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

Abstract

Provided is hot work tool steel powder for additive manufacturing with which an additively manufactured hot work tool steel article having excellent cracking resistance during fabrication can be fabricated.　In this hot work tool steel powder for additive manufacturing and this additively manufactured hot work tool steel article, in terms of mass%, 0.10%≤C≤0.24%, 0.01%≤Si≤0.50%, 0.01%≤Mn≤0.19%, 3.6%≤Cr≤4.4%, 2.0%≤(Mo+1 / 2W)≤3.4% for one or both of Mo and W according to the relationship (Mo+1 / 2W), and 0.2%≤V≤0.9%, the remainder consisting of Fe and unavoidable impurities, and expression (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≤0.63 is satisfied.

Description

Technical Field

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

Background Art

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

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

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

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

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

[0007] As described above, several types of steel for additive manufacturing have been proposed, but cracking sometimes occurs during additive manufacturing depending on the type of additive manufacturing machine, additive manufacturing conditions, and additive manufacturing dimensions. For example, in large additive manufacturing molds or additive manufacturing molds that form complex cavities, there are stress concentration points (recesses), and these areas are particularly prone to stress concentration, making them very susceptible to cracking. Therefore, further improvement in crack resistance is required. Therefore, the object of the present invention is to provide a hot tool steel powder for additive manufacturing that can produce hot tool steel additively manufactured products with improved crack resistance during additive manufacturing. [Means for solving the problem]

[0008] This invention was made in view of the above-mentioned problems. In other words, one aspect of the present invention is a hot work tool steel powder for additive manufacturing that, in mass%, consists of 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo+1 / 2W): 2.0%≦(Mo+1 / 2W)≦3.4%, 0.2%≦V≦0.9%, with the remainder being Fe and unavoidable impurities, and satisfies formula (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≦0.63 (where 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 additively manufactured product that, in mass%, consists of 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo+1 / 2W): 2.0%≦(Mo+1 / 2W)≦3.4%, 0.2%≦V≦0.9%, with the remainder being Fe and unavoidable impurities, satisfying formula (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≦0.63. [Effects of the Invention]

[0010] According to the present invention, it is possible to obtain a hot tool steel powder for additive manufacturing that can be used to manufacture hot tool steel additively manufactured products with particularly excellent crack resistance during additive manufacturing. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of a crack evaluation test specimen used to assess the crackability of the molded part. [Figure 2] This is a graph showing the tempering temperature and hardness of an additively manufactured hot tool steel product according to an example of the present invention. [Figure 3] This graph shows the mechanical properties ((a) 0.2% yield strength, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention at room temperature. [Figure 4] This graph shows the high-temperature mechanical properties ((a) 0.2% yield strength, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention. [Figure 5] This graph shows the Charpy impact values ​​at room temperature for an example of the present invention. [Figure 6] This is a graph showing the thermal conductivity of an example of the present invention. [Modes for carrying out the invention]

[0012] The present invention has a component composition consisting of 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo+1 / 2W): 2.0%≦(Mo+1 / 2W)≦3.4%, 0.2%≦V≦0.9%, with the remainder being Fe and unavoidable impurities. First, the reasons for limiting the composition of the hot tool steel powder for additive manufacturing (hereinafter also referred to as additive manufacturing powder or metal powder) as defined in the present invention will be explained. Unless otherwise specified, "%" represents "mass%". Additive manufacturing may also be simply referred to as "manufacturing". C: 0.10% ≤ C ≤ 0.24% Carbon (C) is a fundamental element of hot work tool steel. Part of it dissolves into the substrate to provide strength, while part of it forms carbides to enhance wear resistance and seizure resistance. Furthermore, C dissolved as interstitial atoms, when added together with substitutional atoms that have a high affinity for C, such as Cr, is expected to contribute to the I(interstitial atom)-S(substitutional atom) effect (acting as drag resistance of solute atoms and increasing the strength of hot work tools), and it is also an element that can improve hardenability. If the C level is too low, a ferrite phase will mainly form during solidification, and since the main phase will remain the ferrite phase until room temperature, quenching, which requires rapid cooling from the austenite phase, becomes impossible. If the main phase remains the ferrite phase from immediately after solidification until room temperature, the mitigation of thermal contraction using martensitic transformation expansion cannot be achieved, making it prone to cracking. However, while a high C level improves hardness, it reduces toughness, which exacerbates cracking during molding. In this invention, the aim is to maintain hardness sufficient for mold use while improving crack resistance, so the C value is set to 0.10% ≤ C ≤ 0.24%. The preferred lower limit of C is 0.13%, more preferably 0.15% or higher, even more preferably 0.16% or higher, and 0.17% or higher. The preferred upper limit of C is 0.23%, more preferably 0.22% or lower.

[0013] Si: 0.01% ≤ Si ≤ 0.50% Si can be used as a deoxidizing agent when adjusting the composition of molten steel, and it is difficult to manufacture without it. Furthermore, the closer one tries to eliminate it, the higher the manufacturing cost becomes, so the amount should be 0.01% or more. The preferred lower limit is 0.03%, and more preferably 0.05% or more. On the other hand, too much Si can lead to the formation of ferrite in the structure after tempering, so the upper limit should be 0.50% or less. The preferred upper limit is 0.30% or less, more preferably 0.20% or less, and 0.15% or less.

[0014] Mn: 0.01% ≤ Mn ≤ 0.19% Mn enhances hardenability, suppresses the formation of ferrite in the structure after tempering, and has the effect of obtaining appropriate hardening and tempering hardness. To obtain these effects, the lower limit of Mn is set at 0.01%. The preferred lower limit is 0.02%, more preferably 0.03% or more, still more preferably 0.04% or more, and particularly 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 at 0.19%. The preferred upper limit is 0.18%, more preferably 0.17% or less, and 0.15% or less.

[0015] Cr: 3.6% ≤ Cr ≤ 4.4% Cr is a basic element of hot work tool steel that enhances hardenability and forms carbides, which is effective in strengthening the matrix, improving wear resistance, and toughness. However, if it is too much, it will cause a decrease in hardenability and high-temperature strength. Therefore, Cr is set at 3.6% ≤ Cr ≤ 4.4%. The preferred lower limit of Cr is 3.7%, more preferably 3.8% or more. The preferred upper limit of Cr is 4.3%, more preferably 4.2% or less.

[0016] (Mo + 1 / 2W) of one or two of Mo and W according to the relational expression: (Mo + 1 / 2W): 2.0% ≤ (Mo + 1 / 2W) ≤ 3.4% Mo and W can be included individually or in combination to impart strength by precipitating or agglomerating fine carbides through tempering, thereby improving softening resistance and high-temperature strength. In this invention, since C is kept low to improve crack resistance, slightly higher concentrations of Mo and W can be expected to complement the strength. The content in this case can be specified together using the Mo equivalent defined by the relationship (Mo + 1 / 2W), since W has approximately twice the atomic weight of Mo (of course, either one or both can be included). To obtain the above effect, the content should be 2.0% or more according to the value of the relationship (Mo + 1 / 2W). A more preferable upper limit is 2.1%, and even more preferable is 2.2% or more. However, too much Mo or W may lead to a decrease in machinability and toughness, resulting in a decrease in crack resistance, and the high melting point makes melting difficult, so large amounts are undesirable from a manufacturing perspective. Therefore, the value obtained by the relationship (Mo + 1 / 2W) should be 3.4% or less. The preferred upper limit is 3.0%, more preferably 2.8% or less, even more preferably 2.6% or less, and 2.5% or less. Here, since W is a more expensive element than Mo, it is preferable to include Mo alone if cost reduction is a priority.

[0017] V: 0.2% ≤ V ≤ 0.9% V forms vanadium carbides, which have the effect of strengthening the matrix, improving wear resistance, and tempering softening resistance. Furthermore, when the additively manufactured product formed in the additive manufacturing process is heated to the quenching temperature for "quenching," the vanadium carbides also act as "pinning particles" that suppress the coarsening of austenite crystal grains during quenching heating, contributing to improved toughness. However, since V has a high carbide-forming ability, if there is too much V, all of the carbon (C) may become vanadium carbides, making it impossible to form other carbides. Hot work tool steel is made up of multiple types of carbides, so it is undesirable for the carbides to be only vanadium carbides. Therefore, the limit is set to 0.2% ≤ V ≤ 0.9%. The preferred lower limit is 0.25%, more preferably 0.30% or more, and even more preferably 0.35% or more. Furthermore, the preferred upper limit is 0.80%, more preferably 0.60% or less, even more preferably 0.50% or less, and 0.45% or less.

[0018] Remainder: Fe and unavoidable impurities The remainder 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), and it is preferable that these elements be present in the lowest possible amounts. However, small amounts may be included for additional effects such as control of inclusion morphology, improvement of other mechanical properties, and improvement of manufacturing efficiency. In this case, a range of Al ≤ 0.04%, Ca ≤ 0.01%, Mg ≤ 0.01%, O ≤ 0.05%, N ≤ 0.05%, and B ≤ 0.05% is sufficient and represents the preferred regulatory upper limit of the present invention. Furthermore, P and S can conform to the JIS steel grade SKD61, for example, P ≤ 0.030% and S ≤ 0.020%.

[0019] In this invention, by achieving an overall balance within the above-described component range, it is possible to obtain hot-work tool steel additively manufactured products with particularly excellent crack resistance during molding.

[0020] Formula (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≦0.63 In addition to specifying the components, one of the features of this invention is that the left side of formula (1) is adjusted to be 0.63 or less. The left side of formula (1) is an improved version of Pcm, which is used as a low-temperature cracking susceptibility index for welding, and C, Si, Mn, Cr, Cu, Ni, Mo, W, and V in formula (1) represent the content (mass%) of each element. Cracking is a problem in welding as well as in additive manufacturing, and since both are molten solidification structures, it was found that this index can be applied to crack suppression in additive manufacturing and was applied to this invention. Here, another cracking index for welding is the high-temperature cracking index HCS, but from preliminary studies, it was found that the cracking in this composition system was large cracks from the surface, and the morphology differed from high-temperature cracks that tend to occur at solidification interfaces, etc. Since the cracking in this composition system is observed in parts where tensile stress due to thermal shrinkage is likely to occur, it is presumed to occur at low temperatures and is similar to low-temperature cracking in welding, and therefore the low-temperature cracking index Pcm was applied in this invention. In this invention, in particular to suppress cracking during additive manufacturing, the left side of formula (1) is set to 0.63 or less. Preferably, it is 0.62 or less, and 0.61 or less.

[0021] Formula (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V≦450 In this invention, in addition to specifying the components, the above formula (2) can be adjusted to be 450 or less. Formula (2) is the relationship between elements excluding Co and the Ms point disclosed in the literature (K. Ishida, Journal of Alloys and Compounds, Volume 220 Issues 1-2 1995 p126-131). If the value of formula (2) is high, the Ms point is expected to be high, so at high temperatures during additive manufacturing, it is likely to transform into martensite, which is brittle at high temperatures, and become prone to cracking due to thermal shrinkage when cooled to room temperature. Furthermore, it is preferable to adjust formula (2) to be 200 or higher. If the value of formula (2) is too low, the Ms point is expected to be low, so if it is too low, the martensite transformation will not be completed, austenite will remain, and the strength will decrease. The preferred upper limit of formula (2) is 440 or less. More preferably, it is 430 or less, and even more preferably, 420 or less. The preferred lower limit of formula (2) is 240 or more. More preferably, it is 260 or higher, and even more preferably 280 or higher. Elements other than those actively added in this invention, such as Cu, Ti, and impurity elements like Al, which may be present in amounts of 0.04% or less, can be calculated as 0%.

[0022] The hot work tool steel powder for additive manufacturing of the present invention can be manufactured by methods such as gas atomization, water atomization, disk atomization, plasma atomization, and rotary electrode methods. In particular, the gas atomization method involves heating and melting a molten raw material prepared to have a desired component composition above its melting point using high-frequency induction heating, and then injecting an inert gas such as argon gas or nitrogen gas into the molten metal that flows out through pores to finely pulverize the molten metal, which is then rapidly cooled and solidified to obtain powder. This gas atomization method can use scrap metal or crude metal raw materials as the molten raw material, and can be manufactured at a lower cost compared to methods such as plasma atomization and rotary electrode methods, which require the preparation of raw materials with a desired component composition and shape in advance, making it a suitable method for obtaining the metal powder for additive manufacturing of the present invention.

[0023] The hot tool steel powder for additive manufacturing of the present invention preferably has a 50% particle size (hereinafter referred to as "D50") of the volume-based cumulative particle size distribution of 10 to 250 μm. By setting the D50 of the metal powder for additive manufacturing of the present invention to 250 μm or less, the powder melting becomes easier, and the formation of internal defects in the additively manufactured product can be suppressed. Furthermore, by setting the D50 of the metal powder for additive manufacturing of the present invention to 10 μm or more, the handling of the metal powder and the atmosphere during additive manufacturing become less susceptible to the effects of moisture and other factors, thereby ensuring good fluidity. Furthermore, the cumulative particle size distribution of the molding powder of the present invention is expressed as a cumulative volume particle size distribution, and its D50 can be expressed by a measurement value obtained by the laser diffraction scattering method as specified in JIS Z 8825.

[0024] The hot tool steel powder for additive manufacturing of the present invention may have its D50 adjusted by sieving classification using a mesh or airflow classification, in accordance with the method described above. For example, in the powder bed method, the metal powder is melted by a laser beam which serves as the heat source, while coarse metal powder that is difficult to melt must be removed in order to minimize the thermally affected area. In addition, fine metal powder with high adhesion must also be removed in order to obtain optimal fluidity to ensure the layability of the metal powder. For this reason, when applying the metal powder of the present invention to the powder bed method, it is preferable to adjust the D50 to a range of 10 to 53 μm. The preferred upper limit of D50 is 40 μm, and the preferred lower limit of D50 is 20 μm. Furthermore, when applying the additive manufacturing powder of the present invention to the laser metal deposition method, it is preferable to adjust the D50 to a range of 50 to 150 μm.

[0025] By additive manufacturing the hot tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described later, a hot tool steel additive manufacturing product (hereinafter also referred to as an additive manufacturing product) can be obtained that, in mass%, consists of 0.10%≦C≦0.24%, 0.01%≦Si≦0.50%, 0.01%≦Mn≦0.19%, 3.6%≦Cr≦4.4%, one or two of Mo and W according to the relationship (Mo+1 / 2W): 2.0%≦(Mo+1 / 2W)≦3.4%, 0.2%≦V≦0.9%, with the remainder being Fe and unavoidable impurities, satisfying equation (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10+≦0.63. This additive manufacturing product is particularly excellent in that it is less prone to cracking (forming cracks) during additive manufacturing.

[0026] The hot-work tool steel additively manufactured product of the present invention is expected to exhibit excellent mechanical properties in addition to the above-mentioned cracking characteristics. For example, the hot-work tool steel additively manufactured product of the present invention preferably has a room temperature (around 20°C) tensile strength of 1000 to 2000 MPa when the temper hardness is adjusted to 45 ± 1 HRC. A more preferable lower limit is 1200 MPa, an even more preferable lower limit is 1300 MPa, and an even more preferable lower limit is 1400 MPa. The room temperature 0.2% yield strength when the temper hardness is adjusted to 45 ± 1 HRC is preferably 800 to 2000 MPa. A more preferable lower limit is 800 MPa, and an even more preferable lower limit is 1000 MPa. The room temperature elongation when the temper hardness is adjusted to 45 ± 1 HRC is preferably 8% or more. A more preferable lower limit is 10%, and an even more preferable lower limit is 12%. When the temper hardness is adjusted to 45±2HRC, the room temperature drop is preferably 30% or more. A more preferable lower limit is 40%, and an even more preferable lower limit is 50%.

[0027] The hot-work tool steel additively manufactured product of the present invention preferably has a high-temperature (approximately 550°C) tensile strength of 600 to 1400 MPa when the temper 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 temper hardness is adjusted to 45 H ± 1 RC is preferably 600 to 1200 MPa. A more preferable lower limit is 700 MPa, and an even more preferable lower limit is 800 MPa. The high-temperature elongation when the temper hardness is adjusted to 45 ± 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 reduction when the temper hardness is adjusted to 45 ± 1 HRC is preferably 30% or more. A more preferable lower limit is 40%, an even more preferable lower limit is 50%, and an even more preferable lower limit is 60%.

[0028] Furthermore, the hot-work tool steel additively manufactured product of the present invention, when tempered to a hardness of 45±1HRC, has a 2mmU-notch Charpy impact value at room temperature of 30 J / cm². 2 It is preferable that the value is above this. A more preferable lower limit is 50 J / cm². 2 A more preferable lower limit is 80 J / cm². 2 A more preferable lower limit is 100 J / cm². 2 Therefore, a particularly preferred lower limit is 150 J / cm². 2 That is the case.

[0029] Furthermore, the additively manufactured hot tool steel product of the present invention preferably has a room-temperature thermal conductivity of 10 W / (m·K) or higher when the temper hardness is adjusted to 45 ± 1 HRC. A more preferable lower limit is 15 W / (m·K), an even more preferable lower limit is 20 W / (m·K), and a particularly preferable lower limit is 25 W / (m·K).

[0030] Next, an example of a manufacturing process that allows obtaining an additively manufactured product of the present invention using the hot tool steel powder for additive manufacturing of the present invention will be described in order. Unless otherwise specified, the manufacturing process described below assumes the powder bed method.

[0031] The manufacturing method according to the present invention involves the steps of: laying down the prepared hot tool steel powder for additive manufacturing of the present invention (hereinafter also referred to as "metal powder") in layers; and sequentially melting and solidifying the laid-down metal powder using a scanning heat source having a diameter larger than the D50 of the metal powder to form a solidified layer. By repeating the steps of laying down the metal powder in layers and forming the solidified layer, multiple layered solidified layers can be formed to produce an additively manufactured object according to the present invention. For example, a laser or an electron beam can be used as the scanning heat source. It is preferable to make the diameter of the scanning heat source larger than the D50 of the metal powder so that the aggregate of metal powder can be melted uniformly.

[0032] In the manufacturing method according to the present invention, when irradiating the aforementioned metal powder with a laser while scanning, the laser output can be set to 50-400 W, the scanning speed to 200-2000 mm / second, and the scanning pitch to 0.02-0.20 mm. Here, if the layer thickness per laser scan is too large, heat will not be easily transferred to the entire surface of the metal powder during laser irradiation, preventing the metal powder from melting sufficiently and promoting the formation of internal defects. On the other hand, if the layer thickness per scan is too small, the number of layers required to achieve the desired size of the additively manufactured product will increase, and the time required for the additive manufacturing process will be longer. For this reason, the layer thickness per scan is preferably 10-200 μm. A more preferable lower limit for the layer thickness is 20 μm, and a more preferable upper limit for the layer thickness is 100 μm. Although a preheating step may be performed before the additive manufacturing process described above, the metal powder of the present invention has particularly improved crack resistance compared to conventional hot tool steel powder. Therefore, for example, if the additive manufactured product is small and has few stress concentration points, it is possible to omit or reduce the preheating step before additive manufacturing.

[0033] In the manufacturing method according to the present invention, it is preferable to subject the additively manufactured component (in the as-additive manufactured state, before heat treatment) to a tempering treatment at a temperature of 500 to 700°C in order to impart the mechanical properties necessary for use as a metal product. By performing tempering, the product can be made into an additively manufactured hot work tool with a predetermined hardness. During this time, the additively manufactured object can be shaped into a hot work tool by various machining processes such as cutting and drilling. In this case, annealing can be performed on the additively manufactured object formed in the additive manufacturing process in order to facilitate machining. Annealing can also be expected to have the effect of refining the vanadium carbides in the microstructure of the additively manufactured hot work tool after tempering. Finishing machining may also be performed after tempering. In some cases, the finishing machining can also be performed on the additively manufactured object after tempering, and the above machining can be performed all at once to finish the additively manufactured hot work tool product. Furthermore, quenching can be performed before the tempering described above. And, regardless of whether or not annealing is performed, or before or after, normalizing can be performed on the additively manufactured object formed in the additive manufacturing process.

[0034] The tempering temperature varies depending on the desired hardness, but is generally around 500-700°C. If quenching is performed before tempering, the quenching temperature is generally around 900-1100°C. For example, in the case of SKD61, a representative grade of hot work tool steel, the quenching temperature is around 1000-1030°C, and the tempering temperature is around 550-650°C. Furthermore, the tempered hardness is preferably 50 HRC (Rockwell hardness) or less, or 520 HV (Vickers hardness) or less. More preferably, it is 48 HRC or less, or 500 HV or less. It is also preferable that it be 40 HRC or more, or 380 HV or more. More preferably, it is 42 HRC or more, or 400 HV or more. In this invention, hardness can be measured in accordance with the measurement method described in JIS Z 2245 "Rockwell hardness test - Test method" or JIS Z 2244-1 "Vickers hardness test - Part 1: Test method", and Rockwell C scale hardness (HRC) or Vickers hardness (HV) can be used. [Examples]

[0035] (Example 1) After preparing each metal crude material to have the component composition shown in Table 1, they were charged into a high-frequency induction melting furnace and melted. The molten metal was then pulverized with argon gas to obtain gas atomized powder. The obtained atomized powder was subjected to sieving classification using a mesh and airflow classification to adjust the particle size, and additive manufacturing powders for the present invention and comparative examples, with a D50 of 35 μm, were obtained. Additively manufactured products were fabricated using an EOS M290 under the fabrication conditions shown in Table 2 with each of the additive manufacturing metal powders obtained above. Table 3 shows the component composition of the fabricated products of Sample No. 3, made from the powder of Sample No. 1, and Sample No. 4, made from the powder of Sample No. 2. Table 4 also shows the values ​​for equation (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10 and equation (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V for samples No. 1 to 4.

[0036] [Table 1]

[0037] [Table 2]

[0038] [Table 3]

[0039] [Table 4]

[0040] (Example 2) To evaluate the cracking susceptibility of additively manufactured parts, crack evaluation test specimens as shown in Figure 1 were fabricated. Specifically, the additive manufacturing powders of samples No. 1 and 2 from Example 1 were additively manufactured under the same manufacturing conditions as in Example 1, and samples No. 5 (composition of sample No. 3) and No. 6 (composition of sample No. 4), which have the same composition as samples No. 3 and 4, were fabricated. These crack evaluation test specimens are 50 mm long, 10 mm wide, and 16 mm high, with a stress concentration zone created with R8 in the middle, and this stress concentration zone is made comb-shaped to make it prone to cracking, simulating an additive manufacturing mold in which complex cavities are formed. After fabrication, the length of the crack at the comb-shaped section of this crack evaluation test specimen can be measured to evaluate the susceptibility of the material to cracking during fabrication. Table 5 shows the crack lengths of the crack test specimens of sample No. 5 (example of the present invention) and sample No. 6 (comparative example). From Table 5, it can be confirmed that the example of the present invention has a shorter crack length than the comparative example, and has even better crack resistance during additive manufacturing than the comparative example.

[0041] [Table 5]

[0042] Next, the tempering behavior of the present invention example was confirmed. A laminated product manufactured under the same conditions as sample No. 3 in Example 1 was subjected to tempering heat treatment for 1 hour twice within the temperature range shown in Figure 2, and the Rockwell hardness was measured according to JIS Z 2245. Figure 1 shows a graph of the hardness at each tempering temperature. It was confirmed that the present invention example can be tempered to a hardness of 40 HRC or higher, which is used in conventional hot work tool steels.

[0043] Furthermore, the mechanical properties and thermal conductivity of the present invention example were confirmed. Additive-formed parts manufactured under the same conditions as sample No. 3 of Example 1 were subjected to tempering heat treatment in the temperature range of 500-650°C to heat the test pieces to 40±1HRC and 45±1HRC, after which tensile tests and 2mm U-notch Charpy impact tests were performed. Additionally, after heat treatment to 45±1HRC, thermal conductivity was measured 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 thermal conductivity measurement results.

[0044] As shown in Figure 3, the additively manufactured product of the present invention had a room-temperature tensile strength of 1200 MPa or more, a room-temperature 0.2% yield strength of 1000 MPa or more, a room-temperature elongation of 13% or more, and a room-temperature reduction of 60% or more at all temper hardness levels of the present invention. Furthermore, as shown in Figure 4, the additively manufactured product of the present invention had a high-temperature tensile strength of 800 MPa or more, a high-temperature 0.2% yield strength of 600 MPa or more, a high-temperature elongation of 13% or more, and a high-temperature reduction of 50% or more at all temper hardness levels of the present invention. In addition, at a hardness of 45±1 HRC, the additively manufactured product of the present invention had a room-temperature tensile strength of 1400 MPa or more, a room-temperature 0.2% yield strength of 1200 MPa or more, a room-temperature elongation of 13% or more, and a room-temperature reduction of 60% or more. Furthermore, the additively manufactured product of the present invention exhibited 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] Figure 5 confirms that the additively manufactured product of the present invention exhibits a Charpy impact value of 60 J / cm² or higher at all temper hardness levels. Even at a hardness of 45±1 HRC, the Charpy impact value was a good 60 J / cm² or higher. Furthermore, Figure 6 confirms that the additively manufactured product of the present invention exhibits a thermal conductivity of 25 W / (m·K) or higher at room temperature at a hardness of 45±1 HRC. As shown in Figures 3 to 6 above, the additively manufactured product of the present invention has properties equivalent to those of hot-work tool steel made from melted materials, and is suitable for applications such as hot-work tools. [Industrial applicability]

[0046] The additive manufacturing hot tool steel powder and additively manufactured products of the present invention are most preferably applied to hot tool applications such as die-casting molds. However, by utilizing the various excellent properties of the additively manufactured products of the present invention, they can also be applied to mold repair using, for example, the powder spray additive manufacturing method. Furthermore, by utilizing the various excellent properties of the additively manufactured products of the present invention, they may also be applicable to molds that require an internal cooling mechanism, such as plastic molds.

Claims

1. Hot work tool steel powder for additive manufacturing, comprising, in mass%, 0.10% ≤ C ≤ 0.24%, 0.01% ≤ Si ≤ 0.50%, 0.01% ≤ Mn ≤ 0.19%, 3.6% ≤ Cr ≤ 4.4%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 2.0% ≤ (Mo + 1 / 2W) ≤ 3.4%, 0.2% ≤ V ≤ 0.9%, with the remainder being Fe and unavoidable impurities, and further satisfying the following formula (1). Formula (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≦0.63 Here, each element symbol in formula (1) indicates the content (mass%) of that element.

2. A hot-work tool steel additively fabricated product having the following mass percentages: 0.10% ≤ C ≤ 0.24%, 0.01% ≤ Si ≤ 0.50%, 0.01% ≤ Mn ≤ 0.19%, 3.6% ≤ Cr ≤ 4.4%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 2.0% ≤ (Mo + 1 / 2W) ≤ 3.4%, 0.2% ≤ V ≤ 0.9%, with the remainder being Fe and unavoidable impurities, and further satisfying formula (1) below. Formula (1): C+Si / 30+(Mn+Cr) / 20+(Mo+1 / 2W) / 15+V / 10≦0.63 Here, each element symbol in formula (1) indicates the content (mass%) of that element.

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