Hot work tool steel powder characterized by a high tempering width and additive manufacturing body using the same

Abstract

To provide a powder for hot work tools used in additive manufacturing that excels in both high thermal conductivity and resistance to molding cracks, and an additively manufactured body produced using this powder. [Solution] A hot work tool steel powder for additive manufacturing, comprising, by mass%, C: 0.10~0.39%, Si: 0.60% or less, Mn: 5.00% or less, Cr: 1.99% or less, Ni: 8.00% or less, Mo: 1.20% or less, W: 2.00% or less (including 0.00%), V: 0.60% or less, Cu: 3.01~less than 6.00%, Al: less than 0.100% (including 0.000%), the remainder being Fe and unavoidable impurities, and Ni+Mn: 8.50% or less, and an additively manufactured body made using this powder.

Description

Technical Field

[0001] The present invention relates to hot work tool steel powder having a high tempering hardening width suitable for laminated molding and a laminated molded body using the same. That is, the present invention relates to a molded body serving as various tools such as a mold by a laminated molding method (also called 3D printer, three-dimensional molding method, Additive Manufacturing, additive manufacturing method, etc.), and hot work tool steel powder suitable for laminated molding. Particularly, even in a large-sized molded body, it relates to hot work tool steel powder that exhibits sufficient hardenability, high resistance to molding cracking, high thermal conductivity, and can also have high mechanical properties.

[0002] Here, the high resistance to molding cracking means that cracks are unlikely to occur in the molded body itself (particularly in the notch part) or at the interface between the molded body and the molding base material due to thermal stress and the like accompanying rapid melting and solidification by laminated molding.

Background Art

[0003] Unlike conventional manufacturing methods, the laminated molding method can manufacture members having complex shapes and three-dimensional structures, and in recent years, remarkable technological development and expansion of the application range have been progressing. Consideration has been made for applying the laminated molding method to various tools, and particularly, practical application is being attempted in a die-cast mold having a complex cooling water pipe with a three-dimensional structure inside.

[0004] Now, tools using hot work tool steel are also used for processing various parts, but their shapes and sizes vary depending on the processing method and part shape. However, the application of laminated molding to tools has been limited to relatively small ones due to the following molding cracks.

[0005] Generally, additive manufacturing involves rapidly melting and solidifying raw materials such as powders or wires by heating them for a short time using a narrowly focused heat source such as a laser or electron beam, and then repeating this process to build up solidified layers, enabling the manufacture of complex three-dimensional parts. In this process, only a portion of the part is heated, melted, and solidified, resulting in thermal stress due to localized solidification shrinkage and thermal expansion / contraction. If the material being fabricated or the base material is hard and brittle, it cannot withstand the generated thermal stress, causing cracks to form in the fabricated object itself or at the interface with the base material.

[0006] Such thermal stresses become even greater when additively manufacturing large objects, resulting in a higher likelihood of cracking during printing. Since tools generally use high-hardness alloys such as JIS standard SKD61, cracking is particularly common. For these reasons, the application of additive manufacturing tools has traditionally been limited to small objects with relatively low thermal stresses.

[0007] The surfaces of hot working tools, including die-casting molds, are in contact with the high-temperature parts (workpieces) being processed, causing their temperature to rise and making them susceptible to damage such as heat checks. Furthermore, areas where the temperature rises particularly sharply are prone to seizing.

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

[0009] Furthermore, in parts machining using hot tools, the tool needs to cool down after machining one part before machining the next. Therefore, being able to efficiently (quickly) cool the tool to the specified temperature has the advantage of shortening the machining cycle for parts and improving the production efficiency of parts.

[0010] As a laminated structure made of hot work tool steel with such high thermal conductivity, the applicant has proposed, for example, a structure made of 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, and the balance being Fe and inevitable impurities, which satisfies the following formulas (1) to (3) (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 - 0.6P > 32.0 ··· Formula (1) T2 = 80.1 + 2.4Mn + 1.6Si + 7.1Cr - 12.0P > 50.0 ··· Formula (2) Average size (μm) of carbides contained in the structure: P < 3.0 ··· Formula (3)

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0012] In hot work tool steel, various alloy elements are added to improve the tempering hardness. The added alloy elements dissolve in the matrix. Then, the alloy elements dissolved in the matrix generally act to lower the thermal conductivity in order to increase the scattering frequency of the conduction electrons in the matrix. Therefore, in order to increase the thermal conductivity, it is required to reduce the alloy elements as much as possible. On the other hand, it is also necessary to ensure the tempering hardness required for die-casting mold steel.

[0013] Patent Document 1 aims to ensure the temper hardness required for die-casting molds by reducing the amount of additive elements compared to SKD61, thereby increasing thermal conductivity. However, when the Fe-based alloy from Patent Document 1 is used as a powder material for additive manufacturing, the as-printed hardness is also high, resulting in low toughness and the possibility of cracking during manufacturing. Furthermore, the temper hardening width is low and insufficient.

[0014] The problem that this invention aims to solve is to provide a hot-working tool powder that is excellent in both high thermal conductivity and resistance to molding cracking, and an additively manufactured body made using this powder. [Means for solving the problem]

[0015] The inventors focused on improving the temper hardening width to enhance resistance to molding cracks. As described in Patent Document 1, aiming to improve temper hardness results in high as-print hardness, which can easily lead to molding cracks.

[0016] As a result of diligent research, we found that while the as-form hardness is comparable to that of conventional hot work tool steels, the tempered hardness is improved, sometimes exceeding the hardness of typical die-casting molds. High as-form hardness can lead to low toughness, making it difficult to withstand the thermal stress generated during molding, which can cause cracking. However, by reducing the as-form hardness, it is possible to reduce cracking during molding.

[0017] First, we discovered that adding Cu improves the tempering hardening width. We also found that optimizing the amounts of Cu and C added makes it possible to suppress the reduction in thermal conductivity. By taking advantage of the improved tempering hardening width provided by Cu and reducing the amount of C, we were able to maintain the hardness required for die-casting molds after tempering while reducing the as-form hardness. Therefore, by strictly defining the amounts of Cu and C added, we discovered that high thermal conductivity and resistance to mold cracking can be achieved at a high level simultaneously, leading to the present invention.

[0018] The first means for solving the problems of the present invention is hot-work tool steel powder for additive manufacturing, which consists of, by mass%, C: 0.10 to 0.39%, Si: 0.60% or less, Mn: 5.00% or less, Cr: 1.99% or less, Ni: 8.00% or less, Mo: 1.20% or less, W: 2.00% or less (including 0.00%), V: 0.60% or less, Cu: more than 3.01% and less than 6.00%, Al: less than 0.100% (including 0.000%), with the balance being Fe and inevitable impurities, and Ni + Mn: 8.50% or less.

[0019] The second means is a shaped body formed by additive manufacturing using the hot-work tool steel powder for additive manufacturing according to the first means.

Advantages of the Invention

[0020] The Fe-based alloy powder according to the means of the present invention maintains a high thermal conductivity of 25.0 W / (m·K) or more, has a low hardness of 48.0 HRC or less as-built, is excellent in crack resistance during manufacturing, and the tempering hardness is 48.0 to 53.0 HRC, which can be suitably applied when additive manufacturing a hot-work tool steel that meets the characteristics required for molds.

[0021] Moreover, according to the means of the present invention, excellent effects can be exerted, such as achieving both high hardenability and high thermal conductivity even when applied to cases where a step of quenching is omitted, small-scale additive manufacturing, or obtaining a shaped body in a preheated additive manufacturing process.

Embodiments for Carrying Out the Invention

[0022] Prior to the description of the embodiments for carrying out the present invention, the reasons for defining the components of the hot-work tool steel powder for additive manufacturing used in the shaped body of the present invention will be described. Note that the % for each component is mass%. The balance of the components is Fe and inevitable impurities.

[0023] C: 0.10 to 0.39% C is a component that strengthens the matrix by solid solution and further forms carbides to promote the precipitation effect. When C is 0.10% or more, sufficient hardening and tempering hardness can be obtained. Therefore, C is preferably 0.10% or more, more preferably 0.15% or more, and still more preferably 0.20% or more. On the other hand, when C exceeds 0.39%, the amount of C dissolved in the matrix increases, reducing the thermal conductivity of the steel. Also, the hardness becomes excessively high as it is, causing shaping cracks and cracks during use in the mold. Therefore, C is set to 0.39% or less, preferably 0.37% or less, and more preferably 0.35% or less.

[0024] Si: 0.60% or less Si is a component that improves hardness by solid solution in the matrix. It also has the effect of improving the softening resistance. Therefore, Si is set to 0 to 0.60% or less. However, when Si exceeds 0.60%, the amount of dissolved Si increases, greatly reducing the thermal conductivity. Therefore, the upper limit of Si is set to 0.60% or less. Preferably, Si is 0.40% or less, and more preferably Si is 0.24% or less. On the other hand, Si may be 0%, but since it is a component that improves hardness and softening resistance, when adding Si, 0.04% or more is preferable, and more preferably 0.10% or more.

[0025] Cr: 2.00% or less Cr is a component that improves hardenability and also improves hardening and tempering hardness even in the deep part of a large-shaped body. It also has the effect of improving the softening resistance. However, when Cr exceeds 2.00%, the amount of dissolved Cr increases, reducing the thermal conductivity, and the reduction effect is relatively large compared to other components. Therefore, Cr is set to 0 to 2.00%. Preferably, Cr is 1.50% or less, and more preferably Cr is 1.15% or less. Cr may be 0%, but since it is a component that improves hardenability and also improves hardening and tempering hardness even in the deep part of a large-shaped body, and it also has the effect of improving the softening resistance, when adding Cr, preferably it is 0.50% or more, and more preferably it is 0.85% or more.

[0026] Ni: 8.00% or less Ni is an essential component for improving hardenability and maintaining hardness even in the deepest parts of the molded object. Furthermore, the addition of Ni prevents martensitic transformation from progressing during molding, allowing the material to remain austenite and thus absorbing thermal deformation strain. For this reason, Ni content of 0.30% or more is preferred, 2.00% or more is more preferred, and 2.30% or more is even more preferred. However, since Ni dissolves in the matrix without forming carbides, adding more than 8.00% significantly reduces the thermal conductivity. Therefore, the amount of Ni should be 8.00% or less, preferably 6.50% or less, and more preferably 5.00% or less.

[0027] Mn: 5.00% or less Mn is a component that improves hardenability and enhances quenching and tempering hardness even in the deep parts of large molded bodies. It also has the effect of improving softening resistance. Therefore, the amount of Mn should be 0 to 5.00% or less. However, if the amount of Mn exceeds 5.00%, the amount of solid-solution Mn increases and the thermal conductivity decreases. Therefore, the amount of Mn should be 5.00% or less. Preferably, the amount of Mn is 1.00% or less, and more preferably, the amount of Mn is 0.41% or less. On the other hand, the amount of Mn may be 0%, but if Mn is added, from the viewpoint of improving deep quenching and tempering hardness and improving softening resistance, the amount of Mn is preferably 0.05% or more, and more preferably, the amount of Mn is 0.11% or more.

[0028] Ni+Mn:8.50% or less Furthermore, Mn is a component that has a similar effect to Ni. Therefore, if the total amount of Mn and Ni exceeds 8.50%, the amount of solid solution in the matrix increases, significantly reducing the thermal conductivity. For this reason, Ni + Mn should be 8.50% or less. Preferably, Ni + Mn is 7.00% or less, and more preferably 5.00% or less.

[0029] Mo: 1.20% or less Mo is a component that promotes secondary hardening during tempering and increases quenched-temper hardness. Although the addition of Mo reduces thermal conductivity, its contribution is small, and its effect on improving hardness is large. Therefore, the amount of Mo is set to 0-1.20%. Preferably, the amount of Mo is 1.05% or less, and more preferably 0.95% or less. Mo may be 0%, but when Mo is added, it promotes secondary hardening during tempering and increases quenched-temper hardness, while the addition reduces thermal conductivity, but its contribution is small, and its effect on improving hardness is large. Therefore, preferably, the amount of Mo is 0.60% or more, and more preferably 0.75% or more.

[0030] W: 2.00% or less (including 0.00%) W is a component that promotes secondary hardening during tempering and increases quenched-temper hardness. Although the addition of W reduces thermal conductivity, its contribution is small, and its effect on improving hardness is large. Therefore, the amount of W should be 0 to 2.00% or less. Preferably, W should be 1.00% or less, and more preferably 0.50% or less. W may be 0%, but since it is a component that promotes secondary hardening during tempering and increases quenched-temper hardness, and although its addition reduces thermal conductivity, its contribution is small, and its effect on improving hardness is large, when W is added, it is preferably 0.05% or more, and more preferably 0.10% or more.

[0031] V:0.60% or less V is a component that promotes secondary hardening during tempering and increases quenched-temper hardness, but excessive addition reduces thermal conductivity. Therefore, V should be 0 to 0.60% or less. Preferably, V is 0.55% or less, and more preferably 0.50% or less. V may be 0%, but since it is a component that promotes secondary hardening during tempering and increases quenched-temper hardness, if added, V is preferably 0.20% or more, and more preferably 0.30% or more.

[0032] Cu: 3.01-6.00% Cu is a component that precipitates from the Fe matrix as an intermetallic compound during heat treatment. This precipitation hardening treatment takes place at a temperature of approximately 500-600°C, which is within the tempering temperature range for tool steel. Cu can further increase the hardness after heat treatment. Furthermore, it suppresses the decrease in thermal conductivity compared to adding other elements to improve hardness. For this reason, the amount of Cu is set to 3.01% or more, preferably 3.25% or more, and more preferably 3.50% or more. However, if the Cu content is 6.00% or higher, the effect of Cu saturates, and the thermal conductivity decreases excessively. Therefore, the Cu content should be less than 6.00%, preferably 5.75% or less, and more preferably 5.50% or less.

[0033] Al: Less than 0.100% (including 0.000%) Al is a component that forms nitrides and suppresses grain coarsening during quenching. However, if Al is added at a concentration of 0.100% or more, the toughness decreases due to the formation of excess Al nitrides. It also reduces thermal conductivity. Therefore, the amount of Al should be between 0% and less than 0.100%. Preferably, Al is 0.070% or less, and more preferably 0.040% or less. Although Al may be 0%, since it is a component that forms nitrides and suppresses grain coarsening during quenching, if Al is added, preferably it is 0.001% or more, and more preferably 0.002% or more. Note that Al may not be intentionally added but may be introduced when the molten metal reacts with refractories used in gas atomization dissolution, but the effect of Al content is the same whether it is added or mixed in.

[0034] (Examples) The raw materials consisting of the chemical components of Examples No. 1 to 20 listed in Table 1 and Comparative Examples No. 1 to 5 listed in Table 2 (values ​​are in mass percent; the remainder is Fe and unavoidable impurities) were gas atomized to obtain a powder. First, each raw material was heated in a vacuum in an alumina crucible using high-frequency induction heating to form a molten alloy. Then, the molten alloy was dropped through a 5 mm diameter nozzle located at the bottom of the crucible, and high-pressure argon gas was injected into the molten metal. This injection refined the molten metal and rapidly cooled it, yielding a large number of fine powders. The obtained powders were classified so that the diameter of each particle was 63 μm or less, yielding Fe-based alloy powders for Examples 1-20 and Comparative Examples 1-5. Note that the underlined portions in Table 2 indicate that they fall outside the component range defined by the present invention.

[0035] [Table 1]

[0036] [Table 2]

[0037] [molding] The Fe-based alloy powder prepared is used to manufacture additively fabricated bodies. In the following examples, powder bed additive manufacturing is used as a representative additive manufacturing method. Of course, bodies can also be obtained using the powder of the present invention with other additive manufacturing methods.

[0038] The powders of Examples 1-20 and Comparative Examples 1-5 were fabricated using a three-dimensional additive manufacturing system (product name "EOS-M290"). This resulted in the production of 10mm x 10mm x 10mm cubes and 15mm wide x 150mm long x 17mm high rectangular prisms. The fabrication conditions were equivalent to MS1 conditions (standard conditions for maraging steel).

[0039] [Evaluation of resistance to mold cracking] A rectangular prism measuring 15 mm wide x 150 mm long x 17 mm high was used to evaluate resistance to build cracks. When this shape is built, the stress caused by the build process is concentrated at the interface with the build plate. Therefore, the presence or absence of build cracks was checked by observing the interface after build, and if cracks were found, their length was measured. Tables 3 and 4 show the results regarding the presence or absence of build cracks.

[0040] [Heat treatment] Each test specimen of the resulting molded body underwent a tempering heat treatment, which involved "holding at 550°C for 60 minutes followed by air cooling," repeated twice.

[0041] [Thermal conductivity measurement] For measuring thermal conductivity, the laser flash method was used. Tempered samples were processed into a disc shape with a diameter of 5 mm and a thickness of 1 mm for testing. The results are shown in Tables 3 and 4.

[0042] [Hardness measurement] The hardness of the printed object was measured using a Rockwell hardness tester, measuring the hardness of the surface perpendicular to the layering direction for both the as-printed and tempered samples. Tables 3 and 4 show the results for as-printed and tempered hardness, respectively.

[0043] [Table 3]

[0044] [Table 4]

[0045] Note that the underlined portions in Table 4 indicate characteristics that fall outside the scope of the present invention.

[0046] Examples 1 to 20 of the present invention are additively fabricated bodies made from Fe-based alloy powder with the chemical composition defined in the present invention. The as-printed hardness was low, at 48.0 HRC or less, and no cracking was observed. Furthermore, the thermal conductivity remained high at 25.0 W / (m·K) or higher, indicating excellent cooling efficiency. Moreover, the tempered hardness met the hardness of 48.0 to 53.0 HRC required for die-casting molds. From these viewpoints, using the hot-work tool steel powder of the present invention yields fabricated bodies suitable for molds.

[0047] The molded objects created with the comparative powder exhibited inferior thermal conductivity, resistance to mold cracking, and temper hardness. For example, Comparative Example 1 had a small amount of added Cu and a small tempering width. As a result, even though it achieved the tempering hardness required for die-casting molds, the as-printed hardness was high, and cracking was observed. Comparative Example 2 had an excessive amount of Cu, resulting in a decrease in thermal conductivity. Comparative Example 3 lacked sufficient carbon content and did not achieve the desired temper hardness. Comparative Example 4 had an excessive amount of carbon, resulting in high hardness in the as-printed state, and cracking was also observed. Comparative Example 5 had excessive amounts of Ni and Ni+Mn, resulting in reduced thermal conductivity. [Industrial applicability]

[0048] The additively fabricated body using the hot tool steel powder of the present invention is suitable as a body for hot stamping and hot molds for die casting.

Claims

1. In mass percent, C: 0.10% to 0.39%, Si: 0.60% or less, Mn: 5.00% or less, Cr: 1.99% or less, Ni: 8.00% or less, Mo: 1.20% or less W: 2.00% or less (including 0.00%) V: 0.60% or less, Cu: 3.01% to less than 6.00% Al: Less than 0.100% (including 0.000%) The remainder consists of Fe and unavoidable impurities, Ni + Mn: 8.50% or less Hot work tool steel powder for additive manufacturing.

2. A fabricated body produced by additive manufacturing using the hot tool steel powder for additive manufacturing described in claim 1.

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