Method for manufacturing a martensitic stainless steel sintered body and martensitic stainless steel sintered body

Abstract

To provide a martensitic stainless steel sinter capable of combining high surface hardness and high corrosion resistance and a manufacturing method thereof.SOLUTION: A manufacturing method of a martensitic stainless steel sinter characterized by comprising steps of: mixing martensitic stainless steel powder containing Nb and an organic binder to obtain a composition; molding the composition to obtain a compact; removing at least a part of the organic binder contained in the compact to obtain a degreased body; performing sintering treatment containing an operation of heating the degreased body at a temperature of 900°C or more and 1100°C or less for more than 30 minutes to obtain a sinter; and performing a nitriding treatment including an operation of heating the sinter at a temperature of 700°C or more and 1300°C or less under a nitrogen atmosphere containing nitrogen.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a martensitic stainless steel sintered body and a martensitic stainless steel sintered body.

Background Art

[0002] Patent Document 1 discloses that after metal injection molding of stainless steel powder equivalent to SUS420J2, nitriding treatment is performed to manufacture wear-resistant parts. By performing nitriding treatment, a large amount of chromium nitride can be left remaining, and the surface hardness can be improved. Thereby, wear resistance can be imparted to the wear-resistant parts.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, chromium nitride causes a decrease in corrosion resistance. Therefore, there is a concern that the wear-resistant parts described in Patent Document 1 may cause a decrease in corrosion resistance while the surface hardness is improved.

Means for Solving the Problems

[0005] The method for manufacturing a martensitic stainless steel sintered body according to an application example of the present invention is as follows: The Nb content is 0.5% by mass or more and 3.0% by mass or less. A composition preparation step of mixing martensitic stainless steel powder and an organic binder to obtain a composition; A molding step of molding the composition to obtain a molded body; A debinding step of removing at least a part of the organic binder contained in the molded body to obtain a debound body; A sintering step to obtain a sintered body by performing a sintering treatment on the degreased body, which includes heating at a temperature of 900°C to 1100°C for 30 minutes or more, The nitriding process involves heating the sintered body in a nitrogen atmosphere containing nitrogen at a temperature of 700°C to 1300°C, and It is characterized by having the following features.

[0006] The martensitic stainless steel sintered body according to an application example of the present invention is The Nb content is 0.5% by mass or more and 3.0% by mass or less. A sintered body of martensitic stainless steel powder, Contains Nb, A nitride layer located on the surface, containing an austenite phase and having an average thickness of 10 μm or more, Located inside the nitrided layer, the base contains a martensite phase, It is characterized by having the following features. [Brief explanation of the drawing]

[0007] [Figure 1] This is a process diagram showing a method for manufacturing a martensitic stainless steel sintered body according to an embodiment. [Figure 2] This is an example of a graph showing the time-dependent change in heating temperature during the sintering process. [Figure 3] This is an example of a graph showing the time change in heating temperature during the nitriding process. [Figure 4] This is a schematic cross-sectional view showing a martensitic stainless steel sintered body according to an embodiment. [Figure 5] The X-ray diffraction pattern P3 was obtained by performing a crystal structure analysis by X-ray diffraction (XRD) on a martensitic stainless steel sintered body according to the embodiment, and the X-ray diffraction pattern P4 was obtained from a sintered body manufactured by a manufacturing method that omits the nitriding treatment. [Figure 6] This graph shows the relationship (hardness distribution) between the depth from the surface and the Vickers hardness in Example 2 and Comparative Example 3. [Figure 7]This graph shows the relationship between potential and current density obtained from the sintered body of Example 2, and the relationship between potential and current density obtained from the sintered body of Comparative Example 3. [Modes for carrying out the invention]

[0008] The method for producing a martensitic stainless steel sintered body and the martensitic stainless steel sintered body of the present invention will be described in detail below based on the embodiments shown in the attached drawings.

[0009] 1. Method for manufacturing a martensitic stainless steel sintered body Figure 1 is a process diagram showing a method for manufacturing a martensitic stainless steel sintered body according to an embodiment.

[0010] The method for manufacturing a martensitic stainless steel sintered body shown in Figure 1 comprises a composition preparation step S102, a molding step S104, a degreasing step S106, a sintering step S108, and a nitriding step S110. Each step will be described in order below.

[0011] 1.1. Composition preparation process In the composition preparation step S102, martensitic stainless steel powder and an organic binder are mixed to obtain a composition. Examples of the composition's form include a kneaded product and granulated powder. This composition is subjected to powder metallurgy, which has the following steps. In powder metallurgy, the composition containing metal powder and an organic binder is molded into a desired shape, and then subjected to degreasing and sintering to obtain a sintered body of the desired shape. This makes it possible to manufacture sintered bodies with complex and fine shapes in a near-net shape, i.e., a shape close to the final shape.

[0012] 1.1.1. Martensitic stainless steel powder For the martensitic stainless steel powder, a powder obtained by adding a predetermined amount of Nb to the composition of the martensitic stainless steel is used. Examples of the composition of the martensitic stainless steel include, for example, the chemical components defined in the JIS standard. In the JIS standard, the steel grades of the martensitic stainless steel are represented by symbols. Examples of such steel grades include SUS403, SUS410, SUS410L, SUS410S, SUS410J1, SUS410F2, SUS416, SUS420J1, SUS420J2, SUS420F, SUS420F2, SUS431, SUS440A, SUS440B, SUS440C, SUS440F, etc.

[0013] In addition, the martensitic stainless steel powder used in this process is a powder of a steel grade obtained by adding Nb to the above-mentioned steel grades. Alternatively, it may be a powder of a steel grade in which the content of some elements among the above-mentioned steel grades is changed, or a powder of a steel grade obtained by adding elements not defined.

[0014] Hereinafter, examples of preferable steel grades will be described. The martensitic stainless steel powder is preferably a powder of an Fe-based alloy having the following chemical components.

[0015] Content of C: 0.42 mass% or more and 1.20 mass% or less Content of Si: 1.00 mass% or less Content of Mn: 1.00 mass% or less Content of Ni: 0.60 mass% or less Content of Cr: 12.0 mass% or more and 18.0 mass% or less Content of Nb: 0.5 mass% or more and 3.0 mass% or less In the above chemical components, the balance is Fe and impurities. Hereinafter, each component will be described.

[0016] 1.1.1.1. C (Carbon) C (Carbon) is used in combination with Nb to precipitate NbC on the particle surface of the martensitic stainless steel powder.

[0017] The carbon (C) content in martensitic stainless steel powder is preferably 0.42% by mass or more and 1.20% by mass or less, more preferably 0.50% by mass or more and 1.00% by mass or less, and even more preferably 0.53% by mass or more and 0.80% by mass or less. If the carbon content falls below the lower limit, the amount of carbon will be insufficient relative to the amount of nitrogen (Nb), and depending on the overall composition ratio, the sintering density may not be sufficiently increased. On the other hand, if the carbon content exceeds the upper limit, the amount of carbon will be excessive relative to the amount of nitrogen (Nb), and depending on the overall composition ratio, the sintering reaction may be inhibited, potentially leading to a decrease in sintering density.

[0018] 1.1.1.2.Si (Silicon) The addition of silicon (Si) increases the oxidation resistance of martensitic stainless steel powder. This improves the sinterability of the martensitic stainless steel powder.

[0019] The Si content in the martensitic stainless steel powder is preferably 1.00% by mass or less, more preferably 0.30% by mass or more and 0.90% by mass or less, and even more preferably 0.50% by mass or more and 0.80% by mass or less.

[0020] 1.1.1.3. Mn (Manganese) The addition of manganese (Mn) increases the affinity of martensitic stainless steel powder for nitrogen (N). This promotes the solid solution of nitrogen into the sintered body during the nitriding process S110, which will be described later.

[0021] The Mn content in the martensitic stainless steel powder is preferably 1.00% by mass or less, more preferably 0.30% by mass or more and 0.90% by mass or less, and even more preferably 0.50% by mass or more and 0.85% by mass or less.

[0022] 1.1.1.4. Ni (Nickel) Nickel (Ni), when used in combination with Cr, enhances the corrosion resistance and heat resistance of sintered bodies.

[0023] The Ni content in the martensitic stainless steel powder is preferably 0.60% by mass or less, more preferably 0.05% by mass or more and 0.40% by mass or less, and even more preferably 0.10% by mass or more and 0.30% by mass or less.

[0024] 1.1.1.5. Cr (Chromium) Chromium (Cr) enhances the corrosion resistance and heat resistance of sintered bodies.

[0025] The Cr content in the martensitic stainless steel powder is preferably 12.0% by mass or more and 18.0% by mass or less, more preferably 12.3% by mass or more and 16.0% by mass or less, and even more preferably 12.5% ​​by mass or more and 14.0% by mass or less. If the Cr content falls below the lower limit, the corrosion resistance of the sintered body may become insufficient depending on the overall composition ratio. On the other hand, if the Cr content exceeds the upper limit, the sinterability of the martensitic stainless steel powder may decrease and the sinter density may decrease, depending on the overall composition ratio.

[0026] 1.1.1.6.Nb (Niobium) When Nb (niobium) is used in combination with C, it precipitates NbC (niobium carbide) on the surface of the metal powder particles. This suppresses the rapid progression of sintering in the degreased body. As a result, it prevents gas generated inside the degreased body from becoming trapped, and the sintering density inside the degreased body can be sufficiently increased, ultimately resulting in a high-density martensitic stainless steel sintered body.

[0027] Furthermore, in the nitriding treatment described later, Nb reacts with N, and NbN (niobium nitride) precipitates. This NbN has the effect of increasing the surface hardness and corrosion resistance of the martensitic stainless steel sintered body. Therefore, by including Nb, a martensitic stainless steel sintered body that achieves both high surface hardness and high corrosion resistance can be obtained.

[0028] Furthermore, the inclusion of Nb increases the sintering start temperature of the martensitic stainless steel powder. This makes it possible to maintain the martensitic stainless steel powder in a temperature range that promotes decarburization before sintering, as will be described later. As a result, the base for the nitriding treatment described later can be effectively formed.

[0029] Furthermore, by increasing the sintering start temperature, the reduction reaction of substances that tend to inhibit sintering can be promoted. Examples of substances that tend to inhibit sintering include oxides such as silicon dioxide and chromium oxide. By reducing these, sintering proceeds more easily in the sintering process described later. As a result, the density of the sintered body can be increased.

[0030] This reduction reaction can be represented by the following reaction equation. SiO2(s) + C(s) → SiO(g) + CO(g) Cr2O3(s)+3C(s)→2Cr(s)+3CO(g)

[0031] In the above equation, (s) represents a solid and (g) represents a gas. In this example, silicon oxide (SiO2) reacts with carbon (C) and is converted into a substance that easily vaporizes and is removed from the molded body. In addition, chromium oxide is reduced to metallic chromium. As a result, oxides that tend to inhibit sintering can be reduced from the molded body, and the density of the sintered body can be increased. Since this reaction tends to occur at temperatures above 800°C, for example, if the sintering start temperature can be increased, the martensitic stainless steel powder can remain in the temperature range suitable for the above reduction reaction for a longer period of time during the sintering process described later.

[0032] The Nb content is preferably 0.5% by mass or more and 3.0% by mass or less, more preferably 1.0% by mass or more and 2.5% by mass or less, and even more preferably 1.2% by mass or more and 1.8% by mass or less. By specifying the Nb content within the above range, it becomes easier to precipitate NbC on the particle surface of the martensitic stainless steel powder. Furthermore, the sintering start temperature of the martensitic stainless steel powder can be increased. This suppresses the trapping of gas generated inside the degreased body and allows the internal sintering density to be sufficiently increased, so that a high-density martensitic stainless steel sintered body can be obtained.

[0033] Furthermore, in martensitic stainless steel powder, if the Nb content falls below the aforementioned lower limit, the amount of Nb will be insufficient relative to the amount of C, and depending on the overall composition ratio, it may not be possible to sufficiently increase the sintering density. On the other hand, if the Nb content exceeds the aforementioned upper limit, the amount of Nb will be excessive relative to the amount of C, and depending on the overall composition ratio, the sintering reaction may be inhibited, potentially leading to a decrease in sintering density.

[0034] Furthermore, when the ratio of C content to Nb content is defined as C / Nb, it is preferable that C / Nb be between 0.18 and 0.90, more preferably between 0.20 and 0.80, and even more preferably between 0.23 and 0.60. This allows for the optimization of the balance between C content and Nb content. As a result, excesses or deficiencies of C and Nb are less likely to occur, making it possible to realize a martensitic stainless steel powder that can produce sintered bodies that achieve a high degree of both high density and high strength.

[0035] Furthermore, when the sum of the C content and Nb content is defined as C+Nb, it is preferable that C+Nb be between 1.5 and 3.5, more preferably between 1.7 and 3.3, and even more preferably between 2.0 and 3.2. This makes it possible to suppress the generation of either too little or too much NbC. As a result, it is possible to realize a martensitic stainless steel powder that can produce sintered bodies that achieve a high degree of both high density and high strength.

[0036] 1.1.1.7. Other Ingredients The martensitic stainless steel powder may optionally contain at least one of Cu, Mo, Co, and Pb.

[0037] The Cu content is preferably 0.20% by mass or less, and more preferably 0.10% by mass or less.

[0038] The Mo content is preferably 0.75% by mass or less, and more preferably 0.30% by mass or less.

[0039] The content of Co and Pb is preferably 0.30% by mass or less, and more preferably 0.10% by mass or less.

[0040] 1.1.1.8. Fe (iron) and impurities Fe (iron) and impurities make up the remainder of the components in martensitic stainless steel powder, excluding those mentioned above. Therefore, the Fe content is the highest in martensitic stainless steel powder. In other words, Fe is the main component of martensitic stainless steel powder and has a significant impact on the properties of the sintered body.

[0041] The Fe content is preferably 50% by mass or more, and more preferably 60% by mass or more.

[0042] Furthermore, the martensitic stainless steel powder may contain any elements other than those mentioned above as impurities. These impurities may be unintentionally present in the raw materials or inevitably introduced during the manufacturing process of the martensitic stainless steel powder. The concentration of each impurity should be lower than the content of each of the components mentioned above, but it is preferable that each be 0.10% by mass or less, and more preferably 0.05% by mass or less.

[0043] 1.1.1.9.Analysis method The chemical composition of martensitic stainless steel powder has been described in detail above, and the above chemical composition can be identified by the following analytical methods.

[0044] Examples of analytical methods include atomic absorption spectrometry for iron and steel as specified in JIS G 1257:2000, ICP emission spectrometry for iron and steel as specified in JIS G 1258:2007, spark discharge emission spectrometry for iron and steel as specified in JIS G 1253:2002, X-ray fluorescence spectrometry for iron and steel as specified in JIS G 1256:1997, and gravimetric titration-absorbance spectrophotometric methods as specified in JIS G 1211 to G 1237.

[0045] Specifically, examples include solid-state emission spectrometers manufactured by SPECTRO, particularly spark discharge emission spectrometers, model: SPECTROLAB, type: LAVMB08A, and the ICP instrument CIROS120 manufactured by Rigaku Corporation.

[0046] Furthermore, in particular, when identifying carbon (C) and sulfur (S), the oxygen-flow combustion (high-frequency induction heating furnace combustion)-infrared absorption method specified in JIS G 1211:2011 is also used. Specifically, the LECO CS-200 carbon-sulfur analyzer is used.

[0047] Furthermore, when specifically identifying N (nitrogen) and O (oxygen), the methods for determining nitrogen in iron and steel as specified in JIS G 1228:1997 and the general rules for determining oxygen in metallic materials as specified in JIS Z 2613:2006 are also used. Specifically, the LECO TC-300 / EF-300 oxygen / nitrogen analyzer is an example.

[0048] 1.1.1.10. Powder Characteristics The average particle size D50 of the martensitic stainless steel powder is not particularly limited, but is preferably 20 μm or less, more preferably 1 μm to 15 μm, and even more preferably 2 μm to 10 μm. Martensitic stainless steel powder with such particle size has good sinterability, making it possible to produce high-density sintered bodies.

[0049] Furthermore, if the average particle size D50 of the martensitic stainless steel powder falls below the lower limit, the powder is prone to agglomeration, which may reduce the sintering density. On the other hand, if the average particle size D50 of the martensitic stainless steel powder exceeds the upper limit, the filling performance during molding will decrease, which may reduce the sintering density.

[0050] The average particle size D50 of martensitic stainless steel powder is the particle size at which the cumulative value from the smallest diameter side reaches 50% in the integrated distribution curve obtained by measuring the volume-based particle size distribution of martensitic stainless steel powder using laser diffraction.

[0051] For martensitic stainless steel powder, the particle size distribution is measured by laser diffraction on a volume basis. In the resulting cumulative distribution curve, if the particle size at which the cumulative value from the smallest diameter side reaches 10% is defined as D10, and the particle size at which the cumulative value from the smallest diameter side reaches 90% is defined as D90, then (D90-D10) / D50 is preferably between 1.0 and 2.5, and more preferably between 1.2 and 2.3. (D90-D10) / D50 is an index indicating the degree of spread of the particle size distribution, and if this index is within the above range, the packing performance of the martensitic stainless steel powder becomes particularly good. As a result, a high-density sintered body can be manufactured.

[0052] 1.1.1.11. Manufacturing method The martensitic stainless steel powder may be manufactured by any method, but it is preferably manufactured by the atomization method, and more preferably by the water atomization method or the rotary water atomization method. The atomization method is a method of producing metal powder by pulverizing and cooling molten metal by colliding it with a liquid or gas sprayed at high speed. By manufacturing martensitic stainless steel powder by the atomization method, extremely fine powder can be produced efficiently.

[0053] 1.1.2. Organic Binder As the organic binder, a resin that can be decomposed in a short time during degreasing or sintering is used. Examples of such resins include polyolefins such as polyethylene, polypropylene, and ethylene-vinyl acetate copolymers; acrylic resins such as polymethyl methacrylate and polybutyl methacrylate; styrene resins such as polystyrene; polyesters such as polyvinyl chloride, polyvinylidene chloride, polyamide, polyethylene terephthalate, and polybutylene terephthalate; polyethers; polyvinyl alcohol; polyvinylpyrrolidone or copolymers thereof; various waxes; paraffin; higher fatty acids; higher alcohols; higher fatty acid esters; higher fatty acid amides; and one or more of these can be used in combination.

[0054] The mixing ratio of the organic binder is preferably 0.2% by mass or more and 20.0% by mass or less of the composition, and more preferably 0.5% by mass or more and 15.0% by mass or less.

[0055] In addition to these, the composition may also contain various additives such as plasticizers, lubricants, antioxidants, degreasing accelerators, and surfactants.

[0056] 1.2. Molding process In molding step S104, the molding composition is molded into the desired shape. This results in a molded body.

[0057] Examples of molding methods include injection molding, compression molding (press molding), extrusion molding, and additive manufacturing. Among these, examples of additive manufacturing methods include material extrusion deposition and binder jetting.

[0058] 1.3. Degreasing process In the degreasing process S106, the molded body is subjected to a degreasing treatment to obtain a degreased body.

[0059] Degreasing treatments include, for example, heating the molded body to decompose the organic binder, or exposing the molded body to a gas that decomposes the organic binder. Through degreasing treatment, all or part of the organic binder in the molded body is removed.

[0060] When using a method of heating the molded body, the heating conditions for the molded body vary slightly depending on the composition and amount of the organic binder, but it is preferable that the temperature is between 100°C and 750°C and the time is between 0.1 hours and 20 hours, and more preferably that the temperature is between 150°C and 600°C and the time is between 0.5 hours and 15 hours.

[0061] The atmosphere used when heating the molded body is not particularly limited and may include an inert atmosphere such as nitrogen or argon, an oxidizing atmosphere such as air, or a reduced-pressure atmosphere obtained by reducing the pressure of these atmospheres.

[0062] One method used to expose a molded body to a gas that decomposes organic binders is acid degreasing. Acid degreasing is a method that utilizes the catalytic action of acid to degrease a molded body by heating it in an acid-containing atmosphere. With acid degreasing, organic binders can be decomposed quickly even at low temperatures, so even large molded bodies can be efficiently degreased.

[0063] An acid-containing atmosphere refers to an atmosphere containing an acid capable of decomposing an organic binder. Examples of such acids include nitric acid, oxalic acid, and ozone, and one or more of these can be used in combination. Alternatively, a mixed gas obtained by mixing these acids with other gases may be used. An example of a mixed gas is fuming nitric acid. The atmospheric pressure may be atmospheric pressure, reduced pressure, or pressurized pressure.

[0064] The heating conditions for molded bodies in an acid-containing atmosphere are lower in temperature or shorter in duration than those described above. This reduces the amount of heat applied to the molded body, making it easier to suppress the oxidation of martensitic stainless steel powder.

[0065] 1.4. Sintering Process In the sintering process S108, the degreased body is subjected to a sintering treatment to obtain a sintered body.

[0066] This sintering process involves heating the degreased body at a temperature of 900°C to 1100°C for 30 minutes or more. By maintaining the body in this temperature range for the aforementioned time or longer, the bonding reaction between carbon (C) and oxygen (O) is promoted on the surface of the degreased body before sintering begins. This bonding reaction is referred to as "decarburization" in this specification. When decarburization occurs, carbon is released from the metal powder in the degreased body as oxides such as carbon monoxide and carbon dioxide. As a result, the carbon content on the surface of the metal powder particles decreases. This carbon-reduced surface layer is called the "decarburized layer." The sintered body obtained through this process will have this decarburized layer. This decarburized layer then serves as a base for promoting the solid solution of high-concentration nitrogen with sufficient thickness when the sintered body is subjected to nitriding in the nitriding process S110 described later.

[0067] The temperature range in the sintering process is lower than the sintering start temperature of the martensitic stainless steel powder. Therefore, by heating the degreased body under these heating conditions, the phenomenon in which carbon oxide produced by decarburization is difficult to remove by sintering can be suppressed. This allows for sufficient decarburization.

[0068] Furthermore, the above temperature range is also a temperature range in which decarburization is likely to occur. Martensitic stainless steel powder, by containing Nb, can raise the sintering initiation temperature as described above, or slow down the sintering process through the precipitation of NbC. This allows for sufficient decarburization before sintering occurs. In addition, in the above temperature range, the substances that tend to inhibit sintering, as mentioned above, are reduced. This makes it easier to increase the density of the sintered body.

[0069] Figure 2 is an example of a graph showing the time change of the heating temperature in the sintering process S108. In the following explanation, this time change of heating temperature will be referred to as the "sintering temperature pattern P1".

[0070] In the example of the sintering temperature pattern P1 shown in Figure 2, the heating temperature is maintained for periods in mainly two temperature ranges. Here, the period during which the temperature is maintained at approximately 1000°C is called the "first temperature holding time t1," and the period during which the temperature is maintained at approximately 1350°C is called the "second temperature holding time t2."

[0071] As shown in Figure 2, the sintering temperature pattern P1 has a first temperature holding time t1. During this first temperature holding time t1, the material is heated at a temperature between 900°C and 1100°C for 30 minutes or more. Therefore, sufficient decarburization can be achieved during the first temperature holding time t1, and the surface is prepared for nitriding treatment.

[0072] The first temperature holding time t1 refers to the time during which the degreased body is continuously held in a temperature range of 900°C to 1100°C, and its length is 30 minutes or more. Preferably, it is 1 hour to 20 hours, and more preferably 2 hours to 10 hours. If the first temperature holding time t1 falls below the lower limit, decarburization will be insufficient, and the surface for nitriding treatment cannot be adequately prepared. On the other hand, the first temperature holding time t1 may exceed the upper limit, but in that case, a decarburized layer may be formed deep inside the sintered body, and the carbon concentration throughout the sintered body may decrease too much. If this occurs, the number of areas where sufficient quenching cannot be performed in the nitriding treatment described later may increase, and the overall mechanical properties may deteriorate.

[0073] Furthermore, it is preferable that the first temperature holding time t1 includes a period of 30 minutes or more in the temperature range of 950°C to 1050°C. This makes the above effect more pronounced.

[0074] During the first temperature holding time t1, the temperature can be constant or change, as long as it is continuously maintained within this temperature range.

[0075] Furthermore, the heating rate from the start of heating to the first temperature holding time t1 is not particularly limited, but is preferably 30°C / hour or more and 300°C / hour or less, and more preferably 50°C / hour or more and 150°C / hour or less. This suppresses temperature unevenness in the degreased material and allows decarburization to proceed uniformly regardless of the shape of the degreased material. In addition, there may be a period of time during this heating process in which the temperature is kept constant.

[0076] The heating atmosphere during the first temperature holding time t1 can be, for example, a reducing atmosphere containing hydrogen, an inert atmosphere containing nitrogen, argon, etc., or a reduced-pressure atmosphere. Of these, an inert atmosphere or a reduced-pressure atmosphere is preferred. This suppresses oxidation of the sintered body. Furthermore, a reduced-pressure atmosphere allows for efficient discharge of gases generated during decarburization, ensuring a sufficient thickness of the decarburized layer and increasing the density of the sintered body.

[0077] The pressure of the reduced-pressure atmosphere is not particularly limited as long as it is below atmospheric pressure, but it is preferably 10 kPa or less, and more preferably 1 kPa or less. In this specification, atmospheric pressure refers to a range of 101 kPa ± 10 kPa.

[0078] The sintering temperature pattern P1 has a second temperature holding time t2, as shown in Figure 2. During this second temperature holding time t2, the degreased body is heated at a higher temperature than the temperature range of the first temperature holding time t1. This allows the degreased body to be sintered.

[0079] The second temperature holding time t2 refers to the time during which the degreased body is continuously held in a temperature range of 1200°C to 1500°C, and its length is preferably 1 hour or more, more preferably 3 hours to 10 hours. If the second temperature holding time t2 falls below the lower limit, sintering may be insufficient, and the density of the sintered body may decrease. On the other hand, if the second temperature holding time t2 exceeds the upper limit, sintering may be excessive, and grain enlargement may occur.

[0080] Furthermore, the second temperature holding time t2 preferably includes a period of one hour or more of holding at a temperature within the aforementioned temperature range and at a temperature 50°C or more higher than the highest temperature held during the first temperature holding time t1, and more preferably includes a period of one hour or more of holding at a temperature range of 1270°C to 1400°C. This makes the above effect more pronounced. Note that if the temperature held during the second temperature holding time t2 falls below the lower limit, sintering may be insufficient. On the other hand, if the temperature held during the second temperature holding time t2 exceeds the upper limit, sintering may be excessive.

[0081] Furthermore, during the second temperature holding time t2, the temperature can be constant or change, as long as it is continuously maintained within this temperature range.

[0082] Furthermore, the heating rate from the first temperature holding time t1 to the second temperature holding time t2 is not particularly limited, but is preferably 30°C / hour or more and 300°C / hour or less, and more preferably 50°C / hour or more and 150°C / hour or less. This suppresses temperature unevenness in the sintered body and allows sintering to proceed uniformly regardless of the shape of the sintered body. In addition, there may be a period during this heating process in which the temperature is kept constant.

[0083] The heating atmosphere during the second temperature holding time t2 can be, for example, a reducing atmosphere containing hydrogen, an inert atmosphere containing nitrogen, argon, etc., or a reduced-pressure atmosphere. Of these, an inert atmosphere or a reduced-pressure atmosphere is preferred. This suppresses oxidation of the sintered body. Furthermore, if a reduced-pressure atmosphere is used, the generated gas can be removed, thereby promoting further decarburization and increasing the density of the sintered body.

[0084] The pressure of the reduced-pressure atmosphere is not particularly limited as long as it is below atmospheric pressure, but it is preferably 10 kPa or less, and more preferably 1 kPa or less.

[0085] After heating for the second temperature holding time t2, the sintered body is cooled by natural cooling or forced cooling to dissipate heat. During heat dissipation, the temperature may be reduced to room temperature or kept higher than room temperature. In other words, between the sintering process S108 and the nitriding process S110 described later, the temperature of the sintered body may be reduced to room temperature, or the two processes may be carried out continuously without reducing the temperature.

[0086] Furthermore, between the degreasing process S106 and the sintering process S108, the temperature of the degreased body may or may not be lowered to room temperature.

[0087] 1.5. Nitriding Process In the nitriding process S110, the obtained sintered body is subjected to nitriding treatment. This yields a martensitic stainless steel sintered body.

[0088] This nitriding treatment involves heating the sintered body in a nitrogen atmosphere at a temperature between 700°C and 1300°C. By heating in this temperature range under a nitrogen atmosphere, nitrogen can be dissolved in the decarburized layer of the sintered body. Since nitrogen, like carbon, is an interstitial solid solution element, it penetrates into the areas where carbon has been lost due to decarburization. In this specification, this solid solution reaction of nitrogen is called "nitriding," and the surface layer in which nitrogen is dissolved is called the "nitrided layer." Since nitrogen is an austenite-forming element, when nitriding occurs, austenitization progresses in the nitrided layer. As a result, high corrosion resistance derived from the austenite phase is obtained on the surface of the sintered body. In this embodiment, since decarburization is performed before the nitriding treatment, a high concentration of nitrogen can be dissolved in the nitrided layer. This allows the corrosion resistance of the nitrided layer to be sufficiently enhanced. In addition, although the decarburized layer has relatively low hardness because it contains a large amount of ferrite phase, its hardness increases due to austenitization. Therefore, the nitrided layer also contributes to improving surface hardness.

[0089] Furthermore, nitriding treatment produces an effect similar to quenching treatment by rapidly cooling the sintered body afterward. Quenching treatment is a process that promotes martensitic formation in the interior of the sintered body rather than in the nitrided layer as the sintered body is rapidly cooled. In martensitic formation, carbon concentration is important, and the higher the carbon concentration, the higher the hardness tends to be. Therefore, martensitic formation is less likely to occur in the nitrided layer, while it is more likely to occur in the interior of the body. Consequently, as a result of nitriding treatment, corrosion resistance and hardness are improved in the nitrided layer due to austenitization, and high hardness is achieved in the interior of the body due to martensitic formation. As a result, a martensitic stainless steel sintered body is obtained that combines the corrosion resistance and high hardness of the nitrided layer itself with high hardness in the interior.

[0090] Figure 3 is an example of a graph showing the time change of the heating temperature in the nitriding process S110. In the following explanation, this time change of heating temperature will be referred to as the "nitriding temperature pattern P2".

[0091] In the example of the nitriding temperature pattern P2 shown in Figure 3, a period of time is provided in which the heating temperature is maintained within one temperature range. Here, this period is referred to as the "third temperature holding time t3". During the third temperature holding time t3, nitrogen can be sufficiently dissolved, and a nitrided layer can be formed.

[0092] The third temperature holding time t3 refers to the time during which the sintered body is continuously held in a temperature range of 700°C to 1300°C, preferably for 30 minutes or more, more preferably for 1 hour to 20 hours, and even more preferably for 3 hours to 10 hours. If the third temperature holding time t3 falls below the lower limit, nitriding may be insufficient, and the corrosion resistance may not be sufficiently improved. On the other hand, the third temperature holding time t3 may exceed the upper limit, but in that case, the quenching may be excessive, and the mechanical properties of the martensitic stainless steel sintered body may deteriorate.

[0093] The third temperature holding time t3 preferably includes a period of 30 minutes or more of holding at a temperature range of 1000°C to 1300°C, and more preferably includes a period of 30 minutes or more of holding at a temperature range of 1100°C to 1300°C. This makes the above effect more pronounced.

[0094] The heating atmosphere during the third temperature holding time t3 is a nitrogen atmosphere containing nitrogen. The nitrogen concentration in the nitrogen atmosphere is not particularly limited, but is preferably 80% by volume or higher. The total pressure of the nitrogen atmosphere may be atmospheric pressure or reduced pressure, but is preferably pressurized (above atmospheric pressure). This promotes the solid solution of nitrogen and allows for uniform nitriding. Furthermore, a sufficiently thick nitrided layer can be formed. As a result, a martensitic stainless steel sintered body with excellent corrosion resistance can be obtained. The total pressure of the pressurized nitrogen atmosphere is not particularly limited as long as it is above atmospheric pressure, but is preferably 120 kPa (0.12 MPa) or higher, and more preferably 150 kPa (0.15 MPa) or higher and 500 kPa (0.50 MPa) or lower. Note that if the pressure of the nitrogen atmosphere falls below the lower limit, the solid solution of nitrogen may become uneven depending on the shape of the sintered body. On the other hand, the pressure of the nitrogen atmosphere may exceed the upper limit, but in that case, the cost of the equipment may increase.

[0095] Furthermore, during the third temperature holding time t3, the temperature can be constant or change, as long as it is continuously maintained within this temperature range.

[0096] Furthermore, after heating for the third temperature holding time t3, the martensitic stainless steel sintered body is cooled by natural cooling or forced cooling. Hardening can be performed by forced cooling. Examples of forced cooling methods include water cooling and oil cooling.

[0097] The rate of cooling during forced cooling is not particularly limited, but is preferably 50°C / second or higher, and more preferably 100°C / second or higher. This allows for the maintenance of the martensite phase in the nitrided layer while particularly promoting martensite formation within the layer.

[0098] Furthermore, the resulting martensitic stainless steel sintered body may be subjected to sub-zero treatment, tempering, etc., as needed. This allows for the creation of a stable martensitic structure.

[0099] Sub-zero treatment is a process in which the crystal structure of residual austenite that did not undergo martensite formation during the quenching process is transformed into martensite through cooling. While residual austenite crystal structure often transforms into martensite over time, this process involves a change in the volume of the sintered body, which can cause the dimensions of the sintered body to change over time. Therefore, by performing sub-zero treatment after quenching, the crystal structure of residual austenite is semi-forcibly transformed into martensite. This prevents the occurrence of dimensional changes over time.

[0100] The sub-zero treatment temperature is preferably 0°C or lower, and the duration is preferably between 0.2 hours and 3 hours. For cooling the sintered body, dry ice, carbon dioxide, liquid nitrogen, etc., are used.

[0101] Tempering is a process in which a sintered body, after quenching, is heated again at a lower temperature than the quenching process. This process reduces the hardness of the sintered body while imparting toughness to it.

[0102] The tempering process is preferably performed at a temperature of approximately 100°C to 250°C for a duration of approximately 0.3 hours to 5 hours.

[0103] 1.6. Effects of the method for manufacturing a martensitic stainless steel sintered body according to the embodiment As described above, the method for manufacturing a martensitic stainless steel sintered body according to the embodiment comprises a composition preparation step S102, a molding step S104, a degreasing step S106, a sintering step S108, and a nitriding step S110. In the composition preparation step S102, martensitic stainless steel powder containing Nb and an organic binder are mixed to obtain a composition. In the molding step S104, the composition is molded to obtain a molded body. In the degreasing step S106, at least a portion of the organic binder contained in the molded body is removed to obtain a degreased body. In the sintering step S108, the degreased body is subjected to a sintering treatment including heating at a temperature of 900°C to 1100°C for 30 minutes or more to obtain a sintered body. In the nitriding step S110, the sintered body is subjected to a nitriding treatment including heating at a temperature of 700°C to 1300°C in a nitrogen atmosphere containing nitrogen.

[0104] This manufacturing method allows decarburization to occur before the sintering of the degreased body begins during the sintering process. This forms a decarburized layer. This decarburized layer can then be used as a base for nitriding, allowing for the solid solution of high-concentration nitrogen during the nitriding process, thereby significantly improving the corrosion resistance of the martensitic stainless steel sintered body. Furthermore, since nitriding has a similar effect to quenching, martensitic formation can be promoted within the sintered body. Consequently, corrosion resistance is improved in the nitrided layer through austenitization, while higher hardness is achieved further within the body through martensitic formation. As a result, according to this embodiment, a martensitic stainless steel sintered body can be manufactured that achieves both high corrosion resistance and high surface hardness through the combination of the corrosion resistance and high hardness of the nitrided layer itself and the high hardness of the interior.

[0105] Furthermore, because the martensitic stainless steel powder contains Nb, it can suppress the rapid sintering process in the degreased body and raise the sintering start temperature. As a result, decarburization proceeds evenly, and a nitrided layer of sufficient thickness can be formed.

[0106] Furthermore, by increasing the sintering start temperature, substances that tend to inhibit sintering are more easily reduced. This makes it possible to increase the density of the sintered body.

[0107] Furthermore, it is preferable that the total pressure of the nitrogen atmosphere during the nitriding process is greater than atmospheric pressure. This promotes the solid solution of nitrogen, allowing for uniform nitriding. It also enables the formation of a sufficiently thick nitrided layer. As a result, a martensitic stainless steel sintered body with excellent corrosion resistance can be obtained.

[0108] Furthermore, the Nb content of the martensitic stainless steel powder is preferably 0.5% by mass or more and 3.0% by mass or less. This facilitates the deposition of NbC on the particle surface of the martensitic stainless steel powder and allows for a higher sintering start temperature. As a result, a high-density martensitic stainless steel sintered body can be obtained.

[0109] Furthermore, it is preferable to carry out the sintering process under an inert atmosphere and at a pressure below atmospheric pressure. By performing the sintering process under these conditions, oxidation of the sintered body is suppressed, and the gases generated during decarburization can be efficiently discharged. This ensures a sufficient thickness of the decarburized layer and increases the density of the sintered body.

[0110] Furthermore, the nitriding process involves rapidly cooling the sintered body. This causes the nitriding process to have a similar effect to quenching. As a result, martensitic formation progresses further inside the nitrided layer, increasing the surface hardness of the martensitic stainless steel sintered body.

[0111] 2. Martensitic stainless steel sintered body A martensitic stainless steel sintered body according to an embodiment will be described.

[0112] Figure 4 is a schematic cross-sectional view showing a martensitic stainless steel sintered body according to an embodiment.

[0113] The martensitic stainless steel sintered body 1 shown in Figure 4 is a sintered body of martensitic stainless steel powder having a composition containing Nb. It has a nitrided layer 2 located on the surface and containing an austenite phase with an average thickness of 10 μm or more, and a base portion 3 located inside the nitrided layer 2 and containing a martensite phase.

[0114] In such a martensitic stainless steel sintered body 1, the nitrided layer 2 exhibits good corrosion resistance and high hardness derived from the austenite phase, while the base 3 achieves high hardness derived from the martensitic phase. Therefore, the martensitic stainless steel sintered body 1 having the nitrided layer 2 and the base 3 achieves both high corrosion resistance and high surface hardness.

[0115] Nitrided layer 2 is a surface layer containing austenite phase with dissolved nitrogen. Nitrided layer 2 has an average thickness of 10 μm or more. Because the austenite phase has excellent corrosion resistance, the presence of nitrided layer 2 on the surface enhances the corrosion resistance of the martensitic stainless steel sintered body 1. The nitrogen dissolved in nitrided layer 2 is an austenite-forming element and therefore contributes to the stability of the austenite phase. Furthermore, the presence of dissolved nitrogen increases hardness compared to a layer without dissolved nitrogen. Consequently, nitrided layer 2 has higher hardness than, for example, a layer containing a ferrite phase.

[0116] The austenite and martensite phases present in the martensitic stainless steel sintered body 1 can be identified by crystal structure analysis using X-ray diffraction.

[0117] Figure 5 shows the X-ray diffraction pattern P3 obtained by performing a crystal structure analysis using X-ray diffraction (XRD) on the martensitic stainless steel sintered body 1 according to the embodiment. The martensitic stainless steel sintered body 1 according to the embodiment was manufactured by the method for manufacturing martensitic stainless steel sintered bodies described above. Therefore, the X-ray diffraction pattern P3 was obtained from a sintered body manufactured by a manufacturing method that includes nitriding treatment. Figure 5 also shows the X-ray diffraction pattern P4 obtained from a sintered body manufactured by a manufacturing method that omits the nitriding treatment.

[0118] The X-ray diffraction pattern P3 shown in Figure 5 includes not only the peak pm originating from the martensite phase, but also the peak pa originating from the austenite phase. There are multiple peaks pm, one of which is located at 2θ = 80-82°. This peak pm located at 2θ = 80-82° is specifically designated as peak pk1 (first peak). There are also multiple peaks pa, one of which is located at 2θ = 50-52°. This peak pa located at 2θ = 50-52° is specifically designated as peak pk2 (second peak).

[0119] When the height of peak pk1 (first peak) is set to 100, the height of peak pk2 (second peak) is preferably between 3 and 100, more preferably between 5 and 50, and even more preferably between 10 and 30. The ratio of the height of peak pk2 to the height of peak pk1, pk2 / pk1, represents the volume ratio of the austenite phase to the martensitic phase. In other words, the ratio pk2 / pk1 corresponds to the thickness of the nitrided layer 2. The martensitic phase contributes to the surface hardness of the martensitic stainless steel sintered body 1, and the austenite phase contributes to both surface hardness and corrosion resistance. Therefore, when this ratio pk2 / pk1 is within the above range, the balance of these contributions is optimized, and high corrosion resistance and high surface hardness can be achieved particularly well in the martensitic stainless steel sintered body 1.

[0120] Furthermore, the average thickness of the nitrided layer 2 is set to 10 μm or more, preferably 20 μm to 500 μm, and more preferably 30 μm to 300 μm. By setting the average thickness of the nitrided layer 2 within the above range, sufficient corrosion resistance can be imparted to the martensitic stainless steel sintered body 1, while the surface can benefit from the high hardness of both the nitrided layer 2 and the base 3.

[0121] The average thickness of the nitrided layer 2 is the average value obtained by observing the cross-section of the martensitic stainless steel sintered body 1 under magnification and measuring the thickness of the nitrided layer 2 at five or more locations.

[0122] The base portion 3 is located inside the nitride layer 2 in the martensitic stainless steel sintered body 1 and contains the martensitic phase. Because the martensitic phase has high hardness, it can further increase the surface hardness of the martensitic stainless steel sintered body 1.

[0123] The Vickers hardness of the nitrided layer 2 is preferably between 400 and 800, and more preferably between 480 and 700. If the Vickers hardness of the nitrided layer 2 is within the above range, the wear resistance of the martensitic stainless steel sintered body 1 can be sufficiently improved.

[0124] The Vickers hardness of nitride layer 2 is measured as follows: First, the martensitic stainless steel sintered body 1 is cut and the cross-section is polished. Next, the Vickers hardness of the cross-section of nitride layer 2 is measured using a micro-Vickers hardness tester. The measurement position is set to the middle of the thickness of nitride layer 2. The indentation load of the indenter during the test is set to 1.96 N.

[0125] For the manufacture of the martensitic stainless steel sintered body 1, martensitic stainless steel powder having the composition described above is preferably used. In this case, the composition of the martensitic stainless steel sintered body 1 will be the same as the composition of the martensitic stainless steel powder described above.

[0126] The martensitic stainless steel sintered body 1 preferably has a Nb content of 0.5% by mass or more and 3.0% by mass or less, more preferably 1.0% by mass or more and 2.5% by mass or less, and even more preferably 1.2% by mass or more and 1.8% by mass or less. By specifying the Nb content within the above range, it is possible to achieve a higher degree of both high corrosion resistance and high surface hardness in the martensitic stainless steel sintered body 1.

[0127] Furthermore, if the Nb content is within the aforementioned range, NbC and NbN are more likely to precipitate on the surface of the martensitic stainless steel sintered body 1. NbC contributes to increasing the density of the martensitic stainless steel sintered body 1. NbN contributes to improving the corrosion resistance and surface hardness of the martensitic stainless steel sintered body 1.

[0128] As described above, the martensitic stainless steel sintered body 1 according to the embodiment is a sintered body of martensitic stainless steel powder, containing Nb, and having a nitrided layer 2 and a base 3. The nitrided layer 2 is located on the surface and is a layer containing the austenite phase with an average thickness of 10 μm or more. The base 3 is located inside the nitrided layer 2 and is a portion containing the martensite phase.

[0129] With this configuration, the nitride layer 2 is located on the surface, thus providing the martensitic stainless steel sintered body 1 with high corrosion resistance and high surface hardness. Furthermore, since the base 3 is located inside, the nitride layer 2 is supported by the base 3, which can compensate for the surface hardness of the nitride layer 2. As a result, a martensitic stainless steel sintered body 1 that achieves both high corrosion resistance and high surface hardness is obtained.

[0130] Furthermore, the Vickers hardness of the nitrided layer 2 is preferably between 400 and 800. This allows for a sufficiently high level of wear resistance of the martensitic stainless steel sintered body 1.

[0131] Furthermore, the Nb content of the martensitic stainless steel sintered body 1 is preferably 0.5% by mass or more and 3.0% by mass or less. This makes it possible to achieve a higher level of both high corrosion resistance and high surface hardness in the martensitic stainless steel sintered body 1.

[0132] Furthermore, when the martensitic stainless steel sintered body 1 is subjected to crystal structure analysis by X-ray diffraction, the resulting X-ray diffraction pattern P3 includes a peak pk1 (first peak) originating from the martensite phase located at 2θ = 80 to 82° and a peak pk2 (second peak) originating from the austenite phase located at 2θ = 50 to 52°. When the height of peak pk1 is set to 100, the height of peak pk2 is preferably between 3 and 100.

[0133] The ratio of the height of peak pk2 to the height of peak pk1, pk2 / pk1, represents the volume ratio of the austenite phase to the martensite phase. When the ratio pk2 / pk1 is within the aforementioned range, the martensitic stainless steel sintered body 1 can achieve a particularly good balance between high corrosion resistance and high surface hardness.

[0134] The martensitic stainless steel sintered body 1 described above can be used as a material to constitute all or part of the following: parts for transportation equipment such as automobile parts, bicycle parts, railway vehicle parts, ship parts, aircraft parts, and space transport vehicle parts; parts for electronic equipment such as personal computer parts, mobile phone terminal parts, tablet terminal parts, and wearable device parts; parts for electrical equipment such as refrigerators, washing machines, and air conditioners; parts for machinery such as machine tools and semiconductor manufacturing equipment; parts for plants such as nuclear power plants, thermal power plants, hydroelectric power plants, oil refineries, and chemical complexes; parts for watches; metal tableware, jewelry, and eyeglass frames.

[0135] The method for manufacturing a martensitic stainless steel sintered body and the martensitic stainless steel sintered body of the present invention have been described above based on preferred embodiments, but the present invention is not limited thereto. For example, the martensitic stainless steel sintered body of the present invention may be manufactured by a manufacturing method other than the method for manufacturing a martensitic stainless steel sintered body of the present invention. Furthermore, the method for manufacturing a martensitic stainless steel sintered body of the present invention may have additional steps added to the above embodiments for any purpose. [Examples]

[0136] Next, embodiments of the present invention will be described. 3. Manufacturing of martensitic stainless steel sintered bodies 3.1. Example 1 First, a mixture (composition) containing martensitic stainless steel powder produced by the water atomization method and an organic binder was prepared. The metal powder used was martensitic stainless steel powder with an average particle size of 6.0 μm. The organic binder was a mixture of polypropylene and wax. The mixing ratio of the organic binder in the mixture was 8% by mass.

[0137] Next, the kneaded mixture was molded using an injection molding machine to obtain a molded body. The molded body was a rectangular parallelepiped with dimensions of 15 mm in length, 15 mm in width, and 3 mm in height.

[0138] Next, the molded body was degreased to obtain a degreased body. The degreasing process involved heating the molded body at 450°C for 2 hours under a nitrogen atmosphere.

[0139] Next, the degreased body was subjected to a sintering treatment to obtain a sintered body. The sintering treatment was carried out according to the sintering temperature pattern P1 shown in Figure 2. Specifically, the time for holding the degreased body in the temperature range of 900°C to 1100°C (first temperature holding time t1) was set to 7 hours. In particular, the time for holding the degreased body in the temperature range of 900°C to 1100°C was set to 5 hours. Of that, the time for holding the degreased body in the temperature range of 950°C to 1050°C was set to 3 hours. In addition, the time for holding the degreased body in the temperature range of 1200°C to 1500°C (second temperature holding time t2) was set to 3 hours. The atmosphere for the sintering treatment was a reduced-pressure argon gas atmosphere.

[0140] Next, the sintered body was subjected to nitriding treatment to obtain a martensitic stainless steel sintered body. The nitriding treatment was carried out according to the nitriding temperature pattern P2 shown in Figure 3. Specifically, the time for holding the sintered body in the temperature range of 700°C to 1300°C (third temperature holding time t3) was set to 6 hours. In particular, the time for holding the sintered body in the temperature range of 1100°C to 1300°C was set to 5 hours. After heating in this temperature range, the sintered body was rapidly cooled by water cooling. The atmosphere for the nitriding treatment was a pressurized nitrogen atmosphere. The total pressure of the atmosphere was set to 0.20 MPa, and only nitrogen gas with a purity of 99% or higher was used.

[0141] 3.2. Examples 2-26 A martensitic stainless steel sintered body was obtained in the same manner as in Example 1, except that the composition ratio of the martensitic stainless steel powder was changed as shown in Tables 1 to 3.

[0142] 3.3. Comparative Examples 1-9 A sintered body was obtained in the same manner as in Example 1, except that the composition ratio of the martensitic stainless steel powder was changed as shown in Tables 1 to 3.

[0143] [Table 1]

[0144] [Table 2]

[0145] [Table 3]

[0146] 4. Evaluation of martensitic stainless steel sintered bodies 4.1. Relative Density The relative density of the sintered bodies obtained in each example and comparative example was calculated according to the method specified in JIS Z 2501:2000. The calculation results are shown in Tables 4 to 6.

[0147] 4.2. Peak height ratio of XRD For the sintered bodies obtained in each example and comparative example, the ratio of the height of peak pk2 to the height of peak pk1, pk2 / pk1, was calculated using the method described above. The calculation results are shown in Tables 4 to 6.

[0148] 4.3. Tensile strength The tensile strength of the sintered bodies obtained in each example and comparative example was measured in accordance with the tensile testing method for metallic materials specified in JIS Z 2241:2011. The tensile test speed was set to 5 mm / min. The measured tensile strength was then evaluated against the following evaluation criteria.

[0149] A: The tensile strength of the sintered body is 1800 MPa or higher. B: The tensile strength of the sintered body is 1600 MPa or more and less than 1800 MPa. C: The tensile strength of the sintered body is 1400 MPa or more and less than 1600 MPa. D: The tensile strength of the sintered body is 1200 MPa or more and less than 1400 MPa. E: The tensile strength of the sintered body is between 1000 MPa and 1200 MPa. F: The tensile strength of the sintered body is 800 MPa or more and less than 1000 MPa. G: The tensile strength of the sintered body is less than 800 MPa. The evaluation results are shown in Tables 4 to 6.

[0150] 4.4. Vickers hardness The sintered bodies obtained in each example and comparative example were cut using a fine cutter. The Vickers hardness was then measured at two points on the cut surface: 0.2 mm from the surface and 0.5 mm from the surface, in accordance with the Vickers hardness test method specified in JIS Z 2244:2009. The measurement load was 200 gf. The measured Vickers hardness was then evaluated against the following evaluation criteria.

[0151] A: The Vickers hardness is 480 or higher. B: The Vickers hardness is between 450 and 480. C: Vickers hardness is between 420 and 450. D: Vickers hardness is between 400 and 420. E: Vickers hardness is between 360 and 400. F: Vickers hardness is less than 360. The evaluation results are shown in Tables 4 to 6.

[0152] Furthermore, for the sintered body of Example 2, the Vickers hardness was measured at 0.050 mm intervals from a depth of 0.050 mm to a depth of 1.000 mm from the surface. In addition, for the sintered body of Comparative Example 3, the Vickers hardness was measured at 0.050 mm intervals from a depth of 0.050 mm to a depth of 0.500 mm from the surface. The relationship between the depth from the surface of the sintered body and the Vickers hardness, i.e., the hardness distribution, is shown as a graph in Figure 6.

[0153] As shown in Figure 6, the sintered body of Example 2 exhibited high Vickers hardness from the surface to the interior, with little variation. In contrast, the sintered body of Comparative Example 3 showed high Vickers hardness internally, but decreased Vickers hardness on the surface. Therefore, it was found that nitriding treatment can increase both the internal hardness and surface hardness of a sintered body.

[0154] 4.5. Corrosion resistance The pitting potential of the sintered bodies obtained in each example and comparative example was measured according to Method B of the method for measuring pitting potential of stainless steel specified in JIS G 0577:2014. Method B is a method for measuring pitting potential using the potentiometric method in a 3.5 mass% sodium chloride aqueous solution. The current density was 100 μA / cm². 2 The potential at which corrosion begins was conveniently defined as the pitting potential, i.e., the potential at which corrosion starts to progress. The pitting potential was set to the saturated calomel electrode (SCE) reference value. The pH of the sodium chloride aqueous solution was set to 7, and the temperature to 30°C. The potential sweep rate was set to 20 mV / min. The measured pitting potential was then evaluated against the following evaluation criteria.

[0155] A: The pitting potential is -0.05V or higher. B: The pitting potential is between -0.10V and -0.05V. C: Pitting potential is between -0.20V and -0.10V D: The pitting potential is between -0.30V and -0.20V. E: Pitting potential is between -0.40V and -0.30V. F: Pitting potential is less than -0.40V The evaluation results are shown in Tables 4 to 6.

[0156] Furthermore, Figure 7 shows the relationship between the potential and current density obtained from the sintered body of Example 2, and the relationship between the potential and current density obtained from the sintered body of Comparative Example 3, as graphs. Note that in Figure 7, the current density is 100 μA / cm². 2 A dashed line was drawn at the position indicated. The potentials corresponding to the intersection of this dashed line with the graphs of Example 2 and Comparative Example 3 are the respective pitting potentials.

[0157] As shown in the graph in Figure 7, the pitting potential obtained from the sintered body of Example 2 was significantly higher than the pitting potential obtained from the sintered body of Comparative Example 3. The pitting potential of the former is comparable to that of typical austenitic stainless steel.

[0158] [Table 4]

[0159] [Table 5]

[0160] [Table 6]

[0161] As shown in Tables 4 to 6, the martensitic stainless steel sintered bodies produced by the manufacturing methods of the examples were found to possess both high weather resistance and high surface hardness. Furthermore, it was found that high-density and high-strength martensitic stainless steel sintered bodies can be produced by optimizing the alloy composition.

[0162] In contrast, when Nb addition was omitted, when decarburization was omitted, and when nitriding was omitted, it was not possible to form a nitrided layer with a thickness of 10 μm or more. As a result, the corrosion resistance and surface hardness were insufficient. [Explanation of symbols]

[0163] 1...Martensitic stainless steel sintered body, 2...Nitrided layer, 3...Base, P1...Sintering temperature pattern, P2...Nitrided temperature pattern, P3...X-ray diffraction pattern, P4...X-ray diffraction pattern, pa...Peak, pk1...Peak, pm...Peak, pk2...Peak, S102...Composition preparation process, S104...Molding process, S106...Degreasing process, S108...Sintering process, S110...Nitrided process, t1...First temperature holding time, t2...Second temperature holding time, t3...Third temperature holding time

Claims

1. A composition preparation step of mixing martensitic stainless steel powder having an Nb content of 0.5% by mass or more and 3.0% by mass or less with an organic binder to obtain a composition, A molding step of molding the above composition to obtain a molded body, A degreasing step to remove at least a portion of the organic binder contained in the molded body to obtain a degreased body, A sintering step to obtain a sintered body by performing a sintering treatment on the degreased body, which includes heating at a temperature of 900°C to 1100°C for 30 minutes or more, The nitriding process involves heating the sintered body in a nitrogen atmosphere containing nitrogen at a temperature of 700°C to 1300°C, and A method for producing a martensitic stainless steel sintered body, characterized by having the following features.

2. The method for manufacturing a martensitic stainless steel sintered body according to claim 1, wherein the total pressure of the nitrogen atmosphere is greater than atmospheric pressure.

3. The method for manufacturing a martensitic stainless steel sintered body according to claim 1 or 2, wherein the sintering process is carried out under an inert atmosphere and at a pressure below atmospheric pressure.

4. The method for producing a martensitic stainless steel sintered body according to any one of claims 1 to 3, wherein the nitriding treatment includes an operation to rapidly cool the sintered body.

5. A sintered body of martensitic stainless steel powder having an Nb content of 0.5% by mass or more and 3.0% by mass or less, Contains Nb, A nitride layer located on the surface, containing an austenite phase and having an average thickness of 10 μm or more, Located inside the nitrided layer, the base contains a martensite phase, A martensitic stainless steel sintered body characterized by having the following properties.

6. The martensitic stainless steel sintered body according to claim 5, wherein the Vickers hardness of the nitrided layer is 400 or more and 800 or less.

7. When subjected to crystal structure analysis by X-ray diffraction, the resulting X-ray diffraction pattern includes a first peak originating from the martensite phase located at 2θ = 80–82° and a second peak originating from the austenite phase located at 2θ = 50–52°. The martensitic stainless steel sintered body according to claim 5 or 6, wherein when the height of the first peak is 100, the height of the second peak is 3 or more and 100 or less.

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