JP7633066B2 - How to evaluate materials - Google Patents

Description

The present invention relates to a method for evaluating materials.

Patent Document 1 describes an invention related to a method for evaluating the metal structure of rare earth-containing alloys. Specifically, it describes drawing a straight line in a specific direction in a microscopic image of a cross section of a rare earth-containing alloy, and distinguishing between the main phase and the R-rich phase from the brightness of the main phase and the R-rich phase that intersect with the line. In the evaluation method of Patent Document 1, the R-rich phase interval is the number of times the brightness of the main phase changes to the brightness of the R-rich phase divided by the length of the line.

JP 2008-058323 A

The present invention aims to provide an evaluation method that can objectively identify the microstructure of a material.

In order to achieve the above object, the material evaluation method according to the present invention comprises the steps of:
obtaining an image of a cross-section of the material;
Dividing the obtained image into a number of sections;
measuring the luminance of a plurality of pixels included in each partition;
obtaining a luminance histogram for each section from the luminance of each pixel;
and identifying the fine structure of each section from the luminance histogram of each section.

The material may be an alloy.

The alloy may be an R-T-B type permanent magnet alloy, where R is a rare earth element, T is a transition metal element, and B is boron.

The type of microstructure of each section may be identified from the peak position and peak standard deviation in the brightness histogram of each section.

The method may further include a step of calculating the total area percentage of the sections having the specific microstructure.

When creating the luminance histogram in 256 gradations with the minimum luminance at 0 and the maximum luminance at 255, the total area ratio of the sections in which the peak position is between 130 and 200 and the peak standard deviation is between 20 and 40 may be calculated.

embedding two or more image preparation plates in a substrate to contain them within the same field of view of an electron microscope;
creating an unadjusted luminance histogram for the field of view;
adjusting the positions of the luminance peaks originating from each thin plate in the unadjusted luminance histogram to a predetermined position and adjusting the difference between the positions of the luminance peaks originating from each thin plate to a predetermined size;
An image of a cross section of the material may be obtained using an electron microscope adjusted using a method for adjusting an electron microscope comprising:

This is a backscattered electron image of normal tissue. This is a backscattered electron image of chill crystal structure. 1 is a backscattered electron image of a structure containing fine dotted R-rich phase. 1 is a backscattered electron image of a structure containing fine dotted R-rich phase. 1 is a backscattered electron image of a structure containing dot-like R-rich phases. 1 is a backscattered electron image of a structure containing fine linear R-rich phases. 1 is a backscattered electron image of a structure containing fine linear R-rich phases. This is a backscattered electron image of a structure containing large dot-like R-rich phases. This is a backscattered electron image of an evaluation sample divided into sections. 10 is a luminance histogram for one section of FIG. 9. 1 is a graph showing differences in peak position and σ of brightness histograms depending on tissue. FIG.

The present invention will now be described with reference to the embodiments shown in the drawings.

The material evaluation method according to the present embodiment includes the steps of:
obtaining an image of a cross-section of the material;
Dividing the obtained image into a plurality of regions;
measuring the luminance of a plurality of pixels included in each region;
obtaining a luminance histogram for each region from the luminance of each pixel;
The method further comprises a step of identifying the fine structure of each region from the shape of the luminance histogram of each region.

The material evaluation method according to this embodiment makes it possible to identify which compartment of a material has which type of tissue in an objective manner that is not dependent on the experience of an individual. This method is simple in that it requires a short time to identify the tissue and is easy to automate. Furthermore, this method is accurate in that it identifies the tissue in an objective manner, eliminating errors caused by differences between operators.

The following describes the case of R-T-B permanent magnet alloys, which are a type of alloy material and to which the evaluation method according to this embodiment can be suitably applied.

<Structure of R-T-B based permanent magnet alloy>
The R-T-B system permanent magnet alloy to which the evaluation method according to this embodiment is applied is an R-T-B system permanent magnet alloy having R, T and B, where R is a rare earth element, T is a transition metal element, and B is boron.

In general, an RTB based permanent magnet alloy contains a main phase which is a columnar crystal having an R ₂ T ₁₄ B type crystal structure, and an R-rich phase which has a higher R content than the main phase.

As shown in the backscattered electron image of Figure 1, in the cross section of an R-T-B permanent magnet alloy, a clear difference can be confirmed between the main phase 11 and the R-rich phase 13, and the majority of the R-rich phase 13 is linear in shape (structure 7, hereafter referred to as normal structure 7). Most of the main phase 11 contained in normal structure 7 is columnar. In normal structure 7, the spacing between linear R-rich phases (hereafter referred to as linear R-rich phases) is 2 μm or more on average. Furthermore, normal structure 7 is a structure in which primary dendrites extend in the direction of solidification of the alloy, in other words, in the thickness direction of the alloy.

In addition to the normal structure 7, R-T-B alloys for permanent magnets also contain a structure that includes a main phase that is not columnar. A main phase that is not columnar is called a non-columnar crystal, and a structure that includes non-columnar crystals is called a non-columnar crystal structure. A non-columnar crystal structure is also a structure in which the primary dendrites do not necessarily extend in the direction of solidification of the alloy.

The non-columnar crystal structures contained in R-T-B permanent magnet alloys are classified into six types when distinguishing between structure 1a and structure 1b, which will be described later. Below, the six types of non-columnar crystal structures are explained using drawings (backscattered electron images).

Figure 2 includes chill crystal structure 1, which includes structures 1a and 1b. Structure 1a is more different from normal structure 7 than structure 1b. Structure 1a shows no clear difference between the main phase and the R-rich phase. The brightness of structure 1a in the backscattered electron image is intermediate between that of the main phase and the R-rich phase. Furthermore, structure 1a shows smooth variations in shading in the backscattered electron image.

Structure 1b is closer to normal structure 7 than structure 1a. However, in structure 1b, no clear difference is observed between the main phase and the R-rich phase. The brightness of structure 1b in the backscattered electron image is intermediate between the main phase and the R-rich phase. Furthermore, the variation in shading in the backscattered electron image of structure 1b is smooth. Also, unlike structure 1a, structure 1b partially contains linear R-rich phase.

In the following description, when simply referring to chill crystal structure, no distinction is made between structure 1a and structure 1b.

Figures 3 and 4 contain a fine dot-like R-rich phase-containing structure 3 (hereinafter, sometimes referred to as structure 3). Note that different alloy pieces are observed in Figures 3 and 4. In structure 3, a clear difference is seen between the main phase and the R-rich phase. However, compared to the normal structure 7 shown in Figure 1, in structure 3, the R-rich phase is aggregated in dots and is excessively dense. Furthermore, compared to the dot-like R-rich phase-containing structure 4 described below, the dot-like R-rich phase is finer.

For example, the dot-like R-rich phase contained in the structure 3 may have a size of 0.3 to 1.5 μm in terms of circle equivalent diameter, and the number of dot-like R-rich phases per unit area may be 0.25 pieces/ ^μm2 or more.

Furthermore, the structure 3 can be cut into sections where the circle equivalent diameter and density of the dot-like R-rich phase are subtly different locally. The actual cut state is shown by dotted lines in Figure 4. The cut polygonal region may have a long axis of 10 μm to 120 μm and a short axis of 5 μm to 80 μm. The long axis refers to the longest distance between two parallel lines that contact the polygon from both sides, and the short axis refers to the shortest distance between two parallel lines that contact the polygon from both sides.

Figure 5 includes dot-like R-rich phase-containing structure 4 (hereinafter, sometimes referred to as structure 4). Structure 4 has a larger dot-like R-rich phase itself than structure 3. Furthermore, in the area where the dot-like R-rich phase aggregates, a phase that is darker than the main phase is present.

Figures 6 and 7 contain a fine linear R-rich phase-containing structure 5 (hereinafter sometimes referred to as structure 5). Note that different alloy pieces are observed in Figures 6 and 7. In structure 5, clear differences can be seen in the main phase and the R-rich phase. However, compared to the normal structure 7 shown in Figure 1, in structure 5 the linear R-rich phase is thin and excessively dense.

For example, the length of the linear R-rich phase contained in the structure 5 may be 4 to 125 μm, the width may be 0.3 to 10 μm, and the spacing between the linear R-rich phases may be 1.0 to 1.8 μm.

Furthermore, the structure 5 can be cut into sections where the length, width, and density of the linear R-rich phase are subtly different locally. The actual cut state is shown by dotted lines in Figure 7. The cut polygonal regions may have a long axis of 30 μm or more and 200 μm or less, and a short axis of 5 μm or more and 150 μm or less.

Figure 8 includes a large dot-like R-rich phase-containing structure 6 (hereinafter sometimes referred to as structure 6). In structure 6, the R-rich phase is aggregated to a large extent. The brightness of the main phase surrounding the aggregated R-rich phase is slightly higher. In addition, linear R-rich phases may be sandwiched between the large aggregated R-rich phases.

The evaluation method according to this embodiment allows the area ratio of non-columnar crystal structures in the cross section of an R-T-B permanent magnet alloy to be evaluated by a method other than visual inspection, i.e., a method that does not rely on human labor.

<Method for evaluating the area ratio of non-columnar crystal structure>
In the evaluation method according to the present embodiment, brightness analysis of a backscattered electron image is used to evaluate the microstructure of a material. Below, a method of brightness analysis will be described in particular for evaluating the area ratio of a non-columnar crystal structure in a cross section of an R-T-B system permanent magnet alloy, but various conditions may be appropriately changed depending on the type of material.

First, adjust the brightness and contrast of the electron microscope in preparation for brightness analysis.

First, prepare an R-T-B permanent magnet alloy plate as the material to be used for the standard sample. The R-T-B permanent magnet alloy plate may be used as is, or a heat-treated plate obtained by subjecting the R-T-B permanent magnet alloy plate to heat treatment may be used. By carrying out the heat treatment, the process of bringing only the main phase 11 into the field of view of the electron microscope becomes easier. As a result, it also becomes easier to adjust the brightness and contrast of the electron microscope. There are no particular limitations on the heat treatment time and temperature when carrying out the heat treatment. For example, heat treatment is carried out at 800 to 1000°C for 30 to 120 minutes.

Next, a plurality of thin metal plates for image adjustment, that is, thin metal plates for adjusting brightness and contrast, are prepared. For example, Ni thin plates, Cu thin plates, and Zn thin plates are prepared. Then, the R-T-B system permanent magnet alloy plate, Ni thin plate, Cu thin plate, and Zn thin plate are embedded in a substrate. An example of the substrate is an embedding resin for electron microscope observation. At this time, the plates are arranged so that the cross sections parallel to the thickness direction are aligned, and a standard sample is prepared. At this time, there is no particular restriction on the type of thin plate other than the R-T-B system permanent magnet alloy plate, and at least two types of thin plates are sufficient. The luminance peak position of each metal and the difference between the luminance peak positions of each metal, which will be described later, may be appropriately set depending on the type of thin plate. In the following description, the case where Ni thin plate, Cu thin plate, and Zn thin plate are used will be described.

Next, the cross section of the standard sample is mirror-polished and gold is vapor-deposited. By mirror-polishing the cross section and vapor-depositing gold, the brightness and contrast can be appropriately adjusted.

Next, the standard sample is set in the electron microscope. The imaging mode is backscattered electron imaging. There is no particular limit to the number of pixels, which varies depending on the material being observed. For example, 1280 x 960 pixels. There is no particular limit to the magnification, but the magnification should be such that the Ni thin plate, Cu thin plate, and Zn thin plate are in the same field of view.

Next, the field of view is shifted so that the Ni thin plate, Cu thin plate, and Zn thin plate are included in the same field of view in that order.

Next, a pre-adjustment luminance histogram is created for the field of view with 256 gradations, with the minimum luminance being 0 and the maximum luminance being 255. After that, the position of the luminance peak derived from each thin metal plate is adjusted to a predetermined position depending on the type of material and the type of thin metal plate. Then, the difference in the position of the luminance peak derived from each thin metal plate is controlled to a predetermined magnitude. In this embodiment, the brightness is adjusted so that the position of the luminance peak derived from the Cu thin plate is about 150 (145 to 155). Furthermore, the contrast is adjusted so that the difference between the position of the luminance peak derived from the Ni thin plate and the position of the luminance peak derived from the Cu thin plate is about 55 (45 to 65), and the difference between the position of the luminance peak derived from the Cu thin plate and the position of the luminance peak derived from the Zn thin plate is about 45 (35 to 55). When adjusting the contrast, the brightness is adjusted as necessary to maintain the position of the luminance peak derived from the Cu thin plate at about 150.

Next, while maintaining the contrast, the field of view of the electron microscope is moved to a position where the R-T-B permanent magnet alloy plate is located. The brightness is appropriately controlled depending on the type of material and the type of thin metal plate. In this embodiment, the magnification is increased so that the white phase (R-rich phase 13) does not appear in the field of view and only the main phase 11 appears in the field of view. The magnification may be up to about 10,000 times. Furthermore, the brightness is adjusted so that the peak position of the brightness histogram is about 110 (105 to 115). Finally, the standard sample is retrieved from the electron microscope.

Next, we will explain how to perform brightness analysis.

First, prepare the material to be subjected to the brightness analysis. Below, we will explain the case where the material is an alloy for an R-T-B system permanent magnet. Next, process the alloy for an R-T-B system permanent magnet so that its cross section can be observed, and prepare an evaluation sample. Multiple evaluation samples may be prepared and the results obtained may be averaged.

Next, the imaging mode is set to backscattered electron imaging, and the observation range is set. Measurement conditions vary depending on the type of material and the microstructure to be evaluated, but when evaluating the above-mentioned microstructure of an R-T-B-based permanent magnet alloy, the observation range is set at a magnification of 350x and a pixel count of 1280 x 960 pixels. Note that with the above-mentioned magnification and pixel count, the observation range is 360 μm x 270 μm.

Next, the portion of the evaluation sample included in the above observation range is divided into sections at regular intervals. There is no particular limit to the size of each section, and it varies depending on the type of material and the microstructure to be evaluated. When evaluating the above microstructure of an R-T-B permanent magnet alloy, it is preferable to set the size to 40 pixels or more and 60 pixels or less, and it is particularly preferable to set it to 50 pixels. Figure 9 shows an image obtained by actually observing the cross section of the evaluation sample and dividing it into sections with a size of 50 pixels. A cross section parallel to the thickness direction of the R-T-B permanent magnet alloy was observed, and the size of the image in Figure 9 is 1280 pixels (= 360 μm) horizontally and 960 pixels (= 270 μm) vertically for the entire image in Figure 9.

Next, a luminance histogram is created for each section. For example, Figure 10 shows the result of creating a luminance histogram for the seventh section from the top and the third section from the left in Figure 9. The fine structure of each section is then identified from the shape of the luminance histogram.

When evaluating the above-mentioned microstructure of an R-T-B permanent magnet alloy, the shape of the brightness histogram is used to determine whether each section has a normal structure or a non-columnar structure. Specifically, the peak position and standard deviation (σ) of the brightness histogram are obtained for each section, and the result is determined from the obtained peak position and σ. A specific example of an R-T-B permanent magnet alloy is described below.

The inventors observed the cross sections of many R-T-B permanent magnet alloys. They then visually observed the backscattered electron images of the cross sections obtained by the above method and classified them into normal structures and six types of non-columnar crystal structures. They then observed the peak positions and σ trends when creating brightness histograms for each structure.

As a result, the brightness histograms of most sections with non-columnar crystal structure had peak positions between 130 and 200, and σ between 20 and 40. In contrast, the brightness histograms of most sections with normal structure 7 had σ above 40.

Figure 11 shows the average peak positions and average σ for the sections corresponding to normal structure and the sections corresponding to six types of non-columnar crystal structure. Most of the sections corresponding to structure 1a, structure 1b, and structure 3 to structure 6 had peak positions between 130 and 200, and σ between 20 and 40.

From the above, sections in which the peak position of the brightness histogram is between 130 and 200 and σ is between 20 and 40 can be considered to have a non-columnar crystal structure. The ratio of the number of sections in which the peak position of the brightness histogram and σ are within the above ranges relative to all sections can be considered to be the area ratio of the non-columnar crystal structure.

It was found that R-T-B permanent magnets made using R-T-B permanent magnet alloys in which the area ratio of the non-columnar crystal structure identified by the above method is 1.0% or more and 30.0% or less have good magnetic properties. In particular, it was found that when it is 1.0% or more, HcJ at high temperatures tends to be high. It was also found that when it is 30% or less, Hk/HcJ at room temperature tends to be high. It was found that it is preferable for the area ratio of the non-columnar crystal structure to be 4.0% or more and 30.0% or less.

Furthermore, a section in which the peak position of the brightness histogram is between 130 and 200 and σ is between 30 and 40 can be regarded as a structure other than the chilled crystal structure in the non-columnar crystal structure. It was also found that the area ratio of the structure other than the chilled crystal structure in the non-columnar crystal structure may be 50% or more, preferably 85% or more and 95% or less, and more preferably 87% or more and 91% or less.

When the dimensions of each section used to create a brightness histogram are too large, two or more different types of tissues will often be contained within the same section. When the dimensions of each section are too small, the number of pixels contained in each section will decrease, reducing the accuracy of the brightness histogram created for each section. As a result, the peak position and σ of the brightness histogram will become unclear. In other words, there is an appropriate range for the dimensions of each section depending on the type of material, etc.

<Composition of R-T-B based permanent magnet alloy>
There is no particular restriction on the composition of the R-T-B permanent magnet alloy. R represents at least one rare earth element. The rare earth elements refer to Sc, Y, and lanthanoid elements belonging to Group 3 of the long periodic table. The lanthanoid elements include, for example, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The rare earth elements are classified into light rare earth elements and heavy rare earth elements, and the heavy rare earth elements refer to Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, and the light rare earth elements are rare earth elements other than the heavy rare earth elements. In this embodiment, from the viewpoint of suitably controlling the manufacturing cost and magnetic properties, Nd and/or Pr may be included as R. In addition, from the viewpoint of improving HcJ in particular, both the light rare earth elements and the heavy rare earth elements may be included. The content of the heavy rare earth element is not particularly limited, and the heavy rare earth element may not be contained. The content of the heavy rare earth element is, for example, 5 mass % or less (including 0 mass %).

The R content may be 29.0 mass% or more and 33.5 mass% or less. If the R content is small, the HcJ of the resulting R-T-B system permanent magnet tends to decrease. If the R content is large, the Br tends to decrease.

The B content may be 0.70% by mass or more, or 0.80% by mass or more. The upper limit of the B content may be less than 1.0% by mass, less than 0.96% by mass, or 0.90% by mass or less. When the B content is smaller than the stoichiometric ratio, specifically when it is less than 1.0% by mass, the area ratio of the non-columnar crystal structure is likely to be 1.0 to 30.0%. When the B content is small, Hk/HcJ is likely to decrease. Furthermore, abnormal grain growth is likely to occur when sintering at high temperatures. Furthermore, Hk/HcJ is difficult to increase when sintering at low temperatures. When the B content is large, abnormal grain growth is likely to occur. And Br is likely to decrease.

T is a transition metal element. It may be Fe alone or Fe and Co. The Co content may be 0% by mass or more and 2.0% by mass or less. The smaller the Co content, the more likely abnormal grain growth occurs when sintering at high temperatures. Also, when sintering at low temperatures, it is difficult to increase Hk/HcJ. When the Co content is large, Br and HcJ decrease. Also, R-T-B permanent magnet alloys tend to be expensive.

The R-T-B system permanent magnet alloy may contain M. M is one or more selected from Cu, Ga, Zr and Al. There is no particular limit to the total content of M. It may be 0.1% by mass or more and 2.0% by mass or less.

There is no particular limit to the Cu content. For example, it may be 0.05% by mass or more and 0.50% by mass or less. By including 0.05% by mass or more of Cu, abnormal grain growth is less likely to occur when sintering at high temperatures. In addition, even when sintering at low temperatures, Hk/HcJ tends to be sufficiently high. By including 0.50% by mass or less of Cu, Br tends to be improved.

There is no particular limit to the Ga content. For example, it may be 0% by mass or more and 1.0% by mass or less. By including Ga, abnormal grain growth is less likely to occur when sintering at high temperatures. Furthermore, even when sintering at low temperatures, Hk/HcJ tends to be sufficiently high. Furthermore, HcJ also tends to be improved. By including 1.0% by mass or less of Ga, Br tends to be improved.

There is no particular limit to the Al content. For example, it may be 0.05% by mass or more and 1.00% by mass or less. By including 0.05% by mass or more of Al, HcJ is more likely to be improved. In addition, abnormal grain growth is less likely to occur even when sintering at high temperatures. Furthermore, Hk/HcJ tends to be sufficiently high even when sintering at low temperatures. By including 1.00% by mass or less of Al, Br is more likely to be improved.

There is no particular limit to the Zr content. For example, it may be 0.05% by mass or more and 1.00% by mass or less. By including 0.05% by mass or more of Zr, Hk/HcJ tends to be sufficiently high even when sintering at low temperatures. In addition, abnormal grain growth is less likely to occur even when sintering at high temperatures. By including 1.00% by mass or less of Zr, Br tends to be improved.

The Fe content is the substantial balance in the components of the R-T-B permanent magnet alloy. The Fe content being the substantial balance means that the total content of elements other than R, Fe, Co, B, and M is 1.0 mass% or less.

The R-T-B system permanent magnets produced using the above-mentioned alloys for R-T-B system permanent magnets can be processed into any shape before use. The shape of the R-T-B system permanent magnet is not particularly limited, and can be any shape, such as a rectangular parallelepiped, hexahedron, flat plate, columnar such as a square prism, or a C-shaped cylinder in cross section.

In addition, R-T-B permanent magnets include both magnet products that are made by processing and magnetizing the magnet, and magnet products that are not magnetized.

<Method of manufacturing R-T-B based permanent magnet alloy>
As an example of a method for producing an RTB system alloy for permanent magnets, a strip casting method using a casting apparatus shown in FIG. 12 will be described.

The casting apparatus shown in FIG. 12 has a chill roll 21 and a tundish 23. Although not shown in FIG. 12, the casting apparatus may also have well-known components of a casting apparatus, such as a refractory crucible and a collection container. There is no particular limit to the type of refractory crucible. Examples include an alumina crucible, a mullite crucible, and a zirconia crucible. There is no particular limit to the material of the chill roll 21. Examples include copper, copper alloys, copper with a plated or sprayed surface, and copper alloys with a plated or sprayed surface.

First, the raw metals are weighed out to obtain the desired alloy composition for the R-T-B permanent magnet alloy, and then mixed to obtain a raw material mixture.

Next, the obtained raw material mixture is loaded into a refractory crucible, and the loaded raw material mixture is melted to obtain a molten alloy. There are no particular limitations on the method for melting the raw material mixture. For example, a method in which the refractory crucible loaded with the raw material mixture is placed in a high-frequency vacuum induction furnace and heated can be mentioned.

Then, the obtained molten alloy is cast by a strip casting method using the casting device shown in FIG. 12 to obtain a cast alloy ribbon (alloy for R-T-B type permanent magnets). Specifically, the molten alloy 25 is supplied to a cooling roll 21, the inside of which is water-cooled, via a tundish 23. The molten alloy 25 supplied onto the cooling roll 21 is cooled and detached from the cooling roll 21 on the opposite side of the tundish 23, and is recovered as a cast alloy ribbon.

The surface of the cast alloy ribbon that contacts the chill roll 21 is the roll surface, and the surface opposite the roll surface is the free surface. The roll surface is cooled more rapidly by the chill roll 21 than the free surface. Here, non-columnar crystal structures are more likely to form when the cooling rate is faster than that of normal structures. Among non-columnar crystal structures, chill crystal structures are more likely to form when the cooling rate is particularly fast. Therefore, chill crystal structures and non-columnar crystal structures are more likely to form on the roll surface than on the free surface. Note that the roll surface is on the left side in all of Figures 1 to 9.

The area ratio of the non-columnar crystal structure (chill crystal structure) varies mainly depending on the temperature (casting temperature) of the molten alloy 25, the composition of the molten alloy 25, the roll surface roughness Rz of the chill roll 21, the shape of the irregularities that affect the roll surface roughness Rz, the molten alloy head pressure 27, the roll circumferential speed of the chill roll 21, and the supply speed of the molten alloy 25 to the chill roll 21 (the supply speed per unit contact width between the molten alloy 25 and the chill roll 21). The temperature of the molten alloy 25 is, for example, 1300 to 1600°C. The roll surface roughness Rz of the chill roll 21 is, for example, 10 to 50 μm. The roll circumferential speed of the chill roll 21 is, for example, 0.5 to 2.5 m/s. The supply speed of the molten alloy 25 to the chill roll 21 is, for example, 0.5 to 2.5 kg/(min·cm).

The molten metal head pressure 27 will be explained below. The molten metal head pressure 27 refers to the depth of the molten alloy 25 in contact with the chill roll 21. The higher the molten metal head pressure 27, the stronger the pressure with which the molten alloy 25 is pressed against the chill roll 21 by its own weight. Therefore, the higher the molten metal head pressure 27, the stronger the adhesion between the molten alloy 25 and the chill roll 21, and the higher the heat transfer rate from the molten alloy 25 to the chill roll 21. At the same time, the length of the molten alloy 25 in contact with the chill roll 21 also changes. Therefore, if other conditions are the same, the higher the molten metal head pressure 27, the thicker the cast alloy ribbon will be. In practice, the molten metal head pressure 27 is selected appropriately in accordance with other casting conditions.

As mentioned above, the faster the cooling rate, the easier it is for non-columnar crystal structure (chilled crystal structure) to form. Therefore, the greater the molten metal head pressure 27, the greater the area proportion of non-columnar crystal structure (chilled crystal structure).

One method for increasing the molten metal head pressure 27 is to increase the amount of molten alloy 25 in the tundish 23. One method for increasing the amount of molten alloy 25 in the tundish 23 is to increase the speed at which the molten alloy 25 is supplied to the tundish 23. If the speed at which the molten alloy 25 is supplied to the tundish 23 is increased, the cast alloy ribbon will become thicker. One method for increasing the molten metal head pressure 27 without changing the thickness of the cast alloy ribbon is to increase the peripheral speed of the cooling roll 21.

In other words, when producing a cast alloy ribbon of the same thickness, if the supply rate of the molten alloy 25 is fast and the peripheral speed of the chill roll 21 is fast, the molten metal head pressure 27 will be large, and if the supply rate of the molten alloy 25 is slow and the peripheral speed of the chill roll 21 is slow, the molten metal head pressure 27 will be small.

There are no particular limitations on the method for producing R-T-B permanent magnets using the R-T-B permanent magnet alloy produced by the above method.

The present invention is not limited to the above-described embodiment, and various modifications can be made within the scope of the present invention. By changing various measurement conditions, the method can also be used to identify the microstructure of alloys other than R-T-B permanent magnet alloys and the microstructure of materials other than alloys.

<Method of manufacturing R-T-B based permanent magnets>
There is no particular limitation on the method for producing an R-T-B system permanent magnet using the R-T-B system permanent magnet alloy produced by the above method. For example, a method having the following steps may be mentioned.
(a) a crushing process for crushing an alloy for an R-T-B system permanent magnet; (b) a molding process for molding the obtained alloy powder; (c) a sintering process for sintering the molded body to obtain an R-T-B system permanent magnet; (d) an aging treatment process for aging the R-T-B system permanent magnet; (e) a cooling process for cooling the R-T-B system permanent magnet; (f) a processing process for processing the R-T-B system permanent magnet; (g) a grain boundary diffusion process for diffusing a heavy rare earth element into the grain boundaries of the R-T-B system permanent magnet; and (h) a surface treatment process for surface-treating the R-T-B system permanent magnet.

[Crushing process]
The R-T-B based permanent magnet alloy is pulverized (pulverization process). The pulverization process includes a coarse pulverization process in which the alloy is pulverized to a particle size of several hundred μm to several mm, and a fine pulverization process in which the alloy is pulverized to a particle size of about several μm.

(Coarse grinding process)
The R-T-B system permanent magnet alloy is coarsely pulverized until the particle size is about several hundred μm to several mm (coarse pulverization step). This produces a coarsely pulverized powder of the R-T-B system permanent magnet alloy. The coarse pulverization can be performed, for example, by absorbing hydrogen into the R-T-B system permanent magnet alloy, and then releasing hydrogen based on the difference in the amount of hydrogen absorbed between different phases, thereby causing self-destructive pulverization by dehydrogenation (hydrogen absorption pulverization).

In addition to using hydrogen absorption pulverization as described above, the coarse pulverization process may be performed in an inert gas atmosphere using a coarse pulverizer such as a stamp mill, jaw crusher, or Braun mill.

The oxygen concentration is adjusted by controlling the atmosphere in each manufacturing process. From the viewpoint of obtaining high magnetic properties, the amount of oxygen in the final R-T-B permanent magnet may be reduced. To achieve this, the oxygen concentration in each process from the crushing process to the sintering process described below may be set to 100 ppm or less.

(Fine pulverization process)
After the R-T-B permanent magnet alloy is coarsely pulverized, the obtained coarsely pulverized powder of the R-T-B permanent magnet alloy is finely pulverized until the average particle size is about several μm (fine pulverization step). In this way, finely pulverized powder of the R-T-B permanent magnet alloy is obtained. By further finely pulverizing the coarsely pulverized powder, finely pulverized powder having particles of, for example, 1 μm to 10 μm, or 3 μm to 5 μm can be obtained.

Fine pulverization is performed by further pulverizing the coarsely pulverized powder using a fine pulverizer such as a jet mill, a ball mill, a vibration mill, a wet attritor, etc., while appropriately adjusting conditions such as pulverization time. A jet mill is a method in which a high-pressure inert gas (e.g., _N2 gas) is released from a narrow nozzle to generate a high-speed gas flow, and the high-speed gas flow accelerates the coarsely pulverized powder of the raw alloy, causing collisions between the coarsely pulverized powder of the raw alloy and with a target or a container wall, thereby pulverizing the powder.

When the coarsely crushed powder of the raw alloy is finely crushed, adding a grinding aid such as zinc stearate, urea, stearic acid amide, or oleic acid amide can produce a finely crushed powder with a high degree of orientation during molding.

[Molding process]
The finely pulverized powder is molded into a desired shape (molding process). In the molding process, the finely pulverized powder is filled into a die surrounded by an electromagnet and pressurized to mold the finely pulverized powder into a desired shape. This is done while applying a magnetic field, which causes a specific orientation in the finely pulverized powder, and molding is performed in the magnetic field with the crystal axes oriented. This results in a molded body. The obtained molded body is oriented in a specific direction, resulting in an R-T-B system permanent magnet with stronger magnetic anisotropy.

The pressure applied during molding may be 30 MPa to 300 MPa. The magnetic field applied may be 950 kA/m to 1600 kA/m. The magnetic field applied is not limited to a static magnetic field, and may also be a pulsed magnetic field. A static magnetic field and a pulsed magnetic field may also be used in combination.

As for the molding method, in addition to the dry molding in which the finely pulverized powder is molded as is as described above, wet molding can also be used in which a slurry in which the finely pulverized powder is dispersed in a solvent such as oil is molded.

The shape of the compact obtained by compacting the finely pulverized powder is not particularly limited, and can be any shape depending on the desired shape of the R-T-B system permanent magnet, such as a rectangular parallelepiped, flat plate, column, ring, etc.

[Sintering process]
The compact obtained by molding in a magnetic field and forming into a desired shape is sintered in a vacuum or inert gas atmosphere to obtain an R-T-B system permanent magnet (sintering process). The sintering temperature needs to be adjusted depending on various conditions such as the composition, the pulverization method, the particle size and particle size distribution, etc. The compact is sintered, for example, by heating in a vacuum or in the presence of an inert gas at 1000°C to 1200°C for 1 hour to 48 hours. This causes liquid phase sintering of the finely pulverized powder, and an R-T-B system permanent magnet (a sintered body of an R-T-B system magnet) with an improved volume ratio of main phase particles is obtained. After obtaining the sintered body by sintering the compact, the sintered body may be quenched from the viewpoint of improving production efficiency.

[Aging treatment process]
After sintering the compact, the R-T-B system permanent magnet is subjected to aging treatment (aging treatment step). After sintering, the R-T-B system permanent magnet obtained is subjected to aging treatment by holding it at a temperature lower than that at the time of sintering. The aging treatment may be, for example, a two-stage heating at a temperature of 700°C or higher and 1000°C or lower for 10 minutes to 6 hours, and then a one-stage heating at a temperature of 500°C to 700°C for 10 minutes to 6 hours, or a one-stage heating at a temperature around 600°C for 10 minutes to 6 hours, and the treatment conditions are appropriately adjusted depending on the number of times the aging treatment is performed. Such aging treatment can improve the magnetic properties of the R-T-B system permanent magnet. The aging treatment step may also be performed after the processing step described below.

[Cooling process]
After the R-T-B system permanent magnet is subjected to aging treatment, the R-T-B system permanent magnet is rapidly cooled in an Ar gas atmosphere (cooling step). This makes it possible to obtain the above-mentioned R-T-B system permanent magnet. The cooling rate is not particularly limited, and may be 30°C/min or more.

[Processing process]
The obtained R-T-B system permanent magnet may be processed into a desired shape as necessary (processing step). Processing methods include, for example, shaping such as cutting and grinding, and chamfering such as barrel polishing.

[Grain boundary diffusion process]
A heavy rare earth element may be further diffused into the grain boundaries of the processed R-T-B system permanent magnet (grain boundary diffusion step). There are no particular limitations on the method of grain boundary diffusion. For example, it may be carried out by attaching a compound containing a heavy rare earth element to the surface of the R-T-B system permanent magnet by coating or vapor deposition, etc., and then carrying out a heat treatment. It may also be carried out by carrying out a heat treatment on the R-T-B system permanent magnet in an atmosphere containing vapor of a heavy rare earth element. Grain boundary diffusion can further improve the HcJ of the R-T-B system permanent magnet.

[Surface treatment process]
The RTB system permanent magnet obtained by the above steps may be subjected to a surface treatment such as plating, resin coating, oxidation treatment, or chemical conversion treatment (surface treatment step).

In this embodiment, a processing step, a grain boundary diffusion step, and a surface treatment step are performed, but these steps do not necessarily have to be performed.

The R-T-B permanent magnet obtained in this manner has good magnetic properties and a wide sintering temperature range. As a result, the R-T-B permanent magnet described above is a magnet that can be produced stably.

The R-T-B permanent magnet obtained in this manner is suitable for use as a magnet in, for example, a surface permanent magnet (SPM) rotating machine with a magnet attached to the rotor surface, an interior permanent magnet (IPM) rotating machine such as an inner rotor brushless motor, or a permanent magnet reluctance motor (PRM). Specifically, the R-T-B permanent magnet is suitable for use in spindle motors and voice coil motors for rotating hard disks in hard disk drives, motors for electric and hybrid cars, motors for electric power steering in automobiles, servo motors for machine tools, motors for vibrators in mobile phones, motors for printers, motors for generators, and the like.

The invention will be described in more detail below with reference to examples, but the invention is not limited to these examples.

(Preparation of R-T-B based permanent magnet alloy)
Nd metal (purity 99% by mass or more), an alloy of Nd and Pr (didymium, purity 99% by mass or more), Tb metal (purity 99% by mass or more), ferroboron (Fe content 80% by mass, B content 20% by mass), Fe metal (purity 99% by mass or more), Co metal (purity 99% by mass or more), Zr metal (purity 99% by mass or more), Cu metal (purity 99% by mass or more), Al metal (purity 99% by mass or more), and Ga metal (purity 99% by mass or more) were weighed out to obtain the alloy composition shown in Table 1 below and mixed to obtain a raw material mixture. In Table 1, "T.RE" is the total content (mass%) of rare earth elements (Nd, Pr, Dy, and Tb), and "T.RL" is the total content (mass%) of light rare earth elements (Nd and Pr). "bal." is the remainder. The Fe content is expressed as "bal." to indicate that the Fe content is the remainder when the entire R-T-B system permanent magnet alloy containing elements other than the elements listed in Table 1 is taken as 100 mass %.

The obtained raw material mixture was loaded into an alumina crucible. The alumina crucible loaded with the raw material mixture was placed in a high-frequency vacuum induction furnace, and the atmosphere inside the high-frequency vacuum induction furnace was then replaced with Ar. The high-frequency vacuum induction furnace was then heated to melt the raw material mixture loaded into the alumina crucible, thereby obtaining a molten alloy. The obtained molten alloy was cast by a strip casting method using the casting device shown in Figure 12, to obtain a cast alloy ribbon (R-T-B system alloy for permanent magnets). Casting was performed in an Ar atmosphere.

The temperature of the molten alloy, the roll surface roughness Rz of the chill roll, and the molten metal head pressure were appropriately selected so as to obtain the R-T-B rare permanent magnet alloy shown in Table 2. Specifically, the temperature of the molten alloy (casting temperature) was selected within the range of 1300 to 1600°C, the roll surface roughness Rz of the chill roll was selected within the range of 5 to 50 μm, and the molten metal head pressure was selected within the range of 5 to 25 mm. The material of the chill roll was a copper alloy. The roll peripheral speed of the chill roll and the supply speed of the molten metal to the chill roll (supply speed per unit contact width between the molten metal and the chill roll) were adjusted to the values shown in Table 2 below.

(Adjusting the electron microscope)
A cross section of the obtained R-T-B system permanent magnet alloy was observed with an electron microscope (manufactured by JEOL Ltd.) and the brightness and contrast of the electron microscope were adjusted in preparation for calculating the area ratio of the non-columnar crystal structure by brightness analysis.

First, the obtained R-T-B permanent magnet alloy was heat-treated at 1000°C for 120 minutes to obtain two heat-treated plates for incorporation into the standard sample.

Next, a 0.1 mm thick Ni thin plate, a 0.1 mm thick Cu thin plate, and a 0.1 mm thick Zn thin plate were prepared. Next, the heat-treated plate, Ni thin plate, Cu thin plate, and Zn thin plate were embedded in an embedding resin for electron microscope observation. At this time, a standard sample was prepared by arranging the heat-treated plate, Ni thin plate, Cu thin plate, Zn thin plate, and heat-treated plate in this order so that the cross sections of each plate were parallel to the thickness direction.

Next, the cross section of the standard sample was mirror polished and gold was vapor-deposited.

The standard sample was then placed in the electron microscope. The imaging mode was backscattered electron imaging, with a magnification of 150x and a pixel count of 1280 x 960 pixels.

Next, the field of view was adjusted so that the Ni thin plate, Cu thin plate, and Zn thin plate were in the same field of view in that order.

Next, a pre-adjustment luminance histogram was created with 256 gradations, with the minimum luminance at 0 and the maximum luminance at 255, and the brightness was adjusted so that the position of the Cu luminance peak was approximately 150 (145-155). Furthermore, the contrast was adjusted so that the difference between the positions of the Ni luminance peak and the Cu luminance peak was approximately 55 (45-65), and the difference between the positions of the Cu luminance peak and the Zn luminance peak was approximately 45 (35-55). When adjusting the contrast, the brightness was additionally adjusted as necessary to maintain the position of the Cu luminance peak at approximately 150.

Next, while maintaining the contrast, the field of view was moved to a position where the heat-treated product was located. Then, the magnification was increased so that the white phase (R-rich phase 13) was not visible and only the main phase 11 was visible. The maximum magnification was about 10,000 times. Furthermore, the brightness was adjusted so that the peak position of the brightness histogram was about 110 (105 to 115). Finally, the standard sample was retrieved from the electron microscope.

(Evaluation of R-T-B based permanent magnet alloys)
The microstructure of the RTB system permanent magnet alloy was evaluated using an electron microscope whose brightness and contrast were adjusted by the above method.

First, samples for evaluation were taken. Specifically, in the process in which one production lot of R-T-B permanent magnet alloy is supplied by the above-mentioned strip casting method, alloy pieces were sampled from the R-T-B permanent magnet alloy supplied at regular time intervals. Thirty alloy pieces were randomly taken from the R-T-B permanent magnet alloy, and each alloy piece was used as an evaluation sample.

Then, the thickness of the evaluation samples was measured.

Of the 30 evaluation samples, the second, third, and fourth thickest samples were selected as thick samples, the 14th, 15th, and 16th thickest samples were selected as near-average samples, and the 27th, 28th, and 29th thickest samples were selected as thin samples.

Next, the nine selected evaluation samples were attached with instant adhesive and processed so that a cross section parallel to the thickness direction could be observed. When attaching the evaluation samples, they were arranged so that it was clear which evaluation samples were thick samples, near-average samples, or thin samples.

Next, the nine evaluation samples attached with instant adhesive were embedded in resin together to form one embedded resin sample. The cross section parallel to the thickness direction was then mirror-finished to create an observation surface.

Next, the imaging mode was set to backscattered electron imaging, the magnification was set to 350x, and the number of pixels was set to 1280 x 960 pixels, and the observation range was set. The observation range was selected from the observation surface of each evaluation sample in an average state. Next, the area ratio of the non-columnar crystal structure in the observation range was calculated by analysis.

The specific analysis procedure is explained below. First, the above observation range was divided into sections at regular intervals. The dimensions of each section were 50 pixels.

Next, a luminance histogram was created for each section. The peak position and σ of the luminance histogram were then obtained for each section.

Next, each section was judged as to whether it had a non-columnar crystal structure. Specifically, sections in which the peak position of the brightness histogram was between 130 and 200 and σ was between 20 and 40 were deemed to have a non-columnar crystal structure. Sections in which the peak position of the brightness histogram or σ was outside the above range were deemed not to have a non-columnar crystal structure. All sections within the observation range were judged as to whether they had a non-columnar crystal structure, and the number of sections in the observation range that had a non-columnar crystal structure was divided by the number of all sections to determine the area ratio of the non-columnar crystal structure in the cross section of each evaluation sample.

The area ratio of the non-columnar crystal structure in the cross section of each evaluation sample was then averaged to calculate the area ratio of the non-columnar crystal structure in the cross section of one production lot of R-T-B permanent magnet alloy. The results are shown in Table 2.

(Preparation of R-T-B based permanent magnets)
The R-T-B alloy for permanent magnets was allowed to absorb hydrogen at room temperature, and then subjected to a hydrogen pulverization treatment (coarse pulverization) in which hydrogen was removed in a vacuum at 600° C. for 1 hour, to obtain an alloy powder (coarse pulverized powder). In this example, each step from the hydrogen pulverization treatment to sintering (fine pulverization and molding) was performed in an Ar atmosphere with an oxygen concentration of less than 50 ppm or in a vacuum.

Next, zinc stearate and stearic acid amide were added to the alloy powder as grinding aids and mixed using a Nauta mixer. The amount of zinc stearate added was 0.05 parts by mass per 100 parts by mass of the coarsely ground powder. The amount of stearic acid amide added was 0.05 parts by mass per 100 parts by mass of the coarsely ground powder. After that, fine grinding was performed using a jet mill to produce a finely ground powder with an average particle size of about 3.0 μm.

The resulting finely pulverized powder was filled into a die placed inside an electromagnet, and a magnetic field of 1200 kA/m was applied while a pressure of 120 MPa was applied to perform magnetic field molding, resulting in a compact.

The resulting compact was then sintered in a vacuum at 1050°C for 4 hours and then rapidly cooled to obtain a sintered body having the magnet composition shown in Table 1. The resulting sintered body was then subjected to a two-stage aging treatment at 900°C for 1 hour and at 500°C for 1 hour (both in an Ar atmosphere) to obtain an R-T-B system permanent magnet (R-T-B system sintered magnet).

[Magnetic properties]
The Br, HcJ and Hk/HcJ of each sample were measured using a B-H tracer. Br was measured for all samples at room temperature. HcJ and Hk/HcJ were measured at room temperature for sample numbers 1 to 5. HcJ and Hk/HcJ were measured at 150°C for the other samples. Note that Hk in this example is the value of the magnetic field when the magnetization is Br x 0.9 in the second quadrant of the demagnetization curve.

The R-T-B permanent magnets of sample numbers 1 to 4, which were made using alloys for R-T-B permanent magnets in which the area ratio of the non-columnar crystal structure, i.e., the area ratio of the sections where the peak position is 130 to 200 and σ is 20 to 40, is 1.0% or more and 30.0% or less, have higher Br, similar HcJ, and higher Hk/HcJ than the R-T-B permanent magnet of sample number 5, which was made using an alloy for R-T-B permanent magnets made under the same conditions except that the area ratio of the non-columnar crystal structure is outside the above range. In particular, the Hk/HcJ at room temperature was higher.

The R-T-B permanent magnets of sample numbers 7 to 12, which were made using R-T-B permanent magnet alloys with an area ratio of non-columnar crystal structure of 4.0% or more and 30.0% or less, had higher HcJ than sample number 6, 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%.

From samples 7 to 12, R-T-B permanent magnets produced under the same conditions except that the area ratio of the non-columnar crystal structure in the R-T-B permanent magnet alloy was changed from 4.0% to 30.0% showed a tendency for the HcJ to increase as the area ratio of the non-columnar crystal structure in the R-T-B permanent magnet alloy increased.

Sample No. 13 has a B content of 0.96 mass%, which is higher than the other samples. Therefore, even under casting conditions similar to those of the other samples, the area ratio of the non-columnar crystal structure was reduced. As a result, HcJ was reduced compared to the other samples.

In addition, it was confirmed that in all samples in which the area ratio of non-columnar crystal structures was 1.0% or more and 30.0% or less, the area ratio of structures other than chill crystal structures in the non-columnar crystal structures was 85% or more and 95% or less.

Reference Signs List 1: Chilled crystal structure 3: Micro-dot R-rich phase-containing structure 4: Dotted R-rich phase-containing structure 5: Micro-linear R-rich phase-containing structure 6: Large dotted R-rich phase-containing structure 7: Normal structure 11: Main phase 13: R-rich phase 21: Chill roll 23: Tundish 25: Molten alloy 27: Molten metal head pressure

Claims (4)

obtaining an image of a cross-section of the material;
Dividing the obtained image into a number of sections;
measuring the luminance of a plurality of pixels included in each partition;
obtaining a luminance histogram for each section from the luminance of each pixel;
Identifying the type of microstructure of each section from the peak position and peak standard deviation in the brightness histogram of each section;
Calculating the total area percentage of the sections having the specific microstructure ,
A method for evaluating a material, in which a luminance histogram is created in 256 gradations, with the minimum luminance being 0 and the maximum luminance being 255, and the total area ratio of sections in which the peak position is 130 to 200 and the peak standard deviation is 20 to 40 is calculated . The method for evaluating a material according to claim 1, wherein the material is an alloy. The material evaluation method according to claim 2, wherein the alloy is an R-T-B type permanent magnet alloy, R is a rare earth element, T is a transition metal element, and B is boron. embedding two or more image preparation plates in a substrate to contain them within the same field of view of an electron microscope;
creating an unadjusted luminance histogram for the field of view;
adjusting the positions of the luminance peaks originating from each thin plate in the unadjusted luminance histogram to a predetermined position and adjusting the difference between the positions of the luminance peaks originating from each thin plate to a predetermined size;
The method for evaluating a material according to any one of claims 1 to 3 , wherein an image of a cross section of the material is obtained using an electron microscope adjusted using a method for adjusting an electron microscope comprising the steps of:

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