Method for manufacturing oriented electromagnetic steel sheet

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

Manufacturing method of grain-oriented electrical steel sheet

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

[0002] Grain-oriented electrical steel sheets are soft magnetic materials in which the crystal orientation of Fe—Si polycrystals is concentrated in the {110}<001> orientation (hereinafter referred to as the "Goss orientation") using secondary recrystallization, thereby aligning the axis of easy magnetization in the rolling direction. Grain-oriented electrical steel sheets have low iron loss at commercial frequencies and can achieve high magnetic flux density with a low excitation field, making them primarily used as iron core materials for electrical equipment such as transformers. The iron loss of grain-oriented electrical steel sheets is expressed as the sum of hysteresis loss, which depends on the crystal orientation and purity of the steel sheet, and eddy current loss, which depends on the sheet thickness, resistivity, and magnetic domain size. One known method for reducing hysteresis loss is to increase the concentration of Goss orientation to improve magnetic flux density. Other known methods for reducing eddy current loss include increasing the content of silicon, which increases electrical resistance, reducing the steel sheet thickness, and subdividing magnetic domains.

[0003] Among these methods for reducing iron loss, a method has been put into industrial use in which a precipitate called an inhibitor is used to differentiate the mobility of grain boundaries during finish annealing, thereby increasing the degree of concentration of grain-oriented electrical steel sheets in the Goss orientation. Patent Document 1 discloses a method using AlN as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor. These methods require heating the steel slab to a high temperature of over 1,300°C in order to completely dissolve the constituent elements of the inhibitor in the steel. This has posed problems in terms of the energy consumption required for high-temperature heating and the cost of equipment.

[0004] To address these issues, Patent Document 3 discloses a method for secondary recrystallization of Goss-oriented grains without using inhibitors, by using highly purified materials and trace amounts of nitrogen to reveal the grain boundary energy dependency of grain boundaries during primary recrystallization on the grain boundary misorientation angle. Patent Document 4 also discloses a method for producing grain-oriented electrical steel sheets with excellent magnetic properties, using a composition system that actively avoids inhibitors, by minimizing Al, S, N, and Se, which are elements that can form inhibitors, but which cannot be completely removed in industrial-scale production. This method is said to achieve excellent magnetic properties by hot-rolled sheet annealing, which involves setting an average heating rate from room temperature to 400°C at 50°C / s or more, setting the time required to reach 900°C from 400°C at 100 seconds or less, and soaking at a temperature of 950°C or higher.

[0005] Japanese Patent Publication No. 40-15644 Japanese Patent Laid-Open No. 49-61019 Japanese Patent Laid-Open No. 2000-129356 Japanese Patent Laid-Open No. 2017-160489

[0006] J. Kunze, Pungun O, K. Friedrich, J. Mater. Sci. Lett., 5(1986) 815-818.

[0007] The manufacturing methods of grain-oriented electrical steel sheets disclosed in Patent Documents 3 and 4 do not use any inhibitors at all or do not actively use inhibitors, thereby successfully reducing the heating temperature of the steel slab to 1,300°C or less. However, because these grain-oriented electrical steel sheets do not actively use inhibitors, depending on the manufacturing conditions, the degree of concentration in the Goss orientation may not necessarily be sufficient compared to conventional grain-oriented electrical steel sheets that actively use inhibitors, leaving room for improvement.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a method for producing a grain-oriented electrical steel sheet that has excellent Goss orientation concentration and magnetic properties, and that can reduce the heating temperature of the steel slab to 1300°C or less.

[0009] The inventors have searched for new heat treatment conditions for further increasing the concentration of Goss orientation and stably obtaining excellent magnetic properties in the hot-rolled sheet annealing performed on the hot-rolled steel sheet obtained by hot rolling, among the methods for producing grain-oriented electrical steel sheet disclosed in Patent Document 4. As a result, they have found that grain-oriented electrical steel sheet with excellent magnetic properties can be produced by performing one or more very short soaking periods in a temperature range of 350°C or higher and 1000°C or lower before the conventional soaking period, and have completed the present invention.

[0010] The gist and configuration of the present invention are as follows.

[0011] [1] In mass ratio, C: 0.0020% or more and 0.100% or less, Si: 2.0% or more and 6.5% or less, Mn: 0.020% or more and 1.00% or less, sol. A method for producing a grain-oriented electrical steel sheet, comprising: preparing a steel slab having a chemical composition containing Al: 10 ppm or more but less than 100 ppm, N: 10 ppm or more but less than 50 ppm, and S: 10 ppm or more but less than 50 ppm, with the balance consisting of Fe and unavoidable impurities; heating the steel slab to a temperature of 1300°C or less, and then hot rolling the steel slab to obtain a hot-rolled steel sheet; hot-rolling the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet; cold-rolling the hot-rolled annealed steel sheet once or two or more times with intermediate annealing between them to obtain a cold-rolled steel sheet having a final thickness; primary recrystallization annealing the cold-rolled steel sheet to obtain a primary recrystallization annealed steel sheet; applying an annealing separator to the surface of the primary recrystallization annealed steel sheet, and then secondary recrystallization annealing the cold-rolled steel sheet, wherein in the hot-rolled annealing a first-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less, and a second-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and 1100°C or less. [2] A method for producing a grain-oriented electrical steel sheet according to [1] above, wherein, in the hot-rolled sheet annealing, the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C is 50°C / s or more.[3] The composition further includes, in mass ratio, Sb: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.50% or less, Ni: 0.005% or more and 1.5% or less, Cu: 0.005% or more and 1.5% or less, Cr: 0.005% or more and 0.10% or less, P: 0.005% or more and 0.50% or less, Mo: 0.005% or more and 0.50% or less, Ti: 0.0005% or more and 0.10% or less, Nb: 0.0005% or more and 0.10% or less, Bi: 0.005% or more and 0.10% or less, Se: 10 ppm or more and 50 ppm 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.

[0012] According to the production method of the present invention, the heating temperature of the steel slab can be reduced to 1300°C or less by using a component composition containing trace amounts of sol. Al, N, and S, which are elements that form inhibitors. In addition, by performing two-stage soaking in the hot-rolled sheet annealing, a grain-oriented electrical steel sheet with excellent magnetic properties can be produced.

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

[0014] Hereinafter, embodiments of the present invention will be described in detail.

[0015] In one embodiment, the present invention provides a steel sheet containing, in mass ratios, C: 0.0020% or more and 0.100% or less, Si: 2.0% or more and 6.5% or less, Mn: 0.020% or more and 1.00% or less, sol. A method for producing a grain-oriented electrical steel sheet, comprising: preparing a steel slab having a chemical composition containing Al: 10 ppm or more but less than 100 ppm, N: 10 ppm or more but less than 50 ppm, and S: 10 ppm or more but less than 50 ppm, with the balance consisting of Fe and unavoidable impurities; heating the steel slab to a temperature of 1300°C or less, and then hot rolling the steel slab to obtain a hot-rolled steel sheet; hot-rolling the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet; cold-rolling the hot-rolled annealed steel sheet once or two or more times with intermediate annealing between them to obtain a cold-rolled steel sheet having a final thickness; primary recrystallization annealing the cold-rolled steel sheet to obtain a primary recrystallization annealed steel sheet; applying an annealing separator to the surface of the primary recrystallization annealed steel sheet, and then secondary recrystallization annealing the cold-rolled steel sheet, wherein in the hot-rolled annealing The invention relates to a method for manufacturing a grain-oriented electrical steel sheet, characterized in that a first-stage soaking is performed once or more, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or higher and 1000°C or lower for a soaking time of 3.5 seconds or higher and 120 seconds or lower, and a second-stage soaking is performed, in which the hot-rolled steel sheet is held at a soaking temperature of over 1000°C and 1100°C or lower.

[0016] <Component Composition> The contents of elements contained in the steel slab prepared in the above embodiment will be described. In this specification, the contents of elements are expressed as mass ratios. The symbol "%" indicates percentages based on mass ratios. The symbol "ppm" indicates parts per million based on mass ratios.

[0017] 1. Essential Elements C: 0.0020% or More, 0.100% or Less C is an element necessary for preventing brittle fracture when steel is heated to high temperatures. If the C content is 0.0020% or more, embrittlement at high temperatures is suppressed, thereby preventing brittle fracture of steel slabs during casting and hot rolling. If the C content is 0.100% or less, the C content can be reduced to 0.005% or less by decarburization treatment. This prevents magnetic aging due to incomplete decarburization. For this reason, the C content is set to 0.0020% or more, 0.100% or less. The C content is preferably 0.020% or more.

[0018] Si: 2.0% or more, 6.5% or less Si is an element necessary for increasing the resistivity of steel and reducing iron loss. If the Si content is 2.0% or more, it is effective in reducing iron loss. If the Si content is 6.5% or less, hot rolling and cold rolling can be easily performed. Therefore, the Si content is set to 2.0% or more, 6.5% or less. The Si content is preferably 2.5% or more. The Si content is preferably 4.0% or less.

[0019] Mn: 0.020% or more, 1.00% or less Mn is an element necessary for improving 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. Therefore, the Mn content is set to 0.020% or more, 1.00% or less. 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 combines with N dissolved in the steel to form AlN and precipitate, functioning as an inhibitor that suppresses normal grain growth of primary recrystallization grains during primary recrystallization annealing. However, in order for AlN to function as an inhibitor, it is necessary to prevent Al segregation in the steel slab and dissolve Al evenly throughout the steel. In conventional techniques, the Al content was set to 100 ppm or more in order to actively utilize AlN as an inhibitor. In this case, the steel slab had to be heated to a high temperature of over 1300°C in order to dissolve Al in the steel.

[0021] In the present 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 that of the prior art. Therefore, a heating temperature of 1300°C or less is sufficient for dissolving Al. Al is classified as acid-soluble Al or acid-insoluble Al depending on the difficulty of dissolving it in acid. Acid-soluble Al is used to improve the properties of steel sheets by being contained in steel in the form of solid-solution Al or AlN. Acid-insoluble Al is Al ２ O ３ However, since the amount is so small that it has little effect on the properties, the content of acid-soluble Al is specified in the present invention.

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

[0023] N: 10 ppm or more, 50 ppm or less As described above, N combines with Al and precipitates to form AlN, which acts as an inhibitor. If the N content is 10 ppm or more, a necessary 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 N contained in the steel slab separating as nitrogen gas during hot rolling and causing blistering. Therefore, the N content is set to 10 ppm or more, 50 ppm or less. The N content is preferably 25 ppm or less.

[0024] S: 10 ppm or more and 50 ppm 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 deterioration of the function of the inhibitor due to Ostwald ripening can be prevented. Therefore, the S content is set to 10 ppm or more and 50 ppm or less. The S content is preferably 25 ppm or less.

[0025] The steel slab prepared in the above embodiment has a chemical composition in which the balance other than the above elements is Fe and unavoidable impurities.

[0026] 2. Additional Components In a preferred embodiment, the present invention relates to a steel sheet having the composition further including, by mass ratio, Sb: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.50% or less, Ni: 0.005% or more and 1.5% or less, Cu: 0.005% or more and 1.5% or less, Cr: 0.005% or more and 0.10% or less, P: 0.005% or more and 0.50% or less, Mo: 0.005% or more and 0.50% or less, Ti: 0.0005% or more and 0.10% or less, Nb: 0.0005% or more and 0.10% or less, Bi: 0.005% or more and 0.10% or less, Se: 10 ppm or more and 50 ppm or less, The present invention relates to a method for producing a grain-oriented electrical steel sheet, which contains one or more elements selected from the group consisting of 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.

[0027] All of these elements are useful for improving magnetic properties. When the content of each is equal to or greater than the lower limit of the above range, the magnetic properties are improved. When the content of each is equal to or less than 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. When Se is 10 ppm or more, the formed MnSe functions as an inhibitor, improving the magnetic flux density of the steel sheet. When Se is 50 ppm or less, the function degradation of the inhibitor due to Ostwald ripening 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 of the grain-oriented electrical steel sheet in the above embodiment will be described.

[0029] <Steel Slab> In the method for producing a grain-oriented electrical steel sheet according to the present invention, first, a steel slab having the chemical composition described above is prepared. The steel slab can be prepared by a general production method. The steel slab can be produced by an ingot casting method in which molten steel having a predetermined chemical composition is poured into a mold and then cooled and solidified. Alternatively, the steel slab may be produced by a continuous casting method in which molten steel is once received in a tundish, poured into a water-cooled mold to solidify, and the steel slab is continuously withdrawn from the bottom of the mold. The method for producing the steel slab prepared in the present invention may be any of the above methods.

[0030] Of the essential elements described above, the contents of C, Si, and Mn can be adjusted by changing the compounding ratio of raw materials used when producing molten steel in various steelmaking furnaces. The contents of C, Si, and Mn can be further adjusted, as necessary, by adding additional additives to the molten steel once received in the ladle from the steelmaking furnace. Since it is difficult to add the above-mentioned additive elements during steelmaking, it is also preferable to add them to the molten steel once received in the ladle.

[0031] On the other hand, among the above-mentioned essential elements, the contents of sol. Al, N, and S are often already contained as inevitable impurities in the raw materials used when producing molten steel. When the contents of sol. Al, N, and S originally contained in molten steel satisfy the numerical ranges of the contents specified in the present invention, steel slabs can be produced without adjusting the contents of these essential elements. When the contents of sol. Al, N, or S are lower than the above-mentioned lower limits, the elements can be adjusted so that the contents are equal to or higher than the lower limits by adding additional additives to the molten steel received in the ladle.

[0032] Among the essential components described above, ferrosilicon and ferromanganese may be used as raw materials for Si and Mn. If the amounts of C and S contained in the ferrosilicon and ferromanganese are large, the amounts of C and S contained in the steel slab may exceed the upper limits described above. In such cases, it is preferable to adjust the composition of the molten steel using high-purity ferrosilicon and ferromanganese with low contents of C and S.

[0033] <Hot Rolling> In the method for producing a grain-oriented electrical steel sheet according to the present invention, the prepared steel slab is then heated to a temperature of 1300°C or less, and then hot-rolled to produce a hot-rolled steel sheet. As described above, in the method for producing a grain-oriented electrical steel sheet according to the present invention, the contents of sol. Al, N, and S, which are elements that form inhibitors, are kept low among the chemical compositions contained in the steel slab. Therefore, even if the heating temperature of the steel slab is 1300°C or less, these elements can be sufficiently dissolved in the steel, thereby reducing the cost of heating the steel slab. The heating temperature of the steel slab is preferably 1100°C or more. Known means such as a gas furnace, an induction heating furnace, or an electric furnace can be used to heat the steel slab.

[0034] In hot rolling of a heated steel slab, it is preferable to first perform one or more passes of rough rolling at a temperature of 1100°C or higher and 1300°C or lower, and then perform two or more passes of finish rolling at a temperature of 800°C or higher and 1100°C or lower, from the viewpoint of controlling the structure of the hot-rolled steel sheet. The total reduction in finish rolling is preferably 80% or higher. By setting the total reduction 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. Dislocations serve as nucleation sites for precipitates, contributing to the formation of fine, high-density precipitates and improving magnetic properties. The temperature in hot rolling is based on the temperature of the steel sheet surface.

[0035] The hot-rolled steel sheet obtained by hot rolling is preferably coiled into a coil shape for easy handling. The coiling temperature of the hot-rolled steel sheet is preferably 400°C or higher and 750°C or lower from the viewpoints of both controlling the structure of carbides in the hot-rolled steel sheet and preventing defects such as cracks. The coiling temperature is more preferably 500°C or higher. The coiling temperature is more preferably 700°C or lower. The coiling temperature of the hot-rolled steel sheet is based on the temperature of the steel sheet surface immediately before coiling.

[0036] As described above, in the method for producing grain-oriented electrical steel sheet according to the present invention, the content of sol. Al in the steel slab is reduced to less than 100 ppm. Therefore, during the hot rolling process, the N in the steel slab hardly combines with the sol. Al, but combines with the Si contained in the steel slab in large amounts to form silicon nitride (Si). ３ N ４ Non-Patent Document 1 describes the results of investigating the solubility of nitrogen in Fe—Si alloys. ３ N ４ The maximum temperature at which Si can exist stably is expressed as a function of the Si content in the Fe-Si alloy. For example, when the Si content is 3%, Si ３ N ４ is thought to be stable in the temperature range of approximately 900°C or less.

[0037] <Hot-rolled sheet annealing> In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is then subjected to hot-rolled sheet annealing to obtain an annealed hot-rolled sheet. The hot-rolled sheet annealing in the present invention is carried out to remove Si formed in the steel during the hot rolling process. ３ N ４The purpose of this process is to replace Si contained in the precipitates 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 holds 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 seconds or higher and 120 seconds or lower, and then subjected to a second-stage soaking hold in which it is held at a soaking temperature of higher than 1000°C and 1100°C or lower. The heat treatment conditions in each temperature range will be described in detail below with appropriate reference to Figure 1.

[0038] 1. Temperature range from 50°C to 350°C In the method for producing a grain-oriented electrical steel sheet according to the present invention, preferably, in the hot-rolled sheet annealing, the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C is 50°C / s or more. In this specification, the "average heating rate" refers to the average heating rate in a certain temperature range, and more specifically, refers to the value obtained by dividing the difference between the minimum and maximum 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 soaking stage described below. By quickly heating the hot-rolled steel sheet in this temperature range at an average heating rate of 50°C / s or more, Si ３ N ４ coarsening of Si is prevented. ３ N ４ The first stage of soaking can be started while maintaining the state in which the fine precipitates are densely distributed in the hot-rolled steel sheet. This allows the magnetic flux density of the steel sheet to be improved. There is no particular upper limit to the average heating rate when heating from 50°C to 350°C, but the average heating rate can be 500°C / s or less.

[0039] In the present invention, the temperature rise rate in the temperature range of the hot-rolled steel sheet below 50°C is ３ N ４ Therefore, even when the temperature increase is started from a temperature below 50°C, the lower limit of the temperature range for evaluating the average temperature increase rate may be 50°C.

[0040] 2. Temperature Range of 350°C or More and 1000°C or Less In the method for producing grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is then subjected to a first-stage soaking at least once in the hot-rolled sheet annealing step, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less. As shown in FIG. 1 , the first-stage soaking is performed after the previous temperature increase step from 50°C to 350°C is completed, in which the hot-rolled steel sheet is held at a soaking temperature T １ The temperature is raised to t １ Hold for . １ If the temperature is less than 350°C, the diffusion of Al is difficult to proceed and the precipitation of AlN does not proceed, resulting in a decrease in magnetic flux density. １ When the temperature exceeds 1000°C, Si precipitates before AlN precipitates. ３ N ４ As a result, the magnetic flux density of the steel sheet decreases. １ The soaking temperature T is 350°C or higher and 1000°C or lower. １ The soaking temperature T is preferably 400°C or higher, and more preferably 500°C or higher. １ is preferably 900°C or less.

[0041] When the first stage is not held at all or the first stage is held at t １ When is less than 3.5s, Si ３ N ４ The magnetic flux density of the steel sheet decreases because there is insufficient time for the substitution of sol. Al for Si contained in the precipitates to proceed. １ If the first-stage soaking time t exceeds 120 s, excessive precipitation and coarsening of AlN will occur, resulting in a decrease in the magnetic flux density of the steel sheet. １ The soaking time t is 3.5 seconds or more and 120 seconds or less. １ The soaking time t is preferably 5.0 seconds or more. １ is preferably 60 seconds or less, more preferably 30 seconds or less.

[0042] The reason why the magnetic flux density of the steel sheet is improved when the soaking is performed for a short time compared to when the temperature is increased at a constant average rate in the temperature range of 350°C or more and 1000°C or less is not clearly understood, but the inventors believe as follows. ３ N ４ The substitution of sol. Al for Si contained in the AlN alloy progresses, and when sol. Al reaches saturation precipitation, Ostwald ripening occurs. This results in a uniform AlN grain size and improves magnetic flux density. On the other hand, when the temperature is increased, Si ３ N ４ Since the solid solution of Si contained in AlN is promoted with increasing temperature, the substitution by sol. Al proceeds quickly. In this case, the AlN grain size is nonuniform from the beginning, making it more difficult to homogenize the AlN grain size in the subsequent process than by soaking. As a result, the magnetic flux density decreases. Therefore, to improve the magnetic flux density, it is essential to maintain soaking, rather than continuously increasing the temperature in the temperature range of 350°C or higher and 1000°C or lower.

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

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

[0045] In addition, in the temperature range of hot-rolled sheet annealing, Si ３ N ４AlN is more stable than precipitates. ３ N ４ The substitution reaction of Si contained in the precipitate with sol. Al occurs irreversibly, and once formed, AlN is replaced by Si. ３ N ４ It does not return to precipitates. ３ N ４ The opportunity to generate fine AlN from precipitates is limited to the process of annealing the hot-rolled sheet.

[0046] 3. Temperature Range of More Than 1000°C and Not More Than 1100°C In the method for producing grain-oriented electrical steel sheet according to the present invention, the hot-rolled steel sheet is then subjected to a second-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and not more than 1100°C after the first-stage soaking, which is the previous step, is completed. ２ The temperature is raised to t ２ This holding allows the precipitate diameter of AlN to be optimized by Ostwald ripening. ２ If the soaking temperature T is 1000° C. or less, the adjustment of the precipitate diameter by Ostwald ripening is insufficient, so the inhibitor function deteriorates and the magnetic flux density decreases. ２ If the soaking temperature T exceeds 1100°C, the AlN will be excessively coarsened or will be redissolved, which will deteriorate its inhibitor function and reduce the magnetic flux density. ２ The soaking temperature T is set to be more than 1000°C and not more than 1100°C. ２ The second stage soaking time t is preferably 1050°C or less. ２ is not particularly limited. ２ The soaking time t is preferably 10 seconds or more. ２ is preferably 60 seconds or less.

[0047] 4. Shape of hot-rolled steel sheet in 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 soaking. When the hot-rolled steel sheet after hot rolling is coiled into a coil shape, the hot-rolled steel sheet can be rapidly heated or cooled by unwinding the coil to return it to its original shape and performing hot-rolled sheet annealing using a known continuous annealing furnace. Performing hot-rolled sheet annealing in the shape of a hot-rolled steel sheet is also preferable in terms of performing appropriate temperature control with a small temperature fluctuation range during soaking after rapid heating or cooling.

[0048] <Cold Rolling> In the method for producing a grain-oriented electrical steel sheet according to the present invention, the hot-rolled and annealed sheet is then cold-rolled once or twice or more times with intermediate annealing in between to produce a cold-rolled steel sheet having a final thickness. The final thickness of the cold-rolled steel sheet is preferably 0.30 mm or less. If the final thickness of the cold-rolled steel sheet is 0.30 mm or less, eddy current loss can be reduced. The final thickness of the cold-rolled steel sheet is more preferably 0.23 mm or less, and even more preferably 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 passes may be one or more. When cold rolling is performed two or more times, intermediate annealing is performed between cold rolling passes. The intermediate annealing is preferably performed under the condition that the annealing temperature is 900°C or higher and 1200°C or lower. When the annealing temperature is 900°C or higher, the recrystallized grains do not become too fine, and the number of nuclei having the Goss orientation in the primary recrystallized structure increases, thereby improving the magnetic flux density of the steel sheet. When the annealing temperature is 1200°C or lower, the recrystallized grains do not become coarse, and a primary recrystallized structure with a uniform grain size can be realized, thereby also improving the magnetic flux density of the steel sheet. In the final stage of cold rolling, it is preferable to perform warm rolling by heating the steel sheet to a temperature of 100°C or higher and 300°C or lower, or to perform aging treatment one or more times at a temperature of 100°C or higher and 300°C or lower between passes, as this increases the concentration of the recrystallized texture and improves the magnetic flux density of the steel sheet.

[0050] In the cold rolling, it is preferable to perform rolling at least once with a reduction ratio of 80% or more. By performing cold rolling with a reduction ratio of 80% or more, the concentration of recrystallized texture increases, which is advantageous in that the magnetic flux density of the steel sheet can be improved.

[0051] <Primary recrystallization annealing> In the method for producing a grain-oriented electrical steel sheet according to the present invention, the cold-rolled steel sheet is then subjected to primary recrystallization annealing to obtain a primarily recrystallization annealed sheet. The primary recrystallization annealing may also serve as decarburization annealing. The annealing temperature for the primary recrystallization annealing is preferably 800°C or higher and 900°C or lower, and the atmosphere is preferably a moist atmosphere, in order to perform decarburization. However, if the C content in the steel slab is 0.0050% or lower, there is no need to further reduce the C content, and therefore the atmosphere for the primary recrystallization annealing may be other than the above. The average heating rate to the holding temperature in the primary recrystallization annealing is preferably 50°C / s or higher and 400°C / s or lower, in order to increase the magnetic flux density of the steel sheet.

[0052] <Secondary Recrystallization Annealing> In the method for producing a grain-oriented electrical steel sheet according to the present invention, an annealing separator is next applied to the surface of the primarily recrystallized annealed sheet, followed by secondary recrystallization annealing. An annealing separator mainly composed of MgO is used as the annealing separator. Secondary recrystallization annealing allows secondary recrystallization grains having a Goss orientation to develop and allows a forsterite film to be formed on the surface of the steel sheet. It is preferable to perform 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 perform secondary recrystallization annealing at a temperature of 1200°C or higher.

[0053] <Post-treatment> After secondary recrystallization annealing, the steel sheet is washed with water, brushed, or pickled to remove the annealing separator adhering to the surface of the steel sheet. Thereafter, the steel sheet is further subjected to flattening annealing to correct the shape, which is effective for reducing iron loss.

[0054] When steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheet before or after planarization annealing in order to improve iron loss. In this case, applying a coating that can impart tension to the steel sheet is preferable in terms of reducing iron loss. Examples of coating methods that can impart tension to the steel sheet include a tension coating application method in which a binder is used in the coating, and a coating method in which an inorganic substance is deposited on the surface layer of the steel sheet by physical vapor deposition or chemical vapor deposition. These coatings are preferred because they have excellent adhesion and an excellent iron loss reduction effect.

[0055] To further reduce iron loss, it is preferable to perform magnetic domain refinement. As a method for magnetic domain refinement, a commonly used method such as applying strain to a final product sheet using an electron beam or laser can be adopted. The target for magnetic domain refinement is not only a final product sheet, but also an intermediate product such as a cold-rolled steel sheet that has reached its final thickness.

[0056] Examples of the present invention will be described below. Note that the embodiments of the present invention are not limited to the following examples and can be modified as desired without departing from the gist of the present invention.

[0057] Example 1 A steel slab having a composition containing, by mass, 0.055% C, 3.2% Si, 0.12% Mn, 80 ppm sol. Al, 35 ppm N, and 32 ppm S, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1200°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet having 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-stage soaking time t １ is fixed at 30 s, and the soaking temperature T １ The second stage soaking temperature T ２ The temperature is set to 1030°C, and the soaking time is t ２The hot-rolled sheet annealing was performed in a dry nitrogen atmosphere. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was cold-rolled to obtain a cold-rolled steel sheet having a final thickness of 0.23 mm.

[0058] Next, the obtained cold-rolled steel sheet was subjected to 50 vol % N ２ -50vol% H ２ The resulting sheets were subjected to primary recrystallization annealing, which also served as decarburization annealing, at 830°C for 150 seconds in a humid atmosphere with a dew point of 50°C, to obtain primarily recrystallized annealed sheets. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primarily 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 B of the obtained samples was measured using the method specified in Japanese Industrial Standard JIS C 2500 when a magnetic field strength of 800 A / m was applied to the samples. ８ The first stage soaking temperature T １ and magnetic flux density B ８ The relationship between these is shown in Figure 2.

[0059] According to FIG. 2, the first stage soaking temperature T １ In the range indicated by the dashed line where the temperature is 350°C or higher and 1000°C or lower, an excellent magnetic flux density B of 1.925T or higher is obtained. ８ It can be seen that the following was obtained.

[0060] Example 2 A steel slab having a composition containing, by mass, 0.048% C, 3.3% Si, 0.12% Mn, 83 ppm sol. Al, 41 ppm N, and 30 ppm S, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1230°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet having 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-stage soaking temperature T １ is fixed at 750 ° C., and the soaking time t １ The second stage soaking temperature T ２ The temperature is set to 1010°C, and the soaking time is t ２The hot-rolled sheet annealing was performed in a dry nitrogen atmosphere. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was cold-rolled to obtain a cold-rolled steel sheet having a final thickness of 0.23 mm.

[0061] Next, the obtained cold-rolled steel sheets were subjected to primary recrystallization annealing 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 first-stage soaking holding times. Next, the magnetic flux density B ８ The first stage soaking time t １ and magnetic flux density B ８ The relationship between the temperature and the temperature is shown in Figure 3. In Figure 3, the horizontal axis indicates the soaking time t １ The scale was logarithmic.

[0062] According to FIG. 3, the first stage soaking time t １ In the range indicated by the dashed line, where the time is 3.5 seconds or more and 120 seconds or less, an excellent magnetic flux density B of 1.925 T or more is obtained. ８ It can be seen that the following was obtained.

[0063] Example 3 A steel slab having a composition containing, by mass, 0.035% C, 3.3% Si, 0.13% Mn, 75 ppm sol. Al, 42 ppm N, 11 ppm S, and 0.075% Sb, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1160°C for 60 minutes, and then hot-rolled to form 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 changed within a range from 20°C / s to 100°C / s. Table 1 shows the average heating rate from 50°C to 350°C, the first-stage soaking temperature T １ and soaking time t １ The second soaking temperature T2 and the second soaking time t2 are shown. The atmosphere for annealing the hot-rolled sheet was a dry nitrogen atmosphere. After the hot-rolled sheet annealing, the surface scale of the annealed hot-rolled sheet was removed by pickling, and then the sheet was cold-rolled to a thickness of 1.6 mm. Next, a 70 vol% N ２ -30vol%H ２The steel sheets were then subjected to intermediate annealing in a humid atmosphere with a dew point of 40°C at 40°C, and then cold-rolled to obtain cold-rolled steel sheets with a final thickness of 0.20 mm.

[0064] Next, the obtained cold-rolled steel sheet was subjected to 50 vol % N ２ -50vol%H ２ The resulting sheets were subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 60 seconds in a humid atmosphere with a dew point of 50°C, to obtain primarily recrystallized annealed sheets. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primarily recrystallized annealed sheets, and secondary recrystallization annealing was performed in a hydrogen atmosphere at 1220°C for 5 hours to obtain 40 types of grain-oriented electrical steel sheet samples with different manufacturing conditions. Next, the magnetic flux density B ８ The results are shown in Table 1.

[0065]

[0066] In addition, for the results shown in Table 1, excluding samples No. 2, No. 29, and No. 35, the horizontal axis represents the soaking time t １ , the vertical axis is the soaking temperature T １ The results are plotted on a graph shown in Figure 4. In Figure 4, the horizontal axis represents the soaking time t １ The scale is a logarithmic scale. In FIG. 4, the white squares represent the magnetic flux density B ８ The black squares represent examples of the invention where the magnetic flux density B ８ 4 also plots the inventive example and comparative example of Example 1 as white circles and black circles, and the inventive example and comparative example of Example 2 as white triangles and black triangles, respectively.

[0067] According to FIG. 4, the first stage soaking temperature T １ is 350 ° C or more and 1000 ° C or less, and the soaking time t １ In the range indicated by the dashed line, where the time is 3.5 seconds or more and 120 seconds or less, an excellent magnetic flux density B of 1.925 T or more is obtained. ８ It can be seen that the following was obtained.

[0068] Example 4 A steel slab containing the essential and additional components shown in Table 2 in mass ratios, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1200°C for 60 minutes, and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.5 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-stage soaking temperature T １ is set to 750°C, and the soaking time is t １ The second stage soaking temperature T ２ The temperature is set to 1030°C, and the soaking time is t ２ The atmosphere for annealing the hot-rolled sheet was 80 vol% N ２ -20vol%CO ２ The atmosphere was a humid atmosphere with a dew point of 30° C. After the hot-rolled sheet annealing, scale on the surface of the annealed hot-rolled sheet was removed by pickling, and then the sheet was subjected to warm rolling at 150° C. and cold rolling 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 a 40 vol % N ２ -60vol%H ２ The resulting sheets were subjected to primary recrystallization annealing, which also served as decarburization annealing, at 850°C for 180 seconds in a humid atmosphere with a dew point of 50°C, to obtain primarily recrystallized annealed sheets. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primarily recrystallized annealed sheets, and secondary recrystallization annealing was performed in a hydrogen atmosphere at 1175°C for 15 hours to obtain 25 types of grain-oriented electrical steel sheet samples with different component compositions. Next, the magnetic flux density B ８ The results are shown in Table 2.

[0070]

[0071] According to Table 2, the grain-oriented electrical steel sheet samples manufactured by the manufacturing method specified in the present invention using steel slabs having the composition specified in the present invention or the preferred composition have an excellent magnetic flux density B of 1.925 T or more. ８ It can be seen that the following was obtained.

[0072] T １ First stage soaking temperature t １First stage heating time T ２ Second stage soaking temperature t ２ Second stage soaking time

Claims

1. In mass ratio, C: 0.0020% or more and 0.100% or less, Si: 2.0% or more and 6.5% or less, Mn: 0.020% or more and 1.00% or less, sol. A method for producing a grain-oriented electrical steel sheet, comprising: preparing a steel slab having a chemical composition containing Al: 10 ppm or more but less than 100 ppm, N: 10 ppm or more but less than 50 ppm, and S: 10 ppm or more but less than 50 ppm, with the balance consisting of Fe and unavoidable impurities; heating the steel slab to a temperature of 1300°C or less, and then hot rolling the steel slab to obtain a hot-rolled steel sheet; hot-rolling the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet; cold-rolling the hot-rolled annealed steel sheet once or two or more times with intermediate annealing between them to obtain a cold-rolled steel sheet having a final thickness; primary recrystallization annealing the cold-rolled steel sheet to obtain a primary recrystallization annealed steel sheet; applying an annealing separator to the surface of the primary recrystallization annealed steel sheet, and then secondary recrystallization annealing the cold-rolled steel sheet, wherein in the hot-rolled annealing a first-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 1000°C or less for a soaking time of 3.5 seconds or more and 120 seconds or less, is performed one or more times; and a second-stage soaking in which the hot-rolled steel sheet is held at a soaking temperature of more than 1000°C and 1100°C or less.

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

3. The composition further includes, in mass ratio, Sb: 0.01% or more and 0.50% or less, Sn: 0.01% or more and 0.50% or less, Ni: 0.005% or more and 1.5% or less, Cu: 0.005% or more and 1.5% or less, Cr: 0.005% or more and 0.10% or less, P: 0.005% or more and 0.50% or less, Mo: 0.005% or more and 0.50% or less, Ti: 0.0005% or more and 0.10% or less, Nb: 0.0005% or more and 0.10% or less, Bi: 0.005% or more and 0.10% or less, Se: 10 ppm or more and 50 ppm or less, Ca: 0.0005% or more and 0.0050% or less, B 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel sheet contains one or more elements selected from the group consisting of: C: 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.

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