Production method for grain-oriented electrical steel sheet

Abstract

Provided is a production method for a grain-oriented electrical steel sheet, wherein: oxygen content of the steel sheet after a decarburization annealing step is not more than 2.4g / m2; a final annealing step has a temperature-raising process and a soaking process; in at least one period between the start of secondary recrystallization in the temperature-raising process and the completion of the secondary recrystallization, a temperature gradient of not less than 0.5°C / cm is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region; a nitriding treatment step is performed at at least one stage from among during the decarburization annealing step, between the decarburization annealing step and the final annealing step, and during the temperature-raising process of the final annealing step up to the start of the secondary recrystallization; and the nitrogen content of the steel sheet after the nitriding treatment step is not less than 0.021 mass%.

Description

Manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet. This application claims priority to Japanese Patent Application No. 2024-101517, filed on June 24, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets (also called unidirectional silicon steel sheets) are soft magnetic materials primarily used as transformer core materials. Therefore, grain-oriented electrical steel sheets are required to have low energy loss (low iron loss). For example, magnetic flux density B8 (magnetic flux density in a magnetic field of 800 A / m) is the most important factor affecting iron loss characteristics. It is known that higher magnetic flux density B8 values ​​result in lower iron loss and better iron loss characteristics. Furthermore, higher magnetic flux density B8 values ​​allow for smaller iron cores, which is advantageous in terms of both the device configuration and manufacturing costs of transformers. To increase the magnetic flux density B8 value, it is important to achieve a highly uniform crystal orientation. This crystal orientation control is achieved by utilizing a catastrophic grain growth phenomenon known as secondary recrystallization.

[0003] Many inventions relating to high magnetic flux density grain-oriented electrical steel sheets have been proposed over the years. However, investigations into industrially produced grain-oriented silicon steel sheets have revealed that their magnetic flux density (B8) does not reach the theoretical upper limit for silicon steel, and there is still room for significant improvement.

[0004] Regarding the improvement of magnetic flux density, for example, Patent Document 1 discloses a method for producing a grain-oriented electrical steel sheet with high magnetic flux density, which comprises heating a silicon steel slab containing 0.015% or less C, 4% or less Si, 0.012% or less S, 0.020 to 0.065% acid-soluble Al, and 0.0030 to 0.0095% T.N. to 1270°C or less, hot working the slab to form a hot-rolled sheet, coiling the hot-rolled sheet at 700 to 950°C, and cold rolling the hot-rolled sheet at a reduction of 65% or more. The hot-rolled sheet is then subjected to primary recrystallization annealing for a short period of time, followed by high-temperature finish annealing, which includes a process for growing secondary recrystallized grains while applying a temperature gradient of 2°C / cm or more to the steel sheet at the boundary between the primary recrystallization region and the secondary recrystallization region.

[0005] Furthermore, Patent Document 2 discloses a method for producing ultra-low iron loss grain-oriented silicon steel sheet at low cost by achieving crystal orientation control through secondary recrystallization and smoothing of the steel sheet surface, even for thin material (e.g., 0.13 mm) that has been difficult to produce in the past. Patent Document 2 discloses that in order to ensure a temperature gradient of at least 2°C / cm, it is necessary to raise the temperature to 1000 to 1100°C at a heating rate of 50°C / hr or more during finish annealing.

[0006] Furthermore, for example, Patent Document 3 discloses a method for producing a grain-oriented silicon steel sheet (strip) having a high magnetic flux density, characterized in that in the manufacturing process of the grain-oriented silicon steel sheet (strip), secondary recrystallization is advanced while a temperature gradient is applied to the steel sheet (strip) in a boundary region between a primary recrystallization region and a secondary recrystallization region.

[0007] Furthermore, for example, Patent Documents 4 and 5 disclose equipment and methods for applying a temperature gradient to a coiled steel sheet.

[0008] Japanese Patent Publication No. 59-41488 Publication of Japanese Patent Publication No. 5-311238 Publication of Japanese Special Publication No. 58-50295 Publication of Japanese Patent Publication No. 57-164935 Publication of Japanese Patent Publication No. 58-1019

[0009] As described above, the techniques described in Patent Documents 1 and 2 indicate that a temperature gradient of 2°C / cm or more is required during finish annealing. When applying a temperature gradient, it is possible to apply the equipment and methods disclosed in Patent Documents 4, 5, etc., but the equipment and methods disclosed therein make it difficult to control the temperature gradient to be constant throughout the entire longitudinal and width directions of an industrial-scale coil, or to control it to be high throughout the entire region. As a result, there are regions with a low temperature gradient (small temperature gradient) of approximately 0.5°C / cm. In such low temperature gradient regions, it is difficult to achieve a sufficient improvement in magnetic flux density. When applying a temperature gradient to a coil, especially when attempting to apply a large temperature gradient, the temperature gradient tends to be smaller inside the coil than outside. That is, even if the temperature gradient is 2°C / cm or more outside the coil, some regions, such as the inside of the coil, will have a low temperature gradient of less than 2°C / cm. Furthermore, when applying a temperature gradient in the width direction of the coil, the temperature gradient tends to be smaller at the low temperature end than at the high temperature end. For example, even if the high-temperature end of the coil has a temperature gradient of 2°C / cm or more, some regions, such as the low-temperature end, may have a low temperature gradient of less than 2°C / cm. Therefore, it is difficult to achieve a temperature gradient of 2°C / cm or more over the entire coil. For this reason, a method has been desired that can achieve a magnetic flux density improvement effect even with a smaller temperature gradient in order to obtain a sufficient magnetic flux density improvement effect over the entire coil.

[0010] Patent Document 3 discloses that applying a temperature gradient of 0.5°C / cm has an effect of improving the B8 characteristics. However, Patent Document 3 also indicates that a significant effect can be obtained when the temperature gradient is 2°C / cm or more. In fact, the B8 value of a grain-oriented electrical steel sheet with a Si content of 2.95% at a temperature gradient of 0.5°C / cm is approximately 1.92 T. Although a certain degree of improvement in magnetic flux density is achieved, this is not sufficient to meet the increasingly sophisticated requirements of recent years.

[0011] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for manufacturing a grain-oriented electrical steel sheet that produces a grain-oriented electrical steel sheet with a high magnetic flux density by performing finish annealing while applying a temperature gradient to the boundary region between the primary recrystallized region and the secondary recrystallized region, and that can produce a grain-oriented electrical steel sheet with a stable high magnetic flux density throughout the coil by achieving a sufficient effect of improving the magnetic flux density even with a small temperature gradient.

[0012] The present inventors investigated a method in which finish annealing is performed while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region, and found that a sufficient improvement in magnetic flux density can be achieved even with a relatively small temperature gradient (e.g., 0.5°C / cm). As a result, they found that the lower limit of the temperature gradient at which a high magnetic flux density can be achieved is wider than conventional methods when the oxygen content of the steel sheet after the decarburization annealing step is controlled and the nitrogen content of the steel sheet before secondary recrystallization is increased to 0.021 mass% or more.

[0013] The present invention has been made in light of the above findings. The gist of the present invention is as follows: [1] A method for producing a grain-oriented electrical steel sheet according to one aspect of the present invention comprises, in mass%, Si: 0.8 to 7.0%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, S and Se: 0 to 0.015% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5% %, Ni: 0 to 1.0%, O: 0 to 0.10%, Mo: 0 to 0.10%, Nb: 0 to 0.2%, Ti: 0 to 0.015%, V: 0 to 0.15%, B: 0 to 0.008%, Sn: 0 to 0.04%, Sb: 0 to 0.04%, and the balance being Fe and impurities, is heated to a temperature of 1280°C or less, and then a hot rolling step of hot rolling to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet as needed, a cold rolling step of subjecting the hot-rolled sheet after the hot rolling step or the hot-rolled sheet annealing step to a single cold rolling or multiple cold rolling steps via annealing with a final rolling reduction of 85% or more to obtain a steel sheet of a final sheet thickness, a decarburization annealing step of decarburizing annealing the steel sheet after the cold rolling step, an annealing separator application step of applying an annealing separator to the steel sheet after the decarburization annealing step and then winding it into a coil, a nitriding treatment step of increasing the nitrogen content of the steel sheet, and a finish annealing step of finish annealing the coiled steel sheet, wherein the oxygen content of the steel sheet after the decarburization annealing step is 2.4 g / m ２ [2] In the method for producing a grain-oriented electrical steel sheet according to [1], the finish annealing step may have a temperature rise step and a soaking step, and a temperature gradient of 0.5°C / cm or more is generated in a boundary region between the primary recrystallization region and the secondary recrystallization region at least for a period from the start of secondary recrystallization to the completion of secondary recrystallization in the temperature rise step, and the nitriding step is carried out in at least one stage of the decarburization annealing step, between the decarburization annealing step and the finish annealing step, or during the temperature rise step in the finish annealing step up to the start of secondary recrystallization, and the nitrogen content of the steel sheet after the nitriding step may be 0.021 mass% or more.

[0014] According to the above aspect of the present invention, there can be provided a method for producing a grain-oriented electrical steel sheet with a high magnetic flux density by performing finish annealing while applying a temperature gradient to the boundary region between the primary recrystallized region and the secondary recrystallized region, and the method can provide a method for producing a grain-oriented electrical steel sheet that can achieve a sufficient improvement in magnetic flux density even if the temperature gradient is small (even if the lower limit of the temperature gradient is small when there are regions with a large temperature gradient and regions with a small temperature gradient).In other words, according to the above aspect of the present invention, there can be provided a method for producing a grain-oriented electrical steel sheet that can produce a grain-oriented electrical steel sheet with a stable high magnetic flux density throughout the entire coil, even if the temperature gradient is made substantially uniformly low throughout the coil, or even if there are regions with a partial low temperature gradient (small temperature gradient) in the coil.

[0015] The effect of nitrogen content (mass%) and oxygen content (g / m) on magnetic flux density (B8) under the condition of low temperature gradient (0.5°C / cm) ２ ) is shown in FIG.

[0016] A method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention (the method for manufacturing a grain-oriented electrical steel sheet according to this embodiment) will be described below. The method for manufacturing a grain-oriented electrical steel sheet according to this embodiment includes: (i) a hot rolling step in which a silicon steel material having a predetermined chemical composition is heated to a temperature of 1280°C or less and then hot-rolled to obtain a hot-rolled sheet; (ii) a hot-rolled sheet annealing step in which the hot-rolled sheet is annealed as necessary; (iii) a cold rolling step in which the hot-rolled sheet after the hot rolling step or the hot-rolled sheet annealing step is subjected to a single cold rolling or multiple cold rolling steps via annealing, with a final reduction of 85% or more, to obtain a steel sheet of a final thickness; (iv) a decarburization annealing step in which the steel sheet after the cold rolling step is decarburized and annealed; (v) an annealing separator application step in which an annealing separator is applied to the steel sheet after the decarburization annealing step and then wound into a coil; and (vi) a nitriding treatment step in which the nitrogen content of the steel sheet is increased. (vii) a finish annealing step of finish annealing the steel sheet wound into a coil. Conditions for each step will be explained below.

[0017] [Hot Rolling Process] In the hot rolling process, a silicon steel material such as a slab having the chemical composition described below is heated to a temperature of 1280°C or less and then hot-rolled to obtain a hot-rolled sheet (hot-rolled steel sheet). In a typical industrial method for producing grain-oriented electrical steel sheets, fine precipitates known as inhibitors are completely dissolved during heating of the silicon steel material before hot rolling, and then fine precipitates are formed during hot rolling and the subsequent annealing process. This method requires heating at a high temperature of 1350°C or more to completely dissolve the precipitates. However, this temperature is approximately 200°C higher than the slab heating temperature for ordinary steel, requiring a dedicated heating furnace for this purpose and resulting in problems such as a large amount of melt scale. Therefore, in the method for producing grain-oriented electrical steel sheets according to this embodiment, the heating temperature is set to 1280°C or less to avoid the problems associated with high-temperature heating. The heating temperature is preferably 1000°C or higher in order to perform hot rolling. Hot rolling conditions other than the heating temperature are not limited and may be determined within a known range depending on the required properties, etc.

[0018] The silicon steel material to be subjected to hot rolling is obtained by melting steel in a converter or electric furnace, subjecting the molten steel to vacuum degassing treatment as necessary, and then continuously casting or blooming the resulting product. The silicon steel material has a chemical composition containing, in mass%, Si: 0.8 to 7.0%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, S and Se: 0 to 0.015% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.0%, O: 0 to 0.10%, Mo: 0 to 0.10%, Nb: 0 to 0.2%, Ti: 0 to 0.015%, V: 0 to 0.15%, B: 0 to 0.008%, Sn: 0 to 0.04%, Sb: 0 to 0.04%, and the balance being Fe and impurities. The reasons for limiting the chemical composition (content of each element) of the silicon steel material will be explained below. The percentage of the content of each element is mass %.

[0019] (Si: 0.8 to 7.0%) If the Si content is less than 0.8%, γ transformation occurs during finish annealing, damaging the crystal orientation of the steel sheet. Furthermore, the inclusion of Si increases electrical resistance and improves iron loss characteristics. Therefore, the Si content in the silicon steel material is set to 0.8% or more. The Si content is preferably 1.5% or more, more preferably 2.0% or more, and even more preferably 2.5% or more. On the other hand, if the Si content exceeds 7.0%, cold rolling becomes extremely difficult and there is a risk of cracking during rolling. Therefore, the Si content is set to 7.0% or less. As a range suitable for industrial production, the Si content may be 4.8% or less, or may be 4.0% or less.

[0020] (C: 0.085% or less) C is an effective element for controlling the primary recrystallization structure, but it has a negative effect on magnetic properties, so it is necessary to decarburize it before final annealing. If the C content in a silicon steel material exceeds 0.085%, the decarburization annealing time becomes long, impairing productivity in industrial production. Therefore, the C content is set to 0.085% or less. A low C content is preferable, and 0% is acceptable, but when considering productivity in industrial production and the magnetic properties of the product, the substantial lower limit of the C content is 0.020%.

[0021] (Acid-soluble Al: 0.010 to 0.065%) Acid-soluble Al (sol. Al) is an element that combines with N to form AlN or (Al, Si) N, and functions as an inhibitor. The acid-soluble Al content is set to 0.010 to 0.065%, as the range in which secondary recrystallization is stable.

[0022] (N: 0.004 to 0.012%) N (nitrogen) is an element that combines with Al and the like to form AlN or (Al, Si)N, which function as inhibitors. If the N content is less than 0.004%, the formation of AlN or (Al, Si)N will be insufficient, so the N content is set to 0.004% or more. The N content is preferably 0.006% or more, more preferably 0.007% or more. On the other hand, if the N content exceeds 0.012%, voids called blisters may occur in the steel sheet during cold rolling, so the N content is set to 0.012% or less. The N content is preferably 0.010% or less, and more preferably 0.009% or less.

[0023] (Mn: 0 to 1.00%) Mn (manganese), like Si, is an element that is effective in increasing electrical resistance and reducing iron loss. Therefore, it may be contained. To obtain the above effect, the Mn content is preferably 0.01% or more. Furthermore, Mn is an element that bonds with S or Se to function as an inhibitor. To obtain this effect, the Mn content is preferably 0.05% or more, more preferably 0.08% or more, and even more preferably 0.09% or more. On the other hand, if the Mn content is excessive, the magnetic flux density decreases. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.50% or less, and more preferably 0.20% or less.

[0024] (S and Se: 0 to 0.015% in total) S (sulfur) and Se (selenium) are elements that bond with Mn and the like to exert an inhibitor effect. Therefore, they may be contained. To obtain this effect, it is preferable that the total content of S and Se is 0.005% or more before finish annealing, for example, at the slab stage. However, if these elements remain as impurities in the final product and the total of S and Se exceeds 0.015%, it will have an adverse effect on the magnetic properties. Therefore, the total content of S and Se is set to 0.015% or less. It is preferable that the total content of S and Se is 0.010% or less, or 0.005% or less.

[0025] In the chemical composition of the silicon steel material, the balance other than the above may be Fe and impurities. On the other hand, in order to improve various properties, one or more elements selected from Cr, Cu, P, and Ni may be contained in place of a part of Fe.

[0026] (Cr: 0 to 0.30%) Cr is an element that favorably adjusts the composition and amount of the oxide layer during decarburization annealing and promotes the formation of a glass film. Therefore, Cr may be added. On the other hand, if the Cr content exceeds 0.30%, decarburization is inhibited and magnetic properties deteriorate. Therefore, the Cr content is set to 0.30% or less.

[0027] (Cu: 0 to 0.4%) Cu is an element that is effective in increasing resistivity and reducing iron loss. Therefore, it may be contained. On the other hand, if the Cu content exceeds 0.4%, the iron loss reduction effect saturates and it becomes a cause of surface defects called "copper scuffs" during hot rolling. Therefore, the Cu content is set to 0.4% or less.

[0028] (P: 0 to 0.5%) P is an element that is effective in increasing resistivity and reducing iron loss. Therefore, it may be contained. On the other hand, if the P content exceeds 0.5%, the rollability decreases. Therefore, the P content is set to 0.5% or less.

[0029] (Ni: 0 to 1.0%) Ni is an element effective for increasing resistivity and reducing iron loss. It is also an element effective for controlling the metal structure of the hot-rolled sheet and improving the magnetic properties. Therefore, it may be contained. On the other hand, if the Ni content exceeds 1.0%, secondary recrystallization becomes unstable. Therefore, the Ni content is set to 1.0% or less.

[0030] (O: 0 to 0.10%) O is an impurity that may be unavoidably contained due to contamination from raw materials or during the manufacturing process. O forms oxides in steel and deteriorates iron loss characteristics, so the O content is set to 0.10% or less. It is preferably 0.05% or less, and more preferably 0.01% or less. On the other hand, the lower the O content, the better, and 0% is acceptable, but from the viewpoint of avoiding an increase in costs, the O content is preferably 0.001% or more, and may be 0.01% or more.

[0031] (Mo: 0 to 0.10%) Mo (molybdenum) is also an element effective in increasing electrical resistance and reducing iron loss. Therefore, Mo may be contained. To obtain the above effects, the Mo content is preferably 0.005% or more, and more preferably 0.01% or more. On the other hand, if the Mo content exceeds 0.10%, problems may occur in the rollability of the steel sheet. Therefore, the Mo content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0032] (V: 0 to 0.15%) (Nb: 0 to 0.20%) (Ti: 0 to 0.015%) V, Nb, and Ti are effective elements that bond with N and C to function as inhibitors. Therefore, V, Nb, and / or Ti may be contained. To obtain the above effects, the V content is preferably 0.002% or more, and more preferably 0.01% or more. The Nb content is preferably 0.005% or more, and more preferably 0.020% or more. The Ti content is preferably 0.002% or more, and more preferably 0.004% or more. On the other hand, if these elements remain in the final product and the V content exceeds 0.15%, the Nb content exceeds 0.20%, or the Ti content exceeds 0.015%, there is a risk of deterioration of the magnetic properties. Therefore, the V content is set to 0.15% or less, the Nb content to 0.20% or less, and the Ti content to 0.015% or less. The V content is preferably set to 0.10% or less, and more preferably set to 0.05% or less. The Nb content is preferably set to 0.10% or less, and more preferably set to 0.08% or less. The Ti content is preferably set to 0.010% or less, and more preferably set to 0.008% or less.

[0033] (B: 0 to 0.008%) B is an effective element that exhibits an inhibitor effect as BN. Therefore, B may be contained. To obtain the above effect, the B content is preferably 0.0005% or more, and more preferably 0.001% or more. On the other hand, if the B content exceeds 0.008%, there is a risk of deterioration of magnetic properties. Therefore, the B content is set to 0.008% or less. The B content is preferably 0.005% or less, and more preferably 0.003% or less.

[0034] (Sn: 0 to 0.04%) (Sb: 0 to 0.04%) Sn and Sb are effective elements that function as grain boundary segregation inhibitors. Therefore, Sn and / or Sb may be contained. To obtain the above effects, the Sn content and Sb content are preferably 0.01% or more. On the other hand, if the Sn and / or Sb content exceeds 0.04%, the grain boundary migration rate decreases, grain growth is suppressed, and magnetic properties may deteriorate. Therefore, the respective contents are set to 0.04% or less.

[0035] In addition to the elements described above, the grain-oriented electrical steel sheet may contain impurities such as 0.0015% or less Mg, 0.10% or less each of As, Pb, W, Co, and Bi, 0.003% or less Zr, 0.02% or less each of REM, Zn, Ba, Cd, Pt, Au, In, Ga, Ge, Sc, Hf, and Se, 0.005% or less each of Te, and / or 0.10% or less Ca. The term "impurities" refers to elements that are mixed in from the raw materials or during the manufacturing process and that do not clearly affect the properties of the grain-oriented electrical steel sheet according to this embodiment.

[0036] The above-mentioned chemical compositions may be measured by a general steel analysis method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Al may be measured as acid-soluble Al by ICP-AES using the filtrate obtained by thermally decomposing a sample with acid. In addition, Si may be measured using the silicon dioxide gravimetric method, C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0037] [Hot-rolled sheet annealing process] After the hot rolling process, the hot-rolled sheet obtained by hot rolling may be annealed (hot-rolled sheet annealing) to improve magnetic properties. The annealing conditions are not limited, but may be, for example, conditions of holding at 900 to 1200°C for 30 seconds to 30 minutes.

[0038] [Cold Rolling Step] In the cold rolling step, the hot-rolled sheet after the hot rolling step or the hot-rolled sheet annealing step is cold-rolled to form a steel sheet (cold-rolled sheet) having the same thickness as the final sheet thickness (the sheet thickness when the hot-rolled sheet is made into a grain-oriented electrical steel sheet (however, if a glass coating or an insulating coating is formed on the surface, the sheet thickness of the base steel sheet excluding these)). The cold rolling can be a single cold rolling (a series of cold rolling steps without intermediate annealing in between) or multiple cold rolling steps with annealing (intermediate annealing). In cold rolling, the final reduction is set to 85% or more in order to develop a preferred primary recrystallization orientation such as {411} during decarburization annealing. The final reduction is the cumulative reduction of cold rolling, and in the case of intermediate annealing, it is the cumulative reduction of cold rolling after the final intermediate annealing.

[0039] [Decarburization Annealing Step] In the decarburization annealing step, the steel sheet after the cold rolling step is decarburized and annealed. In the decarburization annealing step, the steel sheet undergoes primary recrystallization and C, which adversely affects magnetic properties, is removed from the steel sheet. In the method for producing a grain-oriented electrical steel sheet according to this embodiment, the amount of oxygen in the steel sheet after the decarburization annealing step is set to 2.4 g / m ２ The oxygen content of the steel sheet after the decarburization annealing step is controlled to be 2.4 g / m ２ By setting the nitrogen content of the steel sheet after the nitriding treatment process described below to 0.021 mass % or more, a high magnetic flux density improvement effect can be obtained even with a relatively small temperature gradient in the finish annealing process. ２ Hereinafter, it is considered that by setting the nitrogen content to 0.021 mass % or more, the decomposition of the (Al, Si)N inhibitor formed by the nitriding treatment is suppressed, and the steel sheet is thermally stabilized. ２If the nitrogen content of the steel sheet after the nitriding treatment step is more than 0.021 mass %, a sufficient effect of improving the magnetic flux density cannot be obtained. Since the oxygen content of the steel sheet after the decarburization annealing step is mostly oxygen contained in the oxide layer formed on the surface of the steel sheet during the decarburization annealing, the atmosphere during the decarburization annealing (oxidation degree (PH, which is the ratio of the hydrogen partial pressure to the water vapor partial pressure)) is not affected by the amount of oxygen. ２ O / PH ２ The oxygen content can be controlled by adjusting the oxidation degree of the atmospheric gas, such as the temperature, dew point, annealing temperature, annealing time, and heating rate. For example, the oxygen content can be adjusted by controlling the following conditions: the oxidation degree of the atmospheric gas is greater than 0.15 and not greater than 1.10, the annealing temperature is 770 to 900°C, the annealing time (holding time) is 60 to 300 seconds, and the heating rate from a range of 600°C or less to the annealing temperature is 40°C / second or more. The oxidation degree is preferably in the range of 0.18 to 1.00.

[0040] The oxygen content of the steel sheet after the decarburization annealing process was 2.4 g / m ２ When the amount of oxygen in the steel sheet after the decarburization annealing process exceeds 2.4 g / m, the effect of improving the magnetic flux density is saturated at about 0.018 mass % even if the amount of nitrogen after the nitriding process is increased. ２ In the following cases, by setting the nitrogen content after the nitriding treatment step to 0.021 mass % or more, the (Al, Si)N inhibitor is stabilized and a greater effect of improving magnetic flux density can be obtained. In particular, the manufacturing method of the grain-oriented electrical steel sheet according to this embodiment achieves a previously unexpected effect of improving magnetic flux density under low temperature gradient conditions.

[0041] The oxygen content of the steel sheet after the decarburization annealing step can be measured by a known method, for example, an inert gas fusion-non-dispersive infrared absorption method.

[0042] [Annealing separator application process] In the annealing separator application process, an annealing separator is applied to the steel sheet after the decarburization annealing process, and then the steel sheet is wound into a coil. The annealing separator to be applied may be a known one, but one containing magnesia as its main component is preferred. By applying the annealing separator containing magnesia as its main component and then performing finish annealing, a glass coating (forsterite coating) is formed on the surface of the steel sheet.

[0043] [Nitriding Process] The nitriding process increases the nitrogen content of the steel sheet. The nitriding process is performed at least in one of the following stages: during the decarburization annealing process, between the decarburization annealing process and the finish annealing process, or during the temperature increase process of the finish annealing process up to the start of secondary recrystallization. However, regardless of the stage, the nitrogen content of the steel sheet after nitriding (or after the final nitriding process if multiple nitriding processes are performed) must be 0.021 mass% (210 ppm) or more by mass. Increasing the nitrogen content of the steel sheet before the start of secondary recrystallization increases the amount of inhibitors, making them thermally stable. As a result, even with a relatively small temperature gradient, a sufficient magnetic flux density improvement effect can be achieved. On the other hand, if the nitrogen content exceeds 0.035 mass% (350 ppm), the effect saturates and may be detrimental to purification after secondary recrystallization. Therefore, it is preferable to set the nitrogen content after nitriding to 0.035 mass% or less. Conventionally, when finish annealing is performed with a temperature gradient, the nitrogen content of the steel sheet has typically been set to 200 ppm or less. For example, Japanese Patent Laid-Open Publication No. 59-215419 discloses that when secondary recrystallization annealing is performed while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region in final annealing, the nitrogen content in the steel sheet at the start of secondary recrystallization is set to 130 to 200 ppm. Japanese Patent Laid-Open Publication No. 59-215419 also discloses that the effect of improving B8 saturates at a nitrogen content of 180 to 200 ppm. In contrast, the present inventors have found that when the oxygen content of the steel sheet after the decarburization annealing process is controlled and the nitrogen content is increased to 0.021 mass% or more, the lower limit of the temperature gradient at which a high magnetic flux density can be achieved is expanded compared to conventional methods (high B8 can be obtained even with a temperature gradient of approximately 0.5°C / cm). When the nitrogen content of the steel sheet after the nitriding process is 0.021 mass % or more, a large improvement in magnetic flux density can be achieved even when the temperature gradient is relatively small, such as about 0.5° C. / cm.

[0044] An example of a method for increasing the nitrogen content of a steel sheet is a method of controlling the nitrogen content of the steel sheet by annealing in an atmosphere containing a nitriding gas. Furthermore, when this is performed during the temperature rise process of the finish annealing process, an example of a method is to add a nitriding powder such as MnN to the annealing separator. The nitrogen content of the steel sheet after nitriding can be measured by a known method, for example, using an oxygen, nitrogen, and hydrogen analyzer (EMGA-930) manufactured by Horiba, Ltd. or an equivalent device. Known methods include general analytical methods such as the inert gas fusion-thermal conductivity method. A sample of any size can be taken from the steel sheet after the nitriding process during the manufacturing process, and the measurement can be performed using these devices and methods.

[0045] [Finish annealing step] In the finish annealing step, the coiled steel sheet is finish annealed. The finish annealing step includes a temperature increase step in which the steel sheet is heated to a finish annealing temperature to cause secondary recrystallization, and a soaking step in which the steel sheet is maintained at the finish annealing temperature.

[0046] <Temperature-raising process> In this finish annealing process, in the above-mentioned nitriding process, the nitrogen content of the steel sheet is controlled to be 0.021 mass % or more by mass, and a temperature gradient of 0.5°C / cm or more is generated in the boundary region between the primary recrystallized region and the secondary recrystallized region at least for a period from the start of secondary recrystallization to the completion of secondary recrystallization in the temperature-raising process, thereby causing preferential growth of {110}<001> orientation grains by secondary recrystallization.

[0047] During finish annealing, secondary recrystallized grains are formed in the portion heated to or above the secondary recrystallization temperature. When a steel sheet is heated in a state where a temperature gradient exists in a certain direction, secondary recrystallization progresses from the region where the temperature exceeds the secondary recrystallization temperature, and a region where primary recrystallized grains and secondary recrystallized grains coexist (a boundary region) is formed along the isothermal line between the region where the temperature has not yet reached the secondary recrystallization temperature and the region where the primary recrystallized structure remains. As the steel sheet is heated and the temperature increases, this boundary region moves along the temperature gradient toward the region where the primary recrystallized structure remains, expanding the region where the secondary recrystallized structure has formed, and ultimately the entire steel sheet is covered with secondary recrystallized grains. Throughout this process, the temperature of the boundary region is kept relatively constant. Regarding the direction of the temperature gradient, since a coil-shaped grain-oriented electrical steel sheet is usually placed in a furnace so as to form a cylindrical shape during finish annealing, it is preferable to provide a temperature gradient in the width direction of the steel sheet.

[0048] As described above, it is not easy to create a temperature gradient of 2.0°C / cm or more throughout the entire coil, which may result in problems such as productivity and variations in properties within the steel sheet. However, in the manufacturing method of a grain-oriented electrical steel sheet according to this embodiment, the nitrogen content of the steel sheet is 0.021 mass% or more at the start of secondary recrystallization. Therefore, the amount of inhibitor increases, and the inhibitor becomes thermally stable, thereby achieving a sufficient improvement in magnetic flux density even with a relatively small temperature gradient. If the temperature gradient varies, the lower limit of the temperature gradient at which a sufficient improvement in magnetic flux density can be achieved can be reduced. However, if the temperature gradient is less than 0.5°C / cm, the improvement in magnetic flux density cannot be achieved sufficiently. Therefore, the temperature gradient is set to 0.5°C / cm or more. If the temperature gradient varies across the coil or steel sheet, the minimum temperature gradient across the entire coil or steel sheet is set to 0.5°C / cm or more. Although there is no upper limit to the temperature gradient, if the temperature gradient exceeds 10.0°C / cm, the effect saturates and the equipment load increases, so the temperature gradient across the entire coil may be 10.0°C / cm or less. In the present application, a sufficient magnetic flux density improvement effect can be obtained even with a relatively small temperature gradient, so the temperature gradient across the entire coil may be 5.0°C / cm or less, or 2.0°C / cm or less. In particular, if a relatively uniform temperature gradient is provided, it may be further set to 1.5°C / cm or 1.0°C / cm or less. If the temperature gradient varies across each portion of the coil or steel sheet, the minimum temperature gradient across the entire coil or steel sheet may be 5.0°C / cm or less, or 2.0°C / cm or less, or further set to 1.5°C / cm or 1.0°C / cm or less. The temperature gradient can be imparted by raising the temperature by creating a temperature difference within the coil in a finish annealing furnace. Regarding the magnitude of the temperature gradient, for example, if a temperature gradient is applied in the width direction of the coil, the temperature gradient at each location in the steel sheet can be calculated by measuring the temperature history with sensors such as thermocouples arranged at regular intervals in the width direction (intervals at which the temperature gradient can be measured, for example, 100 mm intervals). By calculating the temperature gradient at each location, the minimum value of the temperature gradient over the entire coil (overall coil area) can be obtained.

[0049] Furthermore, the temperature gradient varies depending on the size of the furnace, the temperature difference within the furnace, the size and weight of the coil, etc. In such cases, as described above, physical properties such as thermal diffusivity may be calculated using the results of actually measuring the temperature history of multiple parts of the coil, and the temperature gradient at each part of the coil may be calculated by simulation using, for example, Fluent (registered trademark) manufactured by ANSYS, Inc., with the furnace wall temperature set as a boundary condition. In the simulation, by setting various conditions, it is possible to calculate the temperature gradient at each part of the coil (for example, within any range of 100 mm intervals in the coil width direction) taking into account the temperature gradient variation. By calculating the temperature gradient at each part, the minimum value of the temperature gradient throughout the coil can be obtained.

[0050] Regarding the application of a temperature gradient, the temperature at the boundary region varies depending on the type of steel sheet and the annealing conditions. However, the temperature of the boundary region can be determined by confirming the temperature at which secondary recrystallization occurs under the expected type of steel sheet and annealing conditions through preliminary experiments. For example, in a preliminary experiment, a 20 cm x 60 cm sample is annealed under a temperature gradient, the annealing is stopped and cooled in a temperature range expected to be the secondary recrystallization temperature, and the crystal grain structure is observed, thereby deriving the temperature of the boundary region between the primary and secondary recrystallization regions. For example, in the case of a steel sheet with an Si content of approximately 3 mass% and containing MnS and AlN as inhibitors, the temperature of the boundary region will be approximately 950 to 1100°C, although this will vary depending on the amount of inhibitor. In order to achieve the effects of the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, it is necessary to apply a temperature gradient to each portion of the coil at least near the temperature of the boundary region. By applying a temperature gradient in a temperature range in which each portion of the coil is at least at or above the temperature of the boundary region, a temperature gradient can be applied to the boundary region between the primary and secondary recrystallization regions. For example, when applying a temperature gradient, the high-temperature end of the coil is heated to a temperature equal to or higher than the boundary region temperature, and then each portion of the coil toward the low-temperature end is sequentially heated to a temperature equal to or higher than the boundary region temperature, thereby applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region throughout the coil. When the boundary region is not clearly defined, a temperature gradient may be applied to a wider area or the entire coil (steel sheet). While applying a temperature gradient to the boundary region during at least a period from the generation to growth of secondary recrystallized grains can be effective, to obtain a sufficient effect, it is preferable to apply a temperature gradient to the boundary region from the start of secondary recrystallization until the entire steel sheet is covered with secondary recrystallization grains (until the completion of secondary recrystallization). In other words, a temperature gradient may be applied throughout the entire heating process of the final annealing (until the soaking temperature is reached).

[0051] Furthermore, if a temperature gradient varies in the radial direction of the coil, sensors such as thermocouples can be arranged at regular intervals (at intervals that allow the difference in temperature gradient to be measured, for example, 100 mm intervals) in the width direction at multiple locations in the radial direction of the coil to measure the temperature history in the width direction at each location, thereby calculating the difference in the radial temperature gradient. The minimum temperature gradient of the entire coil can be obtained from the temperature gradient calculated at each measurement location in the radial direction of the coil. For example, when a temperature gradient is applied in the width direction of the coil, the measurement locations for the temperature history in the radial direction of the coil can be three or more locations in total, including one or more measurement locations in the longitudinal direction of the coil on the steel plate located at the outermost position of the coil, the steel plate located in the middle portion of the coil in the radial direction, and the steel plate located at the innermost position. Similarly, the width direction temperature gradient at multiple locations in the radial direction of the coil (for example, at positions spaced 100 mm apart in the radial direction of the coil, or at the outermost, middle, and innermost positions in the radial direction of the coil) can also be calculated by simulation. When the temperature gradient varies within the coil, the temperature gradient tends to be relatively small at the low temperature end of the coil and also at the radially innermost position of the coil. Therefore, the temperature gradient measured or calculated by simulation at the low temperature end and the radially innermost position of the coil may be taken as the smallest temperature gradient throughout the coil.

[0052] <Soaking Process> In the soaking process, impurities that harm magnetic properties, such as N, S, and Se, are purified (removed). For this reason, the finish annealing temperature (soaking temperature) is preferably 1150 to 1250°C. In addition, the annealing time (soaking time) is preferably 10 to 30 hours after the low-temperature side of the coil's temperature gradient reaches the soaking temperature.

[0053] [Insulating Coating Forming Step] The method for producing a grain-oriented electrical steel sheet according to this embodiment may further include an insulating coating forming step of forming an insulating coating on the surface of the steel sheet. The insulating coating to be formed is not limited, and may be a known coating.

[0054] [Magnetic Domain Refinement Step] The manufacturing method of the grain-oriented electrical steel sheet according to this embodiment may further include a magnetic domain refinement step of refining the magnetic domains of the steel sheet. Performing the magnetic domain refinement process can further reduce the iron loss of the grain-oriented electrical steel sheet. The method of magnetic domain refinement is not limited, but examples include a method of narrowing the width of 180° magnetic domains (refining 180° magnetic domains) by forming linear or point-like grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction, or a method of narrowing the width of 180° magnetic domains (refining 180° magnetic domains) by forming linear or point-like stress-strain portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction. Laser beam irradiation, electron beam irradiation, and the like can be used to form stress-strain portions. Furthermore, a mechanical groove formation method using a gear or the like, a chemical groove formation method using electrolytic etching, and a thermal groove formation method using laser irradiation can be used to form grooves. If the insulating coating is damaged by the formation of stress-strained portions or grooves, and the insulating properties and other characteristics are deteriorated, the insulating coating may be formed again to repair the damage.

[0055] Example 1: A silicon steel material containing, by mass, 3.3% Si, 0.053% C, 0.027% acid-soluble Al, 0.008% N, 0.1% Mn, and 0.005% S, with the balance being Fe and impurity elements, was obtained. The silicon steel material was heated to 1150°C, held for 30 minutes, and then hot-rolled to obtain a hot-rolled sheet having a thickness of 2.3 mm. The hot-rolled sheet was annealed at 1100°C for 30 seconds. The hot-rolled sheet was then cold-rolled at a 90% reduction to obtain a steel sheet having a final thickness of 0.23 mm (the same thickness as the base steel sheet excluding the glass coating and insulating coating of the grain-oriented electrical steel sheet). A steel sheet (sample steel sheet) measuring 200 mm in the rolling direction and 600 mm in the width direction was cut from the steel sheet. These steel sheets are heated in a wet atmospheric gas containing, by volume, 75% hydrogen and 25% nitrogen from 550°C to 830°C at a heating rate of 100°C / s, and then annealed at 830°C for 100 seconds (decarburization annealing). The oxidation degree of the atmospheric gas is adjusted in the range of 0.18 to 0.67, and the oxygen content of the steel sheets is adjusted to 1.4 to 2.8 g / m ２The steel sheets after decarburization annealing are subjected to nitriding in an atmospheric gas containing hydrogen, nitrogen, and ammonia, and the nitrogen content of the steel sheets is adjusted to 0.017 to 0.039 mass% by changing the ammonia content. Each steel sheet is coated with an annealing separator mainly composed of magnesia, and then placed in a finish annealing furnace. During finish annealing, the steel sheets are heated to 1200°C in a mixed gas containing, by volume, 75% hydrogen and 25% nitrogen, and then the atmosphere is switched to a 100% hydrogen atmosphere and held for 20 hours. During this heating process, a temperature gradient of 0.5°C / cm is applied in the direction perpendicular to the rolling direction (width direction) to the boundary region between the primary and secondary recrystallization regions from the start of secondary recrystallization to the completion of secondary recrystallization. The temperature gradient is applied uniformly throughout the steel sheet. The temperature gradient is applied by raising the temperature with a temperature difference within the furnace. The magnitude of the temperature gradient is controlled by increasing the temperature while measuring the temperature at regular intervals (for example, 100 mm intervals) in the width direction of the steel sheet. The temperature at the boundary region is determined in a preliminary experiment to confirm the temperature at which secondary recrystallization occurs for the assumed type of steel sheet and annealing conditions, and a temperature gradient is applied to the boundary region between the primary recrystallization region and the secondary recrystallization region by applying a temperature gradient at a position where the temperature is close to the temperature of the boundary region thus determined.

[0056] After the finish annealing, a known insulating coating is formed on the steel sheet to apply tension to the steel sheet (tension coating is applied), and then laser irradiation is performed to perform magnetic domain refining treatment.

[0057] A sample of 60 mm in the width direction and 200 mm in the length direction is taken from the obtained steel plate, and magnetic measurements are performed on this sample using the SST method in accordance with Appendix JA of JIS C2556:2015.

[0058] The magnetic flux density B8 values ​​obtained by the above magnetic measurement are shown in Figure 1. As can be seen from Figure 1, the magnetic flux density B8 tends to increase when the nitrogen content is increased, but when the oxygen content is high (2.8 g / m ２ ) is saturated when the nitrogen content is 0.021 mass% or more. ２When the nitrogen content is set to 0.021 mass% or more, the magnetic flux density B8 increases by increasing the nitrogen content, and it is saturated at a nitrogen content of 0.033 mass%. ２ By setting the nitrogen content of the steel sheet after the nitriding treatment step to 0.021 mass % or more in addition to the above, the B8 becomes 1.93 T or more even under a low temperature gradient condition of 0.5°C / cm in the finish annealing step, and excellent magnetic properties can be obtained.

[0059] Example 2 A silicon steel material having the same chemical composition as in Example 1 was heated to 1150°C, held for 30 minutes, and then hot-rolled to obtain a hot-rolled sheet having a thickness of 2.3 mm, as shown in Tables 1-1 and 1-2. This hot-rolled sheet was then annealed at 1100°C for 30 seconds. This hot-rolled sheet was then cold-rolled at a 90% reduction to obtain a steel sheet with a final thickness of 0.23 mm. Steel sheets (sample steel sheets) measuring 200 mm in the rolling direction and 600 mm in the width direction were cut out from this steel sheet. These steel sheets were then annealed (decarburization annealing) in a moist atmosphere gas containing, by volume, 75% hydrogen and 25% nitrogen, by heating from 500°C to the annealing temperature (830°C) at a heating rate adjusted within a range of 20 to 100°C / sec, and held at the annealing temperature for 100 seconds. The oxidation degree of the atmospheric gas is adjusted to 0.18, and the oxygen content of the steel sheet is adjusted to 1.4 to 2.8 g / m ２ The steel sheets after decarburization annealing (between the decarburization annealing process and the finish annealing process—referred to as timing (ii) in the table) were subjected to nitriding in an atmospheric gas containing hydrogen, nitrogen, and ammonia, and the nitrogen content of the steel sheets was adjusted to 0.025 mass% by changing the ammonia content. Each steel sheet was coated with an annealing separator primarily composed of magnesia and then placed in a finish annealing furnace. During the finish annealing, the steel sheets were heated to 1200°C in a mixed gas containing, by volume, 75% hydrogen and 25% nitrogen, and then the atmosphere was switched to a 100% hydrogen atmosphere and held for 20 hours. During this heating process, a temperature gradient of 0.5 to 14.0°C / cm was applied in the direction perpendicular to the rolling direction to the boundary region between the primary and secondary recrystallization regions from the start of secondary recrystallization to the completion of secondary recrystallization. The temperature gradient was uniformly applied throughout the steel sheet. However, for some steel sheets, no temperature gradient was applied (temperature gradient: 0.0°C / cm).

[0060] After the finish annealing process, a known insulating coating is formed on the steel sheet to apply tension to the steel sheet (tension coating is applied), and then laser irradiation is performed to perform magnetic domain refining treatment.

[0061] A sample measuring 60 mm in width and 200 mm in length was taken from the obtained steel sheet, and magnetic measurements were performed on this sample using the SST method in accordance with Appendix JA of JIS C2556:2015. The magnetic flux density (B8) values ​​obtained by the above magnetic measurements are shown in Table 1-2. If B8 is 1.93 T or more, it is determined that an excellent magnetic flux density has been obtained.

[0062]

[0063]

[0064] As can be seen from Tables 1-1 and 1-2, in the final annealing step, B8 is 1.93 T or more in a temperature gradient range of 0.5°C / cm or more, and excellent magnetic properties are obtained. As the temperature gradient increases, the magnetic flux density tends to improve, but it is found that it saturates at 10.0°C / cm or more.

[0065] Example 3: A silicon steel material containing, by mass%, 3.3% Si, 0.053% C, 0.027% acid-soluble Al, 0.008% N, 0.1% Mn, and 0.005% S, with the balance being Fe and impurity elements, is obtained. This silicon steel material is heated to 1100 to 1200°C, as shown in Tables 2-1 and 2-2, held for 30 minutes, and then hot-rolled to obtain a hot-rolled sheet having a thickness of 2.3 mm. This hot-rolled sheet is annealed at 1100°C for 30 seconds. This hot-rolled sheet is then cold-rolled at a 90% reduction to obtain a steel sheet with a final thickness of 0.23 mm. A steel sheet (sample steel sheet) measuring 200 mm in the rolling direction and 600 mm in the width direction is cut from this steel sheet. This steel sheet is heated in a wet atmosphere gas containing, by volume, 75% hydrogen and 25% nitrogen and having an oxidation degree of 0.20 to 1.00 from a temperature of 400 to 650°C to an annealing temperature of 830 to 840°C at a heating rate of 40°C / sec or more, and annealed at the annealing temperature for 100 to 300 seconds, so that the oxygen content of the steel sheet is 1.4 to 2.6 g / m ２The temperature is adjusted to 100°C. After applying an annealing separator containing magnesia as the main component to each steel sheet, the steel sheets are placed in a finish annealing furnace. During the finish annealing, the steel sheets are heated to 1100-1250°C in a mixed gas containing, by volume, 75% hydrogen and 25% nitrogen, and then the atmosphere is switched to a 100% hydrogen atmosphere and held for 10-30 hours. During the temperature increase process, a temperature gradient of 0.5°C / cm is applied in the direction perpendicular to the rolling direction to the boundary region between the primary and secondary recrystallization regions from the start of secondary recrystallization to its completion. The temperature gradient is applied by increasing the temperature with a temperature difference within the furnace or by heating and / or cooling the edges of the steel sheet. The temperature gradient is applied uniformly throughout the steel sheet.

[0066] Nitriding is performed at one or more of the following stages (timings) so that the amount of nitrogen after the final nitriding treatment is as shown in Table 2-2: (i) During the decarburization annealing process (ii) Between the decarburization annealing process and the finish annealing process (iii) During the temperature rise process in the finish annealing process Nitriding is performed in an atmospheric gas with nitriding ability, such as ammonia, or by adding a nitride with nitriding ability to an annealing separator.

[0067] After the finish annealing process, a known insulating coating is formed on the steel sheet to apply tension to the steel sheet (tension coating is applied), and then laser irradiation is performed to perform magnetic domain refining treatment.

[0068] A sample measuring 60 mm in width and 200 mm in length was taken from the obtained steel sheet, and magnetic measurements were performed on this sample using the SST method in accordance with Appendix JA of JIS C2556:2015. The magnetic flux density (B8) values ​​obtained by the above magnetic measurements are shown in Table 2-2. If B8 is 1.93 T or more, it is determined that an excellent magnetic flux density has been obtained.

[0069]

[0070]

[0071] As can be seen from Tables 2-1 and 2-2, the oxygen content of the steel sheet after the decarburization annealing process was 2.4 g / m ２By performing nitriding treatment at one or more stages (timings) of (i) to (iii) as follows, and setting the nitrogen content of the steel sheet after the final nitriding treatment to 0.021 mass% or more, in the finish annealing step, B8 becomes 1.93 T or more even under a low temperature gradient condition of 0.5°C / cm, and excellent magnetic properties can be obtained.

[0072] Example 4 A silicon steel material having the chemical composition shown in Tables 3-1 and 3-2 is heated to 1100°C and hot-rolled to obtain a hot-rolled sheet having a thickness of 2.0 mm. This hot-rolled sheet is annealed at 1120°C for 30 seconds. This hot-rolled sheet is then cold-rolled at a rolling reduction of 90% to obtain a steel sheet having a final thickness of 0.20 mm. Steel sheets (sample steel sheets) measuring 200 mm in the rolling direction and 600 mm in the width direction are cut out from this steel sheet. These steel sheets are heated in a moist atmosphere gas containing, by volume, 75% hydrogen and 25% nitrogen from 400°C to 840°C at a heating rate of 150°C / sec, and annealed at 840°C for 90 seconds. The oxidation degree of the atmosphere gas is adjusted to obtain an oxygen content of the steel sheet of 1.5 to 1.6 g / m. ２ The steel sheets after decarburization annealing are nitrided in an atmospheric gas containing hydrogen, nitrogen, and ammonia, and the ammonia content is adjusted to 0.023 to 0.024 mass%. Each steel sheet is coated with an annealing separator primarily composed of magnesia and then placed in a finish annealing furnace. During finish annealing, the steel sheets are heated to 1200°C in a mixed gas containing, by volume, 75% hydrogen and 25% nitrogen, and then the atmosphere is switched to a 100% hydrogen atmosphere and held for 20 hours. During this heating process, a temperature gradient of 0.5°C / cm is applied in the direction perpendicular to the rolling direction to the boundary region between the primary and secondary recrystallization regions from the start of secondary recrystallization to its completion. The temperature gradient is applied by raising the temperature with a temperature difference within the furnace or by heating and / or cooling the edges of the steel sheet. The temperature gradient is applied uniformly throughout the steel sheet.

[0073] After the finish annealing process, a known insulating coating is formed on the steel sheet to apply tension to the steel sheet (tension coating is applied), and then laser irradiation is performed to perform magnetic domain refining treatment.

[0074] A sample measuring 60 mm wide and 200 mm long was taken from the obtained steel sheet, and magnetic measurements were performed on this sample using the SST method in accordance with Appendix JA of JIS C2556:2015. The magnetic flux density (B8) values ​​obtained by the above magnetic measurements are shown in Table 3-2. If B8 is 1.93 T or higher, it is determined that an excellent magnetic flux density has been obtained.

[0075]

[0076]

[0077] As can be seen from Tables 3-1 and 3-2, when the chemical composition contains Si: 0.8 to 7.0%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, S and Se: 0 to 0.015% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.0%, O: 0 to 0.10%, Mo: 0 to 0.10%, Nb: 0 to 0.2%, Ti: 0 to 0.015%, V: 0 to 0.15%, B: 0 to 0.008%, Sn: 0 to 0.04%, and Sb: 0 to 0.04%, B8 is 1.93 T or more, and excellent magnetic properties are obtained.

[0078] According to the present invention, it is possible to provide a method for manufacturing a grain-oriented electrical steel sheet that can produce a grain-oriented electrical steel sheet having a stable high magnetic flux density over the entire coil, even when the entire coil has a substantially uniformly low temperature gradient, or when the coil has regions with a partial low temperature gradient (small temperature gradient), and therefore has high industrial applicability.

Claims

1. In mass%, Si: 0.8 to 7.0%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, S and Se: 0 to 0.015% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.0%, O: 0 to 0.10%, Mo a hot rolling process in which a silicon steel material having a chemical composition containing 0 to 0.10%, Nb: 0 to 0.2%, Ti: 0 to 0.015%, V: 0 to 0.15%, B: 0 to 0.008%, Sn: 0 to 0.04%, Sb: 0 to 0.04%, and the balance being Fe and impurities is heated to a temperature of 1280°C or less and then hot rolled to obtain a hot-rolled sheet; a hot-rolled sheet annealing step of annealing the hot-rolled sheet as necessary; a cold-rolling step of subjecting the hot-rolled sheet after the hot-rolling step or the hot-rolled sheet annealing step to a single cold rolling or multiple cold rolling steps via annealing with a final rolling reduction of 85% or more to produce a steel sheet of a final thickness; a decarburization annealing step of decarburizing annealing the steel sheet after the cold-rolling step; an annealing separator application step of applying an annealing separator to the steel sheet after the decarburization annealing step and then winding it into a coil; a nitriding treatment step of increasing the nitrogen content of the steel sheet; and a finish annealing step of finish-annealing the coiled steel sheet, wherein the oxygen content of the steel sheet after the decarburization annealing step is 2.4 g / m or more. ２ the finish annealing step includes a temperature rise step and a soaking step, and a temperature gradient of 0.5°C / cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least for a period from the start of secondary recrystallization to the completion of secondary recrystallization in the temperature rise step, and the nitriding treatment step is carried out in at least one stage of the following: during the decarburization annealing step, between the decarburization annealing step and the finish annealing step, or during the temperature rise step in the finish annealing step up to the start of secondary recrystallization, and the nitrogen content of the steel sheet after the nitriding treatment step is 0.021 mass % or more.

2. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the temperature gradient in the finish annealing step is 0.5 to 10.0°C / cm.

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