JP6946846B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet.

The grain-oriented electrical steel sheet is a steel sheet in which Si is contained in an amount of about 0.5 to 7% by mass and the crystal orientation is integrated in the {110} <001> orientation (goth orientation). A catastrophic grain growth phenomenon called secondary recrystallization is used to control the crystal orientation. Fine precipitates called inhibitors are used for secondary recrystallization.

There are the following two methods for controlling secondary recrystallization. In the first method, the material (slab) is heated at a temperature of 1280 ° C. or higher to dissolve the inhibitor almost completely. Then, hot rolling, cold rolling, annealing and the like are performed on the heated steel material to precipitate an inhibitor during hot rolling and annealing. In the second method, after the material (slab) is heated at a temperature of less than 1280 ° C., hot rolling, cold rolling, decarburization annealing, nitriding treatment, finish annealing and the like are performed, and during decarburization annealing and nitriding treatment, the material (slab) is subjected to decarburization annealing and nitriding treatment. Precipitate AlN or the like as an inhibitor.

In the method for producing grain-oriented electrical steel sheets in which secondary recrystallization is expressed by the _second method, it is preferable that the decarburization annealing time can be shortened from the viewpoint of reducing CO 2 emissions. Therefore, the C content of the slab, which is the material of the grain-oriented electrical steel sheet, is reduced. Specifically, the decarburization annealing time is shortened by setting the C content to 0.060% or less.

However, when the C content of the slab is reduced, the variation in magnetic characteristics (magnetic characteristic deviation) depending on the portion becomes large in the manufactured grain-oriented electrical steel sheet.

A method for manufacturing a grain-oriented electrical steel sheet that suppresses such variations in magnetic properties is disclosed in International Publication No. 2011/102456 (Patent Document 1). In Patent Document 1, Si: 2.5% by mass to 4.0% by mass, C: 0.01% by mass to 0.060% by mass, Mn: 0.05% by mass to 0.20% by mass, acid-soluble Al. : 0.020% by mass to 0.040% by mass, N: 0.002% by mass to 0.012% by mass, S: 0.001% by mass to 0.010% by mass, and P: 0.01% by mass to It contains 0.08% by mass, and further contains at least one selected from the group consisting of Ti: 0.0020% by mass to 0.010% by mass and Cu: 0.010% by mass to 0.50% by mass. Then, a step of hot-rolling the steel whose balance is composed of Fe and unavoidable impurities to obtain a hot-rolled steel sheet, a step of quenching the hot-rolled steel sheet to obtain a burnt steel sheet, and a step of cold-rolling the tempered steel sheet are performed. A step of obtaining a cold-rolled steel sheet, a step of decarburizing the cold-rolled steel sheet at a temperature of 800 ° C. to 950 ° C. It has a step of obtaining a nitrided steel sheet by performing at ° C. and a step of performing finish rolling of the nitrided steel sheet.

In Patent Document 1, a predetermined amount of Ti and Cu is contained in a steel (slab) to precipitate TiN and CuS precipitates. By utilizing TiN and CuS precipitates as inhibitors, the grain growth due to secondary recrystallization during finish annealing can be made uniform, and thus the deviation of the magnetic properties of the grain-oriented electrical steel sheet can be reduced.

International Publication No. 2011/102456

Also in the method for manufacturing grain-oriented electrical steel sheets of Patent Document 1, variations in magnetic properties can be suppressed. However, it is preferable that the variation in magnetic characteristics can be suppressed by other methods as well. In addition, excellent magnetic properties are also required for grain-oriented electrical steel sheets.

An object of the present invention is to provide a method for manufacturing a grain-oriented electrical steel sheet, which has excellent magnetic properties and can suppress variations in magnetic properties.

The method for producing a directional electromagnetic steel sheet according to the present invention includes a hot rolling step, a hot rolled sheet annealing step, a cold rolling step, a decarburization annealing step, a nitriding treatment step, and a finish annealing step.
The hot rolling step includes a rolling step and a post-rolling cooling step. The rolling step is mass%, Si: 2.5 to 4.0%, C: 0.010 to 0.060%, Mn: 0.05 to 0.20%, sol. Al: 0.020~0.040%, N: 0.002~0.012 %, S: 0.001 0 ~0.010%, P: 0.01 0 ~0.08%, Ti: 0. 0028 to 0.010%, Cu: 0.010 to 0.50%, Cr: 0 to 0.20%, Sn: 0 to 0.20%, Sb: 0 to 0.20%, Ni: 0 to 0 .20%, Se: 0 to 0.020%, Bi: 0 to 0.020%, Pb: 0 to 0.020%, B: 0 to 0.020%, V: 0 to 0.020%, Mo A steel material having a chemical composition of 0 to 0.020%, As: 0 to 0.020%, and the balance of Fe and impurities and having a chemical composition satisfying the formula (1) is hot-rolled to obtain a steel plate. do. When the temperature range of 900 to 750 ° C. is defined as the "specific temperature range", the post-rolling cooling step cools the steel sheet after the rolling process to obtain a hot-rolled steel sheet, and the average cooling rate of the steel sheet in the specific temperature range is 30. The temperature is ~ 150 ° C./sec.
The hot-rolled plate annealing step includes a first annealing step, a second annealing step, and a second post-annealing cooling step. In the first annealing step, the hot-rolled steel sheet is held at 1080 to 1200 ° C. for 70 to 120 seconds. In the second annealing step, after the first annealing step, the hot-rolled steel sheet is cooled to 900 to 980 ° C., and the hot-rolled steel sheet is held at 900 to 980 ° C. for 30 to 450 seconds. In the second post-annealing cooling step, after the second annealing step, the hot-rolled steel sheet is cooled to obtain an annealed steel sheet, and the average cooling rate of the hot-rolled steel sheet in a specific temperature range is 30 to 150 ° C./sec.
In the cold rolling process, the annealed steel sheet is cold-rolled to produce a cold-rolled steel sheet.
The decarburization annealing step includes a decarburization step and a post-decarburization cooling step. In the decarburization step, the cold-rolled steel sheet is decarburized and annealed at 800 to 950 ° C. In the post-decarburization cooling step, the steel sheet after the decarburization step is cooled to obtain a decarburized annealed steel sheet, and the average cooling rate of the steel sheet in a specific temperature range is 30 to 150 ° C./sec.
The nitriding treatment step includes a nitriding step and a post-nitriding cooling step. In the nitriding step, the decarburized annealed steel sheet is subjected to nitriding treatment at 700 to 850 ° C. In the post-nitriding cooling step, the steel sheet after the nitriding step is cooled to obtain a nitrided steel sheet in which CuS precipitates in the steel are 5 ppm or less in mass%, and the average cooling rate of the steel sheet in a specific temperature range is 30 to 150 ° C. / Second.
In the finish annealing step, an annealing separator containing MgO is applied to the surface of the nitrided steel sheet, and the nitrided steel sheet coated with the annealing separator is subjected to finish annealing at 1150 to 1250 ° C. for 5 to 30 hours. do.
20 × Ti + Cu ≦ 0.18 Equation (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The method for manufacturing a grain-oriented electrical steel sheet according to the present invention can suppress variations in magnetic properties at a portion of the grain-oriented electrical steel sheet manufactured.

FIG. 1 is a flow chart of a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention. FIG. 2 is a flow chart for explaining the details of the hot-rolled plate annealing step in FIG.

The present inventors have investigated and studied the improvement of the magnetic properties of grain-oriented electrical steel sheets and the suppression of variations in the magnetic properties.

In the conventional method for producing grain-oriented electrical steel sheets, AlN is precipitated as an inhibitor in the hot-rolled sheet annealing step, and MnS, TiN and CuS precipitates, which are inhibitors different from AlN, are precipitated. It has been thought that these inhibitors promote the expression of secondary recrystallization during finish annealing.

However, when CuS precipitates are used as inhibitors, CuS precipitates may remain on the final product, grain-oriented electrical steel sheets. In this case, the CuS precipitate inhibits the domain wall movement and the magnetic properties deteriorate.

In the present specification, the CuS precipitate means a compound containing Cu and S. The stoichiometric ratio of Cu and S in the CuS precipitate may vary, but as described later, in the present specification, the CuS precipitate amount in the nitrided steel plate is calculated as the atomic ratio of Cu to S. The content is calculated as 2/1. Details will be described later.

Therefore, the present inventors have conducted a detailed verification of the effects of these inhibitors on the expression of secondary recrystallization. As a result, the following new findings were obtained. This finding also describes the estimation mechanism. However, even if this estimation mechanism is denied after filing the application, the effect of the present invention (reduction of variation in magnetic characteristics and improvement of magnetic characteristics) does not change.

In the hot-rolled plate annealing step, the solid solution Cu promotes the precipitation of fine MnS in the temperature range where MnS is precipitated. That is, the solid solution Cu increases the amount of fine MnS deposited. MnS can be a precipitation nucleus of AlN. Therefore, in the hot-rolled plate annealing step, first, the fine MnS is precipitated in the steel by the solid solution Cu, and after the fine MnS is sufficiently precipitated, the heat treatment conditions are such that the fine AlN is precipitated with the fine MnS as the precipitation core. If this is controlled, the amount of fine AlN that functions as an inhibitor is significantly increased and uniformly dispersed in the steel. If the amount of fine AlN is increased and uniformly dispersed, secondary recrystallization is likely to occur uniformly at each site of the steel sheet during finish annealing. As a result, it is possible to suppress variations in the magnetic characteristics of the grain-oriented electrical steel sheet.

By utilizing the above mechanism, it is considered that secondary recrystallization can be uniformly expressed with a sufficient amount of fine AlN without utilizing CuS precipitate as an inhibitor. In this case, since the CuS precipitate is not utilized as an inhibitor, it is not necessary to precipitate the CuS precipitate as in the prior art. Therefore, the precipitation of CuS precipitates can be suppressed, and the deterioration of magnetic properties due to the remaining CuS precipitates in the steel can be suppressed.

Therefore, the present inventors manufacture a hot rolling step, a hot-rolled sheet annealing step, a decarburization annealing step, and a nitriding treatment step for expressing the above-mentioned mechanism and suppressing the formation of CuS precipitates. The conditions were further examined. As a result, if the cooling conditions in the hot rolling step, the decarburization annealing step, and the nitriding treatment step are as shown below, and the following three steps are carried out in the hot rolled sheet annealing step, fine MnS is precipitated. It was found that the precipitation amount of fine AlN can be increased and the precipitation of CuS precipitate can be suppressed.

[Cooling conditions in hot rolling process]
The hot rolling step includes a rolling step of rolling a steel material into a steel sheet and a post-rolling cooling step of cooling the steel sheet after the rolling step. In the post-rolling cooling step, the average cooling rate from 900 ° C. to 750 ° C. for the surface temperature of the steel sheet is set to 30 to 150 ° C./sec. In the following description, the temperature range of 900 to 750 ° C. is referred to as a "specific temperature range". By setting the average cooling rate of the steel sheet in a specific temperature range to 30 to 150 ° C./sec, it is possible to suppress the precipitation of CuS precipitates in the hot-rolled steel sheet. In this case, the first annealing step (hot-rolled plate annealing step), which is the next step, can be carried out in a state where the solid solution Cu is sufficiently present in the steel. Therefore, a large amount of fine MnS can be precipitated with a sufficient amount of solid solution Cu.

[3 steps in the hot-rolled sheet annealing process]
The hot-rolled plate annealing step includes the following three steps.
First annealing step: The hot-rolled steel sheet is heated so that the temperature of the hot-rolled steel sheet becomes the first annealing temperature (1080 to 1200 ° C.). Then, the hot-rolled steel sheet that has reached the first annealing temperature is held for the first holding time (70 to 120 seconds) to perform annealing. As a result, the precipitation of fine MnS is promoted by utilizing the solid solution Cu while suppressing the precipitation of CuS precipitates in the hot-rolled steel sheet.
Second annealing step: After the first annealing step, the temperature of the hot-rolled steel sheet is lowered to the second annealing temperature (900 to 980 ° C.). Then, the annealing is carried out by holding the second holding time (30 seconds to 450 seconds) at the second annealing temperature. As a result, a sufficient amount of fine AlN is precipitated using the fine MnS generated in the first annealing step as a nucleation nucleus.
Cooling step after the second annealing: The hot-rolled steel sheet after the second annealing step is cooled to obtain an annealed steel sheet. At this time, when the temperature of the hot-rolled steel sheet during cooling is in the range of 900 to 750 ° C. (specific temperature range), cooling is performed at an average cooling rate of 30 to 150 ° C./sec. By performing cooling at the above average cooling rate in a specific temperature range, precipitation of CuS precipitates in steel is suppressed, and fine AlN is additionally precipitated.

[Cooling conditions in decarburization annealing process]
The decarburization annealing step includes a decarburization step of decarburizing and annealing a cold-rolled steel sheet and a post-decarburization cooling step of cooling the steel sheet after the decarburization step to obtain a decarburized annealed steel sheet. The average cooling rate of the steel sheet in the cooling process after decarburization in a specific temperature range is 30 to 150 ° C./sec. By setting the average cooling rate of the steel sheet in a specific temperature range to 30 to 150 ° C./sec, it is possible to suppress the precipitation of CuS precipitates in the decarburized annealed steel sheet after the decarburization and cooling steps.

[Cooling conditions in the nitriding process]
The nitriding treatment step includes a nitriding step of nitriding a decarburized and tempered steel plate and a post-nitriding cooling step of cooling the steel plate after the nitriding step to obtain a nitriding treated steel plate. In the post-nitriding cooling step, the average cooling rate of the steel sheet in a specific temperature range (900 to 750 ° C.) is set to 30 to 150 ° C./sec. By setting the average cooling rate of the steel sheet in a specific temperature range to 30 to 150 ° C./sec, it is possible to suppress the precipitation of CuS precipitates in the nitrided steel sheet.

By satisfying all the above production conditions, the amount of CuS precipitates in the nitrided steel sheet becomes 5 ppm by mass%. In this case, the amount of CuS precipitates can be reduced and the magnetic properties are enhanced in the grain-oriented electrical steel sheet after finish annealing.

As described above, the present inventors do not deposit Cu in the directional electromagnetic steel plate as a CuS precipitate and utilize it as an inhibitor as in the conventional case, but to promote the precipitation of fine MnS by Cu. The present invention has been completed based on a completely different technical idea from the conventional one, in which a large amount of fine AlN is precipitated by using fine MnS as a precipitation nucleus and the formation of CuS precipitate is suppressed by utilizing it as solid-dissolved Cu. I let you.

The method for manufacturing a directional electromagnetic steel sheet according to the present invention, which is completed based on the above technical idea, is a hot rolling step, a hot rolling plate annealing step, a cold rolling step, a decarburization annealing step, and a nitriding treatment step. And a finish nitriding process.
The hot rolling step includes a rolling step and a post-rolling cooling step. The rolling step is mass%, Si: 2.5 to 4.0%, C: 0.010 to 0.060%, Mn: 0.05 to 0.20%, sol. Al: 0.020~0.040%, N: 0.002~0.012 %, S: 0.001 0 ~0.010%, P: 0.01 0 ~0.08%, Ti: 0. 0028 to 0.010%, Cu: 0.010 to 0.50%, Cr: 0 to 0.20%, Sn: 0 to 0.20%, Sb: 0 to 0.20%, Ni: 0 to 0 .20%, Se: 0 to 0.020%, Bi: 0 to 0.020%, Pb: 0 to 0.020%, B: 0 to 0.020%, V: 0 to 0.020%, Mo A steel material having a chemical composition of 0 to 0.020%, As: 0 to 0.020%, and the balance of Fe and impurities and having a chemical composition satisfying the formula (1) is hot-rolled to obtain a steel plate. do. When the temperature range of 900 to 750 ° C. is defined as the "specific temperature range", the post-rolling cooling step cools the steel sheet after the rolling process to obtain a hot-rolled steel sheet, and the average cooling rate of the steel sheet in the specific temperature range is 30. The temperature is ~ 150 ° C./sec.
The hot-rolled plate annealing step includes a first annealing step, a second annealing step, and a second post-annealing cooling step. In the first annealing step, the hot-rolled steel sheet is held at 1080 to 1200 ° C. for 70 to 120 seconds. In the second annealing step, after the first annealing step, the hot-rolled steel sheet is cooled to 900 to 980 ° C., and the hot-rolled steel sheet is held at 900 to 980 ° C. for 30 to 450 seconds. In the second post-annealing cooling step, after the second annealing step, the hot-rolled steel sheet is cooled to obtain an annealed steel sheet, and the average cooling rate of the hot-rolled steel sheet in a specific temperature range is 30 to 150 ° C./sec.
In the cold rolling process, the annealed steel sheet is cold-rolled to produce a cold-rolled steel sheet.
The decarburization annealing step includes a decarburization step and a post-decarburization cooling step. In the decarburization step, the cold-rolled steel sheet is decarburized and annealed at 800 to 950 ° C. In the post-decarburization cooling step, the steel sheet after the decarburization step is cooled to obtain a decarburized annealed steel sheet, and the average cooling rate of the cold-rolled steel sheet in a specific temperature range is 30 to 150 ° C./sec.
The nitriding treatment step includes a nitriding step and a post-nitriding cooling step. In the nitriding step, the decarburized annealed steel sheet is subjected to nitriding treatment at 700 to 850 ° C. In the post-nitriding cooling step, the steel sheet after the nitriding step is cooled to obtain a nitrided steel sheet in which CuS precipitates in the steel are 5 ppm or less in mass%, and the average cooling rate of the decarburized annealed steel sheet in a specific temperature range is 30. The temperature is ~ 150 ° C./sec.
In the finish annealing step, an annealing separator containing MgO is applied to the surface of the nitrided steel sheet, and the nitrided steel sheet coated with the annealing separator is subjected to finish annealing at 1150 to 1250 ° C. for 5 to 30 hours. do.
20 × Ti + Cu ≦ 0.18 Equation (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

The chemical composition of the steel material may contain one or more selected from the group consisting of Cr: 0.01 to 0.20% and Sn: 0.01 to 0.20%. The chemical composition of the steel material is Sb: 0.01 to 0.20%, Ni: 0.01 to 0.20%, Se: 0.005 to 0.020%, Bi: 0.005 to 0. 020%, Pb: 0.005 to 0.020%, B: 0.005 to 0.020%, V: 0.005 to 0.020%, Mo: 0.005 to 0.020%, and As. : One or more selected from the group consisting of 0.005 to 0.020% may be contained.

Hereinafter, the method for manufacturing a grain-oriented electrical steel sheet according to the present invention will be described in detail. In the present specification,% with respect to the content of the element means mass% unless otherwise specified.

[Manufacturing process flow]
FIG. 1 is a flow chart of a method for manufacturing a grain-oriented electrical steel sheet according to the present invention. With reference to FIG. 1, the present manufacturing method involves a hot rolling step (S1) of performing hot rolling on a steel material to produce a hot-rolled steel sheet, and two-step annealing and cooling on the hot-rolled steel sheet. A hot-rolled sheet annealing step (S2) for producing a hard-rolled steel sheet, a cold rolling step (S3) for cold-rolling a cold-rolled steel sheet to produce a cold-rolled steel sheet, and a cold-rolled steel sheet. A decarburization annealing step (S4) for producing a decarburized annealed steel sheet by carrying out decarburization annealing, and a nitriding process (S5) for producing a nitrided steel sheet by performing a nitride treatment for the decarburized annealed step. , A finish annealing step (S6) of carrying out finish annealing on the nitrided steel sheet is included. Hereinafter, each steps S1 to S6 will be described.

[Hot rolling process (S1)]
The hot rolling step (S1) includes a rolling step (S11) and a post-rolling cooling step (S12). In the rolling step (S11), the prepared steel material is hot-rolled to produce a steel sheet. In the post-rolling cooling step (S12), the steel sheet after the rolling step is cooled. Through the above steps, a hot-rolled steel sheet is manufactured in the hot rolling step. Hereinafter, the rolling process and the post-rolling cooling process will be described.

[Rolling process (S11)]
In the rolling process, the prepared steel material is hot-rolled to produce a steel sheet. The steel material corresponds to the material of the grain-oriented electrical steel sheet, for example, a slab. The chemical composition of steel contains the following elements.

[Essential elements in the chemical composition of steel materials]
Si: 2.5-4.0%
Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces the eddy current loss that forms part of the iron loss. If the Si content is less than 2.5%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 4.0%, the cold workability of the steel is lowered. Therefore, the Si content is 2.5-4.0%. The lower limit of the Si content is preferably 2.8%, more preferably 3.0%. The preferred upper limit of the Si content is 3.7%, more preferably 3.5%.

C: 0.010 to 0.060%
Carbon (C) is effective for structure control until the completion of the decarburization annealing step during the manufacturing process. However, if the C content is less than 0.010%, the above effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.060%, the time required for decarburization annealing becomes too long, and the _{amount of CO 2} emitted increases. If decarburization annealing is insufficient, it is difficult to obtain good magnetic characteristics. Therefore, the C content is 0.010 to 0.060%. The lower limit of the C content is preferably 0.030%, more preferably 0.040%. The preferred upper limit of the C content is 0.055%, more preferably 0.050%.

Mn: 0.05 to 0.20%
Manganese (Mn) increases the specific resistance of grain-oriented electrical steel sheets and reduces iron loss. Mn further enhances hot workability and suppresses the occurrence of cracks in hot rolling. Mn further combines with S to form fine MnS in the hot-rolled plate annealing step. The fine MnS becomes a precipitation nucleus of fine AlN utilized as an inhibitor. Therefore, in the hot-rolled plate annealing step, if the amount of fine MnS deposited is large, a sufficient amount of fine AlN can be obtained. If the Mn content is less than 0.05%, a sufficient amount of fine MnS will not be precipitated. On the other hand, if the Mn content exceeds 0.20%, the magnetic flux density of the grain-oriented electrical steel sheet decreases. Therefore, the Mn content is 0.05 to 0.20%. The preferred lower limit of the Mn content is 0.07%, more preferably 0.08%. The preferred upper limit of the Mn content is 0.17%, more preferably 0.15%.

sol. Al: 0.020 to 0.040%
Aluminum (Al) combines with N to form AlN during the manufacturing process of grain-oriented electrical steel sheets, and functions as an inhibitor. sol. If the Al content is less than 0.020%, a sufficient amount of AlN that functions as an inhibitor cannot be obtained. On the other hand, sol. If the Al content exceeds 0.040%, AlN becomes coarse and the inhibitor strength decreases. Therefore, sol. The Al content is 0.020 to 0.040%. sol. The lower limit of the Al content is preferably 0.022%, more preferably 0.025%. sol. The preferred upper limit of the Al content is 0.035%, more preferably 0.030%. In addition, in this specification, sol. Al means acid-soluble Al. Therefore, sol. The Al content is the content of acid-soluble Al.

N: 0.002 to 0.012%
Nitrogen (N) combines with Al to form AlN during the manufacturing process of grain-oriented electrical steel sheets, and functions as an inhibitor. As will be described later, in the method for manufacturing grain-oriented electrical steel sheets according to the present embodiment, a nitriding process is performed after the cold rolling process. Therefore, it is not necessary for the steel material to contain a large amount of N. However, in order to reduce the N content to less than 0.002%, excessive refining is required in the steelmaking process. Therefore, the lower limit of the N content is 0.002%. On the other hand, if the N content in the steel material exceeds 0.012%, a large number of blister (vacancy) is likely to be generated in the steel sheet during cold rolling. Therefore, the N content is 0.002 to 0.012%. The preferred lower limit of the N content is 0.005%, more preferably 0.006%. The preferred upper limit of the N content is 0.010%, more preferably 0.009%.

S: 0.001 0 to 0.010%
Sulfur (S) combines with Mn during the manufacturing process to form the fine MnS described above. If S content is less than 0.001 0%, no sufficient amount of fine MnS can be obtained. On the other hand, if the S content exceeds 0.010%, MnS may remain even in the steel sheet after finish annealing. In this case, the magnetic characteristics deteriorate. Thus, S content is 0.001 0 to 0.010%. The preferable lower limit of the S content is 0.005%. The preferred upper limit of the S content is 0.009%.

P: 0.01 0 to 0.08%
Phosphorus (P) increases the specific resistance of the grain-oriented electrical steel sheet and reduces iron loss. If P content is less than 0.01 0%, this effect can not be obtained sufficiently. On the other hand, if the P content exceeds 0.08%, the cold workability of the steel sheet is lowered. Accordingly, P content is 0.01 from 0 to 0.08% (0.010% to 0.080%). The preferable lower limit of the P content is 0.015%. The preferred upper limit of the P content is 0.04%.

Ti: 0.0028 to 0.010%
Titanium (Ti) combines with N to form TiN (precipitate). TiN acts as an inhibitor to express secondary recrystallization in the finish annealing step. TiN particularly suppresses the variation in grain growth in the high temperature region in the finish annealing step and reduces the deviation of the magnetic characteristics of the grain-oriented electrical steel sheet. If the Ti content is less than 0.0028%, these effects cannot be sufficiently obtained. On the other hand, if the Ti content exceeds 0.010%, TiN precipitates are excessively formed and remain even after finish annealing. If the TiN precipitate remains after finish annealing, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, the Ti content is 0.0028 to 0.010%. The preferable lower limit of the Ti content is 0.0030%. The preferred upper limit of the Ti content is 0.0035%.

Cu: 0.010 to 0.50%
Copper (Cu) promotes the precipitation of fine MnS, which is the nucleation of AlN, in the hot-rolled plate annealing step. If the Cu content is less than 0.010%, the above effect cannot be sufficiently obtained. On the other hand, if the Cu content is too high, CuS precipitates may precipitate and the CuS precipitates may remain even after finish annealing. If CuS precipitates remain in the steel, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, the Cu content is 0.010 to 0.50%. The lower limit of the Cu content is preferably 0.05%, more preferably 0.10%. The preferred upper limit of the Cu content is 0.40%, more preferably 0.30%.

In the conventional method for producing grain-oriented electrical steel sheets, CuS precipitates are positively generated in the hot-rolled sheet annealing step, and the CuS precipitates are utilized as inhibitors of secondary recrystallization. However, as described above, if the CuS precipitate remains in the steel, the magnetic properties of the grain-oriented electrical steel sheet are deteriorated. Therefore, in the present invention, the precipitation of CuS precipitates is suppressed as much as possible, and the content of CuS precipitates in the steel sheet (nitriding steel sheet) after the nitriding treatment step is set to 5 ppm or less in mass%.

The balance of the chemical composition of the steel material according to the present invention consists of Fe and impurities. Here, the impurities are those mixed from ore, scrap, manufacturing environment, etc. as raw materials when the steel material which is the material of the grain-oriented electrical steel sheet is industrially manufactured, or are completely purified by purification annealing. It means the following elements, etc. that remain in the steel without being allowed, as long as they do not adversely affect the grain-oriented electrical steel sheet manufactured by the manufacturing method of the present invention.

[About equation (1)]
The chemical composition of the steel material described above further satisfies the formula (1).
20 x Ti + Cu ≤ 0.18 (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1).

Formula (1) is an index of the amount of TiN and CuS precipitates deposited in steel. It is defined as F1 = 20 × Ti + Cu. When F1 satisfies the formula (1), the TiN and CuS precipitates in the steel sheet are sufficiently reduced after the finish annealing step, and the influence of these precipitates on the magnetic properties is sufficiently suppressed. Therefore, the magnetic characteristics of the grain-oriented electrical steel sheet are enhanced.

[Arbitrary elements in the chemical composition of steel materials]
The above-mentioned steel material may contain one or more selected from the group consisting of Cr and Sn instead of a part of Fe.

Cr: 0 to 0.20%
Sn: 0 to 0.20%
Both chromium (Cr) and tin (Sn) improve the properties of the oxide layer formed during the decarburization annealing step, and also improve the properties of the primary coating formed using this oxide layer during the finish annealing step. Further, Cr and Sn improve the magnetic characteristics of the grain-oriented electrical steel sheet by realizing the stabilization of the formation of the oxide layer and the primary coating, and suppress the variation in the magnetic characteristics. Sn is also a grain boundary segregation element, which stabilizes secondary recrystallization. However, if the Cr content exceeds 0.20%, the formation of the primary coating may become unstable. Further, if the Sn content exceeds 0.20%, the surface of the steel sheet is less likely to be oxidized, and the formation of the primary coating may be insufficient. Therefore, the Cr content is 0 to 0.20% and the Sn content is 0 to 0.20%. The preferable lower limit of the Cr content for more effectively obtaining the above effect is 0.01%. The preferable lower limit of the Sn content for more effectively obtaining the above effect is 0.01%. A more preferable lower limit of the Cr content is 0.05%, and a further preferable upper limit is 0.15%. A more preferable lower limit of the Sn content is 0.04%, and a further preferable upper limit is 0.15%.

The above-mentioned steel material may further contain one or more selected from the group consisting of Sb, Ni, Se, Bi, Pb, B, V, Mo and As instead of a part of Fe. .. Both of these elements enhance the inhibitor.
Sb: 0 to 0.20%,
Ni: 0-0.20%,
Se: 0-0.020%,
Bi: 0-0.020%,
Pb: 0 to 0.020%,
B: 0 to 0.020%,
V: 0 to 0.020%,
Mo: 0-0.020%,
As: 0-0.020%,
Antimony (Sb), nickel (Ni), selenium (Se), bismuth (Bi), lead (Pb), boron (B), vanadium (V), molybdenum (Mo) and arsenic (As) are all sub-inhibitors. Acts as a stabilizer for secondary recrystallization. However, if these elements are contained in excess, the effect is saturated. Therefore, the Sb content is 0 to 0.20%, the Ni content is 0 to 0.20%, the Se content is 0 to 0.020%, the Bi content is 0 to 0.020%, and the Pb content is 0 to 0.020%. 0 to 0.020%, B content 0 to 0.020%, V content 0 to 0.020%, Mo content 0 to 0.020%, As content 0 to 0.020% And may contain one or more selected from the group consisting of these elements. The preferable lower limit of the Sb content is 0.010%. The preferable lower limit of the Ni content is 0.010%. The preferable lower limit of the Se content is 0.005%. The preferred lower limit of the Bi content is 0.005%. The preferable lower limit of the Pb content is 0.005%. The preferable lower limit of the B content is 0.005%. The preferable lower limit of the V content is 0.005%. The preferable lower limit of the Mo content is 0.005%. The preferable lower limit of the As content is 0.005%.

The steel material having the above chemical composition is, for example, a slab as described above. An example of a method for manufacturing a steel material is as follows. A molten steel having the above chemical composition is produced (melted). Manufacture steel materials using molten steel. Steel materials may be manufactured by a continuous casting method. An ingot may be manufactured using molten steel, and the ingot may be lump-rolled to produce a steel material. Steel materials may be produced by other methods.

The prepared steel material (slab) is hot-rolled using a hot-rolling machine to produce a steel plate (hot-rolled steel plate). First, the steel material is heated. For example, the slab is placed in a well-known heating furnace or a well-known soaking furnace to heat it. The preferred heating temperature of the slab is less than 1280 ° C, more preferably 1100 to 1250 ° C.

A steel sheet (hot-rolled steel sheet) is manufactured by performing hot rolling on a heated steel material (slab) using a hot rolling machine. The hot rolling mill includes a rough rolling mill and a finishing rolling mill located downstream of the rough rolling mill. The rough rolling mill includes a row of rough rolling stands. Each rough rolling stand contains a plurality of rolls arranged one above the other. The finish rolling mill is also equipped with a row of finish rolling stands. Each finish rolling stand contains a plurality of rolls arranged one above the other. The heated steel material is rolled by a rough rolling mill and then further rolled by a finishing rolling mill to produce a hot-rolled steel sheet.

The thickness of the hot-rolled steel sheet produced by hot rolling is not particularly limited, and may be a known thickness. The finishing temperature in the hot rolling process (the temperature of the steel sheet on the outlet side of the finishing rolling stand that finally rolls the steel sheet in the finishing rolling mill) is, for example, 900 to 1000 ° C. A steel sheet is manufactured by the above rolling process.

[Cooling step after rolling (S12)]
In the post-rolling cooling step (S12), the steel sheet after the rolling step (S11) is cooled to obtain a hot-rolled steel sheet. At this time, the average cooling rate from 900 ° C. to 750 ° C. (that is, the steel sheet temperature is in a specific temperature range) is set to 30 to 150 ° C./sec. Here, the steel plate temperature is specified by the following method. A plurality of thermometers (radiation thermometer, thermography, etc.) capable of measuring the temperature at the center of the width of the steel sheet are attached to the outlet side of the rolling stand that is rolled down at the end of the finishing rolling mill and downstream thereof. .. Each temperature gauge measures the surface temperature of the steel sheet at the center in the width direction of the steel sheet. Steel sheet temperature based on the surface temperature of the steel sheet measured by each temperature gauge, the placement position of each temperature gauge, the passing speed of the hot-rolled steel sheet after the outlet side of the rolling stand that is pressed at the end of the finish rolling mill, etc. The time required for the temperature to change from 900 ° C. to 750 ° C. is determined, and the average cooling rate is determined based on the determined time.

If the average cooling rate in the specific temperature range is less than 30 ° C./sec, a large amount of CuS precipitates are generated in the hot-rolled steel sheet after the rolling and cooling steps. In this case, since the solid solution Cu is insufficient in the first annealing step (S21) of the next step, sufficient fine MnS is not generated. Further, since the CuS precipitates generated in the cooling step after rolling tend to remain even after finish annealing, the CuS precipitates remain in the grain-oriented electrical steel sheet, and the magnetic properties are deteriorated. On the other hand, if the average cooling rate exceeds 150 ° C./sec, AlN is unevenly precipitated. Therefore, the magnetic characteristics are deteriorated.

When the average cooling rate in a specific temperature range is 30 to 150 ° C./sec, the formation of CuS precipitates on the hot-rolled steel sheet can be sufficiently suppressed.

Cooling in a specific temperature range is performed, for example, as follows. For example, a cooling device is arranged after the finish rolling mill. The steel sheet after finish rolling is charged into the cooling device as it is and is continuously cooled. After cooling, the steel sheet is wound into a coil. In the cooling device, the cooling fluid may be injected onto the surface of the steel sheet to cool the steel sheet at the above-mentioned average cooling rate. The cooling fluid is, for example, water or a mixed fluid of water and air. Further, the steel sheet may be cooled by passing the steel sheet through a cooling tank in which the cooling fluid is stored.

In the hot rolling step (S1), a hot-rolled steel sheet is manufactured by the above-mentioned rolling step (S11) and post-rolling cooling step (S12).

[Hot rolled plate annealing step (S2)]
An annealed hot-rolled steel sheet is manufactured by performing a hot-rolled sheet annealing step on the manufactured hot-rolled steel sheet. FIG. 2 is a flow chart showing details of the hot-rolled plate annealing step in FIG. With reference to FIG. 2, the hot-rolled plate annealing step (S2) includes a first annealing step (S21), a second annealing step (S22), and a second post-annealing cooling step (S23). In the hot-rolled plate annealing step, each step is carried out in the order of the first annealing step (S21), the second annealing step (S22), and the second annealing post-annealing cooling step (S23). In the first annealing step (S21), Cu is mainly used to precipitate a large number of fine MnS. In the second annealing step (S22), a large number of fine AlNs are mainly precipitated with the fine MnS as the precipitation nuclei. In the second post-annealing cooling step (S23), fine AlN is further added and precipitated, and the precipitation of CuS precipitate is suppressed. Hereinafter, each steps S21 to S23 will be described in detail.

[First annealing step (S21)]
In the first annealing step, the hot-rolled steel sheet is heated by using a heat treatment furnace so that the temperature of the hot-rolled steel sheet becomes the first annealing temperature (1080-1200 ° C.). Then, the hot-rolled steel sheet that has reached the first annealing temperature is held for the first holding time (70 to 120 seconds) to perform annealing. As a result, fine MnS, which is a precipitation core of fine AlN, is precipitated in the hot-rolled steel sheet. The solid solution Cu in the steel promotes the precipitation of fine MnS in the first annealing step. The production conditions in the first annealing step (S21) are as follows.

First annealing temperature: 1080-1200 ° C
First holding time: 70 to 120 seconds The first annealing temperature corresponds to the furnace temperature of the heat treatment furnace. If the first annealing temperature is less than 1080 ° C., a sufficient amount of fine MnS for precipitating fine AlN will not be precipitated. On the other hand, if the first annealing temperature exceeds 1200 ° C., the precipitated MnS becomes coarse. If MnS becomes coarse, the precipitation behavior of AlN changes, and a sufficient amount of fine AlN does not precipitate in the second annealing step (S22). When the first annealing temperature is 1080 to 1200 ° C., a sufficient amount of fine MnS is precipitated on the premise that the first holding time is appropriate.

Further, if the first holding time is less than 70 seconds, a sufficient amount of fine MnS for precipitating fine AlN will not be precipitated. On the other hand, if the first holding time exceeds 120 seconds, MnS becomes coarse. As a result, in the second annealing step (S22), a sufficient amount of fine AlN is not precipitated. When the first holding time is 70 to 120 seconds, a sufficient amount of fine MnS is precipitated on the premise that other conditions in the hot-rolled plate annealing step are appropriate.

[Second annealing step (S22)]
After the first annealing step (S21), the second annealing step is subsequently carried out. In the second annealing step, the temperature of the hot-rolled steel sheet is lowered to the second annealing temperature (900 to 980 ° C.) in the same heat treatment furnace as in the first annealing step. Specifically, after the first annealing step (S21), the furnace temperature is lowered from the first annealing temperature (1080 to 1200 ° C.) to the second annealing temperature (900 to 980 ° C.). After lowering the furnace temperature to set the temperature of the hot-rolled steel sheet to the second annealing temperature, annealing is performed by holding the hot-rolled steel sheet at the second annealing temperature for a second holding time (30 to 450 seconds). By carrying out the second annealing step (S22), a sufficient amount of fine AlN is precipitated using the fine MnS generated in the first annealing step (S21) as a precipitation nucleus. In the second annealing step (S22), fine TiN that functions as an inhibitor is also precipitated. The production conditions in the second annealing step (S22) are as follows.

Second annealing temperature: 900-980 ° C
Second holding time: 30 to 450 seconds The second annealing temperature corresponds to the furnace temperature of the heat treatment furnace. When the second annealing temperature is less than 900 ° C., a sufficient amount of fine AlN is not precipitated, and a very fine AlN is unstablely precipitated in a small amount. In this case, AlN is unlikely to function as an inhibitor. On the other hand, when the second annealing temperature exceeds 980 ° C., although a small amount of coarse AlN is precipitated, it is difficult to precipitate an appropriate amount of fine AlN. When the second annealing temperature is 900 to 980 ° C., a sufficient amount of fine AlN is precipitated on the premise that other conditions in the hot-rolled plate annealing step are appropriate.

If the second holding time is less than 30 seconds, a sufficient amount of fine AlN will not be deposited. On the other hand, if the second retention time exceeds 450 seconds, AlN becomes coarse and it is difficult to function as an inhibitor. When the second holding time is 30 to 450 seconds, a sufficient amount of fine AlN is precipitated on the premise that each condition in the first annealing step is appropriate and the second holding temperature is appropriate.

[Cooling step after second annealing (S23)]
After the second annealing step (S22), the second post-annealing cooling step (S23) is carried out. In the second post-annealing cooling step (S23), the steel sheet (hot-rolled steel sheet) after the second annealing step (S22) is cooled. At this time, when the steel sheet temperature is in the range of 900 to 750 ° C. (specific temperature range), the steel sheet is cooled at an average cooling rate of 30 to 150 ° C./sec. By performing cooling at the above average cooling rate in a specific temperature range, fine AlN is additionally precipitated. Furthermore, since CuS precipitates are likely to be formed in a specific temperature range, the precipitation of CuS precipitates is suppressed by cooling the steel sheet at the above-mentioned average cooling rate. The production conditions in the second post-annealing cooling step (S23) are as follows.

Average cooling rate in a specific temperature range: 30 to 150 ° C./sec When the average cooling rate in a specific temperature range is less than 30 ° C./sec, excess CuS precipitates are deposited. In this case, CuS precipitates remain in the steel even after finish annealing. Therefore, the magnetic characteristics are deteriorated. On the other hand, when the average cooling rate in a specific temperature range exceeds 150 ° C./sec, the cooling of the steel sheet becomes non-uniform. In this case, fine AlN is unevenly additionally deposited in a specific temperature range. Therefore, secondary recrystallization is less likely to occur uniformly, and the variation in magnetic characteristics of the grain-oriented electrical steel sheet becomes large. When the average cooling rate in a specific temperature range is 30 to 150 ° C./sec, it becomes easy to uniformly precipitate fine AlN in the steel sheet in a specific temperature range, and excessive precipitation of CuS precipitates can be suppressed. ..

Cooling in a specific temperature range is performed, for example, as follows. A cooling device is arranged after the heat treatment furnace. The steel sheet extracted from the heat treatment furnace is directly charged into the cooling device and continuously cooled. After cooling, the steel sheet is wound into a coil. In the cooling device, the cooling fluid may be injected onto the surface of the steel sheet to cool the steel sheet at the above-mentioned average cooling rate. The cooling fluid is, for example, water or a mixed fluid of water and air. Further, the steel sheet may be cooled by passing the steel sheet through a cooling tank in which the cooling fluid is stored.

The average cooling rate in a specific temperature range can be measured by the following method. The temperature of the steel sheet is determined by measuring the surface temperature of the steel sheet with a temperature sensor such as a thermography or a radiation thermometer. Based on the measured temperature, the time required for the temperature of the steel sheet to drop from 900 ° C. to 750 ° C. is determined, and the average cooling rate (° C./sec) is determined based on the determined time.

The method for cooling the steel sheet in the temperature range of 980 to 900 ° C. or higher and the temperature range of less than 750 ° C. is not particularly limited, but the cooling is preferably performed at 10 ° C./sec or higher.

Through the above steps, the hot-rolled sheet annealing step is carried out to manufacture an annealed steel sheet. In the hot-rolled plate annealing step, a large number of fine AlNs having fine MnS as precipitation nuclei are precipitated by carrying out the above-mentioned three steps. As a result, in the finish annealing step, secondary recrystallization can be easily expressed uniformly, and variations in the magnetic properties of the grain-oriented electrical steel sheet can be suppressed. Further, it suppresses the precipitation of CuS precipitates. As a result, the content of CuS precipitates in the steel sheet after the nitriding treatment step is 5 ppm or less in mass% on the premise that other production conditions are also satisfied. Therefore, deterioration of the magnetic characteristics of the grain-oriented electrical steel sheet can be suppressed.

[Cold rolling process (S3)]
In the cold rolling step (S3), the produced annealed steel sheet is cold-rolled to produce a cold-rolled steel sheet. Cold rolling is carried out using a cold rolling machine. The cold rolling mill comprises a plurality of cold rolling stands arranged in a row. Each cold rolling stand includes a plurality of cold rolling rolls.

In the cold rolling step, the cold rolling may be carried out only once or may be carried out a plurality of times. When cold rolling is carried out a plurality of times, cold rolling is carried out, intermediate annealing for the purpose of softening is carried out, and then cold rolling is carried out. A known method is used for the intermediate annealing conditions. When intermediate annealing is performed, the average cooling rate of the steel sheet after annealing in a specific temperature range (900 to 750 ° C.) is set to 30 to 150 ° C./sec. By intermediate annealing, the strain introduced into the steel sheet in the cold rolling in the previous stage is reduced (the steel sheet is softened), and then the cold rolling in the next stage is carried out.

When a plurality of cold rolling steps are carried out without carrying out the intermediate annealing step, it may be difficult to obtain uniform characteristics in the manufactured grain-oriented electrical steel sheet. On the other hand, when the cold rolling steps are carried out a plurality of times and the intermediate annealing step is carried out between the cold rolling steps, the magnetic flux density may be low in the manufactured directional electromagnetic steel sheet. Therefore, the number of cold rolling steps and the presence or absence of an intermediate annealing step are determined according to the characteristics and manufacturing cost required for the grain-oriented electrical steel sheet to be finally manufactured.

The preferred cumulative cold rolling rate for one or more cold rollings is 80-95%. Here, the cumulative cold spread rate (%) is defined as follows.
Cold rolling ratio (%) = 1-Thickness of cold-rolled steel sheet after the last cold rolling / Thickness of annealed steel sheet before the start of the first cold rolling x 100

The annealed steel sheet may be pickled before being cold-rolled. The manufactured cold-rolled steel sheet is wound into a coil.

[Decarburization annealing step (S4)]
In the decarburization annealing step (S4), the cold-rolled steel sheet produced in the cold rolling step (S3) is subjected to decarburization annealing to develop primary recrystallization to produce a decarburized annealed steel sheet. The decarburization annealing step (S4) includes a decarburization step (S41) and a decarburization post-cooling step (S42). In the decarburization step (S41), the cold-rolled steel sheet is decarburized to develop primary recrystallization. In the post-decarburization cooling step (S42), the decarburized steel sheet is cooled. Hereinafter, the decarburization step (S41) and the post-decarburization cooling step (S42) will be described.

[Decarburization step (S41)]
In the decarburization step (S41), the cold-rolled steel sheet produced in the cold rolling step (S3) is subjected to decarburization annealing to develop primary recrystallization. Decarburization annealing is carried out, for example, by the following method. The cold-rolled steel sheet is charged into a heat treatment furnace. The temperature of the heat treatment furnace (decarburization annealing temperature) is set to 800 to 950 ° C., and the atmosphere of the heat treatment furnace is set to a moist atmosphere containing hydrogen and nitrogen. By carrying out decarburization annealing, carbon in the steel sheet is removed from the steel sheet, and primary recrystallization develops. The manufacturing conditions for the decarburization annealing process are as follows.

Decarburization annealing temperature: 800-950 ° C
As described above, the decarburization annealing temperature corresponds to the furnace temperature of the heat treatment furnace and corresponds to the temperature of the cold-rolled steel sheet during decarburization annealing. If the decarburization annealing temperature is less than 800 ° C., the crystal grains of the decarburized annealing steel sheet after the development of primary recrystallization are too small. In this case, secondary recrystallization is not sufficiently expressed in the finish annealing step (S6). On the other hand, if the decarburization annealing temperature exceeds 950 ° C., the crystal grains of the decarburized annealing steel sheet after the development of primary recrystallization are too large. In this case as well, secondary recrystallization is not sufficiently expressed in the finish annealing step (S6). When the decarburization annealing temperature is 800 to 950 ° C., the crystal grains of the decarburized annealed steel sheet after the primary recrystallization have an appropriate size, and the secondary recrystallization is sufficiently developed in the finish annealing step (S6).

The holding time at the decarburization annealing temperature in the decarburization annealing step (S4) is not particularly limited, but is, for example, 15 to 150 seconds.

[Cooling step after decarburization (S42)]
In the post-decarburization cooling step (S42), the steel sheet after the decarburization step (S41) is cooled. In the post-decarburization cooling step, the average cooling rate from 900 ° C. to 750 ° C. (that is, the steel sheet temperature is in a specific temperature range) is set to 30 to 150 ° C./sec.

Cooling in a specific temperature range is performed, for example, as follows. A cooling device is arranged after the heat treatment furnace. The steel sheet extracted from the heat treatment furnace is directly charged into the cooling device and continuously cooled. After cooling, the steel sheet is wound into a coil. In the cooling device, the cooling fluid may be injected onto the surface of the steel sheet to cool the steel sheet at the above-mentioned average cooling rate. The cooling fluid is, for example, water or a mixed fluid of water and air. Further, the steel sheet may be cooled by passing the steel sheet through a cooling tank in which the cooling fluid is stored.

The average cooling rate in a specific temperature range can be measured by the following method. The temperature of the steel plate is obtained by measuring the surface temperature of the steel plate with a temperature sensor such as a thermography or a radiation thermometer installed in a cooling device, for example. Based on the measured temperature, the time required for the temperature of the steel sheet to drop from 900 ° C. to 750 ° C. is determined, and the average cooling rate (° C./sec) is determined based on the determined time.

If the average cooling rate of the steel sheet in the specific temperature range is less than 30 ° C./sec, a large amount of CuS precipitates are generated in the steel sheet after the decarburization and cooling step (S42). Since CuS precipitates tend to remain even after finish annealing, CuS precipitates remain in the grain-oriented electrical steel sheet, which deteriorates the magnetic properties. On the other hand, if the average cooling rate exceeds 150 ° C./sec, AlN is unevenly precipitated. Therefore, the magnetic characteristics are deteriorated.

When the average cooling rate of the steel sheet in a specific temperature range is 30 to 150 ° C./sec, the formation of CuS precipitates on the decarburized annealed steel sheet can be sufficiently suppressed.

In the decarburization annealing step (S4), the decarburization annealing steel sheet is manufactured by the above-mentioned decarburization step (S41) and post-decarburization cooling step (S42).

[Nitriding process (S5)]
In the nitriding treatment step (S5), the decarburized annealed steel sheet after the decarburization annealing step (S4) is subjected to a nitriding treatment to produce a nitriding treated steel sheet. The nitriding treatment step (S5) includes a nitriding step (S51) and a post-nitriding cooling step (S52). In the nitriding step (S51), the decarburized annealed steel sheet is nitrided. In the post-nitriding cooling step (S52), the steel sheet after the nitriding step is cooled. Hereinafter, the nitriding step (S51) and the post-nitriding cooling step (S52) will be described.

[Nitriding step (S51)]
In the nitriding step (S51), the nitriding treatment is performed on the decarburized annealed steel sheet after the decarburization annealing step (S4). As a result, finer AlN is deposited in the steel by the time of secondary recrystallization. Preferred nitriding treatment conditions are as follows.

Nitriding temperature: 700-850 ° C
Atmosphere in the nitriding furnace (nitriding atmosphere): Atmosphere containing gases having nitriding ability such as hydrogen, nitrogen and ammonia If the nitriding treatment temperature is less than 700 ° C. or if the nitriding treatment temperature exceeds 850 ° C. In the nitriding process, nitrogen does not easily penetrate into the steel plate. In this case, the amount of nitrogen inside the steel sheet is insufficient in the nitriding step. Therefore, the total amount of precipitation with the amount of fine AlN immediately before the secondary recrystallization is insufficient. As a result, the secondary recrystallization in the finish annealing step (S6) is not sufficiently expressed.

When the nitriding treatment temperature is 700 to 850 ° C., fine AlN before the secondary recrystallization can be sufficiently obtained. Therefore, the total amount of the fine AlN precipitated by the decarburization annealing step (S4) and the fine AlN amount obtained by the nitriding treatment step is a sufficient amount. As a result, secondary recrystallization is sufficiently expressed in the finish annealing step.

The holding time at the nitriding treatment temperature in the nitriding treatment step (S5) is not particularly limited, but is, for example, 10 to 60 seconds.

[Cooling step after nitriding (S52)]
In the post-nitriding cooling step (S52), the steel sheet after the nitriding step (S51) is cooled. In the post-nitriding cooling step, the average cooling rate from 900 ° C. to 750 ° C. (that is, the steel sheet temperature is in a specific temperature range) is set to 30 to 150 ° C./sec.

Cooling in a specific temperature range is performed, for example, as follows. A cooling device is arranged after the nitriding furnace. The steel sheet extracted from the nitriding furnace is directly charged into the cooling device and continuously cooled. After cooling, the steel sheet is wound into a coil. In the cooling device, the cooling fluid may be injected onto the surface of the steel sheet to cool the steel sheet at the above-mentioned average cooling rate. The cooling fluid is, for example, water or a mixed fluid of water and air. Further, the steel sheet may be cooled by passing the steel sheet through a cooling tank in which the cooling fluid is stored.

The average cooling rate in a specific temperature range can be measured by the following method. The temperature of the steel plate is obtained by measuring the surface temperature of the steel plate with a temperature sensor such as a thermography or a radiation thermometer installed in a cooling device, for example. Based on the measured temperature, the time required for the temperature of the steel sheet to drop from 900 ° C. to 750 ° C. is determined, and the average cooling rate (° C./sec) is determined based on the determined time.

If the average cooling rate in the specific temperature range is less than 30 ° C./sec, a large amount of CuS precipitates are generated in the decarburized annealed steel sheet after the decarburized cooling step (S42). Since CuS precipitates tend to remain even after finish annealing, CuS precipitates remain in the grain-oriented electrical steel sheet, which deteriorates the magnetic properties. On the other hand, if the average cooling rate exceeds 150 ° C./sec, AlN is unevenly precipitated. Therefore, the magnetic characteristics are deteriorated.

When the average cooling rate of the steel sheet in a specific temperature range is 30 to 150 ° C./sec, the formation of CuS precipitates on the nitrided steel sheet can be sufficiently suppressed.

In the nitriding steel sheet after the above-mentioned nitriding treatment step (S5) and before the finish annealing step (S6) described later, the amount of CuS precipitates in the steel is 5 ppm or less in mass%. Here, the amount of CuS precipitates in the nitrided steel sheet is measured by the following method.

[Measuring method of CuS precipitate amount in nitrided steel sheet]
First, a method for collecting a sample for chemical analysis used for measuring the mass of CuS precipitate will be described. First, the test material is collected from the central portion of the nitrided steel sheet in the width direction. The oxide film such as scale formed on the surface of the test material is removed by chemical polishing or mechanical polishing treatment. After that, the test material is electrolyzed to separate inclusions and precipitates from the test material and recovered as a residue. This recovered residue is used as a sample.

After collecting the sample, the mass of Cu contained in the sample is quantified. Further, the atomic ratio (Cu / S) of Cu and S is set to 2/1, and the S mass with respect to the quantified Cu mass is obtained. The total mass of S and the mass of Cu obtained are defined as the amount of CuS precipitated substance in the test material. Based on the obtained amount of CuS precipitate substance and the mass of the test material, the content (ppm) of the amount of CuS precipitate in the nitrided steel sheet is determined.

The Cu contained in the above-mentioned sample may contain Cu that is not contained in the precipitate that is strictly determined to be a CuS precipitate based on the crystal structure, composition, etc., but the amount of CuS precipitate in the present invention is calculated. In the case of this, it is converted into the mass of S including such Cu. However, the amount of Cu other than the CuS precipitate in the sample is extremely small.

[Finish annealing process (S6)]
A finish annealing step (S6) is performed on the nitriding steel sheet after the nitriding treatment step (S5). In the finish annealing step (S6), first, an aqueous slurry containing an annealing separator is applied to the surface of the nitrided steel sheet. Then, annealing (finish annealing) is performed on the steel sheet coated with the aqueous slurry.

The aqueous slurry is purified by adding water to an annealing separator described later and stirring the mixture. The annealing separator contains magnesium oxide (MgO). Preferably, MgO is the main component of the annealing separator. Here, the "main component" means that the MgO content in the annealing separator is 60.0% or more in mass%. The annealing separator may contain a well-known additive in addition to MgO.

The finish annealing step (S6) is carried out under the following conditions, for example. Prior to finish annealing, a baking process is performed. First, an annealing separator for an aqueous slurry is applied on the surface of the nitrided steel sheet. A nitrided steel sheet coated with an annealing separator on the surface is wound into a coil. The coiled nitrided steel sheet is placed in a furnace kept at 400 to 1000 ° C. and held (baking treatment). As a result, the annealing separator applied on the surface of the nitrided steel sheet dries. The holding time is, for example, 10 to 90 seconds.

After the annealing separator is dried, finish annealing is performed on the coiled nitrided steel sheet. Finish annealing is carried out using a heat treatment furnace. The manufacturing conditions for finish annealing are, for example, as follows. The atmosphere inside the furnace in finish annealing is a well-known atmosphere.

Finish annealing temperature: 1150 to 1250 ° C
Retention time at finish annealing temperature: 5 to 30 hours If the finish annealing temperature is less than 1150 ° C, sufficient secondary recrystallization does not occur, and purification to remove the precipitate used for secondary recrystallization is sufficient. is not it. Therefore, the magnetic characteristics of the manufactured grain-oriented electrical steel sheet are inferior. On the other hand, even if the finish annealing temperature exceeds 1250 ° C., the effect on secondary recrystallization and purification is low, and problems such as deformation of the steel sheet occur. When the finishing temperature is 1150 to 1250 ° C., sufficient secondary recrystallization is developed and the magnetic properties are enhanced on the premise that the holding time is appropriate. Further, a primary film containing forsterite is soundly formed on the surface of the steel sheet.

By the finish annealing step (S6), each element of the chemical composition of the nitrided steel sheet is removed from the components in the steel to some extent. In particular, S, Al, N and the like that function as inhibitors are largely removed.

By the above manufacturing process, the grain-oriented electrical steel sheet according to the present invention is manufactured. In the manufactured grain-oriented electrical steel sheet, a sufficient amount of fine AlN functioning as an inhibitor is precipitated in the hot-rolled sheet annealing step (S2) and the nitriding treatment step (S5). Further, in the hot-rolled plate annealing step (S2), the formation of CuS precipitates that deteriorate the magnetic properties is suppressed. As a result, a grain-oriented electrical steel sheet having excellent magnetic characteristics and suppressing variations in magnetic characteristics is manufactured.

A primary coating containing forsterite is formed on the surface of the grain-oriented electrical steel sheet after finish annealing.

[Other manufacturing processes]
The method for manufacturing grain-oriented electrical steel sheets according to the present invention may further carry out the following manufacturing steps, if necessary.

[Secondary film forming process]
In the method for producing grain-oriented electrical steel sheets according to the present embodiment, a secondary film forming step may be further carried out after the finish annealing step. In the secondary coating forming step, an insulating coating agent mainly composed of colloidal silica and phosphate is applied to the surface (on the primary coating) of the grain-oriented electrical steel sheet after the temperature of finish annealing is lowered, and then baking is performed. As a result, a secondary coating, which is a tension insulating coating, is formed on the primary coating.

[Magnetic domain subdivision processing process]
The grain-oriented electrical steel sheet according to the present embodiment may be further subjected to a magnetic domain subdivision treatment step after the finish annealing step or the secondary coating forming step. In the magnetic domain subdivision treatment step, the surface of the grain-oriented electrical steel sheet is irradiated with a laser beam having a magnetic domain subdivision effect, or grooves are formed on the surface. In this case, a grain-oriented electrical steel sheet having further excellent magnetic characteristics can be manufactured.

Hereinafter, aspects of the present invention will be specifically described with reference to Examples. These examples are examples for confirming the effect of the present invention, and do not limit the present invention.

[Manufacturing of grain-oriented electrical steel sheets for each test number]
The molten steel having the chemical composition shown in Table 1 was produced in a vacuum melting furnace. A steel material (slab) was manufactured by a continuous casting method using the manufactured molten steel. The F1 value in Table 1 was a calculated value of the left side (F1 = 20 × Ti + Cu) of the equation (1).

The steel material (slab) of each test number shown in Table 2 was heated at the heating temperature (° C.) shown in Table 2. A hot rolling process was carried out on the heated steel material to produce a hot-rolled steel sheet having a plate thickness of 2.3 mm. The finish rolling temperature (° C.) was in the range of 900 to 1000 ° C. in all test numbers. Further, in the post-rolling cooling step, the average cooling rate (° C./sec) from 900 ° C. to 750 ° C. was shown in Table 2.

The hot-rolled steel sheet was annealed by performing a hot-rolled sheet annealing step under the conditions shown in Table 2 to produce a hard-rolled steel sheet. Specifically, the first annealing step was carried out by holding the first holding time (seconds) at the first annealing temperature (° C.) shown in Table 2. After the first annealing step, the second annealing step was carried out by holding the second annealing temperature (° C.) for the second holding time (seconds). After the second annealing step, a cooling step after the second annealing was carried out. In the second post-annealing cooling step, the average cooling rate (° C./sec) in a specific temperature range is as shown in Table 2.

A cold rolling step was carried out on the annealed steel sheet to produce a cold-rolled steel sheet having a sheet thickness of 0.22 mm. In each test number, the cold spread rate was 91 to 95%.

A decarburized annealing step was carried out on the cold-rolled steel sheet to produce a decarburized annealed steel sheet. Specifically, decarburization annealing was carried out at the decarburization annealing temperature (° C.) shown in Table 2. The holding time at the decarburization annealing temperature was 100 seconds for all test numbers. Further, in the cooling step after decarburization, the average cooling rate (° C./sec) from 900 ° C. to 750 ° C. was shown in Table 2.

A nitriding process was carried out on the decarburized annealed steel sheet to produce a nitriding steel sheet. Specifically, the nitriding treatment was carried out at the nitriding treatment temperature (° C.) shown in Table 2. The holding time at the nitriding treatment temperature was 30 seconds for all test numbers. Further, in the post-nitriding cooling step, the average cooling rate (° C./sec) from 900 ° C. to 750 ° C. was shown in Table 2.

A finish annealing step was carried out on the nitrided steel sheet. First, a sintering separator (water slurry) containing MgO as a main component was applied onto the nitrided steel sheet, and a baking treatment was carried out in which the steel sheet was held at 500 ° C. for 10 seconds. Then, at the finish annealing temperature (° C.) shown in Table 2, the holding time (hours) steel sheet shown in Table 2 was held, and finish annealing was performed.

A well-known insulating coating agent mainly composed of colloidal silica and phosphate was applied to the steel sheet after finish annealing, and then a baking treatment was carried out at 900 ° C. for 200 seconds to form a secondary film. ..

Through the above manufacturing process, grain-oriented electrical steel sheets with each test number were manufactured.

[Evaluation test]
[Measurement test of CuS precipitate content after nitriding process]
In each test number, in the nitriding steel sheet after the nitriding process and before the finish annealing process, the test material was collected from the central portion in the width direction of the nitriding steel sheet. The CuS precipitate content (mass%) in the steel was measured by the above-mentioned method using the collected test material.

[Magnetic characterization test]
The magnetic properties of the grain-oriented electrical steel sheets of each test number were evaluated by the following method. Specifically, 15 samples having a length of 300 mm and a width of 60 mm in the rolling direction were collected from the grain-oriented electrical steel sheets of each test number. Specifically, when the width of the grain-oriented electrical steel sheet is defined as t (mm), the pitch is 2000 mm in the rolling direction (distance between the centers of gravity of the sample) at a position of t / 2 in the width direction from the side edge of the grain-oriented electrical steel sheet. ), 5 samples were taken. Further, five samples were taken at a position of t / 4 in the width direction from the side edge of the grain-oriented electrical steel sheet at a pitch of 2000 mm in the rolling direction. Further, five samples were taken at a pitch of 2000 mm in the rolling direction at a position of 3 t / 4 from the side edge of the grain-oriented electrical steel sheet. Through the above sampling steps, 15 samples were sampled from different parts of the grain-oriented electrical steel sheet.

A magnetic field of 800 A / m was applied to the 15 samples collected to determine the magnetic flux density B8 (T). Of the magnetic flux densities obtained from the 15 samples, the highest value was defined as B8max (T), the lowest value was defined as B8min (T), and the average value was defined as B8ave (T). Then, the difference value between the maximum value B8max and the minimum value B8min is defined as the deviation ΔB8 (T). Further, an SST (Single Sheet Tester) test was carried out on the same 15 samples to determine the iron loss W _17/50 (W / kg) at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T.

[Test results]
The obtained test results are shown in Table 2. With reference to Table 2, in Test Nos. 1 to 15, the chemical composition of the steel material was appropriate, and the conditions in each manufacturing process were also appropriate. As a result, the content of CuS precipitates in the nitriding steel sheet after the nitriding treatment step and before the finish annealing step was 5 ppm or less in all the test numbers. As a result, the average magnetic characteristic B8ave was as high as 1.920 T or more, and showed excellent magnetic characteristics. Further, the deviation ΔB8 was 0.015 T or less, and the variation in the magnetic characteristics was suppressed. Further, the iron loss W _17/50 was 0.82 W / kg or less.

On the other hand, in test number 16, the Si content of the steel material was too low. Therefore, the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristics were low.

In test number 17, the C content of the steel material was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 18, the Mn content of the steel material was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 19, the Mn content of the steel material was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 20, the steel material sol. Al content was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 21, the steel material sol. Al content was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 22, the S content of the steel material was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 23, the S content of the steel material was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 24, the P content of the steel material was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 25, the Ti content of the steel material was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 26, the Ti content of the steel material was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 27, the Cu content of the steel material was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 28, the Cu content of the steel material was too high. Therefore, the amount of CuS precipitates in the nitrided steel sheet before the finish annealing step exceeded 5 ppm. As a result, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 29 and test number 30, the F1 value was too high. Therefore, the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristics were low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 31, although the chemical composition of the steel material was appropriate, the average cooling rate of the steel sheet temperature in the specific temperature range (900 to 750 ° C.) was too slow in the post-rolling cooling step of the hot rolling step. Therefore, the content of CuS precipitates in the nitrided steel sheet before the finish annealing step exceeded 5 ppm. As a result, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 32, although the chemical composition of the steel material was appropriate, the average cooling rate of the steel sheet temperature in the specific temperature range (900 to 750 ° C.) was too fast in the post-rolling cooling step of the hot rolling step. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 33, although the chemical composition of the steel material was appropriate, the first annealing temperature in the first annealing step was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 34, although the chemical composition of the steel material was appropriate, the first annealing temperature in the first annealing step was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 35, although the chemical composition of the steel material was appropriate, the first holding time in the first annealing step was too short. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 36, although the chemical composition of the steel material was appropriate, the first holding time in the first annealing step was too long. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 37, although the chemical composition of the steel material was appropriate, the second annealing temperature in the second annealing step was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 38, although the chemical composition of the steel material was appropriate, the second annealing temperature in the second annealing step was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 39, although the chemical composition of the steel material was appropriate, the second holding time in the second annealing step was too short. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 40, although the chemical composition of the steel material was appropriate, the second holding time in the second annealing step was too long. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test Nos. 41 and 42, although the chemical composition of the steel material was appropriate, the average cooling rate in the specific temperature range in the second post-annealing cooling step was too slow. Therefore, the content of CuS precipitates in the nitrided steel sheet before the finish annealing step exceeded 5 ppm in any of the test numbers. As a result, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 43, although the chemical composition of the steel material was appropriate, the average cooling rate in the specific temperature range in the second post-annealing cooling step was too fast. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 44, although the chemical composition of the steel material was appropriate, the decarburization annealing temperature in the decarburization annealing step was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In test number 45, although the chemical composition of the steel material was appropriate, the decarburization annealing temperature in the decarburization annealing step was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 46, although the chemical composition of the steel material was appropriate, the average cooling rate in a specific temperature range in the post-decarburization cooling step of the decarburization annealing step was too slow. Therefore, the content of CuS precipitates in the nitrided steel sheet before the finish annealing step exceeded 5 ppm. As a result, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 47, although the chemical composition of the steel material was appropriate, the average cooling rate in a specific temperature range in the post-decarburization cooling step of the decarburization annealing step was too fast. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 48, although the chemical composition of the steel material was appropriate, the nitriding treatment temperature in the nitriding treatment step was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 49, although the chemical composition of the steel material was appropriate, the nitriding treatment temperature in the nitriding treatment step was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 50, although the chemical composition of the steel material was appropriate, the average cooling rate in the specific temperature range in the post-nitriding cooling step was too slow. Therefore, the content of CuS precipitates in the nitrided steel sheet before the finish annealing step exceeded 5 ppm. As a result, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 51, although the chemical composition of the steel material was appropriate, the average cooling rate in the specific temperature range in the post-nitriding cooling step was too fast. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 52, although the chemical composition of the steel material was appropriate, the finish annealing temperature in the finish annealing step was too low. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015 T, and the variation in magnetic characteristics was large.

In Test No. 53, although the chemical composition of the steel material was appropriate, the finish annealing temperature in the finish annealing step was too high. Therefore, the average magnetic characteristic B8ave was less than 1.920 T, and the iron loss W _17/50 exceeded 0.82 W / kg, and the magnetic characteristic was low. Further, the deviation ΔB8 exceeded 0.015T, and the variation in magnetic characteristics was large.

The embodiments of the present invention have been described above. However, the embodiments described above are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be appropriately modified and implemented within a range that does not deviate from the gist thereof.

Claims (3)

By mass%
Si: 2.5-4.0%,
C: 0.010 to 0.060%,
Mn: 0.05 to 0.20%,
sol. Al: 0.020 to 0.040%,
N: 0.002 to 0.012%,
S: 0.001 0 ~0.010%,
P: 0.01 0 to 0.08%,
Ti: 0.0028-0.010%,
Cu: 0.010 to 0.50%,
Cr: 0-0.20%,
Sn: 0 to 0.20%,
Sb: 0 to 0.20%,
Ni: 0-0.20%,
Se: 0-0.020%,
Bi: 0-0.020%,
Pb: 0 to 0.020%,
B: 0 to 0.020%,
V: 0 to 0.020%,
Mo: 0-0.020%,
As: 0 to 0.020% and
A rolling process in which a steel material having a chemical composition satisfying the formula (1), which is composed of Fe and impurities and has a chemical composition, is hot-rolled to form a steel plate, and a temperature range of 900 to 750 ° C. is defined as a "specific temperature range". When defined , heat includes a post-rolling cooling step in which the steel sheet after the rolling step is cooled to obtain a hot-rolled steel sheet and the average cooling rate of the steel sheet in the specific temperature range is 30 to 150 ° C./sec. Inter-rolling process and
After the first annealing step of holding the hot-rolled steel sheet at 1080-1200 ° C. for 70 to 120 seconds and the first annealing step, the hot-rolled steel sheet is cooled to 900 to 980 ° C., and the hot-rolled steel sheet is 900 to 980. After the second annealing step of holding at ° C. for 30 to 450 seconds and the second annealing step, the hot-rolled steel sheet is cooled to obtain an annealed steel sheet, and the average cooling rate of the hot-rolled steel sheet in the specific temperature range is 30 to 30 to. A hot-rolled plate annealing step, including a second post-annealing cooling step at 150 ° C./sec,
A cold rolling process for producing a cold-rolled steel sheet by performing cold rolling on the annealed steel sheet,
A decarburization step of decarburizing and annealing the cold-rolled steel sheet at 800 to 950 ° C. and a decarburization-annealed steel sheet by cooling the cold-rolled steel sheet after the decarburization step are obtained in the specific temperature range of the cold-rolled steel sheet. A decarburization annealing step including a post-decarburization cooling step with an average cooling rate of 30 to 150 ° C./sec.
The nitriding step of performing the nitriding treatment on the decarburized annealed steel sheet at 700 to 850 ° C. and the cooling of the decarburized annealed steel sheet after the nitriding step are carried out so that the CuS precipitate in the steel is 5 ppm or less in mass%. A nitriding step, which comprises a post-nitriding cooling step of nitriding a steel sheet and setting the average cooling rate of the decarburized annealed steel in the specific temperature range to 30 to 150 ° C./sec.
An annealing separator containing MgO is applied to the surface of the nitrided steel sheet, and the nitrided steel sheet coated with the annealing separator is subjected to finish annealing at 1150 to 1250 ° C. for 5 to 30 hours. Annealing process and
A method for manufacturing a grain-oriented electrical steel sheet.
20 × Ti + Cu ≦ 0.18 Equation (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). The method for manufacturing a grain-oriented electrical steel sheet according to claim 1.
The chemical composition of the steel material is
Cr: 0.01 to 0.20%, and
Sn: A method for producing a grain-oriented electrical steel sheet, which contains at least one selected from the group consisting of 0.01 to 0.20%. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2.
The chemical composition is
Sb: 0.01 to 0.20%,
Ni: 0.01 to 0.20%,
Se: 0.005 to 0.020%,
Bi: 0.005 to 0.020%,
Pb: 0.005 to 0.020%,
B: 0.005 to 0.020%,
V: 0.005 to 0.020%,
Mo: 0.005 to 0.020% and
As: A method for producing a grain-oriented electrical steel sheet, which contains one or more selected from the group consisting of 0.005 to 0.020%.

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