JP2022072078A - Metal powder - Google Patents

Abstract

To provide a metal powder that has low thermal conductivity and also has the amount of addition of Co reduced to the level of impurities, offering good handleability.SOLUTION: A metal powder contains 0.1≤C≤0.4 mass%, 1.8≤Si≤2.5 mass%, 0.3≤Mn≤1.2 mass%, 2.0≤Cr≤6.0 mass%, 0.5≤Mo≤3.5 mass%, and 0.1≤V≤1.0 mass% with the balance being Fe and inevitable impurities. Preferably, the metal powder further contains 0.01≤Ni≤4.0 mass% and/or 0.01≤Cu≤4.0 mass%.SELECTED DRAWING: Figure 1

Description

The present invention relates to a metal powder, and more specifically, when applied to a laminated molding, a metal capable of obtaining a laminated molded product having less cracking and warpage, and having an appropriate hardness and an appropriate thermal conductivity. Regarding powder.

In recent years, metal laminated molding technology has been attracting attention. this is,
(A) A metal part with a complicated shape can be molded into a shape close to the final shape.
(B) The degree of freedom in design is improved.
(C) The shaving margin is smaller than that of the conventional shaving process.
This is because there are advantages such as.

Here, the "laminated modeling method" is to produce a three-dimensional structure by laminating flaky layers corresponding to a structure in which a three-dimensional structure is sliced in the horizontal direction by various methods. The way to do it. As a method of laminating the flaky layer, for example,
(A) A method of repeating a step of forming a thin layer made of metal powder and a step of locally melting and solidifying the powder layer by irradiating an energy beam such as a laser beam or an electron beam.
(B) There is a method of superimposing thin plates having a predetermined shape and performing diffusion joining.

Among these, the layered manufacturing method of irradiating the spread metal powder with laser light to locally melt and solidify the powder layer is also called "SLM (Selective Laser Melting) method". The additive manufacturing method of the SLM method has an advantage that a complicated three-dimensional shape can be easily formed only by changing the irradiation position of the laser beam. Therefore, when this is applied to, for example, the production of a die casting mold, a non-linear or three-dimensional water cooling circuit can be freely arranged inside the mold.

When laminating modeling is performed using an SLM type 3D printer, since only the upper surface of the modeled object is rapidly heated, residual tensile stress is generated on the upper surface of the modeled object after cooling. As a result, the modeled object is easily deformed so as to be convex downward. When the deformation of the modeled object becomes large, not only the dimensional accuracy of the modeled object decreases, but also it becomes difficult to take out the modeled object from the 3D printer after the modeling. Therefore, conventionally, it has been common to use maraging steel powder as the powder for laminated molding (for example, Patent Document 1).

When maraging steel is rapidly cooled, it undergoes martensitic transformation and expands in volume. Further, the maraging steel has a low hardness at the time when martensitic transformation occurs and can be hardened by aging. Therefore, when this is used for laminated modeling, there is an advantage that cracks and distortions during modeling are reduced and it is easy to perform laminated modeling. In addition, the required hardness can be obtained by aging after the laminated molding.

Further, Patent Document 2 discloses a steel powder containing a predetermined amount of C, Si, Cr, Mn, Mo, V, and N, and the balance is Fe and unavoidable impurities.
In the same document,
(A) Conventional mold steels such as SKD61, SUS420J2, and maraging steel have high temperature strength, but contain a large amount of elements such as Si, Cr, Ni, and Co that are easily dissolved in the matrix phase. Because of its low thermal conductivity,
(B) In this type of high alloy steel, if the content of the alloy component that lowers the thermal conductivity is reduced and the amount of Cr is optimized, high thermal conductivity can be achieved while maintaining high corrosion resistance. (C) It is described that such a steel powder is suitable as a powder for laminated molding.

Here, in the mold, not only the part where the cooling efficiency is desired to be high (hereinafter, this is also referred to as "high cooling efficiency part"), but rather the part where the cooling efficiency is desired to be low (hereinafter, this is referred to as "low cooling efficiency part"). Also called) may be included. Examples of the low cooling efficiency portion include a portion for forming a thin-walled portion of a die-cast product. When the steel powder described in Patent Document 2 is used as the powder for laminated modeling in the case of modeling such a thin-walled portion, it is difficult to fabricate the low cooling efficiency portion because the thermal conductivity of the steel powder is too high. Is. Therefore, if maraging steel powder is used as the powder for laminated molding, a low cooling efficiency portion can be produced. However, maraging steel contains Co, which is regulated by the Specified Chemical Substance Prevention Regulations, and when handling materials containing Co, it is obligatory to take environmental measures and carry out regular environmental measurements. Therefore, maraging steel is complicated in use.

International Publication No. 2011/149101 Japanese Patent No. 6601051

An object to be solved by the present invention is to provide a metal powder having a low thermal conductivity and excellent handleability by suppressing the addition amount of Co to an impurity level.

In order to solve the above problems, the metal powder according to the present invention is
0.1 ≤ C ≤ 0.4 mass%,
1.8 ≤ Si ≤ 2.5 mass%,
0.3 ≤ Mn ≤ 1.2 mass%,
2.0 ≤ Cr ≤ 6.0 mass%,
0.5 ≤ Mo ≤ 3.5 mass%, and
0.1 ≤ V ≤ 1.0 mass%
The gist is that the balance consists of Fe and unavoidable impurities.

The metal powder according to the present invention has a low thermal conductivity and is excellent in handleability because Co is suppressed to an impurity level.

It is a schematic diagram of the manufacturing method of the material for evaluation. FIG. 2A is a schematic diagram of a method for producing a thermal fatigue test piece. FIG. 2B is a schematic diagram of the thermal fatigue test.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Metal powder]
[1.1. Main constituent elements]
The metal powder according to the present invention contains the following elements, and the balance consists of Fe and unavoidable impurities. The types of additive elements, their component ranges, and the reasons for their limitation are as follows.

(1) 0.1 ≤ C ≤ 0.4 mass%:
C is an element necessary for increasing the hardness at the time of quenching and obtaining the strength of the modeled product. If the C content is too low, the strength will be insufficient when applied to molds and machine parts. Therefore, the C content needs to be 0.1 mass% or more. The C content is preferably 0.15 mass% or more, more preferably 0.2 mass% or more.
On the other hand, if the C content is excessive, the hardness becomes too high during modeling with a 3D printer, and cracks are likely to occur. Therefore, the C content needs to be 0.4 mass% or less. The C content is preferably 0.35 mass% or less, more preferably 0.3 mass% or less.

(2) 1.8 ≤ Si ≤ 2.5 mass%:
In the present invention, Si has a very important role, and the thermal conductivity can be effectively lowered by increasing the Si content. Even in ordinary hot die steel, about 1 mass% of Si is added. However, in order to reduce the thermal conductivity to the same level as maraging steel, the Si content needs to be 1.8 mass% or more. The Si content is preferably 1.9 mass% or more, more preferably 2.0 mass% or more.
On the other hand, if the Si content is excessive, austenite single phase cannot be obtained during quenching, which may cause poor quenching structure. Therefore, the Si content needs to be 2.5 mass% or less. The Si content is preferably 2.3 mass% or less, more preferably 2.2 mass% or less.

(3) 0.3 ≤ Mn ≤ 1.2 mass%:
Mn is necessary to ensure hardenability. It is preferable that the metal powder has a hardenability to the extent that the powder melted by a heat source of a 3D printer such as a laser at the time of modeling is solidified and cooled to be hardened. For that purpose, the Mn content needs to be 0.3 mass% or more. The Mn content is preferably 0.4 mass% or more, more preferably 0.5 mass% or more.
On the other hand, when the Mn content becomes excessive, not only the effect is saturated, but also a large amount of residual γ is generated after quenching, and the hardness may be rather lowered. Therefore, the Mn content needs to be 1.2 mass% or less. The Mn content is preferably 1.0 mass% or less, more preferably 0.8 mass% or less.

(4) 2.0 ≤ Cr ≤ 6.0 mass%:
Cr is an element necessary for ensuring hardenability like Mn. Further, Cr combines with carbon and precipitates as a carbide, causing secondary curing at 500 to 600 ° C. Due to this secondary hardening, the hot die steel has a characteristic that it is difficult to soften at a high temperature. In order to obtain such an effect, the Cr content needs to be 2.0 mass% or more. The Cr content is preferably 2.5 mass% or more, and more preferably 3.0 mass% or more.
On the other hand, if the Cr content becomes excessive, the precipitation of carbides may be accelerated, and the softening resistance at high temperatures may decrease. Therefore, the Cr content needs to be 6.0 mass% or less. The Cr content is preferably 5.5 mass% or less, more preferably 5.0 mass% or less.

(5) 0.5 ≤ Mo ≤ 3.5 mass%:
Mo is an element that forms carbides like Cr and exhibits secondary curing when heated at 500 to 600 ° C. In order to obtain such an effect, the Mo content needs to be 0.5 mass% or more. The Mo content is preferably 0.8 mass% or more, more preferably 1.0 mass% or more.
On the other hand, Mo is more expensive than other elements. Therefore, adding more Mo than necessary is economically disadvantageous. Therefore, the Mo content needs to be 3.5 mass% or less. The Mo content is preferably 3.2 mass% or less, more preferably 3.0 mass% or less.

(6) 0.1 ≤ V ≤ 1.0 mass%:
V is also an element that forms carbides like Mo and exhibits secondary curing when heated at 500 to 600 ° C. Further, it is necessary to carry out re-quenching when the hardness adjustment fails in tempering after molding, but it is also an element necessary for suppressing the coarsening of crystal grains at the time of re-quenching. In order to obtain such an effect, the V content needs to be 0.1 mass% or more. The V content is preferably 0.2 mass% or more, more preferably 0.3 mass% or more.
On the other hand, if V is added more than necessary, it is economically disadvantageous. Therefore, the V content needs to be 1.0 mass% or less. The V content is preferably 0.9 mass% or less, more preferably 0.8 mass% or less.

[1.2. Sub-constituents]
The metal powder according to the present invention may further contain one or more of the following elements in addition to the above-mentioned elements. The types of additive elements, their component ranges, and the reasons for their limitation are as follows.

(7) 0.01 ≤ Ni ≤ 4.0 mass%:
Ni is an element that improves hardenability and toughness of steel. In order to obtain such an effect, the Ni content is preferably 0.01 mass% or more. The Ni content is preferably 1.0 mass% or more, and more preferably 2.0 mass% or more.
On the other hand, if the Ni content is excessive, a large amount of residual γ may be generated during quenching after molding, and the hardness may decrease. Therefore, the Ni content is preferably 4.0 mass% or less. The Ni content is preferably 3.5 mass% or less, more preferably 3.0 mass% or less.

(8) 0.01 ≤ Cu ≤ 4.0 mass%:
Cu is an element that improves hardenability and toughness of steel. In order to obtain such an effect, the Cu content is preferably 0.01 mass% or more. The Cu content is preferably 1.0 mass% or more, more preferably 2.0 mass% or more.
On the other hand, if the Cu content is excessive, a large amount of residual γ may be generated during quenching after molding, and the hardness may decrease. Further, if the Cu content is excessive, the hot workability may be deteriorated. Therefore, the Cu content is preferably 4.0 mass% or less. The Cu content is preferably 3.5 mass% or less, more preferably 3.0 mass% or less.
The metal powder according to the present invention may contain either Ni or Cu, or may contain both.

(9) 0.01 ≤ Al ≤ 2.0 mass%:
Al is an element required as a dispersant. Al is also an element having an effect of refining crystal grains. In order to obtain such an effect, the Al content is preferably 0.01 mass% or more. The Al content is preferably 0.015 mass% or more, more preferably 0.02 mass% or more.
On the other hand, even if Al is added more than necessary, the effect is saturated. Therefore, the Al content is preferably 2.0 mass% or less. The Al content is preferably 0.8 mass% or less, more preferably 0.6 mass% or less.

(10) 0.01 ≤ Nb ≤ 1.0 mass%:
Nb has the effect of forming fine carbides and making crystal grains finer. In order to obtain such an effect, the Nb content is preferably 0.01 mass% or more. The Nb content is preferably 0.015 mass% or more, more preferably 0.02 mass% or more.
On the other hand, when the Nb content becomes excessive, the carbide becomes coarse, which acts as a starting point of fracture and may deteriorate the characteristics of the product. Therefore, the Nb content is preferably 1.0 mass% or less. The Nb content is preferably 0.8 mass% or less, more preferably 0.6 mass% or less.

(11) 0.01 ≤ Ti ≤ 1.0 mass%:
Like Nb, Ti has the effect of forming fine carbides and making crystal grains finer. In order to obtain such an effect, the Ti content is preferably 0.01 mass% or more. The Ti content is preferably 0.015 mass% or more, more preferably 0.02 mass% or more.
On the other hand, when the Ti content becomes excessive, the carbide becomes coarse, which acts as a starting point of fracture and may deteriorate the characteristics of the product. Therefore, the Ti content is preferably 1.0 mass% or less. The Ti content is preferably 0.8 mass% or less, more preferably 0.6 mass% or less.
The metal powder according to the present invention may contain any one of Al, Nb or Ti, or may contain two or more of them.

[1.3. Use]
The metal powder according to the present invention can be used for various purposes, but is particularly suitable as a metal powder for laminated modeling.

[2. Metal powder manufacturing method]
In the present invention, the method for producing a metal powder is not particularly limited. Examples of the method for producing the metal powder include a gas atomizing method, a water atomizing method, a plasma atomizing method, a plasma rotating electrode method, and a centrifugal force atomizing method.
For example, when a metal powder is produced by using the gas atomizing method, high-pressure gas is blown onto the molten metal while dropping the molten metal from the bottom of the tundish to crush and solidify the molten metal. In this case, an inert gas such as nitrogen, argon or helium is used as the high-pressure gas. When powder is produced by the gas atomizing method, impurities such as P, S, Cu, Co, Ti and Nb may be inevitably mixed.
Further, two or more kinds of metal powders may be mixed to produce the metal powder by a method such as mechanical alloying.

Further, after producing the metal powder by any method, the metal powder may be further subjected to a spheroidizing treatment by reducing thermal plasma. Alternatively, in order to improve the fluidity of the metal powder, an appropriate amount of nanoparticles may be coated on the surface after the powder is produced. Further, the particle size distribution of the metal powder can be controlled by the production conditions, but can also be controlled by using a classification method such as a wet cyclone, a dry cyclone, a dry sieve, and an ultrasonic sieve.

[3. Action]
Laminated modeling with a metal 3D printer has a high degree of freedom in modeling, and is characterized by being able to model complex shapes that could not be modeled by conventional cutting. Taking advantage of this feature, application of metal 3D printers to molds is being actively studied. Currently, the main application of metal 3D printers is the manufacture of die casting dies with water cooling holes. Many of the conventional application examples aim at the effect of reducing the cooling unevenness of the mold by freely arranging the water cooling holes in the die casting mold, preventing seizure on the die casting product, and improving the life of the mold. ..

On the other hand, as the metal powder used in the metal 3D printer, maraging steel has been mainly used in the past. However, since maraging steel has a low thermal conductivity, it is not suitable as a material for a mold that wants to form water cooling holes and cool it effectively. Further, when a modeling portion is formed on the surface of a base portion made of die steel, if maraging steel powder is used as the powder for laminated molding, peeling may occur at the boundary portion between the base portion and the molding portion, or the base portion may be separated. There was a problem that it was difficult to match the hardness of the shaped part. In order to solve these problems, a powder for a metal 3D printer has been developed by improving SKD61, which is widely used in die casting dies, and improving thermal conductivity (for example, Patent Document 1 or 2).

Here, the mold may include not only a portion where the cooling efficiency is desired to be high (high cooling efficiency portion) such as around a water cooling hole, but rather a portion where the cooling efficiency is desired to be lowered (low cooling efficiency portion). Although it has not been achieved with the current metal 3D printer, when a 3D printer that can form a mold with water cooling holes using multiple types of powder is put into practical use, high thermal conductivity powder and low thermal conductivity powder are suitable materials. It will be possible to manufacture high-performance molds by using them in the right places. However, the conventional powder for laminated molding has been material-designed solely for the purpose of increasing the thermal conductivity, and there has been no conventional example in which a metal powder suitable for laminated molding of a low cooling efficiency portion has been proposed.

On the other hand, the metal powder according to the present invention contains elements necessary for ensuring hardenability and has a higher Si content than general-purpose die steel. Therefore, the metal powder according to the present invention has a coefficient of linear expansion and a heat treatment temperature required to obtain a predetermined hardness, which is equivalent to that of general-purpose die steel, and has a lower thermal conductivity than general-purpose die steel. Equivalent to aging steel.
Further, the metal powder according to the present invention is superior in handleability to the maraging steel containing Co because the amount of Co added is suppressed to the impurity level.
Further, in the case of forming a modeling portion on a base portion made of general-purpose die steel, if the metal powder according to the present invention is used as the powder for laminated molding, a low cooling efficiency portion can be formed on the surface of the base portion. can. Moreover, since the metal powder according to the present invention has the same linear expansion coefficient and heat treatment temperature as the base portion, peeling at the boundary portion is suppressed, and the hardness of the base portion and the shaped portion can be easily matched.

(Examples 1 to 9, comparative examples 1 to 3)
[1. Preparation of sample]
[1.1. Preparation of metal powder]
Using the gas atomizing method, 12 kinds of metal powders shown in Table 1 were prepared. Comparative Example 1 corresponds to hot tool steel (JIS SKD61), Comparative Example 2 corresponds to hot tool steel (JIS SKD7), and Comparative Example 3 corresponds to maraging steel.

[1.2. Fabrication of laminated model]
FIG. 1 shows a schematic diagram of a method for producing an evaluation material. First, a base portion 12 made of SKD61 tempered to 48HRC was prepared. Next, using various metal powders shown in Table 1 and an SLM-type 3D printer, the modeling portion 14 was laminated on the base portion 12 to obtain an evaluation material 10. As the powder for laminated molding, a powder having an average particle size of 25 to 53 μm and an avalanche angle of 45 ° or less was used. After the laminated molding, the evaluation material 10 was tempered so that the hardness of the molding portion 14 was 48HRC.

[2. Test method]
[2.1. Hardness]
After tempering the modeling portion 14, test pieces for hardness measurement were cut out from the base portion 12 and the modeling portion 14, respectively (see FIG. 1). Rockwell hardness (JIS Z2245) was measured using the obtained test piece.

[2.2. Thermal conductivity]
After tempering the modeling unit 14, a test piece (φ10 mm × 2 mm) for measuring thermal conductivity was cut out from the modeling unit 14 (see FIG. 1). The specific heat and thermal diffusivity of the test piece were measured at 25 ° C. using a laser flash method. At the time of measurement, carbon spray was applied to both sides of the test piece. In addition, the density of the test piece was measured using the Archimedes method (JIS Z8807). Furthermore, the thermal conductivity was calculated using the following formula (5).
Thermal conductivity = thermal diffusivity x specific heat x density ... (5)

[2.3. Thermal fatigue test]
FIG. 2A shows a schematic diagram of a method for producing a thermal fatigue test piece. FIG. 2B shows a schematic diagram of the thermal fatigue test. First, from the evaluation material 10, a test piece 20 having a diameter of 70 mm × 20 mm including the boundary portion 16 between the base portion 12 and the modeling portion 14 was produced.
Next, a thermal fatigue test was carried out in which the upper end surface of the test piece 20 was heated to 580 ° C. by high frequency heating, and then the operation of spraying cold shower water on the upper end surface for 3 seconds was repeated 10,000 times in total. After the thermal fatigue test, the presence or absence of cracks at the boundary portion 16 was evaluated.

[3. result]
Table 1 shows the results. Table 1 also shows the composition of the metal powder.
Regarding the results of the thermal fatigue test, "○" indicates that no cracks were found at the boundary even after the test, and "x" indicates that cracks occurred at the boundary after the test. From Table 1, the following can be seen.

(1) In Examples 1 to 9, the thermal conductivity was lower than that of Comparative Example 1 (SKD61), and the thermal conductivity was as low as that of Comparative Example 3 (maraging steel). Further, even when the hardness of the modeling portion 14 was tempered with the aim of 48HRC, the hardness of the base portion 12 was 47 to 48HRC, and no significant softening was observed. Furthermore, in the thermal fatigue test, no cracks were observed at the boundary portion 16, showing good characteristics.
(2) Comparative Example 1 and Comparative Example 2 (SKD7) had higher thermal conductivity than Examples 1 to 9 and Comparative Example 3.
(3) Comparative Example 3 has a low thermal conductivity. However, when the hardness of the molded portion 14 was tempered with the aim of 48 HRC, the hardness of the base portion 12 was significantly reduced to 45.3 HRC. Further, as a result of the thermal fatigue test, cracks were generated at the boundary portion 16, and the thermal fatigue strength, which is an important characteristic of the mold, was lowered.

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 can be made without departing from the gist of the present invention.

The metal powder according to the present invention is used as a powder raw material for producing a mold that requires cooling (for example, a die for die casting, a die for hot stamping, a die for tailored die quenching) by using a laminated molding method. Can be used.

Claims (4)

0.1 ≤ C ≤ 0.4 mass%,
1.8 ≤ Si ≤ 2.5 mass%,
0.3 ≤ Mn ≤ 1.2 mass%,
2.0 ≤ Cr ≤ 6.0 mass%,
0.5 ≤ Mo ≤ 3.5 mass%, and
0.1 ≤ V ≤ 1.0 mass%
A metal powder containing Fe and the balance consisting of Fe and unavoidable impurities. 0.01 ≤ Ni ≤ 4.0 mass% and / or
0.01 ≤ Cu ≤ 4.0 mass%
The metal powder according to claim 1, further comprising. 0.01 ≤ Al ≤ 2.0 mass%,
0.01 ≤ Nb ≤ 1.0 mass%, and 0.01 ≤ Ti ≤ 1.0 mass%
The metal powder according to claim 1 or 2, further comprising any one or more elements selected from the group consisting of. The metal powder according to any one of claims 1 to 3 used for laminated molding.

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