Low-nitrogen-content sintered neodymium-iron-boron magnet and preparation method thereof

Abstract

The invention discloses a preparation method of a low-nitrogen-content sintered neodymium-iron-boron magnet. In the steps of hydrogen demolishing, jet milling or oriented pressing, MgH2 and CaO mixed powder is added. On the basis of not changing the original preparation process of the sintered neodymium-iron-boron magnet, the nitrogen content in the magnet can be greatly reduced by introducing a small amount of magnesium element and calcium element, so that the magnetic performance of the sintered neodymium-iron-boron magnet is effectively improved, and the method has the characteristics of simplicity and convenience in operation, low cost and high controllability.

Description

Technical Field

[0001] This invention belongs to the field of sintered NdFeB magnets, specifically relating to a low-nitrogen-content sintered NdFeB magnet and its preparation method. Background Technology

[0002] As a key high-performance rare-earth permanent magnet material, sintered NdFeB magnets have seen a significant increase in market demand in recent years, driven by the booming development of strategic emerging industries such as new energy vehicles, industrial servo motors, humanoid robots, and wind power generation. Currently, the requirements for the comprehensive performance of sintered NdFeB magnets in various fields are also increasing simultaneously. Against this backdrop, the control of impurity elements introduced during magnet manufacturing has become increasingly prominent. Among these, nitrogen impurities have a particularly significant negative effect on the comprehensive performance of magnets (such as magnetic properties, corrosion resistance, and thermal stability), and have become one of the key bottleneck factors restricting the development and industrialization of high-grade, high-performance NdFeB magnets.

[0003] Studies have confirmed that the introduction of nitrogen in the entire process of sintering NdFeB magnets exhibits a multi-path characteristic. Specifically: in the smelting stage, although it is usually carried out under an argon protective atmosphere, trace amounts of nitrogen components remaining in industrial-grade argon can still react with molten rare earth metals; in the hydrogen breaking stage: if the purity of the hydrogen used is not up to standard, the nitrogen impurities entrained in it will be adsorbed by the surface of highly active rare earth alloy powder. This nitrogen is difficult to remove effectively during subsequent dehydrogenation and tends to form stable rare earth nitrides. For air jet milling and pressing, these two stages constitute the main sources of nitrogen pollution because traditional processes commonly use nitrogen as the grinding protective atmosphere and powder transport medium. This results in submicron / micron-sized powders with extremely high specific surface areas being exposed to nitrogen for extended periods, leading to significant enrichment of nitrogen content on the powder surface. During sintering, if the vacuum level in the sintering furnace is insufficient or the sintering exhaust process parameters are improperly set, residual nitrogen in the furnace or nitrogen desorbed from the interior of the green body will react with residual hydrogen generated during the hydrogen breakdown stage at high temperatures to synthesize ammonia. The generated ammonia then reacts with the main phase of the magnet (such as Nd₂Fe₂). 14 B) The reaction leads to the pulverization of the preform. The synergistic effect of the above-mentioned multi-path contamination mechanisms results in the nitrogen content of magnets prepared by conventional processes generally being in the range of 300-600 ppm. If the preparation process is abnormal, it may even reach more than 1000 ppm, which significantly exceeds the technical upper limit of the high-performance requirements of high-performance materials (ideal threshold < 150 ppm).

[0004] Excessive nitrogen content in magnets has a multi-dimensional negative effect on the performance of sintered NdFeB magnets, mainly manifested in the following aspects: (1) Deterioration of magnetic properties: Nitrogen atoms react with rare earth elements (Nd, Pr) in the grain boundary region to form rare earth nitrides (such as NdN, PrN). These nitrides are non-ferromagnetic phases, which not only consume the rare earth resources that should participate in the formation of the main phase, but also directly reduce the volume fraction of the ferromagnetic phase. At the same time, the nitride phases segregated at the grain boundary act as antimagnetization nucleation points at the microscale, significantly weakening the exchange coupling effect between the main phase grains. Studies have confirmed that when the nitrogen content increases from 200ppm to 1000ppm, it directly causes a decrease in remanence of 0.2kGs and an increase in intrinsic coercivity of more than 2kOe. (2) Degradation of grain boundary structure and increased corrosion sensitivity: As a weak area of ​​the magnet, the grain boundary is highly sensitive to nitrogen pollution. Excessive nitrogen intrusion reacts with rare earth-rich phases (such as Nd-rich phases) at grain boundaries to generate hard and brittle Nd-ON compounds, replacing the original beneficial low-melting-point phases such as Nd-Cu / Al. This phase transformation leads to the loss of grain boundary ductility, inducing microcracks during sintering cooling or machining. In addition, the electrochemical potential of the nitride phase is significantly lower than that of the main phase matrix, and it preferentially dissolves as an anode in humid environments, accelerating the electrochemical corrosion process of grain boundaries. (3) Decreased thermal stability: Under high-temperature conditions, the thermal instability of grain boundary nitrides becomes a key factor in the decay of magnetic properties. When the temperature exceeds the critical threshold, some nitrides undergo decomposition reactions, and the released gas accumulates in the grain boundary region and forms submicron-sized pores. Such defects not only reduce the density of the magnet, but also become the nucleation source of microcracks. In summary, in order to meet the market demand for high-performance sintered NdFeB materials, it is necessary to develop NdFeB magnets with low nitrogen content to improve the overall performance of the magnets. Summary of the Invention

[0005] This invention addresses the problem of impurity elements introduced during magnet manufacturing by disclosing a method for preparing low-nitrogen-content sintered NdFeB magnets. The aim is to effectively control the nitrogen content of the magnets through process optimization. This invention significantly reduces the nitrogen content in the magnets by introducing a mixture of MgH2 and CaO powders with optimized proportions and particle size, thereby effectively improving the overall performance of the sintered NdFeB magnets.

[0006] The technical solution adopted in this invention is: A method for preparing a low-nitrogen-content sintered NdFeB magnet, the method comprising the following steps: The raw materials for neodymium iron boron magnets are vacuum melted to obtain rapidly solidified flakes. The rapidly solidified flakes are hydrogen-crushed to obtain medium powder. The medium powder is air-jet milled to obtain fine powder. The fine powder is oriented and pressed and isostatically pressed to obtain a compact. The compact is vacuum sintered and aged to obtain a neodymium iron boron magnet. In the hydrogen-crushing, air-jet milling or oriented pressing steps, a mixed powder of MgH2 and CaO is added. In the MgH2 and CaO mixed powder, the mass ratio of MgH2 to CaO is 2~4:1, preferably 3:1.

[0007] In the MgH2 and CaO mixed powder, the particle size of MgH2 is 50-250nm, preferably 100-200nm; and the particle size of CaO is 1-5μm.

[0008] The addition ratio of the MgH2 and CaO mixed powder is 0.1~0.5% of the total weight of the NdFeB magnet, preferably 0.3~0.4%.

[0009] Furthermore, when adding the mixed powder of MgH2 and CaO in the hydrogenation process, the specific steps are as follows: mix the quick-setting flakes with the mixed powder of MgH2 and CaO, and then perform hydrogenation to obtain medium powder.

[0010] Furthermore, when adding the mixed powder of MgH2 and CaO in the air jet milling step, the specific steps are as follows: mix the medium powder with the mixed powder of MgH2 and CaO, and then perform air jet milling to obtain fine powder.

[0011] Furthermore, when adding the mixed powder of MgH2 and CaO in the orientation pressing step, the specific steps are as follows: mix the fine powder with the mixed powder of MgH2 and CaO, and then perform orientation pressing.

[0012] Furthermore, the raw materials for neodymium iron boron magnets include the following components by mass percentage: Re: 29.5~35%, where Re is one or more of Pr, Nd, Dy, Tb, Gd, Ho, and Y; B: 0.86~1.02%; M, where M is at least one or more of Al, Cu, Ga, Zr, Ti, and Nb, and the mass content of M is greater than 0 and less than or equal to 3.5%; the balance element is Fe, or Fe and Co, wherein Fe accounts for more than 90% of the balance element.

[0013] The vacuum melting, hydrogen breaking, air jet milling, orientation forming, isostatic pressing, vacuum sintering, and aging treatment of this invention can all be performed using conventional processes in the art. Commonly used processes are as follows: In the hydrogen breakdown step, the hydrogen pressure is typically 0.05~0.1 MPa. During the hydrogen absorption reaction, the pressure change inside the reactor should not exceed 0.5% within 10 minutes, indicating the end of the hydrogen absorption process. After the hydrogen absorption reaction, the temperature is raised to 400~600℃ while simultaneously evacuating the vacuum, and held at this temperature for 2~6 hours to remove the hydrogen from the alloy sheet. The sheet is then cooled to obtain intermediate powder. The particle size of the intermediate powder is typically 80-300 μm.

[0014] In the air jet milling step, nitrogen is used as the protective gas, and the powder particle size is controlled by adjusting the separating wheel and cyclone separator of the air jet mill. The average particle size of the fine powder obtained by air jet milling is 2~5μm, preferably 3~4μm.

[0015] The fine powder produced after air jet milling is typically mixed with lubricant and antioxidant, and then molded under an orientation magnetic field. Commercially available magnetic powder lubricant or antioxidant is sufficient. The antioxidant can also be added before air jet milling. The amount of lubricant added can be 0.03~0.2% of the powder mass, and the amount of antioxidant can be 0.03~0.15% of the powder mass.

[0016] The orientation magnetic field is typically 1.8~6T, and the forming pressure is 5~7MPa. The oriented compact can then undergo further cold isostatic pressing at a pressure of 150~220MPa. The density of the oriented compact is 3.6~4.2g / cm³. 3 The density of the compact after cold isostatic pressing is approximately 4.4~4.6 g / cm³. 3 .

[0017] Vacuum sintering process is generally: 10 3 ~10 4 Under a vacuum of Pa, the sintering temperature is 1050–1150℃, and the holding time is 3–24 hours. Then, it is cooled to room temperature.

[0018] Aging treatment is generally carried out at 700~920℃ for the first stage of aging, with a holding time of 2~8 hours, and then at 400~650℃ for the second stage of aging, with a holding time of 2~8 hours.

[0019] The present invention also provides sintered NdFeB magnets with low nitrogen content prepared by the above method.

[0020] In the low-nitrogen sintered NdFeB magnet, the nitrogen content is <300ppm, preferably <150ppm.

[0021] This invention adds a powder mixture of MgH2 and CaO as a nitrogen content regulator for magnets. MgH2 powder provides high activity at low temperatures, while CaO powder covers the high-temperature denitrification blind zone, achieving nitrogen removal across the entire temperature range. MgH2 decomposes without leaving any metal residue, and the CaO reaction products precipitate in other phase forms, without affecting domain wall movement. Furthermore, this invention optimizes the method of adding the mixed powder and related process parameters. The addition steps, amount, component ratios, and particle size of the mixed powder all significantly affect denitrification efficiency; only by achieving synergistic coupling among these process parameters can a high denitrification effect be obtained.

[0022] This invention significantly reduces the nitrogen content in the magnet by introducing small amounts of magnesium and calcium elements without altering the original sintered NdFeB magnet preparation process, thereby effectively improving the magnetic properties of sintered NdFeB magnets. It features simple operation, low cost, and high controllability. Detailed Implementation

[0023] The technical solution of the present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0024] Examples 1-3 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation (PrNd) 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75 (by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm. A mixed powder of MgH2 and CaO was added to the flakes, and the addition amount, particle size and ratio of the two are shown in Table 1. After addition, the mixture was mixed for 3 hours to ensure that the powder and the flakes were in full contact. Hydrogenation was performed to obtain medium powder with a particle size of 80-300 μm. The hydrogen absorption pressure during hydrogenation was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 hours, and the powder was cooled for 3 hours after dehydrogenation. The medium powder after hydrogenation was subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. 0.1% of the alloy mass of antioxidant and 0.15% of lubricant were added before and after air jet milling, respectively, and the mixture was stirred for 3 hours after each addition. The mixed fine powder was then oriented and pressed into shape with a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0025] Examples 4-6 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75(by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with particle sizes of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. 0.1% of the alloy mass of an antioxidant and a mixed powder of MgH2 and CaO were added to the medium powder. The addition amount, particle size, and ratio of the latter two are shown in Table 1. After addition, the mixture was mixed for 3 h. The mixed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. After air jet milling, 0.15% of the alloy mass of a lubricant was added, and the mixture was mixed for 3 h. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0026] Examples 7-9 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75 (by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with particle sizes of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. 0.1% of the alloy mass of an antioxidant was added to the medium powder, and the mixture was stirred for 3 h after the addition. The mixed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. After air jet milling, 0.15% of the alloy mass of a lubricant and a mixed powder of MgH2 and CaO were added, respectively. The addition amount, particle size, and ratio of the latter two are shown in Table 1. The mixture was stirred for 3 h after the addition. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0027] Comparative Example 1 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75 (by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with a particle size of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. The hydrogen-annealed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. 0.1% of an antioxidant and 0.15% of a lubricant by mass of the alloy were added before and after air jet milling, respectively, and the mixtures were mixed for 3 h after each addition. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T, with a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0028] Comparative Examples 2-3 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr0.1 Ti 0.1 Fe 67.75 (by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with a particle size of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. 0.1% of the alloy mass of an antioxidant and one of MgH2 or CaO powder was added to the medium powder. The addition amount, particle size, and ratio of the latter two are shown in Table 1. After addition, the mixture was mixed for 3 h. The mixed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. After air jet milling, 0.15% of the alloy mass of a lubricant was added, and the mixture was mixed for 3 h. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0029] Comparative Examples 4-7 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75(by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with particle sizes of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. 0.1% of the alloy mass of an antioxidant and a mixed powder of MgH2 and CaO were added to the medium powder. The addition amount, particle size, and ratio of the latter two are shown in Table 1. The difference from the example is that the particle size of the powder mixture was changed. After adding the additives, the mixture was mixed for 3 h. The mixed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. After air jet milling, 0.15% of the alloy mass of a lubricant was added, and the mixture was mixed for 3 h. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3 Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0030] Comparative Examples 8-9 The preparation process of neodymium iron boron magnets is as follows: Sintered NdFeB raw material formulation composition PrNd 30 B 0.95 Co 0.5 Al 0.2 Cu 0.2 Ga 0.2 Zr 0.1 Ti 0.1 Fe 67.75 (by mass percentage) The above-mentioned alloy composition was smelted to obtain rapidly solidified flakes with a thickness of 0.25±0.05 mm; hydrogen annealing was performed to obtain medium powder with particle sizes of 80-300 μm. The hydrogen absorption pressure during hydrogen annealing was 0.096 MPa, the dehydrogenation temperature was 560℃, the dehydrogenation time was 3 h, and the powder was cooled for 3 h after dehydrogenation. 0.1% of the alloy mass of an antioxidant and a mixed powder of MgH2 and CaO were added to the medium powder. The addition amount, particle size, and ratio of the latter two are shown in Table 1. The difference from the example is that the addition ratio of the powder mixture was changed. After addition, the mixture was mixed for 3 h. The mixed medium powder was then subjected to air jet milling to obtain fine powder with an average particle size of 3.00±0.05 μm. After air jet milling, 0.15% of the alloy mass of a lubricant was added, and the mixture was mixed for 3 h. The mixed fine powder was then oriented and pressed into shape under a magnetic field strength of 2.0 T and a pressing density of 3.9 g / cm³. 3Then, the compact is subjected to cold isostatic pressing to further eliminate internal defects. The isostatic pressing pressure is 200 MPa, and the density of the compact after isostatic pressing is 4.5 g / cm³. 3 The pressed billets are sintered and aged in a sintering furnace with a vacuum level of 10. -3 Pa, sintering temperature 1090℃, sintering time 8h; first aging temperature 900℃, aging time 3h; second aging temperature 500℃, aging time 4h; after completion, rapidly cool to room temperature to obtain sintered NdFeB magnets.

[0031] Table 1. Addition process of Examples 1-9 and Comparative Examples 1-9 project Add timing Amount added, particle size (wt.%) <![CDATA[MgH2: CaO addition ratio]]> Example 1 Before hydrogen breakdown <![CDATA[150nm / 0.12%MgH2+3μm / 0.04%CaO]]> 3：1 Example 2 Before hydrogen breakdown <![CDATA[150nm / 0.24%MgH2+3μm / 0.08%CaO]]> 3：1 Example 3 Before hydrogen breakdown <![CDATA[150nm / 0.36%MgH2+3μm / 0.12%CaO]]> 3：1 Example 4 Before airflow mill <![CDATA[150nm / 0.12%MgH2+3μm / 0.04%CaO]]> 3：1 Example 5 Before airflow mill <![CDATA[150nm / 0.24%MgH2+3μm / 0.08%CaO]]> 3：1 Example 6 Before airflow mill <![CDATA[150nm / 0.36%MgH2+3μm / 0.12%CaO]]> 3：1 Example 7 Before molding <![CDATA[150nm / 0.12%MgH2+3μm / 0.04%CaO]]> 3：1 Example 8 Before molding <![CDATA[150nm / 0.24%MgH2+3μm / 0.08%CaO]]> 3：1 Example 9 Before molding <![CDATA[150nm / 0.36%MgH2+3μm / 0.12%CaO]]> 3：1 Comparative Example 1 No addition - - Comparative Example 2 Before airflow mill <![CDATA[150nm / 0.32%MgH2]]> - Comparative Example 3 Before airflow mill 3μm / 0.32%CaO - Comparative Example 4 Before airflow mill <![CDATA[20nm / 0.24%MgH2+0.5μm / 0.08%CaO]]> 3：1 Comparative Example 5 Before airflow mill <![CDATA[20nm / 0.24%MgH2+10μm / 0.08%CaO]]> 3：1 Comparative Example 6 Before airflow mill <![CDATA[300nm / 0.24%MgH2+0.5μm / 0.08%CaO]]> 3：1 Comparative Example 7 Before airflow mill <![CDATA[300nm / 0.24%MgH2+10μm / 0.08%CaO]]> 3：1 Comparative Example 8 Before airflow mill <![CDATA[150nm / 0.16%MgH2+3μm / 0.16%CaO]]> 1：1 Comparative Example 9 Before airflow mill <![CDATA[150nm / 0.28%MgH2+3μm / 0.04%CaO]]> 7：1 The sintered NdFeB magnets obtained in the comparative and examples above were subjected to performance tests and staged nitrogen content tests. The testing process is as follows: Magnet blanks from each example and comparative example were taken and machined into cylindrical samples of Ф10*10mm using wire cutting. Magnetic properties were tested using a NIM62000. Each sample was tested three times, and the average value of the test results was taken. Powder or magnet center samples from each example and comparative example after hydrogen annealing, air jet milling, and sintering were taken, and the nitrogen content of the samples at each preparation stage was tested using a nitrogen and oxygen analyzer. The magnet composition was detected using ICP. The test results are shown in Table 2.

[0032] Table 2. Test results of Examples 1-9 and Comparative Examples 1-9 project Nitrogen content of medium-strength flour (ppm) Nitrogen content of fine powder (ppm) Nitrogen content of the magnet (ppm) <![CDATA[B r (kGs)]]> <![CDATA[H cj (no)]]> Mg content in magnet (wt.%) Ca content in magnet (wt.%) Example 1 246 400 287 14.09 14.30 0.003 0.021 Example 2 132 276 116 14.25 15.28 0.005 0.042 Example 3 130 271 112 14.15 14.74 0.008 0.065 Example 4 299 450 268 14.11 14.28 0.004 0.016 Example 5 287 407 78 14.31 15.54 0.005 0.034 Example 6 307 399 85 14.21 15.01 0.009 0.063 Example 7 286 488 299 14.06 14.27 0.004 0.019 Example 8 303 514 144 14.21 15.37 0.006 0.041 Example 9 268 491 108 14.12 14.76 0.009 0.069 Comparative Example 1 305 499 421 14.05 14.01 - - Comparative Example 2 311 457 211 14.11 14.34 0.014 - Comparative Example 3 287 505 359 13.98 14.17 - 0.187 Comparative Example 4 307 460 387 13.96 13.97 0.007 0.041 Comparative Example 5 292 427 366 13.95 13.65 0.008 0.046 Comparative Example 6 314 439 347 14.02 13.84 0.013 0.043 Comparative Example 7 300 491 466 13.83 13.26 0.015 0.047 Comparative Example 8 296 432 217 14.13 13.95 0.008 0.095 Comparative Example 9 279 396 208 13.97 14.42 0.013 0.019 Examples 1-3, 4-6, and 7-9 respectively added MgH2 and CaO powder mixtures with the same particle size and increasing proportions before hydrogen annealing, before air jet milling, and before molding. Compared to Comparative Example 1 without any added substances, the addition of MgH2 and CaO powder mixtures at different preparation stages improved the nitrogen content and magnetic properties of the resulting magnets to varying degrees, demonstrating the universality of the invention; however, the improvement in nitrogen content and magnetic properties of the magnets exhibited a non-linear characteristic. When the addition amounts were 150 nm / 0.24% MgH2 and 3 μm / 0.08% CaO, the nitrogen content and residual Mg / Ca content of the magnets were both at low levels, and the magnetic properties were optimal. For example, in Example 5, the nitrogen content in the magnet was 78 ppm, B r and H cj The optimal values ​​of 14.31 kGs and 15.54 kOe were achieved, respectively. Continuing to increase the amount of additive, although the nitrogen content of the magnet tends to decrease further, the grain boundary impurity phase introduced by the excessive additive will also increase simultaneously, resulting in a slight decrease in magnetic properties compared to when the amount of additive is appropriate.

[0033] In Comparative Examples 2-3, when only MgH2 or CaO powder was added, although the nitrogen content of the magnet showed a decreasing trend, the nitrogen fixation capacity and reduction level of the single component were insufficient, and the denitrification effect in the temperature zone could not be achieved, resulting in low denitrification efficiency and incomplete denitrification, and the magnetic properties were not substantially improved. Comparative Examples 4-7 changed the particle size of the powder mixture. The results showed that the agglomeration effect or the reduction of effective contact surface caused by particle size mismatch was not ideal for the subsequent denitrification effect of the magnet, and also had a deteriorating effect on the magnetic properties. Comparative Examples 8-9 changed the addition ratio of the powder mixture. The results showed that the phase composition change or the increase of impurity ion concentration caused by the imbalance of the addition ratio also had a certain negative impact on the magnetic properties.

Claims

1. A method for preparing a low-nitrogen-content sintered NdFeB magnet, the method comprising the following steps: The process involves vacuum melting of NdFeB magnet raw materials to obtain rapidly solidified flakes, hydrogen-crushing of the flakes to obtain medium powder, air-jet milling of the medium powder to obtain fine powder, orientation pressing and isostatic pressing of the fine powder to obtain a compact, and vacuum sintering and tempering of the compact to obtain the NdFeB magnet. Its characteristic is... In the hydrogen breaking, air jet milling or orientation pressing steps, a mixed powder of MgH2 and CaO is added.

2. The method as described in claim 1, characterized in that... In the MgH2 and CaO mixed powder, the mass ratio of MgH2 to CaO is 2~4:

1.

3. The method as described in claim 1, characterized in that... In the MgH2 and CaO mixed powder, the particle size of MgH2 is 50-250 nm, and the particle size of CaO is 1-5 μm.

4. The method as described in claim 1, characterized in that... The addition ratio of the MgH2 and CaO mixed powder is 0.1~0.5% of the total weight of the NdFeB magnet.

5. The method as described in claim 1, characterized in that... In the hydrogenation process, a mixture of MgH2 and CaO powder is added. The steps are as follows: the quick-setting flakes are mixed with the MgH2 and CaO powder, and then hydrogenation is performed to obtain medium powder.

6. The method as described in claim 1, characterized in that... Adding MgH2 and CaO mixed powder to the air jet milling step involves mixing the medium powder with the MgH2 and CaO mixed powder, and then performing an air jet milling process to obtain fine powder.

7. The method as described in claim 1, characterized in that... Adding a mixture of MgH2 and CaO powder in the orientation pressing step involves mixing the fine powder with the MgH2 and CaO powder, followed by orientation pressing.

8. The method as described in claim 1, characterized in that... The NdFeB magnet raw material comprises the following components by mass percentage: Re: 29.5~35%, where Re is one or more of Pr, Nd, Dy, Tb, Gd, Ho, and Y; B: 0.86~1.02%; M, where M is at least one or more of Al, Cu, Ga, Zr, Ti, and Nb, and the mass content of M is greater than 0 and less than or equal to 3.5%; the balance element is Fe, or Fe and Co, wherein Fe accounts for more than 90% of the balance element.

9. The low-nitrogen-content sintered NdFeB magnet prepared by the method according to any one of claims 1 to 8.

10. The low-nitrogen-content sintered NdFeB magnet as described in claim 9, wherein the nitrogen content is <300ppm.