WO2025005169A1 - Cold-rolled steel sheet - Google Patents

Abstract

In this cold-rolled steel sheet for grain-oriented electromagnetic steel sheets, when a region of 100 mm × 100 mm on the plate surface has been defined into divided sections and it has been confirmed for at least 100 of the divided sections whether Goss-oriented grains are included in the divided sections, divided sections that contain at least one Goss-oriented grain having a grain diameter of 5 μm or more constitute 30 area% or more with respect to all of the divided sections.

Description

Cold rolled steel sheet

The present invention relates to a cold-rolled steel sheet for use in a grain-oriented electrical steel sheet.
This application claims priority based on Japanese Patent Application No. 2023-106555, filed on June 29, 2023, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheet contains silicon, and the crystal orientation of its crystal grains is concentrated near the Goss orientation (cubic {110}<001>), with the <001> orientation, which is the axis of easy magnetization, being nearly aligned with the rolling direction in the steel sheet manufacturing process. Such grain-oriented electrical steel sheet is extremely useful as a material for the iron cores of transformers, etc. Among the magnetic properties of grain-oriented electrical steel sheet, the most important are magnetic flux density and core loss.

The magnetic flux density of a grain-oriented electrical steel sheet when a certain magnetizing force is applied tends to be higher as the degree to which the magnetization easy axis of the crystal grains is aligned in the rolling direction of the steel sheet, i.e., the orientation of the crystal orientation, increases. Magnetic flux density _B8 is generally used as an index of magnetic flux density. Magnetic flux density _B8 is the value of the magnetic flux density of a magnetized grain-oriented electrical steel sheet when a magnetizing force of 800 A/m is applied in the rolling direction. In other words, a grain-oriented electrical steel sheet with a higher magnetic flux density _B8 value is more easily magnetized with a certain magnetizing force and has a higher magnetic flux density, making it suitable for small, highly efficient transformers.

Moreover, the iron loss W17/ ₅₀ is generally used as an index of iron loss. The iron loss W17 _/50 is the iron loss when a grain-oriented electrical steel sheet is excited with an AC current under conditions of a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz. It can be said that the smaller the value of the iron loss W17/ ₅₀ of a grain-oriented electrical steel sheet, the lower the energy loss and the more suitable it is for a transformer.

There are several methods for reducing iron loss, including increasing electrical resistance by adding Si, which is effective in reducing eddy current loss, reducing the thickness of the steel sheet, reducing the grain size, and aligning the orientation of the grains, which is effective in reducing hysteresis loss. That is, the larger the magnetic flux density _B8 , the more the crystal orientation is aligned to the Goss orientation, and the less the hysteresis loss. Therefore, the larger the magnetic flux density _B8 , the smaller the value of the iron loss W17 _/50 . On the other hand, when the development of Goss orientation grains is promoted to improve the degree of integration of the crystal orientation, the grain size tends to become larger, so that the eddy current loss becomes larger, and there is a trade-off between the fact that the iron loss deteriorates when the grains become too large even if the value of _B8 is high.

In recent years, methods have been developed to artificially subdivide the magnetic domain width by introducing distortion through irradiation with laser light or plasma jet, or by forming grooves mechanically or by etching, to control the magnetic domain. By using such methods, in a steel sheet in which the degree of integration of crystal orientation is increased and the magnetic flux density is increased, even if the secondary recrystallized grain size is large, the eddy current loss can be reduced and the iron loss can be sufficiently reduced. However, when distortion is introduced into the steel sheet, the magnetostriction (λ _P-P ) increases due to the distortion. In addition, if heat treatment is performed at a temperature of about 800°C after the magnetic domain subdivision, the iron loss reduction effect disappears, so the steel sheet cannot be used for applications requiring distortion relief annealing at 800°C or higher after irradiation. On the other hand, the magnetic domain control method using grooves has a problem of a decrease in magnetic flux density B ₈ .

Generally, grain-oriented electrical steel sheets are manufactured as follows. A steel sheet material (slab) containing a specified amount of Si is hot-rolled, annealed, and cold-rolled to obtain a steel sheet of the desired thickness. The cold-rolled steel sheet is then annealed (also called primary annealing or decarburization annealing). This annealing causes primary recrystallization, and within the primary recrystallized grains, crystal grains are formed that have a crystal orientation with an angle of deviation of 10° or less from the strict Goss orientation described by {110}<001>, with the axis of easy magnetization aligned in the rolling direction (hereinafter referred to as Goss orientation grains). This annealing also serves as decarburization annealing. After that, an annealing separator mainly composed of MgO is applied to the surface of the steel sheet where primary recrystallization has occurred. Next, the steel sheet coated with the annealing separator is wound up to produce a steel sheet coil, and this steel sheet coil is subjected to batch annealing (also called secondary recrystallization annealing or finish annealing). This annealing causes the Goss orientation grains to eat away at other crystal grains, causing secondary recrystallization and forming a so-called glass coating on the surface of the steel sheet. During secondary recrystallization, the Goss orientation grains grow preferentially due to the influence of inhibitors contained in the steel sheet, and the grain size of larger grains may be 100 mm or more. Next, while the steel sheet coil is unwound, annealing is performed to flatten the steel sheet where secondary recrystallization has occurred, and an insulating coating is formed, etc.

As described above, grain-oriented electrical steel sheets obtain predetermined magnetic properties by inducing secondary recrystallization during final annealing to obtain a crystal structure consisting of Goss-oriented grains accumulated in the {110}<001> orientation. In order to obtain this crystal structure, it is effective to increase the frequency of minor Goss-oriented grains and to increase the frequency of grains that have good crystal lattice matching with the Goss-oriented grains after primary annealing. For example, crystal grains of {778}<447>(≒{111}<112>) orientation and {411}<148> orientation (called corresponding orientation grains), which have high crystal lattice matching with the {110}<001> orientation and have a Σ9 corresponding orientation relationship with the Goss orientation, are crystal grains that are easily encroached upon by the Goss-oriented grains, and it is effective to include a large number of them in the primary recrystallized structure. However, when controlling the crystal orientation of the crystal grains generated by the primary annealing, it is difficult to simultaneously increase the frequency of the Goss orientation grains and the frequency of the crystal grains having the corresponding orientation relationship. For example, in the region where the cold rolling reduction is 85% or more, the frequency of the corresponding orientation grains that are easily encroached by the Goss orientation grains increases with an increase in the cold rolling reduction, whereas the frequency of the Goss orientation grains decreases with an increase in the cold rolling reduction. In addition, the frequency of the Goss orientation grains, which are the minor orientation in the primary recrystallization texture, increases as the heating rate of the primary annealing increases, whereas the frequency of the corresponding orientation grains, which have the {111}<112> orientation, which is the major orientation in the primary recrystallization texture, decreases as the heating rate of the primary annealing increases.
That is, under conventional process control conditions, it has not been possible to independently control the frequency of existence of Goss-oriented grains and the frequency of existence of grains of a corresponding orientation that are easily encroached upon by the Goss-oriented grains.

Here, the deviation angle of the crystal orientation observed in the grain-oriented electrical steel sheet from the ideal {110}<001> orientation around the normal direction Z (ND) of the rolling surface is defined as deviation angle α, the deviation angle from the ideal {110}<001> orientation around the direction perpendicular to the rolling direction C (TD) is defined as deviation angle β, and the deviation angle from the ideal {110}<001> orientation around the rolling direction L (RD) is defined as deviation angle γ. The angular deviation θ of the crystal orientation observed in the grain-oriented electrical steel sheet from the ideal {110}<001> orientation is defined as the angular deviation θ obtained from the deviation angles α, β, and γ around the ND, TD, and RD by the following (Equation 1):
θ=(α ² +β ² +γ ² ) ^1/2 (Formula 1)

As mentioned above, the Goss-oriented grains are not crystal grains (called ideal Goss-oriented grains) that have a Goss orientation (ideal Goss orientation) in the strict sense. The magnetization easy axis direction (cubic {110}<001>) of each crystal grain does not necessarily coincide completely with the rolling direction, and an angle deviation θ exists between the magnetization easy axis direction and the rolling direction. Crystal grains whose angle deviation θ from the ideal Goss orientation based on (Equation 1) is 10° or less are called Goss-oriented grains (practical Goss-oriented grains). When the angle deviation θ of the practical Goss-oriented grains increases, the orientation of the crystal orientation decreases, and the magnetic flux density _B8 decreases.

　Generally, the Goss-oriented grains contained in steel sheets after primary recrystallization also contain grains with a large angle deviation θ. However, when grains grow due to secondary recrystallization, excluding the effect of the size advantage of the grain size in the early stages of secondary recrystallization, the smaller the angle deviation θ among the Goss-oriented grains, the greater the driving force for grain growth and the more likely they are to continue growing preferentially until secondary recrystallization is complete. Therefore, the smaller the angle deviation θ of Goss-oriented grains, the more likely they are to have a larger grain size after secondary recrystallization. In other words, in manufacturing methods that highly orient secondary recrystallized grains in the Goss orientation, certain grains are preferentially grown, which inevitably makes it easier for the grain size to increase.

In addition, during secondary recrystallization, the steel sheet is not flat but curved as it is wound into a coil, but the crystal grains grow while maintaining the linearity of the crystal orientation. Therefore, when the coiled steel sheet is unwound and flattened after secondary recrystallization, there are parts within the crystal grains where the direction of the easy axis of magnetization is not parallel to the surface of the grain-oriented electrical steel sheet. This coil unwinding mainly increases the value of the deviation angle β and also increases the angle deviation θ. The increase in angle deviation θ due to coil unwinding is more noticeable the larger the crystal grain size. In other words, Goss-oriented grains with a small angle deviation θ tend to grow preferentially during secondary recrystallization, but if the secondary recrystallized grain size becomes coarse, the angle deviation θ increases due to coil unwinding.

Patent Document 1 discloses a technique in which an etching process is performed on the surface of a steel sheet after final cold rolling to form linear grooves that satisfy certain conditions, and then in the primary annealing, the heating rate in the steel sheet temperature range of 500°C to 750°C is made faster in the linear groove parts compared to parts other than the linear grooves. In Patent Document 1, by artificially creating parts with fast and slow heating rates, parts with a high heating rate (linear groove parts) and parts with a high heating rate (parts other than the linear grooves) are arranged, where the frequency of (110)[001] oriented grains is actively increased.

Patent Document 2 discloses a technique in which a laser beam is irradiated multiple times at a specified interval PL in the rolling direction toward the surface of a silicon steel sheet between cold rolling and finish annealing. When secondary recrystallization occurs in this silicon steel sheet by finish annealing, the positions along the trajectory of the laser beam become grain boundaries that penetrate the front and back of the silicon steel sheet. In Patent Document 2, after finish annealing, the length of the crystal grains in the rolling direction is at most about 30 mm, which corresponds to the irradiation interval PL, and the angular deviation between the magnetization easy axis direction (cubic {110}<001>) and the rolling direction is within the range of 0° to 6°.

Patent Document 3 discloses a method for manufacturing grain-oriented electrical steel sheet that is based on a high-temperature slab heating process, has a small difference in the degree of orientation concentration in the Goss orientation at the inner and outer periphery of a coil with different curvatures, and has a high magnetic flux density. In Patent Document 3, linear local heating is performed on a cold-rolled steel sheet, and this linear heating portion acts as a barrier to the growth of secondary recrystallized grains, suppressing the growth of secondary recrystallized grains.

Japanese Patent Application Publication No. 2007-169762 International Publication No. WO2012/014290 Japanese Patent Application Publication No. 2022-161269

As mentioned above, attempts have been made to improve the magnetic properties of grain-oriented electrical steel sheets. However, the improvement in magnetic properties achieved by conventional techniques cannot be said to be sufficient.

The present invention has been made in consideration of the above problems. The object of the present invention is to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density. Specifically, the object of the present invention is to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density while suppressing the coarsening of secondary recrystallized grain sizes.

In other words, the gist of the present invention is as follows:

[1] A cold-rolled steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention comprises:
The deviation angle from the ideal Goss orientation with the normal direction of the rolling surface as the rotation axis is defined as α,
The deviation angle from the ideal Goss orientation with the direction perpendicular to the rolling as the rotation axis is defined as β.
The deviation angle of the crystal orientation measured at the measurement point on the plate surface is represented as (α β),
The angular deviation at the measurement point is defined as φ=(α ² +β ² ) ^1/2 ;
A crystal grain having an angle deviation φ of 10° or less is defined as a Goss oriented grain,
A 100 mm x 100 mm area on the plate surface is defined as a divided section.
When it is confirmed whether the Goss oriented grains are included in the divided sections in at least 100 divided sections,
The divided sections containing at least one Goss oriented grain having a grain size of 5 μm or more account for 30% or more by area of all the divided sections.

According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density. Specifically, it is possible to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density while suppressing the coarsening of secondary recrystallized grains.

For example, by subjecting the raw sheet for oriented electrical steel sheet before the primary annealing for manufacturing the oriented electrical steel sheet to localized rapid heating and forming Goss orientation grains of 5 μm or more in the cold-rolled steel sheet, the magnetic flux density of the oriented electrical steel sheet can be increased. Furthermore, by suitably distributing the areas where localized rapid heating is performed within the cold-rolled steel sheet, the magnetic flux density can be increased while suppressing the coarsening of the secondary recrystallized grain size of the oriented electrical steel sheet.

FIG. 2 is a diagram showing an example of spot current heating used as a local rapid heating method, and is a schematic diagram showing an example of the arrangement of local heating regions in a cold-rolled steel sheet for a grain-oriented electrical steel sheet according to the present embodiment. FIG. 2 is a schematic diagram showing the distribution of Goss-oriented grains in a cold-rolled steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention.

A preferred embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention. In addition, the numerical ranges described below include lower and upper limits. Numerical values indicated as "greater than" or "less than" are not included in the numerical range. In addition, "%" in relation to chemical composition means "mass %" unless otherwise specified.

As mentioned above, the frequency of practical Goss orientation grains, which have small angular deviations θ and φ from the ideal Goss orientation {110}<001> orientation, increases as the heating rate of the primary annealing (decarburization annealing) increases. In contrast, crystal grains that are easily encroached upon by Goss orientation grains, such as the {778}<447> (≒{111}<112>) orientation and the {411}<148> orientation, which have a corresponding orientation relationship of Σ9 with the Goss orientation, (called corresponding orientation grains) conversely decrease as the heating rate of the primary annealing increases. For this reason, it has been difficult to increase the proportion of practical Goss orientation grains while also increasing the proportion of corresponding orientation grains during primary annealing.

The technology described in Patent Document 1 forms linear grooves on the surface of a steel sheet by etching, and increases the heating rate of the linear grooves during primary annealing to arrange a portion with a high ratio of Goss orientation grains (linear groove portion) and a portion with a high ratio of corresponding orientation grains (portion other than the linear grooves). However, with this method, it is difficult to increase the difference in heating rate during primary annealing between the linear groove portion and other portions, and the effect obtained is limited. Specifically, the difference in heating rate between the linear groove portion and the portion other than the linear grooves is not so large. For example, even if the entire steel sheet is heated at 100°C/sec, the degree to which the sheet thickness of this portion is thinned by the formation of the grooves does not exceed about 10% of the original sheet thickness (because the normal groove depth is about 20 μm or less, which is about 10% or less of the sheet thickness), and considering the thermal conductivity of the steel sheet, the heating rate ratio is less than the sheet thickness ratio for the same heat input, so the ratio of Goss orientation grains in the linear groove portion is difficult to increase compared to the portion other than the linear grooves. In addition, this method increases the heating rate of the linear groove portion during the primary annealing, so the grain size of the Goss-oriented grains formed in the linear groove portion after the primary annealing does not increase in size compared to the overall average primary recrystallized grain size, but remains the same or finer.

In addition, the technology described in Patent Document 2 irradiates a laser beam toward the steel sheet surface before finish annealing. However, with this method, the laser beam irradiation area is only controlled to the grain boundaries of secondary recrystallization, and the orientation of the Goss orientation grains generated by primary recrystallization is not controlled. In addition, the area heated by laser irradiation is only the area near the surface where the laser is irradiated, and there is significant thermal diffusion to the surrounding areas, so it is usually not possible to ensure a thermal history that will recrystallize the steel sheet.

In addition, the technology described in Patent Document 3 irradiates a laser beam toward the surface of the steel sheet after cold rolling. However, in this method, as in the above, the laser beam irradiation portion is only controlled to the grain boundary of secondary recrystallization, and the heating rate is not controlled, so the orientation of the Goss orientation grains generated by primary recrystallization is not controlled. In Patent Document 3, the laser beam irradiation portion acts as a barrier to the growth of secondary recrystallized grains, thereby only reducing the grain size in the rolling direction of the secondary recrystallized grains, and a grain growth temperature range in the local heating region that is effective for increasing the diameter of Goss orientation grains in the local heating region is not secured. This method may be able to reduce the difference in the degree of orientation accumulation in the Goss orientation at the inner and outer periphery of the coil with different curvatures, but it does not obtain Goss orientation grains whose crystal orientation is close to the ideal Goss orientation. In other words, this method suppresses the decrease in magnetic flux density due to the increase in the grain size in the rolling direction of secondary recrystallized grains, but does not aim to fundamentally improve the orientation of the Goss orientation grains.

In this embodiment, it has been discovered for the first time that a grain-oriented electrical steel sheet with excellent quality can be obtained by locally and rapidly heating the steel sheet prior to the primary annealing (decarburization annealing) to form a locally heated region. The locally heated region formed prior to the primary annealing is arranged locally on the surface of the steel sheet. The crystalline structure of this locally heated region consists of one or both of a recrystallized structure and a recovered structure at the end of the localized rapid heating. The crystalline structure of the non-locally heated region of the steel sheet surface other than the locally heated region remains as it is cold-rolled and therefore consists of a cold-worked structure.

The above-mentioned localized heating region preferably has a heating rate of 500°C/sec or more, preferably 2000°C/sec or more, more preferably 5000°C/sec or more, and even more preferably 10000°C/sec or more. In this way, it was found that when a localized heating region is formed on the surface of a steel sheet before primary annealing, crystal grains having a practical Goss orientation are formed in the localized heating region. It was also found that, among the Goss-oriented grains formed, the proportion of Goss-oriented grains with smaller angular deviations θ and φ from the ideal Goss orientation increases, and the grain size of the Goss-oriented grains increases.

The frequency of Goss-oriented grains in locally heated regions is greater than the frequency of Goss-oriented grains in non-locally heated regions. This characteristic is due to the fact that the non-locally heated regions retain the processed structure. Specifically, when local rapid heating is performed, recovery and recrystallization occur in the locally heated regions, resulting in the formation of practical Goss-oriented grains and facilitating grain growth, but recovery and recrystallization do not occur in non-locally heated regions.

　Prior to the primary annealing, localized rapid heating is used to scatter locally heated areas with the above characteristics on the steel sheet surface, after which the primary annealing and secondary annealing are performed. The Goss-oriented grains formed in the locally heated areas grow preferentially during the secondary annealing after the primary annealing. Goss-oriented grains are formed in the locally heated areas, and among these grains are Goss-oriented grains with large crystal grain size that are close to the ideal Goss orientation with particularly small angle deviations θ and φ, and these Goss-oriented grains grow preferentially during the secondary recrystallization.

Since the Goss orientation grains that serve as the nuclei for secondary recrystallization are supplied to the locally heated region, there is no need to form Goss orientation grains in the primary annealing in the matrix parts other than the locally heated region, i.e., the non-locally heated region other than the locally heated region. Therefore, as the heat treatment conditions for the primary annealing, a slow heating condition with a low heating rate may be adopted, and primary annealing conditions may be adopted that form many corresponding orientation grains with the easily eroded {111}<112> and {411}<148> orientations in the non-locally heated region. For example, by setting the heating rate for the primary annealing to 300°C/sec or less, many Goss orientation grains with large crystal grain size and close to the ideal Goss orientation are formed in the locally heated region, while almost no Goss orientation grains are formed in the non-locally heated region, and conversely, many corresponding orientation grains can be formed.

As described above, in the locally heated region of a steel sheet (decarburized annealed steel sheet) that has completed localized rapid heating and primary annealing, there are many Goss-oriented grains that are close to the ideal Goss orientation and have a large grain size. On the other hand, there are almost no large-sized Goss-oriented grains in the matrix portion (non-locally heated region), and it is also possible to have many corresponding orientation grains that are easily eroded. When such a steel sheet is subjected to secondary annealing, the Goss-oriented grains that are close to the ideal Goss orientation and that are large in grain size, which are present in the locally heated region, begin to grow preferentially, and grow toward the non-locally heated region.

As one form of localized rapid heating, we will explain the case where spot current heating by electrical resistance heating is used. In spot current heating, spot electrodes are pressed against both sides of a steel sheet facing each other, and current is passed between the spot electrodes to spot current heating the electrode holding portion of the steel sheet. A steel having the composition shown in Table 1 was used, and a cold-rolled steel sheet with a sheet thickness of 0.22 mm was obtained by hot rolling and cold rolling. This steel sheet was subjected to spot current heating. The spot current heating was performed using a copper electrode with a diameter of 3 mmφ, under the following conditions: the current was 5.0 kA or less, which is a range in which the steel sheet surface does not melt, the current time was 20 ms to 80 ms, the electrode pressure was 50 kgf to 150 kgf, and the electrode holding time after current was 0.2 seconds. Spot current heating was performed on the surface of the cold-rolled steel sheet in a grid pattern with a pitch of 30 mm in the rolling direction and a pitch of 30 mm in the width direction (perpendicular to the rolling direction), forming a localized heating area.

Figure 1 shows an example of the arrangement of localized heating regions in a cold-rolled steel sheet for grain-oriented electrical steel sheet according to this embodiment. In Figure 1, a steel sheet 1, localized heating regions 2, rolling direction 21, and width direction (direction perpendicular to rolling) 22 are shown diagrammatically.

In order to prepare an observation surface for EBSD (Electron Back Scattering Diffraction pattern) on the surface of the cold-rolled steel sheet (cold-rolled steel sheet after localized rapid heating and before primary annealing), a smooth surface was created by mechanical polishing, and electrolytic polishing was performed to remove processing distortion on the surface. The IQ (Image Quality) value and crystal orientation were measured in 0.5 μm steps in the area including the locally heated area by EBSD. Figure 2 is a schematic diagram showing the distribution of Goss orientation grains in the cold-rolled steel sheet for grain-oriented electrical steel sheet according to this embodiment.

Figure 2 shows an observation area including a locally heated region 2 and a non-locally heated region (matrix portion) 3. The non-locally heated region 3 is a cold-worked structure. Therefore, when the IQ value is measured by EBSD after the above-mentioned pretreatment of the steel plate surface, the non-locally heated region 3 exhibits an IQ value of a cold-worked structure. The locally heated region 2 is a recovered structure and a recrystallized structure, so its IQ value is higher than that of a cold-worked structure. By comparing the IQ values in this way, the locally heated region 2 and the non-locally heated region 3 can be identified, and the boundary between the locally heated region 2 and the non-locally heated region 3 (locally heated region boundary 4) can be confirmed. Note that when it is not easy to distinguish the local heated region boundary 4 by microstructural observation, the region containing Goss-oriented grains with a grain size of 5 μm or more can be regarded as the locally heated region 2.

The current-carrying electrode used for spot current heating is circular, and its diameter is preferably 0.5 mmφ to 10 mmφ, and more preferably 1 mmφ to 5 mmφ, in terms of circle equivalent diameter. Note that, although a circular current-carrying electrode was used in the above, it is sufficient to locally increase the heating rate of the cold-rolled steel sheet, form Goss-oriented grains in this region, and allow the grains to grow; the electrode shape may be other shapes, for example, elliptical or linear.

2, based on the definition of the angle deviation φ=( ^α2 + ^β2 ) ^1/2 , crystal grains (Goss oriented grains 14) whose angle deviation φ from the ideal Goss orientation is 10° or less are indicated by "○". Furthermore, from among the crystal grains whose angle deviation φ is 10° or less, crystal grains (coarse Goss oriented grains 15) whose grain size is 5 μm or more are indicated by "◎".

As shown in FIG. 2, the cold-rolled steel sheet (steel sheet 1) for grain-oriented electrical steel sheet according to this embodiment contains Goss-oriented grains 14 and also contains coarse Goss-oriented grains 15. In particular, the region containing the coarse Goss-oriented grains 15 corresponds to the locally heated region 2. Note that almost no Goss-oriented grains are observed in the matrix portion 3 (non-locally heated region 3) where no local heating has been performed.

The above cold-rolled steel sheet was used for primary annealing (decarburization annealing). In the primary annealing, the cold-rolled steel sheet was heated at a heating rate of 100°C/sec and held at 830°C for 90 seconds. In the decarburization annealed steel sheet after the primary annealing, many Goss-oriented grains were present in the locally heated areas, and these crystal grains had also grown. On the other hand, almost no Goss-oriented grains were observed in the matrix portion (non-locally heated area).

The above decarburized annealed steel sheet was further subjected to nitriding treatment, and then an annealing separator mainly composed of MgO was applied, followed by secondary annealing. The secondary annealing conditions were a hydrogen-nitrogen atmosphere, a heating rate of 15°C/hour, and retention at 1200°C for 20 hours. The obtained steel sheet was subjected to macro-etching to reveal the grain boundaries. When the steel sheet after macro-etching was observed at a location corresponding to the local heating region, it was observed that a plurality of secondary recrystallized grains had grown from the location corresponding to the local heating region, and that secondary recrystallized grains had spread from the center of the location corresponding to the local heating region. In addition, in the location corresponding to the local heating region, fine grains of 2 mm or less were sometimes observed as traces of the local heating region, and sometimes not. Although the secondary recrystallized macrostructure may change slightly depending on the local heating conditions, the degree of integration of the Goss orientation can be increased compared to the case where the local heating treatment was not performed, and as a result, the magnetic flux density B ₈ of the steel sheet was improved. It was confirmed that the crystal orientation of the secondary recrystallized grains thus obtained had an extremely small deviation from the ideal Goss orientation ( _B8 in this steel sheet improved from 1.911 T to 1.943 T). As described above, the cold-rolled steel sheet according to this embodiment contains Goss orientation grains 14 as well as coarse Goss orientation grains 15. It is believed that the formation of coarse Goss orientation grains 15 close to the ideal Goss orientation with a size advantage is the cause of the good secondary recrystallization orientation.

The characteristics of the cold-rolled steel sheet for the grain-oriented electrical steel sheet according to this embodiment will be specifically described. The cold-rolled steel sheet shown in Figure 2 satisfies the following characteristics.

The cold-rolled steel sheet for the grain-oriented electrical steel sheet according to this embodiment is
The deviation angle from the ideal Goss orientation with the normal direction of the rolling surface as the rotation axis is defined as α,
The deviation angle from the ideal Goss orientation with the direction perpendicular to the rolling as the rotation axis is defined as β.
The deviation angle of the crystal orientation measured at the measurement point on the plate surface is represented as (α β),
The angular deviation at the measurement point is defined as φ=(α ² +β ² ) ^1/2 ,
The crystal grains having the angle deviation φ of 10° or less are defined as Goss oriented grains,
A 100 mm x 100 mm area on the plate surface is defined as a divided section.
When it is confirmed whether the Goss oriented grains are included in the divided sections of at least 100 sections,
The area percentage of the divided sections containing at least one Goss oriented grain having a grain size of 5 μm or more is 30% or more of all the divided sections.

For example, a cold-rolled steel sheet 1000 mm long can be divided into the above compartments, and the sum of the areas of the compartments containing at least one Goss-oriented grain with a grain size of 5 μm or more (coarse Goss-oriented grain) can be calculated, and the ratio of this sum to the area of all the compartments can be calculated.

In this embodiment, the above grain size is determined by recognizing a grain boundary as a point where the crystal orientation difference between adjacent measurement points is 1° or more. Also, the above grain size refers to the circle equivalent diameter.

In addition, in this embodiment, the angle deviation φ is defined by the deviation angles α and β. In general, the angle deviation θ is often evaluated by three components, the deviation angles α, β, and γ. However, in this embodiment, since the aim is particularly to improve the magnetic flux density, the angle deviation φ is defined by two components, the deviation angles α and β, excluding the deviation angle γ, which has a small effect on the magnetic flux density.

The above-mentioned coarse Goss-oriented grains are defined as Goss-oriented grains having a grain size of 5 μm or more, but there is no particular upper limit to the grain size. For example, the maximum grain size of the coarse Goss-oriented grains may be, for example, 100 μm.

In the cold-rolled steel sheet according to this embodiment, at least one coarse Goss-oriented grain (a coarse Goss-oriented grain close to the ideal Goss orientation) is contained within a 100 mm x 100 mm divided section of the cold-rolled steel sheet, and when the total area of the divided sections containing such coarse Goss-oriented grains is 30% or more of the area of all divided sections, the Goss-oriented grains grow in a preferred form during the secondary recrystallization stage, and a grain-oriented electrical steel sheet with good magnetic properties can be obtained.

In order to form coarse Goss-oriented grains in cold-rolled steel sheets in this way, it is necessary to accelerate the heating rate of the local heating region to promote the formation of Goss-oriented grains, and to ensure that the recrystallized grains generated in the local heating region have a residence time in a temperature range where they can grow after recrystallization. The above phenomenon (recrystallization and grain growth of Goss-oriented grains) does not occur under the conditions of laser irradiation or electron beam irradiation performed in normal magnetic domain control. In particular, in order to make it easier for coarse Goss-oriented grains to remain even after the primary recrystallization in the local heating region, it is effective to extend the time from when the maximum temperature is reached to when the temperature is cooled to 700°C during local rapid heating. For example, it is effective to hold the temperature from when the maximum temperature is reached to when the temperature is cooled to 700°C for 0.1 seconds or more, preferably 0.2 seconds or more, and more preferably 0.3 seconds or more. For example, during localized rapid heating, the heated spot electrode suppresses heat removal from the steel sheet, promoting the grain growth of the generated Goss orientation grains and promoting the generation of Goss orientation grains that have a size effect suitable for secondary recrystallization and have a crystal orientation close to the ideal Goss orientation. As described above, when the current-carrying electrode has a diameter of 0.5 mmφ to 10 mmφ in equivalent circle diameter, it is easy to ensure a preferable holding time from when the maximum temperature is reached until cooling to 700°C. Under normal laser irradiation or electron beam irradiation conditions, it is difficult to ensure the above-mentioned holding time from when the maximum temperature is reached until cooling to 700°C.

It is preferable that the divided sections containing at least one coarse Goss-oriented grain are uniformly present throughout the entire cold-rolled steel sheet. However, the above condition does not have to be satisfied over the entire surface of the cold-rolled steel sheet. For example, even if the divided sections containing at least one coarse Goss-oriented grain are unevenly distributed in a part of the cold-rolled steel sheet, as long as they account for 30% or more of the area of all the divided sections, a grain-oriented electrical steel sheet with good magnetic properties can ultimately be obtained.

As mentioned above, when localized rapid heating is performed on a steel sheet after cold rolling, coarse Goss-oriented grains 15 are formed in the localized heated region 2 as shown in Figure 2. On the other hand, coarse Goss-oriented grains are not formed in the non-locally heated region 3. Therefore, by performing localized rapid heating so that the localized heated regions 2 are scattered on the surface of the cold-rolled steel sheet, the distribution of coarse Goss-oriented grains 15 on the steel sheet surface can be controlled.

The divided sections containing at least one coarse Goss-oriented grain (a Goss-oriented grain having a grain size of 5 μm or more) preferably account for 50% or more of the area of all divided sections, more preferably 70% or more of the area, and even more preferably 90% or more of the area.

When the heat cycle during annealing in the conventional secondary recrystallization is optimized, Goss-oriented grains having good orientation grow to a grain size of about 100 mm. However, since Goss-oriented grains having good orientation are rarely present among primary recrystallized grains, the conventional technology achieves a magnetic flux density _B8 of only about 1.911 T. On the other hand, in the present embodiment, as described above, by performing local heating on 30% or more of the area of all divided sections, the divided sections including at least one coarse Goss-oriented grain become 30% or more of the area of all divided sections. Therefore, in the present embodiment, Goss-oriented grains having good orientation grow preferentially, and the magnetic flux density is improved compared to the conventional technology. Naturally, the magnetic flux density is higher when the divided sections including at least one coarse Goss-oriented grain are 100% of all divided sections.

Furthermore, if at least one locally heated region is included in each divided section, the magnetic flux density _B8 is improved. However, the fewer the number of locally heated regions in a divided section, the larger the grain size after secondary recrystallization. Although such a steel sheet has improved magnetic flux density, it is difficult to improve iron loss unless magnetic domain control techniques such as laser irradiation or linear groove formation are applied. Therefore, it is preferable to form, for example, an average of three or more locally heated regions, preferably five or more, and more preferably nine or more, in each divided section of 100 mm x 100 mm.

When the cold-rolled steel sheet according to this embodiment is used, during secondary recrystallization, among the Goss-oriented grains in the local heating region, the coarse Goss-oriented grains close to the ideal Goss orientation grow preferentially. Therefore, even if the secondary recrystallized grains are not grown large, the steel sheet can be dominated by secondary recrystallized grains close to the ideal Goss orientation. For example, when the local heating regions are scattered on the steel sheet surface, the size, shape, and arrangement of the crystal grains after secondary annealing can be controlled by appropriately arranging the local heating regions on the steel sheet surface. For example, if the grain growth of a Goss-oriented grain that starts growing from one local heating region collides with another Goss-oriented grain that starts growing from an adjacent local heating region, the grain size of the Goss-oriented grains at the end of secondary annealing will be small, and a steel sheet with excellent iron loss can be obtained. In particular, as described above, in the Goss-oriented grains that grow preferentially from the local heating region in the early stage of secondary annealing, the crystals close to the ideal Goss orientation grow preferentially, so that good magnetic properties can be obtained despite the small secondary recrystallized grain size.

That is, when the cold-rolled steel sheet according to the present embodiment is used, the magnetic flux density can be increased while suppressing the coarsening of the secondary recrystallized grain size, so that the magnetic domain width refinement by strain introduction or groove formation can be unnecessary or reduced. Specifically, since the magnetic domain refinement effect can be preferably obtained by the grain boundaries that increase with the refinement of the secondary recrystallized grain size, the magnetic domains are refined without applying the conventional magnetic domain control technology. Therefore, since it is not necessary to introduce strain into the steel sheet as in the conventional magnetic domain control technology, it is possible to prevent an increase in magnetostriction (λ _P-P ). In addition, since it is not necessary to form grooves in the steel sheet as in the conventional magnetic domain control technology, it is possible to prevent a decrease in the magnetic flux density B _8. In addition, the grain-oriented electrical steel sheet using the cold-rolled steel sheet according to the present embodiment can be used for applications requiring strain relief annealing at 800 ° C. or more.

As described above, in the cold-rolled steel sheet according to this embodiment, the coarse Goss-oriented grains contained in the divided sections need only be Goss-oriented grains with a grain size of 5 μm or more. However, in order to preferentially grow the ideal Goss orientation with a size advantage, it is preferable that the coarse Goss-oriented grains contained in the divided sections are Goss-oriented grains with a grain size of 8 μm or more. Specifically, it is preferable that the divided sections containing at least one Goss-oriented grain with a grain size of 8 μm or more account for 30 area% or more of all the divided sections. It is preferable that this divided section account for 50 area% or more of all the divided sections, more preferably 70 area% or more, and even more preferably 90 area% or more.

The above-mentioned grain size of the Goss orientation can be measured by the EBSD method. If the area to be measured is larger than the area that can be measured by EBSD, the area to be measured can be divided and EBSD measurements can be performed multiple times. For example, the step size for EBSD measurement can be set to 0.5 μm.

Next, we will explain the preferred chemical composition of the cold-rolled steel sheet according to this embodiment.

First, the chemical composition of the grain-oriented electrical steel sheet as the final product produced using the cold-rolled steel sheet according to this embodiment will be described.
The grain-oriented electrical steel sheet as a final product may have a chemical composition, in mass fraction, of 2.0% to 7.0% Si, with the remainder being Fe and impurities.

In addition, this grain-oriented electrical steel sheet may contain publicly known optional elements in place of a portion of Fe in order to improve the magnetic properties. There is no need to set a lower limit for the optional elements, and the lower limit may be 0%. In addition, the upper limit for the optional elements may be set to a range in which no significant decrease in magnetic flux density or iron loss occurs. The approximate upper limit for each optional element is listed below.

The chemical composition of the final product, grain-oriented electrical steel sheet (base steel sheet), is, in mass%, as follows:
C: 0.005% or less,
Si: 2.0 to 7.0%,
Mn: 1.00% or less,
S and Se: 0.015% or less in total;
Al: 0.065% or less,
N: 0.005% or less,
Nb, V, Mo, Ta, and W: 0.050% or less in total;
Cu: 0.40% or less,
Bi: 0.010% or less,
B: 0.080% or less,
P: 0.50% or less,
Ti: 0.015% or less,
Sn: 0.10% or less,
Sb: 0.10% or less,
Cr: 0.30% or less,
Ni: 1.00% or less.

Since these optional elements may be included according to known purposes, there is no need to set a lower limit for the content of the optional elements, and the lower limit may be 0%. The total content of S and Se means that at least one of S and Se is included, and that the total content is the total content of the elements. Similarly, the total content of one or more of Nb, V, Mo, Ta, and W means that at least one of Nb, V, Mo, Ta, and W is included, and that the total content is the total content of the elements.

In addition, in grain-oriented electrical steel sheets, the decarburization annealing and purification annealing during secondary recrystallization cause relatively large changes in the chemical composition (reduction in content). Depending on the element, the content can be reduced to 50 ppm or less, and if purification annealing is carried out sufficiently, it can reach a level that cannot be detected by general analysis (1 ppm or less).

Furthermore, even if the above-mentioned optional elements are contained as impurities, the effect of this embodiment is not impaired. Note that impurities refer to elements that are inevitably mixed in from raw materials such as ore and scrap, or from the manufacturing environment, when industrially manufacturing steel sheets. The upper limit of the total impurity content may be, for example, 5%.

The above chemical composition may be measured using a general analytical method for steel. For example, the chemical composition may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition is determined by measuring a 35 mm square test piece taken from the steel plate using an ICP-AES or similar (measuring device) under conditions based on a previously prepared calibration curve. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

The chemical composition of the cold-rolled steel sheet according to this embodiment contains, for example, the following elements:

The cold-rolled steel sheet according to this embodiment has a chemical composition, in mass%,
C: 0 to 0.0850%,
Si: 2.0 to 7.0%,
Mn: 0.05-1.0%,
S and Se: 0.003 to 0.035% in total,
Al: 0.010-0.0650%,
N: 0 to 0.012%,
Nb, V, Mo, Ta, and W: 0 to 0.050% in total;
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0 to 1.0%
and the remainder being Fe and impurities.

C: 0-0.0850%
Carbon (C) is an effective element for controlling the primary recrystallized structure in the manufacturing process, but if its content in the final product is excessive, it will have a detrimental effect on the magnetic properties. The preferred upper limit of the C content is 0.085% or less. The preferred upper limit of the C content is 0.075%. C is purified in the decarburization annealing process and the final annealing process described below, and is added after the final annealing process to suppress the occurrence of magnetic aging. In order to effectively suppress magnetic aging, the C content is preferably 0.003% or less, and more preferably 0.002% or less. Considering the productivity in production, the lower limit of the C content may be more than 0%. However, since the C content depends on the decarburization annealing time, etc., in consideration of the annealing cost, etc., it is practically preferable to set the lower limit at 0%. It may be 0.0001%. Furthermore, if the manufacturing cost is taken into consideration, it may be 0.0005%.

Si: 2.0-7.0%
Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, γ transformation occurs during final annealing, and the crystallization of the grain-oriented electrical steel sheets is reduced. On the other hand, if the Si content exceeds 7.0%, the cold workability decreases and cracks tend to occur during cold rolling. The upper limit of the Si content is preferably 4.5%, and more preferably 4.0%.

Mn: 0.05-1.0%
Manganese (Mn) combines with S or Se to produce MnS or MnSe, and functions as an inhibitor. When Mn is contained, the Mn content is within the range of 0.05 to 1.0%. In this case, the secondary recrystallization is stabilized. It is possible that part of the inhibitor function is performed by nitrides of Nb group elements (Nb, V, Mo, Ta, and W). In this case, The strength of MnS or MnSe as a general inhibitor is controlled to be weak, and therefore the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

S and Se: 0.003 to 0.035% in total
Sulfur (S) and selenium (Se) combine with Mn to produce MnS or MnSe, which functions as an inhibitor. When at least one of S and Se is contained, if the total content of S and Se is 0.003 to 0.035%, secondary recrystallization is stable. It is possible to use nitrides of Nb group elements to play a part of the inhibitor function. In this case, the strength of MnS or MnSe as a general inhibitor is controlled to be weak. For this reason, in the case of a manufacturing method using low-temperature slab heating, the preferred upper limit of the total content of S and Se is 0.025%, and more preferably 0.010%. If S and Se remain after final annealing, they form compounds and deteriorate iron loss. Therefore, it is preferable to reduce S and Se as much as possible by purification during final annealing.

Here, the total content of S and Se means that at least one of S and Se is included, and is the total content.

Al: 0.010-0.0650%
Aluminum (Al) combines with N to precipitate as (Al,Si)N, and functions as an inhibitor. When Al is contained, the Al content is within the range of 0.010 to 0.0650%. In addition, AlN formed as an inhibitor by nitriding, which will be described later, expands the secondary recrystallization temperature range, and secondary recrystallization is particularly stable in the high temperature range. Therefore, the Al content is 0.010 to 0.0650 The lower limit of the Al content is preferably 0.020%, and more preferably 0.025%. From the viewpoint of the stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%. and more preferably 0.035%.

N: 0-0.012%
Nitrogen (N) combines with Al to function as an inhibitor. Since N can be incorporated by nitriding during the manufacturing process, no lower limit is specified. For example, the lower limit of the N content is set to more than 0%. On the other hand, when N is contained, if the N content exceeds 0.012%, blisters, which are a type of defect, are likely to occur in the steel sheet. The upper limit of the N content is preferably 0.010%, and more preferably 0.009%. N is purified in the final annealing process, and the N content becomes 0.005% or less after the final annealing process.

The balance of the above chemical composition consists of Fe and impurities. Note that "impurities" refers to elements that are mixed in from raw materials such as ore and scrap during industrial production of steel, or from the production environment, etc. The upper limit of the total impurity content may be, for example, 5%.

Total of Nb, V, Mo, Ta, and W: 0 to 0.050%
The total content of Nb (niobium), V (vanadium), Mo (molybdenum), Ta (tantalum), and W (tungsten) may be 0.050% or less. When Nb group elements (one or more of Nb, V, Mo, Ta, and W) are utilized as a part of the inhibitor, the effect of improving _B8 is exhibited. On the other hand, if the Nb group elements remain excessively in the base steel sheet, it may have a negative effect on the magnetic properties. Therefore, the total content of Nb, V, Mo, Ta, and W may be 0.050% or less. Furthermore, if the total content of Nb group elements is 0.030% or less (preferably 0.003% or more and 0.030% or less), it is preferable because secondary recrystallization is started at an appropriate timing. In addition, the orientation of the secondary recrystallized grains that occur becomes very favorable, and it can be finally controlled to a structure favorable for the magnetic properties. In particular, Nb and Ta are preferable because their effects are strong. There is no need to set a lower limit for the total content of Nb group elements, and the lower limit may be 0%. The lower limit is preferably 0.003%.

The total content of Nb group elements is more preferably 0.004 to 0.020%. Even more preferably, it is 0.005 to 0.010%.

Here, the total content of Nb group elements means the total content including at least one of Nb, V, Mo, Ta, and W.

In addition, the cold-rolled steel sheet according to this embodiment contains, as optional elements, in mass%:
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0 to 1.0%
may contain at least one of the following.
Since these optional elements may be contained according to known purposes, there is no need to set a lower limit for the content of the optional elements, and the lower limit may be 0%.

The chemical composition of the cold-rolled steel sheet according to this embodiment can be measured by the above-mentioned analytical method, in the same way as the chemical composition of the grain-oriented electrical steel sheet as the final product.

Next, we will explain one embodiment of a preferred method for manufacturing the cold-rolled steel sheet for the grain-oriented electrical steel sheet according to this embodiment.

Note that the method for manufacturing the cold-rolled steel sheet according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing the cold-rolled steel sheet according to this embodiment.

Furthermore, the steps and quantitative conditions for each step shown below are examples adopted to demonstrate the feasibility of this embodiment, and this embodiment is not limited to these steps and quantitative values. The manufacturing method for cold-rolled steel sheet according to this embodiment may adopt various conditions as long as they do not deviate from the gist of this embodiment and achieve the purpose of this embodiment.

The manufacturing method of the cold-rolled steel sheet according to this embodiment can apply the conventional known manufacturing method of grain-oriented electrical steel sheet as a basic process. For example, examples of conventional manufacturing methods of grain-oriented electrical steel sheet include a manufacturing method in which inhibitors such as MnS and AlN are formed by high-temperature slab heating, and a manufacturing method in which AlN inhibitors are formed by low-temperature slab heating and nitriding treatment. The manufacturing method of the cold-rolled steel sheet according to this embodiment is not limited to a specific manufacturing method. Below, a method of applying nitriding treatment as a low-temperature slab heating process will be described.

(Casting process)
In the casting process, a slab is prepared. An example of a method for producing a slab is as follows. Molten steel is produced (smelted). A slab is produced using the molten steel. The slab may be produced by a continuous casting method. An ingot may be produced using the molten steel, and the ingot may be bloomed to produce a slab. The thickness of the slab is not particularly limited. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. As the slab, a so-called thin slab having a thickness of 10 to 70 mm may be used. When a thin slab is used, rough rolling before finish rolling can be omitted in the hot rolling process.

For example, the above slab may contain the following elements in its chemical composition:

C: 0.085% or less,
Carbon (C) is an effective element for controlling the primary recrystallized structure in the manufacturing process, but if its content in the final product is excessive, it will have a detrimental effect on the magnetic properties. The preferred upper limit of the C content is 0.075%. C is purified in the decarburization annealing process and the finish annealing process to be 0.005% or less. When C is contained, Considering the productivity, the lower limit of the C content may be more than 0%, or may be 0.001%.

Si: 2.0-7.0%
Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, γ transformation occurs during final annealing, and the crystallization of the grain-oriented electrical steel sheets is reduced. On the other hand, if the Si content exceeds 7.0%, the cold workability decreases and cracks tend to occur during cold rolling. The upper limit of the Si content is preferably 4.5%, and more preferably 4.0%.

Mn: 0.05-1.00%
Manganese (Mn) combines with S or Se to produce MnS or MnSe, and functions as an inhibitor. When Mn is contained, the Mn content is within the range of 0.05 to 1.00%. In this case, the secondary recrystallization is stable. It is possible for part of the inhibitor function to be performed by a nitride of an Nb group element. In this case, the strength of MnS or MnSe as a general inhibitor is Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

At least one of S and Se: 0.003 to 0.035% in total
Sulfur (S) and selenium (Se) combine with Mn to produce MnS or MnSe, which functions as an inhibitor. When at least one of S and Se is contained, if the total content of S and Se is 0.003 to 0.035%, secondary recrystallization is stable. It is possible for a part of the inhibitor function to be borne by a nitride of an Nb group element. In this case, the strength of MnS or MnSe as a general inhibitor is controlled to be weak. For this reason, the preferred upper limit of the total content of S and Se is 0.025%, and more preferably 0.010%. If S and Se remain after the final annealing, they form compounds and deteriorate the iron loss. Therefore, it is preferable to reduce S and Se as much as possible by purification during the final annealing.

Here, the total content of S and Se means that at least one of S and Se is included, and is the total content.

Al: 0.010-0.065%
Aluminum (Al) combines with N to precipitate as (Al,Si)N, and functions as an inhibitor. When Al is contained, the Al content is within the range of 0.010 to 0.065%. In addition, AlN formed as an inhibitor by nitriding, which will be described later, expands the secondary recrystallization temperature range, and secondary recrystallization is particularly stable in the high temperature range. Therefore, the Al content is 0.010 to 0.065 The lower limit of the Al content is preferably 0.020%, and more preferably 0.025%. From the viewpoint of the stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%. and more preferably 0.035%.

N: 0.012% or less Nitrogen (N) combines with Al to function as an inhibitor. Since N can be contained by nitriding during the manufacturing process, no lower limit is specified. For example, the lower limit of the N content may be more than 0%, or may be 0.001%. On the other hand, when N is contained, if the N content exceeds 0.012%, blisters, which are a type of defect, tend to occur in the steel sheet. The preferred upper limit of the N content is 0.010%, and more preferably 0.009%. N is purified in the final annealing process, and becomes 0.005% or less after the final annealing process.

The balance of the above chemical composition consists of Fe and impurities. Note that "impurities" refers to elements that are mixed in from raw materials such as ore and scrap during industrial production of steel, or from the production environment, etc. The upper limit of the total impurity content may be, for example, 5%.

The above-mentioned chemical composition may contain known optional elements in place of part of Fe, taking into consideration not only solving manufacturing problems, but also strengthening the inhibitor function through compound formation and the effect on magnetic properties. Examples of optional elements that may be contained in place of part of Fe include the following elements:

Nb group elements: 0.050% or less The total content of Nb group elements (one or more of Nb, V, Mo, Ta, and W) may be 0.050% or less. When using Nb group elements as a part of the inhibitor, it is preferable that the total content of Nb group elements is 0.030% or less, since secondary recrystallization is started at an appropriate timing. In addition, the orientation of the secondary recrystallized grains generated becomes very favorable, and the structure can be finally controlled to be favorable for magnetic properties. In particular, Nb and Ta have a strong effect and are preferable. There is no need to set a lower limit for the total content of Nb group elements, and the lower limit may be 0%. This lower limit is preferably 0.003%.

The total content of Nb group elements is more preferably 0.004 to 0.020%. Even more preferably, it is 0.005 to 0.010%.

Here, the total content of Nb group elements means the total content including at least one of Nb, V, Mo, Ta, and W.

The slab also contains, as optional elements, in mass %:
Cu: 0.40% or less,
Bi: 0.010% or less,
B: 0.080% or less,
P: 0.50% or less,
Ti: 0.015% or less,
Sn: 0.10% or less,
Sb: 0.10% or less,
Cr: 0.30% or less,
Ni: 1.00% or less.
Since these optional elements may be contained according to known purposes, there is no need to set a lower limit for the content of the optional elements, and the lower limit may be 0%.

The chemical composition of the above slabs can be measured by the above analytical method, just like the chemical composition of the final product, grain-oriented electrical steel sheet.

(Hot rolling process)
The hot rolling process is a process in which a slab heated to a predetermined temperature (e.g., 1100 to 1400°C) is hot-rolled to obtain a hot-rolled steel sheet. In the hot rolling process, for example, the silicon steel material (slab) heated in the heating process is roughly rolled, and then finish-rolled to obtain a hot-rolled steel sheet with a predetermined thickness, for example, 1.8 to 3.5 mm. After the finish rolling is completed, the hot-rolled steel sheet is coiled at a predetermined temperature.

In the case of a process that includes a nitriding step during or after decarburization annealing, the strength of MnS as an inhibitor is not so necessary, so if productivity is taken into consideration, the slab heating temperature should be 1100°C to 1280°C.

(Hot-rolled sheet annealing process)
The hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet obtained in the hot rolling process is annealed under a predetermined temperature condition (for example, at 750 to 1200 ° C for 30 seconds to 10 minutes) to obtain an annealed steel sheet. Hot-rolled sheet annealing is generally performed to control the steel sheet structure such as the recrystallization rate, residual strain, and crystal grain size by annealing the hot-rolled steel sheet after the hot rolling process, and to preferably adjust the precipitate form in the steel. For example, in a high-temperature slab heating process, it is a process for finally controlling the form of precipitates such as AlN, and conditions are adjusted so that they precipitate uniformly and finely. As the hot-rolled sheet annealing, for example, after heating the steel sheet to 1050 ° C to 1150 ° C, in order to properly precipitate AlN, etc., it may be cooled slowly to an intermediate temperature (850 ° C to 950 ° C) for 50 to 150 seconds, and then cooled with water.

(Cold rolling process)
The cold rolling process is a process in which the annealed steel sheet obtained in the hot-rolled sheet annealing process is subjected to one cold rolling or multiple cold rolling (two or more times) via annealing (intermediate annealing) to obtain a cold-rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm. In order to improve the magnetic properties, for example, the interpass temperature of the cold rolling may be set to about 100° C. to 300° C.

The total cold rolling reduction is defined as follows.
Total cold rolling rate (%) = (1 - thickness of steel plate after cold rolling / thickness of steel plate before cold rolling) x 100

(Local rapid heating process)
In this embodiment, local rapid heating is performed on the cold-rolled steel sheet after the cold rolling process to form a local heating region. The rapid heating method is not particularly limited as long as it can locally heat the steel sheet. For example, a method of heating by contacting a spot electrode with the front and back surfaces of the steel sheet and passing an electric current through the steel sheet, a method of heating by irradiating the surface of the steel sheet with laser light or an electron beam, a method of locally heating by induction heating, a method of heating by contacting a hot piece, etc. can be adopted. The size of each local heating region can be a dot-like region with a diameter of about 10 μm to 10 mm, or a linear region with a width of about 10 μm to 10 mm. In addition, the electrode used for electric heating may be a circular electrode as described above, or a linear electrode.

Regarding the size of the locally heated region, the minimum diameter and minimum width depend on the shape of the spot electrode and the technology for reducing the focused diameter of the laser light or electron beam. In principle, it is possible to further reduce the size of the locally heated region, but considering that the current size of the nuclei of recrystallized grains is about 1 μm, it is believed that an effect can be obtained if the size of the locally heated region is 1 μm or more. However, since the objective of this embodiment is to form Goss-oriented grains with a grain size of 5 μm or more, the above minimum diameter and minimum width should be 10 μm or more. Regarding the size of the locally heated region, if the maximum diameter or maximum width exceeds 10 mm, the area ratio of Goss-oriented grains with relatively large angle deviations θ and φ that cannot grow outside the locally heated region increases. In this case, it becomes difficult to obtain the effect of local rapid heating.

In localized rapid heating, the heating rate should be 500°C/sec or more in the center of the locally heated region as viewed in the thickness direction and at 1/5 of the thickness of the steel plate. If the heating rate in the above region is 500°C/sec or more, the structure of the locally heated region can be preferably controlled to a recovered structure or a recrystallized structure. The above heating rate is preferably 2000°C/sec or more, and more preferably 10000°C/sec or more. On the other hand, the upper limit of the above heating rate is not particularly limited, and may be, for example, 1,000,000°C/sec.

In addition, it is preferable to control the above heating rate in conjunction with the heating rate during decarburization annealing, which will be described later. For example, if the heating rate during decarburization annealing is faster than the heating rate during localized rapid heating, it is difficult to obtain the desired effect. Therefore, although it depends on the magnitude of the heating rate, it is preferable to set the heating rate during localized rapid heating to be equal to or greater than the heating rate during decarburization annealing.

Note that the 1/5 part of the thickness of the steel plate mentioned above refers to a depth equivalent to 1/5 of the thickness of the steel plate from the surface of the steel plate in the thickness direction.

In addition, in localized rapid heating, the maximum temperature reached should be 700°C or higher in the center of the locally heated region when viewed from the plate thickness direction and at 1/5 of the plate thickness of the steel plate. If the maximum temperature reached in the above region is 700°C or higher, the structure of the locally heated region can be preferably controlled to a recovered structure or a recrystallized structure. The above maximum temperature reached is preferably 800°C or higher, and more preferably 900°C or higher. On the other hand, the above maximum temperature reached should be below the melting point of the steel plate, for example, 1400°C or lower.

In addition, in the case of localized rapid heating, the holding time from reaching the maximum temperature to cooling to 700°C at the center of the locally heated region as viewed from the thickness direction and at 1/5 of the thickness of the steel plate should be 0.1 seconds or more. If the holding time in the above region is 0.1 seconds or more, the structure of the locally heated region can be preferably controlled to a recovered structure or a recrystallized structure. The holding time is preferably 0.2 seconds or more, more preferably 0.3 seconds or more, and even more preferably 0.4 seconds or more. On the other hand, there is no particular limit to the upper limit of the holding time, and it may be, for example, 10 seconds or less. The holding time from reaching the maximum temperature to cooling to 700°C has a fundamental effect on the formation of coarse Goss orientation grains close to the ideal Goss orientation.

In localized rapid heating, in addition to the heating rate and maximum temperature reached, it is important to control the holding time. During this holding time, subgrain structures and recrystallization nuclei that are advantageous for the formation of coarse Goss-oriented grains in the locally heated area can be formed, and as a result, Goss-oriented grains with a grain size of 5 μm or more can be formed in the locally heated area after localized rapid heating. By controlling the holding time in the locally heated area as described above, favorable structural control can be achieved in subsequent processes.

As mentioned above, the method of local rapid heating is not particularly limited. For example, local rapid heating may be performed by spot electrical current heating, laser irradiation, or electron beam irradiation. Regardless of the local rapid heating method, it is sufficient to control the heating rate, maximum temperature, and holding time described above so as to satisfy the above. A person skilled in the art can combine the various conditions of the local rapid heating method to control the desired heating rate, maximum temperature, and holding time.

For example, a person skilled in the art can perform a heat conduction analysis using the finite element method to control the heating rate and maximum temperature reached in the center of the localized heating area when viewed from the plate thickness direction and at 1/5 of the plate thickness of the steel plate. In addition, a person skilled in the art can control the above-mentioned holding time by adjusting the heat removal conditions after the maximum temperature is reached. For example, in the case of spot current heating, the electrode holding time after current is controlled, or the shape of the spot electrode can be changed to a shape suitable for heat removal. In addition, in the case of laser irradiation or electron beam irradiation, the irradiation speed can be adjusted, or the shape of the irradiation area can be changed to an ellipse or the like to gradient the irradiation energy from the center to the outer edge of the irradiation area.

In general, the heating by laser irradiation preferentially heats only the vicinity of the irradiated steel sheet surface, and the inside of the steel sheet is not easily heated. In addition, since the heating by laser irradiation is non-contact heating, the heat is removed quickly after heating, and the temperature is not easily maintained. Therefore, when performing laser irradiation heating as local rapid heating, in order to recover or recrystallize the cold-worked structure in the laser irradiated region, it is necessary to control the conditions so that not only the surface of the steel sheet but also the inside of the steel sheet is heated, and the cooling rate is slowed down to relatively coarsen the subgrain structure in the laser irradiated region. The laser irradiation conditions for performing local rapid heating are completely different from the laser irradiation conditions for performing magnetic domain refinement. For example, even if the laser irradiation conditions for performing magnetic domain refinement (usually 0.5 to 50 mJ/mm ² ) are applied to this embodiment, the structure of the locally heated region is not favorably controlled, and coarse Goss orientation grains close to the ideal Goss orientation are not formed in the cold-rolled steel sheet. The same applies to electron beam irradiation. Even if the electron beam irradiation conditions for magnetic domain refinement are applied to this embodiment, the structure of the locally heated region is not favorably controlled, and coarse Goss-oriented grains close to the ideal Goss orientation are not formed in the cold-rolled steel sheet.

It should be noted that simply increasing the input power of laser or electron beam irradiation makes it difficult to form coarse Goss-oriented grains close to the ideal Goss orientation. For example, when performing rapid heating by laser or electron beam irradiation, it is preferable to make the beam shape elliptical and adjust the scan speed, etc., so that a temperature range can be secured for the crystal grains in that region to grow after rapid heating.

Conventionally, laser irradiation or electron beam irradiation has been performed on electrical steel sheets to reduce iron loss by controlling the magnetic domains of the steel sheets after secondary recrystallization. Under these laser irradiation or electron beam irradiation conditions, the steel sheet is heated to a depth of about 20 μm from the surface, so heat is removed quickly after heating. Therefore, under conventional conditions, it is difficult to ensure the holding time from reaching the maximum temperature as described above until cooling to 700°C. In other words, under conventional laser irradiation or electron beam irradiation conditions, it may be possible to make the irradiated area act as a barrier to the growth of secondary recrystallized grains, but it is difficult to form coarse Goss-oriented grains with an orientation close to the ideal Goss orientation, as in this embodiment. In order to form coarse Goss-oriented grains close to the ideal Goss orientation by laser irradiation or electron beam irradiation, it is necessary to consider the conditions of laser irradiation or electron beam irradiation as described above.

The above steps allow the manufacture of the cold-rolled steel sheet for grain-oriented electrical steel sheet according to this embodiment. The cold-rolled steel sheet according to this embodiment, which is manufactured by controlling the conditions of each step in a complex and inseparable manner, has coarse Goss-oriented grains formed in a preferred form within the steel sheet.

Below, we will explain the method for manufacturing grain-oriented electrical steel sheet using the cold-rolled steel sheet according to this embodiment.

(Decarburization annealing process)
In the decarburization annealing process, the cold-rolled steel sheet after the local rapid heating process is subjected to decarburization annealing (primary annealing). This decarburization annealing increases the frequency of Goss-oriented grains formed in the local heating region, increases the grain size of the Goss-oriented grains, and favorably grows coarse Goss-oriented grains having an orientation close to the ideal Goss orientation. The decarburization annealing conditions may be, for example, an annealing temperature of 700 to 900°C and an annealing time of 1 to 3 minutes. By subjecting the cold-rolled steel sheet to decarburization annealing, C contained in the cold-rolled steel sheet is removed. Decarburization annealing is preferably performed in a moist atmosphere in order to remove "C" contained in the cold-rolled steel sheet.

In addition, it is effective to reduce the primary recrystallized grain size by controlling the conditions of the hot rolling and hot-rolled sheet annealing described above, or by lowering the decarburization annealing temperature as necessary. The primary recrystallized grain size is not particularly limited, but is preferably 8 to 30 μm.

In addition, in this embodiment, coarse Goss orientation grains with an orientation close to the ideal Goss orientation are formed in the locally heated region. Therefore, there is no need to form Goss orientation grains again by decarburization annealing. Therefore, there is no need to increase the heating rate of the decarburization annealing. Rather, it is preferable to slow down the heating rate of the decarburization annealing to form {111}<112> orientation grains and {411}<148> orientation grains that are easily eroded into the matrix portion (non-locally heated region).

Furthermore, the amount of decarbonation and the state of the surface oxide layer affect the formation of the glass film, so in decarburization annealing, the degree of oxidation (PH ₂ O/PH ₂ ) in the annealing atmosphere (furnace atmosphere) may be appropriately adjusted.

(Nitriding)
Nitriding is an effective treatment in a low-temperature slab heating process in which the slab heating temperature is 1280°C or less, and is an important step for adjusting the strength of the inhibitor in secondary recrystallization. Nitriding increases the nitrogen content of the steel sheet by about 40 to 200 ppm between the start of the decarburization treatment and the start of secondary recrystallization in the finish annealing. Examples of nitriding include a treatment of annealing in an atmosphere containing a gas with nitriding ability such as ammonia, and a treatment of finish annealing a decarburized annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN. The amount of nitriding after the nitriding treatment is preferably 130 to 350 ppm, and more preferably 150 to 250 ppm.

(Annealing separator application process)
The annealing separator application process is a process of applying an annealing separator to the decarburized annealed steel sheet. As the annealing separator, for example, an annealing separator mainly composed of MgO can be used. The decarburized annealed steel sheet after application of the annealing separator is wound into a coil and finish-annealed in the next finish-annealing process.

(Finish annealing process)
The final annealing process is a process in which the decarburized annealed steel sheet coated with the annealing separator is subjected to final annealing (secondary annealing) to cause secondary recrystallization. In this process, the growth of the primary recrystallized grains is suppressed by an inhibitor, and the secondary recrystallization is allowed to proceed, thereby preferentially growing {100}<001> oriented grains and dramatically improving the magnetic flux density.

The heating rate during the heating process of the final annealing step is not particularly limited. For example, the temperature may be increased at a heating rate of 3 to 20°C/hour. In addition, slow heating may be performed in the temperature range of 1000°C to 1200°C during the temperature increase. For example, the heating rate in the temperature range of 1000°C to 1200°C during the temperature increase is preferably 3 to 12°C/hour, more preferably 3 to 9°C/hour, and even more preferably 3 to 7°C/hour. Alternatively, the temperature increase may be stopped once during the heating process of the final annealing step and the temperature may be held. For example, the temperature increase may be stopped once in the temperature range of 1050°C to 1100°C during the temperature increase, and the temperature may be held for 5 hours to 15 hours. For example, adjusting the heating rate in the secondary recrystallization temperature range affects the decomposition rate of the inhibitor and can improve the magnetic flux density.

After the heating process in the final annealing step, the holding process (purification annealing) may involve holding the material in a temperature range of 1000°C to 1300°C for 10 hours to 60 hours. The atmosphere during the final annealing may be, for example, a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen.

(Insulating film formation process)
A coating solution containing phosphoric acid or a phosphate, chromic anhydride or a chromate, and colloidal silica is applied to the steel sheet and baked (for example, at 350°C to 1150°C for 5 to 300 seconds) to form an insulating coating. do.

(others)
The grain-oriented electrical steel sheet may be subjected to a magnetic domain refinement treatment for forming localized micro-distorted regions or grooves by a known method such as laser, plasma, mechanical method, etching, etc. However, the grain-oriented electrical steel sheet manufactured using the cold-rolled steel sheet according to the present embodiment has excellent magnetic flux density, but the coarsening of secondary recrystallized grains is suppressed, so that the magnetic domains are refined even without the magnetic domain refinement treatment.

Next, the effects of the present invention will be specifically explained in detail using examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

Cold-rolled steel sheets with the chemical compositions shown in Table 2 were manufactured using slabs with adjusted chemical compositions. These chemical compositions were measured based on the method described above. In Table 2, "-" indicates that no control or manufacturing was performed with the content in mind, and that no content measurements were performed.

When manufacturing the above cold-rolled steel sheets, the slabs were heated to 1150°C and subjected to hot rolling to obtain hot-rolled steel sheets with a thickness of 2.6 mm. The hot-rolled steel sheets were then heated to 1100°C and subsequently annealed at 900°C for hot-rolled sheet annealing, after which pickling was performed to remove the scale formed on the surface. These steel sheets were subjected to one cold rolling or multiple cold rolling with intermediate annealing in between to obtain cold-rolled steel sheets with a final thickness of 0.22 mm.

The above cold-rolled steel sheets were subjected to localized rapid heating under the conditions shown in Tables 3 to 7. In the spot current heating, a copper electrode with a diameter of 3 mm was used in the contact area with the steel sheet, and the electrode shape, electrode pressure, current, current duration, and electrode holding time after current were varied in a composite manner to control the heating rate, maximum temperature, and holding time from reaching the maximum temperature until cooling to 700°C (holding time above 700°C). In the laser heating, a fiber laser was used, and the diameter of the focused spot of the laser light in the rolling direction (i.e., the diameter including 86% of the laser output) was set to 30 μm (except for Tests No. 85 and No. 86). The heating rate, maximum temperature, and holding time above 700°C were controlled by varying the laser irradiation energy density, laser scanning speed, and shape of the laser irradiation area in a composite manner. Those skilled in the art can combine the conditions of the localized rapid heating method to control the desired heating rate, maximum temperature, and holding time above 700°C.

In addition, in test No. 83, the laser beam was elongated in the scanning direction, and the holding time from reaching the maximum temperature to cooling to 700 ° C. during local rapid heating was controlled to 0.2 seconds, so that coarse Goss orientation grains having an orientation close to the ideal Goss orientation were formed in the locally heated area. In test No. 84, general laser irradiation conditions for magnetic domain refinement were applied. In test No. 85, the focusing spot diameter was set to 0.5 mm, the laser irradiation energy density was set to 2.0 J/mm ² , and the laser beam irradiation portion was controlled so that it would eventually become the grain boundary of the secondary recrystallized grain. In test No. 86, the focusing spot diameter was set to 0.5 mm, the laser irradiation energy density was set to 30.0 J/mm ² , and the laser beam irradiation portion was controlled so that it would finally become the grain boundary of the secondary recrystallized grain.

For the manufactured cold-rolled steel sheet, the divided sections containing at least one Goss-oriented grain with a grain size of 5 μm or more were measured based on the above-mentioned method. The results are shown in Tables 8 to 12. EBSD measurements were performed on a surface parallel to the steel sheet surface and 20 μm in the sheet thickness direction. From the EBSD IQ value, it was confirmed that the crystal structure of the locally heated region was a recrystallized structure and a recovered structure, and that the crystal structure of the non-locally heated region other than the locally heated region was a processed structure.

In the table, "No treatment" for the heating method of localized rapid heating indicates that localized rapid heating was not performed. The localized heating conditions "Linear or dotted" indicate the shape of the localized heated area on the surface of the cold-rolled steel sheet, and the localized heating conditions "Rolling direction spacing" and "Width direction spacing" indicate the spacing in the rolling direction and the direction perpendicular to the rolling direction at which the localized heated areas are arranged on the surface of the cold-rolled steel sheet. The localized heated areas are arranged at equal intervals in the rolling direction and the direction perpendicular to the rolling direction. For example, if both the "Rolling direction spacing" and the "Width direction spacing" are 100 mm or less, localized heated areas are arranged in all divided sections. On the other hand, if either the "Rolling direction spacing" or the "Width direction spacing" exceeds 100 mm, there are divided sections in which localized heated areas are not arranged. Note that, among all the divided sections, the divided sections in which localized heated areas are provided are arranged so as not to be unevenly distributed.

In addition, in the table, the "locally heated section ratio" of the local heating conditions represents the area ratio of the divided sections in which the locally heated areas are located on the plate surface of the cold-rolled steel plate. Specifically, a 100 mm x 100 mm area on the plate surface is defined as a divided section, and when it is confirmed in at least 100 divided sections whether a locally heated area is located within this divided section, the sum of the areas of the divided sections in which the locally heated areas are located divided by the area of all divided sections corresponds to the "locally heated section ratio," and the value is shown as a percentage in the table.

Similarly, in the table, the "area ratio of the divided sections containing Goss oriented grains of 5 μm or more" of the manufacturing results represents the area ratio of the divided sections containing coarse Goss oriented grains on the sheet surface of the cold-rolled steel sheet. Specifically, a 100 mm × 100 mm area on the sheet surface is defined as a ^divided section, and at least 100 divided sections are checked to see whether they contain Goss oriented grains (coarse Goss oriented grains) with a grain equivalent diameter of 5 μm or more, determined by recognizing the boundary where the ^angle deviation φ = ( ^α 2 + β 2 ) 1/2 is 10° or less and the crystal orientation difference is 1° or more as a grain boundary. The sum of the areas of the divided sections containing coarse Goss oriented grains divided by the area of all the divided sections corresponds to the "area ratio of the divided sections containing Goss oriented grains of 5 μm or more," and the value is shown as a percentage in the table. In addition, almost no Goss oriented grains of 5 μm or more were observed in the matrix portion other than the locally heated region (non-locally heated region).

The produced cold-rolled steel sheets were subjected to decarburization annealing under the conditions shown in Tables 8 to 12. In the decarburization annealing, the degree of oxidation (PH ₂ O/PH ₂ ) in the annealing atmosphere (furnace atmosphere) was set to 0.13.

The decarburized annealed steel sheet produced was subjected to nitriding treatment at 750°C in a nitrogen-hydrogen-ammonia atmosphere to set the steel sheet nitrogen content to 220 ppm. Furthermore, an annealing separator mainly composed of MgO was applied, and finish annealing was performed. In the finish annealing, the steel sheet was heated to 1000°C (the "Step" below is 1070°C) at a heating rate of 15°C/hour in a mixed atmosphere of hydrogen and nitrogen, and then heated to 1200°C under any of the following conditions, and held at 1200°C for 20 hours in a hydrogen atmosphere.
Normal: 1000°C to 1200°C at 15°C/hour Slow heating 1: 1000°C to 1200°C at 10°C/hour Slow heating: 1000°C to 1200°C at 7.5°C/hour Slow heating 2: 1000°C to 1200°C at 5.0°C/hour Step: Hold at 1070°C for 10 hours

After final annealing, the steel sheet is coated with an insulating coating solution made mainly of colloidal silica and phosphate, with chromic anhydride added as needed, and baked to form an insulating coating.

The properties of the resulting grain-oriented electrical steel sheets were evaluated. The evaluation results are shown in Tables 8 to 12.

The magnetic properties of the grain-oriented electrical steel sheets were measured based on the single sheet magnetic properties test method (SST: Single Sheet Tester) specified in JIS C 2556:2015.

From the obtained grain-oriented electrical steel sheet, 20 samples for single sheet magnetic measurement, each having a size of 100 mm x 500 mm, were prepared, and single sheet magnetic measurement was performed. As a magnetic property, the magnetic flux density B ₈ (T) in the rolling direction of the steel sheet when excited at 800 A/m was measured. The magnetic flux density B ₈ was judged to pass or fail based on the Si content of the steel sheet and the finish annealing conditions. Specifically, the steel sheets were divided into Steels A to Z, each having a Si content of 3.3 to 3.5%, Steel AA, each having a Si content of 2.5%, and Steel AB, each having a Si content of 4.1%, and compared under the same finish annealing conditions. When the magnetic flux density B ₈ was 0.010 _T or more, it was judged to pass. For example, in Test No. 1, in which the steel type was "Steel A", the finish annealing conditions were "normal", and the local rapid heating was "untreated", the steel sheet was "normal" and the local rapid heating was "untreated", the steel sheet was "normal". Since the magnetic flux density _B8 of No. 1 is 1.911 T, it was determined that the effect is obtained when the magnetic flux density _B8 of the steel sheet having the steel type "Steel A" and the finish annealing conditions "normal" is 1.921 T or more by localized rapid heating.

As a reference for the magnetic properties, iron loss W _17/50 (W/kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was measured under conditions of an AC frequency of 50 Hz and an excitation magnetic flux density of 1.7 T.

Among Nos. 1 to 89, the cold-rolled steel sheets according to the present invention had the "area ratio of divided sections containing Goss-oriented grains of 5 μm or more" preferably controlled. All of these examples according to the present invention, as grain-oriented electrical steel sheets, showed excellent magnetic flux density despite the suppression of coarsening of the secondary recrystallized grain size. Furthermore, because the grain-oriented electrical steel sheets had suppressed coarsening of the secondary recrystallized grain size, they also had excellent iron loss.

In addition, although not shown in the table, among Nos. 1 to 89, in the cold-rolled steel sheets that are examples of the present invention, the area ratio of the divided sections that contained at least one Goss-oriented grain with a grain size of 8 μm or more was the same as the area ratio of the divided sections that contained at least one Goss-oriented grain with a grain size of 5 μm or more.

On the other hand, among Nos. 1 to 89, the comparative cold-rolled steel sheets did not have the "area ratio of divided sections containing Goss-oriented grains of 5 μm or more" controlled appropriately. These comparative examples did not provide a desirable magnetic flux density as grain-oriented electrical steel sheets.

According to the above aspect of the present invention, it is possible to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density. Specifically, it is possible to provide a cold-rolled steel sheet for grain-oriented electrical steel sheet that can increase the magnetic flux density while suppressing the coarsening of secondary recrystallized grains. Therefore, it has a high industrial applicability.

1 Steel plate 2 Locally heated area 3 Non-locally heated area (matrix part)
4 Locally heated region boundary 14 Goss oriented grain 15 Coarse Goss oriented grain (Goss oriented grain with a grain size of 5 μm or more)
21 rolling direction 22 width direction (direction perpendicular to rolling)

Claims (1)

A cold-rolled steel sheet for grain-oriented electrical steel sheet,
The deviation angle from the ideal Goss orientation with the normal direction of the rolling surface as the rotation axis is defined as α,
The deviation angle from the ideal Goss orientation with the direction perpendicular to the rolling as the rotation axis is defined as β.
The deviation angle of the crystal orientation measured at the measurement point on the plate surface is represented as (α β),
The angular deviation at the measurement point is defined as φ=(α ² +β ² ) ^1/2 ;
A crystal grain having an angle deviation φ of 10° or less is defined as a Goss oriented grain,
A 100 mm x 100 mm area on the plate surface is defined as a divided section.
When it is confirmed whether the Goss oriented grains are included in the divided sections in at least 100 divided sections,
A cold-rolled steel sheet for use in grain-oriented electrical steel sheet, characterized in that a divided section containing at least one Goss-oriented grain having a grain size of 5 μm or more accounts for 30 area % or more of all the divided sections.

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