technical field
[One]
It relates to a grain-oriented electrical steel sheet and a method for manufacturing the same. More specifically, it relates to a grain-oriented electrical steel sheet having improved magnetic properties and improved adhesion to an insulating coating layer by appropriately forming an island by removing Fe—O oxide formed on the surface after forming a groove, and a method for manufacturing the same.
background
[2]
Since grain-oriented electrical steel sheet is used as an iron core material for electronic products such as transformers, in order to improve energy conversion efficiency by reducing power loss in electrical equipment, a steel sheet with excellent iron loss and a high space factor during lamination and winding is required. .
[3]
Grain-oriented electrical steel sheet refers to a functional steel sheet having a texture (also called "Goss Texture") in which crystal grains secondary recrystallized through hot rolling, cold rolling and annealing processes are oriented in the {110}<001> direction in the rolling direction.
[4]
As a method of lowering the iron loss of a grain-oriented electrical steel sheet, a magnetic domain refining method is known. That is, the size of the large magnetic domain of the grain-oriented electrical steel sheet is miniaturized by scratching the magnetic domain or applying an energy impact. In this case, when the magnetic domain is magnetized and its direction is changed, energy consumption can be reduced compared to when the size of the magnetic domain is large. As the magnetic domain refining method, there are permanent domain refining, which maintains the effect by improving magnetic properties even after heat treatment, and temporary domain refining, which is not.
[5]
The permanent magnetic domain refining method that shows the effect of improving iron loss even after stress relaxation heat treatment above the heat treatment temperature at which recovery occurs can be divided into etching method, roll method and laser method. Since the etching method forms a groove (groove) on the surface of the steel sheet through a selective electrochemical reaction in the solution, it is difficult to control the shape of the groove, and it is difficult to uniformly secure the iron loss characteristics of the final product in the width direction. In addition, it has a disadvantage that may cause environmental pollution due to an acid solution used as a solvent.
[6]
The permanent magnetic domain refining method using a roll forms a groove with a certain width and depth on the plate surface by processing the protrusion on the roll and pressing the roll or plate, and then annealing to improve iron loss by partially causing recrystallization at the bottom of the groove. It is a magnetic domain refinement technique that represents. The roll method has disadvantages in that it is difficult to obtain stability against machining and stable iron loss according to thickness, and the process is complicated, and the iron loss and magnetic flux density characteristics deteriorate immediately after groove formation (before stress relief annealing).
[7]
The permanent magnetic domain refining method by laser uses a method of irradiating a high-power laser to the surface of an electrical steel sheet moving at high speed, and forming a groove accompanied by melting of the base portion by laser irradiation. However, it is difficult to refine the magnetic domain to the minimum size even in this permanent magnetic domain refining method.
[8]
In the case of temporary domain refining, we do not try to irradiate the laser with an intensity higher than a certain level because we are researching the direction of not applying the coating once more after applying the laser in the coated state. This is because it is difficult to properly exert the tension effect due to damage to the coating if applied over a certain level.
[9]
In the case of permanent magnetic domain refinement, a groove is dug to widen the free charge area that can receive static magnetic energy, so a groove depth as deep as possible is required. Of course, side effects such as a decrease in magnetic flux density also occur due to the deep groove depth. Therefore, in order to reduce the deterioration of the magnetic flux density, an appropriate groove depth is maintained.
[10]
On the other hand, grain-oriented electrical steel sheets manufactured by magnetic domain refining technology are manufactured into products such as transformer iron cores through forming and heat treatment processes. In addition, since the product is used in a relatively high temperature environment, it is necessary to secure adhesion with the insulating coating layer as well as the iron loss characteristics.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[11]
An embodiment of the present invention provides a grain-oriented electrical steel sheet and a method for manufacturing the same. Specifically, in one embodiment of the present invention, after forming the groove, by removing the Fe-O oxide formed on the surface to form an island appropriately, the grain-oriented electrical steel sheet with improved magnetic properties and adhesion with the insulating coating layer, and manufacturing thereof provide a way
means of solving the problem
[12]
A grain-oriented electrical steel sheet according to an embodiment of the present invention includes a groove located on the surface of the electrical steel sheet, a metal oxide layer located on the groove, and a metal oxide-based island that is discontinuously distributed and located below the groove.
[13]
The average particle diameter of the islands positioned below the groove may be 0.5 to 5 μm.
[14]
The density of the islands positioned under the groove may be 0.5 pieces/μm 2 or less.
[15]
When bending an electrical steel sheet into a rod-shaped cylinder, the minimum diameter that does not cause peeling or cracking of the insulating coating layer may be less than 25 mm.
[16]
In the electrical steel sheet, R / H hill-up may be 0.02 to 1.0.
[17]
A method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention comprises the steps of manufacturing a cold-rolled sheet; forming a groove in the cold-rolled sheet; removing the Fe-O oxide formed on the surface of the cold-rolled sheet; primary recrystallization annealing the cold-rolled sheet; and applying an annealing separator to the primary recrystallized cold-rolled sheet, and performing secondary recrystallization annealing, wherein the adhesive coefficient calculated by the following Equation 1 is 0.016 to 1.13.
[18]
[Equation 1]
[19]

[20]
(In Equation 1, R represents the average roughness (㎛) of the surface of the cold-rolled sheet after the step of removing the oxide, and H hill-up is the average height of the hill-up existing on the surface of the cold-rolled sheet after the step of removing the oxide (μm) indicates.)
[21]
After the step of removing the oxide, the average roughness (R) of the surface of the cold-rolled sheet may be 3.0 μm or less.
[22]
After the step of removing the oxide, the average height of the hill-up present on the surface of the cold-rolled sheet (H hill-up ) may be 5.0 μm or less.
[23]
In the step of forming the groove, the cold-rolled sheet may be irradiated with a laser or plasma to form the groove.
[24]
In the step of forming the groove, a re-solidification layer may be formed under the groove.
[25]
As for the roughness before the step of removing the oxide, the average roughness (R) of the surface of the cold-rolled sheet may be 1.2 μm or more.
Effects of the Invention
[26]
According to one embodiment of the present invention, by appropriately controlling the adhesion coefficient and appropriately forming the island under the groove, adhesion and corrosion resistance can be improved.
Brief description of the drawing
[27]
1 is a schematic diagram of a rolled surface (ND surface) of a grain-oriented electrical steel sheet according to an embodiment of the present invention.
[28]
2 is a schematic diagram of a groove according to an embodiment of the present invention.
[29]
3 is a schematic diagram of a cross-section of a groove according to an embodiment of the present invention.
Modes for carrying out the invention
[30]
Terms such as first, second and third are used to describe, but are not limited to, various parts, components, regions, layers and/or sections. These terms are used only to distinguish one part, component, region, layer or section from another part, component, region, layer or section. Accordingly, a first part, component, region, layer or section described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present invention.
[31]
The terminology used herein is for the purpose of referring to specific embodiments only, and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. As used herein, the meaning of “comprising” specifies a particular characteristic, region, integer, step, operation, element and/or component, and the presence or absence of another characteristic, region, integer, step, operation, element and/or component; It does not exclude additions.
[32]
When a part is referred to as being “on” or “on” another part, it may be directly on or on the other part, or the other part may be involved in between. In contrast, when a part refers to being “directly above” another part, the other part is not interposed therebetween.
[33]
Although not defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Commonly used terms defined in the dictionary are additionally interpreted as having a meaning consistent with the related technical literature and the presently disclosed content, and unless defined, they are not interpreted in an ideal or very formal meaning.
[34]
Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.
[35]
1 shows a schematic diagram of a grain-oriented electrical steel sheet 10 that has been refined into magnetic domains according to an embodiment of the present invention.
[36]
As shown in FIG. 1, the grain-oriented electrical steel sheet 10 according to an embodiment of the present invention has a linear groove 20 formed in a direction crossing the rolling direction (RD direction) on one or both sides of the electrical steel sheet; is formed has been
[37]
Hereinafter, each step will be described in detail.
[38]
First, a cold-rolled sheet is manufactured. In an embodiment of the present invention, as a characteristic of the magnetic domain refining method after manufacturing the cold rolled sheet, the cold rolled sheet to be subjected to domain refining may be a cold rolled sheet used in the grain-oriented electrical steel sheet field without limitation. In particular, the effect of the present invention is expressed regardless of the alloy composition of the grain-oriented electrical steel sheet. Therefore, a detailed description of the alloy composition of the grain-oriented electrical steel sheet will be omitted. As an example, the cold-rolled sheet is, by weight%, C: 0.07% or less, Si: 1.0 to 6.5%, Mn: 0.005 to 3.0%, Nb+V+Ti: 0.050% or less, Cr+Sn: 1.0% or less, Al : 3.0% or less, P+S: 0.08% or less, and 0.3% or less of the total of rare earth and other impurities, and the remainder may contain Fe.
[39]
The cold-rolled sheet manufacturing method used in the grain-oriented electrical steel sheet field may be used without limitation as to the cold-rolled sheet manufacturing method, and a detailed description thereof will be omitted.
[40]
Next, a groove is formed in the cold-rolled sheet.
[41]
In the step of forming the grooves, 2 to 10 grooves may be intermittently formed with respect to the rolling vertical direction. 1 shows an example in which four grooves are intermittently formed with respect to the vertical direction of rolling. However, the present invention is not limited thereto, and it is also possible to continuously form the groove.
[42]
1 and 2 , the longitudinal direction (RD direction in FIG. 1 , X direction in FIG. 2 ) and the rolling direction (RD direction) of the groove 20 may form an angle of 75 to 88°. When the groove 20 is formed at the above-described angle, it can contribute to improving the iron loss of the grain-oriented electrical steel sheet.
[43]
The width (W) of the groove may be 10 to 200㎛. If the width of the groove 20 is too narrow or large, it may not be possible to obtain an appropriate magnetic domain refinement effect.
[44]
Also, the depth H of the groove may be 30 μm or less. If the depth (H) of the groove is too deep, the structure properties of the steel sheet 10 may be greatly changed due to strong laser irradiation, or the magnetism may be deteriorated by forming a large amount of heel-up and spatter. Therefore, the depth of the groove 20 can be controlled within the above-described range. More specifically, the depth of the groove may be 3 to 30㎛.
[45]
In the step of forming the groove, the cold-rolled sheet may be irradiated with a laser or plasma to form the groove.
[46]
When using a laser, the groove can be formed by irradiating the surface of the cold-rolled sheet with a TEMoo (M 2 ≤1.25) laser beam with an average output of 500W to 10KW on the surface of the cold-rolled sheet. The laser oscillation method can be used without limitation. That is, continuous oscillation or pulsed mode can be used. In this way, the laser is irradiated so that the surface beam absorption rate is greater than or equal to the heat of fusion of the steel sheet, thereby forming the groove 20 shown in FIGS. 1 and 2 . In FIG. 2 , the X direction represents the longitudinal direction of the groove 20 .
[47]
In this way, when a laser or plasma is used, a re-solidification layer may be formed under the groove by heat emitted from the laser or plasma. The re-solidification layer is distinguished because the overall structure and grain size of the electrical steel sheet being manufactured are different. The thickness of the re-solidification layer may be formed to be 5.0㎛ or less. When the thickness of the re-solidification layer is too thick, a metal oxide layer to be described later is formed thick, and adhesion and corrosion resistance between the metal oxide layer and the matrix may be deteriorated.
[48]
After the step of forming the groove, the surface of the steel sheet may be partially oxidized by heat generated from laser or plasma, oxygen and moisture in the air, and oxygen and moisture in the blowing gas, so that Fe—O oxide may be present.
[49]
In one embodiment of the present invention, the Fe-O oxide formed on the surface of the cold-rolled sheet is removed. A method for removing the Fe—O oxide is not particularly limited, and a dry or wet polishing method may be used. After polishing, since Fe-O oxide may be introduced into the groove, a rinsing process may be performed to remove it.
[50]
The Fe-O oxide means an iron oxide such as Fe 2 O 3 and Fe 3 O 4 . The Fe-O oxide can remove all or part of it.
[51]
Before removing the Fe-O oxide, the average roughness (R) of the surface of the cold-rolled sheet is 1.2 μm or more. At this time, if the subsequent process is performed without removing the Fe—O oxide, the metal oxide layer of the groove portion is formed unstable, and adhesion and corrosion resistance may be deteriorated.
[52]
After the Fe—O oxide is removed, the average roughness (R) of the surface of the cold-rolled sheet may be 3.0 μm or less. By removing the Fe—O oxide in the above range, the metal oxide layer is stably formed, and adhesion and corrosion resistance can be improved. Preferably, the average roughness (R) of the surface of the cold-rolled sheet may be 0.05 to 0.30 μm.
[53]
In the process of removing the Fe—O oxide, some heal-up generated in the process of forming the groove may also be partially removed. When the heel-up is formed too high, the oxide layer is unstable, and adhesion and corrosion resistance may be inferior. Specifically, after the step of removing the oxide, the average height of the hill-up present on the surface of the cold-rolled sheet (H hill-up ) may be 5.0 μm or less.
[54]
Next, the cold-rolled sheet is subjected to primary recrystallization annealing.
[55]
Since the step of primary recrystallization annealing is widely known in the field of grain-oriented electrical steel sheet, a detailed description thereof will be omitted. It may include decarburization or decarburization and nitridation in the primary recrystallization annealing process, and may be annealed in a wet atmosphere for decarburization or decarburization and nitridation. The cracking temperature in the primary recrystallization annealing step may be 800 to 950 ℃.
[56]
Next, an annealing separator is applied, followed by secondary recrystallization annealing. Since the annealing separator is widely known, a detailed description thereof will be omitted. As an example, an annealing separator containing MgO as a main component may be used.
[57]
In an embodiment of the present invention, the coefficient of adhesion calculated by Equation 1 is 0.016 to 1.13.
[58]
[Equation 1]
[59]

[60]
(In Equation 1, R represents the average roughness (㎛) of the surface of the cold-rolled sheet after the step of removing the oxide, and H hill-up is the average height of the hill-up existing on the surface of the cold-rolled sheet after the step of removing the oxide (μm) indicates.)
[61]
When the adhesive coefficient satisfies the above range, excellent adhesiveness and corrosion resistance can be ensured.
[62]
The purpose of the secondary recrystallization annealing is broadly to form a {110}<001> texture by secondary recrystallization, and to form a metal oxide (vitreous) film by the reaction of the oxide layer formed during the primary recrystallization annealing and MgO to provide insulation and magnetic properties It is the removal of impurities that harm the As a method of secondary recrystallization annealing, in the temperature increase section before secondary recrystallization occurs, a mixed gas of nitrogen and hydrogen is maintained to protect nitride, which is a grain growth inhibitor, so that secondary recrystallization can develop well, and secondary recrystallization is completed. In the post-cracking stage, impurities are removed by maintaining it in a 100% hydrogen atmosphere for a long time.
[63]
The secondary recrystallization annealing may be performed at a cracking temperature of 900 to 1210 °C.
[64]
In the secondary recrystallization annealing process, the MgO component in the annealing separator may react with the oxide layer formed on the surface of the steel sheet to form a metal oxide layer (forsterite layer) on the surface of the steel sheet and the groove. 3 schematically shows the metal oxide layer 30 . In an embodiment of the present invention, since the groove is formed before the secondary recrystallization annealing, the metal oxide layer 30 may be formed on the surface of the groove as well as the steel sheet.
[65]
In one embodiment of the present invention, since the Fe—O oxide is removed from the surface of the steel sheet after the groove is formed, MgO in the annealing separator penetrates or passes into the steel sheet to form an island 40 under the metal oxide layer 30 . can This island 40 comprises a metal oxide. more specifically forsterite.
[66]
In FIG. 3, the island 40 is schematically indicated. As shown in FIG. 3 , the island 40 may be formed under the metal oxide layer 30 by being separated from the metal oxide layer 30 . Since the island 40 is made of an alloy component similar to the metal oxide layer 30, it is distinguished from the electrical steel sheet matrix structure.
[67]
By appropriately forming the island 40 discontinuously, it can contribute to improving the adhesion between the metal oxide layer 30 and the steel sheet. Specifically, the density of the islands including the metal oxide under the groove may be 0.5 pieces/μm 2 or less. In this case, the reference means the density of the island with respect to a depth area within 5 μm below the groove 20 in the cross section (TD surface) including the steel sheet rolling direction (RD direction) and the thickness direction (ND direction).
[68]
The island 40 positioned under the groove 20 may have an average particle diameter of 0.5 to 5 μm. In this case, the reference may be a cross-section (TD surface) including a steel sheet rolling direction (RD direction) and a thickness direction (ND direction). The particle size assumes an imaginary circle having the same area as the area of ​​the island 40 measured on the TD plane, and means the diameter of the circle. The average particle diameter of the island 40 is the average particle diameter of the island 40 located below the groove 20, and the island 40 located below the surface on which the groove 20 is not formed is calculated in the above-mentioned average particle size. Exclude. By controlling the average particle diameter of the island 40, it is possible to improve magnetic properties and improve adhesion to the insulating coating layer. More specifically, the island 40 positioned below the groove 20 may have an average particle diameter of 0.75 to 3 μm.
[69]
After the secondary recrystallization annealing, the method may further include forming an insulating coating layer on the metal oxide layer.
[70]
A method of forming the insulating coating layer may be used without particular limitation, and, for example, the insulating coating layer may be formed by applying an insulating coating solution containing a phosphate. As the insulating coating solution, it is preferable to use a coating solution containing colloidal silica and metal phosphate. In this case, the metal phosphate may be Al phosphate, Mg phosphate, or a combination thereof, and the content of Al, Mg, or a combination thereof relative to the weight of the insulating coating solution may be 15% by weight or more.
[71]
The grain-oriented electrical steel sheet according to an embodiment of the present invention has a groove 20 located on the surface of the electrical steel sheet 10, a metal oxide layer 30 located on the groove 20, and an island 40 located under the groove. ) is included.
[72]
The average particle diameter of the island 40 positioned below the groove may be 0.5 to 5 μm. If the metal oxide layer is too thin, the island average particle diameter also becomes too small, resulting in poor adhesion. According to the present invention, by controlling the average particle diameter of the island 40 , it is possible to improve the magnetic properties and improve the insulating coating of the metal oxide layer and the adhesion to the matrix structure. Preferably, the island 40 positioned below the groove 20 may have an average particle diameter of 0.75 to 3 μm.
[73]
The density of the islands 40 in the lower portion of the groove 20 may be 0.5 pieces/μm 2 or less. In this case, the reference means the density of the island with respect to a depth area within 5 μm below the groove 20 in the cross section (TD surface) including the steel sheet rolling direction (RD direction) and the thickness direction (ND direction). Preferably, the density of the islands 40 in the lower portion of the groove 20 may be 0.1 pieces/μm 2 or less.
[74]
Hereinafter, the present invention will be described in more detail through examples. However, these examples are only for illustrating the present invention, and the present invention is not limited thereto.
[75]
Example
[76]
A cold-rolled sheet having a thickness of 0.23 mm was prepared. The cold-rolled sheet was irradiated with a continuous wave laser of 2.0 kW in Gaussian mode at a scanning speed of 10 m/s to form a groove at an angle of 85° to the RD direction. Thereafter, the entire surface of the steel sheet was polished using a polishing cloth to remove Fe-O oxide. Thereafter, primary recrystallization annealing was performed, and secondary recrystallization was performed after applying an MgO annealing separator. Then, an insulating coating layer was formed.
[77]
For adhesion, the minimum diameter that the insulating coating layer does not peel or crack is indicated by bending the plate into a rod-shaped cylinder with various diameters. As the adhesion is excellent, the diameter of the rod shape gradually decreases. Preferably, the minimum diameter of the cylinder at which the insulating coating layer does not peel and crack should be less than 25 mm. If it is more than 25mm, the adhesion is deteriorated and the corrosion resistance is also reduced due to the decrease in adhesion. (Cylinder minimum diameter 20mm, 24mm)
[78]
Corrosion resistance was measured by natural corrosion current density through an anode polarization test in 3.5 wt% NaCl aqueous solution at 30°C. The corrosion resistance is preferably 1.6x10 -9 A/cm 2 or less.
[79]
The adhesive coefficient of the electrical steel sheet according to the present invention is preferably 0.016 to 1.13. If the adhesion coefficient is less than 0.016, corrosion resistance may be rapidly deteriorated, and if the adhesion coefficient is more than 1.13, corrosion resistance may be deteriorated. The formula for calculating the adhesion coefficient is as follows.
[80]
As for the viscosity of an annealing separator, 10-84 are preferable. Because, when the viscosity is less than 10, the annealing separator may flow down, and when it exceeds 84, the thickness becomes too thick, and the consumption of the annealing separator increases. Therefore, considering the viscosity of a typical annealing separator, the R/H hill-up of the electrical steel sheet of the present invention is preferably 0.02 to 1.0.
[81]
[Equation 1]
[82]

[83]
(In Equation 1, R represents the average roughness (㎛) of the surface of the cold-rolled sheet after the step of removing the oxide, and H hill-up is the average height of the hill-up existing on the surface of the cold-rolled sheet after the step of removing the oxide (μm) indicates.)
[84]
[Table 1]
[85]
As shown in Table 1, it can be confirmed that the grain-oriented electrical steel sheet manufactured by appropriately controlling the adhesion coefficient after the formation of the groove has excellent adhesion and corrosion resistance. On the other hand, it can be seen that the comparative example in which the adhesion coefficient is not properly controlled has relatively poor adhesion and corrosion resistance.
[86]
In addition, it was confirmed that the average particle diameter range of the islands 40 located under the grooves of Examples 1 to 10 was 0.5 to 5.0 μm. In addition, it was confirmed that the density of the islands 40 was 0.5 pieces/μm 2 or less.
[87]
On the other hand, in the comparative example, it was confirmed that the average particle diameter of the islands 40 was less than 0.5 μm, and it was also confirmed that a plurality of islands 40 were formed with a density of more than 0.5 pieces/μm 2 .
[88]
The present invention is not limited to the embodiments, but can be manufactured in various different forms, and those of ordinary skill in the art to which the present invention pertains can use other specific forms without changing the technical spirit or essential features of the present invention. It will be appreciated that this may be practiced. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.
[89]
[Explanation of code]
[90]
10: grain-oriented electrical steel sheet,
[91]
20: Groove,
[92]
30: metal oxide layer;
[93]
40: Ireland
Claims
[Claim 1]
a groove positioned on the surface of the electrical steel sheet, a metal oxide layer positioned on the groove, and discontinuously distributed and distributed metal oxide-based islands positioned under the groove, wherein the island positioned under the groove has an average particle diameter of 0.5 to 5㎛ grain-oriented electrical steel sheet.
[Claim 2]
The grain-oriented electrical steel sheet according to claim 1, wherein a density of islands positioned under the groove is 0.5 pieces/㎛ 2 or less.
[Claim 3]
The grain-oriented electrical steel sheet according to claim 1, wherein, when the electrical steel sheet is bent into a rod-shaped cylinder, the minimum diameter that does not cause peeling or cracking of the insulating coating layer is less than 25 mm.
[Claim 4]
The grain-oriented electrical steel sheet according to claim 1, wherein in the electrical steel sheet, R / H hill-up is 0.02 to 1.0. (However, R represents the average roughness (μm) of the surface of the cold-rolled sheet after the step of removing the oxide, and H hill-up is the average height of the hill-up present on the surface of the cold-rolled sheet after the step of removing the oxide)
[Claim 5]
manufacturing a cold-rolled sheet; forming a groove in the cold-rolled sheet; removing the Fe-O oxide formed on the surface of the cold-rolled sheet; primary recrystallization annealing the cold-rolled sheet; and applying an annealing separator to the first recrystallized cold-rolled sheet and performing secondary recrystallization annealing, wherein the adhesive coefficient calculated by the following Equation 1 is 0.016 to 1.13. [Equation 1] (In Equation 1, R represents the average roughness (μm) of the surface of the cold-rolled sheet after the step of removing the oxide , and H hill-up is the average of the hill-up present on the surface of the cold-rolled sheet after the step of removing the oxide. It represents the height (μm).)
[Claim 6]
[Claim 6] The method of claim 5, wherein after removing the oxide, the average roughness (R) of the surface of the cold-rolled sheet is 3.0 μm or less.
[Claim 7]
The method of claim 5 , wherein, after removing the oxide, the average height of the hill-up (H hill-up ) present on the surface of the cold-rolled sheet is 5.0 μm or less.
[Claim 8]
The method of claim 5, wherein in the forming of the groove, the cold-rolled sheet is irradiated with laser or plasma to form the groove.
[Claim 9]
The method of claim 5, wherein in the forming of the groove, a re-solidification layer is formed under the groove.
[Claim 10]
[Claim 6] The method of claim 5, wherein the average roughness (R) of the surface of the cold-rolled sheet before the step of removing the oxide is 1.2 μm or more.