JP5895493B2 - Rolling bearing manufacturing method, induction heat treatment apparatus - Google Patents

Description

  The present invention relates to an industrial rolling bearing, particularly a large rolling bearing having an outer ring with an outer diameter of 180 mm or more, such as a rolling bearing suitable for a rolling mill or a windmill in the steel industry, a manufacturing method of the rolling bearing, and a manufacturing method thereof. The present invention relates to a high-frequency heat treatment apparatus used.

Rolling bearings require life and toughness. Industrial rolling bearings, particularly large rolling bearings having an outer ring with an outer diameter of 180 mm or more, such as rolling bearings suitable for rolling mills and windmills in the steel industry, may be subjected to high loads or impact loads. In many cases, the balance between life and toughness is important.
Separation that causes the rolling life of a rolling bearing can be broadly classified into internal origin type separation and surface origin type separation. Since the internal origin type peeling starts from non-metallic inclusions contained in the steel, the life has been extended by a method of reducing the oxygen content of the steel material. Until now, the oxygen content has been reduced by improving various steelmaking processes, but it is desirable that the chemical component has a high carbon content with respect to the reduction of the oxygen content. In fact, it is known that bearing steel represented by SUJ2 exhibits higher cleanliness than S53C, which is medium carbon steel. On the other hand, surface-origin type peeling is caused by stress concentration at the edge of the indentation caused by biting of foreign matter such as metal powder contained in the lubricant, and therefore, the method of reducing the stress concentration by controlling the amount of retained austenite As a result, a longer life has been achieved.

  In general, the surface-origin type peeling is obviously short-lived compared to the internal-origin type, and therefore development of a long-life rolling bearing is often performed by suppressing surface-origin type peeling. However, in order to precipitate a large amount of retained austenite, it is necessary to form a carbon or nitrogen enriched region on the surface of the steel. For this purpose, quenching in a special gas atmosphere such as carburizing or carbonitriding is required. Necessary. Further, the precipitation of a large amount of retained austenite brings about a decrease in the surface hardness that is most necessary for rolling bearings. Therefore, it is necessary to supplement the surface hardness with a hard carbonitride, which is why expensive alloy elements such as molybdenum are added. Sometimes added. Therefore, the production cost is increased.

  On the other hand, regarding toughness, there is a trade-off between the hardness of the material. Therefore, in order to improve toughness, basically, it is necessary to secure as many regions with low hardness as possible. Based on this concept, carburized bearings have been developed in which only the surface is hardened by carburizing or carbonitriding the low / medium carbon steel. However, carburized steel is often used for relatively large bearings such as steel bearings, and the addition of relatively expensive alloy elements such as nickel, molybdenum, and chromium is the mainstream to ensure hardenability. Together with the complicated heat treatment such as carburizing, the production cost is increased.

  On the other hand, in recent years, as shown in Patent Document 1, high-frequency heat treatment for quenching and hardening only a surface portion that requires hardness has attracted attention. This method creates life and toughness by creating a hardened layer (hardened surface part) that can withstand high surface pressure and a non-hardened layer (unhardened core) with excellent toughness in one part. It is a method to make it compatible. Moreover, by having a non-hardened layer, a compressive residual stress is provided to the hardened layer on the surface, which is effective in improving the life and suppressing the occurrence of cracks.

  In addition, since the heat treatment can control the hardness by the heat treatment, high carbon steel represented by general-purpose bearing steel having excellent cleanliness can be used instead of high alloy low carbon steel. Furthermore, although retained austenite is effective against surface damage, according to the technique of induction heat treatment, the current density is high on the surface of induction hardening, so the carbon concentration of the base material is about 0.7% by mass. If it exists, the retained austenite equivalent to carburized steel can be ensured only in the extreme surface layer.

A large amount of retained austenite can cause dimensional changes, but when viewed in the depth direction, the amount of retained austenite decreases rapidly, so that a large amount of retained austenite is present in the surface side portion from the maximum shear stress depth. There is also an advantage that the total amount can be kept low. That is, by subjecting the bearing steel to induction hardening, it is possible to obtain a rolling bearing having an excellent life with respect to both internal fatigue and surface fatigue, and excellent crack resistance.
As a high-frequency heat treatment method and apparatus for a ring-shaped part that can raise the temperature uniformly in a short time and that requires a relatively simple apparatus, for example, the one shown in Patent Document 2 is known.

Further, as a high-frequency heat treatment method, for example, the one shown in Patent Document 3 is known. In Patent Document 3, in a method of subjecting a bearing part made of steel having a carbon content of 0.5% by mass or more to induction hardening, a quenching process in which induction heating is performed and before this quenching process is performed. at least once, after the carbon was dissolve into a green body by holding the high-frequency heating beyond the a ₃ transformation point at a constant temperature, induction hardening method and a step of cooling below the a ₁ transformation point disclosed Has been.

Further, as a high-frequency heat treatment method, for example, the one shown in Patent Document 4 is known. In Patent Document 4, C: 0.1 to 0.45% by weight, Cr: 0.3 to 1.5% by weight, Mn: 0.2 to 1.5% by weight, Si: 0.5 ˜2 wt%, or Al: 0.25 to 1.5 wt%, or (Si + Al): 0.5 to 3.0 wt%, inevitable impurity elements of P, S, N, O containing the balance method for manufacturing a rolling contact member using a steel consisting of Fe, the heating temperature such that the average Cr concentration of a segment of the in the steel material is 2.5 to 10% by weight a ₁ temperature - [Cementite + austenite] at 900 ° C. Cr concentration treatment process, which is a heat treatment in a two-phase region, and induction heating from a temperature below A ₁ to a quenching temperature of 900 to 1100 ° C. with a heating time of 10 seconds or less and rapid cooling Induction hardening process and tempering at a temperature of 100 to 300 ° C. And a processing step and before the induction hardening process, has a pre-heat treatment step of preheating at 300 ° C. to A ₁ temperature below the heating temperature, the A ₁ temperature below the temperature of the high-frequency quenching step 900 The manufacturing method of the rolling member whose heating rate to the quenching temperature of -1100 degreeC is 150 degreeC / sec or more is disclosed.

Further, as a high-frequency heat treatment method, for example, the one shown in Patent Document 5 is known. In Patent Document 5, after making an intermediate material having a raceway surface on any surface from a steel material containing 0.95% by mass or more of C, a surface layer portion of the raceway surface is formed on the intermediate material. In the method of manufacturing a bearing ring for rolling bearings, which is subjected to a heat treatment step for increasing the amount of retained austenite, the raceway surface until the area ratio of the spheroidized carbide structure existing on the raceway surface becomes 3% or less as the heat treatment step. the high-frequency heating, followed by a first step of cooling by a cooling rate higher than 50 ° C. / s to below the a ₁ transformation point, perform quenching in a furnace atmosphere, then, a second step of performing tempering A method for manufacturing a bearing ring for a rolling bearing is disclosed.

Japanese Patent Laid-Open No. 11-37163 JP 2005-325409 A Japanese Patent No. 4208426 Japanese Patent No. 4390526 JP 2009-270172 A

By the way, in the induction hardening, the hardness gradient in the boundary region between the hardened surface portion and the unquenched core is abrupt compared to the carburizing treatment. There is a problem that internal origin destruction (so-called case crash) occurs due to the action.
In other words, industrial rolling bearings are often subjected to higher loads than automotive rolling bearings, etc., and shear stress acts even inside the steel. It is necessary to make the hardness gradient in consideration of acting inside. However, if the depth of the hardened layer is increased more than necessary in order to obtain a hardness gradient that takes into account that the shear stress acts on the inside, sufficient toughness cannot be obtained.

Here, in the inner ring of the rolling bearing and the heat treatment method described in Patent Document 1, a hardened layer having a gradient is imparted to the raceway surface by induction hardening, and further, the hardness of the raceway surface, the inner diameter surface, and the core portion. However, the hardness gradient for suppressing internal origin fracture and cracking (hardness considering the fact that shear stress acts on the inside of rolling bearings with high loads such as industrial rolling bearings) There is no description about (slope).
In addition, in the high-frequency heat treatment method and apparatus for a ring-shaped component described in Patent Document 2, there is no description regarding a hardness gradient for suppressing internal origin fracture and cracking.

On the other hand, in the induction heat treatment method described in Patent Document 3, and high-frequency heating before the quenching step Ru hardening at least once, it exceeds the A ₃ transformation point, i.e. [cementite + austenite] biphasic Heating is performed beyond the region to the austenite single phase region. However, since carbides are dispersed in the bearing steel and the carbides maintain a fine structure due to the pinning effect, significant grain growth occurs when heating to the austenite single-phase region, and carbon is the base. This causes an inconvenience that the austenite is excessively dissolved and residual austenite remains excessively and the hardness decreases. Accordingly, when high-frequency heating is performed on hypereutectoid steel represented by general-purpose bearing steel, the holding temperature should be limited to heating in the [cementite + austenite] two-phase region.

On the other hand, when induction hardening is performed on bearing steel, it is common to perform tempering treatment (quenching and tempering) or normalizing treatment as a pretreatment. This is because the bearing steel contains a large amount of Cr, is spheroidized for the purpose of improving cutting characteristics, and the spheroidizing carbide concentrates Cr to inhibit carbon melting during high-temperature heating. , Is due to. Since the Cr concentration in the carbide greatly depends on the holding time in the temperature range of 500 ° C. to A ₁ transformation point or less, reconcentration of Cr that causes a problem does not occur if it is cooled at a speed higher than air cooling.
Therefore, when induction-hardening bearing steel, as a pretreatment, it is heated to a [cementite + austenite] two-phase region, quenched and tempered, or air-cooled to reduce the Cr concentration in the carbide, and subsequently [cementite + Austenite] It is desirable to quench by induction heating to the two-phase region.

In Patent Document 4, to lower the Cr concentration in the steel material, as a pretreatment for induction hardening, the heating temperature is subjected to heat treatment at a A ₁ temperature to 900 ° C. [cementite + austenite] two phase region . However, the heat treatment is performed by heating in the furnace, and there is a limit to improving the productivity.
Furthermore, in Patent Document 5, this raceway surface is heated at a high frequency until the area ratio of the spheroidized carbide structure existing on the raceway surface becomes 3% or less as a pretreatment before the quenching treatment. As in Patent Document 4, there is a limit to improvement in productivity.

Here, when heating in the furnace is performed in the pretreatment and quenching treatment, there is a problem that heat treatment deformation occurs in addition to a limit in improving productivity. Heat treatment deformation directly affects the grinding time is later step, the heat treatment deformation becomes more remarkable as regions count and austenitizing beyond the A ₁ transformation point increases. In addition, when ordinary bearing steel is used as the material, two phase transformations may occur, so the austenitic region should be kept to a minimum, and high-frequency heating is applied to both the pretreatment and the quenching treatment. desirable. Furthermore, it is necessary to harden the entire surface portion of the bearing, and when pretreatment is performed by heating in the furnace, there is a problem that the amount of retained austenate increases not only on the raceway surface but also on the mating surface and the end surface. In bearings, retained austenite contributes to a longer life on the raceway surface, but in other parts, it is preferable that the remaining austenite be a cause of dimensional change.

  Therefore, the present invention has been made in view of the above-mentioned problems, and its purpose is to appropriately define a hardness gradient for suppressing internal origin fracture and cracking, and to obtain sufficient toughness, It is an object of the present invention to provide a rolling bearing, a rolling bearing manufacturing method, and a high-frequency heat treatment apparatus used in the manufacturing method that do not easily cause internal origin fracture and have a long life.

In order to solve the above problems, the present invention provides an inner ring having a raceway surface, an outer ring having a raceway surface facing the raceway surface of the inner ring, and a raceway surface between the inner ring and a raceway surface of the outer ring. A rolling bearing provided with a plurality of movably arranged rolling elements, and a method of manufacturing a race ring comprising at least one of the inner ring and the outer ring, wherein the following (a) to (c) are satisfied: As a pretreatment, the bearing ring obtained from the steel material to be subjected to heat treatment is subjected to a normalization treatment in which only the raceway surface is heated to a [cementite + austenite] two-phase region and then cooled by high-frequency heating, and then, Rolling bearings characterized by induction hardening to the [cementite + austenite] two-phase region at all outer circumferences of the bearing cross section so that the raceway surface satisfies the heat treatment quality specified in (d) to (f) below. Manufacturing method I will provide a.
(a) The carbon content is 0.5% by mass or more and optionally contains alloying elements of Cr, Mn, Si, Ni, and Mo, and is first precipitated in the prior austenite grain boundaries when heated to the austenite single phase region and gradually cooled. This is a hypereutectoid steel in which ferrite does not precipitate.
(b) The DI value defined by the following formula (1) is 1.0 or more.
DI = D0 * f (Si) * f (Mn) * f (Ni) * f (Cr) * f (Mo) (1)
Here, D0 = 0.2 × (% C) +0.14
f (Si) = 1 + 0.64 × (% Si)
f (Mn) = 1 + 4.1 × (% Mn)
f (Ni) = 1 + 0.52 × (% Ni)
f (Cr) = 1 + 2.33 × (% Cr)
f (Mo) = 1 + 3.14 × (% Mo)
However,% C is mass% of carbon,% Mn is mass% of manganese,% Ni is mass% of nickel,% Cr is mass% of chromium, and% Mo is mass% of molybdenum.
(c) Steel material that has been spheroidized.
(d) Yo (mm) which is the effective hardened layer depth (Hv550) of the raceway surface is in a range satisfying the following formula (2).
0.07 Dw ≦ Yo ≦ 0.07 Dw + 5 (2)
Where Dw: rolling element diameter (mm)
(e) The surface hardness of the raceway surface is Hv650 or more, and the surface retained austenite is 12 to 40% by volume.
(f) The crystal grain size of prior austenite on the raceway surface is 30 μm or less.
Further, a first rolling bearing that solves the above problems includes an inner ring having a raceway surface, an outer ring having a raceway surface facing the raceway surface of the inner ring, and a raceway surface of the inner ring and a raceway surface of the outer ring. and a plurality of rolling elements rotatably disposed rolling, at a rolling bearing at least one of which is manufactured using a steel of the inner ring and the outer ring, it meets the following conditions 1-6.

1. The steel material has a carbon content of 0.5% by mass or more and optionally contains an alloy element of Cr, Mn, Si, Ni, and Mo. When the steel material is heated to the austenite single-phase region and gradually cooled, it is the first to the prior austenite grain boundary. This is a hypereutectoid steel in which no precipitated ferrite precipitates.
2. The steel material has a DI value defined by the following formula (1) of 1.0 or more.
DI = D0 * f (Si) * f (Mn) * f (Ni) * f (Cr) * f (Mo) (1)
Here, D0 = 0.2 × (% C) +0.14
f (Si) = 1 + 0.64 × (% Si)
f (Mn) = 1 + 4.1 × (% Mn)
f (Ni) = 1 + 0.52 × (% Ni)
f (Cr) = 1 + 2.33 × (% Cr)
f (Mo) = 1 + 3.14 × (% Mo)
However,% C is mass% of carbon,% Mn is mass% of manganese,% Ni is mass% of nickel,% Cr is mass% of chromium, and% Mo is mass% of molybdenum.

3. The steel material is heat-treated by induction hardening, and at least one raceway surface of the obtained inner ring and outer ring has a heat treatment quality defined in the following 4 to 6.
4). The effective hardened layer depth (Hv550) Yo (mm) of at least one of the inner ring and the outer ring obtained is in a range satisfying the following expression (2).
0.07Dw ≦ Yo ≦ 0.07Dw + 5 (2)
Where Dw: rolling element diameter (mm)
5. The surface hardness of the raceway surface is Hv650 or more, and the surface retained austenite is 12 to 40% by volume.
6). The crystal grain size of the prior austenite on the raceway surface is 30 μm or less.

Further, the second rolling bearing is the first rolling bearing, wherein the entire surface other than the raceway surface of at least one of the inner ring and the outer ring that can come into contact with other parts while the rolling bearing is rotating is induction-hardened. The surface hardness is Hv500 or higher, and the portion cured to Hv500 or higher is 0.1 mm or more from the surface.
Here, the steel material from which at least one of the inner ring and the outer ring is manufactured is a hypereutectoid steel in which pro-eutectoid ferrite does not precipitate at the prior austenite grain boundaries when heated to the austenite single phase region and gradually cooled. Thereby, the growth of crystal grains during heating during quenching can be prevented. If it is a hypereutectoid steel, carbides remain when the base is completely austenitized, and grain growth can be prevented.

  Moreover, the carbon content of steel materials is 0.5 mass% or more. When the alloy component of the steel material is only carbon, the eutectoid composition is 0.77% by mass, but by adding an alloy element (optionally adding an alloy element of Cr, Mn, Si, Ni, Mo), The carbon content of the eutectoid composition varies. Moreover, when a plurality of alloy elements are combined, the carbon content of the eutectoid composition can be calculated by calculation using Thermocalc. Experimentally, it was confirmed that proeutectoid ferrite does not precipitate at the prior austenite grain boundaries when cooled in the furnace after being heated to 950 ° C. As for the lower limit of the carbon content, the hardness after quenching is governed by the carbon amount regardless of the type and amount of the alloying elements. Therefore, in order to ensure the hardness HRC58 or more required for the rolling bearing, 0.5% by mass That is all.

  Further, the steel material has a DI value defined by the above formula (1) of 1.0 or more. The DI value is an index representing quench hardening characteristics. When steel with a low DI value is quenched, an incompletely quenched structure is generated, which adversely affects hardness and rolling life. The incompletely quenched structure was investigated when the DI value of the steel material was changed and quenched. According to the preliminary experiment, when the volume ratio of the incompletely hardened structure exceeds 5%, an adverse effect on the hardness starts to appear. Therefore, the case where the volume ratio of the incompletely hardened structure is 2% or less is considered good for safety. . As a result, when the DI value of the steel material was 1.0 or more, the volume ratio of the incompletely quenched structure was 2% or less.

  Further, by setting the effective hardened layer depth (Hv550) Yo of at least one of the inner ring and the outer ring to 0.07 Dw or more, a hardness gradient (industrial grade for suppressing internal origin fracture and cracking). In a rolling bearing such as a rolling bearing that is loaded with a high load, a hardness gradient that takes into account that shear stress is applied to the inside is properly defined, and the service life is increased. If Yo is less than 0.07 Dw, the effective hardened layer depth of the raceway surface becomes shallow, the stress applied to the core side exceeds the strength of the material, and internal origin fracture occurs. Moreover, the core part required for high toughness is obtained by making the effective hardened layer depth (Hv550) Yo of a raceway surface into 0.07 Dw + 5 or less. When Yo is larger than 0.07 Dw + 5, the toughness is lowered. Therefore, when the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface satisfies the above formula (2), it is possible to provide a long-life rolling bearing that is less likely to cause internal origin fracture.

  Further, since the surface hardness of the raceway surface is Hv650 or more and the surface retained austenite is 12 to 40% by volume, it is difficult to cause destruction due to the surface starting point, and the life can be extended. When the surface hardness of the raceway surface is less than Hv650, breakage due to the surface starting point is likely to occur. Moreover, even if the surface retained austenite is less than 12%, breakage due to the surface starting point is likely to occur. In addition, if the surface retained austenite is larger than 40% by volume, there is a case where burn cracking may occur, so the surface retained austenite is made 40% by volume or less.

Furthermore, since the crystal grain size of the prior austenite on the raceway surface is 30 μm or less, good life and impact resistance can be obtained. If the crystal grain size is larger than 30 μm, the crystal grains are coarsened, resulting in a decrease in material strength, resulting in a decrease in life and impact resistance.
Further, the entire surface other than the raceway surface of at least one of the inner ring and the outer ring that can be in contact with other components during rotation of the rolling bearing is hardened by heat treatment including induction hardening, and the surface hardness is Hv 500 or more, Since the portion hardened to Hv500 or more is 0.1 mm or more from the surface, it is a rolling bearing having a long life even when used in an environment in which wear due to contact with other parts or fretting of fitting parts occurs. can do.

Further, the third rolling bearing has a hardness gradient in the depth direction between the Hv650 portion and the Hv400 portion of the hardened layer of the raceway surface in the first or second rolling bearing. , 1000 or less. As a result, the hardness gradient of the hardened layer on the raceway surface is gentler or similar to the distribution of shear stress acting on the inner ring and outer ring (shear stress gradient). Can do.

  As a result, the inner and outer rings of the raceway surface are excellent (there is no incompletely quenched structure, the surface retained austenite is 12 to 40% by volume, and the crystal grain size of the prior austenite of the raceway is 30 μm or less). A rolling bearing is obtained. In addition, since both the pretreatment and the quenching treatment are high-frequency heating, the productivity is excellent, the dimensional deformation due to the heat treatment of the bearing is small, and the retained austenite on the fitting surface and the end face of the bearing can be avoided.

The high-frequency heat treatment apparatus of the present invention is an induction heat treatment apparatus used in the production process of the rolling bearing of the present invention, two or more coils partially arranged on the circumference of the inner ring and the outer ring, One or more of the coils are annular coils that heat the entire outer peripheral portion of the bearing cross section, and the other of the coils is a planar coil that heats only the raceway surface.

By using this high-frequency heat treatment apparatus, the rolling bearing manufacturing method of the present invention can be appropriately realized. In other words, pre-treatment is performed only on the raceway surface mainly using a planar coil that heats only the raceway surface, and all the outer periphery portion of the bearing cross section is quenched by an annular coil that heats all the outer circumference portion of the bearing cross section. An inner ring having a thickened hardened layer and a favorable structure of the raceway surface (there is no incompletely quenched structure, the surface retained austenite is 12 to 40% by volume, and the crystal grain size of the prior austenite on the raceway is 30 μm or less); A rolling bearing having an outer ring can be obtained.
Among various coil shapes, the annular coil that arranges the coil so as to surround the entire work section is put into practical use as a coil suitable for uniform heating, but the dependency of the coil gap on the hardened layer depth is small, and the core However, it is difficult to make fine adjustments to make the hardened layer only on the raceway surface thick. On the other hand, a pre-treatment is performed only on the raceway surface by a planar coil that heats only the raceway surface, and a quenching treatment is performed on the entire outer periphery portion of the bearing cross section using an annular coil that heats the entire outer circumference portion of the bearing cross section. The hardened layer only on the surface can be easily thickened.

Further, the fifth rolling bearing is a rolling bearing manufactured by the method of manufacturing a rolling bearing according to the present invention , wherein the retained austenite of the raceway surface is 12 to 40% by volume, and the retained austenite is 0 volume inside the bearing. %, And the retained austenite of the mating surface is 10 volume% or less lower than the retained austenite of the raceway surface.
Thereby, it can be set as a rolling bearing which secured the same level dimensional stability as a carburized bearing with a small dimensional change.

  According to the rolling bearing, the manufacturing method of the rolling bearing, and the high-frequency heat treatment apparatus used in the manufacturing method according to the present invention, the hardness gradient for suppressing internal origin fracture and cracking is appropriately defined and sufficient toughness is provided. By obtaining the above, it is possible to provide a rolling bearing having a long life that is unlikely to cause internal origin fracture, a method for manufacturing the rolling bearing, and a high-frequency heat treatment apparatus used in the manufacturing method.

It is a fragmentary sectional view of the cylindrical roller bearing which is one Embodiment of the rolling bearing which concerns on this invention. It is sectional drawing of the inner ring | wheel of the tapered roller bearing which is other embodiment of the rolling bearing which concerns on this invention. It is a schematic explanatory drawing of an example of the high frequency heat processing apparatus which concerns on this invention. It is a graph which shows the relationship between the hardness in Example 1 and Comparative Example 1 and the distance from the surface. It is a graph which shows the relationship between L10 life ratio and effective hardened layer depth in each Example and Comparative Examples 1, 3, and 4. FIG. It is a graph which shows the relationship between the L10 life ratio in each Example and Comparative Examples 2, 3, and 4 and the crystal grain size of prior austenite. It is a graph which shows the relationship between L10 life ratio and residual austenite amount in each Example and Comparative Examples 5, 6, and 7. It is a graph which shows the relationship between the crushing strength ratio and effective hardened layer depth in each Example and Comparative Examples 8, 9, and 10. It is a graph which shows the relationship between the effective hardened layer depth and diameter in each Example and Comparative Examples 11 and 12. It is a graph which shows the relationship between the heat processing method and roundness, when various heat processing methods are given about the inner ring | wheel of the bearing number: NU2326.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a partial sectional view of a cylindrical roller bearing which is an embodiment of a rolling bearing according to the present invention.
A cylindrical roller bearing 1 shown in FIG. 1 is used for industrial rolling bearings, particularly large-sized rolling bearings having an outer ring with an outer diameter of 180 mm or more, such as rolling mills and windmills in the steel industry.
The cylindrical roller bearing 1 includes an inner ring 11 having a raceway surface 11a on the outer peripheral side, an outer ring 12 having a raceway surface 12a facing the raceway surface 11a of the inner ring 11 on the inner peripheral side, and a raceway surface 11a and an outer ring 12 of the inner ring 11. A plurality of rolling elements (cylindrical rollers) 13 are provided so as to freely roll between the raceway surface 12a. A cage 14 that holds the rolling elements 13 is provided between the inner ring 11 and the outer ring 12. Lubrication between the raceway surfaces 11a and 12a and the rolling surface 13a of the rolling element 13 is performed by a lubricant (not shown) such as grease or lubricating oil. In addition, the holder | retainer 14 does not need to be provided. Moreover, the cylindrical roller bearing 1 may be provided with sealing devices (not shown), such as a seal and a shield.

  Here, both the inner ring | wheel 11 and the outer ring | wheel 12 are manufactured using steel materials. This steel material is a hypereutectoid steel having a carbon content of 0.5% by mass or more and optionally containing alloy elements of Cr, Mn, Si, Ni, and Mo. Hypereutectoid steel is a carbon steel with a composition equal to or greater than the eutectoid composition defined by the equilibrium diagram, and experimentally, when it is heated to an austenite single phase region and annealed, proeutectoid ferrite is formed at the prior austenite grain boundaries. It is a steel that does not precipitate.

  In this steel material, the carbon content is 0.5 mass% or more. When the alloy component of the steel material is only carbon, the eutectoid composition is 0.77% by mass, but by adding an alloy element (optionally adding an alloy element of Cr, Mn, Si, Ni, Mo), The carbon content of the eutectoid composition varies. The reason why the lower limit of the carbon content is set to 0.5% by mass is to secure a hardness HRC58 or more necessary for the rolling bearing. On the other hand, if the carbon content exceeds 2% by mass, the austenite single phase cannot be formed in the raw material production process, and primary crystal cementite remains, which may cause adverse effects on the life. The upper limit is preferably 2% by mass.

  In the steel material, the Cr content is preferably 4% by mass or less. Cr is an important element for ensuring hardenability, and the bearing steel usually contains 1 to 1.5% by mass. On the other hand, if the content exceeds 4% by mass, it becomes easy to generate huge carbides at the time of melting, which tends to cause deterioration of rolling fatigue characteristics, so the upper limit of Cr content may be 4% by mass. preferable.

Moreover, in this steel material, it is preferable that content of Mn shall be 2 mass% or less. Mn is an element that ensures hardenability and is usually contained in the bearing steel by 1% by mass. On the other hand, if the addition is excessive, the production of residual austenate becomes excessive and it is difficult to obtain hardness, so the upper limit of the Mn content is preferably 2% by mass.
Furthermore, in this steel material, the Si content is preferably 2% by mass or less. Si is an element that can impart temper softening resistance and is an effective element against rolling fatigue. However, it also has the drawbacks that the hardenability of ferrite is high and it is difficult to reduce the hardness during spheroidizing annealing. Therefore, it is preferable to set the upper limit of the Si content to 2% by mass in consideration of productivity.

In this steel material, the Ni content is preferably 3.5% by mass or less. Ni is an element that ensures hardenability and is an effective element that improves toughness that is insufficient for bearing steel. However, since it is an expensive element, it is preferable that the upper limit of the Ni content is 3.5% by mass in consideration of economy.
Furthermore, in this steel material, the Mo content is preferably 1.5% by mass or less. Mo is an element that ensures hardenability and is an important element that moves the eutectoid composition to the low carbon side. On the other hand, since it is an expensive element like Ni, it is preferable to set the upper limit of the Mo content to 1.5 mass 5 in consideration of economy.

Further, the steel material has a DI value defined by the following formula (1) of 1.0 or more.
DI = D0 * f (Si) * f (Mn) * f (Ni) * f (Cr) * f (Mo) (1)
Here, D0 = 0.2 × (% C) +0.14
f (Si) = 1 + 0.64 × (% Si)
f (Mn) = 1 + 4.1 × (% Mn)
f (Ni) = 1 + 0.52 × (% Ni)
f (Cr) = 1 + 2.33 × (% Cr)
f (Mo) = 1 + 3.14 × (% Mo)
However,% C is mass% of carbon,% Mn is mass% of manganese,% Ni is mass% of nickel,% Cr is mass% of chromium, and% Mo is mass% of molybdenum.

  Thus, the reason why the DI value is 1.0 or more is as follows. That is, the DI value is an index representing quench hardening characteristics. When steel with a low DI value is quenched, an incompletely quenched structure is generated, which adversely affects hardness and rolling life. For this reason, DI value was 1.0 or more. As will be described later, when the DI value of the steel material is 1.0 or more, the volume ratio of the incompletely quenched structure is 2% or less, and good results are obtained.

Moreover, the rolling element 13 is comprised by bearing steel, such as SUJ3.
Moreover, the steel materials of the inner ring 11 and the outer ring 12 are heat-treated by induction hardening, and the obtained inner ring and outer ring raceway surfaces 11a and 12a have the heat treatment quality defined in the following 1-3.
1. The effective hardened layer depth (Hv550) Yo (mm) of the raceway surfaces 11a and 12a of the inner ring 11 and the outer ring 12 is adjusted to a range satisfying the following expression (2).
0.07Dw ≦ Yo ≦ 0.07Dw + 5 (2)
Where Dw: rolling element diameter (mm)

  Thus, by setting the effective hardened layer depth (Hv550) Yo (mm) of the raceway surfaces 11a and 12a to 0.07 Dw or more, a hardness gradient (industrial rolling) for suppressing internal origin fracture and cracking. In a rolling bearing that is loaded with a high load such as a bearing, a long-life cylindrical roller bearing 1 can be obtained by appropriately defining a hardness gradient that takes into account that shear stress acts to the inside. Here, if the effective hardened layer depth (Hv550) of the raceway surfaces 11a and 12a is less than 0.07 Dw, the effective hardened layer depth of the raceway surfaces 11a and 12a becomes shallow, and the stress applied to the core side is a material. The internal origin fracture occurs. Moreover, the core part required for high toughness is obtained by making the effective hardened layer depth (Hv550) Yo (mm) of the raceway surfaces 11a and 12a 0.07 Dw + 5 or less. When the effective hardened layer depth (Hv550) Yo of the raceway surfaces 11a and 12a is larger than 0.07 Dw + 5, the toughness of the core is lowered. Thereby, it is possible to provide the long-life cylindrical roller bearing 1 that is less likely to cause internal origin fracture.

2. The surface hardness of the raceway surfaces 11a and 12a is Hv650 or more, and the surface retained austenite is 12 to 40% by volume.
Thereby, it becomes difficult to produce the destruction by the surface starting point, and it can be made long life. When the surface hardness of the raceway surfaces 11a and 12a is less than Hv650, the surface start point is liable to break. Moreover, even if the surface retained austenite is less than 12% by volume, breakage due to the surface origin is likely to occur. In addition, if the surface retained austenite is larger than 40% by volume, there is a case where burn cracking may occur, so the surface retained austenite is made 40% by volume or less.

3. The crystal grain size of prior austenite on the raceway surfaces 11a and 12a is 30 μm or less.
Thereby, a favorable lifetime and impact resistance can be obtained. If the crystal grain size is larger than 30 μm, the crystal grains are coarsened, resulting in a decrease in material strength, resulting in a decrease in life and impact resistance.
Further, the entire surface other than at least one of the raceway surfaces 11a and 12a (the inner surface 15 of the inner ring 11, the end surface 17 of the inner ring 11, the outer ring) of the inner ring 11 and the outer ring 12 with which the cylindrical roller bearing 1 can come into contact with other parts during rotation. The end face 17 of 12 and the outer diameter surface 16) of the outer ring 12 are hardened by heat treatment including induction hardening, and the surface hardness is Hv500 or more, and the part hardened to Hv500 or more is 0.1 mm or more from the surface. It is preferable that Thereby, even if it uses in the environment where the abrasion by contact with other components, the fretting of a fitting part, etc. generate | occur | produce, it can be set as the long life cylindrical roller bearing 1. FIG.

In addition, as shown in FIG. 1, other parts such as a seal during the rotation of the rolling bearing, such as the flange inner periphery 18 side of the outer ring 12 and the flange end 22 of the inner ring 21 of the tapered roller bearing as shown in FIG. In a portion where contact cannot occur, wear or fretting does not occur, and thus the surface may not be cured.
Further, the hardness of at least one of the inner ring 11 and the outer ring 12 is preferably Hv 500 or less, and in particular, if it is Hv 300 or less, better toughness can be obtained.
The crystal grain size of the prior austenite on the entire surface of the cylindrical roller bearing 1 excluding at least one of the raceway surface 11a of the inner ring 11 and the raceway surface 12a of the outer ring 12 is preferably 30 μm or less. Thereby, favorable impact resistance can be obtained.

  Furthermore, in the hardened layer on the raceway surfaces 11a and 12a, it is preferable that the hardness gradient in the depth direction between the Hv650 portion and the Hv400 portion is 1000 or less. As a result, the hardness gradient of the hardened layer on the raceway surfaces 11a and 12a is more or less than the distribution of the shear stress acting on the inner ring 11 and the outer ring 12 (shear stress gradient). Can be achieved.

Next, a manufacturing method of the rolling bearing 1 shown in FIG. 1, that is, a heat treatment method will be described.
First, the high-frequency heat treatment apparatus used in the heat treatment method will be described. As shown in FIG. 3, all the outer peripheral portions of the bearing cross section are heated on the circumferences of the inner ring 11 and the outer ring 12 (only the inner ring is shown in FIG. 3). One annular coil 30 and one planar coil 31 for heating only the raceway surface are arranged. That is, the annular coil 30 and the planar coil 31 having different roles are arranged on the inner ring 11 or the like that is one workpiece. Here, the number of turns of the annular coil 30 depends on the inner ring 11 and the outer ring 12 which are workpieces. The annular coil 30 and the planar coil 31 may not be a pair, and the number can be increased according to the workpiece size.

Next, the heat treatment method will be described. First, the inner ring 11 (outer ring 12) before heat treatment obtained from the spheroidized steel material as the workpiece is placed on the plurality of rolls 32 of the high-frequency treatment apparatus shown in FIG. While rotating the roller 32 and rotating the inner ring 11 (outer ring 12) on the roller 32, only the raceway surfaces 11a and 12a are heated at a high frequency, heated to the [cementite + austenite] two-phase region, and then cooled. Perform leveling (preprocessing). In the high frequency heating of only the raceway surfaces 11a and 12a, the planar coil 31 is mainly heated. Raceway surface 11a, the portion other than the 12a may be used as an auxiliary annular coil 30 is in a range that does not exceed the _point A. Here, when cooling, it is preferable to cool so that the volume ratio of martensite is 50% or less at a cooling rate of air cooling or less. Further, in this pretreatment, when heating to the [cementite + austenite] two-phase region, it is preferable to leave the cementite at 3% or more by volume.

  Next, while rotating the inner ring 11 (outer ring 12) on the roller 32, [cementite + austenite] for all the outer peripheral parts of the bearing cross section so that the raceway surfaces 11a and 12a satisfy the heat treatment quality specified in the above 1-3. Induction heating to the phase region and quenching. In this quenching process, the annular coil 30 and the planar coil 31 are simultaneously used to perform induction hardening on the entire outer peripheral portion of the bearing cross section of the inner ring 11 (outer ring 12) that is a workpiece.

  Thereby, at least one of the orbital surfaces 11a and 12a is in a good structure state (there is no incompletely quenched structure, the surface retained austenite is 12 to 40% by volume, and the crystal grain size of the prior austenite on the orbital surface is 30 μm or less. A rolling bearing 1 having an inner ring 11 and an outer ring 12 is obtained. In addition, since both the pre-treatment and the quenching treatment are high-frequency heating, the productivity is excellent, and dimensional deformation due to heat treatment of at least one of the inner ring 11 and the outer ring 12 is small, and the fitting surface of at least one of the inner ring 11 and the outer ring 12 is fitted. And it can avoid that the retained austenite of an end surface increases. Note that the normalizing process may be omitted if the structure of the raceway surfaces 11a and 12a is good.

In addition, although DI value and generation | occurrence | production of incomplete hardening structure are related, it is known that generation | occurrence | production of incomplete hardening structure also has coolant dependency. Induction hardening uses water (hot water) as the main coolant, so the cooling rate is generally faster than oil quenching, but rapid cooling can induce quenching cracks, especially in high-carbon steel, so a cooling rate modifier. As a mixture of polymers.
The high-frequency heat treatment apparatus used in the heat treatment method has two or more coils partially arranged on the circumference of at least one of the inner ring 11 and the outer ring 12, and at least one of the coils is an outer peripheral portion of the bearing cross section. It is an annular coil 30 that heats everything, and the other coil is a planar coil 31 that heats only the track surface.

  By using this high-frequency heat treatment apparatus, the planar coil 31 that heats only the raceway surfaces 11a and 12a is preliminarily treated only on the raceway surface, and the annular coil 30 and the planar coil that simultaneously heats the entire outer periphery of the bearing cross section are used. The entire outer periphery of the bearing cross-section is hardened by use, the hardened layer of only the raceway surfaces 11a and 12a is thickened, and the structure of the raceway surfaces 11a and 12a is good (there is no incompletely quenched structure and the surface retained austenite is 12 It is possible to obtain a rolling bearing having at least one of an inner ring and an outer ring that is ˜40% by volume and the crystal grain size of the prior austenite on the raceway surface is 30 μm or less. Among various coil shapes, the annular coil that arranges the coil so as to surround the entire work section is put into practical use as a coil suitable for uniform heating, but the dependency of the coil gap on the hardened layer depth is small, and the core However, it is difficult to make fine adjustments to make the hardened layer only on the raceway surface thick. On the other hand, by using the planar coil 31 that heats only the raceway surfaces 11a and 12a, the hardened layer of only the raceway surfaces 11a and 12a can be easily thickened. In addition, when the pretreatment is performed on the raceway surfaces 11a and 12a using the planar coil 31, good quality can be secured more easily.

  Further, in the rolling bearing manufactured by the above-described rolling bearing manufacturing method, not only the retained austenite of the raceway surfaces 11a and 12a is 12 to 40% by volume, but also retained austenite inside the bearings (inner ring 11 and outer ring 12). Is preferably 0 volume%, and the retained austenite of the mating surfaces is preferably 10 volume% lower than the retained austenite of the raceway surfaces 11a and 12a. Thereby, it can be set as the rolling bearing 1 with which the dimensional change was small and the dimensional stability of the same level as a carburized bearing was ensured.

Further, the volume ratio of the incompletely hardened structure in the region up to 1/4 of the effective hardened layer depth (Hv550) Yo (mm) of the raceway surfaces 11a and 12a of the inner ring 11 and the outer ring 12 is 2% or less. It is desirable to prevent peeling from the tissue change part.
In addition, although DI value and generation | occurrence | production of incomplete hardening structure are related, it is known that generation | occurrence | production of incomplete hardening structure also has coolant dependency. Induction hardening uses water (hot water) as the main coolant, so the cooling rate is generally faster than oil quenching, but rapid cooling can induce quenching cracks, especially in high-carbon steel, so a cooling rate modifier. As a mixture of polymers.

As mentioned above, although embodiment of this invention has been described, this invention is not limited to this, A various change and improvement can be performed.
For example, although a cylindrical roller bearing has been described as an example of a rolling bearing, the present invention is not limited thereto, and the present invention can also be applied to other types of rolling bearings. For example, radial type rolling bearings such as deep groove ball bearings, angular contact ball bearings, self-aligning ball bearings, self-aligning roller bearings, tapered roller bearings and needle roller bearings, and thrust types such as thrust ball bearings and thrust roller bearings The present invention can be applied to a rolling bearing.

  A cylindrical roller bearing (nominal number: NU2210ET, roller diameter Dw: 11 mm) having substantially the same configuration as that of the cylindrical roller bearing 1 of the present embodiment was prepared, and its life was evaluated. However, the present invention is suitable for industrial large bearings having an outer diameter of the outer ring of 180 mm or more. Therefore, in order to reproduce the average thickness of the large bearing, the inner diameter of the inner ring is 30 mm, the outer diameter is 60 mm, The wall thickness (diameter thickness) was 15 mm.

First, the manufacturing method of the cylindrical roller bearing used for the test is demonstrated.
The inner ring was formed into a predetermined shape by roughing a spheroidized annealing material made of SUJ3, then subjected to induction hardening and tempering, and further subjected to post-processing such as polishing to complete the inner ring. The conditions of induction hardening are a frequency of 5 to 100 kHz, a heating time of 5 to 200 seconds, and a workpiece rotation speed of 20 to 100 min ⁻¹ . Further, the tempering condition is to hold at 180 ° C. for 2 hours and then to cool. A cylindrical roller bearing was manufactured by assembling the inner ring thus obtained and a general outer ring made of SUJ2.

  The rotation test of the cylindrical roller bearing manufactured in this way is performed under a clean lubrication environment in which no foreign matter is mixed in the lubricant (hereinafter referred to as a clean lubrication environment) and in a lubrication environment in which foreign matter is mixed in the lubricant ( Hereinafter, the measurement was performed under each of the foreign-material-mixed lubrication environment, and the time until peeling occurred was measured. Then, seven rotation tests were performed for one type of bearing, a Weibull plot was created, the L10 life was obtained from the result of the Weibull distribution, and this was defined as the life. Tables 1 and 2 show the results (lifetime of Examples 1 to 12 and Comparative Examples 1 to 7). The life is shown as a relative value (L10 life ratio) when the life of Comparative Example 3 (submerged quenching) in each lubrication environment is 1.0.

The conditions of the rotation test are as follows.
A. Conditions of rotation test under clean lubrication environment Radial load: 50kN (P / C = 0.6)
Rotational speed: 1000 min ^-1
Lubricating oil: Lubricating oil whose ISO viscosity grade is ISO VG68. Rotation test conditions in a lubrication environment with foreign matter Radial load: 25kN (P / C = 0.3)
Rotational speed: 1000 min ^-1
Lubricating oil: Lubricating oil whose ISO viscosity grade is ISO VG68 Foreign matter: Instead of mixing foreign matter into the lubricant, 8 pseudo indentations are formed with the Rockwell hardness meter at the center of the inner raceway surface in the width direction.

  Examples 1 to 12 and Comparative Examples 1 to 2 and 5 were obtained by subjecting the inner ring to induction hardening as described above, and Comparative Examples 3 and 6 were obtained by roughing the inner ring and the spheroidized annealing material made of SUJ3. In Comparative Examples 4 and 7, the inner ring was carburized after being formed into a predetermined shape by roughing a spheroidized annealing material made of SUJ3. Heat treatment quality of Examples 1-12 and Comparative Examples 1-7 (Effective hardened layer depth (Hv550) Yo of raceway surface, surface hardness of raceway surface, retained austenite (γR%) on raceway surface, and old raceway surface Table 1 and Table 2 show the crystal grain size of austenite.

Moreover, the hardness profile of the comparative example 1 and Example 1, and the durable hardness profile which converted the shear stress which arises when the radial load of 50 kN (P / C = 0.6) was provided into durable hardness are shown in FIG. Shown in
The effective hardened layer depth (Hv550) Yo, the hardness profile, and the surface hardness shown in Table 1, Table 2, and FIG. 4 were measured with a Vickers hardness meter. Further, the measurement of retained austenite (γR%) was analyzed by an X-ray analysis method. At that time, the surface (orbital surface) of the inner ring was electropolished to remove the work-affected layer and measured. Further, the inner ring was cut and the cut surface was subjected to mirror finishing and corrosion treatment, and then the crystal grain size of the prior austenite on the raceway surface was measured by microscopic observation of the cut surface.

First, the result of the rotation test in a lubrication environment will be described with reference to Table 1 and FIGS. 4 to 6.
For Example 1, the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface is 0.9 mm as shown in Table 1 and FIG. It exceeds 07Dw (roller diameter Dw = 11mm). On the other hand, in Comparative Example 1, the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface is 0.7 mm, which is lower than the lower limit value 0.07 Dw (roller diameter Dw = 11 mm) of Yo. . For this reason, about the comparative example 1, as shown in Table 1 and FIG. 5, L10 life ratio became 0.6 and short life. The reason for this is that the effective hardened layer depth (Hv550) Yo of the raceway surface in Comparative Example 1 was shallow, so as shown in FIG. 4, the stress applied to the core side exceeded the hardness of the material, This is thought to be due to the destruction. In Examples 2 to 10, the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface is 1.5 mm or more, no internal breakage is observed, and the L10 life ratio is 2.5 or more. And long life. Thereby, in order to sufficiently suppress the destruction from the inside, it is necessary to satisfy the relationship that the hardness of the material is larger than the stress applied, and the effective hardened layer depth of the raceway surface of the present invention ( It was found that the lower limit (0.07 Dw) of Hv550) Yo (mm) is necessary.

Next, as shown in FIG. 6, when the crystal grain size of the prior austenite on the raceway surface becomes 30 μm or more, it can be seen that the lifetime is rapidly shortened. In the case of Comparative Example 2, as shown in Table 1 and FIG. 6, the crystal grain size is 33 μm, and the L10 life ratio is 0.2 and the life is rapidly shortened. It is considered that when the crystal grain size is 30 μm or more, the crystal grains become coarse, thereby reducing the material strength.
In the case of Comparative Example 3, the heat treatment is continuous quenching.
Moreover, in the case of the comparative example 4, heat processing is a carburizing process, and it is long life like Examples 1-10.

Next, the result of the rotation test in the foreign matter mixed lubrication environment will be described with reference to Table 2 and FIG.
Residual austenite is effective in extending the life in a foreign matter-contaminated lubricating environment. As shown in Table 2 and FIG. 7, in Examples 11 and 12, the amount of retained austenite exceeds 12% by volume, and the L10 life ratio is Long life was achieved at 2.5 or more. In the case of Comparative Example 5, as shown in Table 2 and FIG. 7, the amount of retained austenite was 10% by volume, the L10 life ratio was 0.8, and the life was short.

  Next, a crushing test for evaluating toughness was performed. A pre-crack was formed on the raceway surface of the inner ring to a depth of 1 m by wire cutting, and the inner ring was mounted on a crushing test apparatus with the pre-crack extending direction horizontal, and the inner ring was compressed by applying a load from above. And the load which the crack propagated from the pre-crack was made into crushing strength. Examples 13 and 14 and Comparative Example 8 were obtained by induction hardening of the inner ring as described above, and Comparative Example 9 was formed after the inner ring was formed into a predetermined shape by roughing a SUJ3 spheroidized annealing material. In the comparative example 11 formed by quenching, the inner ring was carburized after the SUJ3 made spheroidized annealed material was formed into a predetermined shape by roughing. Heat treatment quality of Examples 13 and 14 and Comparative Examples 8 and 9 (effective hardened layer depth (Hv550) Yo of raceway surface, surface hardness of raceway surface, retained austenite (γR%) on raceway surface, and old raceway surface Austenite crystal grain size) and test results are shown in Table 3 and FIG. The crushing strength is shown as a relative value (crushing strength ratio) when the crushing strength of Comparative Example 9 (submerged quenching) is 1.0.

  As a result of the crushing test, for Examples 13 and 14, the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface is 2.1 mm and 5.3 mm, respectively, as shown in Table 3 and FIG. It is below the upper limit 0.07 Dw + 5 (roller diameter Dw = 11 mm) of Yo (mm) of the present invention. On the other hand, in Comparative Example 8, the effective hardened layer depth (Hv550) Yo of the raceway surface is 6.8 mm, which exceeds the upper limit value 0.07 Dw + 5 (roller diameter Dw = 11 mm) of Yo (mm). . For this reason, about the comparative example 8, as shown in Table 3 and FIG. 8, the crushing strength ratio was 1.2 and became short life. The reason for this is that the presence of a high-toughness core is considered as a factor that affects the crushing strength, and the crushing strength is considered to be determined by the influence of the ratio of the high-toughness core. Comparative Example 8 This is because, when the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface exceeds the upper limit value 0.07 Dw + 5, it is considered that the ratio of the core portion with high toughness decreases.

  Next, a cylindrical roller bearing (nominal number: NU2326EM, roller diameter Dw: 40 mm) is prepared by changing the roller diameter Dw from a cylindrical roller bearing with a roller diameter Dw: 11 mm, and induction hardening and tempering are performed in the same manner as (NU2210ET). The inner ring was manufactured and subjected to a rotation test in a clean lubrication environment. Heat treatment quality (effective hardened layer depth (Hv550) Yo (mm) of raceway surface, surface hardness of raceway surface, retained austenite (γR) on raceway surface of produced inner rings (Examples 15 to 17 and Comparative Examples 11 and 12) %) And the crystal grain size of the prior austenite on the raceway surface) and the test results are shown in Table 4 and FIG. In addition, the lifetime which is a test result is shown by the relative value (L10 lifetime ratio) when the lifetime of the comparative example 3 which carried out quenching in a clean lubrication environment is set to 1.0.

  Referring to Table 4 and FIG. 9, even when the roller diameter Dw is changed from 11 mm to 40 mm, the effective hardened layer depth (Hv550) Yo (mm) of the raceway surface is not less than the lower limit value 0.07 Dw. It was found that the L10 life ratio was 2.0 or more.

  Next, the generation | occurrence | production state of the incomplete hardening structure when hardening with respect to the steel grade which changed DI value of steel materials was confirmed. In this study, a disk sample having a thickness of 15 mm and a diameter of 60 mm was prepared for SUJ2 to SUJ5, steel types A to C, and SK5 having the DI values and alloy components shown in Table 5, and held at 840 ° C. for 1 hour. Then, it hardened in the oil at 60 degreeC, and confirmed the generation | occurrence | production state of the incompletely hardened structure | tissue. The results are shown in Table 5. The DI value is an index representing quench hardening characteristics. When steel with a low DI value is quenched, an incompletely quenched structure is generated, which adversely affects hardness and rolling life. According to the preliminary experiment, when the volume ratio of the incompletely hardened structure exceeds 5%, an adverse effect on the hardness starts to appear. Therefore, the case where the volume ratio of the incompletely hardened structure is 2% or less is considered good for safety. .

All samples were hypereutectoid steel in calculation, and no pro-eutectoid ferrite was observed in the annealed structure from 950 ° C.
Further, SUJ2 (DI value 4.9), SUJ3 (DI value 8.2), SUJ4 (DI value 7.3), SUJ5 (DI value 13.1), DI values 2.2 to 4.9, For steel type A (3.4) and steel type B (DI value 2.2), there was no incompletely quenched structure.
Furthermore, in SK5 (DI value 0.7) and steel type C (DI value 1.1), the occurrence of an incompletely quenched structure is observed, but in steel type C, the volume ratio of the incompletely quenched structure is 1%. There is no effect on the characteristics.
Therefore, it can be seen that a steel material having a DI value of 1.0 or more is a good steel material with little incompletely quenched structure.

Next, the roundness of the inner ring when the following heat treatments A to C were performed on the inner ring having the following dimension: NU2326 was investigated.
Inner ring dimensions Inner diameter: 130mm
Outer diameter: 167mm
Width: 90mm
Heat treatment method A: Normalizing by furnace heating as pretreatment → Induction hardening as quenching treatment B: Inner and outer diameter surface normalizing by induction heating as pretreatment → Induction hardening as quenching treatment C: Only raceway surface by induction heating as pretreatment Normalizing → induction hardening as quenching treatment

In the induction hardening, heat treatment was performed for each of the above A to C so that the hardened layer depth was 4.2 mm for the raceway surface and 1.5 mm for the fitting surface. After quenching, tempering was performed immediately at 200 ° C.
The result is shown in FIG. Referring to FIG. 10, the roundness when the heat treatment method is A = the roundness when the heat treatment method is B> the roundness when the heat treatment method is C, and when the heat treatment method is C, That is, it can be seen that the effect of reducing the amount of heat treatment deformation is great when normalizing only the raceway surface by high-frequency heating as pretreatment.

  Further, for the inner ring having the above-mentioned dimensions: NU2326, when the heat treatments A to C are performed, the retained austenite amount on the raceway surface of the inner ring, the minimum retained austenite amount inside the inner ring, and the fitting of the inner ring The amount of retained austenite on the mating surface, the average amount of retained austenite on the inner ring, and the dimensional change rate when held at 200 ° C. for 300 hours were investigated. The results are shown in Table 6.

Referring to Table 6, the dimensional change rate when the heat treatment method is A≈the dimensional change rate when the heat treatment method is B> the dimensional change rate when the heat treatment method is C, and the surface fatigue resistance due to residual austenite It can be seen that in order to obtain a bearing that is difficult to change in size while maintaining the above, it is optimal that the heat treatment method is C, that is, the normalization of only the raceway surface by high-frequency heating is performed as a pretreatment.
Next, based on the results of Table 6, the heat treatment method C is used to ensure that the amount of retained austenite on the raceway surface is 12% by volume or more, and the high-frequency heat treatment conditions are set so that the core of the non-quenched portion remains. The dimensional stability of the inner ring of adjusted nominal number: NU2326 was evaluated. The evaluation results are shown in Table 7.

  In Table 7, Comparative Example 13 was carburized and quenched in heat treatment, Comparative Example 14 was heat-treated by induction hardening, the retained austenite of the raceway surface was 20% by volume, and the retained austenite of the bearing core was 0% by volume. The retained austenite on the mating surface is 14% by volume, and the retained austenite on the mating surface is 6% by volume lower than the retained austenite on the raceway surface.

  Referring to Table 7, in Examples 18 to 21, the heat treatment was induction hardening, the retained austenite of the raceway surface was 15 to 28% by volume, the retained austenite of the bearing core was 0% by volume, and the mating surface The retained austenite is 5 to 11% by volume, and the retained austenite on the mating surface is 10 to 21% by volume lower than the retained austenite on the raceway surface. In this case, it can be seen that the dimensional change rate of the inner ring is 0.04 to 0.05%, which is as low as the dimensional change rate of the inner ring in Comparative Example 13.

On the other hand, in Comparative Example 14, the retained austenite on the mating surface is 6% by volume lower than the retained austenite on the raceway surface. In this case, the dimensional change rate of the inner ring is as large as 0.07%.
Therefore, the retained austenite of the raceway surface is 12 to 40% by volume, the retained austenite is included in the bearing inside of 0% by volume, and the retained austenite of the mating surface is 10% by volume or more than the retained austenite of the raceway surface. It can be seen that the low dimensional stability of the inner ring can be secured to the same extent as the dimensional stability of the carburized and quenched inner ring.
Next, the presence or absence of pretreatment and the effect of steel type on the thermal quality treatment were verified using flat plate samples. The results are shown in Table 8.

The present invention aims at a long life in a lubricating environment containing foreign matter, and the amount of retained austenite is larger than that of the cased steel shown in Comparative Example 17, that is, the retained austenite on the raceway surface is 11 as shown in Table 8. It is necessary to set the volume% or more.
As can be seen from Table 8, in the same steel type (SUJ2-5, steel types A, B, C: the alloy components of each steel type are as shown in Table 5), when normalizing as a pretreatment, It can be seen that as the amount of retained austenite increases, the prior austenite grain size of the raceway surface tends to become finer. In particular, in SUJ2 to SUJ5 containing a large amount of Cr, it can be seen that the retained austenite on the raceway surface is difficult to ensure unless normalization is performed as a pretreatment. When normalizing as a pretreatment is not performed in SUJ2, the retained austenite of the raceway surface is as low as 7% by volume (Comparative Example 15). When normalizing as a pretreatment is not performed in SUJ4, Residual austenite is as low as 6% by volume (Comparative Example 16). Therefore, in the present invention, normalization is performed as preprocessing (Examples 22, 24, 25, 27, 29, 31, 33). However, as shown in Table 5, SUJ3, SUJ5, etc. have a Cr content of around 1% by mass, and bearing steel containing a large amount of Mn, which is an austenite stabilizing element, does not require normalization as a pretreatment. When the retained austenite on the raceway surface can be secured, normalization is not necessarily performed (Examples 23 and 26). Moreover, in steel types with a small amount of Cr, such as steel types A to C, it is not always necessary to perform normalization as a pretreatment (Examples 28, 30, and 32).

  Therefore, as can be seen from Table 8, at least one of the inner ring and the outer ring before heat treatment obtained from the spheroidized steel material is subjected to high-frequency heating only to the [cementite + austenite] two-phase region. After the heating, a pre-treatment for cooling that is cooled is performed, and then the entire circumference of the bearing cross-section is high-frequency heated to the [cementite + austenite] two-phase region so that the raceway surface satisfies the heat treatment quality of claim 1 By applying the quenching treatment, the structure state of the raceway surface is good (there is no incompletely quenched structure, the surface retained austenite is 12 to 40% by volume, and the crystal grain size of the prior austenite on the raceway surface is 30 μm or less). A rolling bearing having an inner ring and an outer ring is obtained.

  In addition, it is desirable that the cooling rate in normalizing the raceway surface in the pretreatment is a gentle cooling that is equal to or higher than air cooling. Specifically, it is desirable to cool so that the martensite volume ratio is 50% or less. The maximum heating temperature is determined by the amount of spheroidized carbide, and is a temperature at which the volume fraction of the spheroidized carbide remains at 3% or more. This is because if the carbide is further reduced, the crystal grains may be partially coarsened.

DESCRIPTION OF SYMBOLS 1 Rolling bearing 11 Inner ring 11a Raceway surface 12 Outer ring 12a Raceway surface 13 Rolling element (cylindrical roller)
13a Rolling surface 14 Cage 15 Inner ring inner diameter surface 16 Outer ring inner diameter surface 17 Inner ring, outer ring end face 18 Outer ring collar inner circumference 21 Tapered roller bearing inner ring 22 Inner ring collar end

Claims (4)

An inner ring having a raceway surface, an outer ring having a raceway surface opposite to the raceway surface of the inner ring, and a plurality of rolling elements arranged to roll between the raceway surface of the inner ring and the raceway surface of the outer ring, A rolling bearing comprising a bearing ring made of at least one of the inner ring and the outer ring,
For the raceway before heat treatment obtained from a steel material satisfying the following (a) to (c):
As pre-treatment, normalizing treatment is performed in which only the raceway surface is heated to a [cementite + austenite] two-phase region and then cooled by high-frequency heating.
Next, in order to satisfy the heat treatment quality specified in (d) to (f) below, all the outer periphery of the bearing cross section is subjected to induction hardening to the [cementite + austenite] two-phase region. A method of manufacturing a rolling bearing.
(a) The carbon content is 0.5% by mass or more and optionally contains alloying elements of Cr, Mn, Si, Ni, and Mo, and is first precipitated in the prior austenite grain boundaries when heated to the austenite single phase region and gradually cooled. This is a hypereutectoid steel in which ferrite does not precipitate.
(b) The DI value defined by the following formula (1) is 1.0 or more.
DI = D0 * f (Si) * f (Mn) * f (Ni) * f (Cr) * f (Mo) (1)
Here, D0 = 0.2 × (% C) +0.14
f (Si) = 1 + 0.64 × (% Si)
f (Mn) = 1 + 4.1 × (% Mn)
f (Ni) = 1 + 0.52 × (% Ni)
f (Cr) = 1 + 2.33 × (% Cr)
f (Mo) = 1 + 3.14 × (% Mo)
However,% C is mass% of carbon,% Mn is mass% of manganese,% Ni is mass% of nickel,% Cr is mass% of chromium, and% Mo is mass% of molybdenum.
(c) Steel material that has been spheroidized.
(d) Yo (mm) which is the effective hardened layer depth (Hv550) of the raceway surface is in a range satisfying the following formula (2).
0.07 Dw ≦ Yo ≦ 0.07 Dw + 5 (2)
Where Dw: rolling element diameter (mm)
(e) The surface hardness of the raceway surface is Hv650 or more, and the surface retained austenite is 12 to 40% by volume.
(f) The crystal grain size of prior austenite on the raceway surface is 30 μm or less. The entire surface other than the raceway surface of the bearing ring during rotation of the rolling bearing may contact with other components, that is cured by heat treatment comprising induction hardening, as well as the surface hardness than Hv 500, 0.1 mm from the surface The method for manufacturing a rolling bearing according to claim 1 , wherein the above portion is set to Hv 500 or more . The method for manufacturing a rolling bearing according to claim 1 or 2, wherein a hardness gradient in a depth direction between a portion of Hv650 and a portion of Hv400 in the hardened layer of the raceway surface is set to 1000 Hv / mm or less. . An induction heat treatment apparatus used in the method for manufacturing a rolling bearing according to any one of claims 1 to 3 ,
Two or more coils are partly arranged on the circumference of the raceway , and one or more of the coils are annular coils that heat the entire outer periphery of the bearing cross section, and the other of the coils is only a raceway surface. A high-frequency heat treatment apparatus characterized by being a planar coil for heating.

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