metallic powder

Abstract

The present invention relates to a metal powder comprising: 0.001 mass% ≤ C ≤ 0.45 mass%, 0.01 mass% ≤ Si ≤ 3.50 mass%, Mn ≤ 2.0 mass%, 7.5 mass% ≤ Cr ≤ 21.0 mass%, 1.5 mass% ≤ Ni ≤ 7.0 mass%, Mo ≤ 1.3 mass%, 0.05 mass% ≤ V ≤ 2.0 mass%, Al ≤ 0.015 mass%, and N ≤ 0.20 mass%, with the balance being Fe and unavoidable impurities, satisfying 0.05 mass% ≤ C+N ≤ 0.58 mass%, and satisfying: 10 < 15C+Mn+0.5Cr+Ni < 20 and Cr eq / Ni eq < 5.6, where Cr eq = Cr+Mo+1.5Si+0.5Nb, and Ni eq = Ni+30C+30N+0.5Mn.

Description

TECHNICAL FIELD

[0001] The present application relates to a metal powder, and more particularly to a metal powder from which a product having less cracks or warping and having excellent heat resistance can be obtained through additive manufacturing. BACKGROUND

[0002] In recent years, metal additive manufacturing technology has been attracting attention. This is because the metal additive manufacturing technology has the following advantages:

[0003] (a) a metal part having a complex shape can be formed so that the shape is close to the final shape;

[0004] (b) the degree of freedom of design is improved; and

[0005] (c) the cutting allowance is smaller than the cutting allowance in the prior art.

[0006] Herein, the "additive manufacturing method" refers to a method of preparing a three-dimensional structure by stacking thin layers using various methods, the thin layers corresponding to a structure obtained by cutting the three-dimensional structure in a horizontal direction. Examples of the method for stacking thin layers include:

[0007] (a) a method in which the steps of forming a thin powder layer made of a metal powder and the step of locally melting and solidifying the powder layer by irradiating with an energy beam such as a laser beam or an electron beam are repeatedly performed, and

[0008] (b) a method in which thin plates each having a given shape are stacked and diffusion-bonded.

[0009] Among them, the additive manufacturing method in which a spread metal powder layer is irradiated with a laser beam to locally melt and solidify the powder layer is also called "selective laser melting (SLM)". The SLM additive manufacturing method has an advantage in that a complex three-dimensional shape can be easily formed simply by changing the irradiation position of the laser beam. Therefore, when the SLM additive manufacturing method is applied to, for example, the manufacture of a die casting mold, a non-linear cooling water circuit or a three-dimensional cooling water circuit can be freely provided inside the mold.

[0010] In the case of additive manufacturing by using an SLM 3D printer, only the upper surface of the manufacturing object is rapidly heated, and thus residual tensile stress is generated on the upper surface of the manufacturing object after cooling. As a result, the manufacturing object is easily deformed to be convex downward. When the deformation of the manufacturing object is large, the dimensional accuracy of the manufacturing object is deteriorated, and it is also difficult to take out the manufacturing object from the 3D printer after manufacturing.

[0011] Therefore, in order to solve this problem in the related art, various proposals have been made.

[0012] For example, Patent Literature 1 discloses a method of manufacturing an article by additive manufacturing, the method including:

[0013] a re-coating step of forming a material layer made of a carbon steel or a martensitic stainless steel powder in a manufacturing region;

[0014] a solidification step of irradiating a predetermined irradiation region of the material layer with a laser beam to form a solidified layer; and

[0015] a temperature adjustment step of adjusting a temperature of the solidified layer so that T1→T2→T1 (where T1≥Mf (a martensitic phase transformation end temperature of the solidified layer), T1>T2, and T2≤Ms (a martensitic phase transformation start temperature of the solidified layer)).

[0016] The same document also discloses the following points:

[0017] (a) In additive manufacturing, a tensile stress is present in the solidified layer because the volume of the solidified layer generally shrinks during the cooling process of the solidified layer;

[0018] (b) In the case of additive manufacturing using a material that has undergone a martensitic phase transformation, the volume shrinkage that occurs during the cooling of the solidified layer and the resulting tensile stress are both reduced because of the volume expansion that occurs when the solidified layer undergoes a martensitic phase transformation, thereby enabling deformation of the manufactured object to be prevented;

[0019] (c) By controlling T1 and T2 in the temperature adjustment step, it is possible to control the amount of deformation (= amount of expansion) of the solidified layer; and

[0020] (d) Since Ms and Mf fluctuate depending on the carbon content of the material, by adjusting the carbon content of the material, the method described in the document can be applied to various materials.

[0021] Patent Literature 2 discloses a metal powder containing predetermined amounts of Cr, Ni, C, Si, Mn, N, Mo, Cu, Nb, P, and S, with the balance being Fe and unavoidable impurities.

[0022] The document discloses that in the case of additive manufacturing using a stainless steel powder, when the amounts of each element contained in the stainless steel powder are adjusted within predetermined ranges and the amounts of P and S are controlled, solidification cracks are less likely to occur, and thus good manufacturing performance is exhibited.

[0023] Furthermore, Patent Literature 3 does not disclose a metal powder for additive manufacturing, but rather discloses a stainless steel containing predetermined amounts of C, Si, Mn, Cr, Mo, V, and N, with the balance being Fe and impurities.

[0024] This document discloses 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.

[0025] Patent Document 1 discloses that when the temperature of the solidified layer is raised and lowered around the Ms point, the residual tensile stress generated during the cooling process after manufacturing can be mitigated by the volume expansion caused by the martensitic transformation, and thus a manufacturing object with little deformation can be obtained. However, in the current 3D printer, due to the equipment limitation, there is an upper limit to the accessible temperature of the manufacturing area. Therefore, the method disclosed in Patent Document 1 can only be applied to steels with Ms point below about 300°C.

[0026] In addition, for applications requiring heat resistance, heat-resistant martensitic steels such as SUH1, SUH3, and SUH11 are used. In the case of using such heat-resistant steels, in order to improve heat resistance, the carbon content is generally high. When a powder having such a high carbon content is applied to additive manufacturing, the hardness of the manufactured object tends to be very high. As a result, cracks can easily occur in the manufacturing object, making it difficult to prepare the manufacturing object.

[0027] To solve this problem, it can be considered to reduce the carbon content in the heat-resistant steel. However, since the reduction of the carbon content would cause an increase in the Ms point, the Ms point can be higher than the accessible temperature range of the manufacturing device. As a result, there is a problem that when additive manufacturing is performed by using a heat-resistant steel powder with a lower carbon content, the residual stress of the manufacturing object increases.

[0028] Patent Document 1: Japanese Patent No. 6295001

[0029] Patent Document 2: JP 2019-119913 A

[0030] Patent Document 3: JP 2020-536169 T SUMMARY

[0031] An object of the present application is to provide a metal powder from which a manufactured object having less cracks or warping and having excellent heat resistance can be obtained by additive manufacturing.

[0032] To solve the above problem, the metal powder according to the present application contains:

[0033] 0.001 mass% ≤ C ≤ 0.45 mass%,

[0034] 0.01 mass% ≤ Si ≤ 3.50 mass%,

[0035] Mn ≤ 2.0 mass%,

[0036] 7.5 mass% ≤ Cr ≤ 21.0 mass%,

[0037] 1.5 mass% ≤ Ni ≤ 7.0 mass%,

[0038] Mo ≤ 1.3 mass%,

[0039] 0.05 mass% ≤ V ≤ 2.0 mass%,

[0040] Al ≤ 0.015 mass% and

[0041] N ≤ 0.20 mass%,

[0042] the balance being Fe and unavoidable impurities,

[0043] satisfies:

[0044] 0.05 mass% ≤ C + N ≤ 0.58 mass%, and

[0045] also satisfies the following expression (1) and expression (2):

[0046] 10 < 15C + Mn + 0.5Cr + Ni < 20 (1)

[0047] Cr eq / Ni eq < 5.6 (2)

[0048] wherein

[0049] Cr eq = Cr + Mo + 1.5Si + 0.5Nb

[0050] Ni eq = Ni + 30C + 30N + 0.5Mn.

[0051] When additive manufacturing is performed using a metal powder containing predetermined elements and satisfying expression (1), residual tensile stress generated in a cooling process after manufacturing is moderated by volume expansion caused by a martensitic transformation. As a result, a product having less cracking and less deformation can be obtained by additive manufacturing.

[0052] Further, by optimizing the components of the metal powder to satisfy expression (2) (in particular, optimizing the amounts of Si, Cr, and Ni), a product having excellent heat resistance can be obtained by additive manufacturing. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 A graph for showing an example of the relationship between temperature change and dimensional change when a sample for a phase transition point determination is cooled at a predetermined cooling rate.

[0054] Figure 2A A schematic view of a sample for measuring a warping amount.

[0055] Figure 2B A case where the radius of curvature R is almost 0 is shown.

[0056] Figure 2C A case where the radius of curvature R is less than 0 is shown.

[0057] Figure 2D A case where the radius of curvature R is greater than 0 is shown.

[0058] Figure 3 A graph showing the relationship between the variable A and the Ms point.

[0059] Figure 4 A graph showing the relationship between the Ms point and the deformation after manufacturing. DETAILED DESCRIPTION

[0060] Hereinafter, embodiments of the present application will be described in detail.

[0061] [1. Metal powder]

[0062] [1.1. Main constituent elements]

[0063] The metal powder according to the present application contains the following elements, with the balance being Fe and unavoidable impurities. The kind of added elements, the composition range thereof, and the reason for the limitation are as follows.

[0064] (1) 0.001 mass% ≤ C ≤ 0.45 mass%:

[0065] C is an element that forms carbide with various elements, and thus can effectively increase hardness and strength. Furthermore, C is also an effective element for lowering the Ms point. In order to obtain such effects, the amount of C needs to be 0.001 mass% or more. The amount of C is preferably 0.01 mass% or more, and more preferably 0.05 mass% or more.

[0066] On the other hand, when additive manufacturing is performed by using the metal powder, the instantaneous hardness of the manufactured object after the additive manufacturing is proportional to the total amount of C and N, and thus, in the case where the amount of C is too much, the instantaneous hardness of the manufactured object after the additive manufacturing is too high, which can cause cracks. Therefore, the amount of C needs to be 0.45 mass% or less. The amount of C is preferably 0.40 mass% or less. The amount of C is more preferably 0.09 mass% or less, and further preferably 0.06 mass% or less.

[0067] (2) 0.01 mass% ≤ Si ≤ 3.50 mass%:

[0068] Si is an effective deoxidizing element. In addition, Si is also an effective element for improving the heat resistance and oxidation resistance of the manufactured object. In order to obtain such effects, the amount of Si needs to be 0.01 mass% or more. The amount of Si is preferably 0.1 mass% or more, and more preferably 0.3 mass% or more.

[0069] On the other hand, in the case where the amount of Si is too much, the toughness of the manufactured object can decrease. Therefore, the amount of Si needs to be 3.50 mass% or less. The amount of Si is preferably 3.2 mass% or less, more preferably 2.00 mass% or less, and even more preferably 1.00 mass% or less.

[0070] (3) Mn ≤ 2.0 mass%:

[0071] Mn is an effective element as a deoxidizing element and a desulfurizing element. In addition, Mn is an effective element for improving the toughness and tensile strength. Furthermore, Mn is also an effective element for lowering the Ms point. Therefore, the metal powder can contain Mn as needed. In order to obtain the above effects, the amount of Mn is preferably 0.01 mass% or more. The amount of Mn is more preferably 0.2 mass% or more.

[0072] On the other hand, Mn is also an austenite stabilizing element. Therefore, in the case where the amount of Mn is too much, the amount of residual austenite is too much, and thus the hardness and corrosion resistance of the manufactured object can decrease. Therefore, the amount of Mn needs to be 2.0 mass% or less. The amount of Mn is preferably 1.8 mass% or less, more preferably 1.6 mass% or less, even more preferably 1.00 mass% or less, and further preferably 0.70 mass% or less.

[0073] (4) 7.5 mass% ≤ Cr ≤ 21.0 mass%:

[0074] Cr is an effective element for improving high-temperature oxidation resistance. In addition, Cr is an effective element for improving the quenching hardness of the manufactured object by forming carbides. Furthermore, Cr is also an effective element for ensuring the corrosion resistance by forming a passivation film on the surface of the manufactured object. In order to obtain such effects, the amount of Cr needs to be 7.5 mass% or more. The amount of Cr is preferably 10.5 mass% or more, and more preferably 11.5 mass% or more.

[0075] On the other hand, in the case where the amount of Cr is too much, ferrite structure can even remain in the quenched structure, thereby decreasing the high-temperature strength. Therefore, the amount of Cr needs to be 21.0 mass% or less. The amount of Cr is preferably 20.0 mass% or less, more preferably 18.5 mass% or less, even more preferably 16.0 mass% or less, and further preferably 15.0 mass% or less.

[0076] (5) 1.5 mass% ≤ Ni ≤ 7.0 mass%:

[0077] Ni is an effective element for improving corrosion resistance and thermal strength. In addition, Ni is also an element effective in lowering the Ms point. In order to obtain such effects, the amount of Ni needs to be 1.5 mass% or more, more preferably 3.00 mass% or more, even more preferably 4.00 mass% or more, and further preferably 5.00 mass% or more.

[0078] On the other hand, in the case where the amount of Ni is excessively large, the Ms point is greatly lowered. As a result, the amount of retained austenite can be excessively large, and thus the temper hardness can be lowered. Therefore, the amount of Ni needs to be 7.0 mass% or less.

[0079] (6) Mo ≤ 1.3 mass%:

[0080] Mo has an effect of promoting regeneration of a passivation film, thereby improving corrosion resistance. Therefore, the metal powder can contain Mo as needed. In order to obtain such effects, the amount of Mo is preferably 0.01 mass% or more. The amount of Mo is more preferably 0.1 mass% or more.

[0081] On the other hand, in the case where the amount of Mo is excessively large, the fracture toughness of the manufactured object can be lowered. Therefore, the amount of Mo needs to be 1.3 mass% or less. The amount of Mo is preferably 1.1 mass% or less, more preferably 0.90 mass% or less, and further preferably 0.50 mass% or less.

[0082] (7) 0.05 mass% ≤ V ≤ 2.0 mass%:

[0083] V is an element that combines with C and / or N to form carbides and / or nitrides, thereby contributing to an increase in hardness. In addition, V is also an element that prevents grains from becoming coarse during quenching and contributes to an increase in toughness. In order to obtain such effects, the amount of V needs to be 0.05 mass% or more. The amount of V is preferably 0.1 mass% or more.

[0084] On the other hand, in the case where the amount of V is excessively large, a large amount of carbides and / or nitrides can remain, and the toughness can be lowered. Therefore, the amount of V needs to be 2.0 mass% or less. The amount of V is preferably 1.3 mass% or less, more preferably 0.8 mass% or less, and further preferably 0.5 mass% or less.

[0085] (8) Al ≤ 0.015 mass%:

[0086] Al is an effective element as a deoxidizing element. In addition, a small amount of Al can prevent grains from becoming coarse during quenching and contribute to an improvement in toughness. Therefore, the metal powder can contain Al as needed. In order to obtain such effects, the amount of Al is preferably greater than 0.002 mass%. The amount of Al is more preferably 0.005 mass% or more.

[0087] On the other hand, in the case where the amount of Al is too much, coarse AlN is formed, which can cause serious deterioration of toughness and fatigue properties. Therefore, the amount of Al needs to be 0.015 mass% or less.

[0088] (9) N ≤ 0.20 mass%:

[0089] N is an element that is mixed into the metal powder when molten metal is powderized by nitrogen atomization. In addition, N has an effect of increasing the immediate hardness of the manufacturing object after additive manufacturing. Therefore, the metal powder can contain N as needed. In order to obtain such an effect, the amount of N is preferably 0.01 mass% or more.

[0090] On the other hand, in the case where the amount of N is too much, the immediate hardness of the manufacturing object after additive manufacturing is too high, which can cause cracks. Therefore, the amount of N needs to be 0.20 mass% or less. The amount of N is preferably 0.1 mass% or less, and more preferably 0.07 mass% or less.

[0091] (10) 0.05 mass% ≤ C + N ≤ 0.58 mass%:

[0092] Both C and N are effective elements for increasing hardness and strength. In addition, C and N are also effective elements for lowering the Ms point. In the case where C + N is too small, the desired hardness cannot be obtained. Therefore, C + N needs to be 0.05 mass% or more.

[0093] On the other hand, in the case where C + N is too large, the immediate hardness of the manufacturing object after additive manufacturing is too high, which can cause cracks. Therefore, C + N needs to be 0.58 mass% or less. C + N is preferably 0.45 mass% or less.

[0094] (11) Unavoidable impurities:

[0095] The metal powder according to the present application can contain the components shown below in the amounts shown below. In this case, these components are regarded as unavoidable impurities in the present application.

[0096] Cu ≤ 0.30 mass%, O ≤ 0.1 mass%, Co ≤ 0.3 mass%, Ta ≤ 0.05 mass%, Ti ≤ 0.05 mass%, Zr ≤ 0.05 mass%, B ≤ 0.005 mass%, Ca ≤ 0.005 mass%, Se ≤ 0.03 mass%, Te ≤ 0.005 mass%, Bi ≤ 0.01 mass%, Pb ≤ 0.03 mass%, Mg ≤ 0.02 mass%, and REM (rare earth metal) ≤ 0.01 mass%.

[0097] [1.2. Auxiliary constituent elements]

[0098] The metal powder according to the present application can contain one or more of the following elements in addition to the above-mentioned main constituent elements. The kind of added element, its composition range, and the reason for the limitation are as follows.

[0099] (1) 0.1 mass% ≤ Nb ≤ 1.0 mass%:

[0100] Similar to V, Nb is also an element that binds with C and / or N to form carbides and / or nitrides to contribute to the increase in hardness. In addition, Nb is also an element that prevents the grains from becoming coarse during quenching and contributes to the increase in toughness. In order to obtain this effect, the amount of Nb is preferably 0.1 mass% or more. Nb can be contained as an unavoidable impurity in an amount less than 0.1 mass%.

[0101] On the other hand, in the case where the amount of Nb is excessive, coarse carbides and / or nitrides can be precipitated to cause quenching cracks. Therefore, the amount of Nb is preferably 1.0 mass% or less.

[0102] (2) 0.1 mass% ≤ W ≤ 1.5 mass%:

[0103] W has the effect of improving corrosion resistance. In order to obtain this effect, the amount of W is preferably 0.1 mass% or more. W can be contained as an unavoidable impurity in an amount less than 0.1 mass%.

[0104] On the other hand, in the case where the amount of W is excessive, the cost increases, and coarse M6C-type carbides can be formed, which can cause quenching cracks. Therefore, the amount of W is preferably 1.5 mass% or less.

[0105] (3) P ≤ 0.03 mass%:

[0106] P is an unavoidable impurity, and from the viewpoint of preventing solidification cracks, it is preferable to reduce the amount of P as much as possible. In order to prevent solidification cracks, the amount of P as an unavoidable impurity is preferably 0.03 mass% or less.

[0107] However, excessive reduction in the amount of P can lead to an increase in production cost. Therefore, the optimal amount of P is preferably selected taking these aspects into consideration.

[0108] (4) S ≤ 0.03 mass%:

[0109] S is an unavoidable impurity, and from the viewpoint of preventing solidification cracks, it is preferable to reduce the amount of S as much as possible. In order to prevent solidification cracks, the amount of S as an unavoidable impurity is preferably 0.03 mass% or less.

[0110] However, excessive reduction in the amount of S can lead to an increase in production cost. Therefore, the optimal amount of S is preferably selected taking these aspects into consideration.

[0111] [1.3. Component balance]

[0112] The metal powder according to the present application needs to satisfy the following expression (1) and expression (2).

[0113] 10 < 15C + Mn + 0.5Cr + Ni < 20 (1)

[0114] Cr eq / Ni eq < 5.6 (2)

[0115] wherein

[0116] Cr eq = Cr + Mo + 1.5Si + 0.5Nb

[0117] Ni eq = Ni + 30C + 30N + 0.5Mn

[0118] [1.3.1. Expression (1)]

[0119] "15C + Mn + 0.5Cr + Ni" in expression (1) (hereinafter also referred to as "variable A") is related to the Ms point of the metal powder. All of the elements in variable A have an effect of lowering the Ms point. In the metal powder according to the present application, when variable A is optimized to satisfy expression (1), the Ms point of the metal powder can be set in a range suitable for additive manufacturing (specifically, about 50°C to about 280°C).

[0120] Variable A is obtained by multiplying the content (mass%) of each element by a predetermined coefficient and adding them. In the case where the metal powder does not contain some of the elements constituting variable A, the content of the element is assumed to be zero to calculate variable A.

[0121] In the case where the Ms point of the metal powder is too low, the amount of retained austenite after additive manufacturing is too much, and thus sufficient hardness cannot be obtained. In addition, even if the material is cooled to room temperature after additive manufacturing, the amount of martensitic transformation is low, and thus it can not be possible to obtain the effect of reducing deformation due to phase transformation expansion. Therefore, the Ms point is preferably 50°C or higher. Variable A is preferably less than 20 in order to achieve an Ms point equal to or greater than the value.

[0122] On the other hand, in order to obtain the effect of reducing the deformation due to the phase transformation expansion, it is necessary to heat the manufacturing object to a temperature lower than the Ms point and higher than the temperature at which the martensitic transformation is completed (the Mf point) after the additive manufacturing. Due to the equipment limitation, the existing additive manufacturing equipment can only be heated to 200°C. In the case where the heating temperature for the manufacturing object is 200°C and the Ms point of the metal powder is higher than 280°C, the heating temperature is too low, and thus the martensitic transformation is almost completed immediately after the additive manufacturing, and the effect of reducing the deformation due to the phase transformation expansion cannot be obtained.

[0123] Even if the additive manufacturing object can be heated to 200°C or more, in the case where the Ms point is 280°C or more, the temperature required for heating is equal to or higher than the temperature at which the bainitic transformation occurs. Thus, the expansion due to the bainitic transformation occurs during the manufacturing process, and the effect of reducing the deformation due to the phase transformation expansion cannot be obtained. Therefore, the Ms point is preferably 280°C or less. The variable A is preferably greater than 10 so as to achieve the Ms point equal to or less than the value.

[0124] [1.3.2. Expression (2)]

[0125] Expression (2) represents the ratio (hereinafter also referred to as "equivalent ratio") of the Cr equivalent (Cr eq ) to the Ni equivalent (Ni eq ). "Cr eq " is an index indicating the easiness of forming ferrite in the stainless steel. In addition, "Ni eq " is an index indicating the easiness of forming austenite in the stainless steel.

[0126] Cr eq is obtained by multiplying the content (mass%) of each ferrite stabilizing element by a predetermined coefficient and adding them. In addition, Ni eq is obtained by multiplying the content (mass%) of each austenite stabilizing element by a predetermined coefficient and adding them. In the case where an element constituting Cr eq or Ni eq is not contained in the metal powder, the content of the element is assumed to be zero and Cr eq or Ni eq is calculated.

[0127] In the case where the equivalent ratio is too large, ferrite structure can be formed in all phases, and the high-temperature strength can be reduced. Thus, the equivalent ratio needs to be less than 5.6. The equivalent ratio preferably satisfies the following expression (2').

[0128] Cr eq / Ni eq < 2.5 (2')

[0129] On the other hand, in the case where the equivalence ratio is too small, the amount of residual austenite is too much, and the hardness and corrosion resistance of the manufacturing object can be reduced. Therefore, the equivalence ratio is preferably 0.3 or more. The equivalence ratio is more preferably 0.4 or more, and still more preferably 0.6 or more.

[0130] [1.4. Powder characteristics]

[0131] In the SLM 3D printer, the metal powder is uniformly spread before being manufactured with a laser. The flowability of the metal powder is important for uniformly spreading the metal powder. In order to ensure the flowability, the powder characteristics (in particular, the number frequency D 50 and the avalanche angle) of the metal powder can be optimized.

[0132] [1.4.1. Number frequency D 50 ]

[0133] The term "number frequency D 50 (μm)" means the 50% number cumulative particle size (median diameter) of the powder. Examples of the method of measuring D 50 include:

[0134] (a) a measurement method using a particle distribution measuring device based on a laser diffraction / scattering method;

[0135] (b) a measurement method using a particle image analyzer;

[0136] (c) a measurement method using a Coulter counter; and the like.

[0137] The term "D 50 " in the present invention means the median diameter measured by a particle image analyzer.

[0138] Generally, as D 50 decreases, the content of fine powder (particles having a particle diameter of 10 μm or less) relatively increases. As the particle diameter decreases, the adhesion force such as van der Waals force and electrostatic force generated between the particles increases. Therefore, in the case where D 50 is too small, the powder tends to be agglomerated, resulting in poor flowability. Therefore, D 50 is preferably 10 μm or more. D 50 is more preferably 20 μm or more, and still more preferably 30 μm or more.

[0139] On the other hand, in the case where D 50 is too large, the frictional force generated on the surface of the powder is more dominant than the adhesion force generated between the particles. Therefore, the shear resistance during the flow of the powder increases, and thus the flowability is inhibited. Therefore, D 50 is preferably 50 μm or less.

[0140] [1.4.2. Avalanche angle]

[0141] Examples of the method for evaluating the flowability of the metal powder include:

[0142] (a) a metal powder-flowability test method prescribed in JIS Z2502:2012;

[0143] (b) a standard test method for flow rate of metal powder using a Hall flow funnel according to ASTM B213;

[0144] (c) a standard test method for flow rate of metal powder using a Carney funnel according to ASTM B964; and the like.

[0145] On the other hand, the avalanche angle determined with a rotary powder analyzer manufactured by Mercury Scientific can be used as an index for evaluating the flowability in the powder spreading step in a metal 3D printer. In the present application, the avalanche angle is used as an index for the flowability of the metal powder.

[0146] Generally, a small avalanche angle indicates a small adhesion force between particles and a good flowability of the metal powder. The avalanche angle usually takes a value of 30° to 60° depending on the D 50 of the metal powder. The avalanche angle is preferably 45° or less so as to spread the metal powder more uniformly in the manufacturing area of the 3D printer. The avalanche angle is more preferably 43° or less, further preferably 40° or less, and also preferably 35° or less.

[0147] [1.4.3. Apparent density, tap density, and Hausner Ratio]

[0148] Examples of the method for determining the apparent density include:

[0149] (a) a metal powder-apparent density test method prescribed in JIS Z2504:2012;

[0150] (b) a method for apparent density of free-flowing metal powders according to a standard test method of ASTM B212 using a Hall flow funnel; and the like.

[0151] In the present application, the term “apparent density p 堆积 (ρ bulk ” refers to a value obtained by the metal powder-apparent density test method prescribed in JIS Z2504:2012. The apparent density of the metal powder can usually take a value of about 3.0 g / cc to about 6.0 g / cc.

[0152] Examples of the method for determining the tap density include:

[0153] (a) The method for determining the tap density of metal powders as specified in JIS Z2512:2012;

[0154] (b) Test methods for tap density of metal powders and compounds according to ASTM B527 standard test methods; etc.

[0155] In this invention, the term "tap density ρ" is used. 振实 (ρ tapped ")" refers to the value obtained by the metal powder tap density determination method specified in JIS Z2512:2012. The tap density of metal powder is usually taken as a value of about 3.0 g / cc to about 6.0 g / cc.

[0156] The term "housenabi" refers to the tap density (ρ) of metal powder. 振实 ) and apparent density (ρ 堆积 The ratio of ρ 振实 / ρ 堆积 A low Hausner ratio typically indicates weak interparticle interactions and high flowability. Conversely, a high Hausner ratio typically indicates strong interparticle interactions and poor flowability. The Hausner ratio of metal powders can usually be below approximately 1.25.

[0157] [1.4.4. Particle Shape]

[0158] Metal powder particles can be spherical or irregular in shape. Generally, metal powder composed of spherical particles exhibits higher flowability than metal powder composed of irregularly shaped particles.

[0159] [1.5. Applications]

[0160] The metal powder according to the present invention can be used for a variety of purposes, and is particularly suitable as a metal powder for additive manufacturing.

[0161] The hardness of articles manufactured by additive manufacturing using the metal powder according to the invention depends primarily on the composition of the metal powder. If the hardness during manufacturing is too high, cracks may occur. To prevent cracking during manufacturing, the hardness during manufacturing is preferably 58 HRC or less. More preferably, it is 55 HRC or less. By optimizing the composition of the metal powder, articles with such hardness during manufacturing can be obtained through additive manufacturing.

[0162] [2. Methods for manufacturing metal powders]

[0163] In this invention, there are no particular limitations on the method for manufacturing metal powder. Examples of methods for manufacturing metal powder include gas atomization, water atomization, plasma atomization, plasma rotating electrode method, centrifugal atomization, etc.

[0164] For example, in the case of manufacturing a metal powder by using a gas atomization method, a high-pressure gas is blown onto molten metal while the molten metal falls from the bottom of a tundish, thereby pulverizing and solidifying the molten metal. In this case, an inert gas such as nitrogen, argon, or helium is used as the high-pressure gas. In the case of manufacturing a metal powder by using a gas atomization method, impurities such as P, S, Cu, Co, Ti, Zr, and Nb are inevitably mixed.

[0165] Further, a metal powder can be manufactured by mixing two or more metal powders and using, for example, a mechanical alloying method.

[0166] Further, after manufacturing a metal powder using any of the above-described methods, the metal powder can be subjected to a spheroidization treatment using a reducing thermal plasma. Alternatively, in order to improve the flowability of the metal powder, after manufacturing the powder, the particle surfaces can be coated with an appropriate amount of nanoparticles. The particle size distribution of the metal powder can be controlled by the manufacturing conditions, or can be controlled by a classification method such as a wet cyclone separator, a dry cyclone separator, a dry sieve, and an ultrasonic sieve.

[0167] [3. Method of producing an article by additive manufacturing]

[0168] Examples of the additive manufacturing method using a metal 3D printer include a powder bed fusion method, a directed energy deposition method, a binder jetting method, and the like. Further, the object of additive manufacturing can be cut during the manufacturing. The metal powder according to the present application can be applied to any of the above-described methods.

[0169] For example, in the case of a selective laser melting method (SLM method), which is one of the powder bed fusion methods, additive manufacturing can be performed by the following steps:

[0170] (a) creating slice data in units of several tens of pm based on three-dimensional shape data (for example, STL data) generated by 3D-CAD or the like, and

[0171] (b) based on the slice data, selectively scanning and irradiating a powder bed with a heat source to obtain a sintered layer, and stacking the sintered layers.

[0172] [4. Effects]

[0173] Mn, Cr, and Ni in steel each have an effect of lowering the Ms point. Therefore, when the amount of C is relatively reduced and the contents of these elements are optimized to satisfy Expression (1), the Ms point can be maintained in a range (about 50°C to about 280°C) suitable for slight deformation of the object of additive manufacturing, without excessively increasing the hardness at the time of manufacture.

[0174] Further, when the content of the elements is optimized to satisfy Expression (2), the formation of ferrite phase can be hindered, which decreases the heat resistance. Among the elements in Expression (2), Si, Cr, and Ni particularly have an effect of increasing the heat resistance of the additive manufacturing object. Therefore, by optimizing the content of the elements in Expression (2) while satisfying Expression (2), it is possible to maintain the heat resistance of the additive manufacturing object.

[0175] EMBODIMENT

[0176] (Examples 1 to 24 and Comparative Examples 1 to 7)

[0177] [1. Preparation of Sample]

[0178] [1.1. Preparation of Metal Powder]

[0179] By using a gas atomization method, 31 kinds of steel powder having the composition shown in Tables 1 and 2 (balance: Fe) were prepared. The steel powder listed in Tables 1 and 2 can contain, in addition to Fe as the balance, elements not listed in the table within the above-specified amount range as impurities. Further, the blank spaces in Tables 1 and 2 indicate that Mn, Ni, Mo, V, S, Nb, and W are less than 0.01 mass%, and Al is less than 0.002 mass%. Further, Comparative Example 1 corresponds to a heat-resistant martensitic steel (SUH11), and Comparative Example 2 corresponds to a heat-resistant martensitic steel (SUH1).

[0180]

[0181]

[0182] [1.2. Preparation of Additive Manufacturing Object]

[0183] An article (a cube of 15 mm x 15 mm x 15 mm) for measuring the Ms point and the hardness at the time of manufacture was prepared by additive manufacturing using a metal 3D printer M2 manufactured by Concept Laser.

[0184] Further, by additive manufacturing using a metal 3D printer M2 manufactured by Concept Laser, an article (a cuboid of 18 mm long, 30 mm wide, and 10 mm high) was formed on a strip-shaped substrate (20 mm long, 150 mm wide, and 15 mm high) to prepare a test piece for measuring the amount of warping after manufacture.

[0185] Additive manufacturing was performed by preheating the additive manufacturing object to a temperature of Ms-30°C to Ms-80°C using a heater. The atmosphere during the manufacturing process was a nitrogen atmosphere.

[0186] [2. Test Method]

[0187] [2.1. Ms Point]

[0188] Cutting a test piece for phase transition point measurement from an additive manufacturing object in a finished state Heating the test piece to 1,000°C to 1,300°C, then cooling to 20°C at a cooling rate of 100°C / min, and measuring the temperature change and the dimensional change during the cooling.

[0189] Figure 1 An example showing the relationship between the temperature change and the dimensional change when a sample for phase transition point measurement is cooled at a predetermined cooling rate. As shown in Figure 1 The temperature at which the thermal shrinkage changes to thermal expansion is shown as the martensitic phase transition start temperature (Ms).

[0190] [2.2. Hardness at the time of finishing]

[0191] A test piece for hardness measurement is cut from the vicinity of the center portion of an additive manufacturing object in a finished state. The Rockwell hardness (JIS Z2245:2016) of the resulting test piece is measured.

[0192] [2.3. Deformation after manufacturing and presence or absence of cracks]

[0193] Figure 2A A schematic view of a test piece for measuring the amount of warping. After the completion of additive manufacturing, the substrate having the additive manufacturing object is taken out from the metal 3D printer M2, and the substrate having the additive manufacturing object is placed on a flat plate. An appearance photograph of the entire manufacturing object is taken from the horizontal direction. The curvature radius R and the thickness t of the additive manufacturing object are calculated by image analysis of the appearance photograph. Then, the deformation after manufacturing is calculated by the following equation (3).

[0194] Deformation after manufacturing (%) = t x 100 / (2R + t) (3)

[0195] Figure 2B A case where the curvature radius R is almost 0 is shown, Figure 2C a case where the curvature radius R is less than 0 is shown, and Figure 2D a case where the curvature radius R is greater than 0 is shown.

[0196] The curvature radius R can also be calculated by placing the substrate on a flat plate, measuring the distance from the flat plate at certain intervals along the longitudinal direction of the manufacturing object with a laser displacement meter or a stylus type dimension measuring instrument, and approximating the displacement as a circle.

[0197] The manufacturing object for which the amount of warping has been measured is divided into five equal portions parallel to the plane perpendicular to the stacking direction. The cross sections of the five samples are polished, and then observed with an optical microscope to confirm the presence or absence of (A) cracks (B).

[0198] [3. Results]

[0199] The results are shown in Table 3. Figure 3 The relationship between the variable A and the Ms point is shown. Figure 4 The relationship between the Ms point after manufacturing and the deformation is shown.

[0200] [Table 3]

[0201] Ms point (°C) Hardness at manufacture Cracking Deformation after manufacture (%) Example 1 233 44 A 0.06 Example 2 205 36 A 0.01 Example 3 217 47 A 0.09 Example 4 214 45 A 0.14 Example 5 209 50 A 0.15 Example 6 184 38 A 0.16 Example 7 224 52 A 0.09 Example 8 244 44 A -0.16 Example 9 239 46 A -0.10 Example 10 141 59 A 0.12 Example 11 207 50 A 0.10 Example 12 228 48 A 0.03 Example 13 153 47 A 0.25 Example 14 75 49 A -0.08 Example 15 192 58 A 0.14 Example 16 170 58 A 0.13 Example 17 147 55 A 0.06 Example 18 127 54 A 0.06 Example 19 222 36 A -0.06 Example 20 214 42 A 0.10 Example 21 221 45 A 0.09 Example 22 221 48 A 0.05 Example 23 186 52 A 0.20 Example 24 220 45 A 0.13 Comparative Example 1 179 Unable to manufacture B - Comparative Example 2 194 60 B 0.18 Comparative Example 3 308 36 A -0.52 Comparative Example 4 42 36 A -0.34 Comparative Example 5 135 35 A 0.05 Comparative Example 6 5 37 A -0.51 Comparative Example 7 284 59 B -0.35

[0202] Table 3, Figure 3 and Figure 4 shows the following.

[0203] (1) In Comparative Example 1, a large crack occurred in the additive manufactured object, and thus the additive manufactured object could not be manufactured. It is considered that this is because the amount of C was large, and thus the hardness at the time of manufacturing was very high, and thus a crack easily occurred during solidification.

[0204] (2) In Comparative Example 2, a crack occurred in the additive manufactured object. It is considered that this is because the amount of C was large, and thus the hardness at the time of manufacturing was high, and thus a crack easily occurred during solidification.

[0205] (3) In Comparative Example 3, the Ms point was higher than 280°C. It is considered that this is because the amount of Ni was low, and the value of the variable A was less than 10. In addition, in Comparative Example 3, the absolute value of the deformation after manufacturing was greater than 0.3%. It is considered that this is because the Ms point was high, and thus the martensitic transformation ended at the same time as the end of the irradiation of the heat source in the device in which the heating temperature was 200°C, and thus the effect of reducing the deformation due to the transformation expansion could not be effectively obtained.

[0206] (4) In Comparative Example 4, the Ms point was lower than 50°C. It is considered that this is because the amount of Ni was too much, and thus the variable A was greater than 20. In addition, in Comparative Example 4, the absolute value of the deformation after manufacturing was greater than 0.3%. It is considered that this is because the Ms point was close to room temperature, and thus the martensitic transformation did not proceed during the manufacturing, and the manufactured object was deformed to be convex downward due to thermal shrinkage.

[0207] (5) In Comparative Example 5, the structure of the manufactured object was a full-phase ferrite structure. It is considered that this is because the amount of Cr was too much, and the equivalent ratio (Cr eq / Ni eq ) was greater than 5.6.

[0208] (6) In Comparative Example 6, the Ms point was lower than 50°C. It is considered that this is because the amounts of Si and Mn were too much, and thus the variable A was greater than 20. In addition, in Comparative Example 6, the absolute value of the deformation after manufacturing was greater than 0.3%. It is considered that this is because the Ms point was close to room temperature, and thus the martensitic transformation did not proceed during the manufacturing, and the manufactured object was deformed to be convex downward due to thermal shrinkage.

[0209] (7) In Comparative Example 7, the Ms point was higher than 280°C. It is considered that this is because the amount of Cr was low, and therefore the value of the variable A was less than 10. In addition, in Comparative Example 7, the absolute value of the deformation after the manufacture was greater than 0.3%. It is considered that this is because the Ms point was high, and therefore in the device in which the heating temperature was 200°C, the martensitic transformation ended at the same time as the irradiation of the heat source ended, and therefore the effect of reducing the deformation due to the transformation expansion could not be effectively obtained.

[0210] In addition, in Comparative Example 7, a crack occurred in the manufactured object. It is considered that this is because the Ms point was higher than 280°C, and therefore the hardness at the time of the manufacture was as high as 59 HRC. It is considered that this is because in the device in which the heating temperature was 200°C, the martensitic transformation ended at the same time as the irradiation of the heat source ended, and therefore a crack was likely to occur during solidification.

[0211] (8) In all of Examples 1 to 24, the absolute value of the deformation after the manufacture was within 0.3%, and no cracks were observed. It is considered that this is because the composition was optimized, and therefore the variable A was within the prescribed range, and the Ms point was within the appropriate range.

[0212] Although the embodiments of the present application have been described in detail above, the present application is not limited to the above-described embodiments, and various modifications can be made within the scope of the gist of the present application.

[0213] This application is based on Japanese Patent Application No. 2021-183725 filed on November 10, 2021, and Japanese Patent Application No. 2022-087488 filed on May 30, 2022, the contents of which are incorporated herein by reference.

[0214] Industrial Applicability

[0215] The metal powder according to the present application can be used as a powder raw material for manufacturing a mold (for example, a die casting mold, a hot stamping mold, a special press quenching mold) that needs to be cooled by an additive manufacturing method.

Claims

1. A metal powder comprising: 0.22 mass% ≤ C ≤ 0.45 mass%, 0.3 mass% ≤ Si ≤ 3.50 mass%, Mn ≤ 2.0 mass%, 7.5 mass% ≤ Cr ≤ 21.0 mass%, 1.5 mass% ≤ Ni ≤ 7.0 mass%, Mo ≤ 1.3 mass%, 0.05 mass% ≤ V ≤ 2.0 mass%, Al ≤ 0.015 mass% and N ≤ 0.20 mass%, with the balance being Fe and unavoidable impurities, satisfying: 0.05 mass% ≤ C + N ≤ 0.58 mass%, and satisfying the following expression (1) and expression (2): 10 < 15C + Mn + 0.5Cr + Ni < 20 (1), and Cr eq / Ni eq <5.6 (2) wherein Cr eq = Cr + Mo + 1.5 Si + 0.5 Nb, and Ni eq = Ni + 30C + 30N + 0.5Mn.

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

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

4. The metal powder according to claim 1, further comprising at least one of the following components: P ≤ 0.03 mass%, and S ≤ 0.03 mass%.

5. The metal powder according to claim 2, further comprising at least one of the following components: P ≤ 0.03 mass%, and S ≤ 0.03 mass%.

6. The metal powder according to claim 3, further comprising at least one of the following components: P ≤ 0.03 mass%, and S ≤ 0.03 mass%.

7. The metal powder according to any one of claims 1, 2 and 4, further satisfying the following expression (2'): Cr eq / Ni eq <2.5(2').

8. The metal powder according to claim 3, further satisfying the following expression (2'): Cr eq / Ni eq <2.5(2') 9. The metal powder according to claim 5, further satisfying the following expression (2'): Cr eq / Ni eq <2.5 (2').

10. The metal powder according to claim 6, further satisfying the following expression (2'): Cr eq / Ni eq <2.5(2').

11. The metal powder according to any one of claims 1, 2, 4, 5 and 9, for use in additive manufacturing.

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