Maraging steel and powder

Abstract

To provide maraging steel exhibiting high thermal conductivity while being substantially free of cobalt, and to provide powder composed of the maraging steel.SOLUTION: The maraging steel contains 9.0≤Ni≤12.0 mass%, 4.0≤Mo≤7.0 mass%, and 0.3≤Al+Ti≤1.4 mass%, with the remainder consisting of Fe and unavoidable impurities. The maraging steel preferably further contains at least one element selected from the group consisting of 0.05≤Nb≤0.3 mass%, 0.1≤V≤1.0 mass%, and 0.5≤W≤5.0 mass%. The powder is produced from the maraging steel.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to maraging steels and powders, and more particularly to maraging steels having relatively high thermal conductivity and powders made from such maraging steels. [Background technology]

[0002] Overlay welding is sometimes performed to repair or change the shape of dies. The materials used for overlay welding are mainly SCM steel, SKD61 steel, and maraging steel. Of these, maraging steel does not contain carbon and has a soft structure of 30-35 HRC in the as-welded state, making it less likely to crack during overlay welding. For this reason, maraging steel is widely used for welding repair of dies.

[0003] Various proposals have been made regarding such maraging steels. For example, Patent Document 1 states: a first member made of a maraging steel containing predetermined amounts of Ni, Co, Mo, Ti, Al, C+N, Si, Mn, P, and S, with the balance being Fe and unavoidable impurities; The second member is made of maraging steel with a different Mo content from the first member. A welded structure obtained by butt welding is disclosed.

[0004] The same document states: (A) Generally, when maraging steel is welded, a Mo-enriched region may be formed in the weld, and in the enriched region, austenite may not completely transform into martensite during the cooling process, resulting in residual austenite; and (B) When welding maraging steel, if the composition of the two base materials to be butt-welded is selected so that the Mo content in the weld is low, the amount of retained austenite in the weld can be reduced to 20% or less. is stated.

[0005] Patent Document 2 discloses a maraging steel containing predetermined amounts of Ni, Mo, Ti, Al, C, and Mn, with the balance being Fe and unavoidable impurities. The document describes that by optimizing the components, high toughness and strength can be obtained even without Co, which has conventionally been considered essential.

[0006] In aluminum die casting and plastic injection molding, using molds made of steel with high thermal conductivity has the advantage of allowing for faster heat removal and shorter molding cycle times. However, conventional maraging steel has lower thermal conductivity than hot die steel and P20 series steel for plastic molds. Therefore, while conventional maraging steel is advantageous in terms of weldability, it has the problem of slow heat removal from the mold when used as a mold.

[0007] In addition, substances containing 1 mass% or more of cobalt are subject to "health hazard prevention measures" under the Regulations on Prevention of Harm from Specified Chemical Substances (Regulations on Prevention of Harm from Specified Chemical Substances), making them difficult to handle. On the other hand, a cobalt-free maraging steel is proposed in Patent Document 2. However, the maraging steel described in this document has a problem in that it has lower thermal conductivity than conventional cobalt-containing maraging steels. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 60-063352 [Patent Document 2] Special Publication No. 01-043016 Summary of the Invention [Problem to be solved by the invention]

[0009] The problem to be solved by the present invention is to provide a maraging steel that is substantially free of cobalt and has high thermal conductivity. Another object of the present invention is to provide a powder made of such a maraging steel. [Means for solving the problem]

[0010] In order to solve the above problems, the maraging steel according to the present invention comprises: 9.0≦Ni≦12.0mass%, 4.0≦Mo≦7.0mass%, and 0.3≦Al+Ti≦1.4mass% and the balance is Fe and unavoidable impurities.

[0011] The powder according to the present invention consists of the maraging steel according to the present invention. [Effects of the Invention]

[0012] In cobalt-free maraging steel, if the Ni content is relatively reduced, (a) The hardness before aging treatment (hardness after solution treatment, hardness as welded) is equivalent to that of conventional maraging steel. (b) The hardness after aging treatment is equivalent to that of conventional maraging steel, and (c) The thermal conductivity is equivalent to that of conventional hot work die steel. Therefore, when such maraging steel is used in die-casting molds or plastic injection molding molds in which cooling circuits are formed, or for repairing such molds, the cycle time can be shortened compared to molds made with conventional maraging steel. [Brief explanation of the drawings]

[0013] [Figure 1] The temperature dependence of the thermal conductivity of various steel materials. [Figure 2] 1 shows the hardness of the steel materials obtained in Example 1, Comparative Example 1, and Comparative Example 3 as they are after solution treatment (ST) and after aging treatment (AG). DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described in detail below. [1. Maraging steel] [1.1. Main constituent elements] The maraging steel according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0015] (1) 9.0≦Ni≦12.0mass%: Ni forms the intermetallic compound Ni3(Al,Ti,Mo) with Ti, Al, Mo, etc. during aging treatment, and has the effect of increasing aging hardness. To obtain this effect, the Ni content must be 9.0 mass% or more. The Ni content is preferably 9.5 mass% or more. On the other hand, if the Ni content is excessive, the thermal conductivity may decrease. Therefore, the Ni content must be 12.0 mass% or less. The Ni content is preferably 11.0 mass% or less.

[0016] (2) 4.0≦Mo≦7.0mass% Mo forms an intermetallic compound with Ni and has the effect of increasing aging hardness. To obtain this effect, the Mo content must be 4.0 mass% or more. The Mo content is preferably 4.5 mass% or more, or 5.5 mass% or more. On the other hand, if the Mo content is excessive, the amount of intermetallic compounds will be excessive, which may result in a decrease in toughness. Therefore, the Mo content must be 7.0 mass% or less. The Mo content is preferably 6.5 mass% or less.

[0017] (3) 0.3≦Al+Ti≦1.4mass%: The maraging steel according to the present invention may contain either Al or Ti, or may contain both. Both Al and Ti form intermetallic compounds with Ni, which increase the aging hardness. To achieve this effect, the total content of Al and Ti must be 0.3 mass% or more. The total content is preferably 0.4 mass% or more, or 0.45 mass% or more. On the other hand, if the total amount of Al and Ti is excessive, the amount of intermetallic compounds will be excessive, which may reduce toughness. Therefore, the total amount must be 1.4 mass% or less. The total amount is preferably 1.3 mass% or less, or 1.2 mass% or less.

[0018] (4) 0.05≦Al≦0.5mass% As mentioned above, Al forms an intermetallic compound with Ni and has the effect of increasing aging hardness. When the total amount of Al and Ti is within the above range, further optimizing the Al amount may further increase the aging hardness. To obtain this effect, the Al amount is preferably 0.05 mass% or more. On the other hand, if the Al content is excessive, the amount of intermetallic compounds will be excessive, which may result in a decrease in toughness. Therefore, the Al content is preferably 0.5 mass% or less.

[0019] (5) 0.1≦Ti≦1.0mass%: As mentioned above, Ti forms an intermetallic compound with Ni and has the effect of increasing aging hardness. When the total amount of Al and Ti is within the above range, further optimizing the Ti amount may further increase the aging hardness. To achieve this effect, the Ti amount is preferably 0.1 mass% or more. The Ti amount is more preferably 0.3 mass% or more. On the other hand, if the Ti content is excessive, the amount of intermetallic compounds will be excessive, which may result in a decrease in toughness. Therefore, the Ti content is preferably 1.0 mass% or less. The Ti content is more preferably 0.6 mass% or less.

[0020] [1.2. Sub-constituent elements (1)] In addition to the above-mentioned main constituent elements, the maraging steel according to the present invention may further contain the following elements of group A and / or group B. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0021] [1.2.1. Group A] (1) 0.05≦Nb≦0.3mass% Nb also forms an intermetallic compound with Ni and has the effect of increasing aging hardness. To obtain this effect, the Nb content is preferably 0.05 mass% or more. The Nb content is more preferably 0.1 mass% or more. On the other hand, if the Nb content is excessive, the amount of intermetallic compounds becomes excessive, which may result in a decrease in toughness. Therefore, the Nb content is preferably 0.3 mass% or less.

[0022] (2) 0.1≦V≦1.0 mass%: V also forms an intermetallic compound with Ni and has the effect of increasing aging hardness. To obtain this effect, the V content is preferably 0.1 mass% or more. The V content is more preferably 0.3 mass% or more. On the other hand, if the V content is excessive, the amount of intermetallic compounds becomes excessive, which may result in a decrease in toughness. Therefore, the V content is preferably 1.0 mass% or less. The V content is more preferably 0.8 mass% or less.

[0023] (3) 0.5≦W≦5.0mass%: W also forms an intermetallic compound with Ni and has the effect of increasing aging hardness. To obtain this effect, the W content is preferably 0.5 mass% or more. On the other hand, if the W content is excessive, the amount of intermetallic compounds becomes excessive, which may result in a decrease in toughness. Therefore, the W content is preferably 5.0 mass% or less. The W content is more preferably 4.0 mass% or less. The maraging steel according to the present invention may contain one or more of Nb, V, and W as the A group elements.

[0024] [1.2.2. Group B] (4) 0.001≦B≦0.010mass%: When intermetallic compounds precipitate at grain boundaries, intergranular fracture occurs, which can reduce toughness. B has the effect of suppressing the precipitation of intermetallic compounds at grain boundaries and improving the toughness of steel. To achieve this effect, the B content is preferably 0.001 mass% or more. On the other hand, even if more B is added than necessary, the effect saturates and there is no practical benefit, so the B content is preferably 0.010 mass% or less. The maraging steel according to the present invention may contain a B group element instead of an A group element, or may further contain a B group element in addition to an A group element.

[0025] [1.3. Sub-constituent elements (2)] The maraging steel according to the present invention may further contain one or more of the following elements in addition to the above-mentioned main constituent elements and sub-constituent elements (1). The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0026] (1) C≦0.03 mass%: If maraging steel contains a relatively large amount of C, a large amount of carbides may be formed, which may reduce the toughness of the steel after aging treatment. In order to obtain a highly tough martensite structure, it is preferable to limit the C content to 0.03 mass% or less.

[0027] (2) Si≦0.3 mass%: If maraging steel contains a relatively large amount of Si, the toughness and thermal conductivity of the steel may decrease after aging treatment. In order to obtain a steel with high toughness and high thermal conductivity, it is preferable to limit the Si content to 0.3 mass% or less.

[0028] (3) Other impurity elements: The maraging steel according to the present invention may contain the following elements as other impurity elements. Mn<0.2mass%, Cr<0.5mass%, Co<0.1mass%, Nb<0.05mass%, W<0.5mass%, V<0.1mass%, B<0.001mass%

[0029] [1.3. Usage] The maraging steel according to the present invention comprises: (a) mold material; (b) Welding rods for various types of welding, such as build-up welding, (c) powders used in additive manufacturing; It can be used for the following purposes:

[0030] [2. Powder] The powder according to the present invention consists of the maraging steel according to the present invention.

[0031] 2.1. Maraging steel The details of the maraging steel, which is the raw material of the powder, are as described above, and therefore will not be described here.

[0032] 2.2. Number frequency D 50 ] "Number Frequency D 50 "(μm)" refers to the cumulative 50% particle diameter (median diameter) of the powder. 50 As a measurement method, for example, (a) A method of measuring using a particle distribution measuring device based on the laser diffraction / scattering method, (b) A method of measuring using a particle image analyzer; (c) Measurement using a Coulter counter; etc. In the present invention, "D 50 " refers to the median diameter measured by a particle image analyzer.

[0033] In the present invention, the powder D 50 is not particularly limited, and an optimum value can be selected depending on the purpose. In general, D 50The smaller the particle size, the higher the content of fine powder (powder with a particle size of 10 μm or less). The smaller the particle size, the stronger the adhesive forces that occur between particles, such as van der Waals forces and electrostatic forces. Therefore, D 50 If D is too small, the powder tends to aggregate and the flowability decreases. 50 is preferably 10 μm or more. 50 is preferably 20 μm or more, and more preferably 30 μm or more. On the other hand, D 50 If D becomes too large, the frictional force on the powder surface becomes more dominant than the adhesive force between particles. As a result, the shear resistance during powder flow increases, hindering fluidity. 50 is preferably 50 μm or less.

[0034] [2.3. Particle shape] The powder particles may be spherical or irregular in shape. Generally, metal powders consisting of an aggregate of spherical particles exhibit higher fluidity than metal powders consisting of an aggregate of irregularly shaped particles.

[0035] [2.4. Usage] The metal powder according to the present invention can be used for various purposes, but is particularly suitable as a powder for additive manufacturing.

[0036] [3. Manufacturing method of maraging steel] The maraging steel according to the present invention comprises: (a) Melt and cast raw materials that are mixed to form the desired composition, (b) subjecting the steel ingot to a homogenizing heat treatment; (c) subjecting the homogenized heat-treated body to hot working; (d) If necessary, low-temperature annealing is performed on the hot-worked body; (e) If necessary, cold working is performed on the hot-worked body or low-temperature annealed body, (f) subjecting the hot-worked body, low-temperature annealed body, or cold-worked body to a solution heat treatment; (g) Aging the solution-treated body It can be produced by

[0037] [3.1. Melting and Casting Process] First, raw materials mixed to form the desired composition are melted and cast. There are no particular restrictions on the melting and casting conditions, and optimal conditions can be selected depending on the purpose.

[0038] [3.2. Homogenization heat treatment process] Next, the steel ingot is subjected to homogenization heat treatment. Homogenization heat treatment is performed to homogenize the components of the steel ingot. The conditions for the homogenization heat treatment are not particularly limited, and optimal conditions can be selected depending on the purpose.

[0039] 3.3. Hot working process Next, the homogenized heat-treated body is subjected to hot working. The hot working is performed to process the homogenized heat-treated body into a final product shape or to process the homogenized heat-treated body into a raw material suitable for cold working. The conditions for the hot working are not particularly limited, and optimal conditions can be selected depending on the purpose.

[0040] [3.4. Low-temperature annealing process] Next, if necessary, the hot-worked body is subjected to low-temperature annealing. Low-temperature annealing is performed when it is necessary to impart cold workability to the hot-worked body. Therefore, if cold working is not performed or if the hot-worked body has sufficient cold workability, the low-temperature annealing step can be omitted. The conditions for low-temperature annealing are not particularly limited, and optimal conditions can be selected depending on the purpose.

[0041] 3.5. Cold working process Next, if necessary, the hot-worked body or low-temperature annealed body is subjected to cold working. Cold working is performed particularly when high dimensional accuracy is required for the final product. Therefore, when high dimensional accuracy is not required for the final product, cold working can be omitted. The conditions for cold working are not particularly limited, and optimal conditions can be selected depending on the purpose.

[0042] [3.6. Solution Heat Treatment Process] Next, the hot-worked body, the low-temperature annealed body, or the cold-worked body is subjected to a solution heat treatment. The conditions for the solution heat treatment are preferably selected optimally depending on the composition of the steel. Generally, if the solution heat treatment temperature is too low, the precipitates may not be sufficiently dissolved, so the solution heat treatment temperature is preferably 1000°C or higher. On the other hand, if the solution heat treatment temperature is too high, the crystal grains may become coarse during the solution heat treatment, so the solution heat treatment temperature is preferably 1100°C or less.

[0043] The solution heat treatment time can be optimally selected depending on the solution heat treatment temperature. Generally, the higher the solution heat treatment temperature, the shorter the time required to dissolve precipitates. The solution heat treatment time is usually 0.5 to 5 hours.

[0044] 3.7. Aging Treatment Process Next, the solution-heat-treated body is subjected to aging treatment. This causes intermetallic compounds to precipitate in the matrix, increasing hardness. It is preferable to select optimal conditions for the aging treatment depending on the composition of the steel. Generally, if the aging temperature is too low, precipitation of intermetallic compounds may be insufficient, so the aging temperature is preferably 500°C or higher. On the other hand, if the aging temperature is too high, the intermetallic compounds may become coarse. Therefore, the aging temperature is preferably 600°C or less.

[0045] The optimum aging treatment time can be selected depending on the aging treatment temperature. Generally, the higher the aging treatment temperature, the more precipitates can be precipitated in a short time. The aging treatment time is usually 1 hour to 10 hours.

[0046] 4. Powder manufacturing method In the present invention, the method for producing the powder made of maraging steel is not particularly limited, and examples of the method for producing the powder include gas atomization, water atomization, plasma atomization, plasma rotating electrode atomization, and centrifugal atomization. For example, when producing powder using the gas atomization method, high-pressure gas is sprayed onto the molten metal as it falls from the bottom of a tundish, pulverizing and solidifying it. In this case, the high-pressure gas used is an inert gas such as nitrogen, argon, or helium. When producing powder using the gas atomization method, impurities such as P, S, O, N, Cu, Co, Ti, Zr, and Nb can inevitably be mixed in. Alternatively, two or more types of metal powders may be mixed and subjected to mechanical alloying or other methods to produce powder made of maraging steel.

[0047] After producing the powder using either method, the powder may be further subjected to a spheroidizing treatment using reducing thermal plasma. Alternatively, to improve the flowability of the powder, the surface of the powder may be coated with an appropriate amount of nanoparticles after production. Furthermore, the particle size distribution of the powder can be controlled by the production conditions, but it can also be controlled using classification methods such as wet cyclones, dry cyclones, dry sieves, and ultrasonic sieves.

[0048] [5. Effect] In cobalt-free maraging steel, if the Ni content is relatively reduced, (a) The hardness before aging treatment (hardness after solution heat treatment, hardness as welded) is equivalent to that of conventional maraging steel. (b) The hardness after aging treatment is equivalent to that of conventional maraging steel, and (c) The thermal conductivity is equivalent to that of conventional hot work die steel. Therefore, when such maraging steel is used in die-casting molds or plastic injection molding molds in which cooling circuits are formed, or for repairing such molds, the cycle time can be shortened compared to molds made with conventional maraging steel. [Example]

[0049] (Examples 1 to 8, Comparative Examples 1 to 5) 1. Sample Preparation The raw materials mixed to obtain the composition shown in Table 1 were melted in a vacuum induction furnace to obtain a 50 kg ingot. The obtained ingot was subjected to homogenization heat treatment by heating at 1200°C for 10 hours. The ingot was then hot forged into a square bar with a cross section of 60 mm x 60 mm, and then slowly cooled. Next, the cooled forged products were subjected to low-temperature annealing at 700°C for 8 hours.Furthermore, the low-temperature annealed materials were machined to prepare test specimens.

[0050] 2. Test Method 2.1. Evaluation of aging hardness After annealing, square bars measuring 15 mm x 15 mm x 10 mm were cut from the material. The square bars were then soaked in a vacuum furnace at 1,050°C for 2 hours, and then gas-cooled at a cooling rate equivalent to oil quenching (solution heat treatment). They were then aged at 550°C for 5 hours in the same vacuum furnace. The measurement surface and the contact surface of the test piece after solution heat treatment (ST) and the test piece after aging treatment (AG) were each mechanically polished to #220. Using the obtained test pieces, the Rockwell C scale hardness after solution heat treatment (as-ST hardness) and the Rockwell C scale hardness after aging treatment (hardness after AG) were measured at room temperature.

[0051] 2.2. Evaluation of thermal conductivity After annealing, round bars of approximately φ12 mm x 30 mm were machined from the material. Then, solution treatment and aging treatment were carried out under the same conditions as for the evaluation of aging hardness. After aging, the round bars were machined to prepare test specimens measuring φ10 mm x 2 mm. The thermal conductivity of the test specimens was measured using the xenon flash method. The thermal conductivity was measured at room temperature, 100°C, 200°C, 300°C, 400°C, and 500°C, and the average value was calculated.

[0052] [3. Results] The results are shown in Table 1. Table 1 also shows the composition of each sample. Figure 1 shows the temperature dependence of thermal conductivity of various steel materials. Figure 2 shows the hardness as solution treated (ST) and hardness after aging treatment (AG) of the steel materials obtained in Example 1, Comparative Example 1, and Comparative Example 3. The following can be seen from Table 1 and Figures 1 and 2.

[0053] (1) Comparative Example 1 is a maraging steel containing a large amount of Co. Because Comparative Example 1 contains Co, it is difficult to handle and has poor production efficiency. Comparative Example 1 also has a lower thermal conductivity than SKD61 (see Figure 1). This is thought to be due to the excessive amount of Ni. (2) Comparative Example 2 is a maraging steel that does not contain Co. Comparative Example 2 had a reduced thermal conductivity. This is thought to be due to the absence of Co and the excessive amount of Ni.

[0054] (3) Comparative Example 3 is a maraging steel in which the Co content and Ni content are reduced compared to Comparative Example 1. Comparative Example 3 exhibited thermal conductivity equivalent to that of SKD61. However, since Comparative Example 3 contains Co, it is difficult to handle and has poor production efficiency. (4) In Comparative Example 4, the hardness after AG decreased. This is thought to be due to the small total amount of Al and Ti.

[0055] (5) In Comparative Example 5, the as-ST hardness was 38HRC. This as-ST hardness was close to the hardness of the as-solidified portion of the weld measured separately. When the as-solidified portion has high hardness, it is highly susceptible to cracking, and there is a possibility that cracking will occur during welding. (6) In Examples 1 to 8, the hardness after ST was 32 HRC or less and the hardness after AG was 42 HRC or more. Furthermore, in Examples 1 to 8, the average thermal conductivity was 27 W / (m·K) or more.

[0056] (7) The average thermal conductivity of Example 1 was slightly lower than that of Comparative Example 3, but was higher than that of Comparative Example 1 and equivalent to that of SKD61. See FIG. The hardness of Example 1 as-received during ST was almost the same as that of Comparative Example 3. Furthermore, the hardness of Example 1 after AG was slightly lower than that of Comparative Examples 1 and 3, but still had sufficient hardness for use as a material for a mold. See Figure 2.

[0057] [Table 1]

[0058] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0059] The maraging steel according to the present invention comprises: (a) Materials for molds that require cooling (e.g., die casting molds, hot stamping molds, tailored die quench molds), (b) Welding rods used in repairing such molds; (b) Powder for manufacturing such molds using additive manufacturing methods It can be used for the following purposes:

Claims

1. 9.0≦Ni≦12.0mass%, 4.0≦Mo≦7.0 mass%, and 0.3≦Al+Ti≦1.4mass% and the balance being Fe and unavoidable impurities.

2. 2. The maraging steel according to claim 1, further comprising the following elements of group A and / or group B: Group A: 0.05≦Nb≦0.3mass%, 0.1≦V≦1.0 mass%, and 0.5≦W≦5.0mass% At least one element selected from the group consisting of: Group B: 0.001≦B≦0.010mass%

3. 3. The maraging steel according to claim 1, which is used as a welding rod.

4. 3. Powder for use in additive manufacturing, comprising the maraging steel according to claim 1 or 2.

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