# Patent

## Title
Manufacturing method of grain-oriented electrical steel sheet

## Abstract

A silicon steel material is heated in a predetermined temperature range according to contents of B, N, Mn, S, and Se (step S1), and is subjected to hot rolling (step S2). Further, a finish temperature Tf of finish rolling in the hot rolling is performed in a predetermined temperature range according to the content of B. Through these treatments, a certain amount of BN is made to precipitate compositely on MnS and/or MnSe.

## Background
TECHNICAL FIELD
The present invention relates to a manufacturing method of a grain-oriented electrical steel sheet suitable for an iron core or the like of an electrical apparatus.
BACKGROUND ART
A grain-oriented electrical steel sheet is a soft magnetic material, and is used for an iron core or the like of an electrical apparatus such as a transformer. In the grain-oriented electrical steel sheet, Si of about 7 mass % or less is contained. Crystal grains of the grain-oriented electrical steel sheet are highly integrated in the {110}<001> orientation by Miller indices. The orientation of the crystal grains is controlled by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.
For controlling the secondary recrystallization, it is important to adjust a structure (primary recrystallization structure) obtained by primary recrystallization before the secondary recrystallization and to adjust a fine precipitate called an inhibitor or a grain boundary segregation element. The inhibitor has a function to preferentially grow, in the primary recrystallization structure, the crystal grains in the {110}<001> orientation and suppress growth of the other crystal grains.
Then, conventionally, there have been made various proposals aimed at precipitating an inhibitor effectively.
However, in conventional techniques, it has been difficult to manufacture a grain-oriented electrical steel sheet having a high magnetic flux density industrially stably.
CITATION LIST
Patent Literature


    Patent Literature 1: Japanese Examined Patent Application Publication No. 30-003651
    Patent Literature 2: Japanese Examined Patent Application Publication No. 33-004710
    Patent Literature 3: Japanese Examined Patent Application Publication No. 51-013469
    Patent Literature 4: Japanese Examined Patent Application Publication No. 62-045285
    Patent Literature 5: Japanese Laid-open Patent Publication No. 03-002324
    Patent Literature 6: U.S. Pat. No. 3,905,842
    Patent Literature 7: U.S. Pat. No. 3,905,843
    Patent Literature 8: Japanese Laid-open Patent Publication No. 01-230721
    Patent Literature 9: Japanese Laid-open Patent Publication No. 01-283324
    Patent Literature 10: Japanese Laid-open Patent Publication No. 10-140243
    Patent Literature 11: Japanese Laid-open Patent Publication No. 2001-152250
    Patent Literature 12: Japanese Laid-open Patent Publication No. 2-258929


Non-Patent Literature


    Non-Patent Literature 1: Trans. Met. Soc. AIME, 212 (1958) p 769/781
    Non-Patent Literature 2: Journal of The Japan Institute of Metals 27 (1963) p 186
    Non-Patent Literature 3: Testu-to-Hagane 53 (1967) p 1007/1023
    Non-Patent Literature 4: Journal of The Japan Institute of Metals 43 (1979) p 175/181, Journal of The Japan Institute of Metals 44 (1980) p 419/424
    Non-Patent Literature 5: Materials Science Forum 204-206 (1996) p 593/598
    Non-Patent Literature 6: IEEE Trans. Mag. MAG-13 p 1427


SUMMARY OF THE INVENTION
Technical Problem
The present invention has an object to provide a manufacturing method of a grain-oriented electrical steel sheet capable of manufacturing a grain-oriented electrical steel sheet having a high magnetic flux density industrially stably.
Solution to Problem
A manufacturing method of a grain-oriented electrical steel sheet according to a first aspect of the present invention includes: at a predetermined temperature, heating a silicon steel material containing Si: 0.8 mass % to 7 mass %, acid-soluble Al: 0.01 mass % to 0.065 mass %, N: 0.004 mass % to 0.012 mass %, Mn: 0.05 mass % to 1 mass %, and B: 0.0005 mass % to 0.0080 mass %, the silicon steel material further containing at least one element selected from a group consisting of S and Se being 0.003 mass % to 0.015 mass % in total amount, a C content being 0.085 mass % or less, and a balance being composed of Fe and inevitable impurities; hot rolling the heated silicon steel material so as to obtain a hot-rolled steel strip; annealing the hot-rolled steel strip so as to obtain an annealed steel strip; cold rolling the annealed steel strip one time or more so as to obtain a cold-rolled steel strip; decarburization annealing the cold-rolled steel strip so as to obtain a decarburization-annealed steel strip in which primary recrystallization is caused; coating an annealing separating agent containing MgO as its main component on the decarburization-annealed steel strip; and causing secondary recrystallization by finish annealing the decarburization-annealed steel strip, wherein the method further comprises performing a nitriding treatment in which an N content of the decarburization-annealed steel strip is increased between start of the decarburization annealing and occurrence of the secondary recrystallization in the finish annealing, the predetermined temperature is, in a case when S and Se are contained in the silicon steel material, a temperature T1 (° C.) or lower, a temperature T2 (° C.) or lower, and a temperature T3 (° C.) or lower, the temperature T1 being expressed by equation (1) below, the temperature T2 being expressed by equation (2) below, and the temperature T3 being expressed by equation (3) below, in a case when no Se is contained in the silicon steel material, the temperature T1 (° C.) or lower, and the temperature T3 (° C.) or lower, in a case when no S is contained in the silicon steel material, the temperature T2 (° C.) or lower, and the temperature T3 (° C.) or lower, a finish temperature Tf of finish rolling in the hot rolling satisfies inequation (4) below, and amounts of BN, MnS, and MnSe in the hot-rolled steel strip satisfy inequations (5), (6), and (7) below.

T1=14855/(6.82−log([Mn]×[S]))−273  (1)

T2=10733/(4.08−log([Mn]×[Se]))−273  (2)

T3=16000/(5.92−log([B]×[N]))−273  (3)

Tf≦1000−10000×[B]  (4)

BasBN≧0.0005  (5)

[B]−BasBN≦0.001  (6)

SasMnS+0.5×SeasMnSe≧0.002  (7)

Here, [Mn] represents a Mn content (mass %) of the silicon steel material, [S] represents an S content (mass %) of the silicon steel material, [Se] represents a Se content (mass %) of the silicon steel material, [B] represents a B content (mass %) of the silicon steel material, [N] represents an N content (mass %) of the silicon steel material, BasBN represents an amount of B (mass %) that has precipitated as BN in the hot-rolled steel strip, SasMnS represents an amount of S (mass %) that has precipitated as MnS in the hot-rolled steel strip, and SeasMnSe represents an amount of Se (mass %) that has precipitated as MnSe in the hot-rolled steel strip.
In a manufacturing method of a grain-oriented electrical steel sheet according to a second aspect of the present invention, in the method according to the first aspect, the nitriding treatment is performed under a condition that an N content [N] of a steel strip obtained after the nitriding treatment satisfies inequation (8) below.

[N]≧14/27[Al]+14/11[B]+14/47[Ti]  (8)

Here, [N] represents the N content (mass %) of the steel strip obtained after the nitriding treatment, [Al] represents an acid-soluble Al content (mass %) of the steel strip obtained after the nitriding treatment, and [Ti] represents a Ti content (mass %) of the steel strip obtained after the nitriding treatment.
In a manufacturing method of a grain-oriented electrical steel sheet according to a third aspect of the present invention, in the method according to the first aspect, the nitriding treatment is performed under a condition that an N content [N] of a steel strip obtained after the nitriding treatment satisfies inequation (9) below.

[N]≧2/3[Al]+14/11[B]+14/47[Ti]  (9)

Here, [N] represents the N content (mass %) of the steel strip obtained after the nitriding treatment, [Al] represents an acid-soluble Al content (mass %) of the steel strip obtained after the nitriding treatment, and [Ti] represents a Ti content (mass %) of the steel strip obtained after the nitriding treatment.
Advantageous Effects of Invention
According to the present invention, it is possible to make BN precipitate compositely on MnS and/or MnSe appropriately and to form appropriate inhibitors, so that a high magnetic flux density can be obtained. Further, these processes can be executed industrially stably.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a flow chart showing a manufacturing method of a grain-oriented electrical steel sheet;
FIG. 2 is a view showing a result of a first experiment (a relationship between precipitates in a hot-rolled steel strip and a magnetic property after finish annealing);
FIG. 3 is a view showing the result of the first experiment (a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing);
FIG. 4 is a view showing the result of the first experiment (a relationship between a Mn content, a condition of hot rolling, and the magnetic property after the finish annealing);
FIG. 5 is a view showing the result of the first experiment (a relationship between a B content, the condition of the hot rolling, and the magnetic property after the finish annealing);
FIG. 6 is a view showing the result of the first experiment (a relationship between a condition of finish rolling and the magnetic property after the finish annealing);
FIG. 7 is a view showing a result of a second experiment (a relationship between precipitates in a hot-rolled steel strip and a magnetic property after finish annealing);
FIG. 8 is a view showing the result of the second experiment (a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing);
FIG. 9 is a view showing the result of the second experiment (a relationship between a Mn content, a condition of hot rolling, and the magnetic property after the finish annealing);
FIG. 10 is a view showing the result of the second experiment (a relationship between a B content, the condition of the hot rolling, and the magnetic property after the finish annealing);
FIG. 11 is a view showing the result of the second experiment (a relationship between a condition of finish rolling and the magnetic property after the finish annealing);
FIG. 12 is a view showing a result of a third experiment (a relationship between precipitates in a hot-rolled steel strip and a magnetic property after finish annealing);
FIG. 13 is a view showing the result of the third experiment (a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing);
FIG. 14 is a view showing the result of the third experiment (a relationship between a Mn content, a condition of hot rolling, and the magnetic property after the finish annealing);
FIG. 15 is a view showing the result of the third experiment (a relationship between a B content, the condition of the hot rolling, and the magnetic property after the finish annealing); and
FIG. 16 is a view showing the result of the third experiment (a relationship between a condition of finish rolling and the magnetic property after the finish annealing).



DESCRIPTION OF EMBODIMENTS
The present inventors thought that in the case of manufacturing a grain-oriented electrical steel sheet from a silicon steel material having a predetermined composition containing B, a precipitated form of B may affect behavior of secondary recrystallization, and thus conducted various experiments. Here, an outline of a manufacturing method of a grain-oriented electrical steel sheet will be explained. FIG. 1 is a flow chart showing the manufacturing method of the grain-oriented electrical steel sheet.
First, as illustrated in FIG. 1, in step S1, a silicon steel material (slab) having a predetermined composition containing B is heated to a predetermined temperature, and in step S2, hot rolling of the heated silicon steel material is performed. By the hot rolling, a hot-rolled steel strip is obtained. Thereafter, in step S3, annealing of the hot-rolled steel strip is performed to normalize a structure in the hot-rolled steel strip and to adjust precipitation of inhibitors. By the annealing, an annealed steel strip is obtained. Subsequently, in step S4, cold rolling of the annealed steel strip is performed. The cold rolling may be performed only one time, or may also be performed a plurality of times with intermediate annealing being performed therebetween. By the cold rolling, a cold-rolled steel strip is obtained. Incidentally, in the case of the intermediate annealing being performed, it is also possible to omit the annealing of the hot-rolled steel strip before the cold rolling to perform the annealing (step S3) in the intermediate annealing. That is, the annealing (step S3) may be performed on the hot-rolled steel strip, or may also be performed on a steel strip obtained after being cold rolled one time and before being cold rolled finally.
After the cold rolling, in step S5, decarburization annealing of the cold-rolled steel strip is performed. In the decarburization annealing, primary recrystallization occurs. Further, by the decarburization annealing, a decarburization-annealed steel strip is obtained. Next, in step S6, an annealing separating agent containing MgO (magnesia) as its main component is coated on the surface of the decarburization-annealed steel strip and finish annealing is performed. In the finish annealing, secondary recrystallization occurs, and a glass film containing forsterite as its main component is formed on the surface of the steel strip and is purified. As a result of the secondary recrystallization, a secondary recrystallization structure arranged in the Goss orientation is obtained. By the finish annealing, a finish-annealed steel strip is obtained. Further, between start of the decarburization annealing and occurrence of the secondary recrystallization in the finish annealing, a nitriding treatment in which a nitrogen amount of the steel strip is increased is performed (step S7).
In this manner, the grain-oriented electrical steel sheet can be obtained.
Further, details will be described later, but as the silicon steel material, there is used one containing Si: 0.8 mass % to 7 mass %, acid-soluble Al: 0.01 mass % to 0.065 mass %, N: 0.004 mass % to 0.012 mass %, and Mn: 0.05 mass % to 1 mass %, and further containing predetermined amounts of S and/or Se, and B, a C content being 0.085 mass % or less, and a balance being composed of Fe and inevitable impurities.
Then, as a result of the various experiments, the present inventors found that it is important to adjust conditions of slab heating (step S1) and the hot rolling (step S2) to then generate precipitates in a form effective as inhibitors in the hot-rolled steel strip. Concretely, the present inventors found that when B in the silicon steel material precipitates mainly as BN precipitates compositely on MnS and/or MnSe by adjusting the conditions of the slab heating and the hot rolling, the inhibitors are thermally stabilized and grains of a grain structure of the primary recrystallization are homogeneously arranged. Then, the present inventors obtained the knowledge capable of manufacturing the grain-oriented electrical steel sheet having a good magnetic property stably, and completed the present invention.
Here, the experiments conducted by the present inventors will be explained.
First Experiment
In the first experiment, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.05 mass % to 0.19 mass %, S: 0.007 mass %, and B: 0.0010 mass % to 0.0035 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1100° C. to 1250° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1000° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a speed of 15° C./s, and were subjected to decarburization annealing at a temperature of 840° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between precipitates in the hot-rolled steel strip and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 2. In FIG. 2, the horizontal axis indicates a value (mass %) obtained by converting a precipitation amount of MnS into an amount of S, and the vertical axis indicates a value (mass %) obtained by converting a precipitation amount of BN into B. The horizontal axis corresponds to an amount of S that has precipitated as MnS (mass %). Further, white circles each indicate that a magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 2, in the samples each having the precipitation amounts of MnS and BN each being less than a certain value, the magnetic flux density B8 was low. This indicates that secondary recrystallization was unstable.
Further, a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 3. In FIG. 3, the horizontal axis indicates a B content (mass %), and the vertical axis indicates the value (mass %) obtained by converting the precipitation amount of BN into B. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 3, in the samples each having the amount of B that has not precipitated as BN being a certain value or more, the magnetic flux density B8 was low. This indicates that the secondary recrystallization was unstable.
Further, as a result of examination of a form of the precipitates in the samples each having the good magnetic property, it turned out that MnS becomes a nucleus and BN precipitates compositely on MnS. Such composite precipitates are effective as inhibitors that stabilize the secondary recrystallization.
Further, a relationship between a condition of the hot rolling and the magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 4 and FIG. 5. In FIG. 4, the horizontal axis indicates a Mn content (mass %) and the vertical axis indicates a temperature (° C.) of slab heating at the time of hot rolling. In FIG. 5, the horizontal axis indicates the B content (mass %) and the vertical axis indicates the temperature (° C.) of the slab heating at the time of hot rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. Further, a curve in FIG. 4 indicates a solution temperature T1 (° C.) of MnS expressed by equation (1) below, and a curve in FIG. 5 indicates a solution temperature T3 (° C.) of BN expressed by equation (3) below. As illustrated in FIG. 4, it turned out that in the samples in which the slab heating is performed at a temperature determined according to the Mn content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T1 of MnS. Further, as illustrated in FIG. 5, it also turned out that in the samples in which the slab heating is performed at a temperature determined according to the B content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T3 of BN. That is, it turned out that it is effective to perform the slab heating in a temperature zone where MnS and BN are not completely solid-dissolved.

T1=14855/(6.82−log([Mn]×[S]))−273  (1)

T3=16000/(5.92−log([B]×[N]))−273  (3)

Here, [Mn] represents the Mn content (mass %), [S] represents an S content (mass %), [B] represents the B content (mass %), and [N] represents an N content (mass %).
Further, as a result of examination of precipitation behavior of BN, it turned out that a precipitation temperature zone of BN is 800° C. to 1000° C.
Further, the present inventors examined a finish temperature of the finish rolling in the hot rolling. Generally, in the finish rolling of the hot rolling, the rolling is performed a plurality of times and thereby a hot-rolled steel strip having a predetermined thickness is obtained. Here, the finish temperature of the finish rolling means the temperature of the hot-rolled steel strip after the final rolling among a plurality of times of rolling. In the examination, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.007 mass %, and B: 0.001 mass % to 0.004 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1150° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1020° C. to 900° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a rate of 15° C./s, and were subjected to decarburization annealing at a temperature of 840° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between the finish temperature of the finish rolling in the hot rolling and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 6. In FIG. 6, the horizontal axis indicates a B content (mass %), and the vertical axis indicates a finish temperature Tf of the finish rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.91 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.91 T. As illustrated in FIG. 6, it turned out that when the finish temperature Tf of the finish rolling satisfies inequation (4) below, the high magnetic flux density B8 is obtained. This is conceivably because by controlling the finish temperature Tf of the finish rolling, the precipitation of BN was further promoted.

Tf≦1000−10000×[B]  (4)

Second Experiment
In the second experiment, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.007 mass %, Mn: 0.05 mass % to 0.20 mass %, Se: 0.007 mass %, and B: 0.0010 mass % to 0.0035 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1100° C. to 1250° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1000° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a rate of 15° C./s, and were subjected to decarburization annealing at a temperature of 850° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between precipitates in the hot-rolled steel strip and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 7. In FIG. 7, the horizontal axis indicates a value (mass %) obtained by converting a precipitation amount of MnSe into an amount of Se, and the vertical axis indicates a value (mass %) obtained by converting a precipitation amount of BN into B. The horizontal axis corresponds to an amount of Se that has precipitated as MnSe (mass %). Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 7, in the samples each having the precipitation amounts of MnSe and BN each being less than a certain value, the magnetic flux density B8 was low. This indicates that secondary recrystallization was unstable.
Further, a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 8. In FIG. 8, the horizontal axis indicates a B content (mass %), and the vertical axis indicates the value (mass %) obtained by converting the precipitation amount of BN into B. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 8, in the samples each having the amount of B that has not precipitated as BN being a certain value or more, the magnetic flux density B8 was low. This indicates that the secondary recrystallization was unstable.
Further, as a result of examination of a form of the precipitates in the samples each having the good magnetic property, it turned out that MnSe becomes a nucleus and BN precipitates compositely on MnSe. Such composite precipitates are effective as inhibitors that stabilize the secondary recrystallization.
Further, a relationship between a condition of the hot rolling and the magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 9 and FIG. 10. In FIG. 9, the horizontal axis indicates a Mn content (mass %) and the vertical axis indicates a temperature (° C.) of slab heating at the time of hot rolling. In FIG. 10, the horizontal axis indicates the B content (mass %) and the vertical axis indicates the temperature (° C.) of the slab heating at the time of hot rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. Further, a curve in FIG. 9 indicates a solution temperature T2 (° C.) of MnSe expressed by equation (2) below, and a curve in FIG. 10 indicates the solution temperature T3 (° C.) of BN expressed by equation (3). As illustrated in FIG. 9, it turned out that in the samples in which the slab heating is performed at a temperature determined according to the Mn content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T2 of MnSe. Further, as illustrated in FIG. 10, it also turned out that in the samples in which the slab heating is performed at a temperature determined according to the B content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T3 of BN. That is, it turned out that it is effective to perform the slab heating in a temperature zone where MnSe and BN are not completely solid-dissolved.

T2=10733/(4.08−log([Mn]×[Se]))−273  (2)

Here, [Se] represents a Se content (mass %).
Further, as a result of examination of precipitation behavior of BN, it turned out that a precipitation temperature zone of BN is 800° C. to 1000° C.
Further, the present inventors examined a finish temperature of the finish rolling in the hot rolling. In the examination, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.007 mass %, Mn: 0.1 mass %, Se: 0.007 mass %, and B: 0.001 mass % to 0.004 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1150° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1020° C. to 900° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a rate of 15° C./s, and were subjected to decarburization annealing at a temperature of 850° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between the finish temperature of the finish rolling in the hot rolling and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 11. In FIG. 11, the horizontal axis indicates a B content (mass %), and the vertical axis indicates the finish temperature Tf of the finish rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.91 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.91 T. As illustrated in FIG. 11, it turned out that when the finish temperature Tf of the finish rolling satisfies inequation (4), the high magnetic flux density B8 is obtained. This is conceivably because by controlling the finish temperature Tf of the finish rolling, the precipitation of BN was further promoted.
Third Experiment
In the third experiment, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.026 mass %, N: 0.009 mass %, Mn: 0.05 mass % to 0.20 mass %, S: 0.005 mass %, Se: 0.007 mass %, and B: 0.0010 mass % to 0.0035 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1100° C. to 1250° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1000° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a rate of 15° C./s, and were subjected to decarburization annealing at a temperature of 850° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.021 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between precipitates in the hot-rolled steel strip and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 12. In FIG. 12, the horizontal axis indicates the sum (mass %) of a value obtained by converting a precipitation amount of MnS into an amount of S and a value obtained by multiplying a value obtained by converting a precipitation amount of MnSe into an amount of Se by 0.5, and the vertical axis indicates a value (mass %) obtained by converting a precipitation amount of BN into B. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 12, in the samples each having the precipitation amounts of MnS, MnSe, and BN each being less than a certain value, the magnetic flux density B8 was low. This indicates that secondary recrystallization was unstable.
Further, a relationship between an amount of B that has not precipitated as BN and the magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 13. In FIG. 13, the horizontal axis indicates a B content (mass %), and the vertical axis indicates the value (mass %) obtained by converting the precipitation amount of BN into B. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. As illustrated in FIG. 13, in the samples each having the amount of B that has not precipitated as BN being a certain value or more, the magnetic flux density B8 was low. This indicates that the secondary recrystallization was unstable.
Further, as a result of examination of a form of the precipitates in the samples each having the good magnetic property, it turned out that MnS or MnSe becomes a nucleus and BN precipitates compositely on MnS or MnSe. Such composite precipitates are effective as inhibitors that stabilize the secondary recrystallization.
Further, a relationship between a condition of the hot rolling and the magnetic property after the finish annealing was examined A result of the examination is illustrated in FIG. 14 and FIG. 15. In FIG. 14, the horizontal axis indicates a Mn content (mass %) and the vertical axis indicates a temperature (° C.) of slab heating at the time of hot rolling. In FIG. 15, the horizontal axis indicates the B content (mass %) and the vertical axis indicates the temperature (° C.) of the slab heating at the time of hot rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.88 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.88 T. Further, two curves in FIG. 14 indicate the solution temperature T1 (° C.) of MnS expressed by equation (1) and the solution temperature T2 (° C.) of MnSe expressed by equation (2), and a curve in FIG. 15 indicates the solution temperature T3 (° C.) of BN expressed by equation (3). As illustrated in FIG. 14, it turned out that in the samples in which the slab heating is performed at a temperature determined according to the Mn content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T1 of MnS and the solution temperature T2 of MnSe. Further, as illustrated in FIG. 15, it also turned out that in the samples in which the slab heating is performed at a temperature determined according to the B content or lower, the high magnetic flux density B8 is obtained. Further, it also turned out that the temperature approximately agrees with the solution temperature T3 of BN. That is, it turned out that it is effective to perform the slab heating in a temperature zone where MnS, MnSe, and BN are not completely solid-dissolved.
Further, as a result of examination of precipitation behavior of BN, it turned out that a precipitation temperature zone of BN is 800° C. to 1000° C.
Further, the present inventors examined a finish temperature of the finish rolling in the hot rolling. In the examination, first, various silicon steel slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.026 mass %, N: 0.009 mass %, Mn: 0.1 mass %, S: 0.005 mass %, Se: 0.007 mass %, and B: 0.001 mass % to 0.004 mass %, and a balance being composed of Fe and inevitable impurities were obtained. Next, the silicon steel slabs were heated at a temperature of 1150° C. and were subjected to hot rolling. In the hot rolling, rough rolling was performed at 1050° C. and then finish rolling was performed at 1020° C. to 900° C., and thereby hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Then, cooling water was jetted onto the hot-rolled steel strips to then let the hot-rolled steel strips cool down to 550° C., and thereafter the hot-rolled steel strips were cooled down in the atmosphere. Subsequently, annealing of the hot-rolled steel strips was performed. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, the cold-rolled steel strips were heated at a rate of 15° C./s, and were subjected to decarburization annealing at a temperature of 850° C., and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.021 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips and finish annealing was performed. In this manner, various samples were manufactured.
Then, a relationship between the finish temperature of the finish rolling in the hot rolling and a magnetic property after the finish annealing was examined. A result of the examination is illustrated in FIG. 16. In FIG. 16, the horizontal axis indicates a B content (mass %), and the vertical axis indicates the finish temperature Tf of the finish rolling. Further, white circles each indicate that the magnetic flux density B8 was 1.91 T or more, and black squares each indicate that the magnetic flux density B8 was less than 1.91 T. As illustrated in FIG. 16, it turned out that when the finish temperature Tf of the finish rolling satisfies inequation (4), the high magnetic flux density B8 is obtained. This is conceivably because by controlling the finish temperature Tf of the finish rolling, the precipitation of BN was further promoted.
According to these results of the first to third experiments, it is found that controlling the precipitated form of BN makes it possible to stably improve the magnetic property of the grain-oriented electrical steel sheet. The reason why the secondary recrystallization becomes unstable, thereby making it impossible to obtain the good magnetic property in the case when B does not precipitate compositely on MnS or MnSe as BN has not been clarified yet so for, but is considered as follows.
Generally, B in a solid solution state is likely to segregate in grain boundaries, and BN that has precipitated independently after the hot rolling is often fine. B in a solid solution state and fine BN suppress grain growth at the time of primary recrystallization as strong inhibitors in a low-temperature zone where the decarburization annealing is performed, and in a high-temperature zone where the finish annealing is performed, B in a solid solution state and fine BN do not function as inhibitors locally, thereby turning the grain structure into a mixed grain structure with coarse grains. Thus, in the low-temperature zone, primary recrystallized grains are small, so that the magnetic flux density of the grain-oriented electrical steel sheet is reduced. Further, in the high-temperature zone, the grain structure is turned into the mixed grain structure with coarse grains, so that the secondary recrystallization becomes unstable.
Next, an embodiment of the present invention made on the knowledge will be explained.
First, limitation reasons of the components of the silicon steel material will be explained.
The silicon steel material used in this embodiment contains Si: 0.8 mass % to 7 mass %, acid-soluble Al: 0.01 mass % to 0.065 mass %, N: 0.004 mass % to 0.012 mass %, Mn: 0.05 mass % to 1 mass %, S and Se: 0.003 mass % to 0.015 mass % in total amount, and B: 0.0005 mass % to 0.0080 mass %, and a C content being 0.085 mass % or less, and a balance being composed of Fe and inevitable impurities.
Si increases electrical resistance to reduce a core loss. However, when a Si content exceeds 7 mass %, the cold rolling becomes difficult to be performed, and a crack is likely to be caused at the time of cold rolling. Thus, the Si content is set to 7 mass % or less, and is preferably 4.5 mass % or less, and is more preferably 4 mass % or less. Further, when the Si content is less than 0.8 mass %, a γ transformation is caused at the time of finish annealing to thereby make a crystal orientation of the grain-oriented electrical steel sheet deteriorate. Thus, the Si content is set to 0.8 mass % or more, and is preferably 2 mass % or more, and is more preferably 2.5 mass % or more.
C is an element effective for controlling the primary recrystallization structure, but adversely affects the magnetic property. Thus, in this embodiment, before the finish annealing (step S6), the decarburization annealing is performed (step S5). However, when the C content exceeds 0.085 mass %, a time taken for the decarburization annealing becomes long, and productivity in industrial production is impaired. Thus, the C content is set to 0.85 mass % or less, and is preferably 0.07 mass % or less.
Acid-soluble Al bonds to N to precipitate as (Al, Si)N and functions as an inhibitor. In the case when a content of acid-soluble Al falls within a range of 0.01 mass % to 0.065 mass %, the secondary recrystallization is stabilized. Thus, the content of acid-soluble Al is set to be not less than 0.01 mass % nor more than 0.065 mass %. Further, the content of acid-soluble Al is preferably 0.02 mass % or more, and is more preferably 0.025 mass % or more. Further, the content of acid-soluble Al is preferably 0.04 mass % or less, and is more preferably 0.03 mass % or less.
B bonds to N to precipitate compositely on MnS or MnSe as BN and functions as an inhibitor. In the case when a B content falls within a range of 0.0005 mass % to 0.0080 mass %, the secondary recrystallization is stabilized. Thus, the B content is set to be not less than 0.0005 mass % nor more than 0.0080 mass %. Further, the B content is preferably 0.001% or more, and is more preferably 0.0015% or more. Further, the B content is preferably 0.0040% or less, and is more preferably 0.0030% or less.
N bonds to B or Al to function as an inhibitor. When an N content is less than 0.004 mass %, it is not possible to obtain a sufficient amount of the inhibitor. Thus, the N content is set to 0.004 mass % or more, and is preferably 0.006 mass % or more, and is more preferably 0.007 mass % or more. On the other hand, when the N content exceeds 0.012 mass %, a hole called a blister occurs in the steel strip at the time of cold rolling. Thus, the N content is set to 0.012 mass % or less, and is preferably 0.010 mass % or less, and is more preferably 0.009 mass % or less.
Mn, S and Se produce MnS and MnSe to be a nucleus on which BN precipitates compositely, and composite precipitates function as an inhibitor. In the case when a Mn content falls within a range of 0.05 mass % to 1 mass %, the secondary recrystallization is stabilized. Thus, the Mn content is set to be not less than 0.05 mass % nor more than 1 mass %. Further, the Mn content is preferably 0.08 mass % or more, and is more preferably 0.09 mass % or more. Further, the Mn content is preferably 0.50 mass % or less, and is more preferably 0.2 mass % or less.
Further, in the case when a content of S and Se falls within a range of 0.003 mass % to 0.015 mass % in total amount, the secondary recrystallization is stabilized. Thus, the content of S and Se is set to be not less than 0.003 mass % nor more than 0.015 mass % in total amount. Further, in terms of preventing occurrence of a crack in the hot rolling, inequation (10) below is preferably satisfied. Incidentally, only either S or Se may be contained in the silicon steel material, or both S and Se may also be contained in the silicon steel material. In the case when both S and Se are contained, it is possible to promote the precipitation of BN more stably and to improve the magnetic property stably.

[Mn]/([S]+[Se])≧4  (10)

Ti forms coarse TiN to affect the precipitation amounts of BN and (Al, Si)N functioning as an inhibitor. When a Ti content exceeds 0.004 mass %, the good magnetic property is not easily obtained. Thus, the Ti content is preferably 0.004 mass % or less.
Further, one or more element(s) selected from a group consisting of Cr, Cu, Ni, P, Mo, Sn, Sb, and Bi may also be contained in the silicon steel material in ranges below.
Cr improves an oxide layer formed at the time of decarburization annealing, and is effective for forming the glass film made by reaction of the oxide layer and MgO being the main component of the annealing separating agent at the time of finish annealing. However, when a Cr content exceeds 0.3 mass %, decarburization is noticeably prevented. Thus, the Cr content may be set to 0.3 mass % or less.
Cu increases specific resistance to reduce a core loss. However, when a Cu content exceeds 0.4 mass %, the effect is saturated. Further, a surface flaw called “copper scab” is sometimes caused at the time of hot rolling. Thus, the Cu content may be set to 0.4 mass % or less.
Ni increases specific resistance to reduce a core loss. Further, Ni controls a metallic structure of the hot-rolled steel strip to improve the magnetic property. However, when a Ni content exceeds 1 mass %, the secondary recrystallization becomes unstable. Thus, the Ni content may be set to 1 mass % or less.
P increases specific resistance to reduce a core loss. However, when a P content exceeds 0.5 mass %, a fracture occurs easily at the time of cold rolling due to embrittlement. Thus, the P content may be set to 0.5 mass % or less.
Mo improves a surface property at the time of hot rolling. However, when a Mo content exceeds 0.1 mass %, the effect is saturated. Thus, the Mo content may be set to 0.1 mass % or less.
Sn and Sb are grain boundary segregation elements. The silicon steel material used in this embodiment contains Al, so that there is sometimes a case that Al is oxidized by moisture released from the annealing separating agent depending on the condition of the finish annealing. In this case, variations in inhibitor strength occur depending on the position in the grain-oriented electrical steel sheet, and the magnetic property also sometimes varies. However, in the case when the grain boundary segregation elements are contained, the oxidation of Al can be suppressed. That is, Sn and Sb suppress the oxidation of Al to suppress the variations in the magnetic property. However, when a content of Sn and Sb exceeds 0.30 mass % in total amount, the oxide layer is not easily formed at the time of decarburization annealing, and thereby the formation of the glass film made by the reaction of the oxide layer and MgO being the main component of the annealing separating agent at the time of finish annealing becomes insufficient. Further, the decarburization is noticeably prevented. Thus, the content of Sn and Sb may be set to 0.3 mass % or less in total amount.
Bi stabilizes precipitates such as sulfides to strengthen the function as an inhibitor. However, when a Bi content exceeds 0.01 mass %, the formation of the glass film is adversely affected. Thus, the Bi content may be set to 0.01 mass % or less.
Next, each treatment in this embodiment will be explained.
The silicon steel material (slab) having the above-described components may be manufactured in a manner that, for example, steel is melted in a converter, an electric furnace, or the like, and the molten steel is subjected to a vacuum degassing treatment according to need, and next is subjected to continuous casting. Further, the silicon steel material may also be manufactured in a manner that in place of the continuous casting, an ingot is made to then be bloomed. The thickness of the silicon steel slab is set to, for example, 150 mm to 350 mm, and is preferably set to 220 mm to 280 mm. Further, what is called a thin slab having a thickness of 30 mm to 70 mm may also be manufactured. In the case when the thin slab is manufactured, the rough rolling performed when obtaining the hot-rolled steel strip may be omitted.
After the silicon steel slab is manufactured, the slab heating is performed (step S1), and the hot rolling (step S2) is performed. Then, in this embodiment, the conditions of the slab heating and the hot rolling are set such that BN is made to precipitate compositely on MnS and/or MnSe, and that the precipitation amounts of BN, MnS, and MnSe in the hot-rolled steel strip satisfy inequations (5) to (7) below.

BasBN≧0.0005  (5)

[B]−BasBN≦0.001  (6)

SasMnS+0.5×SeasMnSe≧0.002  (7)

Here, “BasBN” represents the amount of B that has precipitated as BN (mass %), “SasMnS” represents the amount of S that has precipitated as MnS (mass %), and “SeasMnSe” represents the amount of Se that has precipitated as MnSe (mass %).
As for B, a precipitation amount and a solid solution amount of B are controlled such that inequation (5) and inequation (6) are satisfied. A certain amount or more of BN is made to precipitate in order to secure an amount of the inhibitors. Further, in the case when the amount of solid-dissolved B is large, there is sometimes a case that unstable fine precipitates are formed in the subsequent processes to adversely affect the primary recrystallization structure.
MnS and MnSe each function as a nucleus on which BN precipitates compositely. Thus, in order to make BN precipitate sufficiently to thereby improve the magnetic property, the precipitation amounts of MnS and MnSe are controlled such that inequation (7) is satisfied.
The condition expressed in inequation (6) is derived from FIG. 3, FIG. 8, and FIG. 13. It is found from FIG. 3, FIG. 8, and FIG. 13 that in the case of [B]−BasBN being 0.001 mass % or less, the good magnetic flux density, being the magnetic flux density B8 of 1.88 T or more, is obtained.
The conditions expressed in inequation (5) and inequation (7) are derived from FIG. 2, FIG. 7, and FIG. 12. It is found that in the case when BasBN is 0.0005 mass % or more and SasMnS is 0.002 mass % or more, the good magnetic flux density, being the magnetic flux density B8 of 1.88 T or more, is obtained from FIG. 2. Similarly, it is found that in the case when BasBN is 0.0005 mass % or more and SeasMnSe is 0.004 mass % or more, the good magnetic flux density, being the magnetic flux density B8 of 1.88 T or more, is obtained from FIG. 7. Similarly, it is found that in the case when BasBN is 0.0005 mass % or more and SasMnS+0.5×SeasMnSe is 0.002 mass % or more, the good magnetic flux density, being the magnetic flux density B8 of 1.88 T or more, is obtained from FIG. 12. Then, as long as SasMnS is 0.002 mass % or more, SasMnS+0.5×SeasMnSe becomes 0.002 mass % or more inevitably, and as long as SeasMnSe is 0.004 mass % or more, SasMnS+0.5×SeasMnSe becomes 0.002 mass % or more inevitably. Thus, it is important that SasMnS+0.5×SasMnS+0.5×SeasMnSe is 0.002 mass % or more.
Further, the temperature of the slab heating (step S1) is set so as to satisfy the following conditions.
(i) in the case of S and Se being contained in the silicon steel slab
the temperature T1 (° C.) expressed by equation (1) or lower, the temperature T2 (° C.) expressed by equation (2) or lower, and the temperature T3 (° C.) expressed by equation (3) or lower
(ii) in the case of no Se being contained in the silicon steel slab
the temperature T1 (° C.) expressed by equation (1) or lower and the temperature T3 (° C.) expressed by equation (3) or lower
(iii) in the case of no S being contained in the silicon steel slab
the temperature T2 (° C.) expressed by equation (2) or lower and the temperature T3 (° C.) expressed by equation (3) or lower

T1=14855/(6.82−log([Mn]×[S]))−273  (1)

T2=10733/(4.08−log([Mn]×[Se]))−273  (2)

T3=16000/(5.92−log([B]×[N]))−273  (3)

This is because when the slab heating is performed at such temperatures, BN, MnS, and MnSe are not completely solid-dissolved at the time of slab heating, and the precipitations of BN, MnS, and MnSe are promoted during the hot rolling. As is clear from FIG. 4, FIG. 9, and FIG. 14, the solution temperatures T1 and T2 approximately agree with the upper limit of the slab heating temperature capable of obtaining the magnetic flux density B8 of 1.88 or more. Further, as is clear from FIG. 5, FIG. 10, and FIG. 15, the solution temperature T3 approximately agrees with the upper limit of the slab heating temperature capable of obtaining the magnetic flux density B8 of 1.88 or more.
Further, the temperature of the slab heating is more preferably set so as to satisfy the following conditions as well. This is to make a preferable amount of MnS or MnSe precipitate during the slab heating.
(i) in the case of no Se being contained in the silicon steel slab
a temperature T4 (° C.) expressed by equation (11) below or lower
(ii) in the case of no S being contained in the silicon steel slab
a temperature T5 (° C.) expressed by equation (12) below or lower

T4=14855/(6.82−log(([Mn]−0.0034)×([S]−0.002)))−273  (11)

T5=10733/(4.08−log(([Mn]−0.0028)×([Se]−0.004)))−273  (12)

In the case when the temperature of the slab heating is too high, BN, MnS, and/or MnSe are sometimes solid-dissolved completely. In this case, it becomes difficult to make BN, MnS, and/or MnSe precipitate at the time of hot rolling. Thus, the slab heating is preferably performed at the temperature T1 and/or the temperature T2 or lower, and at the temperature T3 or lower. Further, if the temperature of the slab heating is the temperature T4 or T5 or lower, a preferable amount of MnS or MnSe precipitates during the slab heating, and thus it becomes possible to make BN precipitate compositely on MnS or MnSe to form effective inhibitors easily.
Further, as for B, the finish temperature Tf of the finish rolling in the hot rolling is set such that inequation (4) below is satisfied. This is to promote the precipitation of BN.

Tf≦1000−10000×[B]  (4)

As is clear from FIG. 6, FIG. 11, and FIG. 16, the condition expressed in inequation (4) approximately agrees with the condition capable of obtaining the magnetic flux density B8 of 1.91 T or more. Further, the finish temperature Tf of the finish rolling is preferably set to 800° C. or higher in terms of the precipitation of BN.
After the hot rolling (step S2), the annealing of the hot-rolled steel strip is performed (step S3). Next, the cold rolling is performed (step S4). As described above, the cold rolling may be performed only one time, or may also be performed a plurality of times with the intermediate annealing being performed therebetween. In the cold rolling, the final cold rolling rate is preferably set to 80% or more. This is to develop a good primary recrystallization aggregate structure.
Thereafter, the decarburization annealing is performed (step S5). As a result, C contained in the steel strip is removed. The decarburization annealing is performed in a moist atmosphere, for example. Further, the decarburization annealing is preferably performed at a time such that, for example, a grain diameter obtained by the primary recrystallization becomes 15 μm or more in a temperature zone of 770° C. to 950° C. This is to obtain the good magnetic property. Subsequently, the coating of the annealing separating agent and the finish annealing are performed (step S6). As a result, the grains oriented in the {110}<001> orientation preferentially grow by the secondary recrystallization.
Further, the nitriding treatment is performed between start of the decarburization annealing and occurrence of the secondary recrystallization in the finish annealing (step S7). This is to form an inhibitor of (Al, Si)N. The nitriding treatment may be performed during the decarburization annealing (step S5), or may also be performed during the finish annealing (step S6). In the case when the nitriding treatment is performed during the decarburization annealing, the annealing may be performed in an atmosphere containing a gas having nitriding capability such as ammonia, for example. Further, the nitriding treatment may be performed during a heating zone or a soaking zone in a continuous annealing furnace, or the nitriding treatment may also be performed at a stage after the soaking zone. In the case when the nitriding treatment is performed during the finish annealing, a powder having nitriding capability such as MnN, for example, may be added to the annealing separating agent.
In order to perform the secondary recrystallization more stably, it is desirable to adjust the degree of nitriding in the nitriding treatment (step S7) and to adjust the compositions of (Al, Si)N in the steel strip after the nitriding treatment. For example, according to the Al content, the B content, and the content of Ti existing inevitably, the degree of nitriding is preferably controlled so as to satisfy inequation (8) below, and the degree of nitriding is more preferably controlled so as to satisfy inequation (9) below. Inequation (8) and inequation (9) indicate an amount of N that is preferable to fix B as BN effective as an inhibitor and an amount of N that is preferable to fix Al as AlN or (Al, Si)N effective as an inhibitor.

[N]≧14/27[Al]+14/11[B]+14/47[Ti]  (8)

[N]≧2/3[Al]+14/11[B]+14/47[Ti]  (9)

Here, [N] represents an N content (mass %) of a steel strip obtained after the nitriding treatment, [Al] represents an acid-soluble Al content (mass %) of the steel strip obtained after the nitriding treatment, [B] represents a B content (mass %) of the steel strip obtained after the nitriding treatment, and [Ti] represents a Ti content (mass %) of the steel strip obtained after the nitriding treatment.
The method of the finish annealing (step S6) is also not limited in particular. It should be noted that, in this embodiment, the inhibitors are strengthened by BN, so that a heating rate in a temperature range of 1000° C. to 1100° C. is preferably set to 15° C./h or less in a heating process of the finish annealing. Further, in place of controlling the heating rate, it is also effective to perform isothermal annealing in which the steel strip is maintained in the temperature range of 1000° C. to 1100° C. for 10 hours or longer.
According to this embodiment as above, it is possible to stably manufacture the grain-oriented electrical steel sheet excellent in the magnetic property.
Example
Next, experiments conducted by the present inventors will be explained. The conditions and so on in the experiments are examples employed for confirming the practicability and the effects of the present invention, and the present invention is not limited to those examples.
Fourth Experiment
In the fourth experiment, the effect of the B content in the case of no Se being contained was confirmed.
In the fourth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B having an amount listed in Table 1 (0 mass % to 0.0045 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, a magnetic property (the magnetic flux density B8) after the finish annealing was measured. The magnetic property (magnetic flux density B8) was measured based on JIS C2556. A result of the measurement is listed in Table 1.












TABLE 1












MAGNETIC



SLAB HEATING


PROPERTY













HEATING

NITRIDING

MAGNETIC



TEMPER-

TREATMENT
PRECIPITATES
FLUX



















B CONTENT
ATURE
T1
T3
N CONTENT
BasBN
[B] − BasBN
SasMnS
DENSITY



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
B8 (T)





















COMPAR-
1A
0
1100
1206
—
0.023
0
0
0.005
1.898


ATIVE


EXAMPLE


EXAMPLE
1B
0.0008
1100
1206
1167
0.023
0.0008
0
0.005
1.917



1C
0.0019
1100
1206
1217
0.023
0.0018
0
0.005
1.929



1D
0.0031
1100
1206
1247
0.023
0.0030
0.0001
0.005
1.928



1E
0.0045
1100
1206
1271
0.023
0.0043
0.0002
0.005
1.923









As listed in Table 1, in Comparative Example No. 1A having no B contained in the slab, the magnetic flux density was low, but in Examples No. 1B to No. 1E each having an appropriate amount of B contained in the slab, the good magnetic flux density was obtained.
Fifth Experiment
In the fifth experiment, the effects of the B content and the slab heating temperature in the case of no Se being contained were confirmed.
In the fifth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, Cr: 0.1 mass %, P: 0.03 mass %, Sn: 0.06 mass %, and B having an amount listed in Table 2 (0 mass % to 0.0045 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1180° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 2.












TABLE 2












MAGNETIC



SLAB HEATING


PROPERTY













HEATING

NITRIDING

MAGNETIC



TEMPER-

TREATMENT
PRECIPITATES
FLUX



















B CONTENT
ATURE
T1
T3
N CONTENT
BasBN
[B] − BasBN
SasMnS
DENSITY



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
B8 (T)





















COMPAR-
2A
0
1180
1206
—
0.023
0
0
0.025
1.893


ATIVE
2B
0.0008
1180
1206
1167
0.023
0.0002
0.0006
0.025
1.634


EXAMPLE


EXAMPLE
2C
0.0019
1180
1206
1217
0.023
0.0012
0.0007
0.025
1.922



2D
0.0031
1180
1206
1247
0.023
0.0024
0.0007
0.025
1.927



2E
0.0045
1180
1206
1271
0.023
0.0036
0.0009
0.025
1.920









As listed in Table 2, in Comparative Example No. 2A having no B contained in the slab and Comparative Example No. 2B having the slab heating temperature higher than the temperature T3, the magnetic flux density was low. On the other hand, in Examples No. 2C to No. 2E each having an appropriate amount of B contained in the slab and having the slab heating temperature being the temperature T1 or lower and the temperature T3 or lower, the good magnetic flux density was obtained.
Sixth Experiment
In the sixth experiment, the effects of the Mn content and the slab heating temperature in the case of no Se being contained were confirmed.
In the sixth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.009 mass %, S: 0.007 mass %, B: 0.002 mass %, and Mn having an amount listed in Table (0.05 mass % to 0.20 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1200° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 3.












TABLE 3












MAGNETIC



SLAB HEATING


PROPERTY













HEATING

NITRIDING

MAGNETIC



TEMPER-

TREATMENT
PRECIPITATES
FLUX



















Mn CONTENT
ATURE
T1
T3
N CONTENT
BasBN
[B] − BasBN
SasMnS
DENSITY



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
B8 (T)





















COMPAR-
3A
0.05
1200
1173
1227
0.022
0.0012
0.0008
0.001
1.824


ATIVE


EXAMPLE


EXAMPLE
3B
0.10
1200
1216
1227
0.022
0.0014
0.0006
0.002
1.923



3C
0.14
1200
1238
1227
0.022
0.0015
0.0005
0.004
1.931



3D
0.20
1200
1263
1227
0.022
0.0016
0.0004
0.005
1.925









As listed in Table 3, in Comparative Example No. 3A having the slab heating temperature higher than the temperature T1, the magnetic flux density was low. On the other hand, in Examples No. 3B to No. 3D each having the slab heating temperature being the temperature T1 or lower and the temperature T3 or lower, the good magnetic flux density was obtained.
Seventh Experiment
In the seventh experiment, the effect of the finish temperature Tf of the finish rolling in the hot rolling in the case of no Se being contained was confirmed.
In the seventh experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B: 0.002 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1180° C., and thereafter were subjected to finish rolling at the finish temperature Tf listed in Table 4 (800° C. to 1000° C.). In this manner, hot-rolled steel strips each having a thickness of 2 3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.020 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 4.













TABLE 4













MAGNETIC



SLAB HEATING
FINISH ROLLING


PROPERTY















HEATING

FINISH
RIGHT
NITRIDING

MAGNETIC



TEMPER-

TEMPER-
SIDE OF
TREATMENT
PRECIPITATES
FLUX




















ATURE
T1
T3
ATURE Tf
EXPRES-
N CONTENT
BasBN
[B] − BasBN
SasMnS
DENSITY



No.
(° C.)
(° C.)
(° C.)
(° C.)
SION (4)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
B8 (T)






















EXAMPLE
4A
1180
1206
1220
800
980
0.020
0.0015
0.0005
0.003
1.929



4B
1180
1206
1220
850
980
0.020
0.0013
0.0007
0.003
1.927



4C
1180
1206
1220
900
980
0.020
0.0012
0.0006
0.002
1.924


COMPAR-
4D
1180
1206
1220
1000
980
0.020
0.0011
0.0009
0.002
1.895


ATIVE


EXAMPLE









In the case of the B content being 0.002 mass % (20 ppm), the finish temperature Tf is necessary to be 980° C. or lower based on inequation (4). Then, as listed in Table 4, in Examples No. 4A to 4C each satisfying the condition, the good magnetic flux density was obtained, but in Comparative Example No. 4D not satisfying the condition, the magnetic flux density was low.
Eighth Experiment
In the eighth experiment, the effect of the N content after the nitriding treatment in the case of no Se being contained was confirmed.
In the eighth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B: 0.002 mass %, a content of Ti that is an impurity being 0.0014 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to an amount listed in Table 5 (0.012 mass % to 0.028 mass %). Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 5.











TABLE 5









SLAB HEATING
FINISH ROLLING
NITRIDING TREATMENT

















HEATING


FINISH
RIGHT SIDE OF

RIGHT SIDE OF




TEMPERATURE
T1
T3
TEMPERATURE
EXPRESSION
N CONTENT
EXPRESSION



No.
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)
(8)





EXAMPLE
5A
1150
1206
1220
900
980
0.012
0.018



5B
1150
1206
1220
900
980
0.017
0.018



5C
1150
1206
1220
900
980
0.022
0.018



5D
1150
1206
1220
900
980
0.028
0.018
















MAGNETIC



NITRIDING

PROPERTY



TREATMENT

MAGNETIC



RIGHT SIDE OF
PRECIPITATES
FLUX

















EXPRESSION
BasBN
[B] − BasBN
SasMnS
DENSITY B8




No.
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
5A
0.022
0.0017
0.0003
0.005
1.888




5B
0.022
0.0017
0.0003
0.005
1.905




5C
0.022
0.0017
0.0003
0.005
1.925




5D
0.022
0.0017
0.0003
0.005
1.927










As listed in Table 5, in Examples No. 5C and No. 5D in which an N content after the nitriding treatment satisfied the relation of inequation (8) and the relation of inequation (9), the particularly good magnetic flux density was obtained. On the other hand, in Examples No. 5A and No. 5B in which an N content after the nitriding treatment did not satisfy the relation of inequation (8) and the relation of inequation (9), the magnetic flux density was slightly lower than those in Examples No. 5C and No. 5D.
Ninth Experiment
In the ninth experiment, the effect of the condition of the finish annealing in the case of no Se being contained was confirmed.
In the ninth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B: 0.002 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1000° C. at a rate of 15° C./h, and further were heated up to 1200° C. at a rate listed in Table 6 (5° C./h to 30° C./h) and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 6.












TABLE 6









FINISH

FINISH ROLLING
NITRIDING













ANNEALING
SLAB HEATING

RIGHT SIDE
TREATMENT

















HEATING
HEATING


FINISH
OF
N




SPEED
TEMPERATURE
T1
T3
TEMPERATURE
EXPRESSION
CONTENT



No.
(° C./h)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)





EXAMPLE
6A
5
1150
1206
1220
900
980
0.024



6B
10
1150
1206
1220
900
980
0.024



6C
15
1150
1206
1220
900
980
0.024



6D
30
1150
1206
1220
900
980
0.024
















MAGNETIC



NITRIDING TREATMENT

PROPERTY












RIGHT SIDE
RIGHT SIDE

MAGNETIC



OF
OF
PRECIPITATES
FLUX


















EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SasMnS
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
6A
0.017
0.021
0.0017
0.0003
0.005
1.933




6B
0.017
0.021
0.0017
0.0003
0.005
1.927




6C
0.017
0.021
0.0017
0.0003
0.005
1.924




6D
0.017
0.021
0.0017
0.0003
0.005
1.893










As listed in Table 6, in Examples No. 6A to No. 6C, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 6D, the heating rate in the temperature range exceeded 15° C./h, so that the magnetic flux density was slightly lower than those in Examples No. 6A to No. 6C.
Tenth Experiment
In the tenth experiment, the effect of the condition of the finish annealing in the case of no Se being contained was confirmed.
In the tenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B: 0.002 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips. Then, in Example No. 7A, the steel strip was heated up to 1200° C. at a rate of 15° C./h and was finish annealed. Further, in Examples No. 7B to No. 7E, the steel strips were heated up to a temperature listed in Table 7 (1000° C. to 1150° C.) at a rate of 30° C./h and were kept for 10 hours at the temperature, and thereafter were heated up to 1200° C. at a rate of 30° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table V.












TABLE 7









FINISH

FINISH ROLLING














ANNEALING
SLAB HEATING

RIGHT SIDE
NITRIDING

















MAINTAINING
HEATING


FINISH
OF
TREATMENT




TEMPERATURE
TEMPERATURE
T1
T3
TEMPERATURE Tf
EXPRESSION
N CONTENT



No.
(° C.)
(° C.)
(° C.)
(° C.)
(° C.)
(4)
(MASS %)





EXAMPLE
7A
—
1150
1206
1220
900
980
0.024



7B
1000
1150
1206
1220
900
980
0.024



7C
1050
1150
1206
1220
900
980
0.024



7D
1100
1150
1206
1220
900
980
0.024



7E
1150
1150
1206
1220
900
980
0.024
















MAGNETIC



NITRIDING TREATMENT

PROPERTY












RIGHT SIDE
RIGHT SIDE

MAGNETIC



OF
OF
PRECIPITATES
FLUX


















EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SasMnS
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
7A
0.017
0.021
0.0017
0.0003
0.005
1.908




7B
0.017
0.021
0.0017
0.0003
0.005
1.928




7C
0.017
0.021
0.0017
0.0003
0.005
1.931




7D
0.017
0.021
0.0017
0.0003
0.005
1.927




7E
0.017
0.021
0.0017
0.0003
0.005
1.881










As listed in Table 7, in Example No. 7A, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. Further, in Examples No. 7B to 7D, the steel strips were kept in the temperature range of 1000° C. to 1100° C. for 10 hours, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 7E, the temperature at which the steel strip was kept for 10 hours exceeded 1100° C., so that the magnetic flux density was slightly lower than those in Examples No. 7A to No. 7D.
Eleventh Experiment
In the eleventh experiment, the effect of the slab heating temperature in the case of no Se being contained was confirmed.
In the eleventh experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, and B: 0.0017 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at a temperature listed in Table 8 (1100° C. to 1300° C.), and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.021 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h, and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 8.












TABLE 8












MAGNETIC






PROPERTY



SLAB HEATING
NITRIDING

MAGNETIC














HEATING


TREATMENT
PRECIPITATES
FLUX


















TEMPERATURE
T1
T3
N CONTENT
BasBN
[B] − BasBN
SasMnS
DENSITY B8



No.
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)




















EXAMPLE
8A
1100
1206
1210
0.021
0.0016
0.0001
0.006
1.926



8B
1150
1206
1210
0.021
0.0013
0.0004
0.005
1.925



8C
1200
1206
1210
0.021
0.0011
0.0006
0.002
1.903


COMPARATIVE
8D
1250
1206
1210
0.021
0.0005
0.0012
0.001
1.773


EXAMPLE
8E
1300
1206
1210
0.021
0.0002
0.0015
0.001
1.623









As listed in Table 8, in Examples No. 8A to No. 8C each having the slab heating temperature being the temperature T1 or lower and the temperature T3 or lower, the good magnetic flux density was obtained. On the other hand, in Comparative Examples No. 8D and No. 8E each having the slab heating temperature higher than the temperature T1 and the temperature T3, the magnetic flux density was low.
Twelfth Experiment
In the twelfth experiment, the effect of the components of the slab in the case of no Se being contained was confirmed.
In the twelfth experiment, first, slabs containing components listed in Table 9 and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 10.









TABLE 9









COMPOSITION OF SILICON STEEL MATERIAL (MASS %)
























No.
Si
C
Al
N
Mn
S
B
Cr
Cu
Ni
P
Mo
Sn
Sb
Bi



























EXAMPLE
9A
3.3
0.06
0.028
0.008
0.1
0.006
0.002
—
—
—
—
—
—
—
—



9B
3.2
0.06
0.027
0.007
0.1
0.007
0.002
0.15
—
—
—
—
—
—
—



9C
3.4
0.06
0.025
0.008
0.1
0.008
0.002
—
0.2 
—
—
—
—
—
—



9D
3.3
0.06
0.027
0.008
0.1
0.006
0.002
—
—
0.1
—
—
—
—
—



9E
3.3
0.06
0.024
0.007
0.1
0.006
0.002
—
—
0.4
—
—
—
—
—



9F
3.3
0.06
0.027
0.009
0.1
0.007
0.002
—
—
1.0
—
—
—
—
—



9G
3.4
0.06
0.028
0.007
0.1
0.007
0.002
—
—
—
0.03
—
—
—
—



9H
3.2
0.06
0.027
0.008
0.1
0.006
0.002
—
—
—
—
0.005
—
—
—



9I
3.3
0.06
0.028
0.008
0.1
0.007
0.002
—
—
—
—
—
0.04
—
—



9J
3.3
0.06
0.025
0.008
0.1
0.006
0.002
—
—
—
—
—
—
0.04
—



9K
3.3
0.06
0.024
0.009
0.1
0.008
0.002
—
—
—
—
—
—
—
0.003



9L
3.2
0.06
0.030
0.008
0.1
0.006
0.002
0.10
—
—
0.03
—
0.06
—
—



9M
3.8
0.06
0.027
0.008
0.1
0.007
0.002
0.05
0.15
0.1
0.02
—
0.04
—
—



9N
3.3
0.06
0.028
0.006
0.1
0.006
0.002
0.08
—
—
—
0.003
0.05
—
0.001



9O
2.8
0.06
0.022
0.008
0.1
0.006
0.002
—
—
—
—
—
—
—
—


COMPARATIVE
9P
3.3
0.06
0.035
0.007
0.1
0.002
0.002
—
—
—
—
—
—
—
—


EXAMPLE



















TABLE 10










MAGNETIC



PRECIPITATES
PROPERTY














BasBN
[B] − BasBN
SasMnS
MAGNETIC FLUX



No.
(MASS %)
(MASS %)
(MASS %)
DENSITY B8 (T)
















EXAMPLE
9A
0.0018
0.0002
0.005
1.923



9B
0.0019
0.0001
0.006
1.924



9C
0.0019
0.0001
0.007
1.929



9D
0.0018
0.0002
0.005
1.925



9E
0.0019
0.0001
0.005
1.920



9F
0.0019
0.0001
0.006
1.881



9G
0.0018
0.0002
0.006
1.929



9H
0.0019
0.0001
0.005
1.925



9I
0.0018
0.0002
0.007
1.926



9J
0.0019
0.0001
0.005
1.924



9K
0.0019
0.0001
0.007
1.928



9L
0.0018
0.0002
0.005
1.929



9M
0.0019
0.0001
0.006
1.928



9N
0.0018
0.0002
0.005
1.926



9O
0.0018
0.0002
0.005
1.938


COMPARATIVE
9P
0.0018
0.0002
0.001
1.621


EXAMPLE









As listed in Table 10, in Examples No. 9A to No. 9O each using the slab having the appropriate composition, the good magnetic flux density was obtained, but in Comparative Example No. 9P having an S content being less than the lower limit of the present invention range, the magnetic flux density was low.
Thirteenth Experiment
In the thirteenth experiment, the effect of the nitriding treatment in the case of no Se being contained was confirmed.
In the thirteenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.007 mass %, Mn: 0.14 mass %, S: 0.006 mass %, and B: 0.0015 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained.
Thereafter, as for a sample of Comparative Example No. 10A, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby a decarburization-annealed steel strip was obtained. Further, as for a sample of Example No. 10B, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and further annealing was performed in an ammonia containing atmosphere, and thereby a decarburization-annealed steel strip having an N content of 0.021 mass % was obtained. Further, as for a sample of Example No. 10C, decarburization annealing was performed in a moist atmosphere gas at 860° C. for 100 seconds, and thereby a decarburization-annealed steel strip having an N content of 0.021 mass % was obtained. In this manner, three types of the decarburization-annealed steel strips were obtained.
Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 11.











TABLE 11











NITRIDING TREATMENT












APPLICATION OR
SLAB HEATING

RIGHT SIDE
















NO APPLICATION
HEATING


N
OF




OF NITRIDING
TEMPERATURE
T1
T3
CONTENT
EXPRESSION



No.
TREATMENT
(° C.)
(° C.)
(° C.)
(MASS %)
(3)





COMPARATIVE
10A
NOT APPLIED
1150
1228
1195
0.007
0.016


EXAMPLE


EXAMPLE
10B
APPLIED
1150
1228
1195
0.021
0.016



10C
APPLIED
1150
1228
1195
0.021
0.016














NITRIDING

MAGNETIC



TREATMENT

PROPERTY



RIGHT SIDE

MAGNETIC



OF
PRECIPITATES
FLUX

















EXPRESSION
BasBN
[B] − BasBN
SasMnS
DENSITY B8




No.
(4)
(MASS %)
(MASS %)
(MASS %)
(T)







COMPARATIVE
10A
0.020
0.0013
0.0002
0.005
1.564



EXAMPLE



EXAMPLE
10B
0.020
0.0013
0.0002
0.005
1.927




10C
0.020
0.0013
0.0002
0.005
1.925










As listed in Table 11, in Example No. 10B in which the nitriding treatment was performed after the decarburization annealing, and Example No. 10C in which the nitriding treatment was performed during the decarburization annealing, the good magnetic flux density was obtained. However, in Comparative Example No. 10A in which no nitriding treatment was performed, the magnetic flux density was low. Incidentally, the numerical value in the section of “NITRIDING TREATMENT” of Comparative Example No. 10A in Table 11 is a value obtained from the composition of the decarburization-annealed steel strip.
Fourteenth Experiment
In the fourteenth experiment, the effect of the B content in the case of no S being contained was confirmed.
In the fourteenth experiment, first, slabs containing Si: 3.2 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.12 mass %, Se: 0.008 mass %, and B having an amount listed in Table (0 mass % to 0.0043 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 12.












TABLE 12












MAGNETIC






PROPERTY



SLAB HEATING
NITRIDING

MAGNETIC















B
HEATING


TREATMENT
PRECIPITATES
FLUX



















CONTENT
TEMPERATURE
T2
T3
N CONTENT
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)





















COMPARATIVE
11A
0
1100
1239
—
0.024
0
0
0.0069
1.895


EXAMPLE


EXAMPLE
11B
0.0009
1100
1239
1173
0.024
0.0007
0.0002
0.0068
1.919



11C
0.0017
1100
1239
1210
0.024
0.0015
0.0002
0.0070
1.928



11D
0.0029
1100
1239
1243
0.024
0.0026
0.0003
0.0069
1.925



11E
0.0043
1100
1239
1268
0.024
0.0038
0.0005
0.0071
1.926









As listed in Table 12, in Comparative Example No. 11A having no B contained in the slab, the magnetic flux density was low, but in Examples No. 11B to No. 11E each having an appropriate amount of B contained in the slab, the good magnetic flux density was obtained.
Fifteenth Experiment
In the fifteenth experiment, the effects of the B content and the slab heating temperature in the case of no S being contained were confirmed.
In the fifteenth experiment, first, slabs containing Si: 3.2 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.12 mass %, Se: 0.008 mass %, and B having an amount listed in Table (0 mass % to 0.0043 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1180° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 13.












TABLE 13












MAGNETIC






PROPERTY



SLAB HEATING
NITRIDING

MAGNETIC















B
HEATING


TREATMENT
PRECIPITATES
FLUX



















CONTENT
TEMPERATURE
T2
T3
N CONTENT
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)





















COMPARATIVE
12A
0
1180
1239
—
0.023
0
0
0.0042
1.892


EXAMPLE
12B
0.0009
1180
1239
1173
0.023
0.0003
0.0006
0.0043
1.634


EXAMPLE
12C
0.0017
1180
1239
1210
0.023
0.0013
0.0004
0.0044
1.922



12D
0.0029
1180
1239
1243
0.023
0.0021
0.0008
0.0045
1.927



12E
0.0043
1180
1239
1268
0.023
0.0034
0.0009
0.0043
1.920









As listed in Table 13, in Comparative Example No. 12A having no B contained in the slab and Comparative Example No. 12B having the slab heating temperature higher than the temperature T3, the magnetic flux density was low. On the other hand, in Examples No. 12C to No. 12E each having an appropriate amount of B contained in the slab and having the slab heating temperature being the temperature T2 or lower and the temperature T3 or lower, the good magnetic flux density was obtained.
Sixteenth Experiment
In the sixteenth experiment, the effects of the Mn content and the slab heating temperature in the case of no S being contained were confirmed.
In the sixteenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Se: 0.007 mass %, B: 0.0018 mass %, and Mn having an amount listed in Table (0.04 mass % to 0.2 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 14.












TABLE 14












MAGNETIC






PROPERTY



SLAB HEATING
NITRIDING

MAGNETIC















Mn
HEATING


TREATMENT
PRECIPITATES
FLUX



















CONTENT
TEMPERATURE
T2
T3
N CONTENT
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)





















COMPARATIVE
13A
0.04
1150
1133
1214
0.022
0.0014
0.0004
0.0007
1.612


EXAMPLE


EXAMPLE
13B
0.11
1150
1219
1214
0.022
0.0015
0.0003
0.0042
1.924



13C
0.15
1150
1248
1214
0.022
0.0014
0.0004
0.0051
1.929



13D
0.20
1150
1275
1214
0.022
0.0015
0.0003
0.0057
1.924









As listed in Table 14, in Comparative Example No. 13A having a Mn content being less than the lower limit of the present invention range, the magnetic flux density was low, but in Examples No. 13B to No. 13D each having an appropriate amount of Mn contained in the slab, the good magnetic flux density was obtained.
Seventeenth Experiment
In the seventeenth experiment, the effect of the finish temperature Tf of the finish rolling in the hot rolling in the case of no S being contained was confirmed.
In the seventeenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.026 mass %, N: 0.008 mass %, Mn: 0.15 mass %, Se: 0.006 mass %, and B: 0.002 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at the finish temperature Tf listed in Table 15 (800° C. to 1000° C.). In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.020 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 15.









TABLE 15









FINISH ROLLING











SLAB HEATING

RIGHT SIDE















HEATING


FINISH
OF




TEMPERATURE
T2
T3
TEMPERATURE
EXPRESSION



No.
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)





EXAMPLE
14A
1150
1233
1220
800
980



14B
1150
1233
1220
850
980



14C
1150
1233
1220
900
980


COMPARATIVE
14D
1150
1233
1220
1000
980


EXAMPLE
















MAGNETIC



NITRIDING

PROPERTY



TREATMENT

MAGNETIC



N
PRECIPITATES
FLUX















CONTENT
BasBN
[B] − BasBN
SeasMnSe
DENSITY



No.
(MASS %)
(MASS %)
(MASS %)
(MASS %)
B8 (T)





EXAMPLE
14A
0.020
0.0018
0.0002
0.0045
1.920



14B
0.020
0.0017
0.0003
0.0044
1.923



14C
0.020
0.0017
0.0003
0.0044
1.922


COMPARATIVE
14D
0.020
0.0014
0.0006
0.0042
1.901


EXAMPLE









In the case of the B content being 0.002 mass % (20 ppm), the finish temperature Tf is necessary to be 980° C. or lower based on inequation (4). Then, as listed in Table 15, in Examples No. 14A to 14C each satisfying the condition, the good magnetic flux density was obtained, but in Comparative Example No. 14D not satisfying the condition, the magnetic flux density was low.
Eighteenth Experiment
In the eighteenth experiment, the effect of the N content after the nitriding treatment in the case of no S being contained was confirmed.
In the eighteenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.12 mass %, Se: 0.007 mass %, and B: 0.0016 mass %, a content of Ti that is an impurity being 0.0013 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to an amount listed in Table 16 (0.011 mass % to 0.029 mass %). Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 16.











TABLE 16









SLAB HEATING
FINISH ROLLING
NITRIDING TREATMENT

















HEATING


FINISH
RIGHT SIDE OF

RIGHT SIDE OF




TEMPERATURE
T2
T3
TEMPERATURE Tf
EXPRESSION
N CONTENT
EXPRESSION



No.
(° C.)
(° C.)
(° C.)
(° C.)
(4)
(MASS %)
(8)





EXAMPLE
15A
1100
1227
1207
900
984
0.011
0.016



15B
1100
1227
1207
900
984
0.019
0.016



15C
1100
1227
1207
900
984
0.023
0.016



15D
1100
1227
1207
900
984
0.029
0.016
















MAGNETIC



NITRIDING

PROPERTY



TREATMENT

MAGNETIC



RIGHT SIDE OF
PRECIPITATES
FLUX

















EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
15A
0.020
0.0015
0.0001
0.0059
1.887




15B
0.020
0.0015
0.0001
0.0059
1.918




15C
0.020
0.0015
0.0001
0.0059
1.924




15D
0.020
0.0015
0.0001
0.0059
1.929










As listed in Table 16, in Examples No. 15C and No. 15D in which an N content after the nitriding treatment satisfied the relation of inequation (8) and the relation of inequation (9), the particularly good magnetic flux density was obtained. On the other hand, in Example No. 15B in which an N content after the nitriding treatment satisfied the relation of inequation (8) but did not satisfy the relation of inequation (9), the magnetic flux density was slightly lower than those in Examples No. 15C and No. 15D. Further, in Example No. 15A in which an N content after the nitriding treatment did not satisfy the relation of inequation (8) and the relation of inequation (9), the magnetic flux density was slightly lower than that in Example No. 15B.
Nineteenth Experiment
In the nineteenth experiment, the effect of the condition of the finish annealing in the case of no S being contained was confirmed.
In the nineteenth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, Se: 0.006 mass %, and B: 0.0022 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 840° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1000° C. at a rate of 15° C./h, and further were heated up to 1200° C. at a rate listed in Table 17 (5° C./h to 30° C./h) and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 17.












TABLE 17









FINISH

FINISH ROLLING
NITRIDING













ANNEALING
SLAB HEATING

RIGHT SIDE
TREATMENT

















HEATING
HEATING


FINISH
OF
N




SPEED
TEMPERATURE
T2
T3
TEMPERATURE
EXPRESSION
CONTENT



No.
(° C./h)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)





EXAMPLE
16A
5
1100
1197
1226
900
978
0.024



16B
10
1100
1197
1226
900
978
0.024



16C
15
1100
1197
1226
900
978
0.024



16D
30
1100
1197
1226
900
978
0.024
















MAGNETIC



NITRIDING TREATMENT

PROPERTY












RIGHT SIDE
RIGHT SIDE

MAGNETIC



OF
OF
PRECIPITATES
FLUX


















EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
16A
0.017
0.022
0.0020
0.0002
0.0047
1.935




16B
0.017
0.022
0.0020
0.0002
0.0047
1.928




16C
0.017
0.022
0.0020
0.0002
0.0047
1.922




16D
0.017
0.022
0.0020
0.0002
0.0047
1.882










As listed in Table 17, in Examples No. 16A to No. 16C, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 16D, the heating rate in the temperature range exceeded 15° C./h, so that the magnetic flux density was slightly lower than those in Examples No. 16A to No. 16C.
Twentieth Experiment
In the twentieth experiment, the effect of the condition of the finish annealing in the case of no S being contained was confirmed.
In the twentieth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, Se: 0.006 mass %, and B: 0.0022 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 840° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips. Then, in Example No. 17A, the steel strip was heated up to 1200° C. at a rate of 15° C./h and was finish annealed. Further, in Examples No. 17B to No. 17E, the steel strips were heated up to a temperature listed in Table 18 (1000° C. to 1150° C.) at a rate of 30° C./h and were kept for 10 hours at the temperature, and thereafter were heated up to 1200° C. at a rate of 30° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 18.












TABLE 18









FINISH

FINISH ROLLING














ANNEALING
SLAB HEATING

RIGHT SIDE
NITRIDING

















MAINTAINING
HEATING


FINISH
OF
TREATMENT




TEMPERATURE
TEMPERATURE
T2
T3
TEMPERATURE
EXPRESSION
N CONTENT



No.
(° C.)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)





EXAMPLE
17A
—
1100
1197
1226
900
978
0.024



17B
1000
1100
1197
1226
900
978
0.024



17C
1050
1100
1197
1226
900
978
0.024



17D
1100
1100
1197
1226
900
978
0.024



17E
1150
1100
1197
1226
900
978
0.024
















MAGNETIC



NITRIDING TREATMENT

PROPERTY












RIGHT SIDE
RIGHT SIDE

MAGNETIC



OF
OF
PRECIPITATES
FLUX


















EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
17A
0.017
0.022
0.0020
0.0002
0.0047
1.922




17B
0.017
0.022
0.0020
0.0002
0.0047
1.930




17C
0.017
0.022
0.0020
0.0002
0.0047
1.933




17D
0.017
0.022
0.0020
0.0002
0.0047
1.927




17E
0.017
0.022
0.0020
0.0002
0.0047
1.880










As listed in Table 18, in Example No. 17A, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. Further, in Examples No. 17B to 17D, the steel strips were kept in the temperature range of 1000° C. to 1100° C. for 10 hours, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 17E, the temperature at which the steel strip was kept for 10 hours exceeded 1100° C., so that the magnetic flux density was slightly lower than those in Examples No. 17A to No. 17D.
Twenty-First Experiment
In the twenty-first experiment, the effect of the slab heating temperature in the case of no S being contained was confirmed.
In the twenty-first experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.12 mass %, Se: 0.008 mass %, and B: 0.0019 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at a temperature listed in Table 19 (1100° C. to 1300° C.), and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h, and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 19.












TABLE 19












MAGNETIC




NITRIDING

PROPERTY



SLAB HEATING
TREATMENT
PRECIPITATES
MAGNETIC


















HEATING


N

[B] −

FLUX




TEMPERATURE
T2
T3
CONTENT
BasBN
BasBN
SeasMnSe
DENSITY B8



No.
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)




















EXAMPLE
18A
1100
1239
1217
0.022
0.0018
0.0001
0.0070
1.929



18B
1150
1239
1217
0.022
0.0016
0.0003
0.0058
1.927



18C
1200
1239
1217
0.022
0.0011
0.0008
0.0040
1.917


COMPARATIVE
18D
1250
1239
1217
0.022
0.0004
0.0015
0.0008
1.691


EXAMPLE
18E
1300
1239
1217
0.022
0.0002
0.0017
0.0005
1.553









As listed in Table 19, in Examples No. 18A to No. 18C each having the slab heating temperature being the temperature T2 or lower and the temperature T3 or lower, the good magnetic flux density was obtained. On the other hand, in Comparative Examples No. 18D and No. 18E each having the slab heating temperature higher than the temperature T2 and the temperature T3, the magnetic flux density was low.
Twenty-Second Experiment
In the twenty-second experiment, the effect of the components of the slab in the case of no S being contained was confirmed.
In the twenty-second experiment, first, slabs containing components listed in Table 20 and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 21.









TABLE 20









COMPOSITION OF SILICON STEEL MATERIAL (MASS %)
























No.
Si
C
Al
N
Mn
Se
B
Cr
Cu
Ni
P
Mo
Sn
Sb
Bi



























EXAMPLE
19A
3.3
0.06
0.027
0.008
0.15
0.006
0.002
—
—
—
—
—
—
—
—



19B
3.3
0.06
0.027
0.007
0.12
0.007
0.002
0.13
—
—
—
—
—
—
—



19C
3.4
0.06
0.025
0.008
0.12
0.007
0.002
—
0.22
—
—
—
—
—
—



19D
3.2
0.06
0.028
0.008
0.14
0.008
0.002
—
—
0.1
—
—
—
—
—



19E
3.4
0.06
0.027
0.007
0.11
0.006
0.002
—
—
0.4
—
—
—
—
—



19F
3.1
0.06
0.024
0.006
0.13
0.007
0.002
—
—
1.0
—
—
—
—
—



19G
3.3
0.06
0.029
0.007
0.10
0.008
0.002
—
—
—
0.04
—
—
—
—



19H
3.4
0.06
0.027
0.008
0.11
0.006
0.002
—
—
—
—
0.005
—
—
—



19I
3.1
0.06
0.028
0.008
0.13
0.007
0.002
—
—
—
—
—
0.06
—
—



19J
3.3
0.06
0.028
0.008
0.10
0.006
0.002
—
—
—
—
—
—
0.05
—



19K
3.3
0.06
0.030
0.009
0.10
0.008
0.002
—
—
—
—
—
—
—
0.002



19L
3.2
0.06
0.024
0.008
0.13
0.007
0.002
0.10
—
—
0.03
—
0.05
—
—



19M
3.7
0.06
0.027
0.008
0.10
0.007
0.002
0.08
0.17
0.1
0.02
—
0.07
—
—



19N
3.2
0.06
0.034
0.006
0.12
0.006
0.002
0.12
—
—
—
0.003
0.06
—
0.001



19O
2.8
0.06
0.021
0.007
0.10
0.006
0.002
—
—
—
—
—
—
—
—


COMPARATIVE
19P
3.1
0.06
0.030
0.009
0.10
0.002
0.002
—
—
—
—
—
—
—
—


EXAMPLE



















TABLE 21










MAGNETIC



PRECIPITATES
PROPERTY














BasBN
[B] − BasBN
SeasMnSe
MAGNETIC FLUX



No.
(MASS %)
(MASS %)
(MASS %)
DENSITY B8 (T)
















EXAMPLE
19A
0.0018
0.0002
0.0054
1.923



19B
0.0019
0.0001
0.0060
1.924



19C
0.0019
0.0001
0.0061
1.929



19D
0.0018
0.0002
0.0071
1.925



19E
0.0019
0.0001
0.0048
1.920



19F
0.0019
0.0001
0.0061
1.883



19G
0.0018
0.0002
0.0068
1.929



19H
0.0019
0.0001
0.0049
1.925



19I
0.0018
0.0002
0.0062
1.926



19J
0.0019
0.0001
0.0046
1.924



19K
0.0019
0.0001
0.0067
1.928



19L
0.0018
0.0002
0.0060
1.929



19M
0.0019
0.0001
0.0058
1.928



19N
0.0018
0.0002
0.0049
1.926



19O
0.0018
0.0002
0.0046
1.938


COMPARATIVE
19P
0.0018
0.0002
0.0014
1.567


EXAMPLE









As listed in Table 21, in Examples No. 19A to No. 19O each using the slab having the appropriate composition, the good magnetic flux density was obtained, but in Comparative Example No. 19P having a Se content being less than the lower limit of the present invention range, the magnetic flux density was low.
Twenty-Third Experiment
In the twenty-third experiment, the effect of the nitriding treatment in the case of no S being contained was confirmed.
In the twenty-third experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.007 mass %, Mn: 0.12 mass %, Se: 0.007 mass %, and B: 0.0015 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained.
Thereafter, as for a sample of Comparative Example No. 20A, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby a decarburization-annealed steel strip was obtained. Further, as for a sample of Example No. 20B, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and further annealing was performed in an ammonia containing atmosphere, and thereby a decarburization-annealed steel strip having an N content of 0.023 mass % was obtained. Further, as for a sample of Example No. 20C, decarburization annealing was performed in a moist atmosphere gas at 860° C. for 100 seconds, and thereby a decarburization-annealed steel strip having an N content of 0.023 mass % was obtained. In this manner, three types of the decarburization-annealed steel strips were obtained.
Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 22.











TABLE 22











MAGNETIC



NITRIDING TREATMENT

PROPERTY















APPLICATION
SLAB HEATING

RIGHT
RIGHT
PRECIPITATES
MAGNETIC





















OR NO
HEATING



SIDE OF
SIDE OF

[B] −

FLUX




APPLICATION
TEMPER-


N
EXPRES-
EXPRES-
BasBN
BasBN
SeasMnSe
DENSITY




OF NITRIDING
ATURE
T2
T3
CONTENT
SION
SION
(MASS
(MASS
(MASS
B8



No.
TREATMENT
(° C.)
(° C.)
(° C.)
(MASS %)
(3)
(4)
%)
%)
%)
(T)























COM-
20A
NOT APPLIED
1100
1227
1195
0.007
0.016
0.020
0.0014
0.0001
0.0061
1.578


PARATIVE


EXAMPLE


EXAMPLE
20B
APPLIED
1100
1227
1195
0.023
0.016
0.020
0.0014
0.0001
0.0061
1.930



20C
APPLIED
1100
1227
1195
0.023
0.016
0.020
0.0014
0.0001
0.0061
1.927









As listed in Table 22, in Example No. 20B in which the nitriding treatment was performed after the decarburization annealing, and Example No. 20C in which the nitriding treatment was performed during the decarburization annealing, the good magnetic flux density was obtained. However, in Comparative Example No. 20A in which no nitriding treatment was performed, the magnetic flux density was low. Incidentally, the numerical value in the section of “NITRIDING TREATMENT” of Comparative Example No. 20A in Table 22 is a value obtained from the composition of the decarburization-annealed steel strip.
Twenty-Fourth Experiment
In the twenty-fourth experiment, the effect of the B content in the case of S and Se being contained was confirmed.
In the twenty-fourth experiment, first, slabs containing Si: 3.2 mass %, C: 0.05 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, Se: 0.006 mass %, and B having an amount listed in Table 23 (0 mass % to 0.0045 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 23.












TABLE 23












MAGNETIC




NITRIDING

PROPERTY




TREAT-

MAGNETIC



SLAB HEATING
MENT
PRECIPITATES
FLUX




















B
HEATING



N
BasBN
[B] −
SasMnS +
DENSITY




CONTENT
TEMPERATURE
T1
T2
T3
CONTENT
(MASS
BasBN
0.5 × SeasMnSe
B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(° C.)
(MASS %)
%)
(MASS %)
(MASS %)
(T)






















COMPARATIVE
21A
0
1100
1206
1197
—
0.023
0
0
0.007
1.882


EXAMPLE


EXAMPLE
21B
0.0009
1100
1206
1197
1173
0.023
0.0009
0
0.007
1.919



21C
0.0018
1100
1206
1197
1214
0.023
0.0017
0.0001
0.007
1.931



21D
0.0028
1100
1206
1197
1241
0.023
0.0027
0.0001
0.007
1.929



21E
0.0045
1100
1206
1197
1271
0.023
0.0044
0.0001
0.007
1.925









As listed in Table 23, in Comparative Example No. 21A having no B contained in the slab, the magnetic flux density was low, but in Examples No. 21B to No. 21E each having an appropriate amount of B contained in the slab, the good magnetic flux density was obtained.
Twenty-Fifth Experiment
In the twenty-fifth experiment, the effects of the B content and the slab heating temperature in the case of S and Se being contained were confirmed.
In the twenty-fifth experiment, first, slabs containing Si: 3.2 mass %, C: 0.05 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.1 mass %, S: 0.006 mass %, Se: 0.006 mass %, and B having an amount listed in Table 24 (0 mass % to 0.0045 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1180° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 24.












TABLE 24












MAGNETIC




NITRIDING

PROPERTY




TREAT-

MAGNETIC



SLAB HEATING
MENT
PRECIPITATES
FLUX




















B
HEATING



N
BasBN
[B] −
SasMnS +
DENSITY




CONTENT
TEMPERATURE
T1
T2
T3
CONTENT
(MASS
BasBN
0.5 × SeasMnSe
B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(° C.)
(MASS %)
%)
(MASS %)
(MASS %)
(T)






















COMPARATIVE
22A
0
1180
1206
1197
—
0.023
0
0
0.003
1.879


EXAMPLE
22B
0.0009
1180
1206
1197
1173
0.023
0.0003
0.0006
0.003
1.634


EXAMPLE
22C
0.0018
1180
1206
1197
1214
0.023
0.0013
0.0005
0.003
1.922



22D
0.0028
1180
1206
1197
1241
0.023
0.0023
0.0005
0.003
1.927



22E
0.0045
1180
1206
1197
1271
0.023
0.0038
0.0007
0.003
1.920









As listed in Table 24, in Comparative Example No. 22A having no B contained in the slab and Comparative Example No. 22B having the slab heating temperature higher than the temperature T3, the magnetic flux density was low. On the other hand, in Examples No. 22C to No. 22E each having an appropriate amount of B contained in the slab and having the slab heating temperature being the temperature T1 or lower, the temperature T2 or lower, and the temperature T3 or lower, the good magnetic flux density was obtained.
Twenty-Sixth Experiment
In the twenty-sixth experiment, the effects of the Mn content and the slab heating temperature in the case of S and Se being contained were confirmed.
In the twenty-sixth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.009 mass %, S: 0.006 mass %, Se: 0.004 mass %, B: 0.002 mass %, and Mn having an amount listed in Table 25 (0.05 mass % to 0.20 mass %), and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1200° C., and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 25.












TABLE 25












MAGNETIC




NITRIDING

PROPERTY




TREAT-

MAGNETIC



SLAB HEATING
MENT
PRECIPITATES
FLUX




















Mn
HEATING



N
BasBN
[B] −
SasMnS +
DENSITY




CONTENT
TEMPERATURE
T1
T2
T3
CONTENT
(MASS
BasBN
0.5 × SeasMnSe
B8



No.
(MASS %)
(° C.)
(° C.)
(° C.)
(° C.)
(MASS %)
%)
(MASS %)
(MASS %)
(T)






















COMPARATIVE
23A
0.05
1200
1163
1107
1227
0.022
0.0011
0.0009
0.001
1.824


EXAMPLE
23B
0.08
1200
1192
1144
1227
0.022
0.0012
0.0008
0.001
1.835


EXAMPLE
23C
0.16
1200
1237
1203
1227
0.022
0.0016
0.0004
0.004
1.931



23D
0.20
1200
1252
1222
1227
0.022
0.0017
0.0003
0.005
1.925









As listed in Table 25, in Comparative Examples No. 23A and No. 23B each having the slab heating temperature higher than the temperature T1 and the temperature T2, the magnetic flux density was low. On the other hand, in Examples No. 23C and No. 23D each having the slab heating temperature being the temperature T1 or lower, the temperature T2 or lower, and the temperature T3 or lower, the good magnetic flux density was obtained.
Twenty-Seventh Experiment
In the twenty-seventh experiment, the effect of the finish temperature Tf of the finish rolling in the hot rolling in the case of S and Se being contained was confirmed.
In the twenty-seventh experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.12 mass %, S: 0.005 mass %, Se: 0.005 mass %, and B: 0.002 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1180° C., and thereafter were subjected to finish rolling at the finish temperature Tf listed in Table 26 (800° C. to 1000° C.). In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.022 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 26.












TABLE 26












MAGNETIC



FINISH ROLLING
NITRIDING

PROPERTY














SLAB HEATING
FINISH

TREAT-
PRECIPITATES
MAGNETIC





















HEATING



TEMPER-
RIGHT SIDE
MENT

[B]−

FLUX




TEMPER-



ATURE
OF
N
BasBN
BasBN
SasMnS +
DENSITY




ATURE
T1
T2
T3
Tf
EXPRESSION
CONTENT
(MASS
(MASS
0.5 × SeasMnSe
B8



No.
(° C.)
(° C.)
(° C.)
(° C.)
(° C.)
(4)
(MASS %)
%)
%)
(MASS %)
(T)























EXAMPLE
24A
1180
1206
1197
1220
800
980
0.022
0.0016
0.0004
0.003
1.929



24B
1180
1206
1197
1220
850
980
0.022
0.0016
0.0004
0.003
1.930



24C
1180
1206
1197
1220
900
980
0.022
0.0015
0.0005
0.003
1.928


COM-
24D
1180
1206
1197
1220
1000
980
0.022
0.0012
0.0008
0.003
1.895


PARATIVE


EXAMPLE









In the case of the B content being 0.002 mass % (20 ppm), the finish temperature Tf is necessary to be 980° C. or lower based on inequation (4). Then, as listed in Table 26, in Examples No. 24A to 24C each satisfying the condition, the good magnetic flux density was obtained, but in Comparative Example No. 24D not satisfying the condition, the magnetic flux density was low.
Twenty-Eighth Experiment
In the twenty-eighth experiment, the effect of the N content after the nitriding treatment in the case of S and Se being contained was confirmed.
In the twenty-eighth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.14 mass %, S: 0.005 mass %, Se: 0.005 mass %, and B: 0.002 mass %, a content of Ti that is an impurity being 0.0018 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to an amount listed in Table 27 (0.012 mass % to 0.028 mass %). Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 27.










TABLE 27









FINISH ROLLING
NITRIDING TREATMENT













SLAB HEATING

RIGHT SIDE

RIGHT SIDE


















HEATING



FINISH
OF
N
OF




TEMPERATURE
T1
T2
T3
TEMPERATURE
EXPRESSION
CONTENT
EXPRESSION



No.
(° C.)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)
(8)





EXAMPLE
25A
1150
1216
1211
1220
900
980
0.012
0.018



25B
1150
1216
1211
1220
900
980
0.017
0.018



25C
1150
1216
1211
1220
900
980
0.022
0.018



25D
1150
1216
1211
1220
900
980
0.028
0.018
















MAGNETIC



NITRIDING TREATMENT

PROPERTY



RIGHT SIDE
PRECIPITATES
MAGNETIC

















OF


SasMnS + 0.5 ×
FLUX





EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
25A
0.022
0.0018
0.0002
0.004
1.883




25B
0.022
0.0018
0.0002
0.004
1.911




25C
0.022
0.0018
0.0002
0.004
1.926




25D
0.022
0.0018
0.0002
0.004
1.928










As listed in Table 27, in Examples No. 25C and No. 25D in which an N content after the nitriding treatment satisfied the relation of inequation (8) and the relation of inequation (9), the particularly good magnetic flux density was obtained. On the other hand, in Examples No. 25A and No. 25B in which an N content after the nitriding treatment did not satisfy the relation of inequation (8) and the relation of inequation (9), the magnetic flux density was slightly lower than those in Examples No. 25C and No. 25D.
Twenty-Ninth Experiment
In the twenty-ninth experiment, the effect of the condition of the finish annealing in the case of S and Se being contained was confirmed.
In the twenty-ninth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.14 mass %, S: 0.005 mass %, Se: 0.005 mass %, and B: 0.002 mass %, a content of Ti that is an impurity being 0.0018 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1000° C. at a rate of 15° C./h, and further were heated up to 1200° C. at a rate listed in Table 28 (5° C./h to 30° C./h) and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 28.












TABLE 28









FINISH

FINISH ROLLING
NITRIDING













ANNEALING
SLAB HEATING

RIGHT SIDE
TREATMENT


















HEATING
HEATING



FINISH
OF
N




SPEED
TEMPERATURE
T1
T2
T3
TEMPERATURE
EXPRESSION
CONTENT



No.
(° C./h)
(° C.)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)





EXAMPLE
26A
5
1150
1216
1211
1220
900
980
0.023



26B
10
1150
1216
1211
1220
900
980
0.023



26C
15
1150
1216
1211
1220
900
980
0.023



26D
30
1150
1216
1211
1220
900
980
0.023
















MAGNETIC



NITRIDING TREATMENT
PRECIPITATES
PROPERTY


















RIGHT SIDE
RIGHT SIDE


SasMnS +
MAGNETIC





OF
OF


0.5 ×
FLUX





EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
26A
0.018
0.022
0.0018
0.0002
0.004
1.932




26B
0.018
0.022
0.0018
0.0002
0.004
1.928




26C
0.018
0.022
0.0018
0.0002
0.004
1.922




26D
0.018
0.022
0.0018
0.0002
0.004
1.899










As listed in Table 28, in Examples No. 26A to No. 26C, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 26D, the heating rate in the temperature range exceeded 15° C./h, so that the magnetic flux density was slightly lower than those in Examples No. 26A to No. 26C.
Thirtieth Experiment
In the thirtieth experiment, the effect of the condition of the finish annealing in the case of S and Se being contained was confirmed.
In the thirtieth experiment, first, slabs containing Si: 3.3 mass %, C: 0.06 mass %, acid-soluble Al: 0.028 mass %, N: 0.008 mass %, Mn: 0.14 mass %, S: 0.005 mass %, Se: 0.005 mass %, and B: 0.002 mass %, a content of Ti that is an impurity being 0.0018 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.024 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips. Then, in Example No. 27A, the steel strip was heated up to 1200° C. at a rate of 15° C./h and was finish annealed. Further, in Examples No. 27B to No. 27E, the steel strips were heated up to a temperature listed in Table 29 (1000° C. to 1150° C.) at a rate of 30° C./h and were kept for 10 hours at the temperature, and thereafter were heated up to 1200° C. at a rate of 30° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 29.












TABLE 29









FINISH

FINISH ROLLING














ANNEALING
SLAB HEATING

RIGHT SIDE
NITRIDING


















MAINTAINING
HEATING



FINISH
OF
TREATMENT




TEMPERATURE
TEMPERATURE
T1
T2
T3
TEMPERATURE
EXPRESSION
N CONTENT



No.
(° C.)
(° C.)
(° C.)
(° C.)
(° C.)
Tf (° C.)
(4)
(MASS %)





EXAMPLE
27A
—
1150
1216
1211
1220
900
980
0.024



27B
1000
1150
1216
1211
1220
900
980
0.024



27C
1050
1150
1216
1211
1220
900
980
0.024



27D
1100
1150
1216
1211
1220
900
980
0.024



27E
1150
1150
1216
1211
1220
900
980
0.024
















MAGNETIC



NITRIDING TREATMENT
PRECIPITATES
PROPERTY


















RIGHT SIDE
RIGHT SIDE


SasMnS +
MAGNETIC





OF
OF


0.5 ×
FLUX





EXPRESSION
EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8




No.
(8)
(9)
(MASS %)
(MASS %)
(MASS %)
(T)







EXAMPLE
27A
0.018
0.022
0.0018
0.0002
0.004
1.907




27B
0.018
0.022
0.0018
0.0002
0.004
1.926




27C
0.018
0.022
0.0018
0.0002
0.004
1.934




27D
0.018
0.022
0.0018
0.0002
0.004
1.928




27E
0.018
0.022
0.0018
0.0002
0.004
1.891










As listed in Table 29, in Example No. 27A, the heating rate in a temperature range of 1000° C. to 1100° C. was set to 15° C./h or less, so that the particularly good magnetic flux density was obtained. Further, in Examples No. 27B to 27D, the steel strips were kept in the temperature range of 1000° C. to 1100° C. for 10 hours, so that the particularly good magnetic flux density was obtained. On the other hand, in Example No. 27E, the temperature at which the steel strip was kept for 10 hours exceeded 1100° C., so that the magnetic flux density was slightly lower than those in Examples No. 27A to No. 27D.
Thirty-First Experiment
In the thirty-first experiment, the effect of the slab heating temperature in the case of S and Se being contained was confirmed.
In the thirty-first experiment, first, slabs containing Si: 3.1 mass %, C: 0.05 mass %, acid-soluble Al: 0.027 mass %, N: 0.008 mass %, Mn: 0.11 mass %, S: 0.006 mass %, Se: 0.007 mass %, and B: 0.0025 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at a temperature listed in Table 30 (1100° C. to 1300° C.), and thereafter were subjected to finish rolling at 950° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.021 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h, and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 30.












TABLE 30












MAGNETIC






PROPERTY



SLAB HEATING
NITRIDING
PRECIPITATES
MAGNETIC



















HEATING



TREATMENT


SasMnS +
FLUX




TEMPERATURE
T1
T2
T3
N CONTENT
BasBN
[B] − BasBN
0.5 × SeasMnSe
DENSITY B8



No.
(° C.)
(° C.)
(° C.)
(° C.)
(MASS %)
(MASS %)
(MASS %)
(MASS %)
(T)





















EXAMPLE
28A
1100
1212
1219
1234
0.021
0.0023
0.0002
0.008
1.931



28B
1150
1212
1219
1234
0.021
0.0021
0.0004
0.006
1.928



28C
1200
1212
1219
1234
0.021
0.0018
0.0007
0.002
1.921


COMPARATIVE
28D
1250
1212
1219
1234
0.021
0.0004
0.0021
0.001
1.772


EXAMPLE
28E
1300
1212
1219
1234
0.021
0.0002
0.0023
0.001
1.654









As listed in Table 30, in Examples No. 28A to No. 28C each having the slab heating temperature being the temperature T1 or lower, the temperature T2 or lower, and the temperature T3 or lower, the good magnetic flux density was obtained. On the other hand, in Comparative Examples No. 28D and No. 28E each having the slab heating temperature higher than the temperature T1, the temperature T2, and the temperature T3, the magnetic flux density was low.
Thirty-Second Experiment
In the thirty-second experiment, the effect of the components of the slab in the case of S and Se being contained was confirmed.
In the thirty-second experiment, first, slabs containing components listed in Table 31 and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1100° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained. Thereafter, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby decarburization-annealed steel strips were obtained. Subsequently, the decarburization-annealed steel strips were annealed in an ammonia containing atmosphere to increase nitrogen in the steel strips up to 0.023 mass %. Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 32.









TABLE 31









COMPOSITION OF SILICON STEEL MATERIAL (MASS %)

























No.
Si
C
Al
N
Mn
S
Se
B
Cr
Cu
Ni
P
Mo
Sn
Sb
Bi




























EXAMPLE
29A
3.3
0.06
0.028
0.008
0.12
0.005
0.007
0.002
—
—
—
—
—
—
—
—



29B
3.2
0.06
0.027
0.009
0.12
0.007
0.005
0.002
0.15
—
—
—
—
—
—
—



29C
3.4
0.06
0.025
0.008
0.12
0.006
0.007
0.002
—
0.2 
—
—
—
—
—
—



29D
3.3
0.06
0.027
0.008
0.12
0.006
0.007
0.002
—
—
0.1
—
—
—
—
—



29E
3.3
0.06
0.024
0.007
0.12
0.006
0.007
0.002
—
—
0.4
—
—
—
—
—


COMPARATIVE
29F
3.1
0.06
0.027
0.009
0.12
0.006
0.007
0.002
—
—
1.3
—
—
—
—
—


EXAMPLE


EXAMPLE
29G
3.4
0.06
0.028
0.007
0.12
0.006
0.007
0.002
—
—
—
0.03
—
—
—
—



29H
3.2
0.06
0.027
0.008
0.12
0.006
0.007
0.002
—
—
—
—
0.005
—
—
—



29I
3.3
0.06
0.028
0.008
0.12
0.006
0.007
0.002
—
—
—
—
—
0.04
—
—



29J
3.3
0.06
0.025
0.008
0.12
0.006
0.007
0.002
—
—
—
—
—
—
0.04
—



29K
3.3
0.06
0.024
0.009
0.12
0.006
0.007
0.002
—
—
—
—
—
—
—
0.003



29L
3.2
0.06
0.030
0.008
0.12
0.006
0.004
0.002
0.10
—
—
0.03
—
0.06
—
—



29M
3.8
0.06
0.027
0.008
0.12
0.005
0.005
0.002
0.05
0.15
 0.05
0.02
—
0.04
—
—



29N
3.3
0.06
0.028
0.009
0.12
0.006
0.004
0.002
0.08
—
—
—
0.003
0.05
—
0.001



29O
2.8
0.06
0.022
0.008
0.12
0.004
0.007
0.002
—
—
—
—
—
—
—
—


COMPARATIVE
29P
3.3
0.06
0.035
0.007
0.12
0.001
 0.0003
0.002
—
—
—
—
—
—
—
—


EXAMPLE



















TABLE 32










MAGNETIC



PRECIPITATES
PROPERTY














BasBN
[B] − BasBN
SasMnS + 0.5 × SeasMnSe
MAGNETIC FLUX



No.
(MASS %)
(MASS %)
(MASS %)
DENSITY B8 (T)
















EXAMPLE
29A
0.0018
0.0002
0.007
1.924



29B
0.0019
0.0001
0.008
1.925



29C
0.0018
0.0002
0.008
1.931



29D
0.0018
0.0002
0.008
1.925



29E
0.0018
0.0002
0.008
1.924


COMPARATIVE
29F
0.0019
0.0001
0.008
1.713


EXAMPLE


EXAMPLE
29G
0.0018
0.0002
0.008
1.931



29H
0.0019
0.0001
0.008
1.924



29I
0.0018
0.0002
0.008
1.924



29J
0.0019
0.0001
0.008
1.927



29K
0.0019
0.0001
0.008
1.926



29L
0.0018
0.0002
0.007
1.932



29M
0.0019
0.0001
0.006
1.930



29N
0.0019
0.0001
0.007
1.927



29O
0.0018
0.0002
0.006
1.939


COMPARATIVE
29P
0.0018
0.0002
0.001
1.578


EXAMPLE









As listed in Table 32, in Examples No. 29A to No. 29E and No. 29G to No. 29O each using the slab having the appropriate composition, the good magnetic flux density was obtained, but in Comparative Example No. 29F having a Ni content higher than the upper limit of the present invention range and Comparative Example No. 29P having a total amount of a content of S and Se being less than the lower limit of the present invention range, the magnetic flux density was low.
Thirty-Third Experiment
In the thirty-third experiment, the effect of the nitriding treatment in the case of S and Se being contained was confirmed.
In the thirty-third experiment, first, slabs containing Si: 3.2 mass %, C: 0.06 mass %, acid-soluble Al: 0.027 mass %, N: 0.007 mass %, Mn: 0.14 mass %, S: 0.006 mass %, Se: 0.005 mass %, and B: 0.0015 mass %, and a balance being composed of Fe and inevitable impurities were manufactured. Next, the slabs were heated at 1150° C., and thereafter were subjected to finish rolling at 900° C. In this manner, hot-rolled steel strips each having a thickness of 2.3 mm were obtained. Subsequently, annealing of the hot-rolled steel strips was performed at 1100° C. Next, cold rolling was performed, and thereby cold-rolled steel strips each having a thickness of 0.22 mm were obtained.
Thereafter, as for a sample of Comparative Example No. 30A, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and thereby a decarburization-annealed steel strip was obtained. Further, as for a sample of Example No. 30B, decarburization annealing was performed in a moist atmosphere gas at 830° C. for 100 seconds, and further annealing was performed in an ammonia containing atmosphere, and thereby a decarburization-annealed steel strip having an N content of 0.021 mass % was obtained. Further, as for a sample of Example No. 30C, decarburization annealing was performed in a moist atmosphere gas at 860° C. for 100 seconds, and thereby a decarburization-annealed steel strip having an N content of 0.021 mass % was obtained. In this manner, three types of the decarburization-annealed steel strips were obtained.
Next, an annealing separating agent containing MgO as its main component was coated on the steel strips, and the steel strips were heated up to 1200° C. at a rate of 15° C./h and were finish annealed. Then, similarly to the fourth experiment, a magnetic property (the magnetic flux density B8) was measured. A result of the measurement is listed in Table 33.











TABLE 33









APPLICATION OR
SLAB HEATING
NITRIDING TREATMENT

















NO APPLICATION
HEATING



N
RIGHT SIDE OF




OF NITRIDING
TEMPERATURE
T1
T2
T3
CONTENT
EXPRESSION



No.
TREATMENT
(° C.)
(° C.)
(° C.)
(° C.)
(MASS %)
(3)





COMPARATIVE
30A
NOT APPLIED
1150
1228
1211
1195
0.007
0.016


EXAMPLE


EXAMPLE
30B
APPLIED
1150
1228
1211
1195
0.021
0.016



30C
APPLIED
1150
1228
1211
1195
0.021
0.016
















MAGNETIC





PROPERTY



NITRIDING TREATMENT
PRECIPITATES
MAGNETIC















RIGHT SIDE OF


SasMnS + 0.5 ×
FLUX




EXPRESSION
BasBN
[B] − BasBN
SeasMnSe
DENSITY B8



No.
(4)
(MASS %)
(MASS %)
(MASS %)
(T)





COMPARATIVE
30A
0.020
0.0014
0.0001
0.006
1.645


EXAMPLE


EXAMPLE
30B
0.020
0.0014
0.0001
0.006
1.932



30C
0.020
0.0014
0.0001
0.006
1.929









As listed in Table 33, in Example No. 30B in which the nitriding treatment was performed after the decarburization annealing, and Example No. 30C in which the nitriding treatment was performed during the decarburization annealing, the good magnetic flux density was obtained. However, in Comparative Example No. 30A in which no nitriding treatment was performed, the magnetic flux density was low. Incidentally, the numerical value in the section of “NITRIDING TREATMENT” of Comparative Example No. 30A in Table 33 is a value obtained from the composition of the decarburization-annealed steel strip.
INDUSTRIAL APPLICABILITY
The present invention can be utilized in, for example, an industry of manufacturing electrical steel sheets and an industry in which electrical steel sheets are used.

## Claims

1. A manufacturing method of a grain-oriented electrical steel sheet, comprising:
at a predetermined temperature, heating a silicon steel material containing Si: 0.8 mass % to 7 mass %, acid-soluble Al: 0.01 mass % to 0.065 mass %, N: 0.004 mass % to 0.012 mass %, Mn: 0.05 mass % to 1 mass %, and B: 0.0005 mass % to 0.0080 mass %, the silicon steel material further containing at least one element selected from a group consisting of S and Se being 0.003 mass % to 0.015 mass % in total amount, a C content being 0.085 mass % or less, and a balance being composed of Fe and inevitable impurities;
hot rolling the heated silicon steel material so as to obtain a hot-rolled steel strip;
annealing the hot-rolled steel strip so as to obtain an annealed steel strip;
cold rolling the annealed steel strip one time or more so as to obtain a cold-rolled steel strip;
decarburization annealing the cold-rolled steel strip so as to obtain a decarburization-annealed steel strip in which primary recrystallization is caused;
coating an annealing separating agent containing MgO as its main component on the decarburization-annealed steel strip; and
causing secondary recrystallization by finish annealing the coated decarburization-annealed steel strip, wherein
the method further comprises performing a nitriding treatment in which an N content of the decarburization-annealed steel strip is increased between start of the decarburization annealing and occurrence of the secondary recrystallization in the finish annealing,

wherein:
in a case when S and Se are both contained in the silicon steel material, the predetermined temperature is lower than each of T1 (° C.), T2 (° C.) and T3 (° C.), the temperature T1 being expressed by equation (1) below, the temperature T2 being expressed by equation (2) below, and the temperature T3 being expressed by equation (3) below;
in a case when S is contained but no Se is contained in the silicon steel material, the predetermined temperature is lower than each of T1 (° C.) and T3 (° C.);
in a case when Se is contained but no S is contained in the silicon steel material, the predetermined temperature is lower than each of T2 (° C.) and T3 (° C.);
a finish temperature Tf of finish rolling in the hot rolling satisfies inequation (4) below, and
amounts of BN, MnS, and MnSe in the hot-rolled steel strip satisfy inequations (5), (6), and (7) below,


T1=14855/(6.82−log([Mn]×[S]))−273  (1)



T2=10733/(4.08−log([Mn]×[Se]))−273  (2)



T3=16000/(5.92−log([B]×[N]))−273  (3)



Tf≦1000−10000×[B]  (4)



BasBN≧0.0005  (5)



[B]−BasBN≦0.001  (6)



SasMnS+0.5×SeasMnSe≧0.002  (7)


wherein, [Mn] represents a Mn content (mass %) of the silicon steel material, [S] represents an S content (mass %) of the silicon steel material, [Se] represents a Se content (mass %) of the silicon steel material, [B] represents a B content (mass %) of the silicon steel material, [N] represents an N content (mass %) of the silicon steel material, BasBN represents an amount of B (mass %) that has precipitated as BN in the hot-rolled steel strip, SasMnS represents an amount of S (mass %) that has precipitated as MnS in the hot-rolled steel strip, and SeasMnSe represents an amount of Se (mass %) that has precipitated as MnSe in the hot-rolled steel strip.

2. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the nitriding treatment is performed under a condition that an N content [N] of a steel strip obtained after the nitriding treatment satisfies inequation (8) below,


[N]≧14/27[Al]+14/11[B]+14/47[Ti]  (8)

wherein, [N] represents the N content (mass %) of the steel strip obtained after the nitriding treatment, [Al] represents an acid-soluble Al content (mass %) of the steel strip obtained after the nitriding treatment, and [Ti] represents a Ti content (mass %) of the steel strip obtained after the nitriding treatment.

3. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the nitriding treatment is performed under a condition that an N content [N] of a steel strip obtained after the nitriding treatment satisfies inequation (9) below,


[N]≧2/3[Al]+14/11[B]+14/47[Ti]  (9)

wherein, [N] represents the N content (mass %) of the steel strip obtained after the nitriding treatment, [Al] represents an acid-soluble Al content (mass %) of the steel strip obtained after the nitriding treatment, and [Ti] represents a Ti content (mass %) of the steel strip obtained after the nitriding treatment.

4. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the step of causing the secondary recrystallization includes heating the coated decarburization-annealed steel strip at a rate of 15° C./h or less in a temperature range of 1000° C. to 1100° C. in the finish annealing.

5. The manufacturing method of the grain-oriented electrical steel sheet according to claim 2, wherein the step of causing the secondary recrystallization includes heating the coated decarburization-annealed steel strip at a rate of 15° C./h or less in a temperature range of 1000° C. to 1100° C. in the finish annealing.

6. The manufacturing method of the grain-oriented electrical steel sheet according to claim 3, wherein the step of causing the secondary recrystallization includes heating the coated decarburization-annealed steel strip at a rate of 15° C./h or less in a temperature range of 1000° C. to 1100° C. in the finish annealing.

7. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the step of causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

8. The manufacturing method of the grain-oriented electrical steel sheet according to claim 2, wherein step of the causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

9. The manufacturing method of the grain-oriented electrical steel sheet according to claim 3, wherein the step of causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

10. The manufacturing method of the grain-oriented electrical steel sheet according to claim 4, wherein the step of causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

11. The manufacturing method of the grain-oriented electrical steel sheet according to claim 5, wherein the step of causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

12. The manufacturing method of the grain-oriented electrical steel sheet according to claim 6, wherein the step of causing the secondary recrystallization includes keeping the coated decarburization-annealed steel strip in a temperature range of 1000° C. to 1100° C. for 10 hours or longer in the finish annealing.

13. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

14. The manufacturing method of the grain-oriented electrical steel sheet according to claim 2, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

15. The manufacturing method of the grain-oriented electrical steel sheet according to claim 3, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

16. The manufacturing method of the grain-oriented electrical steel sheet according to claim 4, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

17. The manufacturing method of the grain-oriented electrical steel sheet according to claim 5, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

18. The manufacturing method of the grain-oriented electrical steel sheet according to claim 6, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

19. The manufacturing method of the grain-oriented electrical steel sheet according to claim 7, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

20. The manufacturing method of the grain-oriented electrical steel sheet according to claim 8, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

21. The manufacturing method of the grain-oriented electrical steel sheet according to claim 9, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

22. The manufacturing method of the grain-oriented electrical steel sheet according to claim 10, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

23. The manufacturing method of the grain-oriented electrical steel sheet according to claim 11, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

24. The manufacturing method of the grain-oriented electrical steel sheet according to claim 12, wherein the silicon steel material further contains at least one element selected from a group consisting of Cr: 0.3 mass % or less, Cu: 0.4 mass % or less, Ni: 1 mass % or less, P: 0.5 mass % or less, Mo: 0.1 mass % or less, Sn: 0.3 mass % or less, Sb: 0.3 mass % or less, and Bi: 0.01 mass % or less.

25. The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, wherein BN precipitates during the step of heating the silicon steel material.