Hot tool steel powder for additive manufacturing and hot tool steel additively manufactured products

Abstract

The present invention provides a hot work tool steel powder for additive manufacturing, with which it is possible to form an additive-manufactured hot work tool steel article that is particularly excellent in terms of cracking resistance during manufacturing.　Provided are: a hot work tool steel powder for additive manufacturing, which contains, in mass%, 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni < 9.0%, 3.5% < Cr < 4.5%, one or both of Mo and W satisfying a relational expression 2.5% ≤ (Mo + 1 / 2 W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, and 0.2% ≤ Al ≤ 0.9%, with the balance being made up of Fe and inevitable impurities, and which satisfies formula (1) C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 ≤ 0.95; and an additive-manufactired hot work tool steel article.

Description

Technical Field

[0001] The present invention relates to hot-work tool steel powder for additive manufacturing and hot-work tool steel additive manufactured products.

Background Art

[0002] Hot-work tool steels such as hot forging dies and die-casting dies come into contact with high-temperature workpieces, so they are required to have properties such as high-temperature strength, toughness, and wear resistance. Conventionally, in order to satisfy these requirements, JIS steel grades such as SKD61 and improved steels of SKD61 have been applied to hot-work tool steels.

[0003] Recently, the additive manufacturing method has attracted attention as a means of easily forming metal products (parts) having complex shapes in a near-net shape. The additive manufacturing method is generally also called 3D printing and is an additive manufacturing technology. And as types of the additive manufacturing method, for example, there are a powder spray method in which a heat source is irradiated onto metal powder while melting it and the powder is laminated, and a powder bed method in which a heat source is irradiated onto the metal powder spread on a stage to melt it, and this operation is repeated to laminate it. According to the additive manufacturing method, a metal product having a complex shape can be produced by greatly omitting the conventional machining process, so a metal material with poor machinability can be used. And since a metal material with poor machinability is also a high-strength metal material, a metal product having a complex shape and a long service life can be produced.

[0004] Furthermore, additively manufactured products have been proposed using hot work tool steel as the metallic material and produced by the additive manufacturing method described above. For example, Patent Document 1 describes a material with a composition in mass%, containing C: 0.3~0.5%, Si: 2.0% or less, Mn: 1.5% or less, P: 0.05% or less, S: 0.05% or less, Cr: 3.0~6.0%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 0.5~3.5%, V: 0.1~1.5%, Ni: 0~1.0%, Co: 0~1.0%, Nb: 0~0.3%, with the remainder being Fe and impurities, and having a cross-sectional area of ​​1 μm² in the direction of layering. 2 A hot-working additive manufacturing tool has been proposed, characterized in that the area ratio of the above defects is 0.6% or less.

[0005] Patent Document 2 discloses a steel powder characterized by having a composition in mass%, for the purpose of achieving both high thermal conductivity and high corrosion resistance, with the following characteristics: 0.10≦C<0.25, 0.005≦Si≦0.600, 2.00≦Cr≦6.00, -0.0125×[Cr]+0.125≦Mn≦-0.100×[Cr]+1.800 ··Formula (a) (where [Cr] in formula (a) represents the mass% content of Cr), 0.01≦Mo≦1.80, -0.00447×[Mo]+0.010≦V≦-0.1117×[Mo]+0.901 ··Formula (b) (where [Mo] in formula (b) represents the mass% content of Mo), 0.0002≦N≦0.3000, with the remainder being Fe and unavoidable impurities. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2019 / 220917 [Patent Document 2] Japanese Patent Publication No. 2016-145407 [Overview of the project] [Problems that the invention aims to solve]

[0007] As described above, several steels for additive manufacturing have been proposed. However, cracks may occur during additive manufacturing depending on the type of additive manufacturing machine, additive manufacturing conditions, additive manufacturing dimensions, and the like. For example, in an additive manufacturing mold for forming a large additive manufacturing mold or a complex cavity, an additive manufacturing mold having a stress concentration portion (concave portion) where stress is particularly concentrated is highly likely to crack, and thus further improvement in crack resistance is required. Therefore, an object of the present invention is to provide a hot work tool steel powder for additive manufacturing that can obtain a hot work tool steel additive manufactured product capable of particularly improving crack resistance during additive manufacturing.

Means for Solving the Problems

[0008] The present invention has been made in view of the above-described problems. That is, one aspect of the present invention is a hot work tool steel powder for additive manufacturing, in mass %, 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni ≤ 9.0%, 3.5% < Cr < 4.5%, one or two of Mo and W according to the relational expression of (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≤ Al ≤ 0.9%, the balance being composed of Fe and inevitable impurities, and satisfying the formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 ≤ 0.95 (each element symbol in formula (1) indicates the content (mass %) of the element).)

[0009] In addition, another aspect of the present invention is a hot work tool steel additive manufacturing product, which in mass %, satisfies 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni ≤ 9.0%, 3.5% < Cr < 4.5%, one or both of Mo and W according to the relational expression of (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≤ Al ≤ 0.9%, and the balance consists of Fe and inevitable impurities, and satisfies the formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 ≤ 0.95.

Advantages of the Invention

[0010] According to the present invention, it is possible to obtain a hot work tool steel powder for additive manufacturing that can form a hot work tool steel additive manufacturing product with particularly excellent crack resistance during additive manufacturing.

Brief Description of the Drawings

[0011] [Figure 1] It is a schematic diagram of a crack evaluation test piece for evaluating the cracking property during additive manufacturing. [Figure 2] It is a graph of the tempering temperature and hardness of the hot work tool steel additive manufacturing products of the example of the present invention and the comparative example. [Figure 3] It is a graph showing the mechanical properties (a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) of the example of the present invention at room temperature. [Figure 4] It is a graph showing the mechanical properties (a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) of the example of the present invention at high temperature. [Figure 5] [[ID=​​​​​​​​​​The present invention has a component composition consisting of 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni ≤ 9.0%, 3.5% < Cr < 4.5%, one or two of Mo and W according to the relational expression of (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≤ Al ≤ 0.9%, and the balance being Fe and inevitable impurities. First, the reasons for limiting the composition of the hot working tool steel powder for additive manufacturing (hereinafter also referred to as the powder for additive manufacturing or metal powder) defined in the present invention will be described. Unless otherwise specified, “%” represents “mass %”. Also, additive manufacturing may be simply referred to as “manufacturing”. C: 0.10% ≤ C ≤ 0.40% C is a basic element of hot working tools, where part of it dissolves in the matrix to impart strength, and part forms carbides to enhance wear resistance and seizure resistance. Also, when C dissolved as an interstitial atom is added together with substitutional atoms with a high affinity for C such as Cr, it is expected to contribute to the I (interstitial atom) - S (substitutional atom) effect (acting as the dragging resistance of solute atoms and enhancing the high strength of hot working tools), and it is also an element that can enhance hardenability. If C is too low, mainly a ferrite phase is formed during solidification, and the main phase becomes a ferrite phase up to room temperature, so hardening that requires rapid cooling from the austenite phase becomes impossible. If the main phase is a ferrite phase from immediately after solidification to room temperature, relaxation of thermal shrinkage using martensitic transformation expansion becomes impossible, making it more prone to cracking. However, as C increases, the hardness improves but the toughness decreases, promoting cracking during manufacturing. In the present invention, aiming to improve crack resistance while maintaining a hardness that can withstand the use of molds, 0.1% ≤ C < 0.40%. The lower limit of preferable C is 0.15% or more, more preferably 0.18% or more, still more preferably 0.20% or more, and 0.21% or more. Also, the upper limit of preferable C is 0.35% or less, more preferably 0.30% or less, still more preferably 0.27% or less, 0.25% or less, and 0.24% or less.

[0013] Si: 0.01% ≤ Si ≤ 0.19% Si can be used as a deoxidizer when adjusting the component composition of molten steel. It is difficult to manufacture without addition, and the closer it is to no addition, the more the manufacturing cost increases. Therefore, the lower limit of Si is set at 0.01% or more. The preferred lower limit is 0.05% or more, and more preferably 0.08% or more. On the other hand, if Si is too much, it will cause the formation of ferrite in the tool structure after annealing, so the upper limit is set at 0.19% or less. The preferred upper limit is 0.17% or less, more preferably 0.15% or less, and 0.13% or less.

[0014] Mn: 0.1% ≤ Mn ≤ 1.0% Mn enhances hardenability, suppresses the formation of ferrite in the tool structure, and has the effect of obtaining appropriate hardening and tempering hardness. To obtain these effects, the lower limit of Mn is set at 0.1% or more. The preferred lower limit is 0.25%, and more preferably 0.40% or more. On the other hand, if Mn is too much, it will increase the viscosity of the matrix and reduce the machinability of the material. Therefore, the upper limit is set at 1.0% or less. The preferred upper limit is 0.7% or less, more preferably 0.6% or less, and even more preferably 0.55% or less.

[0015] Ni: 2.0% ≤ Ni ≤ 9.0% Ni is an element that suppresses the formation of ferrite in the tool structure. Also, together with C, Cr, Mn, Mo, W, etc., it imparts excellent hardenability to the tool material, and is an effective element for forming a structure mainly composed of martensite even when the cooling rate during hardening is slow, preventing a decrease in toughness. Furthermore, it can lower the Ms point. In the case of a laminated manufacturing apparatus without a temperature adjustment mechanism, depending on the manufacturing conditions, the temperature during manufacturing can be near room temperature. However, if the Ms point is lowered and brought closer to room temperature, the effect of relaxing thermal shrinkage due to martensitic transformation expansion can be utilized to reduce deformation due to thermal shrinkage. Therefore, in the present invention, the lower limit of Ni is set to 2.0%. The preferred lower limit is 4.0%. More preferably, it is 5.0% or more, still more preferably 6.0% or more, 7.0% or more, and 7.5% or more. However, excessive Ni is an element that increases the viscosity of the matrix and reduces machinability, and also causes an increase in raw material costs. Therefore, even when Ni is contained, the upper limit is set to 9.0%. The preferred upper limit is 8.5%. When it is desired to greatly improve crack resistance, the particularly preferred range of Ni is 7.5 - 8.5%. Also, when it is desired to ensure crack resistance while greatly improving thermal conductivity, the particularly preferred range of Ni is 3.5 - 4.5%. Furthermore, when it is desired to ensure crack resistance while improving thermal conductivity and yield strength, the particularly preferred range of Ni is 5.5 - 6.5%.

[0016] Cr: 3.5% < Cr < 4.5% Cr is a basic element of hot working tools that enhances hardenability and forms carbides, which is effective for strengthening the matrix, improving wear resistance, and toughness. However, if it is too much, it will cause a decrease in hardenability and high-temperature strength. Therefore, Cr is set to 3.5% < Cr < 4.5%. The preferred lower limit of Cr is 3.6% or more, more preferably 3.7% or more, and 3.8% or more. Also, the preferred upper limit of Cr is 4.4% or less, more preferably 4.3% or less, and 4.2% or less.

[0017] (Mo + 1 / 2W) of one or both of Mo and W according to the relational expression: (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5% Mo and W can be included individually or in combination to impart strength by precipitating or agglomerating fine carbides through tempering, thereby improving softening resistance and high-temperature strength. In this invention, since C is kept low to improve crack resistance, slightly higher concentrations of Mo and W can be expected to complement the strength. The content in this case can be specified together using the Mo equivalent defined by the relationship (Mo + 1 / 2W), since W has approximately twice the atomic weight of Mo (of course, either one or both can be included). To obtain the above effect, the content should be 2.5% or more according to the value of the relationship (Mo + 1 / 2W). The preferred lower limit is 2.7% or more, more preferably 2.9% or more, and 3.0% or more. However, too much Mo or W may lead to a decrease in machinability and toughness, potentially resulting in reduced crack resistance, and the high melting point makes melting difficult, so large amounts are undesirable from a manufacturing perspective. Therefore, the value obtained by the relationship (Mo + 1 / 2W) should be less than 3.5%. The preferred upper limit is 3.4% or less, more preferably 3.3% or less, and 3.2% or less. Here, since W is a more expensive element than Mo, it is preferable to include Mo alone if cost reduction is a priority.

[0018] V: 0.45% ≤ V ≤ 1.0% V forms vanadium carbide, which has the effect of strengthening the matrix, improving wear resistance, and resistance to tempering softening. When the additive manufactured part formed in the additive manufacturing process is heated to the quenching temperature for "quenching", the above-mentioned vanadium carbide also acts as a "pinning particle" that suppresses the coarsening of austenite crystal grains during quenching heating, contributing to the improvement of toughness. However, since V has a high carbide-forming ability, if there is too much V, all C will become vanadium carbide, and other carbides may not be able to be formed. Tool steel is composed of the existence of multiple types of carbides, so it is not desirable for the carbide to be only vanadium carbide. Therefore, 0.45 ≦ V ≦ 1.0%. The preferable lower limit is 0.50%, more preferably 0.52% or more, and 0.55% or more. Also, the preferable upper limit is 0.80%, more preferably 0.70% or less, 0.67% or less, and 0.65% or less.

[0019] Cu: 0.3% < Cu < 0.6% Cu is an element that suppresses the formation of ferrite in the tool structure. Also, together with C, Cr, Mn, Ni, Mo, W, etc., it imparts excellent hardenability to the tool material, and is an effective element for forming a structure mainly composed of martensite even when the cooling rate during quenching is slow, preventing the decrease in toughness. Also, aiming for further improvement in crack resistance, Cu is set to be more than 0.3% (0.3% < Cu). Preferably, it is 0.35% or more. On the other hand, excessive addition of Cu causes Cu to precipitate as a single substance in Fe, reducing toughness. Therefore, Cu is less than 1.0% (Cu < 0.6%). Preferably, it is 0.45% or less of Cu, and 0.42% or less.

[0020] Al: 0.2% ≦ Al ≦ 0.9% Al is an element that forms intermetallic compounds such as Ni3Al with Ni, thereby precipitation-strengthening the metal structure. In this invention, since Ni is added, hardness can be improved by adding Al. Therefore, the lower limit of Al is set to 0.2%. The preferred lower limit is 0.3%, and more preferably 0.4% or more. However, if there is too much Al, the number of nonmetallic inclusions in the metal structure may increase, which may reduce toughness. Therefore, the upper limit of Al is set to 0.9%. The preferred upper limit is 0.8% or less, more preferably 0.7% or less, and even more preferably 0.6% or less.

[0021] Remainder: Fe and unavoidable impurities The remainder consists of Fe and unavoidable impurities. Typical examples of unavoidable impurities include elements such as P, S, Ca, Mg, O (oxygen), N (nitrogen), and B (boron), and it is preferable that these elements be present in the lowest possible amounts. However, small amounts may be included for additional effects such as control of inclusion morphology, improvement of other mechanical properties, and improvement of manufacturing efficiency. In this case, a range of Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.05%, N ≤ 0.05%, and B ≤ 0.05% is sufficient and represents the preferred regulatory upper limit of the present invention. Furthermore, P and S can conform to the JIS steel grade SKD61, for example, P ≤ 0.030% and S ≤ 0.020%.

[0022] In this invention, by achieving an overall balance within the above-described component range, it is possible to obtain hot-work tool steel additively manufactured products with particularly excellent crack resistance during molding.

[0023] Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10≦0.95 In addition to specifying the components, one of the features of this invention is that the left side of the above formula (1) is adjusted to be 0.95 or less. The left side of formula (1) is an improved version of Pcm, which is used as a low-temperature cracking susceptibility index for welding, and C, Si, Mn, Cr, Cu, Ni, Mo, W, and V in formula (1) represent the content (mass%) of each element. Cracking is a problem in welding as well as in additive manufacturing, and since both are molten solidification structures, we found that this index can be applied to crack suppression in additive manufacturing and applied it to this invention. Here, another cracking index for welding is the high-temperature cracking index HCS, but from preliminary studies, it was found that the cracking in this composition system was large cracks from the surface, and the morphology differed from high-temperature cracks that tend to occur at solidification interfaces, etc. Since the cracking in this composition system is observed in parts where tensile stress due to thermal shrinkage is likely to occur, it is presumed to occur at low temperatures and is similar to low-temperature cracking in welding, and therefore the low-temperature cracking index Pcm was applied in this invention. By adjusting each principal component so that the left-hand side of equation (1) is 0.95 or less, a powder for additive manufacturing that can further suppress cracking during molding can be obtained. A more preferable left-hand side of equation (1) is 0.92 or less. Even more preferable is 0.90 or less.

[0024] Formula (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V≦400 In this invention, in addition to specifying the components, the above formula (2) can be adjusted to be 400 or less. Formula (2) is a relationship formula between elements and the Ms point disclosed in the literature (K. Ishida, Journal of Alloys and Compounds, Volume 220 Issues 1-2 1995 pp. 126-131). A higher value of formula (2) is expected to result in a higher Ms point, which can lead to transformation into brittle martensite at high temperatures during additive manufacturing, and a tendency for the material to crack due to thermal shrinkage when cooled to room temperature. It is preferable that formula (2) be 380 or less, more preferably 360 or less, even more preferably 340 or less, particularly preferably 320 or less, and most preferably 300 or less. There is no particular lower limit to formula (2), but if the value of formula (2) is too low, it is expected that the Ms point will be low, and if it is too low, there is a concern that the martensitic transformation will not be completed, austenite will remain, and the strength will decrease. Therefore, it is preferable to adjust equation (2) to be 200 or more. More preferably it is 220 or more, even more preferably 240 or more, and 260 or more.

[0025] The hot work tool steel powder for additive manufacturing of the present invention can be manufactured by methods such as gas atomization, water atomization, disk atomization, plasma atomization, and rotary electrode methods. In particular, the gas atomization method involves heating and melting a molten raw material prepared to have a desired component composition above its melting point using high-frequency induction heating, and then injecting an inert gas such as argon gas or nitrogen gas into the molten metal that flows out through pores to finely pulverize the molten metal, which is then rapidly cooled and solidified to obtain powder. This gas atomization method can use scrap metal or crude metal raw materials as the molten raw material, and can be manufactured at a lower cost compared to methods such as plasma atomization and rotary electrode methods, which require the preparation of raw materials with a desired component composition and shape in advance, making it a suitable method for obtaining the additive manufacturing powder of the present invention.

[0026] The hot tool steel powder for additive manufacturing of the present invention preferably has a 50% particle size (hereinafter referred to as "D50") of the volume-based cumulative particle size distribution of 10 to 250 μm. By setting the D50 of the additive manufacturing powder of the present invention to 250 μm or less, the melting of the metal powder becomes easier, and the formation of internal defects in the additively manufactured product can be suppressed. Furthermore, by setting the D50 of the additive manufacturing powder of the present invention to 10 μm or more, the handling of the metal powder and the effects of humidity and other factors in the atmosphere during additive manufacturing become less susceptible, and good fluidity can be ensured. Furthermore, the cumulative particle size distribution of the additive manufacturing powder of the present invention is expressed as a cumulative volume particle size distribution, and its D50 can be expressed by a measurement value obtained by the laser diffraction scattering method as specified in JIS Z 8825.

[0027] The hot tool steel powder for additive manufacturing of the present invention may have its D50 adjusted by sieving classification using a mesh or airflow classification, in accordance with the method described above. For example, when using additive manufacturing powder in the powder bed method, the powder is melted by a laser beam which serves as the heat source, while coarse additive manufacturing powder that is difficult to melt must be removed in order to minimize the thermally affected area. In addition, fine powder with high adhesion must also be removed in order to obtain optimal fluidity to ensure the powder's spreadability. For this reason, when applying the additive manufacturing powder of the present invention to the powder bed method, it is preferable to adjust the D50 to a range of 10 to 53 μm. The preferred upper limit of D50 is 40 μm, and the preferred lower limit of D50 is 20 μm. When applying the additive manufacturing powder of the present invention to the laser metal deposition method, it is preferable to adjust the D50 to a range of 50 to 150 μm.

[0028] By performing additive manufacturing on the hot-work tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described below, a hot-work tool steel additive manufactured product (hereinafter also referred to as an additive manufactured product) can be obtained, which in terms of mass percentage, satisfies 0.10% ≦ C ≦ 0.40%, 0.01% ≦ Si ≦ 0.19%, 0.1% ≦ Mn ≦ 1.0%, 2.0% ≦ Ni ≦ 9.0%, 3.5% < Cr < 4.5%, one or two of Mo and W according to the relational expression of (Mo + 1 / 2W): 2.5% ≦ (Mo + 1 / 2W) < 3.5%, 0.45% ≦ V ≦ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≦ Al ≦ 0.9%, with the balance being Fe and inevitable impurities, and satisfies the formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 + ≦ 0.95. This additive manufactured product is excellent in that it is particularly resistant to forming cracks.

[0029] The hot-work tool steel additive manufactured product of the present invention preferably has a room temperature (about 20 °C) tensile strength of 500 to 2000 MPa when the quenching and tempering hardness is adjusted to 45HRC ± 2. A more preferable lower limit is 1000 MPa, and an even more preferable lower limit is 1200 MPa. The room temperature 0.2% proof stress when the quenching and tempering hardness is adjusted to 45HRC ± 2 is preferably 500 to 2000 MPa. A more preferable lower limit is 800 MPa, an even more preferable lower limit is 1000 MPa, and an even more preferable lower limit is 1100 MPa. The room temperature elongation when the quenching and tempering hardness is adjusted to 45HRC ± 2 is preferably 5% or more. A more preferable lower limit is 8%, and an even more preferable lower limit is 10%. The room temperature reduction of area when the quenching and tempering hardness is adjusted to 45HRC ± 2 is preferably 20% or more. A more preferable lower limit is 30%, and an even more preferable lower limit is 40%. The room temperature 2-mm U-notch Charpy impact value when the quenching and tempering hardness is adjusted to 45HRC ± 2 is 2 preferably 20 J / cm 2 or more. A more preferable lower limit is 30 J / cm 2 or more, and an even more preferable lower limit is 40 J / cm

[0030] The hot-work tool steel additively manufactured product of the present invention preferably has a high-temperature (approximately 550°C in this embodiment) tensile strength of 400 to 1500 MPa when the temper hardness is adjusted to 45 HRC ± 2. A more preferable lower limit is 500 MPa, and an even more preferable lower limit is 600 MPa. The high-temperature 0.2% yield strength when the temper hardness is adjusted to 45 HRC ± 2 is preferably 300 to 1300 MPa. A more preferable lower limit is 400 MPa, and an even more preferable lower limit is 500 MPa. The high-temperature elongation when the temper hardness is adjusted to 45 HRC ± 2 is preferably 5% or more. A more preferable lower limit is 8%, and an even more preferable lower limit is 10%. The high-temperature reduction when the temper hardness is adjusted to 45 HRC ± 2 is preferably 10% or more. A more preferable lower limit is 20%, and an even more preferable lower limit is 30%. Furthermore, the additively manufactured hot tool steel product of the present invention preferably has a room-temperature thermal conductivity of 5 W / (m·K) or higher when the temper hardness is adjusted to 45 HRC ± 2. A more preferable lower limit is 10 W / (m·K), and an even more preferable lower limit is 15 W / (m·K).

[0031] While die-casting molds are the most preferred application for the additively manufactured parts of the present invention, they may also be applicable to other molds requiring an internal cooling mechanism, such as plastic molds. Furthermore, they may also be applicable to the repair of molds using the powder spray additive manufacturing method. Next, an example of a manufacturing process that allows obtaining an additively manufactured product of the present invention using the hot tool steel powder for additive manufacturing of the present invention will be described in order. Unless otherwise specified, the manufacturing process described below assumes the powder bed method.

[0032] The manufacturing method according to the present invention involves the steps of: laying down the prepared hot work tool steel powder for additive manufacturing of the present invention in layers; and sequentially melting and solidifying the laid-down metal powder using a scanning heat source having a diameter larger than the D50 of the metal powder to form a solidified layer. By repeating the steps of laying down the metal powder in layers and forming the solidified layer, multiple layered solidified layers can be formed to produce an additively manufactured product of the present invention. For example, a laser or an electron beam can be used as the scanning heat source. It is preferable to make the diameter of the scanning heat source larger than the D50 of the metal powder so that the aggregate of metal powder can be melted uniformly.

[0033] In the manufacturing method according to the present invention, when irradiating the aforementioned metal powder with a laser while scanning, the laser output can be set to 50-400 W, the scanning speed to 200-2000 mm / second, and the scanning pitch to 0.02-0.20 mm. Here, if the layer thickness per laser scan is too large, heat will not be easily transferred to the entire surface of the metal powder during laser irradiation, preventing the metal powder from melting sufficiently and promoting the formation of internal defects. On the other hand, if the layer thickness per scan is too small, the number of layers required to achieve the desired size of the additively manufactured product will increase, and the time required for the additive manufacturing process will be longer. For this reason, the layer thickness per scan is preferably 10-200 μm. A more preferable lower limit for the layer thickness is 20 μm, and a more preferable upper limit for the layer thickness is 100 μm. Although a preheating step may be performed before the additive manufacturing process described above, the powder of the present invention has particularly improved crack resistance compared to conventional hot tool steel powder. Therefore, for example, if the additively manufactured product is small and has few stress concentration points, it is possible to omit or reduce the preheating step before additive manufacturing.

[0034] In the manufacturing method according to the present invention, it is preferable to subject the additively manufactured component (in the as-additive manufactured state, before heat treatment) to a tempering treatment at a temperature of 500 to 700°C in order to impart the mechanical properties necessary for use as a metal product. By performing tempering, it is possible to create an "additively manufactured hot tool steel product" with a predetermined hardness. During this time, the additively manufactured product can be shaped into a hot tool by various machining processes such as cutting and drilling. In this case, annealing can be performed on the additively manufactured product formed in the additive manufacturing process in order to facilitate machining. Annealing can also be expected to have the effect of refining the vanadium carbides in the microstructure of the additively manufactured hot tool after tempering. Finishing machining may also be performed after tempering. In some cases, the finishing machining can also be performed on the additively manufactured product after tempering, and the above machining can be performed all at once to finish the additively manufactured hot tool product. Furthermore, quenching can be performed before the tempering described above. And, regardless of whether or not annealing is performed, or before or after, normalizing can be performed on additively manufactured parts formed in the additive manufacturing process.

[0035] The tempering temperature varies depending on the desired hardness, but is generally around 500-700°C. If quenching is performed before tempering, the quenching temperature is generally around 900-1100°C. For example, in the case of SKD61, a representative grade of hot work tool steel, the quenching temperature is around 1000-1030°C, and the tempering temperature is around 550-650°C. Furthermore, the tempered hardness is preferably 50 HRC (Rockwell hardness) or less, or 520 HV (Vickers hardness) or less. More preferably, it is 48 HRC or less, or 500 HV or less. It is also preferable that it be 40 HRC or more, or 380 HV or more. More preferably, it is 42 HRC or more, or 400 HV or more. In this invention, hardness can be measured in accordance with the measurement method described in JIS Z 2245 "Rockwell hardness test - Test method" or JIS Z 2244-1 "Vickers hardness test - Part 1: Test method", and Rockwell C scale hardness (HRC) or Vickers hardness (HV) can be used. [Examples]

[0036] (Example 1) After preparing each metal crude material to have the component composition shown in Table 1, the materials were charged into a high-frequency induction melting furnace and melted. The molten metal was then pulverized with argon gas to obtain gas atomized powder. The obtained atomized powder was then subjected to sieving classification using a mesh and airflow classification to adjust the particle size, resulting in additive manufacturing powders for the present invention and comparative examples, each with a D50 of 35 μm. Additively manufactured products were fabricated using an EOS M290 printer with the fabrication conditions shown in Table 2. Table 3 shows the component composition of the additively manufactured products of Sample No. 5 (made from the powder of Sample No. 1), Sample No. 6 (made from the powder of Sample No. 2), Sample No. 7 (made from the powder of Sample No. 3), and Sample No. 8 (made from the powder of Sample No. 4). Table 4 also shows the values ​​for equation (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10 and equation (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V for samples No. 1 to 8.

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] [Table 4]

[0041] (Example 2) To evaluate the cracking susceptibility of additively manufactured parts, crack evaluation test specimens with the shape shown in Figure 1 were fabricated. Specifically, the additive manufacturing powders of samples No. 1 to 4 from Example 1 were additively manufactured under the same manufacturing conditions as in Example 1 to produce samples No. 9 (composition of sample No. 5), No. 10 (composition of sample No. 6), No. 11 (composition of sample No. 7), and No. 12 (composition of sample No. 8), which have the same composition as samples No. 5 to 8. These crack evaluation test specimens are 50 mm long, 10 mm wide, and 16 mm high, with a stress concentration zone created with R8 in the middle, and this stress concentration zone is made comb-shaped to make it prone to cracking, simulating an additive manufacturing mold in which complex cavities are formed. After fabrication, the length of the crack at the comb-shaped section of this crack evaluation test specimen can be measured to evaluate the susceptibility of the material to cracking during fabrication. Table 5 shows the crack lengths of the crack test specimens for samples No. 9, 11, and 12 (examples of the present invention) and sample No. 10 (comparative example). From Table 5, it can be confirmed that the examples of the present invention have significantly shorter crack lengths than the comparative example, and that they have particularly superior crack resistance during molding. In particular, sample No. 9 had a crack length of 1.0 mm or less, and was confirmed to have the best crack resistance.

[0042] [Table 5]

[0043] (Example 3) Next, the tempering behavior of the present invention examples and comparative examples was examined. Tempering heat treatment was performed on samples No. 5 to 8 shown in Example 1 within the temperature range shown in Figure 2, and the Rockwell hardness was measured according to JIS Z 2245. Figure 2 shows graphs of each tempering temperature and hardness. From Figure 2, it was confirmed that samples No. 5, 7, and 8, which are examples of the present invention, can be tempered to a higher hardness than sample No. 6, which is a comparative example.

[0044] Furthermore, the mechanical properties and thermal conductivity of the examples of the present invention were confirmed. Additive-formed products manufactured under the same conditions as samples No. 5, No. 7, and No. 8 of Example 1 (all examples of the present invention) were subjected to tempering heat treatment in the temperature range of 500 to 700°C to heat the test pieces to 40±2HRC, 45±2HRC, and 52±2HRC, respectively. Then, tensile tests, 2mm U-notch Charpy impact tests, and thermal conductivity measurements by laser flash method were performed. Figure 3 shows the results of the room temperature (22°C) tensile test, Figure 4 shows the results of the high temperature (550°C) tensile test, Figure 5 shows the results of the Charpy impact test, and Figure 6 shows the results of the thermal conductivity measurement.

[0045] As shown in Figure 3, the additively manufactured product of the present invention had a room-temperature tensile strength of 1100 MPa or more, a room-temperature 0.2% yield strength of 600 MPa or more, a room-temperature elongation of 12% or more, and a room-temperature reduction of 50% or more at all temper hardness levels. Furthermore, as shown in Figure 4, the additively manufactured product of the present invention had a high-temperature tensile strength of 700 MPa or more, a high-temperature 0.2% yield strength of 400 MPa or more, a high-temperature elongation of 13% or more, and a high-temperature reduction of 35% or more at all temper hardness levels of the present invention. Furthermore, the additively manufactured product of the present invention, at a hardness of 45±2HRC, had a room-temperature tensile strength of 1200MPa or more, a room-temperature 0.2% yield strength of 1100MPa or more, a room-temperature elongation of 13% or more, and a room-temperature reduction of 50% or more. In addition, the additively manufactured product of the present invention, at a hardness of 45±2HRC, had a high-temperature tensile strength of 800MPa or more, a high-temperature 0.2% yield strength of 500MPa or more, a high-temperature elongation of 15% or more, and a high-temperature reduction of 35% or more.

[0046] As shown in Figure 5, the additively fabricated product of the present invention has a Charpy impact value of 50 J / cm² at all temper hardness levels. 2 The above was confirmed. Furthermore, from Figure 6, it was confirmed that the additively manufactured product of the present invention has a thermal conductivity of 15 W / (m·K) or higher at room temperature. In particular, sample No. 7 had a thermal conductivity of 20 W / (m·K) or higher at room temperature, and was confirmed to have the best thermal conductivity characteristics. As shown in Figures 3 to 6 above, the additively manufactured product of the present invention has properties equivalent to those of hot-work tool steel made from melted materials, and is suitable for applications such as hot-work tools.

Claims

1. Hot work tool steel powder for additive manufacturing, comprising, in mass%, 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni ≤ 9.0%, 3.5% < Cr < 4.5%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≤ Al ≤ 0.9%, with the remainder being Fe and unavoidable impurities, and further satisfying the following formula (1). Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10≦0.95 Here, each element symbol in formula (1) indicates the content (mass%) of that element.

2. A hot-work tool steel additively fabricated product having the following mass percentages: 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni ≤ 9.0%, 3.5% < Cr < 4.5%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 2.5% ≤ (Mo + 1 / 2W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, 0.2% ≤ Al ≤ 0.9%, with the remainder being Fe and unavoidable impurities, and further satisfying formula (1) below. Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10≦0.95 Here, each element symbol in formula (1) indicates the content (mass%) of that element.

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