Manufacturing method of grain-oriented electrical steel sheet

Abstract

We propose a method for producing grain-oriented electrical steel sheet with high magnetic flux density, preventing edge cracking and achieving high yield, using steel material with a chemical composition containing few inhibitors. The method includes a hot rolling process, an optional hot-rolled sheet annealing process, a cold rolling process, a decarburization annealing process, and a finish annealing process. In the hot rolling process, 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, and width pressing is performed with a width reduction of 100 to 400 mm. Four or more rough rolling processes are then performed, with the first rough rolling process having a reduction ratio of 20 to 55%, and the second and subsequent rough rolling processes each having a reduction ratio of 50% or less, followed by finish rolling.

Description

[Technical Field]

[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. [Background technology]

[0002] Grain-oriented electrical steel is a soft magnetic material that is mainly used as a transformer core material, and has the easy axis of magnetization of iron. <001> The orientation must be highly aligned in the rolling direction of the steel sheet, and it is required to have high magnetic flux density and low iron loss. Due to the need for energy conservation in recent years, demand for grain-oriented electrical steel sheets that reduce energy loss in transformers is increasing, and it is necessary 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 steel material with adjusted chemical composition at high temperatures and then hot-rolling it to produce hot-rolled steel sheets. The hot-rolled steel sheets are optionally annealed, and then cold-rolled once or twice or more times with intermediate annealing to produce cold-rolled steel sheets of the final thickness. The cold-rolled steel sheets are then subjected to decarburization annealing, which also serves as primary recrystallization annealing, followed by coating the steel sheet surface with an annealing separator and final annealing. Grain-oriented electrical steel sheets manufactured using inhibitors have an issue 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 applying strain to coarse grains at the edge of a steel sheet by changing the rough rolling reduction schedule in hot rolling. 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 steel material side surface to the C content of the slab side surface and slab center, and then performing controlled rolling. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Special Publication No. 57-004690 [Patent Document 2] Japanese Patent Application Publication No. 55-062124 [Patent Document 3] Japanese Patent Application Publication No. 57-165102 [Patent Document 4] Special Publication No. 64-003564 [Patent Document 5] Special Publication No. 03-006842 [Patent Document 6] Japanese Patent Application Laid-Open No. 2006-075885 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the prior art disclosed in the above patent document has the following problems. Grain-oriented electrical steel sheets manufactured from slabs with inhibitor-free composition are produced by heating the slab at a temperature of 1250°C or less 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 from steel materials with 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 using 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 alleviate 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. [Means for solving the problem]

[0009] The inventors have conducted extensive research to solve the above-mentioned problems. As a result, they have discovered that the edge cracks are caused by the surface oxidation of the steel material for inhibitor-free grain-oriented electrical steel sheet, which causes the inhibitor, which is contained in small amounts, to decompose from the surface layer. This reduces the inhibitor's suppressive power, causing coarsening of crystal grains at the edges of the steel material, resulting in edge cracks during hot rolling. The inventors have conducted further extensive research to prevent the occurrence of coarse grains. As a result, they have found that the following four points A to D are important. a) By heating inhibitor-free slabs for electrical steel sheets in a specified atmosphere before hot rolling, it is possible to suppress the surface decomposition of the inhibitor during slab heating and prevent edge cracks due to grain coarsening at the edges. (a) By applying predetermined conditions to the slab heating temperature, it is possible to prevent edge cracking due to grain coarsening at the edge caused by high-temperature recrystallization. (c) By performing width pressing under specified conditions before rough rolling of the slab, it is possible to refine the coarse grains that occur at the edges and prevent edge cracks. d) When rough rolling is performed four or more times, the first rough rolling is performed with a reduction ratio of 20 to 55% to eliminate the dog-bone shape in cross section formed by width pressing, and the second and subsequent rough rollings are performed with a reduction ratio of 50% or less. This forms a texture that facilitates the growth of highly oriented Goss grains, thereby increasing the magnetic flux density.

[0010] 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. [1] A hot rolling process 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 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 the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or twice or more times with intermediate annealing in between to form a cold-rolled steel sheet having a final thickness; and a decarburization annealing process in which the cold-rolled steel sheet is subjected to primary recrystallization annealing or primary recrystallization annealing. and a finish annealing step of coating the surface of the decarburization-annealed sheet with an annealing separator and subjecting it 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 atmosphere having an oxygen concentration of 3.0% by volume or less, and the steel material is width-pressed with a width reduction of 100 to 400 mm. Subsequently, the steel material is subjected to four or more rough rolling passes, with the first rough rolling pass having a reduction ratio of 20 to 55% and the second and subsequent rough rolling passes each having a reduction ratio of 50% or less, followed by finish rolling. [2] In the above [1], the method for producing a grain-oriented electrical steel sheet is such that the steel material further contains, by mass, at least one component selected from the following groups A to D in addition to the above-mentioned chemical composition: Group A: at least one selected from the group consisting of S and Se, either or both of which in total are 0.010% or less, Sn and Sb, either or both of which in total are 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. [Effects of the Invention]

[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 with high magnetic flux density at low cost and with high yield. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described. 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, a hot-rolled sheet annealing step in which the hot-rolled steel sheet is annealed to produce a hot-rolled annealed sheet is also included. 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 twice or more times with intermediate annealing between them to produce a cold-rolled steel sheet having a final thickness. The method also includes a decarburization annealing step in which the cold-rolled steel sheet is subjected to decarburization annealing that also serves as primary recrystallization annealing, or to a combination of primary recrystallization annealing and decarburization annealing to produce a decarburization-annealed sheet. The process includes a finish annealing process in which the surface of the decarburized annealed sheet is coated with an annealing separator and subjected to secondary recrystallization annealing. In the hot rolling process, 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. The heated steel material is subjected to width pressing with a width reduction of 100 to 400 mm. Next, four or more rough rolling processes are performed, with the first rough rolling process having a reduction ratio of 20 to 55% and the second and subsequent rough rolling processes having a reduction ratio of 50% or less, followed by finish rolling. Here, the rough rolling reduction ratio is expressed as a percentage by dividing the amount of thickness reduction due to rolling for each rough rolling process by the thickness before rolling. The width reduction refers to the difference between the width of the steel material before width pressing and the width of the steel material after width pressing.

[0013] First, the experiment that led to the development of the present invention will be described. <Experiment 1> A steel slab (slab thickness: 208 mm, width: 1321 mm) containing, by mass, 0.028% C, 3.2% Si, 0.09% 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 then heated to 1080°C in a gas furnace with an oxygen concentration of 0.1 to 5.0% by volume. It was then width-pressed with a width reduction of 250 mm. Four rough rolling passes were then performed: the first rough rolling pass had a reduction of 45%, the second rough rolling pass had a reduction of 35%, the third rough rolling pass had a reduction of 30%, and the fourth rough rolling pass had a reduction of 30%. The slab was then finish-rolled to a hot-rolled steel sheet with a thickness of 2.3 mm.

[0014] The depth of edge cracks on the hot-rolled steel sheets was measured. Those without edge cracks were subjected to hot-rolled sheet annealing at 1050°C for 40 seconds, followed by primary cold rolling to an intermediate thickness of 1.7 mm and intermediate annealing at 1110°C for 30 seconds. Final cold rolling was then performed to produce cold-rolled steel sheets with a final thickness of 0.23 mm. The steel sheets were then heated at a heating rate of 60°C / s. They were then subjected to decarburization annealing at 850°C for 50 seconds in a mixed atmosphere of H2 and N2. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface, dried, and then final annealed for 15 hours in a hydrogen atmosphere at a maximum temperature of 1200°C.

[0015] The magnetic flux density B8 (magnetic flux density at a magnetizing force of 800 A / m) of the steel sheets obtained after the finish annealing as described above was measured using the method described in JIS C2556:2015. The results are shown in Table 1. Table 1 shows that steel materials heated in a gas furnace with an oxygen concentration in the furnace of 0.1 to 3.0 volume % suppress the occurrence of edge cracks and produce grain-oriented electrical steel sheets with a high magnetic flux density of 1.900 T or more.

[0016] [Table 1]

[0017] <Experiment 2> A steel slab (slab thickness: 225 mm, width: 1180 mm) containing, by mass, 0.025% C, 3.0% Si, 0.07% Mn, 0.006% acid-soluble Al, and 0.003% N, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The oxygen concentration in the furnace was set to 2.1% by volume, and the steel slab was heated to 940 to 1300°C. Next, width pressing was performed with a width reduction of 300 mm. Subsequently, five rough rolling passes were performed: the first rough rolling pass had a reduction of 25%, the second rough rolling pass had a reduction of 30%, the third rough rolling pass had a reduction of 35%, the fourth rough rolling pass had a reduction of 18%, and the fifth rough rolling pass had a reduction of 35%. The slab was then finish-rolled to a hot-rolled steel sheet with a thickness of 2.4 mm.

[0018] The depth of edge cracks on the hot-rolled steel sheets was measured. Those without edge cracks were subjected to primary cold rolling to an intermediate thickness of 1.7 mm, followed by intermediate annealing at 1110°C for 30 seconds. This was followed by final cold rolling to produce cold-rolled steel sheets with a final thickness of 0.23 mm. The steel sheets were then heated at a heating rate of 70°C / s. They were then subjected to decarburization annealing at 820°C for 100 seconds in a mixed atmosphere of H2 and N2. Next, an annealing separator primarily composed of MgO was applied to the steel sheet surface, dried, and then subjected to final annealing for 11 hours in a hydrogen atmosphere at a maximum temperature of 1210°C.

[0019] The magnetic flux density B8 of the steel sheets obtained after the finish annealing as described above was measured using the method described in JIS C2556:2015. The results are shown in Table 2. Table 2 shows that when the 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] [Table 2]

[0021] <Experiment 3> A steel slab (slab thickness: 205 mm, width: 1349 mm) containing, by mass, 0.026% C, 3.2% Si, 0.05% Mn, 0.006% acid-soluble Al, and 0.003% N, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The oxygen concentration in the furnace was set to 1.2% by volume, and the steel slab was heated to 1050°C. Next, width pressing was performed with a width reduction of 0 to 500 mm. Subsequently, five rough rolling passes were performed: the first rough rolling pass had a reduction of 30%, the second rough rolling pass had a reduction of 29%, the third rough rolling pass had a reduction of 27%, the fourth rough rolling pass had a reduction of 25%, and the fifth rough rolling pass had a reduction of 30%. The slab was then finished by rolling to produce a hot-rolled steel sheet with a thickness of 2.4 mm.

[0022] The depth of edge cracks on the hot-rolled steel sheets was measured. Those without edge cracks were cold-rolled to a final thickness of 0.23 mm. The steel sheets were then heated at a heating rate of 100°C / s. They were then subjected to decarburization annealing at 850°C for 110 seconds in a mixed atmosphere of H2 and N2. An annealing separator primarily composed of MgO was then applied to the steel sheet surface, dried, and then subjected to finish annealing for 16 hours in a hydrogen atmosphere at a maximum temperature of 1190°C.

[0023] The magnetic flux density B8 of the steel sheets obtained after the finish annealing as described above was measured using the method described in JIS C25562015. The results are shown in Table 3. It can be seen from this table that when the width reduction is 100 to 400 mm, 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.

[0024] [Table 3]

[0025] <Experiment 4> A steel slab (slab thickness: 214 mm, width: 1287 mm) containing, by mass%, 0.029% C, 3.3% Si, 0.06% 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 oxygen concentration in the furnace was set to 1.5% by volume, and the steel slab was heated to 1150°C. Next, width pressing was performed with a width reduction of 200 mm. After that, rough rolling was performed under the conditions shown in Table 4. Subsequently, finish rolling was performed to produce a hot-rolled steel sheet with a thickness of 2.3 mm.

[0026] The depth of edge cracks on the hot-rolled steel sheets was measured. Those without edge cracks were cold-rolled to a final thickness of 0.23 mm. The steel sheets were then heated at a heating rate of 90°C / s. They were then subjected to decarburization annealing at 850°C for 90 seconds in a mixed atmosphere of H2 and N2. An annealing separator primarily composed of MgO was then applied to the steel sheet surface, dried, and then subjected to finish annealing for 18 hours in a hydrogen atmosphere at a maximum temperature of 1190°C.

[0027] The magnetic flux density B8 of the steel sheets obtained after finish annealing as described above was measured using the method described in JIS C2556:2015. The results are shown in Table 4. Table 4 shows that when rough rolling is performed three or more times, with the first rough rolling having a reduction ratio of 20 to 55% and the second and subsequent rough rolling having reduction ratios of 50% or less, a grain-oriented electrical steel sheet having a high magnetic flux density of 1.900 T or more can be obtained.

[0028] [Table 4]

[0029] The inventors believe that the reason why edge cracks were suppressed at the edge of the steel sheet and good magnetic flux density were obtained under the above conditions is as follows: Inhibitor-free steel materials, 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 pressing under the above conditions can refine any coarse grains that have already formed. By setting the reduction ratio for the first rough rolling to 20 to 55%, the dog-bone shape in cross section formed by width pressing can be eliminated, while appropriate strain can be introduced to form a texture that facilitates the recrystallization of highly oriented Goss grains. Next, by applying the same conditions to the second and subsequent rough rolling, appropriate strain can be introduced to form a texture that facilitates the recrystallization of highly oriented Goss grains, thereby increasing the magnetic flux density.

[0031] Based on these findings, it is believed that even with inhibitor-free materials, it is possible to suppress the generation of coarse grains during hot rolling and refine the crystal grains, thereby preventing edge cracks and producing 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 sheets> First, the chemical composition of the steel material (steel slab) used in manufacturing the grain-oriented electrical steel sheet of this embodiment and the reasons for limiting it 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 to 0.045% If the C content is less than 0.010%, the grain boundary strengthening effect of C is lost, resulting in defects that hinder manufacturing, such as cracks in the slab. On the other hand, if the C content exceeds 0.045%, a recrystallized structure develops, which reduces the orientation of Goss grains. This results in a decrease in magnetic flux density B8. Furthermore, it becomes difficult to reduce the C content to 0.005% or less, the level at which magnetic aging does not occur, by decarburization annealing. Therefore, the C content is set to a range of 0.010 to 0.045%. More preferably, the C content is set to a 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 sheets by rolling. Therefore, the Si content is set to the range of 2.8 to 4.5%. More preferably, the Si content is set to 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 set to 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 the amount of acid-soluble Al, and more preferably, the amount of acid-soluble Al is controlled to 0.008% or less.

[0037] N: 0.008% or less Nitrogen can cause defects such as blisters when steel materials are heated. Therefore, the N content must be limited to 0.008% or less. The N content is more preferably limited to 0.005% or less, and even more preferably limited 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 may be further contained as an optional component.

[0039] Group A: at least one selected from the group consisting of S and / or Se in total: 0.010% or less, Sn and / or Sb in total: 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 excess of the upper limits mentioned above may saturate the effect and result in excessive manufacturing costs. To obtain the effects of the additions, it is preferable to add at least one selected from the total of either or both of S and Se: 0.005% or more, either or both of Sn and Sb: 0.005% or more, Cr: 0.005% or more, Cu: 0.005% or more, Ni: 0.005% or more, Bi: 0.005% or more, P: 0.005% or more, and Mo: 0.005% or more. 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 Addition of B, Nb, Ti, V, and Co in excess of the upper limits mentioned above may saturate the effect and result in excessive manufacturing 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 excess of the upper limits mentioned above may saturate the effect and may result in excessive manufacturing costs. To obtain the effects of the addition, it is preferable to add at least one element 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 excess of the upper limits mentioned above may saturate the effect and result in excessive manufacturing costs. To obtain the effects of the additions, it is preferable to add at least one element 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 of manufacturing grain-oriented electrical steel sheets> Next, a method for manufacturing the 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 slab) is produced using a conventional ingot-making / blooming rolling method or continuous casting method. The steel material is then 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% by volume, oxidative decomposition of inhibitors on the slab surface proceeds, resulting in coarsening of grains at the slab edge and causing edge cracks 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, keeping the oxygen concentration below 0.1% by volume increases energy costs and increases production costs. Therefore, the lower limit for the oxygen concentration in the furnace is preferably about 0.1% by volume.

[0043] When the above 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] Next, width pressing is performed with a width reduction in the range of 100 to 400 mm. The dimensions of the steel material before width pressing are preferably a thickness in the range of 200 to 230 mm and a width in the range of 1150 to 1400 mm. Here, if the width reduction is less than 100 mm, strain is not sufficiently introduced into the generated coarse grains, and the grains at the edges remain coarse, resulting in edge cracks. If the width reduction is more than 400 mm, the dog-bone shape in cross section formed by width pressing becomes prominent. As a result, strain is not sufficiently introduced in the width center by rough rolling, making it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. A more preferable width reduction in width pressing is in the range of 100 to 350 mm.

[0045] Next, rough rolling is performed four or more times, with the first rough rolling having a reduction of 20 to 55% and the second and subsequent rough rolling having a reduction of 50% or less. If the first rough rolling has a reduction of less than 20%, the dogbone shape caused by the width press process cannot be eliminated, and strain is not sufficiently introduced at the width center, making it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. If the first rough rolling has a reduction of more than 55%, excessive strain is introduced, making it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. If the subsequent rough rolling has a reduction of more than 50%, it makes it difficult to recrystallize grains with a crystal orientation favorable for the growth of highly oriented Goss grains. More preferably, rough rolling is performed four or more times, with the first rough rolling having a reduction of 25 to 55% and the second and subsequent rough rolling having a reduction of 45% or less. Then, finish rolling is performed to produce a hot-rolled steel sheet.

[0046] 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 uniformly sized 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 annealing the hot-rolled sheet may become too coarse, making it difficult to obtain a uniformly sized primary recrystallized structure.

[0047] Cold rolling process A hot-rolled steel sheet or a hot-rolled annealed steel sheet is subjected to one cold rolling or two or more cold rollings 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 furthermore, 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 in the case of 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"), it is preferable to perform aging treatment one or more times at a temperature of 100 to 300°C during the cold rolling to improve the primary recrystallized texture and enhance the magnetic properties.

[0048] Decarburization annealing process The cold-rolled steel sheet having the final thickness is subjected to decarburization annealing, which also serves as primary recrystallization annealing. From the viewpoint of decarburization, the decarburization annealing is preferably performed at an annealing temperature in the range of 800 to 900°C, and the atmosphere is preferably a mixed atmosphere of hydrogen and nitrogen and a moist atmosphere in order to improve the controllability of the amount of decarburization. When nitriding the steel sheet, a mixed gas of hydrogen, nitrogen, and ammonia is preferably used. Note that the primary recrystallization annealing may be performed at a different timing from the decarburization annealing.

[0049] Finishing annealing process In the final annealing, the surface of the decarburization-annealed steel sheet is coated with an annealing separator, and then secondary recrystallization annealing is performed. When a forsterite film is to be formed on the decarburization-annealed steel sheet with emphasis on iron loss characteristics, for example, an annealing separator mainly composed of MgO is applied to the steel sheet surface, dried, and then final annealing is performed. In the steel sheet that has been subjected to the final annealing, a secondary recrystallization structure highly concentrated in the Goss orientation is developed and a forsterite film is formed.

[0050] 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.

[0051] The annealing temperature for the final annealing is preferably 800°C or higher to induce secondary recrystallization and form a forsterite film. Furthermore, 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 final annealing is preferably in the range of 850 to 950°C. Furthermore, it is possible to complete the final annealing by simply holding the steel in this temperature range for several hours or more.

[0052] 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.

[0053] 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.

[0054] 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. [Example]

[0055] 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.

[0056] Example 1 A steel slab (slab thickness: 219 mm, width: 1250 mm) containing, by mass, 0.036% C, 3.1% Si, 0.07% Mn, 0.006% 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 in a gas furnace, with the oxygen concentration in the furnace and the heating temperature of the steel slab varied as shown in Table 5. The heated steel slab was then width-pressed with a width reduction of 150 mm. Four rough rolling passes were then performed: the first rough rolling pass had a reduction of 45%, the second rough rolling pass had a reduction of 34%, the third rough rolling pass had a reduction of 28%, and the fourth rough rolling pass had a reduction of 28%. This was followed by finish rolling to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The edge crack depth was then measured. The results are shown in Table 5. The hot-rolled steel sheets that did not develop edge cracks were then annealed at 1070°C for 60 seconds, followed by primary cold rolling to an intermediate thickness of 1.8 mm and intermediate annealing at 1060°C for 45 seconds. These were then cold-rolled to a final thickness of 0.23 mm. Next, the sheets were decarburized at 870°C for 85 seconds in a mixed atmosphere of H2 and N2. The decarburization annealing process was carried out in an induction furnace at a heating rate of 120°C / s. An MgO-based annealing separator was then applied to the steel sheet surface, dried, and then final annealed for 15 hours in a hydrogen atmosphere at a maximum temperature of 1220°C.

[0057] The magnetic flux density B8 of the steel sheets obtained after the finish annealing as described above was measured using the method described in JIS C2556:2015. 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 high magnetic flux density.

[0058] [Table 5]

[0059] Example 2 A steel slab (221 mm thick, 1279 mm wide) containing, by mass, 0.030% C, 3.2% Si, 0.05% Mn, 0.005% 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 heated in a gas furnace at an oxygen concentration of 2.1% by volume and 1090°C. The heated slab was then width-pressed and rough-rolled as shown in Table 6. Finish rolling was then performed to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The depth of edge cracks was then investigated. The results are shown in Table 6. The hot-rolled steel sheet without edge cracks was then annealed at 1000°C for 80 seconds and cold-rolled to a final thickness of 0.23 mm. Next, the steel sheets were subjected to decarburization annealing at 870°C for 80 seconds in a mixed atmosphere of H2 and N2. The decarburization annealing temperature increase process was carried out in an induction heating furnace at a heating rate of 230°C / s. Next, an annealing separator mainly composed of MgO was applied to the steel sheet surface, and after drying, the steel sheets were subjected to final annealing for 16 hours in a hydrogen atmosphere at a maximum temperature of 1220°C.

[0060] The magnetic flux density B8 of the steel sheets obtained after finish annealing as described above was measured using the method described in JIS C2556. The results are shown in Table 6. It can be seen from this table that the invention examples, in which the width pressing conditions and rough rolling conditions were within appropriate ranges, prevented the occurrence of edge cracks and produced grain-oriented electrical steel sheets with high magnetic flux density.

[0061] [Table 6]

[0062] Example 3 A steel slab (slab thickness: 222 mm, width: 1192 mm) having the chemical composition shown in Table 7, with the remainder consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was then heated to 1120°C in a gas furnace with an oxygen concentration of 0.5% by volume. The heated slab was then width-pressed with a width reduction of 220 mm. Five rough rolling passes were then performed: the first rough rolling pass had a reduction of 35%, the second rough rolling pass had a reduction of 30%, the third rough rolling pass had a reduction of 25%, the fourth rough rolling pass had a reduction of 27%, and the fifth rough rolling pass had a reduction of 25%. This was followed by finish rolling to produce a hot-rolled steel sheet with a thickness of 2.4 mm. The depth of edge cracks was then measured. The results are shown in Table 7. The hot-rolled steel sheets that did not develop edge cracks were then subjected to a primary cold rolling process to reduce the intermediate thickness to 1.8 mm, followed by intermediate annealing at 1030°C for 60 seconds. They were then cold-rolled to a final thickness of 0.23 mm. Next, they underwent decarburization annealing at 840°C for 120 seconds in a mixed atmosphere of H2 and N2. The decarburization annealing temperature increase process was carried out using an induction heating furnace at a heating rate of 90°C / s. Next, an annealing separator primarily composed of MgO was applied to the steel sheet surface, which was then dried and then subjected to final annealing for 14 hours in a hydrogen atmosphere at a maximum temperature of 1190°C.

[0063] The magnetic flux density B8 of the steel sheets obtained after the finish annealing as described above was measured using the method described in JIS C2556:2015. The results are shown in Table 7. Table 7 shows that the invention examples, whose composition is within the appropriate range, prevent edge cracking and provide grain-oriented electrical steel sheets with high magnetic flux density.

[0064] [Table 7] [Industrial Applicability]

[0065] The technology of the present invention can also be applied to improving the yield of metal material production.

Claims

1. By mass, C: 0.010-0.045%, Si: 2.8-4.5%, Mn: 0.01-1.00%, Acid-soluble Al: 0.010% or less, and N: 0.008% or less a hot rolling step of hot rolling a steel material having a component composition containing the above and the balance being Fe and unavoidable impurities to obtain a hot-rolled steel sheet; Optionally, a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed sheet; A cold rolling process in which the hot-rolled steel sheet or the hot-rolled annealed sheet is subjected to cold rolling once or two or more times with intermediate annealing therebetween to form a cold-rolled steel sheet having a final thickness; a decarburization annealing step of subjecting the cold-rolled steel sheet to decarburization annealing which also serves as primary recrystallization annealing, or primary recrystallization annealing and decarburization annealing, to obtain 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; Including, 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 in an atmosphere with an oxygen concentration of 3.0% by volume or less, and the steel material is subjected to width pressing with a width reduction of 100 to 400 mm. Then, rough rolling is performed four or more times, with the first rough rolling having a reduction ratio in the range of 20 to 55% and the second and subsequent rough rolling having reduction ratios of 50% or less, followed by finish rolling.

2. 2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the steel material further contains, in addition to the above-mentioned 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 group consisting of S and / or Se in total: 0.010% or less, Sn and / or Sb in total: 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.