Grain-oriented electrical steel sheet and method for producing same

Abstract

Disclosed is a grain-oriented electrical steel sheet exhibiting low hysteresis loss and low coercive force, in which an increase in hysteresis loss due to laser irradiation or electron beam irradiation, which has been a conventional concern, is effectively inhibited. The grain-oriented electrical steel sheet has closure domain regions (X) formed to divide the magnetic domains in a rolling direction, from one end to the other in the width direction of the steel sheet, provided that Expression (1) is satisfied:
(500t80)s+230w(500t80)s+330 Expression (1),
where t represents a sheet thickness (mm); w represents a smaller one of the widths (m) of the regions measured on the front and rear surfaces of the steel sheet, respectively, by using a Bitter method; and s represents an average number of the regions present within one crystal grain.

Claims

1. A grain-oriented electrical steel sheet comprising closure domain regions X formed by electron beam irradiation to divide magnetic domains of the steel sheet in a rolling direction, from one end to the other in the width direction of the steel sheet, in a linear or curved manner, and periodically in the rolling direction, provided that Expression (1) is satisfied:
(500t80)s+230w(500t80)s+330 Expression (1), where t represents a sheet thickness in millimeters; w represents a smaller one of the widths in micrometers of the regions X measured on front and rear surfaces of the steel sheet, respectively, by using a Bitter method; and s represents an average number of the regions X present within one crystal grain, wherein s is about 0.3 to about 5.0, w is about 30 m to about 320 m, and t is 0.27 mm or less.

2. The grain-oriented electrical steel sheet according to claim 1, wherein a change of hysteresis loss is 0.003 W/kg.

3. The grain-oriented electrical steel sheet according to claim 1, wherein t is 0.18 mm to 0.27 mm.

4. The grain-oriented electrical steel sheet according to claim 1, wherein t is 0.18 mm to 0.24 mm.

5. The grain-oriented electrical steel sheet according to claim 1, wherein w is 120 m to 295 m and s is 1.0 to 5.0.

6. The grain-oriented electrical steel sheet according to claim 5, wherein t is 0.18 mm to 0.27 mm.

7. The grain-oriented electrical steel sheet according to claim 5, wherein t is 0.18 mm to 0.24 mm.

8. The grain-oriented electrical steel sheet according to claim 5, wherein w is 195 m to 295 m.

9. The grain-oriented electrical steel sheet according to claim 8, wherein t is 0.18 mm to 0.27 mm.

10. The grain-oriented electrical steel sheet according to claim 8, wherein t is 0.18 mm to 0.24 mm.

11. A grain-oriented electrical steel sheet comprising closure domain regions X formed by electron beam irradiation to divide magnetic domains of the steel sheet in a rolling direction, from one end to the other in the width direction of the steel sheet, in a linear or curved manner, and periodically in the rolling direction, provided that Expression (1) is satisfied:
(500t80)s+230w(500t80)s+330 Expression (1), where t represents a sheet thickness and is 0.27 mm or less; w represents a smaller one of the widths in micrometers of the regions X measured on front and rear surfaces of the steel sheet and is about 30 m to about 320 m, respectively, by using a Bitter method; and s represents an average number of the regions X present within one crystal grain and is about 0.3 to about 5.0, wherein a change of hysteresis loss is 0.003 W/kg.

12. The grain-oriented electrical steel sheet according to claim 11, wherein t is 0.18 mm to 0.27 mm.

13. The grain-oriented electrical steel sheet according to claim 11, wherein t is 0.18 mm to 0.24 mm.

14. The grain-oriented electrical steel sheet according to claim 11, w is 120 m to 295 m and s is 1.0 to 5.0.

15. The grain-oriented electrical steel sheet according to claim 14, wherein w is 195 m to 295 m.

16. A method for producing the grain-oriented electrical steel sheet according to claim 1, the method comprising, in irradiating one surface of the steel sheet with an electron beam, adjusting, depending on an average grain size of the steel sheet, at least any one of a periodic irradiation interval L in the rolling direction, irradiation energy E, and a beam diameter a, so that closure domain regions X are formed to divide magnetic domains of the steel sheet in a rolling direction, from one end to the other in the width direction of the steel sheet, in a linear or curved manner, and periodically in the rolling direction, wherein the electron beam radiating repeated along a scanning line so that a long irradiation time and a short irradiation time alternate, and a dot pitch between repetitions of the irradiation is 0.05 mm to 0.6 mm, and an irradiation frequency is 10 kHz to 25 kHz.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will be further described below with reference to the accompanying drawings, wherein:

(2) FIG. 1 illustrates the formation of a closure domain region X; and

(3) FIG. 2 is a graph showing how the width w of closure domain regions X and an average number s of closure domain regions X present within one crystal grain affect magnetic domain refining and hysteresis loss.

DESCRIPTION OF EMBODIMENTS

(4) The present invention will now be described in detail below.

(5) The present invention is applied to a grain oriented electrical steel sheet. The grain oriented electrical steel sheet may be coated with an insulating coating and the like, or have a coating partially coming off from its surface, or even no coating thereon.

(6) In addition, the electrical steel sheet according to the present invention has closure domain regions X formed to divide magnetic domains of the steel sheet, from one end to the other in the width direction of the steel sheet, in a linear or curved manner, and periodically in a rolling direction. Here, the irradiation in the width direction may not necessarily be performed in a continuous and linear manner, but may also be performed in a discontinuous manner, such as once every several hundred millimeters. That is, for example, the irradiation may be repeated at intervals with appropriate shift as shown in FIG. 1. Note that crystal grain boundaries are not included in the aforementioned closure domain regions formed to divide magnetic domains in the rolling direction.

(7) Considering the amount of change in iron loss before and after the aforementioned closure domain regions X are applied, it is generally believed that a reduction in eddy current loss corresponding to magnetic domain refining and an increase in hysteresis loss with increasing closure domains will be more pronounced with a larger width w of the regions X and with a larger average number s of regions X present within one crystal grain.

(8) The present inventors, however, have found that hysteresis loss decreases when the aforementioned s and w as well as the sheet thickness t satisfy a certain relationship.

(9) Here, an average number s of regions X present within one crystal grain was defined by (i=1, N) S.sub.in.sub.i, where S.sub.i is the measured area ratio of a crystal grain i present within a sample for magnetometry (where i=1 to N; N is the total number of crystal grains) and n.sub.i is the measured number of regions X present within that crystal grain. While the coating may be detached using hydrochloric acid, nitric acid and the like until crystal grains can be detected visually if they cannot be observed easily through the coating, excessive detachment causes elution of the steel substrate and brings about a change in the width of regions X from that with the coating. Consequently, the width of the regions X is preferably measured together with the coating in advance. In addition, the width of the regions X may differ whether measured on the front or rear surface of the steel sheet, and thus was defined by a smaller one, indicated by w. When regions X are observed only on one surface, however, w represents the width on that surface. When w considerably fluctuates in the width direction, the width of regions X is determined by averaging the results obtained in the width direction.

(10) Note that the width of closure domain regions X is measured by using a Bitter method.

(11) Here, the Bitter method is used to observe domain walls and the like by using magnetic colloids, which tend to be attracted to areas where the magnetization state changes greatly.

(12) The present inventors have experimentally determined, through optimization of the aforementioned w and s, the condition under which magnetic domains can be subdivided to reduce eddy current loss, and furthermore, to reduce hysteresis loss as compared to that prior to the irradiation.

(13) FIG. 2 shows the results of investigating how w and s, in the case of electron beam irradiation, affect magnetic domain refining and hysteresis loss.

(14) As shown in the figure, it was revealed that the condition under which magnetic domains are subdivided to reduce hysteresis loss as compared to that prior to the irradiation is defined by:
(500t80)s+230w(500t80)s+330Expression (1)

(15) Note that if w<(500t80)s+230, then the domains that are originally present in the steel sheet cannot be reduced by irradiation and a hysteresis loss reduction effect is insufficient; or if (500t80)s+330<w, then closure domains increase by irradiation too much to realize a reduction in hysteresis loss.

(16) For example, assuming that the aforementioned sheet thickness t is 0.22 mm, the condition under which hysteresis loss becomes lower than that prior to the irradiation is given by:
30s+230w30s+330Expression (2)

(17) Note that if w<30s+230, then the closure domains that are originally present in the steel sheet cannot be reduced by irradiation and a hysteresis loss reduction effect is insufficient; or if 30s+330<w, then closure domains increase by irradiation too much to realize a reduction in hysteresis loss.

(18) In addition, it was found that the range of w within which hysteresis loss can be reduced becomes narrower with increasing sheet thickness t. Presumably, this is because a small sheet thickness t provides small domain wall energy, which allows magnetic domain refining to readily occur upon irradiation with a laser beam, an electron beam and the like and magnetostatic energy to decrease, with the result that lancet domains, which would otherwise be formed for the purpose of reducing magnetostatic energy, are no longer required and thus are removed. Therefore, from the perspective of maximizing the effect of reducing hysteresis loss, the sheet thickness t is preferably 0.27 mm or less.

(19) The present inventors have also found that there is a tendency that as s increases, hysteresis loss is increased excessively. Although the details of the mechanism of this phenomenon are not known exactly, it can be presumed that this phenomenon arose in response to the fact that since almost all the closure domains originally present within a grain will be removed even when s is still small, larger s provides a larger heat-affected zone and ends up with larger hysteresis loss, despite being less effective for reducing closure domains. On the other hand, if s is too small, the resulting hysteresis loss reducing effect is insufficient.

(20) Therefore, an average number s of regions X present within one crystal grain is preferably about 0.3 to about 10.

(21) In addition, the width w of closure domain regions X is preferably about 30 m to about 320 m.

(22) The present inventors have further found that a grain-oriented electrical steel sheet exhibiting low hysteresis loss and low coercive force as described above may be produced by, in irradiating one surface of the steel sheet with a laser beam or an electron beam, adjusting, depending on an average grain size of the steel sheet, at least any one of a periodic irradiation interval L in the rolling direction, irradiation energy E, and a beam diameter a, so that the aforementioned closure domain regions X are formed.

(23) For example, assuming an average grain size D of the steel sheet in the rolling direction is defined by D= (i=1, N) S.sub.id.sub.i, where d.sub.i is the maximum length in the rolling direction of the i.sup.th crystal grain, then, with a sufficiently large number of crystal grains, the following holds:
s=[D/L] or [D/L+1],

(24) where [x] refers to a maximum integer not exceeding x.

(25) It follows that the width w of regions X and the irradiation interval L may be adjusted so that the s satisfies Expression (1). The width w of regions X, which is in high correlation with the irradiation energy E and the beam diameter a, increases with larger E and, for irradiation at the same energy, increases with smaller a. Thus, it is possible to control w by adjusting E and a, provided that the relations among w and E, a are derived beforehand through test irradiation.

(26) In addition, considering a measurement variation of about 0.002 W/kg of hysteresis loss, the amount of change by which the hysteresis loss is determined as being reduced by irradiation upon detection was set as: (pre-irradiation hysteresis loss)(post-irradiation hysteresis loss)0.003 W/kg.

(27) Regions X may be applied by, for example, scribing with a tool such as a ballpoint pen, a knife and the like, heat/light/particle beam irradiation, and so on. When regions X are applied by scribing with a ballpoint pen, a knife or the like, however, more strain is applied and the hysteresis loss tends to increase. Thus, heat/light/particle beam irradiation, such as laser irradiation, electron beam irradiation, and plasma flame irradiation, is preferred.

EXAMPLES

Example 1

(28) The material used in this experiment were grain-oriented electrical steel sheets, each having a measured sheet thickness of 0.22 mm and a flux density B.sub.8 in the rolling direction of 1.85 T to 1.95 T, and having a dual-layer coating on its surfaces, including a vitreous coating, which is mainly composed of Mg.sub.2SiO.sub.4, and a coating (a phosphate-based coating), which is formed by baking an inorganic treatment solution thereon.

(29) Electron beam irradiation and laser irradiation were used to apply closure domain regions X. In each irradiation run, an electron beam and a laser beam were scanned linearly over the entire sheet width so that the electron beam irradiation portions and the laser irradiation portions extend across the steel sheet in the transverse direction (a direction orthogonal to the rolling direction) of the steel sheet.

(30) For electron beam irradiation, the irradiation was repeated along the scanning line so that a long irradiation time (s.sub.1) and a short irradiation time (s.sub.2) alternate, and a distance interval (dot pitch) between repetitions of the irradiation was set to be 0.05 mm to 0.6 mm. In addition, since s.sub.2 is generally small enough to be ignored as compared with s.sub.1, the inverse of s.sub.1 can be considered as the irradiation frequency, which was set to be 10 kHz to 250 kHz. Further, the scanning rate was set to be 4 m/s to 80 m/s and the interval between repetitions of the irradiation in the rolling direction was set to 3 mm to 50 mm. Note that in electron beam irradiation, the shortest distance from the center of a converging coil to the irradiated material was set to 700 mm and the pressure in the working chamber was set to be 2 Pa or less.

(31) On the other hand, for laser irradiation, the irradiation was carried out by continuous irradiation (dot pitch: 0) or intermittent pulse irradiation (pulse interval: 0.3 mm), in which the scanning rate was set to be 10 m/s and the interval between repetitions of the irradiation in the rolling direction was set to be 3 mm to 50 mm. As the laser, a fiber laser was used for continuous irradiation and a YAG laser was used for pulse irradiation; in either case the wavelength was set to be 1064 nm.

(32) After the application of closure domain regions X according to the aforementioned method, the width of the regions X was measured from the front and rear surfaces of each steel sheet by a Bitter method using a magnetic viewer (MV-95, manufactured by Sigma Hi-Chemical Inc.) to determine w. Then, the iron loss was measured. Subsequently, the coating was detached by using an aqueous solution, which was obtained by mixing 500 mL of a 47% hydrogen fluoride solution with an aqueous solution obtained by diluting 5 L of a 35% hydrochloric acid solution with 20 L of water, and an aqueous solution, which was obtained by diluting 500 mL of a 67.5% sulfuric acid solution with 10 L of water.

(33) The regions X present within each crystal grain in each sample from which the coating was detached were observed and counted using the magnetic viewer to determine s.

(34) Table 1 shows the width w of closure domain regions X and the number s of closure domain regions X.

(35) Table 1 also shows the results of measuring the pre-irradiation hysteresis loss Wh.sub.17/50, the post-irradiation improvement in hysteresis loss Wh.sub.17/50 (pre-irradiation minus post-irradiation score), and the post-irradiation improvement in eddy current loss We.sub.17/50 (pre-irradiation minus post-irradiation score).

(36) Table 1 further shows the results of measuring the pre-irradiation coercive force Hc and the post-irradiation improvement in coercive force Hc (pre-irradiation minus post-irradiation score).

(37) Note that the tension applied by coating is labeled as A, B, or C in Table 1, where A denotes a tension in the range of over 10 MPa to 15 MPa or less, B denotes a tension in the range of over 5 MPa to 10 MPa or less, and C denotes a tension of 5 MPa or less.

(38) TABLE-US-00001 TABLE 1 Improve- Improve- ment Improve- Hys- ment in in Eddy Coer- ment in teresis Hysteresis Current cive Coercive Conditional Loss Loss Loss Force force Region X Coating w s Expression Wh.sub.17/50 Wh.sub.17/50 We.sub.17/50 Hc Hc No. Applied by Tension* (m) (conunts) Applicability (W/kg) (W/kg) (W/kg) (A/m) (A/m) Remarks 1 Electron Beam A 325 1.1 Not Applicable 0.306 0.003 0.065 5.74 0.22 Comparative Example 2 Electron Beam A 305 1.2 Not Applicable 0.300 0.002 0.060 5.54 0.31 Comparative Example 3 Electron Beam A 295 1.0 Applicable 0.283 0.003 0.064 5.48 0.37 Inventive Example 4 Electron Beam B 270 1.3 Applicable 0.261 0.004 0.076 5.58 0.26 Inventive Example 5 Electron Beam A 235 1.3 Applicable 0.286 0.008 0.071 5.78 0.38 Inventive Example 6 Electron Beam A 290 1.7 Not Applicable 0.294 0.003 0.072 5.73 0.21 Comparative Example 7 Electron Beam C 270 2.0 Applicable 0.284 0.011 0.078 5.59 0.52 Inventive Example 8 Electron Beam A 210 2.3 Applicable 0.292 0.012 0.105 5.57 0.45 Inventive Example 9 Electron Beam A 250 2.9 Not Applicable 0.305 0.004 0.095 5.67 0.28 Comparative Example 10 Electron Beam A 195 3.1 Applicable 0.278 0.006 0.085 5.54 0.28 Inventive Example 11 Electron Beam A 220 4.2 Not Applicable 0.294 0.015 0.086 6.17 0.17 Comparative Example 12 Electron Beam A 210 3.8 Applicable 0.268 0.003 0.075 5.48 0.25 Inventive Example 13 Electron Beam A 200 5.0 Not Applicable 0.278 0.021 0.114 6.36 0.46 Comparative Example 14 Electron Beam A 190 4.8 Not Applicable 0.276 0.014 0.116 6.12 0.29 Comparative Example 15 Laser A 150 4.9 Applicable 0.246 0.004 0.062 5.40 0.21 Inventive Example 16 Laser C 170 2.1 Applicable 0.255 0.010 0.062 5.62 0.50 Inventive Example 17 Laser A 150 1.2 Not Applicable 0.251 0.001 0.043 5.56 0.15 Comparative Example 18 Laser A 105 3.7 Not Applicable 0.267 0.000 0.053 5.68 0.16 Comparative Example 19 Laser A 120 4.7 Applicable 0.262 0.004 0.066 5.48 0.21 Inventive Example 20 Laser A 85 3.4 Not Applicable 0.273 0.001 0.041 5.72 0.20 Comparative Example 21 Electron Beam A 265 1.8 Applicable 0.285 0.013 0.132 5.30 0.65 Inventive Example 22 Electron Beam A 255 2.2 Applicable 0.287 0.012 0.138 5.23 0.69 Inventive Example *A: over 10 MPa to 15 MPa or less, B: over 5 MPa to 10 MPa or less, C: 5 MPa or less

(39) It can be seen from Table 1 that the eddy current loss was reduced and the magnetic domains were subdivided in any of the cases shown, but the hysteresis loss improved only in those cases where Expression (1) is satisfied. It is also understood that the coercive force Hc was also reduced in the latter cases, allowing for excitation with a small external magnetic field.

(40) It was further found that the improvement in hysteresis loss Wh.sub.17/50 and the improvement in coercive force Hc tend to be more pronounced with a lower coating tension. Presumably, the reason is that as the coating tension increases, fewer lancet domains are present before electron beam irradiation or laser irradiation, and consequently, a higher coating tension results in less significant improvement achieved by irradiation.

Example 2

(41) Electron beam irradiation was performed under the same conditions as described in Example 1, except that grain oriented electrical steel sheets having measured sheet thicknesses of 0.18 mm, 0.19 mm, and 0.24 mm were used.

(42) The measurement results thereof are shown in Table 2.

(43) TABLE-US-00002 TABLE 2 Improve- Improve- ment ment Improve- Hys- in in Eddy ment in Sheet teresis Hysteresis Current Coersive Coersive Thick- Conditional Loss Loss Loss Force Force Resion X ness Coating w s Expression Wh.sub.17/50 Wh.sub.17/50 We.sub.17/50 Hc Hc No. Applied by (mm) Tension* (m) (counts) Applicability (W/kg) (W/kg) (W/kg) (A/m) (A/m) Remarks 23 Electron Beam 0.18 A 280 2.5 Applicable 0.304 0.018 0.207 6.02 0.77 Inventive Example 24 Electron Beam 0.18 A 210 5.0 Applicable 0.295 0.010 0.222 6.21 0.61 Inventive Example 25 Electron Beam 0.19 A 345 5.0 Not Applicable 0.298 0.002 0.263 6.12 0.32 Comparative Example 26 Electron Beam 0.19 A 260 1.3 Applicable 0.280 0.012 0.136 5.59 0.66 Inventive Example 27 Electron Beam 0.19 A 260 2.5 Applicable 0.286 0.013 0.201 5.89 0.39 Inventive Example 28 Electron Beam 0.19 A 220 4.0 Applicable 0.284 0.007 0.194 6.12 0.11 Inventive Example 29 Electron Beam 0.24 A 270 1.4 Applicable 0.286 0.004 0.120 5.22 0.04 Inventive Example 30 Electron Beam 0.24 A 270 2.2 Not Applicable 0.273 0.012 0.142 5.27 0.13 Comparative Example 31 Electron Beam 0.24 A 270 3.3 Not Applicable 0.272 0.018 0.162 5.40 0.18 Comparative Example 32 Electron Beam 0.24 A 210 5.0 Not Applicable 0.268 0.028 0.174 5.42 0.23 Comparative Example *A: over 10 MPa to 15 MPa or less, B: over 5 MPa to 10 MPa or less, C: 5 MPa or less

(44) It can be seen from Table 2 that those steel sheets having a sheet thickness other than 0.22 mm and satisfying Expression (2) also exhibited improvements in hysteresis loss and coercive force, resulting in low hysteresis loss and low coercive force.

Example 3

(45) Further, 100-mm wide steel sheets subjected to magnetic domain refining were used to produce model transformers, each being 500 mm square and simulating a transformer with an iron core of stacked three-phase tripod type, and the model transformers thus obtained were subjected to noise measurements.

(46) The model transformers were formed from a stack of steel sheets that were sheared to have beveled edges, with a stack thickness of about 15 mm and an iron core weight of about 20 kg. The transformers were excited with the three phases being 120 degrees out of phase with one another, where noise was measured under excitation at 1.7 T, 50 Hz. A microphone was used to measure noise at (two) positions 20 cm away from the iron core surface, in which noise levels were represented in units of dBA with A-scale frequency weighting (JIS C 1509).

(47) Table 3 shows the measurement results.

(48) TABLE-US-00003 TABLE 3 Steel Sheet Transformer Noise (dBA) No. Before Irradiation After Irradiation Remarks 13 36 38 Comparative Example 22 35 34 Inventive Example 27 34 33 Inventive Example

(49) A steel sheet of No. 13, which is a comparative example, exhibited an increase in noise after being subjected to magnetic domain refining treatment. Presumably, the reason is that closure domains formed excessively in the steel sheet and magnetic strain increased accordingly.

(50) In contrast, steel sheets of No. 22 and No. 27, which are inventive examples, exhibited a reduction in noise after being subjected to magnetic domain refining treatment. It is believed that while closure domains X applied by irradiation cause, similar to lancet domains, an increase in magnetic strain, increase in closure domains applied by irradiation is more than offset by reduction in lancet domains, resulting in an advantageous condition for reducing magnetic strain as a whole.