Non oriented electrical steel sheet, iron core, manufacturing method of iron core, motor, and manufacturing method of motor

This application is a Continuation of PCT International Application No. PCT/JP2022/029070, filed on Jul. 28, 2022, which claims priority under 35 U.S.C. 119(a) to Patent Application No. 2021-126290, filed in Japan on Jul. 30, 2021, all of which are hereby expressly incorporated by reference into the present application.

The present invention relates to a non oriented electrical steel sheet. More specifically, the present invention relates to a non oriented electrical steel sheet, which is suitable for iron cores of motors for electric vehicles, hybrid vehicles, or the like, an iron core, a manufacturing method of the iron core, a motor, and a manufacturing method of the motor.

Priority is claimed on Japanese Patent Application No. 2021-126290 filed on Jul. 30, 2021, the content of which is incorporated herein by reference.

Due to a need to reduce global warming gases, products with less energy consumption have been developed in industrial fields. For instance, in a field of an automobile, there are fuel-efficient vehicles such as hybrid-driven vehicles that combine a gasoline engine and a motor, and motor-driven electric vehicles. A technology common to these fuel-efficient vehicles is a motor, and increasing an efficiency of the motor has become an important technology.

In General, a motor includes a stator and a rotor. As an iron core for the stator, there are an integrally punched iron core and a segmented iron core. For the integrally punched iron core and the segmented iron core, there is a demand for a non oriented electrical steel sheet having excellent magnetic characteristics in a rolling direction (hereinafter referred to as “L direction”) and in a transverse direction (hereinafter referred to as “C direction”).

As an iron core for the rotor, for instance, there is the iron core for the rotor of an interior permanent magnet motor (IPM motor). The permanent magnets are embedded inside the rotor core of the IPM motor. Thus, for this iron core, there is a demand for a non oriented electrical steel sheet having excellent mechanical properties in addition to excellent magnetic characteristics.

In addition, the motor shows excellent performance in a case where a gap between the stator and the rotor becomes smaller as an internal structure of the motor. Thus, each component of the motor is required to have a high shape accuracy. For instance, the above integrally punched iron core is formed by punching a steel sheet blank to be a hollow disc shape, and the above iron core for the rotor of the IPM motor is formed by punching the steel sheet blank to be a disc shape. However, in a case where the steel sheet blank is punched to be the hollow disc shape or the disc shape, the shape accuracy after punching may be deteriorated due to mechanical anisotropy of the steel sheet blank. Therefore, for these iron cores, it is desired for the non oriented electrical steel sheet to have small mechanical anisotropy.

For instance, Patent Document 1 discloses a technique related to a non oriented electrical steel sheet having excellent magnetic characteristics. Patent Document 2 discloses a technique related to a non oriented electrical steel sheet that can improve efficiency of a motor including a segmented iron core. Patent Document 3 discloses a technique related to a non oriented electrical steel sheet having excellent magnetic characteristics.

[Patent Document 1] Japanese Patent (Granted) Publication No. 5447167

[Patent Document 2] Japanese Patent (Granted) Publication No. 5716315

[Patent Document 3] PCT International Publication No. WO2013/069754

The present invention has been made in consideration of the above mentioned situations. An object of the invention is to provide a non oriented electrical steel sheet with small magnetic anisotropy and small mechanical anisotropy for an iron core of motor, an iron core, a manufacturing method of the iron core, a motor, and a manufacturing method of the motor.

An aspect of the present invention employs the following.

( 1 ) A non oriented electrical steel sheet according to an aspect of the present invention includes a chemical composition containing, by mass %,

0.005% or less of C,

1.0% or more and 5.0% or less of Si,

less than 2.5% of sol. Al,

3.0% or less of Mn,

0.3% or less of P,

0.01% or less of S,

0.01% or less of N,

0.10% or less of B,

0.10% or less of O,

0.10% or less of Mg,

0.01% or less of Ca,

0.10% or less of Ti,

0.10% or less of V,

5.0% or less of Cr,

5.0% or less of Ni,

5.0% or less of Cu,

0.10% or less of Zr,

0.10% or less of Sn,

0.10% or less of Sb,

0.10% or less of Ce,

0.10% or less of Nd,

0.10% or less of Bi,

0.10% or less of W,

0.10% or less of Mo,

0.10% or less of Nb,

0.10% or less of Y, and

a balance consisting of Fe and impurities, wherein

a sheet thickness is 0.10 mm or more and 0.35 mm or less,

an average grain size is 30 μm or more and 200 μm or less,

an X1 value defined by the following expression 1 is less than 0.845,

an X2 value defined by the following expression 2 is 0.800 or more, and

an iron loss W 10/1k when excited so as to be a magnetic flux density of 1.0 T at a frequency of 1 kHz is 80 W/kg or less,

where the expression 1 is X1=(2×B 50L +B 50C )/(3×I S ),

where the expression 2 is X2=(B 50L +2×B 50D +B 50C )/(4×I S ), and

where B 50L denotes a magnetic flux density in a rolling direction when magnetized with a magnetizing force of 5000 A/m, B 50C denotes a magnetic flux density in a transverse direction when magnetized with a magnetizing force of 5000 A/m, B 50D denotes a magnetic flux density in a direction making an angle of 45° with the rolling direction when magnetized with a magnetizing force of 5000 A/m, and I S denotes a spontaneous magnetization at room temperature.

(2) In the non oriented electrical steel according to (1), the chemical composition may include, by mass %, more than 3.25% and 5.0% or less of Si.

(3) In the non oriented electrical steel according to (1) or (2), the chemical composition may include, by mass %, at least one of

0.0010% or more and 0.005% or less of C,

0.10% or more and less than 2.5% of sol. Al,

0.0010% or more and 3.0% or less of Mn,

0.0010% or more and 0.3% or less of P,

0.0001% or more and 0.01% or less of S,

0.0015% or more and 0.01% or less of N,

0.0001% or more and 0.10% or less of B,

0.0001% or more and 0.10% or less of O,

0.0001% or more and 0.10% or less of Mg,

0.0003% or more and 0.01% or less of Ca,

0.0001% or more and 0.10% or less of Ti,

0.0001% or more and 0.10% or less of V,

0.0010% or more and 5.0% or less of Cr,

0.0010% or more and 5.0% or less of Ni,

0.0010% or more and 5.0% or less of Cu,

0.0002% or more and 0.10% or less of Zr,

0.0010% or more and 0.10% or less of Sn,

0.0010% or more and 0.10% or less of Sb,

0.001% or more and 0.10% or less of Ce,

0.002% or more and 0.10% or less of Nd,

0.002% or more and 0.10% or less of Bi,

0.002% or more and 0.10% or less of W,

0.002% or more and 0.10% or less of Mo,

0.0001% or more and 0.10% or less of Nb, and

0.002% or more and 0.10% or less of Y.

(4) In the non oriented electrical steel according to any one of (1) to (3), the chemical composition may include, by mass %, more than 4.0% in total of Si and sol. Al.

(5) In the non oriented electrical steel according to any one of (1) to (4), the X1 value may be 0.800 or more and less than 0.830.

(6) In the non oriented electrical steel according to any one of (1) to (5), the X2 value may be 0.805 or more and 0.825 or less.

(7) An iron core according to an aspect of the present invention may include the non oriented electrical steel sheet according to any one of (1) to (6).

(8) A manufacturing method of an iron core according to an aspect of the present invention may include a process of punching and laminating the non oriented electrical steel sheet according to any one of (1) to (6).

(9) A motor according to an aspect of the present invention may include the iron core according to (7).

(10) A manufacturing method of a motor may include

a process of preparing an iron core by punching and laminating the non oriented electrical steel sheet according to any one of (1) to (6) and

a process of assembling the motor using the iron core.

According to the above aspects of the present invention, it is possible to provide the non oriented electrical steel sheet with small magnetic anisotropy and small mechanical anisotropy for the iron core of motor, the iron core, the manufacturing method of the iron core, the motor, and the manufacturing method of the motor.

FIG. 1 is a schematic illustration of a non oriented electrical steel sheet according to an embodiment of the present invention.

Hereinafter, a preferable embodiment of the present invention is described in detail. However, the present invention is not limited only to the configuration which is disclosed in the embodiment, and various modifications are possible without departing from the aspect of the present invention. In addition, the limitation range as described below includes a lower limit and an upper limit thereof. However, the value expressed by “more than” or “less than” does not include in the limitation range. “%” of the amount of respective elements expresses “mass %”.

FIG. 1 shows a schematic illustration of a non oriented electrical steel sheet according to the embodiment of the present invention.

(Chemical composition)

Limitation reasons of the chemical composition of the non oriented electrical steel sheet according to the present embodiment are explained.

As a chemical composition, the non oriented electrical steel sheet according to the present embodiment contains Si, optional elements as necessary, and a balance consisting of Fe and impurities. Hereinafter, each element is explained.

C: 0% or more and 0.005% or less

C (carbon) is an element contained as an impurity and deteriorates the magnetic characteristics. Thus, the C content is to be 0.005% or less. Preferably, the C content is 0.003% or less. Since it is preferable that the C content is low, a lower limit does not need to be limited, and the lower limit may be 0%. However, it is not easy to industrially control the content to be 0%, and thus, the lower limit may be more than 0% or 0.0010%.

Si: 1.0% or more and 5.0% or less

Si (silicon) is an element that is effective in increasing electrical resistivity of the steel sheet and reducing iron loss. Thus, the Si content is to be 1.0% or more. Moreover, Si is an effective element for the non oriented electrical steel sheet for the iron core of motor to achieve small magnetic anisotropy and small mechanical anisotropy. In this case, the Si content is preferably more than 3.25%, more preferably 3.27% or more, further more preferably 3.30% or more, and further more preferably 3.40% or more. On the other hand, when the Si content is excessive, a magnetic flux density deteriorates significantly. Thus, the Si content is to be 5.0% or less. The Si content is preferably 4.0% or less, and more preferably 3.5% or less.

sol. Al: 0% or more and less than 2.5%

Al (aluminum) is an optional element that is effective in increasing the electrical resistivity of the steel sheet and reducing the iron loss. However, when the content is excessive, the magnetic flux density deteriorates significantly. Thus, the sol. Al content is to be less than 2.5%. A lower limit of sol. Al does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the sol. Al content is preferably 0.10% or more. Herein, the sol. Al expresses acid-soluble aluminum.

Moreover, Si and Al are elements effective in achieving small magnetic anisotropy and small mechanical anisotropy. Thus, the total amount of Si and sol. Al is preferably more than 4.0%, more preferably more than 4.10%, and further more preferably more than 4.15%. On the other hand, Si and Al have a strong effect on solid solution strengthening. When the content is excessive, cold rolling becomes difficult to be performed. Thus, the total amount of Si and sol. Al is preferably less than 5.5%.

Mn: 0% or more and 3.0% or less

Mn (manganese) is an optional element that is effective in increasing the electrical resistivity of the steel sheet and reducing the iron loss. However, since an alloying cost of is higher than Si or Al, an increase in the Mn content is economically disadvantageous. Thus, the Mn content is to be 3.0% or less. Preferably, the Mn content is 2.5% or less. A lower limit of Mn does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Mn content is preferably 0.0010% or more, and more preferably 0.010% or more.

P: 0% or more and 0.3% or less

P (phosphorus) is an element generally contained as an impurity. However, P has an effect in improving texture of the non oriented electrical steel sheet and thereby improving the magnetic characteristics. Thus, P may be included as necessary. However, P is a solid solution strengthening element. When the P content is excessive, the steel sheet is hardened and thereby the cold rolling becomes difficult to be performed. Thus, the P content is to be 0.3% or less. The P content is preferably 0.2% or less. A lower limit of P does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the P content is preferably 0.0010% or more, and more preferably 0.015% or more.

S: 0% or more and 0.01% or less

S (sulfur) is contained as an impurity and forms fine MnS by bonding to Mn in steel. As a result, S suppresses grain growth during annealing and deteriorates the magnetic characteristics of the non oriented electrical steel sheet. Thus, the S content is to be 0.01% or less. The S content is preferably 0.005% or less, and more preferably or less. Since it is preferable that the S content is low, a lower limit does not need to be limited, and the lower limit may be 0%. However, it is not easy to industrially control the content to be 0%, and thus, the lower limit may be 0.0001%.

N: 0% or more and 0.01% or less

N (nitrogen) is contained as an impurity and forms fine AlN by bonding to Al in steel. As a result, N suppresses the grain growth during annealing and deteriorates the magnetic characteristics. Thus, the N content is to be 0.01% or less. The N content is preferably 0.005% or less, and more preferably 0.003% or less. Since it is preferable that the N content is low, a lower limit does not need to be limited, and the lower limit may be 0%. However, it is not easy to industrially control the content to be 0%, and thus, the lower limit may be 0.0001% or more, may be more than 0.0015%, or may be 0.0025% or more.

Sn: 0% or more and 0.10% or less

Sb: 0% or more and 0.10% or less

Sn (tin) and Sb (antimony) are optional elements having effect in improving the texture of the non oriented electrical steel sheet and thereby improving the magnetic characteristics (for instance, magnetic flux density). Thus, Sn and Sb may be included as necessary. However, when the content is excessive, the steel may become brittle and fracture may occur during cold rolling. Moreover, the magnetic characteristics are deteriorated. Thus, the Sn content and the Sb content are to be 0.10% or less, respectively. Lower limits of Sn and Sb do not need to be limited, and the lower limits may be 0%. In order to reliably obtain the above effect, the Sn content is preferably or more, and more preferably 0.01% or more. Moreover, the Sb content is preferably 0.0010% or more, more preferably 0.002% or more, further more preferably or more, and further more preferably more than 0.025%.

Ca: 0% or more and 0.01% or less

Ca (calcium) is an optional element that suppresses precipitation of fine sulfides (MnS, Cu 2 S, or the like) by forming coarse sulfides. When the Ca content is favorable, inclusions are controlled, the grain growth is improved during annealing, and thereby, the magnetic characteristics (for instance, iron loss) are improved. However, when the content is excessive, the effect thereof is saturated, and the cost increases. Thus, the Ca content is to be 0.01% or less. The Ca content is preferably 0.008% or less, and more preferably 0.005% or less. A lower limit of Ca does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Ca content is preferably 0.0003% or more. The Ca content is preferably 0.001% or more, and more preferably or more.

Cr: 0% or more and 5.0% or less

Cr (chromium) is an optional element that increases the electrical resistivity and improves the magnetic characteristics (for instance, iron loss). However, when the content is excessive, a saturation magnetic flux density may decrease, the effect thereof is saturated, and the cost increases. Thus, the Cr content is to be 5.0% or less. The Cr content is preferably 0.5% or less, and more preferably 0.1% or less. A lower limit of Cr does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Cr content is preferably 0.0010% or more.

Ni: 0% or more and 5.0% or less

Ni (nickel) is an optional element that improves the magnetic characteristics (for instance, saturation magnetic flux density). However, when the content is excessive, the effect thereof is saturated, and the cost increases. Thus, the Ni content is to be 5.0% or less. The Ni content is preferably 0.5% or less, and more preferably 0.1% or less. A lower limit of Ni does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Ni content is preferably 0.0010% or more.

Cu: 0% or more and 5.0% or less

Cu (copper) is an optional element that improves strength of the steel sheet. However, when the content is excessive, the saturation magnetic flux density may decrease, the effect thereof is saturated, and the cost increases. Thus, the Cu content is to be 5.0% or less. The Cu content is preferably 0.1% or less. A lower limit of Cu does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Cu content is preferably 0.0010% or more.

Ce: 0% or more and 0.10% or less

Ce (cerium) is an optional element that suppresses the precipitation of fine sulfides (MnS, Cu 2 S, or the like) by forming coarse sulfides coarse oxysulfides, or the like. As a result, the grain growth is improved, and the iron loss is improved. However, when the content is excessive, the iron loss may be deteriorated by forming oxides in addition to sulfides and oxysulfides, and the effect thereof is saturated, and the cost increases. Thus, the Ce content is to be 0.10% or less. The Ce content is preferably 0.01% or less, more preferably 0.009% or less, and further more preferably 0.008% or less. A lower limit of Ce does not need to be limited, and the lower limit may be 0%. In order to reliably obtain the above effect, the Ce content is preferably 0.001% or more. The Ce content is more preferably 0.002% or more, more preferably 0.003% or more, and further more preferably 0.005% or more.

In addition to the above elements, the non oriented electrical steel sheet according to the present embodiment may contain, as a chemical composition, the optional elements such as B, O, Mg, Ti, V, Zr, Nd, Bi, W, Mo, Nb, and Y. Amounts of these optional elements may be controlled on the basis of known knowledge. For instance, the amounts of these optional elements may be as follows.

B: 0% or more and 0.10% or less

O: 0% or more and 0.10% or less

Mg: 0% or more and 0.10% or less

Ti: 0% or more and 0.10% or less

V: 0% or more and 0.10% or less

Zr: 0% or more and 0.10% or less

Nd: 0% or more and 0.10% or less

Bi: 0% or more and 0.10% or less

W: 0% or more and 0.10% or less

Mo: 0% or more and 0.10% or less

Nb: 0% or more and 0.10% or less

Y: 0% or more and 0.10% or less

Moreover, the non oriented electrical steel sheet according to the present embodiment may contain, as a chemical composition, by mass %, at least one of

0.0010% or more and 0.005% or less of C,

0.10% or more and less than 2.5% of sol. Al,

0.0010% or more and 3.0% or less of Mn,

0.0010% or more and 0.3% or less of P,

0.0001% or more and 0.01% or less of S,

0.0015% or more and 0.01% or less of N,

0.0001% or more and 0.10% or less of B,

0.0001% or more and 0.10% or less of O,

0.0001% or more and 0.10% or less of Mg,

0.0003% or more and 0.01% or less of Ca,

0.0001% or more and 0.10% or less of Ti,

0.0001% or more and 0.10% or less of V,

0.0010% or more and 5.0% or less of Cr,

0.0010% or more and 5.0% or less of Ni,

0.0010% or more and 5.0% or less of Cu,

0.0002% or more and 0.10% or less of Zr,

0.0010% or more and 0.10% or less of Sn,

0.0010% or more and 0.10% or less of Sb,

0.001% or more and 0.10% or less of Ce,

0.002% or more and 0.10% or less of Nd,

0.002% or more and 0.10% or less of Bi,

0.002% or more and 0.10% or less of W,

0.002% or more and 0.10% or less of Mo,

0.0001% or more and 0.10% or less of Nb, and

0.002% or more and 0.10% or less of Y.

The B content is preferably 0.01% or less, the O content is preferably 0.01% or less, the Mg content is preferably 0.005% or less, the Ti content is preferably 0.002% or less, the V content is preferably 0.002% or less, the Zr content is preferably 0.002% or less, the Nd content is preferably 0.01% or less, the Bi content is preferably 0.01% or less, the W content is preferably 0.01% or less, the Mo content is preferably 0.01% or less, the Nb content is preferably 0.002% or less, and the Y content is preferably 0.01% or less. Moreover, the Ti content is preferably 0.001% or more, the V content is preferably 0.002% or more, and the Nb content is preferably 0.002% or more.

The chemical composition as described above may be measured by typical analytical methods for the steel. For instance, the chemical composition may be measured by using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometer: inductively coupled plasma emission spectroscopy spectrometry). Herein, the acid soluble Al may be measured by ICP-AES using filtrate after heating and dissolving the sample in acid. In addition, C and S may be measured by the infrared absorption method after combustion, N may be measured by the thermal conductometric method after fusion in a current of inert gas, and O may be measured by, for instance, the non-dispersive infrared absorption method after fusion in a current of inert gas.

The above chemical composition is that of the non oriented electrical steel sheet without insulation coating. When the non oriented electrical steel sheet to be the measurement sample has the insulation coating and the like on the surface, the chemical composition is measured after removing the coating. For instance, the insulation coating may be removed by the following method. First, the non oriented electrical steel sheet having the insulation coating and the like is immersed in sodium hydroxide aqueous solution, sulfuric acid aqueous solution, and nitric acid aqueous solution in this order. The steel sheet after the immersion is washed. Finally, the steel sheet is dried with warm air. Thereby, it is possible to obtain the non oriented electrical steel sheet from which the insulation coating is removed. Alternatively, the insulation coating may be removed by grinding.

(Magnetic characteristics)

As the magnetic flux density, an X1 value defined by the following expression 1 is to be less than 0.845. Moreover, an X2 value defined by the following expression 2 is to be 0.800 or more.

Expression 1: X1=(2×B 50L +B 50C )/(3×I S ).

Expression 2: X2=(B 50L +2×B 50D +B 50C )/(4×I S ).

Herein,

B 50L denotes the magnetic flux density in a rolling direction when magnetized with magnetizing force of 5000 A/m,

B 50C denotes the magnetic flux density in a transverse direction when magnetized with magnetizing force of 5000 A/m,

B 50D denotes the magnetic flux density in a direction making an angle of 45° with the rolling direction when magnetized with magnetizing force of 5000 A/m, and

I S denotes spontaneous magnetization at room temperature.

I S in the expression 1 and expression 2 may be obtained by the following expression 3 and expression 4. The expression 3 is for obtaining the spontaneous magnetization assuming that the spontaneous magnetization of the steel sheet is simply attenuated by elements other than Fe. Density of the steel sheet in the expression 3 may be measured on the basis of JIS Z 8807:2012. In a case where the insulation coating is applied, the density may be measured by the above described method under condition such that the insulation coating exists, and the same value of the density is also used at the time of evaluating the magnetic characteristics described later. Density of Fe in the expression 3 may be 7.873 g/cm 3 .

Expression 3: I S =2.16×{(density of steel sheet)/(density of Fe)}×[Fe content (mass %)]/100

Expression 4: Fe content (mass %)=100 (mass %)−[total amount (mass %) of C, Si, Mn, sol. Al, P, S, N, B, O, Mg, Ca, Ti, V, Cr, Ni, Cu, Zr, Sn, Sb, Ce, Nd, Bi, W, Mo, Nb, and Y]

In order to achieve small magnetic anisotropy and small mechanical anisotropy as the non oriented electrical steel sheet for the iron core of motor, the X1 value is preferably 0.800 or more, and is preferably less than 0.830. Moreover, in order to further improve a roundness, the X2 value is preferably 0.805 or more, and is preferably 0.825 or less.

As the iron loss, an iron loss W 10/1k when excited so as to have the magnetic flux density of 1.0 T at frequency of 1 kHz is to be 80 W/kg or less. The iron loss W 10/1k is preferably 70 W/kg or less, and more preferably 49 W/kg or less. Although a lower limit of the iron loss W 10/1k does not need to be limited, the lower limit may be 30 W/kg as necessary.

The magnetic characteristics may be measured on the basis of the single sheet tester (SST) method regulated by JIS C 2556: 2015. Instead of taking a test piece with size regulated by JIS, a test piece with smaller size, for instance, a test piece of width 55 mm×length 55 mm, may be taken and measured on the basis of the single sheet tester. In a case where the test piece of width 55 mm×length 55 mm is hardly taken, the measurement based on the single sheet tester may be performed using two test pieces of width 8 mm×length 16 mm as a test piece of width 16 mm×length 16 mm. At that time, it is preferable to use an Epstein equivalent value which is converted so as to correspond to a measurement value with an Epstein tester regulated in JIS C 2550:2011.

(Average grain size)

When grain size is excessively coarse or fine, the iron loss under high frequency may deteriorates. Thus, the average grain size is to be 30 μm or more and 200 μm or less.

The average grain size may be measured on the basis of an intercept method regulated by JIS G 0551:2020. For instance, in a longitudinal sectional micrograph, an average value of grain sizes may be measured by the intercept method along sheet thickness direction and rolling direction. As the longitudinal sectional micrograph, an optical micrograph may be used, and for instance, a micrograph obtained at a magnification of 50-fold may be used.

(Sheet thickness)

Sheet thickness is to be 0.35 mm or less. The sheet thickness is preferably 0.30 mm or less. On the other hand, when the sheet thickness is excessively thin, productivity of the steel sheet and motor deteriorates significantly. Thus, the sheet thickness is to be 0.10 mm or more. The sheet thickness is preferably 0.15 mm or more.

The sheet thickness may be measured by a micrometer. When the non oriented electrical steel sheet to be the measurement sample has the insulation coating and the like on the surface, the sheet thickness is measured after removing the coating. The method for removing the insulation coating is as described above.

In addition, as mechanical anisotropy, the roundness after punching to be true circle is preferably more than 0.9997 and 1.0000 or less. Specifically, when the punching is conducted using a die with a disc shape of an outer diameter of 79.5 mm and when 60 sheets of punched pieces are laminated and fastened to be formed, a value (roundness) obtained by dividing a minimum value of a diameter of an outer circumference of the formed piece by a maximum value of a diameter of the outer circumference of the formed piece is preferably more than 0.9997 and 1.0000 or less.

When the above roundness is more than 0.9997, a shape accuracy of the punched piece can be regarded as high. As a result, when it is used for the motor, an increase in cogging torque and an increase in vibration noise can be favorably suppressed. The above roundness is preferably more than 0.9998, and more preferably 0.9999 or more.

The roundness may be measured by the following method. The non oriented electrical steel sheet is punched using the die with the disc shape (true circle) of the outer diameter of 79.5 mm at a punching speed of 250 strokes/min by a 25 t continuous progressive press-working apparatus. The 60 sheets of punched pieces are laminated and fastened to form the core. The obtained disc-shaped core simulates the iron core for the rotor of the motor, and the roundness of the outer circumference can be used as an index of accuracy of an air gap with a stator core. Diameters of the outer circumference of the obtained disc-shaped core are measured at plural positions, and a ratio of a minimum value to a maximum value of the measured diameters is regarded as the roundness. Specifically, when the punching is conducted using the die with the disc shape (true circle) of the outer diameter of 79.5 mm and when the 60 sheets of punched pieces are laminated and fastened to be formed, the value obtained by dividing the minimum value of the diameter of the outer circumference of the formed piece by the maximum value of the diameter of the outer circumference of the formed piece is regarded as the roundness.

The non oriented electrical steel sheet according to the present embodiment has small magnetic anisotropy and small mechanical anisotropy for the iron core of motor. For instance, the non oriented electrical steel sheet according to the present embodiment satisfies the X1 value of less than 0.845, the X2 value of 0.800 or more, and the iron loss W 10/1k of 80 W/kg or less, and as a result, it is possible to obtain the effect such that the roundness is excellent. Moreover, when the chemical composition and manufacturing conditions are favorably controlled, the non oriented electrical steel sheet according to the present embodiment satisfies the X1 value of less than 0.830, the X2 value of 0.800 or more, and the iron loss W 10/1k of 49 W/kg or less, and as a result, it is possible to obtain the effect such that the roundness is more excellent. In this case, it can be judged that small magnetic anisotropy and small mechanical anisotropy are achieved for the iron core of motor.

(Iron core and motor)

Since the non oriented electrical steel sheet according to the present embodiment has small magnetic anisotropy and small mechanical anisotropy, it is suitable for the iron core of motor for electric vehicles, hybrid vehicles, or the like. Thus, an iron core including the non oriented electrical steel sheet according to the present embodiment exhibits excellent performance Moreover, since the non oriented electrical steel sheet according to the present embodiment is suitable for the iron core of motor, a motor including the iron core exhibits excellent performance.

(Manufacturing method)

Hereinafter, an instance of a manufacturing method of the non oriented electrical steel sheet according to the present embodiment is explained below. The non oriented electrical steel sheet according to the present embodiment is not particularly limited in the manufacturing method as long as the above features are included. The following manufacturing method is an instance for manufacturing the non oriented electrical steel sheet according to the present embodiment and a favorable manufacturing method of the non oriented electrical steel sheet according to the present embodiment.

The manufacturing method of the non oriented electrical steel sheet according to the present embodiment includes the following processes (A) to (D).

(A) A first cold rolling process of subjecting a hot rolled steel sheet having the chemical composition described above to cold rolling under conditions such that a rolling reduction is 10% or larger and 75% or smaller.

(B) An intermediate annealing process of subjecting the cold rolled steel sheet obtained in the first cold rolling process to intermediate annealing under condition such that an average heating rate from 500° C. to 650° C. is 300° C/second or faster and 1000° C./second or slower, a retention temperature is 700° C. or higher and 1100° C. or lower, a retention time is 10 seconds or longer and 300 seconds or shorter (0.0028 hours or longer and 0.0833 hours or shorter), and an average cooling rate from 700° C. to 500° C. is 25° C./second or faster.

(C) A second cold rolling process of subjecting the intermediate-annealed steel sheet obtained in the intermediate annealing process to cold rolling under conditions such that a rolling reduction is 50% or larger and 85% or smaller to obtain a sheet thickness of 0.10 mm or more and 0.35 mm or less.

(D) A final annealing process of subjecting the cold rolled steel sheet obtained in the second cold rolling process to final annealing under conditions such that a temperature range to be retained is 900° C. or higher and 1200° C. or lower.

Each process is explained below.

(First cold rolling process)

In the first cold rolling process, the hot rolled steel sheet having the above chemical composition is subjected to cold rolling at the rolling reduction (cumulative rolling reduction) of 10% or larger and 75% or smaller.

When the rolling reduction in the first cold rolling process is smaller than 10% or larger than 75%, the intended magnetic characteristics and roundness may not be obtained. Thus, the rolling reduction in the first cold rolling process is to be 10% or larger and 75% or smaller.

Conditions of the cold rolling other than the above, such as a steel sheet temperature during cold rolling and a diameter of the rolling roll, are not particularly limited, and are appropriately selected depending on the chemical composition of the hot rolled steel sheet, the intended sheet thickness of the steel sheet, or the like.

In general, the hot rolled steel sheet is subject to cold rolling after a scale formed on a surface of the steel sheet during hot rolling is removed by pickling. As described later, in a case where the hot rolled steel sheet is subjected to hot-band annealing, the hot rolled steel sheet may be pickled either before the hot-band annealing or after the hot-band annealing.

(Intermediate annealing process)

In the intermediate annealing process, the cold rolled steel sheet obtained in the above first cold rolling process is subjected to intermediate annealing under condition such that an average heating rate from 500° C. to 650° C. is 300° C./second or faster and 1000° C./second or slower, a retention temperature is 700° C. or higher and 1100° C. or lower, a retention time is 10 seconds or longer and 300 seconds or shorter (0.0028 hours or longer and 0.0833 hours or shorter), and an average cooling rate from 700° C. to 500° C. is 25° C./second or faster.

When the above conditions are not satisfied in the intermediate annealing process, the intended magnetic characteristics and roundness may not be obtained. Conditions of the intermediate annealing other than the above are not particularly limited.

Although an upper limit of the average cooling rate from 700° C. to 500° C. does not need to be limited, the upper limit may be 70° C./second as necessary.

The retention temperature is preferably 850° C. or higher. The retention time is preferably 180 seconds or shorter (0.05 hours or shorter). The average cooling rate from 700° C. to 500° C. is preferably 28° C./second or faster. In particular, when the Si content of more than 3.25%, the average heating rate from 500° C. to 650° C. of 300° C./second or faster, the retention temperature of 850° C. or higher, the retention time of 180 seconds or shorter, and the average cooling rate from 700° C. to 500° C. of 33° C./second or faster are simultaneously satisfied in addition to satisfying the conditions of the present embodiment, it is possible to obtain the non oriented electrical steel sheet in which both desirable magnetic characteristics and roundness are simultaneously and preferably achieved.

(Second cold rolling process)

In the second cold rolling process, the intermediate-annealed steel sheet obtained in the above intermediate annealing process is subjected to cold rolling at the rolling reduction (cumulative rolling reduction) of 50% or larger and 85% or smaller to obtain the sheet thickness of 0.10 mm or more and 0.35 mm or less.

When the rolling reduction in the second cold rolling process is smaller than 50% or larger than 85%, the intended magnetic characteristics and roundness may not be obtained. Thus, the rolling reduction in the second cold rolling process is to be 50% or larger and 85% or smaller.

The sheet thickness is to be 0.10 mm or more and 0.35 mm or less. The sheet thickness is preferably 0.15 mm or more and 0.30 mm or less.

Conditions of the cold rolling other than the above, such as a steel sheet temperature during cold rolling and a diameter of the rolling roll, are not particularly limited, and are appropriately selected depending on the chemical composition of the steel sheet, the intended sheet thickness of the steel sheet, or the like.

(Final annealing process)

In the final annealing process, the cold rolled steel sheet obtained in the above second cold rolling process is subjected to final annealing at the temperature range to be retained of 900° C. or higher and 1200° C. or lower.

When the final annealing temperature in the final annealing process is lower than 900° C., the average grain size may become less than 30 μm due to insufficient grain growth, and thereby sufficient magnetic characteristics may not be obtained. Thus, the final annealing temperature is to be 900° C. or higher. On the other hand, when the final annealing temperature is higher than 1200° C., the grain growth may proceed excessively, the average grain size may become more than 200 μm, and thereby sufficient magnetic characteristics may not be obtained. Thus, the final annealing temperature is to be 1200° C. or lower.

The final annealing time for retaining the cold rolled steel sheet in the temperature range of 900° C. or higher and 1200° C. or lower may not be particularly specified, but it is preferably 1 second or longer to more reliably obtain favorable magnetic characteristics. On the other hand, from a productive standpoint, the final annealing time is preferably 120 seconds or shorter.

Conditions of the final annealing other than the above are not particularly limited.

(Hot-band annealing process)

The hot rolled steel sheet to be subjected to the above first cold rolling process may be subjected to hot-band annealing. When the hot rolled steel sheet is subjected to hot-band annealing, it is possible to obtain favorable magnetic characteristics.

The hot-band annealing may be performed by either box annealing or continuous annealing. When the box annealing is performed, the hot rolled steel sheet is preferably retained in a temperature range of 700° C. or higher and 900° C. or lower for 1 hour or longer and 20 hours or shorter. When the continuous annealing is performed, the hot rolled steel sheet is preferably retained in a temperature range of 850° C. or higher and 1100° C. or lower for 1 second or longer and 180 seconds or shorter.

Conditions of the hot-band annealing other than the above are not particularly limited.

(Hot rolling process)

The hot rolled steel sheet to be subjected to the first cold rolling process can be obtained by subjecting a steel ingot or steel piece (hereinafter referred to as “slab”) having the above chemical composition to hot rolling.

In the hot rolling, a steel having the above chemical composition is made into the slab by typical methods such as continuous casting or blooming the steel ingot. The slab is put into a heating furnace and then subjected to hot rolling. At this time, when the slab temperature is high, the hot rolling may be performed without putting the slab into the heating furnace.

Conditions of the hot-band annealing are not particularly limited.

(Other processes)

After the final annealing process, a coating process of applying an insulation coating including only an organic component, only an inorganic component, or an organic-inorganic compound to a surface of the steel sheet may be performed by typical methods. From a standpoint of reducing an environmental load, an insulation coating that does not include chromium may be applied. Moreover, the coating process may be a process of applying an insulation coating that is adhesiveness by heating and pressurizing. As coating material exhibiting adhesiveness, an acrylic resin, a phenol resin, an epoxy resin, a melamine resin, or the like can be used.

(Manufacturing method of iron core and manufacturing method of motor)

An iron core may be manufactured using the non oriented electrical steel sheet according to the present embodiment obtained as described above. A manufacturing method of the iron core may include a process of punching and laminating the above non oriented electrical steel sheet. Moreover, a motor may be manufactured using the iron core. A manufacturing method of the motor may include a process of preparing an iron core by punching and laminating the above non oriented electrical steel sheet, and a process of assembling the motor using the iron core.

The effects of an aspect of the present invention are described in detail with reference to the following examples. However, the condition in the examples is an example condition employed to confirm the operability and the effects of the present invention, so that the present invention is not limited to the example condition. The present invention can employ various types of conditions as long as the conditions do not depart from the scope of the present invention and can achieve the object of the present invention. Hereinafter, the present invention is explained in detail with reference to examples and comparative examples.

Non oriented electrical steel sheets were prepared by performing each process under the conditions shown in Tables 1 to 16 using slabs whose chemical compositions were adjusted. Moreover, in a case where hot-band annealing was not performed, pickling was performed after hot rolling. In a case where hot-band annealing was performed, pickling for Test No. 1, 7, and 19 was performed before the hot-band annealing, and pickling for the others was performed after the hot-band annealing. Moreover, a retention time of the final annealing was 30 seconds.

A chemical composition, a sheet thickness, an average grain size, an X1 value and an X2 value related to a magnetic flux density, an iron loss W 10/1k , and a roundness of the prepared non oriented electrical steel sheet were measured. Measurement methods thereof are as described above. Measurement results are shown in Tables 1 to 16. Herein, a chemical composition of the prepared non oriented electrical steel sheet was substantially the same as a chemical composition of the slab. The element represented by “-” in the table indicates that it was not consciously controlled and prepared. Moreover, the Si content indicated by “3.3” in the table was more than 3.25%. Moreover, the manufacturing condition represented by “-” in the table indicates that it was not controlled. Moreover, a sheet thickness of the prepared non oriented electrical steel sheet was the same as a final sheet thickness after second cold rolling process.

Moreover, as the mechanical anisotropy, a roundness was defined as a ratio of maximum and minimum values of a diameter of an outer circumference of the above disc-shaped core, and the roundness was evaluated using the following criteria.

Excellent: Roundness is 0.9999 or more and 1.0000 or less.

Very Good: Roundness is more than 0.9998 and less than 0.9999.

Good: Roundness is more than 0.9997 and 0.9998 or less.

Poor: Roundness is 0.9997 or less.

As shown in Tables 1 to 16, among Test Nos. 1 to 91, inventive examples showed small magnetic anisotropy and small mechanical anisotropy as the non oriented electrical steel sheet. On the other hand, among Test Nos. 1 to 91, comparative examples were not excellent in at least one of the magnetic characteristics and the roundness.

According to the above aspects of the present invention, it is possible to provide the non oriented electrical steel sheet with small magnetic anisotropy and small mechanical anisotropy for the iron core of motor, the iron core, the manufacturing method of the iron core, the motor, and the manufacturing method of the motor. Accordingly, the present invention has significant industrial applicability.

1 : NON ORIENTED ELECTRICAL STEEL SHEET

D: DIRECTION MAKING ANGLE OF 45° WITH ROLLING DIRECTION