Steel bar

Abstract

To provide a steel bar that exhibits superior surface fatigue strength.SOLUTION: A steel bar having a chemical composition comprising, in mass%: 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: 0.02-0.35%, V: 0.02-0.35%, Al: 0.005-1.000%, and N: 0.0250% or less, wherein the contents of Si, Mn, Cr, V, and Al satisfy the following formula (1) (where each element symbol in the formula indicates the content of the corresponding element in mass%), the remainder being Fe and impurities; the metallographic structure contains 30% or more in area fraction of bainite and martensite in total; and the Vickers hardness is from 180 to 300 HV. Formula (1): 24≤(Mn+3Cr+2 V+6Al) / Si.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel bars. [Background technology]

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

[0003] In recent years, the automotive industry has been promoting the use of electric power sources, replacing internal combustion engines, in order to reduce CO2 emissions. As a result, the characteristics required for steel bars, such as gears, especially power transmission components, may change dramatically in the future. For example, electric vehicles (EVs) often incorporate reducers directly below the motor that powers them to ensure torque. Miniaturizing these units leads to lighter vehicles and greater design flexibility, making this an area where demand is expected to grow further. However, since EVs are quieter due to the lack of noise and vibrations caused by internal combustion engines, gear noise from reducers becomes more pronounced. Therefore, in addition to the increased strength required for miniaturization, reducer gears will require larger grinding allowances than ever before to achieve the desired shape and reduce noise and vibration, raising concerns about increased grinding costs.

[0004] Carburizing and quenching is a method of heating steel bars to the austenite region (approximately 900-950°C) in the Fe-C binary system, increasing the C (carbon) concentration in the surface layer of the part, and then rapidly cooling it to form hard martensite on the surface of the part, resulting in high surface fatigue strength. However, carburizing and quenching is a heat treatment that hardens the part to its core, and as this tends to result in significant distortion after treatment (heat treatment distortion), subsequent grinding costs increase. As mentioned above, there are concerns about increased grinding costs, particularly for reducer gears for EVs, so there is a strong need to switch to a heat treatment method that contributes to reducing grinding costs.

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

[0006] Nitriding is a surface hardening heat treatment in which steel bars are heated to the ferrite region of the Fe-N binary system (approximately 400-600°C) to allow nitrogen to penetrate the steel surface, while nitrocarburizing is a surface hardening heat treatment in which nitrogen and carbon penetrate the steel surface. Media used for nitriding and nitrocarburizing include gas, salt bath, and plasma. Gas nitriding and gas nitrocarburizing, which are excellent in productivity, are mainly used for automotive parts.

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

[0008] The diffusion layer is a layer hardened by the solid solution strengthening mechanism of intercalated nitrogen and carbon, and by the particle dispersion strengthening mechanism of nitrides. Increasing the hardness and depth of the diffusion layer improves the surface fatigue strength of the part. However, because nitriding and nitrocarburizing 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 increased above 600°C to shorten the treatment time, the hardened layer and the deeper unhardened layer (hereinafter sometimes abbreviated as the "core") will soften, reducing their hardness and making it impossible to achieve sufficient surface fatigue strength. In recent years, attention has begun to be paid to the application of nitriding and nitrocarburizing quenching, a heat treatment that has the potential to overcome the drawbacks of nitriding and nitrocarburizing. This process takes advantage of the fact that the A1 and A3 points in the Fe-N binary system are lower than those in the Fe-C binary system, resulting in less heat treatment distortion than carburizing and quenching, and creating a deeper hardened layer than nitriding or nitrocarburizing. Nitriding and quenching is a method in which steel bars are heated to the austenite region of the Fe-N binary system (around 700-1000°C) to increase the N concentration in the surface layer of the part, and then rapidly cooled, thereby generating hard martensite on the surface layer of the part.

[0009] In recent years, a method of increasing hardness by adjusting nitride-forming elements has been proposed.

[0010] Patent Document 1 discloses a steel for soft nitriding in which the amounts of Si, Cr, Mo, and V are adjusted to achieve both an effective hardened layer depth and high surface hardness.

[0011] Patent Document 2 discloses a steel material having a compound layer, a retained austenite layer, a martensite layer, and a steel base material in this order, and the ratio of the thickness of the retained austenite layer to the thickness of the nitrogen compound layer in a cross-sectional view is 2.5:7.5 to 5.3:4.7. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Publication No. 06-264178 [Patent Document 2] Patent Publication No. 2021-085055 Summary of the Invention [Problem to be solved by the invention]

[0013] In the nitrocarburizing steel disclosed in Patent Document 1, the hardness of the surface layer can be increased due to the high Cr content, but if the content of other nitride-forming elements is high, the N diffusion depth tends to be shallow, and sufficient surface fatigue strength may not be obtained.

[0014] The steel material disclosed in Patent Document 2 specifies a structure formed by nitriding at high temperatures, but it is based on JIS standard steel, and there is a possibility that sufficient surface fatigue strength may not be obtained.

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

[0016] The gist of the means for solving the above problems is as follows. <1> In mass%, C: 0.30~0.50%, Si: 0.05 to 0.25%, Mn: 1.50-2.60%, P: 0.030% or less, S: 0.100% or less, Cr: 0.50~0.95%, Mo: 0.02 to 0.35%, V: 0.02~0.35%, Al: 0.005 to 1.000%, N: 0.0250% or less, wherein Si, Mn, Cr, V, and Al satisfy the following formula (1), and the balance is Fe and impurities, The metal structure has an area ratio of bainite and martensite totaling 30% or more, Steel bar with a Vickers hardness of 180 to 300 HV. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %. <2> In mass%, C: 0.30~0.50%, Si: 0.05 to 0.25%, Mn: 1.50-2.60%, P: 0.030% or less, S: 0.100% or less, Cr: 0.50~0.95%, Mo: 0.02 to 0.35%, V: 0.02~0.35%, Al: 0.005 to 1.000%, N: 0.0250% or less, wherein Si, Mn, Cr, V, and Al satisfy the following formula (1), and further contain one or more elements selected from the group consisting of the following Group A to Group E, with the balance being Fe and impurities; The metal structure has an area ratio of bainite and martensite totaling 30% or more, Steel bar with a Vickers hardness of 180 to 300 HV. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %. [Group A] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group B] Ti: 0.100% or less, and Nb: 0.050% or less, one or two selected from the group consisting of [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, and Sb: 0.100% or less, one or more selected from the group consisting of [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.020% or less, one or more selected from the group consisting of <3> The chemical composition includes the group A <2> The steel bar described in <4> The chemical composition includes the B group <2> or <3> The steel bar described in <5> The chemical composition includes the C group <2> ~ <4> 2. The steel bar according to claim 1, <6> The chemical composition includes the D group <2> ~ <5> Any one of the steel bars described in <7> The treatment temperature is 680 to 1000°C, and the nitriding potential K is calculated by the following formula (2) N is a steel bar that is nitriding quenched at a temperature of 0.05 to 0.60. <1> ~ <6> 2. The steel bar according to claim 1, K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ] Formula (2) [Effects of the Invention]

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

[0018] [Figure 1] FIG. 1 is a diagram showing an example of a small roller test piece for a roller pitching test. [Figure 2] FIG. 1 is a diagram showing an example of a large roller test piece for a roller pitching test. [Figure 3] FIG. 1 is a schematic diagram of a roller pitching test. DETAILED DESCRIPTION OF THE INVENTION

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

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

[0021] Although the application and manufacturing method of the steel bar according to the present disclosure are not particularly limited, a nitriding-quenched steel bar for use in parts is suitable. Hereinafter, a nitriding-quenched steel bar for use in parts (hereinafter, sometimes simply referred to as "steel bar") will be described as a representative example (one embodiment) of the steel bar according to the present disclosure.

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

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

[0024] (a) When steel is nitrided at higher temperatures than usual, such as 680°C or higher, the hardness of the hardened layer and core decreases. Therefore, it is necessary to ensure that the hardness of the steel bar before nitriding is within a certain range. Adding Mn, Cr, V, and Al is effective in improving the hardness of the hardened layer, and adding C, Si, Mn, Cr, and V is effective in improving the hardness of the core. (b) To increase the age hardenability of the hardened layer and core before and after nitriding and quenching, it is necessary to increase the fraction of bainite or martensite in the base material before nitriding and quenching. Increasing the Mn content is effective in increasing the fraction of bainite or martensite.

[0025] (c) The surface hardness after nitriding and quenching increases with increasing Mn, Cr, V, and Al contents. V is particularly effective in improving the age hardening ability of the core.

[0026] (d) The surface hardness after nitriding and quenching decreases with increasing Si content.

[0027] (e) When nitriding is performed at a higher temperature than usual, the reduction in hardness of the hardened layer and core becomes large. Therefore, it is necessary to ensure that the hardness of the steel bar before nitriding and quenching is within a certain range. (f) In order to obtain a high-N martensite structure by rapid cooling from high temperatures, it is essential to add elements that improve hardenability, such as Mn and Mo.

[0028] The steel bar according to the present disclosure has been completed based on the above findings.

[0029] First, the chemical composition of the steel bar according to the present disclosure will be described.

[0030] [Chemical composition] The chemical composition (component elements) of the steel bar according to the present disclosure will be described. The "%" for the content of each component element means "mass %" unless otherwise specified. The chemical composition of the steel bar according to the present disclosure is the same as the chemical composition of the steel material (base material) before nitriding and quenching treatment.

[0031] [C: 0.30~0.50%] C is an element necessary to ensure the core hardness of steel bars and parts. To achieve these effects, 0.30% or more of C is required. On the other hand, if the C content exceeds 0.50%, the strength after hot forging becomes too high, significantly reducing machinability. The preferred range of C content is 0.33 to 0.47%, and more preferably 0.35 to 0.45%.

[0032] [Si: 0.05 to 0.25%] Silicon forms fine nitrides (Si3N4) in compound layers and diffusion layers during nitriding and quenching, which increases hardness and is therefore effective in improving surface fatigue strength. Silicon is also an element that increases core hardness through solid solution strengthening. To achieve these effects, a Si content of 0.05% or more is required. On the other hand, if the Si content exceeds 0.25%, the amount of N penetrating during nitriding and quenching decreases, which can prevent a sufficient effective layer depth from being obtained. Furthermore, the strength of steel bars, wire rods, and after hot forging becomes too high, significantly reducing machinability. The preferred range of Si content is 0.10 to 0.20%.

[0033] [Mn: 1.50~2.60%] Mn forms fine nitrides (Mn3N2) in the compound layer and diffusion layer during nitriding and quenching, increasing hardness. It also increases core hardness through solid solution strengthening. Additionally, Mn is an element that enhances hardenability. By increasing the bainite and martensite fraction in the matrix prior to nitriding and quenching, Mn enhances the age-hardenability of the hardened layer and core during nitriding and quenching, effectively improving contact fatigue strength. To achieve these effects, a Mn content of 1.50% or more is required. On the other hand, if the Mn content exceeds 2.60%, not only does the effect of enhancing contact fatigue strength saturate, but the hardness of the steel bar, wire rod, and hot forged material becomes too high, significantly reducing machinability. The preferred Mn content range is 1.70 to 2.40%.

[0034] [P:0.030% or less] P is an impurity that segregates at grain boundaries and embrittles parts, so a low content is preferable. If the P content exceeds 0.030%, contact fatigue strength may decrease. To prevent a decrease in contact fatigue strength, the upper limit of the P content is preferably 0.020%. The P content may be 0, but it is difficult to achieve a completely 0% content, so a P content of 0.001% or more may be used.

[0035] [S:0.100% or less] S is an element that combines with Mn to form MnS, improving machinability. However, a high S content tends to produce coarse MnS, significantly reducing surface fatigue strength. Therefore, the S content is set to 0.100% or less. The preferred upper limit of the S content is 0.080%, 0.060%, or 0.040%. Although the S content may be 0%, excessive reduction of S leads to increased costs for desulfurization. Therefore, taking into account the economic efficiency of refining, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[0036] [Cr: 0.50~0.95%] Cr is an effective element for improving surface fatigue strength because it forms fine nitrides (CrN) in the compound layer and diffusion layer during nitriding and quenching, increasing hardness. To achieve these effects, 0.50% or more Cr is required. On the other hand, if the Cr content exceeds 0.95%, the depth of the hardened layer may become shallow, and the hardness of the raw material, such as steel bar and wire rod, and after hot forging, becomes too high, significantly reducing machinability. The preferred range of Cr content is 0.60 to 0.85%.

[0037] [Mo: 0.02-0.35%] Mo forms fine nitrides (MoN) in the compound layer and diffusion layer during nitriding and quenching, increasing hardness. It also increases core hardness through solid solution strengthening. Additionally, Mo is an element that improves hardenability. By increasing the bainite and martensite structure fraction in the matrix prior to nitriding and quenching, it enhances the age hardening ability of the hardened layer and core during nitriding and quenching, effectively improving surface fatigue strength. To achieve these effects, Mo must contain 0.02% or more. On the other hand, if the Mo content exceeds 0.35%, the hardness of the steel bar, wire rod, and hot forged steel becomes too high, significantly reducing machinability. The preferred Mo content range is 0.05 to 0.30%.

[0038] [V:0.02~0.35%] V is an element that is effective in improving contact fatigue strength because it forms fine nitrides (VN) in compound layers and diffusion layers during nitriding and quenching, increasing hardness. To achieve these effects, a V content of 0.02% or more is required. On the other hand, if the V content exceeds 0.35%, not only does the effect of improving contact fatigue strength saturate, but the hardness of the raw material, such as steel bars and wire rods, and after hot forging, becomes too high, significantly reducing machinability. The preferred V content range is 0.05 to 0.30%.

[0039] [Al: 0.005 to 1.000%] Al is an effective element for improving surface fatigue strength because it forms fine nitrides (AlN) in compound and diffusion layers during nitriding and quenching, increasing hardness. Additionally, AlN has a pinning effect on austenite grains, suppressing their coarsening during nitriding and quenching, thereby reducing the variability of mechanical properties of parts. To achieve this effect, an Al content of 0.005% or more is required. On the other hand, an Al content exceeding 1.000% can result in a shallow hardened layer and excessive hardness after hot forging, significantly reducing machinability. The preferred Al content range is 0.010 to 0.900%.

[0040] [N:0.0250% or less] Nitrogen (N) combines with Si, Mn, Cr, V, and Al to form Si3N4, Mn3N2, CrN, VN, and AlN, respectively. AlN, in particular, has the effect of refining the structure of the base material before nitriding and quenching by pinning austenite grains, and reducing the variation in mechanical properties of nitriding and quenching-treated parts. To achieve this effect, the N content may be 0.0010% or more, 0.0015% or more, 0.0020% or more, 0.0025% or more, 0.0030% or more, 0.0035% or more, or 0.0040% or more. On the other hand, if the N content is high, coarse AlN is likely to be formed, which may significantly reduce the contact fatigue strength. Furthermore, if the N content is excessively high, the desired contact fatigue strength may not be obtained even if other requirements are met, so the N content is set to 0.0250% or less. Preferably, the N content may be set to 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0041] [Regarding formula (1)] The chemical composition of the steel bar according to the present disclosure further satisfies formula (1), provided that the content of each element is within the range of the present disclosure described above. 24≦(Mn+3Cr+2V+6Al) / Si (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1). Equation (1) is an index showing the penetration depth of N in the surface layer during nitriding and quenching treatment in the manufacturing process of producing nitriding and quenching parts using steel bars as raw materials. In the chemical composition of the steel bar according to the present disclosure, Mn, Cr, V, and Al all have high affinity with N, making it easy to form alloy nitrides in the surface layer 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 in the surface layer of the steel bar during nitriding and quenching. Therefore, in the chemical composition of the steel bar according to the present disclosure, the amount of alloy nitrides in the surface layer of the steel bar during nitriding and quenching can be stabilized by appropriately adjusting the contents of Mn, Cr, V, Al, and Si, provided that the contents of each element are within the ranges of the present disclosure.

[0042] In the chemical composition of the steel bar according to the present disclosure, provided that the content of each element is within the range of the present disclosure, if the value of (Mn+3Cr+2V+6Al) / Si in formula (1) (hereinafter abbreviated as "the value of formula (1)") is less than 24, the Si content is excessively high relative to the contents of Mn, Cr, V, and Al. In this case, alloy nitrides are unlikely to form in the surface layer of the steel bar during nitriding and quenching. In this case, the hardness and depth of the hardened layer after nitriding and quenching are reduced, and sufficient surface fatigue strength may not be obtained. When the value of formula (1) is 24 or more, the Si content relative to the Mn, Cr, V, and Al contents is within an appropriate range. Therefore, an appropriate amount of N penetrates into the steel bar during the nitriding and quenching treatment. As a result, nitriding and quenching parts manufactured using the steel bar as a raw 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. The value of formula (1) is obtained by rounding the calculated value to one decimal place. Although there is no particular upper limit to the value of formula (1), if the Si content is excessively low relative to the contents of Mn, Cr, and V, the N concentration on the steel bar surface becomes excessively high during nitriding, which reduces the N penetration depth and may prevent sufficient surface fatigue strength from being obtained. For this reason, it is preferable that the value of formula (1) be 200 or less.

[0043] In the chemical composition of the steel bar according to the present disclosure, the balance other than the above elements essentially consists of Fe and impurities. Impurities are components contained in raw materials or components mixed in during the manufacturing process, and include elements that are not intentionally added, and are permissible as long as they do not impair the properties of the steel bar according to the present disclosure.

[0044] The steel bar according to the present disclosure may further contain the following optional elements. In other words, the steel bar according to the present disclosure may contain the following elements in place of a portion of Fe. However, the parts according to the present disclosure can solve the problems without containing the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[0045] [Cu: 0-0.50%] Cu is an element that can ensure hardenability and increase core hardness through solid solution strengthening. To ensure this effect, the Cu content can be set to 0.01% or more. On the other hand, if the Cu content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, the Cu content should be set to 0.50% or less. When Cu is contained, the Cu content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Cu content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0046] [Ni: 0-0.50%] Ni is an element that ensures hardenability and increases core hardness through solid solution strengthening. To ensure this effect, the Ni content may be set to 0.01% or more. On the other hand, if the Ni content is too high, the base material will become too hard after hot working, significantly reducing its machinability, so the Ni content should be set to 0.50% or less. When Ni is contained, the Ni content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Ni content is preferably 0.40% or less, 0.35% or less, 0.30% or less, or 0.25% or less.

[0047] [W:0~0.50%] W has the effect of increasing hardenability and improving the strength of steel. To ensure the effects of W, the W content should be 0.01% or more. On the other hand, if the W content is too high, the base material will become too hard after hot working, significantly reducing the machinability of the base material, so the W content should be 0.50% or less. When W is contained, the W content is preferably 0.05% or more, 0.10% or more, or 0.15% or more.Similarly, the W content is preferably 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0048] [Co: 0-0.100%] Co is an element that increases core hardness through solid solution strengthening. To ensure this effect, the Co content should be 0.001% or more. On the other hand, if the Co content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, the Co content should be 0.100% or less. When Co is contained, the Co content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Co content may be preferably 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0049] [B: 0~0.0100%] In addition to improving hardenability, solute B suppresses the grain boundary segregation of P and improves toughness. Furthermore, BN, which combines with N to precipitate, improves machinability. To ensure these effects, the B content should be 0.0005% (5 ppm) or more. On the other hand, a high B content promotes the segregation of large amounts of BN, which can lead to cracking of the steel, so the B content should be 0.0100% or less. When B is contained, the B content is preferably 0.0010% or more, and similarly, the B content is preferably 0.0050% or less.

[0050] [Ti: 0~0.100%] Ti combines with N to form TiN, which has a pinning effect that suppresses coarsening of austenite grains during nitriding and quenching, thereby suppressing variations in the mechanical properties of parts. To achieve this effect, the Ti content is preferably 0.005% or more. On the other hand, if the Ti content exceeds 0.100%, the effect of improving the core hardness and surface hardness saturates, and the alloy cost increases. The preferred range of Ti content is 0.080% or less.

[0051] [Nb: 0~0.050%] Nb combines with C and N to form NbC and NbN, and its pinning action suppresses the coarsening of austenite grains during nitriding and quenching, thereby suppressing variations in the mechanical properties of parts. To achieve this effect, the Nb content is preferably 0.010% or more. On the other hand, if the Nb content exceeds 0.050%, coarse NbC and NbN are formed, making it difficult to achieve the above effect. The preferred range of N content is 0.040% or less.

[0052] [Bi: 0-0.100%] Bi has the effect of reducing cutting resistance and extending the tool life. To ensure this effect, the Bi content should be 0.001% or more. On the other hand, a high Bi content makes the material more susceptible to cracks and scratches during hot working, so the Bi content should be 0.100% or less. When Bi is contained, the Bi content is preferably 0.005% or more, 0.010% or more, or 0.015% or more. Similarly, the Bi content may be preferably 0.090% or less, 0.080% or less, 0.070% or less, or 0.060% or less.

[0053] [Te: 0~0.100%] Te reduces cutting resistance and extends the tool life. To ensure the effects of Te, the Te content should be 0.010% or more. However, if the Te content is too high, the effect saturates and the cost becomes less efficient, so the Te content should be 0.100% or less. When Te is contained, the Te content is preferably 0.020% or more, or 0.030% or more, or 0.040% or more, and similarly, the Te content is preferably 0.090% or less, 0.080% or less, or 0.070% or less.

[0054] [Pb: 0~0.09%] Pb reduces cutting resistance and extends the life of tools, but if the Pb content is increased, the effect saturates, reducing economic viability and also increasing environmental impact, so the Pb content should be kept below 0.09%. When Pb is contained, the Pb content is preferably 0.08% or less, 0.07% or less, 0.06% or less, or 0.05% or less in terms of suppressing environmental load.

[0055] [Sn: 0~0.100%] Sn reduces cutting resistance and extends the tool life. To ensure this effect, the Sn content should be 0.001% or more. However, if the Sn content is too high, the effect saturates and the cost becomes less efficient, so the Sn content should be 0.100% or less. When Sn is contained, the Sn content is preferably 0.005% or more, or 0.010% or more, and similarly, the Sn content is preferably 0.090% or less, or 0.080% or less.

[0056] [Sb: 0~0.100%] Sb reduces cutting resistance and extends the tool life. To ensure this effect, the Sb content should be 0.0010% or more. However, if the Sb content is too high, the effect saturates and the cost becomes uneconomical, so the Sb content should be 0.100% or less. When Sb is contained, the Sb content may be preferably 0.0050% or more, or 0.0060% or more, and similarly, the Sb content may be preferably 0.090% or less, or 0.080% or less.

[0057] [Ca: 0~0.0100%] Calcium refines MnS and improves contact fatigue strength. To ensure this effect, the calcium content should be 0.0010% or more. However, if the calcium content is too high, the effect saturates and the cost becomes uneconomical, so the calcium content should be 0.0100% or less. When Ca is contained, the Ca content is preferably 0.0020% or more, or 0.0030% or more, or 0.0040% or more.Similarly, the Ca content is preferably 0.0090% or less, or 0.0080% or less.

[0058] [Mg: 0~0.0100%] Mg has the effect of refining MnS and improving contact fatigue strength. To ensure the effect of Mg, the Mg content should be 0.0010% or more. On the other hand, if the Mg content is too high, the effect saturates and the cost becomes less efficient, so the Mg content should be 0.0100% or less. When Mg is contained, the Mg content is preferably 0.0020% or more, or 0.0030% or more, or 0.0040% or more, and similarly, the Mg content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0059] [REM:0~0.020%] REM (rare earth elements) refers to a total of 17 elements consisting of Sc, Y, and lanthanides. In this disclosure, "REM" refers to one or more elements selected from these rare earth elements, and the REM content refers to the total content of these 17 elements. When lanthanides are used as REM, industrially, the REM is added in the form of misch metal.

[0060] REM works to refine MnS and improve contact fatigue strength. To ensure the effect of REM, the REM content should be 0.001% or more. On the other hand, if the REM content is too high, the effect saturates and economic efficiency is compromised, so the REM content should be 0.020% or less. When REM is contained, the REM content is preferably 0.002% or more, 0.003% or more, or 0.004% or more, and similarly, the REM content is preferably 0.018% or less, 0.016% or less, or 0.014% or less.

[0061] The above optional elements can be divided into the following groups A to D from the viewpoint of their functions and effects. The steel bar according to the present disclosure may contain one or more elements selected from these groups. [Group A] Improved 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 B] Suppression of coarsening of austenite grains Ti:0.100% or less Nb: 0.050% or less [Group C] Tool life extension Bi:0.100% or less Te: 0.100% or less Pb: 0.09% or less Sn: 0.100% or less Sb: 0.100% or less [Group D] Improved surface fatigue strength due to refined MnS Ca:0.0100% or less Mg: 0.0100% or less REM: 0.020% or less

[0062] Next, the configuration of the steel bar according to the present disclosure other than the chemical composition will be described.

[0063] [Metal structure: total area ratio of bainite and martensite is 30% or more] The metal structure affects the hardness of the hardened layer and core after nitriding and quenching. If the total area ratio of bainite and martensite is 30% or more, nitride-forming elements such as Mn, Cr, and V are primarily present in the hardened layer, while carbide-forming elements such as V are present in the core, and these elements are solid-dissolved in the matrix before nitriding and quenching. These elements then precipitate as fine nitrides and carbides during nitriding and quenching, increasing hardness. If the total area ratio of bainite and martensite is less than 30%, even if the material has sufficient hardness before nitriding and quenching, the hardness of the hardened layer and core after nitriding and quenching cannot be ensured, and sufficient surface fatigue strength may not be achieved. The preferred range for the total area ratio of bainite and martensite is 35% or more, and more preferably 40% or more. The area ratio of the metallographic structure is measured using the following method. A cross section perpendicular to the longitudinal direction of the steel bar is mirror-polished and etched with a 3% nital solution for 20 to 30 seconds. The center of the cross section is then observed under an optical microscope (200x magnification), and optical microscope images of five randomly selected fields are obtained. The metallographic structure is separated from each optical microscope image, and the area ratio of each metallographic structure is determined using the point counting method. The average of the area ratios obtained from five optical microscope images for each metal structure is calculated and used as the area ratio of the metal structure of the steel for nitriding and quenching.

[0064] [Vickers hardness: 180~300HV] In this disclosure, Vickers hardness refers to the hardness of a steel bar before surface hardening, such as by nitriding and quenching. Specifically, the base material is cut perpendicular to the processing direction (rolling or forging direction) after hot working or after heat treatment after hot working, and the resulting cross section is mirror-polished. The Vickers hardness is measured at 10 arbitrary points excluding a region up to 1 mm from the surface using a test force of 1.96 N. The Vickers hardness 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 Vickers hardness in this disclosure. This hardness before nitriding and quenching affects the hardness of the hardened layer and core after nitriding and quenching. In other words, it affects the surface fatigue strength. If the Vickers hardness is less than 180 HV, sufficient hardness cannot be obtained, and the desired surface fatigue strength cannot be achieved. On the other hand, if the Vickers hardness exceeds 300 HV, the machinability of the material significantly decreases. For this reason, the Vickers hardness is set to 180 to 300 HV. The Vickers hardness is preferably 190 to 290 HV, and more preferably 200 to 280 HV.

[0065] [Holding temperature during nitriding: 680-1000℃] The steel bar according to the present disclosure is a steel bar whose composition is designed to accommodate quenching from high temperatures in the austenite range. If the holding temperature for the nitriding and quenching treatment is less than 680°C, the austenite fraction will be insufficient, and sufficient hardness of the hardened layer may not be obtained. On the other hand, if the holding temperature exceeds 1000°C, it will be difficult to increase the NH3 partial pressure, making it difficult to increase the N concentration in the surface layer, and the hardened layer after quenching may not achieve sufficient hardness. Therefore, the nitriding and quenching temperature in the present disclosure is preferably 680 to 1000°C. The preferred range of the nitriding and quenching temperature is 700 to 980°C, and more preferably 720 to 960°C.

[0066] [Manufacturing method for steel bars for nitriding and quenching] An example of a method for manufacturing a steel bar according to the present disclosure will be described below. The method for manufacturing a steel bar described below is an example for manufacturing a steel bar according to the present disclosure. Therefore, a steel bar having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel bar according to the present disclosure.

[0067] An example of a method for manufacturing a steel bar according to the present disclosure includes a step of preparing a raw material (raw material preparation step) and a step of manufacturing a steel bar by hot working the raw material (hot working step). Each step will be described below.

[0068] [Material preparation process] In the material preparation process, a material for the steel bar according to the present disclosure is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of the present disclosure and satisfies formula (1). The refining method is not particularly limited, and a well-known method may be used. For example, molten iron produced by a well-known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to well-known secondary refining. In the secondary refining, alloy elements are added to adjust the composition, and molten steel is produced in which the content of each element is within the range of the present disclosure and which has a chemical composition that satisfies formula (1).

[0069] The molten steel produced by the above-described refining method is used to produce a steel bar material by a well-known casting method. For example, the molten steel is used to produce an ingot by an ingot casting method. Alternatively, the molten steel may be used to produce a bloom by a continuous casting method. By the above-described methods, a steel bar material (ingot or bloom) is produced.

[0070] [Hot processing process] In the hot working step, hot working is performed on the material (ingot or bloom) prepared in the material preparation step to produce a steel bar (e.g., steel bar) according to the present disclosure. The hot working method may be hot forging or hot rolling. In the following explanation, a case where the hot working is hot rolling will be described. In this case, the hot working step includes, for example, a blooming step and a finish rolling step.

[0071] (Bulking rolling process) In the blooming process, a material is hot rolled to produce billets. Specifically, in the blooming process, a billet is produced by hot rolling the material using a blooming mill. If a continuous rolling mill is located downstream of the blooming mill, the billet after blooming may be further hot rolled using the continuous rolling mill to produce a smaller billet. The heating temperature in the blooming process may be within a known range. The heating temperature is, for example, 1000 to 1300°C.

[0072] (Finishing rolling process) In the finish rolling process, the billet produced in the blooming process is hot rolled using a continuous rolling mill to produce a steel bar. The heating temperature in the finish rolling process can be a known temperature. The heating temperature is, for example, 900 to 1250°C. After hot rolling, the steel material is cooled to room temperature. The cooling method is not particularly limited, but for example, it is natural cooling.

[0073] The steel bar according to the present disclosure is manufactured by the above manufacturing method. Note that the above manufacturing method is one example of a manufacturing method for manufacturing the steel bar according to the present disclosure. Therefore, the steel bar according to the present disclosure may be manufactured by a method other than the above manufacturing method. In other words, the manufacturing method is not limited as long as the content of each element in the chemical composition is within the range of the present disclosure and the steel bar satisfies formula (1).

[0074] In the example of the manufacturing method described above, a hot working step is performed after the material preparation step. However, in the manufacturing method of a steel bar according to the present disclosure, the hot working step does not have to be performed after the material preparation step. In other words, the steel bar according to the present disclosure may be a cast material (ingot, bloom, or billet).

[0075] Furthermore, the steel material after the material preparation process or the steel material after the hot working process may be subjected to normalizing and / or annealing in accordance with JIS B 6911:2010 "Normalizing and annealing of steel."

[0076] When the obtained rolled material is directly machined, the finishing temperature and cooling rate of the hot rolling are adjusted so that the metallographic structure after hot rolling becomes a predetermined structure. Specifically, the finishing temperature of the hot rolling is set to 1000°C or higher, and the material is slowly cooled from 1000°C to 500°C at a cooling rate of 0.1 to 0.5°C / sec. This cooling rate can be easily achieved by, for example, covering the steel bar after hot rolling with a heat-insulating cover when cooling it, and does not significantly affect the production rate. However, the hot rolling conditions described here are only one example for adjusting the microstructure of the steel bar, and these conditions are not necessarily required as long as the area ratio of the metallographic structure is within the range specified in the present disclosure.

[0077] [Manufacturing method for nitriding and quenching treated parts] An example of a method for manufacturing a nitriding-quenched component using a steel bar according to the present disclosure will be described below. The method for manufacturing a nitriding-quenched component described below is an example for manufacturing a nitriding-quenched component using a steel bar according to the present disclosure as a raw material. Therefore, a nitriding-quenched component having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a nitriding-quenched component using a steel bar according to the present disclosure.

[0078] The method for manufacturing a nitriding-quenched component includes a hot working step or a cold working step, a machining step, and a heat treatment step. Either the hot working step or the cold working step is performed.

[0079] [Hot processing process] When a hot working step is performed, the steel bar according to the present disclosure is hot worked to form it into a predetermined shape to produce an intermediate product. The hot working is, for example, hot forging. The heating temperature in the hot working step is, for example, 1000 to 1300°C. After the hot working, the steel bar is allowed to cool naturally (air-cooled).

[0080] [Cold working process] When a cold working step is performed, the steel bar according to the present disclosure is subjected to well-known normalizing or spheroidizing annealing, and then cold working is performed to form it into a predetermined shape to produce an intermediate product. The cold working is, for example, cold forging. The conditions for the cold working are not particularly limited.

[0081] [Machining process] The intermediate product after the hot working process or cold working process may be further machined. In the machining process, the intermediate product is machined to produce an intermediate product of a predetermined shape. By performing machining, it is possible to give the nitriding-quenched part a precise shape that would be difficult to achieve by the hot working process or cold working process alone. Machining is, for example, cutting or drilling. If the part is a gear, it is machined by, for example, broaching.

[0082] [Heat treatment process] The intermediate product is then subjected to a heat treatment. Here, the term "heat treatment" includes a nitriding and quenching treatment process. The nitriding and quenching treatment process will be described below as an example of the heat treatment process.

[0083] [Nitriding and quenching process] A nitriding and quenching treatment is performed on a machined part having a predetermined shape. The nitriding and quenching treatment method for the nitriding and quenching part in this disclosure is not particularly limited, and well-known gas nitriding and quenching methods can be applied. The gas used in the nitriding and quenching treatment may be NH3 alone, or a mixed gas containing NH3, N2, and H2. These gases may also contain a carburizing gas. Furthermore, chemical treatments such as coating removal and oxidation treatments may be performed as pre-treatments or post-treatments of the nitriding and quenching treatment, as long as the temperature does not exceed the nitriding and quenching treatment temperature. However, in nitriding and quenching treatments that contain carburizing gases, the compound layer formed on the surface of the part after nitriding and quenching tends to be a brittle single ε phase, so it is preferable to perform nitriding and quenching in an atmosphere that does not contain carburizing gases. The coolant used during nitriding and quenching is preferably a well-known quenching oil or water, which has sufficient cooling capacity, thereby making it possible to produce nitriding and quenching parts with high hardness and excellent surface fatigue strength. The conditions for the nitriding and quenching treatment will be described below as an example.

[0084] (Nitriding and quenching treatment time: 0.5 to 10 hours) The nitriding and quenching treatment is carried out in an atmosphere containing NH3, H2, and N2. The total time for the nitriding and quenching treatment, i.e., the time from the start to the end of the nitriding and quenching treatment (treatment time), is correlated with the formation and decomposition of the compound layer and the diffusion and penetration of nitrogen, and affects the hardness of the diffusion layer. It also affects the hardness of the core. If the treatment time is too short, a compound layer of sufficient thickness may not be obtained. On the other hand, if the treatment time is too long, the hardness of the diffusion layer and core will decrease, and the surface fatigue strength will decrease. If the treatment time is too long, the manufacturing cost will further increase. Therefore, the total treatment time for the nitriding and quenching treatment is 0.5 to 10 hours, and preferably 1.0 to 8 hours.

[0085] The atmosphere for the nitriding and quenching treatment in the present disclosure contains NH3, H2, and N2, as well as unavoidably impurities such as oxygen and carbon dioxide. A preferred atmosphere is one in which the total of NH3, H2, and N2 is 99.5% (volume %) or more.

[0086] (Gas conditions for nitriding and quenching treatment) In the nitriding and quenching method of the present disclosure, it is preferable to control the nitriding potential, which increases the area ratio of the γ' phase in the compound layer and allows for the production of nitriding and quenching-treated components with high contact fatigue strength.

[0087] Nitriding potential K of nitriding and quenching treatment N is defined by the following formula:

[0088] K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ]

[0089] The partial pressures of NH3 and H2 in the atmosphere during nitriding and quenching treatment can be controlled by adjusting the flow rates of the gases.

[0090] By setting the average nitriding potential during nitriding and quenching to 0.05 to 0.60, the formation of a void layer on the part surface can be suppressed, resulting in a nitriding and quenching-treated part with high surface fatigue strength. If the average value exceeds 0.60, a thick compound layer containing many voids will be formed, while if it is lower than 0.05, the N concentration in the surface layer will be low, and sufficient surface hardness may not be obtained. Note that as long as the average value is within the above range, the target nitriding potential may be changed significantly during the treatment, and may be changed multiple times. The preferred range for the average nitriding potential during nitriding and quenching is 0.10 to 0.50.

[0091] By subjecting a steel bar having the chemical composition according to the present disclosure to nitriding and quenching treatment while controlling the nitriding and quenching temperature and nitriding potential as described above, it is possible to manufacture nitriding and quenching treated components using the steel bar according to the present disclosure. [Example]

[0092] The effects of the steel bar of the present disclosure will be explained in more detail below using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel bar of the present disclosure. Therefore, the steel bar of the present disclosure is not limited to this one example of conditions.

[0093] [Steel bar manufacturing] Molten steel having the chemical composition shown in Table 1 was produced.

[0094] [Table 1]

[0095] Ingots were produced by ingot casting using molten steel adjusted to the chemical composition shown in Table 1. The cross section perpendicular to the longitudinal direction of the ingot was a rectangle measuring 180 mm x 180 mm. The produced ingots were allowed to cool to room temperature. Note that underlines in Table 1 indicate compositions outside the scope of the present disclosure, and blank spaces mean that the content of the corresponding element is 0% in significant figures (numbers down to the least significant digit) as specified in the embodiment. The remainder of the chemical composition is Fe and impurities.

[0096] The obtained ingots were heated at 1200°C for 2 hours. The heated ingots were subjected to hot working (hot forging) to produce steel materials (steel bars) with a diameter of 40 mm and a length of 1000 mm. The hot-worked steel materials were allowed to cool to room temperature. Some of the steel materials were then subjected to normalizing or isothermal annealing. The normalizing temperature was 925°C, and the holding time at the temperature was 90 minutes. After the holding time had elapsed, the steel materials were allowed to cool. The cooling rate of the steel materials during natural cooling was 0.3 to 0.9°C / second. The isothermal annealing temperature was 925°C, and the holding time at the temperature was 90 minutes. The furnace temperature was then lowered to 650°C, after which the holding time was further increased to 60 minutes. After the holding time had elapsed, the steel materials were allowed to cool. Steel materials (steel bars) of each test number were produced using the above process.

[0097] [Manufacturing nitriding and quenching treated parts] The steel materials produced with the respective test numbers were used to prepare the following nitriding and quenching treatment test pieces for each test number.

[0098] (1) Small roller test piece Figure 1 shows a side view of the small roller test piece produced in this example. The numbers in Figure 1 indicate dimensions (unit: mm). "φ" in Figure 1 refers to diameter. The inverted triangle symbol in Figure 1 refers to the "finish code" indicating the surface roughness listed in Table 1 of JIS B 0601 (1982). The "G" attached to the finish code refers to the abbreviation for the processing method indicating grinding specified in JIS B 0122 (1978). The small roller test piece is a test piece for measuring surface fatigue strength. Multiple small roller test pieces were prepared for each test number.

[0099] Specifically, first, the steel material of each test number was machined to produce a rough test piece having the rough shape of the small roller test piece. The central axis of the rough test piece was coaxial with the central axis of the steel bar. The rough test piece was subjected to nitriding quenching treatment. Table 2 shows the correspondence between each test piece and the nitriding quenching treatment. After nitriding quenching treatment, the test piece was oil-cooled using oil at 80°C. For the nitriding and quenching treatment, the test pieces were placed in a nitriding and quenching furnace, and NH3, H2, and N2 gases were introduced into the furnace.

[0100] The H2 partial pressure in the atmosphere was measured using a thermal conductivity H2 sensor attached directly to the nitriding and quenching furnace body. The difference in thermal conductivity between the standard gas and the measured gas was converted into gas concentration. The H2 partial pressure was measured continuously during the nitriding and quenching process.

[0101] The NH3 partial pressure was measured using an infrared absorption NH3 analyzer installed outside the furnace, and was continuously measured during the nitriding and quenching treatment.

[0102] Nitriding potential K of nitriding and quenching treatment N is defined by the following formula: K N (atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ]

[0103] Nitriding potential K calculated in the device N The NH3 flow rate and N2 flow rate were controlled so that the nitriding potential K converged to the target value. N Record the K measured during the treatment time. N The average value was calculated.

[0104] After nitriding and quenching, the gripping parts were finished to remove any heat treatment distortion, and then each was used as a roller pitting test piece.In the actual roller pitting test using the small roller test piece, the cylindrical part (test surface) with a diameter of 26 mm was brought into contact with the large roller, and a specified surface pressure was applied before rotation.

[0105] Small roller test pieces, which serve as the strength reference for the contact fatigue strength evaluation test described below, were manufactured using the following method. A rough test piece with the rough shape of the small roller test piece was machined from a 40 mm diameter cylindrical material with a chemical composition equivalent to SCr420 specified in JIS G 4805 (2019). The central axis of the rough test piece was coaxial with the central axis of the steel bar. The rough test piece was subjected to the following standard steel heat treatment pattern (well-known gas carburizing and quenching). Specifically, the test piece was held at 930°C for 180 minutes in an atmosphere with a carbon potential CP of 1.0% (carburizing process). Then, the carbon potential CP was set to 0.8% and held at 930°C for 120 minutes (diffusion process). The temperature was then lowered to 870°C, held at 870°C for 30 minutes, and then oil-quenched in 60°C oil (quenching process). The crude test pieces after oil cooling were tempered. The tempering temperature was 180°C, and the holding time at the tempering temperature was 120 minutes. After carburizing and quenching, the gripping parts were finished to remove any heat treatment distortion. Small roller test pieces for strength standards were produced using the above manufacturing process.

[0106] (2) Test piece for hardened layer investigation Two test pieces for each test number were prepared for the hardened layer investigation. The test pieces for the hardened layer investigation were cylindrical test pieces with a diameter of 26 mm and a length of 100 mm.

[0107] Specifically, the steel material of each test number was first machined to prepare two cylindrical rough test pieces with a diameter of 26 mm and a length of 100 mm. The central axis of the rough test pieces was aligned with the central axis of the steel bar. The rough test pieces were then subjected to the same nitriding and quenching treatment as the small rollers.

[0108] Through the above manufacturing process, test pieces for investigating the hardened layer were prepared.

[0109] [Manufacturing large roller test pieces for two-roller rolling fatigue tests] Furthermore, large roller test pieces to be used in a two-cylinder rolling fatigue test for measuring surface fatigue strength were prepared by the following method. A rough specimen for the large roller test piece, with the shape shown in Figure 2, was cut from a cylindrical material with a diameter of 140 mm and a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2008). The numbers in Figure 2 indicate dimensions (unit: mm). The inverted triangle symbol in Figure 2 represents the "finish symbol" indicating the surface roughness listed in Table 1 of JIS B 0601 (1982). The "G" attached to the finish symbol represents the abbreviation for the processing method, indicating grinding as specified in JIS B 0122 (1978).

[0110] The cut-out rough test pieces were quenched. The quenching temperature was 870°C, and the holding time at the quenching temperature was 90 minutes. After the holding time had elapsed, the pieces were quenched in oil at 60°C. The outer peripheral surfaces of the quenched rough test pieces were then finished by cutting. The outer peripheral surfaces were finished so that the arithmetic mean roughness Ra was 0.6 to 0.8 μm and the maximum height Rz was 2.0 to 4.0 μm. Large roller test pieces were produced using the above manufacturing process.

[0111] [Surface fatigue strength evaluation test] The following evaluation tests were carried out using the above-mentioned various test pieces. A two-cylinder rolling fatigue test was carried out using the small roller test piece and the large roller test piece to determine the surface fatigue strength as follows. The test machine used was a roller pitting tester "RP201" manufactured by Komatsu Engineering Co., Ltd.

[0112] As shown in FIG. 3, the small roller test piece 10 was rolled while being in contact with the cylindrical portion with a diameter of 26 mm and the central position of the outer circumferential surface of the large roller test piece 20 (the outer circumferential portion with a diameter of 130 mm).

[0113] The contact pressure was 1600 to 3000 MPa in Hertzian contact pressure. The rotation speed of the small roller test piece 10 was 2000 rpm. The peripheral speed of the small roller test piece 10 was 163 m / min, and the peripheral speed of the large roller test piece 10 was 229 m / min. During the test, lubricating oil was supplied to the contact area between the small roller test piece and the large roller test piece. The lubricating oil was automatic transmission oil, with an oil temperature of 100°C and an oil flow rate of 1.0 L / min. The slip ratio was -40%. Table 2 shows the test conditions.

[0114] [Table 2]

[0115] The number of repeated cycles in the test was 2.0 × 10, which indicates the fatigue limit of general steel. 7 The small roller test piece was tested for 2.0 × 10 7 The maximum surface pressure (MPa) reached at this time was taken as the fatigue limit of the small roller test piece.

[0116] The occurrence of pitting was detected using a vibration meter attached to the testing machine. After vibration was generated, the rotation of both the small roller test piece and the large roller test piece was stopped, and the occurrence of pitting and the number of rotations were confirmed.

[0117] In this example, assuming application to gear components, the fatigue limit of a small roller test piece made of steel (reference steel) that meets the aforementioned SCr420 standard was used as the reference value. If the fatigue limit was 1.05 times or more that of the reference steel, the test piece was judged to have excellent contact fatigue strength (marked "○" in the "Contact fatigue strength judgment" column in Table 3). On the other hand, if the fatigue limit was less than 1.05 times that of the reference steel, the test piece was judged to have low contact fatigue strength (marked "X" in the "Contact fatigue strength judgment" column in Table 3).

[0118] [Test Results] The results are shown in Table 3. The underlined values ​​in Table 3 indicate values ​​outside the scope of this disclosure.

[0119] [Table 3]

[0120] Test numbers 1 to 17 met the requirements of the present disclosure, and excellent results were obtained in terms of contact fatigue strength. The embodiments and examples of the present disclosure have been described above. However, the above-described embodiments and examples are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments and examples, and can be modified and implemented as appropriate within the scope of the present disclosure. [Industrial Applicability]

[0121] The steel bars according to the present disclosure can be used in a wide range of industrial fields as materials for automobiles and industrial machinery, particularly gears for machines powered by electric motors. [Explanation of symbols]

[0122] 10 Small roller test piece (evaluation material) 20 Large roller test piece (mating material)

Claims

1. In mass%, 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: 0.02-0.35%, V: 0.02-0.35%, Al: 0.005-1.000%, N: 0.0250% or less, wherein Si, Mn, Cr, V, and Al satisfy the following formula (1), and the balance is Fe and impurities, The metal structure has an area ratio of bainite and martensite totaling 30% or more, A steel bar with a Vickers hardness of 180 to 300 HV. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %.

2. In mass%, 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: 0.02-0.35%, V: 0.02-0.35%, Al: 0.005-1.000%, N: 0.0250% or less, wherein Si, Mn, Cr, V, and Al satisfy the following formula (1), and further contain one or more elements selected from the group consisting of the following Group A to Group E, with the balance being Fe and impurities: The metal structure has an area ratio of bainite and martensite totaling 30% or more, A steel bar with a Vickers hardness of 180 to 300 HV. 24≦(Mn+3Cr+2V+6Al) / Si Formula (1) However, each element symbol in the above formula (1) represents the content of the element in mass %. [Group A] Cu: 0.50% or less, Ni: 0.50% or less, W: 0.50% or less, Co: 0.100% or less, and B: 0.0100% or less of one or more selected from the group consisting of [Group B] Ti: 0.100% or less, and Nb: 0.050% or less, one or two selected from the group consisting of [Group C] Bi: 0.100% or less, Te: 0.100% or less, Pb: 0.09% or less, Sn: 0.100% or less, and Sb: 0.100% or less, one or more selected from the group consisting of [Group D] Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.020% or less, one or more selected from the group consisting of

3. The steel bar according to claim 2 , wherein the chemical composition includes the Group A alloy.

4. The steel bar according to claim 2 , wherein the chemical composition includes the B group.

5. The steel bar according to claim 2 , wherein the chemical composition includes the C group.

6. The steel bar according to claim 2 , wherein the chemical composition includes the D group.

7. The treatment temperature is 680 to 1000°C, and the nitriding potential K is calculated by the following formula (2): Ｎ The steel bar according to any one of claims 1 to 6, which is a steel bar that is subjected to nitriding and quenching treatment at a temperature of 0.05 to 0.

60. K Ｎ (atm －１／２ ) = (NH ３ Partial pressure (atm)) / [(H ２ Partial pressure (atm) ３／２ ] Formula (2)

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