RU2791867C1 - Sheet from non-oriented electrical steel and method for its manufacture - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to non-oriented electrical steel sheet with controlled chemical composition and steel sheet microstructure, having suitable iron loss characteristics at high frequencies and high fatigue strength. Sheet of non-oriented electrical steel, having a chemical composition, % by weight: C 0.0100 or less, P 0.100 or less, Si 2.00 or more and 5.00 or less, Mn 5.00 or less, Al 3.00 or less, S 0.0100 or less, N 0.0050 or less, Zn 0.0005 or more and 0.0030 or less, Mo 0.001 or more and 0.100 or less, Cu 0.2 or less, Nb 0.010 or less, O 0.0050 or less, if necessary, at least one of: Cr 0.010 or more and 5.000 or less, Ca 0.0010 or more and 0.0050 or less, Sn 0.001 or more and 0.100 or less, Sb 0.0010 or more and 0.1000 or less, and the rest is Fe and unavoidable impurities. The sheet has a microstructure in which the proportion of non-recrystallized microstructure is 5% or more and 70% or less, and the number of inclusions with a diameter of 5 mcm or more is not more than 5 per mm².

EFFECT: sheet is suitable for use in an internal permanent magnet motor rotor, which has suitable magnetic flux density B₅₀ and high frequency iron loss characteristics, high tensile strength and fatigue strength.

2 cl, 2 dwg, 66 ex, 4 tbl

Description

Technical field

The present invention relates to a non-oriented electrical steel sheet with a controlled chemical composition and microstructure of the steel sheet, wherein the non-oriented electrical steel sheet has suitable high frequency iron loss characteristics and high fatigue strength. The present disclosure also relates to a method for manufacturing a non-oriented electrical steel sheet.

State of the art

The motor for electric vehicles and air conditioners, which achieves great energy savings, usually uses an internal permanent magnet motor (IPM motor). The rotor of the IPM motor has a slotted part made of electrical steel sheet and a permanent magnet built into it. When the rotor rotates at a high speed, the centrifugal force exerts a significant effect on the web section of the slot portion in which the permanent magnet is held, resulting in the web section breaking.

From the point of view of increasing the strength of the web section, the width of the web should be increased, but when the width of the web is increased, the leakage flux of the permanent magnet increases, which reduces the efficiency of the motor.

Thus, the width of the web is designed to be as narrow as possible within the range over which the strength of the rotor can be achieved.

In addition, in a motor for an electric vehicle, the web section of the rotor is subjected to repeated loads when starting, accelerating, and stopping the vehicle.

Therefore, the web section must have strength to withstand centrifugal force during rapid rotation and withstand repeated loads.

Accordingly, the non-oriented electrical steel sheet used as the rotor material must have a strength capable of withstanding centrifugal force when rotating at high speed, i.e. suitable tensile strength and tensile strength that can withstand repeated loads, i.e., adequate fatigue strength.

In addition, in a motor using a permanent magnet, the tooth section is driven by a magnetic flux of up to 1.50 T or more, so that the magnetic flux density B ₅₀ must be 1.50 T or more. Moreover, since iron losses occur on the surface of the rotor due to higher harmonics, the electrical steel sheet used for the rotor must have low iron losses at high frequencies.

As a material for such a rotor, for example, JP 5445194 B (PTL 1) discloses a high-strength electrical steel sheet having a chemical composition containing Si: 0.2% or more and 3.5% or less, Al: 2.50%. or less, and Nb: 0.05% or more and 8.0% or less.

JP 2005-113252 A (PTL 2) discloses a high strength electrical steel sheet whose chemical composition contains Si: 2.0% or more and 3.5% or less, Al: 0.02% or more and 3.0% or less, and N: 0.005% or more and 0.020% or less, and an average grain size of 10 µm or less in a region up to 10 µm at a depth from the surface of the finished sheet.

List of sources cited

Patent Literature

PTL 1: JP 5445194 B

PTL 2: JP 2005-113252 A

Disclosure of the essence of the invention

Technical problem

However, the electrical steel sheet disclosed in PTL 1 uses Nb precipitation hardening, which improves strength but increases iron loss.

The electrical steel sheet disclosed in PTL 2 has a high nitrogen content, which also increases the iron loss.

In addition, it is known that, in order to improve the tensile strength and fatigue strength of an electrical steel sheet, it is effective to reduce the content of impurity elements such as C, S, and N while refining the crystal grains.

However, in this method, since the crystal grain size changes depending on the slight change in the amounts of C, S and N, the change in tensile strength among the manufactured electrical steel sheets becomes large and must be suppressed.

Thus, it may be useful to provide a non-oriented electrical steel sheet suitable for use in an IPM motor rotor, which has a suitable magnetic flux density B ₅₀ and high frequency iron loss characteristics, high tensile strength and fatigue strength, and little change in limit tensile strength, as well as a method of its manufacture.

Solution

As a result of intensive research carried out to solve the above problems, we have found that by appropriately controlling the amount of Zn and Mo added to a non-oriented electrical steel sheet having a non-recrystallized microstructure and reducing inclusions in the steel, it is possible to make the magnetic flux density B ₅₀ and high-frequency iron loss suitable, and the fatigue strength and tensile strength are high, and reduce the change in tensile strength.

The present disclosure has been completed based on the above data and its main features are described below.

1. Non-oriented electrical steel sheet, including: the chemical composition of the steel sheet, containing (consisting of) in wt.%, C: 0% or more and 0.0100% or less, P: 0% or more and 0.100% or less , Si: 2.00% or more and 5.00% or less, Mn: 0% or more and 5.00% or less, Al: 0% or more and 3.00% or less, S: 0% or less more and 0.0100% or less, N: 0% or more and 0.0050% or less, Zn: 0.0005% or more and 0.0030% or less, Mo: 0.001% or more and 0.100% or less , Cu: 0% or more and 0.2% or less, Nb: 0% or more and 0.010% or less, and O: 0% or more and 0.0050% or less, the rest is Fe and unavoidable impurities; and a microstructure in which the proportion of non-recrystallized microstructure is 5% or more and 70% or less, and the number of inclusions with a diameter of 5 μm or more is not more than 5 imp/mm ² .

2. The non-oriented electrical steel sheet according to claim 1, further including Cr: 0.010 mass% or more and 5,000 mass% or less in the chemical composition of the steel sheet.

3. The non-oriented electrical steel sheet according to claim 1 or 2, further including Ca: 0.0010 wt.% or more and 0.0050 wt.% or less in the chemical composition of the steel sheet.

4. Sheet of non-oriented electrical steel according to paragraphs. 1 to 3, further comprising Sn: 0.001 mass% or more and 0.100 mass% or less and/or Sb: 0.0010 mass% or more and 0.1000 mass% or less in the chemical composition of the steel sheet.

5. A method of manufacturing a sheet of non-oriented electrical steel according to paragraphs. 1 - 4, including the supply of the deoxidizer during vacuum degassing and subsequent mixing for 10 minutes or more.

Preferential effect

According to the present disclosure, the rotor material for high speed motors not only has suitable B ₅₀ magnetic flux density and high frequency iron loss, but also high fatigue strength and low high tensile strength spread.

Brief description of the drawings

On the attached drawings:

Fig. 1 illustrates the relationship between the proportion of non-recrystallized microstructure and fatigue strength; And

Fig. 2 illustrates the relationship between the number of inclusions with a diameter of 5 µm or more and fatigue strength.

Implementation of the invention

The following will describe the history of how the present disclosure was made, as well as the details and reasons for the limitations of the present disclosure.

Some non-oriented electrical steel sheets have a non-recrystallized microstructure. A non-recrystallized microstructure in a non-oriented electrical steel sheet is a microstructure in which recrystallization is not completed. Recrystallization means that crystalline grains with a much lower dislocation density are formed and grow when the material is kept at high temperatures, and recrystallized grains can be distinguished from non-crystallized grains by observation with optical microscopy. When the proportion of non-recrystallized microstructure in steel is large, fatigue strength increases and iron loss tends to deteriorate.

First, in order to investigate the effect of non-recrystallized microstructure on fatigue strength, we smelted steel in the laboratory having a wt% C: 0.0015%, Si: 3.7%, Al: 0.5%, Mn: 0.7 % , P: 0.01%, S: 0.001%, N: 0.0016%, Zn: 0.0012% and O: 0.0015% (hereinafter, % for steel components means wt.%), and subjected to hot rolling to obtain hot-rolled sheets with a thickness of 1.6 mm. The hot-rolled sheets are subjected to hot-rolled sheet annealing at 950°C for 30 seconds in an atmosphere of 100% vol. N ₂ followed by acid cleaning, cold rolling to a sheet thickness of 0.25 mm and final annealing at 550°C or more and 800°C or less for 10 s in an atmosphere of 20% vol. H ₂ - 80% vol. N ₂ to obtain final annealed sheets.

A fatigue test piece having a parallel portion 5 mm wide and 150 mm long is taken from each final annealed sheet in the rolling direction and the transverse direction (direction orthogonal to the rolling direction). The parallel part is polished to a mirror finish and additionally polished with sandpaper No. 800 in the longitudinal direction. The fatigue test is carried out with a tension factor of 0.1 and a frequency of 20 Hz, and the stress amplitude at which no fracture occurs after 10 ⁷ repetitions is determined as the fatigue strength of the final annealed sheet.

The “mirror polished” condition is achieved by polishing the surface with a No. 500 diamond grinding stone.

Fig. 1 illustrates the effect of a non-recrystallized microstructure on the fatigue strength of a steel sheet. Here, the fatigue strength in the present disclosure indicates the average value of the fatigue strength in the rolling direction and in the transverse direction (the direction orthogonal to the rolling direction). To observe the microstructure, the cross section in the rolling direction of each steel sheet is polished, etched with nital, and then examined by optical microscopy. 1 m ² steel sheet is randomly sampled at five locations to determine the proportion of non-recrystallized grains in a measurable area of 1 cm ² at each location. The proportion of non-recrystallized grains in the present invention represents the proportion or area fraction in the overall microstructure, and such proportion is the result of using this measurement technique.

As can be seen from FIG. 1, when the proportion of non-recrystallized microstructure in the steel microstructure is 5% or more, a fatigue strength of 550 MPa or more, which is required for a high-speed rotor material of a HEV/EV motor (IPM motor), can be obtained. This is believed to be because the non-recrystallized microstructure has more dislocations, resulting in dislocation clustering and higher fatigue strength.

From the above results, it can be seen that the fatigue strength of the steel sheet becomes high when the proportion of non-recrystallized microstructure is 5% or more, preferably 15% or more. This is because fatigue strength and tensile strength improve as the proportion of non-recrystallized microstructure increases. On the other hand, if the proportion of non-recrystallized microstructure in the steel microstructure exceeds 70%, the iron loss becomes markedly high. Therefore, the proportion of non-recrystallized microstructure should be 70% or less, preferably 60% or less.

Then, in order to investigate the process stability, focusing on the change in inclusions, steel is smelted in the laboratory containing C: 0.0018%, Si: 3.5%, Al: 0.65%, Mn: 0.4%, P: 0 .01%, S: 0.0008%, N: 0.0016%, and Zn: 0.0011% for 10 charges and hot rolled to a sheet thickness of 2 mm to obtain hot rolled sheets. Hot-rolled sheets are subjected to hot-rolled sheet annealing at 1000° C. for 30 seconds in an atmosphere of 100% vol. N ₂ followed by acid cleaning, cold rolling to a sheet thickness of 0.25 mm and final annealing at 650°C for 10 s in an atmosphere of 20% vol. H ₂ - 80% vol. _N2 .

When evaluating the fatigue strength of the steel sheets thus obtained, it was found that some materials have a significantly low fatigue strength.

To investigate the cause of the problem, after polishing the surface of each steel sheet with aluminum oxide and etching with nital, scanning electron microscope (SEM) observation was carried out, and large inclusions were found. These large inclusions were considered sources of cracks in the steel sheet when measuring fatigue strength, resulting in a significant reduction in fatigue strength. Inclusions prevent the growth of crystalline grains in non-oriented electrical steel sheet. Known as inclusions are, for example, sulfides such as copper sulfide (CuS) and manganese sulfide (MnS), oxides such as silicon dioxide (SiO ₂ ) and alumina (Al ₂ O ₃ ), and nitrides such as aluminum nitride. (AlN) and titanium nitride (TiN). As used herein, inclusions refer to non-metallic precipitates such as oxides, sulfides and nitrides.

To study the influence of the number of inclusions on the fatigue strength, the surface of the steel sheet is polished to a depth of 50 μm and to the central part in the thickness direction of the sheet from the surface, and the number of inclusions is determined and the average value is calculated. Fig. 2 illustrates the effect of the number of inclusions on fatigue strength. Here, the size and number of inclusions are estimated as follows. For the size of the inclusions, the equivalent circular diameter of each inclusion is determined. To determine the number of inclusions, 1 m ² of each steel sheet is randomly sampled at twenty locations, the number of inclusions is counted in a field of view of 0.1 mm ² at each sampling location and multiplied by 10 to obtain the number of inclusions per 1 mm ² of steel sheet. As inclusions, inclusions with a diameter of 5 μm or more are taken into account, affecting the fatigue strength. The number of inclusions in the present invention represents the average number of inclusions in the entire microstructure, and this number is the result obtained using this measurement technique.

As can be seen from FIG. 2, the fatigue strength is significantly reduced when the number of inclusions with a diameter of 5 μm or more exceeds 5/mm ² . In ordinary electrical steel sheets, the fatigue strength is not so strongly affected by inclusions, but in high-strength electrical steel sheets, it is considered that the fatigue strength decreases when cracks caused by inclusions appear, and therefore it is necessary to reduce the number of large inclusions. Inclusions smaller than 5 µm in diameter are considered to have little effect as they do not act as a crack initiator.

In view of the above, it can be seen that the number of inclusions with a diameter of 5 μm or more should be 5/mm ² or less to stably maintain a fatigue strength of 550 MPa or more.

The oxygen concentration of the materials used in the tests to obtain the results shown in FIG. 2 has been determined to be 10 ppm or more and 100 ppm or less. In general, it is known that the number of inclusions in steel decreases as the oxygen content decreases. However, in the above experiments, the number of inclusions with a diameter of 5 µm or more does not necessarily correlate with the oxygen content. Thus, it has been found that simply limiting the oxygen content is not enough to suppress the change in fatigue strength and that it is important to control the inclusions.

In the present disclosure, the components of the steel sheet (hereinafter, % for the components of the steel sheet refers to wt%) should be in the following range.

Si: 2.00% or more and 5.00% or less

Si is an effective element for increasing the resistivity of steel sheet. Thus, the lower limit of the Si content is 2.00%, and the Si content is preferably 3.50% or more. On the other hand, a Si content exceeding 5.00% leads to a decrease in the saturation magnetic flux density and a corresponding decrease in the magnetic flux density, as well as a deterioration in fatigue strength. Thus, the upper limit of the Si content is 5.00%.

Al: 3.00% or less

Al, like Si, is an effective element for increasing resistivity. However, an Al content exceeding 3.00% leads to a decrease in the saturation magnetic flux density and a corresponding decrease in the magnetic flux density, as well as to a deterioration in fatigue strength. Thus, the upper limit of the Al content is 3.00%. The lower limit of the Al content is not specifically set, but the Al content is preferably 0.30% or more, and more preferably 0.50% or more in terms of iron loss.

C: 0.0100% or less

C increases iron loss due to carbide precipitation. Therefore, the C content is set to 0.0100% or less, preferably 0.0050% or less, in order to suppress an increase in iron loss. The lower limit is not particularly limited and may be 0%.

Mn: 5.00% or less

Mn is an effective element for increasing the resistivity of steel sheet. However, the Mn content exceeding 5.00% lowers the magnetic flux density, and thus the upper limit of the Mn content is 5.00%. The lower limit of the Mn content is not specifically set, but the Mn content is preferably 0.10% or more in order to reduce iron loss.

P: 0.100% or less

P makes the steel sheet brittle if added more than 0.100%, making cold rolling difficult. Thus, the P content is set to 0.200% or less. The lower limit is not particularly limited and may be 0%.

N: 0.0050% or less

N causes more nitride release if added in large amounts and increases iron losses. Thus, the N content is set to 0.0050% or less. The lower limit is not particularly limited and may be 0%.

O: 0.0050% or less

The content of O exceeding 0.0050% leads to the formation of oxides and degrades the magnetic properties. Thus, the upper limit of the O content is 0.0050%. The lower limit is not particularly limited and may be 0%.

S: 0.0100% or less

S causes an increase in iron losses due to the release of MnS if its content exceeds 0.0100%. Thus, the upper limit of the S content is 0.0100%. The lower limit is not particularly limited and may be 0%.

Mo: 0.001% or more and 0.100% or less

Mo stabilizes the non-recrystallized microstructure, so the lower limit of Mo content is 0.001%. On the other hand, a Mo content exceeding 0.100% greatly increases the iron loss. Thus, the upper limit of the Mo content is 0.100%, preferably 0.010%.

Cu: 0.2% or less

The Cu content exceeding 0.2% leads to a decrease in the magnetic flux density. Thus, the upper limit of the copper content is 0.2% and more preferably 0.1%. The lower limit is not particularly limited and may be 0%.

Nb: 0.010% or less

Nb is an element that forms precipitates such as NbC and contributes to strength increase, but the formation of precipitates greatly increases iron losses. Thus, the upper limit of the Nb content is 0.010%, and preferably 0.005% or less. The lower limit is not limited, but it should be 0.001% or more to improve strength.

Zn: 0.0005% or more and 0.0030% or less

Zn is an element that is usually not added to electrical steel sheet due to its high vapor pressure and high volatility. However, if the temperature of the molten steel is adjusted after deoxidation by adding scrap, Zn can be introduced into the steel. For general purpose electrical steel sheets, materials that have been sufficiently recrystallized are supplied as products so that the strength properties remain stable even if Zn is added to them. However, in a material in which an unrecrystallized microstructure remains, as in the present disclosure, the addition of Zn causes a change in strength.

Here, in order to investigate the effect of Zn on the change in the strength of a steel sheet, a steel having a chemical composition containing C: 0.0021%, Si: 3.7%, Al: 0.6%, Mn: 0.8%, is prepared in the laboratory, P: 0.01%, S: 0.001%, N: 0.0013%, and Zn: 0.0003% or more and 0.0060% or less, and O: 0.0015%, the rest is Fe and unavoidable impurities, and subjected to hot rolling to obtain hot-rolled steel sheets with a thickness of 2 mm. Then hot-rolled steel is subjected to annealing at 1000°C for 30 s in an atmosphere of 100% vol. N ₂ followed by acid cleaning, cold rolling to a sheet thickness of 0.25 mm and final annealing at 680°C for 10 s in an atmosphere of 20% vol. H ₂ - 80% vol. _N2 . Twenty JIS No. 5 test specimens were taken from each steel sheet after final annealing and subjected to tensile testing. From the results of the tensile tests, the standard deviation σ is determined and a graph is plotted between the Zn content and σ. When the Zn content exceeds 0.0030%, σ increases to 20 MPa or more, and the fluctuation becomes large. This is believed to be because Zn was introduced to make the recrystallization behavior unstable, resulting in a change in the degree of recrystallization among twenty test specimens.

For this reason, the upper limit of the Zn content is 0.0030%. As a result, the standard deviation σ can be reduced to 20 MPa or less in an electrical steel sheet in which a non-recrystallized microstructure is retained. Although the lower limit of the Zn content is preferably as low as possible, stabilizing the Zn content to 0.0005% or less increases the cost, and thus the lower limit of the Zn content is 0.0005%.

The rest, except for the components of the steel sheet of the present disclosure, includes Fe and unavoidable impurities.

In addition to the above steel sheet components, the following components may be included in the present disclosure as needed.

Cr: 0.010% or more and 5.000% or less

Cr, like Si, is an effective element for increasing resistivity. Therefore, when Cr is added, the lower limit is 0.010%. Preferably the lower limit is 0.100%. On the other hand, a Cr content exceeding 5.000% leads to a decrease in the saturation magnetic flux density and a corresponding decrease in the magnetic flux density. Thus, the upper limit of the Cr content is 5.000%.

Ca: 0.0010% or more and 0.0050% or less.

Ca is the element that fixes S as CaS and helps reduce iron loss. Therefore, when Ca is added, the lower limit is 0.0010%. On the other hand, since the Ca content exceeding 0.0050% causes saturation of the effect and unnecessarily increases the cost, the upper limit is 0.0050%.

Sn: 0.001% or more and 0.100% or less and/or Sb: 0.0010% or more and 0.1000% or less.

Sn is an effective element for improving the magnetic flux density by improving the texture. Therefore, when Sn is added, the lower limit is 0.001%. On the other hand, the Sn content exceeding 0.100% makes the material brittle, so the upper limit of the Sn content is 0.100%. Sb is also an effective element for improving the magnetic flux density by improving the texture. Therefore, when Sb is added, the lower limit is 0.0010%. On the other hand, the Sb content exceeding 0.1000% makes the material brittle, so the upper limit of the Sb content is 0.1000%.

Here, when the rotor rotates at an extremely high speed, the web section is subjected to undue stress and can be immediately broken. To prevent this immediate rupture, the material must have a sufficiently high tensile strength. In particular, for HEV/EV motors operating at high speeds, a tensile strength of 700 MPa or more is required. A tensile strength of 700 MPa or more can be stably obtained according to the present invention.

In addition, effective iron loss reduction to improve motor efficiency at high speed rotation, and, subject to the present disclosure, a suitable high frequency iron loss property, or W _5/3K , of 120.0 W/kg or less can be proposed.

In addition, in a motor with internal permanent magnets, iron losses tend to increase as current increases, and a higher magnetic flux density is required to prevent this. According to the present disclosure, a high magnetic flux density of 1.50 T or more for B ₅₀ can be obtained.

Now, the manufacturing method of the steel sheet of the present invention will be described.

In the present disclosure, the components constituting the steel sheet should be within the scope of the present disclosure, and the manufacturing method should be further limited as follows.

In the present disclosure, molten steel is subjected to purging in a converter, followed by degassing (secondary refining), casting and hot rolling. Degassing (vacuum degassing) or re-refining is carried out using Ruhrstahl-Heraeus (RH) or Dortmund-Horde (DH). At this time, in order to sufficiently separate and float the inclusions, deoxidizing elements (deoxidizer) such as Al are introduced, and the steel is stirred for 10 minutes or more (also called RH vacuum circulation time or DH circulation vacuum time in the present specification) . In order to avoid the introduction of Zn with scrap or iron alloy, it is preferable to mix for at least 5 minutes after feeding the scrap or iron alloy to evaporate Zn.

The final annealing temperature and the coiling temperature during hot rolling are not specifically indicated. Annealing of the hot-rolled sheet may or may not be performed after hot rolling. Then, cold rolling is performed once, twice, or repeatedly with intermediate annealing in between to achieve a predetermined sheet thickness, and then final annealing is performed to obtain a non-oriented electrical steel sheet according to the present disclosure. The final annealing after cold rolling should be carried out at 620° C. or higher and 720° C. or lower in order to properly retain unrecrystallized grains in the microstructure of the steel sheet, which is a feature of the present disclosure.

Examples

Molten steel is subjected to blowing in the converter, followed by vacuum degassing treatment. The deoxidizer is fed during degassing, and then the steel is stirred for the time (vacuum circulation time) indicated in Table 2 and Table 4, and then cast so as to obtain the steel components listed in Table 1 and Table 3. The term "traces" in tables means that the corresponding element is contained, but in extremely small quantities (not zero).

After said casting, the slab is heated in the mode of 1140°C×1h and hot rolling. The hot rolling end temperature is 800°C and the winding temperature is 610°C. After winding, the hot-rolled sheet is subjected to annealing in an atmosphere of 100 vol.% N ₂ . Then, cold rolling is carried out to a sheet thickness of 0.30 mm, and then final annealing is carried out in an atmosphere of 20 vol.% H ₂ - 80% vol. N ₂ and at the final annealing temperatures indicated in tables 2 and 4 to obtain annealed sheets.

The measurement of magnetic properties is carried out in accordance with the Epstein method on Epstein samples cut from the final annealed sheets in the rolling direction and the transverse direction (direction orthogonal to the rolling direction).

The fatigue test is carried out with a tension factor of 0.1 and a frequency of 20 Hz, and the stress amplitude at which no fracture occurs after 10 ⁷ repetitions is defined as the fatigue strength. A fatigue test specimen having a parallel section 5 mm wide and 150 mm long is taken from each final annealed sheet in the rolling direction and the transverse direction (direction orthogonal to the rolling direction). The parallel part is polished to a mirror finish and additionally polished with sandpaper No. 800 in the longitudinal direction.

Tensile tests are carried out in accordance with JIS Z 2241 using JIS No. 5 test specimens, with their longitudinal direction being the rolling direction. In addition, to study the change (σ) in tensile strength (TS), twenty JIS No. 5 test specimens were taken and subjected to tensile tests to obtain a standard deviation σ.

In addition, for the magnetic flux density, the magnetic flux density B ₅₀ is measured at a magnetic field strength of 5000 A/m. The iron loss W _5/3K is measured when each steel sheet is excited at a frequency of 3 kHz and a magnetic flux density of 0.5 Tesla.

The test results are shown in Table 2 and Table 4.

As can be seen from Table 2 and Table 4, all steel sheets meeting the conditions of the present invention have suitable properties of fatigue strength of 550 MPa or more, tensile strength of 700 MPa or more, standard deviation σ of tensile strength of 20 MPa or less, density magnetic flux B ₅₀ 1.5 T or more and iron loss W _5/3K 120 W/kg or less.

Claims (21)

1. Sheet of non-oriented electrical steel having a chemical composition containing, in wt.%: C: 0.0100 or less P: 0.100 or less Si: 2.00 or more and 5.00 or less, Mn: 5.00 or less Al: 3.00 or less S: 0.0100 or less N: 0.0050 or less Zn: 0.0005 or more and 0.0030 or less, Mo: 0.001 or more and 0.100 or less, Cu: 0.2 or less Nb: 0.010 or less O: 0.0050 or less if necessary, at least one of: Cr: 0.010 or more and 5.000 or less, Ca: 0.0010 or more and 0.0050 or less, Sn: 0.001 or more and 0.100 or less, Sb: 0.0010 or more and 0.1000 or less and the rest is Fe and inevitable impurities; And having a microstructure in which the proportion of non-recrystallized microstructure is 5% or more and 70% or less, and the number of inclusions having a diameter of 5 μm or more is not more than 5 per mm ² . 2. A method for manufacturing a non-oriented electrical steel sheet according to claim 1, including steel smelting, vacuum degassing of the steel being smelted with simultaneous supply of a deoxidizer and subsequent mixing for 10 minutes or more, casting steel into a slab, hot rolling of a slab, annealing hot-rolled sheets, cold rolling and final annealing of cold-rolled sheets.

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