Method of manufacturing grain-oriented electrical steel sheet exhibiting low iron loss

Abstract

A method of manufacturing a grain-oriented electrical steel sheet is provided. When irradiating the surface of a grain-oriented electrical steel sheet having a sheet thickness t with an electron beam in a direction intersecting a rolling direction, the irradiation energy E(t) of the electron beam is adjusted to satisfy Ewmin(0.23)(1.612.83t (mm))E(t)Ewmin(0.23)(1.783.12t (mm)) (Expression (1)) using the value of the irradiation energy Ewmin(0.23) that minimizes iron loss for material with a sheet thickness of 0.23 mm.

Claims

1. A method of manufacturing a grain-oriented electrical steel sheet, the method comprising: measuring a thickness t, in millimeters, and a flux density of a first steel sheet; determining an irradiation energy per unit scanning length Ewmin(0.23), in Joules per meter, of a second steel sheet having a thickness of 0.23 millimeters that produces a minimum amount of iron loss from the second steel sheet, the flux density of the second steel sheet being the same as the flux density of the first steel sheet; adjusting an irradiation energy per unit scanning length E(t), in Joules per meter, of an electron beam to satisfy the following Expression (1) for the first steel sheet,
Ewmin(0.23)(1.612.83t)E(t)Ewmin(0.23)(1.783.12t) Expression (1); and thereafter irradiating a surface of the first a-steel sheet to obtain a grain-oriented electrical steel sheet from the first steel sheet with an adjusted electron beam in a direction intersecting a rolling direction.

2. The method of claim 1, wherein the sheet thickness t is 0.23 mm or less.

3. A method of manufacturing a grain-oriented electrical steel sheet, the method comprising: measuring a thickness t, in millimeters, and a flux density of a first steel sheet, wherein the thickness t of the first steel sheet is 0.23 mm or more; determining a line spacing smin(0.23), in millimeters, for a second steel sheet having a thickness of 0.23 millimeters that produces a minimum amount of iron loss from the second steel sheet, the flux density of the second steel sheet being the same as the flux density of the first steel sheet; adjusting a line spacing s(t) in millimeters of an electron beam to satisfy the following Expression (2),
s min(0.23)/(1.783.12t)s(t)s min(0.23)/(1.612.83t)Expression (2); and thereafter irradiating a surface of the first steel sheet to obtain a grain-oriented electrical steel sheet from the first steel sheet with an adjusted electron beam in a direction intersecting a rolling direction.

Description

BRIEF DESCRIPTION OF DRAWINGS

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

(2) FIG. 1 illustrates the effect of the strain ratio on the iron loss after electron beam irradiation of materials with a sheet thickness of 0.20 mm and of 0.23 mm;

(3) FIG. 2(a) illustrates the relationship between irradiation energy and the amount of change in iron loss for an electron beam method, FIG. 2(b) the relationship between irradiation energy and the amount of change in hysteresis loss for an electron beam method, and FIG. 2(c) the relationship between irradiation energy and the amount of change in eddy current loss for an electron beam method, each figure showing the investigation results for each sheet thickness; and

(4) FIG. 3 illustrates the results of investigation into the effect of sheet thickness on the appropriate irradiation energy.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(5) The present invention will be described in detail below with reference to exemplary embodiments.

(6) The present invention provides a method of manufacturing a grain-oriented electrical steel sheet by irradiation with an electron beam in order to reduce iron loss. An insulating coating may be formed on the electrical steel sheet irradiated with an electron beam, yet omitting the insulating coating poses no problem. The present invention may be applied to any conventionally known grain-oriented electrical steel sheet, for example regardless of whether inhibitor components are included.

(7) Based on the results illustrated in FIGS. 2(a) to 2(c) and FIG. 3, in the present invention the appropriate energy range at each sheet thickness (t) is set to 5% of the value Ewmin(t) at which the iron loss is minimized. The reason is that in this range of 5% of Ewmin(t), the attained iron loss exhibits almost no variation. In this context, energy refers to the irradiation energy per unit scanning length and can be expressed as beam power/scanning rate.

(8) Next, using the results illustrated in FIGS. 2(a) to 2(c) and FIG. 3, the irradiation energy was calculated as an amount of change from the appropriate energy Ewmin(0.23) at which the iron loss is minimized for material with a thickness of 0.23 mm as follows:
283t (mm)+61[amount of change in appropriate irradiation energy from 0.23 mm material](%)312t (mm)+78.

(9) Using the expression above to calculate the appropriate energy range E(t) at each sheet thickness (t) yields Expression (1) below.
Ewmin(0.23)(1.612.83t (mm))E(t)Ewmin(0.23)(1.783.12t (mm))(Expression (1))

(10) Accordingly, without adjusting the beam diameter or line spacing of the electron beam, satisfying Expression (1) allows for suppression of a reduction in productivity caused by optical system adjustment operations or by shortening of line spacing.

(11) The reason why Expression (1) is preferably applied to a steel sheet of 0.23 mm or less is that, as described below, for a thickness of 0.23 or more, reducing iron loss by increasing the line spacing is advantageous from the perspective of productivity.

(12) Furthermore, in the case of thick sheet material that is 0.23 mm or more, based on the results in the above-described FIGS. 2(a) to 2(c) and FIG. 3, the RD line interval s(t) is preferably widened, and in conjunction with the effect of the amount of energy irradiated per unit area (E/s) on iron loss, Expression (2) below is preferably satisfied.
s min(0.23)/(1.783.12t (mm))s(t)s min(0.23)/(1.612.83t (mm)) (Expression(2))

(13) In the present invention, the preferable generation conditions for the electron beam are as follows.

(14) [Acceleration Voltage Va: 30 kV to 300 kV]

(15) If the acceleration voltage Va falls below 30 kV, it becomes difficult to focus the beam diameter, and the effect of reducing iron loss is lessened. Conversely, an acceleration voltage Va exceeding 300 kV not only shortens the life of the equipment, such as the filament, but also causes the size of a device for preventing x-ray leakage to increase excessively, thus reducing maintainability and productivity. Accordingly, the acceleration voltage Va is preferably in a range of 30 kV to 300 kV.

(16) [Beam Diameter: 50 m to 500 m]

(17) If the electron beam diameter is less than 50 m, measures must be taken such as dramatically reducing the distance between the steel sheet and the deflection coil. In this case, the distance at which deflection irradiation with one electron beam source is possible is greatly reduced. As a result, in order to irradiate a wide coil of about 1200 mm, multiple electron guns become necessary, reducing maintainability and productivity.

(18) Conversely, if the beam diameter exceeds 500 m, a sufficient effect of reducing iron loss cannot be obtained. The reason is that the area of the steel sheet irradiated by the beam (the volume of the portion where strain is formed) increases excessively, and hysteresis loss worsens.

(19) Accordingly, the electron beam diameter is preferably in a range of 50 m to 500 m. Note that the full width at half maximum of the beam profile obtained by a slit method was measured as the beam diameter.

(20) [Beam Scanning Rate: 20 m/s or More]

(21) If the beam scanning rate is less than 20 m/s, the production volume of steel sheets decreases. Accordingly, the beam scanning rate is preferably 20 m/s or more. While there is no restriction on the upper limit of the beam scanning rate, in terms of equipment constraints, an upper limit of approximately 1000 m/s is realistic.

(22) [RD Line Spacing: 3 mm to 12 mm]

(23) In the present invention, the steel sheet is irradiated with the electron beam in a straight line from one edge in the width direction to the other edge, and the irradiation is repeated periodically in the rolling direction. The spacing (line spacing) is preferably 3 mm to 12 mm. The reason is that if the line spacing is narrower than 3 mm, the strain region formed in the steel becomes excessively large, and not only does iron loss (hysteresis loss) worsen, but also productivity worsens. On the other hand, if the line spacing is wider than 12 mm, the magnetic domain refining effect lessens no matter how much the closure domain extends in the depth direction, and iron loss does not improve.

(24) [Line Angle: 60 to 120]

(25) In the present invention, when irradiating the steel sheet with the electron beam in a straight line from one edge in the width direction to the other edge, the direction from the starting point to the ending point is set to be from 60 to 120 with respect to the rolling direction. The reason is that upon deviating from a direction of 60 to 120, the volume of the portion where strain is introduced increases excessively, and hysteresis loss worsens. The direction is preferably 90 with respect to the rolling direction.

(26) [Processing Chamber Pressure: 3 Pa or Less]

(27) The reason for this range is that if the pressure of the processing chamber for irradiating with an electron beam is higher than 3 Pa, electrons emitted from the electron gun scatter, and the energy of the electrons forming the closure domain in the portion irradiated by the electron beam is reduced. As a result, the magnetic domain of the steel sheet is not sufficiently refined, and iron loss properties do not improve.

(28) [Beam Focusing]

(29) When irradiating by deflecting the electron beam in the width direction of the steel sheet, the focusing conditions (focusing current and the like) are of course preferably adjusted in advance to optimal conditions so that the beam is uniform in the width direction.

EXAMPLES

(30) In the present examples, four 1500 m grain-oriented electrical steel sheet coils at each nominal sheet thickness (t) of 0.23 mm, 0.27 mm, 0.30 mm, and 0.20 mm were joined tip to tail and subjected to electron beam irradiation.

(31) The electron beam irradiation was performed under the conditions of an acceleration voltage of 60 kV, beam diameter of 250 beam scanning rate of 90 m/s, line angle of 90, and processing chamber pressure of 0.1 Pa, and the electron beam irradiation time for each coil was recorded. Note that 4 m at the tip/tail portion of the coil of each sheet thickness were designated as a region not subjected to electron beam irradiation (non-irradiated portion).

(32) After irradiation, 60 SST samples each were taken from the portion subjected to electron beam irradiation (irradiated portion) and the non-irradiated portion in the coil of each sheet thickness, and iron loss was measured.

(33) Table 1 lists electron beam irradiation conditions along with the measurement results for iron loss.

(34) TABLE-US-00001 TABLE 1 Irradiation Iron loss W.sub.17/50(W/kg) energy per (upper tier: non-irradiated portion; unit lower tier: irradiated portion) Coil irradiation time (min) Total RD line scanning 0.23 0.27 0.30 0.20 0.23 0.27 0.30 0.20 irradiation No. spacing length mm mm mm mm mm mm mm mm time (min) Notes 1 Fixed at Fixed at 0.837/ 0.847/ 0.946/ 0.840/ 25 25 25 25 100 Conventional 5 mm Ewmin(0.23) 0.693 0.758 0.860 0.668 example 2 Fixed at Expression 0.837/ 0.847/ 0.946 0.840/ 25 25 25 25 100 Inventive 5 mm (1) applied 0.693 0.754 0.852 0.663 example 3 Expression Fixed at 0.837/ 0.847/ 0.946/ 0.840/ 25 21 19 25 90 Inventive (2) applied Ewmin(0.23) 0.693 0.752 0.852 0.669 example

(35) Table 1 shows that applying the present technique yielded a maximum improvement of nearly 1% in iron loss for material with a thickness of 0.20 mm, 0.27 mm, and 0.30 mm under conditions that use the beam current to optimize the irradiation energy for each sheet thickness (No. 2).

(36) It is also clear that the present technique yielded a maximum improvement of nearly 1% in iron loss for material with a thickness of 0.27 mm and 0.30 mm under conditions that use line spacing to optimize the irradiation energy (No. 3) and furthermore achieved excellent productivity by reducing the irradiation time by nearly 10%.