Fe-based alloys and metal powders for melt-solidification molding

Abstract

To provide an Fe-based alloy for melt-solidification-shaping which can give a shaped object having small deformation during shaping, a high processing efficiency in an as-shaped state and a low content of expensive alloy elements when applied to melt-solidification-shaping, and a metal powder having a composition that is equivalent to that of the Fe-based alloy.SOLUTION: An Fe-based alloy for melt-solidification-shaping contains 0.05≤C≤0.25 mass%, 0.01≤Si≤2.0 mass%, 0.05≤Mn≤2.5 mass%, 2.5≤Ni≤9.0 mass%, 0.1≤Cr≤8.0 mass%, and 0.005≤N≤0.200 mass%, with the balance being Fe and inevitable impurities, satisfying 11.5<15C+Mn+0.5Cr+Ni<20. There is also provided a metal powder having an average composition that is equivalent to that of the Fe-based alloy.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an Fe-based alloy and metal powder for melt solidification forming, and more specifically to an Fe-based alloy for melt solidification forming that, when applied to melt solidification forming such as build-up welding and additive manufacturing, produces a shaped product with little deformation during forming, high processing efficiency in the as-formed state, and a low content of expensive alloy elements, and a metal powder having an average composition equivalent to the alloy. [Background technology]

[0002] In recent years, metal additive manufacturing technology has been attracting attention. (a) Complex metal parts can be formed into shapes close to their final shape. (b) Increased design freedom; (c) The cutting allowance is smaller than that of conventional machining. This is because of the following advantages:

[0003] Here, the term "additive manufacturing" refers to a method of manufacturing a three-dimensional structure by stacking thin flake layers, which correspond to structures obtained by horizontally slicing a three-dimensional structure, using various methods. Examples of methods for stacking thin flake layers include: (a) A method of repeating a step of forming a thin layer of metal powder and a step of locally melting and solidifying the powder layer by irradiating it with an energy beam such as a laser beam or an electron beam; (b) A method of overlapping thin plates of a predetermined shape and diffusing them together etc.

[0004] Among these, the additive manufacturing method in which a spread-out metal powder is irradiated with laser light to locally melt and solidify the powder layer is also known as the "SLM (Selective Laser Melting) method." The SLM additive manufacturing method has the advantage of being able to easily form complex three-dimensional shapes simply by changing the irradiation position of the laser light. In addition, the additive manufacturing method, in which a laser or electron beam is irradiated while supplying metal powder to selectively deposit molten metal onto an existing material or substrate, is also called the "directed energy deposition (DED) method." When this additive manufacturing method is applied to the production of die-casting molds or plastic molding molds, for example, it becomes possible to freely arrange non-linear or three-dimensional water cooling circuits inside the mold.

[0005] Various proposals have been made for metal powders used in additive manufacturing. For example, Patent Document 1 discloses a steel powder containing predetermined amounts of C, Si, Cr, Mn, Mo, V, and N, with the balance being Fe and unavoidable impurities. The same document states: (a) Conventional mold steels such as SKD61, SUS420J2, and maraging steel have high-temperature strength, but they have low thermal conductivity because they contain large amounts of elements such as Si, Cr, Ni, and Co, which are easily dissolved in the matrix. (b) In this type of high alloy steel, if the content of alloying elements that reduce thermal conductivity is reduced and the Cr content is optimized, high thermal conductivity can be achieved while maintaining high corrosion resistance; and (c) Such steel powder is suitable for additive manufacturing. is stated.

[0006] Patent Document 2 describes a maraging steel containing predetermined amounts of C, Ni, Co, Mo, Ti, and Al, with the balance being Fe and unavoidable impurities, and a median diameter D 50 A metal powder for additive manufacturing having a particle size of 200 μm or less is disclosed. The same document states: (a) When additive manufacturing is performed using metal powder made of maraging steel containing Ti, Ti segregates linearly, which tends to reduce the toughness of the additively manufactured object; and (b) When the Ti content is set to 0.1 to 5.0 mass%, Ti segregation in the additive manufacturing product can be suppressed, and the toughness of the additive manufacturing product can be improved. is stated.

[0007] To shorten delivery times and improve molding quality by introducing complex water cooling circuits, additive manufacturing (AM) technology is being used to manufacture plastic molding dies. In AM (SLM, DED) using metal powders, powders such as SUS420J2, maraging steel, and SKD61 have traditionally been used. The metal powder disclosed in Patent Document 1 has excellent thermal conductivity and corrosion resistance, making it suitable for AM of plastic molding dies. Similarly, the maraging steel powder described in Patent Document 2 has excellent toughness, making it suitable for AM of plastic molding dies.

[0008] To improve production capacity, additive manufacturing of plastic molding dies often uses additive manufacturing equipment that combines additive manufacturing and cutting functions. In this case, cutting is performed on the as-built part. Metal powders used in such additive manufacturing equipment preferably meet the following three conditions: (a) Deformation during molding is small, and efficiency during finishing is high (small machining allowance). (b) The hardness of the as-formed part is low, and the processing efficiency is high in the as-formed state. (c) The content of expensive alloying elements such as Ni, Co, and Mo is low, so the manufacturing cost is low.

[0009] However, no metal powder that satisfies all three of the above conditions has been proposed to date. For example, SKD61 and the metal powder described in Patent Document 1 are prone to significant deformation of the additively manufactured object due to thermal stress during additive manufacturing. If deformation is significant, the dimensional accuracy of the molded product may deteriorate, and the machining allowance during finish processing may increase. Alternatively, the mold must be designed in advance taking deformation into account, which may result in a decrease in processing efficiency.

[0010] In addition, the deformation during additive manufacturing of SUS420J and maraging steel is smaller than that of SKD61. However, SUS420J2 has high hardness as-formed, which causes severe wear of cutting tools during cutting, resulting in low processing efficiency. On the other hand, maraging steel has low hardness as-formed, making it relatively easy to process in its as-formed state, but its manufacturing costs are high due to its high content of expensive rare metals such as Ni, Co, and Mo. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Patent No. 6601051 [Patent Document 2] Japanese Patent Application Publication No. 2020-045567 Summary of the Invention [Problem to be solved by the invention]

[0012] The problem to be solved by the present invention is to provide an Fe-based alloy for melt solidification forming, which, when applied to melt solidification forming such as build-up welding and additive manufacturing, produces a shaped product with little deformation during forming, high processing efficiency in the as-formed state, and a low content of expensive alloy elements. Another problem to be solved by the present invention is to provide a metal powder having an average composition equivalent to that of such an Fe-based alloy for melt-solidification forming. [Means for solving the problem]

[0013] In order to solve the above problems, the Fe-based alloy for melt solidification forming according to the present invention comprises: 0.11≦ C≦0.25mass%, 0.01≦Si≦2.0mass%, 0.05≦Mn≦2.5mass%, 2.5≦Ni≦9.0mass%, 0.1≦Cr≦8.0mass%, and 0.005≦N≦0.200mass% and the balance being Fe and unavoidable impurities, The following formula (1) is satisfied. 11.5<15C+Mn+0.5Cr+Ni<20…(1)

[0014] The metal powder according to the present invention has an average composition equivalent to that of the Fe-based alloy for melt solidification forming according to the present invention. [Effects of the Invention]

[0015] When additive manufacturing is performed using a metal powder that contains the specified elements and satisfies formula (1), the residual tensile stress generated during the cooling process after manufacturing is alleviated by volume expansion due to martensitic transformation, thereby suppressing deformation during additive manufacturing. Furthermore, the metal powder according to the present invention has a lower carbon content than SUS420J2, so the hardness of the as-formed material is lower, and the processing efficiency in the as-formed state is higher.

[0016] Furthermore, the metal powder according to the present invention has a higher carbon content and lower Ni, Mo, and Co contents than maraging steel. Therefore, the metal powder according to the present invention is less expensive than maraging steel. Furthermore, the decrease in hardness caused by the reduced Ni, Mo, and Co contents can be compensated for by the increase in hardness caused by the increased carbon content (strengthening due to martensitic transformation, precipitation strengthening due to carbide precipitation, etc.). [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 10 is a diagram illustrating the relationship between the variable A and the Ms point. [Figure 2] FIG. 10 is a diagram showing the relationship between the Ms point and distortion after fabrication. [Figure 3] FIG. 10 is a diagram illustrating the relationship between the variable A and distortion after modeling. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of the present invention will be described in detail below. [1. Fe-based alloys for melt-solidification molding] [1.1. Main constituent elements] The Fe-based alloy for melt-solidification forming according to the present invention (hereinafter simply referred to as "Fe-based alloy") contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0019] (1) 0.05≦C≦0.25mass%: The C content affects the hardness of martensite immediately after build-up welding or immediately after additive manufacturing (hereinafter, these are also collectively referred to as "immediately after manufacturing"). Generally, the higher the C content, the higher the hardness of martensite immediately after manufacturing. C is also an element that can effectively lower the Ms point. If the C content is too low, the hardness immediately after manufacturing may decrease or the Ms point may increase. Therefore, the C content must be 0.05 mass% or more. The C content is preferably 0.08 mass% or more, and more preferably 0.11 mass% or more. On the other hand, if the C content is excessive, the hardness immediately after molding becomes excessively high, which may reduce the efficiency of cutting. Therefore, the C content must be 0.25 mass% or less. The C content is preferably 0.22 mass% or less, and more preferably 0.19 mass% or less.

[0020] (2) 0.01≦Si≦2.0mass%: Si has the effect of improving machinability during cutting. Since additively manufactured products are manufactured in a shape close to the finished shape, the amount of material that can be cut during cutting is small. Therefore, high machinability is not necessarily required. However, reducing the amount of Si more than necessary increases the refining cost during manufacturing, which is not economical. Therefore, the amount of Si must be 0.01 mass% or more. The amount of Si is preferably 0.05 mass% or more, and more preferably 0.10 mass% or more. On the other hand, if the Si content is excessive, the toughness may decrease. Therefore, the Si content must be 2.0 mass% or less. The Si content is preferably 1.5 mass% or less, more preferably 1.2 mass% or less, and even more preferably 0.5 mass% or less.

[0021] (3) 0.05≦Mn≦2.5mass% Mn is an element necessary for ensuring hardenability. Mn is also an element that effectively lowers the Ms point. If the Mn content is too low, the hardenability may decrease or the Ms point may increase. Therefore, the Mn content must be 0.05 mass% or more. The Mn content is preferably 0.1 mass% or more, and more preferably 0.3 mass% or more. On the other hand, if the Mn content is excessive, the Ms point will drop significantly, which may result in greater deformation of the additive manufacturing product. Therefore, the Mn content must be 2.5 mass% or less. The Mn content is preferably 2.3 mass% or less, and more preferably 1.9 mass% or less.

[0022] (4) 2.5≦Ni≦9.0mass% Ni is an element necessary for ensuring hardenability. Ni is also an element that effectively lowers the Ms point. If the Ni content is too low, it may be difficult to lower the Ms point. Therefore, the Ni content must be 2.5 mass% or more. The Ni content is preferably 2.8 mass% or more, more preferably 4.0 mass% or more, and even more preferably 5.0 mass% or more. On the other hand, if the Ni content is excessive, the Ms point will drop significantly, which may result in greater deformation of the additive manufacturing product. Therefore, the Ni content must be 9.0 mass% or less. The Ni content is preferably 8.0 mass% or less, and more preferably 7.0 mass% or less.

[0023] (5) 0.1≦Cr≦8.0mass% Cr forms carbides and nitrides, which are finely dispersed in steel, contributing to improvements in hardness and wear resistance. Cr is also an element necessary for ensuring hardenability and corrosion resistance. If the Cr content is too low, hardenability and hardness may decrease. Therefore, the Cr content must be 0.1 mass% or more. The Cr content is preferably 0.5 mass% or more, and more preferably 0.8 mass% or more. On the other hand, adding more Cr than necessary is not practical because the effect on hardenability saturates. Therefore, the Cr content must be 8.0 mass% or less. The Cr content is preferably 6.5 mass% or less, and more preferably 5.5 mass% or less.

[0024] (6) 0.005≦N≦0.200mass%: N is an element that is mixed in when the molten metal is pulverized by nitrogen atomization. Reducing the N content more than necessary leads to a significant increase in manufacturing costs. Therefore, the N content must be 0.005 mass% or more. The N content is preferably 0.010 mass% or more, and more preferably 0.015 mass% or more. On the other hand, excessive N content may promote the formation of nitrides, resulting in a significant decrease in toughness. Therefore, the N content must be 0.200 mass% or less. The N content is preferably 0.100 mass% or less, and more preferably 0.050 mass% or less.

[0025] (7) Inevitable impurities: The Fe-based alloy according to the present invention may contain the following components in the amounts shown below. In such cases, these components are treated as unavoidable impurities in the present invention. P≦0.05mass%, S≦0.01mass%, O≦0.08mass%, Mo<0.20mass%, W≦0.20mass%, V<0.05mass%, Al<0.30mass%, Ti≦0.20mass%, Cu<0.50mass%, Co≦0.05mass%, Sn≦0.05mass%, Nb≦0.05mass%, Ta≦0.05mass%, Zr≦0.05mass%, B≦0.01mass%, Ca≦0.01mass%, Se≦0.03mass%, Te≦0.01mass%, Bi≦0.01mass%, Pb≦0.05mass%, Mg≦0.02mass%, REM≦0.01 mass%.

[0026] [1.2. Sub-constituent elements] The Fe-based alloy according to the present invention may further contain one or more of the following elements in addition to the above-mentioned main constituent elements. The types of additional elements, their component ranges, and the reasons for their limitations are as follows:

[0027] (1) 0.5≦Cu≦3.0mass%: Cu is an element that contributes to improving hardness by being precipitated and dispersed in steel as fine Cu particles through aging treatment. To achieve this effect, the Cu content is preferably 0.5 mass% or more. The Cu content is more preferably 0.6 mass% or more. On the other hand, if the Cu content is excessive, not only does the effect of contributing to improving hardness saturate, but also the manufacturing cost increases. Therefore, the Cu content is preferably 3.0 mass% or less. The Cu content is more preferably 2.5 mass% or less, and even more preferably 1.5 mass% or less.

[0028] (2) 0.2≦Mo≦2.0 mass%: Mo is an element that forms carbides and nitrides and disperses finely in steel, thereby contributing to improving hardness and wear resistance. To achieve this effect, the Mo content is preferably 0.2 mass% or more. The Mo content is more preferably 0.3 mass% or more. On the other hand, if the Mo content is excessive, the amount of carbides or Laves phases precipitated increases, which may reduce toughness. Therefore, the Mo content is preferably 2.0 mass% or less. The Mo content is more preferably 1.8 mass% or less, and even more preferably 1.2 mass% or less.

[0029] (3) 0.05≦V≦0.1 mass%: V, like Mo, is an element that forms carbides and nitrides and disperses finely in steel, thereby contributing to improving hardness and wear resistance. To obtain such effects, the V content is preferably 0.05 mass% or more. On the other hand, if the V content is excessive, carbides may be formed, which may increase tool wear during cutting. Therefore, the V content is preferably 0.1 mass% or less. The V content is more preferably 0.08 mass% or less. The metal powder may contain either Mo or V, or may contain both.

[0030] (4) 0.3≦Al≦1.5mass% Al forms intermetallic compounds with Ni during tempering and precipitates in the steel. The precipitation of intermetallic compounds contributes to improving hardness. To obtain this effect, the Al content is preferably 0.3 mass% or more. On the other hand, if the Al content is excessive, the amount of intermetallic compounds and nitrides increases excessively, which may reduce toughness. Therefore, the Al content is preferably 1.5 mass% or less. The Al content is preferably 1.3 mass% or less, and more preferably 1.2 mass% or less.

[0031] [1.3. Ingredient Balance] The Fe-based alloy according to the present invention satisfies the following formula (1). 11.5<15C+Mn+0.5Cr+Ni<20…(1)

[0032] "15C+Mn+0.5Cr+Ni" in formula (1) (hereinafter also referred to as "variable A") correlates with the Ms point of an Fe-based alloy. All elements included in variable A have the effect of lowering the Ms point. In the Fe-based alloy according to the present invention, by optimizing variable A so as to satisfy formula (1), the Ms point of the Fe-based alloy can be set within a range suitable for additive manufacturing (specifically, approximately 50°C to 280°C). The variable A is obtained by multiplying the content (mass%) of each element by a predetermined coefficient and adding them together.

[0033] If the Ms point of an Fe-based alloy is too low, the amount of retained austenite immediately after additive manufacturing will be excessive, making it impossible to obtain the required hardness. Furthermore, even if the alloy is cooled to room temperature immediately after additive manufacturing, the amount of martensite transformation will be small, so the effect of reducing strain due to transformation expansion may not be obtained. Therefore, the Ms point is preferably 50°C or higher. To achieve an Ms point equivalent to or higher than this, the variable A is preferably less than 20.

[0034] On the other hand, to obtain the distortion reduction effect due to transformation expansion, the additively manufactured object must be heated immediately after additive manufacturing to a temperature lower than the Ms point and higher than the temperature at which martensitic transformation is completely completed (Mf point). Current additive manufacturing devices can only heat up to 200°C due to equipment limitations. When the heating temperature of the additively manufactured object is 200°C and the Ms point of the metal powder is higher than 280°C, the heating temperature is too low, so the martensitic transformation is almost complete immediately after additive manufacturing, and the distortion reduction effect due to transformation expansion cannot be obtained.

[0035] Furthermore, even if it becomes possible to heat the additively manufactured object to 200°C or higher, if the Ms point is 280°C or higher, the temperature required for heating is higher than the temperature at which bainite transformation occurs, causing expansion due to bainite transformation during additive manufacturing, and the distortion reduction effect due to transformation expansion cannot be obtained. Therefore, the Ms point is preferably 280°C or lower. To achieve an Ms point equivalent to or lower than this, the variable A is preferably greater than 11.5.

[0036] [1.4. Shape] In the present invention, the shape of the Fe-based alloy is not particularly limited. The Fe-based alloy may be in the form of a lump, a rod, a tube, a wire, a powder, etc. Powder is particularly suitable as a raw material for melt-solidification molding.

[0037] [2. Metal powder] The metal powder according to the present invention has an average composition equivalent to that of the Fe-based alloy for melt solidification forming according to the present invention.

[0038] 2.1. Ingredients "The average composition is equivalent to that of an Fe-based alloy for melt solidification molding" means (a) The metal powder is composed of an aggregate of one type of metal particles having the same composition, and each metal particle is within the above-mentioned composition range; (b) The metal powder is a mixture of two or more metal particles having different compositions, and each of the individual metal particles is within the above-mentioned range of composition, or (c) The metal powder is a mixture of two or more types of metal particles having different compositions, and one or more types of metal particles are not within the above-mentioned component ranges, but the average value of the composition of the entire metal powder is within the above-mentioned component ranges; This refers to

[0039] When a metal powder is a mixture of two or more types of metal particles having different compositions, each metal particle may be a pure metal particle containing a single metal element, or may be an alloy particle containing two or more metal elements. When the metal powder is a mixture, its average composition can be obtained by, for example, extracting about 10 g of a sample from the mixture and analyzing it using methods such as X-ray fluorescence analysis, combustion infrared absorption spectroscopy, or plasma optical emission spectroscopy. The details of the composition (average composition) of the metal powder are the same as those of the Fe-based alloy described above, so a detailed explanation will be omitted.

[0040] [2.2. Average particle size] "Average particle size" means the number frequency D 50 (μm), that is, the cumulative 50% particle diameter (median diameter) of the powder. 50 As a measurement method, for example, (a) A method of measuring using a particle distribution measuring device based on the laser diffraction / scattering method, (b) A method of measuring using a particle image analyzer; (c) Measurement using a Coulter counter; etc. In the present invention, "D 50 " refers to the median diameter measured using a particle distribution measuring device based on the laser diffraction / scattering method. The average particle size and particle size distribution of the metal powder can be controlled by the production conditions of the metal powder and the classification conditions of the metal powder.

[0041] In general, D 50 The smaller the particle size, the higher the content of fine powder (powder with a particle size of 10 μm or less). The smaller the particle size, the stronger the adhesive forces that occur between particles, such as van der Waals forces and electrostatic forces. Therefore, D 50 If D is too small, the powder tends to aggregate and the flowability decreases. 50 is preferably 10 μm or more. 50 is preferably 20 μm or more, and more preferably 30 μm or more.

[0042] On the other hand, D 50 If D becomes too large, the frictional force on the powder surface becomes more dominant than the adhesive force between particles. As a result, the shear resistance during powder flow increases, hindering fluidity. 50 is preferably 50 μm or less.

[0043] [2.3. Particle shape] The particle shape of each metal particle contained in the metal powder is not particularly limited. The metal particles may be spherical or irregularly shaped. To obtain high fluidity, spherical metal particles are preferred.

[0044] 2.4. Surface Coating The surface of the metal particles may be coated with nanoparticles. "Nanoparticles" refer to particles of inorganic compounds with a diameter of 1 nm or more and 100 nm or less. Coating the surfaces of metal particles with certain nanoparticles can sometimes suppress the aggregation of metal particles. Examples of nanoparticles that have the effect of suppressing the aggregation of metal particles include metal oxides such as silica (SiO2), alumina (Al2O3), manganese oxide (MnO), iron oxide (Fe2O3), calcium oxide (CaO), and magnesium oxide (MgO).

[0045] When the surface of metal particles is coated with nanoparticles, if the amount of coating is too small, aggregation of the metal particles may not be sufficiently suppressed. Therefore, the content of nanoparticles is preferably 0.005 mass% or more. On the other hand, if the amount of nanoparticles coated is excessive, the nanoparticles may become inclusions, which may reduce the strength and / or toughness of the shaped product when melt-solidified molding is performed. Therefore, the content of nanoparticles is preferably 0.05 mass% or less.

[0046] [2.5. Usage] The metal powder according to the present invention can be used as a raw material powder for melt-solidification molding. Here, the term "melt solidification molding method" refers to a method of melting metal powder using various heat sources, and then solidifying and depositing the molten metal powder to form all or part of a shaped object. "Forming the entire shaped object" means that the entire shaped object is formed only by melting, solidifying, and depositing the metal powder. "Forming a part of a shaped object" means laminating a new layer that constitutes another part of the shaped object on the surface of a base material that constitutes a part of the shaped object by melting, solidifying, and depositing metal powder (for example, repairing a mold).

[0047] Among the melt solidification molding methods, representative examples include: (a) Direct Energy Deposition (DED) method, (b) Powder bed fusion; (c) Plasma overlay welding method, etc.

[0048] Of these, "Directed Energy Deposition (DED)" is a method in which a laser or electron beam is irradiated while metal powder is supplied, selectively depositing molten metal onto an existing component, substrate, or other cladding material. The DED method allows for repeated deposition of metal layers, and can deposit various shapes, including linear, wall, and block shapes. By using equipment that uses a laser as the heat source, it is possible to narrow the volume of the molten liquid being deposited, preventing quality degradation due to mixing of components that occurs at the interface with the cladding material. Therefore, a variety of materials, such as Fe-based alloys, Ni-based alloys, and Co-based alloys, can be used for cladding materials.

[0049] "Powder bed fusion" is a manufacturing method in which slice data in units of several tens of microns is created based on three-dimensional data (for example, STL data) generated by 3D-CAD, etc., and the resulting slice data is used to selectively scan and irradiate a powder bed with a laser or electron beam, thereby stacking sintered layers. The SLM method is a type of powder bed fusion method. "Plasma build-up welding" refers to a method in which a plasma arc is generated between an electrode and a substrate, metal powder is introduced into the plasma arc, and the metal powder is melted to build up a metal build-up on the surface of the substrate.

[0050] 2. Metal Powder Manufacturing Methods In the present invention, the method for producing the metal powder is not particularly limited, and examples of the method for producing the metal powder include gas atomization, water atomization, plasma atomization, plasma rotating electrode atomization, and centrifugal atomization. For example, when producing metal powder using the gas atomization method, high-pressure gas is sprayed onto the molten metal as it falls from the bottom of a tundish, pulverizing and solidifying it. In this case, the high-pressure gas used is an inert gas such as nitrogen, argon, or helium. When producing powder using the gas atomization method, impurities such as P, S, Cu, Co, Ti, R, and Nb can inevitably be mixed in. Furthermore, two or more types of metal powders may be mixed and then subjected to a method such as mechanical alloying to produce the metal powder.

[0051] Furthermore, after producing the metal powder using either method, the metal powder may be further subjected to a spheroidizing treatment using reducing thermal plasma. Alternatively, to improve the flowability of the metal powder, the surface of the powder may be coated with an appropriate amount of nanoparticles after the powder is produced. Furthermore, the particle size distribution of the metal powder can be controlled by the production conditions, but it can also be controlled using classification methods such as wet cyclones, dry cyclones, dry sieves, and ultrasonic sieves.

[0052] [4. Effect] When additive manufacturing is performed using a metal powder that contains the specified elements and satisfies formula (1), the residual tensile stress generated during the cooling process after manufacturing is alleviated by volume expansion due to martensitic transformation, thereby suppressing deformation during additive manufacturing. Furthermore, the metal powder according to the present invention has a lower carbon content than SUS420J2, so the hardness of the as-formed material is lower, and the processing efficiency in the as-formed state is higher.

[0053] Furthermore, maraging steel is an age-hardening ultra-high strength steel made by adding large amounts of age-hardening elements such as Mo and Co to low-carbon 18% Ni steel. Additive manufacturing using maraging steel powder and aging the resulting product can achieve high hardness. However, Ni, Mo, and Co are all expensive elements. On the other hand, reducing the content of these elements to reduce manufacturing costs can sometimes result in the required hardness not being achieved.

[0054] In contrast, the metal powder according to the present invention has a higher carbon content and lower Ni, Mo, and Co contents than maraging steel. Therefore, the metal powder according to the present invention is less expensive than maraging steel. Furthermore, the decrease in hardness caused by the reduced Ni, Mo, and Co contents can be compensated for by the increase in hardness caused by the increased carbon content (strengthening due to martensitic transformation, precipitation strengthening due to carbide precipitation, etc.). [Example]

[0055] ( Examples 1 to 2, Reference Example 3, Examples 4 to 8, Reference Example 9, Example 10 , Comparative Examples 1 to 8) 1. Sample Preparation 1.1. Preparation of metal powder Eighteen types of steel powders shown in Table 1 were produced using a gas atomization method. Note that the steel powders listed in Table 1 may contain elements not listed in the table within the ranges specified as impurities. Comparative Example 2 corresponds to hot work tool steel (JIS SKD61), Comparative Example 3 corresponds to martensitic stainless steel (JIS SUS420J2), and Comparative Example 4 corresponds to 18Ni maraging steel.

[0056] [Table 1]

[0057] [1.2. Fabrication of additive manufacturing objects] Using a metal 3D printer M2 manufactured by Concept Laser, an additive manufacturing object (a 15 x 15 x 15 mm cube) was produced to measure the Ms point and hardness. The additive manufacturing conditions were the same as those used in the evaluation test for moldability described below.

[0058] 2. Test Method [2.1. Ms point] Test pieces (φ4mm x 10mm) for measuring the transformation point were cut out from the as-formed additive manufacturing object. The test pieces were heated to 1000-1300°C, then cooled to 20°C at a rate of 100°C / min, and the temperature and dimensional changes during cooling were measured.

[0059] [2.2. Hardness as-printed] A test piece for hardness measurement was cut out from the center of the as-formed additive manufacturing object.The Rockwell hardness (JIS Z2245) was measured using the obtained test piece.

[0060] [2.3. Distortion after printing] Using a Concept Laser metal 3D printer M2, an additive manufacturing object measuring 18 mm long, 30 mm wide, and 10 mm high was produced on a rectangular base plate measuring 20 mm long, 150 mm wide, and 15 mm high. The energy density was 85 J / mm2 The additive manufacturing was performed while preheating the additive manufacturing object to a temperature of Ms-30°C to Ms-80°C using a heater. The atmosphere during manufacturing was a nitrogen atmosphere.

[0061] After the additive manufacturing process was completed, the base plate with the additively manufactured object was removed from the metal 3D printer and placed on a surface plate. From an external photograph taken horizontally so that the entire additively manufactured object was visible, the radius of curvature R and thickness t of the additively manufactured object were calculated using image analysis. The distortion after manufacturing was then calculated using the following equation (2). Distortion after molding (%) = t × 100 / (2R + t) … (2) Here, in equation (2), when the additive manufacturing object is deformed so that it is convex downward (i.e., convex toward the base plate), the radius of curvature R is defined as a negative value, and when the additive manufacturing object is deformed so that it is convex upward, the radius of curvature R is defined as a positive value. The radius of curvature R can also be calculated by placing the base plate on a surface plate, measuring the dimensions from the surface plate at regular intervals in the longitudinal direction of the object using a laser displacement meter or a stylus-type dimension measuring instrument, and approximating these displacements as a circle.

[0062] [3. Results] The results are shown in Table 2. Figure 1 shows the relationship between variable A and the Ms point. Figure 2 shows the relationship between the Ms point and distortion after printing. Furthermore, Figure 3 shows the relationship between variable A and distortion after printing. The following can be seen from Table 2 and Figures 1 to 3.

[0063] [Table 2]

[0064] (1) In Comparative Examples 1, 5, and 8, the distortion after molding was large, and the absolute value of the distortion exceeded 0.3%. This is thought to be because the variable A was too small, causing the Ms point to exceed 280°C. (2) Comparative Examples 2 and 3 had high as-formed hardness exceeding 50 HRC. This is thought to be due to the excessive C content. (3) Comparative Examples 4 and 6 have low as-formed hardness and small distortion after forming. However, Comparative Example 4 contains large amounts of Ni, Mo, and Co, and Comparative Example 6 contains large amounts of Ni and Mo, so both are expensive. (4) In Comparative Example 7, the distortion after molding was somewhat large, with the absolute value of the distortion exceeding 0.3%. This is thought to be because the variable A was excessively large, causing the Ms point to be less than 50°C. (5) Examples 1 to 2, Reference Example 3, Examples 4 to 8, Reference Example 9, Example 10 All of these materials have appropriate hardness as they are molded and have little distortion after molding. In addition, the amounts of Ni, Mo, and Co are relatively small, so they are low in cost.

[0065] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0066] The metal powder according to the present invention can be used as a powder raw material for manufacturing molds that require cooling (e.g., molds for plastic molding, molds for die casting, molds for hot stamping, and molds for tailored die quenching) using an additive manufacturing method. Furthermore, the metal powder according to the present invention can be applied not only to additive manufacturing, which requires strict shape control of the object, but also to build-up welding, which does not require strict shape control of the object.

Claims

1. 0.11≦C≦0.25 mass%, 0.01≦Si≦2.0mass%, 0.05≦Mn≦2.5mass%, 2.5≦Ni≦9.0mass%, 0.1≦Cr≦8.0 mass%, and 0.005≦N≦0.200mass% and the balance being Fe and unavoidable impurities, An Fe-based alloy for melt-solidification forming that satisfies the following formula (1): 11.5<15C+Mn+0.5Cr+Ni<20...(1)

2. 0.5≦Cu≦3.0mass% The Fe-based alloy for melt-solidification forming according to claim 1, further comprising:

3. 0.2≦Mo≦2.0 mass%, and / or 0.05≦V≦0.1mass%, The Fe-based alloy for melt solidification forming according to claim 1 or 2, further comprising:

4. 0.3≦Al≦1.5mass% The Fe-based alloy for melt-solidification forming according to any one of claims 1 to 3, further comprising:

5. A metal powder having an average composition equivalent to that of the Fe-based alloy for melt solidification forming according to any one of claims 1 to 4. However, the above-mentioned "average composition is equivalent to that of the Fe-based alloy for melt solidification forming" means that (a) The metal powder is composed of an aggregate of one type of metal particles having the same composition, and each of the metal particles is within the composition range defined in any one of claims 1 to 4; (b) The metal powder is a mixture of two or more types of metal particles having different compositions, and each of the metal particles is within the composition range described in any one of claims 1 to 4, or (c) The metal powder is composed of a mixture of two or more types of metal particles having different compositions, and one or more types of the metal particles are not within the composition range described in any one of claims 1 to 4, but the average value of the composition of the entire metal powder is within the composition range described in any one of claims 1 to 4; This refers to

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