Steel product and method for modifying steel product

Abstract

A method for modifying a steel product according to the present invention comprises: a nitriding step for performing a nitriding treatment on a steel material and forming, on the surface of the steel material, a compound layer L1 in which iron and nitrogen are combined, and a diffusion layer L2 in which an alloy nitride is precipitated; and a compression step for applying a compressive load to the surface of the nitrided steel material.

Description

Steel products and methods for modifying steel products

[0001] The present invention relates to a steel product and a method for modifying the same.

[0002] Nitriding has been known as a surface modification technique for steel materials (see, for example, Patent Document 1). Nitriding forms a compound layer on the surface of a sample. A porous layer with numerous cavities is formed on the surface of the compound layer due to desorption of nitrogen gas. It has been found that the cavities in the porous layer act as defects and reduce fatigue strength, including pitting strength.

[0003] Japanese Patent Application Publication No. 7-300662

[0004] The present invention has been made in view of the above circumstances, and has as its object to improve the fatigue strength of nitrided materials.

[0005] One embodiment of the present invention is a steel product having, on its surface, a compound layer in which iron and nitride are combined, and a diffusion layer in which alloy nitrides are precipitated, wherein the compound layer has a porous layer in its surface layer portion having a plurality of cavities, and the area ratio of the cavities to the compound layer in a depth direction cross section is 3.0% or less.

[0006] One embodiment of the present invention is a method for modifying a steel product, comprising: a nitriding step of nitriding a steel material to form a compound layer including a porous layer having a plurality of cavities on a surface of the steel material; and a compressing step of applying a compressive load to the surface of the nitrided steel material.

[0007] One embodiment of the present invention is a steel product having, on its surface, a compound layer in which iron and nitrogen are combined and a diffusion layer in which alloy nitrides are precipitated, and the following formula (1) is satisfied in the diffusion layer: HV > 2 / 3 × τ 0°max ... (1) where τ 0°max : Maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface HV: τ 0°max Vickers hardness at the depth where the

[0008] One embodiment of the present invention is a method for modifying a steel product, comprising: a nitriding step of nitriding a steel material to form a compound layer of iron and nitrogen and a diffusion layer in which alloy nitrides are precipitated on the surface of the steel material; and a compression step of applying a compressive load to the surface of the nitrided steel material, wherein the compressive load is applied so that the following equation (1) is satisfied in the diffusion layer: HV > 2 / 3 × τ 0°max ... (1) where τ 0°max HV: Maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface. τ 0°max Vickers hardness at the depth where the

[0009] According to the present invention, the fatigue strength of a nitrided material can be improved.

[0010] 10 is a flowchart showing the steps of a method for manufacturing a steel product according to an embodiment. FIG. 10 is a structural photograph of the cross section of the outermost surface of a steel material after nitriding treatment. FIG. 11 is a diagram showing evaluation results of an example. FIG. 12 is a diagram showing the thickness in the depth direction of the compound layer and porous layer of an example. FIG. 13 is a diagram showing a structural photograph of the cross section of a sample taken with an SEM (electron microscope) of an example. FIG. 14 is a diagram for explaining a roller pitting test. FIG. 15 is a diagram showing evaluation results of pitting resistance properties of an example. FIG. 15 is a cross-sectional view showing an eccentric oscillating reducer of an application example of the present invention. FIG. 16 is a diagram showing evaluation results of time strength of a modified example. FIG. 17 is an S-N diagram comparing the test results of the example of FIG. 9 with those of a carburized material and a nitrided material. FIG. 18 is a graph showing the relationship between Vickers hardness and nitriding time when SCM440 and 31CrMoV9 are gas nitrided at 530°C. FIG. 19 is a chemical composition table of SCM440 and 31CrMoV9.

[0011] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0012] [Method for manufacturing steel products] In the method for manufacturing steel products according to this embodiment, the surface of the steel material is hardened by a surface modification method including nitriding treatment. This aims to improve fatigue strength including pitting strength. In the following description, the steel product (steel material) to be treated may be referred to as a "workpiece."

[0013] Fig. 1 is a flowchart showing the steps of the method for manufacturing a steel product according to this embodiment, and Fig. 2 is a photograph of the structure of the outermost surface cross section of a steel product after nitriding. As shown in Fig. 1, in the method for manufacturing a steel product according to this embodiment, in the case of a workpiece such as a gear member, the workpiece is first roughly machined until a predetermined finishing allowance is left on the tooth flank (step S1). Next, the tooth flank of the workpiece is finished using, for example, a polishing machine (step S2). Note that required heat treatments, etc. may be performed before, after, or between steps S1 and S2.

[0014] Next, the surface of the workpiece (e.g., the tooth surface) is subjected to nitriding (nitriding step) (step S3). Nitriding is a surface hardening method in which a steel workpiece is heated and nitrogen atoms diffuse and penetrate from the surface into the interior. Specifically, the workpiece is exposed to an atmosphere containing ammonia or nitrogen and heated at a predetermined temperature for several hours. As a result, as shown in FIG. 2, a compound layer L1 in which iron and nitrogen combine is formed on the outermost surface of the workpiece, and a diffusion layer L2 in which alloy nitrides precipitate in a structure in which iron and nitrogen form a solid solution is formed deeper (inside the material). Here, alloy nitrides are nitrides in which nitrogen is combined with elements added to the steel material. The added elements are, for example, Cr, Mn, and Si, but other elements may also be used. The compound layer L1 includes a porous layer Lp having multiple cavities (pores) H formed in its surface layer and a core layer Lc deeper than the porous layer Lp.

[0015] Next, a compression process is performed in which a compressive load is applied to the surface (e.g., tooth surface) of the workpiece (step S4). This closes the cavities H in the porous layer Lp of the workpiece (compression process). Here, "closing" refers to reducing the size of at least some of the cavities H in the porous layer Lp. Unlike conventional compression processes, the compression process does not actively peel (destroy) the porous layer Lp, but rather applies a compressive load to the surface of the workpiece to close the cavities H in the porous layer Lp. This prevents adverse effects caused by cavities in the porous layer Lp while retaining the properties of the compound layer. Specific compression methods include, but are not limited to, roll loading, shot peening, and HIP (hot isostatic pressing). Roll loading is a method in which a load roller is pressed against a cylindrical workpiece at a predetermined load and rotated. For example, roll loading may be performed using a roller pitching tester (see FIG. 6) described below. In addition, a break-in operation may be performed in which a load is applied to the tooth surface of the workpiece.

[0016] This closes the cavities H in the compound layer L1 formed on the surface of the workpiece by the nitriding treatment. This improves fatigue strength including pitting resistance. This is expected to increase the strength of sliding parts, for example, as a surface hardening treatment instead of carburizing.

[0017] [Evaluation of Surface Modification] The present inventors have found that a good modification effect can be obtained by setting the area ratio of the voids H in the compound layer L1 in the depth direction cross section to 3.0% or less. Hereinafter, examples in which the surface modification effect of the above-mentioned manufacturing method (modification method) for steel products will be described.

[0018] <Evaluation Details> Here, samples were prepared for three nitrided materials (nitriding examples 1 to 3) under different modification conditions, and the "hardness ratio" and "pore ratio" of each sample were evaluated. The modification conditions refer to the compressive load (surface pressure) conditions in the compression treatment in step S4 above. Nitriding examples 1 and 2 were evaluated under three load conditions A to C (A < B < C), and Nitriding example 3 was evaluated under one load condition, C (see Figure 3). The "hardness ratio" was evaluated as the ratio of indentation hardness (hardness) between the porous layer Lp and the core layer Lc. The measurement value of the porous layer Lp was calculated by averaging measurements taken at multiple points (three or more) within a range of 5 μm from the surface of the depth direction cross section (a cross section approximately perpendicular to the surface). The measurement value of the core layer Lc was also calculated by averaging measurements taken at multiple points (three or more) within a predetermined depth range. The indentation hardness measurement was performed using a nanoindenter, with an indentation size of 1 μm. 2 The measurement was performed at 2 mN so as to obtain the following. The "pore ratio" was evaluated as the area ratio of the voids (pores) H in the compound layer L1 in the cross section of the sample in the depth direction. The areas of the compound layer L1 and the voids H were calculated by image analysis from SEM (electron microscope) images of the cross section of each sample in the depth direction. In the image analysis, the area ratio was calculated within 0.01 μm 2 The area ratio was calculated for the above-mentioned cavity portions.

[0019] <Evaluation Results> The evaluation results are shown in Figure 3, the depth thicknesses of the compound layer L1 and porous layer Lp are shown in Figure 4, and a SEM micrograph of the sample cross section is shown in Figure 5. The following can be concluded from Figures 3 to 5. Under all modification conditions, improvements in the closure of the cavities H and the pore ratio were confirmed compared to the initial state (state after nitriding). Generally, the higher the compressive load (surface pressure), the better the modification results. Generally, good results were obtained, with a pore ratio of 3.0% or less and a hardness ratio of approximately 1.0 (e.g., within the range of 1.0 ± 0.1). The depth thickness of the compound layer L1 is preferably within the range of 10 to 20 μm. In other words, maintaining a depth thickness of the compound layer L1 of 10 μm or more by performing a compression treatment without actively peeling off the porous layer Lp improves pitting resistance. The depth thickness of the porous layer Lp is preferably within the range of 3 to 10 μm. In other words, the pitting resistance is improved by maintaining the thickness of the porous layer L1 in the depth direction to 3 μm or more by performing a compression treatment without actively peeling off the porous layer Lp. The thickness ratio of the porous layer Lp in the depth direction to the compound layer L1 is preferably less than 0.5.

[0020] [Evaluation of Pitting Resistance Property] Next, an evaluation test for confirming the change in pitting resistance property due to the above-described manufacturing method of the steel material will be described.

[0021] <Evaluation Details> Using samples under modification conditions A and C of Nitriding Example 2 in the above example and in their initial state, the pitting resistance characteristics of each were evaluated by a roller pitting fatigue test. In the roller pitting test, as shown in Figure 6, a large roller R2 was pressed against a small roller R1 formed from a workpiece (sample), and they were rotated at a predetermined slip ratio, causing pitting on the small roller R1. The main test conditions are as follows: Slip ratio: -40% Small roller diameter: φ26 mm Small roller rotation speed: 1500 rpm Large roller diameter: φ130 mm Large roller test piece: SUJ2 (hardened abrasive material) Large roller crowning amount: 300R Test surface pressure: 3.02 GPa (fatigue limit surface pressure of the sample: 2.96 GPa)

[0022] <Evaluation Results> The evaluation results are shown in Fig. 7. As shown in this figure, under the modification conditions of Nitriding Example 1, the pitting life was 6.9 × 10 ４ On the other hand, under the modification conditions of Nitriding Example 2, the pore ratio was 3.0% or less, and the pitting life was reduced to 1.0 × 10 7 It was confirmed that the pitting life was significantly improved. In other words, the closure of the cavity H can improve the pitting resistance characteristics.

[0023] [Modification of the manufacturing method of steel products] In the compression treatment (compression step) of step S4 described above, it is preferable to increase the hardness of the diffusion layer L2 (and the base material) by work hardening so that the following formula (1) is satisfied: HV > 2 / 3 × τ 0°max ... (1) where τ 0°max is the maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface, and HV is τ 0°max is the Vickers hardness at the depth where the

[0024] This increases the hardness of the diffusion layer L2 (and the base material) through work hardening, thereby improving the long-term strength of the workpiece. That is, according to this modification, not only is the fatigue strength (fatigue limit) improved, but the long-term strength can also be improved.

[0025] Here, HV and τ in equation (1) 0°ma The relationship of x is based on the relationship between hardness and strength in steel materials. That is, it is generally known that in steel materials, the Vickers hardness (HV) and tensile strength σ [MPa] are related by the following formula (2): σ = 3 × HV (2) Furthermore, the tensile strength σ and shear strength τ are roughly related by the following formula (3): σ = 2 × τ (3) From formulas (2) and (3), the relationship between Vickers hardness and shear strength (maximum shear stress) can be obtained by the following formula (4): HV = 2 / 3 × τ 　...(4) Therefore, by work-hardening the diffusion layer L2 so as to satisfy the above formula (1), it is possible to suppress destruction of the diffusion layer L2 due to shear stress, and to expect improvement in long-term strength.

[0026] [Evaluation of Strength over Time] Next, an evaluation test will be described in which changes in strength over time due to the modified manufacturing method of the steel material described above were confirmed.

[0027] <Evaluation Details> Samples of the example treated with a compressive load (2.5 GPa) for the purpose of work hardening after nitriding, a sample as-nitrided (Comparative Example 1), and a sample treated with a compressive load (2.0 GPa) after nitriding that did not achieve sufficient work hardening and did not satisfy formula (1) (Comparative Example 2) were prepared, and the time strength (pitting life) of each was evaluated by a roller pitting fatigue test. In the roller pitting fatigue test, the test surface pressure was set to 3.2 GPa, and the other conditions were the same as those for the above-mentioned pitting resistance property evaluation.

[0028] <Evaluation Results> The evaluation results are shown in Figure 9. The maximum horizontal shear stress τ 0°max is 809 MPa, and τ 0°max The depth position D at which max 9, the pitting life of Comparative Example 2, which was compressed to a range that did not satisfy Equation (1), was longer than that of Comparative Example 1, which was not compressed, but the improvement was only slight. In contrast, the example in which the diffusion layer L2 was work-hardened so that Equation (1) was satisfied showed a significant improvement in the pitting life compared to Comparative Example 2.

[0029] The S-N diagram comparing the test results of the above example with those of carburized and nitrided materials is shown in Figure 10. As shown in this figure, the example in which the diffusion layer L2 was work-hardened exhibited significantly improved long-term strength compared to the nitrided material without compression treatment.

[0030] [Regarding Nitriding Time] Because nitriding produces a shallower hardened layer than carburizing, simply deepening the hardened layer requires either a longer treatment time or the use of steel containing more expensive alloying elements, resulting in increased costs. SCM440 and 31CrMoV9 are known materials commonly used for nitriding. 31CrMoV9 is a European DIN-standard material and is a nitrided steel containing more expensive alloying elements than SCM440. Figure 12 shows the chemical compositions of SCM440 and 31CrMoV9. Figure 11 is a graph showing the relationship between Vickers hardness and nitriding time when SCM440 and 31CrMoV9 are gas-nitrided at 530°C. The Vickers hardness on the vertical axis of Figure 11 is measured at a maximum horizontal shear stress depth of 0.3 mm, the same conditions as in the example shown in Figure 9. In the example shown in Fig. 11, in order to satisfy formula (1) at a depth position under the same conditions as in the example of Fig. 9, nitriding for nearly 20 hours is required even for 31CrMoV9, and an even longer nitriding time is required for SCM440. In this regard, in this embodiment, the nitriding can be performed in a short time by work hardening through compression treatment, and as a result, a nitrided material with excellent long-term strength can be obtained at low cost.

[0031] [Others] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. For example, in the above embodiments, a cylindrical (columnar) workpiece was described as an example, but the shape of the steel material (steel product) that is the workpiece is not particularly limited, and may be, for example, flat. Furthermore, the type of steel material is not particularly limited as long as it is a steel-based material that undergoes surface hardening by nitriding.

[0032] Furthermore, the present invention is not particularly limited, but can be suitably applied to components (e.g., eccentric shafts, bearings, etc.) that are subject to sliding fatigue under high surface pressure. For example, as shown in FIG. 8 , the present invention can be suitably applied to an eccentric shaft 41, a curved plate (externally toothed gear) 42, and an internal gear (a housing 45 having an outer pin 44 and an outer pin groove 45a) of a small-sized precision reducer (the illustration shows an example of an eccentric oscillating reducer). Furthermore, the present invention is not limited to small-sized precision reducers, and may be applied to any device, such as an injection molding machine or an excavator. Furthermore, the details shown in the above embodiment can be modified as appropriate without departing from the spirit of the invention.

[0033] As described above, the present invention is useful for improving the fatigue strength of nitrided materials.

[0034] L1 Compound layer L2 Diffusion layer Lc Core layer Lp Porous layer H Hollow part (pore) R1 Small roller R2 Large roller τ 0°max Maximum horizontal shear stress HV τ 0°max Vickers hardness at the depth where the

Claims

1. A steel product having on its surface a compound layer in which iron and nitrogen are combined and a diffusion layer in which alloy nitrides are precipitated, said compound layer having a porous layer at its surface having a plurality of cavities, and the area ratio of the cavities in said compound layer in a depth direction cross section is 3.0% or less.

2. The steel product according to claim 1, wherein the compound layer has a thickness in the depth direction of 10 to 20 μm.

3. The steel product according to claim 1, wherein the porous layer has a thickness in the depth direction of 3 to 10 μm.

4. The steel product according to claim 1, wherein the porous layer has a thickness ratio in the depth direction to the compound layer of less than 0.

5.

5. The steel product according to claim 1, wherein the compound layer includes a core layer deeper than the porous layer, and the ratio of indentation hardness of the core layer to that of the porous layer is within 1.0±0.

1.

6. The steel product according to claim 1, wherein the following formula (1) is satisfied in the diffusion layer: HV > 2 / 3 × τ 0°max　 ... (1) where τ 0°max HV: Maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface τ 0°max Vickers hardness at the depth where 7. A steel product having a surface layer of a compound of iron and nitrogen and a diffusion layer in which alloy nitrides are precipitated, and the following formula (1) is satisfied in the diffusion layer. HV > 2 / 3 x τ 0°max ... (1) where τ 0°max HV: Maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface τ 0°max Vickers hardness at the depth where 8. A method for modifying a steel product, comprising: a nitriding step of nitriding a steel material to form a compound layer including a porous layer having a plurality of cavities on a surface of the steel material; and a compression step of applying a compressive load to the surface of the nitrided steel material to close the cavities in the porous layer.

9. A method for modifying a steel product, comprising: a nitriding step of nitriding a steel material to form a compound layer of iron and nitrogen and a diffusion layer in which alloy nitrides are precipitated on the surface of the steel material; and a compression step of applying a compressive load to the surface of the nitrided steel material, wherein the compression step applies the compressive load so that the following formula (1) is satisfied in the diffusion layer. HV > 2 / 3 × τ 0°max ... (1) where τ 0°max HV: Maximum horizontal shear stress [MPa] that occurs inside when a load is applied to the surface τ 0°max Vickers hardness at the depth where

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