Nitrided induction hardening steel and nitrided induction hardening parts

Abstract

To provide a steel for nitriding and induction hardening and a nitrided and induction-hardened part having excellent surface fatigue strength and high temperature hardness.SOLUTION: The present invention provides a steel for nitriding and induction hardening, comprising in mass%, C: 0.25% or more and 0.57% or less, Si: 0.10% or more and less than 0.45%, Mn: 0.40% or more and 1.00% or less, P: 0.030% or less, S: 0.095% or less, Cr: 1.15% or more and less than 1.95%, V: more than 0.05% and less than 0.50%, Al: 0.100% or less, and N: 0.0250% or less, Cr, V satisfying the following formula (1) with the balance being Fe and impurities. In the formula (1): 1.90≤Cr+3V≤3.00, each element symbol represents the content of the element in mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a nitrided induction hardening steel and a nitrided induction hardening part. [Background technology]

[0002] Steel parts used in automobiles and various industrial machines, such as transmission gears, CVT (Continuously Variable Transmission) pulleys, bearings, and other power transmission parts, require high surface fatigue strength, bending fatigue strength, wear resistance, etc. For this reason, these parts are made from mechanical structural alloy steels such as JIS SCr420, SCM420, and SNCM420, which are processed into the required shape and then carburized and quenched to create a hardened layer on the surface, thereby improving fatigue strength.

[0003] In recent years, the automotive industry has been promoting the use of electric power, replacing internal combustion engines, in order to reduce CO2 emissions. This means that the properties required for steel components such as gears, especially power transmission components, may change dramatically in the future. For example, electric vehicles (EVs) often incorporate a reduction gear directly below the motor that powers them to ensure torque. Miniaturizing these units leads to lighter vehicle bodies and greater design flexibility, and is therefore expected to further expand demand. However, when considering both the need to increase the rotation speed of motors while still maintaining output while miniaturizing gears, frictional heat generation on the tooth surfaces due to contact between gears in EV reduction gears is a greater concern than in gasoline-powered vehicle transmissions. This suggests that ensuring fatigue strength in high-temperature environments will be a key challenge for steel components such as gears.

[0004] Carburizing and quenching is a method of heating a steel part to the austenite region, increasing the carbon (C) concentration in the surface layer of the part, and then rapidly cooling it to form hard martensite on the surface of the part, resulting in high surface fatigue strength. However, carburizing and quenching is a heat treatment that hardens the part to its core, which tends to result in large distortion after treatment (heat treatment distortion), resulting in high grinding costs in subsequent processes. In addition, when parts hardened with a martensite structure with a high carbon concentration are used in high-temperature environments, cementite precipitates, reducing the carbon concentration in the martensite structure, thereby reducing the hardness of the part. Therefore, as the above-mentioned issues related to frictional heat become more apparent, it may become difficult to ensure fatigue strength, especially surface fatigue strength, in carburized parts.

[0005] Against this background, in recent years, attention has been focused on nitriding and induction hardening, which are surface hardening heat treatments that cause less heat treatment distortion than carburizing and quenching.

[0006] Nitriding is a method of heating a steel part to the ferrite temperature range below the A1 point, increasing the N (nitrogen) concentration in the surface layer of the part and causing the precipitation of nitrogen compounds. The resulting hardened layer containing these nitrogen compounds increases the hardness of the surface layer of the part. Because nitriding does not involve phase transformation, it minimizes heat treatment distortion. Furthermore, because nitriding utilizes the precipitation of nitrogen compounds at temperatures between 450 and 600°C, the hardened layer exhibits higher thermal stability than the hardened layer formed by carburizing and quenching. The hardened layer formed by nitriding consists of a nitrogen diffusion layer and a nitrogen compound layer several to several tens of micrometers thick that forms on the surface side of the part relative to the diffusion layer. The diffusion layer is hardened by the solid solution strengthening of the infiltrated nitrogen and carbon and the particle dispersion strengthening of the nitrides. The nitrogen compound layer is primarily composed of iron nitrides, Fe2N, Fe3N (ε phase), and Fe4N (γ' phase), and is significantly harder than the parent phase (non-nitrided layer). However, because nitriding is a heat treatment performed at a relatively low temperature, the hardened layer depth is small and the surface fatigue strength is inferior to that of carburizing and quenching. For this reason, nitriding is difficult to apply to parts that require high fatigue strength. Furthermore, nitrided parts are prone to a decrease in surface fatigue strength due to the presence of brittle compound layers and voids formed near the surface of the compound layers.

[0007] Induction hardening is a heat treatment method in which steel parts are rapidly heated to the austenite region in a short period of time and hardened, and it results in less quenching distortion than carburizing quenching. However, unlike carburizing quenching, induction hardening is not a heat treatment method that increases the carbon concentration in the surface layer of the part, so it is not suitable for parts that require high surface fatigue strength.

[0008] For these reasons, simply applying nitriding or induction hardening to steel parts used in automobiles, various industrial machines, etc. is not enough to improve the surface fatigue strength of steel parts to enable them to be made lighter, smaller, and to cope with high stress loads. Therefore, in recent years, attempts have been made to combine nitriding and induction hardening as a method to improve fatigue strength, particularly surface fatigue strength, which is a drawback of nitriding and induction hardening.

[0009] Patent Document 1 discloses a steel for soft nitriding in which the area ratio of the bainite phase is more than 50% and core hardness after soft nitriding is ensured by adding Al, V, and Nb.

[0010] Patent Document 2 proposes a nitrided induction hardening steel that has high surface fatigue strength and excellent wear resistance. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] International Publication No. 2020 / 090739 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-208250 Summary of the Invention [Problem to be solved by the invention]

[0012] The main strengthening mechanisms of the nitriding steels and nitrocarburizing steels disclosed in Patent Document 1 and elsewhere are solid solution strengthening by N (nitrogen) that penetrates and diffuses into the surface layer of the part, and particle dispersion strengthening by alloy nitrides. Therefore, these nitriding steels require the addition of large amounts of nitride-forming elements such as Al and V. On the other hand, the surface layer of steel parts obtained by a combined heat treatment that combines nitriding and induction hardening is strengthened by a martensitic structure containing supersaturated solid solution of C (carbon) and N (nitrogen), and therefore does not require large amounts of the above-mentioned nitride-forming elements. Therefore, there is a need for a steel for combined heat treatment that is suitable for nitriding induction hardening.

[0013] Furthermore, in the nitrided induction hardening steel disclosed in Patent Document 2, the Al content must be more than 0.5% in order to obtain high surface fatigue strength, and there is room for improvement in terms of manufacturability and alloy costs.

[0014] The present invention has been made to solve the above problems, and has as its object to provide a nitride induction hardening steel and a nitride induction hardening part that are excellent in surface fatigue strength and high-temperature hardness. [Means for solving the problem]

[0015] In order to solve the above-mentioned problems, the inventors independently changed the various alloying elements in a steel for nitriding induction hardening, and investigated the effects of each element on the N concentration, hardness, and surface fatigue strength in the surface layer of a part after nitriding induction hardening, and obtained the following findings (a) to (f).

[0016] (a) As mentioned above, there is concern about a decrease in fatigue strength in high-temperature environments for power transmission parts. In particular, frictional heat can cause the surface temperature of sliding parts, such as gears used in automobiles, to rise to nearly 300°C. For example, to ensure driving range and power output, EVs require smaller sliding parts with higher rotation speeds, raising concerns about frictional heat exceeding 300°C on the surface of the parts. Therefore, to provide gear parts suitable for gasoline-powered vehicles and EVs, it is necessary to suppress softening of the surface layer of the parts due to frictional heat and maintain hardness at high temperatures (high-temperature hardness). Compared to parts made of quenched carbon steel with a martensitic structure, parts made of carbon steel with a high-N concentration that are nitrided to form a solid solution in the surface layer and then quenched again often have higher surface fatigue strength.

[0017] (b) A major factor behind this is the high tempering softening resistance exhibited by martensitic structures with a high N concentration. Generally, the surface layer of the sliding part of a part after a contact fatigue strength evaluation softens due to frictional heat. However, compared to martensitic structures with a surface layer of high C concentration, martensitic structures with a high N concentration ensure hardness at around 300°C due to the precipitation of iron nitrides.

[0018] (c) In addition, in the surface layer of a carbon steel part that has been quenched to form a martensite structure after nitriding to dissolve N in the surface layer, the precipitation of alloy nitrides such as CrN often increases the hardness at around 400°C. Therefore, increasing the Cr content of the material is effective in improving softening resistance, that is, contact fatigue strength when frictional heat generation becomes greater.

[0019] (d) V is an element that forms nitrides (VN) and carbides (VC), so it is effective in improving not only the hardness of the nitrided layer but also the hardness of the non-nitrided layer (parent phase). Therefore, increasing the V content in the material is effective in improving the contact fatigue strength when frictional heat generation increases.

[0020] (e) The shear stress generated by Hertzian contact between parts, although it varies depending on the shape and applied load, generally reaches its maximum at a depth of approximately 0.1 mm from the surface of the part, and the area around this depth is most vulnerable to pitting. Therefore, it is important to ensure hardness at a depth of 0.1 mm from the surface of the part.

[0021] (f) In addition, in order to achieve the softening resistance at higher temperatures due to the precipitation of nitrides mentioned above, it is necessary to ensure a certain level of N concentration in the region from the surface of the part to a depth of 0.1 mm.

[0022] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0023] (1) The nitrided induction hardening steel according to one aspect of the present invention comprises, in mass %, C: 0.25% or more, 0.57% or less, Si: 0.10% or more, less than 0.45% Mn: 0.40% or more, 1.00% or less, P: 0.030% or less, S: 0.095% or less, Cr: 1.15% or more, less than 1.95% V: More than 0.05% and less than 0.50% Al: 0.100% or less, and N: 0.0250% or less The composition contains Cr and V, which satisfy the following formula (1), and the balance being Fe and impurities. 1.90≦Cr+3V≦3.00 ··· Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %. (2) The nitrided induction hardening steel according to (1) further comprises, in mass %, Cu: less than 0.10% Ni: less than 0.05% Mo: less than 0.15% W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Ti: 0.100% or less, Nb: 0.100% or less, and B: 0.0020% or less may contain one or more of the above. (3) The nitrided induction hardening steel according to (1) or (2) further comprises, in mass %, Ca: 0.0010% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.08% or less, Sn: 0.150% or less, Sb: 0.0100% or less, and REM: 0.0100% or less may contain one or more of the above.

[0024] (4) In one aspect of the present invention, the nitrided induction hardened part has a core component, which comprises, in mass %, C: 0.25% or more, 0.57% or less, Si: 0.10% or more, less than 0.45% Mn: 0.40% or more, 1.00% or less, P: 0.030% or less, S: 0.095% or less, Cr: 1.15% or more, less than 1.95% V: More than 0.05% and less than 0.50% Al: 0.100% or less, and N: 0.0250% or less wherein Cr and V satisfy the following formula (2), and the balance is Fe and impurities, The thickness of the nitrogen compound layer formed on the surface layer and containing at least Fe and N is 5 μm or less, The Vickers hardness at a depth of 0.10 mm from the surface is 670 HV or more. The average N concentration Ns in the region from the surface to a depth of 0.1 mm is 1.60 atomic % or more, With respect to the depth position where the N concentration is 0.3Ns (atomic %), the average N concentration Nh and C concentration Ch in a region 0.05 mm before and after in the depth direction satisfy the following formula (3). 1.90≦Cr+3V≦3.00 ··· Formula (2) Nh+Ch>1.80...Equation (3) However, each element symbol in the above formula (2) represents the content of the element in mass %. (5) In the nitrided induction hardened part described in (4) above, the components of the core portion further comprise, in mass %, Cu: less than 0.10% Ni: less than 0.05% Mo: less than 0.15% W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Ti: 0.100% or less, Nb: 0.100% or less, and B: 0.0020% or less may contain one or more of the above. (6) In the nitrided induction hardened part according to (4) or (5), the components of the core portion are further comprised, in mass%, Ca: 0.0010% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.08% or less, Sn: 0.150% or less, Sb: 0.0100% or less, and REM: 0.0100% or less may contain one or more of the above. [Effects of the Invention]

[0025] The present invention provides a nitrided induction-hardened steel and nitrided induction-hardened parts that are excellent in surface fatigue strength and high-temperature hardness. Therefore, the nitrided induction-hardened steel of the present invention is suitable as a material for, for example, automobiles and industrial machinery, particularly gears in machines powered by electric motors. [Brief explanation of the drawings]

[0026] [Figure 1] Figure 1 shows an example of a small roller for roller pitching tests, with dimensions in the figure in millimeters. [Figure 2] Fig. 2 is a diagram showing an example of a large roller for roller pitching tests, where the dimensions are in mm. DETAILED DESCRIPTION OF THE INVENTION

[0027] Hereinafter, a description will be given of a nitride induction hardening steel and a nitride induction hardening part according to one embodiment of the present invention.

[0028] The nitrided induction-hardened part according to this embodiment (hereinafter, sometimes simply referred to as the "part") is a part that has been nitrided induction-hardened using the nitrided induction-hardening steel of this embodiment, and has a core (hereinafter, sometimes simply referred to as the "core") that is the central region in the depth direction of the part, and a hardened layer located closer to the surface of the part than the core. Here, the core refers to the area where nitrogen did not penetrate during nitriding and the area where the structure did not transform to martensitic form during induction hardening. In other words, the core is the area where the chemical composition and metal structure remain unchanged or where the change is negligible despite the nitriding and induction hardening processes, and has the same chemical composition as the nitride induction hardening steel that is the base material of the part. The composition of the core can also be said to be the composition at a depth of 2.0 mm from the surface of the part, for example. The hardened layer refers to the area where nitrogen has penetrated due to nitriding and the area where the structure has been transformed into martensitic due to induction hardening. In other words, the hardened layer is the area where the chemical composition or metal structure has changed due to the effects of nitriding and induction hardening. For example, it refers to the area from the surface of the part to a depth of approximately 1.0 mm.

[0029] [Component composition] The components of the nitride induction hardening steel (hereinafter sometimes simply referred to as "steel") of this embodiment will be described. Normally, the core of a nitride induction hardening part has the same components as the steel, so unless otherwise specified, the components of the steel and the core of the part are equivalent. Note that "%" for the content of each component element in the steel means "mass %" unless otherwise specified.

[0030] [C: 0.25% or more, 0.57% or less] Carbon (C) is an element necessary for ensuring the hardening ability of the surface layer during induction hardening and core strength. If the C content is less than 0.25%, the above effects are insufficient. Furthermore, if the C content exceeds 0.57%, cementite is likely to form during tempering at high temperatures (e.g., around 400°C). As a result, nitride-forming elements such as Cr, which exhibit high softening resistance, concentrate in cementite, which may result in insufficient high-temperature hardness. In addition, a thick ε phase with low deformability may form in the surface layer after nitriding. This ε phase is likely to serve as a crack initiation point, and if it persists after induction hardening, it reduces surface fatigue strength. Furthermore, if the C content is excessively high, the strength of steel bars and wire rods used as component materials, as well as the strength after hot working, will be too high, significantly reducing machinability. Therefore, the C content of steel materials is set to 0.25% or more and 0.57% or less. More preferably, the C content may be set to 0.30% or more, or 0.35% or more. Similarly, the C content may be set to 0.55% or less, 0.50% or less, or 0.45% or less.

[0031] [Si: 0.10% or more, less than 0.45%] Si has the effect of improving hardenability. Furthermore, Si suppresses the aggregation and coarsening of carbides and nitrides during tempering at high temperatures (for example, about 400°C), thereby improving temper softening resistance. To achieve these effects, the Si content is set to 0.10% or more. On the other hand, if the Si content is excessively high, the amount of N penetrating into the surface layer during nitriding treatment may decrease, resulting in a decrease in temper softening resistance, so the Si content is set to less than 0.45%. More preferably, the Si content may be set to 0.15% or more, or 0.20% or more, and similarly, the Si content may be set to 0.40% or less, or 0.35% or less.

[0032] [Mn: 0.40% or more, 1.00% or less] Mn is an element that is effective in increasing contact fatigue strength because it has the effect of improving hardenability. However, if its content is less than 0.40%, this effect is insufficient. On the other hand, if the Mn content exceeds 1.00%, not only does the effect of increasing contact fatigue strength saturate, but the strength of the steel bar and wire rod used as the component material, as well as the strength after hot working, becomes too high, significantly reducing machinability. Therefore, the Mn content of steel is set to 0.40% or more and 1.00% or less. More preferably, the Mn content may be set to 0.50% or more, or 0.60% or more, and similarly, the Mn content may be set to 0.90% or less, or 0.80% or less.

[0033] [P:0.030% or less] P is an impurity that segregates at grain boundaries and embrittles parts. Excessive P content can reduce contact fatigue strength, so the P content is set to 0.030% or less. The lower the P content, the better. The upper limit of the P content is preferably 0.018%, 0.015%, 0.013%, or 0.010%. While the P content may be 0%, excessive reduction of P increases the cost of dephosphorization. Therefore, taking into account the economics of refining, the P content may be set to 0.001% or more, 0.005% or more, or 0.008% or more.

[0034] [S:0.095% or less] S combines with Mn to form MnS, improving machinability. However, a high S content tends to produce coarse MnS, significantly reducing surface fatigue strength. Therefore, the S content is set to 0.095% or less. The preferred upper limit of the S content is 0.080%, 0.060%, or 0.040%. Although the S content may be 0%, excessive reduction of S leads to increased costs for desulfurization. Therefore, taking into account the economics of refining, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[0035] [Cr: 1.15% or more, less than 1.95%] Cr not only enhances hardenability, but also contributes to suppressing deterioration of high-temperature hardness by combining with N in a high-N martensite structure during tempering at high temperatures (e.g., around 400°C) to precipitate fine CrN particles, thereby exhibiting high softening resistance. However, if the Cr content is less than 1.15%, this effect is insufficient. On the other hand, if the Cr content exceeds 1.95%, not only does the effect of enhancing softening resistance saturate, but the N concentration in the surface layer increases, resulting in a high amount of retained austenite in the surface layer after nitriding induction hardening, resulting in a decrease in hardness and a decrease in surface fatigue strength. In addition, the strength of the raw material steel bar and wire rod, as well as the strength after hot working, becomes too high, significantly reducing machinability. Therefore, the Cr content in steel materials is set to be 1.15% or more but less than 1.95%. More preferably, the Cr content may be set to 1.25% or more, or 1.35% or more, and similarly, the Cr content may be set to 1.85% or less, or 1.75% or less.

[0036] [V: Over 0.05% and less than 0.50%] V not only enhances hardenability, but also contributes to suppressing deterioration of high-temperature hardness by combining with N in a high-N martensite structure during tempering at high temperatures (e.g., around 400°C) to precipitate fine VN, thereby exhibiting high softening resistance. Furthermore, V combines with C in the matrix structure at the nitriding temperature to precipitate VC, thereby improving the hardness of the matrix. Because V thus enhances the overall hardness profile, it is an effective element for enhancing contact fatigue strength. However, if its content is 0.05% or less, the above effect is insufficient. On the other hand, a high V content increases the cost of the steel and significantly reduces the machinability of the base material due to excessive hardness after hot working. Therefore, the V content in the steel is set to more than 0.05% and less than 0.50%. More preferably, the V content may be set to 0.10% or more, or 0.15% or more, and similarly, the V content may be set to 0.40% or less, or 0.30% or less.

[0037] [Al:0.100% or less] Al is a deoxidizing element, but it is not necessarily contained. However, Al combines with N to form AlN, which has the effect of pinning austenite grains, refining the structure of the base material before nitriding, and reducing the variation in mechanical properties of nitrided parts. To achieve this effect, the Al content is preferably 0.001% or more, and more preferably 0.002% or more, 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, Al is an element that easily forms hard oxide-based inclusions, and a high Al content may significantly reduce contact fatigue strength. Furthermore, an excessively high Al content may make it impossible to obtain the desired contact fatigue strength even if other requirements are met, so the Al content is set to 0.100% or less. To prevent a decrease in contact fatigue strength, the preferred upper limit of the Al content is 0.050% or 0.040%.

[0038] [N:0.0250% or less] N (nitrogen) does not necessarily have to be contained. However, N bonds with Mn, Cr, and Al to form Mn3N2, CrN, and AlN, respectively. AlN, in particular, has the effect of refining the structure of the base material before nitriding by pinning austenite grains, and reducing the variation in mechanical properties of nitrided parts. To achieve this effect, the N content may be 0.0010% or more, 0.0015% or more, 0.0020% or more, 0.0025% or more, 0.0030% or more, 0.0035% or more, or 0.0040% or more. On the other hand, if the N content is high, coarse AlN is likely to form, which may significantly reduce the contact fatigue strength. Furthermore, if the N content is excessively high, the desired contact fatigue strength may not be obtained even if other requirements are met, so the N content should be set to 0.0250% or less. Preferably, the N content may be set to 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0039] [1.90≦Cr+3V≦3.00] As described above, Cr and V are elements that increase the contact fatigue strength and softening resistance of parts in high-temperature environments (e.g., around 400°C). To ensure tempering softening resistance, it is necessary to ensure that the combined content (Cr + 3V) of these two elements is equal to or greater than a predetermined amount, in addition to the lower limits of the individual Cr and V contents. On the other hand, if the Cr and V contents are too high, N is trapped in the surface layer, increasing the amount of N in the surface layer. This increases the amount of retained austenite in the surface layer after induction hardening, inhibiting nitrogen diffusion to the interior and reducing contact fatigue strength. Therefore, to ensure contact fatigue strength, it is necessary to set upper limits for the individual Cr and V contents as well as the combined content (Cr + 3V) of these two elements. In this embodiment, the steel composition is specified to satisfy formula (1) for Cr and V. Specifically, formula (1) is set to be 1.90 or more and 3.00 or less. Within this range, formula (1) can stably increase contact fatigue strength and tempering softening resistance in high-temperature environments. If formula (1) is less than 1.90, temper softening resistance cannot be ensured. If formula (1) exceeds 3.00, the N content becomes high only in the surface layer, and the penetration depth of N becomes significantly smaller, resulting in a decrease in contact fatigue strength. Preferably, the lower limit of formula (1) may be 2.10 or more, 2.30 or more, or 2.80 or more, and the upper limit may be 2.80 or less, 2.60 or less, or 2.30 or less.

[0040] 1.90≦Cr+3V≦3.00 ··· Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %.

[0041] In the composition of the nitride induction hardening steel of this embodiment, the balance other than the above elements essentially consists of Fe and impurities. The impurities are components contained in raw materials or components mixed in during the manufacturing process, and also include elements that are not intentionally added. Impurities are permissible as long as they do not impair the properties of the nitride induction hardening steel and nitride induction hardening parts according to this embodiment.

[0042] The steel material of this embodiment, i.e., the base material of the part, may further contain the following optional elements. In other words, the steel material of this embodiment may contain the following elements in place of a portion of Fe. However, the steel material and parts of this embodiment can solve the problem without containing the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[0043] [Cu: less than 0.10%] Cu is an element that can ensure hardenability and increase core hardness through solid solution strengthening. To ensure this effect, the Cu content can be set to 0.01% or more. On the other hand, if the Cu content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, it is recommended that the Cu content be less than 0.10%. When Cu is contained, the Cu content is preferably 0.02% or more, or 0.03% or more, and similarly, the Cu content is preferably 0.09% or less, or 0.08% or less.

[0044] [Ni: less than 0.05%] Ni is an element that can ensure hardenability and increase core hardness through solid solution strengthening. To ensure this effect, the Ni content can be set to 0.01% or more. On the other hand, if the Ni content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, it is recommended that the Ni content be less than 0.05%. When Ni is contained, the Ni content is preferably 0.02% or more, and similarly, the Ni content is preferably 0.04% or less, or 0.03% or less.

[0045] [Mo: less than 0.15%] Mo not only enhances hardenability, but also combines with N in the high-N martensite structure during tempering at high temperatures (e.g., about 400°C) to precipitate fine MoN, improving softening resistance. To achieve these effects, the Mo content may be 0.01% or more. However, a high Mo content increases the cost of the steel and significantly reduces the machinability of the base material due to excessive hardness after hot working. Therefore, the Mo content should be less than 0.15%. When Mo is contained, the Mo content is preferably 0.03% or more, or 0.05% or more, and similarly, the Mo content is preferably 0.10% or less, or 0.08% or less.

[0046] [W:0.50% or less] W has the effect of improving core hardness through solid solution strengthening. To ensure the effect of W, the W content should be 0.01% or more. On the other hand, if the W content is too high, the hardness of the base material after hot working becomes too high, significantly reducing the machinability of the base material, so the W content should be 0.50% or less. When W is contained, the W content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the W content is preferably 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0047] [Bi:0.50% or less] Bi has the effect of reducing cutting resistance and extending the tool life. To ensure this effect, the Bi content should be 0.01% or more. On the other hand, a high Bi content makes the material more susceptible to cracks and scratches during hot working, so the Bi content should be 0.50% or less. When Bi is contained, the Bi content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Bi content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0048] [Co:0.50% or less] Co is an element that increases core hardness through solid solution strengthening. To ensure this effect, the Co content should be 0.01% or more. On the other hand, if the Co content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, the Co content should be 0.50% or less. When Co is contained, the Co content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Co content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0049] [Ti:0.100% or less] Ti combines with N that penetrates the surface layer of the base material during nitriding and with C in the base material to form fine nitrides (TiN) and carbides (TiC), improving surface and core hardness. To ensure this effect, the Ti content should be 0.005% or more. On the other hand, a high Ti content makes it easier for coarse nitrides and carbides to form, so the Ti content should be 0.100% or less. When Ti is contained, the Ti content is preferably 0.007% or more, 0.010% or more, 0.015% or more, or 0.020% or more, and similarly, the Ti content is preferably 0.075% or less, 0.050% or less, or 0.025% or less.

[0050] [Nb:0.100% or less] Nb combines with N that penetrates the surface layer of the steel during nitriding and with C in the parent phase to form fine nitrides (NbN) and carbides (NbC), which has the effect of improving surface hardness and core hardness. To ensure this effect, the Nb content should be 0.005% or more. On the other hand, a high Nb content makes it easier for coarse nitrides and carbonitrides to form, so the Nb content should be 0.100% or less. When Nb is contained, the Nb content is preferably 0.010% or more, and similarly, the Nb content is preferably 0.050% or less.

[0051] [B:0.0020% or less] In addition to improving hardenability, solute B suppresses the grain boundary segregation of P and improves toughness. Furthermore, BN, which combines with N to precipitate, improves machinability. To ensure these effects, the B content should be 0.0005% (5 ppm) or more. On the other hand, a high B content promotes the segregation of large amounts of BN, which can lead to cracking of the steel, so the B content should be 0.0020% or less. When B is contained, the B content is preferably 0.0010% or more, and similarly, the B content is preferably 0.0015% or less.

[0052] [Ca:0.0010% or less] Calcium refines MnS and improves contact fatigue strength. To ensure this effect, the calcium content should be 0.0001% or more. However, if the calcium content is too high, the effect saturates and the cost becomes uneconomical, so the calcium content should be 0.0010% or less. When Ca is contained, the Ca content is preferably 0.0002% or more, 0.0003% or more, or 0.0004% or more, and similarly, the Ca content is preferably 0.0009% or less, 0.0008% or less, or 0.0007% or less.

[0053] [Mg:0.0100% or less] Mg has the effect of refining MnS and improving contact fatigue strength. To ensure the effect of Mg, the Mg content should be 0.0010% or more. On the other hand, if the Mg content is too high, the effect saturates and the cost becomes less efficient, so the Mg content should be 0.0100% or less. When Mg is contained, the Mg content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more, and similarly, the Mg content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0054] [Te:0.100% or less] Te refines MnS and improves contact fatigue strength. To ensure the effect of Te, the Te content should be 0.010% or more. On the other hand, if the Te content is too high, the effect saturates and the cost becomes less efficient, so the Te content should be 0.100% or less. When Te is contained, the Te content is preferably 0.020% or more, 0.030% or more, or 0.040% or more, and similarly, the Te content is preferably 0.090% or less, 0.080% or less, or 0.070% or less.

[0055] [Pb:0.08% or less] Pb has the effect of reducing cutting resistance and extending the life of tools, but if the Pb content is increased, the effect saturates, reducing economic viability and also leading to environmental impacts, so the Pb content should be kept below 0.08%. When Pb is contained, the Pb content is preferably 0.07% or less, 0.06% or less, or 0.05% or less in terms of reducing the environmental load.

[0056] [Sn:0.150% or less] Sn has the effect of reducing cutting resistance and extending the life of tools. To ensure this effect, the Sn content should be 0.0100% or more. On the other hand, if the Sn content is too high, the effect saturates and the cost becomes less efficient, so the Sn content should be 0.150% or less. When Sn is contained, the Sn content is preferably 0.020% or more, 0.030% or more, or 0.040% or more, and similarly, the Sn content is preferably 0.120% or less, 0.100% or less, or 0.080% or less.

[0057] [Sb:0.0100% or less] Sb has the effect of reducing cutting resistance and extending the life of tools. To ensure this effect, the Sb content should be 0.0010% or more. On the other hand, if the Sb content is too high, the effect saturates and the cost becomes less efficient, so the Sb content should be 0.0100% or less. When Sb is contained, the Sb content may be preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Sb content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0058] [REM:0.0100% or less] REM (rare earth elements) refers to a total of 17 elements consisting of Sc, Y, and lanthanoids. In this embodiment, "REM" refers to one or more elements selected from these rare earth elements, and the REM content refers to the total content of these 17 elements. When lanthanoids are used as REMs, they are industrially added in the form of misch metals.

[0059] REM works to refine MnS and improve contact fatigue strength. To ensure the effect of REM, the REM content should be 0.0010% or more. On the other hand, if the REM content is too high, the effect saturates and economic efficiency is compromised, so the REM content should be 0.0100% or less. When REM is contained, the REM content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more, and similarly, the REM content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0060] Next, the nitride induction hardened part of this embodiment will be described.

[0061] There is a certain correlation between the surface hardness of a part and its surface fatigue strength. However, as a result of the inventors' research, they found that the higher the surface temperature of the part (e.g., the tooth surface temperature in the case of a gear), the softer the surface becomes, so the surface fatigue strength of the part cannot be accurately evaluated by surface hardness alone. Furthermore, they found that a certain amount of N concentration is required in the surface layer of the part to increase its surface fatigue strength. Furthermore, although the deeper regions are not as affected by temperature as the surface, shear stress due to Hertzian contact still occurs. Therefore, the hardness in the regions with low N penetration due to nitriding must be ensured by the carbon (C) of the base material. Furthermore, if the nitrogen compound layer formed during nitriding remains thick even after induction hardening, a soft austenite structure may form directly below it, reducing surface fatigue strength. Based on the new knowledge gained above, the properties required for nitrided induction hardened parts are listed below.

[0062] [Nitrogen compound layer after nitriding induction hardening] The thickness of the nitrogen compound layer containing at least Fe and N on the surface of the part is 5 μm or less. The "nitrogen compound layer" here refers to a compound layer whose main constituent phases are Fe3N and Fe4N, and may also contain nitrogen compounds consisting of Fe, N, and alloying elements of the base material. When steel is nitrided, nitrogen penetrates through the steel surface, forming a nitrogen compound layer on the outermost surface, and a diffusion layer (a layer of nitrogen diffused into the steel matrix) is formed inside (toward the core) the nitrogen compound layer. The nitrogen compound layer formed on the steel surface during nitriding acts as a nitrogen supply source for the matrix during induction hardening, reducing its thickness and, in some cases, disappearing. However, if a thick nitrogen compound layer remains after induction hardening, a thick retained austenite layer will form directly below the nitrogen compound layer. Because the hardness of this retained austenite layer is lower than its surroundings (the nitrogen compound layer and the hardened layer directly below the retained austenite layer), the surface fatigue strength of the nitrided part is reduced. If the nitrogen compound layer after induction hardening is 5 μm or less, the retained austenite layer is hardly formed and good surface fatigue strength is obtained. Preferably, it is 3 μm or less. Note that there is no problem even if the nitrogen compound layer does not exist on the part after induction hardening, and therefore the nitrogen compound layer thickness may be 0 μm.

[0063] [Vickers hardness of the surface layer after nitriding induction hardening (surface hardness)] In a component, the Vickers hardness (surface layer hardness) at a depth of 0.10 mm from the surface is set to 670 HV or more. This surface layer hardness affects the surface fatigue strength of the component. A surface layer hardness of 670 HV or more provides good surface fatigue strength. On the other hand, a surface layer hardness of less than 670 HV results in low surface fatigue strength. Therefore, the surface layer hardness of a nitrided induction hardened component is set to 670 HV or more. A preferred lower limit of the surface layer hardness is 690 HV or more, and more preferably 710 HV or more. Note that the Vickers hardness in this embodiment refers to the Vickers hardness (HV) in accordance with JIS Z 2244:2009 "Vickers Hardness Test - Test Method." In addition, in this embodiment, "a position 0.10 mm deep from the surface" is located within the hardened layer.

[0064] Surface hardness refers to the average Vickers hardness calculated by cutting a part perpendicular to the main axis or longitudinal direction after nitriding induction hardening, mirror-polishing the resulting cross section, and measuring 10 random points at a depth of 0.10 mm (100 μm) from the part surface (positions perpendicular to the part surface) with a test force of 1.96 N.

[0065] [N concentration in the surface layer after nitriding induction hardening (Ns)] In the component, the average N concentration Ns (atomic %) in the region from the surface to a depth of 0.1 mm is 1.60% or more. The N concentration in the surface layer of a part after nitriding induction hardening refers to the N content (atomic %) measured by chemical analysis of chips collected by turning on a lathe from the area up to a depth of 0.1 mm from the surface of the part after nitriding induction hardening, and is referred to as Ns in this embodiment.

[0066] This Ns affects the contact fatigue strength of the part. If Ns is 1.60 atomic % or more, the N concentration in the martensite structure in the surface layer of the part is sufficiently high, resulting in high hardness after quenching and high contact fatigue strength. In addition, when the surface layer of the part is tempered at about 400°C, alloy nitrides precipitate, suppressing softening due to tempering. Therefore, Ns should be 1.60 atomic % or more. The preferred lower limit of Ns is 1.80 atomic % or more, and more preferably 2.00 atomic % or more. The upper limit of Ns is not particularly limited and may be 5.00 atomic % or less. Note that, if the part has a nitrogen compound layer on the surface layer, Ns also includes the N content in the nitrogen compound layer.

[0067] [The sum of N and C concentrations in the core of the hardened layer after nitriding induction hardening] When measured from the surface of the component, the average N concentration Nh and C concentration Ch in a region 0.05 mm before and after the depth position where the N concentration is 0.3 times the Ns (depth position where it is 0.3Ns) satisfy the following formula (2).

[0068] Nh+Ch>1.80...Equation (2) However, the element symbols in formula (2) indicate the content (atomic %) of the element.

[0069] The sum of the N and C concentrations in the core side of the hardened layer after nitriding induction hardening refers to the sum of the average N and C concentrations in a region of 0.1 mm, centered on the depth where the N concentration reaches 0.3 Ns, which is 0.05 mm from the surface side of the part and 0.05 mm from the core side. In this embodiment, these N and C concentrations are referred to as Nh and Ch. These values ​​affect high-temperature hardness and surface fatigue strength. In the region of the hardened layer where the amount of N penetration is small, i.e., the region of the hardened layer closer to the core side of the part, the hardness of the martensite structure increases due to the superposition of the penetrated N and the C concentration in the base material. In other words, although the amount of N penetration decreases toward the core side of the hardened layer, the hardness of the martensite can be ensured by the C in the base material. Therefore, sufficient high-temperature hardness and surface fatigue strength can be obtained if Nh + Ch satisfies the following formula (2). On the other hand, if formula (2) is not satisfied due to a low N penetration amount or a low C concentration in the base material, the high-temperature hardness and surface fatigue strength may be reduced even if the N concentration and surface hardness at 0.1 mm from the part surface are sufficiently high. The preferred lower limit of Nh + Ch is 2.00 atomic % or more, and more preferably 2.20 atomic % or more.

[0070] [Manufacturing method for nitride induction hardened parts] One embodiment of the nitride induction hardening steel according to the present embodiment and a method for manufacturing a part using the same will be described below, although the nitride induction hardening steel according to the present embodiment and the method for manufacturing the part using the same are not limited to this embodiment.

[0071] First, steel having the above-mentioned chemical composition is melted by a conventional method to produce steel materials such as ingots, slabs, billets, etc. These steel materials are then processed and heat treated by, for example, the following methods to produce parts.

[0072] [Hot processing] After cutting the steel material having the above-mentioned chemical composition to an appropriate size, it is heated to and maintained at a temperature in the range of 1050 to 1250°C, and then hot worked into a rough shape. Note that the heating and holding time is preferably 0.5 hours (30 minutes) to 4.0 hours to prevent insufficient uniform heating of the steel and to reduce the surface oxide film. The hot working is mainly hot rolling or hot forging, but is not limited to these working methods, and hot working may be performed in accordance with the above-mentioned methods.

[0073] In order to homogenize the crystal grain size of the structure after hot working, normalizing may be performed in accordance with JIS B 6911:2010 "Normalizing and annealing of steel" before cutting, which will be described later. The structure after hot working or normalizing is a mixed structure of ferrite + pearlite or ferrite + pearlite + bainite, and the average Vickers hardness is preferably 210 to 290 HV.

[0074] [Cutting] After hot working, the steel is cut using a lathe or other machine, and then processed into the desired part shape. For example, if the part is a gear, it is processed using broaching or other methods.

[0075] [Nitriding] The parts are machined into a predetermined shape and then nitrided. The nitriding method in this embodiment is not particularly limited, and well-known methods such as gas nitriding, gas soft nitriding, salt bath soft nitriding, and plasma nitriding can be used. The gas used in the nitriding may be NH3 alone, or a mixed gas containing NH3, N2, H2, and CO2. Alternatively, the part may be heat-treated (e.g., nitriding quenching) at a temperature significantly above the A1 point (approximately 590°C) of the Fe-N binary system to allow N to penetrate into the surface layer, followed by induction hardening, as described below. The treatment time (holding time) in the nitriding treatment is also not particularly limited, and may be, for example, 0.5 to 10.0 hours. Furthermore, chemical treatments such as coating removal or oxidation treatments may be performed as pre-treatments or post-treatments of the nitriding treatment, as long as the temperature does not exceed the nitriding temperature.

[0076] In addition, in the case of gas nitriding or nitriding quenching, in order to suppress voids in the surface layer, the nitriding potential K calculated by the following formula (3) is N (atm -1 / 2 ) should be in the range of 0.15 to 0.40. K N =P NH3 / P H2 3 / 2 ··· Equation (3) Here, P in equation (3) NH3 is the partial pressure of the atmospheric gas NH3 [atm], and P H2 is the H2 partial pressure of the atmospheric gas [atm].

[0077] [High-frequency hardening treatment] Induction hardening may be performed as a post-nitriding process to enhance surface fatigue strength. Induction hardening transforms the surface layer into a hardened layer composed of a high-N martensite structure containing nitride-forming elements in solid solution. Therefore, in the temperature range (approximately 200 to 400°C) of the part due to contact friction during surface fatigue, alloys such as Cr and V form nitride clusters, which reduces hardness and ensures high surface fatigue strength. Additionally, the deeper hardened layer reduces internally initiated fracture. To achieve these effects, induction hardening is preferably performed to a depth of 100 μm or more from the surface of the part. Furthermore, the heating temperature for induction hardening should be between 1000°C and 1200°C, and the time required to heat from room temperature to the heating temperature should be within 4 seconds. The time the steel is held in the temperature range between 1000°C and 1200°C should be between 0.2 and 2 seconds.

[0078] [Tempering treatment] The nitrided induction-hardened parts according to this embodiment may be manufactured by performing a tempering process after induction hardening. When performing tempering, it is preferable to hold the part at a temperature of 200°C or less for 60 to 150 minutes. Tempering at a temperature above 400°C is not preferable because it results in a loss of surface hardness. Tempering is not essential, and the part can be used without undergoing tempering.

[0079] The nitrided induction-hardened steel and nitrided induction-hardened parts of this embodiment have been described above, and this embodiment can provide nitrided induction-hardened steel and nitrided induction-hardened parts that are excellent in surface fatigue strength and softening resistance in high-temperature environments (e.g., about 400°C). Therefore, the nitrided induction-hardened steel of this embodiment is suitable as a material for, for example, automobiles and industrial machinery, particularly gears in machines powered by electric motors. [Example]

[0080] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0081] Steels a to ai having the chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace and cast to produce ingots. Of the components of steels a to ai shown in Table 1, the balance other than those shown in Table 1 consists of Fe and impurities. All steels a to ai contained approximately 10 ppm of O (oxygen) as an impurity. Blank cells in Table 1 indicate that no alloying elements were intentionally added, and underlined cells indicate that the elements are outside the scope of the present invention.

[0082] Ingots of each steel shown in Table 1 were hot forged into round bars with a diameter of 40 mm. For hot forging, the ingots were held in a heating furnace at a temperature between 1100°C and 1200°C for 2 hours, then forged into round bars with a diameter of 40 mm, and then allowed to cool in the air after forging.

[0083] Next, each hot-forged round bar was machined to produce small rollers for roller pitting tests to evaluate surface fatigue strength, as shown in Figure 1. Multiple small rollers were produced from one ingot for the roller pitting test, but in order to investigate the hardness of the roller cross section, more small rollers than were required for the roller pitting test were produced. When producing the small rollers from the ingot, the small rollers were cut so that their longitudinal direction coincided with the longitudinal direction of the round bar.

[0084] The collected small rollers (test specimens) were subjected to gas nitriding, gas soft nitriding, plasma nitriding, carbonitriding quenching, and nitriding quenching. Table 2 shows the heat treatment conditions. In all heat treatments, the temperature was 550 to 850°C, and the nitriding time was 1 to 10 hours. After nitriding, the test specimens were oil-cooled using 80°C oil.

[0085] Of the nitriding treatments, for gas nitriding and nitriding quenching, the test pieces were placed in a gas nitriding furnace and NH3, H2, and N2 gases were introduced into the furnace, and for gas soft nitriding, in addition to these gases, CO2 gas was introduced at a volume ratio of 3%.

[0086] The H2 partial pressure in the atmosphere was measured using a thermal conductivity H2 sensor directly attached to the gas nitriding furnace body. The difference in thermal conductivity between the standard gas and the measurement gas was converted into gas concentration. The H2 partial pressure was measured continuously during the gas nitriding process.

[0087] The NH3 partial pressure was measured using an infrared absorption NH3 analyzer installed outside the furnace. The NH3 partial pressure was measured continuously during the gas nitriding treatment. Note that for test number 5, which was performed in an atmosphere containing CO2 gas, (NH4)2CO3 was precipitated in the infrared absorption NH3 analyzer, which could have caused the device to break down, so the NH3 partial pressure was measured every 10 minutes using a glass tube NH3 analyzer.

[0088] Average nitriding potential K for gas nitriding, nitriding quenching, and gas soft nitriding N (atm -1 / 2 ) is defined by the following formula (3). K N =P NH3 / P H2 3 / 2 ··· Equation (3) Here, P in equation (3) NH3 is the partial pressure of NH3 in the atmosphere (atm), and P H2 is the H2 partial pressure (atm) of the atmospheric gas.

[0089] Nitriding potential K calculated in the device NThe NH3 flow rate and N2 flow rate were controlled so that the nitriding potential K converged to the target value. N Record the K measured during the treatment time. N The average value was calculated. For the plasma nitriding treatment, the test piece was placed in the plasma nitriding equipment, and H2 and N2 gases were introduced into the furnace, with the gas flow rate controlled so that the partial pressure ratio of H2 gas to N2 gas was kept constant at 3:1.

[0090] These nitrided test pieces were subjected to induction hardening. In all treatments, the heating temperature was 100 to 1200°C, the time required to raise the temperature from room temperature to the heating temperature was 3 seconds, and the nitrided test pieces were held for 1 to 10 seconds. Immediately after induction hardening, the test pieces were quenched in room temperature water. They were then tempered at 170°C for 1.5 hours.

[0091] [Measurement of the thickness of the compound layer on the surface] The cross section of the small roller that had been subjected to the induction hardening treatment described above, perpendicular to the longitudinal direction, was mirror-polished and then etched. The etched cross section was observed using a scanning electron microscope (SEM) to measure the thickness of the nitrogen compound layer. Etching was performed using a 3% nital solution for 20 to 30 seconds.

[0092] The nitrogen compound layer can be confirmed as a white, uncorroded layer on the surface. 2 μm 2 The thickness of the compound layer was measured at three points every 10 μm. The average value of the 30 measured points was defined as the thickness of the nitrogen compound (μm).

[0093] [Measurement of N and C concentrations] The surface layer of the test section (φ26 mm) of each small roller that had undergone the above-mentioned nitriding induction hardening treatment, which was the region from the surface to a depth of 1.5 mm, was lathe-machined and chips were collected at 0.05 mm intervals. The N and C contents (atomic %) were measured by chemical analysis. The average N concentration Ns was then calculated for the region from the surface to a depth of 0.1 mm. Similarly, at the depth where the N concentration was 0.3Ns, the sum of the average N concentration Nh and C concentration Ch was calculated for the region 0.05 mm before and after that depth.

[0094] [Surface hardness measurement] A sample (thickness: 10 mm) with a cross section perpendicular to the longitudinal direction was taken from the test section (φ26 mm) of each small roller that had been subjected to the above-mentioned nitriding induction hardening treatment, and the cross section was then mirror-polished.The Vickers hardness was then measured at 10 random points 0.1 mm (100 μm) deep from the cross section (polished surface) using a micro Vickers hardness tester (Shimadzu Corporation; HMV-G31-FA) under a test force of 1.96 N.The average value of these 10 points was defined as the surface hardness.

[0095] [Surface fatigue strength evaluation test] The surface fatigue strength was evaluated using a roller pitting tester (Komatsu Equipment Co., Ltd.; RP102) according to the following method. Small rollers for the roller pitting test were made using steels a to ai. The gripping parts were then finish-machined to remove heat treatment strain, and each was then used as a roller pitting test specimen. The shape after finish machining is shown in Figure 1.

[0096] The small roller, which is the roller pitting test specimen, has a central test surface with a diameter of 26 mm and a width of 28 mm, and gripping parts with a diameter of 22 mm on both sides, as shown in Figure 1. In the roller pitting test, the test surface was brought into contact with the large roller, and a predetermined surface pressure was applied before the roller was rotated. The roller pitting test (two-cylinder rolling fatigue test) was carried out under the conditions shown in Table 3 using a combination of the small roller for roller pitting test described above and a large roller for roller pitting test having the shape shown in FIG. The dimensions in Figures 1 and 2 are in mm.

[0097] The large rollers for roller pitting tests were made using steel that met the SCM420 standard of JIS G 4053:2016, using the general manufacturing process of "normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → low-temperature tempering → polishing." The Vickers hardness HV at a depth of 0.05 mm (50 μm) from the surface was 740 to 760, and the depth at which the Vickers hardness Hv was 550 or more was in the range of 0.8 to 1.0 mm.

[0098] Table 3 shows the test conditions for evaluating the surface fatigue strength. The test was stopped at a cycle count of 2.0 × 10, which indicates the fatigue limit of general steel. 7 The small roller test piece was tested for 2 x 10 7 The maximum surface pressure reached was taken as the fatigue limit of the small roller test piece.

[0099] The occurrence of pitting was detected using a vibration meter attached to the testing machine. After vibration occurred, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed. In this example, assuming application to gear components, the reference values ​​used as evaluation criteria for surface fatigue strength were as follows: First, small rollers were produced using steel meeting the SCr420 standard of JIS G 4053:2016, following a typical manufacturing process: normalizing, processing test pieces, eutectoid carburizing in a gas carburizing furnace, and low-temperature tempering. Next, the roller pitting test was performed using the small rollers, and the fatigue limit obtained was used as the reference value for surface fatigue strength in this example. When the fatigue limit was 1.10 times or more the standard value, the contact fatigue strength was judged to be excellent (marked with "○" in Table 2). On the other hand, when the fatigue limit was less than 1.10 times the standard value, the contact fatigue strength was judged to be low (marked with "X" in Table 2).

[0100] [High temperature hardness evaluation test] In this example, assuming that the parts would be used in a high-temperature environment, the high-temperature hardness was evaluated based on the hardness after tempering at 400°C for 24 hours. Each small roller subjected to the nitriding induction hardening process was heat-treated in an atmospheric annealing furnace at a holding temperature of 400°C for 24 hours. The rollers were then cut into 10 mm lengths perpendicular to the longitudinal center and mirror-polished. The Vickers hardness was then measured at 10 random points 0.1 mm (100 μm) deep from the surface using a micro-Vickers hardness tester (Shimadzu Corporation; HMV-G31-FA) under a test force of 1.96 N. The average value of these 10 points was defined as the "surface hardness after 400°C tempering." In this example, the target surface hardness after 400°C tempering was 710 HV or higher.

[0101] [Test Results] The results are shown in Table 2. In Table 2, the underlined values ​​indicate values ​​outside the scope of the present invention, and the "*" mark indicates that the target of the present invention has not been achieved.

[0102] Test Nos. 1 to 19 had steel compositions within the range of the present invention, and therefore had high surface N concentrations Ns and surface hardness after nitriding induction hardening, and excellent surface hardness and surface fatigue strength after tempering at 400°C.

[0103] On the other hand, test numbers 20 to 35 had steel compositions outside the range of the present invention, and the surface N concentration Ns and surface hardness after nitriding induction hardening were low, and did not achieve the desired surface hardness and surface fatigue strength after tempering at 400°C.

[0104] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

[0105] [Table 1]

[0106] [Table 2]

[0107] [Table 3] [Industrial Applicability]

[0108] The present invention can be used in a wide range of industrial fields as a material for automobiles and industrial machinery, particularly gears for machines powered by electric motors.

Claims

1. In mass%, C: 0.25% or more, 0.57% or less, Si: 0.10% or more and less than 0.45% Mn: 0.40% or more, 1.00% or less, P: 0.030% or less, S: 0.095% or less, Cr: 1.15% or more and less than 1.95% V: more than 0.05% and less than 0.50%; Al: 0.100% or less, and N: 0.0250% or less wherein Cr and V satisfy the following formula (1), and the balance is Fe and impurities: 1.90≦Cr+3V≦3.00... Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %.

2. Furthermore, in mass%, Cu: less than 0.10% Ni: less than 0.05% Mo: less than 0.15% W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Ti: 0.100% or less, Nb: 0.100% or less, and B: 0.0020% or less 2. The steel for nitride induction hardening according to claim 1, characterized in that it contains one or more of the following:

3. Furthermore, in mass%, Ca: 0.0010% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.08% or less, Sn: 0.150% or less, Sb: 0.0100% or less, and REM: 0.0100% or less 3. The steel for nitride induction hardening according to claim 1, further comprising one or more of the following:

4. A part that has been nitrided and induction hardened, The composition of the core is, by mass%, C: 0.25% or more, 0.57% or less, Si: 0.10% or more and less than 0.45% Mn: 0.40% or more, 1.00% or less, P: 0.030% or less, S: 0.095% or less, Cr: 1.15% or more and less than 1.95% V: more than 0.05% and less than 0.50%; Al: 0.100% or less, and N: 0.0250% or less wherein Cr and V satisfy the following formula (2), and the balance is Fe and impurities, The thickness of the nitrogen compound layer formed on the surface layer and containing at least Fe and N is 5 μm or less, The Vickers hardness at a depth of 0.10 mm from the surface is 670 HV or more, The average N concentration Ns in the region from the surface to a depth of 0.1 mm is 1.60 atomic % or more, A nitrided induction hardened component characterized in that the average N concentration Nh and C concentration Ch in a region 0.05 mm before and after the depth position where the N concentration is 0.3Ns (atomic%) satisfy the following formula (3). 1.90≦Cr+3V≦3.00... Formula (2) Nh+Ch>1.80...Formula (3) However, each element symbol in the above formula (2) represents the content of the element in mass %.

5. The components of the core portion are further, in mass%, Cu: less than 0.10% Ni: less than 0.05% Mo: less than 0.15% W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Ti: 0.100% or less, Nb: 0.100% or less, and B: 0.0020% or less 5. The nitride induction hardened part according to claim 4, wherein the nitride induction hardened part contains one or more of the following:

6. The components of the core portion are further, in mass%, Ca: 0.0010% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.08% or less, Sn: 0.150% or less, Sb: 0.0100% or less, and REM: 0.0100% or less 6. The nitride induction hardened part according to claim 4, wherein the nitride induction hardened part contains one or more of the following:

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