EP2520681B1

EP2520681B1 - Non-oriented electrical steel sheet having superior magnetic properties and a production method therefor

Info

Publication number: EP2520681B1
Application number: EP10841218.0A
Authority: EP
Inventors: Jae-Hoon Kim; Jae-Kwan Kim; Yong-Soo Kim; Won-Seog Bong
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2009-12-28
Filing date: 2010-12-28
Publication date: 2018-10-24
Anticipated expiration: 2030-12-28
Also published as: US20120267015A1; EP2520681A4; WO2011081386A3; CN102906289A; EP2520681A2; JP5642195B2; CN102906289B; WO2011081386A2; JP2013515170A

Description

Technical Field

The present invention relates to the production of a non-oriented electrical steel sheet, and particularly to a non-oriented electrical steel sheet of the highest quality, wherein the components of steel are optimally designed to increase the distribution density of coarse inclusions in steel and to improve growth of grains and mobility of domain walls, so that magnetic properties are enhanced, and low hardness is ensured, thus improving productivity and punchability, and to a method of producing the same.

Background Art

The present invention pertains to the production of a non-oriented electrical steel sheet useful as a material for iron cores of rotation devices. This non-oriented electrical steel sheet is essential in terms of converting electrical energy into mechanical energy, and thus the magnetic properties thereof are regarded as very important. The magnetic properties mainly include core loss and magnetic flux density. Because the core loss is energy that disappears in the form of heat in the course of converting energy, it is good for it to be as low as possible. The magnetic flux density is a power source of a rotator. The higher the magnetic flux density, the more favorable the energy efficiency. As an example, JP 2001 192 788 A discloses a non-oriented silicon steel sheet used for automotive electrical components such as a starter and a starter generator.
Typically, a non-oriented electrical steel sheet is composed mainly of Si in order to reduce core loss. When the amount of Si increases, the magnetic flux density decreases. If the amount of Si is excessively increased, processability is decreased making it difficult to perform cold rolling. Furthermore, the lifetime of a mold may decrease upon punching by the customer. Hence, attempts are made to decrease the amount of Si and increase the amount of Al so as to improve magnetic properties and mechanical properties. However, the magnetic properties of non-oriented electrical steel sheet of the highest quality are not obtained, and such sheets have not yet been actually produced because of difficulties in mass producing them.
Meanwhile, to obtain a non-oriented electrical steel sheet with good magnetic properties, impurities including C, S, N, Ti and so on such as fine inclusions present in steel are controlled to be minimal and thus the growth of grains needs to be increased. However, the control of impurities to the minimum is not easy in a typical production process of electrical steel sheets, and the cost of a steel making process may undesirably increase.
The impurities which were not removed in the steel making process are present in the form of nitrides or sulfides in a slab upon continuous casting. As the slab is re-heated to 1,100°C or higher for hot rolling, inclusions such as nitrides or sulfides may be re-dissolved and then finely precipitated again upon termination of hot rolling.
The inclusions that are precipitated in typical non-oriented electrical steel sheets include MnS and AlN, which are observed to have a small average size of about 50 nm, and such fine inclusions may hinder the growth of grains upon annealing thus increasing hysteresis loss and obstructing the movement of domain walls upon magnetization, undesirably lowering permeability.
Therefore, in the process of producing the non-oriented electrical steel sheet, impurities are appropriately controlled from the steel making process so that such fine inclusions are not present, and the residual inclusions should be prevented from being more finely precipitated via redissolution upon hot rolling.

Disclosure

Technical Problem

Accordingly, the present invention has been made keeping in mind the above problems occurring in the related art, and an object of the present invention is to provide a non-oriented electrical steel sheet of the highest quality, wherein the proportions of Al, Si and Mn which are alloy elements of steel and N and S which are impurity elements of steel are optimally controlled so that the distribution density of coarse inclusions in steel is increased and the formation of fine inclusions is decreased, thus enhancing the growth of grains and the mobility of domain walls to thereby manifest excellent magnetic properties, and also superior productivity and punchability because of low hardness.

Technical Solution

In order to accomplish the above object, an aspect of the present invention provides a non-oriented electrical steel sheet having superior magnetic properties, comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 - 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt%, and satisfying Condition (2) below, wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet, and a distribution density of the inclusion having an average size of 300 nm or is equal to or greater than 0.02 number/mm², wherein: Condition (2): {[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006, 300≤{([Al]+[Mn])/([N]+[S])}≤1,400; wherein [Al], [Si], [Mn], [N] and [S] indicate amounts (wt%) of Al, Si, Mn, N and S, respectively.
In the non-oriented electrical steel sheet which satisfies Condition (2), the amounts of Al, Si and Mn may satisfy Relation (2) and the following Relations (3) and (4), and a cross-sectional Vickers hardness (Hv1) may be 190 or less. $1.7 \leq \{[Al] + [Si] + [Mn] / 2\} \leq 5.5$
$0.6 \leq [Al] / [Si] \leq 4.0$
Another aspect of the present invention provides a method of producing the non-oriented electrical steel sheet having superior magnetic properties, comprising subjecting a slab comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt% and satisfying Condition (2) below to heating, hot rolling, cold rolling, and final annealing at 750 ∼ 1100°C, wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet subjected to final annealing, and a distribution density of the inclusion having an average size of 300 nm or is equal to or greater than 0.02 number/mm², wherein: Condition (2): {[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006, 300≤{([Al]+[Mn])/([N]+[S])}≤1,400; wherein [Al], [Si], [Mn], [N] and [S] indicate amounts (wt%) of Al, Si, Mn, N and S, respectively.
The slab may be prepared by adding 0.3 ∼ 0.5% of Al to perform deoxidation, adding remaining alloy elements, and maintaining a temperature at 1,500 ∼ 1,600°C.
Further disclosed is a non-oriented electrical steel sheet slab, comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt%, and satisfying Condition (2), wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet, and a distribution density of the inclusion having an average size of 300 nm or is equal to or greater than 0.02 number/mm², wherein.
Further disclosed is a method of producing the non-oriented electrical steel sheet slab, comprising adding 0.3 ~ 0.5% of Al to molten steel to perform deoxidation, adding a remainder of Al and Si and Mn, and maintaining the temperature of the molten steel at 1,500 ~ 1,600°C, thus obtaining the slab comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt%, and satisfying Condition (2), wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet, and a distribution density of the inclusion having an average size of 300 nm or is equal to or greater than 0.02 number/mm², wherein.

Advantageous Effects

According to the present invention, the proportions of alloy elements such as Al, Si and Mn and of impurity elements such as N and S can be appropriately controlled so as to increase the distribution density of coarse inclusions, thus enhancing the growth of grains and the mobility of domain walls. Thereby, a non-oriented electrical steel sheet of the highest quality having excellent magnetic properties and very low hardness can be stably produced. Also customer workability and productivity are superior, and the unit cost of production of products can be decreased, thus reducing the cost.

Description of Drawings

FIG. 1 is a view showing composite inclusions which are present in a non-oriented electrical steel sheet according to the present invention;
FIG. 2 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 0.5 ~ 2.5% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis;
FIG. 3 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 0.2 ∼ 1.0% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis; and
FIG. 4 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 2.3 ~ 3.5% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis.

Mode for Invention

To solve the technical problems as mentioned above, the present inventors have examined the effects of alloy elements and impurity elements in steel and of the relation between respective elements on forming the inclusions and also the effects thereof on magnetic properties and processability, resulted in the finding that among alloy elements of steel, the amounts of Al, Si and Mn and the amounts of impurity elements such as N and S may be appropriately adjusted and Al/Si and Al/Mn, Al+Si+Mn/2, Al+Mn, N+S and (Al+Mn)/(N+S) may be optimally controlled so that the hardness of a steel sheet is decreased and the distribution density of coarse composite inclusions having an average size of 300 nm or more in the steel sheet is increased, thereby drastically enhancing magnetic properties and improving productivity and punchability, which culminates in the present invention.
The present invention is directed to a non-oriented electrical steel sheet of the highest quality according to claim 1.
Further disclosed is a non-oriented electrical steel sheet of the highest quality, comprising 0.7 ∼ 3.0% of Al, 0.2 ∼ 3.5% of Si, 0.2 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, and a balance of Fe and other inevitable impurities by wt%, wherein Al, Si, Mn, N and S are contained so as to satisfy at least one of the following Conditions (1), (2) and (3), and thus the distribution density of 300 nm or more sized coarse inclusions having combinations of nitrides and sulfides is increased to be equal to or greater than 0.02 number/mm², resulting in high magnetic properties and low hardness.

① Condition (1): 0.7≤[Al]≤2.7, 0.2≤[Si]≤1.0, 0.2≤[Mn]≤1.7, {[Al]+[Mn]}≤2.0, 0.002≤{[N]+[S]}≤0.006, 230≤{([Al]+[Mn])/([N]+[S])}1,000
② Condition (2): 1.0≤[Al]≤3.0, 0.5≤[Si]≤2.5, 0.5≤[Mn]≤2.0, {[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006, 300≤{([Al]+[Mn])/([N]+[S])≤1,400
③ Condition (3): 1.0≤[Al]≤3.0, 2.3≤[Si]≤3.5, 0.5≤[Mn]≤2.0, {[Al]+[Mn]}≤3.5, 0.002≤{[N]+[S]}≤0.006, 300≤{([Al]+[Mn])/([N]+[S])}≤1,400

As such, [Al], [Si], [Mn], [N] and [S] indicate the amounts (wt%) of Al, Si, Mn, N and S, respectively.
In addition, the present invention is directed to the production of the non-oriented electrical steel sheet which is superior in both magnetic properties and processability according to claim 6.
In addition, disclosed is the production of the non-oriented electrical steel sheet which is superior in both magnetic properties and processability, by adding 0.3 ~ 0.5% of Al to molten steel to perform deoxidation in a steel making process, adding remaining alloy elements, and then maintaining the temperature of the molten steel at 1,500 ~ 1,600°C thus manufacturing a slab having the composition that satisfies at least one of Conditions (1), (2) and (3), followed by heating the slab to 1,100 ∼ 1,250°C and then performing hot rolling wherein finish hot rolling is conducted at 800°C or higher, carrying out cold rolling, and then finally annealing the cold rolled sheet at 750 ∼ 1,100°C.
The alloy elements of steel, namely, Al, Si and Mn are described below. These alloy elements are added to reduce the core loss of an electrical steel sheet. As the amounts thereof increase, the magnetic flux density may decrease and the processability of a material may deteriorate. Hence, the amounts of such alloy components are appropriately designed to improve not only the core loss but also the magnetic flux density, and also hardness needs to be maintained to an appropriate level or less.
Furthermore, Al and Mn combine with N and S which are impurity elements to form inclusions such as nitrides or sulfides. Such inclusions greatly affect magnetic properties and thus the formation of inclusions that minimize the deterioration of magnetic properties should be increased.
The present inventors were the first to discover that coarse composite inclusions comprising combinations of nitrides or sulfides may be formed when the amounts of Al, Mn, Si, N and S are adapted for specific conditions, and have found the fact that the distribution density of such composite inclusions to a predetermined level or more is ensured, and thereby magnetic properties may be drastically improved despite the addition of minimum amounts of alloy elements that deteriorate processability, and thus devised the present invention.
The reason why the ranges of component elements of the present invention and the amount ratios of the component elements are limited is described below.

[Al: 1.0 ∼ 3.0 wt%]

Al functions to increase resistivity of a material to reduce core loss and to form a nitride, and is added in an amount of 1.0 ∼ 3.0% so as to form a coarse nitride. If the amount of Al is less than 1.0%, inclusions may not be sufficiently grown. In contrast, if the amount thereof exceeds 3.0%, processability may deteriorate and all processes including steel making, continuous casting and so on may be problematic, making it impossible to produce a steel sheet in the typical manner.

[Si: 0.5 ~ 2.5 wt%]

Si functions to increase resistivity of a material to reduce core loss. If the amount of Si is less than 0.5%, it is difficult to expect reduction effects of core loss. In contrast, if the amount thereof exceeds 2.5%, the hardness of a material may increase, undesirably deteriorating productivity and punchability.

[Mn: 0.5 ∼ 2.0 wt%]

Mn functions to increase resistivity of a material to reduce core loss and to form a sulfide, and is added in an amount of 0.5% or more. If the amount thereof exceeds 2.0%, the formation of [111] texture that is unfavorable for magnetic properties may be facilitated. Hence, the amount of Mn is limited to 0.5 ~ 2.0%.

[Sn: 0.005 ~ 0.2 wt%]

Sn is preferentially segregated on the surface and the grain boundaries and may reduce accumulated strain energy upon hot rolling and cold rolling, so that the strength in {100} orientation that is favorable for magnetic properties may increase whereas the strength in {111} orientation that is unfavorable for magnetic properties may decrease, thus achieving improvements in texture. Hence, Sn is added in the range of 0.005 ~ 0.2%. Furthermore, Sn is preferentially formed on the surface during welding to thus suppress surface oxidation and enhance the weld properties thereby increasing productivity of continuous lines. Also, the formation of Al-based oxides and nitrides on the surface or the layer under the surface may be suppressed during heat treatment, thus enhancing magnetic properties. Upon punching by a customer, the increase in hardness of the layer under the surface due to nitrides may be inhibited to improve punchability.
Hence, Sn is added in the range of 0.005% or more. In contrast, if the amount of Sn exceeds 0.2%, improvements in magnetic properties based on such an additional use thereof are insignificant, and fine inclusions and deposits may be formed in steel, rather than preferential segregation on the surface and the grain boundaries, negatively affecting the magnetic properties. Also, cold rollability and punchability may decrease and the Erichsen number that represents the weld properties is 5 mm or less, making it impossible to perform welding of the same species. Thus, a low-graded material having the sum of Si and Al of less than 2 should be undesirably used as a connection material for continuous line working. Hence, the amount of Sn is limited to 0.005 ∼ 0.2%.

[Sb: 0.005 ∼ 0.1 wt%]

Sb is preferentially segregated on the surface and the grain boundaries and may reduce accumulated strain energy upon hot rolling and cold rolling, so that the strength in {100} orientation that is favorable for magnetic properties may increase and the strength in {111} orientation that is unfavorable for magnetic properties may decrease, thus attaining improvements in texture. Hence, Sb is added in the range of 0.1% or less. Furthermore, Sb is preferentially formed on the surface during welding to thus suppress surface oxidation and enhance weld properties thereby increasing productivity of continuous lines. Also, the formation of Al-based oxides and nitrides on the surface or the layer under the surface may be suppressed during heat treatment, thus improving magnetic properties. Upon punching by the customer, the increase in hardness of the layer under the surface due to nitrides may be inhibited to improve punchability.
Hence, Sb is added in the range of 0.005% or more. In contrast, if the amount of Sb exceeds 0.1%, improvements in magnetic properties based on such an additional use thereof are insignificant, and fine inclusions and deposits may be formed in steel, rather than preferential segregation on the surface and the grain boundaries, undesirably aggravating the magnetic properties. Also, cold rollability and punchability may decrease and the Erichsen number that represents the weld properties is 5mm or less, making it impossible to perform welding of the same species. Thus, a low-graded material having the sum of Si and Al of less than 2 should be undesirably used as a connection material for continuous line working. Hence, the amount of Sb is limited to 0.005 ~ 0.1%.

[P: 0.2 wt% or less]

When P is added in the range of 0.2% or less, texture that is favorable for magnetic properties may be formed, and in-plane anisotropy and processability are improved. If the amount thereof exceeds 0.2%, cold rollability may decrease and processability may deteriorate. Hence, the amount of P is limited to 0.2% or less.

[N: 0.001 ∼ 0.004 wt%]

N is an impurity element, and may form a fine nitride during the production process to suppress the growth of grains undesirably deteriorating core loss. Although the formation of nitrides is suppressed, an additional high cost and long process time are required, and thus monetary benefits are negatively affected. Therefore, it is preferred that an element having high affinity for the impurity element N is positively utilized to coarsely grow inclusions so as to reduce an influence on the growth of grains. To coarsely grow the inclusions in this way, the amount of N is essentially controlled in the range of 0.001 ~ 0.004%. If the amount of N exceeds 0.004%, the inclusions may not be coarsely formed undesirably increasing core loss. More preferably, the amount of N is limited to 0.003% or less.

[S: 0.0005 ~ 0.004 wt%]

S is an impurity element, and may form a fine sulfide during the production process to thus suppress the growth of grains and deteriorate core loss. Although the formation of sulfides is suppressed, an additional high cost and long process time are required, and thus monetary benefits are negatively affected. Thus, it is preferred that an element having high affinity for the impurity element S is positively utilized to coarsely grow inclusions so as to reduce the influence on the growth of grains. To coarsely grow the inclusions in this way, the amount of S is essentially controlled in the range of 0.0005 ∼ 0.004%. If the amount of S exceeds 0.004%, the inclusions may not be coarsely formed undesirably increasing core loss. More preferably, the amount of S is limited to 0.003% or less.
In addition to the above impurity elements, inevitable impurities such as C, Ti may be incorporated. C may cause magnetic aging, and the amount thereof is thus limited in the range of 0.004% or less, and more preferably 0.003% or less. Ti may promote the growth of [111] texture that is unfavorable for a non-oriented electrical steel sheet, and the amount thereof is thus limited in the range of 0.004% or less, and preferably 0.002% or less.
In the non-oriented electrical steel sheet that satisfies Condition (1), the sum ([Al]+[Mn]) of Al and Mn by wt% is limited to 2.0% or less. If the sum of Al and Mn exceeds 2.0% in steel comprising 0.7 ∼ 2.7% of Al, 0.2 ∼ 1.0% of Si and 0.2 ∼ 1.7% of Mn, the fraction of [111] texture that is unfavorable for magnetic properties may increase, undesirably deteriorating the magnetic properties. In the case of the non-oriented electrical steel sheet that satisfies Condition (1), if the sum of Al and Mn is less than 0.9%, nitrides, sulfides or composite inclusions of these two are not coarsely formed, thus deteriorating the magnetic properties. However, in the non-oriented electrical steel sheet that satisfies Condition (1), which is not inventive, Al is contained in an amount of 0.7% or more and Mn is contained in an amount of 0.2% or more, so that the sum of Al and Mn is 0.9% or more, thereby preventing the deterioration of the magnetic properties.
In the non-oriented electrical steel sheet that satisfies Condition (2) or (3), the sum ([Al]+[Mn]) of Al and Mn by wt% is limited to 3.5% or less. If the sum of Al and Mn exceeds 3.5% in steel comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 3.5% of Si and 0.5 ∼ 2.0% of Mn, the fraction of [111] texture that is unfavorable for magnetic properties may increase undesirably deteriorating the magnetic properties. In the non-oriented electrical steel sheet that satisfies Condition (2) or (3), if the sum of Al and Mn is less than 1.5%, nitrides, sulfides or composite inclusions of these two are not coarsely formed, thus deteriorating the magnetic properties. However, in the non-oriented electrical steel sheet that satisfies Condition (2) or (3), Al is contained in an amount of 1.0% or more and Mn is contained in an amount of 0.5% or more, so that the sum of Al and Mn is 1.5% or more, thereby preventing the deterioration of the magnetic properties.
In the present invention, the sum ([N]+[S]) of N and S is limited to 0.002 ~ 0.006%. This is because inclusions are coarsely formed in the above range. If the sum of N and S exceeds 0.006%, the fraction of fine inclusions may be increased, undesirably deteriorating the magnetic properties.
Also in the present invention, the ratio of the sum ([Al]+[Mn]) of Al and Mn by wt% to the sum ([N]+[S]) of N and S by wt% is regarded as important.
The present inventors have appreciated that, in order for the distribution density of 300 nm or more sized coarse composite inclusions of nitrides and sulfides to increase and become equal to or greater than 0.02 number/mm², ([Al]+[Mn])/([N]+[S]) should be appropriately adjusted, and the proper range of ([Al]+[Mn])/([N]+[S]) may vary depending on the amounts of Si, Al and Mn.
Under Condition (1), which is not inventive wherein the amounts of Al, Si and Mn are slightly low, when the ratio of ([Al]+[Mn])/([N]+[S]) is slightly low to the extent of 230 ~ 1000, the formation of composite inclusions may be effectively increased. The inclusions may be coarsely formed within the above range and thus the distribution density of coarse composite inclusions may be increased thus improving core loss. However, if the ratio thereof falls outside the above range, the inclusions may not be coarsely formed and the formation of coarse composite inclusions is low and texture that is unfavorable for magnetic properties is formed.
In the case where the amounts of Al, Si and Mn are given as in Condition (2) or (3), when the ratio of ([Al]+[Mn])/([N]+[S]) is 300 ∼ 1400, the formation of composite inclusions may be effectively increased. Specifically, when the ratio of ([Al]+[Mn])/([N]+[S]) falls in the range of 300 ∼ 1400 under Condition (2) or (3), inclusions may be coarsely formed thus increasing the distribution density of coarse composite inclusions. In contrast, when the ratio thereof falls outside the above range, the inclusions are not coarsely formed and the formation of coarse composite inclusions is low and texture that is unfavorable for magnetic properties is formed.
FIG. 1 shows composite inclusions which are present in the non-oriented electrical steel sheet according to the present invention. When the amounts of Al, Mn, N and S are controlled in the optimal ranges, inclusions are grown several times or more compared to when using typical materials, thus increasing the formation of coarse composite inclusions having an average size of 300 nm or more. Accordingly, the formation of fine inclusions having an average size of about 50 nm may decrease, thereby improving magnetic properties. The present inventors have appreciated that, when the distribution density of coarse composite inclusions as shown in FIG. 1 is equal to or greater than 0.02 number/mm², the magnetic properties of the non-oriented electrical steel sheet may be remarkably improved.
The accurate mechanism for forming such coarse composite inclusions has not yet been revealed, but is assumed to take place in the steel making process. Specifically upon initial addition of Al in the steel making process, Al-based oxides and nitrides may be formed due to deoxidation, and in the composition that additionally includes the alloy elements such as Al and Mn and satisfies the amounts of Al, Mn, Si, N and S as designed in the present invention upon bubbling, Al-based oxides and nitrides are grown and Mn-based sulfides may also be precipitated thereon.
FIG. 2 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 0.5 ∼ 2.5% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis.
As shown in FIG. 2, in the range (within the thick line) of the present invention that satisfies Condition (2), namely, wherein the sum ([Al]+[Mn]) of Al and Mn by wt% is 3.5% or less and the sum ([N]+[S]) of N and S by wt% is 0.002 ~ 0.006 and the ratio of the sum of Al and Mn to the sum of N and S ([Al]+[Mn])/([N]+[S]) falls in the range of 300 ~ 1,400, inclusions are coarsely formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is greater than 0.02 number/mm², thus exhibiting superior magnetic properties. However, in the range falling outside the present invention (outside the thick line), coarse inclusions are not formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is less than 0.02 number/mm², thus deteriorating texture and magnetic properties.
FIG. 3 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 0.2 ~ 1.0% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis.
As shown in FIG. 3, in the range (within the thick line) that satisfies Condition (1), namely, wherein the sum ([Al]+[Mn]) of Al and Mn by wt% is 2.0% or less and the sum ([N]+[S]) of N and S by wt% is 0.002 ∼ 0.006 and the ratio of the sum of Al and Mn to the sum of N and S ([Al]+[Mn])/([N]+[S]) falls in the range of 230 ∼ 1,000, inclusions are coarsely formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is greater than 0.02 number/mm², thus exhibiting superior magnetic properties. However, in the range falling outside the thick line, coarse inclusions are not formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is less than 0.02 number/mm², thus deteriorating texture and magnetic properties.
FIG. 4 is a graph showing whether the distribution density of coarse composite inclusions having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² in the non-oriented electrical steel sheet containing 2.3 ∼ 3.5% of Si wherein [N]+[S] is represented on a horizontal axis and [Al]+[Mn] is represented on a vertical axis.
As shown in FIG. 4, in the range (within the thick line) of the present invention that satisfies Condition (3), namely, wherein the sum ([Al]+[Mn]) of Al and Mn by wt% is 3.5% or less and the sum ([N]+[S]) of N and S by wt% is 0.002 ∼ 0.006 and the ratio of the sum of Al and Mn to the sum of N and S ([Al]+[Mn])/([N]+[S]) falls in the range of 300 ∼ 1,400, inclusions are coarsely formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is greater than 0.02 number/mm², thus exhibiting superior magnetic properties. However, in the range falling outside the present invention (outside the thick line), coarse inclusions are not formed and the distribution density of coarse composite inclusions having an average size of 300 nm or more is less than 0.02 number/mm², thus deteriorating texture and magnetic properties.
Although the coarse inclusions are mainly observed to be combinations of nitrides and sulfides having an average size of 300 nm or more, the examples thereof may include combinations of a plurality of nitrides or combinations of a plurality of sulfides having an average size of 300 nm or more, and those having nitrides or sulfides alone having a size of 300 nm or more. Herein, the average size of the inclusions is determined by measuring the longest length and the shortest length of the inclusions when viewed in the cross-section of the steel sheet and averaging the measured values.
Also in the non-oriented electrical steel sheet that satisfies Condition (2), the amount ratio of Al to Si ([Al]/[Si]) is limited to 0.6 ∼ 4.0. In the case where the amount ratio of Al to Si is 0.6 ∼ 4.0, the grains may effectively grow and the hardness of a material may decrease thus improving productivity and punchability. If the ratio of [Al]/[Si] is less than 0.6, inclusions do not greatly grow undesirably decreasing the growth of grains and deteriorating magnetic properties. Furthermore, the amount of Si may increase, undesirably enhancing hardness. If the ratio of [Al]/[Si] exceeds 4.0, texture of a material may become poor undesirably deteriorating the magnetic flux density.
In the present invention, the ratio of Al to Mn ([Al]/[Mn]) is preferably limited to 1 ∼ 8. When the ratio of Al to Mn is 1 ∼ 8, the inclusions may effectively grow thus exhibiting superior core loss properties. In contrast, if the ratio thereof falls outside the above range, the growth of inclusions may decrease and the fraction of texture that is favorable for magnetic properties may decrease.
The limited ratio of alloy components related to resistivity is described below. Recently, as the demand for environmentally friendly automobiles drastically increases, there is a high need for non-oriented electrical steel sheets usable for highly rotatable motors. The motors used in the environmentally friendly automobiles should greatly increase their number of rotations. When the number of rotations of the motor is increased, the fraction of eddy current loss in the inner core loss may be drastically increased. To reduce such eddy current loss, resistivity should increase.
The relation between the amounts of alloy elements of the non-oriented electrical steel sheet and the intrinsic resistance is represented below. $ρ = 13.25 + 11.3 ([Al] + [Si] + [Mn] / 2) (ρ : resistivity, Ω \cdot m)$
In the present invention that satisfies Condition (3), [Al]+[Si]+[Mn]/2 is limited to 3.0 or more so as to ensure resistivity of 47 or more.
Despite the recent development of cold rolling techniques, the case where the resistivity exceeds 87 may increase the amounts of alloy elements and may deteriorate processability. Because the production of steel sheets is impossible via typical cold rolling, the resistivity should be set to 87 or less.
In the present invention that satisfies Condition (3), [Al]+[Si]+[Mn]/2 is controlled in the range of 3.0 ∼ 6.5% so that the resistivity is 47 ∼ 87 (Ω·m) and Vickers hardness (Hv1) is 225 or less.
In the present invention that satisfies Condition (2), [Al]+[Si]+[Mn]/2 is limited to 1.7 or more so as to ensure the resistivity of 32 or more. Furthermore, in the present invention that satisfies Condition (2), [Al]+[Si]+[Mn]/2 is controlled to 5.5% or less so that resistivity (intrinsic resistance) is maintained to 75 or less and Vickers hardness (Hv1) is 190 or less.
Also the demand for high magnetic flux density products is drastically increasing these days to achieve high efficiency of motors. Accordingly, there is an urgent requirement for non-oriented electrical steel sheets having lowered resistivity and improved magnetic flux density. In the case where magnetic flux density properties are regarded as important, resistivity (intrinsic resistance) is decreased to 36 or less to increase the magnetic flux density. Moreover to correspond to high-speed rotations, the resistivity should be controlled to at least 25.
Thus in the disclosure that satisfies Condition (1), [Al]+[Si]+[Mn]/2 is controlled to 1.0 ∼ 2.0% so that the resistivity is 25 ∼ 36 (Ω·m) and Vickers hardness (Hv1) is very low to the extent of 140 or less.
Below is a description of a method of producing the non-oriented electrical steel. The method of producing the non-oriented electrical steel sheet preferably includes adding 0.3 - 0.5% of Al, corresponding to a portion of the total amount of added Al, in the steel making process, so that deoxidation of steel sufficiently occurs, and adding the remaining alloy elements. Subsequently, the temperature of molten steel is maintained at 1,500 ∼ 1,600°C so that inclusions in steel are sufficiently grown, after which the resultant steel is solidified in a continuous casting process thus manufacturing a slab.
Subsequently, the slab is placed in a furnace so that it is re-heated to 1,100 ∼ 1,250°C. If the slab is heated to a temperature exceeding 1,250°C, deposits that negatively affect the magnetic properties may be re-dissolved, hot rolled and then finely deposited, and thus the slab is heated to 1,250°C or less.
Subsequently, the heated slab is hot rolled. Upon hot rolling, finish hot rolling is preferably carried out at 800°C or more. The hot rolled sheet is annealed at 850 ∼ 1,100°C. If the annealing temperature of the hot rolled sheet is lower than 850°C, texture does not grow or finally grows, and thus the extent of increasing the magnetic flux density is low. In contrast, if the annealing temperature of the hot rolled sheet is higher than 1,100°C, magnetic properties may deteriorate instead, and rolling workability may decrease due to plate transformation. Hence, the temperature range thereof is limited to 850 ∼ 1,100°C. More preferably the annealing temperature of the hot rolled sheet is 950 ∼ 1,100°C. The annealing of the hot rolled sheet may be carried out to increase the grain orientation favorable for magnetic properties, as necessary, but may be omitted.
Subsequently, the hot rolled sheet which was annealed or not is pickled, and cold rolled to a reduction of 70 ∼ 95% to obtain a predetermined sheet thickness.
The amounts of added Si, Mn and Al alloy elements that affect cold rollability are appropriately controlled thus attaining superior cold rollability and high reduction. Thus, one cold rolling makes it possible to form a thin sheet having a thickness of about 0.15 mm. Upon cold rolling, two cold rolling operations including intermediate annealing may be conducted, as necessary, or two annealing operations may be applied.
Subsequently, the cold rolled sheet is subjected to final annealing. If the final annealing temperature is lower than 750°C, recrystallization does not sufficiently occur. In contrast, if the final annealing temperature exceeds 1,100°C, the surface oxide layer is deeply formed, undesirably deteriorating magnetic properties. Hence, final annealing is preferably conducted at 750 ∼ 1,100°C.
The finally annealed steel sheet is subjected to insulation coating treatment using typical methods and is then discharged to customers. Upon insulation coating, the application of a typical coating material is possible, and either Cr-type or Cr-free type may be used without limitation.
Below, the present invention is described by the following examples. Unless otherwise stated, the amounts of components are represented by wt% in the following examples.

[Example 1]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 1 below. As such, the amount of each of impurity elements C, S, N, Ti was controlled to 0.002%, and 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 2 below. A sample for use in observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 1]

Steel	Al	Si	Mn	C	S	N	Ti
A1	3.0	0.5	1.0	0.002	0.002	0.002	0.002
A2	2.5	0.5	1.0	0.002	0.002	0.002	0.002
A3	1.0	0.5	1.0	0.002	0.002	0.002	0.002
A4	3.0	1.0	1.0	0.002	0.002	0.002	0.002
A5	2.0	1.0	1.0	0.002	0.002	0.002	0.002
A6	1.0	1.0	1.0	0.002	0.002	0.002	0.002
A7	0.5	1.0	1.0	0.002	0.002	0.002	0.002
A8*	3.5	1.5	1.0	0.002	0.002	0.002	0.002
A9	2.5	1.5	1.0	0.002	0.002	0.002	0.002
A10	1.5	1.5	1.0	0.002	0.002	0.002	0.002
A11	3.0	2.0	1.0	0.002	0.002	0.002	0.002
A12	1.5	2.0	1.0	0.002	0.002	0.002	0.002
A13	3.0	2.5	1.0	0.002	0.002	0.002	0.002
A14	2.5	2.5	1.0	0.002	0.002	0.002	0.002
A15	1.0	2.5	1.0	0.002	0.002	0.002	0.002

* not inventive

[Table 2]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15/ 50)	Magnetic Flux Density (B50)	Hard.	Note
A1*	6.0	3.0	4.0	0.0040	1000	4.0	250	0	2.2	1.62	165	unsuitable
A2*	5.0	2.5	3.5	0.0040	875	3.5	200	0	2.3	1.62	160	unsuitable
A3	2.0	1.0	2.0	0.0040	500	2.0	300	0.02	2.5	1.72	140	suitable
A4*	3.0	3.0	4.0	0.0040	1000	4.5	250	0	2.4	1.62	157	unsuitable
A5	2.0	2.0	3.0	0.0040	750	3.5	500	0.07	2.0	1.67	155	suitable
A6	1.0	1.0	2.0	0.0040	500	2.5	450	0.05	2.1	1.68	150	suitable
A7*	0.5	0.5	1.5	0.0040	375	2.0	50	0	2.5	1.66	145	unsuitable
A8*	2.3	3.5	4.5	0.0040	1125	5.5	75	0	2.5	1.64	190	unsuitable
A9	1.7	2.5	3.5	0.0040	875	4.5	400	0.05	2.0	1.67	185	suitable
A10	1.0	1.5	2.5	0.0040	625	3.5	600	0.08	2.0	1.68	170	suitable
A11*	1.5	3.0	4.0	0.0040	1000	5.5	250	0	2.3	1.62	195	unsuitable
A12	0.8	1.5	2.5	0.0040	625	4.0	400	0.04	2.0	1.68	183	suitable
A13*	1.2	3.0	4.0	0.0040	1000	6.0	75	0	2.0	1.61	210	unsuitable
A14	1.0	2.5	3.5	0.0040	875	5.5	400	0.03	1.9	1.65	190	suitable
A15*	0.4	1.0	2.0	0.0040	500	4.0	60	0	2.4	1.67	195	unsuitable

*not inventive

As is apparent from Table 2, steels A3, A5, A6, A9, A10, A12 and A14 were inventive examples that satisfy Condition (2), wherein coarse composite inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02 (1/mm²) thus exhibiting superior magnetic properties. The Vickers hardness (Hv1) was as low as 190 or less thus obtaining superior productivity and customer punchability.
Whereas in steel A1, the ratio of Al/Si and Al+Mn did not satisfy Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. Also, in steels A2 and A15, the ratio of Al/Si did not satisfy Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. Also in steels A4, A8, A11 and A13, Al+Mn did not satisfy Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. Also in steel A7, the ratio of Al/Si and the ratio of Al/Mn did not satisfy Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated.

[Example 2]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 3 below. As such, the components of steel were controlled while variously adjusting the amounts of impurity elements N and S, and 0.3 ~ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 4 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 3]

Steel	Al	Si	Mn	C	S	N	Ti
B1	1.0	0.5	0.5	0.002	0.001	0.001	0.002
B2	1.0	0.5	0.5	0.002	0.003	0.003	0.002
B3	1.0	0.5	0.5	0.002	0.0005	0.001	0.002
B4	1.0	0.5	1.0	0.002	0.002	0.003	0.002
B5	1.2	0.5	1.2	0.002	0.0015	0.002	0.002
B6	1.2	0.5	1.0	0.002	0.0005	0.0005	0.002
B7	1.2	0.5	1.0	0.002	0.003	0.003	0.002
B8	2.0	0.5	2.0	0.002	0.001	0.003	0.002
B9	2.0	0.5	1.5	0.002	0.001	0.0015	0.002
B10	2.0	0.5	1.5	0.002	0.001	0.003	0.002
B11	2.0	0.5	1.0	0.002	0.003	0.004	0.002
B12	2.0	1.0	1.5	0.002	0.0005	0.0015	0.002
B13	2.0	1.0	1.5	0.002	0.002	0.004	0.002
B14	1.5	1.0	1.5	0.002	0.002	0.0025	0.002
B15	2.5	1.0	1.0	0.002	0.0005	0.0005	0.002

[Table 4]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
B1	2.0	2.0	1.5	0.0020	750	1.8	350	0.03	2.6	1.74	135	suitable
B2*	2.0	2.0	1.5	0.0060	250	1.8	75	0	3.2	1.72	135	unsuitable
B3*	2.0	2.0	1.5	0.0015	1000	1.8	120	0	2.9	1.71	135	unsuitable
B4	2.0	1.0	2	0.0050	400	2.0	400	0.04	2.6	1.70	140	suitable
B5	2.4	1.0	2.4	0.0035	686	2.3	450	0.03	2.2	1.69	150	suitable
B6*	2.4	1.2	2.2	0.0010	2200	2.2	50	0	2.4	1.67	150	unsuitable
B7	2.4	1.2	2.2	0.0060	367	2.2	350	0.02	2.3	1.70	165	suitable
B8*	4.0	1.0	4.0	0.0040	1000	3.5	250	0	2.3	1.62	185	unsuitable
B9	4.0	1.3	3.5	0.0025	1400	3.3	450	0.05	2	1.67	170	suitable
B10	4.0	1.3	3.5	0.0040	875	3.3	550	0.08	2	1.68	170	suitable
B11*	4.0	2.0	3	0.0070	429	3.0	250	0	2.2	1.65	170	unsuitable
B12*	2.0	1.3	3.5	0.0020	1750	3.8	80	0	2.3	1.65	165	unsuitable
B13	2.0	1.3	3.5	0.0060	583	3.8	500	0.07	2	1.68	175	suitable
B14	1.5	1.0	3	0.0045	667	3.3	600	0.07	2	1.68	170	suitable
B15*	2.5	2.5	3.5	0.0010	3500	4.0	50	0	2.2	1.65	165	unsuitable

*not inventive

As is apparent from Table 4, steels B1, B4, B5, B7, B9, B10, B13 and B14 were inventive examples that satisfy Condition (2), wherein coarse composite inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02(1/mm²) thus manifesting excellent magnetic properties. The hardness was low thus obtaining superior productivity and customer punchability.
However in steels B3, B6, B11 and B15, N+S fell outside Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. Also in steel B8, Al+Mn fell outside Condition (2), and in steels B2 and B12, the ratio of (Al+Mn)/(N+S) fell outside Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated.

[Example 3]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 5 below. As such, 0.3 ~ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si, Mn and P were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so as to form sheets having different thicknesses in the range of 0.15 ~ 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec. The core loss and magnetic flux density of respective sheets having different thicknesses were measured. The results are shown in Table 6 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 5]

Steel	Al	Si	Mn	P	C	S	N	Ti
C1*	1	3	0.2	0.03	0.002	0.002	0.002	0.002
C2	2.2	1	0.8	0.05	0.002	0.002	0.002	0.002
C3	2	1.5	1.5	0.05	0.002	0.002	0.002	0.002
C4	1.8	1.3	1.2	0.05	0.002	0.002	0.002	0.002
C5	1.3	1.8	0.6	0.08	0.002	0.002	0.002	0.002
C6	2.2	1.5	0.6	0.1	0.002	0.002	0.002	0.002
C7	1.8	1.2	1.2	0.1	0.002	0.002	0.002	0.002

*not inventive

[Table 6]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) / (N+S)	Al+Si +Mn/2	Magnetic Properties	Thickness (mm)					Note
Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) / (N+S)	Al+Si +Mn/2	Magnetic Properties	0.35	0.3	0.25	0.2	0.15	Note
C1*	0.3	5.0	1.2	0.004	300	4.1	B50	1.65	1.64	1.63	1.62	1.61	unsuitable
C1*	0.3	5.0	1.2	0.004	300	4.1	W10/400	20.2	17.8	15.7	13.4	12.3	unsuitable
C2	2.2	2.8	3.0	0.004	750	3.6	B50	1.67	1.66	1.65	1.64	1.63	suitable
C2	2.2	2.8	3.0	0.004	750	3.6	W10/400	18.2	15.6	13.4	11.2	9.7	suitable
C3	1.3	1.3	3.5	0.004	875	4.25	B50	1.68	1.68	1.65	1.64	1.64	suitable
C3	1.3	1.3	3.5	0.004	875	4.25	W10/400	18.0	15	13.6	11.5	10.1	suitable
C4	1.4	1.5	3.0	0.004	750	3.7	B50	1.68	1.65	1.66	1.65	1.63	suitable
C4	1.4	1.5	3.0	0.004	750	3.7	W10/400	17.8	15.3	13.3	11.1	9.4	suitable
C5	0.7	2.2	1.9	0.004	475	3.4	B50	1.67	1.66	1.65	1.64	1.63	suitable
C5	0.7	2.2	1.9	0.004	475	3.4	W10/400	18.1	15.5	13.4	11.2	9.6	suitable
C6	1.5	3.7	2.8	0.004	700	4	B50	1.67	1.66	1.65	1.64	1.64	suitable
C6	1.5	3.7	2.8	0.004	700	4	W10/400	18.2	15.6	13.5	11.4	9.8	suitable
C7	1.5	1.5	3.0	0.004	750	3.6	B50	1.68	1.68	1.67	1.66	1.65	suitable
C7	1.5	1.5	3.0	0.004	750	3.6	W10/400	19.3	16.5	14.1	11.7	10	suitable

*not inventive

As is apparent from Table 6, steels C2 ~ C7 were inventive examples that satisfy Condition (2), wherein the magnetic flux density was high and the core loss was low. This is considered to be because the composition according to the present invention had coarsely grown inclusions and the distribution density of coarse composite inclusions was greater than 0.02(1/mm²), and also the texture was stable. The radio-frequency core loss (W10/400) is surely correlated with the thickness of steel sheet. Specifically, as the thickness of the steel sheet decreases, the properties thereof may be improved. Compared to the steel sheet having a thickness of 0.35 mm, the core loss of the steel sheet having a thickness of 0.15 mm was improved by about 50%. In steel C1, Al+Mn and Al/Si did not satisfy Condition (2), and thus core loss (W10/400) and magnetic flux density (B50) were deteriorated.

[Example 4]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 7 below. As such, 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si, Mn and P were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec.
The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density, the Erichsen number and hardness were measured. The results are shown in Table 8 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

While the welding part of the hot rolled sheet was pushed-up using a steel ball having a diameter of 20 mm at room temperature, the height until the sheet broken was determined, which is referred to as the Erichsen number. The case where the Erichsen number is typically 5mm or more makes it possible to produce continuous lines via welding of the same species.

[Table 7]

Steel	Al	Si	Mn	P	Sn	Sb	C	S	N	Ti
D1*	1.0	2.5	0.5	0.01	-	-	0.002	0.002	0.002	0.002
D2	2.5	0.8	0.8	0.11	0.03	-	0.002	0.002	0.002	0.002
D3	2.0	1.3	0.8	0.08	-	0.005	0.002	0.002	0.002	0.002
D4	2.0	1.3	0.8	0.08	-	0.03	0.002	0.002	0.002	0.002
D5	2.0	1.3	0.8	0.08	-	0.07	0.002	0.002	0.002	0.002
D6	2.0	1.3	0.8	0.08	-	0.1	0.002	0.002	0.002	0.002
D7*	2.0	1.3	0.8	0.08	-	0.15	0.002	0.002	0.002	0.002
D8	1.7	1.6	0.8	0.08	0.005	-	0.002	0.002	0.002	0.002
D9	1.7	1.6	0.8	0.08	0.03	-	0.002	0.002	0.002	0.002
D10	1.7	1.6	0.8	0.08	0.07	-	0.002	0.002	0.002	0.002
D11	1.7	1.6	0.8	0.08	0.15	-	0.002	0.002	0.002	0.002
D12	1.7	1.6	0.8	0.08	0.18	-	0.002	0.002	0.002	0.002
D13*	1.7	1.6	0.8	0.08	0.25	-	0.002	0.002	0.002	0.002
D14	1.3	2.0	0.8	0.08	0.03	-	0.002	0.002	0.002	0.002
D15	2.2	1.6	0.6	0.05	-	0.03	0.002	0.002	0.002	0.002
D16*	2.2	1.6	0.6	0.05	0.23	-	0.002	0.002	0.002	0.002
D17	1.5	1.0	1.2	0.19	0.05	-	0.002	0.002	0.002	0.002
D18*	1.5	1.0	1.2	0.19	-	0.2	0.002	0.002	0.002	0.002

*not inventive

[Table 8]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) / (N+S)	Al+Si +Mn/2	Size of Inclusion (nm)	Distri. Density of Inclusion (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Erichsen (mm)	Hard.	Note
D1*	0.4	2	1.5	0.004	375	3.75	50	0	2.2	1.66	3	204	Comp.
D2	3.1	3.1	3.3	0.004	825	3.7	600	0.06	2.1	1.67	7	163	Invent.
D3	1.5	2.5	2.8	0.004	700	3.7	500	0.04	1.9	1.68	7	171	Invent.
D4	1.5	2.5	2.8	0.004	700	3.7	540	0.04	1.9	1.68	9	168	Invent.
D5	1.5	2.5	2.8	0.004	700	3.7	600	0.07	1.9	1.68	11	175	Invent.
D6	1.5	2.5	2.8	0.004	700	3.7	650	0.09	1.9	1.68	8	172	Invent.
D7*	1.5	2.5	2.8	0.004	700	3.7	450	0.03	2.1	1.66	4	180	Comp.
D8	1.1	2.1	2.5	0.004	625	3.7	650	0.06	2.1	1.68	8	174	Invent.
D9	1.1	2.1	2.5	0.004	625	3.7	500	0.05	2.0	1.68	10	175	Invent.
D10	1.1	2.1	2.5	0.004	625	3.7	600	0.08	1.9	1.68	11	177	Invent.
D11	1.1	2.1	2.5	0.004	625	3.7	700	0.05	2.0	1.68	9	174	Invent.
D12	1.1	2.1	2.5	0.004	625	3.7	650	0.04	2.0	1.68	7	179	Invent.
D13*	1.1	2.1	2.5	0.004	625	3.7	300	0.02	2.2	1.68	3	180	Comp.
D14	0.7	1.6	2.1	0.004	525	3.7	400	0.03	2.0	1.68	8	183	Invent.
D15	1.4	3.7	2.8	0.004	700	4.1	800	0.12	2.1	1.66	9	178	Invent.
D16*	1.4	3.7	2.8	0.004	700	4.1	350	0.03	2.2	1.67	4	185	Comp.
D17	1.5	1.3	2.7	0.004	675	3.1	550	0.07	2.1	1.69	12	165	Invent.
D18*	1.5	1.3	2.7	0.004	675	3.1	300	0.02	2.2	1.68	4	170	Comp.

As is apparent from Table 8, steels D2∼6, D8∼12, D14, D15 and D17 were inventive examples which satisfy Condition (2) and in which 0.005 ∼ 0.2% of Sn or 0.005 ∼ 0.1% of Sb is added, and thus, the distribution density of coarse inclusions having a size of 300 nm or more was greater than 0.02(1/mm²), and upon final annealing, the oxide layer and the nitride layer of the surface were reduced thus improving core loss and magnetic flux density. Also, the Erichsen number was high and the Vickers hardness (Hv1) was low, thus exhibiting superior weldability, productivity and customer punchability.
Whereas in steel D1, the ratio of Al/Si fell outside Condition (2), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. Also because Sn and Sb were not added, the Erichsen number was low and weldability was decreased and hardness was high undesirably deteriorating processability. In steels D7 and D18, the amount of Sb exceeded 0.1%, and in steels D13 and D16, the amount of Sn exceeded 0.2%, and thus the Erichsen number was low and hardness was high, resulting in decreased weldability, poor productivity and customer punchability and inferior magnetic properties.

[Example 5]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 9 below. As such, 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.50 mm, followed by carrying out final annealing at 900°C for 30 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 10 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 9]

Steel	Al	Si	Mn	C	S	N	Ti
E1	1.5	0.2	0.2	0.002	0.002	0.002	0.002
E2	1.5	0.2	0.5	0.002	0.002	0.002	0.002
E3	0.7	0.2	0.5	0.002	0.002	0.002	0.002
E4	2.7	0.5	0.3	0.002	0.002	0.002	0.002
E5	1.7	0.5	0.3	0.002	0.002	0.002	0.002
E6	0.7	0.5	0.3	0.002	0.002	0.002	0.002
E7	0.5	0.5	0.5	0.002	0.002	0.002	0.002
E8	0.5	0.5	0.5	0.002	0.002	0.002	0.002
E9	2.2	0.5	0.2	0.002	0.002	0.002	0.002
E10	1.2	0.5	0.2	0.002	0.002	0.002	0.002
E11	1.0	0.1	0.2	0.002	0.002	0.002	0.002
E12	1.2	0.2	0.2	0.002	0.002	0.002	0.002
E13	1.0	0.2	0.2	0.002	0.002	0.002	0.002
E14	2.2	0.7	0.2	0.002	0.002	0.002	0.002
E15	0.7	0.7	0.2	0.002	0.002	0.002	0.002
E16	1.3	0.2	0.7	0.002	0.002	0.002	0.002
E17	1.5	0.2	1.0	0.002	0.002	0.002	0.002
E18	1.2	0.2	1.0	0.002	0.002	0.002	0.002
E19	0.9	0.5	1.0	0.002	0.002	0.002	0.002
E20	0.9	0.7	0.8	0.002	0.002	0.002	0.002
E21	1.0	0.5	0.8	0.002	0.002	0.002	0.002

[Table 10]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
E1	7.5	7.5	1.7	0.0040	425	1.8	450	0.40	3.2	1.73	140	suitable
E2	7.5	3.0	2.0	0.0040	500	2.0	500	0.35	3.0	1.73	140	suitable
E3	3.5	1.4	1.2	0.0040	300	1.2	300	0.30	4.0	1.74	110	suitable
E4*	5.4	9.0	3.0	0.0040	750	3.4	250	0.01	3.0	1.68	157	unsuitable
E5*	3.4	5.7	2.0	0.0040	500	2.4	250	0.01	2.9	1.69	145	unsuitable
E6	1.4	2.3	1.0	0.0040	250	1.4	450	0.05	3.5	1.74	115	suitable
E7*	1.0	1.0	1.0	0.0040	250	1.3	50	0.01	4.5	1.74	110	unsuitable
E8*	1.0	1.0	1.0	0.0040	250	1.3	75	0.01	4.5	1.74	110	unsuitable
E9*	4.4	11.0	2.4	0.0040	600	2.8	400	0.01	2.8	1.68	150	unsuitable
E10	2.4	6.0	1.4	0.0040	350	1.8	600	0.15	3.2	1.73	130	suitable
E11*	10	5.0	1.2	0.0040	300	1.2	250	0.01	4.5	1.74	105	unsuitable
E12	6.0	6.0	1.4	0.0040	350	1.5	400	0.20	3.5	1.74	105	suitable
E13	5.0	5.0	1.2	0.0040	300	1.3	300	0.18	3.6	1.74	110	suitable
E14*	3.1	11.0	2.4	0.0040	600	3.0	400	0.01	2.8	1.69	160	unsuitable
E15*	1.0	3.5	0.9	0.0040	225	1.5	150	0.01	3.9	1.74	130	unsuitable
E16	6.5	1.9	2.0	0.0040	500	1.9	350	0.25	2.9	1.72	130	suitable
E17*	7.5	1.5	2.5	0.0040	625	2.2	250	0.01	2.8	1.69	140	unsuitable
E18*	6.0	1.2	2.2	0.0040	550	1.9	250	0.01	2.9	1.70	130	unsuitable
E19*	1.8	0.9	1.9	0.0040	475	1.9	200	0.01	3.2	1.70	135	unsuitable
E20	1.3	1.1	1.7	0.0040	425	2.0	350	0.05	3.5	1.73	140	suitable
E21	2.0	1.3	1.8	0.0040	450	1.9	400	0.05	3.3	1.73	140	suitable

*not inventive

As is apparent from Table 10, steels E1∼E3, E6, E10, E12, E13, E16, E20 and E21 were inventive examples that satisfy Condition (1), wherein the coarse inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02(1/mm²) thus exhibiting superior magnetic properties, and the Vickers hardness (Hv1) was 140 or less, resulting in good productivity and customer punchability.
Whereas in steels E4, E9 and E14, the ratio of Al/Mn and the amount of Al+Mn fell outside Condition (1) and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steels E17 and E18, the amount of Al+Mn did not satisfy Condition (1), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steel E19, the ratio of Al/Mn did not satisfy Condition (1), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steels E4, E5, E9 and E14, Al+Si+Mn/2 did not satisfy Condition (1), and thus hardness was high thereby obtaining poor productivity and punchability.

[Example 6]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 11 below. As such, 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.3 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.50 mm, followed by carrying out final annealing at 900°C for 30 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 12 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 11]

Steel	Al	Si	Mn	C	S	N	Ti
F1	1.0	0.5	0.3	0.0030	0.0010	0.0010	0.0020
F2*	0.7	0.3	0.2	0.0030	0.0030	0.0030	0.0020
F3	0.7	0.3	0.5	0.0030	0.0020	0.0030	0.0020
F4	0.7	0.5	0.3	0.0030	0.0010	0.0025	0.0020
F5*	1.0	0.3	0.7	0.0030	0.0005	0.0005	0.0020
F6	1.0	0.3	0.7	0.0030	0.0040	0.0020	0.0020
F7*	1.2	0.5	1.0	0.0030	0.0020	0.0020	0.0020
F8	1.2	0.2	0.3	0.0030	0.0015	0.0010	0.0020
F9	0.9	0.5	0.8	0.0030	0.0020	0.0020	0.0020
F10*	0.9	0.5	0.8	0.0030	0.0040	0.0030	0.0020
F11	0.9	0.5	0.5	0.0030	0.0030	0.0030	0.0020
F12	0.9	0.5	0.5	0.0030	0.0020	0.0025	0.0020
F13*	0.9	0.5	0.5	0.0030	0.0005	0.0005	0.0020

*not inventive

[Table 12]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
F1	2.0	3.3	1.3	0.0020	650	1.7	350	0.150	3.2	1.73	135	suitable
F2*	2.3	3.5	0.9	0.0060	150	1.1	200	0.010	4.2	1.71	130	unsuitable
F3	2.3	1.4	1.2	0.0050	240	1.3	300	0.200	3.5	1.74	130	suitable
F4	1.4	2.3	1	0.0035	286	1.4	450	0.050	3.4	1.73	130	suitable
F5*	3.3	1.4	1.7	0.0010	1700	1.7	50	0.010	3.5	1.69	140	unsuitable
F6	3.3	1.4	1.7	0.0060	283	1.7	350	0.200	3.2	1.74	140	suitable
F7*	2.4	1.2	2.2	0.0040	550	2.2	250	0.010	2.9	1.68	140	unsuitable
F8	6.0	4.0	1.5	0.0025	600	1.6	450	0.070	3.3	1.74	140	suitable
F9	1.8	1.1	1.7	0.0040	425	1.8	550	0.080	3.1	1.73	135	suitable
F10*	1.8	1.1	1.7	0.0070	243	1.8	250	0.010	3.5	1.69	135	unsuitable
F11	1.8	1.8	1.4	0.0060	233	1.7	500	0.150	3.2	1.73	135	suitable
F12	1.8	1.8	1.4	0.0045	311	1.7	600	0.180	3.2	1.74	135	suitable
F13*	1.8	1.8	1.4	0.0010	1400	1.7	50	0.018	3.7	1.72	135	unsuitable

*not inventive

As is apparent from Table 12, steels F1, F3, F4, F6, F8, F9, F11 and F12 were inventive examples that satisfy Condition (1), wherein the coarse inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02(1/mm²) thus exhibiting superior magnetic properties, and hardness was low, resulting in good productivity and customer punchability.
Whereas in steels F5, F10 and F13, the amount of N+S fell outside Condition (1) and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steel F7, the amount of Al+Mn did not satisfy Condition (1), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated.

[Example 7]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 13 below. As such, 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 14 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 13]

Steel	Al	Si	Mn	C	S	N	Ti
G1	3.0	2.3	1.0	0.002	0.002	0.002	0.002
G2	2.5	1.7	1.0	0.002	0.002	0.002	0.002
G3	1.0	2.3	1.0	0.002	0.002	0.002	0.002
G4	1.5	2.3	0.8	0.002	0.002	0.002	0.002
G5	2.0	2.7	0.8	0.002	0.002	0.002	0.002
G6	1.0	2.7	0.8	0.002	0.002	0.002	0.002
G7	0.5	2.7	0.8	0.002	0.002	0.002	0.002
G8	3.5	3.0	0.8	0.002	0.002	0.002	0.002
G9	2.5	3.0	0.8	0.002	0.002	0.002	0.002
G10	1.5	3.0	1.0	0.002	0.002	0.002	0.002
G11	3.0	3.2	1.0	0.002	0.002	0.002	0.002
G12	1.5	3.2	1.0	0.002	0.002	0.002	0.002
G13	3.0	2.5	1.0	0.002	0.002	0.002	0.002
G14	2.5	2.5	1.0	0.002	0.002	0.002	0.002
G15	1.0	2.5	1.0	0.002	0.002	0.002	0.002

[Table 14]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
G1*	1.3	3.0	4.0	0.0040	1000	5.8	250	0.01	2.0	1.62	225	unsuitable
G2*	1.5	2.5	3.5	0.0040	875	4.7	200	0.01	2.3	1.63	195	unsuitable
G3	0.4	1.0	2	0.0040	500	3.8	300	0.10	2.2	1.67	200	suitable
G4	0.7	1.9	2.3	0.0040	575	4.2	400	0.20	2.2	1.66	205	suitable
G5	0.7	2.5	2.8	0.0040	700	5.1	500	0.15	2.0	1.67	200	suitable
G6	0.4	1.3	1.8	0.0040	450	4.1	450	0.09	2.1	1.66	195	suitable
G7*	0.2	0.6	1.3	0.0040	325	3.6	50	0.01	2.5	1.66	190	unsuitable
G8*	1.2	4.4	4.3	0.0040	1075	6.9	75	0.01	2.0	1.62	230	unsuitable
G9	0.8	3.1	3.3	0.0040	825	5.9	400	0.25	2.1	1.66	220	suitable
G10	0.5	1.5	2.5	0.0040	625	5.0	600	0.10	2.1	1.67	225	suitable
G11*	0.9	3.0	4.0	0.0040	1000	6.7	250	0.005	2.3	1.62	230	unsuitable
G12	0.5	1.5	2.5	0.0040	625	5.2	400	0.15	2.0	1.66	220	suitable
G13*	1.2	3.0	4.0	0.0040	1000	6.0	75	0.01	2.0	1.62	220	unsuitable
G14	1.0	2.5	3.5	0.0040	875	5.5	400	0.10	2.1	1.64	225	suitable
G15	0.4	1.0	2.0	0.0040	500	4.0	350	0.15	2.1	1.67	210	suitable

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
G1*	1.3	3.0	4.0	0.0040	1000	5.8	250	0.01	2.0	1.62	225	Comp.
G2*	1.5	2.5	3.5	0.0040	875	4.7	200	0.01	2.3	1.63	195	Comp.
G3	0.4	1.0	2	0.0040	500	3.8	300	0.10	2.2	1.67	200	Invent.
G4	0.7	1.9	2.3	0.0040	575	4.2	400	0.20	2.2	1.66	205	Invent.
G5	0.7	2.5	2.8	0.0040	700	5.1	500	0.15	2.0	1.67	200	Invent.
G6	0.4	1.3	1.8	0.0040	450	4.1	450	0.09	2.1	1.66	195	Invent.
G7*	0.2	0.6	1.3	0.0040	325	3.6	50	0.01	2.5	1.66	190	Comp.
G8*	1.2	4.4	4.3	0.0040	1075	6.9	75	0.01	2.0	1.62	230	Comp.
G9	0.8	3.1	3.3	0.0040	825	5.9	400	0.25	2.1	1.66	220	Invent.
G10	0.5	1.5	2.5	0.0040	625	5.0	600	0.10	2.1	1.67	225	Invent.
G11*	0.9	3.0	4.0	0.0040	1000	6.7	250	0.005	2.3	1.62	230	Comp.
G12	0.5	1.5	2.5	0.0040	625	5.2	400	0.15	2.0	1.66	220	Invent.
G13*	1.2	3.0	4.0	0.0040	1000	6.0	75	0.01	2.0	1.62	220	Comp.
G14	1.0	2.5	3.5	0.0040	875	5.5	400	0.10	2.1	1.64	225	Invent.
G15	0.4	1.0	2.0	0.0040	500	4.0	350	0.15	2.1	1.67	210	Invent.

*not inventive

As is apparent from Table 14, steels G3∼G6, G9, G10, G12, G14 and G15 were inventive examples that satisfy Condition (3), wherein the coarse inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02(1/mm²) thus exhibiting superior magnetic properties, and the Vickers hardness was as low as 225 or less.
Whereas in steels G1, G8, G11 and G13, the amount of Al+Mn fell outside Condition (3) and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steel G2, the ratio of Al/Si did not satisfy Condition (3), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steel G7, Al/Si, Al/Mn, and Al+Mn did not satisfy Condition (3), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steels G8 and G11, Al+Si+Mn/2 did not satisfy Condition (3), and thus hardness was high, thereby deteriorating productivity and punchability.

[Example 8]

Vacuum melting was performed in a laboratory, thus preparing steel ingots having the components shown in Table 15 below. As such, 0.3 ∼ 0.5% of Al was added to molten steel to facilitate the formation of inclusions, after which the remainder of Al, and Si and Mn were added thus making steel ingots. Each of the ingots was heated to 1,150°C, and finish hot rolled at 850°C thus manufacturing a hot rolled sheet having a thickness of 2.0 mm. The hot rolled sheet was annealed at 1,050°C for 4 min and then pickled. Subsequently, cold rolling was conducted so that the thickness of the sheet was 0.35 mm, followed by carrying out final annealing at 1,050°C for 38 sec.

The size and distribution density of inclusions of respective sheets, the core loss, the magnetic flux density and hardness were measured. The results are shown in Table 16 below. A sample for observing the inclusions was manufactured using a replica method that is typical in the steel industry, and a transmission electron microscope was used therefor. As such, the acceleration voltage of 200 kV was applied.

[Table 15]

Steel	Al	Si	Mn	C	S	N	Ti
H1	1.0	2.3	0.5	0.0030	0.0010	0.0010	0.0020
H2*	1.0	2.3	0.5	0.0030	0.0030	0.0030	0.0020
H3	1.0	2.5	1.0	0.0030	0.0020	0.0030	0.0020
H4	1.2	2.5	1.2	0.0030	0.0015	0.0020	0.0020
H5*	1.2	2.7	1.0	0.0030	0.0005	0.0005	0.0020
H6	1.2	2.7	1.0	0.0030	0.0020	0.0040	0.0020
H7*	2.0	2.7	2.0	0.0030	0.0020	0.0020	0.0020
H8	2.0	3.2	1.5	0.0030	0.0010	0.0015	0.0020
H9	2.0	3.2	1.5	0.0030	0.0020	0.0020	0.0020
H10*	2.0	3.2	1.0	0.0030	0.0030	0.0040	0.0020
H11	2.0	3.2	1.5	0.0030	0.0030	0.0030	0.0020
H12	1.5	3.5	1.5	0.0030	0.0020	0.0025	0.0020
H13*	2.5	3.5	1.0	0.0030	0.0005	0.0005	0.0020

[Table 16]

Steel	Al /Si	Al /Mn	Al +Mn	N+S	(Al +Mn) /(N+S)	Al+Si +Mn/2	Size of Inclusions (nm)	Distri. Density of Inclusions (1/mm²)	Core Loss (W15 /50)	Magnetic Flux Density (B50)	Hard.	Note
H1	0.4	2.0	1.5	0.0020	750	3.6	350	0.15	2.2	1.67	190	suitable
H2*	0.4	2.0	1.5	0.0060	250	3.6	75	0.01	2.3	1.65	190	unsuitable
H3	0.4	1.0	2	0.0050	400	4.0	400	0.20	2.1	1.67	190	suitable
H4	0.5	1.0	2.4	0.0035	686	4.3	450	0.08	2.1	1.67	195	suitable
H5*	0.4	1.2	2.2	0.0010	2200	4.4	50	0.01	2.3	1.65	200	unsuitable
H6	0.4	1.2	2.2	0.0060	367	4.4	350	0.20	2.2	1.67	200	suitable
H7*	0.7	1.0	4.0	0.0040	1000	5.7	250	0.01	2.1	1.63	220	unsuitable
H8	0.6	1.3	3.5	0.0025	1400	6.0	450	0.12	2.0	1.65	225	suitable
H9	0.6	1.3	3.5	0.0040	875	6.0	550	0.09	2.0	1.65	225	suitable
H10*	0.6	2.0	3.0	0.0070	429	5.7	250	0.01	2.2	1.63	220	unsuitable
H11	0.6	1.3	3.5	0.0060	583	6.0	500	0.15	2.0	1.65	225	suitable
H12	0.4	1.0	3	0.0045	667	5.8	600	0.20	2.1	1.65	225	suitable
H13*	0.7	2.5	3.5	0.0010	3500	6.5	50	0.01	2.1	1.62	225	unsuitable

*not inventive

As is apparent from Table 16, steels H1, H3, H4, H6, H8, H9, H11 and H12 were inventive examples that satisfy Condition (3), wherein the coarse inclusions having a size of 300 nm or more were observed, and the distribution density thereof was greater than 0.02 (1/mm²) thus exhibiting superior magnetic properties.
Whereas in steels H5, H10 and H13, N+S did not satisfy Condition (3) and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steel H7, Al+Mn did not satisfy Condition (3), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated. In steels H2, H5 and H13, (Al+Mn)/(N+S) did not satisfy Condition (3), and thus inclusions having a size of 300 nm or more were not observed, and core loss and magnetic flux density were deteriorated.

Claims

A non-oriented electrical steel sheet having superior magnetic properties, comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt%, and satisfying Condition (2) below,
wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet, and a distribution density of the inclusion having an average size of 300 nm or more is equal to or greater than 0.02 number/mm² ; and $\begin{array}{l} \{[Al] + [Mn]\} \leq 3.5, \\ 0.002 \leq \{[N] + [S]\} \leq 0.006, \\ 300 \leq \{([Al] + [Mn]) / ([N] + [S])\} \leq 1,400; \end{array}$
wherein [Al], [Si], [Mn], [N] and [S] indicate amounts (wt%) of Al, Si, Mn, N and S, respectively.
The non-oriented electrical steel sheet of claim 1, wherein the amounts of Al and Mn satisfy Relation (2) below. $1 \leq [Al] / [Mn] \leq 8$
The non-oriented electrical steel sheet of claim 1, which satisfies Condition (2) and wherein the amounts of Al, Si and Mn satisfy Relation (3) below. $1.7 \leq \{[Al] + [Si] + [Mn] / 2\} \leq 5.5$
The non-oriented electrical steel sheet of claim 1, which satisfies Condition (2) and wherein the amounts of Al and Si satisfy Relation (4) below. $0.6 \leq [Al] / [Si] \leq 4.0$
The non-oriented electrical steel sheet of claim 3, wherein a cross-sectional Vickers hardness (Hv1) is 190 or less.
A method of producing a non-oriented electrical steel sheet having superior magnetic properties, comprising subjecting a slab comprising 1.0 ∼ 3.0% of Al, 0.5 ∼ 2.5% of Si, 0.5 ∼ 2.0% of Mn, 0.001 ∼ 0.004% of N, 0.0005 ∼ 0.004% of S, 0.004% or less of C, optionally 0.2% or less of P and/or at least one of 0.005 ∼ 0.2% of Sn and 0.005 ∼ 0.1% of Sb and a balance of Fe and other inevitable impurities by wt% and satisfying Condition (2) below to heating, hot rolling, cold rolling, and final annealing at 750 ∼ 1100°C:
wherein an inclusion comprising a nitride and a sulfide alone or a combination thereof is formed in the steel sheet subjected to final annealing, and a distribution density of the inclusion having an average size of 300 nm or more is equal to or greater than 0.02 number/mm²: $\begin{array}{l} \{[Al] + [Mn]\} \leq 3.5, \\ 0.002 \leq \{[N] + [S]\} \leq 0.006, \\ 300 \leq \{([Al] + [Mn]) / ([N] + [S])\} \leq 1,400; \end{array}$
and
wherein [Al], [Si], [Mn], [N] and [S] indicate amounts (wt%) of Al, Si, Mn, N and S, respectively.
The method of claim 6, wherein the amounts of Al and Mn satisfy Relation (2) below. $1 \leq [Al] / [Mn] \leq 8$
The method of claim 6, wherein the slab satisfies Condition (2) and the amounts of Al, Si and Mn satisfy Relation (3) below. $1.7 \leq \{[Al] + [Si] + [Mn] / 2\} \leq 5.5$
The method of claim 6, wherein the slab satisfies Condition (2) and the amounts of Al and Si satisfy Relation (4) below. $0.6 \leq [Al] / [Si] \leq 4.0$
The method of any one of claims 6 to 9 , wherein the slab is prepared by adding 0.3 ∼ 0.5% of Al to perform deoxidation, adding remaining alloy elements, and maintaining a temperature at 1,500 ∼ 1,600°C.
The method of any one of claims 6 to 9, wherein annealing of a hot rolled sheet is performed between the hot rolling and the cold rolling.