JP5397928B2 - Machine parts - Google Patents

Description

  The present invention relates to a machine part, and more specifically to a machine part made of steel containing 4 mass% or more of chromium and having a nitrogen-enriched layer formed on the surface layer portion.

  For the purpose of improving the strength of the surface layer portion of a machine part made of steel having a carbon content of 0.15% by mass or less, for example, by performing a carburizing process, the surface layer portion has a region with a higher carbon concentration than other regions. After the formation, a process for forming a nitrogen-enriched layer, which is a layer having a higher nitrogen concentration than other areas, in a region including the surface, for example, a nitriding process may be performed. As a conventional steel nitriding treatment method, nitriding treatment in which nitrogen is introduced into the surface layer portion of steel by heating the steel in an atmosphere containing a gas serving as a nitrogen source such as ammonia is typical. However, in a machine part made of steel having a high chromium content, for example, steel containing 4% by mass or more of chromium, a chemically stable oxide film is formed on the surface layer portion. Therefore, even if the nitriding treatment is performed on a mechanical component made of steel having a high chromium content, there is a problem that nitrogen does not enter the surface layer portion and a nitrogen-enriched layer is not formed.

On the other hand, the object to be processed made of steel is arranged in a furnace having a reduced pressure, and after introducing a gas containing a gas to be a nitrogen source into the furnace, the object to be processed is arranged to face the object to be processed. A process (plasma nitriding process) has been proposed in which a potential difference is generated between the formed member, for example, a furnace wall, a glow discharge is generated, and nitrogen is penetrated into a surface layer portion of steel constituting the workpiece (for example, plasma nitriding process). , See Patent Document 1). With respect to the control of the plasma nitriding treatment, for example, a method based on spectroscopic analysis of glow discharge and a method based on the current density of the current flowing through the workpiece have been proposed (see, for example, Patent Documents 2 and 3). ). This makes it possible to form a nitrogen-enriched layer on the surface layer portion of a machine part made of steel containing 4 mass% or more of chromium.
Japanese Patent Laid-Open No. 2-57675 JP-A-7-118826 Japanese Patent Laid-Open No. 9-3646

  However, even when a nitrogen-enriched layer is formed on the surface layer portion of a mechanical component made of steel containing 4% by mass or more of chromium as described above, the characteristics of the mechanical component may not be sufficiently improved. That is, when stress is repeatedly applied to the mechanical parts as described above, peeling or fracture may occur at an early stage (decrease in fatigue strength). Further, when a shocking stress is applied to the above-described mechanical component, breakage may easily occur (decrease in toughness). In other words, in mechanical parts made of steel containing 4% by mass or more of chromium, merely forming a nitrogen-enriched layer increases the hardness of the surface layer part, but it is not always sufficient particularly in terms of fatigue strength and toughness. There is a problem in that it may not be possible to obtain proper characteristics.

  Accordingly, an object of the present invention is to provide a mechanical component that is made of steel containing 4% by mass or more of chromium, has a nitrogen-enriched layer formed in the surface layer portion, and has sufficient fatigue strength and toughness. It is to be.

  The mechanical component according to the present invention includes 0.11% by mass to 0.15% by mass carbon, 0.1% by mass to 0.25% by mass silicon, and 0.15% by mass to 0.35% by mass. Manganese of not more than mass%, nickel of not less than 3.2 mass% and not more than 3.6 mass%, chromium of not less than 4 mass% and not more than 4.25 mass%, and molybdenum of not less than 4 mass% and not more than 4.5 mass% 1.13 mass% or more and 1.33 mass% or less of vanadium, and the balance iron and impurities are used. In the region including the surface, a nitrogen enriched layer having a nitrogen concentration of 0.05% by mass or more is formed. And the total value of the carbon concentration and nitrogen concentration in a nitrogen enriched layer is 0.55 mass% or more and 1.9 mass% or less.

  The present inventor has made a detailed study on the cause of a decrease in fatigue strength and toughness when a nitrogen-enriched layer is formed on a mechanical part made of steel containing 4% by mass or more of chromium. As a result, it was found that the fatigue strength and toughness of the machine parts are reduced due to the following phenomenon.

That is, when a nitrogen-enriched layer is formed on a mechanical part made of steel containing 4% by mass or more of chromium by plasma nitriding as described above, the amount of nitrogen in the surface layer portion is the solid solubility limit of the steel constituting the mechanical part. (Solution limit including nitrogen contained in the precipitate) is exceeded. Therefore, iron nitrides (Fe ₃ N, Fe ₄ N, etc.) that precipitate along the grain boundaries are formed in the steel constituting the machine part. An iron nitride formed with a length of not less than 2 μm and a length of not less than 7.5 μm (hereinafter referred to as having an aspect ratio of not less than 2 and a length of not less than 7.5 μm along the grain boundary) The formed iron nitride is referred to as grain boundary precipitate), which may be the starting point of peeling and fracture.

  More specifically, when stress is repeatedly applied to a machine part on which grain boundary precipitates are formed, the grain boundary precipitates may become a stress concentration source, and cracks may occur. And since this crack progresses and it leads to peeling and a fracture | rupture, the fatigue strength of a machine component falls. In addition, when shock stress is applied to the machine part on which the grain boundary precipitate is formed, the grain boundary precipitate promotes the generation and progress of cracks, and thus the toughness may be reduced. That is, as a result of an excessive amount of nitrogen entering the surface layer portion of the machine part, grain boundary precipitates are formed, which may cause a decrease in fatigue strength and toughness of the machine part.

  In contrast, in the machine part of the present invention, a nitrogen-enriched layer of 0.05% by mass or more is formed in a region including the surface of the machine part made of steel having an appropriate component composition, and then the nitrogen-enriched layer is formed. By making the total value of the carbon concentration and the nitrogen concentration in the layer within an appropriate range, formation of grain boundary precipitates can be suppressed. As a result, according to the mechanical component of the present invention, it is made of steel containing 4% by mass or more of chromium, a nitrogen-enriched layer is formed in the surface layer portion, and fatigue strength and toughness are sufficiently secured. Mechanical parts can be provided. Hereinafter, the reason why the component range of steel constituting the mechanical component of the present invention and the nitrogen and carbon concentrations in the nitrogen-enriched layer are limited to the above ranges will be described.

Carbon: 0.11% by mass or more and 0.15% by mass or less In the steel constituting the machine part, if the carbon is less than 0.11% by mass, there may be a problem that the manufacturing cost of the steel increases. On the other hand, when carbon exceeds 0.15 mass%, problems such as an increase in core hardness and a decrease in toughness may occur. Therefore, carbon needs to be 0.11 mass% or more and 0.15 mass% or less.

Silicon: 0.1 mass% or more and 0.25 mass% or less In the steel which comprises a machine part, if silicon is less than 0.1 mass%, the problem of the manufacturing cost rise of steel may generate | occur | produce. On the other hand, when silicon exceeds 0.25 mass%, the problem that the hardness of a raw material will rise and cold workability will fall may generate | occur | produce. Therefore, silicon needs to be 0.1 mass% or more and 0.25 mass% or less.

Manganese: 0.15% by mass or more and 0.35% by mass or less In the steel constituting the machine part, if manganese is less than 0.15% by mass, there may be a problem of an increase in the manufacturing cost of the steel. On the other hand, when manganese exceeds 0.35 mass%, the problem that the hardness of a raw material will rise and cold workability will fall may generate | occur | produce. Therefore, manganese needs to be 0.15 mass% or more and 0.35 mass% or less.

Nickel: 3.2 mass% or more and 3.6 mass% or less In steel constituting machine parts, if nickel is less than 3.2 mass%, there may be a problem that the effect of improving corrosion resistance, hardness and toughness is reduced. . On the other hand, when nickel exceeds 3.6 mass%, the problem of an increase in the amount of retained austenite may occur. Therefore, nickel needs to be 3.2 mass% or more and 3.6 mass% or less.

Chromium: 4% by mass or more and 4.25% by mass or less In the steel constituting the machine part, if chromium is less than 4% by mass, there may be a problem that the temper softening resistance is lowered. On the other hand, when chromium exceeds 4.25 mass%, the problem of inhibiting the solid solution of a carbide | carbonized_material may generate | occur | produce. Therefore, chromium needs to be 4 mass% or more and 4.25 mass% or less.

Molybdenum: 4 mass% or more and 4.5 mass% or less In the steel which comprises a machine part, if molybdenum is less than 4 mass%, the problem that a temper softening resistance fall may generate | occur | produce. On the other hand, when molybdenum exceeds 4.5 mass%, the problem that the manufacturing cost of steel rises may generate | occur | produce. Therefore, molybdenum needs to be 4 mass% or more and 4.5 mass% or less.

Vanadium: 1.13 mass% or more and 1.33 mass% or less In the steel constituting the machine part, when vanadium is less than 1.13 mass%, the effect of reducing the temper softening resistance and refining the microstructure by adding vanadium is obtained. The problem of fewer may occur. On the other hand, when vanadium exceeds 1.33 mass%, the problem that the manufacturing cost of steel rises may generate | occur | produce. Therefore, vanadium needs to be 1.13 mass% or more and 1.33 mass% or less.

Nitrogen concentration in the nitrogen-enriched layer: 0.05 mass% or more In the machine part made of the above steel, in order to give sufficient hardness to the surface layer portion to ensure wear resistance, the nitrogen concentration in the region including the surface It is necessary to form a nitrogen-enriched layer with 0.05% by mass or more. In order to further improve the wear resistance and the like, the nitrogen concentration on the surface of the machine part is preferably 0.15% by mass or more.

Total value of nitrogen concentration and carbon concentration in the nitrogen-enriched layer: 0.55% by mass or more and 1.9% by mass or less In the machine part made of the above steel, the surface layer portion is given sufficient hardness to provide wear resistance and the like. In order to secure it, it is important to manage not only the nitrogen concentration but also the carbon concentration. And, if the total value of the nitrogen concentration and the carbon concentration in the nitrogen-enriched layer is less than 0.55 mass, it is difficult to impart sufficient hardness to the surface layer portion to ensure wear resistance and the like. Found. Therefore, the total value of the nitrogen concentration and the carbon concentration in the nitrogen-enriched layer needs to be 0.55% by mass or more. Further, in order to make it easy to provide sufficient hardness to the surface layer portion to ensure wear resistance and the like, the total value of the nitrogen concentration and the carbon concentration in the nitrogen-enriched layer is 0.7% by mass or more. It is preferable that

  On the other hand, in the machine parts made of steel, when the nitrogen concentration in the surface layer portion becomes high, grain boundary precipitates are easily formed, and when the carbon concentration becomes high, the tendency becomes stronger. And this inventor discovered that it will become difficult to suppress formation of a grain-boundary precipitate when the total value of the nitrogen concentration and carbon concentration in a nitrogen enriched layer exceeds 1.9 mass%. Therefore, the total value of the nitrogen concentration and the carbon concentration in the nitrogen enriched layer in the nitrogen enriched layer needs to be 1.9% by mass or less. Moreover, in order to further suppress the formation of grain boundary precipitates, the total value of the nitrogen concentration and the carbon concentration in the nitrogen-enriched layer is preferably 1.7% by mass or less. The carbon concentration and the nitrogen concentration refer to concentrations in the substrate (matrix) which is a region other than carbides and nitrides such as iron and chromium.

  In the machine part, preferably, the thickness of the nitrogen-enriched layer is 0.11 mm or more. In mechanical parts such as bearings, hubs, constant velocity universal joints, and gears, the strength of the surface and the area immediately below the surface, specifically, the distance within 0.11 mm from the surface is often important. Therefore, by setting the thickness of the nitrogen-enriched layer to 0.11 mm or more, it is possible to impart sufficient strength to the machine part. In order to further increase the strength of the mechanical component, the thickness of the nitrogen-enriched layer is preferably 0.15 mm or more.

  Preferably, in the mechanical component, the nitrogen-enriched layer has a hardness of 800 HV or higher. By setting the hardness of the nitrogen-enriched layer formed on the surface layer portion to 800 HV or more, it is possible to ensure the strength of the machine part more reliably.

  Preferably, in the mechanical part, when the nitrogen-enriched layer is observed with a microscope, the number of iron nitrides having an aspect ratio of 2 or more and a length of 7.5 μm or more is 1 or less in a 5 field of view of a square region having a side of 150 μm. It is.

  As described above, grain boundary precipitates, which are iron nitrides having an aspect ratio of 2 or more and a length of 7.5 μm or more, may reduce the characteristics such as fatigue strength and toughness of mechanical parts. And when the inventor investigated the relationship between the fatigue strength of the machine part and the number density of the grain boundary precipitates for the machine part made of steel having the above component composition, when the nitrogen-enriched layer was observed with a microscope, It has been found that when grain boundary precipitates are present at a number density of more than one in five fields of a square region having a side of 150 μm, the fatigue strength of the machine part is lowered. Therefore, when the nitrogen-enriched layer is observed with a microscope, the number of grain boundary precipitates is 1 or less in a 5 field of view of a square region having a side of 150 μm, so that the fatigue strength of mechanical parts can be improved. In order to further improve the fatigue strength of the machine part, the number of grain boundary precipitates is preferably 1 or less in the field of view of the square region 60 having a side of 150 μm.

  The mechanical component of the present invention may be used as a component constituting a bearing. The mechanical component of the present invention, which is strengthened by nitriding the surface layer portion and suppresses the occurrence of grain boundary precipitates, is a component that constitutes a bearing that is a mechanical component that requires fatigue strength, wear resistance, etc. Is preferred.

  In addition, you may comprise the rolling bearing provided with the above-mentioned machine component and the rolling element which contacts a bearing ring and contacts a bearing ring, and is arrange | positioned on an annular | circular shaped raceway. That is, at least one of the bearing ring and the rolling element, preferably both are the above-described machine parts. By providing the mechanical part of the present invention in which the surface layer portion is strengthened by nitriding and the occurrence of grain boundary precipitates is suppressed, the rolling bearing provides a long-life rolling bearing. Can do.

  The concentration of nitrogen and carbon in the nitrogen-enriched layer can be investigated by, for example, EPMA (Electron Probe Micro Analysis). The number density of the iron nitride (grain boundary precipitates) can be investigated, for example, as follows. That is, first, a machine part is cut in a cross section perpendicular to the surface, and the cross section is polished. Thereafter, the cross section is corroded with an appropriate corrosive solution, and then the nitrogen-enriched layer is observed with a scanning electron microscope (SEM) or an optical microscope to take a photograph. Then, a square visual field having a side of 150 μm whose surface is defined as one side of the visual field is analyzed by an image analyzer, and the number of grain boundary precipitates is investigated. This is performed randomly in five or more visual fields, and the number of grain boundary precipitates per five visual fields is calculated.

  As is apparent from the above description, according to the mechanical component of the present invention, it is made of steel containing 4 mass% or more of chromium, a nitrogen-enriched layer is formed in the surface layer portion, and fatigue strength and toughness are formed. However, it is possible to provide a sufficiently secured mechanical component.

  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.

  FIG. 1 is a schematic cross-sectional view showing a configuration of a deep groove ball bearing as a rolling bearing provided with mechanical parts according to an embodiment of the present invention. FIG. 2 is a schematic partial cross-sectional view showing an enlarged main part of FIG. With reference to FIG. 1 and FIG. 2, the deep groove ball bearing as a rolling bearing in one embodiment of this invention is demonstrated.

  Referring to FIG. 1, a deep groove ball bearing 1 is arranged between an annular outer ring 11, an annular inner ring 12 arranged inside the outer ring 11, and between the outer ring 11 and the inner ring 12. 14 and a plurality of balls 13 as rolling elements held by 14. An outer ring rolling surface 11 </ b> A is formed on the inner circumferential surface of the outer ring 11, and an inner ring rolling surface 12 </ b> A is formed on the outer circumferential surface of the inner ring 12. And the outer ring | wheel 11 and the inner ring | wheel 12 are arrange | positioned so that 12A of inner ring | wheel rolling surfaces and 11A of outer ring | wheels may mutually oppose. Further, the plurality of balls 13 are in contact with the inner ring rolling surface 12A and the outer ring rolling surface 11A on the ball rolling surface 13A, and are arranged at a predetermined pitch in the circumferential direction by the cage 14, thereby forming an annular shape. It is held so that it can roll on the track. With the above configuration, the outer ring 11 and the inner ring 12 of the deep groove ball bearing 1 are rotatable relative to each other.

  Here, the outer ring 11, the inner ring 12 and the ball 13, which are mechanical parts, are 0.11% by mass to 0.15% by mass of carbon, 0.1% by mass to 0.25% by mass of silicon, .15 mass% to 0.35 mass% manganese, 3.2 mass% to 3.6 mass% nickel, 4 mass% to 4.25 mass% chromium, 4 mass% to 4 mass% It is composed of a steel containing 0.5% by mass or less of molybdenum and 1.13% by mass or more and 1.33% by mass or less of vanadium, the balance being iron and impurities. Then, referring to FIG. 2, in the region including outer ring rolling surface 11 </ b> A, inner ring rolling surface 12 </ b> A and ball rolling surface 13 </ b> A which are the surfaces of outer ring 11, inner ring 12 and ball 13, the nitrogen concentration is 0.05. The outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B, and the ball nitrogen-enriched layer 13B that are at least mass% are formed. Furthermore, the total value of the carbon concentration and the nitrogen concentration in the outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B, and the ball nitrogen-enriched layer 13B is 0.55 mass% or more and 1.9 mass% or less. Here, the impurities include inevitable impurities such as those derived from steel raw materials or those mixed in the manufacturing process.

  In outer ring 11, inner ring 12 and ball 13 which are machine parts in the present embodiment, outer ring rolling surface 11A, inner ring rolling surface 12A and ball made of steel having the above-mentioned appropriate component composition are formed. An outer ring nitrogen-enriched layer 11B, an inner ring nitrogen-enriched layer 12B and a ball nitrogen-enriched layer 13B having a nitrogen concentration of 0.05% by mass or more are formed in a region including the rolling surface 13A. The total value of the carbon concentration and the nitrogen concentration in the outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B, and the ball nitrogen-enriched layer 13B is in an appropriate range of 0.55% by mass to 1.9% by mass. As a result, sufficient hardness is imparted to the surface layer portion, and formation of grain boundary precipitates is suppressed. As a result, the outer ring 11, the inner ring 12 and the ball 13 which are machine parts in the present embodiment are made of steel containing 4% by mass or more of chromium, and a nitrogen-enriched layer is formed in the surface layer portion, and It is a machine part with sufficient fatigue strength and toughness. Further, the deep groove ball bearing 1 which is a rolling bearing provided with the outer ring 11, the inner ring 12 and the ball 13 is a long-life rolling bearing.

  Furthermore, the thicknesses of the outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B and the ball nitrogen-enriched layer 13B formed on the outer ring 11, the inner ring 12 and the ball 13 are preferably 0.11 mm or more. Thereby, sufficient strength is imparted to the outer ring 11, the inner ring 12 and the ball 13.

  Furthermore, the outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B, and the ball nitrogen-enriched layer 13B preferably have a hardness of 800 HV or higher. As a result, the strength of the outer ring 11, the inner ring 12 and the ball 13 can be more reliably ensured.

  Further, when the outer ring nitrogen-enriched layer 11B, the inner ring nitrogen-enriched layer 12B, and the ball nitrogen-enriched layer 13B are observed with a microscope, the number of iron nitrides having an aspect ratio of 2 or more and a length of 7.5 μm or more is 150 μm on a side. It is preferable that the number is 1 or less in the 5 fields of the square area. Thereby, the fatigue strength of the outer ring 11, the inner ring 12 and the ball 13 can be improved.

  FIG. 3 is a schematic cross-sectional view showing a configuration of a thrust needle roller bearing as a rolling bearing provided with a mechanical component that is a first modification of the present embodiment. FIG. 4 is a schematic partial cross-sectional view showing an enlarged main part of FIG. With reference to FIG. 3 and FIG. 4, the thrust needle roller bearing which is a 1st modification is demonstrated.

  Referring to FIG. 3, thrust needle roller bearing 2 has a disk-like shape, and a pair of race rings 21 as a rolling member arranged so that one main surface faces each other, and a rolling member As a plurality of needle rollers 23 and an annular retainer 24. The plurality of needle rollers 23 are in contact with the raceway rolling surface 21 </ b> A formed on the main surfaces of the pair of raceways 21 facing each other at the roller raceway 23 </ b> A that is the outer circumferential surface thereof, and are circumferentially moved by the cage 24. Are held at a predetermined pitch so as to be able to roll on an annular track. With the above configuration, the pair of race rings 21 of the thrust needle roller bearing 2 can rotate relative to each other.

  Here, referring to FIG. 4, the bearing ring 21 of the thrust needle roller bearing 2 in this modification corresponds to the outer ring 11 and the inner ring 12 of the deep groove ball bearing 1, and the needle roller 23 corresponds to the ball 13. Have the same effect. That is, the bearing ring 21 and the needle roller 23 which are mechanical parts are 0.11 mass% or more and 0.15 mass% or less carbon, 0.1 mass% or more and 0.25 mass% or less silicon, and 0.15 mass%. Manganese of not less than 0.3% by mass and not more than 0.35% by mass, nickel of not less than 3.2% by mass and not more than 3.6% by mass, chromium of not less than 4% by mass and not more than 4.25% by mass, and not less than 4% by mass of 4.5% It is composed of a steel containing the remainder of iron and impurities, containing molybdenum in an amount of not more than mass% and vanadium in an amount of not less than 1.13 mass% and not more than 1.33 mass%. And with reference to FIG. 4, in the area | region including 21 A of raceway rolling surfaces and the roller rolling surface 23A which are the surfaces of the raceway ring 21 and the needle roller 23, the raceway ring whose nitrogen concentration is 0.05 mass% or more. A nitrogen enriched layer 21B and a roller nitrogen enriched layer 23B are formed. Further, the total value of the carbon concentration and the nitrogen concentration in the raceway ring nitrogen-enriched layer 21B and the roller nitrogen-enriched layer 23B is 0.55 mass% or more and 1.9 mass% or less.

  In the raceway ring 21 and the needle roller 23 which are mechanical parts in the present modification, the raceway 21 and the needle roller 23 are made of steel having the above-mentioned appropriate composition, and in a region including the raceway rolling surface 21A and the roller rolling surface 23A formed on the surface. A raceway nitrogen-enriched layer 21B and a roller nitrogen-enriched layer 23B having a nitrogen concentration of 0.05% by mass or more are formed. Then, the total value of the carbon concentration and the nitrogen concentration in the raceway ring nitrogen-enriched layer 21B and the roller nitrogen-enriched layer 23B is set to an appropriate range of 0.55 mass% or more and 1.9 mass% or less, Sufficient hardness is imparted to the surface layer portion, and formation of grain boundary precipitates is suppressed. As a result, the bearing ring 21 and the needle roller 23 which are machine parts in this modification are made of steel containing 4% by mass or more of chromium, a nitrogen-enriched layer is formed in the surface layer portion, and fatigue strength is obtained. In addition, it is a machine part with sufficient toughness. Moreover, the thrust needle roller bearing 2 which is a rolling bearing provided with the bearing ring 21 and the needle roller 23 is a long-life rolling bearing.

  FIG. 5 is a schematic cross-sectional view showing a configuration of a constant velocity joint provided with a mechanical component which is a second modification of the present embodiment. FIG. 6 is a schematic sectional view taken along line VI-VI in FIG. FIG. 7 is a schematic sectional view showing a state where the constant velocity joint of FIG. 5 forms an angle. FIG. 8 is a schematic partial cross-sectional view showing an enlarged main part of FIG. FIG. 9 is a schematic partial cross-sectional view showing an enlarged main part of FIG. 5 corresponds to a schematic cross-sectional view along the line VV in FIG. With reference to FIGS. 5-9, the constant velocity joint which is a 2nd modification is demonstrated.

  Referring to FIGS. 5 and 6, constant velocity joint 3 includes inner race 31 connected to shaft 35, outer race 32 arranged to surround the outer peripheral side of inner race 31, and connected to shaft 36. A torque transmitting ball 33 disposed between the inner race 31 and the outer race 32 and a cage 34 for holding the ball 33 are provided. The ball 33 is disposed in contact with the inner race ball groove 31A formed on the outer peripheral surface of the inner race 31 and the outer race ball groove 32A formed on the inner peripheral surface of the outer race 32 on the ball rolling surface 33A. The cage 34 is held so as not to fall off.

  As shown in FIG. 3, the inner race ball groove 31A and the outer race ball groove 32A formed on the outer peripheral surface of the inner race 31 and the inner peripheral surface of the outer race 32 pass through the centers of the shaft 35 and the shaft 36, respectively. In a state where the axes are in a straight line, each of them is formed in a curve (arc) shape having a curvature center at points A and B that are equidistant from the joint center O on the axis to the left and right on the axis. That is, the trajectory of the center P of the ball 33 that rolls in contact with the inner race ball groove 31A and the outer race ball groove 32A is centered on the point A (inner race center A) and point B (outer race center B). Each of the inner race ball groove 31A and the outer race ball groove 32A is formed so as to have a curved line (arc). As a result, even when the constant velocity joint makes an angle (when the constant velocity joint operates so that the axes passing through the centers of the shaft 35 and the shaft 36 intersect), the ball 33 always has the shaft 35 and the shaft 36. Located on the bisector of the angle (∠AOB) formed by the axis passing through the center.

  Next, the operation of the constant velocity joint 3 will be described. 5 and 6, in constant velocity joint 3, when rotation around the shaft is transmitted to one of shafts 35 and 36, the ball fitted in inner race ball groove 31A and outer race ball groove 32A. The rotation is transmitted to the other of the shafts 35 and 36 via 33. Here, when the shafts 35 and 36 form an angle θ as shown in FIG. 7, the ball 33 includes the inner race ball groove 31 </ b> A and the outer race ball having the centers of curvature at the inner race center A and the outer race center B described above. Guided by the groove 32A, the center P is held at a position on the bisector of ∠AOB. Here, since the inner race ball groove 31A and the outer race ball groove 32A are formed so that the distance from the joint center O to the inner race center A is equal to the distance from the outer race center B, the ball 33 The distances from the center P to the inner race center A and the outer race center B are equal, and the triangle OAP and the triangle OBP are congruent. As a result, the distances L from the center P of the ball 33 to the shafts 35 and 36 are equal to each other, and when one of the shafts 35 and 36 rotates around the axis, the other also rotates at a constant speed. Thus, the constant velocity joint 3 can ensure constant velocity even when the shafts 35 and 36 form an angle. The cage 34, together with the inner race ball groove 31A and the outer race ball groove 32A, prevents the balls 33 from jumping out from the inner race ball groove 31A and the outer race ball groove 32A when the shafts 35 and 36 rotate. It performs the function of determining the joint center O of the constant velocity joint 3.

  Here, the inner race 31 and the outer race 32 of the constant velocity joint 3 in this modification correspond to the outer ring 11 and the inner ring 12 of the deep groove ball bearing 1, and the ball 33 corresponds to the ball 13 and has the same configuration. Have the same effect. That is, the inner race 31, the outer race 32, and the ball 33, which are mechanical parts, include 0.11% by mass to 0.15% by mass of carbon, 0.1% by mass to 0.25% by mass of silicon, 0.15 mass% or more and 0.35 mass% or less of manganese, 3.2 mass% or more and 3.6 mass% or less of nickel, 4 mass% or more and 4.25 mass% or less of chromium, and 4 mass% or more It is composed of steel containing 4.5% by mass or less of molybdenum and 1.13% by mass or more and 1.33% by mass or less of vanadium, the balance being iron and impurities. 8 and 9, the surface of inner race ball groove 31A formed on the surfaces of inner race 31, outer race 32 and ball 33, the surface of outer race ball groove 32A and ball rolling surface 33A are shown. In the included region, an inner race nitrogen-enriched layer 31B, an outer race nitrogen-enriched layer 32B, and a ball nitrogen-enriched layer 33B having a nitrogen concentration of 0.05% by mass or more are formed. Furthermore, the total value of the carbon concentration and the nitrogen concentration in the inner race nitrogen enriched layer 31B, the outer race nitrogen enriched layer 32B, and the ball nitrogen enriched layer 33B is 0.55 mass% or more and 1.9 mass% or less.

  In the inner race 31, outer race 32, and ball 33, which are machine parts in this modification, the surface of the inner race ball groove 31A formed on the surface, the outer race ball groove, and the outer race ball groove are made of steel having the appropriate component composition described above. Inner race nitrogen-enriched layer 31B, outer race nitrogen-enriched layer 32B and ball nitrogen-enriched layer 33B having a nitrogen concentration of 0.05% by mass or more are formed in a region including the surface of 32A and ball rolling surface 33A. Yes. The total value of the carbon concentration and the nitrogen concentration in the inner race nitrogen enriched layer 31B, the outer race nitrogen enriched layer 32B, and the ball nitrogen enriched layer 33B is in an appropriate range of 0.55% by mass or more and 1.9% by mass. % Or less, sufficient hardness is imparted to the surface layer portion, and formation of grain boundary precipitates is suppressed. As a result, the inner race 31, the outer race 32, and the ball 33, which are mechanical parts in the present modification, are made of steel containing 4% by mass or more of chromium, and a nitrogen-enriched layer is formed in the surface layer portion. In addition, the mechanical part has sufficiently secured fatigue strength and toughness. In addition, the constant velocity joint 3 that is a universal joint including the inner race 31, the outer race 32, and the ball 33 is a long-lived constant velocity universal joint.

  Next, a description will be given of a method for manufacturing machine components in one embodiment of the present invention, and machine elements such as rolling bearings and constant velocity joints equipped with the machine components. FIG. 10 is a diagram illustrating an outline of a machine component and a method of manufacturing a machine element including the machine component according to an embodiment of the present invention.

  Referring to FIG. 10, first, 0.11% by mass to 0.15% by mass carbon, 0.1% by mass to 0.25% by mass silicon, and 0.15% by mass to 0.35% by mass. Manganese of not more than mass%, nickel of not less than 3.2 mass% and not more than 3.6 mass%, chromium of not less than 4 mass% and not more than 4.25 mass%, and molybdenum of not less than 4 mass% and not more than 4.5 mass% A steel member preparation step is carried out for preparing a steel member that is made of steel consisting of the balance iron and impurities and containing 1.13% by mass to 1.33% by mass of vanadium. Is done. Specifically, for example, a steel bar or steel wire containing the above components is used as a raw material, and the steel bar or steel wire is subjected to cutting, forging, turning, or the like, so that an outer ring as a machine part is obtained. 11, steel members formed into the general shape of machine parts such as the race 21 and the inner race 31 are prepared.

  Next, the above-mentioned steel member prepared in the steel member preparation step is subjected to a heat treatment step including a heat treatment including a quenching treatment and a nitriding treatment. Details of this heat treatment step will be described later.

  Next, a finishing process in which finishing or the like is performed on the steel member that has been subjected to the heat treatment process is performed. Specifically, for example, polishing is performed on the inner ring rolling surface 12A, the raceway rolling surface 21A, the outer race ball groove 32A, and the like of the steel member that has been subjected to the heat treatment process. Thereby, the machine part in this Embodiment is completed and the manufacturing method of the machine part in this Embodiment is completed.

  Further, an assembly process is performed in which the machine elements are assembled by combining the completed machine parts. Specifically, the deep groove ball bearing 1 is assembled by combining the outer ring 11, the inner ring 12, the ball 13 and the cage 14 which are the machine parts of the present invention manufactured by the above-described process. Thereby, the machine element provided with the machine part of the present invention is manufactured.

  Next, details of the above-described heat treatment step will be described. FIG. 11 is a diagram for explaining the details of the heat treatment step included in the method of manufacturing a mechanical component in the present embodiment. In FIG. 11, the horizontal direction indicates time, and the time elapses toward the right. In FIG. 11, the vertical direction indicates the temperature, and the higher the temperature, the higher the temperature. With reference to FIG. 10 and FIG. 11, the detail of the heat processing process included in the manufacturing method of the machine component in this Embodiment is demonstrated.

Referring to FIG. 10, in the heat treatment step of the method for manufacturing a machine part in the present embodiment, first, a carburization step in which a steel member as a workpiece is carburized is performed. Specifically, referring to FIG. 11, for example, a steel member is heated to a temperature T ₁ that is a temperature equal to or higher than the A ₁ transformation point in an atmosphere of carburizing gas containing carbon monoxide and hydrogen, and for a time t ₁ . By being held, carbon enters the surface layer portion of the steel member. Thereby, a carburized layer having a high carbon concentration is formed in the region including the surface of the steel member as compared with the internal region which is a region other than the region including the surface.

Next, referring to FIG. 10, a quenching process is performed in which the steel member subjected to the carburizing process is quenched. Specifically, with reference to FIG. 11, the steel members, by being cooled from temperature T ₁ of a temperature not lower than the A ₁ transformation point M _S point below the temperature, it is quench-hardened.

Here, the point A ₁ in the case of heating the steel refers to a point that the structure of the steel corresponds to the temperature to start the transformation from ferrite to austenite. Further, the M _s point means a point corresponding to a temperature at which martensite formation starts when the austenitized steel is cooled.

Next, with reference to FIG. 10, the 1st tempering process which performs a tempering process with respect to the steel member in which the hardening process was implemented is implemented. Specifically, referring to FIG. 11, for example, after a steel member is heated to a temperature T ₂ that is a temperature lower than the A ₁ transformation point in a reduced-pressure atmosphere (in a vacuum) and held for a time t ₂ , Tempering is performed by cooling. Thereby, the residual stress due to the quenching treatment of the steel member is relaxed, and the effect of suppressing the strain due to the heat treatment can be obtained.

Next, with reference to FIG. 10, the 1st subzero process which performs a subzero process is implemented with respect to the steel member in which the 1st tempering process was implemented. Specifically, with reference to FIG. 11, the steel member, for example liquid nitrogen is sprayed is cooled to a temperature T ₃ at a temperature of less than 0 ° C., is sub-zero treatment by being held for a time t ₃ The Thereby, the residual austenite produced | generated by the hardening process of the steel member transforms into a martensite, and effects, such as stabilization of the structure of steel, are acquired.

Next, with reference to FIG. 10, the 2nd tempering process which performs a tempering process with respect to the steel member in which the 1st subzero process was implemented is implemented. Specifically, referring to FIG. 11, for example, a steel member is heated to a temperature T ₄ that is a temperature lower than the A ₁ transformation point in a vacuum, held for a time t ₄ , and then cooled. Tempered. Thereby, the residual stress due to the sub-zero treatment of the steel member is relaxed, and effects such as suppression of strain can be obtained.

Next, with reference to FIG. 10, the 2nd subzero process which performs a subzero process again is implemented with respect to the steel member in which the 2nd tempering process was implemented. Specifically, with reference to FIG. 11, the steel member, for example liquid nitrogen is sprayed is cooled to a temperature T ₅ at a temperature of less than 0 ° C., is sub-zero treatment by being held for a time t ₅ The Thereby, the residual austenite produced | generated by the hardening process of the steel member further transforms into a martensite, and the effect that the structure | tissue of steel is stabilized more is acquired.

Next, with reference to FIG. 10, the 3rd tempering process which performs a tempering process again is implemented with respect to the steel member in which the 2nd subzero process was implemented. Specifically, with reference to FIG. 11, similarly to the second tempering step, the steel members are heated to a temperature T ₆ is a temperature lower than the A ₁ transformation point in a vacuum, held for a time t ₆ And then tempering by cooling. Here, the temperatures T ₆ and t ₆ can be the same conditions as the temperatures T ₄ and t _{4 in} the second tempering step. As a result, the residual stress that can be generated by the sub-zero treatment of the steel member in the second sub-zero process is relaxed, and effects such as suppression of strain can be obtained.

Next, with reference to FIG. 10, a plasma nitriding step for performing a plasma nitriding process is performed on the steel member on which the third tempering step has been performed. Specifically, referring to FIG. 11, for example, selected from the group consisting of nitrogen (N ₂ ), hydrogen (H ₂ ), methane (CH ₄ ), and argon (Ar) so that the pressure is 50 Pa or more and 5000 Pa or less. A steel member is inserted into a plasma nitriding furnace into which at least one of the above is introduced and heated to a temperature T ₇ under conditions of a discharge voltage of 50 V to 1000 V and a discharge current of 0.001 A to 100 A. after being held for a time t _7, the steel member is treated plasma nitriding by being cooled. Thereby, nitrogen penetrate | invades into the surface layer part of a steel member, a nitrogen enriched layer is formed, and the intensity | strength of the said surface layer part improves. Here, the temperature T ₇ can be, for example, 300 ° C. or more and 550 ° C. or less, and the time t ₇ can be 1 hour or more and 80 hours or less. The heat treatment conditions such as the temperature T ₇ and the time t ₇ take into account the allowance in the finishing process performed in the finishing process, and the grain boundary precipitate layer (grain boundary precipitate is formed in the plasma nitriding process). The thickness of the layer) can be determined to be equal to or less than the thickness that can be removed in the finishing process.

When the steel constituting the steel member is AMS standard 6278 (AISI standard M50NiL), the pressure in the plasma nitriding step is 50 Pa to 1000 Pa, the discharge voltage is 50 V to 600 V, and the discharge current is 0.001 A to 300 A. The temperature T ₇ is preferably 350 ° C. or higher and 450 ° C. or lower, and the time t ₇ is preferably 1 hour or longer and 50 hours or shorter.

Next, referring to FIG. 10, a diffusion process for performing a diffusion process is performed on the steel member that has been subjected to the plasma nitriding process. Specifically, referring to FIG. 11, for example, the steel member is diffusion-treated by being heated to a temperature T ₈ in a vacuum and held for a time t ₈ . Here, the temperature T ₈ can be 300 ° C. or higher and 480 ° C. or lower, preferably 300 ° C. or higher and 430 ° C. or lower, and the time t ₈ can be 50 hours or longer and 300 hours or shorter. Thereby, nitrogen which penetrate | invaded steel can be made to reach | attain a desired area | region, suppressing that the hardness increase of the surface layer part by nitrided layer formation is canceled. And by carrying out this diffusion process, even if the depth of penetration of nitrogen in the plasma nitriding process is limited to a range in which the grain boundary precipitate layer can be removed in the finishing process, the nitrogen that has entered the steel is desired. It is possible to reach even the area. Through the above steps, the heat treatment step in this embodiment is completed.

  As described above, according to the heat treatment method for steel in the present embodiment, a surface layer portion of steel containing 4% by mass or more of chromium is nitrided to form a high hardness nitrogen-enriched layer, and grain boundaries The generation of precipitates can be suppressed.

  Further, according to the method of manufacturing a mechanical component in the above embodiment, the steel part is made of steel containing 4% by mass or more of chromium, and the surface layer portion is nitrided to form a high-hardness nitrogen-enriched layer. Machine parts (outer ring 11, track ring 21, inner race 31, etc.) in which the generation of boundary precipitates is suppressed can be manufactured. As a result, as described above, the surfaces (the outer ring rolling surface 11A, the bearing ring rolling surface 21A, and the inner race ball groove 31A) of the machine parts (the outer ring 11, the race ring 21, the inner race 31 and the like) in the present embodiment. Etc.) in which the nitrogen concentration is 0.05 mass% or more, the total value of the carbon concentration and the nitrogen concentration is 0.55 mass% or more and 1.9 mass% or less, the thickness is 0.11 mm or more, and the hardness is 800 HV or more. A nitrogen-enriched layer is formed, the nitrogen-enriched layer is cut in a cross section perpendicular to the surface, and the cross-section is randomly observed in five fields of view with a 150 μm side including the surface using an optical microscope or SEM. In this case, the number of detected grain boundary precipitates can be set to 1 or less. Here, the carbon concentration and the nitrogen concentration in the nitrogen-enriched layer are controlled, for example, by adjusting the processing time of plasma nitriding performed in the plasma nitriding step and the processing time of diffusion processing performed in the diffusing step. Can do.

  In the above embodiment, as an example of the mechanical component of the present invention, a deep groove ball bearing, a thrust needle roller bearing, and a mechanical component constituting a constant velocity joint have been described. However, the mechanical component of the present invention is not limited to this. Further, it may be a machine part that requires fatigue strength, wear resistance, etc. of the surface layer part, for example, a machine part constituting a hub, a gear, a shaft, or the like.

  Embodiment 1 of the present invention will be described below. A sample having the same configuration as the mechanical component of the present invention is actually produced by a mechanical component manufacturing method adopting the steel heat treatment method in the above embodiment, and generation of grain boundary precipitates in the surface layer portion is suppressed. An experiment was conducted to confirm that the The experimental procedure is as follows.

  First, 0.11% by mass to 0.15% by mass of carbon, 0.1% by mass to 0.25% by mass of silicon, 0.15% by mass to 0.35% by mass of manganese, 3.2 mass% or more and 3.6 mass% or less nickel, 4 mass% or more and 4.25 mass% or less chromium, 4 mass% or more and 4.5 mass% or less molybdenum, and 1.13 mass% or more A steel material of AMS standard 6278 (AISI standard M50NiL), which contains 1.33 mass% or less of vanadium and is composed of the remaining iron and impurities, is prepared and processed to have an outer diameter of 40 mm, an inner diameter of 30 mm, and a thickness. A test piece of t16 mm was produced.

Next, a heat treatment step using the steel heat treatment method described with reference to FIG. 11 in the above embodiment was performed on the test piece. Here, T ₁ , t ₁ , T ₂ , t ₂ , T ₃ , t ₃ , T ₄ , t ₄ , T ₅ , t ₅ , T ₆ and t ₆ are the test pieces after the third tempering step. The hardness was determined to be 58 to 65 HRC, T ₇ was 430 ° C., t ₇ was 10 hours, T ₈ was 430 ° C., and t ₈ was 160 hours. In the plasma nitriding step, as the treatment temperature _{T 7} during plasma nitridation is 430 ° C., the discharge voltage 200V or 450V or less controlled the discharge current at 5A below the range of 1A. Further, in the plasma nitriding process, gas is introduced into the furnace at a ratio of nitrogen (N ₂ ): hydrogen (H ₂ ) = 1: 1 so that the pressure in the furnace during plasma nitriding is 267 Pa or more and 400 Pa or less. did.

  Furthermore, in the diffusion step, the diffusion treatment is performed so that the test piece is heated in an atmosphere furnace adjusted to a nitrogen atmosphere, and the sum of the carbon concentration and the nitrogen concentration on the surface of the test piece is 1.9% by mass or less. It was implemented. The test piece in which the heat treatment method for steel of the present invention was carried out as described above was used as a sample of an example of the present invention (Example A).

On the other hand, the heat treatment process which abbreviate | omitted the diffusion process was implemented from the heat treatment method of the steel demonstrated based on FIG. 11 in the said embodiment with respect to the test piece which consists of AMS specification 6278 similarly produced. Here, T ₁ , t ₁ , T ₂ , t ₂ , T ₃ , t ₃ , T ₄ , t ₄ , T ₅ , t ₅ , T ₆ and t ₆ are the test pieces after the third tempering step. The hardness was determined to be 58 to 65 HRC, T ₇ was 480 ° C., and t ₇ was 30 hours. In the plasma nitriding step, the discharge voltage was controlled in the range of 200 V to 450 V and the discharge current in the range of 1 A to 5 A so that the processing temperature T ₇ during plasma nitriding was 480 ° C. Further, in the plasma nitriding step, nitrogen (N ₂ ): hydrogen (H ₂ ): methane (CH ₄ ) = 79: 80: 1 is set so that the pressure in the furnace during plasma nitriding is 267 Pa or more and 400 Pa or less. Gas was introduced into the furnace at a rate. The test piece subjected to the above heat treatment method was used as a sample of a comparative example of the present invention (Comparative Example A).

  And the sample of Example A and Comparative Example A produced as described above was cut in a cross section perpendicular to the surface, and the cross section was polished. Further, after the polished cross section was corroded with a corrosive liquid, five fields of view of a square having a side of 150 μm including the surface were randomly observed.

  Next, experimental results will be described. FIG. 12 is an optical micrograph of the microstructure near the surface of Example A. FIG. 13 is a diagram showing the hardness distribution in the vicinity of the surface of Example A. FIG. 14 is a graph showing the distribution of carbon and nitrogen concentrations in the vicinity of the surface of Example A. FIG. 15 is an optical micrograph of the microstructure near the surface of Comparative Example A. FIG. 16 is a diagram showing the hardness distribution near the surface of Comparative Example A. FIG. 17 is a graph showing the distribution of carbon and nitrogen concentrations in the vicinity of the surface of Comparative Example A. 12 and 15, the upper part of the photograph corresponds to the surface side of the sample. 13 and 16, the horizontal axis indicates the depth (distance) from the surface, and the vertical axis indicates the hardness (unit is Vickers hardness). 14 and 17, the horizontal axis indicates the depth (distance) from the surface, the vertical axis indicates the carbon and nitrogen concentrations, the thin line indicates the carbon concentration, and the thick line indicates the nitrogen concentration.

Referring to FIG. 12, in the surface layer portion of the sample of Example A of the present invention, grain boundary precipitates (iron nitride formed with an aspect ratio of 2 or more and a length of 7.5 μm or more) It is not observed and has a good microstructure. Referring to FIGS. 13 and 14, the region within a depth of 0.05 mm from the surface of the sample of Example A has a sufficient hardness of 950 HV or more, and a sufficient amount of nitrogen is present. Invaded. Therefore, by performing finishing such as polishing on the surface of the steel member subjected to the same heat treatment as in Example A, the nitrogen concentration is 0.05% by mass or more, and the total value of the carbon concentration and the nitrogen concentration is 0. When a nitrogen-enriched layer having a thickness of 0.11 mm or more, a thickness of 0.11 mm or more, and a hardness of 800 HV or more is formed and the nitrogen-enriched layer is observed with a microscope, grain boundary precipitates are formed. One or less machine parts can be manufactured within 5 fields of view of a square area of 150 μm per side.

On the other hand, with reference to FIG. 15, many grain boundary precipitates 90 are observed in the surface layer portion of the sample of Comparative Example A that is outside the scope of the present invention. Further, referring to FIGS. 16 and 17, the region within a depth of 0.05 mm from the surface of the sample of Comparative Example A has a sufficient hardness of 950 HV or more as in Example A. A sufficient amount of nitrogen has penetrated. Therefore, even when a finishing process such as polishing is performed on the surface of the steel member that has been subjected to the same heat treatment as in Comparative Example A, a high-hardness surface layer portion is formed, but grain boundary precipitates remain in the surface layer portion. Machine parts to be obtained. Such mechanical parts cannot be said to have sufficient fatigue strength and toughness as described above.

  As mentioned above, according to the manufacturing method of the machine parts which employ | adopted the heat processing method of the steel in the said embodiment, while it consists of steel containing 4 mass% or more of chromium, the nitrogen enriched layer is formed in the surface layer part. In addition, it was confirmed that it is possible to manufacture the mechanical component of the present invention in which fatigue strength and toughness are sufficiently ensured.

  Embodiment 2 of the present invention will be described below. An experiment was conducted to investigate an appropriate heating temperature range in the diffusion step of the steel heat treatment method described in the above embodiment. The experimental procedure is as follows.

  First, 0.11% by mass to 0.15% by mass of carbon, 0.1% by mass to 0.25% by mass of silicon, 0.15% by mass to 0.35% by mass of manganese, 3.2 mass% or more and 3.6 mass% or less nickel, 4 mass% or more and 4.25 mass% or less chromium, 4 mass% or more and 4.5 mass% or less molybdenum, and 1.13 mass% or more A steel material of AMS standard 6278 (AISI standard M50NiL), which contains 1.33 mass% or less of vanadium and is composed of the remaining iron and impurities, is prepared and processed to have an outer diameter of 40 mm, an inner diameter of 30 mm, and a thickness. A test piece of t16 mm was produced.

  Next, of the heat treatment process using the steel heat treatment method described with reference to FIG. 11 in the above embodiment, the carburization process to the third tempering process are performed on the test piece. It carried out like the case of A. And the process similar to a diffusion process was implemented by hold | maintaining the said test piece at the temperature of 430 degreeC-570 degreeC for various time, and the hardness of the carburized layer was measured. More specifically, the hardness was measured at 9 points in a region where the distance from the surface of the test piece was 0.2 mm or more and 0.4 mm or less, and the minimum hardness was calculated. Furthermore, the measurement result was analyzed based on the reaction kinetics, and the relationship between the heat treatment time (diffusion time) at each heating temperature in the diffusion step and the hardness of the carburized layer was calculated.

On the other hand, after carrying out the same test piece from the carburizing step to the third tempering step in the same manner as in Example A of Example 1, the plasma nitriding step and the diffusion step were actually carried out, Experiments were also conducted to confirm the hardness distribution. In the plasma nitriding step, the discharge voltage is controlled in the range of 200 V to 450 V, the discharge current is controlled in the range of 1 A to 5 A and maintained for 1 hour so that the processing temperature T ₇ during plasma nitridation is 480 ° C. Nitriding was performed. Furthermore, in the plasma nitriding step, gas was introduced into the furnace at a ratio of nitrogen (N ₂ ): hydrogen (H ₂ ) = 1: 1 so that the pressure in the furnace during plasma nitriding was 267 Pa or more and 400 Pa or less. . Further, a diffusion step of holding at 480 ° C. for 50 hours was performed on the test piece for which the plasma nitriding step was completed. And the hardness distribution in the surface layer part of the test piece before and behind implementing a diffusion process was measured.

  Next, the results of the experiment will be described. FIG. 18 is a diagram (Abramip plot) showing the relationship between the heat treatment time (diffusion time) at each heating temperature in the diffusion step and the hardness of the carburized layer, obtained as a result of the analysis based on the reaction kinetics. In FIG. 18, the horizontal axis indicates the heat treatment time (diffusion time), and the vertical axis indicates the hardness of the carburized layer. Moreover, FIG. 19 is a figure which shows the hardness distribution of the surface layer part of the test piece which performed the diffusion process which hold | maintained at 480 degreeC for 50 hours, and the test piece before performing a diffusion process. In FIG. 19, the horizontal axis indicates the depth (distance) from the surface, and the vertical axis indicates the hardness. Moreover, in FIG. 19, the rhombus indicates the hardness of the test piece before the diffusion step, and the square indicates the hardness of the test piece after the diffusion step held at 480 ° C. for 50 hours.

  Referring to FIG. 18, the hardness of the carburized layer of the test piece decreases in a shorter time as the diffusion temperature is higher, but when the diffusion temperature reaches 480 ° C., the hardness decreases even when the diffusion treatment is performed for 200 hours. The width becomes 50 HV or less, and the influence of the decrease in the hardness of the base material (the hardness in the region of the carburized layer where there is no influence of nitrogen intrusion due to plasma nitriding) on the hardness of the surface layer portion is reduced. Further, when the diffusion temperature is 460 ° C., even when the diffusion treatment is performed for 200 hours, the decrease in hardness is 30 HV or less, and the influence of the decrease in the hardness of the base material on the hardness of the surface layer is further reduced. Further, when the diffusion temperature is 430 ° C., even when the diffusion treatment is performed for 200 hours, the decrease in hardness becomes 10 HV or less, and the decrease in the hardness of the base material hardly affects the hardness of the surface layer portion.

  On the other hand, referring to FIG. 19, when the diffusion step of holding at 480 ° C. for 50 hours is performed, the actual decrease in the base material hardness is almost the same as the analysis result of FIG. 18, and the analysis result of FIG. Is considered to be consistent with the actual heat treatment results.

  From the above experimental results, the heating temperature (diffusion temperature) in the diffusion step is a viewpoint that allows the nitrogen that has entered the steel to reach the desired region while suppressing the influence of the decrease in the hardness of the base material on the hardness of the surface layer portion. Therefore, it is necessary to set the temperature to 480 ° C. or lower, and preferably 460 ° C. or lower. Furthermore, by setting the heating temperature to 430 ° C. or lower, the diffusion step can be carried out with almost no influence on the hardness of the surface layer portion due to the decrease in the hardness of the base material. In addition, from the viewpoint of suppressing the influence of the decrease in the hardness of the base material on the hardness of the surface layer portion, it is preferable to lower the heating temperature in the diffusion step, but the nitrogen that has entered the steel reaches the desired region. Therefore, the heating temperature is preferably set to 300 ° C. or higher in order to avoid the time required for the operation from exceeding the allowable limit in the actual production process.

  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 mechanical component of the present invention can be applied particularly advantageously to a mechanical component made of steel containing 4% by mass or more of chromium and having a nitrogen-enriched layer formed on the surface layer portion.

It is a schematic sectional drawing which shows the structure of the deep groove ball bearing provided with the machine component in one embodiment of this invention. It is a schematic fragmentary sectional view which expands and shows the principal part of FIG. It is a schematic sectional drawing which shows the structure of the thrust needle roller bearing provided with the machine component which is a 1st modification. FIG. 4 is a schematic partial cross-sectional view showing an enlarged main part of FIG. 3. It is a schematic sectional drawing which shows the structure of the constant velocity joint provided with the machine component which is a 2nd modification. It is a schematic sectional drawing in alignment with line segment VI-VI of FIG. It is a schematic sectional drawing which shows the state in which the constant velocity joint of FIG. 5 made the angle. FIG. 6 is a schematic partial cross-sectional view showing an enlarged main part of FIG. 5. It is a schematic fragmentary sectional view which expands and shows the principal part of FIG. It is a figure which shows the outline of the manufacturing method of the machine component and the machine element provided with the said machine component in one embodiment of this invention. It is a figure for demonstrating the detail of the heat treatment process included in the manufacturing method of mechanical components. 2 is an optical micrograph of the microstructure near the surface of Example A. FIG. It is a figure which shows the hardness distribution in the surface vicinity of Example A. FIG. 4 is a graph showing the distribution of carbon and nitrogen concentrations in the vicinity of the surface of Example A. 4 is an optical micrograph of a microstructure in the vicinity of the surface of Comparative Example A. 6 is a diagram showing a hardness distribution in the vicinity of the surface of Comparative Example A. FIG. 6 is a graph showing the distribution of carbon and nitrogen concentrations in the vicinity of the surface of Comparative Example A. FIG. It is a figure (Abami plot) which shows the relationship between the heat processing time in each heating temperature of a spreading | diffusion process, and the hardness of a carburized layer. It is a figure which shows the hardness distribution of the surface layer part of a test piece.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Deep groove ball bearing, 2 Thrust needle roller bearing, 3 Constant velocity joint, 11 Outer ring, 11A Outer ring rolling surface, 11B Outer ring nitrogen enriched layer, 12 Inner ring, 12A Inner ring rolling surface, 12B Inner ring nitrogen enriched layer, 13 balls , 13A ball rolling surface, 13B ball nitrogen enriched layer, 14, 24 cage, 21 race ring, 21A race ring rolling surface, 21B race ring nitrogen enriched layer, 23A roller rolling surface, 23B roller nitrogen enriched layer 31 inner race, 31A inner race ball groove, 31B inner race nitrogen enriched layer, 32 outer race, 32A outer race ball groove, 32B outer race nitrogen enriched layer, 33 balls, 33A ball rolling surface, 33B ball nitrogen enriched Layer, 34 cage, 35, 36 axes, 90 grain boundary precipitates.

Claims (4)

2. 0.11% by mass to 0.15% by mass of carbon, 0.1% by mass to 0.25% by mass of silicon, 0.15% by mass to 0.35% by mass of manganese, 2 mass% or more and 3.6 mass% or less nickel, 4 mass% or more and 4.25 mass% or less chromium, 4 mass% or more and 4.5 mass% or less molybdenum, 1.13 mass% or more 1. Containing not more than 33% by mass of vanadium, and composed of steel consisting of the remaining iron and impurities,
In the region including the surface, a nitrogen enriched layer having a nitrogen concentration of 0.05% by mass or more is formed,
Ri total value der less 1.9 wt% to 0.55 wt% of carbon concentrations and the nitrogen concentrations in the nitrogen-enriched layer,
Wherein when viewed nitrogen-enriched layer through a microscope, the aspect ratio of 2 or more, nitriding the number of the above iron length 7.5μm is Ru der 1 or less in a square area 5 within the field of view of one side 150 [mu] m, mechanical parts.   The machine part according to claim 1, wherein the nitrogen-enriched layer has a thickness of 0.11 mm or more.   The machine part according to claim 1 or 2, wherein the nitrogen-enriched layer has a hardness of 800 HV or more. The machine part according to any one of claims 1 to 3, which is used as a part constituting a bearing.