Steel component

Abstract

To provide a steel component which is excellent in surface fatigue strength.SOLUTION: A steel component includes a steel core part, a compound layer, and a nitrogen diffusion layer existing between the steel core part and the compound layer, wherein the steel core part has a predetermined chemical composition, thickness t of the compound layer is 5 to 30 μm, an area ratio of a γ' phase in the compound layer at a depth of 0 to t / 2 from the surface of the compound layer is 70% or more, the remainder is an ε phase, an area ratio of the ε phase in the compound layer at a depth of t / 2 to t from the surface of the compound layer is 70% or more, the remainder is the γ' phase, Vickers hardness at a depth position of 0.05 mm from the surface of the nitrogen diffusion layer is 630 HV or more, and Vickers hardness at a depth position of 1.00 mm from the surface of the nitrogen diffusion layer is 210 HV or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel components. [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 and wear resistance. For this reason, these parts are made from alloy steels for machine structures, 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 characteristics required for steel parts such as gears, especially power transmission parts, may change dramatically in the future. For example, electric vehicles (EVs) often incorporate reducers directly below the motor that powers them to ensure torque. Miniaturizing these units leads to lighter vehicles and greater design freedom, making demand for them a promising area for future growth. However, because EVs are quieter while running, free from the noise and vibrations of internal combustion engines, gear noise from reducers becomes more pronounced. Therefore, in addition to the increased strength required for miniaturization, reducer gears will require larger grinding allowances than ever before to reduce noise and vibration and achieve a desired shape, raising concerns about increased grinding costs.

[0004] Carburizing and quenching is a method of heating steel parts 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, and as such, distortion after treatment (heat treatment distortion) is likely to be large, which increases grinding costs in subsequent processes. As mentioned above, there are concerns about increased grinding costs, particularly for reducer gears for EVs, so there is a high need to switch to a heat treatment method that contributes to reducing grinding costs.

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

[0006] Nitriding is a surface hardening heat treatment that causes nitrogen to penetrate the surface of steel, while soft nitriding is a surface hardening heat treatment that causes nitrogen and carbon to penetrate the surface of steel. Media used for nitriding and soft nitriding include gas, salt bath, and plasma. Gas nitriding and gas soft nitriding, which are excellent in productivity, are mainly used for automotive parts.

[0007] The hardened layer formed by gas nitriding and gas soft nitriding consists of a nitrogen diffusion layer (hereinafter sometimes abbreviated as "diffusion layer") and a compound layer several to several tens of μm thick formed on the surface side of the diffusion layer.

[0008] The diffusion layer is a layer hardened by the solid solution strengthening mechanism of intercalated nitrogen and carbon, and the particle dispersion strengthening mechanism of nitrides. Increasing the hardness and depth of the diffusion layer improves the surface fatigue strength of the part. Much research has been done on increasing the hardness and depth of the diffusion layer.

[0009] The compound layer is mainly composed of iron nitrides, Fe2N-Fe3N (ε phase) and Fe4N (γ' phase), and is extremely hard compared to the parent phase. Therefore, the compound layer is effective in improving wear resistance. The ε phase has a larger solid solubility range of C and a faster growth rate than the γ' phase. For this reason, soft nitriding, which involves mixing carburizing gas, tends to form a compound layer mainly composed of the ε phase. Therefore, soft nitriding can produce a thick compound layer in a shorter time than nitriding, and regardless of the type of steel used for the part. For this reason, soft nitriding has long been used to improve the wear resistance of parts. On the other hand, nitriding is a heat treatment performed at a relatively low temperature, resulting in a small hardened layer depth and inferior surface fatigue strength compared to carburizing and quenching. For this reason, nitriding is difficult to apply to parts that require high fatigue strength. Furthermore, nitrided parts are prone to reduced surface fatigue strength due to the presence of brittle compound layers and voids formed near the surface of the compound layers. Therefore, in recent years, a method has been proposed in which the morphology of the compound layer is controlled by controlling the nitriding atmosphere, thereby increasing fatigue strength.

[0010] Patent Document 1 discloses a nitrided part and a nitriding method in which the fatigue strength is improved by setting the γ' phase ratio in the compound layer to 30 mol % or more.

[0011] Patent Document 2 proposes a nitrided steel member in which the compound layer has a thickness of 5 μm to 50 μm and the volume ratio of the γ' phase in the compound layer is 20% or more. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-117412 [Patent Document 2] Japanese Patent Publication No. 2022-068375 Summary of the Invention [Problem to be solved by the invention]

[0013] The nitriding method disclosed in Patent Document 1 is a gas soft nitriding method using CO2 as the atmospheric gas, and therefore the surface side of the compound layer is likely to become the ε phase, and therefore the surface fatigue strength is thought to be insufficient.

[0014] In the nitrided steel member disclosed in Patent Document 2, the nitriding treatment is performed in the austenite region, so the hardness of the diffusion layer surface is lower than that of general nitriding in the ferrite region, and it is thought that sufficient surface fatigue strength cannot be obtained.

[0015] The present disclosure has been made to solve the above-mentioned problems, and has an object to provide a steel part having excellent contact fatigue strength. [Means for solving the problem]

[0016] The gist of the means for solving the above problems is as follows. <1> a steel core portion, a compound layer containing iron nitride, and a nitrogen diffusion layer present between the steel core portion and the compound layer, The steel core portion is, in mass%, C: 0.05 to 0.35%, Si: 0.05 to 2.00%, Mn: 0.60~2.50% P: 0.030% or less, S: 0.100% or less, Cr: 0.20~2.50%, V: 0.02~1.30%, Al: 0.100% or less, and N: 0.0250% or less, and the balance being Fe and impurities, The thickness of the compound layer is 5 to 30 μm, When the thickness of the compound layer is t, the area ratio of the γ' phase in the compound layer in a depth range from 0 to t / 2 from the surface of the compound layer is 70% or more, and the remainder is the ε phase, an area ratio of the ε phase in the compound layer in a depth range from t / 2 to t from the surface of the compound layer is 70% or more, and the remainder is the γ' phase, The Vickers hardness at a position 0.05 mm deep from the surface of the nitrogen diffusion layer is 630 HV or more, A steel part in which the Vickers hardness at a position 1.00 mm deep from the surface of the nitrogen diffusion layer is 210 HV or more. <2> a steel core portion, a compound layer containing iron nitride, and a nitrogen diffusion layer present between the steel core portion and the compound layer, The steel core portion is, in mass%, C: 0.05 to 0.35%, Si: 0.05 to 2.00%, Mn: 0.60~2.50% P: 0.030% or less, S: 0.100% or less, Cr: 0.20~2.50%, V: 0.02~1.30%, Al: 0.100% or less, and N: 0.0250% or less, and further containing one or more elements selected from the group consisting of the following Group A to Group D, with the balance being Fe and impurities, [Group A] Mo: 1.50% or less Ti: 0.100% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group B] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, and Sb: 0.100% or less, one or more selected from the group consisting of [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.020% or less, one or more selected from the group consisting of The thickness of the compound layer is 5 to 30 μm, When the thickness of the compound layer is t, the area ratio of the γ' phase in the compound layer in a depth range from 0 to t / 2 from the surface of the compound layer is 70% or more, and the remainder is the ε phase, an area ratio of the ε phase in the compound layer in a depth range from t / 2 to t from the surface of the compound layer is 70% or more, and the remainder is the γ' phase, The Vickers hardness at a position 0.05 mm deep from the surface of the nitrogen diffusion layer is 630 HV or more, A steel part in which the Vickers hardness at a position 1.00 mm deep from the surface of the nitrogen diffusion layer is 210 HV or more. <3> The chemical composition of the steel core contains the A group <2> The steel parts described in <4> The chemical composition of the steel core contains the B group <2> or <3> The steel parts described in <5> The chemical composition of the steel core contains the C group <2> ~ <4> 1. Steel parts according to any one of the preceding items. <6> The chemical composition of the steel core contains the D group <2> ~ <5> 1. Steel parts according to any one of the preceding items. [Effects of the Invention]

[0017] According to the present disclosure, a steel part having excellent contact fatigue strength can be provided. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing an example of a small roller test piece for a roller pitching test. [Figure 2] FIG. 1 is a diagram showing an example of a large roller test piece for a roller pitching test. [Figure 3] FIG. 1 is a schematic diagram of a roller pitching test. [Figure 4] FIG. 1 shows an example of the results of SEM-EBSD analysis of a cross-sectional surface layer of nitrided steel. DETAILED DESCRIPTION OF THE INVENTION

[0019] Hereinafter, a steel part according to an embodiment of the present disclosure will be described.

[0020] In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. However, when the numerical values ​​written before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values ​​as the lower or upper limit. The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (for example, C amount, Si amount, etc.). With respect to the content of elements in the chemical composition, "%" means "mass %." When the content of an element in the chemical composition is stated as "0~", it means that the element does not have to be contained. In addition, in the numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an Example. Furthermore, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an Example. Furthermore, the term "process" does not only refer to an independent process, but also includes processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0021] Although the use and manufacturing method of the steel part according to the present disclosure are not particularly limited, a gas-nitrided steel part is preferable. Hereinafter, a gas-nitrided steel part (hereinafter sometimes simply referred to as a "part") will be described as a representative example (one embodiment) of the steel part according to the present disclosure.

[0022] The steel part according to the present disclosure is, for example, a part obtained by nitriding steel material, and has a steel core (hereinafter sometimes simply referred to as the "core") which 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 portion that was not penetrated by nitrogen during the nitriding treatment. In other words, the core is a region where, despite having undergone the nitriding treatment, there is no change in chemical composition or the change is so small that it can be ignored, and it is a portion that has the same chemical composition (sometimes simply referred to as "composition") as the base material of the part (the steel before the nitriding treatment). The composition of the core can also be said to be the composition at a depth of 1.0 mm or more from the surface of the part, for example. The hardened layer refers to the area where nitrogen has penetrated due to nitriding. In other words, the hardened layer is the area where the chemical composition and / or metal structure has changed due to the effects of nitriding. For example, it refers to the area from the surface of the part to a depth of less than approximately 1.0 mm. The deeper the layer, the smaller the effect of the penetrated N due to nitriding, so the hardness approaches that of the core.

[0023] In order to solve the above-mentioned problems, the inventors of the present disclosure changed various alloy components and nitriding treatment conditions, and investigated the effects of each condition on the morphology of the compound layer in the surface layer of the part, the hardness of the part, and the contact fatigue strength, and obtained the following findings.

[0024] (a) By providing a highly tough γ'-phase-based layer on the surface side of the compound layer and a highly hard ε-phase-based layer on the diffusion layer side of the compound layer, the γ'-phase can suppress the propagation of cracks during tooth surface fatigue, and the ε-phase directly below can prevent the compound layer from being completely worn away. (b) Furthermore, if there is a large difference in hardness between the ε phase and the diffusion layer immediately below it, this can become the starting point for crack initiation. Therefore, ensuring the hardness of the surface layer by using a specified alloy composition is effective in improving surface fatigue strength. (c) In addition, by specifying the hardness at a position 1.0 mm from the surface, internal yielding in surface fatigue can be suppressed, and surface fatigue strength can be increased. (d) In order to stably control the constituent phases of the compound layer, it is important to maintain the treatment atmosphere at a predetermined value before the start of the nitriding treatment and to nitride the steel in the target atmosphere immediately after the start of the treatment. In addition, it is important to keep the upper and lower limits and average of the atmosphere within the predetermined ranges during the treatment.

[0025] The steel parts according to the present disclosure have been completed based on the above findings.

[0026] First, the chemical composition of the steel part according to the present disclosure will be described.

[0027] [Chemical composition] The chemical composition (component elements) of the core of the steel part according to the present disclosure will be described. The "%" for the content of each component element means "mass %" unless otherwise specified. The chemical composition of the core of the steel part according to the present disclosure is the same as the chemical composition of the steel material (base material) before nitriding treatment.

[0028] [C: 0.05~0.35%] C is an element necessary for stabilizing the ε phase in the compound layer and ensuring the core hardness of the part. To achieve these effects, 0.05% or more of C is required. On the other hand, if the C content exceeds 0.35%, the proportion of the ε phase becomes too high, resulting in a significant decrease in surface fatigue strength. Furthermore, the strength after hot forging becomes too high, resulting in a significant decrease in machinability. The preferred range of C content is 0.07 to 0.25%, and more preferably 0.10 to 0.20%.

[0029] [Si: 0.05~2.00%] Silicon is effective in improving surface fatigue strength because it forms fine nitrides (Si3N4) in compound layers and diffusion layers during nitriding treatment, increasing hardness. Silicon is also an element that increases core hardness through solid solution strengthening. To achieve these effects, 0.05% or more of silicon is required. On the other hand, if the silicon content exceeds 2.00%, the strength of steel bars, wire rods, and after hot forging becomes too high, significantly reducing machinability. The preferred range of silicon content is 0.10 to 1.50%.

[0030] [Mn: 0.60~2.50%] Mn is an element effective in improving contact fatigue strength because it forms fine nitrides (Mn3N2) in compound layers and diffusion layers during nitriding, increasing hardness. It also increases core hardness through solid solution strengthening. To achieve these effects, Mn content must be 0.60% or more. On the other hand, if the Mn content exceeds 2.50%, not only does the effect of increasing contact fatigue strength saturate, but the hardness of the steel bar and wire rod used as raw material, and after hot forging, becomes too high, significantly reducing machinability. The preferred range of Mn content is 0.80 to 2.30%.

[0031] [P:0.030% or less] P is an impurity that segregates at grain boundaries and embrittles parts, so a low content is preferable. If the P content exceeds 0.030%, contact fatigue strength may decrease. To prevent a decrease in contact fatigue strength, the upper limit of the P content is preferably 0.020%. The P content may be 0, but it is difficult to achieve a completely 0% content, so a P content of 0.001% or more may be used.

[0032] [S:0.100% or less] S is an element that 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 0.100% 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 economic efficiency of refining, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[0033] [Cr: 0.20~2.50%] Cr is an effective element for improving contact fatigue strength because it forms fine nitrides (CrN) in compound layers and diffusion layers during nitriding, increasing hardness. To achieve these effects, 0.20% or more Cr is required. On the other hand, if the Cr content exceeds 2.50%, not only does the effect of improving contact fatigue strength saturate, but the hardness of the raw material, such as steel bars and wire rods, and after hot forging, becomes too high, significantly reducing machinability. The preferred range of Cr content is 0.50 to 2.20%.

[0034] [V:0.02~1.30%] V is an element that is effective in improving contact fatigue strength because it forms fine nitrides (VN) in compound layers and diffusion layers during nitriding, increasing hardness. To achieve these effects, a V content of 0.02% or more is required. On the other hand, if the V content exceeds 1.30%, not only does the effect of improving contact fatigue strength saturate, but the hardness of the raw material, such as steel bars and wire rods, and after hot forging, becomes too high, significantly reducing machinability. The preferred V content range is 0.05 to 1.00%.

[0035] [Al:0.100% or less] Al is a deoxidizing element. In addition, Al combines with N to form AlN, which has the effect of refining the structure of the base material before nitriding through the pinning action of austenite grains 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%.

[0036] [N:0.0250% or less] Nitrogen (N) bonds with Si, Mn, Cr, V, and Al to form Si3N4, Mn3N2, CrN, VN, and AlN, respectively, and AlN in particular has the effect of refining the structure of the base material before nitriding through the pinning action of 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 be formed, 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 is 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.

[0037] In the chemical composition of the core of the steel part according to the present disclosure, the balance other than the above elements essentially consists of Fe and impurities. Impurities are components contained in raw materials or components mixed in during the manufacturing process, and include elements that are not intentionally added, and are acceptable as long as they do not impair the properties of the steel material and steel part according to the present disclosure.

[0038] The core of the steel part according to the present disclosure may further contain the following optional elements. That is, the core of the steel part according to the present disclosure may contain the following elements in place of a portion of Fe. However, the part according to the present disclosure can solve the problem without containing the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[0039] [Mo: 0-1.50%] Mo stabilizes the ε phase in the compound layer and also forms fine nitrides (MoN) in the compound layer and diffusion layer, increasing hardness, making it an effective element for improving contact fatigue strength. To achieve these effects, Mo content is preferably 0.01% or more. On the other hand, if the Mo content exceeds 1.50%, not only does the ratio of γ' phase decrease, but the hardness of the raw material, such as steel bar and wire rod, and after hot forging, becomes too high, significantly reducing machinability. The preferred range of Mo content is 1.00% or less.

[0040] [Ti: 0~0.100%] Ti combines with N to form TiN, improving the core hardness and surface hardness. To achieve this effect, the Ti content is preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the effect of improving the core hardness and surface hardness saturates and the alloy cost increases. The preferred range of Ti content is 0.080% or less.

[0041] [Nb: 0~0.050%] Nb combines with C and N to form NbC and NbN, which have the effect of pinning austenite grains, refining the structure of the steel before nitriding, and reducing the variation in the mechanical properties of nitrided parts. To achieve this effect, the Nb content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, coarse NbC and NbN are formed, making it difficult to achieve the above effect. The preferred range of N content is 0.040% or less.

[0042] [Cu: 0-0.50%] 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, the Cu content should be set to 0.50% or less. When Cu is contained, the Cu content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Cu content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0043] [Ni: 0-0.50%] Ni is an element that ensures hardenability and increases core hardness through solid solution strengthening. To ensure this effect, the Ni content may 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, so the Ni content should be set to 0.50% or less. When Ni is contained, the Ni content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Ni content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0044] [W:0~0.50%] W has the effect of increasing hardenability and improving the strength of steel. To ensure the effects of W, the W content should be 0.01% or more. On the other hand, if the W content is too high, the base material will become too hard after hot working, 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.

[0045] [Co: 0-0.100%] Co is an element that increases core hardness through solid solution strengthening. To ensure this effect, the Co content should be 0.001% 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 0.100% or less. When Co is contained, the Co content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Co content may be preferably 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0046] [B: 0~0.0100%] 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 0.0100% or less. When B is contained, the B content is preferably 0.0010% or more, and similarly, the B content is preferably 0.0050% or less.

[0047] [Bi: 0-0.100%] Bi has the effect of reducing cutting resistance and extending the tool life. To ensure this effect, the Bi content should be 0.001% 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 0.100% or less. When Bi is contained, the Bi content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Bi content may be preferably 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0048] [Te: 0~0.100%] Te reduces cutting resistance and extends the tool life. To ensure the effects of Te, the Te content should be 0.010% or more. However, 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, or 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.

[0049] [Pb: 0~0.09%] Pb reduces cutting resistance and extends the life of tools, but if the Pb content is increased, the effect saturates, reducing economic viability and also increasing environmental impact, so the Pb content should be kept below 0.09%. When Pb is contained, the Pb content is preferably 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less from the viewpoint of suppressing the environmental load.

[0050] [Sn: 0~0.100%] Sn reduces cutting resistance and extends the tool life. To ensure this effect, the Sn content should be 0.001% or more. However, if the Sn content is too high, the effect saturates and the cost becomes less efficient, so the Sn content should be 0.100% or less. When Sn is contained, the Sn content is preferably 0.005% or more, or 0.010% or more, and similarly, the Sn content is preferably 0.090% or less, or 0.080% or less.

[0051] [Sb: 0~0.100%] Sb reduces cutting resistance and extends the tool life. To ensure this effect, the Sb content should be 0.001% or more. However, if the Sb content is too high, the effect saturates and the cost becomes uneconomical, so the Sb content should be 0.100% or less. When Sb is contained, the Sb content may be preferably 0.005% or more, or 0.006% or more, and similarly, the Sb content may be preferably 0.090% or less, or 0.080% or less.

[0052] [Ca: 0~0.0100%] Calcium refines MnS and improves contact fatigue strength. To ensure this effect, the calcium content should be 0.0010% or more. However, if the calcium content is too high, the effect saturates and the cost becomes uneconomical, so the calcium content should be 0.0100% or less. When Ca is contained, the Ca content is preferably 0.0020% or more, or 0.0030% or more, or 0.0040% or more, and similarly, the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0053] [Mg: 0~0.0100%] 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, or 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] [REM:0~0.020%] REM (rare earth elements) refers to a total of 17 elements consisting of Sc, Y, and lanthanides. In this disclosure, "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 lanthanides are used as REM, industrially, the REM is added in the form of misch metal.

[0055] REM works to refine MnS and improve contact fatigue strength. To ensure the effect of REM, the REM content should be 0.001% 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 0.020% or less. When REM is contained, the REM content is preferably 0.002% or more, 0.003% or more, or 0.004% or more, and similarly, the REM content is preferably 0.018% or less, 0.016% or less, or 0.014% or less.

[0056] The above optional elements can be divided into the following groups A to D from the viewpoint of their functions and effects. The core of the steel part according to the present disclosure may contain one or more elements selected from these groups. [Group A] Improved hardness of nitrided layer Mo: 1.50% or less Ti:0.100% or less Nb: 0.050% or less [Group B] Improved core hardness through solid solution strengthening Cu: 0.50% or less Ni: 0.50% or less W: 0.50% or less Co:0.100% or less B: 0.0100% or less [Group C] Tool life extension Bi:0.100% or less Te: 0.100% or less Pb: 0.09% or less Sn: 0.100% or less Sb: 0.100% or less [Group D] Improved surface fatigue strength due to refined MnS Ca:0.0100% or less Mg: 0.0100% or less REM: 0.020% or less

[0057] Next, the configuration of the core of the steel part according to the present disclosure other than the chemical composition will be described.

[0058] [Compound layer thickness (t): 5 to 30 μm] The compound layer is an iron nitride layer formed by nitriding, and its thickness affects the surface fatigue strength of the nitrided component. If the compound layer is too thick, it is likely to become the starting point of surface fatigue (pitting damage). If the compound layer is too thin, sufficient surface fatigue strength may not be obtained. In the nitrided component according to the present disclosure, the thickness of the compound layer is set to 5 to 30 μm from the viewpoint of ensuring surface fatigue strength. The preferred range of the thickness of the compound layer is 8 to 25 μm.

[0059] The thickness of the compound layer is measured by observing a cross section in the thickness direction of the steel part. For example, after gas nitriding a steel material having the chemical composition of the core of the part according to the present disclosure, the vertical cross section of the obtained test material is polished, etched, and observed with a scanning electron microscope (SEM) for measurement. Etching is performed with a 3% nital solution for 20 to 30 seconds. The compound layer is present on the surface layer of the part according to the present disclosure and is observed as a white, uncorroded layer. A structural photograph taken at 4000x magnification was taken of 10 fields of view (field area: 6.6 × 10 2 μm 2 ) and measure the thickness of the compound layer at three points every 10 μm in the horizontal direction. The average value of the 30 measured points is defined as the compound layer thickness (μm).

[0060] [Area ratio of γ' phase in the compound layer in the depth range from 0 to t / 2 from the surface: 70% or more] The γ' phase has an fcc structure and is tougher than the ε phase, which has a hcp structure. In addition, because the driving force for pore formation is smaller than that of the ε phase, the formation of a porous layer on the surface can be suppressed, thereby suppressing pitting in surface fatigue. To achieve this effect, the area ratio of the γ' phase within the thickness t / 2 range from the surface must be 70% or more. It is preferably 80% or more, and more preferably 90% or more. The remainder is the ε phase.

[0061] [Area ratio of ε phase in the compound layer in the depth range from t / 2 to t from the surface: 70% or more] The ε phase has a wider solid solution range for N and C than the γ' phase, and is therefore harder. Therefore, even if the γ' phase on the surface side wears away during contact fatigue, the presence of the ε phase directly below it can prevent the compound layer from being worn away, thereby suppressing pitting during contact fatigue. To achieve this effect, the area ratio of the ε phase in the thickness range from t / 2 to t must be 70% or more. Preferably, it is 80% or more, and more preferably, it is 90% or more. The remainder is the γ' phase.

[0062] The area ratios of the γ' and ε phases are determined by image processing of the structural photograph. Specifically, 10 fields of view (field area: 6.6 × 10) of cross-sectional structural photographs of the surface layer of the nitrided material (part) were taken at 4000x magnification using electron backscatter diffraction (EBSD, manufactured by EDAX). 2 μm 2 ), the range from the surface to t / 2 and the range from t / 2 to t are selected, respectively, and phase maps based on the differences in crystal structure are created. The γ' phase (fcc) and ε phase (hcp) in the compound layer are identified. For this purpose, it is preferable to use an EBSD camera with binning settings of, for example, 4x4, 8x8, or 11x11, and adjust the SEM accelerating voltage, probe current, and focal length to achieve a Kikuchi line Confidence Index (CI value) of 0.1 or greater or a Fit value of less than 1°. The area ratios of the γ' and ε phases in the compound layer are then binarized using image processing. The average area ratios of the γ' and ε phases in the measured 10 fields of view are then defined as the area fractions (%) of the γ' and ε phases, respectively.

[0063] [Diffusion layer hardness: 630HV or more] Although the presence of the compound layer described above improves the surface fatigue strength of the component, if the hardness of the diffusion layer directly below the compound layer is lower than that of the compound layer, peeling may occur at the interface between the compound layer and the diffusion layer, resulting in early pitting. To improve the hardness of the surface of the diffusion layer, a steel material with the composition specified in this disclosure is nitrided to precipitate nitrides such as CrN and VN in the diffusion layer, thereby improving the hardness of the surface of the diffusion layer. Specifically, when the surface of the diffusion layer, i.e., the interface between the compound layer and the diffusion layer, is considered to be the surface of the diffusion layer, a Vickers hardness (sometimes referred to as the "diffusion layer hardness" in this disclosure) of 630 HV or higher at a depth of 0.05 mm from the surface of the diffusion layer indicates excellent surface fatigue strength. The hardness of the diffusion layer is preferably 650 HV or higher, and more preferably 670 HV or higher. The hardness of the diffusion layer is measured on a cross section in the thickness direction of the steel part. For example, after gas nitriding a steel material having the chemical composition of the core of the part according to the present disclosure, the vertical cross section of the resulting test material is mirror-polished, and the Vickers hardness is measured at 10 arbitrary points at the depth position described above 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 is defined as the hardness of the diffusion layer.

[0064] [Core hardness: 210HV or more] In surface fatigue, shear stress is distributed in the depth direction of about 1 mm from the surface of a steel part. Therefore, if hardness is not ensured even in the region (referred to as the core) where the amount of nitrogen diffusion is small or where nitrogen diffusion does not reach, for example, the interior may yield and pitting damage may occur from the interior. To improve the hardness of the core, a steel material with the composition specified in this disclosure is nitrided to utilize solid solution strengthening of substitutional elements and precipitation strengthening of alloy carbides. In this disclosure, a hardness (of the core) of 210 HV or higher at 1.00 mm from the surface of the diffusion layer will have excellent surface fatigue strength. The hardness of the core is preferably 230 HV or higher, and more preferably 250 HV or higher. The hardness of the core is measured on a cross section in the thickness direction of the steel part. For example, after gas nitriding a steel material having the chemical composition of the core of the part according to the present disclosure, the vertical cross section of the obtained test material is mirror-polished, and the Vickers hardness is measured at 10 arbitrary points at the depth position described above 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 is defined as the hardness of the core.

[0065] [Method of manufacturing steel parts] Next, an example of a method for manufacturing a steel part (nitrided part) according to the present disclosure will be described.

[0066] In one example of a method for manufacturing a steel part (nitrided part) according to the present disclosure, a steel material having the above-described chemical composition of the core is subjected to gas nitriding treatment at a treatment temperature of 530 to 630°C, and the total treatment time of the gas nitriding treatment is 0.5 to 15 hours.

[0067] [Processing temperature: 530~630℃] The temperature of gas nitriding (nitriding temperature) is mainly correlated with the diffusion rate of nitrogen, and affects the thickness of the compound layer, the hardness of the diffusion layer, and the hardness of the core. If the nitriding temperature is too low, a sufficiently thick compound layer cannot be obtained. On the other hand, if the nitriding temperature is A C1 If the temperature exceeds this point, an austenite phase (γ phase), which has a slower nitrogen diffusion rate than a ferrite phase (α phase), is generated in the steel, and the compound layer becomes too thick, resulting in a thick porous layer on the surface and a decrease in the surface hardness and core hardness of the diffusion layer. Therefore, in the present disclosure, the nitriding temperature is 530 to 630°C, which is around the ferrite temperature range. In this case, a decrease in surface hardness can be suppressed, and a decrease in the hardened layer depth can be suppressed. A temperature of 550 to 610°C is preferable.

[0068] [Total gas nitriding treatment time: 0.5 to 15 hours] The gas nitriding treatment is carried out in an atmosphere containing NH3, H2, and N2. The total time for the nitriding treatment, that is, the time from the start to the end of the nitriding treatment (treatment time), is correlated with the formation and decomposition of the compound layer and the diffusion and penetration of nitrogen, and affects the hardness of the diffusion layer. It also affects the hardness of the core. If the treatment time is too short, a compound layer of sufficient thickness may not be obtained. On the other hand, if the treatment time is too long, the hardness of the diffusion layer and core will decrease, and the surface fatigue strength will decrease. If the treatment time is too long, the manufacturing cost will further increase. Therefore, the total treatment time for the nitriding treatment is 0.5 to 15 hours, and preferably 1.0 to 10 hours.

[0069] In addition, the atmosphere for the gas nitriding treatment in the present disclosure contains NH3, H2, and N2, as well as unavoidably impurities such as oxygen and carbon dioxide. A preferred atmosphere is one in which the total of NH3, H2, and N2 is 99.5% (vol %) or more.

[0070] [Nitriding gas conditions] In the nitriding method of the present disclosure, the nitriding potential is controlled, whereby the thickness of the compound layer is 5 to 30 μm, and where the thickness of the compound layer is t, the area ratio of the γ' phase in the compound layer in the depth range from 0 to t / 2 from the surface of the compound layer is 70% or more, and the remainder is the ε phase, and the area ratio of the ε phase in the depth range from t / 2 to t from the surface of the compound layer is 70% or more, and the remainder is the γ' phase, A steel part is obtained in which the nitrogen diffusion layer has a Vickers hardness of 630 HV or more at a position 0.05 mm deep from the surface, and a Vickers hardness of 210 HV or more at a position 1.00 mm deep from the surface.

[0071] Nitriding potential K of gas nitriding treatment N is defined by the following formula:

[0072] K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ]

[0073] The partial pressures of NH3 and H2 in the gas nitriding atmosphere can be controlled by adjusting the flow rates of the gases.

[0074] As a result of investigations by the inventors of the present disclosure, it has been found that the target thickness and constituent phases can be achieved by controlling the nitriding potential of the gas nitriding treatment as follows. (1) Average value of nitriding potential before starting nitriding treatment: 0.20 to 1.50 When nitriding a part, it is important to preheat the furnace and bring the atmosphere within the specified range before starting to hold the part (nitriding treatment), which is necessary to obtain the thickness and structure of the compound layer specified in the present disclosure. A desired compound layer can be obtained if the average nitriding potential from at least 10 minutes before the start of nitriding treatment to the start of nitriding treatment is 0.20 to 1.50. If the nitriding potential before the start of nitriding treatment is too low, a compound layer of sufficient thickness cannot be obtained. Conversely, if it is too high, the compound layer may become too thick or the area ratio of the γ' phase on the surface side may become small. The average nitriding potential before the start of nitriding treatment is preferably 0.40 to 1.30.

[0075] (2) Lower limit of nitriding potential during nitriding treatment: 0.10 During nitriding (holding), it is important to set the lower limit of the nitriding potential to 0.10 or more. If it is below 0.10, the compound layer will decompose, making it impossible to obtain a compound layer of sufficient thickness, which may result in a decrease in contact fatigue strength. The lower limit during nitriding is preferably 0.15.

[0076] (3) Upper limit of nitriding potential during nitriding: 1.20 During the nitriding treatment (holding), it is important to set the upper limit of the nitriding potential to 1.20 or less. If it is below 1.20, the compound layer will be mainly composed of the ε phase, and the compound layer and porous layer will grow rapidly, which may make it difficult to obtain sufficient surface fatigue strength. The upper limit during the nitriding treatment is preferably 1.00.

[0077] (4) Average nitriding potential during nitriding: 0.15 to 0.70 By keeping the average nitriding potential during nitriding within the above range, the desired compound can be obtained. If the average value exceeds 0.70, the compound layer will be thick and contain many pores, and if it is lower than 0.15, the compound layer will be thin and sufficient surface fatigue strength will not be obtained. Note that as long as the average and upper and lower limits are within the above range, the target nitriding potential may be changed significantly during treatment, and may be changed any number of times. The preferred range for the average nitriding potential during nitriding is 0.20 to 0.60.

[0078] The components according to the present disclosure can be manufactured by gas nitriding a steel material having the chemical composition of the core of the components according to the present disclosure while controlling the nitriding potential as described above. When nitriding a steel with the composition according to the present disclosure in this way, it is possible to make the compound layer 5 to 30 μm thick, the surface side of the compound layer mainly made of γ' phase, and the diffusion layer side mainly made of ε phase, without complicating the nitriding conditions. Furthermore, by combining this with the composition according to the present disclosure, the effect of increasing the hardness of the surface layer of the diffusion layer and the hardness of the core is synergistically increased, resulting in high contact fatigue strength. [Example]

[0079] [Steel manufacturing] Ingots were produced by ingot casting using the molten steel shown in Table 1. The cross section of the ingot perpendicular to the longitudinal direction was a rectangle measuring 180 mm x 180 mm. The produced ingots were allowed to cool to room temperature. Note that underlines in Table 1 indicate compositions outside the scope of the present disclosure. Blank spaces mean that the content of the corresponding element is 0% in significant figures (numbers down to the least significant digit) as defined in the present disclosure. The remainder is Fe and impurities.

[0080] [Table 1]

[0081] The obtained ingot was heated at 1200°C for 2 hours. The heated ingot was subjected to hot working (hot forging) to produce a steel material (steel bar) with a diameter of 40 mm and a length of 1000 mm. The hot-worked steel material was allowed to cool to room temperature.

[0082] [Manufacturing steel part test pieces] The produced steel materials were used to prepare the following two types of steel part test pieces (small roller test piece, test piece for investigating hardened layer).

[0083] (1) Small roller test piece Figure 1 shows a side view of the small roller test piece produced in this example. The numbers in Figure 1 indicate dimensions (unit: mm). "φ" in Figure 1 refers to diameter. The inverted triangle symbol in Figure 1 refers to the "finish code" indicating the surface roughness listed in Table 1 of JIS B 0601:1982. The "G" attached to the finish code refers to the abbreviation for the processing method indicating grinding specified in JIS B 0122:1978. The small roller test piece is a test piece for measuring surface fatigue strength. Multiple small roller test pieces were prepared for each test number.

[0084] Specifically, first, the steel material of each test number was machined to produce a small roller test piece. The central axis of the test piece was coaxial with the central axis of the steel bar, and the surface of the cylindrical part (test surface) with a diameter of 26 mm shown in Figure 1 was finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm in accordance with JIS B 0601:2001. The small roller test piece was subjected to a gas nitriding treatment. The conditions for the gas nitriding treatment are shown in Table 3 below.

[0085] The test pieces were placed in a gas nitriding furnace, and NH3, H2, and N2 gases were introduced into the furnace. After nitriding, the test pieces were cooled with N2 gas.

[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 two minutes using a glass tube NH3 analyzer.

[0088] Average nitriding potential K of gas nitriding process N (atm -1 / 2 ) is defined by the following formula (I): K N =P NH3 / P H2 3 / 2 Formula (I) Here, P in formula (I) NH3 is the partial pressure of the atmospheric gas NH3 [atm], and P H2 is the H2 partial pressure of the atmospheric gas [atm].

[0089] Nitriding potential K calculated in the device N The 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. After gas nitriding, the gripping parts were finished to remove any heat treatment strain, and then each was used as a roller pitting test piece.In the actual roller pitting test using the small roller test piece, the cylindrical part (test surface) with a diameter of 26 mm was brought into contact with the large roller, and a specified surface pressure was applied before rotation.

[0090] (2) Test piece for hardened layer investigation Two test pieces for each test number were prepared for the hardened layer investigation. The test pieces for the hardened layer investigation were cylindrical test pieces with a diameter of 26 mm and a length of 100 mm.

[0091] Specifically, first, the steel material of each test number was machined to prepare two cylindrical test pieces with a diameter of 26 mm and a length of 100 mm. The central axis of the test pieces was aligned with the central axis of the steel bar. The test pieces were then subjected to the same gas nitriding treatment as the small roller test pieces. Test pieces for hardened layer investigation were prepared using the above manufacturing process.

[0092] (Manufacturing large roller test pieces for two-roller rolling fatigue tests) Furthermore, large roller test pieces to be used in a two-cylinder rolling fatigue test for measuring surface fatigue strength were prepared by the following method. A rough specimen for the large roller test piece, with the shape shown in Figure 2, was cut from a cylindrical material with a diameter of 140 mm and a chemical composition equivalent to SUJ2 as specified in JIS G 4805:2008. The numbers in Figure 2 indicate dimensions (unit: mm). "φ" in Figure 2 indicates diameter. The inverted triangle symbol in Figure 2 represents the "finish symbol" indicating the surface roughness listed in Table 1 of JIS B 0601:1982. The "G" attached to the finish symbol represents the abbreviation for the processing method, indicating grinding as specified in JIS B 0122:1978.

[0093] The cut-out rough test pieces were quenched. The quenching temperature was 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time had elapsed, the pieces were quenched in oil at 60°C. The outer peripheral surfaces of the quenched rough test pieces were then finished by cutting. The outer peripheral surfaces were finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. Large roller test pieces were produced using the above manufacturing process.

[0094] [Measurement of the thickness of the compound layer on the surface] The cross section perpendicular to the longitudinal direction of the small roller test piece subjected to the gas nitriding treatment was mirror-polished and 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.

[0095] 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 (μm) of the nitrogen compound layer.

[0096] [Measurement of γ' and ε phase ratios] The ratios of γ' and ε phases were determined by image processing of the microstructure photographs of the hardened specimens. Specifically, cross-sectional images of the surface of the nitrided specimens (hardened specimens) were analyzed at 4000x magnification using electron backscatter diffraction (EBSD, manufactured by EDAX) to create phase maps. The thickness of the compound layer was defined as t. Ten phase maps were then selected, each covering a region from the surface to t / 2 and a region from t / 2 to t. The γ' and ε phases in the compound layer were identified, and their area ratios in the compound layer were calculated by image processing. The average area ratios of the γ' and ε phases in the ten measured fields were defined as the area fractions (%) of the γ' and ε phases.

[0097] Figure 4 shows an example of the results of SEM-EBSD analysis of the cross-sectional surface layer of nitrided steel. Note that while Figure 4 shows a grayscale image, the actual analysis image is displayed in color, making it easy to distinguish between the compound layer and the diffusion layer.

[0098] [Hardness measurement] A sample (thickness: 10 mm) having a cross section perpendicular to the longitudinal direction was taken from the test portion (φ26 mm) of each small roller that had been subjected to the gas nitriding treatment, and the cut surface was then mirror-polished. The Vickers hardness was then measured at 10 random points at a depth position described below 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.

[0099] [Hardness at 0.05 mm from the surface of the diffusion layer] The hardness at a position 0.05 mm from the surface of the diffusion layer (surface layer) was calculated by cutting the part after nitriding (nitrided part) perpendicular to the main axis or longitudinal direction, mirror-polishing the resulting cross section, and measuring 10 arbitrary points at a depth of 0.05 mm from the surface (interface between the compound layer and the diffusion layer) (positions perpendicular to the surface) with a test force of 1.96 N, and calculating the average Vickers hardness.

[0100] [Hardness at 1.00 mm from the surface of the diffusion layer] The hardness at a position 1.00 mm from the surface of the diffusion layer (core hardness) was calculated by cutting the part after nitriding (nitrided part) perpendicular to the main axis or longitudinal direction, mirror-polishing the exposed cross section, and measuring 10 random points at a depth of 1.00 mm from the surface (interface between the compound layer and the diffusion layer) with a test force of 1.96 N, and averaging the Vickers hardness values.

[0101] [Evaluation test] The following evaluation tests were carried out using the above-mentioned various test pieces. In this example, application to gear parts was assumed, and the reference values ​​used as evaluation criteria for surface fatigue strength and rotation were set as follows. For the evaluation of surface fatigue strength and rotation, test specimens were prepared using a steel (reference steel) that met the SCr420 standard of JIS G 4053:2016, following a general manufacturing process: normalizing → test specimen processing → eutectoid carburizing in a gas carburizing furnace → low-temperature tempering. Next, the test specimens were subjected to the roller pitting test and rotating bending fatigue test, and the obtained fatigue limits were used as the reference values ​​for surface fatigue strength and rotation in this example.

[0102] (Surface fatigue strength measurement test (two-cylinder rolling fatigue test)) A two-cylinder rolling fatigue test was carried out using the small roller test piece and the large roller test piece to determine the surface fatigue strength as follows. The test machine used was a roller pitting tester "RP201" manufactured by Komatsu Engineering Co., Ltd.

[0103] As shown in FIG. 3, the small roller test piece 10 was rolled while being in contact with the cylindrical portion with a diameter of 26 mm and the central position of the outer circumferential surface of the large roller test piece 20 (the outer circumferential portion with a diameter of 130 mm).

[0104] The contact pressure was 2000 to 3500 MPa in Hertzian contact pressure. The rotation speed of the small roller test piece 10 was 2000 rpm. The peripheral speed of the small roller test piece 10 was 163 m / min, and the peripheral speed of the large roller test piece 10 was 229 m / min. During the test, lubricating oil was supplied to the contact area between the small roller test piece and the large roller test piece. The lubricating oil was automatic transmission oil, with an oil temperature of 100°C and an oil flow rate of 1.0 L / min. The slip ratio was -40%. Table 2 shows the test conditions.

[0105] [Table 2]

[0106] The number of repeated cycles in the test was 2.0 × 10, which indicates the fatigue limit of general steel. 7 The small roller test piece was tested for 2.0 × 10 7 The maximum surface pressure (MPa) reached at this time was taken as the fatigue limit of the small roller test piece.

[0107] The occurrence of pitting was detected using a vibration meter attached to the testing machine. After vibration was generated, 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.

[0108] In this example, assuming application to gear components, the fatigue limit of a small roller test piece made of steel (reference steel) that meets the aforementioned SCr420 standard was used as the reference value. If the fatigue limit was 1.1 times or more that of the reference steel, the test piece was judged to have excellent contact fatigue strength (marked "○" in the "Contact fatigue strength judgment" column in Table 3). On the other hand, if the fatigue limit was less than 1.1 times that of the reference steel, the test piece was judged to have low contact fatigue strength (marked "X" in the "Contact fatigue strength judgment" column in Table 3).

[0109] [Test Results] The results are shown in Table 3. The underlined values ​​in Table 3 indicate values ​​outside the scope of this disclosure. In Table 3, the "surface side t / 2 γ' phase ratio" of the compound layer means the area ratio of the γ' phase in the compound layer in the depth range from 0 to t / 2 from the surface of the compound layer, and the "diffusion layer side t / 2 ε phase ratio" means the area ratio of the ε phase in the compound layer in the depth range from t / 2 to t from the surface of the compound layer. In addition, the "hardness at 0.05 mm" of the diffusion layer means the Vickers hardness at a position 0.05 mm deep from the surface of the diffusion layer, and the "hardness at 1.00 mm" of the parent phase (core) means the Vickers hardness at a position 1.00 mm deep from the surface of the diffusion layer.

[0110] [Table 3]

[0111] Test numbers 1 to 17 met the requirements of the present disclosure, and excellent results were obtained in terms of contact fatigue strength.

[0112] In test numbers 18 to 22, the steel compositions were all outside the range of the present disclosure, and at least one of the ratio of the γ' phase on the surface side of the compound layer, the hardness of the diffusion layer, and the hardness of the core did not meet the requirements of the present disclosure, and the desired surface fatigue strength was not achieved. In test number 23, the steel composition was within the range of the present disclosure, but the compound layer was too thick and the proportion of γ' phase on the surface side was insufficient, so the desired contact fatigue strength was not achieved. In test number 24, the steel composition was within the range of the present disclosure, but the proportion of the ε phase on the diffusion layer side of the compound layer was insufficient, and the desired contact fatigue strength was not achieved. In test number 25, the steel composition was within the range of the present disclosure, but the proportion of the ε phase on the diffusion layer side of the compound layer was insufficient, and the desired contact fatigue strength was not achieved. In test number 26, the steel composition was within the range of the present disclosure, but the proportion of γ' phase on the surface side of the compound layer was insufficient, and the desired contact fatigue strength was not achieved. In test number 27, the steel composition was within the range of the present disclosure, but the compound layer was too thick and the proportion of γ' phase on the surface side was insufficient, so the desired contact fatigue strength was not achieved. In test number 28, the steel composition was within the range of the present disclosure, but the thickness of the compound layer was too small, and the proportion of the ε phase on the diffusion layer side was insufficient, so the desired contact fatigue strength was not achieved.

[0113] The embodiments and examples of the present disclosure have been described above. However, the above-described embodiments and examples are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments and examples, and can be modified and implemented as appropriate within the scope of the present disclosure. [Industrial Applicability]

[0114] The steel parts according to the present disclosure can be used in a wide range of industrial fields as materials for automobiles and industrial machinery, particularly gears for machines powered by electric motors. [Explanation of symbols]

[0115] 10 Small roller test piece (evaluation material) 20 Large roller test piece (mating material)

Claims

1. a steel core portion, a compound layer containing iron nitride, and a nitrogen diffusion layer present between the steel core portion and the compound layer, The steel core portion comprises, in mass %, C: 0.05-0.35%, Si: 0.05-2.00%, Mn: 0.60 to 2.50%, P: 0.030% or less, S: 0.100% or less, Cr: 0.20-2.50%, V: 0.02 to 1.30%, Al: 0.100% or less, and N: 0.0250% or less, and the balance being Fe and impurities, the thickness of the compound layer is 5 to 30 μm, where t is a thickness of the compound layer, an area ratio of the γ' phase in the compound layer within a depth range from 0 to t / 2 from the surface of the compound layer is 70% or more, and the remainder is the ε phase, an area ratio of the ε phase in the compound layer in a depth range from t / 2 to t from the surface of the compound layer is 70% or more, and the remainder is the γ' phase, the Vickers hardness at a position 0.05 mm deep from the surface of the nitrogen diffusion layer is 630 HV or more; A steel part in which the nitrogen diffusion layer has a Vickers hardness of 210 HV or more at a depth of 1.00 mm from the surface.

2. a steel core portion, a compound layer containing iron nitride, and a nitrogen diffusion layer present between the steel core portion and the compound layer, The steel core portion comprises, in mass %, C: 0.05-0.35%, Si: 0.05-2.00%, Mn: 0.60 to 2.50%, P: 0.030% or less, S: 0.100% or less, Cr: 0.20-2.50%, V: 0.02 to 1.30%, Al: 0.100% or less, and N: 0.0250% or less, and further containing one or more elements selected from the group consisting of the following Groups A to D, with the balance being Fe and impurities: [Group A] Mo: 1.50% or less, Ti: 0.100% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group B] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, and Sb: 0.100% or less, one or more selected from the group consisting of [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.020% or less, one or more selected from the group consisting of the thickness of the compound layer is 5 to 30 μm, where t is a thickness of the compound layer, an area ratio of the γ' phase in the compound layer within a depth range from 0 to t / 2 from the surface of the compound layer is 70% or more, and the remainder is the ε phase, an area ratio of the ε phase in the compound layer in a depth range from t / 2 to t from the surface of the compound layer is 70% or more, and the remainder is the γ' phase, the Vickers hardness at a position 0.05 mm deep from the surface of the nitrogen diffusion layer is 630 HV or more; A steel part in which the nitrogen diffusion layer has a Vickers hardness of 210 HV or more at a depth of 1.00 mm from the surface.

3. The steel part according to claim 2, wherein the chemical composition of the steel core contains the group A elements.

4. The steel part according to claim 2 , wherein the chemical composition of the steel core contains the B group.

5. The steel part according to claim 2 , wherein the chemical composition of the steel core contains the C group.

6. The steel part according to claim 2 , wherein the chemical composition of the steel core contains the D group elements.

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