Sm-Fe-N-based magnetic powder with good heat resistance and method for producing the same

Abstract

When heated to approximately 300°C under an inert gas atmosphere, the magnetic properties (especially coercivity H) are... c To provide an Sm-Fe-N-based magnetic powder with good heat resistance and minimal degradation of (the material). [Solution] A powder consisting of particles mainly composed of Sm, Fe, and N, wherein the molar ratio of Sm to Fe (Sm / Fe) is 0.09 or more and 0.25 or less, and the molar ratio of N to Fe (N / Fe) is 0.06 or more and 0.30 or less, and the cumulative 50% particle size in the volume-based particle size distribution by laser diffraction-scattering method D 50 A Sm-Fe-N magnetic powder having a particle size of 0.5 μm or more and 5.0 μm or less, and in depth profiling from the outermost particle surface by AES (Auger electron spectroscopy), the average O content in the depth region of 2.0 nm to 5.0 nm (in terms of SiO2) is 23.0 atomic percent or less out of a total of 100 atomic percent of Sm, Fe, N, and O.

Description

[Technical Field]

[0001] The present invention relates to a heat-resistant Sm-Fe-N magnetic powder that improves the reduction in magnetic properties when heated in an inert gas atmosphere, and to a method for producing the same. [Background technology]

[0002] Sm2Fe 17 A substance in which nitrogen has been introduced into an intermetallic compound (a typical chemical formula is Sm2Fe) 17 N3) is known to be a ferromagnetic material exhibiting excellent hard magnetism. In this specification, Sm2Fe 17 A powder of a ferromagnetic material obtained by introducing nitrogen into an Sm-Fe alloy at or near its stoichiometric composition is called "Sm-Fe-N magnetic powder." Sm-Fe-N magnetic powder is useful as a material for bonded magnets.

[0003] For bonded magnets, Sm-Fe-N magnetic powders generally have a phosphate-based coating formed on the surface of the powder particles. The technique of forming a phosphate-based coating on Sm-Fe-N magnetic powders is widely known (for example, Patent Document 1). Having this type of coating ensures weather resistance when the Sm-Fe-N magnetic powder is exposed to the atmosphere, and improves handling during the manufacturing of bonded magnets. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2003-7521 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] During the manufacturing of bonded magnets, the magnetic powder particles are typically heated to around 200°C in an inert gas atmosphere. Depending on the type of resin, heating to around 300°C may be required.

[0006] Conventional Sm-Fe-N-based magnetic powder having a phosphate film has a coercive force H when heated to about 300 ° C even in an inert gas atmosphere c decreases, and it is difficult to fully exhibit the original excellent magnetic properties of the Sm-Fe-N-based magnetic powder. According to the inventors' studies, the above phosphate film is considered to be mainly composed of iron(III) phosphate formed by the reaction of phosphate and Fe, which is a constituent element of magnetic particles. When the iron(III) phosphate is heated during the production of bonded magnets, it may be reduced by Sm to form a nano-sized αFe phase. The formation of this αFe phase is a factor that reduces the magnetic properties (especially the coercive force H c ) of the bonded magnet.

[0007] On the other hand, if a production line that can maintain the entire process from the synthesis of Sm-Fe-N-based magnetic powder to the production of bonded magnets in an environment where surface oxidation does not occur is used, the need to apply a phosphate film is avoided. However, Sm-Fe-N-based magnetic powder for bonded magnets is mostly synthesized by the reduction diffusion method. In the reduction diffusion method, a washing process is required to remove by-products derived from the reducing agent. Usually, since water is used in this washing process, an increase in the oxygen concentration on the particle surface is inevitable. Even when a phosphate film is not formed, the increase in the oxygen concentration on the surface can be a factor that causes a decrease in magnetic properties due to heating during the production of bonded magnets.

[0008] An object of the present invention is to provide a heat-resistant Sm-Fe-N-based magnetic powder with little decrease in magnetic properties (especially the coercive force H c ) when heated to about 300 ° C in an inert gas atmosphere.

Means for Solving the Problems

[0009] The above object is achieved by the following invention. [1] A powder composed of particles mainly containing Sm, Fe, and N, having a composition where the molar ratio of Sm to Fe, Sm / Fe, is 0.09 or more and 0.25 or less, and the molar ratio of N to Fe, N / Fe, is 0.06 or more and 0.30 or less, and the cumulative 50% particle diameter D 50 in the volume-based particle size distribution by laser diffraction / scattering method is 0.5 μm or more and 5.0 μm or less, and in the depth-direction analysis from the outermost surface of the particles by AES (Auger electron spectroscopy), the average O content in the depth region of 2.0 nm or more and 5.0 nm or less in terms of SiO2 conversion, accounting for 100 atomic% of Sm, Fe, N, and O, is 23.0 atomic% or less. A Sm-Fe-N-based magnetic powder.

[0010] The Sm-Fe-N-based magnetic powder of the above [1] can be produced, for example, by the following method. [2] A gas atomization step of obtaining a powder of a Sm-Fe-based alloy with a Sm / Fe molar ratio of 0.09 or more and 0.25 or less by the gas atomization method, and A heat treatment step of heating the powder obtained in the gas atomization step to a temperature of 900 °C or more and 1200 °C or less to coarsen the crystal grains of the particles of the powder, and A pulverization step of pulverizing the powder of the Sm-Fe-based alloy whose crystal grains have been coarsened by the heat treatment step, thereby refining the particles of the powder by fracture including intra-crystalline fracture, and A nitriding step of introducing nitrogen into the particles of the powder by heating and holding the powder of the Sm-Fe-based alloy refined by the pulverization step in a nitrogen compound or a non-oxidizing gas atmosphere containing nitrogen at a temperature range of 500 °C or less, and Adopting a production process having the above steps, and setting the oxygen concentration in the atmosphere to which the powder is exposed throughout all steps from the gas atomization step to the nitriding step to 10 ppm or less. A method for producing a Sm-Fe-N-based magnetic powder. [3] A surface treatment step of forming a fatty acid film on the particle surface of the powder after the nitriding step, and Further having the above step, and setting the oxygen concentration in the atmosphere to which the powder is exposed throughout all steps from the gas atomization step to the surface treatment step to 10 ppm or less. The method for producing a Sm-Fe-N-based magnetic powder according to the above [2]. [Effects of the Invention]

[0011] According to the present invention, when heated to about 300°C in an inert gas atmosphere, the magnetic properties (especially the coercivity H) are improved. c We have succeeded in creating an Sm-Fe-N magnetic powder with minimal degradation and good heat resistance. This Sm-Fe-N magnetic powder is extremely useful for constructing high-performance bonded magnets using high-performance resins such as PPS resin, which has excellent heat resistance and chemical resistance and therefore requires heating to around 300°C when manufacturing bonded magnets due to its high melting point. [Brief explanation of the drawing]

[0012] [Figure 1] A schematic diagram showing the configuration of the gas atomization device used in the example. [Figure 2] A schematic diagram showing the cross-sectional structure near the bottom of the crucible of the gas atomization device used in the example. [Figure 3] Elemental concentration profiles in the depth direction obtained by AES measurement for the Sm-Fe-N magnetic powder obtained in Example 1. [Figure 4] Elemental concentration profiles in the depth direction obtained by AES measurement for the Sm-Fe-N magnetic powder obtained in Comparative Example 1. [Figure 5] Elemental concentration profiles in the depth direction obtained by AES measurement for the Sm-Fe-N magnetic powder obtained in Comparative Example 2. [Modes for carrying out the invention]

[0013] [Composition of powder] This invention relates to an Sm-Fe-N magnetic powder having a composition in which particles mainly composed of Sm (samarium), Fe (iron), and N (nitrogen) are present, with a molar ratio of Sm to Fe (Sm / Fe) of 0.09 to 0.25 and a molar ratio of N to Fe (N / Fe) of 0.06 to 0.30. "Particles mainly composed of Sm, Fe, and N" refers to particles in which, when the elements contained in the particles are arranged in order of decreasing mass content, the top three elements are Sm, Fe, and N.

[0014] In the Sm-Fe-N-based magnetic powder, the magnetic phase is Th2Zn 17 type crystal structure Sm2Fe 17 It is mainly composed of a magnetic phase in which N (nitrogen) atoms are introduced into the crystal lattice. N atoms are in Sm2Fe 17 It is considered to enter the interstitial position of the crystal lattice, and the Th2Zn 17 type crystal structure is maintained even after the introduction of N atoms. When N atoms are introduced into Sm2Fe 17 , the crystal magnetic anisotropy changes from in-plane type to uniaxial type and the Curie point rises, becoming a practical magnet material. As is well known, the typical composition of Sm-Fe-N-based magnetic powder with excellent magnetic properties is Sm2Fe 17 N3. The Sm / Fe molar ratio in the Sm2Fe 17 N3 composition is 0.118 and the N / Fe molar ratio is 0.176. It is considered that the closer to the stoichiometric composition of Sm2Fe 17 N3, the more advantageous it is in terms of magnetic properties, but it also exhibits hard magnetism in the surrounding composition range. In the present invention, considering that good hard magnetism can be realized, the Sm-Fe-N-based magnetic powder in the composition range where the Sm / Fe molar ratio is 0.09 or more and 0.25 or less, and the N / Fe molar ratio is 0.06 or more and 0.30 or less is targeted. A more preferable range of the Sm / Fe molar ratio is 0.10 or more and 0.20 or less, and a more preferable range of the N / Fe molar ratio is 0.11 or more and 0.27 or less. In addition, in the Sm-Fe-N-based magnetic powder targeted by the present invention, in addition to the Sm2Fe 17 phase containing N, there may be cases where foreign phases such as the SmFe7 phase with a TbCu7-type crystal structure are mixed in, but the presence of foreign phases is allowed as long as it does not inhibit the object of the present invention.

[0015] [Particle size distribution of powder] Considering realizing excellent magnetic properties as an aggregate of Sm-Fe-N-based magnetic particles such as bonded magnets, in the present invention, the cumulative 50% particle diameter D 50 in the volume-based particle size distribution of the Sm-Fe-N-based magnetic powder by the laser diffraction / scattering method is defined to be 0.5 μm or more and 5.0 μm or less.

[0016] [Oxygen content in the surface layer of powder particles] According to the inventors' research, reducing the O content present in the particle surface layer of Sm-Fe-N magnetic powder improves the magnetic properties (especially coercivity H) after heating to around 300°C in an inert atmosphere. c It has been found to be extremely effective in maintaining a high oxygen concentration. Specifically, in depth profiling from the outermost surface of the particles by AES (Auger electron spectroscopy), the average oxygen content in the depth region of 2.0 nm to 5.0 nm (in terms of SiO2) is limited to 23.0 atomic percent or less of the total 100 atomic percent of Sm, Fe, N, and O. An average oxygen content of 21.0 atomic percent or less is even more effective. With conventional techniques that form a phosphate-based film on the surface of particles, it is difficult to realize Sm-Fe-N magnetic powders with such low oxygen concentrations in the surface layer of the particles as described above.

[0017] [Manufacturing Process] A preferred manufacturing process for obtaining the aforementioned Sm-Fe-N magnetic powder with good heat resistance is "gas atomization → heat treatment → pulverization → nitriding treatment".

[0018] [Gas atomization] Gas atomization is a powder formation method in which molten metal discharged into a gas phase space is rapidly blown with gas to break it down into fine liquid phase particles, and these liquid phase particles are rapidly cooled and solidified while in flight in the gas phase space. As metal raw materials for generating the molten metal to be used in gas atomization, pre-melted Sm-Fe-based master alloys with known compositions, metallic Sm, metallic Fe, etc., can be used. (Sm2Fe) 17 The stoichiometric Sm / Fe molar ratio is 0.118. The composition of the molten metal is Sm2Fe 17It is desirable to adjust the composition to a value relatively close to the stoichiometric composition, specifically, to a Sm / Fe molar ratio in the range of 0.09 to 0.25. The inclusion of metal elements other than Sm and Fe is permissible as long as it does not impair the required properties of the final magnetic powder, but the total content of Sm and Fe in the molten metal is preferably 95.0% by mass or more, and more preferably 98.0% by mass or more. It is desirable to produce the molten metal in an inert gas atmosphere excluding nitrogen or in a vacuum.

[0019] Molten metal, maintained at a predetermined temperature and thoroughly homogenized, is discharged from a nozzle into the gas phase space, and a cooling gas is forcefully blown onto the molten metal immediately after discharge. This causes the molten metal to become fine liquid phase particles that fly through the gas phase space and solidify. The molten metal temperature during discharge should be set within the range of 1400 to 1900°C. The particle size can be controlled by the atomization conditions, but if necessary, the gas atomized powder may be graded using a sieve or the like to adjust the particle size before being used in subsequent processes. Considering that the gas atomized powder will be ground into fine particles with an average particle diameter of a few μm or less in subsequent processes, the particle size of the gas atomized powder should be such that the cumulative 50% particle size D is measured in the volume-based particle size distribution by laser diffraction and scattering method. 50 For example, it is preferable that the particle size be 70.0 μm or less, more preferably 50.0 μm or less, and even more preferably 25.0 μm or less. There is no particular limit to the lower limit of the particle size of the gas atomizing powder, however, as described above D 50 However, since it is currently very difficult to industrially synthesize particles smaller than 5.0 μm, the above D 50 The particle size should be adjusted to a range of 5.0 μm or larger, or it may be controlled to a range of 10.0 μm or larger.

[0020] In the gas atomization process, it is desirable that the pressurizing gas used to discharge the molten metal, the propellant gas sprayed onto the molten metal, and the atmospheric gas in the gas phase space through which the liquid phase particles fly are all inert gases excluding nitrogen. If these gases contain nitrogen, the gas atomized powder will be incompletely nitrided, making it difficult to achieve highly uniform nitriding in subsequent nitriding processes.

[0021] [Heat treatment] The crystal grains of gas-atomized powder of Sm-Fe alloy (hereinafter sometimes referred to as "Sm-Fe gas-atomized powder") are coarsened by heat treatment at high temperatures. Specifically, by heating to 900°C or higher, the crystal grains can be coarsened to a size such that the average crystal grain size in the equivalent circle diameter of the particle cross-sectional structure is approximately 3 to 15 μm. This coarsening of the crystal grains makes it easier for intra-grain fracture to occur during subsequent grinding processes, making it easier to obtain fine Sm-Fe alloy powder with a high proportion of particles consisting of single crystal grains. Heating to 930°C or higher is more preferable. Since excessive heating is uneconomical, the heating temperature is preferably set in the range of 1200°C or lower, and may be controlled in the range of 1100°C or lower, or 1000°C or lower. The holding time in the temperature range of 900°C to 1200°C can be, for example, 10 seconds to 10 minutes, or it may be set in the range of 30 seconds to 5 minutes. The heating atmosphere should preferably be an inert gas atmosphere excluding nitrogen, or a vacuum. Before raising the temperature to the above-mentioned high temperature, a hydrogen treatment can be performed, in which the material is heated and held at a temperature of 250°C to 500°C in a hydrogen gas atmosphere. The holding time at this temperature can be, for example, 1 to 60 minutes.

[0022] In Sm-Fe gas atomization powders, it is possible that a Sm-Fe metallic phase with a TbCu7-type crystal structure may be formed depending on the conditions. The TbCu7-type Sm-Fe metallic phase is Th2Zn 17 Compared to the mold, the anisotropic magnetic field after nitriding is small and it is difficult to obtain high coercivity, so it is desirable that the amount of Sm-Fe-based metallic phase in the TbCu7 type be as small as possible. Upon heating to the above temperature range, the crystalline phase of the TbCu7 type becomes Th2Zn 17 Because it transforms the structure, this heat treatment is also effective in homogenizing the microstructure.

[0023] [Crush] When a powder of Sm-Fe alloy, whose grains have been coarsened by the above heat treatment, is subjected to mechanical grinding, individual particles fracture not only at grain boundaries (intergranular fracture) but also within grains (intragranular fracture). When the powder particles are refined by this "fracture including intragranular fracture," a fine Sm-Fe alloy powder with a high proportion of particles consisting of single grains can be obtained. Suitable grinding means for this invention include, for example, wet ball mills and jet mills.

[0024] From the perspective of fragmenting particles by fracture including intragranular fracture, the particle size after grinding is the cumulative 50% particle size D in the volume-based particle size distribution measured by laser diffraction and scattering. 50 It is preferable to adjust the particle size to 5.0 μm or less, and more preferably to 3.0 μm or less. Excessive refinement may increase crystal lattice strain due to the application of excessive external force, potentially adversely affecting the magnetic properties. Typically, the above particle size D 50 It is preferable to grind the material to a size of 0.5 μm or larger, but grinding to a size of 1.0 μm or larger is also acceptable. Furthermore, the particle size distribution may be optimized by classifying the powder obtained by grinding and removing coarse particles, or, if necessary, excessively small fine particles.

[0025] [nitriding treatment] Next, a nitriding treatment is performed to obtain Sm-Fe-N magnetic powder. The nitriding treatment can be carried out by heating and holding the finely ground Sm-Fe alloy powder in a nitrogen compound or a nitrogen-containing non-oxidizing gas atmosphere. If the heating temperature is too high, Sm2Fe 17 Sm2Fe crystal with nitrogen atoms embedded within it 17Because the N3-based structure becomes unstable, nitriding becomes difficult. It is desirable to keep the heating temperature for nitriding below 500°C. At very low temperatures, nitriding takes a long time to proceed, which is disadvantageous for uniformly diffusing nitrogen atoms into the interior of the particles. Heating at 300°C or higher is effective. For the atmospheric gas of the nitriding treatment, a reducing atmosphere consisting of a mixture of ammonia (NH3) and hydrogen (H2) is practical. For example, the mixing ratio of ammonia to hydrogen, NH3:H2, can be in the range of 10:90 to 60:40. Other atmospheric gases that can be used for nitriding treatment include a mixture of hydrogen, ammonia, and nitrogen (N2), a mixture of hydrogen, ammonia, and argon (Ar), ammonia alone, a mixture of ammonia and nitrogen, a mixture of ammonia and argon, nitrogen gas, and a mixture of nitrogen and hydrogen, and these are used to create a non-oxidizing atmosphere. The optimal time for nitriding treatment varies somewhat depending on the average particle size of the powder, the composition of the atmospheric gas, and the temperature, but typically, an optimal time can be found within the range of 15 to 240 minutes when the N / Fe molar ratio falls within the predetermined range mentioned above. After nitriding, classification may be performed to adjust the particle size distribution to an appropriate level for the intended application.

[0026] As described above, Sm-Fe-N magnetic powder can be obtained. However, in order to achieve the aforementioned Sm-Fe-N magnetic powder with good heat resistance, it is extremely important to strictly control the oxygen concentration of the atmosphere to which the powder is exposed throughout all processes, from the gas atomization process to the nitriding process, to 10 ppm or less.

[0027] [Surface treatment] If a manufacturing line can be used that can control the oxygen concentration of the atmosphere to which the powder is exposed after nitriding and before mixing with resin in the manufacturing process of bonded magnets to a minimum of 10 ppm, the deterioration of the magnetic properties of the Sm-Fe-N magnetic powder when heated to about 300°C in an inert gas atmosphere together with the resin can be significantly suppressed. However, to improve handling, it is effective to apply a surface treatment to form a fatty acid film on the particle surface of the powder after the nitriding process. Specifically, a method can be employed in which the nitrided Sm-Fe-N magnetic powder and one or more fatty acids having 12 to 20 carbon atoms per molecule are mixed in a non-oxidizing atmosphere. Examples of such fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, dihomo-γ-linolenic acid, arachidonic acid, and eicosapentaenoic acid. As the non-oxidizing atmosphere, for example, an inert atmosphere such as nitrogen, argon, or helium, or a reducing atmosphere such as hydrogen or carbon monoxide can be applied. From a cost and safety standpoint, a nitrogen gas atmosphere is preferably used. As a mixing method, grinding or mixing equipment such as a vibrating mill, sample mill, Henschel mixer, or fluidized bed mixer can be used. However, it is important to control the oxygen concentration of the atmosphere to which the powder is exposed throughout all processes from the gas atomization process to the surface treatment process to 10 ppm or less.

[0028] Such a fatty acid coating prevents external oxygen from reacting with the Sm-Fe-N magnetic phase due to steric hindrance by the fatty acid molecules. Therefore, Sm-Fe-N magnetic powder treated with a fatty acid surface can be exposed to ambient air at room temperature during handling until it is mixed with resin in the manufacturing process of bonded magnets. Furthermore, unlike phosphates, fatty acids have fewer oxygen atoms in their molecules, so Sm-Fe-N magnetic powder with a fatty acid coating can sufficiently satisfy the surface layer morphology, in the depth profiling analysis from the outermost particle surface by AES described above, where the average oxygen content in the depth region of Sm, Fe, N, and O in terms of SiO2 is 23.0 atomic percent or less in the total 100 atomic percent, and the oxygen atoms in the fatty acids do not cause a decrease in heat resistance. [Examples]

[0029] In each of the following examples, elemental analysis, particle size distribution measurement, magnetic measurement, X-ray diffraction measurement, and AES (Auger electron spectroscopy) measurement were performed using the following methods.

[0030] (Elemental analysis) The analysis of metallic elements was performed by heating and dissolving the sample in hydrochloric acid in a glove box filled with argon (Ar) gas, then diluting it to prepare an analytical sample solution, which was then analyzed using an ICP emission spectrometer (Agilent 720, manufactured by Agilent Technologies). Nitrogen analysis was performed using an oxygen-nitrogen analyzer (Horiba EMGA-920) by the inert gas fusion-thermal conductivity method.

[0031] (Measurement of particle size distribution of powders) A laser diffraction particle size distribution analyzer (Sympatec, Helos / Rodos) was used to determine the cumulative 50% particle size D of the volume-based particle size distribution. 50 They sought it.

[0032] (Magnetic measurement of powders) A sample cell containing 20 mg of sample powder and paraffin was placed in the center of an electromagnet, and the sample cell was heated to 80°C for 2 minutes using a hot air generator. The amount of paraffin used was such that the sample powder and paraffin filled the container of the sample cell. Next, while the sample cell was still heated at 80°C, an external magnetic field of 1.0 T (Tesla) was applied to the sample cell by the electromagnet for 2 minutes. Then, while the 1.0 T magnetic field was still applied, the sample cell was cooled to room temperature. In this way, a measurement sample oriented in a magnetic field was obtained. This measurement sample was placed in a VSM (VSM-5HSC, manufactured by Toei Kogyo Co., Ltd.) so that the direction of the applied magnetic field was parallel to the magnetic field orientation direction of the measurement sample, and the coercivity H c The following measurements were taken. The measurement conditions were a maximum applied magnetic field of 4.79 MA / m and a sweep rate of 8 kA / m·sec.

[0033] (X-ray diffraction measurement) For powder samples, the X-ray diffraction pattern was measured using Co-Kα radiation at a tube voltage of 45kV and a tube current of 40mA.

[0034] (AES measurement) Elemental concentrations in the depth direction from the outermost surface of powder particles were investigated using a field emission Auger electron spectrometer (FE-AES) (JEOL Ltd., JAMP-9500F). The powder sample, placed on a copper plate, was set in the sample holder. The measurement position of the powder sample was confirmed and set using SEM imaging. Elemental analysis in the depth direction was performed by etching using Ar gas under conditions where the etching rate (SiO2 equivalent) was 11 nm / min. Measurements at each depth position were performed with an acceleration voltage of 10 kV and an irradiation current of 1 × 10⁻¹⁰ -8 The procedure was performed at location A. The beam diameter was set to 0 μm using the instrument's software, which allows setting integer values ​​between 0 μm and 100 μm. Assuming the presence of O, Sm, Fe, and N, the atomic percentages were calculated using the instrument's software, and a depth-direction elemental concentration profile was obtained where the sum of Sm, Fe, N, and O equaled 100 atomic percent. Based on this elemental concentration profile, the average O content (atomic percent) within the total 100 atomic percent of Sm, Fe, N, and O in the depth region between 2.0 nm and 5.0 nm (in SiO2 equivalent) was determined.

[0035] [Example 1] (Synthesis of Sm-Fe-based powders by gas atomization method) Figure 1 schematically shows the configuration of the gas atomization apparatus used in this example. Inside the chamber, there are two independent spaces, upper and lower, which can be evacuated by a vacuum exhaust device 10. These spaces can be made into gas phase spaces with a predetermined gas atmosphere by introducing gas from atmospheric gas supply sources 11a and 11b. The upper space contains a crucible 1, in which the raw material is melted by induction heating with a high-frequency coil 4 to form molten metal 5. A molten metal discharge nozzle member 2 is attached to the bottom of the crucible 1 to discharge the molten metal 5 into the lower gas phase space. The molten metal flow path is blocked by pressing the stopper 3 against the molten metal discharge nozzle member 2 until the molten metal 5 is discharged. After the molten metal 5 is sufficiently homogenized and a predetermined temperature is obtained, the gas supply device 13 for molten metal discharge supplies gas at a predetermined pressure to the surface of the molten metal in the crucible 1, and the stopper 3 is raised to discharge the molten metal 5 from the tip of the molten metal discharge nozzle member 2 into the lower gas phase space. The lower gas phase space is equipped with a gas injection nozzle 6 for blowing gas onto the discharged molten metal 5. Before discharge begins, gas is supplied from the gas supply device 12 to the gas injection nozzle 6, and the gas is injected from the gas injection nozzle 6 at a high pressure. By applying this strong jet of injected gas to the molten metal 5, fine particles of the molten metal 5 are formed, and these fine particles are rapidly cooled and solidified. The solidified metal particles 7 accumulate at the bottom of the lower gas phase space.

[0036] Figure 2 schematically shows an example of the cross-sectional structure near the bottom of the crucible of a gas atomizing device. The molten metal discharge nozzle member 2 attached to the bottom of the crucible 1 has a discharge port 21, which is the opening at the tip of the nozzle, and a stopper contact surface 22. The stopper 3 is movable in the vertical direction and has the function of blocking the flow path of the nozzle by contacting the stopper contact surface 22 of the molten metal discharge nozzle member 2, and opening the flow path of the nozzle by moving away from the stopper contact surface 22 when molten metal is discharged. In this example, the entire crucible 1 is made of CaO-containing ZrO2, the entire molten metal discharge nozzle member 2 is made of CaO-containing ZrO2, and at least the portion of the stopper 3 that is immersed in the molten metal 5 is made of thermoalloy. The inner diameter of the nozzle of the molten metal discharge nozzle member 2 is 3.0 mm.

[0037] Sm metal, Fe metal, and Sm-Fe alloy were used as raw materials. Sm metal, Fe metal, and Sm-Fe alloy were mixed so that the Sm / Fe molar ratio was 0.16. 1700.0 g of these raw materials were placed in a crucible and melted by high-frequency induction heating in an Ar atmosphere with an oxygen concentration of 10 ppm or less. After the raw material alloy was completely molten, at 48 minutes from the start of heating, the entire amount of molten metal at 1550°C was discharged from a nozzle into the lower gas phase space. The maximum supply pressure of the gas for molten metal discharge was set to a differential pressure of 57 kPa compared to the atmospheric gas pressure. Ar was used as the ejection gas. The lower gas phase space was also an Ar atmosphere with an oxygen concentration of 10 ppm or less. The generated powder was collected, and coarse particles were removed using a 500 μm sieve in a glove box under a nitrogen atmosphere (oxygen concentration of 10 ppm or less). Gas atomized powder was obtained in this way. Elemental analysis of the gas atomized powder revealed a Sm / Fe molar ratio of 0.15, which was equivalent to that of the raw material formulation. Furthermore, the cumulative 50% particle size D of the volume-based particle size distribution of the gas atomized powder, as determined by laser diffraction and scattering, was also analyzed. 50 It was 45.2 μm.

[0038] (Heat treatment) An electrically heated tubular furnace filled with nitrogen gas was used as the heat treatment furnace. The above Sm-Fe gas atomized powder was placed in a sealed container filled with nitrogen gas (oxygen concentration of 10 ppm or less) and transferred to the glove box (oxygen concentration of 10 ppm or less), and then charged into the tubular furnace without exposure to the atmosphere. The tubular furnace was heated from room temperature to 300°C at a rate of 10°C / min while hydrogen gas was flowed through it, and then held at 300°C for 30 minutes. After that, the temperature was heated from 300°C to 990°C at a rate of 10°C / min while argon gas was flowed through it, and then held at 990°C for 10 minutes, and then cooled to below 50°C while argon gas was flowed through it to obtain the heat-treated powder. The oxygen concentration inside the tubular furnace was controlled to 10 ppm or less.

[0039] (Crush) The obtained heat-treated powder was placed in an airtight container filled with nitrogen gas (oxygen concentration 10 ppm or less) and transferred from the tubular furnace to another glove box filled with nitrogen gas (oxygen concentration 10 ppm or less). Inside this glove box, 500 g of heat-treated powder, 11.25 kg of 3.2 mm diameter stainless steel balls, and 7.5 mL of isopropyl alcohol as a lubricant were placed in a 3000 mL stainless steel mill pot and sealed. Next, the mill pot containing the heat-treated powder was placed in a vibratory mill (YAMP-6SND, manufactured by Eurastechno Co., Ltd.), and grinding was performed with an amplitude of ±2.5 mm, a vibration frequency of 60 Hz, and an operating time of 84 minutes. Next, the contents of the mill pot were passed through a sieve to remove the stainless steel balls on the sieve, and the powder that passed through the sieve was collected. The above loading and collection of the mill pot and sieving operations were performed inside a glove box with a nitrogen atmosphere (oxygen concentration 10 ppm or less).

[0040] (nitriding treatment) The pulverized powder obtained as described above was transferred to an electrically heated tubular furnace under a nitrogen atmosphere. A mixed gas consisting of 35% by volume of ammonia (NH3) gas and 65% by volume of hydrogen (H2) gas was then flowed into the tubular furnace to replace the gas inside. Subsequently, while flowing the mixed gas, the temperature was raised to 390°C at a rate of 10°C / min and held at 390°C for 240 minutes to perform nitriding. Next, the gas flowing into the tubular furnace was changed to hydrogen (H2) gas and held at 390°C for another 130 minutes. Then, the gas flowing into the tubular furnace was changed to argon gas and held at 390°C for 90 minutes. After that, heating was stopped and the furnace was cooled to near room temperature while flowing argon gas to obtain nitrided powder. The oxygen concentration inside the tubular furnace was always maintained at 10 ppm or less.

[0041] (Surface treatment) In a nitrogen gas-atmosphere glove box (oxygen concentration of 10 ppm or less), 100 g of nitrided powder (Sm-Fe-N-based magnetic powder) obtained as described above and 5 g of stearic acid were placed in a sample mill (Kyōritsu Rikou Co., Ltd., SK-M10) and mixed at 9000 rpm for 120 seconds to coat the surface of the magnetic particles with fatty acids. The amount of fatty acid added per 100 parts by mass of nitrided powder was 5 parts by mass. As described above, the Sm-Fe-N magnetic powder used as the test material in this example was obtained. Throughout the entire process from the gas atomization step to the nitriding step, the oxygen concentration of the atmosphere to which the powder was exposed was controlled to 10 ppm or less.

[0042] (Powder properties) The properties of the obtained Sm-Fe-N magnetic powder (test powder) were investigated using the method described above. X-ray diffraction measurements revealed that the test powder is Th2Zn. 17 It was confirmed that it possesses a type crystal structure. The cumulative 50% particle size D of this test powder, obtained by laser diffraction and scattering method, shows the volume-based particle size distribution. 50 It was 3.8 μm. Compositional analysis revealed that the powder used in this example had an Sm / Fe molar ratio of 0.15 and an N / Fe molar ratio of 0.19. Figure 3 shows the elemental concentration profile in the depth direction obtained by AES measurement for the test powder in this example. In this example, the average oxygen content of the test powder was 19.1 atomic percent out of a total of 100 atomic percent of Sm, Fe, N, and O in the depth region from 2.0 nm to 5.0 nm in terms of SiO2. The results of the magnetic measurement showed that the coercivity H of the powder used in this example is c The current was 693 kA / m (8.7 kOe).

[0043] (Evaluation of heat resistance in an inert gas) In a glove box filled with nitrogen gas, 3.0 g of the test powder sample was transferred to an electrically heated tubular furnace under a nitrogen atmosphere. Nitrogen gas was then flowed into the tubular furnace, and it was heated and maintained at 300°C for 30 minutes. After that, heating was stopped, and the sample was cooled to near room temperature while nitrogen gas was flowed through it to obtain a powder sample. 0.09 g of stearic acid was added to this powder sample, and it was placed in a 70 mL capacity mixer with a stainless steel propeller, where it was mixed at 25,000 rpm for 60 seconds. Magnetic measurements were performed on the powder sample after the heating test using the same method as described above. The result showed that the coercivity H after the heating test was c1 The current was 665 kA / m (8.4 kOe). The coercivity retention rate obtained from this heating test was calculated using the following equation (1). Coercive force retention rate=100×H c1 / H c …(1) Here, H c The coercivity of the powder before the heating test (kA / m), H c1 This is the coercivity (kA / m) of the powder after the heating test. The coercivity retention rate of the tested powder in this example was 100 × 665 / 693 ≈ 96%. The results are shown in Table 1 (the same applies to each of the following examples).

[0044] [Comparative Example 1] In this example, the above-mentioned properties were evaluated using Sm-Fe-N magnetic powder synthesized by a known method utilizing reduction-diffusion as the test powder. Specifically, the Sm-Fe-N magnetic powder, which was the test powder, was obtained through a process that included obtaining a composite hydroxide of Sm-Fe by neutralization reaction, obtaining a composite oxide of Sm-Fe by heat treatment, obtaining a partial oxide with a reducing gas, a reduction process by reduction-diffusion method, a nitriding process, a washing process to remove by-products derived from the reducing agent, and a surface treatment process to protect the particle surface with a phosphoric acid compound.

[0045] X-ray diffraction measurements revealed that the powder used in this example is Th2Zn. 17 It was confirmed that it possesses a type crystal structure. The cumulative 50% particle size D of this test powder, obtained by laser diffraction and scattering method, shows the volume-based particle size distribution. 50 It was 2.7 μm. Compositional analysis revealed that the test powder in this example had an Sm / Fe molar ratio of 0.12 and an N / Fe molar ratio of 0.17. Figure 4 shows the elemental concentration profile in the depth direction obtained by AES measurement for the test powder in this example. In this example, the average oxygen content of the test powder was 26.2 atomic percent out of a total of 100 atomic percent of Sm, Fe, N, and O in the depth region from 2.0 nm to 5.0 nm in terms of SiO2. The results of the magnetic measurement showed that the coercivity H of the test powder in this example before the heating test was c The coercivity after heating test was 961 kA / m (12.1 kOe), H c1 The current was 518 kA / m (6.5 kOe), and the coercivity maintenance rate according to equation (1) above was 100 × 518 / 961 ≈ 54%.

[0046] [Comparative Example 2] In this example, the above-mentioned properties were evaluated using Sm-Fe-N magnetic powder synthesized by a known method utilizing reduction-diffusion as the test powder. Specifically, the Sm-Fe-N magnetic powder, which was the test powder, was obtained through a process of mixing Sm raw material, Fe raw material, and reducing agent, a reduction process by reduction-diffusion method, a nitriding process, a washing process to remove by-products derived from the reducing agent, and a grinding process to refine the particles and protect the particle surface with a phosphate compound.

[0047] X-ray diffraction measurements revealed that the powder used in this example is Th2Zn. 17It was confirmed that it possesses a type crystal structure. The cumulative 50% particle size D of this test powder, obtained by laser diffraction and scattering method, shows the volume-based particle size distribution. 50 It was 2.1 μm. Compositional analysis revealed that the test powder in this example had an Sm / Fe molar ratio of 0.12 and an N / Fe molar ratio of 0.17. Figure 5 shows the elemental concentration profile in the depth direction obtained by AES measurement for the test powder in this example. In this example, the average oxygen content of the test powder was 30.9 atomic percent out of a total of 100 atomic percent of Sm, Fe, N, and O in the depth region between 2.0 nm and 5.0 nm in terms of SiO2. The results of the magnetic measurement showed that the coercivity H of the test powder in this example before the heating test was c The coercivity after heating test was 920 kA / m (11.6 kOe), H c1 The current was 271 kA / m (3.4 kOe), and the coercivity maintenance rate according to equation (1) above was 100 × 271 / 920 ≈ 29%.

[0048] [Table 1]

[0049] The Sm-Fe-N magnetic powders obtained in the examples using a manufacturing process that controlled the oxygen concentration to a low level with the gas atomization method had a low oxygen content in the particle surface layer, and the decrease in coercivity due to heat treatment by heating and holding at 300°C in an inert gas atmosphere was very small. In contrast, the comparative Sm-Fe-N magnetic powder obtained by a conventional manufacturing process using the reduction-diffusion method had a higher oxygen content in the particle surface layer than that of the above example, and showed a greater decrease in coercivity due to heat treatment by heating and holding at 300°C in an inert gas atmosphere. [Explanation of symbols]

[0050] 1 crucible 2. Molten metal discharge nozzle component 3 Stopper 4. High-frequency coil 5. Molten metal 6 Gas injection nozzle 7. Solidified metal particles 10 Vacuum exhaust system 11a, 11b Atmosphere gas supply source 12. Injection gas supply device 13. Gas supply device for molten metal discharge 21 Discharge port 22 Stopper contact surface

Claims

1. A powder consisting mainly of particles composed of Sm, Fe, and N, wherein the molar ratio of Sm to Fe (Sm / Fe) is 0.09 or more and 0.25 or less, and the molar ratio of N to Fe (N / Fe) is 0.06 or more and 0.30 or less, and the cumulative 50% particle size D in the volume-based particle size distribution measured by laser diffraction and scattering method. ５０ The particle size is between 0.5 μm and 5.0 μm, and in depth profiling from the outermost surface of the particle by AES (Auger electron spectroscopy), SiO ２ An Sm-Fe-N magnetic powder in which the average O content is 23.0 atomic percent or less of the total 100 atomic percent of Sm, Fe, N, and O in the depth region of 2.0 nm to 5.0 nm (converted).

2. A gas atomization process to obtain powder of an Sm-Fe alloy having an Sm / Fe molar ratio of 0.09 or more and 0.25 or less by gas atomization, A heat treatment step is performed in which the powder obtained in the gas atomization step is heated to a temperature of 900°C to 1200°C to coarse the crystalline grains of the powder particles, A grinding step is performed to pulverize the powder of the Sm-Fe alloy whose crystal grains have been coarsened by the heat treatment step, thereby refining the powder particles by fracture including intra-grain fracture, A nitriding step is performed in which nitrogen is introduced into the particles of the Sm-Fe alloy powder, which has been finely ground by the aforementioned grinding step, by heating and holding it in a non-oxidizing gas atmosphere containing a nitrogen compound or nitrogen at a temperature range of 500°C or less. A method for producing Sm-Fe-N magnetic powder, comprising employing a manufacturing process having the above-mentioned gas atomization process and maintaining an oxygen concentration of 10 ppm or less in the atmosphere to which the powder is exposed throughout all processes from the gas atomization process to the nitriding process.

3. A surface treatment step in which a fatty acid film is formed on the particle surface of the powder after the nitriding step, The method for producing Sm-Fe-N-based magnetic powder according to claim 2, further comprising the above, wherein the oxygen concentration of the atmosphere to which the powder is exposed throughout all steps from the gas atomization step to the surface treatment step is 10 ppm or less.

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