NON-ORIENTED ELECTRICAL STEEL SHEET

Abstract

A non-oriented electrical steel sheet is proposed that has low iron loss and excellent tensile strength and fatigue resistance, having a chemical composition comprising C: not more than 0.005% by mass, Si: 3 to 5% by mass, Mn: not more than 5% by mass, P: not more than 0.1% by mass, S: not more than 0.01% by mass, Al: not more than 3% by mass, N: not more than 0.005% by mass, Zn: 0.0005% by mass to 0.003% by mass, and the remainder being Fe and unavoidable impurities, an average crystal grain size of not more than 40 µm, the number of inclusions having a diameter of not less than 5 µm of not more than 5 / mm², a tensile strength of not less than 600 MPa, and a fatigue strength of not less than 450 MPa.

Description

NON-ORIENTED ELECTRICAL STEEL SHEET FIELD OF INVENTION This invention relates to a non-oriented electrical steel sheet, and more specifically to a non-oriented electrical steel sheet preferably used for a rotor core in an electric motor. BACKGROUND OF THE INVENTION Electric motors for electric vehicles and air conditioners, which require high efficiency, typically use an internal permanent magnet (IPM) motor. The rotor core of an IPM motor has a permanent magnet embedded in a slotted section, and when it rotates at high speed, centrifugal force exerts a large amount of stress on a bridge section. To ensure rotor strength, the bridge width should be increased, but this would increase the leakage flux from the permanent magnet, thus decreasing the motor's efficiency. Therefore, the bridge width is designed to be as narrow as possible within the range where the required rotor strength can be achieved.Therefore, the non-oriented electrical steel lamination used for the electric motor core must have a tensile strength that can withstand centrifugal force at high-speed rotation and a fatigue strength that can cope with repeated loads. Furthermore, iron loss resulting from higher harmonic waves occurs on the surface of the rotor core in a concentrated-winding electric motor, so the electrical steel lamination used for the rotor core must have low iron loss at high frequencies. As a raw material for a rotor core that meets these requirements, for example, Patent Document 1 discloses a high-strength electrical steel sheet having a chemical composition of Si: 0.2 to 3.5% by mass, Al: not more than 2.50% by mass, and Nb: 0.05 to 8.0% by mass. Furthermore, Patent Document 2 discloses a high-strength electrical steel sheet having a chemical composition of Si: 2.0 to 3.5% by mass, Al: 0.02 to 3.0% by mass, and N: 0.005 to 0.020% by mass, and an average crystal grain size of not more than 10 µm from the surface of the product plate to a depth of 10 µm. List of Appointments Patent Documents Patent Document 1: JP-A-201 0-1 59494 Patent Document 2: JP-A-2005-113252 BRIEF DESCRIPTION OF THE INVENTION Technical Problem However, the method disclosed in Patent Document 1 uses Nb precipitation hardening, which can cause the problem of high strength but increased iron loss. The method disclosed in Patent Document 2 also has the problem of increased iron loss due to a high nitrogen content. Furthermore, reducing impurity elements such as C, S, and N while making these elements finer is effective as a means of improving the tensile strength and fatigue strength of an electrical steel sheet, but the resulting strength varies considerably. This invention is made taking into account the aforementioned problems inherent in the FR / ynn / Lznz / e / YiAi conventional methods, and the objective thereof is to provide a non-oriented electrical steel sheet that has a low iron loss and is excellent in tensile strength and fatigue resistance. Solution to the Problem The inventors have conducted several studies to solve the aforementioned problems. Consequently, they have discovered that it is possible to increase tensile strength and fatigue strength while simultaneously achieving low iron loss by reducing inclusions and the zinc content (which is present as an impurity) in the steel, and furthermore, reducing strength variation. The invention has been realized. That is, the present invention is a non-oriented electrical steel sheet having a chemical composition comprising C: not more than 0.005% by mass, Si: 3 to 5% by mass, Mn: not more than 5% by mass, P: not more than 0.1% by mass, S: not more than 0.01% by mass, Al: not more than 3% by mass, N: not more than 0.005% by mass, Zn: 0.0005 to 0.003% by mass, and the remainder being Fe and unavoidable impurities, an average crystal grain size of not more than 40 µm, the number of inclusions having a diameter of not less than 5 µm of not more than 5 / mm², a tensile strength of not less than 600 MPa, and a fatigue strength of not less than 450 MPa. The non-oriented electrical steel sheet according to the invention additionally contains Cr in 0.1 to 5% by mass in addition to the above chemical composition. The non-oriented electrical steel sheet according to the invention additionally contains Ca in 0.001 to 0.005% by mass in addition to the above chemical composition. The non-oriented electrical steel sheet according to the invention additionally contains one or two selected from Sn: 0.001 to 0.1% by mass and Sb: 0.001 to 0.1% by mass in addition to the above chemical composition. The non-oriented electrical steel sheet according to the invention further contains at least one selected from Ni: 0.1 to 2% by mass, Mo: 0.001 to 0.05% by mass, Cu: 0.01 to 0.2% by mass, Mg: 0.001 to 0.005% by mass, REM: 0.001 to 0.005% by mass, and (Ti + V): 0.005 to 0.05% by mass. Advantageous Effects of the Invention This invention can provide a non-oriented electrical steel sheet that has low iron loss and excellent tensile strength and fatigue resistance. Furthermore, the invention can reliably provide a rotor core material for high-speed rotation that exhibits excellent iron loss properties at high frequencies through the use of the non-oriented electrical steel sheet. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a graph showing a relationship between average crystal grain size and fatigue strength. Figure 2 is a graph showing a relationship between the number of inclusions that have a diameter of not less than 5 pm equivalent to a circle and fatigue strength. Figure 3 is a graph showing a relationship between Zn content and deviation FR / ynn / Lznz / e / YiAi standard σ of a breaking stress TS. DETAILED DESCRIPTION OF THE INVENTION Description of Modalities An experiment that led to the development of the invention will be explained. <Experimento 1> First, in order to study the influence of crystal grain size on fatigue strength, a steel containing C: 0.0018% by mass, Si: 3.4% by mass, Mn: 0.6% by mass, P: 0.01% by mass, S: 0.002% by mass, Al: 0.9% by mass, N: 0.0013% by mass, Zn: 0.0012% by mass, and O: 0.0020% by mass is melted in a laboratory, cast into a steel ingot, and hot-rolled to form a hot-rolled sheet with a thickness of 2 mm. The hot-rolled sheet is then subjected to hot-band annealing at 1000 sC for 30 seconds under a 100% by volume atmosphere. of N2, pickling, and cold rolling to form a cold-rolled sheet with a thickness of 0.25 mm. The cold-rolled sheet is then subjected to a finishing annealing under an atmosphere of 20% by volume of H2 - 80% by volume.of N2 for a temperature holding time of 10 seconds when varying the temperature holding temperature within the range of 650 to 1000eC. Next, a fatigue strength test specimen, consisting of a parallel body 5 mm wide and 150 mm long, is cut from the finished annealed steel sheet along the longitudinal direction (the rolling direction) and subjected to a fatigue test. In this test, the parallel body is finished to a surface roughness of VVV:3.2S (JIS B0601 (1970), and the same applies hereafter), and polished with 800-grit sandpaper along the longitudinal direction. The fatigue test is carried out under pulsating tensile loading conditions, a stress ratio of 0.1, and a frequency of 20 Hz, and the stress amplitude at which no fracture occurs after 10⁷ repetitions is determined as the fatigue limit.In addition, the average crystal grain size of the test samples is measured according to JIS G0551 after polishing a cross-section in the rolling direction and subjecting it to chemical etching with Nital. Figure 1 shows the relationship between average crystal grain size and fatigue strength. As seen in Figure 1, the fatigue strength is improved by reducing the average crystal grain size. Specifically, the fatigue strength increases to at least 450 MPa when the average crystal grain size is reduced to no more than 40 µm. At the same time, a tensile strength of at least 600 MPa can be achieved by reducing the average crystal grain size to no more than 40 µm. Based on the above result, the average crystal grain size of a product sheet according to the invention is defined as no more than 40 µm, preferably no more than 30 µm. Here, the fatigue strength of 450 MPa is the lower limit at which a rotor bridge portion required for a rotor material of an HEV / EV electric motor will not break under repeated use.The preferred fatigue limit is not less than 470 MPa, and the preferred breaking strength is not less than 650 MPa. <Experimento 2> In order to study productivity, 10 charges of a steel containing C: 0.0020% by mass, Si: 3.5% by mass, Mn: 0.4% by mass, P: 0.01% by mass, S: 0.001% by mass, Al: 0.7% by mass, N: 0.0016% by mass, and Zn: 0.0011% by mass are melted in a laboratory, poured into a FR / ynn / Lznz / e / YiAi steel ingot is hot-rolled to form a hot-rolled sheet with a thickness of 2 mm. The hot-rolled sheet is then subjected to hot strip annealing at 1000°C for 30 seconds under a 100% vol. N2 atmosphere, pickling, and cold rolling to form a cold-rolled sheet with a thickness of 0.25 mm. The cold-rolled sheet is then subjected to final annealing under a 20% vol. H2 - 80% vol. N2 atmosphere at 800°C for 10 seconds. Each of the fatigue properties of the steel sheets subjected to finish annealing was evaluated using 10 loads. Among them, some sheets subjected to finish annealing showed significantly low fatigue limits. To investigate the cause, one surface of each steel sheet was polished to 50 µm and observed using a scanning electron microscope (SEM). This revealed coarse inclusions in material with a low fatigue limit. Based on the results, it was concluded that these coarse inclusions act as crack initiation points during the fatigue test, thus reducing the fatigue limit. To investigate the influence of inclusions on the fatigue limit, the size (circle-equivalent diameter) and number of inclusions are observed within a 0.1 mm² field of view on the surface after polishing, and the number generated within that field of view is recorded. Here, a circle-equivalent diameter refers to the diameter of a circle that has the same area as the observed inclusions. Figure 2 shows the relationship between the number of inclusions with a diameter of at least 5 µm (equivalent to a circle) and the fatigue limit. As shown in Figure 2, the fatigue limit decreases rapidly when the number of inclusions with a diameter of at least 5 µm (equivalent to a circle) exceeds 5 / mm². The influence of inclusions on the fatigue limit is small in a low-strength electrical steel sheet, whereas the influence of coarse inclusions on the fatigue limit becomes evident in a high-strength electrical steel sheet due to the increased susceptibility to cracking. Based on the above results, the number of inclusions with a diameter of at least 5 µm (equivalent to a circle) is limited to no more than 5 / mm² in the present invention. Preferably, it is no more than 3 / mm². The oxygen (O) content in each steel sheet used in Experiment 2 was measured and showed a value within the range of 0.0010 to 0.0100% by mass (10 to 100 ppm by mass). It was determined that the number of inclusions decreased due to a reduction in the amount of oxygen, while the number of inclusions with a diameter of at least 5 pm (equivalent to a circle) was not necessarily correlated with the oxygen content. From this result, the inventors discovered that limiting the oxygen content in the steel was insufficient to reduce the number of inclusions with a diameter of at least 5 pm (equivalent to a circle) and decrease the variation in fatigue strength. Therefore, it was determined that it was important to implement a control, described later, to reduce the number of inclusions with a diameter greater than 5 pm during the refining process. <Experimento 3> Next, an experiment is carried out to investigate the influence of Zn on the variation of the breaking stress TS. FR / ynn / Lznz / e / YiAi Zinc (Zn) is generally not incorporated into steel due to its high vapor pressure, but it can be incorporated when scrap is added to molten steel after deoxidation in a temperature-controlled refining process or similar. Since the grains grow sufficiently in a normal low-strength electrical steel sheet, the strength properties are not affected by the mixing of Zn with the steel. However, in materials with fine crystal grains, such as a high-strength electrical steel sheet, which is the object of this invention, Zn is considered to be the cause of the strength variation. In the previous experiment, a steel containing C: 0.0025% by mass, Si: 3.6% by mass, Mn: 0.8% by mass, P: 0.01% by mass, S: 0.001% by mass, Al: 0.6% by mass, N: 0.0015% by mass, O: 0.0015% by mass, and Zn varying within the range of 0.0003 to 0.0060% by mass is melted in a laboratory, cast into a steel ingot, and hot-rolled to form a hot-rolled sheet with a thickness of 2 mm. The hot-rolled sheet is then subjected to hot-band annealing at 1000°C for 30 seconds under a 100% by volume atmosphere. of N2, pickling, and cold rolling to form a cold-rolled sheet with a thickness of 0.25mm. The cold-rolled sheet is then subjected to a finishing annealing under an atmosphere of 20% by volume of H2 - 80% by volume of N2 at 800°C for 10 seconds. Next, 20 JIS No. 5 test samples are taken from each product sheet, both from the rolling direction and the tensile direction, and tensile tests are performed according to JIS Z 2241 to measure the tensile strength (TS). The standard deviation (σ) of these 20 tensile strengths (TS) is then determined. The results are shown in Figure 3, which reveals that when the Zn content exceeds 0.003% by mass, the variation in tensile strength (TS) is greater, and the standard deviation (σ) reaches at least 15 MPa. The cause of this is considered to be the fact that when Zn is incorporated into steel, the recrystallization behavior during finishing annealing becomes unstable, causing a variation in the crystal grain size. Therefore, in the present invention, the content of Zn incorporated as an unavoidable impurity is limited to no more than 0.003% by mass. The present invention has been developed based on prior novel knowledge. The reason for limiting the chemical composition of the non-oriented electrical steel sheet according to the invention will be explained below. C: no more than 0.005% by mass Carbon is a harmful element that forms a carbide which will precipitate and has the effect of increasing iron loss. Therefore, carbon is limited to no more than 0.005% by mass, preferably no more than 0.003% by mass. Yes: 3 to 5% by mass Silicon (Si) is an effective element for increasing the specific strength of steel sheets, thereby reducing iron loss, and is therefore contained at no less than 3% by mass. When the Si content exceeds 5% by mass, the saturated magnetic flux density decreases, and the magnetic flux density also decreases. Therefore, the upper limit for Si is 5% by mass. Preferably, the Si content falls within the range of 3.5 to 4.5% by mass. Mn: no more than 5% by mass FR / ynn / Lznz / e / YiAi Mn is an effective element for increasing the specific strength of steel sheet. A manganese content exceeding 5% by mass reduces the magnetic flux density; therefore, the upper limit for Mn is 5% by mass. The Mn content is preferably no more than 2% by mass. The lower limit for Mn is not specifically defined, but Mn is preferably contained at no less than 0.1% by mass to improve hot workability and minimize iron loss. P: no more than 0.1% by mass Phosphorus (P) is an effective element for increasing the strength of steel and improving its formability. However, an addition of P exceeding 0.1% by mass causes embrittlement of the steel and also makes cold rolling difficult. Therefore, its use is limited to no more than 0.1% by mass. Preferably, P falls within the range of 0.002 to 0.01% by mass. S: no more than 0.01% by mass Sulfur (S) is a harmful element that forms MnS with precipitated Mn, leading to increased iron loss. In particular, when the S content exceeds 0.01% by mass, this harmful effect becomes evident. Therefore, S is limited to no more than 0.01% by mass, and preferably no more than 0.005% by mass. Al: no more than 3% by mass Aluminum (Al) is an effective element for reducing iron loss by increasing specific strength, as is silicon (Si). When the content exceeds 3% by mass, however, the saturated magnetic flux density decreases, and the magnetic flux density also decreases. Therefore, the Al content is limited to no more than 3% by mass, preferably no more than 2% by mass. The lower limit for Al is not particularly defined; however, it is preferably no less than 0.3% by mass, and more preferably 0.5% by mass, from the standpoint of improving iron loss reduction. N: no more than 0.005% by mass Nitrogen (N) is a harmful element that forms a nitride which precipitates and increases iron loss. In particular, when the content exceeds 0.005% by mass, the harmful effect becomes evident. Therefore, the N content is limited to 0.005% by mass, preferably 0.002% by mass. Zn: 0.0005 to 0.003% by mass Zinc (Zn) is a detrimental element that increases the variation in tensile strength through changes in recrystallization behavior, as described above, and is limited to no more than 0.003% by mass, preferably no more than 0.002% by mass, to minimize the variation in tensile strength. Although a lower Zn content is preferable for the reasons stated above, adding a small amount of Zn suppresses nitriding, thus improving iron loss. Furthermore, when the Zn content is excessively reduced, the raw materials and scrap used must be carefully selected, leading to increased costs. Therefore, the lower limit for Zn content is approximately 0.0005% by mass. It is preferable that the non-oriented electrical steel sheet according to the invention contains the following ingredients, in addition to the above ingredients. Cr: 0.1 to 5% by mass FR / ynn / Lznz / e / YiAi Cr is an effective element for increasing specific strength, thereby decreasing iron loss, as is Si. Therefore, it is preferable that Cr be present at no less than 0.1% by mass. However, an addition exceeding 5% by mass causes a decrease in saturated magnetic flux density and a corresponding decrease in magnetic flux density. Therefore, it is preferable that Cr be present at no more than 5% by mass. Ca: 0.001 to 0.005% by mass Calcium (Ca) is an element that forms CaS to fix sulfur (S) and contributes to a decrease in iron loss. Therefore, it is preferable that the Ca content be no less than 0.001% by mass. However, when the amount added exceeds 0.005% by mass, the aforementioned effect is saturated, resulting only in an increase in production costs. Thus, the upper limit is 0.005% by mass. Sn: 0.001 to 0.1% by mass, Sb: 0.001 to 0.1% by mass Sn and Sb are effective elements in improving texture and magnetic flux density. Therefore, it is preferable to contain each element at no less than 0.001% by mass. Furthermore, when either element exceeds 0.1% by mass, the aforementioned effect will become saturated. Therefore, the upper limit is 0.1% by mass. Ni: 0.1 to 2% by mass Ni is an effective element for improving magnetic flux density. To achieve this effect, it is preferable to add Ni at a concentration of no less than 0.1% by mass. Additions exceeding 2% by mass will only saturate the effect and increase the cost of raw materials. Therefore, the upper limit is preferably 2% by mass. Mo: 0.001 to 0.05% by mass Molybdenum (Mo) is an element that forms fine carbide, which precipitates and increases the strength of steel. To achieve this effect, it is preferable to add Mo at a concentration of no less than 0.001% by mass. When the amount added exceeds 0.05% by mass, the iron loss increases significantly. Therefore, the upper limit is preferably 0.05% by mass. Cu: 0.01 to 0.2% by mass Copper (Cu) is an element that improves texture and increases magnetic flux density. To achieve this effect, it is preferable to add chromium (Cr) at a concentration of no less than 0.01% by mass. However, concentrations above 0.2% by mass saturate the effect, resulting only in an increased raw material cost. Therefore, the upper limit for copper is preferably 0.2% by mass. Mg: 0.001 to 0.005% by mass Magnesium (Mg) is an element that lubricates inclusions and contributes to a decrease in iron loss. To achieve this effect, it is preferable to add Mg at a concentration of no less than 0.001% by mass. However, when the amount added exceeds 0.005% by mass, the effect is saturated, resulting only in an increase in raw material costs. Therefore, the upper limit for Mg is preferably 0.005% by mass. REM: 0.001 to 0.005% by mass REM is an element that lubricates sulfide-based inclusions and contributes to a decrease in iron loss. To obtain the above effect, it is preferable to add REM in no FR / ynn / Lznz / e / YiAi less than 0.001% by mass. On the other hand, when the amount of addition is greater than 0.005% by mass, the above effect becomes saturated, only causing an increase in the cost of raw materials. Therefore, the upper limit is preferably 0.005% by mass. (Ti + V): 0.005 to 0.05% by mass Titanium (Ti) and vanadium (V) are elements that form fine nitrides which precipitate and increase the strength of steel. To achieve this effect, it is preferable to add either Ti alone or V alone, or Ti and V together, in a total amount of no less than 0.005% by mass. When the amount of added Ti alone or V alone, or Ti and V together, exceeds 0.05% by mass, the iron loss increases significantly. Therefore, the upper limit for either Ti alone or V alone, or the total amount of Ti and V, is preferably 0.05% by mass. The residue other than the elements mentioned above is Fe and unavoidable impurities in the non-oriented electrical steel sheet according to the invention. Since it is a harmful element that forms oxide-based inclusions and increases iron loss, O is preferably reduced as much as possible and is limited to no more than 0.005% by mass. A method for producing a non-oriented electrical steel sheet in accordance with the invention will be described below. A method for producing a non-oriented electrical steel sheet according to the invention can be carried out using a normal method, provided that the steel sheet has a chemical composition in accordance with the invention. That is, a non-oriented electrical steel sheet according to the invention can be produced by subjecting molten steel, after blowing in a converter, to secondary refining by vacuum degassing or similar treatment to adjust the chemical composition to the specified one, forming a steel material (block) by a continuous casting method or a roughing and ingot making method, subjecting the steel material to hot rolling, then to hot strip annealing as required, and cold rolling the sheet to form a cold-rolled sheet of a final thickness followed by finish annealing. In order to reduce the number of inclusions with a diameter of at least 5 pm (equivalent to a circle on the steel sheet) to no more than 5 per 1 mm², it is preferable that the recirculation time after the addition of a deoxidizer in a vacuum degassing treatment of secondary refining be at least 10 minutes. It is also preferable that the recirculation time after the addition of scrap or ferroalloy be at least 5 minutes to evaporate and remove the zinc mixed in with the scrap or ferroalloy. The apparatus used for the vacuum degassing treatment can be based on either the RH or DH method. The final rolling temperature (FDT) and the winding temperature (CT) in hot rolling can be based on conventional methods and are not particularly restricted. Hot strip annealing can be performed after hot rolling, but it is not required. Furthermore, cold rolling can be carried out once, twice, or more times with an intermediate annealing between cold rollings, and it is not particularly restricted. It is preferable that the final annealing after cold rolling be carried out at a holding temperature not exceeding 850°C for a holding time of 5 to 120 seconds to achieve finer crystal grains. FR / ynn / Lznz / e / YiAi The steel sheet, after final annealing, is formed into a finished product sheet by applying an insulating coating as required. It is preferable that the insulating film be selected from inorganic, organic, or mixed inorganic / organic coatings, appropriately chosen according to the intended purpose. EXAMPLE Molten steel, after being blown in a converter, undergoes vacuum degassing to produce steel with a composition of Nos. 1 to 62, as shown in Table 1. Each steel is then continuously cast and formed into a steel block. In this process, the recirculation time after deoxidation in the vacuum degassing treatment is varied as shown in Table 2. The block is then reheated to 1140°C, held for 1 hour, hot-rolled to a final rolling temperature of 800°C to form a hot-rolled sheet, and then coiled at 61°C. Subsequently, the hot-rolled sheet is subjected to hot-band annealing at 950°C for 30 seconds under a 100% by volume atmosphere. of N2, pickling, cold rolling to form a cold rolled sheet with a thickness of 0.25mm, and finish annealing in an atmosphere of 20% vol. H2 and 80% vol. N2 under the conditions shown in Table 2 to form a product sheet. From the resulting product sheet, Epstein test samples are taken in the rolling direction and in a direction perpendicular to the rolling direction to measure a magnetic flux density B50 and a high-frequency iron loss Ws / 3k using the Epstein method. A sample for texture observation is taken from the product sheet, and the cross-section in the rolling direction is polished and chemically etched with Nital to measure the crystal grain size according to JIS G0551, thereby determining an average crystal grain size. Additionally, the number of inclusions with a diameter of at least 5 µm (equivalent to one circle per 1 mm²) is determined by polishing the steel sheet surface to 50 µm and observing 10 fields within an area of ​​0.1 mm² using SEM. From the previous product sheet, 20 JIS No. 5 test samples are extracted with the tensile direction in the rolling direction, each of which is subjected to a tensile test according to JIS Z 2241 to measure the breaking stress TS, thereby obtaining the average value and standard deviation σ of the 20 tensile tests. Furthermore, a fatigue test specimen, consisting of a parallel body 5 mm wide and 150 mm long, is extracted from the product sheet along the rolling direction. This specimen is subjected to a fatigue test under pulsating tensile loading conditions, a stress ratio of 0.1, and a frequency of 20 Hz to measure the stress amplitude (fatigue limit) at which no fracture occurs after 107 repetitions. In this test, the fatigue test specimen is finished to a VVV surface roughness and further polished along the longitudinal direction using 800-grit sandpaper. The results of the above measurements are also shown in Table 2. As can be seen from the results, the non-oriented electrical steel sheet produced using a steel material having a chemical composition according to the present invention not only has excellent magnetic properties, but also excellent tensile strength and fatigue resistance. 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Claims

1. A non-oriented electrical steel sheet characterized in that it has a chemical composition comprising C: not more than 0.005% by mass, Si: 3 to 5% by mass, Mn: not more than 5% by mass, P: not more than 0.1% by mass, S: not more than 0.01% by mass, Al: not more than 3% by mass, N: not more than 0.005% by mass, Zn: 0.0005 to 0.003% by mass, and the remainder being Fe and unavoidable impurities, an average crystal grain size of not more than 40 pm, a number of inclusions having a diameter of not less than 5 pm that is not more than 5 / mm2, a tensile strength of not less than 600 MPa, and a fatigue strength of not less than 450 MPa.

2. The non-oriented electrical steel sheet according to claim 1, further characterized in that it additionally contains Cr in 0.1 to 5% by mass in addition to the above chemical composition.

3. The non-oriented electrical steel sheet according to claim 1 or 2, further characterized in that it additionally contains Ca in 0.001 to 0.005% by mass in addition to the above chemical composition.

4. The non-oriented electrical steel sheet according to any of claims 1 to 3, further characterized in that it additionally contains one or two selected from Sn: 0.001 to 0.1% by mass and Sb: 0.001 to 0.1% by mass in addition to the above chemical composition.

5. The non-oriented electrical steel sheet according to any of claims 1 to 4, further characterized in that it additionally contains at least one selected from Ni: 0.1 to 2% by mass, Mo: 0.001 to 0.05% by mass, Cu: 0.01 to 0.2% by mass, Mg: 0.001 to 0.005% by mass, REM: 0.001 to 0.005% by mass, and (Ti + V): 0.005 to 0.05% by mass.