Method for producing grain-oriented electrical steel sheet, and induction heating device

Abstract

A method for manufacturing a grain-oriented electrical steel sheet having uniform and excellent magnetic characteristics in the sheet width direction is proposed, wherein, when a grain-oriented electrical steel sheet is produced by subjecting a steel material to hot rolling to form a hot-rolled sheet, cold-rolling the hot-rolled sheet to form a rolled sheet having a final sheet thickness, subjecting the cold-rolled sheet to decarburization annealing serving also as primary recrystallization annealing, and then performing finishing annealing for secondary recrystallization, the hot-rolled sheet is rolled at least one pass in a steel sheet temperature range between 150°C and 350°C (both inclusive) in a final cold rolling during the cold rolling, and, in the decarburization annealing, the average temperature increase rate T (°C / s) between 500°C and 700°C in a temperature raising process is 250°C / s or greater, and the time in which the temperature increase rate is reduced to 150°C / s or less in the range between 500°C and 700°C is changed in accordance with the value of x / w at each position in the sheet width direction (x being the distance (mm) from the sheet width center, and w being 1 / 2 of the sheet width (mm)). Also, an induction heating device for use in decarburization annealing in the above method is provided.

Description

Method for manufacturing grain-oriented electrical steel sheet and induction heating device

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

[0002] Grain-oriented electrical steel sheet is a soft magnetic material primarily used as the iron core material for transformers and generators. It has a crystalline structure in which the {110}<001> orientation (Goss orientation), the axis of easy magnetization of iron, is highly aligned in the rolling direction of the steel sheet, resulting in excellent magnetic properties such as low core loss and high magnetic flux density.

[0003] One way to further reduce iron loss in grain-oriented electrical steel sheets is to highly concentrate the crystal grains in the Goss orientation after secondary recrystallization annealing. To increase the concentration of secondary recrystallized grains in the Goss orientation, it is important to form many Goss-oriented grains in the steel sheet structure after primary recrystallization and to differentiate the grain boundary mobility so that only sharp Goss-oriented grains grow preferentially during secondary recrystallization. In other words, it is important to optimize the texture of the steel sheet after primary recrystallization.

[0004] Primary recrystallized structures in which only sharp Goss-oriented grains can grow preferentially include {111}<112>-oriented grains and {411}<148>-oriented grains. By allowing these to exist in a balanced manner and at a high frequency in the primary recrystallized structure, it is possible to highly accumulate Goss-oriented grains in the rolling direction during secondary recrystallization annealing.

[0005] As a method for increasing the abundance ratio of Goss orientation grains in the primary recrystallized structure, for example, Patent Document 1 discloses a method in which a cold-rolled sheet is heat-treated at a low temperature during cold rolling and then aged. Patent Document 2 also discloses a method in which the cooling rate during intermediate annealing before hot-rolled sheet annealing or cold rolling to the final sheet thickness (final cold rolling) is 30°C / s or more, and interpass aging is performed two or more times during final cold rolling, in which the steel sheet is held at a temperature of 150 to 300°C for 2 minutes or more. Patent Document 3 also discloses a technique in which warm rolling is performed, in which the steel sheet temperature is increased during cold rolling.

[0006] The techniques of Patent Documents 1 to 3 all aim to improve the rolling texture by increasing the steel sheet temperature to an appropriate temperature before, during, or between cold rolling passes, thereby promoting the diffusion of solute elements such as carbon (C) and nitrogen (N) to lock dislocations introduced during cold rolling, suppressing dislocation movement during subsequent rolling, and promoting shear deformation. This is based on the idea that nuclei of Goss-oriented grains in the primary recrystallized structure emerge from shear bands introduced into a worked structure with a {111}<112> orientation. The application of these techniques makes it possible to introduce numerous shear bands into the {111}<112> worked structure, thereby forming numerous Goss-oriented grains in the primary recrystallized structure.

[0007] Furthermore, the formation of Goss-oriented grains in the primary recrystallized structure can also be promoted by increasing the heating rate during the heating process of decarburization annealing. For example, Patent Document 4 discloses a method of rapid heating during the heating process of decarburization annealing. This technology aims to suppress the development of a gamma fiber structure ({111} / / ND), which is preferentially formed at a normal heating rate, by heating the steel from room temperature to near the recrystallization temperature in a short period of time using electrical heating or induction heating, and promote the formation of Goss-oriented grains, which serve as nuclei for secondary recrystallized grains.

[0008] Furthermore, Patent Document 5 discloses a method in which, during the heating process of decarburization annealing, rapid heating is performed between 550 and 700°C at an average heating rate of 50°C / s or more, and the heating rate is maintained at 10°C / s or less for 1 to 10 seconds in any temperature range between 250 and 550°C. This technology aims to promote the recovery of the {111} worked structure, suppress recrystallization, and relatively increase the abundance ratio of Goss orientation grains by maintaining the material in the recovery temperature range of 250 to 550°C for a short time.

[0009] JP 50-016610 JP 08-253816 JP 01-215925 JP 04-160114 JP 2014-152393

[0010] Incidentally, the primary recrystallization texture is often not uniform in the width direction of a steel sheet. The reasons for this include the fact that the cold rolling reduction is not uniform in the width direction due to edge drop formed during hot rolling, and the fact that the width direction cannot be uniformly heated by annealing the hot-rolled sheet, resulting in non-uniform grain sizes in the width direction of the steel sheet before cold rolling. Furthermore, when warm rolling is applied to cold rolling as in the techniques disclosed in Patent Documents 1 to 3, the temperature drop at the edge of the steel sheet due to heat dissipation is large, and the diffusion distances of carbon and nitrogen differ in the width direction. This is also thought to be one of the reasons for the change in texture.

[0011] If the primary recrystallization texture varies across the sheet width, the secondary recrystallization behavior will also vary across the sheet width, causing the magnetic properties of the final product to vary across the sheet width. To prevent this, trimming the steel sheet edges after hot rolling or cold rolling is one possible method, but this inevitably reduces the yield.

[0012] The present invention has been made in consideration of the above-mentioned problems of the conventional technology, and an object of the present invention is to propose a method for manufacturing grain-oriented electrical steel sheet having uniform and excellent magnetic properties in the sheet width direction, and to provide an induction heating device for decarburization annealing to be used in the method.

[0013] In order to solve the above problems, the inventors have conducted extensive research into methods for homogenizing the primary recrystallization texture in the sheet width direction. As a result, they have found that the primary recrystallization texture after decarburization annealing can be homogenized in the sheet width direction by performing at least one pass of rolling at a steel sheet temperature of 150°C or higher in the final cold rolling of cold rolling, and by temporarily reducing the heating rate during rapid heating from 500°C to 700°C in the heating process and varying the time for this reduction in the sheet width direction in the decarburization annealing, which has led to the development of the present invention.

[0014] The present invention based on the above findings provides a method for producing a grain-oriented electrical steel sheet, which comprises hot-rolling a steel material to form a hot-rolled sheet, cold-rolling the hot-rolled sheet once or cold-rolling two or more times with intermediate annealing in between to form a cold-rolled sheet of a final sheet thickness, and subjecting the cold-rolled sheet to decarburization annealing that also serves as primary recrystallization annealing, followed by finish annealing for secondary recrystallization; in the final cold rolling of the one or more cold rollings, the steel sheet is rolled through at least one pass in a temperature range of 150°C or higher and 350°C or lower, and in the decarburization annealing, the average heating rate T (°C / s) between 500°C and 700°C during the heating process is set to 250°C / s or higher, and the heating rate at each position in the sheet width direction of the steel sheet in any temperature range between 500°C and 700°C is calculated according to the following formula (1) in accordance with the value of x / w at each position: 200 / T×0.2(1−x / w)≦t≦200 / T×0.8(1−x / w) (1) where x is the distance from the sheet width center (mm), w is 1 / 2 of the sheet width (mm), and we propose a method for producing a grain-oriented electrical steel sheet, characterized in that the temperature is reduced to 150°C / s or less for a time t (s) that satisfies the following condition: 0≦x≦0.9w.

[0015] The method for producing the grain-oriented electrical steel sheet of the present invention is characterized in that the final cold rolling comprises rolling at least one pass in a temperature range of 30°C or higher and 130°C or lower, and then rolling at least one pass in a temperature range of 150°C or higher and 350°C or lower.

[0016] The steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention is characterized by having a component composition containing 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 containing at least one of S and Se: 0.01 to 0.05 mass% in total, with the balance being Fe and unavoidable impurities.

[0017] The steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention is characterized by having a component composition containing 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 the balance being Fe and unavoidable impurities.

[0018] Furthermore, the steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention further contains, in addition to the above-described component composition, 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%.

[0019] The method for producing the grain-oriented electrical steel sheet of the present invention is characterized in that the rapid heating in the decarburization annealing is carried out using a transverse type induction heating device.

[0020] The present invention also provides a transverse type induction heating device used for rapid heating in decarburization annealing in the above-mentioned method for producing grain-oriented electrical steel sheet.

[0021] According to the present invention, it is possible to stably produce grain-oriented electrical steel sheets that have uniform and excellent magnetic properties in the width direction of the steel sheet, which greatly contributes to improving the quality and yield of the finished sheet.

[0022] 1 is a diagram illustrating a range of a temperature rise rate reduction time in the plate width direction that is suitable for the present invention. 2 is another diagram illustrating a range of a temperature rise rate reduction time in the plate width direction that is suitable for the present invention. 3 is a schematic diagram illustrating a transverse type induction heating device.

[0023] First, the experiments that led to the development of the present invention will be described. <Experiment 1> A steel slab having a composition containing 0.033 mass% C, 3.4 mass% Si, 0.07 mass% Mn, 0.0081 mass% sol. Al, 0.0052 mass% N, 0.0030 mass% S, and 0.0030 mass% Se, with the balance consisting of Fe and unavoidable impurities, was heated to 1220°C and then hot-rolled to a hot-rolled sheet with a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, and then cold-rolled once to a final thickness of 0.20 mm. Note that the cold rolling was warm-rolled, in which the steel sheet temperature was increased to 200°C at a position just before the roll bite by induction heating. Next, a sample material was taken from the cold-rolled sheet, and multiple samples of Epstein test piece size (30 mm × length 280 mm) were taken from five positions in the sheet width direction of the sample material, where x = 0, 0.2w, 0.4w, 0.6w, and 0.8w, where x (mm) is the distance from the sheet width center (x = 0 at the sheet width center) and w (mm) is half the sheet width.

[0024] Next, each of the samples was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. During this process, the average heating rate from 500°C to 700°C during the decarburization annealing was 300°C / s. Furthermore, under some conditions, the heating rate was reduced to 120°C / s for the time shown in Table 1 upon reaching 600°C. Next, an annealing separator primarily composed of MgO was applied to the sample surface after the decarburization annealing, followed by finish annealing to induce secondary recrystallization. An insulating coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was then applied to the sample surface after the finish annealing, and baked at 800°C for 30 seconds, simulating planarization annealing, to obtain a product sheet sample. The average temperature rise rate of 300°C / s from 500°C to 700°C is the average temperature rise rate excluding the time during which the temperature rise rate is reduced.

[0025] The product plate sample of the size of the Epstein test piece obtained as described above was subjected to iron loss W in accordance with JIS Z 2550. １７／５０The difference between the maximum and minimum iron loss values ​​in the sheet width direction was determined, and the results are shown in Table 1.

[0026]

[0027] From Table 1, it was found that when the sheet was rapidly heated between 500 and 700°C, a period of time was provided during which the rate of temperature rise was temporarily reduced between the above temperatures, and the period of time during which the rate of temperature rise was reduced was made longer in the central part of the sheet width and shorter at the end parts of the sheet width, and the difference between the maximum and minimum values ​​of the iron loss value in the sheet width direction was 0.04 W / kg or less, and uniform magnetic properties were obtained in the sheet width direction.

[0028] As described above, the inventors believe that the reason why uniform magnetic properties were obtained in the sheet width direction under the condition that the time for temporarily reducing the temperature rise rate during rapid heating was long in the center of the sheet width and short at the end sides of the sheet width is as follows.

[0029] The cold rolling reduction ratio at the width edges is lower than that at the width center due to edge drop during hot rolling and other factors. This means that the amount of deformation during cold rolling is smaller at the width edges. Furthermore, the steel sheet temperature during cold rolling is likely to be lower at the width edges than at the width center due to heat dissipation. As a result, the diffusion distance of carbon and nitrogen in the steel at the width edges is shorter, making it difficult for dislocations formed during rolling to become pinned. Therefore, the cold-rolled structure at the width edges has fewer shear bands, which serve as the formation sites for Goss-oriented grains during primary recrystallization, than at the width center. Therefore, at the width edges, shortening the time for slowing the heating rate promotes the recrystallization of Goss-oriented grains. Conversely, extending the time for slowing the heating rate at the width center suppresses the recrystallization of Goss-oriented grains. As a result, the number of Goss-oriented grains after primary recrystallization annealing is thought to be uniform across the width.

[0030] Next, based on the above experimental results, the inventors conducted an experiment to investigate the appropriate time for reducing the heating rate depending on the position in the sheet width direction. <Experiment 2> Five types of Epstein test piece size samples, which were obtained from the cold-rolled sheets obtained in Experiment 1 and had different width-direction positions, were subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. In this case, the average heating rate from 500°C to 700°C during the heating process of decarburization annealing was 300°C / s, and the time in the sheet width direction for reducing the heating rate to 110°C / s upon reaching 650°C was varied among the eight conditions shown in Figure 1. Next, an annealing separator mainly composed of MgO was applied to the surface of the sample after the decarburization annealing, and the sample was subjected to finish annealing to cause secondary recrystallization. Next, an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the sample after the above-mentioned finish annealing, and baked by heat treatment simulating flattening annealing at 800°C for 30 seconds to obtain a product plate sample.

[0031] The product plate sample of the size of the Epstein test piece obtained as described above was subjected to iron loss W in accordance with JIS Z 2550. １７／５０ The difference between the maximum and minimum iron loss values ​​in the sheet width direction is shown in Table 2.

[0032]

[0033] From Table 2, it can be seen that the time t required to reduce the heating temperature at all positions x=0, 0.2w, 0.4w, 0.6w, and 0.8w in the sheet width direction is determined by the following formula (1): 200 / T×0.2(1-x / w)≦t≦200 / T×0.8(1-x / w) (1) where x is the distance from the sheet width center (mm) and w is half the sheet width (mm), provided that 0≦x≦0.9w is satisfied. It was found that under this condition, the difference between the maximum and minimum iron loss values ​​in the sheet width direction is 0.04 W / kg or less, and uniform magnetic properties are obtained in the sheet width direction. The present invention was completed through further investigation based on the above novel findings.

[0034] Next, the chemical composition of the steel material (slab) used in the production of the grain-oriented electrical steel sheet of the present invention will be described. While the steel material used in the present invention can be any known steel material that has been used in the production of grain-oriented electrical steel sheets, from the viewpoint of obtaining excellent magnetic properties, it is preferable that the steel material has the chemical composition shown below.

[0035] C: 0.01 to 0.10 mass% C is an element that contributes to improving the primary recrystallization texture by precipitating as fine carbides. However, if the C content is less than 0.01 mass%, the amount of precipitated fine carbides may be insufficient, and the texture improvement effect may be insufficient. On the other hand, if the C content exceeds 0.10 mass%, it may be difficult to reduce the C content to 0.0050 mass% or less, at which magnetic aging does not occur during decarburization annealing. Therefore, the C content is preferably in the range of 0.01 to 0.10 mass%, and more preferably in the range of 0.015 to 0.08 mass%.

[0036] Si: 2.0 to 4.5 mass% Si is an element effective in increasing the resistivity of steel and improving iron loss. However, if the Si content is less than 2.0 mass%, the above-mentioned iron loss reduction effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 4.5 mass%, the workability significantly decreases, making it difficult to manufacture by rolling. Therefore, the Si content is preferably in the range of 2.0 to 4.5 mass%, and more preferably in the range of 2.5 to 4.0 mass%.

[0037] Mn: 0.01 to 0.50 mass% Mn is an element necessary for improving hot workability. If the Mn content is less than 0.01 mass%, it becomes difficult to obtain the above-mentioned effect of improving hot workability. On the other hand, if the Mn content exceeds 0.50 mass%, the primary recrystallization texture deteriorates, and it may become difficult to obtain secondary recrystallized grains in which the Goss orientation is highly concentrated. Therefore, the Mn content is preferably in the range of 0.01 to 0.50 mass%, and more preferably in the range of 0.03 to 0.45 mass%.

[0038] The components other than the above C, Si, and Mn differ depending on whether or not an inhibitor is used in secondary recrystallization. Specifically, when an inhibitor is used in secondary recrystallization and AlN is used as the inhibitor, it is preferable to contain Al and N in the ranges of Al: 0.0100 to 0.0400 mass% and N: 0.0050 to 0.0120 mass% in addition to the above C, Si, and Mn. If the Al content and N content are below the above lower limit values, it becomes difficult to obtain the desired inhibitor effect. On the other hand, if the content exceeds the above upper limit values, the dispersion state of the precipitates becomes non-uniform, and it also becomes difficult to obtain the desired inhibitor effect.

[0039] In addition to the above AlN, sulfides (MnS, Cu ２ S, etc.) and selenides (MnSe, Cu ２ When using an inhibitor such as Al or N, it is preferable to further contain at least one element selected from S and Se in a total amount of 0.0100 to 0.0500 mass% as an inhibitor-forming component in addition to the Al and N. If the total content of S and Se is less than the lower limit, it becomes difficult to obtain a sufficient inhibitor effect. On the other hand, if the total content exceeds the upper limit, the dispersion of the precipitates becomes non-uniform, again making it difficult to obtain a sufficient inhibitor effect. The sulfides and selenides may be precipitated in combination.

[0040] On the other hand, when no inhibitor is used in the secondary recrystallization, it is preferable to reduce the components that form the inhibitor as much as possible. Specifically, it is preferable to set the Al content to less than 0.0100 mass%, N to 0.0050 mass% or less, S to less than 0.0100 mass%, and Se to less than 0.0100 mass%.

[0041] The steel material used to manufacture the grain-oriented electrical steel sheet of the present invention contains the above-mentioned basic components, and the remainder is essentially Fe and unavoidable impurities. However, for the purpose of improving the magnetic properties, in addition to the above components, at least one selected from 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% may be contained. Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi are elements useful for improving magnetic properties, and within the above ranges, the effect of improving magnetic properties can be obtained without inhibiting the development of secondary recrystallized grains.

[0042] Next, a method for producing the grain-oriented electrical steel sheet of the present invention will be described. The steel material (slab) used for producing the grain-oriented electrical steel sheet of the present invention is preferably produced by melting steel having the above-described composition in a commonly known refining process in which molten steel obtained from a converter, electric furnace, or the like is subjected to secondary refining such as vacuum degassing, and then converting the steel material into a steel material by a commonly known continuous casting method, ingot making-slabbing method, or the like.

[0043] Next, the steel material (slab) is heated to a predetermined temperature and then hot-rolled to form a hot-rolled sheet. The heating temperature of the slab is preferably about 1050°C or higher in order to ensure hot-rollability. There is no particular upper limit to the heating temperature, but if it exceeds 1450°C, the temperature approaches the melting point of the steel, making it difficult to maintain the shape of the slab, so it is preferably 1450°C or lower. The hot rolling following slab heating can be performed under commonly known conditions and is not particularly limited.

[0044] Next, the steel sheet (hot-rolled sheet) after the above-mentioned hot rolling may be subjected to hot-rolled sheet annealing as necessary. When hot-rolled sheet annealing is performed, commonly known conditions can be applied, and there are no particular limitations.

[0045] The above-mentioned hot-rolled steel sheet or hot-rolled steel sheet annealed is descaled by pickling, mechanical methods, or the like as necessary, and then cold-rolled to a cold-rolled sheet of a final thickness (product thickness). The cold rolling may be performed in a single cold rolling step to produce a cold-rolled sheet of the final thickness, or in two or more cold rolling steps with intermediate annealing in between to produce a cold-rolled sheet of the final thickness. The final thickness is preferably in the range of 0.1 mm to 1.0 mm.

[0046] Furthermore, the reduction ratio of the final cold rolling in the cold rolling is preferably in the range of 60% to 95%. Here, the final cold rolling refers to the cold rolling that is performed last among one or more cold rollings. For example, when cold rolling is performed only once, the one rolling is the final cold rolling, and when cold rolling is performed two or more times, the final rolling is the final cold rolling.

[0047] Here, in the present invention, the final cold rolling must be performed at least once at a rolling temperature of 150°C to 350°C. Performing one or more passes of cold rolling at the above-mentioned temperatures promotes the diffusion of solute carbon and the pinning of dislocations, efficiently introducing shear bands that serve as nucleation sites for Goss-oriented grains in primary recrystallization, and further improving magnetic properties. If the rolling temperature is below 150°C, the pinning of dislocations by solute carbon is insufficient, and the effect of increasing the number of Goss-oriented grains in primary recrystallization cannot be expected. On the other hand, if the rolling temperature exceeds 350°C, the lubrication state will be significantly deteriorated due to evaporation of the rolling oil used for lubrication. The preferred rolling temperature is in the range of 180°C to 300°C. The rolling temperature refers to the temperature of the steel sheet immediately before the roll bite.

[0048] Furthermore, in the final cold rolling, at least one pass of rolling is performed at a low rolling temperature of 30°C to 130°C, followed by at least one pass of rolling at a high rolling temperature of 150°C to 350°C, thereby further enhancing the effect of increasing the number of Goss-oriented grains in primary recrystallization. Since the {111}<112> orientation is a stable rolling orientation whose orientation does not change with rolling, first rolling at a low temperature to develop the {111}<112> texture, and then further rolling at a high temperature, can efficiently increase the number of shear bands formed in the {111}<112> worked texture, which serve as nucleation sites for Goss-oriented grains in primary recrystallization. If the rolling temperature in the low temperature range is less than 30°C, sheet cracking may occur, significantly reducing productivity. On the other hand, if the rolling temperature exceeds 130°C, the {111}<112> texture will be reduced. A preferred rolling temperature in the low temperature range is 40°C or higher and 100°C or lower.

[0049] Furthermore, when final cold rolling is performed in three or more passes, the positions of the passes in the low temperature region and the high temperature region are not limited, as long as rolling in the high temperature region is performed after rolling in the low temperature region. For example, if rolling in a low temperature region of 30°C or higher and 130°C or lower is referred to as low temperature, and rolling in a region of 150°C or higher and 350°C or lower is referred to as high temperature, when final cold rolling is performed in three passes, rolling may be performed in any of the following orders: low temperature-high temperature-high temperature, high temperature-low temperature-high temperature, low temperature-low temperature-high temperature, or low temperature-high temperature-low temperature. However, when rolling is performed in the order of high temperature-low temperature-low temperature or high temperature-high temperature-low temperature, the effect of increasing the Goss orientation cannot be expected.

[0050] Next, the cold-rolled sheet having reached the final thickness is subjected to decarburization annealing, which also serves as primary recrystallization annealing. The decarburization conditions (soaking conditions) in this decarburization annealing are not particularly limited and may be any known conditions, but are preferably, for example, 720 to 870°C x 60 to 150 seconds in a wet hydrogen atmosphere. This decarburization annealing reduces the C content in the steel sheet to 0.0050 mass% or less, at which point magnetic aging does not occur.

[0051] However, in this decarburization annealing, it is important that the heating from 500°C to 700°C during the temperature rise process up to the soaking temperature be rapid heating at an average heating rate of 250°C / s or more. Here, the average heating rate between 500°C and 700°C in the present invention is the average heating rate for the time excluding the time during which the heating rate is temporarily reduced, as described below. If the average heating rate is less than 250°C / s, the Goss orientation grains after primary recrystallization will be insufficient, and good iron loss properties will not be obtained. A preferred average heating rate is 300°C / s or more. Rapid heating may be performed in a temperature range other than 500°C to 700°C.

[0052] Furthermore, during the temperature rise process of the decarburization annealing, it is necessary to provide a time period during which the temperature rise rate is temporarily reduced to 150°C / min or less at any temperature between 500°C and 700°C where the rapid heating is performed. If the steel sheet temperature at which the temperature rise rate is reduced is less than 500°C, reducing the temperature rise rate does not change the recrystallization behavior of Goss-oriented grains, and the effect of adjusting the number of Goss-oriented grains in primary recrystallization cannot be obtained. On the other hand, if the temperature exceeds 700°C, recrystallization is almost complete even if the temperature rise rate is reduced, so the effect of adjusting the number of Goss-oriented grains in primary recrystallization cannot be obtained.

[0053] Furthermore, in the present invention, it is necessary to vary the time for which the heating rate is temporarily reduced depending on the position in the sheet width direction. The present invention is a technology that eliminates differences in the steel sheet structure in the sheet width direction, which arise due to various manufacturing conditions up to decarburization annealing, by varying the time for which the heating rate is temporarily reduced during rapid heating in decarburization annealing in the sheet width direction, and thereby causing the recrystallization of Goss-oriented grains in the primary recrystallization to occur uniformly in the sheet width direction. In particular, when warm rolling is used for the final cold rolling, temperature differences occur in the sheet width direction, which tends to make the primary recrystallization texture non-uniform in the sheet width direction. Therefore, this technology is preferably applied when warm rolling is performed.

[0054] The time t during which the heating rate is temporarily reduced to 150°C / s or less during the rapid heating process is longer near the center of the sheet width and shorter near the edges of the sheet width. Specifically, it satisfies the following formula (1): 200 / T×0.2(1-x / w)≦t≦200 / T×0.8(1-x / w) (1), where x is the distance from the center of the sheet width (mm), w is 1 / 2 the sheet width (mm), and it is important to satisfy the following condition: 0≦x≦0.9w. If t is shorter than the left side of formula (1), the Goss-oriented grains in that area will be too numerous, resulting in a local decrease in core loss. Conversely, if t is longer than the right side of formula (1), recrystallization of the Goss-oriented grains in that area will be suppressed, resulting in a local increase in core loss. As a result, uniform magnetic properties will not be obtained across the sheet width. The reason why the width range satisfying the above formula (1) is set to (0≦x≦0.9w) is that when transverse induction heating is used for rapid heating, the induced current flows concentratedly at the widthwise ends of the sheet, and there is a risk that formula (1) may not be satisfied over the entire width. Of course, it is preferable that formula (1) be satisfied over the entire width.

[0055] Furthermore, the temperature rise rate that is temporarily reduced must be 150°C / s or less. At a temperature rise rate higher than this, the effect of suppressing the recrystallization of Goss-oriented grains becomes insufficient. The lower limit of the temperature rise rate that is reduced is not particularly limited, but is preferably 10°C / s or more. The time period for which the temperature rise rate is temporarily reduced can be obtained by measuring the steel sheet temperature during the heating process using a thermocouple or the like and differentiating the steel sheet temperature at each time with respect to time.

[0056] Here, rapid heating in the temperature rising process of the decarburization annealing and the reduction in the temperature rise rate therebetween can be achieved by arranging two or more rapid heating devices, such as electric heating devices or solenoid-type induction heating devices, in series in the direction in which the steel sheet passes, designating any section between the two or more devices as a temperature rise rate reduction section, and appropriately adjusting the output of the rapid heating devices and the steel sheet passing speed (line speed). Furthermore, an edge heater or the like may be provided in the temperature rise rate reduction section from the viewpoint of preventing heat radiation from the sheet width ends.

[0057] Installing two or more rapid heating devices in series as described above poses problems of high cost and space requirements. However, when using a transverse-type induction heating device, as shown in FIG. 3, in which heating coils wound around an iron core are arranged above and below the steel sheet, and alternating magnetic flux generated within the iron core penetrates the steel sheet in the thickness direction, heating the steel sheet by the action of the magnetic field, the induced current flows within the steel sheet along the shape of the heating coil, but not in the steel sheet facing the iron core. Therefore, the heating rate temporarily decreases as the steel sheet passes through the iron core. This phenomenon can be utilized to slow the heating rate. Furthermore, the time during which the heating rate slows can be adjusted by changing the output power or line speed of the induction heating device. Furthermore, since the induced current flows at the widthwise edges of the steel sheet, heat dissipation at the widthwise edges can be suppressed. Moreover, since the temperature-rise rate slows down within a single induction heating device, there are no space limitations. Therefore, a transverse-type induction heating device is suitable for use in the present invention.

[0058] In the transverse type induction heating device, the temperature rise rate reduction time can be varied in the width direction by increasing the coil diameter in the width direction at the center of the plate and gradually decreasing it toward the width ends. The shape of the heating coil in the transverse type induction heating device may be round, rectangular, elliptical, or the like, but it is preferable to vary the coil diameter in the width direction as described above.

[0059] Next, the cold-rolled sheet that has been subjected to the decarburization annealing is subjected to finish annealing for secondary recrystallization after applying an annealing separator to the surface of the steel sheet. As the annealing separator, a known one can be used, and there is no particular limitation. For example, ２ Those with the addition of auxiliary agents such as SiO ２ and Al ２ O ３ Examples of the main component include:

[0060] After the above-mentioned finish annealing, the steel sheet is preferably subjected to removal of any unreacted annealing separator remaining on the steel sheet surface, followed by application of an insulating coating liquid to the surface of the steel sheet and baking in a flattening annealing process that also corrects the shape of the steel sheet that has been distorted by the finish annealing, to produce a finished steel sheet. The application of the insulating coating may be performed in a separate line. The type of insulating coating is not particularly limited, but when a tension-applying insulating coating that applies tensile tension to the steel sheet surface is formed, it is preferable to apply a slurry containing phosphate and colloidal silica, as disclosed in JP-A-50-79442, JP-A-48-39338, and JP-A-56-75579, and bake the slurry at a temperature of about 800°C.

[0061] If it is desired to further reduce iron loss, magnetic domain refining treatment may be performed by a known method, such as forming grooves on the steel sheet surface in any step after the cold rolling, mechanically forming strain regions on the steel sheet surface after finish annealing, or forming thermally strained regions by irradiating the steel sheet with a laser beam, an electron beam, or the like.

[0062] 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. A steel slab having a component composition was heated to 1260 ° C. and then hot-rolled to a hot-rolled sheet having a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000 ° C. × 60 s, and then cold-rolled once to a final thickness of 0.20 mm. The cold rolling was performed by warm-rolling in which the steel sheet temperature was heated to 250 ° C. by induction heating.

[0063] Next, the cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. During the temperature rise process of the decarburization annealing, the average temperature rise rate T (°C / s) from 500°C to 700°C was varied as shown in Table 3. In some steel sheets, when the temperature reached 620°C, the temperature rise rate at each position in the sheet width direction was reduced by the time t expressed by the following equation (2): t = 200 / T × 0.5 × (1 - x / w) ... (2) where x is the distance from the sheet width center (mm), w is 1 / 2 the sheet width (mm), and 0 ≦ x ≦ 0.9w, so as to achieve the "reduced temperature rise rate" shown in Table 3. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, and the steel sheet was subjected to finish annealing to cause secondary recrystallization. Next, an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the above-mentioned finish annealing, and the steel sheet was baked by planarization annealing at 800°C for 30 seconds to obtain a product sheet.

[0064] Epstein test pieces of 30 mm width x 280 mm length were taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w and 0.8w (x: distance from the center of the plate width (mm), w: 1 / 2 of the plate width (mm)) in the plate width direction of the product plate obtained in this way, and the iron loss W １７／５０ The average value of the iron loss in the sheet width direction and the difference between the maximum and minimum values ​​were determined, and the results are shown in Table 3. Table 3 shows that under the conditions where the average heating rate is 250°C / s or more and the reduced heating rate is 150°C / s or less, the average iron loss is low at 0.84 W / kg or less, and the iron loss difference is suppressed to 0.04 W / kg or less.

[0065]

[0066]

[0067]

[0068] A steel slab containing inhibitor-forming components, having a composition of 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 consisting of Fe and unavoidable impurities, was heated to 1400°C and then hot-rolled to a hot-rolled sheet having a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to a first cold rolling to obtain an intermediate thickness of 1.2 mm. Next, N ２ : 75 vol% + H ２ After intermediate annealing at 1100°C for 80 seconds in an atmosphere of 25 vol% Cr, 0.25% Cr, and a dew point of 46°C, the steel sheet was subjected to a second cold rolling (final cold rolling) using a tandem rolling mill to obtain a cold-rolled sheet with a final thickness of 0.20 mm. At this time, the flow rate of coolant sprayed onto the steel sheet was adjusted so that the steel sheet temperature during final cold rolling was 160°C to 250°C.

[0069] Next, the cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. During the temperature-raising process of the decarburization annealing, a transverse induction heating device was used, and rapid heating was performed from 500°C to 700°C at an average heating rate of 300°C / s. The output of the induction heating device and the line speed were adjusted so that the time in the sheet width direction at which the heating rate T reached 100°C / s when the steel sheet temperature reached 650°C during the induction heating was the sixth condition shown in Figure 2 . Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after decarburization annealing, and the steel sheet was subjected to finish annealing to cause secondary recrystallization. Next, an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after finish annealing, and the steel sheet was baked by planarization annealing at 800°C for 30 seconds to obtain a product sheet.

[0070] Epstein test pieces of 30 mm width x 280 mm length were taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w and 0.8w (x: distance from the center of the plate width (mm), w: 1 / 2 of the plate width (mm)) in the plate width direction of the product plate obtained in this way, and the iron loss W １７／５０The average value of iron loss in the sheet width direction and the difference between the maximum and minimum values ​​were determined, and the results are shown in Table 4. From Table 4, it can be seen that for steel sheets in which the heating rate was reduced during rapid heating under the condition that the following formula (1) was satisfied at all positions in the sheet width direction: 200 / T×0.2(1−x / w)≦t≦200 / T×0.8(1−x / w) (1), where x is the distance from the sheet width center (mm), w is 1 / 2 the sheet width (mm), and 0≦x≦0.9w, the differences between the maximum and minimum values ​​of iron loss in the sheet width direction were all 0.04 W / kg or less. Therefore, it can be seen that even when grain-oriented electrical steel sheets are manufactured using materials containing inhibitor-forming components, the magnetic properties in the sheet width direction can be made uniform by applying the present invention.

[0071]

[0072] Steel slab A had a composition containing no inhibitor-forming components, namely, 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 consisting of Fe and unavoidable impurities; Steel slab B having a composition containing inhibitor-forming components, including Al: 0.0250 mass%, N: 0.0095 mass%, S: 0.01 mass%, and Se: 0.01 mass%, with the balance being Fe and unavoidable impurities, was heated to a temperature of 1300°C and then hot-rolled to a hot-rolled sheet having a thickness of 2.0 mm. Next, the hot-rolled sheet produced from the steel slab A was subjected to hot-rolled sheet annealing at 1000°C x 60 s, and then cold-rolled once to a final thickness of 0.20 mm. On the other hand, the hot-rolled sheet produced from the steel slab B was subjected to hot-rolled sheet annealing at 1000°C x 60 s, and then cold-rolled once to a final thickness of 1.2 mm. ２ : 75 vol% + H ２After intermediate annealing at 1100°C for 80 seconds in an atmosphere of 25vol% Cr, 0.25% by weight, and a dew point of 46°C, the cold-rolled sheets were subjected to a second cold rolling to obtain cold-rolled sheets with a final thickness of 0.20 mm. The cold rolling to obtain the final thickness (final cold rolling) of the above cold rolling processes was performed in four passes at the steel sheet temperatures shown in Table 5.

[0073] Next, the cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. During this decarburization annealing, the average heating rate from 500°C to 700°C was 300°C / s, and upon reaching 600°C, the heating rate at each position in the sheet width direction was reduced to 100°C / s for a time t calculated using the following formula (2): t = 200 / T × 0.5 × (1 - x / w) ... (2) where x is the distance from the sheet width center (mm), w is 1 / 2 the sheet width (mm), and 0 ≤ x ≤ 0.9w. The steel sheets of Conditions Nos. 323, 324, 325, and 326 in Table 5 were rapidly heated at a heating rate of 300°C / s (without reducing the heating rate). Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, and the steel sheet was subjected to finish annealing to cause secondary recrystallization. Next, an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finish annealing, and the steel sheet was baked by planarization annealing at 800°C for 30 seconds to obtain a product sheet.

[0074] Epstein test pieces of 30 mm width x 280 mm length were taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w and 0.8w (x: distance from the center of the plate width (mm), w: 1 / 2 of the plate width (mm)) in the plate width direction of the product plate obtained in this way, and the iron loss W １７／５０ The average value of the iron loss in the sheet width direction and the difference between the maximum and minimum values ​​were calculated, and the results are shown in Table 5. From Table 5, it can be seen that the steel sheets that were subjected to at least one pass of rolling at a steel sheet temperature of 150°C or higher and 350°C or lower in the final cold rolling had a low iron loss W １７／５０In addition, the steel sheet that was rolled at least once at a temperature of 30°C to 130°C and then rolled at least once at a temperature of 150°C to 350°C had a good iron loss W of 0.84 W / kg or less. １７／５０ On the other hand, in the steel sheets that did not satisfy the temperature rise conditions of the present invention during decarburization annealing even after warm rolling, the differences between the maximum and minimum iron loss values ​​in the sheet width direction were all 0.06 W / kg or more, which shows a large variation.

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083] A steel having a composition containing 0.036 mass% C, 3.4 mass% Si, 0.06 mass% Mn, 0.0072 mass% sol.Al, 0.0050 mass% N, 0.0031 mass% S, and 0.0031 mass% Se, and other elements such as Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi in the compositions shown in Table 6, with the balance being Fe and unavoidable impurities, and containing no inhibitor-forming elements, was produced into a steel slab. The steel slab was then heated to 1210°C and hot-rolled to a thickness of 2.0 mm. Next, the hot-rolled sheet was subjected to hot-rolled sheet annealing at 1000°C for 60 seconds, and then cold-rolled to a final thickness of 0.20 mm using a tandem rolling mill (final cold rolling). In this final cold rolling, the flow rate of the coolant sprayed onto the steel sheet was adjusted so that the steel sheet temperature during rolling was 250°C to 300°C.

[0084] Next, the cold-rolled sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 850°C and a soaking time of 100 seconds. During the temperature-raising process of the decarburization annealing, a transverse induction heating device was used, as in Example 2, and the steel sheet was rapidly heated from 500°C to 700°C at an average heating rate of 300°C / s. The time in the width direction for reducing the heating rate to 110°C / s when the steel sheet temperature reached 650°C was set to be the same as that of No. 1 in Figure 2. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheet after the decarburization annealing, and then finish annealing was performed to induce secondary recrystallization. Next, an insulating coating solution containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied to the surface of the steel sheet after the finish annealing, and the steel sheet was baked by planarization annealing at 800°C for 30 seconds to obtain a product sheet.

[0085] Epstein test pieces of 30 mm width x 280 mm length were taken from each of the positions x = 0, 0.2w, 0.4w, 0.6w and 0.8w (x: distance from the center of the plate width (mm), w: 1 / 2 of the plate width (mm)) in the plate width direction of the product plate obtained in this way, and the iron loss W １７／５０ The results are shown in Table 6. From Table 6, it can be seen that the product sheets manufactured under conditions conforming to the method of the present invention using a slab containing at least one element selected from Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi as the steel material and using a transverse induction heating device during the temperature rise process of decarburization annealing all had an average widthwise iron loss value of 0.80 W / kg or less and a difference between the maximum and minimum widthwise iron loss values ​​of 0.04 W / kg or less, and thus had uniform and excellent magnetic properties in the widthwise direction.

[0086]

Claims

1. A method for producing grain-oriented electrical steel sheet, comprising the steps of hot rolling a steel material to obtain a hot-rolled sheet, cold rolling the hot-rolled sheet once or cold rolling two or more times with intermediate annealing in between to obtain a cold-rolled sheet of a final sheet thickness, subjecting the cold-rolled sheet to decarburization annealing which also serves as primary recrystallization annealing, and then performing finish annealing for secondary recrystallization, wherein in the final cold rolling of the one or more cold rollings, the steel sheet is rolled for at least one pass in a temperature range of 150°C to 350°C, and in the decarburization annealing, the average heating rate T (°C / s) during the heating process between 500°C and 700°C is set to 250°C / s or more, and the heating rate at each position in the sheet width direction of the steel sheet is reduced to 150°C / s or less for a time t (s) which satisfies the following formula (1) in any of the temperature ranges between 500°C and 700°C: 200 / T×0.2(1−x / w)≦t≦200 / T×0.8(1−x / w) (1) where, x: distance from the center of the plate width (mm), w: 1 / 2 of the plate width (mm), provided that 0≦x≦0.9w 2. The method for producing grain-oriented electrical steel sheet according to claim 1, characterized in that the final cold rolling comprises rolling at least one pass in a temperature range of 30°C or higher and 130°C or lower, followed by rolling at least one pass in a temperature range of 150°C or higher and 350°C or lower.

3. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that the steel material has a component composition containing 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%, N: 0.0050 to 0.0120 mass%, and further containing at least one of S and Se: 0.01 to 0.05 mass% in total, with the balance being Fe and unavoidable impurities.

4. The method for producing grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that the steel material has a component composition containing 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%, Se: less than 0.0100 mass%, and the balance being Fe and unavoidable impurities.

5. The method for producing a grain-oriented electrical steel sheet according to claim 3 or 4, characterized in that the steel material further contains, in addition to the above-mentioned chemical composition, 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%.

6. The method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 5, characterized in that the rapid heating in the decarburization annealing is carried out using a transverse type induction heating device.

7. A transverse type induction heating device used for rapid heating in decarburization annealing in the method for producing grain-oriented electrical steel sheet according to any one of claims 1 to 5.

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