Manufacturing method of grain-oriented electrical steel sheet

Abstract

To provide a production method of a directional electromagnetic steel sheet, by which a directional electromagnetic steel sheet having little film defect and low iron loss can be produced.SOLUTION: To solve the problem, according to one aspect of the present invention, provided is a method for producing a directional electromagnetic steel sheet, which includes: a cold rolling step for producing a cold rolled steel sheet; a finish annealing step for performing finish annealing to induce secondary recrystallization on the cold rolled steel sheet; a groove forming step for forming grooves in a linear shape in a direction crossing the rolling direction of the cold rolled steel sheet before or after the finish annealing step; and a tension imparting step for forming a tensile film on the groove formed surface of the cold rolled steel sheet by applying and baking a coating solution containing a compound of phosphoric acid, phosphate, chromic anhydride, chromate, alumina or silica while the groove formed surface of the cold rolled steel sheet is directed downward.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Grain-oriented electrical steel sheets are used as magnetic cores in many electrical devices. Grain-oriented electrical steel sheets contain 0.8% to 4.8% silicon and have a crystal orientation of {110} <001> It is a steel sheet with a highly concentrated orientation. Its magnetic properties require high magnetic flux density, represented by the B8 value, and low iron loss, represented by W17 / 50. In particular, there has been a growing demand recently for reduced power loss from the perspective of energy conservation.

[0003] In response to this demand, a technology for refining magnetic domains has been developed as a means of reducing the iron loss of grain-oriented electrical steel sheets. Hereinafter, this technology, i.e., the technology for refining magnetic domains, will be referred to as "domain control technology," and the effect of domain control technology will also be referred to as "domain control effect." For example, Patent Document 1 discloses a method of reducing iron loss by irradiating a steel sheet after finish annealing with a laser beam to subdivide magnetic domains. However, because the reduction in iron loss achieved by this method is due to strain introduced by the laser irradiation, this method cannot be used for wound core transformers, which require stress relief annealing (SRA) after the transformer is formed.

[0004] That is, wound core transformers, which are primarily used in small and medium-sized transformers, are often manufactured using core manufacturing methods that involve mechanical bending, for example. In this manufacturing method, stress relief annealing (for example, at 800°C for 2 to 4 hours) is generally performed to eliminate iron loss deterioration due to processing strain introduced by the bending process. This stress relief annealing eliminates the strain introduced for magnetic domain refining. For this reason, methods of refining magnetic domains by introducing strain cannot be applied to wound core transformers.

[0005] A widely known SRA-resistant magnetic domain control technology that does not lose its magnetic domain control effect even after the above-mentioned stress relief annealing is a "groove-introducing magnetic domain control technology" that forms linear grooves periodically in a direction intersecting the rolling direction. Known groove-introducing magnetic domain control technologies include a groove-forming technology using machining, a groove-forming technology using etching, and a groove-forming technology using laser irradiation. For example, Patent Document 2 discloses a groove-forming technology using laser irradiation. However, these groove-forming methods alone are insufficient to meet the increasing demand for iron loss improvement in recent years.

[0006] Meanwhile, a technique has been proposed for forming an insulating coating on the surface of a steel sheet on which grooves have been formed (i.e., the groove-formed surface), as disclosed in Patent Document 3. Specifically, Patent Document 3 describes forming an insulating coating made of colloidal silica and magnesium phosphate on the groove-formed surface of the steel sheet. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 58-26405 [Patent Document 2] Patent No. 4384451 [Patent Document 3] Patent No. 5742294 Summary of the Invention [Problem to be solved by the invention]

[0008] However, it has been found that when an insulating coating is formed as in the technique disclosed in Patent Document 3, defects in the coating can result in insufficient insulation and insufficient iron loss.

[0009] The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for manufacturing a grain-oriented electrical steel sheet that can produce a grain-oriented electrical steel sheet with few coating defects and low iron loss. [Means for solving the problem]

[0010] The aspects of the present invention are as follows. (1) One aspect of the present invention is a method for manufacturing a cold-rolled steel sheet, comprising: a cold rolling process for producing a cold-rolled steel sheet; a finish annealing process for performing finish annealing involving secondary recrystallization on the cold-rolled steel sheet; and a method for forming a surface roughness on only one side of the cold-rolled steel sheet before or after the finish annealing process in a direction intersecting the rolling direction of the cold-rolled steel sheet. by laser, plasma, mechanical methods or etching The method includes a groove forming step of forming linear grooves, and a tension coating step of applying a coating solution containing a compound of phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or silica to the groove-formed surface of the cold-rolled steel sheet with the groove-formed surface facing downward, and baking the coating solution to form a tension coating on the groove-formed surface. The width of the groove is 20 μm or more, and the depth of the groove is 15 μm or more. This is a method for manufacturing grain-oriented electrical steel sheets.

[0011] (2) In the manufacturing method of the grain-oriented electrical steel sheet described in (1) above, in the tension coating application process, the thickness of the tension coating formed inside the groove may be adjusted so that the thickness of the tension coating formed inside the groove is 1 / 2 or less of the depth of the groove and is 2 times or less of the thickness of the tension coating formed on the flat surface of the cold-rolled steel sheet.

[0012] (3) The method for producing a grain-oriented electrical steel sheet according to (1) or (2) above may further include an annealing separator application step of applying an annealing separator to the cold-rolled steel sheet after the cold rolling step and before the finish annealing step, and the annealing separator may contain magnesia. [Effects of the Invention]

[0013] According to the above-described aspects of the present invention, it is possible to manufacture grain-oriented electrical steel sheets with few coating defects and low core loss. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a flowchart illustrating an example of a method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment. [Figure 2]FIG. 2 is a plan view showing grooves formed in a finish-annealed steel sheet. [Figure 3] FIG. 2 is a schematic cross-sectional view illustrating the configuration of the vicinity of a groove in a grain-oriented electromagnetic steel sheet. [Figure 4] FIG. 10 is a schematic cross-sectional view illustrating a configuration in the vicinity of a groove of a grain-oriented electrical steel sheet according to a modified example. [Figure 5] 1 is a schematic cross-sectional view illustrating a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention, particularly illustrating a tension coating step. FIG. [Figure 6] FIG. 1 is a schematic cross-sectional view for explaining problems with a conventional method for manufacturing a grain-oriented electrical steel sheet. [Figure 7] 1 is an SEM photograph for explaining problems with a conventional method for manufacturing a grain-oriented electrical steel sheet. DETAILED DESCRIPTION OF THE INVENTION

[0015] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In this embodiment, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. "%" means mass % unless otherwise specified.

[0016] The present inventors have conducted extensive research into techniques for forming an insulating coating on the grooved surface of a steel sheet. The insulating coating reduces iron loss by applying tension within the steel sheet surface. From this perspective, the insulating coating in this embodiment may also be referred to as a tension coating.

[0017] For example, there is already a technology for forming an insulating coating on the grooved surface of a steel sheet, as disclosed in Patent Document 3. However, the technology proposed so far has sometimes resulted in insufficient insulation due to coating defects in the grain-oriented electrical steel sheet, and has not been able to sufficiently reduce iron loss.

[0018] The inventors believe that the reason for this lies in the process of forming the insulating coating. As will be described in detail later, conventionally, the coating solution is applied to the grooved surface of the steel sheet with the grooved surface facing "upward," and then the sheet is baked. This makes it easy for the coating solution to accumulate in the grooves. In other words, some of the coating solution applied to the grooved surface remains inside the grooves in a pooled state. If baking is performed in this state, the insulating coating inside the grooves will be thicker than the insulating coating on other parts (the so-called flat surface).

[0019] As described above, the insulating coating inside the grooves tends to be formed to be excessively thicker than the insulating coating on the flat surfaces. Furthermore, an insulating coating that is formed to be excessively thick is likely to peel off from the base steel sheet. In other words, the insulating coating formed inside the grooves is prone to peeling, which may result in insufficient insulation and insufficient iron loss reduction.

[0020] In grooved grain-oriented electrical steel sheets, magnetic flux that reaches one of the groove walls through the steel sheet leaks from the domain wall (i.e., due to magnetic flux leakage), increasing magnetostatic energy. The main magnetic domains are subdivided to reduce this magnetostatic energy, resulting in the magnetic domain control effect. However, when the steel sheet is not tensioned, closure domains are generated near the groove walls, suppressing the increase in magnetostatic energy and resulting in insufficient magnetic domain control. The isotropic tension of the insulating coating makes these closure domains energetically unstable (due to the adverse effect of magnetostriction), restoring magnetic flux leakage and improving the magnetic domain control effect. A thick insulating coating formed in the grooves is likely to peel off, which is thought to be insufficient to apply sufficient tension to the steel sheet or to apply stress in a direction that destabilizes the closure domains.

[0021] In particular, when the coating solution is applied to the grooved surface of the steel sheet, gravity causes a concave (or convex) catenary on the upper surface of the steel sheet. In this case, liquid pools tend to form in the grooves, and a thicker insulating coating forms inside the grooves. This makes the insulating coating inside the grooves more likely to peel off, significantly reducing the iron loss reduction effect.

[0022] Based on the above investigations, the inventors have found that adopting a method in which a coating solution is applied to the grooved surface of a steel sheet with the grooved surface facing "downward" and then baked is effective for thinning the insulating coating formed inside the grooves, and that this makes it possible to produce a grain-oriented electrical steel sheet with few coating defects and low iron loss.

[0023] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described with reference to the flowchart shown in FIG. The flowchart shown in FIG. 1 is merely an example of the method for manufacturing a grain-oriented electrical steel sheet according to this embodiment, and may be arbitrarily modified within the scope that does not impair the effects of this embodiment. That is, the method for manufacturing a grain-oriented electrical steel sheet according to this embodiment may include at least a cold rolling step for producing a cold-rolled steel sheet, a finish annealing step for performing finish annealing involving secondary recrystallization on the cold-rolled steel sheet, a groove forming step for forming linear grooves in the cold-rolled steel sheet before or after the finish annealing step in a direction intersecting the rolling direction of the cold-rolled steel sheet, and a tensile coating step for forming a tensile coating on the groove-formed surface of the cold-rolled steel sheet by applying and baking a coating solution containing a compound of phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or silica with the groove-formed surface of the cold-rolled steel sheet facing downward.

[0024] (Casting process S1) In the casting step S1, a slab is prepared. An example of a method for producing a slab is as follows. First, molten steel is produced (smelted). Next, a slab is produced using the molten steel. The method for producing the slab is not particularly limited, and for example, the slab may be produced by continuous casting. Alternatively, an ingot may be produced using the molten steel, and the ingot may be bloomed to produce a slab. The thickness of the slab is not particularly limited. The thickness of the slab may be, for example, 150 mm to 350 mm. The thickness of the slab is preferably 220 mm to 280 mm. A so-called thin slab having a thickness of 10 mm to 70 mm may be used as the slab. When a thin slab is used, rough rolling before finish rolling can be omitted in the hot rolling step S2.

[0025] The composition of the slab may be any composition that allows secondary recrystallization to occur. The basic components and optional elements of the slab are specifically described below. Note that the notation % used for the components means mass %.

[0026] Si is an important element for increasing electrical resistance and reducing iron loss. If the Si content exceeds 4.8%, the material will be prone to cracking during cold rolling, making rolling impossible. On the other hand, if the Si content is reduced, α→γ transformation will occur during final annealing, impairing the crystal orientation. Therefore, the lower limit may be set at 0.8%, which does not affect the crystal orientation during final annealing. Therefore, the Si content may be 0.8 to 4.8%.

[0027] Although C is an effective element for controlling the primary recrystallization structure in the manufacturing process, excessive C content in the final product may adversely affect the magnetic properties. Therefore, the C content may be 0.085% or less. A preferred upper limit of the C content is 0.080%. C is purified in the decarburization annealing step S5 and the final annealing step S8 described below, and after the final annealing step S8, the C content is 0.005% or less. When the slab contains C, the lower limit of the C content may be greater than 0%, or even 0.001%, taking into account productivity in industrial production.

[0028] Acid-soluble Al is an element that functions as an inhibitor when it bonds with N to form AlN or (Al,Si)N. The content of acid-soluble Al may be 0.012% to 0.050%, which increases the magnetic flux density.

[0029] If 0.01% or more of N is added during steelmaking, voids called blisters will form in the steel sheet, so the upper limit of the N content may be 0.01%. Since N can be added by nitriding during the manufacturing process, the lower limit is not particularly limited and may be 0%. However, since the detection limit for N is 0.0001%, the practical lower limit is 0.0001%.

[0030] Mn and S precipitate as MnS and act as inhibitors. If the Mn content is less than 0.02% and the S content is less than 0.005%, the required amount of effective MnS inhibitor may not be secured. Furthermore, if the Mn content is more than 0.3% and the S content is more than 0.04%, solutionization during slab heating may be insufficient, and secondary recrystallization may not occur stably. Therefore, the Mn content may be 0.02 to 0.3%, and the S content may be 0.005 to 0.04%.

[0031] Other inhibitor constituent elements such as B, Bi, Se, Pb, Sn, and Ti can also be added to the slab. The amounts added may be adjusted as appropriate, with the upper limit for the B content being 0.080%, the upper limit for the Bi content being 0.010%, the upper limit for the Se content being 0.035%, the upper limit for the Pb content being 0.10%, the upper limit for the Sn content being 0.10%, and the upper limit for the Ti content being 0.015%. These optional additional elements may be added to the slab according to known purposes, so there is no need to set a lower limit for the content of the optional additional elements; for example, the lower limit may be 0%.

[0032] The balance of the chemical composition of the slab is Fe and impurities. Note that the term "impurities" used here refers to components that are mixed into the slab due to raw materials such as ore and scrap, or various factors in the manufacturing process, when the slab is industrially produced, and that are acceptable to the extent that they do not substantially affect the grain-oriented electrical steel sheet according to the present embodiment.

[0033] In addition to solving manufacturing problems, the slab may contain (add) known optional elements in place of a portion of Fe, taking into consideration the effects on magnetic properties and the strengthening of inhibitor function through compound formation. Examples of optional elements that may be contained in the slab in place of a portion of Fe include Cu, P, Sb, Cr, and Ni. Any one or more of these may be added to the slab. The upper limit for the Cu content may be 0.40%, the upper limit for the P content may be 0.50%, the upper limit for the Sb content may be 0.10%, the upper limit for the Cr content may be 0.30%, and the upper limit for the Ni content may be 1.00%. Since these optional additional elements may be contained in the slab according to known purposes, there is no need to set a lower limit for the content of the optional additional elements, and the lower limit may be 0%.

[0034] The chemical composition of a slab can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, a 35 mm square test piece taken from the slab is measured using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-established calibration curve, thereby identifying the chemical composition. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0035] (Hot rolling process S2) The hot rolling step S2 is a step in which a slab heated to a predetermined heating temperature (for example, 1100°C to 1400°C) is hot-rolled to obtain a hot-rolled steel sheet. The heating temperature during hot rolling may be, for example, 1100°C or higher from the viewpoint of ensuring the temperature during hot rolling, or may be 1280°C or lower from the viewpoint of not completely dissolving AlN, which is an inhibitor component. Note that when AlN and MnS are used as main inhibitors, the heating temperature during hot rolling may be 1300°C or higher, at which these inhibitor components completely dissolve.

[0036] (Hot-rolled steel sheet annealing process S3) The hot-rolled steel sheet annealing step S3 is a step in which the hot-rolled steel sheet obtained in the hot-rolling step S2 is annealed immediately or within a short time to obtain an annealed steel sheet. The annealing may be performed in a temperature range of 750°C to 1200°C for 30 seconds to 30 minutes. This annealing is effective in improving the magnetic properties of the product.

[0037] (Cold rolling process S4) The cold rolling step S4 is a step of obtaining a cold-rolled steel sheet by cold rolling the annealed steel sheet obtained in the hot-rolled steel sheet annealing step S3 once, or by cold rolling the annealed steel sheet once or twice (two or more times) via annealing (intermediate annealing) (for example, a total cold rolling rate of 80% to 95%). The thickness of the cold-rolled steel sheet may be, for example, 0.10 mm to 0.50 mm.

[0038] (Decarburization annealing process S5) The decarburization annealing step S5 is a step of performing decarburization annealing on the cold-rolled steel sheet obtained in the cold rolling step S4 to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred (a cold-rolled steel sheet that has been subjected to the decarburization annealing step). The decarburization annealing may be performed, for example, at 700°C to 900°C for 1 minute to 3 minutes.

[0039] By subjecting the cold-rolled steel sheet to decarburization annealing, the C component contained in the cold-rolled steel sheet is removed. The decarburization annealing is preferably performed in a humid atmosphere in order to remove the C component contained in the cold-rolled steel sheet.

[0040] (Nitriding process S6) The nitriding step S6 is a step that is performed as necessary to adjust the strength of the inhibitor in secondary recrystallization. The nitriding is a process that increases the nitrogen content of the cold-rolled steel sheet by approximately 40 ppm to 200 ppm during the period from the start of the decarburization annealing step to the start of secondary recrystallization in the finish annealing step. Examples of nitriding include a process in which the decarburization annealed steel sheet is annealed in an atmosphere containing a nitriding gas such as ammonia, and a process in which an annealing separator containing a nitriding powder such as MnN is applied to the decarburization annealed steel sheet in the annealing separator application step S7 described below.

[0041] (Annealing separator application process S7) The annealing separator application step S7 is a step of applying an annealing separator to the decarburization-annealed steel sheet. As the annealing separator, for example, an annealing separator containing alumina (Al2O3) as a main component can be used. After the annealing separator is applied, the decarburization-annealed steel sheet is wound into a coil and then finish-annealed in the next finish-annealing step S8. When forming a glass coating containing Mg2SiO4, an annealing separator containing magnesia (MgO) as the main component is used.

[0042] (Finishing annealing process S8) The final annealing step S8 is a step in which the decarburized annealed steel sheet coated with the annealing separator is subjected to final annealing to cause secondary recrystallization. This final annealing step S8 involving secondary recrystallization is performed by suppressing the growth of primary recrystallized grains with an inhibitor, thereby forming {100} <001> Oriented grains grow preferentially, dramatically improving magnetic flux density. If magnesia (MgO) is applied in the above-mentioned annealing separator application step S7, a glass coating containing Mg2SiO4 is formed in this finish annealing step S8. In this embodiment, such a glass coating is also included in the base steel sheet (the finish annealed steel sheet described below). Therefore, for example, when a glass coating is formed on a finish annealed steel sheet, the "surface of the finish annealed steel sheet" refers to the surface of the glass coating. It is expected that the formation of a glass coating will further improve the properties of the grain-oriented electrical steel sheet 200.

[0043] (Groove formation process S9) The groove forming step S9 is a step of forming grooves in the steel sheet (finish-annealed steel sheet) after the finish annealing step S8 for the purpose of magnetic domain control (magnetic domain refinement). The grooves can be formed by known methods such as laser, electron beam, plasma, mechanical methods, and etching.

[0044] An example of the groove forming step S9 will be described with reference to Fig. 2, Fig. 3, and Fig. 4. Fig. 2 is a plan view showing a groove G formed in a finish annealed steel sheet (base steel sheet) 110, and Fig. 3 is a schematic cross-sectional view for explaining the configuration of the vicinity of the groove G in a grain-oriented electrical steel sheet 200. Fig. 4 is a schematic cross-sectional view showing a modified example of the grain-oriented electrical steel sheet 200. The cross sections in Figs. 3 and 4 are cross sections perpendicular to the extension direction of the groove G.

[0045] 2, the rolling direction of the finish annealed steel sheet 110 is the X-axis, the width direction of the finish annealed steel sheet 110 is the Y-axis, and the thickness direction of the finish annealed steel sheet 110 is the Z-axis. The direction from the groove-formed surface 110a (the surface on which the grooves G are formed; details will be described later) of the finish annealed steel sheet 110 toward the other surface is the positive Z-axis direction. The definitions of the X, Y, and Z axes shown in other figures are similar.

[0046] 3 illustrates a finish-annealed steel sheet 110 and an insulating coating (tensile coating) 130. That is, the grain-oriented electrical steel sheet 200 according to this embodiment includes the finish-annealed steel sheet 110 and the insulating coating 130. The insulating coating 130 is formed on the groove-forming surface 110a of the finish-annealed steel sheet 110 by a tensile coating application step S10, which will be described later.

[0047] As described above, the grooves G are formed on the finish annealed steel sheet 110 by a known method such as laser, electron beam, plasma, mechanical method, or etching. Of the front and back surfaces of the finish annealed steel sheet 110, the surface on which the grooves G are formed is also referred to as a groove-formed surface 110a. Of the groove-formed surface 110a, the portion on which the grooves G are not formed is referred to as a flat surface 110F.

[0048] The grooves G are formed on the upper surface (top surface) of the finish annealed steel sheet 110, but not on the lower surface. However, in the tension coating step S10 described below, the coating solution is applied to the groove formation surface 110a with the grooves G facing "downward." Therefore, the grooves G may be formed on the lower surface (bottom surface) of the finish annealed steel sheet 110.

[0049] In relation to the effects of this embodiment, the shape of the grooves G is preferably within the following range. In order to specify the shape of the grooves G, it becomes necessary to observe the cross section of the grooves G. In this case, any cross section perpendicular to the extension direction of the grooves G can be machined to have a mirror surface, and this cross section can be observed as an observation cross section using a scanning electron microscope or the like.

[0050] From the viewpoint of reducing iron loss, the extension direction of the grooves G in a plan view is preferably in the range of 90° to 60°, and more preferably in the range of 90° to 75° with respect to the X-axis direction (rolling direction).

[0051] If the extension direction of the grooves G is 60° or more with respect to the rolling direction X, the angle between the wall surfaces (groove wall surfaces) 110G of the grooves G and the rolling direction also becomes large, increasing the need to utilize the effect of this embodiment. In other words, more magnetic flux will leak from the grooves G, so it is necessary to make it easier for this magnetic flux to leak. In other words, it becomes more necessary to make the insulating coating 130G inside the grooves thinner.

[0052] The pitch of the grooves G in the rolling direction (rolling direction pitch) is preferably set in the range of 1 to 20 mm depending on the need for magnetic domain refinement. The rolling direction pitch of the grooves G is more preferably set in the range of 2 to 10 mm. The upper limit of the rolling direction pitch of the grooves G is more preferably 8 mm. The upper limit of the rolling direction pitch of the grooves G is even more preferably 5 mm.

[0053] The rolling direction pitch may be measured, for example, by the following method. That is, attention is focused on any pair of grooves G that are adjacent in a plan view. Next, the distance in the rolling direction between the center points of these grooves G in the width direction is measured at several locations, and the average value of these measurements may be taken as the rolling direction pitch of the pair of grooves G. In this embodiment, it is preferable that any rolling direction pitch measured in this manner is a value within the range of 2 to 10 mm.

[0054] The width w of the groove G is preferably 20 μm or more, and more preferably 30 μm or more, because if the width w is 20 μm or more, it becomes technically easier to control the thickness of the insulating coating 130G inside the groove. The width w is the distance between two flat surfaces 110F adjacent to each other in the rolling direction via a groove G, in a direction perpendicular to the extension direction of the groove G and the plate thickness direction (Z-axis direction).

[0055] The width w of the groove G is preferably 150 μm or less, and more preferably 90 μm or less. A width w of 150 μm or less is suitable from the viewpoint of magnetic domain refinement. Furthermore, although it depends on the depth D of the groove G, the smaller the width w, the more significant the problem of increased iron loss due to the angular difference between the magnetization direction of the grain-oriented electrical steel sheet 200 and the direction of tension along the groove wall surface 110G due to the insulating coating 130G inside the groove. Therefore, there is an increasing need to make the insulating coating 130 thinner. For this reason, the width w of the groove G is preferably 150 μm or less.

[0056] The depth D of the groove G is preferably 5 μm or more, and more preferably 15 μm or more. If the depth D is 5 μm or more, depending on the width w, the problem of increased iron loss due to the angular difference between the magnetization direction of the grain-oriented electrical steel sheet 200 and the direction of tension along the groove wall surface 110G due to the insulating coating 130G inside the groove becomes significant. Therefore, there is an increasing need to make the insulating coating 130 thinner. For this reason, the depth D of the groove G is preferably 5 μm or more. The depth D is the distance (depth direction distance) from the bottom surface 110Ga of the groove G (the deepest part of the observed cross section of the groove G) to the flat surface 110F adjacent to the groove G in the plate thickness direction (Z-axis direction).

[0057] The depth D of the groove G is preferably 50 μm or less, and more preferably 30 μm or less. This is because if the depth D of the groove G is 50 μm or less, it is technically easy to control the thickness t2 of the insulating coating 130G inside the groove. On the other hand, if the depth D of the groove G exceeds 50 μm, the sheet thickness of the finish-annealed steel sheet 110 may be significantly reduced in some parts, making it impossible to obtain the iron loss reduction effect.

[0058] In the example shown in the flowchart of FIG. 1, the groove forming step S9 is performed after the finish annealing step S8. However, the groove forming step S9 may be performed on a steel sheet that has been subjected to the cold rolling step S4 (i.e., a cold-rolled steel sheet). In this case, too, it is possible to maintain the cross-sectional shape of the linear grooves G that is ideal for magnetic domain refinement. Therefore, the groove forming step S9 may be performed either before or after the finish annealing step S8. However, the groove forming step S9 must be performed at least before the tension coating step S10.

[0059] In the modification shown in Fig. 4, a finish-annealed steel sheet 110 has a glass coating 150. This glass coating 150 contains Mg2SiO4. However, even in this case, the definitions and determination methods of parameters related to the groove G (width w, depth D, etc.) remain unchanged.

[0060] (Tension film application step S10) The tension coating process S10 is a process of applying a coating solution to the groove-forming surface 110a of the finish-annealed steel sheet 110 while the groove-forming surface 110a is facing "downward" and baking it to form an insulating coating (tensile coating) 130 on the groove-forming surface 110a.

[0061] The coating solution may contain, for example, phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or a compound of silica. Baking may be performed, for example, at 350°C to 1150°C for 5 to 300 seconds.

[0062] Before describing the tension coating step S10 of this embodiment in detail, first, problems with the conventional tension coating step will be described with reference to FIGS.

[0063] FIG. 6 shows an example of a conventional tensile coating process. In the example shown in FIG. 6, a finish-annealed steel sheet 110 is transported by a transport roller 1000. Here, the groove-formed surface 110a faces upward. A coating solution is then applied to the groove-formed surface 110a from above the finish-annealed steel sheet 110, and baking is performed. This makes the coating solution more likely to accumulate in the grooves G. That is, some of the coating solution applied to the groove-formed surface 110a remains inside the grooves G in a liquid pool state. If baking is performed in this state, the insulating coating 130G inside the grooves will be thicker than the insulating coating 130F on other portions (the so-called flat surfaces 110F). FIG. 7 is an SEM photograph showing the cross-sectional structure of a grain-oriented electrical steel sheet 200 manufactured using a conventional tensile coating process. As can be seen from this SEM photograph, the insulating coating 130G inside the grooves is thicker than the insulating coating 130F on other portions (the so-called flat surfaces 110F).

[0064] The insulating coating 130G inside the grooves tends to be formed to be excessively thicker than the insulating coating 130F on the flat surface 110F, and such excessively thick insulating coating 130G is likely to peel off from the base material, the finish-annealed steel sheet 110. In other words, the insulating coating 130G formed inside the grooves is likely to peel off, which is thought to result in a loss of the iron loss reduction effect of the grain-oriented electrical steel sheet 200.

[0065] In the grain-oriented electrical steel sheet 200 having grooves G formed therein, the magnetic flux that passes through the steel sheet and reaches one of the groove walls leaks from the groove wall (i.e., due to magnetic flux leakage), increasing the magnetostatic energy, and the main magnetic domain is subdivided to reduce this magnetostatic energy, which is the cause of the magnetic domain control effect.

[0066] However, when tension is not applied to the steel sheet, closure-type magnetic domains are generated near the groove wall surfaces, which suppresses the increase in magnetostatic energy described above, and the magnetic domain control effect is not sufficiently achieved. The isotropic tension of the insulating coating makes the closure domains energetically unstable (due to the adverse effect of magnetostriction), which restores magnetic flux leakage and improves the magnetic domain control effect. It is thought that a thick insulating coating formed in the grooves is prone to peeling and therefore is unable to provide sufficient tension to the steel sheet, or is unable to apply stress to the steel sheet in a direction that destabilizes the closure domains.

[0067] In particular, when the coating solution is applied to the groove-forming surface 110a of the finish-annealed steel sheet 110, gravity causes a concave (in other words, a downwardly convex) catenary on the upper surface of the finish-annealed steel sheet 110. In this case, the insulating coating 130G inside the groove tends to be formed with an excessively large thickness compared to the insulating coating 130F on the flat surface 110F. In other words, as shown in FIG. 6, when the coating solution is applied in the "upward facing" state, a liquid pool is likely to form in the groove G, and a thicker insulating coating 130G is formed inside the groove. As a result, the insulating coating 130G formed inside the groove is prone to peeling, which further significantly reduces the iron loss reduction effect.

[0068] In contrast, in the tension coating step S10 according to this embodiment, a coating solution is applied to the groove-formed surface 110a of the finish-annealed steel sheet 110 with the groove-formed surface 110a facing downward, and then baking is performed, as shown in Fig. 5. Note that the finish-annealed steel sheet 110 is transported by transport rollers 1000, as in the conventional case.

[0069] In this embodiment, the coating solution tends to accumulate in the groove G, and gravity causes a concave catenary to form on the upper surface of the finish annealed steel sheet 110, similar to the conventional tension coating process (i.e., the process of applying the coating solution with the groove-forming surface facing upward).

[0070] However, because the grooves G face downward, some of the coating solution accumulated inside the grooves drips due to gravity. The downward convex shape of the catenary is thought to make it easier for the coating solution accumulated inside the grooves to drip. As a result, after baking, the insulating coating 130G inside the grooves is thought to be thinner than in conventional tension coating processes. In other words, as a result of the thinner insulating coating 130G inside the grooves, the insulating coating 130G is less likely to peel inside the grooves, resulting in a reduction in iron loss of the grain-oriented electrical steel sheet 200. Furthermore, because the insulating coating 130G inside the grooves can leak more magnetic flux, an improvement in iron loss reduction is expected. As shown in the examples described below, when the coating solution was applied with the groove-forming surface 110a facing downward and baking was performed, the coating peeling rate in the grooves of the grain-oriented electrical steel sheet 200 was reduced, and iron loss was significantly reduced.

[0071] In the tension coating process S10, it is preferable that the thickness t2 of the insulating coating 130G formed inside the groove be equal to or less than half the depth D of the groove G and equal to or less than twice the thickness t1 of the insulating coating 130F formed on the flat surface 110F of the finish-annealed steel sheet 110 (i.e., the base steel sheet). Hereinafter, this requirement will also be referred to as the "additional requirement for thickness t2." The depth D and thicknesses t1 and t2 are shown in FIG. 3. Here, t1 is the thickness measured in a direction perpendicular to the tangent direction of the steel sheet surface at the thickness measurement point. Similarly, t2 is the thickness measured in a direction perpendicular to the tangent direction along the groove surface.

[0072] Here, the thickness t2 of the insulating coating 130G inside the groove is the average value of the thicknesses t2 measured at multiple points on the cross section of the groove G observed.

[0073] Similarly, the thickness t1 of the insulating coating 130F on the flat surface 110F is the average value of the thicknesses t1 measured at multiple points on the observation cross section. There are no particular restrictions on the specific value of the thickness t1 of the insulating coating 130F, and it may be set appropriately depending on the properties required of the grain-oriented electrical steel sheet 200, but it is preferably 1 μm or more, and more preferably 2 μm or more, for example. This is because if the thickness t1 of the insulating coating 130F is 1 μm or more, the corrosion resistance and insulating properties of the grain-oriented electrical steel sheet 200 can be further improved.

[0074] The thickness t1 of the insulating coating 130F is preferably 10 μm or less, and more preferably 5 μm or less, because if the thickness t1 of the insulating coating 130F is 10 μm or less, a significant decrease in the space factor of the finish-annealed steel sheet 110 can be prevented.

[0075] It is preferable that the thickness t1, thickness t2, and depth D measured (determined) by the above method satisfy the "additional requirement for thickness t2" described above. Note that in the tensile coating step S10, the thickness t2 is thinner than in the conventional tensile coating step in which the groove-forming surface faces upward, as described above, and therefore the "additional requirement for thickness t2" described above may be achieved by appropriately changing conditions such as, for example, one or more of the amount, viscosity, and concentration of the coating solution to be applied, the application method (application methods include application by a roll coater, etc.), the time from application to baking, and air blowing to remove a portion of the coating solution.

[0076] The tension coating step S10 may be performed on a separate line from the other steps (i.e., offline), or on the same line (i.e., online). When the grooves G are formed on the lower surface of the finish annealed steel sheet 110 in the groove forming step S9 described above, the finish annealed steel sheet 110 can be directly subjected to the tension coating step S10. In other words, the tension coating step S10 can be performed inline.

[0077] On the other hand, when the grooves G are formed on the upper surface of the finish annealed steel sheet 110 in the groove forming step S9, the finish annealed steel sheet 110 may be once wound up, then turned upside down, and then subjected to the tension coating step S10. In this case, the tension coating step S10 may be performed offline or inline. [Example]

[0078] In this example, the effects of the manufacturing method of the grain-oriented electrical steel sheet according to the present embodiment described above were verified. Of course, the present invention is not limited to the examples described below. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical idea described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

[0079] First, by carrying out the above-described casting step S1 to finish annealing step S8, a finish annealed steel sheet 110 having a thickness of 0.23 mm and a high magnetic flux density B8 value of 1.93 T at 800 A / m was manufactured.

[0080] Next, a fiber laser having an elliptical beam shape of 20 μm in the rolling direction of the steel sheet and 40 μm in the width direction and a beam power of 2.5 kW was irradiated onto this finish-annealed steel sheet 110, thereby forming grooves G on the surface of the finish-annealed steel sheet 110. Here, the scanning speed was 15 m / s, and linear grooves G with a width w of approximately 50 μm and a depth D of approximately 20 μm were formed at 3 mm intervals in the direction perpendicular to the rolling direction.

[0081] Next, the coating solution was applied and baked in the manner shown in Table 1. Here, the coating solution used contained a mixture containing aluminum phosphate and magnesium phosphate, and contained 40 to 70 parts by mass of colloidal silica per 100 parts by mass of the mixture in terms of solid content. In Table 1, "direction of groove-formed surface at time of baking" indicates the direction (upper or lower) of the groove-formed surface 110a when the coating solution was applied. The coating solution was dropped from above the steel sheet and applied using a normal natural coater.

[0082] The amount of coating solution applied is the mass (g / m) of the solid content of the coating solution applied per unit area of ​​the groove-formed surface 110a in plan view. 2 ) is shown. If you want to measure the amount of coating solution applied from the dried insulating coating, you can dissolve the insulating coating in the alkaline solution by immersing the grain-oriented electrical steel sheet in a heated alkaline solution (such as NaOH solution). Then, the amount of coating solution applied can be determined from the mass of the insulating coating dissolved in the alkaline solution.

[0083] Next, the depth D (μm) of the groove G, the thickness t2 (μm) of the insulating coating 130G inside the groove, the thickness t1 (μm) of the insulating coating 130F on the flat surface 110F, the groove insulating coating peeling rate (area %), the magnetic flux density B8 (T), and the iron loss W17 / 50 (W / kg) were measured. The methods for measuring the depth D (μm) of the groove G, the thickness t2 (μm) of the insulating coating 130G inside the groove, and the thickness t1 (μm) of the insulating coating 130F on the flat surface 110F were as described above. The results are shown in Table 1.

[0084] Here, the "groove insulation coating peeling rate" refers to the area ratio of the portion where the insulation coating peeled off to the total area of ​​the groove G in a plan view. Whether or not the insulation coating had peeled off was determined based on the area ratio of the exposed steel sheet surface. Specifically, an area ratio of 10% was determined to be unacceptable.

[0085] [Table 1]

[0086] As is clear from Table 1, in the grain-oriented electrical steel sheets (Experiments Nos. 1 to 4) manufactured by the method for manufacturing grain-oriented electrical steel sheets according to this embodiment, the coating solution was applied to the finish-annealed steel sheet with the groove-formed surface facing downward, and then baked, resulting in fewer coating defects and lower iron loss. In contrast, in the grain-oriented electrical steel sheets manufactured by the conventional manufacturing method for grain-oriented electrical steel sheets (Experiments Nos. 5 to 7), the coating solution was applied and baked with the groove-forming surface of the finish-annealed steel sheet facing upward, resulting in many coating defects and high iron loss.

[0087] It should be noted that the "additional requirement of thickness t2" was satisfied in Experiments Nos. 1 to 3. This is also considered to be the reason why the iron loss was low in the inventive examples in Experiments Nos. 1 to 3. However, since the thickness of the coating inside the groove was more than twice the thickness formed on the flat surface of the steel sheet, it did not satisfy the "additional requirement for thickness t2.") Compared to Experiments No. 1 to 3, Experiment No. 4 had slightly higher iron loss.

[0088] As described above, according to this embodiment, with the groove-formed surface 110a of the finish-annealed steel sheet 110 (or cold-rolled steel sheet) facing downward, a coating solution containing a compound of phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or silica is applied and baked to form an insulating coating on the groove-formed surface 110a. In the grain-oriented electrical steel sheet 200 manufactured by this manufacturing method, the insulating coating 130G inside the grooves is thin and therefore less likely to peel off. This results in fewer coating defects and reduced iron loss.

[0089] Here, in the tension coating application step S10, the thickness t2 of the insulating coating 130G formed inside the grooves may be adjusted so that the thickness t2 is not more than half the depth D of the grooves G and not more than twice the thickness t1 of the insulating coating 130F formed on the flat surface 110F of the finish-annealed steel sheet 110 (or cold-rolled steel sheet). In this case, the insulating coating 130G inside the grooves becomes even thinner, further reducing the iron loss of the grain-oriented electrical steel sheet 200.

[0090] Furthermore, after the cold rolling step and before the finish annealing step, an annealing separator application step may be further performed in which an annealing separator is applied to the cold-rolled steel sheet. The annealing separator may contain magnesia. In this case, the properties of the grain-oriented electrical steel sheet 200 are further improved.

[0091] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Explanation of symbols]

[0092] 200 grain-oriented electrical steel sheet 110 Finish annealed steel sheet 110F flat surface 110G groove wall surface 130 Insulation coating (tensile coating) 130F Insulation coating on flat surfaces 130G Insulation coating inside the groove

Claims

1. a cold rolling process for producing a cold-rolled steel sheet; A finish annealing process in which the cold-rolled steel sheet is subjected to finish annealing accompanied by secondary recrystallization; a groove forming step of forming linear grooves on only one surface of the cold-rolled steel sheet before or after the finish annealing step in a direction intersecting the rolling direction of the cold-rolled steel sheet by laser, plasma, a mechanical method, or etching; a tension coating step of applying a coating solution containing phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or a compound of silica to the cold-rolled steel sheet with the groove-formed surface facing downward, and baking the coating solution to form a tension coating on the groove-formed surface; The width of the groove is 20 μm or more, and the depth of the groove is 15 μm or more. A method for producing a grain-oriented electrical steel sheet.

2. 2. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein in the tensile coating application step, a thickness of the tensile coating formed inside the groove is adjusted so that the thickness of the tensile coating is not more than half the depth of the groove and is not more than twice the thickness of the tensile coating formed on the flat surface of the cold-rolled steel sheet.

3. Further provided is an annealing separator application step of applying an annealing separator to the cold-rolled steel sheet after the cold rolling step and before the finish annealing step, The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the annealing separator contains magnesia.

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