Method for producing grain-oriented electrical steel sheet

Abstract

This method for producing a grain-oriented electrical steel sheet is provided with a hot rolling step, a hot-rolled sheet annealing step, as required, a cold rolling step, a decarburization annealing step, an annealing separating agent application step, a nitriding treatment step, and a final annealing step that has a temperature rise step and a soaking step, a temperature gradient of 0.5 DEG C / cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region in at least one period from the start of the secondary recrystallization to the end of the secondary recrystallization in the temperature raising process; in the decarburization annealing step, in at least one stage between the decarburization annealing step and the final annealing step, or in the temperature raising step of the final annealing step and before the start of the secondary recrystallization, annealing is performed in an atmosphere containing a gas having nitriding ability. The nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more on a mass basis, and the nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more on a mass basis.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a grain-oriented electromagnetic steel sheet.

[0002] This application claims priority based on Japanese Patent Application No. 2023-005564 filed on January 18, 2023, and incorporates its content herein. Background Art

[0003] A grain-oriented electromagnetic steel sheet (also referred to as a non-oriented silicon steel sheet) is a soft magnetic material and is mainly used as a core material for transformers. Therefore, low energy loss (low iron loss) is required for the grain-oriented electromagnetic steel sheet. For example, the magnetic flux density: B8 (magnetic flux density in a magnetic field of 800 A / m) is the most dominant factor in the iron loss characteristics. It is known that the higher the value of the magnetic flux density: B8, the lower the iron loss and the better the iron loss characteristics. In addition, the higher the value of the magnetic flux density: B8, the more the core can be miniaturized, which is advantageous in terms of the device configuration of the transformer and also in terms of the manufacturing cost of the transformer. In order to increase the value of the magnetic flux density: B8, it is important to make the crystal orientation highly consistent. The control of this crystal orientation is achieved by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.

[0004] Conventionally, many inventions regarding high magnetic flux density grain-oriented electromagnetic steel sheets have been proposed since ancient times. However, as long as the non-oriented silicon steel sheets produced industrially are investigated, their magnetic flux density (B8) does not reach the theoretical upper limit value of silicon steel, and there is still room for significant improvement.

[0005] Regarding the increase in the magnetic flux density, for example, Patent Document 1 discloses a method for manufacturing a non-oriented electromagnetic steel sheet with a high magnetic flux density, characterized in that a silicon steel slab containing C: 0.015% or less, Si: 4% or less, S: 0.012% or less, acid-soluble Al: 0.020 to 0.065%, and T.N: 0.0030 to 0.0095% is heated at 1270°C or lower and then hot-worked into a hot-rolled sheet. After coiling at 700 to 950°C, cold rolling is performed with a reduction ratio of 65% or more. After short-time primary recrystallization annealing of the steel sheet, while imparting 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, a high-temperature final annealing including a treatment for growing secondary recrystallization grains is performed.

[0006] In addition, Patent Document 2 discloses a method for manufacturing an ultra-low iron loss grain-oriented silicon steel sheet. In this method, for materials with a relatively thin thickness (e.g., 0.13 mm) that were difficult to manufacture in the past, crystal orientation control and surface smoothing of the steel sheet were achieved through secondary recrystallization, thereby manufacturing an ultra-low iron loss grain-oriented electromagnetic steel sheet at low cost. In Patent Document 2, it is disclosed that in order to ensure a temperature gradient of at least 2 °C / cm, it is necessary to raise the temperature to 1000 - 1100 °C at a heating rate of 50 °C / hr or more during the final annealing.

[0007] In addition, for example, Patent Document 3 discloses a method for manufacturing a grain-oriented silicon steel sheet (strip) with a high magnetic flux density, characterized in that during the manufacturing process of the grain-oriented silicon steel sheet (strip), a temperature gradient is imparted to the steel sheet (strip) while performing secondary recrystallization at the boundary region between the primary recrystallization region and the secondary recrystallization region.

[0008] In addition, for example, Patent Documents 4 and 5 disclose equipment and methods for imparting a temperature gradient to a coiled steel sheet.

[0009] Prior Art Documents

[0010] Patent Documents

[0011] Patent Document 1: Japanese Patent Publication No. 59-41488

[0012] Patent Document 2: Japanese Patent Laid-Open No. 5-311238

[0013] Patent Document 3: Japanese Patent Publication No. 58-50295

[0014] Patent Document 4: Japanese Patent Laid-Open No. 57-164935

[0015] Patent Document 5: Japanese Patent Laid-Open No. 58-1019 Summary of the Invention

[0016] Problems to be Solved by the Invention

[0017] As described above, in the technologies described in Patent Document 1 and Patent Document 2, it is shown that a temperature gradient of 2 °C / cm or more needs to be imparted during the final annealing. When imparting a temperature gradient, it is considered to apply the equipment and methods disclosed in Patent Documents 4, 5, etc. However, it is difficult to control these disclosed equipment and methods in such a way that a uniform temperature gradient is achieved throughout the length and width directions of an industrial-scale coil. In addition, it is difficult to control the temperature gradient to be high throughout the entire region, and there are locally regions with a low temperature gradient (small temperature gradient) of about 0.5 °C / cm. It is difficult to obtain a sufficient effect of increasing the magnetic flux density through such regions with a low temperature gradient.

[0018] The inventors of the present invention calculated the temperature gradient of each part of the coil using simulation with the well-known heat transfer calculation software Fluent (registered trademark) manufactured by ANSYS Inc. As a result, it was found that when a temperature gradient of 2 °C / cm or more was desired to be imparted, there were also regions with a low temperature gradient (small temperature gradient) of about 0.5 °C / cm in some parts of the coil. In particular, it was found that when a temperature gradient was set in the coil, if a particularly large temperature gradient was to be set, the temperature gradient tended to become smaller inside the coil compared to the outside. That is, for example, even if the temperature gradient on the outside of the coil is 2 °C / cm or more, some regions such as the inside of the coil become a low temperature gradient of less than 2 °C / cm. In addition, in the case of setting a temperature gradient in the width direction of the coil, etc., the temperature gradient tends to become smaller on the low-temperature end side compared to the high-temperature end side. For example, even if the temperature gradient on the high-temperature end side of the coil is 2 °C / cm or more, sometimes some regions such as the low-temperature end side become a low temperature gradient of less than 2 °C / cm. Therefore, it is difficult to set a temperature gradient of 2 °C / cm or more over the entire region of the coil.

[0019] Therefore, in order to obtain a sufficient effect of increasing the magnetic flux density over the entire region of the coil, a method that can obtain the effect of increasing the magnetic flux density even with a smaller temperature gradient is desired.

[0020] In Patent Document 3, it is disclosed that the improvement effect of the B8 characteristic was confirmed by imparting a temperature gradient of 0.5 °C / cm. However, in Patent Document 3, it was shown that a significant effect was obtained at 2 °C / cm or more. In fact, the value of B8 of a grain-oriented electrical steel sheet with a Si content of 2.95% when the temperature gradient is 0.5 °C / cm is approximately 1.92 T. Although a certain effect of increasing the magnetic flux density was obtained, it cannot be said to be sufficient for the recent high-level requirements.

[0021] The present invention has been completed in view of the above problems. The problem of the present invention is to provide a method for manufacturing a grain-oriented electrical steel sheet, which is a method for manufacturing a grain-oriented electrical steel sheet with a high magnetic flux density by performing final annealing while imparting a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region, and in which, even if the temperature gradient is small, a sufficient effect of increasing the magnetic flux density can be obtained, thereby stabilizing and having a high magnetic flux density over the entire region of the coil.

[0022] Means for Solving the Problem

[0023] On the premise of a method of performing final annealing while imparting a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region, the inventors have studied a method capable of obtaining a sufficient effect of increasing the magnetic flux density even when the temperature gradient is relatively small (in the case where there are parts with a large temperature gradient and parts with a small temperature gradient, even if the lower limit of the temperature gradient is small).

[0024] As a result, it was found that by making the nitrogen content of the steel sheet before secondary recrystallization 210 ppm or more, a sufficient effect of increasing the magnetic flux density can be obtained even when the temperature gradient is relatively small.

[0025] The present invention has been completed in view of the above-mentioned findings. The gist of the present invention is as follows.

[0026] [1] A method for manufacturing a grain-oriented electrical steel sheet according to one aspect of the present invention includes: a hot rolling step of heating a silicon steel raw material containing Si: 0.80 to 7.00% by mass to a temperature of 1280°C or lower and then performing 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 cold rolling the hot rolled sheet after the hot rolling step or after the hot rolled sheet annealing step to form a steel sheet having a final sheet thickness; a decarburizing annealing step of performing decarburizing annealing on the steel sheet after the cold rolling step; an annealing separator coating step of coating an annealing separator on the steel sheet after the decarburizing annealing step and then winding it into a coil shape; a nitriding treatment step of increasing the nitrogen content of the steel sheet; and a final annealing step of performing final annealing on the steel sheet wound into a coil shape. In the final annealing step, there is a heating-up process and a soaking process. During at least one period between the start and end of secondary recrystallization in the heating-up process, a temperature gradient of 0.5°C / cm or more is generated in the boundary region between the primary recrystallization region and the secondary recrystallization region. For the nitriding treatment step, it is carried out by annealing in an atmosphere containing a gas having nitriding ability at at least one stage among the decarburizing annealing step, between the decarburizing annealing step and the final annealing step, or in the heating-up process of the final annealing step and before the start of secondary recrystallization, and the nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more on a mass basis.

[0027] [2] In the method for manufacturing a grain-oriented electrical steel sheet according to [1], it is also possible that: in the heating-up process of the final annealing step, the atmosphere in the furnace from the start to the end of secondary recrystallization is set to an atmosphere in which the proportion of N2 gas is 20% or more by mass.

[0028] [3] In the method for manufacturing the grain-oriented electromagnetic steel sheet described in [1] or [2], it is also possible to set the nitrogen content of the steel sheet after the nitriding treatment step to 210 ppm or more and 350 ppm or less on a mass basis.

[0029] [4] In the method for manufacturing the grain-oriented electromagnetic steel sheet described in [1] or [2], it is also possible that when the nitrogen content of the steel sheet after the nitriding treatment step is set to [N] in ppm on a mass basis, the proportion of N2 gas in the atmosphere in the final annealing step is set to [N2] in mass%, and the temperature gradient in the final annealing step is set to [T] in °C / cm, the following formula (1) is satisfied.

[0030] [T] ≥ 1 / (0.0026 × [N]) - 0.005 × [N2] - 0.5 (1)

[0031] [5] In the method for manufacturing the grain-oriented electromagnetic steel sheet described in [3], it is also possible that when the nitrogen content of the steel sheet after the nitriding treatment step is set to [N] in ppm on a mass basis, the proportion of N2 gas in the atmosphere in the final annealing step is set to [N2] in mass%, and the temperature gradient in the final annealing step is set to [T] in °C / cm, the following formula (1) is satisfied.

[0032] [T] ≥ 1 / (0.0026 × [N]) - 0.005 × [N2] - 0.5 (1)

[0033] [6] In the method for manufacturing the grain-oriented electromagnetic steel sheet described in [1] or [2], it is also possible that the chemical composition of the silicon steel raw material contains, on a mass% basis: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

[0034] [7] In the method for manufacturing the grain-oriented electromagnetic steel sheet described in [3], it is also possible that the chemical composition of the silicon steel raw material contains, on a mass% basis: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

[0035] [8] In the method for manufacturing the grain-oriented electrical steel sheet described in [4], it is also possible that the chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

[0036] [9] In the method for manufacturing the grain-oriented electrical steel sheet described in [5], it is also possible that the chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

[0037] Advantages of the Invention

[0038] According to the above solution of the present invention, it is possible to provide a method for manufacturing a grain-oriented electrical steel sheet, which is a method for manufacturing a grain-oriented electrical steel sheet with a high magnetic flux density by performing final annealing while imparting a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region. Even if the temperature gradient is small, a sufficient effect of increasing the magnetic flux density can be obtained. Detailed Embodiment

[0039] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention (the method for manufacturing a grain-oriented electrical steel sheet of the present embodiment) will be described.

[0040] The method for manufacturing a grain-oriented electrical steel sheet of the present embodiment includes:

[0041] (i) Hot rolling process: After heating a silicon steel raw material having a predetermined chemical composition to a temperature of 1280°C or lower, hot rolling is performed to obtain a hot rolled sheet;

[0042] (ii) Hot rolled sheet annealing process: Anneal the above hot rolled sheet as needed;

[0043] (iii) Cold rolling process: Cold roll the hot rolled sheet after the above hot rolling process or the hot rolled sheet annealing process to form a steel sheet with a final sheet thickness;

[0044] (iv) Decarburizing annealing process: Decarburizing annealing is performed on the steel sheet after the above cold rolling process;

[0045] (v) Annealing release agent coating process: After coating an annealing release agent on the steel sheet after the above decarburizing annealing process, it is coiled into a coil shape;

[0046] (vi) Nitriding treatment process: Increasing the nitrogen content of the above steel sheet; and

[0047] (vii) Final annealing process: Final annealing is performed on the steel sheet coiled into a coil shape.

[0048] Hereinafter, the conditions for each process will be described.

[0049] [Hot rolling process]

[0050] In the hot rolling process, a silicon steel raw material such as a slab having the chemical composition described below is heated to a temperature of 1280 °C or lower and then hot rolled to obtain a hot rolled sheet (hot rolled steel sheet).

[0051] In the manufacturing method of a grain-oriented electrical steel sheet, generally, the following method is industrially implemented: When heating the silicon steel raw material before hot rolling, fine precipitates called inhibitors are completely dissolved, and then fine precipitates are formed in the hot rolling and subsequent annealing processes. In this method, in order to completely dissolve the precipitates, heating needs to be performed at a high temperature of 1350 °C or higher. However, this temperature is about 200 °C higher than the slab heating temperature of ordinary steel, so a dedicated heating furnace is required, and there are problems such as a large amount of molten scale.

[0052] Therefore, in the manufacturing method of the grain-oriented electrical steel sheet of the present embodiment, the heating temperature is set to 1280 °C or lower to avoid the problems caused by the above high-temperature heating. In order to prevent cracks caused by hot rolling, the lower limit of the heating temperature is preferably set to 700 °C or higher.

[0053] There are no restrictions on the hot rolling conditions other than the heating temperature, and they can be determined within a known range according to the required characteristics and the like.

[0054] In addition, the silicon steel raw material for hot rolling is obtained as follows: Steel is melted by a converter or an electric furnace, and vacuum degassing treatment is performed on the molten steel as needed, and then continuous casting or ingot casting is performed, followed by blooming rolling.

[0055] The silicon steel raw material contains Si: 0.80 to 7.00% by mass. Preferably, it has the following chemical composition: containing Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015% by mass, and the balance contains Fe and impurities.

[0056] The reasons for the content of each element are explained. Hereinafter, the % of the content is by mass.

[0057] (Si: 0.80 to 7.00%)

[0058] When the Si content is less than 0.80%, γ-phase transformation occurs during final annealing, and the crystal orientation of the steel sheet is damaged. In addition, by containing Si, the resistance becomes higher and the iron loss characteristics are improved. Therefore, the Si content in the silicon steel raw material is set to 0.80% or more. The Si content is preferably 1.50% or more, more preferably 2.00% or more, and further preferably 2.50% or more.

[0059] On the other hand, when the Si content exceeds 7.00%, cold rolling becomes extremely difficult, and cracking may occur during rolling. Therefore, the Si content is set to 7.00% or less. As a range suitable for industrial production, the Si content can be set to 4.80% or less, or can be set to 4.00% or less.

[0060] (C: 0.085% or less)

[0061] C is an element effective in controlling the primary recrystallized structure, but it has an adverse effect on magnetic properties, so decarburization is required before final annealing. When the C content in the silicon steel raw material exceeds 0.085%, the decarburization annealing time becomes longer, and the productivity in industrial production is damaged. Therefore, it is preferable to set the C content to 0.085% or less. The lower limit of the C content is not particularly limited. Considering the productivity in industrial production and the magnetic properties of the product, the C content is preferably 0.020% or more, and more preferably 0.050% or more.

[0062] (Acid-soluble Al: 0.010 to 0.065%)

[0063] Acid-soluble Al (sol.Al) is an element that combines with N and functions as an inhibitor as AlN or (Al, Si)N. As a range where secondary recrystallization is stable, it is preferable to set the acid-soluble Al content to 0.010 to 0.065%. The acid-soluble Al content can be set to 0.040% or less, and further to 0.030% or less.

[0064] (N: 0.004 to 0.012%)

[0065] N is an element that combines with Al and functions as an inhibitor. When the N content is less than 0.004%, a sufficient amount of inhibitor cannot be obtained. Therefore, the N content is preferably set to 0.004% or more. The N content is more preferably 0.006% or more, and further preferably 0.007% or more.

[0066] On the other hand, if the N content exceeds 0.012%, voids called blisters sometimes occur in the steel sheet during cold rolling. Therefore, the N content is preferably set to 0.012% or less.

[0067] In the chemical composition of the silicon steel raw material, the above elements are contained, and the remaining part may be Fe and impurities. On the other hand, in order to improve various properties, the following elements may also be contained.

[0068] For the following elements, as long as they are within the ranges described later, they are also allowed to be contained as impurities.

[0069] (Mn: 0 to 1.00%)

[0070] Mn is an element that has the effect of increasing the resistivity and reducing the iron loss. Therefore, it may also be contained.

[0071] In addition, Mn is an effective element for preventing the occurrence of cracks in hot rolling caused by S and Se. In order to prevent crack occurrence, the Mn content is preferably set in the range that satisfies Mn / (S + Se) ≥ 4 in relation to the total amount of S and Se.

[0072] On the other hand, when the Mn content exceeds 1.00%, the magnetic flux density of the grain-oriented electrical steel sheet decreases. Therefore, the Mn content is preferably set to 1.00% or less.

[0073] (Cr: 0 to 0.30%)

[0074] Cr is an element that makes the composition and amount of the decarburizing annealing oxide layer in a preferred state and promotes the formation of the glass film. Therefore, it may also be contained.

[0075] On the other hand, if the Cr content exceeds 0.30%, decarburization is hindered. Therefore, the Cr content is preferably set to 0.30% or less.

[0076] (Cu: 0 to 0.4%)

[0077] Cu is an effective element for increasing the resistivity and reducing the iron loss. Therefore, it may also be contained.

[0078] On the other hand, if the Cu content exceeds 0.4%, the effect of reducing iron loss saturates and it causes surface defects such as "copper scars" during hot rolling. Therefore, it is preferable to set the Cu content to 0.4% or less.

[0079] (P: 0 - 0.5%)

[0080] P is an effective element for increasing the resistivity and reducing iron loss. Therefore, it can also be contained.

[0081] On the other hand, when the P content exceeds 0.5%, the rollability decreases. Therefore, it is preferable to set the P content to 0.5% or less.

[0082] (Ni: 0 - 1.00%)

[0083] Ni is an effective element for increasing the resistivity and reducing iron loss. In addition, it is an element effective in controlling the microstructure of hot-rolled sheets and improving magnetic properties. Therefore, it can also be contained.

[0084] On the other hand, when the Ni content exceeds 1.00%, secondary recrystallization becomes unstable. Therefore, it is preferable to set the Ni content to 1.00% or less.

[0085] (S and Se: total 0 - 0.015%)

[0086] S and Se are elements that have an adverse effect on magnetic properties. Therefore, it is preferable to set the total content of these elements to 0.015% or less. It is preferable that these elements are less, so it can be 0%.

[0087] In addition to the above elements, as impurities, for example, B, O, Mg, Ca, Ti, Mo, V, Nb, Sn, Sb, Bi can each be contained at 0.10% or less. Impurities refer to elements that are mixed in from raw materials or during the manufacturing process and do not have a clear impact on the properties of the grain-oriented electrical steel sheet.

[0088] The chemical composition of the silicon steel raw material can be measured by a well-known method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Al is measured as acid-soluble Al using the filtrate obtained by heating and decomposing the sample with acid and measuring by ICP-AES. In addition, Si can be measured using the silica gravimetric method, C and S can be measured using combustion-infrared absorption method, N can be measured using inert gas fusion-thermal conductivity method, and O can be measured using inert gas fusion-non-dispersive infrared absorption method.

[0089] The above chemical composition is the composition of the silicon steel sheet as the base material. When the grain-oriented electrical steel sheet that becomes the measurement specimen has a glass film, an insulating film, etc. on its surface, these films are removed by a known method and then the chemical composition is measured.

[0090] [Hot-rolled sheet annealing process]

[0091] In order to improve magnetic properties, after the hot-rolling process, the hot-rolled sheet obtained by hot-rolling can be annealed (hot-rolled sheet annealing). The annealing conditions are not limited. For example, it can be set to the condition of holding at 900 to 1200 °C for 30 seconds to 30 minutes. The annealing temperature can be set to 950 to 1050 °C.

[0092] [Cold-rolling process]

[0093] In the cold-rolling process, cold rolling is performed on the hot-rolled sheet after the hot-rolling process or after the hot-rolled sheet annealing process to produce a steel sheet (cold-rolled sheet) with the same thickness as the final thickness (the thickness when it becomes a grain-oriented electrical steel sheet (wherein, when a glass film and an insulating film are formed on the surface, it is the thickness of the base material steel sheet after removing them)). The cold rolling can be set to single-pass (a series of cold rolling without intermediate annealing) cold rolling or multiple cold rollings with annealing (intermediate annealing).

[0094] As the cold rolling, in order to develop preferred primary recrystallization orientations such as {411} during decarburizing annealing, it is preferred that the final reduction ratio is 80% or more. The final reduction ratio refers to the cumulative reduction ratio of cold rolling, and in the case of performing intermediate annealing, it refers to the cumulative reduction ratio of cold rolling after the final intermediate annealing.

[0095] [Decarburizing annealing process]

[0096] In the decarburizing annealing process, the steel sheet after the cold-rolling process is subjected to decarburizing annealing.

[0097] In the decarburizing annealing, as long as the steel sheet can be primary recrystallized and C that has an adverse effect on magnetic properties can be removed from the steel sheet, the decarburizing annealing conditions are not limited. For example, it can be exemplified that the oxidation degree (PH2O / PH2) in the annealing atmosphere (furnace atmosphere) is set to 0.15 to 1.00, the annealing temperature is set to 770 to 950 °C, and held for 10 to 600 seconds.

[0098] [Annealing release agent coating process]

[0099] In the annealing release agent coating process, after coating the annealing release agent on the steel sheet after the decarburizing annealing process, it is coiled into a coil shape.

[0100] The coated annealing release agent may be a known one, but an annealing release agent mainly composed of magnesium oxide is preferred. The annealing release agent mainly composed of magnesium oxide is coated, and then final annealing is performed, whereby a glass film (forsterite film) is formed on the surface of the steel sheet.

[0101] [Nitriding treatment process]

[0102] In the nitriding treatment process, the nitrogen content of the steel sheet is increased. The nitriding treatment process is carried out at least at one stage among in the decarburizing annealing process, between the decarburizing annealing process and the final annealing process, or from the start of the final annealing process to the heating process of the final annealing process and before the start of secondary recrystallization. The period between the decarburizing annealing process and the final annealing process means the period from the end of the decarburizing annealing process to the start of the final annealing process. The nitriding treatment process is preferably carried out after the decarburizing annealing process is completed and before the annealing release agent coating process starts.

[0103] However, in the case of carrying out at any stage, it is necessary that the nitrogen content of the steel sheet after the final nitriding treatment process is 210 ppm (0.0210 mass%) or more on a mass basis. By increasing the nitrogen content of the steel sheet before the start of secondary recrystallization, the amount of the inhibitor increases and the inhibitor becomes thermally stable. As a result, even for a relatively small temperature gradient, a sufficient effect of increasing the magnetic flux density can be obtained. The nitrogen content can be 250 ppm or more, and further can be 300 ppm or more. On the other hand, if the nitrogen content exceeds 350 ppm, in addition to considering the possibility that the effect of increasing the magnetic flux density may saturate, the possibility that it is disadvantageous for purification after secondary recrystallization is also considered. Therefore, the nitrogen content is preferably set to 350 ppm or less.

[0104] In the past, in the case of performing final annealing while applying a temperature gradient, the nitrogen content of the steel sheet was usually set to 200 ppm or less. For example, in Japanese Patent Laid-Open No. 59-215419, it is disclosed that when performing secondary recrystallization annealing while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region of the final annealing, the nitrogen content in the steel sheet is set to 130 to 200 ppm at the start of secondary recrystallization. In Japanese Patent Laid-Open No. 59-215419, it is described that the effect of increasing the magnetic flux density saturates when the nitrogen content is 180 to 200 ppm.

[0105] In contrast, the present inventors have found that when the nitrogen content is increased to 210 ppm or more, the lower limit of the temperature gradient capable of achieving a high magnetic flux density (for example, B8 is stably 1.940 T or more) is expanded compared to the past (even when the temperature gradient is about 0.5 °C / cm, a high B8 can be stably obtained).

[0106] With respect to the target temperature gradient, the nitrogen content of the steel sheet after the final nitriding treatment process is set to [N] in ppm on a mass basis, the ratio of N2 gas in the atmosphere during the heating process of the subsequent final annealing process is set to [N2] in mass %, and when the temperature gradient during the heating process of the final annealing process is set to [T] in units of °C / cm, the nitrogen content after the nitriding treatment process is preferably set within the range that satisfies the following formula (1).

[0107] In this case, a higher magnetic flux density can be obtained. For example, if the nitrogen content and the ratio of N2 gas in the atmosphere in mass % are determined such that the minimum temperature gradient of the entire coil calculated by simulation or the like satisfies formula (1), a higher magnetic flux density can be stably obtained throughout the coil region.

[0108] In the case of satisfying formula (1), as the reason for obtaining a higher magnetic flux density, it is considered that by increasing the nitrogen content of the steel sheet before the start of secondary recrystallization, the amount of the inhibitor increases and the inhibitor becomes thermally stable. By increasing the ratio of N2 gas in the atmosphere in mass %, the detachment of the inhibitor can be suppressed, and the preferential growth of {110}<001> oriented grains can be further enhanced during secondary recrystallization.

[0109] [T]≥1 / (0.0026×[N])-0.005×[N2]-0.5 (1)

[0110] In addition, as a method for increasing the nitrogen content of the steel sheet, a method of annealing in an atmosphere containing a gas having nitriding ability to control the nitrogen content of the steel sheet can be exemplified.

[0111] In addition to the above, during the heating process of the final annealing process, the nitrogen content of the steel sheet can be increased by adding powders having nitriding ability such as MnN to the annealing separating agent.

[0112] The nitrogen content of the steel sheet after nitriding treatment can be measured by a known method using, for example, an oxygen / nitrogen / hydrogen analyzer (EMGA-930) manufactured by Horiba, Ltd. or an equivalent device. As a known method, a general analysis method such as the inert gas fusion - thermal conductivity method can be used. During the manufacturing process, a sample specimen of any size is collected from the steel sheet after the nitriding treatment process, and measurement can be performed using these devices and methods.

[0113] [Final annealing process]

[0114] In the final annealing process, the steel sheet wound into a coil shape is subjected to final annealing.

[0115] The final annealing process includes a heating process for heating to the final annealing temperature to perform secondary recrystallization and a soaking process for holding the above-mentioned steel sheet at the above-mentioned final annealing temperature.

[0116] <Heating process>

[0117] For this final annealing process, in the above nitriding treatment process, when the nitrogen content in the steel sheet is controlled to be 210 ppm or more on a mass basis, at least one period between the start and end of secondary recrystallization during the heating process is used to generate a temperature gradient of 0.5 °C / cm or more in the boundary region between the primary recrystallization region and the secondary recrystallization region, and the {110}<001> oriented grains preferentially grow through secondary recrystallization. Even if a temperature gradient is applied during a period other than the above, such as before the final annealing, the same effect cannot be obtained.

[0118] The heating rate only needs to be a heating rate that satisfies the above temperature gradient and can be 50 °C / h or less.

[0119] In the final annealing, secondary recrystallization grains are generated in the part heated above the secondary recrystallization temperature. If the steel sheet is heated in a state where there is a temperature gradient in a certain direction, secondary recrystallization starts from the region that has reached above the secondary recrystallization temperature, and between the regions that have not yet reached the secondary recrystallization temperature and are still in the primary recrystallization structure, a region (boundary region) where primary recrystallization grains and secondary recrystallization grains coexist is generated along the isotherm when observed in the plate thickness direction. As the steel sheet is heated and the temperature rises, this boundary region moves along the temperature gradient towards the region in the primary recrystallization structure state, whereby the region that becomes the secondary recrystallization structure expands, and finally the entire surface of the steel sheet is covered with secondary recrystallization grains. Through this process, the temperature of the boundary region remains relatively constant. Regarding the direction of the temperature gradient, in the final annealing, the coiled grain-oriented electrical steel sheet is usually arranged in the furnace in a cylindrical shape, so it is preferable to set the temperature gradient in the width direction of the steel sheet. When the temperature gradient is set in the width direction, the temperature gradient is formed in one direction along the entire width direction of the steel sheet (one end becomes the high-temperature side end and the other end becomes the low-temperature side end).

[0120] As described above, it is not easy to impart a temperature gradient of 2.0 °C / cm or more to the entire area of the coil, and problems such as productivity and variations in the properties within the steel sheet may arise. However, in the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, at the start of secondary recrystallization, the nitrogen content of the steel sheet is 210 ppm or more. Therefore, the amount of the inhibitor increases, and the inhibitor becomes thermally stable, so that even with a relatively small temperature gradient, a sufficient effect of increasing the magnetic flux density can be obtained. If there are variations in the temperature gradient, the lower limit of the temperature gradient at which a sufficient effect of increasing the magnetic flux density can be obtained can be reduced. However, when the temperature gradient is less than 0.5 °C / cm, the effect of increasing the magnetic flux density cannot be obtained sufficiently. Therefore, the temperature gradient is set to 0.5 °C / cm or more. When there are variations in the temperature gradient at each part of the coil or the steel sheet, the minimum temperature gradient in the entire area of the coil or the steel sheet is set to 0.5 °C / cm or more. There is no need to limit the upper limit of the temperature gradient, but even when the temperature gradient exceeds 10.0 °C / cm, the effect saturates and the equipment load increases. Therefore, the temperature gradient in the entire area of the coil can also be set to 10.0 °C / cm or less. In the present application, since a sufficient effect of increasing the magnetic flux density can be obtained even with a relatively small temperature gradient, the temperature gradient in the entire area of the coil can be set to 5.0 °C / cm or less or 2.0 °C / cm or less. In particular, when the temperature gradient is set relatively uniformly, it can be further set to 1.5 °C / cm or less or 1.0 °C / cm or less. When there are variations in the temperature gradient at each part of the coil or the steel sheet, the minimum temperature gradient in the entire area of the coil or the steel sheet can be set to 5.0 °C / cm or less or 2.0 °C / cm or less, and can be further set to 1.5 °C / cm or less or 1.0 °C / cm or less.

[0121] When the nitrogen content of the steel sheet after the nitriding treatment step is set to [N] in ppm based on mass, the proportion of N2 gas in the atmosphere during the heating process of the final annealing step described later is set to [N2] in mass%, and the temperature gradient during the heating process of the final annealing step is set to [T] in units of °C / cm, the temperature gradient is preferably set within the range that satisfies the following formula (1).

[0122] In this case, a higher magnetic flux density can be obtained.

[0123] [T]≥1 / (0.0026×[N])-0.005×[N2]-0.5 (1)

[0124] In addition, the atmosphere in the furnace from the start to the end of secondary recrystallization is preferably an atmosphere in which the proportion of N2 gas is 20% or more by mass. If the proportion of N2 gas (nitrogen gas) in the atmosphere is 20% or more by mass, the detachment of the inhibitor can be suppressed, and thus a more preferable effect can be obtained. The proportion of N2 gas (nitrogen gas) in the furnace atmosphere during the heating process of the final annealing is preferably 25% or more. The proportion of N2 gas (nitrogen gas) can be 100%, but if it is 100%, film defects are likely to occur. Therefore, it is preferable to make the proportion of N2 gas (nitrogen gas) less than 100%. More preferably, it is 75% or less, and still more preferably, it is 50% or less.

[0125] When performing the final annealing while imparting a temperature gradient, secondary recrystallization starts from the high-temperature side. Therefore, from the start to the end of secondary recrystallization means that secondary recrystallization starts from a partial area of the steel sheet on the high-temperature side of the temperature gradient until it ends on the entire area of the steel sheet.

[0126] Regarding the imparting of the temperature gradient, the temperature at the position that becomes the boundary region is not constant depending on the type of steel sheet and the annealing conditions, but by prior experiments, etc., under the assumed type of steel sheet and annealing conditions, the temperature at which secondary recrystallization occurs is confirmed in advance, and thus the temperature of the boundary region can be known. Therefore, by imparting a temperature gradient at a position where the temperature is near the temperature of the boundary region investigated in this way, a temperature gradient can be imparted to the boundary region between the primary recrystallization region and the secondary recrystallization region. For example, in the case of a steel sheet with a Si content of about 3% by mass using MnS and AlN as inhibitors, the temperature of the boundary region is about 950 to 1100 °C.

[0127] When the boundary region is not clear, a temperature gradient can also be imparted to a relatively wide range or the entire coil (steel sheet).

[0128] In addition, an effect can be obtained by imparting a temperature gradient to the boundary region during at least one period from the generation to the growth of secondary recrystallization grains, but in order to obtain a sufficient effect, it is preferable to impart a temperature gradient to the boundary region from the start of secondary recrystallization until the entire surface of the steel sheet is covered with secondary recrystallization grains (until secondary recrystallization ends). That is, it can also be set to generate a temperature gradient from the beginning to the end (until the soaking temperature is reached) of the heating process of the final annealing.

[0129] It can be heated up by setting a temperature difference in the furnace, or by heating and / or cooling the ends of the coil to set a temperature difference in the coil within the final annealing furnace, thereby imparting a temperature gradient. Regarding the magnitude of the temperature gradient, for example, if a temperature gradient is imparted in the width direction of the coil, the temperature change process can be measured by arranging sensors such as thermocouples at regular intervals (intervals at which the temperature gradient can be measured, such as 100 mm intervals) in its width direction, and the temperature gradient of each part within the steel plate can be calculated. Additionally, by calculating the temperature gradient of each part, the minimum value of the temperature gradient of the entire coil (the entire area) can be obtained.

[0130] Furthermore, the temperature gradient varies according to the size of the furnace, the temperature difference within the furnace, the size and weight of the coil, etc. In this case, as described above, the physical property values such as the thermal diffusivity can also be calculated using the results obtained from actually measuring the temperature change process at multiple parts of the coil. For example, by providing the furnace wall temperature as a boundary condition, the temperature gradient of each part of the coil can be calculated through simulation using well-known heat transfer calculation software such as Fluent (registered trademark) manufactured by ANSYS Corporation. In the simulation, by setting various conditions, the temperature gradient of each part of the coil (for example, the range of an arbitrary 100 mm interval in the width direction of the coil) considering the deviation of the temperature gradient can be calculated. By calculating the temperature gradient of each part, the minimum value of the temperature gradient of the entire coil can be obtained.

[0131] In addition, when there are differences in the temperature gradient in the radial direction of the coil, at multiple parts in the radial direction of the coil, sensors such as thermocouples are arranged at regular intervals (intervals at which the difference in the temperature gradient can be measured, such as 100 mm intervals) in its width direction, and by measuring the temperature change process in the width direction of each part, the difference in the temperature gradient in the radial direction can be calculated. Based on the temperature gradient calculated at each measurement part in the radial direction of the coil, the minimum value of the temperature gradient of the entire coil can be obtained. Regarding the measurement parts of the temperature change process in the radial direction of the coil, for example, when a temperature gradient is imparted in the width direction of the coil, in each of the steel plates located on the outermost side of the coil, the steel plate located in the middle part in the radial direction of the coil, and the steel plate located on the innermost side, one or more parts in the length direction of the coil can be set as the measurement parts, and a total of three or more parts can be set. Similarly, the temperature gradient in the width direction of multiple parts in the radial direction of the coil (for example, at each position with a 100 mm interval 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 through simulation.

[0132] In the case where a deviation in the temperature gradient occurs within the coil, the temperature gradient tends to become relatively small on the low-temperature end side of the coil, and further tends to become relatively small at the innermost position in the radial direction of the coil. Therefore, the temperature gradient calculated by measurement or simulation at the position on the low-temperature end side and at the innermost position in the radial direction of the coil can also be used as the minimum temperature gradient of the entire coil.

[0133] <Soaking process>

[0134] During the soaking process, impurities such as N, S, and Se that are harmful to magnetic properties are purified. Therefore, the final 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 temperature on the low-temperature side of the temperature gradient of the coil reaches the soaking temperature.

[0135] [Insulating film forming process]

[0136] In the method for manufacturing the oriented electrical steel sheet of the present embodiment, an insulating film forming process for forming an insulating film on the surface of the above-mentioned steel sheet may further be included. There is no limitation on the formed insulating film, and it may be a known film.

[0137] [Magnetic domain refinement process]

[0138] In the method for manufacturing the oriented electrical steel sheet of the present embodiment, a magnetic domain refinement process for refining the magnetic domains of the above-mentioned steel sheet may further be included.

[0139] By performing the magnetic domain refinement treatment, the iron loss of the oriented electrical steel sheet can be further reduced.

[0140] There is no limitation on the method of the magnetic domain refinement treatment. There is a method of narrowing the width of the 180° magnetic domain (performing the refinement of the 180° magnetic domain) by forming linear or dot-shaped groove portions extending in a direction crossing the rolling direction at a predetermined interval along the rolling direction, and a method of narrowing the width of the 180° magnetic domain (performing the refinement of the 180° magnetic domain) by forming linear or dot-shaped stress and strain portions or groove portions extending in a direction crossing the rolling direction at a predetermined interval along the rolling direction.

[0141] In the case of forming stress and strain portions, laser beam irradiation, electron beam irradiation, etc. can be applied. In addition, in the case of forming groove portions, a mechanical groove forming method based on a gear or the like, a chemical groove forming method based on electrolytic etching, and a thermal groove forming method based on laser irradiation can be applied.

[0142] In the case where damage occurs in the insulating film due to the formation of stress and strain portions or groove portions, resulting in deterioration of characteristics such as insulation, the insulating film can also be formed again to repair the damage.

[0143] Examples

[0144] <Example 1>

[0145] By casting, a silicon steel raw material containing, by mass%, Si: 3.30%, C: 0.052%, acid-soluble Al: 0.027%, N: 0.008%, and the balance comprising Fe and impurity elements was obtained.

[0146] After heating the silicon steel raw material to 1150°C, hot rolling was carried out to produce a hot-rolled sheet with a thickness of 2.3 mm.

[0147] The hot-rolled sheet was annealed by heating to 1100°C and then cooling to 900°C and holding for 30 seconds (hot-rolled sheet annealing).

[0148] The hot-rolled sheet after hot-rolled sheet annealing was cold-rolled until it reached a thickness of 0.22 mm to obtain a steel sheet (cold-rolled sheet). A test steel sheet with a length of 200 mm in the rolling direction and a width of 600 mm was cut out from this steel sheet.

[0149] Then, decarburizing annealing was performed on the test steel sheet to cause primary recrystallization. The decarburizing annealing was carried out by heating to 840°C and holding for 120 seconds in a wet atmosphere gas containing 75% hydrogen and 25% nitrogen. In addition, nitriding treatment was carried out at at least one of the following times: during the heating and soaking process of the decarburizing annealing process, between the decarburizing annealing process and the final annealing process, or during the heating process of the final annealing process and before the start of the secondary recrystallization, and the nitrogen content after the final nitriding treatment process was controlled to be 160 to 380 ppm. In the table, for example, "between the decarburizing annealing process and the final annealing process" means that nitriding treatment was carried out once from "the end of the decarburizing annealing process to the start of the final annealing process", and "between the decarburizing annealing process and the final annealing process and during the heating process of the final annealing process and before the start of the secondary recrystallization" means that nitriding treatment was carried out twice at "the end of the decarburizing annealing process to the start of the final annealing process" and "from the start of the final annealing process to the heating process of the final annealing process and before the start of the secondary recrystallization".

[0150] After applying an annealing parting agent mainly composed of MgO to the steel sheet after decarburizing annealing, the ends of the steel sheet were heated to a high temperature, and final annealing was carried out while applying a temperature gradient of 0 to 5.0°C / cm in the entire region in the direction perpendicular to the rolling direction (width direction) to cause secondary recrystallization. The average heating rate from the start to the end of secondary recrystallization in the boundary region was set to 10°C / hr, the final annealing temperature was set to 1200°C, and the soaking time was set to 30 hours.

[0151] A temperature gradient is uniformly imparted to the entire area of the test steel plate. The temperature gradient is imparted by increasing the temperature by setting a temperature difference in the furnace. Regarding the magnitude of the temperature gradient, it is controlled by measuring the temperature at 100 mm intervals in the width direction of the steel plate while increasing the temperature.

[0152] Samples that are 60 mm in the width direction and 200 mm in the rolling direction are collected from the obtained steel plate. For this sample, magnetic measurement is performed using the SST method (refer to Appendix JA of JIS C2556:2015), and the magnetic flux density B8 in the rolling direction is measured.

[0153] The results are shown in Table 1A and Table 1B.

[0154]

[0155]

[0156]

[0157]

[0158]

[0159]

[0160] As can be seen from Tables 1A to 1F, regardless of the timing and number of nitriding treatments, as long as the nitrogen content after the final nitriding treatment is 210 ppm or more and the temperature gradient is 0.5 °C / cm or more, a high magnetic flux density (magnetic flux density B8 is 1.940 T or more) can be achieved. In addition, under the condition of further satisfying the formula (1), a higher magnetic flux density (magnetic flux density B8 is 1.960 T or more) can be achieved.

[0161] <Example 2>

[0162] By casting, a silicon steel raw material (slab) having the chemical composition shown in Table 2 (unit: mass%, the balance being Fe and impurities) is obtained.

[0163] After heating the silicon steel raw material to 1100 - 1200 °C, hot rolling is performed to produce a hot rolled sheet with a thickness of 2.3 mm.

[0164] The hot rolled sheet is annealed by heating to 1100 °C and then cooling to 900 °C and holding for 30 seconds (hot rolled sheet annealing).

[0165] The hot rolled sheet after hot rolled sheet annealing is cold rolled until it becomes a thickness of 0.22 mm to obtain a steel plate (cold rolled sheet). A test steel plate that is 200 mm in the rolling direction and 600 mm in the width direction is cut out from this steel plate.

[0166] Then, for the test specimen steel sheet, decarburizing annealing is carried out in a humid atmosphere gas containing 75% hydrogen and 25% nitrogen to produce primary recrystallization. Nitriding treatment is carried out between the decarburizing annealing process and the final annealing process, and the nitrogen content is controlled to 210 ppm. The nitriding treatment is carried out once between the end of the decarburizing annealing process and the start of the final annealing process.

[0167] After applying an annealing separating agent mainly composed of MgO on the steel sheet after decarburizing annealing, the end of the steel sheet is made to be at a high temperature, and final annealing is carried out while imparting a temperature gradient of 0.5 °C / cm in the entire region in the direction perpendicular to the rolling direction (width direction) to produce secondary recrystallization. The final annealing temperature is set to 1150 - 1250 °C, and the soaking time is set to 10 - 30 hours. The proportion of N2 gas in the furnace atmosphere from the start to the end of secondary recrystallization during the heating process of the final annealing process is set to 30% by mass.

[0168] A temperature gradient is uniformly imparted to the entire region of the test specimen steel sheet. The temperature gradient is imparted by heating while setting a temperature difference in the furnace. Regarding the magnitude of the temperature gradient, it is controlled by measuring the temperature at 100 mm intervals in the width direction of the steel sheet while heating.

[0169] A sample with a width of 60 mm and a length of 200 mm in the rolling direction is collected from the obtained steel sheet, and for this sample, magnetic measurement is carried out by the SST method (refer to Appendix JA of JIS C2556:2015), and the magnetic flux density B8 in the rolling direction is measured.

[0170] The results are shown in Table 2.

[0171]

[0172] As can be seen from Table 2, excellent magnetic flux density B8 can be obtained in any chemical composition case.

[0173] Industrial availability

[0174] According to the present invention, a manufacturing method of a grain-oriented electromagnetic steel sheet can be provided, which manufactures a grain-oriented electromagnetic steel sheet with a high magnetic flux density by carrying out final annealing while imparting a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region. Even if the temperature gradient is small, a sufficient effect of increasing the magnetic flux density can be obtained. Therefore, the industrial availability of the present invention is high.

Claims

1. A method for manufacturing a directional electromagnetic steel sheet, characterized in that, It has the following steps: Hot rolling step: A silicon steel raw material containing Si: 0.80 to 7.00% by mass is heated to a temperature below 1280 °C and then hot rolled to obtain a hot rolled sheet; Hot rolled sheet annealing step: Anneal the hot rolled sheet as needed; Cold rolling step: Cold roll the hot rolled sheet after the hot rolling step or the hot rolled sheet annealing step to form a steel sheet with a final thickness; Decarburizing annealing step: Decarburizing anneal the steel sheet after the cold rolling step; Annealing release agent coating step: Coat an annealing release agent on the steel sheet after the decarburizing annealing step and then coil it into a coil shape; Nitriding treatment step: Increase the nitrogen content of the steel sheet; and Final annealing step: Perform final annealing on the steel sheet coiled into a coil shape, wherein, in the final annealing step, there is a heating-up process and a soaking process, and at least one period between the start and end of secondary recrystallization in the heating-up process is used to generate a temperature gradient of 0.5 °C / cm or more in the boundary region between the primary recrystallization region and the secondary recrystallization region; For the nitriding treatment step: It is carried out by annealing in an atmosphere containing a gas with nitriding ability in at least one of the decarburizing annealing step, between the decarburizing annealing step and the final annealing step, or in the heating-up process of the final annealing step and before the start of secondary recrystallization, thereby performing the nitriding treatment step, and the nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more based on mass; 2. The manufacturing method of the directional electromagnetic steel sheet according to claim 1, characterized in that, In the heating-up process of the final annealing step, the atmosphere in the furnace from the start to the end of secondary recrystallization is set to an atmosphere with a proportion of N2 gas of 20% or more by mass; 3. The manufacturing method of the directional electromagnetic steel sheet according to claim 1 or 2, characterized in that, The nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more and 350 ppm or less based on mass; 4. The manufacturing method of the directional electromagnetic steel sheet according to claim 1 or 2, characterized in that When the nitrogen content of the steel sheet after the nitriding treatment step is set to [N] in ppm based on mass, the proportion of N2 gas in the atmosphere in the furnace of the final annealing step is set to [N2] in mass%, and the temperature gradient of the final annealing step is set to [T] in °C / cm, the following formula (1) is satisfied, [T] ≥ 1 / (0.0026 × [N]) - 0.005 × [N2] - 0.5 (1).

5. The manufacturing method of the directional electromagnetic steel sheet according to claim 3, characterized in that, When the nitrogen content of the steel sheet after the nitriding treatment step is set to [N] in ppm based on mass, the proportion of N2 gas in the atmosphere in the furnace of the final annealing step is set to [N2] in mass%, and the temperature gradient of the final annealing step is set to [T] in °C / cm, the following formula (1) is satisfied, [T] ≥ 1 / (0.0026 × [N]) - 0.005 × [N2] - 0.5 (1).

6. The manufacturing method of the directional electromagnetic steel sheet according to claim 1 or 2, characterized in that, The chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

7. The manufacturing method of the directional electromagnetic steel sheet according to claim 3, characterized in that, The chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

8. The manufacturing method of the directional electromagnetic steel sheet according to claim 4, characterized in that, The chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

9. The manufacturing method of the directional electromagnetic steel sheet according to claim 5, characterized in that, The chemical composition of the silicon steel raw material contains, by mass%: Si: 0.80 to 7.00%, C: 0.085% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0 to 1.00%, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, S and Se: a total of 0 to 0.015%, and the balance contains Fe and impurities.

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