WO2008047999A1

WO2008047999A1 - Annealing separating agent for grain oriented electrical steel sheet having uniform glass film and excellent magnetic properties and method of manufacturig the same

Info

Publication number: WO2008047999A1
Application number: PCT/KR2007/003544
Authority: WO
Inventors: Jong-Ho Park; Jong-Soo Woo; Song-Kwon Park
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2006-10-18
Filing date: 2007-07-23
Publication date: 2008-04-24
Anticipated expiration: 2009-04-18

Abstract

Disclosed herein are an annealing separator for grain-oriented electrical steel sheets having excellent surface properties and a method for producing grain-oriented electrical steel sheets using the same. More particularly, the annealing separator comprises: 100 parts by weight of MgO, consisting of 40-95% of active MgO and 5-60% of inactive MgO; and 0.01-5 parts by weight of a low-melting- point compound having a melting point lower than 900 °C. The method for producing grain-oriented steel sheets comprises applying said annealing separator in the form of slurry to a steel sheet, after stirring the annealing separator in a mixing tank at a revolution speed of 1500-3000 rpm for more than 10 minutes.

Description

ANNEALING SEPARATING AGENT FOR GRAIN ORIENTED

ELECTRICAL STEEL SHEET HAVING UNIFORM GLASS FILM AND

EXCELLENT MAGNETIC PROPERTIES AND METHOD OF

MANUFACTURIG THE SAME

Technical Field

The present invention relates to an annealing separator for grain-oriented electrical steel sheets having excellent surface properties, and a method of producing grain-oriented electrical steel sheets using the same, and more particularly to an annealing separator for grain-oriented electrical steel sheets having excellent surface properties, which comprises 100 parts by weight of MgO, consisting of 40-95% of active MgO and 5-60% of inactive MgO, and 0.01- 0.5 parts by weight of a low-melting-point compound having a melting point lower than 900 ^°C, and to a method for producing grain-oriented steel sheets, which comprises applying said annealing separator in the form of slurry to a steel sheet after stirring the annealing separator in a mixing tank at a revolution speed of 1500-3000 rpm for more than 10 minutes.

Background Art

To produce a grain-oriented electrical steel sheet, a material slab containing 2.5-4.0 wt% of Si is hot-rolled, and the hot-rolled sheet is annealed and subjected to one-time cold rolling or to a plurality of cold rolling steps with annealing steps therebetween, so as to attain a final sheet thickness. Then, the cold-rolled sheet is subjected to a decarburization annealing process, in which cold-rolling oils or contaminants are removed by burn-off or cleaning treatment, and P_H2_O/P_H2 is controlled in an N₂+H₂ atmosphere, thus forming an oxide film, based on Fe₂SiO₄ and SiO_2, which play an important role in decarburization, primary recrystallization and glass film formation.

Then, an MgO-based annealing separator is stirred in water to make a slurry, and the slurry is applied on the steel sheet using a roll and then dried. Then, the steel sheet is wound into a coil and subjected to finish annealing. Then, the steel sheet coil is applied with an insulating coating agent in a continuous line and subjected to annealing and heat flattening, thus obtaining a final product.

In this grain-oriented electrical steel sheet, it is considered that the (110)<001> crystal, having an <001> axis, preferentially develops and grows to erode other crystal grains which inhibit the growth of normal grains by pinning the grain boundary migration of primary recrystallization grains, such as AlN and MnS, so-called inhibitors which are finely dispersed in the steel, during this secondary recrystallization step.

Thus, in order to produce a grain-oriented electrical steel sheet having excellent glass film and magnetic properties, it is important to select oxide film formation conditions and an annealing separating agent and to control final annealing conditions in the decarburization annealing process, in which a stable and uniform glass film is formed and which influences the dispersion of inhibitors (AlN and MnS) in steel and the secondary recrystallization. Among such factors, the reactivity of the annealing separating agent is very important, because it influences the oxidation or nitrification in coils, which occur during glass film formation in the finish annealing process, and thus it influences not only the formation of the glass film, but also the behavior of inhibitors in secondary recrystallization. This is because the conditions of additives, which are added to improve the properties (particularly, reactivity) of the annealing separator, influence the change in the oxide film formed during decarburization annealing, the temperature of initiation of glass film formation, the rate of glass film formation, the uniformity of the glass film, and the degree of oxidation of an atmosphere between the steel sheet surfaces, and, as a result, such factors also influence the stability of inhibitors in steel, thus influencing secondary recrystallization. Particularly, the activity distribution or particle size distribution of MgO is important in order to smoothly induce a reaction with the decarburized oxide layer. That is, depending on the conditions of additives, which are added to improve reactivity, together with MgO for forming a glass film, the oxide film formed in the decarburization annealing process influences the change in components or shapes in the finish annealing process, and thus is an important factor that influences the temperature of initiation of glass film formation, the rate of film formation, the film uniformity and the oxidation degree of an atmosphere between steel sheet surfaces. Also, such factors influence the stability of inhibitors in steel, and thus influence secondary recrystallization. Thus, the role thereof in a high-temperature annealing process is very important.

In the finish annealing process of grain-oriented electrical steel sheets, the reaction of formation of the glass film refers to a forsterite film, which is formed 5 through a reaction between MgO, which is the main component of the annealing separator, and SiO₂, which is the main component of the oxide film formed in the decarburization annealing process. 2MgO + SiO₂ → Mg₂SiO₄ Herein, if AlN is used as an inhibitor, spinel compounds, such as Al₂O₃, 0 MgO and SiO₂, are formed around the lower portion of the forsterite layer.

Generally, the annealing separator is prepared by dispersing MgO in water, if necessary, together with a reaction promoter, to make a slurry, which is then applied on a steel sheet. As this additive for reaction promotion, oxide, an S compound, a B compound or the like has been used. In this prior art, there is a 5 problem caused by the preparation conditions of MgO. For example, in the case of high-activity MgO, a hydration reaction of MgO — > Mg(OH)₂ occurs during the adjustment (that is, stirring) of slurry, leading to an increase in the content of water in the coil, and thus the dew point between the sheet surfaces is increased, such that a glass film becomes non-uniform in the movement direction O (lengthwise direction) or widthwise direction of the coil. On the other hand, in the case of low-activity MgO, it is difficult to obtain a uniform and sufficient film thickness due to insufficient reactivity. As a result, during temperature elevation in the finish annealing process, a film thinning phenomenon occurs, or additional oxidation or nitrification occurs, thus causing defects, such as pinhole-shaped 5 metal spots or discoloration, in the glass film.

In a solution to the problem associated with hydrated water, Japanese Patent Publication No. Hei 2650817 suggests the use of MgO having a low degree of hydration. This is a technology of using MgO, which has a citric acid activity (CAA) of 100-400 seconds at a final reaction rate of 40%, a citric acid activity of0 1000-4000 seconds at a final reaction rate of 80%, a water content of less than 2.5% at slurry adjustment conditions of 20 ^°C and 60 minutes, an average particle size of less than 2.5 [M, and a particle size of 5% finer than 325 Mesh. The publication indicates that the occurrence of additional oxidation can be inhibited through the use of lower-activity MgO, and thus a uniform film is formed even on5 a unidirectional silicon steel sheet containing Al-Sb. As methods of using additives in annealing separators, Japanese Patent Publication No. Sho 58-006783 discloses a method of using an annealing separator, which is based on MgO and contains S or an S compound and a Sr compound, and Japanese Patent Publication No. Sho 57-032716 discloses a method of using an annealing separator, which is based on MgO and contains an Sr compound and/or a Ti compound. However, the additives suggested in such methods are SrSO₄, Sr(OH)₂ ¹SH₂O and the like, and such Sr compounds are effective in thickening the forsterite film, but have problems in that a carburization phenomenon caused by the generation of decomposed gas occurs in the final annealing process, thus deteriorating the magnetic properties of the sheet.

As an improvement over this technology, Japanese Patent Publication No.

Hei 2895645 discloses a method for producing grain-oriented electrical steel sheets, which comprises adding one or more selected from among SrZrO₃ and

SrSnO₃ in an amount of 0.1-10 wt% (as Sr). The invention disclosed in this publication aims to improve the shortcomings of the above-described Sr compound addition technology and discloses that the film and magnetic properties of the grain-oriented electrical steel sheet were further improved by maximizing the forsterite suspension using an annealing separator containing an Sr-containing compound. However, it is a technology which is still unsatisfactory, because a sufficient effect of improving reactivity is not obtained.

Recently, increasing the size of coils by increasing the coil width and unit weight in a production process in order to increase production efficiency has been conducted. As a result, there are problems in that the quality of steel sheets is likely to be non-uniform either due to the difference in temperature elevation between the portions of a coil or due to the difference in the thermal expansion or thermal shrinkage of a sheet coil. Among these problems, one problem is a peroxide glass film or a local color staining phenomenon, which occur due to differences in thermal history. Another problem is a defect problem, such as the distortion of the inner and outer winding portions of steel sheets, which occurs due to the non-uniform thermal expansion or shrinkage of coils, or "crumpled marks" (embossing defects) in the form of dimples. In recent studies, such defects were proven to have a high correlation with the properties of annealing separators, suggesting that the precise control of annealing separators is important.

Generally, an annealing separator is obtained by controlling the activity or impurity of MgO, and dispersing the MgO, together with a reaction promoter, if necessary, in pure water in a mixing tank using a stirring device equipped with a propeller rotor, thus preparing a slurry, which is then applied on steel sheets. This MgO is fine particulate MgO, which is obtained by rehydrating Mg(OH)₂, obtained from brine water or sea water, and calcining the rehydrated Mg(OH)₂ in a box-type batch kiln or a continuous rotary kiln. As the additive, oxide, an S compound, a B compound or the like has generally been used as a reaction promoter for the formation of a forsterite film.

In this prior art, there is a problem caused by the preparation conditions of MgO. For example, in the case of high-activity MgO, a hydration reaction of MgO —> Mg(OH)₂ occurs during the adjustment (that is, stirring) of slurry, leading to an increase in the content of water in coils, and thus the dew point between the sheet surfaces is increased, such that a glass film becomes non-uniform in the movement direction (lengthwise direction) or widthwise direction of the coil. On the other hand, in the case of low-activity MgO, it is difficult to obtain a uniform and sufficient film thickness due to insufficient reactivity. As a result, during temperature elevation in the finish annealing process, a film thinning phenomenon occurs, or additional oxidation or nitrification occurs, thus causing defects, such as pinhole-shaped metal spots or discoloration, in the glass film.

In a solution to problem associated with hydrated water, Japanese Patent Publication No. Hei 2650817 suggests the use of MgO having a low degree of hydration. This is a technology of using MgO, which has a citric acid activity (CAA) of 100-400 seconds at a final reaction rate of 40%, a citric acid activity of 1000-4000 seconds at a final reaction rate of 80%, a hydrated water content of less than 2.5% after stirring at 20 °C for 60 minutes, an average particle size of less than 2.5 μm, and a particle size of 5% finer than 325 Mesh. The publication indicates that the occurrence of additional oxidation can be inhibited through the use of lower-activity MgO, and thus a uniform film is formed even on a unidirectional silicon steel sheet containing Al-Sb. In this technology, the effect of making the film uniform is acknowledged, but the description of a decrease in reactivity is insufficient, and the description of film thickness and adhesion is also insufficient.

A technology for controlling the particle size distribution of annealing separators is disclosed in Japanese Patent Publication No. Hei 6-116736. This publication discloses that, in a method of forming a forsterite film on unidirectional electrical steel sheets, when (85 + 15 loga)[%]≥X[%]>(55 + 35 loga)[%] ("a" represents particle diameter, and "X%" represents integration amount, and (X[%]<100)) is satisfied at an annealing separator particle size of more than 0.5 μm, and when a temperature elevation rate in the range of 900- 1200 ^°C in the finish annealing process is in the range of 2-30 ^°C/hr, a stable and good forsterite film is obtained. Also, Japanese Patent Laid-Open Publication No. Sho 52-31296 discloses the application of an annealing separator, which has a particle size distribution of 40-70% at less than 3 μm and 10-25% at more than 10 μm and a bulk specific gravity of 0.18-0.30 g/cm³. Also, Japanese Patent Laid- Open Publication No. Sho 58-193373 discloses a method for producing grain- oriented electrical steel sheets having excellent magnetic properties, in which an annealing separator, which has a particle size of 0.08-0.18 μm, as measured by powder X-ray diffraction, is used.

However, in such technologies, there are problems in that a solid-phase reaction does not sufficiently occur, and it is difficult to reliably achieve compactness, uniformity and adhesion of the forsterite film. Also, the publications disclose that it was found that the rate of temperature elevation during finish annealing, together with the particle size distribution of an MgO- based annealing separator, greatly influenced the formation of the forsterite film.

A method of improving a glass film using an additive in an annealing separator is disclosed in Japanese Patent Publication No. Sho 60-14103. This publication discloses a method of producing unidirectional electrical steel sheets, having an insulating film and showing excellent magnetic properties, using an annealing separator containing an Mo compound in an amount of 0.1-10 wt% (as Mo). Also, Japanese Patent Publication No. Hei 2895645 discloses a method for producing grain-oriented electrical steel sheets, which comprises adding one or more selected from among SrZrO₃ and SrSnO₃ in an amount of 0.1-10 wt% (as Sr). The invention disclosed in this publication aims to improve the shortcomings of the above-described Sr compound addition technology, and discloses that the film and magnetic properties of the grain-oriented electrical steel sheet can be further improved by maximizing the suspension of forsterite using an annealing separator containing an Sr-containing compound.

In the improved technologies relating to MgO or additives, there are problems in that a glass film is not stably formed depending on steel sheet components, decarburization annealing conditions or finish annealing conditions, and particularly, in the case of large-sized coils, in which a change in temperature is likely to occur, a uniform glass film is not formed. Also, the above technologies do not aim to solve the coil shape problem, and thus are still unsatisfactory.

Disclosure of the Invention

Technical tasks to be solved by the invention

The present invention has been made in order to solve the above- described problems occurring in the prior art, and it is an object of the present invention to provide an annealing separator for grain-oriented electrical steel sheets, which achieves a remarkable effect of improving the formation of a glass film, inhibits additional oxidation or additional nitrification during a finish annealing process so as to form a uniform and excellent glass film over the entire surface of a coil, and has excellent magnetic properties.

Another object of the present invention is to provide an annealing separator for grain-oriented electrical steel sheets having excellent surface properties, and a method for producing grain-oriented electrical steel sheets using the same, in which low-hydrated MgO, obtained by suitably mixing active MgO with inactive MgO, is used, the mixing conditions of an annealing separator slurry are controlled, an additive having a melting point lower than 900 ^°C is used to improve the reactivity of MgO, a remarkable effect of improving the formation of a glass film is obtained even upon the application of low-hydrated MgO, so as to inhibit additional oxidation or additional nitrification during a finish annealing process, thus providing very good film properties and magnetic properties, and the physical property values of coarse MgO particles in low-hydrated MgO, obtained by suitably mixing active MgO with inactive MgO, are adjusted to be within a suitable range, thus solving appearance problems occurring in the coil annealing process.

Technical Solution

To achieve the above objects, the present invention provides an annealing separator for grain-oriented electrical steel sheets having a uniform film and excellent magnetic properties, which comprises 100 parts by weight of MgO, and 0.01-0.5 parts by weight of a low-melting-point compound having a melting point lower than 900 "C.

Also, the annealing separator further comprises, as an additive for controlling the atmosphere between steel sheet surfaces, 0.5-10 parts by weight, based on 100 parts by weight of MgO, of one or two selected from Ti, V, Nb, Cr and Mn oxides, which have a particle diameter of less than 0.5 μm.

Furthermore, the low-melting-point compound contains a compound having a melting point lower than 750 ^°C , in an amount of 30% based on the total weight of the low-melting-point compound. Also, the low-melting-point compound is one or more selected from the group consisting of antimony compounds, amide compounds, chlorides, chloroxides, chlorates, chromates, oxides, bromides, hydroxides, hydrides, carbonates, nitrates, tellurates, vanadates, fluorides, borates, phosphates, sulfides, sulfates, iodides, and hydroiodides. Moreover, the low-melting-point compound comprises one or more elements selected from the group consisting of H, Li, Na, K, Cu, Rb, Ag, Cs, Ba, Be, Mg, Ca, Zn, Sr, Cd, Ba, B, Al, Ga, Y, In, Tl, Ti, Ge, Sn, P, V, Nb, Sb, Ta, Bi, S, Cr, Mo, Te, W, Mn, Fe, Co, and Ni.

Also, said MgO has a 40% CAA value of 50-120 seconds, and a hydrated water content of 1.0-2.5%.

Hereinafter, preferred embodiments of the present invention will be described in further detail.

In the present invention, as a starting material, a silicon steel slab, containing, in addition to 2.5-4.0% Si, one or more inhibitors selected from MnS, AlN, Al, Cu, Sn, Sb and Mo depending on the intended use of the desired steel material, is hot-rolled according to a known method. The hot-rolled steel sheet is subjected to one-time cold rolling or to a plurality of cold rolling steps with annealing steps therebetween, thus achieving a final sheet thickness. Then, the cold-rolled steel sheet is subjected to decarburization annealing in a continuous line to form a SiO2-based oxide film on the surface of the steel sheet. Meanwhile, the inventive annealing separator is prepared by adding, to MgO, one or more low-melting-point additives, having a melting point lower than 900 ^°C , in an amount of 0.01-0.5 parts by weight based on 100 parts by weight of MgO, and optionally adding 0.5-10 parts by weight of one or more oxides selected from among Ti, B and Nb. The annealing separator is sufficiently stirred in water to make a slurry, and the slurry is applied on the steel sheet and dried at a temperature of about 200-300 °C . Then, the dried steel sheet is wound into a coil. Then, in a finish annealing process, the steel sheet coil is subjected to glass film formation, annealing and secondary recrystallization in a batch-type or continuous furnace at a temperature of about 1170-1220 ⁰C for 15-20 hours while the temperature elevation rate and the amount of nitrogen, as an atmospheric gas, are adjusted.

In a continuous line, the coil thus treated is washed with water to remove excess annealing separator, and is washed with acid. Then, it is coated with an insulating coating agent, and subjected to heat flattening at a temperature of about 850 °C for the annealing of the insulating coating film, shape correction and stress-removing annealing, thus producing a final product.

In grain-oriented electrical steel sheets, the glass film and magnetic properties are determined through such a series of processes, and the formation conditions of the oxide film formation conditions in the decarburization annealing process and the conditions of the annealing separator are very important. In other words, the annealing separator influences not only the formation time, formation rate, formation amount and uniformity of the glass film, but also additional oxidation or nitrification, which influences the formation of the film during temperature elevation in the finish annealing process. As a result, it influences the decomposition rates of important inhibitors, AlN and MnS, or other components in steel sheets, and thus also influences the magnetic properties of the steel products. Particularly, it is important in a high-magnetic-flux-density material containing Al as an inhibitor, because it greatly influences the material. In view of this point, the technology of adding a low-melting-point reaction promoter to MgO, having a hydrated water content of about 1.0-2.5%, as disclosed in the present invention, exhibits excellent synergistic effects. Hereinafter, the reason for limitation will be described. The annealing separator is obtained by adding, to 100 parts by weight of MgO, 0.01-0.5 parts by weight of one or more selected from the group consisting of antimony compounds, amide compounds, chlorides, chloroxides, chlorates, chromates, oxides, bromides, hydroxides, hydrides, carbonates, nitrates, tellurates, vanadates, fluorides, borates, phosphates, sulfides, sulfates, iodides, and hydroiodides, which have a melting point lower than 900 ⁰C . In order to increase the uniformity of the glass film by increasing the reactivity of MgO with the _τdecarburized oxide film and to minimize the effect of MgO on additional oxidation or nitrification during finish annealing, and thus on the thickness, uniformity and color variation of the glass film, and on secondary recrystallization, it is preferable to add an additive that can promote the formation of the glass film before additional oxidation or nitrification occurs. That is, the application of an additive having a melting point lower than 900 "C realizes a very great improvement effect. Also, in a preferred condition, when the low- melting-point additive contains an additive having a melting point lower than 750 ^°C, in an amount of more than 30%, additional oxidation or nitrification does not substantially occur, and thus a uniform glass film is formed over the entire length and width of the coil, and the stabilization of magnetic properties is achieved.

Additional oxidation or nitrification of the coil during finish annealing readily occurs in a portion showing a rapid increase in temperature during the process of annealing the coil, that is, the edge of the coil or the outer winding portion of the coil. The use of a given amount of the low-melting-point reaction promoter, as disclosed in the present invention, can provide the following two effects. First, before the initiation of an oxidation or nitrification reaction during coil temperature elevation, the low-melting-point compound forms a glassy, compact and molten layer on the oxide film of the steel sheet, thus inhibiting a reaction with the oxide film.

Second, there is the effect of initiating the formation of a low-temperature glass film, with which the molten layer, formed by MgO and the low-melting- point compound on the surface of the steel sheet, reacts at lower temperatures. Due to this molten layer and the initial film, formed at low temperatures, the effect of inhibiting additional oxidation or nitrification is obtained.

As a result, it is thought that the formation of metallic glossy spots or a non-uniform film, caused by additional nitrification, which occurs in the prior art, is inhibited. Also, it is thought that, due to the glassy effect of this glass film, the effect of atmospheric gas on inhibitors, or the removal of inhibitors, is suppressed, and thus the improvement in magnetic properties is also achieved. In experimental tests in the present invention, it was found that the addition of a low- melting-point additive, which contained additives having a melting point lower than 750 ^°C, in an amount of more than 30%, was highly advantageous for the stability of a glass film formation reaction, because a glassy layer protecting the oxide film starting from low temperatures was formed, and the reaction of MgO with the decarburized oxide layer progressed in a more reliable and stable manner. Also, it was found that this addition of the low-melting-point compound was more advantageous for increasing the uniformity of the glass film, because, when sulfides, sulfates, chlorides, hydroiodides, bromides, phosphates or hydroxides were added alone, the adverse effects of the elements of the compounds could be inhibited. When the amount of addition of the additive is below 0.01 part by weight based on 100 parts by weight of MgO, the effect of promoting the formation of the low-melting-point glassy layer or the formation of the glass film will not be sufficiently obtained. On the other hand, if it exceeds 0.5 parts by weight, the effect of the low-melting-point additive will excessively occur depending on the conditions of MgO or the finish annealing atmosphere, leading to local melt defects, such as pinholes.

The element of the low-melting-point additive is one or more selected from the group consisting of H, Li, Na, K, Cu, Rb, Ag, Cs, Ba, Mg, Ca, Zn, Sr, Cd, Ba, B, Al, Ga, Y, In, Tl, Ti, Ge, Sn, P, B, Nb, Sb, Ta, Bi, S, Cr, Mo, Te, W, Mn, Fe, Co, and Ni. If a compound containing such elements is used, an excellent glass film and improved magnetic properties can be realized.

Then, as an additive for controlling an atmosphere between steel sheet surfaces, one or more selected from the group consisting of Ti, V, Nb, Cr and Mn, which have a particle diameter of less than 0.5 μm, are added in an amount of 0.5- 10 parts by weight based on 100 parts by weight of MgO. In the formation of a forsterite film by finish annealing, it is very important to reduce the melting points of MgO and a steel sheet SiO₂ layer. However, in the formation of forsterite, when the atmosphere is extremely dry, sufficient reactivity is not obtained, in some cases, over the entire length and width of a coil only by the effect of the low-melt-point compound. As a countermeasure thereto, one or more selected from Ti, V, Nb, Cr and Mn oxides are added. More specifically, one or more selected from among TiO₂, Ti₂O₃, TiO, VO₂, V₂O₅, V₂O₃, VO, Nb₂O₅, Nb₂O₃, NbO, CrO₃, Cr₂O₃, MnO₂, Mn₂O₃ and MnO are used. The temperature elevation process in finish annealing is based on a hydrogen gas-containing atmosphere, and when oxygen is slowly decomposed from this oxide to suitably partially wet an atmosphere between steel sheet surfaces, the fayalite layer of the decarburized oxide layer can be stably maintained until the formation of forsterite is initiated. Also, it is thought that this oxide, together with the low-melting-point compound, causes the effect of promoting the glass film formation. In fact, it was observed by X-ray analysis that this oxide was present as a spinel phase in the glass film.

If this oxide has a particle diameter of more than 0.5 μm, color variation will occur in the film, and pinhole-like defects will also occur. The amount of the oxide that is added is determined depending on the components, thickness, and finish annealing conditions of the applied steel sheet. When the amount of addition of the oxide is below 0.5 parts by weight, the effect of partially wetting the atmosphere or promoting the reaction will not be insufficient. On the other hand, if it exceeds 10 parts by weight, the atmosphere between steel sheet surfaces can be excessively oxidative. Depending on the annealing conditions, the color variation of the steel sheet occurs, leading to deterioration in the quality of the glass film. Also, the inactivation of inhibitors is accelerated, thus deteriorating the magnetic properties.

In the present invention, as the main component of the annealing separator, MgO, showing a 40% CAA value of 50-120 seconds and having a hydrated water content of 1.0-2.5%, is used in combination with the low-melting- point additive. As a result, a glass film having better uniformity, and excellent magnetic properties, are realized. CAA is widely used as an index for evaluating the reactivity of MgO. At a 40% CAA value of less than 50 seconds, the activity of MgO is excessively high and has a high content of hydrated water, making it difficult to stably control the hydrated water content. As a result, the content of water in a coil is increased, and thus color variation in the coil is likely to occur. In the present invention, the use of MgO having a very low degree of hydration is not suitable for the technology of using the effect of the low-melting-point additive. Meanwhile, if the 40% CAA value exceeds 120 seconds, the MgO activity will be greatly reduced, leading to a decrease in the hydration thereof, making it impossible to suitably partially wet the atmosphere between the steel sheet surfaces, and the reactivity of MgO itself will be extremely reduced, thus causing a problem in the film thickness of the coil inner winding portion.

The hydrated water content varies depending on stirring conditions, including the solution temperature, stirring time and stirring speed in the slurry adjustment step, and on drying conditions. When the hydrated water content is less than 1.0%, the oxide film on the steel sheet will be difficult to maintain stable until the time point of formation of forsterite, because the extremely low content of hydrated water makes it difficult to partially wet an atmosphere between the steel sheet surfaces, as described above. For this reason, the formation of a stable glass film is not achieved, even when the low-melting-point additive of the present invention is used. On the other hand, if the hydrated water content exceeds 2.5%, the atmosphere will be excessively wetted, and thus variation caused by the difference in atmosphere between the steel sheet surfaces will occur. Due to this variation, the problem of causing additional local oxidation will occur, leading to non-uniform glass film quality. Also, when the atmosphere between the steel sheet surfaces is excessively wetted, the stabilization of inhibitors will be difficult, leading to a reduction in magnetic properties.

Hereinafter, the present invention will be described in further detail with reference to examples. [Example 1]

A high-magnetic-flux-density, grain-oriented electrical steel sheet material, comprising, in wt %, 0.0078% of C, 3.18% of Si, 0.068% of Mn, 0.024% of S, 0.028% of Al, 0.0080% of N, and a balance of Fe and other unavoidable impurities, was subjected to hot rolling, annealing and cold rolling to a final thickness of 0.30 mm. Then, the cold-rolled sheet was subjected to decarburization annealing at 850 ^°C for 150 seconds in an atmosphere of N₂ 50% + H₂ 50% (63 ^°C DP) in a continuous annealing line. Herein, the content of oxygen in the steel sheet was 750 ppm. Then, a slurry, comprising a base composition, obtained by adding 5 parts by weight of TiO₂ to 100 parts by weight of MgO, having a 40% CAA value of 65 seconds, and low-boiling-point additives shown in Table 1 below, was applied at a ratio of 6.5 g (as dried weight) /m^! of the sheet surface and dried. The resulting steel sheet was wound into a coil.

Then, the coil was subjected to finish annealing at 1200 ^°C for 20 hours. Then, the coil was applied with an insulating coating solution, containing aluminum phosphate and colloidal silica as main components, in a continuous line, and then was subjected to annealing at 850 ^°C .

The magnetic properties and film properties obtained in this Example are shown in Table 1 below. [Table 1 ]

Note 1) adhesion: results of banding test with 20 mmφ

As can be seen from the results in Table 1 above, in the cases of the present invention, in which the low-melting-point compounds were added, a highly glossy, uniform glass film was formed over the entire surface of the coil, suggesting very good film formation results. Also, with respect to the magnetic properties, better results than those of the Comparative Examples were obtained.

Meanwhile, in the case of Comparative Example 1, in which the low- melting-point additives were not added, the glass film was extremely thin, and the adhesion after treatment with the insulating coating film was also very poor.

Furthermore, in the case of Comparative Example 2, in which 0.60% LiClO₃ was added, a slightly hyperoxidative phenomenon occurred, and thus metallic spot defects and pinhole-like defects were present, leading to a reduction in adhesion. From such results, it is considered that the time of glass film formation or the progression of reaction has an effect on secondary recrystallization, and furthermore, on the deterioration of magnetic properties.

[Example 2] A high-magnetic-flux-density, grain-oriented electrical steel sheet material, comprising, in wt %, 0.0060% of C, 3.20% of Si, 0.070% of Mn, 0.029% of Al, and a balance of Fe and other unavoidable impurities, was subjected to hot rolling, annealing and cold rolling to a final thickness of 0.23 mm. Then, the cold-rolled sheet was subjected to decarburization annealing at 850 ^°C for 130 seconds in an atmosphere of N₂ 50% + H₂ 50% (DP 62 ^°C) in a continuous annealing line. Herein, the content of oxygen in the steel sheet was 730 ppm. Then, a slurry, comprising a base composition obtained by adding 5 parts by weight of TiO₂ to 100 parts by weight of MgO, having different 40% CAA value, and low-boiling-point additives shown in Table 2 below, was applied on the steel sheet at a ratio of 6.0 g (as dried weight)/ m^! of the sheet surface and dried. The resulting steel sheet was wound into a coil.

Then, according to the same method as in Example 1, the coil was subjected to finish annealing at 1200 ^°C for 20 hours. Then, the coil was applied with an insulating coating solution, containing aluminum phosphate and colloidal silica as main components, in a continuous line, and was then subjected to annealing at 850 ^°C .

The magnetic properties and film properties obtained in this Example are shown in Table 3 below. [Table 2]

[Table 3]

As can be seen in the results in Table 3 above, the inventions 11-20 employed the annealing separator, in which an additive having a melting point lower than 750 ^°C accounted for more than 30% of the weight of the low-melting- point additive, and the 40% CAA value of MgO was controlled to 50-105 seconds. In such inventions 11-20, the glass films were uniform and had good tension and adhesion. Also, the glass films had very good magnetic properties.

However, in the case in which MgO having a 40% CAA value of 45 seconds (high hydration of 2.9%) or a 40% CAA value of 130 seconds (inactive; low hydration of 0.9%) was used, the glass film was non-uniform or very thin, and the magnetic properties were also poor, even though the low-melting-point additive was used.

Also, in the case of invention 23, in which the additive having a melting point lower than 750 ^°C was not contained in the low-melting-point additive, the glass film was somewhat thin, showed gas-mark-like defects and had poor adhesion. In this case, the magnetic properties were very inferior to those of the inventions 11-23, in which the additive having a melting point lower than 750 ^°C was used. In addition, the cases of Comparative Examples 11 and 12, in which the low-melting-point compound was not added or was added in a very small amount, the glass film properties and magnetic properties were all poor.

[Example 3]

The same starting material as used in Example 2 was rolled to a final thickness of 0.23 mm and subjected to decarburization annealing under the same conditions. Then, the sheet coil was applied with an annealing separator, which had the composition shown in Table 4 below and contained TiO₂, V₂O₅, Nb₂O₄ and/or MnO₂ as additives for adjusting the atmosphere between the steel sheet surfaces. Then, the sheet coil was subjected to finish annealing and insulation coating under the same conditions as in Examples 1 and 2, and was evaluated for glass film properties and magnetic properties. The results obtained in this Example are shown in Table 4 below. [Table 4]

As can be seen from the results in Table 4 above, in the cases of the inventions 32-39, in which 3-10 parts by weight of one or more selected from among TiO₂, V₂O₅, Nb₂O₄ and MnO₂ were added together with the low-melting- point additive, good glass film properties and magnetic properties were obtained. However, in the case of the invention 31, in which only the low-melting- point additive was added, the glass film was uniform, but somewhat thin, and the magnetic properties were also slightly inferior to those of the invention 32-39. Also, in the invention 39, in which the low-melting-point compound was added but the amount of addition of TiO₂ was as large as 15 parts by weight, the glass film showed many pinhole-like metallic glossy defects, and the magnetic properties were also very poor. Meanwhile, in Comparative Example 31, in which only TiO₂ was added, but the low-melting-point additive was not added, the glass film was very thin, and the magnetic properties were also very poor.

From the above results, it was confirmed that, in the present invention, when the low-melting-point additive was used in combination with one or more additives selected from among TiO₂, V₂O₅, Nb₂O₄ and MnO₂ for adjusting the atmosphere between steel sheet surfaces, a more stable and better glass film could be obtained.

In another aspect, the present invention provides an annealing separator for grain-oriented electrical steel sheets having excellent surface properties and a method of producing grain-oriented electrical steel sheets using the same, wherein the annealing separator comprises: 100 parts by weight of MgO, consisting of 40- 95% of active MgO and 5-60% of inactive MgO; and 0.01-0.5 parts by weight of a low-melting-point compound, having a melting point lower than 900 "C and serving as a reaction promoter.

In the present invention, the active MgO has an average particle diameter of less than 5 μm and a 40% CAA value of 35-80 seconds, and the inactive MgO has an average particle diameter of more than 10 μm and a 40% CAA value of 250-1500 seconds. Also, the active MgO comprises coarse MgO particles having a BET value of less than 5 and a particle diameter of more than 30 μm, in an amount of more than 25 wt% based on the total weight of coarse MgO particles, and the inactive MgO is round and granular in shape and has a BET value of less than 10, a bulk specific gravity of more than 0.35 and a particle diameter of 10-100 μm. Moreover, in the annealing separator, the total amount of SO₃ and Cl is less than 1 wt%, and the hydrated water content of MgO is less than 2.5%.

Also, the low-melting-point compound having a melting point lower than

900 ^°C is one or more selected from the group consisting of antimony compounds, amide compounds, chlorides, chloroxides, chlorates, chromates, oxides, bromides, hydroxides, hydrides, carbonates, nitrates, tellurates, vanadates, fluorides, borates, phosphates, sulfides, sulfates, iodides, and hydroiodides.

Furthermore, the low-melting-point compound having a melting point lower than 900 ^°C comprises one or more elements selected from the group consisting of H, Li, Na, K, Cu, Rb, Ag, Cs, Ba, Be, Mg, Ca, Zn, Sr, Cd, Ba, B, Al,

Ga, Y, In, Tl, Ti, Ge, Sn, P, V, Nb, Sb, Ta, Bi, S, Cr, Mo, Te, W, Mn, Fe, Co and

Ni.

In addition, the present invention provides a method for producing a grain-oriented electrical steel sheet, comprising heating a steel slab containing 0.030-0.1 wt% of C and 2.5-4.0 wt% of Si, hot-rolling the heated sheet, subjecting the hot-rolled sheet to one-time cold rolling or to a plurality of cold rolling steps with annealing steps therebetween so as to attain a final sheet thickness, subjecting the cold-rolled sheet to decarburization annealing, coating the sheet with an MgO-based annealing separator, subjecting the coated sheet to finish annealing, and then subjecting the finish-annealed sheet to insulation coating and heat flattening, wherein the annealing separator is in the form of a slurry comprising 100 parts by weight of MgO, consisting of 40-95% of active MgO and

5-60% of inactive MgO, and 0.01-0.5 parts by weight of a low-melting-point compound, having a melting point lower than 900 ^°C and serving as an additive for reaction promotion, and is applied on the steel sheet after it is stirred in a mixing tank at a revolution speed of 1500-3000 rpm for more than 10 minutes.

In the present invention, a silicon steel slab, containing 2.5-4.0 wt% Si and one or more inhibitors selected from MnS, AlN, Al, Cu, Sn, Sb and Mo, depending on the intended use of the desired steel material, is hot-rolled according to a known method. The hot-rolled sheet is subjected to one-time cold rolling or to a plurality of cold rolling steps with annealing steps therebetween, so as to attain a final sheet thickness. Then, the cold-rolled steel sheet is subjected to decarburization annealing in a continuous line so as to form a SiO₂-based oxide film on the surface of the steel sheet.

Then, as an annealing separator, MgO, which consists of active MgO and inactive MgO and has adjusted activity, particle size, hydration and impurity contents, is used as a main component. In a reaction for forming a glass film on the steel sheet, the annealing agent, which is in the form of a slurry comprising 100 parts by weight of MgO and 0.01-0.5 parts by weight of one or more low- melting-point compounds, having a melting point lower than 900 °C and serving as an additive for promoting the reaction, is applied on the steel sheet in a continuous line. Before application to the steel sheet, the slurry is preferably stirred at a high revolution speed of 1500-3000 rpm. Then, the steel sheet is dried at a temperature of about 200-300 ^°C and wound into a coil. Then, in a finish annealing process, the steel sheet coil is maintained in a batch-type or continuous furnace at a temperature of about 1170-1220 ^°C for 15-20 hours, while the temperature elevation rate or the amount of nitrogen as atmospheric gas are adjusted. In this case, glass film formation, annealing and secondary recrystallization occur at the same time.

In a continuous line, the coil thus treated is washed with water to remove an excess of the annealing separator, and washed with acid. Then, it is coated with an insulating coating agent, and subjected to heat flattening at a temperature of about 850 °C for the annealing of the insulating coating film, shape correction, and stress-removing annealing, thus producing a final product.

In grain-oriented electrical steel sheets, the glass film and magnetic properties are determined through such a series of processes, and particularly, the formation conditions of the oxide film in the decarburization annealing process and the conditions in the annealing separator are very important. In other words, the annealing separator influences not only the formation time, formation rate, formation amount and uniformity of the glass film, but also additional oxidation or nitrification, which influence the formation of the film during temperature elevation in the finish annealing process. As a result, it influences the decomposition rates of important inhibitors, AlN and MnS, or other components in steel sheets, and thus also influences the magnetic properties of the steel products. Particularly, it is important in a high-magnetic-flux-density material containing Al as an inhibitor, because it greatly influences the material. In addition, the particle size or sintering property of an annealing separator influences the movement of a steel sheet during the thermal expansion or thermal shrinkage of the steel sheet coil, and thus also influences the shape of the coil.

For such reasons, according to the present invention, when the annealing separator, which comprises the low-melting-point additive, is used in addition to low-hydrated MgO, having controlled activity and particles, and when the slurry dispersion technology is used in combination with the annealing separator, excellent synergistic effects on the improvement of the glass film and on the coil shape are realized.

The reason for limitation will now be described. The annealing separator, which is used in the present invention, comprises

MgO, consisting of a mixture of 40-95 wt% of one or more active MgO particles, having an average particle diameter of less than 5 μm and a 40% CAA value of 35-80 seconds, with 5-60 wt% of one or more inactive MgO particles, having an average particle diameter of more than 10 μm and a 40% CAA value of 250-1500 seconds. The MgO for use in the present invention has a total amount of SO₃ and Cl of less than 1 % and a hydrated water content of less than 2.5 wt%. When the hydrated water content is more than 2.5%, the total water content during high- temperature annealing is increased, and thus local water accumulation in the coil occurs, leading to additional oxidation. The active MgO, having an average particle diameter of less than 5 μm and a 40% CAA value of 35-80 seconds, mainly severs as a raw material for forming a forsterite film in a glass film formation process. Also, a trace amount of hydrated water, which is produced on the MgO surface in the step of adjusting the slurry of the active MgO, serves to partially wet the atmosphere between the coil sheets in the finish annealing process and to maintain the oxide layer on the steel sheet surface stable until the glass film formation step, thus stabilizing the reaction. If the content of the active MgO is less than 40 wt% based on the total weight of MgO, the activity of the fine particles will be reduced, and thus the formation of the glass film becomes insufficient. If the amount of fine MgO particles having a particle diameter of less than 5 μsi is insufficient, the adhesion of the glass film to the steel sheet will be reduced. Thus, the glass film is likely to be thin, or pinhole-like defects are likely to occur.

On the other hand, the content of the active MgO exceeds 95 wt%, it will impart an excessively large amount of hydrated water content to the steel sheets, or will reduce the ventilation between the steel sheets, such that peroxide defects are likely to occur on the outer winding portion or edge of the coil. Also, because the content of coarse particles will be excessively reduced, slippage between the steel sheets will be reduced, leading to an increase in the occurrence of distortion defects and embossing defects, which occur due to thermal expansion and thermal shrinkage during finish annealing.

Also, if the active MgO has a 40% CAA value of less than 35 seconds, it will have excessively strong activity, making it difficult to control hydrated water in the mixing process and making it difficult to reduce impurities. If the 40% CAA value exceeds 80 seconds, the activity of the MgO particles themselves will be reduced, and thus the glass film formation reaction will be unstable.

Inactive MgO having a 40% CAA value of 250-1500 seconds mainly contributes to an increase in the ventilation of the atmosphere between steel sheets and to an increase in the slippage between steel sheets. Due to the inactive coarse particles, the hydrated water between steel sheets is smoothly released or the atmosphere between steel sheets becomes uniform, making the glass film uniform. Also, due to the slippage improvement effect of the coarse particles, the movement of the steel sheets during thermal expansion or shrinkage in the finish annealing process is improved, leading to a reduction in shape defects, such as distortion defects or embossing defects. If the content of the inactive MgO is less than 5 wt%, a sufficient effect of improving said ventilation or slippage can be obtained. On the other hand, the content of the inactive MgO exceeds 60 wt%, the amount of the active MgO will be insufficient, making the glass film thin. If the 40% CAA value is less than 250 seconds, it will have an insignificant effect on the improvement of the shape defect problem, and in addition, will influence the total hydration of MgO, and thus have an unfavorable effect on glass film formation. If the 40% CAA value exceeds 1500 seconds, the appearance improvement effect will not be changed, but the inevitable incorporation of very coarse particles will occur, and in addition, the production cost will be increased.

In the mixture of the active MgO with the inactive MgO, the total amount of SO₃ and Cl impurities is maintained at less than 1%. In a process for preparing MgO, in a conventional seawater method of preparing magnesium hydroxide from seawater, the amount of SO₃ in produced MgO tends to increase. In the present invention, the effects of these impurities were closely examined and, as a result, it was proved that a total amount of SO₃ and Cl of less than 1% was not problematic. If the total amount of SO₃ and Cl exceeds 1%, etching will occur in the high-temperature region of the glass film formation process, thus thinning the glass film or causing surface defects, such as local spots or discoloration.

In the present invention, the inactive MgO particles preferably have a granular shape, a BET value of less than 10 and a particle diameter of 10-100 μm. The particles showing a shape close to a spherical shape, called a "granular shape", act as the above-described space, thus contributing to the improvement of ventilation or slippage, thus providing a better film and improving the glass film appearance. When coarse particles, obtained by grinding particles, obtained by calcining clinker or the like at high temperature, are used, the slippage improvement effect will be extremely reduced compared to that of the coarse granular particles used in the present invention, and fine defects will occur on the steel sheet surface along the cracked plane.

When the BET value is less than 10, an excellent effect of improving the film appearance will be obtained. This is because the BET value is connected with the packing density of the particles, and the hardness of the particles is increased so as to show a stable effect of improving the film appearance. In fact, even when coarse particles, produced by the agglomeration of fine particles, were used, the effect of improving the film appearance was not observed. If the BET value exceeds 10, the effect of improving the film appearance will be extremely reduced, possibly because the hardness of the coarse particles is low. For this reason, the BET value is limited to less than 10.

In the present invention, the particle diameter of MgO particles is 10-100 μm. When the MgO particles have a particle diameter of less than 10 μm, they will not sufficiently exhibit the effect of improving ventilation or film appearance. On the other hand, at more than 100 μm, the coarse particles will cause dent defects on the steel sheet surface.

Also, preferably, the content of MgO particles having a particle diameter of more than 30 μm is more than 25 wt% based on the total weight of coarse MgO particles. This is because MgO particles having a particle diameter of about 30-100 μm are required in a given amount in order to exhibit the above- described spacing effect or slippage improvement effect.

In the present invention, the slurry containing MgO is adjusted by stirring it in a mixing tank at a stirring speed of 1500-3000 rpm for at least 10 minutes, and then the stirred slurry is applied on the steel sheet. In the present invention, the stirring mixer is not specifically limited, as long as it is a conventional tank having stirring propellers therein. At a stirring speed of less than 1500 rpm, due to the physical properties of MgO, the uniform dispersion of MgO particles in the slurry and the increase in the adhesion of MgO to the steel sheet, cannot be sufficiently obtained. This can also lead to a decrease in the MgO reactivity for forming the glass film. On the other hand, when the stirring speed exceeds 3000 rpm, the stirring temperature will be increased due to the friction between MgO particles during the stirring of MgO, making it difficult to control the hydrated water of MgO. According to the present invention, when the slurry containing the mixture of active MgO and inactive MgO is adjusted by stirring it in a mixing tank at a stirring speed of 1500-3000 rpm, sufficient dispersion of the MgO particles will occur, and thus a grain-oriented electrical steel sheet product having an excellent glass film and coil shape will be obtained.

In the present invention, as an additive for promoting glass film formation, one or more selected from the group consisting of antimony compounds, amide compounds, chlorides, chloroxides, chlorates, chromates, oxides, bromides, hydroxides, hydrides, carbonates, nitrates, tellurates, vanadates, fluorides, borates, phosphates, sulfides, sulfates, iodides, and hydroiodides, which have a melting point lower than 900 ^°C, are used in an amount of 0.01-0.05 parts by weight based on 100 parts by weight of MgO.

When an annealing separator not containing the above additive is used, the initiation temperature of glass film formation is generally 900-950 °C . For this reason, additional oxidation or addition nitrification occurs during glass film formation, depending on steel sheet components or finish annealing conditions, thus causing glass film defects at the outer winding portion or edge of the coil. According to the present invention, the additive having a melting point lower than 900 ^°C is added to MgO, the initiation temperature of the glass film formation reaction can be significantly reduced to make it possible to suppress the above- described additional oxidation or additional nitrification, and thus a steel sheet product having a more uniform glass film and magnetic properties can be obtained. When the additive is added in an amount of less than 0.01 parts by weight based on 100 parts by weight of MgO, the effect of reducing the initiation temperature of glass film formation will not be sufficient, and thus the effect of improving the glass film will not be obtained. On the other hand, if the amount of addition of the additive exceeds 0.5 parts by weight, the effect of the low- melting-point additive will be obtained due to MgO or finish annealing conditions, but pinhole-like or island-like local melt defects will occur due to the excessive action of the additive. In severe cases, color spot defects will occur.

This low-melting-point additive comprises one or more elements selected from the group consisting of H, Li, Na, K, Cu, Rb, Ag, Cs, Ba, Be, Mg, Ca, Zn,

Sr, Cd, Ba, B, Al, Ga, Y, In, Tl, Ti, Ge, Sn, P, V, Nb, Sb, Ta, Bi, S, Cr, Mo, Te,

W, Mn, Fe, Co and Ni. When a compound containing such elements is used, an excellent glass film and improved magnetic properties will be obtained.

Hereinafter, the present invention will be described in further detail with reference to the following examples. [Example 4]

A high-magnetic-flux-density grain-oriented electrical steel sheet material, comprising, in wt%, 0.055% of C, 3.12% of Si, 0.065% of Mn, 0.025% of S, 0.026% of Al, 0.0077% of N, and a balance of Fe and other unavoidable impurities, was subjected to hot rolling, annealing and cold rolling so as to have a final thickness of 0.30 mm. Then, the cold-rolled steel sheet was subjected to decarburization annealing in a continuous line in an atmosphere of 50% N₂ + 50% H₂ (65 ⁰C DP) at 850 ^°C for 150 seconds. Herein, the steel sheet contained 765 ppm of oxygen. Meanwhile, as shown in Table 5 below, 5 parts by weight of TiO₂ 5 and 0.3 parts by weight OfNa₂B₄O₇ were added to MgO (prepared using a brine method) consisting of active MgO particles, having different 40% CAA values, and inactive MgO particles, having CAA values adjusted to 50 seconds, 700 seconds and 1000 seconds, thus obtaining annealing separators. Each of the annealing separators was stirred in water at 7 ^°C for 30 minutes to obtain slurry, and the slurry was applied on the steel sheet at a ratio of 6.0 g (as dried weight)/m² of the sheet surface and dried, and then the sheet was wound into a 20- ton coil. [Table 5]

Then, the steel sheet was subjected to finish annealing at 1200 ^°C for 20 hours, and then an insulating coating solution, containing aluminum phosphate and colloidal silica as main components, was applied on the steel sheet in a continuous line. Then, the steel sheet was subjected to annealing decarburization at 850 ^°C .

The magnetic properties and glass film properties of the steel sheet obtained in this Example are shown in Table 6 below. [Table 6]

Note 1): Adhesion: 20 mmφ bending test after insulating coating

As can be seen in the results in Table 1 above, in the cases of the inventions in which the mixture of 45-85wt% of MgO, having a 40% CAA value of 35-65 seconds, with 15-55 wt% of MgO, having a CAA value of 500-1000 seconds, was used, the film appearance was uniform and excellent, the adhesion after insulating coating was excellent, and the magnetic properties were also good.

[Example 5]

A high-magnetic-flux-density grain-oriented electrical steel sheet material, comprising, in wt%, 0.067% of C, 3.15% of Si, 0.070% of Mn, 0.025% of Al, and a balance of Fe and other unavoidable impurities, was subjected to hot rolling, annealing and cold rolling so as to have a final thickness of 0.23 mm. Then, the cold-rolled steel sheet was subjected to decarburization annealing in a continuous line in an atmosphere of 50% N₂ + 50% H₂ (65 ^°C DP) at 850 ^°C for 130 seconds. Herein, the steel sheet contained 715 ppm of oxygen. Meanwhile, as shown in Table 7 below, 5 parts by weight of TiO₂ and 0.3 parts by weight of sodium borate were added to 100 parts by weight of a MgO mixture (prepared using a seawater method) of active MgO, having a 40% CAA value of 55 seconds, with inactive granular MgO, having a BET value of 1-4 and various particle diameters, thus preparing annealing separators. Each of the annealing separators was stirred in a water in a mixing tank at 10 °C, thus obtaining slurry. The slurry was applied on the steel sheet at a ratio of 6.0 g (as dried weight)/m² of the sheet surface and dried, and the steel sheet was wound into a coil. [Table 7]

Note 2): in invention 57, inactive MgO, having an average particle diameter of 55 μm and obtained by grinding MgO clinker, was added. Then, the steel sheet was subjected to finish annealing at 1200 ^°C for 20 hours, and then an insulating coating solution, containing aluminum phosphate and colloidal silica as main components, was applied on the steel sheet in a continuous line. Then, the steel sheet was subjected to annealing decarburization at 850 ^°C .

The magnetic properties and glass film properties of the steel sheet obtained in this Example are shown in Table 8 below. [Table 8]

As can be seen from the results in Table 8 above, in the cases of the inventions in which the mixture of active MgO, having a 40% CAA value of 35- 75 seconds, with granular MgO, having a 40% CAA value of 300-1000 seconds, was used, a good glass film and coil shape were obtained. Particularly, in the case in which the 40% CAA value of active MgO was 55 seconds, the uniformity, color tone and adhesion of the glass film were all good. In the case of the invention 57, in which inactive MgO, which had a rough surface and was obtained by grinding MgO clinker, was used, the film properties were relatively good, but in the coil shape, embossing defects were observed at the inner winding portion. Meanwhile, in the case of Comparative Example 51 , in which active MgO had a 40% CAA value of 25 seconds, and in the case of Comparative Example 52, in which active MgO had a 40% CAA value of 95 seconds, it was observed that the thickness of the film in the widthwise direction of the coil was very thin, and the adhesion of the film was also poor. Also, in the case of Comparative Example 52, in which inactive coarse MgO particles were not used, shape defects occurred at the inner winding portion of the coil, and the magnetic properties were very inferior to those of the inventions.

[Example 6]

The same starting material as used in Example 4 was rolled to a final thickness of 0.30 mm and subjected to decarburization annealing in the same conditions. Meanwhile, 5 parts by weight of TiO₂ and 0.3 parts by weight of Na₂B₄O₇ were added to 100 parts by weight of the same MgO as used in the invention 53 of Example 5, and the mixture was stirred in water at 7 ^°C using the addition conditions and mixing conditions of a reaction promoter, as shown in Table 9, thus obtaining slurry. Then, the slurry was applied on the steel sheet at a ratio of 6.5 g/m² of the sheet surface, and the steel sheet was wound into a coil. [Table 9]

Note 3): In Comparative Examples 61 and 62, the same MgO as used in Comparative Example 43 was used.

Then, the steel sheet was subjected to finish annealing at 1200 ^°C for 20 hours, and then an insulating coating solution, containing aluminum phosphate and colloidal silica as main components, was applied on the steel sheet in a continuous line. Then, the steel sheet was subjected to annealing decarburization at 850 ⁰C .

The magnetic properties and glass film properties of the steel sheet obtained in this Example are shown in Table 10 below. [Table 10]

As can be seen from the results in Table 10 above, in the cases of the inventions 62-68, in which the additive for reaction promotion was added, the color tone, uniformity and adhesion of the film were excellent compared to the cases of Comparative Examples 61 and 62, in which the additive for reaction promotion was not added, suggesting that the addition of the additive for reaction promotion enabled a better glass film to be obtained.

Also, in the cases of the inventions 67 and 68, in which the slurry was obtained by stirring in a mixing tank at a stirring speed of 1500 rpm or 2500 rpm, the uniformity and color tone of the glass film were very good, and the magnetic properties were also very improved compared to the case in which stirring was conducted at a speed of 800 rpm.

Meanwhile, in Comparative Example 62, in which MgO consisting only of active MgO was used, the color tone of the glass film was non-uniform, as in the case of Example 4, and scale defects were generated in a large amount. Also, in Comparative Example 61, in which the additive for reaction promotion was added to the MgO, the glass film was slightly improved, but the film properties and the magnetic properties were all inferior to those of the inventions.

Advantageous Effects

As described above, the inventive annealing separator for grain-oriented electrical steel sheets, having a uniform glass film and excellent magnetic properties, has a low content of hydrated water and contains, as an additive for increasing the reactivity of the MgO, an additive having a melting point lower than 900 ^°C, preferably a low-melting-point additive containing an additive having a melting point lower than 750 ^°C, in an amount of 30 wt% based on the total amount of the low-melting-point additive. Thus, the annealing separator shows a remarkable effect of improving glass film formation, and suppresses the occurrence of additional oxidation or additional nitrification during a finish annealing process, such that a grain-oriented electrical steel sheet having very good film properties and magnetic properties, can be obtained.

Also, through the application of the novel annealing separator, a grain- oriented electrical steel sheet, having a uniform glass film, and excellent magnetic properties and an excellent coil shape, can be obtained.

Industrial Applicability

The present invention provides a grain-oriented electrical steel sheet having a uniform glass film formed thereon, excellent magnetic properties, and an excellent coil shape. Thus, the present invention contributes to industrial development.

Claims

Claims:

1. An annealing separator for grain-oriented electrical steel sheets, having a uniform glass film and excellent magnetic properties, the annealing separator comprising: 100 parts by weight of MgO; and 0.01-0.5 parts by weight of a low- melting-point compound having a melting point lower than 900 ^°C .

2. The annealing separator of Claim 1, wherein said MgO consists of 40- 95% of active MgO and 5-60% of inactive MgO.

3. The annealing separator of Claim 2, wherein said active MgO has an average particle diameter of less than 5 μm and a 40% CAA value of 35-80 seconds, and said inactive MgO has an average particle diameter of more than 10 jum and a 40% CAA value of 250-1500 seconds.

4. The annealing separator of Claim 2 or 3, wherein the active MgO contains coarse particles having a BET value of less than 5 and a particle diameter of more than 30 [M in an amount of more than 25 wt% based on the total weight of the active MgO, and the inactive MgO is round and granular in shape and has a BET value of less than 10, a bulk density of more than 0.35 and a particle diameter of 10-100 μm.

5. The annealing separator of Claim 1, which further comprises, based on 100 parts by weight of said MgO, 0.5-10 parts by weight of one or more selected from among Ti, V, Nb, Cr and Mn, which have a particle diameter of less than 0.5 μm and serve as an additive for controlling an atmosphere between the steel sheet surfaces.

6. The annealing separator of Claim 2, wherein the total amount of SO₃ and Cl as impurities is less than 1%, and the hydrated water content of MgO is less than 2.5%.

7. The annealing separator of Claim 1 or 5, wherein the low-melting-point compound contains a compound having a melting point lower than 750 ^°C, in an amount of 30% based on the total amount of the low-melting-point compound.

8. The annealing separator of Claim 2 or 7, wherein the low-melting-point compound is one or more selected from the group consisting of antimony compounds, amide compounds, chlorides, chloroxides, chlorates, chromates, oxides, bromides, hydroxides, hydrides, carbonates, nitrates, tellurates, vanadates, fluorides, borates, phosphates, sulfides, sulfates, iodides, and hydroiodides.

9. The annealing separator of Claim 8, wherein the low-melting-point compound comprises one or more elements selected from the group consisting of H, Li, Na, K, Cu, Rb, Ag, Cs, Ba, Be, Mg, Ca, Zn, Sr, Cd, Ba, B, Al, Ga, Y, In, Tl, Ti, Ge, Sn, P, V, Nb, Sb, Ta, Bi, S, Cr, Mo, Te, W, Mn, Fe, Co and Ni.

10. The annealing separator of Claim 1 or 5, wherein said MgO has a 40% CAA value of 50-120 seconds and a hydrated water content of 1.0-2.5%.

11. A method for producing a grain-oriented electrical steel sheet, comprising heating a steel slab containing 0.030-0.1 wt% C and 2.5-4.0 wt% Si, hot-rolling the heated sheet, subjecting the hot-rolled sheet to one-time cold rolling or a plurality of cold rolling steps with annealing steps therebetween so as to attain a final sheet thickness, subjecting the cold-rolled sheet to decarburization annealing, coating the sheet with an MgO-based annealing separator, subjecting the coated sheet to finish annealing, and then subjecting the finish-annealed sheet to insulation coating and heat flattening, wherein the annealing separator is in the form of a slurry comprising the components of Claim 1 or 2, and is applied on the steel sheet, after it is stirred in a mixing tank at a revolution speed of 1500-3000 rpm for more than 10 minutes.