Neodymium-iron-boron sintered magnet with high corrosion resistance and excellent magnetic properties and method for producing the same

Abstract

The present invention provides a NdFeB based sintered magnet having high corrosion resistance and excellent magnetic properties, and a manufacturing method thereof, the NdFeB based sintered magnet containing 1.5 wt% to 3.0 wt% of element Co, the NdFeB based sintered magnet containing a main phase and a grain boundary phase, the Co element being distributed in the grain boundary phase of crystal grains and the main phase of crystal grains, the NdFeB based sintered magnet satisfying w1≧w2, where w1 is the content of the Co element distributed in the grain boundary phase of crystal grains, and w2 is the content of the Co element distributed in the main phase of crystal grains. By concentrating the Co element in the grain boundary phase, the Co element can form a Nd3Co phase together with the Nd-rich phase. The formation of the Nd3Co phase improves the electrode potential of the entire grain boundary phase, reduces the degree of electrochemical corrosion, greatly reduces the weight loss of the magnet, and significantly improves the corrosion resistance of the NdFeB magnet as well as the magnetic properties of the NdFeB magnet, thereby achieving both the corrosion resistance and magnetic properties of the NdFeB magnet.

Description

[Technical Field]

[0001] This application claims priority from a prior application, bearing patent application number 2022110983441 and entitled "Neodymium-iron-boron-based sintered magnet with high corrosion resistance and excellent magnetic properties and manufacturing method thereof," filed with the State Intellectual Property Office of the People's Republic of China on September 8, 2022. The entire text of this prior application is incorporated herein by reference.

[0002] The present invention relates to the technical field of NdFeB sintered magnets, and more particularly to a neodymium-iron-boron sintered magnet having high corrosion resistance and excellent magnetic properties, and a method for producing the same. [Background technology]

[0003] Since its discovery, NdFeB material has been widely used in fields such as communications, medicine, automobiles, electronics, and aviation due to its excellent magnetic properties and high cost-effectiveness, making it an ideal material for manufacturing magnetic functional materials with high efficiency, small volume, and light weight, which has had a revolutionary impact on many application fields. However, NdFeB material has poor temperature stability, and at the same time, because its main component contains a relatively high content of rare earth elements, NdFeB material has relatively poor corrosion resistance. These two issues have significantly limited the expansion of its application range.

[0004] In conventional technology, the element Co is typically added to NdFeB materials to improve their corrosion resistance. The greater the amount of Co added, the better the corrosion resistance of the NdFeB material. However, if the amount of Co added exceeds 3 wt%, the magnetic properties of the NdFeB material are affected. Therefore, ultimately, it becomes impossible to improve the corrosion resistance of NdFeB materials by adding Co while maintaining their high magnetic properties. Summary of the Invention

[0005] After extensive research into the above-mentioned problem, the inventors of the present invention found that after adding Co to a NdFeB magnet, most of the Co is distributed in the main phase of the crystal grains, while a small portion is distributed in the grain boundary phase of the crystal grains. The Co distributed in the grain boundary phase of the crystal grains plays a major role in improving the corrosion resistance and magnetic properties of the NdFeB magnet. However, as the Co content in the grain boundary phase increases with continued Co addition, a phenomenon has emerged in which the magnetic properties of the NdFeB magnet deteriorate. Through analysis, the inventors believe that this phenomenon occurs because the continuous addition of Co increases the Co content in the grain boundary phase of the crystal grains, further increasing the Co content in the main phase of the crystal grains, thereby deteriorating the magnetic properties of the NdFeB magnet.

[0006] To overcome the deficiencies of the prior art, the present invention provides a neodymium-iron-boron based sintered magnet with high corrosion resistance and excellent magnetic properties, and a manufacturing method thereof, wherein the neodymium-iron-boron based sintered magnet contains 1.5 wt% to 3.0 wt% of the element Co, which is distributed in the grain boundary phase of the crystal grains and the main phase of the crystal grains, and the content of the element Co distributed in the grain boundary phase of the crystal grains is equal to or greater than the content of the element Co distributed in the main phase of the crystal grains. Since the element Co is mainly concentrated in the grain boundary phase of the neodymium-iron-boron based sintered magnet, the corrosion resistance and magnetic properties of the neodymium-iron-boron based sintered magnet can be significantly improved.

[0007] The technical solution of the present invention is as follows: A neodymium-iron-boron based sintered magnet, the neodymium-iron-boron based sintered magnet containing 1.5 wt% to 3.0 wt% of the element Co, the neodymium-iron-boron based sintered magnet including a main phase and a grain boundary phase, the element Co being distributed in the grain boundary phase of the crystal grains and the main phase of the crystal grains, the neodymium-iron-boron based sintered magnet satisfying w1≧w2, where w1 is the content of the element Co distributed in the grain boundary phase of the crystal grains and w2 is the content of the element Co distributed in the main phase of the crystal grains.

[0008] In the present invention, unless otherwise specified, the above contents all refer to contents in mass percentage.

[0009] According to an embodiment of the present invention, the content w1 of the element Co distributed in the grain boundary phase of the crystal grains is greater than the average content of the element Co in a neodymium-iron-boron-based sintered magnet. The content w2 of the element Co distributed in the main phase of the crystal grains is less than the average content of the element Co in a neodymium-iron-boron-based sintered magnet.

[0010] According to an embodiment of the present invention, the neodymium-iron-boron-based sintered magnet satisfies w1≧1.5×w2, where w1 is the content of the element Co distributed in the grain boundary phase of the crystal grains, and w2 is the content of the element Co distributed in the main phase of the crystal grains.

[0011] According to an embodiment of the present invention, the distribution of the element Co in the main phase and grain boundary phase at any position (for example, the surface and center) of the neodymium-iron-boron sintered magnet satisfies w1≧w2, and preferably satisfies w1≧1.5×w2.

[0012] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the Co-enriched regions and the heavy rare earth-enriched regions of the grain boundary phase incompletely overlap each other.

[0013] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the Co-enriched regions of the grain boundary phase are Fe-deficient regions.

[0014] According to an embodiment of the present invention, in the neodymium-iron-boron-based sintered magnet, the grain boundary phase includes a phase consisting of RFeCoM, where R is a rare earth element, Fe is iron, Co is cobalt, and M is one or more of Ga, Cu, Al, Zr, and Ti.

[0015] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the content of the R element in the grain boundary phase is equal to or greater than the content of the R element in the main phase.

[0016] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the content of the M element in the grain boundary phase is equal to or greater than the content of the M element in the main phase.

[0017] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the Fe element content in the grain boundary phase is equal to or less than the Fe element content in the main phase.

[0018] According to an embodiment of the present invention, the neodymium iron boron based sintered magnet contains 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, 2.6 wt%, 2.7 wt%, 2.8 wt%, 2.9 wt%, or 3.0 wt% of the element Co.

[0019] According to an embodiment of the present invention, the neodymium iron boron based sintered magnet has the chemical formula RFeCoBM, in which R is a rare earth element, Fe is iron, B is boron, and M is one or more of Ga, Cu, Al, Zr, and Ti, in which the R content is 26 wt% to 35 wt%, the B content is 0.8 wt% to 1.3 wt%, the Co content is 1.5 wt% to 3.0 wt%, the Ga content is 0.05 wt% to 0.5 wt%, the Cu content is 0.05 wt% to 0.6 wt%, the Al content is 0 wt% to 1.5 wt%, the Zr content is 0 wt% to 0.5 wt%, the Ti content is 0 wt% to 0.5 wt%, and the remainder is iron and unavoidable impurities.

[0020] According to an embodiment of the present invention, R is a rare earth element, such as at least one of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), yttrium (Y), and scandium (Sc).

[0021] According to an embodiment of the present invention, in the neodymium-iron-boron sintered magnet, the C element content is ≦800 ppm, the O element content is ≦800 ppm, and the N element content is ≦800 ppm.

[0022] According to an embodiment of the present invention, the crystal grain size of the neodymium-iron-boron sintered magnet is ≦8 μm.

[0023] The present invention further provides a method for producing the above-mentioned neodymium-iron-boron-based sintered magnet, 1) obtaining R-Fe-Co-BM alloy fine powder by stripping-hydrogen crushing; 2) jet milling the R-Fe-Co-BM alloy fine powder of step 1) to obtain a magnetic powder, and then pressing the magnetic powder into a powder compact; 3) sintering the green compact of step 2), optionally with or without aging treatment, to obtain a sintered magnet; 4) processing the sintered magnet of step 3) into a required size, disposing a diffusion source on the surface of the processed product, and then heating the product under vacuum conditions. When the heating temperature reaches the diffusion treatment temperature, fill the product with inert gas, keep it warm, and then cool it to room temperature. 5) performing an aging treatment on the magnet after the diffusion treatment in step 4) to produce and obtain the neodymium-iron-boron sintered magnet.

[0024] According to an embodiment of the present invention, in step 1), the R content is 26 wt% to 35 wt%, the B content is 0.8 wt% to 1.3 wt%, the Co content is 1.5 wt% to 3.0 wt%, and M is one or more of Ga, Cu, Al, Zr, and Ti, wherein the Ga content is 0.05 wt% to 0.5 wt%, the Cu content is 0.05 wt% to 0.6 wt%, the Al content is 0 wt% to 1.5 wt%, the Zr content is 0 wt% to 0.5 wt%, the Ti content is 0 wt% to 0.5 wt%, and the remainder is iron and unavoidable impurities.

[0025] According to an embodiment of the present invention, in step 1), R is a rare earth element, and exemplarily, R is at least one of lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu), yttrium (Y), and scandium (Sc).

[0026] According to an embodiment of the present invention, in step (2), the magnetic powder has an average particle size of 2 μm to 5 μm.

[0027] According to an embodiment of the present invention, in step 2), the jet milled magnetic powder is mixed with a lubricant that accounts for 0.1 wt% to 0.5 wt% of the total mass of the jet milled magnetic powder for 0.1 to 3 hours, and then the magnetic powder is pressed into a green compact in an orienting field environment at 20°C to 40°C and with a magnetic field strength of 1 T to 5 T.

[0028] According to an embodiment of the present invention, in step 3), the green compact of step 2) is sintered to obtain a sintered magnet, or the green compact of step 2) is first sintered and then aged to obtain a sintered magnet.

[0029] According to an embodiment of the present invention, in step 3), the sintered material is cooled to room temperature and then subjected to an aging treatment.

[0030] According to an embodiment of the present invention, in step 3), the sintering temperature is 900°C to 1100°C (e.g., 900°C, 950°C, 1000°C, 1050°C, or 1100°C), and the sintering time is 2 hours to 10 hours (e.g., 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, or 10 hours). The sintering is performed under vacuum conditions, and the degree of vacuum is ≦20 Pa.

[0031] According to an embodiment of the present invention, in step 3), the aging treatment is a secondary aging treatment including a first-stage aging treatment and a second-stage aging treatment, in which the temperature of the first-stage aging treatment is 700 to 950°C (for example, 700°C, 750°C, 800°C, 850°C, 900°C, or 950°C), the time of the first-stage aging treatment is 1 to 12 h (for example, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, or 12 h), the temperature of the second-stage aging treatment is 400 to 600°C (for example, 400°C, 450°C, 500°C, 550°C, or 600°C), and the time of the second-stage aging treatment is 1 to 12 h (for example, 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h, 10 h, 11 h, or 12 h). h).

[0032] According to an embodiment of the present invention, in step 3), the raw material after the first stage aging treatment is cooled to room temperature, and then subjected to the second stage aging treatment; According to an embodiment of the present invention, in step 4), the diffusion source is a rare earth element source, exemplarily at least one of a Dy source (e.g., metallic Dy), a Tb source (e.g., metallic Tb), a Ho source (e.g., metallic Ho), a Nd source (e.g., metallic Nd), and a Pr source (e.g., metallic Pr).

[0033] According to an embodiment of the present invention, in step 4), the mass of the diffusion source is 0.2% to 1.5% of the total mass of the sintered magnet, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, or 1.5%.

[0034] According to an embodiment of the present invention, in step 4), the disposing method may be at least one of a thermal spraying method, a coating method, a sputtering method, a dipping method, and the like.

[0035] According to an embodiment of the present invention, in step 4), the vacuum condition has a vacuum level of ≦20 Pa, for example, 1 Pa to 20 Pa, for example, 1 Pa, 2 Pa, 5 Pa, 8 Pa, 10 Pa, 12 Pa, 15 Pa, 18 Pa, or 20 Pa.

[0036] According to an embodiment of the present invention, in step 4), the temperature of the diffusion treatment is 800 to 1000°C, for example, 800°C, 850°C, 900°C, 950°C, or 1000°C.

[0037] According to an embodiment of the present invention, in step 4), when the heating temperature has risen to the temperature of the diffusion treatment, an inert gas of 5 kPa or more is filled, preferably an inert gas of 10 kPa to 30 kPa is filled, for example, an inert gas of 10 kPa, 15 kPa, 20 kPa, 25 kPa, or 30 kPa is filled.

[0038] According to an embodiment of the present invention, in step 4), the inert gas atmosphere is, for example, at least one of argon gas, helium gas, and nitrogen gas.

[0039] According to an embodiment of the present invention, in step 4), the incubation time is 3 hours or more, for example, 3 to 24 hours, for example, 3 hours, 6 hours, 8 hours, 9 hours, 10 hours, 12 hours, 15 hours, 18 hours, 20 hours, 22 hours, or 24 hours.

[0040] According to an embodiment of the present invention, step 4) further includes the step of treating the processed sintered magnet by washing, degreasing, and pickling.

[0041] According to an embodiment of the present invention, in step 5), the temperature of the aging treatment is 400 to 600°C (e.g., 400°C, 450°C, 500°C, 550°C, or 600°C), and the time of the aging treatment is 1 to 12 hours (e.g., 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours, 10 hours, 11 hours, or 12 hours).

[0042] According to an embodiment of the present invention, the method includes the following steps: a) Obtain R-Fe-Co-BM alloy fine powder by stripping-hydrogen crushing, where R is a rare earth element, and the R content is 26 wt%-35 wt%, the B content is 0.8 wt%-1.3 wt%, the Co content is 1.5 wt%-3.0 wt%, and M is one or more of Ga, Cu, Al, Zr, and Ti, where Ga content is 0.05 wt%-0.5 wt%, Cu content is 0.05 wt%-0.6 wt%, Al content is 0 wt%-1.5 wt%, Zr content is 0 wt%-0.5 wt%, Ti content is 0 wt%-0.5 wt%, and the remainder is iron and unavoidable impurities.

[0043] b) The R-Fe-Co-BM alloy fine powder from step a) is jet milled to obtain a magnetic powder, with an average particle size of 2-5 μm. A lubricant is added to the jet milled magnetic powder, accounting for 0.1 wt% to 0.5 wt% of the total mass of the jet milled magnetic powder. The materials are then mixed for 0.1 to 3 hours, and the magnetic powder is then pressed into a green compact.

[0044] c) Sintering the powder compact of step b) under vacuum conditions, the sintering temperature is 900-1100°C, the sintering time is 2-10°C, and the sintering time is 2-10 hours, to obtain a sintered magnet.

[0045] d) The sintered magnet of step c) is subjected to a second aging treatment, in which the temperature of the first aging treatment is 700 to 950°C and the temperature of the second aging treatment is 400 to 600°C.

[0046] e) The magnet from step c) or step d) is processed to the required size, and the processed product is treated by cleaning, degreasing, and pickling. A diffusion source, which is a rare earth element source, is disposed on the surface of the treated product, which may be disposed by thermal spraying, coating, sputtering, or immersion. The product is then heated under vacuum conditions, and when the heating temperature reaches the diffusion treatment temperature, an inert gas is introduced. After keeping the temperature, the product is cooled to room temperature. The diffusion treatment temperature is 800-1000°C, and the keeping time is 3 hours or more.

[0047] f) The magnet after the diffusion treatment in step e) is subjected to an aging treatment, and the aging temperature is 400 to 600°C. Beneficial effects of the present invention: The present invention provides a neodymium-iron-boron based sintered magnet with high corrosion resistance and excellent magnetic properties, and a manufacturing method thereof. The neodymium-iron-boron based sintered magnet contains the element Co, and includes a main phase and a grain boundary phase, with the element Co distributed in the grain boundary phase and the main phase of the crystal grains. The neodymium-iron-boron based sintered magnet satisfies w1≧w2, where w1 is the amount of Co distributed in the grain boundary phase and w2 is the amount of Co distributed in the main phase of the crystal grains.

[0048] NdFeB magnets primarily consist of a main phase and a grain boundary phase, and the potential difference between the two phases causes electrochemical corrosion. Because the grain boundary phase is a Nd-rich phase, its electrode potential is even lower, allowing electrochemical corrosion to occur preferentially. The inventors of the present invention surprisingly discovered that by concentrating Co in the grain boundary phase, the Co can form an Nd-rich phase and an Nd3Co phase. The formation of the Nd3Co phase improves the electrode potential of the entire grain boundary phase, reducing the degree of electrochemical corrosion and significantly reducing the weight loss of the magnet. This significantly improves the corrosion resistance and magnetic properties of the NdFeB magnet, achieving both excellent corrosion resistance and magnetic properties.

[0049] In the manufacturing method of the present invention, a sintered magnet is obtained by smelting, milling, press molding, and sintering processes. After processing the sintered magnet to the required size, a diffusion source is placed on the surface. The sintered magnet is then heated under vacuum conditions. Once the heating temperature has risen to the diffusion treatment temperature, an inert gas is filled in and the magnet is kept at that temperature, after which it is cooled to room temperature. This produces a neodymium iron boron sintered magnet in which the content of the element Co in the grain boundary phase of the crystal grains is equal to or greater than the content of the element Co distributed in the main phase of the crystal grains. The inventors speculate that this is because, during the diffusion process under specific conditions, the diffusion source can enter the grain boundary phase, causing element migration between the diffusing element that has entered the grain boundary phase and the element Co in the main phase, resulting in the element migration of the element Co in the main phase to the grain boundary phase. [Brief explanation of the drawings]

[0050] [Figure 1] 1 is an element distribution diagram after scanning a neodymium-iron-boron sintered magnet in Comparative Example 1 using an EPMA. [Figure 2] 2 is an element distribution map of the neodymium-iron-boron sintered magnet after scanning it using an EPMA in Example 1. FIG. [Figure 3] 1 is a graph showing the change in the content of the element Co after scanning a neodymium-iron-boron sintered magnet using an EPMA in Comparative Example 1. FIG. [Figure 4] FIG. 2 is a graph showing the change in the content of the element Co after scanning the neodymium-iron-boron sintered magnet using an EPMA in Example 1. [Figure 5] FIG. 1 is a structural diagram of a neodymium-iron-boron sintered magnet according to the present invention. [Figure 6] 1 is a flowchart showing the production of a neodymium-iron-boron sintered magnet according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0051] The present invention will be described in more detail below with reference to specific examples. It should be understood that the following examples are merely illustrative of the present invention and should not be construed as limiting the scope of the claims of the present invention. Any technology realized based on the above content of the present invention is included in the scope of the claims of the present invention.

[0052] Unless otherwise specified, the experimental methods used in the following examples are all conventional methods, and the reagents, materials, etc. used in the following examples are all commercially available unless otherwise specified. Example 1 (1) NdPr, Dy, Co, Al, Fe, Cu, Ga, Ti, and ferroboron of at least 99% purity by weight were high-frequency melted in an argon gas atmosphere, and the melt was then quenched by a chill roll to produce an alloy with a mass percentage of 30.5% NdPr, 0.5% Dy, 1.5% Co, 0.1% Al, 0.2% Cu, 0.2% Ga, 0.18% Ti, and 0.98% B, with the remainder being iron and unavoidable impurities. The alloy was hydrogenated and pulverized into a coarse powder. The coarse powder was then jet-milled to produce a magnetic powder with a particle size D50 of 4.0 μm. A lubricant accounting for 0.3 wt% of the total mass of the jet-milled powder was added to the jet-milled powder, and the materials were mixed for 2 h and then pressed into a green compact at room temperature in an orienting field with a magnetic field strength of 2 T. The billet was then placed in a vacuum sintering furnace and sintered at 1070°C for 6 hours. After the temperature was maintained, it was cooled to room temperature, then heated to 900°C, and subjected to a first-stage aging treatment at 900°C for 3 hours. After the temperature was maintained, it was cooled to room temperature, then heated to 520°C, and subjected to a second-stage aging treatment at 520°C for 5 hours. After cooling and removal from the furnace, a neodymium-iron-boron magnet was obtained.

[0053] (2) The magnets were processed into rectangular sheets measuring 25-15-5 mm. The rectangular sheets were degreased and pickled, then subjected to a Tb diffusion treatment under vacuum (<20 Pa) at a diffusion temperature of 900°C. Argon gas was introduced at 15 kPa at the temperature of 900°C and the magnets were maintained at this temperature for 3 hours. After the temperature reached 900°C, the magnets were cooled to room temperature and then heated to 480°C. The magnets were then aged at 480°C for 3 hours. The amount of Tb diffusion was 0.6 wt% of the total mass of the 25-15-5 mm rectangular sheet magnets. The diffusion source (metallic Tb) in this example was deposited using a thermal spraying method. The resulting product was designated A1. Magnetic property and weight loss measurements were performed on the magnets. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 Bar, and treatment time 4 days. Comparative Example 1 The other steps were the same as in Example 1, except that 1.0% Co was added in step (1). The product after the diffusion treatment in step (2) was called B1.

[0054] [Table 1]

[0055] As can be seen from Table 1, Example 1 has the same performance as Comparative Example 1, but the weight loss of the product of Example 1 is smaller and the overall performance of the product is also better.

[0056] Furthermore, Product A1 of Example 1 and Product B1 of Comparative Example 1 were machined into 10-10-5 mm products, with the 5 mm extending in the magnetization direction. After grinding and grinding 0.05 mm along the magnetization direction, a 10-10 mm wide surface was scanned using an EPMA. The structure and composition of the main phase and grain boundary phase were observed by scanning. Figure 1 shows the element distribution map of the NdFeB-based sintered magnet of Comparative Example 1 after scanning with an EPMA. Figure 2 shows the element distribution map of the NdFeB-based sintered magnet of Example 1 after scanning with an EPMA. Figure 3 shows the change in the Co content of the NdFeB-based sintered magnet of Comparative Example 1 after scanning with an EPMA. Figure 4 shows the change in the Co content of the NdFeB-based sintered magnet of Example 1 after scanning with an EPMA.

[0057] 1 and 3 show that the element Co is distributed in the grain boundary phase of the crystal grains and the main phase of the crystal grains, and that the content of the element Co distributed in the grain boundary phase of the crystal grains is lower than the content of the element Co distributed in the main phase of the crystal grains. On the other hand, from Fig. 2 and 4, it can be seen that the element Co is distributed in the grain boundary phase of the crystal grains and the main phase of the crystal grains, but that the content of the element Co distributed in the grain boundary phase of the crystal grains is higher than the content of the element Co distributed in the main phase of the crystal grains. It can also be seen that the content of the element Co in the grain boundary phase is approximately twice the content of the element Co in the main phase of the crystal grains.

[0058] [Table 2]

[0059] In summary, a comparison between Example 1 and Comparative Example 1 shows that if the content of the element Co is not in the range of 1.5 wt% to 3.0 wt%, it is impossible to produce a NdFeB-based sintered magnet having the specific grain boundary structure of the present invention; and that the corrosion resistance of a NdFeB-based sintered magnet can only be ensured by ensuring sufficient distribution of the element Co in the grain boundary phase of the crystal grains of the NdFeB-based sintered magnet. Example 2 (1) NdPr, Co, Al, Fe, Cu, Ga, Ti, and ferroboron of at least 99% purity by weight were induction-melted in an argon gas atmosphere, and the melt was then quenched by a chill roll to produce an alloy with a mass percentage of 30.8% NdPr, 1.5% Co, 0.2% Al, 0.15% Cu, 0.15% Ga, 0.2% Ti, and 0.98% B, with the remainder being iron and unavoidable impurities. The alloy was hydrogenated and pulverized into a coarse powder. The coarse powder was then jet-milled to obtain a magnetic powder with a particle size D50 of 3.7 μm. A lubricant accounting for 0.4 wt% of the total mass of the jet-milled powder was added to the jet-milled powder. The materials were mixed for 2 h and then pressed into a green compact at room temperature in an orienting field environment with a magnetic field strength of 2 T. The billet was then placed in a vacuum sintering furnace and sintered at 1065°C for 6 h. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 800°C, and subjected to a first-stage aging treatment at 800°C for 3 hours. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 500°C, and subjected to a second-stage aging treatment at 500°C for 5 hours. After cooling and removal from the furnace, a neodymium-iron-boron magnet was obtained.

[0060] (2) The magnets were processed into rectangular sheets measuring 25-15-2.5 mm. The rectangular sheets were degreased and pickled, then subjected to Dy diffusion treatment under vacuum (<20 Pa) at a diffusion temperature of 880°C. Argon gas was introduced at 20 kPa at the temperature of 880°C and the magnets were maintained at this temperature for 3 hours. After the temperature reached 880°C, the magnets were cooled to room temperature and then heated to 500°C. The magnets were then aged at 500°C for 3 hours. The amount of Dy diffusion was 0.8 wt% of the total mass of the 25-15-2.5 mm rectangular sheet magnets. The diffusion source (metallic Dy) in this example was applied using a coating method. The resulting product was designated A2. Magnetic property and weight loss measurements were performed on the magnets. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 Bar, and treatment time 4 days. Comparative Example 2 The other steps were the same as in Example 2, except that 3.5% Co was added in step (1). The product after the diffusion treatment in step (2) was called B2.

[0061] [Table 3]

[0062] As can be seen from Table 3, compared to Comparative Example 2, Example 2 has the same weight loss, but its magnetic properties are even better. This is mainly because the magnets manufactured using the methods of Example 2 and Comparative Example 2 can ensure sufficient distribution of Co in the grain boundary phase, thereby improving the corrosion resistance of the product. However, in Comparative Example 2, the amount of Co added is too high, which affects the magnetic properties of the magnet and reduces the overall performance of the product. Example 3 (1) Nd, Co, Al, Fe, Cu, Ga, Zr, and ferroboron with a purity of at least 99% by weight were induction-melted in an argon gas atmosphere, and the melt was then quenched by a chill roll to produce an alloy with a mass percentage of 31.5% Nd, 1.8% Co, 0.15% Al, 0.10% Cu, 0.15% Ga, 0.15% Zr, and 0.98% B, with the remainder being iron and unavoidable impurities. The alloy was hydrogenated and pulverized into a coarse powder. The coarse powder was then jet-milled to obtain a magnetic powder with a particle size D50 of 4.8 μm. A lubricant accounting for 0.2 wt% of the total mass of the jet-milled powder was added to the jet-milled powder. The materials were mixed for 2 h and then pressed into a green compact at room temperature in an orienting field environment with a magnetic field strength of 2 T. The billet was then placed in a vacuum sintering furnace and sintered at 1085°C for 6 h. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 900°C, and subjected to a first-stage aging treatment at 900°C for 3 hours. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 480°C, and subjected to a second-stage aging treatment at 480°C for 5 hours. After cooling and removal from the furnace, a neodymium-iron-boron magnet was obtained.

[0063] (2) The magnet was processed into a 40-10-4 mm square sheet. The square sheet was degreased and pickled, then subjected to a Tb diffusion treatment under vacuum (<20 Pa). The diffusion temperature was 920°C. When the temperature reached 920°C, argon gas was introduced at 25 kPa and the magnet was held at that temperature for 3 hours. After the temperature was reached, the magnet was cooled to room temperature, then heated to 510°C and aged at 510°C for 3 hours. The amount of Tb diffusion was 0.8 wt% of the total mass of the 40-10-4 mm square sheet magnet. The diffusion source (metallic Tb) in this example was applied using a coating method. The resulting product was designated A3. Magnetic property and weight loss measurements were performed on the magnet. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 Bar, and treatment time 4 days. Comparative Example 3 The other steps were the same as in Example 3, with the only difference being that magnetic property measurements and weight loss measurements were performed on the neodymium iron boron magnet B3 obtained after cooling and removing from the furnace in step (1) (i.e., without diffusion treatment).

[0064] [Table 4]

[0065] As can be seen from Table 4, Example 3 has better corrosion resistance and magnetic properties than Comparative Example 3. Although the same amount of Co was added during smelting in both Example 3 and Comparative Example 3, Comparative Example 3, which did not undergo the method of the present invention, was unable to ensure that Co was sufficiently distributed in the grain boundary phase, which affected the corrosion resistance of the product. The magnet of Example 3 underwent an additional diffusion treatment, further improving the magnetic properties of the magnet. Example 4-1 (1) Nd, Co, Al, Fe, Cu, Ga, Ti, and ferroboron with a purity of at least 99% by weight were induction-melted in an argon gas atmosphere, and the melt was then quenched by a chill roll to produce an alloy with a mass percentage of 32% Nd, 1.5% Co, 0.35% Al, 0.15% Cu, 0.10% Ga, 0.15% Ti, and 0.98% B, with the remainder being iron and unavoidable impurities. The alloy was hydrogenated and pulverized into a coarse powder. The coarse powder was then jet-milled to obtain a magnetic powder with a particle size D50 of 4.5 μm. A lubricant accounting for 0.4 wt% of the total mass of the jet-milled powder was added to the jet-milled powder. The materials were mixed for 2 h and then pressed into a green compact at room temperature in an orienting field with a magnetic field strength of 2 T. The billet was then placed in a vacuum sintering furnace and sintered at 1080°C for 6 h. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 850°C, and subjected to a first-stage aging treatment at 850°C for 3 hours. After the temperature was maintained, the magnet was cooled to room temperature, then heated to 510°C, and subjected to a second-stage aging treatment at 510°C for 5 hours. After cooling and removal from the furnace, a neodymium-iron-boron magnet was obtained.

[0066] (2) The magnet was processed into a rectangular sheet measuring 38-23-6 mm. The rectangular sheet was degreased and pickled, then subjected to a Dy diffusion treatment under vacuum (<20 Pa) at a diffusion temperature of 890°C. At the temperature of 890°C, argon gas was introduced at 30 kPa and the magnet was maintained at this temperature for 3 hours. After the temperature reached 890°C, the magnet was cooled to room temperature and then heated to 550°C. The aging treatment was performed at 550°C for 3 hours. The amount of Dy diffusion was 0.7 wt% of the total mass of the 38-23-6 mm rectangular sheet magnet. The diffusion source (metallic Dy) in this example was placed using the immersion method. The resulting product was designated A4-1. Magnetic property and weight loss measurements were performed on the magnet. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 Bar, and treatment time 4 days. Example 4-2 The other steps are the same as in Example 4-1, except for step (2): (2) The magnets were processed into rectangular sheets measuring 38-23-6 mm. The rectangular sheets were degreased and pickled, then subjected to Dy diffusion treatment under vacuum (<20 Pa) at a diffusion temperature of 890°C. Argon gas was introduced at 5 kPa when the temperature reached 890°C and the magnets were maintained at this temperature for 3 hours. After the temperature reached 890°C, the magnets were cooled to room temperature and then heated to 550°C. The magnets were then aged at 550°C for 3 hours. The amount of Dy diffusion was 0.7 wt% of the total mass of the 38-23-6 mm rectangular sheet magnets. The diffusion source (metallic Dy) in this example was placed using the immersion method. The resulting product was designated A4-2. Magnetic property and weight loss measurements were performed on the magnets. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 Bar, and treatment time 4 days. Comparative Example 4 The other steps are the same as in Example 4-1, except for step (2): (2) The magnet was processed into a rectangular sheet measuring 38-23-6 mm. The rectangular sheet was degreased and pickled, then subjected to a Dy diffusion treatment under vacuum (<20 Pa). The diffusion temperature was 890°C. Once the temperature reached 890°C, the magnet was held for 3 hours. After cooling, the magnet was then aged at 550°C for 3 hours. The amount of Dy diffusion was 0.7 wt% of the total mass of the 38-23-6 mm rectangular sheet magnet. The diffusion source (metallic Dy) in this comparative example was placed using the immersion method. The product after the diffusion treatment was designated B4. Magnetic property and weight loss measurements were performed on the product. The weight loss conditions were a temperature of 121°C, humidity of 100%, pressure of 2.0 bar, and treatment time of 4 days.

[0067] [Table 5]

[0068] Product A4-1 of Example 4-1, Product A4-2 of Example 4-2, and Product B4 of Comparative Example 4 were machined into 10-10-6 mm products, with the magnetization direction being 6 mm. After grinding and polishing the products by 0.2 mm along the magnetization direction, a 10-10 mm wide surface was scanned using an EPMA. The structure and composition of the main phase and grain boundary phase were observed by scanning, and the measurement results are shown in Table 6.

[0069] [Table 6]

[0070] Tables 5 and 6 show that, compared with Example 4-2, both NdFeB sintered magnets from Example 4-1 satisfied w1 ≥ w2, thereby improving the corrosion resistance of the product. However, Example 4-1 exhibited even better magnetic properties and corrosion resistance. This is primarily because the magnets produced using the method of Example 4-1 were able to ensure sufficient distribution of the element Co in the grain boundary phase, resulting in NdFeB sintered magnets that satisfied w1 ≥ 1.5 × w2, effectively improving the corrosion resistance of the product. At the same time, the heavy rare earth diffusion process was ensured, further improving the magnetic properties of the product. Furthermore, while the magnets produced using the method of Example 4-2 also had distribution of the element Co in the grain boundary phase, the method of Example 4-2 was unable to ensure sufficient distribution of the element Co in the grain boundary phase (i.e., was unable to satisfy w1 ≥ 1.5 × w2), resulting in an insufficient improvement in the corrosion resistance of the product. In Comparative Example 4, a direct diffusion method was adopted without filling argon gas, and therefore a product in which the grain boundary phase was enriched with element Co was not obtained.

[0071] Furthermore, a comparison of Example 4-1, Example 4-2, and Comparative Example 4 revealed that when argon was filled during the diffusion process, a product with Co enriched in the grain boundary phase was obtained and w1 ≥ w2 was satisfied; and when argon gas of ≥ 10 kPa was filled, a product with Co enriched in the grain boundary phase was obtained and w1 ≥ 1.5 × w2 was satisfied. Research showed that magnets that satisfied w1 ≥ 1.5 × w2 had even better magnetic properties and even better corrosion resistance. Comparative Example 5-1 (1) NdPr, Dy, Co, Al, Fe, Cu, Ga, Ti, and ferroboron of at least 99% purity by weight were induction-melted in an argon gas atmosphere, and the melt was then quenched through a chill roll to produce an alloy with a mass percentage of 29% NdPr, 2.5% Dy, 1.0% Co, 0.2% Al, 0.20% Cu, 0.15% Ga, 0.18% Ti, and 0.96% B, with the remainder being iron and unavoidable impurities. The alloy was hydrogenated and pulverized into a coarse powder. The coarse powder was then jet-milled to produce a magnetic powder with a particle size D50 of 4.2 μm. A lubricant accounting for 0.2 wt% of the total mass of the jet-milled powder was added to the jet-milled powder, and the materials were mixed for 2 h and then pressed into a compact at room temperature in an orienting field with a magnetic field strength of 2 T. The billet was then placed in a vacuum sintering furnace and sintered at 1060°C for 6 hours. After the temperature was maintained, it was cooled to room temperature, then heated to 900°C, and subjected to a first-stage aging treatment at 900°C for 3 hours. After the temperature was maintained, it was cooled to room temperature, then heated to 500°C, and subjected to a second-stage aging treatment at 500°C for 5 hours. After cooling and removal from the furnace, a neodymium-iron-boron magnet was obtained.

[0072] (2) The magnet was processed into a rectangular sheet measuring 41-18-1.8 mm. The rectangular sheet was degreased and pickled, then subjected to a Tb diffusion treatment under vacuum (<20 Pa) to a diffusion temperature of 940°C. Argon gas was introduced at 10 kPa when the temperature reached 940°C and the magnet was maintained at that temperature for 3 hours. After the temperature reached 940°C, the magnet was cooled to room temperature and then heated to 490°C. The aging treatment was performed at 490°C for 3 hours. The amount of Tb diffusion was 1.0 wt% of the total mass of the 41-18-1.8 mm rectangular sheet magnet. The diffusion source (metallic Tb) in this example was deposited using a sputtering method. The resulting product was designated B5-1. Magnetic property and weight loss measurements were performed on the magnet. The weight loss conditions were a temperature of 121°C, humidity of 100%, pressure of 2.0 Bar, and a treatment time of 4 days. Comparative Example 5-2 The other steps are the same as those in Comparative Example 5-1, except for step (2): (2) The magnet was processed into a rectangular sheet measuring 41-18-1.8 mm. The rectangular sheet was degreased and pickled, then subjected to a Tb diffusion treatment under vacuum (<20 Pa). The diffusion temperature was 940°C. Once the temperature reached 940°C, the magnet was held at that temperature for 3 hours. After the temperature was reached, the magnet was cooled to room temperature, then heated to 490°C, and aged at 490°C for 3 hours. The Tb diffusion amount was 1.0 wt% of the total mass of the 41-18-1.8 mm rectangular sheet magnet. The diffusion source (metallic Tb) in this example was deposited using a sputtering method. The resulting product was designated B5-2. Magnetic property and weight loss measurements were performed on the magnet. The weight loss was measured under the following conditions: temperature 121°C, humidity 100%, pressure 2.0 bar, and treatment time 4 days.

[0073] [Table 7]

[0074] As can be seen from Table 7, the corrosion resistance of Comparative Example 5-1 is poorer than that of Comparative Example 5-2. This is mainly because the amount of Co added during smelting is relatively small, which does not ensure sufficient distribution of Co in the grain boundary phase, thereby affecting the corrosion resistance of the product. In addition, the method of Comparative Example 5-2 further reduces the distribution of Co in the grain boundary phase, which further deteriorates the corrosion resistance of the product.

[0075] Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of the claims of the present invention.

Claims

1. A neodymium-iron-boron based sintered magnet, the neodymium-iron-boron based sintered magnet containing 1.5 wt% to 3.0 wt% of the element Co, the neodymium-iron-boron based sintered magnet including a main phase and a grain boundary phase, the element Co being distributed in the grain boundary phase of the crystal grains and the main phase of the crystal grains, the neodymium-iron-boron based sintered magnet satisfying w1≧w2, where w1 is the content of the element Co distributed in the grain boundary phase of the crystal grains and w2 is the content of the element Co distributed in the main phase of the crystal grains, In the neodymium-iron-boron-based sintered magnet, the Co-enriched region and the heavy rare earth-enriched region of the grain boundary phase incompletely overlap each other. Neodymium-iron-boron sintered magnet.

2. The neodymium-iron-boron-based sintered magnet satisfies w1≧1.5×w2, where w1 is the content of the element Co distributed in the grain boundary phase of the crystal grains, and w2 is the content of the element Co distributed in the main phase of the crystal grains.

2. The neodymium-iron-boron sintered magnet according to claim 1.

3. In the neodymium-iron-boron sintered magnet, the Co-enriched regions of the grain boundary phase are Fe-deficient regions, and / or, in the neodymium-iron-boron-based sintered magnet, the grain boundary phase includes a phase consisting of RFeCoM, where R is a rare earth element, Fe is iron, Co is cobalt, and M is one or more of Ga, Cu, Al, Zr, and Ti; and / or, in the neodymium-iron-boron-based sintered magnet, the content of the R element in the grain boundary phase is equal to or greater than the content of the R element in the main phase; and / or, in the neodymium-iron-boron-based sintered magnet, the content of the M element in the grain boundary phase is equal to or greater than the content of the M element in the main phase; and / or, in the neodymium-iron-boron sintered magnet, the content of Fe element in the grain boundary phase is less than or equal to the content of Fe element in the main phase.

2. The neodymium-iron-boron sintered magnet according to claim 1.

4. The chemical formula of the neodymium iron boron based sintered magnet is RFeCoBM, in which R is a rare earth element, Fe is iron, B is boron, and M is one or more of Ga, Cu, Al, Zr, and Ti, in which the R content is 26 wt% to 35 wt%, the B content is 0.8 wt% to 1.3 wt%, the Co content is 1.5 wt% to 3.0 wt%, the Ga content is 0.05 wt% to 0.5 wt%, the Cu content is 0.05 wt% to 0.6 wt%, the Al content is 0 wt% to 1.5 wt%, the Zr content is 0 wt% to 0.5 wt%, the Ti content is 0 wt% to 0.5 wt%, and the remainder is iron and unavoidable impurities.

2. The neodymium-iron-boron sintered magnet according to claim 1.

5. 1) Obtaining R-Fe-Co-BM alloy fine powder by stripping-hydrogen crushing; 2) jet milling the R-Fe-Co-BM alloy fine powder of step 1) to obtain a magnetic powder, and then pressing the magnetic powder into a powder compact; 3) sintering the green compact of step 2), optionally with or without aging treatment, to obtain a sintered magnet; 4) processing the sintered magnet of step 3) into a required size, disposing a diffusion source on the surface of the processed product, and then heating the product under vacuum conditions. When the heating temperature reaches the diffusion treatment temperature, fill the product with inert gas, keep it warm, and then cool it to room temperature. 5) performing an aging treatment on the magnet after the diffusion treatment of step 4) to produce and obtain the neodymium-iron-boron-based sintered magnet.

5. A method for producing a neodymium-iron-boron sintered magnet according to claim 1.

6. In step 2), the jet-milled magnetic powder is mixed with a lubricant that accounts for 0.1 wt% to 0.5 wt% of the total mass of the jet-milled magnetic powder for 0.1 to 3 hours, and then the magnetic powder is pressed into a green compact at 20°C to 40°C in an orienting field environment with a magnetic field strength of 1 T to 5 T; The method of claim 5.

7. In step 3), the sintering temperature is 900°C to 1100°C, and the sintering time is 2 hours to 10 hours; and / or in step 3), the aging treatment is a secondary aging treatment including a first-stage aging treatment and a second-stage aging treatment, in which the temperature of the first-stage aging treatment is 700-950°C and the time of the first-stage aging treatment is 1-12 hours, the temperature of the second-stage aging treatment is 400-600°C and the time of the second-stage aging treatment is 1-12 hours; The method of claim 5.

8. In step 4), the diffusion source is at least one of a rare earth element source; and / or in step 4), filling with an inert gas of 5 kPa or more; The method of claim 5.

9. In step 4), the vacuum condition is a vacuum degree of ≦20 Pa; and / or in step 4), the temperature of the diffusion treatment is 800 to 1000°C; And / or, in step 4), the incubation time is 3 hours or more; The method of claim 5.

10. In step 5), the aging temperature is 400 to 600°C; The method of claim 5.

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