Nitrided component and method of manufacturing the same

Abstract

To provide a nitrided component that is superior in crack resistance to a conventional component which has a nitriding layer and a nitrogen compound layer formed while making a greenhouse effect gas emission amount less in a surface treatment on a ferrous material than when forming a carburization layer through a conventional gas carburization treatment, and a method of manufacturing the same.SOLUTION: A nitrided component uses a ferrous material as its base, and has a nitrided layer and a conformity layer in a surface layer region of the base, wherein the nitrided layer has a mean nitride concentration of 0.3 to 2 mass% up to a thickness of 30 μm from a surface of the nitrided component, and the conformity layer is formed outside the nitrided layer, and has a mean thickness of 5 to 20 μm and includes iron oxide.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a metal part subjected to nitriding treatment, and particularly to a part made of a steel material subjected to nitriding treatment and a method for manufacturing the same.

Background Art

[0002] Sliding / rolling parts such as gears and sprockets in a power transmission device are parts used in a harsh environment where they repeatedly receive mechanical loads, and wear resistance is one of the very important mechanical properties. Sliding / rolling parts usually have a steel material as a base material, and carburizing treatment is often performed as a surface treatment to ensure wear resistance.

[0003] Carburizing treatment for steel materials is currently mainly gas carburizing performed using a gas mainly composed of carbon dioxide (CO 2 ), hydrogen (H 2 ), methane (CH 4 ), etc. However, current gas carburizing treatment is inconvenient for reducing greenhouse gas emissions in recent years because it emits a large amount of greenhouse gases (for example, CO 2 gas). Therefore, an alternative surface treatment technology capable of reducing greenhouse gas emissions is required.

[0004] In response to such a demand, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2017-137547) discloses a nitriding treatment method for infiltrating and diffusing nitrogen into a workpiece made of a steel material, which includes performing induction heating on the workpiece at a frequency such that the current penetration depth becomes 2 mm or more, and spraying a nitriding gas onto the surface of the workpiece.

[0005] According to Patent Document 1, it is possible to suppress the decomposition of the nitriding gas on the surface of the workpiece while maintaining the diffusion rate of nitrogen from the nitriding gas into the inside of the workpiece, and as a result, it is possible to shorten the nitriding treatment time. Further, although not directly described in Patent Document 1, it is expected to reduce the amount of greenhouse gases emitted because it is not carburizing treatment.

[0006] Further, Patent Document 2 (WO 2021 / 181570 A1) discloses a component having a steel core part, a compound layer, and a nitrogen diffusion layer present between the steel core part and the compound layer, wherein the composition of the steel core part contains, in mass%, C: 0.05% to 0.60%, Si: 0.05% to 1.50%, Mn: 0.20% to 2.50%, P: 0.025% or less, S: 0.050% or less, Cr: 0.50% to 2.50%, V: 0.05% to 1.30%, Al: 0.050% or less, and N: 0.0250% or less, with the balance being Fe and impurities. In the composition of the steel core part, the contents of C, Mn, Cr, V, and Mo satisfy a predetermined formula. A gas soft nitriding treated component is disclosed, wherein the thickness of the compound layer is 3 to 20 μm, the compound layer contains more than 50% of the ε phase by area ratio, and the balance is the γ' phase, and in the region from the surface of the compound layer to a depth of 3 μm, the area ratio of voids is less than 12%.

[0007] According to Patent Document 2, it is said that a soft nitriding treated component excellent in rotational bending fatigue strength in addition to wear resistance can be obtained. Also, although not directly described in Patent Document 2, since it is a kind of nitriding treatment, it is expected to reduce the amount of greenhouse gas emissions compared to carburizing treatment.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] As described above, in recent years, there has been a strong demand for reducing greenhouse gas emissions in the manufacturing of industrial products. By changing the conventional carburizing treatment for steel materials to nitriding treatment or gas soft nitriding treatment (hereinafter collectively referred to as nitriding treatment) as disclosed in Patent Documents 1 and 2, it is expected that the amount of greenhouse gas emissions during surface treatment can be reduced.

[0010] On the other hand, the nitrided layer and nitrogen compound layer formed by the conventional nitriding treatment are more likely to have a greater hardness than the carburized layer and are advantageous from the viewpoint of wear resistance. However, since the diffusion coefficient of nitrogen in the steel material is smaller than that of carbon, the thickness of the formed layer is more likely to be thinner than that of the carburized layer. Therefore, in the mechanical repeated load on sliding / rolling parts, the conventional nitrided layer and nitrogen compound layer are likely to crack due to stress concentration when unexpected foreign matters are caught, and another problem occurs that the durability of the sliding / rolling parts decreases due to factors different from wear.

[0011] The present invention has been made to solve the above-mentioned requirements and problems, and its object is to provide a nitriding treatment part and a manufacturing method thereof that are superior in crack resistance to parts formed with a conventional nitrided layer or nitrogen compound layer while reducing the amount of greenhouse gas emissions in the surface treatment of steel materials compared to the formation of a carburized layer by the conventional gas carburizing treatment.

Means for Solving the Problems

[0012] (I) One aspect of the present invention is a nitriding treatment part having a steel material as a base material, The nitriding treatment part has a nitrided layer and an adapted layer in the surface layer region of the base material, In the nitrided layer, the average nitrogen concentration from the surface of the nitriding treatment part to a depth of 30 μm in the thickness direction is 0.3 mass% or more and 2 mass% or less, The adapted layer is formed on the outer layer side of the nitrided layer and is a layer having an average thickness of 5 μm or more and 20 μm or less and containing iron oxide. A nitriding treatment part is provided.

[0013] In the nitrided component (I) described above, the following improvements and modifications can be added while freely combining them as follows. (i) The nitrided layer has a region with a Vickers hardness of 680 Hv or more on the inner layer side than the conforming layer, and the conforming layer has a Vickers hardness of 500 Hv or less. (ii) The base material has a martensite structure.

[0014] (II) Another aspect of the present invention is a method for manufacturing the nitrided component described above, comprising: a base material preparation step of preparing the base material having a desired shape; a nitrided layer / conforming layer formation step of forming the nitrided layer and the conforming layer in the surface layer region of the base material; a quenching step of forming a martensite structure in the base material on which the nitrided layer and the conforming layer are formed, and the nitrided layer / conforming layer formation step includes a nitriding / oxidation heat treatment sub-step of heating the base material by a high-frequency induction heating method in a predetermined atmosphere to allow nitrogen to penetrate into the surface layer region of the base material and oxidize the iron component of the base material. A method for manufacturing a nitrided component, characterized by the above, is provided.

[0015] In the method for manufacturing the nitrided component (II) described above, the following improvements and modifications can be added while freely combining them as follows. (iii) In the predetermined atmosphere in the nitriding / oxidation heat treatment sub-step, the concentration of the nitriding source gas is 0.5% or more and 10% or less, the concentration of the oxidation source gas is 0.1% or more and 0.5% or less, and the balance consists of an inert gas. The heating by the high-frequency induction heating method is input-controlled so that the temperature of the surface of the base material becomes 650°C or more and 850°C or less. (iv) The quenching step includes heating by the high-frequency induction heating method, and the input is controlled so that the temperature of the surface of the base material becomes 800°C or more and 1100°C or less and is higher than the temperature in the nitriding / oxidation heat treatment sub-step.

Advantages of the Invention

[0016] According to the present invention, in the surface treatment of steel materials, it is possible to provide a nitrided part that is superior in crack resistance to parts with a conventional nitrided layer or nitrogen compound layer while reducing the greenhouse gas emissions compared to the formation of a carburized layer by a conventional gas carburizing treatment, and a method for manufacturing the same. Regarding problems, configurations, and effects other than those described above, they will be clarified by the description of the embodiments below.

Brief Description of the Drawings

[0017]

Figure 1

Figure 2

Figure 3

Figure 4

Modes for Carrying Out the Invention

[0018] Hereinafter, embodiments according to the present invention will be specifically described along the manufacturing procedure with reference to the drawings. However, the present invention is not limited to the embodiments taken up here, and it is possible to appropriately combine with known technologies or improve based on known technologies without departing from the technical idea of the invention.

[0019] [Method for Manufacturing Nitrided Parts] FIG. 1 is a process diagram showing an example of a method for manufacturing a nitrided part according to the present invention. As shown in FIG. 1, the method for manufacturing a nitrided part of the present invention generally includes a base material preparation step S1, a nitrided layer / adapted layer formation step S2, and a quenching step S3. This manufacturing method has the greatest feature in the nitrided layer / adapted layer formation step S2.

[0020] Hereinafter, each step will be described more specifically.

[0021] The base material preparation step S1 is a step of preparing a base material made of a steel material and having a desired shape. There is no particular limitation on the steel material used as the raw material, and steel materials that have been conventionally used as the raw materials for sliding / rolling parts can be appropriately used. For example, low alloy steel mainly composed of iron, containing 0.02 to 2.14% by mass of carbon and containing other alloy elements in a total amount of 5% by mass or less can be preferably used. Also, there is no particular limitation on the method of forming the raw material into a desired shape, and conventional methods can be appropriately used. For example, forging methods, powder metallurgy methods, etc. can be preferably used.

[0022] The nitrided layer and adapted layer formation step S2 is a step of forming a nitrided layer and an adapted layer in the surface layer region of the base material prepared in step S1, and has a nitriding and oxidation heat treatment sub-step S2a of heating the base material in a predetermined atmosphere to allow nitrogen to penetrate into the base material and oxidizing the iron component of the base material. The predetermined atmosphere preferably has a concentration of a nitriding source gas (for example, ammonia gas) of 0.5% or more and 10% or less, and a concentration of an oxidation source gas (for example, oxygen gas or water vapor) of 0.1% or more and 0.5% or less. The remainder of the atmosphere is preferably a gas that does not directly contribute to nitriding and oxidation (for example, an inert gas such as argon gas or nitrogen gas) from the viewpoint of the controllability of nitriding and oxidation.

[0023] There is no particular limitation on the method of heating the base material in the nitriding and oxidation heat treatment sub-step S2a, and conventional heating methods (for example, high-frequency induction heating method, resistance heating method, infrared heating method, gas combustion heating method) can be appropriately used. The heating is preferably input-controlled so that the temperature of the surface of the base material becomes 650°C or more and 850°C or less (regarding the surface temperature of the base material as the heating temperature). The heating temperature is more preferably 700°C or more and 800°C or less. Thereby, nitrogen can be made to penetrate into the surface layer region of the base material and the iron component of the base material can be oxidized to form a nitrided layer and an adapted layer.

[0024] The thickness of the nitrided layer, the nitrogen concentration in the nitrided layer, and the thickness of the bonding layer can be controlled by adjusting the heating temperature and the heating holding time. In the present invention, the nitrided layer preferably has an average thickness of 30 μm or more and 100 μm or less, and an average nitrogen concentration of 0.3% by mass or more and 2% by mass or less in the thickness direction from the outermost surface of the nitrided part to 30 μm, and the bonding layer preferably has an average thickness of 5 μm or more and 20 μm or less and contains iron oxide.

[0025] From the viewpoint of wear resistance, the nitrided layer preferably has a region with a Vickers hardness of 680 Hv or more on the inner layer side than the bonding layer, and more preferably 700 Hv or more. On the other hand, from the viewpoint of crack resistance, the Vickers hardness of the bonding layer is preferably 500 Hv or less, and more preferably 460 Hv or less.

[0026] The quenching step S3 is a step of forming a martensite structure in the base material on which the nitrided layer and the bonding layer are formed. There is no particular limitation on the quenching method, and the conventional method can be appropriately used. From the viewpoint of working efficiency, it is preferable to use the high-frequency induction heating method in the same manner as in the elementary step S2a. The heating by the high-frequency induction heating method is preferably input-controlled so that the temperature of the surface of the base material is 800°C or more and 1100°C or less and becomes higher than that in the elementary step S2a.

[0027] Note that between the elementary step S2a and the step S3, the temperature of the base material may be temporarily lowered, or the temperature may be raised continuously following the elementary step S2a without lowering the temperature of the base material. Further, in the step S3, there is no particular limitation on the atmosphere, and the conventional atmosphere in the quenching treatment for steel materials can be appropriately used.

Example

[0028] Hereinafter, the present invention will be described more specifically by various experiments. However, the present invention is not limited to the configurations and structures described in these experiments.

[0029] [Experiment 1] (Production of Example 1) First, as a base material, a commercially available chromium molybdenum steel rod (JIS standard: SCM420, diameter 22 mm × length 40 mm) was prepared (base material preparation step).

[0030] Prior to the actual nitriding and oxidation heat treatment sub - process, a control curve of the base material heating temperature by high - frequency induction heating method was created using a dummy sample with a thermocouple attached to the base material surface.

[0031] In an atmosphere consisting of a nitriding source gas concentration (ammonia gas concentration) of 5%, an oxidation source gas concentration (oxygen gas concentration) of 0.3%, and the balance being an inert gas (nitrogen gas), the base material was heated by the high - frequency induction heating method based on the created control curve. The input of the high - frequency was controlled so that the heating temperature reached 780°C and held for 5 minutes (nitriding and oxidation heat treatment sub - process). Then, the heating temperature was controlled to reach 900°C and held for 1 minute, and the input of the high - frequency was cut off to cool the base material (quenching process). Thus, the sample of Example 1 was prepared.

[0032] [Experiment 2] [Investigation of the properties of Example 1] The properties of Example 1 prepared in Experiment 1 were investigated.

[0033] Using an X - ray diffraction (XRD) apparatus (Rigaku Corporation, model Ultima III), XRD measurement of the surface of Example 1 was performed. As a result, diffraction peaks of iron oxides (FeO, Fe 2 O 3 , Fe 3 O 4 ) were detected, and it was confirmed that a layer containing iron oxide was formed. Also, since diffraction peaks of a body - centered cubic structure (BCC) were detected, it was confirmed that the base material parent phase had become a martensite phase.

[0034] Using an optical microscope (Olympus Corporation, model BX53M) and an electron probe microanalyzer (EPMA) device (JEOL Ltd., model JXA-8530F), the microstructure observation, oxygen concentration analysis, and nitrogen concentration analysis of the cross-section of the surface layer region of Example 1 were carried out. Also, using a micro Vickers hardness tester (Matsuzawa Corporation, model AMT-X), the Vickers hardness of the cross-section was measured.

[0035] Figure 2 is an example of an optical microscope observation image of the cross-section of the surface layer region of Example 1. Figure 3 is an example of oxygen element mapping of the cross-section of the surface layer region of Example 1. Figure 4 is a graph showing an example of the relationship between the depth direction distance from the surface and the nitrogen concentration in Example 1.

[0036] From the results of Figures 2 to 4, in Example 1, nitrogen elements have penetrated and diffused up to a depth of about 80 μm in the depth direction from the surface (see Figure 4, the average nitrogen concentration from the surface to a thickness of 30 μm in the thickness direction is about 1.1 mass%), the oxygen element concentration is relatively high on the outer layer side (about 15 μm in the depth direction from the surface) (see Figure 3), and it is confirmed that layers with different microstructures are formed (see Figure 2).

[0037] As a result of the Vickers hardness measurement of the cross-section, the hardness of the internal region with a martensite structure is 440 Hv, the hardness of the region where nitrogen atoms have penetrated and diffused on the outer layer side is 770 Hv, and it is further confirmed that the hardness of the region with a relatively high oxygen element concentration on the outer layer side is 460 Hv.

[0038] Considering the integration of the above property investigation results, it can be said that in Example 1, a nitrided layer with a nitrogen element concentration higher than that of the base material and a Vickers hardness higher than that of the base material is formed in the surface layer region of the base material having a martensite structure, and an adherent layer containing iron oxide and having a Vickers hardness lower than that of the nitrided layer is formed on the outer layer side.

[0039] From this, in Example 1, even if unexpected foreign substances or the like are caught when applied as a sliding / rolling part, since an adaptation layer is formed on the outer layer side of the nitride layer, stress concentration due to foreign substances or the like can be suppressed and alleviated, and crack resistance can be ensured. Also, in Example 1, since the treatment time in the nitride / oxidation heat treatment sub-process is short, it can be said that the amount of greenhouse gas emissions can be reduced.

[0040] [Experiment 3] (Preparation of Examples 2 - 8, Comparative Examples 1 - 4, and Reference Sample) As the base material, an SCM420 rod (diameter 10 mm × length 10 mm) similar to that in Experiment 1 was prepared. Next, the nitride / oxidation heat treatment sub-process was performed by variously adjusting the heating temperature, holding time, ammonia gas concentration, and oxygen gas concentration in the high-frequency induction heating. The quenching process was carried out in the same manner as in Experiment 1 to prepare samples of Examples 2 - 8 and Comparative Examples 1 - 4.

[0041] Also, as a reference sample, a conventional gas soft nitriding treatment material was separately prepared. The preparation conditions were as follows: using the same SCM420 rod (diameter 10 mm × length 10 mm) as the base material, holding at a heating temperature of 580°C for 3 hours in an atmosphere consisting of 10% ammonia gas and the balance nitrogen gas, followed by quenching with the same temperature profile as in Experiment 1.

[0042] The details of the heat treatment conditions are summarized in Table 1 described later. Note that the heat treatment conditions of Example 1 are also listed in Table 1.

[0043] [Experiment 4] (Investigation of the Properties of Examples 2 - 8, Comparative Examples 1 - 4, and Reference Sample) For the obtained Examples 2 - 8, Comparative Examples 1 - 4, and reference sample, in the same manner as in Experiment 2, XRD measurement of the surface, oxygen analysis and nitrogen analysis by EPMA, and Vickers hardness measurement of the cross-section were performed.

[0044] In the sample into which a nitriding source gas (ammonia gas) was introduced as the heat treatment atmosphere, it was confirmed that a nitrided layer was formed in the surface layer region. In the sample into which an oxidation source gas (oxygen gas) was intentionally introduced as the heat treatment atmosphere, it was confirmed that an adherent layer (a layer containing iron oxide and having a Vickers hardness lower than that of the nitrided layer) was formed in the surface layer region. In other words, it was confirmed that no adherent layer was formed in the sample into which oxygen gas was not introduced as the heat treatment atmosphere, and no nitrided layer was formed in the sample into which ammonia gas was not introduced as the heat treatment atmosphere. The results are also shown in Table 1.

[0045] (Investigation of sliding durability in Examples 1 to 8, Comparative Examples 1 to 4, and reference sample) For Examples 1 to 8, Comparative Examples 1 to 4, and the reference sample prepared in Experiment 1 and Experiment 3, a reciprocating sliding pin-on-disk test was conducted to investigate the sliding durability. The disk test piece was prepared by subjecting a commercially available spherical graphite cast iron plate (JIS standard: FCD600) to the same gas soft nitriding treatment as the reference sample. Examples 1 to 8, Comparative Examples 1 to 4, and the reference sample were used as pin test pieces, respectively, and reciprocally slid in the axial direction of the pin test pieces.

[0046] For other test conditions, the temperature of the disk test piece was set to 100 °C, 0.5 mL of industrial lubricating oil (viscosity: ISO VG32) was applied to the sliding surface, the sliding was a sine wave (amplitude 0.1 mm, frequency 10 Hz), and the friction coefficient was measured while increasing the load stepwise by 0.1 kN every 120 s from an initial value of 0.1 kN. When the friction coefficient began to increase during the reciprocating sliding test, it was determined that seizure, which is a pre-seizure stage, occurred, and the load at that time was obtained as the seizure load value.

[0047] For the evaluation of the sliding durability, when it was 1.0 times or less compared to the seizure load value of the reference sample, it was determined as "failed", when it was more than 1.0 times and 1.5 times or less, it was determined as "passed", and when it was more than 1.5 times, it was determined as "excellent". The results are also shown in Table 1.

[0048]

Table 1

[0049] As shown in Table 1, in Comparative Example 1 where ammonia gas was not introduced during the heat treatment, no nitrided layer was formed and the sliding durability was unqualified. In Comparative Example 2 where ammonia gas was excessively introduced during the heat treatment, the average nitrogen concentration exceeded the specification of the present invention and the sliding durability was unqualified. This is considered to be due to the relative decrease in the amount of martensite structure in the base material (the relative increase in the amount of retained austenite phase) caused by the excessive amount of nitrogen.

[0050] Also, in Comparative Example 3 where oxygen gas was not introduced during the heat treatment, no conforming layer was formed and the sliding durability was unqualified. In Comparative Example 4 where oxygen gas was excessively introduced during the heat treatment, the thickness of the conforming layer exceeded the specification of the present invention and the sliding durability was unqualified. This is considered to be due to the relative decrease in the amount of the nitrided layer having a high hardness caused by the excessive formation of the conforming layer with a low Vickers hardness.

[0051] In contrast, Examples 1 to 8 according to the present invention were evaluated as having qualified or excellent sliding durability. That is, it was confirmed that the nitrided parts according to the present invention exhibit higher sliding durability than the parts having a nitrogen compound layer formed by the conventional gas soft nitriding treatment.

[0052] The above-described embodiments and experiments have been described to assist in the understanding of the present invention, and the present invention is not limited to the specific configurations described. For example, a part of the configuration of the embodiment can be replaced with a configuration within the common general knowledge of those skilled in the art, and a configuration within the common general knowledge of those skilled in the art can also be added to the configuration of the embodiment. That is, the present invention can delete, replace with other configurations, or add other configurations to a part of the configurations of the embodiments and experiments in this specification without departing from the technical idea of the invention.

Claims

1. A nitrided component with a steel material as the base material, wherein the nitrided component has a nitride layer and an adapted layer in the surface layer region of the base material, the average nitrogen concentration from the surface of the nitrided component to a depth of 30 μm in the thickness direction is 0.3% by mass or more and 2% by mass or less, the adapted layer is formed on the outer layer side of the nitride layer and is a layer with an average thickness of 5 μm or more and 20 μm or less and containing iron oxide, characterized in that it is a nitrided component.

2. In the nitrided component according to Claim 1, the nitride layer has a region with a Vickers hardness of 680 Hv or more on the inner layer side than the adapted layer, the adapted layer has a Vickers hardness of 500 Hv or less, characterized in that it is a nitrided component.

3. In the nitrided component according to Claim 1, the base material has a martensite structure, characterized in that it is a nitrided component.

4. In the nitrided component according to Claim 2, the base material has a martensite structure, characterized in that it is a nitrided component.

5. A method for manufacturing a nitrided component according to any one of Claims 1 to 4, including a base material preparation step of preparing the base material having a desired shape, a nitride layer / adapted layer formation step of forming the nitride layer and the adapted layer in the surface layer region of the base material, and a quenching step of forming a martensite structure on the base material on which the nitride layer and the adapted layer are formed, wherein the nitride layer / adapted layer formation step has a nitriding / oxidation heat treatment sub-step of heating the base material by a high-frequency induction heating method in a predetermined atmosphere to allow nitrogen to penetrate into the surface layer region of the base material and oxidize the iron component of the base material, characterized in that it is a method for manufacturing a nitrided component.

6. In the method for manufacturing a nitrided component according to Claim 5, in the predetermined atmosphere in the nitriding / oxidation heat treatment sub-step, the concentration of the nitriding source gas is 0.5% or more and 10% or less, the concentration of the oxidation source gas is 0.1% or more and 0.5% or less, and the balance is composed of an inert gas, the heating by the high-frequency induction heating method is input-controlled so that the temperature of the surface of the base material is 650°C or more and 850°C or less, characterized in that it is a method for manufacturing a nitrided component.

7. In the method for manufacturing a nitrided component according to Claim 6, The quenching step includes heating by a high-frequency induction heating method, and the input is controlled so that the temperature of the surface of the base material becomes 800°C or higher and 1100°C or lower by the high-frequency induction heating method and becomes higher than the nitriding / oxidation heat treatment sub-step. A method for manufacturing a nitrided part, characterized by this.

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