EP3395959A1

EP3395959A1 - Oriented electrical steel sheet and manufacturing method therefor

Info

Publication number: EP3395959A1
Application number: EP16879299.2A
Authority: EP
Inventors: Kyu-Seok Han; Hyun-Seok Ko
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2015-12-21
Filing date: 2016-12-20
Publication date: 2018-10-31
Anticipated expiration: 2036-12-20
Also published as: CN108431244B; JP2019505671A; EP3395959A4; KR101675318B1; PL3395959T3; EP3395959B1; US20180371571A1; JP6622919B2; CN108431244A; US20220106657A1; WO2017111432A1

Abstract

A method for manufacturing an oriented electrical steel sheet according to an exemplary embodiment of the present invention includes: providing a slab including, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at 0.001 % to 0.4 %, and Mn at 0.001 % to 2.0 %, and including a balance including Fe and inevitably mixed and input impurities; reheating the slab; manufacturing a hot steel sheet by hot-rolling the slab; performing hot-rolled steel sheet annealing to the hot steel sheet; primarily cold-rolling the hot-rolled steel sheet annealed hot steel sheet; decarburization-annealing the cold-rolled steel sheet; secondarily cold-rolling the decarburization-annealed steel sheet; and finally annealing the cold-rolled steel sheet, wherein, regarding the finally annealed steel sheet, a size 2L of a magnetic domain existing in a grain is less than a thickness D of the steel sheet (2L < D).

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean Patent Application No. 10-2015-0182839 filed in the Korean Intellectual Property Office on December 21, 2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to an oriented electrical steel sheet and a manufacturing method thereof.

(b) Description of the Related Art

An oriented electrical steel sheet is a soft ferrite material that has an excellent magnetism characteristic in a rolling direction and includes grains having a crystal orientation of a steel sheet of {110}<001>, a so-called Goss orientation. The oriented electrical steel sheet is manufactured by rolling the same to a final thickness of 0.15 to 0.35 mm through hot-rolling, hot-rolled steel sheet annealing, and cold-rolling after heating a slab, and then allowing the same to undergo high-temperature annealing for primary recrystallization annealing and secondary recrystallization formation. In this instance, it is known that a degree of integration of the Goss orientation that is secondarily recrystallized as a temperature raising rate is slower at the time of a high-temperature annealing, and the magnetism is excellent. The temperature raising rate during conventional high-temperature annealing of an oriented electrical steel sheet is equal to or less than 15 °C per hour, the temperature raising requires two to three days, and a purification annealing process of more than forty hours is needed, so it is a process that consumes a large amount of energy. Further, the present final high-temperature annealing process performs a batch-type annealing in a coiled state, so subsequent difficulties during the process are generated as follows. First, a temperature deviation of an external winding portion and an internal winding portion of the coil caused by a heat treatment in a coiled state occurs, as the same heat treatment pattern may not be applied to respective portions, so a magnetism deviation of the external winding portion and the internal winding portion is generated. Second, MgO is coated on the surface after decarburization-annealing, and various surface defects are generated during a process for forming a base coating during high-temperature annealing, thereby lowering an actual yield. Third, as the decarburization-annealed decarburization plate is wound in a coil form, it undergoes flattening annealing and insulating coating after high-temperature annealing, so a production process is divided into three steps and the actual yield is reduced. The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. SUMMARY OF THE INVENTION
The present invention has been made in an effort to provide a method for manufacturing an oriented electrical steel sheet and an oriented electrical steel sheet manufactured by the same.
An exemplary embodiment of the present invention provides a method for manufacturing an oriented electrical steel sheet that includes: providing a slab including, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at 0.001 % to 0.4 %, and Mn at 0.001 to 2.0 %, and including a balance including Fe and inevitably mixed and input impurities; reheating the slab; manufacturing a hot steel sheet by hot-rolling the slab; performing hot-rolled steel sheet annealing to the hot steel sheet; primarily cold-rolling the hot-rolled steel sheet annealed hot steel sheet; decarburization-annealing the cold-rolled steel sheet; secondarily cold-rolling the decarburization-annealed steel sheet; and finally annealing the cold-rolled steel sheet, wherein, regarding the finally annealed steel sheet, a size 2L of a magnetic domain existing in a grain is less than a thickness D of the steel sheet (2L < D).
The slab may include Si at equal to or less than 1 wt% (excluding 0 wt%).
The slab may further include Al at equal to or less than 0.01 wt% (excluding 0 wt%).
A reheating temperature of the slab may be 1050 °C to 1350 °C.
Reduction rates in the primarily cold-rolling and the secondarily cold-rolling may respectively be 50 % to 70 %.
The decarburization-annealing of the cold-rolled steel sheet and the secondarily cold-rolling of the decarburization-annealed steel sheet may be repeated at least twice.
The decarburization-annealing may be performed in an atmosphere including hydrogen with a dew point temperature of 0 °C at a temperature of 800 °C to 1150 °C.
The finally annealing may include a first step for performing the same in an atmosphere with a dew point temperature of 10 °C to 70 °C at a temperature of 850 °C to 1150 °C, and a second step for performing the same in a mixed gas atmosphere including hydrogen and nitrogen with a dew point temperature that is equal to or less than 10 °C at a temperature of 900 °C to 1200 °C.
The first step may be performed for equal to or less than 300 seconds, and the second step may be performed for 60 seconds to 300 seconds.
The finally annealing may be continuously performed after the cold-rolling.
An amount of carbon in the electrical steel sheet may be equal to or less than 0.003 wt% (excluding 0 wt%) after the finally annealing.
Regarding the finally annealed steel sheet, a volumetric fraction of a grain with an orientation that is within 15 degrees from an orientation {110}<001> may be equal to or greater than 50 %.
Regarding the finally annealed steel sheet, a volumetric fraction of a grain with a particle diameter of 20 µm to 1000 µm may be equal to or greater than 50 %.
Another embodiment of the present invention provides an oriented electrical steel sheet including, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at equal to or less than 0.003 % (excluding 0 %), and Mn at 0.001 to 2.0 %, and a balance including Fe and an impurity that is inevitably mixed and input, wherein a size 2L of a magnetic domain existing in a grain is less than a thickness (D) of a steel sheet.
Si may be included to be equal to or less than 1.0 wt% (excluding 0 wt%).
Al may be further included to be equal to or less than 0.01 wt% (excluding 0 wt%).
A size 2L of a magnetic domain existing in a grain may be 10 to 500 µm.
A volumetric fraction of a grain with an orientation that is within 15 degrees from an orientation {110}<001> may be equal to or greater than 50 %.
A volumetric fraction of a grain with a particle diameter of 20 µm to 1000 µm may be equal to or greater than 50 %.
According to an exemplary embodiment of the present invention, the method for manufacturing an oriented electrical steel sheet by performing continuous annealing without performing batch-type annealing in a coiled state in the case of the final annealing is provided.
Further, according to an exemplary embodiment of the present invention, an oriented electrical steel sheet may be manufactured by short-time annealing.
Also, according to an exemplary embodiment of the present invention, an oriented electrical steel sheet manufactured by not using a grain growth control agent may be provided.
In addition, according to an exemplary embodiment of the present invention, the nitriding-annealing may be omitted.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a photograph for indicating a microstructure and a magnetic domain of an oriented electrical steel sheet manufactured in Example 1.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers, and/or sections, they are not limited thereto. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention.
The technical terms used herein are to simply mention a particular exemplary 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 terms such as "including", "having", etc., are intended to indicate the existence of specific features, regions, numbers, stages, operations, elements, components, or combinations thereof disclosed in the specification, and are not intended to preclude the possibility that one or more other specific features, regions, numbers, operations, elements, components, or combinations thereof may exist or may be added.
When a part is referred to as being "on" another part, it can be directly on the other part or intervening parts may also be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements therebetween.
Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by those with ordinary knowledge in the field of art to which the present invention belongs. Such terms as those defined in a generally used dictionary are to be interpreted to have the same meanings as contextual meanings in the relevant field of art, and are not to be interpreted to have idealized or excessively formal meanings unless clearly defined in the present application.
Further, as used herein, % means wt%, unless the context clearly indicates otherwise, and 1 ppm is 0.0001 wt%.
The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.
In general, required characteristics of an oriented electrical steel sheet used to transform electric power as a core material of a transformer are a high magnetic flux density characteristic and a low iron loss characteristic. The high magnetic flux density characteristic may increase electric power transforming efficiency and may increase design magnetic flux density, so it has a merit of reducing a size of the transformer by using a small core material. Further, in the case of iron loss that is generated by the oriented electrical steel sheet in a process of changing electric power, it has a merit of reducing a no-load loss of the transformer.
Studies and technical developments on the oriented electrical steel sheet have mostly progressed so as to reduce the iron loss. The iron loss of the oriented electrical steel sheet is generally divided into a hysteresis loss, a classical eddy current loss, and an anomalous eddy current loss.
The hysteresis loss is a loss of the electrical steel sheet generated by a magnetization degree of the oriented electrical steel sheet, and the loss is reduced when the oriented electrical steel sheet has no impurity or defects and the degree of integration of the Goss orientation is high.
The classical eddy current loss is a loss generated by an eddy current generated to the steel sheet for a magnetization process of the oriented electrical steel sheet, and efforts for reducing the loss by minimizing the eddy current of the steel sheet have been made by increasing the amount of Si and reducing a thickness of the steel sheet. Another abnormal eddy current loss is a loss relating to a movement and shift of a magnetic domain of the oriented electrical steel sheet in an AC in which the transformer is operated, and it has a characteristic in that the loss is reduced as the magnetic domain size (2L) becomes minute. Studies for improving the abnormal eddy current loss have been relatively recently progressed compared to studies on the hysteresis loss and the classical eddy current loss, and a method for applying a partial stress to a surface of the steel sheet and temporarily miniaturizing the magnetic domain by irradiating laser beams to the surface of the steel sheet and a method for permanently miniaturizing the magnetic domain through a structural change of the magnetic domain by providing a curve with a predetermined pattern to the surface of the steel sheet have been developed. Regarding another method for miniaturizing a magnetic domain, a method for providing a tension caused by a difference of expansion coefficients to the surface of the steel sheet and miniaturizing the magnetic domain by applying a coating material with a different expansion coefficient to the surface of the steel sheet has been developed.
The present inventors found that, after having repeatedly studied the reduction of the abnormal eddy current loss of the oriented electrical steel sheet, the size of the magnetic domain may be reduced when the size of grains of the oriented electrical steel sheet is reduced, and it is accordingly possible to substantially reduce the entire iron loss of the oriented electrical steel sheet.
A conventional size of the magnetic domain has a relationship with the size of the grain as expressed in Equation 1. $Magnetic domain size (2 L) \propto {(Gain size)}^{1 / 2}$
That is, as the grains become smaller, the magnetic domain becomes small, and the abnormal eddy current loss is resultantly reduced.
The abnormal eddy current loss has a relationship of Equation 2 with the classical eddy current loss. $Wea = [1.63 * (2 L / d) - 1] * Wec$
Here, Wea is an abnormal eddy current loss, Wec is a classical eddy current loss, 2L is a size of the magnetic domain, and d is a thickness of the steel sheet.
As expressed in Equation 2, when the size of the magnetic domain is reduced in the assumption that the thickness of the steel sheet is constant, the abnormal eddy current loss is also reduced.
When the size of the Goss orientation grains is reduced, it is possible to substantially reduce the size of the magnetic domain based on Equation 1 for expressing the relationship between the size of the grains and the size of the magnetic domain, and accordingly, the iron loss of the oriented electrical steel sheet may be substantially reduced.
To sum up, in order to reduce the iron loss of the oriented electrical steel sheet, it is needed to reduce the hysteresis loss according to the excellent magnetization characteristic through formation of recrystallized grains of the Goss orientation, reduce the classical eddy current loss according to an increase of an amount of Si and a reduction of the thickness of the steel sheet, and reduce the abnormal eddy current loss by miniaturizing the size of the Goss orientation grain and miniaturizing the size of the magnetic domain. It is desirable to reduce the hysteresis loss, the classical eddy current loss, and the abnormal eddy current loss so as to reduce the entire loss of the oriented electrical steel sheet, but depending on the cases, the oriented electrical steel sheet that is easy to produce and has an excellent magnetism characteristic may be manufactured by minimizing the size of the Goss orientation grain and thereby substantially improving the abnormal eddy current loss without a big improvement of the hysteresis loss or the classical eddy current loss.
A method for manufacturing an oriented electrical steel sheet according to an exemplary embodiment of the present invention includes: providing a slab including, as wt , Si at equal to or less than 4.0 % (excluding 0 %), C at 0.001 % to 0.4 %, and Mn at 0.001 % to 2.0 %, and including a balance including Fe and an impurity that is inevitably mixed and input; reheating the slab; manufacturing a hot steel sheet by hot-rolling the slab; performing hot-rolled steel sheet annealing on the hot steel sheet; primarily cold-rolling the hot-rolled steel sheet annealed hot steel sheet; decarburization-annealing the cold-rolled steel sheet; secondarily cold-rolling the decarburization-annealed steel sheet; and finally annealing the cold-rolled steel sheet. In addition, if needed, the method for manufacturing an oriented electrical steel sheet may include other steps.
The respective steps will now be described in detail.
A slab including, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at 0.001 % to 0.4 %, and Mn at 0.001 % to 2.0 %, and a balance including Fe and an impurity that is inevitably mixed and input, is provided.
Reasons for limiting the compositions are as follows.
Silicon (Si) improves the iron loss by reducing magnetic anisotropy of the oriented electrical steel sheet and increasing specific resistance. An exemplary embodiment of the present invention has a characteristic of substantially reducing the abnormal eddy current loss by reducing the size of the grain of the final product, but the iron loss may be further improved when more Si is added, so it may be effective to add more than a predetermined amount. Therefore, the content of Si may be added up to the range of 4 wt% indicating a cold-rolling allowable content. When there is a high content of Si, a brittleness property increases in the case of the cold rolling, and the cold rolling may become impossible. In detail, Si may be included at equal to or less than 1 wt% (excluding 0 wt%).
Carbon (C) is an element for catalyzing austenite phase transformation, and is an important element for making a hot-rolled structure of the oriented electrical steel sheet uniform, catalyzing a formation of grains of the Goss orientation in the case of cold rolling, and thereby manufacturing an oriented electrical steel sheet with excellent magnetism. However, when the carbon C exists in the final product, a magnetic aging phenomenon is generated to deteriorate the magnetism characteristic, so the carbon C must exist at equal to or less than 0.003 wt% in the finally manufactured electrical steel sheet. To catalyze the phase transformation by addition of carbon C and recrystallization of the Goss orientation grains, an effect will be generated when the carbon C is added at equal to or greater than 0.001 wt% in the slab, and when the content is less than the above-noted case, secondary recrystallization is unstably formed because of a non-uniform hot-rolled structure. However, when carbon C is added to the slab at greater than 0.4 wt%, the primary recrystallized grain becomes minute by formation of a minute hot-rolled structure caused by an austenite phase transformation at the time of hot rolling, a coarse carbide may be formed during a cooling process after a spiral-winding process or hot-rolled steel sheet annealing after the hot rolling is finished, and non-uniformity may be generated to tissues by forming Fe₃C (cementite) at room temperature. In addition, there is a drawback that the annealing time increases during decarburization to equal to or less than 0.003 wt% in the decarburization process and the final annealing process. Therefore, the content of carbon C in the slab may be limited to 0.001 to 0.4 wt%.
Manganese (Mn), in a like manner of Si, increases specific resistance to reduce the iron loss, and in a like manner of C, it is an important element for catalyzing the austenite phase transformation to miniaturize a particle diameter of the grain during the hot rolling and annealing process. When such Mn is added to be less than 0.001 wt%, the phase transformation is not sufficiently performed like with the effect of carbon C, so the slab and the hot-rolled structure become coarse, the particle diameter of the grain of the final product does not become minute, and the effect of the improvement of the iron loss caused by an increase of specific resistance becomes small. In addition, when Mn is added to be greater than 2.0 wt%, a manganese oxide (Mn Oxide) as well as Fe₂SiO₄ is formed on the surface of the steel sheet, and decarburization is not fluently performed in the final annealing process. Therefore, a preferable added amount of Mn may be 0.001 to 2.0 wt%. In detail, the added amount of Mn may be 0.01 to 1.0 wt%.
According to an exemplary embodiment of the present invention, aluminum (Al) is treated as an inevitable impurity. That is, the content of Al may be minimized in the slab and the steel sheet. In detail, when Al is further added, its range may be provided to be equal to or less than 0.01 wt%.
The above-noted components form a basic configuration of the present invention, and when other alloy elements for improving the magnetism characteristic are added or inevitably included, the effect of improving the iron loss caused by miniaturization of the Goss orientation grain that is the characteristic of the present invention may not be weakened.
A method for manufacturing a slab from the molten steel of the above-described composition includes a blooming method, a continuous casting method, a thin slab casting method, or a strip casting method.
The slab may be reheated. The slab reheating temperature may be 1050 °C to 1350 °C. When the slab is reheated and the temperature is low, a rolling load increases, and when the temperature is high, a slab washing phenomenon may be generated by formation of a high-temperature oxide with a low melting point to deteriorate the actual yield, and the hot-rolled structure may also become coarse to provide a bad effect to the magnetism. Therefore, the slab reheating temperature may be controlled within the above-described range.
The reheated slab is hot-rolled to manufacture a hot steel sheet. At the time of hot-rolling, a hot-rolled steel sheet may be manufactured by applying hot-rolling within the temperature range where the austenite phase exists. At a low temperature where no austenite phase exists, the rolling load increases, and the effect of miniaturizing grains caused by a phase transformation may be acquired.
The hot steel sheet undergoes hot-rolled steel sheet annealing. The hot-rolled steel sheet may undergo the hot-rolled steel sheet annealing at a temperature that is higher than the temperature at which the recrystallization and the phase transformation are allowable. In detail, to prevent the production of an oxidation layer with a low melting point caused by a high-temperature heating, the hot-rolled steel sheet annealing may be performed at a temperature of 850 to 1150 °C. An atmosphere at the time of the hot-rolled steel sheet annealing may be an atmosphere having a dew point temperature which is equal to or greater than 0 °C, for generating a decarburization reaction of the hot-rolled steel sheet, and hydrogen gas.
The hot-rolled steel sheet annealed hot steel sheet is primarily cold-rolled. After performing the hot-rolled steel sheet annealing, the steel sheet may be pickled and cold-rolled. At the time of a cold-rolling, a reduction rate may be 50 % to 70 %.
The cold-rolled steel sheet is then decarburization-annealed. The cold-rolled steel sheet undergoes annealing for recrystallization, and in this instance, in order to generate a decarburization reaction, annealing is performed in the atmosphere having a dew point temperature that is equal to or greater than 0 °C and including hydrogen gas at a temperature of 800 °C to 1150 °C. When the temperature is very low, it is difficult to perform decarburization, and when the temperature is very high, a thick oxidation layer may be formed and the decarburization reaction may be deteriorated. When the dew point temperature is very low, the decarburization reaction may be deteriorated. In detail, the dew point temperature may be 10 to 70 °C.
The decarburization-annealed steel sheet is secondarily cold-rolled. At the time of cold-rolling, the reduction rate may be 50 % to 70 %. The step for decarburization-annealing the cold-rolled steel sheet and the step for secondarily cold-rolling the decarburization-annealed steel sheet may be repeated a plurality of times. For example, when the steps are repeated twice, they may be performed in order of performing primary cold-rolling, performing decarburization-annealing, performing secondary cold-rolling, performing decarburization-annealing, performing third cold-rolling, and performing final annealing. In this instance, the cold-rolling is performed up to the thickness of the final product in the step of performing final cold-rolling, each decarburization process performs annealing in the atmosphere having the dew point temperature that is equal to or greater than 0 °C and including hydrogen gas at the temperature of 800 °C to 1150 °C so as to generate a decarburization reaction.
The cold-rolled steel sheet then undergoes final annealing.
The method for manufacturing an oriented electrical steel sheet according to an exemplary embodiment of the present invention may consecutively perform final annealing in succession to the secondary cold-rolling, differing from the existing batch method.
The final annealing step may include a first step to be performed in the atmosphere with the dew point temperature of 10 °C to 70 °C at the temperature of 850 °C to 1150 °C and a second step to be performed in the atmosphere of a mixed gas including hydrogen and nitrogen with the dew point temperature that is equal to or less than 10 °C at a temperature of 900 °C to 1200 °C. The first step may be performed for under 300 seconds, and the second step may be performed for 60 to 300 seconds.
The cold-rolled steel sheet, before it is finally annealed, undergoes decarburization-annealing so that an amount of silicon steel carbon of 40 wt% to 60 wt% remains with respect to the carbon amount of the minimum slab. Therefore, in the final annealing, in the first step, carbon escapes, and the grains formed on the surface portion are diffused to the inside. The first step may perform decarburization so that the amount of carbon in the steel sheet may be equal to or less than 0.01 wt%.
In the second step, a texture with the Goss orientation diffused in the first step grows. Regarding the method for manufacturing an oriented electrical steel sheet according to an exemplary embodiment of the present invention, a particle diameter of the grain of the Goss texture may be within 1 mm, differing from the conventional case in which the grains grow by the growth of abnormal particles. Therefore, compared to the conventional oriented electrical steel sheet, a microstructure made of Goss orientation grains with a very small particle diameter of the grain may be provided.
The amount of carbon in the finally annealed electrical steel sheet may be equal to or less than 0.003 wt%.
The finally annealed oriented electrical steel sheet may be dried, if necessary, when an insulating coating liquid is applied thereto.
Further, an annealing separating agent with MgO as a major component is coated during the final annealing in the conventional batch type, so a MgO coating layer exists, but the oriented electrical steel sheet according to an exemplary embodiment of the present invention may undergo final annealing not in a batch type but in a continuous manner, and accordingly may not have a MgO coating layer.
The grains of the Goss orientation (an orientation within 15 degrees from the orientation of {110}<001>) produced through an exemplary embodiment of the present invention tend to further increase as the cold rolling and the decarburization-annealing are repeated, and when the cold-rolling and the decarburization-annealing are performed at least twice, a volumetric fraction of the grain having the Goss orientation in the steel sheet increases by at least 50 %.
The grains produced through an exemplary embodiment of the present invention have a particle diameter of less than 5 mm, and the volumetric fraction of the grain to be 20 µm to 1000 µm becomes equal to or greater than 50 %. As a result, the size of the magnetic domain existing in the grain becomes very small. The size of the magnetic domain shown in the conventional oriented electrical steel sheet is greater than the thickness of the conventional steel sheet, but regarding the steel sheet produced through an exemplary embodiment of the present invention, the magnetic domain size (2L) existing in the grain is formed to be less than the thickness (D) of the steel sheet.
The oriented electrical steel sheet according to an exemplary embodiment of the present invention includes, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at equal to or less than 0.003 % (excluding 0 %), and Mn at 0.001 to 2.0 %, the balance includes Fe and impurities that are inevitably mixed, and the size (2L) of the magnetic domain existing in the grain is less than the thickness (D) of the steel sheet.
A composition on the oriented electrical steel sheet corresponds to the composition of the above-noted slab, and a composition range in the process for manufacturing an oriented electrical steel sheet does not substantially change, so no repeated description will be provided. As described above, carbon is decarburized in the decarburization-annealing and the final annealing, so the content of carbon becomes equal to or less than 0.003 wt%.
Regarding the oriented electrical steel sheet according to an exemplary embodiment of the present invention, the volumetric fraction of the grain with the Goss orientation in the steel sheet increases by at least 50 % to provide excellent iron loss and magnetic flux density, the particle diameter of 20 to 1000 um of the grain in the oriented electrical steel sheet is equal to or greater than 50 %, the size is not greater than 5 mm at a maximum, and the size of the magnetic domain existing in the grain becomes less than the thickness of the steel sheet. Because of the minute magnetic domain structure, the abnormal eddy current loss of the steel sheet produced according to the present invention is substantially reduced compared to the abnormal eddy current loss of the oriented electrical steel sheet produced by the prior art, thereby substantially improving the iron loss.
In detail, the size (2L) of the magnetic domain existing in the grain may be 10 to 500 µm.
The present invention will now be described in detail through examples. The examples exemplify the present invention, and the present invention is not limited thereto.

Example 1

A slab including, as wt%, Si at 2.0 %, C at 0.15 %, and Mn at 0.05 %, and including a balance including Fe and inevitable impurities, is heated at a temperature of 1100 °C, it is hot rolled with a thickness of 3 mm, hot-rolled steel sheet annealing is performed at an annealing temperature of 1000 °C, it is cooled, it is pickled, and a cold rolling is performed up to the thickness of 0.27 mm. When the cold rolling is performed up to the final thickness, a method for performing cold rolling up to the final thickness without including decarburization-annealing between cold rolling and cold rolling, and a method for including decarburization-annealing between cold rolling and cold rolling at least once and performing cold rolling with a plurality of steps, are provided. The decarburization-annealing is performed in the atmosphere (with the dew point temperature of 60 °C) of a wet mixed gas of hydrogen and nitrogen at the temperature of 1000 °C.
At the time of final annealing, annealing is performed for two minutes in the atmosphere (with the dew point temperature 60 °C) of a wet mixed gas of hydrogen and nitrogen at the temperature of 1000 °C, and annealing is performed for three minutes in the dry atmosphere of a mixed gas of hydrogen and nitrogen at the temperature (with a dew point temperature 0 °C) of 1100 °C.
A relationship between a fraction of the Goss orientation grain from the finally annealed steel sheet and a magnetism characteristic is compared and is expressed in Table 1.
Here, a method for estimating a fraction of the Goss orientation grain includes measuring a volume fraction of grains of an orientation indicating an error of within 15 degrees from the ideal orientation of {110}<001> by using a conventional method for measuring a crystal orientation.

In addition, the method includes measuring an average size of the magnetic domain through observation of a magnetic domain while the electrical steel sheet is demagnetized, by using Kerr microscopy.

(Table 1)

Number of times of cold rolling to final thickness	Goss orientation grain fraction (%)	Magnetic domain size (µm)	Magnetic flux density (B10)	Iron loss (W17/50)	Etc.
1	32	31	1.65	1.88	Comparative material
2	53	55	1.89	0.99	Exemplary embodiment
3	85	40	1.92	0.95	Exemplary embodiment
4	87	86	1.95	0.91	Exemplary embodiment

When the process for performing cold rolling up to the final thickness after hot-rolled steel sheet annealing is performed as expressed in Table 1 includes intermediate annealing generating decarburization at least once, the fraction of the Goss orientation grains may be acquired for the final product of equal to or greater than 50 %, and a minute magnetic domain size may be obtained. The characteristics of an excellent magnetic flux density and a low iron loss may be acquired from the final product by the high Goss orientation fraction and the minute magnetic domain size.

Example 2

A slab including, as wt%, C at 0.2 % and Mn at 0.05 %, and including a balance of Fe and inevitable impurities, is manufactured by varying the content of Si, as expressed in Table 2. The slab is heated at a temperature of 1150 °C, it is hot rolled to a thickness of 3 mm, hot-rolled steel sheet annealing is performed at a annealing temperature of 950 °C, it is cooled, it is pickled, and cold rolling is performed with a reduction rate of 60 %. The cold-rolled sheet is recrystallized and decarburization-annealed in the mixed gas atmosphere of hydrogen and nitrogen with a dew point temperature 60 °C at a temperature of 900 °C. The same cold-rolling and decarburization-annealing are repeated twice. Cold rolling is performed up to the thickness of the steel sheet of 0.23 mm, decarburization-annealing is performed for 180 seconds in the mixed gas atmosphere of hydrogen and nitrogen with the dew point temperature of 60 °C at the temperature of 950 °C (first step), and heat treatment is performed for 100 seconds in the dry (the dew point of 0 °C) hydrogen atmosphere at the temperature of 1000 °C (second step). Magnetism characteristics of the final annealing steel sheet according to changes of the content of Si are expressed in

Table 2.

(Table 2)

Si content (%)	Particle diameter of grain (um)	Magnetic domain size (um)	Magnetic flux density (B10)	Iron loss (W17/50)	Etc.
0.0005	210	84	1.90	0.97	Exemplary embodiment
0.1	156	56	1.89	0.99	Exemplary embodiment
0.5	365	137	1.91	0.97	Exemplary embodiment
1.0	423	181	1.89	0.99	Exemplary embodiment
1.5	510	229	1.92	0.91	Exemplary embodiment
2.0	198	91	1.93	0.93	Exemplary embodiment
3.0	257	173	1.91	0.91	Exemplary embodiment
3.5	454	125	1.90	0.92	Exemplary embodiment
4.0	781	89	1.89	0.90	Exemplary embodiment
4.3	15	23	1.68	1.22	Comparative material

As expressed in Table 2, when the content of Si is equal to or less than 4 wt%, a microstructure with the final particle diameter of the grain that is equal to or less than 1000 µm is acquired through a plurality of cold rolling and decarburization-annealing steps, and in this instance, the size of the magnetic domain that is less than the thickness of the steel sheet is acquired, thereby acquiring excellent iron loss. When the content of Si is greater than 4 wt%, a brittleness property increases, so it is difficult to perform cold rolling up to the final thickness because of a strip breakage when performing cold rolling, and decarburization is not performed during a decarburization-annealing time, thereby showing a very small particle diameter of the grain and a deteriorated magnetism characteristic.

Example 3

A slab including, as wt%, Si at 3.0 %, C at 0.25 %, and Mn at 0.5 %, and a balance including Fe and inevitable impurities, is heated at the temperature of 1200 °C, it is hot rolled with the thickness of 2.5 mm, hot-rolled steel sheet annealing is performed in the mixed gas atmosphere of hydrogen and nitrogen with the dew point temperature of 40 °C at the annealing temperature 1100 °C, it is cooled, it is pickled, and it is primarily cold rolled with the reduction rate of 65 %. The cold-rolled sheet is decarburization-annealed in the wet mixed gas atmosphere of hydrogen and nitrogen with the dew point temperature of 60 °C at the temperature of 1050 °C. The primarily decarburization-annealed sheet is secondarily cold rolled up to the thickness of 0.30 mm, and it is finally annealed. The final annealing changes the temperature of annealing and performs decarburization-annealing as expressed in Table 3 in the wet mixed gas atmosphere of hydrogen and nitrogen with the dew point temperature of 65 °C so that the content of carbon may be equal to or less than 0.003 wt% (1step). The temperature is increased following the decarburization-annealing to perform a finishing heat treatment in the dry hydrogen atmosphere with the dew point of 0 °C at the temperature of 1150 °C (2step). The particle diameter of the grain of the finally annealed steel sheet and the magnetic domain size were measured using Kerr microscopy, and are shown in Table 3 in comparison with the magnetism characteristic.

(Table 3)

first-step final annealing temperature (°C)	Particle diameter of grain (um)	Grain ratio of 20 to 1000 µm (%)	Magnetic domain size (um)	Magnetic flux density (B10)	Iron loss (W17/50)	Etc.
830	18	43	8	1.68	1.95	Comparative material
850	25	51	21	1.89	1.05	Exemplary embodiment
870	50	58	45	1.91	0.97	Exemplary embodiment
890	128	67	108	1.90	1.00	Exemplary embodiment
910	253	85	117	1.89	0.99	Exemplary embodiment
930	391	92	196	1.90	0.97	Exemplary embodiment
950	510	97	207	1.92	0.99	Exemplary embodiment
1000	732	99	266	1.91	0.98	Exemplary embodiment
1080	805	98	295	1.92	0.91	Exemplary embodiment
1170	1038	48	505	1.82	1.52	Comparative material

As expressed in Table 3, when the final annealing temperature (1step) is 850 to 1150 °C, the particle diameter of the grain of the final product is shown to be 20 to 1000 µm, and the ratio is shown to be equal to or greater than 50 %, and accordingly, the magnetic domain size is shown to be less than the thickness of the steel sheet, thereby indicating an excellent iron loss characteristic. When the decarburization-annealing temperature is less than 850 °C, the magnetic domain size is shown to be very small, and the reason that the magnetism characteristic is deteriorated is that the fraction of the Goss orientation among the grains is equal to or less than 50 %. On the contrary, when the same is greater than 1150 °C, the particle diameter of the grain becomes coarse, and the size of the magnetic domain is greater than the thickness of the steel sheet, so the iron loss is not improved.
While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. Therefore, the embodiments described above are only examples and should not be construed as being limitative in any respects.

Claims

A method for manufacturing an oriented electrical steel sheet, comprising:
providing a slab including, as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at 0.001 % to 0.4 %, and Mn at 0.001 to 2.0 %, and including a balance including Fe and inevitably mixed and input impurities;

reheating the slab;

manufacturing a hot steel sheet by hot-rolling the slab;

performing hot-rolled steel sheet annealing to the hot steel sheet;

primarily cold-rolling the hot-rolled steel sheet annealed hot steel sheet;

decarburization-annealing the cold-rolled steel sheet;

secondarily cold-rolling the decarburization-annealed steel sheet; and

finally annealing the cold-rolled steel sheet,

wherein, regarding the finally annealed steel sheet, a size 2L of a magnetic domain existing in a grain is less than a thickness D of the steel sheet.
The method of claim 1, wherein
the slab includes Si at equal to or less than 1 wt% (excluding 0 wt%).
The method of claim 1, wherein
the slab further includes Al at equal to or less than 0.01 wt% (excluding 0 wt%).
The method of claim 1, wherein
a reheating temperature of the slab is 1050 °C to 1350 °C.
The method of claim 1, wherein
reduction rates in the primarily cold-rolling and the secondarily cold-rolling are respectively 50 % to 70 %.
The method of claim 1, wherein
the decarburization-annealing of the cold-rolled steel sheet and the secondarily cold-rolling of the decarburization-annealed steel sheet are repeated at least twice.
The method of claim 1, wherein
the decarburization-annealing is performed in an atmosphere including hydrogen with a dew point temperature of 0 °C at a temperature of 800 °C to 1150 °C.
The method of claim 1, wherein
the finally annealing includes a first step for performing the same in an atmosphere with the dew point temperature of 10 °C to 70 °C at the temperature of 850 °C to 1150 °C, and a second step for performing the same in a mixed gas atmosphere including hydrogen and nitrogen with a dew point temperature that is equal to or less than 10 °C at a temperature of 900 °C to 1200 °C.
The method of claim 8, wherein
the first step is performed for equal to or less than 300 seconds, and the second step is performed for 60 seconds to 300 seconds.
The method of claim 1, wherein
the finally annealing is continuously performed after the cold-rolling.
The method of claim 1, wherein
an amount of carbon in the electrical steel sheet is equal to or less than 0.003 wt% (excluding 0 wt%) after the finally annealing.
The method of claim 1, wherein
regarding the finally annealed steel sheet, a volumetric fraction of a grain with an orientation that is within 15 degrees from an orientation {110}<001> is equal to or greater than 50 %.
The method of claim 1, wherein
regarding the finally annealed steel sheet, a volumetric fraction of a grain with a particle diameter of 20 µm to 1000 µm is equal to or greater than 50 %.
An oriented electrical steel sheet comprising,
as wt%, Si at equal to or less than 4.0 % (excluding 0 %), C at equal to or less than 0.003 % (excluding 0 %), and Mn at 0.001 to 2.0 %, and a balance including Fe and an impurity that is inevitably mixed and input,
wherein a size 2L of a magnetic domain existing in a grain is less than a thickness (D) of a steel sheet.
The oriented electrical steel sheet of claim 14, wherein
Si is included to be equal to or less than 1.0 wt% (excluding 0 wt%).
The oriented electrical steel sheet of claim 14, wherein
Al is further included to be equal to or less than 0.01 wt% (excluding 0 wt%).
The oriented electrical steel sheet of claim 14, wherein
a size 2L of a magnetic domain existing in a grain is 10 to 500 µm.
The oriented electrical steel sheet of claim 14, wherein
a volumetric fraction of a grain with an orientation that is within 15 degrees from an orientation {110}<001> is equal to or greater than 50 %.
The oriented electrical steel sheet of claim 14, wherein
a volumetric fraction of a grain with a particle diameter of 20 µm to 1000 µm is equal to or greater than 50 %.