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

Abstract

A method for producing a grain-oriented electromagnetic steel sheet, the method comprising: hot-rolling a steel material to obtain a hot-rolled sheet; subjecting the hot-rolled sheet to cold rolling once or subjecting the hot-rolled sheet to cold rolling two or more times and to process annealing interposed therebetween, thereby obtaining a cold-rolled sheet having a final sheet thickness; and subjecting the cold-rolled sheet to decarburization annealing serving also as primary recrystallization annealing and then to finish annealing. The decarburization annealing is conducted such that in the course of temperature rising, the cold-rolled sheet is rapidly heated from 400°C to a temperature T (°C) between 700°C and 900°C at an average heating rate of 250 °C / s or higher and that a time period of 0.10 s or longer but shorter than 1.00 s is set during which the heating rate for any temperatures between 500°C and 700°C is not higher than 2 / 3 the average heating rate. Thus, a grain-oriented electromagnetic steel sheet having excellent magnetic properties is produced. The rapid heating in the decarburization annealing is conducted using a transverse-type induction heater.

Description

[Technical Field]

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

[0002] Grain-oriented electrical steel is a soft magnetic material widely used as the core material for transformers and generators. <001> This steel sheet has excellent magnetic properties and a crystalline structure in which the orientation (Goss orientation) is highly aligned in the rolling direction of the steel sheet.

[0003] One way to reduce iron loss in grain-oriented electrical steel sheets is to concentrate the crystal grains in the Goss orientation to a high degree 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. In other words, it is important to optimize the texture of the steel sheet after primary recrystallization.

[0004] The primary recrystallization structure in which only sharp Goss-oriented grains can grow preferentially is {111} <112> Oriented grains and <148> By having these grains present in a balanced manner and at a high frequency, it is possible to highly concentrate Goss orientation grains in the rolling direction during secondary recrystallization annealing.

[0005] As a method for increasing the proportion of Goss-oriented grains in the steel sheet structure after primary recrystallization, 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 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 further, during final cold rolling, the steel sheet is held at a temperature of 150 to 300°C for 2 minutes or more during interpass aging, which is performed two or more times. Patent Document 3 discloses a technology that utilizes dynamic strain aging, in which dislocations introduced during rolling are immediately fixed with C or N by warm rolling, 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 temperature of the steel sheet to an appropriate temperature before, during, or between passes of cold rolling, thereby promoting the diffusion of dissolved carbon (C) and nitrogen (N) to pin dislocations introduced during cold rolling and suppressing dislocation movement, thereby promoting shear deformation in subsequent rolling. This is because the nuclei of Goss-oriented grains in the primary recrystallized structure are formed in the {111} <112> The idea is that they emerge from shear bands introduced into the oriented processed structure. <112> It becomes possible to introduce many shear bands into the processed structure, and many Goss-oriented grains can be formed 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 normal heating rates, 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 generation of Goss-oriented grains, which serve as nuclei for secondary recrystallized grains.

[0008] Patent Document 5 discloses a method in which, during the temperature rise 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 then a holding treatment is performed in which the heating rate is reduced to 10°C / s or less for 1 to 10 seconds in a 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 holding the material for a short time in the recovery temperature range of 250 to 550°C. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 50-016610 [Patent Document 2] Japanese Patent Application Publication No. 08-253816 [Patent Document 3] Japanese Patent Application Publication No. 01-215925 [Patent Document 4] Japanese Patent Application Publication No. 04-160114 [Patent Document 5] Japanese Patent Application Laid-Open No. 2014-152393 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the above-mentioned Patent Documents 1 to 3 disclose the method of forming a {111} <112> The technique of fixing dislocations in the processed structure with C and N and then introducing many shear bands into the processed structure by cold rolling increases the number of Goss-oriented grains when excessive shear bands are introduced, but the {111} <112> The problem is that the number of oriented grains decreases too much, making it difficult for sharp Goss oriented grains to grow during secondary recrystallization. Therefore, the techniques of Patent Documents 1 to 3 have limitations on the improvement of magnetic properties, and it is becoming difficult to fully meet the increasingly stringent demands for energy conservation in recent years.

[0011] In addition, the method disclosed in Patent Document 4, in which rapid heating is performed during decarburization annealing, increases the number of Goss-oriented grains, but the {111} <112> The improvement in magnetic properties was insufficient due to the decrease in oriented grains. <112> This can hinder the recrystallization of Goss-oriented grains in the processed structure, which can result in the problem that the desired improvement in magnetic properties cannot be stably achieved.

[0012] The present invention has been made in view of the above-mentioned problems associated with the prior art, and has as its object to propose a method for producing grain-oriented electrical steel sheets that solves the above-mentioned problems and enables the stable production of grain-oriented electrical steel sheets with excellent magnetic properties, as well as to provide an induction heating device for decarburization annealing to be used in the method. [Means for solving the problem]

[0013] In order to solve the above problems, the inventors focused on the holding treatment conditions carried out during the rapid heating of decarburization annealing disclosed in the above Patent Document 5, and discovered that the Goss oriented grains and the {111} of the matrix are present in the primary recrystallized structure. <112> As a result, we discovered that the above-mentioned problems can be solved by optimizing the conditions for the holding treatment performed during the rapid heating process in the temperature rise process of decarburization annealing, which led to the development of the present invention.

[0014] Based on the above findings, the present invention proposes a method for producing 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 it twice or more times with intermediate annealing in between to form a cold-rolled sheet of a final thickness, and subjecting the cold-rolled sheet to decarburization annealing which also serves as primary recrystallization annealing, followed by finish annealing, characterized in that, during a temperature-rising process of the decarburization annealing, the steel is rapidly heated from 400°C to a temperature T (°C) between 700 and 900°C at an average heating rate of 250°C / s or more, and setting a time of 0.10 s or more and less than 1.00 s during which the heating rate becomes two-thirds or less of the average heating rate at any temperature between 500°C and 700°C during the heating process.

[0015] The steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention is characterized by having a composition containing components of the following group A or group B, with the balance being Fe and unavoidable impurities. Note Group A: 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 at least one of S and Se: 0.01 to 0.05 mass% in total Group B: 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: 0.0070 mass% or less, and Se: 0.0070 mass% or less

[0016] Furthermore, the steel material used in the method for producing the grain-oriented electrical steel sheet of the present invention is characterized by containing, in addition to the above-mentioned chemical composition, at least one of Sb: 0.500 mass% or less, Cu: 1.50 mass% or less, P: 0.500 mass% or less, Cr: 1.50 mass% or less, Ni: 1.500 mass% or less, Sn: 0.50 mass% or less, Nb: 0.0100 mass% or less, Mo: 0.50 mass% or less, B: 0.0070 mass% or less, and Bi: 0.0500 mass% or less.

[0017] 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.

[0018] The present invention also relates to a transverse induction heating device used in the method for manufacturing the above-described grain-oriented electrical steel sheet, wherein the heating coil has a shape of a rounded rectangle composed of two equal-length parallel lines along the sheet width direction and two semi-circles. When the maximum inner diameter of the heating coil in the sheet width direction is R1 (m), the maximum inner diameter of the heating coil in the sheet passing direction is R2 (m), the width of the steel sheet is w (m), and the sheet passing speed of the steel sheet is v (m / s), the induction heating device is characterized by satisfying the relationship of R1≧w and R2<v.

Advantages of the Invention

[0019] According to the present invention, it becomes possible to stably manufacture a grain-oriented electrical steel sheet having excellent magnetic properties, which greatly contributes to energy saving of electrical equipment.

Brief Description of the Drawings

[0020] [Figure 1] It is a graph showing the influence of the average heating rate and the temporarily decreasing heating rate in decarburizing annealing on the iron loss. [Figure 2] It is a graph showing the influence of the time for temporarily decreasing the heating rate during rapid heating in decarburizing annealing on the iron loss. [Figure 3] It is a diagram showing an example of a transverse induction heating device used for rapid heating in decarburizing annealing, where (a) is a plan view and (b) is a cross-sectional view in the width direction. [Figure 4] It is another graph showing the influence of the average heating rate and the temporarily decreasing heating rate in decarburizing annealing on the iron loss. [Figure 5] It is another graph showing the influence of the time for temporarily decreasing the heating rate during rapid heating in decarburizing annealing on the iron loss. [Figure 6] It is a graph showing the influence of the rapid heating end temperature T on the iron loss. [[ID=3,3]]

Embodiments for Carrying Out the Invention

[0021] First, the experiments that led to the development of the present invention will be described. In order to solve the above-mentioned problems of the prior art, the inventors have developed a method for producing Goss-oriented grains and {111} of the matrix in the steel sheet structure after primary recrystallization. <112> The following experiment was carried out to investigate the temperature rise conditions for decarburization annealing to form oriented grains in a balanced manner with a high frequency.

[0022] <Experiment 1> A steel slab containing 0.035 mass% C, 3.4 mass% Si, 0.05 mass% Mn, 0.0086 mass% Al, 0.0050 mass% N, 0.0031 mass% S, and 0.0031 mass% Se, with the remainder being Fe and unavoidable impurities and containing no inhibitor-forming elements, was heated to 1210°C and hot-rolled to a 2.0 mm thick hot-rolled sheet. Test specimens were then hot-annealed at 1000°C for 60 seconds and cold-rolled to a final thickness of 0.20 mm using a 5-stand tandem rolling mill.

[0023] The cold-rolled steel sheets were then subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. As shown in Table 1, the average heating rate from 400°C to 750°C was varied during the decarburization annealing process, and for some samples, the heating rate was temporarily reduced upon reaching 600°C under the conditions shown in Table 1. An annealing separator primarily composed of MgO was then applied to the surface of the steel sheets after the decarburization annealing, followed by finish annealing to induce secondary recrystallization. Unreacted annealing separator was then removed from the surface of the steel sheets after the finish annealing. An insulating coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was then applied, and the steel sheets were subjected to flattening annealing at 800°C for 30 seconds to bake the coating and correct the shape, resulting in a finished steel sheet.

[0024] Epstein test pieces were taken from the product plates thus obtained, and iron loss W was measured in accordance with JIS C 2550. 17 / 50 The iron loss per unit mass at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T was measured, and the results are shown in Table 1.

[0025] [Table 1]

[0026] From Table 1, when the average heating rate from 400°C to 750°C during the temperature rise process of decarburization annealing is rapid heating of 250°C / s or more, and the heating rate is temporarily reduced during the rapid heating, the iron loss W 17 / 50 It can be seen that the iron loss value of 0.89 W / kg is reduced to 0.89 W / kg or less. Here, the iron loss value of 0.89 W / kg is the reference value of the present invention for judging the quality of the iron loss characteristics of a grain-oriented electrical steel sheet with a sheet thickness of 0.20 mm. The reference value depends on the sheet thickness, and increases as the sheet thickness increases.

[0027] As mentioned above, the reason why iron loss is reduced by setting the average heating rate to 250°C / s or more during the heating process of decarburization annealing and temporarily slowing down the heating rate during the rapid heating is not fully understood at present. However, it is thought that the rapid heating promotes the recrystallization of Goss-oriented grains and the recrystallized {111} <112> We believe this is because the development of oriented grains was well balanced.

[0028] Next, the inventors conducted the following experiment to investigate the influence on iron loss characteristics of the average heating rate of rapid heating in the heating process of decarburization annealing and the heating rate reduced during the rapid heating.

[0029] <Experiment 2> The cold-rolled sheet produced in the above <Experiment 1> was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During this process, the average heating rate during the temperature rise process of the decarburization annealing from 400°C to 750°C was varied between 200 and 500°C / s, and the heating rate was also varied between 25 and 500°C / s for 0.50 seconds when the steel sheet temperature reached 600°C. Next, as in the above <Experiment 1>, an annealing separator was applied to the steel sheet after the decarburization annealing, and the steel sheet was then subjected to finish annealing and flattening annealing to obtain a product sheet. Epstein test pieces were taken from the product sheet, and the iron loss W 17 / 50 was measured.

[0030] The results of the above measurements were compared with the average heating rate from 400°C to 750°C and the heating rate reduced by 0.10 s, and the iron loss W 17 / 50 The relationship between the iron loss and the iron loss is shown in Figure 1. 17 / 50 is below the standard value of 0.89W / kg (good iron loss), and those marked with "▲" are iron loss W 17 / 50 This indicates that the iron loss is higher than the standard value of 0.89 W / kg (poor iron loss).

[0031] From Figure 1, it can be seen that when the average heating rate between 400°C and 750°C is set to 250°C / s or more and the heating rate at 600°C is reduced to 2 / 3 or less of the average heating rate between 400°C and 750°C, the iron loss W 17 / 50 It can be seen that this has been reduced to below the standard value of 0.89 W / kg.

[0032] Furthermore, the inventors conducted the following experiment to investigate the time required to reduce the temperature rise rate in the temperature rise process of decarburization annealing in order to reduce iron loss.

[0033] <Experiment 3> The cold-rolled sheet produced in the above <Experiment 1> was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the heating process, the average heating rate from 400°C to 750°C was changed to two levels, 250°C / s and 300°C / s, and the heating rate was reduced to 50°C / s or 150°C / s when the steel sheet temperature reached 600°C. During this process, the time for reducing the heating rate was varied variously between 0 and 1.2 seconds. Next, as in the above <Experiment 1>, an annealing separator was applied to the steel sheet after the decarburization annealing, and then the steel sheet was subjected to finish annealing and flattening annealing to obtain a product sheet. Epstein test pieces were taken from the product sheet, and the iron loss W 17 / 50 was measured.

[0034] The results of the above measurements are shown in Figure 2. From this figure, it can be seen that the iron loss W 17 / 50 It can be seen that this has been reduced to below the standard value of 0.89 W / kg.

[0035] The present invention was completed based on the above novel findings and further investigations.

[0036] Next, the chemical composition of the steel material used to manufacture the grain-oriented electrical steel sheet of the present invention will be described. The steel material used in the present invention is not particularly limited as long as it has a known chemical composition for grain-oriented electrical steel sheets. However, from the viewpoint of stably producing grain-oriented electrical steel sheets with excellent magnetic properties, it is preferable that the steel material contains C, Si and Mn in the following ranges.

[0037] C: 0.01 to 0.10 mass% C is an austenite-forming element and is useful for increasing the maximum fraction of the γ phase and refining the slab structure. However, if the C content is less than 0.01 mass%, the γ phase fraction decreases, and the slab structure is not sufficiently refined. On the other hand, if the C content exceeds 0.10 mass%, it becomes difficult to reduce it to 0.0050 mass% or less, at which point 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 0.02 to 0.08 mass%.

[0038] Si: 2.0 to 4.5 mass% Si is an element that is effective in increasing the resistivity of steel and reducing 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%.

[0039] 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%, the effect of improving the hot workability cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 0.50 mass%, the primary recrystallization texture deteriorates, and it may become difficult to obtain a secondary recrystallization texture highly concentrated in the Goss orientation. 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 0.30 mass%.

[0040] Furthermore, when AlN is used as an inhibitor to induce secondary recrystallization during finish annealing, the steel material used in the present invention preferably contains, in addition to the above-mentioned C, Si, and Mn, 0.0100 to 0.0400 mass% Al and 0.0050 to 0.0120 mass% N as inhibitor-forming elements. If the Al content and N content are below the above-mentioned lower limits, it becomes difficult to fully achieve the desired inhibitor effect. On the other hand, if the Al content and N content exceed the above-mentioned upper limits, the dispersion state of the precipitated inhibitor becomes non-uniform, again making it difficult to achieve the initial inhibitor effect.

[0041] Furthermore, in addition to the inhibitor AlN described above, Mn sulfides (MnS, CuS, etc.) or selenides (MnSe, CuSe, etc.) may be used as inhibitors, or the sulfides and selenides may be used in combination. When Mn sulfides or selenides are used as additional inhibitors, it is preferable to contain at least one of S and Se in a total amount ranging from 0.01 to 0.05 mass%. If the total content of S and Se is less than the above lower limit, it becomes difficult to obtain a sufficient inhibitor effect. On the other hand, if the total content exceeds the above upper limit, the dispersion of precipitates becomes non-uniform, again making it difficult to obtain a sufficient inhibitor effect.

[0042] On the other hand, when no inhibitor is used to induce secondary recrystallization during finish annealing, it is desirable to reduce the contents of inhibitor-forming components as much as possible, specifically, Al: less than 0.0100 mass%, N: 0.0050 mass% or less, S: 0.0070 mass% or less, and Se: 0.0070 mass% or less.

[0043] In addition to the above components, the steel material used in the present invention may further contain at least one element selected from the group consisting of 0.500 mass% or less Sb, 1.50 mass% or less Cu, 0.500 mass% or less P, 1.50 mass% or less Cr, 1.500 mass% or less Ni, 0.50 mass% or less Sn, 0.0100 mass% or less Nb, 0.50 mass% or less Mo, 0.0070 mass% or less B, and 0.0500 mass% or less Bi. Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi are all useful elements 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. In order to reliably obtain the above-mentioned effects of addition, it is preferable to add Sb: 0.005 mass% or more, Cu: 0.01 mass% or more, P: 0.005 mass% or more, Cr: 0.01 mass% or more, Ni: 0.005 mass% or more, Sn: 0.01 mass% or more, Nb: 0.0005 mass% or more, Mo: 0.01 mass% or more, B: 0.0010 mass% or more, and Bi: 0.0005 mass% or more.

[0044] The steel material used in the present invention contains the remainder other than the above-mentioned components, namely Fe and unavoidable impurities.

[0045] Next, a method for producing the grain-oriented electrical steel sheet of the present invention will be described. The steel material (slab) used for the grain-oriented electrical steel sheet of the present invention is preferably produced by melting steel adjusted to the above-described chemical composition in a commonly known refining process in which molten steel obtained in a converter, electric furnace, or the like is subjected to secondary refining such as vacuum degassing, and then by a commonly known continuous casting method, ingot making-slabbing method, or the like.

[0046] Next, the steel material (slab) is heated to a predetermined temperature and then hot-rolled to obtain a hot-rolled sheet. The heating temperature of the slab is preferably about 1050°C or higher from the viewpoint of ensuring hot-rollability if the slab does not contain an inhibitor-forming component. Furthermore, if the slab contains an inhibitor-forming component, the heating temperature is preferably about 1200°C or higher from the viewpoint of dissolving the inhibitor-forming component in the steel. The upper limit of the heating temperature is not particularly limited, 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 and increasing scale loss. Therefore, the heating temperature is preferably 1450°C or lower. Other hot-rolling conditions may be those generally known and are not particularly limited.

[0047] Next, the hot-rolled steel sheet (hot-rolled sheet) may be subjected to hot-rolled sheet annealing, if necessary. This hot-rolled sheet annealing may be performed under known conditions, and is not particularly limited.

[0048] Next, the hot-rolled sheet or the hot-rolled sheet annealed steel sheet is descaled by pickling or the like, and then cold-rolled to a cold-rolled sheet of the final thickness (product thickness). This cold-rolling may be performed in a single cold rolling step, or may be performed in two or more cold rolling steps with intermediate annealing in between to obtain a cold-rolled sheet of the final thickness.

[0049] In the present invention, the cold rolling to achieve the final plate thickness, specifically, when the final plate thickness is achieved by a single cold rolling pass, that cold rolling pass is referred to, and when the final plate thickness is achieved by two or more cold rolling passes with intermediate annealing in between, the final cold rolling pass is referred to as "final cold rolling." The rolling mill used for cold rolling is not particularly limited, and known rolling mills such as tandem rolling mills, single-stand reversing rolling mills, Sendzimir rolling mills, and planetary rolling mills can be used.

[0050] The reduction ratio of the final cold rolling is not particularly limited, but is preferably 60% or more and 95% or less from the viewpoint of improving the primary recrystallization texture. If it is less than 60%, the {111} <112> The development of oriented grains becomes insufficient, making it difficult for Goss-oriented grains to grow during secondary recrystallization. On the other hand, if it exceeds 95%, cold rolling becomes difficult due to work hardening. Furthermore, the final plate thickness (product plate thickness) is preferably in the range of 0.1 to 1.0 mm. If it is less than 0.1 mm, productivity decreases, and the product plate lacks rigidity, making it difficult to handle when processed into a transformer core. On the other hand, if it exceeds 1.0 mm, eddy current loss increases, resulting in increased iron loss, which is undesirable.

[0051] In the above-mentioned cold rolling, particularly the final cold rolling, in order to form a large number of recrystallization nuclei of Goss-oriented grains in the primary recrystallized structure, it is preferable to perform a heat treatment such as interpass aging or to employ warm rolling, as described in Patent Documents 1 to 3 of the prior art.

[0052] The cold-rolled sheet having the final thickness is then subjected to decarburization annealing, which also serves as primary recrystallization annealing, to reduce the C content to 0.0050 mass% or less, at which point magnetic aging is unlikely to occur. The decarburization conditions (soaking conditions) in this decarburization annealing are not particularly limited and may be any known conditions, but it is preferable to perform annealing in a wet hydrogen atmosphere at 750 to 950°C for 30 to 180 seconds, for example.

[0053] Here, what is important in the present invention is that in this decarburization annealing, it is necessary to rapidly heat the steel from 400°C to a temperature T (°C) between 700 and 900°C at an average heating rate of 250°C / s or more during the heating process up to the soaking temperature. If the average heating rate is less than 250°C / s, primary recrystallization of Goss-oriented grains will be insufficient, and good iron loss properties will not be obtained. A preferred average heating rate is 300°C / s or more. Note that the average heating rate in the present invention is a heating rate including the time during which the heating rate is temporarily reduced, as described below.

[0054] The reason why the temperature T (°C) at which the rapid heating is terminated is set between 700 and 900°C is that if the upper limit of the rapid heating section is less than 700°C, the primary recrystallization of Goss-oriented grains will be insufficient, and the effect of rapid heating will not be obtained. On the other hand, if the temperature exceeds 900°C, the decomposition of the inhibitor (AlN) that occurs at high temperatures will inhibit secondary recrystallization, making it impossible to obtain good iron loss characteristics. The preferred temperature T is in the range of 700 to 850°C.

[0055] Furthermore, during the temperature rise process of the decarburization annealing, at any temperature between 500°C and 700°C during the rapid heating, it is necessary to provide a time of 0.10 seconds or more and less than 1.00 seconds during which the temperature rise rate is reduced to or below the average temperature rise rate from 400°C to T (°C).

[0056] If the temperature at which the heating rate is reduced is less than 500°C, the driving force for recrystallization of Goss-oriented nuclei is reduced by recovery, resulting in insufficient recrystallization of Goss-oriented grains and poor iron loss characteristics. On the other hand, at temperatures above 700°C, the recrystallization rate is already high, so even if the heating rate is reduced, the {111} <112> The effect of promoting the development of oriented grains is not sufficiently achieved.

[0057] As can be seen from Figure 2, the time for reducing the temperature rise rate must be 0.10 seconds or more and less than 1.00 seconds. If it is less than 0.10 seconds, the time for reducing is too short to obtain the effect of reducing the temperature rise rate. On the other hand, if it is 1.00 seconds or more, the {111} <112> The growth of oriented grains becomes excessive, which inhibits the subsequent recrystallization of Goss-oriented grains, and thus it becomes impossible to obtain good core loss properties.The preferred time is 0.20 s or more and 0.70 s or less.

[0058] The rate of temperature rise that is temporarily reduced must be 2 / 3 or less of the average rate of temperature rise between 500°C and T (°C). At a rate of temperature rise higher than this, the {111} <112> The effect of promoting the development of oriented grains cannot be enhanced. Preferably, it is 1 / 2 or less. There is no particular restriction on the lower limit of the reduced heating rate, but since the average heating rate between 400°C and T (°C) needs to be 250°C / s or more, it is necessary to appropriately determine the heating rate, including the time for which the heating rate is reduced. A preferred lower limit of the heating rate is 0°C / s. The heating rate to be temporarily reduced can be determined by measuring the steel sheet temperature during the heating process using a thermocouple or radiation thermometer capable of high-speed response and differentiating the measured temperature with respect to time.

[0059] Here, rapid heating in the temperature rise process of decarburization annealing and a temporary decrease in the temperature rise rate during the process can be achieved by arranging two or more rapid heating devices, such as electric heating devices or solenoid-type induction heating devices, in series on a line, designating any one of the sections between the two or more devices as a section where the temperature rise rate is decreased, and appropriately adjusting the output of the rapid heating devices and the steel sheet passing speed (line speed).

[0060] However, installing two or more rapid heating devices as described above requires a large amount of space. Therefore, it is preferable to use a transverse-type induction heating device as the rapid heating device. The device has heating coils wound around an iron core, which are positioned above and below the steel sheet. The alternating magnetic flux generated within the iron core penetrates the steel sheet in the thickness direction, heating the steel sheet through the magnetic field. In this induction heating device, the induced current flows along the shape of the heating coil within the sheet surface, but does not flow through the portion of the steel sheet facing the iron core. Therefore, the heating rate temporarily decreases as the steel sheet passes through the iron core. Moreover, since this decrease in heating rate occurs within a single induction heating device, there is no problem with installation space. Therefore, a transverse-type induction heating device is preferably suited to the present invention.

[0061] The shape of the heating coil of the above transverse induction heating device may be any of circular, rectangular, elliptical, etc., and is not particularly limited. In FIG. 3, a heating coil having a shape of a rounded rectangle composed of two parallel lines of equal length and two semi - circles is shown as an example. When using a transverse induction heating device having a heating coil of such a shape, when the maximum inner diameter in the plate width direction of the heating coil is R1 (m), the maximum inner diameter in the plate passing direction of the heating coil (the inner diameter at the center position of the steel plate width in FIG. 3) is R2 (m), the width of the steel plate is w (m), and the plate passing speed of the steel plate is v (m / s), it is preferable to satisfy the relationship of R1≧w and R2<v. R1≧w is a necessary condition for generating induced current on the entire surface of the steel plate, and R2<v is a necessary condition for suppressing the temperature rise rate reduction time to less than 1.00 s.

[0062] Next, the cold - rolled sheet subjected to the above decarburization annealing is subjected to finish annealing for secondary recrystallization after applying an annealing release agent to the steel plate surface. As the annealing release agent, known ones can be used and are not particularly limited. For example, those having MgO as the main component and additives such as TiO2 added as necessary, or those having SiO2 or Al2O3 as the main component, etc. can be mentioned.

[0063] The steel plate subjected to the above finish annealing is preferably made into a product plate by removing the unreacted annealing release agent remaining on the steel plate surface, then applying an insulating coating liquid to the steel plate surface, and performing flattening annealing that also serves to bake the above coating and correct the steel plate shape deteriorated by the finish annealing. Note that the formation of the insulating coating may be performed on a separate line. The type of the insulating coating is not particularly limited. However, when forming a tension - applying type insulating coating that applies tensile stress to the steel plate surface, it is preferable to apply a slurry containing phosphate - colloidal silica disclosed in JP - A - 50 - 79442, JP - A - 48 - 39338, JP - A - 56 - 75579, etc. and bake it at a temperature of about 800°C.

[0064] If an even lower iron loss is desired, 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. [Example]

[0065] A steel slab containing inhibitor-forming elements (C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, Al: 0.0250 mass%, N: 0.0090 mass%, S: 0.01 mass%, Se: 0.01 mass%), with the balance consisting of Fe and unavoidable impurities, was heated to 1400°C and hot-rolled to a 2.0 mm thick hot-rolled sheet. The hot-rolled sheet was then subjected to a first cold-rolling to a 1.2 mm intermediate thickness, intermediate annealing at 1100°C for 80 seconds in an atmosphere containing 75 vol% N2 and 25 vol% H2 with a dew point of 46°C, and then cold-rolled (final cold-rolling) to a 0.20 mm thick cold-rolled sheet.

[0066] The cold-rolled sheets were then subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the decarburization annealing, the average heating rate was varied from 400°C to 770°C, as shown in Table 2. For some cold-rolled sheets, the heating rate was temporarily reduced when the steel sheet temperature reached 550°C, as shown in Table 2. The cold-rolled sheets that had undergone the decarburization annealing were then coated with an annealing separator primarily composed of MgO on the surface, followed by finish annealing for secondary recrystallization. After the finish annealing, unreacted annealing separator was removed from the steel sheet surface. An insulating coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was then applied, and the sheet was subjected to flattening annealing at 800°C for 30 seconds to bake the coating and correct the shape, resulting in a finished sheet.

[0067] Epstein test pieces were taken from the product plates thus obtained, and iron loss W was measured in accordance with JIS C 2550. 17 / 50 The results are shown in Table 2.

[0068] From Table 2, it can be seen that even when grain-oriented electrical steel sheets are manufactured using steel slabs containing inhibitor-forming elements, and even when intermediate annealing is performed in the cold rolling process, the iron loss W can be reduced by setting the average heating rate from 400°C to 770°C during the decarburization annealing heating process to 250°C / s or more, and by slowing the heating rate for a short period of time between 0.10 s and 1.00 s during the heating process. 17 / 50 It was confirmed that the radiation energy consumption could be reduced to below the standard value of 0.89 W / kg.

[0069] [Table 2] [Example]

[0070] The cold-rolled sheet having the final thickness produced in Example 1 above was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the decarburization annealing process, the average heating rate from 400°C to 800°C was varied between 200 and 500°C / s, and when the steel sheet temperature reached 650°C, the heating rate was reduced to various rates between 25 and 500°C / s for 0.10 seconds. Next, an annealing separator primarily composed of MgO was applied to the surface of the decarburized, annealed cold-rolled sheet for secondary recrystallization. After the final annealing, unreacted annealing separator was removed from the steel sheet surface, and an insulating coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was applied. The steel sheet was then subjected to planarization annealing at 800°C for 30 seconds to produce a finished sheet.

[0071] Epstein test pieces were taken from the product plates thus obtained, and iron loss W was measured in accordance with JIS C 2550. 17 / 50 The results are shown in Figure 4 as a relationship between the average heating rate during decarburization annealing from 400°C to 800°C and the heating rate reduced by 0.10 seconds, and the iron loss. In the figure, the marks "○" indicate the iron loss W 17 / 50 is the standard value of 0.89W / kg or less, and those marked with "▲" are iron loss W 17 / 50This indicates that the value is higher than the standard value of 0.89 W / kg.

[0072] From Figure 4, it can be seen that even when using a steel slab containing inhibitor-forming elements or when intermediate annealing is performed in the cold rolling process, the iron loss W 17 / 50 It can be seen that the value has been reduced to below the standard value of 0.89 W / kg. [Example]

[0073] The cold-rolled sheet having the final thickness produced in Example 1 above was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the decarburization annealing, the average heating rates from 400°C to 750°C during the heating process were set to two conditions: 250°C / s and 300°C / s. Furthermore, during the heating process, when the steel sheet temperature reached 500°C, the heating rate was reduced to 50°C / s or 100°C / s, and the time for reducing the heating rate was varied in the range of 0 to 1.2 seconds. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface of the decarburization-annealed cold-rolled sheet, and finish annealing was performed to induce secondary recrystallization. Next, after removing unreacted annealing separator from the surface of the steel sheet after the above-mentioned finish annealing, an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied, and the steel sheet was subjected to flattening annealing at 800°C for 30 seconds to obtain a product sheet.

[0074] Epstein test pieces were taken from the product plates thus obtained, and iron loss W was measured in accordance with JIS C 2550. 17 / 50 The results are shown in Figure 5. From this figure, it can be seen that the steel sheets in which the temperature rise rate was reduced during the rapid heating of decarburization annealing for a time period of 0.10 s or more and less than 1.00 s all had low iron loss W 17 / 50 It can be seen that the value has been reduced to below the standard value of 0.89 W / kg. [Example]

[0075] The following two types of steel slabs, A and B, were heated to 1300°C and then hot-rolled to form hot-rolled sheets with a thickness of 2.0 mm. Steel slab A: A steel slab containing C: 0.035 mass%, Si: 3.3 mass%, Mn: 0.05 mass%, Al: 0.0084 mass%, N: 0.0051 mass%, S: 0.0031 mass%, and Se: 0.0031 mass%, with the remainder consisting of Fe and unavoidable impurities, and not containing any inhibitor-forming components. Steel slab B: A steel slab containing inhibitor-forming components, having a composition containing C: 0.06 mass%, Si: 3.4 mass%, Mn: 0.06 mass%, 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.

[0076] Next, test specimens were taken from the hot-rolled sheet produced from the steel slab A, and after annealing at 1000°C for 60 seconds, they were cold-rolled once using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm. On the other hand, test specimens were also taken from the hot-rolled sheet produced from the steel slab B, and after the first cold-rolling to produce a 1.2 mm intermediate thickness, they were annealed at 1100°C for 80 seconds in an atmosphere of 75 vol% N2 + 25 vol% H2 with a dew point of 46°C. After that, they were cold-rolled twice using a Sendzimir rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0077] Next, the cold-rolled steel sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the temperature rise process of the decarburization annealing, the steel sheet was rapidly heated from 400°C to temperature T at an average heating rate of 300°C / s, and the temperature T was varied within a range of 650°C to 950°C. The output of the heating device was adjusted so that, when the steel sheet temperature reached 550°C during the rapid heating, the heating rate became 100°C / s for 0.2 seconds. When the temperature T was 860°C or higher, the steel sheet was rapidly heated to temperature T, cooled to 840°C with nitrogen gas, and then soaked at 840°C for 100 seconds.

[0078] Next, the cold-rolled sheet after the decarburization annealing was subjected to secondary recrystallization by applying an annealing separator to the steel sheet surface, and then subjected to finish annealing, as in Example 1. After that, unreacted annealing separator was removed from the steel sheet surface after the finish annealing, and an insulating coating liquid containing phosphate-chromate-colloidal silica in a mass ratio of 3:1:2 was applied, followed by flattening annealing at 800°C for 30 seconds to obtain a product sheet.

[0079] From the product plate thus obtained, an Epstein test piece was taken and measured for iron loss W in accordance with JIS C 2550. 17 / 50 The results are shown in Figure 6. From this figure, it can be seen that the steel sheets heated under the condition that the rapid heating end temperature T was set between 700 and 900°C all had low iron loss W 17 / 50 It can be seen that this has been reduced to below the standard value of 0.89 W / kg. [Example]

[0080] A steel containing 0.036 mass% C, 3.4 mass% Si, 0.06 mass% Mn, 0.0072 mass% Al, 0.0050 mass% N, 0.0031 mass% S, and 0.0031 mass% Se, with other elements including Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B, and Bi, in the amounts shown in Table 3, with the remainder consisting of Fe and unavoidable impurities, and containing no inhibitor-forming elements, was melted and formed into a steel slab. The slab was then heated to 1210°C and 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 for 60 seconds, and then cold-rolled once (final cold rolling) using a tandem rolling mill to produce a cold-rolled sheet with a final thickness of 0.20 mm.

[0081] The cold-rolled sheet was then subjected to decarburization annealing, which also served as primary recrystallization annealing, with a soaking temperature of 840°C and a soaking time of 100 seconds. During the decarburization annealing process, the steel sheet was rapidly heated from 400°C to 710°C at an average heating rate of 260°C / s using a transverse induction heating device (see Figure 3). Once the steel sheet reached 550°C, the heating rate was reduced to 100°C / s for 0.2 seconds. An annealing separator primarily composed of MgO was then applied to the surface of the decarburized cold-rolled sheet, followed by a finish annealing to induce secondary recrystallization. After the finish annealing, unreacted annealing separator was removed from the steel sheet surface. An insulating coating solution containing phosphate, chromate, and colloidal silica in a mass ratio of 3:1:2 was then applied, followed by planarization annealing at 800°C for 30 seconds to produce the final sheet.

[0082] Epstein test pieces were taken from the product plates thus obtained, and iron loss W was measured in accordance with JIS C 2550. 17 / 50 The results are shown in Table 3. From this table, it can be seen that the steel sheets, which were made from steel slabs containing at least one element selected from Sb, Cu, P, Cr, Ni, Sn, Nb, Mo, B and Bi, and which were rapidly heated under conditions conforming to the present invention using a transverse type induction heating device during the temperature rise process of decarburization annealing, all had iron loss W 17 / 50 is below the standard value of 0.82 W / kg, indicating that it has excellent magnetic properties.

[0083] [Table 3]

Claims

1. A method for manufacturing grain-oriented electrical steel sheet, comprising hot-rolling a steel material having a composition containing elements of Group A or Group B below, with the remainder consisting of Fe and unavoidable impurities, to form a hot-rolled sheet, cold-rolling the hot-rolled sheet once or cold-rolling it two or more times with intermediate annealing in between to form a cold-rolled sheet of final thickness, and subjecting the cold-rolled sheet to decarburization annealing which also serves as primary recrystallization annealing, followed by finish annealing, In the temperature rising process of the decarburization annealing, the temperature is rapidly raised from 400°C to a temperature T (°C) between 700 and 900°C at an average temperature rising rate of 250°C / s or more, a time period during which the heating rate is not more than two-thirds of the average heating rate at any temperature between 550°C and 700°C during the heating process is set to be 0.10 seconds or more and less than 1.00 seconds. Note Group A: 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 at least one of S and Se: 0.01 to 0.05 mass% in total Group B: 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: 0.0070 mass% or less, and Se: 0.0070 mass% or less

2. 2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel material further contains, in addition to the above-mentioned chemical composition, at least one of Sb: 0.500 mass% or less, Cu: 1.50 mass% or less, P: 0.500 mass% or less, Cr: 1.50 mass% or less, Ni: 1.500 mass% or less, Sn: 0.50 mass% or less, Nb: 0.0100 mass% or less, Mo: 0.50 mass% or less, B: 0.0070 mass% or less, and Bi: 0.0500 mass% or less.

3. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the rapid heating in the decarburization annealing is carried out using a transverse type induction heating device.

4. A transverse type induction heating apparatus used in the method for producing a grain-oriented electrical steel sheet according to claim 3, The heating coil has a rounded rectangular shape consisting of two parallel lines of equal length along the plate width direction and two semicircles, The maximum inner diameter of the heating coil in the plate width direction is R １ (m), the maximum inner diameter of the heating coil in the sheet passing direction is R ２ (m), the width of the steel plate is w (m) and the threading speed of the steel plate is v (m / s), R １ ≧w and R ２ <v is satisfied.

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