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 invention comprises a steel core and a hardened layer on the steel core, wherein the steel core has the following composition in mass%, C: 0.30~0.60%, 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%, Mo: 0.12~1.30%, V: 0.02~1.30%, Al: 0.005~1.000%, and N: 0.0 A steel part and a method for manufacturing the same, having a chemical composition containing 250% or less of [unspecified element] with the remainder being Fe and impurities, a microstructure at a depth of 0.10 mm from the surface containing 40% or less by volume of retained austenite, an average N concentration in the region from the surface to a depth of 0.10 mm being 1.80 atomic percent or more, and a Vickers hardness at a depth of 0.10 mm from the surface being 650 HV or more.

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 heating steel bars to the ferrite region (approximately 400-600°C) in a Fe-N binary system to allow nitrogen to penetrate the steel surface. Soft nitriding is a surface hardening heat treatment that involves penetrating nitrogen and carbon into the steel surface. 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 compound layer with a thickness of several to tens of micrometers (hereinafter sometimes abbreviated as "compound layer") that forms on the outermost surface, and a nitrogen diffusion layer (hereinafter sometimes abbreviated as "diffusion layer") that forms directly beneath the compound layer.

[0008] The diffusion layer is a layer hardened by a solid solution strengthening mechanism due to invading nitrogen and carbon, as well as a particle dispersion strengthening mechanism of nitrides. Increasing the hardness and depth of the diffusion layer improves the surface fatigue strength of the part. On the other hand, since nitriding and soft nitriding are heat treatments performed at lower temperatures than carburizing and quenching, a long heat treatment time is required to achieve a hardened layer depth comparable to that of carburizing and quenching. In addition, if the nitriding temperature is raised above 600°C to shorten the treatment time, the hardened layer and the unhardened layer (hereinafter sometimes abbreviated as "core") in deeper regions soften, reducing their hardness and preventing the acquisition of sufficient surface fatigue strength. In recent years, as a heat treatment that could overcome the shortcomings of such nitriding and soft nitriding treatments, attention has begun to be drawn to the application of nitrogen quenching, which takes advantage of the fact that the A1 and A3 points in the Fe-N binary system are at lower temperatures than those in the Fe-C binary system. This treatment reduces heat treatment distortion compared to carburizing and quenching, and creates a deeper hardened layer than nitriding or soft nitriding. Nitrogen quenching is a method in which a steel bar is heated to the austenite region (around 700-1000°C) in the Fe-N binary system, the N concentration on the surface of the part is increased, and then it is rapidly cooled to generate hard martensite on the surface of the part. Therefore, in recent years, methods have been proposed to increase fatigue strength by controlling the temperature and atmosphere of the nitriding quenching process.

[0009] 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.

[0010] Patent Document 2 proposes a steel part having a component composition that satisfies predetermined component parameters, wherein the structure from the surface of the part to a depth of 100 μm has a volume fraction of retained austenite of 30% or less, with the remainder consisting of either or both martensite and bainite, and the structure outside the nitrified area contains ferrite at an area fraction of 5-60% and either or both martensite and bainite at a total area fraction of 40-95%. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2021-085055 [Patent Document 2] Japanese Patent Publication No. 2018-141217 [Overview of the project] [Problems that the invention aims to solve]

[0012] In the components disclosed in Patent Document 1, the target is carbon steel for mechanical structures and alloy steel for mechanical structures. Since the content of nitride-forming elements is limited, there is room for improvement in the effect of improving surface fatigue strength.

[0013] In the components disclosed in Patent Document 2, the N concentration in the surface layer is not mentioned, and the improvement effect of surface fatigue is considered to be limited.

[0014] The present disclosure has been made to solve the above problems, and an object thereof is to provide a steel component excellent in surface fatigue strength and a method for manufacturing the same.

Means for Solving the Problems

[0015] The gist of the means for solving the above problems is as follows. <1> A steel core part and a hardened layer on the steel core part are provided. The steel core part has, in mass%, C: 0.30 to 0.60%, 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%, Mo: 0.12 to 1.30%, V: 0.02 to 1.30%, Al: 0.005 to 1.000%, and N: 0.0250% or less, contains a chemical composition in which the balance is Fe and impurities, the microstructure at a depth of 0.10 mm from the surface contains 40% by volume or less of retained austenite, the average N concentration in the region from the surface to a depth of 0.10 mm is 1.80 atomic% or more, and the Vickers hardness at a depth of 0.10 mm from the surface is 650 HV or more, a steel component. <2> A steel core part and a hardened layer on the steel core part are provided. The steel core portion is, by mass %, C: 0.30~0.60%, 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%, Mo: 0.12~1.30%, V: 0.02~1.30%, Al: 0.005~1.000%, and 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] Ti: 0.100% or less, One or two types selected from the group consisting of Nb: 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 microstructure at a depth of 0.10 mm from the surface contains less than 40 volume percent of retained austenite. The average N concentration in the region from the surface to a depth of 0.10 mm is 1.80 atomic percent or more. A steel component having a Vickers hardness of 650 HV or higher at a depth of 0.10 mm from the surface. <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, V, and Al satisfy the following formula (1): <1> Steel parts as described above. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in formula (1) represents the mass percentage content of that element. <8> In the chemical composition of the steel core, Si, Mn, Cr, V, and Al satisfy the following formula (1): <2> ~ <6> A steel part listed in any one of the following. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in formula (1) 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 above chemical composition, the nitriding and quenching treatment temperature is 680 to 1000°C, and the nitriding potential K is calculated using the following formula (2). N is between 0.03 and 1.00, and K N A method for manufacturing steel parts, comprising a step of performing a nitriding quenching treatment under the condition that the average of the values ​​is 0.05 to 0.60. 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]

[0016] According to this disclosure, it is possible to provide a steel part with excellent surface fatigue strength and a method for manufacturing the same. [Brief explanation of the drawing]

[0017] [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. [Modes for carrying out the invention]

[0018] A steel component according to one embodiment of this disclosure will be described below.

[0019] 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.

[0020] The uses and manufacturing methods of the steel parts relating to this disclosure are not particularly limited, but nitriding-quenched steel parts are preferred. Hereinafter, a representative example (one embodiment) of the steel parts relating to this disclosure, a nitriding-quenched steel part (hereinafter sometimes simply referred to as "part"), will be described.

[0021] The steel component relating to this disclosure is, for example, a component that has been nitrogen-quenched from a 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 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 reach during the nitriding and quenching process. In other words, the core is the region where, despite undergoing nitriding and quenching, there is no change in chemical composition, or the change is negligibly small, and it has a chemical composition (sometimes simply referred to as "composition") equivalent to that of the base material of the part (the steel before nitriding and quenching). 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 quenching process. In other words, the hardened layer is the region where the chemical composition and / or metal structure has changed due to the effects of the nitriding quenching 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 quenching decreases, and the hardness of the hardened layer relatively approaches that of the core.

[0022] To solve the aforementioned problems, the inventors of this disclosure investigated the effects of varying various alloy components and nitriding quenching treatment conditions on the morphology of the compound layer on the surface of the part, the hardness of the part, and the surface fatigue strength of the part, and obtained the following findings.

[0023] (a) To improve surface fatigue strength, it is important to ensure hardness at the 0.10 mm position, where shear stress in the depth direction of the part can be maximum. In addition, by ensuring a constant amount of N concentration at the 0.10 mm position from the surface, softening of the surface layer during surface fatigue can be suppressed, and high surface fatigue strength can be obtained. (b) Furthermore, by maintaining a certain amount of residual austenite on the surface, processing-induced transformations during subsequent surface hardening treatments such as shot peening can be fully utilized, thereby improving surface fatigue strength. (c) When steel is nitrified at temperatures above 680°C, which is higher than normal, the A1 and A3 points decrease in the nitrified region, resulting in a single-phase austenite structure. However, in areas with low nitrogen penetration or in non-nitrified regions such as the core, a two-phase region is present, and sufficient hardness may not be obtained after heat treatment. Therefore, it is important to design the composition to achieve sufficient hardness even after heat treatment. 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, while Si addition is not always advantageous. Therefore, it is important to appropriately control the ratios of these components.

[0024] The steel components relating to this disclosure were completed based on the above-mentioned findings.

[0025] First, the chemical composition of the steel component relating to this disclosure will be explained.

[0026] [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 means "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 and quenching treatment.

[0027] [C:0.30~0.60%] Carbon (C) is an essential element for ensuring the hardness of the hardened layer and core of a component. To achieve these effects, a C content of 0.30% or more is required. On the other hand, if the C content exceeds 0.60%, the strength after hot forging becomes too high, significantly reducing machinability. The preferred range for C content is 0.35 to 0.55%, and more preferably 0.40 to 0.50%.

[0028] [Si: 0.05~2.00%] Si (silicon) is effective in improving surface fatigue strength because it increases hardness by forming fine nitrides (Si3N4) in the hardened layer through nitriding and quenching treatment. 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 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%.

[0029] [Mn: 1.50~3.00%] Mn is an effective element for improving surface fatigue strength because it ensures hardenability and, through nitriding quenching, forms fine nitrides (Mn3N2) in the hardened layer, thereby 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%.

[0030] [P:0.030% or less] P is an impurity that causes grain boundary segregation and embrittlement of the component, so a low 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.

[0031] [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. Preferred upper limits for S content are 0.080%, 0.060%, or 0.040%. While 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.

[0032] [Cr: 0.05~2.00%] Cr is an effective element for improving surface fatigue strength because it ensures hardenability and, through nitriding quenching, forms fine nitrides (CrN) in the hardened layer, thereby 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, resulting in a significant decrease in machinability. The preferred range for Cr content is 0.15 to 1.70%.

[0033] [Mo: 0.12~1.30%] Mo is an effective element for improving surface fatigue strength because it ensures hardenability and forms fine nitrides (Mo2N) in the hardened layer, thereby increasing hardness. To obtain these effects, it is preferable to have a Mo content of 0.12% or more. On the other hand, if the Mo content exceeds 1.30%, 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 Mo content is 0.15 to 1.00% or less.

[0034] [V: 0.02~1.30%] V is an effective element for improving surface fatigue strength because it increases hardness by forming fine nitrides (VN) in the hardened layer through nitriding and quenching treatment. To obtain these effects, V must be present 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, resulting in a significant decrease in machinability. The preferred range for V content is 0.05 to 1.00%.

[0035] [Al:0.005~1.000%] Al is a deoxidizing element. In addition, through nitriding and quenching, it forms fine nitrides (AlN) in the hardened layer, increasing hardness and thus being an effective element for improving surface fatigue strength. Furthermore, the pinning action of austenite grains refines the microstructure of the base material before nitriding and quenching, reducing variations in the mechanical properties of nitriding and quenching treated parts. To obtain this effect, it is preferable to have an Al content of 0.005% or more. On the other hand, if the Al content exceeds 1.000%, there is a risk that the surface fatigue strength will decrease significantly due to the formation of hard Al oxides. The preferred range for Al content is 0.010 to 0.800% and 0.030 to 0.600%.

[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 and quenching, thereby reducing variations in the mechanical properties of nitriding and quenching treated 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: 0.30~0.50%, Si: 0.05~0.25%, Mn: 1.50~2.60%, P: 0.030% or less, S: 0.100% or less, Cr: 0.50~0.95%, Mo: 0.12~0.35%, V: 0.02~0.35%, Al: 0.005~1.000%, 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] [Ti: 0~0.100%] Ti combines with N to form TiN, improving the hardness of the core and the hardened layer. 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 the hardness of the core and the hardened layer saturates, and the alloy cost increases. The preferred range for the Ti content is 0.080% or less.

[0041] [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 and quenching, thereby reducing variations in the mechanical properties of nitriding and quenching treated 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.

[0042] [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.

[0043] [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.

[0044] [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.

[0045] [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.

[0046] [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.

[0047] [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.

[0048] [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.

[0049] [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.

[0050] [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.

[0051] [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.

[0052] [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.

[0053] [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.

[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" 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.

[0055] 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.

[0056] 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 core hardness and hardened layer hardness by nitride formation. 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

[0057] [Regarding equation (1)] The chemical composition of the core of the component relating to this disclosure is preferably such that, provided that the content of each element is within the range described above in this disclosure, it also satisfies formula (1). 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) Here, each element symbol in equation (1) is substituted with the mass percentage content of the corresponding element. Formula (1) is an index indicating the penetration depth of N in the surface layer during the nitriding quenching process in the manufacturing process for producing nitriding quenched parts using steel materials, such as steel bars, that will be the material for the parts according to this disclosure. In the chemical composition of the core of the component according to this disclosure, Mn, Cr, V, and Al all have high affinity with N, making it easy to form alloy nitrides on the surface of the steel bar during nitriding and quenching. 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 bar during nitriding and quenching. 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 bar during nitriding and quenching can be stabilized by appropriately adjusting the content of Mn, Cr, V, Al, and Si.

[0058] 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+6Al) / 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, V, and Al content. In this case, alloy nitrides are less likely to form on the surface of the steel bar during the nitriding quenching treatment. In this case, the hardness and depth of the hardened layer after nitriding quenching may be reduced, which may prevent sufficient surface fatigue strength from being obtained. If the value of formula (1) is 24 or greater, the Si content is within an appropriate range relative to the Mn, Cr, V, and Al content. Therefore, an appropriate amount of N penetrates the steel bar during the nitriding quenching process. As a result, nitriding quenched parts manufactured using steel bars as the material have high surface fatigue strength. The preferred lower limit of the value of formula (1) is 26, more preferably 28, even more preferably 30, and even more preferably 32. Note that the value of formula (1) is obtained by rounding the first decimal place of the calculated value. While there is no particular upper limit to the value of equation (1), if the Si content is excessively low compared to the Mn, Cr, and V content, the N concentration on the surface of the steel bar may become excessively high during the nitriding quenching treatment, resulting in a shallow N penetration depth and insufficient surface fatigue strength. For this reason, it is preferable that the value of equation (1) be 200 or less.

[0059] Next, we will describe the composition of the core of the steel component relating to this disclosure, other than its chemical composition.

[0060] [Microstructure of the hardened layer] The hardened layer extends 0.10 mm from the surface of the steel part. The microstructure at a depth of 0.10 mm from the surface of the steel part contains 40% or less by volume of retained austenite. For example, if the microstructure at a depth of 0.10 mm from the surface of the steel part consists of martensite and retained austenite, the volume percentage of retained austenite is 40% or less. Generally, the microstructure in the hardened layer of a nitriding-quenched steel part is mainly martensite and / or bainite.

[0061] [Methods for observing microtissues] The microstructure at a depth of 0.10 mm from the surface of a steel part can be observed, for example, by the following method: The cross-sectional surface of the steel part is mirror-polished to serve as the observation surface. The mirror-polished observation surface is then etched using 3% nitric acid alcohol (Nital etching solution). From the etched observation surface, five arbitrary observation fields (240 μm × 180 μm) within the measurement area are observed with a 500x optical microscope to distinguish between martensite and / or bainite structures. Note that in this disclosure, it is not necessary to distinguish between martensite and bainite.

[0062] If the volume fraction of retained austenite at a depth of 0.10 mm from the surface of a steel part exceeds 40%, the hardness of the hardened layer is too low. In this case, the surface fatigue strength of the steel part will be low. If the volume fraction of retained austenite at a depth of 0.10 mm from the surface of a steel part is between 0% and 40%, the hardened layer will have sufficient hardness. Therefore, the surface fatigue strength of the steel part will be high. The preferred upper limit for the volume fraction of retained austenite at a depth of 0.10 mm from the surface of a steel part is 35%, more preferably 30%, and still more preferably 25%. A lower volume fraction of retained austenite is preferable. However, it is difficult to reduce retained austenite to 0%, and manufacturing costs will also be high. Therefore, considering industrial production, the lower limit for retained austenite is greater than 0%, and preferably 1%.

[0063] [Method for measuring the volume fraction of retained austenite] The volume fraction of retained austenite is measured by the following method: The surface of the steel part is polished by electrolytic polishing to a depth of 0.10 mm, exposing the surface at a depth of 0.10 mm. An X-ray diffractometer is used to irradiate an arbitrary position on the exposed surface with X-rays, and the volume fraction (%) of retained austenite is measured. The volume fraction of retained austenite is calculated from the ratio (integral intensity ratio) of the integral intensity of the diffraction peak of (211)bcc obtained by X-ray diffraction to the integral intensity of the diffraction peak of (220)fcc. Specifically, the volume fraction (%) of retained austenite can be calculated from the following formula, where Iα is the integral intensity of (211)bcc (α phase) and Iγ is the integral intensity of (220)fcc (γ phase). Volume fraction of retained austenite = Iγ / (RIα+Iγ) Here, R = 0.36746.

[0064] [N concentration in the hardened layer: 1.80 atomic percent or higher] The average nitrogen (N) concentration in the region from the surface of the steel part to a depth of 0.10 mm is 1.80 atomic percent or higher. If the N concentration in this region is 1.80 atomic percent or higher, sufficient surface fatigue strength can be obtained in surface fatigue accompanied by frictional heat generation on the surface of the part due to repeated contact, by suppressing the decrease in hardness of the part surface through precipitation strengthening by alloy nitrides and solid solution strengthening by N. The average N concentration in the region from the surface to a depth of 0.10 mm is preferably 2.00 atomic percent or more, and more preferably 2.20 atomic percent or more.

[0065] [Method for measuring the nitrogen concentration in the hardened layer] The average nitrogen (N) concentration in the region from the surface of a steel part to a depth of 0.10 mm can be measured by the following method: Machining is performed on the steel part to a depth of 0.10 mm from the surface, and chips are collected from the surface region up to a depth of 0.10 mm. Chemical analysis is performed using the collected chips. Specifically, the N concentration is obtained by performing a well-known combustion method (combustion-infrared absorption method) on the collected chips. Specifically, the above-mentioned chips are burned in a graphite crucible in an inert gas atmosphere by resistance heating, and the N2 gas generated by the reduction reaction is detected. The N concentration is calculated from the difference in thermal conductivity. The obtained N concentration is defined as the average N concentration (atomic %) in the region from the surface of the steel part to a depth of 0.10 mm.

[0066] [Hardness of the hardened layer: 650HV or higher] The Vickers hardness at a depth of 0.10 mm from the surface is 650 HV or higher. If the hardness at this position is 650 HV or higher, the occurrence of initial cracks on the surface of the part can be suppressed, and sufficient surface fatigue strength can be obtained. The hardness of the hardened layer is preferably 670 HV or higher, and more preferably 690 HV or higher. The hardness of the hardened layer is measured in the cross-section along the thickness direction of the steel part. The vertical cross-section of the steel part 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 of these 10 points is defined as the hardness of the hardened layer.

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

[0068] In one example of a manufacturing method for steel parts (nitriding-quenched parts) according to this disclosure, a steel material having the chemical composition of the core described above is subjected to nitriding-quenching. The processing temperature for nitriding-quenching is 680 to 1000°C, and the total processing time for nitriding-quenching is 0.5 to 10 hours.

[0069] [Processing temperature: 680~1000℃] The steel parts according to this disclosure can be manufactured using steel bars whose composition is designed to accommodate quenching from temperatures close to the austenite single-phase region. If the holding temperature for the nitriding quenching treatment is less than 680°C, quenching will occur from the ferrite + austenite two-phase region, which may result in insufficient hardness of the hardened layer. On the other hand, if the temperature exceeds 1000°C, many pores may form on the surface, and it may be difficult to increase the N concentration on the surface due to the low NH3 partial pressure, which may result in insufficient hardness of the hardened layer after quenching. Therefore, the nitriding quenching temperature in this disclosure is preferably between 680 and 1000°C. The preferred range for the nitriding quenching temperature is between 700 and 980°C, and more preferably between 720 and 960°C.

[0070] [Total processing time for nitriding and quenching: 0.5-10 hours] The nitrogen quenching treatment is carried out in an atmosphere containing NH3, H2, and N2. The total time of the nitrogen quenching treatment, that is, the time from the start to the end of the 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 the core will be low, and the surface fatigue strength will decrease. Furthermore, if the treatment time is too long, the manufacturing cost will also increase. Therefore, the total treatment time for the nitrogen quenching treatment is 0.5 to 10 hours. Preferably, it is 1.0 to 8 hours.

[0071] The atmosphere used in the nitriding quenching 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 nitrogen quenching treatment] In the nitriding quenching method described herein, it is preferable to control the nitriding potential. This increases the area ratio of the γ' phase in the compound layer, making it possible to obtain a nitriding quenched part with high surface fatigue strength.

[0073] Nitriding potential K in nitrocarburizing heat treatment N is defined by the following formula (2).

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

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

[0076] As a result of the studies by the inventors of the present disclosure, it has been found that a target hardened layer can be obtained by controlling the nitriding potential of the nitrocarburizing heat treatment as follows. (1) Average value of nitriding potential before starting nitrocarburizing heat treatment: 0.15 to 1.20 When nitrocarburizing a steel material (for example, a round steel) which is a material of a part, it is important to preheat the furnace in advance and bring the atmosphere into a predetermined range before starting the holding (nitrocarburizing heat treatment) of the steel material, which is necessary to obtain the configuration defined in the present disclosure. When the average value of the nitriding potential from at least 10 minutes before starting the nitrocarburizing heat treatment to the start of the nitrocarburizing heat treatment is 0.15 to 1.20, a desired hardened layer can be obtained. If the nitriding potential before starting the nitrocarburizing heat treatment is too low, a sufficient N concentration cannot be obtained. On the contrary, if it is too high, a large number of pores may be formed on the surface layer, or the amount of residual γ in the hardened layer may become too large. The average value of the nitriding potential before starting the nitrocarburizing heat treatment is preferably 0.30 to 1.00.

[0077] (2) Lower limit of nitriding potential during nitrocarburizing heat treatment: 0.03 During the nitrocarburizing heat treatment (holding), it is important to set the lower limit of the nitriding potential to 0.03 or more. If it is less than 0.03, the N concentration on the surface layer of the part may not increase sufficiently, and the surface fatigue strength may decrease. The lower limit during the nitrocarburizing heat treatment is preferably 0.07.

[0078] (3) Upper limit of nitriding potential during nitrocarburizing heat treatment: 1.00 During the nitriding quenching process (holding), it is important to keep the nitriding potential below 1.00. If it exceeds 1.00, numerous pores may form on the surface, or the amount of residual γ in the hardened layer may become too large, resulting in insufficient surface fatigue strength. The upper limit during the nitriding quenching process is preferably 0.80.

[0079] (4) Average nitriding potential during nitriding quenching: 0.05~0.60 By setting the average nitriding potential during the nitriding quenching process within the above range, a desired hardened layer can be obtained. If the average value exceeds 0.60, numerous pores will form on the surface, or the amount of residual γ will become too high, resulting in insufficient surface fatigue strength. On the other hand, if the average value is lower than 0.05, the N concentration will be too low, resulting in insufficient surface fatigue strength. As long as the average and upper and lower limits fall within the above range, the target nitriding potential can be significantly changed during the process, and can be changed multiple times. The preferred range for the average nitriding potential during the nitriding quenching process is 0.10 to 0.50.

[0080] The part according to this disclosure can be manufactured by performing a nitriding quenching treatment on a steel material having the chemical composition of the core of the part according to this disclosure, while controlling the nitriding potential as described above. When nitriding quenching a steel with the composition system according to this disclosure in this way, high surface fatigue strength can be obtained by creating a good hardened layer without complicating the nitriding quenching 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, rough test specimens with the rough shape of small roller test specimens were manufactured by machining the steel material for each test number. The central axis of the rough test specimen was made coaxial with the central axis of the steel bar. The surface of the cylindrical part (test surface) with a diameter of 26 mm 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. Nitrogen quenching treatment was performed on the rough test specimens. The conditions for each nitrogen quenching treatment are shown in Table 3 below.

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

[0088] The partial pressure of H2 in the atmosphere was measured using a heat conduction type H2 sensor directly attached to the 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 nitriding quenching 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 nitriding and quenching process. 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 nitriding quenching 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 N The 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 central cylindrical portion of the rough test specimen was ground to create a cylindrical portion (test surface) with a diameter of 26 mm as shown in Figure 1. At this time, the surface of the 26 mm diameter cylindrical portion was finished to have an arithmetic mean roughness Ra of 0.6 to 0.8 μm and a maximum height Rz of 2.0 to 4.0 μm, in accordance with JIS B 0601:2001. In the actual roller pitting test using the small roller test specimen, the 26 mm diameter cylindrical portion (test surface) is brought into contact with the large roller, a predetermined surface pressure is applied, and then it is 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 rough test specimens, 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 rough test specimens was coaxial with the central axis of the steel bar. The rough test specimens were subjected to the same heat treatment (nitriding quenching) as the small roller test specimens. Then, similar to the small roller test specimens, the outer surface of the rough test specimens was ground to finish it. At this time, the outer surface of the 26 mm diameter rough test specimens 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. 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 the volume fraction of retained austenite] Using the hardened layer investigation specimens for each test number, the volume fraction of retained austenite at a depth of 0.10 mm from the surface of the nitriding-quenched parts was determined by the following method. The surface of a steel part was polished by electrolytic polishing to a depth of 0.10 mm, exposing the surface at a depth of 0.10 mm. Using an X-ray diffractometer, X-rays were irradiated at an arbitrary position on the exposed surface, and the volume fraction (%) of retained austenite was measured. The volume fraction of retained austenite was calculated from the ratio (integral intensity ratio) of the integral intensity of the diffraction peak of (211)bcc obtained by X-ray diffraction to the integral intensity of the diffraction peak of (220)fcc. Specifically, the volume fraction (%) of retained austenite was obtained from the following formula, where Iα is the integral intensity of (211)bcc (α phase) and Iγ is the integral intensity of (220)fcc (γ phase). Volume fraction of retained austenite = Iγ / (RIα+Iγ) Here, R = 0.36746.

[0098] [Measurement of remaining tissue] Furthermore, using the hardened layer investigation specimens for each test number, the steel parts were buffed to a depth of 0.10 mm from the surface. The observation surface after polishing was etched using 3% nitric acid alcohol (Nital etching solution). From the etched observation surface, five arbitrary observation fields (240 μm × 180 μm) within the measurement area were observed with a 500x optical microscope to determine the remaining microstructure.

[0099] [Measurement of N concentration in the hardened layer] Using test specimens for hardening layer investigation for each test number, the average N concentration in the region from the surface to a depth of 0.10 mm of the nitrogen-quenched part was determined by the following method: Machining was performed on the steel part to a depth of 0.10 mm from the surface, and chips were collected from the surface region up to a depth of 0.10 mm. Chemical analysis was performed on the collected chips. Specifically, the N concentration was obtained by performing a well-known combustion method (combustion-infrared absorption method) on the collected chips. Specifically, the above chips were burned by resistance heating in a graphite crucible in an inert gas atmosphere, and the N2 gas generated by the reduction reaction was detected, and the N concentration was calculated from the difference in thermal conductivity. The obtained N concentration was defined as the average N concentration (atomic %) in the region from the surface to a depth of 0.10 mm of the steel part. In Table 3, "Average N concentration at a depth of 0.10 mm from the surface" refers to the average N concentration in the region from the surface to a depth of 0.10 mm of the steel part.

[0100] [Measurement of hardened layer hardness] From the test section (φ26mm) of each small roller that had undergone the above-described nitriding quenching 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 a depth of 0.10mm from the surface of the polished surface (position perpendicular to the surface) was measured 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 hardened layer hardness.

[0101] [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.

[0102] (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.

[0103] 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).

[0104] 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.

[0105] [Table 2]

[0106] The number of cycles to be terminated in the test was 2.0 × 10⁻⁶, which is the fatigue limit for typical steel. 7The 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.

[0107] 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.

[0108] 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).

[0109] [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.

[0110] [Table 3]

[0111] Tests 1 through 20 met the requirements of this disclosure and yielded excellent results in terms of surface fatigue strength.

[0112] Tests 21-30 failed to meet any of the requirements of this disclosure and therefore did not achieve the desired surface fatigue strength.

[0113] 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]

[0114] 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]

[0115] 10 Small roller test specimens (evaluation material) 20 Large roller test specimens (counter material)

Claims

1. It comprises a steel core and a hardened layer on the steel core, The steel core portion is, by mass %, C: 0.30-0.60%, 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%, Mo: 0.12 to 1.30%, V: 0.02 to 1.30%, Al: 0.005 to 1.000%, and N: 0.0250% or less, It has a chemical composition containing, with the remainder being Fe and impurities. The microstructure at a depth of 0.10 mm from the surface contains 40% or less of retained austenite. The average N concentration in the region from the surface to a depth of 0.10 mm is 1.80 atomic percent or more. A steel component having a Vickers hardness of 650 HV or higher at a depth of 0.10 mm from the surface.

2. It comprises a steel core and a hardened layer on the steel core, The steel core portion is, by mass %, C: 0.30-0.60%, 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%, Mo: 0.12 to 1.30%, V: 0.02 to 1.30%, Al: 0.005 to 1.000%, and 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] Ti: 0.100% or less, Nb: One or two 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 microstructure at a depth of 0.10 mm from the surface contains less than 40 volume percent of retained austenite. The average N concentration in the region from the surface to a depth of 0.10 mm is 1.80 atomic percent or more. A steel component having a Vickers hardness of 650 HV or higher at a depth of 0.10 mm from the surface.

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+6Al) / Si Formula (1) However, each element symbol in formula (1) 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+6Al) / Si Formula (1) However, each element symbol in formula (1) 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 and quenching treatment temperature is 680 to 1000°C, and the nitriding potential K is calculated using the following formula (2). Ｎ is 0.03 to 1.00, and K Ｎ A method for manufacturing steel parts, comprising a step of performing a nitrogen quenching treatment under the condition that the average of the values ​​is 0.05 to 0.

60. K Ｎ (atm －１／２ ) = (NH ３ Partial pressure (atm) / [(H ２ Partial pressure (atm) ３／２ ] Formula (2) In formula (2), NH ３ partial pressure (atm) and H ２ partial pressure (atm) are partial pressures of the processing atmosphere gas.

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