EP3225701A1

EP3225701A1 - Annealing separator composition for oriented electrical steel sheet, and method for manufacturing oriented electrical steel sheet using same

Info

Publication number: EP3225701A1
Application number: EP15863386.7A
Authority: EP
Inventors: Chang Soo Park; Min Soo Han; Byung-Deug HONG; Soon-Bok Park
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2014-11-26
Filing date: 2015-11-25
Publication date: 2017-10-04
Also published as: JP6535739B2; JP2018504517A; CN107002158A; KR20160063244A; EP3225701A4; CN107002158B; KR101696627B1

Abstract

Disclosed are an annealing separating agent composition for a directional electrical steel sheet and a method for manufacturing a directional electrical steel sheet using the same. Specifically, there are provided an annealing separating agent composition for a directional electrical steel sheet including magnesium oxide or magnesium hydroxide, metal iodide, and a solvent, and a method for manufacturing a directional electrical steel sheet using the annealing separating agent composition as an annealing separating agent in a high-temperature annealing process.

Description

[Technical Field]

The present invention relates to an annealing separating agent composition for a directional electrical steel sheet and a method for manufacturing a directional electrical steel sheet using the same.

[Background Art]

In general, a directional electrical steel sheet contains an Si component of about 3.2% and has a texture of the bearings of crystal grains arranged in the 110 direction. The directional electrical steel sheet has a very excellent magnetic characteristic in the rolling direction and is chiefly used for transformers, motors, generators, other electronic devices, and other core materials using the characteristic.
As a directional electrical steel sheet of a high magnetic flux density grade is recently commercially available, there is a need for a material having a small iron loss. In this connection, research is carried out on a method of forming an insulation film having a high tension characteristic on a surface of the directional electrical steel sheet.
More specifically, the insulation film is commonly formed on a Forsterite (Mg₂SiO₄)-series coating (hereinafter referred to as a "base coating"), that is, the base film of the steel sheet. Accordingly, the method corresponds to a technology for promoting an effect of reducing an iron loss by adding tension stress to the steel sheet based on a difference in the coefficient of thermal expansion between the insulation film formed on the base coating and the steel sheet.
However, the directional electrical steel sheet fabricated using the method has a limit to the improvement of a magnetic characteristic. The limit is caused by the base coating present between the insulation film and the steel sheet. The reason for this is that the base coating acts as a pinning point that hinders a flow of a magnetic domain moving along the surface of the steel sheet.
Accordingly, there is a need for a glassless technology for removing such a base coating.

[DISCLOSURE]

[Technical Problem]

The present invention has been made in an effort to provide a technology for introducing metal iodide into an annealing separating agent composition in order to induce a base coating to be spontaneously removed during a high-temperature annealing process.
The detailed contents of the technology are as follows. One embodiment of the present invention may provide an annealing separating agent composition for a directional electrical steel sheet, including magnesium oxide or magnesium hydroxide, metal iodide and a solvent.
Another embodiment of the present invention may provide a method for manufacturing a directional electrical steel sheet using the annealing separating agent composition at the time of high-temperature annealing in a series of processes of fabricating a directional electrical steel sheet by performing hot rolling-cold rolling-decarbonizing and nitriding processing-high-temperature annealing after preparing a steel slab.

[Technical Solution]

One embodiment of the present invention provides an annealing separating agent composition for a base coating-free directional electrical steel sheet, including magnesium oxide or magnesium hydroxide and metal iodide.
More specifically, the annealing separating agent composition may be an annealing separating agent composition for a base coating-free directional electrical steel sheet.
A related description is as follows. A composition of the annealing separating agent includes 5 to 20 parts by weight of the metal iodide with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide, and the solvent is added to the extent that it can properly disperse the components. In this case, 11 to 20 parts by weight of the metal iodide is preferred with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
Each of components include in the annealing separating agent composition is described in detail below.
The metal forming the metal iodide may be any one metal selected from the group consisting of Ag, Co. Cu, Mo and a combination thereof.
The magnesium oxide or magnesium hydroxide may include magnesium oxide (MgO).
Another embodiment of the present invention provides a method for manufacturing a directional electrical steel sheet, including preparing a steel slab including Si: 0.5 - 4.5 wt% and other inevitable impurities and including a remainder of Fe, heating the steel slab 1,300 °C or less, fabricating a hot-rolled plate by performing a hot rolling on the heated steel slab, fabricating a cold-rolled plate by performing two or more cold rollings including one cold rolling or middle annealing on the hot-rolled plate, performing decarbonizing annealing and nitriding processing on the cold-rolled plate, coating an annealing separating agent on a surface of the decarbonizing annealing- and nitrifying-processed steel sheet, performing high-temperature annealing on the steel sheet on which the annealing separating agent has been coated, and obtaining a directional electrical steel sheet, wherein the annealing separating agent is a slurry including magnesium oxide or magnesium hydroxide, metal iodide and a solvent.
More specifically, the annealing separating agent composition may be an annealing separating agent composition for a base coating-free directional electrical steel sheet. A related description is as follows. A composition of the annealing separating agent includes 5 to 20 parts by weight of the metal iodide with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide, and the solvent is added to the extent that it can properly disperse the components. In this case, 11 to 20 parts by weight of the metal iodide is preferred with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
Hereinafter, each of components included in the annealing separating agent is as follows.
The metal forming the metal iodide may be any one metal selected from the group consisting of Ag, Co. Cu, Mo and a combination thereof.
The magnesium oxide or magnesium hydroxide may comprise magnesium oxide (MgO).
Performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated is described in detail below.
Performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated may be performed in a temperature range of 650 to 1200°C.
Specifically, performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated may include heating the steel sheet at a temperature-rising rate of 0.1 to 20°C/hr from 650°C to 1200°C and then maintaining the steel sheet in the temperature range of 1150 to 1250°C for 20 hours after the temperature 1200°C is reached. More specifically, performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated may include performing the high-temperature annealing in a mixed gas atmosphere in which a volume ratio of hydrogen to nitrogen is 15 to 40 % and starting to, by a base coating of the steel sheet on which the annealing separating agent has been coated, be delaminated when a temperature range of 1000°C or more is reached.
In some embodiments, performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated may include performing the high-temperature annealing in a mixed gas atmosphere in which a volume ratio of hydrogen to nitrogen is 40 to 75% and starting to, by a base coating of the steel sheet on which the annealing separating agent has been coated, be delaminated when a temperature range of 950°C or more is reached. Surface roughness and a coercive force in 1.7T/50Hz of the high-temperature-annealed steel sheet may satisfy a relationship expressed in Equation 1. [Equation 1] 3 ≤ (surface roughness (um) X coercive force (A/m)) ≤ 9
Brilliance of the high-temperature-annealed steel sheet may be 150 GU or more.
Drying the steel sheet on which the annealing separating agent has been coated may be performed in a temperature range of 300 to 700 °C.

[Advantageous Effects]

One embodiment of the present invention can provide the annealing separating agent composition for a directional electrical steel sheet, which can derive the spontaneous delamination of a base coating before a temperature at which secondary re-crystallization is initiated is reached upon high-temperature annealing by the metal iodide.
Another embodiment of the present invention can provide the method for manufacturing a base coating-free directional electrical steel sheet having an excellent magnetic characteristic due to the effective removal of a base coating and a reduced iron loss because the high-temperature annealing process is performed by the annealing separating agent composition.

[Description of the Drawings]

FIG. 1 shows an Ellingham diagram of several materials according to partial pressure of iodide ions.

[Mode for Invention]

Hereinafter, embodiments of the present invention are described in detail. However, the embodiments are only illustrative, the present invention is not restricted by the embodiments, and the present invention is defined by only the category of the claims to be described later.
One embodiment of the present invention provides an annealing separating agent composition for a base coating-free directional electrical steel sheet, including magnesium oxide or magnesium hydroxide and metal iodide.
The annealing separating agent composition is used in a high-temperature annealing process of processes of manufacturing a directional electrical steel sheet (i.e., a series of processes for manufacturing a directional electrical steel sheet by performing hot rolling-cold rolling-decarbonizing annealing and nitriding processing-high-temperature annealing after a steel slab is prepared), and contributes to the fabrication of a base coating-free directional electrical steel sheet by deriving a phenomenon in which a base coating formed in the high-temperature annealing process is spontaneously delaminated.
That is, the annealing separating agent composition may be an annealing separating agent composition for a base coating-free directional electrical steel sheet. A directional electrical steel sheet fabricated using the annealing separating agent composition may have a reduced iron loss and an excellent magnetic characteristic because a base coating layer is removed.
A commonly known annealing separating agent forms a base coating (i.e., a base coating expressed into a chemical formula Mg₂SiO₄) through a reaction with an oxidation film essentially formed on a surface of a decarbonizing annealing- and nitrifying-processed steel sheet because it includes magnesium oxide (MgO). As described above, the base coating needs to be removed because it acts as a so-called pinning point by hindering a flow of a magnetic domain that move along the surface of the steel sheet.
In this connection, the annealing separating agent composition provided by one embodiment of the present invention forms a base coating by the magnesium oxide or magnesium hydroxide in the first half of a high-temperature annealing process, but can derive the spontaneous delamination of the formed base coating by the metal iodide in the second half of the high-temperature annealing process.
Hereinafter, an action of the annealing separating agent composition for a directional electrical steel sheet provided by one embodiment of the present invention is described in more detail in association with a method for manufacturing the directional electrical steel sheet.
In general, a decarbonizing annealing and nitriding processing process corresponds to a process required to generate an inhibitor in order to remove carbon included in a cold-rolled steel sheet (i.e., a cold-rolled plate) and also to properly control the growth of secondary re-crystal grains in a high-temperature annealing process, that is, a subsequent process.
In general, the process is performed by setting a temperature within a furnace to about 800 to 950°C under a humidity atmosphere including a mixed gas of ammonia, hydrogen and nitrogen.
The reason for this is that there is a possibility that crystals may grow in an undesirable bearing at the time of high-temperature annealing because decarbonizing annealing and nitriding processing are rarely performed in an excessively low temperature, and a crystal grain maintains a fine state, whereas there is a possibility that primarily re-crystallized crystal grains may excessively grow in a too high temperature.
As a steel sheet passes through a furnace controlled under the atmosphere, SiO₂ is formed on a surface of the steel sheet because silicon (Si), that is, a component having the highest oxygen affinity within the steel sheet, reacts to oxygen. When oxygen gradually penetrates the steel sheet, Fe-series oxide is further formed.
That is, in the decarbonizing annealing and nitriding processing process, an oxidation film including the SiO₂ and the Fe-series oxide is inevitably formed on a surface of the steel sheet.
In this case, in the decarbonizing annealing and nitriding processing process, decarbonizing and nitride may be simultaneously performed or decarbonizing annealing and nitriding processing may be sequentially performed.
After the decarbonizing annealing and nitriding processing process, a process of coating an annealing separating agent chiefly including MgO on the surface of the steel sheet and then performing high-temperature annealing is performed. In this case, SiO₂ within the oxidation film reacts to the MgO. Such a reaction may be expressed into Chemical Reaction Formula 1. Chemical Reaction Formula 1 corresponds to a reaction for forming Mg₂SiO₄, that is, a base coating.

[Chemical Reaction Formula 1] 2Mg (OH)₂ + SiO₂ → Mg₂SiO₄ (base coating) + 2H₂O
In general, the base coating has been known to have effects of preventing coalescence between steel sheets wound in a coil state, reducing an iron loss by assigning tension to the steel sheet, and providing insulation.
However, it is necessary to pay attention to a magnetic property lost in a surface of a directional electrical steel sheet by taking into consideration that a demand for the directional electrical steel sheet having the characteristics of a low iron loss and high magnetic flux density is recently increased.
As described above, the base coating may act as a pinning point that hinders a flow of a magnetic domain moving along a surface of the directional electrical steel sheet. Accordingly, there is a need for a glassless technology for removing the base coating.
The annealing separating agent composition developed for this purpose can remove the base coating by the metal iodide included in the annealing separating agent even without including a process that is complex and does not have economic feasibility, such as acid cleaning or chemical polishing.
More specifically, the metal iodide can derive the delamination of the base coating in such a manner that it forms an Fel₂ film through a reaction with a surface of a steel sheet during high-temperature annealing and is then evaporated from the surface.
Metal chloride can also remove the base coating like the metal iodide, but has a drawback in that it is vulnerable to the improvement of the magnetic characteristic of the finally obtained directional electrical steel sheet.
For example, in the case of BiCl₃, that is, a kind of metal chloride, Cl atoms (i.e., Cl atoms of BiCl₃) are diffused toward a surface of a steel sheet again rather than exiting to the outside of the steel sheet due to pressure within a furnace at the time of high-temperature annealing. As a result, BiCl₃ causes the following chemical reaction at the boundary surface of the steel sheet and a base coating thereof.

[Chemical Reaction Formula 2] Fe + 2Cl → FeCl₂
Since the evaporation point of the generated FeCl₂ is 1025°C,it is theoretically possible to delaminate the base coating from the surface of the steel sheet as FeCl-₂ is gasified in the high-temperature annealing process.
However, FeCl₂ may derive a reaction expressed into Chemical Reaction Formula 3 below because hydrogen and nitrogen are mixed within the furnace of an actual high-temperature annealing process.

[Chemical Reaction Formula 3] FeCl₂ + H₂ → 2HCl + Fe
If the reaction of Chemical Reaction Formula 3 is generated prior to 1025°C, that is, the evaporation temperature of FeCl₂, HCl gas is generated at the interface of the steel sheet and the base coating. The HCl gas can delaminate the oxidation film.
If the base coating is delaminated less than 1025°C, that is, the evaporation temperature of FeCl₂, as described above, the magnetic characteristic of the finally obtained directional electrical steel sheet is inevitably deteriorated.
Specifically, secondary re-crystal grains are formed during the high-temperature annealing process. The secondary re-crystal grains play an important role in a reduction of an iron loss and the improvement of magnetic flux density of the directional electrical steel sheet. However, a temperature of less than the evaporation temperature (i.e., 1025°C) of FeCl₂ is an excessively low temperature at which sufficient secondary re-crystallization is generated by taking into consideration that the secondary re-crystallization phenomenon is started between about 1050 to 1100°C.
More specifically, it is necessary to suppress the growth of crystal grains by making a precipitate, such as AIN and MnS within the steel sheet, stably present prior to a temperature region in which the secondary re-crystallization is generated.
If a base coating is present, the decomposition of the precipitate may be suppressed by preventing gases, such as hydrogen and nitrogen within a furnace, from having direct contact with the steel sheet. If the base coating is already eliminated by HCl gas prior to a temperature at which secondary re-crystallization is initiated, the decomposition of the precipitate is caused in an exposed surface of the steel sheet. Accordingly, the growth of the crystal grains is suppressed and thus secondary re-crystal grains are not properly formed.
Furthermore, the HCl gas has a danger of corroding a furnace because it has great reactivity with a metal material and also has an environmentally-harmful drawback because it corresponds to a poisonous gas.
In contrast, if metal iodide not the metal chloride is used, Fel-₂ instead of FeCl₂ is generated in the steel sheet and an oxidation film interface thereof and experiences the following chemical reaction due to the influence of an atmosphere within the furnace.

[Chemical Reaction Formula 4] Fel₂+ H₂ → 2HI + Fe
Even in this case, the generated HI gas eliminates the base coating while it exits to the outside of the steel sheet, but the base coating may be eliminated at a high temperature of about 80°C compared to a case where metal chloride is used regardless of partial pressure of hydrogen and nitrogen within the furnace.
Particularly, if a ratio of hydrogen and nitrogen is 0.25:0.75, it is checked that a temperature at which the base coating is eliminated from a surface of the steel sheet is about 1045°C. The temperature corresponds to a temperature almost similar to the temperature at which secondary re-crystallization is initiated.
Accordingly, the precipitate, such as AIN or MnS within the steel sheet, may be stably present up to a relatively higher temperature than metal chloride when metal iodide is used as an annealing separating agent.
That is, the metal iodide is a material more advantageous than metal chloride in that it derives secondary re-crystallization having an excellent iron loss characteristic and has a safer characteristic in terms of the corrosion of a high-temperature annealing furnace and toxicity.
A difference in the effect according to use of the metal iodide not the metal chloride within the annealing separating agent can be supported in more detail through examples.
A composition of the annealing separating agent composition for a base coating-free directional electrical steel sheet provided by one embodiment of the present invention and the components of the composition are described in detail below.
First, in the composition of the annealing separating agent, the metal iodide may be 5 to 20 parts by weight and the solvent may be 800 to 750 parts by weight with respect to 100 parts by weight of the magnesium oxide or the magnesium hydroxide. In this case, the solvent is sufficient to the extent that it can properly disperse components. Furthermore, the metal iodide may be preferably 11 to 20 parts by weight.
In this connection, the reaction of Chemical Reaction Formula 1 is caused to form a base coating during a high-temperature annealing process due to the magnesium oxide or magnesium. However, after a proper temperature range is reached, the reaction of Chemical Reaction Formula 4 is caused by the metal iodide of 5 to 20 parts by weight, preferably, 11 to 20 parts by weight, thereby being capable of eliminating the formed base coating.
If metal iodide of less than 5 parts by weight is contained, however, a base coating-free degree may be reduced because the reaction of Chemical Reaction Formula 4 is not sufficient. If metal iodide of more than 20 parts by weight is contained, the decomposition of a precipitate is performed prior to a temperature at which secondary re-crystallization is initiated because the base coating is not smoothly formed at the beginning of the high-temperature annealing process, and thus poor magnetism may be obtained. Accordingly, the range of the metal iodide is limited as described above.
Each of the components included in the annealing separating agent composition is described in detail below.
A metal that forming the metal iodide may be any one metal selected from the group consisting of Ag, Co. Cu, Mo and a combination thereof.
This means that a phenomenon in which a base coating caused by Chemical Reaction Formula 4 is eliminated may be generated by the iodide of a specific metal other than all of metals.
Specifically, the reason for this is that if iodide ions (I^-) forming metal iodide form HI at a low temperature through a direct reaction with hydrogen within a furnace, the elimination of the base coating may be already caused prior to a temperature at which secondary re-crystallization is initiated.
Accordingly, as in Chemical Reaction Formula 4, after Fel₂ is formed at the beginning of high-temperature annealing, HI needs to be formed in a temperature region in which secondary re-crystallization within the steel sheet is initiated and thereafter the base coating needs to be eliminated. To this end, it is necessary to select metal iodide, which is thermodynamically more stable than HI, but is more unstable than Fel₂.
FIG. 1 shows an Ellingham diagram of several materials according to partial pressure of iodide ions in order to confirm such a fact. In this case, the Ellingham diagram is a tool indicative of the direction of a chemical reaction. In a given temperature, a reaction having a low free energy value (ΔG) is a more stable state. Accordingly, the form of a compound changes to a reaction having lower energy on the Ellingham diagram.
Specifically, FIG. 1 shows temperatures (Kelvin) in a horizontal axis and free energy (KJ/mol) in a vertical axis and shows results that satisfy the following chemical reaction formula 5 for each material.

[Chemical Reaction Formula 5] aM + bl₂ → cMI_x (wherein x is 1 or 2)
In FIG. 1, it is necessary to select metal iodide if a region in which an energy value according to a temperature is smaller than that of HI, but is greater than that of Fel₂ is present. The condition may be satisfied if the metal forming metal iodide is Ag, Co, Cu or Mo.
The magnesium oxide or magnesium hydroxide may be magnesium oxide (MgO). The magnesium oxide (MgO) has been widely known, and a detailed description thereof is omitted.
Furthermore, the solvent may be water (H₂O). If the solvent is water, the annealing separating agent composition may have a form of a slurry including the magnesium oxide or magnesium hydroxide and the metal iodide.
Another embodiment of the present invention provides a method for manufacturing the base coating-free directional electrical steel sheet, including preparing a steel slab; heating the steel slab 1,300 °C or less; fabricating a hot-rolled plate by hot-rolling the heated steel slab; fabricating a cold-rolled plate by cold-rolling the hot-rolled plate; performing decarbonizing annealing and nitriding processing on the cold-rolled plate; coating an annealing separating agent on a surface of the decarbonizing-annealed steel sheet; and performing high-temperature annealing on the steel sheet on which the annealing separating agent has been coated. The annealing separating agent includes magnesium oxide or magnesium hydroxide, metal iodide and a solvent.
The method corresponds to a method for manufacturing a base coating-free directional electrical steel sheet, which does not include a base coating and has a significantly reduced iron loss and improved magnetic flux density because the annealing separating agent is used in the high-temperature annealing process.
A description of the annealing separating agent and a high-temperature annealing process using the annealing separating agent is the same as that described above, and thus a manufacturing process other them is described below.
First, after a steel slab which includes Si: 0.5 - 4.5 wt% and other inevitable impurities and whose remainder is Fe is prepared, the prepared slab is heated. In this case, the slab is heated using a low-temperature slab method at a temperature of 1,300°C or less.
After the heated slab is hot-rolled under a common condition, hot-rolled plate annealing is performed on the heated slab or omitted. Thereafter, after two or more cold rollings including one cold rolling or middle annealing is performed, decarbonizing annealing and nitriding processing are performed on the heated slab.
The decarbonizing annealing and the nitriding processing may be simultaneously performed or the nitriding processing may be performed after the decarbonizing annealing is performed.
As described above, after the annealing separating agent is coated on the steel sheet on which the decarbonizing annealing and nitriding processing have been performed, high-temperature annealing is performed under a condition to be described below. Thereafter, an insulation film may be formed if necessary, or a magnetic domain refining process may be optionally performed.
The optional process may be performed in accordance with a typical method of a directional electrical steel sheet, and thus a detailed description thereof is omitted.
In this case, in the composition of the annealing separating agent, the metal iodide may be 5 to 20 parts by weight and the solvent may be 800 to 750 parts by weight with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide. In this case, the metal iodide may be preferably 11 to 20 parts by weight.
A metal forming the metal iodide may be any one metal selected from the group consisting of Ag, Co. Cu, Mo and a combination thereof.
The magnesium oxide or magnesium hydroxide may be magnesium oxide (MgO).
Performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated is described in detail below.
Specifically, in a high-temperature annealing step of raising a temperature from room temperature to 1200°C, the steel sheet is heated at a temperature-rising rate of 0.1 to 20 °C/hr in the range of 650 °C to 1200 °C . After the temperature of 1200°C is reached, the steel sheet is maintained in a temperature range of 1150 to 1250 °C for 20 hours or more.
The lowest limit range of the temperature-rising rate is not particularly defined. If the temperature-rising rate is 0.1 °C/hr or less, there may be a problem in producibility because time is taken too long. In a temperature-rising rate of 20 °C/hr or more, secondary re-crystal grains may not smoothly grow because the instability of a precipitate, such as AIN or MnS, is increased.
Furthermore, the reason why 20 hours or more are maintained after the temperature of 1200°C is reached is that a sufficient time is necessary to derive the smoothness of an externally exposed surface of the steel sheet and to remove impurities, such as nitrogen or carbon present within the steel sheet.
Particularly, by using the metal iodide other than the metal chloride, the base coating can be delaminated at a temperature or more at which secondary re-crystallization within the steel sheet is initiated regardless of a gas atmosphere in which such a process is performed. Accordingly, the growth of crystal grains can be smoothly suppressed because a precipitate, such as AIN or MNS within the steel sheet, is stably present, and thus secondary re-crystallization can be induced to be well formed as described above.
More specifically, the high-temperature annealing process is performed in a mixed gas atmosphere in which the volume ratio of hydrogen to nitrogen is 15 to 40%. When a temperature range of 1000°C or more is reached, the base coating layer of the steel sheet on which the annealing separating agent has been coated may start to be delaminated.
This means that if the high-temperature annealing process is controlled under a gas atmosphere of nitrogen and hydrogen mixed at the volume ratio, Fel₂ is generated by the metal iodide as described above, the reaction of Chemical Reaction Formula 4 is caused after the temperature range of 1000°C or more is reached, and thus the delamination of the base coating is induced.
Specifically, as supported through an example, if the volume ratio of hydrogen: nitrogen is 0.25:0.75, it is checked that the delamination of the base coating is performed about 1045 °C .
In some embodiments, the high-temperature annealing process may be performed in a mixed gas atmosphere in which the volume ratio of hydrogen to nitrogen is 40 to 75 %. When a temperature range of 950°C or more is reached, the base coating layer of the steel sheet on which the annealing separating agent has been coated may start to be delaminated.
Specifically, as supported by an example, if the volume ratio of hydrogen:nitrogen is 0.50:0.50, it is checked that the delamination of the base coating is performed about 984 °C .
Surface roughness of the high-temperature-annealed steel sheet and a coercive force in 1.7 T/50Hz may satisfy a relationship shown in Equation 1. $3 \leq (surface roughness (um) X coercive force (A / m)) \leq 9$
In this case, the coercive force means the intensity of a reverse magnetic field which is applied to make 0 the degree of magnetization of a magnetized substance. In general, a history loss increases as the coercive force increases and decreases as the coercive force decreases.
The high-temperature-annealed steel sheet has a beautiful surface. Particularly, a pinning point that hinders a movement of a magnetic domain has been removed from the high-temperature-annealed steel sheet. Accordingly, such a change can be known by measuring the coercive force.
More specifically, the coercive force of the high-temperature-annealed steel sheet can satisfy Equation 1 in a 1.7T/50Hz region. This corresponds to lower coercive force compared to a case where the metal chloride is used. This is supported by an example.
Brilliance of the high-temperature-annealed steel sheet may be 150 GU or more.
Brilliance is the amount that expresses the degree of light reflected by a surface. In general, a steel sheet including a base coating has brilliance of less than 30. After the base coating is fully removed as described above, the steel sheet may have a value of 150 GU or more due to the improvement of surface roughness and increased reflectance.
The method may further include drying the steel sheet on which the annealing separating agent has been coated after coating the annealing separating agent on the surface of the decarbonizing annealing- and nitrifying-processed steel sheet.
More specifically, drying the steel sheet on which the annealing separating agent has been coated may be performed in a temperature range of 300 to 700 °C .
If the temperature range exceeds 700°C, there is a problem in that the reoxidation of a surface of the steel sheet is caused due to moisture included in the annealing separating agent. If the temperature range is less than 300°C, there is a problem in that the steel sheet is not sufficiently dried. For the reasons, the dry temperature is limited.
Examples of the present invention and comparative examples are described below. However, the examples are only exemplary embodiments of the present invention and the present invention is not restricted by the examples.

Evaluation of temperature at which base coating is delaminated

Example 1: Simulation of HI generation reaction temperature by metal iodide

A temperature at which the HI generation reaction indicated by Chemical Reaction Formula 4 (i.e., Fel₂ + H₂ → 2HI + Fe) is generated was simulated.
This was for predicting how an HI generation reaction temperature according to a gas atmosphere of a high-temperature annealing furnace would be if a composition including metal oxide (MgO), metal iodide and water (H₂O) was used as an annealing separating agent composition for a base coating-free directional electrical steel sheet.
More specifically, assuming that pressure within a furnace was 1 atmospheric pressure, the simulation was performed to predict a temperature at which the HI generation reaction indicated by Chemical Reaction Formula 4 was generated while a mixed gas atmosphere composition of hydrogen and nitrogen was changed like Table 1 using FactSage, that is, a commercial program capable of thermodynamic calculation for the given reaction. The results of the simulation were recorded on Table 1. (Table 1)

HYDROGEN (ATM) NITROGEN (ATM) REACTION TEMPERATURE (°C)

0.75 0.25 950.58

0.50 0.50 983.80

0.25 0.75 1044.75

Comparative Example 1: Simulation for HCl generation reaction temperature by metal chloride

A temperature at which the HCl generation reaction indicated by Chemical Reaction Formula 3 (i.e., FeCl₂ + H₂ → 2HCl + Fe) is generated was simulated.
This was for predicting how an HCl generation reaction temperature according to a gas atmosphere of a high-temperature annealing furnace would be if metal chloride was used instead of metal iodide of Example 1.
Specifically, assuming that pressure within a furnace was 1 atmospheric pressure, a temperature at which the HCl generation reaction indicated by Chemical Reaction Formula 3 (i.e., FeCl₂ + H₂ → 2HCl + Fe) was generated was predicted while a mixed gas atmosphere composition of hydrogen and nitrogen was changed like Table 2 using the same program as that of Example 1. The results of the simulation were recorded on Table 2. (Table 2)

HYDROGEN (ATM) NITROGEN (ATM) REACTION TEMPERATURE (°C)

0.75 0.25 871.55

0.50 0.50 903.07

0.25 0.75 961.40

Evaluation Example 1: comparison between Example 1 and Comparative Example 1

It revealed that in both Example 1 and Comparative Example 1, the reaction temperatures of Chemical Reaction Formula 4 (In the case of Example 1) and Chemical Reaction Formula 3 (In the case of Comparative Example 1) were changed depending on the composition of hydrogen and nitrogen within the furnace.
From the results of Comparative Example 1 (Table 2), it was found that the reaction of Chemical Reaction Formula 3 was generated prior to 1025°C, that is, the evaporation temperature of FeCl₂, regardless of a composition of hydrogen and nitrogen within the furnace.
This means that a base coating starts to be eliminated in a relatively low temperature range of less than about 962°C due to HCl generated according to Chemical Reaction Formula 3. The temperature range corresponds to a temperature before secondary re-crystallization is initiated.
In contrast, from the results of Example 1 (Table 1), it was found that the reaction of Chemical Reaction Formula 4 was generally generated in a high temperature range of about 80°C compared to Comparative Example 1 regardless of a composition of hydrogen and nitrogen within the furnace.
Particularly, if the volume ratio of hydrogen: nitrogen is 50:50, a reaction temperature in Example 1 is expected to be higher than the highest reaction temperature of Comparative Example 1. Furthermore, if the volume ratio of hydrogen: nitrogen is 0.25:0.75, it is deduced that a base coating will be delaminated about 1045°C. This corresponds to a temperature almost similar to a temperature at which secondary re-crystal grains within a steel sheet is initiated.
From such a comparison of the results, it may be seen that a precipitate, such as AIN or MnS within a steel sheet, can be stably present up to a relatively high temperature and it is more advantageous to derive secondary re-crystallization having an excellent iron loss characteristic if metal iodide is used rather than metal chloride.

Evaluation of base coating-free degree and magnetic characteristic

Example 2: Fabrication of base coating-free directional electrical steel sheet using metal iodide

A base coating-free directional electrical steel sheet was fabricated using a composition including metal oxide (MgO), metal iodide and water (H₂O) in a high-temperature annealing process, and a base coating-free degree and magnetic characteristic thereof were checked.
A steel slab, including C: 0.05 %, Si: 3.3 %, Mn: 0.01 %, Sn: 0.05 %, Al: 0.03 % and N: 0.003 % in wt% and including the remainders of Fe and other inevitably included impurities, was prepared.
After the steel slab was heated 1200°C, a hot-rolled plate of 2.3 mm in thickness was fabricated by performing a hot rolling on the steel slab.
After the hot-rolled plate was cracked 900 °C for 180 seconds, the hot-rolled plate was annealed and then subjected to cooling and acid cleaning. Thereafter, a cold-rolled plate of 0.23 mm in thickness was fabricated by performing a cold rolling.
Decarbonizing annealing and nitriding processing were simultaneously performed on the cold-rolled plate in a mixed gas atmosphere including 840°C, humidity of 58 and the weight ratio of hydrogen:nitrogen of 50: 50.
An annealing separating agent including metal iodide indicated as an "invention material" in Table 3 was coated on a surface of the decarbonizing-annealed steel sheet and then dried 500°C for 10 seconds.
More specifically, the annealing separating agent was fabricated in a slurry form by mixing 15 parts by weight of metal iodide and water with respect to 100 parts by weight of magnesium oxide (MgO).
A temperature was raised at an average of 50°C/h up to 650°C with respect to the steel sheet on which the annealing separating agent was coated and which was then dried. Thereafter, the temperature was raised from 650°C to 1200°C at an average of 15°C/h in a mixed gas atmosphere including the weight ratio of hydrogen: nitrogen of 50:50. After the temperature reached 1200°C, the same temperature was maintained for 20 hours and cooling was then performed.
Accordingly, a base coating-free directional electrical steel sheet could be fabricated.

Comparative Example 2: Fabrication of base coating-free directional electrical steel sheet using metal chloride

A base coating-free directional electrical steel sheet was fabricated using metal chloride instead of the metal iodide of Example 2 in a high-temperature annealing process, and a base coating-free degree and a magnetic characteristic thereof were checked.
To this end, the base coating-free directional electrical steel sheet was fabricated using the same method as that of Example 2 except that an additive (i.e., metal chloride or metal iodide) indicated as a "comparative material" instead of the metal iodide indicated as the "invention materials" in Table 3.

Evaluation Example 2: Comparison between Example 2 and Comparative Example 2

The directional electrical steel sheets finally obtained in Example 2 and Comparative Example 2 were subjected to surface cleaning and then subjected to planarization annealing 830°C for 10 seconds while applying tension of 5 MPa.
Thereafter, a base coating-free degree, magnetic flux density and an iron loss of each of the directional electrical steel sheets were evaluated, and the results of the evaluations were shown in Table 3.
Specifically, the base coating-free degree was evaluated based on brilliance of a surface. If the brilliance was 150 GU or more, it was marked by O. If the brilliance was 30 GU or less, it was marked by X. If the brilliance had a middle value between O and X, it was marked by Δ.

In the case of the magnetic flux density, the intensity of a magnetic field was measured under an 800 A/m condition using a Single Sheet measurement system. The iron loss was evaluated under a 50 Hz condition in 1.7T.

(Table 3)

ADDITIVE	BASE COATING-FREE DEGREE (BRILLIANCE, GU)	MAGNETIC CHARACTERISTIC		NOTE
		Magnetic flux density B8	Iron loss (W17/50)
-	X 5	1.93	0.81	Comparative material
BiCl₃	○ 183	1.92	0.82
Bil₃	X 16	1.92	0.85
Mgl₂	Δ 72	1.90	0.84
AgI₂	○ 176	1.93	0.76	Invention material
CoI₂	○ 200	1.92	0.75
CuI	○ 200	1.94	0.73
Mol₂	○ 181	1.93	0.75

In accordance with Table 3, if a metal forming metal iodide was Ag, Co, Cu or Mo (i.e., the invention material), an iron loss value lower than that of BiCl₃, that is, metal chloride, was measured. If the metal forming metal iodide was Bi or Mg, base coating-free was not properly performed. It may be seen that the metal has a higher iron loss value than that of the invention material.
Accordingly, it may be deduced that it is better to use an annealing separating agent including metal iodide together with magnesium oxide (MgO) other than metal chloride in order to reinforce the magnetic characteristic of a base coating-free directional electrical steel sheet and even in this case, the metal forming metal iodide needs to be Ag, Co, Cu or Mo other than Bi or Mg.

Evaluation of magnetic characteristic and coercive force

Example 3: Fabrication of base coating-free directional electrical steel sheet using metal iodide

A base coating-free directional electrical steel sheet was fabricated using a composition including metal oxide (MgO), metal iodide and water (H₂O) in a high-temperature annealing process, and a magnetic characteristic and coercive force thereof were checked.
A steel slab, including C: 0.06 %, Si: 3.2 %, Mn: 0.1 %, Sn: 0.05 %, Al: 0.04 % and N: 0.004 % in wt% and including the remainders of Fe and other inevitably included impurities, was prepared.
The steel slab was heated 1250°C. Thereafter, a hot-rolled plate of 2.6 mm in thickness was fabricated by performing a hot rolling on the steel slab.
The hot-rolled plate was cracked 930°C for 150 seconds and then subjected to cooling and acid cleaning. After the hot-rolled plate was annealed, a cold-rolled plate of 0.30 mm in thickness was fabricated by performing a cold rolling.
Decarbonizing annealing and nitriding processing were performed on the cold-rolled plate in a mixed gas atmosphere including 820°C, humidity of 55, and the weight ratio of hydrogen:nitrogen of 50: 50.
An annealing separating agent including metal iodide indicated as the "invention material" in Table 3 was coated on a surface of the decarbonizing annealing- and nitrifying-processed steel sheet and then dried 450°C for 12 seconds.
More specifically, the annealing separating agent was fabricated in a slurry form by mixing 3 parts by weight of metal iodide with 24 parts by weight of water with respect to 100 parts by weight of magnesium oxide (MgO).
A temperature was raised at an average of 50°C/h up to 650°C with respect to the steel sheet on which the annealing separating agent was coated and which was then dried. Thereafter, the temperature was raised from 650°C to 1200°C at an average of 10°C/h in a mixed gas atmosphere including the weight ratio of hydrogen: nitrogen of 50:50. After the temperature reached 1200°C, the same temperature was maintained for 20 hours and cooling was then performed.
Accordingly, the base coating-free directional electrical steel sheet could be fabricated.

Comparative Example 3: Fabrication of base coating-free directional electrical steel sheet using metal chloride

A base coating-free directional electrical steel sheet was fabricated using metal chloride instead of the metal iodide of Example 3 in a high-temperature annealing process, and a magnetic characteristic and coercive force thereof were checked.
To this end, the base coating-free directional electrical steel sheet was fabricated using the same method as that of Example 3 except that an additive (i.e., metal chloride or metal iodide) indicated as the "comparative material" instead of the metal iodide indicated as the "invention materials" in Table 3.

Evaluation Example 3: Example 3 and Comparative Example 3
The directional electrical steel sheets finally obtained in Example 3 and Comparative Example 3 were subjected to surface cleaning. Thereafter, the magnetic flux density, an iron loss, surface roughness and coercive force of each of the directional electrical steel sheets were measured in the state in which an insulation film was not coated on a surface. The results of the measurements were shown in Table 4.
Specifically, in the case of the magnetic flux density, the intensity of a magnetic field was measured under an 800 A/m condition using a Single Sheet measurement system. The iron loss was evaluated under a 50 Hz condition in 1.7T.

The surface roughness was measured using a surface roughness tester (model name: Surftest-SJ-500). The coercive force was measured in 1.7T, 50Hz, and the product of the surface roughness and coercive force measured in each case was shown in Table 4.

(Table 4)

ADDITIVE		MAGNETIC CHARACTERISTIC		SUR FACE ROUGHNESS (Ra, um) X COERCIVE FORCE (A/m)	NOTE
ype	Addition (based on parts by weight, MgO 100 parts by weight)	Magnetic flux density (B8)	Iron loss (W17/50)
iCl ₃	10	1. 91	0. 96	9.6	Comparative material
uCl
₂	10	1. 90	0. 98	11.2	Comparative material
gl₂	3	1. 90	0. 98	10.9	Comparative material
gl₂	5	1. 91	0. 92	8.8	Invention material
gl₂	11	1. 91	0. 89	7.9	Invention material
gl₂	15	1. 92	0. 86	6.2	Invention material
gl₂	20	1. 92	0. 88	6.8	Invention material
ol₂	5	1. 92	0. 92	6.5	Invention material
ol₂	12	1. 93	0. 93	7.8	Invention material
ol₂	15	1. 93	0. 87	7.1	Invention material
ol
₂	20	1. 92	0. 88	6.3	Invention material
ul	5	1. 91	0. 89	6.2	Invention material
ul	12	1. 92	0. 87	5.2	Invention material
ul	15	1. 93	0. 84	4.4	Invention material
ul
	20	1. 93	0. 90	5.7	Invention material
ul	24	1. 91	0. 97	10.2	Comparative material
ol₂	3	1. 90	0. 99	10.8	Comparative material
ol₂	5	1.90	0.91	8.4	Invention material
ol₂	12	1. 92	0. 88	6.5	Invention material
ol₂	15	1. 92	0. 87	6.0	Invention material
ol
₂	20	1. 91	0. 88	6.9	Invention material
ol₂	24	1. 89	0. 99	11.2	Comparative material

In accordance with Table 4, a higher iron loss value was measured in metal iodide content of less than 5 parts by weight or more than 20 parts by weight with respect to 100 parts by weight of magnesium oxide than in metal iodide content of 5 to 20 parts by weight (i.e., the invention material).
Furthermore, it could be checked that the product of the surface roughness and coercive force (1.7T, 50Hz condition) of the base coating-free directional electrical steel sheet was 9 or less in all of the invention materials, but exceeded 9 in all of the comparative materials.
That is, this means that if metal chloride was used or the metal iodide content was less than 5 parts by weight or more than 20 parts by weight with respect to 100 parts by weight of magnesium oxide (MgO), it had a great coercive force compared to the invention material, a surface was not beautiful because a history loss was high, and a pinning point (i.e., a base coating) that hinders a movement of a magnetic domain was not removed.
Accordingly, it may be deduced that it is better to use an annealing separating agent including metal iodide together with magnesium oxide (MgO) other than metal chloride and even in this case, it is necessary to control content of metal iodide to 5 to 20 parts by weight with respect to 100 parts by weight of magnesium oxide (MgO) in order to reinforce a magnetic characteristic by completely removing a base coating of a base coating-free directional electrical steel sheet.
Furthermore, it could be seen that the best magnetic characteristic was obtained and a surface was beautiful when content of metal iodide content was controlled to 11 to 20 parts by weight with respect to 100 parts by weight of magnesium oxide (MgO).
The present invention is not limited to the exemplary embodiments and may be modified in other various forms. Those skilled in the art to which the present invention pertains will appreciate that the present invention may be implemented in other detailed forms without departing from the technological spirit or essential characteristics of the present invention. Accordingly, the aforementioned embodiments should be construed as being only illustrative not as being restrictive from all aspects.

Claims

An annealing separating agent composition for a directional electrical steel sheet, comprising:
magnesium oxide or magnesium hydroxide;

metal iodide; and

a solvent.
The annealing separating agent composition of claim 1, wherein the annealing separating agent composition comprises an annealing separating agent composition for a base coating-free directional electrical steel sheet.
The annealing separating agent composition of claim 2, wherein a composition of the annealing separating agent comprises 5 to 20 parts by weight of the metal iodide with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
The annealing separating agent composition of claim 3, wherein 11 to 20 parts by weight of the metal iodide is included with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
The annealing separating agent composition of any one of claims 1 to 4, wherein a metal forming the metal iodide comprises any one metal selected from a group consisting of Ag, Co. Cu, Mo and a combination thereof.
The annealing separating agent composition of claim 5, wherein the magnesium oxide or magnesium hydroxide comprises magnesium oxide (MgO).
A method for manufacturing a directional electrical steel sheet, comprising:
preparing a steel slab comprising Si: 0.5 - 4.5 wt% and other inevitable impurities and comprising a remainder of Fe;

heating the steel slab 1,300 °C or less;

fabricating a hot-rolled plate by performing a hot rolling on the heated steel slab;

fabricating a cold-rolled plate by performing two or more cold rollings comprising one cold rolling or middle annealing on the hot-rolled plate;

performing decarbonizing annealing and nitriding processing on the cold-rolled plate;

coating an annealing separating agent on a surface of the decarbonizing annealing- and nitrifying-processed steel sheet;

performing high-temperature annealing on the steel sheet on which the annealing separating agent has been coated; and

obtaining a directional electrical steel sheet,

wherein the annealing separating agent is a slurry comprising magnesium oxide or magnesium hydroxide, metal iodide and a solvent.
The method of claim 7, wherein the manufactured directional electrical steel sheet comprises a base coating-free directional electrical steel sheet.
The method of claim 8, wherein a composition of the annealing separating agent comprises 5 to 20 parts by weight of the metal iodide with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
The method of claim 9, wherein 11 to 20 parts by weight of the metal iodide is included with respect to 100 parts by weight of the magnesium oxide or magnesium hydroxide.
The method of any one of claims 7 to 10, wherein a metal forming the metal iodide comprises any one metal selected from a group consisting of Ag, Co. Cu, Mo and a combination thereof.
The method of claim 11, wherein the magnesium oxide or magnesium hydroxide comprises magnesium oxide (MgO).
The method of claim 11, wherein performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated comprises:
heating the steel sheet at a temperature-rising rate of 0.1 to 20°C/hr from 650°C to 1200°C, and

maintaining the steel sheet in a temperature range of 1150 to 1250°C for 20 hours after the temperature 1200°C is reached.
The method of claim 13, wherein performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated comprises:
performing the high-temperature annealing in a mixed gas atmosphere in which a volume ratio of hydrogen to nitrogen is 15 to 40 %, and

starting to, by a base coating of the steel sheet on which the annealing separating agent has been coated, be delaminated when a temperature range of 1000°C or more is reached.
The method of claim 13, wherein performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated comprises:
performing the high-temperature annealing in a mixed gas atmosphere in which a volume ratio of hydrogen to nitrogen is 40 to 75%, and

starting to, by a base coating of the steel sheet on which the annealing separating agent has been coated, be delaminated when a temperature range of 950°C or more is reached.
The method of claim 13, wherein in performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated, surface roughness and a coercive force in 1.7T/50Hz of the high-temperature-annealed steel sheet satisfy a relationship expressed in Equation 1. $3 \leq \leq (surface roughness (um) X coercive force (A / m)) \leq \leq 9.$
The method of claim 13, wherein in performing the high-temperature annealing on the steel sheet on which the annealing separating agent has been coated, brilliance of the high-temperature-annealed steel sheet is 150 GU or more.
The method of claim 13, wherein drying the steel sheet on which the annealing separating agent has been coated is performed in a temperature range of 300 to 700 °C .