Patent Description:
Grain-oriented electrical steel sheets are soft magnetic material used as an iron core material for transformers, electric generators and the like. Having a crystal structure where <<NUM>> orientation being a magnetization easy axis of iron is highly aligned in a rolling direction of a steel sheet, such a grain-oriented steel sheet is characterized by being excellent in magnetic properties. The crystal structure is formed, in a finish annealing of the production process of the grain-oriented electrical steel sheet, by using grain boundary energy to preferentially cause secondary recrystallization of crystal grains of {<NUM>}<<NUM>> orientation, so-called Goss orientation and achieve enormous growth thereof.

Common methods for causing the secondary recrystallization generally include a technique of utilizing precipitates called as an inhibitor. For example, Patent Literature <NUM> discloses a method of utilizing AlN or MnS as the inhibitor, and Patent Literature <NUM> discloses a method of utilizing MnS or MnSe as the inhibitor, both of which are industrially put into practice.

In the techniques of utilizing the inhibitor, a method of improving a texture by increasing a cooling rate in a hot-band annealing and an intermediate annealing to control precipitation of carbide is proposed as a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties. For example, Patent Literature <NUM> proposes increasing a cooling rate in a hot-band annealing to increase C in a solid-solution state in cold rolling. In this technique, however, the cooling rate in an example is <NUM>/s at maximum, and rapid cooling of not less than <NUM>/s is not conducted. It is assumed due to the fact that the cooling rate of less than <NUM>/s has been considered sufficient for the control of the carbide and there has been no cooling device capable of attaining a cooling rate of more than the above value.

Whereas, developments of the cooling technique for thin steel sheets have been advanced in recent years. For example, Patent Literature <NUM> discloses a quench-hardening device capable of suppressing a slowdown of a cooling rate for a metal plate while preventing shape failure generated in the metal plate during the quench-hardening in a continuous annealing installation for continuously threading the metal plate to conduct the annealing. The quench-hardening device aims to provide a high-strength steel sheet having a desired strength by performing rapid cooling to control the structure. However, the rapid cooling has not been applied to grain-oriented electrical steel sheets, where high strength is not required.

<CIT>
discloses a method for producing a grain-oriented electrical steel sheet, the method comprising the steps of: heating a steel slab comprising C: <NUM> to <NUM> mass%, Si: <NUM> to <NUM> mass%, Mn: <NUM> to <NUM> mass%, Al: <NUM> to <NUM> mass%, N: <NUM> to <NUM> mass%, S and Se: <NUM> to <NUM> mass%, the remainder being Fe and inevitable impurities to a temperature between <NUM> and <NUM>; hot rolling the slab to form a hot-rolled sheet; subjecting the hot-rolled sheet to a hot-band annealing followed by a single cold rolling to form a cold-rolled steel sheet, alternatively two or more cold rollings having an intermediate annealing between each cold rolling, to form a cold-rolled steel sheet; subjecting the cold-rolled sheet to a decarburization annealing which is typically combined with a primary recrystallization annealing; applying an annealing separator onto a surface of the steel sheet; and subjecting the steel sheet to a finish annealing, wherein the cooling process is conducted from a maximum achieving temperature of <NUM> or higher at a cooling rate of <NUM>-<NUM>/s from <NUM> to <NUM> and a cooling rate of <NUM>-<NUM>/s from <NUM> to <NUM>.

It is, therefore, an object of the invention to propose a method for producing a grain-oriented electrical steel sheet having very excellent magnetic properties comprising.

The inventors have made various studies on an influence of a cooling rate in a hot-band annealing or the like upon magnetic properties of a grain-oriented electrical steel sheet, in a production method of the grain-oriented electrical steel sheet using a raw material containing an inhibitor-forming ingredient. As a result, it has been found out that, by increasing the cooling rate in the hot-band annealing, intermediate annealing and the like before cold rolling as compared to conventional ones, concretely increasing the cooling rate from <NUM> to <NUM> to not less than <NUM>/s, slip system of dislocation in the cold rolling is changed to improve primary recrystallization texture, whereby the magnetic properties are largely improved, and thus the invention has been accomplished.

That is, the method of the present invention is defined in the claims.

According to the invention, a grain-oriented electrical steel sheet having excellent magnetic properties can be produced stably by using a raw material containing an inhibitor-forming ingredient and using a high-speed cooling effect different from the conventional methods of increasing the solid-soluted C or finely dividing carbide, which has a significant effect on industry.

Explanation will be made to experiments leading to the invention.

A steel slab containing C: <NUM> mass%, Si: <NUM> mass%, Mn: <NUM> mass%, sol. Al: <NUM> mass%, N: <NUM> mass%, S: <NUM> mass% and Se: <NUM> mass% is produced by a continuous casting method, reheated to a temperature of <NUM> and hot rolled to form a hot-rolled sheet having a sheet thickness of <NUM>. Then, the hot-rolled sheet is subjected to a hot-band annealing with a maximum achieving temperature (soaking temperature) of <NUM>. In this case, a cooling process in the hot-band annealing from <NUM> to room temperature is divided into three zones of <NUM> to <NUM>, <NUM> to <NUM> and <NUM> to <NUM>, and cooling is performed by changing an average cooling rate of each zone, as shown in Table <NUM>. Then, the steel sheet is subjected to the first cold rolling to have a middle sheet thickness of <NUM>, an intermediate annealing at <NUM> and the second cold rolling to form a cold-rolled sheet having a final sheet thickness of <NUM>. The cold-rolled sheet is thereafter subjected to a primary recrystallization annealing combined with a decarburization annealing in a wet atmosphere of <NUM> vol% H<NUM> - <NUM> vol% N<NUM> with a dew point of <NUM> at a soaking temperature of <NUM> for a soaking time of <NUM> seconds. Then, the steel sheet is coated on its a surface with an annealing separator composed mainly of MgO and subjected to a finish annealing of heating (no temperature holding) between <NUM> and <NUM> at a heating rate of <NUM>/hr to develop a secondary recrystallization, heating to <NUM> at a heating rate of <NUM>/hr between <NUM> and <NUM> to complete the secondary recrystallization and then performing a purification treatment of holding at such a temperature in a hydrogen atmosphere for <NUM> hours.

A sample is taken out from the thus-obtained steel sheet after the finish annealing to measure a magnetic flux density B<NUM> (magnetic flux density in excitation at <NUM> A/m) by a method described in JIS C2550, and the result is also shown in Table <NUM>. As seen from the result, the magnetic flux density is largely increased by conducting rapid cooling at an average cooling rate from <NUM> to <NUM> of not less than <NUM>/s in the cooling process of the hot-band annealing.

Although the mechanism of the increase in the magnetic flux density caused by increasing the average cooling rate from <NUM> to <NUM> to not less than <NUM>/s in the cooling process of the hot-band annealing as mentioned above when using raw materials containing an inhibitor-forming ingredient has not been clear yet, the inventors consider it as follows.

The temperature zone from <NUM> to <NUM> in the cooling process of the hot-band annealing has a large influence on the precipitation state of carbide, and thus cooling has been conducted at about <NUM>/s in the temperature zone for the purpose of increasing solid-soluted C or increasing fine carbide. However, the above mechanism of improving the magnetic properties is considered not to be due to the increase of the solid-soluted C or fine carbide.

The steel sheet having been subjected to the hot-band annealing is before the process of decarburization annealing (primary recrystallization annealing) and has a high C content, and thus part of the steel sheet causes reversible transformation due to heating in the annealing and is changed from α-phase to γ-phase. The γ-phase after the transformation is different from the surrounding α-phase in crystal structure (γ-phase is FCC and α-phase is BCC) as well as thermal expansion coefficient. When rapid cooling is performed from such a state at not less than <NUM>/s, the γ-phase is shrunk to remain due to the supercooling without transforming into α-phase. Therefore, unusual strain is caused in a phase interface between γ-phase and α-phase due to the difference in thermal expansion coefficient. As a result, the slip system of dislocation in the subsequent cold rolling process is changed to increase {<NUM>} orientation grains of the steel sheet after the primary recrystallization annealing (decarburization annealing) and improve the texture, which is considered to improve the magnetic properties. Moreover, it is considered that strain is caused in the phase interface even at a cooling rate of not more than <NUM>/s, but the above effect cannot be obtained sufficiently because the strain is easily eliminated due to the slow cooling rate.

On the other hand, further improvement in magnetic properties is recognized by conducting cooling from <NUM> to <NUM> subsequent to the above rapid cooling at an average cooling rate within <NUM> to <NUM>/s. This is considered due to the fact that martensite transformation of the residual γ-phase is caused by such a slow cooling to introduce higher strain and thereby more improve the primary recrystallization texture. It is well-known that the martensite transformation of γ-phase is caused by rapid cooling. When the cooling to lower than <NUM> is conducted by the rapid cooling of not less than <NUM>/s, the steel sheet is supercooled at the state of γ-phase, and hence it is thought that the martensite transformation is rather hard to be caused.

A steel having a component composition comprising C: <NUM> mass%, Si: <NUM> mass%, Mn: <NUM> mass%, sol. Al: <NUM> mass%, N: <NUM> mass%, S: <NUM> mass%, Se: <NUM> mass% and the remainder being Fe and inevitable impurities is melted in a vacuum melting furnace and cast into a steel ingot. The steel ingot is heated to a temperature of <NUM> and hot rolled to form a hot-rolled sheet having a sheet thickness of <NUM>. The hot-sheet is subjected to a hot-band annealing with a maximum achieving temperature of <NUM>. Then, the sheet is subjected to the first cold rolling to roll to a middle sheet thickness of <NUM> and an intermediate annealing with a maximum achieving temperature of <NUM>. The cooling process from <NUM> of the intermediate annealing to room temperature is conducted at an average cooling rate of <NUM>/s between <NUM> and <NUM>, and then at the average cooling rate of <NUM>/s between <NUM> and <NUM>, and variously changing the average cooling rate between <NUM> and <NUM> of the above temperature zone as shown in Table <NUM>. Thereafter, the second cold rolling (final cold rolling) is conducted to obtain a cold-rolled sheet having a final sheet thickness of <NUM>, and the cold-rolled sheet is subjected to a primary recrystallization annealing combined with a decarburization annealing in a wet atmosphere of <NUM> vol% H<NUM> - <NUM> vol% N<NUM> with a dew point of <NUM> at a soaking temperature of <NUM> for a soaking time of <NUM> seconds. In this case, the average heating rate between <NUM> and <NUM> in the heating process of the primary recrystallization annealing is changed within the range of <NUM> to <NUM>/s. An annealing separator composed mainly of MgO is applied to the steel sheet surface, and then the sheet is subjected to a finish annealing by heating (no temperature-holding) between <NUM> and <NUM> at a heating rate of <NUM>/hr to develop secondary recrystallization, subsequently heating to <NUM> at a heating rate of <NUM>/hr between <NUM> and <NUM> to complete secondary recrystallization and then performing a purification treatment of holding the sheet at the temperature in a hydrogen atmosphere for <NUM> hours.

A sample is taken out from the thus-obtained steel sheet after the finish annealing, and a magnetic flux density B<NUM> (magnetic flux density in the excitation at <NUM> A/m) thereof is measured by a method described in JIS C2550, and the measurement results are also shown in Table <NUM>. As seen from the results, the magnetic flux density is largely increased by conducting the rapid cooling at an average cooling rate of not less than <NUM>/s between <NUM> and <NUM> in the cooling process of the intermediate annealing and heating at a heating rate of not less than <NUM>/s between <NUM> and <NUM> in the heating process of the primary recrystallization annealing subsequent to cold rolling.

Although the mechanism of largely increasing the magnetic flux density by increasing the average cooling rate from <NUM> to <NUM> in the cooling process of the intermediate annealing to not less than <NUM>/s and heating at the heating rate of not less than <NUM>/s between <NUM> and <NUM> in the heating process of the primary recrystallization annealing as mentioned above has not been yet clear sufficiently, the inventors consider as follows.

When the average cooling rate from <NUM> to <NUM> in the cooling process of the intermediate annealing is increased to not less than <NUM>/s, it is considered, as mentioned in Experiment <NUM>, that unusual strain is caused in the phase interface between γ-phase and α-phase. The cold rolling conducted at such a state supposedly causes a deformation band different from usual ones. In this deformation band, nucleation of {<NUM>} orientation grains having a high recrystallization temperature is easily caused, and hence to increase the heating rate in the heating process of the primary recrystallization annealing to such a very fast rate as not less than <NUM>/s is considered to further increase {<NUM>} orientation grains to improve the texture, thereby causing great improvement in the magnetic properties.

The invention is developed based on the above novel knowledge.

Explanation will be made on the reason for limiting the component composition of the raw steel material (slab) used in the production of a grain-oriented electrical steel sheet according to the invention.

When a C content is less than <NUM> mass%, the structure turns α single phase in casting or hot rolling, so that steel is embrittled to cause cracking in the slab or cause an edge cracking in the steel sheet after the hot rolling, which brings about difficulties in production. On the other hand, when the C exceeds <NUM> mass%, it is difficult to reduce the C content to not more than <NUM> mass% where no magnetic aging occurs in the decarburization annealing. Therefore, the C content is in the range of <NUM> to <NUM> mass%. Preferably, it is in the range of <NUM> to <NUM> mass%.

Si is an element required for increasing a specific resistance of steel to thus improve iron loss. When it is less than <NUM> mass%, the above effect is not sufficient, while when it exceeds <NUM> mass%, the workability of steel is deteriorated to cause it difficult to produce the sheet by rolling. Therefore, the Si content is set in the range of <NUM> to <NUM> mass%. Preferably, it is set in the range of <NUM> to <NUM> mass%.

Mn is an element required for improving hot workability of steel. When the Mn content is less than <NUM> mass%, the above effect is not sufficient, while when it exceeds <NUM> mass%, the magnetic flux density of the product sheet lowers. Therefore, the Mn content is set in the range of <NUM> to <NUM> mass%.

As an element that forms AlN to be precipitates and acts as an inhibitor for suppressing normal grain growth in the finish annealing where secondary recrystallization is caused, Al is an important element in the production of a grain-oriented electrical steel sheet. However, when the Al content is less than <NUM> mass% as an acid-soluble Al (sol. Al), an absolute amount of the inhibitor is insufficient, leading to a lack of the power for suppressing normal grain growth. On the other hand, when the Al content exceeds <NUM> mass%, AlN is coarsened by Ostwald growth, also leading to a lack of the power for suppressing normal grain growth. Therefore, the Al content is, as sol. Al, in the range of <NUM> to <NUM> mass%, preferably in the range of <NUM> to <NUM> mass%.

N bonds with Al to form AlN to be an inhibitor and precipitated. When the N content is less than <NUM> mass%, the absolute amount of the inhibitor is insufficient so that the power for suppressing the normal grain growth is insufficient. On the other hand, when the N content exceeds <NUM> mass%, the slab may cause blister in hot rolling. Therefore, the N content is in the range of <NUM> to <NUM> mass%, preferably in the range of <NUM> to <NUM> mass%.

S and Se bond with Mn and form MnS and MnSe, which work as an inhibitor. However, when the contents of S and Se are less than <NUM> mass% alone or in total, the inhibitor effect cannot be obtained sufficiently. On the other hand, when it exceeds <NUM> mass%, the inhibitor is coarsened by Ostwald growth, and the power for suppressing the normal grain growth is insufficient. Therefore, the contents of S and Se are in the range of <NUM> to <NUM> mass% in total. Preferably, it is in a range of <NUM> to <NUM> mass%.

The remainder other than the above component composition of the raw steel material (slab) used in the production of a grain-oriented electrical steel sheet according to the invention is Fe and inevitable impurities. For the purpose of improving the magnetic properties, however, the raw steel material may contain one or more selected from Cr: <NUM> o <NUM> mass%, Cu: <NUM> to <NUM> mass%, Ni: <NUM> to <NUM> mass%, Bi: <NUM> to <NUM> mass%, B: <NUM> to <NUM> mass%, Nb: <NUM> to <NUM> mass%, Sn: <NUM> to <NUM> mass%, Sb: <NUM> to <NUM> mass%, Mo: <NUM> to <NUM> mass%, P: <NUM> to <NUM> mass%, V: <NUM> to <NUM> mass% and Ti: <NUM> to <NUM> mass% in addition to the above component composition. Each element has an effect of improving the magnetic properties of the grain-oriented electrical steel sheet. However, when each content is smaller than the lower limit, the effect of improving the magnetic properties cannot be obtained sufficiently. On the other hand, when each content exceeds the upper limit, the development of the secondary recrystallized grains is suppressed and the magnetic properties may rather deteriorated.

There will be described the method for producing a grain-oriented electrical steel sheet according to the invention below.

A grain-oriented electrical steel sheet according to the invention can be produced by a method for producing a grain-oriented electrical steel sheet comprising a series of steps of.

The raw steel material (slab) can be produced by a usual continuous casting method or ingot making-blooming method after a steel that has been adjusted to have the aforementioned component composition is melted by a usual refining process. Also, a thin cast slab having a thickness of not more than <NUM> may be produced by a direct casting method.

Then, the slab is heated to a temperature of higher than <NUM> and hot rolled to form a hot-rolled sheet having a given sheet thickness. When the heating temperature for the slab is not higher than <NUM>, the added inhibitor-forming ingredients are not brought into the solid-solution state in steel sufficiently. A preferable slab heating temperature is not lower than <NUM>. As means for heating the slab, well-known means such as a gas furnace, an induction heating furnace, an electric furnace and so on can be used. Moreover, the hot rolling subsequent to the slab heating may be conducted under conventionally well-known conditions and is not particularly limited.

Next, the hot-rolled sheet obtained by the hot rolling is subjected to a hot-band annealing for the purpose of complete recrystallization of the structure of the hot-rolled sheet. The maximum achieving temperature in the hot-bad annealing is preferable to be not lower than <NUM> from a viewpoint of surely obtaining the above effect. More preferably, it is not lower than <NUM>. On the other hand, when the maximum achieving temperature exceeds <NUM>, crystal grains after the hot-band annealing are coarsened, which makes it difficult to provide a primary recrystallization texture of size-regulated grains. Accordingly, the temperature is limited to not higher than <NUM>. More preferably, it is not higher than <NUM>. Moreover, the duration for holding the maximum achieving temperature is preferable to fall within the range of <NUM> to <NUM> seconds from a viewpoint of sufficiently obtaining the effect of the hot-band annealing and ensuring productivity.

Then, the hot-rolled sheet after the hot-band annealing is subjected to pickling for descaling and then to a single cold rolling or two or more cold rollings having an intermediate annealing between each cold rolling to form a cold-rolled sheet having a final sheet thickness. When two or more cold rollings are to be conducted, an annealing temperature in the intermediate annealing is preferable to fall within the range of <NUM> to <NUM>. When the annealing temperature is lower than <NUM>, it is difficult to complete recrystallization, while when it exceeds <NUM>, crystal grains after the annealing are coarsened, and hence it is difficult to obtain primary recrystallization texture of size-regulated grains. More preferably, it falls within the range of <NUM> to <NUM>. Moreover, a soaking time of the intermediate annealing is preferable to be in the range of <NUM> to <NUM> seconds from a viewpoint of sufficiently obtaining the effect of annealing and ensuring productivity.

It is most important in the invention that, in the annealing before the cold rolling, concretely in at least one of the hot-band annealing and the intermediate annealing, it is necessary to conduct a rapid cooling at an average cooling rate of not less than <NUM>/s between <NUM> and <NUM> in the cooling process from the maximum achieving temperature. As described above, cooling at the average cooling rate of not less than <NUM>/s in the above temperature range causes large strain to be introduced into the interior of the steel sheet after the cooling and leads to an improvement in the texture of the steel sheet after the primary recrystallization annealing, whereby the magnetic properties of the product sheet can be improved. The average cooling rate is preferably not less than <NUM>/s. In order to industrially attain the cooling rate, the rapid cooling device for jetting water as described in the above Patent Literature <NUM> and the like can be used favorably. Although the upper limit of the cooling rate is not particularly defined, the upper limit of the cooling rate in the above rapid cooling device is about <NUM>/s.

Next, it is important in the invention that the cooling from <NUM> to <NUM> subsequent to the rapid cooling between <NUM> and <NUM> is preferably conducted at an average cooling rate of <NUM> to <NUM>/s. Thus, strain quantity in the steel sheet after the annealing can be more increased to further improve the magnetic properties. More preferably, the average cooling rate falls within the range of <NUM> to <NUM>/s.

Thereafter, the steel sheet with the final sheet thickness after the cold rolling (cold-rolled sheet) is subjected to a primary recrystallization annealing combined with a decarburization annealing. The primary recrystallization annealing is preferable to be conducted at a soaking temperature of <NUM> to <NUM> for a soaking time <NUM> to <NUM> seconds, from a viewpoint of securing decarburization property. The annealing atmosphere is preferable to be a wet atmosphere from a viewpoint of securing the decarburization property. The decarburization annealing allows the C content in the steel sheet to be reduced to not more than <NUM> mass%. Further, the texture is further improved by increasing the temperature at a heating rate of not less than <NUM>/s between <NUM> and <NUM> being the recrystallization temperature zone in the heating process of the primary recrystallization annealing to thus improve the magnetic properties. Desirably, the heating rate is not less than <NUM>/s.

Then, the steel sheet after the primary recrystallization annealing is, in a case where a forsterite coating is to be formed in a finish annealing, coated with an annealing separator composed mainly of MgO on the steel sheet surface and thereafter subjected to the finish annealing of causing a secondary recrystallization and conducting a purification treatment. Whereas, in a case where blanking workability is considered important and thus the forsterite coating is not to be formed, the annealing separator is not applied or an annealing separator composed mainly of silica, alumina or the like is applied to the steel sheet surface and then the finish annealing is conducted.

It is preferable to conduct a temperature holding treatment of holding an arbitrary temperature between <NUM> and <NUM> for <NUM> to <NUM> hours in the heating process of the finish annealing. Alternatively, it is preferable to heat between <NUM> and <NUM> at an average heating rate of not more than <NUM>/hr to develop secondary recrystallization, subsequently, or after lowering the temperature to not higher than <NUM> once, reheat, increase the temperature between <NUM> and <NUM> at an average heating rate of <NUM> to <NUM>/hr up to not lower than <NUM> to complete the secondary recrystallization, and thereafter conduct a purification treatment of holding the temperature for not less than <NUM> hours. The purification treatment allows Al, N, S and Se in the steel sheet to be decreased to the level of inevitable impurities.

A preferable temperature holding time between <NUM> and <NUM> is <NUM> to <NUM> hours, and a preferable average heating rate between <NUM> and <NUM> is <NUM> to <NUM>/hr. Also, a preferable average heating rate between <NUM> and <NUM> is <NUM> to <NUM>/hr, and a preferable temperature and a preferable holding time in the purification treatment are <NUM> to <NUM> and <NUM> to <NUM> hours, respectively. Moreover, an atmosphere of the purification treatment in the finish annealing is preferable to be H<NUM> atmosphere.

The steel sheet after the finish annealing is subjected to a water washing, a brushing, a pickling or the like to remove unreacted annealing separator, and then subjected to a flattening annealing for a shape correction, which is effective for reducing the iron loss. When the steel sheets are laminated for use, it is preferable to apply an insulation coating onto the steel sheet surface in the flattening annealing or before or after the flattening annealing, in order to improve the iron loss. Moreover, it is preferable to use a tension-imparting coating as the insulation coating to further reduce the iron loss. In this case, it is possible to adopt a method of forming the tension-imparting coating through a binder, or a method of depositing an inorganic matter onto the steel sheet surface by a physical vapor deposition method or a chemical vapor deposition method to use as the tension-imparting coating. In order to further reduce the iron loss, it is preferable to conduct a magnetic domain subdividing treatment by irradiating a laser beam, plasma beam or the like onto the surface of the product sheet to apply heat strain or impact strain, or by forming grooves in the steel sheet surface.

A steel slab having a component composition shown in Table <NUM> and the remainder being Fe and inevitable impurities is produced by a continuous casting method, reheated to a temperature of <NUM>, hot rolled to form a hot-rolled sheet having a sheet thickness of <NUM>, and then subjected to a hot-band annealing at <NUM> for <NUM> seconds. In this case, average cooling rates between <NUM> and <NUM> and between <NUM> and <NUM> in the cooling process of the hot-band annealing and intermediate annealing are varied as shown in Table <NUM>. The hot-rolled sheet is thereafter subjected to pickling, the first cold rolling to roll to a middle sheet thickness of <NUM>, an intermediate annealing at <NUM> for <NUM> seconds, and then the second cold rolling to form a cold-rolled sheet having a final sheet thickness of <NUM>. The cold-rolled sheet is subjected to a primary recrystallization annealing combined with a decarburization annealing at <NUM> in a wet atmosphere of <NUM> vol% H<NUM> - <NUM> vol% N<NUM> with a dew point of <NUM> for <NUM> seconds. In this case, the average heating rate between <NUM> and <NUM> in the heating process is <NUM>/s.

Next, an annealing separator composed mainly of MgO is applied onto the surface of the steel sheet after the primary recrystallization annealing, and thereafter the steel sheet is subjected to a finish annealing by heating (no temperature holding) between <NUM> and <NUM> at a heating rate of <NUM>/hr to develop secondary recrystallization, subsequently heating to <NUM> at a heating rate of <NUM>/hr between <NUM> and <NUM> to complete secondary recrystallization and conducting a purification treatment of holding at such a temperature in a hydrogen atmosphere for <NUM> hours.

A test specimen is taken out from the thus-obtained steel sheet after the finish annealing and a magnetic flux density B<NUM> (magnetic flux density excited at <NUM> A/m) thereof is measured by a method described in JIS C2550 to obtain results shown in Table <NUM>. As seen from Table <NUM>, all of the steel sheets obtained by using the raw steel material having the component composition adapted to the invention and performing the rapid cooling in the hot-band annealing and/or the intermediate annealing under the conditions adapted to the invention have an excellent magnetic flux density, and particularly the faster the cooling rate between <NUM> and <NUM>, the more excellent the magnetic flux density.

A steel slab containing C: <NUM> mass%, Si: <NUM> mass%, Mn: <NUM> mass%, sol. Al: <NUM> mass%, N: <NUM> mass%, S: <NUM> mass% and the remainder being Fe and inevitable impurities is produced by a continuous casting method, reheated to a temperature of <NUM> and hot rolled to form a hot-rolled sheet having a sheet thickness of <NUM>. The hot-rolled sheet is subjected to a hot-band annealing at <NUM> for <NUM> seconds. In this case, average cooling rates between <NUM> and <NUM> and between <NUM> and <NUM> in the cooling process of the hot-band annealing are varied as shown in Table <NUM>. The sheet is thereafter subjected to a single cold rolling to form a cold-rolled sheet having a final sheet thickness of <NUM>. The cold-rolled sheet is subjected to a primary recrystallization annealing combined with a decarburization annealing at <NUM> in a wet atmosphere of <NUM> vol% H<NUM> - <NUM> vol% N<NUM> with a dew point of <NUM> for <NUM> seconds. In this case, the average heating rate between <NUM> and <NUM> in the heating process is <NUM>/s.

An annealing separator composed mainly of MgO is applied to the steel sheet surface after the primary recrystallization annealing. Then, the steel sheet is subjected to a finish annealing comprising heating (no temperature holding) between <NUM> and <NUM> at a heating rate of <NUM>/hr to develop secondary recrystallization, subsequently heating to <NUM> at a heating rate of <NUM>/hr between <NUM> and <NUM> to complete the secondary recrystallization and performing a purification treatment of holding such a temperature in a hydrogen atmosphere for <NUM> hours.

A test specimen is taken out from the thus-obtained steel sheet after the finish annealing, and a magnetic flux density B<NUM> (magnetic flux density excited at <NUM> A/m) thereof is measured by a method described in JIS C2550 to obtain results shown in Table <NUM>. As seen from Table <NUM>, all of the steel sheets obtained by using the raw steel material having the component composition adapted to the invention and performing the hot-band annealing under the conditions adapted to the invention are excellent in the magnetic flux density.

A steel slab comprising C: <NUM> mass%, Si: <NUM> mass%, Mn: <NUM> mass%, sol. Al: <NUM> mass%, N: <NUM> mass%, S: <NUM> mass% and the remainder being Fe and inevitable impurities as used in Example <NUM> is produced by a continuous casting method, reheated to a temperature of <NUM> and hot rolled to form a hot-rolled sheet having a sheet thickness of <NUM>. The hot-rolled sheet is subjected to a hot-band annealing at <NUM> for <NUM> seconds. In this case, average cooling rates between <NUM> and <NUM> and between <NUM> and <NUM> in the cooling process of the hot-band annealing are varied as shown in Table <NUM>. Thereafter, the steel sheet is subjected to the first cold rolling to roll to a middle sheet thickness of <NUM>, an intermediate annealing at <NUM> for <NUM> seconds and the second cold rolling to form a cold-rolled sheet having a final sheet thickness of <NUM>. In this case, the average cooling rate between <NUM> and <NUM> in the cooling process of the intermediate annealing is <NUM>/s.

Then, the cold-rolled sheet is subjected to a primary recrystallization annealing combined with a decarburization annealing at <NUM> in a wet atmosphere of <NUM> vol% H<NUM> - <NUM> vol% N<NUM> with a dew point of <NUM> for <NUM> seconds. In this case, the average heating rates between <NUM> and <NUM> in the heating process are varied as shown in Table <NUM>. An annealing separator composed mainly of MgO is applied onto the surface of the steel sheet after the primary recrystallization annealing, and the steel sheet is subjected to a finish annealing of completing the secondary recrystallization and then performing a purification treatment of holding at a temperature of <NUM> in a hydrogen atmosphere for <NUM> hours. In this case, heating conditions for completing the secondary recrystallization in the finish annealing (heating conditions for developing secondary recrystallization between <NUM> and <NUM>, presence or absence of subsequent temperature dropping to <NUM>, and average heating rate between <NUM> and <NUM>) are varied as shown in Table <NUM>.

A test sample is taken out from the thus-obtained steel sheet after the finish annealing, and a magnetic flux density B<NUM> (magnetic flux density excited at <NUM> A/m) thereof is measured by a method described in JIS C2550 to obtain results shown in Table <NUM>. As seen from Table <NUM>, the magnetic flux density of the product sheet is more increased by performing the temperature holding treatment for not less than <NUM> hours between <NUM> and <NUM> or by raising the temperature at not more than <NUM>/s between <NUM> and <NUM> in the heating process of the finish annealing, regardless of the presence or absence of subsequent temperature dropping to <NUM>. Also, the magnetic flux density is further increased by increasing the average heating rate between <NUM> and <NUM> in the heating process of the primary recrystallization annealing to not less than <NUM>/s.

Claim 1:
A method for producing a grain-oriented electrical steel sheet comprising a series of steps of
heating a steel slab having a component composition comprising C: <NUM> to <NUM> mass%, Si: <NUM> to <NUM> mass%, Mn: <NUM> to <NUM> mass%, sol. Al: <NUM> to <NUM> mass%, N: <NUM> to <NUM> mass%, one or two selected from S and Se: <NUM> to <NUM> mass% in total, optionally one or more selected from
Cr: <NUM> to <NUM> mass%,
Cu: <NUM> to <NUM> mass%,
Ni: <NUM> to <NUM> mass%,
Bi: <NUM> to <NUM> mass%,
B: <NUM> to <NUM> mass%,
Nb: <NUM> to <NUM> mass%,
Sn: <NUM> to <NUM> mass%,
Sb: <NUM> to <NUM> mass%,
Mo: <NUM> to <NUM> mass%,
P: <NUM> to <NUM> mass%,
V: <NUM> to <NUM> mass% and
Ti: <NUM> to <NUM> mass%, and the remainder being Fe and inevitable impurities to a temperature higher than <NUM>,
hot rolling the slab to form a hot-rolled sheet,
subjecting the hot-rolled sheet to a hot-band annealing and then a single cold rolling or two or more cold rollings having an intermediate annealing between each cold rolling to form a cold-rolled sheet having a final sheet thickness,
subjecting the cold-rolled sheet to a primary recrystallization annealing combined with a decarburization annealing,
applying an annealing separator onto a surface of the steel sheet, and
subjecting the steel sheet to a finish annealing and a flattening annealing, characterized in that
a rapid cooling is conducted at an average cooling rate of not less than <NUM>/s from <NUM> to <NUM> in a cooling process from a maximum achieving temperature in at least one process of the hot-band annealing and the intermediate annealing.