Method for manufacturing a grain-oriented electrical steel sheet

Abstract

Provided is a method for manufacturing a grain-oriented electrical steel sheet. The method comprises: hot rolling a slab to obtain a hot rolled sheet; subjecting the hot rolled sheet to hot band annealing as necessary; subjecting the hot rolled sheet to cold rolling; subjecting the cold rolled sheet to decarburization annealing; applying an annealing separator having MgO as a main component onto a surface of the decarburization annealed sheet and subjecting the decarburization annealed sheet to final annealing to form the forsterite film; and applying an insulating coating treatment liquid onto the final annealed sheet and subjecting the final annealed sheet to flattening annealing to form a tension-applying insulating coating. A difference in total tensions between one and opposite surfaces of the sheet is less than 0.5 MPa. A difference in tensions between the forsterite films in one and opposite surfaces of the sheet is 0.5 MPa or more.

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet having a forsterite film and a tension-applying insulating coating at both of one surface and an opposite surface thereof, the method comprising: hot rolling a slab to obtain a hot rolled sheet; optionally subjecting the hot rolled sheet to hot band annealing; subsequently subjecting the hot rolled sheet to cold rolling once, or twice or more with intermediate annealing in-between, to obtain a cold rolled sheet of final sheet thickness; subsequently subjecting the cold rolled sheet to decarburization annealing to obtain a decarburization annealed sheet; subsequently applying an annealing separator having MgO as a main component onto both of one surface and an opposite surface of the decarburization annealed sheet and then subjecting the decarburization annealed sheet to final annealing to form the forsterite film and obtain a final annealed sheet; and subsequently applying an insulating coating treatment liquid onto both of one surface and an opposite surface of the final annealed sheet and then subjecting the final annealed sheet to flattening annealing that also serves as baking to form a tension-applying insulating coating on both of the one surface and the opposite surface of the final annealed sheet to obtain the grain-oriented electrical steel sheet, wherein in formation of the forsterite film and the tension-applying insulating coating in both of the one surface and the opposite surface of the sheet, an absolute value of a difference between total tension of the forsterite film and the tension-applying insulating coating in the one surface of the sheet and total tension of the forsterite film and the tension-applying insulating coating in the opposite surface of the sheet is adjusted to less than 0.5 MPa, an absolute value of a difference between tension of the forsterite film in the one surface of the sheet and tension of the forsterite film in the opposite surface of the sheet is adjusted to 0.5 MPa or more, and an absolute value of a difference between tension of the tension-applying insulating coating in the one surface of the sheet and tension of the tension-applying insulating coating in the opposite surface of the sheet is more than 0 MPa.

2. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein tension of the forsterite film is adjusted through at least one of: changing formation conditions of an internal oxidation layer at front and rear steel sheet surfaces in the decarburization annealing; changing the annealing separator in terms of type; changing the annealing separator in terms of application amount; and changing an amount of electrodeposition in electrodeposition treatment performed before the final annealing.

3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein tension of the tension-applying insulating coating is adjusted through at least one of: changing the tension-applying insulating coating in terms of thickness; and changing the tension-applying insulating coating in terms of composition.

4. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein tension of the tension-applying insulating coating is adjusted through at least one of: changing the tension-applying insulating coating in terms of thickness; and changing the tension-applying insulating coating in terms of composition.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) In the accompanying drawings:

(2) FIG. 1 illustrates an example of a magnetostrictive vibration velocity level;

(3) FIG. 2 illustrates a relationship between the number of acceleration/deceleration points and actual transformer noise;

(4) FIG. 3 illustrates the magnitude of velocity level change between adjacent velocity level change points of magnetostrictive vibration;

(5) FIG. 4 illustrates a relationship between a maximum value of the magnitude of velocity level change and noise;

(6) FIG. 5 illustrates a relationship between the number of acceleration/deceleration points and actual transformer noise;

(7) FIG. 6 illustrates a relationship between film tension differences and a magnetostrictive property (number of acceleration/deceleration points in one period of magnetostrictive vibration);

(8) FIG. 7 illustrates a relationship between film tension differences and a magnetostrictive property (magnitude of velocity level change between adjacent velocity level change points);

(9) FIG. 8 illustrates a relationship between the magnitude of deflection of a steel sheet and a magnetostrictive property (number of acceleration/deceleration points in one period of magnetostrictive vibration);

(10) FIG. 9 illustrates a relationship between the magnitude of deflection of a steel sheet and a magnetostrictive property (magnitude of velocity level change between adjacent velocity level change points);

(11) FIG. 10 illustrates results of evaluation of the magnitude of iron loss deterioration performed through magnetic measurement of samples in Experiment 2;

(12) FIG. 11 illustrates a relationship between forsterite film tension difference and a magnetostrictive property (magnitude of velocity level change between adjacent velocity level change points) for various front/rear differences in total tension (total difference) of a forsterite film and an insulating coating;

(13) FIG. 12 illustrates a relationship between forsterite film tension difference and a magnetostrictive property (number of acceleration/deceleration points in one period of magnetostrictive vibration) for various front/rear differences in total tension (total difference) of a forsterite film and an insulating coating; and

(14) FIG. 13 illustrates a relationship between forsterite film tension difference and iron loss for various front/rear differences in total tension (total difference) of a forsterite film and an insulating coating.

DETAILED DESCRIPTION

(15) The following provides a detailed description of our techniques.

(16) First, the reasons for limitations placed on the features of this disclosure are explained.

(17) 1) We discovered that velocity change of magnetostrictive vibration has at least as great an influence on the noise of an actual transformer as conventionally known parameters.

(18) Although the reason for this discovery is not clear, we presume that expansion and contraction of a steel sheet with repeated acceleration and deceleration requires a greater amount of energy and that this increased expansion and contraction energy is a major cause of increased noise.

(19) Herein, the number of acceleration/deceleration points in a magnetostriction velocity level dλ/dt (i.e., the number of instances of acceleration/deceleration at which there is a very large velocity change) in one period of magnetostrictive vibration is limited to 4, which is the minimum number of acceleration/deceleration points that occur even in the case of ideal sinusoidal wave magnetostrictive vibration. Moreover, the magnitude of velocity level change between adjacent velocity level change points in an acceleration zone or deceleration zone is preferably small and is thus limited to 3.0×10.sup.−4 sec.sup.−1 or less.

(20) Although a smaller magnitude of velocity level change is better, a magnitude of velocity level change of approximately 1.0×10.sup.−5 sec.sup.−1 or more is preferable from an industrial viewpoint.

(21) 2) With regards to a rolling direction at the front and rear of a steel sheet, a front/rear difference in total tension of a forsterite film and an insulating coating is preferably less than 0.5 MPa, whereas a front/rear difference in forsterite film tension is preferably 0.5 MPa or more. Herein, the phrase “front/rear difference in tension” refers to the absolute value of the difference between tension at the front surface of the steel sheet and tension at the rear surface of the steel sheet.

(22) Although the front/rear difference in total tension of the forsterite film and the insulating coating does not have a specific lower limit, a lower limit of approximately 0.1 MPa is preferable from an industrial viewpoint. Moreover, although the front/rear difference in forsterite film tension and the front/rear difference in insulating coating tension do not have a specific upper limit, an upper limit of approximately 10 MPa is preferable from an industrial viewpoint.

(23) The difference in total tension of the forsterite film and the insulating coating between the surfaces of the steel sheet is limited to less than 0.5 MPa because stress introduced through a larger magnitude of deflection causes deterioration of iron loss. On the other hand, the front/rear difference in forsterite film tension is limited to 0.5 MPa or more because magnetostrictive properties are significantly enhanced by creating a difference in tension of the forsterite film between the front and rear of the steel sheet.

(24) The front/rear difference in tension of the entire steel sheet can be kept to less than 0.5 MPa by applying higher insulating coating tension at a side of the steel sheet that has lower forsterite film tension. We presume that in a situation in which a surface having high forsterite film tension and a surface having high insulating coating tension are set as opposite surfaces, the direction of deflection caused by the forsterite film and the direction of deflection caused by the tension coating are in direct opposition, and thus the magnetostrictive property enhancement effects thereof act to cancel each other out. However, since the levels of these enhancement effects differ, a magnetostriction enhancement effect by the forsterite film remains even after this canceling out, resulting in enhanced magnetostrictive properties.

(25) Next, means for changing the forsterite film tension and the insulating coating tension are explained. Conventional and commonly known methods may be used such as those described in JP 2009-235472 A and JP 2009-235473 A.

(26) First, methods for controlling forsterite film tension are described. A cold rolled sheet for a grain-oriented electrical steel sheet manufactured by a normal method is used as a material. No specific limitations are placed on the material composition and manufacturing conditions up to cold rolling, which may be the same as those in conventional and commonly known methods. The cold rolled sheet is subsequently subjected to decarburization annealing to obtain a decarburization annealed sheet. Thereafter, an annealing separator composed mainly of MgO is applied onto the surface of the decarburization annealed sheet, which is then subjected to final annealing to form a forsterite film. The tension of the forsterite film is changed between the front and rear surfaces of the steel sheet by changing the film properties between the front and rear surfaces according to any of the following methods.

(27) First, in a situation in which grinding is performed in pretreatment, the grinding may be performed such that the grinding amounts at the front and rear surfaces differ. Although grinding is normally performed to achieve a residual oxygen coating amount of 0.2 g/m.sup.2 or less, it is preferable that grinding is performed more strongly at one surface to create a difference in the residual oxygen coating amount between the front and rear surfaces of 0.05 g/m.sup.2 or more. In a situation in which electrodeposition treatment is performed in pretreatment, a front/rear difference in tension may be created through the amount of electrodeposition. The electrodeposited material may be a metal such as Cu, Ni, Co, or Sn as disclosed, for example, in JP H9-87744 A, and may be electrodeposited in a greater amount at a coil bend inner surface side such as to create a difference in the amount of electrodeposition between the front and rear surfaces of 0.2 mg/m.sup.2 or more. This electrodeposition treatment is not limited to being performed before decarburization annealing, and may alternatively be performed after decarburization annealing.

(28) Herein, a separator composed mainly of MgO is used as an annealing separator. Examples of commonly known additives that may be used with the annealing separator include compounds containing an element such as Ti, Sb, Mg, Ca, Sr, Sn, B, Na, K, Cl, F, or Br.

(29) Such additives may also be used to change the forsterite film tension between the front and rear surfaces. In other words, the forsterite film tension may be changed by adding such additives in different amounts at the front and rear surfaces. The conditions for creating a front/rear difference in tension vary depending on the type of additive that is used. In the case of Ti, Sb, Mg, Ca, Sr, or Sn, it is preferable to create a difference, in terms of metal, of approximately 0.2 parts by mass per 100 parts by mass of MgO, whereas in the case of B, Na, K, Cl, F, or Br, it is preferable to create a difference of 0.001 parts by mass or more per 100 parts by mass of MgO.

(30) Next, methods for controlling insulating coating tension are described. A final annealed sheet for a grain-oriented electrical steel sheet obtained after secondary recrystallization by coil annealing is used as a material. No specific limitations are placed on the composition of the material. Since final annealing is normally performed over a long period, such as several days, it is standard practice for the annealing to be performed with the steel sheet wound up in coil form. Herein, final annealing is preferably also performed by this conventional method.

(31) The surface of the coil obtained after final annealing is cleaned by water washing or phosphoric acid pickling prior to application of an insulating coating treatment liquid. This cleaning can also be performed by a conventional method. After the cleaning, the insulating coating treatment liquid is applied. Although the coating itself may be a conventional tension-applying coating, it is a feature of this disclosure that a higher insulating coating tension is applied at the opposite surface to the surface at which a higher forsterite film tension is applied.

(32) The following describes a suitable tension-applying coating. The most commonly used type of coating is a phosphate-silica-based coating. In terms of composition, the phosphate-silica-based coating preferably contains, by solid content ratio, approximately 10 parts by mass to 80 parts by mass of one or more phosphates of Al, Mg, Ca, Fe, Mn, or the like per 20 parts by mass of colloidal silica.

(33) Moisture absorption resistance is insufficient if the phosphate is contained in too small a proportion, whereas the tension is reduced and the iron loss reducing effect is weakened if the phosphate is contained in too large a proportion because this reduces the relative amount of the colloidal silica. Chromic anhydride and/or a chromic acid compound may be compounded in a total amount of 3 parts by mass to 20 parts by mass with the aim of enhancing moisture absorption. Furthermore, inorganic mineral particles (a powder or the like) such as silica or alumina may be used since compounding thereof enhances sticking resistance. The mixing ratio of these inorganic mineral particles is preferably no greater than approximately 1 part by mass so as not to reduce the stacking factor.

(34) Techniques that do not use chromium have recently been developed for providing environmentally friendly coatings. When such techniques are adopted, a metal sulfate, chloride, colloidal oxide, borate, or the like of Mg, Al, Fe, Bi, Co, Mn, Zn, Ca, Ba, Sr, Ni, or the like is used in place of the chromic anhydride or chromic acid compound. The total amount thereof is preferably approximately 3 parts by mass to 30 parts by mass.

(35) Herein, examples of means for changing the insulating coating tension include changing the insulating coating in terms of thickness and/or composition (i.e., a method in which the application amount of the coating is changed and a method in which the type of the coating is changed). Conventionally, the application amount of the coating is approximately 2 g/m.sup.2 to 8 g/m.sup.2 per surface and approximately 4 g/m.sup.2 to 16 g/m.sup.2 in total for both surfaces. Even in a situation in which the application amount is changed, the total application amount for both surfaces is preferably the same as conventionally used. An excessively large application amount reduces the stacking factor and leads to deterioration of magnetic properties, whereas an excessively small application amount reduces the tension and prevents the achievement of good magnetic properties.

(36) Examples of methods for changing the type of the coating include a method in which the type of phosphate is changed and a method in which the mixing ratio of the coating is changed as disclosed, for example, in “IEEE Transactions on Magnetics, Vol. Mag-15, No. 6, November 1979”. After application and drying of the coating, flattening annealing that also serves as baking is performed. In the flattening annealing, it is important that the annealing temperature and tension applied to the steel sheet are controlled such that residual coil bending caused by the final annealing is as small as possible. The reason for this is that if coil bending remains, it is not possible to obtain the desired magnetostrictive properties even when the forsterite film tension and the insulating coating tension are controlled to within the scope of this disclosure. It is when the influence of coil bending is at a negligible level that the desired magnetostrictive properties are achieved through the tension control disclosed herein.

(37) Through our techniques, it is possible to predict a noise property of a transformer in which a grain-oriented electrical steel sheet is used using the number of acceleration/deceleration points in a magnetostriction velocity level dλ/dt (i.e., the number of instances of acceleration/deceleration at which a large velocity change occurs) in one period of magnetostrictive vibration (magnetostrictive property III) and the magnitude of velocity level change between adjacent velocity level change points in an acceleration zone or deceleration zone (magnetostrictive property VI).

(38) The following describes an example of an acceleration/deceleration point measurement method.

(39) First, a higher-harmonic is superimposed on an excitation voltage to change magnetostrictive properties. For example, a 300 kVA actual transformer is assembled using a grain-oriented electrical steel sheet of 0.27 mm in thickness and the noise thereof is then measured at 50 Hz and 1.7 T. Acceleration/deceleration points (magnetostrictive property III) can be determined by counting the number of acceleration/deceleration points in one period of magnetostrictive vibration. Magnetostrictive vibration can be measured by a strain gauge method, or using a laser displacement meter or a laser Doppler vibrometer. Herein, magnetostrictive vibration properties are preferably evaluated using a laser Doppler vibrometer due to the simplicity thereof.

(40) Next, a method for determining the magnitude of velocity level change between adjacent velocity level change points in an acceleration zone or deceleration zone is described.

(41) In our techniques, the difference between adjacent local maximum and minimum values is investigated as an evaluation parameter for velocity change. This focuses on velocity change within an acceleration/deceleration zone and does not include velocity change spanning across both acceleration and deceleration zones.

(42) As illustrated in the previously described FIG. 3, a number of local maximum values and local minimum values are present in one period of a magnetostriction waveform.

(43) In our techniques, the magnitude of velocity level change between adjacent velocity level change points in an acceleration zone or deceleration zone (magnetostrictive property VI) can be determined by investigating a maximum value of differences between adjacent local maximum and minimum values of the magnetostriction velocity level (magnitude of velocity level change).

(44) Velocity change points occurring at positions indicated by stars in FIG. 3 are points at which:

(45) 1) an increasing rate of acceleration changes to a decreasing rate of acceleration;

(46) 2) a decreasing rate of acceleration changes to an increasing rate of acceleration;

(47) 3) an increasing rate of deceleration changes to a decreasing rate of deceleration; or

(48) 4) a decreasing rate of deceleration changes to an increasing rate of deceleration.

(49) The number of instances of acceleration/deceleration and the magnitude of velocity level change are determined by measuring the velocity change behavior of magnetostrictive vibration as described above. It is then confirmed whether or not the number of instances of acceleration/deceleration (magnetostrictive property III) is 4, and whether or not the magnitude of velocity level change (magnetostrictive property VI) is 3.0×10.sup.−4 sec.sup.−1 or less.

(50) The transformer is determined is have a good noise property if the magnetostrictive property III and the magnetostrictive property VI both satisfy the above conditions (i.e., if the magnetostrictive property III is 4 and the magnetostrictive property VI is 3.0×10.sup.−4 sec.sup.−1 or less).

EXAMPLES

Example 1

(51) A plurality of decarburization annealed sheets for grain-oriented electrical steel sheets of differing compositions that had been manufactured by a commonly known method up to decarburization annealing and that had a thickness of 0.3 mm were used (chemical compositions of materials A and B are shown in Table 2; % values in Table 2 are in mass % and ppm values in Table 2 are in mass ppm, the balance being Fe and incidental impurities). Grinding of front and rear surfaces of each of the decarburization annealed sheets was performed to change the oxygen coating amount to 0.9 g/m.sup.2 to 2.0 g/m.sup.2. Thereafter, Ni was electrodeposited on the front and rear surfaces in a range of 0 mg/m.sup.2 to 0.4 mg/m.sup.2 and decarburization annealing was performed for 2 minutes at 860° C. in a humid H.sub.2 atmosphere.

(52) Next, an annealing separator having a mixing ratio of 100 parts by mass of MgO and 5 parts by mass of TiO.sub.2 was applied onto the front and rear surfaces in an amount of 8 g/m.sup.2 on each surface and then final annealing was performed at a heating rate of 10° C./hour and with 5 hours of soaking at 1200° C.

(53) Thereafter, excess annealing separator was removed. The type of insulating coating was changed at the front and rear as shown in Table 3. “Magnesium phosphate” in Table 3 indicates an insulating coating treatment liquid having a mixing ratio of 50 mass % of magnesium phosphate, 40 mass % of colloidal silica, 9.5 mass % of chromic anhydride, and 0.5 mass % of silica powder. “Aluminum phosphate” in Table 3 indicates an insulating coating treatment liquid having a mixing ratio of 50 mass % of aluminum phosphate, 40 mass % of colloidal silica, 9.5 mass % of chromic anhydride, and 0.5 mass % of silica powder. These insulating coating treatment liquids were applied such that the dried mass on each surface was 5 g/m.sup.2 to 8 g/m.sup.2, and were dried at 300° C. for 1 minute. Thereafter, flattening annealing was performed in a dry N.sub.2 atmosphere at 850° C. for 2 minutes under conditions such that the applied tension was 13 MPa to obtain a grain-oriented electrical steel sheet.

(54) Magnetic properties and magnetostrictive properties of each grain-oriented electrical steel sheet obtained in this manner were measured. Additionally, differences between the surfaces of the steel sheet in terms of forsterite film tension, insulating coating tension, and total tension thereof were determined. Each electrical steel sheet was also used as a material for manufacturing a 1,000 kVA transformer, a noise property of which was then evaluated at 1.5 T and 60 Hz.

(55) Table 3 shows details of the manufacturing conditions, such as the oxygen coating amount, and also shows the steel sheet tension, iron loss, and magnetostrictive properties. Note that tension differences in Table 3 each represent the real value of the front/rear difference in tension, but in comparison with front/rear differences in tension that are prescribed in the present disclosure, it is the absolute values thereof that are compared.

(56) TABLE-US-00002 TABLE 2 Com- postion C (%) Si (%) Mn (%) Al (ppm) N (ppm) S (ppm) Se (ppm) Ni (%) Cu (%) P (%) Mo (%) Cr (%) Sb (ppm) Sn (ppm) A 0.07 3.4 0.06 250 90 15 170 0.01 0.1 0.05 0.002 0.01 400 10 B 0.04 3.4 0.07 10 20 10 200 0.01 0.07 0.05 0.002 0.01 300 10

(57) TABLE-US-00003 TABLE 3 Oxygen Ni coating amount electrodepostion Main agent of insulating coating (g/m.sup.2) amount (g/m.sup.2) Front surface Rear surface Front Rear Front Rear Front application Rear application No. Material surface surface surface surface surface amount (g/m.sup.2) surface amount (g/m.sup.2) 1 A 1.8 1.8 0 0 Aluminum 6 Aluminum 6 phosphate phosphate 2 A 1.8 1.1 0.2 0 Aluminum 6 Aluminum 6 phosphate phosphate 3 A 1.5 1.1 0.3 0.1 Aluminum 6 Magnesium 6 phosphate phosphate 4 A 0.9 1.8 0 0.1 Aluminum 6 Magnesium 6.5 phosphate phosphate 5 A 0.7 2.0 0 0.5 Magnesium 7 Aluminum 5 phosphate phosphate 6 B 1.8 1.8 0 0 Aluminum 6 Aluminum 6 phosphate phosphate 7 B 0.7 2.0 0 0.5 Magnesium 7 Aluminum 5 phosphate phosphate 8 B 0.9 1.8 0 0.1 Aluminum 6 Magnesium 6.5 phosphate phosphate Insulating Magnitude Forsterite coating Total of velocity film tension tension tension level Acceleration/ difference difference difference change deceleration W.sub.17/50 Transformer No. (MPa) (MPa) (MPa) (sec.sup.−1) *1 points *2 (W/kg) noise (dB) Remarks 1 0 0 0 9.2 × 10.sup.−4 8 0.95 58 dB Comparative Example 2 0.7 0 0.7 0.5 × 10.sup.−4 4 1.09 42 dB Comparative Example 3 0.7 −0.5 −0.2 2.2 × 10.sup.−4 4 0.94 42 dB Example 4 −0.7 −0.7 −1.4 0.4 × 10.sup.−4 4 1.12 43 dB Comparative Example 5 −1.1 1.2 0.1 2.6 × 10.sup.−4 4 0.95 43 dB Example 6 0 0 0 2.8 × 10.sup.−3 12 1.01 68 dB Comparative Example 7 −1.1 1.2 0.1 5.5 × 10.sup.−4 6 0.95 65 dB Comparative Example 8 −0.7 −0.7 −1.4 4.5 × 10.sup.−4 4 1.18 65 dB Comparative Example *1 Magnitude of velocity level change between adjacent magnetostriction velocity level change points in acceleration zone or deceleration zone of magnetostrictive vibration *2 Number of acceleration/deceleration points in one period of magnetostrictive vibration

(58) Upon comparison of No. 1 and No. 6 in which there were no tension differences as conventionally, it can be concluded that a feature of materials A and B is that material B has higher magnetostrictive property parameters and a poorer iron loss property than material A. Accordingly, material A has better values for parameters, other than tension, that are conventionally known to influence magnetostrictive properties and iron loss (for example, magnetic flux density and crystal orientation), and thus material A has better magnetic properties and magnetostrictive properties than material B prior to adoption of our techniques.

(59) It can be seen from Table 3 that No. 3 and No. 5, which are within the scope of this disclosure, achieved both transformer noise reduction and iron loss reduction. On the other hand, although sample No. 7 in which film tensions were within the scope of this disclosure had slightly enhanced magnetostrictive properties compared to samples outside the scope of this disclosure, magnetostrictive properties were not enhanced sufficiently to achieve a good noise property since the initial magnetostrictive properties (sample No. 6) were poor.

Example 2

(60) A plurality of decarburization annealed sheets for grain-oriented electrical steel sheets of differing compositions that had been manufactured by a commonly known method up to decarburization annealing and that had a thickness of 0.23 mm were used (chemical compositions of materials C and D are shown in Table 4; % values in Table 4 are in mass % and ppm values in Table 4 are in mass ppm, the balance being Fe and incidental impurities). An annealing separator having a mixing ratio of 100 parts by mass of MgO and 4.5 parts by mass of TiO.sub.2 was applied onto front and rear surfaces of each decarburization annealed sheet in an amount of 8 g/m.sup.2 on each surface. Thereafter, sodium borate aqueous solution was applied onto the front surface and the rear surface by spraying such that the amounts of B and Na were each adjusted to 0 parts by mass to 0.03 parts by mass per 100 parts by mass of MgO at both surfaces. Final annealing was then performed at a heating rate of 120° C./hour and with 20 hours of soaking at 1200° C.

(61) Next, excess annealing separator was removed and an insulating coating treatment liquid having a mixing ratio of 50 mass % of magnesium phosphate, 40 mass % of colloidal silica, 9.5 mass % of chromic anhydride, and 0.5 mass % of silica powder was applied such as to have a dry mass of 5 g/m.sup.2 to 10 g/m.sup.2 on each surface. Drying was performed for 1 minute at 250° C., and then flattening annealing was performed in a dry N.sub.2 atmosphere at 820° C. for 2 minutes and under conditions such that the applied tension was 15 MPa to obtain a grain-oriented electrical steel sheet.

(62) Magnetic properties and magnetostrictive properties of each grain-oriented electrical steel sheet obtained in this manner were measured. Additionally, differences between the surfaces of the steel sheet in terms of forsterite film tension, insulating coating tension, and total tension thereof were determined. Each electrical steel sheet was also used as a material for manufacturing a 750 kVA transformer, a noise property of which was then evaluated at 1.6 T and 60 Hz.

(63) Table 5 shows details of the film applying conditions, such as the amounts of B and Na per 100 parts by mass of MgO, and also shows the steel sheet tension, iron loss, and magnetostrictive properties. Note that tension differences in Table 5 each represent the real value of the front/rear difference in tension, but in comparison with front/rear differences in tension that are prescribed in the present disclosure, it is the absolute values thereof that are compared.

(64) TABLE-US-00004 TABLE 4 Com- postion C (%) Si (%) Mn (%) Al (ppm) N (ppm) S (ppm) Se (ppm) Ni (%) Cu (%) P (%) Mo (%) Cr (%) Sb (ppm) Sn (ppm) C 0.07 3.2 0.05 250 100 10 100 0.1 0.01 0.01 0.001 0.05 250 100 D 0.06 3.2 0.05 20 15 10 120 0.1 0.01 0.01 0.001 0.05 250 100

(65) TABLE-US-00005 TABLE 5 Forsterite Na and B content Insulating coating film (parts by mass per 100 parts by mass of MgO) Front surface Rear surface tension Na at front Na at rear B at front B at rear application application difference No. Material surface surface surface surface amount (g/m.sup.2) amount (g/m.sup.2) (MPa) 1 C 0 0 0 0 6 6 0 2 C 0.025 0 0.01 0 6 6 0.8 3 C 0.025 0.01 0.025 0.01 4 6 0.7 4 C 0.01 0.025 0.01 0.025 4 6 −0.7 5 C 0 0.025 0 0.025 8 6 −1.1 6 D 0 0 0 0 6 6 0 7 D 0 0.025 0 0.025 8 6 −1.1 8 D 0.01 0.025 0.01 0.025 4.5 6.5 −0.7 Insulating coating Total Magnitude tension tension of velocity Acceleration/ difference difference level change deceleration W.sub.17/50 Transformer No. (MPa) (MPa) (sec.sup.−1) *1 points *2 (W/kg) noise (dB) Remarks 1 0 0 7.2 × 10.sup.−4 8 0.81 53 dB Comparative Example 2 0 0.8 1.1 × 10.sup.−4 4 0.92 41 dB Comparative Example 3 −0.5 −0.2 2.4 × 10.sup.−4 4 0.82 40 dB Example 4 −0.7 −1.4 0.8 × 10.sup.−4 4 0.91 39 dB Comparative Example 5 1.2 0.1 1.9 × 10.sup.−4 4 0.81 41 dB Example 6 0 0 1.8 × 10.sup.−3 12 0.88 53 dB Comparative Example 7 1.2 0.1 4.9 × 10.sup.−4 6 0.88 51 dB Comparative Example 8 −0.7 −1.4 2.8 × 10.sup.−4 4 0.98 41 dB Comparative Example *1 Magnitude of velocity level change between adjacent magnetostriction velocity level change points in acceleration zone or deceleration zone of magnetostrictive vibration *2 Number of acceleration/deceleration points in one period of magnetostrictive vibration

(66) In the same way as in Example 1, upon comparison of No. 1 and No. 6 in which there were no tension differences as conventionally, it can be concluded that a feature of materials C and D is that material D has higher magnetostrictive property parameters and a poorer iron loss property than material C. Accordingly, material C has better values for parameters, other than tension, that are conventionally known to influence magnetostrictive properties and iron loss (for example, magnetic flux density and crystal orientation), and thus material C has better magnetic properties and magnetostrictive properties than material D prior to adoption of our techniques.

(67) It can be seen from Table 5 that No. 3 and No. 5, which are within the scope of this disclosure, achieved both good transformer noise and good iron loss. On the other hand, although sample No. 7 in which film tensions were within the scope of this disclosure had slightly enhanced magnetostrictive properties compared to samples outside the scope of this disclosure, magnetostrictive properties were not enhanced sufficiently to achieve a good noise property since the initial magnetostrictive properties (sample No. 6) were poor.

(68) Furthermore, it can be seen that when the surface at which forsterite film tension is high and the surface at which insulating coating tension is high are the same surface as in No. 8, although magnetostrictive properties that enhance the noise level are obtained because there is no canceling out of magnetostriction enhancement effects, deflection of the steel sheet arises, and this deflection causes significant deterioration of the iron loss property and prevents good magnetostriction and good iron loss from being achieved together.