JP7744564B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to a method for manufacturing grain-oriented electrical steel sheets.

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 highly concentrated crystal orientation in the {110}<001> direction. Their magnetic properties require high magnetic flux density, as represented by the _B8 value, and low iron loss, as represented by the W17/50 value. In particular, there has been a growing demand for reduced power loss in recent years to conserve energy.

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 that require stress relief annealing (SRA) after the transformer is formed.

In other words, 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 2-4 hours) is generally performed to eliminate iron loss degradation caused by the processing strain introduced by the bending process. However, 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.

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

On the other hand, as disclosed in Patent Document 3, a technology 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). 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.

Special Publication No. 58-26405 Patent No. 4384451 Patent No. 5742294

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

The present invention was made in consideration of the above problems, and its object is to provide a method for manufacturing grain-oriented electrical steel sheets that can produce grain-oriented electrical steel sheets with few coating defects and low iron loss.

The aspects of the present invention are as follows.
(1) One aspect of the present invention is a method for producing a grain-oriented electrical steel sheet, the method comprising: 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 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; and a tensile coating application step for applying a coating solution containing a compound of phosphoric acid, phosphate, chromic anhydride, chromate, alumina, or silica to the groove-formed surface with the surface facing downward, and baking the coating solution, wherein the width of the groove is 20 μm or more, and the depth of the groove is 15 μm or more .

(2) In the method for manufacturing a 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 the thickness of the tension coating formed on the flat surface of the cold-rolled steel sheet.

(3) The method for producing a grain-oriented electrical steel sheet described in (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.

According to the above-described aspects of the present invention, grain-oriented electrical steel sheets with few coating defects and low iron loss can be manufactured.

1 is a flowchart illustrating an example of a method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment. FIG. 2 is a plan view showing grooves formed in a finish-annealed steel sheet. FIG. 2 is a schematic cross-sectional view illustrating the configuration of the vicinity of a groove in a grain-oriented electromagnetic steel sheet. 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. 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. FIG. 1 is a schematic cross-sectional view for explaining problems with a conventional method for manufacturing a grain-oriented electrical steel sheet. 1 is a SEM photograph for explaining problems with a conventional method for manufacturing a grain-oriented electrical steel sheet.

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

The 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 is sometimes referred to as a tension coating.

For example, as disclosed in Patent Document 3, technology already exists for forming an insulating coating on the grooved surface of a steel sheet. However, with the technology proposed so far, there are cases where insulation is insufficient due to coating defects in the grain-oriented electrical steel sheet, and iron loss has not been sufficiently reduced.

The inventors believe that the reason for this lies in the process of forming the insulating coating. As will be explained in more detail below, conventionally, the coating solution was applied to the grooved surface of the steel sheet with the grooved surface facing "upward," and then the sheet was 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 the sheet was baked in this state, the insulating coating inside the grooves would be thicker than the insulating coating on other parts (the so-called flat surface).

As such, the insulating coating inside the grooves tends to be formed excessively thicker than the insulating coating on the flat surfaces. Furthermore, an insulating coating that is formed too thick is prone to peeling off from the base steel plate. In other words, the insulating coating formed inside the grooves is prone to peeling, which may result in insufficient insulation and an inability to sufficiently reduce iron loss.

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 tension is not applied to the steel sheet, closure domains form near the groove walls, suppressing the increase in magnetostatic energy described above and preventing sufficient 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 prone to peeling, and is therefore thought to be unable to provide sufficient tension to the steel sheet, or to apply stress in a direction that destabilizes the closure domains.

In particular, when the coating solution is applied to the grooved surface of the steel plate, gravity causes a concave (or convex) catenary to form on the upper surface of the steel plate. In this case, liquid pools are more likely to form in the groove, and a thicker insulating coating is formed inside the groove. As a result, the insulating coating formed inside the groove is more likely to peel off, significantly reducing its iron loss reduction effect.

Based on the above considerations, the inventors discovered that applying a coating solution to the grooved surface of the steel sheet with the grooved surface facing "downward" and then baking it is effective in thinning the insulating coating formed inside the grooves, and that this makes it possible to produce grain-oriented electrical steel sheet with few coating defects and low iron loss.

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 changed as desired 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.

(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. The molten steel may be used to produce an ingot, and the ingot may then 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 in the hot rolling step S2 can be omitted.

The chemical composition of the slab may be any composition that allows secondary recrystallization to occur. Specific details of the basic components and optional elements of the slab are as follows. Note that the percentages used for the components refer to mass percent.

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

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

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

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 through nitriding during the manufacturing process, there is no particular lower limit, and it may even be 0%. However, since the detection limit for N is 0.0001%, the effective lower limit is 0.0001%.

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%, it may not be possible to ensure the required amount of effective MnS inhibitor. Furthermore, if the Mn content is more than 0.3% and the S content is more than 0.04%, solution treatment during slab heating may be insufficient, and secondary recrystallization may not occur stably. Therefore, the Mn content may be 0.02-0.3%, and the S content may be 0.005-0.04%.

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 B content being 0.080%, Bi content being 0.010%, Se content being 0.035%, Pb content being 0.10%, Sn content being 0.10%, and Ti content being 0.015%. Since these optional elements can be added to the slab according to known purposes, there is no need to set a lower limit for the content of the optional elements; for example, the lower limit may be 0%.

The remainder of the slab's chemical composition consists of Fe and impurities. Note that "impurities" here refer to components that are mixed into the slab during industrial production of the slab due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are acceptable to the extent that they do not substantially affect the grain-oriented electrical steel sheet according to this embodiment.

In addition to resolving manufacturing issues, the slab may contain (add) known optional elements in place of a portion of the Fe, taking into consideration strengthening the inhibitor function through compound formation and the impact on magnetic properties. Examples of optional elements that can be contained in the slab in place of a portion of the 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%. Because these optional added 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 added elements; the lower limit may be 0%.

The chemical composition of a slab can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition can be determined by measuring a 35 mm square test piece taken from the slab using a Shimadzu ICPS-8100 or other measuring device under conditions based on a pre-created calibration curve. 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.

(Hot rolling process S2)
The hot rolling step S2 is a step of hot rolling a slab heated to a predetermined heating temperature (e.g., 1100°C to 1400°C) 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.

(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 for 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.

(Cold rolling step 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 multiple times (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.

(Decarburization annealing step 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.

By subjecting cold-rolled steel sheet to decarburization annealing, the carbon contained in the cold-rolled steel sheet is removed. Decarburization annealing is preferably performed in a humid atmosphere to remove the carbon contained in the cold-rolled steel sheet.

(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.

(Annealing separator application step 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 mainly composed of alumina (Al ₂ O ₃ ) can be used. After the annealing separator application step S7, 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 Mg ₂ SiO ₄ , an annealing separator containing magnesia (MgO) as the main component is used.

(Finishing annealing step 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 allows the growth of primary recrystallized grains to proceed while the growth of primary recrystallized grains is suppressed by an inhibitor, thereby preferentially growing {100}<001> oriented grains and 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 properties of the grain-oriented electrical steel sheet 200 will be further improved by forming a glass coating.

(Groove formation step 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 method, etching, etc.

An example of the groove forming process S9 will be described with reference to Figures 2, 3, and 4. Note that Figure 2 is a plan view showing a groove G formed in a finish-annealed steel sheet (base steel sheet) 110, and Figure 3 is a schematic cross-sectional view illustrating the configuration of the groove G and its vicinity in a grain-oriented electrical steel sheet 200. Figure 4 is a schematic cross-sectional view showing a modified example of the grain-oriented electrical steel sheet 200. The cross sections in Figures 3 and 4 are perpendicular to the extension direction of the groove G.

In Figure 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 formation surface 110a (the surface on which the groove G is 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.

Figure 3 shows a finish-annealed steel sheet 110 and an insulating coating (tensile coating) 130. In other words, the grain-oriented electrical steel sheet 200 according to this embodiment comprises a finish-annealed steel sheet 110 and an insulating coating 130. The insulating coating 130 is formed on the groove-forming surface 110a of the finish-annealed steel sheet 110 by the tensile coating application process S10, which will be described later.

As described above, the grooves G are formed on the finish annealed steel sheet 110 by known methods such as laser, electron beam, plasma, mechanical methods, and 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 the groove-formed surface 110a. The portion of the groove-formed surface 110a on which the grooves G are not formed is referred to as the flat surface 110F.

The grooves G are formed on the upper surface (top surface) of the finish annealed steel sheet 110, not on the lower surface. However, in the tension coating process 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 also be formed on the lower surface (bottom surface) of the finish annealed steel sheet 110.

In relation to the effects of this embodiment, the shape of the groove G is preferably within the following range. It should be noted that, in order to specify the shape of the groove G, it may be necessary to observe the cross section of the groove G. In this case, any cross section perpendicular to the extension direction of the groove G may be machined to have a mirror finish, and this cross section may then be observed using a scanning electron microscope or the like.

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

If the extension direction of the groove G is 60° or more relative to the rolling direction X, the angle between the wall surface (groove wall surface) 110G of the groove G and the rolling direction becomes larger, increasing the need to utilize the effects of this embodiment. In other words, more magnetic flux will leak from the groove G, so it is necessary to make it easier for this magnetic flux to leak. In other words, there is an increased need to make the insulating coating 130G inside the groove thinner.

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. It is even more preferable to set the rolling direction pitch of the grooves G in the range of 2 to 10 mm. It is even more preferable that the upper limit of the rolling direction pitch of the grooves G is 8 mm. It is even more preferable that the upper limit of the rolling direction pitch of the grooves G is 5 mm.

The rolling direction pitch may be measured, for example, by the following method. That is, focus is placed on any pair of adjacent grooves G in a planar view. Next, the rolling direction distance between the widthwise center points of these grooves G is measured at several locations, and the average value of these measurements is taken as the rolling direction pitch for that 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.

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).

The width w of the groove G is preferably 150 μm or less, and more preferably 90 μm or less. A groove G width w of 150 μm or less is suitable from the perspective of magnetic domain refinement. Furthermore, although this depends on the depth D of the groove G, the smaller the width w, the more pronounced 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 caused by the insulating coating 130G inside the groove. Therefore, there is a greater need to make the insulating coating 130 thinner. For this reason, the width w of the groove G is preferably 150 μm or less.

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) in the thickness direction (Z-axis direction) 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.

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. Furthermore, if the depth D of the groove G exceeds 50 μm, the thickness of the finish-annealed steel sheet 110 may be significantly reduced in some areas, making it impossible to achieve the iron loss reduction effect.

In the example shown in the flowchart of Figure 1, the groove formation process S9 is performed after the finish annealing process S8. However, the groove formation process S9 can also be performed on steel sheet that has undergone the cold rolling process S4 (i.e., cold-rolled steel sheet). In this case, too, the cross-sectional shape of the linear grooves G, which is ideal for magnetic domain refinement, can be maintained. Therefore, the groove formation process S9 can be performed either before or after the finish annealing process S8. However, the groove formation process S9 must be performed at least before the tension coating process S10.

In the modification shown in Fig. 4, the 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.

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

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

Before explaining the tension coating process S10 of this embodiment in detail, we will first explain the problems with the conventional tension coating process with reference to Figures 6 and 7.

Figure 6 shows an example of a conventional tension coating process. In the example shown in Figure 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 in a liquid pool inside the grooves G. 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). Figure 7 is an SEM photograph showing the cross-sectional structure of a grain-oriented electrical steel sheet 200 manufactured using a conventional tension 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).

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 impair the iron loss reduction effect of the grain-oriented electromagnetic steel sheet 200.

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

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.

In particular, when the coating solution is applied to the groove-forming surface 110a of the finish-annealed steel sheet 110, gravity creates a concave (or downward-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 Figure 6, when the coating solution is applied "facing upward," liquid pools are 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, further significantly compromising the iron loss reduction effect.

In contrast, in the tension coating process S10 according to this embodiment, as shown in FIG. 5, the groove-formed surface 110a of the finish-annealed steel sheet 110 is "facing downward" and the coating solution is applied to the groove-formed surface 110a, followed by baking. Note that the finish-annealed steel sheet 110 is transported by the transport rollers 1000, as in the conventional method.

In this embodiment, the coating solution tends to accumulate in the grooves G, and gravity creates a concave catenary on the top surface of the finish-annealed steel sheet 110, just like in conventional tension coating processes (i.e., processes in which the coating solution is applied with the groove-forming surface facing upward).

However, because the grooves G face downward, some of the coating solution accumulated inside the grooves will drip due to gravity. Because the catenary has a downward convex shape, it is believed that the coating solution accumulated inside the grooves will drip more easily. 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 in the grain-oriented electrical steel sheet 200. Furthermore, because the insulating coating 130G inside the grooves can leak more magnetic flux, an improved iron loss reduction effect can be expected. As shown in the examples described below, when coating solution is applied with the groove-forming surface 110a facing downward and baking is performed, the rate of coating peeling in the grooves of the grain-oriented electrical steel sheet 200 can be reduced, and iron loss is significantly reduced.

Here, in the tension coating application process S10, it is preferable that the thickness t2 of the insulating coating 130G formed inside the groove be no more than half the depth D of the groove G and no more 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." Note that the depth D and thicknesses t1 and t2 are shown in Figure 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.

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 observed cross section of the groove G.

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.
The specific value of the thickness t1 of the insulating coating 130F is not particularly limited and may be set appropriately depending on the characteristics required of the grain-oriented electrical steel sheet 200, but 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.

The thickness t1 of the insulating coating 130F is preferably 10 μm or less, and more preferably 5 μm or less. This is 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.

It is preferable that the thickness t1, thickness t2, and depth D measured (determined) using the above method satisfy the "additional requirement for thickness t2" described above. Note that in the tensile coating process S10, thickness t2 is thinner than in conventional tensile coating processes in which the groove-forming surface faces upward, as described above. 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 using a roll coater, etc.), the time from application to baking, and the use of air blowing to remove some of the coating solution.

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

On the other hand, if the groove G is formed on the upper surface of the finish-annealed steel sheet 110 in the groove forming process S9, the finish-annealed steel sheet 110 can be wound up, then turned upside down, and then subjected to the tension coating process S10. In this case, the tension coating process S10 may be performed offline or inline.

In this example, the effects of the manufacturing method for grain-oriented electrical steel sheet according to the present embodiment described above were verified. Of course, the present invention is not limited to the example described below. It is clear that a person with ordinary skill in the technical field to which the present invention pertains can conceive of various modified or altered examples within the scope of the technical ideas set forth in the claims, and it is understood that these naturally fall within the technical scope of the present invention.

First, by carrying out the above-described casting process S1 to finish annealing process S8, a high magnetic flux density finish annealed steel sheet 110 having a plate thickness of 0.23 mm and a magnetic flux density _B8 value of 1.93 T at 800 A/m was produced.

Next, grooves G were formed on the surface of the finish-annealed steel sheet 110 by irradiating it with 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. 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.

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 application was performed using a normal natural coater.

The coating solution application amount indicates the mass (g/ ^m2 ) of the solid content of the coating solution applied per unit area in a plan view of the groove-formed surface 110a. To measure the coating solution application amount from the dried insulating coating, the grain-oriented electrical steel sheet is immersed in a heated alkaline solution (such as an NaOH solution) to dissolve the insulating coating in the alkaline solution. The coating solution application amount can then be determined from the mass of the insulating coating dissolved in the alkaline solution.

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.

Here, "groove insulation coating peeling rate" refers to the area ratio of the portion where the insulation coating has peeled off to the total area of the groove G when viewed from above. Whether or not the insulation coating had peeled off was determined based on the area ratio of the exposed steel plate surface. Specifically, an area ratio of 10% was considered a failure.

As is clear from Table 1, in the grain-oriented electrical steel sheets (Experiments Nos. 1 to 4) manufactured by the method for manufacturing a grain-oriented electrical steel sheet 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 (Experiments Nos. 5 to 7) manufactured by the conventional manufacturing method for grain-oriented electrical steel sheets, the coating solution was applied and baked with the groove-formed surface of the finish-annealed steel sheet facing upward, resulting in many coating defects and high iron loss.

It should be noted that the "additional requirement of thickness t2" was satisfied in Experiments No. 1 to 3. This is also considered to be the reason why the iron loss was low in the inventive examples in Experiments No. 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, the "additional requirement for thickness t2" was not met.) Compared to Experiments Nos. 1 to 3, Experiment No. 4 had slightly higher iron loss.

As explained 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.

Here, in the tension coating application process S10, the thickness t2 of the insulating coating 130G formed inside the groove may be adjusted so that the thickness t2 is no more than half the depth D of the groove G and no 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 groove becomes even thinner, further reducing the iron loss of the grain-oriented electrical steel sheet 200.

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

While 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 with ordinary skill in the technical field to which the present invention pertains could conceive of various modifications or 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.

200 Grain-oriented electromagnetic steel sheet 110 Finish annealed steel sheet 110F Flat surface 110G Groove wall surface 130 Insulating coating (tensile coating)
130F: Insulation coating on flat surface 130G: Insulation coating inside groove

Claims (3)

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. The method for manufacturing grain-oriented electrical steel sheet described in claim 1, characterized in that in the tension coating application process, the thickness of the tension coating formed inside the groove is adjusted so that the thickness of the tension coating formed inside the groove is no more than half the depth of the groove and no more than twice the thickness of the tension coating formed on the flat surface of the cold-rolled steel sheet. The method further includes 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.