Ferritic stainless steel powder, ferritic stainless steel components, and manufacturing methods of ferritic stainless steel components

Abstract

This invention provides ferritic stainless steel powder capable of manufacturing ferritic stainless steel components with excellent shapeability, oxidation resistance, and corrosion resistance. A ferritic stainless steel powder comprises, by mass%, C: 0.003–0.200%, Si: 0.01–2.00%, Mn: 0.05–2.00%, P: less than 0.040%, S: less than 0.010%, Cr: 15.0–35.0%, Ni: 0.10–2.50%, Mo: 0.50–3.00%, Nb: more than 0.10% and less than 0.70%, N: less than 0.030%, and O: less than 0.200%, and satisfies the following formula (1), with the balance consisting of Fe and unavoidable impurities, and a median diameter D. 50 The micrometer size is greater than 10 μm and less than 200 μm, with an apparent density of 3.5 Mg / m³. 3 Above and 5.0Mg / m 3 the following. (Ni+Mo)≥2.00…(1)

Description

Technical Field

[0001] This invention relates to ferritic stainless steel powder, and more particularly to ferritic stainless steel powder suitable for manufacturing complex-shaped components such as heat exchangers that require excellent corrosion resistance. Furthermore, this invention also relates to ferritic stainless steel components obtained by molding using the ferritic stainless steel powder, and a method for manufacturing the same. Background Technology

[0002] In recent years, the application of metal additive manufacturing has been expanding. Currently, the most popular modeling method is powder additive manufacturing (also known as powder bed manufacturing). Powder additive manufacturing is a method that uses laser or electron beam as a heat source to melt metal powder as raw material while simultaneously performing additive manufacturing.

[0003] The most significant characteristic of additive manufacturing is its ability to create shapes that are impossible to form using conventional processing methods, even while considering both surface and internal structure. Furthermore, because additive manufacturing does not require consideration of material machinability, even materials that were previously difficult to process and had limited shape, such as Ni-based alloys and Ti-based alloys, can be shaped into any form.

[0004] Leveraging these characteristics, the industrial application of metal additive manufacturing is advancing in the aerospace and medical fields. For example, for components with complex shapes and high heat resistance requirements, such as turbine blades, additive manufacturing of Ni alloy components offers advantages such as shape optimization and reduced component count. Furthermore, in the medical field, additive manufacturing is also being utilized due to its ability to create artificial joints and implants tailored to individual patients.

[0005] Additive manufacturing is currently limited to aerospace and medical applications due to low mass production capabilities and high styling costs. However, it is believed that if these issues are resolved, its applications will expand to sectors such as automotive, where productivity and cost are paramount. Particularly in the automotive industry, applying additive manufacturing to the manufacture of complex exhaust system components and heat exchangers could potentially add value, such as shape optimization or improved overall component strength without the need for brazing or welding. Therefore, Fe-based alloys, such as stainless steel, which are less expensive than Ni-based and Ti-based alloys, are gaining attention as materials for additive manufacturing.

[0006] Stainless steel powder for additive manufacturing has been disclosed in Patent Documents 1-6.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: International Publication No. 2019 / 139017

[0010] Patent Document 2: International Publication No. 2019 / 235014

[0011] Patent Document 3: International Publication No. 2020 / 110498

[0012] Patent Document 4: International Publication No. 2021 / 214958

[0013] Patent Document 5: Japanese Patent No. 6270563

[0014] Patent Document 6: Japanese Patent No. 6985940 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] In recent years, from the perspective of global environmental protection, there has been a demand to further improve fuel efficiency and enhance exhaust gas purification in automobiles. Therefore, there is a growing trend towards higher exhaust gas temperatures, and the application of automotive heat exchangers such as waste heat recovery units and EGR (Exhaust Gas Recirculation) coolers is expanding rapidly.

[0017] Here, the waste heat recovery unit is a device that uses the heat from exhaust gases to heat the engine's cooling water, shortening the engine's warm-up time during startup and thus improving fuel efficiency. Typically, the waste heat recovery unit is located between the catalytic converter and the muffler, and consists of a heat exchanger section combining tubes, plates, fins, and side plates, as well as an inlet side pipe section and an outlet side pipe section. Exhaust gases enter the heat exchanger section through the inlet side pipe, and their heat is transferred to the cooling water via the heat transfer surfaces such as the fins, before being discharged from the outlet side pipe. Furthermore, the plates and fins of the heat exchanger section constituting this type of waste heat recovery unit are primarily bonded and assembled using brazing rather than welding.

[0018] Additionally, the EGR cooler is a device used to recirculate low-oxygen exhaust gas back to the engine's intake side to lower the fuel combustion temperature and thus suppress nitrogen oxides (NOx) that are easily formed at high temperatures. If the high-temperature exhaust gas were directly recirculated back to the engine, the fuel would burn at an inappropriate time, resulting in abnormal vibrations known as knock. Therefore, the EGR cooler consists of a pipe that introduces a portion of the exhaust gas, a heat exchanger that cools the introduced exhaust gas, and a pipe that returns the cooled exhaust gas to the engine's intake side. The exhaust gas temperature is above 600°C at the EGR cooler's inlet side, but it is cooled in the heat exchanger and reaches below 100°C at the outlet side. Furthermore, for reasons of weight reduction, miniaturization, and cost reduction, the heat exchanger portion of the EGR cooler is constructed by stacking thin plates into fins, and their joining and assembly are mainly done by brazing.

[0019] Thus, the various parts of the waste heat recovery unit and EGR cooler are assembled by brazing, which can lead to problems such as component sagging during the high-temperature brazing heat treatment. Furthermore, due to exposure to high-temperature exhaust gases, excellent oxidation resistance is required. Moreover, in addition to NOx, the exhaust gases contain sulfur oxides (SOx) and hydrocarbons (HC), which condense inside the heat exchanger, forming highly corrosive acidic condensate. Therefore, the heat exchanger also requires excellent corrosion resistance. Especially when the exhaust gas temperature reaches above 600°C, it is necessary to prevent Cr at the grain boundaries from reacting with C and N to form Cr carbonitrides, creating a Cr-depleted layer with poor corrosion resistance around it—a process known as sensitization—to ensure corrosion resistance.

[0020] For the reasons mentioned above, the heat exchanger sections of waste heat recovery units and EGR coolers typically use austenitic stainless steels such as SUS316L or SUS304L, which prevent sensitization by reducing carbon content. However, austenitic stainless steels suffer from high costs due to their high Ni content, severe vibration at high temperatures due to large thermal expansion, and low thermal fatigue characteristics in constrained operating environments. Therefore, they are being gradually replaced by ferritic stainless steels such as SUS444 (18Cr-2Mo) and SUS430J1L (18Cr-Cu).

[0021] On the other hand, stainless steel powder for additive manufacturing has been disclosed in Patent Documents 1-6. However, the materials disclosed in Patent Documents 1-6 do not meet the requirements for sufficient oxidation and corrosion resistance when envisioned for application in automotive exhaust system components, particularly heat exchangers such as waste heat recovery units and EGR coolers. Therefore, it is necessary to develop stainless steel powder that meets these requirements. Furthermore, for components manufactured through additive manufacturing, it is required that no cracks occur even under temperature changes such as rapid solidification and rapid heating characteristic of additive manufacturing; that is, excellent formability is required.

[0022] The present invention was made in view of the above circumstances, and its object is to provide ferritic stainless steel powder capable of manufacturing ferritic stainless steel components with excellent shapeability, oxidation resistance and corrosion resistance.

[0023] In this invention, excellent oxidation resistance means that, for test pieces obtained by polishing the surface of ferritic stainless steel components with #600 abrasive paper, after an oxidation test in the atmosphere at 600°C for 400 hours, the oxidation increment is 20 g / m³. 2 The following conditions were observed without any peeling of the oxidized film.

[0024] Furthermore, in this invention, excellent corrosion resistance means that a 20mm square test piece is cut from a ferritic stainless steel component, and according to JIS G 0577:2014, for test pieces obtained by polishing the surface with #600 abrasive paper, after leaving an 11mm square test surface, it is covered with a sealing material, and then the test piece is immersed in a 3.5% by mass NaCl solution at 30°C, and the pitting potential Vc'10 is measured to be above 200mV (vs SCE).

[0025] Methods for solving problems

[0026] To achieve the aforementioned objectives, the inventors conducted in-depth research and discovered that excellent oxidation resistance and corrosion resistance can be achieved by increasing the Cr content to 15.0% by mass or more, containing 0.10% by mass or more of Ni and 0.50% by mass or more of Mo, and further specifying the total content of Ni and Mo. Furthermore, it was found that by optimizing the composition containing these components and specifying the properties of the steel powder (particle size and apparent density), ferritic stainless steel powder capable of manufacturing ferritic stainless steel components with excellent formability, oxidation resistance, and corrosion resistance can be obtained.

[0027] By manufacturing ferritic stainless steel powder as described above and using it as a material for additive manufacturing, ferritic stainless steel components with complex shapes and extremely excellent properties can be obtained.

[0028] Especially considering its application in EGR coolers, excellent oxidation resistance and superior corrosion resistance compared to SUS430J1L are desired. More specifically, a pitting potential Vc'10 of 200 mV (vs SCE) or higher is expected.

[0029] This invention is based on the above insights and has been completed through further research. The main points of this invention are as follows.

[0030] [1] A ferritic stainless steel powder having, by mass %, the following composition: C: 0.003–0.200%, Si: 0.01–2.00%, Mn: 0.05–2.00%, P: less than 0.040%, S: less than 0.010%, Cr: 15.0–35.0%, Ni: 0.10–2.50%, Mo: 0.50–3.00%, Nb: more than 0.10% and less than 0.70%, N: less than 0.030%, and O: less than 0.200%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities, and a median diameter D 50 The micrometer size is greater than 10 μm and less than 200 μm, with an apparent density of 3.5 Mg / m³. 3 Above and 5.0Mg / m 3 the following.

[0031] (Ni+Mo)≥2.00 …(1)

[0032] In equation (1), Ni and Mo represent the contents (mass %) of Ni and Mo, respectively.

[0033] [2] The ferritic stainless steel powder according to [1], wherein the composition, by mass %, further contains one or more of the following: Al: less than 0.30%, Ti: less than 0.30%, V: less than 0.50%, Zr: less than 0.50%, Cu: less than 1.00%, Co: less than 0.50%, B: less than 0.0100%, Ca: less than 0.0100%, Mg: less than 0.0050%, REM: less than 0.50%, Sn: less than 0.50%, and Sb: less than 0.50%.

[0034] [3] A ferritic stainless steel component comprising, by mass%, C: 0.003–0.200%, Si: 0.01–2.00%, Mn: 0.05–2.00%, P: less than 0.040%, S: less than 0.010%, Cr: 15.0–35.0%, Ni: 0.10–2.50%, Mo: 0.50–3.00%, Nb: more than 0.10% and less than 0.70%, N: less than 0.030%, and O: less than 0.200%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities, and the number of MnS per unit area N MnS It satisfies the following equation (2).

[0035] (Ni+Mo)≥2.00 …(1)

[0036] N MnS ≤100 pieces / mm 2 …(2)

[0037] In equation (1), Ni and Mo represent the contents (mass %) of Ni and Mo, respectively.

[0038] [4] The ferritic stainless steel component according to [3], wherein the composition, by mass %, further contains one or more of the following: Al: less than 0.30%, Ti: less than 0.30%, V: less than 0.50%, Zr: less than 0.50%, Cu: less than 1.00%, Co: less than 0.50%, B: less than 0.0100%, Ca: less than 0.0100%, Mg: less than 0.0050%, REM: less than 0.50%, Sn: less than 0.50%, and Sb: less than 0.50%.

[0039] [5] A method for manufacturing a ferritic stainless steel component, wherein the ferritic stainless steel component is manufactured by additive manufacturing using the ferritic stainless steel powder described in [1] or [2].

[0040] Invention Effects

[0041] According to the present invention, ferritic stainless steel powder can be provided that can manufacture ferritic stainless steel components with excellent shapeability, oxidation resistance and corrosion resistance.

[0042] According to the present invention, ferritic stainless steel powder suitable for manufacturing ferritic stainless steel components, particularly for manufacturing ferritic stainless steel components by additive manufacturing, can be provided. The ferritic stainless steel powder of the present invention is suitable as a material for ferritic stainless steel components with complex shapes and requiring excellent oxidation and corrosion resistance, such as automotive exhaust system components and heat exchangers, and is particularly suitable as a material for manufacturing such ferritic stainless steel components by additive manufacturing.

[0043] Furthermore, ferritic stainless steel components manufactured using the ferritic stainless steel powder of the present invention exhibit excellent oxidation and corrosion resistance. Moreover, they exhibit excellent formability because they do not crack due to rapid temperature changes during additive manufacturing. The ferritic stainless steel components obtained by the present invention are particularly suitable for use as components with complex shapes and requiring oxidation and corrosion resistance, such as automotive exhaust system components and heat exchangers. Detailed Implementation

[0044] The present invention will be described based on the following embodiments.

[0045] First, the composition of the ferritic stainless steel powder (hereinafter, also referred to as stainless steel powder) and the ferritic stainless steel component (hereinafter, also referred to as stainless steel component) of the present invention will be described. It should be noted that the unit in the composition is "mass%", and unless otherwise specified, it will be expressed as "%".

[0046] C: 0.003~0.200%

[0047] C has the effect of forming carbides with elements such as Nb, thereby increasing the strength of steel. Here, to obtain sufficient strength, the C content is set to 0.003% or more. The C content is preferably set to 0.030% or more. On the other hand, when the C content exceeds 0.200%, the steel becomes excessively hardened, and the toughness of stainless steel components decreases. Furthermore, when the C content exceeds 0.200%, solidification cracks are easily generated during additive manufacturing, reducing shapeability. Therefore, the C content is set to 0.200% or less. The C content is preferably 0.150% or less, and more preferably 0.100% or less.

[0048] Si: 0.01~2.00%

[0049] Si has the effect of improving the oxidation resistance of steel. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably set to 0.25% or more. However, when the Si content exceeds 2.00%, the steel becomes overly hardened, and its toughness decreases. Therefore, the Si content is set to 2.00% or less. The Si content is preferably 1.50% or less, and more preferably 1.00% or less.

[0050] Mn: 0.05~2.00%

[0051] Mn has the effect of reducing the oxidation rate and preventing oxide scale peeling. To achieve this effect, the Mn content is set to 0.05% or more. The Mn content is preferably set to 0.25% or more. On the other hand, Mn is an austenite phase-forming element; therefore, when the Mn content exceeds 2.00%, an austenite phase will form, thereby reducing oxidation resistance. Therefore, the Mn content is set to 2.00% or less. The Mn content is preferably 1.50% or less, and more preferably 1.00% or less.

[0052] P: below 0.040%

[0053] Phosphorus (P) is an element that reduces corrosion resistance, therefore it is desirable to reduce its content, with the upper limit of P content set at 0.040%. The P content is preferably 0.030% or less, more preferably 0.020% or less, and even more preferably 0.010% or less. It should be noted that there is no particular limitation on the lower limit of P content. However, excessive P removal will lead to increased costs, therefore the P content is preferably 0.005% or more.

[0054] S: below 0.010%

[0055] Sulfur (S) is an element that reduces corrosion resistance, so it is desirable to reduce its content, and the upper limit of S content is set at 0.010%. The S content is preferably 0.005% or less, more preferably 0.003% or less. It should be noted that there is no particular limitation on the lower limit of S content. However, excessive S removal will lead to increased costs, so the S content is preferably 0.0005% or more.

[0056] Cr: 15.0~35.0%

[0057] Cr is an important element for improving the corrosion resistance and oxidation resistance of steel. To achieve the desired corrosion resistance and oxidation resistance, the Cr content needs to be 15.0% or more. However, when the Cr content exceeds 35.0%, the toughness of stainless steel components decreases. Therefore, the Cr content is set in the range of 15.0% to 35.0%. The Cr content is preferably 18.0% or more, more preferably 20.0% or more. Furthermore, the Cr content is preferably 32.0% or less, more preferably 30.0% or less, and even more preferably 28.0% or less.

[0058] Ni: 0.10~2.50%

[0059] Ni has the effect of improving the corrosion resistance of stainless steel components. To achieve this effect, the Ni content is set to 0.10% or more. Preferably, the Ni content is 0.20% or more, and more preferably 0.50% or more. On the other hand, Ni is an element that promotes the formation of the austenite phase. When the Ni content exceeds 2.50%, the austenite phase will form, thereby reducing oxidation resistance. Therefore, the Ni content is set to 2.50% or less. Preferably, the Ni content is 2.30% or less.

[0060] Mo: 0.50–3.00%

[0061] Mo stabilizes the passivation film on stainless steel, thereby improving its corrosion resistance and oxidation resistance. This effect is achieved when the Mo content is 0.50% or more. However, when the Mo content exceeds 3.00%, intermetallic compounds precipitate, resulting in reduced toughness. Therefore, the Mo content is set in the range of 0.50% to 3.00%. The Mo content is preferably 1.25% or more, more preferably 1.50% or more, and even more preferably 1.80% or more. Furthermore, the Mo content is preferably 2.50% or less, more preferably 2.00% or less.

[0062] Nb: ≥0.10% and <0.70%

[0063] Nitrogen (Nb) acts as a solid solution strengthening element, improving the high-temperature resistance of steel. Furthermore, it also enhances the high-temperature resistance of steel by forming carbides and nitrides with carbon (C) and nitrogen (N) in the steel. These effects are achieved when the Nb content is 0.10% or higher. However, when the Nb content reaches 0.70% or higher, intermetallic compound phases are formed, reducing the toughness of stainless steel components. Therefore, the Nb content is set in the range of 0.10% or higher and less than 0.70%. The Nb content is preferably 0.20% or higher, more preferably 0.30% or higher. Furthermore, the Nb content is preferably 0.60% or lower, more preferably 0.50% or lower, and even more preferably 0.40% or lower.

[0064] N: below 0.030%

[0065] Nitrogen (N) acts as a solid solution strengthening element, improving the high-temperature resistance of steel. Furthermore, it increases the strength of steel by forming nitrides with elements such as Nb. To achieve this effect, the N content is preferably 0.0030% or more, more preferably 0.0050% or more. On the other hand, when the N content exceeds 0.030%, solidification cracks are easily generated during additive manufacturing, reducing formability. Therefore, the N content is set to 0.030% or less. The N content is preferably 0.020% or less.

[0066] O: Below 0.200%

[0067] The stainless steel powder of the present invention can be manufactured by gas atomization or water atomization, but metal powders manufactured by these methods are prone to contain oxide (O). To reduce the amount of oxide inclusions generated in the molded product, the O content is set to 0.200% or less. The O content is preferably 0.070% or less, more preferably 0.050% or less. However, excessive O removal leads to increased costs; therefore, the O content is preferably 0.020% or more.

[0068] (Ni+Mo)≥2.00 …(1)

[0069] In equation (1), Ni and Mo represent the contents (mass %) of Ni and Mo, respectively.

[0070] Ni and Mo have the effect of improving corrosion resistance by solidifying in the parent phase. To obtain this effect, it is necessary to contain one or both of Ni and Mo in a total of 2.00% or more. That is, it is necessary to satisfy the above formula (1). The total content of one or both of Ni and Mo is preferably 3.00% or more. In addition, the above total content is 5.50% or less, preferably 4.80% or less, and more preferably 4.30% or less.

[0071] The ferritic stainless steel powder and stainless steel components of the present invention have a composition containing the above-mentioned components, with the balance consisting of Fe and unavoidable impurities.

[0072] The stainless steel powder and stainless steel components of the present invention may, in addition to the above-mentioned components, contain one or more components selected from Al: less than 0.30%, Ti: less than 0.30%, V: less than 0.50%, Zr: less than 0.50%, Cu: less than 1.00%, Co: less than 0.50%, B: less than 0.0100%, Ca: less than 0.0100%, Mg: less than 0.0050%, REM: less than 0.50%, Sn: less than 0.50%, and Sb: less than 0.50%.

[0073] Al: below 0.30%

[0074] Al significantly improves the oxidation resistance of steel by forming an Al oxide film on the surface. To achieve this effect, the Al content is preferably set to 0.03% or more. However, when the Al content exceeds 0.30%, the toughness of stainless steel components decreases. Therefore, in the case of Al content, the Al content is set to 0.30% or less.

[0075] Ti: below 0.30%

[0076] Ti has the effect of forming carbides and nitrides with C and N in steel, thereby increasing the strength of the steel. To achieve this effect, the Ti content is preferably set to 0.01% or more. However, when the Ti content exceeds 0.30%, the steel becomes excessively hardened, and the toughness of stainless steel components decreases. Therefore, when Ti is present, the Ti content is set to 0.30% or less. The Ti content is preferably 0.20% or less.

[0077] V: Below 0.50%

[0078] V (V) has the effect of forming carbides and nitrides with C and N in steel, thereby increasing the strength of the steel. To achieve this effect, the V content is preferably set to 0.01% or more. However, when the V content exceeds 0.50%, the steel becomes excessively hardened, and the toughness of stainless steel components decreases. Therefore, when V is present, the V content is set to 0.50% or less. The V content is preferably 0.30% or less.

[0079] Zr: below 0.50%

[0080] Zr improves the adhesion of oxide films and enhances oxidation resistance. To achieve this effect, the Zr content is preferably 0.01% or more. However, when the Zr content exceeds 0.50%, intermetallic compound phases precipitate, which reduces oxidation resistance. Therefore, when Zr is present, the Zr content is set to 0.50% or less. The Zr content is preferably 0.30% or less, and more preferably 0.10% or less.

[0081] Cu: below 1.00%

[0082] Cu is an element that improves corrosion resistance. To achieve this effect, the Cu content is preferably set to 0.01% or more. However, when the Cu content exceeds 1.00%, the steel becomes excessively hardened, and the toughness of stainless steel components decreases. Therefore, when Cu is present, the Cu content is set to 1.00% or less. The Cu content is preferably 0.80% or less, and more preferably 0.50% or less.

[0083] Co: below 0.50%

[0084] Co is an element that improves the toughness of stainless steel components. To achieve this effect, the Co content is preferably set at 0.01% or higher. However, when the Co content exceeds 0.50%, the steel becomes overly hardened, which reduces toughness. Therefore, in the case of Co content, the Co content is set to a range of 0.50% or lower.

[0085] B: Below 0.0100%

[0086] Boron (B) strengthens grain boundaries and improves toughness. To achieve this effect, the B content is preferably 0.0002% or more. However, when the B content exceeds 0.0100%, the steel becomes excessively hardened, which reduces the toughness of stainless steel components. Therefore, when B is present, the B content is set to 0.0100% or less. A B content of 0.0005% or more is more preferred. Furthermore, a B content of 0.0050% or less is preferable.

[0087] Ca: below 0.0100%

[0088] Ca has the effect of lowering the melting point of oxide inclusions and reducing inclusions in molten steel when manufacturing metal powders. To achieve this effect, the Ca content is preferably set to 0.0002% or more. However, when the Ca content exceeds 0.0100%, the toughness of stainless steel components decreases. Therefore, when Ca is present, the Ca content is set to a range of 0.0100% or less. The Ca content is more preferably 0.0005% or more. Furthermore, the Ca content is preferably 0.0050% or less, and more preferably 0.0030% or less.

[0089] Mg: below 0.0050%

[0090] Mg is an element that improves corrosion resistance. This effect is achieved by containing 0.0002% or more of Mg. Therefore, when Mg is present, the Mg content is preferably in the range of 0.0002% or more. However, when the Mg content exceeds 0.0050%, the toughness of the stainless steel component decreases. Therefore, when Mg is present, the Mg content is set to be in the range of 0.005% or less. The Mg content is more preferably 0.0005% or more. Furthermore, the Mg content is preferably 0.0035% or less, and more preferably 0.0020% or less.

[0091] REM: below 0.50%

[0092] Rare earth elements (REMs) improve the adhesion of oxide films and enhance oxidation resistance. To achieve this effect, the REM content is preferably 0.01% or higher. However, when the REM content exceeds 0.50%, the toughness of stainless steel components decreases. Therefore, when REMs are present, the REM content is set to 0.50% or lower. More preferably, the REM content is 0.05% or higher, and even more preferably 0.10% or higher. Furthermore, the REM content is preferably 0.30% or lower, and more preferably 0.20% or lower. It should be noted that REM is a collective term for 15 elements, including Sc, Y, and lanthanum (La) from atomic number 57 to lutetium (Lu) from atomic number 71; the REM content described herein refers to the total content of these elements.

[0093] Sn: below 0.50%

[0094] Sn has the effect of preventing surface roughness when grinding the surface of the molded object. To achieve this effect, the Sn content is preferably 0.01% or more. However, when the Sn content exceeds 0.50%, solidification cracks are easily generated, reducing the moldability. Therefore, when Sn is present, the Sn content is set to 0.50% or less. The Sn content is more preferably 0.03% or more, and even more preferably 0.05% or more. Furthermore, the Sn content is preferably 0.30% or less, and more preferably 0.10% or less.

[0095] Sb: below 0.50%

[0096] Sb has the effect of preventing surface roughness when grinding the surface of the molded object. To achieve this effect, the Sb content is preferably 0.01% or more. However, when the Sb content exceeds 0.50%, solidification cracks are easily generated, reducing the moldability. Therefore, when Sb is present, the Sb content is set to 0.50% or less. The Sb content is more preferably 0.03% or more, and even more preferably 0.05% or more. Furthermore, the Sb content is preferably 0.30% or less, and more preferably 0.10% or less.

[0097] Next, the properties (particle size and apparent density) of the stainless steel powder of the present invention will be described.

[0098] The median diameter D of the stainless steel powder of the present invention as a volume reference 50 The particle size is greater than 10 μm and less than 200 μm. Furthermore, the apparent density is 3.5 Mg / m³. 3 Above and 5.0Mg / m 3 the following.

[0099] Median diameter D 50 : 10μm and above and 200μm

[0100] The median diameter D of the stainless steel powder of the present invention as a volume reference 50 The median diameter D of stainless steel powder is between 10 μm and 200 μm. 50 When the powder is too small, its flowability decreases, leading to uneven powder filling and causing defects such as voids during additive manufacturing. Consequently, the strength, low-temperature toughness, and corrosion resistance of steel components manufactured from this powder may be reduced. The median diameter D of the stainless steel powder... 50 When the diameter is above 10 μm, it is easier to suppress the generation of defects such as voids during additive manufacturing. Therefore, the median diameter D of stainless steel powder is... 50 Set to 10 μm or larger. Median diameter D of stainless steel powder. 50 Preferably, it is 20 μm or more, more preferably 30 μm or more. On the other hand, the median diameter D of the stainless steel powder... 50 When the diameter is too large, it can cause defects such as voids during additive manufacturing. As a result, steel components manufactured using this steel powder may suffer from reduced strength, low-temperature toughness, and corrosion resistance due to these defects. The median diameter D of stainless steel powder... 50 When the diameter is below 200 μm, it is easier to suppress the generation of defects such as voids during additive manufacturing. Therefore, the median diameter D of stainless steel powder is... 50 Set to below 200 μm. Median diameter D of stainless steel powder. 50 Preferably, it is 150 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less.

[0101] It should be noted that the median diameter D of the stainless steel powder 50 The median diameter (50% particle size) refers to the cumulative volume distribution of stainless steel powder. The median diameter of stainless steel powder can be determined using a laser diffraction particle size analyzer. In this invention, the median diameter D of stainless steel powder is determined by the method described below. 50 .

[0102] As a laser diffraction particle size analyzer, there is the LA-950V2 manufactured by Horiba Manufacturing Co., Ltd. Of course, other devices can be used, but for accurate measurements, it is preferable to use a device with a lower limit of 0.1 μm and an upper limit of 200 μm or more for the measurable particle size range. In the laser diffraction particle size analyzer, a solvent containing dispersed stainless steel powder is irradiated with a laser, and the particle size distribution and average particle size (median diameter) of the stainless steel powder are determined based on the diffraction and scattering intensity of the laser. As the solvent for dispersing the stainless steel powder, ethanol, which has good dispersibility and is easy to handle, is preferred. When using solvents with high van der Waals forces and poor dispersibility, such as water, the powder may aggregate during measurement, sometimes resulting in a measurement result larger than the original average particle size, which is therefore undesirable. Therefore, for the ethanol solution containing stainless steel powder, it is preferable to perform ultrasonic dispersion treatment before measurement.

[0103] It should be noted that, since the appropriate dispersion time varies depending on the type of stainless steel powder being measured, the dispersion treatment time was divided into seven stages at 10-minute intervals, ranging from 0 to 60 minutes. The particle size distribution and average particle size (median diameter) of the stainless steel powder were measured after each dispersion treatment. During each measurement, the solvent was stirred while the measurement was performed to prevent aggregation of the stainless steel powder. Then, the minimum average particle size (median diameter) obtained from the seven measurements with varying dispersion times at 10-minute intervals was taken as the median diameter D of the stainless steel powder of this invention. 50 use.

[0104] The median diameter D was controlled as described above. 50 The stainless steel powder of the present invention further controls the apparent density to a high value. This is achieved by controlling the median diameter D of the stainless steel powder. 50 Both apparent density and apparent density contribute to better shapeability. The apparent density is set at 3.5 mg / m³. 3 The above. Apparent density is less than 3.5 Mg / m³. 3 Sometimes, a decrease in fluidity can cause defects such as voids during additive manufacturing. The apparent density is preferably set to 4.0 Mg / m³. 3 That's all. In addition, the apparent density of the stainless steel powder was set to 5.0 Mg / m³. 3 The following is because the apparent density exceeds 5.0 Mg / m³. 3 From an industrial perspective, it is sometimes difficult to reliably control apparent density.

[0105] It should be noted that the apparent density is the value obtained by the test method specified in JIS Z 2504:2020.

[0106] Next, the properties (number of MnS per unit area) of the stainless steel component of the present invention will be described.

[0107] The number N of MnS per unit area in the stainless steel component of the present invention MnS It satisfies the following equation (2).

[0108] N MnS ≤100 pieces / mm 2 …(2)

[0109] In the steel microstructure of the stainless steel component of the present invention, the number of MnS atoms per unit area (N) MnS ) is 100 pieces / mm 2The following is a summary of the findings. MnS, an inclusion composed of Mn and S, is a corrosion initiation point; therefore, the lower its quantity, the better the corrosion resistance. The inventors have discovered that even stainless steel components with the same composition, steel components manufactured using additive manufacturing (additive moldings) exhibit superior corrosion resistance compared to steel components manufactured from machined steel plates. The reason for this is not yet clear, but it can be considered that the rapid solidification unique to additive manufacturing prevents MnS from precipitating during cooling, thus reducing the amount of N in the steel structure. MnS Reduced. Further detailed research revealed that N MnS 100 pieces / mm 2 Excellent corrosion resistance can be obtained under the following conditions. N MnS More preferably 50 pieces / mm 2 The following is a further preferred value: 30 pieces / mm 2 Below. In addition, N MnS The lower limit is not specifically defined, N MnS It can be 0 / mm 2 .

[0110] The number N of MnS per unit area of ​​stainless steel components MnS The measurement method is as follows. A test piece is cut so that the observation surface is perpendicular to the height (additive manufacturing) direction of the stainless steel component. Resin embedding and mirror polishing are then performed. The observation surface can be set at 1 / 2 the height of the stainless steel component. Then, the 1.0 mm section of the mirror-polished observation surface is observed using SEM-EDX (scanning electron microscope-energy dispersive spectroscopy). 2 The range is defined as MnS, which contains at least 25 atomic percent Mn and at least 25 atomic percent S, and has a particle size (maximum diameter) of at least 0.05 μm. The particle size of these inclusions is set to at least 0.05 μm because inclusions larger than this size are detrimental to corrosion resistance, while inclusions smaller than 0.05 μm do not affect corrosion resistance. Three fields of view are used for SEM-EDX observation, and the average number of MnS inclusions in the three fields is taken as the sample's area per unit area (mm²). 2 The number N of MnS MnS .

[0111] Next, a preferred embodiment of the method for manufacturing stainless steel powder of the present invention will be described.

[0112] The stainless steel powder of the present invention is provided as the final material form through a series of manufacturing processes. For example, the stainless steel powder of the present invention is manufactured through processes such as melting, ingot formation, master ingot remelting, and powder production using an atomization process.

[0113] First, in the smelting-ingot forming process, the aforementioned elements are smelted and alloyed in a high-frequency vacuum smelting furnace in specified amounts, and then cast into an ingot (master ingot). At this time, smelting is preferably carried out under reduced pressure in an Ar atmosphere and at a smelting temperature of 1600°C or higher. The reason for setting this condition is as follows: if the smelting temperature is too low, the molten steel will solidify when dripping from the nozzle, causing nozzle blockage. Furthermore, from the viewpoint of preventing oxidation of the molten steel, smelting is preferably carried out in a reduced pressure Ar atmosphere. It should be noted that the smelting furnace used in this process is not limited to a high-frequency vacuum smelting furnace; other smelting furnaces (e.g., direct-electric heating smelting furnaces) can also be used in this invention.

[0114] Next, in the mother ingot remelting-atomization process, the mother ingot obtained from casting is used as raw material and remelted in a melting furnace such as a high-frequency or induction furnace. Low oxygen stainless steel powder is obtained by using gas atomization with inert gas Ar or He.

[0115] Subsequently, these stainless steel powders were graded to achieve the aforementioned median diameter D. 50 Apparent density is provided as the stainless steel powder of this invention. It should be noted that classification can be performed using sieves or other methods such as air classification. Furthermore, water atomization can be used instead of gas atomization.

[0116] Next, one embodiment of the method for manufacturing the stainless steel component of the present invention will be described.

[0117] First, using the stainless steel powder of the present invention as a material, a stainless steel additive model (three-dimensional structure) is created, for example, by additive manufacturing (metal powder additive manufacturing). As an additive manufacturing method, 3D printing can be used, for example. Here, a laser-type powder bed 3D printer is used. In particular, the settings for the 3D printer are not specified. From the viewpoint of preventing over-melting or under-melting, it is preferable, for example, to set the laser output power of the 3D printer to 150–300 W and the scanning speed to 700–1100 mm / s. Next, the modeled component is heat-treated as needed to obtain the stainless steel component of the present invention. In this heat treatment process, the temperature is held at a range of 850–1200°C for at least 1 minute, and then air-cooled. Preferably, the temperature is held at a range of 1000–1200°C for at least 10 minutes before air-cooling.

[0118] Example

[0119] The present invention will be described below through examples. First, a raw material powder having the composition shown in Table 1 was manufactured by gas atomization. Then, it was graded to obtain an apparent density adjusted to 3.4–4.7 Mg / m³. 3 And the median diameter D of the volume datum50 Stainless steel powder with a thickness of 21–220 μm was used as the material to manufacture stainless steel components via powder bed molding. An M290 molding apparatus manufactured by EOS Corporation was used. A laser was used as the heat source for melting the stainless steel powder, and molding was performed at a scanning speed of 800 mm / s, an output power of 200 W, and a powder additive thickness of 40 μm per layer. The molded object was a plate with a width of 35 mm, a length of 180 mm, and an additive direction height (thickness) of 4 mm. The molded object was subjected to a heat treatment of holding it in the atmosphere at 1000°C for 1 hour followed by air cooling. Test pieces were then cut for the tests described later for characteristic evaluation. However, test pieces rated × in the shape evaluation shown below were not subsequently evaluated for characteristics. Furthermore, No. 41 is a 2 mm thick cold-rolled annealed plate with essentially the same composition as No. 1, manufactured using conventional small-scale ingot melting, hot rolling, hot-rolled plate annealing, cold rolling, and cold-rolled plate annealing processes. The annealing temperature for the hot-rolled sheet was set to 1000℃, and the annealing temperature for the cold-rolled sheet was set to 900℃. For the cold-rolled annealed sheet, its corrosion resistance and oxidation resistance were evaluated as comparative materials. Furthermore, for stainless steel components No. 1 to 41, the number of MnS atoms (N) per unit area was determined using the method described above. MnS Regarding the observation surfaces of the steel structure, No. 1 to 40 are set at the half-height position in the additive direction, and No. 41 is set at the center of the plate thickness. The results are shown in Table 2.

[0120] (1) Form

[0121] The shapeability is evaluated by checking for cracks in the additively manufactured stainless steel components. First, the surface of the heat-treated shape (the two largest surfaces) is ground down by 1 mm. Then, a penetrant testing test is performed on this ground surface according to the method described in JIS Z2343-1:2017 to confirm the presence of cracks.

[0122] Cases where no cracks longer than 1.0 mm are identified are rated as ○, cases where cracks longer than 1.0 mm are identified are rated as ×, and cases rated as ○ are rated as having excellent shape.

[0123] (2) Corrosion resistance

[0124] For No. 1 to No. 40, 20 mm square test pieces were cut from the heat-treated molded objects; for No. 41, 20 mm square test pieces were cut from steel plates. According to JIS G 0577:2014, for test pieces obtained by polishing the surface with #600 abrasive paper, after leaving an 11 mm square test surface, the piece was covered with sealing material and then immersed in a 3.5% (w / w) NaCl solution at 30°C. Then, after holding under electrostatic potential for 10 minutes, the potential was scanned to 1.1 mA / cm at a scan rate of 20 mV / min. 2 Achieving a current density of 10 μA / cm 2 The potential at time Vc'10 is taken as the pitting potential.

[0125] Regarding the results of pitting potential measurement, Vc'10 is marked as × when it is less than 200mV (vs SCE), marked as ○ when it is above 200mV (vs SCE) but less than 400mV (vs SCE), and marked as ◎ when it is above 400mV (vs SCE). If it is ○ or ◎, it is evaluated as having excellent corrosion resistance.

[0126] (3) Oxidation resistance

[0127] Oxidation resistance was evaluated through oxidation tests. For No. 1 to No. 40, 1 mm of thickness was removed from the two largest surfaces of the heat-treated molded object to obtain a 2 mm thick plate-shaped test piece. Anodized test pieces measuring 20 mm × 30 mm × 2.0 mm were cut from this plate. For No. 41, 0.1 mm of thickness was removed from the two largest surfaces of the steel plate to obtain a 1.8 mm thick plate-shaped test piece. Anodized test pieces measuring 20 mm × 30 mm × 1.8 mm were cut from this plate. These surfaces were then polished with #600 abrasive paper to obtain the anodized test pieces. Oxidation resistance was evaluated by maintaining these test pieces in a high-temperature atmosphere for 400 hours. The test temperatures were set at 600 °C and 800 °C. After the oxidation test, an oxidation increment exceeding 20 g / m² was considered acceptable. 2 If the oxide film on the surface has peeled off, it is considered unqualified, and the oxide increment is set at 20 g / m². 2 The following conditions, where no oxide film peeling occurs, are considered acceptable.

[0128] Regarding oxidation resistance, cases that fail the tests at both 600℃ and 800℃ are marked as ×, cases that pass the test at 600℃ but fail the test at 800℃ are marked as ○, and cases that pass the tests at both 600℃ and 800℃ are marked as ◎. If the result is ○ or ◎, the oxidation resistance is evaluated as excellent.

[0129]

[0130] The evaluation results of (1) to (3) are shown in Table 2. As can be seen from Table 2, No.1 to 21 and No.31 to 40, which are examples of inventions, all have good shapeability, corrosion resistance and oxidation resistance.

[0131] In contrast, composition and properties (median diameter D) 50 Comparative examples No. 22-30, whose apparent density is outside the appropriate range, cannot simultaneously satisfy good formability, corrosion resistance, and oxidation resistance. Furthermore, the steel plate of No. 41 failed to achieve good corrosion resistance.

[0132] More specifically, in Comparative Example No. 22 (steel powder designation B1), the Ni content exceeded the upper limit of the present invention, thus austenitic phase precipitated and good oxidation resistance was not obtained.

[0133] In Comparative Example No. 23 (steel powder designation B2), the Mn content exceeded the upper limit of the present invention, thus austenitic phase precipitated and good oxidation resistance was not obtained.

[0134] In Comparative Example No. 24 (steel powder designation B3), the carbon content exceeded the upper limit of the present invention, thus failing to achieve good formability. Since Comparative Example No. 24 did not achieve the desired formability, subsequent evaluation of corrosion resistance and oxidation resistance was not conducted (evaluation was not possible).

[0135] In Comparative Example No. 25 (steel powder designation B4), the nitrogen content exceeded the upper limit of the present invention, and therefore good formability was not obtained. Since Comparative Example No. 25 did not achieve the desired formability, subsequent evaluation of corrosion resistance and oxidation resistance was not conducted (evaluation was not possible).

[0136] In Comparative Example No. 26 (steel powder designation B5), the Cr content was less than the lower limit of the present invention, and therefore good corrosion resistance and oxidation resistance were not obtained.

[0137] In Comparative Example No. 27 (steel powder designation B6), the Mo content was less than the lower limit of the present invention, and therefore good corrosion resistance and oxidation resistance were not obtained.

[0138] In Comparative Example No. 28 (steel powder designation B7), the Ni content was less than the lower limit of the present invention, and therefore good corrosion resistance was not obtained.

[0139] In Comparative Example No. 29 (steel powder designation B8), although the contents of Ni and Mo were within the range of the present invention, (Ni+Mo) was less than 2.00, and therefore good corrosion resistance was not obtained.

[0140] In Comparative Example No. 30 (steel powder designation B9), although the composition is the same as that of Invention Example No. 1 (steel powder designation A1), the median diameter D is different. 50 (μm) is 220, apparent density (Mg / m 3 The coefficient of performance (COP) was 3.4, therefore good shapeability was not achieved. Since Comparative Example No. 30 did not achieve the desired shapeability, subsequent evaluation of corrosion resistance and oxidation resistance was not conducted (evaluation was not possible).

[0141] Although the steel plate of Comparative Example No. 41 (steel plate designation C1) has a composition that is basically the same as that of Invention Example No. 1 (steel powder designation A1), the steel plate per unit area (mm²) has a different composition. 2 The high number of MnS molecules resulted in poor corrosion resistance.

[0142] Industrial availability

[0143] By using the stainless steel powder of the present invention, stainless steel components with excellent corrosion resistance and oxidation resistance can be molded without solidification cracking. The stainless steel components obtained by the present invention are particularly suitable for use in automotive heat exchangers and exhaust system components. Furthermore, they can be applied to various components beyond these applications. The stainless steel powder of the present invention is suitable as a material for ferritic stainless steel components with complex shapes and requiring excellent oxidation and corrosion resistance, such as automotive exhaust system components and heat exchangers, and is particularly suitable as a material for manufacturing such stainless steel components by additive manufacturing methods.

Claims

1. A ferritic stainless steel powder, comprising, by mass%, C: 0.003–0.200%, Si: 0.01–2.00%, Mn: 0.05–2.00%, P: less than 0.040%, S: less than 0.010%, Cr: 15.0–35.0%, Ni: 0.10–2.50%, Mo: 0.50–3.00%, Nb: more than 0.10% and less than 0.70%, N: less than 0.030%, and O: less than 0.200%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities. Median diameter D 50 The micrometer size is greater than 10 μm and less than 200 μm, with an apparent density of 3.5 Mg / m³. 3 Above and 5.0Mg / m 3 the following, (Ni+Mo)≥2.00 …(1) in, In formula (1), Ni and Mo are the mass percentage contents of Ni and Mo, respectively.

2. The ferritic stainless steel powder according to claim 1, wherein, The composition, by mass%, further contains one or more of the following: Al: less than 0.30%, Ti: less than 0.30%, V: less than 0.50%, Zr: less than 0.50%, Cu: less than 1.00%, Co: less than 0.50%, B: less than 0.0100%, Ca: less than 0.0100%, Mg: less than 0.0050%, REM: less than 0.50%, Sn: less than 0.50%, and Sb: less than 0.50%.

3. A ferritic stainless steel component comprising, by mass%, C: 0.003–0.200%, Si: 0.01–2.00%, Mn: 0.05–2.00%, P: less than 0.040%, S: less than 0.010%, Cr: 15.0–35.0%, Ni: 0.10–2.50%, Mo: 0.50–3.00%, Nb: more than 0.10% and less than 0.70%, N: less than 0.030%, and O: less than 0.200%, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities. The number of MnS molecules per unit area N MnS Satisfy the following equation (2), (Ni+Mo)≥2.00 …(1) N MnS ≤100 pieces / mm 2 …(2) in, In formula (1), Ni and Mo are the mass percentage contents of Ni and Mo, respectively.

4. The ferritic stainless steel component according to claim 3, wherein, The composition, by mass%, further contains one or more of the following: Al: less than 0.30%, Ti: less than 0.30%, V: less than 0.50%, Zr: less than 0.50%, Cu: less than 1.00%, Co: less than 0.50%, B: less than 0.0100%, Ca: less than 0.0100%, Mg: less than 0.0050%, REM: less than 0.50%, Sn: less than 0.50%, and Sb: less than 0.50%.

5. A method for manufacturing ferritic stainless steel components, wherein, Ferritic stainless steel components are manufactured using the ferritic stainless steel powder as described in claim 1 or 2 via additive manufacturing.

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