WO2025187780A1 - Hot-rolled annealed steel sheet for grain-oriented electrical steel sheet - Google Patents

Abstract

This hot-rolled annealed steel sheet for a grain-oriented electrical steel sheet has, regarding the particle diameter-number density distribution of precipitates having an equivalent circle diameter D of 50-1000 nm, a mode diameter Dp of 100-300 nm, a number density f(Dp) of the mode diameter of 1,000,000/g or more, and a value Wp/Dp obtained by dividing the half-value width Wp of the mode diameter by the mode diameter Dp of 1.0-2.0, and the average particle diameter of the hot-rolled annealed steel sheet is 20.0-21.5 μm.

Description

Hot-rolled annealed steel sheets for grain-oriented electrical steel sheets

The present invention relates to a hot-rolled and annealed steel sheet for use in a grain-oriented electrical steel sheet.
This application claims priority based on Japanese Patent Application No. 2024-034126, filed on March 6, 2024, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheet contains 7% or less by mass of Si and has a secondary recrystallization texture concentrated in the {110}<001> orientation (Goss orientation). Note that the {110}<001> orientation means that the {110} plane of the crystal is aligned parallel to the rolling surface, and the <001> axis of the crystal is aligned parallel to the rolling direction.

The magnetic properties of grain-oriented electrical steel sheets are greatly affected by the degree of concentration in the {110}<001> orientation. In particular, the relationship between the rolling direction of the steel sheet, which is the main magnetization direction when the steel sheet is in use, and the crystal's <001> direction, which is the direction of easy magnetization, is considered important. For this reason, in recent years, the angle between the crystal's <001> direction and the rolling direction in practical grain-oriented electrical steel sheets is controlled to be within a range of around 5°.

Such precise crystal orientation control is achieved by dispersing fine precipitates called inhibitors in the steel appropriately before final annealing, and by holding the steel sheet at high temperatures during final annealing. For example, inhibitors increase the selective growth of Goss-oriented grains, which results in secondary recrystallization progressing during final annealing so that Goss-oriented grains grow preferentially. Previously, attempts have been made to highly control inhibitors in order to precisely control crystal orientation.

For example, Patent Document 1 discloses using MnS as an inhibitor and performing two cold rolling steps.
Patent Documents 2 and 3 disclose the use of inhibitors to control MnS+AlN and MnS (and/or MnSe)+Sb, respectively.
Patent Document 4 discloses a technique for preferably controlling inhibitors in order to lower the slab heating temperature for the purpose of reducing production costs.

Patent Document 5 discloses controlling the primary recrystallized grain size and its dispersion related to an inhibitor.
Patent Documents 6 to 8 disclose the addition of Nb, V, etc. to grain-oriented electrical steel sheets.

Furthermore, Patent Documents 9 to 11 disclose techniques for improving magnetostriction by precisely controlling the atmosphere and residence time during finish annealing to form subgrain boundaries within secondary recrystallized grains. These techniques demonstrate the technical concept of expanding the temperature range over which secondary recrystallization progresses in order to form subgrain boundaries, and at the same time, they also show that improvements in magnetic flux density can be expected.

Japan Special Publication No. 30-3651 Japan Special Publication No. 40-15644 Japan Special Publication No. 51-13469 Japanese Patent Publication No. 62-40315 Japanese Patent Publication No. 2008-261022 Japanese Patent Publication No. 52-024116 Japanese Patent Application Publication No. 02-200732 Japanese Patent No. 4962516 International Publication No. 2020/027215 International Publication No. 2020/027218 International Publication No. 2020/027219

In recent years, amid a global trend toward environmental conservation, including power and energy conservation, there has been an increasing demand for more efficient transformers. In this social environment, there is also a demand for improved performance in grain-oriented electrical steel sheets, which are used as iron core materials in transformers. In particular, there is a demand for increased magnetic flux density in grain-oriented electrical steel sheets.

As a result of the inventors' investigations, it was determined that the conventional inhibitor control technologies disclosed in the above-mentioned Patent Documents 1 to 8 do not fully meet the requirements for grain-oriented electrical steel sheets, and that even higher magnetic flux densities are necessary.

One aspect of the present invention was made in consideration of the above-mentioned problems. Given the current demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, one aspect of the present invention aims to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheets that can increase the magnetic flux density.

The gist of the present invention is as follows:

(1) A hot-rolled annealed steel sheet for grain-oriented electrical steel sheet according to one aspect of the present invention is
In mass%,
C: 0.0010 to 0.10%,
Si: 2.0 to 7.0%,
Mn: 0.050 to 1.0%,
S: 0 to 0.0350%,
Se: 0 to 0.0350%,
S+Se total content: 0.0030-0.0350%,
Al: 0.010-0.0650%,
N: 0.0040-0.0120%,
Nb: 0 to 0.030%,
V: 0 to 0.030%,
Mo: 0 to 0.030%,
Ta: 0 to 0.030%,
W: 0-0.030%,
Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%,
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0-1.0%,
and the balance being Fe and impurities,
Regarding the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among the precipitates that are residues obtained by electrolytic extraction of the hot-rolled annealed steel sheet,
The mode diameter is Dp in units of nm,
The number density of the most frequent diameter is represented by f(Dp) in units of particles/g,
When the half width of the mode diameter is Wp in the unit of nm,
Dp is 100 to 300 nm,
f(Dp) is 1,000,000 particles/g or more;
Wp/Dp is 1.0 to 2.0,
The hot-rolled and annealed steel sheet has an average grain size of 20.0 to 21.5 μm.

The above aspect of the present invention provides a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density.

FIG. 1 is a schematic diagram of particle size-number density distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm. 1 is a flowchart of a method for manufacturing a hot-rolled annealed steel sheet for a grain-oriented electrical steel sheet according to an embodiment of the present invention.

A preferred embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention. Furthermore, the numerical ranges listed 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.

Furthermore, in the following explanation, the term "inhibitor" is primarily used in explanations relating to the secondary recrystallization mechanism for the precipitates in the steel that characterize this embodiment, and the term "precipitates" is primarily used in explanations relating to the compound phases observed in the structure. However, in this embodiment, the terms "inhibitor" and "precipitates" are not used with the intention of strictly distinguishing them.

As mentioned above, there is currently a demand to increase the magnetic flux density of grain-oriented electrical steel sheets.

The inventors therefore focused on the technical concept of "expanding the secondary recrystallization progression temperature range" disclosed in the above-mentioned Patent Documents 9 to 11. In these Patent Documents 9 to 11, the technology of "expanding the secondary recrystallization progression temperature range" is primarily used to form subgrain boundaries within secondary recrystallized grains and to reduce noise associated with this. The inventors believed that if this technology of "expanding the secondary recrystallization progression temperature range" could be optimized to increase crystal orientation selectivity, it would be possible to further improve magnetic flux density.

Specifically, the researchers investigated how to more effectively expand the temperature range over which secondary recrystallization progresses by appropriately controlling the inhibitor morphology in the steel, and how to preferentially grow grains with a preferred crystal orientation during the secondary recrystallization process over this expanded temperature range. As a result, they discovered that if the morphology of precipitates contained in hot-rolled annealed steel sheets is optimally controlled during the manufacturing process of grain-oriented electrical steel sheets, the temperature range over which secondary recrystallization progresses can be expanded during finish annealing, allowing Goss-oriented grains to grow preferentially, and ultimately increasing the magnetic flux density of the grain-oriented electrical steel sheets obtained compared to conventional technology.

Generally, inhibitors are tiny precipitates in steel with a diameter of approximately 1000 nm or less. These inhibitors have a pinning effect on grain boundaries, suppressing grain growth. When the temperature reaches approximately 1000°C or higher during final annealing, these inhibitors dissolve into the α-Fe phase, which is the parent phase, and the pinning effect on the grain boundaries weakens. As a result, abnormal grain growth occurs, a phenomenon known as secondary recrystallization.

For example, sulfides and selenides are used as Mn-based precipitates, and nitrides are used as Al-based precipitates. Mn-based inhibitors and Al-based inhibitors (Al-based inhibitors controlled before cold rolling) are primarily used in manufacturing methods in which the slab heating temperature before hot rolling is 1300°C or higher (hereinafter referred to as the "high-temperature slab heating process"). Al-based inhibitors (Al-based inhibitors controlled after cold rolling) are primarily used in manufacturing methods in which the slab heating temperature before hot rolling is 1280°C or lower, and nitriding is performed after cold rolling and before finish annealing (hereinafter referred to as the "low-temperature slab heating process"). In addition to the above inhibitors, carbides and nitrides of Nb, V, Mo, Ta, W, etc. are sometimes used as secondary inhibitors.

In the past, when manufacturing grain-oriented electrical steel sheets, the steel composition and manufacturing conditions have been controlled in order to form inhibitors with the appropriate functions within the steel. In particular, the steel composition, hot rolling conditions, and decarburization annealing conditions are recognized as manufacturing conditions that have a significant impact on the morphology of the inhibitor, and these conditions have been precisely controlled.

In this embodiment, by controlling the size and distribution of the precipitates (inhibitors) contained in the hot-rolled and annealed steel sheet within an appropriate range, the temperature range in which secondary recrystallization progresses is expanded during the subsequent finish annealing process, and the selectivity of crystal orientation associated with the progress of secondary recrystallization is increased. Specifically, the above effects are achieved by allowing relatively fine inhibitors and relatively coarse inhibitors to coexist in the hot-rolled and annealed steel sheet with appropriate sizes and distributions.

The inventors speculate that the mechanism by which the above-mentioned effects are achieved is as follows:

First, we speculate on the reason why the temperature range over which secondary recrystallization progresses expands. As mentioned above, secondary recrystallization occurs because the pinning effect of grain boundaries weakens as inhibitors dissolve. During final annealing, fine inhibitors are thought to dissolve and disappear earlier than coarse inhibitors. Therefore, when fine and coarse inhibitors coexist, the fine inhibitors are thought to disappear preferentially early in the temperature rise process of final annealing. In particular, when at least one element selected from Nb, V, Mo, Ta, and W is added, it becomes possible to favorably control the fine inhibitors, which decompose at lower temperatures than conventional inhibitors such as AlN.

As fine inhibitors dissolve, coarse inhibitors may grow, similar to Ostwald ripening. However, the increase in pinning force that accompanies the growth of coarse inhibitors is thought to have a smaller effect than the decrease in pinning force that accompanies the disappearance of fine inhibitors. Therefore, if fine and coarse inhibitors coexist and the fine inhibitors dissolve earlier than the coarse inhibitors, secondary recrystallization is thought to begin at a relatively low temperature during the temperature rise process of final annealing.

In addition, it is believed that coarse inhibitors remain dissolved in a non-equilibrium state until relatively high temperatures during the temperature rise process of final annealing, and that their pinning effect is maintained up to high temperatures. Therefore, if fine inhibitors and coarse inhibitors coexist and the coarse inhibitors remain up to high temperatures, it is believed that the pinning effect is maintained up to high temperatures and secondary recrystallization continues up to relatively high temperatures.

In other words, when fine inhibitors and coarse inhibitors coexist, secondary recrystallization begins at a relatively low temperature during the temperature rise process of finish annealing and continues up to a relatively high temperature, which is thought to expand the temperature range over which secondary recrystallization progresses.

Next, we speculate on the reasons for the improved selectivity of crystal orientation. As mentioned above, secondary recrystallization progresses through the preferential growth of Goss-oriented grains. This preferential growth of Goss-oriented grains is thought to result from the unique grain boundary characteristics (grain boundary character) and unique crystal grain size (size advantage) of Goss-oriented grains.

However, the driving force for the preferential growth of Goss-oriented grains is not very strong. Therefore, when grain boundary migration occurs relatively easily in grains other than Goss-oriented grains during secondary recrystallization, for example, when the decomposition of the inhibitor is rapid and the pinning effect on grain growth is weakened, resulting in a relatively high grain growth rate (when the driving force for grain growth is relatively high), crystal grains other than Goss-oriented grains also grow easily. In this case, the preferential growth of Goss-oriented grains is inhibited.

Therefore, to preferentially grow Goss-oriented grains, the inhibitor decomposition rate should be kept as slow as possible, the grain growth rate during secondary recrystallization should be made relatively fast compared to the inhibitor decomposition rate, and secondary recrystallization should be sustained for a long period of time. For example, the rate of temperature rise in the temperature range where the inhibitor strength weakens (the temperature range where the inhibitor dissolves) should be slowed down, the inhibitor dissolution rate should be slowed, and the resulting growth rate of secondary recrystallized grains should be made relatively fast compared to the inhibitor decomposition rate. However, this method inevitably results in a longer total finish annealing time, resulting in a decrease in productivity.

In cases where it is difficult to extend the final annealing time industrially (when it is difficult to change the heating rate if the maximum temperature reached is the same), even if the heating rate is constant, if the temperature range over which secondary recrystallization progresses can be expanded by slowing the decomposition rate of the inhibitor, the time over which secondary recrystallization progresses can be extended without reducing productivity, the growth rate of secondary recrystallized grains can be made relatively fast, and the preferential growth of secondary recrystallized grains can be increased. For example, considering that the entire surface of a grain-oriented electrical steel sheet is ultimately occupied by secondary recrystallized grains, it can be understood that extending the time over which secondary recrystallization progresses will lead to an increase in the growth rate of secondary recrystallized grains relative to the decomposition rate of the inhibitor.

In other words, when fine inhibitors and coarse inhibitors coexist, the temperature range in which the inhibitor decomposition rate is slow expands, and the secondary recrystallization progression temperature range in which the growth rate of secondary recrystallization grains is relatively fast compared to the inhibitor decomposition rate expands, making it easier for Goss-oriented grains to grow preferentially. As a result, it is thought that it will ultimately be possible to increase the magnetic flux density.

In this embodiment, by controlling the steel composition, casting conditions, hot rolling conditions, and hot-rolled sheet annealing conditions in a composite and inseparable manner, relatively fine precipitates and relatively coarse precipitates of appropriate size and distribution are allowed to coexist in the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process. Furthermore, in this embodiment, the morphology of the precipitates is preferably controlled by adding auxiliary inhibitor-forming elements.

In this embodiment, the morphology of the above precipitates is defined based on hot-rolled annealed steel sheet (steel sheet immediately before cold rolling).

The hot-rolled annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment will be described in detail below.

The hot-rolled annealed steel sheet according to this embodiment has, in mass%,
C: 0.0010 to 0.10%,
Si: 2.0 to 7.0%,
Mn: 0.050 to 1.0%,
S: 0 to 0.0350%,
Se: 0 to 0.0350%,
S+Se total content: 0.0030-0.0350%,
Al: 0.010-0.0650%,
N: 0.0040-0.0120%,
Nb: 0 to 0.030%,
V: 0 to 0.030%,
Mo: 0 to 0.030%,
Ta: 0 to 0.030%,
W: 0-0.030%,
Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%,
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0-1.0%,
and the balance being Fe and impurities,
Regarding the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among the precipitates that are residues obtained by electrolytic extraction of hot-rolled annealed steel sheets,
The mode diameter is Dp in units of nm,
The number density of the most frequent diameter is represented by f(Dp) in units of particles/g.
When the half width of the mode diameter is Wp in units of nm,
Dp is 100 to 300 nm,
f(Dp) is 1,000,000 particles/g or more;
Wp/Dp is 1.0 to 2.0,
The hot-rolled and annealed steel sheet has an average grain size of 20.0 to 21.5 μm.

1. Chemical Composition The chemical composition of the hot-rolled and annealed steel sheet according to this embodiment may be a general chemical composition used in grain-oriented electrical steel sheets.

It should be noted that publicly available literature on grain-oriented electrical steel sheets rarely discloses the chemical composition of hot-rolled and annealed steel sheets, which are intermediate products. However, since the steel composition hardly changes during the process from slab to decarburization annealing, the chemical composition of hot-rolled and annealed steel sheets can basically be considered to be the same as the chemical composition of slabs disclosed in publicly available literature.

The hot-rolled annealed steel sheet according to this embodiment has a chemical composition that includes basic elements, optional elements as needed, and the balance being Fe and impurities.

The hot-rolled annealed steel sheet according to this embodiment contains, as base elements (major alloying elements), the following mass fractions: C: 0.0010-0.10%, Si: 2.0-7.0%, Mn: 0.050-1.0%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.0040-0.0120%, and Nb+V+Mo+Ta+W total content: 0.0030-0.030%.

C: 0.0010-0.10%
Carbon (C) is an element effective in controlling the primary recrystallization structure during the manufacturing process. However, excessive C content in the final product adversely affects the magnetic properties. Therefore, the C content of the hot-rolled annealed steel sheet may be 0.0010 to 0.10%. The preferred upper limit of the C content is 0.0850%, or 0.0750%. Note that C is purified in the decarburization annealing process and the finish annealing process described below, and after the finish annealing process, the C content is 0.0050% or less. When C is contained, the C content may be more than 0% or may be 0.0010% or more, taking into account productivity in industrial production.

Si: 2.0-7.0%
Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content of hot-rolled annealed steel sheets should be 2.0 to 7.0%. The preferred lower limit of the Si content is 2.50%, more preferably 3.0%. The preferred upper limit of the Si content is 4.50%, more preferably 4.0%.

Mn: 0.050-1.0%
Manganese (Mn) combines with S and Se to precipitate as MnS or MnSe, functioning as an inhibitor. To favorably control the form of these inhibitors (precipitates), the Mn content of the hot-rolled annealed steel sheet should be 0.050 to 1.0%. If the Mn content is below 0.050%, the amount of precipitated MnS and MnSe, which function as inhibitors, is insufficient, thereby inhibiting the appropriate progress of secondary recrystallization. Furthermore, if the Mn content exceeds 1.0%, the amount of precipitated MnS and MnSe, which function as inhibitors, becomes excessive, thereby inhibiting the appropriate progress of secondary recrystallization. In this embodiment, part of the inhibitor function may be performed by carbides, nitrides, carbonitrides, or the like of Nb group elements. In this case, the amount of precipitated MnS and MnSe, which act as inhibitors, may be controlled to be small. Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

S: 0-0.0350%
Se: 0-0.0350%
S+Se total content: 0.0030-0.0350%
Sulfur (S) and selenium (Se) combine with Mn to precipitate as MnS or MnSe, functioning as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the S content of the hot-rolled annealed steel sheet should be 0 to 0.0350%, the Se content should be 0 to 0.0350%, and the total S + Se content should be 0.0030 to 0.0350%. A total S and Se content of 0.0030 to 0.0350% is preferable because it stabilizes secondary recrystallization. Furthermore, in this embodiment, part of the inhibitor function may be performed by carbides, nitrides, or carbonitrides of Nb-group elements. In this case, the precipitation amount of the inhibitors, MnS and MnSe, may be controlled to be low. Therefore, the upper limit of the total S and Se content is preferably 0.0250%, more preferably 0.010%. If S and Se remain in the steel after final annealing, they may form compounds that deteriorate the iron loss. Therefore, it is preferable to reduce the content of S and Se by purifying the steel during final annealing to remove them from the steel.

Here, "the total content of S and Se is 0.0030 to 0.0350%" means that the hot-rolled annealed steel sheet may contain only one of S or Se in its chemical composition, with the content being 0.0030 to 0.0350%. Alternatively, it means that the hot-rolled annealed steel sheet may contain both S and Se, with the total content being 0.0030 to 0.0350%.

Al: 0.010-0.0650%
Aluminum (Al) combines with N to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Al content of the hot-rolled annealed steel sheet should be 0.010 to 0.0650%. When the Al content is 0.010% or more, AlN or (Al,Si)N precipitates in a favorable form through nitriding in the low-temperature slab heating process, stabilizing secondary recrystallization, particularly in the high-temperature range. When the Al content is below 0.010%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, is insufficient, hindering the proper progress of secondary recrystallization. Furthermore, when the Al content exceeds 0.0650%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, becomes excessive, hindering the proper progress of secondary recrystallization. The lower limit of the Al content is preferably 0.020%, more preferably 0.0250%. From the viewpoint of stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%, and more preferably 0.030%.

N:0.0040~0.0120%
Nitrogen (N) combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor. The N content of hot-rolled annealed steel sheet may be 0.0040 to 0.0120%. Note that in the low-temperature slab heating process, N may be added to the steel by nitriding during the manufacturing process. If the N content exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.010%, more preferably 0.0090%. N is purified in the finish annealing process, and after the finish annealing process, the N content is 0.0050% or less.

Nb+V+Mo+Ta+W total content: 0.0030-0.030%
Nb: 0-0.030%
V: 0-0.030%
Mo: 0-0.030%
Ta: 0-0.030%
W: 0-0.030%
Niobium (Nb), vanadium (V), molybdenum (Mo), tantalum (Ta), and tungsten (W) precipitate as auxiliary inhibitors, such as carbides, nitrides, and carbonitrides, and preferably function as inhibitors. Specifically, they preferably expand the temperature range in which secondary recrystallization progresses. Therefore, the Nb content is set to 0 to 0.030%, the V content is set to 0 to 0.030%, the Mo content is set to 0 to 0.030%, the Ta content is set to 0 to 0.030%, the W content is set to 0 to 0.030%, and the total content of Nb + V + Mo + Ta + W is set to 0.0030 to 0.030%. The lower limit of the content of Nb, V, Mo, Ta, and/or W is preferably 0.0040%, and more preferably 0.0050%. The upper limit of the content of Nb, V, Mo, Ta, and/or W is preferably 0.020%, more preferably 0.010%.

In this embodiment, Nb, V, Mo, Ta, and W may be collectively referred to as "Nb group elements."

The hot-rolled annealed steel sheet according to this embodiment contains one or more Nb group elements selected from the Nb group consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.

When Nb group element precipitates are used as inhibitors, when the total content of Nb group elements in the hot-rolled annealed steel sheet is 0.030% or less (preferably 0.0030% or more and 0.030% or less), the morphology of the Nb group element precipitates is favorably controlled and the secondary recrystallization progression temperature range is favorably expanded. As a result, Goss-oriented grains grow favorably, and the magnetic flux density of the final grain-oriented electrical steel sheet is favorably increased.

The reason why precipitates of Nb group elements function favorably as inhibitors is not clear, but is thought to be as follows. Carbides, nitrides, or carbonitrides of Nb group elements precipitate non-equilibrium during cooling from high temperatures and are thought to act as precipitation nuclei for the subsequent precipitation of MnS and AlN. Therefore, compared to when Nb group elements are not contained, when Nb group elements are contained, there are more precipitation sites for MnS and AlN, and as a result, MnS and AlN are thought to be more likely to form as fine precipitates. In the hot-rolled annealed steel sheet according to this embodiment, the coexistence of fine inhibitors and coarse inhibitors expands the secondary recrystallization progression temperature range, and precipitates of Nb group elements are thought to be particularly effective in expanding the secondary recrystallization progression temperature range toward the lower temperature side.

The total content of Nb group elements is preferably 0.0040% or more, and more preferably 0.0050% or more. Furthermore, the total content of Nb group elements is preferably 0.020% or less, and more preferably 0.010% or less. If the total content of Nb group elements is below 0.0030%, there will be a shortage of Nb group element precipitates that act as precipitation nuclei, making it difficult to refine MnS and AlN. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of Nb group element precipitates will be too high, making the Nb group element precipitates likely to become coarse and low-density. Furthermore, the difference between the precipitation temperature range of Nb group element precipitates and the precipitation temperature range of MnS and AlN will be large, making it difficult for the Nb group element precipitates to effectively function as precipitation nuclei for refining MnS and AlN.

Here, "the total content of Nb group elements is 0.0030 to 0.030%" means that the hot-rolled annealed steel sheet may contain at least one element selected from the group consisting of Nb, V, Mo, Ta, and W in its chemical composition, with the content being 0.0030 to 0.030%. Alternatively, it means that the hot-rolled annealed steel sheet may contain at least two elements selected from the group consisting of Nb, V, Mo, Ta, and W, with the total content being 0.0030 to 0.030%.

The hot-rolled annealed steel sheet according to this embodiment may contain impurities as part of its chemical composition. "Impurities" refer to elements that are mixed in from raw materials such as ore or scrap during the industrial production of steel, or from the production environment, etc. The upper limit of the total impurity content may be, for example, 5%.

Furthermore, the hot-rolled annealed steel sheet according to this embodiment may contain optional elements in addition to the basic elements and impurities described above. For example, instead of a portion of the remaining Fe, optional elements such as Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be contained. These optional elements may be contained according to their intended purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are contained as impurities, the above-mentioned effects are not impaired.

Cu: 0-0.40%
Bi: 0~0.010%
B: 0-0.080%
P: 0 to 0.50%
Ti: 0~0.0150%
Sn: 0-0.10%
Sb: 0-0.10%
Cr: 0-0.30%
Ni: 0 to 1.0%
Copper (Cu), bismuth (Bi), boron (B), phosphorus (P), titanium (Ti), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be contained according to known purposes. There is no need to set a lower limit for the content of these optional elements, and the lower limit may be 0%.

In addition, with grain-oriented electrical steel sheet, relatively large changes in chemical composition (reduction in content) occur as a result of decarburization annealing and purification annealing during secondary recrystallization. Depending on the element, purification annealing can reduce the content to a level that cannot be detected by general analytical methods (1 ppm or less). However, the chemical composition listed above is that of hot-rolled annealed steel sheet. The steel composition changes very little from the process from slab to before decarburization annealing.

The chemical composition of the hot-rolled annealed steel sheet according to this embodiment may be measured using a general steel analysis method. For example, the chemical composition of the hot-rolled annealed steel sheet 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 hot-rolled annealed steel sheet using ICP-AES under conditions based on a pre-prepared calibration curve. C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas fusion-thermal conductivity method.

2. Precipitates Next, the precipitates contained in the hot-rolled annealed steel sheet according to this embodiment will be described.

The precipitates (inhibitors) contained in the hot-rolled annealed steel sheet according to this embodiment may be of any type, as long as the precipitation form of the precipitates is controlled. The precipitates may be formed from elements contained in the hot-rolled annealed steel sheet. For example, Mn-based precipitates (Mn-containing precipitates) may be sulfides or selenides, Al-based precipitates (Al-containing precipitates) may be nitrides, and Nb-group element-containing precipitates may be carbides, nitrides, or carbonitrides. In addition to these inhibitors, compounds of optional elements such as Bi and B, and complex compounds with the above elements may also be included.

Note that while the type (composition) of precipitates may contribute to some extent to the above-described effects obtained in this embodiment, the inventors' investigations have revealed that the above-described effects obtained in this embodiment are primarily due to the size and distribution of the precipitates contained in the hot-rolled annealed steel sheet. Therefore, the size and distribution of the precipitates are specified in the hot-rolled annealed steel sheet according to this embodiment.

The precipitates that should be controlled in the hot-rolled annealed steel sheet according to this embodiment are precipitates with an equivalent circle diameter D of 50 to 1000 nm. Note that "equivalent circle diameter" refers to the diameter of a circle when the area of a precipitate is converted into a circle with the same area. This equivalent circle diameter is the same as the equivalent sphere diameter.

Currently, precipitates in hot-rolled and annealed steel sheets with an equivalent circle diameter D of less than 50 nm have little effect in expanding the secondary recrystallization progression temperature range. While the reason for this is unclear, it is thought that precipitates with an equivalent circle diameter D of less than 50 nm at the time of hot-rolled and annealed steel sheets change or disappear in subsequent processes, making them less likely to function as inhibitors during finish annealing. Therefore, in the hot-rolled and annealed steel sheets according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 nm or more are controlled. It is anticipated that by considering processes subsequent to the hot-rolled sheet annealing process, precipitates with an equivalent circle diameter D of less than 50 nm will be able to function as inhibitors.

Furthermore, precipitates with an excessively large equivalent circle diameter D may have a negative effect on the growth of secondary recrystallized grains in the final stage of secondary recrystallization. Furthermore, the formation of precipitates with an excessively large equivalent circle diameter D may reduce the number of precipitates (number density) contained in the hot-rolled annealed steel sheet. Furthermore, precipitates with an excessively large equivalent circle diameter D are less likely to function as inhibitors. Therefore, it is preferable that the average equivalent circle diameter D of the precipitates is 1000 nm or less. In the hot-rolled annealed steel sheet according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 to 1000 nm are controlled as precipitates that have the effect of expanding the secondary recrystallization progression temperature range.

In the hot-rolled annealed steel sheet according to this embodiment, among the precipitates that are residues obtained by electrolytic extraction of the hot-rolled annealed steel sheet, the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm is as follows:
The mode diameter is Dp in units of nm,
The number density of the most frequent diameter is represented by f(Dp) in units of particles/g.
When the half width of the mode diameter is Wp in units of nm,
Dp is 100 to 300 nm,
f(Dp) is 1,000,000 particles/g or more;
Wp/Dp is 1.0 to 2.0,
Satisfy.

Figure 1 shows a schematic diagram of the particle size-number density distribution of precipitates with a circular equivalent diameter D of 50 to 1000 nm. Figure 1 shows examples of Dp, f(Dp), and Wp.

Dp exceeding 300 nm is inappropriate because it results in fewer fine precipitates for expanding the secondary recrystallization temperature range. The upper limit of Dp is preferably 275 nm, and more preferably 250 nm. On the other hand, when Dp is 100 nm or more, the secondary recrystallization proceeding temperature range is preferably expanded. The lower limit of Dp is preferably 125 nm, and more preferably 150 nm.

When f(Dp) is 1,000,000 particles/g or more, the precipitates necessary for secondary recrystallization to occur are sufficiently precipitated, and the pinning effect is preferably achieved. On the other hand, the upper limit of f(Dp) is not particularly limited, but it may be set to, for example, 50,000,000 particles/g.

When Wp/Dp is 1.0 or greater, the ratio of Wp to Dp becomes a favorable value, and the secondary recrystallization temperature range is preferably expanded. The lower limit of Wp/Dp is preferably 1.2. On the other hand, when Wp/Dp is 2.0 or less, the ratio of Wp to Dp becomes a favorable value, and primary recrystallized grains grow uniformly during normal grain growth, resulting in a favorable grain structure before secondary recrystallization. The upper limit of Wp/Dp is preferably 1.75.

The particle size-number density distribution of precipitates with a circular equivalent diameter D of 50 to 1000 nm can be determined as follows:

For example, this can be done using the method described in Japanese Patent No. 6572598. First, precipitates are electrolytically extracted from hot-rolled annealed steel sheet. Electrolytic extraction can be performed under constant current electrolytic extraction (500 mA - 2 hours) using an acetylacetone-based electrolyte to which a dispersant such as a surfactant (for example, sodium dodecyl sulfate with a molecular weight of 288.38 g/mol) has been added in advance. Electrolytic extraction can be performed by electrolysis so that the amount of electrolysis of the hot-rolled annealed steel sheet is 1 g or more.

The extraction residue (precipitate) can be recovered from the electrolytic extraction solution. The size and distribution of the recovered precipitates can be measured using the FFF (Field Flow Fractionation) method. For details on the measurement method using the FFF method, please refer to the above-mentioned Japanese Patent No. 6572598.

Note that each parameter can be changed depending on the particle size and type to be measured. An example is shown below. The FFF device used may be a Wyatt Eclipse AF4 device (Wyatt Technology Europe, Germany). The measurement sample dispersion solution may be an aqueous solution of sodium dodecyl sulfate at a concentration of 300 mg/mL. The cell may have a channel length of 275 mm and an asymmetric diamond-shaped channel spacer with a thickness of 350 μm. The separation membrane may be a regenerated cellulose ultrafiltration membrane with a molecular weight of 30 kDa.

Before adding the extraction residue (precipitate) recovered from the electrolytic extraction solution, it is necessary to create a calibration curve using standard samples with known particle sizes, correlating particle size with the time it takes for particles to be detected. The type and number of standard samples can be selected according to the particle size distribution of the extraction residue to be measured; for example, polystyrene latex standard particles with a particle size of 29 to 500 nm can be selected.

The size of the standard particles must be directly confirmed in advance using a TEM (Transmission Electron Microscope) or similar. It is sufficient to measure 500 or more particles. The long side of each standard particle is measured and the average value calculated. Furthermore, six different particle diameters, for example, 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm, can be used for the standard particles.

Actual separation conditions can be as follows. First, for stabilization before focusing, the FFF device eluent (hereinafter referred to as channel flow) should be set to 1.0 mL/min, the cross flow to 0.5 mL/min, and the time should be 1 minute. Then, for focusing before sample injection, the focus flow should be set to 3.0 mL/min and the time should be 1 minute. Next, for focusing, the sample should be injected at 0.2 mL/min for 2 minutes. After sample injection, the focusing time should be 1 minute. Then, the flow path is switched, the focus flow is stopped, and the channel flow is set to 1.0 mL/min and the cross flow is reduced from 0.5 mL/min to 0.05 mL/min over 35 minutes while the flow rate is reduced in direct proportion. A calibration curve can be created by using the time when the flow started as the reference point and correlating the time until particles are detected with the average particle size of the standard particles measured in advance. The maximum time for particle detection is 35 minutes, and the injection volume of the liquid in which the sample is dispersed is 0.1 to 0.4 mL.

After creating the calibration curve as described above, add the extraction residue (precipitate) recovered from the electrolytic extraction solution back into the device. The device setting parameters should be the same as above.

In this way, the particle size of the nanoparticles contained in the nanoparticle dispersion sample to be measured can be measured.

In addition, the effluent from the FFF device (a solution containing precipitates separated by size) can be analyzed for components using a conventional ICP (Inductively Coupled Plasma) mass spectrometer.

Using the particle size distribution data measured using the FFF method, the particle size can be divided into 0.5 nm intervals, and the number density can be calculated in units/g from the number of precipitates within this particle size interval and the amount of electrolysis used in the extraction electrolysis, and a histogram of particle size and number density can be created. Dp, f(Dp), and Wp can then be determined from this histogram.

In this embodiment, the "mode diameter" corresponds to the particle diameter (particle diameter category) at which the number density value is greatest in the above-mentioned histogram of particle diameter and number density (particle diameter-number density distribution of precipitates).

It is preferable to calculate the above Dp, f(Dp), Wp, etc. after smoothing the measurement data obtained using the FFF method. For example, the simple moving average method can be used to smooth the measurement data obtained using the FFF method. The value of f(Dp) can be calculated by considering the top three digits as significant.

3. Average Grain Size Next, the average grain size of the hot-rolled and annealed steel sheet according to this embodiment will be described.

The average grain size of the hot-rolled annealed steel sheet according to this embodiment is 20.0 to 21.5 μm. In the hot-rolled annealed steel sheet according to this embodiment, relatively fine precipitates and relatively coarse precipitates coexist with appropriate sizes and distribution, and the average grain size of the hot-rolled annealed steel sheet (steel sheet after hot-rolled sheet annealing) is an appropriate grain size. This is achieved by lowering the first-stage annealing temperature and also lowering the second-stage annealing temperature during hot-rolled sheet annealing.

The average grain size of hot-rolled annealed steel sheet is preferably 21.5 μm or less, and more preferably 21.0 μm. The lower limit of the average grain size is not particularly limited in terms of the effect of broadening the precipitate size distribution. However, in order to reduce the average grain size, it is better to use a higher second-stage annealing temperature, but in that case, fine precipitates will be scarce and the precipitate size distribution will not be sufficiently broadened. For this reason, for example, the average grain size of hot-rolled annealed steel sheet may be 20.0 μm or more.

The average grain size of the hot-rolled annealed steel sheet according to this embodiment can be determined based on the intersecting method of JIS G0551:2013. For example, an L-section (a cross section normal to the direction perpendicular to the rolling direction) of the hot-rolled annealed steel sheet can be photographed using an optical microscope at a magnification of 200x, and the grain size of the cross-sectional structure can be measured along the thickness direction of the sheet based on the intersecting method described above. This measurement can be carried out at least five times at different measurement locations to determine the average grain size.

3. Sheet Thickness The sheet thickness of the hot-rolled annealed steel sheet according to this embodiment is not particularly limited. The hot-rolled annealed steel sheet according to this embodiment is subjected to a subsequent cold rolling process and is finally finished into a grain-oriented electrical steel sheet. Therefore, taking into account the manufacturing conditions of general grain-oriented electrical steel sheets, the sheet thickness of the hot-rolled annealed steel sheet may be 1.8 to 3.5 mm. However, the sheet thickness is not limited to this, and any known sheet thickness or a sheet thickness that is practically used may be adopted.

4. Manufacturing Method Next, a method for manufacturing a hot-rolled and annealed steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention will be described. Note that the method for manufacturing the hot-rolled and annealed steel sheet according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing the hot-rolled and annealed steel sheet according to this embodiment.

Figure 2 is a flow chart illustrating the manufacturing process for hot-rolled annealed steel sheet according to this embodiment. Figure 2 also shows the manufacturing process for grain-oriented electrical steel sheet using this hot-rolled annealed steel sheet. As shown in Figure 2, the manufacturing method for hot-rolled annealed steel sheet according to this embodiment comprises a casting process, a hot-rolling process, and a hot-rolled sheet annealing process. The conditions controlled in these processes will be described in detail below.

Furthermore, the processes subsequent to the cold rolling process shown in Figure 2, i.e., the cold rolling process, decarburization annealing process, annealing separator application process, and finish annealing process, are manufacturing processes for grain-oriented electrical steel sheet (finish-annealed steel sheet). The effects of the hot-rolled annealed steel sheet according to this embodiment can be confirmed in the final product, which is grain-oriented electrical steel sheet, so the conditions for controlling these processes will also be described later.

The method for producing a hot-rolled annealed steel sheet according to this embodiment includes a casting process, a hot-rolling process, and a hot-rolled sheet annealing process,
In the casting process,
In mass%,
C: 0.0010 to 0.10%,
Si: 2.0 to 7.0%,
Mn: 0.050 to 1.0%,
S: 0 to 0.0350%,
Se: 0 to 0.0350%,
S+Se total content: 0.0030-0.0350%,
Al: 0.010-0.0650%,
N: 0.0040-0.0120%,
Nb: 0 to 0.030%,
V: 0 to 0.030%,
Mo: 0 to 0.030%,
Ta: 0 to 0.030%,
W: 0-0.030%,
Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%,
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0-1.0%,
and the balance being Fe and impurities, and forming the molten steel into a slab;
In the hot rolling process, the slab after the casting process is heated, roughly rolled, and then finish rolled to form a hot-rolled steel plate.
In the hot-rolled sheet annealing process, the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing in a temperature range of 1040 to 1080°C, and second-stage annealing in a lower temperature range of 810 to 880°C, and then cooled at an average cooling rate of 5 to 80°C/second to obtain a hot-rolled annealed steel sheet.

In addition, in the manufacturing method of the hot-rolled annealed steel sheet according to this embodiment,
In the hot rolling process,
When heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030 ° C. and less than 1180 ° C., so that a portion of the precipitates contained in the slab is preferably solutionized (for example, 12 to 85 volume % of the precipitates are solutionized based on the precipitates contained in the slab after the casting process), and in order to make this solution state uniform within the slab, the soaking time of the slab is set to more than 70 minutes; and
During rough rolling, the rolling temperature is set to 940 to 1070° C., and the rolling reduction is set to 82 to 95%.

In order to control the size and distribution of precipitates contained in hot-rolled and annealed steel sheet, it is necessary to control the steel composition, casting conditions, hot-rolling conditions, and hot-rolled sheet annealing conditions, and it is particularly important to control the steel composition, slab heating conditions (solution state of precipitates before rough rolling), rough rolling temperature, rough rolling reduction, and hot-rolled sheet annealing conditions. Furthermore, in order to control the above-mentioned "solution state of precipitates before rough rolling," it is important to control the steel composition and slab heating conditions.

The above slab heating can be performed by soaking at a specified temperature for a specified time without temporarily increasing the heating temperature during the slab heating process. In this case, the above slab soaking temperature refers to the surface temperature of the slab, and the slab soaking time refers to the holding time after the slab surface temperature reaches the above soaking temperature. Although it is affected by, for example, the steel composition and heating rate, if the slab surface temperature reaches the above soaking temperature during slab heating, the solution state of the precipitates on the slab surface will be preferably controlled. Furthermore, if the slab surface temperature reaches the above soaking temperature and is held for the above soaking time after reaching the above soaking temperature, the solution state of the precipitates will be preferably controlled all the way to the center of the slab.

Below, important manufacturing conditions for the manufacturing method of hot-rolled annealed steel sheet according to this embodiment will be explained. Other manufacturing conditions may be the same as those for conventionally known grain-oriented electrical steel sheets.

(Casting process)
In the casting process, a slab is prepared. As described above, the chemical composition of the slab hardly changes during the process from slab to decarburization annealing, so the chemical composition of the slab is set to the chemical composition of the target hot-rolled annealed steel sheet (the chemical composition of the hot-rolled annealed steel sheet described above).

The chemical composition of the slab affects the "solution state of precipitates before rough rolling" mentioned above. As will be explained in more detail later, the chemical composition of the slab must not only satisfy the chemical composition requirements for hot-rolled annealed steel sheet mentioned above, but must also be controlled in conjunction with other manufacturing conditions that affect the "solution state of precipitates before rough rolling."

An example of a method for manufacturing slabs is as follows: Molten steel is produced (smelted). Slabs are manufactured using this molten steel. For example, slabs may be manufactured using continuous casting. Alternatively, ingots may be manufactured using the molten steel, and the ingots may be bloomed to manufacture slabs. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. So-called thin slabs with a thickness of 10 to 70 mm may also be used as slabs.

(Hot rolling process)
The hot rolling step is a step in which a slab is heated to a predetermined temperature and hot rolled (rough rolling and finish rolling) to obtain a hot-rolled steel sheet.

For example, in the hot rolling process, the slab after the casting process is heated, rough rolled, and then finish rolled to produce a hot-rolled steel sheet with a specified thickness of 1.8 to 3.5 mm. After finish rolling is complete, the hot-rolled steel sheet is coiled at the specified temperature.

When heating the slab after the casting process during the hot rolling process, the following conditions must be met:

When heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, which preferably brings some of the precipitates contained in the slab into solution (for example, 12 to 85 volume % of the precipitates are brought into solution, based on the precipitates contained in the slab at room temperature after the casting process), and in order to make this solution state uniform within the slab, the slab should be heated for a soaking time of more than 70 minutes.

Preferably bringing some of the precipitates contained in the slab into solution before rough rolling is necessary to achieve a favorable final balance between the amounts of relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitated precipitates).

Furthermore, the Wp value, which is the main technical feature of this embodiment, can be increased by controlling the size difference between the relatively coarse precipitates that remain precipitated during the slab heating stage (residual precipitates) and the relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitated precipitates).

The above "solution state of precipitates before rough rolling" refers to the "solution state of precipitates before rough rolling" in an equilibrium state, not a non-equilibrium state. In a non-equilibrium state, for example, the solution state of precipitates will be uneven near the surface and near the center in the thickness direction. If a slab in this non-equilibrium state is subjected to rough rolling, it will ultimately become difficult to control the size and distribution of precipitates contained in the steel sheet after the hot-rolled sheet annealing process.

For example, in order to bring the solution state of the precipitates closer to equilibrium, it is preferable to keep the value obtained by subtracting the temperature at the center of the slab from the temperature at the surface of the slab within the range of more than -10°C and less than 50°C during slab heating and extraction. In particular, if the temperature difference is less than -10°C, the steel plate surface will be less likely to stretch, resulting in significant defects. Furthermore, if the temperature difference is more than 50°C, the solution state of the precipitates will be uneven in the thickness direction, making it difficult to control the size of the precipitates.

Note that, although different from the slab heating method in this embodiment, the heating temperature may be temporarily increased during slab heating in order to shorten the soaking time. In this case, it is effective to keep the difference between the surface temperature at the maximum temperature reached and the surface temperature at the time of heating and extraction of the slab to 80°C or less. In this case, after the temperature is reduced from the maximum temperature reached, it is preferable to hold the slab in a low-temperature region of the slab heating furnace for at least 20 minutes or more, so that the difference between the surface temperature and the central temperature at the time of extraction from the slab heating furnace is less than 50°C. It is even more preferable to keep the difference between the surface temperature and the central temperature of the slab to be 0 to 30°C.

In conventional technology, known as the low-temperature slab heating process, which involves heating a slab at temperatures below 1280°C, there was no technical concept of solutionizing only a specific percentage of the precipitates contained in the slab, nor any knowledge that the solution state of these precipitates needed to approach equilibrium. In the manufacturing method for hot-rolled annealed steel sheet according to this embodiment, the solution state of the precipitates is suitably controlled, and a slab in which the solution state of the precipitates is in equilibrium is subjected to rough rolling.

The above-mentioned "solution state of precipitates before rough rolling" is a characteristic that is affected by both the steel composition and the hot rolling conditions (slab heating conditions). To control this "solution state of precipitates before rough rolling," it is necessary to control each manufacturing condition in a composite and inseparable manner, taking into consideration the effect that the above manufacturing conditions have on the "solution state of precipitates." For example, those skilled in the art can control material properties, including precipitation behavior, and so as long as they understand the effect that each of the above conditions has on the "solution state," they can combine the above conditions to control the "solution state."

For example, as described above, the "solution state of the precipitates before rough rolling" may be controlled by temporarily increasing the heating temperature during the slab heating process and then holding the temperature for a certain period of time after cooling. However, in the manufacturing method for hot-rolled annealed steel sheet according to this embodiment, as an example, a method is shown in which the "solution state of the precipitates before rough rolling" is controlled by soaking at a predetermined temperature for a predetermined period of time without temporarily increasing the heating temperature during the slab heating process.

In the hot rolling process, when heating the slab before rough rolling, the soaking temperature during slab heating should be greater than 1030°C and less than 1180°C, and the soaking time should be greater than 70 minutes. In this case, it is easy to favorably bring some of the precipitates contained in the slab into solution (for example, it is easy to bring 12 to 85 volume % of the precipitates into solution, based on the precipitates contained in the slab at room temperature after the casting process).

When the content of Nb-group elements is within the above range, it is ultimately possible to have both fine and coarse inhibitors coexist, even if the slab heating temperature is 1100°C or higher. For example, if the slab heating temperature is high and solution formation of AlN, MnS, etc. is promoted during the slab heating stage, these AlN and MnS are likely to re-precipitate as coarse particles in subsequent processes. However, when the content of Nb-group elements is within the above range, the precipitates of Nb-group elements act as precipitation nuclei for MnS and AlN, reducing the size of the re-precipitated AlN and MnS. Furthermore, because the precipitation nose of Nb-group element precipitates (carbonitrides) is located at a lower temperature than the precipitation noses of AlN and MnS, the precipitates of Nb-group elements themselves are more likely to precipitate as finer precipitates than AlN, etc.

For this reason, when the content of Nb group elements is within the above range, the upper limit temperature during slab soaking needs to be less than 1180°C. As the soaking temperature increases, the solution of precipitates is also promoted, and when the content of Nb group elements is within the above range, it is easier to preferably bring some of the precipitates contained in the slab into solution (for example, the upper limit of the solution rate of precipitates should be 85% by volume). When these conditions are met, the effect of the precipitates of Nb group elements described above ultimately makes it easier for fine inhibitors and coarse inhibitors to coexist.

Similarly, when the content of Nb group elements is within the above range, the lower limit temperature during slab soaking may be above 1030°C. Note that as the soaking temperature decreases, the solution of precipitates is also suppressed, but when the content of Nb group elements is within the above range, it is easier to preferably bring some of the precipitates contained in the slab into solution (for example, the lower limit of the solution rate of precipitates may be 12% by volume). When these conditions are met, it is ultimately possible to achieve the coexistence of fine inhibitors and coarse inhibitors.

The mechanism by which the above effects are achieved is thought to be related to the fact that precipitates (carbonitrides) of Nb group elements precipitate more easily than MnS or AlN (MnS in particular does not precipitate easily without support such as dislocation multiplication caused by rolling, and when it does precipitate, its size increases), and that precipitates of Nb group elements function as precipitation nuclei for the precipitation of MnS and AlN, suppressing the coarsening of the re-precipitated AlN and MnS.

Furthermore, when the content of Nb group elements is within the above range, a soaking time of 70 minutes or less is too short, making it difficult to control the solution state of the precipitates to an equilibrium state. There is no particular upper limit to the soaking time, but considering productivity in industrial production, it may be set to 2 hours.

The soaking time during slab heating may be relaxed by changing the hot-rolled sheet annealing conditions described below. For example, by favorably changing the hot-rolled sheet annealing conditions, appropriate precipitate control may be possible even with a soaking time of 70 minutes or less. For example, even if the soaking time is 60 minutes or more and 70 minutes or less, desired properties may be obtained (e.g., magnetic flux density _B8 of 1.935 T or more) by controlling other conditions and hot-rolled sheet annealing conditions within appropriate ranges.

Controlling the "solution state of precipitates before rough rolling" to the above conditions is necessary to achieve a favorable final balance between the amount of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage and the amount of relatively fine precipitates (re-precipitated precipitates) that precipitate after hot rolling.

The above-mentioned slab soaking temperature refers to the surface temperature of the slab, and the slab soaking time refers to the time the slab's surface temperature is maintained after it reaches the soaking temperature. For example, this is affected by the steel composition and heating rate, but if the slab's surface temperature reaches the soaking temperature during slab heating, the solution state of the precipitates on the slab surface will be suitably controlled. Furthermore, if the slab's surface temperature is maintained for the soaking time after reaching the soaking temperature, the solution state of the precipitates will be suitably controlled all the way to the center of the slab.

There are no particular limitations on the specific value of the solution ratio. As described above, the "solution state of precipitates before rough rolling" can be favorably controlled by controlling the steel composition and slab heating conditions. However, if necessary, the specific value of the solution ratio can be determined using integrated thermodynamic calculation software. For example, "Thermo-Calc" is known as a commonly available integrated thermodynamic calculation software. In this embodiment, the solution ratio was calculated from the chemical composition and temperature of the slab using "Thermo-Calc" (2019a ver.) and used as a reference.

In the hot rolling process, hot rolling is carried out following the slab heating described above. Hot rolling is generally divided into rough rolling and finish rolling. In this embodiment, in order to control the size and distribution of precipitates contained in the steel sheet after the hot-rolled sheet annealing process, it is important to control the rolling temperature and reduction rate of the rough rolling after controlling the "solution state of the precipitates before rough rolling" described above.

In the hot rolling process, the following conditions must be met when rough rolling the slab after heating.

When rough rolling the heated slab, the rolling temperature should be controlled to 940-1070°C and the reduction ratio to 82-95%.

By setting the reduction rate within the above range, stress-induced precipitation occurs, making it possible to precipitate fine, large amounts of precipitates. If the rough rolling reduction rate is smaller than the above lower limit, fewer dislocations are introduced by the rolling process, and there are fewer precipitation sites available for stress-induced precipitation, resulting in larger precipitate particle sizes and a smaller Wp value. On the other hand, the upper limit of the rough rolling reduction rate is not particularly limited, but it may be set to 95%, taking into account the performance of the rolling mill, etc.

The rough rolling reduction described above means the cumulative rolling reduction in the rough rolling. Specifically, the rough rolling reduction is defined as follows:
Rough rolling reduction (cumulative reduction) (%) = (1 - "steel plate thickness after rough rolling" / "steel plate thickness before rough rolling") x 100

Furthermore, if the rolling temperature for rough rolling is higher than the above upper limit, deformation-induced precipitation of precipitates of MnS, AlN, Nb-group elements, etc. will occur on the higher temperature side of the precipitation nose or near the nose, resulting in a larger critical precipitation radius for the precipitates that re-precipitate during hot rolling. This reduces the size difference with the relatively coarse precipitates (residual precipitates) that have been precipitated since the slab heating stage, resulting in a smaller Wp value. On the other hand, there is no particular lower limit for the rolling temperature for rough rolling, but since a lower temperature will harden the slab and reduce rollability, it is sufficient to roll at, for example, 940°C or higher. The rough rolling temperature is defined as the average of the start and end temperatures of rough rolling.

Furthermore, when Nb group elements are suitably contained in the chemical composition, in addition to MnS and AlN, precipitates of Nb group elements (particularly carbides and nitrides) will precipitate during rough rolling. These Nb group element precipitates act as precipitation nuclei for the subsequent precipitation of MnS and AlN, resulting in finer re-precipitation of MnS and AlN. Therefore, when Nb group elements are suitably contained in the chemical composition, it is sufficient to control the various control conditions, such as the solution state of the precipitates (for example, the solution rate of the precipitates before rough rolling), the rough rolling temperature, and the rough rolling reduction, as described above.

The reason why the conditions of the hot rolling process can be controlled as described above when Nb group elements are suitably contained is thought to be as follows: When Nb group elements are contained, MnS and AlN are reprecipitated more finely due to the precipitation of Nb group elements, resulting in a smaller Dp compared to when Nb group elements are not contained. On the other hand, even if Nb group elements are contained, the value of Wp does not change significantly. Therefore, when Nb group elements are contained, Wp/Dp is larger compared to when Nb group elements are not contained. Therefore, in order to suitably control the values of Dp and Wp/Dp, it is thought that the conditions of the hot rolling process can be controlled as described above.

For example, if the solution state of the precipitates is not controlled appropriately when an Nb group element is contained (for example, if the "solution rate of precipitates before rough rolling" is lower than 12% by volume), the precipitates will not be fully dissolved when the slab is heated, just as in the case where an Nb group element is not contained, and therefore fewer fine precipitates will re-precipitate during hot rolling. This reduces Wp and makes it impossible to sufficiently expand the secondary recrystallization progression temperature range during finish annealing. Furthermore, if the solution state of the precipitates is not controlled appropriately when an Nb group element is contained (for example, if the "solution rate of precipitates before rough rolling" is higher than 85% by volume), then most of the precipitates will be dissolved when the slab is heated, just as in the case where an Nb group element is not contained, and therefore fewer relatively coarse precipitates (undissolved precipitates) will remain in the slab. This makes it impossible to sufficiently expand the secondary recrystallization progression temperature range during finish annealing.

Furthermore, the reason why the rough rolling reduction rate should be controlled as above when Nb group elements are suitably contained, compared to when Nb group elements are not suitably contained, is thought to be as follows: When Nb group elements are contained, precipitates of Nb group elements are more likely to precipitate finely in the steel, so the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when Nb group elements are not contained. Therefore, when Nb group elements are contained, there are more precipitation sites for precipitates, and deformation-induced precipitation is more likely to occur even when the reduction rate is lowered. For this reason, it is thought that the rough rolling reduction rate should be controlled as above.

When Nb group elements are contained, if the rough rolling reduction is less than 82%, as in the case where Nb group elements are not contained, fewer dislocations are introduced by the rolling process, and there are fewer precipitation sites available for processing-induced precipitation, resulting in larger precipitate particle sizes and a smaller Wp value. When Nb group elements are contained, the upper limit of the rough rolling reduction is preferably 93%.

Furthermore, the reason why the rough rolling temperature should be controlled as described above when Nb group elements are suitably contained, compared to when Nb group elements are not suitably contained, is thought to be as follows. When Nb group elements are contained, as mentioned above, the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when Nb group elements are not contained. Therefore, when Nb group elements are contained, there are more precipitation sites for precipitates, and the precipitates that re-precipitate during hot rolling are more likely to be fine. For this reason, it is thought that the rough rolling temperature should be controlled as described above.

When Nb group elements are contained, if the rolling temperature for rough rolling is higher than 1070°C, all precipitates of MnS, AlN, and Nb group elements will undergo processing-induced precipitation at temperatures higher than the precipitation nose of the precipitate, and the critical precipitation radius of the precipitates that re-precipitate during hot rolling will become larger. As a result, the size difference between these precipitates and the relatively coarse precipitates (residual precipitates) that have been precipitated since the slab heating stage will become smaller, resulting in a smaller Wp value. When Nb group elements are contained, the upper limit of the rolling temperature for rough rolling is preferably 1065°C, and more preferably 1040°C.

In addition, the reason why Nb group elements promote the fine precipitation of precipitates is not clear, but it is thought to be as follows.

During rough rolling, the steel sheet temperature drops rapidly over time. For this reason, the rough rolling process is considered to be in a non-equilibrium state. Even in temperature ranges where MnS and AlN would all precipitate in an equilibrium state, dissolved MnS and AlN may exist in a non-equilibrium state. For example, because the rough rolling process is in a non-equilibrium state, dissolved MnS and AlN are thought to exist even in temperature ranges where Nb group element precipitates. Therefore, when Nb group element precipitates precipitate during rough rolling, these Nb group element precipitates are thought to act as precipitation nuclei for the subsequent precipitation of MnS and AlN, causing the fine precipitation of MnS and AlN. Specifically, compared to when Nb group element precipitates are not present, when Nb group element precipitates are present, there are more precipitation sites for MnS and AlN, resulting in fine precipitation of MnS and AlN.

Furthermore, when precipitates of Nb group elements that acted as precipitation nuclei for MnS or AlN are covered with MnS or AlN, further growth of the Nb group element precipitates is suppressed. In this case, it is thought that the Nb group elements that would have been consumed in the growth of the precipitates precipitate finely as new precipitates. These new fine precipitates of Nb group elements act as new precipitation nuclei for MnS or AlN, contributing to further fine precipitation of MnS and AlN. In this way, precipitates of Nb group elements are thought to contribute synergistically to the fine precipitation of MnS and AlN.

When the slab heating conditions and rough rolling conditions in the hot rolling process satisfy the above conditions, the size and distribution of precipitates are favorably controlled. As a result, after the hot-rolled sheet annealing process, the particle size-number density distribution of precipitates is controlled within the above range.

The slab soaking temperature during slab heating before rough rolling and the rolling temperature during rough rolling are controlled with a purpose. These temperatures are not caused by a natural temperature drop that occurs when the slab is removed from the slab heating furnace and subjected to rough rolling. For example, in typical operations, the slab soaking temperature and rough rolling temperature are not controlled with a purpose. Typically, if the slab soaking temperature is high, the rough rolling temperature will also be high, and if the slab soaking temperature is low, the rough rolling temperature will also be low. On the other hand, in this embodiment, the slab soaking temperature and rough rolling temperature are controlled with a purpose. For example, even if the slab soaking temperature is high within the above range, the rough rolling temperature is controlled to be within the above range. Similarly, even if the slab soaking temperature is low within the above range, the rough rolling temperature is controlled to be within the above range.

The conditions for finish rolling in the hot rolling process are not particularly limited, and normal hot rolling conditions may be used.

(Hot-rolled sheet annealing process)
The hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet after the hot-rolling process is annealed to obtain a hot-rolled annealed steel sheet. The hot-rolled sheet annealing process is generally performed to control the steel sheet structure, such as the recrystallization rate, residual strain, and crystal grain size, and to preferably adjust the morphology of precipitates in the steel by annealing the hot-rolled steel sheet after the hot-rolling process.

The annealing conditions for the hot-rolled sheet annealing process may be the hot-rolled sheet annealing conditions described below. In this embodiment, the precipitates contained in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process are controlled to have a particle size-number density distribution within the ranges described above.

For example, in this embodiment, the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing for recrystallization in a temperature range of 1040 to 1080°C. Second-stage annealing is then performed at a lower temperature range of 810 to 880°C. The steel sheet is then cooled at an average cooling rate of 5 to 80°C/s. The first-stage annealing temperature is preferably 1040 to 1060°C. The second-stage annealing temperature is preferably 830 to 870°C. The average heating rate to the first-stage annealing temperature is preferably 5°C/s or more. The steel sheet is preferably held for 20 seconds or more during second-stage annealing. The average cooling rate after second-stage annealing is preferably 10°C/s or more, and more preferably 20°C/s or more. While there is no particular upper limit to the average cooling rate, to prevent breakage during cold rolling, the average cooling rate is preferably 50°C/s or less, and more preferably less than 40°C/s. The average cooling rate mentioned above refers to the temperature range from the second annealing temperature to 500°C divided by the time required for cooling.

By setting the first-stage annealing temperature to 1040-1080°C, it is possible to preferably increase the number of precipitates, mainly due to Nb-group elements, that precipitate during cooling after second-stage annealing, and therefore to preferably increase Wp. For example, annealing at a temperature higher than 1080°C is likely to increase the amount of AlN that goes into solution in the first-stage annealing temperature range, and the number of fine AlN particles that precipitate during second-stage annealing is likely to increase. In this case, Nb-group elements are likely to attach to the fine AlN and precipitate during cooling after second-stage annealing, which may result in a small Wp. Furthermore, for example, annealing at a temperature lower than 1040°C makes it difficult for precipitates due to Nb-group elements to go into solution sufficiently, and the number of precipitates due to Nb-group elements that precipitate during cooling after second-stage annealing is likely to decrease, which may result in a small Wp.

By setting the second-stage annealing temperature to 810-880°C, the precipitates of Nb-group elements that precipitate during subsequent cooling can be preferably finely precipitated, thereby preferably increasing Wp. For example, if annealing is performed at a temperature higher than 880°C, the subsequent cooling begins in the high-temperature range, making it easier for large Nb-group elements to precipitate during cooling, which can result in a small Wp. Also, for example, if annealing is performed at a temperature lower than 840°C, the number of relatively large Nb-group element precipitates that precipitate during second-stage annealing increases, and the number of fine Nb-group element precipitates that precipitate during subsequent cooling is likely to decrease, which can result in a small Wp.

Furthermore, by controlling the annealing conditions as described above in the hot-rolled sheet annealing process, the average grain size of the hot-rolled annealed steel sheet (steel sheet after hot-rolled sheet annealing) can be controlled to preferably 20.0 to 21.5 μm.

As described above, the method for manufacturing hot-rolled annealed steel sheet according to this embodiment comprises a casting process, a hot-rolling process, and a hot-rolled sheet annealing process. Hot-rolled annealed steel sheet manufactured by comprehensively controlling the above conditions in each process has favorably controlled precipitate size and distribution, and the particle size-number density distribution of the precipitates is controlled within the above range. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

For example, as described above, in the manufacturing method of hot-rolled annealed steel sheet according to this embodiment, the amount of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage is controlled primarily by the slab soaking temperature and time during slab heating before rough rolling, and the amount of relatively fine precipitates (re-precipitated precipitates) is controlled by the subsequent manufacturing conditions, thereby controlling the characteristics of the hot-rolled annealed steel sheet within the above-mentioned ranges. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

5. Method of Using Hot-Rolled Annealed Steel Sheet The effects of the hot-rolled annealed steel sheet according to this embodiment can be confirmed in the final product, that is, grain-oriented electrical steel sheet. Therefore, from the perspective of a method of using the hot-rolled annealed steel sheet according to this embodiment, the manufacturing process of the grain-oriented electrical steel sheet subsequent to the hot-rolled sheet annealing process will be described.

The manufacturing method for grain-oriented electrical steel sheet includes a cold rolling process, a decarburization annealing process, an annealing separator application process, and a finish annealing process. If necessary, it may also include an insulating coating formation process and a magnetic domain control process. These processes may employ well-known, general process conditions. Below, we will explain an example of a manufacturing method that applies nitriding treatment as a low-temperature slab heating process.

(Cold rolling process)
The cold rolling step is a step of cold rolling the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step once, or cold rolling the hot-rolled annealed sheet multiple times (two or more times) via annealing (intermediate annealing) (for example, a total cold rolling rate of 80 to 95%) to obtain a cold-rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm.

(Decarburization annealing process)
The decarburization annealing step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarburization annealing (for example, at 700 to 900°C for 1 to 3 minutes) to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred. By subjecting the cold-rolled steel sheet to decarburization annealing, C contained in the cold-rolled steel sheet is removed. The decarburization annealing is preferably performed in a humid atmosphere in order to remove "C" contained in the cold-rolled steel sheet.

(nitriding treatment)
The nitriding treatment is carried out to adjust the strength of the inhibitor in secondary recrystallization. In the nitriding treatment, the nitrogen content of the steel sheet may be increased to about 40 to 300 ppm at any timing between the start of the above-mentioned decarburization annealing and the start of secondary recrystallization in the finish annealing described below. Examples of nitriding treatment include a treatment in which a steel sheet is annealed in an atmosphere containing a gas with nitriding ability such as ammonia, and a treatment in which a decarburization-annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN is finish-annealed.

(Annealing separator application process)
The annealing separator application step is a step of applying an annealing separator to the decarburized annealed steel sheet. As the annealing separator, for example, an annealing separator containing MgO as a main component or an annealing separator containing alumina 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 next finish-annealing process.

(Finish annealing process)
The final annealing step is a step in which the decarburized annealed steel sheet coated with the annealing separator is subjected to final annealing to cause secondary recrystallization. In this step, the growth of primary recrystallized grains is suppressed by an inhibitor, and secondary recrystallization is allowed to proceed, thereby preferentially growing {110}<001> oriented grains and improving magnetic flux density.

When the hot-rolled annealed steel sheet according to this embodiment is used, the temperature range in which secondary recrystallization progresses expands during finish annealing, causing preferential growth of {100}<011> oriented grains to an extent not previously seen, resulting in a dramatic improvement in magnetic flux density. Furthermore, abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and after finish annealing, the secondary recrystallized grains occupy the entire sheet surface. The few secondary recrystallized grains cover the entire steel sheet surface, and the grain size of each secondary recrystallized grain increases.

Furthermore, in the finish annealing process, the finish annealing conditions for "expanding the secondary recrystallization progression temperature range" disclosed in the above-mentioned Patent Documents 9 to 11 may be applied as needed. By using the hot-rolled annealed steel sheet according to this embodiment and applying the finish annealing conditions disclosed in Patent Documents 9 to 11, the secondary recrystallization progression temperature range can be expanded even more favorably.

The following insulating coating formation process and magnetic domain control process are not necessary from the viewpoint of concentrating the crystal orientation in {110}<001>, but are steps that are adopted for general grain-oriented electrical steel sheets to improve practical magnetic properties.
(Insulating film formation process)
The insulating coating formation step is a step of forming an insulating coating on the grain-oriented electrical steel sheet (finish-annealed steel sheet) after the finish-annealing step. An insulating coating mainly composed of phosphate and colloidal silica or an insulating coating mainly composed of alumina sol and boric acid may be formed on the steel sheet after the finish-annealing step.

(Magnetic domain control process)
The magnetic domain control step is a step of subdividing the magnetic domains of the grain-oriented electrical steel sheet. This step is carried out at an appropriate timing after cold rolling. For example, localized micro-strains or localized grooves may be formed in the grain-oriented electrical steel sheet by a known method such as laser, plasma, mechanical method, or etching.

6. Grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to this embodiment A brief description will be given of the grain-oriented electrical steel sheet produced using the hot-rolled annealed steel sheet according to this embodiment.

The hot-rolled annealed steel sheet according to this embodiment has both relatively fine precipitates and relatively coarse precipitates of a preferred size and distribution. Therefore, in the grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to this embodiment, Goss-oriented grains grow preferentially, resulting in a preferred increase in magnetic flux density. Furthermore, the grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment does not suffer from a deterioration in other properties due to the increased magnetic flux density, and can therefore be used in the same applications as conventional ones.

The grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment contains, as a base element (major alloying element), 2.0 to 7.0% Si (silicon) by mass fraction.

It may also contain impurities. "Impurities" refer to elements that are mixed in from raw materials such as ore or scrap during industrial steel production, or from the manufacturing environment, etc. The upper limit for the total impurity content may be, for example, 5%.

Furthermore, in addition to the basic elements and impurities described above, optional elements may be contained. For example, instead of a portion of the remaining Fe, optional elements such as Nb, V, Mo, Ta, W, C, Mn, S, Se, Al, N, Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be contained. These optional elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. These optional elements may also be contained as impurities.

In addition, grain-oriented electrical steel sheet undergoes relatively large changes in chemical composition (reduction in content) through decarburization annealing and purification annealing during secondary recrystallization. Purification annealing can reduce the content of some elements to a level that cannot be detected by standard analytical methods (1 ppm or less). Generally, the chemical composition of the final product differs from that of the starting material, the slab, but the optional elements listed above are elements contained in the slab that remain in the final product, and the content of each element will not exceed the content range stated above for the slab, but will be within a range that depends on the content in the slab and the subsequent manufacturing process.

The above chemical composition is that of grain-oriented electrical steel sheet. If the grain-oriented electrical steel sheet to be measured has an insulating coating on its surface, remove the coating using a known method before measuring the chemical composition.

The grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment may have an intermediate layer disposed in contact with the grain-oriented electrical steel sheet (silicon steel sheet), and an insulating coating disposed in contact with the intermediate layer.

For example, the intermediate layer may be a layer mainly made of oxide, a layer mainly made of carbide, a layer mainly made of nitride, a layer mainly made of boride, a layer mainly made of silicide, a layer mainly made of phosphide, a layer mainly made of sulfide, or a layer mainly made of an intermetallic compound. These intermediate layers are formed primarily to ensure adhesion between the silicon steel sheet and the insulating coating, and may be any known layer formed by heat treatment in an atmosphere with controlled oxidation-reduction, chemical vapor deposition (CVD), physical vapor deposition (PVD), or the like.

Typical examples of the insulating coating include an insulating coating that is primarily composed of phosphate and colloidal silica and has an average thickness of 0.1 to 10 μm, and an insulating coating that is primarily composed of alumina sol and boric acid and has an average thickness of 0.5 to 8 μm.

Next, the effects of the present invention will be explained in detail using examples. The conditions used in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the objectives of the present invention.

Hot-rolled and annealed steel sheets were manufactured using slabs with the chemical compositions shown in Tables 1 and 2. The chemical compositions of the manufactured hot-rolled and annealed steel sheets were equivalent to the chemical compositions of the slabs shown in Tables 1 and 2. These chemical compositions were measured based on the method described above. In Tables 1 and 2, "-" indicates that no control or manufacturing was carried out with the content in mind, and that the content was not measured.

The above hot-rolled and annealed steel sheets were manufactured based on the manufacturing conditions shown in Tables 3 to 9. Slab heating was performed by soaking at a specified temperature for a specified time without temporarily increasing the heating temperature during the slab heating process. The soaking temperature shown in the table indicates the surface temperature of the slab after heating, and the soaking time shown in the table indicates the slab heating time from when the slab surface temperature reached the soaking temperature.

Although not shown in the table, when the total content of Nb group elements is 0.0030 to 0.030 mass%, by setting the slab soaking temperature to more than 1030°C and less than 1180°C and the slab soaking time to more than 70 minutes, 12 to 85 volume % of the precipitates will be solutionized, based on the precipitates contained in the slab after the casting process.

In the hot-rolled sheet annealing process, the hot-rolled steel sheet after the hot rolling process was annealed. In Examples other than No. 100, the hot-rolled steel sheet after the hot rolling process was annealed under the annealing conditions shown in Tables 3 to 9. In this case, the heating rate to the first-stage annealing temperature was an average of 5°C/second or more, and the holding time in second-stage annealing was 20 seconds or more. In Example No. 100, second-stage annealing was not performed.

The average grain size and precipitation morphology of the produced hot-rolled annealed steel sheets were investigated using the methods described above. The precipitation morphology of precipitates with a circular equivalent diameter D of 50 to 1000 nm is shown in Tables 10 to 16. In the tables, Dp represents the most frequent diameter, f(Dp) represents the number density of the most frequent diameter, and Wp represents the half-width of the most frequent diameter.

The hot-rolled and annealed steel sheets were then cold-rolled and decarburized under known conditions. The cold-rolling reduction was 90.7% and the sheet thickness was 0.26 mm. The decarburization annealing temperature was between 830 and 860°C, and the steel sheets were annealed for 90 seconds. After decarburization annealing, the steel sheets were then nitrided in a mixed atmosphere of hydrogen, nitrogen, and ammonia, resulting in a nitrogen content of 0.020 to 0.023 mass% (200 to 230 ppm).

Furthermore, an annealing separator primarily composed of MgO was applied to the steel sheet, which was then subjected to finish annealing. In the final stage of the finish annealing, the steel sheet was held in a hydrogen atmosphere at 1200°C for 20 hours (purification annealing), and then naturally cooled.

A coating solution for forming an insulating coating, primarily composed of phosphate and colloidal silica with chromium, was applied to the primary coating (intermediate layer) formed on the surface of the manufactured grain-oriented electrical steel sheet (finish-annealed steel sheet). The sheet was then heated and held in an atmosphere of 75% by volume hydrogen:25% by volume nitrogen, and then cooled to form an insulating coating.

When viewed on a cross section with the cutting direction parallel to the thickness direction, the manufactured grain-oriented electrical steel sheet had an intermediate layer placed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating placed in contact with this intermediate layer. The intermediate layer was a forsterite coating with an average thickness of 2 μm, and the insulating coating was an insulating coating primarily composed of phosphate and colloidal silica with an average thickness of 1 μm.

The obtained grain-oriented electrical steel sheets were evaluated for various properties. The evaluation results are shown in Tables 10 to 16.

(1) 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.

As magnetic properties, the magnetic flux density _B8 (T) in the rolling direction of the steel sheet when excited at 800 A/m was measured. A magnetic flux density _B8 of 1.945 T or more was judged to be acceptable. For reference, iron loss W17 _/50 (W/kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was also measured under conditions of an AC frequency of 50 Hz and an excitation magnetic flux density of 1.7 T.

Among Nos. 1 to 103, the particle size-number density distribution of the precipitates contained in the hot-rolled annealed steel sheet was well controlled in the inventive examples, and all of them exhibited excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, among Nos. 1 to 103, the comparative examples did not have well controlled particle size-number density distribution of the precipitates contained in the hot-rolled annealed steel sheet, and did not achieve the magnetic flux density desirable for grain-oriented electrical steel sheets.

The above aspects of the present invention make it possible to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density, as well as a manufacturing method thereof, and therefore have high industrial applicability.

Claims (1)

In hot-rolled annealed steel sheets for grain-oriented electrical steel sheets,
The hot-rolled annealed steel sheet comprises, in mass%,
C: 0.0010 to 0.10%,
Si: 2.0 to 7.0%,
Mn: 0.050 to 1.0%,
S: 0 to 0.0350%,
Se: 0 to 0.0350%,
S+Se total content: 0.0030-0.0350%,
Al: 0.010-0.0650%,
N: 0.0040-0.0120%,
Nb: 0 to 0.030%,
V: 0 to 0.030%,
Mo: 0 to 0.030%,
Ta: 0 to 0.030%,
W: 0-0.030%,
Nb + V + Mo + Ta + W total content: 0.0030 to 0.030%,
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0-0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0-1.0%,
and the balance being Fe and impurities,
Regarding the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among the precipitates that are residues obtained by electrolytic extraction of the hot-rolled annealed steel sheet,
The mode diameter is Dp in units of nm,
The number density of the most frequent diameter is represented by f(Dp) in units of particles/g,
When the half width of the mode diameter is Wp in the unit of nm,
Dp is 100 to 300 nm,
f(Dp) is 1,000,000 particles/g or more;
Wp/Dp is 1.0 to 2.0,
Fulfilling
The average grain size of the hot-rolled annealed steel sheet is 20.0 to 21.5 μm.
A hot-rolled annealed steel sheet for grain-oriented electrical steel sheet, characterized in that