WO2025197625A1 - Hot work tool steel powder for additive manufacturing and additively manufactured hot work tool steel article - Google Patents

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

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

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

Hot work tool steels used in hot forging dies, die casting dies, and other tools come into contact with high-temperature workpieces, and therefore require high-temperature strength, toughness, and wear resistance. Traditionally, to meet these requirements, JIS steel grade SKD61 and improved versions of SKD61 have been used for hot work tool steels.

Recently, additive manufacturing has been attracting attention as a means of easily creating near-net-shape metal products (components) with complex shapes. Additive manufacturing, commonly known as 3D printing, is an additive manufacturing technology. Types of additive manufacturing include the powder spray method, in which a heat source is applied to metal powder to melt it and then layer it on top, and the powder bed method, in which a heat source is applied to metal powder spread on a stage to melt it, then the powder is solidified and layered repeatedly. Additive manufacturing allows metal products with complex shapes to be produced without requiring much of the traditional machining process, making it possible to use metal materials that are difficult to process. Furthermore, because difficult-to-process metal materials are primarily high-strength metal materials, it is possible to produce metal products with complex shapes and long durability.

Furthermore, additively manufactured products have been proposed using hot work tool steel as the metal material and the additive manufacturing method described above. For example, Patent Document 1 proposes an additively manufactured hot work tool having a component composition containing, by mass%, 0.3 to 0.5% C, 2.0% or less Si, 1.5% or less Mn, 0.05% or less P, 0.05% or less S, 3.0 to 6.0% Cr, 0.5 to 3.5% of one or two of Mo and W according to the relationship (Mo + 1/2W), 0.1 to 1.5% V, 0 to 1.0% Ni, 0 to 1.0% Co, 0 to 1.0% Nb, and the balance being Fe and impurities, and characterized in that the area ratio of defects having an area of 1 ^μm2 or more in a cross section parallel to the stacking direction is 0.6% or less.

Patent Document 2 discloses a steel powder that aims to achieve both high thermal conductivity and high corrosion resistance and is characterized by a composition, in mass%, of 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 mass % Cr content), 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 mass % Mo content), 0.0002≦N≦0.3000, with the balance being Fe and unavoidable impurities.

International Publication No. 2019/220917 JP 2016-145407 A

As described above, several steels for AM have been proposed, but cracks may occur during AM depending on the type of AM machine, AM conditions, AM dimensions, etc. For example, large AM molds or AM molds that form complex cavities have stress concentration areas (concave portions), and because these areas are particularly prone to stress concentration and cracks, further improvements in crack resistance are 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 can improve crack resistance during additive manufacturing.

The present invention has been made in view of the above-mentioned problems.
That is, one aspect of the present invention is a hot work tool steel powder for additive manufacturing that contains, 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 formula (Mo+½W): 2.0%≦(Mo+½W)≦3.4%, 0.2%≦V≦0.9%, and the remainder being Fe and unavoidable impurities, and satisfies formula (1): C+Si/30+(Mn+Cr)/20+(Mo+½W)/15+V/10≦0.63 (each element symbol in formula (1) indicates the content (mass%) of that element).

Another aspect of the present invention is a hot work tool steel additive manufactured product that contains, 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%, and the remainder consisting of Fe and unavoidable impurities, and satisfies formula (1): C+Si/30+(Mn+Cr)/20+(Mo+1/2W)/15+V/10≦0.63.

The present invention makes it possible to obtain hot work tool steel powder for additive manufacturing, which can be used to produce hot work tool steel additive manufactured products that have particularly excellent crack resistance during additive manufacturing.

FIG. 1 is a schematic diagram of a crack evaluation test piece for evaluating molding crack resistance. 1 is a graph showing the tempering temperature and hardness of a hot work tool steel additive manufactured product of an example of the present invention. 1 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. 1 is a graph showing 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 high temperatures. 1 is a graph showing Charpy impact values at room temperature of examples of the present invention. 1 is a graph showing the thermal conductivity of an example of the present invention.

The present invention has a chemical 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 + ½W): 2.0%≦(Mo + ½W)≦3.4%, 0.2%≦V≦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, "%" represents "mass %." Additive manufacturing may also be referred to simply as "manufacturing."
C: 0.10%≦C≦0.24%
Carbon (C) is a fundamental element of hot-work tool steels, partially dissolving in the matrix to impart strength, while partially forming carbides to enhance wear resistance and seizure resistance. Furthermore, when added together with substitutional atoms with high affinity for C, such as Cr, C dissolved as an interstitial atom is expected to contribute to the I (interstitial atom)-S (substitutional atom) effect (which acts as drag resistance for solute atoms and increases the strength of hot-work tools). It can also enhance hardenability. If the C content is too low, the ferrite phase is primarily formed 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 the dominant phase immediately after solidification until room temperature, it becomes impossible to mitigate thermal contraction by utilizing martensitic transformation expansion, making the steel more susceptible to cracking. However, increasing the C content increases hardness but decreases toughness, promoting cracking during molding. In the present invention, the C content is set to 0.10%≦C≦0.24% in order to improve crack resistance while maintaining a hardness sufficient for use in dies. The lower limit of C is preferably 0.13%, more preferably 0.15% or more, even more preferably 0.16% or more, and even more preferably 0.17% or more. The upper limit of C is preferably 0.23%, more preferably 0.22% or less.

Si: 0.01%≦Si≦0.50%
Si can be used as a deoxidizer when adjusting the chemical composition of molten steel. It is difficult to eliminate Si from the steel in production, and the closer to eliminating it, the higher the manufacturing cost. Therefore, the Si content is set to 0.01% or more. The preferred lower limit is 0.03%, and more preferably 0.05% or more. On the other hand, if the Si content is too high, ferrite is formed in the structure after tempering, so the upper limit is set to 0.50% or less. The preferred upper limit is 0.30% or less, and more preferably 0.20% or less, and 0.15% or less.

Mn: 0.01%≦Mn≦0.19%
Mn has the effects of improving hardenability, suppressing the formation of ferrite in the structure after tempering, and achieving appropriate quench-and-temper hardness. To achieve these effects, the lower limit of Mn is set to 0.01%. A preferred lower limit is 0.02%, more preferably 0.03% or more, even more preferably 0.04% or more, and 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 to 0.19%. A preferred upper limit is 0.18%, more preferably 0.17% or less, and 0.15% or less.

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

One or both of Mo and W according to the relationship (Mo+½W): (Mo+½W): 2.0%≦(Mo+½W)≦3.4%
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 an atomic weight approximately twice that of Mo, the Mo content can be determined together with the Mo equivalent defined by the relationship (Mo + 1/2W). (Naturally, only one of them may be added, or both may be added.) To achieve the above effect, the content should be 2.0% or more, as determined by the relationship (Mo + 1/2W). A more preferred upper limit is 2.1%, and even more preferably 2.2% or more. However, excessive Mo and W may result in reduced machinability and toughness, resulting in reduced cracking resistance. Furthermore, their high melting points make them difficult to melt, making their inclusion in large amounts undesirable from a manufacturing perspective. Therefore, the value according to the relational expression (Mo + 1/2W) is set to 3.4% or less. The upper limit is preferably 3.0%, more preferably 2.8%, and even more preferably 2.6%, and even more preferably 2.5%. Here, since W is a more expensive element than Mo, it is preferable to contain Mo alone when cost reduction is important.

V: 0.2%≦V≦0.9%
V forms vanadium carbides, which strengthen the matrix and improve its wear resistance and temper softening resistance. When an additive manufacturing process involves heating an additively manufactured product to a quenching temperature for quenching, the vanadium carbides also function as "pinning particles" that suppress coarsening of austenite grains during quenching, contributing to improved toughness. However, because V has a high carbide-forming ability, excessive V may convert all C to vanadium carbide, preventing the formation of other carbides. Because hot work tool steels are made up of multiple types of carbides, vanadium carbide alone is not desirable. Therefore, the V content is 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. The upper limit is preferably 0.80%, more preferably 0.60% or less, further preferably 0.50% or less, and particularly preferably 0.45% or less.

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

In the present invention, by achieving an overall balance within the above-mentioned component ranges, it is possible to obtain hot work tool steel additive manufactured products that have particularly excellent crack resistance during manufacturing.

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 the present invention is adjusting the left side of the above formula (1) to be 0.63 or less. The left side of formula (1) is an improved version of Pcm, which is used as a cold cracking susceptibility index for welding. C, Si, Mn, Cr, Cu, Ni, Mo, W, and V in formula (1) represent the content (mass%) of each element. Like additive manufacturing, cracking is a problem in welding, and both are molten solidification structures. Therefore, it was found that this index can be applied to suppress cracking in additive manufacturing, and was applied to the present invention. Another known index for welding cracking is the hot cracking index HCS. However, prior investigations revealed that the molding cracks in this composition system were largely cracked from the surface and had a different morphology from the hot cracks that tend to occur at the solidification interface, etc. Since the molding cracks in this composition system are found in areas prone to tensile stress due to thermal contraction and therefore occur at low temperatures, it was assumed that they were similar to cold cracks in welding. Therefore, the cold cracking index Pcm was applied in the present invention. In the present invention, particularly in order to suppress cracking during additive manufacturing, the left side of formula (1) is set to 0.63 or less, preferably 0.62 or less, and more preferably 0.61 or less.

Formula (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V≦450
In the present invention, in addition to the component specifications, the above formula (2) can be adjusted to 450 or less. Formula (2) is a relationship formula between elements excluding Co and the Ms point, as disclosed in a 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, at high temperatures during additive manufacturing, the alloy will transform into brittle martensite, which tends to be prone to cracking due to thermal contraction when cooled to room temperature. It is also preferable to adjust formula (2) to 200 or more. If the value of formula (2) is too low, the Ms point is expected to be low. If the value is too low, the martensitic transformation will not be complete, and austenite will remain, resulting in reduced strength. The preferred upper limit of formula (2) is 440 or less. It is more preferably 430 or less, and even more preferably 420 or less. The preferred lower limit of formula (2) is 240 or more. It is more preferably 260 or more, and even more preferably 280 or more. Elements other than those positively added in the present invention, such as Cu and Ti, and Al which may be contained in a range of 0.04% or less as an impurity element, may be calculated as zero%.

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, and the like. Among these, gas atomization is a method in which a molten raw material prepared to have the desired composition is heated to above its melting point by high-frequency induction heating, melted, and then the molten metal that flows out through fine holes is finely crushed by injecting an inert gas such as argon gas or nitrogen gas onto the molten metal, which is then rapidly cooled and solidified to obtain powder. This gas atomization method allows scrap metal, raw metal raw materials, and the like to be used as the molten raw material. Compared to plasma atomization and rotating electrode methods, which require the preparation of raw materials with the desired composition and shape in advance, gas atomization can be produced at a lower cost, making it an ideal method for obtaining the metal powder for additive manufacturing of the present invention.

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 on a volume basis 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 additively manufactured product can be suppressed.
Furthermore, by making the metal powder for additive manufacturing of the present invention have a D50 of 10 μm or more, the metal powder is less susceptible to the effects of moisture and other factors in the atmosphere during handling and additive manufacturing, ensuring good fluidity.
The cumulative particle size distribution of the powder for molding of the present invention is expressed as a cumulative volumetric particle size distribution, and its D50 can be expressed as a value measured by the laser diffraction scattering method specified in JIS Z 8825.

The D50 of the hot work tool steel powder for additive manufacturing of the present invention may be adjusted by mesh sieving or air classification, etc., depending on the method described above. For example, metal powders used in the powder bed method are melted by a laser beam, which serves as the heat source, but coarse metal powder that is difficult to melt must be removed to minimize the area affected by the heat. Furthermore, highly adhesive fine metal powder must also be removed to achieve optimal fluidity for ensuring the spreadability 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 the range of 10 to 53 μm. The preferred upper limit of D50 is 40 μm, and the preferred lower limit of D50 is 20 μm. Furthermore, when applying the powder for additive manufacturing of the present invention to the laser metal deposition method, it is preferable to adjust the D50 to the range of 50 to 150 μm.

By additively manufacturing the hot work tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described below, it is possible to obtain a hot work tool steel additively manufactured product (hereinafter also referred to as an additively manufactured product) that consists, by mass, 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+½W): 2.0%≦(Mo+½W)≦3.4%, 0.2%≦V≦0.9%, and the remainder being Fe and unavoidable impurities, and that satisfies formula (1): C+Si/30+(Mn+Cr)/20+(Mo+½W)/15+V/10+≦0.63. This additively manufactured product is particularly advantageous in that it is less susceptible to cracking (manufacturing cracking) during additive manufacturing.

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

The hot work tool steel additive manufactured product of the present invention preferably has a high-temperature (approximately 550°C) tensile strength of 600 to 1400 MPa when the tempered hardness is adjusted to 45±1 HRC. A more preferred lower limit is 800 MPa, and an even more preferred lower limit is 900 MPa. The high-temperature 0.2% yield strength when the tempered hardness is adjusted to 45H±1RC is preferably 600 to 1200 MPa. A more preferred lower limit is 700 MPa, and an even more preferred lower limit is 800 MPa. The high-temperature elongation when the tempered hardness is adjusted to 45±1 HRC is preferably 10% or more. A more preferred lower limit is 13%, and an even more preferred lower limit is 16%. The high-temperature drawing ability when the tempered hardness is adjusted to 45±1 HRC is preferably 30% or more. A more preferred lower limit is 40%, an even more preferred lower limit is 50%, and an even more preferred lower limit is 60%.

Furthermore, the hot work tool steel additive manufactured product of the present invention preferably has a 2 mm U-notch Charpy impact value at room temperature of 30 J/ ^cm2 or more when the tempered hardness is adjusted to 45±1 HRC. A more preferable lower limit is 50 J/ ^cm2 , an even more preferable lower limit is 80 J/ ^cm2 , an even more preferable lower limit is 100 J/ ^cm2 , and an especially preferable lower limit is 150 J/ ^cm2 .

Furthermore, the room temperature thermal conductivity of the hot work tool steel additive manufactured product of the present invention, when the tempered hardness is adjusted to 45±1 HRC, is preferably 10 W/(m·K) or more. 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).

Next, we will explain one example of a manufacturing process that can produce an additive manufacturing product of the present invention using the hot work tool steel powder for additive manufacturing of the present invention. Note that the manufacturing process described below assumes the powder bed method, unless otherwise specified.

The manufacturing method of the present invention involves the steps of: spreading the prepared hot work tool steel powder for additive manufacturing of the present invention (hereinafter also referred to as "metal powder") in layers; and successively 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 steps of spreading the metal powder in layers and 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. Setting the diameter of the scanning heat source to be larger than the D50 of the metal powder is preferable because it allows the metal powder clusters to be melted evenly.

In the manufacturing method of the present invention, when the laser is irradiated onto the above-mentioned metal powder while scanning, 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, 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 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 preferable lower limit for the layer thickness is 20 μm, and a more preferable upper limit for the layer thickness is 100 μm. A preheating step may be carried out before the additive manufacturing process described above. However, because the metal powder of the present invention has improved crack resistance compared to conventional hot work tool steel powders, for example, if the additive manufacturing product is small and has few stress concentration areas, it is possible to omit the preheating step before additive manufacturing or to use a lower temperature.

In the manufacturing method according to the present invention, it is preferable to subject the as-AM (i.e., the as-AM component (i.e., the component has not been heat-treated 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 allows the AM hot work tool to be shaped with a predetermined hardness. During this process, the AM product can be shaped into the hot work tool shape by various machining processes, such as cutting and drilling. In this case, the AM product formed in the AM process can be annealed to facilitate machining. Annealing can also be expected to refine the vanadium carbide in the structure of the tempered AM hot work tool. Finishing machining may then be performed after tempering. In some cases, the AM hot work tool can be finished by simultaneously performing the above-described machining processes on the tempered AM hot work tool.
It should be noted that quenching can be performed before the tempering. Regardless of whether or not the annealing is performed, or before or after the annealing, the layered object formed in the layered manufacturing process can be normalized.

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. Also, 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.

Example 1
Each metal raw material was prepared to have the 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 powders for additive manufacturing (AM) of the present invention and comparative examples with a D50 of 35 μm. Using each of the AM metal powders obtained above, AM products were fabricated using an EOS M290 under the fabrication conditions shown in Table 2. Table 3 shows the composition of the molded products of Sample No. 3, fabricated from powder Sample No. 1, and Sample No. 4, fabricated from powder Sample No. 2.
Table 4 also shows the values of formula (1): C + Si/30 + (Mn + Cr)/20 + (Mo + ½W)/15 + V/10 for Samples No. 1 to 4, and the values of formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V.

Example 2
To evaluate the susceptibility of AM products to cracking, crack evaluation test pieces as shown in Figure 1 were fabricated. Specifically, AM powders No. 1 and 2 from Example 1 were AM processed under the same fabrication conditions as in Example 1 to produce Sample No. 5 (Sample No. 3 composition) and Sample No. 6 (Sample No. 4 composition), which have the same compositions as Samples No. 3 and 4. These crack evaluation test pieces were 50 mm long, 10 mm wide, and 16 mm high, with a stress concentration area (R8) formed along the middle. This stress concentration area was comb-shaped to facilitate cracking, and was designed to simulate an AM mold for forming complex cavities. After fabrication, the crack evaluation test pieces were measured for the length of the cracks in the comb-shaped area, allowing the susceptibility of the material to crack during fabrication to be evaluated. Table 5 shows the crack lengths of the crack test pieces for Sample No. 5 (an example of the present invention) and Sample No. 6 (a comparative example). 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 had even better crack resistance during additive manufacturing than the comparative example.

Next, the tempering behavior of the inventive example was confirmed. An additively manufactured product manufactured under the same conditions as Sample No. 3 in Example 1 was subjected to tempering heat treatment twice, for 1 hour each, within the temperature range shown in Figure 2, and the Rockwell hardness was measured in accordance with JIS Z 2245. Figure 1 shows a graph of each tempering temperature and hardness. It was confirmed that the inventive example could be tempered to a hardness of 40 HRC or higher, which is the hardness used in conventional hot work tool steels.

Furthermore, the mechanical properties and thermal conductivity of the inventive example were confirmed. An additive manufacturing product manufactured under the same conditions as Sample No. 3 in Example 1 was subjected to tempering heat treatment in the temperature range of 500 to 650°C, and test specimens were tempered to 40±1 HRC and 45±1 HRC, after which tensile tests and 2mm U-notch Charpy impact tests were conducted. Furthermore, after tempering the test specimens to 45±1 HRC, thermal conductivity measurements were conducted using the laser flash method. Figure 3 shows the results of the room temperature (22°C) tensile test, Figure 4 the results of the high temperature (550°C) tensile test, Figure 5 the results of the Charpy impact test, and Figure 6 the results of the thermal conductivity measurements.

As can be seen from Figure 3, the additively manufactured products of the present invention had room temperature tensile strength of 1200 MPa or more, room temperature 0.2% yield strength of 1000 MPa or more, room temperature elongation of 13% or more, and room temperature drawing of 60% or more at all tempered hardnesses of the present invention. Furthermore, as can be seen from Figure 4, the additively manufactured products of the present invention had high temperature tensile strength of 800 MPa or more, high temperature 0.2% yield strength of 600 MPa or more, high temperature elongation of 13% or more, and high temperature drawing of 50% or more at all tempered hardnesses of the present invention. Furthermore, at 45±1 HRC hardness, the additively manufactured products of the present invention had room temperature tensile strength of 1400 MPa or more, room temperature 0.2% yield strength of 1200 MPa or more, room temperature elongation of 13% or more, and room temperature drawing of 60% or more. Furthermore, at a hardness of 45±1 HRC, the additively manufactured product of the present invention had a high-temperature tensile strength of 900 MPa or more, a high-temperature 0.2% yield strength of 700 MPa or more, a high-temperature elongation of 16% or more, and a high-temperature reduction of 60% or more.

Figure 5 confirms that the additively manufactured products of the present invention had Charpy impact values of 60 J/cm or more at all tempered hardness levels. Even at a hardness of 45±1 HRC, the Charpy impact value was a favorable value of 60 J/cm or more. Figure 6 also confirms that the additively manufactured products of the present invention had thermal conductivity of 25 W/(m·K) or more at room temperature at a hardness of 45±1 HRC.
As described above, from FIGS. 3 to 6, it was confirmed that the additively manufactured product of the present invention has properties at the same level as those of ingot hot work tool steel, and is suitable for use in hot work tools, for example.

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

Claims (2)

A hot work tool steel powder for additive manufacturing, which contains, 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 formula (Mo+½W): 2.0%≦(Mo+½W)≦3.4%, 0.2%≦V≦0.9%, and the balance 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 the element.
A hot work tool steel additive manufactured product 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+½W): 2.0%≦(Mo+½W)≦3.4%, 0.2%≦V≦0.9%, the balance 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 the element.