Hot work tool steel powder for additive manufacturing and hot work tool steel additive manufacturing product

Abstract

Provided is a hot work tool steel powder for additive manufacturing that makes it possible to manufacture a hot work tool steel additive manufacturing product having high softening resistance.　Provided are: a hot work tool steel powder for additive manufacturing composed of, by mass%, 0.10%≤C≤0.24%, 0.01%≤Si≤0.50%, 0.01%≤Mn≤0.19%, 0.5%≤Ni≤1.5%, 3.6%≤Cr≤4.4%, 2.1%≤(Mo+1 / 2W)≤3.0% of one or both among Mo and W according to the relationship expression (Mo+1 / 2W), 0.20%≤V≤1.0%, and 0.1≤Nb≤0.4, with the remainder being Fe and unavoidable impurities, said hot work tool steel powder satisfying expression (1): C+Si / 30+(Mn+Cr) / 20+Ni / 60+(Mo+1 / 2W) / 15+V / 10+Nb / 20≤0.75; and a hot work tool steel additive manufacturing product.

Description

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

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

[0002] Hot work tool steels for hot forging dies, die casting dies, etc., are required to have high-temperature strength, toughness, and wear resistance because they come into contact with high-temperature workpieces. To meet these requirements, JIS steel grade SKD61 and improved versions of SKD61 have been used as hot work tool steels.

[0003] Recently, additive manufacturing (AM) has been attracting attention as a means for easily forming metal products (components) with complex shapes in near-net shapes. AM, commonly referred to as 3D printing, is an additive manufacturing technology. Examples of AM methods include the powder spray method, in which metal powder is irradiated with a heat source to melt and layer the powder, and the powder bed method, in which metal powder spread on a stage is irradiated with a heat source to melt and solidify the powder, layering the powder. AM allows for the production of metal products with complex shapes while largely eliminating the need for conventional machining processes, allowing the use of difficult-to-process metal materials. Furthermore, since difficult-to-process metal materials are primarily high-strength metal materials, it is possible to produce metal products with complex shapes and long durability.

[0004] Furthermore, an additive manufacturing product has been proposed that uses hot work tool steel as the metal material and is produced by the additive manufacturing method described above. For example, Patent Document 1 discloses an additive manufacturing product having a component composition containing, in mass %, C: 0.3 to 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 to 6.0%, one or two of Mo and W according to the relationship formula (Mo + 1 / 2W): 0.5 to 3.5%, V: 0.1 to 1.5%, Ni: 0 to 1.0%, Co: 0 to 1.0%, Nb: 0 to 0.3%, and the balance being Fe and impurities, and having an area of ​​1 μm in a cross section parallel to the lamination direction. ２ An additive manufacturing hot 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 having the following composition, in mass%, for the purpose of achieving both high thermal conductivity and high corrosion resistance: 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) (wherein, [Cr] in formula (a) represents the Cr content in mass%), 0.01≦Mo≦1.80, −0.00447×[Mo]+0.010≦V≦−0.1117×[Mo]+0.901...formula (b) (wherein, [Mo] in formula (b) represents the Mo content in mass%), 0.0002≦N≦0.3000, with the balance being Fe and unavoidable impurities.

[0006] International Publication No. 2019 / 220917 Japanese Patent Application Laid-Open No. 2016-145407

[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, large additive manufacturing molds or additive manufacturing molds that form complex cavities tend to be prone to cracking due to the presence of stress concentration areas (recesses). Furthermore, because hot work tools manufactured by additive manufacturing can form three-dimensional internal cooling circuits, they are likely to be used in higher temperature environments, raising concerns about an increased risk of softening. Therefore, an object of the present invention is to provide a hot work tool steel powder for additive manufacturing that can produce hot work tool steel additive manufactured products with high softening resistance.

[0008] The present invention has been made in view of the above-mentioned problems. That is, one aspect of the present invention is a steel sheet having, in mass %, 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relational expression (Mo+½W): 2.1%≦(Mo+½W)≦3.0%, 0.20%≦V≦ The hot work tool steel powder for additive manufacturing comprises C+Si / 30+(Mn+Cr) / 20+Ni / 60+(Mo+½W) / 15+V / 10+Nb / 20≦0.75 (each element symbol in formula (1) indicates the content (mass %) of that element).

[0009] Another aspect of the present invention is a hot work tool steel additive manufactured product having, in mass%, 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship formula (Mo+½W): 2.1%≦(Mo+½W)≦3.0%, 0.20%≦V≦1.0%, 0.1%≦Nb≦1.0%, and the balance being Fe and unavoidable impurities, and satisfying formula (1): C+Si / 30+(Mn+Cr) / 20+Ni / 60+(Mo+½W) / 15+V / 10+Nb / 20≦0.75.

[0010] According to the present invention, it is possible to obtain a hot work tool steel powder for additive manufacturing that can be used to manufacture hot work tool steel additive manufactured products that have high softening resistance.

[0011] 1 is a graph showing the tempering temperature and hardness of hot work tool steel additive manufactured products of an example of the present invention and a comparative example; FIG. 2 is a graph showing the cumulative holding time at 600°C and hardness of hot work tool steel additive manufactured products of an example of the present invention and a comparative example; FIG. 3 is a graph showing the mechanical properties ((a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention at room temperature; FIG. 4 is a graph showing the mechanical properties ((a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention at high temperatures; FIG. 5 is a graph showing the Charpy impact value of an example of the present invention at room temperature; FIG. 6 is a graph showing the thermal conductivity of an example of the present invention.

[0012] The present invention has a chemical composition consisting of 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo + ½W): 2.1%≦(Mo + ½W)≦3.0%, 0.20%≦V≦1.0%, 0.1%≦Nb≦1.0%, and the balance being Fe and unavoidable impurities. First, the reasons for the compositional limitations of the hot work tool steel powder for additive manufacturing (hereinafter also referred to as additive manufacturing powder or metal powder) specified in the present invention will be described. Unless otherwise specified, "%" refers to "mass %." Also, additive manufacturing may be simply referred to as "manufacturing." C: 0.10%≦C≦0.24%. Carbon (C) is a basic element of hot-work tool steels. Some carbon dissolves in the matrix to provide strength, while others form carbides, enhancing wear resistance and seizure resistance. Furthermore, when added together with substitutional atoms with high affinity for C, such as Cr, C dissolved as an interstitial atom is expected to contribute to the I (interstitial atom)-S (substitutional atom) effect (which acts as drag resistance for solute atoms and increases the strength of hot-work tools). It can also enhance hardenability. If the C content is too low, the ferrite phase forms primarily during solidification, and the ferrite phase remains the dominant phase until room temperature, making quenching, which requires rapid cooling from the austenite phase, impossible. If the ferrite phase remains dominant immediately after solidification until room temperature, thermal contraction cannot be alleviated by utilizing martensitic transformation expansion, making the steel more susceptible to cracking. However, increasing the C content increases hardness but decreases toughness, promoting cracking during molding. In the present invention, the range of C is 0.10%≦C≦0.24% in order to improve crack resistance while maintaining hardness sufficient for use in dies. The lower limit of C is preferably 0.12%, more preferably 0.15% or more, and even more preferably 0.18% or more. The upper limit of C is preferably 0.23% or less.

[0013] Si: 0.01%≦Si≦0.50% Si can be used as a deoxidizer when adjusting the chemical composition of molten steel. It is difficult to eliminate Si from the steel in production, and the closer we try to eliminate it, the higher the production costs become. Therefore, the lower limit of Si is set to 0.01%. A preferred lower limit is 0.05%, and more preferably 0.08% or more. On the other hand, excessive Si leads to the formation of ferrite in the structure after tempering, so the upper limit is set to 0.50%. A preferred upper limit is 0.25%, more preferably 0.17% or less, even more preferably 0.15% or less, and even more preferably 0.13% or less.

[0014] Mn: 0.01%≦Mn≦0.19% Mn has the effects of improving hardenability, suppressing the formation of ferrite in the structure after tempering, and achieving appropriate quench-and-temper hardness. To achieve these effects, the lower limit of Mn is set to 0.01%. A preferred lower limit is 0.02%, more preferably 0.03% or more, even more preferably 0.04% or more, and even more preferably 0.05% or more. On the other hand, if Mn is too much, it increases the viscosity of the matrix and reduces the machinability of the material. Therefore, the upper limit is set to 0.19%. A preferred upper limit is 0.18%, more preferably 0.17% or less, and even more preferably 0.15% or less.

[0015] Ni: 0.5%≦Ni≦1.5% Ni is an element that suppresses the formation of ferrite in the structure after tempering. In addition, together with C, Cr, Mn, Mo, W, etc., Ni imparts excellent hardenability to tool materials and, even when the cooling rate during quenching is slow, forms a martensite-based structure, effectively preventing a decrease in toughness. Furthermore, Ni also improves the intrinsic toughness of the matrix, so in the present invention, the lower limit of Ni is set to 0.5%. A preferred lower limit is 0.6%, i.e., 0.7% or more. However, excessive Ni increases the viscosity of the matrix and reduces machinability. Therefore, the upper limit of Ni is set to 1.5%. A preferred upper limit is 1.2%, more preferably 1.0% or less, and even more preferably 0.9% or less.

[0016] Cr: 3.6%≦Cr≦4.4% Cr is a basic element of hot work tool steel that improves hardenability and forms carbides, thereby strengthening the matrix and improving wear resistance and toughness. However, too much Cr can lead to a decrease in hardenability and high-temperature strength. Therefore, the Cr content is set to 3.6%≦Cr≦4.4%. The preferred lower limit of Cr is 3.7%, and more preferably 3.8% or more. The preferred upper limit of Cr is 4.3%, and more preferably 4.2% or less.

[0017] One or both of Mo and W according to the formula (Mo + ½W): (Mo + ½W): 2.1%≦(Mo + ½W)≦3.0%. Mo and W can be added alone or in combination to impart strength by precipitating or agglomerating fine carbides during tempering, thereby improving softening resistance and high-temperature strength. In the present invention, the C content is reduced to improve crack resistance, so a slightly higher Mo and W content can be expected to complement the strength. In this case, since W has approximately twice the atomic weight of Mo, the content can be determined together with the Mo equivalent defined by the formula (Mo + ½W). (Naturally, either one or both may be added.) To achieve the above effect, the content must be 2.1% or more according to the formula (Mo + ½W). The preferred lower limit is 2.2%. However, excessive Mo or W may cause a decrease in machinability and toughness, resulting in a decrease in crack resistance, and also increases the difficulty of melting due to their high melting points, so a large content is not preferable in terms of manufacturing. Therefore, the value according to the relationship (Mo + 1 / 2W) is set to 3.0% or less. The preferred upper limit is 2.8%, more preferably 2.6% or less, and even more preferably 2.5% or less. Here, since W is a more expensive element than Mo, it is preferable to contain Mo alone when cost reduction is important.

[0018] V: 0.20%≦V≦1.0% V forms vanadium carbides, which strengthen the matrix and improve wear resistance and temper softening resistance. When an AM product formed in an AM process is quenched by heating it to a quenching temperature, the vanadium carbides function as "pinning particles" that suppress coarsening of austenite grains during quenching, contributing to improved toughness. However, because V has a high carbide-forming ability, excessive V may convert all C to vanadium carbide, preventing the formation of other carbides. Because hot work tool steels are made up of multiple types of carbides, it is undesirable for the carbide to be solely vanadium carbide. Therefore, the V content is 0.20%≦V≦1.0%. The preferred lower limit is 0.22%, and more preferably 0.24% or more. The upper limit is preferably 0.90%, more preferably 0.70% or less, further preferably 0.50% or less, 0.40% or less, or 0.34% or less.

[0019] Nb: 0.1%≦Nb≦1.0% Nb forms niobium carbide, which strengthens the matrix and improves wear resistance and temper softening resistance. When an additively manufactured product formed in an additive manufacturing process is heated to a quenching temperature for quenching, the niobium carbide acts as a "pinning particle" that suppresses coarsening of austenite grains during quenching heating, contributing to improved toughness. Nb is also an element that contributes to improved softening resistance. However, because Nb has a high carbide-forming ability, if there is too much Nb, all of the C may become niobium carbide, making it impossible to form other carbides. Because tool steels are made up of multiple types of carbides, it is not desirable for the carbide to be solely niobium carbide. Therefore, the Nb content is 0.1%≦Nb≦1.0%. The preferred lower limit is 0.15%. The upper limit is preferably 0.5%, more preferably 0.40% or less, further preferably 0.35% or less, 0.30% or less, or 0.25% or less.

[0020] Balance: Fe and unavoidable impurities. The balance consists of Fe and unavoidable impurities. Typical examples of unavoidable impurities include elements such as P, S, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen), and B (boron). The lowest possible content of these elements is preferable. However, small amounts may be included due to additional effects such as controlling the shape of inclusions, improving other mechanical properties, and improving manufacturing efficiency. In this case, the ranges of Al≦0.04%, Ca≦0.01%, Mg≦0.01%, O≦0.05%, N≦0.05%, and B≦0.05% are sufficient and are the preferred upper limits of the present invention. Furthermore, P and S can conform to the JIS steel grade SKD61, e.g., P≦0.030% and S≦0.020%.

[0021] In the present invention, the overall balance within the above-mentioned component ranges is expected to improve crack resistance during molding, and a hot work tool steel additive manufactured product with excellent softening resistance can be obtained.

[0022] Equation (1): C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + Nb / 20 ≦ 0.75 In addition to the specified components, one of the features of the present invention is adjusting the left side of equation (1) to 0.75 or less. The left side of equation (1) is an improved version of Pcm, which is used as a cold cracking susceptibility index for welding. In equation (1), C, Si, Mn, Cr, Cu, Ni, Mo, W, and V represent the content (mass%) of each element. Similar to additive manufacturing, cracking is a problem in welding. Since both processes involve molten solidification structures, this index was found to be applicable to crack suppression in additive manufacturing and was applied to the present invention. Another known index for welding cracking is the hot cracking index HCS. However, prior research showed that the cracking occurring during manufacturing with this composition system was largely from the surface, differing in form from hot cracking that tends to occur at the solidification interface, etc. Since the cracks observed during molding with this composition are likely to occur in areas where tensile stress due to thermal contraction is likely to occur, and therefore occur at low temperatures, it is assumed that they are similar to cold cracks in welding. Therefore, the cold cracking index Pcm was applied in this invention. By adjusting each main component so that the left side of formula (1) is 0.75 or less, a powder that can further suppress cracks during molding can be obtained. The left side of formula (1) is more preferably 0.72 or less. It is even more preferably 0.70 or less, 0.68 or less, or 0.66 or less.

[0023] Formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ≦ 440 In the present invention, in addition to the component specifications, the above formula (2) can be adjusted to 440 or less. Formula (2) is a relationship formula between elements excluding Co and the Ms point disclosed in the literature (K. Ishida, Journal of Alloys and Compounds, Volume 220 Issue 1-2 1995 pp. 126-131). It is expected that a high value of formula (2) will increase the Ms point, and therefore, during additive manufacturing at high temperatures, the alloy will transform into brittle martensite, which tends to crack easily due to thermal contraction when cooled to room temperature. It is also preferable to adjust formula (2) to 300 or more. If the value of formula (2) is too low, the Ms point is expected to be low. If the value is too low, the martensitic transformation will not be completed, and austenite will remain, resulting in a decrease in strength. A preferred upper limit of formula (2) is 430 or less. More preferably, it is 420 or less, even more preferably, it is 410 or less, and particularly preferably, it is 400 or less. A preferred lower limit of formula (2) is 320 or more. More preferably, it is 340 or more, and even more preferably, it is 360 or more. Elements other than those actively added in the present invention, such as Cu and Ti, and Al, which may be contained in a range of 0.04% or less as an impurity element, may be calculated as zero%.

[0024] The hot work tool steel powder for additive manufacturing of the present invention can be produced by, for example, gas atomization, water atomization, disk atomization, plasma atomization, rotating electrode atomization, etc. Among these, gas atomization is a method in which a melted raw material prepared to have a desired composition is heated to above its melting point by high-frequency induction heating, melted, and then the molten metal that flows out through a fine hole is finely pulverized by injecting an inert gas such as argon gas or nitrogen gas into the melted metal, which is then rapidly cooled and solidified to obtain a powder. This gas atomization method can use scrap metal, raw metal materials, etc. as the melted raw material. Compared to plasma atomization and rotating electrode atomization, which require the preparation of a raw material with a desired composition and shape in advance, this method can be produced at a lower cost and is therefore suitable as a method for obtaining the powder for additive manufacturing of the present invention.

[0025] The hot work tool steel powder for additive manufacturing of the present invention preferably has a 50% particle size (hereinafter referred to as "D50") of a cumulative particle size distribution based on volume of 10 to 250 μm. By setting the D50 of the additive manufacturing powder of the present invention to 250 μm or less, the powder can be easily melted, and the formation of internal defects in the additive manufacturing product can be suppressed. Furthermore, by setting the D50 of the additive manufacturing powder of the present invention to 10 μm or more, the additive manufacturing powder is less susceptible to the influence of humidity and other factors in the atmosphere during handling and additive manufacturing, ensuring good flowability. The cumulative particle size distribution of the additive manufacturing powder of the present invention is expressed as a cumulative volumetric particle size distribution, and the D50 can be expressed as a measurement value obtained by the laser diffraction scattering method specified in JIS Z 8825.

[0026] The D50 of the hot work tool steel powder for additive manufacturing of the present invention may be adjusted by mesh sieving or airflow classification, etc., in accordance with the above-mentioned method. For example, for metal powders used in the powder bed method, while the metal powder is melted by a laser beam as a heat source, coarse metal powder that is difficult to melt must be removed to minimize the area of ​​thermal influence. Furthermore, to obtain optimal fluidity to ensure the laying of the metal powder, highly adhesive fine metal powder must also be removed. For this reason, when applying the metal powder of the present invention to the powder bed method, it is preferable to adjust the D50 to the 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 powder for additive manufacturing of the present invention to a laser metal deposition method, it is preferable to adjust the D50 to the range of 50 to 150 μm.

[0027] By additively manufacturing the hot working tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described below, it is possible to obtain a hot working tool steel powder for additive manufacturing having the following components by mass: 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, and one or two of Mo and W according to the relationship formula (Mo+½W): 2.1 It is possible to obtain a hot work tool steel additive manufactured product (hereinafter also referred to as an additive manufactured product) consisting of C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + 1 / 2 W) / 15 + V / 10 + Nb / 20 ≦ 0.75, where C + Si is 0.20% ≦ (Mo + 1 / 2 W) ≦ 3.0%, 0.20% ≦ V ≦ 1.0%, 0.1 ≦ Nb ≦ 0.4, and the remainder being Fe and unavoidable impurities, and which satisfies formula (1): C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + 1 / 2 W) / 15 + V / 10 + Nb / 20 ≦ 0.75. This additive manufactured product is excellent in that it has high softening resistance.

[0028] The hot work tool steel additive manufactured product of the present invention is expected to exhibit excellent mechanical properties in addition to the softening resistance described above. For example, the hot work tool steel additive manufactured product of the present invention preferably has a room temperature (approximately 20°C) tensile strength of 1000 to 2000 MPa when the tempered hardness is adjusted to 45±1 HRC. A more preferred lower limit is 1200 MPa, an even more preferred lower limit is 1300 MPa, and an even more preferred lower limit is 1400 MPa. The room temperature 0.2% yield strength when the tempered hardness is adjusted to 45±1 HRC is preferably 800 to 2000 MPa. A more preferred lower limit is 800 MPa, and an even more preferred lower limit is 1000 MPa. The room temperature elongation when the tempered hardness is adjusted to 45±1 HRC is preferably 8% or more. A more preferred lower limit is 10%, and an even more preferred lower limit is 12%. When the tempered hardness is adjusted to 45±2 HRC, the room temperature drawing is preferably 30% or more, with the lower limit being more preferably 40%, and even more preferably 50%.

[0029] The hot work tool steel additive manufactured product of the present invention preferably has a high-temperature (approximately 550°C) tensile strength of 600 to 1400 MPa when the tempered hardness is adjusted to 45±1 HRC. 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 tempered hardness is adjusted to 45H±1RC 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 tempered hardness is adjusted to 45±1 HRC is preferably 10% or more. A more preferable lower limit is 13%, and an even more preferable lower limit is 16%. The high-temperature drawing ability when the tempered hardness is adjusted to 45±1 HRC is preferably 30% or more. A more preferred lower limit is 40%, an even more preferred lower limit is 50%, and an even more preferred lower limit is 60%.

[0030] Furthermore, the hot work tool steel additive manufactured product of the present invention has a 2 mm U-notch Charpy impact value of 20 J / cm at room temperature when the tempered hardness is adjusted to 45±1 HRC. ２ A more preferable lower limit is 30 J / cm ２ and a more preferable lower limit is 40 J / cm ２ and an even more preferable lower limit is 50 J / cm ２ is.

[0031] Furthermore, the hot work tool steel additive manufactured product of the present invention preferably has a room temperature thermal conductivity of 10 W / (m·K) or more when the tempered hardness is adjusted to 45±1 HRC. A more preferred lower limit is 15 W / (m·K), an even more preferred lower limit is 20 W / (m·K), and an especially preferred lower limit is 25 W / (m·K).

[0032] The manufacturing method of the present invention includes the steps of: spreading the prepared hot work tool steel powder for additive manufacturing of the present invention in layers; and sequentially melting and solidifying the spread metal powder using a scanning heat source having a diameter larger than the D50 of the metal powder to form solidified layers. The step of spreading the metal powder in layers and the step of forming the solidified layers are then repeated to form multiple solidified layers, thereby producing the additively manufactured product of the present invention. The scanning heat source may be, for example, a laser or an electron beam. Using a scanning heat source with a diameter larger than the D50 of the metal powder is preferable because it allows for uniform melting of the metal powder clusters.

[0033] In the manufacturing method according to the present invention, when the metal powder is irradiated with a laser while being scanned, the laser output can be set to 50 to 400 W, the scanning speed to 200 to 2000 mm / sec, and the scanning pitch to 0.02 to 0.20 mm. Here, if the layer thickness per laser scan is too large, heat is not easily transferred to the entire spread metal powder during laser irradiation, causing the metal powder to not melt sufficiently, which promotes 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 additive manufacturing product increases, lengthening the time required for the additive manufacturing process. For this reason, the layer thickness per scan is preferably set to 10 to 200 μm. A more preferred lower limit of the layer thickness is 20 μm, and a more preferred upper limit of the layer thickness is 100 μm. A preheating step may be carried out before the additive manufacturing process described above. However, since the metal powder of the present invention has improved crack resistance compared to conventional hot work tool steel powders, it is possible to omit or lower the preheating step before additive manufacturing, for example, if the additive manufacturing product is small and has few stress concentration areas.

[0034] In the manufacturing method according to the present invention, it is preferable to subject the as-AM (i.e., the AM product without heat treatment after AM) component to a tempering treatment at a temperature of 500 to 700°C to impart the mechanical properties necessary for use as a metal product. Tempering allows for the creation of an "AM hot work tool" with a predetermined hardness. During this process, the AM product can be shaped into the shape of the hot work tool by various machining processes, such as cutting and drilling. In this case, the AM product formed in the AM process can be annealed to facilitate machining. Annealing can also be expected to refine the vanadium carbide in the structure of the tempered AM hot work tool. Finishing machining may then be performed after tempering. In some cases, this finishing machining can also be performed on the tempered AM product, and the above-mentioned machining processes can be performed all at once to create an AM hot work tool. Note that quenching can be performed before the tempering. Regardless of whether or not the above-mentioned annealing is performed, normalizing can be performed on the additive manufacturing product formed in the additive manufacturing process.

[0035] The tempering temperature varies depending on the target hardness, etc., but is generally around 500 to 700°C. If quenching is performed before tempering, the quenching temperature is generally around 900 to 1100°C. For example, in the case of SKD61, a typical hot work tool steel, the quenching temperature is around 1000 to 1030°C, and the tempering temperature is around 550 to 650°C. 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. Furthermore, it is preferably 40 HRC or more or 380 HV or more. More preferably, it is 42 HRC or more or 400 HV or more. In the present invention, the 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.

[0036] Example 1: Each metal raw material was prepared to have the component composition shown in Table 1, then charged into a high-frequency induction melting furnace and melted. The molten metal was pulverized with argon gas to obtain a gas-atomized powder. The resulting atomized powder was subjected to mesh sieving and airflow classification to adjust the particle size, resulting in powders for additive manufacturing (AM) of the present invention and comparative examples with a D50 of 35 μm. Using each of the AM powders obtained above, AM products were fabricated using an EOS M290 under the fabrication conditions shown in Table 2. Table 3 shows the component compositions of the AM products of Sample No. 3 fabricated from the powder of Sample No. 1 and Sample No. 4 fabricated from the powder of Sample No. 2. Table 4 also shows the component compositions of Sample No. 1 and Sample No. 4 fabricated from the powder of Sample No. 2. The values ​​of formula (1): C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + Nb / 20 in 1 to 4, and the values ​​of formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V are shown.

[0037]

[0038]

[0039]

[0040]

[0041] Next, the tempering behavior of the inventive examples and comparative examples was confirmed. Samples No. 3 and 4 were subjected to tempering heat treatment within the temperature range shown in Figure 1, and their Rockwell hardness was measured in accordance with JIS Z 2245. Figure 1 shows a graph of each tempering temperature and hardness. From Figure 1, it was confirmed that the inventive examples had higher hardness at high temperatures, i.e., higher softening resistance, than the comparative examples.

[0042] (Example 2) Next, the softening behavior of the inventive example and comparative example when held at high temperatures was confirmed. Samples No. 3 and No. 4 from Example 1 described above were subjected to tempering heat treatment and tempered to 45±1 HRC. Then, the samples were held in a heat treatment furnace at 600°C for a given time, removed from the furnace, and cooled to room temperature. Their Rockwell hardness was measured based on JIS Z 2245. This procedure was repeated several times to measure the cumulative holding time at 600°C and hardness, i.e., softening resistance at 600°C. Figure 2 shows the relationship between the cumulative holding time at 600°C and hardness. Figure 2 confirms that the inventive example showed less decrease in hardness than the comparative example even when held at 600°C for a long time, i.e., had high softening resistance even when held in a high-temperature environment for a long time.

[0043] Furthermore, the mechanical properties and thermal conductivity of the inventive examples were confirmed. Additive manufacturing products fabricated under the same conditions as Sample No. 3 in Example 1 were subjected to tempering heat treatment in the temperature range of 500 to 700°C and tempered to 40±1 HRC, 45±1 HRC, and 48±1 HRC. Tensile tests and 2mm U-notch Charpy impact tests were then conducted. Furthermore, after tempering the test specimens to 45±1 HRC, thermal conductivity measurements were conducted using the laser flash method. 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 measurements.

[0044] As shown in Figure 3, the additively manufactured products of the present invention had room-temperature tensile strengths of 1200 MPa or more, room-temperature 0.2% proof stresses of 1000 MPa or more, room-temperature elongations of 13% or more, and room-temperature drawing ratios of 50% or more at all tempered hardnesses. Also, as shown in Figure 4, the additively manufactured products of the present invention had high-temperature tensile strengths of 800 MPa or more, high-temperature 0.2% proof stresses of 600 MPa or more, high-temperature elongations of 13% or more, and room-temperature drawing ratios of 50% or more at all tempered hardnesses. Furthermore, at 45±1 HRC hardness, the additively manufactured products of the present invention had room-temperature tensile strengths of 1400 MPa or more, room-temperature 0.2% proof stresses of 1200 MPa or more, room-temperature elongations of 13% or more, and room-temperature drawing ratios of 60% or more. Furthermore, the additive manufacturing products of the present invention had a high-temperature tensile strength of 900 MPa or more, a high-temperature 0.2% yield strength of 700 MPa or more, a high-temperature elongation of 16% or more, and a high-temperature reduction of 60% or more at a hardness of 45±1 HRC.

[0045] As can be seen from FIG. 5, the additively manufactured products of the present invention have a Charpy impact value of 60 J / cm for all tempered hardnesses. ２ It was confirmed that the Charpy impact value was 60 J / cm even at a hardness of 45±1 HRC. ２ These were favorable values. Furthermore, it was confirmed from Figure 6 that the additively manufactured product of the present invention had a thermal conductivity of 25 W / (m K) or more at room temperature at a hardness of 45±1 HRC. From Figures 3 to 6, it was confirmed that the additively manufactured product of the present invention has properties at the same level as ingot hot work tool steel, and is suitable for hot work tool applications, for example.

[0046] The hot work tool steel powder for additive manufacturing and the additive manufactured product of the present invention are most preferably applied to hot work tool applications such as die-casting molds, but by taking advantage of the various excellent properties of the additive manufactured product of the present invention, it can also be used to repair molds, for example, by using additive manufacturing by powder spraying.Furthermore, by taking advantage of the various excellent properties of the additive manufactured product of the present invention, it may also be applied to molds that require an internal cooling mechanism, such as plastic molds.

Claims

1. A hot work tool steel powder for additive manufacturing, comprising, by mass%, 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship formula (Mo+½W): 2.1%≦(Mo+½W)≦3.0%, 0.20%≦V≦1.0%, 0.1%≦Nb≦1.0%, the balance being Fe and unavoidable impurities, and further satisfying the following formula (1): Formula (1): C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + Nb / 20 ≦ 0.75 Here, each element symbol in formula (1) indicates the content (mass%) of the element.

2. A hot work tool steel additive manufactured product having, in mass%, 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 0.5%≦Ni≦1.5%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo+½W): 2.1%≦(Mo+½W)≦3.0%, 0.20%≦V≦1.0%, 0.1%≦Nb≦1.0%, the balance being Fe and unavoidable impurities, and further satisfying the following formula (1): Formula (1): C + Si / 30 + (Mn + Cr) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + Nb / 20 ≦ 0.75 Here, each element symbol in formula (1) indicates the content (mass%) of the element.

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