Ferritic stainless steel powder, ferritic stainless steel member, and method for producing ferritic stainless steel member

Abstract

Provided is a ferritic stainless steel powder with which it is possible to manufacture a ferritic stainless steel member that is excellent in formability, oxidation resistance and corrosion resistance.　The ferritic stainless steel powder has a component composition of, in mass%, C: 0.003-0.200%, Si: 0.01-2.00%, Mn: 0.05-2.00%, P: 0.040% or less, S: 0.010% or less, and Cr: 15.0-35.0%, Ni: 0.10-2.50%, Mo: 0.50-3.00%, Nb: at least 0.10% but less than 0.70%, and N: 0.030% or less, and O: 0.200% or less, the balance being Fe and unavoidable impurities. The ferritic stainless steel powder satisfies formula (1) and has a median diameter D50 of 10-200 μm, and an apparent density of 3.5 Mg / m3-5.0 Mg / m3. Formula (1): (Ni + Mo) ≥ 2.00

Description

Ferritic stainless steel powder, ferritic stainless steel member, and method for producing ferritic stainless steel member

[0001] The present invention relates to a ferritic stainless steel powder, and in particular to a ferritic stainless steel powder suitable for producing components having complex shapes and requiring excellent corrosion resistance, such as heat exchangers. The present invention also relates to a ferritic stainless steel component produced using the ferritic stainless steel powder and a method for producing the same.

[0002] In recent years, the use of metal additive manufacturing has been expanding. The most widely used manufacturing method is powder bed fusion (also known as the powder bed method). Powder bed fusion is a method of additive manufacturing that uses a laser or electron beam as a heat source to melt the raw material metal powder while creating a material.

[0003] The greatest feature of additive manufacturing is that it can produce objects with shapes that were impossible to form using conventional processing methods, taking into account the surface and internal structure. Furthermore, because additive manufacturing does not require consideration of the processability of the material, it can be used to form any shape even from materials such as Ni-based alloys and Ti-based alloys, which were previously difficult to process and had limited product shapes.

[0004] Taking advantage of these characteristics, metal additive manufacturing is increasingly being used industrially in the aerospace and medical fields. For example, for components with complex shapes and requiring high heat resistance, such as turbine blades, manufacturing Ni alloy components using additive manufacturing offers benefits such as shape optimization and a reduction in the number of components. Furthermore, in the medical field, additive manufacturing is increasingly being used due to its advantages, such as the ability to manufacture artificial joints and implants with shapes adapted to each patient.

[0005] At present, additive manufacturing is limited to applications in the aerospace and medical fields due to its low mass-productivity and high manufacturing costs. However, if these issues can be resolved, it is expected that its application will expand to fields such as automobiles, where productivity and cost are important. In particular, in the automotive field, applying additive manufacturing to the manufacture of exhaust system components and heat exchangers with complex shapes may add value by enabling shape optimization and eliminating the need for joining such as brazing or welding, thereby improving the overall strength of the component. For this reason, Fe-based alloys such as stainless steel, which are cheaper than Ni-based alloys and Ti-based alloys, are attracting attention as materials to be used in additive manufacturing.

[0006] Patent documents 1 to 6 disclose stainless steel powders for additive manufacturing.

[0007] International Publication No. 2019 / 139017 International Publication No. 2019 / 235014 International Publication No. 2020 / 110498 International Publication No. 2021 / 214958 Japanese Patent No. 6270563 Japanese Patent No. 6985940

[0008] In recent years, from the standpoint of protecting the global environment, there has been a demand for further improvements in fuel efficiency and enhanced exhaust gas purification for automobiles. As a result, exhaust gas temperatures are becoming higher, and the application of automotive heat exchangers such as exhaust heat recovery units and EGR (Exhaust Gas Recirculation) coolers is expanding.

[0009] Here, a waste heat recovery device is a device that improves fuel efficiency by warming engine coolant with the heat of exhaust gas and shortening the warm-up time at engine start. Generally, a waste heat recovery device is installed between the catalytic converter and the muffler and is composed of a heat exchanger section that combines pipes, plates, fins, side plates, etc., an inlet pipe section, and an outlet pipe section. Exhaust gas enters the heat exchanger section from the inlet pipe, where it transfers its heat to the coolant via heat transfer surfaces such as fins, and is then discharged from the outlet pipe. Furthermore, brazing, rather than welding, is mainly used to bond and assemble the plates and fins that make up the heat exchanger section of such waste heat recovery devices.

[0010] In addition, the EGR cooler recirculates exhaust gas with a low oxygen concentration to the intake side of the engine, lowering the fuel combustion temperature and reducing nitrogen oxides (NO Ｘ ) is a device for suppressing the temperature rise. If high-temperature exhaust gas is directly returned to the engine, fuel will burn at an inappropriate time, resulting in abnormal vibrations known as knocking. For this reason, EGR coolers are composed of a pipe that takes in some of the exhaust gas, a heat exchanger that cools the taken-in exhaust gas, and a pipe that returns the cooled exhaust gas to the intake side of the engine. The temperature of the exhaust gas is over 600°C at the inlet side of the EGR cooler, but is cooled by the heat exchanger to below 100°C at the outlet side. In addition, the heat exchanger portion of the EGR cooler is made by stacking thin plates in a fin-like shape for reasons of weight reduction, compactness, and cost reduction, and brazing is also mainly used for joining and assembling these parts.

[0011] As described above, the components of the exhaust heat recovery device and EGR cooler are bonded and assembled by brazing, which can cause problems such as sagging of the components during the high-temperature brazing heat treatment. Also, since they are exposed to high-temperature exhaust gases, they require excellent oxidation resistance. Furthermore, the exhaust gas contains NOx, which is a major component of the exhaust gas. Ｘ In addition to sulfur oxides (SO Ｘ ), and hydrocarbons (HC), which condense inside the heat exchanger and become highly corrosive, acidic condensed water. For this reason, heat exchangers are also required to have excellent corrosion resistance. In particular, when the exhaust gas temperature rises to 600°C or higher, Cr at the grain boundaries reacts with C and N to form Cr carbonitrides, creating a Cr-depleted layer with poor corrosion resistance around them. This phenomenon, known as sensitization, must be prevented to ensure corrosion resistance.

[0012] For these reasons, austenitic stainless steels such as SUS316L or SUS304L, which have a reduced carbon content to prevent sensitization, have typically been used for the heat exchanger components of exhaust heat recovery devices and EGR coolers. However, austenitic stainless steels have problems, such as high cost due to their large Ni content and poor fatigue properties at high temperatures in operating environments where they are subjected to restraint forces due to severe vibrations due to their large thermal expansion. For this reason, replacement of austenitic stainless steels with ferritic stainless steels such as SUS444 (18Cr-2Mo) and SUS430J1L (18Cr-Cu) is progressing.

[0013] Meanwhile, Patent Documents 1 to 6 disclose stainless steel powders for additive manufacturing, but the materials disclosed in Patent Documents 1 to 6 do not satisfy the sufficient oxidation resistance and corrosion resistance required when applying them to automobile exhaust system components, particularly heat exchangers such as exhaust heat recovery devices and EGR coolers, and there is a need for the development of stainless steel powders that satisfy these required properties. Furthermore, components manufactured by additive manufacturing are required to be free from cracks even when subjected to temperature changes such as rapid solidification and rapid heating that are unique to additive manufacturing, i.e., to have excellent formability.

[0014] The present invention has been made in view of the above circumstances, and aims to provide a ferritic stainless steel powder that can be used to produce ferritic stainless steel members that have excellent formability, oxidation resistance, and corrosion resistance.

[0015] In the present invention, excellent oxidation resistance means that a test piece obtained by polishing the surface of a ferritic stainless steel member with #600 abrasive paper is subjected to an oxidation test in which the test piece is held in air at 600°C for 400 hours, and then an oxidation weight gain of 20 g / m is achieved. ２ This means that the oxide film is not peeled off and the temperature is below 100°C.

[0016] In the present invention, "excellent corrosion resistance" refers to a test piece obtained by taking a 20 mm square specimen from a ferritic stainless steel member, polishing the surface with #600 abrasive paper in accordance with JIS G 0577:2014, covering the obtained specimen with a sealing material except for an 11 mm square measurement surface, and immersing the specimen in a 3.5 mass % NaCl solution at 30°C, and measuring the pitting corrosion potential Vc'10, which is 200 mV (vs. SCE) or more.

[0017] The present inventors conducted extensive research to achieve the above object and found that excellent oxidation resistance and corrosion resistance can be achieved by increasing the Cr content to 15.0 mass% or more, adding 0.10 mass% or more of Ni and 0.50 mass% or more of Mo, and further specifying the total content of Ni and Mo. Furthermore, they found that by optimizing the component composition including these components and specifying the properties of the steel powder (particle size and apparent density), a ferritic stainless steel powder can be obtained that can be used to produce ferritic stainless steel members that are excellent in formability and have excellent oxidation resistance and corrosion resistance.

[0018] By producing the above-described ferritic stainless steel powder and using it as a material for additive manufacturing, it is possible to obtain ferritic stainless steel parts with complex shapes and extremely excellent properties.

[0019] In particular, when considering application to an EGR cooler, it is desirable for the steel to have excellent oxidation resistance and corrosion resistance superior to that of SUS430J1L. More specifically, it is desirable for the pitting corrosion potential Vc'10 to be 200 mV (vs. SCE) or higher.

[0020] The present invention has been completed based on the above findings and further investigations. The gist of the present invention is as follows.

[0021] [1] Contains, in mass%, C: 0.003 to 0.200%, Si: 0.01 to 2.00%, Mn: 0.05 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0 to 35.0%, Ni: 0.10 to 2.50%, Mo: 0.50 to 3.00%, Nb: 0.10% or more but less than 0.70%, N: 0.030% or less, and O: 0.200% or less, and satisfies the following formula (1), with the balance being Fe and unavoidable impurities, and has a component composition, ５０ is 10 μm or more and 200 μm or less, and the apparent density is 3.5 Mg / m ３ 5.0Mg / m or more ３ ferritic stainless steel powder, wherein the composition satisfies the following formula: (Ni+Mo)≧2.00 (1), where Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively. [2] The ferritic stainless steel powder according to [1], wherein the composition further contains, in mass%, one or more elements selected from Al: 0.30% or less, Ti: 0.30% or less, V: 0.50% or less, Zr: 0.50% or less, Cu: 1.00% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0050% or less, REM: 0.50% or less, Sn: 0.50% or less, and Sb: 0.50% or less. [3] Contains, in mass%, C: 0.003 to 0.200%, Si: 0.01 to 2.00%, Mn: 0.05 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0 to 35.0%, Ni: 0.10 to 2.50%, Mo: 0.50 to 3.00%, Nb: 0.10% or more but less than 0.70%, N: 0.030% or less, and O: 0.200% or less, and satisfies the following formula (1), with the balance consisting of Fe and unavoidable impurities, and the number N of MnS per unit area is ＭｎＳ A ferritic stainless steel member that satisfies the following formula (2): (Ni+Mo)≧2.00 (1) ＭｎＳ ≦ 100 pieces / mm ２... (2) However, Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively. [4] The ferritic stainless steel member according to [3], wherein the chemical composition further contains, in mass%, one or more elements selected from Al: 0.30% or less, Ti: 0.30% or less, V: 0.50% or less, Zr: 0.50% or less, Cu: 1.00% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0050% or less, REM: 0.50% or less, Sn: 0.50% or less, and Sb: 0.50% or less. [5] A method for producing a ferritic stainless steel member, comprising producing the ferritic stainless steel member by an additive manufacturing method using the ferritic stainless steel powder according to [1] or [2].

[0022] According to the present invention, it is possible to provide a ferritic stainless steel powder that can be used to produce a ferritic stainless steel member that has excellent formability, oxidation resistance, and corrosion resistance.

[0023] According to the present invention, a ferritic stainless steel powder suitable for use in the production of ferritic stainless steel members, particularly those produced by additive manufacturing, can be provided. The ferritic stainless steel powder of the present invention is suitable as a material for ferritic stainless steel members that have complex shapes and are required to have excellent oxidation resistance and corrosion resistance, such as automobile exhaust system members and heat exchangers, and is particularly suitable as a material for producing the ferritic stainless steel members by additive manufacturing.

[0024] Furthermore, ferritic stainless steel members produced using the ferritic stainless steel powder of the present invention have excellent oxidation resistance and corrosion resistance. Furthermore, they have excellent formability because they are not subject to cracking due to sudden temperature changes during additive manufacturing. The ferritic stainless steel members obtained by the present invention are particularly suitable for use as components having complex shapes and requiring oxidation resistance and corrosion resistance, such as automotive exhaust system components and heat exchangers.

[0025] The present invention will be described based on the following embodiments.

[0026] First, the composition of the ferritic stainless steel powder (hereinafter also simply referred to as stainless steel powder) and the ferritic stainless steel member (hereinafter also simply referred to as stainless steel member) of the present invention will be described. Note that the unit of composition is always "mass %", but hereinafter, unless otherwise specified, it will be simply expressed as "%".

[0027] C: 0.003 to 0.200% C has the effect of increasing the strength of steel by forming carbides with elements such as Nb. Here, C is set to 0.003% or more to obtain sufficient strength. The C content is preferably set to 0.030% or more. On the other hand, if the C content exceeds 0.200%, the steel becomes excessively hard, and the toughness of the stainless steel member decreases. Furthermore, if the C content exceeds 0.200%, solidification cracking is more likely to occur during additive manufacturing, and formability decreases. 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.

[0028] Si: 0.01 to 2.00% Si has the effect of improving the oxidation resistance of steel. To obtain this effect, the Si content is set to 0.01% or more. The Si content is preferably set to 0.25% or more. However, if the Si content exceeds 2.00%, the steel becomes excessively hard and the 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.

[0029] Mn: 0.05 to 2.00% Mn has the effect of reducing the oxidation rate and preventing scale spalling. 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, since Mn is an austenite phase-forming element, if the Mn content exceeds 2.00%, an austenite phase is formed and oxidation resistance decreases. 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.

[0030] P: 0.040% or less Since P is an element that reduces corrosion resistance, it is desirable to reduce it, and the upper limit of the P content is set to 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. The lower limit of the P content is not particularly limited. However, since excessive dephosphorization leads to an increase in costs, the P content is preferably 0.005% or more.

[0031] S: 0.010% or less S is an element that reduces corrosion resistance, so it is desirable to reduce its content, and the upper limit of the S content is set to 0.010%. The S content is preferably 0.005% or less, and more preferably 0.003% or less. There is no particular restriction on the lower limit of the S content. However, since excessive desulfurization leads to an increase in costs, the S content is preferably 0.0005% or more.

[0032] Cr: 15.0 to 35.0% Cr is an important element that improves the corrosion resistance and oxidation resistance of steel. To achieve the targeted corrosion resistance and oxidation resistance, the Cr content must be 15.0% or more. However, if the Cr content exceeds 35.0%, the toughness of the stainless steel member decreases. Therefore, the Cr content is set to 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.

[0033] Ni: 0.10 to 2.50% Ni has the effect of improving the corrosion resistance of stainless steel members. To achieve this effect, the Ni content is set to 0.10% or more. The Ni content is preferably 0.20% or more, and more preferably 0.50% or more. On the other hand, Ni is an element that promotes the formation of an austenite phase. If the Ni content exceeds 2.50%, an austenite phase is formed, resulting in a decrease in oxidation resistance. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably 2.30% or less.

[0034] Mo: 0.50 to 3.00% Mo stabilizes the passivation film of stainless steel, improving corrosion resistance and oxidation resistance. This effect is achieved when the Mo content is 0.50% or more. However, if the Mo content exceeds 3.00%, intermetallic compounds precipitate, reducing toughness. Therefore, the Mo content is set to 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. The Mo content is also preferably 2.50% or less, more preferably 2.00% or less.

[0035] Nb: 0.10% or more and less than 0.70% Nb acts as a solid solution strengthening element and has the effect of increasing the high-temperature yield strength of steel. It also forms carbides and nitrides with C and N in steel, thereby increasing the high-temperature yield strength of steel. These effects are achieved when the Nb content is 0.10% or more. However, when the Nb content is 0.70% or more, an intermetallic compound phase is formed, reducing the toughness of the stainless steel member. Therefore, the Nb content is set to a range of 0.10% or more and less than 0.70%. The Nb content is preferably 0.20% or more, more preferably 0.30% or more. The Nb content is also preferably 0.60% or less, more preferably 0.50% or less, and even more preferably 0.40% or less.

[0036] N: 0.030% or less N acts as a solid solution strengthening element and has the effect of increasing the high-temperature yield strength of steel. It also has the effect of increasing 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, and more preferably 0.0050% or more. On the other hand, if the N content exceeds 0.030%, solidification cracking is more likely to occur during additive manufacturing, resulting in a decrease in formability. Therefore, the N content is set to 0.030% or less. The N content is preferably 0.020% or less.

[0037] O: 0.200% or less The stainless steel powder of the present invention can be produced by gas atomization or water atomization, but metal powders produced by these methods are prone to O contamination. In order to reduce the amount of oxide-based inclusions formed in the shaped product, the O content is set to 0.200% or less. The O content is preferably 0.070% or less, and more preferably 0.050% or less. However, since excessive deoxidation increases costs, the O content is preferably 0.020% or more.

[0038] (Ni + Mo) ≧ 2.00 (1) where Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively. Ni and Mo have the effect of dissolving in the matrix to enhance corrosion resistance. To achieve this effect, it is necessary to contain one or both of Ni and Mo in a total content of 2.00% or more. In other words, 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. Furthermore, the total content is 5.50% or less, preferably 4.80% or less, and more preferably 4.30% or less.

[0039] The ferritic stainless steel powder and stainless steel member of the present invention have a composition containing the above-mentioned components, with the balance being Fe and unavoidable impurities.

[0040] In addition to the above components, the stainless steel powder and stainless steel member of the present invention may further contain one or more elements selected from Al: 0.30% or less, Ti: 0.30% or less, V: 0.50% or less, Zr: 0.50% or less, Cu: 1.00% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0050% or less, REM: 0.50% or less, Sn: 0.50% or less, and Sb: 0.50% or less.

[0041] Al: 0.30% or less Al has the effect of forming an Al oxide film on the surface, significantly improving the oxidation resistance of steel. To achieve this effect, the Al content is preferably 0.03% or more. However, if the Al content exceeds 0.30%, the toughness of the stainless steel member decreases. Therefore, if Al is contained, the Al content should be 0.30% or less.

[0042] Ti: 0.30% or less Ti has the effect of increasing the strength of steel by forming carbides or nitrides with C and N in the steel. To achieve this effect, the Ti content is preferably 0.01% or more. However, if the Ti content exceeds 0.30%, the steel becomes excessively hard and the toughness of the stainless steel member decreases. Therefore, if Ti is contained, the Ti content is set to 0.30% or less. The Ti content is preferably 0.20% or less.

[0043] V: 0.50% or less V has the effect of increasing the strength of steel by forming carbides or nitrides with C and N in the steel. To obtain this effect, the V content is preferably 0.01% or more. However, if the V content exceeds 0.50%, the steel becomes excessively hard and the toughness of the stainless steel member decreases. Therefore, when V is contained, the V content is set to 0.50% or less. The V content is preferably 0.30% or less.

[0044] Zr: 0.50% or less Zr has the effect of improving the adhesion of an oxide film and enhancing oxidation resistance. To achieve this effect, the Zr content is preferably 0.01% or more. However, if the Zr content exceeds 0.50%, an intermetallic compound phase precipitates, which actually reduces oxidation resistance. Therefore, if Zr is contained, 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.

[0045] Cu: 1.00% or less Cu is an element that enhances corrosion resistance. To achieve this effect, the Cu content is preferably 0.01% or more. However, if the Cu content exceeds 1.00%, the steel becomes excessively hard and the toughness of the stainless steel member decreases. Therefore, when Cu is contained, 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.

[0046] Co: 0.50% or less Co is an element that improves the toughness of stainless steel members. To achieve this effect, the Co content is preferably 0.01% or more. However, if the Co content exceeds 0.50%, the steel becomes excessively hard and the toughness decreases. Therefore, when Co is contained, the Co content is set to 0.50% or less.

[0047] B: 0.0100% or less B has the effect of strengthening grain boundaries and improving toughness. To achieve this effect, the B content is preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the steel becomes excessively hard, and the toughness of the stainless steel member decreases. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is more preferably 0.0005% or more. Furthermore, the B content is preferably 0.0050% or less.

[0048] Ca: 0.0100% or less Ca has the effect of lowering the melting point of oxide-based inclusions and reducing the number of inclusions in molten steel when producing metal powder. To achieve this effect, the Ca content is preferably 0.0002% or more. However, if the Ca content exceeds 0.0100%, the toughness of the stainless steel member decreases. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is more preferably 0.0005% or more. Furthermore, the Ca content is preferably 0.0050% or less, more preferably 0.0030% or less.

[0049] Mg: 0.0050% or less Mg is an element that has the effect of improving corrosion resistance. This effect is obtained when the Mg content is 0.0002% or more. Therefore, when Mg is contained, the Mg content is preferably 0.0002% or more. However, if the Mg content exceeds 0.0050%, the toughness of the stainless steel member decreases. Therefore, when Mg is contained, the Mg content is set to 0.0050% or less. The Mg content is more preferably 0.0005% or more. Furthermore, the Mg content is preferably 0.0035% or less, more preferably 0.0020% or less.

[0050] REM: 0.50% or less. REM (rare earth elements) have the effect of improving the adhesion of oxide films and enhancing oxidation resistance. To achieve this effect, the REM content is preferably 0.01% or more. However, if the REM content exceeds 0.50%, the toughness of the stainless steel member decreases. Therefore, when REM is contained, the REM content is set to 0.50% or less. The REM content is more preferably 0.05% or more, and even more preferably 0.10% or more. The REM content is preferably 0.30% or less, and more preferably 0.20% or less. REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content here refers to the total content of these elements.

[0051] Sn: 0.50% or less Sn has the effect of preventing roughness when the surface of a molded object is polished. To achieve this effect, the Sn content is preferably 0.01% or more. However, if the Sn content exceeds 0.50%, solidification cracking is more likely to occur, resulting in a decrease in moldability. Therefore, when Sn is contained, 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.

[0052] Sb: 0.50% or less Sb has the effect of preventing roughness when the surface of a molded object is polished. To achieve this effect, the Sb content is preferably 0.01% or more. However, if the Sb content exceeds 0.50%, solidification cracking is more likely to occur, resulting in a decrease in moldability. Therefore, when Sb is contained, 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.

[0053] Next, the properties (particle size and apparent density) of the stainless steel powder of the present invention will be described.

[0054] The stainless steel powder of the present invention has a volume-based median diameter D ５０ The apparent density is 3.5 Mg / m ３ 5.0Mg / m or more ３ The following is the result.

[0055] Median diameter D ５０ The stainless steel powder of the present invention has a volume-based median diameter D ５０ The median diameter D of the stainless steel powder is 10 μm or more and 200 μm or less. ５０ If the median diameter D of the stainless steel powder is too small, the fluidity of the powder will decrease, resulting in uneven powder filling, which will cause defects such as voids to form during additive manufacturing. As a result, the strength, low-temperature toughness, and corrosion resistance of the steel member manufactured using such steel powder as a raw material may decrease. ５０ When the median diameter D of the stainless steel powder is 10 μm or more, it becomes easier to suppress the generation of defects such as voids during additive manufacturing. ５０ The median diameter D of the stainless steel powder is 10 μm or more. ５０ is preferably 20 μm or more, more preferably 30 μm or more. ５０If the median diameter D of the stainless steel powder is too large, it may cause defects such as voids to be generated during additive manufacturing. As a result, in a steel member manufactured using such a steel powder as a raw material, the defects may lead to a decrease in strength, low-temperature toughness, and corrosion resistance. ５０ When the median diameter D of the stainless steel powder is 200 μm or less, it is easy to suppress the generation of defects such as voids during additive manufacturing. ５０ The median diameter D of the stainless steel powder is 200 μm or less. ５０ is preferably 150 μm or less, more preferably 100 μm or less, and even more preferably 50 μm or less.

[0056] The median diameter D of the stainless steel powder ５０ refers to the median diameter (50% particle diameter) of the cumulative volume distribution of the stainless steel powder. A laser diffraction particle size measurement device can be used to measure the median diameter of the stainless steel powder. In the present invention, the median diameter D of the stainless steel powder is measured by the method described below. ５０ Measure.

[0057] Examples of laser diffraction particle size analyzers include the LA-950V2 manufactured by Horiba, Ltd. While other devices may be used, it is preferable to use one with a measurable particle size range with a lower limit of 0.1 μm or less and an upper limit of 200 μm or more for accurate measurements. With a laser diffraction particle size analyzer, a laser beam is irradiated onto a solvent containing dispersed stainless steel powder, and the particle size distribution and average particle size (median diameter) of the stainless steel powder are measured based on the diffraction and scattering intensity of the laser beam. Ethanol is preferably used as the solvent for dispersing the stainless steel powder, as it has good powder dispersibility and is easy to handle. Using a solvent with high van der Waals forces and low dispersibility, such as water, is undesirable because the powder may aggregate during measurement, resulting in a larger measured average particle size than the actual average particle size. Therefore, it is preferable to subject the ethanol solution containing the stainless steel powder to ultrasonic dispersion treatment before measurement.

[0058] Since the appropriate dispersion treatment time varies depending on the stainless steel powder to be measured, the dispersion treatment time is varied between 0 and 60 minutes in seven steps at 10-minute intervals, and the particle size distribution and average particle diameter (median diameter) of the stainless steel powder are measured after each dispersion treatment. During each measurement, the solvent is stirred to prevent aggregation of the stainless steel powder. The smallest average particle diameter (median diameter) obtained from the seven measurements, which were performed with the dispersion treatment time varied at 10-minute intervals, is defined as the median diameter D of the stainless steel powder of the present invention. ５０ Used as.

[0059] As mentioned above, the median diameter D ５０ The stainless steel powder of the present invention, in which the median diameter D of the stainless steel powder is controlled, further has a high apparent density. ５０ By controlling both the apparent density and the apparent density, better molding properties can be obtained. ３ The apparent density is 3.5 Mg / m or more. ３ If the apparent density is less than 4.0 Mg / m, the fluidity may decrease, which may cause defects such as voids to be generated during additive manufacturing. ３ The apparent density of the stainless steel powder is 5.0 Mg / m or more. ３ The apparent density is 5.0 Mg / m or less. ３ If the apparent density exceeds this range, it may be difficult to stably control the apparent density from an industrial point of view.

[0060] The apparent density is a value measured by the test method specified in JIS Z 2504:2020.

[0061] Next, the properties (number of MnS per unit area) of the stainless steel member of the present invention will be described.

[0062] The stainless steel member of the present invention has a number N of MnS per unit area. ＭｎＳ satisfies the following formula (2): ＭｎＳ ≦ 100 pieces / mm ２ ... (2) In the steel structure of the stainless steel member of the present invention, the number of MnS per unit area (N ＭｎＳ) is 100 pieces / mm ２ The reason for this is as follows. MnS is an inclusion made of Mn and S and serves as the starting point for corrosion, so the fewer MnS there are, the better the corrosion resistance. The inventors have discovered that even for stainless steel members with the same component composition, steel members (laminated manufactured objects) manufactured by additive manufacturing have better corrosion resistance than steel members manufactured by processing steel plates. The reason for this is not clear, but in additive manufactured objects, MnS is less likely to precipitate during cooling due to the rapid solidification that occurs during additive manufacturing, and the N in the steel structure is more likely to precipitate. ＭｎＳ This is thought to be due to the fact that N ＭｎＳ 100 pieces / mm ２ It has been found that excellent corrosion resistance can be obtained when the N ＭｎＳ is more preferably 50 pieces / mm ２ More preferably, 30 pieces / mm ２ The following is true. ＭｎＳ There is no particular limitation on the lower limit of N ＭｎＳ is 0 pieces / mm ２ may be.

[0063] Number of MnS per unit area of ​​stainless steel member N ＭｎＳ The measurement method is as follows: A test piece is taken so that the surface perpendicular to the height (lamination) direction of the stainless steel member is the observation surface, and then the test piece is embedded in resin and mirror-polished. The observation surface can be located at half the height of the stainless steel member. Then, a 1.0 mm ２ The range was observed by SEM / EDX (Scanning Electron Microscope / Energy Dispersive X-ray Spectrometry), and inclusions containing 25 at% or more of Mn and S and having a particle size (maximum diameter) of 0.05 μm or more were defined as MnS, and the number of such inclusions was counted. The particle size of the inclusions was set to 0.05 μm or more because inclusions of this size or more are harmful to corrosion resistance, while inclusions of less than 0.05 μm do not affect corrosion resistance. The number of visual fields for SEM / EDX observation was three, and the average number of MnS in the three visual fields was calculated as the average number per unit area (mm ２ ) Number of MnS per ＭｎＳIt was decided.

[0064] Next, a preferred embodiment of the method for producing stainless steel powder of the present invention will be described.

[0065] The stainless steel powder of the present invention is provided in the final material form through the following series of manufacturing steps: for example, melting, forming an ingot, remelting the master ingot, and producing powder through an atomization process.

[0066] First, in the melting-ingot formation process, predetermined amounts of the above-mentioned elements are melted as materials in a high-frequency vacuum melting furnace, alloyed, and cast to produce an ingot (master ingot). In this process, melting is preferably performed under conditions of a reduced-pressure Ar atmosphere and a melting temperature of 1600°C or higher. The reason for these conditions is as follows: if the melting temperature is too low, the molten steel will solidify and clog the nozzle when it is dropped from the nozzle. Furthermore, from the viewpoint of preventing oxidation of the molten steel, it is desirable to melt in a reduced-pressure Ar atmosphere. The melting furnace used in this process is not limited to a high-frequency vacuum melting furnace; other melting furnaces (e.g., direct-current heating melting furnaces) can also be used in the present invention.

[0067] Next, in the master ingot remelting-atomization process, the cast master ingot is used as the raw material and remelted in a melting furnace such as a high-frequency or induction furnace, and then gas atomization is performed using an inert gas such as Ar or He to obtain stainless steel powder with a low oxygen content.

[0068] These stainless steel powders are then mixed to form a powder having the above-mentioned median diameter D ５０ The powder is classified to have an apparent density of 1000 kJ / cm2 and is used as the stainless steel powder of the present invention. The classification may be performed using a sieve or by other methods such as air classification. Furthermore, water atomization may be used instead of gas atomization.

[0069] Next, one embodiment of the method for manufacturing a stainless steel member of the present invention will be described.

[0070] First, a stainless steel additive manufacturing object (three-dimensional structure) is fabricated using the stainless steel powder of the present invention described above as a material, for example, by additive manufacturing (metal powder additive manufacturing). As the additive manufacturing method, for example, a 3D printer method can be used. Here, a laser-type powder bed 3D printer is used. No particular setting conditions for the 3D printer are specified. To prevent excessive or insufficient melting, for example, the laser output of the 3D printer is preferably 150 to 300 W and the scan speed is preferably 700 to 1100 mm / s. Next, the fabricated member is subjected to heat treatment as necessary to obtain the stainless steel member of the present invention. In this heat treatment process, the member is held at a temperature in the range of 850 to 1200°C for at least 1 minute, followed by air cooling. Preferably, the member is held at a temperature in the range of 1000 to 1200°C for at least 10 minutes, followed by air cooling.

[0071] The present invention will be explained below with reference to examples. First, raw material powders having the component compositions shown in Table 1 were produced by gas atomization. Then, they were classified to obtain powders having an apparent density of 3.4 to 4.7 Mg / m. ３ and the volume-based median diameter D ５０A stainless steel powder was obtained with a diameter adjusted to 21 to 220 μm. This powder was used as the material to manufacture stainless steel parts using the powder bed method. The molding machine used was an EOS M290. The heat source for melting the stainless steel powder was a laser, and molding was performed under conditions of a scanning speed of 800 mm / s, an output of 200 W, and a powder layer thickness of 40 μm per layer. The molded object had a plate shape with a width of 35 mm, a length of 180 mm, and a height (thickness) in the layering direction of 4 mm. This molded object was subjected to a heat treatment by holding it in air at 1000°C for 1 hour and then air-cooling. Test specimens were then taken for the tests described below and their properties were evaluated. However, specimens that received an X in the moldability evaluation below were not subjected to further property evaluation. Furthermore, No. 41 was a specimen manufactured using the conventional process of small steel ingot melting, hot rolling, hot-rolled sheet annealing, cold rolling, and cold-rolled sheet annealing. The cold-rolled annealed sheets were 2 mm thick and had almost the same chemical composition as No. 1. The annealing temperature for the hot-rolled sheet was 1000°C, and the annealing temperature for the cold-rolled sheet was 900°C. The cold-rolled annealed sheets were used as comparative materials and evaluated for the corrosion resistance and oxidation resistance described below. In addition, for the stainless steel members No. 1 to 41, the number of MnS particles per unit area N ＭｎＳ was measured by the method described above. The observation surface of the steel structure was at a position halfway up the height in the stacking direction for Nos. 1 to 40, and at the center of the plate thickness for No. 41. The results are shown in Table 2.

[0072] (1) Moldability Moldability was evaluated based on the presence or absence of cracks in the stainless steel member after additive manufacturing. First, the surface of the heat-treated molded object (the two widest surfaces) was ground to a thickness of 1 mm. A penetrant test was performed on this ground surface in accordance with the method described in JIS Z2343-1:2017 to confirm the presence or absence of cracks. When no cracks of 1.0 mm or more in length were observed, the evaluation was rated as ◯, and when cracks of 1.0 mm or more in length were observed, the evaluation was rated as ×. ◯ was evaluated as excellent moldability.

[0073] (2) Corrosion Resistance For Nos. 1 to 40, 20 mm square test pieces were taken from the heat-treated molded articles, and for No. 41, a 20 mm square test piece was taken from the steel plate. In accordance with JIS G 0577:2014, the surfaces of the test pieces were polished with #600 abrasive paper, and the test pieces were covered with a sealant, leaving an 11 mm square measurement surface. The test pieces were then immersed in a 3.5 mass % NaCl solution at 30°C. After holding the test pieces at their natural potential for 10 minutes, they were subjected to a voltage sweep of 1.1 mA / cm at a sweep rate of 20 mV / min. ２ The potential was swept up to a current density of 10 μA / cm ２ The potential at which Vc'10 reached this value was defined as the pitting potential Vc'10. The measurement results of the pitting potential were evaluated as follows: x if Vc'10 was less than 200 mV (vs. SCE), ◯ if 200 mV (vs. SCE) or more but less than 400 mV (vs. SCE), and ⊚ if 400 mV (vs. SCE) or more; and ◯ or ⊚ was evaluated as excellent corrosion resistance.

[0074] (3) Oxidation Resistance Oxidation resistance was evaluated by conducting an oxidation test. For Nos. 1 to 40, the surfaces (the two largest surfaces) of the heat-treated molded articles were ground by 1 mm to obtain plate-shaped test pieces with a thickness of 2 mm. Oxidation test pieces measuring 20 mm x 30 mm x 2.0 mm were taken from these test pieces. For No. 41, the surfaces (the two largest surfaces) of the steel plate were ground by 0.1 mm to obtain plate-shaped test pieces with a thickness of 1.8 mm. Oxidation test pieces measuring 20 mm x 30 mm x 1.8 mm were taken from these test pieces. These surfaces were then polished with #600 abrasive paper to obtain oxidation test pieces. An oxidation test was conducted on these oxidation test pieces in a high-temperature atmosphere for 400 hours to evaluate their oxidation resistance. The test temperatures were 600°C and 800°C. After the oxidation test, the oxidation weight gain was 20 g / m. ２ If the oxidation increase is more than 20g / m or peeling of the surface oxide film occurs, it is considered a failure. ２ The oxidation resistance was evaluated as "pass" when the test temperature was below 600°C and no peeling of the oxide film occurred. The oxidation resistance was evaluated as "×" when the test was failed at both 600°C and 800°C, as "◯" when the test was passed at 600°C and failed at 800°C, and as "◎" when the test was passed at both 600°C and 800°C. A rating of "◯" or "◎" indicated that the oxidation resistance was excellent.

[0075]

[0076]

[0077] The evaluation results of (1) to (3) above are shown in Table 2. As can be seen from Table 2, all of the invention examples Nos. 1 to 21 and 31 to 40 had good formability, corrosion resistance, and oxidation resistance.

[0078] On the other hand, the component composition, properties (median diameter D ５０ In Comparative Examples 22 to 30, in which the apparent density (apparent density) was outside the appropriate range, it was not possible to simultaneously satisfy good formability, corrosion resistance, and oxidation resistance. In addition, the steel plate of No. 41 was unable to obtain good corrosion resistance.

[0079] More specifically, in Comparative Example No. 22 (steel powder symbol B1), the Ni content exceeded the upper limit specified in the present invention, resulting in the precipitation of an austenite phase and poor oxidation resistance. In Comparative Example No. 23 (steel powder symbol B2), the Mn content exceeded the upper limit specified in the present invention, resulting in the precipitation of an austenite phase and poor oxidation resistance. In Comparative Example No. 24 (steel powder symbol B3), the C content exceeded the upper limit specified in the present invention, resulting in poor formability. Since Comparative Example No. 24 did not achieve the desired formability, subsequent evaluation of corrosion resistance and oxidation resistance was not performed (evaluation not possible). In Comparative Example No. 25 (steel powder symbol B4), the N content exceeded the upper limit specified in the present invention, resulting in poor formability. Since Comparative Example No. 25 did not achieve the desired formability, subsequent evaluation of corrosion resistance and oxidation resistance was not performed (evaluation not possible). In Comparative Example No. 26 (steel powder symbol B5), the Cr content was below the lower limit of the present invention, so good corrosion resistance and oxidation resistance were not obtained. In Comparative Example No. 27 (steel powder symbol B6), the Mo content was below the lower limit of the present invention, so good corrosion resistance and oxidation resistance were not obtained. In Comparative Example No. 28 (steel powder symbol B7), the Ni content was below the lower limit of the present invention, so good corrosion resistance was not obtained. In Comparative Example No. 29 (steel powder symbol B8), the Ni and Mo contents were both within the ranges of the present invention, but good corrosion resistance was not obtained because (Ni + Mo) was less than 2.00. In Comparative Example No. 30 (steel powder symbol B9), the composition was the same as Inventive Example No. 1 (steel powder symbol A1), but the median diameter D ５０ (μm) is 220, apparent density (Mg / m ３ ) was 3.4, so good formability was not obtained. Comparative Example No. 30 did not achieve the desired formability, so subsequent evaluation of corrosion resistance and oxidation resistance was not performed (evaluation not possible). The steel plate of Comparative Example No. 41 (steel plate symbol C1) has almost the same chemical composition as Invention Example No. 1 (steel powder symbol A1), but the surface area (mm ２ The number of MnS particles per 1000g was large, and good corrosion resistance was not obtained.

[0080] By using the stainless steel powder of the present invention, stainless steel members having excellent corrosion resistance and oxidation resistance can be formed without solidification cracking. The stainless steel members obtained by the present invention are particularly suitable for use in automotive heat exchangers and exhaust system members. Furthermore, they can be applied to a variety of members, not limited to these uses. The stainless steel powder of the present invention is suitable as a material for ferritic stainless steel members that have complex shapes and require excellent oxidation resistance and corrosion resistance, such as automotive exhaust system members and heat exchangers, and is particularly suitable as a material for producing such stainless steel members by additive manufacturing.

Claims

1. A steel sheet containing, by mass%, C: 0.003 to 0.200%, Si: 0.01 to 2.00%, Mn: 0.05 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0 to 35.0%, Ni: 0.10 to 2.50%, Mo: 0.50 to 3.00%, Nb: 0.10% or more and less than 0.70%, N: 0.030% or less, and O: 0.200% or less, and satisfying the following formula (1), with the balance being Fe and unavoidable impurities; and having a component composition of: ５０ is 10 μm or more and 200 μm or less, and the apparent density is 3.5 Mg / m ３ 5.0Mg / m or more ３ A ferritic stainless steel powder having the following formula: (Ni+Mo)≧2.00 (1) where Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively.

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

3. A steel sheet containing, by mass%, C: 0.003 to 0.200%, Si: 0.01 to 2.00%, Mn: 0.05 to 2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0 to 35.0%, Ni: 0.10 to 2.50%, Mo: 0.50 to 3.00%, Nb: 0.10% or more but less than 0.70%, N: 0.030% or less, and O: 0.200% or less, and satisfying the following formula (1), with the balance consisting of Fe and unavoidable impurities; and the number N of MnS per unit area: ＭｎＳ A ferritic stainless steel member that satisfies the following formula (2): (Ni+Mo)≧2.00 (1) N ＭｎＳ ≦ 100 pieces / mm ２ ... (2) where Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively.

4. A ferritic stainless steel member as described in claim 3, wherein the composition further contains, by mass%, one or more selected from the following: Al: 0.30% or less, Ti: 0.30% or less, V: 0.50% or less, Zr: 0.50% or less, Cu: 1.00% or less, Co: 0.50% or less, B: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0050% or less, REM: 0.50% or less, Sn: 0.50% or less, and Sb: 0.50% or less.

5. A method for producing a ferritic stainless steel part, comprising producing the ferritic stainless steel part by an additive manufacturing method using the ferritic stainless steel powder according to claim 1 or 2.

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