NON-ORIENTED ELECTRICAL STEEL SHEET AND MANUFACTURING METHOD THEREFOR

Abstract

A non-oriented electrical steel sheet according to one embodiment of the present disclosure comprises 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfies formula 1 below, wherein a mean size of oxides in the precipitates is larger than a mean size of non-oxides. [ Si ] 1.8 + 1.3 × [ Al ] > 3.7 × [ Mn ] × [ Mn ] Formula   ( 1 ) (Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

Description

TECHNICAL FIELD

The present disclosure relates to a non-oriented electrical steel sheet and a manufacturing method therefor.

BACKGROUND ART

The non-oriented electrical steel sheets are used as core materials in rotational apparatus such as motors, generators, and stationary apparatus such as small transformers, playing an important role in determining the energy efficiency of electrical apparatus. Accordingly, the recent demands for energy saving and compactness of the electric apparatus emphasize improved efficiency of electrical apparatuses, subsequently demanding enhanced properties of the non-oriented electrical steel sheets. Typical properties of electrical steel sheet are core loss and flux density. Lower core loss and higher flux density are more desired, because the lower core loss can reduce the energy loss by heat, and the higher flux density can induce greater magnetic field with the same amount of energy, when electricity is applied to the iron core to induce a magnetic field. Accordingly, it is necessary to develop a technique to manufacture a non-oriented electrical steel sheet with low core loss and high flux density in order to cope with the increasing demands for energy saving, environmentally friendly products.

Representative examples of the method for improving core loss, which is one of the magnetic properties of non-oriented electrical steel sheets, include a method of decreasing the thickness and a method of adding elements with high specific resistivity such as Si and Al. However, the thickness is determined by the characteristics of the product used, and decreased thickness leads to reduced productivity and increased cost. The method of decreasing core loss by increasing the electrical resistivity of a general material by adding an alloy element having a high resistivity such as Si, Al, Mn or the like can reduce core loss. However, this method is contradictory in that, while it can reduce the core loss by adding the alloying element, it inevitably results in reduction of the flux density due to the decreased saturation flux density. In addition, when the Si content is 4% or more, the machinability is deteriorated, thus inhibiting the cold rolling and decreasing the productivity. Increased Al and Mn contents can also contribute to the deteriorated rolling property, in which case the hardness is increased and the machinability is decreased. Accordingly, there is a need for technology to not only improve the magnetic properties, but also reduce cost by way of adding these additive elements in most appropriate proportion.

Meanwhile, there are unavoidable impurities added in the steel such as C, S, N, O, Ti, or the like, and these are combined with the additive elements such as Fe, SI, Al, Mn, or the like, to form fine precipitates that suppress the grain growth and interfere with the migration of the magnetic domains, thus deteriorating the magnetic properties. Such precipitates in steel include carbide, nitride, sulfide, oxide, and the like. These appear individually or in combination. These fine compounds are classified as dross or precipitates according to their size and cause of formation, and it is believed that the dross more than 100 nm in size does not significantly affect the grain growth, while the precipitates that are below 100 nm in size particularly inhibit the grain growth.

When the precipitates are small in size, the quantity of precipitates increases and contribute to suppressing the migration of the magnetic domains or grain growth, and accordingly, it is important to increase the size of the precipitates or to make a composite of two or more precipitates.

DISCLOSURE

Technical Problem

The present invention has been made in an effort to provide a non-oriented electrical steel sheet with improved magnetic properties by facilitating migration of magnetic domains during grain growth and magnetization by way of limiting the amounts of added alloy elements and allowing the precipitates to grow to a larger size, and a manufacturing method therefor.

Technical Solution

The non-oriented electrical steel sheet according to one embodiment of the present disclosure includes 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 or of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfies formula 1 below, wherein a mean size of oxides in the precipitates is larger than a mean size of non-oxides.

$\begin{array}{cc}\frac{\left[\mathrm{Si}\right]}{1.8}+1.3\times \left[\mathrm{Al}\right]>3.7\times \left[\mathrm{Mn}\right]\times \left[\mathrm{Mn}\right]& \mathrm{Formula}\left(1\right)\end{array}$

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

The number of oxides in the precipitates may be larger than that of non-oxides.

0.01 to 0.2 wt % of Sn and Sb may be further included, individually or in combination, respectively.

The number of FeO in the precipitates or precipitates containing FeO may be 40% or more.

The mean particle size may be between 50 and 180 μm.

A manufacturing method for a non-oriented electrical steel sheet according to one embodiment of the present disclosure includes steps of: heating a slab including 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfying formula 1 below, and hot rolling the slab to prepare a hot rolled sheet; winding and cooling the hot rolled sheet; annealing and cooling the hot rolled sheet; cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet; and final annealing and then cooling the cold rolled sheet, in which in the step of winding and cooling the hot rolled sheet includes cooling at 600° C. or higher for 30 minutes or more, the step of annealing and then cooling the hot rolled sheet includes cooling at 600° C. or higher for 5 seconds or more, and the step of final annealing and then cooling the cold rolled sheet includes cooling at 600° C. or higher for 5 seconds or more.

$\begin{array}{cc}\frac{\left[\mathrm{Si}\right]}{1.8}+1.3\times \left[\mathrm{Al}\right]>3.7\times \left[\mathrm{Mn}\right]\times \left[\mathrm{Mn}\right]& \mathrm{Formula}\left(1\right)\end{array}$

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

The slab may further include 0.01 to 0.2 wt % of Sn and Sb, individually or in combination.

The step of preparing a hot rolled sheet may include heating the slab at 1200° C. or less.

The temperature of the winding may be 600 to 800° C. in the step of winding and cooling the hot rolled sheet.

The temperature of the hot rolled sheet annealing may be 850 to 1150° C. in the step of annealing and then cooling the hot rolled sheet.

The step of cold rolling the hot rolled annealing sheet to prepare cold rolled sheet may include cold rolling the sheet to a thickness of 0.1 to 0.7 mm.

In the step of cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet, the cold rolling may include primary cold rolling, intermediate annealing, and secondary cold rolling.

For annealing of the step of final annealing and then cooling the cold rolled sheet, the cracking temperature of the annealing may be 850 to 1100° C.

A mean size of oxides in the precipitates of the manufactured electrical steel sheets may be larger than a mean size of non-oxides.

The number of oxides in the precipitates may be larger than that of non-oxides.

The number of FeO in the precipitates or precipitates containing FeO may be 40% or more.

The mean particle size may be between 50 and 180 μm.

Advantageous Effects

The non-oriented electrical steel sheet according to one embodiment of the present disclosure can improve the magnetic properties by allowing the precipitates to grow to a larger size, thereby facilitating grain growth and migration of magnetic domains during magnetization.

MODE FOR INVENTION

The terms “first”, “second” and “third” as used herein are intended to describe various parts, components, regions, layers and/or sections, but not construed as limiting. These terms are merely used to distinguish any parts, components, regions, layers and/or sections from another parts, components, regions, layers and/or sections. Accordingly, a first part, component, region, layer or section to be described below may be referred to as a second part, component, region, layer or section without departing from the scope of the present disclosure.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the present disclosure. The singular forms used herein include plural forms as long as the phrases do not expressly mean to the contrary. As used herein, the meaning of “comprising” specifies specific features, regions, integers, steps, operations, elements and/or components, and does not exclude the presence or the addition of other features, regions, integers, steps, operations, elements and/or components.

When a portion is referred to as being “above” or “on” another portion, it may be directly on another portion or may be accompanied by yet another portion disposed in between. In contrast, when a portion is referred to as being “directly above” another portion, no other portion is interposed in between.

Unless otherwise defined, all terms including technical and scientific terms used herein have the same meaning as commonly understood by those with ordinary knowledge in the art to which this invention belongs. Terms defined in the general dictionaries are not to be construed as the ideal or very formal meanings unless they are further interpreted and defined as having a meaning consistent with the relevant technical literature and the present disclosure.

In addition, unless otherwise stated, % means wt %, and 1 ppm is 0.0001 wt %.

Hereinafter, preferred embodiments of the present disclosure will be described in detail to help those with ordinary knowledge in the art easily achieve the present disclosure. 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.

The non-oriented electrical steel sheet according to one embodiment of the present disclosure includes 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfies formula 1 below, wherein a mean size of oxides in the precipitates is larger than a mean size of non-oxides.

$\begin{array}{cc}\frac{\left[\mathrm{Si}\right]}{1.8}+1.3\times \left[\mathrm{Al}\right]>3.7\times \left[\mathrm{Mn}\right]\times \left[\mathrm{Mn}\right]& \mathrm{Formula}\left(1\right)\end{array}$

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

In one embodiment of the present invention, among the components of the non-oriented electrical steel sheet, particular components such as Si, Al and Mn were precisely regulated to produce precipitates as large as possible, and also to cause the precipitates to precipitate in a larger size by being in combination with each other, rather than existing individually. In addition, a mean size of oxides in the precipitates is formed larger than a mean size of the non-oxide, to improve the magnetic properties.

In one embodiment of the present disclosure, the elements being added are Si, Mn, Al, P or, if necessary, Sn and Sb, and Fe in the base material. Other added elements are O, C, N, S, and so on, which need to be managed at a low content. Among these, element such as N or C forms nitrides and carbides with other elements, element such as Al, Mn, Si, Fe, and the like forms oxides with O, and element such as Mn and Cu form sulfides, and the like with S, all of which are formed individually or in combination.

In one embodiment of the present disclosure, the precipitates were coarsened, and in particular, the precipitates were precipitated in combination, rather than alone, to further facilitate the growth. Among them, oxide is more easily coarsened, since coarsening the oxide is possible without adding additive elements. As a result, it was confirmed that the magnetic properties of the electrical steel sheet were improved.

In one embodiment of the present invention, the oxide was 50% or more of the total number of precipitates in the precipitates, and in the oxides, FeO in particular accounted for more than 40%. In particular, the influence of oxides greatly contributed to the formation of the complex precipitates. These oxides are considered to be O remaining as oxides in steels after O was lowered in the steelmaking process, or precipitated after annealing. Sulfide was precipitated in a large amount when reheating slabs and cooling after hot rolling, which appeared as CuS, MnS or complex precipitates of these. However, the oxide has more complex precipitates of oxides such as FeO, Al₂O₂, and than those of sulfides, and the bonding of the oxide with the nitride and the carbide is relatively less.

In one embodiment of the present disclosure, the oxides in the precipitates were present individually or in combination, and observed in a mean size of 15 nm to 70 nm and an average quantity of 10,000 to 400,000 precipitates per 1 mm². In addition, the non-oxides in the precipitates were present individually or in combination and observed in a mean size of 10 nm to 50 nm and an average quantity of 5,000 to 200,000 precipitates per 1 mm².

Since the oxides are formed in the precipitates with a larger mean size than that of the non-oxides, the grain growth is facilitated, and specifically, the mean particle size of 50 to 180 μm can be achieved. The ‘particle size’ as used herein refers to the particle size measured by the intercept method commonly used in the field of the electrical steel sheets.

The reason for limiting the components of the non-oriented electrical steel sheet will be described below.

Si: 1.0-4.0 wt %

Silicon (Si) is a major additive element for it is the component that increases the specific resistivity of steel to lower the core loss, that is, the eddy current loss. Si is an element that easily forms oxide. When Si is added in an insufficient amount, it is difficult to obtain low core loss properties, and Si added in an excessive amount can hinder cold rolling. Accordingly, Si may be limited to 1.0-4.0 wt %.

Mn: 0.1-1.0 wt %

Like Si or Al, manganese (Mn) has an effect of increasing specific resistivity that lowers core loss, and accordingly, Mn is added in an amount of 0.1 wt % or more to improve core loss. However, increased amount of Mn causes reduced saturation flux density which in turn results in reduced flux density. In addition, Mn is combined with S to form the fine MnS precipitates, which inhibit grain growth and hinder the magnetic domain wall movement, thus increasing core loss, or more particularly, increasing the hysteresis loss. Accordingly, Mn is added in an amount of 1.0 wt % or less.

Al: 0.15-1.5 wt %

Aluminum (Al) is an element that is inevitably added for steel deoxidation in a steelmaking process, and because Al is a major element that increases the specific resistivity, in many cases, Al is added to lower the core loss. However, when added, Al also serves to reduce the saturation flux density. In addition, the presence of considerably insufficient Al content can cause formation of fine AlN, which may inhibit the grain growth and result in deteriorated magnetic properties. In addition, the presence of too much Al can serve as a cause of decreased flux density. Accordingly, the content of Al may be limited to 0.15-1.5 wt %.

P: 0.2 wt % or Less

Phosphorus (P) increases the specific resistivity to decrease the core loss, and segregated on the grain boundaries to inhibit the formation of texture detrimental to the magnetic properties, while forming a favorable texture {100}. However, if added in an overly large amount, P can deteriorate rolling property. Accordingly, P may be limited to 0.2 wt % or less.

C: 0.005 wt % or Less

When added in an overly large amount, carbon (C) increases the austenite region, increases the phase transformation interval, and inhibits the grain growth of ferrite during annealing, thus resulting in increased core loss. Further, C also binds with Ti, or the like to form carbides that deteriorate magnetic properties, and it increases core loss by magnetism aging as it is processed and used in the final product such as electrical product. Accordingly, C may be limited to 0.005 wt % or less.

N: 0.005 wt % or Less

Nitrogen (N) is an element detrimental to the magnetic properties, because N binds strongly with Al, Ti, or the like to form nitrides to inhibit the grain growth, and so on. Accordingly, content of N is preferably maintained at a low level and may be limited to 0.005 wt % or less.

S: 0.001-0.006 wt %

Sulfur (S) is an element that forms sulfides such as MnS, CuS and (Cu, Mn)S, which are harmful to magnetic properties. Accordingly, the content of S is preferably maintained as low as possible. However, a considerably insufficient amount of S can be rather disadvantageous to the texture formation and result in deteriorated magnetic properties. In addition, the presence of overly large amount of S can cause deteriorated magnetic properties due to increasing presence of fine sulfides. Accordingly, S may be limited to 0.001-0.006 wt %.

Ti: 0.005 wt % or Less

Titanium (Ti) forms fine carbides and nitrides to inhibit the grain growth. An increased content of Ti causes increased presence of fine carbides and nitrides which deteriorate the texture and result in deteriorated magnetic properties. Accordingly, Ti may be limited to 0.005 wt % or less.

O: 0.005 wt % or Less

Content of oxygen (O) may be maintained as low as possible because O forms various oxides that will inhibit grain growth. Accordingly, O may be limited to 0.005 wt % or less.

Sn, Sb: 0.01 to 0.2 wt %

As the segregating elements in the grain boundaries, tin (Sn) and antimony (Sb) suppress the spreading of nitrogen through the grain boundaries and suppress the {111} texture which is detrimental to the magnetic properties. Sn and Sb are added to increase favorable {100} texture and improve the magnetic properties. If Sn or Sb, either individually or in combination, is present in an overly large amount, it may inhibit the grain growth, which will deteriorate the magnetic properties and the rolling properties. Accordingly, when Sn or Sb is added, the content of Sn and Sb, either individually or in combination, may be limited to 0.01 to 0.2 wt %.

In particular, in one embodiment of the present disclosure, the amounts of Si, Mn, and Al are regulated to satisfy formula 1 below in order to ensure that there is not a high amount of Mn, but a high amount of Si, and with the presence of a substantial amount of Al, to suppress AlN, and the like.

$\begin{array}{cc}\frac{\left[\mathrm{Si}\right]}{1.8}+1.3\times \left[\mathrm{Al}\right]>3.7\times \left[\mathrm{Mn}\right]\times \left[\mathrm{Mn}\right]& \mathrm{Formula}\left(1\right)\end{array}$

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

A manufacturing method for a non-oriented electrical steel sheet according to one embodiment of the present disclosure includes steps of: heating a slab including 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfying formula 1 below, and hot rolling the slab to prepare a hot rolled sheet; winding and cooling the hot rolled sheet; annealing and cooling the hot rolled sheet; cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet; and final annealing and then cooling the cold rolled sheet, in which in the step of winding and cooling the hot rolled sheet includes cooling at 600° C. or higher for 30 minutes or more, the step of annealing and then cooling the hot rolled sheet includes cooling at 600° C. or higher for 5 seconds or more, and the step of final annealing and then cooling the cold rolled sheet includes cooling at 600° C. or higher for 5 seconds or more.

In one embodiment of the present disclosure, after preparing the hot rolled sheet, after annealing the hot rolled sheet, and after annealing the cold rolled sheet, cooling is performed slowly to allow time for the precipitates to grow, thereby improving the magnetic properties.

Hereinafter, the process will be described step by step.

First, the slab is heated and then hot rolled to prepare a hot rolled sheet. The reason for limiting the addition ratio of each composition is the same as the reason for limiting the addition ratio of the non-oriented electrical steel sheet described above. Since the composition of the slab does not substantially change during hot rolling, hot rolled sheet annealing, cold rolling, and final annealing, and the like, the composition of the slab is substantially the same as that of the non-oriented electrical steel sheet.

The slab may be charged to a furnace and heated at 1200° C. or less. When heated at an overly high heating temperature, precipitates such as AlN and MnS present in the slab can be re-solved and then formed into fine precipitates during hot rolling, which may inhibit the grain growth and deteriorate the magnetic properties. More specifically, the slab may be heated at 1050° C. to 1200° C.

The heated slab is hot rolled to 1.4 mm to 3 mm to prepare a hot rolled sheet. During hot rolling, the finishing rolling of the finishing milling is completed in the ferrite phase, with the final reduction rate of 20% or less for the purpose of sheet shape control.

Next, the hot rolled sheet is wound and then cooled. The hot rolled sheet is wound at a temperature of 600° C. to 800° C. and then cooled in air or in a separate furnace. The temperature for cooling is allowed to be maintained at 600° C. or higher for at least 30 minutes or more. If the temperature is too low or the time is kept short, growth of precipitates may be difficult and fine precipitates may appear. More specifically, the temperature may be maintained between 600 and 800° C. for 30 minutes to 3 hours.

Next, the hot rolled sheet is annealed and cooled. The hot rolled sheet is annealed to improve the magnetic properties, and hot rolled sheet annealing temperature is 850 to 1150° C. If the hot rolled sheet annealing temperature is too low, the grain growth may be insufficient. If the hot rolled sheet annealing temperature is too high, there may be excessive grain growth, which may cause excessive surface defects.

For cooling after hot rolled sheet annealing, cooling is not quenched, but maintained at 600° C. or higher for 5 seconds or more. If the temperature is too low or the sustaining time is short during cooling, it may be difficult to coarsen precipitates and the sheet may be bent. More specifically, the cooling temperature may be from 600 to 800 ° C., and may be maintained for 5 to 30 seconds.

The hot rolled sheet may be pickled after annealing.

Next, the hot rolled annealing sheet is cold rolled to prepare a cold rolled sheet. The cold rolling may finally roll to a thickness of 0.1 mm to 0.7 mm and may include primary cold rolling, intermediate annealing and secondary cold rolling as necessary, with the final reduction ratio being in the range of 50 to 95%.

Next, the cold rolled sheet is finally annealed and then cooled. During annealing in a cold rolled sheet annealing process, the cracking temperature of the annealing is 850 to 1100° C. When the cold rolled sheet annealing temperature is 850° C. or lower, insufficient grain growth results in increased {111} texture that is detrimental to the magnetic properties. When the cold rolled sheet annealing temperature is 1100° C. or higher, the excessive grain growth can adversely affect the magnetic properties. Accordingly, the cracking temperature of the cold rolled sheet is set at 850 to 1100° C.

For cooling after cold rolled sheet annealing, cooling is not quenched, but maintained at 600° C. or higher for 5 seconds or more. If the temperature is too low or the sustaining time is short during cooling, the fine precipitates can individually appear. More specifically, the cooling temperature may be from 600 to 800° C., and may be maintained for 5 to 30 seconds.

The annealing sheet is shipped to customer after insulation coating treatment. The insulating coating may be treated with an organic, inorganic or organic-inorganic composite coating, or may be treated with other coating agents capable of insulation. The customer may use the steel sheet as it is after processing it.

Hereinafter, the present disclosure is explained in more detail with reference to Examples. However, the Examples are described merely to illustrate the present disclosure, and the present disclosure is not limited thereto.

EXAMPLE 1

A steel ingot was prepared with the compositions shown in Tables 1 and 2 below, from which inventive steels A1 to A7 including Si, Al and Mn contents (in wt %) satisfying formula 1, and comparative steels A8 to A12 including Si, Al and Mn contents (in wt %) not satisfying formula 1 were melted by vacuum melting.

Vacuum melt steels A1 to A7 were prepared by including Si, Al, and Mn in the range of the present disclosure, after which each steel ingot was heated at 1120° C., hot rolled to a thickness of 2.2 mm, and wound, and then slowly cooled in the air and wound as shown in Table 2. The cooled hot rolled steel sheet was then annealed in a nitrogen atmosphere for 5 minutes, followed by slow cooling at a temperature of 600° C. or higher in an atmosphere in which nitrogen and oxygen were mixed, and then finally quenched by spraying water. The annealed hot rolled sheets were pickled and then cold rolled to 0.35 mm thickness and for the final annealing, the cold rolled sheet was annealed for 2 minutes in a 30% hydrogen and 70% nitrogen mixed atmosphere. The cooling bed was cooled at atmosphere of the 40% hydrogen and nitrogen. The final annealing sheet was examined for the size and quantity of oxides, sulfides, carbides, nitrides and their complex precipitates for each specimen and the grains and magnetic properties were measured and listed in Table 3 below.

As a method to analyze the size, type and distribution of precipitates, carbon replica extracted from specimen was observed by TEM and analyzed by EDS. The TEM observation was carried out by analyzing the type of precipitates through the EDS spectrum on the randomly selected areas without bias.

Core loss (W_15/50) was measured as the average loss (W/kg), in the rolling direction and the perpendicular direction to the rolling direction, when flux density of 1.5 Tesla was induced at 50 Hz frequency.

The flux density (B₅₀) was measured by the magnitude of flux density (Tesla) induced when a magnetic field of 5000 Nm was applied.

TABLE 1
Item	C:	Si:	Al:	Mn:	P	S	N	Ti	Sn	Sb
A1	0.0025	1.56	0.25	0.42	0.031	0.0024	0.0014	0.0002	0.026	0.012
A2	0.0028	2.64	0.22	0.4	0.036	0.0021	0.0021	0.0015	0.019	0
A3	0.0025	2.82	0.82	0.8	0.045	0.0028	0.0014	0.0017	0	0
A4	0.0022	2.95	0.78	0.62	0.055	0.0021	0.0012	0.0016	0	0
A5	0.0025	2.82	1.3	0.45	0.032	0.0015	0.0025	0.0011	0	0.031
A6	0.0028	2.91	0.32	0.52	0.031	0.0018	0.0021	0.0011	0.024	0.021
A7	0.0022	3.3	0.25	0.4	0.035	0.0032	0.0026	0.0015	0.036	0.015
A8	0.0021	0.52	0.002	0.45	0.031	0.0024	0.0014	0.0002	0.026	0.012
A9	0.0026	1.43	0.25	0.62	0.045	0.0001	0.0015	0.0019	0.025	0.031
A10	0.0023	2.24	0.12	0.72	0.055	0.0032	0.0018	0.0021	0	0.019
A11	0.0027	2.51	0.45	0.9	0.023	0.0035	0.0021	0.0021	0.035	0
A12	0.0029	2.96	0.74	1.3	0.019	0.0019	0.0019	0.0025	0.043	0

	TABLE 2
	Hot rolled sheet anneal	Cold rolled sheet anneal
	Sustaining		Sustaining
	Cooling after winding	Anneal	time (sec) at	Anneal	time (sec) at
Steel	Satisfy	Temp.	Time	temp.	600° C. or	temp.	600° C. or
grade	Formula 1	(° C.)	(min)	(° C.)	higher	(° C.)	higher	Remarks
A1	◯	700	60	900	10	900	8	Inventive steel 1
A2	◯	650	60	1000	12	1030	15	Inventive steel 2
A3	◯	650	60	1000	10	1030	15	Inventive steel 3
A4	◯	650	60	1000	7	1030	15	Inventive steel 4
A5	◯	650	60	1000	7	1050	15	Inventive steel 5
A6	◯	650	60	1000	10	1050	15	Inventive steel 6
A7	◯	650	60	1000	10	1050	15	Inventive steel 7
A8	X	700	60	900	10	900	8	Comp. steel 1
A9	X	700	60	900	12	900	8	Comp. steel 2
A10	X	650	60	1000	12	1050	15	Comp. steel 3
A11	X	650	60	1000	12	1050	15	Comp. steel 4
A12	X	650	60	1000	12	1050	15	Comp. steel 5

	TABLE 3
		Oxide in		Non-oxide in	Core	Flux
	Particle	precipitate	FeO ratio	precipitate	loss	density
Steel	size	Size	Ratio	(%) in	Size	Ratio	(W15/50)	B50
grade	(μm)	(nm)	(%)	precipitate	(nm)	(%)	W/kg	Tesla	Remarks
A1	60	45	55	45	40	45	3.72	1.78	Inventive steel 1
A2	80	48	60	50	43	40	2.21	1.73	Inventive steel 2
A3	87	60	62	54	45	38	2.12	1.71	Inventive steel 3
A4	120	65	65	48	46	35	1.85	1.69	Inventive steel 4
A5	120	58	70	55	45	30	1.92	1.68	Inventive steel 5
A6	110	45	72	62	40	28	1.95	169	Inventive steel 6
A7	160	48	70	55	35	30	1.93	1.68	Inventive steel 7
A8	40	32	35	28	38	65	6.43	1.71	Comp. steel 1
A9	45	33	40	35	38	60	4.52	1.69	Comp. steel 2
A10	60	31	35	32	37	65	2.52	1.66	Comp. steel 3
A11	65	22	40	36	39	60	2.54	1.65	Comp. steel 4
A12	70	28	45	30	35	55	2.32	1.62	Comp. steel 5

As shown in Table 1 to Table 3, it can be seen that A1 to A7 satisfy the composition ranges of the electrical steel sheet and formula 1, the size of the oxide in the precipitates is larger than the size of the non-oxide, the grain growth is good, and the core loss and flux density are also excellent. On the other hand, it can be seen that A8 to A12 do not satisfy the composition ranges of the electrical steel sheet and formula 1, and some of these exhibit the size of the oxides smaller than the size of the non-oxide in the precipitates. Accordingly, it is apparent that the core loss and the flux density are inferior.

EXAMPLE 2

A steel ingot was prepared with the compositions shown in Tables 4 and 5 below, from which inventive steels A13 to A15 including Si, Al and Mn contents (in wt %) satisfying formula 1 were melted by vacuum melting.

Each steel ingot was heated at 1120° C., hot rolled to a thickness of 2.2 mm, and wound, and then slowly cooled in the air and wound as shown in Table 5. The cooled hot rolled steel sheet was then annealed in a nitrogen atmosphere for 5 minutes, followed by slow cooling at a temperature of 600° C. or higher in an atmosphere in which nitrogen and oxygen were mixed, and then finally quenched by spraying water. The annealed hot rolled sheets were pickled and then cold rolled to 0.35 mm thickness and for the final annealing, the cold rolled sheet was annealed for 2 minutes in a 30% hydrogen and 70% nitrogen mixed atmosphere. The cooling bed was cooled at atmosphere of the 40% hydrogen and nitrogen. The final annealing sheet was examined for the size and amount of oxides, sulfides, carbides, nitrides and their complex precipitates for each specimen and the grains and magnetic properties were measured and listed in Table 6 below.

TABLE 4
Item	C	Si	Al	Mn	P	S	N	Ti	Sn	Sb
A13	0.0035	2.12	0.31	0.2	0.032	0.0044	0.0025	0.0013	0	0.035
A14	0.0024	2.52	0.26	0.21	0.043	0.0022	0.0029	0.0011	0.041	0
A15	0.0021	3.12	0.51	0.8	0.045	0.0045	0.0022	0.0009	0.031	0

	TABLE 5
	Hot rolled sheet anneal	Cold rolled sheet anneal
	& cool	& cool
	Sustaining		Sustaining
	Cooling after winding	Anneal	time (sec)	Anneal	time at
Steel	Satisfy	Temp.	Time	temp	at 600° C.	temp	600° C.
grade	Formula 1	(° C.)	(min)	(° C.)	or higher	(° C.)	or higher	Remarks
A13	◯	650	50	950	12	980	10	Inventive steel 8
A13	◯	650	50	800	2	980	2	Comp. steel 6
A14	◯	620	80	1020	10	1020	11	Inventive steel 9
A14	◯	620	80	1020	2	1020	11	Comp. steel 7
A14	◯	620	1	1020	2	900	2	Comp. steel 8
A14	◯	520	80	1020	2	1020	2	Comp. steel 9
A15	◯	650	30	1020	15	1020	12	Inventive steel 10
A15	◯	650	1	1020	2	1020	2	Comp. steel 10
A15	◯	650	30	1020	2	1020	2	Comp. steel 11

	TABLE 6
		Oxide in		Non-oxide in	Core	Flux
	Particie	precipitate	FeO ratio	precipitate	loss	density
Steel	size	Size	Ratio	(%) in	Size	Ratio	(W15/50)	B50
grade	(μm)	(nm)	(%)	precipitate	(nm)	(%)	W/kg	Tesla	Remarks
A13	75	47	68	52	35	32	2.81	1.75	Inventive steel 8
A13	45	30	41	35	38	59	3.52	1.72	Comp. steel 6
A14	78	52	65	45	46	35	2.23	1.71	Inventive steel 9
A14	60	28	38	38	35	62	2.52	1.68	Comp. steel 7
A14	40	31	35	35	35	65	2.43	1.67	Comp. steel 8
A14	60	28	36	32	36	64	2.61	1.68	Comp. steel 9
A15	120	65	80	57	38	20	2.11	1.71	Inventive steel 10
A15	72	25	45	35	33	55	2.43	1.65	Comp. steel 10
A15	67	21	42	36	33	58	2.64	1.63	Comp. steel 11

As shown in Tables 4 to 6, it can be seen that the inventive steel was given enough cooling time after winding compared to comparative steel, and also given sufficient time at 600° C. or higher after annealing of the hot rolled sheet and the cold rolled sheet. Accordingly, oxides including FeO oxides were well formed, resulting in the good grain growth and excellent magnetic properties.

On the other hand, comparative steel 6 was subjected to a low hot rolled sheet annealing temperature, and during cooling, maintained for a short sustaining time at a temperature of 600° C. or higher, which resulted in small oxide size in precipitates and also small oxide amount in precipitates. Comparative steel 7 was also subjected to cooling for a short cooling time after the hot rolled sheet annealing, thus resulting in relatively smaller oxide size than that of the non-oxides in the precipitates, and also in smaller quantity, and the FeO oxide ratio was also low as 40% or less. Comparative steel 8 was cooled rapidly by water cooling after winding and subjected to cooling at 600° C. or higher after the hot rolled sheet annealing for a short cooling time, and also to a short cooling after the cold rolled sheet annealing, which resulted in the insufficient formation of oxides including FeO in the precipitates. As a result, core loss was relatively high and flux density was low. It can be seen that comparative steel 9, which satisfies the composition, but has a low winding temperature and short annealing time during cooling after the hot rolled sheet annealing, exhibited small oxide such as FeO or small complex precipitates, and the number of the oxides was also smaller as compared with that of non-oxides, which resulted in small particle size and inferior magnetic properties. It can be seen that comparative steel 10 as well as the comparative steel 11 was quenched in water after winding and given a short cooling time after the hot rolled (and cold rolled) sheet annealing, which resulted in a low FeO ratio in the precipitates and insufficient formation of oxides. As a result, the grains were small and the magnetic properties were insufficient.

It will be understood that the present disclosure is not limited to the above embodiments but may be embodied in many different forms from each other and those of ordinary skill in the art to which the present disclosure pertains can implement the invention in other specific forms without changing the technical idea or essential features of the present disclosure. Accordingly, it will be understood that the exemplary embodiments described above are only illustrative, and should not be construed as limiting.

Claims

1. A non-oriented electrical steel sheet, comprising: [ Si ] 1.8 + 1.3 × [ Al ] > 3.7 × [ Mn ] × [ Mn ] Formula   ( 1 )

0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities,

and satisfies formula 1 below,

wherein a mean size of oxides in the precipitates is larger than a mean size of non-oxides.

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

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

a number of oxides in the precipitates is larger than that of non-oxides.

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

further comprising 0.01 to 0.2 wt % of Sn and Sb, individually or in combination, respectively.

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

wherein a number of FeO in the precipitates or precipitates containing FeO is 40% or more.

5. The non-oriented electrical steel sheet of claim 1,

wherein a mean particle size is 50 to 180 μm.

6. A method for manufacturing a non-oriented electrical steel sheet, comprising steps of: [ Si ] 1.8 + 1.3 × [ Al ] > 3.7 × [ Mn ] × [ Mn ] Formula   ( 1 )

heating a slab comprising 0.005 wt % or less of C (excluding 0 wt %), 1.0-4.0 wt % of Si, 0.15-1.5 wt % of Al, 0.1-1.0 wt % of Mn, 0.2 wt % or less of P (excluding 0 wt %), 0.005 wt % or less of N (excluding 0 wt %), 0.001-0.006 wt % of S, 0.005 wt % or less of Ti (excluding 0 wt %), and 0.005 wt % or less of O (excluding 0 wt %), and the remainder being Fe and other inevitable impurities, and satisfying formula 1 below and then hot rolling the slab to prepare a hot rolled sheet;

winding and then cooling the hot rolled sheet;

annealing and then cooling the hot rolled sheet;

cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet; and

final annealing and then cooling the cold rolled sheet,

wherein, the step of winding and cooling the hot rolled sheet comprises cooling at 600° C. or higher for 30 minutes or more,

the step of annealing and then cooling the hot rolled sheet includes cooling at 600° C. or higher for 5 seconds or more, and

the step of annealing and then cooling the cold rolled sheet includes cooling at 600° C. or higher for 5 seconds or more.

(Here, [Si], [Al] and [Mn] represent the contents (in wt %) of Si, Al and Mn, respectively.)

7. The method of claim 6, wherein

wherein the slab further comprises 0.01 to 0.2 wt % of Sn and Sb, individually or in combination.

8. The method of claim 6, wherein

the step of preparing the hot rolled sheet comprises heating the slab at 1200° C. or lower.

9. The method of claim 6, wherein

a temperature of the winding is 600 to 800° C. in the step of winding and then cooling the hot rolled sheet.

10. The method of claim 6, wherein

a temperature of the hot rolled sheet annealing is 850 to 1150° C. in the step of annealing and then cooling the hot rolled sheet.

11. The method of claim 6, wherein

wherein the step of cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet comprises cold rolling to a thickness of 0.1 to 0.7 mm.

12. The method of claim 6, wherein

the step of cold rolling the hot rolled annealing sheet to prepare a cold rolled sheet comprises primary cold rolling, intermediate annealing, and secondary cold rolling.

13. The method of claim 6, wherein

a cracking temperature of the cold rolled sheet annealing during annealing is 850 to 1100° C. in the step of annealing and then cooling the cold rolled sheet.

14. The method of claim 6, wherein

a mean size of oxides in the precipitates of a manufactured electrical steel sheet is larger than a mean size of non-oxides.

15. The method of claim 14, wherein

a number of oxides in the precipitates is larger than that of non-oxides.

16. The method of claim 14, wherein

a number of FeO in the precipitates or precipitates containing FeO is 40% or more.

17. The method of claim 14,

wherein a mean particle size is 50 to 180 μm.