Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

The present invention provides: a grain-oriented electrical steel sheet having an extremely high degree of integration of easy axis <001> of magnetization, which could not be achieved by conventional technologies; and a method for manufacturing the same. According to the present invention, a steel strip which has been subjected to secondary recrystallization annealing and does not have a forsterite film is subjected to plastic deformation of 0.05 or more in true strain by applying a tensile stress thereto in a direction parallel to the rolling direction. Regarding the crystal orientation of the grain-oriented electrical steel sheet after plastic deformation, the average value of the angles α between the rolling direction and a projection straight line obtained by projecting the <001> axis closest to the rolling direction onto the surface is 1.8 degrees or less, the average value of the angles β between the <001> axis and the projection straight line is 1.5 degrees or less, and the average value of the changes ∆β of the angle β per 10 mm in length when viewed in the rolling direction is 0.4 degree or less.

Description

Grain-oriented electrical steel sheet and its manufacturing method

[0001] The present invention is directed to the easy axis of magnetization. <001> The present invention relates to a grain-oriented electrical steel sheet having an extremely high degree of grain concentration in the rolling direction, and a method for manufacturing the same.

[0002] Iron has a body-centered cubic crystal structure at room temperature, <001> It has the property of being easily magnetized in one direction. <001> By orienting the axis in a specific direction, it is possible to improve the magnetic properties when a magnetic field is applied in the direction of the orientation. Grain-oriented electrical steel utilizes a metallurgical phenomenon called secondary recrystallization, and the orientation of the iron crystal grains is the {110} Goss orientation. <001> This steel material has improved magnetic properties in the rolling direction by creating a crystal texture with aligned orientation.

[0003] In the production of oriented electrical steel sheets <001> As a means for increasing the concentration of orientation, a method is known in which precipitates called inhibitors are used to preferentially induce secondary recrystallization of crystal grains having the Goss orientation during final annealing. For example, Patent Document 1 describes a method using AlN as an inhibitor, and Patent Document 2 describes a method using MnS or MnSe as an inhibitor, both of which have been put into practical use industrially. Furthermore, Patent Document 3 describes a method of adding Pb, Sb, Nb, or Te, and Patent Document 4 describes a method of adding Zr, Ti, B, Nb, Ta, V, Cr, or Mo as additive elements that have the effect of strengthening the function of these inhibitors.

[0004] High-temperature solution treatment is required to dissolve inhibitor-forming components and additive elements in the slab. Solution treatment increases manufacturing costs. Therefore, inhibitor-less methods that induce secondary recrystallization without the addition of inhibitor components have been investigated. For example, Patent Document 5 describes a method in which the content of trace elements in highly purified grain-oriented electrical steel sheets is controlled to drive surface energy to preferentially grow the Goss orientation.

[0005] The main use of grain-oriented electromagnetic steel sheets is in the iron cores of transformers. Transformers using grain-oriented electromagnetic steel sheets come in two forms: stacked cores, in which grain-oriented electromagnetic steel sheets are cut into a specified shape and stacked, and wound cores, in which grain-oriented electromagnetic steel sheets are wound into a coil. One of the ways to reduce the iron loss of transformers using grain-oriented electromagnetic steel sheets is to <001> In recent years, with the increasing demand for energy saving, the <001> There is a demand for further increasing the degree of orientation integration.

[0006] Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Publication No. 38-8214 Japanese Patent Laid-Open No. 52-24116 Japanese Patent Laid-Open No. 2000-129356

[0007] In the manufacturing process of grain-oriented electrical steel sheets, annealing to induce secondary recrystallization requires a long heat treatment. For this reason, a steel strip of a predetermined length is usually wound into a coil and then charged into a batch-type annealing furnace for heat treatment. Figure 1 shows the distribution of iron crystal grains in a coil that has undergone secondary recrystallization annealing. <001> 1(a) is a schematic diagram showing the orientation of the Goss orientation. Fig. 1 shows a cross section perpendicular to the central axis of the coil. In the coil immediately after the secondary recrystallization annealing, the grains having the Goss orientation preferentially grew, resulting in the grains shown by the arrows in Fig. 1(a). <001> The orientation is aligned in one direction throughout the entire interior of a single crystal grain. It is also known that the orientation of the aggregate of crystal grains formed immediately after secondary recrystallization annealing varies to a certain extent from the Goss orientation.

[0008] When the coil that has undergone secondary recrystallization annealing is unwound and plastically deformed into a flat steel strip, the shape indicated by the arrows in Figure 1(b) appears. <001> The angle β between the orientation and the rolling direction of the steel strip varies depending on the position within a single crystal grain. As shown in Figure 1(a), the larger the coil diameter during secondary recrystallization, the smaller the change in angle β, and the smaller the coil diameter, the larger the change in angle β. Geometric calculations show that for a coil with a diameter of 3.0 m, the change in angle β per 10 mm in the rolling direction is 0.4 degrees. Because the size of an annealing furnace is finite, the coil diameter cannot be increased beyond 3.0 m. Therefore, as long as the long secondary recrystallization annealing required for grain-oriented electrical steel sheets is performed in coil form, it has been extremely difficult in prior art to reduce the change in angle β per 10 mm in the rolling direction due to coil unwinding to 0.4 degrees or less.

[0009] The cause of deviation of the iron crystal grain orientation from the Goss orientation is not limited to uncoiling, and several other causes are thought to be involved. For example, slight meandering of the steel strip during transportation for rolling can cause the rolling direction of part of the steel strip to deviate by a few degrees from the original direction toward the in-plane direction. As such, deviation of the iron crystal grain orientation from the Goss orientation is due to both variations during metallurgical secondary recrystallization and the manufacturing conditions described above, and therefore could not be completely eliminated even if the steel strip was subjected to long-term secondary recrystallization annealing without being coiled.

[0010] The present invention has been made in view of the above-mentioned problems, and has achieved a magnetization easy axis that was not possible with the prior art. <001> The present invention aims to realize a grain-oriented electrical steel sheet with an extremely high degree of grain integration and to provide a method for manufacturing the same.

[0011] The inventors applied various plastic deformations to a steel strip that had already been subjected to secondary recrystallization annealing, in which the orientation of the crystal grains was shifted from the Goss orientation by several degrees on average, and in which the average change in the angle β per 10 mm of length in the rolling direction was 0.7 degrees, and measured the change in the orientation of the crystal grains before and after the plastic deformation in detail. As a result, they found that applying a tensile stress parallel to the rolling direction to the steel strip and causing plastic deformation in the rolling direction with a true strain of 0.05 or more, preferably 0.06 or more, resulted in the easy axis of magnetization being shifted. <001> It was found that this method is extremely effective in improving the degree of integration. Improvement in the degree of integration was not observed in plastic deformation by rolling, but the phenomenon of increased orientation of the crystal orientation was observed only when plastic deformation by tensile stress was caused. Furthermore, it was found that in order to improve the degree of integration by the above-mentioned plastic deformation, it is preferable to remove the forsterite film formed on the surface of the steel strip after secondary recrystallization, and that there are preferable conditions for the temperature, strain rate, etc. of the steel strip during plastic deformation, and the present invention was completed based on these findings.

[0012] The gist and configuration of the present invention are as follows.

[0013] [1] An area arbitrarily selected from the surface of a grain-oriented electrical steel sheet is 1000 cm 2Regarding the crystal orientation of iron grains at multiple measurement points included in the measurement area, the crystal orientation closest to the rolling direction was <001> the average value of the angle α between the projection line of the axis projected onto the surface and the rolling direction is 1.8 degrees or less, <001> a grain-oriented electrical steel sheet characterized in that the average value of the angle β between the axis and the projection line is 1.5 degrees or less, and the average value of the change Δβ in the angle β per 10 mm length as viewed in the rolling direction is 0.4 degrees or less. [2] The grain-oriented electrical steel sheet according to the above [1], which has a chemical composition containing, in mass % or mass ppm, 2.0 to 4.5% Si, 0.01 to 0.50% Mn, less than 50 ppm each of Se, Te, and O, less than 30 ppm S, and 5 ppm or more but less than 40 ppm of acid-soluble Al, with the balance being Fe and unavoidable impurities. [3] The grain-oriented electrical steel sheet according to [2] above, wherein the chemical composition further contains, in mass % or mass ppm, one or more elements selected from Ni: 1.50% or less, Sn: 0.50% or less, Sb: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, P: 0.50% or less, Cr: 1.50% or less, Co: 100 ppm or less, B: 50 ppm or less, Ti: 50 ppm or less, Nb: 50 ppm or less, Zn: 50 ppm or less, Ga: 50 ppm or less, Bi: 50 ppm or less, W: 30 ppm or less, Pb: 10 ppm or less, and Ge: 10 ppm or less. [4] The grain-oriented electrical steel sheet according to any one of [1] to [3] above, wherein there is at least one location in the measurement region where the thicknesses of two adjacent crystal grains differ by 0.5% or more, and wherein the area ratio of the amount of twinned crystals among the crystal grains in the measurement region is 0% to 20%. [5] The grain-oriented electrical steel sheet according to any one of [1] to [4] above, wherein at least one of the surfaces has been subjected to magnetic domain refinement treatment and at least one of the surfaces has an insulating coating. [6] A method for producing a grain-oriented electrical steel sheet, comprising a tensile stress applying step of applying a tensile stress parallel to the rolling direction to a steel strip that has been subjected to secondary recrystallization annealing and has no forsterite coating, thereby causing plastic deformation of 0.05 or more in true strain. [7] A method for producing a grain-oriented electrical steel sheet according to [6] above, wherein plastic deformation of 0.06 or more in true strain is caused in the tensile stress applying step.[8] The temperature of the steel strip when the plastic deformation is performed is 80°C or higher, or the strain rate in the plastic deformation is 0.5 s -1 The method for producing a grain-oriented electrical steel sheet according to the above [6] or [7], which is as follows:

[0014] According to the present invention, the steel strip after secondary recrystallization is subjected to a tensile stress in a direction parallel to the rolling direction under specific conditions to cause plastic deformation, thereby forming iron crystal grains. <001> The grains can be rotated so that the deviation between the orientation and the rolling direction of the steel strip approaches zero. <001> The deviation of the axis from the rolling direction is 1.8 degrees or less, and the change in angle β per 10 mm of length in the rolling direction is 0.4 degrees or less. <001> Therefore, it is possible to obtain a grain-oriented electrical steel sheet having an extremely high degree of grain integration.

[0015] The grain size of iron in coils subjected to secondary recrystallization annealing <001> Schematic diagrams showing the orientation of the iron crystal grains. (a) shows the state of the coil immediately after secondary recrystallization annealing, and (b) shows the state of the coil after secondary recrystallization annealing. <001> 1 is a schematic diagram showing deviation of orientation from the rolling direction. 2 is a photograph, substituted for a drawing, of the surface of a sample of the grain-oriented electrical steel sheet according to the present invention.

[0016] Hereinafter, embodiments of the present invention will be described in detail.

[0017] <Grain-oriented electrical steel sheet> In one embodiment, the present invention provides a grain-oriented electrical steel sheet having an area of ​​1000 cm2 arbitrarily selected from the surface thereof. 2 Regarding the crystal orientation of iron grains at multiple measurement points included in the measurement area, the one closest to the rolling direction was <001> the average value of the angle α between the projection line of the axis projected onto the surface and the rolling direction is 1.8 degrees or less; <001> The invention relates to a grain-oriented electrical steel sheet characterized in that the average value of the angle β between the axis and the projection line is 1.5 degrees or less, and the average value of the change Δβ in the angle β per 10 mm of length viewed in the rolling direction is 0.4 degrees or less.

[0018] 1. Grain-oriented electrical steel sheet As described below, the grain-oriented electrical steel sheet according to the present invention can be produced by using a grain-oriented electrical steel sheet according to the prior art as a starting material, applying tensile stress parallel to the rolling direction, and then plastically deforming the starting material. The grain-oriented electrical steel sheet used as the starting material may be any material that has a crystal texture in which the deviation angle of the crystal orientation of iron grains from the Goss orientation is within 10 degrees after secondary recrystallization annealing. The starting material may be a material produced for the purpose of implementing the present invention, or may be a grain-oriented electrical steel sheet that is commercially available. The means for aligning the crystal orientation in the starting material may be a method using an inhibitor, or an inhibitorless method that does not use an inhibitor.

[0019] In a preferred embodiment, the grain-oriented electrical steel sheet according to the present invention has a composition containing, in mass % or mass ppm, 2.0 to 4.5% Si, 0.01 to 0.50% Mn, less than 50 ppm each of Se, Te, and O (oxygen), less than 30 ppm S, and 5 ppm or more but less than 40 ppm of acid-soluble Al, with the balance being Fe and unavoidable impurities. These components are the components of typical grain-oriented electrical steel sheets. If the composition of the starting material is within the above range of composition, the extremely high tensile strength of the present invention can be achieved. <001> Orientational integration can be achieved.

[0020] In a more preferred embodiment, the grain-oriented electrical steel sheet according to the present invention further contains, in mass % or mass ppm, one or more elements selected from the group consisting of 1.50% or less Ni, 0.50% or less Sn, 0.50% or less Sb, 0.50% or less Cu, 0.50% or less Mo, 0.50% or less P, 1.50% or less Cr, 100 ppm or less Co, 50 ppm or less B, 50 ppm or less Ti, 50 ppm or less Nb, 50 ppm or less Zn, 50 ppm or less Ga, 50 ppm or less Bi, 30 ppm or less W, 10 ppm or less Pb, and 10 ppm or less Ge. These elements may be added to further improve the magnetic properties.

[0021] 2. Measurement of crystal orientation In the present invention, the crystal orientation of the iron grains contained in the grain-oriented electrical steel sheet that is closest to the rolling direction is measured. <001> The deviation between the axial direction and the rolling direction is evaluated by the method described below. First, an area of ​​1000 cm2 is arbitrarily selected from the surface of a flat grain-oriented electrical steel sheet to be evaluated. 2 Determine the measurement area. The total area of ​​the measurement area is 1000 cm. 2 By setting the measurement area to 1000cm, the evaluation will not be biased towards local trends in the grain-oriented electrical steel sheet, and measurements that represent the entire surface can be obtained. The measurement area may be selected from one location on the surface of the grain-oriented electrical steel sheet, or from multiple locations with a total area of ​​1000cm. 2 However, if the area per location is too small, it may not represent the whole area, so the area per location should be 200cm. 2 It is preferable that the measurement area is set to the above value. The measurement area may be selected from one surface of the grain-oriented electrical steel sheet, or may be selected from both the front and back surfaces. In most cases, the same crystal grains are exposed on both the front and back surfaces of a grain-oriented electrical steel sheet, so when measurement areas are selected from both the front and back surfaces, it is preferable to select them from positions where the two do not overlap.

[0022] Next, the crystal orientation of the iron crystal grains is measured at multiple measurement points included in the measurement area. The measurement points for measuring the crystal orientation can be determined, for example, by drawing a grid with a width of 5 mm in the measurement area and determining the intersections of the grid. The measurement of the crystal orientation of the iron crystal grains is preferably performed using the X-ray Laue method. Because the X-ray beam diameter in the X-ray Laue method is sufficiently small compared to the size of the iron crystal grains, the X-ray beam is often irradiated within the iron crystal grains, allowing for accurate measurement of the crystal orientation. If the X-ray beam accidentally irradiates a grain boundary, preventing accurate measurement of the crystal orientation, the measurement results at that measurement point can be excluded from the data.

[0023] Figure 2 shows the structure of iron crystal grains. <001> 2 is a schematic diagram showing deviation of the crystal orientation from the rolling direction RD of one crystal grain 1a of the grain-oriented electrical steel sheet 1. In FIG. <001> The direction of the axis is indicated by an arrow. <001> The angle formed by a projection line 2, which is a projection of the axial direction onto the surface of the grain-oriented electrical steel sheet 1, and the rolling direction RD is defined as α. <001> The axis swing angle. <001> The angle between the axis and the projection line 2 is defined as β. The angle β is the angle <001> The angle α is the elevation angle of the axis. Both the angle α and the angle β are set to zero or a positive value. Finally, the average value of the angle α and the angle β measured at multiple measurement points is calculated. The closer the average value of the angle α and the angle β is to zero, <001> This means that the axis is well aligned with the rolling direction RD.

[0024] In the grain-oriented electrical steel sheet according to the present invention, the average value of the angle α evaluated by the above-mentioned measurement method is 1.8 degrees or less, and the average value of the angle β is 1.5 degrees or less. In this specification, angles are expressed in degrees. Since the angles α and β of the grain-oriented electrical steel sheet according to the prior art are larger than the above-mentioned average values, the degree of integration of the grain-oriented electrical steel sheet according to the present invention in the Goss orientation is superior to that of the prior art. Since the smaller the average values ​​of the angles α and β, the more preferable, no lower limit is set. As described above, the angles α and β are zero or positive values, so the average values ​​of the angles α and β are zero or greater. Preferably, with respect to the crystal orientation of the iron crystal content, the average value of the angle α is 1.0 degrees or less, and the average value of the angle β is 0.8 degrees or less.

[0025] 3. Measurement of Δβ As explained using Figure 1, the rate of change in the angle β of the same crystal grain as seen in the rolling direction is a good indicator of the deviation in crystal orientation due to unwinding the coil. In the above-described crystal orientation measurement, by setting multiple measurement points parallel to the rolling direction, the change in angle β per 10 mm in the rolling direction, Δβ, can be calculated. As shown in Figure 1(b), the change in angle β of the same crystal grain shows a monotonous increase or decrease, so Δβ is first calculated from a set of such measured values ​​of angle β. Like angles α and β, Δβ is also set to zero or a positive value. Next, the average value of Δβ obtained from multiple sets of measurement points is calculated. The closer Δβ is to zero, the smaller the deviation in angle β due to unwinding the coil.

[0026] In the grain-oriented electrical steel sheet according to the present invention, the average value of the change Δβ in the angle β per 10 mm of length as viewed in the rolling direction, as evaluated by the above-mentioned measurement method, is 0.4 degrees or less. As mentioned above, it was extremely difficult to reduce the Δβ of grain-oriented electrical steel sheets according to the prior art to 0.4 degrees or less due to restrictions on the coil diameter, so the degree of integration of the Goss orientation of the grain-oriented electrical steel sheet according to the present invention is superior to that of the prior art. The smaller the average value of Δβ, the more preferable it is, so no lower limit is set. As mentioned above, Δβ is set to zero or a positive value, so the average value of Δβ is zero or greater. Preferably, the average value of the change Δβ in the angle β per 10 mm of length as viewed in the rolling direction of the crystal grains is 0.3 degrees or less.

[0027] 4. Changes in Grain Thickness: In a preferred embodiment, the grain-oriented electrical steel sheet according to the present invention has at least one location within the measurement region where the thicknesses of two adjacent grains differ by 0.5% or more. Figure 3 is a photograph of the surface of a grain-oriented electrical steel sheet sample according to the present invention, taken using an optical microscope. The horizontal direction of the image corresponds to the rolling direction (RD). The vertical width of the sample is approximately 12 mm. In Figure 3, areas of constant brightness represent the area occupied by a single grain. Differences in contrast occur between different grains due to differences in the crystal orientation of the grains. A step is observed at the grain boundary between adjacent grains due to the difference in thickness between the grains. The difference in thickness between grains can be measured by height measurement using a laser microscope or by known methods such as a micrometer or vernier calipers. According to height measurement using a laser microscope, the maximum size of this step is approximately 2 μm. Since plastic deformation that brings the angle β closer to zero is achieved by slip deformation that reduces the sheet thickness, it is thought that differences in the angles of each grain before tensile deformation result in differences in the amount of thickness reduction, resulting in steps.Since such steps do not occur at the grain boundaries in grain-oriented electrical steel sheets according to the prior art, steps that occur at the grain boundaries can be said to be one piece of evidence that slip deformation has occurred in the thickness reduction direction due to tensile deformation.

[0028] 5. Twin crystals In a preferred embodiment, the grain-oriented electrical steel sheet according to the present invention has an area ratio of crystal grains having twin crystals among the crystal grains included in the measurement area of ​​0% or more and 20% or less. When iron crystal grains have twin crystals, the orientation within the twin crystals is <110> Since the axis is parallel to the rolling direction, <001> It becomes difficult to rotate the crystal so that the axis is parallel to the rolling direction. Therefore, it is preferable that the grain-oriented electrical steel sheet according to the present invention contains as few twins as possible. When twins are present in the grains after plastic deformation, the twins appear as white streaks when observed with an optical microscope, making it easy to identify the grains containing twins. 2The area ratio of the twinned crystal grains in the measurement area can be determined visually. If visual determination is difficult, the crystal orientation of the crystal grains can be evaluated using EBSD, X-rays, etc., and the crystal orientation of the twinned portion is generally {411} <122> From {100} <011> Since the orientation is between 1 and 2, it is possible to accurately evaluate whether twins exist within the crystal grains based on the evaluation of the crystal orientation. The conditions for plastic deformation that are effective in preventing the occurrence of twins will be described later.

[0029] 6. Magnetic Domain Refinement In a preferred embodiment, the grain-oriented electrical steel sheet according to the present invention is subjected to a magnetic domain refinement treatment on at least one of its surfaces. It is known that the effect of magnetic domain refinement becomes more pronounced as the angle β of the crystal grains contained in the grain-oriented electrical steel sheet becomes smaller. Since the grain-oriented electrical steel sheet according to the present invention has a smaller angle β than the prior art, the effect of reducing iron loss obtained by magnetic domain refinement becomes more pronounced. Known methods for magnetic domain refinement can be used, such as forming physical grooves or introducing thermal strain using a laser, electron beam, plasma jet, or the like. As mentioned above, the steps at the grain boundaries of the grain-oriented electrical steel sheet according to the present invention also have a slight magnetic domain refinement effect.

[0030] 7. Insulation Coating: In a preferred embodiment, the grain-oriented electrical steel sheet according to the present invention has an insulation coating on at least one of its surfaces. When manufacturing a transformer with a stacked or wound core using the grain-oriented electrical steel sheet according to the present invention, an insulation coating treatment liquid is applied to the surface of the grain-oriented electrical steel sheet, followed by baking. The type of insulation coating treatment liquid to be applied is not particularly limited, and any known insulation coating treatment liquid and insulation coating are suitable. For example, a method in which a coating liquid containing phosphate, chromate, and colloidal silica is applied to the steel sheet and baked at approximately 800°C is suitable. In addition, various insulation coatings, such as coatings using aluminum borate or TiN, can also be applied. When the insulation coating is baked at a high temperature, it can also be performed as stress relief annealing after plastic deformation, simplifying the manufacturing process.

[0031] <Method for manufacturing grain-oriented electrical steel sheet> In another embodiment, the present invention provides a method for manufacturing a grain-oriented electrical steel sheet, comprising a tensile stress application step of applying a tensile stress parallel to the rolling direction to a steel strip that has been subjected to secondary recrystallization annealing and has no forsterite coating, thereby subjecting the steel strip to plastic deformation with a true strain of 0.05 or more.

[0032] 1. Plastic Deformation In the manufacturing method of grain-oriented electrical steel sheet according to the present invention, secondary recrystallization annealing is performed on a steel strip that does not have a forsterite coating, and plastic deformation is performed by applying tensile stress parallel to the rolling direction. This plastic deformation is due to slip deformation of polycrystalline bodies. The iron crystal grains undergo crystal rotation as the slip deformation occurs, and the angles α and β approach zero, improving the degree of integration into the Goss orientation. To induce this crystal rotation, it is necessary to elongate the steel strip by applying tensile stress parallel to the rolling direction. Even with the same plastic deformation, the degree of crystal orientation is not improved by extending the length in the rolling direction through rolling processing.

[0033] As a specific method for carrying out plastic deformation, for example, a method can be used in which the coil after secondary recrystallization annealing is unwound to form a steel strip, and then the strip is sent to a tension leveler to apply tensile stress in a direction parallel to the rolling direction. Also, when a material having a short length in the rolling direction is used as the starting material, a method can be used in which tensile stress is applied using an Amsler or tensile testing device.

[0034] The exact reason why tensile deformation parallel to the rolling direction causes crystal rotation such that the angles α and β approach zero is not known, but the following mechanism is thought to be at work.

[0035] Plastic deformation of metals can be classified into slip deformation due to slip and twin deformation due to twinning. Slip deformation generally occurs when atoms shift in the slip direction in which they are most densely arranged on the slip plane where atoms are most densely arranged in the crystal lattice. In the body-centered cubic lattice, which is the crystal lattice of iron, the slip plane where atoms are most densely arranged is the {110} plane, but slip often occurs on the {112} and {123} planes as well. The slip direction is <111> is.

[0036] After secondary recrystallization annealing, the iron grains in the steel strip are aligned in the {110} <001> When a steel strip is rolled, plastic deformation occurs with the {112} plane as the slip plane, and the steel strip elongates in the rolling direction, and at the same time, crystal rotation occurs. As a result, the crystal orientation of the steel strip becomes {111} <112> This means that the Goss orientation is not stable during rolling.

[0037] On the other hand, when a steel strip is tensile deformed parallel to the rolling direction, there are four {112} slip systems that are completely equivalent to each other, up, down, left, and right, as viewed from the rolling direction, i.e., the direction of the stress axis. When the Schmid factor, which is an index of the ease of slip deformation, is calculated for these four {112} slip systems, the <001> When the axis is deviated from the rolling direction, the Schmid factor of the {112} slip system, which causes crystal rotation in the direction where the misalignment angles α and β approach zero, shows the highest value. In tensile deformation parallel to the rolling direction, the activation of such {112} slip systems results in <001> It is believed that the crystal rotation occurs in the direction in which the axis is parallel to the rolling direction.

[0038] As a result of crystal rotation, <001> After the axis is completely aligned with the rolling direction, the Schmid factor of the previously active {112} slip system becomes equal to the Schmid factor of the {112} slip system located symmetrically to it, causing crystal rotation to stop and the grain to stabilize in the Goss orientation. In other words, the Goss orientation is stable during tensile processing parallel to the rolling direction. When a pair of symmetrically located {112} slip systems is simultaneously activated and slip deformation without crystal rotation progresses, the grain deformation is reduced in the thickness direction when a pair of upper and lower {112} slip systems is activated, and reduced in the width direction when a pair of left and right {112} slip systems is activated.

[0039] In a steel strip after plastic deformation, the coercive force increases due to the introduction of strain. It is preferable to perform stress relief annealing on the steel strip after plastic deformation in order to improve its magnetic properties. Strain relief annealing can be performed under known conditions. Strain relief annealing may be performed as a standalone heat treatment, or may be performed in conjunction with a heat treatment performed after plastic deformation in order to form a forsterite coating or an insulating coating.

[0040] 2. Removal of Forsterite Film In the manufacturing method of grain-oriented electrical steel sheet according to the present invention, the starting material is a steel strip that has been subjected to secondary recrystallization annealing and does not have a forsterite film. If a forsterite film is present on the surface of the steel strip, it may hinder uniform slip deformation during tensile deformation and inhibit improvement of the degree of orientation due to crystal rotation. If a forsterite film is formed on the surface of the steel strip after secondary recrystallization annealing, the forsterite film is removed.

[0041] The coating can be removed by chemical or physical methods. Specifically, the simplest method for removing the coating is to immerse the steel strip in hydrochloric acid. Alternatively, a steel strip that does not form a forsterite coating may be used by using alumina as an annealing separator in secondary recrystallization annealing.

[0042] In addition to the forsterite coating, it is preferable that no surface layer material that would hinder uniform sliding deformation is present on the surface of the steel strip. For example, if surface oxide scale formed by heat treatment remains, it is preferable to remove it by a method such as pickling before plastic deformation. When using a commercially available grain-oriented electrical steel sheet as the starting material, it may have an insulating coating in addition to the forsterite coating. In such cases, it is sufficient to remove both coatings to expose the base steel portion of the grain-oriented electrical steel sheet.

[0043] 3. Magnitude of true strain In plastic deformation due to the application of tensile stress, it is necessary to cause a certain amount of strain through tensile deformation. Since the Schmid factor varies depending on the magnitude of the misalignment angles α and β, in tensile deformation, crystal rotation occurs preferentially from crystal grains with larger Schmid factors. Therefore, in order to cause crystal rotation in all crystal grains with angles α and β that are not zero, it is effective to cause a certain amount of strain without stopping the tensile deformation midway.

[0044] In the method for producing a grain-oriented electrical steel sheet according to the present invention, plastic deformation with a true strain of 0.05 or more is performed in the tensile stress application step. In this case, a grain-oriented electrical steel sheet can be obtained in which the average value of angle α is 1.8 degrees or less, the average value of angle β is 1.5 degrees or less, and the average value of change Δβ in angle β per 10 mm of length as viewed in the rolling direction is 0.4 degrees or less. In the tensile stress application step, plastic deformation with a true strain of 0.06 or more is preferably performed. If the magnitude of true strain exceeds 0.15, the grain-oriented electrical steel sheet tends to be more susceptible to fracture, so the magnitude of true strain is preferably 0.15 or less.

[0045] 4. Temperature and Strain Rate of Steel Sheet In a preferred embodiment, in the method for producing a grain-oriented electrical steel sheet according to the present invention, the temperature of the steel sheet when plastic deformation is performed is 80°C or higher, or the strain rate in plastic deformation is 0.5 s -1 As mentioned above, there are two types of plastic deformation: slip deformation and twin deformation. <110> Since the axis is parallel to the rolling direction, twin deformation occurs. <001> Therefore, in the method for producing a grain-oriented electrical steel sheet according to the present invention, it is preferable to prevent twin deformation as much as possible.

[0046] When the temperature of the steel plate is 80°C or higher, or when the strain rate is 0.5 s -1 If the temperature is below 80°C, slip deformation is likely to occur. On the other hand, the likelihood of twinning does not depend on the temperature or strain rate. When the temperature of the steel strip during plastic deformation is set to 80°C or higher, or the strain rate during plastic deformation is set to 0.5 s -1By setting the temperature of the steel strip to 80°C or higher and the strain rate in plastic deformation to 0.5 s -1 The more preferable temperature of the steel strip is 150°C or more. The more preferable strain rate in the plastic deformation is 0.1 s -1 If the temperature of the steel strip when plastic deformation is performed is too high, creep-induced deformation of the steel strip may occur, making it difficult to achieve the desired crystal rotation. Therefore, the temperature of the steel strip when plastic deformation is performed is preferably 600°C or less. There is no particular lower limit to the strain rate when plastically deforming the steel strip by applying tensile stress parallel to the rolling direction. The strain rate can be the slowest rate possible using the equipment used for plastic deformation. The strain rate can be, for example, 10 -5 s -1 It may be more than that.

[0047] 5. Magnetic Flux Density In a preferred embodiment, the magnetic flux density B8 of the steel strip to which the method for producing a grain-oriented electrical steel sheet according to the present invention is applied is 1.89 T or more. The magnitude of the magnetic flux density B8 increases as the degree of concentration of the crystal orientation in the Goss orientation increases. <001> If the direction of the axis is significantly different from the rolling direction, the Schmid factor of the {112} slip system decreases, and a large amount of strain is required to cause crystal rotation. If a steel strip with a low degree of integration of the crystal orientation in the Goss orientation and a magnetic flux density B8 of less than 1.89 T before plastic deformation is used, the amount of strain required for crystal rotation becomes large, and there is a risk that the steel strip will break during the plastic deformation process. For this reason, it is preferable that the magnetic flux density B8 of the steel strip before plastic deformation be 1.89 T or more.

[0048] Example 1 Seven commercially available grain-oriented electrical steel sheets, A to G, were purchased. Multiple samples were taken from the coils of the purchased materials. The sample locations were randomly selected from the inner diameter side, outer diameter side, and intermediate position of the coil. Some of the obtained samples were immersed in a mixed solution of hydrofluoric acid and hydrochloric acid to remove the insulating coating and forsterite coating. The remaining samples were left unremoved, leaving the coating on the surface. Multiple tensile test specimens were cut from random locations of these multiple samples so that the tensile direction coincided with the rolling direction. The shape of the tensile test specimens was the No. 5 test specimen specified in Japanese Industrial Standard JIS Z 2241. The parallel portion of the tensile test specimens had a length of 60 mm and a width of 25 mm.

[0049] Next, the crystal orientation of the iron crystal grains was measured using an X-ray Laue diffractometer at the intersections of lattices spaced 5 mm apart on the parallel portion of one side of the obtained tensile test specimen, and the angles α, β, and the change Δβ in angle β per 10 mm in length in the rolling direction were determined. This measurement was carried out in a region where the total area of ​​the measurement region was 1000 cm. 2 The measurement was repeated until the magnetic flux density B8 of the tensile test specimen was measured, and the average value was calculated. The magnetic flux density B8 of the tensile test specimen was also measured using an SST measurement frame. The measured values ​​of the tensile test specimen before tensile deformation are shown in Table 1.

[0050] Next, the tensile test specimens were placed in a tensile testing machine and tensile deformation was performed under the conditions shown in Table 1, while varying the magnitude of true strain, the temperature of the tensile test specimen, and the strain rate. The crystal orientation of the tensile test specimens after tensile deformation was then measured in the same manner as described above. The surface of the parallel section of the tensile test specimen was also observed using an optical microscope, and the area ratio of the twinned region, visible as white streaks, was visually determined. Furthermore, the thickness of the tensile test specimen was continuously measured in a 60 mm direction at the center of the parallel section using a laser microscope to determine whether there was a step of 0.5% or more of the thickness. After these measurements, the tensile test specimens were subjected to strain relief annealing at 850°C for 2 minutes in an Ar atmosphere, and the magnetic flux density B8 was measured using an SST measurement frame. The measured values ​​obtained for the tensile test specimens after tensile deformation are shown in Table 1.

[0051]

[0052] As shown in Table 1, in the tensile test pieces of the invention, in which tensile stress was applied to commercially available grain-oriented electrical steel sheets by the method for manufacturing grain-oriented electrical steel sheets according to the present invention, the average values ​​of the angles α, β, and Δβ all decreased due to tensile deformation, and the grain size of the iron was <001> It can be seen that the axis is rotating in the direction of rolling. Commercially available grain-oriented electrical steel sheets have various compositions, but by applying the present invention, it is possible to obtain a uniform grain structure regardless of the composition. <001> It was confirmed that the degree of orientation of the axes was improved. In particular, in the example of the present invention in which tensile deformation was performed under conditions that suppressed the occurrence of twins, <001> The degree of axial orientation was significantly improved, and an improvement in magnetic flux density B8 was observed.

[0053] Example 2 A steel strip was produced having a composition, in mass % or mass ppm, of 3.2% Si, 0.81% Mn, 0.2 ppm Se, 0.1 ppm Te, 8 ppm O, 3 ppm S, and 5 ppm acid-soluble Al, with the balance being Fe and unavoidable impurities. The steel strip had a thickness of 0.23 mm and was subjected to secondary recrystallization annealing and had no forsterite coating. This steel strip was coated with alumina as an annealing separator during secondary recrystallization annealing, wound into a coil, and then subjected to secondary recrystallization annealing. The coil was cut using a slitter to produce two coils each 300 mm wide, 50 m long, and 1.3 m in outer diameter.

[0054] Only one of the two coils obtained was set on a tension leveler, and a tensile stress of approximately 300 MPa was applied to the coil in the longitudinal direction, i.e., the rolling direction, at a temperature of 30°C, with a strain rate of 0.001 s -1 The coils were then subjected to a magnetic field so that a true strain of 0.1 was generated, resulting in plastic deformation. Next, magnesia was applied to the surface of both the coil of the plastically deformed experimental material and the coil of the non-plastically deformed comparative material, and then annealed at 1150°C to form a forsterite coating. A phosphate-based insulating tension coating was then applied, followed by baking at 850°C for 60 seconds to form the insulating coating. Next, magnetic domain refinement was performed using a laser to obtain the coils of the experimental material and the comparative material.

[0055] Next, 12 steel plates each 30 mm wide and 300 mm long were cut out from the completed coil, and the iron loss value W17 / 50 (W / kg) was measured when each test piece was excited to 1.7 T at a frequency of 50 Hz using an SST test frame. 2 The angles α, β, and Δβ were measured for one side of each of the 12 test pieces under the same conditions as in Example 1, and the average values ​​were calculated. Next, the test pieces were immersed in a mixed solution of hydrofluoric acid and hydrochloric acid to remove the insulating coating and forsterite coating, and the presence or absence of steps of 0.5% or more of the thickness and the area ratio of crystal grains containing twins were determined under the same conditions as in Example 1. The results obtained are shown in Table 2.

[0056]

[0057] As shown in Table 2, the average values ​​of angles α and Δβ were smaller for the experimental material than for the comparative material. On the other hand, the average value of angle β was 1.4 degrees for the experimental material and 1.0 degrees for the comparative material. This is thought to be because the average value of angle β in the coil of the comparative material happened to be smaller than the average value of angle β in the coil of the experimental material before tensile deformation. Since the AC magnetic properties of the experimental material were improved compared to the comparative material, it can be seen that, overall, the application of tensile stress has shifted the easy axis of magnetization. <001> It is clear that the degree of integration has improved.

[0058] 1 Grain-oriented electrical steel sheet 1a Grain 2 Projected line RD Rolling direction α Swing angle β Elevation angle

Claims

1. An area of ​​1000 cm² can be arbitrarily selected from the surface of the grain-oriented electrical steel sheet. 2 Regarding the crystal orientation of iron crystal grains at multiple measurement points included in the measurement area, Closest to the rolling direction <001> The average value of the angle α between the projection line obtained by projecting the axis onto the surface and the rolling direction is 1.8 degrees or less. The aforementioned <001> The average value of the angle β between the axis and the projection line is 1.5 degrees or less. A grain-oriented electrical steel sheet characterized in that the average value of the change in angle β per 10 mm in length as viewed in the rolling direction is 0.4 degrees or less.

2. The grain-oriented electrical steel sheet according to claim 1, having a component composition in mass percent or mass ppm of Si: 2.0 to 4.5%, Mn: 0.01 to 0.50%, Se, Te, and O: less than 50 ppm each, S: less than 30 ppm, and acid-soluble Al: 5 ppm or more and less than 40 ppm, with the remainder being Fe and unavoidable impurities.

3. The grain-oriented electrical steel sheet according to claim 2, wherein the aforementioned component composition further contains one or more elements selected from the following in mass percent or mass ppm: Ni: 1.50% or less, Sn: 0.50% or less, Sb: 0.50% or less, Cu: 0.50% or less, Mo: 0.50% or less, P: 0.50% or less, Cr: 1.50% or less, Co: 100 ppm or less, B: 50 ppm or less, Ti: 50 ppm or less, Nb: 50 ppm or less, Zn: 50 ppm or less, Ga: 50 ppm or less, Bi: 50 ppm or less, W: 30 ppm or less, Pb: 10 ppm or less, and Ge: 10 ppm or less.

4. The grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein in the measurement area there is at least one location where the thickness of two adjacent crystal grains differs by 0.5% or more, and in the measurement area the area ratio of crystal grains having twins is 0% or more and 20% or less.

5. The grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein at least one of the surfaces is subjected to magnetic domain refinement treatment, and at least one of the surfaces has an insulating coating.

6. A method for manufacturing grain-oriented electrical steel sheets, characterized by having a tensile stress application step in which tensile stress is applied parallel to the rolling direction to a steel strip that has undergone secondary recrystallization annealing and does not have a forsterite coating, thereby causing plastic deformation of 0.05 or more in true strain.

7. The method for manufacturing grain-oriented electrical steel sheets according to claim 6, wherein in the tensile stress application step, a plastic deformation of 0.06 or more in true strain is applied.

8. The temperature of the steel strip when the aforementioned plastic deformation is applied is 80°C or higher, or the strain rate during the aforementioned plastic deformation is 0.5 s. -1 The method for manufacturing grain-oriented electrical steel sheets according to claim 6 or 7, which is as follows: