WO2008078947A1

WO2008078947A1 - Method of manufacturing grain-oriented electrical steel sheets

Info

Publication number: WO2008078947A1
Application number: PCT/KR2007/006803
Authority: WO
Inventors: Hyung-Don Joo; Jong-Tae Park; Chang-Soo Kim; Kyu-Seok Han; Jae-Kwan Kim; Byeong-Goo Kim; Kyu-Seung Choi; Won-Gul Lee; Seong-Kyu See; Jae-Soo Lim
Original assignee: Posco
Priority date: 2006-12-27
Filing date: 2007-12-24
Publication date: 2008-07-03

Abstract

Disclosed is a method of producing grain-oriented electrical steel sheets, which are used as iron core materials for electrical devices, including large-sized rotating machines such as various transformers and electric generators. More specifically, disclosed is a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties, low iron loss, and high magnetic flux density, or at least one of these properties, the method comprising reheating a slab for the grain-oriented electrical steel sheet, hot-rolling the reheated slab to produce a hot-rolled sheet, optionally annealing the hot-rolled sheet, cold-rolling the resulting sheet, subjecting the cold-rolled sheet to simultaneous decarburization and nitriding annealing, and then subjecting the annealed sheet to secondary recrystallization annealing, wherein a vary small amount of N and S are added to the slab, such that AlN and MnS can be produced and completely dissolved in solution in the slab reheating.

Description

METHOD OF MANUFACTURING GRAIN-ORIENTED ELECTRICAL STEEL SHEETS

Technical Field

The present invention relates to a method of producing grain-oriented electrical steel sheets, which are used as core materials for electrical devices, including various kinds of transformers and large-sized rotating machines such as electric generators, and more particularly to a method for producing a grain- oriented electrical steel sheet having excellent magnetic properties, low iron loss, high magnetic flux density, or at least one of these properties, the method comprising reheating a slab for the grain-oriented electrical steel sheet, hot-rolling the reheated slab to produce a hot-rolled sheet, optionally annealing the hot-rolled sheet, cold-rolling the resulting sheet, subjecting the cold-rolled sheet to simultaneous decarburization and nitriding annealing, and then subjecting the annealed sheet to secondary recrystallization annealing, wherein a vary small amount of N and S are added to the slab, such that AlN and MnS can be produced and completely dissolved in solution in the slab reheating.

Background Art

Grain-oriented electrical steel sheets are soft magnetic materials composed of crystal grains having a so-called Goss texture, expressed by {110}<001> on the Miller index, in which the {110} crystal plane of the grain is parallel to the rolling plane and the <001> crystal direction of the grain is parallel to the rolling direction. Thus, these sheets have excellent magnetic properties in the rolling direction. It is possible to obtain this {110}<001> texture through a combination of various production processes, and it is generally important to very strictly control chemical compositions, slab reheating, hot rolling, hot-band annealing, primary recrystallization annealing, final annealing and the like.

Because such grain-oriented electrical steel sheets show excellent magnetic properties due to the secondary recrystallized texture, obtained by inhibiting the growth of primary recrystallized grains and selectively growing

{110}<001>-oriented grains, among the grains having various orientations, an inhibitor hindering the normal growth of primary recrystallized grains (hereinafter, referred to as "inhibitor") is particularly important. Also, it is important in the technology for producing grain-oriented electrical steel sheets to enable grains having a stable {110}<001> texture, among various grains, to preferentially grow (hereinafter, referred to as "secondary recrystallization") in a final annealing process.

Specifically, as the inhibitor, an artificially formed fine precipitate or segregation element is used, and in order for the growth of all primary recrystallized grains to be inhibited in the final annealing process until secondary recrystallization occurs, such precipitates should be distributed in a sufficient amount and a suitable size and should be thermally stable, so that they should not easily dissolved at temperatures as high as that experienced immediately before secondary recrystallization occurs.

In the final annealing process, secondary recrystallization starts to occur because such inhibitors lose their function of inhibiting the growth of primary recrystallized grains while they grow or are dissolved with an increase in temperature. At this time, the secondary recrystallization occurs in a relatively short time.

Inhibitors that satisfy the above-mentioned conditions, and thus are currently widely used in industrial applications, include MnS, AlN, MnSe and the like. A typical previous technology for producing electrical steel sheets using only MnS, among the inhibitors is disclosed in Japanese Patent Publication No. Sho 30-3651, and the related production method comprises obtaining a stable secondary recrytallized texture through two stage cold rolling with an intermediate annealing However, the method that uses only MnS as the inhibitor has problems in that high magnetic flux density cannot be obtained, and a high production cost is incurred, because the cold rolling is carried out two times. In grain-oriented electrical steel sheets, high magnetic flux density is required, because, when products having high magnetic flux density are used as core materials, the size of electrical devices can be smaller. For this reason, efforts to increase magnetic flux density have been continuously made. Meanwhile, an example of a method of producing grain-oriented electrical steel sheets using both MnS and AlN as inhibitors is disclosed in Japanese Patent Publication Sho 40- 15644. hi this method, products having a high magnetic flux density are obtained at a high reduction rate of more than 80% by one stage cold rolling. Specifically, this method comprises a series of processes, including high- temperature slab reheating, hot rolling, hot-band annealing, cold rolling, decarburization annealing and final annealing. Herein, the final annealing is a process in which secondary recrystallization occurs in a state in which the sheet is wound into a coil so as to develop the {110}<001> texture. In this final annealing process, a MgO-based annealing separator is applied on the surface of steel sheets before annealing so as to prevent the steel sheets from sticking to each other and, at the same time, to allow an oxide layer, formed on the steel sheet surface upon decarburization annealing, to react with the annealing separator, to impart insulation properties to the steel sheets. The steel sheets having the {1 10}<001> texture, obtained through the final annealing, are finally subjected to insulation coating, thus producing final products. Another example of a method of producing grain-oriented electrical steel sheets using MnSe and Sb as inhibitors is disclosed in Japanese Patent Publication No. Sho 51-13469. The production method comprises a series of processes, including high-temperature slab reheating, hot rolling, hot-band annealing, first cold rolling,, intermediate annealing, second cold rolling, decarburization annealing and final annealing. This method has an advantage in that high magnetic flux density can be obtained, but it has problems in that, because cold rolling is carried out two times, and expensive Sb or Se is used as an inhibitor, the production cost is increased, and workability is poor because of the toxicity of these elements. In addition to the above-described problems, these methods have fundamental problems that are very serious. That is, for use as an inhibitor, MnS or AlN contained in a slab for grain-oriented electrical steel sheets should be dissolved in solid solution at a high temperature, such that it can be made into precipitates having a suitable size and distribution during hot rolling. For this purpose, the slab must be reheated to a high temperature. Specifically, it is known that the slab must be reheated at more than 1300 ^°C in the method that uses MnS as an inhibitor, more than 1350 ^°C in the method that uses MnS+AIN as inhibitors, and more than 1320 ^°C in the method that uses MnSe+Sb as inhibitors, such that high magnetic flux density can be obtained. In actual industrial production, the slab is reheated to a temperature of about 1400 ^°C in view of the size of the slab in order to realize uniform temperature distribution throughout the interior of the slab.

When the slab is reheated to a high temperature for a long time, as described above, there are problems in that the production cost is increased because a large amount of heat is used, and in that, because the surface of the slab flows down in a molten state, the cost of repairing the reheating furnace is high, and the life span of the heating furnace is reduced. Particularly, when the columnar structure of the slab grows coarsely by high-temperature heating for a long time, there is a problem in that cracks occur in the width direction of sheets in a subsequent hot-rolling process, thus remarkably reducing production yield.

Accordingly, if grain-oriented electrical steel sheets can be produced through a process of reheating the slab at lower temperatures, many advantageous effects in terms of production cost and yield can be obtained. Thus, new methods that do not use MnS, which is dissolved in solid solution at a high dissolution temperature, have been studied. This does not entirely depend on the elements contained in steel compositions, but became possible thanks to techniques of forming nitrides in a suitable step in a production process using a method known as nitriding treatment. In this process, the above-described problems are solved by reducing the reheating temperature of slabs, and required inhibitors are made through a method of nitriding steel sheets at the final thickness. This process is generally called a "technique of producing grain-oriented electrical steel sheets using a low- temperature reheating process". Nitriding methods include various methods, including nitriding steel sheets in a gas atmosphere having nitriding ability after a decarburization process, applying an annealing separator containing a compound having a nitriding ability on steel sheets, and introducing an atmosphere gas, containing a gas having nitriding ability into the central part of steel sheets during a heating period in a high-temperature annealing process. Among them, the method of nitriding steel sheets in a gas atmosphere having nitriding ability after a decarburization process is most generally used. As a currently used method, a method of supplying nitrogen into steel sheets in a separate nitriding process containing ammonia gas, after decarburizing the steel sheets with Al-based nitride, is disclosed in Japanese Patent Publication Nos. Hei 1-230721 and 1-283324. Meanwhile, a method comprising carrying out decarburization annealing and nitriding annealing at the same time in an economical manner is disclosed in Korean Patent Laid-Open Publication No. 97-43184. Korean Patent Application No. 97-28305 discloses a method comprising carrying out decarburization and nitriding at the same time using chemical compositions different from those of the above patent. Also, with respect to the time point at which nitriding treatment is carried out, Japanese Patent Publication No. Hei 3-2324 discloses a method comprising preferentially carrying out decarburization annealing and carrying out nitriding with ammonia gas after grains grow to an arbitrary size or larger. In all the above-mentioned patents, the slab heating is carried out at a temperature at which AlN, acting as an inhibitor in secondary recrystallization, is partially dissolved in solution. If the slab is reheated only to the temperature at which precipitates in the slab are partially dissolved in solution, there will be a great difference in size distribution between precipitates produced in a casting process and precipitates formed in hot rolling. This difference leads to a difference in the size distribution of grains in primary recrystallized sheets and deteriorate the magnetic properties of steel products subjected to final annealing. In addition, even when AlN is used as a main inhibitor, MnS influences the size of primary recrystallized grains, and whether MnS is dissolved in solution is also important, because it influences the size distribution of primary recrystallized grains. In fact, Japanese Patent Laid-Open Publication No. 2003-201518 discloses the production of grain-oriented electrical steel sheets using the difference in the grain boundary migration rate of primary recrystallized grains with a component system containing no inhibitor-forming element without any inhibitor. This suggests that it is important to partially dissolve precipitates in solution during slab heating. Korean Patent Application No. 2001-0031104 and Japanese Patent Publication No. Hei 12-167963 claim a method for producing electrical steel sheets, in which a slab is reheated to a temperature of more than 1200 °C, and nitriding treatment is carried out between decarburization annealing and the initiation of secondary recrystallization in finish annealing, such that the average grain size of primary recrystallized grains is 7-18 μm. This patent publication discloses that, at a reheating temperature of less than 1200 ^°C, magnetic properties are not ensured, because the size of grains in conditions in which precipitates are completely dissolved in solution is 26.2 μm, at which secondary recrystallization does not occur. When grains become larger, the distribution of grain size can also widen, leading to non-uniform secondary recrystallization, which adversely affects the magnetic properties of the resulting steel sheets. However, in the present invention, electrical steel sheets having excellent magnetic properties can be produced, because simultaneous decarburization and nitriding are carried out and the amount of inhibitors is low, and thus secondary recrystallization readily occurs even at a grain size of 28.5 μm. Unlike Korean Patent Application No. 2001-0031104 and Japanese Patent Publication No. Hei 12-167963, the present invention is characterized in that conditions in which precipitates are completely dissolved in solution are realized while the slab reheating temperature is below 1200 ^°C, simultaneous decarburization and nitriding treatments are carried out, and the average size of primary recrystallized grains is 20-32 μm. Also, Japanese Patent Publication No. Hei 2-294428 discloses a method for producing grain-oriented electrical steel sheets, which comprises reheating a slab to a temperature lower than 1200 ^°C, carrying out simultaneous decarburization and nitriding, and forming an inhibitor based on (Al₅Si)N. However, this patent publication suggests that the slab reheating temperature is a condition in which Al is partially dissolved in solution. In this patent publication, the content of N is limited to the range of 0.0030- 0.010%, and due to the increase in the N content, an inhibitor containing Al, which is partially dissolved in solution, remains. In the present invention, the content of N in a slab is limited to less than 0.0030%, such that precipitates containing Al and Mn can be completely dissolved in solution, and thus an improvement in magnetic properties can be obtained through the uniform distribution of grains and the increase in grain size, as described above. Among the above methods, nitriding with ammonia gas uses a characteristic which ammonia is decomposed into hydrogen and nitrogen at more than about 500 ^°C, and the decomposed nitrogen is introduced into steel sheets. The nitrogen introduced into steel sheets reacts with Al, Si, Mn and the like, present in the steel sheets, to form nitrides which are used as inhibitors. Among the formed nitrides, Al-based nitrides, including AlN and (Al,Si,Mn)N, are used as inhibitors. All of the above-described methods are methods of producing grain-oriented electrical steel sheets by reheating slabs to low temperatures and forming additional precipitates in the steel sheets using a material or gas having nitriding ability. As mentioned above, the gas having nitriding ability is represented by ammonia, and the operation of nitriding using the gas after decarburization annealing, and associated problem, are as follows.

Nitriding via the decomposition of ammonia gas can be achieved at temperatures higher than 500 ^°C, which is the decomposition temperature of ammonia gas. However, at temperatures around 500 ^°C, the diffusion rate of nitrogen in steel sheets is very low, and thus nitriding must be carried out for a long time. At temperatures higher than 800 ^°C, nitriding easily occurs, but primary recrystallized grains grow easily, so that the distribution of grains in steel sheets becomes non-uniform, making the development of secondary recrystallization unstable. Thus, a suitable nitriding temperature range is considered to be 500-800 ^°C . However, when the nitriding temperature is low, the nitriding time should be excessively increased. Therefore, nitriding is carried out in the temperature range of 700-800 ^°C due to problems associated with productivity. A method of carrying out nitriding based on this fact is disclosed in Korean Patent Publication No. 95-4710. In this temperature range, the decomposition of ammonia and the diffusion of nitrogen actively occur, and thus it is required to very strictly control the nitriding conditions in order to introduce nitrogen in the desired amount. That is, the amount of nitriding is determined by ammonia concentration, nitriding temperature and nitriding time, and the suitable amount of nitriding should be determined based on a combination of these conditions. In view of productivity, nitriding should be achieved in a short time, and thus the ammonia concentration and nitriding temperature should be high. In this case, because nitriding is achieved in a short time, the nitrogen concentration is increased mainly in the surface part of steel sheets thereof. Accordingly, the variation in nitrogen concentration through the thickness of steel sheets is greatly increased. The central part of steel sheets is not substantially nitrified, and in the surface part, non-uniform nitriding is very evident. Also, the amount of nitriding is greatly influenced by the conditions of steel sheets, including surface roughness, grain size and chemical composition. When the surface roughness is high, the area of contact with atmospheric gas is increased, thus causing variation in the nitriding amount. When the grain size is small, the area of grain boundaries per unit area are increased, and the diffusion of nitrogen through the grain boundaries occurs faster than the diffusion of nitrogen through the grain interiors, thus causing variation in the nitriding amount. With respect to chemical composition, variation in the amount of nitriding can occur depending on the relative amount of elements which easily make nitrides. This variation in the amount of nitriding ultimately causes surface defects, and this problem can be solved by a combination of a final annealing atmosphere and a heat treatment temperature, as disclosed in Korean Patent Application No. 97-65356.

As mentioned above, the final annealing process is a very important process for obtaining the secondary recrystallized texture having the {110}<001> orientation. Particularly, the method disclosed in Korean Patent Publication No. 95-4710, in which nitriding is carried out after decarburization, comprises a process of transforming precipitates, produced after nitriding annealing, in the final annealing process. Specifically, the precipitates, produced after nitriding annealing, are Si₃N₄ or (Si₅Mn)N, which are easily decomposed due to their thermal instability. Thus, such precipitates cannot be used as inhibitors, because they do not satisfy the conditions of the inhibitors. Accordingly, these precipitates should be converted into thermally stable precipitates such as AlN or (Al₅Si, Mn)N, such that they can function as inhibitors, hi the case where nitrides are formed by nitriding annealing after decarburization, the precipitates should be maintained at a temperature of 700-800 "C for at least 4 hours in the subsequent final annealing process, such that they can be transformed into precipitates that can be used as inhibitors. This means that the final annealing process becomes very lengthy and must be strictly controlled, which is very disadvantageous in terms of production cost, hi an attempt to solve these problems, a method comprising carrying out decarburization and nitriding at the same time is disclosed in Korean Patent Application No. 98-58313.

However, this method has a problem in that, because decarburization and nitriding are carried out at the same time, the grain size of primary recrystallized sheets is reduced compared to the method of carrying out nitriding after decarburization. Accordingly, the temperature of initiation of secondary recrystallization in a final annealing process is lowered, and thus the likelihood that the secondary recrystallization of grains having other orientations, in addition to the {110}<001> orientation, will occur is increased, so that the {110}<001> sharpness of secondary recrystallized grains can be decreased, thus deteriorating the magnetic properties.

Specifically, in the case where decarburization and nitriding are carried out at the same time, carbon and nitrogen, which are interstitial elements, fundamentally interfere with the growth of primary recrystallized grains, and this reduction in the size of primary recrystallized grains influences the temperature of initiation of secondary recrystallization in the final annealing process. More specifically, when the size of primary recrystallized grains is reduced, the temperature of initiation of secondary recrystallization is lowered, so that grains having other orientations, in addition to grains having the {110}<001> orientation, are also subjected to secondary recrystallization, and thus the {110}<001> sharpness of secondary recrystallized grains can be decreased, thus deteriorating the magnetic properties.

Meanwhile, it is considered important to strictly control secondary recrystallization in order to produce grain-oriented electrical steel sheets having excellent magnetic properties. This secondary recrystallization behavior is most easily controlled by controlling the size of primary recrystallized grains, in which secondary recrystallization is completed at a temperature just lower than the temperature region in which precipitates of AlN and (Al,Si,Mn)N, which are inhibitors, rapidly start to become unstable. For this purpose, in a production process comprising simultaneous decarburization and nitriding, a method of growing primary recrystallized grains a little more, or a method of increasing the inhibitory ability required for secondary recrystallization, has been mainly used. Particularly, it has been suggested that an element such as B or Cu be added in order to increase the inhibitory ability required for secondary recrystallization. However, in the case where B is added, it is difficult to obtain a uniform and stable inhibiting force, because a very coarse compound of B and C tends to be formed. On the other hand, in the case where Cu is added, Copper sulfide is formed, but it is unevenly precipitated, so that the variation in iron loss and magnetic flux density is increased, thus reducing the quality of steel products.

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 in the prior art, and it is a main object of the present invention to provide a technology for producing grain-oriented electrical steel sheets having excellent magnetic properties through a low-temperature slab-reheating process, by controlling the content of nitrogen in a slab at a very low level, such that the grain size of primary recrystallized sheets can be increased, even though AlN, which acts as an inhibitor, can be completely dissolved in solution during slab reheating, controlling the content of sulfur in the slab at a very low level, such that MnS, which has an insignificant function as an inhibitor in secondary recrystallization, but influences the size of primary recrystallized grains, can be completely dissolved in solution, so that the size of primary recrystallized grains can be uniform and be increased, and subjecting the resulting steel sheets to simultaneous decarburization and nitriding.

Another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having excellent magnetic properties by performing the production process described in the main object, but adding a ferrite-forming element P to the slab while controlling the content of sulfur and nitrogen in the slab at a very low level, and carrying out simultaneous decarburization and nitriding.

Still another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having low iron loss by performing the production process described in the main object, but adding a ferrite-forming element Cr to the slab while controlling the content of sulfur and nitrogen in the slab at a very low level, and carrying out simultaneous decarburization and nitriding. Still another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having excellent magnetic properties by performing the production process described in the main object, but adding Sb to the slab while controlling the content of sulfur and nitrogen in the slab at a very low level, and carrying out simultaneous decarburization and nitriding.

Still another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having excellent magnetic properties by performing the production process described in the main object, but adding Sn to the slab while controlling the content of sulfur and nitrogen in the slab at a very low level, and carrying out simultaneous decarburization and nitriding.

Still another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having an improved magnetic property, a low iron loss and a high magnetic flux density by performing the production process described above, but adding a fine precipitate-forming element

Cu to the slab while controlling the content of sulfur and nitrogen in the slab at a very level, and carrying out simultaneous decarburization and nitriding.

Yet another object of the present invention is to provide a technology for producing grain-oriented electrical steel sheets having excellent magnetic properties by performing the production process described in the first object, but adding one or more of Sn and Sb to the slab so as to make the size and distribution of precipitates uniform while controlling the content of sulfur and nitrogen in the slab at a very low level, developing the {110}<001>-oriented texture of secondary recrystallized grains, and carrying out simultaneous decarburization and nitriding.

Technical Solution

To achieve the above objects, the present invention provides a method for producing a grain-oriented electrical steel sheet, the method comprising slab reheating, hot-rolling the reheated slab to produce a hot-rolled sheet, optionally hot-band annealing, cold-rolling the resulting sheet, subjecting the cold-rolled sheet to simultaneous decarburization and nitriding annealing, and then subjecting the annealed sheet to secondary recrystallization annealing, wherein a very small amount of N and S are added to the slab, such that AlN and MnS can be produced and completely dissolved in solution in the slab reheating step. The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si,

0.015-0.035% acid-soluble Al, less than 0.20% Mn, and the balance of Fe and other unavoidable impurities ("first electrical steel sheet").

The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.02-0.075% P, and the balance of Fe and other unavoidable impurities ("second electrical steel sheet").

The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.05-0.40% Cr, and the balance of Fe and other unavoidable impurities ("third electrical steel sheet").

The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.01-0.10% Sb, and the balance of Fe and other unavoidable impurities ("fourth electrical steel sheet").

The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.01-0.10% Sn, and the balance of Fe and other unavoidable impurities ("fifth electrical steel sheet"). The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si,

0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.01-0.15% Cu, and the balance of Fe and other unavoidable impurities ("sixth electrical steel sheet").

The grain-oriented electrical steel sheet may contain, in wt%, 2.0-7.0% Si, 0.005-0.040% acid-soluble Al, less than 0.20% Mn, less than 0.005% N, 0.02- 0.07% C, less than 0.005% S, 0.01-0.3% one or more of Sn and Sb, and the balance of Fe and other unavoidable impurities ("seventh electrical steel sheet").

With regard to all of the grain-oriented electrical steel sheets, the simultaneous decarburization and nitriding annealing is preferably carried out at a temperature of 800-950 ^°C in an atmosphere of a mixed gas of ammonia, hydrogen and nitrogen, and the slab reheating temperature is preferably 1100-

1200 ^°C .

The content of each of N and S in the slab is preferably less than 30 ppm in all cases except for the seventh electrical steel sheet, the mole fraction of a N- containing precipitate in a coil after hot rolling and winding is preferably less than 0.015%, and the mole fraction of an S-containing precipitate in the coil is preferably less than 0.007%. The temperature of initiation of secondary recrystallization annealing is preferably determined by controlling the simultaneous decarburization and annealing temperature to control the size of primary recrystallized grains, the size of said primary recrystallized grains is in the range of 18-32 μm, and the primary recrystallized grains preferably have a "standard deviation of grain size/ average grain size" greater than 1.2.

Advantageous Effects As described above, according to the inventive method for producing grain-oriented electrical steel sheets, recrystallized grains can be made uniform and large by controlling the contents of N and S at a very low level and reheating the slab above a temperature at which precipitates in the slab are completely dissolved in solution. Also, in the case where the content of N and S in the slab is low, there is an effect in which the size of initial grains before cold rolling becomes coarse, and thus the number of {110}<001>-oriented grains in primary recrystallized sheets can increase, so that the size of secondary recrystallized grains can decrease. Thus, in this case, grain-oriented electrical steel sheets having a high magnetic flux density, low iron loss and excellent magnetic properties can be produced.

Also, according to the inventive method for producing grain-oriented electrical steel sheets, it is possible to make recrystallized grains uniform and large by controlling the contents of N and S at very low levels while adding a suitable content of P, and heating the slab above a temperature at which inhibitors are completely dissolved in a solid solution. Moreover, in the case where the contents of N and S in the slab are low, the size of initial grains before cold rolling becomes coarse, and thus the number of {110}<001>-oriented grains in primary recrystallized sheets can increase, leading to a decrease in the size of secondary recrystallized grains. The addition of a suitable amount of P allows primary recrystallized grains to grow and changes the primary recrystallized texture, such that secondary recrystallization in the final annealing process can stably occur. Thus, in this case, low-temperature-reheated, grain-oriented electrical steel sheets having a low iron loss and a high magnetic flux density can be produced.

Furthermore, according to the inventive method for producing grain- oriented electrical steel sheets, it is possible to make recrystallized grains uniform and large by adding a suitable content of Cr while controlling the contents of N and S in the slab at very low levels, and heating the slab above the temperature at which precipitates in the slab are completely dissolved in solution.

Also, in the case where the contents of N and S in the slab are low, the size of initial grains before cold rolling becomes coarse, and thus the number of {110}<001>-oriented grains in primary recrystallized sheets can increase, leading to a decrease in the size of secondary recrystallized grains. The addition of a suitable content of Cr allows primary recrystallized grains to grow and increases the temperature of initiation of secondary recrystallization in the secondary recrystallization process. Accordingly, low-temperature-reheated, grain-oriented electrical steel sheets having a low iron loss can be produced.

Also, according to the inventive method for producing grain-oriented electrical steel sheets, it is possible to make recrystallized grains uniform and large by controlling the content of N and S in the slab at very low levels while adding a suitable amount of Sb to the slab, and heating the slab above a temperature at which precipitates in the slab are completely dissolved in solution. Moreover, in the case where the contents of N and S in the slab are low, there is an effect in which the size of initial grains before cold rolling becomes coarse, and thus the number of {110}<001>-oriented grains in primary recrystallized grains can increase, leading to a decrease in the size of secondary recrystallized grains.

A suitable content of Sb, which is a grain-boundary segregation element, interferes with the migration of grain boundaries, thus interfering with the growth of grains. Thus, Sb is not present in the form of a precipitate such as (Al,Si,Mn)N, while it effectively interferes with the growth of grains to assist in the secondary recrystallization of Goss grains. Accordingly, grain-oriented electrical steel sheets having a low iron loss, a high magnetic flux density and excellent magnetic properties can be produced.

Also, according to the inventive method for producing grain-oriented electrical steel sheets, it is possible to make recrystallized grains uniform and large by controlling the contents of N and S in the slab at very low levels while adding a suitable content of Sn to the slab, and heating the slab above a temperature at which precipitates, serving as inhibitors in the slab, are completely dissolved in solution. Also, the size of initial grains before cold rolling can become coarse, and the number of {110}<001>-oriented grains in primary recrystallized sheets can increase, leading to a decrease in the size of secondary recrystallized grains. Also, the size of secondary recrystallized grains can be reduced by increasing the number of secondary nuclei having the {110}<001> orientation, thus improving iron loss characteristics.

Moreover, when Sn, which plays an important role in inhibiting normal grain growth through grain boundary segregation, is added, it can compensate for a decrease in inhibitory strength, which is caused by the coarsening of AlN grains, leading to an increase in the content of Si. Accordingly, the secondary recrystallized texture having the {110}<001> orientation can be successfully formed even with a relatively high content of Si, so that grain-oriented electrical steel sheets having low iron loss and high magnetic flux density can be produced. Furthermore, according to the inventive method for producing grain- oriented electrical steel sheets, it is possible to make recrystallized grains uniform and large by controlling the contents of N and S very low levels and adding a suitable amount of Cu to the slab, even when the slab is heated above the temperature at which precipitates in the slab are completely dissolved in solution.

Moreover, according to the present invention, Cu, added in a suitable amount, bonds with S to form the precipitate Cu₂S, which inhibits the growth of recrystallized grains so as to inhibit grain growth in the primary recrystallization step, thus making the size of primary recrystallized grains uniform. Also, Cu contributes to the solid solution and fine precipitation of AlN by forming the austenite, like Mn. In addition, Cu acts as a nucleus for AlN precipitation, making the distribution of AlN uniform, so that it makes secondary recrystallized grains stable and better, thus increasing the integration of the Goss orientation. Accordingly, through the addition of Cu, a low-temperature-heated, grain-oriented steel sheet, having low iron loss and high magnetic flux density, can be produced. In addition, according to the inventive method for producing grain- oriented electrical steel sheets, one or more of Sn and Sb is added, so that the number of {110}<001>-oriented grains in primary recrystallized sheets increases, making the size of AlN and (Al₅Si₅Mn)N precipitates fine and uniform. Thus, the integration of {110}<001> grains after secondary recrystallization is greatly increased, and the size of secondary recrystallized grains is reduced. Accordingly, grain-oriented electrical steel sheets having a high magnetic flux density and a low iron loss can be produced.

Best Mode for Carrying Out the Invention

Below, the operation of the present invention will be described in detail.

Nitriding is carried out after decarburization or at the same time as decarburization, and nitrogen produced by the decomposition of ammonia gas is introduced into steel sheets to form nitrides. The method of carrying out nitriding following decarburization is performed at a temperature of 700-800 ⁰C , and the method of carrying out decarburization and nitriding at the same time is performed at a temperature of 800-950 ^°C in an atmosphere of ammonia + hydrogen + nitrogen. However, these two methods are based on metallurgically different technical concepts, rather than a mere difference between nitriding methods or annealing temperatures therebetween.

The method of forming precipitates through a separate nitriding process after decarburization is carried out at an annealing temperature of less than 800 ^°C, at which nitrides such as Si₃N₄ and (Si₅Mn)N are formed. Such precipitates are easily formed at low temperatures, but are thermally very unstable. Thus, such precipitates are readily decomposed at high temperatures, and thus cannot be used as inhibitors in grain-oriented electrical steel sheets. Also, because the diffusion of nitrogen is not sufficiently active due to the low annealing temperature, nitrides are formed locally at the surface part of steel sheets. Thus, these precipitates should be decomposed again in the final annealing process, which is a subsequent process, such that they can be re- precipitated with other elements present in steel sheets. The precipitates produced at this time are stable nitrides such as AlN or (Al₉Si)N, which can be inhibitors in grain-oriented electrical steel sheets. The method of forming precipitates through simultaneous decarburization and nitriding requires an annealing temperature of more than 800 ^"C . This temperature is set in consideration of the fact that temperatures of less than 800 ⁰C are not industrially useful, because the annealing time at that temperature is excessively long, and the fact that nitrides can be relatively stably produced by allowing the diffusion of nitrogen to actively occur, hi this temperature range, unstable precipitates such as Si₃N₄ or (Si₅Mn)N cannot be formed, and thermally very stable precipitates such as AlN, (Al₅Si₅Mn)N are formed. Thus, these precipitates can be used as inhibitors without the need to precipitate them again in a subsequent final annealing process.

However, when decarburization and nitriding are carried out simultanuously, carbon and nitrogen, which are interstitial elements, fundamentally interfere with the growth of primary recrystallized grains, and this reduction in the size of primary recrystallized grains influences the temperature of initiation of secondary recrystallization in the final annealing process. More specifically, when the size of primary recrystallized grains is reduced, the temperature of initiation of secondary recrystallization is lowered, so that grains having other orientations, in addition to grains having the {110}<001> orientation, can also be subjected to secondary recrystallization, and thus the {110}<001> sharpness of secondary recrystallization can be decreased, thus deteriorating the magnetic properties. Consequently, it is considered important to strictly control secondary recrystallization in order to produce grain-oriented electrical steel sheets having excellent magnetic properties. This secondary recrystallization behavior is most easily controlled by controlling the size of primary recrystallized grains, in which secondary recrystallization is completed at a temperature just lower than the temperature region in which precipitates of AlN and (Al₅Si₅Mn)N, which are inhibitors, start to become unstable rapidly. For this purpose, in a production process comprising simultaneous decarburization and nitriding, a method of growing primary recrystallized grains a little more without widening the distribution of grain sizes, or a method of increasing the inhibiting force required for secondary recrystallization, has been mainly used.

The present inventors have found that, as a method for preventing a decrease in the size of primary recrystallized grains in a process of carrying out simultaneous decarburization and nitriding, a method of controlling the contents of nitrogen (N) and sulfur (S) in the slab at low levels is very effective. The size of primary recrystallized grains is mainly determined by AlN and MnS precipitates remaining after hot rolling, and when the contents of N and S, which form the precipitates, are maintained at very low levels, the amount of the precipitates can be reduced. In this case, the present inventors have found that, when the contents of N and S in the slab are maintained at low levels and, at the same time, the slab contains P, the ferrite-forming element P allows primary recrystallized grains to grow to be large, and in addition, strongly develops the {111}<112> texture in primary recrystallized sheets, thus reducing the iron loss of final products and increasing the magnetic flux density. In addition, the present inventors have found that P segregates in grain boundaries up to a high temperature of about 1000 ^°C in secondary recrystallization annealing, thus increasing the {110}<001> sharpness of secondary grains in final products. Specifically, in the case where the slab contains P and, at the same time, has reduced contents of N and S, when the size of primary recrystallized grains is suitably controlled, electrical steel sheets having excellent magnetic properties can be produced.

In the above-described case, when the ferrite-forming element Cr is added while the contents of N and S in the slab are controlled at low levels, Cr increases the size of primary recrystallized grains, leading to an increase in secondary recrystallization temperature, which increases the integration of the {110}<001> texture, thus reducing iron loss. Also, the addition of Cr increases the number of {110}<001>-oriented grains in primary recrystallized sheets, and thus reduces the size of secondary recrystallized grains, leading to a decrease in iron loss. Specifically, in the case where Cr is added while the contents of N and S in the slab are reduced, when the size of primary recrystallized grains is suitably controlled, electrical steel sheets having excellent magnetic properties can be produced.

In the above-described case, when the slab contains Sb while the contents of N and S in the slab are maintained at low levels, Sb, which is a grain-boundary segregation element interfering with the growth of grains by interfering with the migration of grain boundaries, is not present in the form of precipitates such as (Al,Si,Mn)N, while it effectively interferes with the growth of grains to assist in the secondary recrystallization of Goss grains. In addition, Sb promotes the growth of Goss grains, so that an increased number of Goss grains participate in secondary recrystallization, thus improving magnetic flux density and iron loss. In the above-described case, when the slab contains Sn, while the contents of N and S in the slab are maintained at low levels, Sn can increase the number of secondary nuclei having the {110}<001> orientation so as to reduce the size of secondary recrystallized grains, thus improving iron loss characteristics. Also, Sn plays an important role in inhibiting normal grain growth through segregation in grain boundaries, and it can compensate for a reduction in inhibitory strength, caused by an increase in the size of AlN particles and an increase in Si content, and thus the successful formation of secondary recrystallized textures having the {ri0}<001> orientation can be ensured even with a relatively high content of Si. Specifically, the Si content can be increased without any reduction in the integration of secondary recrystallized textures having the {110}<001> orientation, and in addition, the final thickness can be reduced.

In the above-described case, when the slab contains Cu, while the contents of N and S in the slab are maintained at low levels, S-containing precipitates can be made uniform and fine, such that grain growth in the primary recrystallization step can be inhibited, making the size of primary recrystallized grains uniform. Thus, secondary recrystallized grains can be stably formed, so that the integration of Goss grains can be increased. Also, in the case where the slab contains Cu, while the contents of N and S in the slab are maintained at low levels, if the size of primary recrystallized grains is suitably controlled, electrical steel sheets having excellent magnetic properties can be produced.

In the above-described case, when grain-boundary segregation elements such as Sn and Sb are added to the slab, while the contents of N and S in the slab are maintained at low levels, grains having the {110}<001> orientation can be increased due to initial coarse grains before rolling, and AlN and (Al,Si,Mn)N, formed by N, introduced into steel through nitriding, are finely and uniformly distributed, secondary recrystallization stably occurs, thus greatly improving the magnetic properties.

In all the above-described cases, the size of primary recrystallized grains is maintained in the range of 18-32 μm. If the size of primary recrystallized grains is less than 18 μm, the grain growth driving force will be increased, so that the temperature of initiation of secondary recrystallization will be lowered, and the growth of grains having orientations other than the Goss orientation will occur, thus deteriorating the magnetic properties and iron loss characteristics of the resulting steel sheets. On the other hand, if the size of primary recrystallized grains is more than 32 μm, the grain growth deriving force will be reduced, and thus secondary recrystallization will not occur, thus deteriorating the magnetic properties.

Meanwhile, if the slab is heated to a temperature at which precipitates in the slab are partially dissolved in solution, there will be a great difference in the distribution of AlN precipitates, leading to a great variation in the size distribution of primary recrystallized grains, thus making the magnetic properties unstable. However, if the content of N and the content of S are very low, the amount of precipitates produced in the slab will be very small, even when the slab is heated to the complete solution temperature, and thus it is possible to obtain uniform and large primary recrystallized grains. Also, in the case where the contents of N and S in the slab are low, there is an effect in which the size of initial grains before cold rolling becomes coarse, and thus the number of {110}<001>-oriented grains in primary recrystallized sheets increases, leading to a decrease in the size of secondary recrystallized grains, thus improving the magnetic properties of final products. Herein, when the primary recrystallized sheet has a standard deviation of grain sizes devided by average grain size smaller than 1.2, grains having orientations other than the Goss orientation will become coarse, thus deteriorating the magnetic properties. For this reason, in order to prevent the magnetic properties from deteriorating, standard deviation of grain sizes devided by average grain size should be greater than 1.2.

Hereinafter, the reasons for the limitation of components in the present invention will be described in further detail.

Si is the basic element of electrical steel sheets, and serves to increase the resistivity of materials and to reduce the core loss of the materials. If the content of Si is less than 2.0%, resistivity will decrease, and the core loss characteristics will consequently be deteriorated, and if it is more than 7.0%, the brittleness of steel will be increased, in which case cold rolling becomes very difficult, and the formation of secondary recrystallized grains becomes unstable. For this reason, the content of Si is limited to 2.0-7.0%.

Al forms nitrides such as AlN, Al,Si,Mn)N, which act as inhibitors. If the content of Al is less than 0.005%, it cannot sufficiently provide the effect of the inhibitors, and if it is excessively high, the temperature required for the complete solution thereof will be increased, thus reducing hot rolling workability. For this reason, the content of Al is limited to 0.005-0.040%. Mn has the effect of increasing resistivity, to thus reduce core loss in the same manner as Si. Also, it reacts with nitrogen, introduced by nitriding treatment together with Si, to form a precipitate of (Al,Si,Mn)N to inhibit the growth of primary recrystallized grains, thus playing an important role in the development of secondary recrystallization. However, when it is added in an amount of more than 0.20%, it will promote austenite phase transformation during hot rolling to thus reduce the size of primary recrystallized grains, thus making secondary recrystallization unstable. Accordingly, the content of Mn is limited to less than 0.20%. hi the case where N is contained in an amount of more than 0.005% in a slab, when the slab is heated to the complete solution temperature, the size of primary recrystallized grains will be reduced, leading to a decrease in the temperature of initiation of secondary recrystallization. Thus, grains having orientations other than the {110}<001> orientation will also be subjected to secondary recrystallization, thus deteriorating the magnetic properties of steel. Meanwhile, if the content of N in steel is 0.005% or lower, even when the slab is heated to complete solution temperature, the amount of precipitates produced will be very low, so that it is possible to obtain uniform and large primary recrystallized grains, and thus a product having excellent magnetic properties can be obtained. Also, if the content of N in steel is 0.005% or lower, the effect of coarsening the size of initial grains before cold rolling will be obtained, and thus the number of grains having the {110}<001> orientation in the primary recrystallized sheet will be increased, so that the size of secondary recrystallized grains will be reduced, thus improving the magnetic properties of final products. Accordingly, the content of N is limited to less than 0.005%.

When C is added in an amount of more than 0.07%, it will promote the austenite phase transformation of steel, so that a hot-rolled texture will made fine during a hot rolling process, thus assisting in the formation of a uniform fine texture. However, when the content thereof is excessively large, coarse carbides will be deposited, and it will be difficult to remove carbon during decarburization.

Accordingly, the content of C is limited to 0.02-0.07%.

If S is contained in the slab in an amount of more than 0.005%, when the slab is heated to a complete solution temperature, the size of primary recrystallized grains will be reduced, leading to a decrease in the temperature of initiation of secondary recrystallization, so that grains having orientations other than the {110}<001> orientation will also be subjected to secondary recrystallization, thus deteriorating the magnetic properties of steel. On the other hand, if the content of S is 0.005% or lower, even when the slab is heated to complete solution temperature, the amount of precipitates produced will be very small, thus making it possible to obtain uniform and large primary recrystallized grains, so that products having excellent magnetic properties can be obtained. Also, if the content of S is 0.005% or lower, the effect of coarsening the size of initial grains before cold rolling will be obtained, and thus the number of grains having the {110}<001> orientation in primary recrystallized sheets will be increased, leading to a decrease in the size of secondary recrystallized grains, thus improving the magnetic properties of final products. Accordingly, the content of S is limited to less than 0.005%.

P is generally contained in low-temperature-heated, grain-oriented steel sheets in an amount of less than 0.02%. P, which is a ferrite-forming element, promotes the growth of primary recrystallized grains, so that it increases the temperature of secondary recrystallization, thus increasing the integration of {110}<001>-oriented grains in final products. Meanwhile, P not only reduces the iron loss of final products by increasing {110}<001>-oriented grains in primary recrystallized sheets, but also increases the integration of {110}<001>- oriented grains in final products by strongly developing the {111}<112> in the primary recrystallized sheets, thus increasing the magnetic flux density of the products. Also, P has the action of segregating in grain boundaries even at a high temperature of about 1000 °C during secondary recrystallization annealing, to thus retard the decomposition of precipitates so as to enhance the inhibitory ability of the precipitates. In order for this action of P to be sufficiently exhibited, P needs to be added in an amount of more than 0.02%. However, if P is added in an amount of more than 0.075%, the size of primary recrystallized grains will be reduced instead of being increased, so that secondary recrystallization will be unstable, and in addition, the brittleness of steel will be increased, thus deteriorating the cold rolling properties of steel. Accordingly, P is limited to 0.02%-0.075%.

Cr, which is a ferrite forming element, acts to grow primary recrystallized grains, so that it increases the secondary recrystallization temperature, and thus increases the integration of the Goss orientation in secondary recrystallization. Also, it has the effect of increasing the number of {110}<001>-oriented grains in primary recrystallized sheets. In order for this action of Cr to be effective, Cr needs to be added in an amount of more than 0.05%. However, if it is added in an excessively large amount, it will form a compact oxide layer at the surface portion of steel sheets during the simultaneous decarburization and nitriding annealing process, thus interfering not only with decarburization, but also with nitriding. Accordingly, the content of Cr is limited to 0.05-0.4%.

Sb is generally not contained in low-temperature-heated, grain-oriented electrical steel sheets.

If Sb is contained while the contents of N and S are controlled at low levels, Sb, which is a grain-boundary segregation element, interferes with the migration of grain boundaries. Thus, it is not present in the form of precipitates such as (Al,Si,Mn)N, and effectively interferes with the growth of primary recrystallized grains so as to assist in the secondary recrystallization of Goss grains. In addition, it is known that Sb promotes the growth of Goss grains, so that an increased number of Goss grains precipitate in secondary recrystallization, thus improving the magnetic flux density and iron loss characteristics of steel.

Sb, which is a grain-boundary segregation element, interferes with the migration of grain boundaries, so that it interferes with the growth of primary recrystallized grains so as to inhibit the excessive growth of the grains and make the size of primary recrystallized grain proper. If Sb is contained in an excessively large amount, the size of primary recrystallized grains becomes excessively small, and thus the integration of the Goss orientation in secondary recrystallization will be reduced, thus deteriorating the magnetic properties of steel sheets. Accordingly, the content of Sb is limited to 0.01-0.1%. If Sn is added to steel sheets, the number of {110}<001>-oriented secondary nuclei can be increased so as to reduce the size of secondary recrystallized grains, thus improving the iron loss characteristics of the steel sheets. Also, Sn plays an important role in inhibiting normal grain growth through grain boundary segregation, so that it compensates for a decrease in inhibitory strength, which is caused by the coarsening of AlN grains, leading to an increase in the content of Si. As a result, even with a relatively high content of Si, the successful formation of secondary textures having the {110}<001> orientation can be ensured. Specifically, without any reduction in the integration of secondary recrystallized structures having the {110}<001> orientation, not only an increase in the content of Si, but also a decrease in final thickness, can be achieved. If Sn is added in an amount of more than 0.1%, an insulation coating film will be worse, the size of primary recrystallized grains will be excessively decreased, and the integration of Goss orientation in secondary recrystallization will be reduced, thus deteriorating the magnetic properties of steel sheets. As a result, the content of Sn is limited to less than 0.1 %.

Cu, which is an austenite-forming element, like Mn, contributes to the solid solution and fine precipitation of AlN and stabilizes secondary recrystallization. Also, Cu bonds with S to form the precipitate Cu₂S, which inhibits the growth of recrystallized grains. In addition, it acts as a nucleus for AlN precipitation to thus make the distribution of AlN more uniform, thus improving secondary recrystallization. Cu in the slab composition of the present invention quickly bonds with S at a temperature lower than the temperature at which MnS is formed, thus forming Cu₂S. Accordingly, Cu has the effect of inhibiting the formation of MnS, having a high solid solution temperature, and prevents the central segregation of S. If it is added in a very small amount, the effect thereof will be insignificant, and thus it is preferably added in an amount of more than 0.01%. If Cu is added in an excessively large amount, it will adversely affect the formation of an insulation film in high-temperature annealing, and if it is added in an amount such that it is partially dissolved in solid solution, primary recrystallized grains will be non-uniform, and secondary recrystallized grains will consequently be unstable, and the orientation of grains will deviate from the <001> orientation, thus deteriorating the magnetic properties of steel sheets. Accordingly Cu is preferably added in an amount of 0.01-0.15%. For this reason, the content of Cu is limited to less than 0.15%. The seventh electrical steel sheet contains one or both of Sn and Sb in an amount of 0.01-0.3%. Herein, Sn is known as a grain growth inhibitor, because it is a grain-boundary segregation element that interferes with the migration of grain boundaries. Sn assists in the development of secondary recrystallization by promoting the production of Goss grains having the {110}<001> orientation, and thus it is an essential element in the present invention, which has the effect of inhibiting the growth of grains and allows Goss grains having a good orientation to be secondarily recrystallized, thus exhibiting high magnetic flux density. If Sn is added in an amount of less than 0.01 %, the effect of addition thereof will be low, and if it is added in an amount of more than 0.3%, severe grain boundary segregation will occur, so that the brittleness of steel sheets will be increased, leading to sheet breakage during rolling. For this reason, the content of Sn is limited to 0.01-0.3%.

Sb, which is a grain-boundary segregation element, like Sn, has the effect of inhibiting the growth of grains. It inhibits the formation of an oxide layer on the surface of a steel sheet during secondary recrystallization, so that it increases the adhesion of an oxide layer to the steel sheet, thus improving the iron loss characteristics of the steel sheet.

Herein, one or both of Sn and Sb is added, such that the grain growth inhibitory effect can be obtained, and an increased number of Goss grains having the {110}<001> orientation can be formed. If the content of one or both of Sn and Sb is less than 0.01%, the effect thereof will be insignificant, and if it is more than 0.3%, the cost of producing the steel sheet will be increased without any increase in the effect thereof. For this reason, the content of one or two of Sn and Sb is preferably 0.01-0.3%. Hereinafter, the process conditions of the present invention will be described.

The slab heating temperature before hot rolling is set to the temperature at which precipitates, serving as inhibitors, are completely dissolved in solid solution. When the heating temperature is the temperature at which the precipitates are partially dissolved in solid solution, there will be a great difference in the size of precipitates produced in casting and the size of precipitates redissolved in solid solution during the slab heating process. This can make the grain size of primary recrystallized sheets non-uniform, thus making the magnetic properties of the steel sheet non-uniform. For this reason, the slab heating temperature is set to a temperature range in which the precipitates are completely dissolved in solid solution.

The electrical steel sheet slab heated as described above is hot-rolled according to a conventional method. In the method that is currently generally used, the final thickness of a hot-rolled sheet is generally 2.0-3.5 mm. The hot- rolled sheet is annealed and then cold-rolled to a final thickness of 0.23-0.35 mm.

Although the hot-rolled sheet annealing can be performed according to various methods, a method that comprises heating the sheet to 1000-1200 ^°C, cracking the sheet at 800-950 "C and then cooling the sheet is used in the present invention.

The cold-rolled sheet is subjected to simultaneous decarburization and nitriding annealing in an atmosphere of a mixed gas of ammonia + hydrogen + nitrogen. The dew point of a mixed gas of hydrogen and nitrogen varies depending on the annealing temperature and the composition of the mixed gas, and is set such that decarburization ability is maximized. Also, the simultaneous decarburization and nitriding annealing is preferably carried out at a temperature of 800-950 ^°C .

If the annealing temperature is below 800 ^°C , a long time will be required for decarburization, and the size of primary recrystallized grains will also be reduced, so that stable secondary recrystallization in final annealing cannot be expected. If the annealing temperature is above 950 "C , it will be difficult to control the nitriding rate, and primary recrystallized grains will grow excessively or become non-uniform, so that it will be difficult to develop a stable secondary recrystallized texture. The time of concurrent decarburization and nitriding annealing is determined by the annealing temperature and the concentration of the added ammonia gas, and the annealing time is generally more than 30 seconds. In a conventional process for producing grain-oriented electrical steel sheets, grain-oriented electrical steel sheets having excellent magnetic properties are produced by applying an MgO-based annealing separator on steel sheets, and then subjecting the steel sheets to final annealing for a long time to induce secondary recrystallization, thus forming a {110}<001> texture in which the {110} plane of the steel sheet is parallel to the rolling plane and the <001> orientation is parallel to the rolling direction. The objects of the final annealing are to impart insulation properties through the formation of a glassy film by the reaction between an oxide layer formed in decarburization and MgO, to form the {110}<001> texture by secondary recrystallization, and to remove impurities, which reduce magnetic properties. In the conventional final annealing method, in a temperature increase zone before secondary recrystallization occurs, the steel sheets are maintained in a mixed gas of nitrogen and hydrogen to protect nitrides, serving as grain growth inhibitors, such that secondary recrystallized grains can develop well. After the completion of secondary recrystallization, the steel sheet is maintained in an atmosphere of 100% hydrogen for a long time to thus remove impurities from the steel sheet. In the method of carrying out nitriding following decarburization, the transformation of precipitates occurs in the final annealing process. In this method, the nitriding temperature is 700-800 ^°C, and after nitriding, Si₃N₄ and (Si,Mn)N are produced on the surface portion of the steel sheet. These precipitates should be re-precipitated as thermally stable nitrides, such as AlN or (Al₅Si)N, in the final annealing process, such that they can be used as inhibitors in grain-oriented electrical steel sheets. On the contrary, nitrides produced in simultaneous decarburization and nitriding annealing are AlN and (Al,Si,Mn)N, which can be used directly as inhibitors without the need to transform them in the final annealing process. The reason why the kinds of nitrides produced differ between the nitriding methods is because the annealing temperatures are different. That is, at temperatures above 800 ^°C, Si₃N₄ or (Si,Mn)N cannot stably exist, and the diffusion of nitrogen also occurs very rapidly.

Mode for Carrying Out the Invention

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

In a slab for grain-oriented electrical steel sheets, which contains, in wt%, 3.18% Si, 0.056% C, 0.062% Mn, 0.0029% S, 0.0020% N, 0.026% soluble Al and the balance of Fe and other unavoidable impurities, the temperature at which AlN is completely dissolved in solution was 1164 ^°C, and the temperature at which MnS is completely dissolved in solution was 1133 ^°C. The slab was heated for 210 minutes at each of 1130 ^°C (at which both AlN and MnS were partially dissolved in solution), 1150 "C (at which MnS was completely dissolved in solution, and AlN was partially dissolved in solution), and 1175 ^°C and 1190 ⁰C, which are higher than 1164 ^°C, at which both AlN and MnS are completely dissolved in solution, for 210 minutes. Then, the heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 "C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The content of nitrogen in the nitrided steel sheets was maintained in the range from 170 ppm to 200 ppm. The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 1. [Table 1 ]

As can be seen in Table 1 above, the inventive materials, the slab heating temperature of which was higher than the temperature at which AlN and MnS were completely dissolved in solution, had higher magnetic flux density and lower iron loss than did the comparative materials, the slab heating temperature of which was the temperature at which both AlN and MnS or only AlN was partially dissolved in solution. [Example 2]

Slabs for grain-oriented electrical steel sheets, which contain, in wt%, 3.21% Si, 0.056% C, 0.055% Mn, 0.0029% S, 0.024% soluble Al, N in the varying amounts shown in Table 2 below, and the balance of Fe and unavoidable impurities, were heated for 210 minutes at a temperature higher than the temperature at which the precipitates are completely dissolved in solution (the temperature at which MnS was completely dissolved in solution: 1125 ^°C ; the temperature at which AlN was completely dissolved in solution: see Table 2). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 2. [Table 2]

As can be seen in Table 2 above, when the slabs were heated to the temperature at which the precipitates were completely dissolved in solution, the inventive materials having an N content of less than 0.0050% had lower magnetic flux density and lower iron loss than those of the inventive materials. [Example 3] Slabs for grain-oriented electrical steel sheets, which contain, in wt%, 3.27% Si, 0.045% C, 0.064% Mn, 0.0015% N, 0.024% soluble Al, and S in the varying amounts shown in Table 3 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at temperatures higher than 1132 ^°C, at which AlN and MnS were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 "C , maintained at 900 °C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 °C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state, hi the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 "C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 3. [Table 3]

As can be seen in Table 3 above, when the slabs were heated to the temperatures at which the precipitates were completely dissolved in solution, the inventive materials having an S content of less than 0.0100% had high magnetic flux density and low iron loss. [Example 4]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.25% Si, 0.057% C, 0.06% Mn, 0.0029% S, 0.0018% N, 0.024% soluble Al and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at temperature 1160 "C which is higher than 1147^°C, at which precipitates were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C , maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^"C, and 1% dry ammonia gas, into a furnace, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The content of N in the nitrided steel sheets was maintained at 180-230 ppm. Also, the simultaneous decarburization and nitriding was carried out at varying temperatures of 780, 810, 865, 940 and 980 ^"C, and the decarburization and nitriding time was changed in the range of 120-240 seconds in order to control the nitriding amount.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 "C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 4. [Table 4]

As can be seen in Table 4 above, the comparative materials, the annealing of which was performed at temperatures lower than 800 ^°C or higher than 950 ^°C , did not show good magnetic properties, even when they were treated so as to have a suitable nitriding amount by controlling the annealing conditions during the simultaneous decarburization and nitriding.

[Example 5]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.27% Si, 0.045% C, 0.074% Mn, 0.024% soluble Al, N and S in the varying amounts shown in Table 5 below, and the balance of Fe and other unavoidable impurities, were heated to 1180 ^°C for 210 minutes. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C, maintained at 900 "C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 °C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. An extraction test was performed in such heat treatment conditions so as to examine the temperature of initiation of secondary recrystallization. The temperatures of initiation of secondary recrystallization, measured for various process conditions, are shown in Table 5. [Table 5]

As can be seen in Table 5 above, when the slabs were heated to the temperatures at which the precipitates were completely dissolved in solution, the grain size was increased to 28.44 μm, but the inventive materials had higher magnetic flux density and lower iron loss than did the comparative materials. Also, the secondary recrystallization temperatures of the inventive materials were much higher than those of the comparative material 3, and thus the Goss integration in the inventive materials was increased, leading to the improvement in the magnetic properties.

[Example 6] Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.23% Si, 0.048% C, 0.071% Mn, 0.024% soluble Al, N and S in the varying amounts shown in Table 6, and the balance of Fe and other unavoidable impurities, were heated at 1180 ^°C for 210 minutes. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C , maintained at 900 ⁰C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C , and 1 % dry ammonia gas, into a furnace at 830-880 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state, hi the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 "C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. An extraction test was performed in such heat treatment conditions so as to examine the temperature of initiation of secondary recrystallization. The average size of primary recrystallized grains, the standard deviation of grain size devided by average grain size, and magnetic properties, measured for various process conditions, are shown in Table 6. [Table 6]

In Table 6 above, the standard derivation of grain size distribution was shown in order to show the uniformity of grain size. A decrease in standard deviation indicates an increase in uniformity. As shown in Table 6, in the average size and standard deviation of grains according to process conditions, the inventive materials had large grain size and small standard deviation, compared to those of the comparative materials. The comparative materials 4-9 showed an increase in standard deviation, and thus an increase in non-uniformity, according to an increase in the average size of grains, when the average grain size was controlled only by the decarburization annealing temperature in similar component conditions. As described above, it can be seen that an increase in the grain size had advantageous effects in terms of magnetic properties, but an excessive increase in the grain size undesirably led to the non-uniformity of the grain size. However, when the contents of N and S were reduced to increase the grain size, the increase in standard deviation was not significant. The "standard deviation of grain size/average grain size" is conveniently used as a parameter for expressing this correlation, because this value is increased when the grain size is large and the standard deviation is low. As shown in Table 6, above, the magnetic properties were excellent when the grain size was more than 20 μm and the standard deviation of grain sizes devided by average grain size was greater than 1.2. [Example 7]

In slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.25% Si, 0.056% C, 0.062% Mn, 0.0028% S, 0.0020% N, 0.026% soluble Al, P in varying amounts of 0.006%, 0.015%, 0.025%, 0.037%, 0.052% and 0.083%, and the balance of Fe and other unavoidable impurities, the temperature at which AlN is completely dissolved in solution was 1164 ⁰C, and the temperature at which MnS is completely dissolved in solution was 1131 "C . The slabs were heated for 210 minutes at 1170 ^°C, at which both AlN and MnS were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^"C , maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm.

(A) The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 ^°C , and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

(B) The cold-rolled sheets were subjected to decarburization annealing for 150 seconds in an atmosphere of a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C , in a furnace maintained at 845 ^°C , and then the sheets were subjected to nitriding treatment by the addition of dry ammonia gas for 30 seconds in the furnace at 770 ^°C .

The content of nitrogen in the steel sheets, nitrided according to each of the methods (A) and (B), was 190-210 ppm. The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 7. [Table 7]

As shown in Table 7 above, in the method (A), in which the simultaneous decarburization and nitriding was carried out, the inventive materials having a P content of 0.02-0.075%, falling in the range of the present invention, had higher magnetic flux density and lower iron loss than did the comparative materials. Meanwhile, in the cases where P was contained, the method (A) of carrying out the simultaneous decarburization and nitriding showed excellent magnetic properties, compared to the method (B) of carrying out nitriding following decarburization.

[Example 8]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.27% Si₅ 0.045% C, 0.074% Mn, 0.0015% N, 0.024% soluble Al, 0.0020% S, and P in the varying amounts shown in Table 8 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1150 ^°C, which was higher than the temperature at which AlN was completely dissolved in solution (1132 "C) and the temperature at which MnS was completely dissolved in solution (1122 "C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 °C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to thicknesses of 0.23 mm and 0.27 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 "C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 8. [Table 8]

As shown in Table 8, regardless of the thickness of the products, the inventive materials 4 and 5, having a P content falling in the range of the present invention, had excellent magnetic properties compared to the comparative materials 10 and 11.

[Example 9] In slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.16% Si, 0.059% C, 0.062% Mn, 0.0028% S, 0.0020% N, 0.026% soluble Al, Cr in varying amounts of 0.03%, 0.15%, 0.27%, 0.36% and 0.55%, and the balance of Fe and other unavoidable impurities, the temperature at which AlN is completely dissolved in solution was 1164 "C , and the temperature at which MnS is completely dissolved in solution was 1075 °C . The slabs were heated for 210 minutes at 1122 ⁰C, at which both AlN and MnS were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm.

(A) The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 °C , and 1% dry ammonia gas, into a furnace at 875 ⁰C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The content of nitrogen in the steel sheets nitrided according to each of the methods (A) and (B) was 190-210 ppm.

The steel sheets were applied with the annealing separator MgO and finally annealed in a coil state, hi the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 9. [Table 9]

As can be seen in Table 9, in the method (A), in which the simultaneous decarburization and nitriding was carried out, the inventive materials having a Cr content of 0.05-0.40%, falling in the range of the present invention, had higher magnetic flux density and lower iron loss than did the comparative materials. Meanwhile, in the cases where Cr was contained in the same amount, the method (A) of carrying out the simultaneous decarburization and nitriding showed excellent magnetic properties compared to the method (B) of carrying out nitriding following decarburization. [Example 10]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.35% Si, 0.054% C, 0.074% Mn, 0.0015% N, 0.024% soluble Al, 0.0020% S, and Cr in the varying amounts shown in Table 10 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1150 ^°C, which was higher than the temperature at which AlN was completely dissolved in solution (1132 ^°C) and the temperature at which MnS was also completely dissolved in solution (1122 "C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to 1100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to thicknesses of 0.23 mm and 0.27 mm. The cold- rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 10. [Table 10]

As shown in Table 10 above, regardless of the thickness of the products, the inventive materials 4-7, having a Cr content falling in the range of the present invention, had excellent magnetic properties compared to the comparative materials having the same thickness.

[Example 11]

In slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.25% Si, 0.056% C, 0.062% Mn, 0.0028% S, 0.0020% N, 0.026% soluble Al, Sb in varying amounts of 0.000%, 0.006%, 0.017%, 0.025%, 0.050%, 0.092% and 0.12%, and the balance of Fe and other unavoidable impurities, the temperature at which AlN is completely dissolved in solution was 1164 ^°C, and the temperature at which MnS is completely dissolved in solution was 1131 °C . The slabs were heated for 210 minutes at 1170 ^°C at which both AlN and MnS were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm.

(A) The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 "C , and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

(B) The cold-rolled sheets were subjected to decarburization annealing for 150 seconds in an atmosphere of a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C , in a furnace maintained at 845 ^°C , and then the sheets were subjected to nitriding treatment by the addition of dry ammonia gas for 30 seconds in the furnace at 770 "C .

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 11.

[Table 11]

As can be seen in Table 11, in the method (A) in which the simultaneous decarburization and nitriding was carried out, the inventive materials having an Sb content of 0.01-0.10% falling in the range of the present invention had higher magnetic flux density and lower iron loss than did the comparative materials. Meanwhile, in the cases where Sb was contained in the same amount, the method (A) of carrying out the simultaneous decarburization and nitriding resulted in excellent magnetic properties, compared to the method (B) of carrying out nitriding following decarburization.

[Example 12]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.27% Si, 0.045% C, 0.074% Mn, 0.0015% N, 0.024% soluble Al, 0.0020% S, Sb in the varying amounts shown in Table 2 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1150 "C, which was higher than the temperature at which AlN was completely dissolved in solution (1132 ^"C) and the temperature at which MnS was completely dissolved in solution (1122 "C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to 1100 ^°C , maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold- rolled to thicknesses of 0.23 mm and 0.27 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 "C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 °C, and after a temperature of 1200 °C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 12. [Table 12]

Sheet thickness Sb content Magnetic flux Iron loss (Wi_7/5o, Remark (wt%) density (B₁₀, W/kg) Tesla)

0.27 mm 0.000 1.91 0.98 Comparative material 4

0.029 1.93 0.93 Inventive material 5

0.053 1.93 0.91 Inventive material 6

0.120 1.89 0.99 Comparative material 4

0.23 mm 0.000 1.91 0.90 Comparative material 6

0.029 1.94 0.83 Inventive material 7

0.053 1.94 0.82 Inventive material 8

0.120 1.90 0.92 Comparative material 7

As shown in Table 12 above, regardless of the thickness of the products, the inventive materials 5 and 6, having an Sb content falling in the range of the present invention, had excellent magnetic properties compared to the comparative materials 4 and 5. [Example 13]

Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.25% Si, 0.056% C, 0.062% Mn, 0.0028% S, 0.0019% N, 0.026% soluble Al, Sn in varying amounts of 0.000%, 0.015%, 0.037%, 0.055%, 0.075% and 0.122%, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1170 ^°C, which was higher than the temperature at which AlN was completely dissolved in solution (1159 ^°C) and the temperature at which MnS was also completely dissolved in solution (1131 ^°C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1 100 ^°C, maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm.

The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 880 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The content of nitrogen in the steel sheets nitrided according to the above method was in the range from 190 ppm to 210 ppm.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 "C, and after a temperature of 1200 ^"C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 13. [Table 13]

As shown in Table 13 above, in the case where the steel sheets were subjected to the simultaneous decarburization and nitriding, the inventive materials 1-4 having an Sn content of 0.01-0.10%, falling in the range of the present invention, had higher magnetic flux density and lower iron loss than did the comparative materials 1 and 2.

[Example 14] Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.27%

Si, 0.045% C, 0.074% Mn, 0.0016% N, 0.024% soluble Al, 0.0020% S, Sn in the varying amounts shown in Table 14 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1150 "C, which was higher than the temperature at which AlN was completely dissolved in solution (1137 "C) and the temperature at which MnS was completely dissolved in solution (1122 "C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to 1100 ^°C , maintained at 900 °C for 90 seconds, quenched in water, washed with acid, and then cold- rolled to thicknesses of 0.23 mm and 0.27 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 °C, and 1% dry ammonia gas, into a furnace at 875 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^"C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 14. [Table 14]

As shown in Table 14 above, regardless of the thickness of the products, the inventive materials 5-8, having an Sn content falling in the range of the present invention, had excellent magnetic properties compared to those of the comparative materials 3 and 4. [Example 15]

In slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.20% Si, 0.058% C, 0.063% Mn, 0.0028% S, 0.0021% N, 0.026% soluble Al, Cu in varying amounts of 0.006%, 0.016%, 0.035%, 0.054%, 0.087%, 0.115%, 0.170% and 0.238%, and the balance of Fe and other unavoidable impurities, the temperature at which AlN is completely dissolved in solution is 1168 ^°C, and the temperature at which MnS is completely dissolved in solution is 1132 ^°C . The slabs were heated for 210 minutes at 1190 ^°C, at which both AlN and MnS were completely dissolved in solution. The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.3 mm. The hot-rolled sheets were heated to a temperature higher than 1100 "C , maintained at 900 ^°C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to a thickness of 0.30 mm.

The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C , and 1 % dry ammonia gas, into a furnace at 860 ^°C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The content of nitrogen in the steel sheets nitrided according to this method was maintained in the range from 190 ppm and 210 ppm.

The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state. In the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 °C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 15.

[Table 15]

As shown in Table 15 above, it can be seen that the inventive materials having a Cu content of 0.01-0.15%, falling in the range of the present invention, had higher magnetic flux density and lower iron loss than did the comparative materials. When the content of Cu was very low, the effect of improving magnetic properties could not be shown, and when Cu was added in an excessively large amount, it was not completely dissolved in solid solution, so that the grain size of the resulting steel sheets was non-uniform, thus deteriorating the magnetic properties of the steel sheets. [Example 16] Slabs for grain-oriented electrical steel sheets, containing, in wt%, 3.27%

Si, 0.052% C, 0.069% Mn, 0.0015% N, 0.025% soluble Al, 0.0024% S, Cu in the varying amounts shown in Table 16 below, and the balance of Fe and other unavoidable impurities, were heated for 210 minutes at 1160 ^°C, which was higher than the temperature at which AlN was completely dissolved in solution (1 135 ^°C) and also higher than the temperature at which MnS was completely dissolved in solution (1131 "C). The heated slabs were hot-rolled to produce hot-rolled sheets having a thickness of 2.2 mm. The hot-rolled sheets were heated to 1100 ^°C, maintained at 900 °C for 90 seconds, quenched in water, washed with acid, and then cold-rolled to thicknesses of 0.23 mm and 0.27 mm. The cold-rolled sheets were subjected to simultaneous decarburization and nitriding by simultaneously introducing a mixed gas of 75% hydrogen and 25% nitrogen, having a dew point of 65 ^°C, and 1% dry ammonia gas, into a furnace at 875 °C, and maintaining the cold-rolled sheets in the atmosphere gas for 180 seconds. The steel sheets were applied with an annealing separator MgO and finally annealed in a coil state, hi the final annealing, the steel sheets were maintained in an atmosphere of a mixed gas of 25% nitrogen + 75% hydrogen up to 1200 ^°C, and after a temperature of 1200 ^°C was reached, the steel sheets were maintained in an atmosphere of 100% hydrogen for more than 10 hours, and then cooled in the furnace. The magnetic properties of the steel sheets, measured for various process conditions, are shown in Table 16. [Table 16]

As can be seen in Table 16 above, regardless of the thickness of the products, the inventive materials having a Cu content falling in the range of the present invention had better magnetic properties than did those of the inventive materials.

[Example 17]

To a base material containing, in wt%, 3.5% Si, 0.040% C, 0.08% Mn, 0.003% S, 0.024% Al and 0.002% N, one or more of Sn and Sb were added and dissolved in a vacuum, thus producing ingots. The ingots were heated to 1200 °C, and then hot-rolled to produce hot-rolled sheets having a thickness of 2.0 mm. The hot-rolled sheets were heat-treated at 900 °C , quenched, washed with acid, and then cold-rolled one time to realize a final thickness of 0.30 mm.

The cold-rolled steel sheets were subjected to decarburization annealing at 850 °C in a wet atmosphere, and, at the same time, subjected to nitriding by introducing nitrogen ions, decomposed from ammonia gas, into the steel sheets, to produce AlN and (Al,Si,Mn)N precipitates. Then, the steel sheets were applied with an annealing separator, and then finally annealed at a high temperature in an atmosphere of a mixed gas of 10% nitrogen + 90% hydrogen. In the high-temperature annealing, the steel sheets were heated at a heating rate of 15 ^°C/hr to 1200 ^°C, and then subjected to final high-temperature annealing for more than 10 hours, such that complete secondary recrystallization occurred. The changes in magnetic flux density with the changes in the contents of Sn and Sb, added in the steel making process, are shown in Table 17 below. [Table 17]

As can be seen in Table 17, the inventive materials 14-19, in which the content of one or two of Sn and Sb was in the range of 0.01-0.3%, showed a magnetic flux density higher than 1.90T and secured a low iron loss of 0.955- 0.993 W/kg. hi contrast, the comparative materials 17 and 22, in which the content of one or two of Sn and Sb was less than 0.01%, secured a magnetic flux density of only about 1.85T, because the effect of the addition of Sn and/or Sb was not apparent. Also, the comparative materials 18-21, in which the content of one or two of Sn and Sb was more than 0.3%, did not secure a magnetic flux density of more than 1.90 T.

Claims

Claims:

1. A method for producing a grain-oriented electrical steel sheet, the method comprising reheating a slab for the grain-oriented electrical steel sheet, hot-rolling the reheated slab to produce a hot-rolled sheet, optionally annealing the hot-rolled sheet, cold-rolling the resulting sheet, subjecting the cold-rolled sheet to simultaneous decarburization and nitriding annealing, and then subjecting the annealed sheet to secondary recrystallization annealing, wherein a very small amount of N and S are added to the slab, such that AlN and MnS can be produced and completely dissolved in solution in the slab reheating.

2. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, and a balance of Fe and other unavoidable impurities.

3. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.02-0.075% P, and a balance of Fe and other unavoidable impurities.

4. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.05-0.40% Cr, and a balance of Fe and other unavoidable impurities.

5. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20%

Mn, 0.01-0.10% Sb, and a balance of Fe and other unavoidable impurities.

6. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.01-0.10% Sn, and a balance of Fe and other unavoidable impurities.

7. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.015-0.035% acid-soluble Al, less than 0.20% Mn, 0.01-0.15% Cu, and a balance of Fe and other unavoidable impurities.

8. The method of Claim 1, wherein the grain-oriented electrical steel sheet contains, in wt%, 2.0-7.0% Si, 0.005-0.040% acid-soluble Al, less than 0.20% Mn, less than 0.005% N, 0.02-0.07% C, less than 0.005% S, 0.01-0.3% one or more of Sn and Sb, and a balance of Fe and other unavoidable impurities.

9. The method of any one of Claims 1 to 8, wherein the simultaneous decarburization and nitriding annealing is carried out at a temperature of 800- 950 °C in an atmosphere of a mixed gas of ammonia, hydrogen and nitrogen.

10. The method of any one of Claims 1 to 8, wherein the slab reheating temperature is 1100-1200 "C.

11. The method of any one of Claims 1 to 7, wherein the content of each of N and S in the slab is less than 30 ppm.

12. The method of any one of Claims 1 to 8, wherein the temperature of initiation of secondary recrystallization annealing is determined by controlling the decarburization annealing temperature to control the size of primary recrystallized grains.

13. The method of Claim 12, wherein the size of said primary recrystallized grains is in a range of 18-32 μm, and the primary recrystallized grains have a standard deviation of grains size divided by average grain size greater than 1.2.