Steel component

Abstract

To provide a steel component that exhibits excellent surface fatigue characteristic.SOLUTION: A steel component includes a nitride compound layer and a nitrogen diffusion layer from the surface to the inside thereof in this order. The nitrogen diffusion layer includes a carbonitride matching with a base phase ferrite, where, of the carbonitride, a carbonitride having a longitudinal length of 3 nm or more and 20 nm or less has the number densities of 1.0×1023 / m3 or more in a position of 50 μm inside from the surface, 0.3×1023 / m3 or more in a position of 200 μm inside from the surface, and 0.1×1023 / m3 or more in a position of 400 μm inside from the surface.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to steel parts, and particularly to steel parts suitable for use in parts such as automobiles and construction machinery, which have a diffusion layer with a large number of fine carbonitrides due to soft nitriding treatment and excellent surface fatigue characteristics.

Background Art

[0002] Mechanical structural parts such as automobile gears are required to have excellent fatigue characteristics such as surface fatigue characteristics and bending fatigue characteristics, so surface hardening treatment is usually performed. As surface hardening treatments, carburizing treatment, high-frequency quenching treatment, nitriding treatment, etc. are well known.

[0003] Among these, carburizing treatment involves infiltrating and diffusing C in the high-temperature austenite region, so a deep hardened layer can be obtained, which is effective in improving fatigue characteristics. However, since heat treatment distortion occurs due to carburizing treatment, its application has been difficult for parts that require strict dimensional accuracy from the viewpoints of quietness, etc.

[0004] In addition, high-frequency quenching treatment is a treatment for quenching the surface layer by high-frequency induction heating, so heat treatment distortion also occurs, and there are problems in terms of dimensional accuracy similar to carburizing treatment.

[0005] On the other hand, nitriding treatment is a treatment for infiltrating and diffusing N in a relatively low temperature range below the Ac1 transformation point to increase the surface hardness, so the heat treatment distortion as described above is small. However, there are problems that the treatment time is as long as 50 to 100 hours, and it is necessary to remove the brittle compound layer on the surface after the treatment.

[0006] Therefore, so-called soft nitriding treatment, which shortens the treatment time at a treatment temperature comparable to nitriding treatment, has been developed and has been widely popularized in recent years for parts such as those for mechanical structures. This soft nitriding treatment involves simultaneously infiltrating N and C in the temperature range of 500 to 600 °C to form a nitrided compound layer with C dissolved in the outermost layer, and further diffusing N into the base metal to form a nitrogen diffusion layer to harden the surface layer. Compared with conventional nitriding treatment, the treatment time can be reduced to less than half.

[0007] However, in the case of the carburizing treatment described above, it is possible to increase the core hardness by quenching and hardening, whereas the soft nitriding treatment is carried out at a temperature below the transformation point of steel, so the core hardness does not increase, and there is a problem that the soft nitrided material is inferior in fatigue characteristics compared with the carburized material.

[0008] Therefore, in order to improve the fatigue characteristics of the soft nitrided material, normalizing and tempering treatments are usually carried out before the soft nitriding treatment to increase the core hardness. However, the obtained fatigue characteristics are hardly sufficient, the manufacturing cost increases, and a decrease in machinability is also inevitable.

[0009] To solve such problems, Patent Document 1 proposes a steel for soft nitriding that enables high bending fatigue characteristics to be obtained after soft nitriding treatment by containing Ni, Cu, Al, Cr, Ti, etc. in steel. That is, this steel is age-hardened with Ni—Al, Ni—Ti-based intermetallic compounds or Cu compounds for the core part by soft nitriding treatment, while for the surface layer part, nitrides and carbides such as Cr, Al, Ti, etc. are precipitated and hardened in the nitrided layer to improve the bending fatigue characteristics.

[0010] Also, Patent Document 2 proposes a steel for soft nitriding that can obtain excellent bending fatigue characteristics after soft nitriding treatment by hot forging and stretching a steel containing 0.5 to 2% Cu, air-cooling it to form a ferrite-based structure with Cu dissolved, precipitating and hardening Cu during soft nitriding treatment at 580 °C for 120 minutes, and further using precipitation hardening of Ti, V, and Nb carbonitrides in combination.

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

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

[0013] Patent Document 5 has proposed a steel component having excellent surface fatigue characteristics by appropriately controlling the compound layer morphology and the hardness of the diffusion layer.

Prior Art Documents

Patent Documents

[0014]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Disclosure of the Invention

Problems to be Solved by the Invention

[0015] However, although the soft nitrided steels described in Patent Documents 1 to 3 are excellent in bending fatigue strength, the surface fatigue strength is not considered. When sliding contact occurs in a component, cracks will occur at the contact portion if the surface fatigue strength is insufficient. The technique described in Patent Document 4 considers the hardness and carbonitrides at a depth of 50 μm from the surface layer, but does not consider the characteristics deeper inside. Furthermore, although the technique described in Patent Document 5 conducts an evaluation of the surface fatigue strength at 2600 MPa, considering the application to members that require a higher surface fatigue strength, a steel material showing an even more excellent surface fatigue strength is desirable.

[0016] The present invention significantly solves the above problems, and particularly aims to provide a steel part having excellent surface fatigue characteristics.

Means for Solving the Problems

[0017] In order to solve the above problems, the inventors intensively studied the optimal microstructure in the nitrogen diffusion layer. As a result, it was found that controlling the number density and morphology of carbonitrides with a longitudinal length of 3 nm or more and 20 nm or less in the nitrogen diffusion layer is effective in improving the surface fatigue characteristics.

[0018] In addition, when the steel part is subjected to a sliding resistance, a shear stress is applied by the vertical resistance. Since this is maximized at a depth of about 400 μm from the outermost surface, in order to exhibit excellent surface fatigue characteristics, not only the surface layer but also the control of the internal structure is important.

[0019] Based on the above findings, the present invention was completed after further study, and the gist configuration of the present invention is as follows.

[0020] 1. Having a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, The nitrogen diffusion layer has carbonitrides that match the matrix ferrite, and the number density of carbonitrides with a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.0×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.3×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.1×10 23 / m 3 or more at a position 400 μm from the surface to the inside. A steel part.

[0021] 2. The steel part according to item 1 above, wherein the steel part is a toothed part and has the nitride compound layer and the nitrogen diffusion layer at least in the tooth portion.

Brief Description of the Drawings

[0022]

Figure 1

Figure 2

Figure 3

Summary of the Invention

Effects of the Invention

[0023] According to the present invention, it is possible to provide a steel component in which the microstructure in the diffusion layer is controlled by soft nitriding treatment to improve the surface fatigue characteristics. Therefore, the steel of the present invention is extremely useful as a material for components for mechanical structures such as automobiles.

Modes for Carrying Out the Invention

[0024] Hereinafter, the present invention will be specifically described. As described above, in order to exhibit excellent surface fatigue characteristics in a steel component, it is important to control not only the surface layer but also the internal structure of the steel component. 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 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.

[0025] That is, in a steel component 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 matching the matrix ferrite, the number density of carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.0×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.3×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.1×10 23 / m 3 or more at a position 400 μm from the surface to the inside, which is important.

[0026] [Carbonitride in the nitrogen diffusion layer] First, the carbonitride in the nitrogen diffusion layer will be described. In the present invention, the carbonitride needs to be coherent with the matrix ferrite. When the carbonitride is coherent with the matrix ferrite, by generating a coherency strain, the resistance to dislocations increases, and it becomes possible to improve the fatigue characteristics.

[0027] Here, being coherent 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 is coherent with (011) of the NaCl - type carbonitride. In order to generate a sufficient coherency 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 carbonitride coherent with the matrix ferrite is a carbonitride in which the said ε is 10% or less. For a carbonitride with ε exceeding 10%, the inhibitory effect on dislocation movement becomes small. ε = |a0 - ap| / ap × 100 (%) …(1) Here, a0 represents the interplanar spacing of ferrite, and ap represents the interplanar spacing of the carbonitride.

[0028] There is no particular limitation on the method for measuring the above - mentioned interplanar spacing, but it can be confirmed by directly observing the interface region by high - resolution electron microscope observation method. For example, FIG. 1 shows a HAADF - STEM (High Angle Annular Dark - Field Scanning Transmission Electron Microscopy) image obtained by <001> incidence of the carbonitride. As shown in FIG. 1, it is possible to directly measure the interplanar spacing ap of the carbonitride from this image.

[0029] The hardness of the above - mentioned carbonitride preferably has a value of 1000 HV or more in order to have sufficient shear strength. If it is a carbonitride precipitated by soft nitriding treatment, this hardness can be satisfied.

[0030] In order to improve the surface fatigue characteristics of steel parts, the number density of carbonitrides (hereinafter also referred to as compatible-diameter carbonitrides) having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides compatible with the matrix ferrite is 1.0×10 23 / m 3 or more at a position 50 μm from the surface to the inside, 0.3×10 23 / m 3 or more at a position 200 μm from the surface to the inside, and 0.1×10 23 / m 3 or more at a position 400 μm from the surface to the inside is important. If the above conditions are not satisfied, excellent surface fatigue characteristics cannot be obtained because dislocation movement cannot be sufficiently inhibited.

[0031] Here, regarding the carbonitrides, the reason for defining the carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less is as follows. First, carbonitrides having a longitudinal length of less than 3 nm are difficult to exhibit a sufficient dislocation movement inhibition effect even if they are compatible with the matrix ferrite. In addition, when the length of the carbonitride exceeds 20 nm, the compatibility with the matrix ferrite deteriorates and the resistance to dislocation movement becomes small.

[0032] Furthermore, the reason for defining the number density of the above-mentioned compatible-diameter carbonitrides at positions 50 μm, 200 μm, and 400 μm from the surface to the inside is as described above for the 400 μm position. On the other hand, at the 50 μm position and the 200 μm position, compressive stress becomes a problem. That is, the compressive stress is maximum on the surface side and gradually decreases. In order to have a structure that can withstand this compressive stress, it is necessary to control the microstructure at depth positions of 50 and 200 μm step by step from the surface. By defining the number density of the compatible-diameter carbonitrides at these two depth positions, it is possible to withstand the gradually changing compressive stress. And the reasons for limiting the number density of the compatible-diameter carbonitrides at each position are as follows.

[0033] [At a position 50 μm from the surface to the inside, 1.0×10 23 / m 3 or more] The number density of the coherent carbide nitride at the position of 50 μm is 1.0×10 23 / m 3 or more to sufficiently ensure the resistance force against the above-described dislocation movement. Although the upper limit does not particularly need to be limited, from the viewpoint of alloy cost, it is preferably 4.0×10 23 / m 3 or less.

[0034] [At the position 200 μm from the surface to the inside, 0.3×10 23 / m 3 or more] The number density of the coherent carbide nitride at the position of 200 μm is 0.3×10 23 / m 3 or more to similarly sufficiently ensure the resistance force against dislocation movement. Although the upper limit does not particularly need to be limited, from the viewpoint of alloy cost, it is preferably 2.5×10 23 / m 3 or less.

[0035] [At the position 400 μm from the surface to the inside, 0.1×10 23 / m 3 or more] The number density of the coherent carbide nitride at the position of 400 μm is 0.1×10 23 / m 3 or more to similarly sufficiently ensure the resistance force against dislocation movement. Although the upper limit does not particularly need to be limited, from the viewpoint of alloy cost, it is preferably 0.6×10 23 / m 3 or less.

[0036] The carbide nitride that matches the above-described matrix ferrite precipitates on the {001} plane of the ferrite that is the matrix structure, satisfying the Baker-Nutting relationship. For this reason, the number density of the carbide nitride is calculated as follows. First, transmission electron microscope (TEM) observations were performed from the {001} plane of the parent-phase ferrite, 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 by electron energy loss spectroscopy (EELS), and the volume of the observation field was calculated. Thereafter, the number density was calculated by dividing the number of carbonitrides by the volume. Here, although the carbonitrides are precipitated on the (001), (010), and (100) planes, for example, when observed with (001) incidence, the carbonitrides on the (001) plane cannot be observed, so only two planes can be observed by TEM observation. For this reason, the number obtained by multiplying the number of carbonitrides observed by TEM by 1.5 was taken as the number of carbonitrides. The measurement of the film thickness was taken as the value obtained by multiplying the relative film thickness measured by EELS by the mean free path λ calculated from the formula of Iakoubovskii et al. JPEG2025112615000002.jpg14170 Here, E0: acceleration voltage, α: convergence angle, β: capture angle, ρ: sample density, F = (1 + E0 / 1022) / (1 + E0 / 511) 2 , θ E = 5.5ρ 0.3 / FE0, θ c = 20mrad was used.

[0037] Next, the steel structures of the compound layer and the diffusion layer of the steel of the present invention will be described. The compound layer and the diffusion layer are formed by 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, on the outermost surface of the steel, a compound layer is formed in which iron nitride (Fe3N or Fe4N) formed by the combination of Fe, which is the main component of the steel to be subjected to the soft nitriding treatment, and N is used as the parent phase, and other contained components are combined with nitrogen and carbon and precipitated as carbonitrides. Note that the compound layer is preferably formed to have a thickness ranging from 3 to 40 μm.

[0038] Furthermore, the diffusion layer is a layer in which nitrogen diffuses into the steel and the nitrogen concentration becomes higher than that before the soft nitriding treatment, and is formed adjacent to the inside of the compound layer. Since the above non-hardened portion is a portion where no nitrogen diffusion occurs, it has the component composition of the non-hardened portion as described above, while the compound layer and the diffusion layer have a component composition with a high N content with respect to the component composition of the non-hardened portion. Note that the structure of the diffusion layer and the non-hardened portion inside thereof forms a structure in which the matrix phase is ferrite or bainite. The diffusion layer is preferably formed to have a thickness of 400 to 1000 μm inside the compound layer.

[0039] The steel component according to the present invention has been described above. The steel component according to the present invention has the shape of various components, preferably components for mechanical structures, and more preferably toothed components such as gears.

[0040] Note that even if it is not a toothed component, for a steel component in which a sliding contact occurs at a site, since the surface fatigue characteristics of this site are important for ensuring the durability of the component, by forming the above-described diffusion layer on such a site, an effect of improving the surface fatigue characteristics can be obtained. Therefore, the steel component of the present invention is not limited to toothed components.

[0041] Next, a method for manufacturing the steel component of the present invention will be described.

[0042] FIG. 2 shows a typical manufacturing process for manufacturing a steel component using a steel for soft nitriding (bar steel). Here, S1 is a manufacturing process for a bar steel (steel for soft nitriding) as a raw material, S2 is a conveying process, and S3 is a manufacturing process for a component (soft nitrided component).

[0043] First, in the bar steel manufacturing process (S1), an ingot is hot-rolled and / or hot-forged into a bar steel, and after quality inspection, it is shipped. Then, after conveying (S2), in the soft nitrided component finishing process (S3), the bar steel is cut into a predetermined size, hot-forged or cold-forged, and if necessary, subjected to cutting processes such as drill piercing and turning to obtain a desired shape (for example, a gear shape or a shaft shape), and then soft nitriding treatment is performed to obtain a product (steel component).

[0044] In addition, the hot-rolled material may be directly finished into a desired shape by cutting processes such as turning or drill piercing, and then subjected to soft nitriding treatment to obtain a product (steel part). In the case of hot forging, cold straightening may be performed after hot forging. Also, in some cases, a coating treatment such as painting or plating is applied to the final product.

[0045] Here, for the soft nitriding treatment, it is desirable to perform the soft nitriding treatment at 520 - 550 °C for 2 hours or more, and then continue the soft nitriding treatment while raising the temperature at 20 - 30 °C / h. After reaching 560 - 590 °C, further perform the soft nitriding treatment for 1 hour or more. By performing such inclined soft nitriding treatment, N and C can be diffused to a deeper region, and carbonitrides can be formed up to a deeper region.

[0046] In the soft nitriding treatment, since N and C are simultaneously introduced into the steel to form a nitrided compound layer in which C is dissolved, and further N is diffused into the base metal to form a nitrogen diffusion layer, 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, may be used for the soft nitriding treatment.

[0047] By performing the above soft nitriding treatment, a compound layer formed from Fe3N or Fe4N, or both, is formed on the surface layer with a thickness ranging from 3 to 40 μm. On the inner side of the compound layer, a nitrogen diffusion layer in which a large number of carbonitrides are precipitated is formed with a thickness of 400 μm or more from the surface of the nitrided compound layer. The nitrogen diffusion layer and the tissue inside it have a matrix that forms a ferrite or pearlite or bainite structure. It is preferable to form the nitrogen diffusion layer with a thickness ranging from 400 to 1000 μm inside the nitrided compound layer.

[0048] By using the above method, it is possible to generate a tissue having the form of high-density carbonitrides as described above. The soft nitriding treatment does not necessarily have to be limited to the above method, and any treatment that can form the structure of the diffusion layer defined in the present invention is acceptable.

Examples

[0049] Hereinafter, embodiments of the present invention will be specifically described. Steels (Steel Grades A to K) having the compositions shown in Table 1 were made into slabs with a cross-section of 300 mm × 400 mm using a continuous casting machine. After soaking this slab at 1250°C for 30 minutes, it was hot-rolled into a steel slab with a rectangular cross-section having a side length of 140 mm. Further, this steel slab was hot-rolled into a round bar with a diameter of 80 mm (raw material as hot-rolled). After holding this round bar at 1200°C for 1 hour, hot forging was performed to obtain a smaller-diameter round bar with a diameter of 35 mm.

[0050]

Table 1

[0051] Furthermore, with respect to the above hot-forged material (round bar), a roller pitching test piece shown in Fig. 3 was sampled parallel to the longitudinal direction (axial direction), and this test piece was subjected to soft nitriding treatment. In order to obtain a desired diffusion layer structure, the soft nitriding temperature and soft nitriding time were adjusted as shown in Table 2.

[0052]

Table 2

[0053] For the obtained soft nitrided material, the number density of carbonitrides and the evaluation of surface fatigue characteristics were measured at 50 μm, 200 μm, and 400 μm from the surface.

[0054] That is, for the measurement of the number density of carbonitrides, three fields of view were photographed at a magnification of 640,000 times using TEM and calculated according to the above-described method. Also, for the conformity, ε obtained according to the above-described method using the HAADF-STEM image obtained by the incidence of the {001} plane of the matrix ferrite in the above soft nitrided material was determined to be in conformity (〇) when it was 10% or less. When ε exceeded 10%, it was determined to be non-conforming (×).

[0055] For the evaluation of surface fatigue characteristics, using the roller pitching test piece after soft nitriding treatment, the fatigue life was measured with a RPT-402 manufactured by Nikkokei. When the fatigue life was 1.0×10 6Those with a fatigue life of 1.0×10 6 or more cycles were considered to have a good fatigue life (〇), and those with less than 1.0×10 6 cycles were considered to have a poor fatigue life (×). The roller pitching test conditions were a maximum contact stress of 3000 MPa, a slip ratio of 40%, using gear oil (BESCO Transaxle) as the lubricating oil, and the test was conducted at an oil temperature of 80°C. The rotational speed during the test was 2000 rpm. For the large roller in contact with the transfer surface, a hardened and tempered product made of SUJ2 with a crowning R of 150 mm was used. The above evaluation results are shown in Table 3.

[0056]

Table 3

Claims

1. having a nitride compound layer and a nitrogen diffusion layer in order from the surface to the inside, The nitrogen diffusion layer has carbonitrides that match the matrix ferrite, and the number density of carbonitrides having a longitudinal length of 3 nm or more and 20 nm or less among the carbonitrides is 1.0×10 ２３ / m ３ or more at a position 50 μm from the surface to the inside, 0.3×10 ２３ / m ３ or more at a position 200 μm from the surface to the inside, and 0.1×10 ２３ / m ３ or more at a position 400 μm from the surface to the inside.

2. The steel component according to claim 1, wherein the steel component is a toothed component and has the nitride compound layer and the nitrogen diffusion layer at least in the tooth portion.

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