JP7557127B2 - Grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to grain-oriented electrical steel sheets.

Grain-oriented electrical steel sheets generally contain 2% to 5% by mass of Si as a steel sheet component, and the orientation of the crystal grains of the steel sheet is highly concentrated in the {110}<001> orientation, known as the Goss orientation. Grain-oriented electrical steel sheets have excellent magnetic properties and are used, for example, as iron core materials for static inductors such as transformers.

Various technologies have been developed to improve the magnetic properties of such grain-oriented electrical steel sheets. In particular, with the recent demand for energy conservation, there is a demand for even lower iron loss in grain-oriented electrical steel sheets. To reduce iron loss in grain-oriented electrical steel sheets, it is effective to increase the concentration of crystal grains in the Goss orientation in the steel sheet to improve magnetic flux density and reduce hysteresis loss.

Grain-oriented electrical steel sheets, which are used as the base material for wound transformers, are particularly required to have even lower iron loss. In electrical steel sheets, magnetic domain refinement is performed to reduce iron loss, but wound transformers are annealed to relieve distortion during the manufacturing process, so when performing magnetic domain refinement, heat-resistant magnetic domain refinement technology is required.

One method for heat-resistant magnetic domain refinement is to form periodic grooves in steel sheet. For example, Patent Document 1 discloses that the iron loss of the resulting grain-oriented electrical steel sheet is reduced by suppressing the N content in the forsterite coating formed after final annealing to 3.0 mass% or less and performing a magnetic domain refinement process by laser irradiation. It also discloses that the iron loss improvement effect is further improved by suppressing compositional fluctuations of each forsterite by controlling the Al and Ti amounts in the forsterite coating and setting the standard deviation of the forsterite grain size to 1.0 times or less the average grain size.

Patent Document 2 describes how, by imparting linear scratches with a spacing of 5 mm or less, a width of 1 mm or less, and a depth of 0.3 to 5.0 μm in Ra value and 10 μm or less in Rmax value to the surface of the steel sheet before or after decarburization annealing, a grain-oriented electrical steel sheet with extremely excellent core loss and coating adhesion can be obtained.

Patent document 3 describes a grain-oriented electrical steel sheet having a steel sheet surface on which grooves are formed that extend in a direction intersecting the rolling direction and whose groove depth direction is the sheet thickness direction. The formed grooves have an asymmetric shape in the groove width direction with respect to the center of the groove width. It is disclosed that an iron loss reduction effect can be obtained by setting the average depth of grooves having such a cross-sectional shape, the arithmetic mean height Ra of the roughness curve that forms the outline of the groove bottom region of the groove, and the average length RSm of the roughness curve elements that form the outline of the groove bottom region within specific ranges.

JP 2012-31512 A Japanese Unexamined Patent Publication No. 1-198429 Re-table No. 2016-171130

Grain-oriented electrical steel sheets used as the base material for wound transformers are required to have even lower iron loss. Heat-resistant magnetic domain refinement is generally achieved by forming linear grooves, but at present this does not provide sufficient iron loss reduction.
If the depth of the linear grooves formed for magnetic domain refinement is not uniform, the magnetic wall area changes due to magnetic wall movement, resulting in a larger pinning effect than would be achieved if there were no grooves, and the iron loss reduction effect decreases. One of the reasons why the depth of the linear grooves is not uniform is thought to be the influence of precipitates in the steel sheet in which the grooves are formed.
In the present invention, it has been discovered that by performing a pickling treatment using a special pickling solution before forming the grooves, the specific heat and heat transfer coefficient of the precipitates on the steel sheet surface are brought closer to those of the base steel, thereby preventing unevenness in the groove shape due to differences in heat transfer.

The present invention has been made in consideration of the above problems. It aims to further reduce the iron loss of the resulting grain-oriented electrical steel sheet by controlling the shape of the linear grooves formed for magnetic domain refinement and the Ra value of the surface roughness of the groove bottom and groove side surfaces within a certain range.

The gist of the present invention is a steel sheet containing, in mass% , Si : 2.50 to 4.50%, Mn: 0.01 to 0.15% , C: 0.085% or less, acid-soluble Al: 0.065% or less, N: 0.012% or less, Cr: 0.3% or less, Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, S: 0.015% or less, Se: 0.015% or less, Bi: 0.02% or less , the balance being Fe and impurities, and having an angle of 0 to 30° with the direction perpendicular to the rolling direction. The grain-oriented electrical steel sheet for wound transformers comprises a steel sheet having a steel sheet surface on which linear grooves extending in a direction forming an angle with respect to the surface are formed at intervals of 1 to 20 mm, the grooves having a depth D of 10 to 50 μm, a groove width W of 20 to 200 μm, a surface roughness Ra value of the groove bottom surface of 0.1 to 5.0 μm, and a surface roughness Ra value of the groove side surface of 0.1 to 5.0 μm.

According to the present invention, the grooves formed by laser irradiation are made uniform, which reduces the pinning effect of the magnetic domain walls and reduces iron loss.

FIG. 1 is a cross-sectional view of a groove in a grain-oriented electrical steel sheet according to the present invention.

The preferred embodiment of the present invention will be described in detail below. Unless otherwise specified, the notation "A to B" for numerical values A and B means "greater than or equal to A and less than or equal to B." In such notation, when a unit is added only to numerical value B, the unit is also applied to numerical value A.

[Steel plate composition]
First, the chemical composition of the steel sheet used in the grain-oriented electrical steel sheet according to the present invention will be described.
In the following, unless otherwise specified, the notation "%" represents "mass %." The balance of the steel sheet other than the elements described below is Fe and impurities.

The steel sheet used in the grain-oriented electrical steel sheet according to the present invention has a composition preferable for controlling the crystal orientation to a Goss texture concentrated in the {110}<001> orientation , and contains Si : 2.50-4.50% and Mn: 0.01-0.15%.

(Si: 2.50-4.50%)
The content of Si (silicon) is 2.50 to 4.50%. Si increases the electrical resistance of the steel sheet, thereby reducing eddy current loss, which is one of the causes of iron loss. If the Si content is less than 2.50%, it is difficult to sufficiently suppress the eddy current loss of the final grain-oriented electrical steel sheet, which is not preferable. However, this is not preferable because it reduces the workability of the grain-oriented electrical steel sheet. Therefore, the Si content is 2.50 to 4.50%, and preferably 2.70 to 4.00%.

(Mn: 0.01-0.15%)
The Mn (manganese) content is 0.01 to 0.15%. Mn forms MnS and MnSe, which are inhibitors that affect secondary recrystallization. If the Mn content is less than 0.15%, the absolute amounts of MnS and MnSe that cause secondary recrystallization are insufficient, which is not preferable. If the Mn content exceeds 0.15%, it becomes difficult for Mn to dissolve in the slab during heating. In addition, if the Mn content exceeds 0.15%, the precipitate size of MnS and MnSe, which act as inhibitors, tends to become coarse, and the optimum size distribution as an inhibitor is impaired, which is also undesirable. The content of is 0.01 to 0.15%, preferably 0.03 to 0.13%.

The components other than Si and Mn may be the components contained in ordinary grain-oriented electrical steel sheets.
For example, as components other than Si and Mn, the alloy may contain, by mass%, C : 0.085 % or less, acid-soluble Al : 0.065 % or less, N : 0.012 % or less, Cr : 0.3 % or less, Cu : 0.4 % or less, P : 0.5% or less, Sn : 0.3% or less, Sb : 0.3% or less, Ni: 1% or less, S : 0.015% or less, Se : 0.015 % or less, and Bi : 0.02 % or less.

The remainder of the steel plate other than the above components is Fe and impurities. Here, impurity elements refer to components contained in the raw materials or components mixed in during the manufacturing process, but not intentionally included in the steel plate.

To subdivide the magnetic domains, grooves are formed on the surface of the steel sheet, which extend in a direction intersecting the rolling direction and whose depth direction is in the sheet thickness direction. The grooves only need to be arranged so as to intersect with the rolling direction, and the groove extension direction does not necessarily have to be perpendicular to the rolling direction, but are arranged in a direction that forms an angle of 0 to 30° with the direction perpendicular to the rolling direction. Furthermore, when viewed from the sheet thickness direction (when the grooves are viewed in plan), the grooves do not necessarily have to have a linear shape, and may have an arched shape. The groove shape measurements described below are performed after removing the insulating coating from the final product by pickling or the like.

The grooves are formed on the surface of the steel sheet at intervals of 1 to 20 mm. If the groove spacing is less than 1 mm, the magnetic domain refinement effect saturates and the effect of reducing eddy current loss is hardly obtained, while hysteresis loss increases due to distortion, resulting in increased iron loss, which is undesirable. If the groove spacing exceeds 20 mm, the magnetic domain refinement effect decreases, resulting in insufficient iron loss improvement, which is undesirable. The preferred groove spacing is 2 to 10 mm.

Figure 1 is a cross-sectional view of a groove in an electromagnetic steel sheet of the present invention. In this figure, the groove is shaped like a trapezoid, but the groove shape may be arched. In one embodiment of the electromagnetic steel sheet of the present invention, the depth D of the groove is in the range of 10 μm to 50 μm. If the depth D is less than 10 μm, the amount of magnetic poles generated from the groove wall surface is small, and sufficient iron loss reduction effect cannot be obtained. If the depth D exceeds 50 μm, the magnetic domains are subdivided, but the decrease in magnetic flux density due to the formation of the groove is large, and sufficient iron loss reduction effect cannot be obtained. The preferred depth is 15 μm to 30 μm.

(Measurement of Groove Depth D)
The method for measuring the "depth D" according to the present invention is as follows.
An arbitrary location on the electrical steel sheet was selected, and the maximum depths of two arbitrary grooves (A) and (B) spaced 3 mm apart as shown in the groove cross-sectional view of Fig. 1 were measured using a laser microscope (a 3D laser microscope using a pinhole confocal optical system), with the deeper being depth d and the shallower being depth d'. Depth D is the average value of these values.

The "groove width W" in this invention refers to the portions indicated by w and w' in FIG. 1. The groove width W is in the range of 20 μm to 200 μm. If the width W is less than 20 μm, magnetic flux leaking from the groove wall enters the opposite groove wall, reducing the amount of magnetic pole generated and failing to obtain a sufficient iron loss reduction effect. If the width W exceeds 200 μm, the iron loss reduction effect saturates, and the laser power required to form the groove increases, resulting in increased manufacturing costs. The preferred width W is 30 to 100 mm.

(Measurement of groove width W)
The method for measuring the "groove width W" according to the present invention is as follows.
An arbitrary location on the electrical steel sheet was selected, and the widths at which the groove depth of two arbitrary points (A) and (B) separated by 3 mm as shown in the groove cross-sectional view of Figure 1 were halved were measured using a laser microscope (a 3D laser microscope using a pinhole confocal optical system), with the wider one being w and the narrower one being w'. The groove width W is the average value of these values.

In the present invention, the "surface roughness Ra value of the groove bottom surface and groove side surface" refers to the value of the arithmetic mean height Ra of the roughness curve of the groove bottom surface and groove side surface between two points (A) and (B) in FIG. 1. The definition of the arithmetic mean height Ra of the roughness curve is in accordance with the Japanese Industrial Standard JIS B 0601 (2013). In the grain-oriented electrical steel sheet of the present invention, each Ra is 0.1 to 5.0 μm, preferably 0.1 to 3.0 μm, and more preferably 0.1 to 1.0 μm. If Ra is less than 0.1 μm, there is no problem with the magnetic properties, but it is difficult to achieve from a manufacturing technology perspective. If Ra exceeds 5.0 μm, the pinning effect of the domain wall becomes large, the hysteresis loss increases, and sufficient iron loss reduction effect cannot be obtained.

In the present invention, the reason why the effect of reducing iron loss is achieved by setting the surface roughness Ra of the groove bottom and groove side surfaces within the range of 0.1 to 5.0 μm is believed to be that pinning of the domain walls by the grooves is almost eliminated, suppressing the increase in hysteresis loss.

(Method of measuring surface roughness Ra of groove bottom surface and groove side surface)
Each Ra was measured using a laser microscope (a 3D laser microscope using a pinhole confocal optical system). An arbitrary location of the electromagnetic steel sheet was selected, and the unevenness of the groove bottom surface of a straight line connecting the maximum depth points d and d' of two groove parts (A) and (B) that are 3 mm apart as shown in the groove cross-sectional view of FIG. 1 was taken as the surface roughness Ra of the groove bottom surface of the present invention. In addition, the Ra and Ra' of the unevenness of the groove side surface of a straight line connecting points on the same side of the maximum depth among the points (d/2, d'/2) that are 1/2 the depth of the maximum depths d and d' of both groove parts were derived, and the average value of these values was taken as the Ra of the groove side surface.

[Method of manufacturing grain-oriented electrical steel sheet]
The manufacturing process of the grain-oriented electrical steel sheet of the present invention will be described below by dividing it into a process for obtaining a cold-rolled steel sheet and a subsequent magnetic domain control process.

[Process from slab to obtaining cold-rolled steel sheet]
A slab containing, in mass % , 2.50% to 4.50% Si , 0.01% to 0.15% Mn, with the balance being Fe and impurities, is hot-rolled to obtain a hot-rolled steel sheet.
Next, the hot-rolled steel sheet is pickled to obtain a pickled sheet, or the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet, and then the hot-rolled annealed sheet is pickled to obtain a pickled sheet. The pickling solution used here contains one or more of Cu, Hg, Ag, Pb, Cd, Co, Zn, and Ni, the total concentration of each element is 0.0001 to 0.1000%, and the pH is -1 to 5. The temperature of the pickling solution is 15°C to 100°C, and the steel sheet is immersed in the pickling solution for 5 seconds to 200 seconds. A pickled sheet is obtained by this pickling process, and the pickled sheet is cold-rolled to obtain a cold-rolled steel sheet.

[Slab composition]
The components of the slab used in the production of the grain-oriented electrical steel sheet according to the present invention contain Si : 2.50 to 4.50%, Mn: 0.01 to 0.15%.

The Si (silicon) content is 2.5-4.5%. Si increases the electrical resistance of the steel sheet, thereby reducing eddy current loss, which is one of the causes of iron loss. If the Si content is less than 2.5%, it is not preferable because it becomes difficult to sufficiently suppress eddy current loss in the final grain-oriented electrical steel sheet. If the Si content is more than 4.5%, it is not preferable because the workability of the grain-oriented electrical steel sheet decreases. Therefore, the Si content is 2.5%-4.5%, and preferably 2.7-4.0%.

The Mn (manganese) content is 0.01 to 0.15%. Mn forms MnS and MnSe, which are inhibitors that affect secondary recrystallization. If the Mn content is less than 0.01%, the absolute amount of MnS and MnSe that cause secondary recrystallization is insufficient, which is not preferable. If the Mn content is more than 0.15%, it is not preferable because it becomes difficult for Mn to form a solid solution during slab heating. Also, if the Mn content is more than 0.15%, it is not preferable because the precipitate size of MnS and MnSe, which are inhibitors, tends to become coarse, which impairs the optimal size distribution as an inhibitor. Therefore, the Mn content is 0.01 to 0.15%, and preferably 0.03 to 0.13%.

The components other than Si and Mn can be the following components.
For example, as components other than Si and Mn, the alloy may contain, in mass%, C: 0.02 to 0.10%, the total of one or two of S and Se: 0.001 to 0.050%, acid-soluble Al: 0.01 to 0.05%, N: 0.002 to 0.015%, Cr : 0.3 % or less, Cu : 0.4 % or less, P : 0.5 % or less, Sn : 0.3 % or less, Sb : 0.3 % or less, Ni: 1% or less, S : 0.015 % or less, Se : 0.015 % or less, and Bi : 0.02 % or less.

The C (carbon) content is 0.02-0.10%. C has various roles, but if the C content is less than 0.02%, the grain size becomes excessively large when the slab is heated, which increases the iron loss value of the final grain-oriented electrical steel sheet, which is not preferable. If the C content exceeds 0.10%, the decarburization time becomes long during decarburization after cold rolling, which increases the manufacturing cost, which is not preferable. Also, if the C content exceeds 0.10%, decarburization is likely to be incomplete, which is not preferable because it may cause magnetic aging in the final grain-oriented electrical steel sheet. Therefore, the C content is 0.02-0.10%, and preferably 0.05-0.09%.

The total content of S (sulfur) and Se (selenium) is 0.001-0.050%. S and Se form inhibitors together with the above-mentioned Mn. Both S and Se may be contained in the slab, but it is sufficient that at least one of them is contained in the slab. If the total content of S and Se is outside the above range, it is not preferable because a sufficient inhibitor effect cannot be obtained. Therefore, the total content of S and Se is 0.001-0.050%, and preferably 0.001-0.040%.

The content of acid-soluble Al (acid-soluble aluminum) is 0.01 to 0.05%. Acid-soluble Al constitutes an inhibitor necessary for producing grain-oriented electrical steel sheets with high magnetic flux density. If the content of acid-soluble Al is less than 0.01%, the amount of acid-soluble Al is insufficient, and the inhibitor strength is insufficient, which is not preferable. If the content of acid-soluble Al is more than 0.05%, the AlN precipitated as an inhibitor becomes coarse, which is not preferable, as it reduces the inhibitor strength. Therefore, the content of acid-soluble Al is 0.01 to 0.05%, and preferably 0.01 to 0.04%.

The N (nitrogen) content is 0.002-0.015%. N forms AlN, an inhibitor, together with the acid-soluble Al mentioned above. If the N content is outside the above range, it is not preferable because a sufficient inhibitor effect cannot be obtained. Therefore, the N content is 0.002-0.015%, and preferably 0.002-0.012%.

In addition, in order to improve magnetic properties, the slab used in the manufacture of the grain-oriented electrical steel sheet according to this embodiment may contain, in mass%, one or more selected from the group consisting of Cu: 0.4% or less, P: 0.5% or less, Sn: 0.3% or less, Sb: 0.3% or less, Ni: 1% or less, S: 0.015% or less, Se: 0.015% or less, and Bi: 0.02% or less, in place of a portion of the remaining Fe, in addition to the above-mentioned elements. In the slab according to one embodiment, the Cr content may be 0.02% or more, the Bi content may be 0.0005% or more, the Sb content may be 0.005% or more, the Se content may be 0.001% or more, and the Mo content may be 0.005% or more, in mass%.

A slab is formed by casting molten steel adjusted to the composition described above. The method of casting the slab is not particularly limited. In research and development, even if a steel ingot is formed in a vacuum melting furnace or the like, the same effects can be confirmed for the above-mentioned components as when a slab is formed.

[Process for producing hot-rolled steel sheet]
The cast slab is heated at a predetermined temperature, and the heated slab is hot-rolled to be processed into a hot-rolled steel sheet. The thickness of the hot-rolled steel sheet after processing may be, for example, 1.8 mm to 3.5 mm. If the thickness of the hot-rolled steel sheet is less than 1.8 mm, the steel sheet temperature after hot rolling becomes low, and the amount of AlN precipitation in the steel sheet increases, making secondary recrystallization unstable, which is not preferable because the magnetic properties of the grain-oriented electrical steel sheet with a final thickness of 0.23 mm or less are reduced. If the thickness of the hot-rolled steel sheet is more than 3.5 mm, the rolling load in the cold rolling process becomes large, which is not preferable.

[Shot blasting process]
Prior to the pickling treatment, defects such as cracks are introduced into the surface of the steel sheet by a treatment such as shot blasting, so that the pickling solution reaches a certain depth range in the subsequent pickling treatment.
The pickling solution is made to penetrate the steel sheet to a certain depth in order to replace or coat the precipitated MnS with CuS, etc. This makes it possible to replace or coat MnS with CuS, etc., and thus makes the heat transfer coefficient of the steel sheet surface constant, thereby suppressing the variation in groove depth.
As a method for introducing defects such as cracks into the surface of a steel sheet, a leveller or the like can be used in addition to shot blasting.

The conditions for shot blasting include, for example, using a mechanical projector to project 1,000 kg of iron balls with a hardness of about Hv500 and a diameter of about 1.5 mm at a projection speed of 50 m/s per minute, but the conditions are not critical as long as fine cracks are introduced into the steel plate that allow the pickling solution to penetrate.

[Pickling process]
Subsequently, the processed hot-rolled steel sheet is pickled or hot-rolled sheet annealed to obtain a hot-rolled annealed sheet, which is then pickled.

The pickling solution contains one or more of Cu, Hg, Ag, Pb, Cd, Co, Zn and Ni, the total concentration of each element is 0.0001-0.1000%, and the pH is -1 or more and 5 or less. The temperature of the pickling solution is 15°C or more and 100°C or less, and the steel sheet is immersed in the pickling solution for 5 seconds or more and 200 seconds or less.

If the total concentration of one or more of Cu, Hg, Ag, Pb, Cd, Co, Zn and Ni in the pickling solution is less than 0.0001%, the inhibitor control effect in the thickness direction will be insufficient, which is not preferable. If the total concentration of one or more of Cu, Hg, Ag, Pb, Cd, Co, Zn and Ni in the pickling solution is more than 0.1000%, the effect of improving magnetic properties will saturate and the cost of the pickling solution will increase, which is not preferable. Therefore, the total concentration of one or more of Cu, Hg, Ag, Pb, Cd, Co, Zn and Ni in the pickling solution is 0.0001 to 0.1000%.

If the pH of the pickling solution is less than -1, it is undesirable because the acidity becomes too strong and the handling of the pickling solution becomes difficult. If the pH of the pickling solution is more than 5, the effect of the pickling process in controlling inhibitors in the thickness direction becomes insufficient, which is undesirable. Therefore, the pH of the pickling solution is from -1 to 5.

If the temperature of the pickling solution is less than 15°C, the effect of the pickling process in controlling inhibitors in the thickness direction will be insufficient, which is not preferable. If the temperature of the pickling solution is more than 100°C, it will be difficult to handle the pickling solution, which is not preferable. Therefore, the temperature of the pickling solution is 15°C or higher and 100°C or lower.

If the time during which the steel sheet is immersed in the pickling solution during the pickling process is less than 5 seconds, the effect of the pickling process in controlling inhibitors in the thickness direction will be insufficient, which is not preferable. If the time during which the steel sheet is immersed in the pickling solution during the pickling process exceeds 200 seconds, the equipment will become long and large, which is not preferable. Therefore, the time during which the steel sheet is immersed in the pickling solution during the pickling process is 5 seconds or more and 200 seconds or less.

When pickling is performed under the conditions of this application, the MnS precipitates in the steel are replaced or coated with sulfides such as CuS, and the heat transfer coefficient of the steel sheet surface containing the precipitates becomes constant. This makes it possible to make the effect of heat from laser irradiation, etc. constant. Therefore, if the sulfides are not sufficiently replaced or coated by pickling, there will be a large variation in the heat transfer coefficient of the steel sheet surface, and the Ra of the groove bottom and side surfaces will increase. On the other hand, even if the pickling time or pickling solution concentration is set to a certain level or higher, the effect of stabilizing the heat transfer coefficient will saturate and no further reduction in Ra can be expected.

[Process for producing cold-rolled steel sheet]
The hot-rolled steel sheet is subjected to pickling and then rolled by one cold rolling or multiple cold rolling with intermediate annealing therebetween to be processed into a cold-rolled steel sheet.
In addition, the steel sheet may be heat-treated at about 300° C. or less between passes of cold rolling, between rolling roll stands, or during rolling. In such a case, the magnetic properties of the final grain-oriented electrical steel sheet can be improved. The hot-rolled steel sheet may be rolled by three or more cold rolling passes, but since multiple cold rolling passes increase the manufacturing cost, it is preferable to roll the hot-rolled steel sheet by one or two cold rolling passes. When cold rolling is performed by reverse rolling such as a Sendzimir mill, the number of passes in each cold rolling pass is not particularly limited, but is preferably 9 or less from the viewpoint of manufacturing cost.
The process from the slab to obtaining a cold-rolled steel sheet has been described above.

Next, decarburization annealing is performed. The cold-rolled steel sheet is subjected to heat treatment (i.e., decarburization annealing) under a predetermined temperature condition (for example, heating at 700 to 900°C for 1 to 3 minutes). When the decarburization annealing is performed, the carbon in the cold-rolled steel sheet is reduced to a predetermined amount or less, and a primary recrystallization structure is formed. In addition, in the decarburization annealing, an oxide layer containing silica (SiO ₂ ) as a main component is formed on the surface of the cold-rolled steel sheet.

Next, an annealing separator is applied to the surface of the cold-rolled steel sheet (the surface of the oxide layer).
Next, the cold-rolled steel sheet coated with the annealing separator is subjected to heat treatment (i.e., finish annealing) under a predetermined temperature condition (for example, heating at 1100 to 1300°C for 20 to 24 hours). When the finish annealing is performed, secondary recrystallization occurs in the cold-rolled steel sheet, and the cold-rolled steel sheet is purified. As a result, a cold-rolled steel sheet is obtained that has the above-mentioned chemical composition of the steel sheet and whose crystal orientation is controlled so that the magnetization easy axis of the crystal grains coincides with the rolling direction X.

Furthermore, when the above-mentioned final annealing treatment is performed, the oxide layer containing silica as a main component reacts with the annealing separator containing magnesia as a main component to form a glass film containing composite oxides such as _forsterite ( _Mg2SiO4 ) on the surface of the steel sheet. In the final annealing process, the final annealing treatment is performed with the steel sheet wound in a coil shape. The formation of a glass film on the surface of the steel sheet during the final annealing treatment can prevent the occurrence of seizure on the coiled steel sheet.

[Step of forming linear grooves on steel sheet surface]
In the subsequent laser irradiation process, a laser is irradiated onto the surface (only one side) of the steel plate on which the glass coating has been formed, to form a plurality of grooves on the surface of the steel plate extending in a direction intersecting the rolling direction at intervals of 1 to 20 mm along the rolling direction.

In the laser irradiation process, the laser irradiation device rotates a polygon mirror to irradiate the surface of the steel plate with laser light, and scans the laser light in a direction that forms an angle of 0 to 30 degrees with the direction perpendicular to the rolling direction.

At the same time as the laser light is irradiated, an assist gas such as air or an inert gas is sprayed onto the area of the steel sheet where the laser light is irradiated. Examples of inert gas include nitrogen or argon. The assist gas serves to remove components that have melted or evaporated from the steel sheet due to the laser irradiation. By spraying the assist gas, the laser light can reach the steel sheet without being obstructed by the melted or evaporated components, so that grooves are formed stably.

As the laser light source, for example, a high-power laser generally used for industrial purposes, such as a fiber laser, a YAG laser, a semiconductor laser, or a _CO2 laser, can be used. In addition, a pulsed laser or a continuous wave laser may be used as the laser light source as long as it can stably form a groove. As the laser light, it is preferable to use a single mode laser that has high light-collecting ability and is suitable for forming a groove.

As the conditions for the laser light irradiation, it is preferable to set, for example, the laser output to 200 W to 3000 W, the focused spot diameter of the laser light in the rolling direction (i.e. the diameter containing 86% of the laser output, hereinafter abbreviated as 86% diameter) to 10 μm to 200 μm, the focused spot diameter of the laser light in the plate width direction (86% diameter) to 10 μm to 1000 μm, and the laser scanning speed to 5 m/s to 50 m/s. These laser irradiation conditions are appropriately adjusted so that a groove depth D of 10 to 50 μm is obtained.

In the final insulating film forming process, an insulating coating liquid containing, for example, colloidal silica and phosphate is applied from above the glass film to the steel sheet surface on which grooves have been formed by the above-mentioned laser irradiation. Heat treatment is then carried out under specified temperature conditions (for example, 840 to 920°C), ultimately obtaining the steel sheet on which grooves have been formed, the glass film, and the insulating film according to the present invention.

The groove depth D, groove width W, and surface roughness Ra of the groove bottom and side surfaces were measured for the groove shape formed in the obtained grain-oriented electrical steel sheet using the measurement method described above.

The grain-oriented electrical steel sheet of the present invention will be described in more detail below, showing examples. Note that the examples shown below are merely examples of the grain-oriented electrical steel sheet according to this embodiment, and the grain-oriented electrical steel sheet according to this embodiment is not limited to the examples shown below.

The slab contained, by mass fraction, 3.0% Si, 0.08% C, 0.05% acid-soluble Al, 0.01% N, 0.12% Mn, 0.05% Cr, 0.04% Cu, 0.01% P, 0.02% Sn, 0.01% Sb, 0.005% Ni, 0.007% S, 0.001% Se, with the remainder being Fe and impurities. Hot rolling was performed on the slab, and a hot-rolled steel sheet with a thickness of 2.3 mm was obtained.

Next, the hot-rolled steel sheets were annealed at 1000°C for 1 minute.

After annealing, defects such as cracks were introduced into the steel plate surface using the shot blasting process described above.

After the shot blasting, the surface of the hot-rolled steel sheet was pickled using the pickling solution and conditions shown in Table 1, and then cold rolling was performed to obtain a cold-rolled steel sheet with a thickness of 0.23 mm. Next, this cold-rolled steel sheet was subjected to a decarburization annealing treatment under the temperature condition of heating at 800°C for 2 minutes, and then an annealing separator containing magnesia (MgO) as the main component was applied to the surface of the cold-rolled steel sheet.

The components, concentrations, pH values, and immersion times of the pickling solutions used are shown in Table 1.

Then, the cold-rolled steel sheet coated with the annealing separator was subjected to a final annealing treatment under the temperature conditions of heating at 1200°C for 20 hours. As a result, a steel sheet was obtained with the above-mentioned chemical composition, a glass film formed on the surface, and a crystal orientation controlled so that the magnetization easy axis of the crystal grains coincided with the rolling direction.

Next, a laser was irradiated onto the surface of the steel plate on which the glass film was formed, forming multiple grooves on the surface of the steel plate that extended in a direction intersecting the rolling direction and at predetermined intervals along the rolling direction.

The laser light irradiation conditions were adjusted so that the desired groove depth D was obtained: laser output in the range of 200 W to 3000 W, the focused spot diameter (86% diameter) of the laser light in the rolling direction in the range of 10 μm to 500 μm, the focused spot diameter (86% diameter) of the laser light in the plate width direction in the range of 10 μm to 1000 μm, the laser scanning speed in the range of 5 m/s to 50 m/s, and the laser scanning pitch (spacing PL) in the range of 1 mm to 20 mm.

As described above, an insulating coating liquid containing colloidal silica and phosphate was applied to the steel sheet with grooves on top of the glass film, and then the steel sheet was heat-treated at 850°C for 1 minute. Finally, a grain-oriented electrical steel sheet with grooves, glass film, and insulating film was obtained.

As a comparative example, the results of pickling using a pickling solution outside the scope of the present invention are shown.

The groove depth D, groove width W, and surface roughness Ra of the groove bottom and side surfaces were measured for the groove shape formed using the measurement method described above. The measurement results are shown in Table 2 together with the pickling conditions and iron loss W17/50.

These results show that by carrying out the pickling treatment of the embodiment (invention example), the surface roughness Ra value of the groove bottom and groove side surfaces was controlled within a certain range, and iron loss was further reduced compared to the comparative example.

Claims (1)

In mass percent,
Si : 2.50 to 4.50%,
Mn: 0.01~0.15% ,
C: 0.085% or less,
Acid-soluble Al: 0.065% or less,
N: 0.012% or less,
Cr: 0.3% or less,
Cu: 0.4% or less,
P: 0.5% or less,
Sn: 0.3% or less,
Sb: 0.3% or less,
Ni: 1% or less,
S: 0.015% or less,
Se: 0.015% or less,
Bi: 0.02% or less
and the balance being Fe and impurities, the grain-oriented electrical steel sheet having a steel sheet surface on which linear grooves extending in a direction forming an angle of 0 to 30° with a rolling direction perpendicular to the rolling direction are formed at intervals of 1 to 20 mm,
The depth D of the groove is 10 to 50 μm,
The groove width W is 20 to 200 μm,
the surface roughness Ra value of the bottom surface of the groove is 0.1 to 5.0 μm;
The grain-oriented electrical steel sheet for wound transformers has a surface roughness Ra value of the groove side surface of 0.1 to 5.0 μm.