Hot work tool steel powder for molding and molded body using same

Abstract

The purpose of the present invention is to provide a hot work tool steel powder for molding and to also provide a molded body including the same, the hot work tool steel powder allowing production of a molded body in which: deep tempering is easy even if the molded body is large; occurrence of cracks in the molded body and / or at the interface between the molded body and the molded base material is suppressed; and the thermal conductivity is high. Provided is a hot work tool steel powder for molding comprising, as essential components in mass%, C at more than 0.10% and less than 0.45% and Ni at more than 2.00% and less than 8.00%, and as optional components, one or more of Si at less than 0.60%, Mn at less than 5.00%, Cr at less than 2.00%, Mo at less than 1.20%, W at less than 2.00%, V at less than 0.60%, and Al at less than 0.10%, the balance being Fe and unavoidable impurities, where Ni + Mn is less than 8.50%.

Description

Hot work tool steel powder for molding and molded body using the same

[0001] The present invention relates to a hot work tool steel powder for shaping and a shaped body using the same. Specifically, the present invention relates to a hot work tool steel powder for shaping and a shaped body that is formed using the same by a shaping method such as an additive manufacturing method (also called a 3D printer method, three-dimensional shaping method, additive manufacturing, or the like) and that becomes a variety of tools (e.g., hot work tools) including dies. In particular, the present invention relates to a hot work tool steel powder for shaping that can be used to produce a shaped body that exhibits sufficient hardenability, high resistance to shaping cracking, and high thermal conductivity, and has high mechanical properties, even when the shaped body is large, and a shaped body using the same.

[0002] In this specification, "high resistance to cracking during molding" means that cracks are less likely to occur in the molded body itself (especially in the areas that become notches) and / or at the interface between the molded body and the molding base material due to thermal stresses associated with rapid melting, cooling, and solidification during molding.

[0003] Unlike conventional manufacturing methods, additive manufacturing is capable of manufacturing components with complex shapes or three-dimensional structures, and in recent years has seen remarkable technological development and an expansion of its range of applications. Attempts are being made to apply additive manufacturing to various tools, and in particular, efforts are being made to put it to practical use in die-casting molds that have complex cooling water pipes with three-dimensional structures inside.

[0004] Now, tools made of hot work tool steel are used to process various parts, and their shapes and sizes vary depending on the processing method or part shape. However, the application of additive manufacturing to tools has been limited to relatively small tools due to the following problems with manufacturing cracks:

[0005] In general, additive manufacturing (AM) involves rapidly melting and solidifying raw material powder or wire using a narrowly focused heat source such as a laser or electron beam for a short period of time, and then repeating this process to build up solidified layers, enabling the production of parts with complex three-dimensional shapes. During this process, only a portion of the part is heated, melted, and solidified, resulting in localized solidification and shrinkage, thermal expansion, and thermal contraction, which generates thermal stress. If the material being molded or the molded base material is hard and brittle, it will not be able to withstand the thermal stress, resulting in cracks in the molded object itself and / or at the interface between the molded object and the molded base material.

[0006] Such thermal stresses become even greater when a large object is additively manufactured, resulting in increased likelihood of cracks during manufacturing. Generally, tools are often made of high-hardness alloys, such as SKD61, which meets the JIS standard, making them prone to cracks during manufacturing. For these reasons, the application of additive manufacturing to tools has traditionally been limited to small tools with relatively low thermal stress.

[0007] The surfaces of hot working tools, including die-casting dies, are susceptible to heat checking and other damage due to the temperature rise caused by contact with the hot parts (workpieces) being processed. Furthermore, areas that experience particularly rapid temperature increases are prone to seizure.

[0008] To avoid these problems, it is important to efficiently cool the surface of the hot work tool. By using an alloy with high thermal conductivity as the material for the hot work tool, the cooling effect of water cooling pipes placed inside the tool can be maximized all the way to the hot work tool surface.

[0009] Furthermore, in part machining using hot tools, the tools must cool after machining one part and before machining the next part. Therefore, being able to efficiently (in a short time) cool the tools to a predetermined temperature has the advantage of shortening the part machining cycle and improving part production efficiency.

[0010] As an additive manufactured body made from such a hot work tool steel with high thermal conductivity, the applicant has proposed a manufactured body that satisfies the following formulas (1) to (3) and is made from an Fe-based alloy powder containing, in mass %, 0.20<C<0.60, Si<0.60, Mn<0.90, Cr<4.00, Ni<2.00, Mo<1.20, W<2.00, V<0.60, Al<0.10, with the remainder being Fe and unavoidable impurities (see Patent Document 1). T1 = 71.7 - 5.9Mn - 6.3Cr - 2.8V - 5.7Mo - 1.1W - 23.1C - 5.8Ni - 1.9Si - 0.5Al - 0.6PC > 32.0 ... formula (1) T2 = 80.1 + 2.4Mn + 1.6Si + 7.1Cr - 12.0PC > 50.0 ... formula (2) Average size (μm) of carbides contained in the shaped body: PC < 3.0 ... formula (3)

[0011] Japanese Patent Application Laid-Open No. 2022-092524

[0012] In the above-mentioned Patent Document 1, since the thermal conductivity decreases with an increase in the amount of each element added, an attempt is made to achieve high thermal conductivity by specifying the amount of each element added and the lower limit of the T1 parameter. In other words, an attempt is made to obtain high thermal conductivity by reducing the amount of each added element compared to SKD61, a general-purpose steel that is widely used for hot working tools.

[0013] However, since the various additive elements also contribute to improving hardenability, a decrease in the amount of these elements added also leads to a decrease in hardenability.

[0014] In the past, due to the risk of cracking due to thermal stress, additive manufacturing was limited to relatively small hot working tools. In other words, when the object to be manufactured was large, even if an alloy with relatively low hardenability was used, which did not achieve a sufficient cooling rate during quenching in the deep part of the large object (i.e., did not achieve sufficient quench-temper hardness), the problem could be avoided if the object to be manufactured was relatively small.

[0015] However, in order to respond to the recent trend to expand the range of applications for tools made by additive manufacturing and the demand to use additively manufactured bodies for larger hot working tools, it is necessary to achieve a high level of both hardenability and high thermal conductivity, which are in a trade-off relationship depending on the amount of various added elements. Conventional alloys have not been able to maintain both properties at a high level.

[0016] Furthermore, because additive manufacturing is a rapid cooling method, the additively manufactured object may be in a pseudo-hardened state after additive manufacturing, so in some cases the additively manufactured object is used after only tempering or stress relief heat treatment without being hardened after additive manufacturing. However, in the center of a large manufactured object, heat accumulates from the heat source of the manufacturing, causing the temperature to rise, making it difficult to achieve the rapid cooling effect despite the rapid cooling method. Again, alloys with low hardenability tend to have low hardness in the deep parts of the manufactured object, which hinders the size of the manufactured object.

[0017] It is also known that preheating the base material during molding can reduce cracking, but due to the design of the molding equipment, a preheating temperature of approximately 80 to 300°C is often used.In this case, the rapid cooling effect during molding is less likely to be achieved not only for large objects but also for small objects, and alloys with low hardenability tend to have low hardness in the deep parts of the object, which is a problem.

[0018] Furthermore, the optimum working hardness of various tools varies depending on the application, with some applications requiring around 40 HRC and others requiring around 50 HRC, but if the hardness differs between the surface and deeper parts due to poor hardenability, it becomes difficult to adjust the entire tool to the optimum working hardness, which can lead to problems. Therefore, in addition to the absolute value of hardness, it is also important to reduce variations in hardness depending on the part.

[0019] In view of the above, the present invention aims to provide a hot working tool steel powder for molding, which can be used to produce a molded body that is easy to heat deep into, even if the molded body is large, is less likely to crack in the molded body itself and / or at the interface between the molded body and the molding base material, and has high thermal conductivity, and a molded body made using the same.

[0020] In light of the above background, the inventors have diligently developed the components of a hot-working tool steel for forming that can produce formed bodies having high hardenability and high thermal conductivity, sufficient to obtain sufficient hardened hardness even when the formed bodies are large.They focused on Ni as an element that can achieve high hardenability while minimizing the decrease in thermal conductivity, and as a result of their research, they found that by strictly specifying the amount of Ni added along with Cr and other elements, both properties can be achieved at a high level, leading to the present invention.

[0021] Furthermore, as described above, when the object to be produced is large, the generated thermal stress increases and cracking becomes more likely to occur during production. However, it has been discovered that by specifying the amounts of Ni, Cr, and C added, the hardness of the as-produced object can be reduced and resistance to cracking during production can be improved. That is, the present invention relates to a hot work tool steel powder for shaping that can be practically applied to larger objects as well, compared to conventional hot work tool steel powders for shaping that have been applied mainly to small objects, and that has optimized ranges of addition of Ni, Cr, and C to improve hardenability and minimize reductions in thermal conductivity, and that can produce objects that have high resistance to cracking during production, even if the object to be produced is large, and to an additive shaping object using the same.

[0022] A first means for solving the problems of the present invention is a hot work tool steel powder for shaping, which contains, in mass %, as essential added components: C: more than 0.10% and less than 0.45%, and Ni: more than 2.00% and less than 8.00%, and as optional added components: one or more of Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Mo: less than 1.20%, W: less than 2.00%, V: less than 0.60%, and Al: less than 0.10%, with the balance consisting of Fe and unavoidable impurities, and Ni + Mn: less than 8.50%.

[0023] A second means for solving the problems of the present invention is a shaped body shaped using the hot working tool steel powder for shaping described in the first means.

[0024] A third means for solving the problems of the present invention is the shaped body according to the second means, in which the shaped body is a layered shaped body.

[0025] According to the present invention, it is possible to provide a hot working tool steel powder for molding and a molded body using the same, which can be easily quenched deep into the molded body even if the molded body is large, and which is less likely to develop cracks in the molded body itself and / or at the interface between the molded body and the molding base material, and which can produce a molded body with high thermal conductivity.

[0026] Furthermore, according to the present invention, excellent effects can be achieved, such as the ability to achieve both high hardenability and high thermal conductivity, even when a shaped body is obtained using a shaping method in which hardening is omitted, a shaping method in which a small shaped body is produced, or a shaping method in which the shaped body base material is preheated during shaping.

[0027] The present invention relates to a hot work tool steel, characterized in that the hot work tool steel contains, by mass%, as essential additional components: more than 0.10% and less than 0.45% C, and more than 2.00% and less than 8.00% Ni, and as optional additional components: one or more of less than 0.60% Si, less than 5.00% Mn, less than 2.00% Cr, less than 1.20% Mo, less than 2.00% W, less than 0.60% V, and less than 0.10% Al, with the balance consisting of Fe and unavoidable impurities, and a Ni+Mn content of less than 8.50%.

[0028] The present invention also relates to a hot work tool steel powder. The hot work tool steel powder of the present invention is a powder made of the hot work tool steel of the present invention. The hot work tool steel powder of the present invention can be used as a material for manufacturing (preferably additive manufacturing).

[0029] The present invention also relates to a hot work tool steel powder for shaping. The hot work tool steel powder for shaping of the present invention is a powder used as a material for shaping (preferably additive manufacturing), and is a powder made of the hot work tool steel of the present invention.

[0030] The present invention also relates to a shaped body. The shaped body of the present invention is a shaped body shaped using the hot work tool steel powder for shaping of the present invention. The shaped body of the present invention is preferably an additively shaped body shaped using the hot work tool steel powder for shaping of the present invention. The shaped body of the present invention is preferably made of the hot work tool steel of the present invention, as well as the hot work tool steel powder for shaping of the present invention. The shaped body of the present invention becomes a hot work tool after being subjected to one or more desired processes (e.g., quenching, tempering, cutting, etc.).

[0031] Hot work tool steel is an alloy steel suitable for hot work tools, such as dies, including die-casting dies such as aluminum die-casting dies.

[0032] Specific examples of methods for producing hot work tool steel powder include water atomization, single roll quenching, twin roll quenching, gas atomization, disk atomization, and centrifugal atomization. From the viewpoint of spheroidization, the hot work tool steel powder is preferably a gas atomized powder. The produced powder may be used as a raw material powder for shaping after adjusting the particle size by sieving or the like.

[0033] A specific example of a method for producing a shaped object is a rapid melting and rapid solidification process, which involves melting and solidifying a hot-work tool steel powder. Specific examples of this process include three-dimensional additive manufacturing (3D AM), thermal spraying, laser coating, and cladding. Specific examples of three-dimensional AM methods include powder bed fusion (powder bed method) and directed energy deposition (powder deposition method). Specific examples of powder bed fusion (powder bed method) methods include selective laser sintering (SLS), selective laser melting (SLM), and electron beam melting (EBM). Hot-work tool steel powder is suitable for three-dimensional AM, particularly powder bed fusion, and can produce large, high-density additive objects.

[0034] For example, a 3D printer can be used for three-dimensional additive manufacturing. In powder bed fusion (powder bed method) additive manufacturing, a laser beam or electron beam is irradiated onto a spread-out hot work tool steel powder. The irradiation causes the particles to rapidly heat and melt. The molten particles then rapidly solidify. This melting and solidification process bonds the particles together. Irradiation is selectively performed on a portion of the spread-out hot work tool steel powder. The unirradiated portions of the spread-out powder do not melt. A bonding layer is formed only in the irradiated portions.

[0035] A thin layer of hot work tool steel powder is spread on top of the bonding layer. A portion of this hot work tool steel powder is irradiated with a laser beam or electron beam. The irradiation causes the particles to rapidly melt. The molten particles then rapidly solidify. This melting and solidification process bonds the particles in the powder together, forming a new bonding layer. The new bonding layer also bonds with the existing bonding layer.

[0036] By repeating the bonding process through irradiation, the aggregate of bonding layers gradually grows. This growth results in an additive manufacturing object with a three-dimensional shape. This additive manufacturing method makes it easy to obtain additive manufacturing objects with complex shapes.

[0037] In this specification, the "quenched and tempered hardness" of a shaped body refers to the hardness of a shaped body that has been quenched after shaping and then tempered. However, because additive manufacturing involves rapid melting, quenching, and solidification, even an additively manufactured body may be in a pseudo-quenched state after additive manufacturing. Therefore, additively manufactured bodies may be used after only tempering or stress relief heat treatment without being quenched after additive manufacturing. In such cases, the "quenched and tempered hardness" of an additively manufactured body refers to the hardness of an additively manufactured body that has been only tempered or stress relief heat treatment without being quenched after additive manufacturing.

[0038] The reasons for specifying the contents of the essential additional elements (C and Ni) and the optional additional elements (one or more of Si, Mn, Cr, Mo, W, V, and Al) of the hot work tool steel of the present invention will be explained below. The "%" for the content of each element is mass %. The balance other than the essential additional elements and the optional additional elements is Fe and unavoidable impurities.

[0039] C: More than 0.10% to Less than 0.45% C is an essential component for achieving high quench-and-temper hardness by dissolving in the martensite matrix and precipitating fine carbides. However, if the C content is 0.10% or less, high quench-and-temper hardness cannot be achieved. Therefore, the lower limit of the C content is set to more than 0.10%. The lower limit of the C content is preferably 0.20% or more, more preferably 0.30% or more. On the other hand, if the C content is 0.45% or more, the hardness of the as-formed object becomes excessively high, which has a relatively large effect on deteriorating resistance to forming cracking compared to other components, and the amount of solute C increases, resulting in a decrease in thermal conductivity. Therefore, the upper limit of the C content is set to less than 0.45%. The upper limit of the C content is preferably 0.42% or less, more preferably 0.40% or less. Each of these upper limits may be combined with any of the above-mentioned lower limits.

[0040] Ni: More than 2.00% to Less than 8.00% Ni is an essential component for improving hardenability and improving quench-temper hardness even in deep portions of large shaped bodies. Its thermal conductivity reduction effect is relatively small compared to other components, making it the most important component in the present invention. Ni also delays martensitic transformation during cooling during shaping, and by maintaining austenite, which is soft and resistant to shaping cracks, at relatively low temperatures, it also improves shaping crack resistance. However, if the Ni content is 2.00% or less, this effect is insufficient. Therefore, the lower limit of the Ni content is set to more than 2.00%. The lower limit of the Ni content is preferably 2.30% or more, more preferably 3.00% or more. On the other hand, if the Ni content is 8.00% or more, the amount of solid solution in the matrix increases, significantly reducing thermal conductivity. Therefore, the upper limit of the Ni content is set to less than 8.00%. The upper limit of the Ni content is preferably 6.50% or less, more preferably 5.00% or less. Each of these upper limits may be combined with any of the lower limits mentioned above.

[0041] Si: Less than 0.60% Si is a component that improves hardness by dissolving in the matrix. Si also has the effect of improving softening resistance. Therefore, the Si content is set to 0 to less than 0.60%. If the Si content is 0.60% or more, the amount of dissolved Si increases, significantly reducing thermal conductivity. Therefore, the upper limit of the Si content is set to less than 0.60%. The upper limit of the Si content is preferably 0.40% or less, more preferably 0.24% or less. The Si content may be 0% or more. Since Si is a component that improves hardness and softening resistance, when Si is added, the lower limit of the Si content is preferably 0.04% or more, more preferably 0.10% or more. These lower limits may be combined with any of the above-mentioned upper limits.

[0042] Mn: Less than 5.00% Mn is a component that improves hardenability and improves quench-tempered hardness even in deep portions of large shaped bodies. Mn also has the effect of improving softening resistance. Therefore, the Mn content is set to 0 to less than 5.00%. If the Mn content is 5.00% or more, the amount of solute Mn increases, reducing thermal conductivity. Therefore, the upper limit of the Mn content is set to less than 5.00%. The upper limit of the Mn content is preferably 1.00% or less, more preferably 0.41% or less. The Mn content may be 0% or more. When Mn is added, from the viewpoint of improving quench-tempered hardness and softening resistance even in deep portions of large shaped bodies, the lower limit of the Mn content is preferably 0.05% or more, more preferably 0.11% or more. These lower limits may be combined with any of the above-mentioned upper limits.

[0043] Ni + Mn: Less than 8.50% Mn is a component that has an effect similar to that of Ni. However, if the total content of Mn and Ni is 8.50% or more, the amount of solid solution in the matrix increases, significantly reducing thermal conductivity. Therefore, the total content of Ni and Mn is set to less than 8.50%. The total content of Ni and Mn is preferably 7.00% or less, more preferably 5.00% or less.

[0044] Cr: Less than 2.00% Cr is a component that improves hardenability and improves quench-temper hardness even in deep portions of large shaped bodies. Cr also has the effect of improving softening resistance. However, if the Cr content is 2.00% or more, the amount of dissolved Cr increases, reducing thermal conductivity, and this reducing effect is relatively large compared to other components. Therefore, the Cr content is set to 0 to less than 2.00%. The upper limit of the Cr content is preferably 1.50% or less, more preferably 1.15% or less. The Cr content may be 0% or more. Cr is a component that improves hardenability and improves quench-temper hardness even in deep portions of large shaped bodies, and also has the effect of improving softening resistance. Therefore, when Cr is added, the lower limit of the Cr content is preferably 0.50% or more, more preferably 0.85% or more. These lower limits may be combined with any of the above-mentioned upper limits.

[0045] Mo: Less than 1.20% Mo is a component that promotes secondary hardening during tempering and increases quench-temper hardness. Although Mo reduces thermal conductivity by its addition, its contribution is minimal, and its hardness-improving effect is significant. Therefore, the Mo content is set to 0 to less than 1.20%. The upper limit of the Mo content is preferably 1.05% or less, more preferably 0.95% or less. The Mo content may be 0% or more. Mo is a component that promotes secondary hardening during tempering and increases quench-temper hardness. Although Mo reduces thermal conductivity by its addition, its contribution is minimal, and its hardness-improving effect is significant. Therefore, when Mo is added, the lower limit of the Mo content is preferably 0.60% or more, more preferably 0.75% or more. These lower limits may be combined with any of the upper limits described above.

[0046] W: Less than 2.00% W is a component that promotes secondary hardening during tempering and increases quench-temper hardness. Although its addition reduces thermal conductivity, its contribution is minimal, and its hardness-improving effect is significant. Therefore, the W content is set to 0 to less than 2.00%. The upper limit of the W content is preferably 1.00% or less, more preferably 0.50% or less. The W content may be 0% or more. W is a component that promotes secondary hardening during tempering and increases quench-temper hardness. Although its addition reduces thermal conductivity, its contribution is minimal, and its hardness-improving effect is significant. Therefore, when W is added, the lower limit of the W content is preferably 0.05% or more, more preferably 0.10% or more. These lower limits may be combined with any of the upper limits described above.

[0047] V: Less than 0.60% V is a component that promotes secondary hardening during tempering and increases quench-temper hardness, but excessive addition reduces thermal conductivity. Therefore, the V content is set to 0 to less than 0.60%. The upper limit of the V content is preferably 0.55% or less, more preferably 0.50% or less. The V content may be 0% or more. Since V is a component that promotes secondary hardening during tempering and increases quench-temper hardness, when V is added, the lower limit of the V content is preferably 0.20% or more, more preferably 0.30% or more. Each of these lower limits may be combined with any of the above-mentioned upper limits.

[0048] Al: Less than 0.10% Al is a component that forms nitrides and suppresses grain coarsening during quenching. However, adding 0.10% or more of Al reduces toughness due to the formation of excess Al nitrides. Thermal conductivity also decreases. Therefore, the Al content is set to 0 to less than 0.10%. The upper limit of the Al content is preferably 0.07% or less, more preferably 0.04% or less. The Al content may be 0% or more. Since Al forms nitrides and suppresses grain coarsening during quenching, when Al is added, the lower limit of the Al content is preferably 0.001% or more, more preferably 0.002% or more. These lower limits may be combined with any of the above-mentioned upper limits. Note that Al may not be intentionally added, but may be unavoidably mixed in from refractories used in the gas atomization melting process. However, the effect of adding Al is similar in either case.

[0049] Table 1 shows the component compositions (units: mass%) of the steel powders of Examples 1 to 19 and Comparative Examples 1 to 6. The balance (Bal.) is Fe and unavoidable impurities. Note that these Examples are merely examples of embodiments of the present invention, and the scope of the present invention is not limited to these Examples.

[0050]

[0051] [Production of Raw Powder] Powders with the component compositions listed in Table 1 were obtained by gas atomization. The component compositions of the additively manufactured bodies using these powders are also identical to those of the powders. The specific powder production procedure is as follows: First, the molten raw materials placed in an alumina crucible were melted by high-frequency heating in a vacuum and argon atmosphere. The molten alloy was poured from a 5 mm diameter nozzle at the bottom of the crucible and immediately sprayed with high-pressure argon gas. This spraying breaks the molten alloy into fine droplets, which cool and solidify as they fall through the tower of the atomization device, becoming alloy powder. The obtained alloy powder was sieved using a 63 μm mesh sieve, and the powder that passed through the sieve at the bottom was used as the raw powder for the subsequent additive manufacturing.

[0052] Additive manufacturing was performed using a laser-heated powder bed type device (EOS-M290 manufactured by EOS Corporation) under the standard manufacturing conditions for maraging steel (MS1 conditions) specified for the device, with a preheating temperature of 180°C.

[0053] The base material for the molding was an annealed S45C plate, and a cylinder with a diameter of 180 mm and a height of 120 mm (with a curvature of R10 on the outer periphery of the joint interface between the plate and the molded body and on the outer periphery of the top surface of the cylinder) and a square pillar with a width of 15 mm, a length of 150 mm, and a height of 17 mm were molded on top of this.

[0054] [Evaluation] Table 2 shows the surface hardness and center hardness of the shaped bodies of the examples and comparative examples, the difference between these hardnesses (surface hardness - center hardness), thermal conductivity, and the presence or absence of cracks during shaping.

[0055]

[0056] To evaluate the quenched and tempered hardness of the surface and deep portions of the large-sized object, a large cylinder measuring 180 mm in diameter and 120 mm in height was cut from a plate using wire cutting. It was quenched and tempered in an atmospheric furnace, and the Rockwell hardness of the test pieces cut from the surface and deep portions was measured. For quenching, the specimens were held at 1,030°C for 1 hour and oil-cooled. Then, for tempering, the specimens were held at 600°C for 4 hours and air-cooled. The same tempering process was performed twice. However, in Examples 18 and 19, only tempering was performed without quenching. After quenching and tempering, 10 mm square blocks were cut from the outermost and central portions at the center of the large cylinder, and the Rockwell hardness of the surfaces parallel to the stacking direction was measured. The Rockwell hardness of the block cut from the outermost portion was designated the "surface hardness," and the Rockwell hardness of the block cut from the central portion was designated the "center hardness." The Rockwell hardness was measured using a Rockwell hardness tester in accordance with JIS Z 2245:2016.

[0057] The optimum working hardness of various tools varies depending on the application, with some applications requiring around 40 HRC and others requiring around 50 HRC, but if the hardness differs between the surface and deeper parts due to poor hardenability, it becomes difficult to adjust the entire tool to the optimum working hardness, which can lead to problems. Therefore, apart from the absolute value of hardness, the variation in hardness depending on the part was evaluated as the difference between the surface hardness and the center hardness (surface hardness - center hardness).

[0058] As described above, thermal conductivity was measured using test pieces cut from the outermost part of the large cylinder at the center of its height after quenching and tempering.The test pieces were finished into a disk shape with a diameter of 5 mm and a thickness of 1 mm, and the thermal conductivity at room temperature was measured using the laser flash method.

[0059] Since additive manufacturing involves rapid melting and rapid cooling and solidification, the additively manufactured body may be in a pseudo-quenched state after additive manufacturing, so there are cases where the additively manufactured body is used after only tempering or strain relief heat treatment without quenching after additive manufacturing. Therefore, in some examples (No. 18 and No. 19), only tempering was performed without quenching.

[0060] To evaluate crack resistance, a rectangular pillar measuring 15 mm wide, 150 mm long, and 17 mm high was used. Because this pillar lacks the curvature of the interface with the plate like a large circular pillar, the interface is perpendicular, and thermal stress concentrates at this location. Furthermore, thermal stress increases in the longitudinal direction, where the amount of thermal deformation is large. As a result, in the case of a molded object with low crack resistance, cracks occur at the interface between the plate and the molded object at the 150 mm end. Therefore, this area was visually inspected with a magnifying glass, and crack resistance was evaluated based on the presence or absence of cracks and their length.

[0061] All of the molded bodies of Examples 1 to 19 of the present invention have excellent surface and center hardness values ​​of 40 HRC or higher. Furthermore, the difference between the surface and center hardness values ​​is within 1.5 HRC, suppressing hardness variation. Furthermore, the thermal conductivity is 25 W / m / K or higher (most are 30 W / m / K or higher), demonstrating excellent hardness and thermal conductivity. Furthermore, most of the molded bodies exhibit no molding cracks, and even when molding cracks are observed, the crack length is limited to 0.5 mm, demonstrating excellent molded crack resistance.

[0062] Comparative Examples No. 1 and No. 2 contained too little Ni, resulting in variations in the surface and central hardness of the molded body, and significant molding cracks were observed. Comparative Example No. 3 contained too much Ni, resulting in low thermal conductivity. Comparative Example No. 4 contained too much Cr, resulting in low thermal conductivity. Comparative Example No. 5 contained too little C, resulting in low hardness of the molded body. Comparative Example No. 6 contained too much C, resulting in significant molding cracks.

Claims

1. A hot work tool steel powder for molding containing, by mass, as required added components: C: more than 0.10% and less than 0.45%, and Ni: more than 2.00% and less than 8.00%, and as optional added components: one or more of Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Mo: less than 1.20%, W: less than 2.00%, V: less than 0.60%, and Al: less than 0.10%, with the balance consisting of Fe and unavoidable impurities, and Ni + Mn: less than 8.50%.

2. A shaped body produced using the hot work tool steel powder for shaping according to claim 1.

3. The shaped body according to claim 2, which is an additive shaped body.

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