via the pipe. It can implement by adjusting with the flow control apparatus with which it was equipped. That is, when the measured undecomposed ammonia concentration is higher than the target undecomposed ammonia concentration, the ammonia supply amount adjusting step can be performed by decreasing the flow rate, and increasing the flow rate when the concentration is low. In this ammonia supply amount adjustment step, when there is a predetermined difference between the measured undecomposed ammonia concentration and the target undecomposed ammonia concentration, the degree of increase or decrease in flow rate is determined experimentally in advance. It can be determined based on the relationship between increase / decrease in gas flow rate and increase / decrease in undecomposed ammonia concentration.

  On the other hand, when the undecomposed ammonia concentration is the target undecomposed ammonia concentration, the undecomposed ammonia concentration measuring step is performed again without performing the ammonia supply amount adjusting step.

In the partial pressure control step, by adjusting the supply amount of propane (C ₃ H ₈ ) gas, butane gas (C ₄ H ₁₀ ), etc. as an enriched gas, at least one of the partial pressures of CO and CO ₂ The partial pressure is controlled and the a _C value is adjusted. Specifically, for example, the partial pressure P _CO2 partial pressure P _CO and carbon monoxide to carbon dioxide in the atmosphere by using an infrared gas concentration measurement device are measured. Based on the measured value, the supply amount of propane (C ₃ H ₈ ) gas, butane gas (C ₄ H ₁₀ ), or the like as the enriched gas is adjusted so that the a _C value becomes a target value.

The value of γ may be controlled by changing the a _C value by the partial pressure control step while keeping the undecomposed ammonia concentration constant by the undecomposed ammonia concentration control step, or conversely, the partial pressure control step In this state, the undecomposed ammonia concentration may be controlled by changing the undecomposed ammonia concentration control step while keeping the a _C value constant. Further, the value of γ may be controlled by changing the undecomposed ammonia concentration and the a _C value by the undecomposed ammonia concentration control step and the partial pressure control step.

In the heating pattern control step, the heating history applied to the steel material as the object to be processed is controlled. Specifically, as shown in FIG. 5, in an atmosphere in which the steel material is controlled by the above-described atmosphere control step and partial pressure control step, a temperature of 800 ° C. or higher and 1000 ° C. or lower, which is a temperature of A ₁ point or higher, for example, It is heated to 850 ° C. and held for 60 minutes to 300 minutes, for example, 150 minutes. As the holding time elapses, the heating pattern control process ends, and at the same time, the atmosphere control process ends.

Thereafter, the steel member by being dipped (oil cooling) in oil, a cooling step is cooled from a temperature of more than ₁ point A to M _S point below the temperature is carried out. Through the above steps, the surface layer portion of the steel material is carbonitrided and hardened and hardened. Thereby, the quench hardening process (S20) of this Embodiment is completed.

  Thus, in the method for manufacturing a bearing component according to the present embodiment, the carbonitriding process is performed so that the nitrogen concentration in the surface layer portion is 0.4 mass% or more in the quench hardening step (S20), and the firing is performed. In the returning step (S30), a tempering process is performed at a temperature of 240 ° C. or higher and 260 ° C. or lower. As a result, the amount of retained austenite in the surface layer portions 11B, 12B, and 13B is 8% by volume or less, and the area ratio of precipitates in the non-nitrided interiors 11C, 12C, and 13C is 11% or more, yield strength and life It is possible to manufacture the outer ring 11, the inner ring 12 and the ball 13 which are excellent in (see FIGS. 1 and 2).

  The following experiment was conducted for the purpose of obtaining a bearing component that was inexpensive and improved in yield strength and life. First, a steel material comprising JIS standard SUJ2 was prepared, and the steel material was subjected to carbonitriding, quenching, and tempering in order, and thereafter subjected to grinding and finishing to produce a bearing part. And the following tests were done in the case where the nitrogen concentration and tempering temperature in the surface layer portion were changed.

  The atmosphere during the carbonitriding treatment was a value of γ of 4.75 and a heating temperature of 850 ° C. When the value of γ is larger than 5, the nitrogen intrusion rate decreases and the region where the nitrogen concentration is high tends to stay on the surface layer, and considering the machining allowance in grinding, the time for carbonitriding becomes very long and is not practical. Absent. Further, when the SUJ2 material is used, a large amount of ammonia is required to keep the undecomposed ammonia fraction high at a temperature significantly higher than 850 ° C., thereby increasing the cost of the process. On the other hand, at a temperature significantly lower than 850 ° C., the diffusion rate of nitrogen into the steel becomes slow and the treatment time becomes long. Therefore, it can be said that a temperature around 850 ° C. is appropriate in the carbonitriding treatment of the SUJ2 material.

(1) Relationship between the nitrogen concentration and tempering temperature in the surface layer portion and the residual indentation depth First, the effect of the nitrogen concentration and tempering temperature in the surface portion on the residual indentation depth was investigated. First, a flat test piece in which the nitrogen concentration and the tempering temperature in the surface layer portion were changed was produced. Specifically, test pieces having the nitrogen concentration of 0 mass% (mass%), 0.1 mass%, 0.25 mass%, and 0.4 mass% were prepared, and 180 ° C., 210 ° C., and 240 ° C. for each test piece. Tempering treatment was performed at a temperature of ℃ and 260 ℃. The residual indentation depth after pressing the ceramic sphere (size: 3/8 inch) against the plane of the test piece with a load having a maximum contact surface pressure of 4.5 GPa (assumed as a complete elastic body) and removing the load. investigated. The value of 4.5 GPa is almost the same value as the maximum value of the maximum contact surface pressure applied to the rolling bearing in the market.

  FIG. 6 shows the results of investigating the relationship between the nitrogen concentration and tempering temperature in the surface layer and the residual indentation depth. In FIG. 6, the horizontal axis indicates the nitrogen concentration (mass%), and the vertical axis indicates the residual indentation depth (μm). As apparent from FIG. 6, the residual indentation depth was greatly reduced when the tempering temperatures were 240 ° C. and 260 ° C., compared to when the tempering temperatures were 180 ° C. and 210 ° C. Further, when the tempering temperature is 180 ° C. and 210 ° C., the residual indentation depth increases as the nitrogen concentration increases, whereas when the tempering temperature is 240 ° C. and 260 ° C. As the nitrogen concentration increased, the residual indentation depth became smaller (harder to plastic deformation). From this result, in the range of the above experiment (tempering temperature is 180 ° C. or higher and 260 ° C. or lower), in the SUJ2 material containing nitrogen, the yield strength increases as the tempering temperature increases, and a high static load capacity is obtained. I found out that

(2) Relationship between Nitrogen Concentration at Surface Layer and Indentation Origination Peeling Life Next, Relationship between Nitrogen Concentration at Surface Layer and Indentation Origination Separation Life at High Temperature Tempering (240 ° C, 260 ° C) investigated. First, a test piece processed into an inner ring shape of a ball bearing (bearing model number: 6206) was produced. And the artificial indentation of 6 degrees equidistant was formed in the rolling-surface groove bottom part of the said test piece, and the rolling fatigue test was implemented. The artificial indentation was formed by loading a Rockwell indenter with a load of 196N. In the rolling fatigue test, the maximum contact surface pressure was 3.2 GPa, the inner ring rotational speed was 3000 rpm, and the lubricating oil was circulating oil supply of turbine oil VG56.

  In FIG. 7, the result of having investigated about the relationship between the nitrogen concentration in a surface layer part and an indentation origin type | mold peeling life is shown. FIG. 7 is a Weibull plot of the indentation origin peeling life, where the horizontal axis represents the life (h) and the vertical axis represents the cumulative failure probability (%). In FIG. 7, the nitrogen concentration and the tempering temperature are indicated by the expression “nitrogen concentration−tempering temperature”. For example, “0.4 mass% −240 ° C.” indicates that the nitrogen concentration is 0.4 mass% and the tempering temperature is 240 ° C. For reference, the results are also shown when the nitrogen concentration is 0.4 mass% and the tempering temperature is 180 ° C. As is apparent from FIG. 7, when the nitrogen concentration is 0.4 mass% and the tempering temperature is 240 ° C., almost the same result as that obtained when the nitrogen concentration is 0 mass% and the tempering temperature is 180 ° C. is obtained. It was. However, as the nitrogen concentration decreased, the indentation origin peel life decreased. Therefore, in order to achieve both improvement in yield strength and improvement in life, the tempering temperature is set to a high temperature (240 ° C. and 260 ° C.) as described above, and the nitrogen concentration in the surface layer portion is set to 0.4 mass% or more. I found it necessary to be high.

(3) Relationship between the nitrogen concentration and tempering temperature in the surface layer portion and the hydrogen embrittlement peeling life Next, the influence of the nitrogen concentration and tempering temperature in the surface layer portion on the hydrogen resistance in rolling fatigue was investigated. Here, the hydrogen source in rolling fatigue is the lubricant itself or water mixed in the lubricant, which decomposes due to slippage between the contact elements and generates hydrogen, and a part of this hydrogen is in the steel. It is thought to invade.

A rolling fatigue test was performed using a bearing ring of a thrust bearing (bearing model number: 51106) as a test piece. As an operation pattern, the rapid acceleration / deceleration pattern shown in FIG. 8 was used. In FIG. 8, the horizontal axis indicates time (sec), and the vertical axis indicates the rotation speed (min ⁻¹ ). Moreover, as shown in FIG. 8, it accelerates to the rotational speed of 2500min < ^-1 > in 0-0.1sec, maintains the said rotational speed for 0.3sec after that, and is 0min < ^-1 > in 0.1sec. Slow down to The test was conducted by repeating this cycle. A ball made of SUS440C was used, and the number was reduced from 17 (standard number) to 12. As the cage, a resin-made one for 12 equal distribution was prepared, and the maximum contact surface pressure was set to 2.3 GPa. A water-soluble polyglycol oil was used as the lubricating oil, and pure water as a hydrogen source was mixed therein (pure water concentration: 40 mass%). The tester automatically stopped when the raceway ring was separated and the vibration increased.

  Here, it has been confirmed in advance that the influence of pure water, which is a hydrogen source, on the life (hydrogen resistance life) does not change when the pure water concentration is 20 mass% or more. After the test, the proportion of pure water in the lubricating oil is slightly reduced. Therefore, the validity of the experiment for evaluating the hydrogen resistance is ensured by measuring the mixing ratio of pure water after the test and confirming that it is 20 mass% or more. In addition, after the said experiment, about 35 mass% pure water remained.

FIG. 9 and Table 1 show the results of investigating the relationship between the surface nitrogen concentration and tempering temperature and the hydrogen embrittlement separation life. FIG. 9 is a Weibull plot of the hydrogen resistance lifetime, where the horizontal axis indicates the lifetime (h) and the vertical axis indicates the cumulative failure probability (%). In FIG. 9, the nitrogen concentration and the tempering temperature are indicated by the display of “nitrogen concentration−tempering temperature” as in the case of FIG. 7. Table 1 shows L ₁₀ life (h), L ₅₀ life (h) and Weibull slope values for each heat treatment condition (nitrogen concentration-tempering temperature).

9 and Table 1, whereas in the case of 0mass% -180 ℃ _{L 10} life was 44h, in the case of 0.4mass% -180 ℃ 138h (3.1-fold) In the case of 0.4 mass% -240 ° C., it is 173 h (3.9 times), and in the case of 0.4 mass% -260 ° C., it is 185 h (4.2 times), and the tempering temperature becomes high. As a result, the service life became longer. As described with reference to the results shown in FIG. 6, this is considered to be caused by the fact that plastic deformation becomes difficult as the tempering temperature increases. It is known that hydrogen acts on plastic deformation by facilitating the movement of dislocations, stabilizing atomic vacancies generated by the interaction between dislocations, and causing proliferation. Therefore, it is considered that the life is extended so that plastic deformation is difficult.

  The longest life is 212 h for 0 mass% -180 ° C., 242 h for 0.4 mass% -180 ° C., 251 h for 0.4 mass% -240 ° C., 0 In the case of 4 mass% -260 ° C., it was 266 h, so there was no significant difference in the longest life. On the other hand, in the case of 0 mass% -180 ° C., the life variation is large, and as a result, the L10 life is shortened, whereas 0.4 mass% -180 ° C., 0.4 mass% -240 ° C. and 0.4 mass%. In the case of −260 ° C., it is considered that all of them are stable and have a long life.

(4) Relationship between tempering temperature and retained austenite (γ) amount Next, the relationship between the tempering temperature and the retained austenite amount was investigated. As described above, the static load capacity and life of the bearing vary depending on the tempering temperature, but it is difficult to obtain the tempering temperature condition directly from the bearing in the product state. However, when the tempering temperature is constant, there is a correlation between the amount of retained austenite and the tempering temperature at a position where the nitrogen concentration is 0.4 mass%, so the tempering temperature is indirectly determined from the amount of retained austenite. It is possible to clarify.

  FIG. 10 shows the relationship between the tempering temperature and the amount of retained austenite. 10, the horizontal axis indicates the tempering temperature (° C.), and the vertical axis indicates the amount of retained austenite (vol.%) At the position where the nitrogen concentration is 0.4 mass%. Further, the holding time of the tempering process is constant at 2 h. As shown in FIG. 10, when the tempering temperature is 240 ° C., the amount of retained austenite is 8% by volume (vol.%) Or less, and when the tempering temperature is 260 ° C., 5 vol. It turned out that it falls to less than%.

(5) Relationship between area ratio of internal precipitates and tempering temperature Next, the relationship between the area ratio of internal precipitates and the tempering temperature was investigated. As a method of indirectly clarifying the tempering temperature from the product state, there is a method of measuring the area ratio of precipitates in addition to the amount of retained austenite. This is because when the heating temperature during quenching (here, 850 ° C., the prior austenite grains are No. 9 to 11 according to JIS standard) and the holding time during tempering are constant. This is because, as the return temperature increases, carbon dissolved in the matrix precipitates and large cementite (Fe ₃ C) is formed.

  FIG. 11 shows the result of investigating the relationship between the area ratio of precipitates in the internal non-nitrided region and the tempering temperature. In FIG. 11, the horizontal axis indicates the tempering temperature (° C.), and the vertical axis indicates the area ratio (%) of the precipitate. Further, the holding time in the tempering process was fixed at 2 hours. 12 to 15 are photographs of a cross section of the steel material when the tempering temperatures are 180 ° C., 210 ° C., 240 ° C. and 260 ° C., and precipitates in the steel material appear black. FIG. 11 shows the relationship between the tempering temperature and the area ratio of precipitates that appear black from the photographs of FIGS. 12 to 15. FIG. 16 is a photograph of the prior austenite crystal grains in the internal non-nitrided region when the heating temperature during the quenching process is 850 ° C.

  From FIG. 11, it was found that the area ratio was 11% or more when the tempering temperature was 240 ° C., and increased to 12% or more when it was 260 ° C. The reason why the area ratio of the precipitate is determined in the internal non-nitrided region is that the area ratio of the precipitate greatly fluctuates because the carbon solid solubility limit is changed in the nitrided surface layer portion. is there. Moreover, although the said area ratio fluctuates also by carbonitriding temperature and quenching heating temperature, these can be estimated from the average grain size of prior austenite crystal grains. As shown in FIG. 16, the grain size number of the prior austenite crystal grains in the unnitrided region when the heating temperature was 850 ° C. was 9.5. From the examination of the above (1) to (5), the steel material made of JIS standard SUJ2 is subjected to carbonitriding and quenching treatment so that the nitrogen concentration on the surface (rolling surface or rolling surface) is 0.4 mass% or more, Further, it has been found that by performing the tempering treatment at a temperature of 240 ° C. or higher, it is possible to obtain a bearing component that is inexpensive and has improved yield strength and life.

  The embodiments and examples disclosed herein are illustrative in all respects and should not be construed as being restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  The bearing component and the rolling bearing of the present invention can be particularly advantageously applied to a bearing component having a nitrided surface layer including a rolling surface or a rolling surface and a rolling bearing including the bearing component.

  DESCRIPTION OF SYMBOLS 1 Deep groove ball bearing, 11 outer ring, 11A outer ring rolling surface, 12 inner ring, 12A inner ring rolling surface, 11B, 12B, 13B surface layer part, 11C, 12C, 13C inside, 11D, 12D, 13D nitrogen enriched layer, 13 balls , 13A Rolling surface, 14 Cage.

Claims (6)

A bearing part made of bearing steel and having a nitrided surface layer including the rolling surface or rolling surface,
The nitrogen concentration in the surface layer is 0.4% by mass or more,
The bearing component whose amount of retained austenite in the surface layer portion is 8% by volume or less.   The bearing component according to claim 1, wherein the amount of retained austenite is 5% by volume or less.   The bearing component according to claim 1 or 2, comprising JIS standard SUJ2.   The bearing component according to any one of claims 1 to 3, wherein a grain size number of the prior austenite crystal grains in the non-nitrided interior is 9 or more and 11 or less. The surface layer portion is nitrided by carbonitriding,
The carbonitriding process, the activity of a _c of carbon in the bearing _part, the undecomposed ammonia concentration of the bearing part in the heat treatment furnace is disposed in the carbonitriding process in case of the C _N, γ = a the value of γ is defined by _{c /} C _N is performed such that the range of 2 to 5, the bearing component according to any one of claims 1 to 4. An outer ring having a rolling surface on the inner peripheral surface;
An inner ring disposed on the inner side of the outer ring, having a rolling surface on the outer peripheral surface;
A plurality of rolling elements arranged in contact with the rolling surface on the rolling surface and arranged side by side on an annular track,
At least any one of the said outer ring | wheel, the said inner ring | wheel, and the said rolling element is a rolling bearing which is a bearing component of any one of Claims 1-5.