Steel parts and their manufacturing methods

Abstract

To provide a steel part with excellent surface fatigue strength and a method for manufacturing the same. [Solution] The device comprises a steel core, a compound layer containing iron nitride, and a nitrogen diffusion layer between the steel core and the compound layer, wherein the steel core has the following composition by mass%, C: 0.05~0.30%, Si: 0.05~2.00%, Mn: 1.50~3.00%, P: 0.030% or less, S: 0.100% or less, Cr: 0.05~2.00%, V: 0.02~1.30%, Al: 0.100% or less, and N: 0.0250% or less. A steel part and a method for manufacturing the same, having a chemical composition containing the following, with the remainder being Fe and impurities, a compound layer thickness of more than 3 to 30 μm, the compound layer containing more than 50% γ' phase by area ratio, with the remainder being ε phase, containing 5 to 40% γ phase by area ratio in the region up to 10 μm from the surface of the nitrogen diffusion layer, and having a Vickers hardness of 580 HV or more at a depth of 0.05 mm from the surface of the nitrogen diffusion layer.

Description

[Technical Field]

[0001] This disclosure relates to steel parts and methods for manufacturing the same. [Background technology]

[0002] Steel parts used in automobiles and various industrial machinery, such as transmission gears, CVT (Continuously Variable Transmission) pulleys, and bearings, require high surface fatigue strength and wear resistance. Therefore, these parts are made from structural alloy steels such as JIS standard SCr420, SCM420, and SNCM420. After being processed into the desired shape, a hardened layer is applied to the surface of the part through carburizing and quenching, thereby improving fatigue strength.

[0003] In recent years, the automotive industry has been promoting the electrification of power sources, replacing internal combustion engines, in order to reduce CO2 emissions. As a result, the characteristics required of steel parts such as gears, especially power transmission components, may change dramatically in the future. For example, in electric vehicles (EVs), it is common to see a reduction gear incorporated directly below the motor, which is the power source, to ensure torque. Miniaturizing these units leads to lighter vehicle bodies and increased design flexibility, so this is an area where demand is expected to grow further in the future. However, because EVs do not produce noise and vibration caused by internal combustion engines and are quiet, gear noise generated by the reduction gear becomes more noticeable. Therefore, in addition to the need for increased strength due to miniaturization, there are concerns about increased grinding costs for the gears of the reduction gear, as grinding allowances must be increased to shape them in order to suppress noise and vibration.

[0004] Carburizing and quenching is a method of heating steel parts to the austenite region to increase the carbon (C) concentration on the surface of the part, and then rapidly cooling it to create hard martensite on the surface of the part, resulting in high surface fatigue strength. On the other hand, carburizing and quenching is a heat treatment that hardens the core of the part, and the resulting distortion (heat treatment distortion) tends to be large, which increases the cost of grinding in subsequent processes. In particular, for speed reducer gears for EVs, as mentioned above, there is a concern about increased grinding costs, so there is a high need to switch to heat treatment methods that contribute to reducing grinding costs.

[0005] Against this backdrop, in recent years, surface hardening heat treatments such as nitriding and soft nitriding, which produce less heat treatment distortion than carburizing and quenching, have been attracting attention.

[0006] Nitriding is a surface hardening heat treatment that involves introducing nitrogen into the surface of steel, while soft nitriding is a surface hardening heat treatment that involves introducing both nitrogen and carbon into the surface of steel. The media used in nitriding and soft nitriding include gas, salt baths, and plasma. For automotive parts, gas nitriding and gas soft nitriding, which offer superior productivity, are primarily used.

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

[0008] The diffusion layer is a layer hardened by solid solution strengthening mechanisms due to invading nitrogen and carbon, as well as particle dispersion strengthening mechanisms of nitrides. Increasing the hardness and depth of the diffusion layer improves the surface fatigue strength of the component. Extensive research has been conducted on increasing the hardness and depth of the diffusion layer.

[0009] The compound layer is mainly composed of iron nitrides of Fe2N~Fe3N (ε phase) and Fe4N (γ' phase), and is extremely hard compared to the matrix phase. Therefore, the compound layer is effective in improving wear resistance. The ε phase has a larger solid solution range for carbon and a higher growth rate compared to the γ' phase. For this reason, in soft nitriding, which involves mixing with a carburizing gas, a compound layer mainly composed of the ε phase is easily formed. Therefore, soft nitriding can produce a thick compound layer in a shorter time than nitriding, and regardless of the type of steel used in the part. For this reason, soft nitriding has been used for a long time to improve the wear resistance of parts. On the other hand, because nitriding is a heat treatment performed at relatively low temperatures, the hardened layer depth is small, resulting in 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, in nitrided parts, the presence of a brittle compound layer and voids formed near the surface of the compound layer tends to reduce surface fatigue strength. Therefore, in recent years, methods have been proposed to increase fatigue strength by controlling the temperature and atmosphere of the nitriding process.

[0010] Patent Document 1 discloses a steel material comprising, in this order, an outermost nitrogen compound layer, a retained austenite layer, a martensite layer, and a steel substrate, wherein the ratio of the thickness of the retained austenite layer to the thickness of the nitrogen compound layer is 2.5:7.5 to 5.0:5.0, and the nitrogen compound layer comprises a porous layer and a non-porous layer from the surface of the steel material.

[0011] Patent Document 2 proposes a nitrided steel member having a nitride compound layer on its surface, a hardened layer having an austenite structure below the nitride compound layer, a diffusion layer in which nitrogen is diffused within the matrix below the hardened layer, the nitride compound layer having a phase distribution in the order of ε phase, γ' phase, ε phase, the volume ratio of the γ' phase in the nitride compound layer being 20% ​​or more, and the nitride compound layer having a thickness of 5 μm to 50 μm from the surface of the nitrided steel member. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2021-085055 [Patent Document 2] Japanese Patent Application Laid-Open No. 2022-068375 [Summary of the Invention] [Problems to be Solved by the Invention]

[0013] In the components disclosed in Patent Document 1, since the target is carbon steel for mechanical structures and alloy steel for mechanical structures and the content of nitride-forming elements is limited, there is room for improvement in the effect of improving surface fatigue strength.

[0014] In the nitrided steel member disclosed in Patent Document 2, since the constituent phase of the compound layer is mainly the ε phase, the improvement in surface fatigue strength is considered to be limited.

[0015] The present disclosure has been made to solve the above problems, and an object thereof is to provide a steel component having excellent surface fatigue strength. [Means for Solving the Problems]

[0016] [[ID=3२]]The gist of the means for solving the above problems is as follows. <1> It includes a steel core part, a compound layer containing iron nitride, and a nitrogen diffusion layer existing between the steel core part and the compound layer. The steel core part has, in mass %, C: 0.05 to 0.30%, Si: 0.05 to 2.00%, Mn: 1.50 to 3.00%, P: 0.030% or less, S: 0.100% or less, Cr: 0.05 to 2.00%, V: 0.02 to 1.30%, Al: 0.100% or less, and N: 0.0250% or less, <000०111>has a chemical composition containing the above and the balance being Fe and impurities. The thickness of the compound layer is more than 3 to 30 μm, The aforementioned compound layer contains more than 50% of the γ' phase by area ratio, with the remainder being the ε phase. In the region up to a depth of 10 μm from the surface of the nitrogen diffusion layer, the γ phase is contained in an area ratio of 5 to 40%, A steel component having a Vickers hardness of 580 HV or more at a depth of 0.05 mm from the surface of the nitrogen diffusion layer. <2> It comprises a steel core, a compound layer containing iron nitride, and a nitrogen diffusion layer existing between the steel core and the compound layer, The steel core portion is, by mass %, C: 0.05~0.30%, Si: 0.05~2.00%, Mn: 1.50~3.00%, P: 0.030% or less, S: 0.100% or less, Cr: 0.05~2.00%, V: 0.02~1.30%, Al: 0.100% or less, N: 0.0250% or less, Furthermore, it has a chemical composition in which it contains one or more selected from the groups A to D below, with the remainder being Fe and impurities. [Group A] Mo: 1.30% or less Ti: 0.100% or less, Nb: One or more species selected from the group consisting of 0.050% or less [Group B] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, B: One or more types selected from the group consisting of 0.0100% or less. [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, and Sb: One or more selected from the group consisting of 0.100% or less [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: One or more species selected from the group consisting of 0.020% or less. The thickness of the compound layer is greater than 3 to 30 μm. The aforementioned compound layer contains more than 50% of the γ' phase by area ratio, with the remainder being the ε phase. In the region up to a depth of 10 μm from the surface of the nitrogen diffusion layer, the γ phase is contained in an area ratio of 5 to 40%, A steel component having a Vickers hardness of 580 HV or more at a depth of 0.05 mm from the surface of the nitrogen diffusion layer. <3> The chemical composition of the steel core contains the A group. <2> Steel parts as described above. <4> The chemical composition of the steel core contains the B group. <2> or <3> Steel parts as described above. <5> The chemical composition of the steel core contains the C group. <2> ~ <4> A steel part listed in any one of the following. <6> The chemical composition of the steel core contains the D group. <2> ~ <5> A steel part listed in any one of the following. <7> In the chemical composition of the steel core, Si, Mn, Cr, and V satisfy the following formula (1): <1> Steel parts as described above. 24≦(Mn+3Cr+2V) / Si≦80 Formula (1) However, each element symbol in formula (1) above represents the mass percentage content of that element. <8> In the chemical composition of the steel core, Si, Mn, Cr, and V satisfy the following formula (1): <2> ~ <6> A steel part listed in any one of the following. 24≦(Mn+3Cr+2V) / Si≦80 Formula (1) However, each element symbol in formula (1) above represents the mass percentage content of that element. <9> <1> ~ <8> A method for manufacturing a steel part as described in any one of the following: For a steel material having the aforementioned chemical composition, the nitriding treatment temperature is 600 to 680°C, and the nitriding potential K is calculated using the following formula (2). N However, K before nitriding treatmentN The average of is 0.20 to 1.50, and K during nitriding treatment N The value is 0.10 to 1.20, and K during nitriding treatment. N A method for manufacturing steel parts, comprising a step of performing nitriding treatment under the condition that the average of the values ​​is 0.15 to 0.70. K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ] Formula (2) In equation (2), the partial pressure of NH3 (atm) and the partial pressure of H2 (atm) are the partial pressures of the treatment atmosphere gas. [Effects of the Invention]

[0017] According to this disclosure, it is possible to provide steel parts with excellent surface fatigue strength. [Brief explanation of the drawing]

[0018] [Figure 1] This figure shows an example of a small roller test piece for roller pitting testing. [Figure 2] This figure shows an example of a large roller test specimen used for roller pitting testing. [Figure 3] This is a schematic diagram of a roller pitting test. [Figure 4] This figure shows an example of SEM-EBSD analysis results on the cross-sectional surface of nitrided steel. [Modes for carrying out the invention]

[0019] A steel component according to one embodiment of this disclosure will be described below.

[0020] In this specification, a numerical range represented by "~" means a range that includes the numbers before and after "~" as the lower and upper limits, respectively. However, if the numbers before and after "~" are preceded by "greater than" or "less than", the numerical range means a range that does not include those numbers as the lower or upper limit. The elemental content in a chemical composition is sometimes expressed by adding the "amount" to the element symbol (for example, C amount, Si amount, etc.). In chemical composition, the percentage "%" indicates "mass percent". When the chemical composition of an element is described as "0~", it means that the element does not need to be included. Furthermore, in the numerical ranges described stepwise within this specification, the upper limit of one stepwise numerical range may be replaced with the upper limit of another stepwise numerical range, or with the value shown in the examples. Similarly, the lower limit of one stepwise numerical range may be replaced with the lower limit of another stepwise numerical range, or with the value shown in the examples. Furthermore, the term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved.

[0021] The uses and manufacturing methods of the steel parts relating to this disclosure are not particularly limited, but gas-nitrided steel parts are preferred. Hereinafter, a representative example (one embodiment) of the steel parts relating to this disclosure, a gas-nitrided steel part (hereinafter sometimes simply referred to as "part"), will be described.

[0022] The steel component relating to this disclosure is, for example, a component made of steel material that has been nitrided, and has a steel core (hereinafter sometimes simply referred to as the "core") which is the central region in the depth direction of the component, and a hardened layer located on the surface side of the component than the core. Here, the core refers to the portion where nitrogen penetration did not occur during the nitriding process. In other words, the core is the region where, despite undergoing nitriding, there is no change in chemical composition, or the change is negligibly small, and which has a chemical composition (sometimes simply referred to as "composition") equivalent to that of the base material of the part (the steel before nitriding). The composition of the core can also be said to be, for example, the composition at a depth of 2.0 mm or more from the surface of the part. The hardened layer refers to the area where nitrogen has penetrated due to the nitriding process. In other words, the hardened layer is the region where the chemical composition and / or metal structure have changed due to the nitriding process. For example, it refers to the region from the surface of the part to a depth of less than approximately 2.0 mm. As the depth increases, the effect of the nitrogen penetration due to nitriding decreases, and the hardness approaches that of the core.

[0023] To solve the aforementioned problems, the inventors of this disclosure investigated the effects of varying various alloy components and nitriding treatment conditions on the morphology of the compound layer on the surface of the component, the hardness of the component, and the surface fatigue strength of the component, and obtained the following findings.

[0024] (a) By making the constituent phase of the compound layer a layer mainly composed of a highly tough γ' phase, crack propagation in tooth surface fatigue can be suppressed by the γ' phase. (b) To obtain a highly tough γ'-phase-dominant layer, it is effective to treat the steel in the two-phase region (600-680°C) above the A1 point in the Fe-N binary system, thereby distributing the carbon in the compound layer to the austenite structure in the two-phase structure directly beneath it. (c) When steel is nitrided at temperatures above 600°C, which is higher than normal, the hardness of the hardened layer and the core decreases. Therefore, maintaining a certain hardness range of the steel bar before nitriding further improves the surface fatigue strength. Adding Mn, Cr, and V is effective in improving the hardness of the hardened layer, and C, Si, Mn, Cr, and V is effective in improving the hardness of the core, however, the addition of Si is not always advantageous. Therefore, it is important to appropriately control the ratio of these components.

[0025] The steel components relating to this disclosure were completed based on the above-mentioned findings.

[0026] First, the chemical composition of the steel component relating to this disclosure will be explained.

[0027] [Chemical composition] This document describes the chemical composition (elemental components) of the core of the steel component relating to this disclosure. Unless otherwise specified, the "%" for the content of each elemental component refers to "mass%". The chemical composition of the core of the steel component relating to this disclosure is equivalent to the chemical composition of the steel material (base material) before nitriding treatment.

[0028] [C:0.05~0.30%] Carbon (C) is an essential element for stabilizing the ε-phase in the compound layer and ensuring core hardness of the component. To achieve these effects, a C content of 0.05% or more is required. On the other hand, if the C content exceeds 0.30%, the proportion of the ε-phase becomes too high, significantly reducing surface fatigue strength. Furthermore, the strength after hot forging becomes too high, significantly reducing machinability. The C content should be 0.30% or less, and may even be 0.29% or less. The preferred range for C content is 0.07-0.25%, and more preferably 0.10-0.20%.

[0029] [Si: 0.05~2.00%] Si is effective in improving surface fatigue strength because nitriding treatment forms fine nitrides (Si3N4) in the compound layer and diffusion layer, increasing hardness. It is also an element that increases core hardness through solid solution strengthening. To obtain these effects, a Si content of 0.05% or more is necessary. On the other hand, if the Si content exceeds 2.00%, the strength of the steel bars, wires, and hot forged materials becomes too high, significantly reducing machinability. The preferred range for Si content is 0.10 to 1.50%.

[0030] [Mn: 1.50~3.00%] Mn is an effective element for improving surface fatigue strength because nitriding treatment forms fine nitrides (Mn3N2) in the compound layer and diffusion layer, increasing hardness. It also increases core hardness through solid solution strengthening. To obtain these effects, a Mn content of 1.50% or more is necessary. On the other hand, if the Mn content exceeds 3.00%, not only does the effect of increasing surface fatigue strength saturate, but the hardness of the raw material, such as steel bars and wires, and the hardness after hot forging becomes too high, significantly reducing machinability. The preferred range for Mn content is 1.70 to 2.80%.

[0031] [P:0.030% or less] P is an impurity that causes grain boundary segregation and embrittlement of the component, so a low P content is preferable. If the P content exceeds 0.030%, the surface fatigue strength may decrease. The preferred upper limit for P content to prevent a decrease in surface fatigue strength is 0.020%. The P content may be 0%, but it is difficult to make it completely 0%, so it may be contained at 0.001% or more.

[0032] [S:0.100% or less] S is an element that combines with Mn to form MnS, improving machinability. However, high S content makes it easier to generate coarse MnS, which significantly reduces surface fatigue strength. Therefore, the S content should be 0.100% or less. The preferred upper limits for S content are 0.080%, 0.060%, or 0.040%. While the S content can be 0%, excessively reducing S leads to increased costs for desulfurization. Therefore, considering the economics of refining, the S content may be 0.001% or more, 0.002% or more, or 0.005% or more.

[0033] [Cr: 0.05~2.00%] Cr is an effective element for improving surface fatigue strength because nitriding treatment forms fine nitrides (CrN) in the compound layer and diffusion layer, increasing hardness. To obtain these effects, a Cr content of 0.05% or more is necessary. On the other hand, if the Cr content exceeds 2.00%, not only does the effect of improving surface fatigue strength saturate, but the hardness of the raw material, such as steel bars and wires, and the hardness after hot forging becomes too high, significantly reducing machinability. The preferred range for Cr content is 0.15 to 1.70%.

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

[0035] [Al:0.100% or less] Al is a deoxidizing element. In addition, it combines with N to form AlN, and through the pinning action of austenite grains, it refines the structure of the base material before nitriding, thereby reducing variations in the mechanical properties of nitrided parts. To obtain 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 readily forms hard oxide inclusions, and a high Al content may significantly reduce surface fatigue strength. Furthermore, if the Al content is excessively high, the desired surface fatigue strength may not be obtained even if other requirements are met, so the Al content should be 0.100% or less. To prevent a decrease in surface fatigue strength, the preferred upper limit for Al content is 0.050%.

[0036] [N:0.0250% or less] Nitrogen (N) combines with Si, Mn, Cr, V, and Al to form Si3N4, Mn3N2, CrN, VN, and AlN, respectively. In particular, AlN, through its pinning action on austenite grains, refines the microstructure of the base material before nitriding, thereby reducing variations in the mechanical properties of nitrided parts. From the viewpoint of obtaining 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 more likely to form, and there is a risk that the surface fatigue strength will decrease significantly due to the coarse AlN. Furthermore, if the N content is excessively high, the desired surface fatigue strength may not be obtained even if other requirements are met, so the N content should be 0.0250% or less. Preferably, the N content may be 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0037] The core of the steel component relating to this disclosure may have, for example, the following chemical composition. C: 0.16~0.29%, Si: 0.05~0.25%, Mn: 1.50~2.60%, P: 0.030% or less, S: 0.100% or less, Cr: 0.65~1.00%, V: 0.05~0.35%, Al: 0.100% or less, N: 0.0250% or less, Remainder: Fe and impurities.

[0038] In the chemical composition of the core of the steel component relating to this disclosure, the remainder of the elements other than those mentioned above basically consists of Fe and impurities. Impurities include elements contained in the raw materials or elements mixed in during the manufacturing process, and are acceptable as long as they do not impair the properties of the steel component relating to this disclosure.

[0039] The following are optional elements that may be further contained in the core of the steel part according to this disclosure. In other words, the core of the steel part according to this disclosure may contain the following elements in place of a portion of Fe. However, the part according to this 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%.

[0040] [Mo: 0~1.30%] Mo is an effective element for improving surface fatigue strength because it stabilizes the ε phase in the compound layer and forms fine nitrides (Mo2N) in the compound layer and diffusion layer, thereby increasing hardness. To obtain these effects, it is preferable that the Mo content be 0.01% or more. On the other hand, if the Mo content exceeds 1.50%, not only does the ratio of the γ' phase decrease, but the hardness of the raw material, such as steel bars and wires, and the hardness after hot forging becomes too high, resulting in a significant decrease in machinability. The preferred range for Mo content is 1.00% or less.

[0041] [Ti: 0~0.100%] Ti combines with N to form TiN, improving both core and surface hardness. To achieve this effect, it is preferable that the Ti content be 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the effect of improving core and surface hardness saturates, and the alloy cost increases. The preferred range for Ti content is 0.080% or less.

[0042] [Nb:0~0.050%] Nb combines with C and N to form NbC and NbN, and through the pinning action of austenite grains, it refines the microstructure of the steel material before nitriding, thereby reducing variations in the mechanical properties of nitrided parts. To obtain this effect, it is preferable that the Nb content be 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 obtain the above effect. The preferred range for Nb content is 0.040% or less.

[0043] [Cu: 0~0.50%] Cu is an element that can ensure hardenability and increase core hardness through solid solution strengthening. To reliably obtain this effect, the Cu content may be increased to 0.01% or more. On the other hand, if the Cu 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 it is best to keep the Cu content below 0.50%. If copper is included, it is preferable that the copper content be 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, it is preferable that the copper content be 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0044] [Ni: 0~0.50%] Ni is an element that can ensure hardenability and increase core hardness through solid solution strengthening. To reliably obtain this effect, the Ni content may be increased to 0.01% or more. On the other hand, if the Ni 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 it is best to keep the Ni content below 0.50%. If Ni is included, it is preferable that the Ni content be 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, it is preferable that the Ni content be 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0045] [W: 0~0.50%] Water (W) has the effect of increasing hardenability and improving the strength of steel. To reliably obtain the effects of W, the W content should be 0.01% or higher. On the other hand, if the W content is too high, the hardness of the base material after hot working becomes too high, and the machinability of the base material decreases significantly, so the W content should be 0.50% or lower. If W is present, it is preferable that the W content be 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, it is preferable that the W content be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0046] [Co: 0~0.100%] Co is an element that increases core hardness through solid solution strengthening. To reliably obtain this effect, the Co content should be 0.001% or higher. On the other hand, if the Co 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 Co content should be 0.100% or lower. If Co is present, it is preferable that the Co content be 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, it is also preferable that the Co content be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0047] [B: 0~0.0100%] Solid solution B has the effect of improving hardenability, suppressing grain boundary segregation of P, and improving toughness. In addition, BN, which precipitates by bonding with N, improves machinability. To reliably obtain these effects, the B content should be 0.0005% (5 ppm) or higher. On the other hand, if the B content is too high, a large amount of BN segregation will be promoted, which may lead to cracking of the steel, so the B content should be 0.0100% or less. If B is present, it is preferable that the B content be 0.0010% or more. Similarly, it is preferable that the B content be 0.0050% or less.

[0048] [Bi: 0~0.100%] Bi (Bicarbonate) has the effect of reducing cutting resistance and extending tool life. To reliably achieve this effect, the Bi content should be 0.001% or higher. On the other hand, if the Bi content is high, cracks and defects are more likely to occur during hot working, so the Bi content should be 0.100% or lower. If Bi is included, it is preferable that the Bi content be 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, it is also preferable that the Bi content be 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0049] [Te:0~0.100%] Te has the effect of reducing cutting resistance and extending tool life. To reliably obtain the effect of Te, it is best to have a Te content of 0.010% or more. On the other hand, if the Te content is too high, the effect will saturate and economic efficiency will be compromised, so it is best to have a Te content of 0.100% or less. If Te is included, it is preferable that the Te content be 0.020% or more, or 0.030% or more, or 0.040% or more. Similarly, it is preferable that the Te content be 0.090% or less, 0.080% or less, or 0.070% or less.

[0050] [Pb: 0~0.09%] Lead (Pb) reduces cutting resistance and extends tool life. To reliably obtain the effects of Pb, the Pb content should be 0.01% or higher. However, increasing the Pb content too much leads to saturation of the effect, diminishing economic viability and contributing to environmental burden, so the Pb content should be kept below 0.09%. When containing Pb, it is preferable to limit the Pb content to 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less, from the standpoint of minimizing environmental impact.

[0051] [Sn: 0~0.100%] Sn has the effect of reducing cutting resistance and extending tool life. To reliably obtain this effect, the Sn content should be 0.001% or higher. On the other hand, if the Sn content is too high, the effect saturates and economic efficiency is lost, so the Sn content should be 0.100% or lower. If Sn is present, it is preferable that the Sn content be 0.005% or more, or 0.010% or more. Similarly, it is preferable that the Sn content be 0.090% or less, or 0.080% or less.

[0052] [Sb: 0~0.100%] Sb has the effect of reducing cutting resistance and extending tool life. To reliably obtain this effect, the Sb content should be 0.001% or higher. On the other hand, if the Sb content is too high, the effect saturates and economic efficiency is lost, so the Sb content should be 0.100% or lower. If Sb is present, the Sb content may preferably be 0.005% or more, or 0.006% or more. Similarly, the Sb content may preferably be 0.090% or less, or 0.080% or less.

[0053] [Ca: 0~0.0100%] Ca has the function of improving surface fatigue strength by refining MnS. To reliably obtain this effect of Ca, it is best to have a Ca content of 0.0010% or more. On the other hand, if the Ca content is too high, the effect saturates and economic efficiency is lost, so it is best to have a Ca content of 0.0100% or less. If Ca is present, it is preferable that the Ca content be 0.0020% or more, or 0.0030% or more, or 0.0040% or more. Similarly, it is preferable that the Ca content be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0054] [Mg: 0~0.0100%] Mg has the effect of refining MnS and improving surface fatigue strength. To reliably obtain the effect of Mg, it is best to have a Mg content of 0.0010% or more. On the other hand, if the Mg content is too high, the effect will saturate and economic efficiency will be compromised, so it is best to have a Mg content of 0.0100% or less. If Mg is present, it is preferable that the Mg content be 0.0020% or more, or 0.0030% or more, or 0.0040% or more. Similarly, it is preferable that the Mg content be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0055] [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" is composed of one or more elements selected from these rare earth elements, and REM content means the total content of these 17 elements. When lanthanides are used as REM, industrially, REM is added in the form of mischmetal.

[0056] REM works by refining MnS to improve surface fatigue strength. To reliably obtain the effect of REM, it is best to have a REM content of 0.001% or more. On the other hand, if the REM content is too high, the effect will saturate and economic efficiency will be compromised, so it is best to have a REM content of 0.020% or less. If REM is included, it is preferable that the REM content be 0.002% or more, 0.003% or more, or 0.004% or more. Similarly, it is preferable that the REM content be 0.018% or less, 0.016% or less, or 0.014% or less.

[0057] The above-mentioned arbitrary elements can be divided into the following groups A to D from the viewpoint of their effects. The core of the steel component according to this disclosure may contain one or more elements selected from these groups. [Group A] Improvement of nitride layer hardness Mo: 1.30% or less Ti:0.100% or less Nb: 0.050% or less [Group B] Improvement of 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] Extending the lifespan of tools 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] Improvement of surface fatigue strength by MnS refinement Ca:0.0100% or less Mg: 0.0100% or less REM: 0.020% or less

[0058] [Regarding equation (1)] The chemical composition of the core of the component relating to this disclosure satisfies formula (1), provided that the content of each element is within the range described above in this disclosure. 24 ≤ (Mn + 3Cr + 2V) / Si ≤ 80 (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. Formula (1) is an index that indicates the penetration depth of N in the surface layer during the nitriding treatment in the manufacturing process for producing nitrided parts using steel materials, such as steel bars, which are the material for the parts relating to this disclosure. In the chemical composition of the core of the component according to this disclosure, Mn, Cr, and V all have high affinity with N, making it easy to form alloy nitrides on the surface of the steel material during nitriding treatment. On the other hand, although Si is a nitride-forming element, it has low affinity with N, making it difficult to form alloy nitrides on the surface of the steel material during nitriding treatment. Therefore, assuming that the content of each element in the chemical composition of the core of the component according to this disclosure is within the range of this disclosure, the amount of alloy nitrides on the surface of the steel material during nitriding treatment can be stabilized by appropriately adjusting the content of Mn, Cr, V, and Si.

[0059] In the chemical composition of the core of the component relating to this disclosure, assuming that the content of each element is within the range of this disclosure, if the value of (Mn+3Cr+2V) / Si in formula (1) (hereinafter abbreviated as "value of formula (1)") is less than 24, the Si content is excessively high compared to the Mn, Cr, and V content. In this case, alloy nitrides are less likely to form on the surface of the steel material during nitriding. In this case, the hardness and depth of the hardened layer after nitriding may be reduced, and sufficient surface fatigue strength may not be obtained. Also, if the value of formula (1) is greater than 80, the Si content is excessively low compared to the Mn, Cr, and V content. In this case, during nitriding, the N concentration on the surface of the steel material becomes excessively high, resulting in a shallow N penetration depth, and sufficient surface fatigue strength may not be obtained. If the value of equation (1) is between 24 and 80, the Si content is within an appropriate range relative to the Mn, Cr, and V content. Therefore, an appropriate amount of N penetrates the steel during the nitriding process. As a result, nitrided parts manufactured using steel as the material have high surface fatigue strength. The preferred range for the value of equation (1) is 26 to 50, and more preferably 28 to 45. Note that the value of equation (1) is obtained by rounding the first decimal place of the calculated value.

[0060] Next, we will describe the composition of the core of the steel component relating to this disclosure, other than its chemical composition.

[0061] [Thickness (t) of the compound layer: more than 3 μm to 30 μm] The compound layer is a layer of iron nitride formed by nitriding treatment, and its thickness affects the surface fatigue strength of the nitrided part. If the compound layer is too thick, it is likely to become a fracture origin of surface fatigue (pitching damage). If the compound layer is too thin, sufficient surface fatigue strength may not be obtained. In the nitrided part according to the present disclosure, from the viewpoint of ensuring surface fatigue strength, the thickness of the compound layer is set to more than 3 μm to 30 μm. The preferable range of the thickness of the compound layer is 5 μm to 27 μm.

[0062] The thickness of the compound layer is measured by observing a cross-section in the thickness direction of the steel part. For example, a perpendicular cross-section is polished on the surface of the part according to the present disclosure, etched, and observed and measured with a scanning electron microscope (SEM). The etching is performed with a 3% nital solution for 20 to 30 seconds. The compound layer exists in the surface layer of the part according to the present disclosure and is observed as a white uncorroded layer. Observe the compound layer from 10 fields of view of the microstructure photograph taken at 4000 times (area of one field of view: 6.6×10 2 μm 2 ), and measure the thickness of the compound layer at three points every 10 μm in the horizontal direction. Then, the average value of the 30 measured points is defined as the compound layer thickness (μm).

[0063] [Area ratio of γ' phase in the compound layer: more than 50%] The γ' phase has a fcc structure and is more ductile than the ε phase having a hcp structure. In addition, since the driving force for pore generation is smaller than that of the ε phase, the formation of a porous layer on the surface can be suppressed, and as a result, pitching in surface fatigue can be suppressed. To obtain this effect, the area ratio of the γ' phase needs to be more than 50%. Preferably it is 60% or more, more preferably 70% or more. The remainder is the ε phase.

[0064] [Area ratio of γ phase in the surface layer of the diffusion layer: 5% to 40%] By having an area ratio of 5% or more of the γ phase in the region up to a depth of 10 μm from the surface of the diffusion layer (sometimes referred to as the "surface layer of the diffusion layer" in this disclosure), the area ratio of the γ' phase in the compound layer can be stably increased. On the other hand, if the area ratio of the γ phase exceeds 40%, the hardness of this layer decreases, resulting in a decrease in surface fatigue strength. The preferred range for the area ratio of the γ phase to obtain this effect is 10 to 30%, and more preferably 15 to 25%. The remaining structure in the surface layer of the diffusion layer is not particularly limited.

[0065] The area ratios of the γ' and ε phases in the compound layer, as well as the area ratio of the γ phase on the surface of the diffusion layer, are determined by image processing of microstructure photographs. Specifically, cross-sectional microstructure photographs of the surface of the nitride material (part) taken at 4000x magnification using electron backscatter diffraction (EBSD, EDAX) are obtained from 10 fields of view (1 field of view area: 6.6 × 10⁻¹⁰). 2 μm 2 For this analysis, the compound layer and the diffusion layer up to a depth of 10 μm from the surface are selected, and the γ' phase (fcc) and ε phase (hcp) in the compound layer, as well as the γ phase (fcc) at a depth of 10 μm from the surface of the diffusion layer, are identified. In this case, for the EBSD camera, for example, the binning is set to 4×4, 8×8, or 11×11, and the SEM acceleration voltage, irradiation current, and focal length are adjusted to perform the analysis under conditions where the Kikuchi line Confidence Index (CI value) is 0.1 or higher, or the Fit value is less than 1°. The area ratios of the γ' phase and ε phase in the compound layer, and the area ratio of the γ phase on the surface of the diffusion layer are determined by binarization using image processing. The average value of the area ratios of the γ' phase and ε phase in the compound layer of the 10 measured fields of view is defined as the area ratios of the γ' phase and ε phase (%), and the average value of the area ratio of the γ phase on the surface of the diffusion layer is defined as the area ratio of the γ phase (%).

[0066] [Diffusion layer hardness: 580HV or higher] 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 beneath the compound layer is lower than that of the compound layer, delamination may occur at the interface between the compound layer and the diffusion layer, leading to premature pitting. To improve the hardness of the surface of the diffusion layer, the steel material of the component system specified in this disclosure is subjected to nitriding treatment, and nitrides such as CrN and VN are precipitated 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 the surface of the diffusion layer, a Vickers hardness (sometimes referred to as "diffusion layer hardness" in this disclosure) at a depth of 0.05 mm from the surface of the diffusion layer of 580 HV or higher results in excellent surface fatigue strength. The hardness of the diffusion layer is preferably 600 HV or higher, and more preferably 620 HV or higher. The hardness of the diffusion layer is measured in the cross-section in the thickness direction of the steel part. For example, the vertical cross-section of the part according to this disclosure is mirror-polished, and the Vickers hardness of 10 arbitrary points at the aforementioned depth positions from the cross-section (polished surface) is measured 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.

[0067] [Manufacturing method for steel parts] Next, an example of a manufacturing method for steel parts (nitrided parts) related to this disclosure will be described.

[0068] In one example of a manufacturing method for steel parts (nitrided parts) according to this disclosure, a steel material having the chemical composition of the core described above is subjected to gas nitriding. The processing temperature for gas nitriding is 600 to 680°C, and the total processing time for gas nitriding is 0.5 to 15 hours.

[0069] [Processing temperature: 600~680℃] The temperature of the gas nitriding treatment (nitriding temperature) is mainly correlated with the nitrogen diffusion rate 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 sufficient thickness of the compound layer cannot be obtained. On the other hand, if the nitriding temperature is too high, the austenite fraction in the nitrogen diffusion layer becomes too high, resulting in low surface hardness of the diffusion layer. Therefore, in this disclosure, the nitriding temperature is 600 to 680°C, which is the temperature range of the ferrite + austenite two-phase region in the Fe-N binary system. In this case, a good compound layer can be obtained and a deep diffusion layer can be formed in a relatively short time. Preferably, it is 620 to 660°C.

[0070] [Total processing time for gas nitriding: 0.5-15 hours] Gas nitriding is carried out in an atmosphere containing NH3, H2, and N2. The total time of the nitriding process, that is, the time from the start to the end of the nitriding process (processing 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 processing time is too short, a compound layer of sufficient thickness may not be obtained. On the other hand, if the processing time is too long, the hardness of the diffusion layer and the core will decrease, and the surface fatigue strength will decrease. Furthermore, if the processing time is too long, the manufacturing cost will also increase. Therefore, the total processing time for nitriding is 0.5 to 15 hours. Preferably, it is 1.0 to 10 hours.

[0071] The atmosphere used in the gas nitriding treatment in this disclosure inevitably contains impurities such as oxygen and carbon dioxide, in addition to NH3, H2, and N2. A preferred atmosphere is one in which the total of NH3, H2, and N2 is 99.5% (by volume) or more.

[0072] [Gas conditions for nitriding treatment] In the nitriding method described herein, the nitriding potential is controlled. As a result, the thickness of the compound layer is greater than 3 to 30 μm, the area ratio of the γ' phase in the compound layer is greater than 50%, and the remainder is the ε phase. In the region up to 10 μm from the surface of the nitrogen diffusion layer, the γ phase is contained in an area ratio of 5 to 40%. A steel part is obtained in which the Vickers hardness at a depth of 0.05 mm from the surface of the nitrogen diffusion layer is 580 HV or higher.

[0073] Nitriding potential K in gas nitriding treatment N It is defined by the following equation (2).

[0074] K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ] Formula (2)

[0075] The partial pressures of NH3 and H2 in the atmosphere during gas nitriding can be controlled by adjusting the gas flow rate.

[0076] As a result of the inventors' investigations, they found that the desired thickness and constituent phase can be achieved by controlling the nitriding potential of the gas nitriding treatment as follows. (1) Average value of nitriding potential before the start of nitriding treatment: 0.20~1.50 When nitriding steel materials (e.g., steel bars) that are the material for parts, it is important to preheat the furnace and bring the atmosphere to a predetermined range before starting the holding (nitriding) of the steel material. This is necessary to obtain the thickness and composition of the compound layer as defined in this disclosure. The desired compound layer can be obtained if the average value of the nitriding potential from at least 10 minutes before the start of the nitriding treatment until the start of the nitriding treatment is 0.20 to 1.50. If the nitriding potential before the start of the nitriding treatment is too low, a compound layer of sufficient thickness cannot be obtained, and 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 value of the nitriding potential before the start of the nitriding treatment is preferably 0.40 to 1.30.

[0077] (2) Lower limit of nitriding potential during nitriding: 0.10 During the nitriding process (holding), it is important to keep the lower limit of the nitriding potential at 0.10 or higher. If it falls below 0.10, the compound layer may decompose, resulting in an insufficient thickness of compound layer and a decrease in surface fatigue strength. The lower limit during the nitriding process is preferably 0.15.

[0078] (3) Upper limit of nitriding potential during nitriding: 1.20 During the nitriding process (holding), it is important to keep the upper limit of the nitriding potential below 1.20. If it falls below 1.20, the compound layer will become predominantly ε-phase, and the compound layer and porous layer may grow rapidly, potentially preventing the acquisition of sufficient surface fatigue strength. The upper limit during the nitriding process is preferably 1.00.

[0079] (4) Average nitriding potential during nitriding: 0.15~0.70 By setting the average nitriding potential during the nitriding process within the above range, the desired compound can be obtained. If the average value exceeds 0.70, a thick compound layer with many pores will be formed, and if it is lower than 0.15, a thin compound layer will be formed, and sufficient surface fatigue strength cannot 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 the process, and may be changed any number of times. The preferred range for the average nitriding potential during the nitriding process is 0.20 to 0.60.

[0080] The component according to this disclosure can be manufactured by gas nitriding a steel material having the chemical composition of the core of the component according to this disclosure, while controlling the nitriding potential as described above. When nitriding a steel with the component system according to this disclosure in this way, high surface fatigue strength can be obtained by creating a good compound layer and diffusion layer without complicating the nitriding treatment conditions. [Examples]

[0081] [Steel manufacturing] Ingots were manufactured by the ingot-making method using the molten steel shown in Table 1. The cross-section of the ingot perpendicular to its longitudinal direction was a rectangle of 180 mm × 180 mm. The manufactured ingots were allowed to cool to room temperature. Underlined elements in Table 1 indicate compositions outside the scope of this disclosure. Blank spaces indicate that the corresponding element content is 0% in terms of significant figures (to the least significant digit) as defined in this disclosure. The remainder consists of Fe and impurities.

[0082] [Table 1]

[0083] The obtained ingot was heated at 1200°C for 2 hours. After heating, the ingot was subjected to hot working (hot forging) to produce a steel bar with a diameter of 40 mm and a length of 1000 mm. The steel bar was allowed to cool to room temperature after hot working.

[0084] [Manufacturing of steel component test pieces] Using the manufactured steel material, the following two types of steel component test pieces (small roller test piece and hardened layer investigation test piece) were prepared.

[0085] (1) Small roller test piece Figure 1 shows a side view of the small roller test specimen prepared in this embodiment. The numbers in Figure 1 indicate dimensions (in mm). "φ" in Figure 1 means diameter. The inverted triangle symbol in Figure 1 represents the "finishing symbol" indicating surface roughness as described in Explanatory Table 1 of JIS B 0601:1982. The "G" attached to the finishing symbol is an abbreviation for the grinding method specified in JIS B 0122:1978. The small roller test specimen is a test specimen used to measure surface fatigue strength. Multiple small roller test specimens were prepared for each test number.

[0086] Specifically, first, small roller test specimens were manufactured by machining the steel materials for each test number. The central axis of the test specimen was coaxial with the central axis of the steel bar. The surface of the cylindrical section (test surface) with a diameter of 26 mm, as shown in Figure 1, was machined to conform to JIS B 0601:2001, with an arithmetic mean roughness Ra of 0.6 to 0.8 μm and a maximum height Rz of 2.0 to 4.0 μm. Gas nitriding treatment was then performed on these small roller test specimens. The conditions for the gas nitriding treatment are shown in Table 3 below.

[0087] The test specimens were placed in a gas nitriding furnace, and NH3, H2, and N2 gases were introduced into the furnace. After the nitriding treatment, the test specimens were cooled with N2 gas.

[0088] The partial pressure of H2 in the atmosphere was measured using a thermal conduction type H2 sensor directly attached to the gas nitriding furnace body. The difference in thermal conductivity between the standard gas and the measured gas was converted to gas concentration for measurement. The partial pressure of H2 was continuously measured throughout the gas nitriding process.

[0089] Furthermore, the NH3 partial pressure was measured using an infrared absorption type NH3 analyzer installed outside the furnace. The NH3 partial pressure was continuously measured throughout the gas nitriding treatment. In the case of test number 5, which was conducted in a CO2 gas-mixed atmosphere, (NH4)2CO3 precipitated inside the infrared absorption type NH3 analyzer, and there was a risk of the device malfunctioning, so the NH3 partial pressure was measured every 2 minutes using a glass tube type NH3 analyzer.

[0090] Average nitriding potential K of gas nitriding treatment N (atm -1 / 2 ) is defined by the following equation (2). K N =P NH3 / P H2 3 / 2 ··· Equation (2) Here, P in equation (2) NH3 This is the partial pressure of NH3 in the ambient gas [atm], and P H2 This is the partial pressure of H2 in the ambient gas [atm].

[0091] Nitride potential K calculated within the device NThe NH3 and N2 flow rates were controlled so that the nitridation potential K converged to the target value. N Record the K measured during the processing time. N The average value was calculated.

[0092] After heat treatment, the gripping portion was finished to remove heat treatment distortion, and then each was used as a roller pitting test specimen. In the actual roller pitting test using the small roller specimen, the cylindrical portion (test surface) with a diameter of 26 mm was brought into contact with the large roller, a predetermined surface pressure was applied, and then it was rotated.

[0093] (2) Test specimens for investigating the hardened layer Two test specimens were prepared for each test number for investigating the hardened layer. The test specimens for investigating the hardened layer were cylindrical, with a diameter of 26 mm and a length of 100 mm.

[0094] Specifically, first, two cylindrical test specimens, each 26 mm in diameter and 100 mm in length, were prepared by machining the steel material for each test number. The central axis of the test specimens was made coaxial with the central axis of the steel bar. The test specimens were subjected to the same heat treatment (nitriding treatment) as the small roller test specimens. Test specimens for hardening layer investigation were prepared through the above manufacturing process.

[0095] (Manufacturing of large roller test specimens used in two-cylinder rolling fatigue tests) Furthermore, large roller test specimens used in two-cylinder rolling fatigue tests to measure surface fatigue strength were manufactured using the following method. A rough specimen of a large roller test piece, as shown in Figure 2, was cut from a cylindrical material with a diameter of 140 mm, having a chemical composition equivalent to SUJ2 as specified in JIS G 4805:2008. The numbers in Figure 2 indicate dimensions (in mm). "φ" in Figure 2 means diameter. The inverted triangle symbol in Figure 2 represents the "finishing symbol" indicating surface roughness as described in Explanatory Table 1 of JIS B 0601:1982. The "G" attached to the finishing symbol is an abbreviation for the grinding method specified in JIS B 0122:1978.

[0096] The cut-out rough test specimens were subjected to quenching. The quenching temperature was set to 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time, they were rapidly cooled in 60°C oil. The outer surface of the quenched rough test specimens was finished by machining. The outer surface 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. Large roller test specimens were manufactured using the above process.

[0097] [Measurement of compound layer thickness on the surface] The cross-section of the small roller test specimen, which had undergone the above quenching treatment, was mirror-polished perpendicular to its longitudinal direction and then etched. The etched cross-section was observed using a scanning electron microscope (SEM), and the thickness of the nitrogen compound layer was measured. Etching was performed with a 3% nital solution for 20-30 seconds.

[0098] The nitrogen compound layer can be identified as a white, uncorroded layer on the surface. (Ten fields of view, 4000x magnification; 1 field of view area: 6.6 × 10) 2 μm 2 The nitrogen compound layer was observed from the ) and the thickness of the compound layer was measured at three points every 10 μm. The average of the 30 measured points was defined as the nitrogen compound layer thickness (μm).

[0099] [Measurement of γ' phase and γ phase ratio] The area ratios of the γ' and ε phases in the compound layer, as well as the area ratio of the γ phase on the surface of the diffusion layer, are determined by image processing of microstructure photographs. Specifically, cross-sectional microstructure photographs of the surface of the nitride material (part) taken at 4000x magnification using electron backscatter diffraction (EBSD, EDAX) are obtained from 10 fields of view (1 field of view area: 6.6 × 10⁻¹⁰). 2 μm 2For each of the following, the compound layer and the diffusion layer up to a depth of 10 μm from the surface were selected, and the γ' phase (fcc) and ε phase (hcp) in the compound layer, as well as the γ phase (fcc) at a depth of 10 μm from the surface of the diffusion layer, were identified. In this case, the EBSD camera was set to binning 4×4, 8×8, and 11×11, and the SEM acceleration voltage, irradiation current, and focal length were adjusted to perform the analysis under conditions where the Kikuchi line's Confidence Index (CI value) was 0.1 or higher, or the Fit value was less than 1°. The area ratios of the γ' phase and ε phase in the compound layer, and the area ratio of the γ phase on the surface of the diffusion layer were determined by binarization using image processing. The average value of the area ratios of the γ' phase and ε phase in the compound layer of the 10 measured fields of view was defined as the area ratio (%) of the γ' phase and ε phase, and the average value of the area ratio of the γ phase on the surface of the diffusion layer was defined as the area ratio (%) of the γ phase.

[0100] Figure 4 shows an example of SEM-EBSD analysis results on the cross-sectional surface of nitrided steel. Although Figure 4 is displayed in grayscale, the actual analysis image is in color, allowing for easy identification of the compound layer and the diffusion layer. Therefore, the surface of the diffusion layer, i.e., the boundary between the compound layer and the diffusion layer, can be easily identified using SEM-EBSD analysis.

[0101] [Hardness measurement] From the test section (φ26mm) of each small roller subjected to the above nitriding treatment, a sample (thickness: 10mm) with a cross-section perpendicular to the longitudinal direction was taken, and its cut surface was mirror-polished. Subsequently, the Vickers hardness of 10 arbitrary points at depth positions described later was measured from the cross-section (polished surface) using a micro-Vickers hardness tester (Shimadzu Corporation; HMV-G31-FA) under a test force of 1.96N. The average of these 10 points was defined as the surface hardness.

[0102] [Hardness at a distance of 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 mirror-polishing the cross-section revealed by cutting the nitrided part (nitrided part) perpendicular to the main axis or longitudinal direction, and measuring the Vickers hardness at 10 arbitrary points at a depth of 0.05 mm from the surface (interface between the compound layer and the diffusion layer) with a test force of 1.96 N. The average value of these Vickers hardness values ​​was then calculated.

[0103] [Hardness at a distance of 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 mirror-polishing the cross-section revealed by cutting the nitrided part perpendicular to the main axis or longitudinal direction, and measuring the Vickers hardness at 10 arbitrary 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.

[0104] [Evaluation Test] The following evaluation tests were conducted using the various test pieces described above. In this embodiment, assuming application to gear components, the reference values ​​for evaluating surface fatigue strength and rotation were set as follows. For surface fatigue strength and rotational fatigue evaluation, test specimens were prepared using steel (reference steel) that meets the SCr420 standard of JIS G 4053:2016, following a general manufacturing process: "normalizing → specimen processing → eutectoid carburizing in a gas carburizing furnace → low-temperature tempering". Subsequently, the roller pitting test and rotational bending fatigue test were performed using these test specimens, and the obtained fatigue limits were used as the reference values ​​for surface fatigue strength and rotational fatigue in this embodiment.

[0105] (Surface fatigue strength measurement test (two-cylinder rolling fatigue test)) Two-cylinder rolling fatigue tests were conducted using small and large roller specimens, and the surface fatigue strength was determined as follows. The testing machine used was the "RP201" roller pitting tester manufactured by Komatsu Engineering Co., Ltd.

[0106] As shown in Figure 3, the cylindrical portion of the small roller test piece 10, with a diameter of 26 mm, was rolled while in contact with the center of the outer surface of the large roller test piece 20 (the outer portion with a diameter of 130 mm).

[0107] The contact pressure was set to 2000-3500 MPa in Hertzian pressure. The rotation speed of the small roller test piece 10 was set to 2000 rpm. The peripheral speed of the small roller test piece 10 was set to 163 m / min, and the peripheral speed of the large roller test piece 10 was set to 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 set to -40%. The test conditions are shown in Table 2.

[0108] [Table 2]

[0109] The number of cycles to be terminated in the test was 2.0 × 10⁻⁶, which is the fatigue limit for typical steel. 7 The number of times was set to 2.0 × 10 in the small roller test specimen without pitting occurring. 7 The maximum surface pressure (MPa) reached was defined as the fatigue limit of the small roller test specimen.

[0110] Pitching was detected using a vibration meter installed on the testing machine. After vibration occurred, the rotation of both the small and large roller test pieces was stopped, and the occurrence of pitting and the rotation speed were confirmed.

[0111] In this embodiment, 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 1.05 times or more that of the reference steel, it was judged to have excellent surface fatigue strength (indicated as "○" in the "Surface Fatigue Strength Judgment" column in Table 3). On the other hand, if the fatigue limit was less than 1.05 times that of the reference steel, it was judged to have low surface fatigue strength (indicated as "×" in the "Surface Fatigue Strength Judgment" column in Table 3).

[0112] [Test Results] The results are shown in Table 3. Underlined text in Table 3 indicates that the content is outside the scope of this disclosure. Furthermore, the "0.05mm position hardness" of the diffusion layer refers to the Vickers hardness at a depth of 0.05mm from the surface of the diffusion layer.

[0113] [Table 3]

[0114] Tests 1 through 17 met the requirements of this disclosure and yielded excellent results in terms of surface fatigue strength.

[0115] Tests 18-27 failed to meet any of the requirements of this disclosure and therefore did not achieve the desired surface fatigue strength.

[0116] The embodiments and examples of the present disclosure have been described above. However, the embodiments and examples described above are merely illustrative examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the embodiments and examples described above, and can be modified as appropriate without departing from the spirit thereof. [Industrial applicability]

[0117] The steel components relating to this disclosure can be used in a wide range of industrial fields as materials for automobiles, industrial machinery, and especially gears in machines powered by electric motors. [Explanation of Symbols]

[0118] 10 Small roller test specimens (evaluation material) 20 Large roller test specimens (counter material)

Claims

1. It comprises a steel core, a compound layer containing iron nitride, and a nitrogen diffusion layer existing between the steel core and the compound layer, The steel core portion is, by mass %, C: 0.05-0.30%, Si: 0.05-2.00%, Mn: 1.50-3.00%, P: 0.030% or less, S: 0.100% or less, Cr: 0.05-2.00%, V: 0.02 to 1.30%, Al: 0.100% or less, N: 0.0250% or less, It has a chemical composition containing, with the remainder being Fe and impurities. The thickness of the compound layer is greater than 3 to 30 μm. The aforementioned compound layer contains more than 50% of the γ' phase by area ratio, with the remainder being the ε phase. In the region up to a depth of 10 μm from the surface of the nitrogen diffusion layer, the γ phase is contained in an area ratio of 5 to 40%, A steel part having a Vickers hardness of 580 HV or more at a depth of 0.05 mm from the surface of the nitrogen diffusion layer.

2. It comprises a steel core, a compound layer containing iron nitride, and a nitrogen diffusion layer existing between the steel core and the compound layer, The steel core portion is, by mass %, C: 0.05-0.30%, Si: 0.05-2.00%, Mn: 1.50-3.00%, P: 0.030% or less, S: 0.100% or less, Cr: 0.05-2.00%, V: 0.02 to 1.30%, Al: 0.100% or less, N: 0.0250% or less, Furthermore, it has a chemical composition in which it contains one or more selected from the groups A to D below, with the remainder being Fe and impurities. [Group A] Mo: 1.30% or less Ti: 0.100% or less, Nb: One or more types selected from the group consisting of 0.050% or less [Group B] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, B: One or more types selected from the group consisting of 0.0100% or less. [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, Sb: One or more selected from the group consisting of 0.100% or less [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, REM: One or more types selected from the group consisting of 0.020% or less. The thickness of the compound layer is greater than 3 to 30 μm. The aforementioned compound layer contains more than 50% of the γ' phase by area ratio, with the remainder being the ε phase. In the region up to a depth of 10 μm from the surface of the nitrogen diffusion layer, the γ phase is contained in an area ratio of 5 to 40%, A steel part having a Vickers hardness of 580 HV or more at a depth of 0.05 mm from the surface of the nitrogen diffusion layer.

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

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

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.

7. The steel part according to claim 1, wherein the chemical composition of the steel core satisfies the following formula (1). 24≦(Mn+3Cr+2V) / Si≦80 Formula (1) However, each element symbol in formula (1) above represents the mass percentage content of that element.

8. The steel part according to claim 2, wherein the chemical composition of the steel core satisfies the following formula (1). 24≦(Mn+3Cr+2V) / Si≦80 Formula (1) However, each element symbol in formula (1) above represents the mass percentage content of that element.

9. A method for manufacturing a steel part according to any one of claims 1 to 8, For a steel material having the above chemical composition, the nitriding treatment temperature is 600 to 680°C, and the nitriding potential K is calculated using the following formula (2). Ｎ However, K before nitriding treatment Ｎ The average of is 0.20 to 1.50, and K during nitriding treatment Ｎ The ratio is 0.10 to 1.20, and K during nitriding treatment. Ｎ A method for manufacturing steel parts, comprising a step of performing nitriding treatment under the condition that the average of is 0.15 to 0.

70. K Ｎ (atm －１／２ ) = (NH ３ partial pressure (atm)) / [(H ２ partial pressure (atm)) ３／２ Equation (2) In equation (2), NH ３ Partial pressure (atm) and H ２ Partial pressure (atm) is the partial pressure of the processing atmosphere gas.

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