RU2843250C2 - Isotropic sheet of electrical steel and method of its production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to sheet of isotropic electrical steel with thickness of 0.1-0.3 mm, used as material of iron core of electric motors. Base metal sheet of isotropic electrical steel has a chemical composition containing, wt.%: C: 0.0050 or less, Si: more than 3.70 and 4.60 or less, Mn: more than 0.20 and 0.50 or less, Al: 0.23-0.75, P: 0.030 or less, S: 0.0018 or less, N: 0.0040 or less, Ti: less than 0.0050, Nb: less than 0.0050, Zr: less than 0.0050, V: less than 0.0050, Cu: less than 0.200, Ni: less than 0.500, Sn: 0.005-0.040, if necessary Sb: 0.040 or less, the rest is Fe and impurities. Chemical composition satisfies expression: 4.2≤Si+Al+0.5×Mn≤4.9. Content of N from surface to 20 mcm in the depth of the base metal is 0.0060% or less. Mean size of basic metal crystal grain makes 50-120 mcm.

EFFECT: sheet has high tensile strength equal to 600 MPa or more, and high magnetic properties, in particular density of magnetic flux of saturation equal to 1.945 T or more.

3 cl, 2 tbl

Description

Field of technology to which the invention relates

[0001] The present invention relates to an isotropic electrical steel sheet and to a method for producing the same.

State of the art

[0002] In recent years, global environmental issues have attracted attention, and the need for energy conservation efforts has increased. In particular, there is a serious need for greater efficiency in electrical equipment. For this reason, there is an increasing need for improved magnetic properties of isotropic electrical steel sheets, which are widely used as iron core materials for electric motors, generators, and the like. This is especially true for drive motors for electric and hybrid vehicles and electric motors for air conditioning compressors.

[0003] The core of the electric motor for various electric motors, as described above, consists of a stator, which is a stationary element, and a rotor, which is a moving element. The stator requires excellent magnetic properties (low iron loss and high magnetic flux density), in particular, low iron loss and high saturation magnetic flux density, while the rotor requires excellent mechanical properties.

[0004] Since the characteristics required for the stator and rotor are different, the required characteristics can be achieved by producing different types of isotropic electrical steel sheets for the stator and rotor. However, producing two types of isotropic electrical steel sheets would result in a decrease in the yield of good products. Therefore, in order to achieve the high strength required for rotors and the low iron loss required for stators, isotropic electrical steel sheets with excellent strength and magnetic properties have been studied in the past.

[0005] For example, attempts have been made to achieve excellent magnetic properties and high strength in Patent Documents 1-4.

List of prior art documents

Patent documents

[0006] Patent Document 1. WO 2019/017426 A1

Patent document 2. WO 2020/091039 A1

Patent document 3. WO 2020/091043 A1

Patent document 4. JP2010-90474 A

The essence of the invention

Technical task

[0007] However, in order to commercialize an isotropic electrical steel sheet having high strength and low iron loss, it is necessary to include a large amount of alloying elements as disclosed in Patent Documents 1 to 4, which results in a decrease in impact toughness and a tendency to fracture during cold rolling. In addition, high alloying may cause a decrease in saturation magnetic flux density.

[0008] The present invention is made to solve these problems, and an object of the present invention is to stably provide an isotropic electrical steel sheet having high strength and excellent magnetic properties.

Solution of the problem

[0009] The essence of the present invention lies in the following isotropic electrical steel sheet and in the method for producing it.

[0010] (1) An isotropic sheet of electrical steel comprising a base metal having a chemical composition containing, by weight %:

C: 0.0050 or less

Si: greater than 3.70 and 4.60 or less

Mn: more than 0.20 and 0.50 or less

Al: 0.23-0.75

P: 0.030 or less

S: 0.0018 or less

N: 0.0040 or less

Ti: less than 0.0050

Nb: less than 0.0050

Zr: less than 0.0050

V: less than 0.0050

Cu: less than 0.200

Ni: less than 0.500

Sn: 0.005-0.040, and

Sb: 0-0.040,

with a balance of Fe and impurities, and

satisfying the following formula (i), and:

- [N]s, the N content from the surface to 20 µm into the base metal is 0.0060% or less,

- the average size of the crystalline grain of the base metal is 50-120 µm,

- the saturation magnetic flux density is 1.945 T or more,

- the tensile strength is 600 MPa or more, and

- sheet thickness is 0.10-0.30 mm:

4.2≤Si+Al+0.5×Mn≤4.9 (i)

where the symbols of the elements in the above formula (i) represent the content (wt%) of each element.

[0011] (2) The isotropic electrical steel sheet described in (1) above, wherein the steel sheet includes an insulating coating on the surface of the base metal.

[0012] (3) The method for producing the isotropic electrical steel sheet described in (1) or (2) above,

the method includes:

- hot rolling stage,

- etching stage,

- a stage of hot strip annealing in a periodic mode, carried out at a holding temperature of 650-780°C for a holding time of 8-36 hours,

- a cold rolling stage to reduce the sheet thickness to 0.10-0.30 mm, and

- a final annealing stage at a holding temperature of 880-1020°C for a holding time of 1 second to 10 minutes,

- wherein the hot rolling stage, the pickling stage, the hot strip annealing stage, the cold rolling stage and the finish annealing stage are sequentially performed for a steel ingot having a chemical composition containing, by weight %:

C: 0.0050 or less

Si: greater than 3.70 and 4.60 or less

Mn: more than 0.20 and 0.50 or less

Al: 0.23-0.75

P: 0.030 or less

S: 0.0018 or less

N: 0.0040 or less

Ti: less than 0.0050

Nb: less than 0.0050

Zr: less than 0.0050

V: less than 0.0050

Cu: less than 0.200

Ni: less than 0.500

Sn: 0.005-0.040, and

Sb: 0-0.040,

with a balance of Fe and impurities, and

satisfying the following formula (i):

4.2≤Si+Al+0.5×Mn≤4.9 (i)

where the element symbols in the above formula represent the content (wt%) of each element.

Effects of invention

[0013] According to the present invention, an isotropic electrical steel sheet having high strength and excellent magnetic properties can be obtained.

Detailed description of embodiments

[0014] The present inventors have come to the following conclusions as a result of careful study in order to solve the above problems.

[0015] In order to achieve high strength, low iron loss and high saturation magnetic flux density while ensuring impact toughness during cold rolling, it is necessary to optimize the content of Si, Mn and Al, which are the main alloying elements.

[0016] In particular, Si, which has the greatest solid solution strengthening ability and the greatest contribution to the increase in electrical resistance, should be contained at more than 3.70% and 4.60% or less. In addition, in order to obtain good crystal grain growth, Al should be contained at 0.23% or more. On the other hand, the Al content should be 0.75% or less in order to suppress the deterioration of the saturation magnetic flux density. In addition, Mn, which has the least solid solution strengthening ability of the three elements but contributes to the increase in electrical resistance with little degradation in toughness, should be contained at more than 0.20%.

[0017] As a result of repeated studies by the present inventors, it has been found that when the surface layer of a steel sheet is nitrided, the iron loss worsens. Although the mechanism is not clear, it has been found that Mn has a negative effect on the nitriding of the surface layer of a steel sheet. In order to suppress the deterioration of the iron loss due to the nitriding of the surface layer of a steel sheet, the content of Mn should be 0.50% or less. In addition, it has also been found that Sn has an effect of suppressing the nitriding of the surface layer of a steel sheet. Therefore, Sn should be contained at 0.005-0.040%.

[0018] In general, in the continuous annealing stage of hot strip, the hot rolled steel sheet with scale is placed in an annealing furnace, which forms scale that is difficult to remove by pickling after annealing, which requires mechanical scale removal such as shot blasting before pickling. However, for high alloy steels such as the steels mentioned above, shot blasting causes deformation twinning on the surface of the steel sheet, and the deformation twinning can easily lead to problems such as sheet fracture and edge cracking during cold rolling.

[0019] In the present invention, a hot-rolled steel sheet is pickled before hot strip annealing and then annealed in a batch furnace. Since the scale on the hot-rolled steel sheet can be easily removed by pickling, shot blasting is not required, and deformation twinning does not occur. As a result, good impact toughness is ensured even in high-alloy steel, and the occurrence of problems such as sheet fracture and edge cracking during cold rolling can be suppressed.

[0020] The present invention has been accomplished based on the above findings. Each requirement of the present invention is described in detail below.

[0021] 1. General configuration

The isotropic electrical steel sheet of the embodiment of the present invention is suitable for both stators and rotors due to its high strength and excellent magnetic properties. In addition, the isotropic electrical steel sheet of the embodiment of the present invention preferably comprises an insulating coating described below on the surface of the base metal.

[0022] 2. Chemical composition of the base metal

The reasons for limiting each element are as follows. In the explanation below, "%" for content means "mass percentage".

[0023] C: 0.0050% or less

C (carbon) is an element that deteriorates the iron loss of the isotropic electrical steel sheet. A C content of more than 0.0050% deteriorates the iron loss of the isotropic electrical steel sheet, and the required magnetic properties cannot be achieved. Therefore, the C content should be 0.0050% or less. The C content is preferably 0.0040% or less, more preferably 0.0035% or less, and still more preferably 0.0030% or less. Since C contributes to the high strength of the isotropic electrical steel sheet, the C content is preferably 0.0005% or more, and more preferably 0.0010% or more when this effect is to be achieved.

[0024] Si: more than 3.70% and 4.60% or less

Si (silicon) is an element that increases the electrical resistance of steel to reduce eddy current loss and improve high-frequency iron loss of isotropic electrical steel sheet. Si is also an effective element for increasing the strength of isotropic electrical steel sheet due to its high solid solution strengthening ability. In order to achieve these effects, the Si content should be greater than 3.70%. The Si content is preferably 3.80% or more, more preferably 3.90% or more, and even more preferably 4.00% or more. On the other hand, if the Si content is excessive, the workability is greatly deteriorated and it becomes difficult to perform cold rolling. Therefore, the Si content should be 4.60% or less. The Si content is preferably 4.50% or less, more preferably 4.40% or less.

[0025] Mn: more than 0.20% and 0.50% or less

Mn (manganese) is an effective element for increasing the electrical resistance of steel so as to reduce the eddy current loss and improve the high-frequency iron loss of isotropic electrical steel sheet. In addition, if the Mn content is too low, the effect of increasing the electrical resistance is small, and the iron loss is worsened due to the precipitation of fine sulfides (Mn) in the steel. Therefore, the Mn content should be greater than 0.20%. The Mn content is preferably 0.25% or more, and more preferably 0.30% or more. On the other hand, if the Mn content is excessive, the nitriding of the surface layer of the steel sheet becomes excessive, and the iron loss is worsened. Therefore, the Mn content should be 0.50% or less. The Mn content is preferably 0.45%, and more preferably 0.40% or less.

[0026] Al: 0.23-0.75%

Al (aluminum) is an element that has the effect of reducing eddy current loss by increasing the electrical resistance of steel and improving the high-frequency iron loss of isotropic electrical steel sheet. Al also has the effect of improving iron loss by improving the texture of steel sheet. In addition, Al is an element that contributes to higher strength of isotropic electrical steel sheet through solution strengthening, although not to the same extent as Si. In addition, adding an appropriate amount of Al has the effect of suppressing the refinement of AlN caused by the bonding of Al with N in steel, improving the growth of crystal grains during finish annealing, and suppressing the deterioration of iron loss caused by the fine AlN itself.

[0027] In order to achieve these effects, the Al content must be 0.23% or more. The Al content is preferably 0.25% or more, and more preferably 0.27% or more. The above-mentioned nitriding of the surface layer of the steel sheet is more likely to occur with a higher Al content. As a result, the iron loss worsens. However, in the present invention, the nitriding of the surface layer of the steel sheet can be suppressed even with a high Al content by controlling the contents of Mn and Sn. Therefore, the advantage of the present invention is exhibited to a greater extent when the Al content is high. In other words, the effect of the present invention is exhibited to a greater extent when the Al content is, for example, more than 0.45% or 0.47% or more. On the other hand, when the Al content is excessive, the impact toughness is deteriorated and the risk of fracture during cold rolling increases. Therefore, the Al content should be 0.75% or less. The Al content is preferably 0.70% or less, and more preferably 0.65% or less.

[0028] In the present embodiment, the electrical resistance of the steel is ensured by appropriately controlling the content of Si, Al and Mn. It is also necessary to properly control the content of Si, Al and Mn from the viewpoint of ensuring the strength. On the other hand, an upper limit is also necessary from the viewpoint of ensuring the saturation magnetic flux density and the impact toughness. Therefore, in addition to the content of Si, Al and Mn being within the above-mentioned ranges, the following formula (i) must be satisfied. The value of the middle portion of the following formula (i) is preferably 4.3 or more, and more preferably 4.4 or more; the value is preferably 4.8 or less, and more preferably 4.7 or less.

[0029] 4.2≤Si+Al+0.5×Mn≤4.9 (i)

Here, the element symbols in the above formula represent the content (wt%) of each element.

[0030] P: 0.030% or less

P (phosphorus) is contained in steel as an impurity, and its excessive content significantly reduces the toughness of the isotropic electrical steel sheet. Therefore, the P content should be 0.030% or less. The P content is preferably 0.025% or less, and more preferably 0.020% or less. Since an extreme decrease in the P content may cause an increase in production costs, the P content is preferably 0.003% or more, and more preferably 0.005% or more.

[0031] S: 0.0018% or less

S (sulfur) is an element that increases iron loss by forming fine MnS inclusions and causes degradation of the magnetic properties of the isotropic electrical steel sheet. Therefore, the S content should be 0.0018% or less. The S content is preferably 0.0016% or less, and more preferably 0.0014% or less. Since an extreme decrease in the S content may cause an increase in production costs, the S content is preferably 0.0001% or more, more preferably 0.0003% or more, and still more preferably 0.0005% or more.

[0032] N: 0.0040% or less

N (nitrogen) is an element inevitably mixed in steel, and forms nitrides that increase iron loss and deteriorate the magnetic properties of isotropic electrical steel sheet. Therefore, the N content should be 0.0040% or less. The N content is preferably 0.0030% or less, and more preferably 0.0020% or less. Since an extreme decrease in the N content may cause an increase in production costs, it is preferable that the N content is 0.0005% or more.

[0033] Ti: less than 0.0050%

Ti (titanium) is an element inevitably mixed in steel, and can combine with carbon or nitrogen to form inclusions (carbides and nitrides). When carbides or nitrides are formed, these inclusions directly cause degradation of the magnetic properties of the isotropic electrical steel sheet. In addition, they disturb the growth of crystal grains during finish annealing, thereby causing degradation of the magnetic properties of the isotropic electrical steel sheet. Therefore, the Ti content should be less than 0.0050%. The Ti content is preferably 0.0040% or less, more preferably 0.0030% or less, and still more preferably 0.0020% or less. Since an extreme decrease in the Ti content may cause an increase in production costs, it is preferable that the Ti content is 0.0005% or more.

[0034] Nb: less than 0.0050%

Nb (niobium) is an element that contributes to high strength by bonding with carbon or nitrogen to form inclusions (carbides and nitrides), but these inclusions directly cause degradation of the magnetic properties of the isotropic electrical steel sheet. Therefore, the Nb content should be less than 0.0050%. The Nb content is preferably 0.0040% or less, more preferably 0.0030% or less, and still more preferably 0.0020% or less. Since an extreme decrease in the Nb content may cause an increase in production costs, it is preferable that the Nb content is 0.0001% or more.

[0035] Zr: less than 0.0050%

Zr (zirconium) is an element that contributes to high strength by bonding with carbon or nitrogen to form precipitates (carbides and nitrides), but these inclusions directly cause degradation of the magnetic properties of the isotropic electrical steel sheet. Therefore, the Zr content should be less than 0.0050%. The Zr content is preferably 0.0040% or less, more preferably 0.0030% or less, and still more preferably 0.0020% or less. Since an extreme decrease in the Zr content may cause an increase in production costs, it is preferable that the Zr content is 0.0001% or more.

[0036] V: less than 0.0050%

V (vanadium) is an element that contributes to high strength by combining with carbon or nitrogen to form inclusions (carbides and nitrides), but these inclusions directly cause degradation of the magnetic properties of the isotropic electrical steel sheet. Therefore, the V content should be less than 0.0050%. The V content is preferably 0.0040% or less, more preferably 0.0030% or less, and still more preferably 0.0020%. Since an extreme decrease in the V content may cause an increase in production costs, it is preferable that the V content is 0.0001% or more.

[0037] Cu: less than 0.200%

Cu (copper) is an element that is inevitably mixed in steel. Intentionally including Cu increases the production cost of isotropic electrical steel sheet. Therefore, in the present embodiment, Cu does not necessarily need to be actively included and may be included at an impurity level. The content of Cu must be less than 0.200%, which is the maximum value that can be inevitably mixed in the production step. The content of Cu is preferably 0.150% or less, and more preferably 0.100% or less. The lower limit of the Cu content is not particularly limited, but an extreme decrease in the Cu content may cause an increase in the production cost. Therefore, the content of Cu is preferably 0.001% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.

[0038] Ni: less than 0.500%

Ni (nickel) is an element that is inevitably mixed into steel. However, since Ni is also an element that increases the strength of the isotropic electrical steel sheet, it may be intentionally included. However, since Ni is expensive, the Ni content should be less than 0.500%. The Ni content is preferably 0.400% or less, and more preferably 0.300% or less. The lower limit of the Ni content is not particularly limited, but an extreme decrease in the Ni content may cause an increase in production costs. Therefore, the Ni content is preferably 0.001% or more, more preferably 0.003% or more, and still more preferably 0.005% or more. In the case of intentional inclusion, the Ni content is preferably 0.200% or more.

[0039] Sn: 0.005-0.040%

Sn (tin) is an element useful for ensuring low iron loss in isotropic electrical steel sheet by segregating on the surface of the base metal and suppressing oxidation and nitriding during annealing. Sn also has the effects of increasing the magnetic flux density of the isotropic electrical steel sheet by segregating at the grain boundaries and improving the texture. In order to achieve these effects, the Sn content should be 0.005% or more. The Sn content is preferably 0.010% or more, and more preferably 0.015% or more. On the other hand, if the Sn content is excessive, the toughness of the steel decreases and it becomes difficult to perform cold rolling. Therefore, the Sn content should be less than 0.040%. The Sn content is preferably less than 0.035%, and more preferably less than 0.030%.

[0040] Sb: 0-0.040%

Sb (antimony), similar to Sn, segregates on the surface of the base metal and suppresses oxidation and nitriding during annealing, and is a useful element for ensuring low iron loss in isotropic electrical steel sheet. Sb also has the effect of increasing the magnetic flux density of the isotropic electrical steel sheet by segregating at the grain boundaries and improving the texture. Therefore, Sb can be included as needed. However, excessive Sb content can reduce the toughness of steel and make cold rolling difficult. Therefore, the Sb content should be 0.040% or less. The Sb content is preferably 0.030% or less. In order to ensure the above effects, the Sb content is preferably 0.005% or more, and more preferably 0.010% or more.

[0041] In the chemical composition of the base metal of the isotropic electrical steel sheet of the present invention, the balance is Fe and impurities. The term "impurities" in this document means components that are mixed in during industrial steelmaking due to various factors in raw materials and the production stage, such as ores, scrap and other raw materials, and which are acceptable to the extent that they do not adversely affect the present invention.

[0042] It should be noted that the content of Cr and Mo as impurity elements is not specified. In the isotropic electrical steel sheet of the present embodiment, the content of these elements in a range of 0.5% or less, respectively, does not particularly affect the properties of the isotropic electrical steel sheet of the present embodiment. Ca and Mg contained in a range of 0.002% or less, respectively, do not particularly affect the properties of the isotropic electrical steel sheet of the present embodiment. Even if rare earth elements (REM) are contained in a range of 0.004% or less, there is no particular effect on the properties of the isotropic electrical steel sheet of the present embodiment. It should be noted that, in the present embodiment, REM means a total of 17 elements consisting of Sc, Y and lanthanides, and that the above-mentioned REM content means the total content of these elements.

[0043] O (oxygen) is also an impurity element, but its inclusion in the range of 0.035% or less does not affect the properties of the isotropic electrical steel sheet of the present embodiment. Since O can be mixed into the steel at the annealing stage, an O content of 0.010% or less at the slab stage (i.e., the value in the volume in the ladle) does not specifically affect the properties of the isotropic electrical steel sheet of the present embodiment.

[0044] In addition to the above elements, elements such as Pb, Bi, As, B and Se may be included as impurity elements, but if the content of each is within the range of 0.0050% or less, the characteristics of the isotropic electrical steel sheet of the present embodiment are not deteriorated.

[0045] Various known measurement methods are available for the chemical composition of the base metal of the isotropic electrical steel sheet of the present embodiment. For example, it can be measured by ICP-based optical emission spectrometry, gravimetric analysis, or spark discharge-based optical emission spectrometry. C and S can be measured by a post-combustion infrared absorption method, N by a thermal conductivity inert gas melting method, and O by a non-dispersive infrared absorption inert gas melting method.

[0046] Furthermore, in the isotropic electrical steel sheet of the present embodiment, [N]s indicating the N content from the surface of the base metal to 20 μm in the depth direction should be 0.0060% or less, from the viewpoint of suppressing nitriding of the surface layer of the steel sheet. If [N]s is 0.0060% or less, deterioration in iron loss can be suppressed. [N]s is preferably 0.0055% or less, and more preferably 0.0050% or less.

[0047] It should be noted that [N]s, which denotes the N content from the surface of the base metal to 20 μm in the depth direction, is measured by the following procedure. First, [N] ₁ , the N content of the isotropic electrical steel sheet in which the insulation coating is removed by a heated alkaline solution, is measured. Then, both surfaces of the isotropic electrical steel sheet are removed by chemical polishing to 20 μm, and [N] ₂ , the N content of the sample after removal, is measured. After that, from the measured [N] ₁ and [N] ₂ and the thickness t (μm) of the isotropic electrical steel sheet, [N]s is obtained by the following formula.

[N]s=(tx [N] ₁ -(t-40) x [N] ₂ )/40

[0048] 3. Crystalline grain size

In the present embodiment, the average crystal grain size of the base metal should be 50-120 μm. By setting the average crystal grain size of the base metal to 50 μm or more, it is possible to suppress the deterioration of the hysteresis loss and improve the magnetic properties. On the other hand, by setting the average crystal grain size to 120 μm or less, the effect of increasing the strength of steel can be achieved, and the deterioration of the iron loss due to increased eddy current loss can be suppressed. The average crystal grain size is preferably 60 μm or more, and more preferably 70 μm or more. In addition, the average crystal grain size is preferably 110 μm or less, and more preferably 100 μm or less.

[0049] In the present invention, the average crystal grain size of the base metal should be determined in accordance with JIS G 0551:2013 "Steel - Microscopic test methods for grain size".

[0050] 4. Magnetic properties

In the isotropic electrical steel sheet according to the present embodiment, the term "excellent magnetic properties" means that the iron loss W _10/400 is low and the saturation magnetic flux density Bs is high.

[0051] Here, the W _10/400 iron loss means the iron loss caused under conditions such that the maximum magnetic flux density is 1.0 T and the frequency is 400 Hz, and should be measured according to the Epstein method specified in JIS C 2550-1:2011. In the present invention, the term "low W _10/400 iron loss" means that the iron loss is 14.5 W/kg or less for a sheet thickness of 0.26 mm or more, 12.5 W/kg or less for a sheet thickness of 0.21-0.25 mm, and 11.0 W/kg or less for a sheet thickness of 0.20 mm or less.

[0052] The saturation magnetic flux density Bs is measured by a vibrating sample magnetometer (VSM). In the isotropic electrical steel sheet according to the present embodiment, the magnetic flux density Bs is 1.945 T or more regardless of the thickness of the sheet.

[0053] 5. Mechanical properties

The isotropic electrical steel sheet according to the present embodiment has high strength; in particular, the tensile strength is 600 MPa or more. The tensile strength is preferably 605 MPa or more, and more preferably 610 MPa or more. Here, the tensile strength should be measured by performing a tensile test according to JIS Z 2241:2011.

[0054] 6. Sheet thickness

In the isotropic electrical steel sheet according to the present embodiment, the sheet thickness should be 0.10 mm or more from the viewpoint of the production cost of cold rolling and finish annealing. On the other hand, from the viewpoint of reducing iron loss, the sheet thickness should be 0.30 mm or less. Therefore, the sheet thickness of the isotropic electrical steel sheet of the present embodiment is 0.10-0.30 mm.

[0055] The effect of nitriding the surface layer of a steel sheet becomes more pronounced the smaller the sheet thickness. In other words, if nitriding the surface layer of a steel sheet is not suppressed, the smaller the sheet thickness, the more the iron loss deteriorates. However, since the present invention suppresses nitriding the surface layer of a steel sheet, the deterioration in iron loss can be suppressed even when the sheet thickness is small. Therefore, when the sheet thickness is, for example, less than 0.25 mm or 0.20 mm or less, the advantage of the present invention is exhibited to a greater extent.

[0056] 7. Insulating coating

In the isotropic electrical steel sheet according to the present embodiment, it is preferable to provide the surface of the base metal with an insulating coating. Since the isotropic electrical steel sheets are stacked on top of each other after the core blanks are punched and then used, providing the surface of the base metal with an insulating coating can reduce eddy currents between the sheets, and the eddy current loss as a core can be reduced.

[0057] The type of the insulation coating is not particularly limited, and any known insulation coating used as an insulation coating for isotropic electrical steel sheets can be used. As such an insulation coating, for example, a composite insulation coating mainly including an inorganic component and further containing an organic component can be mentioned. Here, for example, the composite insulation coating is an insulation coating mainly including at least one of a metal chromate, a metal phosphate or other inorganic chemical compounds such as Zr chemical compounds and Ti chemical compounds in which fine particles of an organic resin are dispersed. In particular, from the viewpoint of reducing the impact on the environment during production, which has become increasingly necessary in recent years, insulation coatings using metal phosphates, Zr or Ti binding agents or carbonates or ammonium salts thereof as raw materials are preferably used.

[0058] The amount of the insulating coating to be applied is not particularly limited, but it is preferably approximately 200 to 1500 mg/ ^m2 per side, for example, and more preferably 300 to 1200 mg/ ^m2 per side. By forming the insulating coating such that the amount of the application is within the above-mentioned range, excellent uniformity can be maintained. In addition, when the amount of the application of the insulating coating is subsequently measured, various known measurement methods can be used. For example, a method using the difference in mass before and after immersion in a sodium hydroxide solution or an X-ray fluorescence method using a calibration curve method can be suitably used.

[0059] 8. Method of production

The isotropic electrical steel sheet of the present embodiment can be produced by performing a hot rolling step, a pickling step, a hot strip annealing step in a batch mode, a cold rolling step, and a finish annealing step in this order under the conditions described below with respect to a steel ingot having the chemical composition described above. In the case of forming an insulating coating on the surface of the base metal, the step of forming the insulating coating is performed after the finish annealing step described above. Each step is described in detail below.

[0060] Hot rolling stage

A steel ingot (slab) having the above-mentioned chemical composition is heated, and hot rolling is performed on the heated steel ingot to obtain a hot-rolled sheet. The heating temperature of the steel ingot for hot rolling is not specified, but, for example, 1050-1250°C is preferable. The thickness of the hot-rolled sheet after hot rolling is also not specified, but is preferably 1.5-3.0 mm, for example, taking into account the final thickness of the base metal.

[0061] Etching stage

The above hot rolled sheet is subjected to pickling to remove the scale layer formed on the surface of the base metal. The pickling conditions such as the acid concentration, the accelerator concentration and the pickling solution temperature are not limited and may be any known pickling conditions.

[0062] Hot strip annealing stage in batch mode

Hot strip annealing is then performed in order to reduce the iron loss of the steel sheet. Hot strip annealing is performed using a batch annealing furnace with a holding temperature of 650-780°C and a holding time of 8-36 hours. By setting the holding time to 8 hours or more, the metallurgical structure is sufficiently homogenized and the inclusions are coarsened, so that sufficient improvement in iron loss can be achieved. On the other hand, if the holding temperature exceeds 780°C or the holding time exceeds 36 hours, the crystal grain size becomes coarser and the toughness decreases, resulting in fracture during cold rolling.

[0063] The atmosphere in the batch annealing furnace is non-oxidizing and may be a mixture of _H2 and _N2 at a ratio of 1 to 100 vol% _H2 (i.e., _H2 + _N2 = 100 vol%). Even in an atmosphere containing _N2 , nitriding of the surface layer of the base metal can be suppressed if the content of Mn and Sn is appropriate. However, from the viewpoint of more reliably suppressing nitriding, it is preferable to use an atmosphere containing 100 vol% _H2 .

[0064] Cold rolling stage

The steel sheet that has undergone the above-mentioned hot strip annealing is subjected to cold rolling. In cold rolling, the base metal is rolled with such a reduction ratio that the final thickness of the base metal is 0.10-0.30 mm.

[0065] Finishing annealing stage

After the above-mentioned cold rolling, the steel sheet is subjected to finish annealing. In the method for producing isotropic electrical steel sheets according to the present embodiment, a continuous annealing furnace is preferably used for the finish annealing. The finish annealing is performed under conditions of a holding temperature of 880-1020°C and a holding time of 1 second to 10 minutes. Preferably, the atmosphere is a mixture of _H2 and _N2 with a ratio of 1 to 100 vol% _H2 (i.e., _H2 + _N2 = 100 vol%) and a dew point of -50 to +10°C.

[0066] If the soaking temperature is less than 880°C, the crystal grain size becomes finer and the iron loss worsens, which is not required. If the soaking temperature exceeds 1020°C, not only is the strength insufficient, but the iron loss also worsens due to nitriding of the surface layer, which is not required. If the soaking time is less than 1 second, sufficient growth of the crystal grains is impossible. On the other hand, if the soaking time exceeds 10 minutes, the production cost must increase.

[0067] Stage of forming the insulating coating

After the finishing annealing described above, a step of forming an insulating coating is performed as necessary. The method for forming the insulating coating is not particularly limited; using known solutions for applying an insulating coating as shown below, the solution can be applied and dried by known methods. As an example of known insulating coatings, a composite insulating coating mainly including an inorganic component and also containing an organic component can be mentioned.

[0068] The composite insulation coating is, for example, an insulation coating mainly comprising at least one of a metal chromate, a metal phosphate or other inorganic chemical compounds such as Zr chemical compounds and Ti chemical compounds, in which fine particles of an organic resin are dispersed. In particular, from the viewpoint of reducing the environmental impact during production, which has become increasingly necessary in recent years, insulation coatings using metal phosphates, Zr or Ti binding agents as a raw material, or insulation coatings using a metal phosphate, carbonate or ammonium salts of phosphates, or Zr or Ti binding agents as a raw material are preferably used.

[0069] The surface of the base metal on which the insulating coating is to be formed may be subjected to any pretreatment, such as degreasing with an alkali or pickling with hydrochloric, sulfuric or phosphoric acid, before the application of the treatment solution. The surface of the base metal may be coated with the treatment solution in a finish-annealed condition even without any pretreatment.

[0070] The isotropic electrical steel sheet obtained by the above-described method has excellent characteristics of high saturation magnetic flux density, low iron loss and high strength, and is suitable as a material for both rotors and stators.

[0071] The invention will now be described in more detail by means of the following examples, but the invention is not limited to these examples.

Example

[0072] Slabs with the chemical compositions shown in Table 1 are heated to 1150°C, hot rolled at a finishing rolling temperature of 850°C to a finishing rolling thickness of 2.0 mm, and coiled at 600°C to produce hot rolled steel sheets. After pickling to remove scale, the hot rolled steel sheets are hot strip annealed in a batch annealing furnace at the holding temperatures and holding times shown in Table 2. The resulting steel sheets are cold rolled to a thickness of 0.20 mm. The steel sheets are then finish annealed in a mixed atmosphere of _H2 :20%, _N2 :80% with a dew point of -30°C for a soaking time of 20 seconds at the soaking temperatures shown in Table 2. After finish annealing, the steel sheets are coated with an insulating coating consisting of an emulsion of aluminum phosphate and an acrylic-styrene copolymer resin with a particle size of 0.2 μm, and then baked at 350°C in air.

[0073] Table 1

Table 1 Steel Chemical composition (wt.%, balance: Fe and impurities) The value of the middle part of formula (i) ^† C Si Mn P S Al N You Nb Zr V Cu Ni Sn Sb A 0.0025 3.80 0.15 0,014 0,0015 0.30 0,0014 0,0012 0,0006 0,0007 0.0002 0.029 0,018 0.031 traces 4.2 B 0.0026 3.80 0.25 0,015 0,0015 0.30 0,0015 0,0013 0,0008 0,0007 0,0016 0,030 0,020 0,030 traces 4.2 C 0,0020 3.90 0.35 0,013 0,0015 0.25 0,0015 0,0012 0,0009 0,0006 0,0005 0.031 0.035 0.029 0.002 4.3 D 0.0021 3.90 0.35 0,012 0.0019 0.25 0,0014 0,0013 0,0007 0,0004 traces 0.028 0.034 0,030 0.003 4.3 E 0.0018 3.86 0.42 0,013 0,0004 0.51 0,0013 0,0016 0,0006 0,0004 0,0006 0.022 0,019 0.003 traces 4.6 F 0.0022 3.87 0.43 0,015 0,0004 0.52 0,0015 0,0015 0,0007 0.0011 0,0009 0,007 0,019 0.006 0,013 4.6 G 0.0029 4.20 0.30 0,015 0,0005 0.40 0,0017 0,0010 0,0014 0,0006 0,0009 0,009 0,005 0.035 0.004 4.8 H 0.0024 4.20 0.30 0,014 0,0005 0.40 0,0010 0,0012 0,0016 0,0006 0,0009 0,005 0,005 0.046 0.003 4.8 I 0.0023 4.11 0.45 0,020 0,0005 0.36 0,0010 0,0014 0,0014 0,0005 traces 0.006 0.006 0,010 0,001 4.7 J 0.0025 4.10 0.55 0,019 0,0005 0.38 0,0017 0.0014 0,0015 0,0004 traces 0.006 0.006 0,007 0,001 4.8 K 0.0036 4.50 0.26 0.008 0,0010 0.25 0,0008 0,0007 0,0004 0,0005 0,0006 0,005 0,011 0,010 0,001 4.9 L 0.0026 4.50 0.36 0.008 0,0010 0.35 0,0015 0,0012 0,0006 traces 0,0006 0,005 0,013 0,010 0,001 5.0 M 0,0020 4.72 0.30 0,012 0,0010 0.30 0,0013 0,0012 0,0006 traces 0,0006 0.006 0,018 0,010 0,005 5.2 N 0,0016 4.02 0.30 0,016 0.0011 0.33 0,0012 0,0017 0,0006 0,0005 0,0004 0,012 0,018 0,030 0,005 4.5 O 0.0028 3.93 0.39 0,018 0,0006 0.29 0,0014 0,0014 0,0005 0,0005 0,0003 0,013 0,019 0,010 0.004 4.4 P 0.0025 3.70 0.30 0,015 0,0008 0.35 0,0012 0,0013 0,0005 0,0005 0,0003 0,012 0,021 0.026 0.003 4.2 Q 0,0030 3.80 0.21 0,016 0,0008 0.23 0,0014 0,0014 0,0008 0,0007 0,0005 0,014 0.022 0.026 0.002 4.1 R 0.0029 3.95 0.23 0,016 0,0010 0.20 0,0015 0,0012 0,0008 0,0007 0,0005 0,012 0,018 0.035 0.003 4.3 S 0,0010 3.78 0.25 0.025 0,0010 0.48 0.0018 0,0013 0,0010 0,0046 0.0041 0,019 0,016 0,018 0.008 4.4 T 0.0032 3.80 0.32 0,009 0,0010 0.69 0,0020 0,0010 0,0007 0,0005 0,0007 0,018 0,019 0.024 traces 4.7 U 0.0026 3.95 0.23 0,010 0,0010 0.77 0,0020 0,0010 0,0006 0,0004 0,0007 0,018 0,018 0.025 traces 4.8 V 0.0011 3.81 0.41 0.008 0,0009 0.62 0.0032 0,0009 0.0042 0,0012 0,0015 0.153 0.203 0,015 0.002 4.6 W 0,0008 3.80 0.26 0,016 0,0007 0.71 0,0011 0.0036 0.0018 0,0012 0.0019 0.181 0,410 0,015 traces 4.6 X 0.0025 3.95 0.25 0.002 0,0012 0.22 0,0015 0,0010 0,0009 0,0006 0,0008 0,017 0,020 0.024 0,001 4.3 4.2≤Si+Al+0.5×Mn≤4.9 (i) "traces" indicate that the value is at or below the measurement limit. Underlining indicates that the value is outside the range of the present invention.

[0074] Table 2

Table 2 Test number Steel Hot strip annealing Finish annealing [N]s (wt.%) Average crystal grain size (µm) Tensile strength (MPa) W _10/400 (W/kg) Bs (T) Notes Holding temperature (°C) Holding time (h) Holding temperature (°C) 1 A 750 10 1000 0.0029 52 610 11.8 1,979 Comparative example 2 B 750 10 1000 0.0027 62 604 10.9 1,977 Example of the invention 3 C 750 10 1000 0.0029 69 612 10.4 1,974 Example of the invention 4 D 750 10 1000 0.0027 57 619 11.5 1,974 Comparative example 5 E 710 10 1000 0.0078 102 604 11.3 1,960 Comparative example 6 F 710 10 1000 0.0033 100 606 10.0 1,959 Example of the invention 7 G 710 10 1000 0.0025 95 643 9.8 1,954 Example of the invention 8 H 710 10 Failure during cold rolling Comparative example 9 I 730 10 1000 0.0051 105 629 9.8 1,957 Example of the invention 10 J 730 10 1000 0,0069 95 632 11.1 1,954 Comparative example 11 K 680 10 1000 0.0035 80 679 10.0 1,951 Example of the invention 12 L 680 10 1000 0,0040 85 682 10.5 1,943 Comparative example 13 M 680 10 Failure during cold rolling Comparative example 14 N 820 10 Failure during cold rolling Comparative example 15 N 780 10 1000 0.0032 80 624 10.1 1,965 Example of the invention 16 N 750 10 1000 0.0033 75 626 10.3 1,965 Example of the invention 17 N 700 10 1000 0,0030 68 629 10.5 1,965 Example of the invention 18 N 640 10 1000 0.0031 48 643 11.8 1,965 Comparative example 19 O 700 10 870 0.0025 45 634 12.2 1,970 Comparative example 20 O 700 10 950 0,0030 70 617 10.3 1,970 Example of the invention 21 O 700 10 1000 0.0045 100 605 10.0 1,970 Example of the invention 22 O 700 10 1050 0.0096 125 599 11.1 1,970 Comparative example 23 P 750 10 950 0,0030 65 593 11.0 1,978 Comparative example 24 Q 750 10 950 0.0035 65 600 11.2 1,982 Comparative example 25 R 720 10 1000 0.0028 48 629 12.0 1,977 Comparative example 26 S 720 10 1000 0.0025 75 601 10.2 1,968 Example of the invention 27 T 720 10 1000 0.0035 85 607 10.0 1,954 Example of the invention 28 U 720 10 Failure during cold rolling Comparative example 29 V 720 10 980 0.0039 80 612 10.3 1,953 Example of the invention 30 W 720 10 980 0.0041 78 614 10.5 1,951 Example of the invention 31 W 780 38 Failure during cold rolling Comparative example 32 W 800 10 Failure during cold rolling Comparative example 33 X 720 10 980 0.0029 52 630 11.5 1,975 Comparative example Underlining indicates that the value is outside the range of the present invention.

[0075] For each obtained sample, the average crystal grain size is measured in a section parallel to the rolling direction of the base metal in accordance with JIS G 0551:2013 "Steel - Microscopic test method for grain size". Epstein test specimens are taken from the rolling direction and the widthwise direction of each sample, and the W _10/400 iron loss is estimated by the Epstein test in accordance with JIS C 2550-1:2011. The saturation magnetic flux density is measured by a vibrating sample magnetometer (VSM).

[0076] Then, in accordance with JIS Z 2241:2011, JIS No. 5 tensile test specimens are taken from each sample in such a way that the longitudinal direction coincides with the rolling direction of the steel sheet. The above test specimens are then subjected to a tensile test in accordance with JIS Z 2241:2011 to measure the tensile strength.

[0077] In addition, [N]s, which denotes the N content from the surface of the base metal to 20 μm in the depth direction, is measured by the following procedure. First, [N] ₁ , the N content of the isotropic electrical steel sheet from which the insulation coating is removed by a heated alkaline solution, is measured. Then, both surfaces of the isotropic electrical steel sheet are removed by chemical polishing to 20 μm, and [N] ₂ , the N content of the sample after removal, is measured. After that, [N]s is obtained from the measured [N] ₁ and [N] ₂ and the thickness t (μm) of the isotropic electrical steel sheet by the following formula.

[N]s=(t × [N] ₁ -(t-40) × [N] ₂ )/40

[0078] The above results are shown in Table 2.

[0079] It was found that in tests Nos. 2, 3, 6, 7, 9, 11, 15-17, 20, 21, 26, 27, 29 and 30, which satisfy the requirements of the present invention, low iron loss, high saturation flux density and high tensile strength of 600 MPa or more are achieved.

[0080] In contrast, comparative examples, in tests with numbers 1, 4, 5, 8, 10, 12-14, 18, 19, 22-25, 28 and 31-33, either worse W _10/400 losses in iron, worse saturation magnetic flux density, or significantly lower impact toughness are obtained, which makes them difficult to manufacture.

[0081] Specifically, in test number 1, the Mn content is below the specified range, resulting in worse iron loss due to a large amount of fine MnS inclusions. In test number 4, the S content is above the specified range, resulting in worse iron loss due to a large amount of MnS inclusions. In test number 5, the Sn content is below the specified range, resulting in worse iron loss due to its higher [N]s. In test number 8, the Sn content is above the specified range, which reduces impact toughness and causes fracture during cold rolling, resulting in the inability to measure tensile strength and magnetic properties.

[0082] In test number 10, the Mn content is higher than the specified range, resulting in worse iron loss due to higher [N]s. In test number 12, the saturation magnetic flux density is worse because Si+Al+0.5 x Mn is higher than the specified range. In test number 13, the Si and Si+Al+0.5 x Mn content are higher than the specified ranges, causing worse impact toughness, causing fracture during cold rolling and making it impossible to measure tensile strength and magnetic properties.

[0083] In test number 14, the annealing temperature for the hot strip annealing is above the specified range, causing failure during cold rolling due to degradation of impact toughness, causing the tensile strength and magnetic properties to be impossible to measure. In test number 18, the hot strip annealing temperature is below the specified range, causing worse iron loss due to an average crystal grain size after finish annealing being smaller than the specified range. In test number 19, the finish annealing temperature is below the specified range, causing worse iron loss due to an average crystal grain size being smaller than the specified range. In test number 22, the finish annealing temperature is above the specified range, the average crystal grain size is larger than the specified range, and [N]s is above the specified range, causing worse iron loss and worse strength.

[0084] In test number 23, the Si content is below the specified range, resulting in poor tensile strength. In test number 24, Si+Al+0.5 x Mn is below the specified range, resulting in poor iron loss. In addition, in test number 25, the average crystal grain size after finish annealing is smaller than the specified range because the Al content is below the specified range, resulting in poor iron loss. In test number 28, the Al content is above the specified range, resulting in poor impact toughness and fracture during cold rolling, resulting in failure to measure tensile strength and magnetic properties.

[0085] In test No. 31, the holding time in the hot strip annealing exceeds the specified range, which causes the impact toughness to deteriorate due to the coarsening of the crystal grain size, resulting in fracture during cold rolling, and making it impossible to measure the tensile strength and magnetic properties. In test No. 32, the holding temperature in the hot strip annealing step is higher than the specified range, and the impact toughness deteriorates due to the coarsening of the crystal grain size, resulting in fracture during cold rolling, making it impossible to measure the tensile strength and magnetic properties. In test No. 33, the Al content is lower than the specified range, resulting in worse iron loss due to the deterioration of the texture and the precipitation of fine AlN inclusions.

Industrial applicability

[0086] As described above, the present invention makes it possible to produce an isotropic electrical steel sheet with high strength and excellent magnetic properties at low cost.

Claims (53)

1. An isotropic sheet of electrical steel containing a base metal having a chemical composition containing, by weight %: C: 0.0050 or less Si: greater than 3.70 and 4.60 or less Mn: more than 0.20 and 0.50 or less Al: 0.23-0.75 P: 0.030 or less S: 0.0018 or less N: 0.0040 or less Ti: less than 0.0050 Nb: less than 0.0050 Zr: less than 0.0050 V: less than 0.0050 Cu: less than 0.200 Ni: less than 0.500 Sn: 0.005-0.040, and if necessary Sb: 0.040 or less, with a balance of Fe and impurities, and satisfying the following formula (i), and: - [N]s, the N content from the surface to 20 µm deep in the base metal is 0.0060% or less, - the average size of the crystalline grain of the base metal is 50-120 µm, - the saturation magnetic flux density is 1.945 T or more, - the tensile strength is 600 MPa or more, and - sheet thickness is 0.10-0.30 mm: 4.2≤Si+Al+0.5×Mn≤4.9 (i) where the symbols of the elements in the above formula (i) represent the content (wt%) of each element. 2. An isotropic electrical steel sheet according to claim 1, wherein the steel sheet comprises an insulating coating on the surface of the base metal. 3. A method for producing an isotropic sheet of electrical steel according to paragraph 1 or 2, wherein the method comprises: - hot rolling stage, - etching stage, - a stage of hot strip annealing in a periodic mode, carried out at a holding temperature of 650-780°C for a holding time of 8-36 hours, - a cold rolling stage to reduce the sheet thickness to 0.10-0.30 mm, and - a final annealing stage at a holding temperature of 880-1020°C for a holding time of 1 second to 10 minutes, - wherein the hot rolling stage, the pickling stage, the hot strip annealing stage, the cold rolling stage and the finish annealing stage are sequentially performed for a steel ingot having a chemical composition containing, by weight %: C: 0.0050 or less Si: greater than 3.70 and 4.60 or less Mn: more than 0.20 and 0.50 or less Al: 0.23-0.75 P: 0.030 or less S: 0.0018 or less N: 0.0040 or less Ti: less than 0.0050 Nb: less than 0.0050 Zr: less than 0.0050 V: less than 0.0050 Cu: less than 0.200 Ni: less than 0.500 Sn: 0.005-0.040, and if necessary Sb: 0.040 or less, with a balance of Fe and impurities, and satisfying the following formula (i): 4.2≤Si+Al+0.5×Mn≤4.9 (i) where the element symbols in the above formula represent the content (wt%) of each element.