RU2843190C2 - Isotropic sheet of electrical steel, electric motor core and methods of their production - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to metallurgy, namely to a sheet of isotropic electrical steel with thickness of 0.1-0.3 mm, used as material of iron cores 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, balance is - Fe and impurities. Content of components in steel sheet satisfies the following ratio: 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 equal to 0.0060 wt.% or less. Mean size of basic metal crystal grain makes 10-30 mcm.

EFFECT: sheet has high tensile strength equal to 680 MPa or more, and high magnetic properties, in particular, losses in iron W_10/400 equal to 20 W/kg or less, and density of saturation magnetic flux equal to 1.945 T or more.

6 cl, 2 tbl, 1 ex

Description

Field of technology to which the invention relates

[0001] The present invention relates to an isotropic electrical steel sheet, to an electric motor core and to methods for producing them.

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 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 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, in Patent Documents 1-4, attempts are made to achieve excellent magnetic properties and high strength.

List of prior art documents

Patent documents

[0006] Patent Document 1. WO 2019/017426

Patent document 2. WO 2020/091039

Patent document 3. WO 2020/091043

Patent document 4. JP2008-50686 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 with 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 core of an electric motor and in methods for their production.

[0010] (1) An isotropic electrical steel sheet comprising a base metal having a chemical composition comprising, in 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%, and

Sb: 0-0.040%,

- with a balance of Fe and impurities, and

- satisfying the following formula (i), while:

- [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 10-30 µm,

- W _10/400 losses in iron are 20.0 W/kg or less,

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

- the tensile strength is 680 MPa or more, and

- sheet thickness is 0.10-0.30 mm:

4.2≤Si+Al+0.5 x Mn≤4.9 (i)

- where the symbols of the elements in the above formula (i) represent the content (in 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 700-830°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, in 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%, and

Sb: 0-0.040%,

- with a balance of Fe and impurities, and

- satisfying the following formula (i):

4.2≤Si+Al+0.5 x Mn≤4.9 (i)

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

(4) The core of an electric motor, which includes a plurality of laminated isotropic sheets of electrical steel,

- in this case, the chemical composition of the base metal of the isotropic sheet of electrical steel contains, in 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%, and

Sb: 0-0.040%,

- with a balance of Fe and impurities, and

- and it satisfies the formula (i) below, while:

- [N]s, the N content from the surface to 20 µm into the base metal is 0.0070% 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 thickness of the sheet of isotropic electrical steel sheet is 0.10-0.30 mm:

4.2≤Si+Al+0.5 x Mn≤4.9 (i)

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

[0013] (5) The core of an electric motor described in (4) above, wherein the isotropic electrical steel sheet includes an insulating coating on the surface of the base metal of the isotropic electrical steel sheet.

[0014] (6) A method for producing a core of an electric motor described in (4) or (5) above,

- the method includes:

- hot rolling stage,

- etching stage,

- a stage of hot strip annealing in a periodic mode, performed 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,

- a final annealing stage at a holding temperature of 700-830°C for a holding time of 1 second to 10 minutes,

- perforation stage,

- the stage of laying the package,

- an annealing stage to relieve mechanical stress at a holding temperature of 750-900°C for 10-180 minutes,

- wherein the hot rolling stage, the pickling stage, the hot strip annealing stage, the cold rolling stage, the finish annealing stage, the perforation stage, the stacking stage and the stress relieving annealing stage are sequentially performed for a steel ingot having a chemical composition containing, in 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%, and

Sb: 0-0.040%,

- with a balance of Fe and impurities, and

- satisfying the following formula (i):

4.2≤Si+Al+0.5 x Mn≤4.9 (i)

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

Advantages of the invention

[0015] According to the present invention, it is possible to obtain an isotropic electrical steel sheet having high strength and excellent magnetic properties.

Description of embodiments

[0016] The present inventors have come to the following conclusions as a result of careful study in order to solve the above problems.

[0017] 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.

[0018] 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%.

[0019] The present inventors have studied and found that iron loss worsens when the surface layer of a steel sheet is nitrided. Although the mechanism is unclear, 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 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 nitriding of the surface layer of a steel sheet. Therefore, Sn should be contained at 0.005-0.040%.

[0020] In the general continuous hot strip annealing, a hot rolled steel sheet with scale is placed in an annealing furnace, which forms a 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, strain twinning occurs on the surface of the steel sheet due to shot blasting, and the strain twinning can easily lead to problems such as sheet fracture and edge cracking during cold rolling.

[0021] 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.

[0022] The present invention has been accomplished based on the above findings. Each requirement of the present invention is described in detail below.

[0023] 1. General configuration

The isotropic electrical steel sheet of the embodiment according to the present invention is suitable for both stators and rotors due to its high strength and excellent magnetic properties.

[0024] In particular, when an isotropic electrical steel sheet is used as a stator core, a reduction in iron loss is highlighted. In such cases, it is preferable to produce electric motor cores by punching and stacking the above-mentioned isotropic electrical steel sheets and then subjecting only the stator core to stress relief annealing. The stator core that undergoes stress relief annealing has reduced iron loss due to the growth of crystal grains, which can significantly improve the efficiency of the electric motor.

[0025] The core of the electric motor according to the embodiment of the present invention is obtained by sequentially applying a punching step, a stacking step, and a stress-relieving annealing step to the above-mentioned isotropic electrical steel sheets. In other words, the core of the electric motor of the embodiment of the present invention is a laminated material made of a plurality of isotropic electrical steel sheets. In the following description, the base metal of the isotropic electrical steel sheets constituting the core of the electric motor is simply referred to as the "base metal of the core of the electric motor."

[0026] In addition, the isotropic electrical steel sheet and the core of the electric motor according to the present embodiment preferably comprise an insulating coating on the surface of the base metal described below.

[0027] 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".

[0028] 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 obtained.

[0029] 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.

[0030] 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 sulfide inclusions (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.

[0031] 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 the iron loss by improving the texture of the steel sheet. In addition, Al is an element that contributes to higher strength of isotropic electrical steel sheet through solid 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, which is caused by combining Al with N in steel, improving the growth of crystal grains during finish annealing and stress relief annealing, and suppressing the deterioration of iron loss caused by fine AlN itself.

[0032] 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 when the Al content is higher. 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 when the Al content is high by controlling the Mn and Sn content. Therefore, the advantage of the present invention is exhibited to a greater extent when the Al content is high. In other words, the advantage 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 toughness is deteriorated and the risk of fracture during cold rolling increases. Therefore, the Al content must be 0.75% or less. The Al content is preferably 0.70% or less, and more preferably 0.65% or less.

[0033] 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.

[0034] 4.2≤Si+Al+0.5 x Mn≤4.9 (i)

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

[0035] 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.

[0036] 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.

[0037] N: 0.0040% or less

N (nitrogen) is an element inevitably mixed with steel, and forms nitrides that increase iron loss and cause degradation of 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.

[0038] Ti: less than 0.0050%

Ti (titanium) is an element inevitably contained in steel, and can combine with carbon or nitrogen to form inclusions (carbides and nitrides). If 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 and stress relief 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.

[0039] Nb: less than 0.0050%

Nb (niobium) 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 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.

[0040] Zr: less than 0.0050%

Zr (zirconium) 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 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.

[0041] 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.

[0042] Cu: less than 0.200%

Cu (copper) is an element that is inevitably mixed in steel. Intentionally including Cu increases the production cost of the isotropic electrical steel sheet. Therefore, in the present embodiment, Cu does not necessarily need to be actively included and may be included at the impurity level. The content of Cu must be less than 0.200% or less, which is the maximum value that can be inevitably mixed in the production stage. 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.

[0043] 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 isotropic electrical steel sheet, it can 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.

[0044] 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 effect of increasing the magnetic flux density of the isotropic steel sheet by segregating at the grain boundaries and improving the texture. In order to obtain 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 cold rolling becomes difficult. 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%.

[0045] 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.

[0046] In the chemical composition of the base metal of the isotropic electrical steel sheet according to 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 in 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.

[0047] It should be noted that the content of Cr and Mo as impurity elements is not specified. In the isotropic electrical steel sheet and the motor core of the present embodiment, the content of these elements in a range of 0.5% or less, respectively, does not specifically affect the properties of the isotropic electrical steel sheet and the motor core of the present embodiment. Ca and Mg contained in a range of 0.002% or less, in particular, do not affect the properties of the isotropic electrical steel sheet and the motor core of the present embodiment. Even if rare earth elements (REM) are contained in a range of 0.004% or less, there is no specific effect on the properties of the isotropic electrical steel sheet and the rotor core of the present embodiment. In the present embodiment, REM means a total of 17 elements consisting of Sc, Y and lanthanides, and the above-mentioned REM content means the total content of these elements.

[0048] 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 and the core of the electric motor of the present embodiment. Since O can be mixed into the steel in the annealing step, the O content of 0.010% or less in the slab step (i.e., the volume value in the ladle) does not specifically affect the properties of the isotropic electrical steel sheet and the core of the electric motor of the present embodiment.

[0049] In addition to the above elements, elements such as Pb, Bi, As, B and Se may be included as impurity elements, and if the content of each is within the range of 0.0050% or less, the characteristics of the isotropic electrical steel sheet and the core of the electric motor of the present embodiment are not deteriorated.

[0050] Various known methods are available for measuring the chemical composition of the base metal of the isotropic electrical steel sheet or the base metal of the electric motor core of the present embodiment. For example, it can be measured, for example, 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 conductive inert gas fusion method, and O by a non-dispersive infrared absorption inert gas fusion method.

[0051] 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.

[0052] On the other hand, in the electric motor core of the present embodiment, nitriding is improved because the above-mentioned isotropic electrical steel sheet is further annealed to relieve mechanical stress. However, in order to suppress deterioration in terms of iron loss, the [N]s in the electric motor core of the present embodiment must be 0.0070% or less. The [N]s in the above-mentioned electric motor core is preferably 0.0065% or less, and more preferably 0.0060% or less.

[0053] It should be noted that [N]s, indicating the N content from the surface of the base metal of the isotropic electrical steel sheet or the base metal of the electric motor core to 20 μm in the depth direction, is measured by the following procedure. First, [N] ₁ , indicating the N content of the base metal of the isotropic electrical steel sheet or the base metal of the electric motor core from which the insulation coating is removed by a heated alkaline solution, is measured. Then, both surfaces of the base metal of the isotropic electrical steel sheet or the base metal of the electric motor core are removed by chemical polishing to 20 μm, and [N] ₂ , indicating the N content of the sample after the removal, is measured. After that, from the measured [N] ₁ and [N] ₂ and the sheet thickness t (μm) of the isotropic electrical steel sheet or the base metal of the electric motor core, [N]s is obtained by the following formula.

[N]s=(tx[N] ₁ -(t-40) x[N] ₂ )/40

[0054] 3. Crystalline grain size

In the isotropic electrical steel sheet according to the present embodiment, the average crystal grain size of the base metal should be 10-30 μm. By setting the average crystal grain size of the base metal of the isotropic electrical steel sheet to 10 μm or more, it is possible to minimize the deterioration in hysteresis loss and improve magnetic properties. On the other hand, by setting the average crystal grain size to 30 μm or less, the effect of increasing the strength of the steel is remarkable. When magnetic properties are important, the average crystal grain size is preferably 15 μm or more, and more preferably 20 μm or more. On the other hand, when strength is important, the average crystal grain size is preferably 25 μm or less, and more preferably 20 μm or less.

[0055] In the core of the electric motor of the present embodiment, the average crystal grain size of the base metal is 50-120 μm. By setting the average crystal grain size of the base metal of the core of the electric motor 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 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. The average crystal grain size is preferably 110 μm or less, and more preferably 100 μm or less.

[0056] In the present invention, the average crystal grain size of the base metal of the isotropic electrical steel sheet and the average crystal grain size of the base metal of the electric motor core should be determined according to JIS G 0551:2013 "Steel - Microscopic test method for crystal grain size".

[0057] 4. Magnetic properties

In the isotropic electrical steel sheet and the core of the electric motor 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.

[0058] Here, the W _10/400 iron loss means the iron loss generated under conditions such that the maximum magnetic flux density is 1.0 T and the frequency is 400 Hz, and shall be measured by the Epstein method specified in JIS C 2550-1:2011.

[0059] In the isotropic electrical steel sheet of the present embodiment, the iron loss W _10/400 is 20.0 W/kg or less. On the other hand, in the motor core of the present embodiment, the iron loss is reduced to a greater extent by stress relief annealing, and the term "low iron loss W _10/400 " means 13.5 W/kg or less for a sheet thickness of 0.26 mm or more, 11.5 W/kg or less for a sheet thickness of 0.21-0.25 mm, and 10.0 W/kg or less for a sheet thickness of 0.20 mm or less.

[0060] It is impossible to take large test specimens, such as Epstein test specimens, from the core of an electric motor. In order to evaluate the iron loss of the core of an electric motor, the laminated iron core is divided into steel sheets, and a small test specimen for magnetic measurement of single sheets with dimensions that can be assembled according to the size of the separated steel sheet is manufactured by electric spark machining. Then, the iron loss value is measured using a small single sheet tester corresponding to the above small test specimen. In this case, the measurement principle adheres to the document "Methods of measurement of the magnetic properties of electrical steel strip and sheet by means of the single sheet tester" (for the single sheet tester: SST) specified in JIS C 2556:2015.

[0061] The above-mentioned Epstein test specimens and small test specimens are selected in advance from various types of isotropic electrical steel sheets, and the iron loss values are measured by the Epstein method and the single sheet magnetic property measurement method, and a conversion formula is derived from the relationship between the two measured values. Then, the iron loss value measured by the single sheet tester is corrected so that it is equivalent to the iron loss value measured by the Epstein method, using the above-mentioned conversion formula.

[0062] In addition, the saturation magnetic flux density Bs is measured by a vibrating sample magnetometer (VSM). In the isotropic electrical steel sheet of the present embodiment, the saturation magnetic flux density Bs is 1.945 T or more. In the core of the electric motor of the present embodiment, the saturation magnetic flux density Bs is 1.945 T or more.

[0063] 5. Mechanical properties

The isotropic electrical steel sheet of the present embodiment has high strength; in particular, the tensile strength is 680 MPa or more. The tensile strength is preferably 690 MPa or more, and more preferably 700 MPa or more. Here, the tensile strength should be measured by performing a tensile test according to JIS Z 2241:2011.

[0064] 6. Sheet thickness

In the isotropic electrical steel sheet of the present embodiment and the isotropic electrical steel sheets constituting the core of the electric motor of the present embodiment, the sheet thickness must 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 must be 0.30 mm or less. Therefore, the thickness of the isotropic electrical steel sheet of the present embodiment or the sheets constituting the core of the electric motor of the present embodiment is 0.10-0.30 mm.

[0065] 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.

[0066] 7. Insulating coating

In the isotropic electrical steel sheet and the core of the electric motor according to the present embodiment, it is preferable to have an insulating coating on the surface of the base metal.

Since isotropic electrical steel sheets are stacked on top of each other after the core blanks are punched and then used, the eddy currents between the sheets and the eddy current losses as the core can be reduced by providing the surface of the base metal with an insulating coating.

[0067] The type of the insulating coating is not particularly limited, and any known insulating coating used as an insulating coating for isotropic electrical steel sheets can be used. For example, a composite insulating coating mainly composed of an inorganic component and also containing an organic component can be mentioned. Here, the composite insulating coating, for example, is an insulating coating mainly including at least one of a metal chromate, a metal phosphate or other inorganic components such as colloidal silica, chemical compounds of Zr and chemical compounds of Ti 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, insulating coatings using metal phosphates, Zr or Ti binding agents or carbonate or ammonium salts thereof as raw materials are preferably used.

[0068] The amount of the insulating coating applied is not particularly limited, but it is preferably about 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 applied is within the above-mentioned range, excellent uniformity can be maintained. In addition, when the amount of the applied 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.

[0069] 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 furnace, a cold rolling step, and a finish annealing step in that order under the conditions shown 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 a 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.

[0070] 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.

[0071] Etching stage

The above hot rolled sheet is pickled 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 temperature of the pickling solution are not limited and may be any known pickling conditions.

[0072] 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 excessively coarse and the toughness decreases, resulting in fracture during cold rolling.

[0073] The atmosphere in the batch annealing furnace is not oxidized 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 .

[0074] Cold rolling stage

The steel sheet that undergoes the above-mentioned hot strip annealing is subjected to cold rolling. In cold rolling, the steel sheet is rolled at such a degree of compression that the final thickness of the base metal is 0.10-0.30 mm.

[0075] Finish 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, it is preferable to use a continuous annealing furnace for finish annealing. The finish annealing is performed under the condition of a holding temperature of 700-830°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.

[0076] If the soaking temperature is below 700°C, the crystal grain size becomes finer and the unrecrystallized structures increase, resulting in a significant deterioration in iron loss, which is undesirable, and if the soaking temperature exceeds 830°C, the strength becomes insufficient, which is undesirable. If the soaking time is less than 1 second, the dislocation density cannot be reduced sufficiently. On the other hand, if the soaking time exceeds 10 minutes, the production cost must increase.

[0077] Stage of formation of insulating coating

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

[0078] The composite insulation coating is, for example, an insulation coating mainly including at least one of a metal chromate, a metal phosphate or other inorganic components such as colloidal silica, Zr compounds and Ti 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 is an increasing demand in recent years, insulation coatings using metal phosphates, Zr or Ti binding agents as raw materials, or insulation coatings using carbonate or ammonium salts among the salts of metal phosphates, Zr or Ti binding agents as raw materials are preferably used.

[0079] The surface of the base metal on which the insulating coating is to be formed may be subjected to any preliminary treatment such as degreasing with an alkali or pickling with hydrochloric acid, sulfuric acid or phosphoric acid before the application of the treatment solution. The surface of the finish-annealed base metal may be coated with the treatment solution even without these preliminary treatments.

[0080] Furthermore, with respect to the electric motor core of the present embodiment, although the production method is not particularly limited, it is possible to produce the electric motor core, for example, by sequentially performing a punching step, a stacking step and a stress relieving annealing step under the conditions shown below, for an isotropic electrical steel sheet obtained through the above steps. As mentioned above, when the isotropic electrical steel sheet is used as a stator core in which low iron loss is important, it is preferable to perform the stress relieving annealing described below. Therefore, it is preferable that the electric motor core of the present embodiment is a stator core. Each step is described in detail below.

[0081] Perforation stage

The isotropic electrical steel sheet obtained as described above is perforated to form materials for the rotor core or stator core. There is no limitation on the perforation conditions, and general methods can be used.

[0082] Package laying stage

A plurality of isotropic sheets of electrical steel, which undergo a perforation stage, are stacked in a package to form the core of an electric motor.

[0083] Annealing stage to relieve mechanical stress

Stress relief annealing is performed on the core of the electric motor. The core of the electric motor that undergoes stress relief annealing reduces iron loss and can greatly improve the efficiency of the electric motor by promoting recrystallization and crystal grain growth.

[0084] In the production method for the cores of an electric motor according to the present embodiment, stress relieving annealing is performed under conditions of a holding temperature of 750-900°C and a holding time of 10-180 minutes. If the proportion of _N2 in the atmosphere is less than 70 vol%, the cost of stress relieving annealing must increase, which is undesirable. The proportion of _N2 in the atmosphere is more preferably 80 vol% or more, still more preferably 90-100 vol%, and particularly 97-100 vol%. An atmospheric gas other than _N2 is not specified, but is generally a reducing mixed gas consisting of hydrogen, carbon dioxide, carbon monoxide, water vapor, methane, etc. Such a gas is usually produced by burning propane or natural gas. The dew point of the atmosphere is preferably between -50 and +10°C.

[0085] Hereinafter, the present invention will be described in more detail by means of the following examples, but the present invention is not limited to these examples.

Example

[0086] Slabs having the chemical compositions shown in Table 1 are heated to 1150°C, hot rolled to a finishing thickness of 2.0 mm at a finishing temperature of 850°C, and coiled at 600°C to produce hot-rolled steel sheets. After pickling to remove scale, the resulting hot-rolled steel sheets are subjected to hot strip annealing in a batch annealing furnace at the holding temperatures shown in Table 2 for a holding time of 10 hours. Such resulting steel sheets are cold rolled to a thickness of 0.20 mm. In addition, the steel sheets are subjected to finish annealing in a mixed atmosphere _{of H2} :15%, _N2 :85% with a dew point of -30°C for a holding time of 20 seconds at the holding temperatures shown in Table 2. After the 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 baked at 350°C in air.

[0087] In addition, the obtained isotropic electrical steel sheets are subjected to stress relieving annealing under conditions of holding at 800°C x 120 min in a nitrogen atmosphere with a dew point of -40°C (the nitrogen ratio in the atmosphere is 99.9% by volume or more).

[0088] Table 1

Table 1

Steel Chemical composition (wt.%, balance: Fe and impurities) The meaning of the middle part of the formula (i)t 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 0,015 0,0012 0.22 0.0015 0,0010 0,0009 0,0006 0,0008 0,017 0,020 0.024 0,001 4.3

t 4.2≤Si+Al+0.5xMn≤4.9 (i)

"traces" indicates that the value is at or below the measurement limit.

Underlining indicates that the value is outside the range of the present invention.

[0089] Table 2

Table 2

Test number Steel Annealing holding temperature of hot strip (°C) Finish annealing holding temperature (°C) After final annealing After annealing to relieve mechanical stress Notes [N]s (wt.%) Average crystal grain size (µm) Tensile strength (MPa) W _10/400 (W/kg) Bs (T) [N]s (wt.%) Average crystal grain size (µm) W _10/400 (W/kg) Bs (T) 1 A 750 750 0.0024 12 698 21.5 1,979 0,0043 45 10.7 1,979 Comparative example 2 B 750 750 0.0022 13 693 19.0 1,977 0.0045 60 9.4 1,977 Example of the invention 3 C 750 750 0.0021 14 698 18.6 1,974 0.0041 62 9.1 1,974 Example of the invention 4 D 750 750 0,0020 12 711 21.8 1,974 0,0044 43 11.0 1,974 Comparative example 5 E 710 750 0.0065 13 745 20.5 1,954 0,0080 64 10.8 1,960 Comparative example 6 F 710 750 0,0030 14 739 18.7 1,954 0.0056 71 9.0 1,959 Example of the invention 7 G 710 750 0.0035 15 734 18.1 1,954 0,0040 78 8.8 1,954 Example of the invention 8 H 710 Failure during cold rolling Comparative example 9 I 730 750 0.0045 15 735 18.0 1,951 0.0052 80 8.6 1,957 Example of the invention 10 J 730 750 0.0061 14 741 20.3 1,949 0.0074 83 10.7 1,954 Comparative example 11 K 680 780 0,0044 19 748 16.8 1,951 0.0053 82 8.5 1,951 Example of the invention 12 L 680 780 0.0048 18 756 17.0 1,943 0.0056 80 8.8 1,943 Comparative example 13 M 680 Failure during cold rolling Comparative example 14 N 820 Failure during cold rolling Comparative example 15 N 780 730 0.0028 12 724 19.3 1,968 0,0040 61 9.5 1,965 Example of the invention 16 N 750 730 0.0027 11 732 19.5 1,968 0.0039 57 9.4 1,965 Example of the invention 17 N 700 730 0.0025 10 740 19.8 1,968 0.0038 53 9.6 1,965 Example of the invention 18 N 640 730 0.0024 9 750 21.5 1,968 0.0039 48 10.2 1,965 Comparative example 19 O 700 680 0.0021 8 782 23.3 1,959 0.0059 61 9.8 1,970 Comparative example 20 O 700 700 0.0023 10 760 19.9 1,959 0.0061 64 9.2 1,970 Example of the invention 21 O 700 820 0,0046 25 692 17.2 1,959 0.0059 75 8.8 1,970 Example of the invention 22 O 700 860 0.0048 34 675 16.3 1,959 0.0079 70 9.3 1,970 Comparative example 23 P 750 750 0,0030 14 677 19.2 1,978 0.0035 75 9.6 1,978 Comparative example 24 Q 750 750 0.0032 14 684 21.2 1,982 0,0040 72 10.2 1,982 Comparative example 25 R 720 750 0.0035 13 708 20.3 1,977 0,0040 45 10.6 1,977 Comparative example 26 S 720 750 0.0036 14 706 18.0 1,969 0.0048 65 9.3 1,968 Example of the invention 27 T 720 750 0.0038 15 709 16.7 1,955 0.0054 75 8.9 1,954 Example of the invention 28 U 720 Failure during cold rolling Comparative example 29 V 740 770 0.0042 15 705 17.5 1,953 0.0052 69 8.9 1,953 Example of the invention 30 W 740 770 0,0044 14 707 17.8 1,950 0.0055 65 9.0 1,951 Example of the invention 31 W 800 Failure during cold rolling Comparative example 32 X 720 730 0,0020 11 718 20.5 1,975 0.0041 51 10.5 1,975 Comparative example

Underlining indicates that the value is outside the range of the present invention.

[0090] The average crystal grain size of each specimen after both the finish annealing and the stress relief annealing is measured in a cross section parallel to the rolling direction of the base metal in accordance with JIS G 0551:2013 "Steel - Microscopic test method for grain size". In addition, Epstein test specimens are taken from the rolling direction and the widthwise direction of the finish annealing of each specimen, and the W _10/400 iron loss is estimated by the Epstein test in accordance with JIS C 2550-1:2011. The estimation of the W _10/400 iron loss after the stress relief annealing is performed for the Epstein test specimens taken from each test specimen after the finish annealing and then subjected to the stress relief annealing. The saturation magnetic flux density is measured using a vibrating sample magnetometer (VSM).

[0091] Thereafter, JIS No. 5 tensile test specimens in accordance with JIS Z 2241:2011 are taken from each sample, which is finish-annealed in such a manner 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.

[0092] In addition, [N]s, denoting the N content from the surface of the base metal to 20 μm in the depth direction of each sample after the finish annealing or after the stress relief annealing, is measured by the following procedure. First, [N] ₁ , denoting the N content of the sample, is measured after the insulation coating is removed by a heated alkaline solution. Then, both surfaces of the sample are removed by chemical polishing to 20 μm, and [N] ₂ , denoting the N content of the sample after the removal, is measured. After that, [N]s is obtained from the measured [N] ₁ and [N] ₂ and the thickness t (μm) of the sample by the following formula.

[N]s=(tx [N] ₁ -(t-40) x [N] ₂ )/40

[0093] The above results are also shown in Table 2.

[0094] Test numbers 2, 3, 6, 7, 9, 11, 15-17, 20, 21, 26, 27, 29 and 30, which satisfy the requirements of the present invention, have low iron loss after finish annealing, high saturation flux density and high tensile strength above 680 MPa, and also have low iron loss after stress relief annealing.

[0095] In contrast, in comparative examples, test numbers 1, 4, 5, 8, 10, 12-14, 18, 19, 22-25, 28, 31 and 32 have either worse W _10/400 iron loss, worse saturation magnetic flux density, or significantly reduced impact toughness, making them difficult to manufacture.

[0096] Specifically, in test number 1, the Mn content is below the specified range, which results in poor iron loss due to a large amount of fine MnS inclusions. In test number 4, the S content is above the specified range, which results in poor iron loss due to a large amount of fine MnS inclusions. In test number 5, the Sn content is below the specified range, which results in higher [N]s and poor iron loss. In test number 8, the Sn content is greater than the specified range, which reduces impact toughness and results in fracture during cold rolling, making it impossible to measure tensile strength and magnetic properties.

[0097] In test No. 10, the Mn content is greater than the specified range, which results in higher [N]s and worse iron loss. In test No. 12, the saturation magnetic flux density is worse because Si+Al+0.5 x Mn is greater than the specified range. In test No. 13, the Si and Si+Al+0.5 x Mn content are higher than the specified ranges, and the impact toughness is worse, which results in fracture during cold rolling and makes it impossible to measure the tensile strength and magnetic properties.

[0098] In test number 14, the hot strip annealing temperature is above the specified range, which results in degradation of toughness and failure during cold rolling and makes it impossible to measure tensile strength and magnetic properties. In test number 18, the hot strip annealing temperature is below the specified range, which results in a smaller average crystal grain size and worse iron loss, both after finish annealing and after stress relief annealing. In test number 19, the finish annealing temperature is below the specified range, and the average crystal grain size after finish annealing is smaller than the specified range, which results in worse iron loss after finish annealing. In test number 22, the finish annealing temperature is above the specified range, which results in worse tensile strength.

[0099] In test number 23, the Si content is below the specified range, which results in poor tensile strength. In test number 24, Si+Al+0.5 x Mn is below the specified range, which results in poor iron loss both after finish annealing and after stress relief annealing. In addition, in test number 25, the Al content is below the specified range, which results in poor iron loss after finish annealing due to AlN microinclusions, and also results in poor iron loss after stress relief annealing due to the small average crystal grain size. In test number 28, the Al content is greater than the specified range, which results in reduced impact toughness and fracture during cold rolling, and makes it impossible to measure tensile strength and magnetic properties.

[0100] In test No. 31, the holding temperature of the hot strip annealing stage is higher than the specified range, and the impact toughness is deteriorated due to the coarsening of the crystal grain size, which leads to fracture during cold rolling, making it impossible to measure the tensile strength and magnetic properties. In test No. 32, the Al content is lower than the specified range, which leads to deterioration of the texture and precipitation of fine AlN, which worsens the iron loss after finish annealing and also after stress relief annealing.

Industrial applicability

[0101] As described above, according to the present invention, an isotropic electrical steel sheet with high strength and excellent magnetic properties can be obtained at low cost.

Claims (110)

1. An isotropic sheet of electrical steel containing a base metal having a chemical composition containing, by weight %: 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 and if necessary Sb: 0.040 or less, - with a balance of Fe and impurities and - satisfying the following formula (i), while: - [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 10-30 µm, - W _10/400 losses in iron are 20.0 W/kg or less, - the saturation magnetic flux density is 1.945 T or more, - the tensile strength is 680 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 (in wt.%) of each element. 2. An isotropic sheet of electrical steel according to paragraph 1, wherein the steel sheet contains 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, 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 700-830°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: 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, 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 symbols of the elements in the above formula (i) represent the content (in wt.%) of each element. 4. The core of the electric motor, containing laminated isotropic sheets of electrical steel, - in this case, the chemical composition of the base metal of the isotropic sheet of electrical steel contains, by weight %: 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 and if necessary Sb: 0.040 or less, - with a balance of Fe and impurities, and - and it satisfies the formula (i) below, while: - [N]s, the N content from the surface to 20 µm into the base metal, is 0.0070 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 thickness of the isotropic sheet of electrical steel 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 (in wt.%) of each element. 5. The core of an electric motor according to claim 4, wherein the isotropic sheet of electrical steel comprises an insulating coating on the surface of the base metal of the isotropic sheet of electrical steel. 6. A method for producing an electric motor core according to paragraph 4 or 5, - 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, - a final annealing stage at a holding temperature of 700-830°C for a holding time of 1 second to 10 minutes, - perforation stage, - the stage of laying the package, - an annealing stage to relieve mechanical stress at a holding temperature of 750-900°C for 10-180 minutes, - wherein the hot rolling stage, the pickling stage, the hot strip annealing stage, the cold rolling stage, the finish annealing stage, the perforation stage, the stacking stage and the stress relieving annealing stage are sequentially performed for a steel ingot having a chemical composition containing, by weight %: 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, 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 symbols of the elements in the above formula (i) represent the content (in wt.%) of each element.