Grain-oriented electrical steel sheet production method

Abstract

Provided is a grain-oriented electrical steel sheet production method with which it is possible to produce a grain-oriented electrical steel sheet having better magnetic properties than conventional products in a component system in which an inhibitor is not actively used. This grain-oriented electrical steel sheet production method involves subjecting a steel slab to hot rolling, hot-rolled sheet annealing, cold rolling, primary recrystallization annealing, and secondary recrystallization annealing. In the final pass of the rough rolling in the hot rolling, the rolling temperature is set to 950-1150°C, and the reduction ratio is set to 25% or higher. In a temperature raising process in the hot-rolled sheet annealing, the average temperature increase rate when heating a hot-rolled steel sheet from 50°C to 350°C is set to 40°C / s or more.

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] 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.

[0007] The present invention has been made in view of the above-mentioned problems, and has an object to provide a method for producing a grain-oriented electrical steel sheet that can produce a grain-oriented electrical steel sheet having magnetic properties superior to conventional ones in a component system that does not actively use an inhibitor.

[0008] The inventors have searched for new conditions for hot rolling and hot-rolled sheet annealing to further increase the degree of concentration in the Goss orientation and stably obtain excellent magnetic properties among the manufacturing methods of grain-oriented electrical steel sheets disclosed in Patent Document 4. As a result, they have found that grain-oriented electrical steel sheets with excellent magnetic properties can be manufactured by setting the rolling temperature and reduction in the final pass of rough rolling in hot rolling and the average heating rate in annealing of the hot-rolled sheet within specific ranges, and have completed the present invention.

[0009] The gist and configuration of the present invention are as follows.

[0010] [1] In mass ratio, C: 0.002% or more and 0.100% or less, Si: 2.0% or more and 6.5% or less, Mn: 0.02% or more and 1.00% or less, sol. A method for producing a grain oriented electrical steel sheet, comprising the steps of: 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 a surface of the primary recrystallization annealed steel sheet, and then secondary recrystallization annealing the cold-rolled steel sheet; a rolling temperature of 950°C or more and 1150°C or less and a rolling reduction of 25% or more in a final pass of rough rolling in the hot rolling; and an average heating rate of 40°C / s or more when heating the hot-rolled steel sheet from 50°C to 350°C in the hot-rolled sheet annealing.

[0011] [2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein, in the temperature-raising process of the hot-rolled sheet annealing, the hot-rolled steel sheet is subjected to one or more soaking cycles in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 950°C or less for 3.0 seconds or more and 100 seconds or less.

[0012] [3] The composition further includes, in mass ratio, Se: 0.001% or more and 0.005% or less, 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, 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.

[0013] According to the manufacturing method of the present invention, it is possible to manufacture grain-oriented electrical steel sheets having magnetic properties superior to those of conventional steel sheets in a component system that does not actively utilize inhibitors.

[0014] 1 is a graph showing the relationship between the reduction rate and magnetic flux density in the final pass of rough rolling in hot rolling.

[0015] First, the experiment that gave rise to the idea for the present invention will be described.

[0016] <Experiment> A steel slab having a composition containing, by mass, 0.055% C, 3.2% Si, 0.12% Mn, 76 ppm sol.Al, 33 ppm N, and 32 ppm S, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting, heated to 1200 ° C for 60 minutes, and then hot rolled to obtain a hot-rolled steel sheet with a thickness of 2.3 mm. In this case, the rolling temperature in the final pass of rough rolling of the hot rolling was set to 1000 ° C, and the rolling reduction was changed in the range from 22% to 43%. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing at 1030 ° C for 10 seconds. In this case, the average heating rate from 50 ° C to 350 ° C was set to two conditions: 20 ° C / s and 50 ° C / s. In addition, some samples were subjected to soaking in which the surface temperature of the hot-rolled steel sheet was held at 800°C for 30 seconds during the temperature increase. 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.

[0017] Next, the obtained cold-rolled steel sheet was subjected to 50 vol % H ２ +50vol%N ２ The samples 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 32 types of grain-oriented electrical steel sheet samples with different manufacturing conditions. Next, the magnetic flux density B when a magnetic field strength of 800 A / m was applied to the obtained samples was measured using the method specified in Japanese Industrial Standard JIS C 2500. ８ The reduction rate and magnetic flux density B of the final pass of rough rolling in hot rolling were measured. ８ The relationship between these is shown in Figure 1.

[0018] According to FIG. 1, when the rolling reduction is 25% or more and the average temperature rise rate from 50°C to 350°C in the hot-rolled sheet annealing is 50°C / s, an excellent magnetic flux density B of 1.925T or more is obtained. ８ In particular, the sample that was subjected to soaking during the temperature rise in the hot-rolled sheet annealing exhibited a superior magnetic flux density B ８ It can be seen that

[0019] Next, an embodiment of the present invention will be described in detail.

[0020] In one embodiment, the present invention provides a steel sheet containing, in mass ratios, C: 0.002% or more and 0.100% or less, Si: 2.0% or more and 6.5% or less, Mn: 0.02% or more and 1.00% or less, sol. A method for producing a grain oriented electrical steel sheet, comprising the steps of: 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 a surface of the primary recrystallization annealed steel sheet, and then secondary recrystallization annealing the cold-rolled steel sheet; the rolling temperature is set to 950°C or more and 1150°C or less and the rolling reduction is set to 25% or more in the final pass of rough rolling in the hot rolling; and the average heating rate when heating the hot-rolled steel sheet from 50°C to 350°C in the hot-rolled sheet annealing is set to 40°C / s or more.

[0021] <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.

[0022] (Essential Elements) C: 0.002% 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 0.002% 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 0.002% or more, 0.100% or less. The C content is preferably 0.020% or more.

[0023] 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.

[0024] Mn: 0.02% or more, 1.00% or less Mn is an element necessary for improving the hot workability of steel. If the Mn content is 0.02% 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 0.02% or more, 1.00% or less. The Mn content is preferably 0.04% or more. The Mn content is preferably 0.30% or less.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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.

[0029] S: 10 ppm or more and 50 ppm or less S combines with Mn to form MnS. When the S content is 10 ppm or more, the formed MnS functions as an inhibitor, improving the magnetic flux density of the steel sheet. When 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 40 ppm or less, more preferably 35 ppm or less.

[0030] 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.

[0031] (Optionally Added Components) In a preferred embodiment, the present invention relates to a steel sheet having the composition further including, in mass ratios, Se: 0.001% or more and 0.005% or less, 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, 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.

[0032] These elements are all useful for improving magnetic properties. When the content of each element is equal to or greater than the lower limit of the above-mentioned range, the magnetic properties are improved. When the content of each element is equal to or less than the upper limit of the above-mentioned range, the formation of texture by secondary recrystallization is not hindered.

[0033] Next, the manufacturing conditions of the grain-oriented electrical steel sheet in the above embodiment will be described.

[0034] <Steel Slab> The steel slab is not particularly limited as long as it has the above-mentioned component composition. The method for producing the steel slab is not particularly limited, and known methods using a converter, an electric furnace, or the like can be used. From the viewpoint of productivity and the like, it is preferable to produce a slab (steel material) by continuous casting after the slab is produced, but the slab may also be produced by known casting methods such as ingot making-blooming rolling or thin slab continuous casting.

[0035] <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.

[0036] From the viewpoint of controlling the structure of the hot-rolled steel sheet, the rough rolling in the hot rolling is performed in one or more passes. Here, one of the major features of the present invention is that in the final pass of the rough rolling in the hot rolling, the rolling temperature is set to 950°C or higher and 1150°C or lower, and the reduction ratio is set to 25% or higher. The temperature range of 950°C or higher and 1150°C or lower corresponds to the temperature range in which the γ phase coexists. By performing the final pass reduction in this temperature range, the introduction of strain into the steel is promoted, and recrystallization of the stable rolling orientation {100}<011>, which is difficult to cause secondary recrystallization, is easily generated. By performing the final pass reduction with a high reduction ratio of 25% or higher in this temperature range, the steel sheet structure at the end of the rough rolling can be made to have a high recrystallization ratio and a fine structure. This promotes the introduction of dislocations into the steel in the subsequent finish rolling, and dislocations are introduced uniformly and densely into the hot-rolled steel sheet. The rolling temperature in the final pass of rough rolling in hot rolling is preferably 980°C or higher and 1080°C or lower. The rolling reduction in the final pass is preferably 30% or higher and 60% or lower. The rolling temperature in rough rolling is based on the temperature of the steel sheet surface.

[0037] By performing rough rolling of hot rolling under the above conditions, the magnetic flux density of the final product is improved. The reason for this is not entirely clear, but the inventors believe it to be as follows: In inhibitor-less materials, MnS and Si are present in the hot-rolled steel sheet after hot rolling. ３ N ４During the annealing of the hot-rolled sheet following hot rolling, precipitates consisting mainly of Si ３ N ４ The Si contained in the alloy is replaced by Al to form AlN, which then functions as an inhibitor. In order to improve the magnetic flux density of the final product, it is important to increase the amount of AlN precipitated and to ensure that AlN is uniformly and finely distributed in the hot-rolled and annealed sheet. ３ N ４ It is necessary to select the hot rolling conditions and the hot-rolled sheet annealing conditions so that the precipitates are uniform and fine.

[0038] As described above, by performing reduction at a low temperature and a high reduction rate in the final pass of rough rolling in hot rolling, dislocations are introduced uniformly and densely into the hot rolled steel sheet after finish rolling. ３ N ４ Since this becomes a nucleation site for Si during the cooling process after hot rolling, ３ N ４ Furthermore, by increasing the temperature rise rate in the subsequent hot-rolled sheet annealing process to prevent diffusion and re-dissolution of N, Si ３ N ４ A uniform fine distribution of Si is achieved in the hot rolled and annealed sheets. ３ N ４ The AlN inhibitor is generated uniformly and finely while maintaining the uniform and fine precipitation state of AlN. As a result, it is thought that the ability of the inhibitor to suppress normal grain growth during secondary recrystallization can be optimized, and a grain-oriented electrical steel sheet with excellent Goss orientation and magnetic properties can be obtained.

[0039] From the viewpoint of controlling the structure of the hot-rolled steel sheet, it is preferable that the finish rolling following the rough rolling of the hot rolling be performed at a rolling temperature of 800°C or more and 1100°C or less, and that the rolling be performed in two or more passes. The rolling temperature in the finish rolling is based on the temperature of the steel sheet surface. It is preferable that the total reduction in the finish rolling be 80% or more. By setting the total reduction in the temperature range of 1100°C or less to 80% or more, dislocations are introduced into the hot-rolled steel sheet at a high density.

[0040] 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 lower limit of the coiling temperature is more preferably 500°C or higher. The upper limit of 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.

[0041] <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 ４ The purpose of this process is to replace Si contained in the precipitates with sol. Al to form AlN. In the hot-rolled sheet annealing, the surface temperature of the hot-rolled steel sheet is raised from 50°C to 350°C at an average heating rate of 40°C / s or more, and then the surface temperature of the hot-rolled steel sheet is soaked at a temperature preferably higher than 950°C and lower than 1100°C.

[0042] Average heating rate from 50°C to 350°C: 40°C / s or more The faster the average heating rate from 50°C to 350°C, the more the diffusion of N is suppressed, and the more Si ３ N ４ In the final pass of rough rolling in hot rolling, rough rolling is performed in the temperature range and reduction rate as described above, and the distribution of Si ３ N ４ By controlling the hot rolling conditions in this way and setting the temperature rising rate of the hot rolled sheet annealing to 40°C / s or more, the precipitation of Si is further promoted. ３ N ４By making the distribution of the inhibitors denser, a larger amount of precipitation can be achieved, resulting in a denser inhibitor precipitation state and improved magnetic flux density. The heating method is not particularly limited, but in order to achieve an average temperature rise rate of 40°C / s or more, induction heating or electric heating may be employed in addition to conventional heating methods using heaters or burners. The average temperature rise rate is more preferably 50°C / s or more. There is no particular upper limit to the average temperature rise rate when raising the temperature from 50°C to 350°C, but the average temperature rise rate can be 500°C / s or less.

[0043] Temperature Range of More Than 950°C and Less Than 1100°C In the method for producing a grain-oriented electrical steel sheet according to the present invention, hot-rolled sheet annealing is performed by soaking the surface temperature of the hot-rolled steel sheet, preferably at a soaking temperature of more than 950°C and less than 1100°C. The soaking is performed by raising the surface temperature of the hot-rolled steel sheet to the soaking temperature after the completion of the previous temperature increase process from 50°C to 350°C, and then holding the temperature for the soaking time. This holding process optimizes the AlN precipitate diameter through Ostwald ripening. If the soaking temperature is 950°C or less, the precipitation diameter is not sufficiently adjusted by Ostwald ripening, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. If the soaking temperature is more than 1100°C, AlN becomes excessively coarse or re-dissolves, resulting in a deterioration of the inhibitor function and a decrease in magnetic flux density. For this reason, the soaking temperature is preferably more than 950°C and less than 1100°C. The soaking temperature is more preferably higher than 1000° C., and more preferably not higher than 1050° C. The soaking time is preferably not shorter than 10 seconds. The soaking time is preferably not longer than 60 seconds.

[0044] 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 surface temperature of a hot-rolled steel sheet during hot-rolled sheet annealing can be measured by a known method.

[0045] Temperature range of 350°C or higher and 950°C or lower In a preferred embodiment, the method for producing a grain-oriented electrical steel sheet according to the present invention performs one or more soaking steps in the temperature-raising process of hot-rolled sheet annealing, in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or higher and 950°C or lower for 3.0 seconds or longer and 100 seconds or shorter. The soaking in this temperature range involves raising the temperature of the surface of the hot-rolled steel sheet to the soaking temperature after the previous temperature-raising step from 50°C to 350°C is completed, and holding the temperature for the soaking time.

[0046] When the soaking temperature is less than 350°C, Si ３ N ４ When the soaking temperature is higher than 950°C, the Si ３ N ４ This causes coarsening or dissolution of AlN, or AlN tends to precipitate excessively, making it difficult to obtain an appropriate suppression of normal grain growth and resulting in a decrease in magnetic flux density. For this reason, the soaking temperature is preferably 350°C or higher and 950°C or lower. The soaking temperature is more preferably 400°C or higher, and even more preferably 500°C or higher. The soaking temperature is preferably 900°C or lower.

[0047] When soaking is not performed at all in the temperature range of 350°C or more and 950°C or less, or when the soaking time is less than 3.0 seconds, Si ３ N ４ When the soaking time is longer than 100 seconds, the Si ３ N ４ As a result, coarsening or dissolution of AlN occurs, or excessive precipitation of AlN occurs, making it difficult to obtain an appropriate suppression of normal grain growth, and the magnetic flux density tends to decrease. For this reason, the soaking time is preferably 3.0 seconds or more and 100 seconds or less. The soaking time is more preferably 5.0 seconds or more. The soaking time is more preferably 60 seconds or less, and even more preferably 30 seconds or less.

[0048] 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 950°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, in order to improve the magnetic flux density, it is preferable to maintain soaking rather than continuously increasing the temperature in the temperature range of 350°C or higher and 950°C or lower.

[0049] In the method for producing a grain-oriented electrical steel sheet according to the present invention, the soaking in the temperature range of 350°C or higher and 950°C or lower may be performed once, or may be repeated two or more times. When soaking in this temperature range is performed two or more times, the soaking temperature from the second time onwards may be the same as the temperature of the first soaking, or may be a temperature 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 950°C or lower. When soaking is performed multiple times, the soaking time for each time is preferably 3.0 seconds or longer and 100 seconds or shorter.

[0050] In addition, in the temperature range of hot-rolled sheet annealing, Si ３ N ４ AlN 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 ４The opportunity to generate fine AlN from precipitates is limited to the process of annealing the hot-rolled sheet.

[0051] <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.

[0052] The number of cold rolling steps may be one or more. When cold rolling is performed two or more times, intermediate annealing is performed between the cold rolling steps. 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 excessively 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 cold rolling, it is effective and preferable to increase the cold rolling temperature from 100°C to 300°C and to perform aging treatment one or more times in the range of 100°C to 300°C during cold rolling in order to change the recrystallized texture and improve the magnetic properties.

[0053] 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.

[0054] <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 0.005% 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. In order to increase the magnetic flux density of the steel sheet, it is preferable that the average heating rate to the holding temperature in the primary recrystallization annealing be 50°C / s or higher and 400°C / s or lower.

[0055] <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. The secondary recrystallization annealing allows secondary recrystallization grains having a Goss orientation to develop, and also allows a forsterite film to be formed on the surface of the steel sheet. The secondary recrystallization annealing is preferably carried out at 800°C or higher to induce secondary recrystallization. Furthermore, in order to complete secondary recrystallization, annealing is preferably carried out at a temperature of 800°C or higher for 20 hours or more. In order to form a forsterite film, it is preferable to raise the temperature to about 1200°C.

[0056] <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.

[0057] 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.

[0058] To further reduce iron loss, it is preferable to perform magnetic domain refinement treatment. A commonly used method for this purpose is to apply strain to the iron crystal lattice of the final product sheet using an electron beam, laser, or the like. Furthermore, grooves may be pre-grooved not only in the final product sheet but also in intermediate products such as cold-rolled sheets that have reached the final thickness.

[0059] 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.

[0060] Example 1 Fourteen types of steel slabs containing the essential elements shown in Table 1, with the balance being Fe and unavoidable impurities, were produced by continuous casting, heated to 1160°C, and then hot-rolled to form hot-rolled steel sheets with a thickness of 2.4 mm. During this process, the rolling temperature and reduction in the final pass of rough rolling in the hot rolling were changed as shown in Table 2. The obtained hot-rolled steel sheets were subjected to hot-rolled sheet annealing at 1030°C for 30 seconds. During this process, the average heating rate from 50°C to 350°C was changed as shown in Table 2. Furthermore, with the exception of some samples, soaking was performed during heating, in which the temperature of the hot-rolled steel sheets was held at the soaking temperature shown in Table 2 for 30 seconds. The hot-rolled sheet annealing atmosphere was 90 vol% N ２ +10vol%CO ２ After the hot-rolled sheet annealing, the surface scale of the annealed hot-rolled sheet was removed by pickling, and the sheet was subjected to a first cold rolling to a thickness of 1.6 mm. ２+30vol%H ２ The steel sheet was subjected to intermediate annealing at 1000°C for 110 seconds in an atmosphere of 0.20 mm, and then subjected to a second cold rolling to obtain a cold-rolled steel sheet having a final thickness of 0.20 mm.

[0061]

[0062] Next, the obtained cold-rolled steel sheet was subjected to 50 vol % N ２ +50vol%H ２ The primary recrystallization annealing, which also served as decarburization annealing, was performed at 850°C for 60 seconds in a humid atmosphere with a dew point of 50°C to obtain a primary recrystallization annealed sheet. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primary recrystallization annealed sheet, and the sheet was annealed at 900°C for 40 hours in N ２ After holding in a hydrogen atmosphere, secondary recrystallization annealing was performed at 1220°C for 5 hours, and 32 types of grain-oriented electrical steel sheet samples with different manufacturing conditions were obtained. Next, the magnetic flux density B ８ The magnetic flux density B ８ The measurement results are shown in Table 2.

[0063]

[0064] According to Table 2, for steel types A to D, when the rolling temperature in the final pass of rough rolling in hot rolling is 950°C or higher and 1150°C or lower, and the reduction rate is 25% or higher, and in addition, the average temperature rise rate from 50°C to 350°C in hot-rolled sheet annealing is 40°C / s or higher, excellent magnetic flux density B of 1.925T or higher is obtained. ８ It can be seen that the following was obtained.

[0065] Example 2 A steel slab containing the essential elements and optional elements shown in Table 3, with the balance being Fe and unavoidable impurities, was produced by continuous casting, heated to 1200°C, and then hot-rolled to form a hot-rolled steel sheet with a thickness of 2.5 mm. The rolling temperature in the final pass of rough rolling in the hot rolling was 980°C, and the reduction was 30%. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing. The average heating rate from 50°C to 350°C was 80°C / s, and the hot-rolled steel sheet was soaked at 800°C for 30 seconds, then held at 1030°C for 30 seconds, and cooled. The hot-rolled sheet annealing atmosphere 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 warm-rolled at 150° C. to obtain a cold-rolled steel sheet with a final thickness of 0.27 mm.

[0066] Next, the obtained cold-rolled steel sheet was subjected to a 40 vol % N ２ +60vol%H ２ The primary recrystallization annealing, which also served as decarburization annealing, was performed at 850°C for 180 seconds in a humid atmosphere with a dew point of 50°C to obtain a primary recrystallization annealed sheet. Next, an annealing separator mainly composed of MgO was applied to the surface of the obtained primary recrystallization annealed sheet, and the sheet was annealed at 870°C for 40 hours in N ２ After holding in a hydrogen atmosphere, secondary recrystallization annealing was performed at 1175°C for 15 hours to obtain 24 types of grain-oriented electrical steel sheet samples with different composition. Next, the magnetic flux density B ８ The magnetic flux density B ８ The measurement results are shown in Table 3.

[0067]

[0068] According to Table 3, even for grain-oriented electrical steel sheets containing one or two optional added elements, an excellent magnetic flux density B of 1.925 T or more is obtained. ８ It can be seen that the following was obtained.

Claims

In mass ratio, C: 0.002% or more, 0.100% or less, Si: 2.0% or more, 6.5% or less, Mn: 0.02% or more, 1.00% or less, sol. Al: 10 ppm or more and less than 100 ppm, N: 10 ppm or more and 50 ppm or less, and S: 10ppm or more, 50ppm or less a steel slab having a composition containing the above and the balance being Fe and unavoidable impurities; The steel slab is heated to a temperature of 1300°C or less, and then hot-rolled to form a hot-rolled steel sheet; The hot-rolled steel sheet is subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet. The hot-rolled annealed sheet is subjected to cold rolling once or twice or more times with intermediate annealing interposed therebetween to obtain a cold-rolled steel sheet having a final sheet thickness; The cold-rolled steel sheet is subjected to primary recrystallization annealing to obtain a primary recrystallization annealed sheet; A method for producing a grain-oriented electrical steel sheet, comprising applying an annealing separator to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing, In the final pass of rough rolling of the hot rolling, the rolling temperature is 950°C or more and 1150°C or less, and the rolling reduction is 25% or more; In the hot-rolled sheet annealing, the average temperature rising rate when the hot-rolled steel sheet is heated from 50°C to 350°C is 40°C / s or more. A method for producing a grain-oriented electrical steel sheet.

2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the hot-rolled steel sheet is subjected to one or more soaking cycles in which the hot-rolled steel sheet is held at a soaking temperature of 350°C or more and 950°C or less for 3.0 seconds or more and 100 seconds or less during a temperature increase process of the hot-rolled steel sheet annealing. 　 The component composition further comprises, in mass ratio: Se: 0.001% or more, 0.005% or less, 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, 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 and 0.010% or less, and Zn: 0.0005% or more, 0.010% or less The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, further comprising the step of:

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