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

Abstract

Provided is a hot work tool steel powder for deposition modeling, with which it is possible to further improve cracking resistance during modeling and to obtain a hot work tool steel deposition model that has excellent toughness.　The present invention specifically provides: a hot work tool steel powder for deposition modeling, which contains, in mass%, 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 0.3% ≤ Ni < 1.0%, 3.5% < Cr < 4.5%, one or both of Mo and W according to the relation (Mo + 1 / 2 W), satisfying a relational expression 2.5% ≤ (Mo + 1 / 2 W) < 3.5%, 0.45% ≤ V ≤ 1.0%, and 0.1% ≤ Cu ≤ 1.0%, 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 ≤ 0.80; and a hot work tool steel deposition model.

Description

[Technical Field]

[0001] This invention relates to hot tool steel powder for additive manufacturing and hot tool steel additively manufactured products. [Background technology]

[0002] Hot work tool steels, such as those used in hot forging dies and die-casting dies, require high-temperature strength, toughness, and wear resistance because they come into contact with high-temperature workpieces. Conventionally, to satisfy these requirements, JIS grade SKD61 and improved versions of SKD61 have been used for hot work tool steels.

[0003] Recently, additive manufacturing has been attracting attention as a means of easily forming metal products (parts) with complex shapes in near-net shape. Additive manufacturing is a type of additive manufacturing technology, also commonly known as 3D printing. Types of additive manufacturing include, for example, the powder spray method, which melts metal powder by irradiating it with a heat source and layering it, and the powder bed method, which melts metal powder spread on a stage by irradiating it with a heat source and then solidifies it, repeating this process to layer the material. With additive manufacturing, metal products with complex shapes can be manufactured by significantly reducing conventional machining processes, and difficult-to-machine metal materials can be used. Furthermore, difficult-to-machine metal materials are often high-strength metal materials, so it is possible to manufacture metal products with complex shapes and long lifespans.

[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%, consisting of 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 with a cross-sectional area of ​​1 μm² parallel to the layering direction. 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, etc. 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 part (concave part) tends to crack during additive manufacturing. Also, depending on the application, particularly high toughness may be required. For example, when additive manufacturing a mold with a complex shape due to the high design of products in recent years, high toughness is required to prevent large cracks in the stress concentration part. Therefore, an object of the present invention is to provide a hot work tool steel powder for additive manufacturing that can further improve crack resistance during additive manufacturing and can obtain a hot work tool steel additive manufactured product having excellent toughness.

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, in mass%, in mass%, 0.10% ≦ C ≦ 0.40%, 0.01% ≦ Si ≦ 0.19%, 0.1% ≦ Mn ≦ 1.0%, 0.3 ≦ Ni < 1.0%, 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%, 0.1% ≦ Cu ≦ 1.0%, the balance being Fe and inevitable impurities, and satisfying the formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 ≦ 0.80 (each element symbol in formula (1) indicates the content (mass%) of the element).), which is a hot work tool steel powder for additive manufacturing.

[0009] In addition, another aspect of the present invention is a hot work tool steel additive manufactured product, which, in mass%, satisfies 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 0.3 ≤ Ni < 1.0%, 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%, 0.1% ≤ Cu ≤ 1.0%, with the balance being Fe and inevitable impurities, and satisfies formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 ≤ 0.80.

Advantages of the Invention

[0010] According to the present invention, it is possible to further improve the cracking resistance during additive manufacturing, and to provide a hot work tool steel powder for additive manufacturing that can obtain a hot work tool steel additive manufactured product having excellent toughness.

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 showing the tempering temperature and hardness in the hot work tool steel additive manufactured products of the examples of the present invention and the comparative examples. [Figure 3] It is a graph showing the mechanical properties ((a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) at room temperature of the examples of the present invention. [Figure 4] It is a graph showing the mechanical properties ((a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) at high temperature of the examples of the present invention. [Figure 5] It is a graph showing the Charpy impact value at room temperature of the examples of the present invention. [Figure 6] It is a graph showing the thermal conductivity of the examples of the present invention.

Embodiments for Carrying Out the Invention

[0012] The present invention has a component composition consisting of 0.10% ≦ C ≦ 0.40%, 0.01% ≦ Si ≦ 0.19%, 0.1% ≦ Mn ≦ 1.0%, 0.3 ≦ Ni < 1.0%, 3.5% < Cr < 4.5%, 2.5% < (Mo + 1 / 2W) < 3.5%, 0.45% ≦ V ≦ 1.0%, 0.1% ≦ Cu ≦ 1.0%, and the balance being Fe and inevitable impurities. First, the reasons for limiting the composition of the hot-work 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-work 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 large 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-work 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 the ferrite phase up to room temperature, so hardening that requires rapid cooling from the austenite phase becomes impossible. If the main phase is the ferrite phase from immediately after solidification to room temperature, relaxation of thermal contraction using martensitic transformation expansion becomes impossible, making it more prone to cracking. However, as C increases, toughness decreases instead of hardness improving, promoting cracking during manufacturing. In the present invention, aiming to improve crack resistance while maintaining a hardness that can withstand the use of molds, 0.10% ≦ C < 0.40%. The lower limit of preferable C is 0.15%, more preferably 0.18% or more, still more preferably 0.20% or more. Also, the upper limit of preferable C is 0.35%, more preferably 0.30% or less, still more preferably 0.27% or less, 0.25% or less, 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 gets to no addition, the higher the manufacturing cost. Therefore, the lower limit of Si is set at 0.01%. The preferred lower limit is 0.05%, and more preferably 0.08% or higher. 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%. The preferred upper limit is 0.17%, more preferably 0.15% or lower, and 0.13% or lower.

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

[0015] Ni: 0.3% ≤ Ni < 1.0% 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 the decrease in toughness. Furthermore, since it also improves the inherent toughness of the matrix, the lower limit of Ni is set at 0.3%. Preferably it is 0.5% or higher, more preferably 0.7% or higher. However, excessive Ni increases the viscosity of the matrix and reduces the machinability, so the upper limit is set at less than 1.0%. Preferably it is 0.90% or lower, and even more preferably 0.85% or lower.

[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 in 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 lower limit of preferable Cr is 3.6%, more preferably 3.7% or more, and 3.8% or more. Also, the upper limit of preferable Cr is 4.4%, more preferably 4.3% or less, and 4.2% or less.

[0017] (Mo + 1 / 2W) of one or two of Mo and W according to the relational expression: 2.5% ≤ (Mo + 1 / 2W) < 3.5% Mo and W can be contained singly or in combination in order to precipitate or aggregate fine carbides by tempering to impart strength and improve softening resistance and high-temperature strength. And in the present invention, in order to aim at improving crack resistance, since C is lowered, by containing one or two of Mo and W slightly more, an effect of complementing strength can be expected. At this time, the content can be defined together by Mo equivalent defined by the relational expression of (Mo + 1 / 2W) because W has an atomic weight about twice that of Mo (it is also possible to contain only one of them or both of them together). And in order to obtain the above effect, the content should be 2.5% or more in terms of the value according to the relational expression of (Mo + 1 / 2W). The lower limit of more preference is 2.7%, more preferably 2.9% or more, and 3.0% or more. However, if there is too much Mo or W, there is a fear of causing a decrease in machinability and toughness, resulting in a decrease in crack resistance, and also because the melting point is high and the difficulty of melting increases, a large amount of content is not preferable in terms of manufacturing. Therefore, the value according to the relational expression of (Mo + 1 / 2W) is less than 3.5%. Preferably it 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, when emphasizing cost reduction, it is preferable to contain Mo alone.

[0018] V: 0.45% ≤ V ≤ 1.0% V forms vanadium carbides, which have the effect of strengthening the matrix, improving wear resistance, and tempering softening resistance. Furthermore, when the additively manufactured product formed in the additive manufacturing process is heated to the quenching temperature for "quenching," the vanadium carbides also act as "pinning particles" that suppress the coarsening of austenite crystal grains during quenching heating, contributing to improved toughness. However, since V has a high carbide-forming ability, if there is too much V, all of the carbon (C) may become vanadium carbides, making it impossible to form other carbides. Tool steel is made up of multiple types of carbides, so it is undesirable for the carbides to consist only of vanadium carbides. Therefore, the limit is set to 0.45 ≤ V ≤ 1.0%. The preferred lower limit is 0.50%, more preferably 0.52% or more, and more preferably 0.55% or more. The preferred upper limit is 0.80%, more preferably 0.70% or less, less than 0.67%, and less than 0.65%.

[0019] Cu: 0.1% ≤ Cu ≤ 1.0% Cu is an element that suppresses the formation of ferrite in the tool structure. It also effectively imparts excellent hardenability to tool materials along with C, Cr, Mn, Ni, Mo, and W, forming a martensite-dominant structure even at slow cooling rates during quenching, thus preventing a decrease in toughness. However, excessive addition of Cu causes elemental precipitation in Fe, reducing toughness. Therefore, the Cu content should be 0.1% ≤ Cu ≤ 1.0%. The preferred lower limit is 0.2% or more, more preferably 0.30% or more, 0.33% or more, and 0.35% or more. The preferred upper limit is 0.7% or less, more preferably 0.5% or less, 0.48% or less, 0.45% or less, and 0.42% or less.

[0020] Remainder: Fe and unavoidable impurities The remainder consists of Fe and unavoidable impurities. Typical examples of unavoidable impurities include elements such as P, S, Al, 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 are acceptable because they have additional effects such as controlling the morphology of inclusions, improving other mechanical properties, and increasing manufacturing efficiency. In this case, a range of Al ≤ 0.04%, 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. For Al, it is more preferable to have 0.025% or less. Furthermore, P and S can conform to the JIS steel grade SKD61, for example, P ≤ 0.030% and S ≤ 0.020%.

[0021] In this invention, by achieving an overall balance within the above-mentioned component range, it is possible to obtain a hot-work tool steel additively manufactured product with particularly excellent toughness.

[0022] Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10≦0.80 In addition to specifying the components, one of the features of this invention is that the left side of formula (1) above is adjusted to be 0.80 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 differed in morphology 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 0.80 or less, a powder can be obtained that can further suppress cracking during molding. A more preferable left-hand side of equation (1) is 0.78 or less. Even more preferable is 0.76 or less.

[0023] Formula (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V≦500 In addition to specifying the components, the present invention allows for adjustment so that formula (2) is 500 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 high value of formula (2) is expected to result in a high 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 450 or less, more preferably 400 or less, and even more preferably 390 or less. The lower limit of formula (2) is not particularly limited, 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 martensite transformation will not be completed, austenite will remain, and the strength will decrease. For this reason, it is preferable to adjust the lower limit of formula (2) to 200 or more. More preferably 250 or more, even more preferably 300 or more, and 350 or more.

[0024] The hot work tool steel powder for additive manufacturing of the present invention can be manufactured by, for example, 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 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 metal powder for additive manufacturing of the present invention.

[0025] 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 metal powder for additive manufacturing of the present invention to 250 μm or less, the powder melting becomes easier, and the formation of internal defects in the additively manufactured product can be suppressed. Furthermore, by setting the D50 of the metal powder for additive manufacturing of the present invention to 10 μm or more, the handling of the metal powder and the atmosphere during additive manufacturing become less susceptible to the effects of moisture and other factors, thereby ensuring good fluidity. Furthermore, the cumulative particle size distribution of the molding 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.

[0026] 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, in additive manufacturing metal powder used in the powder bed method, the metal powder is melted by a laser beam which serves as the heat source, while coarse metal powder that is difficult to melt must be removed in order to minimize the thermally affected area. In addition, fine metal powder with high adhesion must also be removed in order to obtain optimal fluidity to ensure the layability of the metal powder. 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. Furthermore, 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.

[0027] 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 simply referred to as an additive manufactured product) can be obtained, which, in mass%, satisfies 0.10% ≦ C ≦ 0.40%, 0.01% ≦ Si ≦ 0.19%, 0.1% ≦ Mn ≦ 1.0%, 0.3 ≦ Ni < 1.0%, 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%, 0.1% ≦ Cu ≦ 1.0%, 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 + ≦ 0.80. This additive manufactured product is excellent in that it is less likely to crack during manufacturing and has good toughness. Here, the toughness in the present invention can be evaluated by the Charpy impact value. The preferable Charpy impact value is 30 J / cm 2 or more, more preferably 50 J / cm 2 or more, still more preferably 100 J / cm 2 or more. Particularly preferably, it is 200 J / cm 2 or more. Also, preferably, the Charpy impact value at room temperature (about 20 °C) when the quenched and tempered hardness is adjusted to 45 ± 2 HRC is 150 J / cm 2 or more. The above-mentioned Charpy impact value can be measured by a 2 mm U-notch Charpy impact test based on JIS Z 2242.

[0028] The hot-work tool steel additively manufactured product of the present invention is expected to exhibit excellent mechanical properties in addition to the above-mentioned cracking characteristics and toughness. For example, the hot-work tool steel additively manufactured product of the present invention preferably has a room temperature (around 20°C) tensile strength of 1000 to 2000 MPa when the temper hardness is adjusted to 45 ± 2 HRC. A more preferable lower limit is 1200 MPa, an even more preferable lower limit is 1300 MPa, and an even more preferable lower limit is 1400 MPa. The room temperature 0.2% yield strength when the temper hardness is adjusted to 45 ± 2 HRC is preferably 800 to 2000 MPa. A more preferable lower limit is 800 MPa, and an even more preferable lower limit is 1000 MPa. The room temperature elongation when the temper hardness is adjusted to 45 ± 2 HRC is preferably 8% or more. A more preferable lower limit is 10%, and an even more preferable lower limit is 12%. When the temper hardness is adjusted to 45±2HRC, the room temperature drop is preferably 30% or more. A more preferable lower limit is 40%, an even more preferable lower limit is 50%, and an even more preferable lower limit is 60%.

[0029] 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 600 to 1400 MPa when the temper hardness is adjusted to 45±2HRC. A more preferable lower limit is 800 MPa, and an even more preferable lower limit is 900 MPa. The high-temperature 0.2% yield strength when the temper hardness is adjusted to 45H±2RC is preferably 600 to 1200 MPa. A more preferable lower limit is 700 MPa, and an even more preferable lower limit is 800 MPa. The high-temperature elongation when the temper hardness is adjusted to 45±2HRC is preferably 10% or more. A more preferable lower limit is 13%, and an even more preferable lower limit is 16%. The high-temperature reduction when the temper hardness is adjusted to 45±2HRC is preferably 30% or more. A more preferable lower limit is 40%, an even more preferable lower limit is 50%, and an even more preferable lower limit is 60%.

[0030] Furthermore, the additively manufactured hot tool steel product of the present invention preferably has a room-temperature thermal conductivity of 10 W / (m·K) or higher when the temper hardness is adjusted to 45 ± 2 HRC. A more preferable lower limit is 15 W / (m·K), an even more preferable lower limit is 20 W / (m·K), and a particularly preferable lower limit is 25 W / (m·K).

[0031] While die-casting molds are the most preferred application for this additively manufactured product, it may also be applicable to other molds requiring internal cooling mechanisms, such as plastic molds. Furthermore, it may be applicable to the repair of molds manufactured using the powder spray additive manufacturing method. Next, an example of a manufacturing process that allows obtaining a hot tool steel additive manufacturing 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 tool steel powder for additive manufacturing of the present invention (hereinafter also referred to as "metal powder") 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 this 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 improved crack resistance compared to conventional hot tool steel powder. Therefore, for example, if the manufactured product has few stress concentration points and is small, 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 it as a 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, they 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, obtaining additive manufacturing powders for the present invention and comparative examples with a D50 of 35 μm. Additively manufactured products were fabricated using an EOS M290 under the fabrication conditions shown in Table 2 for each of the additive manufacturing metal powders obtained above. Table 3 shows the component composition of the fabricated products of Sample No. 3, made from the powder of Sample No. 1, and Sample No. 4, made from the powder of Sample No. 2. 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 4.

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] [Table 4]

[0041] (Example 2) To evaluate the crackability of additively manufactured parts, crack evaluation test specimens as shown in Figure 1 were fabricated. Specifically, the additive manufacturing powders of samples No. 1 and 2 from Example 1 were additively manufactured under the same manufacturing conditions as in Example 1 to obtain No. 5 (composition of No. 3) of the present invention and No. 6 (composition of No. 4) of the comparative example, which have the same composition as samples No. 3 and No. 4. 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 part of this crack evaluation test specimen can be measured to evaluate the ease with which the material cracks during fabrication. The crack lengths of the crack test specimens of sample No. 5 (present invention example) and sample No. 6 (comparative example) are shown in Table 5. From Table 5, it can be confirmed that the present invention example has a shorter crack length than the comparative example, and is equally or better resistant to cracking than the comparative example.

[0042] [Table 5]

[0043] (Example 3) Next, the tempering behavior of the present invention example and the comparative example was examined. Samples No. 3 and 4, obtained by the manufacturing method described in Example 1 above, were subjected to tempering heat treatment within the temperature range shown in Figure 2, and their 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 sample No. 3 of the present invention example could be tempered to a higher hardness than comparative example No. 4.

[0044] (Example 4) Next, the toughness of the additively manufactured product of the present invention was evaluated. Gas atomized powders having the alloy compositions of sample No. 1 and sample No. 2 obtained by the manufacturing method shown in Example 1 were prepared. Then, using an EOS M290, additively manufactured products of sample No. 7 (same composition as No. 3) of the present invention and sample No. 8 (same composition as No. 4) of the comparative example were fabricated under additive manufacturing conditions of laser output: 300W, scanning speed: 800mm / s, and layer thickness: 40μm. Sample No. 7 was tempered at 605~610℃ to a hardness of 45±2HRC. Sample No. 8 was tempered at 550℃ to a hardness of 45±2HRC, the same as sample No. 4. The hardness was measured using a Rockwell hardness tester in accordance with JIS Z 2245. Subsequently, Charpy impact test specimens were taken from the obtained samples No. 7 and No. 8 in a direction horizontal to the layering direction. Then, a 2mm U-notch Charpy impact test was performed on the obtained test specimens in accordance with JIS Z 2242. The toughness results obtained are shown in Table 6. From the results in Table 6, the present invention example, using a test specimen tempered to a hardness of 45±2HRC, yielded a Charpy impact value of 200 J / cm². 2 In summary, it was confirmed that the toughness was superior to that of the comparative example.

[0045] [Table 6]

[0046] Furthermore, the mechanical properties and thermal conductivity of the examples of the present invention were confirmed. For test pieces fabricated under the same conditions as Sample No. 3 (example of the present invention) of Example 1, tempering heat treatment was carried out in the temperature range of 500 to 700 °C to temper the test pieces to 40 ± 1 HRC, 45 ± 1 HRC, and 49 ± 1 HRC. Then, a tensile test and a 2-mm U-notch Charpy impact test were carried out. Also, after tempering the test pieces to 45 ± l HRC, measurement of thermal conductivity by the laser flash method was carried out. Fig. 3 shows the results of the tensile test at room temperature (22 °C), Fig. 4 shows the results of the tensile test at high temperature (550 °C), Fig. 5 shows the results of the Charpy impact test, and Fig. 6 shows the measurement results of thermal conductivity.

[0047] From Fig. 3, for the laminated molded products of the examples of the present invention, at all tempered hardnesses, the tensile strength at room temperature was 1200 MPa or more, the 0.2% proof stress at room temperature was 1000 MPa or more, the elongation at room temperature was 13% or more, and the reduction of area at room temperature was 60% or more. Also, from Fig. 4, for the laminated molded products of the examples of the present invention, at all tempered hardnesses of the examples of the present invention, the tensile strength at high temperature was more than 800 MPa, the 0.2% proof stress at high temperature was more than 600 MPa, the elongation at high temperature was 13% or more, and the reduction of area at high temperature was 50% or more. And for the laminated molded products of the examples of the present invention at a hardness of 45 ± 1 HRC, the tensile strength at room temperature was 1400 MPa or more, the 0.2% proof stress at room temperature was 1200 MPa or more, the elongation at room temperature was 13% or more, and the reduction of area at room temperature was 60% or more. Also, for the laminated molded products of the examples of the present invention at a hardness of 45 ± 1 HRC, the tensile strength at high temperature was 900 MPa or more, the 0.2% proof stress at high temperature was 700 MPa or more, the elongation at high temperature was 18% or more, and the reduction of area at high temperature was 60% or more.

[0048] [[ID=I2]]From Fig. 5, it was confirmed that for the laminated molded products of the examples of the present invention, at all tempered hardnesses, the Charpy impact value was 100 J / cm 2 or more. Particularly, at hardnesses of 40 ± 1 HRC and 45 ± 1 HRC, the Charpy impact value was 150 J / cm 2 or more, which were good values. Also, from Fig. 6, it was confirmed that for the laminated molded products of the examples of the present invention at a hardness of 45 ± 1 HRC, the thermal conductivity at room temperature was 25 W / (m·K) or more. As shown in Figures 3 to 6 above, the additively manufactured product of the present invention has properties equivalent to those of conventional hot tool steel, and it has been confirmed that it is suitable for applications such as hot 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%, 0.3% ≤ Ni < 1.0%, 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%, 0.1% ≤ Cu ≤ 1.0%, 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≦0.80 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%, 0.3% ≤ Ni < 1.0%, 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%, 0.1% ≤ Cu ≤ 1.0%, 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≦0.80 Here, each element symbol in formula (1) indicates the content (mass%) of that element.

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