Titanium boride dispersion-strengthened steel, method for manufacturing titanium boride dispersion-strengthened steel, and raw material powder for titanium boride dispersion.

Abstract

To provide titanium boride dispersed steel due to 3-D molding that is titanium boride dispersion strengthening steel capable of effectively developing the titanium boride dispersion strengthening, a manufacturing method of the titanium boride dispersion strengthened steel, and raw material powder for dispersing the titanium boride.SOLUTION: Titanium boride dispersed steel contains titanium boride particles having an average particle diameter of 3.0 μm or less in terms of projected area circle equivalent diameter, and the titanium boride particles are present as aggregates, and the titanium boride particles are present in aggregates, and the aggregates have a size of 5 μm or more and 20 μm or less, and the aggregates are scattered throughout the steel. A method for manufacturing titanium boride dispersed steel uses a powder bed melt modeling method, in which a mixed powder of composite powder containing titanium boride and iron and steel powder is used as a raw material, and the titanium boride is molded under the conditions of combining a laser output and a scanning speed that become energy density that does not melt the titanium boride.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to titanium boride dispersion strengthened steel, a method for manufacturing titanium boride dispersion strengthened steel, and a raw material powder for titanium boride dispersion.

Background Art

[0002] The additive manufacturing technology using a 3D printer (three-dimensional shaping device, 3D shaping device) has made remarkable progress in recent years, and has been put into practical use in aircraft parts, automobile parts, artificial limbs of the human body, etc. in Europe and the United States. The materials used include various types such as resins, metals, and ceramics. However, with the increasing demand for parts that require strength in recent years, the proportion of metal powders has gradually increased. Metal powders include stainless steel-based, titanium-based, aluminum-based, cobalt-chromium-based, nickel-based, copper, etc., and the composition and particle size are adjusted according to the model and application of the 3D printer.

[0003] The additive manufacturing technology of 3D printers includes differences in powder supply methods, powder smoothing methods, heat sources for melting (laser beam, electron beam), etc., and even so-called hybrid methods that grind and shape the shape after shaping. There are many types, but when roughly classified by the powder supply method, it can be divided into the PBF method (powder bed fusion method, Powder bed fusion) and DED (directed energy deposition method, Directed energy deposition).

[0004] In the former, after supplying the powder to the table, the layer thickness is adjusted to a constant by a recoater or roller, and then irradiated with a laser to melt it, and this is repeated for additive manufacturing. In additive manufacturing, the laser irradiation conditions, so-called parameters, are very important, and the state of the shaped product changes greatly depending on the level and combination thereof. The following items are listed as the main parameters.

[0005] [1] Laser output (W) [2] Scanning speed (mm / s) [3] Scanning pitch (mm) [4] Lamination thickness (mm)

[0006] These parameters determine the energy density (J / mm²). 3 This determines the molten state of the metal, and since this greatly affects the molten state of the metal, it is extremely important to maintain it at an appropriate level.

[0007] The latter method involves using an airflow of argon (Ar) or similar to transport the powder to the 3D printing nozzle, where it falls onto the base plate and is simultaneously irradiated with a laser to melt and layer the powder. The parameters for this method include the following:

[0008] [1] Laser power (W) [2] Head feed speed (mm / s) [3] Powder feeding rate (g / min)

[0009] Generally, the PBF method offers higher molding accuracy and less dimensional error, while the DED method is less accurate than the PBF method but has advantages such as being suitable for larger objects and being able to perform multi-layer printing by simultaneously using multiple materials and layering different materials in different parts. Therefore, these methods are used differently depending on the application and purpose of the printing. However, while the DED method is fine up to the point of delivering the powder to the printing nozzle with an Ar gas stream, the area around the nozzle is an air atmosphere, so it is impossible to completely avoid air entrapment, and the metal powder is oxidized to some extent during printing. In contrast, the PBF method replaces the inside of the printing chamber with N2 or Ar, and the oxygen concentration can be kept to about 0.1% or less, so this method is more suitable for metals that are easily oxidized.

[0010] The 3D-formed objects produced by the aforementioned 3D printing apparatus, such as 3D-formed steel, are limited to those using steel powder provided by the 3D printer (Patent Documents 1 and 2). These include conventional steels such as stainless steel, maraging steel, Inconel, and carbon steel for machine structures.

[0011] Non-patent document 1 discloses an example of 3D fabrication using a mixture of SUS316L powder and TiB2 powder. In Non-patent document 1, TiB2 powder with a purity of 99.0% and a particle size of 2-12 μm is mixed with SUS316L powder with an average particle size of 45 μm using a ball mill for 8 hours, and an 8 mm x 6 mm cylinder is fabricated using a PBF (Plant-Blowing) 3D printer.

[0012] Furthermore, Patent Document 3 discloses a titanium boride-containing powder containing Fe and TiB2 within its particles, and states that when this powder is used by dispersing it in an alloy or the like, the titanium boride particles do not aggregate in the dispersion medium such as the master alloy and are easily dispersed into a fine dispersion.

[0013] Furthermore, Patent Document 4 discloses a molding powder consisting of a pulverized composite powder made of ceramics and Fe, and a mixed powder of stainless steel powder, and it is shown that the ceramics include TiB2. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] International Publication No. 2019 / 124296 [Patent Document 2] Japanese Patent Publication No. 2017-186653 [Patent Document 3] Japanese Patent Publication No. 2017-88972 [Patent Document 4] Japanese Patent Publication No. 2020-164916 [Non-patent literature]

[0015] [Non-Patent Document 1] B.AlMangour, D.Grzesiak, JMYang, “Rapid fabrication of bulk-form TiB2 / 316L stainless steel nanocomposites with novel Reinforcement architecture and improve performance by selective laser melting” Journal of Alloys and Compounds 680 (2016)480-493. [Non-Patent Document 2] B.AlMangour, D.Grzesiak, JMYang, “Selective laser melting of TiB2 / 316L stainless steel composites: The roles of powder preparation and hot isostatic pressing post-treatment” Powder Technology 309 (2017)37-48. [Non-Patent Document 3] Bandar AlMangoura, Young-Kyun Kimb, Dariusz Grzesiakc, Kee-Ahn Leeb, “Novel TiB2-reinforced 316L stainless steel nanocomposites with excellent room- and high-temperature yield strength developed by additive manufacturing” Composites Part B 156 (2019)51-63. [Overview of the project] [Problems that the invention aims to solve]

[0016] Non-patent Documents 1 to 3 also show the parameters during shaping. However, since these are using pure TiB2 powder, it is difficult to obtain a shaped product with sufficiently dispersed TiB2 just by mixing with SUS316L powder. Therefore, the improvement in hardness is small considering the amount of TiB2 added.

[0017] On the other hand, Patent Document 3 discloses a TiB2 / Fe composite powder, stating that when this is added to a sintered metal, TiB2 can be homogeneously dispersed. In a sintered metal, it is more effective to use the TiB2 / Fe composite powder rather than adding the TiB2 powder as it is. However, TiB2 undergoes some grain growth due to the heat history in the sintered metal process.

[0018] Patent Document 4 discloses that when using a TiB2 / Fe composite powder for shaping by a 3D printer, TiB2 can be more finely dispersed than in the aforementioned sintered metal, and the hardness is improved.

[0019] From the above, it can be understood that the hardness is improved when TiB2 is dispersed, TiB2 can be more homogeneously dispersed when using a TiB2 / Fe composite powder, and the improvement in hardness due to TiB2 dispersion is also recognized in shaping by a 3D printer.

[0020] However, particularly in 3D shaping steel, it is desired to more efficiently perform titanium boride dispersion strengthening. That is, it is desired to exhibit sufficient strengthening with as little titanium boride as possible.

[0021] The present invention has been made in view of the above problems, and aims to provide a titanium boride dispersion steel by 3D shaping, a titanium boride dispersion strengthening steel capable of effectively expressing titanium boride dispersion strengthening, a method for manufacturing the titanium boride dispersion strengthening steel, and a raw material powder for titanium boride dispersion.

Means for Solving the Problems

[0022] The inventors of the present invention discovered that in order to dramatically enhance the dispersion strengthening of titanium boride, it is preferable for the steel to have a structure in which aggregates formed by the aggregation of fine titanium boride particles are dispersed. Furthermore, the inventors of the present invention discovered that, as one method of forming the aforementioned structure, a composite powder containing titanium boride (TiB2) and iron is used in a powder bed molten metallurgy method, and that by molten at a low energy density that can suppress the grain growth of the titanium boride (for example, by molten at a low energy density that does not melt the titanium boride), the grain growth of the titanium boride is suppressed, and desirable titanium boride dispersion strengthened 3D molded steel with finely dispersed titanium boride can be produced. This led to the present invention.

[0023] In other words, the gist of this invention is as follows.

[0024] (1) A titanium boride dispersed steel, characterized in that it contains titanium boride particles with an average particle size of 3.0 μm or less in terms of projected area circle diameter, the titanium boride particles exist in aggregates, the average size of the aggregates is 5 μm or more and 20 μm or less, and the aggregates are scattered in the steel.

[0025] (2) The titanium boride dispersion-strengthened steel according to (1), characterized in that the content (moles) of B and Ti in the titanium boride dispersion-strengthened steel is 1.6 or more and 2.0 or less in molar ratio.

[0026] (3) The titanium boride dispersion-strengthened steel according to (1), characterized in that the content (moles) of B and Ti in the titanium boride dispersion-strengthened steel is 1.7 or more and less than 2.0 in molar ratio.

[0027] (4) A titanium boride dispersion-reinforced steel according to any of (1) to (3) above, characterized in that the steel is one selected from SUS304, SUS316, SUS316L, and SUS630, or a mixture of two or more types.

[0028] (5) A method for producing titanium boride dispersion-reinforced steel by powder bed fusion molding, characterized in that a mixed powder of composite powder containing titanium boride and iron and steel powder is used as raw material, and the steel is fabricated under conditions of a combination of laser output and scanning speed that results in an energy density that suppresses grain growth of the titanium boride.

[0029] (6) A method for producing titanium boride dispersion-reinforced steel by powder bed fusion molding, characterized in that a mixed powder of composite powder containing titanium boride and iron and steel powder is used as raw material, and the molding is performed under conditions of a combination of laser power and scanning speed that results in an energy density that does not melt the titanium boride.

[0030] (7) The energy density is 50 J / mm 3 More than 200J / mm 3 A method for producing titanium boride dispersion-reinforced steel according to (5) or (6) above, characterized in that it is as follows.

[0031] (8) A method for manufacturing titanium boride dispersion-strengthened steel according to any of (5) to (7), characterized in that the laser output is 50 W or more and 100 W or less, and the scanning speed is 50 mm / s or more and 500 mm / s or less.

[0032] (9) A method for producing titanium boride dispersion-reinforced steel according to any of (5) to (8), characterized in that the composite powder containing titanium boride and iron is a pulverized powder with a fluidity of 20 seconds / 50g or less.

[0033] (10) A method for producing titanium boride dispersion-strengthened steel according to any of (5) to (9), characterized in that the composite powder containing titanium boride and iron is a pulverized powder, and is a sieved powder obtained by sieving with a sieve with a mesh size of 5 μm or more and 50 μm or less.

[0034] (11) A method for producing titanium boride dispersion-strengthened steel according to any of (5) to (10) above, characterized in that the titanium boride content is 5% by mass or less.

[0035] (12) A composite powder containing titanium boride and iron, characterized in that the composite powder is a pulverized powder and has a fluidity of 20 seconds / 50g.

[0036] (13) The raw material powder for titanium boride dispersion according to (12), characterized in that the content (moles) of B and Ti is 1.7 or more and less than 2.0 in molar ratio.

[0037] (14) A composite powder containing titanium boride and iron, wherein the composite powder is a pulverized powder and is a sieved powder obtained by sieving with a sieve with a mesh size of 5 μm or more and 50 μm or less, as a raw material powder for dispersing titanium boride according to (12) or (13). [Effects of the Invention]

[0038] According to the present invention, it is possible to obtain titanium boride dispersion-reinforced 3D-shaped steel with significantly improved strength and hardness. [Brief explanation of the drawing]

[0039] [Figure 1] Figure 1 is an electron microscope image showing a typical example of the titanium boride dispersion structure of the present invention. [Figure 2] Figure 2 shows a representative example (electron microscope image) of the internal particle structure of the titanium boride / iron composite powder used in the manufacturing method of the present invention. [Figure 3] Figure 3 shows an example of a fabricated product made from the titanium boride dispersion-reinforced 3D-formed steel of the present invention. [Figure 4] Figure 4 shows an example of a 3D-formed product made from titanium boride dispersion-reinforced steel used in tensile testing. [Figure 5] Figure 5 shows the effect of titanium boride (TiB2) content on the strength improvement rate and hardness improvement rate of 3D-formed steel made of titanium boride-reinforced SUS316L. [Figure 6] Figure 6 shows a comparison of comparative examples and examples of strength improvement rates and hardness improvement rates for titanium boride dispersion-reinforced 3D-formed steel (MS1, SUS304, SUS603). [Modes for carrying out the invention]

[0040] The present invention will be described in detail below.

[0041] The titanium boride dispersion-strengthened steel of the present invention is a 3D-formed steel manufactured by 3D printing, and contains titanium boride particles with an average particle size of 3.0 μm or less in terms of the projected area circle diameter. The titanium boride particles are scattered in the steel as aggregates with a size of 5 μm to 20 μm. Figure 1 is an electron microscope image of the microstructure of a typical titanium boride dispersion steel of the present invention.

[0042] This configuration yields a dramatically different effect from conventional methods, resulting in titanium boride dispersion-reinforced steel with significantly improved strength and hardness.

[0043] Dispersion strengthening can be achieved by dispersing highly hard titanium boride particles into steel. The inventors conducted detailed research on what kind of microstructure should be used to dramatically improve the effect of titanium boride particle dispersion strengthening in steel manufactured by 3D printing (Additive Manufacturing, AM), and concluded that the aforementioned microstructure is necessary.

[0044] The average particle size of the titanium boride particles dispersed in the steel is set to 3.0 μm or less in terms of the diameter of the projected area circle, in order to efficiently obtain the effect of dispersion strengthening. Dispersing finer titanium boride particles makes it easier to obtain the effects of the present invention. However, since it is not easy to form titanium boride particles smaller than 0.1 μm, 0.1 μm may be set as the lower limit.

[0045] Furthermore, the titanium boride particles exist in aggregates with a projected area diameter of 5 μm to 20 μm, meaning they have a hierarchical dispersion structure. By forming a microstructure with these two layers, dispersion strengthening can be achieved more efficiently than ever before. By setting the average size of the aggregates to a projected area diameter of 5 μm to 20 μm, a significant improvement in strength can be obtained.

[0046] The distance between titanium boride aggregates and the degree of their dispersion are not particularly limited. For example, aggregates can be distributed at an area ratio of approximately 1-10%.

[0047] In this invention, the average particle size of titanium boride particles dispersed in steel is determined by observing the titanium boride particles with an electron microscope as shown in Figure 1, and calculating the diameter equivalent to the projected area circle. The average value of 50 particles is obtained by observing 5 fields of view.

[0048] In this invention, the average size of the titanium boride particle aggregate is determined by calculating the diameter equivalent to the projected area circle of the aggregate (particle group) observed with the electron microscope shown in Figure 1, and the average value of 50 particles obtained by observing 5 fields of view is taken.

[0049] In the titanium boride dispersion-strengthened steel of the present invention, it is preferable that the ratio of B to Ti content (moles) (hereinafter referred to as "B / Ti molar ratio") is 1.6 or more and 2.0 or less.

[0050] From the Ti-B phase diagram, if the B / Ti molar ratio exceeds 2.0, it enters a two-phase region consisting of TiB2 phase and B phase, which can lead to the formation of low-melting-point Fe-B (ferroboron), resulting in insufficient dispersion strengthening or poor high-temperature stability. Therefore, a B / Ti molar ratio of 2.0 or less is more preferable. On the other hand, if the B / Ti molar ratio is less than 1.6, a Ti3B4 phase with a lower melting point than the TiB2 phase is also formed, reducing the proportion of the TiB2 phase, which is undesirable. A B / Ti molar ratio of less than 2.0 and 1.7 or more is more preferable.

[0051] As mentioned above, the TiB2 phase is TiB 2-x Since it forms a solid solution of (0≦x≦1.6), strictly speaking it is "TiB 2-x Although it would normally be written as "TiB2", here we will use "TiB2" unless otherwise specified, and include the solid solution composition.

[0052] The steel used in the titanium boride dispersion-strengthened steel of the present invention may be any alloy containing iron (Fe). It is more preferable to use one or more steels selected from SUS304, SUS316, SUS316L, and SUS630, as this makes it easier to obtain the effect of titanium boride dispersion strengthening.

[0053] The titanium boride dispersion-strengthened steel of the present invention can be manufactured by any 3D fabrication method that can produce the above-described microstructure. Examples include powder bed fusion (PBF), directed energy deposition (DED), fused deposition modeling (FDM), arc welding, binder jetting, supersonic deposition, and liquid metal deposition.

[0054] The manufacturing method of titanium boride dispersion-strengthened steel according to the present invention, using the PBF method, is described below.

[0055] In the present invention's method for producing titanium boride-dispersed steel by the PBF method, a mixed powder of composite powder containing titanium boride and iron and steel powder is used as a raw material, and titanium boride-dispersed strengthened steel is produced by fabricating the object under conditions of laser output and scanning speed that result in an energy density that can suppress the grain growth of titanium boride. By suppressing the grain growth of titanium boride and performing 3D fabrication, as described above, fine titanium boride particles can be dispersed in the steel as aggregates, and dispersion strengthening by titanium boride can be effectively achieved. Furthermore, cracks that occur in the 3D fabricated object can also be reduced.

[0056] Furthermore, using a composite powder containing titanium boride and iron as the titanium boride source in the PBF method is also an effective method for obtaining the above effects, and the effects of the present invention can be obtained by combining both methods.

[0057] The composite powder containing titanium boride and iron used in the present invention preferably has a composite structure of an iron matrix containing titanium boride particles with an average particle size of 3.0 μm or less in terms of the projected area circle diameter, as shown in Figure 2.

[0058] One example of a method for producing a composite powder containing titanium boride and iron, which is a raw material for the titanium boride dispersion-strengthened steel of the present invention, is described below.

[0059] Ferrotitanium powder and ferroboron powder are mixed, then calcined in an inert gas atmosphere, and finally pulverized and classified. This yields a composite powder in which titanium boride (TiB2) is uniformly dispersed within the iron (Fe) matrix particles (Figure 2). Because the particle size is adjusted to an appropriate level, there is almost no cohesiveness, and when used as a raw material powder for 3D printers, it can be easily and uniformly mixed with steel powder, such as SUS316L powder.

[0060] On the other hand, pure titanium boride (not a composite), as described in Non-Patent Document 1, does not produce fine particles due to its manufacturing method, and most particles have a diameter of 10 μm or larger. Although it is possible to crush the particles to make them finer, titanium boride is hard and difficult to crush, and even if crushed, measures to avoid dust explosions etc. are necessary. Despite these problems, even when made into fine particles of a few μm, it has strong cohesiveness, requiring a long time to mix with steel particles, such as SUS316L powder, and some aggregated particles remain even after mixing. The composite powder containing titanium boride and iron in the present invention has a completely different form and particle size of titanium boride than the mixed powder described in Non-Patent Document 1, so its melting and solidification behavior during laser irradiation is naturally different.

[0061] Any method is acceptable for manufacturing the titanium boride dispersion-strengthened steel of the present invention, as long as it satisfies the essential requirements of the present invention. For example, one method is to manufacture the steel by 3D molding using a mixed powder of composite powder containing titanium boride and iron and steel powder as raw materials. In the 3D molding process, it is preferable to manufacture the steel by rapid heating and cooling in a manner that can suppress the grain growth of titanium boride. This is because it is easier to obtain steel in which titanium boride is finely dispersed. Furthermore, one 3D molding method that can suppress the grain growth of titanium boride is to supply energy in the 3D molding process under conditions that prevent the titanium boride from melting.

[0062] In the method for producing titanium boride dispersed steel by the PBF method, the energy density is 80 J / mm². 2 More than 200J / mm 3 The following is more preferable. Within this range, the density of the 3D printed object tends to increase, and crack formation becomes less likely. This is especially preferable when the steel particles are SUS316L.

[0063] Furthermore, it is more preferable that the laser output is between 50W and 100W, and the scanning speed is between 50mm / s and 500mm / s. Within this range, the density of the 3D printed object tends to increase and cracks are less likely to occur.

[0064] In PBF (Print-on-Fiber) fabrication, lasers are generally irradiated at an output of over 100W but less than or equal to 300W, and high scanning speeds of over 500mm / s but less than or equal to 2000mm / s are often used to increase productivity. This causes the material to undergo extremely rapid heating and cooling, resulting in rapid melting, thermal expansion and contraction, and solidification, creating residual stress within the material. Even after solidification, the material is repeatedly affected by the heat from the subsequent layering, further increasing the residual stress. As a result, the fabricated object may warp, crack significantly, or develop microcracks. Since residual stress is also affected by the size and shape of the fabricated object, size constraints and shape design considerations are necessary, which are limitations in actual production. If manufacturing is possible but there is a certain probability of cracking, the product acceptance rate decreases and costs increase. It is acceptable if microcracks occur on the surface and can be detected by appearance, but if microcracks occur internally and go undetected, and the product passes through the process, it will be incorporated as a component in the final product, which could lead to failure depending on the application.

[0065] The composite powder containing titanium boride and iron is preferably a pulverized powder with a fluidity of 20 seconds / 50g or less. In 3D printing, it is preferable for the raw material powder to have excellent fluidity, and the fluidity measured according to JIS Z 2502:2020 (metal powder - fluidity measurement method) is more preferably 20 seconds / 50g or less, and even more preferably 16 seconds / 50g or less. Since a lower fluidity is better, there is no lower limit, but in principle, 0 seconds / 50g is impossible, so it will be greater than 0 seconds / 50g.

[0066] The composite powder containing titanium boride and iron is preferably a pulverized powder, and more preferably a sieved powder obtained by sieving with a mesh size (opening) of 50 μm or less and 5 μm or more. That is, when the composite powder containing titanium boride and iron is a pulverized powder, it is more preferable to remove the fine particles as this may result in better fluidity, and therefore it is more preferable to be a sieved powder obtained by sieving with a mesh size of 50 μm or less and 5 μm or more. Even more preferable is a sieved powder obtained by sieving with a mesh size of 25 μm.

[0067] From the viewpoint of preventing crack formation in 3D printed objects, the titanium boride content in titanium boride dispersion-strengthened steel is preferably 10% by mass or less, and more preferably 5% by mass or less. On the other hand, from the viewpoint of improving strength, hardness, and Young's modulus, even a small amount of titanium boride can be effective, but when it exceeds 5% by mass, the effect of improving strength, hardness, and Young's modulus through dispersion strengthening becomes significant. The titanium boride content should be set appropriately according to the application, taking into account the balance between crack formation and dispersion strengthening.

[0068] 3D printing is being used for many types of steel, including SUS316L and maraging steel. When using a single metal material, or a material composed of only multiple metals such as titanium alloys or cobalt-chromium steel, it becomes relatively easy to control the process by properly adjusting the parameters. However, the situation is quite different when titanium boride, a ceramic, is added to a metal, such as SUS316L steel, and the difficulty of 3D printing increases significantly. One reason for this increased difficulty is that cracks are more likely to occur in the 3D printed material.

[0069] One possible explanation for the formation of such cracks is that microcracks initially form at the interface between titanium boride and steel (e.g., SUS316L), then propagate outwards, growing larger and eventually becoming large fissures. However, microscopic observation of cracks in 3D printed objects reveals that the cracks propagate from the outer surface inward, rather than through the interface with titanium boride. The cracks in 3D printed objects are likely a result of brittle fracture rapidly progressing inward when cracks form on the outer surface due to thermal stress during laser irradiation, as the addition of titanium boride increases hardness but reduces the elongation, or ductility, of the base steel, making it brittle.

[0070] While rapid heating and cooling are considered to be the main cause of cracks, ceramics and metals generally have significantly different coefficients of thermal expansion. Therefore, when ceramics are added to a metal, the residual stress becomes much greater than in the case of the metal alone, making it more prone to cracking. For example, the coefficients of thermal expansion of the steel base material SUS316L and titanium boride differ by about three times, which is thought to contribute to strain, deformation, and crack formation during additive manufacturing.

[0071] For example, when fabricating with SUS316L alone, the fabrication parameters have already been established, and various parts have been manufactured and put into practical use. Although the parameters vary depending on the manufacturer and model, in the case of the LMD method, the conditions are generally such that the laser output is between 100W and 300W, the scanning speed is between 500mm / s and 2,500mm / s, the scanning pitch is between 30μm and 90μm, and the layer thickness is between 20μm and 60μm. These parameters are adjusted so that there are no particular problems with the appearance, the dimensional accuracy of the fabricated product is maintained, and the relative density of the fabricated product is generally 99.0% or higher, and even 99.5% or higher. On the other hand, even with the same SUS316L powder, the manufacturing equipment and conditions differ depending on the manufacturer, and there are subtle differences in the particle size range, particle size distribution, shape and surface condition of the obtained powder, so it is necessary to adjust the parameters again for each powder.

[0072] While 3D printing using SUS316L powder has been made practical for creating parts of a certain size through such parameter adjustments, adding a composite powder of titanium boride and iron requires significant parameter adjustments because titanium boride, a type of ceramic, is now incorporated into the metallic SUS316L. The method discovered by the inventors involves adjusting the parameters while gradually increasing the content of titanium boride and iron, using the parameters of SUS316L as a base. This will be explained in detail below.

[0073] When the titanium boride content after mixing titanium boride composite powder and SUS316L powder is 2% by mass or less, the amount added is small, so the parameters are almost problem-free even under normal conditions, and molded parts with no cracks or warping and a relative density of 99.0% or higher can be obtained. However, when the titanium boride content is 4% by mass, the density becomes 70 J / mm². 3At the following low energy densities, cracks do not occur. However, due to insufficient melting of iron and SUS316L, the relative density does not increase. When the energy density is increased to a relative density of 99.0% or higher, cracks and warping occur, resulting in defective molded products. This tendency becomes more pronounced when the titanium boride content reaches 5 mass%, with cracks becoming severe and molded products sometimes breaking into large pieces.

[0074] Even with SUS316L alone, large parts are prone to cracking. This is thought to be due to residual stress caused by thermal expansion due to rapid heating and contraction due to rapid cooling. The same is true for SUS316L containing titanium boride, but in this case, even relatively small parts will begin to crack from a titanium boride content of 4 mass%, as mentioned above. This is thought to be due to the significant difference in thermal expansion coefficients between SUS316L, which is a metal, and titanium boride, which is a ceramic, in addition to residual stress. In other words, the difference in linear expansion coefficients is three times, making it more sensitive to rapid heating and cooling, and thus significantly increasing residual stress.

[0075] While SUS316L's greatest feature is its corrosion resistance, it generally lacks strength and wear resistance, limiting its applications. The manufacturing method of the present invention, which strengthens SUS316L by dispersing titanium boride, maintains its inherent properties while significantly improving Vickers hardness, thereby increasing strength and wear resistance. This is expected to greatly expand the applications of 3D printer parts using SUS316L. For example, with a titanium boride content of 3.5% by mass, the Vickers hardness is approximately twice that of SUS316L alone, and the strength and wear resistance are also roughly doubled, greatly expanding the potential for new applications. [Examples]

[0076] The present invention will be described below based on embodiments. However, the present invention is not limited to these embodiments.

[0077] (Preparation of titanium boride / iron composite powder)

[0078] Ferrotitanium powder (particle size 150 μm or less, Ti content 70% by mass) and ferroboron powder (particle size 63 μm or less, B content 18% by mass) were mixed in a V-type mixer for 30 minutes, then packed into a magnesium casing and charged into a vacuum furnace. After purging with argon and vacuuming to 10⁻⁵ MPa or less, heating was started and maintained at the specified temperature (Table 1, reaction temperature) for 2 hours. After cooling, the mixture was removed and roughly broken into pieces several centimeters in size using a jaw crusher. Then, as a second stage of grinding, ball mill grinding was performed for 5 hours, and as a third stage of grinding, vibratory mill grinding was performed for 10 hours.

[0079] The titanium boride / iron composite powder obtained in this manner was classified by sieving the unsieved powder through a sieve with a mesh size of 75 μm and then sieving the sieved powder through a sieve with a predetermined mesh size (Table 1).

[0080] The fluidity of the titanium boride / iron composite powder was measured using a method conforming to the fluidity measurement method for metal powders specified in JIS Z-2502. Specifically, 50 g of dry powder was supplied to a funnel of a predetermined shape with a sealed outlet, and the time from when the powder began to fall after opening the outlet until the entire amount had fallen was measured.

[0081] (3D printing of titanium boride / iron composite powder and steel powder)

[0082] Using the above-mentioned titanium boride / iron composite powder, atomized powders of each steel (stainless steel SUS316L, SUS304, SUS630, maraging steel MS1) shown in Table 1 were mixed with the titanium boride content in the mixed powder as shown in Table 1 to prepare molding powders.

[0083] Next, the fluidity of these molding powders was measured. Even when mixed with the aforementioned titanium boride / iron composite powder, they exhibited excellent fluidity, posing no problems whatsoever for use as 3D printer or build-up material.

[0084] Next, when we performed additive manufacturing of a cube using the PBF method with these molding powders, the powder supply and layer thickness adjustment by the recoater worked without problems, continuous operation was possible, and we were able to obtain a molded product of the desired shape, as shown in Figure 3. Here, the laser spot diameter of the PBF method was 0.1 mm, the layer thickness was 0.02 mm, and the scanning speed, laser output, and energy density were as shown in Table 1.

[0085] The particle size of titanium boride particles was determined by first determining the area of ​​the observed titanium boride particles using an electron microscope equipped with an energy-dispersive X-ray spectrometer, and then calculating the projected area equivalent diameter by replacing it with the area of ​​a circle. After observing five fields of view and calculating the projected area equivalent diameter of 50 particles, these were averaged to obtain the average particle size of titanium boride. The size of aggregates of titanium boride particles was determined in the same manner.

[0086] The hardness and strength of the molded parts were measured as follows: The cylindrical molded part shown in Figure 3 was cut, and the Vickers hardness was measured on the polished cut surface. Table 2 shows the percentage of hardness (hardness improvement rate) when the Vickers strength of the 3D molded part made of base steel without titanium boride is set to 100. Tensile test specimens shown in Figure 4 were prepared, and the strength of each molded part was measured. Table 2 shows the percentage of strength (strength improvement rate) when the strength of the 3D molded part made of base steel without titanium boride is set to 100.

[0087] Therefore, the effectiveness of the present invention is judged by whether, for steel with the same titanium boride TiB2 content, superior strength improvement and hardness improvement rates were obtained compared to conventional titanium boride dispersion-reinforced steel (Figures 5 and 6). In Figures 5 and 6, black circles represent examples, and triangles represent comparative examples. White circles indicate the standard strength when titanium boride is not included.

[0088] The improvement in strength and hardness due to dispersion strengthening varies depending on the base material, and we determined that particularly significant effects were obtained in the following cases.

[0089] SUS316L: Hardness improvement rate of 105% or more and strength improvement rate of 165% or more MS1: Hardness improvement rate of 105% or more and strength improvement rate of 115% or more SUS304: Hardness improvement rate of 200% or more and strength improvement rate of 180% or more SUS630: Hardness improvement rate of 115% or more, and strength improvement rate of 105% or more

[0090] In titanium boride-dispersed steel, it was confirmed that if the microstructure of the present invention is met, the hardness and strength are improved when comparing 3D-formed products with the same titanium boride content. In other words, it was found that to obtain the same dispersion strengthening effect, only a small amount of titanium boride is needed in titanium boride-dispersed steel with the microstructure of the present invention (Examples 19-28, 30 and Comparative Examples 6-7, 8; Examples 31, 32, 33, 34 and Comparative Examples 9, 10, 11, 12).

[0091] When titanium boride particles have an average diameter of 3.0 μm or less and are distributed in aggregates between 5 μm and 20 μm in size, the hardness and strength of the base steel are significantly improved, indicating a high effect of dispersion strengthening. On the other hand, comparative examples 6 and 7, which did not have the above-mentioned dispersed titanium boride particle structure, showed a low dispersion effect of titanium boride particles.

[0092] Furthermore, in molded products with a titanium boride content exceeding 5% by mass, tensile tests were conducted and the resulting stress-strain curves confirmed a significant improvement in Young's modulus (stiffness).

[0093] Furthermore, in a separate single-layer bead molding test using the DED method, the supply of the aforementioned powder was completely problem-free, and single-layer bead molded products of the predetermined shape were obtained. Moreover, in the microstructure resulting from the present invention, the effect of dispersion strengthening by titanium boride particles was remarkable.

[0094] [Table 1]

[0095] [Table 2] [Industrial applicability]

[0096] According to the present invention, titanium boride dispersion-reinforced 3D-shaped steel with significantly improved strength, hardness, and rigidity can be obtained. Therefore, it is possible to manufacture steel structures with a 3D structure that has not been possible before, and their strength, hardness, and rigidity can be made superior to those previously unseen.

Claims

1. Titanium boride dispersed steel, It contains titanium boride particles with an average particle size of 0.1 μm or more and 3.0 μm or less in terms of the projected area circle diameter. The aforementioned titanium boride particles exist as aggregates, The average size of the aggregate is 5 μm or more and 20 μm or less. The aforementioned aggregates are scattered throughout the steel. A titanium boride dispersion-reinforced steel characterized by the following features.

2. The titanium boride dispersion-strengthened steel according to claim 1, characterized in that the content (moles) of B and Ti in the titanium boride dispersion-strengthened steel is 1.6 or more and 2.0 or less in molar ratio.

3. The titanium boride dispersion-strengthened steel according to claim 1, characterized in that the content (moles) of B and Ti in the titanium boride dispersion-strengthened steel is 1.7 or more and less than 2.0 in molar ratio.

4. The titanium boride dispersion-reinforced steel according to any one of claims 1 to 3, characterized in that the steel is one selected from SUS304, SUS316, SUS316L, and SUS630, or a mixture of two or more types of steel.

5. A method for producing titanium boride dispersion-reinforced steel by powder bed fusion molding, The raw material is a mixed powder of titanium boride and iron-containing composite powder and steel powder. The titanium boride is fabricated using a combination of laser power and scanning speed that provides an energy density that suppresses grain growth. A method for producing titanium boride dispersion-reinforced steel, characterized by the above.

6. A method for producing titanium boride dispersion-reinforced steel by powder bed fusion molding, The raw material is a mixed powder of titanium boride and iron-containing composite powder and steel powder. The titanium boride is fabricated using a combination of laser power and scanning speed that results in an energy density that does not melt the aforementioned titanium boride. A method for producing titanium boride dispersion-reinforced steel, characterized by the above.

7. The aforementioned energy density is 50 J / mm². 3 More than 200J / mm 3 A method for producing titanium boride dispersion-reinforced steel according to claim 5 or 6, characterized in that it is as follows:

8. A method for producing titanium boride dispersion-reinforced steel according to any one of claims 5 to 7, characterized in that the laser output is 50 W or more and 100 W or less, and the scanning speed is 50 mm / s or more and 500 mm / s or less.

9. A method for producing titanium boride-dispersed reinforced steel according to any one of claims 5 to 8, characterized in that the composite powder containing titanium boride and iron is a pulverized powder with a fluidity of 20 seconds / 50 g or less.

10. The method for producing titanium boride-dispersed strengthened steel according to any one of claims 5 to 9, characterized in that the composite powder containing titanium boride and iron is a pulverized powder, and is a sieved powder obtained by sieving with a sieve with a mesh size of 5 μm or more and 50 μm or less.

11. A method for producing titanium boride-dispersed strengthened steel according to any one of claims 5 to 10, characterized in that the titanium boride content is 5% by mass or less.

12. A composite powder containing titanium boride and iron, characterized in that the composite powder is a pulverized powder and the fluidity of the composite powder is 20 seconds / 50 g.

13. The raw material powder for titanium boride dispersion according to claim 12, characterized in that the content (moles) of B and Ti is 1.7 or more and less than 2.0 in molar ratio.

14. A composite powder containing titanium boride and iron, wherein the composite powder is a pulverized powder and is a sieved powder obtained by sieving with a sieve with a mesh size of 5 μm or more and 50 μm or less, as described in 12 or 13.

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