Manufacturing method of grain-oriented electrical steel sheet

Abstract

Proposed is a method for producing a grain-oriented electrical steel sheet having a low iron loss and a high magnetic flux density, by using a steel material that contains no inhibitor-forming components. The present invention involves: heating, at a temperature of 1150-1250°C, a steel material containing, in mass%, 0.002-0.10% of C, 2.0-8.0% of Si, 0.005-1.0% of Mn, less than 0.010% of Al, less than 0.0050% of N, less than 0.0070% of Se, and less than 0.0050% of S; hot rolling and then cold rolling the steel material to obtain a cold-rolled sheet having a final thickness; subjecting the cold-rolled sheet to decarburization annealing that also serves as primary recrystallization annealing; applying an annealing separation agent to a surface of the steel sheet; subjecting the steel sheet to finishing annealing; and, in order to form an insulating coating, subjecting the steel sheet to rapid heating at 100-1000°C / s during between 500°C and 700°C of the heating process of the decarburization annealing, and subjecting the steel sheet to gradual heating for at least 10 hours at a temperature increase rate of 0.5-4.0°C / hour in the 800-900°C section of the heating process of the finishing annealing.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet with low core loss and high magnetic flux density. [Background technology]

[0002] Grain-oriented electrical steel is a soft magnetic material used as the iron core material for transformers and generators. <001> This texture is characterized by a crystalline structure with a highly aligned orientation in the rolling direction of the steel sheet. This texture causes secondary recrystallization during the finish annealing process in the manufacturing process of grain-oriented electrical steel sheets, resulting in a {110} Goss orientation. <001> It is formed by preferentially growing crystal grains of the same orientation.

[0003] To induce the secondary recrystallization, a technique using fine precipitates called inhibitors is commonly used. For example, Patent Document 1 discloses a method using MnS or MnSe as an inhibitor, and Patent Document 2 discloses a method using AlN or MnS as an inhibitor, both of which have been put into industrial use. These inhibitor-based methods require heating the slab to extremely high temperatures of 1300°C or higher, but are very effective methods for stably inducing secondary recrystallization.

[0004] As techniques for enhancing the action of these inhibitors, Patent Document 3 discloses a method of adding Pb, Sb, Nb, or Te, and Patent Document 4 discloses a method of adding Zr, Ti, B, Nb, Ta, V, Cr, and Mo. Furthermore, Patent Document 5 proposes a method in which acid-soluble Al (sol.Al) is added to a steel material in an amount of 0.010 to 0.060%, the slab heating temperature is kept low, and nitriding is performed in an appropriate atmosphere in the decarburization annealing process, thereby precipitating (Al,Si)N during secondary recrystallization in finish annealing and using the precipitated (Al,Si)N as an inhibitor.

[0005] On the other hand, Patent Document 6 and other publications disclose a technique for developing Goss-oriented grains by secondary recrystallization using a steel material that does not contain inhibitor-forming elements. This technique minimizes impurities such as inhibitor-forming elements and reveals the grain boundary misorientation angle dependence of the grain boundary energy of the grain boundaries during primary recrystallization, thereby enabling secondary recrystallization of Goss-oriented grains without the use of inhibitors. This effect is also known as texture inhibition. This method offers significant advantages in terms of cost and manufacturing, such as eliminating the need for a purification process for inhibitor-forming elements, thereby eliminating the need for high-temperature finish annealing. Furthermore, since it does not require the fine dispersion of inhibitors, it also eliminates the need for high-temperature slab heating, which is essential for solid-solubilizing the inhibitor-forming elements.

[0006] Furthermore, to improve the performance of transformers, it is necessary to reduce no-load loss (energy loss), and it is therefore essential that the steel sheet material used in the iron core has low iron loss.Methods known to be effective in reducing iron loss in grain-oriented electrical steel include increasing the silicon content, reducing sheet thickness, improving the crystal orientation, applying tension to the steel sheet, smoothing the steel sheet surface, and refining the grain size of the secondary recrystallization structure.

[0007] Among the above methods, a method has been proposed for improving the primary recrystallization texture by rapid heating during decarburization annealing as a technique for refining secondary recrystallized grains. For example, Patent Document 7 discloses a technique for obtaining a grain-oriented electrical steel sheet with low iron loss by limiting the amount of N as AlN in the hot-rolled sheet to 25 massppm or less and heating it to 700°C or higher at a heating rate of 80°C / s or more during decarburization annealing. Furthermore, Patent Document 8 discloses a technique for obtaining a grain-oriented electrical steel sheet with low iron loss by limiting the amount of N as AlN in the hot-rolled sheet to 25 massppm or less and heating it to 700°C or higher at a heating rate of 80°C / s or more during decarburization annealing. H2O / P H2 This publication discloses a technique for obtaining grain-oriented electrical steel sheets with low iron loss by rapidly heating to a temperature of 700°C or higher at a rate of 100°C / s or higher in a non-oxidizing atmosphere with a modulus of 0.2 or lower. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 51-013469 [Patent Document 2] Special Publication No. 40-015644 [Patent Document 3] Special Publication No. 38-008214 [Patent Document 4] Japanese Patent Application Publication No. 52-024116 [Patent Document 5] Japanese Patent Application Publication No. 03-002324 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-129356 [Patent Document 7] Japanese Patent Application Publication No. 10-130729 [Patent Document 8] Japanese Patent Application Publication No. 07-062436 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the techniques disclosed in Patent Documents 7 and 8 have the problem that rapid heating during decarburization annealing reduces iron loss but also reduces magnetic flux density. Therefore, an object of the present invention is to propose a method for producing grain-oriented electrical steel sheet with low iron loss and high magnetic flux density by preventing the reduction in magnetic flux density caused by rapid heating during decarburization annealing when using a steel material that does not contain inhibitor-forming components. [Means for solving the problem]

[0010] The inventors conducted extensive research into the cause of the decrease in magnetic flux density due to rapid heating during the decarburization annealing. As a result, they discovered that when the temperature rise rate during the heating process of decarburization annealing is increased, slow heating in the temperature range where secondary recrystallization occurs during finish annealing and grain growth of only Goss-oriented crystal grains can be performed, thereby realizing low iron loss without incurring a decrease in magnetic flux density, and have developed the present invention.

[0011] Based on the above findings, the present invention provides a steel material having a chemical composition containing C: 0.002 to 0.10 mass%, Si: 2.0 to 8.0 mass%, Mn: 0.005 to 1.0 mass%, Al: less than 0.010 mass%, N: less than 0.0050 mass%, Se: less than 0.0070 mass%, and S: less than 0.0050 mass%, with the balance being Fe and unavoidable impurities, the steel material being heated to a temperature of 1150°C or higher and 1250°C or lower, and then hot-rolled to form a hot-rolled sheet, the hot-rolled sheet being annealed, and then 1 The present invention proposes a method for producing a grain-oriented electrical steel sheet, which comprises cold rolling once or cold rolling two or more times with intermediate annealing in between to produce a cold-rolled sheet of final thickness, subjecting the cold-rolled sheet to decarburization annealing which also serves as primary recrystallization annealing, applying an annealing separator to the steel sheet surface, and then subjecting the steel sheet to finish annealing. In order to form an insulating coating, the method comprises rapidly heating the steel sheet between 500 and 700°C at a rate of 100 to 1000°C / s in the heating process of the decarburization annealing, and then gradually heating the steel sheet between 800 and 900°C for at least 10 hours at a heating rate of 0.5 to 4.0°C / hr in the heating process of the finish annealing.

[0012] The steel material used in the method for producing a 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 Ni: 0.01 to 1.50 mass%, Cr: 0.01 to 0.50 mass%, Cu: 0.005 to 1.000 mass%, P: 0.005 to 0.500 mass%, Sb: 0.005 to 0.500 mass%, Sn: 0.005 to 0.500 mass%, Bi: 0.005 to 0.500 mass%, Mo: 0.005 to 0.500 mass%, Nb: 0.0010 to 0.0100 mass%, Ta: 0.001 to 0.010 mass%, and Ti: 0.001 to 0.0100 mass%.

[0013] The method for producing a grain-oriented electrical steel sheet of the present invention is characterized in that, in any step after the cold rolling, grooves are formed on the surface of the steel sheet in a direction intersecting the rolling direction to perform magnetic domain refinement treatment.

[0014] The method for producing a grain-oriented electrical steel sheet of the present invention is characterized in that the surface of the steel sheet coated with the insulating coating is subjected to magnetic domain refinement treatment by irradiating the surface with an electron beam or laser beam in a direction intersecting the rolling direction. [Effects of the Invention]

[0015] According to the present invention, the heating rate in the temperature range where secondary recrystallization occurs during finish annealing is optimized, and secondary recrystallization of crystal grains that are misoriented from the Goss orientation is suppressed. This makes it possible to produce a grain-oriented electrical steel sheet with low iron loss without causing a decrease in magnetic flux density, even when rapid heating is performed during decarburization annealing. DETAILED DESCRIPTION OF THE INVENTION

[0016] First, the experiments that led to the development of the present invention will be described. A steel slab containing 0.009 mass% C, 2.92 mass% Si, 0.081 mass% Mn, 0.001 mass% Al, 0.0001 mass% N, 0.0005 mass% Se, and 0.0040 mass% S, with the remainder being Fe and unavoidable impurities, was produced by continuous casting. The slab was then heated to 1200°C and hot-rolled to a 2.4 mm thick hot-rolled sheet. The hot-rolled sheet was then annealed at 1100°C for 30 seconds, followed by a first cold-rolling to a 1.7 mm intermediate thickness, an intermediate annealing at 1020°C for 100 seconds, and a second cold-rolling in a reverse rolling mill to a 0.23 mm thick cold-rolled sheet. The cold-rolled steel sheets were then subjected to decarburization annealing, which also served as primary recrystallization annealing, at 860°C for 60 seconds in a 50vol% H2-50vol% N2 wet atmosphere. The heating rates between 500 and 700°C were set to two levels: 30°C / s and 200°C / s. An MgO-based annealing separator was then applied to the steel sheet surface. The steel sheets were then wound into coils and heated to 850°C at 25°C / hr in an N2 atmosphere. After holding at this temperature for 20 hours, the steel sheets were heated to 900°C at 25°C / hr. The steel sheets were then further heated to 1180°C at 25°C / hr in a mixed atmosphere of H2:50vol%-N2:50vol%. The steel sheets were then subjected to final annealing, which involved a purification treatment, by holding at this temperature for 5 hours in an H2 atmosphere.

[0017] Test pieces were taken from the steel sheets after the final annealing, and the iron loss W 17 / 50 The iron loss (iron loss when excited at 1.7 T at a frequency of 50 Hz) and magnetic flux density B8 (magnetic flux density at a magnetizing force of 800 A / m) were measured using the method described in JIS C 2550. Test specimens were taken from three locations: both ends and the center of the product coil, and the worst (highest) iron loss value and the lowest (worst) magnetic flux density value were taken as representative values ​​for that coil. The results of the above measurements are shown in Table 1 below. It can be seen that when the heating rate between 500 and 700°C during the decarburization annealing heating process was 200°C / s, the iron loss value tended to improve compared to a heating rate of 30°C / s, but the magnetic flux density tended to decrease.

[0018] [Table 1]

[0019] Therefore, experiments were carried out in which the heating rate between 500 and 700°C in the heating process of decarburization annealing was set to 200°C / s (constant), and the heating rate between 800 and 900°C in the heating process of finish annealing was varied as shown in Table 2. 17 / 50 The temperature rise rate in the final annealing was found to have a suitable range, specifically 0.5 to 4.0°C / hr, that can achieve both low iron loss and high magnetic flux density.

[0020] [Table 2]

[0021] As described above, the reason why the iron loss reduction effect of rapid heating in decarburization annealing can be achieved without causing a decrease in magnetic flux density by optimizing the temperature rise rate between 800 and 900°C in finish annealing has not yet been fully elucidated, but the inventors believe it to be as follows. Increasing the heating rate during decarburization annealing increases the number of Goss-oriented and nearby recrystallized grains in the primary recrystallized structure. As a result, the increased Goss-oriented and nearby primary recrystallized grains undergo secondary recrystallization during final annealing, resulting in a refined secondary recrystallized structure. Here, as proposed by the present invention, when slowly heated at 0.5 to 4.0°C / hr in the temperature range of 800 to 900°C, the grain growth of Goss-oriented grains relative to their neighboring grains becomes dominant, increasing the orientation sharpness of the secondary recrystallized structure and increasing the magnetic flux density B8. In contrast, when the heating rate is less than 0.5°C / hr, the difference in grain growth between Goss-oriented grains and their neighboring grains is small, resulting in a decrease in the orientation sharpness of the secondary recrystallized structure. On the other hand, when the heating rate exceeds 4.0°C / hr, the growth rates of both the Goss-oriented grains and their neighboring grains increase significantly, again resulting in a decrease in the orientation sharpness of the secondary recrystallized structure. The present invention was developed based on the above novel findings.

[0022] Next, the chemical composition of the steel material (slab) used in manufacturing the grain-oriented electrical steel sheet of the present invention will be described. C: 0.002 to 0.10 mass% If the C content is less than 0.002 mass%, the grain boundary strengthening effect of C is lost, causing cracks in the slab, which may hinder manufacturing or result in surface defects. On the other hand, if the C content exceeds 0.10 mass%, it becomes difficult to reduce the C content to 0.005 mass% or less, at which point magnetic aging does not occur during decarburization annealing. Therefore, the C content is set to the range of 0.002 to 0.10 mass%, preferably 0.010 to 0.080 mass%.

[0023] Si: 2.0 to 8.0 mass% Si is an essential component for increasing the resistivity of steel and reducing iron loss, and if its content is less than 2.0 mass%, the above effect is insufficient, while if its content exceeds 8.0 mass%, workability decreases and rolling becomes difficult. Therefore, the Si content is set to the range of 2.0 to 8.0 mass%, and preferably to the range of 2.5 to 4.5 mass%.

[0024] Mn: 0.005 to 1.0 mass% Mn improves the hot workability of steel, and must be contained in a content of 0.005 mass% or more. On the other hand, if the content exceeds 1.0 mass%, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is set to the range of 0.005 to 1.0 mass%, and preferably to the range of 0.02 to 0.20 mass%.

[0025] Since the present invention is a method for producing grain-oriented electrical steel sheet by inducing secondary recrystallization using a steel slab that does not contain inhibitor-forming components, it is desirable to reduce the inhibitor-forming components Al, N, S, and Se in the steel material as much as possible, specifically, Al: less than 0.010 mass%, N: less than 0.0050 mass%, Se: less than 0.0070 mass%, and S: less than 0.0050 mass%. Preferably, Al: 0.007 mass% or less, N: 0.0040 mass% or less, Se: 0.0050 mass% or less, and S: 0.0040 mass% or less.

[0026] The steel material used in the present invention contains the above-mentioned components, with the remainder being Fe and unavoidable impurities. However, for the purpose of improving iron loss characteristics, in addition to the above-mentioned components, the steel material may further contain one or more elements selected from the group consisting of 0.01 to 1.50 mass% Ni, 0.01 to 0.50 mass% Cr, 0.005 to 1.000 mass% Cu, 0.005 to 0.500 mass% P, 0.005 to 0.500 mass% Sb, 0.005 to 0.500 mass% Sn, 0.005 to 0.500 mass% Bi, 0.005 to 0.500 mass% Mo, 0.0010 to 0.0100 mass% Nb, 0.001 to 0.010 mass% Ta, and 0.001 to 0.0100 mass% Ti.

[0027] Next, a method for producing the grain-oriented electrical steel sheet of the present invention will be described. The method for producing a grain-oriented electrical steel sheet of the present invention comprises a series of steps: heating a steel material (slab) having the above-described chemical composition to a predetermined temperature, hot-rolling the slab to form a hot-rolled sheet, annealing the hot-rolled sheet, cold-rolling the slab once or two or more times with intermediate annealing between them to form a cold-rolled sheet of a final thickness, subjecting the cold-rolled sheet to decarburization annealing that also serves as primary recrystallization annealing, applying an annealing separator to the surface of the steel sheet, followed by finish annealing, and then forming an insulating coating. This process will be described in detail below.

[0028] The above-mentioned steel material (slab) is preferably produced by melting steel adjusted to the above-mentioned composition by a commonly known refining process and then by a continuous casting method or an ingot casting-blooming rolling method. Note that thin slabs with a thickness of 100 mm or less may also be produced by a direct casting method.

[0029] Next, the steel slab is heated to a temperature of 1150°C or higher and 1250°C or lower, and then subjected to hot rolling. If the slab heating temperature is lower than 1150°C, the load during hot rolling may increase, or the specified hot rolling temperature may not be ensured. On the other hand, if the slab is heated to a temperature higher than 1250°C, the thermal energy cost increases. The preferred range is 1180 to 1235°C. The hot rolling conditions are not particularly limited and may be the usual conditions for hot rolling grain-oriented electrical steel sheets. In addition, hot rolling may be omitted in the case of thin slabs.

[0030] Next, the hot-rolled sheet after the hot rolling is subjected to hot-rolled sheet annealing in order to improve the magnetic properties. This hot-rolled sheet annealing may be carried out under commonly known conditions, and there are no particular limitations.

[0031] Next, the hot-rolled sheet after the hot rolling is pickled and descaled, and then cold-rolled to the final thickness (product thickness) by one cold rolling or two or more cold rollings with intermediate annealing in between, as is commonly known.

[0032] The cold-rolled sheet having the final thickness is then subjected to decarburization annealing, which also serves as primary recrystallization annealing, to obtain a desirable primary recrystallized structure and reduce the C content in the steel sheet to 0.0050 mass% or less, at which point magnetic aging does not occur. It is important to rapidly heat the steel sheet during the decarburization annealing process from 500°C to 700°C at a heating rate of 100 to 1000°C / s to improve the primary recrystallized structure and refine the secondary recrystallized grains. A heating rate of less than 100°C / s reduces the number of Goss-oriented grains, which serve as the nuclei for secondary recrystallization, in the primary recrystallized structure. A heating rate of more than 1000°C / s increases the number of grains other than Goss-oriented grains, resulting in deterioration of magnetic properties in either case. A heating rate of 150 to 700°C / s is preferred. In order to promote decarburization, the soaking conditions for decarburization annealing are preferably in a humid atmosphere at 800 to 900° C. for 60 to 300 seconds.

[0033] Next, the decarburization-annealed steel sheet is coated with an annealing separator, dried, and then wound into a coil. It is then subjected to finish annealing in the coiled state to cause secondary recrystallization. During the finish annealing, it is important to gradually heat the steel sheet at a rate of 0.5 to 4.0°C / hr in the range of 800 to 900°C during the heating process in order to preferentially grow Goss-oriented grains and suppress the increase of secondary recrystallized grains that deviate from the Goss orientation. If the heating rate is less than 0.5°C / hr, the sharpness of the Goss orientation in the secondary recrystallized structure decreases. On the other hand, if the heating rate exceeds 4.0°C / hr, the growth rate of the Goss orientation and its neighboring grains increases excessively, resulting in a significant decrease in the concentration of secondary recrystallized grains in the Goss orientation. The heating rate is preferably in the range of 0.7 to 2.0°C / hr. To achieve the above effects, the time for gradually heating the steel sheet at a rate of 0.5 to 4.0°C / hr must be at least 10 hours. The period is preferably 20 hours or more. The period for gradual heating may be a part of the range from 800 to 900° C., provided that gradual heating can be performed for 10 hours or more. The atmosphere in this period is preferably nitrogen, argon, or a mixed atmosphere of nitrogen and argon.

[0034] Following the slow heating process, the steel sheet is heated to 1150-1250°C and then maintained at that temperature for 5-20 hours for purification treatment to remove impurities from the steel sheet. This reduces the impurities in the steel sheet to the level of unavoidable impurities. The atmosphere used during the purification treatment is preferably hydrogen, but nitrogen or argon can also be used if necessary. The rate of temperature increase up to the purification treatment temperature is preferably 5°C / hr or more. The atmosphere used during this treatment can be nitrogen, argon, or a mixture of nitrogen, argon, and hydrogen. The purification treatment may be omitted if the steel material contains sufficient amounts of Al, N, S, Se, and other unavoidable impurities.

[0035] Next, the steel sheet after the finish annealing has unreacted annealing separator removed from the surface, is subjected to planarization annealing, and is then coated with an insulating coating to obtain a finished steel sheet. The insulating coating is preferably a tension-imparting insulating coating from the viewpoint of reducing iron loss.

[0036] Furthermore, from the viewpoint of further reducing iron loss, the non-oriented electrical steel sheet of the present invention is preferably subjected to a magnetic domain refinement treatment. A conventionally known method can be used for magnetic domain refinement. For example, in any step after cold rolling, grooves can be formed on the steel sheet surface by etching or the like, continuously or intermittently in a direction intersecting the rolling direction and at predetermined intervals in the rolling direction, or a method can be used in which an electron beam or a laser beam is irradiated onto the steel sheet surface after the insulating coating is formed, continuously or intermittently in a direction intersecting the rolling direction and at predetermined intervals in the rolling direction. [Example]

[0037] A steel slab containing 0.02 mass% C, 3.40 mass% Si, 0.70 mass% Mn, 0.007 mass% Al, 0.0001 mass% N, 0.0022 mass% Se, and 0.0042 mass% S, with the remainder being Fe and unavoidable impurities, was produced by continuous casting. The slab was then heated to 1200°C and hot-rolled to a 2.4 mm thick hot-rolled sheet. The hot-rolled sheet was then subjected to hot-rolled sheet annealing at 1100°C for 30 seconds, followed by a first cold-rolling to an intermediate thickness of 1.7 mm, intermediate annealing at 1020°C for 100 seconds, and a second cold-rolling in a reverse rolling mill to a final cold-rolled sheet with a thickness of 0.23 mm. Next, the cold-rolled steel sheet was subjected to decarburization annealing, which also served as primary recrystallization annealing, at 860°C for 60 seconds in a 50 vol% H2-50 vol% N2 wet atmosphere. The heating rate during the heating process from 500 to 700°C was 600°C / s. An annealing separator mainly composed of MgO was then applied to the steel sheet surface, which was then wound into a coil and subjected to finish annealing. The heating conditions and annealing atmosphere during the heating process of the finish annealing, from 800 to 900°C, were varied as shown in Table 3.

[0038] Test pieces were taken from the steel sheets after the final annealing, and the iron loss W 17 / 50 The iron loss (iron loss when excited at 1.7 T at a frequency of 50 Hz) and magnetic flux density B8 (magnetic flux density at a magnetizing force of 800 A / m) were measured using the method described in JIS C 2550. Test specimens were taken from three locations, at both ends and in the center of the product coil, and the highest (worst) iron loss value and the lowest (worst) magnetic flux density value were taken as representative values ​​for that coil. The results of the above measurements are shown in Table 3 below. These results show that steel sheets with low iron loss and high magnetic flux density can be obtained when the heating rate in the 800-900°C section of the final annealing is in the range of 0.5-4.0°C / hr.

[0039] [Table 3] [Example]

[0040] Steel slabs having various chemical compositions shown in Table 4 were produced by continuous casting. The slabs were then heated to 1200°C and hot-rolled to a thickness of 2.4 mm. The hot-rolled sheets were then subjected to hot-rolled annealing at 1100°C for 30 seconds, and then to a first cold rolling process to produce cold-rolled sheets with a final thickness of 0.23 mm. The cold-rolled sheets were then subjected to decarburization annealing, which also served as primary recrystallization annealing, at 860°C for 60 seconds in a 50 vol% H2-50 vol% N2 moist atmosphere. The heating rate during this process between 500 and 700°C was 350°C / s. An annealing separator mainly composed of MgO was then applied to the surface of the steel sheet, and the coiled steel sheet was subjected to finish annealing. In this case, the heating in the gradual heating section from 800 to 900°C was performed in an N2 atmosphere at a temperature increase rate of 1.0°C / hr between 820 and 860°C for 40 hours, and the temperature increase rate at other temperatures was 15°C / hr. After that, the temperature was increased to 1180°C at 25°C / hr in a mixed atmosphere of 75 vol% H2 and 25 vol% N2, and a purification treatment was performed by holding at that temperature for 30 hours in an H2 atmosphere.

[0041] [Table 4]

[0042] Test pieces were taken from the steel sheets after the final annealing, and the iron loss W 17 / 50 The iron loss (iron loss when excited at 1.7 T at a frequency of 50 Hz) and magnetic flux density B8 (magnetic flux density at a magnetizing force of 800 A / m) were measured using the method described in JIS C 2550. Test specimens were taken from three locations: both ends and the center of the product coil, and the highest (worst) iron loss value and the lowest (worst) magnetic flux density value were taken as representative values ​​for that coil. The results of the above measurements are also shown in Table 4. These results demonstrate that by using a steel material that satisfies the present invention and applying conditions that are compatible with the present invention, grain-oriented electrical steel sheets with low iron loss and high magnetic flux density can be consistently obtained.

Claims

1. A steel material having a component composition containing C: 0.002 to 0.10 mass%, Si: 2.0 to 8.0 mass%, Mn: 0.005 to 1.0 mass%, Al: less than 0.010 mass% (including 0 mass%), N: less than 0.0050 mass% (including 0 mass%), Se: less than 0.0070 mass% (including 0 mass%), and S: less than 0.0050 mass% (including 0 mass%), with the balance being Fe and unavoidable impurities, is heated to a temperature of 1150°C or higher and 1250°C or lower, The steel sheet is hot-rolled to form a hot-rolled sheet, and the hot-rolled sheet is then subjected to hot-rolled sheet annealing. After that, the hot-rolled sheet is cold-rolled once or twice or more times with intermediate annealing sandwiched therebetween to form a cold-rolled sheet of a final sheet thickness. The cold-rolled sheet is then subjected to decarburization annealing which also serves as primary recrystallization annealing. After that, an annealing separator is applied to the surface of the steel sheet, and finish annealing is performed. Then, an insulating coating is formed. In the heating process of the decarburization annealing, the temperature is rapidly heated between 500 and 700°C at a rate of 100 to 1000°C / s, In the 800 to 900 ° C. section of the heating process of the above-mentioned finish annealing, raising the temperature at a temperature rising rate of 0.5 to 4.0 ° C. / hr is called slow heating, and when the section in which the temperature is raised at a temperature rising rate of 0.5 to 4.0 ° C. / hr within the 800 to 900 ° C. section is defined as the slow heating section, A method for manufacturing a grain-oriented electrical steel sheet, wherein the time for slow heating in the slow heating section is at least 10 hours. However, this does not apply when holding treatment is performed after slow heating during the heating process of the above-mentioned finish annealing.

2. The steel material further contains, in addition to the above-described component composition, at least one of Ni: 0.01 to 1.50 mass%, Cr: 0.01 to 0.50 mass%, Cu: 0.005 to 1.000 mass%, P: 0.005 to 0.500 mass%, Sb: 0.005 to 0.500 mass%, Sn: 0.005 to 0.500 mass%, Bi: 0.005 to 0.500 mass%, Mo: 0.005 to 0.500 mass%, Nb: 0.0010 to 0.0100 mass%, Ta: 0.001 to 0.010 mass%, and Ti: 0.001 to 0.0100 mass%.

3. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein in any step after the cold rolling, grooves are formed on the surface of the steel sheet in a direction intersecting the rolling direction to perform magnetic domain refinement treatment.

4. 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the surface of the steel sheet on which the insulating coating is formed is subjected to a magnetic domain refinement treatment by irradiating the surface with an electron beam or a laser beam in a direction perpendicular to the rolling direction.

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