WO2024154774A1 - Method for manufacturing grain-oriented electromagnetic steel sheet - Google Patents

Abstract

This method for manufacturing a grain-oriented electromagnetic steel sheet comprises: a hot rolling step; a hot-rolled sheet annealing step; a cool rolling step; a decarburization annealing step; an annealing separator application step; a nitriding step; and a finishing annealing step. The finishing annealing step includes a heating process and a homogenizing process, and at least in a certain period from the start of secondary recrystallization in the heating process until the completion of the secondary recrystallization, a temperature gradient of 0.5℃/cm or more is generated in a boundary area between a primary recrystallization area and a secondary recrystallization area. The nitriding step is performed through annealing in an atmosphere containing nitriding gas in at least one stage during the decarburization annealing step, between the decarburization annealing step and the finishing annealing step, or until the start of the secondary recrystallization in the heating process of the finishing annealing step. The nitrogen content of the steel sheet after the nitriding step is, on a mass basis, 210 ppm or more.

Description

Manufacturing method of grain-oriented electrical steel sheet

The present invention relates to a method for producing a grain-oriented electrical steel sheet.
This application claims priority based on Japanese Patent Application No. 2023-005565, filed on January 18, 2023, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheets (also called unidirectional silicon steel sheets) are soft magnetic materials and are mainly used as iron core materials for transformers. For this reason, grain-oriented electrical steel sheets are required to have low energy loss (low iron loss). For example, magnetic flux density: B8 (magnetic flux density in a magnetic field of 800 A/m) is the largest governing factor of iron loss characteristics. It is known that the higher the magnetic flux density: B8 value, the lower the iron loss and the better the iron loss characteristics. In addition, the higher the magnetic flux density: B8 value, the more compact the iron core can be, which is advantageous in terms of the device configuration of the transformer and also in terms of the manufacturing costs of the transformer. In order to increase the magnetic flux density: B8 value, it is important to highly align the crystal orientation. This control of the crystal orientation is achieved by utilizing a catastrophic grain growth phenomenon called secondary recrystallization.

Many inventions related to high magnetic flux density grain-oriented electrical steel sheets have been proposed since ancient times, but research into industrially produced grain-oriented silicon steel sheets has shown that their magnetic flux density (B8) does not reach the theoretical upper limit for silicon steel, and there is still room for significant improvement.

Regarding the improvement of magnetic flux density, for example, Patent Document 1 discloses a method for manufacturing grain-oriented electrical steel sheet with high magnetic flux density, which is characterized by heating a silicon steel slab containing C: 0.015% or less, Si: 4% or less, S: 0.012% or less, acid-soluble Al: 0.020-0.065%, and T.N: 0.0030-0.0095% to 1270°C or less, hot-rolling it into a hot-rolled sheet, coiling it at 700-950°C, and cold-rolling it with a reduction of 65% or more, subjecting this steel sheet to primary recrystallization annealing for a short period of time, and then subjecting the steel sheet to high-temperature finish annealing, which includes a process for growing secondary recrystallized grains while applying a temperature gradient of 2°C/cm or more to the steel sheet at the boundary between the primary recrystallization region and the secondary recrystallization region.

Patent Document 2 also discloses a method for producing ultra-low iron loss grain-oriented silicon steel sheet, which achieves crystal orientation control through secondary recrystallization and smoothing of the steel sheet surface, even with thin-thickness materials (e.g., 0.13 mm), which have been difficult to produce in the past, to produce ultra-low iron loss grain-oriented electrical steel sheet at low cost. Patent Document 2 discloses that in order to ensure a temperature gradient of at least 2°C/cm, it is necessary to raise the temperature to 1000-1100°C during finish annealing at a heating rate of 50°C/hr or more.

For example, Patent Document 3 discloses a method for manufacturing grain-oriented silicon steel sheet (strip) with high magnetic flux density, characterized in that in the manufacturing process of the grain-oriented silicon steel sheet (strip), secondary recrystallization is advanced while a temperature gradient is applied to the steel sheet (strip) in the boundary region between the primary recrystallization region and the secondary recrystallization region.

Furthermore, for example, Patent Documents 4 and 5 disclose equipment and methods for imparting a temperature gradient to a coiled steel sheet.

Japan Special Publication No. 59-41488 Japanese Patent Application Publication No. 5-311238 Japan Special Publication No. 58-50295 Japanese Patent Publication No. 57-164935 Japanese Patent Publication No. 58-1019

As described above, the techniques described in Patent Documents 1 and 2 indicate that a temperature gradient of 2° C./cm or more must be applied during finish annealing. When applying a temperature gradient, it is possible to apply the equipment and methods disclosed in Patent Documents 4 and 5, etc., but with the equipment and methods disclosed therein, it is difficult to control the temperature gradient to be constant over the entire area in the longitudinal and transverse directions of an industrial-scale coil, and to control the temperature gradient to be high over the entire area, and there are regions with low temperature gradients of about 0.5° C./cm (small temperature gradients). In such regions with low temperature gradients, it is difficult to obtain a sufficient effect of improving the magnetic flux density.
When the inventors calculated the temperature gradient of each part of the coil by simulation using Fluent (registered trademark) manufactured by ANSYS, a known heat transfer calculation software, it was found that even if a temperature gradient of 2 ° C./cm or more is applied, some parts of the coil have a low temperature gradient of about 0.5 ° C./cm (small temperature gradient). In particular, when a temperature gradient is applied to the coil, it was found that the temperature gradient is likely to be small inside the coil compared to the outside, especially when a large temperature gradient is applied. That is, even if the temperature gradient is 2 ° C./cm or more outside the coil, some areas such as the inside of the coil have a low temperature gradient of less than 2 ° C./cm. In addition, when a temperature gradient is applied in the width direction of the coil, the temperature gradient is likely to be small on the low temperature end side compared to the high temperature end side. For example, even if the temperature gradient is 2 ° C./cm or more on the high temperature end side of the coil, some areas such as the low temperature end side may have a low temperature gradient of less than 2 ° C./cm. Therefore, it is difficult to provide a temperature gradient of 2 ° C./cm or more over the entire coil.
For these reasons, in order to obtain a sufficient improvement in magnetic flux density over the entire coil, a method has been desired that can obtain the effect of improving magnetic flux density even with a smaller temperature gradient.

Patent Document 3 discloses that applying a temperature gradient of 0.5°C/cm improves the B8 characteristics. However, Patent Document 3 also indicates that a significant effect is obtained at 2°C/cm or more. In fact, the B8 value of a grain-oriented electrical steel sheet with a Si content of 2.95% at a temperature gradient of 0.5°C/cm is roughly 1.92T, and although a certain degree of improvement in magnetic flux density is obtained, this is not sufficient to meet the increasingly sophisticated requirements of recent years.

The present invention was made in consideration of the above problems. The present invention is a manufacturing method for grain-oriented electrical steel sheets that produces grain-oriented electrical steel sheets with high magnetic flux density by performing finish annealing while applying a temperature gradient to the boundary region between the primary recrystallized region and the secondary recrystallized region, and the objective of the present invention is to provide a manufacturing method for grain-oriented electrical steel sheets with stable high magnetic flux density throughout the entire coil by achieving a sufficient magnetic flux density improvement effect even with a small temperature gradient.

The inventors have investigated a method in which finish annealing is performed while applying a temperature gradient to the boundary region between the primary recrystallized region and the secondary recrystallized region, which can provide a sufficient improvement in magnetic flux density even if the temperature gradient is relatively small (even if the lower limit of the temperature gradient is small, in the case where there are regions with a large temperature gradient and regions with a small temperature gradient).
As a result, it was found that by setting the primary recrystallized grain size to 15 μm or less and setting the nitrogen content of the steel sheet before secondary recrystallization to 210 ppm or more by a nitriding treatment process, a sufficient improvement in magnetic flux density can be obtained even if the temperature gradient is relatively small.

The present invention has been made in view of the above findings.
[1] A method for producing a grain-oriented electrical steel sheet according to one embodiment of the present invention includes a hot rolling process in which a silicon steel material containing, by mass%, Si: 0.80 to 7.00% is heated to a temperature of more than 1300°C and then hot-rolled to obtain a hot-rolled sheet, a hot-rolled sheet annealing process in which the hot-rolled sheet is annealed, a cold rolling process in which the hot-rolled sheet after the hot rolling process or after the hot-rolled sheet annealing process is cold-rolled to obtain a steel sheet of a final sheet thickness, a decarburization annealing process in which the steel sheet after the cold rolling process is decarburized and annealed to obtain a steel sheet having a primary recrystallized grain size of 15 μm or less, an annealing separator application process in which an annealing separator is applied to the steel sheet after the decarburization annealing process and then wound into a coil, a nitriding treatment process in which the amount of nitrogen in the steel sheet is increased, and a nitriding treatment process in which the steel sheet is wound into a coil. and a finish annealing process in which the steel sheet subjected to the nitriding process is finish-annealed, the finish annealing process having a temperature rise process and a soaking process, in which a temperature gradient of 0.5°C/cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least for a period from the start of secondary recrystallization to the completion of the secondary recrystallization in the temperature rise process, the nitriding process is performed by annealing in an atmosphere containing a gas capable of nitriding in at least one stage of the following: during the decarburization annealing process, between the decarburization annealing process and the finish annealing process, or during the temperature rise process of the finish annealing process up to the start of secondary recrystallization, and the nitrogen content of the steel sheet after the nitriding process is set to 210 ppm or more by mass.
[2] In the method for producing a grain-oriented electrical steel sheet according to [1], the chemical composition of the silicon steel material may contain, in mass%, Si: 0.80 to 7.00%, C: 0.15% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0.01 to 0.50%, S and Se: 0.01 to 0.05% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, and the balance being Fe and impurities.

According to the above aspect of the present invention, a method for manufacturing grain-oriented electrical steel sheets that produces grain-oriented electrical steel sheets with high magnetic flux density by performing finish annealing while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region can be provided, and a method for manufacturing grain-oriented electrical steel sheets that can achieve a sufficient increase in magnetic flux density even with a small temperature gradient can be provided.

Hereinafter, a method for producing a grain-oriented electrical steel sheet according to an embodiment of the present invention (a method for producing a grain-oriented electrical steel sheet according to the present embodiment) will be described.
The method for producing the grain-oriented electrical steel sheet according to this embodiment is as follows:
(i) a hot rolling step of heating a silicon steel material having a predetermined chemical composition to a temperature exceeding 1300°C and then hot rolling the material to obtain a hot-rolled sheet;
(ii) a hot-rolled sheet annealing step of annealing the hot-rolled sheet;
(iii) a cold rolling process in which the hot-rolled sheet after the hot rolling process or the hot-rolled sheet annealing process is subjected to cold rolling to obtain a steel sheet having a final plate thickness;
(iv) a decarburization annealing step of decarburizing the steel sheet after the cold rolling step to obtain a steel sheet having a primary recrystallized grain size of 15 μm or less;
(v) an annealing separator application step of applying an annealing separator to the steel sheet after the decarburization annealing step, and then winding the steel sheet into a coil shape;
(vi) a nitriding process for increasing the nitrogen content of the steel plate;
(vii) a finish annealing step of finish annealing the steel sheet wound into a coil shape;
Equipped with.
The conditions for each step will be described below.

[Hot rolling process]
In the hot rolling process, a silicon steel material such as a slab having a chemical composition described below is heated to a temperature of more than 1300° C. and then hot rolled to obtain a hot-rolled sheet.
By setting the heating temperature to more than 1300° C., the precipitates acting as inhibitors are completely solidified and finely precipitated in the hot rolling and the subsequent annealing process, thereby suppressing grain growth from the decarburization annealing process to the finish annealing process. The heating temperature may be 1310° C. or higher, or 1350° C. or higher.
Although there is no upper limit to the heating temperature, if the heating temperature is excessively high, good magnetic properties may not be obtained due to secondary recrystallization defects. Therefore, the heating temperature is preferably 1450° C. or less.
The hot rolling conditions other than the heating temperature are not limited, and may be determined within known ranges depending on the required properties, etc.

The silicon steel material to be subjected to hot rolling is obtained by melting steel in a converter or an electric furnace or the like, subjecting the molten steel to vacuum degassing treatment as necessary, and then continuously casting or making the steel into an ingot and then blooming and rolling it.
This silicon steel material contains, by mass%, 0.80 to 7.00% Si. Preferably, the chemical composition contains, by mass%, 0.80 to 7.00% Si, 0.15% or less C, 0.010 to 0.065% acid-soluble Al, 0.004 to 0.012% N, 0.01 to 0.50% Mn, 0.01 to 0.05% S and Se in total, 0 to 0.30% Cr, 0 to 0.4% Cu, 0 to 0.5% P, 0 to 1.00% Ni, with the balance being Fe and impurities.
The reason for the content of each element will be explained below. In the following, % regarding the content is mass %.

(Si: 0.80-7.00%)
If the Si content is less than 0.80%, γ transformation occurs during final annealing, damaging the crystal orientation of the steel sheet. In addition, the inclusion of Si increases electrical resistance and improves iron loss characteristics. Therefore, the Si content in silicon steel material is set to 0.80% or more. The Si content is preferably 1.50% or more, more preferably 2.00% or more, and even more preferably 2.50% or more. That's all.
On the other hand, if the Si content exceeds 7.00%, cold rolling becomes extremely difficult and there is a risk of cracking during rolling. Therefore, the Si content is set to 7.00% or less. Therefore, the Si content may be set to 4.80% or less, or 4.00% or less.

(C: 0.15% or less)
C is an effective element for controlling the primary recrystallization structure, but it has a negative effect on magnetic properties, so it is necessary to decarburize it before final annealing. If the C content exceeds 0.5%, the decarburization annealing time becomes long, which impairs the productivity in industrial production. Therefore, the C content is preferably 0.15% or less. The C content is more preferably 0.5% or less. The lower limit of the C content is not particularly limited, but considering the productivity in industrial production and the magnetic properties of the product, the C content is preferably 0.02% or more. , more preferably 0.03% or more, and further preferably 0.05% or more.

(Acid-soluble Al: 0.010-0.065%)
Acid-soluble Al (sol. Al) is an element that combines with N to form AlN or (Al,Si)N, and functions as an inhibitor. The range in which secondary recrystallization is stable is The amount of acid-soluble Al is preferably 0.010 to 0.065%. The acid-soluble Al content may be 0.040% or less, and further may be 0.030% or less.

(N: 0.004-0.012%)
N is an element that combines with Al and functions as an inhibitor. If the N content is less than 0.004%, a sufficient amount of inhibitor cannot be obtained. Therefore, the N content is set to 0. The N content is preferably 0.004% or more, more preferably 0.006% or more, and further preferably 0.007% or more.
On the other hand, if the N content exceeds 0.012%, voids called blisters may occur in the steel sheet during cold rolling, so the N content is preferably 0.012% or less.

The chemical composition of the silicon steel material may contain the above elements with the balance being Fe and impurities. On the other hand, in order to improve various properties, the following elements may be further contained.
The following elements may be contained as impurities within the ranges described below.

(Mn: 0.01-0.50%)
Mn is an element that functions as an inhibitor by becoming MnS and MnSe. If the Mn content is less than 0.01%, a sufficient amount of inhibitor cannot be obtained. Therefore, the Mn content is set to 0.01% or less. The Mn content is preferably 0.03% or more, more preferably 0.06% or more.
On the other hand, if the Mn content exceeds 0.50%, it is not preferable because it becomes difficult for Mn to dissolve in the silicon steel material when it is heated. Also, if the Mn content exceeds 0.50%, the inhibitor MnS However, this is not preferred because the precipitate size of MnSe is likely to become coarse, and the optimum size distribution as an inhibitor is lost. Therefore, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0. It is preferably 30% or less, and more preferably 0.28% or less.
Mn is an element that has the effect of increasing resistivity and reducing iron loss. Mn is also an element that is effective in preventing the occurrence of cracks during hot rolling that are caused by S and Se. In order to prevent the occurrence of cracks, it is preferable that the Mn content be in a range that satisfies Mn/(S+Se)≧4 in relation to the total amount of S and Se.

(S and Se: 0.01 to 0.05% in total)
S and Se form an inhibitor together with the above-mentioned Mn. If the total content of S and Se is less than 0.01%, a sufficient amount of inhibitor cannot be obtained. Therefore, it is preferable that the total content of S and Se is 0.01% or more. It is more preferable that the total content of S and Se is 0.02% or more.
On the other hand, if the total content of S and Se exceeds 0.05%, it causes hot brittleness and makes rolling extremely difficult. Therefore, the total content of S and Se is preferably 0.05% or less. The total content of S and Se is preferably 0.04% or less.

(Cr: 0-0.30%)
Cr is an element that adjusts the composition and amount of the oxide layer produced by decarburization annealing to a preferable state and promotes the formation of a glass film, and therefore may be contained.
On the other hand, if the Cr content exceeds 0.30%, decarburization is inhibited, so the Cr content is preferably set to 0.30% or less.

(Cu: 0-0.4%)
Cu is an element that is effective for increasing resistivity and reducing core loss, and therefore may be contained.
On the other hand, if the Cu content exceeds 0.4%, the iron loss reduction effect is saturated and it becomes a cause of surface defects called "copper scuffs" during hot rolling. Therefore, the Cu content is set to 0.4% or less. It is preferred.

(P: 0-0.5%)
P is an element that is effective in increasing resistivity and reducing core loss, and therefore may be contained.
On the other hand, if the P content exceeds 0.5%, the rollability decreases, so the P content is preferably set to 0.5% or less.

(Ni: 0-1.00%)
Ni is an effective element for increasing resistivity and reducing iron loss. It is also an effective element for controlling the metal structure of the hot-rolled sheet and improving the magnetic properties. This is also fine.
On the other hand, if the Ni content exceeds 1.00%, the secondary recrystallization becomes unstable, so the Ni content is preferably 1.00% or less.

In addition to the elements mentioned above, impurities such as B, O, Mg, Ca, Ti, Mo, V, Nb, Sn, Sb, and Bi may be contained in an amount of 0.10% or less each. Impurities refer to elements that are mixed in from the raw materials or during the manufacturing process and do not have a clear effect on the properties of the grain-oriented electrical steel sheet obtained by the manufacturing method for grain-oriented electrical steel sheet according to this embodiment.

The chemical composition of the silicon steel material may be measured by a known method.
For example, the measurement may be performed using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Al may be measured as acid-soluble Al by ICP-AES using the filtrate obtained by thermally decomposing a sample with acid. In addition, Si may be measured using the silicon dioxide gravimetric method, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method. O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
The above chemical composition is that of the silicon steel sheet as the base material. If the grain-oriented electrical steel sheet to be measured has a glass coating or an insulating coating on the surface, the coating is removed by a known method before measuring the chemical composition.

[Hot-rolled sheet annealing process]
After the hot rolling process, the hot rolled sheet obtained by hot rolling is annealed (hot rolled sheet annealing) to improve the magnetic properties. If hot rolled sheet annealing is not performed, sufficient magnetic properties cannot be obtained. The annealing conditions may be, for example, 900 to 1200°C and holding for 30 seconds to 30 minutes. The annealing temperature may be 950 to 1050°C.

[Cold rolling process]
In the cold rolling process, the hot-rolled sheet after the hot rolling process or after the hot-rolled sheet annealing process is cold-rolled to produce a steel sheet (cold-rolled sheet) with the same thickness as the final sheet thickness (the sheet thickness when it becomes a grain-oriented electrical steel sheet (however, if a glass coating or insulating coating is formed on the surface, the sheet thickness of the base steel sheet excluding these)). Cold rolling can be a single cold rolling (a series of cold rolling without intermediate annealing in between) or multiple cold rolling with annealing (intermediate annealing) in between.
In cold rolling, in order to develop a preferred primary recrystallization orientation, the final reduction is preferably 80% or more. The final reduction is the cumulative reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative reduction of cold rolling after the final intermediate annealing.

[Decarburization annealing process]
In the decarburization annealing process, the steel sheet after the cold rolling process is decarburized and annealed.
In the decarburization annealing, the steel sheet is primarily recrystallized and C, which adversely affects the magnetic properties, is removed from the steel sheet. In the manufacturing method of the grain-oriented electrical steel sheet according to the present embodiment, the primary recrystallized grain size (grain size of the primary recrystallized grains) is set to 15 μm or less. The primary recrystallized grain size may be set to 13 μm or less, or further to 10 μm or less.
In order to manufacture a steel sheet with a high magnetic flux density by applying a temperature gradient, it is necessary to preferentially grow only {110}<001> oriented grains with good magnetic properties along the temperature gradient from the high temperature side to the low temperature side, and for this purpose, it is necessary to appropriately control the grain growth by the primary recrystallized grain size and the inhibitor. By reducing the primary recrystallized grain size (grain size of the primary recrystallized grains), the driving force of grain growth during secondary recrystallization is increased. In addition, the inhibitor can be thermally stabilized by nitriding the steel sheet by the nitriding treatment described later. By increasing the driving force by setting the primary recrystallized grain size to 15 μm or less, and by setting the nitrogen content of the steel sheet to 210 ppm (0.0210 mass%) or more by mass basis to thermally stabilize the inhibitor, it becomes possible to preferentially grow only {110}<001> oriented grains with good magnetic properties even with a low temperature gradient.
In order to reduce the primary recrystallized grain size, the heating temperature of the silicon steel material in the above-mentioned hot rolling process is set to more than 1300° C., and then the annealing temperature and time in the decarburization annealing process are controlled. The annealing temperature in the decarburization annealing process is not limited, but may be, for example, 700° C. to 850° C., or may be 750° C. or more or 800° C. or less. The holding time at the annealing temperature is also not limited, but may be 10 to 600 seconds.

The primary recrystallized grain size after the decarburization annealing step is measured by the following method.
A sample is taken from the steel sheet after the decarburization annealing process and before the finish annealing process, and a cross section of the sample parallel to the rolling direction and parallel to the sheet thickness direction is observed by an optical microscope, and the average grain size (circle equivalent diameter) of the primary recrystallized grains in the entire thickness of the cross section is obtained by image analysis, and the average value is the primary recrystallized grain size. At this time, one or multiple fields of view are observed, and 500 or more grains are observed in order to suppress variation. When multiple fields of view are observed, it is desirable to observe two or more positions separated by a linear distance of 50 mm or more in order to suppress the influence of variation in the primary recrystallized grain size due to the position of the steel sheet.

[Annealing separator application process]
In the annealing separator application process, an annealing separator is applied to the steel sheet after the decarburization annealing process, and then the steel sheet is wound into a coil.
The annealing separator to be applied may be a known one, but is preferably one containing magnesia as a main component. By applying the annealing separator containing magnesia as a main component and then performing finish annealing, a glass coating (forsterite coating) is formed on the surface of the steel sheet.

[Nitriding process]
In the nitriding process, the nitrogen content of the steel sheet is increased. The nitriding process is carried out in at least one stage of the following: during the decarburization annealing process, between the decarburization annealing process and the finish annealing process, or from the start of the finish annealing process to the start of secondary recrystallization during the temperature increase process of the finish annealing process. Between the decarburization annealing process and the finish annealing process means the period from the completion of the decarburization annealing process to the start of the finish annealing process. The nitriding process is preferably carried out after the completion of the decarburization annealing process and before the start of the annealing separator application process.
However, regardless of the stage at which the nitriding is performed, the nitrogen content of the steel sheet after the final nitriding process must be 210 ppm (0.0210 mass%) or more on a mass basis. By increasing the nitrogen content of the steel sheet before the start of secondary recrystallization, the amount of inhibitor increases and the inhibitor becomes thermally stable. As a result, even with a relatively small temperature gradient, a sufficient magnetic flux density improvement effect can be obtained. The nitrogen content can be 250 ppm or more, and even 300 ppm or more. On the other hand, if the nitrogen content exceeds 350 ppm, the magnetic flux density improvement effect may be saturated, and it may be disadvantageous to purification after secondary recrystallization, so the nitrogen content is preferably 350 ppm or less.
Conventionally, when finish annealing is performed while applying a temperature gradient, the nitrogen content of the steel sheet is usually set to 200 ppm or less. For example, JP-A-59-215419 discloses that when secondary recrystallization annealing is performed while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region in final annealing, the nitrogen content in the steel sheet is set to 130 to 200 ppm at the start of secondary recrystallization. JP-A-59-215419 also describes that the effect of improving magnetic flux density is saturated at a nitrogen content of 180 to 200 ppm.
Furthermore, in the past, when silicon steel material was heated to high temperatures to utilize MnS and MnSe as inhibitors, a nitriding process was not usually carried out.
In response to this, the present inventors have found that, even in the case of high-temperature heating, when the primary recrystallized grain size is set to 15 μm or less and the nitrogen content is increased to 210 ppm or more, the lower limit of the temperature gradient at which a high magnetic flux density (for example, B8 is stably 1.940 T or more) can be achieved is expanded compared to the conventional case (high B8 can be stably obtained even with a temperature gradient of about 0.5° C./cm).

As a method for increasing the nitrogen content of a steel sheet, for example, there is a method in which the nitrogen content of the steel sheet is controlled by annealing the steel sheet in an atmosphere containing a gas having a nitriding ability.
In addition to the above, the nitrogen content of the steel sheet may be increased by adding a powder having nitriding ability, such as MnN, to the annealing separator during the temperature increase process of the final annealing step.
The nitrogen content of the steel sheet after nitriding can be measured by a known method using, for example, an oxygen/nitrogen/hydrogen analyzer (EMGA-930) manufactured by Horiba, Ltd. or an equivalent device. Known methods include general analytical methods such as inert gas fusion-thermal conductivity method. A sample of any size can be taken from the steel sheet after the nitriding process during the manufacturing process, and the nitrogen content can be measured using these devices and methods.

[Finish annealing process]
In the final annealing process, the coiled steel sheet is final annealed.
The final annealing step includes a temperature increasing process in which the steel sheet is heated to a final annealing temperature to cause secondary recrystallization, and a soaking process in which the steel sheet is held at the final annealing temperature.

<Temperature rise process>
In this final annealing step, in the above-mentioned nitriding step, a temperature gradient of 0.5°C/cm or more is generated in the boundary region between the primary recrystallized region and the secondary recrystallized region at least for a period from the start of secondary recrystallization to the completion of secondary recrystallization in the heating process, with the nitrogen content of the steel sheet controlled to 210 ppm or more by mass, and {110}<001> orientation grains are preferentially grown by secondary recrystallization. The same effect cannot be obtained if a temperature gradient is applied at a time other than the above, for example, before final annealing.
The temperature rise rate is not limited as long as it satisfies the above temperature gradient, but may be 50° C./h or less.

In finish annealing, secondary recrystallized grains are generated in the parts that are heated to the secondary recrystallization temperature or higher. When a steel sheet is heated in a state where a temperature gradient exists in a certain direction, secondary recrystallization progresses from the area where the temperature is above the secondary recrystallization temperature, and a region (boundary region) where primary recrystallized grains and secondary recrystallized grains are mixed is generated along the isotherm between the area where the primary recrystallized structure remains and the area where the secondary recrystallized structure remains and has not yet reached the secondary recrystallization temperature. As the steel sheet is heated and the temperature increases, this boundary region moves along the temperature gradient toward the area where the primary recrystallized structure remains, and the area where the secondary recrystallized structure has become larger, and finally the entire steel sheet is covered with secondary recrystallized grains. Throughout this process, the temperature of the boundary region is kept relatively constant. Regarding the direction of the temperature gradient, in finish annealing, since a coil-shaped grain-oriented electrical steel sheet is usually arranged in a furnace so that it is cylindrical, it is preferable to set a temperature gradient in the width direction of the steel sheet. When creating a temperature gradient in the width direction, the temperature gradient is formed in one direction across the entire width of the steel plate (so that one end is the high temperature end and the other end is the low temperature end).

As mentioned above, it is not easy to provide a temperature gradient of 2.0°C/cm or more throughout the entire coil, and there is a risk of problems with productivity and variations in properties within the steel sheet. However, in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, the nitrogen content of the steel sheet is 210 ppm or more at the start of secondary recrystallization. Therefore, the amount of inhibitor increases and the inhibitor becomes thermally stable, so that a sufficient magnetic flux density improvement effect can be obtained even with a relatively small temperature gradient. If there is variation in the temperature gradient, the lower limit of the temperature gradient at which a sufficient magnetic flux density improvement effect can be obtained can be reduced. However, if the temperature gradient is less than 0.5°C/cm, the magnetic flux density improvement effect cannot be sufficiently obtained. Therefore, the temperature gradient is set to 0.5°C/cm or more. If there is variation in the temperature gradient in each part of the coil or steel sheet, the minimum temperature gradient throughout the coil or steel sheet is set to 0.5°C/cm or more. There is no need to limit the upper limit of the temperature gradient, but if the temperature gradient exceeds 10.0°C/cm, the effect will saturate and the equipment load will increase, so the temperature gradient over the entire coil may be 10.0°C/cm or less. In the present application, a relatively small temperature gradient can provide a sufficient magnetic flux density improvement effect, so the temperature gradient over the entire coil may be 5.0°C/cm or less, or 2.0°C/cm or less, and if the temperature gradient is particularly uniform, it may be further set to 1.5°C/cm or less, or 1.0°C/cm or less. If the temperature gradient varies at each part of the coil or steel plate, the minimum temperature gradient over the entire coil or steel plate may be 5.0°C/cm or less, or 2.0°C/cm or less, and further set to 1.5°C/cm or less, or 1.0°C/cm or less.

Regarding the application of a temperature gradient, the temperature at the position that becomes the boundary region is not constant depending on the type of steel sheet and the annealing conditions, but the temperature of the boundary region can be known by confirming the temperature at which secondary recrystallization occurs under the assumed type of steel sheet and annealing conditions in a preliminary experiment, etc. Therefore, by applying a temperature gradient at a position that has a temperature close to the temperature of the boundary region thus examined, a temperature gradient can be applied to the boundary region between the primary recrystallization region and the secondary recrystallization region. For example, in the case of a steel sheet with a Si content of about 3 mass% that uses MnS and AlN as inhibitors, the temperature of the boundary region is about 900 to 1100°C.
When the boundary region is not clearly defined, a temperature gradient may be applied to a wider range or to the entire coil (steel sheet).
Although the effect can be obtained by providing a temperature gradient to the boundary region at least for a certain period from the generation to the growth of secondary recrystallized grains, in order to obtain a sufficient effect, it is preferable to provide a temperature gradient to the boundary region from the start of secondary recrystallization until the entire surface of the steel sheet is covered with secondary recrystallized grains (until the completion of secondary recrystallization). In other words, the temperature gradient may be generated from the beginning to the end of the temperature rise process of the finish annealing (until the soaking temperature is reached).

The temperature gradient can be imparted by raising the temperature with a temperature difference in the furnace, or by heating and/or cooling the coil end to raise the temperature with a temperature difference in the coil in the finish annealing furnace. Regarding the magnitude of the temperature gradient, for example, if a temperature gradient is imparted in the width direction of the coil, the temperature gradient of each part in the steel sheet can be calculated by measuring the temperature history by arranging sensors such as thermocouples at regular intervals in the width direction (intervals at which the temperature gradient can be measured, for example, 100 mm intervals). In addition, by calculating the temperature gradient of each part, the minimum value of the temperature gradient in the entire coil (entire area) can be obtained.
The temperature gradient also varies depending on the size of the furnace, the temperature difference in the furnace, the size and weight of the coil, and the like. In that case, as described above, the physical property values such as thermal diffusivity may be calculated using the results of actually measuring the temperature history of multiple parts of the coil, and the temperature gradient of each part of the coil may be calculated by simulation using, for example, Fluent (registered trademark) manufactured by ANSYS, Inc., a known heat transfer calculation software, etc. In the simulation, by setting various conditions, it is possible to calculate the temperature gradient of each part of the coil (for example, within any range of 100 mm intervals in the width direction of the coil) taking into account the variation in the temperature gradient. By calculating the temperature gradient of each part, the minimum value of the temperature gradient in the entire coil can be obtained.

In addition, when a difference in temperature gradient occurs in the radial direction of the coil, sensors such as thermocouples are arranged at regular intervals (intervals at which the difference in temperature gradient can be measured, for example, 100 mm intervals) in the width direction at multiple locations in the radial direction of the coil, and the temperature history in the width direction at each location is measured, so that the difference in the temperature gradient in the radial direction can be calculated. The minimum value of the temperature gradient of the entire coil can be obtained from the temperature gradient calculated at each measurement location in the radial direction of the coil. For example, when a temperature gradient is applied in the width direction of the coil, the measurement locations of the temperature history in the radial direction of the coil can be one or more measurement locations in the longitudinal direction of the coil on the steel plate located at the outermost position of the coil, the steel plate located at the middle part in the radial direction of the coil, and the steel plate located at the innermost position, for a total of three or more measurement locations. Similarly, the temperature gradient in the width direction at multiple locations in the radial direction of the coil (for example, each position at 100 mm intervals in the radial direction of the coil, or each of the outermost, middle, and innermost positions in the radial direction of the coil) can also be calculated by simulation.
In the case where the temperature gradient varies within the coil, the temperature gradient is likely to be relatively small at the low temperature end of the coil and is likely to be relatively small at the radially innermost position of the coil, and therefore the temperature gradient measured or calculated by simulation at the low temperature end of the coil and at the radially innermost position may be taken as the minimum temperature gradient of the entire coil.

<Heat treatment process>
In the soaking process, impurities such as N, S, and Se that are harmful to magnetic properties are purified. For this reason, the finish annealing temperature (soaking temperature) is preferably 1150 to 1250° C. In addition, the annealing time (soaking time) is preferably 10 to 30 hours after the low temperature side of the coil temperature gradient reaches the soaking temperature.

[Insulating film formation process]
The method for producing the grain-oriented electrical steel sheet according to the present embodiment may further include an insulating coating forming step of forming an insulating coating on the surface of the steel sheet. The insulating coating to be formed is not limited, and may be any known insulating coating. The coating may be of the above formula.

[Magnetic domain refining process]
The method for producing a grain-oriented electrical steel sheet according to this embodiment may further include a magnetic domain refining step of subjecting the steel sheet to magnetic domain refining.
By performing magnetic domain refining treatment, it is possible to further reduce the core loss of grain-oriented electrical steel sheet.
The method of magnetic domain subdivision is not limited, but examples of the method include a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction, and a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like stress distortion portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction.
In the case of forming the stress-strained portion, laser beam irradiation, electron beam irradiation, etc. can be applied. In the case of forming the groove portion, a mechanical groove forming method using gears, etc., a chemical groove forming method using electrolytic etching, and a thermal groove forming method using laser irradiation, etc. can be applied.
In cases where damage occurs to the insulating coating due to the formation of stress-distorted portions or grooves, causing deterioration of properties such as insulation, the insulating coating may be formed again to repair the damage.

Example 1
By casting, a silicon steel material is obtained containing, by mass%, 3.30% Si, 0.08% C, 0.027% acid-soluble Al, 0.008% N, 0.08% Mn, 0.02% S + Se, with the balance being Fe and impurity elements.
This silicon steel material is heated to 1280 to 1400° C. and held at that temperature for 1 hour, and then hot-rolled to produce a hot-rolled sheet having a thickness of 2.3 mm.
The hot-rolled sheets except for No. 7 are heated to 1000° C. and annealed for 60 seconds (hot-rolled sheet annealing).
This hot-rolled sheet is cold-rolled to a thickness of 0.22 mm to obtain a steel sheet (cold-rolled sheet). A sample steel sheet of 200 mm in the rolling direction and 600 mm in the width direction is cut out from this steel sheet.
Thereafter, the sample steel sheet is subjected to decarburization annealing by holding at 750 to 900°C for 100 seconds, and the primary recrystallized grain size is controlled to be 5 to 20 μm. (The primary recrystallized grain size is measured after the decarburization annealing step and before the finish annealing.) In addition, nitriding treatment is performed at least at one of the following times: during the temperature rise and soaking process in the decarburization annealing step, between the decarburization annealing step and the finish annealing step, or during the temperature rise process in the finish annealing step until the start of secondary recrystallization, and the nitrogen content after the final nitriding treatment step is controlled to be 160 to 380 ppm. In the table, for example, "between the decarburization annealing step and the finish annealing step" indicates that the nitriding treatment is carried out once "from the completion of the decarburization annealing step to the start of the finish annealing step", and "between the decarburization annealing step and the finish annealing step, and during the temperature rise process of the finish annealing step, up to the start of secondary recrystallization" indicates that the nitriding treatment is carried out twice, "from the completion of the decarburization annealing step to the start of the finish annealing step" and "from the start of the finish annealing step to the start of secondary recrystallization during the temperature rise process of the finish annealing step".
After applying an annealing separator mainly composed of MgO to the steel sheet after decarburization annealing, the steel sheet is subjected to finish annealing while applying a temperature gradient of 0 to 5.0°C/cm in the entire area in the direction perpendicular to the rolling direction (width direction) so that the end portion of the steel sheet is heated to a high temperature, and secondary recrystallization occurs. The average heating rate from the start of secondary recrystallization in the boundary region to the completion of secondary recrystallization is 10°C/hr, the finish annealing temperature is 1200°C, and the soaking time is 30 hours.
The temperature gradient is applied uniformly over the entire area of the sample steel sheet. The temperature gradient is applied by increasing the temperature with a temperature difference in the furnace. The magnitude of the temperature gradient is controlled by increasing the temperature while measuring the temperature at intervals of 100 mm in the width direction of the steel sheet.

A sample of 60 mm in the width direction and 200 mm in the rolling direction is taken from the obtained steel sheet, and magnetic measurements are performed on this sample using the SST method (see JIS C2556:2015 Annex JA) to measure the magnetic flux density B8 in the rolling direction.
The results are shown in Tables 1-1 to 1-3.

As can be seen from Tables 1-1 to 1-3, when the temperature gradient during final annealing is in the range of 0.5°C/cm or more, regardless of the number of nitriding treatments or the timing, if the amount of nitrogen after nitriding is 210 ppm or more and the primary recrystallized grain size is 15 μm or less, a high magnetic flux density (magnetic flux density B8 of 1.940 T or more) can be achieved.

Example 2
By casting, a silicon steel material (slab) having the chemical composition shown in Table 2 (unit: mass %, balance: Fe and impurities) is obtained.
This silicon steel material is heated to 1350° C. and held at that temperature for 1 hour, and then hot-rolled to produce a hot-rolled sheet having a thickness of 2.3 mm.
This hot-rolled sheet is subjected to hot-rolled sheet annealing at an annealing temperature of 950 to 1100° C. for 30 to 120 seconds.
The hot-rolled sheet after annealing is cold-rolled to a thickness of 0.22 mm to obtain a steel sheet (cold-rolled sheet). A sample steel sheet of 200 mm in the rolling direction and 600 mm in the width direction is cut out from this steel sheet.
Thereafter, the sample steel sheet is subjected to decarburization annealing, and the primary recrystallized grain size is controlled to be 10 μm. The decarburization annealing conditions are an annealing temperature of 750 to 800°C and a holding time of 50 to 200 seconds. Nitriding treatment is performed between the decarburization annealing step and the finish annealing step, and the nitrogen content is controlled to be 210 ppm. The nitriding treatment is performed once between the completion of the decarburization annealing step and the start of the finish annealing step.
After the decarburization annealing, the steel sheet is coated with an annealing separator mainly composed of MgO, and then the steel sheet is subjected to finish annealing while applying a temperature gradient of 0.5°C/cm in the entire area in the direction perpendicular to the rolling direction (width direction) so that the end of the steel sheet is heated to a high temperature, thereby causing secondary recrystallization. The finish annealing temperature is 1150 to 1250°C, and the soaking time is 10 to 30 hours. The temperature gradient is applied uniformly in the entire area of the sample steel sheet. The temperature gradient is applied by raising the temperature with a temperature difference in the furnace. The magnitude of the temperature gradient is controlled by raising the temperature while measuring the temperature at intervals of 100 mm in the width direction of the steel sheet.

A sample of 60 mm in the width direction and 200 mm in the rolling direction is taken from the obtained steel sheet, and magnetic measurements are performed on this sample using the SST method (see JIS C2556:2015 Annex JA) to measure the magnetic flux density B8 in the rolling direction.
The results are shown in Table 2.

As can be seen from Table 2, excellent magnetic flux density B8 is obtained regardless of the chemical composition.

The present invention provides a method for manufacturing grain-oriented electrical steel sheets that produce grain-oriented electrical steel sheets with high magnetic flux density by performing finish annealing while applying a temperature gradient to the boundary region between the primary recrystallization region and the secondary recrystallization region, and can provide a method for manufacturing grain-oriented electrical steel sheets that can achieve a sufficient increase in magnetic flux density even with a small temperature gradient. Therefore, the present invention has a high industrial applicability.

Claims (2)

A hot rolling process in which a silicon steel material containing, by mass%, Si: 0.80 to 7.00% is heated to a temperature exceeding 1300°C and then hot rolled to obtain a hot-rolled sheet;
A hot-rolled sheet annealing process for annealing the hot-rolled sheet;
A cold rolling process in which the hot-rolled sheet after the hot rolling process or the hot-rolled sheet annealing process is subjected to cold rolling to obtain a steel sheet having a final plate thickness;
a decarburization annealing step of decarburizing the steel sheet after the cold rolling step to obtain a steel sheet having a primary recrystallized grain size of 15 μm or less;
an annealing separator application process in which an annealing separator is applied to the steel sheet after the decarburization annealing process, and then the steel sheet is wound into a coil;
a nitriding process for increasing the nitrogen content of the steel plate;
A finish annealing process of finish annealing the steel sheet wound into a coil shape;
Equipped with
The finish annealing step includes a temperature rise process and a soaking process, and generates a temperature gradient of 0.5° C./cm or more in a boundary region between a primary recrystallization region and a secondary recrystallization region during at least a period from the start of secondary recrystallization to the completion of secondary recrystallization in the temperature rise process,
The nitriding treatment step is performed by annealing in an atmosphere containing a gas having nitriding ability in at least one stage of the decarburization annealing step, between the decarburization annealing step and the finish annealing step, or during the temperature increase process of the finish annealing step until the start of the secondary recrystallization, and the nitrogen content of the steel sheet after the nitriding treatment step is set to 210 ppm or more on a mass basis.
A method for producing a grain-oriented electrical steel sheet comprising the steps of: The chemical composition of the silicon steel material is, in mass%, Si: 0.80 to 7.00%, C: 0.15% or less, acid-soluble Al: 0.010 to 0.065%, N: 0.004 to 0.012%, Mn: 0.01 to 0.50%, S and Se: 0.01 to 0.05% in total, Cr: 0 to 0.30%, Cu: 0 to 0.4%, P: 0 to 0.5%, Ni: 0 to 1.00%, and the balance being Fe and impurities.
The method for producing a grain-oriented electrical steel sheet according to claim 1 .