NON-ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Abstract

A non-oriented electrical steel sheet according to one embodiment of the present invention includes, in wt %, Si: 1.5 to 4%, Al: 0.1 to 2%, and Mn: 0.05 to 2%, with a remainder being Fe and unavoidable impurities. According to one embodiment of the present invention, the non-oriented electrical steel sheet has an area fraction of crystal grains having an orientation within 15° from {110}<001> of 10% or less.

Description

BACKGROUND OF THE INVENTION

(a) Field of the Invention

An embodiment of the present invention relates to a non-oriented electrical steel sheet and a method for manufacturing the same. Specifically, the embodiment of the present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof, which secures the strength of the steel sheet before a stress removal annealing and secures the magnetism of the steel sheet after the stress removal annealing by suppressing the {110}<001> grain structure by performing a hot rolling twice.

(b) Description of the Related Art

A non-oriented electrical steel is an important core material required to convert an electrical energy into a mechanical energy in a rotating machinery, in order to save an energy, it is important to have magnetic properties, namely low high-frequency iron loss and high magnetic flux density. Here, the iron loss is an energy that is converted into heat during the energy conversion process and lost, so the lower it is, the more efficient it is, and the magnetic flux density is the power that generates a power, so the higher it is, the more efficient it is. In addition, when rotating at thousands or tens of thousands of RPM, it is advantageous to reduce the thickness deviation of the electrical steel sheet along with the strength for a symmetrical core production to suppress the vibration and improve the durability. Recently, as a measure to address the shortage of fossil fuels and reduce greenhouse gases, technology to convert existing internal combustion engine vehicles to hybrid vehicles (HEVs)/electric vehicles (EVs) is rapidly developing. These HEVs/EVs are cars that change a part or all of their driving methods to electric motors, allowing them to reduce the use amount of gasoline or diesel fuel used in the existing internal combustion engines while achieving better fuel efficiency. The motors used in these cars must produce a large torque at low speeds or during an acceleration, and rotate at high speeds when driving at constant speeds or at high speeds. Therefore, the non-oriented electrical steel sheet, which is the motor core material, must have the large magnetic flux density and high strength at low rotation speeds, and the large magnetic flux density and small high-frequency iron loss at high rotation speeds, as well as the small thickness deviation to suppress the vibration. Generally, the high-frequency iron loss refers to an iron loss at frequencies above 200 Hz, but in the non-oriented electrical steel sheets for automobiles, a value of W10/400 is mainly used, and the iron loss and the magnetic flux density in the circumferential direction are particularly important due to the characteristics of a rotating equipment. In general, the rotor of the motor are important for the magnetic flux density, strength, and thickness deviation, and the stator is important for the low high-frequency iron loss. However, when the crystal grains are refined to increase the strength, the iron loss deteriorates, so different materials are used for the rotor and stator, which increases the cost. Accordingly, by manufacturing the stator and the rotor by using the electrical steel with fine grains and then performing a customer heat treatment only on the stator, the rotor may be made to have the high strength and the stator may have the low high-frequency and low iron loss by using the same material. However, when going through this process, the grain structure deteriorates, and the magnetic flux density became excessively low after the customer's heat treatment. In addition, the magnetic flux density is further reduced by the addition of resistive elements such as Si, Al, and Mn to improve the high-frequency iron loss, so it is necessary to have the high magnetic flux density for materials that continuously require a weight reduction, such as eco-friendly electric vehicle drive motors. To solve this problem, a method was proposed to improve the properties by reducing the thickness of the hot-rolled sheet to 2.0 mm or less. In addition, a method of improving the magnetism by including high Al and performing double annealing and double rolling was proposed. In addition, a method of hot-rolling a sheet metal using a thin slab manufacturing method was proposed. However, the method of reducing the hot-rolled thickness has a limitation on a roll force in the hot-rolling process, making it difficult to apply to a mass production, and there is a problem that the thickness deviation increases due to roll bending as the reduction ratio increases in the hot-rolling process, which is performed without a tension. The double annealing and double rolling process shows some improvement in properties, but it is a major cost increase factor and tends to worsen the cylindrical properties due to excessive the development of the Goss texture.

SUMMARY OF THE INVENTION

Technical Problem

In one embodiment of the present invention, a non-oriented electrical steel sheet and a method for manufacturing the same are provided. Specifically, the embodiment of the present invention relates to a non-oriented electrical steel sheet and a manufacturing method thereof, which secures the strength of the steel sheet before a stress removal annealing and secures the magnetism of the steel sheet after the stress removal annealing by suppressing the {110}<001> grain structure by performing a hot rolling twice.

Technical Solution

A non-oriented electrical steel sheet includes Si: 1.5 to 4%, Al: 0.1 to 2%, and Mn: 0.05 to 2% by a weight, with a remainder being Fe and inevitable impurities. An area fraction of grains having an orientation within 15° from {110}<001> is 10% or less.

The non-oriented electrical steel sheet may further include at least one of Cr: 0.5 wt % or less (excluding 0%), Cu: 0.2 wt % or less (excluding 0%), P: 0.1 wt % or less (excluding 0%), Sn: 0.06 wt % or less (excluding 0%), and Sb: 0.06 wt % or less (excluding 0%).

The non-oriented electrical steel sheet may further include 0.005 wt % or less (excluding 0%) of one or more of C, N, S, Ti, Nb, and V.

An average grain particle diameter of the non-oriented electrical steel sheet according to an embodiment may be 10 to 25 μm.

The non-oriented electrical steel sheet according to an embodiment may satisfy Equation 1.

$\mathrm{Yield}\mathrm{strength}\left(\mathrm{MPa}\right)\ge 140+100\times \left[\mathrm{Si}\right]+35\times \left(\left[\mathrm{Al}\right]+\left[\mathrm{Mn}\right]\right)$

(In Formula 1, [Si], [Al] and [Mn] represent contents (weight %) of Si, Al and Mn, respectively.)

The non-oriented electrical steel sheet according to an embodiment may satisfy Equation 2.

$\begin{array}{cc}\mathrm{Circumferential}\mathrm{flux}\mathrm{density}\left(B50,\mathrm{Tesla}\right)\ge 1.880.1\times t-0.067\times \left[\mathrm{Si}\right]-0.0458\times \left[\mathrm{Al}\right]-0.022\times \left[\mathrm{Mn}\right]& \left[\mathrm{Equation}2\right]\end{array}$

(In Equation 2, [Si], [Al] and [Mn] represent contents (in weight %) of Si, Al and Mn, respectively, and t represents a thickness (in mm) of the steel sheet.)

The non-oriented electrical steel sheet according to an embodiment may satisfy Equation 3 after a stress removal annealing at a temperature of 700 to 850° C. for 10 to 300 minutes.

$\begin{array}{cc}\mathrm{circumferential}\mathrm{flux}\mathrm{density}\left(B50,\mathrm{Tesla}\right)\ge 1.850.1\times t-0.067\times \left[\mathrm{Si}\right]-0.0458\times \left[\mathrm{Al}\right]-0.022\times \left[\mathrm{Mn}\right]& \left[\mathrm{Equation}3\right]\end{array}$ $\mathrm{circumferential}\mathrm{iron}\mathrm{loss}\left(W10/400,W/\mathrm{kg}\right)\le 64000\times t2/(1311\times \left(\left[\mathrm{Si}\right]\left[\mathrm{Al}\right]0.5\times \left[\mathrm{Mn}\right]\right)$

(In Equation 3, [Si], [Al] and [Mn] represent the contents (in weight %) of Si, Al and Mn, respectively, and t represents the thickness (in mm) of the steel sheet.)

A method for manufacturing a non-oriented electrical steel sheet according to an embodiment includes first hot rolling of manufacturing a first hot-rolled sheet by hot-rolling a slab including Si: 1.5 to 4% by weight, Al: 0.1 to 2%, and Mn: 0.05 to 2% by a weight, with a remainder being Fe and unavoidable impurities; winding the first hot-rolled sheet; second hot rolling of hot-rolling the first hot-rolled sheet at a temperature of 700 to 1000° C. and a rolling reduction ratio of 20 to 50% to produce a second hot-rolled sheet; cold rolling the second hot-rolled sheet to manufacture a cold-rolled sheet; and annealing the cold rolled sheet at a temperature of 710 to 820° C.

The first hot rolling may include a rough rolling and a finish rolling.

The thickness of the first hot-rolled sheet may be 1.8 to 2.5 mm.

After the winding, the wound the first hot-rolled sheet may be cooled to 700° C. or less.

Annealing the first hot-rolled sheet at a temperature of 700 to 1000° C. after the winding may be further included.

The thickness of the second hot-rolled sheet may be 1.2 to 1.8 mm.

After the second hot rolling, second hot-rolled sheet annealing of annealing the second hot-rolled sheet at a temperature of 850 to 1150° C. may be further included.

After annealing the cold rolled sheet, performing a stress removal annealing at a temperature of 700 to 850° C. for 10 to 300 minutes may be further included.

Advantageous Effects

According to one embodiment of the present invention, the non-oriented electrical steel sheet reduces a width direction thickness deviation due to a shape correction, has excellent strength and circumferential average magnetic flux density before the customer's heat treatment (a stress removal annealing, SRA) through a reduction in a rolling reduction ratio and a suppression of GOSS texture formation, and has an excellent circumferential average magnetic flux density and high-frequency iron loss after the customer's heat treatment (the stress removal annealing, SRA).

Ultimately, the non-oriented electrical steel sheet according to one embodiment of the present invention contributes to the manufacture of eco-friendly automobile motors, high-efficiency home appliance motors, and super premium-grade motor cores by using the same steel sheet as a rotor without the SRA treatment and as a stator after the SRA treatment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It will be understood that, although the terms first, second, third, etc., may be used herein to describe various elements, components, regions, layers, and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another element, component, region, layer, or section. Thus, a first element, component, region, layer, or section described below could be termed a second element, component, region, layer, or section without departing from the teachings of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and “comprising”, when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups.

It will be understood that when an element such as a layer, film, region, or substrate is referred to as being “on” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when a part is said to be “directly on” another part, no other part intervenes.

Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

In one embodiment of the present invention, the inclusion of an additional element means including a remainder of iron (Fe) in an amount equivalent to the additional amount of the additional element.

Hereinafter, embodiments of the present invention will be described in detail so that a person having ordinary knowledge in the technical field to which the present invention pertains can easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

A non-oriented electrical steel sheet according to one embodiment of the present invention includes, in wt %, Si: 1.5 to 4%, Al: 0.1 to 2%, and Mn: 0.05 to 2%, with the remainder of Fe and unavoidable impurities.

First, the reason for the limitation of the composition of the non-oriented electrical steel sheet is described.

Si::1.5 to 4.0 wt %

Silicon (Si) should be added in relatively large quantities because it increases the resistivity of the material and reduces the iron loss. If too little Si is added, the effect of improving the high-frequency iron loss may be minimal. If too much Si is added, the hardness of the material increases, which is not desirable as it reduces a productivity and a punching property. More specifically, it may include 2.5 to 3.7 wt % of Si.

Al: 0.1 to 2.0 wt %

Aluminum (Al) acts together with Si to increase the resistivity of the material and reduce the iron loss, and may form nitrides and oxides that are harmful to the magnetism. If too little Al is added, it may be ineffective in reducing the high-frequency iron loss and fine nitrides may form, which may deteriorate the magnetism. If too much Al is added, it may cause problems by changing the properties of a mold flux during a steelmaking and a continuous casting process, which may significantly reduce the productivity. More specifically, it may include 0.5 to 1.5 wt % of Al.

Mn: 0.05 to 2.00 wt %

Manganese (Mn), together with Si and Al, plays a role in increasing the resistivity of the material, improving the iron loss, and forming sulfides. If too little Mn is added, fine MnS may precipitate, which may deteriorate the magnetism. If too much Mn is added, it promotes the formation of [111] aggregate structure, which is unfavorable for magnetism, and also Mn can reduce the fraction of Fe, which can cause a rapid decrease in the magnetic flux density. More specifically, it may include 0.5 to 1.5 wt % of Mn.

The non-oriented electrical steel sheet according to one embodiment of the present invention may further include at least one of Cr: 0.5 wt % or less, Cu: 0.2 wt % or less, P: 0.1 wt % or less, Sn: 0.06 wt % or less, and Sb: 0.06 wt % or less.

Cr: 0.50 wt % or less

Chromium (Cr) plays a role in improving the iron loss by increasing the resistivity. If too much Cr is included, the magnetic flux density may decrease. More specifically, when Cr is further included, it may be included in an amount of 0.01 to 0.50 wt %. More specifically, it may include 0.050 to 0.20 wt %.

Cu: 0.200 wt % or less

Copper (Cu) plays a role in forming sulfides together with manganese. If too much Cu is added, high-temperature embrittlement may occur, which may cause cracks to form during a casting or a hot rolling. More specifically, it may further include 0.005 to 0.200 wt % of Cu. More specifically, it may include 0.01 to 0.10 wt %.

P: 0.10 wt % or less

Phosphorus (P) is mostly employed in a steel and has the effect of improving the iron loss. If P is added more, too much addition may cause a segregation at grain boundaries, which may lower the toughness of the material and reduce the productivity and the punching property. More specifically, it may include 0.001 to 0.100 wt % of P. More specifically, it may include 0.005 to 0.050 wt %.

Sn: 0.06 wt % or less

Tin (Sn) be added to improve the magnetism because it improves the texture structure of the material by segregating at the grain boundaries and surfaces and suppresses a surface oxidation. If too much Sn is added, the grain boundary segregation becomes severe, the surface quality deteriorates, the hardness increases, and a cold-rolled sheet may fracture, reducing the rollability. Therefore, Sn may be additionally added within the above-mentioned range. More specifically, it may include 0.01 to 0.06 wt % of Sn. More specifically, it may include 0.02 to 0.05 wt %.

Sb: 0.06 wt % or less

Antimony (Sb) may be added to improve the magnetism because it improves the texture structure of the material by being segregated at the grain boundaries and surfaces and suppresses the surface oxidation. If too much Sb is added, the grain boundary segregation becomes severe, the surface quality deteriorates, the hardness increases, and the cold-rolled sheets may fracture, reducing the rollability. Therefore, Sb may be additionally added within the above-mentioned range. More specifically, it may include 0.01 to 0.06 wt % of Sb. More specifically, it may include 0.02 to 0.05 wt %.

The non-oriented electrical steel sheet according to one embodiment of the present invention may further include 0.005 wt % or less of one or more of C, N, S, Ti, Nb, and V.

C, N, and Ti may be limited because they form carbonitrides and hinder a magnetic domain movement, and S may form sulfides and thus lower a grain growth, which may limit its upper limit. Each of these elements may be included in the amount of 0.0050 wt % or less.

N combines with Ti, Nb, and V to form nitrides and plays a role in reducing the grain growth.

C reacts with N, Ti, Nb, V, etc. to form fine carbides, which hinder the grain growth and the domain movement.

S forms sulfides, which impairs the grain growth.

More specifically, it may include one or more of C, S, N, Ti, Nb, and V, each in an amount of 0.0040 wt % or less.

The non-oriented electrical steel sheet according to one embodiment of the present invention may further include at least one of Mo: 0.03 wt % or less, B: 0.0050 wt % or less, V: 0.0050 wt % or less, Ca: 0.0050 wt % or less, Nb: 0.0050 wt % or less, and Mg: 0.0050 wt % or less.

Since these may react with C, S, N, etc., which are inevitably included, to form fine carbides, nitrides, or sulfides, which may adversely affect the magnetism, the upper limit may be limited as described above.

Other Impurities

In addition to the elements mentioned above, impurities that are inevitably mixed in may be included. In one embodiment of the present invention, it is not excluded that additional elements are included in addition to the elements described above, and when additional elements are included, they are included in place of the remainder, Fe.

In one embodiment of the present invention, the non-oriented electrical steel sheet has an area fraction of crystal grains having an orientation within 15° from {110}<001> of 10% or less.

This microstructure is an orientation that has a negative effect on the magnetism, so it reduces this and relatively increases the number of the crystal grains with the orientations favorable to the magnetism, thereby improving the magnetism.

This microstructure is suppressed from being formed by performing the hot rolling process twice. When hot-rolling twice, the thickness of the second hot-rolled sheet becomes thinner after the hot-rolling twice, which reduces the cold rolling reduction rate and suppresses the production. Even after the stress removal annealing process, the fraction of the microstructure remains intact without any additional changes. More specific details are provided in relation to the manufacturing process.

The measurement method is not particularly limited, but may be measured using an X-ray pole figure or an electron backscattering diffraction (EBSD). The measurement reference plane is not particularly limited and may be a plane (a TD plane) perpendicular to a rolling direction.

The average grain particle diameter of the steel sheet may be 10 to 25 μm. If the average grain particle diameter is too small, the magnetism is excessively poor, and even if a stress removal annealing is performed, the desired magnetism cannot be obtained. If the average grain particle diameter is too large, it is difficult to secure the adequate strength. More specifically, the average grain particle diameter of the steel sheet may be 15 to 20 μm. In one embodiment of the present invention, the grain particle diameter may be measured with respect to the rolling vertical plane (the TD plane) of the steel sheet. More specifically, it may be measured at the thickness ranging from ¼t to ¾t with respect to the total thickness t of the steel sheet. The grain particle diameter is determined by assuming a virtual circle with the same area as the particle diameter of the grain, and the particle diameter of that circle is used as the grain particle diameter. The average grain particle diameter may be measured by dividing the number of the grains within the area being measured by the area of the interest. In one embodiment of the present invention, the characteristics such as the average grain particle diameter, a yield strength, etc., without a separate description, refer to the characteristics before SRA.

After a stress removal annealing, the average grain particle diameter may be 30 to 300 μm. The stress removal annealing is a process of punching and laminating the steel sheets and then heat-treating them to remove any remaining stress in the steel sheets when manufacturing the motor from the non-oriented electrical steel sheet. Specifically, it may be performed at a temperature of 700 to 850° C. for 10 to 300 minutes.

As described above, the non-oriented electrical steel sheet according to one embodiment of the present invention has the excellent strength and the circumferential average magnetic flux density before the stress removal annealing, and the excellent circumferential average magnetic flux density and high-frequency iron loss after the stress removal annealing.

Specifically, Equation 1 below may be satisfied with respect to the strength.

$\begin{array}{cc}\mathrm{The}\mathrm{yield}\mathrm{strength}\left(\mathrm{MPa}\right)\ge 140+100\times \left[\mathrm{Si}\right]+35\times \left(\left[\mathrm{Al}\right]+\left[\mathrm{Mn}\right]\right)& \left[\mathrm{Equation}1\right]\end{array}$

(In Equation 1, [Si], [Al] and [Mn] represent the contents (weight %) of Si, Al and Mn, respectively.)

When Si, Al, and Mn are included in the steel sheet, the yield strength is improved. In one embodiment of the present invention, in addition to the addition of Si, Al, and Mn, the yield strength may be additionally improved by suppressing the formation of {110}<001> grains and controlling the average grain particle diameter.

More specifically, the yield strength may be greater than 400 MPa. More specifically, it may be 400 to 600 MPa. More specifically, it may be 450 to 550 MPa. The yield strength may be measured under a 0.2% offset condition by producing three KS-13A specimens and conducting a uniaxial tensile test.

The non-oriented electrical steel sheet according to one embodiment of the present invention may satisfy Equation 2.

$\begin{array}{cc}\mathrm{Circumferential}\mathrm{flux}\mathrm{density}\left(B50,\mathrm{Tesla}\right)\ge 1.880.1\times t-0.067\times \left[\mathrm{Si}\right]-0.0458\times \left[\mathrm{Al}\right]-0.022\times \left[\mathrm{Mn}\right]& \left[\mathrm{Equation}2\right]\end{array}$

(In Equation 2, [Si], [Al] and [Mn] represent the contents (in weight %) of Si, Al and Mn, respectively, and t represents the thickness (mm) of the steel sheet.)

When Si, Al, and Mn are included in the steel sheet, the magnetic flux density decreases. In one embodiment of the present invention, even if a certain amount of Si, Al, and Mn is added, the magnetic flux density may be improved by suppressing the formation of {110}<001> crystal grains and controlling the average grain particle diameter.

More specifically, the circumferential magnetic flux density may be greater than 1.62 T. More specifically, it may be 1.65 to 1.80 T. More specifically, it may be 1.68 to 1.75 T. The magnetic flux density B50 is the magnetic flux density induced in a magnetic field of 5000 A/m. The circumferential direction means a direction of a circumference of a circle, and in one embodiment of the present invention, a ring sample having an outer diameter of 100 mm and an inner diameter of 90 mm is manufactured by an electrical discharge machining, and 10 of them are stacked and then a copper wire is wound to measure the magnetism.

The non-oriented electrical steel sheet according to one embodiment of the present invention may satisfy Equation 3 after the stress removal annealing at a temperature of 700 to 850° C. for 10 to 300 minutes.

$\begin{array}{cc}\mathrm{Circumferential}\mathrm{flux}\mathrm{density}\left(B50,\mathrm{Tesla}\right)\ge 1.850.1\times t-0.067\times \left[\mathrm{Si}\right]-0.0458\times \left[\mathrm{Al}\right]-0.022\times \left[\mathrm{Mn}\right]& \left[\mathrm{Equation}3\right]\end{array}$ $\mathrm{Circumferential}\mathrm{iron}\mathrm{loss}\left(W10/400,W/\mathrm{kg}\right)\le 64000\times t2/(1311\times \left(\left[\mathrm{Si}\right]\left[\mathrm{Al}\right]0.5\times \left[\mathrm{Mn}\right]\right)$

(In Equation 3, [Si], [Al] and [Mn] represent the contents (weight %) of Si, Al and Mn, respectively, and t represents the thickness (mm) of the steel sheet.) When the stress removal annealing is performed, the magnetic flux density and the iron loss tend to decrease. In one embodiment of the present invention, by appropriately controlling the microstructure, the iron loss may be reduced as much as possible while suppressing the deterioration of the magnetic flux density

More specifically, the circumferential magnetic flux density B50 after the stress removal annealing may be 1.59 T or more. More specifically, the circumferential magnetic flux density B50 after the stress removal annealing may be 1.62 to 1.78 T. More specifically, the circumferential magnetic flux density B50 after the stress removal annealing may be 1.65 to 1.75 T.

After the stress removal annealing, the circumferential iron loss W10/400 may be less than 13.2 W/kg. More specifically, it may be 8.0 to 13.0 W/kg. More specifically, it may be 8.5 to 12.5 W/kg.

The non-oriented electrical steel sheet according to one embodiment of the present invention may have a thickness deviation of 2.0% or less. More specifically, the thickness deviation may be between 1.0 and 1.8%. The thickness deviation may be calculated by measuring the thickness at the points 15 mm from the central portion in the width direction and both sides of the width of the steel sheet and dividing the difference by the thickness at the central portion. The thickness of the steel sheet can be 0.10 to 0.35 mm.

The non-oriented electrical steel sheets may have an additional insulating film on top of the base steel sheet. Since the insulating film is widely known, a detailed description is omitted.

A method for manufacturing the non-oriented electrical steel sheet according to one embodiment of the present invention includes first hot-rolling of hot-rolling a slab including, in wt %, Si: 1.5 to 4%, Al: 0.1 to 2%, and Mn: 0.05 to 2%, with the remainder being Fe and unavoidable impurities to manufacture a first hot-rolled sheet; winding the first hot-rolled sheet; second hot-rolling of hot-rolling the first hot-rolled sheet at a temperature of 700 to 1000° C. and a rolling reduction ratio of 20 to 50% to manufacture a second hot-rolled sheet; cold-rolling the second hot-rolled sheet to manufacture a cold-rolled sheet; and annealing of the cold-rolled sheet at a temperature of 710 to 820° C.

Below, each step is explained in detail.

First, the slab is hot rolled to manufacture the first hot-rolled sheet. The reason for limiting the addition ratio of each composition within the slab is the same as the reason for limiting the composition of the non-oriented electrical steel sheet described above, so a repeated explanation is omitted. Since the composition of the slab does not substantially change during the manufacturing processes such as the first hot rolling, the second hot rolling, the cold rolling, the cold-rolled sheet annealing, and the stress removal annealing described later, the composition of the slab and the composition of the non-oriented electrical steel sheet are substantially the same.

The slab may be heated prior to the manufacturing of the first hot rolled sheet. Specifically, the slab is placed in a furnace and heated to 1100 to 1250° C. When being heated at the temperature exceeding 1250° C., the precipitates may be re-dissolved and finely precipitated after the hot rolling. If the temperature is too low, the deformation resistance during the hot rolling may be too high, making it difficult to be hot-rolled to an appropriate thickness.

The first hot rolling may include a rough rolling and a finish rolling. The rough rolling is manufacturing a bar with a thickness of 20 to 50 mm. The hot rolled sheet having the thickness of 1 to 3 mm is manufactured by rolling the finish rolled bar. The rough rolling and the finish rolling are different from the first hot rolling and the second hot rolling of the present invention in that they are performed continuously without the winding. The temperature of the steel sheet during the finish rolling process may be between 80° and 1000° C.

The thickness of the first hot-rolled sheet may be 1.8 to 2.5 mm. If the thickness of the first hot-rolled sheet is too thick, the rolling load in the subsequent second hot-rolling and cold-rolling increases, and a large amount of (111) recrystallized texture is formed, which may have a negative effect on the magnetism. It is difficult to further reduce the thickness with a single hot rolling, and even if the thickness is reduced, the thickness deviation in the width direction increases due to the bending phenomenon of the rolling roll, which affects the thickness deviation of the final manufactured steel sheet and causes shape defects. More specifically, the thickness of the first hot-rolled sheet may be 1.9 to 2.3 mm.

Next, the first hot-rolled sheet is wound. The temperature of the steel sheet during the winding may be 500 to 700° C. After the coiling step, the coiled coil may be cooled to 700° C. or less. By performing the second hot rolling after the cooling, there is an advantage in that an isotropy may be further improved.

The first hot-rolled sheet annealing may further include an annealing at a temperature of 700 to 1000° C. for 1 second to 10 hours after the winding. By suppressing the strong cold-rolled deformation band texture in the subsequent rolling through the first hot-rolled sheet annealing, the anisotropic Goss texture may be suppressed in the subsequent post-rolling annealing process, thereby achieving the isotropy. More specifically, the annealing may be performed for 10 to 600 seconds.

Next, in the second hot rolling, the first hot-rolled sheet is hot-rolled at the temperature of 700 to 1000° C. and the rolling reduction ratio of 20 to 50% to manufacture the second hot-rolled sheet. In one embodiment of the present invention, by adding the manufacturing of the second hot-rolled sheet, the rolling reduction ratio in the cold rolling may be reduced to improve the magnetism, and the width direction thickness deviation of the final product sheet may be reduced by correcting the shape. The reduction ratio may be calculated as (a sheet thickness before the rolling reduction—a sheet thickness after the rolling reduction)/the sheet thickness before the rolling reduction ×100.

If the steel sheet temperature is too low in the second hot-rolled sheet, a high pressing force is required, making it difficult to correct the width-thickness deviation, furthermore, the GOSS texture occurs, and the magnetism of the final product sheet rapidly deteriorates around 45° from the rolling direction, which may deteriorate the cylindrical characteristic. If the steel sheet temperature is too high, the rigidity of the roll itself may be weakened, causing a warpage to become severe, furthermore, the ductility of the hot-rolled sheet itself will be too weak, making it difficult to control, which may actually increase the thickness deviation. More specifically, the steel sheet temperature in the second hot rolling step may be 730 to 980° C. More specifically, it may be 750 to 950° C.

The rolling reduction ratio during the second hot rolling may be 20 to 50%. If the rolling reduction ratio is too small, the effect of performing the second hot rolling may be not sufficiently obtained. If the rolling reduction ratio is too high, the shape control is not easy and the thickness deviation may actually increase. More specifically, the rolling reduction ratio may be between 25 and 45%.

The thickness of the second hot-rolled sheet can be 1.2 to 1.8 mm. If the thickness of the second hot-rolled sheet is too thick, the rolling load in the subsequent cold rolling increases, and a large amount of (111) recrystallized texture is formed, which may have a negative effect on the magnetism. If the thickness is too low, the shape control is not easy and the thickness deviation may actually increase. More specifically, the thickness of the second hot-rolled sheet may be 1.3 to 1.7 mm.

After the second hot rolling step, a second hot-rolled sheet annealing may further be included, in which the second hot-rolled sheet is annealed at a temperature of 850 to 1150° C. for 3 to 600 seconds.

If the annealing temperature of the second hot-rolled sheet is too low, the texture does not grow or grows finely, so the effect of increasing the magnetic flux density is small. If the annealing temperature is too high, the magnetic properties may deteriorate, and the rolling workability may deteriorate due to the deformation of the sheet shape. More specifically, the temperature range may be 950 to 1125° C. The second hot-rolled sheet annealing may be performed to increase the orientation favorable to magnetism as needed, and may be omitted.

Next, the hot-rolled sheet is acid-cleansed and cold-rolled to the predetermined thickness. It may be applied differently depending on the thickness of the hot-rolled sheet, but it may be cold rolled to the final thickness of 0.10 to 0.35 mm by applying the rolling reduction ratio of 70 to 85%. To match the rolling reduction ratio, one cold rolling or two or more cold rolling with the intermediate annealing may be performed. More specifically, the cold rolling may be performed to be the thickness of 0.15 to 0.30 mm.

The cold rolled sheet that is cold rolled undergo the cold rolled sheet annealing.

The annealing temperature in the cold rolled sheet annealing is 710 to 820° C. If the annealing temperature is too low, un-recrystallized portions may remain, making it difficult to secure the sufficient magnetic flux density. If the annealing temperature is too high, the grains may become coarser, making it difficult to secure the adequate strength. More specifically, in the cold rolled sheet annealing, the annealing temperature may be 720 to 800° C., and the annealing time may be 10 to 60 seconds. More specifically, the annealing time may be between 20 and 45 seconds.

After the cold rolled sheet annealing, the un-recrystallized fraction may be 3 to 13 area %. When the non-recrystallization of the cold rolled sheet is properly formed, both magnetism and strength may be secured simultaneously. In one embodiment of the present invention, the un-recrystallized crystals may be distinguished through a lattice orientation distribution using an EBSD. More specifically, the un-recrystallized fraction can be 5 to 10 area %.

Next, the annealed cold rolled sheet is subjected to the stress removal annealing. After the cold rolled sheet annealing, processes of an insulating film formation, a punching, and a lamination may be performed. Since this is widely known, the detailed explanation thereof is omitted. During the punching process, the stress is generated in the non-oriented electrical steel sheet, which adversely affects the magnetism of the non-oriented electrical steel sheet. For the stator, which has the relatively important magnetic characteristic among the motor core, the magnetism of the steel sheet is improved by removing the stress remaining in the steel sheet through the stress removal annealing. On the other hand, for the rotor where the strength characteristic is relatively more important than the magnetism, the stress removal annealing may be omitted.

That is, even if the same steel sheet is used, it may be used for different purposes as the stator and the rotor depending on the presence or absence of the stress removal annealing.

The stress removal annealing may be performed at a temperature of 700 to 850° C. for 10 to 300 minutes.

The motor core according to one embodiment of the present invention includes the rotor formed by laminating the plurality of non-oriented electrical steel sheets and the stator formed by laminating a plurality of non-oriented electrical steel sheets. The rotor may be made of the laminated non-oriented electrical steel sheets prior to the aforementioned stress-relieving annealing, and the stator may be made of the laminated non-oriented electrical steel sheets after the aforementioned stress-relieving annealing.

As for the rotor, the characteristics are the same as those of the non-oriented electrical steel sheet before the SRA annealing, and as for the stator, the characteristics are the same as those of the non-oriented electrical steel sheet after the SRA annealing, so the detailed description thereof is omitted.

In one embodiment of the present invention, a rotor and a stator can be manufactured simultaneously using the same non-oriented electrical steel sheet, thereby further improving manufacturing efficiency.

The motor core may have an insulating film sandwiched between the steel sheets. Since the insulating film is widely known, the detailed description thereof is omitted.

The following examples illustrate the present invention in more detail. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

Embodiment 1

A slab was manufactured with a composition shown in Table 1 below. Except for component listed in Table 1, Ti, Nb, V, etc. were each controlled to less than 0.003 wt %, and the remainder is Fe.

The slab was heated to 1150° C. and first hot rolled to an inlet thickness listed in Table 2 below. A second hot rolling was performed under the conditions shown in Table 2, and a second hot-rolled sheet annealing was performed at 1100° C. for 60 seconds. The hot-rolled and annealed sheets were cold rolled to the thickness listed in Table 2 after an acid-cleansing, and annealed for 1 minute in 20% by volume of hydrogen and 80% by volume of nitrogen at the annealing temperature listed in Table 2 to measure the average magnetic characteristic around the circumference, the strength, and the thickness deviation. Afterwards, heat treatment (stress removal annealing) was performed at 800° C. for 1 hour in a nitrogen atmosphere, and the circumferential average magnetic properties were measured again. The circumference average magnetic characteristic was measured by manufacturing a ring sample with an outer diameter of 100 mm and an inner diameter of 90 mm by an electrical discharge machining, stacking 10 of them, and then winding a copper wire around them. The yield strength was measured by performing a uniaxial tensile test on three KS-13A specimens under a 0.2% offset condition. The thickness deviation was calculated by measuring the thickness at points 15 mm from the center and the width direction both sides of the coil and dividing the difference by the center thickness.

The GOSS fraction was measured (15 degree offset) by Electron Backscattering Diffraction (EBSD) of the steel sheet before the stress removal annealing.

TABLE 1
	Si	Al	Mn	P	C	S	N
component	(wt %)	(wt %)	(wt %)	(wt %)	(wt %)	(wt %)	(wt %)
A	3.2	0.7	0.4	0.05	0.003	0.002	0.0015
B	2.5	0.7	0.2	0.05	0.003	0.002	0.0015

	TABLE 2
	Cold
	rolled
	second hot rolling condition		Production	sheet
	Sheet	Inlet	Outlet	Rolling	Production	sheet	annealing
	temper-	thick-	thick-	reduction	sheet	thickness	temper-
	ature	ness	ness	ratio	thickness	deviation	ature
component	(° C.)	(mm)	(mm)	(%)	(mm)	(%)	(° C.)	remark
A	850	2.3	1.6	30.4	0.25	1.4	680	Comparative
								Material 1
A	850	2.3	1.6	30.4	0.25	1.4	700	Comparative
								Material 2
A	850	2.3	1.6	30.4	0.25	1.4	720	Inventive
								Material 1
A	850	2.3	1.6	30.4	0.25	1.4	800	Inventive
								Material 2
A	850	2.3	1.6	30.4	0.25	1.4	830	Comparative
								Material 3
A	850	2.3	1.6	30.4	0.25	1.4	850	Comparative
								Material 4
A	300	2.3	1.6	30.4	0.25	3	790	Comparative
								Material 5
A	500	2.3	1.6	30.4	0.25	2.6	790	Comparative
								Material 6
A	650	2.3	1.6	30.4	0.25	2.3	790	Comparative
								Material 7
A	750	2.3	1.6	30.4	0.25	1.5	790	Inventive
								Material 3
A	850	2.3	1.6	30.4	0.25	1.4	790	Inventive
								Material 4
A	930	2.3	1.7	26.1	0.25	1.4	790	Inventive
								Material 5
A	950	2.3	1.6	30.4	0.25	1.6	790	Inventive
								Material 6
A	950	2.3	1.3	43.5	0.25	1.7	790	Inventive
								Material 7
A	1050	2.3	1.6	30.4	0.25	2.3	790	Comparative
								Material 8
A	1040	2.3	1	56.5	0.25	3	790	Comparative
								Material 9
A	1020	2.3	1.9	17.4	0.25	2.2	790	Comparative
								Material
								10
A	600	2.3	1.9	17.4	0.25	1.9	790	Comparative
								Material
								11
A	600	2.3	1.3	43.5	0.25	1.8	790	Comparative
								Material
								12
A	750	2.3	2	13	0.25	1.8	790	Comparative
								Material
								13
A	750	2.3	1.7	26.1	0.25	1.7	790	Inventive
								Material 8
A	750	2.3	1.6	30.4	0.25	1.5	790	Inventive
								Material 9
A	750	2.3	1.4	39.1	0.25	1.5	790	Inventive
								Material 10
A	750	2.3	1.2	47.8	0.25	1.8	790	Inventive
								Material 11
A	750	2.3	1	56.5	0.25	2.5	790	Comparative
								Material
								14
A	800	2	1.3	35	0.2	1.5	790	Inventive
								Material 12
A	800	2	1.2	40	0.2	1.8	790	Inventive
								Material 13
A	800	2	0.9	55	0.2	2.5	790	Comparative
								Material
								15
B	650	2.3	1.6	30.4	0.27	2.3	790	Comparative
								Material
								16
B	750	2.3	1.6	30.4	0.27	1.5	790	Inventive
								Material 14
B	950	2.3	1.6	30.4	0.27	1.6	790	Inventive
								Material 15
B	1050	2.3	1.6	30.4	0.27	2.3	790	Comparative
								Material
								17
B	750	2.3	1.6	30.4	0.27	1.5	790	Inventive
								Material 16
B	750	2.3	1.2	47.8	0.27	1.8	790	Inventive
								Material 17
B	750	2.3	1	56.5	0.27	2.5	790	Comparative
								Material
								18
B	800	2.3	1.9	17.4	0.3	2.1	790	Comparative
								Material
								19
B	800	2.3	1.8	21.7	0.3	1.5	790	Inventive
								Material 18
B	800	2.3	1.2	47.8	0.3	1.8	790	Inventive
								Material 19
B	800	2.3	1	56.5	0.3	2.5	790	Comparative
								Material
								20

TABLE 3
Characteristic before stress	Characteristic after stress
removal annealing	removal annealing
		circumferential		circumferential
	grain	average		average	circumferential
	average	magnetic		magnetic	average
Goss	particle	flux	yield	flux	iron loss
fraction	diameter	density	strength	density	W10/400
(are a %)	(μm)	B50 (T)	(MPa)	B50 (T)	(W/kg)
0.8	Non-	1.58	750	1.66	10.2	Comparative
	recrystallization					Material 1
0.7	5	1.62	650	1.66	10.2	Comparative
						Material 2
0.8	12	1.65	590	1.66	10.2	Inventive
						Material 1
0.7	20	1.66	510	1.66	10.2	Inventive
						Material 2
0.8	30	1.66	480	1.66	10.2	Comparative
						Material 3
0.8	40	1.66	470	1.66	10.2	Comparative
						Material 4
23	18	1.64	513	1.59	11.1	Comparative
						Material 5
18	19	1.64	512	1.59	11	Comparative
						Material 6
15	18	1.64	515	1.59	10.9	Comparative
						Material 7
6	19	1.68	512	1.63	10.2	Inventive
						Material 3
0.9	21	1.68	513	1.63	10.2	Inventive
						Material 4
0.2	22	1.68	511	1.63	10.1	Inventive
						Material 5
0.2	24	1.68	513	1.63	10	Inventive
						Material 6
0.1	22	1.69	516	1.64	9.9	Inventive
						Material 7
0.1	19	1.68	511	1.63	9.8	Comparative
						Material 8
0.1	18	1.69	512	1.64	9.7	Comparative
						Material 9
0.2	19	1.66	513	1.61	10.7	Comparative
						Material 10
16	19	1.64	512	1.6	10.6	Comparative
						Material 11
15	18	1.64	530	1.6	10.6	Comparative
						Material 12
6	19	1.64	520	1.6	10.7	Comparative
						Material 13
4	19	1.68	520	1.63	10.2	Inventive
						Material 8
3	18	1.68	521	1.63	10.1	Inventive
						Material 9
4	18	1.68	516	1.63	10	Inventive
						Material 10
3	18	1.69	515	1.64	9.8	Inventive
						Material 11
4	19	1.7	513	1.65	9.7	Comparative
						Material 14
4	19	1.67	513	1.62	8.6	Inventive
						Material 12
5	18	1.68	511	1.63	8.5	Inventive
						Material 13
4	19	1.69	523	1.66	8.6	Comparative
						Material 15
16	18	1.71	432	1.66	12.3	Comparative
						Material 16
5	19	1.73	430	1.68	11.6	Inventive
						Material 14
4	18	1.74	435	1.69	11.7	Inventive
						Material 15
0.2	19	1.75	433	1.7	11.7	Comparative
						Material 17
5	18	1.73	431	1.68	11.6	Inventive
						Material 16
4	19	1.73	432	1.68	10.4	Inventive
						Material 17
4	18	1.74	436	1.69	10	Comparative
						Material 18
5	21	1.69	432	1.64	14.3	Comparative
						Material 19
5	19	1.73	431	1.68	13.1	Inventive
						Material 18
4	18	1.74	432	1.69	13.1	Inventive
						Material 19
4	19	1.75	425	1.7	13	Comparative
						Material 20

As shown in Table 1 to Table 3, when the temperature and the rolling reduction ratio were appropriately controlled during the secondary hot rolling and the temperature was appropriately controlled during the cold-rolled sheet annealing, the {110}<001> texture structure was formed less and the thickness deviation was appropriate. In addition, the magnetic flux density and yield strength before the SRA were excellent, and the magnetic flux density and iron loss after the—SRA were excellent.

On the other hand, when the cold rolled sheet annealing temperature was low, the magnetic flux density before the SRA was inferior. When the cold rolled sheet annealing temperature was high, the yield strength before the SRA was inferior. When the temperature was low during the second hot rolling, the magnetic flux density before and after SRA was inferior. When the temperature was high during the second hot rolling, the thickness deviation increased. When the secondary hot rolling reduction ratio was small, the magnetism before and after SRA was inferior. When the secondary hot rolling reduction ratio was large, the thickness deviation increased.

Embodiment 2

A slab was manufactured with a composition shown in Table 4 below. C, S, N, Ti, Nb, V, etc., other than those listed in Table 4 were each controlled to less than 0.003 wt %, and the remainder was Fe.

The slab was heated to 1130° C. and subjected to the first hot rolling to 2.3 mm. Afterwards, the wound coil was reheated to 730° C. using an induction heating, and then a second hot rolling was performed to 1.6 mm, and the hot-rolled sheet annealing was performed at 1050° C. The hot-rolled and annealed sheet was cold-rolled to 0.25 mm after the acid cleansing, and cold-rolled at 790° C. in 20% by a volume of hydrogen and 80% by a volume of nitrogen for 1 minute was annealed, followed by a heat treatment (a stress removal annealing) in a nitrogen atmosphere at 800° C. for 1 hour, and the magnetism and yield strength before and after SRA were measured in the same manner as in Embodiment 1.

	TABLE 4
	Characteristic before stress
	removal annealing	Characteristic after stress
	ircumfer-	removal annealing
				ential	ircumfer-
				average	ential	ircumfer-
	grain		Yield	magnetic	average	ential
	average		strength	flux	magnetic	average
component	particle	Goss	(MPa)	density	flux	iron
(wt %)	diameter	fraction	B50	B50	density	loss
Si	Al	Mn	(μm)	(area %)	(T)	(T)	(W/kg)	10/400
1.5	0.5	0.2	20	3	330	1.79	1.76	12.5	Inventive
									Material 1
4.2	0.5	0.2	18	Sheet fracture during cold rolling	Comparative
									Material 1
3.2	0.05	0.2	8	4	501	1.66	1.63	12.5	Comparative
									Material 2
3.2	1.2	0.2	20	3	520	1.65	1.62	9.5	Inventive
									Material 2
3.2	1.8	0.2	18	4	550	1.62	1.59	9.5	Inventive
									Material 3
3.2	2.2	0.2	20	5	520	1.59	1.56	11.2	Comparative
									Material 3
3.2	0.7	1	19	4	540	1.65	1.62	9.8	Inventive
									Material 4
3.2	0.7	1.8	18	3	560	1.64	1.61	9.5	Inventive
									Material 5
3.2	0.7	2.5	20	4	550	1.61	1.58	10.5	Comparative
									Material 4
2	0.3	1.9	21	3	440	1.72	1.69	10.5	Inventive
									Material 6
2.3	0.6	1.6	19	4	470	1.71	1.68	10.5	Inventive
									Material 7
2.7	0.9	1.3	20	3	515	1.67	1.64	10	Inventive
									Material 8
3.5	1.7	0.9	21	2	600	1.6	1.57	9.2	Inventive
									Material 9
3.7	1.5	0.7	19	3	620	1.6	1.57	9.1	Inventive
									Material 10
1.3	1.6	0.5	20	4	310	1.76	1.73	12.3	Comparative
									Material 5
3.5	1.6	0.03	4	3	550	1.61	1.58	10.4	Comparative
									Material 6

As shown in Table 4, when the Si, Al, and Mn contents were appropriately controlled, the magnetic flux density and yield strength before-SRA were excellent, and the magnetic flux density and iron loss after the SRA were excellent.

On the other hand, when the Si, Al, and Mn contents were too low or too high, it was confirmed that a sheet fracture occurred, the magnetism before/after the SRA was poor, or the yield strength before the SRA was poor.

The present invention may be embodied in many different forms, and should not be construed as being limited to the disclosed embodiments. In addition, it will be understood by those skilled in the art that various changes in form and details may be made thereto without departing from the technical spirit and essential features of the present invention. Therefore, it is to be understood that the above-described exemplary embodiments are for illustrative purposes only and the scope of the present invention is not limited thereto.

Claims

1. A non-oriented electrical steel sheet comprising:

Si: 1.5 to 4%, Al: 0.1 to 2%, and Mn: 0.05 to 2% by a weight, with a remainder being Fe and inevitable impurities,

wherein an area fraction of grains having an orientation within 15° from {110}<001> is 10% or less.

2. The non-oriented electrical steel sheet of claim 1, further comprising:

at least one of Cr: 0.5 wt % or less (excluding 0%), Cu: 0.2 wt % or less (excluding 0%), P: 0.1 wt % or less (excluding 0%), Sn: 0.06 wt % or less (excluding 0%), and Sb: 0.06 wt % or less (excluding 0%).

3. The non-oriented electrical steel sheet of claim 1, further comprising:

0.005 wt % or less (excluding 0%) of one or more of C, N, S, Ti, Nb, and V.

4. The non-oriented electrical steel sheet of claim 1, wherein:

an average grain particle diameter is 10 to 25 μm.

5. The non-oriented electrical steel sheet of claim 1, wherein: yield ⁢ strength ( MPa ) ≥ 140 + 1 ⁢ 0 ⁢ 0 × [ Si ] + 3 ⁢ 5 × ( [ Al ] + [ Mn ] ) [ Equation ⁢ 1 ]

the non-oriented electrical steel sheet satisfies Equation 1,

(In Formula 1, [Si], [Al] and [Mn] represent contents (weight %) of Si, Al and Mn, respectively).

6. The non-oriented electrical steel sheet of claim 1, wherein: circumferential ⁢ flux ⁢ density ⁢ ( B ⁢ 50, Tesla ) ≥ 1.88 0.1 × t - 0. 0 ⁢ 6 ⁢ 7 × [ Si ] - 0.0458 × [ Al ] - 0. 0 ⁢ 2 ⁢ 2 × [ Mn ] [ Equation ⁢ 2 ]

the non-oriented electrical steel sheet satisfies Equation 2,

(In Equation 2, [Si], [Al] and [Mn] represent contents (in weight %) of Si, Al and Mn, respectively, and t represents a thickness (in mm) of the steel sheet).

7. The non-oriented electrical steel sheet of claim 1, wherein: circumferential ⁢ flux ⁢ density ⁢ ( B ⁢ 50, Tesla ) ≥ 1.85 0.1 × t - 0. 0 ⁢ 6 ⁢ 7 × [ Si ] - 0.0458 × [ Al ] - 0. 0 ⁢ 2 ⁢ 2 × [ Mn ] [ Equation ⁢ 3 ]

the non-oriented electrical steel sheet satisfies Equation 3 after a stress removal annealing at a temperature of 700 to 850° C. for 10 to 300 minutes,

circumferential iron loss (W10/400, W/kg)≤6 4000×12/(13 11×([Si][Al]0.5×[Mn])

(In Equation 3, [Si], [Al] and [Mn] represent the contents (in weight %) of Si, Al and Mn, respectively, and t represents the thickness (in mm) of the steel sheet).

8-15. (canceled)