R-t-b based permanent magnet, method for producing the same, and use thereof

Abstract

The application discloses an R-T-B series permanent magnet and a preparation method and application thereof. The R-T-B series permanent magnet contains X ppm of nitrogen, wherein X satisfies 500<=X<=1300; the average grain size of the R-T-B series permanent magnet is D mu m, wherein D satisfies -2.6lnX+20<=D<=-2.3lnX+19.6; the R-T-B series permanent magnet comprises main phase grains and grain boundary phases; the main phase grains comprise R2Fe 14 B; the grain boundary phases are distributed between the main phase grains; and the volume fraction of R-N enrichment zones in the grain boundary phases is less than 10%. In the R-T-B series permanent magnet, the nitrogen element is uniformly distributed at the grain boundaries, and NdN agglomerates are not formed or the content of the NdN agglomerates is extremely low, so that the coercive force Hcj of the magnet is improved, the high-temperature magnetic loss is reduced, and the corrosion resistance is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to an R-T-B based permanent magnet and a preparation method and application thereof. BACKGROUND

[0002] In the prior art, nitrogen in R-T-B based rare earth permanent magnet material is an inevitable impurity, mainly from three aspects: raw material impurities, process process must add organic additives, nitrogen atmosphere of each process. According to the control ability of nitrogen content, the possible range of nitrogen content in R-T-B based rare earth permanent magnet material is 0.01%-0.15%, and the actual measurement level of products in the field is mostly 0.02%-0.05%. High nitrogen content is easy to form NdN agglomerates, on the one hand, the consumption of rare earth elements causes the decrease of coercivity Hcj, on the other hand, the increase of impurities leads to the decrease of the volume fraction of the main phase, thereby reducing the remanence Br, and the R activity is high, the corrosion resistance of high concentration agglomerates is decreased. In order to improve the performance, the lower the impurity nitrogen is controlled, the better. The prior art CN103875047A reduces the nitrogen content of the magnet by changing the manufacturing process environment atmosphere and using He, Ar and other inert gases instead of N2 atmosphere. But this method has high manufacturing cost and is not suitable for mass production. SUMMARY

[0003] The technical problem to be solved by the present application is to overcome the defects in the prior art that the R-T-B based permanent magnet forms NdN agglomerates at the grain boundary due to high nitrogen content, resulting in the decrease of Hcj of the magnet, and to provide an R-T-B based permanent magnet and a preparation method and application thereof. The R-T-B based permanent magnet of the present application can allow a higher nitrogen content, without intentionally reducing the nitrogen content, overcoming the bias of the prior art. The nitrogen element in the R-T-B based permanent magnet described in the present application is uniformly distributed at the grain boundary, without forming NdN agglomerates or with very low content, thereby improving the coercivity Hcj of the magnet, reducing the high temperature magnetic loss, and improving the corrosion resistance.

[0004] The present application solves the above technical problems by the following technical solutions.

[0005] The present application provides an R-T-B based permanent magnet, the nitrogen content in the R-T-B based permanent magnet is Xppm, and the X satisfies 500≤X≤1300;

[0006] The average grain size of the R-T-B based permanent magnet is Dμm, and the D satisfies -2.6lnX+20≤D≤-2.3lnX+19.6;

[0007] The R-T-B based permanent magnet comprises main phase grains and grain boundary phases; the main phase grains comprise R2Fe 14 B; the grain boundary phases are distributed between the main phase grains; the R-N rich region in the grain boundary phase accounts for less than 10% of the volume fraction of the grain boundary phase.

[0008] In the present application, the volume fraction of the R-N rich region in the grain boundary phase can be calculated by FE-EPMA equipment combined with conventional image processing software in the art (e.g. Image J).

[0009] In the present application, the X preferably satisfies 550≤X≤1200, more preferably 600≤X≤1000.

[0010] In the present application, the D preferably satisfies 1.6≤D≤5.0, more preferably 2.1≤D≤4.8.

[0011] In a preferred embodiment of the present application, the X is 557 and the D is 4.1.

[0012] In a preferred embodiment of the present application, the X is 685 and the D is 3.5.

[0013] In a preferred embodiment of the present application, the X is 812 and the D is 3.2.

[0014] In a preferred embodiment of the present application, the X is 956 and the D is 3.3.

[0015] In a preferred embodiment of the present application, the X is 1105 and the D is 3.

[0016] In the present application, the volume fraction of the R-N rich region in the grain boundary phase is preferably less than 5%, more preferably less than 3%, for example 1% or 2%.

[0017] The present application also provides a preparation method of the R-T-B based permanent magnet, comprising the following steps:

[0018] S1, smelting a raw material composition of the R-T-B based permanent magnet to obtain a molten liquid;

[0019] S2, casting the molten liquid to obtain an alloy sheet;

[0020] S3, hydrogen breaking the alloy sheet to obtain a coarse powder;

[0021] S4, airflow milling the coarse powder to obtain a fine powder;

[0022] S5, shaping the fine powder to obtain a shaped body;

[0023] S6, sintering the shaped body to obtain a sintered body;

[0024] S7, heat treating the sintered body to obtain the R-T-B based permanent magnet.

[0025] In the present application, preferably, the raw material composition of the R-T-B based permanent magnet comprises the following components:

[0026] a light rare earth element RL, RL comprising Nd: 24-30 wt%, Pr: 0 wt%≤Pr≤8 wt%;

[0027] a heavy rare earth element RH, RH comprising: Dy and / or Tb, 0 wt%≤RH≤0.9 wt%;

[0028] Co: 0-1.5 wt%;

[0029] Al: 0.03-0.3 wt%;

[0030] X: 0-0.6 wt%, X being one or more of Zr and Ti;

[0031] Cu: 0.1-0.4 wt%;

[0032] Ga: 0.1-0.4 wt%;

[0033] B: 0.92-0.98 wt%;

[0034] the balance being Fe;

[0035] wherein wt% represents the mass percentage of the R-T-B based permanent magnet, and the total of each component is 100 wt%.

[0036] In step S1, the melting can be prepared according to the conventional method in the art, for example: melting in a high-frequency vacuum induction melting furnace, i.e.

[0037] wherein the vacuum degree of the high-frequency vacuum induction melting furnace can be 5×10 -2 Pa.

[0038] wherein the temperature of the melting can be 1500°C or lower, for example 1480°C.

[0039] wherein the melting is generally carried out in an alumina crucible. The alumina crucible introduces a part of Al into the R-T-B based permanent magnet.

[0040] In step S2, the casting process can be the conventional casting process in the art, for example, including: casting in an Ar atmosphere in a medium-frequency vacuum induction rapid solidification belt casting furnace, quenching to obtain an alloy sheet.

[0041] wherein the pressure of the Ar atmosphere is preferably 5.5×10 4 Pa.

[0042] wherein the cooling speed of the quenching is preferably 10 2 ℃ / second-10 4°C / sec.

[0043] In step S3, the hydrogen decrepitation is performed in a hydrogen decrepitation furnace. The hydrogen decrepitation process can be a conventional hydrogen decrepitation process in the art, for example, including hydrogen absorption, hydrogen desorption, and cooling.

[0044] The hydrogen absorption can be performed under a hydrogen pressure of 0.05-0.25 MPa, for example, 0.15 MPa.

[0045] The hydrogen desorption can be performed under a condition of vacuumizing and heating.

[0046] The cooling can be performed under a nitrogen atmosphere or an argon atmosphere.

[0047] When the cooling is performed under a nitrogen atmosphere, the nitrogen atmosphere can be achieved by introducing nitrogen. The number of times of introducing nitrogen is preferably 1-5, and the pressure of introducing nitrogen is preferably 0.08 MPa.

[0048] In step S4, the jet milling is performed in a jet mill.

[0049] The jet milling can be performed under a nitrogen atmosphere with an oxidizing gas content of 150 ppm or less. The oxidizing gas refers to oxygen and / or moisture.

[0050] The rotation speed of the jet milling is preferably 3500-6000 rpm.

[0051] The nozzle pressure of the jet milling is preferably 0.35-0.45 MPa, for example, 0.38 MPa.

[0052] The time of the jet milling can be 3 hours.

[0053] The particle size D50 of the fine powder obtained after the jet milling is preferably 2.5-5.0 μm.

[0054] Preferably, after the jet milling, a step of uniformly mixing the fine powder with a lubricant is further included; or, before the jet milling, a step of uniformly mixing the coarse powder with a lubricant is further included. The lubricant is, for example, zinc stearate and benzotriazole; the addition amount of the lubricant can be 0.10-0.15% of the weight of the mixed powder, for example, 0.12%. The mixing is preferably performed by using a V-type mixer to mix sufficiently.

[0055] In step S5, the molding process can be a conventional molding process in the art, for example, a magnetic field orientation molding method. The preferred operation of the magnetic field orientation molding method is as follows: using a magnetic field molding machine of a right-angle orientation type, in an orientation magnetic field of 1.6 T, at a pressure of 0.35 ton / cm 2molding pressure, the powder is once-molded, and then demagnetized in a magnetic field of 0.2 T; the once-molded body is sealed, and then subjected to secondary molding using an isostatic press at a pressure of 1.3 ton / cm 2 .

[0056] In step S6, the sintering is performed in a sintering furnace. The sintering process can be a conventional sintering process in the art, for example, including preheating, sintering, and cooling under vacuum.

[0057] The vacuum condition is, for example, 5 x 10 -3 Pa.

[0058] The preheating temperature can be 300 to 600 °C. The preheating time can be 1 to 2 h. Preferably, the preheating is performed at 300 °C and 600 °C for 1 h each.

[0059] The sintering temperature can be a conventional sintering temperature in the art, for example, 1040 to 1090 °C.

[0060] The sintering time can be a conventional sintering time in the art, for example, 4 h.

[0061] The cooling can be performed under a nitrogen atmosphere or an argon atmosphere.

[0062] When the cooling is performed under a nitrogen atmosphere, the nitrogen atmosphere can be achieved by introducing nitrogen. The number of times of introducing nitrogen is preferably 1 to 5, and the pressure of the introduced nitrogen is preferably 0.05 to 0.1 MPa, for example, 0.1 MPa.

[0063] The heat treatment temperature in step S7 is preferably 430 to 600 °C, for example, 500 °C.

[0064] Preferably, the heat treatment is performed under a vacuum of 9 x 10 -3 Pa.

[0065] The heat treatment time can be 3 h.

[0066] The heat treatment further includes a cooling step, and the cooling can be performed under a nitrogen atmosphere or an argon atmosphere.

[0067] In certain preferred embodiments, the cooling in step S3, the cooling in step S6, and the cooling in step S7 are performed under a nitrogen atmosphere in at most one of the steps.

[0068] In a preferred embodiment of the present application, the cooling in step S3 is performed under an argon atmosphere, the cooling in step S6 is performed under an argon atmosphere, and the cooling in step S7 is performed under an argon atmosphere.

[0069] In a preferred embodiment of the application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under argon atmosphere, and the cooling in step S7 is performed under argon atmosphere.

[0070] In a preferred embodiment of the application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, and the cooling in step S7 is performed under argon atmosphere.

[0071] In a preferred embodiment of the application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, and the cooling in step S7 is performed under nitrogen atmosphere.

[0072] In a preferred embodiment of the application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, and the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 4500-5500 rpm, and the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa.

[0073] In a preferred embodiment of the application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under argon atmosphere, and the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 5000-6000 rpm, and the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa.

[0074] In a preferred embodiment of the application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, and the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 5000-6000 rpm, and the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa.

[0075] In a preferred embodiment of the application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, and the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500-6000 rpm, and the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa.

[0076] In a further preferred embodiment of the present application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 4500-5500 rpm, the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa, and the lubricant is added in an amount of 0.25-0.35% by weight of the mixed powder.

[0077] In a further preferred embodiment of the present application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under argon atmosphere, the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 5000-6000 rpm, the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa, and the lubricant is added in an amount of 0.10-0.20% by weight of the mixed powder.

[0078] In a further preferred embodiment of the present application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 5000-6000 rpm, the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa, and the lubricant is added in an amount of 0.10-0.20% by weight of the mixed powder.

[0079] In a further preferred embodiment of the present application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500-6000 rpm, the nozzle pressure of the jet mill in step S4 is 0.35-0.45 MPa, and the lubricant is added in an amount of 0.10-0.20% by weight of the mixed powder.

[0080] In a specific embodiment of the present application, the cooling in step S3 is performed under argon atmosphere, the cooling in step S6 is performed under argon atmosphere, the cooling in step S7 is performed under argon atmosphere, the rotation speed of the jet mill in step S4 is 5000 rpm, the nozzle pressure of the jet mill in step S4 is 0.38 MPa, and the lubricant is added in an amount of 0.30% by weight of the mixed powder.

[0081] In a specific embodiment of the present application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500 rpm, the nozzle pressure of the jet mill in step S4 is 0.38 MPa, and the lubricant is added in an amount of 0.12% of the weight of the mixed powder.

[0082] In a specific embodiment of the present application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500 rpm, the nozzle pressure of the jet mill in step S4 is 0.38 MPa, and the lubricant is added in an amount of 0.12% of the weight of the mixed powder.

[0083] In a specific embodiment of the present application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500 rpm, the nozzle pressure of the jet mill in step S4 is 0.38 MPa, and the lubricant is added in an amount of 0.12% of the weight of the mixed powder.

[0084] In a specific embodiment of the present application, the cooling in step S3 is performed under nitrogen atmosphere, the cooling in step S6 is performed under nitrogen atmosphere, the cooling in step S7 is performed under nitrogen atmosphere, the rotation speed of the jet mill in step S4 is 5500 rpm, the nozzle pressure of the jet mill in step S4 is 0.38 MPa, and the lubricant is added in an amount of 0.12% of the weight of the mixed powder.

[0085] In the present application, preferably, after step S6 and before step S7, the preparation method further comprises a step of grain boundary diffusion.

[0086] In the present application, the grain boundary diffusion treatment can be performed according to conventional processes in the art, for example, by evaporating, coating or sputtering a diffusion source material on the surface of the sintered body obtained after sintering, and then performing a diffusion heat treatment.

[0087] Preferably, the diffusion source material contains heavy rare earth elements. The diffusion source material containing heavy rare earth elements can be a heavy rare earth element metal, or a compound or alloy containing heavy rare earth elements. The heavy rare earth elements preferably include Tb and / or Dy.

[0088] The diffusion heat treatment can be performed at a temperature of 800-950°C, for example, 920°C.

[0089] The diffusion heat treatment can be performed for a time of 12-48h, for example, 24h.

[0090] The application also provides an application of the R-T-B based permanent magnet as an electronic component in an electric machine.

[0091] The above-mentioned preferred conditions can be combined arbitrarily to obtain preferred examples of the application.

[0092] The reagents and raw materials used in the application are commercially available.

[0093] The positive progress effect of the application is that:

[0094] By controlling the relationship between the average grain size and nitrogen content of the R-T-B based permanent magnet, the application ensures the uniform distribution of nitrogen element, avoids or minimizes the formation of R-N rich region, the volume fraction of R-N rich region in the grain boundary phase is less than 10%, even less than 2%, thereby improving the coercivity Hcj (Hcj≥22.6kOe, even as high as 28.5kOe), reducing the high-temperature magnetic loss (the thermal demagnetization at 120°C is not more than 0.5%, the thermal demagnetization at 150°C is not more than 1.0%), and improving the corrosion resistance (in the HAST test, the weight change value ΔM before and after the test is not more than 0.57mg / cm 2 ). BRIEF DESCRIPTION OF DRAWINGS

[0095] Figure 1 The Nd and N distribution map of the R-T-B based permanent magnet prepared in Example 3 is formed by FE-EPMA surface scanning.

[0096] Figure 2 The Nd and N distribution map of the R-T-B based permanent magnet prepared in Comparative Example 3 is formed by FE-EPMA surface scanning, wherein point 1 is the R-N rich region. DETAILED DESCRIPTION

[0097] The application will be further described by way of examples, but the application is not limited in the scope of the examples. The experimental methods in the following examples without specific conditions are selected according to conventional methods and conditions, or according to the instructions of the commodity.

[0098] The preparation method of the R-T-B based permanent magnet is as follows:

[0099] Examples 1, 2, 4, 5 and Comparative Examples 1, 2 and 4

[0100] S1, melting process: The prepared raw material composition was put into an alumina crucible according to the formulation of Table 1, and vacuum melting was performed in a high frequency vacuum melting furnace at 5 x 10 -2 Pa under vacuum and at 1480°C.

[0101] S2, In an intermediate frequency vacuum induction rapid solidification and tape casting furnace, Ar was introduced at a pressure of 5.5 x 10 4 Pa, casting was performed, and quenching was performed at a speed of 10 2 °C / sec to 10 4 °C / sec to obtain an alloy sheet.

[0102] S3, hydrogen decrepitation process: A hydrogen decrepitation furnace in which the alloy sheet was placed was evacuated at room temperature, and then hydrogen gas having a purity of 99.9% was introduced into the hydrogen decrepitation furnace, and the pressure of the hydrogen gas was maintained at 0.15 MPa. After sufficient hydrogen absorption, the hydrogen was removed by evacuation while increasing the temperature, and sufficient dehydrogenation was performed. Thereafter, argon or nitrogen was introduced to cool the hydrogen decrepitation powder, and the hydrogen decrepitation powder was taken out after being crushed.

[0103] S4, jet milling process: The hydrogen decrepitation powder was jet milled for 3 h in a nitrogen atmosphere having an oxygen gas or moisture content of 150 ppm or less under conditions of a certain rotational speed of a jet milling air classifier wheel and a certain nozzle pressure of jet milling pulverization to obtain a fine powder having a D50 of 2.5 to 5.0 μm. A certain amount of a lubricant, zinc stearate and benzotriazole, was added to the fine powder, and the mixture was sufficiently mixed using a V-type mixer.

[0104] S5, magnetic field forming process: A right-angle orientation type magnetic field forming machine was used to form the mixed powder once in an orientation magnetic field of 1.6 T under a forming pressure of 0.35 ton / cm 2 . After the first forming, demagnetization was performed in a magnetic field of 0.2 T. In order to prevent the first formed body from contacting air, the first formed body was sealed, and a second forming was performed using an isostatic pressing machine under a pressure of 1.3 ton / cm 2 .

[0105] S6, sintering process: The formed bodies were moved to a sintering furnace to perform sintering. The sintering was performed under vacuum at 5 x 10 -3 Pa, and the temperature was maintained at 300°C and 600°C for 1 h each, and then sintering was performed at 1040°C for 2 h. Thereafter, argon or nitrogen was introduced to increase the pressure to 0.1 MPa, and then cooling was performed to room temperature.

[0106] S7, heat treatment process: The sintered body was subjected to tempering treatment at a heat treatment temperature of 500°C for 3 h under vacuum at 9 x 10 -3 Pa. Argon or nitrogen was introduced to cool the sintered body to room temperature, and then the sintered body was taken out to obtain an R-T-B based permanent magnet.

[0107] Example 3 and Comparative Example 3

[0108] A grain boundary diffusion step is also included between steps S6 and S7:

[0109] Each group of sintered bodies was processed into a magnet with a length x width of 50 x 39 mm and a thickness of 4.5 mm, with the thickness direction being the magnetic field orientation direction. After surface cleaning, the magnet was coated with a raw material prepared using Tb fluoride, and the coated magnet was dried and diffusion heat treated at a temperature of 920°C for 24 h in a high-purity argon atmosphere, and then cooled to room temperature.

[0110] The raw material formulations and preparation process parameters (lubricant addition amount, dehydrogenation gas used, air flow mill air selection wheel speed, air flow mill pulverization nozzle pressure, sintering cooling gas used, and heat treatment cooling gas used) of the R-T-B based permanent magnets in Examples 1 to 5 and Comparative Examples 1 to 4 are shown in Tables 1 and 2.

[0111] Table 1 Composition and content (wt%) of raw material composition of R-T-B based permanent magnets

[0112] Number TRE Nd Pr Dy Al Cu Ga Co Ti Zr B Fe A 32.2 24 8 0.2 0.3 0.4 0.4 1.5 0.2 / 0.92 bal. B 31.0 27.9 3.1 / 0.2 0.3 0.3 0.5 / 0.4 0.96 bal. C 30.0 30 / / 0.03 0.1 0.1 / 0.12 / 0.98 bal.

[0113] Table 2 Formulation composition and process parameters of Examples 1 to 5 and Comparative Examples 1 to 4

[0114]

[0115] Effect Example 1

[0116] The R-T-B based permanent magnets obtained in Example 3 and Comparative Example 3 were characterized by FE-EPMA equipment, and the results are shown in Figure 1 and Figure 2

[0117] Different colors represent the concentration of each element, as shown by the scale on the right. The darker the color, the lower the concentration; the lighter the color, the higher the concentration.

[0118] As can be seen from Figure 1 , the R-T-B based permanent magnet prepared in Example 3 has a more uniform distribution of Nd and N in the grain boundary phase, with less Nd-N enrichment. As can be seen from Figure 2 , the R-T-B based permanent magnet prepared in Comparative Example 3 has Nd and N gathered in the same area, forming a large amount of Nd-N enrichment area in the grain boundary phase.

[0119] Effect Example 2

[0120] (1) N content test: The surface of the R-T-B based permanent magnets prepared in Examples 1 to 5 and Comparative Examples 1 to 4 was ground to remove the oxide layer, and then a nitrogen content test was performed using an oxygen-nitrogen analyzer (Horiba, EMGA-620W). ​

[0121] (2) Average grain size D test: The vertical orientation surface of the R-T-B based permanent magnet prepared in Examples 1-5 and Comparative Examples 1-4 was polished, etched with 5% nitric acid solution, and then photographed at 1000 times magnification using a metallographic microscope. Three straight lines of length L were drawn on the upper, middle and lower parts of the picture, the number of grains n on each line segment was measured, the grain size = L / n, and the average grain size was the average of the three lines.

[0122] (3) Volume fraction of R-N rich region in grain boundary phase: The vertical orientation surface of the R-T-B based permanent magnet prepared in Examples 1-5 and Comparative Examples 1-4 was polished, and then face scanning was performed using a field emission electron probe microanalyzer FE-EPMA (JEOL, 8530F), and then proportion statistics was performed by using a commonly used image processing software (such as Image J).

[0123] (4) Magnetic property test: The magnetic properties (remanence Br, coercivity Hcj) of the R-T-B based permanent magnet prepared in Examples 1-5 and Comparative Examples 1-4 were detected by using NIM-2000 type permanent magnet material precision measurement system of China Institute of Metrology. The test temperature was 20°C.

[0124] (5) Thermal demagnetization test: The R-T-B based permanent magnet prepared in Examples 1-5 and Comparative Examples 1-4 was processed into 50mm x 39mm x 4.5mm, 4.5mm being the orientation direction, under the condition of half open circuit of 2mm thick iron plate, the permanent magnet was heated to 120°C or 150°C, and then cooled to room temperature after keeping in the oven for 2h. The magnetic flux before and after heating was tested by using a fluxmeter, and the thermal demagnetization = (magnetic flux before heating - magnetic flux after heating) / magnetic flux before heating x 100%.

[0125] (6) Corrosion resistance HAST test: The R-T-B based permanent magnet prepared in Examples and Comparative Examples was processed into 50mm x 39mm x 4.5mm, 4.5mm being the orientation direction, in a high pressure accelerated aging tester, the humidity was set to 95%, the temperature was set to 130°C, and the test was kept for 240h. The weights of the samples before and after the test were measured. The corrosion resistance was measured by the weight change value per unit area (AM), AM = (weight before test - weight after test) / sample surface area.

[0126] The above test results are shown in Table 3.

[0127] As can be seen from Table 3, by changing the amount of lubricant added, the type of dehydrogenation gas used, the rotational speed of the air classifier wheel of the jet mill, the nozzle pressure of the jet mill, the type of gas used for sintering cooling, and the type of gas used for heat treatment cooling, the N content and grain size of the prepared R-T-B based permanent magnet can be changed, thereby affecting the volume fraction of the R-N rich region in the grain boundary phase, the magnetic properties and the corrosion resistance of the permanent magnet.

[0128] From the effect data of Examples 1, 2, 4, 5 and Comparative Examples 1, 2, 4, it can be seen that the volume fraction of R-N rich zone in the grain boundary phase of the four examples is only not more than 2%, while that of Comparative Examples 1, 2 and 4 is as high as 27%, 46% and 58% respectively; for the residual magnetization Br, Example 2 is as high as 13.83 kGs, while that of the comparative examples is only 13.73 kGs at most; for the coercive force Hcj, the four examples are all more than 22 kOe, while the comparative examples are not more than 20.2 kOe; for the thermal demagnetization at 120°C, the four examples are not more than 0.4%, while Comparative Examples 1, 2 and 4 are as high as 4.5%, 4.8% and 4.3% respectively; for the corrosion resistance, the weight change per unit area of the four examples is not more than 0.57 mg / cm 2 , while that of the comparative examples is more than 1.9 mg / cm 2 .

[0129] The R-T-B based permanent magnet prepared by the grain boundary diffusion of Example 3 and Comparative Example 3 also has the same increasing and decreasing trend of the magnet parameters as the above-mentioned R-T-B based permanent magnet which has not been subjected to grain boundary diffusion. Specifically: the volume fraction of R-N rich zone in the grain boundary phase of Example 3 is only 1%, while that of Comparative Example 3 is as high as 38%; for the residual magnetization Br and the coercive force Hcj, Comparative Example 3 is reduced compared with Example 3; for the thermal demagnetization at 150°C, Example 3 is 0.90%, while Comparative Example 3 is as high as 4.2%; for the corrosion resistance, the weight change per unit area of Example 3 is 0.564 mg / cm 2 , while that of Comparative Example 3 is 1.815 mg / cm 2 .

[0130] Table 3 Comparison of the magnet parameters of Examples 1-5 and Comparative Examples 1-4

[0131]

[0132] The present application adjusts the N content and the grain size of the R-T-B based permanent magnet by the above-mentioned process conditions, thereby reducing the volume fraction of R-N rich zone in the grain boundary phase, improving the residual magnetization Br and the coercive force Hcj, reducing the weight change per unit area under the conditions of thermal demagnetization and damp heat, and having a positive influence on the magnetic properties and the corrosion resistance.

Claims

1. An R-T-B based permanent magnet, characterized by, The nitrogen content in the R-T-B based permanent magnet is X ppm, and the X satisfies 956≤X≤1300; The average grain size of the R-T-B based permanent magnet is D μm, and the D satisfies -2.6lnX+20≤D≤-2.3lnX+19.6; The R-T-B based permanent magnet comprises main phase grains and grain boundary phases; the main phase grains comprise R2Fe 14 B; the grain boundary phases are distributed between the main phase grains; a R-N rich region in the grain boundary phases accounts for less than 10% of the volume fraction of the grain boundary phases. The R-T-B based permanent magnet is prepared by the following steps: S1, smelting a raw material composition of the R-T-B based permanent magnet to obtain a molten liquid; S2, casting the molten liquid to obtain an alloy sheet; S3, hydrogen breaking the alloy sheet to obtain a coarse powder; S4, airflow milling the coarse powder to obtain a fine powder; S5, forming the fine powder to obtain a formed body; S6, sintering the formed body to obtain a sintered body; S7, heat treating the sintered body to obtain the R-T-B based permanent magnet; wherein, In step S3, the hydrogen breaking process includes hydrogen absorption, hydrogen desorption and cooling; in step S4, after the airflow milling, the fine powder is uniformly mixed with a lubricant, or before the airflow milling, the coarse powder is uniformly mixed with a lubricant; in step S6, the sintering process includes preheating, sintering and cooling under vacuum; and in step S7, the heat treatment further includes a cooling step; and the following conditions are met: In step S3, the cooling is performed in an argon atmosphere; in step S6, the cooling is performed in a nitrogen atmosphere; and in step S7, the cooling is performed in an argon atmosphere; in step S4, the airflow milling is performed at a speed of 5000-6000 rpm, and the nozzle pressure is 0.35-0.45 MPa; and in step S4, the lubricant is added in an amount of 0.10-0.20% of the weight of the mixed powder; or, in step S3, the cooling is performed in an argon atmosphere; in step S6, the cooling is performed in an argon atmosphere; and in step S7, the cooling is performed in a nitrogen atmosphere; in step S4, the airflow milling is performed at a speed of 5500-6000 rpm, and the nozzle pressure is 0.35-0.45 MPa; and in step S4, the lubricant is added in an amount of 0.10-0.20% of the weight of the mixed powder.

2. The R-T-B based permanent magnet according to claim 1, wherein The volume fraction of the R-N rich region in the grain boundary phase is less than 5%.

3. The R-T-B based permanent magnet according to claim 2, wherein The volume fraction of the R-N rich region in the grain boundary phase is less than 3%.

4. The R-T-B based permanent magnet according to claim 3, wherein The volume fraction of the R-N rich region in the grain boundary phase is 1% or 2%.

5. The R-T-B based permanent magnet according to claim 2, wherein The X is 956, and the D is 3.3; or, the X is 1105, and the D is 3.

6. The R-T-B based permanent magnet of claim 1, wherein In step S1, the smelting is performed in a high-frequency vacuum induction smelting furnace; and / or, in step S2, the casting process includes casting in an Ar atmosphere in a medium-frequency vacuum induction rapid solidification spinning furnace to obtain an alloy sheet; In step S5, the forming process is a magnetic field orientation forming method.

7. The R-T-B based permanent magnet of claim 6, wherein In step S1, the vacuum degree of the high-frequency vacuum induction melting furnace is 5 x 10 -2 Pa; And / or, in step S1, the temperature of the melting is 1500℃ or less; And / or, in step S1, the melting is performed in an alumina crucible; And / or, in step S2, the pressure of the Ar atmosphere is 5.5 x 10 4 Pa; And / or, in step S2, the cooling rate of the quenching is 10 2 °C / sec ~ 10 4 °C / sec; And / or, in step S5, the operation of the magnetic field orientation molding method is as follows: using a right-angle orientation type magnetic field molding machine, a powder is once molded in an orientation magnetic field of 1.6 T under a molding pressure of 0.35 ton / cm 2 , and then demagnetized in a magnetic field of 0.2 T; the once molded body is sealed, and a second molding is performed using an isostatic press molding machine under a pressure of 1.3 ton / cm 2 .

8. The R-T-B based permanent magnet according to claim 7, wherein In step S1, the temperature of the melting is 1480℃.

9. The R-T-B based permanent magnet according to claim 1, wherein In step S3, the hydrogen absorption is performed under a hydrogen pressure of 0.05 to 0.25 MPa. And / or, in step S3, the hydrogen desorption is performed while vacuumizing and heating. And / or, in step S4, the jet mill pulverization is performed in a nitrogen atmosphere having an oxygen gas content of 150 ppm or less. And / or, in step S4, the particle size D50 of the fine powder obtained after the jet mill pulverization is 2.5 to 5.0 μm. And / or, in step S4, the lubricant is zinc stearate and benzotriazole. And / or, in step S4, the mixing is performed by using a V-type mixer to mix sufficiently. And / or, in step S6, the vacuum condition is 5 x 10 -3 Pa; And / or, in step S6, the preheating temperature is 300 to 600℃. And / or, in step S6, the preheating time is 1 to 2 hours. And / or, in step S6, the sintering temperature is 1040 to 1090℃. And / or, in step S7, the heat treatment temperature is 430 to 600℃. And / or, in step S7, the heat treatment is performed under vacuum at 9 x 10 -3 Pa.

10. The R-T-B based permanent magnet according to claim 9, wherein In step S3, the hydrogen absorption is performed under a hydrogen pressure of 0.15 MPa. And / or, in step S4, the oxidizing gas is oxygen and / or moisture. And / or, in step S6, the preheating is performed at 300℃ and 600℃ respectively for 1 hour. And / or, in step S6, when the cooling is performed in a nitrogen atmosphere, the nitrogen atmosphere is achieved by introducing nitrogen. And / or, in step S7, the heat treatment temperature is 500℃.

11. The R-T-B based permanent magnet according to claim 10, wherein In step S6, in the cooling, the number of times of introducing nitrogen is 1 to 5. And / or, in step S6, in the cooling, the pressure of the introduced nitrogen is 0.05 to 0.1 MPa.

12. The R-T-B based permanent magnet according to claim 11, wherein In step S6, in the cooling, the pressure of the introduced nitrogen is 0.1 MPa.

13. The R-T-B based permanent magnet according to claim 1, wherein After step S6 and before step S7, a step of grain boundary diffusion is further included.

14. The R-T-B based permanent magnet according to claim 13, wherein The step of grain boundary diffusion is to vaporize, coat or sputter a diffusion source raw material on the surface of the sintered body, and then perform a diffusion heat treatment.

15. The R-T-B based permanent magnet according to claim 14, wherein The diffusion source raw material is a substance containing heavy rare earth elements; And / or, the diffusion heat treatment temperature is 800 to 950℃. And / or, the diffusion heat treatment time is 12 to 48 hours.

16. The R-T-B based permanent magnet according to claim 15, wherein The substance containing heavy rare earth elements is a heavy rare earth element metal, or a compound or alloy containing heavy rare earth elements. And / or, the heavy rare earth elements include Tb and / or Dy. And / or, the diffusion heat treatment temperature is 920℃. And / or, the diffusion heat treatment time is 24 hours.

17. The R-T-B based permanent magnet according to claim 1, wherein The raw material composition of the R-T-B based permanent magnet includes the following components: light rare earth element RL, RL comprising Nd: 24-30 wt%, Pr: 0 wt%≤ Pr≤ 8 wt%; heavy rare earth element RH, RH comprising Dy and / or Tb: 0 wt%≤ RH≤ 0.9 wt%; Co: 0-1.5 wt%; Al: 0.03-0.3 wt%; X: 0-0.6 wt%, X being one or more of Zr and Ti; Cu: 0.1-0.4 wt%; Ga: 0.1-0.4 wt%; B: 0.92-0.98 wt%; the balance being Fe; wherein wt% means mass percentage of the R-T-B based permanent magnet, and the sum of each component is 100 wt%.

18. Use of the R-T-B based permanent magnet according to any one of claims 1-17 as an electronic component in an electric machine.

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