JP7601280B2 - Grain-oriented electrical steel sheet - Google Patents

Description

The present invention relates to grain-oriented electrical steel sheet suitable as a transformer core material.

Grain-oriented electrical steel sheet is a soft magnetic material used as the iron core material of transformers, and has a crystal structure in which the <001> orientation, the axis of easy magnetization of iron, is highly aligned in the rolling direction of the steel sheet. This texture is formed through a phenomenon called secondary recrystallization, in which crystal grains with the {110}<001> orientation, known as the Goss orientation, are preferentially grown to large sizes during purification annealing in the manufacturing process of grain-oriented electrical steel sheet.

The common technique for forming this alloy is to use a precipitate called an inhibitor to cause secondary recrystallization of grains with Goss orientation during purification annealing. For example, the method using AlN and MnS described in Patent Document 1 and the method using MnS and MnSe described in Patent Document 2 have been put into industrial use.

The method of using these inhibitors is useful for stably developing secondary recrystallized grains, but in order to finely disperse the inhibitors in the steel, it is necessary to heat the slab at high temperatures of 1300°C or higher and dissolve the inhibitor components in solid solution.

On the other hand, a technology for developing Goss-oriented crystal grains by secondary recrystallization in a material that does not contain inhibitor components is disclosed in Patent Document 3 and other publications. This technology makes the grain boundary misorientation angle dependency of the grain boundary energy of the grain boundaries during primary recrystallization apparent by eliminating impurities such as inhibitor components as much as possible, and allows grains with Goss orientation to undergo secondary recrystallization without the use of inhibitors; this effect is called the texture inhibition effect. This method does not require fine dispersion of inhibitors in the steel, and therefore does not require slab heating at high temperatures, which was previously essential, making it a method with great advantages in terms of both cost and maintenance.

Grain-oriented electrical steel sheets are mainly used as transformer cores, and are required to have excellent magnetization characteristics, especially low iron loss. To achieve this, it is important to highly align the secondary recrystallized grains in the steel sheet to the Goss orientation and to reduce impurities in the product sheet. In addition, a technology has been developed to introduce nonuniformity into the surface of the steel sheet by physical methods, subdivide the width of the magnetic domains, and reduce the iron loss, that is, a magnetic domain subdivision technology. For example, Patent Document 4 proposes a technology to reduce the iron loss of the steel sheet by irradiating the final product sheet with a laser, introducing a high dislocation density region into the surface layer of the steel sheet, and narrowing the magnetic domain width. In addition, Patent Document 5 proposes a technology to control the magnetic domain width by irradiating it with an electron beam.

Highly aligning the orientation after the secondary recrystallization mentioned above to the Goss orientation and reducing impurities in the product sheet leads to a reduction in hysteresis loss. In contrast, the application of magnetic domain refinement technology primarily reduces eddy current loss.

Special Publication No. 40-15644 Special Publication No. 51-13469 JP 2000-129356 A Special Publication No. 57-2252 Special Publication No. 6-72266

As mentioned above, grain-oriented electromagnetic steel sheets are mainly used as the iron core of transformers. Generally, there is a discrepancy between the iron loss value of a transformer core and the iron loss value of the grain-oriented electromagnetic steel sheet that is the material, with the iron loss of the transformer core being greater. The iron loss ratio between the two (the iron loss of the transformer core divided by the iron loss value of the material) is called the building factor. In other words, even if the iron loss of the material is good, if the building factor is high, the iron loss of the transformer core will be large, resulting in a problem of not being able to perform adequately. In the carbon-neutral era, what needs to be reduced is the iron loss of the final product, the transformer, and no matter how low the iron loss of the material is, it is meaningless if the building factor is high. The building factor affects not only the design of the transformer but also the characteristics of the material, so there is a demand to lower the building factor along with the iron loss of the material.

In other words, the object of the present invention is to provide a grain-oriented electrical steel sheet having magnetic properties that can sufficiently reduce the building factor.

As a result of extensive research, the inventors have discovered that it is possible to manufacture grain-oriented electrical steel sheet that achieves a low building factor by controlling, within a certain range, the amount of Co in the base steel sheet on which a base coating mainly composed of forsterite is formed, and the amount of Ti in the grain-oriented electrical steel sheet on which the base coating is formed.

The experiments that led to the success of the present invention are described below.
<Experiment 1>
In order to mainly change the Co content, the steel slabs were produced by continuous casting, containing, by mass%, C: 0.050-0.081%, Si: 3.15-3.31%, Mn: 0.07-0.10%, Al: 0.020-0.025%, N: 0.0069-0.0085%, S: 0.0011-0.0031%, Sb: 0.025-0.036%, Co: 0-0.123%, and Ti: 0.0080-0.0090%, with the balance being Fe and unavoidable impurities. The slabs were then heated at 1400°C for 20 minutes, and then hot-rolled to a thickness of 2.4 mm. The slabs were then annealed at 1000°C for 30 seconds in a _N2 atmosphere. Next, the plate was cold-rolled to a thickness of 1.5 mm, and then intermediate annealing was performed at 1000°C for 100 seconds in a 25% H ₂ -75% N ₂ atmosphere. After that, the plate was cold-rolled to a thickness of 0.23 mm, and decarburization annealing was performed at 850°C for 150 seconds in a 50% H ₂ -50% N ₂ moist atmosphere with a dew point of 50°C. Next, an annealing separator mainly composed of MgO was applied to the surface of the base steel plate after decarburization annealing, and purification annealing was performed by holding at 1200°C for 10 hours. At this time, the heating rate up to 1200°C was 20°C/h, and further, in the heating process, the N ₂ atmosphere was used from room temperature to 700°C, the atmosphere was changed in various mixture ratios of N ₂ and H ₂ from 700°C to 1100°C, and the H ₂ atmosphere was used from 1100°C to 1200°C. The holding time was a _H2 atmosphere, and the cooling time was an Ar atmosphere. In this manner, a sample was obtained in which a base coating mainly composed of forsterite (hereinafter, sometimes referred to as a forsterite coating) was formed on the surface of the base steel sheet.

For the samples thus obtained, the iron losses W _17/50 (iron loss when excited up to 1.7 T at 50 Hz) and W _19/50 (iron loss when excited up to 1.9 T at 50 Hz), and the hysteresis losses Wh ₁₇ (hysteresis loss when excited up to 1.7 T) and Wh ₁₉ (hysteresis loss when excited up to 1.9 T) were measured using the method specified in JIS C2550-1.

In addition, to measure the Co content in the base steel plate, a portion of the obtained sample was immersed in a 10% hydrochloric acid aqueous solution at 80°C for 180 seconds to remove the forsterite coating, and the Co content was measured using the method specified in JIS G1222.

Next, a three-phase three-legged model transformer was made from the obtained samples, simulating a transformer with an external shape of 500 mm square and a plate width of 100 mm for each leg and yoke, and the model transformer iron loss WT _17/50 (transformer iron loss when excited to 1.7 T at 50 Hz) was measured. The number of laminated sheets of the sample was 50, and two sheets were alternately stacked. The building factor F17 of the model transformer was calculated as the value (WT _17/50 /W _17/50 ) obtained by dividing the model transformer iron loss WT _17/50 by the sample iron loss W _17/50 . The relationship between the building factor F17 and the Co content in the base steel sheet is shown in Figure 1.

From the results shown in Figure 1, no clear correlation was found between the building factor F17 and the amount of Co. However, Figure 1 shows that the building factor F17 is divided into two groups: good values of 1.25 or less, and high values of 1.30 or more.

Therefore, we investigated whether this difference could be explained from the relationship between the iron loss and hysteresis loss of the samples. As a result, we found that when _R17 is the ratio of hysteresis loss Wh ₁₇ to iron _loss W _17/50 when excited at 1.7 T, and _R19 is the ratio of hysteresis loss Wh ₁₉ to iron loss W _19/50 when excited at 1.9 T, _the samples can be divided into Group A, which has the relationship 0.30≦R17≦R19, and Group B, which does not. Figure 2 shows the result of extracting only Group A from the data in Figure 1 and replotting it.

From the results shown in Figure 2, it can be seen that the alloy belongs to group A, i.e., has the relationship 0.30≦R17≦R19, and exhibits a good building factor of 1.25 or less when the Co content is in the range of 0.005 to 0.050%.

<Experiment 2>
The steel slab, which contains, in mass%, C: 0.037%, Si: 3.05%, Mn: 0.18%, Al: 0.009%, N: 0.0036%, Se: 0.007%, Sn: 0.062%, and Co: 0.0080%, with the balance being Fe and unavoidable impurities, was produced by continuous casting, and after slab heating at 1300°C for 30 minutes, it was finished to a thickness of 2.2 mm by hot rolling. Then, it was annealed at 1100°C for 30 seconds in a _N2 atmosphere. Then, it was cold rolled to a thickness of 0.23 mm, and further decarburization annealing was performed at 840°C for 120 seconds in a moist atmosphere of 40% _H2-60 % _N2 with a dew point of 40°C. Next, an annealing separator in which TiO ₂ was mixed in a range of 0 to 15 parts by mass relative to MgO was applied to the surface of the base steel sheet after decarburization annealing, and purification annealing was performed by holding at 1220 ° C for 5 hours. At this time, the heating rate up to 1220 ° C was 15 ° C / h, and further, in the heating process, the N ₂ atmosphere was used from room temperature to 700 ° C, the atmosphere was changed to various mixture ratios of N ₂ and H ₂ from 700 ° C to 1100 ° C, and the H ₂ atmosphere was used from 1100 ° C to 1220 ° C. In addition, the H ₂ atmosphere was used during holding, and the Ar atmosphere was used during cooling. In this way, a sample was obtained in which a base coating mainly composed of forsterite (hereinafter sometimes referred to as a forsterite coating) was formed on the surface of the base steel sheet.

For the samples thus obtained, the iron losses W _17/50 and W _19/50 and the hysteresis losses Wh ₁₇ and Wh ₁₉ were measured in the same manner as in Experiment 1 according to the method specified in JIS C2550-1.

In addition, the Ti content in the steel plate having a forsterite coating was measured using the method specified in JIS G1223.

Furthermore, to measure the amount of Co in the base steel plate, a part of the obtained sample was immersed in a 10% hydrochloric acid solution at 80°C for 180 seconds to remove the forsterite coating, and the Co amount was measured according to the method specified in JIS G1222. As a result, the Co amount was 0.0080%, which was the same as the content in the steel slab.

Furthermore, similarly to experiment 1, the ratio Wh17/W17 _{/ 50} of the hysteresis loss _Wh17 to the iron loss _W17 /50 when excited at 1.7 T was defined as R17, and the ratio _Wh19 /W19 _{/ 50} of the hysteresis loss _Wh19 to the iron loss W19/50 when excited at 1.9 T was defined as R19, and the steel sheets were divided into Group A where 0.30≦R17≦R19 was satisfied, and Group B where the rest was satisfied. The relationship between the Ti content in the steel sheet having a forsterite coating and whether it belongs to Group A or Group B is shown in Figure 3.

From the results shown in Figure 3, it can be seen that when the Ti content in steel plate having a forsterite coating is 0.0039% or more and 0.0200% or less, it tends to belong to group A.

Furthermore, as in Experiment 1, a three-phase, three-legged model transformer was fabricated with an external shape of 500 mm square and each leg and yoke having a plate width of 100 mm, and the model transformer iron loss WT _17/50 (transformer iron loss when excited up to 1.7 T at 50 Hz) was measured. The building factor F17 of the model transformer was then calculated as the value obtained by dividing the model transformer iron loss WT _17/50 by the sample iron loss W _17/50 (WT _17/50 /W _17/50 ). The relationship between the building factor F17 and the Ti content in the steel sheet with a forsterite coating is shown in Figure 4.

From the results shown in Figure 4, we can see that when the Ti content of the steel plate with a forsterite coating is less than 0.0050%, the building factor F17 is high even if it belongs to group A. In summary, we found that when the Ti content of the steel plate with a forsterite coating is 0.0050-0.0200%, the building factor is low and good. This means that it is good for a certain amount of Ti to be present in the forsterite coating.

As described above, the mechanism by which the building factor of a model transformer is improved depending on the amount of Co in the base steel sheet and the amount of Ti in the steel sheet having a base coating mainly composed of forsterite is not clear, but the inventors believe it to be as follows.
That is, because the yoke and legs of the transformer have a certain width, the magnetic path has different distances on the inside and outside, like an athletics track. For this reason, during excitation, the magnetic flux tends to be biased to the inside where the magnetic path is shorter. Even when the entire steel plate is excited to 1.7 T, the magnetic flux density on the inside exceeds that. Therefore, it is presumed that the more advantageous the high magnetic field characteristics are, the better the transformer characteristics such as the building factor will be. When Co is dissolved in iron, the saturation magnetic flux density of the iron increases, which is expected to improve the high magnetic field characteristics, and therefore it is presumed that the building factor has improved. However, throughout Experiments 1 and 2, there were two cases where the building factor was poor, even when Co was added.

The first point is when the ratio Wh ₁₇ /W _17/50 of the hysteresis loss Wh ₁₇ to the iron loss W _{17/50 when excited at 1.7T, i.e. R17, and the ratio Wh 19 /W 19/50 of the hysteresis loss Wh 19 to the iron loss W 19/50 when excited at 1.9T} , i.e. R19, do not satisfy the relationship 0.30≦R17≦R19. A detailed investigation showed that R17 was below 0.30 in most cases. Hysteresis loss has a high correlation with B ₈ , and it is thought that there is no large fluctuation if the same B ₈ is used, so the above is thought to be a case where the eddy current loss is extremely large. In a transformer, even if it is excited with a sine wave, high-frequency components are superimposed and the waveform is distorted, so it is thought that the eddy current loss, which is highly frequency-dependent, increases. Therefore, it is thought that a high eddy current loss ratio increases the building factor.

The second point is when the Ti content in the steel sheet with a forsterite coating is less than 0.0050% by mass or more than 0.0200% by mass. Although it is only a guess, it is thought that the presence of a certain amount of Ti in the forsterite coating improves the coating properties. For example, if the coating tension is improved, the magnetic domains will become finer, which may reduce eddy current loss. In that case, contrary to the cases of R17 and R19 above, the eddy current loss ratio will decrease, and it is thought that the building factor can be reduced.

Incidentally, JP 2021-509149 A discloses a manufacturing technology for grain-oriented electrical steel sheets containing Co. However, this document mentions a technology for improving the magnetic properties of the electrical steel sheet itself, which is a completely different technology from the present invention, which reduces the building factor by using a technology for including Ti in the forsterite coating in combination.

The present invention is based on the above findings. That is, the gist of the present invention is as follows.

1. A grain-oriented electrical steel sheet having a base steel sheet containing 1.50 to 8.00 mass% Si, 0.02 to 1.00 mass% Mn, and 0.005 to 0.050 mass% Co, and an undercoat film containing forsterite as a main component and formed on the surface of the base steel sheet,
the Ti content in the base steel sheet and the undercoating as a whole is 0.0050 to 0.0200 mass%,
A grain-oriented electrical steel sheet that satisfies the following formula (1), where R17 is the ratio of hysteresis loss _Wh17 to iron _{loss W17/50 when excited at} 1.7 T, and R19 is the ratio of hysteresis loss _Wh19 to iron loss _W19 / ₅₀ when excited at 1.9 T.
0.30≦R17≦R19...(1)

2. The grain-oriented electrical steel sheet described in 1, in which the Ti content in the base steel sheet is 0.0030 mass% or less.

3. A grain-oriented electrical steel sheet as described in 1 or 2, which has an insulating coating on the surface of the base coating.

4. The grain-oriented electrical steel sheet according to any one of 1 to 3, wherein the base steel sheet further contains one or more selected from Sn: 0.500 mass% or less, Cr: 0.500 mass% or less, Cu: 0.50 mass% or less, Ni: 0.50 mass% or less, Bi: 0.500 mass% or less, P: 0.500 mass% or less, Sb: 0.500 mass% or less, Mo: 0.500 mass% or less, B: 25.0 mass ppm or less, Nb: 0.020 mass% or less, V: 0.020 mass% or less, As: 0.0200 mass% or less, Zn: 0.020 mass% or less, Pb: 0.0100 mass% or less, W: 0.0100 mass% or less, Ga: 0.0050 mass% or less, and Ge: 0.0050 mass% or less.

According to the present invention, it is possible to provide a grain-oriented electrical steel sheet having magnetic properties that can sufficiently reduce the building factor.

1 is a graph showing the relationship between the Co content of the base steel plate and the building factor F17 for Experiment 1. 1 is a graph showing the relationship between the Co content of the base steel plate and the building factor F17 for Experiment 1 (only Group A is extracted). 11 is a graph showing the relationship between the Ti content in a steel sheet having a forsterite film and whether the steel sheet belongs to group A or group B, in Experiment 2. 1 is a graph showing the relationship between the Ti content and the building factor F17 of a steel plate having a forsterite coating in Example 2.

Next, the reasons for limiting the constituent elements of the present invention will be described. First, the amount of each element (composition) in the base steel sheet of the grain-oriented electrical steel sheet of the present invention will be explained. Note that the "%" and "ppm" indications regarding the composition of the components respectively indicate "% by mass" and "ppm by mass" unless otherwise specified.

Si: 1.50-8.00%
Silicon is an element necessary for increasing the resistivity of steel and improving iron loss. Silicon is also an element necessary for forming the forsterite film in the steel sheet of the present invention. However, if the amount of silicon is less than 1.50%, there is no effect, and if the amount of silicon exceeds 8.00%, the workability of the steel deteriorates and rolling becomes difficult. For this reason, the amount of silicon is limited to 1.50 to 8.00%. The amount of silicon is preferably 2.50% or more and 4.50% or less.

Mn: 0.02-1.00%
Mn is an element necessary for improving hot workability. However, if the Mn content is less than 0.02%, there is no effect, and if it exceeds 1.00%, the magnetic flux density of the product sheet decreases. Therefore, the Mn content is set to 0.02 to 1.00%. The Mn content is preferably 0.04% or more, and 0.20% or less.

Co: 0.005-0.050%
For the reasons mentioned above, it is essential that Co be contained in an amount in the range of 0.005 to 0.050%. The Co amount is preferably 0.006% or more, and more preferably 0.008% or more. The Co amount is preferably 0.020% or less, and more preferably 0.015% or less.

The base steel sheet of the grain-oriented electrical steel sheet of the present invention may contain, in addition to the basic components (Si, Mn, and Co) described above, C (e.g., 0.020 to 0.100%), Al (e.g., 0.002 to 0.040%), and N (e.g., 0.002 to 0.015%). The base steel sheet may also optionally contain S (e.g., 0.020% or less) and/or Se (e.g., 0.040% or less). In addition to the components described above, the base steel sheet of the grain-oriented electrical steel sheet of the present invention may appropriately contain the components (elements) described below as necessary.

Specifically, for the purpose of improving the magnetic properties, the base steel sheet contains the following: Sn: (over 0%) 0.500% or less, Cr: (over 0%) 0.500% or less, Cu: (over 0%) 0.50% or less, Ni: (over 0%) 0.50% or less, Bi: (over 0%) 0.500% or less, P: (over 0%) 0.500% or less, Sb: (over 0%) 0.500% or less, Mo: (over 0%) 0.500% or less, B: (over 0ppm) 25.0ppm or less One or more of the following may be added in combination: pm or less, Nb: (over 0%) 0.020% or less, V: (over 0%) 0.020% or less, As: (over 0%) 0.0200% or less, Zn: (over 0%) 0.020% or less, Pb: (over 0%) 0.0100% or less, W: (over 0%) 0.0100% or less, Ga: (over 0%) 0.0050% or less, and Ge: (over 0%) 0.0050% or less.
That is, each of the above elements can be contained in the base steel sheet within the above upper limit range in order to further improve the magnetic properties. If the amount (content) of each element exceeds the upper limit, the development of secondary recrystallized grains may be suppressed, resulting in deterioration of the magnetic properties. The lower limit of each element does not need to be particularly limited, but is preferably within the following range.
Sn: 0.005% or more, Cr: 0.005% or more, Cu: 0.01% or more, Ni: 0.01% or more, Bi: 0.005% or more, P: 0.005% or more, Sb: 0.005% or more, Mo: 0.005% or more, B: 0.1 ppm or more, Nb: 0.001% or more, V: 0.001% or more, As: 0.0010% or more, Zn: 0.001% or more, Pb: 0.0001% or more, W: 0.0010% or more, Ga: 0.0001% or more and Ge: 0.0001% or more

In the base steel plate, the remainder other than the above-mentioned components (elements) is Fe and unavoidable impurities.

The above-mentioned composition is the composition of the base steel sheet, i.e., does not take into consideration the undercoating mainly composed of forsterite. In the present invention, the Ti content in the steel sheet having the undercoating mainly composed of forsterite, i.e., the Ti content in the entire base steel sheet and the undercoating, is limited to 0.0050-0.0200% for the reasons described above. The Ti content in the entire base steel sheet and the undercoating is preferably 0.0060% or more and preferably 0.0150% or less.
In addition, the "main component" of the undercoating refers to the component that has the largest mass among the components that make up the undercoating.

Here, when limiting the Ti content in the steel sheet with a base coating to 0.0050-0.0200%, it is preferable that the Ti content in the base steel sheet is 0.0030% or less. This is because if the Ti content in the base steel sheet is 0.0030% or less, it is possible to suppress a significant deterioration in iron loss due to the formation of Ti precipitates in the steel. On the other hand, the Ti content in the steel sheet with a base coating is set to 0.0050% or more. This is because, as mentioned above, it is believed that the presence of a certain amount of Ti in the forsterite coating can improve the coating properties and improve eddy current loss, but if the Ti content is less than 0.0050%, it is presumed that this effect is poor.

Furthermore, in the present invention, as described above, it is necessary to limit the range of parameters calculated from the hysteresis loss and iron loss of the product steel sheet. That is, the ratio R17 (= Wh ₁₇ /W _17/50 ) of the hysteresis loss Wh ₁₇ to the iron loss W _17/50 when excited at 1.7 T and the ratio R19 (= Wh ₁₉ /W _19/50 ) of the hysteresis loss Wh ₁₉ to the iron loss W _19/50 when excited at 1.9 T must satisfy the relationship of 0.30≦R17≦R19. These values can be measured by the method specified in JIS C2550-1. In addition, the hysteresis loss can be calculated by multiplying the energy loss of the iron core for one revolution of the hysteresis loop by 50, which is the excitation frequency, in order to match the iron loss at 50 Hz.

Next, the manufacturing method of the directional magnetic steel sheet of the present invention will be described. The manufacturing method can be a method for manufacturing a general magnetic steel sheet. For example, molten steel with a predetermined composition adjustment may be manufactured into a slab by a normal ingot casting method or a continuous casting method, or a thin cast piece with a thickness of 100 mm or less may be manufactured by a direct casting method. Molten steel may be manufactured by a blast furnace method or an electric furnace method. Since it is difficult to add the various components that can be contained in the base steel sheet described above during the process, it is preferable to add them at the molten steel stage. The slab is heated and hot rolled by a normal method, or is hot rolled immediately after casting without heating. In the case of heating, a low temperature of 1300°C or less is effective for the purpose of reducing costs, since a high-temperature annealing to dissolve the inhibitor is not required in a composition system with a small amount of inhibitor components. In the case of heating, the temperature is preferably 1250°C or less.

Next, hot-rolled sheet annealing is performed as necessary. The temperature for hot-rolled sheet annealing is preferably around 950 to 1150°C. A temperature of 950°C or higher can adequately prevent the remaining unrecrystallized parts, while a temperature of 1150°C or lower can prevent excessive coarsening of the grain size after annealing, resulting in a better primary recrystallization texture. The temperature for hot-rolled sheet annealing is preferably 1000°C or higher, and preferably 1100°C or lower.

After hot rolling or hot-rolled sheet annealing, the steel sheet is cold-rolled once or cold-rolled twice or more with intermediate annealing in between to produce a cold-rolled sheet of the final thickness. The annealing temperature for the intermediate annealing is preferably in the range of 900 to 1200°C. If the annealing temperature is 900°C or higher, it is possible to effectively prevent the recrystallized grains after the intermediate annealing from becoming too fine, and further, it is possible to effectively prevent the deterioration of the magnetic properties of the product sheet due to the reduction in Goss nuclei in the primary recrystallized structure. On the other hand, if the annealing temperature is 1200°C or lower, it is possible to prevent the crystal grains from becoming excessively coarse, as in the case of hot-rolled sheet annealing, and to improve the primary recrystallized structure of the grain size regulation.

The cold-rolled sheet having reached its final thickness is then subjected to primary recrystallization annealing, which also serves as decarburization annealing. If decarburization annealing is also involved, the annealing temperature in this primary recrystallization annealing is preferably in the range of 800 to 900°C in order to allow the decarburization reaction to proceed quickly, and the atmosphere is preferably a humid atmosphere.

Then, by applying an annealing separator mainly made of MgO and then performing purification annealing, it is possible to develop a secondary recrystallized structure and form a forsterite film. Here, "mainly made of MgO" means that the material contains 75 mass% or more of MgO.

Furthermore, Ti can be effectively made to exist in the forsterite coating by adding a Ti compound to the annealing separator and by introducing a _N2 atmosphere during purification annealing, which will be described later. However, Ti may also be made to exist in the forsterite coating by other methods.

Next, secondary recrystallization annealing (purification annealing) is performed. This purification annealing is preferably performed at 800 ° C or higher to manifest secondary recrystallization, and from the viewpoint of purification, it is desirable to raise the temperature to a holding temperature of 1100 ° C or higher. The holding temperature is more preferably 1180 ° C or higher. The longer the holding time, the more purification progresses, but because shape deterioration due to high-temperature creep occurs, it is desirable to hold for 3 hours or more and 15 hours or less. After purification annealing, it is preferable to wash with water, brush, or pickle in order to remove the attached annealing separator. In addition, from the viewpoint of obtaining the desired iron loss characteristics, in the temperature rise process of purification annealing, it is preferable to use an N ₂ atmosphere up to the first intermediate temperature (for example, a temperature selected from the range of 600 to 800 ° C), a mixed atmosphere of N ₂ and H ₂ from the first intermediate temperature to the second intermediate temperature (for example, a temperature selected from the range of 1050 to 1150 ° C), and an H ₂ atmosphere from the second intermediate temperature to the holding temperature.

After that, flattening annealing is performed to correct the shape, which is effective in reducing iron loss. When steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheet before or after flattening annealing in order to improve iron loss. This insulating coating is preferably a coating that can impart tension to the steel sheet in order to reduce iron loss. It is preferable to adopt a method of forming a coating by depositing an inorganic substance on the surface of the steel sheet using a tension coating application method via a binder, a physical vapor deposition method, or a chemical vapor deposition method, as these methods have excellent coating adhesion and are effective in significantly reducing iron loss.

The present invention will be specifically described below with reference to examples. However, the present invention is not limited to these.

Example 1
Steel slab A: C: 0.070%, Si: 3.55%, Mn: 0.07%, Al: 0.0080%, N: 0.0050%, Co: 0.012%, Mo: 0.026%, Ti: 0.025%, the balance being Fe and unavoidable impurities Steel slab B: C: 0.072%, Si: 3.51%, Mn: 0.07%, Al: 0.0080%, N: 0.0047%, Co: 0.011%, Mo: 0.025%, Ti: 0.0025%, the balance being Fe and unavoidable impurities Steel slab C: C: 0.072%, Si: 3.49%, Mn: 0.07%, Al: 0.0090%, N: 0.0051%, Co: 0.002%, Mo: 0.025%, Ti: 0.024%, with the balance being Fe and unavoidable impurities. Steel slab D: C: 0.068%, Si: 3.48%, Mn: 0.07%, Al: 0.0090%, N: 0.0050%, Co: 0.008%, Mo: 0.022%, Ti: 0.0010%, with the balance being Fe and unavoidable impurities. Each of the above steel slabs A to D was produced by continuous casting, and after slab heating by soaking at 1200°C for 40 minutes, was finished to a thickness of 2.2 mm by hot rolling. The strip was then annealed in a _N2 atmosphere at 1000°C for 60 seconds, then cold rolled to a thickness of 0.23 mm, and decarburized in a 60% _H2-40 % _N2 , 60°C dew point, humid atmosphere at 850°C for 90 seconds.

Next, an annealing separator mainly composed of MgO (MgO: 97% by mass) was applied to the surface of the base steel sheet after decarburization annealing, and purification annealing was performed by holding at 1100°C for 25 hours and then at 1200°C for 10 hours. During this temperature increase process, the atmosphere was _N2 from room temperature to 700°C, the atmosphere was variously changed in the mixture ratio of _N2 and _H2 from 700°C to 1100°C, and the atmosphere was _H2 from 1100°C (start of holding) to 1200°C (end of holding). Furthermore, the Ar atmosphere was used during cooling.

The Ti content (Ti content in the entire base steel sheet and base coating) of the thus obtained samples, i.e., steel sheets with a base coating mainly composed of forsterite, was measured according to the method specified in JIS G1223. The results are shown in Table 1.

An insulating coating mainly composed of magnesium phosphate and silica was applied onto the undercoat of the steel sheet. The iron loss W _17/50 (iron loss when excited to 1.7 T at 50 Hz) and W _19/50 (iron loss when excited to 1.9 T at 50 Hz) and hysteresis loss Wh ₁₇ (hysteresis loss when excited to 1.7 T) and Wh ₁₉ (hysteresis loss when excited to 1.9 T) of the thus obtained sample were measured according to the method specified in JIS C2550-1. _{Wh 17} /W _17/50 (i.e. R17) and Wh ₁₉ /W _19/50 (i.e. R19) are also shown in Table 1.

Furthermore, to measure the Co and Ti contents in the base steel sheet, a part of the obtained sample was immersed in a 10% hydrochloric acid solution at 80°C for 180 seconds to remove the undercoat, and then subjected to measurements according to the methods specified in JIS G1222 and JIS G1223. The measurement results are shown in Table 1.

Next, a three-phase three-legged model transformer was made from the sample with the insulating coating, simulating a transformer with an external shape of 500 mm square and each leg and yoke having a plate width of 100 mm, and the model transformer iron loss WT _17/50 (transformer iron loss when excited to 1.7 T at 50 Hz) was measured. The number of laminated sheets of the sample was 50, and they were stacked two by two. The building factor F17 of the model transformer was calculated as the value obtained by dividing the model transformer iron loss WT _17/50 by the sample iron loss W _17/50 (WT _17/50 /W _17/50 ). The results are shown in Table 1.

As is clear from Table 1, the sample (grain-oriented electrical steel sheet) according to the present invention has good iron loss characteristics (building factor).

Example 2
A steel slab containing the components shown in Table 2, with the balance being Fe and unavoidable impurities, was produced by continuous casting, and after slab heating at 1410°C for 20 minutes, it was finished to a thickness of 2.4 mm by hot rolling. Then, it was annealed at 1100°C for 20 seconds in a _N2 atmosphere. Then, it was cold rolled to a thickness of 1.5 mm, and further intermediate annealed at 900°C for 100 seconds in a 25% _H2-75 % _N2 atmosphere. Then, it was cold rolled to a thickness of 0.23 mm, and further decarburization annealed at 825°C for 150 seconds in a moist atmosphere of 40% _H2-60 % _N2 with a dew point of 45°C.

Next, an annealing separator mainly composed of MgO (MgO: 88 mass%) was applied to the surface of the base steel sheet after decarburization annealing. The annealing separator was prepared by adding 5 parts by mass of perhydrated _TiO2 powder, which was obtained by adding _TiO2 powder to hot water at 50°C and stirring for 24 hours.

Further, purification annealing was performed by holding at 1200°C for 10 hours. At this time, the heating rate up to 1200°C was 15°C/h, and further, in the heating process, _N2 atmosphere was used from room temperature to 700°C, atmospheres with various mixture ratios of _N2 and _H2 were used from 700°C to 1100°C, and _H2 atmosphere was used from 1100°C to 1200°C. In addition, the _H2 atmosphere was used during holding, and Ar atmosphere was used during cooling.

The Ti content (Ti content in the entire base steel sheet and base coating) of the thus obtained samples, i.e., steel sheets with a base coating mainly composed of forsterite, was measured according to the method specified in JIS G1223. The measurement results are shown in Table 3.

An insulating coating mainly composed of magnesium phosphate and silica was applied onto the undercoat of the steel sheet. The iron loss W _17/50 (iron loss when excited to 1.7 T at 50 Hz) and W _19/50 (iron loss when excited to 1.9 T at 50 Hz) and hysteresis loss Wh ₁₇ (hysteresis loss when excited to 1.7 T) and Wh ₁₉ (hysteresis loss when excited to 1.9 T) of the thus obtained sample were measured according to the method specified in JIS C2550-1. _{Wh 17} /W _17/50 (i.e. R17) and Wh ₁₉ /W _19/50 (i.e. R19) are shown in Table 3.

Furthermore, to measure the Co and Ti contents in the base steel sheet, a part of the obtained sample was immersed in a 10% hydrochloric acid solution at 80°C for 180 seconds to remove the undercoat, and then subjected to measurements according to the methods specified in JIS G1222 and JIS G1223. The measurement results are shown in Table 3.

Next, a three-phase three-legged model transformer was made from the sample with the insulating coating, simulating a transformer with an external shape of 500 mm square and a plate width of 100 mm for each leg and yoke, and the model transformer iron loss WT _17/50 (transformer iron loss when excited to 1.7 T at 50 Hz) was measured. The number of laminated sheets of the sample was 50, and two sheets were stacked alternately. The building factor F17 of the model transformer was calculated as the value (WT _17/50 /W _17/50 ) obtained by dividing the model transformer iron loss WT _17/50 by the iron loss W _17/50 of the sample. The results are shown in Table 3.

As is clear from Table 3, the sample (grain-oriented electrical steel sheet) according to the present invention has good iron loss characteristics (building factor).

Claims (5)

A grain-oriented electrical steel sheet comprising a base steel sheet containing 1.50 to 4.50 mass% Si, 0.02 to 1.00 mass% Mn, 0.005 to 0.050 mass% Co , with the balance being Fe and unavoidable impurities , and a base coating containing forsterite as a main component and formed on a surface of the base steel sheet,
the Ti content in the base steel sheet is 0.0034% by mass or less, and the Ti content in the base steel sheet and the undercoating as a whole is 0.0050 to 0.0200% by mass,
A grain-oriented electrical steel sheet that satisfies the following formula (1), where R17 is the ratio of hysteresis loss _Wh17 to iron _{loss W17/50 when excited at} 1.7 T, and R19 is the ratio of hysteresis loss _Wh19 to iron loss _W19 / ₅₀ when excited at 1.9 T.
0.30≦R17≦R19...(1) The grain-oriented electrical steel sheet according to claim 1, wherein the amount of Ti in the base steel sheet is 0.0030 mass% or less. The grain-oriented electrical steel sheet according to claim 1 or 2, which has an insulating coating on the surface of the base coating. The grain-oriented electrical steel sheet according to claim 1 or 2, wherein the base steel sheet further contains one or more selected from Sn: 0.500 mass% or less, Cr: 0.500 mass% or less, Cu: 0.50 mass% or less, Ni: 0.50 mass% or less, Bi: 0.500 mass% or less, P: 0.500 mass% or less, Sb: 0.500 mass% or less, Mo: 0.500 mass% or less, B: 25.0 mass ppm or less, Nb: 0.020 mass% or less, V: 0.020 mass% or less, As: 0.0200 mass% or less, Zn: 0.020 mass% or less, Pb: 0.0100 mass% or less, W: 0.0100 mass% or less, Ga: 0.0050 mass% or less, and Ge: 0.0050 mass% or less. The grain-oriented electrical steel sheet according to claim 3, wherein the base steel sheet further contains one or more selected from Sn: 0.500 mass% or less, Cr: 0.500 mass% or less, Cu: 0.50 mass% or less, Ni: 0.50 mass% or less, Bi: 0.500 mass% or less, P: 0.500 mass% or less, Sb: 0.500 mass% or less, Mo: 0.500 mass% or less, B: 25.0 mass ppm or less, Nb: 0.020 mass% or less, V: 0.020 mass% or less, As: 0.0200 mass% or less, Zn: 0.020 mass% or less, Pb: 0.0100 mass% or less, W: 0.0100 mass% or less, Ga: 0.0050 mass% or less, and Ge: 0.0050 mass% or less.

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