R-t-b based alloy for permanent magnet and method for producing r-t-b based permanent magnet

Abstract

The present invention provides an R-T-B based permanent magnet alloy capable of producing an R-T-B based permanent magnet having improved magnetic properties. The R-T-B based permanent magnet alloy has R, T, and B, R being a rare earth element, T being a transition metal element, and B being boron. The area ratio of non-columnar crystal structure in a cross section is 1.0% or more and 30.0% or less.

Description

Technical Field

[0001] This invention relates to alloys for R-T-B permanent magnets and methods for manufacturing R-T-B permanent magnets. Background Technology

[0002] Patent Document 1 discloses an invention relating to alloy sheets for rare-earth magnets, characterized by setting the thickness and surface roughness within a specific range. By setting the surface roughness of the casting rotary roller surface within a specific range, the fine R-phase rich regions in the alloy sheets for rare-earth magnets are reduced, thereby improving magnetic properties.

[0003] Patent Document 2 discloses an invention involving a method for manufacturing rare-earth-containing alloy sheets, characterized by setting the shape of the casting surface of a casting rotary roller to a specific shape and setting the surface roughness of the casting surface of the casting rotary roller within a specific range. The R-T-B alloy sheets produced by this manufacturing method exhibit reduced fine R-phase-rich regions and excellent homogeneity.

[0004] Patent document 3 discloses an invention involving a raw material alloy for R-T-B magnets, characterized in that the volume fraction of the region where secondary dendrite arms are formed is set within a specific range. Due to the formation of secondary dendrite arms, the microstructure is refined, thereby improving the coercivity of R-T-B sintered magnets obtained using this R-T-B magnet raw material alloy as a raw material.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2003-188006

[0008] Patent Document 2: Japanese Patent Application Publication No. 2004-181531

[0009] Patent Document 3: International Publication No. 2014 / 156181 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] The object of this invention is to provide an alloy for R-T-B permanent magnets that can be used to manufacture R-T-B permanent magnets with improved magnetic properties.

[0012] Methods for solving problems

[0013] To achieve the above objectives, the R-T-B series permanent magnet alloy of the present invention has R, T and B, where R is a rare earth element, T is a transition metal element, and B is boron, wherein the area ratio of non-columnar crystalline structure in the cross section is 1.0% or more and 30.0% or less.

[0014] The area ratio of the aforementioned non-columnar crystalline structures can be above 4.0% and below 30.0%.

[0015] The aforementioned non-columnar crystalline structure may include chilled crystalline structure, and the area ratio of the structure other than the chilled crystalline structure in the aforementioned non-columnar crystalline structure may be 50% or more.

[0016] The content of R can be 29.0% by mass or more and 33.5% by mass or less, and the content of B can be 0.70% by mass or more and less than 0.96% by mass.

[0017] The manufacturing method of the R-T-B series permanent magnet of the present invention includes a step of pulverizing the above-mentioned R-T-B series permanent magnet with an alloy. Attached Figure Description

[0018] Figure 1 It is a reflected electron image of ordinary tissue.

[0019] Figure 2 It is a reflected electron image of a chilled crystal structure.

[0020] Figure 3 It is a reflected electron image of a tissue containing fine, dot-like R-phase.

[0021] Figure 4 It is a reflected electron image of a tissue containing fine, dot-like R-phase.

[0022] Figure 5 It is a reflected electron image of a tissue containing dot-like R-phase.

[0023] Figure 6 It is a reflected electron image of a tissue containing fine, linear, R-rich phases.

[0024] Figure 7 It is a reflected electron image of a tissue containing fine, linear, R-rich phases.

[0025] Figure 8 It is a reflected electron image of a tissue containing large, dot-like R-phase.

[0026] Figure 9 It is an evaluation of the reflected electron image of the sample by dividing the region.

[0027] Figure 10 yes Figure 9 A luminance histogram of a region.

[0028] Figure 11 It is a graph that shows the peak position and σ difference of the brightness histogram of the organization.

[0029] Figure 12This is a schematic diagram of a casting apparatus.

[0030] Symbol Explanation

[0031] 1…Quick-cooled crystal structure

[0032] 3…tissues containing fine, punctate R-phase-rich structures

[0033] 4…tissues containing punctate R-rich phases

[0034] 5…tissues containing fine, linear, R-rich phases

[0035] 6…tissues containing large, punctate R-rich phases

[0036] 7…Ordinary Organizations

[0037] 11…Main Phase

[0038] 13…R-phase

[0039] 21…cooling rollers

[0040] 23…Tundish

[0041] 25… Alloy molten liquid

[0042] 27… Melt head pressure. Detailed Implementation

[0043] The present invention will now be described based on the embodiments shown in the accompanying drawings.

[0044] <Microstructure of R-T-B series permanent magnet alloys>

[0045] The R-T-B series permanent magnet alloy of this embodiment has R, T and B, where R is a rare earth element, T is a transition metal element, and B is boron, and the area ratio of non-columnar crystalline structure in the cross section is 1.0% or more and 30.0% or less.

[0046] Generally speaking, R-T-B series permanent magnet alloys contain R2T 14 The main phase of columnar crystals with B-type crystalline structure and the R-rich phase with a higher R content than the main phase.

[0047] In the cross-section of R-T-B series permanent magnet alloys, such as Figure 1As shown in the reflected electron image, a clear difference can be identified between the main phase 11 and the R-rich phase 13, with most of the R-rich phase 13 being linear structures 7 (hereinafter referred to as ordinary structure 7). The main phase 11 contained in ordinary structure 7 is mostly columnar crystals. In ordinary structure 7, the average spacing between the linear R-rich phases (hereinafter referred to as linear R-rich phases) is 2 μm or more. Furthermore, ordinary structure 7 is a structure in which primary dendrites extend along the solidification direction of the alloy, in other words, mainly along the thickness direction of the alloy.

[0048] In addition to the ordinary microstructure 7, R-T-B series permanent magnet alloys also contain microstructures including a main phase that is not columnar. The main phase that is not columnar is called non-columnar, and the microstructure containing non-columnar phases is called non-columnar microstructure. Furthermore, the non-columnar microstructure is a microstructure in which primary dendrites do not necessarily extend along the solidification direction of the alloy.

[0049] The non-columnar crystalline structures contained in R-T-B series permanent magnet alloys are classified into six types, distinguished as structures 1a and 1b as described below. The six types of non-columnar crystalline structures are explained below using accompanying drawings (reflected electron images).

[0050] exist Figure 2 The sample contains chilled crystal structure 1, which includes structures 1a and 1b. Structure 1a differs significantly from ordinary structure 7 compared to structure 1b. No clear difference is observed between the main phase and the R-rich phase in structure 1a. The brightness in the electron reflectance image of structure 1a is intermediate between the main phase and the R-rich phase. Furthermore, the variation in depth in the electron reflectance image of structure 1a is smooth.

[0051] Tissue 1b is closer to ordinary tissue 7 than tissue 1a. However, no clear difference is observed between the main phase and the R-rich phase in tissue 1b. The luminance in the electron reflectance pattern of tissue 1b is intermediate between the main phase and the R-rich phase. In addition, the variations in depth in the electron reflectance pattern of tissue 1b are smooth. Furthermore, unlike tissue 1a, tissue 1b contains a portion of linear R-rich phase.

[0052] In the following description, when simply described as a chilled crystal structure, structure 1a and structure 1b are not distinguished.

[0053] exist Figure 3 , Figure 4 It includes tissue 3 (hereinafter, sometimes referred to as tissue 3) containing fine punctate R-rich phase. Furthermore, in Figure 3 and Figure 4 Different alloy flakes were observed. Clear differences were visible in microstructure 3 between the main phase and the R-rich phase. However, compared to... Figure 1Compared to the ordinary tissue 7 shown, the R-rich phase in tissue 3 is punctately aggregated and very dense. Furthermore, compared to tissue 4, which contains punctate R-rich phases as described later, the punctate R-rich phases are finer.

[0054] For example, the size of the punctate R-rich phase contained in tissue 3, in terms of circle equivalent diameter, can be 0.3–1.5 μm, and the number of punctate R-rich phases per unit area can be 0.25 per μm. 2 above.

[0055] Furthermore, tissue 3 can be segmented according to subtle differences in the circular equivalent diameter and density of locally point-rich R-phase. Figure 4 The dashed lines in the diagram represent the actual division. The resulting polygonal regions can have a major axis greater than or equal to 10 μm and less than 120 μm, and a minor axis greater than or equal to 5 μm and less than 80 μm. Furthermore, the major axis refers to the longest distance between two parallel lines that connect to the two sides of the polygon, and the minor axis refers to the shortest distance between two parallel lines that connect to the two sides of the polygon.

[0056] exist Figure 5 It contains tissue 4 (hereinafter sometimes referred to as tissue 4) containing a punctate R-rich phase. Compared to tissue 3, the punctate R-rich phase of tissue 4 is larger. In addition, there is a darker phase than the main phase in the part where the punctate R-rich phase is condensed.

[0057] exist Figure 6 , Figure 7 It includes tissue 5 (hereinafter, sometimes referred to as tissue 5) containing fine, linear, R-rich phase. Furthermore, in Figure 6 and Figure 7 Different alloy flakes were observed. Microstructure 5 showed clear differences between the main phase and the R-rich phase. However, compared to... Figure 1 Compared to the ordinary tissue 7 shown, the linear R-rich phase of tissue 5 is finer and very dense.

[0058] For example, the linear R-rich phase contained in tissue 5 can have a length of 4–125 μm, a width of 0.3–10 μm, and a spacing of 1.0–1.8 μm between the linear R-rich phases.

[0059] Furthermore, tissue 5 can be segmented according to subtle differences in the length, width, and density of local linear R-rich phases. Figure 7 The dashed lines in the diagram represent the actual segmentation. The resulting polygonal regions can have a major axis of 30μm or more and 200μm or less, and a minor axis of 5μm or more and 150μm or less.

[0060] exist Figure 8The structure contains a tissue 6 (hereinafter, sometimes referred to as tissue 6) containing large, dot-like R-rich phases. In tissue 6, the R-rich phases are largely aggregated. Furthermore, the brightness of the main phase surrounding the aggregated R-rich phases is slightly increased. Additionally, linear R-rich phases are sometimes interspersed among the larger aggregated R-rich phases.

[0061] The aforementioned common structures 7 and 6 non-columnar crystalline structures can be visually distinguished from the reflected electron images of cross-sections of R-T-B series permanent magnet alloys.

[0062] <Method for calculating the area ratio of non-columnar crystalline structures>

[0063] In this embodiment, luminance analysis of reflected electron images is used when calculating the area ratio of non-columnar crystalline structures in the cross-section of the R-T-B system permanent magnet alloy. The method of luminance analysis will be described below.

[0064] First, in preparation for luminance resolution, the brightness and contrast of the electron microscope are adjusted.

[0065] First, prepare an R-T-B series permanent magnet alloy plate for use as a standard sample. An R-T-B series permanent magnet alloy plate can be used directly, or a heat-treated plate that has undergone heat treatment can be used. Heat treatment simplifies the process of bringing only the principal phase 11 into the field of view of the electron microscope. As a result, adjusting the brightness and contrast of the electron microscope becomes easier. There are no particular limitations on the heat treatment time and temperature. For example, heat treatment at 800–1000°C for 30–120 minutes is acceptable.

[0066] Next, Ni, Cu, and Zn thin plates are prepared, and these plates are embedded in embedding resin for electron microscopy observation. Standard samples are then prepared by arranging the plates side-by-side with cross-sections parallel to their thickness direction. There are no particular restrictions on the type of thin plates other than the R-T-B permanent magnet alloy plates; at least two types are acceptable. The brightness peak positions of each metal and the differences between their brightness peak positions (described later) are appropriately set according to the type of thin plate. The use of Ni, Cu, and Zn thin plates will be explained in the following description.

[0067] Next, the cross-section of the standard sample is mirror-polished and then gold-plated.

[0068] Next, the standard sample was placed under an electron microscope. The imaging mode was set to reflectance electron imaging, and the pixel count was set to 1280×960 pixels. There were no particular restrictions on the magnification; it was set to the magnification at which the Ni, Cu, and Zn thin plates entered the same field of view.

[0069] Next, the field of view is moved by having Ni, Cu, and Zn thin plates enter the same field of view in that order.

[0070] Next, a luminance histogram was created using 256 gray levels with the minimum luminance set to 0 and the maximum luminance set to 255. The brightness was adjusted so that the luminance peak of Cu was at level 150 (145–155). Then, the contrast was adjusted so that the difference between the luminance peaks of Ni and Cu was at level 55 (45–65), and the difference between the luminance peaks of Cu and Zn was at level 45 (35–55). While adjusting the contrast, the brightness was adjusted as needed to maintain the luminance peak of Cu at level 150.

[0071] Next, while maintaining contrast, the field of view of the electron microscope is moved to the position where the R-T-B permanent magnet alloy plate enters. Then, the magnification is increased so that only the main phase 11 enters the field of view, without allowing the white phase (R-rich phase 13) to enter. The magnification can be set up to a maximum of 10,000x. In addition, the brightness is adjusted so that the peak position of the luminance histogram is at level 110 (105-115). Finally, the standard sample is recovered from the electron microscope.

[0072] Next, the method for luminance analysis will be explained.

[0073] First, an R-T-B based permanent magnet alloy is prepared for luminance analysis. Next, an evaluation sample is fabricated by processing the alloy in a manner that allows for observation of its cross-section. Alternatively, multiple evaluation samples can be fabricated and the results averaged.

[0074] Next, the shooting mode was set to reflective electronic image, the magnification was set to 350x, the pixel count was set to 1280×960 pixels, and the observation range was set. Furthermore, the observation range, which was set as magnification and pixel count, was 360μm×270μm.

[0075] Next, the evaluation sample within the aforementioned observation range is divided into regions at certain intervals. There is no particular limitation on the size of a region; for example, it can be set to 40 pixels or more and 60 pixels or less. Figure 9 This is an image of a cross-section of a sample used for actual observation and evaluation, divided into regions of 50 pixels. Observing a cross-section of an R-T-B series permanent magnet alloy parallel to its thickness direction... Figure 9 In terms of image size, Figure 9 Overall, it measures 1280 pixels (360 μm) horizontally and 960 pixels (270 μm) vertically.

[0076] Next, luminance histograms are created for each region. For example, Figure 10 Yes Figure 9 The results show the luminance histograms for the 7th region from the top and the 3rd region from the left. Furthermore, the peak positions and standard deviations (σ) of the luminance histograms for each region were obtained.

[0077] The inventors of this invention have made the following new discovery. Regions where the peak position of the luminance histogram is 130 or higher and 200 or lower, and where σ is 20 or higher and 40 or lower, can be considered as non-columnar crystalline structures. Furthermore, the ratio of the number of regions with peak positions and σ within the aforementioned range relative to all regions can be considered as the area ratio of the non-columnar crystalline structure. Moreover, R-T-B permanent magnets made from R-T-B alloys with a non-columnar crystalline structure area ratio of 1.0% or higher and 30.0% or lower exhibit excellent magnetic properties. In particular, it has been determined that when the area ratio is 1.0% or higher, HcJ tends to increase at high temperatures. Additionally, it has been determined that when the area ratio is 30.0% or lower, Hk / HcJ tends to increase at room temperature. It has been determined that the area ratio of the non-columnar crystalline structure is preferably 4.0% or higher and 30.0% or lower.

[0078] Furthermore, regions where the peak position of the luminance histogram is 130 or higher and 200 or lower, and where σ is 30 or higher and 40 or lower, can be considered as structures other than the chilled crystal structure in the aforementioned non-columnar crystal structure. Moreover, it has been determined that the area ratio of structures other than the chilled crystal structure in the aforementioned non-columnar crystal structure can be 50% or higher, preferably 85% or higher and 95% or lower, and more preferably 87% or higher and 91% or lower.

[0079] The following further explains the relationship between the aforementioned luminance histogram and the microstructure of R-T-B based permanent magnet alloys. The inventors of this invention have discovered that R-T-B based permanent magnets made from R-T-B based permanent magnet alloys with a non-columnar microstructure area ratio within a specified range exhibit excellent magnetic properties. However, visually distinguishing between ordinary and non-columnar microstructures in reflected electron images relies on the subjectivity of the observer; therefore, results may sometimes vary depending on the observer.

[0080] The inventors of this invention believe that if the area ratio of non-columnar crystalline structures is calculated using luminance analysis, the area ratio of non-columnar crystalline structures can be calculated independently of the measuring instrument. To this end, the inventors observed cross-sections of several R-T-B system permanent magnet alloys. Then, by visually observing the reflected electron images of the cross-sections, they classified them into ordinary structures and six types of non-columnar crystalline structures. Then, they observed the peak positions and the tendency of σ when creating luminance histograms for each structure.

[0081] The results showed that in the region with a non-columnar crystalline structure, most peaks in the luminance histogram were located between 130 and 200, and σ was between 20 and 40. In contrast, in the region with a common crystalline structure, most peaks and / or σ were outside these ranges. The luminance histogram of the region with a common crystalline structure showed particularly large σ values.

[0082] Figure 11 The diagram illustrates the average peak positions and average σ values ​​in multiple regions corresponding to ordinary microstructures and multiple regions corresponding to six non-columnar microstructures in an R-T-B based permanent magnet alloy with a specific composition. In most regions corresponding to any of microstructures 1a, 1b, and 3 through 6, the peak positions are above 130 and below 200, and the σ values ​​are above 20 and below 40. Even if the composition of the R-T-B based permanent magnet alloy is changed, the average peak positions and average σ values ​​of each microstructure will not change significantly, as long as the method of adjusting the brightness and contrast of the electron microscope remains unchanged.

[0083] Furthermore, the inventors of this invention have discovered that R-T-B series permanent magnets made from R-T-B series permanent magnet alloys with an area ratio of non-columnar crystal structure calculated from luminance analysis of 1.0% or more and 30.0% or less have good magnetic properties.

[0084] Furthermore, regarding the regions used to construct luminance histograms, when a region is too large, there are more cases where it contains two or more different microstructures within the same region. When a region is too small, the number of pixels in each region decreases, thus reducing the accuracy of the luminance histograms constructed for each region. As a result, the peak positions and σ of the luminance histogram become unclear. In other words, the size of a region has an appropriate range depending on factors such as the type of alloy.

[0085] <Composition of R-T-B series permanent magnet alloys>

[0086] The composition of R-T-B series permanent magnet alloys is not particularly limited. R represents at least one rare earth element. Rare earth elements refer to Sc, Y, and lanthanum elements belonging to Group 3 of the long-period periodic table. Lanthanum elements include, for example, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Rare earth elements are classified into light rare earth elements and heavy rare earth elements. Heavy rare earth elements refer to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, while light rare earth elements are rare earth elements other than heavy rare earth elements. In this embodiment, from the viewpoint of appropriately controlling manufacturing costs and magnetic properties, R may contain Nd and / or Pr. Furthermore, particularly from the viewpoint of improving HcJ, both light and heavy rare earth elements may be included. The content of heavy rare earth elements is not particularly limited, and heavy rare earth elements may be absent. The content of heavy rare earth elements is, for example, 5% by mass or less (containing 0% by mass).

[0087] The content of R can be above 29.0% by mass and below 33.5% by mass. When the content of R is low, the HcJ of the resulting R-T-B permanent magnet tends to decrease. When the content of R is high, the Br content tends to decrease.

[0088] The boron (B) content can be 0.70% by mass or more, or 0.80% by mass or more. The upper limit of the B content can be less than 1.0% by mass, less than 0.96% by mass, or less than 0.90% by mass. When the B content is less than the stoichiometric ratio, specifically less than 1.0% by mass, it is easy to set the area ratio of non-columnar crystal structures to 1.0–30.0%. Furthermore, when the B content is low, the Hk / HcJ ratio tends to decrease. Additionally, abnormal grain growth is more likely to occur when sintering at high temperatures. Conversely, when sintering at low temperatures, the Hk / HcJ ratio is less likely to increase. When the B content is high, abnormal grain growth is more likely to occur. Moreover, Br content tends to decrease.

[0089] T is a transition metal element. It can be Fe alone, or Fe and Co. The Co content can be 0% by mass or more and 2.0% by mass or less. The lower the Co content, the easier it is for abnormal grain growth to occur under high-temperature sintering. In addition, under low-temperature sintering, Hk / HcJ is less likely to increase. When the Co content is high, Br and HcJ decrease. In addition, the price of the R-T-B series permanent magnet alloy of this embodiment tends to be higher.

[0090] R-T-B series permanent magnet alloys may contain M. M is selected from one or more of Cu, Ga, Zr, and Al. There is no particular limitation on the total content of M. It can be more than 0.1% by mass and less than 2.0% by mass.

[0091] There are no particular restrictions on the Cu content. For example, it can be 0.05% by mass or more and 0.50% by mass or less. With a Cu content of 0.05% by mass or more, abnormal grain growth is less likely to occur during high-temperature sintering. In addition, even during low-temperature sintering, the Hk / HcJ ratio is easily increased sufficiently. With a Cu content of 0.50% by mass or less, Br is easily increased.

[0092] There are no particular restrictions on the Ga content. For example, it can be above 0% by mass and below 1.0% by mass. By containing Ga, abnormal grain growth is less likely to occur during high-temperature sintering. In addition, even during low-temperature sintering, the Hk / HcJ ratio is easily increased sufficiently. Furthermore, HcJ is also easily increased. By containing less than 1.0% by mass of Ga, Br is easily increased.

[0093] There are no particular restrictions on the Al content. For example, it can be 0.05% by mass or more and 1.00% by mass or less. With Al content of 0.05% by mass or more, HcJ is easily increased. Furthermore, abnormal grain growth is less likely to occur even under high-temperature sintering. Additionally, Hk / HcJ is easily increased even under low-temperature sintering. With Al content of 1.00% by mass or less, Br is easily increased.

[0094] There are no particular restrictions on the Zr content. For example, it can be 0.05% by mass or more and 1.00% by mass or less. By containing more than 0.05% by mass of Zr, the Hk / HcJ ratio can be easily increased sufficiently, even when sintered at low temperatures. In addition, abnormal grain growth is less likely to occur even when sintered at high temperatures. By containing less than 1.00% by mass of Zr, Br can be easily increased.

[0095] The Fe content is the substantial remainder in the composition of R-T-B series permanent magnet alloys. The substantial remainder of Fe content means that the total content of elements other than R, Fe, Co, B, and M is less than 1.0% by mass.

[0096] The R-T-B permanent magnets made of the alloy used in this embodiment can be processed into any shape for use. The shape of the R-T-B permanent magnets is not particularly limited, and for example, they can be formed into any shape such as cuboids, hexahedrons, flat plates, prisms, or cylindrical shapes with a C-shaped cross-section.

[0097] In addition, the R-T-B series permanent magnets include both magnet products that are processed and magnetized and magnet products that are not magnetized.

[0098] <Manufacturing Method of R-T-B Series Permanent Magnet Alloys>

[0099] As an example of a method for manufacturing R-T-B series permanent magnet alloys, this involves using... Figure 12 The method of manufacturing the thin strip continuous casting apparatus shown will be explained.

[0100] Figure 12 The casting apparatus shown includes a cooling roller 21 and an tundish 23. Additionally, although in Figure 12 Not shown, the casting apparatus may include well-known components such as refractory crucibles and collecting containers. There are no particular limitations on the type of refractory crucible. Examples include alumina crucibles, mullite crucibles, and zirconia crucibles. There are no particular limitations on the material of the cooling roller 21. Examples include materials for copper, copper alloys, copper surfaces that have been plated or flame-sprayed, and materials for copper alloy surfaces that have been plated or flame-sprayed.

[0101] First, weigh the raw material metals to make them the alloy composition of the target R-T-B system permanent magnet alloy, mix them, and obtain a raw material mixture.

[0102] Next, the obtained raw material mixture is loaded into a refractory crucible and melted to obtain an alloy melt. There are no particular limitations on the method used to melt the raw material mixture. For example, a method of heating the refractory crucible filled with the raw material mixture in a high-frequency vacuum induction furnace can be cited.

[0103] Next, use Figure 12 The casting apparatus shown casts the obtained alloy molten material using a thin-strip continuous casting method to obtain cast alloy strips (R-T-B series permanent magnet alloys). Specifically, alloy molten material 25 is supplied to a water-cooled cooling roller 21 via an tundish 23. The alloy molten material 25 supplied to the cooling roller 21 is cooled, detaches from the cooling roller 21 on the opposite side of the tundish 23, and is recovered as cast alloy strips.

[0104] In the cast alloy strip, the surface in contact with the cooling roller 21 is called the roller surface, and the opposite side of the roller surface is called the free surface. Compared to the free surface, the roller surface is rapidly cooled by the cooling roller 21. Here, under rapid cooling conditions, non-columnar crystalline structures are more easily formed than ordinary structures. Among non-columnar crystalline structures, under particularly rapid cooling conditions, chilled crystalline structures are easily formed. Therefore, non-columnar crystalline structures are more easily formed on the roller surface than on the free surface. Furthermore, Figures 1-9 All have a roller surface on the left side.

[0105] The area ratio of non-columnar crystalline structures varies primarily based on the temperature (casting temperature) of the molten alloy 25, the composition of the molten alloy 25, the surface roughness Rz of the cooling roller 21, the shape of the unevenness affecting the surface roughness Rz, the molten head pressure 27, the circumferential speed of the cooling roller 21, and the supply speed of the molten alloy 25 to the cooling roller 21 (the supply speed of the molten alloy 25 and the cooling roller 21 per unit contact width). The temperature of the molten alloy 25 is, for example, 1300–1600 °C. The surface roughness Rz of the cooling roller 21 is, for example, 10–50 μm. The circumferential speed of the cooling roller 21 is, for example, 0.5–2.5 m / s. The supply speed of the molten alloy 25 to the cooling roller 21 is, for example, 0.5–2.5 kg / (min·cm).

[0106] The following explains the melt head pressure 27. The melt head pressure 27 is the depth of the alloy molten metal 25 in contact with the cooling roller 21. The greater the melt head pressure 27, the stronger the pressure exerted by the alloy molten metal 25 against the cooling roller 21 due to its own weight. Therefore, the greater the melt head pressure 27, the stronger the adhesion between the alloy molten metal 25 and the cooling roller 21, and the higher the heat transfer rate from the alloy molten metal 25 to the cooling roller 21. Simultaneously, the length of the alloy molten metal 25 in contact with the cooling roller 21 also changes. Therefore, if other conditions are the same, a higher melt head pressure 27 results in a thicker cast alloy strip. In practice, the melt head pressure 27 can be appropriately selected based on other casting conditions.

[0107] As mentioned above, the faster the cooling rate, the easier it is to form a non-columnar crystalline structure (chilled crystal structure). Therefore, there is a trend that the higher the melt head pressure 27, the larger the area ratio of the non-columnar crystalline structure (chilled crystal structure).

[0108] One method to increase the molten head pressure 27 is to increase the amount of molten alloy 25 in the tundish 23. Another method to increase the amount of molten alloy 25 in the tundish 23 is to increase the supply speed of the molten alloy 25 to the tundish 23. If the supply speed of the molten alloy 25 to the tundish 23 is increased, the casting alloy strip becomes thicker. Another method to increase the molten head pressure 27 without changing the thickness of the casting alloy strip is to increase the circumferential speed of the cooling roller 21.

[0109] That is, when producing cast alloy strips of the same thickness, the melt head pressure 27 increases when the supply speed of the alloy melt 25 is fast and the circumferential speed of the cooling roller 21 is fast, and the melt head pressure 27 decreases when the supply speed of the alloy melt 25 is slow and the circumferential speed of the cooling roller 21 is slow.

[0110] <Manufacturing Method of R-T-B Series Permanent Magnets>

[0111] There are no particular limitations on the method for manufacturing R-T-B permanent magnets using the R-T-B alloy obtained by the above method. For example, methods including the following steps can be listed.

[0112] (a) Crushing process of R-T-B series permanent magnets using alloy powder

[0113] (b) The forming process of shaping the obtained alloy powder

[0114] (c) Sintering the molded body to obtain the sintering process of R-T-B system permanent magnet.

[0115] (d) Aging treatment process for R-T-B system permanent magnets

[0116] (e) Cooling process for R-T-B series permanent magnets

[0117] (f) Processing steps for R-T-B series permanent magnets

[0118] (g) Grain boundary diffusion process that diffuses heavy rare earth elements into the grain boundaries of R-T-B system permanent magnets

[0119] (h) Surface treatment process for R-T-B system permanent magnets

[0120] [Grinding Process]

[0121] The R-T-B series permanent magnets are pulverized using an alloy (pulverization process). The pulverization process includes a coarse pulverization process, which pulverizes the magnets to a particle size of several hundred μm to several mm, and a micro pulverization process, which pulverizes the magnets to a particle size of several μm.

[0122] (Coarse grinding process)

[0123] The R-T-B series permanent magnet alloy is coarsely pulverized to a particle size of several hundred μm to several mm (coarse pulverization process). This yields coarsely pulverized R-T-B series permanent magnet alloy powder. Coarse pulverization can be performed, for example, by allowing the R-T-B series permanent magnet alloy to adsorb hydrogen, then releasing the hydrogen based on the difference in hydrogen adsorption amounts between different phases, resulting in dehydrogenation and subsequent self-disintegration pulverization (hydrogen adsorption pulverization).

[0124] In addition to using hydrogen adsorption pulverization as mentioned above, the coarse pulverization process can also be carried out in an inert gas atmosphere using coarse pulverizers such as crushers, jaw crushers, and Brown mills.

[0125] Furthermore, the oxygen concentration is adjusted through atmospheric control in each manufacturing process. From the viewpoint of obtaining high magnetic properties, the oxygen content of the final R-T-B system permanent magnet can be reduced. For this purpose, the oxygen concentration in each process from the pulverizing process to the sintering process described later can be set to 100 ppm or less.

[0126] (Micro-pulverization process)

[0127] After coarsely grinding the R-T-B series permanent magnet alloy, the resulting coarsely ground R-T-B series permanent magnet alloy powder is then micro-ground to an average particle size of several μm (micro-grinding process). This yields micro-ground R-T-B series permanent magnet alloy powder. By further micro-grinding the coarsely ground powder, micro-ground powder with particles of, for example, 1 μm or larger and 10 μm or smaller, or 3 μm or larger and 5 μm or smaller, can be obtained.

[0128] Micropulverization is carried out as follows: Conditions such as pulverization time are appropriately adjusted, and micropulverizers such as spray mills, ball mills, vibratory mills, and wet mills are used to further pulverize the coarsely pulverized powder. Spray mills work by releasing high-pressure, inert gas (e.g., N2) from a narrow nozzle, generating a high-speed airflow. This high-speed airflow accelerates the coarsely pulverized raw material alloy powder, causing collisions between the powder particles and with the target material or container wall, thus pulverizing the powder.

[0129] When the coarse powder of the raw alloy is finely pulverized, by adding pulverizing aids such as zinc stearate, urea, stearamide, and oleamide, a finely pulverized powder with high orientation during molding can be obtained.

[0130] [Molding Process]

[0131] The process involves shaping finely pulverized powder into a target shape (forming process). In this process, the finely pulverized powder is filled into a mold surrounded by an electromagnet and pressurized, thereby shaping the powder into any desired shape. Simultaneously, a magnetic field is applied, inducing a predetermined orientation within the powder, and shaping is performed in a magnetic field while maintaining the crystal axis orientation. This yields a shaped body. The resulting shaped body is oriented in a specific direction, thus producing a stronger, anisotropic R-T-B system permanent magnet.

[0132] The pressure applied during molding can be from 30 MPa to 300 MPa. The applied magnetic field can be from 950 kA / m to 1600 kA / m. The applied magnetic field is not limited to a static magnetic field and can also be a pulsed magnetic field. In addition, a static magnetic field and a pulsed magnetic field can be used together.

[0133] In addition to dry molding, which directly shapes the micronized powder as described above, wet molding, which shapes a slurry in which the micronized powder is dispersed in a solvent such as oil, can also be used as a molding method.

[0134] The shape of the molded body obtained by micronized powder molding is not particularly limited. For example, it can be formed into any shape based on the desired shape of R-T-B permanent magnet, such as cuboid, plate, column, ring, etc.

[0135] [Sintering process]

[0136] The process involves shaping the material in a magnetic field, then sintering the resulting molded body in a vacuum or inert gas atmosphere to obtain an R-T-B permanent magnet (sintering process). The sintering temperature needs to be adjusted based on various conditions such as composition, grinding method, particle size, and particle size distribution. For the molded body, sintering is performed by heating at 1000°C or higher and 1200°C or lower for 1 hour to 48 hours in a vacuum or in the presence of an inert gas. This process results in liquid-phase sintering of the micronized powder, yielding an R-T-B permanent magnet with an increased volume ratio of the main phase particles (sintered body of the R-T-B magnet). From the perspective of improving production efficiency, the sintered body can also be rapidly cooled after sintering the molded body.

[0137] [Aging Process]

[0138] After sintering the molded body, the R-T-B permanent magnets undergo aging treatment (aging process). Following sintering, the R-T-B permanent magnets are aged by holding them at a temperature lower than that used during sintering. The aging treatment can be performed in two stages: heating at a temperature between 700°C and 1000°C for 10 minutes to 6 hours, followed by heating at 500°C to 700°C for 10 minutes to 6 hours; or in one stage: heating at a temperature around 600°C for 10 minutes to 6 hours. The treatment conditions are adjusted appropriately depending on the number of aging treatments performed. This aging treatment improves the magnetic properties of the R-T-B permanent magnets. Alternatively, the aging treatment process can be performed after the processing steps described later.

[0139] [Cooling Process]

[0140] After aging treatment, the R-T-B permanent magnet is quenched (cooling process) in an Ar atmosphere. This yields the R-T-B permanent magnet of this embodiment. The cooling rate is not particularly limited and can be set to 30°C / min or higher.

[0141] [Processing Steps]

[0142] The obtained R-T-B series permanent magnets can be processed into the desired shape as needed (processing steps). Processing methods include, for example, cutting, grinding, and chamfering.

[0143] [Grain boundary diffusion process]

[0144] Grain boundary diffusion can further diffuse heavy rare earth elements relative to the grain boundaries of processed R-T-B permanent magnets (grain boundary diffusion process). There are no particular limitations on the method of grain boundary diffusion. For example, it can be carried out by coating or vapor deposition to attach a compound containing heavy rare earth elements to the surface of the R-T-B permanent magnet, followed by heat treatment. Alternatively, it can be carried out by heat treating the R-T-B permanent magnet in an atmosphere containing vapor of heavy rare earth elements. Through grain boundary diffusion, the HcJ of the R-T-B permanent magnet can be further improved.

[0145] [Surface treatment process]

[0146] The R-T-B series permanent magnets obtained through the above processes can be subjected to surface treatments such as plating, resin coating, oxidation, and chemical surface treatment (surface treatment processes).

[0147] Furthermore, in this embodiment, processing steps, grain boundary diffusion steps, and surface treatment steps are performed, but these steps are not necessarily required.

[0148] The R-T-B permanent magnet of this embodiment obtained as described above has good magnetic properties and a wide sintering temperature range. As a result, the R-T-B permanent magnet of this embodiment becomes a magnet that can be stably produced.

[0149] The R-T-B series permanent magnets obtained in this embodiment are suitable for use as magnets in surface permanent magnet (SPM) rotary machines, interior permanent magnet (IPM) rotary machines (such as brushless motors with internal rotors), and PRM (Permanent magnet Reluctance Motors). Specifically, the R-T-B series permanent magnets of this embodiment can be appropriately used for applications such as spindle motors for hard disk drive rotation drives, voice coil motors, motors for electric vehicles and hybrid vehicles, electric power steering motors for automobiles, servo motors for machine tools, vibrator motors for mobile phones, printer motors, and generator motors.

[0150] Furthermore, the present invention is not limited to the embodiments described above, and various modifications can be made within the scope of the present invention.

[0151] Example

[0152] The invention will now be described in more detail by way of examples, but the invention is not limited to these examples.

[0153] (Making of R-T-B series permanent magnet alloys)

[0154] Weigh Nd metal (purity ≥ 99% by mass), Nd and Pr alloy (neodymium-praseodymium, purity ≥ 99% by mass), Tb metal (purity ≥ 99% by mass), ferroboron-boron alloy (Fe content 80% by mass, B content 20% by mass), Fe metal (purity ≥ 99% by mass), Co metal (purity ≥ 99% by mass), Zr metal (purity ≥ 99% by mass), Cu metal (purity ≥ 99% by mass), Al metal (purity ≥ 99% by mass), and Ga metal (purity ≥ 99% by mass) to obtain the alloy composition shown in Table 1 below. Mix them to obtain a raw material mixture. In Table 1, "T.RE" represents the total content (by mass%) of rare earth elements (Nd, Pr, Dy, and Tb), and "T.RL" represents the total content (by mass%) of light rare earth elements (Nd and Pr). "bal." represents the remainder. Recording the Fe content as "bal." indicates that the Fe content is the remainder when the R-T-B series permanent magnet alloy containing elements other than those listed in Table 1 is set to 100% by mass.

[0155] The obtained raw material mixture was filled into an alumina crucible. The alumina crucible containing the raw material mixture was placed in a high-frequency vacuum induction furnace, and the furnace was then purged with Ar. The raw material mixture in the alumina crucible was then melted by heating the furnace, yielding an alloy melt. This was then used... Figure 12 The thin-strip continuous casting method of the casting apparatus shown casts the obtained alloy molten metal to obtain cast alloy thin strips (R-T-B series permanent magnet alloys). Casting is carried out in an Ar atmosphere.

[0156] The R-T-B series rare-grain permanent magnet alloys listed in Table 2 were obtained by appropriately selecting the temperature of the alloy melt, the surface roughness Rz of the cooling roller, and the melt head pressure. Specifically, the alloy melt temperature (casting temperature) was appropriately selected within the range of 1300–1600 °C, the surface roughness Rz of the cooling roller was 5–50 μm, and the melt head pressure was 5–25 mm. Furthermore, the material of the cooling roller was set to a copper alloy. The circumferential speed of the cooling roller and the supply speed of molten metal to the cooling roller (the supply speed of molten metal per unit contact width of the cooling roller) were adjusted to the values ​​shown in Table 2 below.

[0157] (Adjustment of the electron microscope)

[0158] In preparation for observing the cross-section of the R-T-B series permanent magnet alloy obtained by electron microscopy (JEOL Ltd.), and calculating the area ratio of the non-columnar crystalline structure by luminance analysis, the brightness and contrast of the electron microscope are adjusted.

[0159] First, the obtained R-T-B series permanent magnet alloy was heat-treated at 1000℃ for 120 minutes to obtain two heat-treated plates for assembling standard samples.

[0160] Next, prepare 0.1 mm thick Ni, Cu, and Zn thin plates. Then, embed the heat-treated plates, Ni, Cu, and Zn thin plates in embedding resin for electron microscopy. At this point, arrange the heat-treated plates, Ni, Cu, and Zn thin plates side by side with their cross-sections parallel to the thickness direction in that order to create a standard sample.

[0161] Next, the cross-section of the standard sample is mirror-polished and then plated with gold.

[0162] Next, the standard sample was placed under an electron microscope. The imaging mode was set to reflectance electron imaging, the magnification was set to 150x, and the pixel count was set to 1280×960 pixels.

[0163] Next, the field of view is adjusted by having Ni, Cu, and Zn thin plates enter the same field of view in that order.

[0164] Next, a luminance histogram was created using 256 gray levels with the minimum luminance set to 0 and the maximum luminance set to 255. The brightness was adjusted so that the luminance peak of Cu was at level 150 (145–155). Then, the contrast was adjusted so that the difference between the luminance peaks of Ni and Cu was at level 55 (45–65), and the difference between the luminance peaks of Cu and Zn was at level 45 (35–55). While adjusting the contrast, the brightness was adjusted as needed to maintain the luminance peak of Cu at level 150.

[0165] Next, while maintaining contrast, the field of view was moved to the position where the heat treatment plate entered. Then, the magnification was increased so that only the main phase 11 entered the field of view, without allowing the white phase (R-rich phase 13) to enter. The maximum magnification was set to 10,000. In addition, the brightness was adjusted so that the peak position of the luminance histogram was at level 110 (105-115). Finally, the standard sample was recovered from the electron microscope.

[0166] (Determination of the area ratio of non-columnar crystalline structures)

[0167] Using an electron microscope with brightness and contrast adjusted by the method described above, the area ratio of non-columnar crystalline structures in R-T-B series permanent magnet alloys was determined.

[0168] First, evaluation samples are collected. Specifically, in the process of supplying one manufacturing batch of R-T-B series permanent magnet alloy using the aforementioned thin-strip continuous casting method, alloy sheets are sampled from the R-T-B series permanent magnet alloy supplied at regular time intervals. Thirty alloy sheets are randomly collected from the R-T-B series permanent magnet alloy, and each alloy sheet is designated as an evaluation sample.

[0169] Next, the thickness of the collected evaluation samples was measured. Then, the second, third, and fourth thickest samples out of the 30 evaluation samples were selected as thick samples, the fourteenth, fifteenth, and sixteenth thickest samples were selected as average samples, and the twenty-seventh, twenty-eighth, and twenty-ninth thickest samples were selected as thin samples, and these samples were screened.

[0170] Next, the nine selected evaluation samples were glued together using an instant adhesive, allowing for observation of the cross-section parallel to the thickness direction. Furthermore, the evaluation samples were arranged to identify which were thick, average, or thin samples.

[0171] Next, the nine evaluation samples, bonded with instant adhesive, were uniformly embedded in resin as a single resin-embedded sample. Then, the cross-section parallel to the thickness direction was mirror-finished and set as the observation surface.

[0172] Next, the imaging mode was set to reflective electron imaging, the magnification to 350x, and the pixel count to 1280×960 pixels. The observation range was then set. The average portion of the observation surface of each evaluation sample was selected as the observation range. Next, the area ratio of non-columnar crystalline structures within the observation range was calculated analytically.

[0173] The specific analysis order is explained below. First, the observation area is divided into regions at certain intervals. The size of each region is set to 50 pixels.

[0174] Next, a luminance histogram is created for each region. Then, the peak position and σ of the luminance histogram for each region are obtained.

[0175] Next, it is determined whether each region is a non-columnar crystalline structure. Specifically, regions with a peak position of 130 or higher and 200 or lower in the luminance histogram, and a σ value of 20 or higher and 40 or lower, are defined as non-columnar crystalline structures. Regions with peak positions or σ values ​​outside the above ranges are defined as not being non-columnar crystalline structures. For all regions within the observation range, it is determined whether they are non-columnar crystalline structures. The ratio of the number of non-columnar crystalline regions within the observation range to the total number of regions is used as the area ratio of non-columnar crystalline structures in the cross-section of each evaluation sample.

[0176] Then, the area ratio of non-columnar crystalline structures in the cross-sections of each evaluation sample was averaged to calculate the area ratio of non-columnar crystalline structures in the cross-sections of an R-T-B series permanent magnet alloy for one manufacturing batch. The results are shown in Table 2.

[0177] (The fabrication of R-T-B series permanent magnets)

[0178] After adsorbing hydrogen onto the R-T-B series permanent magnet using an alloy at room temperature, dehydrogenation is performed in a vacuum at 600°C for 1 hour, followed by hydrogen pulverization (coarse pulverization) to obtain alloy powder (coarsely pulverized powder). Furthermore, in this embodiment, all processes from hydrogen pulverization to sintering (micro-pulverization and forming) are performed in an Ar atmosphere with an oxygen concentration below 50 ppm or in a vacuum.

[0179] Next, zinc stearate and stearamide were added to the alloy powder as pulverizing aids, and the mixture was prepared using a Nota mixer. The amount of zinc stearate added was 0.05 parts by weight relative to 100 parts by weight of the coarsely pulverized powder. The amount of stearamide added was also 0.05 parts by weight relative to 100 parts by weight of the coarsely pulverized powder. Then, the powder was micronized using a spray mill to form micronized powder with an average particle size of approximately 3.0 μm.

[0180] The obtained micronized powder is filled into a mold placed in an electromagnet, and the molded body is formed in a magnetic field with a magnetic field of 1200 kA / m and a pressure of 120 MPa applied at the same time to obtain the molded body.

[0181] The resulting molded body was then sintered in a vacuum at 1050°C for 4 hours, followed by rapid cooling to obtain a sintered body with the magnet composition shown in Table 1. The sintered body was then subjected to two stages of aging treatment: one at 900°C for 1 hour and the other at 500°C for 1 hour (both under an Ar atmosphere) to obtain R-T-B permanent magnets (R-T-B sintered magnets).

[0182] [Magnetic properties]

[0183] The Br, HcJ, and Hk / HcJ of each sample were determined using a B-H tracer. Br was measured for all samples at room temperature. In Examples 1-4 and Comparative Example 1, HcJ and Hk / HcJ were measured at room temperature. In other examples and comparative examples, HcJ and Hk / HcJ were measured at 150°C. Furthermore, in this example, Hk is the magnetic field value in the second quadrant of the demagnetization curve when the magnetization is Br × 0.9.

[0184] Table 1

[0185]

[0186] Table 2

[0187]

[0188] According to Examples 1-4 and Comparative Example 1, the R-T-B permanent magnets of Examples 1-4, made using an R-T-B permanent magnet alloy with a non-columnar structure area ratio of 1.0% or more and 30.0% or less, compared with the R-T-B permanent magnet of Comparative Example 1, made using an R-T-B permanent magnet alloy under the same conditions except that the non-columnar structure area ratio is within the aforementioned range, have higher Br, similar HcJ, and higher Hk / HcJ. In particular, Hk / HcJ is higher at room temperature.

[0189] According to Examples 5 to 11, the R-T-B permanent magnets of Examples 6 to 11, which were made using an R-T-B permanent magnet alloy with an area ratio of 4.0% or more and 30.0% or less of non-columnar crystal structure, had a higher HcJ compared to Example 5, which was made under the same conditions except that the area ratio of non-columnar crystal structure was 1.0% or more and less than 4.0%.

[0190] According to Examples 6 to 11, except that the area ratio of non-columnar crystalline structure in the R-T-B permanent magnet alloy is varied to be 4.0% or more and 30.0% or less, there is a trend that the higher the area ratio of non-columnar crystalline structure in the R-T-B permanent magnet alloy, the higher the HcJ.

[0191] The content of B in Comparative Example 2 was 0.96% by mass, which was higher than that in the other examples. Therefore, even with casting conditions similar to those in the other examples, the area ratio of the non-columnar structure was reduced. As a result, HcJ was lower than in the examples.

[0192] In addition, in all embodiments, it was confirmed that the area ratio of the non-columnar crystal structure other than the chilled crystal structure was 85% or more and 95% or less.

Claims

1. An R-T-B series permanent magnet alloy, characterized in that: The alloy contains R, T, and B, where R is a rare earth element, T is a transition metal element, and B is boron. Specifically, the area ratio of non-columnar crystalline structure in a cross-section parallel to the thickness direction of the R-T-B alloy for permanent magnets is 1.0% or more and 30.0% or less. The non-columnar crystalline structure includes chilled crystalline structures, and the area ratio of the structures other than the chilled crystalline structures in the non-columnar crystalline structure is more than 85% and less than 95%.

2. The R-T-B series permanent magnet alloy as described in claim 1, characterized in that: The area ratio of the non-columnar crystalline structure is 4.0% or more and 30.0% or less.

3. The R-T-B series permanent magnet alloy as described in claim 1 or 2, characterized in that: The content of R is 29.0% by mass or more and 33.5% by mass or less, and the content of B is 0.70% by mass or more and less than 0.96% by mass.

4. A method for manufacturing an R-T-B system permanent magnet, characterized in that: The process includes pulverizing the R-T-B series permanent magnets according to any one of claims 1 to 3 using an alloy.

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