JP7464822B2 - Steel for bearing raceways and bearing raceways - Google Patents

Description

The present invention relates to steel materials for bearing raceways and bearing raceways.

Bearing raceways are widely used in automobiles, wind turbines, industrial machinery, etc. Bearing raceways are components that require an excellent rolling fatigue life, and the steel materials used to manufacture bearing raceways are, for example, JIS-SUJ2, SUJ3, etc.

In order to improve the rolling fatigue life of bearing raceways, it is important to suppress early spalling. One of the causes of early spalling is white structure change caused by hydrogen that has penetrated into the steel material.
Here, the white structure is a structure that has changed from martensite to fine ferrite of about several tens of nanometers. This white structure is known to cause spalling damage.

In order to suppress the white structure change, measures such as preventing the generation and infiltration of hydrogen on the lubricating oil side are taken.
However, such measures are often insufficient or difficult to apply, and so measures against early spalling due to white structure change are also required on the steel side.

For example, Patent Document 1 discloses that by setting the Cr concentration in the carbide to 6% or more and the Mn concentration to 5%, the Cr and Mn dissolved in the carbide can suppress the carbide taken by the white structure from dissolving in the matrix in the process of the white structure expanding region. Patent Document 1 also discloses that this can delay the formation of the white structure and provide a bearing raceway steel with excellent rolling fatigue properties in an environment with increased hydrogen penetration.
Moreover, Patent Document 2 discloses a method of suppressing the white structure by adding both Ni and Cu at the same time.

JP 2017-179544 A JP 2013-112834 A

However, in recent years, there has been a demand to further suppress the generation and growth of white structure, which is a cause of spalling damage, as well as to suppress spalling damage and improve rolling fatigue life.

The objective of the present invention is to provide a bearing raceway that suppresses the generation and growth of white structure, which is a cause of spalling damage, and that exhibits excellent rolling fatigue life by suppressing spalling damage, as well as a steel material for the bearing raceway that is the raw material for the bearing raceway.

The above problem can be solved by the following means:

[1]
The composition is, in mass%,
C: 0.15 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.05 to 1.50%,
Cr: 0.57 to 2.00%,
P: 0.025% or less,
S: 0.025% or less,
Al: 0.005 to 0.100%,
N: 0.003 to 0.030%,
O: 0.0015% or less,
Ni: 0.65 to 1.98%, and Cu: 0.03 to 0.20%,
Contains
The balance is Fe and impurities,
The contents of Ni and Cu satisfy formula (1);
Steel for bearing raceways.
Ni<27×Cu (1)
In formula (1), the element symbols indicate the content of each element in mass %.
[2]
In mass percent,
Mo: 2.00% or less,
V: 2.00% or less; and B: 0.0050% or less;
The steel material for bearing raceways according to [1], containing one or more selected from the group consisting of:
[3]
In mass percent,
Nb: 0.100% or less,
Ti: 0.100% or less; and REM: 0.020% or less;
The steel material for bearing raceways according to [1] or [2], containing one or more types selected from the group consisting of:
[4]
A bearing raceway using the steel material for bearing raceway according to any one of [1] to [3],
In a region 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the race or raceway disk,
C content is 0.60 to 1.20% by mass,
A bearing raceway having a structure including, by area ratio, 1 to 15% carbides having an equivalent circle diameter of 300 nm or more, 5 to 40% retained austenite, and the remainder being tempered martensite.
[5]
The bearing raceway according to [4], wherein the Vickers hardness is 720 HV or more in a region 0.10 to 0.50 mm deep from the surface of the portion that becomes the raceway surface of the raceway ring or raceway disk.

The present invention can provide a bearing raceway that suppresses the occurrence and growth of white structure, which is a cause of spalling damage, while also suppressing spalling damage and exhibiting an excellent rolling fatigue life, as well as the steel material for the bearing raceway that is the material for the bearing raceway.

An embodiment of the present invention will be described in detail below.

In this specification, the "%" designation for the content of each element in a chemical composition means "mass %."
The content of each element in a chemical composition may be expressed as the “element amount.” For example, the content of C may be expressed as the C amount.
A numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits.
When the numerical range described before and after "to" is followed by "greater than" or "less than," it means that the numerical range does not include the numerical value as the lower limit or upper limit.
The term "process" includes not only an independent process but also a process that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

<Bearing raceway steel>
The steel material for the bearing raceway according to this embodiment is
The composition is, in mass%,
C: 0.15 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.05 to 1.50%,
Cr: 0.57 to 2.00%,
P: 0.025% or less,
S: 0.025% or less,
Al: 0.005 to 0.100%,
N: 0.003 to 0.030%,
O: 0.0015% or less,
Ni: 0.65 to 1.98%, and Cu: 0.03 to 0.20%,
Contains
The balance is Fe and impurities,
The contents of Ni and Cu satisfy formula (1);
Steel for bearing raceways.
Ni<27×Cu (1)
In formula (1), the element symbols indicate the content of each element in mass %.

The inventors have found that by using a steel material for the bearing raceway containing Ni and Cu in the above-mentioned specified contents as the material for the bearing raceway, the growth of white tissue, which is a cause of spalling damage, is suppressed, along with the occurrence of white tissue in the area of the rolling part of the bearing raceway that receives the rolling load. They have also found that the spalling damage itself can be suppressed by using a steel material for the bearing raceway that has a specified chemical composition. As a result, the inventors have found that the rolling fatigue life of the bearing raceway is improved.

The details of the steel material for the bearing raceway according to this embodiment are described below.

[Chemical composition]
The chemical composition of the steel material for the bearing raceway according to this embodiment is as follows.

(Essential elements)
C: 0.15 to 0.50%
C is an element for obtaining the hardness required for a bearing raceway. If the C content is less than 0.15%, the C content after carburization will also be low, and the hardness required for a bearing raceway will not be obtained. On the other hand, if the C content is more than 0.50%, the area ratio of martensite will decrease in the structure of the surface layer of the bearing raceway due to an increase in the amount of retained austenite. Therefore, the C content is set to 0.15-0.50%. The preferred C content is 0.20-0.45%.

Si: 0.05 to 0.50%
Silicon is an element that functions as a deoxidizer. If the amount of silicon is less than 0.05%, the effect of the deoxidizer cannot be obtained. On the other hand, if the amount of silicon is more than 0.50%, _SiO2 -based inclusions are generated in the steel material, and spalling damage originating from these inclusions occurs. As a result, the rolling fatigue life of the bearing raceway decreases. Therefore, the amount of silicon is set to 0.05 to 0.50%. The preferable amount of silicon is 0.10 to 0.45%.

Mn: 0.05 to 1.50%
Mn is an element that functions as a deoxidizer and a desulfurizer. Mn is also a useful element for ensuring the hardenability of steel. If the Mn content is less than 0.05%, deoxidation is insufficient, oxides are generated, and spalling damage occurs starting from these oxides. Furthermore, the hardenability is insufficient, and the hardness required for a bearing raceway cannot be obtained. As a result, the rolling fatigue life of the bearing raceway is reduced. On the other hand, if the Mn content exceeds 1.50%, the toughness is reduced and quench cracks occur during quenching. Therefore, the Mn content is set to 0.05 to 1.50%. The preferred Mn content is 0.10 to 1.30%.

Cr: 0.57 to 2.00%
Cr is an element effective in ensuring the hardenability of steel. In addition, Cr is an element effective in spheroidizing carbides and increasing the amount of carbides because it is concentrated in cementite and stabilizes cementite in austenite. If the Cr content is less than 0.57%, the effect of improving hardenability cannot be obtained, and the hardness required for a bearing raceway cannot be obtained. On the other hand, if the Cr content exceeds 2.00%, quench cracks occur during quenching. In addition, the forgeability and machinability of the steel are reduced. Furthermore, coarse carbides are generated. Therefore, the Cr content is set to 0.57 to 2.00%. The preferred Cr content is 0.70 to 1.50%.

P: 0.025% or less P is an impurity. P segregates at the austenite grain boundaries and embrittles the prior austenite grain boundaries, which causes peeling damage due to grain boundary cracking, so it is desirable to reduce it as much as possible. For this reason, it is necessary to limit the P content to a range of 0.025% or less. The preferred upper limit of the P content is 0.015%.
In order to suppress an increase in the dephosphorization cost, the lower limit of the P content is preferably set to, for example, 0.003%.

S: 0.025% or less S forms sulfides in steel material, which cause spalling damage originating from the sulfides, and reduces the rolling fatigue properties of the bearing raceway. It is necessary to limit the S content to a range of 0.025% or less. The preferred upper limit of the S content is 0.015%.
In order to suppress an increase in the desulfurization cost, the lower limit of the S content is preferably set to, for example, 0.002%.

Al: 0.005 to 0.100%
Al is an element that deoxidizes steel. If the Al content is too low, deoxidation is insufficient, oxides are generated, and spalling damage occurs starting from these oxides. On the other hand, if the Al content is too high, coarse oxides remain in the steel, spalling damage occurs starting from these oxides, and the rolling fatigue properties of the bearing raceway deteriorate. Therefore, the Al content is set to 0.005 to 0.100%. The preferred Al content is 0.008 to 0.050%.

N: 0.003 to 0.030%
N is an element that combines with Al to form AlN, and suppresses the coarsening of crystal grains in the austenite region, which is a cause of spalling damage. In order to suppress the coarsening of crystal grains, the N content must be 0.003% or more. On the other hand, if N is contained in excess, coarse AlN and coarse BN are generated, spalling damage originating from these coarse AlN and coarse BN occurs, and the rolling fatigue properties of the bearing raceway deteriorate. Therefore, the N content is set to 0.003 to 0.030%. The preferred N content is 0.005 to 0.020%.

O: _0.0015 % or less O produces oxides such as _Al2O3 and _SiO2 , which become a propagation path for fatigue cracks, causing spalling damage and degrading the rolling fatigue properties of the bearing raceway. It is desirable to reduce the O content as much as possible. For this reason, it is necessary to limit the O content to a range of 0.0015% or less. The preferred upper limit of the O content is 0.0012%.
In order to suppress an increase in the cost of deoxidization, the lower limit of the O content is preferably set to, for example, 0.0001%.

Ni: 0.65 to 1.98%
Ni can suppress the generation and growth of white structure in the area directly below the rolling surface that receives the rolling load by suppressing hydrogen penetration into the bearing raceway. Ni also increases the toughness of steel. Furthermore, Ni is an effective element for ensuring the hardenability of steel. If the Ni content is less than 0.65%, the above effect cannot be obtained. On the other hand, if the Ni content exceeds 1.98%, the manufacturing cost becomes high. Therefore, the Ni content is set to 0.65 to 1.98%. The Ni content is preferably 0.70 to 1.70%.

Cu: 0.03 to 0.20%
Cu can suppress the generation and growth of white structure in the area directly below the rolling surface that receives the rolling load by suppressing hydrogen penetration into the steel. It also improves corrosion resistance. In order to obtain the effect of suppressing hydrogen penetration and the effect of improving corrosion resistance, if the Cu content is less than 0.03%, the above effects cannot be obtained. On the other hand, if the Cu content exceeds 0.20%, the hot ductility during rolling decreases. In addition, the toughness of the steel decreases. Therefore, the Cu content is set to 0.03 to 0.20%. The preferable Cu content is 0.05 to 0.15%.

(Formula (1): Ni < 27 × Cu)
Furthermore, in order to effectively suppress the intrusion of hydrogen into the bearing raceway and thereby suppress the generation and growth of white structure in the region immediately below the rolling surface that receives the rolling load, the relationship between the Ni and Cu contents is important.
If the Ni content is 27 times or less the Cu content, hydrogen intrusion into the steel causes the generation and growth of white structure in the region immediately below the rolling surface that receives the rolling load.
Therefore, the content of Ni and Cu has a relationship of formula (1): Ni<27×Cu (wherein in formula (1), the element symbols indicate the content of each element in mass %).

(Remainder)
The balance is Fe and impurities. The impurities refer to those that are mixed in from raw materials such as ores, scraps, or the manufacturing environment when steel materials are industrially manufactured. In addition, impurities other than P, S, and O include Mg, Co, As, Zr, Sb, W, REM (elements with atomic numbers 57 to 71), Hf, Ta, La, Ce, Ca, In, Sn, Pb, Bi, Te, and Zn, and it is desirable to restrict the contents of Mg to 0.02% or less, (Co, As): 0.1% or less, (Sb, W, REM, Hf, Ta, La, and Ce): 0.01% or less, and (Ca, In, Zr, Te, Bi, Pb, Sn, and Zn): 0.01% or less, respectively.

(Optional element)
The steel material for the bearing raceway according to this embodiment may contain the following optional elements.
(1) In mass%, one or more selected from the group consisting of Mo: 2.00% or less, V: 2.00% or less, and B: 0.0050% or less. (2) In mass%, one or more selected from the group consisting of Nb: 0.100% or less, Ti: 0.100% or less, and REM: 0.020% or less.

Mo: 2.00% or less Mo is an effective element for ensuring the hardenability of steel. Mo also promotes the formation of carbides and can increase the hardness of steel. In order to obtain the effect of improving hardness, Mo may be contained in an amount of 2.00% or less. On the other hand, if Mo is contained in an amount exceeding 2.00%, the hot workability and machinability of the steel are reduced, and the manufacturing cost is further increased. Therefore, the Mo content is preferably 2.00% or less, and more preferably 0.20 to 1.50%.

V: 2.00% or less V is an effective element for ensuring the hardenability of steel. V can also increase the hardness of steel. In order to obtain the effect of improving hardness, V may be contained in an amount of 2.00% or less. On the other hand, if V is contained in an amount exceeding 2.00%, it combines with C to generate MC carbides, and the hot workability and machinability of the steel are also reduced. Therefore, the V content is preferably 2.00% or less, and more preferably 0.10 to 1.00%.

B: 0.0050% or less Since B improves the hardenability of steel by dissolving in austenite only slightly, it may be contained in the steel to efficiently obtain martensite during carburizing and quenching. On the other hand, if B is added in excess of 0.0050%, a large amount of BN is formed and N is consumed, resulting in coarsening of austenite grains. Therefore, the B content is preferably 0.0050% or less, and more preferably 0.0007 to 0.0030%.

Nb: 0.100% or less Nb is an element that combines with N and C to form carbonitrides in steel. These carbonitrides pin the austenite grain boundaries, thereby suppressing grain growth and preventing coarsening of the structure. In order to obtain this effect of preventing coarsening of the structure, Nb may be contained in an amount of 0.100% or less. On the other hand, if Nb is contained in an amount exceeding 0.100%, not only will the workability of cutting, forging, etc. deteriorate significantly due to an increase in the material hardness, but the toughness of the steel will also deteriorate. Therefore, the Nb content is preferably 0.100% or less, and more preferably 0.005 to 0.05%.

Ti: 0.100% or less Ti is an element that combines with N and C to form carbonitrides in steel. These carbonitrides pin the austenite grain boundaries, thereby suppressing grain growth and preventing coarsening of the structure. In order to obtain this effect of preventing coarsening of the structure, Ti may be contained in an amount of 0.100% or less. On the other hand, if Ti is contained in an amount exceeding 0.100%, not only will the workability of cutting, forging, etc. be significantly deteriorated due to an increase in the material hardness, but the toughness of the steel will also be deteriorated. Therefore, the Ti content is preferably 0.100% or less, and more preferably 0.005 to 0.05%.

REM: 0.020% or less REM (rare earth elements) is a collective term for 17 elements, including 15 elements from lanthanum with atomic number 57 to lutetium with atomic number 71, scandium with atomic number 21, and yttrium with atomic number 39. When REM is contained in steel, the extension of MnS particles is suppressed during rolling and hot forging. However, when the REM content exceeds 0.020%, a large amount of sulfides containing REM is generated, deteriorating the machinability of the steel. Therefore, the REM content is preferably 0.020% or less, and more preferably 0.005% to 0.015%.
The REM amount is the total amount of 17 elements.

<Bearing raceway>
The bearing raceway in this embodiment refers to a raceway of a ball bearing, a raceway of a roller bearing, or a raceway disk of a thrust bearing. The bearing raceway in this embodiment has the chemical composition of the steel material for the bearing raceway in this embodiment, and in a region 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the raceway or the raceway disk, the C content is 0.60 to 1.20% by mass, and the structure is made up of carbides, retained austenite, and tempered martensite. The structure is made up of carbides with an equivalent circle diameter of 300 nm or more: 1 to 15%, retained austenite: 5 to 40%, and the remainder is tempered martensite, in terms of area ratio.
Furthermore, the bearing raceway according to this embodiment preferably has a Vickers hardness of 720 HV or more in a region 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the raceway ring or race disk.

The structure and hardness of the bearing raceway in this embodiment will be described in detail below. Note that the "area 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the raceway ring or raceway disk" is sometimes referred to as the "surface layer."

(C content in surface layer)
In the bearing raceway according to this embodiment, the C content in the surface layer is 0.60 to 1.20% by mass. This provides high hardness and prevents spalling damage. The preferred C content is 0.70 to 1.10% by mass.

(Surface layer tissue)
In the bearing raceway according to this embodiment, the structure in the surface layer contains, in area ratio, 1 to 15% carbides with an equivalent circle diameter of 300 nm or more, 5 to 40% retained austenite, and the remainder is tempered martensite.
In addition, examples of structures that are inevitably formed include at least one of bainite, pearlite, and ferrite, but these do not affect the mechanical properties of the bearing raceway as long as their area ratio is less than 3%.

By making the structure of the surface layer of the bearing raceway contain, by area ratio, 1-15% carbides with a circle equivalent diameter of 300 nm or more, 5-40% retained austenite, and the remainder tempered martensite, it is possible to obtain high toughness as well as wear resistance, and thus to increase the rolling fatigue life. The area ratio of tempered martensite is preferably 60% or more, and more preferably 70% or more.
Here, the structure with the balance being tempered martensite includes a structure containing, in addition to tempered martensite, bainite, pearlite and ferrite in a total area ratio of 0 to 3% or less.

The area ratio of carbides having an equivalent circle diameter of 300 nm or more is 1 to 15%.
If the area ratio of carbides is less than 1%, the rolling fatigue life is reduced. On the other hand, if the area ratio of carbides exceeds 15%, the forgeability and machinability of the steel are reduced. In addition, the concentration of alloy elements in the carbides reduces the hardenability in localized areas, increasing the risk of the formation of structures such as pearlite and ferrite. Therefore, the area ratio of carbides in the surface layer is set to 15% or less.
The area ratio of carbides is preferably 12% or less.
Here, examples of carbides include _cementite , _M7C3 , _M23C6 , MC, _{and M2C} .

The area ratio of the retained austenite is 5 to 40%.
If the area ratio of the retained austenite is less than 5%, the rolling fatigue life is reduced. On the other hand, if the area ratio of the retained austenite is more than 40%, the presence of a large amount of retained austenite, which is less hard than martensite, causes spalling damage originating from the retained austenite, and the rolling fatigue properties of the bearing raceway are reduced. Therefore, the area ratio of the retained austenite in the surface layer is set to 40% or less.
The area fraction of retained austenite is preferably 30% or less.

The metal structure of the surface layer of the bearing raceway is observed and measured as follows. Specifically, it is as follows:

-carbide-
A sample is taken from a cross section of the raceway or race disk cut perpendicular to the raceway surface, with the observation surface being a surface located in a region 0.10 to 0.50 mm deep from the surface that will become the raceway surface.
The observation surface of the sample is mirror-polished, etched for 10 seconds using a picral solution (a solution in which 4 g of picric acid is dissolved in 100 ml of ethanol), and then washed with water.
Thereafter, five fields of view are observed using a scanning electron microscope at a magnification of 5000 times, starting from 0.10 mm in the depth direction from the surface of the bearing raceway on the observation surface to 0.50 mm at 0.10 mm intervals.
Using a structural photograph taken by a scanning electron microscope, the area ratio of carbides with a circle equivalent diameter of 300 nm or more is measured using a normal image analyzer. Specifically, the area ratio of the carbides relative to the area of the observation field is measured. For example, ImageJ (manufactured by the National Institutes of Health) is used as the image analysis software. The area ratio of the carbides is the average value of five fields of view.
Here, the carbides are identified by electron backscattering analysis and energy dispersive X-ray analysis.

-Retained austenite-
A sample is taken from a cross section of the raceway or race disk cut perpendicular to the raceway surface, with the observation surface being a surface located in a region 0.10 to 0.50 mm deep from the surface that will become the raceway surface.
The observation surface of the sample is subjected to normal mirror polishing and then further polished by 0.02 mm by electrolytic polishing to obtain a distortion-free observation surface.
Thereafter, the amount of retained austenite is measured at five points on the observation surface of the sample, from the surface to a depth of 0.50 mm, at intervals of 0.10 mm in the depth direction of the surface.
The amount of retained austenite is measured by X-ray diffraction. The X-ray diffraction conditions are preferably as follows: A Cr tube is used as the X-ray tube, and the amount of retained austenite is obtained from the diffraction peak intensity of γ-Fe (220). The amount of retained austenite is taken as a five-point average value.
In the X-ray diffraction method, the amount of retained austenite is measured as a volume fraction, and this volume fraction is treated as being equal to the area fraction.

- Tempered martensite -
A sample is taken from a cross section of the raceway or race disk cut perpendicular to the raceway surface, with the observation surface being a surface located in a region 0.10 to 0.50 mm deep from the surface that will become the raceway surface.
The observation surface of the sample is mirror-polished, etched for 10 seconds using a nital solution (a solution prepared by dissolving 3 g of nitric acid in 100 ml of ethanol, with a surfactant added as required), and then washed with water.
Thereafter, five fields of view were observed at 0.10 mm intervals in the depth direction of the surface from the surface of the bearing raceway on the observation surface of the sample to a depth of 0.50 mm using a scanning electron microscope at a magnification of 5000 times.
Bainite, pearlite, and ferrite are marked on a structural photograph taken by a scanning electron microscope, and the area ratio is measured using a conventional image analyzer. Specifically, the total area ratio of bainite, pearlite, and ferrite to the area of the observation field is measured. For example, ImageJ (manufactured by National Institutes of Health) is used as image analysis software. The area ratio of tempered martensite is the average value of five fields of view.
The area ratio of tempered martensite is defined as 100% - area ratio (%) of retained austenite - area ratio (%) of carbide - (total area ratio (%) of bainite, pearlite and ferrite).

(Vickers hardness at the surface layer)
In the bearing raceway according to this embodiment, the surface layer has a Vickers hardness of 720 HV or more.
If the Vickers hardness HV of the surface layer is less than 720 HV, excellent wear resistance cannot be obtained, and furthermore, the rolling fatigue life is also reduced. Therefore, the Vickers hardness HV of the surface layer is set to 720 HV or more.
However, if the Vickers hardness (HV) of the surface layer exceeds 1000HV, microcracks will form.

When such a crack occurs, the crack will be more sensitive to propagation, and the surface-initiated peeling property may be decreased. Therefore, the Vickers hardness (HV) of the surface layer is preferably 1000 HV or less.

A preferable lower limit of the Vickers hardness HV of the surface layer is 735 HV.
On the other hand, the preferred upper limit of the Vickers hardness HV in the surface layer portion is 950 HV.

The Vickers hardness of the surface layer is measured by a method in accordance with JIS Z 2244 (2009). Specifically, the method is as follows.
A sample is taken from the raceway or raceway disk, with the cross section perpendicular to the raceway surface as the measurement surface.
On the measurement surface of this sample, in a region 0.10 to 0.50 mm deep from the surface of the bearing raceway, the Vickers hardness is measured at five positions spaced at a depth of 0.10 mm with a measuring force (load) of 2.94 N at each of five points (total of 25 points).
The calculated average value of the Vickers hardness at the total of 25 points is determined as the Vickers hardness of the surface layer.

As described above, the bearing raceway according to this embodiment optimizes the hardness and structure of the surface layer as well as the chemical composition, thereby suppressing the occurrence and growth of white structure that causes spalling damage, while also suppressing spalling damage, thereby achieving an excellent rolling fatigue life.

<Manufacturing method of bearing raceway>
Hereinafter, an example of a method for manufacturing a bearing raceway according to this embodiment using the steel material for the bearing raceway according to this embodiment will be described in detail.

The manufacturing method of the bearing raceway according to this embodiment can include, for example, a process of manufacturing the steel material for the bearing raceway (the steel material for the bearing raceway according to this embodiment), a process of forming the bearing raceway into a shape, a process of carburizing, quenching and tempering, and a process of performing a finishing process.

(Process for manufacturing steel for bearing raceways)
In the process of manufacturing the steel material for the bearing raceway, first, molten steel having the chemical composition of the steel material for the bearing raceway according to the present embodiment is cast into a slab by a continuous casting method. Alternatively, the molten steel may be cast into an ingot (steel block) by an ingot casting method.
The slab or ingot may then be hot worked to produce a billet, for example by blooming.
Next, the steel slab or cast piece is hot worked, and then a steel material for a bearing raceway, such as a steel bar or wire rod, is manufactured. The hot working may be hot rolling or hot forging (hot forging extension, etc.). If necessary, the steel slab or cast piece before hot rolling may be subjected to a spheroidizing annealing treatment or a soaking diffusion treatment. In addition, if necessary, the manufactured steel material for a bearing raceway may be subjected to a normalizing treatment. The steel material for a bearing raceway can be obtained by the above steps.

(Process for forming bearing raceway shape)
In the process of forming the bearing raceway shape, the steel material for the bearing raceway is processed into a predetermined bearing raceway shape to produce an intermediate product (steel material formed into a bearing raceway shape). Examples of the processing method include hot forging, cold forging, machining (cutting, etc.), etc. However, the processing method is not limited to these, and any processing method can be used as long as it can produce an intermediate product having the predetermined bearing raceway shape.

(Carburizing, quenching and tempering process)
In the carburizing, quenching and tempering process, the intermediate product is subjected to carburizing, quenching and tempering.
Carburizing is performed at a temperature range of 850 to 1100°C. If the carburizing temperature is less than 850°C, a long heat treatment is required to sufficiently diffuse carbon into the steel, which increases the cost. On the other hand, if the carburizing temperature exceeds 1100°C, significant coarsening and mixing of grains occurs.
Therefore, carburizing is carried out in the temperature range of 850 to 1100° C. In order to achieve lower costs, suppression of coarsening and suppression of mixed grains at an even higher level, carburizing is preferably carried out in the temperature range of 900 to 1050° C. Gas carburizing or vacuum carburizing can be used as the carburizing method.
Quenching may be performed after holding at a specified temperature for a certain period of time. The purpose of holding for a certain period of time is to reduce cracking and distortion during quenching. The holding temperature is 800°C or higher for 10 minutes or more. On the other hand, even if the temperature is held at over 900°C for more than 60 minutes, the effects of preventing cracking and reducing distortion during quenching are saturated.

In the tempering process, the tempering temperature is preferably 150°C or higher in order to ensure toughness and adjust hardness. If the tempering temperature is less than 150°C, the toughness of the bearing raceway may not be ensured. On the other hand, if the tempering temperature exceeds 200°C, the hardness of the bearing raceway product may decrease, and the wear resistance may decrease. Therefore, the tempering temperature is preferably 150°C to 200°C.

(Finishing process)
In the process of finishing, the intermediate product that has been carburized, quenched and tempered is subjected to finishing processes such as grinding and polishing.

The above steps are used to manufacture the bearing raceway according to this embodiment.

Next, an embodiment of the present invention will be described, but the conditions shown below are merely examples adopted to confirm the feasibility and effects of the present invention, and the conditions of the present invention are not limited to these examples. In implementing the present invention, various conditions can be adopted as long as they do not deviate from the gist of the present invention and the object is achieved.

<Molding of various test pieces>
(Preparation of steel bars)
Steels (Steel Nos. A to AV) having the chemical compositions shown in Table 1 were melted and hot forged to prepare φ40 mm steel bars. In Table 1, underlined values indicate that the values are outside the range of the present invention. The manufactured steel bars were subjected to spheroidizing annealing treatment. Specifically, the steel bars were held at 780°C for 5 hours. Thereafter, they were slowly cooled to 650°C at 10°C/h, and then air-cooled to room temperature. Through these steps, steel bars were obtained as steel materials for bearing raceways.

(Molding of small roller test specimens)
A small roller test piece of φ26 mm×130 mm was formed by machining from a steel bar of φ40 mm.

(Forming of round bar test pieces)
A round bar test piece of φ26 mm×50 mm for measuring hardness, C concentration, and structure was formed by machining from a steel bar of φ40 mm.

(Molding of large roller test specimens)
A large disc-shaped roller test piece having a diameter of 150 mm was molded using high carbon chromium steel equivalent to SUJ2 specified in JIS G4805 (2008).

<Carburizing, quenching and tempering>
Next, the small roller test pieces and the round bar test pieces were subjected to carburizing, quenching, and tempering.
Production Nos. 1 to 18 and 21 to 42 were subjected to gas carburizing, quenching, and tempering. The gas carburizing conditions were 930°C x 160 minutes.
Production Nos. 19 and 20 were subjected to vacuum carburizing, quenching, and tempering. The vacuum carburizing conditions were 930°C x 160 minutes.
Production No. 43 was subjected to vacuum carburizing, quenching, and tempering. The vacuum carburizing conditions were 1110°C x 60 minutes.
Then, the specimen was held at 830°C for 30 minutes and oil quenched. Then, the specimen was tempered at 180°C for 120 minutes. After tempering, the specimen was air-cooled.
However, in Production No. 42, sub-zero treatment was performed between carburizing and quenching and tempering.

After carburizing, quenching, and tempering, the gripping and testing parts of the small roller test pieces were finished to improve the test accuracy. In this way, small roller test pieces No. 1 to 43 and round bar test pieces No. 1 to 43 were obtained.

<Performance evaluation of each test piece>
(Vickers hardness)
The micro Vickers hardness of each round bar test piece was measured at five points at 0.10 mm intervals in a region of 0.10 to 0.50 mm deep from the surface of the test piece corresponding to the rolling surface in a cross section cut perpendicular to the longitudinal direction. The hardness measurements were performed in accordance with JIS Z 2244 (2009) with a test force of 2.94 N.
The Vickers hardness was calculated and averaged at a total of 25 points. The results are shown in Table 2.

(C content)
The C content is measured using chips. Specifically, chips are taken from the round bar test piece at a depth of 0.10 mm to 0.50 mm (400 μm) from the surface by lathe cutting, and the carbon concentration determined by the high-frequency induction heating furnace combustion method is regarded as the C concentration in the region at a depth of 0.10 to 0.50 mm from the surface. The results are shown in Table 2.

(Organization)
According to the above-mentioned method for observing the structure, the area ratios of carbides having an equivalent circle diameter of 300 nm or more, retained austenite (represented as retained γ in the table), and tempered martensite (represented as tempered MA in the table) were measured. The results are shown in Table 2.
In addition, in all test pieces, the remainder other than carbides having a circle equivalent diameter of 300 nm or more, retained austenite, and tempered martensite was bainite, pearlite, and ferrite. The total area ratio of bainite, pearlite, and ferrite (referred to as Ba + Pa + αFe area ratio in the table)

(Roller pitting (RP) test (presence or absence of peeling, presence or absence of white tissue))
A roller pitting test (two-cylinder rolling fatigue test) was conducted to evaluate the spalling damage of the bearing raceway. The circumferential surface of the large roller test piece was brought into contact with the surface of the small roller test piece at a rotation speed of 1500 rpm and a surface pressure of 2500 MPa, and the test was conducted up to 20 million times to check for the presence or absence of spalling.

After the test, a test piece was cut out from the small roller so that the structure of the surface that had come into contact with the large roller test piece could be observed, with the observation surface being perpendicular to the longitudinal direction of the small roller.
The observation surface of the test piece was mirror-polished, then etched for 10 seconds using a nital solution (a solution prepared by dissolving 3 g of nitric acid in 100 ml of ethanol and adding a surfactant), and then washed with water.
Next, the observation surface of the sample piece was observed with an optical microscope to check the presence or absence of white structure. The presence or absence of white structure was determined when white structure having a length of 5 μm or more was observed at 1000 times magnification in the range up to a depth of 1.0 mm directly below the rolling surface of the small roller. The length of the white structure (the length of the largest white structure) was also determined. Even when it was determined that no white structure was generated, if white structure having a length of less than 5 μm was observed, the length of the white structure (the length of the largest white structure) was also determined.
The results are shown in Table 2. In Table 2, underlined values indicate that the values are outside the range of the present invention.

As is clear from Tables 1 and 2, in manufacturing examples 1 to 20, which were designed to optimize the chemical composition and the hardness and structure of the surface layer of the bearing raceway, no peeling occurred and no white structure was observed, demonstrating that excellent white structure suppression effects and rolling fatigue properties were obtained.

On the other hand, in Manufacturing Example 21, the C content of the steel was low, and the C content of the surface layer of the bearing raceway was also low, so the hardness required for the bearing raceway could not be obtained. Therefore, the roller pitting test was not performed.
In Production Example 22, the steel material had a high C content and a large amount of retained austenite, so the area ratio of retained austenite did not reach the target. Therefore, the roller pitting test was not performed.
In Production Example 23, the Si content was high, and inclusions were formed, causing spalling damage originating from the inclusions.
In Production Example 24, the Mn content was low, the hardenability was insufficient, and the hardness required for the bearing raceway could not be obtained, so the roller pitting test was not performed.
In Production Example 25, the Mn content was high and cracks occurred during quenching, so the roller pitting test was not performed.
In Production Example 26, the Cr content was low, the hardenability was insufficient, and the hardness required for the bearing raceway could not be obtained, so the roller pitting test was not performed.
In Production Example 27, the Cr content was high and coarse carbides were formed, so the carbide area ratio was not achieved and the forgeability and machinability of the steel were poor. Therefore, a roller pitting test was not performed.

In Production Example 28, the P amount was high, and before the white structure was generated or grew, peeling damage occurred due to the generated embrittled part.
In Production Example 29, the S amount was high, and spalling damage occurred starting from the formed sulfides before the white structure was generated or grew.
In Production Example 30, the Al amount was low, and deoxidation was insufficient before the white structure was generated or grew, and spalling damage occurred starting from the generated oxide.
In Production Example 31, the Al amount was high, and spalling damage occurred starting from the generated coarse oxide before the white structure was generated or grew.
In Production Example 32, the N amount was low, and before the white structure was generated or grew, spalling damage occurred starting from the interface between coarsened crystal grains.
In Production Example 33, the N amount was high, and before the white structure was generated or grew, spalling damage occurred starting from the generated coarse AlN.
In Production Example 34, the O amount was high, and spalling damage originating from oxides occurred before white structures were generated or grew.

In Production Example 35, the Ni amount was low, and spalling damage occurred starting from white structures formed due to hydrogen penetration.
In Production Example 36, the Cu content was high, the hot ductility was reduced, and a bearing raceway could not be produced. Therefore, various tests could not be carried out.
In Production Example 37, the Cu amount was low, and spalling damage occurred starting from white structures formed due to hydrogen penetration.
Production Examples 38 to 40 did not satisfy formula (1), and peeling damage occurred starting from white structures formed by hydrogen penetration.
In Production Example 41, the carbide area ratio was low and the carbide area ratio required for the bearing raceway could not be obtained, so spalling damage occurred before the white structure was generated or grew.
In Example 42, the area ratio of retained austenite was low, and the area ratio of retained austenite required for the bearing raceway could not be obtained, so spalling damage occurred before the white structure was generated or grew.
In Production Example 43, the surface layer had a high C content and a large amount of retained austenite, so the area ratio of retained austenite did not reach the target. Therefore, the roller pitting test was not performed.

Claims (7)

The composition is, in mass%,
C: 0.15 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.05 to 1.50%,
Cr: 0.57 to 2.00%,
P: 0.025% or less,
S: 0.025% or less,
Al: 0.005 to 0.100%,
N: 0.003 to 0.030%,
O: 0.0015% or less,
Ni: 0.65 to 1.45% , and Cu: 0.03 to 0.20%,
Contains
The balance is Fe and impurities,
The contents of Ni and Cu satisfy formula (1);
Steel for bearing raceways.
Ni<27×Cu (1)
In formula (1), the element symbols indicate the content of each element in mass %. In mass percent,
Mo: 2.00% or less,
V: 2.00% or less; and B: 0.0050% or less;
2. The steel material for bearing raceways according to claim 1, which contains one or more types selected from the group consisting of: In mass percent,
Nb: 0.100% or less,
Ti: 0.100% or less; and REM: 0.020% or less;
3. The steel material for bearing raceways according to claim 1, which contains one or more selected from the group consisting of: The composition is, in mass%,
C: 0.15 to 0.50%,
Si: 0.05 to 0.50%,
Mn: 0.05 to 1.50%,
Cr: 0.57 to 2.00%,
P: 0.025% or less,
S: 0.025% or less,
Al: 0.005 to 0.100%,
N: 0.003 to 0.030%,
O: 0.0015% or less,
Ni: 0.65 to 1.98%, and
Cu: 0.03 to 0.20%,
Contains
The balance is Fe and impurities,
The contents of Ni and Cu satisfy formula (1);
A bearing raceway using a steel material for the bearing raceway,
In a region 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the race or raceway disk,
C content is 0.60 to 1.20% by mass,
A bearing raceway having a structure including, by area ratio, 1 to 15% carbides having an equivalent circle diameter of 300 nm or more, 5 to 40% retained austenite, and the remainder being tempered martensite.
Ni<27×Cu (1)
In formula (1), the element symbols indicate the content of each element in mass %. The bearing raceway according to claim 4, in which the Vickers hardness is 720 HV or more in a region 0.10 to 0.50 mm deep from the surface of the portion that will become the raceway surface of the raceway ring or raceway disk. The bearing raceway steel material is
In mass percent,
Mo: 2.00% or less,
V: 2.00% or less, and
B: 0.0050% or less,
6. The bearing raceway according to claim 4, which contains one or more selected from the group consisting of: The bearing raceway steel material is
In mass percent,
Nb: 0.100% or less,
Ti: 0.100% or less, and
REM: 0.020% or less,
The bearing raceway according to any one of claims 4 to 6 , comprising one or more selected from the group consisting of: