Steel and steel parts

Abstract

To provide steel excellent in surface fatigue characteristics.SOLUTION: Steel has a component composition characterized in that from the surface to the inside in order, there are a nitride compound layer and a nitrogen diffusion layer, wherein the nitrogen diffusion layer contains carbonitrides that are compatible with the base phase ferrite, the number density of carbonitrides, with a longitudinal length of 3 nm or more and 20 nm or less of the carbonitrides is at least 1.00×1023 / m3 at a position 50 μm inward from the surface, at least 0.30×1023 / m3 at a position 200 μm inward from the surface, and at least 0.10×1023 / m3 at a position 400 μm inward from the surface, the portion excluding the nitride compound layer and nitrogen diffusion layer contains, by mass percentage, C: 0.010% or more and 0.200% or less, Si: not more than 1.00%, Mn: 0.50% or more and 3.00% or less, P: not more than 0.020%, S: 0.020% or more and 0.060% or less, Cr: 0.30% or more and 3.00% or less, and V: 0.02% or more and 0.80% or less, with the remainder consisting of Fe and impurities.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to steel and steel parts, and more particularly to steel and steel parts that have a diffusion layer containing a large number of fine carbonitrides due to soft nitriding treatment, and that have excellent surface fatigue properties, making them suitable for use in parts for automobiles, construction machinery, and the like. [Background technology]

[0002] Automotive gears and other mechanical structural components require excellent fatigue properties, such as surface fatigue characteristics and bending fatigue characteristics, and therefore surface hardening treatments are commonly performed. Well-known surface hardening treatments include carburizing, high-frequency induction hardening, and nitriding.

[0003] Of these methods, carburizing is effective in improving fatigue properties because it allows carbon to penetrate and diffuse in the high-temperature austenite region, resulting in a deep hardened layer. However, since carburizing causes heat treatment distortion, it has been difficult to apply to parts that require strict dimensional accuracy from the standpoint of quietness and other factors.

[0004] Furthermore, since high-frequency induction hardening is a process that hardens the surface layer using high-frequency induction heating, heat treatment distortion still occurs, and there were problems in terms of dimensional accuracy, similar to carburizing.

[0005] On the other hand, nitriding is a process that increases surface hardness by infiltrating and diffusing nitrogen in a relatively low temperature range below the Ac1 transformation point, so the heat treatment distortion described above is small. However, it has the problem of requiring a long processing time of 50 to 100 hours, and the need to remove the brittle compound layer on the surface after processing.

[0006] For this reason, a so-called soft nitriding process has been developed that shortens the processing time at a similar processing temperature to nitriding, and in recent years it has become widely used for mechanical structural parts and the like. This soft nitriding process involves simultaneously introducing N and C at a temperature range of 500-600°C, forming a nitride compound layer with solid-solution C on the outermost surface, and further diffusing N into the base metal to form a nitrogen diffusion layer and harden the surface. This process makes it possible to reduce the processing time to less than half compared to conventional nitriding.

[0007] However, while the aforementioned carburizing treatment can increase core hardness through quenching, the soft nitriding treatment is performed at a temperature below the transformation point of steel, so core hardness does not increase, and soft nitrided materials have the problem of having inferior fatigue properties compared to carburized materials.

[0008] Therefore, in order to improve the fatigue properties of soft-nitrided materials, quenching and tempering treatments are usually performed before soft-nitriding to increase the core hardness. However, the resulting fatigue properties are not always satisfactory, and the manufacturing costs increase, as well as a decrease in machinability, is unavoidable.

[0009] To address these problems, Patent Document 1 proposes a steel for soft nitriding that incorporates elements such as Ni, Cu, Al, Cr, and Ti into the steel, thereby enabling high bending fatigue properties after soft nitriding treatment. Specifically, in this steel, the core is age-hardened with Ni-Al, Ni-Ti intermetallic compounds or Cu compounds through soft nitriding treatment, while the surface layer is precipitation-hardened with nitrides and carbides such as Cr, Al, and Ti within the nitrided layer, thereby improving the bending fatigue properties.

[0010] Furthermore, Patent Document 2 proposes a steel for soft nitriding in which excellent bending fatigue properties are obtained after soft nitriding treatment by hot forging a steel containing 0.5 to 2% Cu, air cooling to obtain a ferrite-dominant structure with dissolved Cu, precipitation hardening of Cu during soft nitriding treatment at 580°C for 120 minutes, and further precipitation hardening of Ti, V and Nb carbonitrides.

[0011] Furthermore, Patent Document 3 has proposed a steel for soft nitriding in which Ti-Mo carbides and carbides containing one or more of Nb, V, and W are further dispersed therein.

[0012] Patent Document 4 has proposed a steel for soft nitriding in which the formation of nitrides during soft nitriding treatment is promoted by forming a dislocation structure at a high density in the steel for soft nitriding.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0014] However, although the soft nitrided steels described in Patent Documents 1 to 3 are excellent in bending fatigue strength, the surface fatigue strength has not been considered. When sliding contact occurs in a component, if the surface fatigue strength is insufficient, cracks will occur from the contact portion. The technique described in Patent Document 4 considers the hardness and carbonitrides within 50 μm from the surface layer, but does not consider the characteristics deeper inside.

[0015] The present invention significantly solves the above problems, and an object thereof is to provide a steel and steel components particularly excellent in surface fatigue characteristics.

Means for Solving the Problems

[0016] To solve the above problems, the inventors diligently investigated the optimal microstructure in the nitrogen diffusion layer. As a result, they found that controlling the number density and morphology of carbonitrides with a longitudinal length of 3 nm to 20 nm in the nitrogen diffusion layer is effective in improving surface fatigue properties.

[0017] Furthermore, when steel components are subjected to resistance involving sliding, shear stress is applied due to the normal force. This stress is greatest at a depth of approximately 400 μm from the surface, so controlling the internal structure as well as the surface is important to achieve excellent surface fatigue characteristics.

[0018] This invention was completed after further consideration based on the above findings. In other words, the gist of this invention is as follows:

[0019] 1. From the surface inward, it has a nitride compound layer and a nitrogen diffusion layer, The nitrogen diffusion layer has carbonitrides that match the matrix ferrite, and the number density of carbonitrides with a longitudinal length of 3 nm to 20 nm is 1.00 × 10⁻¹⁰ at a position 50 μm inward from the surface. 23 / m 3 In summary, at a distance of 200 μm from the surface inward, the measurement is 0.30 × 10⁻⁶. 23 / m 3 In summary, at a distance of 400 μm from the surface inward, the measurement is 0.10 × 10⁻⁶. 23 / m 3 That's all. The portion excluding the nitride compound layer and the nitrogen diffusion layer is, in mass%, C: 0.010% or more and 0.200% or less, Si: 1.00% or less, Mn: 0.50% or more and 3.00% or less, P: 0.020% or less, S: 0.002% or more and 0.060% or less, Cr: 0.30% to 3.00% and V: 0.02% or more and 0.80% or less A steel having a component composition that contains [a certain substance], with the remainder being Fe and impurities.

[0020] 2. The above component composition is further expressed in mass %, Nb: 0.003% or more and 0.150% or less, Al: 0.01% or more and 0.20% or less, Ti: 0.01% to 0.10% and Mo: 0.005% or more and 0.400% or less The steel according to item 1, which contains one or more of the following:

[0021] 3. The steel according to 1 or 2, wherein the carbonitride contains Cr and V as metallic elements that combine with carbon and nitrogen, and the concentration of (V+Cr) in the metallic elements is 50 at% or more.

[0022] 4. A steel component made of the steel described in 1, 2, or 3 above. [Effects of the Invention]

[0023] According to the present invention, it is possible to provide steel and steel parts with improved surface fatigue properties. Therefore, the steel and steel parts of the present invention are extremely useful as materials for mechanical structural parts such as those used in automobiles. [Brief explanation of the drawing]

[0024] [Figure 1] This figure shows a HAADF-STEM image of carbonitrides present in steel. [Figure 2] This diagram shows typical manufacturing methods for soft nitride components. [Figure 3] This is a diagram showing a roller pitting test specimen. [Modes for carrying out the invention]

[0025] The present invention will be described in detail below. As described above, in order to exhibit excellent surface fatigue characteristics in steel, it is important not only to control the surface layer of the steel but also to control the internal structure. In order to improve the surface fatigue characteristics of steel, it is important to precipitate fine carbonitrides at a high density and even inside the steel. The presence of a large amount of carbonitrides in the surface layer inhibits the dislocation movement during surface pressure loading, making it possible to exhibit excellent surface fatigue characteristics.

[0026] That is, in a steel having a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, and the nitrogen diffusion layer having carbonitrides that match the matrix ferrite, the number density of the carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.00×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.30×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.10×10 23 / m 3 or more at a position 400 μm from the surface to the inside. This is essential.

[0027] [Carbonitrides in the Nitrogen Diffusion Layer] First, the carbonitrides in the nitrogen diffusion layer will be described. In the present invention, the carbonitrides need to be aligned with the matrix ferrite. In the case of carbonitrides that are aligned with the matrix ferrite, the resistance to dislocations increases by generating misfit strain, making it possible to improve the fatigue characteristics.

[0028] Here, being aligned with the matrix ferrite means having a crystal orientation relationship called the Baker-nutting relationship with the matrix ferrite. In this case, (001) of ferrite and (011) of the NaCl-type carbonitride are aligned, and in order to generate sufficient misfit strain, the misfit (ε) between the two becomes important. This misfit (ε) is calculated according to the following formula (1). In the present invention, the carbonitrides that are aligned with the matrix ferrite are those in which ε is 10% or less. Carbonitrides with ε exceeding 10% have a reduced effect of inhibiting dislocation movement. ε = |a0 - ap| / ap × 100 (%) …(1) Here, a0 represents the interplanar spacing of the ferrite (=0.2866 nm), and ap represents the interplanar spacing of the carbonitride.

[0029] The method for measuring the interplanar spacing described above is not particularly limited, but it can be confirmed by direct observation of the interface region using high-resolution electron microscopy. For example, Figure 1 shows carbonitrides <001> The image obtained using HAADF-STEM (High Angle Annular Dark-Field Scanning Transmission Electron Microscopy) is shown. As shown in Figure 1, it is possible to directly measure the interplanar spacing ap of the carbonitrides from this image.

[0030] To improve the surface fatigue properties of steel, the number density of carbonitrides that match the matrix ferrite and have a longitudinal length of 3 nm to 20 nm (hereinafter also called compatible diameter carbonitrides) should be 1.0 × 10⁻¹⁶ at a position 50 μm inward from the surface. 23 / m 3 In summary, at a distance of 200 μm from the surface inward, the measurement is 0.3 × 10⁻⁶. 23 / m 3 In summary, at a distance of 400 μm from the surface inward, the measurement is 0.1 × 10⁻⁶. 23 / m 3 It is important that these conditions are met. If these conditions are not met, dislocation movement cannot be sufficiently inhibited, and therefore excellent surface fatigue characteristics cannot be obtained.

[0031] Here, the carbonitrides are defined as having a longitudinal length of 3 nm or more and 20 nm or less because, firstly, carbonitrides with a longitudinal length of less than 3 nm have difficulty exhibiting a sufficient inhibitory effect on dislocation motion even if they are well-matched with the matrix ferrite. Secondly, if the carbonitride is longer than 20 nm, the matching with the matrix ferrite becomes poor, resulting in reduced resistance to dislocation motion.

[0032] Furthermore, the number density of the suitable diameter carbonitrides described above is defined at positions 50 μm, 200 μm, and 400 μm from the surface, as explained above for the 400 μm position. On the other hand, compressive stress becomes a problem at the 50 μm and 200 μm positions. That is, compressive stress is maximum at the surface and gradually decreases, but in order to create a structure that can withstand this compressive stress, microstructure control is required at depths of 50 μm and 200 μm from the surface in stages. By defining the number density of suitable diameter carbonitrides at these two depth positions, it is possible to withstand gradually changing compressive stress. The reason for limiting the number density of suitable diameter carbonitrides at each position is as follows.

[0033] [1.00 × 10 at a position 50 μm from the surface inward] 23 / m 3 [End] The number density of suitable diameter carbonitrides at a position of 50 μm is 1.00 × 10⁻⁶ 23 / m 3 The reason for setting it above is to ensure sufficient resistance to the dislocation motion described above. While there is no particular need to limit the upper limit, from the perspective of alloy cost, 4.00 × 10 23 / m 3 The following is preferable.

[0034] [0.30 × 10 at a position 200 μm from the surface inward] 23 / m 3 [End] The number density of suitable diameter carbonitrides at a position of 200 μm is 0.30 × 10⁻⁶. 23 / m 3 The reason for setting it above is to ensure sufficient resistance to dislocation motion. While there is no particular need to limit the upper limit, from the perspective of alloy cost, 2.50 × 10 23 / m 3 The following is preferable.

[0035] [0.10 × 10 at a position 400 μm from the surface inward] 23 / m 3 [End] The number density of suitable diameter carbonitrides at a position of 400 μm is 0.10 × 10⁻⁶. 23 / m 3 The reason for setting it above is to ensure sufficient resistance to dislocation motion. While there is no particular need to limit the upper limit, from the perspective of alloy cost, 0.60 × 10 23 / m 3 The following is preferable.

[0036] Carbonitrides that are compatible with the above-mentioned ferrite matrix precipitate on the {001} plane of the ferrite matrix, satisfying the Baker-Nutting relationship. Therefore, the number density of carbonitrides is calculated as follows. First, the {001} plane of the matrix ferrite was observed using a transmission electron microscope (TEM), and the number of carbonitrides in the field of view was measured. Next, the relative film thickness of the sample in each observation field was measured using electron energy loss spectroscopy (EELS), and the volume of the observation field was calculated. Then, the number density was calculated by dividing the number of carbonitrides by the volume. Here, carbonitrides precipitate on the (001), (010), and (100) planes, but for example, when observed with (001) incidence, carbonitrides on the (001) plane cannot be observed, so only two planes can be observed with TEM observation. For this reason, the number of carbonitrides was taken by multiplying the number of carbonitrides observed by TEM by 1.5. The film thickness was measured by multiplying the relative film thickness measured by EELS by the mean free path λ calculated from the following formula by Iakoubovskii et al. JPEG0007878376000001.jpg14170 Here, E0: Acceleration voltage, α: angle of convergence, β: angle of entry, ρ: sample density, F = (1 + E0 / 1022) / (1 + E0 / 511) 2 , θ E = 5.5ρ 0.3 / FE0, θ c =20mrad That's what I decided.

[0037] In order to control the number density of the suitable diameter carbonitrides described above to the ranges mentioned above at positions 50 μm, 200 μm, and 400 μm from the surface inward, it is preferable to perform the gradient soft nitriding treatment described later, in addition to the component composition shown below.

[0038] [Component composition of the uncured portion] Next, the reason why the component composition of the non-cured portion, excluding the nitride compound layer (hereinafter also referred to as the compound layer) and the nitrogen diffusion layer (hereinafter also referred to as the diffusion layer), is limited to the range described above will be explained. Note that the "%" used to represent the component composition below means "mass%" unless otherwise specified.

[0039] C: 0.010% or more and 0.200% or less Carbon (C) is necessary to ensure the strength of the unhardened portion, as well as the compound layer and the diffusion layer. Furthermore, C is necessary to form carbides in the steel material before the soft nitriding treatment. These carbides are useful for carbonitride formation during the soft nitriding treatment. If the C content is less than 0.010%, a sufficient amount of carbonitride will not be formed after soft nitriding. Therefore, the C content should be 0.010% or more. More preferably, the C content should be 0.050% or more. On the other hand, if the C content exceeds 0.200%, the carbonitride becomes coarse, and the coherence with the matrix ferrite deteriorates. Therefore, the C content should be 0.200% or less. More preferably, it should be in the range of 0.100% or less.

[0040] Si: 1.00% or less Si is effective in ensuring strength. However, if the Si content exceeds 1.00%, solid solution strengthening degrades machinability; therefore, the Si content should be 1.00% or less. A more preferable Si content is 0.500% or less. From the viewpoint of ensuring the strength of the steel, it is preferable that the Si content be 0.005% or more.

[0041] Mn: 0.50% or more and 3.00% or less Mn forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby strengthening the precipitation of the compound layer and the diffusion layer. If the amount of Mn is less than 0.50%, the amount of carbonitrides formed is insufficient, and the carbonitride number density decreases. Therefore, the amount of Mn should be 0.50% or more, preferably 1.50% or more. On the other hand, if the amount of Mn exceeds 3.00%, carbonitrides precipitate only in the surface layer and cannot precipitate into the interior. Furthermore, the (V+Cr) concentration falls below 50 at%, resulting in poor carbonitride coherence. Therefore, the amount of Mn should be 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0042] P:0.020% or less P is an element that is mixed into steel as an impurity and is known to cause surface cracking in cast slabs. Therefore, it is desirable to minimize the content of P, but it is acceptable up to 0.020%. However, since reducing P to less than 0.001% is costly, it is sufficient to reduce it to 0.001% industrially.

[0043] S: 0.002% or more and 0.060% or less S is an element that is mixed into steel as an impurity, but it also contributes to improving machinability. Specifically, if the amount of S is less than 0.002%, the amount of MnS generated in the steel decreases, and machinability decreases. On the other hand, if the amount of S exceeds 0.060%, the excess precipitated as MnS reduces the amount of solid-solution Mn required during soft nitriding treatment, so the content should be limited to 0.060% or less. Preferably, it is 0.040% or less.

[0044] Cr: 0.30% or more and 3.00% or less Cr forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby strengthening the precipitation of the compound layer and diffusion layer. The compound layer and diffusion layer can be formed by soft nitriding treatment as described later, and they become compound layers and diffusion layers when the N concentration increases compared to the component composition of the material before soft nitriding treatment. In the unhardened portion, which is the part other than the compound layer and diffusion layer, the component composition of the material before soft nitriding is maintained, except for N. Therefore, although the purpose is to strengthen the precipitation of the compound layer and diffusion layer, the Cr content in the unhardened layer should also be 0.30% or more. If the Cr content is less than 0.30%, the amount of CrN that precipitates in the compound layer and diffusion layer during soft nitriding treatment will be insufficient. Furthermore, the concentration of (V+Cr) carbonitrides cannot be made 50 at% or more. Therefore, the Cr content should be 0.30% or more. On the other hand, if it exceeds 3.00%, carbonitrides will only precipitate in the surface layer and will not precipitate in the interior, so the Cr content should be 3.00% or less. Preferably it is 0.50% or more. Furthermore, it is preferably 1.50% or less.

[0045] V: 0.02% or more and 0.80% or less V forms nitrides with nitrogen diffused from the surface during soft nitriding, contributing to an increase in surface hardness. Furthermore, V forms fine precipitates during soft nitriding, significantly contributing to an increase in the number density of the suitable diameter carbonitrides. If the V content is less than 0.02%, the amount of VN precipitated in the compound layer and diffusion layer during soft nitriding will be insufficient. Additionally, the (V+Cr) concentration of the carbonitrides cannot be increased to 50 at% or more, resulting in poor carbonitride coherence. Therefore, the V content should be 0.02% or more. On the other hand, excessive addition will cause carbonitrides to precipitate only in the surface layer, preventing them from precipitation in the interior. Therefore, the V content should be 0.80% or less, preferably 0.50% or less, and more preferably 0.40% or less.

[0046] In addition to the elements described above, one or more of the following elements may be optionally included. The elements that can be optionally included and their preferred content are described below.

[0047] Nb: 0.003% or more and 0.150% or less Nb contributes to increasing the hardness of the surface layer by forming nitrides with nitrogen diffused from the surface during soft nitriding. Furthermore, Nb forms fine precipitates due to the temperature increase during soft nitriding, increasing the hardness of the core. Therefore, the Nb content should be 0.003% or more. On the other hand, excessive addition results in carbonitrides precipitating only on the surface, preventing carbonitrides from precipitating throughout the interior, and further worsening the carbonitride integrity. Therefore, the Nb content should be 0.150% or less, preferably 0.120% or less.

[0048] Al: 0.01% or more and 0.20% or less Al forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby strengthening the precipitation of the compound layer and the diffusion layer. For this purpose, the Al content should be 0.01% or more. On the other hand, if the Al content exceeds 0.20%, carbonitrides will only precipitate in the surface layer, and carbonitrides cannot be precipitated into the interior. Therefore, the Al content should be 0.20% or less, preferably 0.10% or less, and more preferably 0.04% or less.

[0049] Ti: 0.01% or more and 0.10% or less Ti forms carbonitrides with N and C diffused from the surface during soft nitriding, thereby strengthening the compound layer and diffusion layer through precipitation. If the Ti content is less than 0.01%, there will be insufficient carbonitrides to precipitate in the compound layer and diffusion layer during the nitriding process, making it difficult to ensure sufficient strength. Therefore, the Ti content should be 0.01% or more. On the other hand, if it exceeds 0.10%, carbonitrides will only precipitate in the surface layer, and carbonitrides will not precipitate in the interior, further worsening the carbonitride integrity. Therefore, the Ti content should be 0.10% or less.

[0050] Mo: 0.005% or more and 0.400% or less Mo forms nitrides with nitrogen diffused from the surface during soft nitriding, contributing to an increase in surface hardness. Mo also generates bainite, contributing to improved machinability and core hardness. Therefore, the Mo content should be 0.005% or more. On the other hand, excessive addition of Mo causes carbonitrides to precipitate only on the surface, preventing them from precipitation in the interior and further worsening the carbonitride integrity. Therefore, the Mo content should be 0.400% or less, preferably 0.150% or less.

[0051] The component composition of the unhardened portion of the steel of the present invention consists of Fe and impurities, with the remainder being the elements described above. Impurities are those introduced during the industrial production of steel materials from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the properties of this embodiment.

[0052] [Carbonitride composition] Next, we will explain the concentration of the metal elements constituting the carbonitride. When V and Cr form carbonitrides, the interstitial misfit with ferrite is small. Therefore, if the concentration of (V+Cr) in the metal elements of the carbonitride is 50 at% or more, the misfit between the carbonitride and ferrite is reduced, and the carbonitride can be precipitated with good coherence with the ferrite. Carbonitrides with good coherence with ferrite have high resistance to dislocation motion and can achieve excellent surface fatigue properties. For this reason, it is preferable to set the concentration of (V+Cr) to 50 at% or more.

[0053] The metal elements that make up the carbonitride include V and Cr, as well as Mn, Nb, Al, Ti, and Mo, and their total concentration should be 50 at% or less.

[0054] In this invention, the concentration of each metal element constituting the carbonitride can be measured using a three-dimensional atom probe (3DAP). Specifically, the region with an N concentration of 5% or more is defined as the carbonitride, and the average concentration of 20 or more carbonitrides can be quantified to determine the concentration.

[0055] Next, the microstructure of the compound layer and diffusion layer of the steel of the present invention will be described. The compound layer and diffusion layer are formed during the soft nitriding treatment, and are formed by the diffusion of nitrogen and carbon in the soft nitriding treatment atmosphere into the steel. That is, a compound layer is formed on the outermost surface of the steel, with iron nitride (Fe3N or Fe4N), which is formed when Fe, the main component of the steel subjected to the soft nitriding treatment, combines with N, as the matrix phase, and other constituent components precipitated as carbonitrides by combining with nitrogen and carbon. It is preferable that the compound layer be formed over a thickness of 3 to 40 μm.

[0056] Furthermore, the diffusion layer is a layer in which nitrogen diffuses into the steel, resulting in a higher nitrogen concentration than before the soft nitriding treatment. It is formed adjacent to the inside of the compound layer. The unhardened portion is the area where nitrogen diffusion does not occur, and therefore has the component composition of the unhardened portion as described above. In contrast, the compound layer and the diffusion layer have a component composition with a higher nitrogen content compared to the component composition of the unhardened portion. The structure of the diffusion layer and the unhardened portion inside it forms a matrix of ferrite or bainite. It is preferable to form the diffusion layer inside the compound layer with a thickness of 400 to 1000 μm.

[0057] The steel according to the present invention has been described above. Furthermore, the steel parts according to the present invention are in which the steel of the present invention described above is in the shape of various parts, such as machine structural parts. Here, the steel parts of the present invention are particularly preferably toothed parts such as gears, and in this case, it is preferable that the compound layer described above is formed at least on the surface layer of the tooth portion. The teeth of toothed parts such as gears are areas where there is contact accompanied by sliding, and are areas where excellent surface fatigue strength is required. If the compound layer and diffusion layer described above are formed on this tooth portion, it will lead to ensuring the durability of the toothed part.

[0058] Furthermore, even for steel parts that are not toothed, if there are areas where contact involving sliding occurs, the surface fatigue characteristics of these areas are important for ensuring the durability of the part. Therefore, by forming the aforementioned diffusion layer on such areas, the effect of improving surface fatigue characteristics can be obtained. Accordingly, the steel parts of the present invention are not limited to toothed parts.

[0059] [Manufacturing method] Next, the method for manufacturing steel and steel parts according to the present invention will be described. Figure 2 shows a typical manufacturing process for producing steel parts using steel (steel bars). Here, S1 is the manufacturing process for the steel bars (steel for soft nitriding) that serve as the raw material, S2 is the transport process, and S3 is the manufacturing process for the parts (soft nitrided parts, which include soft nitrided steel).

[0060] First, in the steel bar manufacturing process (S1), the steel ingot is hot-rolled and / or hot-forged to form steel bars, which are then shipped after quality inspection. After transportation (S2), in the soft nitriding parts finishing process (S3), the steel bars are cut to predetermined dimensions, hot-forged or cold-forged, and then, if necessary, cut to the desired shape (e.g., gear shape or shaft shape) by drilling, turning, etc. Finally, they undergo soft nitriding treatment to become products (steel parts).

[0061] In addition, hot-rolled material may be shaped into the desired form by machining processes such as turning or drilling, and then subjected to soft nitriding to produce the final product (steel part). In the case of hot forging, cold straightening may be performed after hot forging. Furthermore, the final product may be coated with paint, plating, or other finishes.

[0062] Here, it is desirable to perform soft nitriding at 520-550°C for more than 2 hours, and then continue the soft nitriding process while increasing the temperature by 20-30°C / h until it reaches 560-590°C, after which it is desirable to perform soft nitriding for another hour or more. By performing soft nitriding in this gradient manner, N and C can be diffused to deeper regions, and carbonitrides can be formed in deeper regions.

[0063] In soft nitriding, N and C are simultaneously introduced into the steel to form a nitride compound layer in which C is dissolved, and then N is diffused into the base metal to form a diffusion layer. Therefore, soft nitriding should be performed in a mixed atmosphere of nitrogenous gases such as NH3 and N2, and carburizing gases such as CO2 and CO, for example, an atmosphere of NH3:N2:CO2 = 50:45:5.

[0064] As a result of the above soft nitriding treatment, a compound layer formed of Fe3N, Fe4N, or both is created on the surface with a thickness of 3 to 40 μm, and a diffusion layer with numerous carbonitrides precipitated is created inside the compound layer with a thickness of 400 μm or more from the surface of the compound layer. The structure of the diffusion layer and the structure inside the diffusion layer forms a matrix of ferrite, pearlite, or bainite.

[0065] By using the above method, it is possible to produce a structure having the desired carbonitride material at high density as described above. Note that the soft nitriding treatment is not limited to the above method; any method capable of forming the diffusion layer structure defined in this invention is acceptable. The steel of the present invention or a part made from this steel is obtained through the above manufacturing process. [Examples]

[0066] The following describes specific embodiments of the present invention. Steel (steel grades 1-60) with the compositions shown in Table 1 were cast into slabs with a cross-section of 300 mm x 400 mm using a continuous casting machine. After soaking at 1250°C for 30 minutes, these slabs were hot-rolled into rectangular steel billets with sides of 140 mm. These steel billets were further hot-rolled to produce 80 mmφ steel bars (as-hot-rolled material). These steel bars were held at 1200°C for 1 hour and then hot-forged to produce smaller diameter 35 mmφ steel bars.

[0067] [Table 1] TIFF0007878376000003.tif197170

[0068] Furthermore, roller pitting test specimens, as shown in Figure 3, were taken from the hot-forged material (steel bar) described above, parallel to the longitudinal direction (axial direction), and these specimens were subjected to soft nitriding treatment. The soft nitriding temperature and soft nitriding time were adjusted as shown in Table 2 to obtain the desired diffusion layer structure.

[0069] [Table 2] TIFF0007878376000005.tif210170

[0070] The number density of carbonitrides and the surface fatigue characteristics of the obtained soft-nitrided material were measured at 50 μm, 200 μm, and 400 μm from the surface as follows.

[0071] Specifically, the number density of carbonitrides was measured using a TEM to capture three fields of view at a magnification of 640,000x, and calculated according to the method described above. Furthermore, consistency was determined by using HAADF-STEM images obtained with {001} plane incidence on the matrix ferrite of the soft-nitrided material described above. If ε was 10% or less, it was judged as consistent (○). If ε was greater than 10%, it was judged as inconsistent (×).

[0072] The concentrations of the metal elements constituting the carbonitrides were calculated according to the method described above.

[0073] For surface fatigue characteristics evaluation, the fatigue limit strength was measured by creating an S / N diagram using a Nikko Create RPT-402 on roller pitting test specimens that had undergone soft nitriding treatment. The fatigue limit strength was 10 for N=2 or greater. 7 The maximum stress during the test, exceeding one cycle, was defined as the maximum stress achieved. A fatigue limit strength of 2600 MPa or higher was considered good (○), while a fatigue limit strength of less than 2600 MPa was considered poor (×). The roller pitting test conditions were a slip ratio of 40%, using gear oil (BESCO transaxle) as the lubricant, and an oil temperature of 80°C. The rotational speed during the test was 2000 rpm. For the large rollers in contact with the transfer surface, hardened and tempered SUJ2 rollers with a crowning radius of 150 mm were used. The evaluation results are shown in Table 3.

[0074] [Table 3] TIFF0007878376000007.tif207170

Claims

1. From the surface inward, it has a nitride compound layer and a nitrogen diffusion layer, The nitrogen diffusion layer has carbonitrides that match the matrix ferrite, and the number density of carbonitrides with a longitudinal length of 3 nm to 20 nm is 1.00 × 10⁻¹⁶ at a position 50 μm inward from the surface. ２３ / m ３ In summary, at a position 200 μm from the surface inward, the measurement is 0.30 × 10⁻⁶. ２３ / m ３ In summary, at a distance of 400 μm from the surface inward, the measurement is 0.10 × 10 ２３ / m ３ That's all. The portion excluding the nitride compound layer and the nitrogen diffusion layer is, in mass%, C: 0.010% or more and 0.200% or less, Si: 1.00% or less, Mn: 0.50% or more and 3.00% or less, P: 0.020% or less, S: 0.002% or more and 0.060% or less, Cr: 0.30% to 3.00% and V: 0.02% or more and 0.80% or less A steel having a component composition that contains [a certain substance], with the remainder being Fe and impurities.

2. The aforementioned component composition is further expressed in mass%, Nb: 0.003% or more and 0.150% or less, Al: 0.01% or more and 0.20% or less, Ti: 0.01% or more and 0.10% or less and Mo: 0.005% or more and 0.400% or less The steel according to claim 1, which contains one or more of the following.

3. The steel according to claim 1 or 2, wherein the carbonitride contains Cr and V as metallic elements that combine with carbon and nitrogen, and the concentration of (V + Cr) in the metallic elements is 50 at% or more.

4. A steel component made of the steel described in claim 1 or 2.

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