Metal powder

Abstract

To provide a metal powder which can give an additive-manufactured object having less cracks or warpage and excellent heat resistance when applied to additive manufacturing.SOLUTION: A metal powder contains: 0.001≤C≤0.45 mass%, 0.01≤Si≤3.50 mass%, Mn≤2.0 mass%, 7.5≤Cr≤21.0 mass%, 1.5≤Ni≤7.0 mass%, Mo≤1.3 mass%, 0.05≤V≤2.0 mass%, Al≤0.015 mass%, N≤0.20 mass%, and 0.05≤C+N≤0.58 mass%, with the balance being Fe and inevitable impurities, satisfying 10<15C+Mn+0.5Cr+Ni<20, and Creq / Nieq<5.6, where, Creq=Cr+Mo+1.5Si+0.5 Nb, and Nieq=Ni+30C+30N+0.5Mn.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a metal powder, and more particularly to a metal powder that, when applied to additive manufacturing, can produce additively manufactured products with minimal cracking and warping, and excellent heat resistance. [Background technology]

[0002] In recent years, metal additive manufacturing technology has been attracting attention. This is because (a) It is possible to form complex-shaped metal parts into a shape close to the final shape, (b) Design flexibility is improved, (c) Compared to conventional machining, the amount of material removed is smaller. This is because it offers advantages such as those listed above.

[0003] Here, "additive manufacturing" refers to a method of creating a three-dimensional structure by stacking thin, slice-like layers, which correspond to a horizontal cross-section of a three-dimensional structure, using various methods. For example, methods for stacking these thin layers include: (a) A method that repeatedly involves the steps of forming a thin layer made of metal powder and 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 sheets having a predetermined shape and diffusion bonding them together. These are some examples.

[0004] Among these methods, the additive manufacturing method that uses laser light to irradiate a layer of metal powder, causing localized melting and solidification of the powder layer, is also known as the "SLM (Selective Laser Melting) method." The SLM additive manufacturing method has the advantage of easily forming complex three-dimensional shapes simply by changing the irradiation position of the laser light. Therefore, when applied to the manufacture of die-casting molds, for example, non-linear or three-dimensional water cooling circuits can be freely arranged inside the mold.

[0005] When performing additive manufacturing using an SLM (Scaling-Luminous Manufacturing) 3D printer, only the top surface of the printed object is rapidly heated, resulting in residual tensile stress on the top surface after cooling. As a result, the printed object is prone to deformation, becoming convex downwards. If the deformation of the printed object becomes large, not only will the dimensional accuracy of the object decrease, but it will also become difficult to remove the printed object from the 3D printer after printing.

[0006] Therefore, various proposals have been made to solve this problem. For example, Patent Document 1 contains: A recoating process is performed to form a material layer consisting of powder of carbon steel or martensitic stainless steel in the molding area. A solidification process in which a laser beam is irradiated onto a predetermined irradiation area of ​​the material layer to form a solidified layer, T1→T2→T1 (where T1≧Mf (martensitic transformation completion temperature of the solidified layer), T1>T2, T2≦Ms (martensitic transformation start temperature of the solidified layer), and a temperature control step to adjust the temperature of the solidified layer. A method for manufacturing additively fabricated objects is disclosed.

[0007] The document states: (a) In additive manufacturing, the volume of the solidified layer generally shrinks during the cooling process, resulting in residual tensile stress in the solidified layer. (b) When additive manufacturing is performed using a material that undergoes martensitic transformation, the solidified layer undergoes volume expansion when it undergoes martensitic transformation, which reduces the volume contraction that occurs when the solidified layer cools and the resulting tensile stress, thereby suppressing deformation of the manufactured object. (c) The amount of transformation (=expansion) can be controlled by controlling T1 and T2 in the temperature control process, and (d) Since Ms and Mf fluctuate depending on the carbon content in the material, the method described in the literature can be applied to a variety of materials by adjusting the carbon content in the material. It is stated.

[0008] Patent Document 2 discloses a metal powder containing a predetermined amount of Cr, Ni, C, Si, Mn, N, Mo, Cu, Nb, P, and S, with the balance being Fe and unavoidable impurities. This document describes that when performing additive manufacturing using stainless steel powder, if the amount of each element contained in the stainless steel powder is within a predetermined range and the amounts of P and S are controlled, solidification cracking is less likely to occur and good formability is exhibited.

[0009] Furthermore, Patent Document 3 discloses a stainless steel containing a predetermined amount of C, Si, Mn, Cr, Mo, V, and N, with the balance being Fe and impurities, which is not a metal powder for additive manufacturing. This document describes that when the content of each element is within a predetermined range, a stainless steel having high hardness, high toughness, and good corrosion resistance can be obtained even after high-temperature annealing.

[0010] Patent Document 1 describes that when the temperature of the solidified layer is raised and lowered near the Ms point, the tensile residual stress generated during the cooling process after forming is relaxed by the volume expansion due to the martensitic transformation, and a formed object with less distortion can be obtained. However, in current 3D printers, due to device constraints, there is an upper limit to the temperature of the reachable forming area. Therefore, the method described in Patent Document 1 can be applied only to steel grades with an Ms point of about 300°C or lower.

[0011] Also, for applications requiring heat resistance, martensitic heat-resistant steels such as SUH1, SUH3, and SUH11 are used. In the case of this type of heat-resistant steel, usually, the carbon content is increased to enhance heat resistance. When such a powder with a high carbon content is applied to additive manufacturing, the hardness tends to become very high as it is formed. As a result, cracks may occur in the formed object, making it difficult to manufacture the formed object. To solve this problem, it is also conceivable to reduce the carbon content of the heat-resistant steel. However, since the decrease in the carbon content causes an increase in the Ms point, the Ms point may exceed the reachable temperature range of the shaping device. As a result, when performing laminated shaping using heat-resistant steel powder with a reduced carbon content, there is a problem that the residual stress of the shaped object increases.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0013] The problem to be solved by the present invention is to provide a metal powder that can obtain a laminated shaped object with few cracks and warps and excellent heat resistance when applied to laminated shaping.

Means for Solving the Problems

[0014] In order to solve the above problems, the metal powder according to the present invention is 0.001 ≦ C ≦ 0.45 mass%, 0.28≦ Si ≦ 3.50 mass%, Mn ≦ 2.0 mass%, 7.5 ≦ Cr ≦ 21.0 mass%, 1.5 ≦ Ni ≦ 7.0 mass%, Mo ≦ 1.3 mass%, 0.05 ≦ V ≦ 2.0 mass%, Al ≦ 0.015 mass%, N ≦ 0.20 mass %、 0.05 ≦ C + N ≦ 0.5 8、 P ≤ 0.03 mass%, and, S ≤ 0.03 mass% It contains, with the remainder being Fe and unavoidable impurities. the law of nature, As the aforementioned unavoidable impurities, Cu≦0.30mass%, O≦0.1mass%, Co≦0.3mass%, Ta≦0.05mass%, Ti≦0.05mass%, Zr≦0.05mass%, B≦0.005mass%, Ca≦0.005mass%, Se≦0.03mass%, Te≦0.005mass%, Bi≦0.01mass%, Pb ≤ 0.03 mass%, Mg ≤ 0.02 mass%, and REM ≤ 0.01 mass%, The following equations (1) and (2) are satisfied.

[0015] 10 < 15C + Mn + 0.5Cr + Ni < 20 …(1) Cr eq / Ni eq <5.6 …(2) however, Cr eq =Cr+Mo+1.5Si+0.5Nb Ni eq =Ni+30C+30N+0.5Mn [Effects of the Invention]

[0016] When additive manufacturing is performed using metal powder containing a specified element and satisfying formula (1), the tensile residual stress generated during the cooling process after manufacturing is relieved by volume expansion due to martensitic transformation. As a result, additively manufactured objects with less cracking and distortion can be obtained. Furthermore, by optimizing the composition of the metal powder to satisfy equation (2) (in particular, optimizing the Si, Cr, and Ni content), it is possible to obtain additively fabricated products with excellent heat resistance. [Brief explanation of the drawing]

[0017] [Figure 1] This figure shows an example of the relationship between temperature and dimensional change when a sample for transformation point measurement is cooled at a predetermined cooling rate. [Figure 2]This is a schematic diagram of the method for measuring warpage. [Figure 3] This figure shows the relationship between variable A and the Ms point. [Figure 4] This figure shows the relationship between the Ms point and the distortion after fabrication. [Modes for carrying out the invention]

[0018] One embodiment of the present invention will be described in detail below. [1. Metal powder] [1.1. Main constituent elements] The metal powder according to the present invention contains the following elements, with the remainder being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows.

[0019] (1) 0.001 ≤ C ≤ 0.45 mass%: Carbon (C) forms carbides with various other elements, making it an effective element for improving hardness and strength. Furthermore, carbon is also effective in reducing the Ms point. To achieve these effects, the carbon content must be 0.001 mass% or higher. Preferably, the carbon content is 0.01 mass% or higher, and more preferably, 0.05 mass% or higher. On the other hand, when additive manufacturing is performed using metal powder, the hardness of the manufactured object immediately after additive manufacturing is proportional to the total amount of C and N. Therefore, if the amount of C is excessive, the hardness of the manufactured object immediately after additive manufacturing becomes too high, which may cause cracking. Accordingly, the amount of C needs to be 0.45 mass% or less. Preferably, the amount of C is 0.40 mass% or less. More preferably, the amount of C is 0.09 mass% or less, or 0.06 mass% or less.

[0020] (2) 0.01 ≤ Si ≤ 3.50 mass%: Si is an effective element as a deoxidizing agent. Furthermore, Si is an effective element for improving the heat resistance and oxidation resistance of molded objects. To obtain these effects, the Si content must be 0.01 mass% or more. Preferably, the Si content is 0.1 mass% or more, and more preferably 0.3 mass% or more. On the other hand, if the amount of Si is excessive, the toughness of the fabricated object may decrease. Therefore, the amount of Si needs to be 3.50 mass% or less. Preferably, the amount of Si is 3.2 mass% or less.

[0021] (3) Mn ≤ 2.0 mass %: Mn is an effective element as a deoxidizing and desulfurizing element. Furthermore, Mn is an effective element for improving toughness and tensile strength. In addition, Mn is an effective element for reducing the Ms point. Therefore, metal powder may contain Mn as needed. To obtain the effects described above, the amount of Mn is preferably 0.01 mass% or more. More preferably, the amount of Mn is 0.2 mass% or more. On the other hand, Mn is also an austenite-stabilizing element. Therefore, if the amount of Mn is excessive, the amount of retained austenite will be excessive, which may reduce the hardness and corrosion resistance of the molded object. Accordingly, the amount of Mn needs to be 2.0 mass% or less. Preferably, the amount of Mn is 1.8 mass% or less, and more preferably 1.6 mass% or less.

[0022] (4) 7.5 ≤ Cr ≤ 21.0 mass%: Cr is an effective element for improving high-temperature oxidation resistance. Furthermore, Cr is effective for improving the hardness of molded objects through carbide formation. In addition, Cr is effective in ensuring corrosion resistance by forming a passive film on the surface of molded objects. To obtain these effects, the Cr content must be 7.5 mass% or higher. Preferably, the Cr content is 10.5 mass% or higher. On the other hand, if the amount of Cr is excessive, a ferrite structure may remain even in the quenched structure, which can lead to a decrease in high-temperature strength. Therefore, the amount of Cr needs to be 21.0 mass% or less. Preferably, the amount of Cr is 20.0 mass% or less, and more preferably 18.5 mass% or less.

[0023] (5) 1.5 ≤ Ni ≤ 7.0 mass%: Ni is an effective element for improving corrosion resistance and hot strength. Furthermore, Ni is also an element that effectively lowers the Ms point. To obtain these effects, the Ni content needs to be 1.5 mass% or higher. On the other hand, if the amount of Ni is excessive, the Ms point decreases significantly. As a result, the amount of retained austenite becomes excessive, which can lead to a decrease in temper hardness. Therefore, the amount of Ni needs to be 7.0 mass% or less.

[0024] (6) Mo ≤ 1.3 mass %: Mo promotes the regeneration of the passive film and enhances corrosion resistance. Therefore, the metal powder may contain Mo as needed. To obtain this effect, the amount of Mo is preferably 0.01 mass% or more. More preferably, the amount of Mo is 0.2 mass% or more. On the other hand, if the amount of Mo is excessive, the fracture toughness of the fabricated object may decrease. Therefore, the amount of Mo needs to be 1.3 mass% or less. Preferably, the amount of Mo is 1.1 mass% or less.

[0025] (7) 0.05 ≤ V ≤ 2.0 mass%: V is an element that combines with C and / or N to form carbides and / or nitrides, contributing to improved hardness. Furthermore, V also prevents grain coarsening during quenching, contributing to improved toughness. To achieve these effects, the V content must be 0.05 mass% or higher. Preferably, the V content is 0.1 mass% or higher. On the other hand, if the amount of V is excessive, a large amount of carbides and / or nitrides may remain, which can reduce toughness. Therefore, the amount of V needs to be 2.0 mass% or less. Preferably, the amount of V is 1.3 mass% or less, and more preferably 0.8 mass% or less.

[0026] (8) Al ≤ 0.015 mass %: Al is an effective deoxidizing element. Furthermore, trace amounts of Al can suppress grain coarsening during quenching, contributing to improved toughness. Therefore, metal powder may contain Al as needed. To achieve these effects, an Al content of more than 0.002 mass% is preferable. More preferably, the Al content is 0.005 mass% or more. On the other hand, if the amount of Al is excessive, coarse AlN may be generated, which can lead to a significant decrease in toughness and fatigue properties. Therefore, the amount of Al needs to be 0.015 mass% or less.

[0027] (9) N ≤ 0.20 mass %: Nitrogen (N) is an element that is mixed in when molten metal is powdered using nitrogen spraying. Furthermore, N increases the hardness of the fabricated object immediately after additive manufacturing. Therefore, metal powder may contain N as needed. To achieve this effect, the amount of N is preferably 0.01 mass% or more. On the other hand, if the amount of nitrogen is excessive, the hardness of the fabricated object immediately after additive manufacturing becomes too high, which can cause cracking. Therefore, the amount of nitrogen needs to be 0.20 mass% or less. Preferably, the amount of nitrogen is 0.1 mass% or less, and more preferably 0.07 mass% or less.

[0028] (10) 0.05 ≤ C + N ≤ 0.58 mass%: Both carbon (C) and nitrogen (N) are effective elements for improving hardness and strength. Furthermore, C and N are also effective elements for reducing the Ms point. If the amount of C+N is too low, the required hardness cannot be achieved. Therefore, the amount of C+N needs to be 0.05 mass% or more. On the other hand, if the C+N ratio is excessive, the hardness of the fabricated object immediately after additive manufacturing becomes too high, which can cause cracking. Therefore, the C+N ratio needs to be 0.58 mass% or less. Preferably, the C+N ratio is 0.45 mass% or less.

[0029] (11) Inevitable impurities: In the metal powder according to the present invention, the following components may be present in the amounts shown below. In such cases, these components are treated as unavoidable impurities in the present invention. Cu≦0.30mass%, O≦0.1mass%, Co≦0.3mass%, Ta≦0.05mass%, Ti≦0.05mass%, Zr≦0.05mass%, B≦0.005mass%, Ca≦0.005mass%, Se≦0.03mass%, Te≦0.005mass%, Bi≦0.01mass%, Pb≦0.03mass%, Mg≦0.02mass%, REM≦0.01mass%.

[0030] [1.2. Sub-constituent elements] The metal powder according to the present invention may further contain one or more elements in addition to the main constituent elements described above. The types of additive elements, their component ranges, and the reasons for their limitations are as follows.

[0031] (1) 0.1 ≤ Nb ≤ 1.0 mass%: Like V, Nb combines with C and / or N to form carbides and / or nitrides, contributing to improved hardness. Furthermore, Nb prevents grain coarsening during quenching, contributing to improved toughness. To achieve these effects, a Nb content of 0.1 mass% or more is preferable. On the other hand, if the amount of Nb is excessive, coarse carbides and / or nitrides may precipitate, which can promote quench cracking. Therefore, the amount of Nb is preferably 1.0 mass% or less.

[0032] (2) 0.1 ≤ W ≤ 1.5 mass %: W has the effect of improving corrosion resistance. To obtain this effect, the amount of W is preferably 0.1 mass% or more. On the other hand, if the amount of W is excessive, it not only leads to increased costs but can also generate coarse carbides of type M6C, which can promote quenching cracks. Therefore, the amount of W is preferably 1.5 mass% or less.

[0033] (3) P ≤ 0.03 mass%: Although phosphorus (P) is an unavoidable impurity, it is preferable to reduce it as much as possible from the viewpoint of suppressing solidification cracking. To suppress solidification cracking, the amount of P is preferably 0.03 mass% or less. However, reducing the phosphorus content more than necessary will lead to increased manufacturing costs. Therefore, it is preferable to select an optimal phosphorus content considering these factors.

[0034] (4) S ≤ 0.03 mass %: Although sulfur (S) is an unavoidable impurity, it is preferable to reduce it as much as possible from the viewpoint of suppressing solidification cracking. To suppress solidification cracking, the amount of sulfur is preferably 0.03 mass% or less. However, reducing the amount of sulfur (S) more than necessary will lead to increased manufacturing costs. Therefore, it is preferable to select an optimal amount of sulfur (S) considering these points.

[0035] [1.3. Ingredient Balance] The metal powder according to the present invention must satisfy the following formulas (1) and (2). 10 < 15C + Mn + 0.5Cr + Ni < 20 …(1) Cr eq / Ni eq <5.6 …(2) however, Cr eq =Cr+Mo+1.5Si+0.5Nb Ni eq =Ni+30C+30N+0.5Mn

[0036] [1.3.1. Formula (1)] The expression "15C + Mn + 0.5Cr + Ni" in equation (1) (hereinafter also referred to as "variable A") correlates with the Ms point of the metal powder. All elements included in variable A have the effect of lowering the Ms point. In the metal powder according to the present invention, by optimizing variable A to satisfy equation (1), the Ms point of the metal powder can be set to a range suitable for additive manufacturing (specifically, about 50°C to 280°C). Variable A is calculated by multiplying the mass percentage of each element by a predetermined coefficient and then summing these results. If the metal powder does not contain some of the elements that make up variable A, the content of those elements is set to zero when calculating variable A.

[0037] If the Ms point of the metal powder is too low, excess retained austenite will be present after additive manufacturing, resulting in insufficient hardness. Furthermore, even after cooling to room temperature, insufficient martensitic transformation may prevent the reduction of strain due to transformation expansion. Therefore, an Ms point of 50°C or higher is preferable. To achieve an Ms point equivalent to or higher than this, variable A should preferably be less than 20.

[0038] On the other hand, in order to obtain a strain reduction effect due to transformation expansion, the additively manufactured object must be heated after additive manufacturing to a temperature lower than the Ms point, but higher than the temperature at which martensitic transformation is completely finished (Mf point). Due to equipment limitations, current additive manufacturing equipment can only heat up to 200°C. When the heating temperature of the manufactured object is 200°C, and the Ms point of the metal powder is above 280°C, the heating temperature is too low, causing the martensitic transformation to be almost completed immediately after additive manufacturing, and the strain reduction effect due to transformation expansion cannot be obtained.

[0039] Furthermore, even if the additively fabricated object can be heated to over 200°C, if the Ms point is 280°C or higher, the temperature required for heating will be above the temperature at which bainite transformation occurs. As a result, expansion due to bainite transformation will occur during fabrication, and the distortion reduction effect due to transformation expansion cannot be obtained. Therefore, an Ms point of 280°C or lower is preferable. To achieve an Ms point equivalent to or lower than this, a variable A greater than 10 is preferable.

[0040] [1.3.2. Formula (2)] Formula (2) represents the ratio of the Cr equivalent (Cr eq ) to the Ni equivalent (Ni eq ) (hereinafter, this is also referred to as the "equivalent ratio"). "Cr eq " is an index representing the ease of formation of ferrite in stainless steel. Also, "Ni eq " is an index representing the ease of formation of austenite in stainless steel. Note that Cr eq is obtained by multiplying the content (mass%) of ferrite stabilizing elements by a predetermined coefficient and adding them together. Also, Ni eq is obtained by multiplying the content (mass%) of austenite stabilizing elements by a predetermined coefficient and adding them together. When the metal powder does not contain a part of the elements constituting Cr eq or Ni eq , the content of the element is set to zero and Cr eq or Ni eq is calculated.

[0041] If the equivalent ratio becomes too large, a fully ferritic structure will result, and the high-temperature strength may decrease. Therefore, the equivalent ratio needs to be less than 5.6. The equivalent ratio preferably particularly satisfies the following (2'). Cr eq / Ni eq <2.5 …(2')

[0042] On the other hand, if the equivalent ratio becomes too small, the amount of retained austenite will become excessive, and the hardness and corrosion resistance of the shaped object may decrease. Therefore, the equivalent ratio is preferably 0.3 or more. The equivalent ratio is more preferably 0.4 or more, and even more preferably 0.6 or more.

[0043] [1.4. Powder characteristics] In an SLM method 3D printer, it is necessary to uniformly spread the metal powder before shaping with a laser. In order to uniformly spread the metal powder, the fluidity of the metal powder is important. To ensure this fluidity, the powder characteristics of the metal powder (especially the number frequency D50 The avalanche angle needs to be optimized.

[0044] [1.4.1. Quantity Frequency D 50 ] "Quantity Frequency D" 50 "(μm)" refers to the cumulative particle size (median diameter) of 50 percent of powder particles. 50 For example, as a method of measurement, (a) A method of measurement using a particle distribution analyzer based on laser diffraction and scattering, (b) A method of measurement using a particle image analyzer, (c) Method of measurement using a Coulter counter, These are some examples. In the present invention, "D 50 When this term is used, it refers to the median diameter measured by a particle image analyzer.

[0045] Generally, D 50 The smaller the particle size, the higher the relative content of fine powder (powder with a particle size of 10 μm or less). The adhesive forces between particles, such as van der Waals forces and electrostatic forces, increase as the particle size decreases. Therefore, D 50 If the size becomes too small, the powder will tend to aggregate, and its fluidity will decrease. Therefore, D 50 It is preferable that it be 10 μm or larger. 50 Preferably, the particle size is 20 μm or more, and more preferably 30 μm or more. Meanwhile, D 50 If the ratio becomes too large, the frictional force on the powder surface will dominate the adhesion force between particles. As a result, the shear resistance during powder flow will increase, and flowability will be inhibited. Therefore, D 50 The particle size is preferably 50 μm or less.

[0046] [1.4.2. Avalanche angle] For example, a method for evaluating the fluidity of metal powders is: (a) Metal powder - flowability measurement method specified in JIS Z2502:2012, (b)ASTM B213 Standard Test Methods for Flow Rate of Metal Powders Using the Hall Flowmeter Funnl; (c)ASTM B964 Standard Test Methods for Flow Rate of Metal Powders Using the Carney Funnel; These are some examples. On the other hand, the avalanche angle, which can be measured with Mercury Scientific's Revolution Powder Analyzer, is a suitable method for evaluating fluidity in the powder spreading process in metal 3D printers. In this invention, this avalanche angle is used as an indicator of the fluidity of the metal powder.

[0047] Generally, a small avalanche angle indicates low interparticle adhesion and good fluidity of the metal powder. Furthermore, the avalanche angle is related to the D of the metal powder. 50 Depending on the circumstances, the avalanche angle is usually between 30 and 60 degrees. To more uniformly spread the metal powder in the 3D printer's build area, the avalanche angle is preferably 45 degrees or less. Preferably, the avalanche angle is 43 degrees or less, more preferably 40 degrees or less, and even more preferably 35 degrees or less.

[0048] [1.4.3. Apparent density, tap density, and Hausner ratio] Methods for measuring apparent density include, for example, (a) Method for determining apparent density of metal powder as specified in JIS Z2504:2012, (b) A method in accordance with ASTM B212 Standard Test Method for Apparent Density of Free-Flowing Metal Powders Using the Hall Flowmeter Funnel. These are some examples. In this invention, "apparent density ρ bulkWhen referring to "apparent density," it means the value obtained by the metal powder apparent density measurement method specified in JIS Z2504:2012. In the case of metal powder, the apparent density can usually take values ​​of around 3.0 to 6.0 g / cc.

[0049] For example, a method for measuring tap density is: (a) Method for measuring the density of metal powder taps as specified in JIS Z2512:2012, (b) A method in accordance with ASTM B527 Standard Test Method for Tap Density of Metal Powders and Compounds. These are some examples. In the present invention, "tap density ρ" tapped When referring to "tap density," it means the value obtained by the metal powder tap density measurement method specified in JIS Z2512:2012. In the case of metal powder, the tap density can usually take values ​​of around 3.0 to 6.0 g / cc.

[0050] "Hausner ratio" refers to the apparent density (ρ) of metal powders. bulk ) and tap density (ρ tapped The ratio of (=ρ) tapped / ρ bulk The Hausner ratio is a measure of fluidity. A small Hausner ratio generally indicates weak interparticle interactions and high fluidity. Conversely, a large Hausner ratio generally indicates strong interparticle interactions and low fluidity. In the case of metal powders, the Hausner ratio can typically take a value of ~1.25.

[0051] [1.4.4. Particle shape] The particle shape of the metal powder can be spherical or irregular. Generally, metal powder composed of aggregates of spherical particles exhibits higher fluidity than metal powder composed of aggregates of irregularly shaped particles.

[0052] [1.5. Usage] The metal powder according to the present invention can be used for various applications, but is particularly suitable as a metal powder for additive manufacturing.

[0053] The as-printed hardness of an additively manufactured object using the metal powder according to the present invention mainly depends on the composition of the metal powder. If the as-printed hardness is too high, it can cause cracking. To suppress cracking during manufacturing, the as-printed hardness is preferably 58 HRC or less. More preferably, the as-printed hardness is 55 HRC or less. By optimizing the composition of the metal powder, an additively manufactured object having such as-printed hardness can be obtained.

[0054] [2. Method for producing metal powder] In the present invention, the method for producing metal powder is not particularly limited. Examples of methods for producing metal powder include gas atomization, water atomization, plasma atomization, plasma rotating electrode method, and centrifugal atomization. For example, when manufacturing metal powder using the gas atomization method, molten metal is dropped from the bottom of a tundish while high-pressure gas is blown onto it, causing the molten metal to pulverize and solidify. In this case, inert gases such as nitrogen, argon, and helium are used as the high-pressure gas. When manufacturing powder using the gas atomization method, impurities such as P, S, Cu, Co, Ti, Zr, and Nb may inevitably be mixed in. Furthermore, metal powder may be produced by mixing two or more metal powders and using methods such as mechanical alloying.

[0055] Furthermore, after producing the metal powder using either method, the metal powder may be subjected to a spheroidizing treatment using a reducing thermal plasma. Alternatively, to improve the fluidity of the metal powder, an appropriate amount of nanoparticles may be coated onto the surface after powder production. In addition, the particle size distribution of the metal powder can be controlled by the manufacturing conditions, but it can also be controlled using classification methods such as wet cyclones, dry cyclones, dry sieves, and ultrasonic sieves.

[0056] [3. Method for manufacturing additively fabricated objects] Examples of additive manufacturing methods using metal 3D printers include powder bed fusion, directed energy deposition, and binder jetting. Machining of the additively manufactured object may also be performed during the manufacturing process. The metal powder according to the present invention can be applied to any of these methods. For example, in the case of selective laser fusion (SLM), a type of powder bed fusion method, additive manufacturing is performed as follows: (a) Create slice data in units of tens of micrometers based on 3D shape data (e.g., STL data) generated by 3D-CAD, etc. (b) Using slice data, a heat source is selectively scanned and irradiated onto the powder bed to build up a sintered layer. It is carried out by [this method].

[0057] [4. Effect] C, Mn, Cr, and Ni in steel all have the effect of lowering the Ms point. Therefore, by relatively reducing the amount of C and optimizing the content of these elements to satisfy equation (1), it is possible to maintain the Ms point within a range suitable for low deformation of additively manufactured objects (approximately 50°C to 280°C) without excessively increasing the as-printed hardness.

[0058] Furthermore, optimizing the content of each element to satisfy equation (2) can suppress the formation of the ferrite phase, which reduces heat resistance. In addition, among the elements included in equation (2), Si, Cr, and Ni, in particular, have the effect of improving the heat resistance of the additively manufactured product. Therefore, by satisfying equation (2) while optimizing the content of each element in equation (2), the heat resistance of the additively manufactured product can be maintained. [Examples]

[0059] ( Examples 1-6, Reference Example 7, Examples 8-9, Reference Example 10, Examples 11-13, Reference Example 14, Examples 15-24, Comparative Examples 1-7) [1. Sample Preparation] [1.1. Preparation of Metal Powders] Using the gas atomization method, 31 types of steel powders, as shown in Tables 1 and 2, were prepared. Note that the steel powders listed in Tables 1 and 2 may contain elements not listed in the tables as impurities within the specified range. Furthermore, blank spaces in Tables 1 and 2 indicate less than 0.01 mass% for Mn, Ni, Mo, V, S, Nb, and W, and less than 0.002 mass% for Al. Additionally, Comparative Example 1 corresponds to martensitic heat-resistant steel (SUH11), and Comparative Example 2 corresponds to martensitic heat-resistant steel (SUH1).

[0060] [Table 1]

[0061] [Table 2]

[0062] [1.2. Fabrication of additively manufactured objects] Using a Concept Laser M2 metal 3D printer, we fabricated a layered object (a 15 x 15 x 15 mm cube) for measuring the Ms point and as-printed hardness. Furthermore, using a Concept Laser M2 metal 3D printer, we fabricated a layered object (a rectangular prism measuring 18mm x 30mm x 10mm) on a rectangular base plate measuring 20mm x 150mm x 15mm in height, in order to measure the amount of warping after fabrication. The additive manufacturing process involved preheating the object to a temperature of Ms-30°C to Ms-80°C using a heating element. The manufacturing atmosphere was a nitrogen atmosphere.

[0063] [2. Test Method] [2.1. Ms point] A test specimen (φ4mm × 10mm) for measuring the transformation point was cut from the as-printed additively manufactured object. The test specimen was heated to 1000°C to 1300°C, then cooled to 20°C at a cooling rate of 100°C / min, and the temperature and dimensional changes during cooling were measured. Figure 1 shows an example of the relationship between temperature and dimensional change when a sample for transformation point measurement is cooled at a predetermined cooling rate. As shown in Figure 1, the temperature at which thermal contraction transitions to thermal expansion was defined as the martensitic transformation onset temperature (Ms).

[0064] [2.2. Hardness as it is molded] A test specimen for hardness measurement was cut from near the center of the as-printed additively manufactured object. The Rockwell hardness (JIS Z2245) was measured using the obtained test specimen.

[0065] [2.3. Presence or absence of distortion and cracks after molding] Figure 2 shows a schematic diagram of the method for measuring warpage. After the additive manufacturing process was completed, the base plate with the additively manufactured object was removed from the metal 3D printer M2 and placed on a surface plate. The radius of curvature R and thickness t of the additively manufactured object were calculated by image analysis from an external photograph taken horizontally so that the entire object was visible. Then, the distortion after manufacturing was calculated using the following equation (3). Distortion after molding (%) = t × 100 / (2R + t) …(3)

[0066] The radius of curvature R can also be calculated by placing the base plate on a surface plate and measuring the distance from the surface plate at regular intervals along the longitudinal direction of the molded object using a laser displacement meter or a stylus-type dimension measuring instrument, and then approximating these displacements as a circle. The fabricated object, whose warpage was measured, was divided into five equal sections parallel to a plane perpendicular to the layering direction. After polishing the cross-sections of the five samples, they were observed under an optical microscope to check for the presence or absence of cracks.

[0067] [3. Results] The results are shown in Table 3. Figure 3 shows the relationship between variable A and the Ms point. Figure 4 shows the relationship between the Ms point and the distortion after fabrication. From Table 3 and Figures 3-4, the following can be seen.

[0068] [Table 3]

[0069] (1) In Comparative Example 1, large cracks occurred in the additively fabricated object, making it impossible to fabricate the object. This is thought to be because the high carbon content resulted in a very high hardness in the as-printed state, making it prone to cracking during solidification. (2) In Comparative Example 2, cracks occurred in the additively fabricated object. This is thought to be because the high carbon content resulted in a high initial hardness, making it more prone to cracking during solidification.

[0070] (3) Comparative Example 3 had an Ms point exceeding 280°C. This is thought to be because the amount of Ni was low and the value of variable A was less than 10. Furthermore, Comparative Example 3 had an absolute value of strain after fabrication exceeding 0.3%. This is thought to be because, due to the high Ms point, in equipment with a heating temperature of 200°C, the martensitic transformation was completed at the same time as the irradiation of the heat source was completed, and the strain suppression effect due to transformation expansion was not efficiently obtained.

[0071] (4) Comparative Example 4 had an Ms point of less than 50°C. This is thought to be because the variable A exceeded 20 due to an excess of Ni. Furthermore, Comparative Example 4 had an absolute value of strain after fabrication that exceeded 0.3%. This is thought to be because the Ms point was near room temperature, so the martensitic transformation did not proceed during fabrication, and the fabricated object deformed significantly downwards due to thermal shrinkage.

[0072] (5) In Comparative Example 5, the structure of the fabricated object was an all-phase ferrite structure. This is thought to be because the amount of Cr was excessive and the equivalent ratio exceeded 5.6. (6) Comparative Example 6 had an Ms point of less than 50°C. This is thought to be because the variable A exceeded 20 due to the excessive amounts of Si and Mn. Furthermore, Comparative Example 6 had an absolute value of strain after fabrication that exceeded 0.3%. This is thought to be because, since the Ms point was near room temperature, the martensitic transformation did not proceed during fabrication, and the fabricated object deformed significantly downwards due to thermal shrinkage.

[0073] (7) Comparative Example 7 had an Ms point exceeding 280°C. This is thought to be because the amount of Cr was low and the value of variable A was less than 10. In addition, the absolute value of the distortion after fabrication in Comparative Example 7 exceeded 0.3%. This is thought to be because, due to the high Ms point, in equipment with a heating temperature of 200°C, the martensitic transformation was completed at the same time as the irradiation of the heat source was completed, and the effect of suppressing distortion due to transformation expansion was not efficiently obtained. Furthermore, in Comparative Example 7, cracks occurred in the fabricated object. This is thought to be due to the Ms point exceeding 280°C, in addition to the high as-printed hardness of 59 HRC. In the equipment with a heating temperature of 200°C, the martensitic transformation is completed and hardening occurs simultaneously with the completion of heat source irradiation, which is thought to make cracking more likely during solidification.

[0074] (8) Examples 1-6, Reference Example 7, Examples 8-9, Reference Example 10, Examples 1-13, Reference Example 14, Examples 15-24 In all cases, the absolute value of the distortion after molding was within 0.3%, and no cracks were observed. This is thought to be because the components were optimized so that variable A falls within a predetermined range, and as a result, the Ms point fell within the appropriate range.

[0075] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0076] The metal powder according to the present invention can be used as a powder raw material for manufacturing molds that require cooling (for example, die-casting molds, hot stamping molds, and tailored die-quenching molds) using additive manufacturing.

Claims

1. 0.001≦C≦0.45mass%, 0.28≦Si≦3.50 mass%, Mn≦2.0mass%, 7.5≦Cr≦21.0mass%, 1.5≦Ni≦7.0mass%, Mo≦1.3mass%, 0.05≦V≦2.0mass%, Al≦0.015 mass%, N≦0.20mass%, 0.05 ≤ C + N ≤ 0.58, P ≤ 0.03 mass%, and, S≦0.03mass% It contains, with the remainder consisting of Fe and unavoidable impurities. As the aforementioned unavoidable impurities, Cu≦0.30mass%, O≦0.1mass%, Co≦0.3mass%, Ta≦0.05mass%, Ti≦0.05mass%, Zr≦0.05mass%, B≦0.005mass%, Ca≦0.005 mass%, Se≦0.03 mass%, Te≦0.005mass%, Bi≦0.01mass%, Pb ≤ 0.03 mass%, Mg ≤ 0.02 mass%, and, REM ≤ 0.01 mass% included, A metal powder that satisfies the following equations (1) and (2). 10<15C+Mn+0.5Cr+Ni<20...(1) 10. The eq DR eq 566 ...(2) however, Cr eq =Cr+Mo+1.5Si+0.5Nb Ni eq =N+30C+30\+0.5Mn

2. The metal powder according to claim 1, further comprising 0.1 ≤ Nb ≤ 1.0 mass%.

3. The metal powder according to claim 1 or 2, further comprising 0.1 ≤ W ≤ 1.5 mass%.

4. The metal powder according to claim 1 or 2, satisfying the following formula (2').

10. The eq DR eq 285 ...(2')

5. The metal powder according to claim 3, satisfying the following formula (2').

10. The eq DR eq 285 ...(2')

6. The metal powder according to claim 1 or 2, used in additive manufacturing.

7. The metal powder according to claim 3, used in additive manufacturing.

8. The metal powder according to claim 4, used in additive manufacturing.

9. The metal powder according to claim 5, used in additive manufacturing.

Images