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

Abstract

A ferritic stainless steel powder is provided that can be used to produce a ferritic stainless steel member having excellent formability, oxidation resistance, and corrosion resistance. The alloy contains, 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 satisfies the following formula (1), with the balance being Fe and unavoidable impurities, and has a component composition of 50 is 10μm or more and 200μm or less, and the apparent density is 3.5Mg / m 3 More than 5.0Mg / m 3 Ferritic stainless steel powder, which is: (Ni+Mo)≧2.00 (1)

Description

[Technical field]

[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, etc. The present invention also relates to a ferritic stainless steel component shaped using the ferritic stainless steel powder and a method for producing the same. [Background technology]

[0002] In recent years, the use of metal additive manufacturing has become more widespread. The most widely used additive manufacturing method at present is the powder bed fusion method (also known as the powder bed method). The powder bed fusion method is an additive manufacturing method that uses a laser or electron beam as a heat source to melt the raw metal powder while additive manufacturing.

[0003] The greatest feature of additive manufacturing is that it can produce objects with shapes that were impossible to mold using conventional processing methods, taking into account the surface and internal structures. Furthermore, because additive manufacturing does not require consideration of the workability of the material, it can be used to mold any shape, even with 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, the industrial use of metal additive manufacturing is progressing 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 has the advantage of optimizing the shape and reducing the number of components. In the medical field, additive manufacturing is also being used due to its advantage of being able 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, but if these issues can be resolved, its application is expected to spread to fields such as automobiles where productivity and cost are important. In particular, in the automobile field, applying additive manufacturing to the manufacture of exhaust system components and heat exchangers with complex shapes may add value by optimizing the shape and improving the overall strength of the components by eliminating the need for joining such as brazing or welding. 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. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] International Publication No. 2019 / 139017 [Patent Document 2] International Publication No. 2019 / 235014 [Patent Document 3] International Publication No. 2020 / 110498 [Patent Document 4] International Publication No. 2021 / 214958 [Patent Document 5] Patent No. 6270563 [Patent Document 6] Patent No. 6985940 Summary of the Invention [Problem to be solved by the invention]

[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 devices and EGR (Exhaust Gas Recirculation) coolers is expanding.

[0009] Here, the term "exhaust heat recovery device" refers to 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. In general, the exhaust 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 its heat is transferred to the coolant via heat transfer surfaces such as fins, and is discharged from the outlet pipe. In addition, brazing is mainly used for bonding and assembling the plates and fins that constitute the heat exchanger section of such an exhaust heat recovery device, rather than welding.

[0010] In addition, the EGR cooler recirculates exhaust gas with low oxygen concentration to the intake side of the engine, lowering the fuel combustion temperature and reducing nitrogen oxides (NO X ) is a device that suppresses the recirculation of exhaust gas into the engine. If high-temperature exhaust gas is directly recirculated to the engine, fuel will burn at an inappropriate time, resulting in abnormal vibrations known as knocking. For this reason, EGR coolers are made up of a pipe that takes in some of the exhaust gas, a heat exchanger that cools the 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 it is cooled by the heat exchanger, so that it is below 100°C at the outlet side. In addition, the heat exchanger part of the EGR cooler is made up of thin plates stacked together in a fin shape for reasons such as weight reduction, compactness, and cost reduction, and brazing is mainly used for joining and assembling these parts.

[0011] In this way, the various parts of the exhaust heat recovery device and EGR cooler are bonded and assembled by brazing, which poses problems such as sagging of the components during the brazing heat treatment at high temperatures. In addition, because they are exposed to high-temperature exhaust gases, they require excellent oxidation resistance. Furthermore, exhaust gases contain NO 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 temperature of exhaust gas rises to 600°C or higher, Cr at the grain boundaries reacts with C and N to form Cr carbonitrides, which form Cr-depleted layers with poor corrosion resistance around them. It is necessary to ensure corrosion resistance by preventing this so-called sensitization.

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

[0013] On the other hand, 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 sufficient oxidation resistance and corrosion resistance required for application to automobile exhaust system components, particularly heat exchangers such as exhaust heat recovery devices and EGR coolers, and there is a demand for the development of stainless steel powders that satisfy these required properties. Furthermore, components manufactured by additive manufacturing are required to be crack-free even when subjected to temperature changes such as rapid solidification and rapid heating that are unique to additive manufacturing, that is, to have excellent formability.

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

[0015] In the present invention, "excellent oxidation resistance" refers to a condition in which 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, after which the oxidation weight gain is 20 g / m 2 This means that the oxide film is not peeled off and that the oxide film is not peeled off.

[0016] In the present invention, excellent corrosion resistance refers to a test piece of 20 mm square taken from a ferritic stainless steel member, the surface of which is polished with #600 abrasive paper in accordance with JIS G 0577:2014, the test piece obtained is covered with a sealant except for an 11 mm square measurement surface, and the test piece is further immersed in a 3.5 mass % NaCl solution at 30°C, and the measured pitting corrosion potential Vc'10 is 200 mV (vs SCE) or more. [Means for solving the problem]

[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, containing Ni at 0.10 mass% or more, and Mo at 0.50 mass% or more, and further specifying the total content of Ni and Mo. Furthermore, they found that by optimizing the component composition containing 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 manufacture ferritic stainless steel parts that are excellent in formability and 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 of complex shapes with extremely excellent properties.

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

[0020] The present invention has been completed based on the above findings and further investigations.

[0021] [1] In mass percent, 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: 15.0~35.0%, Ni: 0.10-2.50%, Mo: 0.50-3.00%, Nb: 0.10% or more and less than 0.70% N: 0.030% or less, and O: 0.200% or less, and has a composition that satisfies the following formula (1), with the balance being Fe and unavoidable impurities; Median diameter D 50 is 10μm or more and 200μm or less, and the apparent density is 3.5Mg / m 3 More than 5.0Mg / m 3 Ferritic stainless steel powder, which is: (Ni+Mo)≧2.00 (1) In the formula (1), Ni and Mo are the contents (mass%) of Ni and Mo, respectively. [2] The composition further comprises, in mass%, 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 The ferritic stainless steel powder according to [1], containing one or more selected from the following: [3] In mass percent, 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: 15.0~35.0%, Ni: 0.10-2.50%, Mo: 0.50-3.00%, Nb: 0.10% or more and less than 0.70% N: 0.030% or less, and O: 0.200% or less, and has a composition that satisfies the following formula (1), with the balance being Fe and unavoidable impurities; Number of MnS particles per unit area N MnS A ferritic stainless steel member that satisfies the following formula (2): (Ni+Mo)≧2.00 (1) N MnS ≦ 100 pieces / mm 2 (2) In the formula (1), Ni and Mo are the contents (mass%) of Ni and Mo, respectively. [4] The composition further comprises, in mass%, 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 The ferritic stainless steel member according to [3], comprising one or more selected from the following: [5] A method for producing a ferritic stainless steel part, comprising producing a ferritic stainless steel part by an additive manufacturing method using the ferritic stainless steel powder according to [1] or [2] above. Effect of the Invention

[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 part having excellent formability, oxidation resistance, and corrosion resistance.

[0023] According to the present invention, it is possible to provide a ferritic stainless steel powder that is suitable for use in the manufacture of ferritic stainless steel members, particularly in the manufacture of ferritic stainless steel members by additive manufacturing. The ferritic 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 automobile exhaust system members and heat exchangers, and is particularly suitable as a material when the ferritic stainless steel members are manufactured by additive manufacturing.

[0024] In addition, the ferritic stainless steel member produced using the ferritic stainless steel powder of the present invention has excellent oxidation resistance and corrosion resistance. Furthermore, it has excellent formability because cracks do not occur due to sudden temperature changes during additive manufacturing. The ferritic stainless steel member obtained by the present invention is particularly suitable as a member having a complex shape and requiring oxidation resistance and corrosion resistance, such as an automobile exhaust system member or a heat exchanger. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

[0027] C: 0.003 to 0.200% C has the effect of forming carbides together with elements such as Nb to increase the strength of steel. Here, C is set to 0.003% or more in order 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. Also, if the C content exceeds 0.200%, solidification cracking is likely to occur during additive manufacturing, and the moldability 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] Silicon: 0.01 to 2.00% Silicon has the effect of improving the oxidation resistance of steel. To obtain this effect, the silicon content is set to 0.01% or more. The silicon content is preferably set to 0.25% or more. However, if the silicon content exceeds 2.00%, the steel becomes excessively hard and the toughness decreases. Therefore, the silicon content is set to 2.00% or less. The silicon 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 spalling of scale. To obtain 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%, the austenite phase is formed and the oxidation resistance is reduced. 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 further preferably 0.010% or less. The lower limit of the P content is not particularly limited. However, since excessive de-P leads to an increase in costs, the P content is preferably 0.005% or more.

[0031] S: 0.010% or less Since S is an element that reduces corrosion resistance, it is desirable to reduce the 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. The lower limit of the S content is not particularly limited. However, since excessive de-S leads to an increase in costs, the S content is preferably 0.0005% or more.

[0032] Cr: 15.0~35.0% Cr is an important element that improves the corrosion resistance and oxidation resistance of steel. In order to obtain 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 a range of 15.0 to 35.0%. The Cr content is preferably 18.0% or more, and more preferably 20.0% or more. In addition, the Cr content is preferably 32.0% or less, more preferably 30.0% or less, and further preferably 28.0% or less.

[0033] Ni: 0.10~2.50% Ni has the effect of improving the corrosion resistance of stainless steel members. To obtain 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 and the oxidation resistance decreases. Therefore, the Ni content is set to 2.50% or less. The Ni content is preferably 2.30% or less.

[0034] Mo: 0.50-3.00% Mo stabilizes the passivation film of stainless steel to improve corrosion resistance and oxidation resistance. This effect is obtained when the Mo content is 0.50% or more. However, when the Mo content exceeds 3.00%, intermetallic compounds are precipitated and toughness decreases. 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 further preferably 1.80% or more. Moreover, the Mo content is preferably 2.50% or less, and more preferably 2.00% or less.

[0035] Nb: 0.10% or more and less than 0.70% Nb has the effect of increasing the high-temperature yield strength of steel as a solid solution strengthening element. It also has the effect of increasing the high-temperature yield strength of steel by forming carbides and nitrides with C and N in steel. These effects are obtained when the Nb content is 0.10% or more. However, when the Nb content is 0.70% or more, an intermetallic compound phase is generated and the toughness of the stainless steel member decreases. 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 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 has the effect of increasing the high-temperature resistance of steel as a solid solution strengthening element. It also has the effect of increasing the strength of steel by forming nitrides with elements such as Nb. To obtain 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 likely to occur during additive manufacturing, and the moldability is reduced. 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 contamination with O. In order to reduce the amount of oxide-based inclusions formed in the molded 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 de-O leads to increased costs, the O content is preferably 0.020% or more.

[0038] (Ni+Mo)≧2.00 (1) In the formula (1), Ni and Mo are the contents (mass%) of Ni and Mo, respectively. Ni and Mo have the effect of increasing corrosion resistance by dissolving in the matrix. To obtain this effect, it is necessary to contain at least one of Ni and Mo in a total of 2.00% or more. In other words, it is necessary to satisfy the above formula (1). The total content of at least one of Ni and Mo is preferably 3.00% or more. Moreover, 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 part of the present invention may further contain one or more 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 obtain this effect, it is preferable that the Al content be 0.03% or more. However, if the Al content exceeds 0.30%, the toughness of the stainless steel member decreases. For this reason, if Al is contained, the Al content is set to 0.30% or less.

[0042] Ti: 0.30% or less Ti has the effect of forming carbides and nitrides with C and N in steel to increase the strength of steel. To obtain 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, when Ti is contained, the Ti content is set to a range of 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 and 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 a range of 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 the oxide film and enhancing the oxidation resistance. To obtain 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 reduces the oxidation resistance. Therefore, when 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 obtain 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 obtain 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, if 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 obtain 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 a range of 0.0100% or less. The B content is more preferably 0.0005% or more. Moreover, 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. In order to obtain 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 a range of 0.0100% or less. The Ca content is more preferably 0.0005% or more. Moreover, 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 in the range of 0.0002% or more. However, when 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 be in the range of 0.0050% or less. The Mg content is more preferably 0.0005% or more. Moreover, 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 obtain 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 a range of 0.50% or less. The REM content is more preferably 0.05% or more, and further preferably 0.10% or more. In addition, the REM content is preferably 0.30% or less, and more preferably 0.20% or less. Note that REM is a collective term for 15 elements including Sc, Y, and lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here is 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 obtain this effect, the Sn content is preferably 0.01% or more. However, if the Sn content exceeds 0.50%, solidification cracking is likely to occur, and moldability is reduced. Therefore, when Sn is contained, the Sn content is set to a range of 0.50% or less. The Sn content is more preferably 0.03% or more, and further preferably 0.05% or more. Moreover, 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 obtain this effect, the Sb content is preferably 0.01% or more. However, if the Sb content exceeds 0.50%, solidification cracking is likely to occur, and moldability is reduced. Therefore, when Sb is contained, the Sb content is set to a range of 0.50% or less. The Sb content is more preferably 0.03% or more, and further preferably 0.05% or more. Moreover, 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 More than 5.0Mg / m 3 The following is the result.

[0055] Median diameter D 50 :10μm or more and 200μm or less 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. 50If the median diameter D of the stainless steel powder is too small, the powder will have a reduced fluidity, resulting in uneven powder packing, which may cause defects such as voids 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 be reduced. 50 When the median diameter D of the stainless steel powder is 10 μm or more, 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 10 μm or more. 50 is preferably 20 μm or more, more preferably 30 μm or more. On the other hand, the median diameter D 50 If the median diameter D of the stainless steel powder is excessively large, it may cause defects such as voids 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 of the stainless steel powder is D 50 refers to the median diameter (50% particle diameter) of the cumulative volume distribution of the stainless steel powder. A laser diffraction particle size measuring device can be used to measure the median diameter of the stainless steel powder. In the present invention, the median diameter D 50 Measure.

[0057] Examples of laser diffraction particle size measuring devices include the LA-950V2 manufactured by Horiba, Ltd. Of course, other devices may be used, but it is preferable to use a device 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 in order to perform accurate measurements. In the laser diffraction particle size measuring device, a laser beam is irradiated onto a solvent in which stainless steel powder is dispersed, and the particle size distribution and average particle size (median size) of the stainless steel powder are measured from the diffraction and scattering intensity of the laser beam. As a solvent for dispersing stainless steel powder, it is preferable to use ethanol, which has good dispersibility of the powder and is easy to handle. If a solvent such as water, which has high van der Waals forces and low dispersibility, is used, the powder may aggregate during measurement, resulting in a measurement result that is larger than the original average particle size, which is not preferable. Therefore, it is preferable to subject the ethanol solution containing stainless steel powder to an 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 set to 7 steps at 10-minute intervals between 0 and 60 minutes, 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 the stainless steel powder from agglomerating. The smallest average particle diameter (median diameter) obtained in the seven measurements, which were performed with the dispersion treatment time changed at 10-minute intervals, was determined as the median diameter D of the stainless steel powder of the present invention. 50 Used as.

[0059] As shown 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 viscosity and the apparent density, better moldability can be obtained. The apparent density is 3.5Mg / m 3 Apparent density is 3.5Mg / 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. 3The apparent density of the stainless steel powder is 5.0Mg / m or more. 3 The apparent density is 5.0Mg / 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 shall be the value measured using 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 of MnS per unit area N MnS satisfies the following equation (2). N 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 is as follows. MnS is an inclusion consisting of Mn and S, and serves as the starting point of corrosion, so the fewer the number of MnS, the better the corrosion resistance. The inventors have discovered that, even for stainless steel members having the same component composition, steel members manufactured by additive manufacturing (additively manufactured objects) have better corrosion resistance than steel members manufactured by processing steel plates. The reason for this is not clear, but with additively 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 the number of MnS 100 pieces / mm 2 It has been found that excellent corrosion resistance can be obtained when: MnS More preferably, 50 pieces / mm 2 Less than 30 pieces / mm, more preferably 30 pieces / mm 2 The following is also true. MnS There is no particular lower limit for N MnS is 0 pieces / mm 2 may be also possible.

[0063] Number of MnS per unit area of ​​stainless steel component 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 becomes the observation surface, and is embedded in resin and mirror-polished. The observation surface can be located at 1 / 2 the height of the stainless steel member. Then, a 1.0 mm 2 The range was observed with SEM·EDX (Scanning Electron Microscope·Energy Dispersive X-ray Spectrometry), and inclusions containing 25 at% or more each of Mn and S and with 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 at 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. Three fields of view were used for SEM·EDX observation, and the average number of MnS in the three fields of view was calculated based on the unit area (mm 2 ) Number of MnS particles per MnS It 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, a predetermined amount of the above-mentioned elements is melted as materials in a high-frequency vacuum melting furnace, alloyed, and cast to produce an ingot (master ingot). At this time, it is preferable to melt under the conditions of a reduced pressure Ar atmosphere and a melting temperature of 1600°C or higher. The reason for setting 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. Also, from the viewpoint of preventing oxidation of the molten steel, it is preferable to melt in a reduced pressure Ar atmosphere. Note that the melting furnace used in this process is not limited to a high-frequency vacuum melting furnace, and other melting furnaces (for example, a direct current heating type melting furnace) 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 material to remelt in a melting furnace such as a high-frequency or induction furnace, and stainless steel powder with a low oxygen content is obtained by gas atomization using inert gases such as Ar or He.

[0068] These stainless steel powders are then mixed to obtain the above-mentioned median diameter D 50 The powder is classified to have an apparent density and used as the stainless steel powder of the present invention. The classification may be performed using a sieve or may be performed using other techniques such as air flow classification. Moreover, water atomization may be used instead of gas atomization.

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

[0070] First, the stainless steel powder of the present invention is used as a material to form a stainless steel laminated object (three-dimensional structure) by, for example, an additive manufacturing method (metal powder additive manufacturing method). For example, a 3D printer method can be used as the additive manufacturing method. Here, a laser-type powder bed type 3D printer is used. The setting conditions of the 3D printer are not specified. From the viewpoint of preventing excessive melting or insufficient melting, for example, it is preferable that the laser output of the 3D printer is 150 to 300 W and the scan speed is 700 to 1100 mm / s. Next, the formed member is subjected to heat treatment as necessary to obtain the stainless steel member of the present invention. In this heat treatment step, the stainless steel member is held at a temperature in the range of 850 to 1200 ° C for 1 minute or more, and then air-cooled. Preferably, the stainless steel member is held at a temperature in the range of 1000 to 1200 ° C for 10 minutes or more, and then air-cooled. EXAMPLES

[0071] The present invention will be described below with reference to examples. First, raw material powder having the composition shown in Table 1 was produced by gas atomization. Then, classification was carried out 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 in which the diameter was adjusted to 21 to 220 μm. This was used as a material to manufacture a stainless steel member by the powder bed method. The molding device used was an M290 manufactured by EOS. The heat source for melting the stainless steel powder was a laser, and molding was performed under the 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 shape of the molded object was 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 in which it was held in the air at 1000°C for 1 hour and then air-cooled, and then test pieces to be used for each test described later were taken and the characteristics were evaluated. However, those that were evaluated as × in the moldability evaluation shown below were not subjected to subsequent characteristic evaluation. No. 41 is a cold-rolled annealed sheet having a thickness of 2 mm and having almost the same chemical composition as No. 1, which was produced by the conventional process of small steel ingot melting, hot rolling, hot-rolled sheet annealing, cold rolling and cold-rolled sheet annealing. The annealing temperature for the hot-rolled sheet annealing was 1000°C, and the annealing temperature for the cold-rolled sheet annealing was 900°C. This cold-rolled annealed sheet was evaluated as a comparative material for the following corrosion resistance and oxidation resistance. Furthermore, the number N of MnS per unit area for the stainless steel members Nos. 1 to 41 was calculated. MnS The observation surface of the steel structure was at 1 / 2 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) Formability The moldability was evaluated based on the presence or absence of cracks in the stainless steel parts 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 the ground surface in accordance with the method described in JIS Z2343-1:2017 to confirm the presence or absence of cracks. If no cracks of 1.0 mm or more in length were found, the sample was rated as ◯, and if cracks of 1.0 mm or more in length were found, the sample was rated as ×. If the sample was ◯, the sample was rated as having excellent formability.

[0073] (2) Corrosion resistance For No. 1 to No. 40, 20 mm square test pieces were taken from the molded objects after heat treatment, and for No. 41, a 20 mm square test piece was taken from a steel plate. In accordance with JIS G 0577:2014, the surface of the test piece was polished with #600 abrasive paper, and the test piece was covered with a sealant except for an 11 mm square measurement surface, and the test piece was immersed in a 3.5 mass% NaCl solution at 30°C. After holding at the natural potential for 10 minutes, a voltage of 1.1 mA / cm was applied at a sweep rate of 20 mV / min. 2 The potential was swept up to 10 μA / cm 2 The potential at which this occurred was defined as the pitting potential Vc'10. The pitting potential measurement results were rated as follows: if Vc'10 was less than 200mV (vs. SCE), then it was rated as ×; if it was 200mV (vs. SCE) or more but less than 400mV (vs. SCE), then it was rated as ○; if it was 400mV (vs. SCE) or more, then it was rated as ◎; and if it was ○ or ◎, then it was rated as excellent corrosion resistance.

[0074] (3) Oxidation resistance The oxidation resistance was evaluated by carrying out an oxidation test. For No. 1 to No. 40, the surface (the two widest surfaces) of the heat-treated molded object was ground by 1 mm to obtain a plate-shaped test piece with a thickness of 2 mm. From this test piece, an oxidation test piece with a size of 20 mm x 30 mm x 2.0 mm was taken. For No. 41, the surface (the two widest surfaces) of the steel plate was ground by 0.1 mm to obtain a plate-shaped test piece with a thickness of 1.8 mm. From this test piece, an oxidation test piece with a size of 20 mm x 30 mm x 1.8 mm was taken. Then, these surfaces were polished with #600 abrasive paper to obtain the oxidation test piece. An oxidation test was carried out on this oxidation test piece by holding it in high-temperature air for 400 hours, and the oxidation resistance was evaluated. The test temperatures were set to two conditions: 600°C and 800°C. After the oxidation test, the oxidation weight gain was 20 g / m 2 If the oxide film on the surface peels off, the product is deemed to have failed. The oxidation gain is 20g / m 2 A test piece that did not peel off from the oxide film was rated as "pass." For oxidation resistance, a material was rated as × if it failed both the 600°C and 800°C tests, as ◯ if it passed the 600°C test and failed the 800°C test, and as ◎ if it passed both the 600°C and 800°C tests. A rating of ○ or ◎ was used to evaluate the material as having excellent oxidation resistance.

[0075] [Table 1]

[0076] [Table 2]

[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] In contrast, the composition and properties (median diameter D 50 In the comparative examples Nos. 22 to 30, in which the apparent density (apparent density) was outside the appropriate range, good formability, corrosion resistance and oxidation resistance could not all be satisfied at the same time. In addition, the steel plate No. 41 could not obtain good corrosion resistance.

[0079] More specifically, in Comparative Example No. 22 (steel powder symbol B1), the Ni content exceeded the upper limit of the present invention, and therefore the austenite phase precipitated, failing to provide good oxidation resistance. In Comparative Example No. 23 (steel powder symbol B2), the Mn content exceeded the upper limit of the present invention, and therefore the austenite phase precipitated, failing to provide good oxidation resistance. In Comparative Example No. 24 (steel powder symbol B3), the C content exceeded the upper limit of the present invention, and therefore good moldability was not obtained. Since the desired moldability was not obtained for Comparative Example No. 24, the corrosion resistance and oxidation resistance were not evaluated thereafter (evaluation not possible). In Comparative Example No. 25 (steel powder symbol B4), the N content exceeded the upper limit of the present invention, and therefore good moldability was not obtained. Since the desired moldability was not obtained for Comparative Example No. 25, the corrosion resistance and oxidation resistance were not evaluated thereafter (evaluation not possible). In Comparative Example No. 26 (steel powder symbol B5), the Cr content was below the lower limit of the present invention, and therefore 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, and therefore 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, and therefore good corrosion resistance was not obtained. In Comparative Example No. 29 (steel powder symbol B8), the contents of Ni and Mo 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 is the same as that of 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, and therefore good moldability was not obtained. Comparative Example No. 30 did not obtain the desired moldability, and therefore 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 composition as that of Invention Example No. 1 (steel powder symbol A1), but the unit area (mm 2 ) was large, and good corrosion resistance was not obtained. [Industrial Applicability]

[0080] By using the stainless steel powder of the present invention, a stainless steel member having excellent corrosion resistance and oxidation resistance can be formed without solidification cracking. The stainless steel member obtained by the present invention can be particularly suitably used for automobile heat exchangers and exhaust system members. Furthermore, it can be applied to various members without being limited to these uses. The stainless steel powder of the present invention is suitable as a material for ferritic stainless steel members having complex shapes and requiring excellent oxidation resistance and corrosion resistance, such as automobile exhaust system members and heat exchangers, and is particularly suitable as a material when manufacturing the stainless steel member by an additive manufacturing method.

Claims

1. In mass percent, C: 0.003-0.200%, Si: 0.01 to 2.00%, Mn: 0.05-2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0-35.0%, Ni: 0.10-2.50%, Mo: 0.50-3.00%, Nb: 0.10% or more and less than 0.70%; N: 0.030% or less, and O: 0.200% or less, and has a component composition that satisfies the following formula (1), with the balance being Fe and unavoidable impurities; Median diameter D ５０ is 10 μm or more and 200 μm or less, and the apparent density is 3.5 Mg / m ３ 5.0 Mg / m or more ３ Ferritic stainless steel powder, which is: (Ni+Mo)≧2.00 (1) Here, Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively.

2. The composition further comprises, in mass%, 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 The ferritic stainless steel powder according to claim 1 , comprising one or more selected from the following:

3. In mass percent, C: 0.003-0.200%, Si: 0.01 to 2.00%, Mn: 0.05-2.00%, P: 0.040% or less, S: 0.010% or less, Cr: 15.0-35.0%, Ni: 0.10-2.50%, Mo: 0.50-3.00%, Nb: 0.10% or more and less than 0.70%; N: 0.030% or less, and O: 0.200% or less, and has a component composition that satisfies the following formula (1), with the balance being Fe and unavoidable impurities; Number of MnS per unit area N ＭｎＳ A ferritic stainless steel member, which satisfies the following formula (2): (Ni+Mo)≧2.00...(1) N ＭｎＳ ≦ 100 pieces / mm ２ ...(2) Here, Ni and Mo in formula (1) are the contents (mass%) of Ni and Mo, respectively.

4. The composition further comprises, in mass%, 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 The ferritic stainless steel member according to claim 3, comprising at least one selected from the group consisting of:

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