Hot work tool steel powder for deposition modeling and hot work tool steel deposition model

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

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 AM have been proposed. However, cracks may occur during AM depending on the type of AM machine, AM conditions, AM dimensions, etc. For example, in large AM molds or AM molds for forming complex cavities, AM molds with stress concentration areas (recesses) tend to be prone to cracking during AM. Furthermore, depending on the application, particularly high toughness may be required. For example, when AM molds with complex shapes are manufactured due to the recent trend toward sophisticated product designs, high toughness tends to be required to prevent large cracks in stress concentration areas. Therefore, an object of the present invention is to provide a hot work tool steel powder for AM that can further improve crack resistance during AM and can produce hot work tool steel AM products with excellent toughness.

[0008] The present invention has been made in view of the above-mentioned problems. That is, one aspect of the present invention is a method for producing a cellulose acylate having a viscosity of 1000 ppm or less, the viscosity being 1000 ppm or less, and the viscosity being 1000 ppm or less. This hot work tool steel powder for additive manufacturing 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 two of Mo and W according to the relationship formula (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.1%≦Cu≦1.0%, and the remainder being Fe and unavoidable impurities, and satisfies formula (1): C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+(Mo+½W) / 15+V / 10≦0.80 (each element symbol in formula (1) indicates the content (mass%) of the element in question).

[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%, 0.3≦Ni<1.0%, 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%, 0.1%≦Cu≦1.0%, 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≦0.80.

[0010] The present invention provides a hot work tool steel powder for additive manufacturing, which can further improve crack resistance during additive manufacturing and can produce hot work tool steel additive manufactured products that also have excellent toughness.

[0011] 1 is a schematic diagram of a crack evaluation test piece for evaluating molding crack resistance. 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. 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. 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 temperature. 5 is a graph showing the Charpy impact value of an example of the present invention at room temperature. 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%, 0.3≦Ni<1.0%, 3.5%<Cr<4.5%, 2.5%<(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.1%≦Cu≦1.0%, with the balance being Fe and unavoidable impurities. First, the reasons for the composition 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, "%" represents "mass %." Additive manufacturing may also be referred to simply 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. A preferred lower limit for C is 0.15%, more preferably 0.18% or more, and even more preferably 0.20% or more. A preferred upper limit for C is 0.35%, more preferably 0.30% or less, even more preferably 0.27% or less, 0.25% or less, or 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%. 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 tool structure after tempering, so the upper limit is set to 0.19%. A preferred upper limit is 0.17%, and more preferably 0.15% or less, and 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%. A preferred lower limit is 0.25%, and more preferably 0.40% or more. On the other hand, if there is too much Mn, it increases the viscosity of the matrix and reduces the machinability of the material, so the upper limit is set to 1.0%. The content is preferably 0.7% or less, more preferably 0.6% or less, and even more preferably 0.55% or less.

[0015] Ni: 0.3%≦Ni<1.0% Ni 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, 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. Furthermore, Ni also improves the intrinsic toughness of the matrix, so the lower limit of Ni is set to 0.3%. It is preferably 0.5% or more, and more preferably 0.7% or more. However, because excessive Ni increases the viscosity of the matrix and reduces machinability, the upper limit is set to less than 1.0%. It is preferably 0.90% or less, and more preferably 0.85% or less.

[0016] Cr: 3.5%<Cr<4.5% Cr is a basic element of hot work tools that enhances 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%, more preferably 3.7% or more, and 3.8% or more. The preferred upper limit of Cr is 4.4%, more preferably 4.3% or less, and 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 content of one or both of Mo and W 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). (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). A more preferable lower limit is 2.7%, and even more preferably 2.9% or more, or 3.0% or more. However, if Mo or W is too much, it may cause a decrease in machinability and toughness, resulting in a decrease in crack resistance, and since it has a high melting point and is difficult to melt, a large content is not preferable in manufacturing. Therefore, the value according to the relationship (Mo + 1 / 2W) is set to less than 3.5%. Preferably, it is 3.4% or less, more preferably 3.3% or less, or 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: 0.1%≦Cu≦1.0% 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. On the other hand, excessive Cu addition causes Cu to precipitate as an element in Fe, reducing toughness. Therefore, the Cu content is set to 0.1%≦Cu≦1.0%. The preferred lower limit is 0.2% or more, more preferably 0.30% or more, 0.33% or more, or 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, or 0.42% or less.

[0020] Balance: Fe and unavoidable impurities. The balance consists of Fe and unavoidable impurities. Typical examples of unavoidable impurities include P, S, Al, Ca, Mg, O (oxygen), N (nitrogen), and B (boron). The lowest possible content of these elements is preferable. However, small amounts are acceptable 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. The Al content is more preferably 0.025% or less. 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, 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 having particularly excellent toughness.

[0022] Equation (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 ≦0.80 In addition to specifying the components, one of the features of the present invention is adjusting the left side of equation (1) to 0.80 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 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 0.80 or less, it is possible to obtain a powder that can further suppress cracks during molding. The left side of equation (1) is more preferably 0.78 or less, and even more preferably 0.76 or less.

[0023] Formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V ≦ 500 In the present invention, in addition to the component specifications, the above formula (2) can be adjusted to 500 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). It is expected that a high value of formula (2) will increase the Ms point, and therefore, the alloy will transform into brittle martensite at high temperatures during additive manufacturing, and will tend to crack due to thermal contraction when cooled to room temperature. Formula (2) is preferably 450 or less, more preferably 400 or less, and even more preferably 390 or less. Although the lower limit of formula (2) is not particularly limited, if the value of formula (2) is too low, it is expected that the Ms point will be low. If the value is too low, martensitic transformation will not be completed, and austenite will remain, which may result in a decrease in strength. Therefore, it is preferable to adjust the lower limit of formula (2) to 200 or more. It is more preferably 250 or more, and even more preferably 300 or more, or 350 or more.

[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, and can be produced at a lower cost than plasma atomization or rotating electrode atomization, which require the preparation of a raw material with a desired composition and shape in advance. Therefore, it is suitable as a method for obtaining the metal 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 metal powder for additive manufacturing 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 metal powder for additive manufacturing of the present invention to 10 μm or more, the metal powder is less susceptible to the influence of moisture and other factors in the atmosphere during handling and additive manufacturing, thereby 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 measured by the laser diffraction scattering method specified in JIS Z 8825.

[0026] The D50 of the metal powder 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 a metal powder for additive manufacturing in a powder bed process, the metal powder is melted by a laser beam, which serves as a heat source, while coarse metal powder that is difficult to melt must be removed to minimize the area of ​​thermal influence. Furthermore, in order to obtain optimal fluidity to ensure the layability of the metal powder, highly adhesive fine metal powder must also be removed. Therefore, when using the powder for additive manufacturing of the present invention in a powder bed process, it is preferable to adjust the D50 to a range of 10 to 53 μm. The preferred upper limit of D50 is 40 μm, and the preferred lower limit of D50 is 20 μm. When using the powder for additive manufacturing of the present invention in a laser metal deposition process, it is preferable to adjust the D50 to a 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, the following contents can be obtained: 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%, and one or two of Mo and W according to the relational expression (Mo+1 / 2W): 2.5%≦( It is possible to obtain a hot work tool steel additively manufactured product (hereinafter also referred to simply as additively manufactured product) consisting of C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 + ≦0.80, where C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + 1 / 2W) / 15 + V / 10 + ≦0.80, with the remainder consisting of Fe and unavoidable impurities. This additively manufactured product is excellent in that it is less susceptible to cracking during manufacturing and has good toughness. Here, the toughness in the present invention can be evaluated by the Charpy impact value. A preferred Charpy impact value is 30 J / cm ２ More preferably, 50 J / cm ２ or more, more preferably 100 J / cm ２ More preferably, 200 J / cm ２ Preferably, the Charpy impact value at room temperature (about 20°C) when the tempered hardness is adjusted to 45±2HRC is 150 J / cm ２ 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 additive manufactured product of the present invention is expected to exhibit excellent mechanical properties in addition to the above-mentioned manufacturing cracking properties and toughness. 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±2 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% proof stress when the tempered hardness is adjusted to 45±2 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±2 HRC is preferably 8% or more. A more preferred lower limit is 10%, and an even more preferred lower limit is 12%. The room temperature drawing ratio when the tempered hardness is adjusted to 45±2 HRC is preferably 30% or more, with the lower limit being more preferably 40%, even more preferably 50%, and even more preferably 60%.

[0029] 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 600 to 1400 MPa when the tempered 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 tempered 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 tempered 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 drawing ability when the tempered hardness is adjusted to 45±2HRC 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 preferably has a room temperature thermal conductivity of 10 W / (m·K) or more when the tempered hardness is adjusted to 45±2 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).

[0031] While die-casting molds are the most preferred application example for this additively manufactured product, it may also be applicable to other molds that require internal cooling mechanisms, such as plastic molds. It may also be applicable to the repair of molds using additive manufacturing by the powder spray method. Next, an example of a manufacturing process that can obtain a hot-work tool steel additively manufactured product of the present invention 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 (hereinafter also referred to as "metal powder") 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. Furthermore, it is preferable to use a scanning heat source with a diameter larger than the D50 of the metal powder, since this allows for uniform melting of the metal powder aggregates.

[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 product to be manufactured is small and has few stress concentration areas.

[0034] In the manufacturing method according to the present invention, in order to impart the mechanical properties necessary for use as a metal product, 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. 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, to facilitate machining, the AM product formed in the AM process can be annealed. Annealing can also be expected to have the effect of refining the vanadium carbide in the structure of the AM hot work tool after tempering. 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 produce a 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 obtained metal powders for additive manufacturing, AM products were produced using an EOS M290 under the manufacturing conditions shown in Table 2. Table 3 shows the component compositions of the molded products of Sample No. 3 made from powder Sample No. 1 and Sample No. 4 made from powder Sample No. 2. Table 4 also shows the component compositions of the molded products of Sample No. 1 and Sample No. 4 made from powder Sample No. 2. The values ​​of formula (1): C + Si / 30 + (Mn + Cr + Cu) / 20 + Ni / 60 + (Mo + ½W) / 15 + V / 10 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] Example 2 To evaluate the crack susceptibility of an additively manufactured product, a crack evaluation test piece as shown in Figure 1 was manufactured. Specifically, the additive manufacturing powders of Samples No. 1 and 2 of Example 1 were additively manufactured under the same manufacturing conditions as Example 1, resulting in Example No. 5 (No. 3 composition) of the present invention and Example No. 6 (No. 4 composition) of the comparative example, which have the same compositions as Samples No. 3 and No. 4. 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 along the middle. This stress concentration area was comb-shaped to facilitate 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 at the comb-tooth portion, allowing the susceptibility of the material to crack during manufacturing to be evaluated. The crack lengths of the crack evaluation test pieces of Sample No. 5 (Invention Example) and Sample No. 6 (Comparative Example) are shown in Table 5. From Table 5, it was confirmed that the crack length of the inventive example was shorter than that of the comparative example, and that the inventive example was equally or more resistant to cracking than the comparative example.

[0042]

[0043] (Example 3) Next, the tempering behavior of the inventive example and the comparative example was confirmed. Samples No. 3 and 4 obtained by the manufacturing method shown in Example 1 above were subjected to tempering heat treatment in the temperature range shown in Figure 2, and their Rockwell hardness was measured based on JIS Z 2245. Figure 2 shows a graph of each tempering temperature and hardness. From Figure 2, it was confirmed that inventive example No. 3 could be tempered to a higher hardness than comparative example No. 4.

[0044] Example 4 Next, the toughness of the additively manufactured products 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 were produced under additive manufacturing conditions of laser power: 300 W, scanning speed: 800 mm / s, and layer thickness: 40 μm. Sample No. 7 (similar composition to Sample No. 3) and Comparative Example Sample No. 8 (similar composition to Sample No. 4) were produced. Sample No. 7 was tempered at 605-610°C to achieve a hardness of 45±2 HRC. Sample No. 8 was tempered at 550°C to achieve the same hardness of 45±2 HRC as Sample No. 4. Hardness was measured using a Rockwell hardness tester conforming to JIS Z 2245. Subsequently, the obtained Sample No. Charpy impact test specimens were taken from Sample No. 7 and Sample No. 8 in the direction horizontal to the lamination direction. The obtained test specimens were subjected to a 2 mm U-notch Charpy impact test in accordance with JIS Z 2242. The toughness results obtained are shown in Table 6. From the results in Table 6, it can be seen that the inventive example had a Charpy impact value of 200 J / cm for a test specimen tempered to a hardness of 45±2 HRC. ２ From the above, it was confirmed that the toughness was superior to that of the comparative example.

[0045]

[0046] Furthermore, the mechanical properties and thermal conductivity of the inventive example were confirmed. Test specimens prepared under the same conditions as Sample No. 3 (inventive example) in Example 1 were subjected to tempering heat treatment in the temperature range of 500 to 700°C to temper the specimens to 40±1 HRC, 45±1 HRC, and 49±1 HRC. Tensile tests and 2-mm U-notch Charpy impact tests were then conducted. Furthermore, after tempering the 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.

[0047] As shown in Figure 3, 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 1000 MPa or more, room-temperature elongations of 13% or more, and room-temperature drawing ratios of 60% 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 800 MPa or more, high-temperature 0.2% proof stresses of 600 MPa or more, high-temperature elongations of 13% or more, and high-temperature drawing ratios of 50% or more at all tempered hardnesses. Furthermore, at 45±1 HRC hardness, the additively manufactured products of the examples 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 18% or more, and a high-temperature reduction of 60% or more at a hardness of 45±1 HRC.

[0048] As can be seen from FIG. 5, the additively manufactured products of the present invention have a Charpy impact value of 100 J / cm for all tempered hardnesses. ２ It was confirmed that the Charpy impact value was 150 J / cm or more, especially at hardnesses of 40±1 HRC and 45±1 HRC. ２ These were favorable values. Furthermore, Figure 6 confirms 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 hot work tool steel, which is a conventional ingot material, and is suitable for use in hot work tools, 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%, 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%, 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.

80. 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≦0.40%, 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%, 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 + ½W) / 15 + V / 10 ≦ 0.

80. Here, each element symbol in formula (1) indicates the content (mass%) of the element.

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