Steel processing method and electrical steel sheet

Abstract

The present invention allows an electromagnetic steel sheet having Al appropriately diffused therein to be fabricated at a relatively low cost. Disclosed is a steel material processing method involving subjecting a steel material for the electromagnetic steel sheet to a diffusion process in an oxidizing atmosphere, said steel material having a surface on which an aluminum-containing layer is formed. The steel material preferably contains more than 0.00 mass% and not more than 7.00 mass% of Si, and more than 0.00 mass% and not more than 2.00 mass% of Al. The aluminum-containing layer is preferably in the form of a plating layer having a thickness of 2-30 μm. The composition of the plating layer includes Al and may further include, for example, Fe. The diffusion process is preferably performed under a processing condition for achieving a change gradient such that the Al concentration gradually decreases in the direction from the surface of the steel material toward the inside.

Description

Technical Field

[0001] This disclosure relates to a steel material processing method and an electromagnetic steel sheet.

Background Art

[0002] There is a known technique for reducing the loss of an electromagnetic steel sheet by forming a plating layer on the surface of the electromagnetic steel sheet by aluminaizing treatment (molten aluminum plating treatment) and then creating a state where the Al concentration is inclined from the surface by diffusion treatment in a non-oxidizing atmosphere (see, for example, Patent Documents 1 and 2).

[0003] There is also a known technique for reducing the loss of an electromagnetic steel sheet by applying a slurry containing Al to the surface of the electromagnetic steel sheet and then creating a state where the Al concentration is inclined from the surface by diffusion treatment in a non-oxidizing atmosphere (see, for example, Patent Document 3).

[0004] There is also a known technique for reducing the loss of an electromagnetic steel sheet by forming a plating layer on the surface of the electromagnetic steel sheet by aluminaizing treatment, then allowing Al to penetrate by high-temperature long-time diffusion treatment in an oxidizing atmosphere, and realizing a grain-oriented electromagnetic steel sheet with an increased Al concentration. (see, for example, Patent Document 4).

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, with the conventional technologies described above, it is difficult to manufacture electrical steel sheets with properly diffused Al at a relatively low cost.

[0007] Therefore, in one aspect, this disclosure aims to manufacture electrical steel sheets in which Al is appropriately diffused at a relatively low cost. [Means for solving the problem]

[0008] In one aspect, a steel treatment method is provided, which involves performing a diffusion treatment in an oxidizing atmosphere on a steel material for electrical steel sheets having an aluminum-containing layer formed on its surface. [Effects of the Invention]

[0009] In one respect, this disclosure makes it possible to manufacture electrical steel sheets with properly diffused Al at a relatively low cost. [Brief explanation of the drawing]

[0010] [Figure 1] This flowchart schematically shows the flow of each step included in the steel processing method according to this embodiment. [Figure 2] This diagram schematically shows the cross-sectional structure of steel obtained by aluminizing treatment. [Figure 3] This diagram schematically shows the cross-sectional structure of steel obtained by diffusion treatment. [Figure 4] This is an explanatory diagram of the desired aluminum diffusion mode, and shows an example of the aluminum diffusion mode in steel obtained by the steel processing method of this embodiment. [Figure 5] This flowchart provides a schematic overview of the flow of each step included in the steel processing method according to Comparative Example 1. [Figure 6] This flowchart provides a schematic overview of the flow of each step included in the steel processing method of other comparative examples. [Figure 7] This figure schematically shows the Al concentration profile of the steel obtained by Comparative Example 2. [Figure 8]Explanatory drawing of the method for calculating the Al penetration amount (Part 1). [Figure 9] Explanatory drawing of the method for calculating the Al penetration amount (Part 2). [Figure 10] Plot showing the relationship between Al slope gradient plate thickness ratio × (Al penetration amount plate thickness ratio)^0.5 and magnetic flux density reduction rate. [Figure 11] Plot showing the relationship between Al penetration amount plate thickness ratio and magnetic flux density reduction rate.

Mode for Carrying Out the Invention

[0011] Hereinafter, each embodiment will be described in detail while referring to the accompanying drawings. Note that the dimensional ratios in the drawings are merely examples and are not limited thereto, and the shapes and the like in the drawings may be exaggerated partially for the convenience of explanation. Also, in the following description, various substances may be specified by generally known abbreviations or chemical symbols such that aluminum is referred to as aluminum (or Al).

[0012] FIG. 1 is a flowchart schematically showing the flow of each step included in the steel material processing method according to the present embodiment. FIG. 2 is a diagram schematically showing the cross-sectional structure of a steel material (steel material after aluminizing treatment) obtained by aluminizing treatment.

[0013] The steel material processing method of the present embodiment first includes a step (step S100) of preparing a steel material (base material) having a desired plate thickness. The steel material having a desired plate thickness can be obtained, for example, by rolling a material (for example, in the form of a steel slab or a steel sheet) obtained by casting or the like. The desired plate thickness is arbitrary, but preferably may be substantially the same as the thickness of the final product (for example, an electromagnetic steel sheet) manufactured through this method. In this case, the rolling step after the next aluminizing step becomes unnecessary, and the manufacturing cost can be reduced.

[0014] The desired plate thickness of the steel material is arbitrary, but may be, for example, from 0.20 mm to 0.35 mm, and more preferably from 0.25 mm to 0.30 mm.

[0015] In this embodiment, the steel material is for an electromagnetic steel sheet and preferably contains Si and Al. The reason for adding Si and Al to the steel material is that the crystal magnetic anisotropy is reduced and the hysteresis loss is decreased. Also, the addition of Si and Al increases the electrical resistance of the steel material, thus contributing to the reduction of eddy current loss. Therefore, the addition of Si and Al leads to a reduction in the iron loss (hysteresis loss and eddy current loss) of the steel material. On the other hand, excessive addition of Si and Al not only decreases the magnetic flux density but also deteriorates the workability in the manufacturing process (such as rolling and pressing, etc.) of the steel material. Note that, for example, an electromagnetic steel sheet as a final product can be press-processed into a desired shape to form a rotor core or a stator core of a rotating electrical machine. Considering such conflicting matters, the steel material (base material) preferably contains Si within a range greater than 0.00 mass% and less than or equal to 7.00 mass%, and Al within a range greater than 0.00 mass% and less than or equal to 2.00 mass%.

[0016] Next, the steel material processing method of this embodiment includes an aluminum-containing layer forming step (step S102) of forming an aluminum-containing layer (refer to reference numeral 20 in FIG. 2) on the steel material. The aluminum-containing layer can be formed, for example, by an aluminizing treatment. For example, the aluminum-containing layer is in the form of a plating layer having a thickness within a range of 2 μm or more and 30 μm or less. For example, a plating layer having a thickness within a range of 2 μm or more and 30 μm or less may be formed by gas wiping or the like.

[0017] In a modified example, the aluminum-containing layer may be formed by a process of applying an aluminum-containing slurry instead of the aluminizing treatment. However, in the case of applying the aluminum-containing slurry, it is disadvantageous that it is difficult to make Al penetrate into the interior of the steel sheet in the subsequent diffusion treatment.

[0018] The composition of the aluminum-containing layer (plating layer) may consist solely of aluminum, for example, in the case of an aluminum-containing layer formed by sputtering. Alternatively, the composition of the aluminum-containing layer (plating layer) may include aluminum plus one or more of iron (Fe), silicon (Si), and manganese (Mn). Preferably, it contains Al, Fe, and Si. For example, the plating layer may contain Al in the range of 20% by mass or more and 100% by mass or less, Fe in the range of 0.00% by mass or more and 80% by mass or less, and Si in the range of 0.00% by mass or more and 15% by mass or less. The plating layer may be a single layer or a multi-layer layer.

[0019] Furthermore, the steel processing method of this embodiment, as described above, eliminates or reduces the need to perform a process to reduce the thickness of the steel material with the aluminum-containing layer formed on it by rolling or the like as a pretreatment before the subsequent diffusion treatment process, by preparing a steel material of the desired thickness. However, in a modified example, a process to reduce the thickness of the steel material with the aluminum-containing layer formed on it by rolling or the like may be performed.

[0020] Furthermore, the steel treatment method of this embodiment includes a diffusion treatment step (step S104) in which the steel on which the aluminum-containing layer has been formed undergoes a diffusion treatment in an oxidizing atmosphere.

[0021] The diffusion treatment is a process to create a state in which the Al concentration is gradient from the surface of the steel material, based on the Al contained in the aluminum-containing layer. In other words, the diffusion treatment diffuses the Al contained in the aluminum-containing layer from the surface of the steel material in the thickness direction to form an Al diffusion layer (see reference numeral 32 in Figure 3). As will be described later with reference to Figure 4, the Al diffusion layer has a gradient (gradient change) in Al concentration such that the Al concentration becomes higher towards the surface in the thickness direction.

[0022] Furthermore, in this embodiment, the diffusion treatment in an oxidizing atmosphere can also form an insulating film (see reference numeral 30 in Figure 3) on the surface of the steel material. That is, the diffusion treatment in an oxidizing atmosphere oxidizes the aluminum-containing layer on the surface of the steel material, thereby forming an oxide layer on the surface of the steel material as an insulating film. At this time, an oxide layer based on aluminum and iron contained in the aluminum-containing layer and oxygen contained in the oxidizing atmosphere is formed simultaneously with the Al diffusion layer. The oxide layer is a single or composite oxide of Al, Fe, Si, or Mn, but may also contain other elements depending on the components contained in the aluminum-containing layer. For example, the oxide layer may contain 20 to 80% by mass of Al or Fe. This facilitates the formation of the desired oxide layer described later.

[0023] In this specification, "oxidizing atmosphere" has the same general meaning, but in substance, it is a concept that excludes atmospheres that substantially do not contain oxygen, and includes reactions carried out in air. Therefore, in this embodiment, vacuum equipment is not required. That is, manufacturing costs can be reduced. In addition, continuous processing is possible, and manufacturing efficiency can be increased.

[0024] The diffusion treatment may be carried out under predetermined temperature conditions for a predetermined treatment time (hereinafter also referred to as "diffusion time").

[0025] If the diffusion temperature is too low, not only will the formation of the oxide layer and the penetration and diffusion of Al take a long time, but the desired oxide layer may not be obtained. In this case, sufficient insulation cannot be obtained. Conversely, if the diffusion temperature is too high, the desired oxide layer may not be obtained either. Also, if the diffusion time is too short or too long, the desired aluminum diffusion pattern described later cannot be obtained.

[0026] Therefore, taking these conflicting factors into consideration, the diffusion treatment may preferably be carried out at a temperature in the range of 900°C or higher and 1200°C or lower (hereinafter also referred to as the "diffusion temperature") and for a diffusion time in the range of 0.1 minutes or higher and 1920 minutes or lower. This makes it possible to facilitate both the formation of the desired oxide layer and the realization of the desired aluminum diffusion pattern, as will be described later.

[0027] Furthermore, the details of the manufacturing conditions (temperature conditions and processing time) related to the diffusion treatment may be adapted according to the thickness of the aluminum-containing layer obtained in step S102 (i.e., the plating thickness), etc., so that the desired oxide layer and the desired aluminum diffusion pattern (penetration amount and gradient state) can be obtained simultaneously. Note that the temperature conditions are a concept that includes not only the diffusion temperature, but also the heating rate to the diffusion temperature and the cooling rate from the diffusion temperature.

[0028] The desired thickness of the oxide layer is essentially arbitrary as long as it functions as an insulating film, but it is desirable for the thickness to be as thin as possible. On the other hand, if the oxide layer is too thick, the total magnetic flux of the steel material will decrease. Taking this into consideration, the desired thickness of the oxide layer is within the range of 1 μm or more and 25 μm or less.

[0029] The desired aluminum diffusion mode is basically arbitrary as long as the Al diffusion layer has a gradient (gradient change) in Al concentration. However, preferred ranges for various parameters will be discussed later with reference to Figure 4.

[0030] Figure 4 is an explanatory diagram of the desired aluminum diffusion pattern, and shows an example of the aluminum diffusion pattern in steel obtained by the steel processing method of this embodiment. In Figure 4, the upper part schematically shows a cross-sectional view of the steel, and the lower part shows the aluminum diffusion pattern in the thickness direction of the steel. The aluminum diffusion pattern in the steel is shown as an Al concentration profile of the steel in the thickness direction, with the horizontal axis being the distance from the center of the plate thickness of the steel and the vertical axis being the Al concentration. The Al concentration profile of the steel in the thickness direction may have symmetry with respect to the center of the plate thickness. In Figure 4, d1 represents the thickness of the insulating film, and d2 represents the diffusion depth of Al. Also in Figure 4, α1 represents the Al concentration in the insulating film, and α2 represents the Al concentration on the surface of the Al diffusion layer.

[0031] As shown in Figure 4, in this embodiment, the Al diffusion layer has a gradient (gradient change) in Al concentration such that the Al concentration increases towards the surface in the thickness direction. Specifically, in the Al diffusion layer, the gradient is largest at the surface of the Al diffusion layer (i.e., the interface with the oxide layer) and gradually decreases towards the center of the plate thickness.

[0032] Here, if the Al concentration on the surface of the Al diffusion layer (hereinafter also referred to as "surface Al concentration of the Al diffusion layer") is low, the iron loss reduction effect described above will not be obtained, and if it is too high, as described above, it may lead to a decrease in magnetic flux density and a decrease in workability in the steel manufacturing process (rolling, pressing, etc.). Also, as described above, if the diffusion depth of Al is too shallow, the iron loss reduction effect described above will not be obtained, and if it is too deep, as described above, it may lead to a decrease in magnetic flux density, etc.

[0033] Taking these conflicting factors into consideration, the surface Al concentration of the Al diffusion layer is preferably within the range of 1.50% by mass or more and 10.00% by mass or less. Next, with reference to Figure 5, the effects of this embodiment will be further explained in comparison with a steel processing method according to a comparative example.

[0034] Figure 5 is a flowchart that schematically shows the flow of each step included in the steel processing method according to Comparative Example 1.

[0035] The steel treatment method according to Comparative Example 1 is the same as the steel treatment method according to this embodiment in the preparation step (step S100) and the aluminum-containing layer formation step (step S102), but differs from there onward.

[0036] Specifically, the steel treatment method according to Comparative Example 1 includes a diffusion treatment step (step S104') in which a diffusion treatment is performed in a non-oxidizing atmosphere on the steel on which the aluminum-containing layer has been formed. The diffusion treatment conditions other than the atmosphere may be the same as those of the diffusion treatment step (step S104) described above. Such a diffusion treatment in a non-oxidizing atmosphere differs from the diffusion treatment in an oxidizing atmosphere performed in the steel treatment method according to this embodiment, because it is a non-oxidizing atmosphere, no oxide layer is formed.

[0037] Next, the steel treatment method according to Comparative Example 1 includes a step (step S106') of forming an insulating film on the steel that has undergone diffusion treatment in an oxidation-free atmosphere. In this case, the insulating film may be a different film from the oxide layer.

[0038] In this comparative example 1, vacuum equipment is required for diffusion treatment in an oxidation-free atmosphere.

[0039] In contrast, in this embodiment, as described above, by utilizing diffusion treatment in an oxidizing atmosphere, vacuum equipment and the like are unnecessary, thereby reducing manufacturing costs. Furthermore, continuous processing is possible, which can increase manufacturing efficiency.

[0040] Furthermore, in Comparative Example 1, it is necessary to add a separate step to form an insulating film after the diffusion treatment in an oxidation-free atmosphere.

[0041] In contrast, in this embodiment, as described above, an oxide layer can be formed as an insulating film by diffusion treatment in an oxidizing atmosphere. In other words, diffusion treatment in an oxidizing atmosphere is a process that can simultaneously obtain the desired oxide layer and the desired aluminum diffusion pattern.

[0042] Thus, according to the steel processing method of this embodiment, electrical steel sheets in which Al is appropriately diffused in a manner having a gradient (change slope) of Al concentration can be manufactured at a relatively low cost.

[0043] Figure 6 is a flowchart that schematically shows the flow of each step included in the steel processing method of another comparative example (hereinafter also referred to as "Comparative Example 2").

[0044] The steel treatment method according to Comparative Example 2 is the same as the steel treatment method according to this embodiment in the preparation step (step S100) and the aluminum-containing layer formation step (step S102), but differs from there onward.

[0045] Specifically, the steel treatment method according to Comparative Example 2 includes a treatment step (step S104") in which a grain growth acceleration treatment is performed in an oxidizing atmosphere on the steel material on which the aluminum-containing layer has been formed. The grain growth acceleration treatment differs from the diffusion treatment in an oxidizing atmosphere performed in the steel treatment method according to this embodiment in that it is performed under conditions that allow the Al concentration to diffuse uniformly within the steel material. Specifically, the grain growth acceleration treatment tends to have a significantly higher diffusion temperature and a significantly longer diffusion time compared to the diffusion treatment in an oxidizing atmosphere performed in the steel treatment method according to this embodiment.

[0046] Figure 7 is a schematic diagram showing the Al concentration profile of the steel obtained by Comparative Example 2, and is in contrast to the lower part of Figure 4 in this embodiment.

[0047] As shown in Figure 7, the Al diffusion layer has approximately the same Al concentration at the surface of the Al diffusion layer and at the center of the steel plate thickness. That is, the Al concentration is approximately constant from the surface of the Al diffusion layer to the center of the steel plate thickness (see concentration α3). In such steel materials, a sufficient reduction in iron loss due to Al penetration cannot be obtained.

[0048] In contrast, in this embodiment, as described above, a steel material having a desired oxide layer and a desired aluminum diffusion pattern can be obtained without excessively raising the diffusion temperature or excessively lengthening the diffusion time. In other words, an electrical steel sheet with good properties that is less prone to a decrease in magnetic flux density can be manufactured efficiently from the viewpoint of energy efficiency and manufacturing efficiency.

[0049] Thus, according to the steel processing method of this embodiment, electrical steel sheets in which Al is appropriately diffused in a manner having a gradient (change slope) of Al concentration can be manufactured at a relatively low cost. [Examples]

[0050] Next, we will describe some examples of steel materials that the present inventor has actually processed using the steel processing method according to this embodiment.

[0051] The inventors of this application prepared three types of steel materials (denoted as steel types A, B, and C) having the component characteristics shown in Table 1 below, and performed processing based on the steel material processing method according to this embodiment, resulting in the results shown in Table 2. Specifically, in Example 1, etc., a ring-shaped test piece with an outer diameter of 64 mm and an inner diameter of 50 mm was cut out by laser from a steel plate having the chemical composition of Table 1, and the test piece was subjected to aluminizing treatment to obtain the plating thickness shown in Table 2. Then, after the aluminizing treatment, diffusion treatment was performed under the conditions shown in Table 2. In Example 5, etc., a single-plate test piece was cut out by laser from a steel plate having the chemical composition of Table 1, and the test piece was similarly subjected to aluminizing treatment to obtain the plating thickness shown in Table 2. Then, after the aluminizing treatment, diffusion treatment was performed under the conditions shown in Table 2. Note that there is a difference in the shape of the test piece, whether it is a ring shape or a single plate (rectangular shape with a size of 20 × 50 mm), but this does not substantially affect the test results (e.g., iron loss reduction rate, magnetic flux density reduction rate, etc.).

[0052] Furthermore, the conditions shown in Table 2 indicate whether or not annealing was performed. The annealing conditions were 750°C for 60 minutes followed by furnace cooling, but this is optional. Annealing affects the test results by reducing the effects of laser cutting (e.g., increased iron loss). Note that annealing may be unnecessary for steel plates used in rotors.

[0053] The three types of steel materials (denoted as steel grades A, B, and C) all contain 0-7.00 mass% Si, 0-1.00 mass% Mn, and 0-2.00 mass% Al. While a base material with this composition is preferred, it is not limited to this. In Table 2, "mass%" represents mass percentage.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Table 3]

[0057] [Table 4]

[0058] [Table 5] As a result, the results in Tables 3, 4, and 5 were obtained. Evaluation 1 in Tables 3 and 4 is an evaluation by FE (Field Emission)-EPMA (Electron Probe Micro Analyzer) analysis. The Al concentration and Fe concentration in the oxide layer in Evaluation 1 may be the concentration at or near the center of the oxide layer. Note that the Fe concentration tends to be higher on the surface side of the oxide layer. The iron loss in Evaluation 2 in Table 5 is the result of AC magnetic measurement, and the magnetic flux density in Evaluation 2 is the result of DC magnetic measurement. Note that the diffusion depth-to-plate thickness ratio is the value obtained by dividing the diffusion depth by the plate thickness. In Table 5, magnetic flux density represents the magnetic flux density (B50) at a magnetization force of 5000 A / m, and iron loss represents the iron loss (W10 / 400) at a frequency of 400 Hz and a magnetic flux density of 1.0 T. Also, in Table 5, the iron loss reduction rate is the rate obtained by dividing the reduction amount from the iron loss of the untreated product by the iron loss of the untreated product.

[0059] As can be seen from Evaluation 1 in Tables 3 and 4 and Evaluation 2 in Table 5, each example exhibits a reduction in iron loss greater than 0% at a frequency of 400 Hz and a magnetic flux density of 1 T, and a reduction in magnetic flux density of 25% or less at 5 kA / m, demonstrating excellent characteristics.

[0060] As can be seen from Evaluation 1 in Tables 3 and 4 and Evaluation 2 in Table 5, in Example 3, the magnetic flux density reduction rate (reduction rate compared to the untreated product) was relatively large because the diffusion depth-to-plate thickness ratio was too large, but it remained below 25%. On the other hand, in Comparative Example 1, the magnetic flux density reduction rate was relatively large because the surface Al concentration of the diffusion layer was too high, exceeding 25%. From these evaluation results, it can be seen that in the steel treatment method of this embodiment, in order to suppress the amount of Al penetration, the plating thickness is preferably 50 μm or less, and more preferably 30 μm or less (20% or less of the plate thickness). Furthermore, it can be seen that the surface Al concentration of the diffusion layer is preferably 14.0 mass% or less, and more preferably less than 10.0 mass%.

[0061] Examples 6 and 7 and Comparative Examples 2 and 3 use a low-oxygen potential atmosphere instead of air, as shown in Table 2. A low-oxygen potential atmosphere is an atmosphere containing oxygen, hydrogen, and argon, and has a significantly lower oxygen concentration than air. Therefore, as shown in Evaluation 1 of Tables 3 and 4, the thickness of the oxide layer becomes relatively small. The thickness of the oxide layer is preferably 1 μm or more, as described above, in order to ensure the necessary electrical insulation as an insulating film. The oxide layer may be a single layer or a multi-layer layer, but in the case of a multi-layer layer, it is preferably 1 μm or more overall. It can be expected that the iron loss will be smaller as the oxide layer is thicker (see, for example, the significant difference between Comparative Examples 2 and 3 and Examples 8 and 9). As shown in Tables 3 and 4, the inventors of this application used two evaluation parameters: the Al penetration amount to plate thickness ratio (unit: mass%) and the Al gradient plate thickness ratio × (Al penetration amount to plate thickness ratio)^0.5 (unit: mass%^1.5 / [μm]^2). The symbol "^" in ^0.5 and ^2 represents exponentiation; for example, ^0.5 represents raising to the power of 0.5. The symbol "*" in Table 4 represents multiplication.

[0062] The Al penetration-to-plate thickness ratio is the value obtained by dividing the amount of aluminum penetration in the Al diffusion layer (hereinafter also referred to as "Al penetration amount") by the base plate thickness, based on the aluminum diffusion depth and the aluminum concentration at the center of the base plate thickness (hereinafter also referred to as "central Al concentration"). Specifically, in this test, the Al penetration amount was calculated as follows based on the aluminum diffusion depth, surface Al concentration, and central Al concentration. If the aluminum diffusion depth is less than the base plate thickness / 2, the amount of Al penetration = (surface Al concentration - central Al concentration) × aluminum diffusion depth × 1 / 2 In this case, the amount of Al penetration corresponds to the area S11 of the triangle shown by the dashed line in Figure 8. In this case, the three vertices of the triangle correspond to the position on the base material surface corresponding to the surface Al concentration, the position on the base material surface corresponding to the central Al concentration, and the position inward from the base material surface by the diffusion depth of aluminum corresponding to the central Al concentration, as shown in Figure 8. If the aluminum diffusion depth is greater than or equal to the base material thickness / 2, then the amount of Al penetration = (surface Al concentration + central Al concentration - base material Al concentration × 2) × aluminum diffusion depth × 1 / 2 In this case, the amount of Al penetration corresponds to the area S12 of the trapezoid shown by the dashed line in Figure 9. In this case, the four vertices of the trapezoid correspond to the position on the base material surface corresponding to the surface Al concentration, the position on the base material surface corresponding to the base material Al concentration, the position inward from the base material surface by the diffusion depth of aluminum corresponding to the central Al concentration, and the position inward from the base material surface by the diffusion depth of aluminum corresponding to the base material Al concentration, as shown in Figure 9. The case where the aluminum diffusion depth ≥ base material thickness / 2 corresponds to the case where the diffusion depth-to-thickness ratio is 0.50 or greater in Table 3. The base material Al concentration corresponds to the base material components in Table 2 (see also D0 in Figure 9). The base material thickness corresponds to the thickness in Table 2, and is the total thickness minus the thickness of the oxide layer.

[0063] The Al gradient plate thickness ratio is the gradient of change in aluminum concentration with respect to the change in thickness direction in the Al diffusion layer. The Al gradient plate thickness ratio is the value obtained by dividing the Al gradient by the base plate thickness. Specifically, in this test, the Al gradient was calculated as follows. Al gradient = (Surface Al concentration - Central Al concentration) / Aluminum diffusion depth The base material thickness corresponds to the thickness in Table 2, and is the total thickness minus the thickness of the oxide layer. Furthermore, similar to the Al penetration rate-to-thickness ratio described above, if the aluminum diffusion depth ≥ base material thickness / 2, then the aluminum diffusion depth can be considered equal to the base material thickness / 2.

[0064] Figure 10 is a plot plot showing the results for each example and comparative example, with the horizontal axis plotting Al gradient plate thickness ratio × (Al penetration amount plate thickness ratio)^0.5 and the vertical axis plotting the magnetic flux density reduction rate. Figure 10 also shows the interpolated curves for these plotted points.

[0065] As can be seen from Figure 10, if the ratio of Al gradient plate thickness × (al penetration amount plate thickness ratio)^0.5 is 0.000400 or less, the magnetic flux density reduction rate can be kept to approximately 25% or less. In Figure 10, the two plot points where the magnetic flux density reduction rate exceeds 25% correspond to Comparative Examples 1 and 4. Each plot point where the magnetic flux density reduction rate is 25% or less corresponds to each example.

[0066] Figure 11 is a plot diagram showing the results for each example and comparative example, with the Al penetration amount to plate thickness ratio on the horizontal axis and the magnetic flux density reduction rate on the vertical axis. Figure 11 also shows linear interpolation lines for these plotted points.

[0067] As can be seen from Figure 11, when the Al penetration-to-plate thickness ratio is 5.50 or less, it can be expected that the magnetic flux density reduction rate will be kept to approximately 25% or less. In Figure 11, the two plotted points where the magnetic flux density reduction rate exceeds 25% correspond to Comparative Examples 1 and 4. Each plotted point where the magnetic flux density reduction rate is 25% or less corresponds to each example. The plotted points corresponding to Comparative Examples 1 and 4 also have an Al penetration-to-plate thickness ratio of 5.50 or less, but as mentioned above with reference to Figure 10, for example, they do not satisfy the condition that Al gradient plate thickness ratio × (Al penetration-to-plate thickness ratio)^0.5 is 0.000400 or less, and can therefore be excluded based on this condition.

[0068] Although each embodiment has been described in detail above, the invention is not limited to any particular embodiment, and various modifications and changes are possible within the scope described in the claims. Furthermore, it is possible to combine all or more of the components of the embodiments described above.

[0069] For example, in the embodiments described above, the diffusion treatment may be performed immediately after the formation of the Al-containing layer, or it may be performed after processing such as pressing following the formation of the Al-containing layer. Furthermore, the formation of the Al-containing layer may be achieved not only by aluminizing or slurry, but also by vapor deposition, sputtering, calorizing, shot peening, foil bonding, etc. [Explanation of Symbols]

[0070] 20...Aluminum-containing layer, 30...Insulating coating (oxide layer), 32...Al diffusion layer (aluminum diffusion layer)

Claims

1. The process includes a step of performing a diffusion treatment in an oxidizing atmosphere on a steel material for electrical steel sheets, which has an aluminum-containing layer formed on its surface. The aluminum-containing layer is formed uniformly across the entire surface of the flat steel material. In the diffusion treatment step, temperature conditions and treatment time are selected to form an oxide layer on the surface of the steel material with a thickness of 1 μm or more and 25 μm or less. A method for processing steel, wherein the step of performing the diffusion treatment does not include an additional step of further diffusing the aluminum that has been diffused in the diffusion treatment.

2. The steel material treatment method according to claim 1, wherein the steel material before the formation of the aluminum-containing layer contains silicon (Si) in a range greater than 0.00 mass% and less than or equal to 7.00 mass%, aluminum (Al) in a range greater than 0.00 mass% and less than or equal to 2.00 mass%, and manganese (Mn) in a range greater than 0.00 mass% and less than or equal to 1.00 mass%.

3. The aluminum-containing layer is in the form of a plating layer with a thickness in the range of 2 μm to 30 μm. The steel treatment method according to claim 2, wherein the composition of the plating layer consists of aluminum (Al), or, in addition to aluminum (Al), contains one or more of iron (Fe), silicon (Si), and manganese (Mn).

4. The steel treatment method according to claim 2, wherein the diffusion treatment is performed under treatment conditions such that a gradient of change is obtained in which the aluminum concentration gradually decreases from the surface of the steel material toward the center of the plate thickness.

5. The steel treatment method according to claim 2, wherein the diffusion treatment is performed at a temperature in the range of 900°C to 1200°C and for a time in the range of 0.1 minutes to 1920 minutes.

6. The steel treatment method according to any one of claims 1 to 5, wherein the diffusion treatment simultaneously forms an aluminum diffusion layer based on aluminum contained in the aluminum-containing layer and an oxide layer based on aluminum contained in the aluminum-containing layer and oxygen contained in the oxidizing atmosphere.

7. An oxide layer that forms an insulating film on the surface of a base material, comprising an oxide layer containing one or more of aluminum, iron, silicon, and manganese, It includes an aluminum diffusion layer in which the aluminum concentration gradually decreases from the surface of the base material toward the center of the base material thickness, The oxide layer has a thickness in the range of 1 μm to 25 μm and contains aluminum or iron in the range of 20% to 80% by mass, in an electrical steel sheet.

8. An oxide layer that forms an insulating film on the surface of a base material, comprising an oxide layer containing one or more of aluminum, iron, silicon, and manganese, It includes an aluminum diffusion layer in which the aluminum concentration gradually decreases from the surface of the base material toward the center of the base material thickness, The gradient of change in aluminum concentration with respect to the change in thickness direction in the aluminum diffusion layer is defined as the Al gradient, and the value obtained by dividing the Al gradient by the base plate thickness is defined as the Al gradient plate thickness ratio. Furthermore, when the amount of aluminum penetration in the aluminum diffusion layer is calculated based on the diffusion depth of aluminum in the aluminum diffusion layer and the concentration of aluminum at the surface of the base material and at the center of the base material plate thickness, the value obtained by dividing the amount of penetration by the thickness of the base material plate is defined as the Al penetration amount-to-thickness ratio. Al slope plate thickness ratio × (Al penetration amount plate thickness ratio) １／２ However, the electrical steel sheet is within the range of greater than 0 and less than or equal to 0.000400.

9. The electromagnetic steel sheet according to claim 8, wherein the Al penetration amount to plate thickness ratio is greater than 0 and within the range of 5.50 or less.

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