Manufacturing method of grain-oriented electrical steel sheet

Abstract

We propose a manufacturing method for grain-oriented electrical steel sheet that utilizes an AlN-based inhibitor to further improve magnetic properties and enable stable production. The method includes a hot-rolling process in which a steel material having a composition containing C, Si, Mn, sol.Al, and N, and at least one of S and Se, a hot-rolled sheet annealing process, a cold-rolling process, a primary recrystallization annealing process, and a finish annealing process. In the hot-rolled sheet annealing process, the coil winding temperature after hot rolling is defined as Ct, and the temperature-raising process of the hot-rolled sheet annealing is divided into three temperature sections: a section from room temperature to temperature T1, a section from temperature T1 to temperature T2°C, and a section from temperature T2 to the maximum temperature of the hot-rolled sheet annealing. The heating rates in each section are defined as t1, t2, and t3, respectively, and satisfy the following formulas (1) and (2). t1>t2>t3 and t1≧5×t3 (1) T1≦Ct≦T2 and T2≧T1+150 (2)

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing grain-oriented electrical steel sheets used as iron core materials for transformers. In this specification, "x to y" representing a range of values ​​means x or more and y or less, and includes the boundary values. "Room temperature" is defined as the temperature range of 5 to 35°C as specified in the Japanese Industrial Standards. [Background technology]

[0002] Grain-oriented electrical steel is a soft magnetic material used as the iron core material for transformers and large generators. Grain-oriented electrical steel has a crystal orientation that is the easy axis of magnetization of iron. <001> The steel sheet has a crystallographic texture with its axis highly aligned in the rolling direction. This texture is formed by secondary recrystallization and has a {110} Goss orientation. <001> This is achieved by preferentially growing grains with the Goss orientation. Therefore, the manufacturing method of grain-oriented electrical steel generally uses a precipitate called an inhibitor to induce secondary recrystallization of grains with the Goss orientation during secondary recrystallization annealing.

[0003] Patent Document 1 discloses a method using AlN or MnS, and Patent Document 2 discloses a method using MnS or MnSe, both of which have been put into industrial use. These methods using inhibitors require the steel material to be heated at high temperatures of 1300°C or higher to completely dissolve the inhibitor components. Nevertheless, these methods are extremely useful for stably developing secondary recrystallized grains.

[0004] Furthermore, in order to enhance the action of these inhibitors, Patent Document 3 discloses a method of using Pb, Sb, Nb, and Te, and Patent Document 4 discloses a method of using Zr, Ti, B, Nb, Ta, V, Cr, and Mo.

[0005] Furthermore, Patent Document 5 proposes a method in which acid-soluble Al (sol.Al) is added in an amount of 0.010 to 0.060 mass%, the slab is heated at a low temperature, and nitriding is performed in an appropriate nitriding atmosphere during the decarburization annealing process. As a result, (Al,Si)N precipitates during secondary recrystallization annealing and is used as an inhibitor. Many methods have been proposed, called nitriding methods, in which nitriding treatment is performed in an intermediate process after casting and (Al,Si)N or AlN is used as an inhibitor.

[0006] (A) On the other hand, Patent Document 6 and other publications disclose a technique for developing Goss-oriented crystal grains through secondary recrystallization in a material that does not contain inhibitor components. This technique makes it possible to induce secondary recrystallization of Goss-oriented grains without using inhibitors by eliminating impurities such as inhibitor components as much as possible, thereby making the grain boundary energy dependent on the grain boundary misorientation angle of the grain boundaries during primary recrystallization apparent. This effect is called the texture inhibition effect. (a) This method does not require the fine dispersion of the inhibitor in the steel, and therefore does not require the previously required high temperature heating of the steel material, making it a method with great advantages in terms of both cost and maintenance.

[0007] However, methods using materials that do not contain inhibitor components have occasionally been found to result in inferior final magnetic properties compared to methods that use inhibitor components. This is thought to be because there is no inhibitor that suppresses normal grain growth during primary recrystallization annealing, regulates the grain size to a constant value, and enhances the sharpness of the Goss orientation during secondary recrystallization. As a solution to this problem, Patent Document 7 discloses a technology that enables materials that do not contain inhibitor components to have high magnetic flux density by increasing the temperature rise rate during hot-rolled sheet annealing. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Special Publication No. 40-15644 [Patent Document 2] Special Publication No. 51-13469 [Patent Document 3] Special Publication No. 38-8214 [Patent Document 4] Japanese Patent Application Publication No. 52-24116 [Patent Document 5] Japanese Patent Application Publication No. 03-2324 [Patent Document 6] Japanese Patent Application Laid-Open No. 2000-129356 [Patent Document 7] Japanese Patent Application Laid-Open No. 2017-160489 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the prior art disclosed in the above patent document has the following problems. When the method of increasing the heating rate during annealing of hot-rolled sheet described in Patent Document 7 was applied to a steel type that uses an AlN-based inhibitor, the manufacturing method described in Patent Document 7 did not show any improvement in magnetic properties.

[0010] The present invention has been made in view of the above problems, and aims to solve the above problems by proposing a method for producing grain-oriented electrical steel sheets that utilizes an AlN-based inhibitor and enables further improvement in magnetic properties and stable production. [Means for solving the problem]

[0011] The inventors have conducted extensive research into solving the above problems and have now completed the present invention, and the following describes the experiments that led to this invention.

[0012] <Experiment 1> A steel slab containing, by mass, 0.055% C, 3.41% Si, 0.12% Mn, 0.0075% N, 0.024% sol. Al, and 0.020% Se, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1400°C for 40 minutes, and then hot-rolled to obtain a 2.6 mm thick hot-rolled steel sheet. Cooling immediately after hot-rolling was controlled so that the coiling temperature after hot-rolling was 550°C. The hot-rolled steel sheet was then hot-annealed at 1050°C for 30 seconds to obtain a hot-rolled annealed sheet. During the temperature rise process, the temperature rise rate from room temperature to T1°C was set to 80°C / s, then the temperature rise rate from T1°C to T2°C was set to 10°C / s, and the temperature rise time from T2 to 1050°C was set to 5°C / s, and the temperatures of T1 and T2 were variously changed. After hot-rolled sheet annealing, the surface scale was removed by pickling, and then cold-rolled to obtain a first cold-rolled steel sheet with a thickness of 1.5 mm. Next, intermediate annealing was performed at 1100°C for 150 seconds, followed by cold rolling to obtain a second cold-rolled steel sheet with a thickness of 0.23 mm. The second cold-rolled steel sheet was then subjected to primary recrystallization annealing with decarburization at 840°C for 150 seconds in a humid atmosphere of 55% H2 by volume, 45% N2 by volume, and a dew point of 60°C, to obtain a primarily recrystallized annealed sheet. The primarily recrystallized annealed sheet was then coated with an annealing separator mainly composed of MgO and subjected to secondary recrystallization annealing at 1200°C for 15 hours in a hydrogen atmosphere. The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the obtained sample was measured using the method described in JIS C2550. The results of the obtained magnetic flux density B8, organized by the hot-rolled sheet annealing conditions, are shown in Figure 1. These results show that the magnetic properties are good if T1 is 550°C, i.e., the coil winding temperature or lower, T2 is 550°C, i.e., the coil winding temperature or higher, and T2 ≥ T1 + 150°C.

[0013] <Experiment 2> A steel slab containing, by mass, 0.062% C, 2.99% Si, 0.05% Mn, 0.0080% N, 0.014% sol. Al, 0.005% S, and 0.014% Se, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1350°C for 60 minutes, and then hot-rolled to obtain a 3.0 mm thick hot-rolled steel sheet. Cooling immediately after hot rolling was controlled so that the coiling temperature after hot rolling was 650°C. The hot-rolled steel sheet was then hot-annealed at 900°C for 10 seconds to obtain a hot-rolled annealed sheet. During the heating process, the heating rate from room temperature to T1 = 350°C was defined as t1 (°C / s), the heating rate from T1 = 350°C to T2 = 800°C was defined as t2 (°C / s), and the heating time from T2 = 800°C to 900°C was defined as t3 (°C / s). t2 was fixed at 7.0°C / s, and t1 and t3 were varied. The hot-rolled annealed steel sheet was pickled to remove surface scale, and then cold-rolled to obtain a first cold-rolled steel sheet with a thickness of 2.0 mm. Next, intermediate annealing was performed at 1150°C for 100 seconds, and cold-rolled to obtain a second cold-rolled steel sheet with a thickness of 0.23 mm. The second cold-rolled steel sheet was then subjected to primary recrystallization annealing with decarburization at 830°C for 150 seconds in a 50% H2-50% N2 humid atmosphere with a dew point of 60°C to obtain a primary recrystallization-annealed sheet. The primary recrystallization-annealed sheet was then coated with an MgO-based annealing separator and subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere. The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the resulting sample was measured using the method specified in JIS C2550. The results are shown in Figure 2. These results indicate that good magnetic properties can be obtained when t1 > t2 (= 7.0) > t3 and t1 ≥ 5 × t3.

[0014] Summarizing the results of the two experiments above, it was shown that when the temperature zone during the temperature rise process of hot-rolled sheet annealing is divided into three sections, the lower the temperature section, the faster the temperature rise rate, and when the temperature rise rate is five times or more faster than the highest temperature section, and when the second temperature section includes the coiling temperature after hot rolling, good magnetic properties can be obtained. The reason for this is not entirely clear, but the inventors believe it to be as follows.

[0015] The present invention is an application of the technology described in Patent Document 7. Specifically, as described in Patent Document 7, Si3N4 is formed as a nitride in steel after hot rolling. In particular, cooling after hot rolling involves rapid water cooling down to the coiling temperature. After coiling, the steel is left to cool in the coiled state, resulting in extremely slow cooling. During this process, so-called Ostwald ripening occurs, resulting in significant coarsening of the nitrides themselves. However, the steel type using the inhibitor used in this experiment contains a relatively high amount of Al. Therefore, during hot rolling, not only Si3N4 but also a small amount of AlN precipitates adjacent to the Si3N4, which is different from Patent Document 7. In terms of thermodynamic stability, AlN is more stable than Si3N4. However, it is believed that Si3N4 precipitates preferentially due to the lattice matching, i.e., the difference in elastic energy, between the matrix and the precipitates. In other words, even if AlN precipitates due to the high Al content, it is difficult for it to grow, resulting in the presence of fine composite nitrides of AlN and Si3N4.

[0016] Fine precipitation of the inhibitor results in better final magnetic properties, making it preferable to a low Al content. When hot-rolled strips are subsequently annealed, the temperature range during the heating process is divided into three sections: the lowest section is defined as the temperature range from room temperature to below the coiling temperature, the second section includes the coiling temperature, and the highest section is defined as above the coiling temperature. These three temperature sections, from the lowest, can be considered as the temperature range where Si3N4 nitrides grow, the temperature range where Si3N4 solid-solution occurs, and the temperature range where AlN forms. By increasing the heating rate in the lowest section, we can prevent the nitrides from coarsening, while by slowing the heating rate in the highest section, we can promote the transformation of fine Si3N4 nitrides into AlN, thereby enabling the fine precipitation of AlN.

[0017] The present invention has been made based on this finding, and the method for producing a grain-oriented electrical steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is configured as follows.

[0018] [1] In mass%, C: 0.02 to 0.10%, Si: 2.0 to 5.0%, Mn: 0.03 to 1.00%, sol.Al: 0.010 to 0.040%, and N: 0.0040 to 0.0250%, and the total of one or two selected from S and Se: 0.002 to 0.040%; Optionally, Group A: at least one selected from Sb: 0.005 to 0.500%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%; Group B: at least one selected from B: 0.00001 to 0.0025%, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%, and Group C: containing at least one component selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%, A hot rolling process in which a steel material having a composition with the balance consisting of Fe and unavoidable impurities is reheated to a temperature of 1300°C or higher and hot rolled to form a hot rolled steel sheet; a hot rolled sheet annealing process in which the hot rolled steel sheet is annealed to form a hot rolled annealed sheet; a cold rolling process in which cold rolling is performed once or twice or more times with intermediate annealing in between to form a cold rolled steel sheet; a primary recrystallization annealing process in which the cold rolled steel sheet is subjected to primary recrystallization annealing to form a primary recrystallization annealed sheet; and a primary recrystallization annealing agent mainly composed of MgO is applied to the surface of the primary recrystallization annealed sheet. and a finish annealing step of performing secondary recrystallization annealing, wherein in the hot rolling step, a coil winding temperature after the hot rolling is defined as Ct, and in the hot-rolled sheet annealing step, the temperature rise process of the hot-rolled sheet annealing is divided into three temperature sections: a section from room temperature to T1, a section from T1 to T2, and a section from T2 to the maximum temperature of the hot-rolled sheet annealing, and the temperature rise rates in the respective sections are defined as t1, t2, and t3, and the following relational expressions 1 and 2 are satisfied. (Equation 1) t1>t2>t3 and t1≧5×t3 Here, t1 is the temperature rise rate (°C / s) from room temperature to T1, t2 is the temperature rise rate (°C / s) from T1 to T2, and t3 is the temperature rise rate (°C / s) from T2 to the maximum temperature for hot-rolled sheet annealing. (Equation 2) T1≦Ct≦T2 and T2≧T1+150 Here, Ct is the coil winding temperature after hot rolling (°C), and T1 and T2 are temperatures (°C) at which the temperature rise rate in the annealing of the hot-rolled sheet is changed. [2] In the above-mentioned [1], the method for producing a grain-oriented electrical steel sheet is characterized in that the average heating rate t1 in the temperature range from room temperature to 400°C during the heating process of the hot-rolled sheet annealing step is 50°C / s or more. [3] In the method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], in the temperature-raising process of the hot-rolled sheet annealing step, the temperature is lowered once by 5°C or more in a temperature range including Ct (°C), and then raised again. [4] In any one of the above [1] to [4], the hot-rolled sheet annealing step is a method for producing a grain-oriented electrical steel sheet, in which the temperature is maintained at Ct (°C) or higher for 10 seconds or more during the cooling process after reaching the maximum temperature. [Effects of the Invention]

[0019] According to the present invention, by using a material that utilizes an inhibitor and specifying the coil winding temperature after hot rolling and the temperature rise rate during the temperature rise process of hot-rolled sheet annealing for each temperature range, it is possible to achieve an improvement in magnetic flux density. [Brief explanation of the drawings]

[0020] [Figure 1] 10 is a graph showing the influence of the temperature at which the temperature rise rate of the hot-rolled sheet annealing is changed on the magnetic flux density B8 of the product sheet. [Figure 2] 1 is a graph showing the effect of the temperature rising rate in the annealing of a hot-rolled sheet on the magnetic flux density B8 of a product sheet. DETAILED DESCRIPTION OF THE INVENTION

[0021] The method for producing a grain-oriented electrical steel sheet according to the present embodiment comprises, in mass%, C: 0.02 to 0.10%, Si: 2.0 to 5.0%, Mn: 0.03 to 1.00%, sol. Al: 0.010 to 0.040%, and N: 0.0040 to 0.0250%, and a total of one or two selected from S and Se: 0.002 to 0.040%, Optionally, Group A: at least one selected from Sb: 0.005 to 0.500%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%; Group B: at least one selected from B: 0.00001 to 0.0025%, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%, and Group C: containing at least one component selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%, The method includes a hot rolling step of reheating a steel material having a component composition with the balance being Fe and unavoidable impurities to a temperature of 1300°C or higher and hot rolling it to form a hot-rolled steel sheet; a hot-rolled sheet annealing step of hot-rolling the hot-rolled steel sheet to form a hot-rolled annealed sheet; a cold rolling step of cold-rolling the cold-rolled steel sheet once or two or more times with intermediate annealing in between to form a cold-rolled steel sheet; a primary recrystallization annealing step of primary recrystallization annealing the cold-rolled steel sheet to form a primary recrystallization annealed sheet; and a finish annealing step of applying an annealing separator mainly composed of MgO to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing. In the hot rolling step, the coil winding temperature after the hot rolling is Ct, In the hot-rolled sheet annealing process, the temperature rising process of the hot-rolled sheet annealing is divided into three temperature sections: a section from room temperature to T1, a section from T1 to T2, and a section from T2 to the maximum temperature of the hot-rolled sheet annealing, and the temperature rising rates in each section are t1, t2, and t3, which satisfy the following relational expressions 1 and 2. (Equation 1) t1>t2>t3 and t1≧5×t3 Here, t1 is the temperature rise rate (℃ / s) from room temperature to T1, t2 is the temperature rise rate (℃ / s) from T1 to T2, and t3 is the temperature rise rate (℃ / s) from T2 to the maximum temperature for hot-rolled sheet annealing. is. (Equation 2) T1≦Ct≦T2 and T2≧T1+150 Here, Ct is the coil winding temperature after hot rolling (°C), and T1 and T2 are temperatures (°C) at which the temperature rise rate in the annealing of the hot-rolled sheet is changed.

[0022] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described.

[0023] <Steel material for manufacturing grain-oriented electrical steel sheets> First, the chemical composition of the steel material (steel slab) used in manufacturing the grain-oriented electrical steel sheet of the present invention and the reasons for limiting it will be explained. In the following explanation, unless otherwise specified, the notation "%" in the chemical composition of the steel material means "% by mass."

[0024] C: 0.02 to 0.10% If the C content is less than 0.02%, the precipitation of fine carbides will be insufficient, and the steel structure of the raw material will be a single α phase. This will cause the steel to become embrittled during casting and hot rolling, resulting in defects that will hinder manufacturing. On the other hand, if the C content exceeds 0.10%, it will be difficult to reduce the C content to 0.005% or less by decarburization annealing, at which point magnetic aging will not occur. Therefore, the C content is set to a range of 0.02 to 0.10%. Preferably, the C content is set to a range of 0.025 to 0.08%.

[0025] Si: 2.0 to 5.0% Si is an element necessary for increasing the resistivity of steel and reducing iron loss. The above effects are not sufficient if the Si content is less than 2.0%. On the other hand, if the Si content exceeds 5.0%, workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content is set in the range of 2.0 to 5.0%. Preferably, the Si content is in the range of 2.5 to 4.0%.

[0026] Mn: 0.03 to 1.00% Mn is an element necessary for improving the hot workability of steel. If the Mn content is less than 0.03%, the above effect is insufficient. On the other hand, if the Mn content exceeds 1.00%, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is set to the range of 0.03 to 1.0%. Preferably, the Mn content is in the range of 0.05 to 0.30%.

[0027] sol.Al: 0.010~0.040% In this embodiment, the inclusion of Al is essential because an AlN-based inhibitor is used. Al precipitates as AlN and functions as an inhibitor that suppresses normal grain growth during secondary recrystallization annealing. However, if the Al content is less than 0.010% in terms of acid-soluble Al (sol. Al), the absolute amount of inhibitor is insufficient, resulting in insufficient suppression of normal grain growth. On the other hand, if the Al content exceeds 0.040% in terms of sol. Al, AlN undergoes Ostwald ripening and becomes coarse, again resulting in insufficient suppression of normal grain growth. Therefore, the Al content is set to a range of 0.010 to 0.040% in terms of sol. Al. Preferably, the Al content is set to a range of 0.012 to 0.030% in terms of sol. Al.

[0028] N: 0.0040~0.0250% N combines with Al to form AlN, which acts as an inhibitor and precipitates. If the N content is less than 0.0040%, the absolute amount of inhibitor is insufficient, resulting in insufficient suppression of normal grain growth. On the other hand, if the N content exceeds 0.0250%, there is a risk of slab swelling during hot rolling. Therefore, the N content is set to a range of 0.0040 to 0.0250%. Preferably, the N content is in a range of 0.005 to 0.010%.

[0029] Sum of one or two selected from S and Se: 0.002 to 0.040% S and Se combine with Mn to form MnS and MnSe, which act as inhibitors. However, if the content of each element alone or in combination is less than 0.002%, the inhibitor effect is insufficient. On the other hand, if the content of each element alone or in combination exceeds 0.040%, the inhibitor coarsens due to Ostwald ripening, resulting in insufficient suppression of normal grain growth. Therefore, the content of each element alone or in combination is set to 0.002 to 0.040%. Preferably, the content of each element alone or in combination is set to 0.004 to 0.030%. More preferably, the content of each element alone or in combination is set to 0.005 to 0.027%.

[0030] The above is the basic composition of the steel material for manufacturing the grain-oriented electrical steel sheet according to this embodiment, but optionally, it may further contain at least one component from groups A to C below.

[0031] Group A: at least one selected from Sb: 0.005 to 0.500%, Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%. Sb, Sn, Cr, Cu, Ni, Bi, P, Mo, and Co improve the recrystallization texture, thereby improving the final magnetic properties. Addition of these elements below their respective lower limits results in a small effect. Addition exceeding their respective upper limits saturates the effect, resulting in increased costs. Therefore, the contents of Sb, Sn, Cr, Cu, Ni, Bi, P, Mo, and Co are limited to the above-mentioned ranges.

[0032] Group B: at least one selected from B: 0.00001 to 0.0025%, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%. B, Nb, Ti, V, and W form fine carbides and nitrides of these elements, which refine the crystal grain size after annealing, improving bending properties and reducing strip threading problems. Addition of these elements below the lower limit of their respective contents results in a small effect. On the other hand, addition of these elements above their upper limit results in a saturation of their effects, resulting in a disadvantageous factor of increased costs. Therefore, the contents of B, Nb, Ti, V, and W are limited to the above ranges.

[0033] Group C: at least one selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%. Zn, Zr, Pb, As, Ag, Au, Ga, Ge, Ca, Mg, REM, and Hf strengthen grain boundaries by concentrating at the grain boundaries or forming compounds at the grain boundaries, thereby suppressing defects caused by grain boundary fracture. Addition of these elements below the lower limit of each content is ineffective. On the other hand, addition of these elements above the upper limit of each content saturates the effect, resulting in disadvantages that increase costs. Therefore, the contents of Zn, Zr, Pb, As, Ag, Au, Ga, Ge, Ca, Mg, REM, and Hf are limited to the ranges indicated above.

[0034] The chemical composition of the steel material for manufacturing the grain-oriented electrical steel sheet according to this embodiment contains the above elements, with the balance being Fe and unavoidable impurities.

[0035] <Method of manufacturing grain-oriented electrical steel sheets> Next, a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described. Molten steel whose composition has been adjusted to a predetermined level is made into a steel material, for example, a steel slab, by a normal ingot-making method or continuous casting method. Of the additive elements described above, those that are difficult to add in the intermediate steps after casting are preferably added at the molten steel stage.

[0036] Hot rolling process The steel material (steel slab) is heated by a conventional method and subjected to hot rolling. The component composition of this embodiment contains large amounts of inhibitor-forming elements such as Al, N, Mn, S, and Se, so the steel material needs to be heated at a high temperature of 1300°C or higher to achieve solid solution. The steel material is preferably heated to 1350°C or higher. While there is no particular upper limit to the heating temperature, the melting point of steel containing Si drops to approximately 1460°C, so the heating temperature needs to be below that temperature. For the reasons mentioned above, the coiling temperature after hot rolling needs to have a certain relationship with the temperature during the temperature rise process of the hot-rolled sheet annealing in the subsequent process. Preferably, the coiling temperature is in the range of 500 to 750°C.

[0037] Hot-rolled sheet annealing process The temperature rise process of hot-rolled sheet annealing is divided into three temperature sections: from room temperature to temperature T1, from temperature T1 to temperature T2, and from temperature T2 to the maximum temperature of hot-rolled sheet annealing. The temperature rise rates in each section are designated as t1, t2, and t3, and annealing is performed by determining T1, T2, t1, t2, and t3 so as to satisfy the following relational expressions 1 and 2. (Equation 1) t1>t2>t3 and t1≧5×t3 Here, t1 is the temperature rise rate (°C / s) from room temperature to T1, t2 is the temperature rise rate (°C / s) from T1 to T2, and t3 is the temperature rise rate (°C / s) from T2 to the maximum temperature for hot-rolled sheet annealing. (Equation 2) T1≦Ct≦T2 and T2≧T1+150 Here, Ct is the coil winding temperature after hot rolling (°C), and T1 and T2 are temperatures (°C) at which the temperature rise rate in the annealing of the hot-rolled sheet is changed. During the heating process, the heating rate is faster in lower temperature sections, and the heating rate from room temperature to T1°C is at least five times that of the highest temperature section. The section from T1°C to T2°C includes the coiling temperature after hot rolling, and T2 is at least T1 + 150°C. The three temperature sections, from the lowest, must be the temperature section where Si3N4 nitride grows, the temperature section where Si3N4 dissolves, and the temperature section where AlN forms, respectively. There are no restrictions on the upper limit of temperature T1 or the lower limit of temperature T2. However, considering the above-mentioned mechanism, temperature T1 is preferably 600°C or below, where Si3N4 does not dissolve, and temperature T2 is preferably 800°C or above, where AlN precipitation is promoted.

[0038] t1>t2>t3 and t1≧5×t3 The heating rate required to obtain good magnetic properties is t1 > t2 > t3 and t1 ≥ 5 × t3. A fast heating rate in the lowest temperature range prevents nitride coarsening, while a slow heating rate in the highest temperature range promotes the conversion of fine Si3N4 nitrides to AlN, resulting in the fine precipitation of AlN. Therefore, t1 > t2 > t3 and t1 ≥ 5 × t3 are required. Furthermore, a heating rate of 50 °C / s or higher in the temperature range from room temperature to 400 °C is extremely effective for obtaining good magnetic properties. However, because a high heating rate requires specialized equipment and is costly, it is preferable to limit the heating rate to approximately 300 °C / s. There are no particular limitations on the heating method, but in order to achieve a heating rate of 50 °C / s or higher, induction heating and electric current heating are possible, in addition to conventional heating methods such as heaters and burners. In addition, in order to improve the magnetic properties, it is preferable to cool the material by 5°C or more once in a temperature range including the coil winding temperature during the temperature rise process of hot-rolled sheet annealing, and then raise the temperature again, or to hold the material at a temperature equal to or higher than the coil winding temperature for 10 seconds or more during the cooling process after soaking.

[0039] Cold rolling process After the above-mentioned hot-rolled sheet annealing, the steel sheet is cold-rolled and, if necessary, intermediately annealed, and then cold-rolled again to obtain a cold-rolled steel sheet of the final thickness. In cold rolling, warm rolling in which the steel sheet temperature is raised to 100 to 300°C, or aging treatment in the range of 100 to 300°C once or multiple times during cold rolling, are effective in changing the recrystallization texture and improving magnetic properties. In addition, intermediate annealing is preferably performed at 900°C or higher for 30 seconds or more, as this is effective in improving the structure. More preferably, intermediate annealing is performed at 1000°C or higher for 60 seconds or more.

[0040] Primary recrystallization annealing process The primary recrystallization annealing may also serve as decarburization of the steel sheet. The annealing temperature is preferably in the range of 800 to 900°C from the viewpoint of decarburization. From the viewpoint of decarburization, the atmosphere is preferably a moist atmosphere. Furthermore, the heating rate to the holding temperature in the primary recrystallization annealing is preferably in the range of 50 to 1000°C / s to improve the magnetic properties.

[0041] Finishing annealing process After applying an annealing separator mainly composed of MgO to the surface of a steel sheet, secondary recrystallization annealing is performed to develop secondary grains with Goss orientation and form a forsterite film. In secondary recrystallization annealing, the steel sheet is preferably held at 1100°C or higher for 5 hours or longer to purify the steel sheet and remove inhibitor components from the base steel. More preferably, the purification is performed at 1150°C or higher for 10 hours or longer. Furthermore, since the H2 atmosphere promotes purification, it is preferable to use an atmosphere containing 50% or more of H2 in the high-temperature range. Here, "mainly composed of MgO" means that the MgO content in the annealing separator is 60% by mass or more in terms of solid content. The MgO content in the annealing separator is preferably 80% by mass or more in terms of solid content. MgO is suspended in water to form a slurry solution and applied to the steel sheet. In this case, the slurry solution is preferably maintained at a constant temperature within a range of 5 to 30°C to suppress an increase in viscosity.

[0042] After the secondary recrystallization annealing, it is useful to perform water rinsing, brushing, or pickling to remove any adhering annealing separator. Then, further flattening annealing to correct the shape is effective for reducing iron loss. To further improve iron loss, it is effective to apply an insulating coating to the steel sheet surface before or after flattening annealing. In this case, it is preferable to use a coating that can impart tension to the steel sheet from the perspective of reducing iron loss. In this case, it is preferable to deposit an inorganic substance on the steel sheet surface by physical vapor deposition or chemical vapor deposition, as this provides excellent coating adhesion and significantly reduces iron loss. [Example]

[0043] The embodiments of the present invention will be further explained by way of examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. The embodiments can achieve the desired performance within the scope of the present invention.

[0044] Example 1 Steel slabs containing, by mass, 0.026% C, 3.06% Si, 0.07% Mn, 0.0050% N, 0.013% sol. Al, and 0.005% Se, with the remainder consisting of Fe and unavoidable impurities, were produced by continuous casting. The steel slabs were subjected to slab heating by soaking at 1320°C for 40 minutes, and then hot-rolled to obtain hot-rolled steel sheets with a thickness of 1.8 mm. The coiling temperature Ct after hot rolling was varied as shown in Table 1. The hot-rolled steel sheets were then annealed at 1000°C for 10 seconds to obtain hot-rolled annealed sheets. During the heating process, the heating rate from room temperature to temperature T1 was t1 = 20°C / s, followed by the heating rate from temperature T1 to temperature T2 at t2 = 10°C / s, and the heating time from temperature T2 to 1000°C at t3 = 3°C / s. T1 and T2 were varied as shown in Table 1. The hot-rolled annealed steel sheets were pickled to remove surface scale and then cold-rolled to obtain cold-rolled steel sheets with a thickness of 0.23 mm. The cold-rolled steel sheets were then subjected to primary recrystallization annealing with decarburization in a humid atmosphere of 50% H2 by volume, 50% N2 by volume, and a dew point of 55°C to obtain primary recrystallization annealed steel sheets. The primary recrystallization annealed steel sheets were then coated with an annealing separator mainly composed of MgO and subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere. The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the obtained sample was measured by the method described in JIS C2550. The obtained magnetic flux densities B8 are also shown in Table 1. From Table 1, it can be seen that good magnetic properties can be obtained by using hot-rolled sheet annealing conditions that are compatible with the present invention.

[0045] [Table 1]

[0046] Example 2 A steel slab containing, by mass, 0.080% C, 3.40% Si, 0.09% Mn, 0.0077% N, 0.029% sol. Al, 0.010% S, and 0.015% Se, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1420°C for 30 minutes, and then hot-rolled to obtain a 2.4 mm thick hot-rolled steel sheet. The coiling temperature after hot rolling was controlled to 650°C. The hot-rolled steel sheet was then hot-annealed at 1100°C for 45 seconds to obtain a hot-rolled annealed sheet. During the temperature rise process, the temperature rise rate from room temperature to T1 = 500°C was defined as t1 (°C / s), the subsequent temperature rise rate from T1 = 500°C to T2 = 1000°C was defined as t2 (°C / s), and the temperature rise time from T2 = 1000 to 1100°C was defined as t3 (°C / s). t1, t2, and t3 were varied as shown in Table 2. The hot-rolled annealed steel sheets were pickled to remove surface scale, then cold-rolled to a thickness of 1.5 mm, and then intermediate annealed at 1000°C for 50 seconds, followed by cold rolling to obtain cold-rolled steel sheets with a thickness of 0.20 mm. The cold-rolled steel sheets were then subjected to primary recrystallization annealing, which involved decarburization in a humid atmosphere of 50% H2-50% N2 with a dew point of 55°C, using a temperature pattern of 840°C for 120 seconds followed by 860°C for 30 seconds. This resulted in a primarily recrystallized annealed sheet. The primary recrystallized annealed sheets were then coated with an annealing separator primarily composed of MgO, and subjected to secondary recrystallization annealing, which involved holding the sheets at 1200°C for 10 hours in a hydrogen atmosphere. The magnetic flux density B8 (magnetic flux density when excited at 800 A / m) of the resulting samples was measured using the method described in JIS C2550. The obtained magnetic flux density B8 values ​​are also shown in Table 2. Table 2 demonstrates that excellent magnetic properties can be obtained by using the hot-rolled sheet annealing conditions consistent with the present invention.

[0047] [Table 2]

[0048] Example 3 A steel slab containing, by mass, 0.062% C, 3.15% Si, 0.18% Mn, 0.0045% N, 0.020% sol. Al, and 0.006% S, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1350°C for 30 minutes, and then hot-rolled to obtain a 2.6 mm thick hot-rolled steel sheet. The coiling temperature after hot rolling was controlled to 650°C. The hot-rolled sheet was then annealed at 1030°C for 45 seconds. In this case, during the temperature rise process, the temperature rise rate from room temperature to T1 = 300°C was 20°C / s, then the temperature rise rate from T1 = 300°C to T2 = 1000°C was 10°C / s, and the temperature rise time from T2 = 1000 to 1030°C was 2°C / s. The conditions for proceeding to the next step as is called Condition A.

[0049] Condition B was a modification of condition A, in which the temperature was increased during the annealing of the hot-rolled sheet by rapidly cooling from 600°C to 595°C once, then increasing the temperature again, and controlling the average temperature increase rate from 300°C to 1000°C to be 10°C / s.

[0050] Condition C had the same heating process as condition A, but after soaking at 1030°C for 45 seconds, the cooling process included a temperature hold at 850°C for 15 seconds.

[0051] The hot-rolled annealed steel sheets were pickled to remove surface scale, then cold-rolled to a thickness of 1.7 mm. They were then intermediate-annealed at 1070°C for 100 seconds, followed by cold rolling to obtain cold-rolled steel sheets with a thickness of 0.23 mm. The cold-rolled steel sheets were then subjected to primary recrystallization annealing, involving decarburization in a humid atmosphere of 50% H2 by volume, 50% N2 by volume, and a dew point of 62°C, with a temperature pattern of 150 seconds at 830°C and 30 seconds at 860°C. The primary recrystallization annealed steel sheets were then coated with an MgO-based annealing separator and subjected to secondary recrystallization annealing at 1200°C for 10 hours in a hydrogen atmosphere. The magnetic flux density B8 of the resulting samples (magnetic flux density when excited at 800 A / m) was measured using the method described in JIS C2550. The obtained magnetic flux density B8 values ​​are listed in Table 3. From Table 3, it can be seen that temporary rapid cooling during the temperature rise process of hot-rolled sheet annealing and maintaining the temperature during cooling are effective in improving magnetic properties.

[0052] [Table 3]

[0053] Example 4 A steel slab having the chemical composition shown in Table 4, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was reheated to a temperature of 1425°C and then hot-rolled to obtain a 2.2 mm-thick hot-rolled steel sheet. The coiling temperature after hot rolling was controlled to 650°C. The hot-rolled steel sheet was then annealed at 950°C for 60 seconds to obtain a hot-rolled annealed sheet. During this heating process, the heating rate from room temperature to T1 = 400°C was t1 = 100°C / s, the heating rate from T1 = 400°C to T2 = 900°C was t2 = 10°C / s, and the heating rate from T2 = 900°C to 950°C was t3 = 2°C / s. The hot-rolled annealed steel sheets were pickled to remove surface scale, then cold-rolled to a thickness of 2.0 mm. They were then subjected to intermediate annealing at 1000°C for 30 seconds, followed by cold rolling to obtain cold-rolled steel sheets with a thickness of 0.20 mm. The cold-rolled steel sheets were then subjected to primary recrystallization annealing with decarburization at 835°C for 150 seconds in a humid atmosphere of 50% H2-50% N2 with a dew point of 50°C to obtain primary recrystallization annealed steel sheets. The primary recrystallization annealed steel sheets were then coated with an MgO-based annealing separator and subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere. The magnetic flux density B8 of the obtained samples (magnetic flux density when excited at 800 A / m) was measured using the method described in JIS C2550. The obtained magnetic flux density B8 values ​​are listed in Table 4. From Table 4, it can be seen that by using a steel material having a component composition that is compatible with the present invention, a grain-oriented electrical steel sheet with good magnetic properties can be obtained.

[0054] [Table 4]

Claims

1. In mass percent, C: 0.02-0.10%, Si: 2.0 to 5.0%, Mn: 0.03 to 1.00%, Sol. Al: 0.010–0.040%, and N: Contains 0.0040 to 0.0250%, The sum of one or two types selected from S and Se: 0.002–0.040% It contains, Optionally, Group A; At least one selected from Sb: 0.005–0.500%, Sn: 0.005–0.500%, Cr: 0.005–0.500%, Cu: 0.01–0.50%, Ni: 0.01–0.50%, Bi: 0.005–0.500%, P: 0.005–0.200%, Mo: 0.005–0.500%, and Co: 0.001–0.500%. Group B; At least one selected from B: 0.00001 to 0.0025%, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%, and Group C; At least one selected from Zn: 0.0005–0.020%, Zr: 0.001–0.020%, Pb: 0.0001–0.0100%, As: 0.001–0.020%, Ag: 0.001–0.050%, Au: 0.001–0.050%, Ga: 0.0001–0.0050%, Ge: 0.0001–0.0050%, Ca: 0.0005–0.020%, Mg: 0.0005–0.020%, REM: 0.0005–0.0200%, and Hf: 0.001–0.020%. It contains at least one of the following components: A hot rolling process involves reheating a steel material having a composition consisting of Fe and unavoidable impurities to a temperature of 1300°C or higher and hot rolling it to produce a hot-rolled steel sheet. A hot-rolled sheet annealing process is performed on the aforementioned hot-rolled steel sheet to obtain a hot-rolled annealed sheet. A cold rolling process to produce cold-rolled steel sheet by cold rolling once or twice or more with an intermediate annealing in between, The cold-rolled steel sheet is subjected to a primary recrystallization annealing process to obtain a primary recrystallization annealed sheet, A finishing annealing step is performed in which an annealing separating agent mainly composed of MgO is applied to the surface of the primary recrystallized annealed plate, and secondary recrystallization annealing is performed. Includes, In the hot rolling process, the coil winding temperature after hot rolling is Ct. A method for manufacturing grain-oriented electrical steel sheets, characterized in that the hot-rolled sheet annealing process is divided into three temperature intervals: from room temperature to T1, from T1 to T2, and from T2 to the maximum temperature of the hot-rolled sheet annealing, with the heating rates in each interval being t1, t2, and t3, and satisfying the following relational equations 1 and 2. (Relationship 1) t1 > t2 > t3 and t1 ≥ 5 × t3 Here, t1: the rate of heating from room temperature to T1 (°C / s), t2: Heating rate (°C / s) in the interval from T1 to T2. t3: Heating rate (°C / s) from T2 to the maximum temperature of hot-rolled sheet annealing. That is the case. (Relationship equation 2) T1 ≤ Ct ≤ T2 and T2 ≥ T1 + 150 Here, Ct: coil winding temperature after hot rolling (°C), T1, T2: Temperatures (°C) at which the heating rate is changed during hot-rolled sheet annealing. That is the case.

2. The method for manufacturing grain-oriented electrical steel sheets according to claim 1, characterized in that the heating rate t1 in the temperature range from room temperature to 400°C during the heating process of the hot-rolled sheet annealing step is 50°C / s or more.

3. The method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2, characterized in that, in the heating process of the hot-rolled sheet annealing process, the temperature is lowered by 5°C or more once in a temperature range including Ct (°C), and then the temperature is raised again.

4. The method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2, characterized in that, in the hot-rolled sheet annealing process, the temperature is maintained at a temperature of Ct (°C) or higher for 10 seconds or more during the cooling process after reaching the maximum temperature.

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