Hot 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, which can be easily quenched into a deep part even in a large molded body, is unlikely to crack in the molded body itself and / or at the interface between the molded body and a molding base material, and can produce a molded body having high thermal conductivity; provided are: a hot-work tool steel powder for molding, which contains, in mass%, more than 0.10% to less than 0.45% of C and more than 2.00% to less than 8.00% of Ni as essential additive components, and which contains, as any additive component, one or more of less than 0.60% of Si, less than 5.00% of Mn, less than 2.00% of Cr, less than 1.20% of Mo, less than 2.00% of W, less than 0.60% of V, and less than 0.10% of Al, with the remainder being Fe and unavoidable impurities, and which is characterized in that: the steel powder for molding contains, in terms of mass%, more than 0.10% to less than 0.45% of C and more than 2.00% to less than 8.00% of Ni; and less than 8.50% of Ni + Mn.

Description

Technical Field

[0001] This invention relates to hot work tool steel powder for molding and molding bodies using the same. Specifically, this invention relates to hot work tool steel powder for molding, and molding bodies, primarily molds, used as tools (e.g., hot work tools) formed by molding methods such as layer molding (also known as 3D printing, three-dimensional molding, additive manufacturing, etc.), particularly concerning hot work tool steel powder for molding that can produce molding bodies, even large molding bodies, exhibiting sufficient hardenability, high resistance to molding cracking, high thermal conductivity, and high mechanical properties, and molding bodies using the same.

[0002] In this specification, "high resistance to molding cracking" means that the molding itself (especially the part where notches are formed) and / or the interface between the molding and the base material is unlikely to crack due to thermal stress caused by rapid melting and solidification during molding. Background Technology

[0003] Layered molding differs from existing construction methods in that it can create components with complex shapes or three-dimensional structures. In recent years, significant technological advancements and expanded applications have been driving this development. Research is underway to explore the application of layered molding to various tools, particularly in the practical application of die-casting molds for complex cooling water pipes with internally integrated three-dimensional structures.

[0004] Tools made of hot work tool steel are used for machining various parts, and their shapes and sizes vary depending on the machining method or the shape of the part. However, the application of the layered molding method in tools is limited to relatively small tools due to the following molding cracks.

[0005] Generally, the layering method uses a focused heat source such as a laser or electron beam to heat powder or filament as raw materials for a short time, rapidly melting and solidifying them. This process is repeated to create layers of solidified material, allowing for the manufacture of complex three-dimensional parts. In this process, because only a portion of the part is heated, melting, and solidifying, thermal stress arises due to localized solidification shrinkage, thermal expansion, and thermal contraction. If the material being modeled or the base material is hard and brittle, it cannot withstand the thermal stress, leading to modeling cracks in the model itself and / or at the interface between the model and the base material.

[0006] Such thermal stress becomes even greater when layering large shapes, resulting in a higher likelihood of molding cracks. Generally, tools are made of high-hardness alloys like SKD61 (compliant with JIS standards), which are also prone to molding cracks. For this reason, the application of layering molding methods in toolmaking has historically been limited to small tools with relatively low thermal stress.

[0007] The surfaces of hot-working tools, such as die-casting molds, are susceptible to damage, primarily thermal cracking, due to their contact with the high-temperature parts being processed. Furthermore, melting is particularly likely to occur in areas where the temperature rises sharply.

[0008] To avoid these problems, it is important to efficiently cool the surface of hot work tools. By using alloys with high thermal conductivity as the material of hot work tools, the cooling effect of water cooling pipes and other components inside the tool can be maximized on the surface of the hot work tool.

[0009] In addition, in the machining of parts using hot working tools, since there is a need for tool cooling after machining one part and before machining the next, it is possible to efficiently (in a short time) cool the tool to the specified temperature, which also has the advantages of shortening the machining cycle of parts and improving the production efficiency of parts.

[0010] As a laminated shape formed from hot work tool steel with high thermal conductivity, the applicant proposes a shape made of, for example, Fe-based alloy powder that satisfies the following formulas (1) to (3), wherein the Fe-based alloy powder, by mass%, contains 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 balance consisting of Fe and unavoidable impurities (see Patent Document 1).

[0011] 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...Equation (1)

[0012] T2 = 80.1 + 2.4Mn + 1.6Si + 7.1Cr - 12.0PC > 50.0... Equation (2)

[0013] Average size (μm) of carbides contained in the model body: PC < 3.0… Equation (3)

[0014] Existing technical documents

[0015] Patent documents

[0016] Patent Document 1: Japanese Patent Application Publication No. 2022-092524 Summary of the Invention

[0017] The problem that the invention aims to solve

[0018] In the aforementioned Patent Document 1, the thermal conductivity decreased due to the increased addition of various elements. Therefore, it was desired to achieve high thermal conductivity by specifying the addition amounts of these elements and the lower limit of the T1 parameter. That is, compared to SKD61, a general-purpose steel widely used in hot-working tools, high thermal conductivity was obtained by reducing the amount of each added element.

[0019] However, since various added elements also help improve hardenability, reducing the amount of these added elements will also lead to a decrease in hardenability.

[0020] Previously, based on the viewpoint that thermal stress causes molding cracks, the application of the stacked molding method was actually limited to relatively small hot-working tools. That is, even when the molded object is large, alloys with low hardenability that cannot achieve sufficient cooling rates during quenching (i.e., insufficient quenching and tempering hardness) at their depths in large molded objects will not have problems and can be applied if the molded object is relatively small.

[0021] However, in response to the recent trend of expanding the application of laminated molding in tools and the demand for using laminated molding in larger hot-working tools, it is necessary to achieve a higher level of balance between hardenability and high thermal conductivity, which are opposite properties, by using various additive elements. Currently, it is not possible to maintain both properties to a high degree in existing alloys.

[0022] Furthermore, since layered molding is a rapid cooling construction method, the layered molded body is also in a state of simulated quenching. Therefore, there are also cases where, after layered molding, no quenching treatment is performed, and only tempering or straightening heat treatment is applied before using the layered molded body. However, in the center of large molded bodies, the heat source of the molding causes heat accumulation and temperature rise. Although it is a rapid cooling construction method, it is difficult to achieve a rapid cooling effect. In alloys with low hardenability, the hardness of the deeper parts of the molded body still tends to decrease, thus hindering the large-scale production of molded bodies.

[0023] In addition, it is known that preheating the molding base material during molding can reduce molding cracks. However, in the design of molding equipment, the preheating temperature is usually in the range of 80 to 300°C. In this case, not only large molding bodies, but also small molding bodies, the rapid cooling effect during molding is difficult to achieve. In alloys with low hardenability, there is a tendency for the hardness of the deep part of the molding body to decrease.

[0024] Furthermore, the optimal hardness for various tools varies depending on their intended use; some applications require around 40 HRC, while others prefer around 50 HRC. However, if the hardenability is poor, resulting in differences in hardness between the surface and deeper layers, it becomes difficult to adjust the overall tool to its optimal hardness, leading to malfunctions. Therefore, in addition to the absolute value of the hardness, minimizing hardness variations between different parts is also crucial.

[0025] Based on the above, the object of the present invention is to provide a hot work tool steel powder for molding, which is capable of producing molded shapes that are easy to harden to a deep depth even when the molded shape is large, and is unlikely to crack in the molded shape itself and / or at the interface between the molded shape and the molded base material, and has high thermal conductivity, as well as a molded shape using the powder.

[0026] Problem-solving methods

[0027] Based on the above background, the inventors have focused on developing a composition for hot work tool steel for molding, which can produce molded bodies with sufficient hardenability and high thermal conductivity, even when the molded body is large. Ni was the element that can achieve high hardenability while minimizing the decrease in thermal conductivity. The results of the research showed that by strictly controlling the amount of Cr and other elements added along with Ni, the two properties can be achieved in a high degree, thus completing the present invention.

[0028] Furthermore, it was found that, as mentioned above, when the manufactured molded body is large, the thermal stress increases, making it prone to molding cracks. However, by specifying the amount of C and other materials added along with Ni and Cr, the hardness of the molded body in the molding state can be reduced, and the resistance to molding cracks is also excellent. That is, the present invention relates to hot work tool steel powder for molding and laminated molded bodies using it. The hot work tool steel powder for molding is practically applicable to larger molded bodies compared to existing hot work tool steel powders for molding, which are mainly suitable for small molded bodies. In this invention, the addition range of Cr and C, with Ni as the main component, is optimized to improve hardenability and minimize the decrease in thermal conductivity. In addition, even if the manufactured molded body is large, it is possible to produce a molded body with high resistance to molding cracks.

[0029] A first means for solving the problem of the present invention is a hot work tool steel powder for molding, wherein, as an essential additive, it contains, by mass%, C: more than 0.10% to less than 0.45% and Ni: more than 2.00% to less than 8.00%, and as an optional additive, it contains one or more of the following: 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 being Fe and unavoidable impurities, and Ni + Mn: less than 8.50%.

[0030] A second means for solving the problem of the present invention is a shaped body formed using hot work tool steel powder for shaping as described in the first means.

[0031] A third means for solving the problem of the present invention is a model body according to the second means, wherein the model body is a layered model body.

[0032] The effects of the invention

[0033] According to the present invention, it is possible to provide a hot work tool steel powder for molding and a molding body using the same, which can produce a molding body that is easy to harden to a deep depth, is difficult to crack in the molding body itself and / or at the interface between the molding body and the molding base material, and has high thermal conductivity, even if the molding body is a large molding body.

[0034] Furthermore, according to the present invention, even when a molded body is obtained by a molding method that omits quenching, a molding method that produces small molded bodies, or a molding method that preheats the molding base material during molding, it is possible to achieve excellent effects such as high hardenability and high thermal conductivity. Detailed Implementation

[0035] This invention relates to hot work tool steel. The hot work tool steel of this invention is characterized by, by mass%, as essential additives, containing C: more than 0.10% to less than 0.45% and Ni: more than 2.00% to less than 8.00%; as optional additives, containing one or more of the following: 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%; the balance comprising Fe and unavoidable impurities; and Ni + Mn: less than 8.50%.

[0036] Furthermore, this invention relates to hot work tool steel powder. The hot work tool steel powder of this invention is a powder comprising the hot work tool steel of this invention. The hot work tool steel powder of this invention can be used as a material for molding (preferably layered molding).

[0037] Furthermore, the present invention relates to hot work tool steel powder for molding. The hot work tool steel powder for molding of the present invention is a powder used as a material for molding (preferably layered molding), and is a powder containing the hot work tool steel of the present invention.

[0038] Furthermore, this invention relates to molding bodies. The molding body of this invention is a molding body formed using the hot work tool steel powder of this invention. Preferably, the molding body of this invention is a layered molding body formed by layering the hot work tool steel powder of this invention. Like the hot work tool steel powder of this invention, the molding body of this invention is preferably formed from the hot work tool steel of this invention. The molding body of this invention, after undergoing one or more required processing steps (e.g., quenching, tempering, cutting, etc.), becomes a hot work tool.

[0039] Hot work tool steel is an alloy steel suitable for use in hot work tools. Examples of hot work tools include molds, and examples of molds include die-casting molds such as aluminum die-casting molds.

[0040] Specific examples of methods for producing hot work tool steel powder include water atomization, single-roller quenching, double-roller quenching, gas atomization, disc atomization, and centrifugal atomization. From the viewpoint of spheroidization, gas-atomized powder is preferred for hot work tool steel. After the powder is prepared and its particle size is adjusted by sieving, it can be used as a raw material powder for molding.

[0041] Specific examples of methods for fabricating shapes include rapid melting and solidification processes involving the melting and solidification of hot work tool steel powder. Examples of this process include three-dimensional stacking, thermal spraying, laser cladding, and welding. Specific examples of three-dimensional stacking include powder bed fusion bonding and directional energy deposition (PED). Specific examples of powder bed fusion bonding include selective laser sintering (SLS), selective laser melting (SLM), and electron beam melting (EBM). Hot work tool steel powder is suitable for three-dimensional stacking, especially for powder bed fusion bonding, enabling the high-density formation of large-sized stacked shapes.

[0042] In three-dimensional layered modeling, for example, a 3D printer can be used. In powder-bed fusion bonding layered modeling, a laser beam or electron beam is irradiated onto a bed of hot work tool steel powder. Upon irradiation, the particles are rapidly heated and melted. The molten particles then rapidly solidify. Through this melting and solidification, the particles bond together. Irradiation is selectively applied to a portion of the bed of hot work tool steel powder. Unirradiated portions of the powder do not melt. A bonding layer forms only in the irradiated areas.

[0043] A thin layer of hot work tool steel powder is then laid on top of the bonding layer. A portion of this powder is irradiated with a laser or electron beam. Irradiation causes the particles to melt rapidly. The molten particles then solidify rapidly. Through this melting and solidification process, the particles in the powder bond together to form a new bonding layer. This new bonding layer also bonds with the existing bonding layer.

[0044] Through repeated irradiation, the resulting aggregate of layers gradually grows. This growth process yields a layered model with a three-dimensional shape. Based on this layering method, complex shapes can be easily obtained.

[0045] In this specification, the "quenching and tempering hardness" of a molded object refers to the hardness of an object that has undergone quenching and tempering after molding. However, due to the rapid melting and solidification associated with stacked molding, the stacked object in this state is also in a state that simulates quenching. Therefore, there are cases where the stacked object is used without quenching after molding, but only undergoes tempering or straightening heat treatment. In this case, the "quenching and tempering hardness" of the stacked object refers to the hardness of the stacked object without quenching after molding, but only undergoes tempering or straightening heat treatment.

[0046] The reasons for specifying the content of the essential additives (C and Ni) and the optional additives (one or more of Si, Mn, Cr, Mo, W, V, and Al) in the hot work tool steel of the present invention will be explained below. The percentage (%) for each component is by mass. The balance other than the essential and optional additives is Fe and unavoidable impurities.

[0047] C: Above 0.10% to below 0.45%

[0048] Carbon (C) is an essential component for achieving high quenching and tempering hardness through solid solution in the martensitic phase of the matrix and the precipitation of fine carbides. However, if the C content is below 0.10%, high quenching and tempering hardness cannot be obtained. Therefore, the lower limit of the C content is higher than 0.10%. The lower limit of the C content is preferably 0.20% or more, and more preferably 0.30% or more. On the other hand, if the C content is 0.45% or more, the hardness of the molded body in the molded state increases excessively, resulting in a greater effect on resistance to mold cracking deterioration compared to other components, and the amount of solid-dissolved C increases, leading to a decrease in thermal conductivity. Therefore, the upper limit of the C content is lower than 0.45%. The upper limit of the C content is preferably 0.42% or less, and more preferably 0.40% or less. These upper limits can be arbitrarily combined with the aforementioned lower limits.

[0049] Ni: Above 2.00% to Below 8.00%

[0050] Ni is an essential component for improving hardenability and maintaining high hardness even in the deep layers of large molded bodies after quenching and tempering. Since its effect on reducing thermal conductivity is relatively small compared to other components, it is the most important component in this invention. Furthermore, Ni also delays the martensitic phase transformation during cooling during molding, and by maintaining the soft, crack-resistant austenite at a relatively low temperature, it also improves resistance to molding cracking. However, if the Ni content is below 2.00%, this effect is insufficient. Therefore, the lower limit of the Ni content is higher 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 solid solution content in the matrix increases, resulting in a significant decrease in thermal conductivity. Therefore, the upper limit of the Ni content is lower than 8.00%. The upper limit of the Ni content is preferably 6.50% or less, more preferably 5.00% or less. These upper limits can be arbitrarily combined with the aforementioned lower limits.

[0051] Si: below 0.60%

[0052] Si is a component that increases hardness by being dissolved in the matrix. Additionally, Si also improves resistance to softening. Therefore, the Si content is 0% to less than 0.60%. If the Si content is 0.60% or more, the amount of dissolved Si increases, resulting in a significant decrease in thermal conductivity. Therefore, the upper limit of the Si content is 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 can be 0% or more. Since Si is a component that improves hardness and resistance to softening, when adding Si, the lower limit of the Si content is preferably 0.04% or more, more preferably 0.10% or more. These lower limits can be arbitrarily combined with the aforementioned upper limits.

[0053] Mn: below 5.00%

[0054] Mn is a component that improves hardenability and enhances hardness during quenching and tempering, even in the deep parts of large molded objects. Additionally, Mn also improves resistance to softening. Therefore, the Mn content is 0% to less than 5.00%. If the Mn content is 5.00% or higher, the amount of dissolved Mn increases, leading to a decrease in thermal conductivity. Therefore, the upper limit of the Mn content is 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 can be 0% or higher. When adding Mn, from the viewpoint that it can improve hardness during quenching and tempering, or enhance resistance to softening, even in the deep parts of large molded objects, the lower limit of the Mn content is preferably 0.05% or more, more preferably 0.11% or more. These lower limits can be arbitrarily combined with the aforementioned upper limits.

[0055] Ni + Mn: below 8.50%

[0056] Mn is a component that has effects similar to Ni. However, if the combined content of Mn and Ni is 8.50% or more, the solid solution content in the matrix increases, resulting in a significant decrease in thermal conductivity. Therefore, the combined content of Ni and Mn is less than 8.50%. The combined content of Ni and Mn is preferably 7.00% or less, more preferably 5.00% or less.

[0057] Cr: less than 2.00%

[0058] Cr (Cr) improves hardenability, increasing hardness even in the deep parts of large molded objects after quenching and tempering. It also improves resistance to softening. However, if the Cr content is 2.00% or higher, the increased amount of dissolved Cr leads to a decrease in thermal conductivity, a reduction more significant than that of other components. Therefore, the Cr content is 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 can be 0% or higher. Since Cr improves hardenability, increasing hardness even in the deep parts of large molded objects, and also improves resistance to softening, the lower limit of the Cr content is preferably 0.50% or more, more preferably 0.85% or more. These lower limits can be combined with the aforementioned upper limits in any way.

[0059] Mo: less than 1.20%

[0060] Mo promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials. Furthermore, although adding Mo reduces thermal conductivity, its contribution is small, while its effect on increasing hardness is significant. Therefore, the Mo content is 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 can be 0% or more. Since Mo promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials, and although adding Mo reduces thermal conductivity, its contribution is small, while its effect on increasing hardness is significant, the lower limit of the Mo content is preferably 0.60% or more, more preferably 0.75% or more. These lower limits can be arbitrarily combined with the aforementioned upper limits.

[0061] W: Below 2.00%

[0062] W promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials. Furthermore, although adding W reduces thermal conductivity, its contribution is small, while its effect on increasing hardness is significant. Therefore, the W content is 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 can be 0% or more. W promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials. Although adding W reduces thermal conductivity, its contribution is small, while its effect on increasing hardness is significant. Therefore, when adding W, the lower limit of the W content is preferably 0.05% or more, more preferably 0.10% or more. These lower limits can be arbitrarily combined with the aforementioned upper limits.

[0063] V: Below 0.60%

[0064] Vitamin V promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials, but excessive addition reduces thermal conductivity. Therefore, the V content is 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 can be 0% or more. Since V promotes secondary hardening during tempering and increases the hardness of quenched and tempered materials, the lower limit of the V content is preferably 0.20% or more, more preferably 0.30% or more. These lower limits can be combined with the aforementioned upper limits in any way.

[0065] Al: Below 0.10%

[0066] Al is a component that forms nitrides and suppresses grain coarsening during quenching. However, if more than 0.10% of Al is added, excess Al nitrides are formed, leading to a decrease in toughness. Additionally, thermal conductivity also decreases. Therefore, the Al content is 0% to less than 0.10%. The upper limit of the Al content is preferably less than 0.07%, more preferably less than 0.04%. The Al content can be 0% or higher. Since Al is a component that forms nitrides and suppresses grain coarsening during quenching, the lower limit of the Al content is preferably more than 0.001%, more preferably more than 0.002%. These lower limits can be combined with the aforementioned upper limits in any way. Furthermore, Al may be added unintentionally, possibly due to unavoidable contamination from refractory materials used in gas atomization melting; the effect of containing Al is the same in all cases.

[0067] Example

[0068] Table 1 shows the composition (in mass %) of the steel powders of Examples No. 1-19 and Comparative Examples No. 1-6. The balance (Bal.) is Fe and unavoidable impurities. Furthermore, these examples are illustrative embodiments of the present invention, and the scope of the invention is not limited to these examples.

[0069] Table 1

[0070]

[0071] [Preparation of raw material powder]

[0072] The powders with the compositions shown in Table 1 were obtained by gas atomization. Furthermore, the composition of the laminated bodies formed using these powders is identical to that of the powders themselves. The specific manufacturing process for the powders is as follows: First, molten raw materials loaded into an alumina crucible are melted by high-frequency heating in a vacuum and argon atmosphere. The molten alloy is discharged from a 5mm diameter nozzle at the bottom of the crucible and then sprayed with high-pressure argon gas. The molten alloy is broken into fine droplets by the spray, which fall and cool within the cooling tower of the atomization device, solidifying into alloy powder. The alloy powder obtained by sieving through a 63μm mesh sieve, with the powder passing through the lower side of the sieve, is used as the raw material powder in subsequent laminated molding.

[0073] [Layered Design]

[0074] Using a laser-heated powder bed apparatus (EOS-M290 manufactured by EOS Corporation), layer molding is performed at a preheating temperature of 180°C according to the standard molding conditions (MS1 conditions) for maraging steel specified by the apparatus.

[0075] The sheet material used as the base material is annealed S45C. On it, a cylinder with a diameter of 180mm and a height of 120mm is shaped (with an R10 curvature set on the outer periphery of the interface between the sheet material and the shaped body and the outer periphery of the top surface of the cylinder), and a prism with a width of 15mm, a length of 150mm and a height of 17mm are shaped.

[0076] [evaluate]

[0077] Table 2 shows the surface hardness and center hardness of the molded bodies for the embodiments and comparative examples, the difference in hardness (surface hardness - center hardness), thermal conductivity, and whether cracks occurred during molding.

[0078] Table 2

[0079]

[0080] To evaluate the quenching and tempering hardness of the surface and deep portions of large-scale structures, large cylinders with a diameter of 180 mm and a height of 120 mm were cut from sheet metal by wire cutting, quenched and tempered in an atmospheric furnace, and the Rockwell hardness of test pieces taken from the surface and deep portions was measured. Quenching was performed by holding at 1030°C for 1 hour followed by oil cooling. Tempering was then performed by holding at 600°C for 4 hours followed by air cooling. The same tempering process was repeated twice. However, in Examples No. 18 and 19, only tempering was performed without quenching. 10 mm square blocks were cut from the outermost periphery and the center of the quenched and tempered large cylinder at the center of its height, respectively. The Rockwell hardness of the surface parallel to the stacking direction was measured. The Rockwell hardness of the block cut from the outermost periphery was taken as the "surface hardness," and the Rockwell hardness of the block cut from the center was taken as the "center hardness." Rockwell hardness was measured using a Rockwell hardness tester according to JIS Z 2245:2016.

[0081] The optimal hardness for various tools varies depending on their intended use. While some applications may have a hardness of around 40 HRC and others around 50 HRC, poor hardenability can lead to differences in hardness between the surface and deeper layers. This makes it difficult to adjust the overall hardness of the tool to its optimal level, potentially causing malfunctions. Therefore, in addition to the absolute value of hardness, the deviation in hardness between different areas is evaluated as the difference between surface hardness and core hardness (surface hardness - core hardness).

[0082] Similarly, thermal conductivity was measured using a test piece cut from the outermost periphery at the center of the height of a large, quenched and tempered cylinder. The room temperature thermal conductivity was measured using a test piece machined into a circular plate shape with a diameter of 5 mm and a thickness of 1 mm, employing a laser flash method.

[0083] Furthermore, since layered molding involves rapid melting and solidification, the layered molded body in the layered molding state is also in a state that simulates quenching. Therefore, there are also cases where quenching is not performed after layered molding, but only tempering or straightening heat treatment is performed before using the layered molded body. Therefore, in some embodiments (No. 18, No. 19), quenching is not performed, and only tempering is performed.

[0084] In evaluating resistance to molding cracking, a prism measuring 15mm wide × 150mm long × 17mm high was used. This prism lacks the curvature of a large cylinder at the interface with the sheet metal; the interface is a right angle, causing thermal stress concentration at this location. Furthermore, thermal stress increases along the longitudinal direction where thermal deformation is greatest. As a result, in molded objects with low resistance to molding cracking, molding cracks occur at the 150mm long end of the molded object interface with the sheet metal. Therefore, this area is visually inspected under magnification, and the presence and length of cracks are used to evaluate resistance to molding cracking.

[0085] The molded bodies of Embodiments No. 1 to 19 of the present invention have a surface hardness and a core hardness of 40 HRC or higher, exhibiting excellent hardness. Furthermore, the difference between the surface hardness and the core hardness is within 1.5 HRC, suppressing hardness deviation. In addition, the thermal conductivity is 25 W / m / K or higher (most are 30 W / m / K or higher), demonstrating excellent hardness and thermal conductivity. Moreover, regarding molding cracks, most were not found; even when molding cracks were found, their length remained at 0.5 mm, exhibiting excellent resistance to molding cracking.

[0086] In comparative examples No.1 and No.2, there was too little Ni, resulting in a discrepancy between the surface hardness and the center hardness of the molded object, and large molded cracks were found.

[0087] In Comparative Example No. 3, there is too much Ni, resulting in low thermal conductivity.

[0088] Comparative Example No. 4 has too much Cr, resulting in low thermal conductivity.

[0089] In Comparative Example No. 5, there is too little C, resulting in low hardness of the model.

[0090] In Comparative Example No. 6, there was too much C, and large cracks were found in the shape.

Claims

1. A hot work tool steel powder for molding, wherein, As an essential additive, it contains (by weight%) C: Above 0.10% and below 0.45%, and Ni: Above 2.00% and below 8.00%, As an optional additive, it contains one or more of the following ingredients: Si: below 0.60%, Mn: below 5.00%, Cr: less than 2.00% Mo: less than 1.20%, W: Below 2.00% V: below 0.60%, and Al: below 0.10%, The balance includes Fe and unavoidable impurities. Furthermore, Ni + Mn: less than 8.50%.

2. A molded body formed using hot work tool steel powder as described in claim 1.

3. The shape according to claim 2, wherein, The model is a layered model.

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