JP7159595B2 - Manufacturing method of grain-oriented electrical steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet.

A grain-oriented electrical steel sheet is a steel sheet containing about 0.5 to 7% by mass of Si and having crystal orientations of {110}<001> orientation (Goss orientation). Grain-oriented electrical steel sheets are used as core materials for transformers and other electrical equipment as soft magnetic materials. A catastrophic grain growth phenomenon called secondary recrystallization is used to control the crystal orientation of grain-oriented electrical steel sheets.

A method for producing a grain-oriented electrical steel sheet is as follows. The slab is heated and hot rolled to produce a hot rolled steel sheet. The hot-rolled steel sheet is annealed as necessary. The hot-rolled steel sheet is pickled as necessary. The pickled hot-rolled steel sheet is cold-rolled one or more times at a cumulative cold-rolling rate of 80% or more to produce a cold-rolled steel sheet. The cold-rolled steel sheet is decarburized and annealed to develop primary recrystallization. After the decarburization annealing, the cold-rolled steel sheet is subjected to finish annealing to develop secondary recrystallization. A grain-oriented electrical steel sheet is manufactured by the above steps.

A grain-oriented electrical steel sheet is required to have magnetic properties, particularly excellent excitation properties and iron loss properties. B8, which is the magnetic flux density at a magnetic field strength of 800 A/m, is used as an index indicating the excitation characteristics of a grain-oriented electrical steel sheet. W _17/50 , which is the iron loss per unit mass when magnetized to 1.7 T at 50 Hz, is used as an index indicating the iron loss characteristics of a grain-oriented electrical steel sheet.

In recent years, there has been an increasing demand for further improvements in iron loss properties of grain-oriented electrical steel sheets. This is because the efficiency of generators and transformers is increased by further reducing core loss of grain-oriented electrical steel sheets.

Iron loss consists of hysteresis loss and eddy current loss. Hysteresis loss is affected by the purity, internal strain, crystal orientation, and the like of the grain-oriented electrical steel sheet. The eddy current loss is affected by the electrical resistance, thickness, grain size, size of the magnetic domain, tension of the film formed on the surface of the steel sheet, and the like of the grain-oriented electrical steel sheet.

If the Si content is increased, the electrical resistance of the steel sheet increases, thereby reducing the eddy current loss. Therefore, it is considered effective to increase the Si content in order to reduce iron loss. Techniques for improving iron loss characteristics in grain-oriented electrical steel sheets with an increased Si content have been proposed in Japanese Patent Application Laid-Open Nos. 2001-192733 (Patent Document 1) and 5-345921 (Patent Document 2). there is

In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 1, in mass %, Si: 3.0 to 3.8%, Mn: 0.03 to 0.45%, S, Se: single or combined After heating an electrical steel slab containing 0.15% or less, acid-soluble Al: 0.015 to 0.035%, and N: 0.0035 to 0.012% to a temperature of 1250 ° C. or less After rolling, hot-rolled sheet annealing, cold rolling to obtain the final sheet thickness, decarburization annealing, nitriding treatment, and finish annealing are then performed. Then, when the thickness of the hot-rolled sheet is tA (mm) and the thickness of the final cold-rolled sheet is tC (mm), tA/tC is 3 according to the Si content (Si (%)). .57−0.43×Si(%)≦ln(tA/tC)≦4.58−0.64×Si(%). In Patent Document 1, by adjusting the cold rolling rate (tA/tC) within the range of the above formula, the Goss orientation in the primary recrystallization is effectively increased, thereby increasing the Goss orientation density in the secondary recrystallization. It is described that it is possible to increase the Si content, and as a result, it is possible to obtain iron loss characteristics according to the Si content.

In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 2, in mass%, C: 0.090% or less, Si: 2.5 to 4.5%, Mn: 0.03 to 0.15% , S: 0.010 to 0.050%, acid-soluble Al: 0.010 to 0.050%, N: 0.0045 to 0.012%, Sn: 0.03 to 0.5%, Cu: 0 An electrical steel slab containing 0.02 to 0.3% Ni, 0.05 to 1.0% Ni, and the balance being Fe and unavoidable impurities is heated to 1250 ° C. or higher, hot rolled, precipitation annealed, and finally Cold rolling with a cold rolling rate of 80% or more, decarburization annealing, and finish annealing are applied. Patent Document 2 describes that magnetic properties (magnetic flux density and iron loss) are enhanced by further containing Ni in the chemical composition of grain-oriented electrical steel sheets with a high Si content.

Japanese Patent Laying-Open No. 1-290716 (Patent Document 3) proposes a method for reducing the iron loss of an electrical steel sheet by a method different from that of Patent Documents 1 and 2. Specifically, in Patent Document 3, in decarburization annealing, a steel sheet is heated to 675° C. or higher at a heating rate of 100° C./sec or higher. Patent Document 3 describes that iron loss is reduced by performing rapid heating at 100° C./second or more during decarburization annealing. In the examples, a steel sheet having a Si content of 2.96% and a thickness of 0.25 mm was heated at a temperature rising rate of 1140 to 555° C./sec, and then decarburized and annealed. disclosed.

Japanese Patent Application Laid-Open No. 2001-192733 JP-A-5-345921 JP-A-1-290716

By the way, as one of the methods for reducing the iron loss of the grain-oriented electrical steel sheet, there is a method of reducing the thickness of the steel sheet to reduce the eddy current loss. Recently, further thinning of grain-oriented electrical steel sheets has been studied, and specifically, ultra-thin grain-oriented electrical steel sheets having a thickness of 0.22 mm or less have been studied.

As a method of thinning a steel sheet, there is a method of thinning the steel sheet in a cold rolling process. As described above, cold rolling is performed one or more times in the cold rolling process during the manufacturing process of the grain-oriented electrical steel sheet.

However, when the steel sheet is thinned in the cold rolling process, if the cold rolling reduction in the final cold rolling is increased, the Goss grains of the primary recrystallization are reduced, and as a result, the magnetic properties of the grain-oriented electrical steel sheet are reduced. may be lower. In order to avoid this, it is conceivable to perform cold rolling a plurality of times in the cold rolling process to suppress the cold rolling rate of the final cold rolling. In this case, the magnetic properties can be ensured while reducing the thickness of the steel sheet.

However, if cold rolling is performed multiple times in the cold rolling process, the productivity will decrease. Even when the thickness of the steel sheet is reduced for the purpose of reducing iron loss, it is preferable to be able to suppress the decrease in productivity.

An object of the present disclosure is to provide a method for producing a grain-oriented electrical steel sheet that is extremely thin with a thickness of 0.17 to 0.22 mm and that can increase the productivity of a grain-oriented electrical steel sheet that has excellent magnetic properties. It is to be.

A method for manufacturing a grain-oriented electrical steel sheet according to the present disclosure includes a hot rolling process, a pre-cold rolling annealing process, a cold rolling process, a decarburization annealing process, an annealing separator application process, and a finish annealing process. Prepare.
In the hot rolling process, the chemical composition is mass%, C: 0.020 to 0.100%, Si: 3.20 to 3.70%, Mn: 0.010 to 0.300%, S and / or Se: 0.001 to 0.050% in total, sol. Al: 0.020 to 0.028%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , the remainder: Fe and impurities, and when the thickness of the steel sheet after the cold rolling process is defined as D (mm), the slab that satisfies the formulas (1) and (2) is subjected to hot rolling. to produce steel sheets.
2×D+2.86≦Si≦2×D+3.26 (1)
0.04×D+0.0132≦sol. Al≦0.04×D+0.0192 (2)
Here, the content (% by mass) of the corresponding element in the chemical composition of the slab is substituted for the element symbol in formulas (1) and (2).
In the pre-cold-rolling annealing step, the steel sheet after the hot-rolling step is annealed.
In the cold-rolling process, the steel sheet after the pre-cold-rolling annealing process is cold-rolled at a cold-rolling rate of 89.0 to 93.0% without performing annealing treatment in the middle. A steel plate having a thickness D of 0.17 to 0.22 mm is produced.
The decarburization annealing step includes a temperature raising step and a decarburization step. In the temperature raising step, the steel plate after the cold rolling step is heated to the decarburization annealing temperature, and the steel plate is heated at an average temperature raising rate of 800 to 2400 ° C./sec until the temperature of the steel plate reaches at least 500 to 700 ° C. heat up. In the decarburization step, decarburization annealing is performed while holding the steel sheet at a decarburization annealing temperature of 800 to 950°C.
In the annealing separator application step, an annealing separator is applied to the surface of the steel sheet after the decarburization annealing step.
In the finish annealing step, finish annealing is performed on the steel sheet to which the annealing separator is applied.

The method for producing a grain-oriented electrical steel sheet according to the present disclosure can increase the productivity of a grain-oriented electrical steel sheet that is extremely thin with a thickness of 0.17 to 0.22 mm and has excellent magnetic properties.

FIG. 1 is a flowchart showing manufacturing steps of a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment. FIG. 2 is a schematic diagram showing a heat pattern in the decarburization annealing step in FIG.

In the process of manufacturing an ultra-thin grain-oriented electrical steel sheet having a thickness of 0.17 to 0.22 mm, if cold rolling is performed multiple times, the productivity will decrease. Therefore, the present inventors did not perform cold rolling multiple times in the cold rolling process in the manufacturing process of ultra-thin grain-oriented electrical steel sheets with a thickness of 0.17 to 0.22 mm. was considered to improve productivity by performing only once.

Here, "performing cold rolling once" means that the rolling of the steel sheet in the cold rolling process is completed without performing the annealing treatment during the cold rolling. For example, when cold rolling is performed in a plurality of passes using a reverse rolling mill, one or more passes of cold rolling are performed without interposing an annealing treatment between cold rolling passes. When cold rolling is performed, it corresponds to "performing cold rolling once". Note that, for example, when the annealing treatment is performed once during the cold rolling of multiple passes, it corresponds to "performing the cold rolling twice". Here, the "pass" means that the steel plate is passed through a rolling stand to give a reduction to the steel plate, and one pass means that the steel plate is passed through the rolling stand once to give a reduction to the steel plate. means

Therefore, a method for enhancing the magnetic properties of an ultra-thin grain-oriented electrical steel sheet manufactured by cold rolling only once was investigated. As a result, the present inventors found that by increasing the Si content to 3.20% or more and reducing the Al content to 0.028% or less, an ultra-thin grain-oriented electrical steel sheet has sufficient magnetic properties. I thought I could get the properties. Specifically, the chemical composition is mass% C: 0.020 to 0.100%, Si: 3.20 to 3.70%, Mn: 0.010 to 0.300%, S and / or Se: 0.001 to 0.050% in total, sol. Al: 0.020 to 0.028%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , and the balance: Fe and impurities.

However, it has been found that if the Si content is increased to 3.20 to 3.70% in an ultra-thin grain-oriented electrical steel sheet that is manufactured by performing cold rolling only once, the magnetic properties deteriorate. rice field. Accordingly, the inventors have investigated the cause of the deterioration of the magnetic properties. As a result, it was thought that the magnetic properties would deteriorate for the following reasons.

When the Si content is increased to 3.20% or more, the ferrite ratio increases in the steel sheet during hot rolling. In this case, during hot rolling, an α-fiber orientation group, which is a rolling stable orientation, develops at the center of the steel sheet. The α-fiber orientation group means a crystal grain group in which the <110> axis of the crystal is along the rolling direction. In this embodiment, cold rolling is performed only once. In this case, the α-fiber orientation group generated in the hot rolling process tends to remain even in the steel sheet after the cold rolling process. This α-fiber orientation group deteriorates the primary recrystallized structure, and as a result, selective growth of Goss orientation is suppressed during secondary recrystallization in the final annealing step. Therefore, it is considered that the degree of integration in the Goss orientation decreases, and the magnetic properties decrease.

Therefore, even if the Si content is increased to 3.20 to 3.70% in ultra-thin grain-oriented electrical steel sheets manufactured by performing cold rolling only once, the degree of integration in the Goss orientation does not decrease. The inventors have studied how to suppress this. As a result, the following findings were obtained.

In the heating step of the decarburization annealing step in the manufacturing process of the grain-oriented electrical steel sheet, the average heating rate RR _500-700 in the temperature range of 500-700° C. is set to 800-2400° C./sec. In this case, the primary recrystallized structure is improved, the density of the Goss orientation of the grain-oriented electrical steel sheet is increased, and the magnetic properties are enhanced.

Although the reason for this is not clear, the following reasons are conceivable. By increasing the heating rate in the temperature range of 500 to 700 ° C., in the primary recrystallization, the form of recrystallization (primary recrystallization) from the α-fiber orientation group is changed, and the α-fiber orientation group is changed to the Goss orientation. It increases the production of {411}<148> grains with the corresponding orientation relationship of Σ9. Here, the Σ9 corresponding orientation grains are orientation crystal grains represented by {411} <148>, {111} <112>, etc., which enhance the selective growth of the Goss orientation during secondary recrystallization. grains. It is considered that the increase in grains corresponding to the Σ9 orientation increases the degree of integration of the Goss orientation in the secondary recrystallization, resulting in excellent magnetic properties.

However, even if the temperature rise rate from 500 to 700 ° C. in the decarburization annealing process is 800 ° C. or more, in the ultra-thin grain-oriented electrical steel sheet with the above chemical composition manufactured by cold rolling only once , it was found that the magnetic properties may still be low. Therefore, the present inventors conducted further studies. As a result, in the case of the ultra-thin grain-oriented electrical steel sheet with the chemical composition described above, which is manufactured by performing cold rolling only once, the sheet thickness is thin, so the relationship between the sheet thickness, the Si content, and the Al content was found to affect the magnetic properties. Therefore, as a result of further investigation, in the production of the ultra-thin grain-oriented electrical steel sheet with the chemical composition described above, if the following formulas (1) and (2) are further satisfied, the ultra-thin grain having excellent magnetic properties It was found that a grain-oriented electrical steel sheet can be produced by one cold rolling.
2×D+2.86≦Si≦2×D+3.26 (1)
0.04×D+0.0132≦sol. Al≦0.04×D+0.0192 (2)
Here, the content (% by mass) of the corresponding element in the chemical composition of the slab is substituted for the element symbol in formulas (1) and (2). D is the plate thickness (mm) of the steel plate after the cold rolling process (that is, the cold-rolled steel plate).

The method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment completed based on the above findings includes a hot rolling process, an annealing process before cold rolling, a cold rolling process, a decarburizing annealing process, and an annealing separator application process. and a finish annealing step.
In the hot rolling process, the chemical composition is mass%, C: 0.020 to 0.100%, Si: 3.20 to 3.70%, Mn: 0.010 to 0.300%, S and / or Se: 0.001 to 0.050% in total, sol. Al: 0.020 to 0.028%, N: 0.002 to 0.015%, Sn: 0 to 0.500%, Cr: 0 to 0.500%, Cu: 0 to 0.500%, and , the remainder: Fe and impurities, and when the thickness of the steel sheet after the cold rolling process is defined as D (mm), the slab that satisfies the formulas (1) and (2) is subjected to hot rolling. to produce steel sheets.
2×D+2.86≦Si≦2×D+3.26 (1)
0.04×D+0.0132≦sol. Al≦0.04×D+0.0192 (2)
Here, the content (% by mass) of the corresponding element in the chemical composition of the slab is substituted for the element symbol in formulas (1) and (2).
In the pre-cold-rolling annealing step, the steel sheet after the hot-rolling step is annealed.
In the cold-rolling process, the steel sheet after the pre-cold-rolling annealing process is cold-rolled at a cold-rolling rate of 89.0 to 93.0% without performing annealing treatment in the middle. A steel plate having a thickness D of 0.17 to 0.22 mm is produced.
The decarburization annealing step includes a temperature raising step and a decarburization step. In the temperature raising step, the steel plate after the cold rolling step is heated to the decarburization annealing temperature, and the steel plate is heated at an average temperature raising rate of 800 to 2400 ° C./sec until the temperature of the steel plate reaches at least 500 to 700 ° C. heat up. In the decarburization step, decarburization annealing is performed while holding the steel sheet at a decarburization annealing temperature of 800 to 950°C.
In the annealing separator application step, an annealing separator is applied to the surface of the steel sheet after the decarburization annealing step.
In the finish annealing step, finish annealing is performed on the steel sheet to which the annealing separator is applied.

The chemical composition of the slab is selected from the group consisting of Sn: 0.005 to 0.500%, Cr: 0.010 to 0.500%, and Cu: 0.010 to 0.500%1 It may contain more than one seed.

Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment will be described in detail. In addition, in this specification, % regarding the content of an element means mass %, unless otherwise specified.

[Manufacturing process flow]
FIG. 1 is a flowchart of a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment. Referring to FIG. 1, this manufacturing method includes a hot rolling step (S1) of hot rolling a slab, and an annealing treatment of a steel plate (hot rolled steel plate) after hot rolling. A pre-cold rolling annealing step (S2), a cold rolling step (S3) in which the steel plate after the pre-cold rolling annealing step is cold rolled once, and a steel plate after the cold rolling step (cold A decarburization annealing step (S4) for decarburizing and annealing the rolled steel sheet), an annealing separator application step (S5) for applying an annealing separator to the surface of the steel sheet after the deoxidizing annealing step, and an annealing separator and a finish annealing step (S6) of performing finish annealing on the steel plate to which is applied. Each step S1 to S6 will be described below.

[Hot rolling step (S1)]
In the hot rolling step (S1), the prepared slab is hot rolled to manufacture a hot rolled steel sheet. The chemical composition of the slab contains the following elements:

[Essential elements in the chemical composition of the slab]
C: 0.020-0.100%
Carbon (C) is effective for structure control during the manufacturing process until the decarburization annealing process is completed. However, if the C content is less than 0.020%, the above effect cannot be sufficiently obtained. On the other hand, if the C content exceeds 0.100%, decarburization becomes insufficient and magnetic aging occurs even if the decarburization annealing step described later is performed. In this case, sufficient iron loss characteristics cannot be obtained. Therefore, the C content is 0.020-0.100%. A preferred lower limit for the C content is 0.030%, more preferably 0.040%. A preferable upper limit of the C content is 0.090%, more preferably 0.080%.

Si: 3.20-3.70%
Silicon (Si) increases the electrical resistance of an ultra-thin grain-oriented electrical steel sheet having a thickness of 0.17 to 0.22 mm, and reduces eddy current loss among core losses. If the Si content is less than 3.20%, the above effect cannot be sufficiently obtained. On the other hand, if the Si content exceeds 3.70%, the cold workability of the steel deteriorates. Therefore, the Si content is 3.20-3.70%. A preferred lower limit for the Si content is 3.25%, more preferably 3.30%. A preferable upper limit of the Si content is 3.65%, more preferably 3.60%.

Mn: 0.010-0.300%
Manganese (Mn) increases the specific resistance of grain-oriented electrical steel sheets to reduce core loss. Mn further enhances hot workability and suppresses the occurrence of cracks during hot rolling. Mn further combines with S and/or Se to form fine MnS and/or fine MnSe in the pre-cold rolling annealing step. Fine MnS and fine MnSe serve as precipitation nuclei for fine AlN that is utilized as an inhibitor. Therefore, in the pre-cold-rolling annealing step, if the amounts of fine MnS and fine MnSe precipitated are large, a sufficient amount of fine AlN can be obtained. If the Mn content is less than 0.010%, a sufficient amount of fine MnS and fine MnSe will not precipitate. On the other hand, if the Mn content exceeds 0.300%, the magnetic flux density of the grain-oriented electrical steel sheet decreases. Therefore, the Mn content is 0.010-0.300%. A preferred lower limit for the Mn content is 0.020%, more preferably 0.030%. A preferable upper limit of the Mn content is 0.200%, more preferably 0.150%.

S and / or Se: 0.001 to 0.050% in total
Sulfur (S) and selenium (Se) combine with Mn during the manufacturing process to form fine MnS and/or fine MnSe described above. Fine MnS and fine MnSe serve as precipitation nuclei for fine AlN that is utilized as an inhibitor. Therefore, in the pre-cold-rolling annealing step, if the amounts of fine MnS and fine MnSe precipitated are large, a sufficient amount of fine AlN can be obtained. If the total content of S and/or Se is less than 0.001%, sufficient amounts of fine MnS and fine MnSe cannot be obtained. On the other hand, if the total content of S and/or Se exceeds 0.050%, MnS and/or MnSe may remain even in the steel sheet after the finish annealing process. In this case, the magnetic properties are degraded. Therefore, the total content of S and/or Se is 0.001-0.050%. A preferred lower limit for the total content of S and/or Se is 0.005%. A preferable upper limit of the total content of S and/or Se is 0.040%, more preferably 0.030%.

sol. Al: 0.020-0.028%
Aluminum (Al) combines with N to form AlN and functions as an inhibitor during the manufacturing process of the grain-oriented electrical steel sheet. sol. If the Al content is less than 0.020%, a sufficient amount of AlN that functions as an inhibitor can be obtained when a steel sheet having a thickness D of 0.17 to 0.22 mm is produced by one cold rolling. can't On the other hand, sol. If the Al content exceeds 0.028%, fine AlN decreases when a steel sheet having a thickness D of 0.17 to 0.22 mm is produced by one cold rolling (that is, coarse AlN increases), and the inhibitor strength of AlN decreases when a steel sheet having a thickness D of 0.17 to 0.22 mm is produced by one cold rolling. Therefore, sol. The Al content is 0.020-0.028%. sol. A preferable lower limit of the Al content is 0.021%, more preferably 0.022%. sol. A preferable upper limit of the Al content is 0.027%, more preferably 0.026%. In this specification, sol. Al means acid-soluble Al. Therefore, sol. The Al content is the content of acid-soluble Al.

N: 0.002-0.015%
Nitrogen (N) combines with Al to form AlN and functions as an inhibitor during the manufacturing process of the grain-oriented electrical steel sheet. In order to reduce the N content to less than 0.002%, excessive refining is required in the steelmaking process, which increases production costs. Therefore, the lower limit of the N content is 0.002%. On the other hand, if the N content in the steel material exceeds 0.015%, a large number of blisters (voids) are likely to form in the steel sheet during cold rolling. Therefore, the N content is 0.002-0.015%. A preferable lower limit of the N content is 0.004%, more preferably 0.006%. A preferable upper limit of the N content is 0.012%, more preferably 0.0010%.

The balance of the chemical composition of the slab according to this embodiment consists of Fe and impurities. Here, impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when slabs, which are materials of grain-oriented electrical steel sheets, are industrially manufactured. It means a permissible range that does not adversely affect the grain-oriented electrical steel sheet manufactured by the manufacturing method.

[Arbitrary element in chemical composition of slab]
The chemical composition of the slab described above may contain one or more selected from the group consisting of Sn, Cr and Cu in place of part of Fe.

Sn: 0-0.500%
Tin (Sn) is an optional element and may not be contained. That is, the Sn content may be 0%. When included, Sn improves the properties of the oxide layer formed during the decarburization annealing process, and also improves the properties of the primary coating formed using this oxide layer during the final annealing process. Further, Sn stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. Sn is also a grain boundary segregation element and stabilizes secondary recrystallization. However, if the Sn content exceeds 0.500%, the surface of the steel sheet becomes difficult to oxidize, and the formation of the primary coating may become insufficient. Therefore, the Sn content is 0-0.500%. The lower limit of the Sn content is preferably over 0%, more preferably 0.005%, still more preferably 0.010%, still more preferably 0.020%. A preferable upper limit of the Sn content is 0.300%, more preferably 0.200%.

Cr: 0-0.500%
Chromium (Cr) is an optional element and may not be contained. That is, the Cr content may be 0%. When included, Cr improves the properties of the oxide layer formed during the decarburization annealing process, and also improves the properties of the primary coating formed using this oxide layer during the final annealing process. Furthermore, Cr stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. However, if the Cr content exceeds 0.500%, the formation of the primary coating may become unstable. Therefore, the Cr content is 0-0.500%. A preferable lower limit of the Cr content is more than 0%, more preferably 0.010%, still more preferably 0.020%. A preferable upper limit of the Cr content is 0.200%, more preferably 0.150%.

Cu: 0-0.500%
Copper (Cu) is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu promotes the precipitation of fine MnS that serves as nuclei for the formation of AlN in the pre-cold-rolling annealing step. However, if the Cu content is too high, CuS precipitates are precipitated, and the CuS precipitates may remain even after final annealing. If CuS precipitates remain in the steel, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. Therefore, the Cu content is 0-0.500%. A preferable lower limit of the Cu content is more than 0%, more preferably 0.010%, still more preferably 0.030%, still more preferably 0.050%. A preferable upper limit of the Cu content is 0.400%, more preferably 0.300%.

[Regarding formula (1) and formula (2)]
The chemical composition of the slab described above further satisfies formulas (1) and (2), where D (mm) is the thickness of a steel sheet produced by the cold rolling process (cold-rolled steel sheet).
2×D+2.86≦Si≦2×D+3.26 (1)
0.04×D+0.0132≦sol. Al≦0.04×D+0.0192 (2)
Here, the contents (% by mass) of the corresponding elements are substituted for the element symbols in the formulas (1) and (2).

[Regarding formula (1)]
Formula (1) defines an appropriate Si content for the thickness D of the cold-rolled steel sheet. As described above, Si increases the electrical resistance (specific resistance) of the steel sheet and reduces the eddy current loss, thereby reducing iron loss. However, in the case of an ultra-thin plate with a thickness of 0.17 to 0.22 mm, even if the Si content is 3.20% or more, the Si content is the lower limit of formula (1) (2 × D + 2.86) or more. Otherwise, a sufficiently low iron loss cannot be obtained in the grain-oriented electrical steel sheet manufactured by the manufacturing method of the present embodiment.

On the other hand, in the case of an extremely thin steel sheet with a thickness of 0.17 to 0.22 mm, if the Si content is too high relative to the thickness D, the ferrite ratio in the steel sheet increases during the hot rolling process. In this case, during the hot rolling process, a group of α-fiber orientations develops in the thickness central portion. Even if the Si content is 3.70% or less, if the upper limit (2 × D + 3.26) of formula (1) is exceeded, the α-fiber orientation group will remain excessively even after cold rolling. Even if the decarburization annealing step of the morphology is performed, the primary recrystallized structure is not sufficiently improved. As a result, the Goss-oriented grains do not grow sufficiently in the secondary recrystallization, and the magnetic properties deteriorate.

If the Si content in the chemical composition of the slab is 3.20 to 3.70% and the formula (1) is satisfied, the other production conditions of the present embodiment including the formula (2) are satisfied. A grain-oriented electrical steel sheet having excellent magnetic properties can be obtained even if cold rolling is performed only once.

A preferable lower limit of formula (1) is 2×D+2.88, more preferably 2×D+2.90. A preferable upper limit of formula (1) is 2×D+3.20, more preferably 2×D+3.15.

[Regarding formula (2)]
Formula (2) defines an appropriate Al content for the thickness D of the cold-rolled steel sheet. As described above, by suppressing the Al content to 0.028% or less, fine AlN precipitated in the steel sheet can be maintained. In this case, the difference in grain size between the crystal grains in the surface layer portion and the crystal grains in the thickness center portion can be increased. Specifically, due to the inhibitory action of fine AlN, the crystal grains in the thickness central portion can be kept smaller than the crystal grains in the surface layer portion. As a result, it is possible to ensure the driving force for grain growth during secondary recrystallization caused by the difference in grain size between the crystal grains in the surface layer and the crystal grains in the central portion of the sheet thickness.

Here, the difference in grain size between the crystal grains in the surface layer formed by AlN and the crystal grains in the central portion of the sheet thickness is closely related to the sheet thickness D of the cold-rolled steel sheet. Even if the Al content is 0.028% or less, if the upper limit (0.04 × D + 0.0192) of formula (2) is exceeded, the Al content is too large for the plate thickness D, and the plate thickness D Corresponding fine AlN cannot be sufficiently secured. In this case, it is not possible to ensure a sufficient grain size difference between the crystal grains in the surface layer and the crystal grains in the center of the plate thickness in the steel plate with a thickness D, and the Goss orientation grains are preferentially grown during secondary recrystallization. A sufficient driving force for grain growth cannot be obtained. As a result, Goss-oriented grains do not grow sufficiently in the finish annealing step, and the magnetic properties of the grain-oriented electrical steel sheet deteriorate.

On the other hand, even if the Al content is 0.020% or more, if it is less than the lower limit (0.04 × D + 0.0132) of formula (2), the Al content is too small with respect to the plate thickness D, In the steel sheet of thickness D, the grain size difference between the crystal grains in the surface layer and the crystal grains in the central portion of the thickness becomes excessively large, and the driving force for grain growth is excessively increased. As a result, the grains of the grain orientation other than the grains of the Goss orientation, which are superior in magnetic properties, also undergo secondary recrystallization, and the magnetic properties of the grain-oriented electrical steel sheet deteriorate.

If the Al content in the chemical composition of the slab is 0.020 to 0.028% and the formula (2) is satisfied, then two It is possible to apply the necessary amount of grain growth driving force to preferentially grow the Goss orientation in the next recrystallization. Therefore, Goss-oriented grains can be sufficiently grown in the secondary recrystallization, and the magnetic properties of the grain-oriented electrical steel sheet are enhanced on the premise that the other manufacturing conditions of the present embodiment are satisfied.

A preferable lower limit of formula (2) is 0.04×D+0.0142, more preferably 0.04×D+0.0152. A preferable upper limit of formula (2) is 0.04×D+0.0182, more preferably 0.04×D+0.0172.

[Method for producing slab having the above chemical composition]
An example of a method for manufacturing a slab having the above chemical composition is as follows. Molten steel having the above chemical composition is produced (smelted). Using molten steel, a slab is manufactured by a continuous casting method.

[Hot rolling step (S1) using the above slab]
The prepared slab having the above chemical composition is hot-rolled using a hot rolling mill to produce a steel sheet (hot-rolled steel sheet). First, the steel material is heated. For example, the slab is loaded into a known heating furnace or a known soaking furnace and heated. The preferred heating temperature for the slab is 1300-1400°C, more preferably 1320-1380°C.

The heated slab is hot rolled using a hot rolling mill to produce a steel plate (hot rolled steel plate). The hot rolling mill comprises a roughing mill and a finishing mill arranged downstream of the roughing mill. A roughing mill comprises one or more roughing stands in a row. Each roughing stand includes a plurality of rolls arranged one above the other. Rough rolling may be rolling using one reverse type rough rolling stand, or rolling using a tandem type rough rolling mill having a plurality of rough rolling stands arranged in a line. The finishing mill comprises a plurality of finishing stands arranged in a row. Each finishing stand includes a plurality of rolls arranged one above the other. After the heated slab is rolled by the rough rolling mill, it is further rolled by the tandem type finishing rolling mill to produce a hot-rolled steel sheet.

The thickness of the hot-rolled steel sheet manufactured by hot rolling is not particularly limited, and can be a known thickness. The finishing temperature in the hot rolling process (the temperature of the steel sheet at the delivery side of the finishing rolling stand where the steel sheet is finally reduced in the finishing rolling mill) is, for example, 900 to 1100°C. The finishing temperature is the surface temperature (° C.) of the steel sheet obtained by a thermometer placed on the delivery side of the finishing rolling stand that performs the final reduction. A steel plate is manufactured by the rolling process described above.

[Annealing step before cold rolling (S2)]
In the pre-cold-rolling annealing step, the steel plate (hot-rolled steel plate) after the hot-rolling step is annealed. Well-known conditions are sufficient for the conditions of the annealing treatment in this step. The annealing temperature in the annealing step before cold rolling is, for example, 900 to 1200° C., and the holding time at the annealing temperature is 30 to 180 seconds.

[Cold rolling step (S3)]
In the cold rolling step (S3), the manufactured steel plate is cold rolled at a cold rolling rate of 89.0 to 93.0% without performing annealing treatment in the middle, and the plate thickness D A steel plate (cold-rolled steel plate) with a thickness of 0.17 to 0.22 mm is produced. Here, "without performing annealing treatment in the middle" means performing cold rolling once. “Cold rolling is performed once” means that the rolling of the steel sheet in this step is completed without performing the annealing treatment during the cold rolling. For example, when cold rolling is performed in one pass using a reverse rolling mill, it corresponds to "performing cold rolling once". In addition, when cold rolling is performed in multiple passes using a reverse rolling mill, multiple passes of cold rolling are performed without interposing an annealing treatment between cold rolling passes. If it does, it corresponds to "perform cold rolling once". Note that, for example, when the annealing treatment is performed once during the cold rolling of multiple passes, it corresponds to "performing the cold rolling twice". Here, "pass" means that a steel plate is passed through a rolling stand to give a reduction, and "one pass" means that a steel plate is passed through a rolling stand once and given a reduction.

In the present embodiment, in order to increase productivity, cold rolling is performed only once to produce a cold-rolled steel sheet having a thickness D of 0.17 to 0.22 mm. A preferable upper limit of the plate thickness D is 0.20 mm.

The cold rolling rate in the cold rolling process is 89.0-93.0%. Here, the cold rolling rate (%) is defined as follows.
Cold rolling rate (%) = 100 - thickness of cold-rolled steel sheet after final cold rolling/thickness of steel sheet before the start of first cold rolling x 100

[Decarburization annealing step (S4)]
In the decarburization annealing step (S4), the steel sheet (cold-rolled steel sheet) after the cold rolling step (S3) is subjected to decarburization annealing to develop primary recrystallization. In the present embodiment, when the temperature is raised in the decarburization annealing process, the average temperature increase rate in the temperature range of 500 to 700 ° C. is set to 800 ° C./second or more, so that the ultra-thin grain-oriented electrical steel sheet is cooled once. Excellent magnetic properties can be obtained even by rolling.

FIG. 2 is a schematic diagram showing a heat pattern in the decarburization annealing step (S4). Referring to FIG. 2, the decarburization annealing step (S4) includes a heating step (S41), a decarburization step (S42), and a cooling step (S43). In the temperature raising step (S41), the steel sheet is heated to the decarburization annealing temperature Ta. In the decarburization step (S42), the steel sheet heated to the decarburization annealing temperature Ta is subjected to decarburization annealing to develop primary recrystallization. In the cooling step (S43), the steel sheet after the decarburization step (S42) is cooled by a known method. In the present embodiment, by significantly increasing the temperature increase rate in the temperature range of 500 to 700 ° C. in the temperature increase step (S41), cold rolling is performed on a steel plate having a Si content of 3.20% or more. Even if the cold-rolled steel sheet is made to be an extremely thin steel sheet having a thickness D of 0.17 to 0.22 mm by carrying out the heat treatment only once, the magnetic properties of the grain-oriented electrical steel sheet can be enhanced. Details of each step will be described below.

[Temperature raising step (S41)]
In the temperature raising step (S41), first, the steel plate having the thickness (0.17 to 0.22 mm) after the cold rolling step is charged into a heat treatment furnace. In the heat treatment furnace for decarburization annealing according to the present embodiment, the temperature of the cold-rolled steel sheet is raised to the decarburization annealing temperature by, for example, high-frequency induction heating. The manufacturing conditions in the heating step are as follows.

Average heating rate RR _500-700 : 800 to 2400 ° C./sec In the heating process, the average heating rate from 500 ° C. to 700 ° C. of the steel sheet is the average heating rate RR _500-700 . (°C/sec). 500 to 700° C. is the temperature range in which primary recrystallization occurs. As described above, the Si content in the chemical composition of the slab of this embodiment is as high as 3.20% or more. Furthermore, in the present embodiment, cold rolling is performed only once in the cold rolling process, and the thickness of the cold-rolled steel sheet after cold rolling is as thin as 0.17 to 0.22 mm. In this case, since the ferrite ratio in the steel sheet increases in the hot rolling process, an α-fiber orientation group develops in the central portion of the sheet thickness, and the α-fiber orientation group may remain even in the cold-rolled steel sheet.

However, if the average heating rate RR _500-700 is set to 800° C./sec or higher in the heating step, {411} <148> recrystallized grains (hereinafter referred to as Σ9-oriented grains) are likely to be generated during the primary recrystallization. The Σ9-oriented grains promote the growth of Goss-oriented grains during secondary recrystallization. Therefore, the primary recrystallization structure can be improved by setting the average heating rate RR _500-700 to 800° C./sec or higher for the cold-rolled steel sheet of the present embodiment. This effect cannot be obtained if the average heating rate RR _500-700 is less than 800° C./sec.

On the other hand, if the average heating rate RR _500-700 exceeds 2400° C./sec, the {111}<112>, which is the Σ9 corresponding orientation different from {411}<148>, recrystallizes from the γ-fiber orientation group. The crystal orientation grains are rather reduced. In this case, the growth of Goss-oriented grains is suppressed in the finish annealing step. As a result, the Goss orientation density (B8/Bs) decreases, and the hysteresis loss among the core losses increases. As a result, excellent iron loss properties cannot be obtained. In addition, since the growth of Goss-oriented grains is suppressed, the magnetic flux density also decreases. Therefore, the upper limit of the average heating rate RR _500-700 is 2400° C./sec.

A preferable lower limit of the average heating rate RR _500-700 is 900° C./second, more preferably 1000° C./second. The preferred upper limit of the average heating rate RR _500-700 is 2100°C/sec.

A well-known atmosphere is sufficient for the atmosphere during the heating process. The atmosphere during the heating step is, for example, a dry nitrogen atmosphere with an oxygen potential (P _H2O /P _H2 ) of 0.1 or less.

Average heating rate RR _500-700 is measured by the following method. A plurality of thermometers are installed in the heat treatment furnace for measuring the surface temperature of the steel sheet. A plurality of thermometers are arranged from upstream to downstream of the heat treatment furnace. Based on the temperature of the steel sheet measured by the thermometer and the time required for the steel sheet temperature to rise from 500°C to 700°C, an average heating rate RR _500-700 is obtained.

In the heating step, the heating rate in temperature ranges other than 500 to 700° C. (room temperature to less than 500° C. and over 700° C. to decarburization annealing temperature) is not particularly limited. Also in these temperature ranges, heating may be performed at the same temperature elevation rate as the average temperature elevation rate RR _500-700 , or may be heated at a temperature elevation rate different from the average temperature elevation rate RR _500-700 . Also, if the temperature is from room temperature to less than 500° C., it may be held at a constant temperature for several seconds.

[Decarburization step (S42)]
In the decarburization step (S42) in the decarburization annealing step (S4), decarburization annealing is performed by holding the steel sheet after the temperature raising step (S41) at the decarburization annealing temperature Ta. This causes the steel sheet to exhibit primary recrystallization. The atmosphere during the decarburization step may be a well-known atmosphere, such as a wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. By performing decarburization annealing, carbon in the steel sheet is removed from the steel sheet, and primary recrystallization occurs. The manufacturing conditions in the decarburization process are as follows.

Decarburization annealing temperature Ta: 800-950°C
The decarburization annealing temperature Ta, as described above, corresponds to the furnace temperature of the heat treatment furnace in which decarburization annealing is performed, and corresponds to the temperature of the steel sheet during decarburization annealing. If the decarburization annealing temperature Ta is less than 800°C, the crystal grains of the steel sheet after primary recrystallization are too small. In this case, the secondary recrystallization does not sufficiently occur in the finish annealing step (S6). On the other hand, if the decarburization annealing temperature Ta exceeds 950° C., the crystal grains of the steel sheet after primary recrystallization are too large. Also in this case, the secondary recrystallization does not sufficiently occur in the finish annealing step (S6). If the decarburization annealing temperature Ta is 800 to 950° C., the crystal grains of the steel sheet after primary recrystallization will have an appropriate size, and secondary recrystallization will sufficiently occur in the finish annealing step (S6).

The holding time at the deoxidizing annealing temperature Ta in the decarburizing step (S42) is not particularly limited. The holding time at the decarburization annealing temperature Ta is, for example, 15 to 150 seconds.

[Cooling step (S43)]
In the cooling step (S43), the steel sheet after the decarburization step (S42) is cooled to normal temperature by a well-known method. The cooling method may be air cooling or water cooling. Preferably, the steel sheet after the decarburization step is allowed to cool. In the decarburization annealing step (S4), the steel sheet is subjected to the decarburization annealing treatment through the above steps.

[Annealing separating agent application step (S5)]
An annealing separator application step (S5) is performed on the steel sheet after the decarburization annealing step (S4). In the annealing separator application step (S5), an annealing separator is applied to the surface of the steel sheet. Specifically, an aqueous slurry containing an annealing separator is applied to the surface of the steel sheet. The aqueous slurry is prepared by adding water to the annealing separator and stirring. The annealing separator contains magnesium oxide (MgO). Preferably, MgO is the main component of the annealing separator. Here, the "main component" means that the content of MgO in the annealing separator is 60.0% or more by mass. The annealing separator may contain well-known additives in addition to MgO.

In the annealing separator application step, an aqueous slurry of an annealing separator is applied onto the surface of the steel sheet. A steel sheet coated with an annealing separator on its surface is wound up into a coil. After coiling the steel sheet, the finish annealing step (S6) is performed.

After coating the surface of the steel sheet with an annealing separating agent in the form of aqueous slurry and forming the steel sheet into a coil shape, the baking treatment may be performed before performing the finish annealing step. In the baking treatment, a coiled steel plate is put into a furnace maintained at 400 to 1000° C. and held there (baking treatment). As a result, the annealing separator applied on the surface of the nitriding steel sheet is dried. The retention time is, for example, 10-90 seconds.

The finish annealing process may be performed on the coiled steel sheet to which the annealing separator is applied without performing the baking treatment.

[Finish annealing step (S6)]
After the annealing separator application step (S5), the steel sheet is subjected to the finish annealing step (S6) to develop secondary recrystallization. A finish annealing process is implemented using a heat treatment furnace. Well-known conditions are sufficient for the manufacturing conditions in the finish annealing step. Manufacturing conditions in the finish annealing step are, for example, as follows. In addition, the atmosphere in the furnace in the finish annealing is a well-known atmosphere.

Finish annealing temperature: 1150-1250°C
Holding time at finish annealing temperature: 5 to 30 hours If the finish annealing temperature is less than 1150°C, sufficient secondary recrystallization does not occur, and the purification to remove precipitates used for secondary recrystallization is sufficient. is not. As a result, the produced grain-oriented electrical steel sheet has low magnetic properties. On the other hand, even if the finish annealing temperature exceeds 1250° C., the effects on secondary recrystallization and purification are low, and problems such as deformation of the steel sheet occur. If the finish annealing temperature is 1150 to 1250° C., assuming that the holding time is appropriate, sufficient secondary recrystallization occurs and the magnetic properties are enhanced. Furthermore, a primary coating containing forsterite is formed soundly on the surface of the steel sheet.

In addition, each element of the chemical composition of the steel sheet is removed to some extent from the components in the steel by the finish annealing step (S6). In particular, S, Al, N, etc., which function as inhibitors, are largely removed. A primary coating containing forsterite is formed on the surface of the grain-oriented electrical steel sheet after the finish annealing process.

The grain-oriented electrical steel sheet according to the present embodiment is manufactured by the above-described manufacturing process. The manufactured grain-oriented electrical steel sheet is an extremely thin steel sheet with a thickness of 0.17 to 0.22 mm, and is obtained by a manufacturing process in which cold rolling is performed only once. It has magnetic properties. Therefore, in the manufacturing method of the present embodiment, it is possible to increase the productivity of the grain-oriented electrical steel sheet which is extremely thin with a thickness of 0.17 to 0.22 mm and has excellent magnetic properties.

[Secondary film forming step]
In the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, a secondary coating forming step may be further carried out after the finish annealing step (S6), if necessary. In the secondary coating forming step, the surface (on the primary coating) of the grain-oriented electrical steel sheet after cooling in the final annealing step is coated with an insulating coating agent mainly composed of colloidal silica and phosphate, and then baked. . As a result, a secondary coating, which is a tension insulating coating, is formed on the primary coating. In the secondary coating forming step, flattening annealing may be performed to apply tension to the steel sheet during baking. Flattening annealing may be performed under well-known conditions. Annealing temperature in flattening annealing is, for example, 800 to 950.degree.

[Magnetic domain refining process]
The grain-oriented electrical steel sheet according to the present embodiment may further be subjected to a magnetic domain refining treatment step after the finish annealing step or the secondary coating forming step, if necessary. In the magnetic domain refining treatment step, the surface of the grain-oriented electrical steel sheet is irradiated with a laser beam having a magnetic domain refining effect, or grooves are formed on the surface. In this case, a grain-oriented electrical steel sheet with even better magnetic properties can be produced.

[EXAMPLES] Below, an Example demonstrates the aspect of this invention concretely. These examples are examples for confirming the effect of the present invention, and do not limit the present invention.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.075%, Si: 3.40%, Mn: 0.075%, S: 0.028%, sol. A slab containing N: 0.008%, further containing Al of the content (% by mass) shown in Table 1, and the balance being Fe and impurities was prepared.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

An annealing step before cold rolling was performed on the hot-rolled steel sheet. In the pre-cold-rolling annealing step, the hot-rolled steel sheet was heated to 1120°C, and then the steel sheet was annealed at an annealing temperature of 900°C for a holding time of 40 seconds.

The steel sheets after the pre-cold-rolling annealing process were cold-rolled once to obtain cold-rolled steel sheets having thicknesses of 0.17 mm, 0.19 mm, and 0.22 mm. The cold rolling rate in the cold rolling process was 90.4-92.6%.

A decarburization annealing process was performed on the cold-rolled steel sheet. In the decarburization annealing step, the decarburization annealing temperature was set to 850°C. The atmosphere in the heat treatment furnace in which the decarburization annealing treatment was performed was a well-known wet atmosphere containing hydrogen and nitrogen. The oxygen potential (P _H2O /P _H2 ) in the atmosphere during the heating process was set to 0.1. Furthermore, the average heating rate from room temperature to the decarburization annealing temperature (850 ° C.) is 800 ° C./sec, and the average heating rate RR _500-700 in the SiO ₂ temperature range (500 to 700 ° C.) during heating is also 800° C./sec.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary film forming process, after applying an insulating coating agent mainly composed of colloidal silica and aluminum phosphate to the surface (on the primary film) of the grain-oriented electrical steel sheet after the finish annealing process, the purpose is baking and shape correction. A flattening annealing was performed. The annealing temperature in the flattening annealing was 900° C.×30 seconds, and the tension applied to the steel sheet during the flattening annealing was the same for all test numbers. A secondary coating, which is a tension insulating coating, was formed on the primary coating by the secondary coating forming step. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
[Magnetic property evaluation test]
The magnetic properties (magnetic flux density B8 and core loss W _17/50 ) of the grain-oriented electrical steel sheets of each test number were evaluated by the following method in accordance with JIS C2556:2015. Specifically, a magnetic field of 800 A/m was applied to each sample, and the magnetic flux density B8(T) was measured. Also, iron loss W _17/50 (W/kg) was measured at a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T.

[Test results]
Table 1 shows the obtained magnetic flux density B8, Goss orientation density B8/Bs, and iron loss _W17 /50. With reference to Table 1, in test numbers 2 to 5, 10 to 12, and 17 to 20, the chemical composition of the slab is appropriate, the Si content (3.40%) satisfies formula (1), The Al content satisfied the formula (2), and the conditions in each manufacturing process were also appropriate. As a result, in any test number, the magnetic flux density B8 was as high as 1.914 T or more, and the iron loss _W17 /50 was as low as 0.806 W/kg or less. Therefore, with these test numbers, an ultra-thin grain-oriented electrical steel sheet with excellent magnetic properties was obtained even with one cold rolling.

On the other hand, in test numbers 1, 8 and 15, the Al content was too high. Therefore, the magnetic flux density B8 was less than 1.914T. Furthermore, iron loss W _17/50 exceeded 0.806 W/kg. During the primary recrystallization, a sufficient difference in grain size between the crystal grains in the surface layer and the crystal grains in the central part of the plate thickness was not obtained, and as a result of the insufficient driving force for grain growth, sufficient Goss-oriented grains were obtained during the secondary recrystallization. presumably did not grow to

In test numbers 7, 14 and 21, the Al content was too low. Therefore, the magnetic flux density B8 was less than 1.914T. Furthermore, iron loss W _17/50 exceeded 0.806 W/kg. As a result of the excessive stabilization of the secondary recrystallization, crystal grains other than the Goss-oriented grains also grew, and as a result, excellent magnetic properties (magnetic flux density B8 and iron loss W _17/50 ) were not obtained. .

In test numbers 6 and 13, the Al content was less than the lower limit of formula (2). Therefore, the magnetic flux density B8 was less than 1.914T. Furthermore, iron loss W _17/50 exceeded 0.806 W/kg. As a result of the excessive stabilization of the secondary recrystallization, crystal grains other than the Goss-oriented grains also grew, and as a result, excellent magnetic properties (magnetic flux density B8 and iron loss W _17/50 ) were not obtained. .

On the other hand, in test numbers 9 and 16, the Al content exceeded the upper limit of formula (2). Therefore, the magnetic flux density B8 was less than 1.914T. Furthermore, iron loss W _17/50 exceeded 0.806 W/kg. During the primary recrystallization, a sufficient difference in grain size between the crystal grains in the surface layer and the crystal grains in the central part of the plate thickness was not obtained, and as a result of the insufficient driving force for grain growth, sufficient Goss-oriented grains were obtained during the secondary recrystallization. presumably did not grow to

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition, in mass%, contains C: 0.075%, Mn: 0.075%, S: 0.028%, N: 0.008%, and the contents shown in Table 2 (mass% ) Si and sol. A slab containing Al with the balance being Fe and impurities was prepared.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

An annealing step before cold rolling was performed on the hot-rolled steel sheet. In the pre-cold-rolling annealing step, the hot-rolled steel sheet was heated to 1120°C, and then annealed at an annealing temperature of 900°C for a holding time of 40 seconds.

After the pre-cold-rolling annealing process, the steel sheet was subjected to cold rolling once to obtain a cold-rolled steel sheet having a thickness of 0.17 mm and 0.22 mm. The cold rolling rate in the cold rolling process was 90.4-92.6%.

A decarburization annealing process was performed on the cold-rolled steel sheet. In the decarburization annealing step, the decarburization annealing temperature was set to 850°C. The atmosphere in the heat treatment furnace in which the decarburization annealing treatment was performed was a well-known wet atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Furthermore, the average heating rate from room temperature to the decarburization annealing temperature (850 ° C.) is 800 ° C./sec, and the average heating rate RR _500-700 in the temperature range of 500 to 700 ° C. during heating is also 800 ° C./sec. seconds.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary film forming process, after applying an insulating coating agent mainly composed of colloidal silica and aluminum phosphate to the surface (on the primary film) of the grain-oriented electrical steel sheet after the finish annealing process, the purpose is baking and shape correction. A flattening annealing was performed. The annealing temperature in the flattening annealing was 900° C.×30 seconds, and the tension applied to the steel sheet during the flattening annealing was the same for all test numbers. A secondary coating, which is a tension insulating coating, was formed on the primary coating by the secondary coating forming step. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8 and iron loss W _17/50 of each test number were obtained. Further, in this example, the saturation magnetic flux density Bs was obtained based on the following equation.
Bs = 2.2032 - 0.0581 Si
Here, the Si content (% by mass) of each test number is substituted for Si in the formula. Based on the obtained magnetic flux density B8 and saturation magnetic flux density Bs, the Goss orientation integration degree (B8/Bs), which is the ratio of the magnetic flux density B8 to the saturation magnetic flux density Bs, was determined.

[Test results]
Table 2 shows the test results obtained. With reference to Table 2, in test numbers 1 to 4, 6 to 9, 11 to 14, 17 to 20, 22 to 25, and 27 to 30, the chemical composition of the slab is appropriate, and the Si content is the formula (1) was satisfied, the Al content satisfied formula (2), and the conditions in each manufacturing process were also appropriate. As a result, in any test number, the magnetic flux density B8 was as high as 1.907 T or more, the Goss direction accumulation degree B8/Bs was as high as 0.952 or more, and the iron loss _W17 /50 was also 0.808 W/kg or less. was low. Therefore, with these test numbers, an ultra-thin grain-oriented electrical steel sheet with excellent magnetic properties was obtained even with one cold rolling.

On the other hand, in test numbers 5, 10 and 15, the Si content was less than the lower limit of formula (1). Therefore, iron loss W _17/50 exceeded 0.808 W/kg. It is believed that the Si content was too low for the plate thickness D, resulting in a low specific resistance of the steel plate and, as a result, a high iron loss.

In test numbers 16, 21, and 26, the Si content exceeded the upper limit of formula (1). Therefore, the magnetic flux density B8 was less than 1.907 T, the Goss orientation density B8/Bs was less than 0.951, and the iron loss _W17 /50 exceeded 0.808 W/kg. The Si content was too high for the sheet thickness D, and the α-faber orientation group generated in the hot rolling process remained even in the cold-rolled steel sheet. It is considered that the growth of oriented grains was insufficient and excellent magnetic properties were not obtained.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition contains, in mass%, C: 0.077%, Si: 3.45%, Mn: 0.075%, S: 0.028%, N: 0.008%, and Table 3 The content (% by mass) sol. A slab containing Al with the balance being Fe and impurities was prepared.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

An annealing step before cold rolling was performed on the hot-rolled steel sheet. In the pre-cold-rolling annealing step, the hot-rolled steel sheet was heated to 1120°C, and then annealed at an annealing temperature of 900°C for a holding time of 40 seconds.

After the pre-cold-rolling annealing process, the steel sheet was cold-rolled once to produce a cold-rolled steel sheet with a thickness of 0.17 mm and 0.22 mm. The cumulative cold rolling reduction in the cold rolling process was 90.4-92.6%.

A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 850°C. The atmosphere in the heat treatment furnace in which the decarburization annealing treatment was performed was a well-known wet atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Furthermore, the values shown in Table 3 were taken as the average temperature increase rate RR _500-700 at 500 to 700°C during temperature increase. In this example, the temperature increase rate was the same as the average temperature increase rate RR _500-700 in the temperature range from room temperature to less than 500° C. and from over 700° C. to the decarburization annealing temperature.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary film forming process, after applying an insulating coating agent mainly composed of colloidal silica and aluminum phosphate to the surface (on the primary film) of the grain-oriented electrical steel sheet after the finish annealing process, the purpose is baking and shape correction. A flattening annealing was performed. The annealing temperature in the flattening annealing was 900° C.×30 seconds, and the tension applied to the steel sheet during the flattening annealing was the same for all test numbers. A secondary coating, which is a tension insulating coating, was formed on the primary coating by the secondary coating forming step. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8, the Goss orientation density B8/Bs, and the iron loss W17/50 of each test number were obtained.

[Test results]
Table 3 shows the test results obtained. With reference to Table 3, in test numbers 4 to 12 and 19 to 27, the chemical composition of the slab is appropriate, the Si content satisfies formula (1), and the Al content satisfies formula (2). And the conditions in each manufacturing process were also appropriate. As a result, in any test number, the magnetic flux density B8 was as high as 1.918 T or more, and the iron loss _W17 /50 was also as low as 0.803 W/kg or less. Therefore, with these test numbers, an ultra-thin grain-oriented electrical steel sheet with excellent magnetic properties was obtained even with one cold rolling.

On the other hand, in test numbers 1 to 3 and 16 to 18, the chemical composition of the slabs was appropriate and the formulas (1) and (2) were satisfied, but the average temperature rise in the decarburization annealing process was 500 to 700 ° C. Speed RR _500-700 was too slow. Therefore, the magnetic flux density B8 was less than 1.918 T, and the iron loss W _17/50 also exceeded 0.803 W/kg. It is considered that the Σ9 corresponding orientation grains were not sufficiently formed in the primary recrystallization structure after the primary annealing process because the average heating rate RR _500-700 was too slow.

In test numbers 13 to 15 and 28 to 30, the chemical composition of the slabs was appropriate and the formulas (1) and (2) were satisfied. _500-700 was too fast. Therefore, the magnetic flux density B8 was less than 1.918 T, and the iron loss W _17/50 also exceeded 0.803 W/kg. It is considered that the recrystallized grains of {111}<112> corresponding to Σ9 were not sufficiently formed in the primary recrystallized structure after the decarburization annealing process because the average heating rate RR _500-700 was too fast.

[Production of grain-oriented electrical steel sheets for each test number]
The chemical composition is mass %, C: 0.077%, Si: 3.45%, Mn: 0.075%, S: 0.028%, N: 0.008%, Sn: 0.100%, Cu: 0.070%, Cr: 0.050%, and sol. A slab containing Al with the balance being Fe and impurities was prepared.

The slabs of each test number were heated to 1350°C in a heating furnace. A hot-rolling process was performed on the heated slab to produce a hot-rolled steel sheet with a thickness of 2.3 mm. The finish rolling temperature (°C) was within the range of 1000 to 1100°C in any test number.

An annealing step before cold rolling was performed on the hot-rolled steel sheet. In the pre-cold-rolling annealing step, the hot-rolled steel sheet was heated to 1120°C, and then annealed at an annealing temperature of 900°C for a holding time of 40 seconds.

After the pre-cold-rolling annealing process, the steel sheet was cold-rolled once to produce a cold-rolled steel sheet with a thickness of 0.17 mm and 0.22 mm. The cumulative cold rolling reduction in the cold rolling process was 90.4-92.6%.

A decarburization annealing process was performed on the cold-rolled steel sheet after the cold rolling process. In the decarburization annealing step, the decarburization annealing temperature was set to 850°C. The atmosphere in the heat treatment furnace in which the decarburization annealing treatment was performed was a well-known wet atmosphere containing hydrogen and nitrogen. Then, the oxygen potential (P _H2O /P _H2 ) in the atmosphere in the heating process was set to 0.1. Furthermore, the values shown in Table 4 were taken as the average temperature increase rate RR _500-700 at 500 to 700°C during temperature increase. In this example, the temperature increase rate was the same as the average temperature increase rate RR _500-700 in the temperature range from room temperature to less than 500° C. and from over 700° C. to the decarburization annealing temperature.

An annealing separating agent (aqueous slurry) containing MgO as a main component was applied to the surface of the steel sheet after decarburization annealing, and then wound into a coil. Finish annealing was performed on the steel sheet wound into a coil. The finish annealing temperature was set to 1150° C., and the holding time at the finish annealing temperature was set to 10 hours. The steel sheet after finish annealing was allowed to cool.

A secondary coating forming step was performed on the steel sheet after the finish annealing step. In the secondary film forming process, after applying an insulating coating agent mainly composed of colloidal silica and aluminum phosphate to the surface (on the primary film) of the grain-oriented electrical steel sheet after the finish annealing process, the purpose is baking and shape correction. A flattening annealing was performed. The annealing temperature in the flattening annealing was 900° C.×30 seconds, and the tension applied to the steel sheet during the flattening annealing was the same for all test numbers. A secondary coating, which is a tension insulating coating, was formed on the primary coating by the secondary coating forming step. The grain-oriented electrical steel sheets of each test number were manufactured by the manufacturing process described above.

[Evaluation test]
By the same method as in Example 1, the magnetic flux density B8, Goss orientation density B8/Bs, and core loss W _17/50 of each test number were obtained.

[Test results]
Table 4 shows the test results obtained. With reference to Table 4, in test numbers 4 to 12 and 19 to 27, the chemical composition of the slab is appropriate, the Si content satisfies formula (1), and the Al content satisfies formula (2). And the conditions in each manufacturing process were also appropriate. As a result, in any test number, the magnetic flux density B8 was as high as 1.915 T or more, and the iron loss _W17 /50 was also as low as 0.779 W/kg or less. Therefore, with these test numbers, an ultra-thin grain-oriented electrical steel sheet with excellent magnetic properties was obtained even with one cold rolling.

On the other hand, in test numbers 1 to 3 and 16 to 18, the chemical composition of the slabs was appropriate and the formulas (1) and (2) were satisfied, but the average temperature rise in the decarburization annealing process was 500 to 700 ° C. Speed RR _500-700 was too slow. Therefore, the magnetic flux density B8 was as low as less than 1.915 T, and the core loss W _17/50 also exceeded 0.779 W/kg. It is considered that the Σ9 corresponding orientation grains were not sufficiently formed in the primary recrystallization structure after the primary annealing process because the average heating rate RR _500-700 was too slow.

In test numbers 13 to 15 and 28 to 30, the chemical composition of the slabs was appropriate and the formulas (1) and (2) were satisfied. _500-700 was too fast. Therefore, the magnetic flux density B8 was less than 1.915 T, and the iron loss W _17/50 also exceeded 0.779 W/kg. It is considered that the recrystallized grains of {111}<112> corresponding to Σ9 were not sufficiently formed in the primary recrystallized structure after the decarburization annealing process because the average heating rate RR _500-700 was too fast.

The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment without departing from the spirit of the present invention.

Claims (2)

The chemical composition is mass %,
C: 0.020 to 0.100%,
Si: 3.20 to 3.70%,
Mn: 0.010-0.300%,
S and / or Se: 0.001 to 0.050% in total,
sol. Al: 0.020-0.028%,
N: 0.002 to 0.015%,
Sn: 0 to 0.500%,
Cr: 0 to 0.500%,
Cu: 0 to 0.500%, and
Balance: Fe and impurities,
When the plate thickness of the steel plate after the cold rolling process is defined as D (mm), the slab that satisfies the formulas (1) and (2) is hot rolled to produce a steel plate. a rolling step;
A pre-cold rolling annealing step of performing an annealing treatment on the steel plate after the hot rolling step;
The steel sheet after the pre-cold-rolling annealing step is cold-rolled at a cold-rolling rate of 89.0 to 93.0% without performing an annealing treatment in the middle, and the thickness D is A cold rolling process for producing the steel plate of 0.17 to 0.22 mm;
A temperature rising step of heating the steel sheet after the cold rolling step to a decarburization annealing temperature, and heating the steel sheet in a temperature range of 500 to 700 ° C. at an average temperature rising rate of 800 to 2400 ° C./sec; a decarburization annealing step of holding the steel sheet at the decarburization annealing temperature of 800 to 950 ° C. and performing decarburization annealing;
An annealing separator application step of applying an annealing separator to the surface of the steel sheet after the decarburization annealing step;
A finish annealing step of performing finish annealing on the steel plate coated with the annealing separator;
A method for manufacturing a grain-oriented electrical steel sheet.
2×D+2.86≦Si≦2×D+3.26 (1)
0.04×D+0.0132≦sol. Al≦0.04×D+0.0192 (2)
Here, the content (% by mass) of the corresponding element in the chemical composition of the slab is substituted for the element symbols in formulas (1) and (2). A method for manufacturing a grain-oriented electrical steel sheet according to claim 1,
The chemical composition of the slab is
Sn: 0.005 to 0.500%,
Cr: 0.010 to 0.500%, and
Cu: 0.010-0.500%,
A method for producing a grain-oriented electrical steel sheet containing one or more selected from the group consisting of

Images