Patent Description:
Since a grain-oriented electrical steel sheet is used as an iron core material of an electrical device such as a transformer, in order to improve energy conversion efficiency thereof by reducing power loss of the device, it is necessary to provide a steel sheet having excellent iron loss of the iron core material and a high occupying ratio when being stacked and spiral-wound.

The grain-oriented electrical steel sheet refers to a functional material having a texture (referred to as a "GOSS texture") of which a secondary-recrystallized grain is oriented with an azimuth {<NUM>}<<NUM>> in a rolling direction through a hot rolling process, a cold rolling process, and an annealing process.

As a method of reducing the iron loss of the grain-oriented electrical steel sheet, a magnetic domain refining method is known. In other words, it is a method of refining a large magnetic domain contained in a grain-oriented electrical steel sheet by scratching or energizing the magnetic domain. In this case, when the magnetic domain is magnetized and a direction thereof is changed, energy consumption may be reduced more than when the magnetic domain is large. The magnetic domain refining methods include a permanent magnetic domain refining method, which retains an improvement effect even after heat treatment, and a temporary magnetic domain refining method, which does not retain an improvement effect after heat treatment.

The permanent magnetic domain refining method in which iron loss is improved even after stress relaxation heat treatment at a heat treatment temperature or more at which recovery occurs may be classified into an etching method, a roll method, and a laser method. According to the etching method, since a groove is formed on a surface of a steel sheet through selective electrochemical reaction in a solution, it is difficult to control a shape of the groove, and it is difficult to uniformly secure iron loss characteristics of a final product in a width direction thereof. In addition, the etching method has a disadvantage that it is not environmentally friendly due to an acid solution used as a solvent.

The permanent magnetic domain refining method using a roll is a magnetic domain refining technology that provides an effect of improving iron loss that partially causes recrystallization at a bottom of a groove by forming the groove with a certain width and depth on a surface of a plate by pressing the roll or plate by a protrusion formed on the roll and then annealing it. The roll method is disadvantageous in stability in machine processing, in reliability due to difficulty in securing stable iron loss depending on a thickness, in process complexity, and in deterioration of the iron loss and magnetic flux density characteristics immediately after the groove formation (before the stress relaxation annealing).

The permanent magnetic domain refining method using a laser is a method in which a laser beam of high output is irradiated onto a surface portion of an electrical steel sheet moving at a high speed, and a groove accompanied by melting of a base portion is formed by the laser irradiation. However, these permanent magnetic domain refining methods also have difficulty in refining the magnetic domain to a minimum size.

Current technology of the temporary magnetic domain refining method does not focus on performing coating once again after irradiating the laser in a coated state, and thus, the laser is not attempted to be irradiated with a predetermined intensity or higher. This is because when the laser is irradiated with a predetermined intensity or higher, it is difficult to properly obtain a tension effect due to damage to the coating.

Since the permanent magnetic domain refining method is to increase a free charge area that may receive static magnetic energy by forming a groove, a deep groove depth is required as much as possible. In addition, a side effect such as a decrease in magnetic flux density also occurs due to the deep groove depth. Therefore, in order to reduce the magnetic flux density deterioration, the groove is managed with an appropriate depth.

On the other hand, a grain-oriented electrical steel sheet manufactured by a magnetic domain refining technology is manufactured into products such as transformer cores through molding and heat treatment processes. In addition, since a product is used in a relatively high temperature environment, it is necessary to secure not only iron loss characteristics but also a close contacting property to the insulating coating layer.

<CIT> relates to a grain-oriented electrical steel sheet, comprising a base steel plate having a base steel plate which extends in a direction crossing a rolling direction on a surface and has a groove whose depth direction is a plate thickness direction. One or two or more kinds of insulating coating films are formed on a flat part of a surface of the base steel plate where the groove is not formed and a maximum depth of the groove formed on the base steel plate is formed. When the thickness of the mother steel sheet in the flat portion is t, t/<NUM> or more is satisfied, and an insulating coating film of the outermost layer of the one or two or more kinds of insulating films is formed so as to be in contact with the inner surface of the groove formed in the mother steel sheet. The grain-oriented electrical steel sheet is characterized in that a specific element concentration portion having a diameter of <NUM> or less is formed at a density of <NUM> pieces/µm or more at an interface between the base steel plate in the groove and the insulating film on the outermost layer.

The present invention has been made in an effort to provide a grain-oriented electrical steel sheet and a manufacturing method thereof. More specifically, the present invention has been made in an effort to provide a grain-oriented electrical steel sheet and a manufacturing method thereof that may improve magnetism and may improve a close contacting property to an insulating coating layer, by appropriately forming an island by removing a portion of an oxide layer after forming a groove.

An embodiment of the present invention as defined in independent claim <NUM>, provides a grain-oriented electrical steel sheet, including a groove positioned on a surface of an electrical steel sheet in a direction crossing the rolling direction, a metal oxide layer positioned on the groove, and metal oxide-based islands that are discontinuously distributed and positioned under the groove, wherein average sphericity of the islands positioned under the groove is <NUM> to <NUM>.

A density of the islands positioned under the groove is <NUM> pieces/µm<NUM>.

<NUM> to <NUM> grooves may be intermittently present with respect to a rolling vertical direction.

A length direction of the groove and a rolling direction of the steel sheet may form an angle of <NUM> to <NUM>°.

A manufacturing method of the grain-oriented electrical steel sheet according to the embodiment of the present invention as defined in independent claim <NUM>, includes: manufacturing a cold-rolled sheet; forming a groove by irradiating a laser beam on the cold-rolled sheet; and partially removing an oxide layer formed on a surface of the cold-rolled sheet so that a thickness of the oxide layer remains at <NUM> to <NUM>.

In the forming of the groove, a laser beam scanning rate may be <NUM>/s or more.

In the forming of the groove, a gas may be injected toward the groove at an angle of <NUM>° or less with respect to a rolling direction.

A pressure of the injected gas may be <NUM>/cm<NUM> (<NUM>,<NUM> kPa) or more.

A water content of the injected gas may be <NUM> wt% or less.

After the forming of the groove, an oxide layer having a thickness of <NUM> to <NUM> may be present on a surface of the cold-rolled sheet.

According to the embodiment of the present invention, it is possible to improve magnetism and to improve a close contacting property with an insulating coating layer, by partially removing an oxide layer after forming a groove.

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, areas, zones, layers, and/or sections, they are not limited thereto. These terms are only used to distinguish one element, component, region, area, zone, layer, or section from another element, component, region, layer, or section. Therefore, a first part, component, region, area, zone, layer, or section to be described below may be referred to as second part, component, area, layer, or section within the range of the present invention.

The technical terms used herein are to simply mention a particular embodiment and are not meant to limit the present invention. An expression used in the singular encompasses an expression of the plural, unless it has a clearly different meaning in the context. In the specification, it is to be understood that the terms such as "including", "having", etc., are intended to indicate the existence of specific features, regions, numbers, stages, operations, elements, components, and/or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other features, regions, numbers, stages, operations, elements, components, and/or combinations thereof may exist or may be added.

When referring to a part as being "on" or "above" another part, it may be positioned directly on or above another part, or another part may be interposed therebetween. In contrast, when referring to a part being "directly above" another part, no other part is interposed therebetween.

The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown.

<FIG> illustrates a schematic view of a grain-oriented electrical steel sheet <NUM> that is magnetic-domain-refined by an embodiment of the present invention.

As shown in <FIG>, a grain-oriented electrical steel sheet <NUM> according to an embodiment of the present invention is provided with a linear groove <NUM> formed in a direction crossing a rolling direction (RD direction) on one surface or both surfaces of the electrical steel sheet.

In the embodiment of the present invention, a groove is formed through a laser, and a portion of an oxide layer present in a process of forming the groove is removed, so that a uniform metal oxide layer is formed in a secondary recrystallization annealing process, and ultimately, it is possible to improve magnetism and to improve a close contacting property to a insulating coating layer. In this case, the metal oxide layer may be a forsterite (FeMg<NUM>SiO<NUM>) layer.

A manufacturing method of the grain-oriented electrical steel sheet according to the embodiment of the present invention includes: manufacturing a cold-rolled sheet; forming a groove by irradiating a laser beam on the cold-rolled sheet; and removing a portion of an oxide layer formed on a surface of the cold-rolled sheet to maintain a thickness of the oxide layer at <NUM> to <NUM>.

Hereinafter, respective steps will be specifically described.

First, a cold rolled sheet is manufactured. An embodiment of the present invention is characterized in a magnetic domain refining method after the cold-rolled sheet is manufactured, and the cold-rolled sheet to be subjected to magnetic domain refining may be a cold-rolled sheet used in a field of grain-oriented electrical steel sheets without limitation. Particularly, an effect of the present invention is realized regardless of an 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. For example, the cold-rolled sheet may include, in wt%, C at <NUM> % or less, Si at <NUM> to <NUM> %, Mn at <NUM> to <NUM> %, Nb+V+Ti at <NUM> % or less, Cr+Sn at <NUM> % or less, Al at <NUM> % or less, P+S at <NUM> % or less, a total of rare earths and other impurities at <NUM> %, and the balance of Fe.

Manufacturing methods of the cold-rolled sheet used in a grain-oriented electrical steel sheet field may be used for the manufacturing method of the cold-rolled sheet without limitation, and a detailed description thereof will be omitted.

Next, the cold-rolled sheet is irradiated with a laser beam to form a groove.

The groove may be formed by irradiating a TEMoo (M<NUM> ≤ <NUM>) laser beam having an average power of <NUM> W to <NUM> KW on a surface of the cold-rolled sheet. A laser oscillation method may be used without limitation. That is, a continuous oscillation or pulsed mode may be used. In this way, the laser beam is irradiated so that a surface beam absorption rate is greater than or equal to heat of melting the steel sheet, thereby forming the groove <NUM> shown in <FIG> and <FIG>.

In this case, a scanning rate of the laser may be <NUM>/s or more. When the scanning rate of the laser is too low, there may be a problem that the groove is not properly formed. More specifically, the scanning rate of the laser may be <NUM>/s to <NUM>/s.

In the forming of the groove, a gas may be injected toward the groove at an angle of <NUM>° or less with respect to the rolling direction (RD direction). In this case, the angle is an angle based on a rolling vertical surface (TD surface). By properly spraying the gas, it is possible to prevent the melt from solidifying in the groove. When the angle is too high, the melt may not be properly removed.

In this case, a gas pressure may be <NUM>/cm<NUM> (<NUM>,<NUM> kPa) or more. When the gas pressure is too low, the melt may not be properly removed. More specifically, the gas pressure may be <NUM> to <NUM>/cm<NUM> (<NUM>,<NUM> kPa).

A moisture content of the injected gas may be <NUM> wt% or less. When the moisture content is too high, a non-uniform and thick oxide layer may be formed on the surface of the steel sheet due to gas injection. The oxide layer may form a non-uniform metal oxide layer in a process of secondary recrystallization annealing later to ultimately adversely affect close contacting properties and magnetism. More specifically, the moisture content of the injected gas may be <NUM> wt% or less.

As shown in <FIG>, with respect to the rolling vertical direction, <NUM> to <NUM> grooves may be intermittently formed. However, the present invention is not limited thereto, and it is also possible to continuously form grooves.

As shown in <FIG> and <FIG>, a length direction (X direction) and the rolling direction (RD direction) of the groove <NUM> may form an angle of <NUM> to <NUM>°. When forming the above-described angle, it may contribute to improving the iron loss of the grain-oriented electrical steel sheet.

A width W of the groove may be <NUM> to <NUM>. When the width of the groove <NUM> is narrow or wide, it may not be possible to obtain an appropriate magnetic domain refining effect.

In addition, a depth H of the groove may be <NUM> to <NUM> % of the thickness of the steel sheet. When the depth H of the groove is too shallow, it is difficult to obtain a proper iron loss improvement effect. When the depth H of the groove is too deep, texture characteristics of the steel sheet <NUM> are significantly changed due to strong laser irradiation, or a large amount of hill-up and spatter are formed, so that magnetic properties may be deteriorated. Therefore, it is possible to control the depth of the groove <NUM> in the above-described range.

After the forming of the groove, the surface of the steel sheet may be partially oxidized by heat generated from the laser beam, oxygen and moisture in the air, and oxygen and moisture in the injection gas, so that an oxide layer may exist. Specifically, a thickness of the oxide layer may be <NUM> to <NUM>. In addition, the oxide layer may be formed non-uniformly on an entire surface of the steel sheet, and the thickness of the aforementioned oxide layer means an average thickness on the entire surface of the steel sheet.

When the oxide layer is formed too thickly, a problem in that the oxide layer thickly remains may occur even if the oxide layer is removed in an oxide layer removing step described later.

A re-solidification layer may be formed at lower and side portions of the groove due to the thermal effect of the laser beam. The re-solidification layer may have a thickness of <NUM> or less. When the re-solidification layer is formed too thickly, close contacting properties and iron loss may be deteriorated due to an increase in a heat-affected zone. The re-solidification layer includes recrystallization with an average particle diameter of <NUM> to <NUM>, and is distinguished from the overall structure of the electrical steel sheet being manufactured.

Next, a portion of the oxide layer formed on the surface of the cold-rolled sheet is removed, so that the thickness of the oxide layer remains at <NUM> to <NUM>.

When the oxide layer is not removed, the non-uniform oxide layer remains thick, and the metal oxide layer formed in the secondary recrystallization annealing process is formed non-uniform and thick, which causes deterioration of the magnetism, and deterioration of the close contacting property between the metal oxide layer and the basic structure.

Techniques for removing the hill-up or spatter formed during the groove formation process through a brush or pickling are known, but that the melt-solidified hill-up or spatter is removed and that the oxide layer is removed are completely different in terms of removing the oxide layer together in addition to the hill-up or spatter.

As a method of removing the oxide layer, it may be removed through friction between a polishing roll (paper) and the oxide layer by using the polishing paper or the polishing roll.

The thickness of the oxide layer remains at <NUM> to <NUM>. When the thickness of the oxide layer remains too thickly, the metal oxide layer is formed unevenly and thick, which causes deterioration of the magnetism and close contacting property. When the thickness of the oxide layer remains too thin, an appropriate metal oxide layer is not formed, which causes deterioration of the magnetism and close contacting property. More specifically, <NUM> to <NUM> of the oxide layer may remain.

After the remaining of the oxide layer, primary recrystallization annealing the cold-rolled sheet may be further included.

Since the primary recrystallization annealing is widely known in the field of grain-oriented electrical steel sheets, a detailed description thereof is omitted. In the primary recrystallization annealing process, decarburizing, or decarburizing and nitriding may be included, and annealing may be performed in a humid atmosphere for the decarburizing or the decarburizing and nitriding. A soaking temperature in the primary recrystallization annealing may be <NUM> to <NUM>.

After the primary recrystallization annealing, applying an annealing separating agent and secondary recrystallization annealing may be further included. Since the annealing separating agent is widely known, a detailed description will be omitted. For example, the annealing separating agent including MgO as a main component may be used.

The purpose of the secondary recrystallization annealing is largely formation of {<NUM>}<<NUM>> texture by the secondary recrystallization, insulation-imparting by the formation of a glassy film by reaction between the oxide layer formed during the primary recrystallization annealing and MgO, and removal of impurities that degrades magnetic properties. In the method of the secondary recrystallization annealing, in the heating section before the secondary recrystallization occurs, the mixture of nitrogen and hydrogen is maintained to protect the nitride, which is a particle growth inhibitor, so that the secondary recrystallization may develop well, and in the soaking after the secondary recrystallization is completed, impurities are removed by maintaining it in a <NUM> % hydrogen atmosphere for a long time.

The secondary recrystallization annealing may be performed at a soaking temperature of <NUM> to <NUM>.

During the secondary recrystallization annealing process, the MgO component in the annealing separating agent reacts with the oxide layer formed on the surface of the steel sheet, thereby forming a metal oxide layer on the surfaces of the steel sheet and of the groove. In <FIG>, the metal oxide layer <NUM> is schematically shown. In the embodiment of the present invention, since the groove is formed before the secondary recrystallization annealing, the metal oxide layer <NUM> may be formed not only on the steel sheet but also on the surface of the groove.

In the embodiment of the present invention, since the oxide layer is partially removed from the surface of the steel sheet after the groove is formed, the thickness of the oxide layer is thin, so that MgO in the annealing separating agent may penetrate or pass through the oxide layer to form an island <NUM> under the metal oxide layer <NUM>. The island <NUM> may include forsterite.

In <FIG>, the island <NUM> is schematically shown. As shown in <FIG>, the island <NUM> may be formed under the metal oxide layer <NUM> so as to be separated from the metal oxide layer <NUM>. Since the island <NUM> is made of an alloy composition similar to that of the metal oxide layer <NUM>, it is distinct from the electrical steel sheet base structure.

Since the island <NUM> is appropriately discontinuously formed, it may contribute to improving the close contacting property between the metal oxide layer <NUM> and the steel sheet. Specifically, the density of the islands including the metal oxide under the groove may be <NUM> pieces/µm<NUM> or less. In this case, a reference means the density of the islands with respect to a depth area within <NUM> below the groove <NUM> in the cross-section (TD surface) including the steel sheet rolling direction (RD direction) and the thickness direction (ND direction).

The island <NUM> positioned below the groove <NUM> may have average sphericity (short axis/long axis) of <NUM> to <NUM>. In this case, a reference is the cross-section (TD surface) including the steel sheet rolling direction (RD direction) and the thickness direction (ND direction). The island <NUM> positioned below a surface in which the groove <NUM> is not formed is excluded from the calculation of the average particle diameter described above. By controlling the average sphericity of the island <NUM>, it is possible to improve the magnetism and the close contacting property with the insulating coating layer. More specifically, the island <NUM> positioned below the groove <NUM> may have average sphericity (short axis/long axis) of <NUM> to <NUM>.

After the secondary recrystallization annealing, forming an insulating coating layer on the metal oxide layer may be further included.

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. It is preferable to use a coating solution containing colloidal silica and a metal phosphate as the insulating coating solution. In this case, the metal phosphate may be Al phosphate, Mg phosphate, or a combination thereof, and a content of Al, Mg, or a combination may be <NUM> wt% or more with respect to a weight of the insulating coating solution.

The grain-oriented electrical steel sheet according to the embodiment of the present invention includes the groove <NUM> positioned on the surface of the electrical steel sheet <NUM>, the metal oxide layer <NUM> positioned on the groove <NUM>, and the island <NUM> positioned under the groove.

The island <NUM> positioned below the groove may have an average sphericity (short axis/long axis) of <NUM> to <NUM>. By controlling the average sphericity of the island <NUM>, it is possible to improve the magnetism and the close contacting property with the insulating coating layer. More specifically, the island <NUM> positioned below the groove <NUM> may have average sphericity of <NUM> to <NUM>. When the average sphericity is less than <NUM>, as the close contacting property is deteriorated between the forsterite and the base part, a diameter of the cylinder is found to be <NUM> or more during a close contacting property test due to cracking or bursting of the forsterite after the insulating coating.

The density of the islands <NUM> under the groove <NUM> is <NUM> pieces/µm<NUM>. In this case, a reference means the density of the islands with respect to a depth area within <NUM> below the groove <NUM> in the cross-section (TD surface) including the steel sheet rolling direction (RD direction) and the thickness direction (ND direction). More specifically, the density of the islands <NUM> under the groove <NUM> may be <NUM> pieces/µm<NUM> or less.

Hereinafter, the present invention will be described in more detail through examples. However, the examples are only for illustrating the present invention, and the present invention is not limited thereto.

A cold-rolled sheet with a thickness of <NUM> was prepared. The cold-rolled sheet was irradiated with a <NUM> kW Gaussian mode of continuous wave laser beam at a scanning rate of <NUM>/s to form <NUM>° angled grooves with the RD direction. When forming the groove, dry air from which moisture was removed at a pressure of <NUM>/cm<NUM> (<NUM>,<NUM> kPa) was sprayed on an upper portion thereof at an angle of <NUM>° to the rolling direction. Next, the entire surface of the steel sheet was polished by using a polishing cloth, and the thickness of the oxide layer was adjusted to <NUM> or less as shown in Table <NUM> below. When the thickness of the oxide layer exceeds <NUM>, the close contacting property is deteriorated. Next, the primary recrystallization annealing was performed, and then the secondary recrystallization was performed after MgO coating to form the insulating coating layer.

The close contacting property was indicated with the minimum diameter in which the insulating coating layer was not peeled and cracked by bending the product sheet to a rod-shaped cylinder having various diameters. The better the close contacting property, the diameter of the rod gradually decreases.

As shown in Table <NUM>, it can be confirmed that the examples in which the oxide layer was properly removed after the groove was formed had an excellent close contacting property and excellent iron loss compared to the comparative example.

In addition, in Examples <NUM> to <NUM>, it was confirmed that when the average sphericity of the islands <NUM> under the groove was <NUM> to <NUM>, respectively, and the density thereof was <NUM> pieces/µm<NUM> or less, the iron loss and the close contacting property thereof were excellent.

Claim 1:
A grain-oriented electrical steel sheet (<NUM>), comprising
a groove (<NUM>) positioned on a surface of the electrical steel sheet in a direction crossing the rolling direction,
a metal oxide layer (<NUM>) positioned on the groove, and
metal oxide-based islands (<NUM>) that are discontinuously distributed and positioned under the groove (<NUM>),
characterised in that an
average sphericity of the islands (<NUM>) positioned under the groove (<NUM>) is <NUM> to <NUM>, and wherein
a density of the metal oxide-based islands (<NUM>) positioned under the groove (<NUM>) is <NUM> pieces/µm<NUM>.