Method for manufacturing oriented electromagnetic steel sheet, and induction heating device

Abstract

Proposed is a method for producing a grain-oriented electrical steel sheet having excellent and uniform magnetic properties in the sheet width direction by hot-rolling a steel material to form a hot-rolled sheet, subjecting the hot-rolled sheet to cold rolling to have a final thickness, and subjecting the cold-rolled sheet to decarburization annealing serving also as primary recrystallization annealing, followed by finishing annealing to induce secondary recrystallization, and in the decarburization annealing, the average heating rate T (°C / s) in the temperature range of 500°C to 700° during heating is set to 250°C / s or greater, and the period for the reduced heating rate to 150°C / s or less in the temperature range of 500°C to 700°C is set in accordance with the value of x / w (where x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm)) at each position in the sheet width direction. Also provided is an induction heating device for use in the decarburization annealing of the method.

Description

Technical Field

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet, and an induction heating device for use in decarburization annealing of the production method.Background Art

[0002] A grain-oriented electrical steel sheet is a soft magnetic material that is mainly used as an iron core material of a transformer or a generator, etc., and is a steel sheet with excellent magnetic properties including low iron loss and high magnetic flux density as it has a crystal structure in which the {110}<001> orientation (i.e., Goss orientation), which corresponds to the magnetic easy axis of iron, is highly aligned along the rolling direction of the steel sheet.

[0003] Methods for further reducing iron loss of grain-oriented electrical steel sheets include promoting a high degree of crystal grain alignment in the Goss orientation, following secondary recrystallization annealing. To increase the alignment degree of secondary recrystallized grains in the Goss orientation, it is important to form a large number of Goss-oriented grains in the steel sheet texture at the completion of primary recrystallization, and to introduce a difference in grain boundary mobility to preferentially promote the growth of sharp Goss-oriented grains during secondary recrystallization, in other words, to optimize the steel sheet texture after primary recrystallization.

[0004] Examples of primary recrystallization textures that promote the preferential growth of sharp Goss-oriented grains include {111}<112> oriented grains and {411}<148> oriented grains. Ensuring that these grains are present in a primary recrystallization texture in a good balance and with high frequency facilitates strong alignment of Goss-oriented grains in the rolling direction during secondary recrystallization annealing.

[0005] As a method for increasing the fraction of Goss-oriented grains in a primary recrystallization texture, for example, Patent Literature 1 discloses a method of performing aging treatment by applying heat treatment at a low temperature to a cold-rolled sheet during cold rolling. Patent Literature 2 discloses a method in which the cooling rate is set to 30°C / s or greater during either hot-rolled sheet annealing or intermediate annealing (final cold rolling) performed prior to cold rolling to obtain the final thickness, and in which in-pass aging is performed two or more times by holding the steel sheet at a temperature of 150 to 300°C for 2 minutes or longer during the final cold-rolling. Patent Literature 3 discloses a method involving performing warm rolling by raising the temperature of the steel sheet during cold rolling.

[0006] The methods of Patent Literatures 1 to 3 are each aimed to increase the temperature of a steel sheet to an appropriate level before or during cold rolling, or between consecutive passes of cold rolling. This promotes the diffusion of dissolved elements, such as carbon (C) and nitrogen (N), to pin dislocations introduced by cold rolling and thereby suppress the migration of the dislocations during the following rolling process, so as to promote the shear deformation and thus improve the rolled texture. This is based on the view that the nuclei of Goss-oriented grains in a primary recrystallization texture appear from shear bands introduced into a processed texture having a {111}<112> orientation. Applying these methods can introduce a large number of shear bands into the {111}<112> processed texture and thereby form a large number of Goss-oriented grains in the primary recrystallization texture.

[0007] Increasing the heating rate of a steel sheet during heating in decarburization annealing also promotes the formation of Goss-oriented grains in a primary recrystallization texture. For example, Patent Literature 4 discloses a method involving rapidly heating a steel sheet during heating in decarburization annealing. This method is aimed to suppress the development of a γ-fiber texture ({111} / / ND), which is preferentially formed at a regular heating rate. This is achieved by heating a steel sheet from the room temperature to a temperature around the recrystallization temperature in a short time using electric heating, induction heating, etc., promoting the generation of Goss-oriented grains as the nuclei of secondary recrystallized grains.

[0008] Patent Literature 5 discloses a method that involves rapidly heating a steel sheet at an average heating rate of 50°C / s or greater in the temperature range of 550 to 700°C during a heating process of decarburization annealing and maintaining the heating rate of 10°C / s or less for 1 to 10 seconds within a selected temperature range between 250 and 550°C. The method is aimed at promoting the recovery of the {111} processed texture by holding the steel sheet in the recovery temperature range of 250 to 550°C for a short time and thereby suppressing recrystallization to increase the relative fraction of Goss-oriented grains.Citation ListPatent Literature

[0009] Patent Literature 1: JP-A-S50-016610 Patent Literature 2: JP-A-H08-253816 Patent Literature 3: JP-A-H01-215925 Patent Literature 4: JP-A-H04-160114 Patent Literature 5: JP-A-2014-152393 Summary of InventionTechnical Problem

[0010] The primary recrystallization texture of a steel sheet is often not uniform in the sheet width direction. This is considered to be due to factors such as uneven rolling reduction during cold rolling caused by edge drops formed in hot rolling, or non-uniform heating across the sheet width during hot-rolled sheet annealing and the like, resulting in uneven crystal grain sizes across the sheet width before cold rolling. In addition, when warm rolling is applied as the cold rolling as in the methods disclosed in Patent Literatures 1 to 3 above, a large temperature drop occurs at edge portions of the steel sheet due to heat radiation, varying the diffusion distance of carbon and nitrogen in the sheet width direction. This is also considered to be a cause of a change in the texture.

[0011] If the primary recrystallization texture varies across the sheet width, the secondary recrystallization behavior will also vary accordingly, leading to variation in the magnetic properties of the final product across the sheet width.. To prevent this, trimming the edge portions of the steel sheet after hot rolling or cold rolling may be effective. However, this approach would inevitably reduce yield.

[0012] The present invention has been made in view of the foregoing problems of the conventional methods, and it is an object of the present invention to propose a method for producing a grain-oriented electrical steel sheet having excellent and uniform magnetic properties in the sheet width direction, and an induction heating device for decarburization annealing for use in such a production method.Solution to Problem

[0013] To address the above problems, the inventors conducted focused studies on methods for achieving a uniform primary recrystallization texture across the sheet width. As a result, the inventors discovered that such uniform primary recrystallization texture after decarburization annealing across the sheet width can be obtained by: performing at least one rolling pass at a steel sheet temperature of 150 °C or higher during final cold rolling; temporarily reducing the heating rate within the temperature range of 500 °C to 700 °C during rapid heating in decarburization annealing, which serves as primary recrystallization annealing; and varying the period for the reduced heating rate across the sheet width.

[0014] The present invention based on the foregoing findings proposes a method for producing a grain-oriented electrical steel sheet, including hot-rolling a steel material to form a hot-rolled sheet; subjecting the hot-rolled sheet to cold rolling including a single cold rolling step or two or more cold rolling steps with intermediate annealing interposed between each rolling step, to form a cold-rolled sheet with a final thickness; and subjecting the cold-rolled sheet to decarburization annealing also serving as primary recrystallization annealing, followed by finishing annealing to induce secondary recrystallization, characterized in that in the decarburization annealing, an average heating rate T (°C / s) is 250°C / s or greater within a temperature range of 500°C to 700° during heating, and within a selected temperature range between 500°C and 700°, a heating rate at each position across the sheet width is reduced to 150°C / s or less for a period t (s) that satisfies Expression (1) below, in accordance with a value of x / w of the position: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents a distance (mm) from a center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w.

[0015] The steel material used for the method for producing a grain-oriented electrical steel sheet of the present invention has a composition of components including C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: 0.0100 to 0.0400 mass%, and N: 0.0050 to 0.0120 mass%, and further including at least one of S and Se in a total amount of 0.01 to 0.05 mass%, with a balance being Fe and unavoidable impurities.

[0016] The steel material used for the foregoing method for producing a grain-oriented electrical steel sheet of the present invention has a composition of components including C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: less than 0.0100 mass%, and Se: less than 0.0100 mass%, with a balance being Fe and unavoidable impurities.

[0017] The steel material used for the foregoing method for producing a grain-oriented electrical steel sheet of the present invention further includes, in addition to the composition of components, at least one component selected from the group consisting of Sb: 0.005 to 0.500 mass%, Cu: 0.01 to 1.50 mass%, P: 0.005 to 0.500 mass%, Cr: 0.01 to 1.50 mass%, Ni: 0.005 to 1.500 mass%, Sn: 0.01 to 0.50 mass%, Nb: 0.0005 to 0.0100 mass%, Mo: 0.01 to 0.50 mass%, B: 0.0010 to 0.0070 mass%, and Bi: 0.0005 to 0.0500 mass%.

[0018] In the foregoing method for producing a grain-oriented electrical steel sheet of the present invention, rapid heating in the decarburization annealing is performed using a transverse induction heating device.

[0019] The present invention also provides a transverse induction heating device for use in rapid heating of the decarburization annealing in the foregoing method for producing a grain-oriented electrical steel sheet.Advantageous Effects of Invention

[0020] According to the present invention, it is possible to stably produce a grain-oriented electrical steel sheet having magnetic properties excellent and uniform in the sheet width direction, which greatly contributes to improving the quality of a product sheet and increasing yields.Brief Description of Drawings

[0021] [Fig. 1] is a graph illustrating the period during which the heating rate was reduced in the sheet width direction, in accordance with the present invention. [Fig. 2] is another graph illustrating the period during which the heating rate was reduced in the sheet width direction, in accordance with the present invention. [Fig. 3] is a schematic view illustrating a transverse induction heating device. Description of Embodiments

[0022] First, that experiments that led to the development of the present invention will be described.<Experiment 1>

[0023] A steel slab having a composition of components including C: 0.033 mass%, Si: 3.4 mass%, Mn: 0.07 mass%, sol. Al: 0.0081 mass%, N: 0.0052 mass%, S: 0.0030 mass%, and Se: 0.0030 mass%, with the balance being Fe and unavoidable impurities was heated to 1220°C, and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds and then cold rolling once to produce a cold-rolled sheet with a final thickness of 0.20 mm. Thereafter, a sample material was taken from the cold-rolled sheet, and a plurality of samples each having the size of an Epstein test piece (a width of 30 mm × a length of 280 mm) were taken at five positions of x=0, 0.2w, 0.4w, 0.6w, and 0.8w of the sample material in the sheet width direction, provided that the distance from the center in the sheet width direction is represented by x (mm) (where the center in the sheet width direction is x = 0), and 1 / 2 of the sheet width is represented by w (mm).

[0024] Subsequently, each sample was subjected to decarburization annealing also serving as primary recrystallization annealing at a soaking temperature of 850°C for a soaking time of 100 seconds. During heating of the decarburization annealing, the average heating rate within the temperature range of 500°C to 700°C was set to 300°C / s. When the sample temperature reached 600°C under certain conditions, the heating rate was reduced to 120°C / s for the period indicated in Table 1. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the sample after decarburization annealing, followed by finishing annealing to induce secondary recrystallization. After the finishing annealing, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the sample, and heat treatment simulating flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet sample. Note that the average heating rate of 300°C / s within the temperature range of 500°C to 700°C refers to the average heating rate over the duration excluding the period during which the heating rate was being reduced.

[0025] For each product sheet sample sized according to the Epstein test piece and prepared as described above, the iron loss W 17 / 50 was measured in accordance with JIS Z 2550. The difference between the maximum value and the minimum value of the iron loss values in the sheet width direction was determined. The results are shown in Table 1. [Table 1]ConditionsPeriod(s) for reduced heating timeDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarksx=0x=0.2wx=0.4wx=0.6wx=0.8w10.000.000.000.000.000.06Comparative Example20.100.100.100.100.100.07Comparative Example30.300.300.300.300.300.08Comparative Example40.500.500.500.500.500.07Comparative Example50.100.200.300.400.500.09Comparative Example60.200.400.000.300.200.07Comparative Example70.500.400.300.200.100.04Invention Example80.300.300.200.100.050.03Invention Example90.400.300.200.150.100.03Invention Example

[0026] Table 1 demonstrates that, under conditions where a period for temporarily reduced heating rate was provided during rapid heating within the temperature range of 500 to 700 °C, and the duration of said period was set to be longer at a central portion in the sheet width direction and shorter at the edge portions thereof, the difference between the maximum and minimum iron loss values in the sheet width direction was 0.04 W / kg or less. Accordingly, magnetic properties substantially uniform across the sheet width direction were obtained.

[0027] The present inventors consider the following as the reason why magnetic properties were uniform in the sheet width direction under conditions in which the period for temporarily reduced heating rate during rapid heating was set to be longer at the central portion and shorter at the edge portions in the sheet width direction.

[0028] The cold rolling reduction at the edge portions in the sheet width direction is smaller than that at the central portion, due to edge drops caused during hot rolling or the like. Accordingly, the amount of deformation during cold rolling is smaller at the edge portions in the sheet width direction. Furthermore, the temperature of the steel sheet during cold rolling tends to be lower at the edge portions in the sheet width direction than at the central portion, due to heat radiation. This results in a shorter diffusion distance of carbon and nitrogen in the steel at the edge portions in the sheet width direction, making it difficult to pin dislocations formed during rolling. Consequently, the cold-rolled microstructure at the edge portions in the sheet width direction contains a smaller amount of introduced shear bands, which serve as sites for the formation of Goss-oriented grains during primary recrystallization, compared to the central portion in the sheet width direction. Therefore, the recrystallization of Goss-oriented grains is promoted at the edge portions in the sheet width direction by shortening the period for the reduced heating rate, while it is suppressed at the central portion in the sheet width direction by prolonging the said period. Accordingly, it is considered that the number of Goss-oriented grains after primary recrystallization annealing has become uniform across the sheet width.

[0029] Next, based on the experiment results, the inventors conducted an experiment to study the appropriate period for the reduced heating rate in accordance with the position in the sheet width direction.<Experiment 2>

[0030] The five types of samples having the size of an Epstein test piece, taken from different positions in the sheet width direction of the cold-rolled sheet obtained in Experiment 1, were subjected to decarburization annealing serving also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of the decarburization annealing, the average heating rate within the temperature range of 500°C to 700°C was set to 300°C / s. When the temperature of each sample reached 650°C, the period during which the heating rate was reduced to 110°C / s at each position in the sheet width direction was varied in eight conditions as shown in Fig. 1. After the decarburization annealing, an annealing separating agent mainly composed of MgO was applied to the surface of the sample, followed by finishing annealing to induce secondary recrystallization. Subsequently, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the sample after finishing annealing, and heat treatment simulating flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet sample.

[0031] For each product sheet sample having the size of an Epstein test piece thus obtained, the iron loss W 17 / 50 was measured according to JIS Z 2550. Table 2 shows the difference between the maximum value and the minimum value of the iron loss values in the sheet width direction. [Table 2]ConditionsDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarks10.04Invention Example20.03Invention Example30.03Invention Example40.02Invention Example50.07Comparative Example60.06Comparative Example70.10Comparative Example80.08Comparative Example

[0032] Table 2 demonstrates that, under conditions in which a period t of reduced heating rate at each of positions x=0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction satisfies Expression (1) below: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0≤x≤0.9w. the difference between the maximum and minimum iron loss values in the sheet width direction was 0.04 W / kg or less. Accordingly, magnetic properties uniform across the sheet width direction were obtained.

[0033] The present invention has been completed by adding further consideration to the foregoing new findings.

[0034] The component composition of a steel material (slab) used for producing a grain-oriented electrical steel sheet according to the present invention is described. It should be noted that conventionally known steel materials used in the production of grain-oriented electrical steel sheets may also be employed in the present invention. However, from the viewpoint of achieving superior magnetic properties, a steel material having the following composition is preferably used.C: 0.01 to 0.10 mass%

[0035] C is precipitated as fine carbide particles and thereby contributes to improving a primary recrystallization texture. However, if the content of C falls below 0.01 mass%, the precipitation amount of fine carbide particles is insufficient, and the effect of improving the texture may not be fully obtained. Meanwhile, if the content of C exceeds 0.10 mass%, it may be difficult to reduce the content of C to 0.0050 mass% or less, with which no magnetic aging occurs during decarburization annealing. Accordingly, the content of C should be set within the range of 0.01 to 0.10 mass% inclusive. Preferably, the content of C should be set within the range of 0.015 to 0.08 mass% inclusive.Si: 2.0 to 4.5 mass%

[0036] Si is an element effective in increasing the specific resistance of steel and thereby improving iron loss properties. However, if the content of Si is less than 2.0 mass%, the effect of reducing iron loss cannot be fully obtained. Meanwhile, the content of Si exceeding 4.5 mass% significantly deteriorates workability, making it difficult to produce a steel sheet by rolling. Accordingly, the content of Si should be set in the range of 2.0 to 4.5 mass% inclusive. Preferably, it should be set in the range of 2.5 to 4.0 mass% inclusive.Mn: 0.01 to 0.50 mass%

[0037] Mn is an element essential for to improving hot workability. If the content of Mn is less than 0.01 mass%, the effect of improving hot workability can hardly be obtained. Meanwhile, the content of Mn exceeding 0.50 mass% may deteriorate primary recrystallization texture, making it difficult to obtain secondary recrystallized grains with highly aligned Goss-oriented grains. Accordingly, the content of Mn should be set in the range of 0.01 to 0.50 mass% inclusive. Preferably, it should be in the range of 0.03 to 0.45 mass% inclusive.

[0038] Components other than C, Si, and Mn described above vary depending on whether an inhibitor is used for secondary recrystallization.

[0039] Specifically, when an inhibitor is used for secondary recrystallization and AlN is employed as the inhibitor, the steel material should contain Al and N in the range of Al: 0.0100 to 0.0400 mass% inclusive and N: 0.0050 to 0.0120 mass% inclusive in addition to C, Si, and Mn described above. If each of the content of Al and the content of N falls below its lower limit, it becomes difficult to achieve an intended effect of the inhibitor. Meanwhile, if each of the content of Al and the content of N exceeds its upper limit, the precipitates may be unevenly dispersed, which also hinders the predetermined effect of the inhibitor.

[0040] Furthermore, when sulfide (e.g., MnS or Cu 2 S) and / or selenide (e.g., MnSe or Cu 2 Se) are / is used as inhibitors, in addition to AlN described above, at least one of S and Se should be contained in a total amount of 0.0100 to 0.0500 mass%, as inhibitor-forming components, along with Al and N. If the total content of S and Se falls below its lower limit described above, it becomes difficult to fully achieve the effect of the inhibitor. Meanwhile, if the total content of S and Se exceeds its upper limit described above, precipitates are unevenly dispersed, which also hinders the effect of inhibitors. It should be noted that sulfide and selenide described above may undergo complex precipitation.

[0041] Meanwhile, when no inhibitor is used for secondary recrystallization, the inhibitor-forming components should be minimized. Specifically, the following ranges are preferable: Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: less than 0.0100 mass%, and Se: less than 0.0100 mass%.

[0042] The steel material used in the production of a grain-oriented electrical steel sheet according to the present invention has the aforementioned basic component composition, with the balance being substantially Fe and unavoidable impurities. However, in order to enhance magnetic properties, the steel material may further contain, in addition to the aforementioned components, at least one element selected from the group consisting of Sb: 0.005 to 0.500 mass%, Cu: 0.01 to 1.50 mass%, P: 0.005 to 0.500 mass%, Cr: 0.01 to 1.50 mass%, Ni: 0.005 to 1.500 mass%, Sn: 0.01 to 0.50 mass%, Nb: 0.0005 to 0.0100 mass%, Mo: 0.01 to 0.50 mass%, B: 0.0010 to 0.0070 mass%, and Bi: 0.0005 to 0.0500 mass%. Each of Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi is effective in improving magnetic properties. When present within the aforementioned ranges, these components achieve the effect of improving magnetic properties without hindering the development of secondary recrystallized grains.

[0043] Next, a method for producing a grain-oriented electrical steel sheet according to the present invention will be described.

[0044] A steel material (slab) used in the production of a grain-oriented electrical steel sheet according to the present invention is preferably prepared by subjecting molten steel, obtained in a converter, an electric furnace, or the like, to a commonly known refining process involving secondary refining such as vacuum degassing, to form steel having the aforementioned composition of components, followed by a commonly known continuous casting process or ingot making-blooming process.

[0045] Next, the steel material (slab) is heated to a predetermined temperature and then hot-rolled to form a hot-rolled sheet. The slab heating temperature is preferably set approximately at 1050°C or higher to ensure hot rollability. The upper limit of the heating temperature is not limited to a particular value. However, if the heating temperature exceeds 1450°C, it approaches the melting temperature of the steel, making it difficult to maintain the shape of the slab. Therefore, the heating temperature is preferably set at 1450°C or lower.

[0046] The hot rolling following the slab heating may be performed under commonly known conditions, which are not particularly limited.

[0047] Next, the hot-rolled steel sheet (hot-rolled sheet) may be subjected to hot-rolled sheet annealing as appropriate. Note that when hot-rolled sheet annealing is performed, commonly known conditions may be applied, and the conditions are not limited to particular conditions.

[0048] The hot-rolled steel sheet or the hot-rolled-annealed steel sheet is descaled as appropriate by pickling, a mechanical method, or the like and cold-rolled to produce a cold-rolled sheet with a final thickness (thickness of the product sheet). The cold rolling may consist of a single cold-rolling step to obtain a cold-rolled sheet with a final thickness, or two or more cold-rolling steps with intermediate annealing interposed between each step, to obtain a cold-rolled sheet with a final thickness. It should be noted that the final thickness preferably falls within the range of 0.1 mm to 1.0 mm inclusive.

[0049] The rolling reduction in the final cold-rolling step preferably falls within the range of 60% to 95%, inclusive. The term 'final cold-rolling step' refers to the cold-rolling step performed last, whether in a single cold-rolling step or in two or more cold-rolling steps. In cases where cold rolling is performed only once, that step corresponds to the final cold-rolling step. In cases where two or more cold-rolling steps are performed, the cold-rolling step conducted last corresponds to the final cold-rolling step.

[0050] In the cold rolling process, in order to improve magnetic properties by forming numerous recrystallization nuclei of Goss-oriented grains within the primary recrystallized structure, it is preferable to apply heat treatments such as inter-pass aging or to adopt warm rolling, as described in Patent Literatures 1 to 3.

[0051] Next, the cold-rolled sheet with the final thickness is subjected to decarburization annealing, which also serves as primary recrystallization annealing. The decarburization conditions (conditions at soaking) of the decarburization annealing are not limited to particular conditions, and known conditions may be applied. For example, such conditions of 720 to 870°C for 60 to 150 seconds in a wet hydrogen atmosphere are preferably. This decarburization annealing can reduce the content of C in the steel sheet to 0.0050 mass% or less, at which magnetic aging is unlikely to occur.

[0052] Herein, it is important to perform rapid heating at an average heating rate of 250°C / s or greater within the temperature range of 500°C to 700°C during the heating process of the decarburization annealing, until the soaking temperature is reached. In the present invention, the average heating rate within the temperature range of 500°C to 700°C refers to the average heating rate over the time, excluding the period during which the heating rate was being temporarily reduced, which will be described later. If the average heating rate is less than 250°C / s, sufficient Goss-oriented grains are not formed after primary recrystallization, making it unlikely to achieve favorable iron loss. The preferable average heating rate is 300°C / s or greater. It should be noted that rapid heating may be performed in a range other than the temperature range of 500°C to 700°C.

[0053] In the heating process of the decarburization annealing, it is necessary to provide a period during which the heating rate is temporarily reduced to 150°C / s or less within a selected temperature range between 500 and 700°C during the rapid heating. If the temperature of the steel sheet at which the heating rate is reduced is below 500°C, the recrystallization behavior of Goss-oriented grains is unlikely to change, even with a decreased heating rate. As a result, the effect of adjusting the number of Goss-oriented grains through primary recrystallization cannot be obtained. Meanwhile, if the steel sheet temperature at which the heating rate is decreased exceeds 700°C, recrystallization is nearly complete by the time the heating rate is reduced. Therefore, the effect of adjusting the number of Goss-oriented grains through primary recrystallization cannot be achieved.

[0054] Furthermore, in the present invention, it is necessary to vary the period during which the heating rate is temporarily reduced, in accordance with the position in the sheet width direction. The present invention relates to a method for eliminating variation in the microstructure of a steel sheet in the sheet width direction, which occurred due to various production conditions of processes prior to decarburization annealing, by varying the period for temporarily decreasing the heating rate in accordance with a position in the sheet width direction during rapid heating of the decarburization annealing to cause uniform recrystallization of Goss-oriented grains in the sheet width direction in primary recrystallization.

[0055] The period t of temporarily reduced heating rate to 150°C / s or less during the rapid heating is set longer on the side of the central portion in the sheet width direction and shorter on edge portions in the sheet width direction. Specifically, it is important to vary the period t so as to satisfy Expression (1) below: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w.

[0056] If t is shorter than the left-hand side of Expression (1) above, the number of corresponding Goss-oriented grains increases excessively, so that the iron loss properties are partially degrade. Conversely, if t is longer than the right-hand side of Expression (1) above, the recrystallization of Goss-oriented grains is suppressed correspondingly, so that iron loss properties partially increase. Consequently, it becomes impossible to obtain magnetic properties uniform in the sheet width direction. The reason the range of the sheet width that satisfies Expression (1) above is set to (0 ≤ x ≤ 0.9w) is that, when transverse induction heating is used for rapid heating, an induced current tends to concentrate at the edge portions in the sheet width direction, which may make it difficult to satisfy Expression (1) across the full width. Needless to say, it is preferable for Expression (1) to be satisfied across the entire sheet width.

[0057] The temporarily reduced heating rate needs to be 150°C / s or less. If the temporarily reduced heating rate exceeds this value, the effect of suppressing the recrystallization of Goss-oriented grains cannot be sufficiently achieved. It should be noted that the lower limit of the reduced heating rate is not limited to a particular value, but is preferably 10°C / s or greater. The period for the temporarily reduced heating rate can be determined by measuring the steel sheet temperature during the heating process, using a thermocouple, for example, and then calculating the temporal derivative of the temperature over time.

[0058] Herein, the rapid heating performed during the heating process of the decarburization annealing as well as reduction in the heating rate during the rapid heating can be achieved by arranging two or more rapid heating devices, such as electric heating devices or solenoid induction heating devices, in series in the threading direction of the steel sheet. The heating rate can be reduced in the section between any two of these devices, and further adjusted by appropriately controlling the outputs of the heating devices and the threading speed (line speed) of the steel sheet. In addition, an edge heater and the like may be provided in the section where the heating rate is decreased, from the perspective of preventing heat radiation at the edge portions in the sheet width direction.

[0059] Arranging two or more rapid heating devices in series, as described above, presents challenges in terms of cost and space. However, as schematically illustrated in Fig. 3, when using a transverse induction heating device as the rapid heating device, where heating coils wound around an iron core are arranged above and below the steel sheet, an alternating magnetic flux generated within the iron core penetrates the steel sheet in the thickness direction, heating it through the action of the magnetic field. In this configuration, the induced current flows across the sheet plane following the shape of the heating coils, rather than through the portions of the steel sheet directly facing the iron core. Accordingly, when the steel sheet passes in the vicinity of the iron core, the heating rate temporarily decreases. This phenomenon may be utilized to intentionally reduce the heating rate. In addition, the period for the reduced heating rate can be adjusted by controlling the output of the induction heating device, the line speed, or the like. Further, an induced current flows through the edge portions in the sheet width direction, where heat radiation can be suppressed. Furthermore, since the decrease in heating rate occurs within a single induction heating device, there is no concern regarding installation space. Accordingly, a transverse induction heating device is preferably employed in the present invention.

[0060] To vary the period for the reduced heating rate in accordance with a position in the sheet width direction when using the transverse induction heating device, the coil diameter may be set larger at the central portion in the sheet width direction and sequentially reduced toward the steel width edge portions. The shape of the heating coil of the transverse induction heating device may be round, quadrangular, or elliptical shapes, for example. However, as described above, the coil diameter in the threading direction is preferably varied across the sheet width.

[0061] Thereafter, an annealing separating agent is applied to the surface of the cold-rolled steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. The annealing separating agent can may be any known type and is not limited to a particular one. Examples of the annealing separating agent include an agent mainly composed of MgO and also containing an auxiliary agent such as TiO 2 , as appropriate, and an agent mainly composed of SiO 2 or Al 2 O 3 .

[0062] Unreacted portions of the annealing separating agent remaining on the surface of the steel sheet surface after the finishing annealing, are removed. Thereafter, it is preferable to apply an insulation coating solution to the surface of the steel sheet, followed by flattening annealing to simultaneously bake the coating and correct the shape of the steel sheet, which may have been deformed during finishing annealing, thereby obtaining a product sheet. It should be noted that the insulation coating may be formed through a different line. The type of the insulation coating is not limited to a particular type. However, to form a tension-imparting insulation coating, which imparts tension to the surface of the steel sheet, it is preferable to apply slurry containing phosphate and colloidal silica to the surface of the steel sheet and then bake the slurry at a temperature of about 800°C as disclosed in Japanese Patent Laid-Open No. 50-79442, Japanese Patent Laid-Open No. 48-39338, Japanese Patent Laid-Open No. 56-75579, and the like.

[0063] When a further reduction of iron loss is desired, magnetic domain subdividing treatment may be performed using a known method. Such methods include forming grooves in the surface of the steel sheet during any of the process following the cold rolling, mechanically introducing strain regions into the surface, or creating thermal strain regions by irradiating the surface with a laser beam or an electron beam, for example, after finishing annealing.Example 1

[0064] A Steel slab having a composition of components including C: 0.035 mass%, Si: 3.3 mass%, Mn: 0.05 mass%, sol. Al: 0.0084 mass%, N: 0.0051 mass%, S: 0.0031 mass%, and Se: 0.0031 mass%, with the balance being Fe and unavoidable impurities was heated to 1260°C and then hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0065] Thereafter, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During a heating process of the decarburization annealing, the average heating rate T (°C / s) was varied in the temperature range from 500°C to 700°C, as shown in Table 3. For certain steel sheets, when the temperature of the steel sheet reached 620°C, the heating rate at each position in the sheet width direction was reduced for a period t, defined by Expression (2) below, in order to achieve the "reduced heating rate" shown in Table 3: t = 200 / T × 0.5 × 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w. Thereafter, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. An insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing, followed by flattening annealing at 800°C for 30 seconds to bake the coating and thereby obtain a product sheet.

[0066] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 3 presents the results, demonstrating that under conditions in which the average heating rate is 250 °C / s or greater and the reduced heating rate is 150 °C / s or less, the mean iron loss value is as low as 0.87 W / kg or less, and the variation in iron loss is suppressed to 0.04 W / kg or less. [Table 3-1]ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values1200500.910.07Comparative Example2225500.880.07Comparative Example3250500.830.02Invention Example4260500.860.02Invention Example5270500.820.04Invention Example6280500.840.01Invention Example7290500.830.02Invention Example8300500.840.02Invention Example9350500.850.02Invention Example10400500.840.01Invention Example11500500.840.03Invention Example12600500.830.02Invention Example13200750.890.08Comparative Example14225750.990.08Comparative Example15250750.830.02Invention Example16260750.840.04Invention Example17270750.860.01Invention Example18280750.860.01Invention Example19290750.850.03Invention Example20300750.840.01Invention Example21350750.860.02Invention Example22400750.820.03Invention Example23500750.830.01Invention Example24600750.860.01Invention Example252001000.960.09Comparative Example262251000.910.09Comparative Example272501000.830.01Invention Example282601000.830.02Invention Example292701000.840.02Invention Example302801000.820.01Invention Example312901000.830.02Invention Example323001000.850.01Invention Example333501000.840.03Invention Example344001000.850.02Invention Example355001000.820.02Invention Example366001000.840.02Invention Example372001100.900.07Comparative Example382251101.010.10Comparative Example392501100.820.04Invention Example402601100.840.01Invention Example412701100.840.02Invention Example422801100.850.01Invention Example432901100.820.04Invention Example443001100.820.03Invention Example453501100.830.03Invention Example464001100.850.01Invention Example475001100.830.04Invention Example486001100.850.04Invention Example [Table 3-2] ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values492001200.940.07Comparative Example502251200.930.07Comparative Example512501200.840.03Invention Example522601200.850.01Invention Example532701200.840.02Invention Example542801200.820.02Invention Example552901200.850.01Invention Example563001200.830.03Invention Example573501200.840.02Invention Example584001200.830.01Invention Example595001200.860.02Invention Example606001200.830.04Invention Example612001300.940.06Comparative Example622251301.010.06Comparative Example632501300.830.01Invention Example642601300.850.01Invention Example652701300.860.04Invention Example662801300.840.03Invention Example672901300.850.04Invention Example683001300.850.03Invention Example693501300.850.02Invention Example704001300.840.02Invention Example715001300.850.04Invention Example726001300.820.01Invention Example732001400.900.06Comparative Example742251400.890.08Comparative Example752501400.820.01Invention Example762601400.820.02Invention Example772701400.830.04Invention Example782801400.840.02Invention Example792901400.860.01Invention Example803001400.840.01Invention Example813501400.830.01Invention Example824001400.840.03Invention Example835001400.830.01Invention Example846001400.850.04Invention Example852001500.990.07Comparative Example862251500.970.10Comparative Example872501500.840.04Invention Example882601500.830.01Invention Example892701500.830.04Invention Example902801500.820.02Invention Example912901500.850.02Invention Example923001500.820.02Invention Example933501500.840.02Invention Example944001500.820.03Invention Example955001500.850.04Invention Example966001500.850.02Invention Example [Table 3-3] ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values972001600.880.08Comparative Example982251600.910.07Comparative Example992501600.830.09Comparative Example1002601600.850.07Comparative Example1012701600.840.06Comparative Example1022801600.840.09Comparative Example1032901600.840.10Comparative Example1043001600.850.08Comparative Example1053501600.850.08Comparative Example1064001600.840.10Comparative Example1075001600.850.10Comparative Example1086001600.830.08Comparative Example1092001700.970.06Comparative Example1102251700.900.10Comparative Example1112501700.840.06Comparative Example1122601700.830.07Comparative Example1132701700.850.09Comparative Example1142801700.850.07Comparative Example1152901700.820.08Comparative Example1163001700.840.08Comparative Example1173501700.850.09Comparative Example1184001700.840.09Comparative Example1195001700.840.07Comparative Example1206001700.860.07Comparative Example1212001800.890.10Comparative Example1222251800.900.08Comparative Example1232501800.860.08Comparative Example1242601800.850.09Comparative Example1252701800.850.09Comparative Example1262801800.840.07Comparative Example1272901800.830.08Comparative Example1283001800.860.07Comparative Example1293501800.860.09Comparative Example1304001800.860.09Comparative Example1315001800.820.09Comparative Example1326001800.830.09Comparative Example1332001900.940.09Comparative Example1342251900.920.10Comparative Example1352501900.850.08Comparative Example1362601900.840.09Comparative Example1372701900.830.08Comparative Example1382801900.860.08Comparative Example1392901900.860.09Comparative Example1403001900.840.10Comparative Example1413501900.860.10Comparative Example1424001900.820.08Comparative Example1435001900.840.06Comparative Example1446001900.820.09Comparative Example Example 2

[0067] A steel slab, which contains inhibitor-forming components and has a composition of components including C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol. Al: 0.0250 mass%, N: 0.0090 mass%, S: 0.01 mass%, and Se: 0.01 mass%, with the balance being Fe and unavoidable impurities, was heated to 1400°C and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to the first cold rolling to reach an intermediate thickness of 1.2 mm. Then, the resulting sheet was subjected to intermediate annealing at 1100°C for 80 seconds in an atmosphere consisting of N 2 : 75 vol% and H 2 : 25 vol% with a dew point of 46°C, and then the second cold rolling (final cold rolling) using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0068] Subsequently, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of decarburization annealing, rapid heating was performed using a transverse induction heating device at an average heating rate of 300 °C / s within the temperature range of 500°C to 700 °C. When the steel sheet temperature reached 650 °C during the induction heating, parameters such as the output of the induction heating device and the line speed were adjusted so that the period for reduced heating, during which the heating rate T reached 100 °C / s in the sheet width direction, satisfied the six conditions shown in Fig. 2. After the decarburization annealing, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet, followed by finishing annealing to induce secondary recrystallization. Thereafter, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing, followed by flattening annealing at 800°C for 30 seconds to bake the coating and thereby obtain a product sheet.

[0069] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 4 presents the results, demonstrating that the steel sheets formed by reducing the heating rate during rapid heating, under the condition that Expression (1) below is satisfied at all positions across the sheet width, exhibit a difference of 0.04 W / kg or less between the maximum and minimum iron loss values in the sheet width direction: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w,

[0070] Accordingly, even when a grain-oriented electrical steel sheet is produced using a material containing inhibitor-forming components, uniform magnetic properties across the sheet width can be achieved by applying the present invention. [Table 4]ConditionsDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarks10.02Invention Example20.03Invention Example30.02Invention Example40.06Comparative Example50.07Comparative Example60.05Comparative Example Example 3

[0071] Steel that contains no inhibitor-forming components and has a composition of components including C: 0.036 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol.Al: 0.0072 mass%, N: 0.0050 mass%, S: 0.0031 mass%, Se: 0.0031 mass% as well as the other components including Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, with the composition presented in Table 5, with the balance being Fe and unavoidable impurities, was melted to form a steel slab. The steel slab was heated to 1210°C and hot-rolled to produce a hot-rolled sheet with a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once (final cold-rolling step) using a tandem rolling mill to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0072] The cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of the decarburization annealing, as in Example 2, rapid heating was performed using a transverse induction heating device at the average heating rate in the temperature range of 500°C to 700°C of 300°C / s, and when the temperature of the steel sheet reached 650°C, the period during which the heating rate was reduced to 110°C / s at each position in the sheet width direction was set to satisfy the same condition as the condition of No. 1 in Fig. 2. An annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. Subsequently, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing. Thereafter, flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet.

[0073] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 5 presents the results, in which the product sheets produced using, as a steel material, a slab containing at least one element selected from the group consisting of Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, and subjected to heating during the decarburization annealing using a transverse induction heating device under the conditions compliant with the method of the present invention, exhibit a mean iron loss value in the sheet width direction of 0.82 W / kg or less, and the difference between the maximum and minimum values of the iron loss values in the sheet width direction of 0.04 W / kg or less, thereby demonstrating excellent and uniform magnetic properties across the sheet width. [Table 5]Steel symbolOther steel components (mass%)Iron loss W 17 / 50 in sheet width directionRemarksSbCuPCrNiSnNbMoBBiMean value (W / kg)Difference (W / kg) between maximum and minimum valuesA----------0.870.04Invention ExampleB0.010.050.05-------0.810.02Invention ExampleC0.01---0.050-0.005---0.800.02Invention ExampleD0.14------0.220.004-0.800.04Invention ExampleE0.350.220.12--0.12----0.810.02Invention ExampleF-0.050.050.02------0.820.03Invention ExampleG-0.78---0.43-0.37--0.820.04Invention ExampleH-1.31-0.270.420-----0.790.03Invention ExampleI--0.30---0.002-0.0050.00200.810.01Invention ExampleJ---1.01------0.810.01Invention ExampleK0.42--1.310.890-----0.790.03Invention ExampleL----1.350-----0.790.01Invention ExampleM--0.42-0.007--0.11-0.00100.810.03Invention ExampleN---------0.00070.820.03Invention Example0----0.0500.020.005---0.800.03Invention ExampleP-------0.020.0020.01000.810.02Invention ExampleQ-0.050.05-0.050-----0.800.04Invention ExampleR-----0.020.005--0.01000.810.03Invention ExampleS---0.02---0.020.002-0.820.04Invention ExampleT-0.05---0.02---0.01000.810.04Invention ExampleU---0.02-0.02-0.02--0.800.02Invention ExampleV--0.05-------0.820.03Invention Examplew----0.050-----0.820.03Invention ExampleX--------0.002-0.810.02Invention Example

Examples

example 1

[0064]A Steel slab having a composition of components including C: 0.035 mass%, Si: 3.3 mass%, Mn: 0.05 mass%, sol. Al: 0.0084 mass%, N: 0.0051 mass%, S: 0.0031 mass%, and Se: 0.0031 mass%, with the balance being Fe and unavoidable impurities was heated to 1260°C and then hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0065]Thereafter, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During a heating process of the decarburization annealing, the average heating rate T (°C / s) was varied in the temperature range from 500°C to 700°C, as shown in Table 3. For certain steel sheets, when the temperature of the steel sheet reached 620°C, the heating ...

example 2

[0067]A steel slab, which contains inhibitor-forming components and has a composition of components including C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol. Al: 0.0250 mass%, N: 0.0090 mass%, S: 0.01 mass%, and Se: 0.01 mass%, with the balance being Fe and unavoidable impurities, was heated to 1400°C and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to the first cold rolling to reach an intermediate thickness of 1.2 mm. Then, the resulting sheet was subjected to intermediate annealing at 1100°C for 80 seconds in an atmosphere consisting of N 2 : 75 vol% and H 2 : 25 vol% with a dew point of 46°C, and then the second cold rolling (final cold rolling) using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0068]Subsequently, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for...

example 3

[0071]Steel that contains no inhibitor-forming components and has a composition of components including C: 0.036 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol.Al: 0.0072 mass%, N: 0.0050 mass%, S: 0.0031 mass%, Se: 0.0031 mass% as well as the other components including Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, with the composition presented in Table 5, with the balance being Fe and unavoidable impurities, was melted to form a steel slab. The steel slab was heated to 1210°C and hot-rolled to produce a hot-rolled sheet with a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once (final cold-rolling step) using a tandem rolling mill to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0072]The cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. Du...

Technical Field

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet, and an induction heating device for use in decarburization annealing of the production method.Background Art

[0002] A grain-oriented electrical steel sheet is a soft magnetic material that is mainly used as an iron core material of a transformer or a generator, etc., and is a steel sheet with excellent magnetic properties including low iron loss and high magnetic flux density as it has a crystal structure in which the {110}<001> orientation (i.e., Goss orientation), which corresponds to the magnetic easy axis of iron, is highly aligned along the rolling direction of the steel sheet.

[0003] Methods for further reducing iron loss of grain-oriented electrical steel sheets include promoting a high degree of crystal grain alignment in the Goss orientation, following secondary recrystallization annealing. To increase the alignment degree of secondary recrystallized grains in the Goss orientation, it is important to form a large number of Goss-oriented grains in the steel sheet texture at the completion of primary recrystallization, and to introduce a difference in grain boundary mobility to preferentially promote the growth of sharp Goss-oriented grains during secondary recrystallization, in other words, to optimize the steel sheet texture after primary recrystallization.

[0004] Examples of primary recrystallization textures that promote the preferential growth of sharp Goss-oriented grains include {111}<112> oriented grains and {411}<148> oriented grains. Ensuring that these grains are present in a primary recrystallization texture in a good balance and with high frequency facilitates strong alignment of Goss-oriented grains in the rolling direction during secondary recrystallization annealing.

[0005] As a method for increasing the fraction of Goss-oriented grains in a primary recrystallization texture, for example, Patent Literature 1 discloses a method of performing aging treatment by applying heat treatment at a low temperature to a cold-rolled sheet during cold rolling. Patent Literature 2 discloses a method in which the cooling rate is set to 30°C / s or greater during either hot-rolled sheet annealing or intermediate annealing (final cold rolling) performed prior to cold rolling to obtain the final thickness, and in which in-pass aging is performed two or more times by holding the steel sheet at a temperature of 150 to 300°C for 2 minutes or longer during the final cold-rolling. Patent Literature 3 discloses a method involving performing warm rolling by raising the temperature of the steel sheet during cold rolling.

[0006] The methods of Patent Literatures 1 to 3 are each aimed to increase the temperature of a steel sheet to an appropriate level before or during cold rolling, or between consecutive passes of cold rolling. This promotes the diffusion of dissolved elements, such as carbon (C) and nitrogen (N), to pin dislocations introduced by cold rolling and thereby suppress the migration of the dislocations during the following rolling process, so as to promote the shear deformation and thus improve the rolled texture. This is based on the view that the nuclei of Goss-oriented grains in a primary recrystallization texture appear from shear bands introduced into a processed texture having a {111}<112> orientation. Applying these methods can introduce a large number of shear bands into the {111}<112> processed texture and thereby form a large number of Goss-oriented grains in the primary recrystallization texture.

[0007] Increasing the heating rate of a steel sheet during heating in decarburization annealing also promotes the formation of Goss-oriented grains in a primary recrystallization texture. For example, Patent Literature 4 discloses a method involving rapidly heating a steel sheet during heating in decarburization annealing. This method is aimed to suppress the development of a γ-fiber texture ({111} / / ND), which is preferentially formed at a regular heating rate. This is achieved by heating a steel sheet from the room temperature to a temperature around the recrystallization temperature in a short time using electric heating, induction heating, etc., promoting the generation of Goss-oriented grains as the nuclei of secondary recrystallized grains.

[0008] Patent Literature 5 discloses a method that involves rapidly heating a steel sheet at an average heating rate of 50°C / s or greater in the temperature range of 550 to 700°C during a heating process of decarburization annealing and maintaining the heating rate of 10°C / s or less for 1 to 10 seconds within a selected temperature range between 250 and 550°C. The method is aimed at promoting the recovery of the {111} processed texture by holding the steel sheet in the recovery temperature range of 250 to 550°C for a short time and thereby suppressing recrystallization to increase the relative fraction of Goss-oriented grains.Citation ListPatent Literature

[0009] Patent Literature 1: JP-A-S50-016610 Patent Literature 2: JP-A-H08-253816 Patent Literature 3: JP-A-H01-215925 Patent Literature 4: JP-A-H04-160114 Patent Literature 5: JP-A-2014-152393 Summary of InventionTechnical Problem

[0010] The primary recrystallization texture of a steel sheet is often not uniform in the sheet width direction. This is considered to be due to factors such as uneven rolling reduction during cold rolling caused by edge drops formed in hot rolling, or non-uniform heating across the sheet width during hot-rolled sheet annealing and the like, resulting in uneven crystal grain sizes across the sheet width before cold rolling. In addition, when warm rolling is applied as the cold rolling as in the methods disclosed in Patent Literatures 1 to 3 above, a large temperature drop occurs at edge portions of the steel sheet due to heat radiation, varying the diffusion distance of carbon and nitrogen in the sheet width direction. This is also considered to be a cause of a change in the texture.

[0011] If the primary recrystallization texture varies across the sheet width, the secondary recrystallization behavior will also vary accordingly, leading to variation in the magnetic properties of the final product across the sheet width.. To prevent this, trimming the edge portions of the steel sheet after hot rolling or cold rolling may be effective. However, this approach would inevitably reduce yield.

[0012] The present invention has been made in view of the foregoing problems of the conventional methods, and it is an object of the present invention to propose a method for producing a grain-oriented electrical steel sheet having excellent and uniform magnetic properties in the sheet width direction, and an induction heating device for decarburization annealing for use in such a production method.Solution to Problem

[0013] To address the above problems, the inventors conducted focused studies on methods for achieving a uniform primary recrystallization texture across the sheet width. As a result, the inventors discovered that such uniform primary recrystallization texture after decarburization annealing across the sheet width can be obtained by: performing at least one rolling pass at a steel sheet temperature of 150 °C or higher during final cold rolling; temporarily reducing the heating rate within the temperature range of 500 °C to 700 °C during rapid heating in decarburization annealing, which serves as primary recrystallization annealing; and varying the period for the reduced heating rate across the sheet width.

[0014] The present invention based on the foregoing findings proposes a method for producing a grain-oriented electrical steel sheet, including hot-rolling a steel material to form a hot-rolled sheet; subjecting the hot-rolled sheet to cold rolling including a single cold rolling step or two or more cold rolling steps with intermediate annealing interposed between each rolling step, to form a cold-rolled sheet with a final thickness; and subjecting the cold-rolled sheet to decarburization annealing also serving as primary recrystallization annealing, followed by finishing annealing to induce secondary recrystallization, characterized in that in the decarburization annealing, an average heating rate T (°C / s) is 250°C / s or greater within a temperature range of 500°C to 700° during heating, and within a selected temperature range between 500°C and 700°, a heating rate at each position across the sheet width is reduced to 150°C / s or less for a period t (s) that satisfies Expression (1) below, in accordance with a value of x / w of the position: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents a distance (mm) from a center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w.

[0015] The steel material used for the method for producing a grain-oriented electrical steel sheet of the present invention has a composition of components including C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: 0.0100 to 0.0400 mass%, and N: 0.0050 to 0.0120 mass%, and further including at least one of S and Se in a total amount of 0.01 to 0.05 mass%, with a balance being Fe and unavoidable impurities.

[0016] The steel material used for the foregoing method for producing a grain-oriented electrical steel sheet of the present invention has a composition of components including C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: less than 0.0100 mass%, and Se: less than 0.0100 mass%, with a balance being Fe and unavoidable impurities.

[0017] The steel material used for the foregoing method for producing a grain-oriented electrical steel sheet of the present invention further includes, in addition to the composition of components, at least one component selected from the group consisting of Sb: 0.005 to 0.500 mass%, Cu: 0.01 to 1.50 mass%, P: 0.005 to 0.500 mass%, Cr: 0.01 to 1.50 mass%, Ni: 0.005 to 1.500 mass%, Sn: 0.01 to 0.50 mass%, Nb: 0.0005 to 0.0100 mass%, Mo: 0.01 to 0.50 mass%, B: 0.0010 to 0.0070 mass%, and Bi: 0.0005 to 0.0500 mass%.

[0018] In the foregoing method for producing a grain-oriented electrical steel sheet of the present invention, rapid heating in the decarburization annealing is performed using a transverse induction heating device.

[0019] The present invention also provides a transverse induction heating device for use in rapid heating of the decarburization annealing in the foregoing method for producing a grain-oriented electrical steel sheet.Advantageous Effects of Invention

[0020] According to the present invention, it is possible to stably produce a grain-oriented electrical steel sheet having magnetic properties excellent and uniform in the sheet width direction, which greatly contributes to improving the quality of a product sheet and increasing yields.Brief Description of Drawings

[0021] [Fig. 1] is a graph illustrating the period during which the heating rate was reduced in the sheet width direction, in accordance with the present invention. [Fig. 2] is another graph illustrating the period during which the heating rate was reduced in the sheet width direction, in accordance with the present invention. [Fig. 3] is a schematic view illustrating a transverse induction heating device. Description of Embodiments

[0022] First, that experiments that led to the development of the present invention will be described.<Experiment 1>

[0023] A steel slab having a composition of components including C: 0.033 mass%, Si: 3.4 mass%, Mn: 0.07 mass%, sol. Al: 0.0081 mass%, N: 0.0052 mass%, S: 0.0030 mass%, and Se: 0.0030 mass%, with the balance being Fe and unavoidable impurities was heated to 1220°C, and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds and then cold rolling once to produce a cold-rolled sheet with a final thickness of 0.20 mm. Thereafter, a sample material was taken from the cold-rolled sheet, and a plurality of samples each having the size of an Epstein test piece (a width of 30 mm × a length of 280 mm) were taken at five positions of x=0, 0.2w, 0.4w, 0.6w, and 0.8w of the sample material in the sheet width direction, provided that the distance from the center in the sheet width direction is represented by x (mm) (where the center in the sheet width direction is x = 0), and 1 / 2 of the sheet width is represented by w (mm).

[0024] Subsequently, each sample was subjected to decarburization annealing also serving as primary recrystallization annealing at a soaking temperature of 850°C for a soaking time of 100 seconds. During heating of the decarburization annealing, the average heating rate within the temperature range of 500°C to 700°C was set to 300°C / s. When the sample temperature reached 600°C under certain conditions, the heating rate was reduced to 120°C / s for the period indicated in Table 1. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the sample after decarburization annealing, followed by finishing annealing to induce secondary recrystallization. After the finishing annealing, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the sample, and heat treatment simulating flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet sample. Note that the average heating rate of 300°C / s within the temperature range of 500°C to 700°C refers to the average heating rate over the duration excluding the period during which the heating rate was being reduced.

[0025] For each product sheet sample sized according to the Epstein test piece and prepared as described above, the iron loss W 17 / 50 was measured in accordance with JIS Z 2550. The difference between the maximum value and the minimum value of the iron loss values in the sheet width direction was determined. The results are shown in Table 1. [Table 1]ConditionsPeriod(s) for reduced heating timeDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarksx=0x=0.2wx=0.4wx=0.6wx=0.8w10.000.000.000.000.000.06Comparative Example20.100.100.100.100.100.07Comparative Example30.300.300.300.300.300.08Comparative Example40.500.500.500.500.500.07Comparative Example50.100.200.300.400.500.09Comparative Example60.200.400.000.300.200.07Comparative Example70.500.400.300.200.100.04Invention Example80.300.300.200.100.050.03Invention Example90.400.300.200.150.100.03Invention Example

[0026] Table 1 demonstrates that, under conditions where a period for temporarily reduced heating rate was provided during rapid heating within the temperature range of 500 to 700 °C, and the duration of said period was set to be longer at a central portion in the sheet width direction and shorter at the edge portions thereof, the difference between the maximum and minimum iron loss values in the sheet width direction was 0.04 W / kg or less. Accordingly, magnetic properties substantially uniform across the sheet width direction were obtained.

[0027] The present inventors consider the following as the reason why magnetic properties were uniform in the sheet width direction under conditions in which the period for temporarily reduced heating rate during rapid heating was set to be longer at the central portion and shorter at the edge portions in the sheet width direction.

[0028] The cold rolling reduction at the edge portions in the sheet width direction is smaller than that at the central portion, due to edge drops caused during hot rolling or the like. Accordingly, the amount of deformation during cold rolling is smaller at the edge portions in the sheet width direction. Furthermore, the temperature of the steel sheet during cold rolling tends to be lower at the edge portions in the sheet width direction than at the central portion, due to heat radiation. This results in a shorter diffusion distance of carbon and nitrogen in the steel at the edge portions in the sheet width direction, making it difficult to pin dislocations formed during rolling. Consequently, the cold-rolled microstructure at the edge portions in the sheet width direction contains a smaller amount of introduced shear bands, which serve as sites for the formation of Goss-oriented grains during primary recrystallization, compared to the central portion in the sheet width direction. Therefore, the recrystallization of Goss-oriented grains is promoted at the edge portions in the sheet width direction by shortening the period for the reduced heating rate, while it is suppressed at the central portion in the sheet width direction by prolonging the said period. Accordingly, it is considered that the number of Goss-oriented grains after primary recrystallization annealing has become uniform across the sheet width.

[0029] Next, based on the experiment results, the inventors conducted an experiment to study the appropriate period for the reduced heating rate in accordance with the position in the sheet width direction.<Experiment 2>

[0030] The five types of samples having the size of an Epstein test piece, taken from different positions in the sheet width direction of the cold-rolled sheet obtained in Experiment 1, were subjected to decarburization annealing serving also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of the decarburization annealing, the average heating rate within the temperature range of 500°C to 700°C was set to 300°C / s. When the temperature of each sample reached 650°C, the period during which the heating rate was reduced to 110°C / s at each position in the sheet width direction was varied in eight conditions as shown in Fig. 1. After the decarburization annealing, an annealing separating agent mainly composed of MgO was applied to the surface of the sample, followed by finishing annealing to induce secondary recrystallization. Subsequently, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the sample after finishing annealing, and heat treatment simulating flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet sample.

[0031] For each product sheet sample having the size of an Epstein test piece thus obtained, the iron loss W 17 / 50 was measured according to JIS Z 2550. Table 2 shows the difference between the maximum value and the minimum value of the iron loss values in the sheet width direction. [Table 2]ConditionsDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarks10.04Invention Example20.03Invention Example30.03Invention Example40.02Invention Example50.07Comparative Example60.06Comparative Example70.10Comparative Example80.08Comparative Example

[0032] Table 2 demonstrates that, under conditions in which a period t of reduced heating rate at each of positions x=0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction satisfies Expression (1) below: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0≤x≤0.9w. the difference between the maximum and minimum iron loss values in the sheet width direction was 0.04 W / kg or less. Accordingly, magnetic properties uniform across the sheet width direction were obtained.

[0033] The present invention has been completed by adding further consideration to the foregoing new findings.

[0034] The component composition of a steel material (slab) used for producing a grain-oriented electrical steel sheet according to the present invention is described. It should be noted that conventionally known steel materials used in the production of grain-oriented electrical steel sheets may also be employed in the present invention. However, from the viewpoint of achieving superior magnetic properties, a steel material having the following composition is preferably used.C: 0.01 to 0.10 mass%

[0035] C is precipitated as fine carbide particles and thereby contributes to improving a primary recrystallization texture. However, if the content of C falls below 0.01 mass%, the precipitation amount of fine carbide particles is insufficient, and the effect of improving the texture may not be fully obtained. Meanwhile, if the content of C exceeds 0.10 mass%, it may be difficult to reduce the content of C to 0.0050 mass% or less, with which no magnetic aging occurs during decarburization annealing. Accordingly, the content of C should be set within the range of 0.01 to 0.10 mass% inclusive. Preferably, the content of C should be set within the range of 0.015 to 0.08 mass% inclusive.Si: 2.0 to 4.5 mass%

[0036] Si is an element effective in increasing the specific resistance of steel and thereby improving iron loss properties. However, if the content of Si is less than 2.0 mass%, the effect of reducing iron loss cannot be fully obtained. Meanwhile, the content of Si exceeding 4.5 mass% significantly deteriorates workability, making it difficult to produce a steel sheet by rolling. Accordingly, the content of Si should be set in the range of 2.0 to 4.5 mass% inclusive. Preferably, it should be set in the range of 2.5 to 4.0 mass% inclusive.Mn: 0.01 to 0.50 mass%

[0037] Mn is an element essential for to improving hot workability. If the content of Mn is less than 0.01 mass%, the effect of improving hot workability can hardly be obtained. Meanwhile, the content of Mn exceeding 0.50 mass% may deteriorate primary recrystallization texture, making it difficult to obtain secondary recrystallized grains with highly aligned Goss-oriented grains. Accordingly, the content of Mn should be set in the range of 0.01 to 0.50 mass% inclusive. Preferably, it should be in the range of 0.03 to 0.45 mass% inclusive.

[0038] Components other than C, Si, and Mn described above vary depending on whether an inhibitor is used for secondary recrystallization.

[0039] Specifically, when an inhibitor is used for secondary recrystallization and AlN is employed as the inhibitor, the steel material should contain Al and N in the range of Al: 0.0100 to 0.0400 mass% inclusive and N: 0.0050 to 0.0120 mass% inclusive in addition to C, Si, and Mn described above. If each of the content of Al and the content of N falls below its lower limit, it becomes difficult to achieve an intended effect of the inhibitor. Meanwhile, if each of the content of Al and the content of N exceeds its upper limit, the precipitates may be unevenly dispersed, which also hinders the predetermined effect of the inhibitor.

[0040] Furthermore, when sulfide (e.g., MnS or Cu 2 S) and / or selenide (e.g., MnSe or Cu 2 Se) are / is used as inhibitors, in addition to AlN described above, at least one of S and Se should be contained in a total amount of 0.0100 to 0.0500 mass%, as inhibitor-forming components, along with Al and N. If the total content of S and Se falls below its lower limit described above, it becomes difficult to fully achieve the effect of the inhibitor. Meanwhile, if the total content of S and Se exceeds its upper limit described above, precipitates are unevenly dispersed, which also hinders the effect of inhibitors. It should be noted that sulfide and selenide described above may undergo complex precipitation.

[0041] Meanwhile, when no inhibitor is used for secondary recrystallization, the inhibitor-forming components should be minimized. Specifically, the following ranges are preferable: Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: less than 0.0100 mass%, and Se: less than 0.0100 mass%.

[0042] The steel material used in the production of a grain-oriented electrical steel sheet according to the present invention has the aforementioned basic component composition, with the balance being substantially Fe and unavoidable impurities. However, in order to enhance magnetic properties, the steel material may further contain, in addition to the aforementioned components, at least one element selected from the group consisting of Sb: 0.005 to 0.500 mass%, Cu: 0.01 to 1.50 mass%, P: 0.005 to 0.500 mass%, Cr: 0.01 to 1.50 mass%, Ni: 0.005 to 1.500 mass%, Sn: 0.01 to 0.50 mass%, Nb: 0.0005 to 0.0100 mass%, Mo: 0.01 to 0.50 mass%, B: 0.0010 to 0.0070 mass%, and Bi: 0.0005 to 0.0500 mass%. Each of Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi is effective in improving magnetic properties. When present within the aforementioned ranges, these components achieve the effect of improving magnetic properties without hindering the development of secondary recrystallized grains.

[0043] Next, a method for producing a grain-oriented electrical steel sheet according to the present invention will be described.

[0044] A steel material (slab) used in the production of a grain-oriented electrical steel sheet according to the present invention is preferably prepared by subjecting molten steel, obtained in a converter, an electric furnace, or the like, to a commonly known refining process involving secondary refining such as vacuum degassing, to form steel having the aforementioned composition of components, followed by a commonly known continuous casting process or ingot making-blooming process.

[0045] Next, the steel material (slab) is heated to a predetermined temperature and then hot-rolled to form a hot-rolled sheet. The slab heating temperature is preferably set approximately at 1050°C or higher to ensure hot rollability. The upper limit of the heating temperature is not limited to a particular value. However, if the heating temperature exceeds 1450°C, it approaches the melting temperature of the steel, making it difficult to maintain the shape of the slab. Therefore, the heating temperature is preferably set at 1450°C or lower.

[0046] The hot rolling following the slab heating may be performed under commonly known conditions, which are not particularly limited.

[0047] Next, the hot-rolled steel sheet (hot-rolled sheet) may be subjected to hot-rolled sheet annealing as appropriate. Note that when hot-rolled sheet annealing is performed, commonly known conditions may be applied, and the conditions are not limited to particular conditions.

[0048] The hot-rolled steel sheet or the hot-rolled-annealed steel sheet is descaled as appropriate by pickling, a mechanical method, or the like and cold-rolled to produce a cold-rolled sheet with a final thickness (thickness of the product sheet). The cold rolling may consist of a single cold-rolling step to obtain a cold-rolled sheet with a final thickness, or two or more cold-rolling steps with intermediate annealing interposed between each step, to obtain a cold-rolled sheet with a final thickness. It should be noted that the final thickness preferably falls within the range of 0.1 mm to 1.0 mm inclusive.

[0049] The rolling reduction in the final cold-rolling step preferably falls within the range of 60% to 95%, inclusive. The term 'final cold-rolling step' refers to the cold-rolling step performed last, whether in a single cold-rolling step or in two or more cold-rolling steps. In cases where cold rolling is performed only once, that step corresponds to the final cold-rolling step. In cases where two or more cold-rolling steps are performed, the cold-rolling step conducted last corresponds to the final cold-rolling step.

[0050] In the cold rolling process, in order to improve magnetic properties by forming numerous recrystallization nuclei of Goss-oriented grains within the primary recrystallized structure, it is preferable to apply heat treatments such as inter-pass aging or to adopt warm rolling, as described in Patent Literatures 1 to 3.

[0051] Next, the cold-rolled sheet with the final thickness is subjected to decarburization annealing, which also serves as primary recrystallization annealing. The decarburization conditions (conditions at soaking) of the decarburization annealing are not limited to particular conditions, and known conditions may be applied. For example, such conditions of 720 to 870°C for 60 to 150 seconds in a wet hydrogen atmosphere are preferably. This decarburization annealing can reduce the content of C in the steel sheet to 0.0050 mass% or less, at which magnetic aging is unlikely to occur.

[0052] Herein, it is important to perform rapid heating at an average heating rate of 250°C / s or greater within the temperature range of 500°C to 700°C during the heating process of the decarburization annealing, until the soaking temperature is reached. In the present invention, the average heating rate within the temperature range of 500°C to 700°C refers to the average heating rate over the time, excluding the period during which the heating rate was being temporarily reduced, which will be described later. If the average heating rate is less than 250°C / s, sufficient Goss-oriented grains are not formed after primary recrystallization, making it unlikely to achieve favorable iron loss. The preferable average heating rate is 300°C / s or greater. It should be noted that rapid heating may be performed in a range other than the temperature range of 500°C to 700°C.

[0053] In the heating process of the decarburization annealing, it is necessary to provide a period during which the heating rate is temporarily reduced to 150°C / s or less within a selected temperature range between 500 and 700°C during the rapid heating. If the temperature of the steel sheet at which the heating rate is reduced is below 500°C, the recrystallization behavior of Goss-oriented grains is unlikely to change, even with a decreased heating rate. As a result, the effect of adjusting the number of Goss-oriented grains through primary recrystallization cannot be obtained. Meanwhile, if the steel sheet temperature at which the heating rate is decreased exceeds 700°C, recrystallization is nearly complete by the time the heating rate is reduced. Therefore, the effect of adjusting the number of Goss-oriented grains through primary recrystallization cannot be achieved.

[0054] Furthermore, in the present invention, it is necessary to vary the period during which the heating rate is temporarily reduced, in accordance with the position in the sheet width direction. The present invention relates to a method for eliminating variation in the microstructure of a steel sheet in the sheet width direction, which occurred due to various production conditions of processes prior to decarburization annealing, by varying the period for temporarily decreasing the heating rate in accordance with a position in the sheet width direction during rapid heating of the decarburization annealing to cause uniform recrystallization of Goss-oriented grains in the sheet width direction in primary recrystallization.

[0055] The period t of temporarily reduced heating rate to 150°C / s or less during the rapid heating is set longer on the side of the central portion in the sheet width direction and shorter on edge portions in the sheet width direction. Specifically, it is important to vary the period t so as to satisfy Expression (1) below: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w.

[0056] If t is shorter than the left-hand side of Expression (1) above, the number of corresponding Goss-oriented grains increases excessively, so that the iron loss properties are partially degrade. Conversely, if t is longer than the right-hand side of Expression (1) above, the recrystallization of Goss-oriented grains is suppressed correspondingly, so that iron loss properties partially increase. Consequently, it becomes impossible to obtain magnetic properties uniform in the sheet width direction. The reason the range of the sheet width that satisfies Expression (1) above is set to (0 ≤ x ≤ 0.9w) is that, when transverse induction heating is used for rapid heating, an induced current tends to concentrate at the edge portions in the sheet width direction, which may make it difficult to satisfy Expression (1) across the full width. Needless to say, it is preferable for Expression (1) to be satisfied across the entire sheet width.

[0057] The temporarily reduced heating rate needs to be 150°C / s or less. If the temporarily reduced heating rate exceeds this value, the effect of suppressing the recrystallization of Goss-oriented grains cannot be sufficiently achieved. It should be noted that the lower limit of the reduced heating rate is not limited to a particular value, but is preferably 10°C / s or greater. The period for the temporarily reduced heating rate can be determined by measuring the steel sheet temperature during the heating process, using a thermocouple, for example, and then calculating the temporal derivative of the temperature over time.

[0058] Herein, the rapid heating performed during the heating process of the decarburization annealing as well as reduction in the heating rate during the rapid heating can be achieved by arranging two or more rapid heating devices, such as electric heating devices or solenoid induction heating devices, in series in the threading direction of the steel sheet. The heating rate can be reduced in the section between any two of these devices, and further adjusted by appropriately controlling the outputs of the heating devices and the threading speed (line speed) of the steel sheet. In addition, an edge heater and the like may be provided in the section where the heating rate is decreased, from the perspective of preventing heat radiation at the edge portions in the sheet width direction.

[0059] Arranging two or more rapid heating devices in series, as described above, presents challenges in terms of cost and space. However, as schematically illustrated in Fig. 3, when using a transverse induction heating device as the rapid heating device, where heating coils wound around an iron core are arranged above and below the steel sheet, an alternating magnetic flux generated within the iron core penetrates the steel sheet in the thickness direction, heating it through the action of the magnetic field. In this configuration, the induced current flows across the sheet plane following the shape of the heating coils, rather than through the portions of the steel sheet directly facing the iron core. Accordingly, when the steel sheet passes in the vicinity of the iron core, the heating rate temporarily decreases. This phenomenon may be utilized to intentionally reduce the heating rate. In addition, the period for the reduced heating rate can be adjusted by controlling the output of the induction heating device, the line speed, or the like. Further, an induced current flows through the edge portions in the sheet width direction, where heat radiation can be suppressed. Furthermore, since the decrease in heating rate occurs within a single induction heating device, there is no concern regarding installation space. Accordingly, a transverse induction heating device is preferably employed in the present invention.

[0060] To vary the period for the reduced heating rate in accordance with a position in the sheet width direction when using the transverse induction heating device, the coil diameter may be set larger at the central portion in the sheet width direction and sequentially reduced toward the steel width edge portions. The shape of the heating coil of the transverse induction heating device may be round, quadrangular, or elliptical shapes, for example. However, as described above, the coil diameter in the threading direction is preferably varied across the sheet width.

[0061] Thereafter, an annealing separating agent is applied to the surface of the cold-rolled steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. The annealing separating agent can may be any known type and is not limited to a particular one. Examples of the annealing separating agent include an agent mainly composed of MgO and also containing an auxiliary agent such as TiO 2 , as appropriate, and an agent mainly composed of SiO 2 or Al 2 O 3 .

[0062] Unreacted portions of the annealing separating agent remaining on the surface of the steel sheet surface after the finishing annealing, are removed. Thereafter, it is preferable to apply an insulation coating solution to the surface of the steel sheet, followed by flattening annealing to simultaneously bake the coating and correct the shape of the steel sheet, which may have been deformed during finishing annealing, thereby obtaining a product sheet. It should be noted that the insulation coating may be formed through a different line. The type of the insulation coating is not limited to a particular type. However, to form a tension-imparting insulation coating, which imparts tension to the surface of the steel sheet, it is preferable to apply slurry containing phosphate and colloidal silica to the surface of the steel sheet and then bake the slurry at a temperature of about 800°C as disclosed in Japanese Patent Laid-Open No. 50-79442, Japanese Patent Laid-Open No. 48-39338, Japanese Patent Laid-Open No. 56-75579, and the like.

[0063] When a further reduction of iron loss is desired, magnetic domain subdividing treatment may be performed using a known method. Such methods include forming grooves in the surface of the steel sheet during any of the process following the cold rolling, mechanically introducing strain regions into the surface, or creating thermal strain regions by irradiating the surface with a laser beam or an electron beam, for example, after finishing annealing.Example 1

[0064] A Steel slab having a composition of components including C: 0.035 mass%, Si: 3.3 mass%, Mn: 0.05 mass%, sol. Al: 0.0084 mass%, N: 0.0051 mass%, S: 0.0031 mass%, and Se: 0.0031 mass%, with the balance being Fe and unavoidable impurities was heated to 1260°C and then hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0065] Thereafter, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During a heating process of the decarburization annealing, the average heating rate T (°C / s) was varied in the temperature range from 500°C to 700°C, as shown in Table 3. For certain steel sheets, when the temperature of the steel sheet reached 620°C, the heating rate at each position in the sheet width direction was reduced for a period t, defined by Expression (2) below, in order to achieve the "reduced heating rate" shown in Table 3: t = 200 / T × 0.5 × 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w. Thereafter, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. An insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing, followed by flattening annealing at 800°C for 30 seconds to bake the coating and thereby obtain a product sheet.

[0066] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 3 presents the results, demonstrating that under conditions in which the average heating rate is 250 °C / s or greater and the reduced heating rate is 150 °C / s or less, the mean iron loss value is as low as 0.87 W / kg or less, and the variation in iron loss is suppressed to 0.04 W / kg or less. [Table 3-1]ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values1200500.910.07Comparative Example2225500.880.07Comparative Example3250500.830.02Invention Example4260500.860.02Invention Example5270500.820.04Invention Example6280500.840.01Invention Example7290500.830.02Invention Example8300500.840.02Invention Example9350500.850.02Invention Example10400500.840.01Invention Example11500500.840.03Invention Example12600500.830.02Invention Example13200750.890.08Comparative Example14225750.990.08Comparative Example15250750.830.02Invention Example16260750.840.04Invention Example17270750.860.01Invention Example18280750.860.01Invention Example19290750.850.03Invention Example20300750.840.01Invention Example21350750.860.02Invention Example22400750.820.03Invention Example23500750.830.01Invention Example24600750.860.01Invention Example252001000.960.09Comparative Example262251000.910.09Comparative Example272501000.830.01Invention Example282601000.830.02Invention Example292701000.840.02Invention Example302801000.820.01Invention Example312901000.830.02Invention Example323001000.850.01Invention Example333501000.840.03Invention Example344001000.850.02Invention Example355001000.820.02Invention Example366001000.840.02Invention Example372001100.900.07Comparative Example382251101.010.10Comparative Example392501100.820.04Invention Example402601100.840.01Invention Example412701100.840.02Invention Example422801100.850.01Invention Example432901100.820.04Invention Example443001100.820.03Invention Example453501100.830.03Invention Example464001100.850.01Invention Example475001100.830.04Invention Example486001100.850.04Invention Example [Table 3-2] ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values492001200.940.07Comparative Example502251200.930.07Comparative Example512501200.840.03Invention Example522601200.850.01Invention Example532701200.840.02Invention Example542801200.820.02Invention Example552901200.850.01Invention Example563001200.830.03Invention Example573501200.840.02Invention Example584001200.830.01Invention Example595001200.860.02Invention Example606001200.830.04Invention Example612001300.940.06Comparative Example622251301.010.06Comparative Example632501300.830.01Invention Example642601300.850.01Invention Example652701300.860.04Invention Example662801300.840.03Invention Example672901300.850.04Invention Example683001300.850.03Invention Example693501300.850.02Invention Example704001300.840.02Invention Example715001300.850.04Invention Example726001300.820.01Invention Example732001400.900.06Comparative Example742251400.890.08Comparative Example752501400.820.01Invention Example762601400.820.02Invention Example772701400.830.04Invention Example782801400.840.02Invention Example792901400.860.01Invention Example803001400.840.01Invention Example813501400.830.01Invention Example824001400.840.03Invention Example835001400.830.01Invention Example846001400.850.04Invention Example852001500.990.07Comparative Example862251500.970.10Comparative Example872501500.840.04Invention Example882601500.830.01Invention Example892701500.830.04Invention Example902801500.820.02Invention Example912901500.850.02Invention Example923001500.820.02Invention Example933501500.840.02Invention Example944001500.820.03Invention Example955001500.850.04Invention Example966001500.850.02Invention Example [Table 3-3] ConditionsHeating rate during decarburization annealingIron loss W 17 / 50 in sheet width directionRemarksAverage heating rate (°C / s) from 500°C to 700°CReduced heating rate (°C / s)Mean Value (W / kg)Difference (W / kg) between maximum and minimum values972001600.880.08Comparative Example982251600.910.07Comparative Example992501600.830.09Comparative Example1002601600.850.07Comparative Example1012701600.840.06Comparative Example1022801600.840.09Comparative Example1032901600.840.10Comparative Example1043001600.850.08Comparative Example1053501600.850.08Comparative Example1064001600.840.10Comparative Example1075001600.850.10Comparative Example1086001600.830.08Comparative Example1092001700.970.06Comparative Example1102251700.900.10Comparative Example1112501700.840.06Comparative Example1122601700.830.07Comparative Example1132701700.850.09Comparative Example1142801700.850.07Comparative Example1152901700.820.08Comparative Example1163001700.840.08Comparative Example1173501700.850.09Comparative Example1184001700.840.09Comparative Example1195001700.840.07Comparative Example1206001700.860.07Comparative Example1212001800.890.10Comparative Example1222251800.900.08Comparative Example1232501800.860.08Comparative Example1242601800.850.09Comparative Example1252701800.850.09Comparative Example1262801800.840.07Comparative Example1272901800.830.08Comparative Example1283001800.860.07Comparative Example1293501800.860.09Comparative Example1304001800.860.09Comparative Example1315001800.820.09Comparative Example1326001800.830.09Comparative Example1332001900.940.09Comparative Example1342251900.920.10Comparative Example1352501900.850.08Comparative Example1362601900.840.09Comparative Example1372701900.830.08Comparative Example1382801900.860.08Comparative Example1392901900.860.09Comparative Example1403001900.840.10Comparative Example1413501900.860.10Comparative Example1424001900.820.08Comparative Example1435001900.840.06Comparative Example1446001900.820.09Comparative Example Example 2

[0067] A steel slab, which contains inhibitor-forming components and has a composition of components including C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol. Al: 0.0250 mass%, N: 0.0090 mass%, S: 0.01 mass%, and Se: 0.01 mass%, with the balance being Fe and unavoidable impurities, was heated to 1400°C and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to the first cold rolling to reach an intermediate thickness of 1.2 mm. Then, the resulting sheet was subjected to intermediate annealing at 1100°C for 80 seconds in an atmosphere consisting of N 2 : 75 vol% and H 2 : 25 vol% with a dew point of 46°C, and then the second cold rolling (final cold rolling) using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0068] Subsequently, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of decarburization annealing, rapid heating was performed using a transverse induction heating device at an average heating rate of 300 °C / s within the temperature range of 500°C to 700 °C. When the steel sheet temperature reached 650 °C during the induction heating, parameters such as the output of the induction heating device and the line speed were adjusted so that the period for reduced heating, during which the heating rate T reached 100 °C / s in the sheet width direction, satisfied the six conditions shown in Fig. 2. After the decarburization annealing, an annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet, followed by finishing annealing to induce secondary recrystallization. Thereafter, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing, followed by flattening annealing at 800°C for 30 seconds to bake the coating and thereby obtain a product sheet.

[0069] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 4 presents the results, demonstrating that the steel sheets formed by reducing the heating rate during rapid heating, under the condition that Expression (1) below is satisfied at all positions across the sheet width, exhibit a difference of 0.04 W / kg or less between the maximum and minimum iron loss values in the sheet width direction: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents the distance (mm) from the center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w,

[0070] Accordingly, even when a grain-oriented electrical steel sheet is produced using a material containing inhibitor-forming components, uniform magnetic properties across the sheet width can be achieved by applying the present invention. [Table 4]ConditionsDifference (W / kg) between maximum and minimum values of iron loss W 17 / 50 in sheet width directionRemarks10.02Invention Example20.03Invention Example30.02Invention Example40.06Comparative Example50.07Comparative Example60.05Comparative Example Example 3

[0071] Steel that contains no inhibitor-forming components and has a composition of components including C: 0.036 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol.Al: 0.0072 mass%, N: 0.0050 mass%, S: 0.0031 mass%, Se: 0.0031 mass% as well as the other components including Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, with the composition presented in Table 5, with the balance being Fe and unavoidable impurities, was melted to form a steel slab. The steel slab was heated to 1210°C and hot-rolled to produce a hot-rolled sheet with a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once (final cold-rolling step) using a tandem rolling mill to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0072] The cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During the heating process of the decarburization annealing, as in Example 2, rapid heating was performed using a transverse induction heating device at the average heating rate in the temperature range of 500°C to 700°C of 300°C / s, and when the temperature of the steel sheet reached 650°C, the period during which the heating rate was reduced to 110°C / s at each position in the sheet width direction was set to satisfy the same condition as the condition of No. 1 in Fig. 2. An annealing separating agent mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, followed by finishing annealing to induce secondary recrystallization. Subsequently, an insulation coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finishing annealing. Thereafter, flattening annealing was performed at 800°C for 30 seconds to bake the coating, thereby producing a product sheet.

[0073] An Epstein test piece having a width of 30 mm and a length of 280 mm was taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction of the product sheet thus obtained, where x represents the distance (mm) from the center in the sheet width direction and w represents half the sheet width (mm). The iron loss W 17 / 50 was measured in accordance with JIS Z 2550 to determine both the mean value of the iron loss across the sheet width and the difference between the maximum and minimum values. Table 5 presents the results, in which the product sheets produced using, as a steel material, a slab containing at least one element selected from the group consisting of Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, and subjected to heating during the decarburization annealing using a transverse induction heating device under the conditions compliant with the method of the present invention, exhibit a mean iron loss value in the sheet width direction of 0.82 W / kg or less, and the difference between the maximum and minimum values of the iron loss values in the sheet width direction of 0.04 W / kg or less, thereby demonstrating excellent and uniform magnetic properties across the sheet width. [Table 5]Steel symbolOther steel components (mass%)Iron loss W 17 / 50 in sheet width directionRemarksSbCuPCrNiSnNbMoBBiMean value (W / kg)Difference (W / kg) between maximum and minimum valuesA----------0.870.04Invention ExampleB0.010.050.05-------0.810.02Invention ExampleC0.01---0.050-0.005---0.800.02Invention ExampleD0.14------0.220.004-0.800.04Invention ExampleE0.350.220.12--0.12----0.810.02Invention ExampleF-0.050.050.02------0.820.03Invention ExampleG-0.78---0.43-0.37--0.820.04Invention ExampleH-1.31-0.270.420-----0.790.03Invention ExampleI--0.30---0.002-0.0050.00200.810.01Invention ExampleJ---1.01------0.810.01Invention ExampleK0.42--1.310.890-----0.790.03Invention ExampleL----1.350-----0.790.01Invention ExampleM--0.42-0.007--0.11-0.00100.810.03Invention ExampleN---------0.00070.820.03Invention Example0----0.0500.020.005---0.800.03Invention ExampleP-------0.020.0020.01000.810.02Invention ExampleQ-0.050.05-0.050-----0.800.04Invention ExampleR-----0.020.005--0.01000.810.03Invention ExampleS---0.02---0.020.002-0.820.04Invention ExampleT-0.05---0.02---0.01000.810.04Invention ExampleU---0.02-0.02-0.02--0.800.02Invention ExampleV--0.05-------0.820.03Invention Examplew----0.050-----0.820.03Invention ExampleX--------0.002-0.810.02Invention Example

Examples

example 1

[0064]A Steel slab having a composition of components including C: 0.035 mass%, Si: 3.3 mass%, Mn: 0.05 mass%, sol. Al: 0.0084 mass%, N: 0.0051 mass%, S: 0.0031 mass%, and Se: 0.0031 mass%, with the balance being Fe and unavoidable impurities was heated to 1260°C and then hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. The hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0065]Thereafter, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. During a heating process of the decarburization annealing, the average heating rate T (°C / s) was varied in the temperature range from 500°C to 700°C, as shown in Table 3. For certain steel sheets, when the temperature of the steel sheet reached 620°C, the heating ...

example 2

[0067]A steel slab, which contains inhibitor-forming components and has a composition of components including C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol. Al: 0.0250 mass%, N: 0.0090 mass%, S: 0.01 mass%, and Se: 0.01 mass%, with the balance being Fe and unavoidable impurities, was heated to 1400°C and hot-rolled to form a hot-rolled sheet with a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to the first cold rolling to reach an intermediate thickness of 1.2 mm. Then, the resulting sheet was subjected to intermediate annealing at 1100°C for 80 seconds in an atmosphere consisting of N 2 : 75 vol% and H 2 : 25 vol% with a dew point of 46°C, and then the second cold rolling (final cold rolling) using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0068]Subsequently, the cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for...

example 3

[0071]Steel that contains no inhibitor-forming components and has a composition of components including C: 0.036 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, sol.Al: 0.0072 mass%, N: 0.0050 mass%, S: 0.0031 mass%, Se: 0.0031 mass% as well as the other components including Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, with the composition presented in Table 5, with the balance being Fe and unavoidable impurities, was melted to form a steel slab. The steel slab was heated to 1210°C and hot-rolled to produce a hot-rolled sheet with a thickness of 2.0 mm. Thereafter, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, followed by cold rolling once (final cold-rolling step) using a tandem rolling mill to form a cold-rolled sheet with a final thickness of 0.20 mm.

[0072]The cold-rolled sheet was subjected to decarburization annealing, which serves also as primary recrystallization annealing, at a soaking temperature of 850°C for a soaking time of 100 seconds. Du...

Claims

1. A method for producing a grain-oriented electrical steel sheet, comprising: hot-rolling a steel material to form a hot-rolled sheet; subjecting the hot-rolled sheet to cold rolling including a single cold rolling step or two or more cold rolling steps with intermediate annealing interposed between each rolling step, to form a cold-rolled sheet with a final thickness; and subjecting the cold-rolled sheet to decarburization annealing serving also as primary recrystallization annealing, followed by finishing annealing to induce secondary recrystallization, characterized in that in the decarburization annealing, an average heating rate T (°C / s) is 250°C / s or greater within a temperature range of 500°C to 700° during heating, and within a selected temperature range between 500°C and 700°, a heating rate at each position across the sheet width is reduced to 150°C / s or less for a period t (s) that satisfies Expression (1) below, in accordance with a value of x / w of the position: 200 / T × 0.2 1 − x / w ≤ t ≤ 200 / T × 0.8 1 − x / w provided that x represents a distance (mm) from a center in the sheet width direction, and w represents 1 / 2 of the sheet width (mm), where 0 ≤ x ≤ 0.9w.

2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel material has a composition of components comprising C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: 0.0100 to 0.0400 mass%, and N: 0.0050 to 0.0120 mass%, and further comprising at least one of S and Se in a total amount of 0.01 to 0.05 mass%, with a balance being Fe and unavoidable impurities.

3. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the steel material has a composition of components comprising C: 0.01 to 0.10 mass%, Si: 2.0 to 4.5 mass%, Mn: 0.01 to 0.50 mass%, Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: less than 0.0100 mass%, and Se: less than 0.0100 mass%, with a balance being Fe and unavoidable impurities.

4. The method for producing a grain-oriented electrical steel sheet according to claim 2 or 3, wherein the steel material further includes, in addition to the composition of components, at least one component selected from the group consisting of Sb: 0.005 to 0.500 mass%, Cu: 0.01 to 1.50 mass%, P: 0.005 to 0.500 mass%, Cr: 0.01 to 1.50 mass%, Ni: 0.005 to 1.500 mass%, Sn: 0.01 to 0.50 mass%, Nb: 0.0005 to 0.0100 mass%, Mo: 0.01 to 0.50 mass%, B: 0.0010 to 0.0070 mass%, and Bi: 0.0005 to 0.0500 mass%.

5. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein rapid heating in the decarburization annealing is performed using a transverse induction heating device.

6. A transverse induction heating device for use in rapid heating of the decarburization annealing in the method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 4.

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