JP5397740B2 - Metal balls for rolling elements - Google Patents

Description

  The present invention relates to a rolling element metal ball used for a rolling bearing or the like.

  Conventionally, steel balls for rolling elements used for ball bearings, etc., are formed into a spherical shape by compressing a piece of steel wire cut to a certain length from both sides with female and male molds with hemispherical ball seats. Next, it is rolled between two hard casting machines by applying pressure to remove burrs, and then heat treatment for adjusting the structure is performed and fine polishing is performed (for example, non-casting) Patent Document 1).

However, the forging method described above is uneconomical because the smaller the ball, the longer the manufacturing time. Thus, for example, a method of producing a metal ball by extruding a molten metal from an orifice provided at the bottom of a crucible by applying pressure and vibration to a molten metal or alloy desired, and rapidly solidifying the molten molten metal is a uniform droplet method. It is said that it is a method suitable for manufacturing the metal balls for rolling elements described above. Patent Document 1 discloses a rolling element metal sphere made of an Fe-based alloy containing B: 1.5% to 9.0% by mass% using this method.
Tengu Steel Ball Manufacturing Co., Ltd. website (2004) (Internet <URL: http://www.aksball.co.jp/seir.htm>) JP 2007-217722 A

  However, when Fe-B alloy spheres were actually produced as a rolling element by the uniform liquid method, there was a problem that a perfect spherical shape without defects was generated depending on the production conditions and composition. Moreover, it was found that even in a solidified sphere with no defects, the internal hardness greatly varies, and the characteristics are insufficient for use as a rolling element.

Conventional metal balls for rolling elements manufactured through forging and wire drawing have a structure in which carbides are uniformly dispersed. Therefore, it is difficult for the mechanical properties of the material to be biased. The property depends not only on the composition but also on the cooling rate. In particular, at a cooling rate of about 10 ² to 10 ⁴ K / sec, large supercooling occurs in the molten metal droplets, resulting in unexpected metastable phase crystallization and abnormal growth. Even if the composition is close to a crystal, coarse shrinkage cavities may remain in the solidified spheres, and even if there is no shrinkage cavities, the hardness inside the metal spheres may vary greatly.

  An object of the present invention is to provide a metal sphere for a rolling element with high uniformity of solidified structure, no coarse shrinkage nest, and less hardness variation inside the metal sphere.

  In view of the above circumstances, the present inventor has studied Fe-B eutectic alloys in various ways. As a result, the solidification structure is composed of a composite structure of a network α-Fe dendritic crystal and an Fe-B compound. It has been found that metal spheres having a highly uniform structure can be obtained by adding Si and C to a -B eutectic alloy and solidifying under rapid cooling.

  That is, the present invention contains Si: 4.0% to 6.0%, B: 2.5% to 3.0%, and C: 0.1% to 0.6% by mass%, with the balance being Fe. An alloy composed of inevitable impurities is solidified in a spherical shape, and 80% or more of the structure is composed of a composite structure of α-Fe dendrite and Fe-B compound having a distance between secondary arms of 5 μm or less. Is a metal sphere for rolling elements having 700 HV or more.

The Fe-B compound is preferably composed of Fe ₂ B compound.
Moreover, it is preferable that the metal sphere for rolling elements of this invention contains Mo: 0.1%-2% in the mass%.
Moreover, the metal sphere for rolling elements of the present invention is particularly effective when the particle size is 0.05 mm or more and 3 mm or less.

  According to the present invention, an alloy adjusted to a specific composition is solidified under rapid cooling and adjusted to a composite structure of fine α-Fe dendrites and Fe—B compounds, so that there is no coarse shrinkage nest and a metal. A metal sphere with less variation in hardness inside the sphere can be obtained. Therefore, it is possible to maintain the necessary mechanical characteristics without going through the conventional forging process, and to provide a metal ball that can be used for easy final polishing work. This technology is effective in reducing the manufacturing cost of metal balls for rolling elements. It becomes.

  An important feature of the present invention is that the crystallization of a metastable phase due to supercooling is suppressed by adjusting the composition so as to be a composite structure of fine α-Fe dendrites and Fe—B compounds. Details will be described below.

In the present invention, by adding Si to the Fe—B eutectic alloy and adjusting the amount of B, α-Fe dendrites are preferentially grown and the rate of solidification is reduced, thereby generating shrinkage nests. In addition to reducing the degree of supercooling, the Fe ₂ B compound was crystallized from the remaining Fe—B alloy-based melt to reduce hardness variation. Since the distance between the secondary arms of the dendrites correlates with the cooling rate, the network-like α-Fe dendrites according to the present invention have a distance between secondary arms of 5 μm or less and more than 80% of the metal spheres. It is necessary to occupy. More preferably, a structure in which the network α-Fe dendrites occupy 90% or more of the metal spheres with a secondary arm distance of 3 μm or less is preferable.

  In the present invention, the composite structure of the α-Fe dendrites and the Fe—B compound is included in the eutectic structure 1 of the Fe—B compound and α-Fe as shown in the optical micrograph and schematic diagram of FIG. It is a structure in which α-Fe dendrites 2 which are primary crystals are developed. The inter-secondary arm distance 5 is a distance between the centers of the adjacent secondary arms 4 extending perpendicularly from the primary arm 3 of the α-Fe dendrite 2, and two or more of them are arranged side by side. Obtained from the average value of two or more of the center-to-center distance 5 in the next arm 4. The value is measured at three points for one sphere cross section, and the average is taken. Moreover, 80% or more of the structure | tissue shows that the ratio of the structure | tissue which the alpha-Fe resinous crystal 2 accounts with respect to a cross-sectional area when it observes in the cross section which passes along the center of a sphere is 80% or more.

In addition, since the Fe ₂ B compound is a compound that crystallizes as an eutectic component with Fe after the primary α-Fe dendrite, as a result, a metastable phase such as Fe ₃ B also crystallizes at the same time. Conceivable. This metastable phase is considered to cause variation in the hardness inside the metal sphere, and in the present invention, in order to reduce the hardness variation due to a homogeneous structure, metastable such as Fe ₃ (B, C). It is desirable to reduce the phase as much as possible.

B in the metal sphere for rolling element of the present invention is an element that determines the mechanical properties of the metal sphere as a rolling element by being dispersed in the metal sphere as a fine boride, and is the other main component Fe. And an important element that forms a eutectic with Si. In a state where a large amount of B is contained and the boride crystallizes from the molten metal as the primary crystal, the boride grows coarsely in the molten metal, so that the fatigue resistance and toughness of the solidified metal spheres are reduced. In addition, even if the addition amount is very close to eutectic on the equilibrium diagram, the liquid droplets are likely to be supercooled under the manufacturing conditions where they are rapidly cooled, so that the metastable phase crystallizes and causes hardness variations. Therefore, the amount of B added is 3.0% by mass or less.
On the other hand, when the amount of addition of B is reduced, a hypoeutectic composition is obtained, so that the eutectic melt to fill the gaps between the finely grown α-Fe dendrites is relatively reduced. Since a minute shrinkage nest is formed in the gap, the amount of B added needs to be 2.5% by mass or more. Desirably, 2.6 to 2.8 mass% is good.

  Si in the metal sphere for rolling element of the present invention is an important element for stably growing an α-Fe dendritic crystal and forming an Fe—Si—B ternary eutectic. If the amount added is small, α-Fe does not grow stably, and shrinkage cavities remain in the solidified metal spheres. As a result, it becomes difficult to form an extremely smooth surface required for rolling elements by polishing. Therefore, the amount of Si added is required to be 4.0% by mass or more. Further, if the amount of Si added is large, the amount of Si dissolved in α-Fe increases, which may reduce the fatigue resistance and toughness of the solidified metal spheres. 0 mass% or less. Desirably, 4.5-5.5 mass% is good.

C in the metal spheres for rolling elements of the present invention is not only dispersed in the metal spheres as fine carbides but also has a strengthening mechanism for the base material, and is important for satisfying the mechanical properties required for the rolling elements. It is an element. If added excessively, the metastable phase Fe ₃ (B, C) type compound is preferentially crystallized, which is considered to cause the hardness variation inside the metal sphere. Up to 6% by mass. Moreover, since the hardness of a metal sphere will fall if there is little addition amount and a fatigue resistance characteristic when using as a rolling element will fall, C addition amount needs to be 0.1 mass% or more. Desirably, 0.3-0.5 mass% is good.

It is preferable that Mo is added to the metal sphere for rolling elements of the present invention. Although there is a part that is not clear about the action due to the addition of Mo, segregation of solid solution C in the structure is prevented by strongly attracting C in the molten droplet, and the crystal of the aforementioned Fe ₃ (B, C) type compound. It is considered that the variation in hardness is remarkably reduced by suppressing the protrusion. In the metal sphere for rolling elements of the present invention, the effect can be obtained by adding at least 0.1% by mass of Mo in order to further reduce the hardness variation.
However, addition of a large amount causes abnormal growth of shrinkage cavities and compounds, so the Mo addition amount is desirably 2.0% by mass or less. More desirably, the content is 0.3 to 1.5% by mass.

  When the metal sphere for rolling elements of the present invention is used in a rolling element, it prevents deformation and damage of the metal sphere by suppressing friction and wear with a rail or holder that holds the metal sphere, and is good for a long period of time. From the purpose of ensuring a good rolling characteristic, it has a Vickers hardness of 700 HV or more. Furthermore, in order to obtain a long life, the Vickers hardness is preferably 800 HV or more. The hardness of the metal sphere refers to the hardness of the cross section of the metal sphere.

  In addition, as a result of significant improvement in processing technology for peripheral parts that hold metal balls in combination with consumer needs for miniaturization of equipment, products using metal balls having a particle diameter of 1 mm or less are increasing. In view of the situation in recent years, the diameter of the metal sphere for rolling elements of the present invention is preferably 0.05 mm or more and 3 mm or less. When the particle diameter is less than 0.05 mm, it is difficult to control the molten metal to solidify into a spherical shape, and the productivity is poor. On the other hand, when the particle size exceeds 3 mm, the cooling time from the molten metal to solidification becomes long, so that coarsening of the structure and crystallization of an undesired phase easily occur, resulting in poor productivity. Considering manufacturability, the particle size is more preferably 0.2 mm or more and 1.5 mm or less.

  In addition, the metal balls for rolling elements solidified into a spherical shape of the present invention may be subjected to a treatment for improving the structure or characteristics such as heat treatment, if necessary, in addition to the solidified structure. . That is, the manufactured solid metal sphere can be subjected to an appropriate heat treatment to maximize its characteristics. For example, the toughness can be improved by removing the solidification strain by annealing, and the strength can be improved by fine precipitation of the compound.

  The metal balls for rolling elements of the present invention can be applied by any manufacturing method that can solidify a molten alloy into a spherical shape. Preferably, the uniform droplet method as described above is used. In the molten alloy, the diffusion of all the constituents occurs at a very high speed compared to solids, so by creating droplets directly from the homogeneously mixed molten alloy and solidifying them into metal spheres The component ratio for every metal sphere is equal. Therefore, it is possible to stably produce metal spheres with little composition variation, which is difficult to obtain by a conventional production method using a thin wire produced from an ingot that has already been segregated. Further, since the droplets are directly formed and solidified from the molten metal, plastic processing is not required, and it is possible to manufacture a metal sphere for a rolling element having a high alloy.

  The atmosphere in which the molten alloy is solidified and the metal spheres are recovered is that the surface of the metal spheres is excessively oxidized in the air, thereby inhibiting spheroidization due to the surface tension of the molten metal. It is desirable to cool and solidify inside. In particular, He has a high thermal conductivity among inert gases, and can coagulate droplets in a short time and a short distance. Therefore, He is desirable in terms of the cost of the manufacturing apparatus and the mass productivity of metal spheres. In addition, in order to obtain a high cooling rate, water or quenching oil, a polymer-based coolant, or a liquid refrigerant such as a liquefied gas may be used to cool them by bringing them into contact with the droplets.

A molten metal having the composition shown in Table 1 was extruded from a sapphire nozzle provided at the bottom of the crucible by the above-described uniform liquid method, and the molten molten metal was rapidly cooled and solidified to produce metal balls. At this time, as shown in Table 1, the nozzle diameter was adjusted, and the molten metal was extruded with the molten metal temperature set at 1360 ° C. when the molten metal was extruded from the nozzle. The extruded droplets (a) to (o) were spheroidized and solidified in He gas and recovered, and the droplets (p) to (r) were quenched oil at room temperature (# 125, JIS, manufactured by Daido Chemical Industries). In K2242 type 1 No. 2), each was spheroidized, solidified and recovered.
The presence or absence of shrinkage nests of the obtained metal balls was confirmed. The shrinkage nest shown in Table 1 is when there are holes (including the outside and inside) exceeding 5% of the longest diameter of the metal sphere when the metal sphere is observed with a transmission X-ray apparatus MH-3160-D made by Pony Industries. Is indicated by ×, and when there is no hole, it is indicated by ○.
As shown in Table 1, no shrinkage cavities were observed in any of the metal spheres of the present invention examples (f) to (r) whose compositions were adjusted to satisfy predetermined amounts of Si, B, C, and Mo. On the other hand, shrinkage nests were observed in the metal balls of Comparative Examples (a) to (e) that did not satisfy the predetermined amounts of Si, B, C, and Mo. As a representative example, FIG. 1 shows a transmission X-ray image of a metal sphere. In Example (f) of the present invention, no vacant holes, i.e., shrinkage cavities, were observed. On the other hand, in the transmission X-ray image of the metal sphere of the comparative example (d), vacancies in which a part of the metal sphere appears light, that is, shrinkage nests were confirmed.

Next, particle size measurement was performed on the metal spheres of the invention example (j) in Table 1. The result is shown in FIG. As shown in FIG. 2, the metal spheres had a median value of φ612 μm and a standard deviation of 3.4, confirming monodispersion.
In addition, the appearance of the metal spheres of the invention example (j) in Table 1 was observed with a scanning electron microscope. The result is shown in FIG. As shown in FIG. 3, it was confirmed that there was no shrinkage nest in the appearance and an extremely smooth surface.

  The metal spheres of the present invention examples (f) to (r), which were produced as described above, were adjusted by fine polishing, cross-sectional observation was performed with an optical microscope, and the uniformity of the tissue was investigated. As a result, as shown in Table 1, in the metal spheres of Examples (f) to (r) of the present invention, α-Fe dendrites having a structure in which 80% or more of the metal spheres have a distance between secondary arms of 5 μm or less. It was confirmed that it was present. Moreover, as shown in FIG. 4, in the metal sphere of the invention example (j) in Table 1, it can be confirmed that the cross-sectional structure exhibits a homogeneous structure composed of a fine Fe-B compound and α-Fe. It was.

Next, X-ray diffraction was performed using a Rigaku X-ray diffractometer RINT2500PC, and the constituent materials of the metal spheres were identified. Cu-Kα rays were used as the radiation source. As shown in Table 1, in the inventive examples (f) to (j) and (m) to (r), the metal sphere is made of α-Fe, Fe ₂ B, Fe ₃ (B, C). I was able to confirm. As an example, as shown in FIG. 5, in the present invention example (g), Fe ₃ (B, C) had a slight peak intensity compared to Fe ₂ B. Further, in the inventive examples (k) and (l), the metal sphere was composed of α-Fe and Fe ₂ B, and Fe ₃ (B, C) could not be confirmed at all.

The metal spheres prepared above were adjusted by fine polishing, and the hardness at the center of the cross section was measured with a load of 1.96 N (200 gf) using a Vickers hardness meter MUK-E3 manufactured by Akashi. The hardness in Table 1 is an average value obtained by measuring 10 metal balls. As a result of measuring the hardness, it was confirmed that the metal spheres of Examples (f) to (r) of the present invention showed 800 HV or more and had sufficient strength to serve as a rolling element material.
Next, in the cross section passing through the center of the metal sphere of the invention examples (f) to (l), 4 points at a position of 0.1 mm from the end in the direction of 12 o'clock, 3 o'clock, 6 o'clock, 9 o'clock, The hardness was measured using a total of five points at one point as a measurement position for one metal ball. At this time, the variation width (maximum value−minimum value) was calculated for five metal balls. The calculation result of the average value of the variation width is shown in FIG. As shown in FIG. 6, especially in the present invention example (j), the variation is reduced as compared with the other present invention examples, and the structure inside the metal sphere is extremely homogeneous, which is sufficient for serving as a rolling element material. It has been confirmed that it has excellent characteristics.

It is a transmission X-ray photograph which shows the shrinkage nest which exists in the surface or inside of the metal sphere for rolling elements of the example of the present invention and a comparative example. It is a graph which shows the particle size distribution of the metal sphere for rolling elements of the example of this invention. It is a microscope picture which shows the surface form of the metal sphere for rolling elements of the example of this invention. It is a microscope picture which shows the cross-sectional structure of the metal sphere for rolling elements of the example of this invention. It is an X-ray diffraction spectrum which shows the constituent material of the metal sphere for rolling elements of the example of this invention. It is a graph which shows the hardness variation in the inside of the sphere of the metal ball for rolling elements of the example of the present invention.

Explanation of symbols

1 Eutectic structure 2 α-Fe dendrite 3 Primary arm 4 Secondary arm 5 Distance between secondary arms

Claims (4)

Si: 4.0% to 6.0% by mass%, B: 2.5% to 3.0%, C: 0.1% to 0.6%, and the remainder from Fe and inevitable impurities The ratio of the structure occupied by the α-Fe dendrite composed of a composite structure of α-Fe dendrite and Fe-B compound having a distance between secondary arms of 5 μm or less. Is a metal sphere for rolling elements, characterized by having a hardness of not less than 80% and a hardness of not less than 700 HV. The Fe-B compound, rolling metal ball according to claim 1, characterized in that it consists of Fe 2 _B compound.   The metal sphere for rolling elements according to claim 1, wherein Mo: 0.1% to 2% is contained in mass%.   The metal sphere for a rolling element according to any one of claims 1 to 3, wherein the metal sphere has a particle diameter of 0.05 mm or more and 3 mm or less.