Decarburized annealed steel sheet for grain-oriented electromagnetic steel sheet

Abstract

In this decarburized annealed steel sheet for a grain-oriented electromagnetic steel sheet, the area ratio of divided sub-section group I, for which the value of FC ÷ aveFC is 1.3 or more, is 0.05-50% relative to the area of a divided section that is at least 100 mm × 100 mm on the sheet surface, and the value obtained by dividing the area ratio of Goss-oriented grains contained within the divided sub-section group I relative to the area of the divided sub-section group I by the area ratio of Goss-oriented grains contained within the divided section relative to the area of the divided section is 1.3 or more.

Description

Decarburized annealed steel sheets for grain-oriented electrical steel sheets

[0001] The present invention relates to a decarburization annealed steel sheet for grain-oriented electrical steel sheet. This application claims priority to Japanese Patent Application No. 2023-106859, filed on June 29, 2023, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets contain silicon, and the crystal orientation of the silicon 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 during the steel sheet manufacturing process. Such grain-oriented electrical steel sheets are excellent materials for transformer cores, etc. Among the magnetic properties of grain-oriented electrical steel sheets, the magnetic flux density and core loss are particularly important.

[0003] The magnetic flux density of a grain-oriented electrical steel sheet when a predetermined magnetizing force is applied tends to increase 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. ８ is used. Magnetic flux density B ８ is the value of the magnetic flux density of the magnetized grain-oriented electrical steel sheet when a magnetizing force of 800 A / m is applied in the rolling direction. That is, the magnetic flux density B ８ The larger the value of σ, the easier it is to magnetize with a certain magnetizing force and the greater the magnetic flux density, making it suitable for small, highly efficient transformers.

[0004] In addition, iron loss W is generally used as an index to represent iron loss. １７／５０ is used. Iron loss W １７／５０ is the iron loss when the grain-oriented electrical steel sheet is excited by AC under the conditions of a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz. １７／５０ The smaller the value of grain-oriented electrical steel sheet, the lower the energy loss and the more suitable it is for transformers.

[0005] 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 grain orientation, which is effective in reducing hysteresis loss. ８The larger the value of B, the more the crystal orientation is aligned with the Goss orientation, and the lower the hysteresis loss. ８ The larger the value of W, the greater the iron loss １７／５０ On the other hand, when the development of Goss oriented grains is promoted to improve the degree of integration of the crystal orientation, the grain size tends to become larger, which increases the eddy current loss and decreases the B ８ Even if the value of is high, if the grains become too large, there is a trade-off in that the core loss deteriorates.

[0006] In recent years, methods have been developed to artificially subdivide the magnetic domain width by introducing strain through irradiation with laser light or plasma jet, or by forming grooves mechanically or by etching, to control the magnetic domains. By using such methods, in steel sheets in which the degree of integration of crystal orientation is increased and the magnetic flux density is increased, eddy current loss can be reduced and iron loss can be sufficiently reduced even when the secondary recrystallized grain size is large. However, when strain is introduced into steel sheets, magnetostriction (λ Ｐ－Ｐ ) increases. Furthermore, if heat treatment is performed at a temperature of about 800°C after magnetic domain refinement, the iron loss reduction effect disappears, so this method cannot be used in applications that require stress relief annealing at 800°C or higher after irradiation. On the other hand, in the magnetic domain control method using grooves, the magnetic flux density B ８ There is a problem of a decrease in

[0007] Generally, grain-oriented electrical steel sheets are manufactured as follows. A steel sheet material (slab) containing a predetermined 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 referred to as primary annealing or decarburization annealing). This annealing causes primary recrystallization, and crystal grains with a crystal orientation whose easy axis of magnetization is aligned in the rolling direction and whose deviation angle from the strict Goss orientation described by {110}<001> is 10° or less (hereinafter referred to as Goss-oriented grains) are formed within the primary recrystallized grains. This annealing also serves as decarburization annealing. Then, 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 Goss-oriented grains to encroach on other crystal grains, resulting in secondary recrystallization and the formation of a so-called glass coating on the surface of the steel sheet. During secondary recrystallization, Goss-oriented grains grow preferentially due to the influence of inhibitors contained in the steel sheet, and in some cases, the grain size can reach 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.

[0008] As described above, grain-oriented electrical steel sheets achieve desired magnetic properties by undergoing secondary recrystallization during final annealing to obtain a crystalline structure consisting of Goss-oriented grains concentrated in the {110}<001> orientation. To obtain this crystalline structure, it is effective to increase the frequency of minor Goss-oriented grains after primary annealing or to increase the frequency of grains that have good lattice matching with the Goss-oriented grains. For example, grains with {778}<447> (≒{111}<112>) and {411}<148> orientations (called "matching orientation grains"), which have high lattice matching with the {110}<001> orientation and a Σ9 matching orientation relationship with the Goss orientation, are easily encroached upon by Goss-oriented grains. Therefore, it is effective to include many 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 Goss-oriented grains and the frequency of grains having a corresponding orientation relationship. For example, in the region where the cold-rolling reduction is 85% or more, the frequency of corresponding-oriented grains, which are easily encroached upon by Goss-oriented grains, increases with increasing cold-rolling reduction, while the frequency of Goss-oriented grains decreases with increasing cold-rolling reduction. Furthermore, the frequency of Goss-oriented grains, which are the minor orientation in the primary recrystallization texture, increases with increasing heating rate during primary annealing, while the frequency of corresponding-oriented grains, which have the {111}<112> orientation, which is the major orientation in the primary recrystallization texture, decreases with increasing heating rate during primary annealing. In other words, conventional process control conditions have not been able to independently control the frequency of Goss-oriented grains and the frequency of corresponding-oriented grains, which are easily encroached upon by Goss-oriented grains.

[0009] 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) to 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 from the ideal {110}<001> orientation of the crystal orientation observed in the grain-oriented electrical steel sheet 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=(α２ +β ２ +γ ２ ) １／２ (Formula 1)

[0010] As mentioned above, 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 easy axis direction of magnetization (cubic {110}<001>) of each crystal grain does not necessarily coincide perfectly with the rolling direction, and there is an angular deviation θ between the easy axis direction of magnetization and the rolling direction. Crystal grains whose angular 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 angular deviation θ of practical Goss-oriented grains increases, the orientation of the crystal orientation decreases, and the magnetic flux density B ８ decreases.

[0011] Generally, the Goss-oriented grains contained in steel sheets after primary recrystallization include grains with a large angular deviation θ. However, when grains grow due to secondary recrystallization, excluding the effect of the grain size advantage in the early stages of secondary recrystallization, the smaller the angular deviation θ among the Goss-oriented grains, the stronger the driving force for grain growth and the more likely they are to grow preferentially until secondary recrystallization is complete. Therefore, the smaller the angular deviation θ, the more likely the grain size after secondary recrystallization is to become large. In other words, in manufacturing methods that highly orient secondary recrystallized grains in the Goss orientation, specific grains are preferentially grown, which inevitably leads to an increase in grain size.

[0012] Furthermore, during secondary recrystallization, the steel sheet is not flat but curved due to being wound into a coil, but the crystal grains grow while maintaining the linearity of their crystal orientation. Therefore, when the coiled steel sheet is unwound and flattened after secondary recrystallization, portions of the crystal grains will have an orientation where the axis of easy magnetization is not parallel to the surface of the grain-oriented electrical steel sheet. This coil unwinding mainly increases the value of the misalignment angle β, and also increases the angle deviation θ. The increase in the angle deviation θ due to coil unwinding becomes more pronounced as the crystal grain size increases. 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.

[0013] Patent Document 1 discloses a technique in which an etching treatment is performed on the surface of a steel sheet after final cold rolling to form linear grooves that satisfy predetermined conditions, and then, in the subsequent primary annealing, the heating rate in the linear groove portions is made faster than in portions other than the linear grooves in a temperature range in which the steel sheet temperature is 500° C. or higher and 750° C. In Patent Document 1, by artificially creating portions with a fast heating rate and portions with a slow heating rate, portions (linear groove portions) where the frequency of (110)

[001] oriented grains is increased and portions (portions other than the linear grooves) where the frequency of (111)

[112] oriented grains is increased are arranged.

[0014] Patent Document 2 discloses a technique in which, between cold rolling and finish annealing, a laser beam is irradiated onto the surface of a silicon steel sheet multiple times at a predetermined interval PL in the rolling direction. 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 a range of 0° to 6°.

[0015] Patent Document 3 discloses a method for manufacturing a grain-oriented electrical steel sheet that uses a high-temperature slab heating process, has a small difference in the degree of orientation concentration in the Goss orientation between the inner and outer peripheries of a coil with different curvatures, and has a high magnetic flux density. In Patent Document 3, a cold-rolled steel sheet is subjected to linear local heating, and this linearly heated portion acts as a barrier to the growth of secondary recrystallized grains, thereby suppressing the growth of secondary recrystallized grains.

[0016] Japanese Patent Publication No. 2007-169762 International Publication No. WO2012 / 014290 Japanese Patent Publication No. 2022-161269

[0017] As mentioned above, attempts have been made to improve the magnetic properties of grain-oriented electrical steel sheets, but the improvements in magnetic properties achieved by conventional techniques have not been sufficient.

[0018] The present invention has been made in view of the above-mentioned problems. An object of the present invention is to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density. Specifically, an object of the present invention is to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density while suppressing coarsening of secondary recrystallized grains.

[0019] That is, the gist of the present invention is as follows.

[0020] [1] In a decarburization annealed steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention, an area of ​​at least 100 mm x 100 mm on the sheet surface is defined as a divided section, and each divided section is subdivided into 1 mm x 1 mm areas, each of which is defined as a divided small section; the average grain size of crystal grains contained in each divided section is defined as aveD in μm; and the ratio of the number of crystal grains contained in one divided small section whose grain size is 3.0 x aveD or more to the number of crystal grains contained in this divided small section is defined as F. Ｃ aveF is defined as the ratio of the number of crystal grains having a grain size of 3.0×aveD or more to the number of crystal grains contained in the divided section. Ｃ and F Ｃ ÷aveF Ｃ When the divided subdivisions having a value of 1.3 or more are collectively defined as divided subdivision group I, the area ratio of the divided subdivision group I to the area of ​​the divided subdivision is 0.05% or more and 50% or less, and the deviation angle from the ideal Goss orientation with the normal direction to the rolling surface as the rotation axis is defined as α, and the deviation angle from the ideal Goss orientation with the direction perpendicular to the rolling as the rotation axis is defined as β, and the deviation angle of the crystal orientation measured at a measurement point on the sheet surface is represented as (αβ), and the angular deviation at the measurement point is expressed 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, and the area ratio of the Goss-oriented grains contained in the divided small section group I to the area of ​​the divided small section group I is defined as A Ｉ ＧＯＳＳ and the area ratio of the Goss oriented grains contained in the divided section to the area of ​​the divided section is defined as aveA ＧＯＳＳ When defined as follows, AＩ ＧＯＳＳ ÷aveA ＧＯＳＳ [2] In the decarburization annealed steel sheet for grain-oriented electrical steel sheet according to the above item [1], when crystal grains having an orientation difference of 10° or less from a crystal orientation having a Σ9 corresponding orientation relationship with the ideal Goss orientation are defined as corresponding orientation grains, and when the divided subdivisions excluding the divided subdivision group I among the divided subdivisions are defined as the divided subdivision group II, the area ratio of the corresponding orientation grains contained in the divided subdivision group II to the area of ​​the divided subdivision group II may be 10% or more and 50% or less.

[0021] According to the above aspect of the present invention, it is possible to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density. Specifically, it is possible to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density while suppressing coarsening of secondary recrystallized grains.

[0022] 1 is a diagram showing an example of spot current heating as a local rapid heating method, and is a schematic diagram showing an example of the arrangement of local heat affected zones in a decarburization annealed steel sheet for a grain-oriented electrical steel sheet according to an embodiment of the present invention.

[0023] 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 within the scope of the present invention. Furthermore, the numerical ranges described below include the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Furthermore, "%" in relation to chemical composition means "mass %" unless otherwise specified.

[0024] 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>, 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 those with {778}<447> (≒{111}<112>) and {411}<148> orientations, which have a Σ9 correspondence orientation relationship with the Goss orientation, (referred to as "corresponding orientation grains"), decrease as the heating rate of the primary annealing increases. Therefore, it has been difficult to increase the proportion of practical Goss orientation grains while also increasing the proportion of corresponding orientation grains during the primary annealing.

[0025] The technology described in Patent Document 1 forms linear grooves on the surface of a steel sheet by etching, and then increases the heating rate of the linear grooves during primary annealing to create a region with a high proportion of Goss-oriented grains (the linear grooves) and a region with a high proportion of correspondingly oriented grains (the regions 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 grooves and other regions, and the resulting effect is limited. Specifically, the difference in heating rate between the linear grooves and the regions other than the linear grooves is not significant. For example, even if the entire steel sheet is heated at 100°C / s, the thickness reduction in this region due to the groove formation does not exceed about 10% of the original thickness (because a typical groove depth is about 20 μm or less, or about 10% of the thickness). Therefore, considering the thermal conductivity of the steel sheet, the heating rate ratio is less than the thickness ratio for the same heat input, and therefore the ratio of Goss-oriented grains in the linear grooves is unlikely to increase compared to the regions other than the linear grooves. Furthermore, in this method, the heating rate of the linear groove portion is increased during the primary annealing, so that the grain size of the Goss-oriented grains formed in the linear groove portion after the primary annealing does not increase compared to the overall average primary recrystallization grain size, but remains the same or finer.

[0026] In addition, the technology described in Patent Document 2 irradiates a laser beam onto the surface of a steel sheet before finish annealing. However, this method only controls the laser beam irradiation area to the grain boundary of secondary recrystallization, and does not control the orientation of Goss-oriented grains generated by primary recrystallization. Furthermore, the area heated by laser irradiation is limited to the area near the surface where the laser is irradiated, and heat diffusion to the surrounding area is significant, so it is usually not possible to ensure a thermal history sufficient to recrystallize the steel sheet.

[0027] In addition, the technology described in Patent Document 3 irradiates a laser beam onto the surface of a steel sheet after cold rolling. However, as with the above, this method only controls the laser beam irradiation area to the grain boundaries of secondary recrystallization, but does not control the heating rate or the orientation of Goss-oriented grains generated by primary recrystallization. Patent Document 3 only reduces the grain size of secondary recrystallized grains in the rolling direction by using the laser beam irradiation area as a barrier to the growth of secondary recrystallized grains. It does not ensure a grain growth temperature range in the locally heated region that is effective for increasing the diameter of Goss-oriented grains in the locally heated region. While this method may be able to reduce the difference in the degree of orientation concentration of Goss orientation at the inner and outer peripheries of a coil with different curvatures, it does not produce Goss-oriented grains with a crystal orientation 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 of secondary recrystallized grains in the rolling direction, but does not aim to fundamentally improve the orientation of Goss-oriented grains.

[0028] 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 performing local rapid heating on a steel sheet to form a locally heated region, followed by primary annealing (decarburization annealing). The locally heated region formed before the primary annealing is arranged locally on the surface of the steel sheet. The crystalline structure of this locally heated region comprises one or both of a recrystallized structure and a recovered structure at the end of the local rapid heating. The crystalline structure of the non-rapidly heated portion of the steel sheet surface other than the locally heated region comprises a cold-worked structure that remains as cold-rolled.

[0029] The localized rapid heating is preferably performed at a heating rate of 500°C / s or more, preferably 2000°C / s or more, and more preferably 10000°C / s or more. Thus, it has been found that when a locally heated region is formed on the surface of a steel sheet before primary annealing and then primary annealing is performed on this steel sheet, many crystal grains having the practical Goss orientation are formed in the region heat-affected by the localized rapid heating (localized heat-affected region). It has also been found that, among the formed Goss-oriented grains, the proportion of Goss-oriented grains having smaller angular deviations θ and φ from the ideal Goss orientation increases, and the grain size of the Goss-oriented grains after primary annealing also increases.

[0030] The frequency of coarse Goss-oriented grains in the local heat-affected zone is higher than that obtained by conventional rapid heating of the entire sheet. This characteristic is thought to be due to the fact that the peripheral area of ​​the locally heated zone has a deformed structure that retains a lot of strain. Specifically, when local rapid heating is performed, the practical Goss-oriented grains generated in the locally heated zone preferentially encroach on the peripheral unrecrystallized region, which retains a lot of strain, during primary annealing, thereby increasing in diameter. However, when the entire steel sheet is heated, the formation and growth of recrystallized grains occurs uniformly within the steel sheet, so the above-mentioned enlargement of grain diameter is thought to be prevented.

[0031] After the primary annealing has scattered local heat-affected zones with the above characteristics on the steel sheet surface, secondary annealing is then performed. The Goss-oriented grains formed in the local heat-affected zone grow preferentially during secondary annealing. Many Goss-oriented grains are formed in the local heat-affected zone, including Goss-oriented grains with particularly small angular deviations θ and φ, which are close to the ideal Goss orientation and have large crystal grain sizes after the primary annealing. These Goss-oriented grains grow preferentially during secondary recrystallization.

[0032] Because Goss-oriented grains, which serve as nuclei for secondary recrystallization, are supplied to the local heat-affected zone, there is no need to form Goss-oriented grains in the matrix portion other than the local heat-affected zone, i.e., the local heat-unaffected zone, during primary annealing. Therefore, the primary annealing conditions may be slow heating conditions with a low heating rate, which allows for the formation of many corresponding-oriented grains with the {111}<112> and {411}<148> orientations, which are susceptible to corrosion, in the local heat-unaffected zone. For example, by setting the heating rate of the primary annealing to 300°C / sec or less, many Goss-oriented grains with large grain sizes and close to the ideal Goss orientation are formed in the local heat-affected zone, while almost no Goss-oriented grains are formed in the local heat-unaffected zone, resulting in the formation of many corresponding-oriented grains.

[0033] As described above, in the local heat-affected zone of a steel sheet (decarburized annealed steel sheet) that has undergone local rapid heating and primary annealing, many Goss-oriented grains with a grain size close to the ideal Goss orientation are present. On the other hand, the matrix portion (region unaffected by local heating) contains almost no large-sized Goss-oriented grains, and many grains with a corresponding orientation that are susceptible to corrosion can be present. When such a steel sheet is subjected to secondary annealing, the Goss-oriented grains with a grain size close to the ideal Goss orientation that are present in the local heat-affected zone begin to grow preferentially, growing toward the region unaffected by local heating.

[0034] As one embodiment of localized rapid heating, spot resistance heating using electrical resistance heating will be described. In spot resistance heating, spot electrodes are pressure-bonded to both sides of a steel sheet, and current is passed between the spot electrodes to spot-heat the electrode-holding portion of the steel sheet. A steel having the composition shown in Table 1 was hot-rolled and cold-rolled to a thickness of 0.22 mm. This steel sheet was subjected to spot resistance heating. Spot resistance heating was performed using copper electrodes with a diameter of 3 mm under the following conditions: a current of 5.0 kA or less, which is within a range that does not melt the steel sheet surface; a current duration of 20 to 80 milliseconds; an electrode pressure of 50 kgf to 500 kgf; and an electrode holding time after current application of 0.2 seconds. Spot resistance heating was performed on the surface of the cold-rolled steel sheet in a grid pattern with a 20 mm pitch in the rolling direction and a 30 mm pitch in the width direction (perpendicular to the rolling direction), forming locally heated regions. This cold-rolled steel sheet was then subjected to decarburization annealing (primary annealing). In the decarburization annealing, the cold-rolled steel sheet was heated at a heating rate of 20°C / sec and held at 830°C for 90 seconds.

[0035] An example of the arrangement of the local heat affected zone in a decarburization annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment is shown in Figure 1. In Figure 1, a steel sheet 1, a local heat affected zone 2, a rolling direction 21, and a width direction (direction perpendicular to the rolling direction) 22 are schematically shown.

[0036]

[0037] To prepare an observation surface for EBSD (Electron Back Scattering Diffraction pattern) on the surface of the decarburized annealed steel sheet (steel sheet after local rapid heating and primary annealing), a smooth surface was created by mechanical polishing, and electrolytic polishing was performed to remove processing strain on the surface. The IQ (Image Quality) value and crystal orientation were measured in 0.5 μm steps in a region including the locally heated region by EBSD. Fig. 2 is a schematic diagram showing the distribution of Goss orientation grains in the decarburized annealed steel sheet for use in the grain-oriented electrical steel sheet according to this embodiment.

[0038] The current-carrying electrode used in spot resistance heating is circular, and its diameter, in terms of circle equivalent diameter, is preferably 0.5 mmφ to 10 mmφ, more preferably 1 mmφ to 5 mmφ. 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 cause the grains to grow, and the electrode shape may be other shapes, for example, elliptical or linear.

[0039] In FIG. 2, the angle deviation φ=(α ２ +β ２ ) １／２ Based on the definition of "aveD", crystal grains (Goss-oriented grains 14) with an angular deviation φ of 10° or less from the ideal Goss orientation were marked with a "○". Furthermore, from among the crystal grains with an angular deviation φ of 10° or less, crystal grains with a coarse grain size (coarse Goss-oriented grains 15) were marked with a "◎". Note that coarse Goss-oriented grains 15 refer to Goss-oriented grains with a grain size three times or more larger than the average grain size (aveD).

[0040] As shown in Figure 2, the decarburization annealed steel sheet (steel sheet 1) for use in a grain-oriented electrical steel sheet according to this embodiment contains Goss-oriented grains 14 and coarse Goss-oriented grains 15. In particular, the region containing the coarse Goss-oriented grains 15 corresponds to the local heat-affected zone 2. Note that although Figure 2 schematically illustrates the local heat-affected zone boundary 4 and the local heat-affected zone boundary 5, it is not easy to distinguish these boundaries by observing the structure. However, in this embodiment, the region containing the coarse Goss-oriented grains 15 can be considered the local heat-affected zone 2.

[0041] The above-mentioned 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 / hr (°C / hr), and a holding time of 1200°C for 20 hr (hr). 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 heat-affected zone, it was observed that multiple secondary recrystallized grains had grown from the location corresponding to the local heat-affected zone, and that secondary recrystallized grains had spread from the center of the location corresponding to the local heat-affected zone. Furthermore, in the location corresponding to the local heat-affected zone, fine grains of 2 mm or less were sometimes observed as traces of the local heat-affected zone, and in other cases they were 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 when local heat treatment is not performed, resulting in a higher magnetic flux density B of the steel sheet. ８ It was confirmed that the crystal orientation of the secondary recrystallized grains thus obtained had an extremely small deviation from the ideal Goss orientation (B ８ (Improved to 1.96T) As described above, the decarburization annealed steel sheet according to this embodiment contains Goss orientation grains 14 as well as coarse Goss orientation grains 15. The formation of coarse Goss orientation grains 15 that have a size advantage and are close to the ideal Goss orientation is thought to be the reason for the favorable secondary recrystallization orientation.

[0042] The characteristics of the decarburization annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment will be specifically described. The decarburization annealed steel sheet shown in FIG. 2 satisfies the following characteristics.

[0043] In the decarburization annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment, an area of ​​at least 100 mm x 100 mm on the sheet surface is defined as a divided section, and this divided section is further divided into 1 mm x 1 mm areas, each of which is defined as a divided small section; the average grain size of crystal grains contained in the divided section is defined as aveD in μm; and the ratio of the number of crystal grains contained in this divided small section whose grain size is 3.0 x aveD or more to the number of crystal grains contained in this divided small section is defined as F. ＣaveF is defined as the ratio of the number of crystal grains having a grain size of 3.0×aveD or more to the number of crystal grains contained in the divided section. Ｃ and F Ｃ ÷aveF Ｃ When the divided small sections having a value of 1.3 or more are collectively defined as divided small section group I, the area ratio of the divided small section group I to the area of ​​the divided section is 0.05% or more and 50% or less, and the deviation angle from the ideal Goss orientation with the normal direction to 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 a measurement point on the sheet surface is represented as (α β), and the angle deviation at the measurement point is expressed 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, and the area ratio of the Goss-oriented grains contained in the divided small section group I to the area of ​​the divided small section group I is defined as A Ｉ ＧＯＳＳ and the area ratio of the Goss orientation grains contained in the divided section to the area of ​​the divided section is defined as aveA ＧＯＳＳ When defined as follows, A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ The value of is 1.3 or more.

[0044] In this embodiment, the above-mentioned crystal grains are determined as crystal grain boundaries when the crystal orientation difference between adjacent measurement points is 1° or more. The above-mentioned grain size refers to the circle-equivalent diameter. The above-mentioned average crystal grain size refers to the number-average diameter.

[0045] In this embodiment, the angular deviation φ is defined by the deviation angles α and β. Generally, the angular 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 angular deviation φ is defined by two components: the deviation angles α and β, excluding the deviation angle γ, which has little effect on the magnetic flux density.

[0046] In the decarburization annealed steel sheet according to this embodiment, the area ratio of the divided small section group I to the area of ​​the divided section is 0.05% or more, and A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ When the value of is 1.3 or more, Goss oriented grains grow in a preferable form in the secondary recrystallization stage, and a grain-oriented electrical steel sheet with good magnetic properties can be obtained.

[0047] For example, when the area ratio of the divided small section group I to the area of ​​the divided section is 0.05% or more, when an area of ​​100 mm x 100 mm is set as the divided section, the divided small section (F Ｃ ÷aveF Ｃ This means that 10,000 divided sub-partitions contain 5 or more divided sub-partitions with a value of 1.3 or more. Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ A value of 1.3 or more means that the area proportion of Goss oriented grains contained in the divided subdivisions (divided subdivision group I) where coarse grains are preferably present is significantly larger than the area proportion of Goss oriented grains contained in the matrix. In other words, a decarburization annealed steel sheet satisfying these conditions means that there is a region (divided subdivision group I) in the matrix that contains coarse grains and has a large area proportion of Goss oriented grains.

[0048] In addition, a decarburized annealed steel sheet that satisfies the above conditions means that each of the divided sections arbitrarily provided within the steel sheet contains, on average, divided small sections (divided small section group I) that contain coarse crystal grains and have a large area ratio of Goss orientation grains.

[0049] Therefore, in the decarburization-annealed steel sheet according to this embodiment, during secondary recrystallization, coarse Goss-oriented grains (Goss-oriented grains with large crystal grain sizes) close to the ideal Goss orientation begin preferential growth uniformly over a wide area within the steel sheet. The preferentially grown Goss-oriented grains collide with other preferentially grown Goss-oriented grains, halting grain growth and resulting in a smaller grain size of the Goss-oriented grains (secondary recrystallized grains). This grain-oriented electrical steel sheet can increase magnetic flux density while suppressing coarsening of the secondary recrystallized grain size compared to conventional techniques. Furthermore, because coarsening of the secondary recrystallized grain size is suppressed, this grain-oriented electrical steel sheet also exhibits excellent iron loss and magnetostriction.

[0050] The area ratio of the divided small section group I to the area of ​​the divided section is preferably 0.2% or more, more preferably 0.5% or more, and even more preferably 1% or more. On the other hand, the above area ratio is preferably 50% or less because a region (matrix) is required for the coarse Goss orientation grains to preferentially undergo secondary recrystallization. The upper limit of this area ratio may be 40%, or may be 30%.

[0051] In addition, F Ｃ ÷aveF Ｃ The fact that the value of F is 1.3 or more means that, as described above, there is preferably a region containing coarse crystal grains in the matrix. According to the investigations conducted by the present inventors, when normal heating of the entire plate is performed without local heating, even if the heating rate is increased to, for example, 10,000°C / sec, F Ｃ ÷aveF Ｃ The value of is only about 1.1 at most. This is because, as mentioned above, when local heating is performed before decarburization annealing, recrystallized grains (e.g., Goss oriented grains) generated in the locally heated region grow by preferentially encroaching on the surrounding unrecrystallized region during decarburization annealing, whereas when the entire plate is heated, recrystallization occurs evenly within the steel plate even if the heating rate is increased, making it difficult for specific crystal grains (e.g., Goss oriented grains) to grow preferentially. Ｃ ÷aveF Ｃ The upper limit of the value is not particularly limited, and may be 100, for example.

[0052] Similarly, A Ｉ ＧＯＳＳ ÷aveAＧＯＳＳ The value of 1.3 or more means that the area ratio of Goss orientation grains contained in the divided small section group I is preferably large, as described above. According to the investigation by the inventors, when normal heating of the entire plate is performed without local heating, even if the heating rate is increased to, for example, 10,000°C / sec, the F Ｃ ÷aveF Ｃ Since no divided sub-regions having a value of 1.3 or more are formed, the divided sub-region group I is not formed either. Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ The value of A does not exceed 1.3. Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ The upper limit of the value is not particularly limited, and may be 100, for example.

[0053] It is preferable that the divided sections including the divided small section group I that satisfies the above condition are uniformly present over the entire decarburization-annealed steel sheet. However, the above condition does not have to be satisfied over the entire sheet surface of the decarburization-annealed steel sheet. For example, when 10 divided sections spaced apart from one another are arbitrarily set on the sheet surface of the decarburization-annealed steel sheet and the above condition is evaluated for each of these 10 divided sections, it is possible that the above condition (the area ratio of the divided small section group I to the area of ​​the divided sections is 0.05% or more and A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ In this case, it is possible to obtain a grain-oriented electrical steel sheet with good magnetic properties.

[0054] The number of divided sections that satisfy the above conditions is preferably 7 or more out of 10, and more preferably 8 or more. The upper limit of the number of divided sections that satisfy the above conditions is 10, as the higher the number the better.

[0055] When the decarburization-annealed steel sheet according to this embodiment is used, during secondary recrystallization, among the numerous Goss-oriented grains in the local heat-affected zone, coarse Goss-oriented grains (Goss-oriented grains with large grain sizes) close to the ideal Goss orientation preferentially grow. Therefore, a steel sheet dominated by secondary recrystallized grains close to the ideal Goss orientation can be obtained without requiring the secondary recrystallized grains to grow large. For example, when scattering the local heat-affected zones on the steel sheet surface, the size, shape, and arrangement of the grains after secondary annealing can be controlled by appropriately arranging the local heat-affected zones on the steel sheet surface. For example, if a Goss-oriented grain that started growing from one local heat-affected zone collides with another Goss-oriented grain that started growing from an adjacent local heat-affected zone, the grain size of the Goss-oriented grains at the end of secondary annealing will be small, resulting in a steel sheet with excellent core loss. In particular, as mentioned above, in the early stage of secondary annealing, the Goss-oriented grains that grow preferentially from the local heat-affected zone have a crystal orientation close to the ideal Goss orientation, so that good magnetic flux density can be obtained despite the small secondary recrystallized grain size.

[0056] In other words, when the decarburization annealed steel sheet according to this embodiment is used, the magnetic flux density can be increased while suppressing the coarsening of the secondary recrystallized grain size, so that the need for or reduction of the magnetic domain width refinement by introducing strain or forming grooves can be eliminated. Specifically, the magnetic domain refinement effect can be obtained favorably by the grain boundaries that increase with the refinement of the secondary recrystallized grain size, so that the magnetic domains are refined without applying conventional magnetic domain control techniques. Therefore, unlike conventional magnetic domain control techniques, there is no need to introduce strain into the steel sheet, and magnetostriction (λ Ｐ－Ｐ ) can be prevented from increasing. In addition, unlike conventional magnetic domain control technology, there is no need to form grooves in the steel sheet, so the magnetic flux density B ８ Furthermore, a grain-oriented electrical steel sheet using the decarburization annealed steel sheet according to this embodiment can be used in applications requiring stress relief annealing at 800°C or higher.

[0057] Additionally, in the decarburization annealed steel sheet according to this embodiment, in order to preferentially grow the coarse Goss orientation grains during secondary annealing, it is preferable that the area ratio of the corresponding orientation grains is high.

[0058] For example, in this embodiment, crystal grains whose orientation difference from a crystal orientation that has a Σ9 corresponding orientation relationship with the ideal Goss orientation are defined as corresponding orientation grains, and when the divided subdivisions among the divided subdivisions excluding the divided subdivision group I are defined as divided subdivision group II, it is preferable that the area ratio of the corresponding orientation grains contained within the divided subdivision group II to the area of ​​the divided subdivision group II is 10% or more and 50% or less.

[0059] If the area ratio of the corresponding orientation grains is 10% or more, the coarse Goss orientation grains can preferably grow preferentially during secondary annealing. The area ratio of the corresponding orientation grains is preferably 12% or more, and more preferably 15% or more. On the other hand, there is no particular upper limit to the area ratio of the corresponding orientation grains, and for example, this upper limit may be 40% or 30%.

[0060] The area ratio of corresponding orientation grains means the total area ratio of crystal grains having a crystal orientation with a crystal orientation difference (disorientation) of 10° or less from a crystal orientation having a corresponding orientation relationship of Σ9 with the ideal Goss orientation {110}<001>, and corresponds to, for example, the total area ratio of {778}<447> orientation grains, {111}<112> orientation grains, and {411}<148> orientation grains.

[0061] In addition, the average particle diameter aveD and the number ratio F Ｃ , number ratio aveF Ｃ , deviation angle α, deviation angle β, area ratio A of Goss orientation grains Ｉ ＧＯＳＳ , the area ratio of Goss-oriented grains aveA ＧＯＳＳ The area ratio of the corresponding orientation grains can be measured by the EBSD method described above. 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.

[0062] Next, a preferred chemical composition of the decarburized annealed steel sheet according to this embodiment will be described.

[0063] First, the chemical composition of the grain-oriented electrical steel sheet as a final product manufactured using the decarburization annealed steel sheet according to this embodiment will be described. The grain-oriented electrical steel sheet as a final product may contain, in mass fraction, 2.0% to 7.0% Si, with the balance being Fe and impurities.

[0064] Furthermore, this grain-oriented electrical steel sheet may contain known optional elements in place of a portion of Fe for the purpose of improving magnetic properties. There is no need to set a lower limit for the optional elements, and the lower limit may be 0%. Furthermore, the upper limit for the optional elements may be set within a range in which a significant decrease in magnetic flux density or iron loss does not occur. The upper limit values ​​serving as a guide for each optional element are listed below.

[0065] The grain-oriented electrical steel sheet (base steel sheet) as a final product may contain, in mass %, as a chemical composition at least one of the following: 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.

[0066] 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%. Note that the total content of S and Se means that at least one of S and Se is included and the total content thereof. 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 the total content thereof.

[0067] In grain-oriented electrical steel sheets, 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 may be reduced to 50 ppm or less, and if purification annealing is performed sufficiently, the content may reach a level that is undetectable by general analysis (1 ppm or less).

[0068] Furthermore, even if the above-mentioned optional elements are contained as impurities, the effects of this embodiment are not impaired. Note that the 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 a steel sheet. The upper limit of the total content of impurities may be, for example, 5%.

[0069] The above chemical composition may be measured by a general steel analysis method. 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 pre-created calibration curve. C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0070] The chemical composition of the decarburized annealed steel sheet according to this embodiment contains, for example, the following elements.

[0071] The decarburization annealed steel sheet according to this embodiment has a chemical composition, in mass %, of C: 0 to 0.0050%, Si: 2.0 to 7.0%, Mn: 0.05 to 1.0%, S and Se: 0.003 to 0.035% in total, Al: 0.010 to 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 to 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 may be Fe and impurities.

[0072] C: 0 to 0.0050% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process, but excessive C content in the final product adversely affects the magnetic properties. For example, the C content of a slab is 0.085% or less. However, C is purified in the decarburization annealing process and the finish annealing process to 0.0050% or less. Considering productivity in industrial production, the lower limit of the C content may be more than 0%, or even 0.001%.

[0073] Si: 2.0 to 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 finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheets. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. The preferred lower limit of the Si content is 2.5%, more preferably 3.0%. The preferred upper limit of the Si content is 4.5%, more preferably 4.0%.

[0074] Mn: 0.05 to 1.0% Manganese (Mn) combines with S or Se to produce MnS or MnSe, which functions as an inhibitor. When Mn is contained, secondary recrystallization is stabilized when the Mn content is within the range of 0.05 to 1.0%. Note that part of the inhibitor function can be 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. For this reason, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

[0075] S and Se: 0.003 to 0.035% in total. Sulfur (S) and selenium (Se) combine with Mn to form MnS or MnSe, functioning as inhibitors. When at least one of S and Se is contained, secondary recrystallization is stabilized if the total content of S and Se is 0.003 to 0.035%. It is possible for nitrides of Nb group elements to partially fulfill the inhibitor function. 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 S and Se content is 0.025%, more preferably 0.010%. If S and Se remain after final annealing, they form compounds that deteriorate iron loss. Therefore, it is preferable to reduce S and Se as much as possible through purification during final annealing.

[0076] Here, the total content of S and Se means that at least one of S and Se is included and the total content thereof is included.

[0077] Al: 0.010 to 0.0650% Aluminum (Al) combines with N to precipitate as (Al, Si)N, functioning as an inhibitor. When Al is contained, if the Al content is within the range of 0.010 to 0.0650%, AlN, which acts as an inhibitor formed by nitriding (described later), expands the secondary recrystallization temperature range, stabilizing secondary recrystallization particularly in the high-temperature range. Therefore, the Al content is 0.010 to 0.0650%. The preferred lower limit of the Al content is 0.020%, more preferably 0.025%. From the viewpoint of secondary recrystallization stability, the preferred upper limit of the Al content is 0.040%, more preferably 0.035%.

[0078] N: 0 to 0.012% Nitrogen (N) combines with Al and functions 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 may be greater than 0%, or even 0.001%. On the other hand, when N is incorporated, if the N content exceeds 0.012%, blisters, 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 after the final annealing process, the N content is 0.005% or less.

[0079] The balance of the above chemical composition is composed of Fe and impurities. The term "impurities" refers to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial production of steel. The upper limit of the total content of impurities may be, for example, 5%.

[0080] The above-mentioned chemical composition may contain known optional elements in place of a portion of Fe, taking into consideration the enhancement of the inhibitor function through compound formation and the influence on magnetic properties. Examples of optional elements that can be contained in place of a portion of Fe include the following elements:

[0081] 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 an Nb group element (one or more of Nb, V, Mo, Ta, and W) is used as a part of the inhibitor, B ８This exhibits an effect of improving the magnetic properties. On the other hand, if an excessive amount of Nb group elements remains in the base steel sheet, it may have an adverse 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), secondary recrystallization is initiated at an appropriate timing, which is preferable. In addition, the orientation of the secondary recrystallized grains that occur becomes very favorable, and the final structure can be controlled to be favorable for the 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%.

[0082] The total content of Nb group elements is more preferably 0.004 to 0.020%, and even more preferably 0.005 to 0.010%.

[0083] Here, the total content of Nb group elements means the total content including at least one of Nb, V, Mo, Ta, and W.

[0084] Furthermore, the decarburization annealed steel sheet according to this embodiment may contain, in mass %, at least one of the following optional elements: Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, and Ni: 0 to 1.0%. These optional elements may be contained according to known purposes, so there is no need to set a lower limit for the content of the optional elements, and the lower limit may be 0%.

[0085] The chemical composition of the decarburized annealed steel sheet according to this embodiment may be measured by the above-mentioned analytical method, similar to the chemical composition of the grain-oriented electrical steel sheet as the final product.

[0086] Next, one embodiment of a preferred method for producing a decarburization annealed steel sheet for use in a grain-oriented electrical steel sheet according to this embodiment will be described.

[0087] The method for producing the decarburization annealed steel sheet according to the present embodiment is not limited to the method described below. The following production method is one example for producing the decarburization annealed steel sheet according to the present embodiment.

[0088] 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 method for producing a decarburized annealed steel sheet according to this embodiment can adopt various conditions as long as they do not deviate from the gist of this embodiment and the object of this embodiment is achieved.

[0089] The manufacturing method of the decarburization annealed steel sheet according to the present embodiment can apply, as a basic step, a conventionally known manufacturing method of grain-oriented electrical steel sheet. 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 decarburization annealed steel sheet according to the present embodiment is not limited to a specific manufacturing method. Below, a method in which nitriding treatment is applied as the low-temperature slab heating process will be described.

[0090] (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 continuous casting. The molten steel may be used to produce an ingot, 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. A so-called thin slab having a thickness of 10 to 70 mm may be used as the slab. When a thin slab is used, rough rolling before finish rolling can be omitted in the hot rolling process.

[0091] For example, the above slab may contain the following elements as its chemical composition:

[0092] C: 0.085% or less. Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process, but excessive C content in the final product adversely affects the magnetic properties. Therefore, 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 finish annealing process, resulting in a C content of 0.005% or less. When C is contained, taking into account productivity in industrial production, the lower limit of the C content may be more than 0%, or even 0.001%.

[0093] Si: 2.0 to 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 finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheets. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. The preferred lower limit of the Si content is 2.5%, more preferably 3.0%. The preferred upper limit of the Si content is 4.5%, more preferably 4.0%.

[0094] Mn: 0.05 to 1.00% Manganese (Mn) combines with S or Se to form MnS or MnSe, which functions as an inhibitor. When Mn is contained, secondary recrystallization is stabilized when the Mn content is within the range of 0.05 to 1.00%. Note that part of the inhibitor function can be performed by nitrides of Nb group elements. 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 Mn content is 0.50%, and more preferably 0.20%.

[0095] At least one of S and Se: 0.003 to 0.035% in total. Sulfur (S) and selenium (Se) combine with Mn to form MnS or MnSe, functioning as an inhibitor. When at least one of S and Se is contained, secondary recrystallization is stabilized if the total content of S and Se is 0.003 to 0.035%. It is possible for part of the inhibitor function to be performed by nitrides of Nb group elements. 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 S and Se content is 0.025%, more preferably 0.010%. If S and Se remain after final annealing, they form compounds that deteriorate iron loss. Therefore, it is preferable to reduce S and Se as much as possible through purification during final annealing.

[0096] Here, the total content of S and Se means that at least one of S and Se is included and the total content thereof is included.

[0097] Al: 0.010 to 0.065% Aluminum (Al) combines with N to precipitate as (Al, Si)N, functioning as an inhibitor. When Al is contained, if the Al content is within the range of 0.010 to 0.065%, AlN, which acts as an inhibitor formed by nitriding (described later), expands the secondary recrystallization temperature range, stabilizing secondary recrystallization particularly in the high-temperature range. Therefore, the Al content is 0.010 to 0.065%. The preferred lower limit of the Al content is 0.020%, and more preferably 0.025%. From the viewpoint of secondary recrystallization stability, the preferred upper limit of the Al content is 0.040%, and more preferably 0.035%.

[0098] N: 0.012% or less Nitrogen (N) combines with Al and functions 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 may be greater than 0%, or even 0.001%. On the other hand, when N is incorporated, if the N content exceeds 0.012%, blisters, a type of defect, tend to occur in the steel sheet. The preferred upper limit of the N content is 0.010%, more preferably 0.009%. N is purified in the final annealing process, and the N content is 0.005% or less after the final annealing process.

[0099] The balance of the above chemical composition is composed of Fe and impurities. The term "impurities" refers to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial production of steel. The upper limit of the total content of impurities may be, for example, 5%.

[0100] The above-mentioned chemical composition may contain known optional elements in place of a portion of Fe, taking into consideration not only solving problems in production but also strengthening the inhibitor function by compound formation and the influence on magnetic properties. Examples of optional elements that can be contained in place of a portion of Fe include the following elements.

[0101] 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 Nb group elements are used as part of the inhibitor, a total content of Nb group elements of 0.030% or less is preferable because it initiates secondary recrystallization at an appropriate timing. In addition, the orientation of the secondary recrystallized grains that occur becomes very favorable, and ultimately, it is possible to control the structure 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; the lower limit may be 0%. This lower limit is preferably 0.003%.

[0102] The total content of Nb group elements is more preferably 0.004 to 0.020%, and even more preferably 0.005 to 0.010%.

[0103] Here, the total content of Nb group elements means the total content including at least one of Nb, V, Mo, Ta, and W.

[0104] The slab may also contain, as optional elements, at least one of the following 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, and Ni: 1.00% or less. These optional elements may be contained according to known purposes, so there is no need to set a lower limit for the content of the optional elements, and the lower limit may be 0%.

[0105] The chemical composition of the slab may be measured by the above-mentioned analytical method, in the same manner as the chemical composition of the grain-oriented electrical steel sheet as the final product.

[0106] (Hot Rolling Process) The hot rolling process is a process in which a slab heated to a predetermined temperature (for example, 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 rough-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.

[0107] 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 in consideration of productivity, the slab heating temperature should be set to 1100°C to 1280°C.

[0108] (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 predetermined temperature conditions (e.g., 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 favorably adjust the precipitate morphology in the steel. For example, in a high-temperature slab heating process, this is a process that ultimately controls the morphology of precipitates such as AlN, and conditions are adjusted to precipitate uniformly and finely. In hot-rolled sheet annealing, for example, the steel sheet is heated to 1050°C to 1150°C, and then slowly cooled to an intermediate temperature (850°C to 950°C) over 50 to 150 seconds to properly precipitate AlN, etc., followed by water cooling.

[0109] (Cold Rolling Step) The cold rolling step is a step in which the annealed steel sheet obtained in the hot-rolled sheet annealing step is subjected to a single cold rolling or multiple (two or more) cold rolling steps (e.g., a total cold rolling rate of 80 to 95%) 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 in the cold rolling may be set to about 100°C to 300°C.

[0110] The total cold rolling reduction is defined as follows: Total cold rolling reduction (%) = (1 - thickness of steel sheet after cold rolling / thickness of steel sheet before cold rolling) x 100

[0111] (Localized Rapid Heating Process) In this embodiment, localized rapid heating is performed on the cold-rolled steel sheet after the cold rolling process to form a locally heated 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 spot electrodes to 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 steel sheet surface with laser light or an electron beam, a method of locally heating by induction heating, or a method of heating by contacting a hot piece can be used. The size of each locally heated 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. Furthermore, the electrodes used for electrical heating may be circular electrodes as described above, or linear electrodes.

[0112] Regarding the size of the locally heated region, the minimum diameter and width depend on the shape of the spot electrode and the technology for reducing the focused diameter of the laser light or electron beam. While it is theoretically possible to further reduce the size of the locally heated region, considering that the current size of recrystallized grain nuclei is approximately 1 μm, it is believed that a locally heated region of 1 μm or greater will be effective. Industrially, the minimum diameter and width should be 10 μm or greater. Regarding the size of the locally heated region, if the maximum diameter or width exceeds 10 mm, the area proportion of Goss-oriented grains with relatively large angular deviations θ and φ that cannot grow outside the locally heated region increases. In this case, the effect of local rapid heating is less likely to be achieved.

[0113] In localized rapid heating, the heating rate should be 500°C / s or more at 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 / s or more, the structure of the locally heated region can be preferably controlled to a recovered structure or a recrystallized structure. The heating rate is preferably 2000°C / s or more, and more preferably 10000°C / s or more. On the other hand, the upper limit of the heating rate is not particularly limited, and may be, for example, 1000000°C / s.

[0114] Furthermore, it is preferable to control the above heating rate in combination with and inseparably with the heating rate during decarburization annealing, which will be described later. For example, in this embodiment, the aim is to have coarse Goss-oriented grains in the local heat-affected zone after decarburization annealing. However, if the heating rate during decarburization annealing is faster than the heating rate during local rapid heating, it is difficult to achieve the above objective. Therefore, although it depends on the magnitude of the heating rate, it is preferable to set the heating rate during local rapid heating to be equal to or greater than the heating rate during decarburization annealing. Those skilled in the art will be able to preferably control the heating rates during local rapid heating and decarburization annealing if they understand the aim of having coarse Goss-oriented grains in the local heat-affected zone after decarburization annealing.

[0115] The above-mentioned 1 / 5 part of the plate thickness of the steel plate means a depth corresponding to 1 / 5 of the plate thickness of the steel plate from the surface of the steel plate in the plate thickness direction.

[0116] Furthermore, in localized rapid heating, it is sufficient that the maximum temperature reached is 700°C or higher in the center of the locally heated region as viewed from the thickness direction and at 1 / 5 of the 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 may be lower than the melting point of the steel plate, for example, 1400°C or lower.

[0117] Furthermore, in the 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 in the thickness direction and at 1 / 5 of the thickness of the steel plate is 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 limitation on 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-oriented grains close to the ideal Goss orientation.

[0118] In localized rapid heating, it is important to control the heating rate and maximum temperature, as well as 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 region can be formed. Controlling the subgrain structure in the locally heated region by the holding time allows for favorable microstructural control in subsequent processes.

[0119] As described above, the method of local rapid heating is not particularly limited. For example, local rapid heating may be performed by spot 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 as described above. A person skilled in the art can combine the conditions of the local rapid heating method to control the desired heating rate, maximum temperature, and holding time.

[0120] For example, a person skilled in the art can perform heat conduction analysis using the finite element method to control the heating rate and the maximum temperature reached 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. Additionally, 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, this can be achieved by controlling the electrode holding time after current application or by changing the shape of the spot electrode to a shape suitable for heat removal. Furthermore, in the case of laser irradiation or electron beam irradiation, the irradiation speed can be adjusted or the shape of the irradiation region can be changed to an ellipse or the like to gradient the irradiation energy from the center to the outer edge of the irradiation region.

[0121] In general, heating by laser irradiation preferentially heats only the vicinity of the irradiated steel sheet surface, and the interior of the steel sheet is less likely to be heated. Furthermore, since heating by laser irradiation is non-contact heating, heat is removed quickly after heating and the temperature is difficult to maintain. Therefore, when laser irradiation heating is performed as localized rapid heating, in order to recover or recrystallize the cold-worked structure in the laser-irradiated area, it is necessary to control the conditions so that not only the surface of the steel sheet but also the interior of the steel sheet is heated, and the cooling rate is slowed to relatively coarsen the subgrain structure in the laser-irradiated area. The laser irradiation conditions for localized rapid heating are completely different from the laser irradiation conditions for magnetic domain refinement. For example, the laser irradiation conditions for magnetic domain refinement (usually 0.5 to 50 mJ / mm ２ ) is applied to this embodiment, the structure of the locally heated region is not controlled favorably, and coarse Goss-oriented grains close to the ideal Goss orientation are not formed in the steel sheet after the decarburization annealing process. The same applies to electron beam irradiation as to laser 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 controlled favorably, and coarse Goss-oriented grains close to the ideal Goss orientation are not formed in the steel sheet after the decarburization annealing process.

[0122] However, 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 rapid heating is performed by laser or electron beam irradiation, it is preferable to make the beam shape elliptical or adjust the scan speed, etc., so as to ensure a temperature range in which the crystal grains in that region can grow after rapid heating.

[0123] Conventionally, laser irradiation or electron beam irradiation has been performed on electrical steel sheets to control magnetic domains after secondary recrystallization and reduce iron loss. Under these laser or electron beam irradiation conditions, the steel sheet is heated to a depth of approximately 20 μm from the surface, resulting in rapid heat dissipation after heating. Therefore, under conventional conditions, it is difficult to ensure the required holding time from reaching the maximum temperature to cooling to 700°C. In other words, under conventional laser or electron beam irradiation conditions, although the irradiated area may be able to act as a barrier to the growth of secondary recrystallized grains, it is difficult to form coarse Goss-oriented grains with an orientation close to the ideal Goss orientation, as in the present embodiment. To form coarse Goss-oriented grains close to the ideal Goss orientation by laser or electron beam irradiation, the conditions for laser or electron beam irradiation must be considered as described above.

[0124] (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 heat-affected zone, increases the grain size of the Goss-oriented grains, and preferably forms coarse Goss-oriented grains with an orientation close to the ideal Goss orientation. The decarburization annealing conditions may be a heating rate of 10 to 10,000°C / s, an annealing temperature of 700 to 900°C, and an annealing time of 10 to 600 seconds. Furthermore, 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 humid atmosphere to remove "C" contained in the cold-rolled steel sheet.

[0125] It is also 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.

[0126] Furthermore, in this embodiment, many coarse Goss-oriented grains with orientations close to the ideal Goss orientation are formed in the local heat-affected zone. Therefore, there is no need to form Goss-oriented grains by decarburization annealing. As described above, the heating rate for decarburization annealing may be 10,000°C / s or less. However, it is preferable to use a slower heating rate for decarburization annealing to form {111}<112>-oriented grains and {411}<148>-oriented grains that are easily eroded into the matrix portion (local heat-unaffected zone). For example, the heating rate for decarburization annealing is preferably 300°C / s or less, more preferably 200°C / s or less, and even more preferably 100°C / s or less.

[0127] The amount of decarbonation and the state of the surface oxide layer affect the formation of the glass film. Therefore, in decarburization annealing, the degree of oxidation (PH) in the annealing atmosphere (furnace atmosphere) is important. ２ O / PH ２ ) may be set to 0.01 to 0.15. ２ O / PH ２ is the partial pressure of water vapor in the atmosphere, PH ２ O and hydrogen partial pressure PH ２ It can be defined as the ratio of

[0128] The decarburization-annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment can be manufactured by the above-described steps. The decarburization-annealed steel sheet according to this embodiment, which is manufactured by controlling the conditions of each step in a composite and inseparable manner, has coarse Goss-oriented grains formed in the steel sheet in a preferred form.

[0129] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet using the decarburization annealed steel sheet according to this embodiment will be described.

[0130] (Nitriding) Nitriding is an effective treatment in low-temperature slab heating processes where 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 approximately 40 to 200 ppm between the start of decarburization and the start of secondary recrystallization in finish annealing. Examples of nitriding include annealing in an atmosphere containing a nitriding gas such as ammonia, and finish annealing of a decarburized annealed steel sheet coated with an annealing separator containing a nitriding powder such as MnN. The amount of nitride after nitriding is preferably 130 to 350 ppm, and more preferably 150 to 250 ppm.

[0131] (Annealing separator application step) The annealing separator application step is a step of applying an annealing separator to a decarburized annealed steel sheet. As the annealing separator, for example, an annealing separator containing MgO as a main component can be used. After the annealing separator is applied, the decarburized annealed steel sheet is wound into a coil and then finish-annealed in the subsequent finish-annealing step.

[0132] (Finish annealing process) The finish annealing process is a process in which the decarburized annealed steel sheet coated with the annealing separator is subjected to finish annealing (secondary annealing) to cause secondary recrystallization. In this process, the growth of primary recrystallized grains is suppressed by an inhibitor, and secondary recrystallization is allowed to proceed, thereby preferentially growing {100}<001> oriented grains and dramatically improving magnetic flux density.

[0133] The heating rate during the heating process in the finish annealing step is not particularly limited, but is preferably 3 to 7°C / hr (°C / hour). Furthermore, the finish annealing may be performed by holding the steel sheet in a temperature range of 1000°C to 1300°C for 10 hours to 60 hours. The atmosphere during the finish annealing may be, for example, a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen.

[0134] (Insulating Coating Forming Step) 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.

[0135] (Other) The grain-oriented electrical steel sheet may be subjected to a magnetic domain refinement treatment to form localized micro-strain 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 decarburization annealed steel sheet according to this embodiment has excellent magnetic flux density, but because coarsening of secondary recrystallized grains is suppressed, the magnetic domains are refined even without the magnetic domain refinement treatment.

[0136] Next, the effects of the present invention will be specifically described 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. The present invention can adopt various conditions as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0137] Using the slabs with adjusted chemical compositions as raw materials, decarburized annealed steel sheets having the chemical compositions shown in Table 2 were manufactured. These chemical compositions were measured based on the above-mentioned method. In Table 2, "-" indicates that the content was not measured, and that control and manufacturing were not carried out with the content in mind.

[0138] When producing the above-mentioned decarburized annealed steel sheet, a slab was heated to 1150°C and subjected to hot rolling to obtain a hot-rolled steel sheet having a thickness of 2.6 mm. This hot-rolled steel sheet was then subjected to hot-rolled sheet annealing by heating to 1100°C and subsequently annealing at 900°C, and then subjected to pickling to remove scale formed on the surface. These steel sheets were subjected to a single cold rolling or multiple cold rollings with intermediate annealing in between to obtain a cold-rolled steel sheet having a final thickness of 0.22 mm.

[0139] The above cold-rolled steel sheets were subjected to localized rapid heating under the conditions shown in Tables 3 to 6. In spot current heating, a copper electrode with a diameter of 1 mm was used in contact with the steel sheet. The electrode shape, electrode pressure, current, current application time, and electrode holding time after current application 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 at 700°C or higher). In laser heating, a fiber laser was used, and the focused spot diameter of the laser beam in the rolling direction (i.e., the diameter containing 86% of the laser output) was set to 30 μm (except for Tests No. 43 and No. 44). The laser irradiation energy density, laser scanning speed, and shape of the laser irradiation region were varied in a composite manner to control the heating rate, maximum temperature, and holding time at 700°C or higher. 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 at 700°C or higher.

[0140] In Test No. 41, the laser beam was elongated in the scanning direction, and the holding time from reaching the maximum temperature until cooling to 700°C during local rapid heating was controlled to 0.2 seconds, so that coarse Goss-oriented grains with an orientation close to the ideal Goss orientation were formed in the locally heated region. In Test No. 42, general laser irradiation conditions for magnetic domain refinement were applied. In Test No. 43, the focused spot diameter was set to 0.5 mm, and the laser irradiation energy density was set to 2.0 J / mm. ２ In Test No. 44, the laser beam was focused at a spot diameter of 0.5 mm and a laser irradiation energy density of 30.0 J / mm. ２ The laser beam was controlled so that the laser beam irradiated area would ultimately be the grain boundary of the secondary recrystallized grains.

[0141] In the table, "untreated" indicates that the localized rapid heating method was not performed. The localized heating conditions "linear or spotted" represent the shape of the localized heated region on the surface of the cold-rolled steel sheet, and the localized heating conditions "rolling direction spacing" and "width direction spacing" represent the spacing in the rolling direction and transverse direction at which the localized heated region is arranged on the surface of the cold-rolled steel sheet. The localized heated region is evenly spaced at equal intervals in the rolling direction and transverse direction. For example, if both the "rolling direction spacing" and the "width direction spacing" are 100 mm or less, a localized heated region is arranged in all divided sections. On the other hand, if either the "rolling direction spacing" or the "width direction spacing" exceeds 100 mm, there is a divided section in which a localized heated region is not arranged. Note that, among all divided sections, the divided sections in which a localized heated region is provided are arranged so as not to be unevenly distributed.

[0142] The cold-rolled steel sheets after localized rapid heating were subjected to decarburization annealing under the conditions shown in Tables 3 to 6. In the decarburization annealing, the degree of oxidation (PH) in the annealing atmosphere (furnace atmosphere) was ２ O / PH ２ ) was set to 0.13.

[0143] For the manufactured decarburized annealed steel sheet, the area ratio of the divided small section group I to the area of ​​the divided section, A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ The values ​​of , and the area ratio of the corresponding orientation grains contained in the divided subdivision group II to the area of ​​the divided subdivision group II were measured based on the above-mentioned method. The results are shown in Tables 7 to 10.

[0144] The produced decarburized annealed steel sheets were nitrided at 750°C in a nitrogen-hydrogen-ammonia atmosphere to adjust the nitrogen content of the steel sheets to 220 ppm. Furthermore, an annealing separator mainly composed of MgO was applied, and then finish annealing was performed. In the finish annealing, the steel sheets were heated to 1000°C at a rate of 15°C / hr (°C / hr) in a mixed atmosphere of hydrogen and nitrogen, then heated to 1200°C at the heating rates shown in Tables 3 to 6, and held at 1200°C for 20 hr (hours) in a hydrogen atmosphere.

[0145] After the finish annealing, the steel sheet was coated with a coating solution for an insulating film, which was mainly composed of colloidal silica and phosphate, with chromic anhydride added as needed, and baked to form an insulating film.

[0146] The obtained grain-oriented electrical steel sheets were evaluated for various properties. The evaluation results are shown in Tables 7 to 10. In addition, the chemical composition of the base steel sheet of the obtained grain-oriented electrical steel sheets was analyzed. The results are shown in Table 11.

[0147] (1) Secondary recrystallized grain size of grain-oriented electrical steel sheet The secondary recrystallized grain size of the grain-oriented electrical steel sheet was measured based on the cutting method specified in JIS G0551:2020. Note that the secondary recrystallized grain size (average grain size) in the rolling direction was measured as the secondary recrystallized grain size. A secondary recrystallized grain size (average grain size) in the rolling direction of 65 μm or less was judged to be acceptable.

[0148] (2) Magnetic Properties of Grain-Oriented Electrical Steel Sheets The magnetic properties of the grain-oriented electrical steel sheets were measured based on the Single Sheet Tester (SST) specified in JIS C 2556:2015.

[0149] From the obtained grain-oriented electrical steel sheets, 20 samples for single sheet magnetic measurement, each measuring 100 mm x 500 mm, were prepared, and single sheet magnetic measurement was carried out. As the magnetic properties, the magnetic flux density B ８ (T) was measured. ８ For reference, the iron loss W, defined as the power loss per unit weight (1 kg) of the steel sheet under the conditions of AC frequency: 50 Hz, excitation magnetic flux density: 1.7 T, was １７／５０ (W / kg) was measured.

[0150] Among Nos. 1 to 84, the decarburization annealed steel sheets according to the present invention have the following characteristics: the area ratio of the divided small section group I to the area of ​​the divided section and A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳThe values ​​of were preferably controlled. All of these examples of the present invention, as grain-oriented electrical steel sheets, exhibited excellent magnetic flux density despite the suppression of coarsening of the secondary recrystallized grains. Furthermore, because the grain-oriented electrical steel sheets suppressed the suppression of coarsening of the secondary recrystallized grains, they also exhibited excellent iron loss and magnetostriction.

[0151] On the other hand, among Nos. 1 to 84, the decarburization annealed steel sheets as comparative examples have the following characteristics: the area ratio of the divided small section group I to the area of ​​the divided sections and the area ratio A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ The value of was not controlled favorably. In these comparative examples, favorable magnetic flux density was not obtained as a grain-oriented electrical steel sheet.

[0152]

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159]

[0160]

[0161]

[0162] According to the above-described aspects of the present invention, it is possible to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density while suppressing coarsening of secondary recrystallized grains, and therefore has high industrial applicability.

[0163] REFERENCE SIGNS LIST 1 Steel plate 2 Local heat affected zone 3 Local heat unaffected zone (matrix part) 4 Boundary of local heat affected zone 5 Boundary of local heat affected zone 14 Goss oriented grain 15 Coarse Goss oriented grain 21 Rolling direction 22 Width direction (direction perpendicular to rolling)

Claims

1. A decarburization annealed steel sheet for grain-oriented electrical steel sheet, wherein an area of ​​at least 100 mm x 100 mm on the sheet surface is defined as a divided section, and the divided section is divided into 1 mm x 1 mm areas, each of which is defined as a divided small section, the average grain size of the crystal grains contained in the divided small section is defined as aveD in μm, and the ratio of the number of crystal grains having a grain size of 3.0 x aveD or more to the number of crystal grains contained in one divided small section is defined as F. Ｃ aveF is defined as the ratio of the number of crystal grains having a grain size of 3.0×aveD or more to the number of crystal grains contained in the divided section. Ｃ Define F Ｃ ÷aveF Ｃ When the divided small sections having a value of 1.3 or more are collectively defined as divided small section group I, the area ratio of the divided small section group I to the area of ​​the divided sections is 0.05% or more and 50% or less, and the deviation angle from the ideal Goss orientation about the normal direction to the rolling surface as the rotation axis is defined as α, the deviation angle from the ideal Goss orientation about the direction perpendicular to the rolling surface as the rotation axis is defined as β, the deviation angle of the crystal orientation measured at a measurement point on the sheet surface is represented as (α β), and the angle deviation at the measurement point is represented 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, and an area ratio of the Goss oriented grains contained in the divided small section group I to an area of ​​the divided small section group I is defined as A Ｉ ＧＯＳＳ and the area ratio of the Goss oriented grains contained in the divided section to the area of ​​the divided section is defined as aveA ＧＯＳＳ When defined as above, A Ｉ ＧＯＳＳ ÷aveA ＧＯＳＳ A decarburization annealed steel sheet for grain-oriented electrical steel sheet, characterized in that the value of is 1.3 or more.

2. A decarburization annealed steel sheet for grain-oriented electrical steel sheet as described in claim 1, characterized in that when crystal grains whose orientation difference from a crystal orientation that has a Σ9 corresponding orientation relationship with the ideal Goss orientation are within 10° are defined as corresponding orientation grains, and when the divided subdivisions among the divided subdivisions excluding divided subdivision group I are defined as divided subdivision group II, the area ratio of the corresponding orientation grains contained within divided subdivision group II to the area of ​​divided subdivision group II is 10% or more and 50% or less.

Images