Hot work tool steel powder and molded article for additive manufacturing using the same

Abstract

To provide a hot work tool steel powder exhibiting superior thermal conductivity and superior resistance to molding cracking, and a molded article for additive manufacturing that is formed from the powder.SOLUTION: An Fe-based alloy powder comprises, in mass%, C: more than 0.10% to less than 0.45%, Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Ni: less than 8.00%, Mo: less than 2.30%, W: less than 2.00%, V: more than 0.6% to less than 1.50%, and Al: less than 0.10%, with the balance being Fe and inevitable impurities, and satisfying Ni+Mn: less than 8.50%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a shaped body obtained by rapidly melting and rapidly solidifying an Fe-based alloy powder, i.e., a shaped body formed by a process for producing a shaped body, such as three-dimensional additive manufacturing, thermal spraying, laser coating, cladding, or hot isostatic pressing. [Background technology]

[0002] In recent years, additive manufacturing (also known as 3D printers, three-dimensional modeling, additive manufacturing, etc.) has begun to be used to create shaped objects made of metal. In additive manufacturing, metal powder is irradiated with a laser beam or electron beam, causing rapid melting and solidification. On top of the formed bonding layer, further irradiation causes the metal particles to melt and solidify, forming a new bonding layer. By repeating this process, the aggregate of bonding layers gradually grows. This growth results in a shaped object with a three-dimensional shape. Using such additive manufacturing methods, it is easy to create shaped objects with complex shapes.

[0003] Attention is now being paid to the technology of manufacturing molds using metal additive manufacturing, and attempts are being made to use steel types such as maraging steel and SKD61 as specific powder materials. This makes it possible to obtain molds with complex shapes that were previously difficult to manufacture.

[0004] SKD61, specified by the JIS (Japanese Industrial Standards), is a typical mold steel used in hot working. However, it has been discovered that when powder with the SKD61 composition is applied to additive manufacturing, cracks occur during manufacturing when the object to be manufactured is large.

[0005] Therefore, in order to utilize hot work tool steel in additive manufacturing, it is required to have both excellent thermal conductivity and resistance to cracking during manufacturing.

[0006] When the thermal conductivity is high, the cooling rate can be increased when applying it to molds that require cooling mechanisms such as die casting. As a result, the production cycle speed can be improved.

[0007] When the resistance to shape cracking is high, cracks are less likely to occur in the shaped body itself or at the interface between the shaped body and the shaping plate. As a result, even large-sized shaped bodies can be produced without cracking.

[0008] As a mold steel applicable to metal additive manufacturing, for example, a shaped body containing 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 in mass% and the balance being Fe and inevitable impurities and satisfying the following formulas (1) to (3) has been proposed (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 C > 32.0 ··· Formula (1) T2 = 80.1 + 2.4Mn + 1.6Si + 7.1Cr - 12.0P C > 50.0 ··· Formula (2) Average size (μm) of carbides contained in the shaped body: P C < 3.0 ··· Formula (3)

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] In hot work tool steels, various alloying elements are added to improve tempered hardness. The added alloying elements dissolve in the matrix. As a result, the alloying elements dissolved in the matrix increase the scattering frequency of conduction electrons in the matrix, which generally reduces thermal conductivity. Therefore, in order to increase thermal conductivity, it is necessary to reduce the amount of alloying elements as much as possible, while at the same time ensuring the tempered hardness required for die-casting mold steel.

[0011] Patent Document 1 aims to ensure the tempered hardness required for die-casting dies, and aims to increase thermal conductivity by reducing the amount of added elements compared to SKD61. However, when used as a powder material for additive manufacturing, the as-formed hardness is also high, which raises concerns about cracking during manufacturing due to low toughness.

[0012] The problem to be solved by the present invention is to provide a powder for hot working tools that has both high thermal conductivity and excellent resistance to cracking during molding, and an additive manufacturing body produced using this powder. [Means for solving the problem]

[0013] The inventors focused on improving the temper hardening width to improve molding crack resistance. Aiming to improve temper hardness as in Patent Document 1, the as-formed hardness increases, leading to molding cracks. After extensive research, they discovered that while the as-formed hardness is comparable to that of conventional hot work tool steels, the tempered hardness can be improved to exceed the hardness of typical die-casting dies. In other words, they came up with the idea that by taking advantage of the improved temper hardening width and reducing the C content, it is possible to reduce the as-formed hardness while ensuring the hardness required for the die-casting die after tempering.

[0014] If the as-formed hardness is high, the toughness is low and the part may not be able to withstand the thermal stress generated during the forming process, which can lead to cracks. However, by reducing the as-formed hardness, it is possible to reduce cracks during the forming process.

[0015] As a result of investigations based on the above points, it was found that increasing the amount of Mo and V added improves the temper-hardening width. It was also found that optimizing the amount of Mo, V, Cr, and C added makes it possible to maintain thermal conductivity. It was found that by strictly limiting the amount of these elements added, both properties can be achieved at a high level, which led to the present invention.

[0016] Therefore, a first means for solving the problems of the present invention is an Fe-based alloy powder containing, by mass%, C: more than 0.10% and less than 0.45%, Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Ni: less than 8.00%, Mo: less than 2.30%, W: less than 2.00%, V: more than 0.6% and less than 1.50%, Al: less than 0.10%, with the remainder consisting of Fe and unavoidable impurities, and Ni+Mn: less than 8.50%.

[0017] The second means is an Fe-based alloy powder containing, in mass%, C: more than 0.10% and less than 0.45%, Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Ni: less than 8.00%, Mo: more than 1.20% and less than 2.30%, W: less than 2.00%, V: less than 1.50%, Al: less than 0.10%, with the remainder consisting of Fe and unavoidable impurities, and Ni+Mn: less than 8.50%.

[0018] A third means is the Fe-based alloy powder according to the first or second means, which satisfies the requirement of Cr: less than 1.20% by mass.

[0019] The fourth means is a layered manufactured body produced using the alloy powder of any one of the first to third means. [Effects of the Invention]

[0020] When the Fe-based alloy powder of the present invention is used in additive manufacturing, the resulting additive manufactured body has a high thermal conductivity of 30.0 W / (m·k) or more, a hardness of less than 48.0 HRC as-built, which means high toughness and excellent resistance to cracking during manufacturing, and a tempered hardness of 43.0 to 50.0 HRC, making it suitable as a hot-work tool steel for additive manufacturing. Furthermore, additive manufactured bodies using the Fe-based alloy powder of the present invention are resistant to cracking during manufacturing and are suitable for hot-work molds. DETAILED DESCRIPTION OF THE INVENTION

[0021] Before describing the embodiments of the present invention, we will first explain the range of chemical components of the Fe-based alloy powder used to fabricate an additive manufacturing object and the reasons for specifying it. The remainder of the chemical components is Fe and unavoidable impurities. The percentages in the chemical components below are by mass.

[0022] C: Over 0.10% to less than 0.45% C is an element that strengthens the matrix by dissolving in solid solution and further forms carbides to promote the precipitation effect. A C content of more than 0.10% can provide sufficient quench-and-temper hardness. From this perspective, C is set to more than 0.10%. Preferably, C is set to more than 0.15%, and more preferably, C is set to more than 0.20%. On the other hand, excessive C increases the amount of C dissolved in the matrix, reducing the thermal conductivity of the steel. Furthermore, the as-formed hardness and tempered hardness become excessively high, which can cause cracks during forming and mold use. From this perspective, C is set to less than 0.45%, preferably less than 0.40%, and more preferably less than 0.35%.

[0023] Si: Less than 0.60% Silicon is an element that improves hardness by dissolving in the matrix. It also has the effect of improving softening resistance. However, if the Si content is 0.60% or more, the silicon dissolves in the matrix without forming carbides, significantly reducing thermal conductivity. Therefore, the Si content is set to less than 0.60%, and preferably less than 0.30%.

[0024] Cr: Less than 2.00% Cr is an element that improves hardenability and suppresses the decrease in toughness due to bainite formation. It also has the effect of improving softening resistance. However, if Cr is 2.00% or more, it dissolves in the matrix and reduces thermal conductivity. Therefore, the Cr content is set to less than 2.00%, preferably less than 1.50%, and more preferably less than 1.20% from the viewpoint of thermal conductivity.

[0025] Ni: Less than 8.00% Ni is an element necessary for improving hardenability and maintaining hardness deep within the shaped body. Furthermore, adding Ni prevents martensitic transformation during shaping, and the austenite remains, absorbing strain from thermal deformation. Therefore, Ni content is preferably greater than 0.30%, more preferably greater than 2.00%, and even more preferably greater than 2.30%. However, because Ni dissolves in the matrix without forming carbides, adding more than 8.00% significantly reduces thermal conductivity. Therefore, Ni content is set to less than 8.00%, preferably less than 6.50%, and more preferably less than 5.00%.

[0026] Mn: Less than 5.00% Mn is an element that improves hardenability and suppresses the decrease in toughness due to bainite formation. It also has the effect of improving softening resistance. Therefore, Mn content is preferably more than 0.05%, more preferably more than 0.11%. However, if Mn is 5.00% or more, it dissolves in the matrix and reduces thermal conductivity. Therefore, Mn content is set to less than 5.00%, preferably less than 1.00%, and more preferably less than 0.41%.

[0027] Ni+Mn: Less than 8.50% Ni and Mn are components with similar effects. Therefore, if the total of Ni and Mn is 8.50% or more, they dissolve in the matrix, significantly reducing thermal conductivity. Therefore, Ni+Mn should be less than 8.50%, preferably less than 5.00%, and more preferably less than 3.00%.

[0028] Mo: Less than 2.30% Mo is an element that promotes secondary hardening during tempering and increases quenching and tempering hardness. Furthermore, Mo contributes less to a decrease in thermal conductivity than other elements, and is highly effective in improving hardness. From these perspectives, Mo content is preferably greater than 0.70%, and more preferably greater than 1.20%. However, if Mo content is 2.30% or more, the amount of Mo remaining in the matrix increases, resulting in a decrease in thermal conductivity. Therefore, Mo content is set to less than 2.30%, and preferably less than 1.90%.

[0029] W: Less than 2.00% W is an element that promotes secondary hardening during tempering and increases the hardness after quenching and tempering. However, if W is 2.00% or more, the amount of W remaining in the matrix increases, and the thermal conductivity decreases. Therefore, W is set to 0 to less than 2.00%.

[0030] V: Less than 1.50% V is an element that promotes secondary hardening during tempering and increases quenching and tempering hardness. Furthermore, V contributes less to a decrease in thermal conductivity than other elements, and has a significant effect in improving hardness. From these perspectives, V is preferably more than 0.40%, more preferably more than 0.60%. However, if V is 1.50% or more, the amount of V remaining in the matrix increases, resulting in a decrease in thermal conductivity. Therefore, V is set to less than 1.50%, and preferably less than 1.20%.

[0031] Al: Less than 0.10% Al is an element that forms nitrides and suppresses coarsening of crystal grains during quenching. However, if Al is added in an amount of 0.10% or more, the toughness decreases due to the formation of excess Al nitrides. Therefore, the Al content is set to 0 to less than 0.10%.

[0032] [Powder preparation] Fe-based alloy powders were obtained by gas atomizing raw materials consisting of the chemical components of Examples 1 to 20 shown in Table 1 and Comparative Examples 1 to 9 shown in Table 2, with the balance being Fe and unavoidable impurities. First, the raw materials were heated in a vacuum in an alumina crucible by high-frequency induction heating to form a molten alloy, and then the molten alloy was dropped from a 5 mm diameter nozzle installed at the bottom of the crucible. High-pressure argon gas was sprayed onto the molten metal, which atomized the molten metal and rapidly cooled it to obtain a large number of fine powders. The obtained powders were classified so that the particle diameters were 63 μm or less, and the Fe-based alloy powders of Examples 1 to 20 and Comparative Examples 1 to 9 were obtained. The underlined parts in Table 2 indicate that the components fall outside the ranges specified by the present invention.

[0033] [Table 1]

[0034] [Table 2]

[0035] [molding] The produced Fe-based alloy powder is used to manufacture an additive manufacturing object. In the following examples, additive manufacturing using the powder bed method is used as an example of a typical additive manufacturing method. Of course, an additive manufacturing object can also be obtained using the powder of the present invention with other additive manufacturing methods.

[0036] The powders of Examples 1 to 20 and Comparative Examples 1 to 9 were molded using a three-dimensional additive manufacturing device (product name "EOS-M290"). As a result, cubes measuring 10 mm x 10 mm x 10 mm and prisms measuring 15 mm wide x 150 mm long x 17 mm high were obtained. The molding conditions were equivalent to MS1 conditions (standard conditions for maraging steel).

[0037] [Evaluation of molding crack resistance] To evaluate the resistance to cracking during molding, a square pillar measuring 15 mm wide x 150 mm long x 17 mm high was used. When molding with this shape, the stress caused by molding is concentrated at the interface with the molding plate. Therefore, the presence or absence of cracking during molding was confirmed by observing the interface after molding, and if any cracks were present, their length was measured.

[0038] [Heat treatment] Each test piece of the resulting additive manufacturing body was subjected to a tempering heat treatment of "holding at 600°C for 60 minutes followed by air cooling" twice.

[0039] [Thermal conductivity measurement] The thermal conductivity was measured using the laser flash method, and the tempered samples were finished into disks with a diameter of 5 mm and a thickness of 1 mm and then subjected to the test. The results are shown in Tables 3 and 4.

[0040] [Hardness measurement] The hardness of the molded body was measured using a Rockwell hardness tester on the surface perpendicular to the lamination direction for both the as-molded sample before heat treatment and the tempered sample. Tables 3 and 4 show the hardness results for the as-molded and tempered samples, respectively.

[0041] [Table 3]

[0042] [Table 4]

[0043] The underlined parts in Table 4 indicate that the properties were outside the range of the present invention.

[0044] The additive manufacturing bodies produced using Examples 1 to 20, which are the Fe-based alloy powders of the present invention, have excellent thermal conductivity of 30.3 W / (m·k) or more, and when used in dies, they have excellent cooling efficiency. Furthermore, since the as-built hardness is low at 44.0 HRC or less, no cracking was observed during manufacturing. Furthermore, the tempered hardness is 46.5 to 47.9 HRC, which meets the tempered hardness required for die-casting dies. Therefore, additive manufacturing bodies produced using the Fe-based alloy powder of the present invention are suitable for dies without cracking during manufacturing.

[0045] On the other hand, the molded bodies made from the powders of the comparative examples were inferior in thermal conductivity, as-molded hardness, molded crack resistance, or tempered hardness. As underlined, those with a conductivity below 30.0 W / (m·k), those with an as-formed hardness exceeding 48.0 HRC, those with a tempered hardness less than 43.0 HRC or exceeding 50.0 HRC, and those in which forming cracks were observed were evaluated as inferior to the present invention.

[0046] For example, Comparative Example 1 had a small amount of both Mo and V added, and a small temper-hardening range. Therefore, even though the tempered hardness was the level required for a die-casting die, the as-molded hardness was high, and molding cracks were observed. In Comparative Example 2, the amount of Mo was excessive, and the thermal conductivity was reduced. In Comparative Example 3, the amount of V was excessive, and the thermal conductivity was reduced. In Comparative Example 4, the amount of Cr was excessive, and the thermal conductivity was reduced. In Comparative Example 5, the amount of C was insufficient, and the tempered hardness was not improved to the desired level. In Comparative Example 6, the amount of C was excessive, and the as-formed hardness and tempered hardness were high, and forming cracks were also observed. In Comparative Example 7, the Ni amount and the total content of Ni+Mn were excessive, and the thermal conductivity was reduced. In Comparative Example 8, the total content of Ni+Mn was excessive, and the thermal conductivity was reduced. In Comparative Example 9, the amount of Mn and the total content of Ni+Mn were excessive, and the thermal conductivity was reduced. [Industrial Applicability]

[0047] The layered shaped body of the present invention is suitable for use in hot stamping or hot molds for die casting.

Claims

1. An Fe-based alloy powder containing, by mass%, C: more than 0.10% and less than 0.45%, Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Ni: less than 8.00%, Mo: less than 2.30%, W: less than 2.00%, V: more than 0.6% and less than 1.50%, Al: less than 0.10%, with the balance being Fe and unavoidable impurities, and Ni+Mn: less than 8.50%.

2. An Fe-based alloy powder containing, by mass%, C: more than 0.10% and less than 0.45%, Si: less than 0.60%, Mn: less than 5.00%, Cr: less than 2.00%, Ni: less than 8.00%, Mo: more than 1.20% and less than 2.30%, W: less than 2.00%, V: less than 1.50%, Al: less than 0.10%, with the balance being Fe and unavoidable impurities, and Ni+Mn: less than 8.50%.

3. The Fe-based alloy powder according to the first or second aspect of the present invention, which satisfies the requirement of Cr: less than 1.20% by mass.

4. A layered product produced using the alloy powder according to any one of claims 1 to 3.

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