JP4478796B2 - Thermal reforming method for electrical steel sheet - Google Patents

Description

  The present invention relates to a method for thermal reforming of an electrical steel sheet used in high-speed rotating electrical equipment such as a motor and a generator.

Electrical steel sheets are widely used as iron core materials for electrical equipment such as motors and generators.
In recent years, the demand for prevention of global warming has further increased, and it has become impossible to avoid increasing the efficiency of electrical equipment. Therefore, an electrical steel sheet having a low high-frequency iron loss has been required. On the other hand, high-speed rotation is being promoted in machine tools, HDD motors, micro gas turbines, and the like. On the other hand, there is a demand for a magnetic steel sheet having high strength that can withstand centrifugal force during high-speed rotation, in other words, high yield strength.
For this reason, it is necessary to develop an electrical steel sheet having low high-frequency iron loss (hereinafter, sometimes simply referred to as iron loss) and high yield strength.
However, in the field of cold-rolled steel sheets, it is generally said that both characteristics are in a trade-off relationship.

From the viewpoint of increasing the yield strength, various methods for adding an element having a large solid solution strengthening have been studied. However, these methods cannot always sufficiently reduce high-frequency iron loss.
In view of this point, a high-tensile non-oriented electrical steel sheet excellent in high-frequency iron loss containing a predetermined amount of Nb, Ni, and Mn has been proposed (see, for example, Patent Document 1 and Non-Patent Document 1). In this case, it is said that the addition of Nb, Ni, and Mn can increase the tension of the steel, and can further reduce the former eddy current loss among the eddy current loss and hysteresis loss that constitute high frequency iron loss with Ni. Yes.
JP 2003-342698 A Kubota et al., Materia, Vol. 42, No. 3, 2003, pp. 242-244

  However, in the above-described electrical steel sheet, the yield strength is twice that of the conventional silicon steel sheet, but the iron loss is doubled and the required characteristics are not sufficiently satisfied.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a thermal reforming method for an electrical steel sheet having low high-frequency iron loss and high yield strength.

In order to achieve the above object, the method for thermal reforming of an electrical steel sheet according to the present invention is a heating / cooling method in which the electrical steel sheet after final rolling is rapidly heated to the temperature of the austenite region and then rapidly cooled to a temperature below the austenite region. Heat treatment is repeated 3 to 6 times to adjust the average crystal grain size to 3 to 15 μm.

The method for thermal reforming of an electrical steel sheet according to the present invention is characterized in that the heat treatment is repeated using a high frequency power source or a laser or a combination of both as a heat source .

Moreover, the thermal reforming method of the electrical steel sheet according to the present invention is characterized in that the Si content of the obtained electrical steel sheet is 2 to 4% .

Moreover, the thermal reforming method of the electrical steel sheet according to the present invention is characterized in that the yield strength of the obtained electrical steel sheet is 440 MPa or more .

  In the method for thermal reforming of the electrical steel sheet of the present invention, since the heating / cooling process of rapidly cooling to the temperature below the austenite region is performed at least three times after being rapidly heated to the temperature of the austenite region, A magnetic steel sheet having a low and high yield strength can be obtained.

  Embodiments of the present invention will be described below.

  As is known from Hall Petch's law, the yield strength is inversely proportional to the square root of the grain size. Therefore, in order to improve the yield strength, it is better that the particle size is small. However, the particle size is also correlated with the iron loss, and the smaller the particle size, the higher the electrical resistance and the eddy current loss constituting the iron loss, but the hysteresis loss constituting the iron loss increases. The present inventors have found the present invention while examining the balance of the above three factors of yield strength, eddy current loss and hysteresis loss.

In the method for thermal reforming of the electrical steel sheet of the present invention, after the annealing, the material before the surface treatment is rapidly heated to the temperature of the austenite region as shown in the rapid heating / rapid cooling pattern example of FIG. A heat treatment in which a heating / cooling step of rapidly cooling to a temperature below the austenite region is repeated at least three times is performed. In FIG. 1, Ts is the lower limit temperature of the austenite region, and Tf is the upper limit temperature of the austenite region.
Heating / cooling may be repeated with respect to the entire electromagnetic steel sheet, or may be repeated with respect to a specific specific portion.
Thereby, an electrical steel sheet having lower iron loss (high-frequency iron loss) than that of a conventional electrical steel sheet and high yield strength can be obtained. In addition, it is possible to reduce the crystal grain size of the electrical steel sheet which is usually about several tens of μm or more. Electrical equipment using this electrical steel sheet is highly efficient, small in size, and can handle high-speed rotation. In addition, when the number of repetitions of the heating / cooling process is less than 3, neither the iron loss nor the yield strength can be significantly improved.

The number of repetitions of the heating / cooling step is preferably 6 or less.
If the number of repetitions exceeds 6, the yield strength increases further, but the iron loss tends to increase.

The average crystal grain size is preferably adjusted to 3 to 15 μm by optimizing the heat treatment conditions according to the steel type conditions and the like.
When the average crystal grain size is less than 3 μm, the yield loss increases further, but the iron loss tends to increase. On the other hand, when the average crystal grain size exceeds 15 μm, both the iron loss and the yield strength have a remarkable improvement effect. Can't get.

The heat treatment is preferably performed using a high frequency power source or a laser as a heating source. As the laser, a carbon dioxide gas or a YAG laser can be suitably used.
In this case, the heat treatment may be repeated as many times as necessary using only one of the heating sources, or, for example, the first three out of the four times of repetition may be the last one using a high frequency power source as the heating source. For example, a laser may be used as a heat source, and the heat treatment may be repeated as many times as necessary by appropriately combining two heat sources.

Moreover, in the thermal reforming method of the electrical steel sheet of the present invention, the obtained electrical steel sheet is preferably an iron alloy containing 2 to 4% of Si.
When the Si content is less than 2%, the iron loss reducing effect and the yield strength improving effect due to the presence of Si are impaired. It is not necessary to make the value more than 4%, and when the Si content exceeds 4%, the austenite region disappears, so that the heat treatment effect of the present invention cannot be obtained.

  Moreover, in the thermal reforming method of the electrical steel sheet according to the present invention, the yield strength of the obtained electrical steel sheet is preferably 440 MPa or more.

FIG. 2 shows an example in which the present invention is applied to the 3% Si—Fe electromagnetic steel sheet 10 that has come out from the final rolling process using the laser beam 12.
The electromagnetic steel sheet 10 has a thickness of 0.20 mm and a width of 1000 mm. While winding the electromagnetic steel sheet 10 with the roller 14, heating by the laser beam 12 and water cooling by the water cooling mechanism 16 were repeated alternately. The heating was performed to a temperature at which the structure of the electrical steel sheet reached the austenite region, and the cooling was performed to a temperature at which the structure of the electrical steel sheet reached the austenite region or lower, that is, the region in which austenite and ferrite co-deposited.
The galvano scan method was applied as the laser scanning method in the width direction. As the laser, a carbon dioxide laser (output 2 kW) was used to irradiate a portion having a width of 1.5 mm at a speed of 300 mm / second.

The iron loss of the electrical steel sheet after the heat treatment was measured at a frequency of 10 kHz, and a tensile tester was used for measuring the yield strength. The microstructure was observed with a microscope after corrosion.
The results are shown in Table 1. In the embodiment in which the laser beam is irradiated three times or more, it can be seen that the iron loss is smaller and the yield strength is higher than in the comparative example in which the laser beam is not irradiated or the number of irradiation is two or less. The average particle size of the ferrite of the example was in the range of 2 to 15 μm. Sample No. 7 has significantly higher yield strength, although the iron loss is slightly larger than that of the comparative example. For this reason, it can be applied even if the average particle size of Sample No. 7 is 2 μm or less depending on the application.

It is a figure which shows the temperature pattern example of rapid heating rapid cooling in the thermal reforming method of the electrical steel sheet of this invention. It is a figure for demonstrating the process of the thermal reforming method of the electrical steel sheet of this invention.

Explanation of symbols

10 Magnetic Steel Sheet 12 Laser Beam 14 Roller 16 Water Cooling Mechanism

Claims (4)

The steel sheet after the final rolling is rapidly heated to the temperature of the austenite region, and then subjected to a heat treatment that repeats the heating / cooling step of rapidly cooling to a temperature below the austenite region 3 to 6 times, so that the average grain size is 3 to 15 μm. A method for thermal reforming of an electrical steel sheet, characterized by comprising:   The method for thermal reforming of an electrical steel sheet according to claim 1, wherein the heat treatment is repeated using a high frequency power source or a laser or a combination of both as a heat source.   The method for thermal reforming of an electrical steel sheet according to claim 1 or 2, wherein the obtained electrical steel sheet has a Si content of 2 to 4%.   The method for thermal reforming of an electrical steel sheet according to any one of claims 1 to 3, wherein the yield strength of the obtained electrical steel sheet is 440 MPa or more.

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