Manufacturing method of grain-oriented electrical steel sheets

Abstract

Provided is a method for manufacturing an oriented electromagnetic steel sheet having excellent magnetic characteristics and capable of reducing the heating temperature of a steel slab to at most 1,300ºC. This method for manufacturing an oriented electromagnetic steel sheet comprises subjecting a steel slab to hot rolling, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and secondary recrystallization annealing, wherein, in the hot-rolled sheet annealing, first-stage soaking holding in which the hot-rolled steel sheet is held at a soaking temperature of 350-1,000ºC for a soaking time of 3.5-120 s is performed at least once, and second-stage soaking holding in which the hot-rolled steel sheet is held at a soaking temperature of higher than 1,000ºC and at most 1,100ºC is performed.

Description

Technical Field

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

Background Art

[0002] A grain-oriented electrical steel sheet is a soft magnetic material in which the direction of the magnetization easy axis is aligned with the rolling direction by utilizing secondary recrystallization to accumulate the crystal orientations of Fe-Si polycrystals in the {110}<001> orientation (hereinafter referred to as the "Goss orientation"). Since the grain-oriented electrical steel sheet has low iron loss at commercial frequencies and can obtain a high magnetic flux density with a low excitation field, it is mainly used as a core material for electrical equipment such as transformers. The iron loss of the grain-oriented electrical steel sheet is represented by the sum of the hysteresis loss that depends on the crystal orientation and steel sheet purity, etc., and the eddy current loss that depends on the sheet thickness, specific resistance, size of magnetic domains, etc. As a method for reducing the hysteresis loss, a method of improving the magnetic flux density by increasing the degree of accumulation in the Goss orientation is known. As a method for reducing the eddy current loss, methods such as increasing the content of Si, etc. that increase the electrical resistance, reducing the sheet thickness of the steel sheet, and subdividing the magnetic domains are known.

[0003] Among these methods for reducing iron loss, a method of increasing the degree of accumulation of the grain-oriented electrical steel sheet in the Goss orientation by creating a difference in the mobility of grain boundaries in finish annealing using precipitates called inhibitors has been industrially put into practical use. Patent Document 1 discloses a method of using AlN as an inhibitor, and Patent Document 2 discloses a method of using MnS and MnSe as inhibitors. In these methods, it is necessary to heat the steel slab to a temperature exceeding 1300°C in order to completely dissolve the component elements constituting the inhibitor in the steel. Therefore, there are problems in terms of the consumption of energy required for high-temperature heating and the cost of equipment.

[0004] To address these challenges, Patent Document 3 discloses a method for secondary recrystallization of grains with Goss orientation without using inhibitors, by making the dependence of grain boundary energy on the grain boundary orientation difference angle during primary recrystallization apparent through high-purity material and the action of trace amounts of nitrogen. Furthermore, Patent Document 4 discloses a method for producing grain-oriented electrical steel sheets with excellent magnetic properties in a component system that actively avoids the use of inhibitors, by removing as much as possible Al, S, N, and Se, which are elements that can form inhibitors, while utilizing these elements, which cannot be completely removed in industrial-scale manufacturing. In this method, it is said that excellent magnetic properties can be obtained by performing hot-rolled sheet annealing with an average heating rate of 50°C / s or more from room temperature to 400°C, a time of 100 s or less to reach 900°C from 400°C, and uniform heating and holding at a temperature of 950°C or higher. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Special Publication No. 40-15644 [Patent Document 2] Japanese Patent Application Publication No. 49-61019 [Patent Document 3] Japanese Patent Publication No. 2000-129356 [Patent Document 4] Japanese Patent Publication No. 2017-160489 [Non-patent literature]

[0006] [Non-Patent Document 1] J. Kunze, Pungun O, K. Friedrich, J. Mater. Sci. Lett., 5(1986) 815-818. [Overview of the project] [Problems that the invention aims to solve]

[0007] The manufacturing methods for grain-oriented electrical steel sheets disclosed in Patent Documents 3 and 4 have succeeded in reducing the heating temperature of the steel slab to 1300°C or below by either not using inhibitors at all or not actively using inhibitors. However, because these grain-oriented electrical steel sheets do not actively use inhibitors, depending on the manufacturing conditions, the degree of accumulation in the Goss orientation may not be sufficient compared to conventional grain-oriented electrical steel sheets that actively use inhibitors, and there was room for improvement.

[0008] The present invention has been made in view of the above problems, and aims to provide a method for manufacturing grain-oriented electrical steel sheets that has excellent integration in the Goth orientation and magnetic properties, and that can reduce the heating temperature of the steel slab to 1300°C or less. [Means for solving the problem]

[0009] The inventors, in relation to the hot-rolled sheet annealing process performed on hot-rolled steel sheets obtained by hot-rolling, as disclosed in Patent Document 4, explored novel heat treatment conditions to further increase the degree of concentration towards the Goss orientation and stably obtain excellent magnetic properties. As a result, they discovered that by performing a soaking-in-the-grain electrical steel sheet with excellent magnetic properties one or more times for an extremely short time in a temperature range of 350°C or higher and 1000°C or lower, before the conventional soaking-in-the-grain holding process, it is possible to manufacture a grain-oriented electrical steel sheet with excellent magnetic properties, thus completing the present invention.

[0010] The gist of the present invention is as follows:

[0011] [1] In mass ratio, C: 0.0020% or more, 0.100% or less, Si: 2.0% or more, 6.5% or less, Mn: 0.020% or more, 1.00% or less, sol.Al: 10 ppm or more, less than 100 ppm, N: 10 ppm or more, 50 ppm or less, S: 10ppm or more, 50ppm or less A steel slab is prepared having a composition in which the remainder consists of Fe and unavoidable impurities, After heating the steel slab to a temperature of 1300°C or lower, it is subjected to hot rolling to form a hot-rolled steel sheet. The aforementioned hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet. The aforementioned hot-rolled sheet and annealed sheet are subjected to cold rolling once or two or more times with an intermediate annealing in between to obtain a cold-rolled steel sheet having a final thickness. The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallized annealed sheet. A method for manufacturing grain-oriented electrical steel sheets, comprising applying an annealing separating agent to the surface of the primary recrystallized annealed sheet and then performing secondary recrystallization annealing, In the aforementioned hot-rolled sheet annealing, The hot-rolled steel sheet is subjected to a first stage of soaking, in which it is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 s or higher and 120 s or lower, at least once. The hot-rolled steel sheet is subjected to a second stage of uniform heating, maintaining it at a uniform temperature between 1000°C and 1100°C. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features. [2] The method for manufacturing grain-oriented electrical steel sheets according to [1] above, wherein the average heating rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is 50°C / s or more during the hot-rolled sheet annealing process. [3] The above component composition is further, in terms of mass ratio, Sb: 0.01% or more, 0.50% or less, Sn: 0.01% or more, 0.50% or less, Ni: 0.005% or more, 1.5% or less, Cu: 0.005% or more, 1.5% or less, Cr: 0.005% or more, 0.10% or less, P: 0.005% or more, 0.50% or less, Mo: 0.005% or more, 0.50% or less, Ti: 0.0005% or more, 0.10% or less, Nb: 0.0005% or more, 0.10% or less, Bi: 0.005% or more, 0.10% or less, Se: 10ppm or more, 50ppm or less, Ca: 0.0005% or more and 0.0050% or less, B: 0.0001% or more and 0.0020% or less, V: 0.0005% or more and 0.10% or less, Pb: 0.0002% or more and 0.050% or less, As: 0.0005% or more and 0.010% or less, and Zn: 0.0005% or more and 0.010% or less The manufacturing method of the grain-oriented electrical steel sheet according to [1] or [2] above, containing one or more selected from the group consisting of the above.

Advantages of the Invention

[0012] According to the manufacturing method according to the present invention, by setting the component composition to contain trace amounts of sol.Al, N, and S, which are elements forming inhibitors, the heating temperature of the steel slab can be reduced to 1300°C or less. Further, by performing two-stage soaking during the hot rolling sheet annealing of the hot rolled steel sheet, a grain-oriented electrical steel sheet excellent in magnetic properties can be manufactured.

Brief Description of the Drawings

[0013] [Figure 1] It is a graph for explaining the temperature profile of hot rolling sheet annealing. [Figure 2] It is a graph showing the relationship between the soaking temperature in the first stage and the magnetic flux density. [Figure 3] It is a graph showing the relationship between the soaking time in the first stage and the magnetic flux density. [Figure 4] It is a graph showing the relationship between the soaking temperature and soaking time in the first stage and the magnetic flux density.

Embodiments for Carrying Out the Invention

[0014] Hereinafter, embodiments for carrying out the present invention will be described in detail.

[0015] In one embodiment, the present invention In terms of mass ratio, C: 0.0020% or more and 0.100% or less, Si: 2.0% or more, 6.5% or less, Mn: 0.020% or more, 1.00% or less, sol.Al: 10 ppm or more, less than 100 ppm, N: 10 ppm or more, 50 ppm or less, S: 10ppm or more, 50ppm or less A steel slab is prepared having a composition in which the remainder consists of Fe and unavoidable impurities, After heating the steel slab to a temperature of 1300°C or lower, it is subjected to hot rolling to form a hot-rolled steel sheet. The aforementioned hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet. The aforementioned hot-rolled sheet and annealed sheet are subjected to cold rolling once or two or more times with an intermediate annealing in between to obtain a cold-rolled steel sheet having a final thickness. The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallized annealed sheet. A method for manufacturing grain-oriented electrical steel sheets, comprising applying an annealing separating agent to the surface of the primary recrystallized annealed sheet and then performing secondary recrystallization annealing, In the aforementioned hot-rolled sheet annealing, The hot-rolled steel sheet is subjected to a first stage of soaking, in which it is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 s or higher and 120 s or lower, at least once. The hot-rolled steel sheet is subjected to a second stage of uniform heating, maintaining it at a uniform temperature between 1000°C and 1100°C. This invention relates to a method for manufacturing grain-oriented electrical steel sheets, characterized by the following features.

[0016] <Component composition> The elemental content of the steel slab prepared in the above embodiment will be described below. In this specification, elemental content is expressed as a mass ratio. The symbol "%" indicates a percentage by mass ratio. The symbol "ppm" indicates parts per million by mass ratio.

[0017] 1.Essential ingredients C: 0.0020% or more, 0.100% or less Carbon (C) is an essential element for preventing brittle fracture when steel is heated to high temperatures. If the C content is 0.0020% or higher, embrittlement at high temperatures is suppressed, thus preventing brittle fracture of steel slabs during casting and hot rolling. If the C content is 0.100% or lower, it can be reduced to 0.005% or lower through decarburization treatment. This avoids the occurrence of magnetic aging due to incomplete decarburization. Therefore, the C content should be between 0.0020% and 0.100%. Preferably, the C content is 0.020% or higher.

[0018] Si: 2.0% or more, 6.5% or less Si is an element necessary to increase the resistivity of steel and reduce iron loss. A Si content of 2.0% or more is effective in reducing iron loss. A Si content of 6.5% or less facilitates both hot rolling and cold rolling. Therefore, the Si content should be between 2.0% and 6.5%. A Si content of 2.5% or more is preferred. A Si content of 4.0% or less is preferred.

[0019] Mn: 0.020% or more, 1.00% or less Mn is an element necessary to improve the hot workability of steel. If the Mn content is 0.020% or more, the hot workability is improved. If the Mn content is 1.00% or less, the magnetic flux density of the grain-oriented electrical steel sheet does not decrease significantly. For this reason, the Mn content should be between 0.020% and 1.00%. The Mn content is preferably 0.040% or more. The Mn content is preferably 0.30% or less.

[0020] sol.Al: 10 ppm or more, less than 100 ppm Al is an important element in grain-oriented electrical steel sheets because it precipitates by combining with nitrogen dissolved in steel to form AlN, and functions as an inhibitor that suppresses normal grain growth of primary recrystallized grains during primary recrystallization annealing. However, in order for AlN to function as an inhibitor, segregation of Al in the steel slab must be prevented and Al must be evenly dissolved in the steel. In conventional technology, the Al content was set to 100 ppm or more in order to actively utilize AlN as an inhibitor. In this case, it was necessary to heat the steel slab to a high temperature of over 1300°C in order to dissolve Al in the steel.

[0021] In this invention, AlN is also used as an inhibitor, but since the content of acid-soluble Al in the steel slab is set to 10 ppm or more and less than 100 ppm, the amount of AlN is less than in the conventional technology. For this reason, a heating temperature of 1300°C or less is sufficient to solid-solve Al. Al is classified into acid-soluble Al and acid-insoluble Al depending on the difficulty of acid dissolution. Acid-soluble Al is used to improve the properties of steel sheets by being included in steel in the form of solid-solution Al or AlN. Acid-insoluble Al is included in steel in the form of Al2O3, etc., but because it is in trace amounts, it has little effect on the properties. For this reason, the content of acid-soluble Al is specified in this invention.

[0022] If the content of acid-soluble Al (hereinafter referred to as "sol.Al") is 10 ppm or more, the required amount of AlN will precipitate as the inhibitor mentioned above, improving the magnetic flux density of the steel plate. If the content of sol.Al is less than 100 ppm, as described above, Al can be dissolved in the steel slab by heating to 1300°C or less. Therefore, the content of sol.Al should be between 10 ppm and 100 ppm. Preferably, the content of sol.Al is 80 ppm or less. The content of sol.Al contained in the steel slab can be measured, for example, by the method specified in Japanese Industrial Standard JIS G 1257-10-2 (2013) or by other known methods.

[0023] N: 10ppm or more, 50ppm or less As mentioned above, N combines with Al and precipitates to form AlN, which acts as an inhibitor. If the N content is 10 ppm or more, the required amount of AlN precipitates as the inhibitor, improving the magnetic flux density of the steel sheet. If the N content is 50 ppm or less, there is no risk of the N contained in the steel slab separating as nitrogen gas during hot rolling and causing blistering. For this reason, the N content should be between 10 ppm and 50 ppm. Preferably, the N content is 25 ppm or less.

[0024] S: 10ppm or more, 50ppm or less S combines with Mn to form MnS. If the S content is 10 ppm or more, the formed MnS functions as an inhibitor, improving the magnetic flux density of the steel sheet. If the S content is 50 ppm or less, the functional degradation of the inhibitor due to Ostwald growth can be prevented. Therefore, the S content should be between 10 ppm and 50 ppm. Preferably, the S content is 25 ppm or less.

[0025] The composition of the steel slab prepared in the above embodiment consists of Fe and unavoidable impurities, with the remainder being the elements mentioned above.

[0026] 2.Additional ingredients In a preferred embodiment, the present invention further provides that the above component composition is, by mass ratio, Sb: 0.01% or more, 0.50% or less, Sn: 0.01% or more, 0.50% or less, Ni: 0.005% or more, 1.5% or less, Cu: 0.005% or more, 1.5% or less, Cr: 0.005% or more, 0.10% or less, P: 0.005% or more, 0.50% or less, Mo: 0.005% or more, 0.50% or less, Ti: 0.0005% or more, 0.10% or less, Nb: 0.0005% or more, 0.10% or less, Bi: 0.005% or more, 0.10% or less, Se: 10ppm or more, 50ppm or less, Ca: 0.0005% or more, 0.0050% or less, B: 0.0001% or more, 0.0020% or less, V: 0.0005% or more, 0.10% or less, Pb: 0.0002% or more, 0.050% or less, As: 0.0005% or more, 0.010% or less, and Zn: 0.0005% or more, 0.010% or less This invention relates to a method for manufacturing grain-oriented electrical steel sheets, which contains one or more selected from the group consisting of the following.

[0027] All of these elements are useful for improving magnetic properties. If the content of each element is above the lower limit of the above range, an improvement in magnetic properties can be obtained. If the content of each element is below the upper limit of the above range, the formation of texture by secondary recrystallization is not hindered. Among the added components, Se combines with Mn to form MnSe. If the Se content is 10 ppm or more, the formed MnSe functions as an inhibitor, improving the magnetic flux density of the steel sheet. If the Se content is 50 ppm or less, the functional degradation of the above inhibitor due to Ostwald growth can be prevented. Therefore, the Se content is preferably 10 ppm or more and 50 ppm or less. The Se content is more preferably 25 ppm or less.

[0028] Next, the manufacturing conditions for grain-oriented electrical steel sheets in the above embodiment will be described.

[0029] <Steel slab> The method for manufacturing grain-oriented electrical steel sheets according to the present invention first involves preparing a steel slab having the component composition described above. The steel slab can be prepared by a general manufacturing method. The steel slab can be manufactured by an ingot-making method in which molten steel with predetermined component adjustments is poured into a mold and then cooled and solidified. Alternatively, the steel slab may be manufactured by a continuous casting method in which molten steel is first received in a tundish, then poured into a water-cooled mold and solidified, and the steel slab is continuously drawn out from the bottom of the mold. The method for manufacturing the steel slab prepared in the present invention may be any of the above methods.

[0030] Of the essential components mentioned above, the content of C, Si, and Mn can be adjusted by changing the mixing ratio of the raw materials used when producing molten steel in various steelmaking furnaces. The content of C, Si, and Mn can be further adjusted as needed by adding additional additives to the molten steel that has been received in the ladle from the steelmaking furnace. Since it is difficult to add the aforementioned additive components during steelmaking, it is also preferable to add them to the molten steel received in the ladle.

[0031] On the other hand, among the essential components mentioned above, the content of sol.Al, N, and S is often already present as unavoidable impurities in the raw materials used to produce molten steel. If the content of sol.Al, N, and S originally present in the molten steel meets the numerical range of content specified in this invention, steel slabs can be produced without adjusting the content of these essential components. If the content of sol.Al, N, or S is less than the lower limit mentioned above, the components can be adjusted so that the content is equal to or above the lower limit by adding additional additives to the molten steel received in the ladle.

[0032] Ferrosilicon and ferromanganese are sometimes used as raw materials for Si and Mn, which are among the essential components mentioned above. If the amount of C and S contained in ferrosilicon and ferromanganese is high, the amount of C and S contained in the steel slab may exceed the upper limit mentioned above. In such cases, it is preferable to adjust the composition of the molten steel using high-purity ferrosilicon and ferromanganese with low C and S content.

[0033] <Hot rolling> The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves heating the prepared steel slab to a temperature of 1300°C or lower, and then hot-rolling it to produce a hot-rolled steel sheet. As described above, in the method for manufacturing grain-oriented electrical steel sheets according to the present invention, the content of sol.Al, N, and S, which are inhibitor-forming elements, in the component composition of the steel slab is kept low. Therefore, even if the heating temperature of the steel slab is 1300°C or lower, these elements can be sufficiently dissolved in the steel, thus reducing the cost of heating the steel slab. The heating temperature of the steel slab is preferably 1100°C or higher. Known means such as gas furnaces, induction heating furnaces, and electric furnaces can be used to heat the steel slab.

[0034] In hot rolling of heated steel slabs, it is preferable from the viewpoint of controlling the microstructure of the hot-rolled steel sheet to first perform rough rolling at a temperature of 1100°C or higher and 1300°C or lower for one or more passes, and then perform finish rolling at a temperature of 800°C or higher and 1100°C or lower for two or more passes. The total reduction ratio in finish rolling is preferably 80% or higher. By setting the total reduction ratio in the temperature range of 1100°C or lower to 80% or higher, dislocations are introduced into the hot-rolled steel sheet at a high density. Since dislocations serve as nucleation sites for precipitates, they contribute to the formation of fine and high-density precipitates, which improves the magnetic properties. The temperature in hot rolling is based on the temperature of the steel sheet surface.

[0035] Hot-rolled steel sheets obtained by hot rolling are preferably wound into coils to facilitate handling. The winding temperature of the hot-rolled steel sheets is preferably 400°C or higher and 750°C or lower, from the viewpoint of both controlling the carbide structure in the hot-rolled steel sheets and preventing defects such as cracks. A winding temperature of 500°C or higher is more preferable. A winding temperature of 700°C or lower is even more preferable. The winding temperature of the hot-rolled steel sheets is based on the temperature of the steel sheet surface immediately before winding.

[0036] As described above, in the method for manufacturing grain-oriented electrical steel sheets according to the present invention, the content of sol.Al dissolved in the steel slab is reduced to less than 100 ppm. Therefore, during the hot rolling process, the N dissolved in the steel slab hardly combines with sol.Al, but instead combines with the Si which is abundant in the steel slab to form silicon nitride Si3N4. Non-patent document 1 describes the results of an investigation into the solubility of nitrogen in Fe-Si alloys. According to this document, the maximum temperature at which Si3N4 can stably exist in an Fe-Si alloy is expressed as a function of the Si content in the Fe-Si alloy. For example, when the Si content is 3%, Si3N4 is considered to exist stably in a temperature range of approximately 900°C or lower.

[0037] <Hot-rolled sheet annealing> The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves first subjecting the hot-rolled steel sheet to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet. The purpose of hot-rolled sheet annealing in the present invention is to replace the Si contained in the Si3N4 precipitate formed in the steel during the hot-rolling process with sol.Al to form AlN. Figure 1 is a graph illustrating the temperature profile of hot-rolled sheet annealing. As shown in Figure 1, in hot-rolled sheet annealing, the hot-rolled steel sheet is subjected to one or more first-stage soaking and holding processes, where it is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 s or higher and 120 s or lower. Subsequently, the hot-rolled steel sheet is subjected to a second-stage soaking and holding process, where it is held at a soaking temperature of over 1000°C and 1100°C or lower. The heat treatment conditions in each temperature range will be described in detail below with reference to Figure 1 as appropriate.

[0038] Temperature range from 1.50°C to 350°C In the method for manufacturing grain-oriented electrical steel sheets according to the present invention, preferably, in the hot-rolled sheet annealing, the average heating rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is set to 50°C / s or higher. In this specification, "average heating rate" refers to the averaged heating rate in a certain temperature range, and specifically, it refers to the value obtained by dividing the difference between the lowest and highest temperatures in that temperature range by the time required for heating. The temperature range from 50°C to 350°C corresponds to the stage immediately before the first stage of uniform heating, which will be described later. By rapidly heating the hot-rolled steel sheet in this temperature range at an average heating rate of 50°C / s or higher, the coarsening of Si3N4 is prevented, and the first stage of uniform heating can be started while maintaining a state in which fine precipitates of Si3N4 are densely distributed in the hot-rolled steel sheet. This makes it possible to improve the magnetic flux density of the steel sheet. There is no particular upper limit to the average heating rate when raising the temperature from 50°C to 350°C, but the average heating rate may be 500°C / s or less.

[0039] In this invention, the heating rate of the hot-rolled steel sheet in the temperature range below 50°C has little effect on the morphology of the Si3N4 precipitate. Therefore, even if heating is started from a temperature below 50°C, the lower limit of the temperature range for evaluating the average heating rate may be 50°C.

[0040] 2. Temperature range between 350°C and 1000°C The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves, in the hot-rolled sheet annealing process, first performing a soaking stage at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 s (seconds) or higher and 120 s or lower, once or more times. As shown in Figure 1, the first soaking stage is performed after the preceding step of raising the temperature from 50°C to 350°C is completed, by raising the temperature of the hot-rolled steel sheet to a soaking temperature T1 and holding it for a soaking time t1. If T1 is less than 350°C, the diffusion of Al does not proceed easily, and the precipitation of AlN does not proceed, so the magnetic flux density decreases. If T1 is greater than 1000°C, the dissolution of Si3N4 occurs before the precipitation of AlN, so the magnetic flux density of the steel sheet decreases. For this reason, the soaking temperature T1 is set to 350°C or higher and 1000°C or lower. The soaking temperature T1 is preferably 400°C or higher, and more preferably 500°C or higher. The soaking temperature T1 is preferably 900°C or lower.

[0041] If the first stage of soaking is not performed at all, or if the soaking time t1 of the first stage is less than 3.5 s, the magnetic flux density of the steel sheet will decrease because there is not enough time for the substitution of Si in the Si3N4 precipitate with sol.Al to proceed. If the soaking time t1 of the first stage exceeds 120 s, the magnetic flux density of the steel sheet will decrease due to excessive deposition and coarsening of AlN. For this reason, the soaking time t1 of the first stage should be 3.5 s or more and 120 s or less. The soaking time t1 is preferably 5.0 s or more. The soaking time t1 is preferably 60 s or less, more preferably 30 s or less.

[0042] The reason why the magnetic flux density of a steel plate improves when it is heated for a short period of time while maintaining a uniform temperature compared to when it is heated at a constant average heating rate in the temperature range of 350°C to 1000°C is not entirely clear, but the inventors believe the following: During uniform heating, the substitution of Si contained in Si3N4 with sol.Al proceeds for a certain period of time, and when sol.Al reaches saturated deposition, Ostwald growth occurs. This makes the AlN grain size uniform, improving the magnetic flux density. On the other hand, during heating, the solid solution of Si contained in Si3N4 is accelerated as the temperature rises, so substitution with sol.Al proceeds earlier. In this case, the AlN grain size is non-uniform from the beginning, making it more difficult to uniformize the AlN grain size in subsequent processes than during uniform heating. As a result, the magnetic flux density decreases. Therefore, maintaining a uniform temperature rather than continuously heating in the temperature range of 350°C to 1000°C is essential for improving the magnetic flux density.

[0043] In this specification, "soaking" refers to maintaining the temperature of the hot-rolled steel sheet at a constant target temperature during hot-rolled sheet annealing. In this specification, a fluctuation of ±5.0°C from the target temperature is considered "soaking" for the duration that the steel sheet temperature is within that range. The temperature of the hot-rolled steel sheet during hot-rolled sheet annealing can be measured by known methods.

[0044] In the method for manufacturing grain-oriented electrical steel sheets according to the present invention, the first stage of soaking and holding may be performed once or repeated two or more times. When the first stage of soaking and holding is repeated two or more times, a cooling period may be provided between soaking and holding. The temperature of the second and subsequent soaking and holding may be the same as the temperature of the first soaking and holding, or it may be a higher or lower temperature than the first soaking and holding, as long as it is within the temperature range of 350°C or higher and 1000°C or lower. When the first stage of soaking and holding is performed multiple times, the soaking time for each time shall be 3.5 s or more and 120 s or less.

[0045] Furthermore, in the temperature range of hot-rolled sheet annealing, AlN is more stable than Si3N4 precipitates. Therefore, the substitution reaction of Si in the Si3N4 precipitates to sol.Al occurs irreversibly, and once AlN is formed, it cannot revert back to Si3N4 precipitates. Consequently, the opportunity to generate fine AlN from Si3N4 precipitates is limited to the hot-rolled sheet annealing process.

[0046] 3. Temperature range above 1000℃ and below 1100℃ The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves a second stage of soaking and holding the hot-rolled steel sheet at a soaking temperature of over 1000°C and 1100°C or less during the hot-rolled steel sheet annealing process. As shown in Figure 1, the second stage of soaking and holding is performed after the completion of the first stage of soaking and holding, in which the hot-rolled steel sheet is heated to a soaking temperature T2 and held for a soaking time t2. This holding allows the precipitation diameter of AlN to be optimized by Ostwald growth. If the soaking temperature T2 is 1000°C or less, the adjustment of the precipitation diameter by Ostwald growth is insufficient, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. If the soaking temperature T2 is over 1100°C, the AlN becomes excessively coarse or redissolves, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. For this reason, the soaking temperature T2 is set to be over 1000°C and 1100°C or less. The soaking temperature T2 is preferably 1050°C or lower. The soaking time t2 for the second stage is not particularly limited. The soaking time t2 is preferably 10 seconds or more. The soaking time t2 is preferably 60 seconds or less.

[0047] 4. Shape of hot-rolled steel sheet during hot-rolled sheet annealing. As described above, in hot-rolled sheet annealing, the hot-rolled steel sheet is repeatedly subjected to rapid heating or cooling and uniform heat retention. If the hot-rolled steel sheet is wound into a coil shape after hot rolling, the coil is unwound to return it to the original shape of the hot-rolled steel sheet, and the hot-rolled sheet annealing is performed using a known continuous annealing furnace, thereby enabling rapid heating or cooling of the hot-rolled steel sheet. Performing hot-rolled sheet annealing in the shape of the hot-rolled steel sheet is also preferable for achieving appropriate temperature control with minimal temperature fluctuations during uniform heat retention after rapid heating or cooling.

[0048] <Cold rolling> The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves cold rolling the hot-rolled sheet and annealed sheet once or twice or more with an intermediate annealing in between to obtain a cold-rolled steel sheet having a final thickness. Preferably, the final thickness of the cold-rolled steel sheet is 0.30 mm or less. If the final thickness of the cold-rolled steel sheet is 0.30 mm or less, eddy current losses can be reduced. A more preferable final thickness for the cold-rolled steel sheet is 0.23 mm or less, and an even more preferable final thickness is 0.20 mm or less. There is no particular lower limit for the final thickness of the cold-rolled steel sheet, but the final thickness in cold rolling is technically limited to approximately 0.10 mm or more.

[0049] The number of cold rolling steps may be one or two or more. If cold rolling is performed two or more times, intermediate annealing is performed between cold rolling steps. Intermediate annealing is preferably performed at an annealing temperature of 900°C or higher and 1200°C or lower. If the annealing temperature is 900°C or higher, the recrystallized grains do not become too fine, and the number of nuclei with a Goss orientation in the primary recrystallized structure increases, improving the magnetic flux density of the steel sheet. If the annealing temperature is 1200°C or lower, the recrystallized grains do not become coarse, and a primary recrystallized structure with uniform grain size can be achieved, also improving the magnetic flux density of the steel sheet. In the final stage of cold rolling, it is preferable to heat the steel sheet to 100°C or higher and 300°C or lower and perform warm rolling, or to perform aging treatment once or multiple times at a temperature of 100°C or higher and 300°C or lower between passes, as this increases the degree of accumulation of the recrystallized texture and improves the magnetic flux density of the steel sheet.

[0050] In cold rolling, it is preferable to perform at least one rolling operation with a reduction ratio of 80% or more. Performing cold rolling with a reduction ratio of 80% or more is advantageous because it increases the degree of accumulation of the recrystallized texture, thereby improving the magnetic flux density of the steel sheet.

[0051] <Primary recrystallization annealing> The method for manufacturing a grain-oriented electrical steel sheet according to the present invention involves first subjecting the cold-rolled steel sheet to primary recrystallization annealing to obtain a primary recrystallized annealed sheet. Primary recrystallization annealing may also serve as decarburization annealing. The annealing temperature for primary recrystallization annealing should be 800°C or higher and 900°C or lower, and a humid atmosphere is preferable for decarburization. However, if the carbon content in the steel slab is 0.0050% or less, it is not necessary to further reduce the carbon content, so the atmosphere for primary recrystallization annealing may be an atmosphere other than that described above. The average heating rate to the holding temperature during primary recrystallization annealing should be 50°C / s or higher and 400°C / s or lower to increase the magnetic flux density of the steel sheet.

[0052] <Secondary recrystallization annealing> The method for manufacturing grain-oriented electrical steel sheets according to the present invention involves first applying an annealing separation agent to the surface of the primary recrystallized annealed sheet, and then performing secondary recrystallization annealing. An annealing separation agent mainly composed of MgO is used. Secondary recrystallization annealing develops secondary recrystallized grains having a Goss orientation and forms a forsterite film on the surface of the steel sheet. It is preferable to perform the secondary recrystallization annealing at a temperature of 800°C or higher for 20 hours or more in order to induce and complete secondary recrystallization. To form a forsterite film, it is preferable to set the temperature of the secondary recrystallization annealing to 1200°C or higher.

[0053] <Post-processing> After secondary recrystallization annealing, the annealing release agent adhering to the surface of the steel sheet is removed by washing with water, brushing, or pickling. Subsequently, correcting the shape of the steel sheet by performing flattening annealing is effective in reducing iron loss.

[0054] When using laminated steel plates, it is effective to apply an insulating coating to the surface of the steel plates before or after planar annealing in order to improve iron loss. In this case, applying a coating that can impart tension to the steel plates is preferable for reducing iron loss. As a coating method that can impart tension to the steel plates, for example, a tension coating application method using a binder, or a coating method in which inorganic material is deposited on the surface layer of the steel plate by physical vapor deposition or chemical vapor deposition can be employed. These coatings are preferable because they have excellent adhesion and an excellent effect in reducing iron loss.

[0055] To further reduce iron loss, it is preferable to perform magnetic domain subdivision treatment. As a method for magnetic domain subdivision treatment, commonly used methods such as applying strain to the final product sheet using an electron beam or laser can be employed. The target of magnetic domain subdivision treatment may be not only the final product sheet, but also intermediate products such as cold-rolled steel sheets that have reached their final thickness. [Examples]

[0056] The following describes embodiments of the present invention. However, the embodiments of the present invention are not limited to the following embodiments and can be modified as appropriate without departing from the spirit of the invention.

[0057] <Example 1> A steel slab with a composition of C:0.055%, Si:3.2%, Mn:0.12%, sol.Al:80ppm, N:35ppm, and S:32ppm by mass ratio, with the remainder being Fe and unavoidable impurities, was manufactured by continuous casting. After heating to 1200°C for 60 minutes, it was hot-rolled to obtain a hot-rolled steel sheet with a thickness of 2.3 mm. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C / s. The soaking time t1 for the first stage was fixed at 30 s, and the soaking temperature T1 was varied from 300°C to 1020°C. The soaking temperature T2 for the second stage was set to 1030°C, and the soaking time t2 was 30 s. The atmosphere for hot-rolled sheet annealing was a dry nitrogen atmosphere. After removing the scale from the surface of the hot-rolled and annealed sheet by pickling, the sheet was cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm.

[0058] Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 830°C for 150 s in a humid atmosphere of 50 vol% N2 - 50 vol% H2 with a dew point of 50°C, to obtain primary recrystallized annealed sheets. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the obtained primary recrystallized annealed sheets, and secondary recrystallization annealing was performed at 1200°C for 5 hours in a hydrogen atmosphere to obtain 17 types of grain-oriented electrical steel sheet samples with different first-stage soaking temperatures. Next, the magnetic flux density B8 of the obtained samples was measured when the strength of the magnetic field applied to the sample was 800 A / m, according to the method specified in Japanese Industrial Standard JIS C 2500. The relationship between the first-stage soaking temperature T1 and the magnetic flux density B8 is shown in Figure 2.

[0059] As shown in Figure 2, an excellent magnetic flux density B8 of 1.925T or higher was obtained within the dashed line range where the first stage soaking temperature T1 is between 350°C and 1000°C.

[0060] <Example 2> A steel slab with a composition of C:0.048%, Si:3.3%, Mn:0.12%, sol.Al:83ppm, N:41ppm, and S:30ppm by mass ratio, with the remainder being Fe and unavoidable impurities, was manufactured by continuous casting. After heating to 1230°C for 60 minutes, it was hot-rolled to obtain a hot-rolled steel sheet with a thickness of 2.3 mm. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C / s. The first soaking temperature T1 was fixed at 750°C, and the soaking time t1 was varied from 0 s to 120 s. The second soaking temperature T2 was set to 1010°C, and the soaking time t2 was set to 30 s. The atmosphere for hot-rolled sheet annealing was a dry nitrogen atmosphere. After removing the scale from the surface of the hot-rolled and annealed sheet by pickling, the sheet was cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm.

[0061] Next, the obtained cold-rolled steel sheets were subjected to primary and secondary recrystallization annealing under the same conditions as in Example 1 to obtain 16 types of grain-oriented electrical steel sheet samples with different soaking times in the first stage. Then, the magnetic flux density B8 was measured for the obtained samples using the same method as in Example 1. Figure 3 shows the relationship between the soaking time t1 and the magnetic flux density B8. In Figure 3, the scale of the soaking time t1 on the horizontal axis is logarithmic.

[0062] As shown in Figure 3, an excellent magnetic flux density B8 of 1.925T or higher was obtained within the dashed line range where the first stage soaking time t1 is between 3.5s and 120s.

[0063] <Example 3> A steel slab with a composition consisting of C:0.035%, Si:3.3%, Mn:0.13%, sol.Al:75ppm, N:42ppm, S:11ppm, and Sb:0.075% by mass ratio, with the remainder being Fe and unavoidable impurities, was manufactured by continuous casting. After heating to 1160°C for 60 minutes, it was hot-rolled to produce a hot-rolled steel sheet with a thickness of 2.4 mm. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. However, unlike Figure 1, the average heating rate from 50°C to 350°C was varied within the range of 20°C / s to 100°C / s. Table 1 shows the average heating rate from 50°C to 350°C, the soaking temperature T1 and soaking time t1 for the first stage, and the soaking temperature T2 and soaking time t2 for the second stage. The hot-rolled sheet was annealed in a dry nitrogen atmosphere. After hot-rolling, the scale on the surface of the annealed sheet was removed by pickling, and then cold-rolled to a thickness of 1.6 mm. Next, intermediate annealing was performed in a humid atmosphere of 70 vol% N2 - 30 vol% H2 with a dew point of 40°C, and then cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.20 mm.

[0064] Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 60 s in a humid atmosphere of 50 vol% N2 - 50 vol% H2 with a dew point of 50°C, to obtain a primary recrystallized annealed sheet. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the obtained primary recrystallized annealed sheet, and secondary recrystallization annealing was performed at 1220°C for 5 s in a hydrogen atmosphere to obtain 40 types of grain-oriented electrical steel sheet samples with different manufacturing conditions. Next, the magnetic flux density B8 was measured for the obtained samples using the same method as in Example 1. The results are shown in Table 1.

[0065] [Table 1]

[0066] Furthermore, Figure 4 shows the results of plotting the results from Table 1, excluding samples No. 2, No. 29, and No. 35, on a graph with the soaking time t1 on the horizontal axis and the soaking temperature T1 on the vertical axis. In Figure 4, the scale of the soaking time t1 on the horizontal axis is logarithmic. In Figure 4, white squares represent inventive examples where the magnetic flux density B8 is 1.925T or higher. Black squares represent comparative examples where the magnetic flux density B8 is less than 1.925T. In Figure 4, the inventive examples and comparative examples of Example 1 are further plotted with white circles and black circles, and the inventive examples and comparative examples of Example 2 are plotted with white triangles and black triangles, respectively.

[0067] As shown in Figure 4, it can be seen that an excellent magnetic flux density B8 of 1.925T or higher was obtained within the dashed range where the first-stage soaking temperature T1 was between 350°C and 1000°C, and the soaking time t1 was between 3.5s and 120s.

[0068] <Example 4> Steel slabs having a composition consisting of essential and additive components in mass ratio as shown in Table 2, with the remainder being Fe and unavoidable impurities, were manufactured by continuous casting. After heating to 1200°C for 60 minutes, they were hot-rolled to obtain hot-rolled steel sheets with a thickness of 2.5 mm. The obtained hot-rolled steel sheets were subjected to hot-rolled sheet annealing according to the temperature pattern shown in Figure 1. The average heating rate from 50°C to 350°C was 50°C / s. The soaking temperature T1 for the first stage was 750°C, and the soaking time t1 was 20 s. The soaking temperature T2 for the second stage was 1030°C, and the soaking time t2 was 30 s. The atmosphere for hot-rolled sheet annealing was a humid atmosphere of 80 vol% N2 - 20 vol% CO2 with a dew point of 30°C. After removing the scale from the surface of the hot-rolled and annealed sheet by pickling, the sheet was cold-rolled at 150°C to obtain a cold-rolled steel sheet with a final thickness of 0.27 mm.

[0069] Next, the obtained cold-rolled steel sheet was subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 180 s in a humid atmosphere of 40 vol% N2 - 60 vol% H2 with a dew point of 50°C, to obtain a primary recrystallized annealed sheet. Next, an annealing separating agent mainly composed of MgO was applied to the surface of the obtained primary recrystallized annealed sheet, and secondary recrystallization annealing was performed at 1175°C for 15 hours in a hydrogen atmosphere to obtain 25 types of grain-oriented electrical steel sheet samples with different component compositions. Next, the magnetic flux density B8 was measured for the obtained samples using the same method as in Example 1. The results are shown in Table 2.

[0070] [Table 2]

[0071] Table 2 shows that in samples of grain-oriented electrical steel sheets manufactured using the manufacturing method specified in the present invention with steel slabs having the component composition specified in the present invention or a preferred component composition, an excellent magnetic flux density B8 of 1.925T or higher was obtained. [Explanation of Symbols]

[0072] T1 first stage soaking temperature t1 First stage soaking time T2 second stage soaking temperature t2 Soaking time for the second stage

Claims

1. In terms of mass ratio, C: 0.0020% or more, 0.100% or less, Si: 2.0% or more, 6.5% or less, Mn: 0.020% or more, 1.00% or less, Sol. Al: 10 ppm or more, less than 100 ppm. N: 10 ppm or more, 50 ppm or less, and S: 10ppm or more, 50ppm or less A steel slab is prepared having a composition in which the remainder consists of Fe and unavoidable impurities, After heating the steel slab to a temperature of 1300°C or lower, it is subjected to hot rolling to form a hot-rolled steel sheet. The aforementioned hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet. The aforementioned hot-rolled sheet and annealed sheet are subjected to cold rolling once or two or more times with intermediate annealing in between to obtain a cold-rolled steel sheet having a final thickness. The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallized annealed sheet. A method for manufacturing grain-oriented electrical steel sheets, comprising applying an annealing separating agent to the surface of the primary recrystallized annealed sheet and then performing secondary recrystallization annealing, In the aforementioned hot-rolled sheet annealing, The hot-rolled steel sheet is subjected to a first stage of soaking, in which it is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 s or higher and 120 s or lower, at least once. The hot-rolled steel sheet is subjected to a second stage of uniform heating, maintaining it at a uniform temperature between 1000°C and 1100°C. A method for manufacturing grain-oriented electrical steel sheets, characterized by producing grain-oriented electrical steel sheets having a magnetic flux density B8 of 1.925T or more.

2. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein, in the hot-rolled sheet annealing, the average heating rate when raising the temperature of the hot-rolled steel sheet from 50°C to 350°C is 50°C / s or more.

3. The aforementioned component composition is further, in terms of mass ratio, Sb: 0.01% or more, 0.50% or less, Sn: 0.01% or more, 0.50% or less, Ni: 0.005% or more, 1.5% or less, Cu: 0.005% or more, 1.5% or less, Cr: 0.005% or more, 0.10% or less, P: 0.005% or more, 0.50% or less, Mo: 0.005% or more, 0.50% or less, Ti: 0.0005% or more, 0.10% or less, Nb: 0.0005% or more, 0.10% or less, Bi: 0.005% or more, 0.10% or less, Se: 10 ppm or more, 50 ppm or less, Ca: 0.0005% or more, 0.0050% or less, B: 0.0001% or more, 0.0020% or less, V: 0.0005% or more, 0.10% or less, Pb: 0.0002% or more, 0.050% or less, As: 0.0005% or more, 0.010% or less, and Zn: 0.0005% or more, 0.010% or less A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2, comprising one or more selected from the group consisting of the following.

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