US20240392422A1

US20240392422A1 - Bearing steel with improved insulation performance

Info

Publication number: US20240392422A1
Application number: US18/232,456
Authority: US
Inventors: Dong Hwi Kim; Soon Woo Kwon; Sang Jae YOON; Min Woo Kang; Chung An LEE; Seung Hyun Hong; Hyun Ki Kim; Dong Ik Kim; Jae Hyeok Shim; Woo Sang JUNG; Dong Jun Choi
Original assignee: Hyundai Motor Co; Korea Institute of Science and Technology KIST; Kia Corp
Current assignee: Hyundai Motor Co; Korea Institute of Science and Technology KIST; Kia Corp
Priority date: 2023-05-26
Filing date: 2023-08-10
Publication date: 2024-11-28
Also published as: KR20240170090A

Abstract

A bearing steel has improved insulation performance by forming a continuous insulating film. The bearing steel includes 0.65 to 1.0 mass % of carbon (C), 1.6 to 2.0 mass % of chromium (Cr), 1.5 to 3.0 mass % of aluminum (Al), 0.8 to 1.2 mass % of manganese (Mn), 0.7 to 1.0 mass % of silicon (Si), 0.1 to 0.3 mass % of nickel (Ni), 0.08 to 0.12 mass % of molybdenum (Mo), and a balance of iron (Fe), in which an oxynitride layer including a nitride layer and an oxide layer is formed on the surface of a material.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims, under 35 U.S.C. § 119(a), the benefit of priority from Korean Patent Application No. 10-2023-0068118, filed on May 26, 2023, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to bearing steel, and more particularly to bearing steel with improved insulation performance by forming a continuous insulating film.

(b) Background Art

In case of driving devices such as reducers, etc., mounted on conventional electric vehicles, grounding is unstable in motors or inverters, so leaked current flows into the shafts of the driving devices, causing electrical erosion in bearings, resulting in bearing damage.
Recently, in order to solve such problems, various improvement methods such as bypass circuit configuration or plastic coating technology are applied to bearings. However, these methods have problems in that electrical erosion prevention performance is deteriorated due to dimensional change (abrasion) during repeated contact or efficiency is lowered due to contact or friction coefficient differences.
Against the above background, development of bearings capable of alleviating bearing damage due to electrical erosion is continuously carried out.

SUMMARY

An object of the present disclosure is to provide bearing steel with improved insulation performance capable of alleviating bearing damage due to electrical erosion by virtue of steel having optimally adjusted components and an insulating film formed on the surface of the steel.
The objects of the present disclosure are not limited to the foregoing. The objects of the present disclosure will be able to be clearly understood through the following description and to be realized by the means described in the claims and combinations thereof.
The present disclosure provides bearing steel including a steel including 0.65 to 1.0 mass % of carbon (C), 1.6 to 2.0 mass % of chromium (Cr), 1.5 to 3.0 mass % of aluminum (Al), 0.8 to 1.2 mass % of manganese (Mn), 0.7 to 1.0 mass % of silicon (Si), 0.1 to 0.3 mass % of nickel (Ni), 0.08 to 0.12 mass % of molybdenum (Mo), and a balance of iron (Fe), and an oxynitride layer including a nitride layer and an oxide layer formed on a surface of the steel.
The steel may include 0.025 mass % or less of phosphorus (P), 0.02 mass % or less of sulfur (S), 0.01 mass % or less of nitrogen (N), and 0.0020 mass % or less of oxygen (O).
The nitride layer may be located between the oxide layer and the surface of the steel, and may include at least one selected from the group consisting of Fe₂N, Fe₃N, Fe₄N, and combinations thereof.
The oxide layer may be located on the uppermost surface of the bearing steel, and may include at least one selected from the group consisting of Fe₃O₄, Fe₂O₃, and combinations thereof.
The thickness of the oxynitride layer may be 10 to 30 μm.
The thickness of the oxide layer may be 4 μm or less.
The steel may be heat-treated at a temperature of 850 to 950° C.
The steel may be configured such that the oxynitride layer is formed on the surface thereof by oxynitriding treatment.
The bearing steel may have surface hardness of 650 Hv or more as measured according to ISO 6507-1.
The bearing steel may have sheet resistance of 50 Ω·cm²or more as measured according to ASTM F1711-96.

BRIEF DESCRIPTION OF THE FIGURES

The above and other features of the present disclosure will now be described in detail with reference to certain exemplary embodiments thereof illustrated in the accompanying drawings, which are given hereinbelow by way of illustration only, and thus are not limitative of the present disclosure, and wherein:

FIGS. 1, 2, and 3 show results of carbide formation depending on temperature in bearing steel according to Examples;

FIG. 4 shows a scanning electron microscope (SEM) image of a cross-section of an oxynitride layer of bearing steel according to Example;

FIG. 5 shows results of measurement of the oxygen distribution of FIG. 4 ; and

FIG. 6 shows an SEM image of a cross-section of an oxynitride layer of bearing steel according to Comparative Example.

DETAILED DESCRIPTION

The above and other objects, features and advantages of the present disclosure will be more clearly understood from the following preferred embodiments taken in conjunction with the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed herein, and may be modified into different forms. These embodiments are provided to thoroughly explain the disclosure and to sufficiently transfer the spirit of the present disclosure to those skilled in the art.
It will be further understood that the terms “comprise”, “include”, “have”, etc., when used in this specification, specify the presence of stated features, integers, steps, operations, elements, components, or combinations thereof, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or combinations thereof. Also, it will be understood that when an element such as a layer, film, area, or sheet is referred to as being “on” another element, it may be directly on the other element, or intervening elements may be present therebetween. Similarly, when an element such as a layer, film, area, or sheet is referred to as being “under” another element, it may be directly under the other element, or intervening elements may be present therebetween.
Unless otherwise specified, all numbers, values, and/or representations that express the amounts of ingredients, reaction conditions, and compositions used herein are to be taken as approximations including various uncertainties affecting measurement that inherently occur in obtaining these values, among others, and thus should be understood to be modified by the term “about” in all cases. Furthermore, when a numerical range is disclosed in this specification, the range is continuous, and includes all values from the minimum value of said range to the maximum value thereof, unless otherwise indicated. Moreover, when such a range pertains to integer values, all integers including the minimum value to the maximum value are included, unless otherwise indicated.
In the present specification, when a range is described for a variable, it will be understood that the variable includes all values including the end points described within the stated range. For example, the range of “5 to 10” will be understood to include any subranges, such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, and the like, as well as individual values of 5, 6, 7, 8, 9 and 10, and will also be understood to include any value between valid integers within the stated range, such as 5.5, 6.5, 7.5, 5.5 to 8.5, 6.5 to 9, and the like. Also, for example, the range of “10% to 30%” will be understood to include subranges, such as 10% to 15%, 12% to 18%, 20% to 30%, etc., as well as all integers including values of 10%, 11%, 12%, 13% and the like up to 30%, and will also be understood to include any value between valid integers within the stated range, such as 10.5%, 15.5%, 25.5%, and the like.
Before describing the present disclosure, there is no particular limitation in manufacturing bearing steel according to the present disclosure, and a conventionally known method may be applied thereto. Specifically, high-strength steel may be manufactured by subjecting a material having a properly controlled steel composition to steelmaking, casting, and rolling, followed by oxynitriding treatment such that the surface of the manufactured steel is processed to be suitable for bearing steel.
The bearing steel according to the present disclosure is configured to include carbon (C), chromium (Cr), aluminum (Al), manganese (Mn), silicon (Si), nickel (Ni), molybdenum (Mo), phosphorus (P), sulfur (S), nitrogen (N), oxygen (O), and a balance of iron (Fe), in which an oxynitride layer including a nitride layer and an oxide layer is formed on its surface.
Preferably, the bearing steel according to the present disclosure includes a steel including 0.65 to 1.0 mass % of carbon (C), 1.6 to 2.0 mass % of chromium (Cr), 1.5 to 3.0 mass % of aluminum (Al), 0.8 to 1.2 mass % of manganese (Mn), 0.7 to 1.0 mass % of silicon (Si), 0.1 to 0.3 mass % of nickel (Ni), 0.08 to 0.12 mass % of molybdenum (Mo), 0.025 mass % or less of phosphorus (P), 0.02 mass % or less of sulfur (S), 0.01 mass % or less of nitrogen (N), and 0.0020 mass % or less of oxygen (O), and the oxynitride layer disposed on the steel.
Below is a detailed description of the configuration of the bearing steel according to the present disclosure.
The bearing steel according to the present disclosure includes the oxynitride layer disposed on the surface of the steel.
The steel may be manufactured by heat-treating an alloy including carbon (C), chromium (Cr), aluminum (Al), manganese (Mn), silicon (Si), nickel (Ni), molybdenum (Mo), phosphorus (P), sulfur (S), nitrogen (N), oxygen (O), and a balance of iron (Fe) at a temperature of 850 to 950° C.
Also, an oxynitride layer may be formed on the surface of the steel through oxynitriding treatment. The oxynitride layer may include a nitride layer and an oxide layer, and the total thickness of the oxynitride layer may be 10 to 30 μm.
Specifically, the nitride layer may be located between the oxide layer and the surface of the steel, and may include at least one selected from the group consisting of Fe₂N, Fe₃N, Fe₄N, and combinations thereof.
The oxide layer may be located on the uppermost surface of the bearing steel and may include at least one selected from the group consisting of Fe₃O₄, Fe₂O₃, and combinations thereof. Here, the thickness of the oxide layer may be 4 μm or less. Specifically, the oxide layer may have a thickness of 0.01 to 4 μm.
In the present disclosure, the oxide layer indicates an “insulation film” that may exhibit superior insulation effects by being formed on the surface of steel to be described later in Examples.
In the present disclosure, “oxynitriding treatment” may be a two-step heat treatment process of steel using gas.
In the present disclosure, the surface of the steel may be subjected to nitriding treatment in a nitrogen-based gas atmosphere such as ammonia gas to form a nitride layer, after which the surface of the nitride layer may be oxidized in an oxidizing atmosphere such as steam or oxygen to form an oxide layer. In general, oxynitriding treatment may be performed at a temperature of 500 to 720° C. for 3 to 7 hours, but the present disclosure is not limited thereto.
Also, the functions and amounts of individual components of the bearing steel according to the present disclosure are described below.

(A) Carbon (C)

Carbon is an essential element for maintaining strength and durability of bearings.
In the present disclosure, since it is expected to exhibit a surface strengthening effect by nitrogen diffusion in the nitriding heat treatment process for forming an insulating film having high durability, carbon may be used in a relatively small amount.
Specifically, carbon may be contained at 0.65 to 1.0 mass % in steel. If the amount of carbon is less than 0.65 mass %, due to a large amount of aluminum, which is a ferrite stabilizing element, high-temperature ferrite may be formed in the general heat treatment temperature range of 850 to 950° C. during manufacture of steel, resulting in lowered strength and surface hardness. On the other hand, if the amount of carbon exceeds 1.0 mass %, intergranular iron (Fe) carbide may be formed, resulting in increased brittleness and thus lowered durability.

(B) Chromium (Cr)

Chromium is an element that effectively enhances strength and durability by improving hardenability of steel and forming fine carbides to refine the structure of steel.
Specifically, chromium may be contained at 1.6 to 2.0 mass % in steel. If the amount of chromium is less than 1.6 mass %, hardenability may not be sufficiently attained, and chromium (Cr) carbide may be limitedly formed, and thus the strengthening effect may not be sufficiently exhibited. On the other hand, if the amount of chromium exceeds 2.0 mass %, intergranular chromium (Cr) carbide may be formed in the general heat treatment temperature range of 850 to 950° C. during manufacture of steel, resulting in increased brittleness and thus lowered durability.

(C) Aluminum (Al)

Aluminum is the most important alloying element that is added to maximize the effect of preventing electrical erosion in the present disclosure. Also, aluminum is a representative lightweight element and an element that forms an oxide. Aluminum may serve as an accelerator for promoting oxide formation in the present disclosure, and a final bearing product may exhibit high electrical resistance due to oxide formation of aluminum (Al).
Specifically, aluminum may be contained at 1.5 to 3.0 mass % in steel. In general, when an excess of aluminum is added, kappa carbide, which is very brittle, is formed, deteriorating properties of steel, but in the present disclosure, when the amount of aluminum is 1.5 mass % or more, formation of a continuous non-conductive oxide is promoted, thus exhibiting an effect of preventing electrical erosion in the present disclosure.
On the other hand, if the amount of aluminum exceeds 3.0 mass %, high-temperature ferrite may be formed in a region where carbon content is 0.8% or less, lowering strength and surface hardness, and kappa carbide may be formed in the high-carbon-content region, rather than saturating the oxide content of aluminum (Al), ultimately deteriorating properties of steel.

(D) Manganese (Mn)

Manganese is a typical austenite-stabilizing alloying element that increases toughness and improves hardenability of steel.
Specifically, in the present disclosure, since aluminum (Al) is added in a large amount to maximize the formation of an oxide film, manganese may be contained at 0.8 to 1.2 mass % in steel. If the amount of manganese is less than 0.8 mass %, ferrite may be formed even at high temperatures, resulting in decreased strength. On the other hand, if the amount of manganese exceeds 1.2 mass %, the effect on a phase diagram may be saturated, making it difficult to exhibit an additional improvement effect.

(E) Silicon (Si)

Silicon is an alloying element that suppresses the formation of harmful inclusions and effectively removes oxygen in a bearing steelmaking process where cleanliness is important. Also, silicon serves to effectively suppress the formation of iron (Fe) carbide because the mutual solubility thereof with cementite is close to 0.
Specifically, silicon may be contained at 0.7 to 1.0 mass % in steel. In the present disclosure, since aluminum (Al) is included as well as iron (Fe) and chromium (Cr) that easily form carbide, at least 0.7 mass % of silicon is required.
On the other hand, if the amount of silicon exceeds 1.0 mass %, the effect of suppressing the formation of iron (Fe) carbide becomes excessive, and supersaturated carbon binds to aluminum (Al) to form kappa carbide, increasing brittleness and lowering durability.

(F) Nickel (Ni)

Nickel is an alloying element that is similar to manganese (Mn) but more effectively stabilizes austenite and improves hardenability and toughness.
Specifically, nickel may be contained at 0.1 to 0.3 mass % in steel. In the present disclosure, since the amounts of chromium (Cr)/silicon (Si)/aluminum (Al), which are representative ferrite stabilizing elements, are high, nickel is added, as well as manganese (Mn), in order to attain stability of high-temperature austenite. Therefore, in order to realize the effects of the present disclosure, at least 0.1 mass % of nickel is required.
On the other hand, if the amount of nickel exceeds 0.3 mass %, it is difficult to exhibit an additional improvement effect due to excessive saturation thereof.

(G) Molybdenum (Mo)

Molybdenum is an alloying element that is known to effectively increase toughness of a material and suppress the formation of unstable low-temperature carbide to improve softening resistance and fatigue performance.
Specifically, molybdenum may be contained at 0.08 to 0.12 mass % in steel. In the present disclosure, at least 0.08 mass % of molybdenum is required to bring about a carbide precipitation effect.
On the other hand, if the amount of molybdenum exceeds 0.12 mass %, formation of iron (Fe)/chromium (Cr) carbide is suppressed, but Mo₆C is formed, making it difficult to exhibit the effects of the present disclosure.

(H) Phosphorus (P), Sulfur (S), Nitrogen (N), Oxygen (O)

Elements such as phosphorus, sulfur, nitrogen, and oxygen correspond to impurities because they impair the characteristics of a bearing material when added in excess. Hence, elements corresponding to the impurities have to be contained at the lowest levels possible.
Specifically, the steel according to the present disclosure may include 0.025mass % or less of phosphorus (P), 0.02 mass % or less of sulfur (S), 0.01 mass % or less of nitrogen (N), and 0.0020 mass % or less of oxygen (O).
In addition, the bearing according to the present disclosure may have surface hardness of 650 Hv or more as measured according to ISO 6507-1. Moreover, the bearing may have sheet resistance of 50 Ω·cm²or more as measured according to ASTM F1711-96.
A better understanding of the present disclosure may be obtained through the following examples. These examples are merely set forth to illustrate the present disclosure, and are not to be construed as limiting the scope of the present disclosure.

Examples 1 to 7 and Comparative Examples 1 to 4

Steel was manufactured through a typical method using components in the amounts shown in Tables 1 and 2 below. Subsequently, the steel was subjected to oxynitriding treatment to afford specimens. Here, the oxynitriding treatment was performed using a known technique, and a general vacuum induction melting furnace was used.

TABLE 1

Component
(mass %)	C	Cr	Al	Mn	Si	Ni	Mo

Example 1	0.89	1.71	2.07	1.12	0.92	0.15	0.11
Example 2	0.91	1.72	2.97	1.11	0.91	0.14	0.10
Example 3	0.91	1.71	1.52	1.12	0.92	0.15	0.11
Example 4	0.98	1.98	2.98	1.13	0.92	0.15	0.11
Example 5	0.66	1.63	1.55	1.10	0.92	0.14	0.11
Example 6	0.88	1.69	2.05	1.09	0.89	0.14	0.07
Example 7	0.91	1.72	2.04	1.12	1.04	0.15	0.10

TABLE 2

Component
(mass %)	C	Cr	Al	Mn	Si	Ni	Mo

Comparative	0.92	1.72	0.03	1.11	0.91	0.15	0.10
Example 1
Comparative	0.91	1.72	1.02	1.12	0.88	0.15	0.10
Example 2
Comparative	0.59	1.72	2.05	1.11	0.89	0.16	0.09
Example 3
Comparative	0.90	1.72	2.03	1.11	0.90	0.14	0.18
Example 4

Test Example

The properties of the manufactured specimens were measured by evaluation methods according to the following items. The results thereof are shown in Tables 3 and 4 below.

Evaluation Methods

(1) Surface hardness: Vickers hardness was measured using a load of 5 kgf according to ISO 6507-1.
(2) Sheet resistance: Since the insulating film is an oxide and has ceramic properties, sheet resistance was measured using a device of ASTM F1711-96 according to the thin film 4-point probe resistance measurement method.

TABLE 3

Evaluation	Sheet resistance	Surface	Intergranular	Kappa-
items	(Ω · cm²)	hardness	carbide	carbide

Example 1	141.3	Hv 711	X	X
Example 2	262.1	Hv 704	X	X
Example 3	65.4	Hv 707	X	X
Example 4	257.9	Hv 715	X	X
Example 5	59.2	Hv 682	X	X
Example 6	135.4	Hv 702	X	X
Example 7	146.5	Hv 689	X	X

TABLE 4

Evaluation	Sheet resistance	Surface	Intergranular	Kappa-
items	(Ω · cm²)	hardness	carbide	carbide

Comparative	0.01↓	Hv 692	X	X
Example 1	(not measured)
Comparative	0.72	Hv 698	X	X
Example 2
Comparative	127.2	Hv 537	X	X
Example 3
Comparative	137.6	Hv 597	X	X
Example 4

As is apparent from the results of Table 3, Examples 1 to 7 exhibited surface hardness of 650 Hv or more and sheet resistance of 50 Ω·cm²or more, so that sheet resistance and surface hardness were balanced by mixing appropriate amounts of the components, resulting in high quality.
In particular, Example 1 had surface hardness of 711 Hv and sheet resistance of 141.3 Ω·cm², indicating an optimal composition.
Also, as is apparent from the results of Table 4, Comparative Example 1 without aluminum (Al) had low sheet resistance and measurement was not possible. Likewise, the sheet resistance of Comparative Example 2 containing less than 1.5 mass % of aluminum (Al) was measured to be very low.
Comparative Example 3 containing less than 0.65 mass % of carbon (C) showed relatively low surface hardness compared to Examples.
Comparative Example 4 containing greater than 0.12 mass % of molybdenum (Mo) showed relatively low surface hardness and sheet resistance compared to Examples.
In addition, with reference to FIGS. 1 to 3 , the results of carbide formation in the bearing depending on temperature were confirmed.
FIG. 1 shows results of carbide formation depending steel on temperature in the bearing according to Example 1.
As shown in FIG. 1 , when the amount of carbon (C) was 0.65 to 1.0 mass %, high-temperature BCC was not formed and intergranular carbide was not formed.
FIG. 2 shows results of carbide formation in the bearing steel depending on temperature by changing only the amount of aluminum (Al) in the composition of Example 1 to 3.2 mass %.
As shown in FIG. 2 , high-temperature ferrite and kappa carbide appeared when an excess of aluminum (Al) was added.
FIG. 3 shows results of carbide formation in the bearing steel depending on temperature by changing only the amount of chromium (Cr) in the composition of Example 1 to 2.1 mass %.
As shown in FIG. 3 , intergranular chromium (Cr) carbide was formed when an excess of chromium (Cr) was added.
In addition, the oxynitride layer of the bearing steel according to the present disclosure was observed in detail using a scanning electron microscope (SEM). Here, FIG. 4 is an SEM image of a cross-section of the oxynitride layer of the bearing steel according to Example 1. FIG. 5 shows results of measurement of the oxygen distribution of FIG. 4 .
With reference to FIGS. 4 and 5 , the bearing steel according to the present disclosure was confirmed to form an oxynitride layer including a nitride layer and an oxide layer with a total thickness of about 15 μm during oxynitriding heat treatment using appropriate amounts of components. Specifically, according to the present disclosure, a continuous oxide layer having a thickness of up to 4 μm composed of Fe₃O₄and Fe₂O₃was formed by including 1.5 mass % or more of aluminum (Al) in the steel. In the present disclosure, it is possible to attain sheet resistance and prevent electrical erosion by such an oxide layer.
FIG. 6 shows an SEM image of a cross-section of an oxynitride layer of the bearing steel according to Comparative Example 2.
As shown in FIG. 6 , when the amount of aluminum (Al) was insufficient, a discontinuous oxide layer was formed rather than the continuous oxide layer, and thus Comparative Example 2 resulted in poor sheet resistance.
Therefore, the bearing according to the present disclosure is configured such that a continuous insulating film is formed on the surface of steel composed of appropriate amounts of components, thereby attaining sheet resistance by the insulating film, ultimately reducing damage to the bearing due to electrical erosion when applied to an electric vehicle.
Moreover, the bearing according to the present disclosure is able to maximize the effect of the insulation film by providing an optimal amount of aluminum capable of forming an insulation film having high durability and high efficiency on the surface of steel.
As is apparent from the above description, bearing steel according to the present disclosure is configured such that a continuous insulating film is formed on the surface of a material in which appropriate amounts of components are mixed, so that sheet resistance is ensured by the insulating film, thereby alleviating bearing damage due to electrical erosion when applied to an electric vehicle.
In addition, the bearing steel according to the present disclosure is capable of maximizing the effect of the insulating film by providing an optimal amount of aluminum capable of forming an insulating film having high durability and high efficiency on the surface of the material.
The effects of the present disclosure are not limited to the above-mentioned effects. It should be understood that the effects of the present disclosure include all effects that can be inferred from the description of the present disclosure.
Although specific embodiments of the present disclosure have been described, those skilled in the art will appreciate that the present disclosure may be embodied in other specific forms without changing the technical spirit or essential features thereof. Thus, the embodiments described above should be understood to be non-limiting and illustrative in every way.

Claims

1. A bearing steel, comprising:

a steel comprising 0.65 to 1.0 mass % of carbon (C), 1.6 to 2.0 mass % of chromium (Cr), 1.5 to 3.0 mass % of aluminum (Al), 0.8 to 1.2 mass % of manganese (Mn), 0.7 to 1.0 mass % of silicon (Si), 0.1 to 0.3 mass % of nickel (Ni), 0.08 to 0.12 mass % of molybdenum (Mo), and a balance of iron (Fe); and

an oxynitride layer comprising a nitride layer and an oxide layer formed on a surface of the steel.

2. The bearing steel of claim 1, wherein the steel comprises 0.025 mass % or less of phosphorus (P), 0.02 mass % or less of sulfur (S), 0.01 mass % or less of nitrogen (N), and 0.0020 mass % or less of oxygen (O).

3. The bearing steel of claim 1, wherein the nitride layer is located between the oxide layer and the surface of the steel, and comprises at least one selected from the group consisting of Fe₂N, Fe₃N, Fe₄N, and combinations thereof.

4. The bearing steel of claim 1, wherein the oxide layer is located on an uppermost surface of the bearing steel, and comprises at least one selected from the group consisting of Fe₃O₄, Fe₂O₃, and combinations thereof.

5. The bearing steel of claim 1, wherein a thickness of the oxynitride layer is 10 to 30 μm.

6. The bearing steel of claim 1, wherein a thickness of the oxide layer is 4 μm or less.

7. The bearing steel of claim 1, wherein the steel is heat-treated at a temperature of 850° C. to 950° C.

8. The bearing steel of claim 1, wherein the steel is configured such that the oxynitride layer is formed on the surface by oxynitriding treatment.

9. The bearing steel of claim 1, wherein the bearing steel has a surface hardness of 650 Hv or more as measured according to ISO 6507-1.

10. The bearing steel of claim 1, wherein the bearing steel has a sheet resistance of 50 Ω·cm²or more as measured according to ASTM F1711-96.