Hot work tool steel powder for additive manufacturing and additive-manufactured hot work tool steel article

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

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, but cracks may occur during additive manufacturing depending on the type of additive manufacturing machine, additive manufacturing conditions, additive manufacturing dimensions, etc. For example, in the case of large additive manufacturing dies or additive manufacturing dies that form complex cavities, additive manufacturing dies that have stress concentration areas (recesses) where stress is particularly concentrated tend to be very prone to cracking, and therefore 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 produce hot work tool steel additive manufactured products that have particularly improved crack resistance during additive manufacturing.

[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 containing, 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 (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.5%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.5%≦C≦0.40%, 0.5%≦V≦1.0%, 0.5%≦C≦0.40%, 0.5%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, 2.5%≦(Mo+½W)<3.5%, 0.4 ... . 3%<Cu<0.6%, 0.2%≦Al≦0.9%, with the remainder consisting of Fe and unavoidable impurities, and satisfying formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+½W) / 15+V / 10≦0.95 (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 that 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 two of Mo and W according to the relationship (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, and the balance being Fe and unavoidable impurities, and that satisfies formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+½W) / 15+V / 10≦0.95.

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

[0011] 1 is a schematic diagram of a crack evaluation test piece for evaluating molding crack resistance. FIG. 2 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. 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≦0.40%, 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+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, 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≦0.40%. Carbon (C) is a fundamental element of hot-working tools. 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-working tools). It also has the potential to 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 tool more susceptible to cracking. However, increasing the C content increases hardness but decreases toughness, promoting cracking during molding. In the present invention, the C content is set to 0.10%≦C<0.40% in order to improve crack resistance while maintaining hardness sufficient for use in dies. The preferred lower limit of C is 0.15% or more, more preferably 0.18% or more, even more preferably 0.20% or more, and 0.21% or more. The preferred upper limit of C is 0.35% or less, more preferably 0.30% or less, even 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 chemical composition of molten steel. It is difficult to eliminate Si from the manufacturing process, and the closer we try to eliminate it, the higher the manufacturing costs increase. Therefore, the lower limit of Si is set to 0.01% or more. A preferred lower limit is 0.05% or more, and more preferably 0.08% or more. On the other hand, excessive Si leads to the formation of ferrite in the tool structure after tempering, so the upper limit is set to 0.19% or less. A preferred upper limit is 0.17% or less, more preferably 0.15% or less, and even 0.13% or less.

[0014] Mn: 0.1%≦Mn≦1.0% Mn has the effects of improving hardenability, suppressing the formation of ferrite in the tool structure, and achieving appropriate quench-and-temper hardness. To achieve these effects, the lower limit of Mn is set to 0.1% or more. A preferred lower limit is 0.25%, and more preferably 0.40% or more. On the other hand, too much Mn increases the viscosity of the matrix and reduces the machinability of the material. Therefore, the upper limit is set to 1.0% or less. A 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. It also, along with C, Cr, Mn, Mo, W, and other elements, 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, it can lower the Ms point. In the case of an additive manufacturing device without a temperature control mechanism, the temperature during manufacturing may be close to room temperature depending on the manufacturing conditions. However, lowering the Ms point to room temperature can utilize the effect of mitigating thermal contraction due to martensitic transformation expansion, thereby reducing deformation due to thermal contraction. For this reason, in the present invention, the lower limit of Ni is set to 2.0%. A preferred lower limit is 4.0%. It is more preferably 5.0% or more, and even more preferably 6.0% or more, 7.0% or more, or 7.5% or more. However, excessive Ni increases the viscosity of the matrix, reducing machinability and also increases 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 significantly improve crack resistance, the particularly preferred Ni range is 7.5 to 8.5%. Furthermore, when it is desired to significantly improve thermal conductivity while also ensuring crack resistance, the particularly preferred Ni range is 3.5 to 4.5%. Furthermore, when it is desired to further improve thermal conductivity and yield strength while also ensuring crack resistance, the particularly preferred Ni range is 5.5 to 6.5%.

[0016] Cr: 3.5%<Cr<4.5% Cr is a basic element of hot work tools 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 3.5%<Cr<4.5%. The preferred lower limit of Cr is 3.6% or more, more preferably 3.7% or more, or 3.8% or more. The preferred upper limit of Cr is 4.4% or less, more preferably 4.3% or less, or 4.2% or less.

[0017] One or both of Mo and W according to the formula (Mo + ½W): (Mo + ½W): 2.5%≦(Mo + ½W)<3.5%. 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, since the C content is reduced to improve cracking resistance, 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 2.5% or more according to the formula (Mo + ½W). The preferred lower limit is 2.7% or more, more preferably 2.9% or more, and even more preferably 3.0% or more. 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 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 contain Mo alone when cost reduction is important.

[0018] V: 0.45%≦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 heated to a quenching temperature for quenching, 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. Tool steels are made up of multiple types of carbides, so vanadium carbide alone is not desirable. Therefore, the V content is 0.45≦V≦1.0%. The preferred lower limit is 0.50%, more preferably 0.52% or more, and even more preferably 0.55% or more. The upper limit is preferably 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. It also imparts excellent hardenability to tool materials, along with C, Cr, Mn, Ni, Mo, W, and other elements. It is an effective element for preventing a decrease in toughness by forming a martensite-based structure even when the cooling rate during quenching is slow. To further improve crack resistance, Cu is set to more than 0.3% (0.3%<Cu). Preferably, it is set to 0.35% or more. On the other hand, excessive Cu addition precipitates as a simple element in Fe, reducing toughness. Therefore, Cu is set to less than 1.0% (Cu<0.6%). Preferably, Cu is set to 0.45% or less, and 0.42% or less.

[0020] Al: 0.2%≦Al≦0.9% Al is Ni and Ni ３Al is an element that forms intermetallic compounds like Al and has the effect of precipitation strengthening the metal structure. Since Ni is added in the present invention, adding Al can improve hardness. Therefore, the lower limit of Al is set to 0.2%. A preferred lower limit is 0.3%, and more preferably 0.4% or more. However, if Al is too much, non-metallic inclusions may increase in the metal structure, reducing toughness. Therefore, the upper limit of Al is set to 0.9%. A preferred upper limit is 0.8% or less, more preferably 0.7% or less, and even more preferably 0.6% or less.

[0021] Balance: Fe and unavoidable impurities. The balance 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). 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 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%.

[0022] In the present invention, by achieving an overall balance within the above-mentioned range of components, it is possible to obtain a hot work tool steel additive manufactured product that is particularly excellent in crack resistance during manufacturing.

[0023] Equation (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 ≦0.95 In addition to specifying the components, one of the features of the present invention is adjusting the left side of equation (1) to 0.95 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 molding cracks of this composition system are observed in areas prone to tensile stress due to thermal contraction and occur at low temperatures, it is assumed that they are similar to cold cracks in welding, and the cold cracking index Pcm was applied in this invention. By adjusting each main component so that the left side of equation (1) is 0.95 or less, it is possible to obtain a powder for additive manufacturing that can further suppress cracks during molding. The left side of equation (1) is more preferably 0.92 or less, and even more preferably 0.90 or less.

[0024] Formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ≦ 400 In the present invention, in addition to the component specifications, the above formula (2) can be adjusted to be 400 or less. Formula (2) is a relationship between elements and Ms point disclosed in the literature (K. Ishida, Journal of Alloys and Compounds, Volume 220 Issue 1-2 1995 pp. 126-131), and it is expected that the Ms point will be high if the value of formula (2) is high. Therefore, it will transform into brittle martensite at high temperatures during additive manufacturing and will tend to be prone to cracking due to thermal contraction when cooled to room temperature. Formula (2) is preferably 380 or less, more preferably 360 or less, even more preferably 340 or less, particularly preferably 320 or less, and extremely preferably 300 or less. There is no particular limitation on the lower limit of formula (2), but if the value of formula (2) is too low, it is expected that the Ms point will be low. If it is too low, martensitic transformation will not be completed, and austenite will remain, which may result in a decrease in strength. Therefore, formula (2) is preferably adjusted to 200 or more. It is more preferably 220 or more, even more preferably 240 or more, or 260 or more.

[0025] 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.

[0026] 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 powder for additive manufacturing of the present invention to 250 μm or less, the metal 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 powder for additive manufacturing of the present invention to 10 μm or more, the powder is less susceptible to the influence of moisture and other factors in the atmosphere during handling and additive manufacturing, ensuring good flowability. The cumulative particle size distribution of the powder for additive manufacturing 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.

[0027] 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, when using an additive manufacturing powder for a powder bed method, the powder is melted by a laser beam, which serves as a heat source, while coarse additive manufacturing powder that is difficult to melt must be removed to minimize the range of thermal effects. Furthermore, to obtain optimal fluidity to ensure powder spreadability, highly adhesive fine powder must also be removed. Therefore, when using the additive manufacturing powder of the present invention for 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 using the additive manufacturing powder of the present invention for laser metal deposition, it is preferable to adjust the D50 to the range of 50 to 150 μm.

[0028] By additively manufacturing the hot working tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described below, the following components can be obtained: 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%, and one or two of Mo and W according to the relationship formula (Mo+1 / 2W): 2.5%≦(Mo+1 / 2W). 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 + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + ≦0.95, where C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + ≦0.95, and where C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 + ≦0.95. This additive manufactured product is particularly excellent in that it is less susceptible to cracking during manufacturing.

[0029] The hot work tool steel additive manufactured product of the present invention preferably has a room temperature (approximately 20°C) tensile strength of 500 to 2000 MPa when the tempered hardness is adjusted to 45 HRC ± 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 tempered hardness is adjusted to 45 HRC ± 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 tempered 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 room temperature drawing when the tempered hardness is adjusted to 45 HRC ± 2 is preferably 20% or more. The lower limit is more preferably 30%, and even more preferably 40%. When the tempered hardness is adjusted to 45HRC±2, the room temperature 2 mm U-notch Charpy impact value is 20 J / cm ２ A more preferable lower limit is 30 J / cm ２ and a more preferable lower limit is 40 J / cm ２ is.

[0030] The hot work tool steel additive 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 tempered hardness is adjusted to 45HRC±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 tempered hardness is adjusted to 45HRC±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 tempered 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 high-temperature reduction when the tempered hardness is adjusted to 45HRC±2 is preferably 10% or more. A more preferable lower limit is 20%, and an even more preferable lower limit is 30%. Furthermore, the hot work tool steel additive manufactured product of the present invention preferably has a room temperature thermal conductivity of 5 W / (m K) or more when the tempered hardness is adjusted to 45 HRC±2. A more preferred lower limit is 10 W / (m K), and an even more preferred lower limit is 15 W / (m K).

[0031] The additively manufactured product of the present invention is most preferably applied to die-casting molds, but may also be applied to other molds that require internal cooling mechanisms, such as plastic molds. It may also be applied to repairing molds using additive manufacturing by the powder spray method. Next, an example of a manufacturing process by which the additively manufactured product of the present invention can be obtained using the hot work tool steel powder for additive manufacturing of the present invention will be described in order. The manufacturing process described below assumes the powder bed method, unless otherwise specified.

[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 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 temperature 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 as-AM component without heat treatment after AM) 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 can produce a "hot work tool steel AM product" with a predetermined hardness. During this process, the AM product can be shaped into the shape of a 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 can 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 can be performed all at once to produce 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 gas-atomized powder. The resulting atomized powder was subjected to mesh sieving and airflow classification to adjust the particle size, resulting in additive manufacturing powders for the present invention and comparative examples with a D50 of 35 μm. Additive manufacturing products were produced using the additive manufacturing powders obtained above using an EOS M290 under the manufacturing conditions shown in Table 2. Table 3 shows the component compositions of the additive manufacturing products of Sample No. 5 produced from powder No. 1, Sample No. 6 produced from powder No. 2, Sample No. 7 produced from powder No. 3, and Sample No. 8 produced from powder No. 4. Table 4 also shows the component compositions of Sample No. 1, Sample No. 8, and Sample No. 5. The values ​​of formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 in Tables 1 to 8, 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] Example 2 To evaluate the crack susceptibility of an additively manufactured product, a crack evaluation test piece having the shape shown in FIG. 1 was manufactured. Specifically, the additive manufacturing powders of Samples No. 1 to 4 of Example 1 were additively manufactured under the same manufacturing conditions as in Example 1 to produce Samples No. 9 (Sample No. 5 composition), Sample No. 10 (Sample No. 6 composition), Sample No. 11 (Sample No. 7 composition), and Sample No. 12 (Sample No. 8 composition), which have the same compositions as Samples No. 5 to 8. This crack evaluation test piece was 50 mm long, 10 mm wide, and 16 mm high, with a stress concentration area with an R8 radius created in the middle. This stress concentration area was comb-shaped to make it more susceptible to cracking, and was designed to simulate an additive manufacturing mold for forming complex cavities. After manufacturing, the crack evaluation test piece was measured for the length of the crack in the comb-tooth portion, allowing the material's susceptibility to cracking during manufacturing to be evaluated. Sample No. The crack lengths of the crack test pieces for Samples No. 9, 11, and 12 (Invention Examples) and Sample No. 10 (Comparative Example) are shown in Table 5. From Table 5, it was confirmed that the Inventive Examples had significantly shorter crack lengths than the Comparative Examples, and had particularly superior crack resistance during molding compared to the Comparative Examples. In particular, Sample No. 9 had a crack length of 1.0 mm or less, confirming that it had the best crack resistance.

[0042]

[0043] (Example 3) Next, the tempering behavior of the inventive example and the comparative example was confirmed. Samples Nos. 5 to 8 shown in Example 1 above were subjected to tempering heat treatment within the temperature range shown in Figure 2, and their Rockwell hardness was measured in accordance with JIS Z 2245. Figure 2 shows a graph of each tempering temperature and hardness. From Figure 2, it was confirmed that Samples Nos. 5, 7, and 8, which are inventive examples, could 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 inventive examples were confirmed. Additive manufacturing products fabricated under the same conditions as Samples No. 5, No. 7, and No. 8 (all inventive examples) in Example 1 were subjected to tempering heat treatment in the temperature range of 500 to 700°C to temper the test specimens to 40±2 HRC, 45±2 HRC, and 52±2 HRC, respectively. Tensile tests, 2mm U-notch Charpy impact tests, and thermal conductivity measurements using the laser flash method were then 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 measurements.

[0045] As shown in Figure 3, the additively manufactured products of the examples of the present invention had room-temperature tensile strengths of 1100 MPa or more, room-temperature 0.2% proof stresses of 600 MPa or more, room-temperature elongations of 12% 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 examples of the present invention had high-temperature tensile strengths of 700 MPa or more, high-temperature 0.2% proof stresses of 400 MPa or more, high-temperature elongations of 13% or more, and high-temperature drawing ratios of 35% or more at all tempered hardnesses. Furthermore, at 45±2 HRC hardness, the additively manufactured products of the examples of the present invention had room-temperature tensile strengths of 1200 MPa or more, room-temperature 0.2% proof stresses of 1100 MPa or more, room-temperature elongations of 13% or more, and room-temperature drawing ratios of 50% or more. Furthermore, the additive manufacturing products of the present invention had a high-temperature tensile strength of 800 MPa or more, a high-temperature 0.2% yield strength of 500 MPa or more, a high-temperature elongation of 15% or more, and a high-temperature reduction of 35% or more at a hardness of 45±2 HRC.

[0046] As can be seen from FIG. 5, the additively manufactured products of the present invention have a Charpy impact value of 50 J / cm for all tempered hardnesses. ２ It was confirmed that the thermal conductivity at room temperature was 15 W / (m K) or more. Furthermore, from Figure 6, it was confirmed that the additively manufactured products of the present invention had a thermal conductivity of 15 W / (m K) or more at room temperature. In particular, it was confirmed that Sample No. 7 had a thermal conductivity of 20 W / (m K) or more at room temperature, and had the most excellent thermal conductivity characteristics. From Figures 3 to 6, it was confirmed that the additively manufactured products of the present invention have properties at the same level as ingot hot work tool steel, and are suitable for hot work tool applications, for example.

Claims

1. A 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+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, the balance being Fe and unavoidable impurities, and further satisfying the following formula (1): Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+½W) / 15+V / 10≦0.95 wherein 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≦0.40%, 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 consisting of Fe and unavoidable impurities, and further satisfying the following formula (1): Formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+½W) / 15+V / 10≦0.95 wherein each element symbol in formula (1) indicates the content (mass%) of the element.

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