Method for manufacturing grain-oriented electrical steel sheet

Abstract

The present invention proposes a method for manufacturing a grain-oriented electrical steel sheet having a high magnetic flux density at a high yield by preventing edge cracking in a steel material having a component composition with less inhibitors. This method for manufacturing a grain-oriented electrical steel sheet comprises a hot rolling step, an optional hot-rolled sheet annealing step, a cold rolling step, a decarburization annealing step, and a finish annealing step. In the hot rolling step, the steel material is heated to a range of 1000-1260°C in a gas furnace having an atmosphere that has an in-furnace oxygen concentration of 3.0 vol% or less, four or more rounds of rough rolling are performed thereon at a rolling reduction of 50% or less, width rolling is performed thereon between rounds of the rough rolling at a width rolling reduction of 1.5-8.2% in the width direction, and then finish rolling is performed thereon.

Description

Manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to a method for producing grain-oriented electrical steel sheets with a high yield, by preventing edge cracks that occur during hot rolling of the grain-oriented electrical steel sheets used for iron cores of transformers and the like.

[0002] Grain-oriented electrical steel sheets are soft magnetic materials primarily used as iron core materials for transformers, and are required to have a high magnetic flux density and low iron loss, with the <001> orientation, the axis of easy magnetization of iron, highly aligned in the rolling direction of the steel sheet. In recent years, the need for energy conservation has led to an increasing demand for grain-oriented electrical steel sheets that reduce energy loss in transformers, and there is a need to manufacture grain-oriented electrical steel sheets with high magnetic flux density and low iron loss at low cost and with high yield.

[0003] Grain-oriented electrical steel sheets manufactured using inhibitors are produced by heating a steel material with adjusted chemical composition at high temperatures and then hot-rolling it to produce a hot-rolled steel sheet. The hot-rolled steel sheet is optionally subjected to hot-rolled sheet annealing, followed by one or more cold rolling steps with intermediate annealing in between to produce a cold-rolled steel sheet of the final thickness. The cold-rolled steel sheet is then subjected to decarburization annealing, which also serves as primary recrystallization annealing, followed by coating the steel sheet surface with an annealing separator and finish annealing. Grain-oriented electrical steel sheets manufactured using inhibitors have a problem with cracks in the edge areas (hereinafter referred to as "edge cracks") caused by coarse crystal grains (hereinafter referred to as "coarse grains") that occur at the edges of the steel sheet during hot rolling.

[0004] As a means for preventing such edge cracks, for example, Patent Document 1 discloses a method of promoting recrystallization by changing the rough rolling reduction schedule in hot rolling to impart strain to coarse grains at the edge of the steel sheet. Patent Document 2 discloses a method of controlling the start and end temperatures of finish rolling in hot rolling, and Patent Document 3 discloses a method of reducing the temperature difference in the longitudinal and width directions of the rolled material before finish rolling in hot rolling. Patent Documents 4 and 5 disclose methods of performing width reduction on a hot-rolled sheet bar. Furthermore, Patent Document 6 discloses a technology for preventing edge cracks by adjusting slab heating conditions, controlling the ratio of the C content of the surface layer of the side surface of the steel material to the C content of the side surface and center of the slab, and then performing controlled rolling.

[0005] Japanese Patent Publication No. 57-004690 Japanese Patent Laid-Open No. 55-062124 Japanese Patent Laid-Open No. 57-165102 Japanese Patent Laid-Open No. 64-003564 Japanese Patent Publication No. 03-006842 Japanese Patent Laid-Open No. 2006-075885

[0006] However, the prior art disclosed in the above-mentioned patent document has the following problems. Grain-oriented electrical steel sheets manufactured from slabs having an inhibitor-free composition are produced by heating the slab at a temperature of 1250°C or less and then hot-rolling it to form a hot-rolled steel sheet. The hot-rolled steel sheet is optionally subjected to hot-rolled sheet annealing and then cold-rolled once or twice or more times with intermediate annealing in between to form a cold-rolled steel sheet of final thickness. The cold-rolled steel sheet is then subjected to decarburization annealing that also serves as primary recrystallization annealing, followed by coating the steel sheet surface with an annealing separator and finish annealing. Grain-oriented electrical steel sheets manufactured from steel materials having an inhibitor-free composition can be produced by heating the slab at a temperature of 1250°C or less, and therefore can be produced at a lower cost than grain-oriented electrical steel sheets manufactured using inhibitors.

[0007] Even when manufacturing grain-oriented electrical steel sheets made from steel materials with inhibitor-free chemical compositions, edge cracks can occur during hot rolling. The method of changing the rough rolling reduction schedule in hot rolling, as described in Patent Document 1, has little effect on the edge portion and does not sufficiently improve edge cracks. The hot rolling methods described in Patent Documents 2 and 3 control the temperature before and after finish rolling. However, since coarse grains are generated at the end of rough rolling, this does not fundamentally solve the edge crack problem. The hot rolling methods described in Patent Documents 4 and 5 are significantly affected by the heating conditions of the slab and do not provide sufficient measures to prevent edge cracks. The hot rolling method described in Patent Document 6 causes grain coarsening due to high-temperature recrystallization in inhibitor-free systems, and its application to inhibitor-free systems actually increases costs.

[0008] 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 prevent edge cracking, have a high yield, and have a high magnetic flux density, using steel materials with a component composition containing few inhibitors.

[0009] The inventors conducted extensive research to solve the above-mentioned problems. As a result, they discovered that the edge cracking occurs when the surface oxidation of the steel material for inhibitor-free grain-oriented electrical steel sheet causes the inhibitor, which is present in small amounts, to decompose from the surface layer. This reduces the inhibitor's inhibitory effect, causing coarsening of crystal grains at the edges of the steel material, resulting in edge cracking during hot rolling. The inventors conducted further extensive research to prevent the occurrence of coarse grains. As a result, they discovered that the following four points, A to D, are important: A) By heating the steel material for inhibitor-free electrical steel sheet in a specified atmosphere before hot rolling, surface decomposition of the inhibitor during heating can be suppressed, preventing edge cracking due to grain coarsening at the edges. B) By applying specified conditions to the steel material heating temperature, edge cracking due to grain coarsening at the edges due to high-temperature recrystallization can be prevented. c) By applying specified conditions to width rolling between each rough rolling, it is possible to refine the coarse grains that occur at the edges and prevent edge cracks. d) By applying specified conditions to rough rolling in conjunction with a higher reduction rate in width rolling, it is possible to form a texture that facilitates the growth of highly oriented Goss grains, thereby increasing the magnetic flux density and enabling the production of grain-oriented electrical steel sheets with high magnetic flux density at a high yield.

[0010] The present invention was 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: [1] A hot rolling step in which a steel material having a composition containing, by mass%, C: 0.010 to 0.045%, Si: 2.8 to 4.5%, Mn: 0.01 to 1.00%, acid-soluble Al: 0.010% or less, and N: 0.008% or less, with the balance being Fe and unavoidable impurities, is hot-rolled to produce a hot-rolled steel sheet, a hot-rolled sheet annealing step in which the hot-rolled steel sheet is optionally annealed to produce a hot-rolled annealed sheet, a cold-rolling step in which the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or two or more times with intermediate annealing between them to produce a cold-rolled steel sheet having a final sheet thickness, and a primary recrystallization step in which the cold-rolled steel sheet is subjected to a primary recrystallization step. [2] A method for producing a grain-oriented electrical steel sheet according to the above item [1], wherein the steel material further contains, by mass, at least one element selected from the group A to D below, in addition to the chemical composition described above:Group A: at least one selected from the total of either or both of S and Se: 0.010% or less, the total of either or both of Sn and Sb: 1.000% or less, Cr: 0.100% or less, Cu: 1.500% or less, Ni: 1.500% or less, Bi: 0.100% or less, P: 0.500% or less, and Mo: 0.500% or less; Group B: at least one selected from B: 25.0 ppm or less, Nb: 0.1000% or less, Ti: 0.1000% or less, V: 0.1000% or less, and Co: 0.050% or less; Group C: at least one selected from As: 0.0200% or less, Pb: 0.0100% or less, W: 0.0100% or less, and Zn: 0.020% or less; Group D: At least one selected from Ag: 0.050% or less, Au: 0.050% or less, Ca: 0.020% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Nd: 0.020% or less, and La: 0.020% or less.

[0011] According to the present invention, it is possible to prevent edge cracks from occurring during hot rolling and to produce grain-oriented electrical steel sheets having high magnetic flux density at low cost and with high yield.

[0012] A method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described below. The method for producing a grain-oriented electrical steel sheet according to this embodiment includes a hot rolling step in which a steel material having a composition containing, by mass%, 0.010 to 0.045% C, 2.8 to 4.5% Si, 0.01 to 1.00% Mn, 0.010% or less acid-soluble Al, and 0.008% or less N, with the balance being Fe and unavoidable impurities, is hot-rolled to produce a hot-rolled steel sheet. Optionally, the method may include a hot-rolled sheet annealing step in which the hot-rolled steel sheet is annealed to produce a hot-rolled annealed sheet. The method also includes a cold rolling step in which the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or two or more times with intermediate annealing between them to produce a cold-rolled steel sheet having a final thickness. The method includes a decarburization annealing step in which a cold-rolled steel sheet is subjected to decarburization annealing that also serves as primary recrystallization annealing, or a combination of primary recrystallization annealing and decarburization annealing, to produce a decarburization-annealed sheet. The method also includes a finish annealing step in which the surface of the decarburization-annealed sheet is coated with an annealing separator and subjected to secondary recrystallization annealing. In the hot rolling step, the steel material is heated to a temperature range of 1000 to 1260°C in a gas furnace with an oxygen concentration of 3.0% by volume or less in the furnace atmosphere, and rough rolling is performed four or more times with a rolling reduction of 50% or less. Between the rough rolling steps, width rolling is performed in the width direction with a width reduction of 1.5 to 8.2%, followed by finish rolling. Here, the rough rolling reduction is expressed as a percentage by dividing the amount of thickness reduction by the sheet thickness before rolling for each rough rolling step. The width reduction rate is the amount of reduction in the sheet width due to each width rolling divided by the sheet width before rolling, and expressed as a percentage.

[0013] First, the experiment that inspired the development of the present invention will be described. <Experiment 1> A steel slab containing, by mass%, 0.032% C, 3.3% Si, 0.07% Mn, 0.007% acid-soluble Al, and 0.003% N, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was then heated to 1200°C in a gas furnace with an oxygen concentration in the furnace of 0.1 to 5.0% by volume. Thereafter, four rough rolling operations were performed: the first rough rolling operation had a reduction ratio of 30%, the second rough rolling operation had a reduction ratio of 33%, the third rough rolling operation had a reduction ratio of 40%, and the fourth rough rolling operation had a reduction ratio of 33%. Width rolling was performed between each rough rolling operation, with a width reduction of 3.2%. A hot-rolled steel sheet with a thickness of 2.5 mm was then obtained by finish rolling.

[0014] The depth of edge cracks of the hot-rolled steel sheets was measured. Those without edge cracks were subjected to hot-rolled sheet annealing at 1000°C for 50 seconds, followed by primary cold rolling to an intermediate sheet thickness of 1.7 mm, and intermediate annealing at 1050°C for 25 seconds. Then, final cold rolling was performed to produce cold-rolled steel sheets with a final thickness of 0.23 mm. Subsequently, the steel sheets were heated at a heating rate of 50°C / s. Then, H ２ and N ２ The steel sheets were then subjected to decarburization annealing at 840°C for 70 seconds in a hydrogen atmosphere containing MgO. An annealing separator mainly composed of MgO was then applied to the steel sheet surface, which was then dried. The steel sheets were then subjected to finish annealing at a maximum temperature of 1220°C for 10 hours in a hydrogen atmosphere.

[0015] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The magnetic flux density (at a magnetizing force of 800 A / m) was measured. The results are shown in Table 1. Table 1 shows that the steel material heated in a gas furnace with an oxygen concentration in the furnace of 0.1 to 3.0 volume % suppresses the occurrence of edge cracks and produces grain-oriented electrical steel sheets with a high magnetic flux density of 1.900 T or more.

[0016]

[0017] <Experiment 2> A steel slab containing, by mass%, 0.027% C, 3.0% Si, 0.06% Mn, 0.007% acid-soluble Al, and 0.004% N, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was heated to 940-1300 °C in a gas furnace with an oxygen concentration of 2.0% by volume. Five rough rolling operations were then performed: the first rough rolling reduction was 21%, the second rough rolling reduction was 29%, the third rough rolling reduction was 42%, the fourth rough rolling reduction was 21%, and the fifth rough rolling reduction was 27%. Width rolling was performed between each rough rolling operation, with a width reduction of 3.0%. A hot-rolled steel sheet with a thickness of 2.4 mm was then obtained by finish rolling.

[0018] The depth of edge cracks of the hot-rolled steel sheets was measured. Those without edge cracks were subjected to primary cold rolling to an intermediate thickness of 1.8 mm, and intermediate annealing was performed at 1030°C for 30 seconds. Thereafter, final cold rolling was performed to produce cold-rolled steel sheets with a final thickness of 0.23 mm. Subsequently, the steel sheets were heated at a heating rate of 80°C / s. Then, H ２ and N ２ The steel sheets were then subjected to decarburization annealing at 840°C for 80 seconds in a mixed atmosphere of MgO and MgO. An annealing separator mainly composed of MgO was then applied to the surface of the steel sheets, which were then dried and then subjected to finish annealing at a maximum temperature of 1220°C for 12 hours in a hydrogen atmosphere.

[0019] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The results are shown in Table 2. From Table 2, it can be seen that when the steel slab heating temperature is set to 1000 to 1260°C, the occurrence of edge cracks is suppressed and grain-oriented electrical steel sheets having a high magnetic flux density of 1.900 T or more can be obtained.

[0020]

[0021] <Experiment 3> A steel slab containing, by mass%, 0.024% C, 3.2% Si, 0.05% Mn, 0.007% acid-soluble Al, and 0.004% N, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was heated to 1080 ° C in a gas furnace with an oxygen concentration of 1.0 vol%. Five rough rolling operations were then performed: the first rough rolling reduction was 18%, the second rough rolling reduction was 31%, the third rough rolling reduction was 27%, the fourth rough rolling reduction was 28%, and the fifth rough rolling reduction was 36%. Between each rough rolling operation, width rolling was performed at the width reduction shown in Table 3. Then, a hot-rolled steel sheet with a thickness of 2.4 mm was obtained by finish rolling.

[0022] The depth of edge cracks of the hot-rolled steel sheets was measured. Those without edge cracks were cold-rolled to produce cold-rolled steel sheets with a final thickness of 0.23 mm. Subsequently, the steel sheets were heated at a temperature-raising rate of 85°C / s. ２ and N ２ The steel sheets were then subjected to decarburization annealing at 850°C for 120 seconds in a mixed atmosphere of MgO and MgO. An annealing separator mainly composed of MgO was then applied to the surface of the steel sheets, which were then dried and then subjected to finish annealing at a maximum temperature of 1190°C for 17 hours in a hydrogen atmosphere.

[0023] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The results are shown in Table 3. It can be seen from the table that in the rolled material in which the sheet bar is width-rolled to a width reduction ratio in the range of 1.5 to 8.2% in the width direction from the width of the steel material before rough rolling, the occurrence of edge cracks is suppressed and a grain-oriented electrical steel sheet having a high magnetic flux density of 1.900 T or more can be obtained.

[0024]

[0025] <Experiment 4> A steel slab containing, by mass%, 0.029% C, 3.2% Si, 0.05% Mn, 0.006% acid-soluble Al, and 0.003% N, with the balance being Fe and unavoidable impurities, was produced by continuous casting. The steel slab was then heated to 1170°C in a gas furnace with an oxygen concentration of 1.5% by volume. Rough rolling was then performed according to Table 4. Width rolling was performed between each rough rolling pass, with a width reduction of 3.8%. A hot-rolled steel sheet with a thickness of 2.4 mm was then obtained by finish rolling.

[0026] The depth of edge cracks of the hot-rolled steel sheets was measured. Those without edge cracks were cold-rolled to produce cold-rolled steel sheets with a final thickness of 0.23 mm. Subsequently, the steel sheets were heated at a temperature-raising rate of 60°C / s. ２ and N ２ The steel sheets were subjected to decarburization annealing at 860°C for 100 seconds in a mixed atmosphere of MgO and MgO. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheets, which were then dried and then subjected to finish annealing at a maximum temperature of 1200°C for 16 hours in a hydrogen atmosphere.

[0027] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The results are shown in Table 4. From Table 4, it can be seen that when rough rolling at a reduction rate of 50% or less was performed four or more times, grain-oriented electrical steel sheets having a high magnetic flux density of 1.900 T or more were obtained.

[0028]

[0029] The inventors believe that the reason why edge cracks were suppressed at the edge of the steel sheet and good magnetic flux density was obtained under the above conditions is as follows: In inhibitor-free steel materials with a low γ phase ratio, coarse grains are likely to occur at the edge of the steel sheet during hot rolling, which causes edge cracks. As described above, by lowering the oxygen concentration in the gas furnace that heats the steel material, it is possible to suppress the decomposition of inhibitors in the surface layer when the steel material is heated, and to prevent coarsening of grains at the edge.

[0030] Next, because the steel has an inhibitor-free composition, the heating temperature of the steel material can be lowered, as described above, and coarsening of grains at the edges due to high-temperature recrystallization can be suppressed. Next, width rolling during rough rolling under the above conditions can refine the coarse grains that have already occurred. By performing rough rolling four or more times, each with a reduction rate of 50% or less, appropriate strain is introduced, forming a texture that facilitates recrystallization of highly oriented Goss grains and increasing the magnetic flux density.

[0031] Based on these findings, it is believed that by suppressing the generation of coarse grains during hot rolling and refining the crystal grains, even in inhibitor-free materials with a low γ phase ratio, it is possible to prevent edge cracking and produce grain-oriented electrical steel sheets with high magnetic flux density at low cost and with high yield.

[0032] <Steel material for manufacturing grain-oriented electrical steel sheet> First, the chemical composition of the steel material (steel slab) used to manufacture the grain-oriented electrical steel sheet according to this embodiment and the reasons for limiting this will be described. In the following description, unless otherwise specified, the notations "%" and "ppm" regarding the chemical composition mean "% by mass" and "ppm by mass", respectively.

[0033] C: 0.010-0.045% If the C content is less than 0.010%, the grain boundary strengthening effect of C is lost, causing defects that hinder manufacturing, such as cracks in the steel material. On the other hand, if the C content exceeds 0.045%, a recrystallized structure develops, which reduces the orientation of Goss grains. This reduces the magnetic flux density B ８ This causes a decrease in the C content. Furthermore, it becomes difficult to reduce the C content to 0.005% or less, which is the level at which magnetic aging does not occur, by decarburization annealing. Therefore, the C content is set to the range of 0.010 to 0.045%. More preferably, the C content is set to the range of 0.015 to 0.040%.

[0034] Si: 2.8 to 4.5% 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 2.8%. On the other hand, if the Si content exceeds 4.5%, workability decreases, making it difficult to produce steel sheet by rolling. Therefore, the Si content is set to the range of 2.8 to 4.5%. More preferably, the Si content is in the range of 3.0 to 4.0%.

[0035] Mn: 0.01 to 1.00% Mn is an element necessary for improving the hot workability of steel. The above effect is not sufficient if the Mn content is less than 0.01%. 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 0.01 to 1.00%. More preferably, the Mn content is in the range of 0.02 to 0.50%.

[0036] Acid-soluble Al: 0.010% or less Since Al forms a dense oxide film on the surface and may inhibit decarburization, the Al content is controlled to 0.010% or less in terms of acid-soluble Al, and more preferably, to 0.008% or less.

[0037] N: 0.008% or less N can cause defects such as blisters when the steel material is heated. Therefore, the N content must be limited to 0.008% or less. More preferably, the N content is limited to 0.005% or less, and even more preferably, to 0.004% or less.

[0038] The above is the basic composition of the steel material for manufacturing grain-oriented electrical steel sheet according to this embodiment. Optionally, for the purpose of improving magnetic properties, at least one component from Groups A to D below can be further contained as an optional component.

[0039] Group A: at least one selected from the total of either one or both of S and Se: 0.010% or less, the total of either one or both of Sn and Sb: 1.000% or less, Cr: 0.100% or less, Cu: 1.500% or less, Ni: 1.500% or less, Bi: 0.100% or less, P: 0.500% or less, and Mo: 0.500% or less. Addition of S, Se, Sn, Sb, Cr, Cu, Ni, Bi, P, and Mo in amounts exceeding the above upper limits saturates the effect and may result in excessively high production costs. To obtain the effects of the addition, it is preferable to add at least one selected from the group consisting of S and / or Se in total at 0.005% or more, Sn and / or Sb in total at 0.005% or more, Cr in total at 0.005% or more, Cu in total at 0.005% or more, Ni in total at 0.005% or more, Bi in total at 0.005% or more, P in total at 0.005% or more, and Mo in total at least one selected from the group B: B in total at 25.0 ppm or less, Nb in total at 0.1000% or less, Ti in total at 0.1000% or less, V in total at 0.1000% or less, and Co in total at 0.050% or less. Addition of B, Nb, Ti, V, and Co in amounts exceeding the upper limits mentioned above may saturate the effect and may result in excessive production costs. To obtain the effects of the addition, it is preferable to add at least one selected from B: 0.1 ppm or more, Nb: 0.0005% or more, Ti: 0.0005% or more, V: 0.0005% or more, and Co: 0.002% or more. Group C: At least one selected from As: 0.0200% or less, Pb: 0.0100% or less, W: 0.0100% or less, and Zn: 0.020% or less. Addition of As, Pb, W, and Zn in amounts exceeding the upper limits mentioned above may saturate the effects and result in excessive manufacturing costs. To obtain the effects of the addition, it is preferable to add at least one selected from As: 0.0010% or more, Pb: 0.0001% or more, W: 0.0010% or more, and Zn: 0.001% or more.Group D: at least one selected from Ag: 0.050% or less, Au: 0.050% or less, Ca: 0.020% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Nd: 0.020% or less, and La: 0.020% or less. Addition of Ag, Au, Ca, Ga, Ge, Nd, and La in amounts exceeding the upper limits saturates the effect and may result in excessive manufacturing costs. To obtain the effects of addition, it is preferable to add at least one selected from Ag: 0.001% or more, Au: 0.001% or more, Ca: 0.001% or more, Ga: 0.0001% or more, Ge: 0.0001% or more, Nd: 0.001% or more, and La: 0.001% or more.

[0040] The chemical composition of the steel material for manufacturing grain-oriented electrical steel sheet according to this embodiment contains the above elements, with the remainder being Fe and inevitable impurities. The inevitable impurities are impurities that are inevitably mixed in from raw materials, the manufacturing process, manufacturing equipment, etc., and are allowed to be present to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, and scrap.

[0041] <Method for manufacturing grain-oriented electrical steel sheet> Next, a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described.

[0042] Hot Rolling Process: After steel having the aforementioned chemical composition is melted using a conventional refining process, a steel material (steel stock) is produced using a conventionally known ingot-blooming and rolling method or continuous casting method. The steel material is heated in a gas furnace and subjected to hot rolling. The oxygen concentration in the gas furnace is set to 3.0% by volume or less. If the oxygen concentration in the furnace exceeds 3.0%, oxidative decomposition of inhibitors on the surface of the steel material progresses, causing grain coarsening at the edges of the steel material and resulting in edge cracking during hot rolling. Therefore, the oxygen concentration in the furnace is set to 3.0% by volume or less. While there is no particular lower limit for the oxygen concentration in the furnace, setting it below 0.1% by volume increases energy costs and increases costs, so it is sufficient to set it to 0.1% by volume or more.

[0043] When the above-mentioned steel material is heated in a gas furnace, if the heating temperature of the steel material is less than 1000°C, the rolling load becomes high during hot rolling, making hot rolling difficult. On the other hand, if the heating temperature of the steel material is more than 1260°C, high-temperature recrystallization will cause grain coarsening at the edge of the steel material and increase costs. Therefore, the heating temperature of the steel material is set to a range of 1000 to 1260°C. More preferably, the heating temperature of the steel material is set to a range of 1060 to 1260°C.

[0044] In the hot rolling, rough rolling with a reduction of 50% or less is performed four or more times. Rough rolling with a reduction of more than 50% introduces excessive strain into the steel sheet, making it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. Next, width rolling with a width reduction of 1.5 to 8.2% is performed in the width direction between each rough rolling. Here, if the width reduction is less than 1.5%, strain is not sufficiently introduced into the generated coarse grains, resulting in the grains at the edges of the steel sheet remaining coarse, causing edge cracks. On the other hand, if the width reduction is more than 8.2%, a dog-bone shape is prominent in the cross-sectional view due to width rolling. As a result, rough rolling insufficiently introduces strain into the width center, making it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. More preferably, the width reduction is in the range of 1.5 to 7.0°C. Then, finish rolling is performed to obtain a hot-rolled steel sheet.

[0045] Hot-rolled sheet annealing process: Optionally, the hot-rolled steel sheet may be annealed. The annealing temperature for this hot-rolled sheet annealing is preferably in the range of 800 to 1150°C in order to obtain good magnetic properties. If the annealing temperature is less than 800°C, the band structure formed by hot rolling will remain, making it difficult to obtain a granular primary recrystallized structure and the development of secondary recrystallization may be inhibited. On the other hand, if the annealing temperature exceeds 1150°C, the grain size after hot-rolled sheet annealing may become too coarse, making it difficult to obtain a granular primary recrystallized structure.

[0046] Cold Rolling Process: The hot-rolled steel sheet or hot-rolled annealed sheet is subjected to one cold rolling or two or more cold rolling processes with intermediate annealing in between to produce a cold-rolled steel sheet of the final thickness. The annealing temperature for the intermediate annealing is preferably in the range of 900 to 1200°C. If the annealing temperature is less than 900°C, the recrystallized grains after the intermediate annealing tend to become finer, and the Goss nuclei in the primary recrystallized structure tend to decrease, resulting in a deterioration in the magnetic properties of the product sheet. On the other hand, if the annealing temperature exceeds 1200°C, as with hot-rolled sheet annealing, the crystal grains may become too coarse, making it difficult to obtain a uniformly sized primary recrystallized structure. Next, in the cold rolling to produce the final thickness (also referred to as "final cold rolling"), aging treatment is preferably performed one or more times at a temperature of 100 to 300°C during cold rolling to improve the primary recrystallized texture and magnetic properties.

[0047] Decarburization annealing process: A cold-rolled steel sheet having a final thickness is subjected to decarburization annealing, which also serves as primary recrystallization annealing. From the viewpoint of decarburization, the annealing temperature for this decarburization annealing is preferably in the range of 800 to 900°C. Furthermore, a mixed atmosphere of hydrogen and nitrogen and a moist atmosphere is preferred to improve controllability of the amount of decarburization. When nitriding the steel sheet, a mixed gas of hydrogen, nitrogen, and ammonia is preferably used. Note that primary recrystallization annealing may be performed at a different timing from decarburization annealing.

[0048] Finish annealing process: Finish annealing involves coating the surface of the decarburized annealed steel sheet with an annealing separator and then subjecting it to secondary recrystallization annealing. When a forsterite film is to be formed on the decarburized annealed steel sheet with emphasis on core loss characteristics, for example, an annealing separator mainly composed of MgO is applied to the steel sheet surface, dried, and then finish annealed. A secondary recrystallized structure highly concentrated in the Goss orientation is developed in the steel sheet that has been subjected to finish annealing, and a forsterite film is formed.

[0049] On the other hand, when emphasis is placed on punching workability and the formation of a forsterite coating is not desired, it is preferable to either not use an annealing separator or to perform finish annealing using an annealing separator mainly composed of silica, alumina, or the like. When a forsterite coating is not desired, electrostatic application of the annealing separator, which does not introduce moisture, is also effective. Alternatively, a heat-resistant inorganic material sheet, such as silica, alumina, or mica, may be used instead of the annealing separator.

[0050] In order to form a forsterite film, the annealing temperature for the finish annealing is preferably 800°C or higher to induce secondary recrystallization. Furthermore, in order to complete the secondary recrystallization, it is preferable to hold the steel at a temperature of 800°C or higher for 15 hours or more. When a purification treatment is performed to emphasize iron loss characteristics, or when a forsterite film is formed to reduce transformer noise, it is preferable to raise the temperature to about 1200°C. On the other hand, when a forsterite film is not to be formed, it is sufficient to complete secondary recrystallization, so the annealing temperature for the finish annealing is preferably in the range of 850 to 950°C. Furthermore, it is also possible to complete the finish annealing by simply holding the steel in this temperature range for several hours or more.

[0051] After finish annealing, the steel sheet is preferably subjected to water washing, brushing, pickling, etc. to remove unreacted annealing separator adhering to the steel sheet surface, followed by flattening annealing to correct the shape, in order to reduce iron loss. The reason for flattening annealing is that finish annealing is generally performed in a coiled state, which can cause the coil to develop a curl, which can cause deterioration of characteristics during iron loss measurement.

[0052] Furthermore, 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 the planarization annealing. In particular, in order to reduce iron loss, it is preferable to apply a tension-applying coating that can apply tension to the steel sheet as the insulating coating. Note that, when forming the tension-applying coating, it is preferable to adopt a method of applying a tension coating via a binder or a method of depositing an inorganic substance on the surface layer of the steel sheet by physical vapor deposition or chemical vapor deposition, because this makes it possible to form an insulating coating that has excellent coating adhesion and a significantly large iron loss reduction effect.

[0053] In addition, to further reduce iron loss, it is preferable to perform a magnetic domain refinement treatment. A commonly used treatment method is to form grooves in the final product sheet. Other methods that can be used include introducing linear or point-shaped thermal strain or impact strain by laser irradiation, electron beam irradiation, or plasma irradiation, and etching the surface of intermediate steel sheets, such as steel sheets cold-rolled to the final thickness, to form grooves. In addition to these, various other treatments can be performed as needed. Examples include pickling, degreasing, and physical surface cleaning.

[0054] 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.

[0055] Example 1 A steel slab containing, by mass%, 0.038% C, 3.4% Si, 0.07% Mn, 0.006% acid-soluble Al, and 0.003% N, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was heated in a gas furnace, with the oxygen concentration in the furnace and the heating temperature of the steel slab changed as shown in Table 5. After heating the steel slab, four rough rolling operations were performed: the first rough rolling operation had a reduction of 35%, the second rough rolling operation had a reduction of 43%, the third rough rolling operation had a reduction of 25%, and the fourth rough rolling operation had a reduction of 35%. Width rolling was performed between each rough rolling operation, with a width reduction of 4.5%. Then, a hot-rolled steel sheet with a thickness of 2.4 mm was obtained by finish rolling. The edge crack depth was then investigated. The results are shown in Table 5. Thereafter, the hot-rolled steel sheets that did not have edge cracks were subjected to hot-rolled sheet annealing at 1040°C for 60 seconds, and then subjected to primary cold rolling to reduce the intermediate thickness to 1.8 mm, followed by intermediate annealing at 1050°C for 30 seconds. Thereafter, the sheets were cold-rolled to a final thickness of 0.23 mm. ２ and N ２The steel sheets were subjected to decarburization annealing at 880°C for 90 seconds in a hydrogen atmosphere containing 1,240°C. The temperature-raising process for the decarburization annealing was carried out using an induction heating furnace at a heating rate of 200°C / s. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheets, which were then dried, and then finish annealed for 13 hours in a hydrogen atmosphere at a maximum temperature of 1,240°C.

[0056] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The results are shown in Table 5. Table 5 shows that the invention examples in which the oxygen concentration in the gas furnace and the heating temperature of the steel slab were within the appropriate ranges prevented edge cracks from occurring during hot rolling and produced grain-oriented electrical steel sheets with a high magnetic flux density of 1.900 T or more.

[0057]

[0058] Example 2 A steel slab containing, by mass%, 0.030% C, 3.2% Si, 0.05% Mn, 0.005% acid-soluble Al, and 0.003% N, with the balance being Fe and unavoidable impurities, was produced by continuous casting. The steel slab was heated to 1090°C using a gas furnace with an oxygen concentration of 2.1% by volume. The heated steel slab was then subjected to hot rolling, with rough rolling and width rolling being performed as shown in Table 6. A hot-rolled steel sheet having a thickness of 2.3 mm was then obtained by finish rolling. The depth of edge cracks was then investigated. The results are shown in Table 6. The hot-rolled steel sheet that did not have edge cracks was then subjected to hot-rolled sheet annealing at 1000°C for 80 seconds, followed by cold rolling to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm. Next, H ２ and N ２ The steel sheets were subjected to decarburization annealing at 870°C for 80 seconds in a hydrogen atmosphere containing 1,220°C. The temperature-raising process for the decarburization annealing was carried out using an induction heating furnace at a heating rate of 200°C / s. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheets, which were then dried, and then finish annealed for 16 hours in a hydrogen atmosphere at a maximum temperature of 1,220°C.

[0059] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８The results are shown in Table 6. It can be seen from Table 6 that the invention examples in which the rough rolling conditions were within the appropriate range prevented the occurrence of edge cracks and produced grain-oriented electrical steel sheets having a high magnetic flux density of 1.900 T or more.

[0060]

[0061] (Example 3) A steel material (steel slab) having the chemical composition shown in Table 7, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was then heated to 1150°C in a gas furnace with an oxygen concentration of 0.8% by volume. Five rough rolling operations were then performed: the first rough rolling operation had a reduction of 21%, the second rough rolling operation had a reduction of 29%, the third rough rolling operation had a reduction of 25%, the fourth rough rolling operation had a reduction of 28%, and the fifth rough rolling operation had a reduction of 31%. Between each rough rolling operation, width rolling was performed with a width reduction of 5.1%. Then, a hot-rolled steel sheet with a thickness of 2.4 mm was obtained by finish rolling. The depth of edge cracks was then investigated. The results are shown in Table 7. The hot-rolled steel sheet without edge cracks was then subjected to primary cold rolling to an intermediate thickness of 1.7 mm and intermediate annealing at 980°C for 50 seconds. Thereafter, the steel sheet was subjected to cold rolling to be finished into a cold-rolled steel sheet having a final thickness of 0.23 mm. ２ and N ２ The steel sheets were subjected to decarburization annealing at 850°C for 100 seconds in a hydrogen atmosphere containing 1,200°C. The temperature-raising process for the decarburization annealing was carried out using an induction heating furnace at a heating rate of 150°C / s. Next, an annealing separator mainly composed of MgO was applied to the surface of the steel sheets, which were then dried and then subjected to finish annealing for 15 hours in a hydrogen atmosphere at a maximum temperature of 1,200°C.

[0062] The steel sheet obtained as above after the final annealing was subjected to the magnetic flux density B ８ The results are shown in Table 7. It can be seen from Table 7 that the invention examples, which have component compositions within the appropriate range, prevent edge cracking and provide grain-oriented electrical steel sheets with a high magnetic flux density of 1.900 T or more.

[0063]

[0064] The technology of the present invention can also be applied to improving the yield of metal material production.

Claims

1. A hot rolling process in which a steel material having a component composition containing, by mass%, C: 0.010 to 0.045%, Si: 2.8 to 4.5%, Mn: 0.01 to 1.00%, acid-soluble Al: 0.010% or less, and N: 0.008% or less, with the balance being Fe and unavoidable impurities, is hot-rolled to form a hot-rolled steel sheet; a hot-rolled sheet annealing process in which the hot-rolled steel sheet is optionally annealed to form a hot-rolled annealed sheet; a cold-rolling process in which the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or two or more times with intermediate annealing in between to form a cold-rolled steel sheet having a final sheet thickness; and a decarburization annealing process in which the cold-rolled steel sheet is subjected to decarburization annealing which also serves as primary recrystallization annealing, or to primary recrystallization annealing and decarburization annealing, to form a decarburization-annealed sheet. a finish annealing step of coating the surface of the decarburized annealed sheet with an annealing separator and performing secondary recrystallization annealing, wherein in the hot rolling step, the steel material is heated to a temperature in the range of 1000 to 1260°C in a gas furnace with an atmosphere having an oxygen concentration of 3.0% by volume or less, and rough rolling is performed four or more times with a rolling reduction of 50% or less, and width rolling is performed in the width direction with a width reduction of 1.5 to 8.2% between the rough rolling steps, and then finish rolling is performed.

2. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the steel material further contains, in addition to the above-mentioned chemical composition, at least one component selected from the following groups A to D on a mass basis: Group A: at least one selected from the total of either or both of S and Se: 0.010% or less, the total of either or both of Sn and Sb: 1.000% or less, Cr: 0.100% or less, Cu: 1.500% or less, Ni: 1.500% or less, Bi: 0.100% or less, P: 0.500% or less, and Mo: 0.500% or less; Group B: at least one selected from B: 25.0 ppm or less, Nb: 0.1000% or less, Ti: 0.1000% or less, V: 0.1000% or less, and Co: 0.050% or less; Group C: at least one selected from As: 0.0200% or less, Pb: 0.0100% or less, W: 0.0100% or less, and Zn: 0.020% or less; Group D: At least one selected from Ag: 0.050% or less, Au: 0.050% or less, Ca: 0.020% or less, Ga: 0.0050% or less, Ge: 0.0050% or less, Nd: 0.020% or less, and La: 0.020% or less.

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