WO2025170067A1 - Hot work tool steel powder for additive manufacturing and additive-manufactured hot work tool steel article - Google Patents

Abstract

The present invention provides a hot work tool steel powder for additive manufacturing, with which it is possible to form an additive-manufactured hot work tool steel article that is particularly excellent in terms of cracking resistance during manufacturing.　Provided are: a hot work tool steel powder for additive manufacturing, which contains, in mass%, 0.10% ≤ C ≤ 0.40%, 0.01% ≤ Si ≤ 0.19%, 0.1% ≤ Mn ≤ 1.0%, 2.0% ≤ Ni < 9.0%, 3.5% < Cr < 4.5%, one or both of Mo and W satisfying a relational expression 2.5% ≤ (Mo + 1/2 W) < 3.5%, 0.45% ≤ V ≤ 1.0%, 0.3% < Cu < 0.6%, and 0.2% ≤ Al ≤ 0.9%, with the balance being made up of Fe and inevitable impurities, and which satisfies formula (1) C + Si/30 + (Mn + Cr + Cu)/20 + Ni/60 + (Mo + 1/2W)/15 + V/10 ≤ 0.95; and an additive-manufactired hot work tool steel article.

Description

Hot work tool steel powder for additive manufacturing and hot work tool steel additive manufactured products

The present invention relates to hot work tool steel powder for additive manufacturing and hot work tool steel additive manufactured products.

Hot work tool steels used in hot forging dies, die casting dies, and other tools come into contact with high-temperature workpieces, and therefore require high-temperature strength, toughness, and wear resistance. Traditionally, to meet these requirements, JIS steel grade SKD61 and improved versions of SKD61 have been used for hot work tool steels.

Recently, additive manufacturing has been attracting attention as a means of easily creating near-net-shape metal products (components) with complex shapes. Additive manufacturing, commonly known as 3D printing, is an additive manufacturing technology. Types of additive manufacturing include the powder spray method, in which a heat source is applied to metal powder to melt it and then layer it on top, and the powder bed method, in which a heat source is applied to metal powder spread on a stage to melt it, then the powder is solidified and layered repeatedly. Additive manufacturing allows metal products with complex shapes to be produced without requiring much of the traditional machining process, making it possible to use metal materials that are difficult to process. Furthermore, because difficult-to-process metal materials are primarily high-strength metal materials, it is possible to produce metal products with complex shapes and long durability.

Furthermore, additively manufactured products have been proposed using hot work tool steel as the metal material and the additive manufacturing method described above. For example, Patent Document 1 proposes an additively manufactured hot work tool having a component composition containing, by mass%, 0.3 to 0.5% C, 2.0% or less Si, 1.5% or less Mn, 0.05% or less P, 0.05% or less S, 3.0 to 6.0% Cr, 0.5 to 3.5% of one or two of Mo and W according to the relationship (Mo + 1/2W), 0.1 to 1.5% V, 0 to 1.0% Ni, 0 to 1.0% Co, 0 to 1.0% Nb, and the balance being Fe and impurities, and characterized in that the area ratio of defects having an area of 1 ^μm2 or more in a cross section parallel to the stacking direction is 0.6% or less.

Patent Document 2 discloses a steel powder that aims to achieve both high thermal conductivity and high corrosion resistance and is characterized by a composition, in mass%, of 0.10≦C<0.25, 0.005≦Si≦0.600, 2.00≦Cr≦6.00, -0.0125×[Cr]+0.125≦Mn≦-0.100×[Cr]+1.800...formula (a) (wherein [Cr] in formula (a) represents the mass % Cr content), 0.01≦Mo≦1.80, -0.00447×[Mo]+0.010≦V≦-0.1117×[Mo]+0.901...formula (b) (wherein [Mo] in formula (b) represents the mass % Mo content), 0.0002≦N≦0.3000, with the balance being Fe and unavoidable impurities.

International Publication No. 2019/220917 JP 2016-145407 A

As described above, several steels for additive manufacturing have been proposed, but cracks may occur during additive manufacturing depending on the type of additive manufacturing machine, additive manufacturing conditions, additive manufacturing dimensions, etc. For example, in the case of large additive manufacturing molds or additive manufacturing molds that form complex cavities, additive manufacturing molds that have stress concentration areas (recesses) where stress is particularly concentrated tend to be very susceptible to cracking, and therefore further improvements in crack resistance are required.
Therefore, an object of the present invention is to provide a hot work tool steel powder for additive manufacturing that can produce hot work tool steel additive manufactured products that have particularly improved crack resistance during additive manufacturing.

The present invention has been made in view of the above-mentioned problems.
That is, one embodiment of the present invention is a steel sheet containing, in mass %, 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, one or two of Mo and W according to the relationship formula (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, ...Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.10%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, . 3%<Cu<0.6%, 0.2%≦Al≦0.9%, with the remainder consisting of Fe and unavoidable impurities, and satisfying formula (1): C+Si/30+(Mn+Cr+Cu)/20+Ni/60+(Mo+½W)/15+V/10≦0.95 (each element symbol in formula (1) indicates the content (mass %) of that element).

Another aspect of the present invention is a hot work tool steel additive manufactured product that contains, in mass%, 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, one or two of Mo and W according to the relationship (Mo+1/2W): 2.5%≦(Mo+1/2W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, with the balance being Fe and unavoidable impurities, and that satisfies formula (1): C+Si/30+(Mn+Cr+Cu)/20+Ni/60+(Mo+1/2W)/15+V/10≦0.95.

The present invention makes it possible to obtain hot work tool steel powder for additive manufacturing, which can be used to produce hot work tool steel additive manufactured products that have particularly excellent crack resistance during additive manufacturing.

FIG. 1 is a schematic diagram of a crack evaluation test piece for evaluating molding crack resistance. 1 is a graph showing the tempering temperature and hardness of hot work tool steel additive manufactured products of an example of the present invention and a comparative example. 1 is a graph showing the mechanical properties ((a) 0.2% proof stress, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention at room temperature. 1 is a graph showing the mechanical properties ((a) 0.2% yield strength, (b) tensile strength, (c) elongation, (d) reduction of area) of an example of the present invention at high temperatures. 1 is a graph showing Charpy impact values at room temperature of examples of the present invention. 1 is a graph showing the thermal conductivity of an example of the present invention.

The present invention has a chemical composition consisting of 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, one or two of Mo and W according to the relationship (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, and the balance being Fe and unavoidable impurities. First, the reasons for the compositional limitations of the hot work tool steel powder for additive manufacturing (hereinafter also referred to as additive manufacturing powder or metal powder) specified in the present invention will be described. Unless otherwise specified, "%" refers to "mass %." Also, additive manufacturing may be simply referred to as "manufacturing."
C: 0.10%≦C≦0.40%
Carbon (C) is a fundamental element of hot-working tools, partially dissolving in the matrix to provide strength, while partially forming carbides to enhance wear resistance and seizure resistance. Furthermore, when added together with substitutional atoms with high affinity for C, such as Cr, C dissolved as an interstitial atom is expected to contribute to the I (interstitial atom)-S (substitutional atom) effect (which acts as drag resistance for solute atoms and increases the strength of hot-working tools). It also has the potential to enhance hardenability. If the C content is too low, the ferrite phase forms primarily during solidification, and the ferrite phase remains the dominant phase until room temperature, making quenching, which requires rapid cooling from the austenite phase, impossible. If the ferrite phase remains dominant immediately after solidification until room temperature, thermal contraction cannot be alleviated by utilizing martensitic transformation expansion, making the tool more susceptible to cracking. However, increasing the C content increases hardness but decreases toughness, promoting cracking during molding. In the present invention, the C content is set to 0.10%≦C<0.40% in order to improve crack resistance while maintaining hardness sufficient for use in dies. The preferred lower limit of C is 0.15% or more, more preferably 0.18% or more, even more preferably 0.20% or more, and 0.21% or more. The preferred upper limit of C is 0.35% or less, more preferably 0.30% or less, even more preferably 0.27% or less, 0.25% or less, and 0.24% or less.

Si: 0.01%≦Si≦0.19%
Silicon can be used as a deoxidizer when adjusting the chemical composition of molten steel. It is difficult to eliminate silicon from the manufacturing process. Furthermore, the closer one tries to eliminate silicon, the higher the manufacturing cost. Therefore, the lower limit of silicon content is set to 0.01% or more. A preferred lower limit is 0.05% or more, and more preferably 0.08% or more. On the other hand, excessive silicon content can lead to the formation of ferrite in the tool structure after tempering, so the upper limit is set to 0.19% or less. A preferred upper limit is 0.17% or less, and more preferably 0.15% or less, and even 0.13% or less.

Mn: 0.1%≦Mn≦1.0%
Mn has the effects of improving hardenability, suppressing the formation of ferrite in the tool structure, and achieving appropriate quench-and-temper hardness. To achieve these effects, the lower limit of Mn is set to 0.1% or more. A preferred lower limit is 0.25%, and more preferably 0.40% or more. On the other hand, if there is too much Mn, it increases the viscosity of the matrix and reduces the machinability of the material. Therefore, the upper limit is set to 1.0% or less. A preferred upper limit is 0.7% or less, more preferably 0.6% or less, and even more preferably 0.55% or less.

Ni: 2.0%≦Ni≦9.0%
Ni is an element that suppresses the formation of ferrite in the tool structure. In addition, together with C, Cr, Mn, Mo, W, etc., it imparts excellent hardenability to tool materials and is an effective element for preventing a decrease in toughness by forming a martensite-based structure even when the cooling rate during quenching is slow. Furthermore, it can lower the Ms point. In the case of an additive manufacturing device without a temperature control mechanism, the temperature during manufacturing may be close to room temperature depending on the manufacturing conditions. However, by lowering the Ms point to room temperature, the effect of mitigating thermal contraction due to martensitic transformation expansion can be utilized, thereby reducing deformation due to thermal contraction. For this reason, in the present invention, the lower limit of Ni is set to 2.0%. A preferred lower limit is 4.0%. It is more preferably 5.0% or more, and even more preferably 6.0% or more, 7.0% or more, or 7.5% or more. However, excessive Ni increases the viscosity of the matrix, reducing machinability and increasing raw material costs. Therefore, even if Ni is contained, the upper limit is set to 9.0%. The preferred upper limit is 8.5%. When it is desired to significantly improve crack resistance, a particularly preferred Ni range is 7.5 to 8.5%. When it is desired to significantly improve thermal conductivity while also ensuring crack resistance, a particularly preferred Ni range is 3.5 to 4.5%. When it is desired to further improve thermal conductivity and yield strength while also ensuring crack resistance, a particularly preferred Ni range is 5.5 to 6.5%.

Cr:3.5%<Cr<4.5%
Cr is a basic element of hot work tools that improves hardenability and forms carbides, thereby strengthening the matrix and improving wear resistance and toughness. However, too much Cr can lead to a decrease in hardenability and high-temperature strength. Therefore, the Cr content is set to 3.5% < Cr < 4.5%. The preferred lower limit of Cr is 3.6% or more, more preferably 3.7% or more, or 3.8% or more. The preferred upper limit of Cr is 4.4% or less, more preferably 4.3% or less, or 4.2% or less.

One or both of Mo and W according to the relationship (Mo+½W): (Mo+½W): 2.5%≦(Mo+½W)<3.5%
Mo and W can be added alone or in combination to impart strength by precipitating or agglomerating fine carbides during tempering, thereby improving softening resistance and high-temperature strength. In the present invention, since the C content is reduced to improve cracking resistance, a slightly higher Mo and W content can be expected to complement the strength. In this case, since W has approximately twice the atomic weight of Mo, the Mo content can be determined together with the Mo equivalent defined by the relationship (Mo + 1/2W). (Naturally, only one of them may be added, or both may be added.) To achieve the above effect, the content should be 2.5% or more, as determined by the relationship (Mo + 1/2W). The preferred lower limit is 2.7% or more, more preferably 2.9% or more, and even 3.0% or more. However, excessive Mo and W may result in reduced machinability and toughness, resulting in reduced cracking resistance. Furthermore, their high melting points make them difficult to melt, making their inclusion in large amounts undesirable from a manufacturing perspective. Therefore, the value according to the relational expression (Mo + 1/2W) is set to less than 3.5%. The preferred upper limit is 3.4% or less, more preferably 3.3% or less, and even more preferably 3.2% or less. Here, since W is a more expensive element than Mo, it is preferable to contain Mo alone when cost reduction is important.

V: 0.45%≦V≦1.0%
V forms vanadium carbides, which strengthen the matrix and improve its wear resistance and temper softening resistance. When an AM product formed in an AM process is quenched by heating it to a quenching temperature, the vanadium carbides also function as "pinning particles" that suppress coarsening of austenite grains during quenching, contributing to improved toughness. However, because V has a high carbide-forming ability, excessive V may convert all C to vanadium carbide, preventing the formation of other carbides. Tool steels are made up of multiple types of carbides, so vanadium carbide alone is not desirable. Therefore, the V content is 0.45% or less and 1.0% or less. The preferred lower limit is 0.50%, more preferably 0.52% or more and 0.55% or more. The preferred upper limit is 0.80%, more preferably 0.70% or less, 0.67% or less, and 0.65% or less.

Cu: 0.3%<Cu<0.6%
Cu is an element that suppresses the formation of ferrite in the tool structure. It also effectively imparts excellent hardenability to tool materials, along with C, Cr, Mn, Ni, Mo, W, etc., and forms a martensite-based structure even when the cooling rate during quenching is slow, thereby preventing a decrease in toughness. To further improve crack resistance, Cu is set to more than 0.3% (0.3% < Cu). Preferably, it is set to 0.35% or more. On the other hand, excessive addition of Cu precipitates as a simple element in Fe, reducing toughness. Therefore, Cu is set to less than 1.0% (Cu < 0.6%). Preferably, Cu is set to 0.45% or less, and 0.42% or less.

Al: 0.2%≦Al≦0.9%
Al is an element that forms intermetallic compounds such as _Ni3Al with Ni and has the effect of precipitation strengthening the metal structure. Since Ni is added in the present invention, adding Al can improve hardness. Therefore, the lower limit of Al is set to 0.2%. A preferred lower limit is 0.3%, and more preferably 0.4% or more. However, if the amount of Al is too much, non-metallic inclusions may increase in the metal structure, reducing toughness. Therefore, the upper limit of Al is set to 0.9%. A preferred upper limit is 0.8% or less, more preferably 0.7% or less, and even more preferably 0.6% or less.

Balance: Fe and unavoidable impurities. The balance consists of Fe and unavoidable impurities. Typical examples of unavoidable impurities include elements such as P, S, Ca, Mg, O (oxygen), N (nitrogen), and B (boron). The lowest possible content of these elements is preferable. However, small amounts may be included due to additional effects such as controlling the shape of inclusions, improving other mechanical properties, and improving manufacturing efficiency. In this case, the ranges of Ca≦0.01%, Mg≦0.01%, O≦0.05%, N≦0.05%, and B≦0.05% are sufficient and are the preferred upper limits of the present invention. Furthermore, P and S can conform to the JIS steel grade SKD61, e.g., P≦0.030% and S≦0.020%.

In the present invention, by achieving an overall balance within the above-mentioned component ranges, it is possible to obtain hot work tool steel additive manufactured products that have particularly excellent crack resistance during manufacturing.

Formula (1): C+Si/30+(Mn+Cr+Cu)/20+Ni/60+(Mo+1/2W)/15+V/10≦0.95
In addition to specifying the components, one of the features of the present invention is adjusting the left side of the above formula (1) to 0.95 or less. The left side of formula (1) is an improved version of Pcm, which is used as a cold cracking susceptibility index for welding. C, Si, Mn, Cr, Cu, Ni, Mo, W, and V in formula (1) represent the content (mass%) of each element. Like additive manufacturing, cracking is a problem in welding, and both are molten solidification structures. Therefore, it was found that this index can be applied to suppress cracking in additive manufacturing, and was applied to the present invention. Another known cracking index for welding is the hot cracking index HCS. However, prior investigations revealed that the molding cracks in this composition system were largely cracked from the surface and had a different morphology from the hot cracks that tend to occur at the solidification interface, etc. Since the molding cracks in this composition system are found in areas prone to tensile stress due to thermal contraction and therefore occur at low temperatures, it was assumed that they were similar to cold cracks in welding. Therefore, the cold cracking index Pcm was applied in the present invention. By adjusting each main component so that the left side of formula (1) is 0.95 or less, it is possible to obtain a powder for additive manufacturing that can further suppress cracking during manufacturing. The left side of formula (1) is more preferably 0.92 or less, and even more preferably 0.90 or less.

Formula (2): 545-330C+2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si+3Ti+4V≦400
In the present invention, in addition to the component specifications, the above formula (2) can be adjusted to 400 or less. Formula (2) is a relationship between elements and Ms point disclosed in a literature (K. Ishida, Journal of Alloys and Compounds, Volume 220, Issues 1-2, 1995, pp. 126-131). It is expected that a high value of formula (2) will result in a high Ms point, which will tend to transform into brittle martensite at high temperatures during additive manufacturing and become more susceptible to cracking due to thermal contraction when cooled to room temperature. Formula (2) is preferably 380 or less, more preferably 360 or less, even more preferably 340 or less, particularly preferably 320 or less, and extremely preferably 300 or less. Although the lower limit of formula (2) is not particularly limited, if the value of formula (2) is too low, it is expected that the Ms point will be low. If the value is too low, martensitic transformation will not be completed, and austenite will remain, which may result in a decrease in strength. Therefore, it is preferable to adjust formula (2) to 200 or more. It is more preferably 220 or more, and even more preferably 240 or more, or 260 or more.

The hot work tool steel powder for additive manufacturing of the present invention can be produced by, for example, gas atomization, water atomization, disk atomization, plasma atomization, rotating electrode atomization, and the like. Among these, gas atomization is a method in which a molten raw material prepared to have the desired composition is heated to above its melting point by high-frequency induction heating, melted, and then the molten metal that flows out through fine holes is finely crushed by injecting an inert gas such as argon gas or nitrogen gas onto the molten metal, which is then rapidly cooled and solidified to obtain powder. This gas atomization method allows scrap metal, raw metal raw materials, etc. to be used as the molten raw material, and can be produced at a lower cost than methods such as plasma atomization and rotating electrode atomization, which require the preparation of raw materials with the desired composition and shape in advance, making it an ideal method for obtaining the powder for additive manufacturing of the present invention.

The hot work tool steel powder for additive manufacturing of the present invention preferably has a 50% particle size (hereinafter referred to as "D50") of a cumulative particle size distribution on a volume basis of 10 to 250 μm. By making the D50 of the powder for additive manufacturing of the present invention 250 μm or less, the metal powder can be easily melted, and the formation of internal defects in the additive manufacturing product can be suppressed.
Furthermore, by making the powder for additive manufacturing of the present invention have a D50 of 10 μm or more, it becomes less susceptible to the effects of moisture and the like in the atmosphere during handling of the metal powder and additive manufacturing, thereby ensuring good fluidity.
The cumulative particle size distribution of the powder for layered manufacturing of the present invention is expressed as a cumulative volumetric particle size distribution, and its D50 can be expressed as a value measured by the laser diffraction scattering method specified in JIS Z 8825.

The D50 of the hot work tool steel powder for additive manufacturing of the present invention may be adjusted by mesh sieving or airflow classification, depending on the method described above. For example, when using additive manufacturing powders for powder bed processes, the powder is melted by a laser beam, which serves as the heat source. However, coarse additive manufacturing powder particles that are difficult to melt must be removed to minimize the area affected by the heat. Furthermore, highly adhesive fine powder particles must also be removed to achieve optimal fluidity for ensuring powder spreadability. For this reason, when using the additive manufacturing powder of the present invention for the powder bed process, it is preferable to adjust the D50 to the range of 10 to 53 μm. The preferred upper limit of D50 is 40 μm, and the preferred lower limit of D50 is 20 μm. Furthermore, when using the additive manufacturing powder of the present invention for laser metal deposition, it is preferable to adjust the D50 to the range of 50 to 150 μm.

By additively manufacturing the hot working tool steel powder for additive manufacturing of the present invention described above using the manufacturing method described below, the following components are obtained by mass %: 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, and one or both of Mo and W according to the relationship formula (Mo+1/2W): 2.5%≦(Mo+1 It is possible to obtain a hot work tool steel additive manufactured product (hereinafter also referred to as an additive manufactured product) consisting of C + Si/30 + (Mn + Cr + Cu)/20 + Ni/60 + (Mo + 1/2W)/15 + V/10 + ≦0.95, with the remainder consisting of C, Si, Mn, Cr, Cu, Mo, and Al (1/2W) < 3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, and the remainder consisting of Fe and unavoidable impurities, and which satisfies formula (1): C + Si/30 + (Mn + Cr + Cu)/20 + Ni/60 + (Mo + 1/2W)/15 + V/10 + ≦0.95. This additive manufactured product is particularly excellent in that it is resistant to cracking during manufacturing.

The hot work tool steel additive manufactured product of the present invention preferably has a room temperature (approximately 20°C) tensile strength of 500 to 2000 MPa when the tempered hardness is adjusted to 45 HRC ± 2. A more preferable lower limit is 1000 MPa, and an even more preferable lower limit is 1200 MPa. The room temperature 0.2% proof stress when the tempered hardness is adjusted to 45 HRC ± 2 is preferably 500 to 2000 MPa. A more preferable lower limit is 800 MPa, an even more preferable lower limit is 1000 MPa, and an even more preferable lower limit is 1100 MPa. The room temperature elongation when the tempered hardness is adjusted to 45 HRC ± 2 is preferably 5% or more. A more preferable lower limit is 8% and an even more preferable lower limit is 10%. The room temperature drawing when the tempered hardness is adjusted to 45 HRC ± 2 is preferably 20% or more. A more preferable lower limit is 30%, and an even more preferable lower limit is 40%. When the tempered hardness is adjusted to 45HRC±2, the room temperature 2 mm U-notch Charpy impact value is preferably 20 J/ ^cm2 or more. A more preferable lower limit is 30 J/ ^cm2 , and an even more preferable lower limit is 40 J/ ^cm2 .

The hot work tool steel additive manufactured product of the present invention preferably has a high-temperature (approximately 550°C in this embodiment) tensile strength of 400 to 1500 MPa when the tempered hardness is adjusted to 45HRC±2. A more preferred lower limit is 500 MPa, and an even more preferred lower limit is 600 MPa. The high-temperature 0.2% yield strength when the tempered hardness is adjusted to 45HRC±2 is preferably 300 to 1300 MPa. A more preferred lower limit is 400 MPa, and an even more preferred lower limit is 500 MPa. The high-temperature elongation when the tempered hardness is adjusted to 45HRC±2 is preferably 5% or more. A more preferred lower limit is 8%, and an even more preferred lower limit is 10%. The high-temperature reduction when the tempered hardness is adjusted to 45HRC±2 is preferably 10% or more. A more preferred lower limit is 20%, and an even more preferred lower limit is 30%. Furthermore, the room temperature thermal conductivity of the hot work tool steel additive manufactured product of the present invention, when the tempered hardness is adjusted to 45HRC±2, is preferably 5 W/(m·K) or more. A more preferred lower limit is 10 W/(m·K), and an even more preferred lower limit is 15 W/(m·K).

The additively manufactured product of the present invention is most preferably applied to die-casting molds, but it may also be applied to other molds that require internal cooling mechanisms, such as plastic molds. It may also be applied to repairing molds that have been manufactured using the powder spray additive manufacturing method.
Next, an example of a manufacturing process for obtaining an additive manufactured product of the present invention using the hot working tool steel powder for additive manufacturing of the present invention will be described in order. Note that the manufacturing process described below is based on the powder bed method unless otherwise specified.

The manufacturing method of the present invention involves the steps of spreading the prepared hot work tool steel powder for additive manufacturing of the present invention in layers, and sequentially melting and solidifying the spread metal powder using a scanning heat source having a diameter larger than the D50 of the metal powder to form solidified layers. The step of spreading the metal powder in layers and the step of forming the solidified layers are then repeated to form multiple solidified layers, thereby producing the additively manufactured product of the present invention. The scanning heat source may be, for example, a laser or electron beam. Setting the diameter of the scanning heat source to be larger than the D50 of the metal powder is preferable because it allows the metal powder clusters to be melted evenly.

In the manufacturing method of the present invention, when the laser is irradiated onto the above-mentioned metal powder while scanning, the laser output can be set to 50 to 400 W, the scanning speed to 200 to 2000 mm/sec, and the scanning pitch to 0.02 to 0.20 mm. Here, if the layer thickness per laser scan is too large, heat is not easily transferred to the entire spread metal powder during laser irradiation, preventing the metal powder from melting sufficiently and promoting the formation of internal defects. On the other hand, if the layer thickness per scan is too small, the number of layers required to achieve the desired size of the additive manufacturing product increases, lengthening the time required for the additive manufacturing process. For this reason, the layer thickness per scan is preferably set to 10 to 200 μm. A more preferable lower limit for the layer thickness is 20 μm, and a more preferable upper limit for the layer thickness is 100 μm. A preheating step may be carried out before the additive manufacturing process described above. However, because the powder of the present invention has improved crack resistance compared to conventional hot work tool steel powders, for example, if the additive manufacturing product is small and has few stress concentration areas, it is possible to omit the preheating step before additive manufacturing or to use a lower temperature.

In the manufacturing method according to the present invention, in order to impart the mechanical properties necessary for use as a metal product, it is preferable to subject the as-AM (i.e., the AM product without heat treatment after AM) component to a tempering treatment at a temperature of 500 to 700°C. Tempering can produce a "hot work tool steel AM product" with a predetermined hardness. During this process, the AM product can be shaped into the shape of a hot work tool by various machining processes such as cutting and drilling. In this case, the AM product formed in the AM process can be annealed to facilitate machining. Annealing can also be expected to refine the vanadium carbide in the structure of the tempered AM hot work tool. Finishing machining can then be performed after tempering. In some cases, this finishing machining can also be performed on the tempered AM product, and the above-mentioned machining processes can be performed all at once to produce an AM hot work tool.
It should be noted that quenching can be performed before the above-mentioned tempering. Regardless of whether or not the above-mentioned annealing is performed, the additive manufacturing product formed in the additive manufacturing process can be subjected to normalizing.

The tempering temperature varies depending on the target hardness, etc., but is generally around 500 to 700°C. If quenching is performed before tempering, the quenching temperature is generally around 900 to 1100°C. For example, in the case of SKD61, a typical hot work tool steel, the quenching temperature is around 1000 to 1030°C, and the tempering temperature is around 550 to 650°C.
The tempered hardness is preferably 50 HRC (Rockwell hardness) or less or 520 HV (Vickers hardness) or less. More preferably, it is 48 HRC or less or 500 HV or less. Also, it is preferably 40 HRC or more or 380 HV or more. More preferably, it is 42 HRC or more or 400 HV or more. In the present invention, the hardness can be measured in accordance with the measurement method described in JIS Z 2245 "Rockwell hardness test - Test method" or JIS Z 2244-1 "Vickers hardness test - Part 1: Test method", and Rockwell C scale hardness (HRC) or Vickers hardness (HV) can be used.

Example 1
Each metal raw material was prepared to have the component composition shown in Table 1, then charged into a high-frequency induction melting furnace and melted. The molten metal was pulverized with argon gas to obtain gas-atomized powder. The resulting atomized powder was subjected to mesh sieving and airflow classification to adjust the particle size, resulting in additive manufacturing powders for the present invention and comparative examples with a D50 of 35 μm. Additive manufacturing products were produced using the additive manufacturing powders obtained above using an EOS M290 under the manufacturing conditions shown in Table 2. Table 3 shows the component compositions of the additive manufacturing products of Sample No. 5 produced from powder No. 1, Sample No. 6 produced from powder No. 2, Sample No. 7 produced from powder No. 3, and Sample No. 8 produced from powder No. 4. Table 4 also shows the component compositions of Sample No. 1, Sample No. 6 produced from powder No. 2, Sample No. 7 produced from powder No. 3, and Sample No. 8 produced from powder No. 4. The values of formula (1): C + Si/30 + (Mn + Cr + Cu)/20 + Ni/60 + (Mo + ½W)/15 + V/10 in Tables 1 to 8, and the values of formula (2): 545-330C + 2Al-14Cr-13Cu-23Mn-5Mo-4Nb-13Ni-7Si + 3Ti + 4V are shown.

Example 2
To evaluate the crack susceptibility of additively manufactured products, crack evaluation test pieces were fabricated in the shape shown in Figure 1. Specifically, the additive manufacturing powders of Samples No. 1 to 4 in Example 1 were additively manufactured under the same manufacturing conditions as in Example 1 to fabricate Samples No. 9 (Sample No. 5 composition), Sample No. 10 (Sample No. 6 composition), Sample No. 11 (Sample No. 7 composition), and Sample No. 12 (Sample No. 8 composition), which have the same compositions as Samples No. 5 to 8. These crack evaluation test pieces were 50 mm long, 10 mm wide, and 16 mm high, with a stress concentration area (R8) created midway. This stress concentration area was comb-shaped to facilitate cracking, and was designed to simulate an additive manufacturing mold for forming complex cavities. After fabrication, the crack length of the comb-tooth portion of these crack evaluation test pieces was measured, allowing the susceptibility of the material to crack during fabrication to be evaluated. Sample No. The crack lengths of the crack test pieces for Samples No. 9, 11, and 12 (Invention Examples) and Sample No. 10 (Comparative Example) are shown in Table 5. From Table 5, it was confirmed that the Inventive Examples had significantly shorter crack lengths than the Comparative Examples, and had particularly superior crack resistance during molding compared to the Comparative Examples. In particular, Sample No. 9 had a crack length of 1.0 mm or less, confirming that it had the best crack resistance.

Example 3
Next, the tempering behavior of the inventive and comparative examples was confirmed. Samples Nos. 5 to 8 shown in Example 1 above were subjected to tempering heat treatment within the temperature range shown in Figure 2, and their Rockwell hardness was measured in accordance with JIS Z 2245. Figure 2 shows a graph of each tempering temperature and hardness. From Figure 2, it was confirmed that inventive samples Nos. 5, 7, and 8 could be tempered to a higher hardness than comparative sample No. 6.

Furthermore, the mechanical properties and thermal conductivity of the inventive examples were confirmed. Additive manufacturing products manufactured under the same conditions as Samples No. 5, No. 7, and No. 8 (all inventive examples) in Example 1 were subjected to tempering heat treatment in the temperature range of 500-700°C to refine the test specimens to 40±2 HRC, 45±2 HRC, and 52±2 HRC, respectively. Tensile tests, 2mm U-notch Charpy impact tests, and thermal conductivity measurements using the laser flash method were then conducted. Figure 3 shows the results of the room temperature (22°C) tensile tests, Figure 4 shows the results of the high temperature (550°C) tensile tests, Figure 5 shows the results of the Charpy impact tests, and Figure 6 shows the results of the thermal conductivity measurements.

3, the additive manufactured products of the examples of the present invention had room temperature tensile strength of 1100 MPa or more, room temperature 0.2% proof stress of 600 MPa or more, room temperature elongation of 12% or more, and room temperature drawing of 50% or more at all tempered hardnesses. Also, as shown in Fig. 4, the additive manufactured products of the examples of the present invention had high temperature tensile strength of 700 MPa or more, high temperature 0.2% proof stress of 400 MPa or more, high temperature elongation of 13% or more, and high temperature drawing of 35% or more at all tempered hardnesses.
The additive manufactured products of the examples of the present invention had a room temperature tensile strength of 1200 MPa or more, a room temperature 0.2% proof stress of 1100 MPa or more, a room temperature elongation of 13% or more, and a room temperature drawing capacity of 50% or more at a hardness of 45±2 HRC.Furthermore, the additive manufactured products of the examples of the present invention had a high temperature tensile strength of 800 MPa or more, a high temperature 0.2% proof stress of 500 MPa or more, a high temperature elongation of 15% or more, and a high temperature drawing capacity of 35% or more at a hardness of 45±2 HRC.

Figure 5 confirms that the additively manufactured products of the present invention have a Charpy impact value of 50 J/ ^cm2 or higher at all tempered hardness levels. Figure 6 also confirms that the additively manufactured products of the present invention have a thermal conductivity of 15 W/(m K) or higher at room temperature. In particular, sample No. 7 had a thermal conductivity of 20 W/(m K) or higher at room temperature, demonstrating the best thermal conductivity characteristics.
As described above, from FIGS. 3 to 6, it was confirmed that the additively manufactured product of the present invention has properties at the same level as those of ingot hot work tool steel, and is suitable for use in hot work tools, for example.

Claims (2)

A hot work tool steel powder for additive manufacturing, which contains, in mass%, 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, one or two of Mo and W according to the relationship (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, and the balance being Fe and unavoidable impurities, and further satisfying the following formula (1):
Formula (1): C+Si/30+(Mn+Cr+Cu)/20+Ni/60+(Mo+1/2W)/15+V/10≦0.95
Here, each element symbol in formula (1) indicates the content (mass %) of the element. A hot work tool steel additive manufactured product comprising, in mass%, 0.10%≦C≦0.40%, 0.01%≦Si≦0.19%, 0.1%≦Mn≦1.0%, 2.0%≦Ni≦9.0%, 3.5%<Cr<4.5%, one or two of Mo and W according to the relationship (Mo+½W): 2.5%≦(Mo+½W)<3.5%, 0.45%≦V≦1.0%, 0.3%<Cu<0.6%, 0.2%≦Al≦0.9%, and the remainder being Fe and unavoidable impurities, and further satisfying the following formula (1):
Formula (1): C+Si/30+(Mn+Cr+Cu)/20+Ni/60+(Mo+1/2W)/15+V/10≦0.95
Here, each element symbol in formula (1) indicates the content (mass %) of the element.