EP3225701A1 - Glühseparatorzusammensetzung für elektrostahlblech und verfahren zur herstellung eines orientierten elektrostahlblechs damit - Google Patents

Glühseparatorzusammensetzung für elektrostahlblech und verfahren zur herstellung eines orientierten elektrostahlblechs damit Download PDF

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EP3225701A1
EP3225701A1 EP15863386.7A EP15863386A EP3225701A1 EP 3225701 A1 EP3225701 A1 EP 3225701A1 EP 15863386 A EP15863386 A EP 15863386A EP 3225701 A1 EP3225701 A1 EP 3225701A1
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Prior art keywords
steel sheet
annealing
separating agent
temperature
annealing separating
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EP15863386.7A
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English (en)
French (fr)
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EP3225701A4 (de
Inventor
Chang Soo Park
Min Soo Han
Byung-Deug HONG
Soon-Bok Park
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Posco Holdings Inc
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Posco Co Ltd
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Priority claimed from PCT/KR2015/012735 external-priority patent/WO2016085257A1/ko
Publication of EP3225701A1 publication Critical patent/EP3225701A1/de
Publication of EP3225701A4 publication Critical patent/EP3225701A4/de
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1277—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties involving a particular surface treatment
    • C21D8/1283—Application of a separating or insulating coating
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
    • C21D1/70—Temporary coatings or embedding materials applied before or during heat treatment while heating or quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
    • C21D1/72—Temporary coatings or embedding materials applied before or during heat treatment during chemical change of surfaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/12—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties
    • C21D8/1244—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of articles with special electromagnetic properties the heat treatment(s) being of interest
    • C21D8/1272—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

  • 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.
  • 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.
  • the insulation film is commonly formed on a Forsterite (Mg 2 SiO 4 )-series coating (hereinafter referred to as a "base coating"), that is, the base film of the steel sheet.
  • a base coating Mg 2 SiO 4 -series coating
  • 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.
  • 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 base coating acts as a pinning point that hinders a flow of a magnetic domain moving along the surface of the steel sheet.
  • 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.
  • 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.
  • 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 may be an annealing separating agent composition for a base coating-free directional electrical steel sheet.
  • 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.
  • 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.
  • the annealing separating agent composition may be an annealing separating agent composition for a base coating-free directional electrical steel sheet.
  • 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 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 may be performed in a temperature range of 650 to 1200°C.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIG. 1 shows an Ellingham diagram of several materials according to partial pressure of iodide ions.
  • 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.
  • 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 Mg 2 SiO 4 ) 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).
  • a base coating i.e., a base coating expressed into a chemical formula Mg 2 SiO 4
  • MgO magnesium oxide
  • 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.
  • 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.
  • 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.
  • 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.
  • SiO 2 As a steel sheet passes through a furnace controlled under the atmosphere, SiO 2 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.
  • decarbonizing and nitride may be simultaneously performed or decarbonizing annealing and nitriding processing may be sequentially performed.
  • Chemical Reaction Formula 1 corresponds to a reaction for forming Mg 2 SiO 4 , that is, a base coating. [Chemical Reaction Formula 1] 2Mg (OH) 2 + SiO 2 ⁇ Mg 2 SiO 4 (base coating) + 2H 2 O
  • 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.
  • 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.
  • the metal iodide can derive the delamination of the base coating in such a manner that it forms an Fel 2 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.
  • BiCl 3 that is, a kind of metal chloride
  • Cl atoms i.e., Cl atoms of BiCl 3
  • BiCl 3 causes the following chemical reaction at the boundary surface of the steel sheet and a base coating thereof.
  • FeCl 2 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 FeCl 2 + H 2 ⁇ 2HCl + Fe
  • HCl gas is generated at the interface of the steel sheet and the base coating.
  • the HCl gas can delaminate the oxidation film.
  • the base coating is delaminated less than 1025°C, that is, the evaporation temperature of FeCl 2 , as described above, the magnetic characteristic of the finally obtained directional electrical steel sheet is inevitably deteriorated.
  • 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.
  • a temperature of less than the evaporation temperature (i.e., 1025°C) of FeCl 2 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.
  • 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.
  • 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.
  • 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.
  • a ratio of hydrogen and nitrogen is 0.25:0.75
  • 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.
  • 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.
  • 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.
  • 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.
  • the metal iodide in the composition of the annealing separating agent, 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • 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.
  • FIG. 1 shows an Ellingham diagram of several materials according to partial pressure of iodide ions in order to confirm such a fact.
  • the Ellingham diagram is a tool indicative of the direction of a chemical reaction.
  • a reaction having a low free energy value ( ⁇ G) is a more stable state. Accordingly, the form of a compound changes to a reaction having lower energy on the Ellingham diagram.
  • 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.
  • metal iodide 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 Fel 2 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).
  • MgO magnesium oxide
  • the magnesium oxide (MgO) has been widely known, and a detailed description thereof is omitted.
  • the solvent may be water (H 2 O). 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.
  • the prepared slab is heated.
  • the slab is heated using a low-temperature slab method at a temperature of 1,300°C or less.
  • 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.
  • 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.
  • the metal iodide in the composition of the annealing separating agent, 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).
  • 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.
  • a precipitate such as AIN or MnS
  • 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.
  • 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.
  • 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%.
  • the base coating layer of the steel sheet on which the annealing separating agent has been coated may start to be delaminated.
  • 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 .
  • 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 %.
  • the base coating layer of the steel sheet on which the annealing separating agent has been coated may start to be delaminated.
  • 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 .
  • 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.
  • 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.
  • 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.
  • 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.
  • 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 .
  • 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.
  • Example 1 Simulation of HI generation reaction temperature by metal iodide
  • 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.
  • Example 1 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.
  • 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.
  • 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 (H 2 O) in a high-temperature annealing process, and a base coating-free degree and magnetic characteristic thereof were checked.
  • MgO metal oxide
  • H 2 O water
  • 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.
  • a hot-rolled plate of 2.3 mm in thickness was fabricated by performing a hot rolling on the steel slab.
  • 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.
  • 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.
  • 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.
  • 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.
  • an additive i.e., metal chloride or metal iodide
  • 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.
  • 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 ⁇ .
  • 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.
  • 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 (H 2 O) in a high-temperature annealing process, and a magnetic characteristic and coercive force thereof were checked.
  • MgO metal oxide
  • H 2 O water
  • 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.
  • 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.
  • the base coating-free directional electrical steel sheet could be fabricated.
  • 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.
  • Example 3 Example 3 and Comparative Example 3
  • 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.
  • 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 gl 2 3 1. 90 0.
  • MgO magnesium oxide
  • an annealing separating agent including metal iodide together with magnesium oxide (MgO) other than metal chloride 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.

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