Grain-oriented electromagnetic steel sheet and method for producing same

Abstract

Provided is a grain-oriented electromagnetic steel sheet in which, when a crystal orientation at measurement points provided at a pitch of 1 mm in a rolling direction and a direction orthogonal to the rolling direction is measured by an X-ray diffraction method on a surface of a base material steel plate, and an average particle diameter Dr of crystal grains in the rolling direction and an average particle diameter Dt of crystal grains in the direction orthogonal to the rolling direction on the surface are obtained, Dr is 20.0 mm or less, a ratio of Dr to Dt (Dt / Dr) is 1.20 or more, and a ratio of a length of grain boundaries having an azimuth difference of 15° or less to a total length of the grain boundaries of the crystal grains is 50% or more.

Description

Grain-oriented electrical steel sheet and its manufacturing method

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same.

[0002] Grain-oriented electrical steel sheets are soft magnetic materials that are primarily used as iron core materials in transformers. Therefore, grain-oriented electrical steel sheets are required to have magnetic properties, such as high magnetization and low iron loss. Iron loss is the power loss consumed as heat energy when an iron core is excited by an AC magnetic field, and from the perspective of energy conservation, iron loss should be as low as possible.

[0003] One of the controlling factors of iron loss characteristics is magnetic flux density (for example, B8: magnetic flux density in a magnetic field of 800 A / m), and the higher the magnetic flux density value, the lower the iron loss tends to be. In grain-oriented electrical steel sheets, to increase the magnetic flux density, the crystal orientation is generally concentrated in the Goss orientation, which is good for magnetic properties, during the manufacturing process (increasing the degree of orientation concentration). Low iron loss is achieved by refining the magnetic domain structure of grain-oriented electrical steel sheets with high magnetic flux density.

[0004] To increase the degree of orientation concentration in the Goss orientation, usually, high-temperature and long-time finish annealing is performed. With this finish annealing, "Goss-oriented grains," which have good magnetic properties and whose longitudinal direction is the <100> direction and whose surface direction is the <110> direction, grow to a size on the order of centimeters while encroaching on surrounding crystal grains, thereby aligning the crystal orientation (increasing the degree of orientation concentration).

[0005] In the conventional technology for improving the degree of orientation concentration in the Goss orientation, the selective growth of Goss orientation grains during the final annealing is increased, and at the same time, the frequency of secondary recrystallization is reduced, resulting in an increase in the diameter of each individual crystal grain.

[0006] However, when grain-oriented electrical steel sheets are manufactured, finish annealing is performed in a coiled state. That is, secondary recrystallized grains grow when the steel sheet has a certain curvature. Therefore, when the steel sheet is uncoiled and flattened, continuous deviations in the crystal orientation occur within the grains according to the curvature of the coil (so-called "coil set" occurs). This deviation causes the easy axis of magnetization, <100>, to deviate from the rolling direction, making magnetization difficult, resulting in a decrease in magnetic flux density and an increase in core loss.

[0007] This deviation becomes larger as the grain size of the secondary recrystallized grains in the longitudinal direction of the coil increases and as the coil curvature (radius of curvature) increases. As described above, in conventional techniques, increasing the degree of orientation concentration in the Goss orientation results in a larger grain size of the secondary recrystallized grains. Therefore, in order to reduce iron loss over the entire length of the coil, a manufacturing method is needed that resolves the trade-off between increasing the degree of orientation concentration in the Goss orientation and reducing the diameter of the secondary recrystallized grains.

[0008] To address this issue, Patent Documents 1 to 4 disclose a method of dividing the crystal grains and shortening the length of the secondary recrystallized grains in the rolling direction by forming grooves in the steel sheet. Patent Documents 5 and 6 also disclose a method of dividing the crystal grains in a product by performing heat treatment using laser beam irradiation either before or after the decarburization annealing step.

[0009] JP 2011-208196 A JP 2002-294416 A International Publication No. 2012 / 033197 JP 7-268474 A International Publication No. 2012 / 014290 JP 2-258928 A

[0010] However, while the techniques of providing grooves as described in Patent Documents 1 to 4 certainly enable control of the crystal grain size, the productivity of groove formation is low. Furthermore, the presence of grooves poses the problem of a reduced space factor when formed into a transformer. Furthermore, the conventional techniques of dividing crystal grains by irradiating them with a laser beam as described in Patent Documents 5 and 6 do not necessarily achieve low iron loss, leaving room for improvement.

[0011] Furthermore, as a result of the inventors' investigations, it was found that even if grain boundaries of secondary recrystallized grains are formed near the laser beam irradiated area, it is difficult to achieve high magnetic flux density and low iron loss if crystal orientation grains (abnormal grains) with inferior magnetic properties are generated in the laser beam irradiated area.

[0012] The present disclosure has been made in view of the above-mentioned problems, and aims to provide a grain-oriented electrical steel sheet having excellent magnetic properties that achieves both an improved orientation concentration in the Goss orientation and a reduced diameter of secondary recrystallized grains.

[0013] The present disclosure relates to the following grain-oriented electrical steel sheet and a method for manufacturing the same.

[0014] (1) A grain-oriented electrical steel sheet, wherein, when the crystal orientations at measurement points arranged at 1 mm pitch in the rolling direction and in a direction perpendicular to the rolling direction on the surface of a base steel sheet are measured by X-ray diffraction, and the average grain size Dr in the rolling direction and the average grain size Dt in the direction perpendicular to the rolling direction of the crystal grains on the surface are determined, Dr is 20.0 mm or less, the ratio Dt / Dr of Dr to Dt is 1.20 or more, and the proportion of the length of grain boundaries with an orientation difference of 15° or less to the total length of the grain boundaries of the crystal grains is 50% or more.

[0015] (2) The grain-oriented electrical steel sheet according to (1) above, wherein the base steel sheet has a thickness of 0.30 mm or less.

[0016] (3) The chemical composition of the base steel plate is, in mass %, Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0001 to 0.010%, C: 0.0005 to 0.010%, sol. 10. The grain-oriented electrical steel sheet according to claim 1, wherein the total of one or more elements selected from S and Se is 0 to 0.010%, Ti is 0 to 0.010%, Ni is 0 to 0.50%, Cu is 0 to 0.50%, Sb is 0 to 0.30%, Sn is 0 to 0.30%, Cr is 0 to 0.50%, P is 0 to 0.05%, Mo is 0 to 0.05%, Ta is 0 to 0.05%, Nb is 0 to 0.010%, V is 0 to 0.50%, B is 0 to 0.010%, Te is 0 to 0.0150%, Bi is 0 to 0.0150%, and the balance is Fe and impurities.

[0017] (4) A hot rolling process of heating a slab and hot rolling the heated slab to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing process of annealing the hot-rolled steel sheet; a cold-rolling process of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing process to obtain a cold-rolled steel sheet; a decarburization annealing process of decarburization annealing the cold-rolled steel sheet to obtain a decarburization annealed steel sheet; a finish annealing process of applying an annealing separator to the decarburization annealed steel sheet and then finish annealing it to obtain a finish annealed sheet; and an insulating coating forming process of forming an insulating coating on a surface of the finish annealed sheet, wherein the decarburization annealing process includes a temperature-raising process and a soaking process, a method for producing a grain-oriented electrical steel sheet, wherein, during the temperature-raising process of the decarburization annealing step, the temperature is raised from a temperature range of 450°C or lower to a temperature range of 750 to 950°C, and the average heating rate from 450 to 750°C is 40 to 2000°C / sec; and after the temperature-raising process and before the soaking process, a laser beam is irradiated in a direction intersecting with the rolling direction to irradiate both the front and back surfaces of the cold-rolled steel sheet within a temperature range of 650 to 850°C, under conditions that satisfy the following formulas (i) and (ii) so that the laser beams overlap when viewed from a direction perpendicular to the surfaces: 0.2≦Up≦22.3 (i) 5≦d≦18 (ii) In the above formula (i), Up is the instantaneous input energy (J / mm) expressed by 4 / π×P / (Dl×Vc) when the average intensity of the laser beam is P (W), the average focused diameter of the focused spot in the rolling direction is Dl (mm), and the average scanning speed of the laser beam is Vc (mm / sec). ２ ) and d in the above formula (ii) is the irradiation interval (mm) of the laser beam.

[0018] (5) The method for producing a grain-oriented electrical steel sheet according to (4) above, wherein, when the cold-rolled steel sheet has a thickness of 0.18 mm or less, the average heating rate during the temperature rise process in the decarburization annealing step is 400 to 2000°C / sec from 450°C to 750°C.

[0019] (6) The chemical composition of the slab is, in mass %, Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0030 to 0.0150%, C: 0.010 to 0.100%, sol. 6. A method for producing a grain-oriented electrical steel sheet according to claim 4 or 5, wherein the content of Al is 0.010 to 0.050%, a total of one or more elements selected from S and Se is 0.010 to 0.050%, Ti: 0 to 0.010%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Sb: 0 to 0.30%, Sn: 0 to 0.30%, Cr: 0 to 0.50%, P: 0 to 0.05%, Mo: 0 to 0.05%, Ta: 0 to 0.05%, Nb: 0 to 0.010%, V: 0 to 0.50%, B: 0 to 0.010%, Te: 0 to 0.0200%, Bi: 0 to 0.0200%, and the balance is Fe and impurities.

[0020] According to the present invention, it is possible to obtain a grain-oriented electrical steel sheet having excellent magnetic properties, which can achieve both an improvement in the degree of orientation concentration in the Goss orientation and a reduction in the diameter of secondary recrystallized grains.

[0021] As a result of investigations conducted by the present inventors to solve the above problems, the following findings were obtained.

[0022] The present inventors have improved the local heat treatment technique using a laser beam in order to simultaneously improve the degree of orientation concentration in the Goss orientation and reduce the diameter of secondary recrystallized grains.

[0023] In the prior art, localized heat treatment using a laser beam was performed after the decarburization annealing process or before the temperature-raising process in the decarburization annealing process. The inventors investigated the reason why iron loss did not improve in this case and found that the prior art was unable to sufficiently increase the low-angle grain boundary ratio, which will be described later. They then discovered that this problem could be solved by performing localized heat treatment using a laser beam after the temperature-raising process and before the soaking process.

[0024] Furthermore, as described above, even if the diameter of secondary recrystallized grains can be reduced in the technology of performing local heat treatment using a laser beam, abnormal grains may occur in the area irradiated with the laser beam, or the diameter of secondary recrystallized grains may not be reduced at all.

[0025] However, the inventors' investigations have revealed that by performing local heat treatment using a laser beam under appropriate conditions, it is possible to reduce the diameter of secondary recrystallized grains while also suppressing the occurrence of abnormal grains.

[0026] The present disclosure has been made based on the above findings. Hereinafter, a grain-oriented electrical steel sheet and a method for manufacturing the same according to one embodiment of the present invention will be described.

[0027] 1. Overall Configuration The grain-oriented electrical steel sheet according to this embodiment has a base steel sheet. The surface of the base steel sheet preferably has a primary coating and a secondary coating. Furthermore, the grain-oriented electrical steel sheet according to this embodiment has excellent magnetic properties because the structure of the crystal grains contained in the base steel sheet is controlled. Note that the grain-oriented electrical steel sheet according to this embodiment may be subjected to a conventionally known magnetic domain refinement process, such as forming grooves on the surface of the steel sheet or irradiating the surface of the steel sheet with a laser beam or electron beam to introduce localized strain.

[0028] 2. Crystal grains of base steel sheet In the grain-oriented electrical steel sheet according to this embodiment, when the average grain size Dr in the rolling direction and the average grain size Dt in the direction perpendicular to the rolling direction of the crystal grains on the surface of the base steel sheet are determined, Dr is 20.0 mm or less, and the ratio Dt / Dr of Dr to Dt is 1.20 or more.

[0029] As described above, in the manufacturing process of grain-oriented electrical steel sheets, finish annealing is performed in a coiled state, and secondary recrystallized grains grow while the steel sheet has a certain curvature. As a result, when the steel sheet is uncoiled, a continuous deviation in crystal orientation occurs according to the curvature of the coil in the coiled state. However, by setting Dr to 20.0 mm or less, it is possible to suppress the occurrence of a continuous deviation in crystal orientation.

[0030] Furthermore, the crystal grains are usually elongated in the rolling direction. That is, Dt / Dr is less than 1.00. However, in this embodiment, the crystal grains are divided by laser beam irradiation to form grain boundaries of secondary recrystallized grains, thereby controlling the crystal grains to be elongated in a direction perpendicular to the rolling direction. As a result, Dt / Dr becomes 1.20 or more.

[0031] By setting Dr to 20.0 mm or less to achieve a reduction in the diameter of secondary recrystallized grains in the rolling direction, and by setting Dt / Dr to 1.20 or more to coarsen the average crystal grain size in the direction perpendicular to the rolling direction, it becomes possible to improve the degree of orientation integration in the Goss orientation while suppressing the occurrence of continuous deviation in crystal orientation.

[0032] If abnormal grains occur in the laser beam irradiated area, it becomes difficult to achieve a Dt / Dr of 1.20 or more. However, in this embodiment, the occurrence of abnormal grains can be suppressed by performing local heat treatment using a laser beam under appropriate conditions.

[0033] Dr is preferably 19.0 mm or less, more preferably 18.0 mm or less. There is no need to set a lower limit for Dr, but it is preferably 5.0 mm or more, more preferably 7.0 mm or more, even more preferably 10.0 mm or more, and even more preferably 12.0 mm or more. There is no particular limitation on Dt as long as Dt / Dr is 1.20 or more, but it is preferably 5.0 mm or more, more preferably 7.0 mm or more, even more preferably 10.0 mm or more, and even more preferably 12.0 mm or more. Furthermore, Dt / Dr is preferably 1.30 or more, and even more preferably 1.40 or more. There is no need to set an upper limit for Dt / Dr, but a substantial upper limit is about 30.0.

[0034] Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, the ratio of the length of grain boundaries with a misorientation of 15° or less to the total length of the grain boundaries (hereinafter also referred to as the "low-angle grain boundary ratio" in this specification) is 50% or more. Increasing the low-angle grain boundary ratio suppresses the generation of auxiliary magnetic domains such as lancets, reduces hysteresis loss, and improves iron loss after laser magnetic domain refinement. The low-angle grain boundary ratio is preferably 55% or more, and more preferably 60% or more.

[0035] In this embodiment, Dr, Dt, and the small-angle grain boundary ratio are determined by measuring the crystal orientation at measurement points arranged at 1 mm pitches in the rolling direction and in a direction perpendicular to the rolling direction on the surface of the base steel sheet by X-ray diffraction.

[0036] More specifically, crystal orientation measurement is performed using the Laue method on a sample measuring 60 mm x 300 mm x thickness. During the measurement, orientation data is obtained from 2,601 measurement points (51 x 51 points) at 1 mm intervals in the center of the sheet. The orientation data is mapped using OIM analysis, and Dr and Dt are calculated using the line segment method. In the line segment method, 10 lines are drawn at equal intervals in both the rolling direction and the direction perpendicular to the rolling direction, and the average grain size is calculated by counting the number of intersections between the lines and the grain boundaries. Furthermore, a crystal grain is defined as having an orientation difference of 1° or more and one or more points, and the proportion of crystal grain boundaries with an orientation difference of 15° or less is calculated. Note that "one or more points" means, for example, that even if there is only one measurement point, if the orientation difference from the surrounding measurement points is 1° or more, the measurement point is determined to be one crystal grain.

[0037] In the present invention, the rolling direction of the base steel sheet is determined by crystal orientation measurement. Specifically, the crystal orientation of the grain-oriented electrical steel sheet is measured at 1° intervals using an X-ray diffractometer while rotating the grain-oriented electrical steel sheet around an axis parallel to the sheet thickness, and the direction in which the deviation angle from {110}<001> is smallest is defined as the rolling direction. In this case, the above measurement is performed at 10 locations on the grain-oriented electrical steel sheet, and the average of the respective directions is defined as the rolling direction.

[0038] 3. Magnetic Properties In the grain-oriented electrical steel sheet according to this embodiment, the magnetic flux density B8 in the rolling direction is 1.930 T or more, and the iron loss W １７／５０ is preferably 0.711 W / kg or less.

[0039] Magnetic flux density B8 and iron loss W １７／５０ are measured by the following method. First, a test piece measuring 60 mm x 300 mm x thickness is prepared from the center of the sheet width of the grain-oriented electrical steel sheet. During preparation, the rolling direction is parallel to the longitudinal direction of the test piece. If the rolling direction is unknown, the rolling direction is determined by the crystal orientation measurement described above, and then the test piece is prepared. Then, the magnetic flux density and iron loss are measured using a small single sheet tester compatible with the test piece. In this case, the measurement principle follows the Single Sheet Tester (SST) method for measuring single sheet magnetic properties specified in JIS C 2556:2015.

[0040] Here, since JIS C 2556:2015 stipulates that the size of the test piece must be 500 mm or more, a correction coefficient is calculated using the following method, and correction is performed by multiplying the magnetic flux density and iron loss values ​​obtained by the above measurements. First, a test piece measuring 500 mm x 500 mm x thickness is taken from a predetermined grain-oriented electrical steel sheet, and the magnetic flux density and iron loss are measured using an SST in accordance with JIS C 2556:2015. Next, eight test pieces measuring 300 mm x 60 mm x thickness are taken from the test piece by electrical discharge machining. The magnetic flux density and iron loss of the obtained eight test pieces are measured using the above-mentioned small single sheet tester, and the average of the eight measured values ​​is calculated. The correction coefficient is then calculated by dividing the measured value using the 500 mm x 500 mm x thickness test piece by the average of the eight measured values.

[0041] In addition, Epstein test pieces measuring 30 mm x 320 mm x thickness were obtained from the above test pieces measuring 500 mm x 500 mm x thickness by electrical discharge machining, and magnetic flux density and iron loss were measured using the Epstein method specified in JIS C 2550-1:2011. A conversion formula was then derived from the relationship between the values ​​measured by SST and those measured by the Epstein method. Using the resulting conversion formula, the values ​​measured by SST were corrected to be equivalent to the magnetic property values ​​measured by the Epstein method.

[0042] When measuring the magnetic flux density and iron loss, the excitation direction is parallel to the rolling direction. Note that the magnetic flux density B8 means the magnetic flux density in a magnetic field of 800 A / m, and the iron loss W １７／５０ means the iron loss that occurs under the conditions of a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz.

[0043] In the grain-oriented electrical steel sheet according to this embodiment, the thickness of the base steel sheet is not particularly limited, but is preferably 0.14 to 0.30 mm, for example. From the viewpoint of reducing iron loss, the thickness of the base steel sheet is preferably 0.18 mm or less.

[0044] 5. Chemical Composition of Base Steel Plate There are no particular restrictions on the chemical composition of the base steel plate, but the following may be used in mass %: Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0001 to 0.010%, C: 0.0005 to 0.010%, sol. It is preferable that the alloy contains 0-0.010% Al, 0-0.010% total of one or more elements selected from S and Se, 0-0.010% Ti, 0-0.010% Ni, 0-0.50% Cu, 0-0.50% Sb, 0-0.30% Sn, 0-0.30% Cr, 0-0.50% P, 0-0.05% Mo, 0-0.05% Ta, 0-0.05% Nb, 0-0.010% V, 0-0.50% B, 0-0.010% Te, and 0-0.0150% Bi. The balance other than the above elements preferably includes Fe and impurities.

[0045] The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0046] Si: 3.00 to 3.70% Si (silicon) is an extremely effective element for increasing the electrical resistance (resistivity) of steel and reducing eddy current loss, which constitutes part of iron loss. By setting the Si content of the base steel sheet to 3.00% or more, eddy current loss can be sufficiently reduced. Furthermore, by suppressing the phase transformation of the steel during secondary recrystallization annealing and sufficiently promoting secondary recrystallization, it is possible to achieve the effect of improving magnetic flux density and iron loss. Therefore, the Si content of the base steel sheet is preferably 3.00% or more, more preferably 3.10% or more, and even more preferably 3.20% or more. On the other hand, by setting the Si content to 3.70% or less, embrittlement of the steel sheet can be suppressed and sheet threadability during the manufacturing process can be improved. Therefore, the Si content of the base steel sheet is preferably 3.70% or less, more preferably 3.60% or less, and even more preferably 3.50% or less.

[0047] Mn: 0.01 to 0.30% Mn (manganese) exists as solute Mn. Solute Mn increases resistivity and therefore has the effect of reducing iron loss. Therefore, the Mn content of the base steel sheet is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. However, solute Mn has a smaller effect of increasing resistivity than Si. Therefore, the Mn content of the base steel sheet is preferably 0.30% or less, more preferably 0.25% or less, and even more preferably 0.20% or less.

[0048] N: 0.0001 to 0.010% N (nitrogen) is an element that reacts with sol. Al to form AlN, which functions as an inhibitor. However, N also has a negative effect on the magnetic properties of grain-oriented electrical steel sheets. Therefore, the N content of the base steel sheet is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.002% or less. While a lower N content of the base steel sheet is preferable, an extreme reduction can increase manufacturing costs. Therefore, the N content of the base steel sheet is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0049] C: 0.0005 to 0.010% C (carbon) is an element that has the effect of improving magnetic flux density. Therefore, the C content of the base steel sheet is preferably 0.0005% or more, more preferably 0.001% or more, and even more preferably 0.002% or more. On the other hand, by setting the C content to 0.010% or less, it is possible to reduce iron loss. Therefore, the C content of the base steel sheet is preferably 0.010% or less, more preferably 0.007% or less, and even more preferably 0.005% or less.

[0050] Sol. Al: 0 to 0.010% Sol. Al (acid-soluble aluminum) is an element that reacts with N to form AlN, which functions as an inhibitor. However, sol. Al also has a negative effect on the magnetic properties of grain-oriented electrical steel sheets. Therefore, the sol. Al content of the base steel sheet is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.002% or less. Since a lower sol. Al content of the base steel sheet is preferable, the lower limit is 0%. However, an extreme reduction can increase manufacturing costs, so the sol. Al content of the base steel sheet is preferably more than 0%, more preferably 0.0001% or more, even more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0051] Total of one or more selected from S and Se: 0 to 0.010%. S (sulfur) and Se (selenium) react with Mn to form inhibitors, MnS and MnSe, respectively. However, these elements adversely affect the magnetic properties of grain-oriented electrical steel sheets. Therefore, the total content of S and Se in the base steel sheet is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.002% or less. Since a lower total content of S and Se in the base steel sheet is preferable, the lower limit is 0%. However, an extreme reduction can increase manufacturing costs, so the total content of S and Se in the base steel sheet is preferably more than 0%, more preferably 0.0001% or more, even more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0052] Ti: 0 to 0.010% Ti (titanium) is an element that can be contained in steel as an impurity, but reducing its content makes it possible to reduce iron loss (hysteresis loss). Therefore, the Ti content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less. The lower the Ti content, the better, so the lower limit is 0%. However, an extreme reduction can increase manufacturing costs, so the Ti content of the base steel sheet is preferably more than 0%, more preferably 0.0001% or more, even more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0053] Ni: 0 to 0.50% Ni (nickel) is an element effective in increasing electrical resistance and reducing iron loss. Ni is also an element effective in controlling the metal structure of hot-rolled steel sheets and improving magnetic properties. Therefore, Ni may be contained as needed. However, excessive Ni content can cause secondary recrystallization to become unstable. Therefore, the Ni content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. There is no need to set a lower limit for the Ni content; it may be 0% or more. To reliably obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0054] Cu: 0 to 0.50% Cu (copper) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallization structure and also contributes to improving the adhesion of the glass coating. Therefore, Cu may be contained as necessary. However, if its content is excessive, the steel sheet may become embrittled during hot rolling. Therefore, the Cu content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Cu content; it may be 0% or more. To reliably obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0055] Sb: 0 to 0.30% Sb (antimony) is an element that has the effect of improving magnetic properties. Therefore, Sb may be contained as necessary. However, if the content is excessive, the adhesion of the glass coating may decrease. Therefore, the Sb content is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Sb content; it may be 0% or more than 0%. To ensure the above effects, the Sb content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0056] Sn: 0 to 0.30% Sn (tin) is an element that has the effect of improving magnetic properties. Therefore, Sb may be contained as needed. However, if the Sn content is excessive, the glass coating may deteriorate and sufficient tension for magnetic domain refinement may not be obtained, which may result in a decrease in core loss characteristics. Therefore, the Sn content is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Sn content; it may be 0% or more. To ensure the above effects, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0057] Cr: 0 to 0.50% Cr (chromium) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallization structure and to improving the adhesion of the glass coating. Therefore, Cr may be added as needed. However, excessive Cr content may result in the formation of Cr oxides, which may degrade the magnetic properties. Therefore, the Cr content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Cr content; it may be 0% or more. To ensure the above effects, the Cr content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0058] P: 0 to 0.05% P (phosphorus) is an element that reduces workability during rolling. By setting the P content to 0.05% or less, excessive deterioration in rolling workability can be suppressed, and breakage during manufacturing can be suppressed. From this perspective, the P content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. The lower limit of the P content is not limited and may include 0%, but P is also an element that has the effect of improving texture and magnetic properties. To achieve this effect, the P content may be more than 0%, 0.005% or more, or 0.01% or more.

[0059] Mo: 0 to 0.05% Mo (molybdenum) is an element that has the effect of improving magnetic properties. Therefore, Mo may be contained as needed. However, if its content is excessive, cold rolling ability may decrease, leading to the risk of fracture. Therefore, the Mo content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. There is no need to set a lower limit for the Mo content; it may be 0% or more than 0%. To ensure the above effects, the Mo content is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.02% or more.

[0060] Ta: 0 to 0.05% Ta (tantalum) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Ta may be contained as necessary. However, if its content is excessive, secondary recrystallization may become unstable. Therefore, the Ta content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. There is no need to set a lower limit for the Ta content; it may be 0%. To reliably obtain the above effect, the Ta content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0061] Nb: 0 to 0.010% Nb (niobium) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Nb may be contained as necessary. However, if the content is excessive, secondary recrystallization may become unstable. Therefore, the Nb content is preferably 0.010% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no need to set a lower limit for the Nb content; it may be 0%. To reliably obtain the above effect, the Nb content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0062] V: 0 to 0.50% V (vanadium) is an effective element that bonds with N or C and functions as an inhibitor. Therefore, V may be contained as needed. However, if its content is excessive, magnetic properties may be degraded. Therefore, the V content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. There is no need to set a lower limit for the V content; it may be 0% or more than 0%. To ensure the above effects, the V content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0063] B: 0 to 0.010% B (boron) is an element that has the effect of stabilizing secondary recrystallization. Therefore, B may be contained as necessary. However, if the content is excessive, secondary recrystallization may become unstable. Therefore, the B content is preferably 0.010% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no need to set a lower limit for the B content; it may be 0%. To reliably obtain the above effect, the B content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0064] Te: 0 to 0.0150% Te (tellurium) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Te may be contained as needed. However, if its content is excessive, hot rolling property and cold rolling property may be deteriorated, which may lead to fracture. Therefore, the Te content is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. There is no need to set a lower limit for the Te content; it may be 0%. To reliably obtain the above effect, the Te content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0065] Bi: 0 to 0.0150% Bi (bismuth) is an element that effectively stabilizes precipitates such as sulfides and enhances their inhibitory function. Therefore, Bi may be added as needed. However, excessive Bi content may reduce the adhesion of the insulating coating or degrade the magnetic properties. Therefore, the Bi content is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. There is no need to set a lower limit for the Bi content; it may be 0% or more. To ensure the above effects, the Bi content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0066] The balance of the chemical composition of the grain-oriented electrical steel sheet of this embodiment is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ore and scrap, and in the production process, and are acceptable within a range that does not adversely affect the present invention.

[0067] Examples of impurities include one or more selected from the group consisting of Ca: 0-0.001%, Mg: 0-0.002%, O: 0-0.01%, As: 0-0.10%, Co: 0-0.10%, Zr: 0-0.003%, W: 0-0.10%, Hf: 0-0.02%, Sr: 0-0.02%, Zn: 0-0.02%, Pb: 0-0.10%, REM: 0-0.02%, Ba: 0-0.02%, Cd: 0-0.02%, Pt: 0-0.02%, Au: 0-0.02%, Ga: 0-0.02%, and Ge: 0-0.02%. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of the REM refers to the total content of these elements. When a plurality of these impurity elements is contained, the total content of the plurality of elements is preferably 0 to 0.05%.

[0068] 4. Manufacturing Method A manufacturing method of the grain-oriented electrical steel sheet according to this embodiment will be described. The manufacturing method of the grain-oriented electrical steel sheet according to this embodiment includes the following steps: (A) a hot rolling step of heating a slab and hot-rolling the heated slab to obtain a hot-rolled steel sheet, (B) a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet, (C) a cold-rolling step of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing step to obtain a cold-rolled steel sheet, (D) a decarburization annealing step of decarburization annealing the cold-rolled steel sheet to obtain a decarburization annealed steel sheet, (E) a finish annealing step of applying an annealing separator to the decarburization annealed steel sheet and then finish annealing it to obtain a finish annealed sheet, and (F) an insulating coating forming step of forming an insulating coating on the surface of the finish annealed sheet.

[0069] In addition to the above steps, the method may further include (G) a magnetic domain refinement step of subjecting the obtained grain-oriented electrical steel sheet to magnetic domain refinement. Each step will be described below. For steps or conditions not described, known steps or conditions can be applied.

[0070] (A) Hot Rolling Process In the hot rolling process, a slab having the chemical composition described below is heated to 1280°C or higher, and the heated slab is hot-rolled to produce a hot-rolled steel sheet. If the heating temperature is lower than 1280°C, inclusions formed in the slab cannot be dissolved, and inhibitors are not sufficiently formed in the hot rolling process or the hot-rolled sheet annealing process. Therefore, the slab heating temperature is set to 1280°C or higher. There is no upper limit for the slab heating temperature, but if the slab is heated to a temperature higher than 1450°C, the slab will melt, making hot rolling difficult. Therefore, the slab heating temperature is preferably 1450°C or lower.

[0071] The hot rolling conditions are not particularly limited and may be appropriately set based on the desired properties. The thickness of the hot-rolled steel sheet obtained by hot rolling is preferably within a range of 1.0 mm to 4.0 mm, for example.

[0072] [Chemical Composition of Slab] There are no particular limitations on the chemical composition of the slab, but the following may be used in mass %: Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0030 to 0.0150%, C: 0.010 to 0.100%, sol. It is preferable that the alloy contains 0.010 to 0.050% Al, 0.010 to 0.050% of a total of one or more elements selected from S and Se, 0 to 0.010% Ti, 0 to 0.010% Ni, 0 to 0.50% Cu, 0 to 0.50% Sb, 0 to 0.30% Sn, 0 to 0.30% Cr, 0 to 0.50% P, 0 to 0.05% Mo, 0 to 0.05% Ta, 0 to 0.05% Nb, 0 to 0.010% V, 0 to 0.50% B, 0 to 0.010% Te, and 0 to 0.0200% Bi. The balance other than the above elements preferably includes Fe and impurities.

[0073] The reasons for limiting the content of each element are as follows: In the following description, "%" in the content means "% by mass."

[0074] Si: 3.00 to 3.70% Si (silicon) is an extremely effective element for increasing the electrical resistance (resistivity) of steel and reducing eddy current loss, which constitutes part of iron loss. By setting the Si content of the slab to 3.00% or more, eddy current loss can be sufficiently reduced. Furthermore, by suppressing the phase transformation of the steel during secondary recrystallization annealing and sufficiently promoting secondary recrystallization, it exhibits the effect of improving magnetic flux density and iron loss. Therefore, the Si content of the slab is preferably 3.00% or more, more preferably 3.10% or more, and even more preferably 3.20% or more. On the other hand, by setting the Si content to 4.00% or less, embrittlement of the steel sheet can be suppressed and sheet threadability during the manufacturing process can be improved. Therefore, the Si content of the slab is preferably 3.70% or less, more preferably 3.60% or less, and even more preferably 3.50% or less.

[0075] Mn: 0.01 to 0.30% Mn (manganese) is an element that forms MnS, one of the main inhibitors. In order to ensure the absolute amount of MnS necessary to cause secondary recrystallization, the Mn content of the slab is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.06% or more. On the other hand, in order to suppress the phase transformation of the steel during secondary recrystallization annealing, sufficiently promote secondary recrystallization, and reliably obtain good magnetic flux density and low iron loss, the Mn content of the slab is preferably 0.30% or less, more preferably 0.28% or less, and even more preferably 0.26% or less.

[0076] N: 0.0030 to 0.0150% N (nitrogen) is an element that reacts with sol. Al to form AlN, which functions as an inhibitor. In order to sufficiently form AlN, which functions as an inhibitor, the N content is preferably 0.0030% or more, more preferably 0.0040% or more, and even more preferably 0.0050% or more. On the other hand, if the N content is excessive, AlN, which functions as an inhibitor, is not sufficiently generated, resulting in insufficient secondary recrystallization. In addition, in order to suppress the formation of blisters (voids) in the steel sheet during cold rolling, suppress an increase in the strength of the steel sheet, and ensure sheet passing properties during production, the N content of the slab is preferably 0.0150% or less, more preferably 0.0120% or less, and even more preferably 0.0100% or less.

[0077] C: 0.010 to 0.100% C (carbon) is an element that improves magnetic flux density. Therefore, the C content of the slab is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more. On the other hand, excessive C content in the slab reduces productivity in the decarburization annealing process. Furthermore, if the C content of the slab is high and decarburization is insufficient, the steel undergoes phase transformation during secondary recrystallization annealing (i.e., finish annealing), and secondary recrystallization does not proceed sufficiently. As a result, good magnetic flux density and low core loss cannot be obtained, and magnetic properties deteriorate due to magnetic aging. Therefore, the C content of the slab is preferably 0.100% or less, more preferably 0.070% or less, and even more preferably 0.050% or less.

[0078] Sol. Al: 0.010 to 0.050% Sol. Al (acid-soluble aluminum) is an element that reacts with N to form AlN, which functions as an inhibitor. In order to generate a sufficient amount of AlN, which functions as an inhibitor, and to obtain sufficient secondary recrystallization, the sol. Al content of the slab is preferably 0.010% or more, and more preferably 0.020% or more. However, if the content is excessive, AlN, which functions as an inhibitor, is not generated sufficiently, and secondary recrystallization becomes insufficient. Therefore, the sol. Al content is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0079] Total of one or more elements selected from S and Se: 0.010 to 0.050%. S (sulfur) and Se (selenium) are elements that react with Mn to form inhibitors MnS and MnSe, respectively. To sufficiently form MnS and / or MnSe, which function as inhibitors, the total content of one or more elements selected from S and Se in the slab is preferably 0.010% or more, more preferably 0.020% or more. However, excessive contents of these elements can result in insufficient production of MnS and MnSe, which function as inhibitors, resulting in insufficient secondary recrystallization. In addition, this can cause hot embrittlement, making hot rolling difficult. Therefore, the total content of S and Se in the slab is preferably 0.050% or less, more preferably 0.040% or less, and even more preferably 0.030% or less.

[0080] Ti: 0 to 0.010% Ti (titanium) is an element that can be contained in slabs as an impurity, but reducing its content makes it possible to reduce iron loss (hysteresis loss). Therefore, the Ti content of the slab is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less. The lower the Ti content, the better, so the lower limit is 0%. However, an extreme reduction can increase manufacturing costs, so the Ti content of the slab is preferably more than 0%, more preferably 0.0001% or more, even more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0081] Ni: 0 to 0.50% Ni (nickel) is an element effective in increasing electrical resistance and reducing iron loss. Ni is also an element effective in controlling the metal structure of hot-rolled steel sheets and improving magnetic properties. Therefore, Ni may be contained as needed. However, excessive Ni content can cause secondary recrystallization to become unstable. Therefore, the Ni content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. There is no need to set a lower limit for the Ni content; it may be 0% or more. To reliably obtain the above effects, the Ni content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0082] Cu: 0 to 0.50% Cu (copper) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallization structure and also contributes to improving the adhesion of the glass coating. Therefore, Cu may be contained as necessary. However, if its content is excessive, the steel sheet may become embrittled during hot rolling. Therefore, the Cu content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Cu content; it may be 0% or more. To reliably obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0083] Sb: 0 to 0.30% Sb (antimony) is an element that has the effect of improving magnetic properties. Therefore, Sb may be contained as necessary. However, if the content is excessive, the adhesion of the glass coating may decrease. Therefore, the Sb content is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Sb content; it may be 0% or more than 0%. To ensure the above effects, the Sb content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0084] Sn: 0 to 0.30% Sn (tin) is an element that has the effect of improving magnetic properties. Therefore, Sb may be contained as needed. However, if the Sn content is excessive, the glass coating may deteriorate and sufficient tension for magnetic domain refinement may not be obtained, which may result in a decrease in core loss characteristics. Therefore, the Sn content is preferably 0.30% or less, more preferably 0.20% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Sn content; it may be 0% or more. To ensure the above effects, the Sn content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0085] Cr: 0 to 0.50% Cr (chromium) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallization structure and to improving the adhesion of the glass coating. Therefore, Cr may be added as needed. However, excessive Cr content may result in the formation of Cr oxides, which may degrade the magnetic properties. Therefore, the Cr content is preferably 0.50% or less, more preferably 0.30% or less, and even more preferably 0.10% or less. There is no need to set a lower limit for the Cr content; it may be 0% or more. To ensure the above effects, the Cr content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0086] P: 0 to 0.05% P (phosphorus) is an element that reduces workability during rolling. By setting the P content to 0.05% or less, excessive deterioration in rolling workability can be suppressed, and breakage during manufacturing can be suppressed. From this perspective, the P content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. The lower limit of the P content is not limited and may include 0%, but P is also an element that has the effect of improving texture and magnetic properties. To achieve this effect, the P content may be more than 0%, 0.005% or more, or 0.01% or more.

[0087] Mo: 0 to 0.05% Mo (molybdenum) is an element that has the effect of improving magnetic properties. Therefore, Mo may be contained as needed. However, if its content is excessive, cold rolling ability may decrease, leading to the risk of fracture. Therefore, the Mo content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. There is no need to set a lower limit for the Mo content; it may be 0% or more than 0%. To ensure the above effects, the Mo content is preferably 0.005% or more, more preferably 0.01% or more, and even more preferably 0.02% or more.

[0088] Ta: 0 to 0.05% Ta (tantalum) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Ta may be contained as necessary. However, if its content is excessive, secondary recrystallization may become unstable. Therefore, the Ta content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. There is no need to set a lower limit for the Ta content; it may be 0%. To reliably obtain the above effect, the Ta content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.001% or more.

[0089] Nb: 0 to 0.010% Nb (niobium) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Nb may be contained as necessary. However, if the content is excessive, secondary recrystallization may become unstable. Therefore, the Nb content is preferably 0.010% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no need to set a lower limit for the Nb content; it may be 0%. To reliably obtain the above effect, the Nb content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0090] V: 0 to 0.50% V (vanadium) is an effective element that bonds with N or C and functions as an inhibitor. Therefore, V may be contained as needed. However, if its content is excessive, magnetic properties may be degraded. Therefore, the V content is preferably 0.50% or less, more preferably 0.40% or less, and even more preferably 0.30% or less. There is no need to set a lower limit for the V content; it may be 0% or more than 0%. To ensure the above effects, the V content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0091] B: 0 to 0.010% B (boron) is an element that has the effect of stabilizing secondary recrystallization. Therefore, B may be contained as necessary. However, if the content is excessive, secondary recrystallization may become unstable. Therefore, the B content is preferably 0.010% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no need to set a lower limit for the B content; it may be 0%. To reliably obtain the above effect, the B content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0092] Te: 0 to 0.0200% Te (tellurium) is an element that has the effect of stabilizing secondary recrystallization. Therefore, Te may be contained as needed. However, if its content is excessive, hot rolling property and cold rolling property may be deteriorated, which may lead to fracture. Therefore, the Te content is preferably 0.0200% or less, more preferably 0.0150% or less, and even more preferably 0.0100% or less. There is no need to set a lower limit for the Te content; it may be 0%. To reliably obtain the above effect, the Te content is preferably more than 0%, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0093] Bi: 0 to 0.0200% Bi (bismuth) is an element that effectively stabilizes precipitates such as sulfides and enhances their inhibitory function. Therefore, Bi may be added as needed. However, excessive Bi content may reduce the adhesion of the insulating coating or degrade the magnetic properties. Therefore, the Bi content is preferably 0.0200% or less, more preferably 0.0150% or less, and even more preferably 0.0100% or less. There is no need to set a lower limit for the Bi content; it may be 0% or more. To ensure the above effects, the Bi content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0094] In the chemical composition of the slab of this embodiment, the balance is Fe and impurities. Here, "impurities" refer to components that are mixed in during industrial steel production due to various factors in raw materials such as ores and scraps, and in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0095] Examples of impurities include one or more selected from the group consisting of Ca: 0-0.001%, Mg: 0-0.002%, O: 0-0.01%, As: 0-0.10%, Co: 0-0.10%, Zr: 0-0.003%, W: 0-0.10%, Hf: 0-0.02%, Sr: 0-0.02%, Zn: 0-0.02%, Pb: 0-0.10%, REM: 0-0.02%, Ba: 0-0.02%, Cd: 0-0.02%, Pt: 0-0.02%, Au: 0-0.02%, Ga: 0-0.02%, and Ge: 0-0.02%. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of the REM refers to the total content of these elements. When a plurality of these impurity elements is contained, the total content of the plurality of elements is preferably 0 to 0.05%.

[0096] (B) Hot-rolled sheet annealing process The hot-rolled sheet annealing process is a process of annealing a hot-rolled steel sheet manufactured through a hot-rolling process. By performing such an annealing treatment, recrystallization occurs in the steel sheet structure, making it possible to achieve good magnetic properties. In the hot-rolled sheet annealing process of this embodiment, the hot-rolled steel sheet manufactured through the hot-rolling process may be annealed according to a known method. The means for heating the hot-rolled steel sheet during annealing is not particularly limited, and known heating methods can be adopted. Furthermore, the annealing conditions are not particularly limited, but for example, the hot-rolled steel sheet can be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.

[0097] (C) Cold Rolling Step In the cold rolling step, the hot-rolled steel sheet after the hot-rolled sheet annealing step is subjected to cold rolling including multiple passes to obtain a cold-rolled steel sheet. The cold rolling may be a single cold rolling, or multiple cold rolling passes may be performed, with the cold rolling interrupted before the final pass of the cold rolling step and at least one or two intermediate annealings performed between the cold rolling passes. When intermediate annealing is performed, it is preferable to hold the steel at a temperature of 1000 to 1200°C for 5 to 180 seconds. The annealing atmosphere is not particularly limited. Considering production costs, the number of intermediate annealings is preferably three or less. Furthermore, before the cold rolling step, the surface of the hot-rolled steel sheet may be pickled under known conditions.

[0098] (D) Decarburization Annealing Step In the decarburization annealing step, the cold-rolled steel sheet is subjected to decarburization annealing to produce a decarburization annealed steel sheet. In the decarburization annealing step, the cold-rolled steel sheet is subjected to primary recrystallization, and C, which adversely affects magnetic properties, is removed from the steel sheet. The decarburization annealing step includes a temperature-raising step and a soaking step. In the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, local heat treatment is performed using a laser beam after the temperature-raising step and before the soaking step.

[0099] During the heating process, the cold-rolled steel sheet is heated in a non-oxidizing atmosphere from a temperature range of 450°C or less to a temperature range of 750 to 950°C, which is the decarburization annealing temperature. This heating promotes the nucleation of Goss-oriented grains. Furthermore, to achieve a low-angle grain boundary ratio of 50% or more, the average heating rate between 450°C and 750°C is set to 40°C / sec or more. When the thickness of the cold-rolled steel sheet is 0.18 mm or less, the average heating rate is preferably set to 400°C / sec or more to achieve a low-angle grain boundary ratio of 50% or more. Furthermore, the average heating rate is set to 2000°C / sec or less. If the average heating rate exceeds 2000°C / sec, the formation of Goss-oriented grain nuclei is promoted, but the corresponding orientations that promote the growth of nuclei are reduced, the preferential growth of Goss orientation is reduced, and the proportion of low-angle grain boundaries formed is actually reduced.

[0100] In addition, if the atmosphere during temperature rise is not a non-oxidizing atmosphere, the surface layer of the steel sheet will have a tight SiO ２In this embodiment, the non-oxidizing atmosphere is a nitrogen atmosphere or a nitrogen / hydrogen mixed atmosphere, and the dew point is an atmosphere of -50°C or more and 0°C or less. The dew point is the SiO ２ From the viewpoint of suppressing the generation of carbon dioxide and promoting decarburization, the dew point is preferably −5° C. or lower, more preferably −10° C. or lower. From the viewpoint of promoting internal oxidation to facilitate process control, the dew point may be −40° C. or higher.

[0101] After the temperature rise process, within a temperature range of 650 to 850°C, both the front and back surfaces of the cold-rolled steel sheet are irradiated with a laser beam in a direction intersecting with the rolling direction so that the laser beams overlap when viewed from a direction perpendicular to the surface, under conditions that satisfy the following formulas (i) and (ii): 0.2≦Up≦22.3 (i) 5≦d≦18 (ii) where Up in the above formula (i) is the instantaneous input energy (J / mm) expressed as 4 / π×P / (Dl×Vc), where P (W) is the average intensity of the laser beam, Dl (mm) is the average focused diameter of the focused spot in the rolling direction, and Vc (mm / sec) is the average scanning speed of the laser beam. ２ ) and d in the above formula (ii) is the irradiation interval (mm) of the laser beam.

[0102] By performing localized heat treatment using a laser beam to generate crystal grains, it is possible to reduce the diameter of secondary recrystallized grains. In addition, as a result of various studies, the inventors have found that by performing the heat treatment after the temperature increase process and before the soaking process within a temperature range of 650 to 850°C, it is possible to control the crystal grains generated by laser beam irradiation so that they are eroded during the finish annealing described below. In other words, the generation of abnormal grains can be suppressed, and a decrease in the low-angle grain boundary ratio can be suppressed. If the temperature of the cold-rolled steel sheet during laser beam irradiation is less than 650°C, it becomes difficult to reduce the diameter of secondary recrystallized grains, making it difficult to achieve a Dr of 20.0 mm or less and a Dt / Dr of 1.20 or more. On the other hand, if the temperature of the cold-rolled steel sheet during laser beam irradiation exceeds 850°C, it becomes difficult to increase Dt, making it difficult to achieve a Dt / Dr of 1.20 or more.

[0103] Furthermore, unless the laser beam is irradiated onto both the front and back surfaces of the cold-rolled steel sheet so as to overlap when viewed from a direction perpendicular to the surface, it becomes difficult to reduce the diameter of the secondary recrystallized grains, and it becomes difficult to achieve Dr of 20.0 mm or less and Dt / Dr of 1.20 or more. Furthermore, the conditions for irradiating the laser beam onto both the front and back surfaces of the cold-rolled steel sheet must both satisfy the above formulas (i) and (ii), and it is preferable that the conditions are the same for the front and back surfaces.

[0104] Furthermore, Up is 0.2 J / mm ２ If Up is less than 22.3 J / mm, the crystal grains are divided and the grain boundaries of secondary recrystallized grains cannot be formed, making it difficult to make Dr 20.0 mm or less and Dt / Dr 1.20 or more. ２ If the value exceeds this limit, Dt cannot be increased, and it becomes difficult to make Dt / Dr 1.20 or more. Furthermore, if it is desired to make the low-angle grain boundary ratio 55% or more, Up must be 11.9 J / mm ２ It is preferable to have the following:

[0105] Similarly, if d is greater than 18 mm, the irradiation interval is too wide, making it difficult to set Dr to 20.0 mm or less and Dt / Dr to 1.20 or more. On the other hand, if d is less than 5 mm, Dt cannot be increased, making it difficult to set Dt / Dr to 1.20 or more. The direction of laser beam irradiation may be any direction intersecting the rolling direction, but is preferably a direction forming an angle of 30 to 150° with respect to the rolling direction. When laser beam irradiation is performed over multiple rows, all irradiation intervals must satisfy the above formula (ii).

[0106] There are no particular restrictions on the atmosphere when performing local heat treatment using a laser beam. However, from the viewpoint of suppressing deterioration of the coating adhesion, the oxygen potential (PH) in the heat treatment atmosphere is ２ O / PH ２ ) is preferably 0.20 or less.

[0107] After the local heat treatment using the laser beam, a soaking process is performed. There are no particular restrictions on the conditions of the soaking process. For example, the oxygen potential (PH) in the annealing atmosphere (furnace atmosphere) is２ O / PH ２ ) is set to 0.15 to 1.0, and the temperature is held in the temperature range of 750 to 950°C for 10 to 600 seconds.

[0108] (E) Finish Annealing Step In the finish annealing step, a predetermined annealing separator is applied to one or both sides of the decarburization-annealed steel sheet obtained in the decarburization-annealing step, and then the steel sheet is subjected to finish annealing to obtain a finish annealed sheet. Finish annealing is generally performed for a long period of time while the steel sheet is wound into a coil. Therefore, prior to the finish annealing, an annealing separator is applied to the decarburization-annealed steel sheet and dried in order to prevent seizure between the inside and outside of the coil winding.

[0109] The annealing separator to be applied is an annealing separator containing MgO as the main component (for example, containing 80% or more by weight). By using an annealing separator containing MgO as the main component, a glass coating can be formed on the surface of the base steel sheet. If MgO is not the main component, the primary coating (glass coating) is not formed. This is because the primary coating contains Mg ２ SiO ４ or MgAl ２ O ４ This is because it is a compound, and if MgO is not the main component, there will be a shortage of Mg, which is necessary for the formation reaction.

[0110] The finish annealing may be carried out, for example, in an atmospheric gas containing hydrogen and nitrogen by raising the temperature to 1150 to 1250° C. and annealing at that temperature for 10 to 60 hours.

[0111] (F) Insulating Coating Forming Step In the insulating coating forming step, excess annealing separator is removed from the surface of the finish-annealed sheet obtained in the finish-annealing step by water washing, pickling, or the like, and then a secondary coating, which is an insulating coating, is formed.

[0112] This insulating coating applies tension to the grain-oriented electrical steel sheet, thereby reducing the iron loss of each individual steel sheet, and also ensures electrical insulation between the steel sheets when the grain-oriented electrical steel sheets are stacked for use, thereby reducing the iron loss of the iron core.

[0113] The insulating coating is formed by applying a coating solution containing aluminum phosphate and colloidal silica as main components to the surface of the finish-annealed sheet, baking it at a temperature of 350 to 600°C, for example, and then heat treating it at a temperature of 800 to 1000°C.

[0114] (G) Magnetic Domain Refinement Treatment Step The grain-oriented electrical steel sheet obtained by the above steps may be further subjected to a magnetic domain refinement treatment. By performing the magnetic domain refinement treatment, it becomes possible to reduce eddy current loss and iron loss. The magnetic domain refinement treatment may be performed using a known method, such as forming grooves on the steel sheet surface by a mechanical method or irradiating the steel sheet surface with a laser beam or electron beam to introduce local strain. Note that the magnetic domain refinement treatment does not change the size of the crystal grains. In other words, the above-mentioned Dt and Dr are the same before and after the magnetic domain refinement treatment.

[0115] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0116] A slab having the chemical composition shown in Table 1 was heated to 1340°C in a heating furnace. The heated slab was subjected to a hot rolling process to produce a hot-rolled steel sheet having a thickness of 2.3 mm. Next, the hot-rolled steel sheet was heated to a temperature of 1100°C to be recrystallized, and then annealed at 900°C for 30 seconds to produce a hot-rolled annealed steel sheet. After the hot-rolled annealing, a cold rolling process was carried out to produce a cold-rolled steel sheet having a thickness shown in Table 2.

[0117]

[0118] After cold rolling, a decarburization annealing process was carried out. In the temperature-raising process of the decarburization annealing process, the temperature was raised to 880°C at the temperature-raising rate shown in Table 2. After the temperature-raising process, the steel was cooled to the temperature shown in Table 2, and then irradiated with a laser beam under the conditions shown in Table 2. Thereafter, the oxygen potential (PH) in the annealing atmosphere was adjusted. ２ O / PH ２ ) was set to 0.50, and decarburization annealing was performed by soaking at 850°C for 150 seconds to obtain decarburization annealed steel sheets.

[0119]

[0120] An annealing separator (water slurry) containing MgO as a main component was applied to the surface of a decarburized annealed steel sheet. The decarburized annealed steel sheet coated with the annealing separator was then wound into a coil. The coil was subjected to finish annealing to produce a finish annealed steel sheet. The finish annealing temperature was 1150°C, and the holding time at the finish annealing temperature was 30 hours.

[0121] After the finish annealing process, the annealing separator was removed from the steel sheets, and then a secondary coating process (insulating coating process) was carried out. Specifically, a secondary coating agent (insulating coating agent) mainly composed of colloidal silica and phosphate was applied to the surface (on the glass coating) of each finish annealed steel sheet, and then the finish annealed steel sheet to which the secondary coating agent had been applied was baked, thereby forming a secondary coating, which was a tension insulating coating, on the primary coating. Grain-oriented electrical steel sheets with each test number were manufactured using the above manufacturing process.

[0122] The chemical composition of the base steel sheet was measured by the following method. First, the primary coating and the secondary coating were removed from the base steel sheet by the following method. Specifically, the grain-oriented electrical steel sheet on which the secondary coating was formed was immersed in a solution of NaOH: 30 mass % + H ２ O: The steel sheet was immersed in a 70% by mass aqueous solution of sodium hydroxide at 80° C. for 5 minutes, and then rinsed with water and dried. Through this process, the secondary coating was removed from the directional potential steel sheet.

[0123] Furthermore, the grain-oriented electrical steel sheet from which the secondary coating had been removed and from which the primary coating remained was immersed in high-temperature hydrochloric acid to remove the primary coating. Specifically, the grain-oriented electrical steel sheet from which the primary coating remained was immersed in 30 mass % hydrochloric acid at 80°C for 1 minute, and after immersion, was washed with water and dried. Through the above steps, a base steel sheet from which the secondary coating and primary coating had been removed was obtained.

[0124] The chemical composition of the obtained steel plate was measured by a method conforming to JIS G 0321:2017. Specifically, chips were first collected from the obtained base steel plate, and the collected chips were dissolved in acid to obtain a solution. Next, the solution was subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content were determined by the well-known high-frequency combustion method (combustion-infrared absorption method). The N content was determined using the well-known inert gas fusion-thermal conductivity method. Specifically, measurements were performed using a component analyzer (trade name: ICPS-8000) manufactured by Shimadzu Corporation.

[0125] The analysis results are shown in Table 3.

[0126]

[0127] Next, test pieces for measuring magnetic properties were taken from the grain-oriented electrical steel sheets with each test number. The taking position was on the inner peripheral side of the coil in the finish annealing process. The magnetic flux density B8 of the taken test pieces for measuring magnetic properties was measured by the method described above. In this example, when the magnetic flux density B8 was 1.930 T or more, the magnetic properties were evaluated as excellent and the test piece was deemed to have passed. Thereafter, the test pieces for measuring magnetic properties were subjected to a magnetic domain refining process by irradiating them with a laser beam. Specifically, the irradiation pitch was set to 4 mm intervals, and the energy density Ua was set to 1.25 mJ / mm ２ The iron loss W of the magnetic property measurement specimen after the magnetic domain refining treatment was measured by the above-mentioned method. １７／５０ In this example, the iron loss W after the magnetic domain refining treatment was measured. １７／５０ However, it was determined that low iron loss was obtained if the iron loss was 0.711 W / kg or less.

[0128] Furthermore, test pieces for grain measurement were taken from the grain-oriented electrical steel sheets of each test number. The taking position was on the inner peripheral side of the coil in the finish annealing process. Using the taken test pieces for grain measurement, Dr, Dt, and the small-angle grain boundary ratio were determined by the above-mentioned method. Note that when the measured value of the magnetic flux density B8 was less than 1.700 T, it was determined that secondary recrystallization was defective, and no crystal orientation measurement was performed.

[0129] Furthermore, a coating adhesion evaluation test was conducted on the grain-oriented electrical steel sheets with each test number listed above. Specifically, test pieces measuring 60 mm in the rolling direction and 15 mm in the direction perpendicular to the rolling direction were taken from the center of the grain-oriented electrical steel sheet width. The obtained test pieces were then wrapped around round bars of various diameters, and the coating adhesion was evaluated based on the smallest diameter at which the coating did not peel off visually (hereinafter referred to as the "bending peeling diameter"). The smaller the bending peeling diameter, the better the coating adhesion. A bending peeling diameter of 40 mm or more increased the risk of coating peeling during core manufacturing, and the test piece was deemed a failure. Note that if the measured magnetic flux density B8 was less than 1.700 T, the coating adhesion evaluation test was not conducted.

[0130] Table 4 shows the measurement results of the magnetic properties and crystal orientation, as well as the evaluation results of the coating adhesion.

[0131]

[0132] For test numbers 1 to 11 and 38 to 41, the manufacturing conditions were all appropriate. As a result, Dr was 20.0 mm or less, and Dt / Dr was 1.20 or more. The low-angle grain boundary ratio was also 50% or more. As a result, the magnetic flux density was 1.930 T or more, and the iron loss was 0.711 W / kg or less, demonstrating good magnetic properties. The coating adhesion was also excellent.

[0133] Test No. 12 had a large Up value. Therefore, excessive heat input caused magnetically inferior grains to form in the laser beam irradiated area. Although Dr was controlled, Dt decreased, resulting in a Dt / Dr ratio of less than 1.20. The low-angle grain boundary ratio was also less than 50%. Therefore, the magnetic flux density B8 was less than 1.930 T, and the iron loss was significantly inferior.

[0134] Test No. 13 had a small Up value. As a result, Dr was not sufficiently controlled and became excessive. As a result, the magnetic flux density B8 was less than 1.930 T due to the coil setting, and the iron loss was also poor.

[0135] Test No. 14 had an excessively long irradiation interval, which resulted in a large Dr, a magnetic flux density B8 of less than 1.930 T due to the coil setting, and poor iron loss.

[0136] In test number 15, the irradiation interval was too short. As a result, excessive heat input caused magnetically inferior grains to form in the laser beam irradiated area. Although Dr was controlled, Dt decreased, resulting in a Dt / Dr ratio of less than 1.20. The low-angle grain boundary ratio was also less than 50%. As a result, the magnetic flux density B8 was less than 1.930 T, and the iron loss was also significantly inferior.

[0137] In test number 16, one side was irradiated, which resulted in a large Dr, and the magnetic flux density B8 was less than 1.930 T due to the coil setting, resulting in poor magnetic properties and poor iron loss.

[0138] Test No. 17 had a high temperature during irradiation. When the temperature during irradiation was high, excessive heat input caused magnetically inferior grains to form in the area irradiated by the laser beam. Although Dr was controlled, Dt decreased, resulting in a Dt / Dr ratio of less than 1.20. The low-angle grain boundary ratio was also less than 50%. As a result, the magnetic flux density B8 was less than 1.930 T, and the iron loss was also significantly inferior.

[0139] In test number 18, the temperature during irradiation was low. When the temperature during irradiation was low, Dr increased. In addition, the small-angle grain boundary ratio was 50% or more. Therefore, the magnetic flux density B8 was less than 1.930 T, and the iron loss was also poor.

[0140] In test number 19, the average heating rate during the temperature rise process in the decarburization annealing step was excessively high. Although Dr and Dt were within the appropriate ranges, the small-angle grain boundary ratio decreased, the magnetic flux density B8 was low, and the iron loss was inferior.

[0141] In test number 20, the average heating rate during the temperature rise process in the decarburization annealing step was too low. Although Dr and Dt were within the appropriate ranges, the small-angle grain boundary ratio was reduced, the magnetic flux density B8 was low, and the iron loss was inferior.

[0142] Test No. 21 had a high Si concentration, broke during threading, and characteristics evaluation was not possible. Test No. 22 had a low Si concentration, and iron loss was poor. Test Nos. 23 to 36 did not undergo secondary recrystallization because the contents of the inhibitor-forming elements were inappropriate. As a result, magnetic flux density B8 was significantly poor, so measurements of crystal orientation, iron loss, etc. were not performed.

[0143] In Test No. 37, the laser beam was irradiated before the temperature was raised, so the effect of increasing the low-angle grain boundary ratio due to rapid heating could not be fully enjoyed, and the low-angle grain boundary ratio was less than 50%. As a result, although Dt / Dr was 1.20 or more and the magnetic flux density B8 was 1.930 T or more, the iron loss was inferior.

[0144] As described above, according to the present invention, it is possible to obtain a grain-oriented electrical steel sheet having excellent magnetic properties, which can achieve both an improvement in the degree of orientation concentration in the Goss orientation and a reduction in the diameter of secondary recrystallized grains.

Claims

1. A grain-oriented electrical steel sheet, wherein, when the crystal orientations at measurement points arranged at 1 mm pitch in the rolling direction and in a direction perpendicular to the rolling direction on the surface of a base steel sheet are measured by X-ray diffraction, and the average grain size Dr in the rolling direction and the average grain size Dt in the direction perpendicular to the rolling direction of the crystal grains on the surface are determined, Dr is 20.0 mm or less, the ratio of Dr to Dt, Dt / Dr, is 1.20 or more, and the proportion of the length of grain boundaries with an orientation difference of 15° or less to the total length of the grain boundaries of the crystal grains is 50% or more.

2. The grain-oriented electrical steel sheet according to claim 1, having a thickness of 0.30 mm or less.

3. The chemical composition of the base steel plate is, in mass %, Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0001 to 0.010%, C: 0.0005 to 0.010%, sol.

3. The grain-oriented electrical steel sheet according to claim 1, wherein the total content of one or more elements selected from S and Se is 0 to 0.010%, Ti: 0 to 0.010%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Sb: 0 to 0.30%, Sn: 0 to 0.30%, Cr: 0 to 0.50%, P: 0 to 0.05%, Mo: 0 to 0.05%, Ta: 0 to 0.05%, Nb: 0 to 0.010%, V: 0 to 0.50%, B: 0 to 0.010%, Te: 0 to 0.0150%, Bi: 0 to 0.0150%, and the balance: Fe and impurities.

4. A method for manufacturing a steel sheet, comprising: a hot rolling process of heating a slab and hot rolling the heated slab to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing process of annealing the hot-rolled steel sheet; a cold rolling process of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing process to obtain a cold-rolled steel sheet; a decarburization annealing process of decarburization annealing the cold-rolled steel sheet to obtain a decarburization annealed steel sheet; a finish annealing process of applying an annealing separator to the decarburization annealed steel sheet and then finish annealing it to obtain a finish annealed sheet; and an insulating coating forming process of forming an insulating coating on the surface of the finish annealed sheet, wherein the decarburization annealing process includes a temperature rise process and a soaking process, and during the temperature rise process of the decarburization annealing process, the temperature is raised from a temperature range of 450°C or less to a temperature range of 750 to 950°C, and the average heating rate from 450 to 750°C is 40 to 2000°C / s, A method for producing a grain-oriented electrical steel sheet, comprising: irradiating both the front and back surfaces of the cold-rolled steel sheet with a laser beam in a direction intersecting the rolling direction, in a temperature range of 650 to 850°C after the temperature increase step and before the soaking step, so that the laser beams overlap when viewed from a direction perpendicular to the front surfaces, under conditions that satisfy the following formulas (i) and (ii): 0.2≦Up≦22.3 (i) 5≦d≦18 (ii) where Up in the formula (i) is the instantaneous input energy (J / mm) expressed as 4 / π×P / (Dl×Vc), where P (W) is the average intensity of the laser beam, Dl (mm) is the average focused diameter of the focused spot in the rolling direction, and Vc (mm / sec) is the average scanning speed of the laser beam. ２ ) and d in the above formula (ii) is the irradiation interval (mm) of the laser beam.

5. The method for producing a grain-oriented electrical steel sheet according to claim 4, wherein, when the cold-rolled steel sheet has a thickness of 0.18 mm or less, the average heating rate during the temperature rise process in the decarburization annealing step from 450°C to 750°C is set to 400 to 2000°C / sec.

6. The chemical composition of the slab is, in mass %, Si: 3.00 to 3.70%, Mn: 0.01 to 0.30%, N: 0.0030 to 0.0150%, C: 0.010 to 0.100%, sol.

6. The method for producing a grain-oriented electrical steel sheet according to claim 4 or 5, wherein the content of Al is 0.010 to 0.050%, a total of one or more elements selected from S and Se is 0.010 to 0.050%, Ti: 0 to 0.010%, Ni: 0 to 0.50%, Cu: 0 to 0.50%, Sb: 0 to 0.30%, Sn: 0 to 0.30%, Cr: 0 to 0.50%, P: 0 to 0.05%, Mo: 0 to 0.05%, Ta: 0 to 0.05%, Nb: 0 to 0.010%, V: 0 to 0.50%, B: 0 to 0.010%, Te: 0 to 0.0200%, Bi: 0 to 0.0200%, and the balance: Fe and impurities.

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