EP4793372A1

EP4793372A1 - Method for producing grain-oriented electrical steel sheet, grain-oriented electrical steel sheet, and industrial product

Info

Publication number: EP4793372A1
Application number: EP24877036.4A
Authority: EP
Inventors: Hideyuki Hamamura; Masato Yasuda; Nobusato Morishige
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-10-11
Filing date: 2024-09-27
Publication date: 2026-08-19
Also published as: CN121889518A; WO2025079437A1; JPWO2025079437A1; KR20260052130A

Abstract

Provided is a method for producing a grain-oriented electrical steel sheet in which a plurality of grooves extending in a direction generally parallel to the width direction is formed in the longitudinal direction of the steel sheet, the method including: a first groove formation step for forming a first groove comprising a plurality of grooves which extends in a direction generally parallel to the width direction on a cold-rolled steel sheet; a secondary recrystallization annealing step for annealing the cold-rolled steel sheet, so as to align the easy axis of magnetization of the cold-rolled steel sheet in the longitudinal direction to form a grain-oriented electrical steel sheet; and a second groove formation step for forming a second groove comprising a plurality of grooves which extends in a direction generally parallel to the width direction on the grain-oriented electrical steel sheet using a contact-type groove formation process.

Description

Technical Field

The present disclosure relates to a method for manufacturing a grain-oriented electrical steel sheet, a grain-oriented electrical steel sheet, and an industrial product.

Background Art

In order to improve electromagnetic properties of a grain-oriented electrical steel sheet, a stress relief annealing (SRA) magnetic domain control method has been used, which enables a magnetic domain control with enhanced heat resistance and iron loss reduction by providing periodic grooves in the longitudinal direction of the grain-oriented electrical steel sheet. In such an SRA magnetic domain control method, several processes and groove forming methods have been proposed.

Citation List

Patent Literature

PTL 1
Japanese Patent Gazette No. S62-53579
PTL 2
Japanese Patent No. 6826604

Summary of Invention

Technical Problem

The magnetic properties required of grain-oriented electrical steel sheets are a large iron loss reduction (low iron loss) effect and a high magnetic flux density, but these are generally contradictory properties. That is, in order to reduce the iron loss, it is desirable to provide a deep groove by a laser or etching that does not cause mechanical wear. However, on the other hand, making the grooves deeper unfortunately increases the loss of the base material, which reduces magnetic flux density. In addition, when grooves are provided by the mechanical processing, the loss of the base material is not large, and therefore the decrease in the magnetic flux density can be suppressed, but there is a problem such that the mechanical processing apparatus wears out due to contact with the steel sheet. The deeper the provided groove are, the greater the wear of the mechanical processing apparatus.
That is, in the case of performing the magnetic domain control, in order to obtain sufficient iron loss reduction, a required groove depth is 20 to 25 µm, and a groove pitch is 3 to 4 mm. However, when a base material is laser-processed according to the forming conditions described above, the magnetic flux density B8 is decreased by the removal of the base material. On the other hand, when a base material is mechanically processed according to the forming conditions, the decrease in the magnetic flux density B8 can be avoided to some extent by the pressing of the base material, but it is difficult to obtain the groove depth required for the iron loss reduction from the viewpoint of wear of the mechanical processing apparatus.
In view of the above-described problems, an object of the present invention is to provide a method for manufacturing a grain-oriented electrical steel sheet that achieves both iron loss reduction and a sufficient magnetic flux density and to provide a grain-oriented electrical steel sheet that achieves both iron loss reduction and a sufficient magnetic flux density.

Solution to Problem

In order to achieve the above object, according to an aspect of the present invention, the following is provided:
A method for manufacturing a grain-oriented electrical steel sheet in which a plurality of grooves are formed in a longitudinal direction of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to a width direction of the grain-oriented electrical steel sheet, the method including:

a first groove forming of forming a first groove composed of a plurality of grooves on a cold-rolled steel sheet serving as a material of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to the width direction;
a secondary recrystallization annealing of obtaining a grain-oriented electrical steel sheet by annealing the cold-rolled steel sheet including the first groove formed thereon to align an easy magnetization axis of the cold-rolled steel sheet with the longitudinal direction; and
a second groove forming of forming, by using groove forming processing of a contact type, a second groove composed of a plurality of grooves on the grain-oriented electrical steel sheet obtained in the secondary recrystallization annealing, the plurality of grooves each extending in a direction substantially parallel to the width direction.

The first groove forming may form the first groove by groove forming processing of a non-contact type.
The groove forming processing of the non-contact type may form the first groove by using any one of a laser, an electron beam, and plasma.
The first groove forming may form the first groove by groove forming processing of a mechanical processing type.
The first groove may be deeper than the second groove.
In order to achieve the above object, according to another aspect of the present invention, the following is provided:

A grain-oriented electrical steel sheet in which a plurality of grooves are formed in a longitudinal direction of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to a width direction of the grain-oriented electrical steel sheet, in which
the plurality of grooves are formed of a first groove in which a fine grain is not formed at a bottom of the first groove and a second groove in which a fine grain is formed at the bottom of the second groove.

The first groove may be deeper than the second groove.
A subgrain boundary factor does not have to be formed in either the first groove or the second groove.
An industrial product manufactured by using the above-described grain-oriented electrical steel sheet is provided.

Advantageous Effects of Invention

The present invention can provide a method for manufacturing a grain-oriented electrical steel sheet that achieves both iron loss reduction and a sufficient magnetic flux density and a grain-oriented electrical steel sheet that achieves both iron loss reduction and a sufficient magnetic flux density.

Brief Description of Drawings

FIG. 1 is a schematic view of a grain-oriented electrical steel sheet according to an embodiment of the present invention;
FIG. 2 is a flowchart for describing a method for manufacturing a grain-oriented electrical steel sheet according to the embodiment of the present invention;
FIGS. 3A and 3B are schematic views of the grain-oriented electrical steel sheet according to the embodiment of the present invention;
FIGS. 4A and 4B are schematic views of the grain-oriented electrical steel sheet according to the embodiment of the present invention;
FIGS. 5A and 5B are schematic views of a grain-oriented electrical steel sheet according to the embodiment of the present invention;
FIG. 6 illustrates the relationship between groove depth and iron loss reduction rate in the case of forming grooves by a laser method alone and the case of forming grooves by a mechanical method alone;
FIG. 7 illustrates the relationship between groove depth and B8 deterioration amount in the case of forming grooves by the laser method alone and the case of forming grooves by the mechanical method alone;
FIG. 8 illustrates the iron loss reduction rate in the case of combining the laser method and the mechanical method according to the embodiment of the present invention;
FIG. 9 illustrates the relationship between groove depth and iron loss reduction rate in combinations according to the embodiment of the present invention;
FIG. 10 illustrates B8 deterioration amount in the case of combining the laser method and the mechanical method according to the embodiment of the present invention; and
FIG. 11 illustrates the relationship between groove depth and B8 deterioration amount in the combinations according to the embodiment of the present invention.

Description of Embodiments

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In the present specification and the drawings, components having substantially the same functional configuration are designated by the same reference numeral to omit overlapping descriptions.
As the details will be described below, in the embodiment of the present invention, grain-oriented electrical steel sheet 10 is manufactured by secondary recrystallization annealing of a cold-rolled steel sheet to align the easy magnetization axis to convert the cold-rolled steel sheet into the grain-oriented electrical steel sheet. Therefore, in the following description, a cold-rolled steel sheet serving as a material of grain-oriented electrical steel sheet 10 and a grain-oriented electrical steel sheet in which the crystal orientation is aligned after the secondary recrystallization may be collectively or individually simply referred to as a steel sheet.

[Grain-oriented electrical steel sheet]

First, grain-oriented electrical steel sheet 10 according to the embodiment of the present invention will be described. Grain-oriented electrical steel sheet 10 according to the embodiment of the present invention is grain-oriented electrical steel sheet 10 in which a plurality of grooves are formed in the longitudinal direction of a steel sheet and each groove extends in a direction substantially parallel to the width direction of the steel sheet. Here, the plurality of grooves are formed of first grooves 20 in which fine grains are not formed at the bottom of the first groove and second grooves 30 in which fine grains are formed at the bottom of the second groove. Here, a fine grain is a crystal grain that has been made finer in size than the periphery of the bottom of the groove by partial mechanical processing or the like. The size of the fine grain is generally 20 to 200 µm, and may be, for example, about 50 µm. As a method for measuring the grain diameter, for example, the grain diameter is measured by a cutting method of JIS G0551:2020 in a plane parallel to a rolling surface at a depth of 1/2 of a sheet thickness from the steel sheet surface. As the grain-oriented electrical steel sheet, for example, a grain-oriented electrical steel strip of JIS C 2553:2012 can be used.
First groove 20 and second groove 30 formed on the surface of grain-oriented electrical steel sheet 10 are each formed to extend in a direction substantially parallel to the width direction of the steel sheet or slightly inclined with respect to the parallel direction in order to reduce the iron loss of grain-oriented electrical steel sheet 10. Here, the substantially parallel can be in a range of 0 degrees or more and ±45 degrees or less, and is typically in a range of 0 degrees or more and ±30 degrees or less. In practice, in order to balance the avoidance of fracture or breakage and the iron loss reduction and the B8 deterioration amount, the range may be +10 degrees or more and +20 degrees or less or -20 degrees or more and -10 degrees or less. The reason why the groove is formed to be inclined in a direction substantially parallel to the width direction of the steel sheet is as follows. When grain-oriented electrical steel sheet 10 is used as a core of a transformer, the grain-oriented electrical steel sheet is wound in a rectangular cross section to form a shape of the core of the transformer. At this time, the extending direction of the groove and the bending direction are made to be different from each other in order to prevent the groove direction and the bending direction to coincide with each other, which would cause grain-oriented electrical steel sheet 10 to fracture along the groove direction from the groove as a starting point. Therefore, the degree (angle of inclination) to which the groove is inclined with respect to the width direction of the steel sheet can be appropriately set in consideration of the risk of fracture that differs depending on the material or thickness of the steel sheet. In addition, first groove 20 and second groove 30 may be formed on the surface or the back surface of grain-oriented electrical steel sheet 10, or may be formed in different surfaces. In addition, first grooves 20 may be formed, for example, by using a laser according to a forming pattern such as a scan speed of 45 m/s, a groove pitch of 3 mm, a beam shape of 25 × 50 µm, and a groove depth of 20 µm, and second grooves 30 may be formed, for example, by using mechanical processing according to a forming pattern such as a groove pitch of 5 mm and a groove depth of 10 µm.
First groove 20 can be formed by a non-contact groove forming method such as laser processing, etching processing, electron beam processing, and plasma processing. In addition, first groove 20 can also be formed by a contact groove forming method such as mechanical processing including press processing of pressing a mold including protrusions such as a gear to the steel sheet surface. In addition, second groove 30 is formed by a contact groove forming method such as mechanical processing.
That is, the groove forming of second groove 30 is a contact method, while the groove forming of first groove 20 can be performed by either a contact method or a non-contact method. However, as will be described below, the contact method generates fine grains at the groove bottom. The non-contact method generates, except etching processing, a factor (hereinafter, simply referred to as a subgrain boundary factor) that causes a subgrain boundary at the groove bottom. However, as will be described below, the fine grains and the subgrain boundary factor disappear by performing the secondary recrystallization annealing between the forming of first groove 20 and the forming of second groove 30, and therefore, there would be no fine grains and subgrain boundary factor at the bottom of first groove 20.
In detail, when a groove is formed by a contact groove forming method, the steel sheet is compressed at the bottom of the groove, so that fine grains are generated at the bottom of the groove immediately after the groove is formed due to the groove forming. On the other hand, when a groove is formed by a non-contact groove forming method, the steel sheet is not subjected to compression or the like, and therefore, fine grains are not generated, but the present inventors have found that but when grooves are formed using a non-contact method such as laser, electron beam, or plasma processing, which involves heat (hereinafter simply referred to as heat processing), a subgrain boundary factor is generated. Even in the non-contact method, when grooves are formed by etching processing or the like which does not involve heat, neither the fine grain nor the subgrain boundary factor is generated.
Here, the above-described subgrain boundary factor is a factor that is the origin (cause) of a subgrain boundary, which is generated by exposure to a high-temperature environment. The subgrain boundary factor has not yet developed into a subgrain boundary at the time of forming the groove, and it is not easy to confirm, but it becomes a subgrain boundary that can be easily confirmed by receiving a heating treatment of about 800°C such as stress relief annealing. In addition, when the subgrain boundary factor grows into the subgrain boundary, this is problematic because it may cause the iron loss deterioration. However, the fine grain and the subgrain boundary (including the subgrain boundary generated from the subgrain boundary factor) disappear by aging when exposed to a high temperature (for example, about 1000°C or more) for a long time such as the secondary recrystallization annealing.
As described above, since first groove 20 is planned to be exposed to a high temperature for a long time after the forming, first groove 20 may be formed by a contact method such as mechanical processing or a non-contact method such as laser processing. However, in the case of mechanical processing, the cold-rolled sheet (base iron) used to form first groove 20 is harder than the steel sheet (grain-oriented electrical steel sheet) after the secondary recrystallization, making it more difficult to form deep grooves and causing the teeth to wear easily. In addition, etching processing does not generate a subgrain boundary factor, so there is no influence on the iron loss deterioration due to the subgrain boundary, but the manufacturing process and the equipment are complicated and expensive. From such a viewpoint, it is considered that a method that involves heat among the non-contact methods is advantageous, and the laser processing is also an advantageous method. Further, when first groove 20 is formed by the heat processing, a groove deeper than that formed by the mechanical processing can be easily formed without concern for wear of the teeth or the like. It is known that, in a depth range of, for example, 25 µm or less, the deeper the groove is, the more effective it is for reducing iron loss. Therefore, when first groove 20 is formed by the heat processing, a deep groove having a high iron loss reduction effect can be relatively easily formed.
On the other hand, when second groove 30 is formed by the contact method, the loss of the base material is smaller than that in the non-contact method, the decrease in the magnetic flux density due to the loss of the base material is suppressed, and the subgrain boundary factor is not generated, so that the subgrain boundaries do not increase even when stress relief annealing is performed after second groove 30 is formed. Therefore, it is also possible to prevent the iron loss (due to the subgrain boundary) of an industrial product such as the transformer using grain-oriented electrical steel sheet 10 from being deteriorated more than the iron loss of grain-oriented electrical steel sheet 10 used as the material.
In addition, first groove 20 may be formed as a relatively deep groove by the heat processing such as the laser processing, so that the loss of the base material is large but the contribution to the iron loss reduction is high, whereas second groove 30 may be formed as a groove shallower than first groove 20 by suppressing the loss of the base material (suppressing the decrease in the magnetic flux density) by the mechanical processing. In this manner, the decrease in the magnetic flux density is large due to first groove 20, but the large iron loss reduction effect is obtained, and the iron loss reduction effect is small due to second groove 30 but the effect of suppressing the decrease in the magnetic flux density is obtained. Therefore, it is possible to achieve both the iron loss reduction and the magnetic flux density at a higher level. Further, when first groove 20 is formed by the laser processing and second groove 30 is formed by the mechanical processing, it is possible to manufacture such a grain-oriented electrical steel sheet at a lower cost.
In the heat processing, the influence of the subgrain boundary factor is large. For example, when grain-oriented electrical steel sheet 10 in which first grooves 20 and second grooves 30 are formed is wound into a transformer shape and then subjected to the stress relief annealing, subgrain boundary factors at the bottoms of second grooves 30 grow into subgrain boundaries in the case of forming second grooves 30 by the heat processing, so that the iron loss is deteriorated. For example, when the iron loss immediately after the groove processing is denoted by W1, the iron loss after the stress relief annealing is denoted by W2, the iron loss deterioration amount due to the processing strain is denoted by ΔW3, and the iron loss deterioration amount due to the generation of the subgrain boundary is denoted by ΔW4, $W 1 < W 2$ (the iron loss W2 after the stress relief annealing is high and deteriorated) $W 2 = W 1 - ΔW 3 + ΔW 4$ $ΔW 3 < ΔW 4$ (the iron loss deterioration amount ΔW4 due to the subgrain boundary is very large)
However, when the heat treatment of 1000°C or higher is performed, ΔW4 disappears, so that the iron loss is reduced to W2 - ΔW4. For such a reason, second groove 30 is preferably formed by the mechanical processing.
Specifically, the steel sheet, in which first grooves 20 have been formed by the groove forming performed after the cold rolling step, is subjected to the secondary recrystallization annealing at a heating temperature of about 1200°C-according to a known method for manufacturing grain-oriented electrical steel sheet 10- so as to grow the crystals and align the easy magnetization axis with the rolling orientation of the steel sheet. Therefore, by performing the secondary recrystallization annealing, the fine grains and/or subgrain boundaries generated at the bottoms of the grooves due to the formation of first grooves 20 are aged and disappear from the bottoms of first grooves 20. As a result, in the final state of grain-oriented electrical steel sheet 10, neither the fine grains nor the subgrain boundaries are formed (do not exist) at the bottom of first groove 20. Therefore, the processing and forming means of first groove 20 are not particularly limited, and can be selected in consideration of the process cost, the equipment cost, and the like.
Thereafter, second grooves 30 are formed on the surface of the steel sheet having been subjected to the secondary recrystallization annealing, with a formation pattern (for example, groove pitch, groove depth, groove angle, and the like) different from that of first grooves 20. Second groove 30 is formed by a contact groove forming method such as the mechanical processing. Therefore, the fine grains are formed at the bottoms of second grooves 30 as traces of the mechanical processing, but the fine grains are not aged until grain-oriented electrical steel sheet 10 is finally obtained, and therefore the fine grains remain (exist) at the bottoms of the grooves corresponding to second grooves 30 in the grooves of final grain-oriented electrical steel sheet 10.
FIG. 1 is a schematic view of grain-oriented electrical steel sheet 10 according to the embodiment of the present invention. Grain-oriented electrical steel sheet 10 according to the embodiment of the present invention has, for example, a surface as illustrated in FIG. 1. As illustrated in FIG. 1, a plurality of first grooves 20-each extending in a direction substantially parallel to the sheet width direction of grain-oriented electrical steel sheet 10-are formed at a predetermined pitch in the rolling direction (longitudinal direction) of grain-oriented electrical steel sheet 10. For example, first grooves 20 are formed by using a laser according to a forming pattern such as a scan speed of 45 m/s, a groove pitch of 3 mm, a beam shape of 25 × 50 µm, and a groove depth of 20 µm. First groove 20 formed by using such a laser is heated by the secondary recrystallization annealing performed later, so that the fine grain and the subgrain boundary do not remain at the bottom of the groove portion.
On the other hand, as illustrated in FIG. 1, plurality of second grooves 30-each extending in a direction substantially parallel to the sheet width direction of grain-oriented electrical steel sheet 10 (not necessarily a direction parallel to the extending direction of the first groove)-are formed at a predetermined pitch in the rolling direction of grain-oriented electrical steel sheet 10. For example, second grooves 30 are formed by using the mechanical processing according to a forming pattern such as a groove pitch of 5 mm and a groove depth of 10 µm.
In this manner, grain-oriented electrical steel sheet 10 in which first groove 20 and second grooves 30 are mixed is generated. In grain-oriented electrical steel sheet 10 generated in this manner, second grooves 30 formed by using the mechanical processing are not subjected to the secondary recrystallization by annealing, and therefore fine grains remain at the bottoms of the groove portions.
In FIG. 1, each of first groove 20 and second groove 30 are drawn in a step shape in three or four portions in the sheet width direction. This indicates that, when the length in the sheet width direction that can be formed by one groove forming apparatus used for forming the groove is smaller than the length of the sheet width of the steel sheet, a plurality of groove forming apparatuses (three or four groove forming apparatuses in the case of FIG. 1) are arranged in the sheet width direction, grooves are respectively formed by using the plurality of groove forming apparatuses, thereby performing the groove forming (with a length that covers the entire sheet width direction of the steel sheet as a whole). Although a repeated explanation will be omitted, this situation also applies to FIGS. 3, 4, and 5, which will be described below.
In this manner, grain-oriented electrical steel sheet 10 in which the plurality of first grooves 20 (with fine grains and subgrain boundaries not formed at the bottoms of the groove portions) and the plurality of second grooves 30 (with fine grains formed at the bottoms of the groove portions) are mixed is manufactured.
According to the embodiment described above, the grain-oriented electrical steel sheet including a plurality of first grooves 20 including no fine grain and no subgrain boundary at the bottoms thereof and a plurality of second grooves 20 including fine grains formed at the bottoms thereof is manufactured. The plurality of first grooves can be formed by using the fact that the fine grains, the subgrain boundary factors, and the subgrain boundaries at the groove bottoms disappear by the secondary recrystallization annealing and the plurality of second grooves can be formed by the contact method. As a result, it is possible to provide the grain-oriented electrical steel sheet that can achieve both the iron loss reduction and the magnetic flux density. That is, first groove 20 does not include the subgrain boundary and the subgrain boundary factor that cause the iron loss deterioration. In addition, second groove 30 is formed by a contact method and thus basically include no subgrain boundary and no subgrain boundary factor and has a small loss of the base material that decreases the magnetic flux density. Therefore, the grain-oriented electrical steel sheet including both first groove 20 and second groove 30 can achieve both the iron loss reduction and the magnetic flux density. In addition, in grain-oriented electrical steel sheet 10 manufactured in this manner, there is no subgrain boundary factor in any groove, and therefore, the iron loss deterioration (which would be caused by a subgrain boundary factor becoming a subgrain boundary) is not generated even when the industrial product such as the transformer is manufactured by using grain-oriented electrical steel sheet 10. Therefore, it is also possible to prevent the manufacturing of the industrial product from deteriorating the iron loss more than after second groove 30 is formed.
In addition, when the depth of first groove 20 is made deeper than the depth of second groove 30, the burden (wear and the like) on the equipment of a contact method such as the mechanical processing apparatus can be reduced. In particular, when first groove 20 is formed by heat processing such as the laser processing, a deep groove can be easily formed. In this manner, the above-described both the iron loss reduction and the magnetic flux density can be achieved at a high level as a whole with the presence of first groove 20 having a high iron loss reduction effect and a sufficient depth and second groove 30 having the iron loss reduction effect while capable of reducing the decrease in the magnetic flux density. In grain-oriented electrical steel sheet 10 according to the present embodiment, subgrain boundaries that cause the iron loss deterioration can be eliminated by the annealing, and thus it is possible to achieve both the iron loss reduction and the magnetic flux density.

[Method for manufacturing grain-oriented electrical steel sheet]

Next, a method for manufacturing grain-oriented electrical steel sheet 10 according to the embodiment of the present invention will be described. The method for manufacturing grain-oriented electrical steel sheet 10 according to the embodiment of the present invention is a method for manufacturing the following grain-oriented electrical steel sheet 10: grain-oriented electrical steel sheet 10 in which a plurality of grooves are formed in the longitudinal direction of a steel sheet and each groove extends in a direction substantially parallel to the width direction of the steel sheet. FIG. 2 is a flowchart for describing the method for manufacturing grain-oriented electrical steel sheet 10 according to the embodiment of the present invention.
As illustrated in FIG. 2, when the manufacturing of grain-oriented electrical steel sheet 10 is started, first groove 20 composed of a plurality of grooves each extending in a direction substantially parallel to the width direction is formed in a cold-rolled steel sheet serving as a material of grain-oriented electrical steel sheet 10 in step S101 (first groove forming step). Typically, the plurality of first grooves 20 each extending in a direction substantially parallel to the width direction of a steel sheet (cold-rolled steel sheet) are formed on the surface of the cold-rolled steel sheet for magnetic domain control-the steel sheet is a material of grain-oriented electrical steel sheet 10, which is discharged from a finishing rolling machine after the sheet is subjected to casting, hot rolling, annealing, and cold rolling. First grooves 20 are formed by using any one of laser processing, etching processing, electron beam processing, plasma processing, and mechanical processing, based on a predetermined groove pitch, groove depth, or groove angle. Here, the groove angle is defined as an angle of the extending direction of a groove (direction in which the groove extends) with respect to the sheet width direction of a steel sheet. In general, as the groove angle increases, the iron loss reduction tends to decrease, and the deterioration amount of the magnetic flux density B8 tends to improve.
For example, first groove 20 may be formed to be deeper than second groove 30. Forming first groove 20 deeper than second groove 30 is effective for reducing iron loss of grain-oriented electrical steel sheet 10. It is preferable to use the laser processing by which the control in the depth direction is easy and the wear due to the contact with the steel sheet does not occur, but the groove forming is not limited to the laser processing. First groove 20 can be formed by various methods including various non-contact methods such as electron beam processing and plasma processing and various methods such as the mechanical processing as long as satisfactory iron loss reduction can be achieved.
FIGS. 3A and 3B are schematic views of grain-oriented electrical steel sheet 10 according to the embodiment of the present invention. For example, first grooves 20 illustrated in FIG. 3A are formed by laser processing at a groove pitch of 3 mm, a groove depth of 20 µm, and a groove angle of 10 degrees.
When the processing of step S101 is completed, the processing proceeds to step S102.
In step S102, the cold-rolled steel sheet on which first grooves 20 are formed is annealed to align the easy magnetization axis of the cold-rolled steel sheet with the longitudinal direction of the steel sheet to obtain the grain-oriented electrical steel sheet (secondary recrystallization annealing step).
To explain the process in more detail, as is well known, the steel sheet on which first grooves 20 are formed is subjected to, for example, decarburization annealing at a heating temperature of 700°C to 900°C for a heating time of 1 to 3 minutes. By the decarburization annealing, the carbon concentration in the steel sheet is adjusted, and an oxide layer mainly composed of silica (SiO₂) is formed on the surface of the decarburized and annealed steel sheet. Subsequently, an annealing separator mainly composed of magnesia (MgO) is applied to the oxide layer on the surface of the decarburized and annealed steel sheet, and the steel sheet is wound into a coil.
The steel sheet having been decarburized and annealed and coated with an annealing separator is inserted into a batch furnace while still in the coil form, and is subjected to a heat treatment at a heating temperature of 1100°C to 1300°C for a heating time of 20 to 24 hours (secondary recrystallization annealing). This heat treatment causes preferential crystal growth of so-called Goss grains, in which the rolling direction of the steel sheet coincides with the axis of easy magnetization (secondary recrystallization occurs). As a result, grain-oriented electrical steel sheet 10 having high crystal orientation (crystal alignment) is obtained after the secondary recrystallization annealing. In addition, during the secondary recrystallization annealing, the oxide layer and the annealing separator react with each other, and a glass coating film composed of forsterite (Mg₂SiO₄) is formed on the surface of the steel sheet, thereby forming grain-oriented electrical steel sheet 10.
Thereafter, grain-oriented electrical steel sheet 10 wound in a coil shape is unwound to be stretched into a sheet shape, and an insulating coating agent (coating liquid) is applied onto the glass coating film formed on the surface of grain-oriented electrical steel sheet 10 in the stretched state. Grain-oriented electrical steel sheet 10 on which the insulating coating agent is applied is annealed at a heating temperature of 800°C to 850°C for a heating time of 10 seconds to 120 seconds to perform baking of the insulating coating agent, thereby forming an insulating coating film on the surface of grain-oriented electrical steel sheet 10. In this manner, grain-oriented electrical steel sheet 10 can be provided with electrical insulation and a predetermined tension on the surface.
When the processing of step S102 is completed, the processing proceeds to step S103.
In step S103, second groove 30 composed of a plurality of grooves each extending in a direction substantially parallel to the width direction is formed in grain-oriented electrical steel sheet 10 by using the mechanical processing (second groove forming step). Second grooves 30 are formed by using the mechanical processing with a gear or the like according to a predetermined groove pitch, groove depth, or groove angle.
For example, second grooves 30 illustrated in FIG. 3B are formed in the steel sheet (on which first grooves 20 illustrated in FIG. 3A are formed) by mechanical processing (press processing) according to a second forming pattern of a groove pitch of 5 mm, a groove depth of 10 µm, and a groove angle of 12 degrees. In the press processing, achieving a groove depth of 20 µm or more places a heavy burden on a blade due to chipping and wear of the blade, making it difficult to carry out this work continuously. Therefore, it is possible to achieve both the iron loss reduction and the magnetic flux density by the following: forming deep grooves (which are easy to reduce the iron loss) as first grooves 20 by using the laser or the like that is easy to process and does not cause wear and the like; and forming second grooves 30 (separately from the formation of the first grooves) by the mechanical processing in which a steel sheet is pressed into the grooves during groove formation so that the steel sheet is not lost, thereby making it difficult for the magnetic flux density to deteriorate (magnetic flux density B8 is difficult to deteriorate). For example, in order to obtain the effect of reducing iron loss, the forming of 20 to 25 µm grooves is required, and when the groove formation is carried out in step S101, the magnetic characteristics (improvement of the iron loss and avoidance of the deterioration of B8) cannot be obtained.
Thereafter, when the insulating coating film formed on the surface of grain-oriented electrical steel sheet 10 is lost due to the formation of second groove 30, the insulating coating agent (coating liquid) is applied again onto the surface of grain-oriented electrical steel sheet 10, and the insulating coating agent is baked at a heating temperature of 800°C to 850°C for a heating time of 10 seconds to 120 seconds to form the insulating coating film on the surface of grain-oriented electrical steel sheet 10, thereby obtaining final grain-oriented electrical steel sheet 10.
Grain-oriented electrical steel sheets 10 manufactured in this manner are shipped and used in any appropriate industrial products, for example, as the materials of the cores of transformers.
In the above description, second grooves 30 are formed in step S103 after the insulating coating film is formed in step S102, but the insulating coating film may be formed after second grooves 30 are formed as long as the secondary recrystallization occurs in step S102.
When a groove is formed by the mechanical processing, the steel sheet is compressed at the bottom of the groove to generate fine grains, but when a groove is formed by the laser processing or the etching processing, fine grains are not generated. On the other hand, when a groove is formed by the laser processing, the thermal strain is introduced into the bottom (or the wall surface) of the groove to generate a subgrain boundary factor. Then, such fine grains or subgrain boundaries disappear, for example, by aging when exposed to a high temperature (about 1000°C or higher) for a long time, such as the secondary recrystallization annealing.
Therefore, there are fine grains at the bottom of second groove 30 of grain-oriented electrical steel sheet 10 (because they are not exposed to a high temperature for a long time after the groove processing), but there are no fine grains, subgrain boundaries, and subgrain boundary factors at the bottom of first groove 20 (because they are exposed to a high temperature related to the secondary recrystallization annealing after the groove processing).
FIG. 4 is a schematic view of the grain-oriented electrical steel sheet according to the embodiment of the present invention. As illustrated in FIG. 4A, in a cross section of grain-oriented electrical steel sheet 10 along the A-A' line, as illustrated in FIG. 4B, the fine grains remain at the bottom of the groove corresponding to second groove 30, while fine grain and subgrain boundary disappear at the bottom of the groove corresponding to first groove 20.
FIGS. 5A and 5B are schematic views of the grain-oriented electrical steel sheet according to the embodiment of the present invention. The forming pattern, such as a groove pitch, a groove depth, and a groove angle, in the case of forming second grooves 30 in the second groove forming step is not particularly limited, but may be, for example, as illustrated in FIGS. 5A and 5B. That is, on the surface of the steel sheet after first grooves 20 are formed (as illustrated in FIG. 5A) in the material steel sheet as described above with reference to FIG. 3A, second grooves 30 may be formed according to a forming pattern different from the forming pattern of first groove 20 as illustrated in FIG. 5B. For example, second grooves 30 illustrated in FIG. 5B are formed by the mechanical processing according to a second forming pattern of a groove pitch of 5 mm, a groove depth of 10 µm, and a groove angle of -12 degrees.
According to the manufacturing method described above, grain-oriented electrical steel sheet 10 in which the plurality of first grooves 20 including no fine grain and no subgrain boundary at the bottom thereof and plurality of second grooves 30 including fine grains at the bottom thereof are mixed can be manufactured. Grain-oriented electrical steel sheet 10 includes first grooves 20 which are formed by the laser processing or the like and have a groove depth effective for reducing iron loss, and second grooves 30 which are formed by the mechanical method and causes a small amount of loss of the steel sheet base material effective for the magnetic flux density. Therefore, it is possible to achieve both the iron loss reduction and the magnetic flux density.

[Examples]

FIG. 6 illustrates the relationship between groove depth and iron loss reduction rate in the case of forming grooves by the laser method alone and the case of forming grooves by the mechanical method alone. In addition, FIG. 7 illustrates the relationship between groove depth and B8 deterioration amount in the case of forming grooves by the laser method alone and the case of forming grooves by the mechanical method alone.
For the groove formation by the laser method alone and the groove formation by the mechanical method alone in the related art, the relationship between groove depth and iron loss reduction rate is shown by a graph as illustrated in FIG. 6. In addition, the relationship between groove depth and B8 deterioration amount is shown by a graph as illustrated in FIG. 7.
Regarding the measurement method for the groove depth, the groove depth is measured by a measuring apparatus (KEYENCE (registered trademark) WI-5000 and WI-001). Specifically, the groove depth can be acquired by using the above-described measuring apparatus to measure the length corresponding to a difference (H0 - H) between reference H0 and H-the reference H0 (> 0) is the measured average height in any range (on the surface of the steel sheet) where there is no groove and H (> 0) is the height of a maximum depth of the groove portion.
In addition, regarding the measurement method for the iron loss and the magnetic flux density B8, the iron loss [W/kg] of a grain-oriented electrical steel sheet is, for example, an iron loss when a magnetic field of a maximum magnetic flux density of 1.7 tesla and 50 Hz is applied to 10 test pieces having a size of 60 [mm] × 300 [mm] and a sheet thickness of 0.23 [mm]. The iron loss can be measured by a single sheet tester (SST) measurer. The average iron loss of material steel sheets is 0.85 [W/kg]. In addition, the magnetic flux density B8 may be, for example, a magnetic flux density generated when the 10 test pieces are magnetized in a magnetic field of 800 A/m. The average magnetic flux density B8 of the material steel sheets is 1.92 [T (tesla)].
In this case, the iron loss reduction rate can be defined as follows. That is, the iron loss reduction rate of a grain-oriented electrical steel sheet is calculated as follows with the iron loss of a material steel sheet as a reference. Iron loss reduction rate [%] = ((iron loss of material steel sheet) - (iron loss of grain-oriented electrical steel sheet)/(iron loss of material steel sheet)) × 100
Here, the material steel sheet means a steel sheet to which the two groove forming steps of forming first groove 20 and second groove 30 described above are not performed in the same base material coil as the grain-oriented electrical steel sheet. That is, first groove 20 and second groove 30 are not formed in the material steel sheet.
In addition, the deterioration amount ΔB8 of the magnetic flux density B8 can be defined as follows. That is, the deterioration amount ΔB8 of the magnetic flux density B8 of the grain-oriented electrical steel sheet is calculated as follows with the magnetic flux density B8 of the material steel sheet as a reference. ΔB8 [gauss] = (magnetic flux density B8 (T) of material steel sheet - magnetic flux density B8 (T) of grain-oriented electrical steel sheet) × 10000
That is, by the groove formation by the laser method alone, the iron loss reduction rate of 11% or more can be achieved at a groove depth of 20 µm or more, but the B8 deterioration amount is 300 gauss or more. The reason therefor can be considered in the case of the laser method that is a non-contact type method as follows: the iron loss can be easily improved because relatively deep grooves can be formed, the leakage magnetic flux is generated from the surface magnetic pole, and the static magnetic energy is increased, so that the main magnetic domain is subdivided; on the other hand, the magnetic flux density B8 is deteriorated because the removal volume (amount) of the steel sheet base material is increased.
On the other hand, by the groove formation by the mechanical method alone, the deterioration amount of the magnetic flux density B8 is 200 gauss or less at a groove depth of 15 to 20 µm, but the iron loss reduction rate of 11% or more cannot be achieved. As a reason why the iron loss reduction rate is not satisfactory, in the groove formation by the contact method, it is difficult to form deep grooves due to wear or chipping of the tooth shape. In addition, as a reason why the magnetic flux density B8 is satisfactory, it is considered that, as can be seen from the generation of fine grains, the base material of the steel sheet is pressed into the bottom of the groove, thereby minimizing the loss of the base material of the steel sheet.
In the present example, first grooves 20 are formed by laser processing according to first forming patterns of a scan speed of 45 m/s, a groove pitch of 3 mm, a beam shape of 25 × 50 µm, and groove depths of 0 µm, 10 µm, 15 µm, 20 µm, and 25 µm by adjusting laser power. In addition, second grooves 30 are formed by press processing according to second forming patterns of a groove pitch of 5 mm and groove depths of 0 µm, 10 µm, 15 µm, and 20 µm by adjusting a pressurization force of the tooth shape.
FIG. 8 illustrates an iron loss reduction rate in the case of combining the laser method and the mechanical method according to the embodiment of the present invention. FIG. 9 illustrates the relationship between groove depth and iron loss reduction rate in each combination according to the embodiment of the present invention. For grain-oriented electrical steel sheet 10 in which first grooves 20 formed by the laser processing and second grooves 30 formed by the press processing are mixed according to the present example, the relationship between each combination of the groove depth by the laser processing and the groove depth by the press processing and the iron loss reduction rate is obtained as experimental results as illustrated in FIGS. 8 and 9. Combinations that achieve the iron loss reduction rate of 11% or more is the seven combinations highlighted in FIG. 8.
FIG. 10 illustrates the B8 deterioration amount in the case of combining the laser method and the mechanical method according to the embodiment of the present invention. FIG. 11 illustrates the relationship between groove depth and B8 deterioration amount in each combination according to the embodiment of the present invention. The relationship between each combination of the groove depth by the laser processing and the groove depth by the press processing and the iron loss reduction rate is obtained as experimental results as illustrated in FIGS. 10 and 11. Among the seven combinations that achieve the iron loss reduction rate of 11% or more, the B8 deterioration amount is 200 gauss or less in three combinations, namely a combination of the groove depth of 10 µm by the laser processing and the groove depth of 15 µm by the press processing (B8 deterioration amount: 180 gauss), a combination of the groove depth of 15 µm by the laser processing and the groove depth of 5 µm by the press processing (B8 deterioration amount: 170 gauss), and a combination of the groove depth of 15 µm by the laser processing and the groove depth of 10 µm by the press processing (B8 deterioration amount: 200 gauss).
Therefore, the combination of the groove depth of 15 µm by the laser processing and the groove depth of 10 µm by the press processing is considered optimal in the present example, and grain-oriented electrical steel sheet 10 formed by this forming pattern of the combination is considered to be capable of achieving the iron loss reduction rate of 14.1% and the B8 deterioration amount of 200 gauss. In this manner, both the iron loss reduction and the magnetic flux density can be achieved.
In addition, a groove having the same depth as in the laser processing may be formed by heat processing that is the same as the laser processing, such as electron beam processing or plasma processing. For example, the groove depth can be controlled by adjusting an acceleration voltage and/or a current in the case of the electron beam, and by adjusting an output current in the case of the plasma.
Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the specific embodiments described above, and various modifications and changes can be made within the spirit and scope of the present disclosure described in the claims.
This application is entitled to and claims the benefit of Japanese Patent Application No. 2023-176056 filed on October 11, 2023 , the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety.

Reference Signs List

10 Grain-oriented electrical steel sheet
20 First groove
30 Second groove

Claims

A method for manufacturing a grain-oriented electrical steel sheet in which a plurality of grooves are formed in a longitudinal direction of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to a width direction of the grain-oriented electrical steel sheet, the method comprising:
a first groove forming of forming a first groove composed of a plurality of grooves on a cold-rolled steel sheet serving as a material of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to the width direction;

a secondary recrystallization annealing of obtaining a grain-oriented electrical steel sheet by annealing the cold-rolled steel sheet including the first groove formed thereon to align an easy magnetization axis of the cold-rolled steel sheet with the longitudinal direction; and

a second groove forming of forming, by using groove forming processing of a contact type, a second groove composed of a plurality of grooves on the grain-oriented electrical steel sheet obtained in the secondary recrystallization annealing, the plurality of grooves each extending in a direction substantially parallel to the width direction.
The method according to claim 1, wherein
the first groove forming forms the first groove by groove forming processing of a non-contact type.
The method according to claim 2, wherein
the groove forming processing of the non-contact type forms the first groove by using any one of a laser, an electron beam, and plasma.
The method according to any one of claims 1 to 3, wherein
the first groove forming forms the first groove by groove forming processing of a mechanical processing type.
The method according to any one of claims 1 to 4, wherein
the first groove is deeper than the second groove.
A grain-oriented electrical steel sheet in which a plurality of grooves are formed in a longitudinal direction of the grain-oriented electrical steel sheet, the plurality of grooves each extending in a direction substantially parallel to a width direction of the grain-oriented electrical steel sheet, wherein
the plurality of grooves are formed of a first groove in which a fine grain is not formed at a bottom of the first groove and a second groove in which a fine grain is formed at the bottom of the second groove.
The grain-oriented electrical steel sheet according to claim 6, wherein
the first groove is deeper than the second groove.
The grain-oriented electrical steel sheet according to claim 6 or 7, wherein
a subgrain boundary factor is not formed in either the first groove or the second groove.
An industrial product manufactured by using the grain-oriented electrical steel sheet according to any one of claims 6 to 8.