RU2778537C1 - Method for manufacturing a sheet of anisotropic electrical steel - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to the production of an anisotropic electrical steel sheet, which can be used as a material for the metal core of a transformer. A method for producing an anisotropic electrical steel sheet includes a process for hot rolling a steel billet containing, wt.%: 0.10 or less C, 0.80-7.00 Si, 0.01-0.07 acid-soluble Al, 0.012 or less N, 1.00 or less Mn, 0.08 or less S, optionally one or more of 0.01-0.50 Cr, 0.01-0.50 Cu and 0.01-0.02 Sn and the rest is Fe and impurities, to obtain a hot-rolled steel sheet; annealing process of hot rolled steel sheet; pickling process; a cold rolling process to obtain a cold rolled steel sheet; a decarburization annealing process and a final annealing process.

EFFECT: obtaining a sheet of anisotropic electrical steel without a forsterite film, low magnetic losses are realized.

4 cl, 5 tbl, 4 ex

Description

FIELD OF TECHNOLOGY

[0001] The present invention relates to a method for manufacturing an anisotropic electrical steel sheet used as a metal core material of a transformer, in particular, a method for manufacturing an anisotropic electrical steel sheet having excellent decarburization properties and no forsterite film. Priority is claimed from Japanese Patent Application No. 2019-005129, filed January 16, 2019, the contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The anisotropic electrical steel sheet is an electrical steel sheet containing 7% or less Si and formed from highly aligned grains in the {110}<001> orientation (hereinafter referred to as the Goss orientation), and is mainly used as a material metal core of the transformer.

[0003] High alignment in the Goss orientation in anisotropic electrical steel sheets is realized by using a grain growth phenomenon called secondary recrystallization. The anisotropic electrical steel sheets should have high magnetic induction (represented by magnetic induction value B8 at a magnetic field strength of 800 A/m) and low magnetic loss (represented by W _17/50 ) as magnetic characteristics. In particular, in recent years there has been an increase in the demand for reducing power losses in terms of energy saving.

[0004] In the anisotropic electrical steel sheets, the magnetic domains change in accordance with the movement of the domain wall under the influence of an alternating magnetic field. When the motion of the domain wall is performed smoothly, it effectively reduces the magnetic loss. However, when the movement of magnetic domains is observed, there are some magnetic domains that do not move, and in order to further reduce the magnetic loss value of the anisotropic electrical steel sheet, it is important to eliminate the bonding effect of interfacial irregularities due to the forsterite (Mg ₂ SiO ₄ ) based film (hereinafter sometimes called "glass film") on the surface of the steel sheet, which prevents the movement of magnetic domains.

[0005] In order to eliminate such a bonding effect, it is effective not to form a glass film on the surface of the steel sheet that impedes the movement of magnetic domains, and as methods for doing so, Patent Documents 1-10 disclose that surface smoothing can be achieved after final annealing by controlling the dew point of the decarburization annealing so that iron-based oxides (Fe ₂ SiO ₄ , FeO or the like) are not formed in the oxide layer formed during the decarburization annealing, and using a material such as alumina that does not react with silica as annealing separator.

[0006] However, in general, since the decarburization reaction is a chemical reaction between atmospheric water vapor and solid solution carbon in the steel sheet, there is a problem that the decarburization properties deteriorate as the oxygen partial pressure decreases. For example, Patent Document 10 proposes a method of performing intermediate annealing in an oxygen partial pressure atmosphere of 0.50-0.88 after the first cold rolling to promote decarburization, and then performing a second cold rolling to realize decarburization annealing in the oxygen partial pressure range of 0.0001 -0.2.

[0007] With this method, the formation of Fe-based oxides can be more strongly suppressed while providing satisfactory decarburization properties, but cold rolling and annealing are carried out twice, which causes a significant problem of increasing costs from an industrial point of view.

[0008] Patent Document 6 proposes a technology for improving decarburization properties in which decarburization annealing is divided into an initial stage and a later stage, and is realized by dividing the holding range of the initial stage and later stage into two stages: T1(°C) represented by the range 770°C≤T1≤860°C, and T2(°C) represented by the range T1+10≤T2≤950°C. However, since the value of the lower limit of oxygen partial pressure adopted in Patent Document 6 is about 0.01, there has been a problem that decarburization properties cannot be secured at a lower oxygen partial pressure, and in particular, difficulties with respect to the decarburization properties of a thick material such as a sheet with a final thickness of 0.23 mm or more.

BIBLIOGRAPHY

PATENT DOCUMENTS

[0009] Patent Document 1: Japanese Patent Application Pending, First Publication No. S64-062417

Patent Document 2: Japanese Unexamined Patent Application First Publication No. H07-118750

Patent Document 3: Japanese Unexamined Patent Application First Publication No. H07-278668

Patent Document 4: Japanese Unexamined Patent Application First Publication No. H07-278669

Patent Document 5: Japanese Unexamined Patent Application First Publication No. 2002-173715

Patent Document 6: Japanese Unexamined Patent Application First Publication No. 2003-055717

Patent Document 7: Japanese Unexamined Patent Application First Publication No. 2003-003213

Patent Document 8: Japanese Unexamined Patent Application First Publication No. 2003-041320

Patent Document 9: Japanese Unexamined Patent Application First Publication No. 2003-247021

Patent Document 10: Japanese Published Translation No. 2011-518253 of PCT International Application.

SUMMARY OF THE INVENTION

PROBLEMS SOLVED BY THE INVENTION

[0010] In view of the problems associated with conventional technologies, an object of the present invention is to provide a method for producing an anisotropic electrical steel sheet having excellent decarburization properties for suitably reducing residual carbon and realizing low magnetic loss even when the thickness of the sheet is large at production of a final annealed anisotropic electrical steel sheet made by removing a glass film by a method such as etching or deliberately preventing its formation, that is, an anisotropic electrical steel sheet having no forsterite film.

REMEDIES FOR SOLVING THE PROBLEM

[0011] In order to solve the above-described problems, the present inventors intensively researched a production method in order to suitably reduce the residual amount of carbon and realize low magnetic loss even when the sheet thickness is large in an anisotropic electrical steel sheet having no forsterite film. .

[0012] As a result, it has been found that the residual amount of carbon can be suitably reduced and low magnetic loss can be realized in the anisotropic electrical steel sheet produced by strictly controlling the decarburization annealing process.

[0013] The present invention has been made based on the findings described above, and the gist of the invention is as follows.

[0014] (1) A method for producing an anisotropic electrical steel sheet, including a hot rolling process in which a steel billet containing 0.10 mass% or less C, 0.80-7.00 mass% Si, 0.01- 0.07 wt% of acid-soluble Al, 0.012 wt% or less of N, 1.00 wt% or less of Mn, and 0.08 wt% or less of S, and including the balance of Fe and impurities, are hot rolled into a hot-rolled steel sheet, an annealing process in which a hot-rolled steel sheet is annealed, a pickling process in which a hot-rolled steel sheet after an annealing process is pickled, a cold rolling process in which a hot-rolled steel sheet after a pickling process is cold-rolled into a cold-rolled steel sheet, a decarburization annealing process, in which the cold-rolled steel sheet is decarburized, and a final annealing process in which the steel sheet after the decarburization annealing process is finally annealed, in which the decarburization annealing process includes (i-1) a first heating heat treatment at an average heating rate HR1 that is 40-500°C/s in a temperature range of 550°C or above and below 720°C, (i-2) a second heat treatment at an average heating rate HR2, which is 5-50°C/s, in the temperature range of 720°C or higher and a temperature of T1°C or lower, which satisfies the following expression (2), after the first heat treatment, and (ii) the first temperature holding annealing treatment T1° C for 50 to 1000 seconds after the second heat treatment, wherein the first heat treatment, the second heat treatment, and the first annealing treatment are performed in an atmosphere with an oxygen partial pressure P1 satisfying the following expression (1), and the amount of C in the steel sheet after the first annealing treatment is 25 ^ppm (ppm) or less.

0.0010≤P1≤0.20 ... (1)

770≤T1 (°C)≤900 ... (2)

[0015] (2) In the production method of the anisotropic electrical steel sheet of the above (1), the decarburization annealing process may further include a second annealing treatment to maintain a temperature of T2°C satisfying the following expression (4) for 3 to 500 seconds in an atmosphere with an oxygen partial pressure P2 satisfying the following expression (3) after the first annealing treatment.

P2 < P1 ... (3)

960≥T2≥T1+10 ... (4)

[0016] (3) The production method of the anisotropic electrical steel sheet of (2) above may further include a third heat treatment between the first annealing treatment and the second annealing treatment, wherein the third heat treatment performs heating at an average heating rate HR3 of 5-50°C/s, in the temperature range from T1°C to T2°C.

[0017] (4) In the production method of the anisotropic electrical steel sheet according to any of (1) to (3) above, the steel billet may contain one or more of 0.01 to 0.50 mass% Cr, 0, 01-0.50 wt% Cu and 0.01-0.02 wt% Sn.

[0018] (5) The method for manufacturing the anisotropic electrical steel sheet according to any of the above (1) to (4) may further include a nitriding treatment process by nitriding the steel sheet after the decarburization annealing process between the decarburizing annealing process and the final annealing process , in which the steel sheet after the nitriding process is finally annealed in the final annealing process.

EFFECTS OF THE INVENTION

[0019] According to the present invention, in a method for producing an anisotropic electrical steel sheet having no forsterite film by intentionally preventing the formation of Fe-based oxides on the surface of the steel sheet by annealing in a low oxygen partial pressure atmosphere in a decarburization annealing process after cold rolling , decarburization annealing can be stably performed without going through two or more cold rolling passes including intermediate annealing, even in an atmosphere with a low oxygen partial pressure in which the generation of Fe-based oxides is suppressed. As a result, the residual amount of carbon can be suitably reduced and low magnetic loss can be achieved even with a large sheet thickness.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

[0020] A method for producing an anisotropic electrical steel sheet having excellent decarburization properties according to one embodiment of the present invention (hereinafter, sometimes referred to as the "proposed production method") includes a hot rolling process in which a steel billet containing 0.10 wt% or less C, 0.80-7.00 wt% Si, 0.01-0.07 wt% acid-soluble Al, 0.012 wt% or less N, 1.00 wt% or less Mn, and 0.08 mass% or less of S, and including the rest of Fe and impurities, is hot rolled into a hot-rolled steel sheet, an annealing process in which the hot-rolled steel sheet is annealed, a pickling process in which the hot-rolled steel sheet after the annealing process is pickled, the process cold rolling, in which the hot-rolled steel sheet after the pickling process is subjected to cold rolling into a cold-rolled steel sheet, the decarburization annealing process, in which the cold-rolled steel sheet is decarburized and a final annealing process in which the steel sheet after the decarburization annealing process is finally annealed, in which the decarburization annealing process includes (i-1) a first heat treatment with an average heating rate HR1 that is 40-500°C/s , in the temperature range of 550°C or above and below 720°C, (i-2) the second heat treatment by heating with an average heating rate HR2, which is 5-50°C/s, in the temperature range of 720°C or above and T1 °C or lower, which satisfies the following expression (2), after the first heat treatment, and (ii) the first annealing treatment while maintaining the temperature T1°C for 50-1000 seconds after the second heat treatment, the first heat treatment, the second heat treatment and the first annealing treatment is performed in an atmosphere with an oxygen partial pressure P1 satisfying the following expression (1), and the amount of C in the steel sheet after the first annealing treatment is 25 ^ppm or less.

0.0010≤P1≤0.20 ... (1)

770≤T1 (°C)≤900 ... (2)

[0021] Next, the proposed production method will be described. In the present embodiment, a range of numerical values expressed using "-" means a range including the numerical values specified before and after "-" as the lower limit and upper limit values.

[0022] First, the reason for limiting the chemical composition of the steel billet (hereinafter referred to as "this steel billet" in some cases) for the production of hot-rolled electrical steel sheet in the present production method will be described. Hereinafter, "%" referring to the chemical composition means "wt %" with respect to the total weight of the steel billet (more specifically, the sample of the steel billet used for chemical composition analysis).

<Chemical composition>

C: 0.10% or less

[0023] When the content of carbon (C) exceeds 0.10%, since the steel undergoes a phase transformation at the time of secondary recrystallization annealing, secondary recrystallization does not proceed sufficiently, and satisfactory magnetic induction and magnetic loss characteristics cannot be obtained, so the content C is set to 0.10% or less. From the viewpoint of magnetic loss reduction, the C content is preferably 0.08% or less, and more preferably 0.06% or less. Since the detection limit for C is about 0.001%, 0.001% is a practical lower limit in a real steel sheet.

Si: 0.80-7.00%

[0024] When the content of silicon (Si) is less than 0.80%, since the steel undergoes a phase transformation at the time of secondary recrystallization annealing, secondary recrystallization does not proceed sufficiently, and satisfactory magnetic induction and magnetic loss characteristics cannot be obtained, therefore the Si content is set to 0.80% or more. The Si content is preferably 1.00% or more, more preferably 2.50% or more, and even more preferably 3.00% or more.

[0025] On the other hand, when the Si content exceeds 7.00%, since the steel sheet becomes brittle and the passability in the manufacturing process deteriorates significantly, the Si content is set to 7.00% or less. The Si content is preferably 4.00% or less, and more preferably 3.75% or less.

Acid-soluble Al: 0.01-0.07%

[0026] In the electrical steel sheet of the present invention, acid-soluble aluminum Al (soluble Al) is an indispensable element as a secondary recrystallization inhibitor. That is, acid-soluble Al is an element that forms AlN and stably causes secondary recrystallization.

[0027] When the content of acid-soluble Al is less than 0.01%, since there is not enough AlN that functions as an inhibitor, the secondary recrystallization is insufficient, and the magnetic loss performance is not improved, the content of acid-soluble Al is set to 0.01% or more . The content of acid-soluble Al is preferably 0.015 mass% or more, and more preferably 0.020 mass% or more.

[0028] On the other hand, when the content of acid-soluble Al exceeds 0.07%, since the steel sheet becomes brittle, and this brittleness is significant especially in the electrical steel sheet of the present invention having a large amount of Si, the content of acid-soluble Al is preferably set to 0, 07% or less. The content of acid-soluble Al is preferably 0.05 mass% or less, and more preferably 0.03 mass% or less.

N: 0.012% or less

[0029] When the nitrogen (N) content exceeds 0.012%, since bubbles (voids) are generated in the steel sheet during cold rolling, the strength of the steel sheet increases and the permeability deteriorates during production, so the N content is set to 0.012% or less. The N content is preferably 0.010% or less, and more preferably 0.009% or less.

[0030] On the other hand, in order for N to combine with Al to form AlN, which acts as an inhibitor, the N content is preferably 0.004% or more. The N content is more preferably 0.006 mass% or more.

Mn: 1.00% or less

[0031] When the manganese (Mn) content exceeds 1.00%, since the steel undergoes a phase transformation at the time of secondary recrystallization annealing, secondary recrystallization does not proceed sufficiently, and satisfactory magnetic induction and magnetic loss characteristics cannot be obtained, so the content Mn is set to 1.00% or less. The Mn content is preferably 0.50% or less, and more preferably 0.20% or less.

[0032] MnS can be used as an inhibitor during secondary recrystallization, but when using AlN as an inhibitor, MnS is not necessary, and therefore the lower limit of the Mn content includes 0%. When MnS is used as an inhibitor, the content of Mn is 0.02% or more. The Mn content is preferably 0.05% or more, and more preferably 0.07% or more.

S: 0.08% or less

[0033] When the sulfur (S) content exceeds 0.08%, hot brittleness makes hot rolling much more difficult, and therefore the S content is set to 0.08% or less. The sulfur content is preferably 0.04 mass% or less, and more preferably 0.03 mass% or less.

[0034] When AlN is used as an inhibitor, MnS is not mandatory, and therefore the lower limit of the S content includes 0%, but when MnS is used as an inhibitor during secondary recrystallization, the S content is set to 0.005% or more. The S content is preferably 0.01% or more, and more preferably 0.02% or more.

[0035] In addition, the S portion may be replaced by selenium (Se) or antimony (Sb), in which case the value converted according to the formulas Seq=S+0.406Se or Seq=S+0.406Sb is used.

[0036] In addition to the above elements, the electrical steel sheet of the present invention may contain one or more of the following elements in order to improve the characteristics of the electrical steel sheet of the present invention.

Cr: 0.01 to 0.50%

[0037] Chromium (Cr) is an element that improves the primary recrystallization texture, stabilizes the secondary recrystallization, and causes a magnetic loss reduction effect. Since the texture improvement effect cannot be sufficiently obtained when the Cr content is less than 0.01%, the Cr content is set to 0.01% or more. The Cr content is preferably 0.03% or more, and more preferably 0.05% or more.

[0038] On the other hand, when the Cr content exceeds 0.50%, since Cr combines with O to form a Cr ₂ O ₃ film and this causes decarburization properties to deteriorate, the Cr content is set to 0.50% or less. The Cr content is preferably 0.30% or less, and more preferably 0.20% or less.

Cu: 0.01-0.50%

[0039] Like Cr, copper (Cu) is an element that improves the primary recrystallization texture, stabilizes the secondary recrystallization, and causes a magnetic loss reduction effect. Since the texture improvement effect cannot be sufficiently obtained when the Cu content is less than 0.01%, the Cu content is set to 0.01% or more. The Cu content is preferably 0.03% or more, and more preferably 0.05% or more.

[0040] On the other hand, when the Cu content exceeds 0.50%, the steel sheet becomes brittle during hot rolling, and therefore the Cu content is set to 0.50% or less. The Cu content is preferably 0.20% or less, and more preferably 0.10% or less.

Sn: 0.01 to 0.20%

[0041] Like Cr and Cu, tin (Sn) is an element that improves the primary recrystallization texture. Since the surface smoothing effect of the steel sheet cannot be sufficiently obtained when the Sn content is less than 0.01%, the Sn content is set to 0.01% or more. The Sn content is preferably 0.02% or more, and more preferably 0.03% or more.

[0042] On the other hand, when the Sn content exceeds 0.20%, the secondary recrystallization becomes unstable, the magnetic characteristics deteriorate, and therefore the Sn content is set to 0.20% or less. The Sn content is preferably 0.15% or less, and more preferably 0.10% or less.

[0043] The rest of the chemical composition of this steel billet is Fe and impurities, but may contain trace elements such as Mo, W, In, Bi, Sb, Ag, Te, Ce, V, Co, Ni, Se, Re, Os , Nb, Zr, Hf, Ta, Y and La in a total amount of 5.0% or less, and preferably 2.0% or less, instead of part of the rest - iron, in order to further improve the magnetic characteristics, improve the characteristics required for structural elements such as strength, corrosion resistance and fatigue performance, improvements in casting properties and permeability, and productivity improvements due to the use of scrap or the like. Impurities include unavoidable impurities that inevitably enter the steel billet during the manufacturing process. Elements other than those listed above may be added to the steel blank as impurities within a range in which the effects of the present invention are not impaired.

[0044] The above-described chemical composition of the steel billet (or base steel sheet in the anisotropic electrical steel sheet) can be measured in detail by a conventional analytical method. For example, steel components can be measured using inductively coupled plasma atomic emission spectrometry (ICP-AES). In addition, C and S can be measured using the infrared absorption method of the combustion spectrum, N can be measured using the thermal conductivity method when melting in an inert gas, and O can be measured using a non-dispersive absorption method in the infrared region when melting in an inert gas. gas.

[0045] Next, the proposed production method will be described.

<Production method>

The method for producing an anisotropic electrical steel sheet having excellent decarburization properties according to the present embodiment includes a hot rolling process in which a steel billet containing 0.10 wt.% or less C, 0.80 to 7.00 wt. % Si, 0.01-0.07 wt.% acid-soluble Al, 0.012 wt.% or less N, 1.00 wt.% or less Mn, and 0.08 wt.% or less S, and including the balance of Fe and impurities, subjected to hot rolling into a hot-rolled steel sheet, an annealing process in which the hot-rolled steel sheet is annealed, a pickling process in which the hot-rolled steel sheet after the annealing process is pickled, a cold rolling process in which the hot-rolled steel sheet after the pickling process is cold-rolled into a cold-rolled steel sheet, a decarburization annealing process in which the cold-rolled steel sheet is decarburized, and a final annealing process in which the steel sheet after the decarburization annealing process ha finally annealed. Here, the decarburization annealing process is a process of decarburizing a cold-rolled steel sheet and controlling the grain size to become preferable for secondary recrystallization (the grain size before secondary recrystallization will be referred to as the primary recrystallization grain size).

[0046] The decarburization annealing process of the proposed production method includes: (i-1) a first heat treatment with an average heating rate HR1 of 40-500°C/s in a temperature range of 550°C or above and below 720°C, (i -2) a second heat treatment by heating at an average heating rate HR2 of 5-50°C/s in a temperature range of 720°C or higher and a temperature of T1°C or lower which satisfies the following expression (2), after the first heat treatment, and ( ii) a first annealing treatment to maintain a temperature of T1°C for 50-1000 seconds, after the second heat treatment. In addition, the first heat treatment, the second heat treatment, and the first annealing treatment are performed in an atmosphere with an oxygen partial pressure P1 satisfying the following expression (1). In addition, the amount of C in the steel sheet after the first annealing treatment is 25 ^ppm or less.

0.0010≤P1≤0.20 ... (1)

770≤T1 (°C)≤900 ... (2)

[0047] Important control factors will now be described.

[0048] Oxygen partial pressure P1 in annealing atmosphere: 0.0010 or more and 0.20 or less. In decarburization annealing, when an oxide film (for example, an oxide film formed from Mn or Si) is formed on the surface of the steel sheet, the decarburization reaction becomes difficult, and therefore it is important to control the formation of the oxide film.

[0049] Factors controlling the formation of an oxide film include the partial pressure of oxygen in the atmosphere in addition to the heating rate (which will be described below) and the holding temperature and holding time (which will also be described below). Since the oxide film is formed due to an oxidation reaction between metal atoms and oxygen, the oxide film formation behavior is particularly sensitive to the oxygen partial pressure (oxidation state) in the annealing atmosphere.

[0050] The partial pressure of oxygen (oxidation state) is defined as the partial pressure of water vapor (H ₂ O) divided by the partial pressure of hydrogen (H ₂ ). That is, the atmosphere with a large partial pressure of oxygen is an atmosphere with a high partial pressure of water vapor. Since decarburization occurs by the reaction C+H ₂ O → H ₂ +CO, the oxygen partial pressure is preferably large.

[0051] The oxygen partial pressure P1 in the decarburization annealing atmosphere of the proposed production method is the oxygen partial pressure satisfying the above-described expression (1).

[0052] When the oxygen partial pressure P1 is less than 0.0010, dense SiO _{2 of} the external oxidation type is rapidly formed, which prevents the decarburization reaction, and therefore the oxygen partial pressure P1 is set to 0.0010 or more. The oxygen partial pressure P1 is preferably 0.010 or more.

[0053] The upper limit of the oxygen partial pressure is not particularly limited in terms of promoting the decarburization reaction, but when the oxygen partial pressure P1 exceeds 0.20, iron-based oxides are easily formed on the surface of the steel sheet, suppressing the occurrence of secondary recrystallization, and therefore the oxygen partial pressure P1 is set to 0.20 or less. The oxygen partial pressure P1 is preferably 0.15 or less.

[0054]

(i-1) The first stage of heating (in some cases called the first heat treatment)

Heating temperature range: 550°C or above and below 720°C

Average heating rate HR1: 40°C/s or faster and 500°C/s or slower

The behavior of the formation of SiO ₂ , which hinders the decarburization reaction, depends not only on the partial pressure of oxygen in the atmosphere, but also on the heating rate. Therefore, it is important that the heating in the decarburization annealing goes through a thermal cycle that avoids the formation of SiO ₂ as much as possible.

[0055] Therefore, the average heating rate HR1 in the temperature range of 550°C or higher, which is the temperature range of formation of SiO ₂ , is set to 40°C/s or higher. The average heating rate HR1 is preferably 70°C/s or higher. When the heating rate is higher, it is more preferable for decarburization, but if the heating rate becomes too high, there is a possibility of overheating of the steel sheet (exceeding the upper limit value T1). When the temperature of the steel sheet exceeds the upper limit, the grains in the texture of the primary recrystallized grains become coarse, and the secondary recrystallization becomes defective. Therefore, the heating rate is set to 500°C/s or less. The heating rate is preferably 200°C/s or less.

[0056] In order to ensure that the temperature achieved by heating at an average heating rate HR1 of 40°C/s or more and 500°C/s or less does not exceed the temperature T1°C that satisfies the above expression (2) , the temperature range to which the above heating rate applies is set lower than 720°C (<770°C [lower limit temperature T1°C]). That is, the heating temperature range with the average heating rate HR1 is set to 550°C or higher and lower than 720°C. In addition, the "average heating rate" in each stage in the present embodiment can be obtained by dividing the temperature range (upper limit−lower limit) in each stage by the time required for heating in each stage. In addition, when measuring the average heating rate of the first heating stage, the upper limit value of the first heating stage is set to 720°C for convenience.

[0057] (i-2) Second heating step (called second heat treatment in some cases)

Heating temperature range: 720°C or higher and temperature T1°C or lower which satisfies the above expression (2)

Average heating rate HR2: 5°C/s or faster and 50°C/s or slower

The heating rate in the heating temperature range of 720°C or higher and T1°C or lower is also important. It is necessary to use a heating rate that ensures that the temperature reached during heating does not exceed the temperature T1°C, which satisfies the above expression (2). Therefore, the average heating rate HR2 in the temperature range of 720°C or higher and the temperature T1°C or lower that satisfies the above expression (2) is set to 5°C/s or more and 50°C/s or less.

[0058] Since the productivity decreases when the average heating rate of HR2 becomes lower than 5°C/s, the average heating rate of HR2 is set to 5°C/s or higher. The average heating rate HR2 is preferably 7°C/s or more. On the other hand, since there are cases in which the temperature reached during heating exceeds the temperature T1°C, when the average heating rate HR2 exceeds 50°C/s, the average heating rate HR2 is set to 50°C/s or lower. The average heating rate HR2 is preferably 20°C/s or less.

[0059] (ii) Decarburizing annealing (in some cases referred to as first annealing treatment)

Annealing temperature T1: Temperature T1°C (770°C or higher and 900°C or lower) which satisfies the above expression (2)

Holding time: 50 seconds or more and 1000 seconds or less

Amount of C in steel sheet: 25 ^ppm or less

The decarburization reaction is highly dependent on the diffusion rate of carbon in the steel, i.e. temperature and time. The decarburization reaction is difficult to proceed when the annealing temperature T1 is 770°C or lower, and therefore the annealing temperature T1 is set to 770°C or higher. The annealing temperature T1 is preferably 800°C or higher, and more preferably 810°C or higher.

[0060] On the other hand, when the annealing temperature T1 exceeds 900°C, since an oxide film of Mn or Si is formed on the surface of the steel sheet, the decarburization reaction shifts from diffusion-controlled to interface-controlled, and the decarburization reaction becomes difficult, the annealing temperature T1 set to 900°C or lower. The annealing temperature T1 is preferably 870°C or less, and more preferably 850°C or less.

[0061] In addition, in the decarburization annealing, the holding time is also important in addition to the annealing temperature. Therefore, the holding time at T1°C is set to 50 seconds or more and 1000 seconds or less. When the holding time is less than 50 seconds, the decarburization reaction does not proceed sufficiently, and the remaining C causes magnetic aging, so the holding time is set to 50 seconds or more. The holding time is preferably 70 seconds or longer, and more preferably 100 seconds or longer.

[0062] On the other hand, when the holding time exceeds 1000 seconds, the primary recrystallized grains become coarse, resulting in secondary recrystallization defects, and therefore the holding time is set to 1000 seconds or less. The holding time is preferably 500 seconds or less, and more preferably 200 seconds or less.

[0063] In the proposed production method, in order to further promote the decarburization reaction, after the above-described annealing (first annealing treatment), it is effective to carry out the second annealing treatment at which the temperature T2°C satisfying the following expression (4) is held for 3 seconds or more and 500 seconds or less in an atmosphere with an oxygen partial pressure P2 satisfying the following expression (3).

P2 < P1 ... (3)

960≥T2≥T1+10 ... (4)

[0064] The rate of decarburization at a temperature of T1°C decreases with time, and the decarburization reaction eventually reaches a thermal equilibrium state. If heated to a higher temperature when the decarburization reaction has reached the thermal equilibrium state, the decarburization reaction can be continued again. Therefore, in order to more appropriately reduce the residual amount of carbon in the produced anisotropic electrical steel sheet, it is preferable in the present production method to perform the second annealing treatment after the first annealing treatment.

[0065] Second annealing treatment

Oxygen partial pressure P2 in annealing atmosphere: less than P1 (expression (3) described above)

Annealing temperature T2: T1+10°C or higher and 960°C or lower (expression (4) described above)

Holding time: 3 seconds or more and 500 seconds or less

When the oxygen partial pressure P2 in the annealing atmosphere of the second annealing treatment is made the same as the oxygen partial pressure P1 (0.0010 or more and 0.20 or less) in the annealing atmosphere of the first annealing treatment, an oxide film is formed on the surface of the steel sheet, and therefore, the oxygen partial pressure P2 in the annealing atmosphere of the second annealing treatment is set to be smaller than the oxygen partial pressure P1. The oxygen partial pressure P2 is preferably P1×0.1 or less.

[0066] Since the oxygen partial pressure P2 in the annealing atmosphere of the second annealing treatment can be suitably set within the range in which the decarburization reaction is achieved while suppressing the formation of an oxide film on the surface of the steel sheet at an oxygen partial pressure of less than P1, its lower limit is not specifically is limited, but the oxygen partial pressure is preferably P1×0.01 or more from the point of view of reliably suppressing the formation of an oxide film.

[0067] When the annealing temperature T2°C in the second annealing treatment is lower than T1 (770°C or higher and 900°C or lower) +10°C, the decarburization reaction is not continued again, and therefore the annealing temperature T2°C in the second annealing treatment, T1°C (770°C or more and 900°C or less) +10°C or more. The annealing temperature T2°C is preferably T1°C (770°C or higher and 900°C or lower) +20°C or higher.

[0068] There is no upper limit of the T2°C annealing temperature from the viewpoint of promoting the decarburization reaction, but the T2°C annealing temperature is set to 960°C or lower from the viewpoint of grain size control. The annealing temperature T2°C is preferably 950°C or lower, and more preferably T1°C+60°C or lower.

[0069] Since the second annealing treatment is an additional annealing treatment after the first annealing treatment step, the holding time is preferably short in terms of productivity. When the holding time is less than 3 seconds, the decarburization reaction practically does not proceed, and the effect of the second annealing treatment cannot be obtained, and therefore the holding time is set to 3 seconds or more. The holding time is preferably 10 seconds or more.

[0070] On the other hand, when the holding time exceeds 500 seconds, the effect of the second annealing treatment saturates and productivity decreases, and therefore the holding time is set to 500 seconds or less. The holding time is preferably 100 seconds or less.

[0071] When changing from the first annealing treatment to the second annealing treatment, it is necessary to suppress the formation of an oxide film on the surface of the steel sheet so as not to hinder the decarburization reaction. Therefore, the average heating rate HR3 during heating (sometimes referred to as the third heat treatment) from the first annealing temperature T1°C to the second annealing temperature T2°C is set to 5°C/s or more and 50°C/s or less.

[0072] When the average heating rate of HR3 is less than 5°C/s, the second annealing treatment temperature T2°C is not reached, and therefore the average heating rate of HR3 is set to 5°C/s or more. The average heating rate of HR3 is preferably 10°C/s or more. On the other hand, when the average heating rate HR3 exceeds 50°C/s, since the temperature reached by heating exceeds the temperature T2°C, the primary recrystallization grain size becomes coarse, resulting in secondary recrystallization defects, and the magnetic induction decreases, the average heating rate HR3 is set to 50°C/s or less. The average heating rate of HR3 is preferably 20°C/s or less.

[0073] Nitriding treatment

The nitriding treatment can be performed on the steel sheet after the decarburization annealing process. The nitriding treatment method is not particularly limited, and an example is a method performed in an atmosphere of a nitriding gas such as ammonia or the like. The treatment time and treatment conditions in the nitriding treatment are not particularly limited, and the nitriding treatment may be performed so that the amount of N in the steel sheet is 0.005% or more, and it is preferable that the ratio (amount of N)/(amount of acid-soluble Al) in the steel sheet is 2/3 or more. This nitriding treatment process is especially effective when the hot rolling process is performed after heating the steel billet at a temperature below 1300° C. (called low temperature slab heating or medium temperature slab heating in some cases). On the other hand, when the hot rolling process is performed after heating the steel billet at a temperature of 1300° C. or higher to almost completely dissolve fine precipitates called inhibitors (called high-temperature slab heating in some cases), the nitriding treatment process cannot be performed.

[0074] As described above, in the present embodiment, an anisotropic electrical steel sheet having no forsterite film is produced by deliberately preventing the formation of Fe-based oxides on the surface of the steel sheet by annealing in a low oxygen partial pressure atmosphere in a decarburization annealing process after cold rolling. In addition, in the present embodiment, decarburization annealing can be stably performed without passing through two or more cold rolling passes with intermediate annealing in between even in a low oxygen partial pressure atmosphere in which the formation of iron-based oxides is suppressed by strictly controlling the decarburization annealing process. As a result, even when the thickness of the sheet is large (for example, when the thickness of the sheet is 0.23 mm or more), the residual amount of carbon can be suitably reduced and low magnetic loss can be realized. In addition, the magnetic induction can be increased (for example, magnetic induction B8 at a magnetic field strength of 800 A/m).

Examples

[0075] Hereinafter, the technical details of the proposed production method will be further described based on examples of the proposed production method. In addition, the conditions in the examples described below are examples of the conditions used to confirm the feasibility and effects of the present invention, and the present invention is not limited to these example conditions. In addition, the present invention can use various conditions, as long as the purpose of the present invention is achieved, without departing from the gist of the present invention.

<Example 1>

[0076] An ingot with the chemical composition shown in Table 1 was melted under vacuum and cast into a steel billet. The steel billet was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. This hot-rolled steel sheet was annealed at 1000-1100°C for 2 minutes, pickled, and then cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.23-0.30 mm.

[0077] [Table 1]

Steel No. C Si Mn Al S N Other Invention steel A1 0.005 3.15 0.08 0.028 0.021 0.007 A2 0.095 3.16 0.08 0.027 0.018 0.008 A3 0.065 1.01 0.09 0.027 0.018 0.007 A4 0.078 6.38 0.11 0.028 0.015 0.009 A5 0.037 2.67 0.03 0.029 0.017 0.008 A6 0.034 2.99 0.95 0.029 0.018 0.007 A7 0.035 3.14 0.06 0.012 0.018 0.008 A8 0.058 3.18 0.45 0.068 0.019 0.009 A9 0.045 3.12 0.09 0.033 0.003 0.008 A10 0.055 3.16 0.07 0.039 0.071 0.006 A11 0.057 3.26 0.08 0.027 0.039 0.002 A12 0.037 3.07 0.12 0.028 0.037 0.011 A13 0.036 3.17 0.09 0.028 0.018 0.010 A14 0.029 3.11 0.07 0.029 0.017 0.004 A15 0.051 3.15 0.09 0.028 0.015 0.007 Cr:0.05 A16 0.055 3.22 0.17 0.029 0.012 0.006 Cr:0.14, Cu:0.08 A17 0.059 3.45 0.08 0.029 0.014 0.007 Cu:0.32, Sn:0.03 Comparative steel a1 0.135 3.05 0.09 0.029 0.018 0.009 a2 0.032 0.77 0.08 0.029 0.015 0.008 a3 0.048 7.12 0.12 0.027 0.019 0.007 a4 0.037 3.17 1.21 0.028 0.017 0.006 a5 0.045 3.11 0.08 0.008 0.015 0.008 a6 0.042 3.07 0.09 0.087 0.014 0.007 a7 0.056 3.17 0.07 0.029 0.091 0.009 a8 0.034 3.26 0.12 0.028 0.018 0.015

[0078] The cold rolled steel sheet was subjected to decarburization annealing at 820°C for 140 seconds in a humid gas containing hydrogen and nitrogen (P1=P _H2O /P _H2 =0.10). The average heating rate of HR1 in the temperature range of 550°C or above and below 720°C was set to 80°C/s, and the average heating rate of HR2 in the temperature range of 720°C or above and 820°C or below (that is, T1: 820 °C) was set to 10°C/s.

[0079] An annealing separator containing alumina as a main component was applied to the steel sheet after decarburization annealing as an aqueous slurry, and final annealing was performed. Final annealing was performed up to 1200° C. at a temperature increase rate of 15° C./hour in an atmosphere containing nitrogen, and the atmosphere was switched to 100% hydrogen at 1200° C. to perform annealing for 20 hours.

[0080] The sample for evaluation was taken from the secondary recrystallized steel sheet subjected to the above processes, then this sample was subjected to stress relief annealing at 800°C for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, after which aging treatment was performed at 150 °C for 300 hours in a nitrogen atmosphere, and then measured the magnetic induction and magnetic loss using a single sheet tester (SST), and further analyzed the residual amount of carbon.

[0081] Table 2 shows magnetic induction B8 (T), magnetic loss (W _17/50 ) and residual carbon ( ^ppm ) after aging treatment.

[0082] [Table 2]

No. Steel No. Residual carbon [C] ( ^ppm ) Magnetic induction B8 (Tl) Magnetic losses W _17/50 (W/kg) Sheet thickness (mm) Example according to the invention B1 A1 22 1.89 0.82 0.30 B2 A2 21 1.91 0.81 0.30 B3 A3 fifteen 1.95 0.84 0.30 B4 A4 eighteen 1.89 0.77 0.30 B5 A5 22 1.96 0.82 0.30 B6 A6 23 1.90 0.79 0.30 B7 A7 fifteen 1.89 0.81 0.30 B8 A8 eighteen 1.89 0.80 0.30 B9 A9 22 1.90 0.80 0.30 B10 A10 21 1.91 0.81 0.30 B11 A11 twenty 1.94 0.82 0.30 B12 A12 22 1.93 0.81 0.30 B13 A13 12 1.93 0.80 0.30 B14 A14 9 1.92 0.82 0.30 B15 A15 fourteen 1.93 0.79 0.30 B16 A16 fourteen 1.94 0.75 0.30 B17 A17 fourteen 1.93 0.75 0.30 B18 A17 17 1.95 0.74 0.27 B19 A17 eighteen 1.94 0.73 0.27 B20 A17 19 1.94 0.73 0.23 B21 A17 21 1.96 0.72 0.23 Comparative Example b1 a1 47 1.67 − 0.30 b2 a2 19 1.55 − 0.30 b3 a3 − − − 0.30 b4 a4 23 1.49 − 0.30 b5 a5 24 1.51 − 0.30 b6 a6 − − − 0.30 b7 a7 − − − 0.30 b8 a8 − − − 0.30

[0083] The carbon residual analysis was performed in accordance with JIS G 1211-4: 2011. In terms of magnetic aging, the target carbon residual was 25 ^ppm or less. SST was performed in accordance with JIS C 2553. Magnetic loss was estimated from magnetic loss W _17/50 (W/kg) at a frequency of 50 Hz and a maximum magnetic induction of 1.7 T. The target value for W _17/50 (W/kg) was 0.85 or less.

[0084] Magnetic induction was evaluated by magnetic induction B8 at a magnetic field strength of 800 A/m. The target B8 value was 1.88 T or higher, and the magnetic loss was not evaluated for a sample with a B8 value of less than 1.88 T. In addition, for a steel sheet that could not be cold rolled, the residual amount of carbon was not analyzed, and the magnetic induction and magnetic loss were not evaluated. In the example of the present invention, a residual carbon amount of 23 ^ppm or less, a magnetic induction B8 of 1.89 T or more, and a magnetic loss W _17/50 (W/kg) of 0.84 or less were obtained.

<Example 2>

[0085] An ingot with the chemical composition shown in Table 1 was melted under vacuum and cast into a steel billet. The steel billet was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was annealed at 1000-1100°C for 2 minutes, pickled, and then cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.23-0.30 mm.

[0086] This cold-rolled steel sheet was subjected to decarburization annealing under the conditions shown in Table 3. HR1 is the average heating rate in the temperature range of 550°C or above and below 720°C, and HR2 is the average heating rate in the temperature range of 720°C or higher and the annealing temperature T1°C or lower. In addition, the "holding time" in Table 3 is the holding time at T1°C during the decarburization annealing.

[0087] [Table 3]

No. Steel No. Decarburizing annealing Residual carbon [C] ( ^ppm ) Magnetic induction B8 (T) Magnetic losses W _17/50 (W/kg) Sheet thickness (mm) Oxygen partial pressure P1 Holding temperature T1 (°C) Holding time (s) HR1 (°C/s) HR2 (°C/s) Example according to the invention C1 A15 0.002 790 150 54 12 22 1.90 0.76 0.30 C2 A15 0.186 788 140 55 ten eighteen 1.91 0.77 0.30 C3 A15 0.005 775 150 60 9 19 1.91 0.78 0.30 C4 A15 0.008 880 140 56 9 eighteen 1.92 0.78 0.30 C5 A15 0.05 803 55 68 eleven twenty 1.92 0.78 0.30 C6 A15 0.075 806 150 55 12 19 1.90 0.76 0.30 C7 A15 0.065 800 120 43 fourteen twenty 1.91 0.76 0.30 C8 A15 0.08 807 120 120 6 13 1.93 0.75 0.30 C9 A17 0.12 825 125 155 12 12 1.94 0.73 0.30 C10 A17 0.11 820 120 150 13 17 1.95 0.71 0.27 C11 A17 0.12 830 125 155 eleven 19 1.96 0.69 0.23 C12 A15 0.11 825 995 65 ten eighteen 1.92 0.77 0.30 C13 A15 0.12 890 120 495 12 17 1.91 0.78 0.30 C14 A15 0.12 775 120 60 45 eighteen 1.92 0.77 0.30 Comparative Example c1 A15 0.0008 786 142 60 12 89 1.90 0.88 0.30 c2 A15 0.235 795 143 59 fifteen 77 1.91 0.89 0.30 c3 A15 0.007 750 145 55 12 86 1.90 0.91 0.30 c4 A15 0.008 920 147 47 fifteen 77 1.89 0.91 0.30 c5 A15 0.007 865 40 45 12 67 1.89 0.88 0.30 c6 A15 0.007 860 1100 43 ten 17 1.56 − 0.30 c7 A15 0.009 871 155 thirty twenty 89 1.90 0.87 0.30 c8 A15 0.005 859 145 55 3 68 1.92 0.95 0.30 c9 A15 0.007 910 120 600 − 73 1.87 − 0.30 c10 A15 0.007 915 120 60 80 69 1.86 − 0.30

[0088] An annealing separator containing alumina as a main component was applied to the steel sheet after decarburization annealing as an aqueous suspension, and final annealing was performed. Final annealing was performed up to 1200°C at a temperature increase rate of 15°C/hour in an atmosphere containing nitrogen, and the atmosphere was switched to 100% hydrogen at 1200°C to perform annealing for 20 hours.

[0089] The sample for the evaluation table was taken from the secondary recrystallized steel sheet subjected to the above processes, then this sample was subjected to stress relief annealing at 800°C for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, after which the aging treatment was performed at 150°C for 300 hours under nitrogen, and then measured the magnetic induction and magnetic loss using a single sheet tester (SST), and further analyzed the residual amount of carbon. The magnetism evaluation method and the residual carbon analysis method were the same as in Example 1.

[0090] Table 3 shows magnetic induction B8 (T), magnetic loss (W _17/50 ) and residual carbon ( ^ppm ) after aging treatment. In the example of the invention, it was found that a residual carbon amount of 22 ^ppm or less, a magnetic induction B8 of 1.90 T or more, and a magnetic loss W _17/50 (W/kg) of 0.78 or less were obtained. In addition, in comparative example c9, the average heating rate HR1 exceeded 500°C/s, and the temperature of the steel sheet exceeded the upper limit value T1 when it was heated at the average heating rate HR1. Therefore, the average heating rate HR2 could not be measured.

<Example 3>

[0091] An ingot with the chemical composition shown in Table 1 was melted under vacuum and cast into a steel billet. The steel billet was heated to 1150° C. and subjected to hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of 2.3 mm. The hot-rolled steel sheet was annealed at 1000-1100°C for 2 minutes, pickled, and then cold-rolled to obtain a cold-rolled steel sheet with a final thickness of 0.23-0.30 mm.

[0092] This cold rolled steel sheet was subjected to decarburization annealing at T1=820°C for 140 seconds in an annealing atmosphere with P1=0.10. Meanwhile, heating to T1 was performed at an average heating rate HR1 of 80°C/s in a temperature range of 550°C or more and below 720°C, and was performed at an average heating rate HR2 of 10°C/s in a temperature range of 720°C or more and 820°C or below.

[0093] The cold rolled steel sheet was subjected to decarburization annealing at 820° C. for 140 seconds and then a second annealing treatment under the conditions shown in Table 4 without lowering the holding temperature.

[0094] [Table 4]

No. Steel No. HR3 (°C/s) Second annealing treatment Residual carbon [C] ( ^ppm ) Magnetic induction B8 (T) Magnetic losses W _17/50 (W/kg) Sheet thickness (mm) Partial pressure of oxygen P2 Holding temperature T2 (°C) Holding time (s) Example according to the invention D1 A15 6 0.056 845 150 5 1.90 0.74 0.30 D2 A15 45 0.035 850 140 3 1.91 0.73 0.30 D3 A15 6 0.081 856 5 four 1.93 0.72 0.30 D4 A15 7 0.041 835 150 2 1.91 0.74 0.30 D5 A15 7 0.038 860 5 four 1.92 0.73 0.30 D6 A15 eight 0.037 870 190 3 1.90 0.72 0.30 D7 A17 17 0.002 865 fifteen 2 1.94 0.70 0.30 D8 A15 3 0.001 875 fifteen ten 1.90 0.76 0.30 D9 A15 four 0.002 871 86 12 1.91 0.76 0.30 D10 A15 four 0.115 889 156 22 1.90 0.79 0.30 D11 A15 four 0.034 820 145 24 1.90 0.80 0.30 D12 A15 65 0.035 960 147 22 1.89 0.79 0.30 D13 A15 71 0.041 894 one 23 1.89 0.79 0.30 D14 A17 16 0.002 860 17 ten 1.94 0.68 0.27 D15 A17 eighteen 0.002 870 16 eighteen 1.97 0.65 0.23 D16 A17 eight 0.002 870 495 3 1.93 0.72 0.30

[0095] An annealing separator containing alumina as a main component was applied to the steel sheet after the second annealing treatment as an aqueous suspension, and a final annealing was performed. Final annealing was performed up to 1200° C. at a temperature increase rate of 15° C./hour in an atmosphere containing nitrogen, and the atmosphere was switched to 100% hydrogen at 1200° C. to perform annealing for 20 hours.

[0096] The sample for evaluation was taken from the secondary recrystallized steel sheet subjected to the above processes, then this sample was subjected to stress relief annealing at 800°C for 2 hours in a dry gas atmosphere containing nitrogen and hydrogen, after which aging treatment was performed at 150 °C for 300 hours in a nitrogen atmosphere, and then measured the magnetic induction and magnetic loss using a single sheet tester (SST), and further analyzed the residual amount of carbon. The magnetism evaluation method and the residual carbon analysis method were the same as in Example 1.

[0097] Table 4 shows magnetic inductions B8 (T), magnetic losses (W _17/50 ) and carbon residuals ( ^ppm ) of steel sheets after aging treatment. It was found that a residual carbon amount of 24 ^ppm or less, a magnetic induction B8 of 1.89 T or more, and a magnetic loss of W _17/50 (W/kg) of 0.80 or less were obtained. In particular, in the inventive examples D1-D7 and D14-D16, which satisfy the preferred conditions of the second annealing treatment and the third heat treatment described in the present embodiment, the residual carbon or magnetic loss tended to be further reduced. In addition, the sheet thicknesses were reduced in Examples D14 and D15, and in this respect, the magnetic loss also became satisfactory.

<Example 4>

[0098] In Example 4, the same treatment was performed as in Inventive Examples D1-D16 of Example 3, except that the nitriding treatment was performed between the second annealing treatment and the final annealing process. Here, the nitriding treatment was performed by holding the steel sheet at 700-800° C. for 30 seconds in an ammonia gas atmosphere. The results are shown in Table 5. In any of the examples of the invention, the residual amount of carbon was 25 ^ppm or less, the magnetic induction B8 was 1.88 T or more, and the magnetic loss W _17/50 (W/kg) was 0.85 or less.

[0099] [Table 5]

No. Steel No. Residual carbon [C] ( ^ppm ) Magnetic induction B8 (Tl) Magnetic losses W _17/50 (W/kg) Sheet thickness (mm) Example according to the invention E1 A15 6 1.91 0.75 0.30 E2 A15 four 1.92 0.74 0.30 E3 A15 5 1.92 0.73 0.30 E4 A15 one 1.91 0.74 0.30 E5 A15 3 1.91 0.74 0.30 E6 A15 four 1.91 0.71 0.30 E7 A17 3 1.93 0.72 0.30 E8 A15 eleven 1.90 0.75 0.30 E9 A15 13 1.91 0.76 0.30 E10 A15 21 1.91 0.78 0.30 E11 A15 23 1.90 0.79 0.30 E12 A15 21 1.89 0.80 0.30 E13 A15 24 1.90 0.79 0.30 E14 A17 eleven 1.93 0.69 0.27 E15 A17 17 1.96 0.65 0.23 E16 A17 four 1.92 0.72 0.30

INDUSTRIAL APPLICABILITY

[0100] As described above, according to the present invention, in a method for producing an anisotropic electrical steel sheet having no forsterite film, by deliberately preventing the formation of Fe-based oxides on the surface of the steel sheet when annealing in an atmosphere of low oxygen partial pressure in a decarburization process annealing after cold rolling, decarburization annealing can be stably performed without going through two or more cold rolling passes, including intermediate annealing, even in an atmosphere with a low oxygen partial pressure in which the formation of Fe-based oxides is suppressed. Therefore, the present invention has high applicability in the electrical steel sheet industry.

Claims (22)

1. A method for producing an anisotropic electrical steel sheet, comprising: a hot rolling process in which a steel billet containing, by mass%: 0.10 or less C, 0.80-7.00 Si, 0.01-0.07 acid-soluble Al, 0.012 or less N, 1.00 or less than Mn, 0.08 or less S, optionally, one or more of 0.01-0.50 Cr, 0.01-0.50 Cu and 0.01-0.02 Sn and the rest is Fe and impurities, subjected to hot rolling into hot rolled steel sheet; an annealing process in which the hot-rolled steel sheet is annealed; a pickling process in which the hot-rolled steel sheet after the annealing process is pickled; a cold rolling process in which the hot-rolled steel sheet after the pickling process is cold-rolled into a cold-rolled steel sheet; a decarburization annealing process in which the cold-rolled steel sheet is decarburized; and a final annealing process in which the steel sheet after the decarburization annealing process is finally annealed, wherein The decarburization annealing process includes: (i-1) a first heat treatment with an average heating rate HR1 of 40-500°C/s in a temperature range of 550°C or higher and lower than 720°C; (i-2) after the first heat treatment, a second heat treatment by heating with an average heating rate HR2 of 5-50°C/s in a temperature range of 720°C or higher and a temperature of T1°C or lower, which satisfies the following expression (2 ); and (ii) a first T1°C annealing treatment for 50-1000 seconds after the second heat treatment, wherein the first heat treatment, the second heat treatment, and the first annealing treatment are performed in an atmosphere with an oxygen partial pressure P1 satisfying the following expression (1), and the amount of C in the steel sheet after the first annealing treatment is 25 ^ppm or less, 0.0010≤P1≤0.20 ... (1), 770≤T1 (°C)≤900 ... (2). 2. The method for manufacturing the anisotropic electrical steel sheet according to claim 1, wherein the decarburization annealing process after the first annealing treatment further includes a second annealing treatment while maintaining a temperature of T2°C satisfying the following expression (4) for 3 to 500 seconds in atmosphere with oxygen partial pressure P2 satisfying the following expression (3), P2 < P1 ... (3), 960≥T2 (°C)≥T1+10 ... (4). 3. The method for producing an anisotropic electrical steel sheet according to claim 2, further comprising a third heat treatment between the first annealing treatment and the second annealing treatment, wherein in the third heat treatment, heating is performed at an average heating rate HR3 of 5-50°C/s in a temperature range of T1°C to T2°C. 4. Method for the production of anisotropic electrical steel sheet according to any one of paragraphs. 1-3 further comprising a nitriding treatment process by nitriding the steel sheet after the decarburization annealing process between the decarburizing annealing process and the final annealing process, wherein the steel sheet after the nitriding process is finally annealed in the final annealing process.