STEEL MATERIAL AS STARTING MATERIAL FOR CARBONITED BEARING COMPONENT

Abstract

Steel material with a chemical composition consisting of, in mass %: C: 0.15 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 0.60% P: 0.015% or less, S: 0.005% or less, Cr: 0.80 to 1.50%, Mon: 0.17 to 0.30%, V: 0.24 to 0.40%, Al: 0.005 to 0.100%, N: 0.0300% or less, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.0050%, Note: 0 to 0.100% Ti: 0 to 0.100% Approximately 0 to 0.0010%, and as residues of iron and impurities, and fulfilling formulas (1) to (4), wherein in its microstructure a total surface area fraction of ferrite and pearlite is 10.0% or more and the remainder is bainite, and a ratio of the content of V (mass-%) in the electrolytic extraction residue to the content of V (mass-%) in the chemical composition is 10.0% or less: 1.50 < 0.4 Cr + 0.4 Mo + 4.5 V < 2.45 2.20 < 2.7 C + 0.4 Si + Mn + 0.45 Ni + 0.8 Cr + Mo + V < 2.80 Mo / V ≥ 0, 58 (Mo + V + Cr) / (Mn + 20 P) ≥ 2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass %).

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a steel material, in particular a steel material as a starting material for a carbonitrided bearing component, that is, a bearing component subjected to a carbonitriding treatment. TECHNICAL BACKGROUND

[0002] Steel materials to be used as starting material for a bearing component are typically specified by SUJ2 in JIS G 4805(2008). These types of steel materials are manufactured into a bearing component through the following process. A steel material is hot forged and / or machined to produce an intermediate product with the desired shape. The intermediate product is subjected to heat treatment to adjust the hardness of the steel material and formulate the microstructure of the steel material. Examples of heat treatment include quenching and tempering, carburizing treatment, and carbonitriding treatment. Through the above processes, a bearing component with the desired bearing properties (wear resistance and toughness of a core region of the bearing component) is manufactured.

[0003] In addition to the heat treatment described above, carbonitriding is also performed in cases where wear resistance is particularly required for bearing performance. Carbonitriding here refers to a treatment that combines carbonitriding with quenching and tempering. During carbonitriding, a carbonitriding layer is formed in the outer layer of a steel material, which hardens the outer layer of the steel material. A bearing component subjected to carbonitriding is referred to herein as a carbonitrided bearing component.

[0004] Techniques for increasing wear resistance, toughness, and the like of a bearing component are proposed in Japanese Patent Application Publication No. H08-49057 A (Patent Literature 1), Japanese Patent Application Publication No. H11-12684 A (Patent Literature 2), and International Application Publication No. JP 2016-17162 A (Patent Literature 3).

[0005] A rolling bearing disclosed in Patent Literature 1 includes a race and a rolling element, at least one of which is a steel obtained by manufacturing a medium-carbon or low-carbon low-alloy steel containing C: 0.1 to 0.7 wt%, Cr: 0.5 to 3.0 wt%, Mn: 0.3 to 1.2 wt%, Si: 0.3 to 1.5 wt%, Mo: 3 wt% or less, and V: 0.8 to 2.0 wt%. A product formed from the starting material is subjected to carburizing treatment or carbonitriding treatment in the heat treatment to satisfy a relationship in which a carbon concentration of a surface of the product is 0.8 to 1.5 wt% and a concentration ratio V / C of the surface is 1 to 2.5. Patent Literature 1 describes that wear resistance of the rolling bearing can be increased by causing precipitation of V-carbide on a surface of the rolling bearing.

[0006] A hardenable steel for cold forging disclosed in Patent Literature 2 has a ferrite + pearlite area ratio of 75% or more, an average ferrite grain diameter of 40 μm or less, and an average pearlite grain diameter of 30 μm or less. Patent Literature 2 describes that the inclusion of the above-mentioned microstructure can increase the wear resistance of this hardenable steel for cold forging.

[0007] A steel for carbonitrided bearings disclosed in Patent Literature 3 includes a chemical composition consisting of, in mass%, C: 0.22 to 0.45%, Si: 0.50% or less, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.30 to 2.0%, Mo: 0.10 to 0.35%, V: 0.20 to 0.40%, Al: 0.005 to 0.10%, N: 0.030% or less, O: 0.0015% or less, B: 0 to 0.0050%, Nb: 0 to 0.10%, and Ti: 0 to 0.10%, the balance being Fe and impurities, and satisfying Formula (1) and Formula (2). Here, formula (1) is 1.20 < 0.4Cr + 0.4Mo + 4.5V < 2.60, and formula (2) is 2.7C + 0.4Si + Mn + 0.8Cr + Mo + V > 2.20. Patent Literature 3 describes that this steel for carbonitrided bearings has excellent hardenability despite not containing Ni, and that the steel has excellent toughness, wear resistance, and surface-initiated exfoliation life after heat treatment.Patent Literature 4 describes a steel with excellent processability and a component made with it. CITATION LISTPATENT LITERATURE Patent Literature 1: Japanese Patent Application Publication No. H08-49057A Patent Literature 2: Japanese Patent Application Publication No. H11-12684 A Patent Literature 3: International Application Publication No. JP 2016- 17 162 A Patent Literature 4: German publication of a translation of the European patent specification No. EP 0 903 418 B1 No. DE 697 18 784 T2 SUMMARY OF THE INVENTION TECHNICAL PROBLEM

[0008] Bearing components are divided into medium or large bearing components, which are used for mining machinery or construction equipment, and small bearing components, which are used for automobiles. Examples of small bearing components are bearing components used in engines. Bearing components for automobiles are often used in environments where lubricants such as engine oil circulate.

[0009] Recently, the viscosity of a lubricant is reduced to reduce frictional resistance and transmission resistance, and the consumption of lubricant in the circuit is reduced to improve fuel efficiency. As a result, the lubricant used may decompose and generate hydrogen. In a case where hydrogen is generated in an environment where a bearing component is used, the hydrogen penetrates into the bearing component from the outside. The penetrating hydrogen causes a structural change, among other things, in a microstructure of the bearing component. The structural change during use of the bearing component reduces a spalling life of the bearing component. Hereinafter, an environment in which hydrogen is generated, causing a structural change, is referred to as a "hydrogen-generating environment" in this specification.

[0010] In Patent Documents 1 to 3 described above, the exfoliation life of a carbonitrided bearing component under a hydrogen-generating environment is not discussed. Furthermore, in the case of a steel material serving as the starting material for a carbonitrided bearing component, an intermediate product after hot forging in a process for manufacturing a carbonitrided bearing component may be subjected to machining to obtain a final shape. In this case, the steel material serving as the starting material for a carbonitrided bearing component must also have excellent machinability.

[0011] An object of the present disclosure is to provide a steel material as a starting material for a carbonitrided bearing component, which has excellent machinability and exhibits excellent wear resistance, toughness of its core portion, and spalling life upon microstructure change under a hydrogen-generating environment of the carbonitrided bearing component after the carbonitriding treatment. SOLUTION TO THE PROBLEM

[0012] A steel material according to the present disclosure comprises a chemical composition consisting of, in mass%: C: 0.15 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 0.60%, P: 0.015% or less, S: 0.005% or less, Cr: 0.80 to 1.50%, Mon: 0.17 to 0.30%, V: 0.24 to 0.40%, Al: 0.005 to 0.100%, N: 0.0300% or less, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.0050%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Ca: 0 to 0.0010%, and as the remainder Fe and impurities, and satisfying formula (1) to formula (4), wherein in its microstructure a total area fraction of ferrite and pearlite is 10.0% or more and the remainder is bainite, and a ratio of a content of V (mass%) in the electrolytic extraction residue to the content of V (mass%) in the chemical composition is 10.0% or less: 1.50<0.4Cr+0.4Mo+4.5V<2.45 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 Mo / V 0≥0.58 (Mo+V+Cr) / (Mn+20P)≥2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass%). ADVANTAGEOUS EFFECT OF THE INVENTION

[0013] The steel material according to the present disclosure has excellent machinability and exhibits excellent wear resistance, core region toughness, and exfoliation life upon microstructure change under a hydrogen-generating environment after carbonitriding treatment. BRIEF DESCRIPTION OF THE DRAWINGS [ Fig. 1] Fig.1 is a graph showing the exfoliation life (Hr) under a hydrogen-generating environment of a bearing component (comparative example) manufactured by performing quenching and tempering treatment on a steel material conforming to SUJ2 specified in JIS G 4805(2008), and bearing components (carbonitrided bearing components: inventive examples of the present invention) each manufactured by performing carbonitriding treatment on a steel material according to the present embodiment, which has the above-described chemical composition and satisfies formulas (1) to (4), and in which a ratio of a content of V (mass %) in electrolytic extraction residue to a content of V (mass %) in the chemical composition is 10.0% or less. [ Fig. 2] Fig.Figure 2 is a diagram illustrating a heating pattern of quenching and tempering performed on test specimens for a hardenability evaluation test and a toughness evaluation test in EXAMPLE. [ Fig. 3] Fig. Figure 3 is a side view of an intermediate small roll sample used in a roll nip test in EXAMPLE. [ Fig. 4] Fig. Figure 4 is a side view of a small roller sample used in a roller test in EXAMPLE. [ Fig. 5] Fig. Figure 5 is a front view of a large roller used in the roller test in EXAMPLE. DESCRIPTION OF THE EMBODIMENT

[0014] The present inventors conducted investigations and studies on machinability of a steel material as a raw material of a carbonitrided bearing component and on wear resistance, toughness of a core portion, and exfoliation life with a microstructure change under a hydrogen-generating environment of the carbonitrided bearing component after the carbonitriding treatment.

[0015] First, the present inventors conducted studies on a chemical composition of a steel material having the properties described above.As a result, the inventors found that when using a steel material having a chemical composition consisting of, in mass%, C: 0.15 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 0.60%, P: 0.015% or less, S: 0.005% or less, Cr: 0.80 to 1.50%, Mo: 0.17 to 0.30%, V: 0.24 to 0.40%, Al: 0.005 to 0.100%, N: 0.0300% or less, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.0050%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Ca: 0 to 0.0010%, and the rest is Fe and impurities, there is a possibility that machinability of the steel material and wear resistance, toughness of the core portion and spalling life can be improved with a change in microstructure under a hydrogen generating environment of the carbonitrided bearing component after carbonitriding treatment.

[0016] However, it was found that a steel material in which the elements only fall within the respective ranges described above does not necessarily improve the above-described properties (machinability and wear resistance, toughness of its core region, and spalling life under the hydrogen-generating environment of its carbonitrided bearing component). Therefore, the present inventors conducted further investigations. As a result, the present inventors found that the above-described properties can be enhanced if the above-described chemical composition of the core region additionally satisfies the following formulas (1) to (4): 1.50<0.4Cr+0.4Mo+4.5V<2.45 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 Mo / V 0≥0.58 (Mo+V+Cr) / (Mn+20P)≥2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass%). [Formula 1)]

[0017] To increase the spalling life of a carbonitrided bearing component under a hydrogen-generating environment, it is effective to incorporate one or more species selected from the group consisting of V carbides with equivalent circular diameters of 150 nm or less, V carbo-nitrides with equivalent circular diameters of 150 nm or less, complex V carbides with equivalent circular diameters of 150 nm or less, and complex V carbo-nitrides with equivalent circular diameters of 150 nm or less in a large amount into the carbonitrided bearing component. Complex V carbides here refer to carbides containing V and Mo. Complex V carbo-nitrides refer to carbo-nitrides containing V and Mo.In the following description, V-carbides and V-carbonitrides are also referred to as "V-carbides and the like," and complex V-carbides and complex V-carbonitrides are also referred to as "complex V-carbides and the like." Furthermore, V-carbides and the like with equivalent circular diameters of 150 nm or less are referred to as "small V-carbides and the like," and complex V-carbides and the like with equivalent circular diameters of 150 nm or less are referred to as "small complex V-carbides and the like." Here, the equivalent circular diameter means a diameter of a circle with the same area as V-carbides and the like or complex V-carbides and the like.

[0018] In a case where V carbides and the like and complex V carbides and the like are small V carbides and the like and small complex V carbides and the like that have equivalent circular diameters of 150 nm or less, the small V carbides and the like and the small complex V carbides and the like trap hydrogen. In addition, small V carbides and the like and small complex V carbides and the like resist cracking due to their small size. Therefore, by sufficiently dispersing small V carbides and the like and small complex V carbides and the like in a carbonitrided bearing component, a change in the microstructure under a hydrogen-generating environment cannot occur, and as a result, the spalling life of the carbonitrided bearing component under the hydrogen-generating environment can be increased.

[0019] Let F1 be defined as F1 = 0.4Cr + 0.4Mo + 4.5V. F1 is an index related to the production amount of small V carbides and the like and small complex V carbides and the like, which capture hydrogen to increase the spalling life of a carbonitrided bearing component under a hydrogen-generating environment. The production of small V carbides and the like and small complex V carbides and the like is accelerated by containing both V and Cr and Mo. Cr produces Fe-based carbide such as cementite or Cr carbide in a temperature range lower than the temperature range in which V carbides and the like and complex V carbides and the like are produced. Mo produces Mo carbide (Mo2C) in a temperature range lower than the temperature range in which V carbides and the like and complex V carbides and the like are produced.As the temperature rises, the Fe-based carbide, the Cr-based carbide, and the Mo carbide are dissolved to serve as nucleation sites for the precipitation of the V carbides and the like and the complex V carbides and the like.

[0020] When F1 is 1.50 or less, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), Cr and Mo are insufficient, and thus the nucleation sites of precipitation for V carbides and the like and complex V carbides and the like become insufficient. Otherwise, a content of V necessary to produce V carbides and the like and complex V carbides and the like themselves becomes insufficient with respect to a content of Cr and a content of Mo. As a result, V carbides and the like and complex V carbides and the like are not sufficiently produced.On the other hand, when F1 is 2.45 or more, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), V carbides and the like with equivalent circular diameters of more than 150 nm and complex V carbides and the like with equivalent circular diameters of more than 150 nm are produced. In the following description, V carbides and the like with equivalent circular diameters of more than 150 nm are also referred to as "coarse V carbides and the like," and complex V carbides and the like with equivalent circular diameters of more than 150 nm are also referred to as "coarse complex V carbides and the like." Coarse V carbides and the like and coarse complex V carbides and the like have poor hydrogen capture performance and are therefore prone to microstructure changes.Therefore, coarse V-carbides and the like and coarse complex V-carbides and the like reduce the spalling life of a carbonitrided bearing component in a hydrogen-generating environment.

[0021] When F1 is more than 1.50 and less than 2.45, provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), small V carbides and the like and small complex V carbides and the like are sufficiently generated in a resulting carbonitrided bearing component, and V carbides and the like and complex V carbides and the like are sufficiently dissolved in a resulting steel material to be a raw material for a carbonitrided bearing component. Therefore, a microstructure change cannot occur under a hydrogen-generating environment, and thus the exfoliation life of the carbonitrided bearing component under the hydrogen-generating environment is increased.Furthermore, when F1 is less than 2.45, the formation of coarse V-carbides and the like and coarse complex V-carbides and the like in the carbonitrided bearing component is prevented or reduced, and a large number of small V-carbides and the like and small complex V-carbides and the like are formed in its outer layer. Therefore, the wear resistance of the carbonitrided bearing component is also improved. [Formula (2)]

[0022] In addition, to increase the exfoliation life of a carbonitrided bearing component in a hydrogen-generating environment, it is effective to increase the strength of a core region of the carbonitrided bearing component. To increase the strength of a core region of a carbonitrided bearing component, it is effective to increase the hardenability of a steel material serving as the starting material for the carbonitrided bearing component. However, if the hardenability of a steel material is excessively increased, the machinability of the steel material will be reduced.

[0023] F2 is defined as F2 = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo + V. The elements shown in F2 (C, Si, Mn, Ni, Cr, Mo, and V) are primary elements that increase the hardenability of the steel, from among the elements of the chemical composition described above. F2 is thus an index of the strength of the core region of a carbonitrided bearing component and of the steel's machinability.

[0024] When F2 is 2.20 or less, the hardenability of a resulting steel material is insufficient even if the contents of elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4). As a result, the strength of a core portion of a resulting carbonitrided bearing component is insufficient, and sufficient exfoliation life of the carbonitrided bearing component under a hydrogen-generating environment is not achieved. On the other hand, when F2 is 2.80 or more, the hardenability of the steel material becomes too high even if the contents of the elements in the chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4). In this case, sufficient machinability of the steel material is not achieved.

[0025] When F2 is more than 2.20 and less than 2.80, provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4), sufficient machinability of the steel material is achieved. Furthermore, the strength of a core portion of a resulting carbonitrided bearing component is sufficiently increased, and the exfoliation life of the carbonitrided bearing component under a hydrogen-generating environment is sufficiently increased. [Formula (3)]

[0026] Mo is an element that accelerates the precipitation of small V carbides and the like and small complex V carbides and the like. In particular, as described above, F1 satisfying formula (1) enables the provision of a total of a V content, a Cr content, and a Mo content necessary to produce small V carbides and the like and small complex V carbides and the like. However, as a result of studies conducted by the present inventors, it has been revealed that the production of sufficiently small V carbides and the like and small complex V carbides and the like still requires the adjustment of a ratio of a Mo content to a V content (= Mo / V).In particular, when the ratio of a Mo content to a V content is too low, Mo carbides serving as nucleation sites of precipitation do not sufficiently precipitate prior to the production of small V carbides and the like and small complex V carbides and the like. In this case, even if a V content, a Cr content, and a Mo content fall within the respective ranges according to the present embodiment and satisfy formula (1), small V carbides and the like and small complex V carbides and the like are not sufficiently produced.

[0027] Let F3 be defined as F3 = Mo / V. When F3 is less than 0.58, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (2), and (4), small V carbides and the like and small complex V carbides and the like are not sufficiently produced. As a result, a sufficient spalling life of the carbonitrided bearing component is not achieved under a hydrogen-generating environment. Provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (2), and (4), when F3 is 0.58 or more, that is, when formula (3) is satisfied, small V carbides and the like and complex V carbides and the like are sufficiently produced.As a result, the spalling life of the carbonitrided bearing component is sufficiently increased under a hydrogen generating environment. [Formula (4)]

[0028] The small V carbides and the like, as well as the complex V carbides and the like described above, not only capture hydrogen but also exert precipitation strengthening to strengthen the interior of the grains. At the same time, if the small V-based precipitates also strengthen the grain boundaries in a carbonitrided bearing component under a hydrogen-generating environment and, in addition, can prevent or reduce hydrogen penetration, the exfoliation life of the carbonitrided bearing component under the hydrogen-generating environment can be further increased through a synergistic effect of three effects: (a) intragranular strengthening, (b) grain boundary strengthening, and (c) hydrogen penetration prevention. The intragranular strengthening indicated as (a) depends on a total of a content of Mo, a content of V, and a content of Cr, as described above.Meanwhile, for grain boundary strengthening, indicated as (b), it is effective to reduce the P content, which is particularly likely to segregate into grain boundaries under the chemical composition described above. Furthermore, a study conducted by the inventors has shown that reducing the Mn content in a steel material is extremely effective for preventing hydrogen penetration, indicated as (c).

[0029] Let F4 be defined as F4 = (Mo+V+Cr) / (Mn+20P). The numerator in F4 (= (Mo+V+Cr)) is an index of intragranular strengthening (equivalent to (a) described above). The denominator in F4 (= (Mn+20P)) is an index of grain boundary embrittlement and hydrogen penetration (equivalent to (b) and (c) described above). A large denominator in F4 means that grain boundary strength is low or that hydrogen can penetrate into a resulting carbonitrided bearing component.Therefore, even if an intragranular strengthening index (the numerator in F4) is large, if the grain boundary embrittlement hydrogen penetration index (the denominator in F4) is large, a synergistic effect of an intragranular strengthening mechanism, a grain boundary strengthening mechanism, and a hydrogen penetration prevention mechanism is not achieved, and therefore, a spalling life of the carbonitrided bearing component under a hydrogen generating environment is not sufficiently improved.

[0030] Provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy the formulas (1) to (3), when F4 is 2.40 or more, the synergistic effect of the intragranular strengthening mechanism, the grain boundary strengthening mechanism, and the hydrogen permeation prevention mechanism is achieved, and a sufficient exfoliation life of a resulting carbonitrided bearing component under a hydrogen-generating environment is achieved.

[0031] In addition, in the steel material according to the present embodiment, the ratio of a content of V (mass %) in the electrolytic extraction residue of the steel material to the content of V (mass %) in the chemical composition of the steel material is 10.0% or less.

[0032] As described above, in order to sufficiently produce small V carbides and the like and small complex V carbides and the like in a carbonitrided bearing component, it is preferable that V carbides and the like and complex V carbides and the like be dissolved as much as possible in a steel material to be a starting material of the carbonitrided bearing component. When V carbides and the like and complex V carbides and the like remain in a steel material, the V carbides and the like and complex V carbides and the like remaining in the steel material coarsen in a manufacturing process of the carbonitrided bearing component. Coarse V carbides and the like and coarse complex V carbides and the like have poor hydrogen capture performance during use of the carbonitrided bearing components under a hydrogen-generating environment.Therefore, coarse V-carbides and the like, as well as coarse complex V-carbides and the like, tend to cause a change in the microstructure and serve as the origin of a crack during use of the carbonitrided bearing component in a hydrogen-generating environment. As a result, the spalling life of the carbonitrided bearing component is reduced.

[0033] Therefore, in the steel material according to the present embodiment, assuming that [V] R denotes a content of V in the electrolytic extraction residue of the steel material and [V]c denotes a content of V in the chemical composition of the steel material, a proportion of the V content in the residue RA V , which is defined by formula (A) shown below, is 10.0% or less. RAV=[V]R / [V]C×100

[0034] If a portion of the V content in the residue RA VWhen the content of V carbides and the like, as well as complex V carbides and the like, is 10.0% or less, it sufficiently dissolves V carbides and the like in a steel material that is a starting material for a carbonitrided bearing component. As a result, a reduction in the spalling life of the carbonitrided bearing component under a hydrogen-generating environment due to coarse V carbides and the like and coarse complex V carbides and the like is prevented or reduced.

[0035] The steel material according to the present embodiment having the above configuration exhibits an extremely good exfoliation life of a carbonitrided bearing component subjected to carbonitriding treatment in a hydrogen-generating environment. Fig.1 is a graph showing the exfoliation life under a hydrogen-generating environment of a bearing component (comparative example) manufactured by quenching and tempering a steel material conforming to SUJ2 specified in JIS G 4805(2008), and bearing components (carbonitrided bearing components: inventive examples of the present invention) each manufactured by performing a carbonitriding treatment on the steel material according to the present embodiment, which has the above-described chemical composition and satisfies formulas (1) to (4) and in which a proportion of the V content in the residue RA V 10.0% or less. A test of exfoliation life under a hydrogen-generating environment was conducted according to a method described below in EXAMPLE. The ordinate axis of Fig.1 shows the ratio of the exfoliation life of each inventive example of the present invention to the exfoliation life of the comparative example (hereinafter referred to as exfoliation life ratio), where the exfoliation life of the comparative example is defined as 1.0 (reference).

[0036] Referring to Fig.1, the exfoliation life under a hydrogen-generating environment of the inventive examples of the present invention is more than at least 2.0 times the exfoliation life under a hydrogen-generating environment of the bearing component having a conventional chemical composition (Comparative Example). That is, the exfoliation life under a hydrogen-generating environment of a carbonitrided bearing component made of the steel material according to the present embodiment is extremely, significantly improved compared to that of the conventional bearing component.

[0037] The steel material as a raw material for a carbonitrided bearing component according to the present embodiment, which was prepared based on the above findings, has the following configuration.

[0038] [1] A steel material with a chemical composition consisting of, in mass%: C: 0.15 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 0.60%, P: 0.015% or less, S: 0.005% or less, Cr: 0.80 to 1.50%, Mon: 0.17 to 0.30%, V: 0.24 to 0.40%, Al: 0.005 to 0.100%, N: 0.0300% or less, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.0050%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Ca: 0 to 0.0010%, and as the remainder Fe and impurities, and satisfying formula (1) to formula (4), wherein in its microstructure a total area fraction of ferrite and pearlite is 10.0% or more and the remainder is bainite, and a ratio of a content of V (mass%) in the electrolytic extraction residue to the content of V (mass%) in the chemical composition is 10.0% or less: 1.50<0.4Cr+0.4Mo+4.5V<2.45 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 Mo / V≥0.58 (Mo+V+Cr) / (Mn+20P)≥2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass%).

[0039] [2] The steel material according to [1], where the chemical composition contains one or more types of elements selected from the group consisting of: Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, B: 0.0001 to 0.0050%, Nb: 0.005 to 0.100%, and Ti: 0.005 to 0.100%.

[0040] [3] The steel material according to [1] or [2], where the chemical composition contains Ca: 0.0001 to 0.0010%.

[0041] The steel material according to the present embodiment is described in detail below. The symbol "%" in reference to elements means mass% unless otherwise specified. [Chemical composition of the steel used as starting material for the carbonitrided bearing component]

[0042] The steel material according to the present embodiment is intended to be a starting material for a carbonitrided bearing component. A chemical composition of the steel material according to the present embodiment includes the following elements. C: 0.15 to 0.45%.

[0043] Carbon (C) increases the hardenability of steel. Therefore, C increases the strength of the core portion of a carbonitrided bearing component made from the steel material according to the present embodiment as a raw material and increases the toughness of the core portion. Furthermore, C increases the wear resistance of the carbonitrided bearing component by forming fine carbides and carbonitrides through the carbonitriding treatment. Furthermore, C forms small V-carbides and the like, and small complex V-carbides and the like, mainly during the carbonitriding treatment. Small V-carbides and the like, and small complex V-carbides and the like, trap hydrogen in the steel material during use of the carbonitrided bearing component under a hydrogen-generating environment.As a result, small V carbides and the like and small complex V carbides and the like increase the spalling life under a hydrogen-generating environment of the carbonitrided bearing component. When the C content is less than 0.15%, the above-described effects are not sufficiently achieved even if the contents of the other elements in the chemical composition are within the respective ranges according to the present embodiment. On the other hand, when the C content is more than 0.45%, V carbides and the like and complex V carbides and the like are not completely dissolved but partially remain in a manufacturing process of the steel material, even if the contents of the other elements fall within the respective ranges according to the present embodiment.The remaining V carbides and the like, as well as complex V carbides and the like, are not sufficiently dissolved even in a manufacturing process of the carbonitrided bearing component. The V carbides and the like, and complex V carbides and the like remaining in the steel material then grow during the manufacturing process of the carbonitrided bearing component and remain in the form of coarse V carbides and the like, and coarse complex V carbides and the like in the carbonitrided bearing component. In this case, a microstructure change occurs during use of the carbonitrided bearing component in a hydrogen-generating environment because the coarse V carbides and the like, and the coarse complex V carbides and the like in the carbonitrided bearing component have poor hydrogen capture performance.The coarse V carbides and the like and the coarse complex V carbides and the like in the carbonitrided bearing component also serve as a cause of cracking. As a result, the exfoliation life of the carbonitrided bearing component is reduced in a hydrogen-generating environment. Therefore, the C content should be 0.15 to 0.45%. A lower limit of the C content is preferably 0.16%, more preferably 0.17%, and even more preferably 0.18%. An upper limit of the C content is preferably 0.40%, more preferably 0.35%, and even more preferably 0.32%. Si: 0.50% or less

[0044] Silicon (Si) is inevitably included. In other words, a Si content greater than 0%. Si increases the hardenability of the steel material and is further dissolved in the ferrite of the steel material to reinforce the ferrite. This increases the strength of the core portion of the carbonitrided bearing component. However, if the Si content is more than 0.50%, the hardness of the steel material becomes too high, which reduces the machinability of the steel material, even if the contents of the other elements are within the respective ranges according to the present embodiment. Therefore, the Si content should be 0.50% or less. A lower limit of the Si content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. An upper limit of the Si content is preferably 0.40%, more preferably 0.35%, even more preferably 0.32%, and even more preferably 0.30%. Mn: 0.20 to 0.60%

[0045] Manganese (Mn) increases the hardenability of the steel material. This increases the strength of the core region of the carbonitrided bearing component, which increases the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment. When the Mn content is less than 0.20%, the above-described effects are not sufficiently achieved even if the contents of the other elements are within the respective ranges according to the present embodiment. On the other hand, when the Mn content is more than 0.60%, the hardness of the steel material becomes excessively high, thereby reducing the machinability of the steel material, even if the contents of the other elements are within the respective ranges according to the present embodiment.A Mn content of more than 0.60% also allows hydrogen to penetrate into the carbonitrided bearing component when the carbonitrided bearing component is used in a hydrogen-generating environment, which reduces the exfoliation life of the carbonitrided bearing component. Therefore, the Mn content should be 0.20 to 0.60%. A lower limit of the Mn content is preferably 0.22%, more preferably 0.24%, and even more preferably 0.26%. An upper limit of the Mn content is preferably 0.55%, more preferably 0.50%, and even more preferably 0.45%. P: 0.015% or less

[0046] Phosphorus (P) is an unavoidable impurity. In other words, a P content greater than 0% causes P to segregate into grain boundaries, thereby decreasing grain boundary strength. When the P content is more than 0.015%, P segregates into the grain boundaries in an excessive amount, thereby decreasing grain boundary strength, even if the contents of other elements are within the respective ranges according to the present embodiment. As a result, the exfoliation life of the carbonitrided bearing component is reduced under a hydrogen-generating environment. Therefore, the P content should be 0.015% or less. An upper limit of the P content is preferably 0.013%, more preferably 0.010%. The P content is preferably as low as possible. However, excessive reduction of the P content increases production costs.Therefore, taking into account normal industrial production, the lower limit of the P content is preferably 0.001%, particularly preferably 0.002%. S: 0.005% or less

[0047] Sulfur (S) is an unavoidable impurity. In other words, an S content greater than 0% generates sulfide inclusions. Coarse sulfide inclusions may cause cracking when the carbonitrided bearing component is used in a hydrogen-generating environment. If the S content is more than 0.005%, the sulfide-based inclusions will coarsen, reducing the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment, even if the contents of other elements are within the respective ranges according to the present embodiment. Therefore, the S content should be 0.005% or less. An upper limit of the S content is preferably 0.004%, more preferably 0.003%. The S content is preferably as low as possible. However, an excessive reduction in the S content increases production costs.Therefore, taking into account normal industrial production, the lower limit of the S content is preferably 0.001%, particularly preferably 0.002%. Cr: 0.80 to 1.50%

[0048] Chromium (Cr) increases the hardenability of the steel material. This increases the strength of the core region of the carbonitrided bearing component. When contained in combination with V and Mo, Cr further accelerates the formation of small V carbides and the like, as well as small complex V carbides and the like, during the carbonitriding treatment. This not only increases the wear resistance of the carbonitrided bearing component but also the exfoliation life of the carbonitrided component under a hydrogen-generating environment. When the Cr content is less than 0.80%, the above-described effects are not sufficiently achieved. On the other hand, when the Cr content is more than 1.50%, the carburizing characteristics of the carbonitriding treatment are reduced, even if the contents of the other elements are within the respective ranges according to the present embodiment.In this case, sufficient wear resistance of the carbonitrided bearing component is not achieved. Therefore, the Cr content should be 0.80 to 1.50%. A lower limit of the Cr content is preferably 0.85%, more preferably 0.88%, and even more preferably 0.90%. An upper limit of the Cr content is preferably 1.45%, more preferably 1.40%, and even more preferably 1.35%. Mon: 0.17 to 0.30%

[0049] Like Cr, molybdenum (Mo) increases the hardenability of the steel. This increases the strength of the core region of the carbonitrided bearing component. In combination with V and Cr, Mo further accelerates the formation of small V carbides and the like, as well as small complex V carbides and the like, during the carbonitriding treatment. This not only increases the wear resistance of the carbonitrided bearing component but also the spalling life of the carbonitrided bearing component under a hydrogen-generating environment. When the Mo content is less than 0.17%, the above-described effects cannot be sufficiently achieved. On the other hand, when the Mo content is above 0.30%, the strength of the steel material becomes too high. In this case, the machinability of the steel is reduced. Therefore, the Mo content should be 0.17 to 0.30%. A lower limit of the Mo content is preferably 0.18%, more preferably 0.19%, and even more preferably 0.20%.An upper limit of the Mo content is preferably 0.29%, more preferably 0.28%, and even more preferably 0.27%. V: 0.24 to 0.40%

[0050] Vanadium (V) forms small V carbides and the like and small complex V carbides and the like in a manufacturing process of the carbonitrided bearing component using the steel material. Small V carbides and the like and small complex V carbides and the like capture hydrogen that penetrates into the carbonitrided bearing component when the carbonitrided bearing component is used in a hydrogen-generating environment. The equivalent circular diameters of the small V carbides and the like and the small complex V carbides and the like in the carbonitrided bearing component are as small as 150 nm or less. Therefore, the small V carbides and the like and the small complex V carbides and the like resist serving as the origin of a microstructure change even after capturing hydrogen. As a result, the spalling life of the carbonitrided bearing component in a hydrogen-generating environment is increased.Furthermore, V increases the wear resistance of the carbonitrided bearing component by forming small V carbides and the like and small complex V carbides and the like in a manufacturing process of the carbonitrided bearing component. When a V content is less than 0.24%, the above-described effects are not sufficiently achieved. On the other hand, when the V content is more than 0.40%, V carbides and complex V carbides and the like are not completely dissolved in a manufacturing process of the steel material, but partially remain, even if the contents of the other elements are within the respective ranges according to the present embodiment.The remaining V carbides and the like and complex V carbides and the like are also not sufficiently dissolved in a manufacturing process of the carbonitrided bearing component and may grow into coarse V carbides and the like and coarse complex V carbides and the like in the manufacturing process of the carbonitrided bearing component. Coarse V carbides and the like and coarse complex V carbides and the like reduce the toughness of a core portion of a carbonitrided bearing component. Furthermore, coarse V carbides and the like and coarse complex V carbides and the like in a carbonitrided bearing component have poor hydrogen capture performance. Therefore, coarse V carbides and the like and coarse complex V carbides and the like are likely to cause a change in the microstructure during use of the carbonitrided bearing component in a hydrogen-generating environment.Coarse V carbides and the like, and coarse complex V carbides and the like, additionally serve as the origin of a crack. Therefore, coarse V carbides and the like, as well as coarse complex V carbides and the like, reduce the spalling life of a carbonitrided bearing component in a hydrogen-generating environment. Therefore, the V content should be 0.24 to 0.40%. A lower limit of the V content is preferably 0.25%, more preferably 0.26%, and even more preferably 0.27%. An upper limit of the V content is preferably 0.39%, more preferably 0.38%, and even more preferably 0.36%. Al: 0.005 to 0.100%

[0051] Aluminum (Al) deoxidizes steel. If the Al content is less than 0.005%, this effect is not sufficiently achieved even if the contents of the other elements are within the respective ranges according to the present embodiment. On the other hand, if the Al content is more than 0.100%, coarse oxide-based inclusions are generated even if the contents of the other elements are within the respective ranges according to the present embodiment. Coarse oxide-based inclusions serve as a cause of fatigue fracture of the carbonitrided bearing component under a hydrogen-generating environment. As a result, the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment is reduced. Therefore, the Al content should be 0.005 to 0.100%. A lower limit of the Al content is preferably 0.008%, more preferably 0.010%.An upper limit of the Al content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%. The Al content as used herein means a total Al content (Total Al). N: 0.0300% or less

[0052] Nitrogen (N) is an unavoidable impurity. In other words, the N content is greater than 0%. N is dissolved in the steel material and reduces the hot formability of the steel material. If the N content is more than 0.0300%, the hot formability of the steel is significantly reduced. Therefore, the N content should be 0.0300% or less. An upper limit of the N content is preferably 0.0250%, more preferably 0.0200%, even more preferably 0.0150%, and even more preferably 0.0130%. The N content is preferably as low as possible. However, excessive reduction of the N content increases production costs. Therefore, considering normal industrial production, a lower limit of the N content is preferably 0.0001%, even more preferably 0.0002%. O (oxygen): 0.0015% or less

[0053] Oxygen (O) is an unavoidable impurity. In other words, an O content greater than 0%. O combines with other elements in the steel to form coarse oxide-based inclusions. Coarse oxide-based inclusions serve as a cause of fatigue fracture of the carbonitrided bearing component under a hydrogen-generating environment. As a result, the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment is reduced. If the O content is more than 0.0015%, the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment is significantly reduced, even if the contents of the other elements are within the respective ranges according to the present embodiment. Therefore, the O content should be 0.0015% or less. An upper limit of the O content is preferably 0.0013%, more preferably 0.0012%.The O content is preferably as low as possible. However, an excessive reduction in the O content increases production costs. Therefore, taking normal industrial production into account, the lower limit of the O content is preferably 0.0001%, more preferably 0.0002%.

[0054] The remainder of the chemical composition of the steel material according to the present embodiment is Fe and impurities. Impurities here are those that are added from ores and scrap as raw materials or from a production environment during the industrial production of the steel material, and which may be present in the steel material within ranges where the impurities do not have an adverse effect on the steel material according to the present embodiment. [Optional elements]

[0055] The chemical composition of the steel material according to the present embodiment may further contain one or more types of elements selected from the group consisting of Cu, Ni, B, Nb, and Ti instead of a portion of Fe. These elements are optional elements and all increase the strength of the carbonitrided bearing component. Cu: 0 to 0.20%

[0056] Copper (Cu) is an optional element and need not be included. In other words, the Cu content can be 0%. When Cu is included, it increases the hardenability of the steel material. This increases the strength of the steel material and the strength of the core portion of the carbonitrided bearing component. A trace content of Cu brings about the effects described above to some extent. However, if the Cu content is more than 0.20%, the strength of the steel material is excessively increased and the machinability of the steel material is reduced, even if the contents of the other elements are within the respective ranges according to the present embodiment. Therefore, the Cu content should be 0 to 0.20%. A lower limit of the Cu content is preferably more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.05%.An upper limit of the Cu content is preferably 0.18%, more preferably 0.16%, and even more preferably 0.15%. Ni: 0 to 0.20%

[0057] Nickel (Ni) is an optional element and need not be included. In other words, the Ni content may be 0%. When included, Ni increases the hardenability of the steel material. This increases the strength of the steel material and the strength of the core portion of the carbonitrided bearing component. A trace content of Ni brings about the above-described effects to some extent. However, if the Ni content is more than 0.20%, the strength of the steel material is excessively increased and the machinability of the steel material is reduced, even if the contents of the other elements are within the respective ranges according to the present embodiment. Therefore, the Ni content should be 0 to 0.20%. A lower limit of the Ni content is preferably more than 0%, more preferably 0.01%, even more preferably 0.02%, even more preferably 0.03%, and even more preferably 0.05%.An upper limit of the Ni content is preferably 0.18%, more preferably 0.16%, and even more preferably 0.15%. B: 0 to 0.0050%.

[0058] Boron (B) is an optional element and is not required. In other words, the B content can be 0%. When included, B increases the hardenability of the steel material. This increases the strength of the steel material and the strength of the core region of the carbonitrided bearing component. In addition, B prevents the segregation of P in grain boundaries. A trace content of B causes the effects described above to some extent. However, if the B content is more than 0.0050%, B nitride (BN) is formed, which reduces the toughness of the core region of the carbonitrided bearing component. Therefore, the B content should be 0 to 0.0050%. A lower limit of the B content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, even more preferably 0.0005%, and even more preferably 0.0010%. An upper limit of the content of B is preferably 0.0030%, more preferably 0.0025%, and even more preferably 0.0020%. Nb: 0 to 0.100%

[0059] Niobium (Nb) is an optional element and is not required. In other words, the Nb content can be as low as 0%. When included, Nb combines with C and N in the steel to form its carbides, nitrides, and carbonitrides. These precipitates exert precipitation hardening, which increases the strength of the carbonitrided bearing component. A trace amount of Nb will, to some extent, produce the effects described above. However, if the Nb content exceeds 0.100%, the toughness of the core portion of the carbonitrided bearing component will be reduced. Therefore, the Nb content should be 0 to 0.100%. The lower limit of the Nb content is preferably greater than 0%, more preferably 0.005%, and even more preferably 0.010%. An upper limit of the content of Nb is preferably 0.080%, more preferably 0.070%, even more preferably 0.050%, and even more preferably 0.040%. Ti: 0 to 0.100%

[0060] Titanium (Ti) is an optional element and is not required. In other words, the Ti content can be 0%. When included, Ti, like Nb, forms its carbide, nitride, and carbonitride, thereby increasing the strength of the carbonitrided bearing component. A trace amount of Ti included will produce the effects described above to some extent. However, if the Ti content is more than 0.100%, the toughness of the core portion of the carbonitrided bearing component decreases. Therefore, the Ti content should be 0 to 0.100%. A lower limit of the Ti content is preferably more than 0%, more preferably 0.005%, and even more preferably 0.010%. An upper limit of the Ti content is preferably 0.080%, more preferably 0.070%, even more preferably 0.050%, and even more preferably 0.040%.

[0061] The chemical composition of the steel material according to the present embodiment may further contain Ca instead of a part of Fe. Ca: 0 bis 0.0010 %.

[0062] Calcium (Ca) is an optional element and is not required. In other words, the Ca content can be as low as 0%. When Ca is present, it dissolves in inclusions in the steel material and refines and spheroidizes into sulfides. In this case, the hot formability of the steel material is enhanced. A trace amount of Ca will produce the effects described above to a certain extent. However, if the Ca content exceeds 0.0010%, coarse oxide-based inclusions will form in the steel material. If coarse oxide-based inclusions trap hydrogen during the service of the carbonitrided bearing component in a hydrogen-generating environment, a microstructure change may occur. The occurrence of a microstructure change will reduce the exfoliation life of the carbonitrided bearing component. Therefore, the Ca content should be between 0 and 0.0010%.A lower limit of the Ca content is preferably more than 0%, more preferably 0.0001%, and even more preferably 0.0003%. An upper limit of the Ca content is preferably 0.0009% and even more preferably 0.0008%. [Formula (1) to Formula (4)]

[0063] The chemical composition of the steel material according to the present embodiment additionally satisfies the following formula (1) to formula (4): 1.50<0.4Cr+0.4Mo+4.5V<2.45 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 Mo / V≥0.58 (Mo+V+Cr) / (Mn+20P)≥2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass%). [Formula 1)]

[0064] The chemical composition of the steel material according to the present embodiment satisfies the formula (1): 1.50<0.4Cr+0.4Mo+4.5V<2.45 where the symbols of the elements in formula (1) are to be replaced by the contents of the corresponding elements (mass%).

[0065] Let F1 be defined as F1 = 0.4Cr + 0.4Mo + 4.5V. F1 is an index related to the production of small V carbides and the like (small V carbides and small V carbonitrides) and small complex V carbides and the like (small complex V carbides and small complex V carbonitrides), which capture hydrogen to increase the spalling life of the carbonitrided bearing component under a hydrogen-generating environment. As described above, the production of small V carbides and the like and small complex V carbides and the like is accelerated by containing both V, Cr, and Mo. Cr produces Fe-based carbide such as cementite or Cr carbide in a temperature range lower than a temperature range in which V carbides and the like and complex V carbides and the like are produced.Mo produces Mo carbide (Mo2C) in a temperature range lower than the temperature range where V carbides and the like and complex V carbides and the like are produced. As the temperature rises, the Fe-based carbide, the Cr-based carbide, and the Mo carbide dissolve to serve as nucleation sites for the precipitation of V carbides and the like and complex V carbides and the like.

[0066] When F1 is 1.50 or less, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), Cr and Mo are insufficient, and thus the nucleation sites of precipitation for V carbides and the like and complex V carbides and the like become insufficient. Otherwise, the V content for producing V carbides and the like and complex V carbides and the like itself becomes insufficient with respect to the Cr content and the Mo content. As a result, small V carbides and the like and small complex V carbides and the like are not sufficiently produced.On the other hand, when F1 is 2.45 or more, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), coarse V carbides and the like and coarse complex V carbides and the like are produced. In this case, the V carbides and the like and the complex V carbides and the like are not sufficiently dissolved in a manufacturing process of the steel material, but partially remain in the steel material. Therefore, the V carbides and the like and complex V carbides and the like remaining in the steel material grow into coarse V carbides and the like and coarse complex V carbides and the like in a manufacturing process of the carbonitrided bearing component. Coarse V carbides and the like and coarse complex V carbides and the like have poor hydrogen capture performance.Therefore, coarse V-carbides and the like, as well as coarse complex V-carbides and the like, tend to cause microstructure changes during use of the carbonitrided bearing component in a hydrogen-generating environment. Coarse V-carbides and the like, as well as coarse complex V-carbides and the like, also serve as the origin of cracks. As a result, the exfoliation life of the carbonitrided bearing component in a hydrogen-generating environment is reduced.

[0067] When F1 is greater than 1.50 and less than 2.45, provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (2) to (4), small V carbides and small complex V carbides are sufficiently generated in a resulting carbonitrided bearing component, and V carbides and the like and complex V carbides and the like are sufficiently dissolved in a resulting steel material. Thus, a microstructure change attributable to hydrogen cracking is not expected during use of the carbonitrided bearing component in a hydrogen-generating environment. As a result, the spalling life of the carbonitrided bearing component in a hydrogen-generating environment is increased.

[0068] A lower limit of F1 is preferably 1.51, more preferably 1.52, even more preferably 1.54, even more preferably 1.55, and even more preferably 1.56. An upper limit of F1 is preferably 2.44, more preferably 2.43, and even more preferably 2.42. A numerical value of F1 shall be a value obtained by rounding F1 to the third decimal place. [Formula (2)]

[0069] The chemical composition of the steel material according to the present embodiment further satisfies the formula (2): 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 where the symbols of the elements in formula (2) are to be replaced by the contents of the corresponding elements (mass%).

[0070] F2 is defined as F2 = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo + V. The elements listed in F2 each increase the hardenability of the steel. F2 is thus an index of the strength of the core area of ​​the carbonitrided bearing component.

[0071] When F2 is 2.20 or less, the hardenability of the resulting steel material is insufficient, even if the contents of the elements in the chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4). Therefore, the strength of the core portion of the carbonitrided bearing component is insufficient. In this case, a sufficient exfoliation life of the carbonitrided bearing component is not achieved under a hydrogen-generating environment. On the other hand, when F2 is 2.80 or more, even if the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4), the hardenability of the steel material becomes excessively high. In this case, in a microstructure of the steel material, a total area ratio of ferrite and pearlite is less than 10.0%.Thus, sufficient workability of the steel is not achieved.

[0072] When F2 is more than 2.20 and less than 2.80, provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (3), and (4), sufficient machinability of the steel material is achieved. Moreover, the strength of a core portion of a resulting carbonitrided bearing component is sufficiently increased, and the exfoliation life of the carbonitrided bearing component under a hydrogen-generating environment is sufficiently increased. A lower limit of F2 is preferably 2.23, more preferably 2.25, even more preferably 2.30, even more preferably 2.35, and even more preferably 2.45. An upper limit of F2 is preferably 2.78, more preferably 2.75, even more preferably 2.73, and even more preferably 2.70. A numeric value of F2 shall be a value obtained by rounding F2 to the third decimal place. [Formula (3)]

[0073] The chemical composition of the steel material according to the present embodiment further satisfies the formula (3). Mo / V≥0.58 where the symbols of the elements in formula (3) are to be replaced by the contents of the corresponding elements (mass%).

[0074] Let F3 be defined as F3 = Mo / V. In the steel material according to the present embodiment as described above, F1 satisfying formula (1) enables the provision of a total of a V content, a Cr content, and a Mo content necessary to produce small V carbides and the like and small complex V carbides and the like. However, the production of sufficiently small V carbides and the like and small complex V carbides and the like still requires adjustment of the ratio of the Mo content to the V content. In particular, if the ratio of the Mo content to the V content (= Mo / V) is too low, the Mo carbides serving as nucleation sites for precipitation do not sufficiently precipitate before producing V carbides and the like and complex V carbides and the like.In this case, even if a V content, a Cr content, and a Mo content fall within the respective ranges according to the present embodiment and satisfy formula (1), small V carbides and the like and small complex V carbides and the like are not sufficiently produced. In particular, when F3 is less than 0.58, even if the contents of elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formula (1), formula (2), and formula (4), small V carbides and the like and small complex V carbides and the like are not sufficiently produced in a resulting carbonitrided bearing component. As a result, a sufficient exfoliation life of the carbonitrided bearing component is not achieved under a hydrogen-generating environment.

[0075] Provided that the contents of elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1), (2), and (4), when F3 is 0.58 or more, that is, formula (3) is satisfied, small V carbides and the like and small complex V carbides and the like are sufficiently generated in a resulting carbonitrided bearing component. As a result, the spalling life of the carbonitrided bearing component under a hydrogen-generating environment is sufficiently increased. A lower limit of F3 is preferably 0.60, more preferably 0.65, even more preferably 0.68, even more preferably 0.70, even more preferably 0.73, and even more preferably 0.76. A numerical value of F3 shall be a value obtained by rounding F3 to the third decimal place. [Formula (4)]

[0076] The chemical composition of the steel material according to the present embodiment further satisfies the formula (4): (Mo+V+Cr) / (Mn+20P)≥2.40 where the symbols of the elements in formula (4) are to be replaced by the contents of the corresponding elements (mass%).

[0077] Let F4 be defined as F4 = (Mo+V+Cr) / (Mn+20P). Small V carbides and the like, as well as small complex V carbides and the like, not only capture hydrogen but also exert precipitation strengthening to strengthen the interior of the grains. If the small V-based precipitates can simultaneously strengthen the grain boundaries in a carbonitrided bearing component under a hydrogen-generating environment, and in addition, hydrogen penetration can be prevented or reduced, the exfoliation life of the carbonitrided bearing component under the hydrogen-generating environment can be further increased through a synergistic effect of three effects: (a) intragranular strengthening, (b) grain boundary strengthening, and (c) hydrogen penetration prevention. The intragranular strengthening indicated as (a) depends on a total of a content of Mo, a content of V, and a content of Cr, as described above.Meanwhile, for grain boundary strengthening, indicated as (b), it is effective to reduce the P content, which is particularly likely to segregate into grain boundaries under the chemical composition described above. Furthermore, for preventing hydrogen penetration, indicated as (c), it is extremely effective to reduce the Mn content in the steel material.

[0078] The numerator in F4 (= (Mo+V+Cr)) is an index of intragranular strengthening (equivalent to (a) described above). The denominator in F4 (= (Mn+20P)) is an index of grain boundary embrittlement and hydrogen penetration (equivalent to (b) and (c) described above). A large denominator in F4 means that grain boundary strength is weak or that hydrogen can penetrate into a resulting carbonitrided bearing component. Therefore, even if an intragranular strengthening index (the numerator in F4) is large, if the grain boundary embrittlement and hydrogen penetration index (the denominator in F4) is large, a synergistic effect of an intragranular strengthening mechanism, a grain boundary strengthening mechanism, and a hydrogen penetration prevention mechanism is not achieved, and therefore, a spalling life of the carbonitrided bearing component under a hydrogen-generating environment is not sufficiently improved.

[0079] Provided that the contents of the elements in a chemical composition fall within the respective ranges according to the present embodiment and satisfy formulas (1) to (3), when F4 is 2.40 or more, the synergistic effect of the intragranular strengthening mechanism, the grain boundary strengthening mechanism, and the hydrogen permeation prevention mechanism is achieved, and a sufficient exfoliation life of a resulting carbonitrided bearing component under a hydrogen-generating environment is achieved. A lower limit of F4 is preferably 2.42, more preferably 2.45, even more preferably 2.47, even more preferably 2.50, and even more preferably 2.52. A numerical value of F4 shall be a value obtained by rounding F4 to the third decimal place. [Microstructure of steel materials]

[0080] A microstructure of the steel material according to the present embodiment means a microstructure in which a total area ratio of ferrite and pearlite is 10.0% or more, and the remainder is bainite. In a case where a C content in a chemical composition is low, in the microstructure of the steel material according to the present embodiment, a total area ratio of ferrite and pearlite is 50.0% or more, and the remainder is bainite. In contrast, in a case where a C content in a chemical composition is high, in the microstructure of the steel material according to the present embodiment, an area ratio of bainite is 50.0% or more, and the remainder is ferrite and pearlite. In the microstructure of the steel material according to the present embodiment, the total area ratio of ferrite and pearlite is at least 10.0% or more, and the remainder is bainite, regardless of variations in the C content.It is preferable that, in the microstructure of the steel material according to the present embodiment, a total area ratio of ferrite and pearlite is 10.0 to 90.0%, and the remainder is bainite. An upper limit of the total area ratio of ferrite and pearlite is preferably 80.0%, and more preferably 78.0%. A lower limit of the total area ratio of ferrite and pearlite is preferably 20.0%, and more preferably 25.0%. Note that, in the microstructure of the steel material, regions other than bainite, ferrite, and pearlite include, for example, retained austenite, precipitates (including cementite), and inclusions; however, a total area ratio of retained austenite, precipitates, and inclusions in the microstructure is negligibly small. [Method for measuring area fractions of ferrite and pearlite]

[0081] The total area ratio (%) of ferrite and pearlite in the microstructure of the steel material according to the present embodiment and the area ratio (%) of bainite in the microstructure are measured according to the following method. A sample is taken from a center position of a radius R connecting a surface and a center axis (R / 2 position) of a cross section of a steel material, which is a steel bar or wire rod, perpendicular to a longitudinal direction (axial direction) of the steel material (hereinafter referred to as a cross section). From the surfaces of the taken sample, a surface corresponding to the cross section is determined as an observation surface. The observation surface is mirror-polished and then etched with 2% nitric acid-alcohol etching (Nital etching).The etched observation surface is viewed under a light microscope at 500x magnification, and photographic images of 20 freely selected fields of view are taken on the etched observation surface. Each field of view is set to 100 µm × 100 µm.

[0082] In each field of view, phases such as bainite, ferrite, and pearlite have their own distinct contrasts. Therefore, the phases are identified based on their respective contrasts. Of the identified phases, a total area of ​​ferrite (µm 2 ) and a total area of ​​pearlite (µm 2). The ratio of the total area of ​​ferrite and pearlite in all fields of view to the total area of ​​all fields of view is defined as the total area fraction (%) of ferrite and pearlite. A bainite area fraction (%) is determined from the total area fraction of ferrite and pearlite using the following method. Bainite area fraction = 100.0 - total area fraction of ferrite and pearlite

[0083] The total area ratio (%) of ferrite and pearlite is a value obtained by rounding the total area ratio (%) of ferrite and pearlite up or down to the second decimal place. [V content in electrolytic extraction residues of steel material]

[0084] Further, in the steel material according to the present embodiment, assuming that [V] Rdenotes a content of V in the electrolytic extraction residue of the steel material and [V]c denotes a content of V in the chemical composition of the steel material, a proportion of the V content in the residue RA V , which is defined by formula (A) shown below, is 10.0% or less. RAV=[V]R / [V]C×100

[0085] In the steel material according to the present embodiment, V carbides and the like and complex V carbides and the like are sufficiently dissolved, and the amount of remaining V carbides and the like and complex V carbides and the like is sufficiently small. Specifically, the proportion of V content in the residue RA V , which is defined by formula (A), 10.0% or less.

[0086] If the proportion of V content in the residue RA Vis more than 10.0%, V carbides and the like and complex V carbides and the like are not sufficiently dissolved in the steel material. In this case, the V carbides and the like and complex V carbides and the like remaining in the steel material grow into coarse V carbides and the like and coarse complex V carbides and the like during a manufacturing process of a carbonitrided bearing component using the steel material as a raw material. Coarse V carbides and the like and coarse complex V carbides and the like have poor hydrogen capture performance. Therefore, coarse V carbides and the like and coarse complex V carbides and the like may cause a microstructure change during use of the carbonitrided bearing component in a hydrogen-generating environment. If a microstructure change occurs, the spalling life of the carbonitrided bearing component in a hydrogen-generating environment decreases.

[0087] If the proportion of V content in the residue RA V 10.0% or less, V carbides and the like, as well as complex V carbides and the like, are sufficiently dissolved in the steel material. As a result, a reduction in the exfoliation life of the carbonitrided bearing component under a hydrogen-generating environment due to coarse V carbides and the like, and coarse complex V carbides and the like, is prevented or reduced. An upper limit of the proportion of V content in the residue RA V is preferably 9.5%, more preferably 9.2%, even more preferably 9.0%, even more preferably 8.5%, even more preferably 8.3%, even more preferably 8.0%, even more preferably 7.5%, even more preferably even 7.0%, even more preferably even 6.5% and even more preferably even 6.0%. [Method for determining the proportion of V content in the residue RAV]

[0088] The V content in the electrolytic extraction residue of the steel material according to the present embodiment can be measured by the following method. First, precipitates and inclusions in the steel material are detected as residues. Cylindrical samples each with a diameter of 6 mm and a length of 50 mm are taken from the steel material. Specifically, three of the above-described cylindrical samples are taken from an R / 2 position of a cross section of the steel material perpendicular to a longitudinal direction (axial direction) of the steel material (hereinafter referred to as the cross section). A surface of each of the taken cylindrical samples is subjected to preliminary electropolishing to polish it by approximately 50 μm, thereby obtaining a new surface. The electropolished cylindrical samples are electrolyzed with an electrolyte (10% acetylacetone + 1% tetraammonium + methanol).After electrolysis, the residues are collected by passing the electrolyte through a 0.2 µm filter. The obtained electrolytic residues are subjected to acid decomposition, and inductively coupled plasma (ICP) optical emission spectrometry is performed to determine the V content in the electrolytic residues in mass%, based on the steel material (base metal), which is assumed to be 100 mass%. An arithmetic mean of the V contents in the electrolytic residue of the cylindrical samples (i.e., an arithmetic mean of three V contents) is defined as the V content in the electrolytic residue of the steel material, [V]. R . The V content in the electrolytic extraction residue, [V] R, is a value obtained by rounding the arithmetic mean described above to the second decimal place. Using the V content in the chemical composition of the steel material, [V]c, and the V content in the electrolytic extraction residue, [V] R , which is obtained by the measurement described above, the proportion of V content in the residue, RA V , determined by formula (A). The proportion of V content in the residue RA V is a value obtained by rounding down the proportion of V content in the residue RA V to the second decimal place. RAV=[V]R / [V]C×100

[0089] In the steel material according to the present embodiment having the above-described configuration, the element contents fall within the above-described respective ranges according to the present embodiment, and F1 to F4 satisfy the formulas (1) to (4). Furthermore, in a microstructure of the steel material, a total area ratio of ferrite and pearlite is 10.0% or more, the remainder is bainite, and furthermore, a ratio of V content in the residue is RA. V10.0% or less. For these reasons, the steel material according to the present embodiment has excellent machinability. Furthermore, a carbonitrided bearing component obtained by performing hot forging and subsequent carbonitriding treatment on the steel material according to the present embodiment offers excellent wear resistance and toughness of its core region, and furthermore, the carbonitrided bearing component offers excellent spalling life under a hydrogen-generating environment. [Method for producing steel material according to the present embodiment]

[0090] An example of a method for manufacturing the steel material according to the present embodiment will be described. The steel material manufacturing method described below is an example of manufacturing the steel material as a starting material for a carbonitrided bearing component according to the present embodiment. Therefore, the steel material having the above-described configuration can be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferable example of the method for manufacturing the steel material according to the present embodiment.

[0091] The example of the method for producing the steel material according to the present embodiment includes a steelmaking process in which molten steel is refined and the molten steel is cast to produce a raw material (casting), and a hot-working process in which hot-working the raw material is performed to produce the steel material. Each of these processes is described below. [Steelmaking process]

[0092] In the steelmaking process, molten steel is first produced, containing the above-described chemical composition, in which the element contents fall within the respective ranges according to the present embodiment, and F1 to F4 satisfy Formula (1) to Formula (4). A refining method is not limited to a specific method, and a known refining method may be used. For example, molten iron produced by a known method is subjected to refining in a converter (primary refining). The molten steel drawn from the converter is subjected to known secondary refining.In secondary refining, alloying elements for component formulation are added to produce a molten steel containing a chemical composition in which the contents of elements fall within the respective ranges according to the present embodiment and F1 to F4 satisfy the formulas (1) to (4).

[0093] Using the molten steel produced by the refining process described above, a starting material is produced by a known casting process. For example, an ingot is produced using the molten steel by an ingot production process. Alternatively, a bloom or billet can be produced using the molten steel by a continuous casting process. The starting material (bloom or ingot) is produced by the above process. [Hot forming process]

[0094] In the hot forming process, the starting material (bloom or ingot) produced in the steelmaking process undergoes hot forming to become steel material. The steel material is a steel bar or wire rod.

[0095] The hot forming process includes a rough rolling process and a finish rolling process. In the rough rolling process, the starting material is hot formed to process it into a billet. For example, a rough rolling mill is used in the rough rolling process. The starting material is rolled into a billet by the rough rolling mill. If a continuous rolling mill is installed downstream of the rough rolling mill, the billet produced by rough rolling can be further hot rolled into a smaller billet by the continuous rolling mill. In the continuous rolling mill, horizontal stands, each containing a pair of horizontal rolls, and vertical stands, each containing a pair of vertical rolls, are arranged alternately in a row. Through the process described above, the starting material is processed into a billet in the rough rolling process.

[0096] In the rough rolling process, a heating temperature and a residence time in a reheating furnace should be as follows. Heating temperature: 1150 to 1300° C

[0097] Retention time at the above heating temperature: 1.5 to 10.0 hours. The heating temperature is the furnace temperature (°C) of the reheating furnace. The residence time is the residence time (hours) for which the furnace temperature of the reheating furnace is set at 1150 to 1300°C.

[0098] If the heating temperature is less than 1150°C or the residence time for which the heating temperature is set at 1150 to 1300°C is less than 1.5 hours, V carbides and complex V carbides in the starting material are not sufficiently dissolved. As a result, the proportion of V content in the residue RA V in the steel material is more than 10.0%. On the other hand, if the heating temperature exceeds 1300°C or the residence time at 1150 to 1300°C exceeds 10.0 hours, the unit demand becomes excessive, increasing production costs.

[0099] When the heating temperature of the rough rolling process is 1150 to 1300°C and the residence time at 1150 to 1300°C is 1.5 to 10.0 hours, V carbides and the like and complex V carbides and the like are sufficiently dissolved in the starting material.

[0100] In the finish rolling process, the billet is first heated in a reheating furnace. The heated billet is then hot-rolled in a continuous rolling mill and processed into a steel bar or wire rod made from the steel material. In the finish rolling process, the heating temperature and residence time in the reheating furnace are selected as follows. Heating temperature: 1150 to 1300° C

[0101] Retention time at the above heating temperature: 1.5 to 5.0 hours. The heating temperature is the furnace temperature (°C) of the reheating furnace. The residence time is the residence time (hours) for which the furnace temperature of the reheating furnace is set at 1150 to 1300°C.

[0102] In the finish rolling process, the precipitation of V-carbides and the like and complex V-carbides and the like is prevented or reduced as much as possible. When the heating temperature in the reheating furnace during the finish rolling process is less than 1150°C or the residence time for 1150 to 1300°C is less than 1.5 hours, a load applied to a rolling mill during finish rolling becomes excessively heavy. On the other hand, when the heating temperature exceeds 1300°C or the residence time for 1150 to 1300°C is more than 5 hours, the unit consumption becomes excessively high, increasing production costs.

[0103] When the heating temperature is 1150 to 1300°C and the residence time at 1150 to 1300°C is 1.5 to 5.0 hours in the finish rolling process, V carbides and the like and complex V carbides and the like are sufficiently dissolved in the starting material.

[0104] The steel material subjected to finish rolling is cooled at a cooling rate not higher than that allowing cooling of the steel material according to the present embodiment. Preferably, an average cooling rate CR for a temperature range in which a temperature of the steel material subjected to finish rolling is 800°C to 500°C is set to 0.1 to 5.0°C / sec. When the temperature of the steel material is 800 to 500°C, a phase transformation from austenite to ferrite, pearlite, or bainite occurs. When the average cooling rate CR for the temperature range in which the temperature of the steel material is 800°C to 500°C is 0.1 to 5.0°C / sec, the microstructure becomes one in which a total area ratio of ferrite and pearlite is 10.0% or more, and the remainder is bainite.

[0105] The average cooling rate CR is measured using the following method. The steel material undergoing finish rolling is transported downstream on a conveyor line. A plurality of thermometers are arranged along the conveyor line to measure the temperature of the steel material at different positions along the conveyor line. Based on the measurement results of the multiple thermometers arranged along the conveyor line, the time required for the temperature of the steel material to drop from 800°C to 500°C is determined, and then the average cooling rate CR (°C / sec) is determined. The average cooling rate CR can be adjusted, for example, by arranging a plurality of slow cooling covers spaced apart along the conveyor line.

[0106] By the above manufacturing method, the steel material according to the present embodiment having the above-described configuration can be manufactured. [Carbonitrided bearing component]

[0107] The steel material according to the present embodiment is used for a carbonitrided bearing component. The carbonitrided bearing component is a bearing component that undergoes carbonitriding treatment. Carbonitriding treatment here refers to a treatment that includes carbonitriding, quenching, and tempering.

[0108] A bearing component is a component of a rolling bearing. Examples of bearing components include a race, a bearing washer, and a rolling element. The race can be an inner ring or an outer ring, and the bearing washer can be a shaft washer, a housing washer, a center washer, or an aligning housing washer. The race and bearing washer are not limited to a specific race and bearing washer, as long as the race and bearing washer are elements that each have a raceway. The rolling element can be a ball or a roller. Examples of the roller include a cylindrical roller, a long cylindrical roller, a needle roller, a tapered roller, and a convex roller.

[0109] A carbonitrided bearing component includes a carbonitrided layer formed in its outer layer by the carbonitriding treatment and a core region located inside the carbonitrided layer. The depth of the carbonitrided layer is not limited to a specific depth; however, an example of the depth from a surface of the carbonitrided layer is 0.2 mm to 5.0 mm. The core region has the same chemical composition as the chemical composition of the steel material according to the present embodiment. [Process for producing a carbonitrided bearing component.]

[0110] An example of a method for manufacturing a carbonitrided bearing component having the above-described configuration is as follows. First, the steel material according to the present embodiment, which is to be a starting material for the carbonitrided bearing component, is machined into a predetermined shape to produce an intermediate product. A method for the machining is, for example, hot forging or machining. The machining is, for example, machining. It is sufficient to perform hot working under known conditions. In hot forging, the heating temperature of the steel material is, for example, 1000 to 1300°C. The intermediate product subjected to hot forging is allowed to cool. After hot forging, a machining process can be performed. The steel material or the intermediate product can be subjected to the known spheroidizing process before the machining process.For machining, it is advantageous that the steel material (the intermediate product) has high machinability. The steel material according to the present embodiment described above has excellent machinability. Therefore, the steel material according to the present embodiment is suitable for machining.

[0111] The resulting intermediate product undergoes carbonitriding treatment to become a carbonitrided bearing component. The carbonitriding treatment includes carbonitriding, quenching, and tempering, as described above. During carbonitriding and quenching, the intermediate product is heated and maintained at a carbonitriding temperature not lower than the Ac3 transformation point in a known atmospheric gas containing a known converted carburizing gas and ammonia gas, and then rapidly cooled. During tempering, the intermediate product subjected to carbonitriding and quenching is maintained at a tempering temperature of 100 to 500°C for a predetermined time. The converted carburizing gas here refers to a known endothermic converted gas (RX gas).The RX gas is a gas produced by mixing a hydrocarbon gas such as butane and propane with air and passing it through a heated Ni catalyst to react with each other; the RX gas is a gaseous mixture containing CO, H2, N2, and the like.

[0112] The surface C concentration, the surface N concentration, and the surface hardness of the carbonitrided bearing component can be adjusted by controlling the conditions for carbonitriding, quenching, and tempering. Specifically, the surface C concentration and the surface N concentration are adjusted by controlling the carbon potential, ammonia concentration, and the like in the atmospheric gas during carbonitriding and quenching.

[0113] In particular, the surface C concentration of the carbonitrided bearing component is mainly controlled by the carbon potential during carbonitriding and quenching, the carbonitriding temperature, and the residence time at the carbonitriding temperature. The surface C concentration increases with an increase in the carbon potential, an increase in the carbonitriding temperature, and an increase in the residence time at the carbonitriding temperature. In contrast, the surface C concentration decreases with a decrease in the carbon potential, a decrease in the carbonitriding temperature, and a decrease in the residence time at the carbonitriding temperature.

[0114] The surface N concentration is mainly controlled by the ammonia concentration during carbonitriding and quenching, the carbonitriding temperature, and the residence time at the carbonitriding temperature. The surface N concentration increases with an increase in the ammonia concentration, a decrease in the carbonitriding temperature, and an increase in the residence time at the carbonitriding temperature. On the other hand, the surface N concentration decreases with a decrease in the ammonia concentration, an increase in the carbonitriding temperature, and a decrease in the residence time at the carbonitriding temperature.

[0115] Surface hardness is related to the surface concentration of C and the surface concentration of N. In particular, surface hardness increases with increasing surface concentration of C and the surface concentration of N. On the other hand, surface hardness decreases with decreasing surface concentration of C and the surface concentration of N.

[0116] Increased surface hardness due to carbonitriding and quenching can be reduced by tempering. The surface hardness of a carbonitrided bearing component is reduced by increasing the tempering temperature and extending the residence time at the tempering temperature. The surface hardness of a carbonitrided bearing component can be maintained at a high level by lowering the tempering temperature and shortening the residence time at the tempering temperature.

[0117] Preferred conditions for carbonitriding and quenching are as follows. Carbon potential CP in atmospheric gas: 0.70 to 1.40

[0118] When the carbon potential CP in the atmospheric gas is 0.70 or more, the carbon concentration on the surface of the carbonitrided bearing component is sufficiently increased; for example, the surface carbon concentration is increased to 0.70% or more by mass. In this case, carbonitriding treatment produces sufficient amounts of carbonitrides, significantly increasing wear resistance. When the carbon potential CP is 1.40 or less, the surface carbon concentration is increased to 1.20% or less, and the formation of coarse carbonitrides is sufficiently prevented or reduced. Therefore, a carbon potential CP of 0.70 to 1.40 is preferable.

[0119] Concentration of ammonia relative to the flow of converted carburizing gas in the atmosphere: 1.00 to 6.00%.

[0120] An ammonia concentration relative to a converted carburizing gas flow in the atmosphere means an ammonia concentration (mass %) relative to the converted carburizing gas flow, which is assumed to be 100%. When the ammonia concentration relative to the converted carburizing gas flow is 1.00% or more, the surface N concentration of the carbonitrided bearing component is sufficiently increased, and the surface N concentration becomes 0.15% or more. In this case, carbonitriding treatment produces carbonitrides in sufficient quantity, which significantly increases wear resistance. When the ammonia concentration relative to the converted carburizing gas flow is 6.00% or less, the surface N concentration of the carbonitrided bearing component becomes 0.60% or less. In this case, the formation of coarse carbonitrides is sufficiently prevented or reduced.Therefore, the concentration of ammonia, based on the flow of converted carburizing gas in the atmosphere, should be 1.00 to 6.00%. Residence temperature during carbonitriding (carbonitriding temperature): 830 to 930 °C Residence time at carbonitriding temperature: 30 to 100 minutes

[0121] If the carbonitriding temperature is too low, the diffusion rates of C and N become low. In this case, the treatment time required to achieve the specified heat treatment properties increases, which increases production costs. On the other hand, if the carbonitriding temperature is too high, ammonia in the atmosphere decomposes, reducing the amount of N penetrating the steel material. Furthermore, the solubilities of C and N penetrating the steel material matrix increase. As a result, carbonitrides are not formed in sufficient quantities, which reduces the wear resistance of the carbonitrided bearing component. The carbonitriding temperature should therefore be between 830 and 930°C.

[0122] The residence time at the carbonitriding temperature is not limited to a specific time, as long as a sufficient concentration of C and a sufficient concentration of N is maintained on the surface of the steel material. The residence time is, for example, 30 to 100 minutes. Quenching temperature: 830 to 930 °C

[0123] If the quenching temperature is too low, carbon will not be sufficiently dissolved in the steel, resulting in a decrease in the steel's hardness. On the other hand, if the quenching temperature is too high, the grains will coarsen, allowing coarse carbonitrides to precipitate along the grain boundaries. Therefore, the quenching temperature should be 830 to 930 °C. Note that the carbonitriding temperature can also be used as a carburizing quenching temperature. Preferred tempering conditions are as follows. Tempering temperature: 150 to 200° C Retention time at tempering temperature: 30 to 240 minutes

[0124] A tempering temperature that is too low does not ensure sufficient toughness of the core area of ​​the carbonitrided bearing component. On the other hand, a tempering temperature that is too high reduces the surface hardness of the carbonitrided bearing component, thereby reducing its wear resistance. Therefore, the tempering temperature should be between 150 and 200 °C.

[0125] A residence time at the tempering temperature that is too short will not result in sufficient toughness in the core area. On the other hand, a residence time that is too long will reduce the surface hardness, thereby reducing the wear resistance of the carbonitrided bearing component. Therefore, the residence time at the tempering temperature should be between 30 and 240 minutes. [C concentration, N concentration and Rockwell hardness C scale of the surface of the carbonitrided bearing component]

[0126] The C concentration, N concentration and Rockwell hardness C scale HRC of a surface of the carbonitrided bearing component produced by the above manufacturing method are as follows. Concentration of C of the surface: 0.70 to 1.20% in mass%

[0127] The C concentration of a surface of the carbonitrided bearing component produced by subjecting the steel material according to the present embodiment to carbonitriding and quenching and tempering under the conditions described above is 0.7 to 1.2%. If the C concentration of the surface is too low, the surface hardness becomes too low, and the wear resistance is reduced. On the other hand, if the C concentration of the surface is too high, coarse carbides, coarse carbonitrides, and the like are generated, which reduces the spalling life under a hydrogen-generating environment. When the C concentration of the surface is 0.70 to 1.20%, the carbonitrided bearing component is excellent in wear resistance and spalling life under a hydrogen-generating environment. A lower limit of the C concentration of the surface is preferably 0.75%, and more preferably 0.80%.An upper limit of the C concentration of the surface is preferably 1.10%, more preferably 1.05%, and even more preferably 1.00%. N concentration of the surface: 0.15 to 0.60% in mass%

[0128] The N concentration of the surface of the carbonitrided bearing component produced by carbonitriding and quenching and tempering the steel material according to the present embodiment under the conditions described above is 0.15 to 0.60%. If the surface N concentration is too low, the formation of fine carbonitrides is prevented or reduced, and thus the wear resistance of the carbonitrided bearing component is reduced. Conversely, if the surface N concentration is too high, excess retained austenite is formed. This leads to a decrease in the surface hardness of the carbonitrided bearing component, thereby reducing the spalling life under a hydrogen-generating environment. When the surface N concentration is 0.15 to 0.60%, the carbonitrided bearing component is excellent in wear resistance and spalling life in a hydrogen-generating environment.A lower limit of the surface N concentration is preferably 0.18%, more preferably 0.20%. An upper limit of the surface N concentration is preferably 0.58%, more preferably 0.56%, and even more preferably 0.54%.

[0129] The C and N concentrations of the surface are measured using the following method. Using an electron probe microanalyzer (EPMA), the C and N concentrations are measured at a freely selectable surface position of the carbonitrided bearing component, from the surface to a depth of 100 µm with a pitch of 1.0 µm. An arithmetic mean of the measured C concentrations is defined as the surface C concentration (mass%). Analogously, an arithmetic mean of the measured N concentrations is defined as the surface N concentration (mass%). Rockwell hardness C-scale HRC of the surface: 58 to 65

[0130] The Rockwell hardness C-scale HRC of the surface of the carbonitrided bearing component should be 58 to 65. If the Rockwell hardness C-scale HRC of the surface is less than 58, the wear resistance of the carbonitrided bearing component will decrease. Conversely, if the Rockwell hardness C-scale of the surface is above 65, fine cracks are easy to form and propagate, and the spalling life in a hydrogen-generating environment will be reduced. If the Rockwell hardness C-scale of the surface is 58 to 65, excellent wear resistance and excellent spalling life under a hydrogen-generating environment are achieved. A lower limit of the Rockwell hardness C-scale of the surface is preferably 59. An upper limit of the Rockwell hardness C-scale of the surface is preferably 64.

[0131] The Rockwell C-scale hardness (HRC) of a carbonitrided bearing component is measured using the following procedure. Four freely selectable measurement positions are defined on a surface of the carbonitrided bearing component. The Rockwell C-scale hardness test is performed at the four specified measurement positions in accordance with JIS Z 2245(2011). The arithmetic mean of the four obtained Rockwell C-scale hardness values ​​is defined as the Rockwell C-scale hardness (HRC) of the surface.

[0132] By the manufacturing method described above, the carbonitrided bearing components are manufactured using the steel material described above as the starting material for a carbonitrided bearing component. The present invention will be specifically described below with EXAMPLE. EXAMPLE

[0133] Molten steels with different chemical compositions were prepared, which are listed in Table 1.

[0134] Empty cells in Table 1 indicate that the content of a corresponding element was below the detection limit of that element. A steel grade Y contained a chemical composition equivalent to that of SUJ2, a conventional steel material specified in JIS G 4805(2008). In this EXAMPLE, steel grade Y is used as the reference steel material for comparison. The molten steels shown in Table 1 were continuously cast into blooms. The blooms were subjected to the rough rolling process. Specifically, the blooms were heated to the heating temperatures (°C) shown in Table 2. The residence times at the heating temperatures were all 3.0 to 3.5 hours.

[0135] The heated blooms were subjected to a rough rolling process to process them into billets with a rectangular cross-section of 160 mm × 160 mm. Furthermore, the billets were subjected to the finish rolling process. During finish rolling, the billets were heated to the heating temperatures (°C) shown in Table 2. The residence times at the heating temperatures were every 2.5 to 3.0 hours. The heated billets were formed into steel bars with a diameter of 60 mm each by hot rolling. The produced billets were cooled at average cooling rates CR (°C / sec) shown in Table 2. Through the above processes, the steel bars were manufactured as steel materials. A steel bar with a diameter of 60 mm was manufactured from the reference steel material for comparison under the same production conditions.For the reference steel, the heating temperature in the rough rolling process was 1250 °C and the residence time was 3.0 hours. During finish rolling, the heating temperature was 1250 °C and the residence time was 2.5 hours. The average cooling rate CR was 1.0 °C / sec. [Evaluation tests]

[0136] The produced steel materials (steel bars) were subjected to a test to observe the microstructure, a test to measure the proportion of V content in the residue RA V , a machinability evaluation test, a toughness evaluation test, a wear resistance evaluation test and a spalling life evaluation test in a hydrogen generating environment. [Microstructure observation test]

[0137] A sample was taken from each test number at an R / 2 position of a cross-section of a steel material (steel bar) perpendicular to a longitudinal direction (axial direction) of the steel material (cross-section). From the sampled areas, an area corresponding to the cross-section was designated as the observation area. The observation area was mirror-polished and then etched with 2% nitric acid alcohol etching (Nital etchant). The etched observation area was observed under a light microscope at 500x magnification, and photographic images of 20 freely selected fields of view were taken on the etched observation area. A size of 100 µm × 100 µm was specified for each field of view.

[0138] In each field of view, the phases (ferrite, pearlite, and bainite) were identified based on their contrast. Of the identified phases, a total area of ​​ferrite (µm 2) and a total area of ​​pearlite (µm 2 ). The ratio of the summed area of ​​the total areas of ferrite and the total areas of pearlite in all fields of view to the total area of ​​all fields of view was defined as the total area ratio (%) of ferrite and pearlite. The total area ratio (%) of ferrite and pearlite was determined as a value obtained by rounding the total area ratio (%) of ferrite and pearlite to the second decimal place. Note that for each test number, the microstructure except for ferrite and pearlite was bainite (excluding inclusions and precipitates). A total area ratio of ferrite and pearlite of each test number is given in the "F+P Total Area Ratio" column in Table 2. [Measurement test of the proportion of V content in the residue RA V ]

[0139] Three cylindrical specimens, each 6 mm in diameter and 50 mm long, were taken from an R / 2 position of a cross-section of the steel material (steel bar) of each test number, perpendicular to a longitudinal direction (axial direction) of the steel material (cross-section). A surface of each of the taken cylindrical specimens was subjected to preliminary electropolishing to polish a length of approximately 50 μm, thereby obtaining a new surface. The electropolished specimens were electrolyzed with an electrolyte (10% acetylacetone + 1% tetraammonium + methanol). After electrolysis, the residue was collected by passing the electrolyte through a 0.2 μm filter.The resulting residues were subjected to acid decomposition, and inductively coupled plasma (ICP) optical emission spectrometry was performed to determine the V content in the electrolytic extraction residue in mass%, based on the steel material (base metal), which was assumed to be 100 mass%. An arithmetic mean of the V contents in the electrolytic extraction residue of the cylindrical samples (i.e., an arithmetic mean of three V contents) was defined as the V content in the electrolytic extraction residue of the steel material, [V]. R . The V content in the electrolytic extraction residue, [V] R , was determined as a value obtained by rounding the arithmetic mean described above to the second decimal place. Using the V content in the chemical composition of the steel material, [V]c, and the V content in the electrolytic extraction residue, [V]R , which was obtained by the measurement described above, the proportion of V content in the residue, RA V (%), determined by formula (A). The proportion of V content in the residue RA V was determined as a value obtained by rounding off the proportion of V content in the residue RA V to the second decimal place. RAV=[V]R / [V]C×100

[0140] The remaining proportions of the V content in the residue RA V (%) are in the column “RA V “ shown in Table 2. [Machinability evaluation test]

[0141] Straight turning was performed on the steel material of each test number (a 60 mm diameter steel bar) to evaluate its chipping life. Specifically, straight turning was performed on the steel bar of each test number under the following conditions. A cutting tool made of a cemented carbide equivalent to P10 specified in JIS B 4053(2013) was used. The cutting speed was set to 150 m / min, the feed rate to 0.15 mm / rev, and the depth of cut to 1.0 mm. Note that no lubricant was used during turning.

[0142] Straight turning was performed under the cutting conditions described above, and the time required for the flank wear width of a cutting tool to reach 0.2 mm was defined as the tool life (Hr). A tool life of the reference steel material was used as a reference for comparison, and a tool life ratio of each test number was determined by the following formula. Tool life ratio = tool life (Hr) of each test number / tool life (Hr) of the reference steel for comparison

[0143] When a tool life ratio of 0.8 or more was achieved, the steel material was classified as having excellent machinability (represented as "E" (Excellent) in the machinability rating column in Table 2). In contrast, when the tool life ratio was less than 0.8, the steel material was classified as having poor machinability (represented as "B" (Poor) in the machinability rating column in Table 2). [Toughness evaluation test]

[0144] The toughness evaluation test was conducted according to the following procedure. The steel bar of each test number was machined (straight-turned) to produce an intermediate product (steel bar) with a diameter of 40 mm. The intermediate product subjected to machining was quenched and tempered in a Fig. 2, which simulates a carbonitriding treatment (simulated carbonitriding treatment). Referring to Fig.2. During the quenching treatment, the quenching temperature was set to 900°C and the residence time was set to 60 minutes in the simulated carbonitriding treatment. After the residence time, the intermediate product (steel bar) was subjected to oil quenching (shown as "OQ" in the drawing). During the tempering treatment, the tempering temperature was set to 180°C and the residence time was set to 120 minutes. After the residence time, the intermediate product (steel bar) was subjected to air cooling (shown as "AC" in the drawing). The steel bar subjected to the simulated carbonitriding treatment described above corresponded to the core region of the carbonitrided bearing component.

[0145] A Charpy specimen with a V-notch was taken from an R / 2 position of the steel bar subjected to the simulated carbonitriding treatment (quenching and tempering) described above. The Charpy specimen was subjected to the Charpy test according to JIS Z 2242(2009) at normal temperature (20 °C ± 15 °C). The absorbed energy resulting from the test was divided by the original cross-sectional area of ​​the notched part (the cross-sectional area of ​​the notched part of the specimen before the test), resulting in the impact value vE. 20 (J / cm 2 ) was determined. The obtained impact values ​​vE 20 are in the column “vE 20 ” are listed in Table 2.

[0146] In addition, a bar tensile specimen was taken from a test coupon No. 4 according to JIS Z 2241(2011) from the steel material (steel bar) subjected to the simulated carbonitriding treatment (quenching and tempering) described above. This specimen was subjected to the tensile test according to JIS Z 2241(2011) in air at normal temperature (20 °C ± 15 °C), and a 0.2% offset yield stress σy (MPa) was determined from the obtained stress-strain curve. The obtained 0.2% offset yield stresses σy are listed in the "σy" column in Table 2.

[0147] A obtained Charpy impact value vE 20 (J / cm 2 ) and a 0.2% yield stress σy (MPa) were used to determine Index, an index for evaluating toughness, using the following formula: Index=σy×(vE20)0.1

[0148] The obtained indices are listed in the "Index" column of Table 2. The index of a carbonitrided bearing component, as described above, must be 950 or more. If a carbonitrided bearing component has an index of 950 or more, the carbonitrided bearing component was rated as excellent in the toughness test (represented as mark "E" in the "Toughness Test" column of Table 2). Conversely, if the carbonitrided bearing component had an index of less than 950, the carbonitrided bearing component was rated as poor in toughness (represented as mark "B" in the toughness rating column of Table 2). [Wear resistance evaluation test]

[0149] The test for evaluating wear resistance was carried out according to the following procedure. A steel bar with a diameter of 60 mm was machined into a Fig.The intermediate product shown in Figure 3 is produced. Fig. 3 is a side view of the intermediate product. The numerical values ​​in Fig. 3 indicate the dimensions (mm) of the corresponding parts of the intermediate product. In Fig. 3 the numerical values ​​marked with “ϕ” indicate the diameters (mm).

[0150] The intermediate product was subjected to a carbonitriding treatment (carbonitriding and quenching as well as tempering) to be produced into a small roll sample, which was Fig.4, simulating the carbonitrided bearing component. At this time, the carbonitriding, quenching, and tempering conditions were adjusted so that the small roller specimens each had a surface C concentration of 0.80%, a surface N concentration of 0.30%, and a surface hardness of 60.0 in the Rockwell hardness C scale (HRC). Specifically, the carbonitriding and quenching treatments were performed with the carbon potentials CP, ammonia concentrations relative to the converted carburizing gas in the atmosphere, heating temperatures (in this EXAMPLE: heating temperature = carbonitriding treatment temperature = quenching temperature), and residence times (= residence time at carbonitriding treatment temperature + residence time at quenching temperature) shown in Table 3, and oil quenching was used as the cooling method.The tempering treatment was carried out at the tempering temperatures and residence times specified in Table 3, and air cooling was performed after the respective residence times. The intermediate product, which had undergone carbonitriding, quenching, and tempering, was subjected to finish machining (machining) to produce a small roll sample with the dimensions shown in Table 3. Fig. 4 shown form. Fig. Figure 4 is a side view of the small roller specimen. The numerical values ​​in Fig. 4 indicate the dimensions (mm) of the corresponding sections of the test specimen. In Fig. 4 the numerical values ​​marked with “ϕ” indicate the diameters (mm). [Table 3] TABLE 3 Test No. Type steel Carbonitriding and quenching Starting CP Ammonia concentration (%) Heating temperature (°C) Retention time (min) Tempering temperature (°C) Retention time (min) 1 A 1,00 3,00 900 60 180 120 2 B 0,90 3,00 900 60 180 120 3 C 1,00 2,00 900 60 180 120 4 D 1,10 3,00 900 60 180 120 5 E 1,00 4,00 900 60 180 120 6 F 1,20 2,00 900 60 180 120 7 G 1,00 3,00 900 60 180 120 8 H 1,10 3,00 900 60 180 120 9 I 1,00 2,00 900 60 180 120 10 J 1,00 4,00 900 60 180 120 11 K 1,10 3,00 900 60 180 120 12 L 1,00 2,00 900 60 180 120 13 M 1,00 3,00 880 60 180 120 14 N 1,10 3,00 910 60 180 120 15 O 1,00 2,00 900 60 180 120 16 P 1,20 3,00 920 60 180 120 17 Q 1,00 3,00 900 60 180 120 18 R 1,00 2,00 900 60 180 120 19 S 1,10 3,00 900 60 180 120 20 T 1,00 3,00 900 60 180 120 21 U 0,90 2,00 880 60 180 120 22 V 1,00 3,00 900 60 180 120 23 W 0,90 3,00 910 60 180 120 24 X 1,00 3,00 900 60 180 120 25 AA 1,00 3,00 900 60 180 120 26 BB 0,90 3,00 880 60 180 120 27 AA 1,00 3,00 900 60 180 120

[0151] As a test to evaluate wear resistance, a roller notch test (two-roller rolling fatigue test) was conducted on the small roller of each test number. As shown in Fig.As shown in Figure 5, a large roll with a diameter of 130 mm and a crown radius of 150 mm was prepared. The raw material of the large roll had the chemical composition of steel type Y, which is shown in Table 1 as the reference steel material for comparison. The raw material of the large roll was subjected to quenching and tempering treatment. During the quenching treatment, the quenching temperature was set at 860 °C and the residence time at the quenching temperature was set at 60 minutes. After the residence time, the raw material was subjected to oil quenching with oil at 80 °C. The raw material that had undergone quenching treatment was subjected to tempering treatment. During the tempering treatment, its tempering temperature was set at 180 °C and its residence time at the tempering temperature was set at 120 minutes.After the quenching treatment and the tempering treatment described above, a finish machining was carried out to obtain the results shown in . Fig. 5 to produce the large roll shown.

[0152] The following roller notch test was conducted on the small roller sample of each test number. Specifically, the small roller sample and the large roller were arranged so that a central axis of the small roller sample and a central axis of the large roller were parallel to each other. Then, the roller notch test was conducted under the following conditions. A surface of the large roller was pressed against a central area of ​​the small roller sample (an area with a diameter of 26 mm). The rotation speed of the small roller was set to 1500 rpm, the rotation directions of the small roller and the large roller at their contact area were set to be the same, and the slip factor was set to 40%. Assuming that V1 (m / sec) denotes the rotation speed of the large roller and V2 (m / sec) denotes the rotation speed of the small roller sample, the slip factor (%) was determined by the following formula: Slip factor=(V2−V1) / V2×100

[0153] During the test, the contact stress between the small roller sample and the large roller was set to 3.0 GPa. During the test, a lubricant (commercial automatic transmission fluid: ATF) was sprayed at a rate of 2 l / min onto the contact area between the large roller and the small roller sample (one surface of a test part) in the opposite direction to the rotational directions, with the oil temperature set to 80°C. The number of cycles was limited to a maximum of 2 × 10 7 set and the test was repeated after the number of cycles of 2 × 10 7 finished.

[0154] Based on the small roll sample subjected to the wear resistance evaluation test, the average wear depth (µm), surface hardness (HRC), surface concentration of C (mass%) and surface concentration of N (mass%) were determined according to the following methods. [Average wear depth]

[0155] After the test, the roughness of a sliding part of the sample was measured. Specifically, a roughness profile was measured on a circumferential surface of the small roller sample at four locations provided with 90° pitches in the circumferential direction. The maximum depth of the roughness profile at the above four locations was defined as the wear depth, and the average of the wear depths at these four locations was defined as the average wear depth (µm). The average wear depths are listed in the "Average Wear Depth" column in Table 2. When the average wear depth was 10µm or less, the carbonitrided bearing component was rated as highly wear-resistant (represented as "E" in the wear resistance rating in Table 2).In contrast, at an average wear depth of more than 10µm, the carbonitrided bearing component was rated as having low wear resistance (represented as “B” in the wear resistance rating in Table 2). [Surface hardness]

[0156] After the test, four measurement positions with 90° pitches in the circumferential direction were established in an area on a surface of the test piece of the small roller specimen other than the sliding part (hereinafter referred to as the non-sliding part). The Rockwell hardness C-scale test was performed at the four specified measurement positions in accordance with JIS Z 2245(2011). The arithmetic mean value of the Rockwell hardness C-scale HRC at the measurement positions was defined as the Rockwell hardness C-scale HRC of the surface. The obtained Rockwell hardness C-scale values ​​are listed in the "HRC" column in Table 2. [Surface concentration of C and surface concentration of N]

[0157] The non-sliding part of the test piece of the small roller sample was cut perpendicular to an axial direction of the small roller sample. A specimen with a cut section including a surface (peripheral surface) of the non-sliding part was taken. The cut section was subjected to embedding polishing. Then, an electron probe microanalyzer (EPMA) was used to measure the C concentration and the N concentration from the surface of the non-sliding part to a depth of 10 μm at a pitch of 0.1 μm. The arithmetic mean of the measured values ​​was defined as the surface concentration of C (mass%) and the surface concentration of N (mass%). The obtained surface concentrations of C (%) and N (%) are shown in Table 2. [Exfoliation test under hydrogen-generating environment]

[0158] From the steel material (a 60 mm diameter steel bar) of each test number, a disc-shaped intermediate product with a diameter of 60 mm and a thickness of 5.5 mm was machined. The thickness of the intermediate product (5.5 mm) corresponded to a longitudinal direction of the steel bar. The intermediate product was subjected to carbonitriding treatment (carbonitriding, quenching, and tempering) to become a carbonitrided bearing component. At this time, carbonitriding, quenching, and tempering were performed so that each carbonitrided bearing component had a surface C concentration of 0.80%, a surface N concentration of 0.30%, and a surface Rockwell hardness C scale HRC of 60.Specifically, the carbonitriding and quenching treatments were performed with the carbon potentials CP, ammonia concentrations relative to the converted carburizing gas in the atmosphere, heating temperatures (in this example: heating temperature = carbonitriding treatment temperature = quenching temperature), and residence times (= residence time at carbonitriding treatment temperature + residence time at quenching temperature) specified in Table 3. Oil quenching was used as the cooling method. The tempering treatment was performed at the tempering temperatures and residence times specified in Table 3, and air cooling was performed after each residence time. A surface of the obtained sample was lapped to prepare a rolling contact fatigue test specimen.

[0159] It should be noted that in the exfoliation test under hydrogen conditions, the Y-type steel used as a reference steel was subjected to the following quenching and tempering treatment instead of the carbonitriding treatment described above. Specifically, a disc-shaped intermediate product with a diameter of 60 mm and a thickness of 5.5 mm was machined from a Y-type steel bar with a diameter of 60 mm. The thickness of the intermediate product (5.5 mm) corresponded to a longitudinal direction of the steel bar. The intermediate product was subjected to quenching treatment. During the quenching treatment, the quenching temperature was set to 860 °C and the residence time at the quenching temperature was set to 60 minutes. After the residence time, the intermediate product was subjected to oil quenching with oil at 80 °C.Note that the furnace atmosphere in a heat treatment furnace used for quenching was formulated to prevent decarburization in the intermediate product undergoing quenching. The intermediate product undergoing quenching was then subjected to tempering. During tempering, the tempering temperature was set at 180°C and the residence time at the tempering temperature was set at 120 minutes. One surface of the obtained specimen was lapped to prepare a rolling contact fatigue test specimen from the reference steel material for comparison purposes.

[0160] The following exfoliation test was conducted using the rolling contact fatigue specimen of each test number and the rolling contact fatigue specimen of the reference steel for comparison (steel type Y). To simulate a hydrogen-generating environment, the rolling contact fatigue specimen was immersed in a 20% aqueous ammonium thiocyanate solution (NH4SCN) and subjected to hydrogen charging. Specifically, the hydrogen charging was performed at an aqueous solution temperature of 50 °C and an immersion time of 24 hours.

[0161] The hydrogen-charged rolling contact fatigue specimen was subjected to rolling contact fatigue testing using a shear rolling contact fatigue tester. A maximum contact interface pressure of 3.0 GPa and a cycle rate of 1800 cycles per minute (cpm) were specified for the test. Turbine oil was used as the lubricant, and a thermally modified SUJ2 steel ball according to JIS G 4805(2008) was used as the steel ball.

[0162] The rolling contact fatigue test results were plotted on Weibull probability paper, and a life L10 with a 10% probability of fracture was defined as the "exfoliation life." The ratio of the exfoliation life L10 of each test number to the exfoliation life L10 of steel type Y was defined as the exfoliation life ratio. In other words, the exfoliation life ratio was determined by the following formula: Flaking life ratio = Flaking life of each test number / Flaking life of steel type Y

[0163] The obtained exfoliation life ratios are listed in the "Exfoliation Life Ratio" column of Table 2. When the obtained exfoliation life ratio was 2.0 or more, the carbonitrided bearing component was rated as excellent in terms of exfoliation life in a hydrogen-generating environment (represented as "E" in the "Rating" column of "Exfoliation Life Ratio" in Table 2). In contrast, when the exfoliation life ratio was less than 2.0, the exfoliation life of the carbonitrided bearing component was rated as poor in a hydrogen-generating environment (represented as "B" in the "Rating" column of "Exfoliation Life Ratio" in Table 2). [Test results]

[0164] Table 2 shows the test results. Table 2 shows that for the chemical compositions of test Nos. 1 to 10, the element contents were appropriate, and F1 to F4 met formulas (1) to (4). Furthermore, their production conditions were also appropriate. Therefore, the total area ratio of ferrite and pearlite in their microstructures was 10.0% or more, the remainder was bainite, and the V content in the residue was RA. Vwas 10.0% or less. As a result, the steel materials each exhibited a tool life ratio of 0.8 or more, thus providing excellent machinability. Furthermore, their indexes after simulated carbonitriding treatment were all 950 or more, and the core areas of their carbonitrided bearing components were expected to exhibit excellent toughness. Furthermore, after carbonitriding treatment, the carbonitrided bearing components exhibited a surface C concentration of 0.70 to 1.20% and a surface N concentration of 0.15 to 0.60%, and the surfaces of the carbonitrided bearing components exhibited a Rockwell hardness C scale HRC of 58 to 65.Furthermore, the average wear depths in the wear resistance evaluation test were 10µm or less, thus demonstrating excellent wear resistance for the carbonitrided bearing components. Furthermore, in the exfoliation test in a hydrogen-generating environment, the carbonitrided bearing components after the carbonitriding treatment showed a surface C concentration of 0.70 to 1.20% and a surface N concentration of 0.15 to 0.60%, and the surfaces of the carbonitrided bearing components had a Rockwell C-scale hardness (HRC) of 58 to 65. Furthermore, their exfoliation lives were 2.0 or more, thus demonstrating excellent exfoliation lives under a hydrogen-generating environment.

[0165] In contrast, the Mn content in Test No. 11 was too low. As a result, the exfoliation life ratio was below 2.0, and thus the exfoliation life in a hydrogen environment was low.

[0166] In Test No. 12, the Mn content was excessively high. As a result, the exfoliation life ratio was less than 2.0, and thus the exfoliation life in a hydrogen environment was poor.

[0167] In Test No. 13, the Mo content was too low. As a result, the average wear depth in the wear resistance test was more than 10 µm, resulting in poor wear resistance. Furthermore, the spalling life ratio was less than 2.0, resulting in poor spalling life in a hydrogen environment.

[0168] In Test No. 14, the Mo content was excessively high. As a result, the tool life ratio of the steel was below 0.8, resulting in poor machinability.

[0169] In Test No. 15, the V content was too low. As a result, the average wear depth in the wear resistance evaluation test was more than 10µm, resulting in poor wear resistance. Furthermore, the spalling life ratio was below 2.0, resulting in poor spalling life in a hydrogen environment.

[0170] In Test No. 16, its V content was excessively high. As a result, the proportion of V content in the residue (RA V ) was more than 10.0%. Consequently, the index after the simulated carbonitriding treatment was below 950, and the toughness was low. In addition, the exfoliation life ratio was less than 2.0, and thus the exfoliation life in a hydrogen environment was poor.

[0171] In Test No. 17, although the element content in the chemical composition was appropriate, F1 was smaller than the lower limit of formula (1). As a result, the average wear depth in the wear resistance evaluation test was more than 10 µm, resulting in poor wear resistance. Furthermore, the spalling life ratio was less than 2.0, resulting in poor spalling life in a hydrogen environment.

[0172] In Test No. 18, the contents of the elements in the chemical composition were appropriate, but F1 was above the upper limit of formula (1). Consequently, the proportion of V content in the residue RA was Vmore than 10.0%. Consequently, its index after the simulated carbonitriding treatment was below 950, and thus its toughness was low. Furthermore, the exfoliation life ratio was less than 2.0, and thus the exfoliation life in a hydrogen environment was poor.

[0173] In Test No. 19, although the element content in the chemical composition was appropriate, F2 was below the lower limit of formula (2). As a result, the exfoliation life ratio was less than 2.0, and thus the exfoliation life in a hydrogen environment was poor.

[0174] In Test No. 20, although the element content in the chemical composition was appropriate, F2 exceeded the upper limit of formula (2). As a result, the total area fraction of ferrite and pearlite was less than 10.0%. Consequently, the tool life ratio of the steel was less than 0.8, making the steel material difficult to machine.

[0175] In tests 21 and 22, although the element contents in the chemical compositions were appropriate, F3 was less than the lower limit of formula (3). As a result, the average wear depths in the wear resistance evaluation tests were more than 10 µm, resulting in poor wear resistance. Furthermore, the spalling life ratios were less than 2.0, resulting in poor spalling life in a hydrogen environment.

[0176] In Tests Nos. 23 and 24, although the element contents in the chemical compositions were appropriate, F4 was less than the lower limit of formula (4). As a result, the exfoliation lifetime ratios were less than 2.0, and thus the exfoliation lifetimes under a hydrogen environment were low.

[0177] In Test No. 25, the element contents in the chemical composition were appropriate, and F1 to F4 satisfied formulas (1) to (4). However, the heating temperature in the rough rolling process was too low. As a result, the proportion of V content in the residue was RA. V more than 10.0%. Therefore, the index after the simulated carbonitriding treatment was below 950, and the toughness was low. Furthermore, the exfoliation life ratio was less than 2.0, and thus the exfoliation life was low under a hydrogen-generating environment.

[0178] In Test No. 26, the element contents in the chemical composition were appropriate, and F1 to F4 met formulas (1) to (4). However, the heating temperature in the finish-rolling process was too low. As a result, the proportion of V content in the residue was RA. V more than 10.0%. Therefore, the index after the simulated carbonitriding treatment was below 950, and the toughness was low. Furthermore, the exfoliation life ratio was less than 2.0, and thus the exfoliation life under a hydrogen environment was poor.

[0179] In Test No. 27, the element contents in the chemical composition were appropriate, and F1 to F4 met formulas (1) to (4). However, the cooling rate CR in the finish rolling process was too high. As a result, the total area ratio of ferrite and pearlite was less than 10.0%. As a result, the tool life ratio of the steel material was below 0.8, and thus the steel material was poorly machinable.

[0180] An embodiment according to the present invention has been described above. However, the above-described embodiment is merely an example for practicing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and practiced without departing from the scope of the present invention.

Claims

[1] Steel material with a chemical composition consisting of, in mass%: C: 0.15 to 0.45%, Si: 0.50% or less, Mn: 0.20 to 0.60%, P: 0.015% or less, S: 0.005% or less, Cr: 0.80 to 1.50%, Mon: 0.17 to 0.30%, V: 0.24 to 0.40%, Al: 0.005 to 0.100%, N: 0.0300% or less, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.0050%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, Ca: 0 to 0.0010%, and as the remainder Fe and impurities, and satisfying formula (1) to formula (4), wherein in its microstructure a total area fraction of ferrite and pearlite is 10.0% or more and the remainder is bainite, and a ratio of a content of V (mass%) in the electrolytic extraction residue to the content of V (mass%) in the chemical composition is 10.0% or less: 1.50<0.4Cr+0.4Mo+4.5V<2.45 2.20<2.7C+0.4Si+Mn+0.45Ni+0.8Cr+Mo+V<2.80 Mo / V≥0.58 (Mo+V+Cr) / (Mn+20P)≥2.40 where each symbol of an element in formula (1) to formula (4) is to be replaced by a content of a corresponding element (mass%). [2] The steel material according to claim 1, wherein the chemical composition contains one or more kinds of elements selected from the group consisting of: Cu: 0.01 to 0.20%, Ni: 0.01 to 0.20%, B: 0.0001 to 0.0050%, Nb: 0.005 to 0.100%, and Ti: 0.005 to 0.100%. [3] Steel material according to claim 1 or claim 2, wherein the chemical composition contains Ca: 0.0001 to 0.0010%.

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