Nitriding steel and nitrided parts

Abstract

To provide a steel for nitriding and a nitriding treatment component which are excellent in machinability even in hot-working, and secure surface fatigue strength and rotation bending fatigue strength after nitriding treatment.SOLUTION: Mn is effective so as to increase hardness before nitriding in association with increase in C content and aggregate machinability, and enhance a bainite structure fraction of a base material so as to increase core part age hardening capability before and after nitriding, and Cr and V are effective so as to increase hardness of a surface layer part after nitriding, and V is effective so as to improve core part age hardening capability. The nitriding steel contains, by mass%, 0.01-0.11% C, 0.01-0.30% Si, 1.20-2.50% Mn, 0.025% or less P, 0.100% or less S, 0.20-0.90% Cr, 0.05-0.50% V, 0.050% or less Al, 0.0250% or less N, and the balance Fe with impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel for nitriding and to a steel part subjected to a gas nitriding treatment. [Background technology]

[0002] Steel parts used in automobiles and various industrial machines are often subjected to surface hardening heat treatments such as carburizing, induction hardening, and nitriding to improve mechanical properties such as fatigue strength, surface fatigue strength, and seizure resistance.

[0003] Carburizing and quenching is a heat treatment that involves infiltrating and diffusing carbon (C) into the surface layer of steel, heating the steel to the austenite region above the Ac3 point, and then cooling it to harden the surface layer through martensitic transformation, resulting in a deep hardened layer and high surface hardness in a relatively short time. However, because the entire part undergoes a phase transformation, there is a problem of significant dimensional change in the part after heat treatment.

[0004] Induction hardening is a heat treatment that rapidly heats only the surface layer of steel to the austenite region in a short period of time, and results in less quenching distortion than carburizing quenching. However, unlike carburizing quenching, induction hardening is not a heat treatment method that increases the C (carbon) concentration in the surface layer, so it is not suitable for parts that require high surface fatigue strength or bending fatigue strength.

[0005] Nitriding and nitrocarburizing processes involve heating steel in the ferrite region below the A1 point and hardening the surface layer through solid solution strengthening by N (nitrogen) that penetrates and diffuses into the surface layer and nitride particle dispersion strengthening. This reduces heat treatment strain without phase transformation. Furthermore, induction hardening after nitriding or nitrocarburizing creates a deep martensite structure with a high N concentration in the surface layer, thereby increasing the surface fatigue strength and bending fatigue strength of the part while minimizing distortion. For this reason, nitriding and nitrocarburizing are often used for parts with high dimensional accuracy or large components, such as gears used in automobile transmissions and crankshafts used in engines. However, in nitriding and nitrocarburizing, in order to increase the surface hardness, the base steel (hereinafter simply referred to as "base material") must contain nitride-forming elements such as Cr and V, or alloying elements must be added to ensure the hardness of the non-nitrided core steel to increase fatigue strength. Therefore, the base material of nitriding steel is relatively harder than that of carburizing and quenching steel, and poor machinability can sometimes be a problem.

[0006] Therefore, in order to achieve both machinability before nitriding and fatigue strength after nitriding, steels for soft nitriding as shown in Patent Documents 1 to 3, for example, have been proposed. Patent Document 1 discloses a steel for nitrocarburizing in which machinability is ensured by including Mo and V within a certain range, and a method for producing a nitrocarburized part. Patent Document 2 proposes a steel for soft nitriding, which contains Mo, V, and Ti within a certain range and is characterized in that the area ratio of bainite structure exceeds 50%. Patent Document 3 proposes a soft nitriding steel that has excellent cold forgeability and a hardness of 150 HV or less as hot rolled or hot forged, by obtaining a precipitation effect and fatigue strength by precipitating Cu in ferrite, and by reducing the C and N contents in the steel and suppressing pearlite. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-193827 [Patent Document 2] International Publication No. 2016 / 152167 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-69571 Summary of the Invention [Problem to be solved by the invention]

[0008] The soft nitriding steel disclosed in Patent Document 1 requires the addition of Mo and B to ensure hardenability, and there is room for improvement in alloy cost and manufacturability. The steel for soft nitriding disclosed in Patent Document 2 has a hardness of 233 to 363 HV before nitriding treatment, and it is difficult to say that it has excellent machinability. The hardness of the nitrocarburizing steel disclosed in Patent Document 3 is low before nitriding, but the base material must contain 0.5 mass% or more of Cu. In addition, this technology is based on cold forging, and in parts that are machined in the hot forged state, the core hardness after nitriding is low, making it difficult to achieve high fatigue strength.

[0009] The present invention aims to solve the above problems and provide a steel for nitriding and a nitrided part that has excellent machinability in the state after hot working (hot rolling, hot forging, etc.) and also ensures surface fatigue strength and rotating bending fatigue strength after nitriding. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems, the inventors independently changed the various alloy components and investigated the effects of each element on the hardness and structural morphology of the base material before nitriding (before nitriding treatment) and on the hardness of the surface and core of the diffusion layer after nitriding (after nitriding treatment), and obtained the findings (a) to (f).

[0011] (a) The lower the surface hardness after hot working (such as hot rolling or hot forging) (before nitriding), the better the machinability. The higher the hardness of the surface and core of the diffusion layer after nitriding, the higher the surface fatigue strength and bending fatigue strength.

[0012] (b) The hardness before nitriding increases significantly with an increase in the C content. Therefore, from the viewpoint of machinability, it is preferable to reduce the C content of the base material.

[0013] (c) In order to increase the age hardenability of the core before and after nitriding, it is necessary to increase the bainite fraction of the base material before nitriding. Increasing the Mn content is effective in increasing the bainite fraction.

[0014] (d) The hardness of the surface of the diffusion layer after nitriding increases with increasing Cr and V contents. V is particularly effective in improving the age hardening ability of the core.

[0015] (e) The hardness of the surface of the diffusion layer after nitriding decreases with increasing C and Si contents.

[0016] (f) As the C and Mo contents in the base material increase, the volume fraction of the ε phase in the compound layer after nitriding increases, resulting in a decrease in the surface fatigue strength and bending fatigue strength.

[0017] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] In mass%, C: 0.01 to 0.11%, Si: 0.01 to 0.30%, Mn: 1.20~2.50% P: 0.025% or less, S: 0.100% or less, Cr: 0.20~0.90%, V: 0.05~0.50%, Al: 0.050% or less, and N: 0.0250% or less, and the balance being Fe and impurities. [2] The nitriding steel according to [1], characterized in that the Vickers hardness of the surface is 220 HV or less. [3] Furthermore, in mass%, Mo: 0.50% or less Cu: 0.45% or less, Ni: 0.50% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.200% or less, Ti: 0.250% or less, and B: 0.0100% or less, The steel for nitriding according to [1] or [2], characterized in that it contains one or two or more of the following: [4] Furthermore, in mass%, Ca: 0.0100% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.090% or less, Sn: 0.100% or less, Sb: 0.0100% or less, and REM: 0.0100% or less The steel for nitriding according to any one of [1] to [3], characterized in that it contains one or more of the following: [5] In a cross section perpendicular to the main axis direction, the components of the core, which is a similar area that shares a centroid with the cross section and has a similarity ratio of 1 / 4 of the cross section, are, in mass%, C: 0.01 to 0.11%, Si: 0.01 to 0.30%, Mn: 1.20~2.50% P: 0.025% or less, S: 0.100% or less, Cr: 0.20~0.90%, V: 0.05~0.50%, Al: 0.050% or less, and N: 0.0250% or less, the balance being Fe and impurities, In the cross section, the N concentration at a depth of 50 μm from the surface is higher than the N concentration in the core portion, The nitrided component is further characterized in that the Vickers hardness at the 50 μm depth position is 700 HV or more, and the Vickers hardness at the core portion is 220 HV or more. [6] Furthermore, the components of the core are, in mass%, Mo: 0.50% or less Cu: 0.45% or less, Ni: 0.50% or less, W: 0.50% or less, Bi: 0.50% or less, Co: 0.50% or less, Nb: 0.200% or less, Ti: 0.250% or less, and B: 0.0100% or less, The nitrided part according to [5], characterized in that it contains one or more of the following: [7] Furthermore, the components of the core are, in mass%, Ca: 0.0100% or less, Mg: 0.0100% or less, Te: 0.100% or less, Pb: 0.090% or less, Sn: 0.100% or less, Sb: 0.0100% or less, and REM: 0.0100% or less The nitrided part according to [5] or [6], characterized in that it contains one or more of the following: [Effects of the Invention]

[0018] According to the present invention, it is possible to obtain a nitriding steel that has good machinability due to its low hardness after hot working and before nitriding, and furthermore, by nitriding, it is possible to obtain nitrided parts that have both high surface fatigue strength and bending fatigue strength, which makes the steel suitable for use as a material for, for example, internal gears for automobiles, which are difficult to process. [Brief explanation of the drawings]

[0019] [Figure 1]Figure 1 shows an example of a small roller for roller pitching tests, with dimensions in the figure in millimeters. [Figure 2] Fig. 2 is a diagram showing an example of a large roller for roller pitching tests, where the dimensions are in mm. [Figure 3] Fig. 3 is a diagram showing an example of a cylindrical test piece for a rotating bending fatigue test, where the dimensions are in mm. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described below using as examples a nitriding steel and a nitrided component according to one embodiment of the present invention. The nitrided component according to this embodiment (a nitrided component, hereinafter sometimes simply referred to as a "component") has a core (hereinafter sometimes simply referred to as a "core") that is the center of the steel material that will become the component, a compound layer located on the surface of the steel material, and a nitrogen diffusion layer (hereinafter sometimes simply referred to as a "diffusion layer") adjacent to the compound layer. Here, the core refers to the portion that nitrogen did not penetrate during the nitriding process. For convenience, it refers to a region of a similar shape (1 / 16 in area) that shares a centroid with the cross section perpendicular to the main axis (e.g., the longitudinal central axis) of the nitrided part and has a similarity ratio of 1 / 4 of the cross section. That is, in the cross section of the nitrided part, it refers to a region centered at a depth of 1 / 2 of the thickness (plate thickness for plate-shaped parts, diameter for rod-shaped parts) from the surface in the direction (depth direction) perpendicular to the surface (hereinafter sometimes referred to as the "1 / 2 position of the thickness (diameter)") and has a width of 1 / 4 of the thickness (diameter) (a runout width on one side from the center (or center line) of ±1 / 8 of the thickness (diameter)). For example, in the case of a cylindrical part with a diameter D, the core is a circular region with a diameter D / 4 from the centroid (center of the circle) of the cross section perpendicular to the axial direction.

[0021] [Base material composition] The composition of the base material will be explained. Normally, the core of a nitrided part has the same composition as the base material, so unless otherwise specified, the composition of the base material and the core of the part are equivalent. Note that the "%" for the content of each component element in the steel and the concentration of the element on the surface of the part means "mass %" unless otherwise specified.

[0022] [C: 0.01 to 0.11%] Carbon (C) is an important element for ensuring the core hardness of nitrided parts. To achieve this effect, the C content should be 0.01% or more. On the other hand, a high C content reduces the hardening ability of the diffusion layer after nitriding, and also increases the volume ratio of the ε phase in the compound layer, resulting in reduced surface fatigue strength and bending fatigue strength. In addition, the hardness of the base material after hot working becomes too high, significantly reducing the machinability of the base material. Furthermore, the higher the C content, the lower the surface hardness of the nitrided part. For these reasons, a lower C content is preferable, with the upper limit set at 0.11%. More preferably, the C content may be 0.02% or more, 0.03% or more, or 0.04% or more. Similarly, the C content may be 0.10% or less, 0.09% or less, 0.08% or less, 0.07% or less, or 0.06% or less.

[0023] [Si: 0.01 to 0.30%] Silicon has a deoxidizing effect. To achieve this effect, it is recommended that the silicon content be 0.01% or more. On the other hand, if the silicon content is too high, the hardening ability of the diffusion layer decreases, and the base material becomes too hard after hot working, significantly reducing the machinability of the base material. Therefore, the silicon content should be 0.30% or less. More preferably, the Si content may be 0.02% or more, 0.03% or more, 0.04% or more, 0.05% or more, 0.06% or more, or 0.07% or more. Similarly, the Si content may be 0.29% or less, 0.28% or less, 0.27% or less, 0.26% or less, 0.25% or less, 0.24% or less, 0.23% or less, 0.22% or less, 0.21% or less, 0.20% or less, 0.19% or less, 0.18% or less, 0.17% or less, 0.16% or less, or 0.15% or less.

[0024] [Mn: 1.20~2.50%] Mn is an element that increases core hardness through solid solution strengthening. It also improves hardenability and increases the bainite fraction of the base material after hot working. Additionally, Mn forms fine nitrides (Mn3N2) in compound and diffusion layers during nitriding, improving surface fatigue strength and bending fatigue strength. To achieve these effects, the Mn content should be 1.20% or more. On the other hand, if the Mn content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, it is recommended that the Mn content be 2.50% or less. More preferably, the Mn content may be 1.25% or more, 1.30% or more, 1.35% or more, 1.40% or more, 1.45% or more, 1.50% or more, 1.55% or more, 1.60% or more, 1.65% or more, 1.70% or more, or 1.75% or more. Similarly, the Mn content may be 2.45% or less, 2.40% or less, 2.35% or less, 2.30% or less, 2.25% or less, 2.20% or less, 2.15% or less, 2.10% or less, or 2.05% or less.

[0025] [P:0.025% or less] P is an impurity that segregates at grain boundaries, embrittling parts and reducing surface fatigue strength and bending fatigue strength. Therefore, the P content should be 0.025% or less. The lowest P content possible is preferable. The preferred upper limit of the P content is 0.018%, 0.015%, 0.013%, or 0.010%. While the P content may be 0%, this increases the cost of dephosphorization, so considering the economics of refining, the P content may be 0.001% or more, 0.005% or more, or 0.008% or more.

[0026] [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 and bending fatigue strength, so the S content should be set to 0.100%. The preferred upper limit of the S content is 0.080%, 0.060%, or 0.040%. Although the S content may be 0%, this increases the cost of desulfurization, so considering the economics of refining, the S content may be set to 0.001% or more, 0.002% or more, or 0.005% or more.

[0027] [Cr: 0.20~0.90%] Cr is an element that forms fine nitrides (CrN) in compound layers and diffusion layers during nitriding, improving contact fatigue strength and bending fatigue strength. To achieve these effects, the Cr content should be 0.20% or more. On the other hand, if the Cr content is too high, not only will the effect of improving contact fatigue strength and bending fatigue strength saturate, but the base material will become too hard after hot working, significantly reducing its machinability. Therefore, the Si content should be 0.90% or less. More preferably, the Cr content may be 0.25% or more, 0.30% or more, 0.35% or more, 0.40% or more, 0.45% or more, or 0.50% or more. Similarly, the Cr content may be 0.85% or less, 0.80% or less, 0.75% or less, 0.70% or less, 0.65% or less, or 0.60% or less.

[0028] [V:0.05~0.50%] V is an element that, during nitriding, forms fine nitrides (VN) in compound layers and diffusion layers, and carbides (VC) in the core, thereby increasing the surface fatigue strength and bending fatigue strength of parts. To achieve these effects, the V content should be 0.05% or more. However, if the V content is too high, not only does the effect of improving surface fatigue strength and bending fatigue strength saturate, but the base material becomes too hard after hot working, significantly reducing its machinability. Therefore, the V content should be 0.50% or less. More preferably, the V content may be 0.06% or more, 0.08% or more, 0.10% or more, 0.12% or more, 0.15% or more, 0.18% or more, or 0.20% or more. Similarly, the V content may be 0.48% or less, 0.45% or less, 0.43% or less, 0.40% or less, 0.38% or less, or 0.35% or less.

[0029] [Al:0.050% or less] Al is a deoxidizing element, but it is not necessarily contained. However, Al combines with N to form AlN, which has the effect of refining the structure of the base material before nitriding through its austenite grain pinning action, thereby reducing the variation in mechanical properties of nitrided parts. To achieve this effect, the Al content is preferably 0.001% or more, and more preferably 0.002% or more, 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, Al tends to form hard oxide-based inclusions, and a high Al content significantly reduces bending fatigue strength, making it impossible to obtain the desired bending fatigue strength even if other requirements are met. Therefore, the Al content should be set to 0.050% or less. To prevent a decrease in bending fatigue strength, the preferred upper limit of the Al content is 0.040%, 0.030%, or 0.020%.

[0030] [N:0.0250% or less] N (nitrogen) does not necessarily have to be contained. However, N combines with Mn, Cr, Al, and V to form Mn3N2, CrN, AlN, and VN. In particular, Al and V have the effect of refining the structure of the base material before nitriding through the pinning action of austenite grains, and reducing the variation in mechanical properties of nitrided parts. 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 and VN are likely to be formed, which significantly reduces the bending fatigue strength and may make it impossible to obtain the desired surface fatigue strength or bending fatigue strength even if other requirements are met, so the N content should be 0.0250% or less, preferably 0.0200% or less, 0.0150% or less, or 0.0100% or less.

[0031] The remainder of the above elements essentially consists of Fe and impurities. Impurities are components contained in the 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 nitriding steel and nitrided parts according to this embodiment.

[0032] The base material may further contain the following elements in place of part of Fe. However, the part according to this embodiment can solve the problem without including the elements exemplified below. Therefore, the lower limit of the content of the elements exemplified below is 0%.

[0033] [Mo: 0.50% or less] Mo is an element that increases core hardness through solid solution strengthening. It also improves hardenability and increases the bainite fraction of the base material after hot working. Additionally, Mn forms fine nitrides (MoN) in the compound and diffusion layers during nitriding, and carbides (MoC) in the core, thereby improving surface fatigue strength and bending fatigue strength. To achieve these effects, the Mo content can be set to 0.01% or more. However, a high Mo content tends to increase the volume fraction of the ε phase in the compound layer after nitriding, reducing surface fatigue strength and bending fatigue strength. Therefore, the Mo content should be set to 0.50% or less. Furthermore, the base material becomes too hard after hot working, significantly reducing its machinability. When Mo is contained, the Mo content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Mo content is preferably 0.45% or less, 0.40% or less, or 0.30% or less.

[0034] [Cu:0.45% or less] Cu is an element that increases core hardness through solid solution strengthening. To ensure this effect, the Cu content may be set to 0.01% or more. However, 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 0.45% 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.

[0035] [Ni: 0.50% or less] Ni is an element that 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. Therefore, the Ni content should be set to 0.50% or less. When Ni is contained, the Ni content may be preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Ni content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0036] [W:0.50% or less] W improves core and surface hardness through solid solution strengthening and the precipitation of carbides (WC and W2C). To ensure the effects of W, it is best to keep the W content at 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 its machinability, so it is best to keep the W content at 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.

[0037] [Bi:0.50% or less] Bi has the effect of reducing cutting resistance and extending the tool life. To ensure this effect, the Bi content should be 0.01% or more. On the other hand, a high Bi content makes the material more susceptible to cracks and scratches during hot working, so the Bi content should be 0.50% or less. When Bi is contained, the Bi content is preferably 0.05% or more, 0.10% or more, or 0.15% or more, and similarly, the Bi content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0038] [Co:0.50% or less] Co is an element that increases core hardness through solid solution strengthening. To ensure this effect, the Co content should be 0.01% or more. On the other hand, if the Co content is too high, the base material will become too hard after hot working, significantly reducing its machinability. Therefore, the Co content should be 0.50% or less. When Co is contained, the Co content is preferably 0.05% or more, 0.10% or more, or 0.15% or more. Similarly, the Co content may be 0.45% or less, 0.40% or less, 0.35% or less, or 0.30% or less.

[0039] [Nb:0.200% or less] Nb combines with N that penetrates the surface layer of the steel during nitriding and with C in the parent phase to form fine nitrides (NbN) and carbides (NbC), improving surface and core hardness. To ensure this effect, the Nb content should be 0.010% or more. On the other hand, a high Nb content makes it easier for coarse nitrides and carbonitrides to form, so the Nb content should be 0.200% or less. When Nb is contained, the Nb content is preferably 0.015% or more, 0.020% or more, 0.025% or more, or 0.030% or more. Similarly, the Nb content is preferably 0.175% or less, 0.150% or less, 0.125% or less, or 0.100% or less.

[0040] [Ti:0.250% or less] Ti combines with N that penetrates the surface layer of the base material during nitriding and with C in the base material to form fine nitrides (TiN) and carbides (TiC), improving surface and core hardness. To ensure this effect, the Ti content should be 0.005% or more. On the other hand, a high Ti content makes it easier for coarse nitrides and carbides to form, so the Ti content should be 0.250% or less. When Ti is contained, the Ti content is preferably 0.007% or more, 0.010% or more, 0.015% or more, or 0.020% or more. Similarly, the Ti content is preferably 0.200% or less, 0.150% or less, or 0.100% or less.

[0041] [B:0.0100% or less] Solute B has the effect of suppressing the grain boundary segregation of P and improving toughness. In addition, BN, which combines with N and precipitates, 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 may 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 0.0008% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more. Similarly, the B content is preferably 0.0080% or less, 0.0070% or less, or 0.0060% or less.

[0042] [Ca:0.0100% or less] Calcium refines MnS and improves contact fatigue strength. To ensure this effect, the Ca content should be 0.0010% or more. However, if the Ca content is too high, the effect saturates and economic efficiency is compromised, so the Ca content should be 0.0100% or less. When Ca is contained, the Ca content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more, and similarly, the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0043] [Mg:0.0100% or less] Mg has the function 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 uneconomical, so the Mg content should be 0.0100% or less. When Mg is contained, the Mg content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more, and similarly, the Mg content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0044] [Te:0.100% or less] Te refines MnS and improves contact fatigue strength. To ensure the effect of Te, the Te content should be 0.010% or more. On the other hand, if the Te content is too high, the effect saturates and the cost becomes uneconomical, so the Te content should be 0.0100% or less. When Te is contained, the Te content is preferably 0.020% or more, 0.030% or more, or 0.040% or more, and similarly, the Te content is preferably 0.090% or less, 0.080% or less, or 0.070% or less.

[0045] [Pb:0.090% or less] Pb has the effect of reducing cutting resistance and extending the life of tools, but if the Pb content is increased, the effect saturates and economic efficiency is impaired, so the Pb content should be kept at 0.090% or less. When Pb is contained, the Pb content is preferably 0.080% or less, 0.070% or less, 0.060% or less, or 0.050% or less.

[0046] [Sn:0.100% or less] Sn has the effect of reducing cutting resistance and extending the life of tools. To ensure this effect, the Sn content should be 0.010% or more. On the other hand, if the Sn content is too high, the effect saturates and the cost becomes less efficient, so the Sn content should be 0.100% or less. When Sn is contained, the Sn content is preferably 0.020% or more, 0.030% or more, or 0.040% or more, and similarly, the Sn content is preferably 0.090% or less, 0.080% or less, or 0.070% or less.

[0047] [Sb:0.0100% or less] Sb has the effect of reducing cutting resistance and extending the life of tools. To ensure this effect, the Sb content should be 0.0010% or more. On the other hand, if the Sb content is too high, the effect saturates and the cost becomes uneconomical, so the Sb content should be 0.0100% or less. When Sb is contained, the Sb content may be preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more. Similarly, the Sb content may be 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0048] [REM:0.0100% or less] REM (rare earth elements) refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these 17 elements. When lanthanides are used as REMs, industrially, the REMs are added in the form of misch metals. REM refines MnS and improves contact fatigue strength. To ensure the effect of REM, the REM content should be 0.0010% or more. On the other hand, if the REM content is too high, the effect saturates and economic efficiency is compromised, so the REM content should be 0.0100% or less. When REM is contained, the REM content is preferably 0.0020% or more, 0.0030% or more, or 0.0040% or more, and similarly, the REM content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

[0049] [Hardness] The nitriding steel according to the present invention is hot-worked (such as hot rolling or hot forging) after melting, then machined into a desired part shape such as a gear by cutting or other processes, and then nitrided to produce a nitrided part. In one embodiment of the present invention, we have found that there is a certain correlation between the hardness after hot working (hereinafter referred to as "pre-nitriding hardness") and machinability, and between the hardness of the surface layer of the diffusion layer after nitriding (hereinafter referred to as "surface hardness") and the hardness of the core (hereinafter referred to as "core hardness") and the contact fatigue strength and bending fatigue strength. Therefore, we will explain the pre-nitriding hardness, which is an indicator of the machinability of the nitriding steel, and the surface hardness and core hardness, which are indicators of the fatigue strength of the nitrided part. Note that hardness in this invention refers to Vickers hardness (HV) in accordance with JIS Z 2244:2009 "Vickers Hardness Test - Test Method."

[0050] [Hardness of nitriding steel after hot working (hardness before nitriding)] The pre-nitriding hardness of the nitriding steel according to the present invention refers to the average value of the Vickers hardness at any 10 points excluding the area up to 1 mm from the surface, measured with a test force of 1.96 N, after the base material after hot working is cut perpendicular to the processing direction (rolling or forging direction) and the resulting cross section is mirror-polished. This pre-nitriding hardness affects the machinability of parts, and it is empirically known that if the pre-nitriding hardness is 220 HV or less, the machinability of the parts is good. On the other hand, if the pre-nitriding hardness exceeds 220 HV, the machinability of the parts becomes poor. For this reason, the hardness of the nitriding steel after hot working (before nitriding) is set to 220 HV or less. The post-hot working (before nitriding) hardness is preferably 210 HV, and more preferably 200 HV or less.

[0051] [Hardness of the surface of the diffusion layer after nitriding (surface hardness)] Surface layer hardness refers to the average Vickers hardness calculated by cutting a nitrided part (nitrided part) perpendicular to the main axis or longitudinal direction, mirror-polishing the exposed cross section, and measuring 10 random points at a depth of 0.05 mm (50 μm) from the surface (positions perpendicular to the surface) with a test force of 1.96 N. This surface layer hardness affects the contact fatigue strength and bending fatigue strength of the part. If the surface layer hardness is 700 HV or more, the contact fatigue strength and bending fatigue strength of the part are good. On the other hand, if the surface layer hardness is less than 700 HV, the contact fatigue strength and bending fatigue strength of the part will be low. Therefore, it is preferable that the surface layer hardness of the nitrided part is 700 HV or more. The preferred lower limit of the surface layer hardness is 720 HV, and more preferably 740 HV.

[0052] [Hardness of the core after nitriding (core hardness)] Core hardness refers to the average Vickers hardness calculated by cutting a nitrided part (nitrided part) perpendicular to the main axis or longitudinal direction, mirror-polishing the exposed cross section, and measuring 10 random points in the core with a test force of 1.96 N. The reason for selecting the core is as follows. While it is difficult to precisely identify the boundary between the diffusion layer where nitrogen penetrated during nitriding and the remaining area, the core, which is located at half the thickness (diameter), is hardly affected by nitrogen penetration due to nitriding. Therefore, by measuring the hardness of the core, it is possible to measure the hardness of the core without being affected by the chemical composition of the nitriding. Core hardness affects the surface fatigue strength and bending fatigue strength of the part. A core hardness of 220 HV or higher ensures good surface fatigue strength and bending fatigue strength. On the other hand, if the core hardness is less than 220 HV, plastic deformation occurs in the non-nitrided surface layer, which has the same hardness as the core. This can lead to cracks near the interface with the diffusion layer or release of compressive residual stress in the diffusion layer, resulting in low surface fatigue strength and bending fatigue strength. Therefore, it is recommended that the core hardness of nitrided parts be 220 HV or higher. The lower limit of the core hardness is preferably 230 HV, and more preferably 240 HV. On the other hand, since the core hardness is not harder than the diffusion layer, the upper limit of the core hardness may be 700 HV.

[0053] [Manufacturing method for nitrided parts] One embodiment of the nitriding steel and the method for manufacturing a nitrided component using the same according to the present invention will be described below. The nitriding steel and the method for manufacturing a nitrided component according to the present invention are not limited to this embodiment. First, steel having the above-mentioned chemical composition is melted by a conventional method to produce steel materials such as ingots, slabs, billets, etc. These steel materials can then be processed and heat treated by, for example, the following methods to produce the steel.

[0054] [Hot processing] After cutting the steel material having the above-mentioned chemical composition to an appropriate size, it is heated and held at 1050 to 1250°C, and then hot worked into a rough shape. Note that the heating and holding time is preferably 0.5 to 4.0 hours to prevent insufficient uniform heating of the steel and to reduce the surface oxide film. The hot working is mainly hot rolling or hot forging, but is not limited to these working methods, and hot working may be performed in accordance with the above-mentioned methods. In order to make the crystal grain size of the structure uniform after hot working, normalizing in accordance with JIS B 6911:2010 "Normalizing and annealing of steel" may be performed before cutting, which will be described later.

[0055] [Cutting] The rough product after hot working is cut using a lathe or the like, and then in the case of gears, for example, it is machined into a desired part shape by broaching or the like.

[0056] [Nitriding] The machined part having the desired shape is then subjected to nitriding. The nitriding method for the nitrided part in this embodiment is not particularly limited, and well-known methods such as gas nitriding, gas soft-nitriding, salt bath soft-nitriding, and plasma nitriding can be used. The gas used for the nitriding may be NH3 alone, or a mixed gas containing NH3, N2, and H2. Furthermore, the soft-nitriding may be performed using these gases containing 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 treatment, as long as the temperature does not exceed the nitriding temperature. However, in nitriding treatments that contain carburizing gases (e.g., gas nitrocarburizing and salt bath nitrocarburizing), the compound layer formed on the surface of the part after nitriding tends to be a brittle single ε phase. Therefore, it is preferable to perform nitriding treatment in an atmosphere that does not contain carburizing gases. In order to stably form the compound layer and diffusion layer and ensure the surface fatigue strength and bending fatigue strength, the nitriding temperature is preferably 550 to 650°C and the nitriding holding time is preferably 1 to 10 hours.

[0057] [High-frequency hardening treatment] Induction hardening may be performed as a post-nitriding process to enhance surface fatigue strength and bending fatigue strength. Induction hardening transforms the surface layer into a hardened layer with a high-N martensite structure, where nitride-forming elements are dissolved. Therefore, in the tooth surface temperature range (approximately 200 to 400°C) due to contact friction during surface fatigue, alloys such as Cr and V form nitride clusters, which reduces hardness loss and achieves high surface fatigue strength. Additionally, the deeper hardened layer reduces internally initiated fracture during bending fatigue. To achieve these effects, induction hardening is preferably performed to a depth of 100 μm or more from the surface of the part. Furthermore, the heating temperature for induction hardening should be between 1000°C and 1200°C, and the time required to heat from room temperature to the heating temperature should be within 4 seconds. The steel should be held in the temperature range between 1000°C and 1200°C for between 0.2 and 2 seconds. [Example]

[0058] EXAMPLES The following examples are provided as examples of the present invention, but the present invention is not limited to the following examples. Steels a to ad having the chemical compositions shown in Table 1 were melted in a 50 kg vacuum melting furnace and cast to produce ingots. Of the compositions of steels a to o shown in Table 1, the remainder other than the components shown in Table 1 consisted of Fe and impurities. All steels contained approximately 10 ppm of O (oxygen) as an impurity.

[0059] Ingots of each steel shown in Table 1 were hot forged into round bars with a diameter of 40 mm. In the hot forging, the ingots were held in a heating furnace at a temperature between 1100°C and 1200°C for 2 hours, and then forged into round bars with a diameter of 40 mm. After forging, the bars were allowed to cool in the air. [Hardness measurement before nitriding] After hot forging, each round bar was cut perpendicular to the longitudinal direction at the center of the longitudinal direction, and the cut surface was mirror-polished. The Vickers hardness was then measured at 10 random points on the cross section using a micro Vickers hardness tester (Shimadzu Corporation; HMV-G31-FA) under a test force of 1.96 N. The average value of the 10 points was defined as the hardness before nitriding.

[0060] [Machinability evaluation test] After hot forging, each round bar was machined to a diameter of 20 mm and a length of 300 mm, and then a cutting test was carried out using a cemented carbide tool (K10 grade, TiN coated) at a feed rate of 0.20 mm / rev, a peripheral speed of 100 m / min, and a depth of cut of 0.9 mm, and the cutting resistance was measured. As an evaluation index for machinability, steels meeting the SCr420 standard of JIS G 4053:2016 were machined to the same shape and subjected to the cutting test described above. The cutting resistance of SCr420 steel was set as the reference cutting resistance, and if the ratio of the cutting resistance of each steel to the reference cutting resistance (specific cutting resistance) was 1.40 or less, the machinability was evaluated as good.

[0061] Next, each hot-forged round bar was machined to produce small rollers for roller pitting tests to evaluate surface fatigue strength, as shown in Figure 1. Multiple small rollers were produced from one ingot for the roller pitting test, but more small rollers than were necessary for the roller pitting test were produced in anticipation of investigating the hardness at the cross section. Furthermore, cylindrical test pieces for evaluating rotating bending fatigue strength, as shown in Figure 3, were produced using the same round bars. Multiple cylindrical test pieces were also produced from one ingot for the rotating bending fatigue test.

[0062] The collected test specimens were subjected to gas nitriding, gas soft nitriding, and plasma nitriding. Table 2 shows the relationship between each test specimen and the nitriding treatment. In all nitriding treatments, the nitriding temperature was 590°C, the nitriding time was 5 hours, and the test specimens after nitriding were oil-cooled using 80°C oil. For the gas nitriding treatment, the test pieces were placed in a gas nitriding furnace and NH3, H2, and N2 gases were introduced into the furnace. For the gas soft nitriding test, in addition to these gases, CO2 gas was introduced at a volume ratio of 3%.

[0063] The H2 partial pressure in the atmosphere was measured using a thermal conductivity H2 sensor directly attached to the gas nitriding furnace body. The difference in thermal conductivity between the standard gas and the measurement gas was converted into gas concentration. The H2 partial pressure was measured continuously during the gas nitriding process.

[0064] The NH3 partial pressure was measured using an infrared absorption NH3 analyzer installed outside the furnace. The NH3 partial pressure was measured continuously during the gas nitriding treatment. Note that for test number 5, which was performed in an atmosphere containing CO2 gas, (NH4)2CO3 was precipitated in the infrared absorption NH3 analyzer, which could have caused the device to break down, so the NH3 partial pressure was measured every 10 minutes using a glass tube NH3 analyzer.

[0065] The nitriding potential KN of the gas nitriding treatment is defined by the following formula: KN(atm -1 / 2 ) = (NH3 partial pressure (atm)) / [(H2 partial pressure (atm)) 3 / 2 ]

[0066] The NH3 and N2 flow rates were controlled so that the nitriding potential KN calculated within the equipment converged to the target value. The nitriding potential KN was recorded every 10 minutes, and the average value of KN measured during the treatment time was calculated. For the plasma nitriding treatment, the test piece was placed in the plasma nitriding equipment, and H2 and N2 gases were introduced into the furnace, with the gas flow rate controlled so that the partial pressure ratio of H2 gas to N2 gas was kept constant at 3:1. A portion of the nitrided test pieces was subjected to induction hardening. In each treatment, the heating temperature was 1100°C, the time required to raise the temperature from room temperature to the heating temperature was 3 seconds, and the steel material was held at 1100°C for 1 second. After induction hardening, the steel material was immediately quenched in room temperature water. Then, it was tempered at 170°C for 1.5 hours.

[0067] [Measurement of surface hardness and core hardness] Each small roller that had been subjected to the nitriding or nitride induction hardening treatment was cut into 10 mm lengths along a plane perpendicular to the longitudinal center, and the cut surface was then mirror-polished. The Vickers hardness was then measured at 10 random points 0.05 mm (50 μm) deep from the surface using a micro Vickers hardness tester (Shimadzu Corporation; HMV-G31-FA) under a test force of 1.96 N. The average value of these 10 points was defined as the surface hardness. Similarly, using the test piece for which the surface hardness had been measured, the Vickers hardness of 10 arbitrary points in the core was measured using the same device under a test force of 1.96 N, and the average value of the 10 points was defined as the core hardness.

[0068] [Surface fatigue strength evaluation test] The surface fatigue strength was evaluated using a roller pitting tester (Komatsu Equipment Co., Ltd.; RP102) according to the following method. The small rollers for the roller pitting test were subjected to finish machining of the gripping parts to remove heat treatment strain, and then used as roller pitting test specimens. The shape after finish machining is shown in Figure 1.

[0069] The small roller, which is the roller pitting test specimen, has a central test surface with a diameter of 26 mm and a width of 28 mm, and gripping parts with a diameter of 22 mm on both sides, as shown in Figure 1. In the roller pitting test, the test surface was brought into contact with the large roller, and a predetermined surface pressure was applied before the roller was rotated. The roller pitting test was carried out under the conditions shown in Table 3 using a combination of the small roller for roller pitting test described above and a large roller for roller pitting test having the shape shown in FIG.

[0070] The dimensions in Figures 1 and 2 are in mm. The large rollers for roller pitting tests were made using steel that met the SCM420 standard of JIS G 4053:2016, using the general manufacturing process of "normalizing → test piece processing → eutectoid carburizing in a gas carburizing furnace → low-temperature tempering → polishing." The Vickers hardness HV at a depth of 0.05 mm (50 μm) from the surface was 740 to 760, and the depth at which the Vickers hardness Hv was 550 or more was in the range of 0.8 to 1.0 mm.

[0071] Table 3 shows the test conditions under which the surface fatigue strength was evaluated. The test was stopped at 2.0 × 10 cycles, which indicates the fatigue limit of general steel. 7 The small roller test piece was tested for 2 x 10 7 The maximum contact pressure reached in 1000 tests was defined as the fatigue limit of the small roller test piece. In the roller pitting test, the contact pressure was increased in 50 MPa increments, particularly near the fatigue limit. In other words, the pitting strength values ​​shown in Table 2 indicate that, for the target test number, pitting did not occur in the small roller test piece tested under the same contact pressure, but pitting did occur in the small roller test piece tested under a contact pressure 50 MPa higher than the same contact pressure.

[0072] The occurrence of pitting was detected using a vibration meter attached to the testing machine, and 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 rotation speed were confirmed. In this example, assuming application to gear parts, the target was a contact pressure of 2000 MPa or more at the fatigue limit in the roller pitting test shown in Table 3.

[0073] [Rotating bending fatigue strength evaluation test] The cylindrical test pieces subjected to gas nitriding were subjected to an Ono-type rotating bending fatigue test in accordance with JIS Z 2274:1978 "Rotating bending fatigue test method for metallic materials." The rotation speed was 3000 rpm, and the test cutoff cycle number was 1 × 10, which indicates the fatigue limit of general steel. 7 The rotating bending fatigue test piece was tested for 1×10 7 The maximum stress reached at each test was defined as the fatigue limit of the rotating bending fatigue test specimen. In the rotating bending fatigue test, the stress was increased in 10 MPa increments, especially near the fatigue limit. In other words, the rotating bending fatigue strength values ​​shown in Table 2 indicate that, for the target test number, no fracture occurred in the cylindrical test specimen tested under the same stress, but fracture occurred in the cylindrical test specimen tested under a stress 10 MPa higher than the same stress.

[0074] The part of the present invention is intended for application to gear parts, and the target stress at the fatigue limit in the Ono-type rotating bending fatigue test is 550 MPa or more.

[0075] [Test Results] The results are shown in Table 2. Test Nos. 1 to 19 had steel compositions within the range of the present invention, and therefore had low hardness before nitriding and excellent machinability, and also had high surface and core hardness after nitriding or nitriding induction hardening, providing both surface fatigue strength and rotating bending fatigue strength.

[0076] On the other hand, in test numbers 20 to 35, the steel compositions were outside the range of the present invention, and the hardness before nitriding was high, resulting in poor machinability, or the surface hardness and core hardness after nitriding or nitriding induction hardening were low, so the desired surface fatigue strength and rotating bending fatigue strength did not reach the targets of the present invention.

[0077] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

[0078] [Table 1]

[0079] [Table 2]

[0080] [Table 3] [Industrial Applicability]

[0081] The present invention can be widely used in industrial fields, such as for mechanical parts such as gears, and for transmissions and camshafts of transportation machines.

Claims

1. In mass%, C: 0.01-0.09%, Si: 0.01-0.28%, Mn: 1.20-2.50%, P: 0.025% or less, S: 0.100% or less, Cr: 0.20-0.90%, V: 0.05-0.50%, Al: 0 to 0.050%, N: 0 to 0.0250%, Mo: 0 to 0.50%, Cu: 0 to 0.45%, Ni: 0 to 0.50%, W: 0 to 0.50%, Bi: 0 to 0.50%, Co: 0 to 0.50%, Nb: 0 to 0.200%, B: 0 to 0.0100%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Te: 0 to 0.100%, Pb: 0 to 0.090%, Sn: 0 to 0.100%, and REM: 0~0.0100% and the balance being Fe and impurities, A nitriding steel characterized in that the average value of Vickers hardness at any 10 points on a cross section perpendicular to the processing direction of the nitriding steel, excluding a region up to 1 mm from the surface, is 220 HV or less.

2. In mass%, Mo: 0.01-0.50%, Cu: 0.05-0.45%, Ni: 0.05-0.50%, W: 0.05-0.50%, Bi: 0.05-0.50%, Co: 0.05-0.50%, Nb: 0.015 to 0.200%, and B: 0.0008-0.0100%, 2. The nitriding steel according to claim 1, further comprising one or more of the following:

3. In mass%, Ca: 0.0020-0.0100%, Mg: 0.0020-0.0100%, Te: 0.0020 to 0.100%, Pb: 0.042-0.090%, Sn: 0.010 to 0.100%, and REM: 0.0020 to 0.0100% or less 3. The steel for nitriding according to claim 1, further comprising one or more of the following:

4. In a cross section perpendicular to the main axis direction, the components of the core portion, which is a similar area that shares a centroid with the cross section and has a similarity ratio of 1 / 4 of the cross section, are, in mass%, C: 0.01-0.09%, Si: 0.01-0.26%, Mn: 1.20-2.50%, P: 0.025% or less, S: 0.100% or less, Cr: 0.20-0.90%, V: 0.05-0.50%, Al: 0 to 0.050%, N: 0 to 0.0250%, Mo: 0 to 0.50%, Cu: 0 to 0.45%, Ni: 0 to 0.50%, W: 0 to 0.50%, Bi: 0 to 0.50%, Co: 0 to 0.50%, Nb: 0 to 0.200%, B: 0 to 0.0100%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, Te: 0 to 0.100%, Pb: 0 to 0.090%, Sn: 0 to 0.100%, and REM: 0~0.0100% with the remainder being Fe and impurities, In the cross section, the N concentration at a depth of 50 μm from the surface is higher than the N concentration in the core portion, The nitrided component is further characterized in that the Vickers hardness at the 50 μm depth position is 700 HV or more, and the Vickers hardness at the core portion is 220 HV or more.

5. The components of the core are, in mass%, Mo: 0.01-0.50%, Cu: 0.05-0.45%, Ni: 0.05-0.50%, W: 0.05-0.50%, Bi: 0.05-0.50%, Co: 0.05-0.50%, Nb: 0.015 to 0.200%, and B: 0.0008-0.0100%, 5. The nitrided part according to claim 4, further comprising one or more of the following:

6. The components of the core are, in mass%, Ca: 0.0020-0.0100%, Mg: 0.0020-0.0100%, Te: 0.0020 to 0.100%, Pb: 0.042-0.090%, Sn: 0.010 to 0.100%, and REM: 0.0020 to 0.0100% or less 6. The nitrided part according to claim 4, further comprising one or more of the following:

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