High thermal conductivity hot work tool steel powder for additive manufacturing with excellent hardenability and mold cracking resistance and lamination molded body therewith

Abstract

To provide hot work tool steel powder for additive manufacturing, which is easy to harden deep inside even in a large sized additive manufacturing, is difficult to generate cracks in a molded body and a boundary surface of the molded body and a base material for additive manufacturing, and has high thermal conductivity and a lamination molded body therewith.SOLUTION: Hot work tool steel powder for additive manufacturing comprising in mass %, as essential additive components, C: more than 0.10% and less than 0.45%, Ni: more than 2.00% and less than 8.00%, as optional additive components, 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%, Al: less than 0.10%, and the remainder consisting of Fe and unavoidable impurities, and Ni+Mn: less than 8.50%, and a molded body that is lamination-molded using this powder.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a hot work tool steel powder with high thermal conductivity for use in additive manufacturing and an additively manufactured object using the same. That is, the present invention relates to a manufactured object and raw material powder that can be used to manufacture various tools, including dies, using additive manufacturing methods (also known as 3D printers, three-dimensional modeling methods, additive manufacturing, etc.), and in particular to a hot work tool steel that exhibits sufficient hardenability, high resistance to molding cracking, high thermal conductivity, and excellent mechanical properties, even for large manufactured objects.

[0002] Here, high resistance to cracking during molding refers to the fact that cracks are less likely to occur in the molded body itself (especially in areas where notches will form) or at the interface between the molded body and the molding base material due to thermal stresses associated with rapid melting and solidification during additive manufacturing. [Background technology]

[0003] Unlike conventional manufacturing methods, additive manufacturing is capable of manufacturing components with complex shapes and 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 a variety of parts, and their shapes and sizes vary depending on the processing method and part shape. However, the application of additive manufacturing to tools has had to be limited to relatively small ones due to the following problems with manufacturing cracks:

[0005] In general, additive manufacturing involves rapidly melting and solidifying raw material powder or wire by heating it 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, making it possible to manufacture parts with complex three-dimensional shapes. During this process, only a portion of the part is heated, melted, and solidified, which generates thermal stress due to local solidification shrinkage and thermal expansion and contraction. If the material being molded or the base material is hard and brittle, it will not be able to withstand the thermal stress that occurs, resulting in cracks in the molded object itself or at the interface with the base material.

[0006] This thermal stress becomes even greater when large objects are additively manufactured, resulting in increased likelihood of cracks during manufacturing. Generally, high-hardness alloys such as JIS standard SKD61 are used for tools, which makes 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 small thermal stresses.

[0007] The surfaces of hot tools, including die-casting dies, come into contact with the high-temperature parts (workpieces) being processed, causing temperatures to rise and making them susceptible to damage such as heat checking. 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, when machining parts using hot tools, the tools must cool down after machining one part and before machining the next part. Therefore, being able to efficiently (in a short time) cool the tools to a specified temperature has the advantage of shortening the part machining cycle and improving part production efficiency.

[0010] As a laminated formed body made of hot work tool steel with such high thermal conductivity, the applicant has proposed, for example, a formed body made of an Fe-based alloy powder 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 %, with 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 formed body: 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 the above Patent Document 1, since the thermal conductivity decreases due to an increase in the addition amounts of various elements, it is attempted to achieve high thermal conductivity by defining the upper limits of the addition amounts of these elements and the T1 parameter. That is, compared with SKD61, which is a general-purpose steel widely used as a hot work tool, it is attempted to obtain high thermal conductivity by reducing the amount of each added element.

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

[0014] In the past, due to the risk of cracks occurring in the mold due to thermal stress, additive manufacturing was limited to relatively small hot working tools. Therefore, in the deep parts of large tools, the cooling rate during quenching was not sufficient (sufficient quench-temper hardness was not obtained). Even alloys with relatively low hardenability could be used without any problems arising because the molded bodies were 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 molded parts may be in a pseudo-hardened state, so they may be used after only tempering or distortion relief heat treatment without further hardening after manufacturing. However, heat accumulates in the center of large molded parts from the heat source during manufacturing, causing the temperature to rise, making it difficult to achieve the rapid cooling effect despite the rapid cooling method. Also, alloys with low hardenability tend to have low hardness in the deeper parts of the molded parts, which hinders the ability to make them larger.

[0017] Furthermore, it is known that preheating the base material during molding reduces 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 realized not only for large but also for small objects, and alloys with low hardenability tend to have low hardness deep in the molded object, which is a problem.

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

[0019] In light of the above, the present invention aims to provide a hot working tool steel powder for additive manufacturing, which is easy to harden deep even in large-scale additive manufacturing, is less likely to crack at the interface between the molded body and the additive base material, and has high thermal conductivity, and an additive manufacturing body made using the same. [Means for solving the problem]

[0020] In light of the above background, the inventors have diligently developed the components of a hot work tool steel for additive manufacturing that has sufficient hardenability and high thermal conductivity to ensure sufficient hardenability even in large molded bodies. 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 investigations, they discovered that by strictly specifying the amounts of Ni and Cr added, both properties can be achieved at a high level, leading to the present invention.

[0021] Furthermore, as mentioned above, large-scale additive manufacturing generates large thermal stresses, making the alloy more susceptible to cracking during manufacturing, but it was discovered that by specifying the amounts of Ni, Cr, and C added, the hardness of the molded object can be reduced and resistance to cracking during manufacturing is also excellent. In other words, the present invention is practically applicable to larger molded objects compared to conventional methods that have mainly been applied to small molded objects, and has been improved to an alloy that improves hardenability by optimizing the range of addition of Ni, Cr, and C, minimizes the decrease in thermal conductivity, and is less susceptible to cracking during manufacturing, even when large-scale manufacturing is performed.

[0022] Therefore, the first means for solving the problems of the present invention is a hot work tool steel powder for additive manufacturing, which contains, in mass%, as essential added components: C: more than 0.10% and less than 0.45%, Ni: more than 2.00% and less than 8.00%, and as optional added components: 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%, Al: less than 0.10%, with the remainder consisting of Fe and unavoidable impurities, and Ni+Mn: less than 8.50%.

[0023] The second aspect of the present invention is a shaped body produced by additive manufacturing using the hot working tool steel powder for additive manufacturing described in the first aspect. [Effects of the Invention]

[0024] According to the means of the present invention, it is possible to provide a hot working tool steel powder for additive manufacturing, and an additive manufacturing body using the same, which is easy to harden deep even in large-scale additive manufacturing, is less likely to develop cracks at the interface between the molded body and the additive base material, and has high thermal conductivity.

[0025] Furthermore, the means of the present invention can exhibit excellent effects, such as being able to achieve both high hardenability and high thermal conductivity, even when applied to processes in which hardening is omitted, small-scale molding, or when obtaining a molded body through a preheated molding process. DETAILED DESCRIPTION OF THE INVENTION

[0026] Before describing the embodiments of the present invention, the reasons for specifying the components of the hot work tool steel powder for additive manufacturing used in the shaped body of the present invention will be explained. Note that % for each component is mass %. The remainder of the components is Fe and unavoidable impurities.

[0027] C: Over 0.10% to less than 0.45% Carbon is an essential element for achieving high quench-and-temper hardness by dissolving in the martensite phase, which is the matrix, and precipitating fine carbides. However, if the carbon content is 0.10% or less, high quench-and-temper hardness cannot be achieved. From this perspective, the carbon content is set to more than 0.10%. Preferably, the carbon content is more than 0.20%, and more preferably, the carbon content is more than 0.30%. On the other hand, if the carbon content is 0.45% or more, the as-formed hardness becomes excessively high, which has a relatively large effect on deteriorating forming crack resistance compared to other elements, and the amount of dissolved carbon increases, resulting in a decrease in thermal conductivity. Therefore, the carbon content is set to less than 0.45%. Preferably, the carbon content is less than 0.42%, and more preferably, the carbon content is less than 0.40%.

[0028] Ni: Over 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-sized molded 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 molding, and by maintaining austenite, which is soft and resistant to molding cracks, at relatively low temperatures, it also improves molding crack resistance. However, this effect is insufficient when Ni is 2.00% or less. Therefore, Ni is set to exceed 2.00%. From this perspective, Ni is preferably set to exceed 2.30%, more preferably exceed 3.00%. On the other hand, Ni content of 8.00% or more increases the amount of solid solution in the matrix, significantly reducing thermal conductivity. Therefore, Ni is set to less than 8.00%. Ni is preferably set to less than 6.50%, more preferably less than 5.00%.

[0029] Si: Less than 0.60% Si is a component that improves hardness by dissolving in the matrix. It also has the effect of improving softening resistance. Therefore, the Si content is set to 0 to less than 0.60%. However, if the Si content exceeds 0.60%, the amount of solute Si increases, significantly reducing thermal conductivity. Therefore, the upper limit of Si is set to less than 0.60%. Preferably, Si is less than 0.40%, and more preferably, Si is less than 0.24%. On the other hand, although Si may be 0%, because Si is a component that improves hardness and softening resistance, when Si is added, it is preferably more than 0.04%, and more preferably more than 0.10%.

[0030] Mn: Less than 5.00% Mn is a component that improves hardenability and improves quench and temper hardness even in deep parts of large shaped bodies. It also has the effect of improving softening resistance. Therefore, the Mn content is set to 0 to less than 5.00%. However, if the Mn content is 5.00% or more, the amount of solute Mn increases, reducing thermal conductivity. Therefore, the Mn content is set to less than 5.00%. Preferably, the Mn content is less than 1.00%, and more preferably, the Mn content is less than 0.41%. On the other hand, while the Mn content may be 0%, when Mn is added, from the viewpoint of improving deep quench and temper hardness and improving softening resistance, the Mn content is preferably more than 0.05%, and more preferably, more than 0.11%.

[0031] Ni+Mn: Less than 8.50% Furthermore, Mn is a component that has an effect similar to that of Ni. Therefore, if the total amount of Mn and Ni is 8.50% or more, the amount of solid solution in the matrix increases, significantly reducing thermal conductivity. Therefore, Ni + Mn is set to less than 8.50%. Preferably, Ni + Mn is less than 7.00%, and more preferably less than 5.00%.

[0032] Cr: Less than 2.00% Cr is a component that improves hardenability and improves quench and temper hardness even in deep portions of large shaped objects. It also has the effect of improving softening resistance. However, when Cr exceeds 2.00%, the amount of dissolved Cr increases, reducing thermal conductivity, and this reducing effect is relatively large compared to other components. Therefore, Cr is set to 0 to less than 2.00%. Preferably, Cr is less than 1.50%, and more preferably, Cr is less than 1.15%. Cr may be 0%, but because Cr is a component that improves hardenability and improves quench and temper hardness even in deep portions of large shaped objects, and also has the effect of improving softening resistance, when Cr is added, it is preferably more than 0.50%, and more preferably more than 0.85%.

[0033] Mo: Less than 1.20% Mo is a component that promotes secondary hardening during tempering and increases quench and temper hardness. When added, Mo reduces thermal conductivity, but its contribution is small, while its effect on improving hardness is significant. Therefore, Mo content is set to 0 to less than 1.20%. Mo content is preferably less than 1.05%, and more preferably less than 0.95%. Although Mo may be 0%, when Mo is added, it promotes secondary hardening during tempering and increases quench and temper hardness, but its addition reduces thermal conductivity, but its contribution is small, while its effect on improving hardness is significant. Therefore, Mo content is preferably set to more than 0.60%, and more preferably more than 0.75%.

[0034] W: Less than 2.00% W is a component that promotes secondary hardening during tempering and increases quench and temper hardness. Although the addition of W reduces thermal conductivity, it makes little contribution and is effective in increasing hardness. Therefore, the W content is set to 0 to less than 2.00%. W is preferably less than 1.00%, and more preferably less than 0.50%. Although W may be 0%, W is a component that promotes secondary hardening during tempering and increases quench and temper hardness, and although the addition reduces thermal conductivity, it makes little contribution and is effective in increasing hardness. Therefore, when W is added, W is preferably more than 0.05%, and more preferably more than 0.10%.

[0035] 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, V content is set to 0 to less than 0.60%. V is preferably less than 0.55%, and more preferably less than 0.50%. V may be 0%, but since V is a component that promotes secondary hardening during tempering and increases quench-temper hardness, when V is added, V content is preferably more than 0.20%, and more preferably more than 0.30%.

[0036] Al: Less than 0.10% Al is a component that forms nitrides and suppresses grain coarsening 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. It also reduces thermal conductivity. Therefore, the Al content is set to 0 to less than 0.10%. Preferably, Al is less than 0.07%, and more preferably less than 0.04%. Although Al may be 0%, Al is a component that forms nitrides and suppresses grain coarsening during quenching. Therefore, if Al is added, it is preferably more than 0.001%, and more preferably more than 0.002%. Note that Al may not be intentionally added, but may be mixed in from refractories used in the melting process for gas atomization. However, the effect of adding Al is the same in either case.

[0037] (Example) Table 1 shows the component compositions of the steel powders of Examples Nos. 1 to 19 of the present invention and Comparative Examples Nos. 1 to 6 (values ​​are in mass %). The balance consists of Fe and unavoidable impurities. These examples are merely examples of embodiments of the present invention, and the scope of the claims is not limited to these examples.

[0038] [Table 1]

[0039] [Production of raw powder] The hot work tool steel powder for additive manufacturing of the present invention was obtained by gas atomization with the chemical composition shown in Table 1. The chemical composition of the molded body after additive manufacturing using these powders was also the same as that of the powders. The specific manufacturing procedure for the powder was as follows: First, the 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 broke the molten alloy into fine droplets, which cooled and solidified as they fell through the tower of the atomization device, becoming alloy powder. This alloy powder was sieved through a mesh with 63 μm openings, and the powder that passed through the sieve at the bottom was used as the raw material powder for the subsequent additive manufacturing.

[0040] [Additive Manufacturing] The additive manufacturing was carried out using a laser-heated powder bed type device (product name: EOS-M290) with a preheating temperature of 180°C, which corresponds to the standard manufacturing conditions (MS1 conditions) for maraging steel specified for the device.

[0041] The base material for the molding was an annealed S45C plate, on which 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 molding 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.

[0042] [evaluation] Table 2 shows the hardness of the surface layer and the center of the molded body of each example and comparative example, the difference between these hardnesses, the thermal conductivity, and whether or not cracks occurred during molding.

[0043] [Table 2]

[0044] To evaluate the quenched and tempered hardness of the surface and deep portions of the large-sized objects, large cylinders measuring 180 mm in diameter and 120 mm in height were cut from the plate using wire cutting. These were quenched and tempered in an atmospheric furnace, and the Rockwell hardness of the test pieces cut from the surface and deep portions was evaluated. For quenching, the cylinders were held at 1,030°C for one hour and oil-cooled. For tempering, the cylinders were then held at 600°C for four hours and air-cooled. The same tempering process was repeated twice. (Examples Nos. 18 and 19 were not quenched, but only tempered.) 10 mm square blocks were cut from the outermost and central portions of the quenched and tempered large cylinders at the center of their height, and their Rockwell hardness was measured.

[0045] Furthermore, the optimum working hardness for various tools varies depending on the application, with some applications requiring a hardness of around 40HRC and others requiring a hardness of around 50HRC. However, 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 malfunctions. Therefore, apart from the absolute hardness value, the variation in hardness depending on the part was evaluated as the difference in hardness between the surface and center.

[0046] Similarly, 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.Thermal conductivity at room temperature was measured using the laser flash method using test pieces finished into a disk shape with a diameter of 5 mm and a thickness of 1 mm.

[0047] In addition, since the alloys were rapidly cooled and solidified, in some examples (Nos. 18 and 19), the hardness and thermal conductivity were determined by only tempering without the above-mentioned quenching.

[0048] To evaluate crack resistance, a rectangular pillar measuring 15 mm wide, 150 mm long, and 17 mm high was used. Unlike large circular pillars, this pillar does not have a curvature at the interface with the plate, so the interface is at a right angle, and thermal stress is concentrated at this location. Furthermore, thermal stress increases in the longitudinal direction, where the amount of thermal deformation is greater. 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 end of the 150 mm length. 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.

[0049] The molded bodies of Examples 1 to 19 of the present invention all had excellent surface and center hardness of 40 HRC or more, with the difference in hardness between the surface and center being within 1.5 HRC, suppressing hardness variation, and also had excellent thermal conductivity of 25 W / m / K or more (most of them 30 W / m / K or more).In addition, with regard to molding cracks, most were not observed, and even when they were observed, the length of the cracks was limited to 0.5 mm, demonstrating excellent resistance to molding cracks.

[0050] Comparative Examples Nos. 1 and 2 contained insufficient Ni, and the hardness of the center and surface of the molded body varied, resulting in large molding cracks. Comparative Example No. 3 contained an excessive amount of Ni, resulting in low thermal conductivity. Comparative Example No. 4 contained an excessive amount of Cr, resulting in low thermal conductivity. Comparative Example No. 5 had an insufficient amount of C, resulting in a shaped body with low hardness. Comparative Example No. 6 contained excessive C, and large cracks were observed in the molded product.

Claims

1. In mass%, As an essential ingredient, C: more than 0.10% to less than 0.45%; Ni: more than 2.00% to less than 8.00%; As optional additional ingredients, 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% Al: less than 0.10% The balance consists of Fe and unavoidable impurities, and Ni+Mn: less than 8.50% Hot work tool steel powder for additive manufacturing.

2. A shaped body produced by additive manufacturing using the hot working tool steel powder for additive manufacturing according to claim 1.

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